Ink Jetting of Photochromic Ink Thesis

Thesis for the Degree of Doctor of Technology Ink Jetting of Photochromic Ink Towards the Design of a Smart Textile Sensor Sina Seipel S #$"$ N VA ENMÄRK Trycksak 3041 0234 E T %EB&<KK@E>F=,?FKF:?IFD@:%EBa0FN8I;JK?< <J@>EF=8/D8IK0<OK@C</<EJFI .<J<8I:?/:?FFCF=0<OK@C<J8E;"8J?@FE "8:LCKPF=0<OK@C<J !E>@E<<I@E>8E;LJ@E<JJ 1E@M<IJ@KPF=FITJ FGPI@>?KZ/@E8/<@G<C ,I@EK<;@E/N<;<E9P/K<D8/G<:@8CKIP:B FITJ KIP:BK G;= %//* 4 /BI@=K<I=ITE$W>JBFC8E@FITJ EI !C<:KIFE@:M<IJ@FE?KKG LIE B9 J< I<JFCM<LIELIEE9EJ<?9;@M8 /D8IK K<OK@C<J ?8M< 9<<E 8 ?FK KFG@: @E I<J<8I:? =FI J<M<I8C ;<:8;<J ?FN<M? F= JD8IK 8E; =LE:K@FE8C K<OK@C<J N?@:? 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N@:?K@>@D(<9<E@JKLE;;8JJD8EE@:?KM<JJ<E;8I=;<E@EE<I<E==<EI8LJQLC8JJ<E @@ Och sist men inte minst, tack Benjamin, min bästa vän, älskade man och världens finaste papa till Ava att du har stått ut med mig senaste åren och trott på mig i de tuffaste stunderna. Tack att du har dagligen gett mig mening och sett till att ha kul i livet. iii List of Appended Papers $"%$ $ !! !"# Paper I &5*6 S. Seipel, J. Yu and V. A. Nierstrasz, (2020) Influence of physical parameters and temperature on ink jetting of UV-curable photochromic inks for smart textile applications. Manuscript submitted /@E8 /<@G<C GC8EE<; K?< <OG<I@D<EKJ N@K? K?< :F 8LK?FIJ :FE;L:K<; 8CC <OG<I@D<EKJ 8E;NIFK<K?<D8@EG8IKF=K?<8IK@:C< =@E8C@Q@E>@KN@K?K?<:F 8LK?FIJ Paper II /@E8 /<@G<C GC8EE<; K?< <OG<I@D<EKJ N@K? K?< :F 8LK?FIJ :FE;L:K<; 8CC <OG<I@D<EKJ 8E;NIFK<K?<D8@EG8IKF=K?<8IK@:C< =@E8C@Q@E>@KN@K?K?<:F 8LK?FIJ S. Seipel, J. Yu, A. Periyasamy, M. Viková, M. Vik and V. A. Nierstrasz, (2018) Inkjet printing and UV-LED curing of photochromic dyes for functional and smart textile applications, RSC Advances, 8, 28395-28404. Paper III S. Seipel, J. Yu, M. Viková, M. Vik, M. Koldinská, A. Havelka and V. A. Nierstrasz, (2019) Colour performance, durability and handle of inkjet-printed and UV-cured photochromic textiles for multi-coloured applications, Fibers and Polymers, 20, 14241435. Paper IV/Book chapter S. Seipel, J. Yu, A. Periyasamy, M. Viková, M. Vik and V. A. Nierstrasz, (2018) Resource-efficient production of a smart textile UV sensor using photochromic dyes: Characterization and optimization, in: Prof. Dr. Yordan Kyosev et al. (Ed.) Narrow and Smart Textiles, Springer Publishing Company, pp. 251-257. Paper V J. Yu, S. Seipel and V.A. Nierstrasz, (2018) Digital inkjet functionalization of waterrepellent textile for smart textile application, Journal of Materials Science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aper VI M. Tadesse Abate, S. Seipel, J. Yu, M. Viková, M. Vik, A. Ferri, G. Jinping, G. Chen, and V. A. Nierstrasz, (2020) Supercritical CO2 dyeing of polyester fabric with photochromic dyes to fabricate UV sensing smart textiles. 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M@@ Figure 16: Absorbance spectra of photo-initiator TPO-L, Ruby red in ethyl acetate and Sea green in hexane. ............................................................................................ 40 Figure 17: Viscosity of monomer-oligomer mixtures with ratio 7:1 at a shear rate of 10000 s-1 and 20°C. ............................................................................................. 41 Figure 18: Surface tension of monomer-oligomer mixtures with ratio 7:1 measured at 20°C. .................................................................................................................... 42 Figure 19: Influence of different ratios of ink components on the surface tension (¿) and the viscosity (¢) of the ink formulations at 20 °C. ...................................... 43 Figure 20: Viscosity of RR ink, SG ink and ink varnish as function of (a) shear rate from 1 to 10000 s-1 at 20 °C and of (b) temperature from 20 to 40 °C at 10000 s-1. ............................................................................................................................. 44 Figure 21: Drop formation upon jetting at 35 °C between 50 and 200 μs of (a) RR ink and (b) SG ink. .................................................................................................... 46 Figure 22: Comparison of FTIR spectra from reference plain-woven PET fabric and inkjet-printed plain-woven PET fabric with RR ink. .......................................... 47 Figure 23: FTIR spectra with cleavage of the C=C double bond at 1619 and 1636 cm-1 for liquid ink compared to a weak and strong curing setting with UVA light and UV LED light, respectively. ................................................................................ 47 Figure 24: Melting temperature Tm as function of lamp intensity during UV curing with belt speed of 50 mm s-1 (¢) and 300 mm s-1 () of unwashed 10-pass prints. ... 48 Figure 25: Melting temperature Tm of unprinted PET treated with different intensities () of UV-LED light and at different distances between the conveyor belt and the UV lamp (p) compared to untreated PET (¢). .................................................. 49 Figure 26: Storage modulus as function of frequency of PET reference fabric and prints cured with different intensities on PET ............................................................... 50 Figure 27: Coloration data (n) and first-order curve fitting (–) according to Equation 8 of a photochromic print with 19 g m-2 deposited ink, cured at belt speed of 50 mm s-1 and 1% lamp intensity in cycle 1 with a fitting value R2 < 0.45. ................... 51 Figure 28: Activation cycles 1-5 (i-v) of a print with 19 g m-2 deposited ink cured at 300 mm s-1 and 1%. Measured K/S (■) includes decay (– ·) and coloration (–). Calculated sum of decay and coloration (– –) validates the model. 133 ............... 53 Figure 29: (a) Color yield ΔK/S, (b) coloration rate constant kcol, (c) decoloration rate constant kdecol of printed (■) and washed (£) RR prints as function of printing passes and (d) Inactive (I), printed (P) and washed (W) RR prints with 1, 5 and 10 printing passes cured at 50 mm s-1 and 80%. ...................................................... 55 Figure 30: Schematic molecular rearrangement of photochromic dyes upon ringopening and closing. ............................................................................................ 56 Figure 31: Change in kcol before and after washing of RR prints as function of curing intensity with low (,), intermediate (p,r) and strong (¢, £) curing. ....... 56 Figure 32: Effects of the factors curing intensity (¢), dye type () and treatment (p) on (a) ΔK/S, (b) kcol and (c) kdecol.142 ................................................................... 58 Figure 33: Effect of abrasion cycles 5000, 10000, 15000 and 20000 on ΔK/S of RR (a) and SG (b) prints cured at 50 mm s-1 and 80% (50_80) and 300 mm s-1 and 1% (300_1) on and on kcol of RR (c) and SG (d) prints cured at 50 mm s-1 and 80% (50_80) and 300 mm s-1 and 1% (300_1).142 ....................................................... 59 Figure 34: Effect of 0, 1 and 10 washing cycles on ΔK/S, kcol and kdecol of (a) RR prints cured at 50 mm s-1 and 80% (█,■,□) and 300 mm s-1 and 1% (█,▲,∆) and of (b) SG prints cured at 50 mm s-1 and 80% (█,■,□) and 300 mm s-1 and 1% (█,▲,∆).142 ............................................................................................................................. 60 Figure 35: PET fabric printed with 19 g m-2 of RR ink cured at 100 mm s-1 belt speed and 25% lamp intensity (a, c) compared to PET reference (b, d). 142.................. 62 Figure 36: Fitted curves of K/S values as function of time of the coloration and decoloration reaction of PET dyed with spirooxazine SG and naphthopyran RR (I) before and (II) after washing. .............................................................................. 63 Figure 37: Color difference ΔE for stabilized samples with whey powder, wax and HALS before and after washing. ......................................................................... 65 Figure 38: Color difference ΔE for impregnated samples with amylose before and after washing. ............................................................................................................... 65 Figure 39: Color difference ΔE of stabilized prints over multiple days of activation before washing..................................................................................................... 66 Figure 40: Color difference ΔE of stabilized prints over multiple days of activation after washing. ............................................................................................................... 66 viii ix $$ '786&(8 . 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At that time photochromic materials experienced a sudden increase of scientific interest, along with the development of physical methods (IR, NMR, Xray, UV, time-resolved and flash spectroscopy) and organic synthesis 57. However, it was not until the 1980s where the development of more fatigue-resistant compounds have made applications such as ophthalmic lenses possible. Photochromism is a reversible transformation of a chemical species between two isomers upon photoirradiation 59. As illustrated in Figure 1, the two forms, ring-closed form A and ringopened form B, show different absorption spectra. The activation occurs through highenergy photons hv1 from the near UV range. Due to the changes in the electron density, material B is able to absorb low-energy photons hv2 from the visible part of the electromagnetic spectrum giving the coloration. 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'*!+& &!+%+ -(+(+(+ !!$!! Figure 1: Change of absorbance spectra upon isomerization between colorless form A and colored form B The photochromic effect, i.e. a visible color change of organic photochromic compounds results through the stimulation of UV light. UV light activates the compounds from their thermodynamically stable form A to form B; isomerization occurs. There are various forms of isomerization, which determine the photochromic process depending on the photochromic compound. The categories of isomerization imply heterolytic cleavage, homolytic cleavage, triplet-triplet isomerization, trans-cis isomerization, tautomerism and photo-dimerization 35, 40, 55. The photochromic reaction of pyrans follows heterolytic cleavage, where breakage of the covalent bond of the excited molecule results in conformations with quinoidal or zwitterionic structures 6062 . 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I<JLCKJ@EJKIFE>:FCFIP@<C; @@C<8JKJFCM<EKKFO@:@KPGFJJ@9C<8E;@@@JK89@C@KPKFN8I;J K<DG<I8KLI<=FI8GGC@:8K@FE@EN<8I89C<J 0?<G?FKF:?IFD@:K<OK@C<JJ?FLC;J?FNJK89C< :FCFI8K@FE@E8GI8:K@:8CI8E><F=K<DG<I8KLI<8E;<JG<:@8CCP8IFLE; YKF9<JL@K89C< @EN<8I89C<8GGC@:8K@FEJ @=;<J@I<; "@>LI<J?FNJK?<89JFI98E:<F==@M<:FDD<I:@8C F7B7C +!&', 9-*$*&!+ 0?<:FD9@E8K@FEF=:FDGFE<EKJ@EK?<12 :LI89C<M8IE@J?N8JJ<C<:K<;98J<;FEK?<@I G?PJ@:8CGIFG<IK@<JKF<EJLI<@EBA<KK89@C@KP JL==@:@<EKI<8:K@M@KP=FI:LI@E>F=K?<@EB =C<O@9@C@KP <C8JK@:@KP @E I<>8I;J KF K?< GI@EK89C< K<OK@C< JL9JKI8K< 8E; I<C8K@M< I<J@E ?8I;E<JJ=FI8;LI89C<<E;GIF;L:K %E12 :LI89C<@EBA<KGI@EK@E>K?<D8@E:?8CC<E><@J KF 98C8E:< K?< FC@>FD? @EB M@J:FJ@KP 9LK E<:<JJ8IP:LI@E><==@:@<E:P 3?<I<8JK?<KPG<F=FC@>FD<I@J><E<I8CCP;<:@J@M<=FIK?< D<:?8E@:8CGIFG<IK@<JF=K?<:LI<;I<J@E K?<KPG<F=DFEFD<I?8JC8I><JK@DG8:KFEK?< :LI<I<8:K@M@KPF=K?<I<J@E8E;@KJ8DFLEK:8E;@CLK<K?<@EBKF8:?@<M<8CFN<IM@J:FJ@KP J =FI K?< G?FKF @E@K@8KFI 12 (! G?FKF @E@K@8KFIJ 8I< LJ<; =FI <EP J8M@E> 8E; <==@:@<EK:LI@E>F=K?<I<J@E 0?<D8@EGIFG<IK@<JF=@EBA<K@EBJ N?@:?E<<;KF9<D<K8::FI;@E>KFJG<:@=@:8K@FEJF= K?<:?FJ<EGI@EK?<8; 8I<K?<M@J:FJ@KP JLI=8:<K<EJ@FE G8IK@:C<J@Q<@=8GGC@:89C<8E; JK89@C@KP F= K?< @EB %EB =FIDLC8K@FEJ LJ@E> K?< D8K<I@8C C@JK<; @E 089C< ?8M< 9<<E 8;ALJK<;KFD<<KK?<J<I<HL@I<D<EKJ $D6 !+,'%,*!$+-+,''*%-$,9-*$.*&!+ 7 64)9(83&2* -484.3.8.&846 1.,42*6 4342*6 ,# Figure 12: Absorbance at maximum wavelength of commercial and pure photochromic dyes in hexane, toluene and acetonitrile as function of temperature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he Xennia ink fluid development system is fitted with one print head that is mounted on a rotating plate. It is movable in x-, y- and z-direction whilst the printing substrate is static (Figure 13a and Figure 14a). The printable substrate is placed on a porous board, which is connected to a vacuum system to prohibit a shift of the substrate while printing. Compliant with the printable image data, the head moves along the substrate in x- and y-direction to print the imagery in multi-pass mode. The resolution of the printed image is adjustable up to 300 dpi. -*2.(&1 $E6 !+,'%,*!$+-+'* %!$&( 1+!$+,!$!2,!'&'( ',' *'%!(*!&,+7 64)9(83&2* 0@ELM@E !5*(.+.(&8.437 /" $@E;<I<;8D@E<C@>?K JK89@C@Q<I$(/ /K8I *LKI@K@FE /@>D8 C;I@:? 3?<PGFN;CF9LC@E #I<<EC8E;N8O "ASCCISM<E DPCFJ< /@>D8 C;I@:? 3?<PGIFK<@E@JFC8K< ,IFK<@E:FE:<EKI8K<=IFD 9FM@E<D@CB G8I8==@EN8O 9<<N8O /K8I:?=IFDGFK8KF Figure 13: (a) Xennia ink fluid development system with UV protection box and (b) modular inkjet printing system with UV curing bridge. The modular inkjet printing system is a continuous inkjet printer with two bridges for printing of aqueous, solvent or UV-curable inks and one bridge for curing with a UVLED lamp (Figure 13b and Figure 14b). 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esign of UV-Curable Photochromic Inkjet Inks G7D7D &&!&$,*'&!*'+'(1;< /!)@J:FDDFECPLJ<;KF8E8CPQ<K?<DFIG?FCF>P8E;JLI=8:<GIFG<IK@<JF=K<OK@C< JKIL:KLI<J %EK?<:?8I8:K<I@Q8K@FEF=G?FKF:?IFD@:GI@EKJ@EG8G<I%%% K?<@DG8:KF=K?< @EBA<K=LE:K@FE8C@Q8K@FEFEK?<@EK8&!+( &/) " %D8><J N<I< K8B<E 8K 8E 8::<C<I8K@FE MFCK8>< F= B2 8E; 8 NFIB@E> ;@JK8E:<F=DD G7E,,!+,!$&$1+!+ /:@<EK@=@:I<JLCKJ@EK?@JNFIB8I<JK8K@JK@:8CCPLE;<IG@EE<;LJ@E>*+28E8CPJ<J8K8 :FE=@;<E:<@EK<IM8CF= 7. Summary of Results Naphthopyran dye Ruby red (RR) and spiroozaxine dye Sea green (SG) were chosen as dyes for the design of UV-curable photochromic inks. Both RR and SG were relatively stable over the practical range of temperatures between 0 and 30 °C (as seen in Figure 12). RR shows stronger absorbance in polar and SG in non-polar solvents. Color analyses of the photochromic dyes as prints on textile are made at wavelengths, which correspond to the dyes’ absorbance maxima in solvents: 500 nm for RR in ethyl acetate and 620 nm for SG in hexane as shown in Figure 16. Prerequisite for fully functional UV-curable photochromic ink is that the photo-initiator used in the ink formulation conforms to the UV-light source, which is used to initiate photo-polymerization. In Figure 16, it can be seen that the photo-initiator TPO-L has an absorption peak between 340 and 410 nm, which conforms to the LED-UV lamp used for curing with emission wavelengths between 380 and 420 nm. Figure 16: Absorbance spectra of photo-initiator TPO-L, Ruby red in ethyl acetate and Sea green in hexane. 40 • • • Surface tension: 30 – 35 mN m-1 Viscosity: 0.008 – 0.020 Pa s (ideally 0.010 – 0.014 Pa s) Particle size: < 0.1 μm As mentioned earlier, no particles are used in the formulation, but filtration using syringe filters has been initially made without an affecting the ink. The viscosity had been identified as the most crucial physical property, which is decisive for the jettability of the formulated ink. Combinations of different monomers and oligomers (as listed in Table 3) have been tested initially to evaluate, which combination is suitable for the development of photochromic ink. According to Figure 17, mixtures with oligomers Genomer 3497 and Genomer 5275 resulted in too high viscosity (> 15 mPas) and mixtures with monomer ACLA in too low viscosity (< 10 mPas). It was also observed that the polymer mixtures with ACLA were turbid and could not be homogenized sufficiently. Genomer 3497 Genomer 5275 Ebecryl 81 Ebecryl 853 Viscosity (mPa s) 30 according to print head requirements. Eventually, the mixture of monomer DPGDA and oligomer Ebecryl 81 was chosen as basis for the varnish for further development of photochromic ink. This combination fulfilled requirements in regards to the surface tension, viscosity and showed best preliminary curing efficiency when combining with the photo-initiator TPO-L. Genomer 3497 Genomer 5275 Ebecryl 853 35 30 25 20 15 10 5 0 DPGDA DDDA DMGDA ACLA Figure 18: Surface tension of monomer-oligomer mixtures with ratio 7:1 measured at 20°C. When adding photo-initiator TPO-L and the dyestuff (RR) or larger amount of monomer, the surface tension decreases (Figure 19). However, all formulations are within the required range of 30 – 35 mN m-1. In terms of viscosity, it was observed that when adding the photo-initiator and the dye, a viscosity of 22.4 mPas was reached, which is too high for jetting with the Sapphire QS 10 and Starfire SA print heads. Accordingly, the ratio of monomer to the other components had to be increased in order to achieve ink with a lower viscosity. This resulted in an adjustment of 21 parts of DPGDA compared to initial 7 parts (see Figure 19). 25 20 15 10 5 0 Ebecryl 81 40 Surface tension (mN/m) For ink formulation, the physical properties of the ink have to follow the print head requirements. This means that the following has to be met for Fujifilm Dimatix print heads Sapphire QS 10 and Starfire SA: DPGDA DDDA DMGDA ACLA Figure 17: Viscosity of monomer-oligomer mixtures with ratio 7:1 at a shear rate of 10000 s-1 and 20°C. As can be seen in Figure 18 and Figure 19, different combinations of monomers and oligomers and different ratios of ink components do not largely affect the surface tension of the ink. With the exception of the combinations of Genomer 3497/ DPGDA and Genomer 3497/ DDDA, all other combinations were within the jettable range 41 42 Surface tension Viscosity 30 25 35 20 30 15 10 25 Viscosity (mPas) Surface tension (mN/m) 40 5 20 A GD DP :1 17 +8 A GD DP L OTP + 1 +8 u +R d Re by :1 7:1 A GD DP L OTP + 1 +8 1 :1: 21 A GD DP L OTP + 1 +8 u +R Re by :1 1:1 d2 0 Figure 19: Influence of different ratios of ink components on the surface tension (¿) and the viscosity (¢) of the ink formulations at 20 °C. Eventually, the formulation of monomer DPGDA, oligomer Ebecryl 81 and photoinitiator TPO-L was finalized for printing and curing studies (paper II, III and IV) with the ratio of 21:1:1 and RR/SG photochromic dyes with concentrations of 2.5 and 4 g L1 in ethyl acetate and hexane, respectively. As can be seen in Figure 20a, the viscosity of the varnish and RR and SG inks decreases as function of increasing shear rate from 1 to 10000 s-1. The UV-curable varnish as mixture of polymers with different chain length may be responsible for the shearthinning behavior of the ink. Thus, UV-curable photochromic inks behave as nonNewtonian fluids. In comparison, functional waterborne ink containing polysiloxane shows Newtonian behavior and weak temperature-dependence, which is discussed in paper V. An increase of 20 °C affects the functional waterborne ink with a viscosity decrease of ca. 15% (from ca. 0.012 Pas to 0.010 Pas). Photochromic UV inks are more affected by temperature with a 30% decrease in viscosity from 20 °C to 40 °C as seen in Figure 20b (from ca. 0.015 Pas to 0.0105 Pas). 43 Figure 20: Viscosity of RR ink, SG ink and ink varnish as function of (a) shear rate from 1 to 10000 s-1 at 20 °C and of (b) temperature from 20 to 40 °C at 10000 s-1. Despite similar viscosity and surface tension of RR and SG inks, differences were observed in relation to the velocity of the droplets upon jetting and the drop volume. SG ink drops exhibit remarkably lower velocity at room temperature and generally lower drop volume than RR ink. Such difference can be explained by the distinct molecular structures of the dyes. With SG being non-planar in the ring-closed state, jetting of the ink can be similarly affected as the dye kinetics in a UV resin with different crosslinking densities as discussed in papers II and III. During jetting, higher temperatures enable more flexibility in the ink and smoother ink flow. The bulky structure of spirooxazines could cause a bottleneck at the orifice and result in smaller drop volumes. 44 J ;<J:I@9<; @E 0- '66#$+.+6; @E :?8GK<I A<KK89@C@KP F= @EB :8E 9< <M8CL8K<; 9P :8C:LC8K@FEJF=;@D<EJ@FEC<JJELD9<IJ!' '8E;" N?@:?8I<98J<;FEK?<A<KK89C< =CL@;Jd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a b Figure 21: Drop formation upon jetting at 35 °C between 50 and 200 μs of (a) RR ink and (b) SG ink. 7.2 Degree of Cure of the Inkjet-Printed and UV-Cured Ink on Textile After printing of the ink, curing via a UV light source results, which necessitates the analysis and specific quantification of the degree of cure of the ink in order to optimize the system. The settings in the technical process of UV curing affect the chemical process of polymer crosslinking. There both the degree of polymer crosslinking and the crosslinking density of the resin are influenced. However, within this thesis no differentiation between the degree and density of crosslinking is aimed at. The degree of cure is relevant in three respects – i) impact of the resin rigidity on the dye kinetics, ii) influence of the degree of cure on the durability of a photochromic print on textile and iii) health concern of the UV resin in liquid form. Usually, FTIR is a common method to determine the degree of cure as cleavage of the C=C in UV-curable material. However, in the case of our material FTIR was not suitable to determine the degree of cure of the ink-jetted photochromic ink on the plainwoven PET fabric. As seen in Figure 22, the PET reference and printed PET show equal FTIR-spectra, which shows that the thin layer of photochromic ink cannot be detected by FTIR on the porous textile structure. As seen in Figure 23, monomer cleavage of the C=C double bond at 1636 and 1619 cm-1 is observed for a drop of ink with a volume of 1 μl on a glass plate. Here, it can be seen that the amount of broken C=C increases with increasing curing intensity as compared to the liquid photochromic ink drop, which allows qualitative analysis. However, the amount of a 1-μl ink drop on glass and the ink-jetted material on plain-woven PET fabric are quantitatively incomparable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igure 22: Comparison of FTIR spectra from reference plain-woven PET fabric and inkjetprinted plain-woven PET fabric with RR ink. !-*CE6$,!&,%(*,-* +-&,!'&'$%(!&,&+!,1-*!&-*!&/!, $, +('FA%%+9B;<&DAA%%+9B;<'-&/+ BA9(++(*!&,+7 Figure 23: FTIR spectra with cleavage of the C=C double bond at 1619 and 1636 cm-1 for liquid ink compared to a weak and strong curing setting with UVA light and UV LED light, respectively. 47 "LIK?<ID<8JLI<D<EKJ <O:<<;@E>K?<FLK:FD<F=G8G<I%%J?FNK?8K12:LI@E>F=K?< LEGI@EK<;,!0N@K?;@==<I<EK:LI@E>@EK<EJ@K@<J @ < DDJ DDJ 8E; DDJ 8E;8K;@==<I<EK;@JK8E:<JF=K?<:FEM<PFI9<CKKFK?<12 (! C8DG @ < DD DD8E;DD @E:I<8J<JK?</F=,!08JJL:?J<<"@>LI< 0?<F9J<IM<;@E:I<8J<F=/F=,!08::FLEKJ=FI8GGIFO Y8E;K?<I<N@K?@E;@:8K<J 1600 PES + print cured at 50 mm/s + 80% 1400 Storage Modulus [MPa] that UV radiation influences the textile material by itself. However, the increase of Tm of UV cured ink on PET accounts for a successive increase of 1 °C of the melting peak, with an observed systematic increase with increasing curing intensity. Because the extended measurements of UV-treated unprinted PET are not statistically representable, but indicate a factor, which was not regarded in paper II, other methods for quantitative analysis might be necessary to clarify the degree of cure of the photochromic ink on a textile as a result of curing intensity. 1200 PES + print cured at 300 mm/s + 1% 1000 800 600 400 200 PES reference 0 2 4 6 Frequency [Hz] 8 10 12 Figure 26: Storage modulus as function of frequency of PET reference fabric and prints cured with different intensities on PET 7.3 Development of an Extended Kinetic Model Figure 25: Melting temperature Tm of unprinted PET treated with different intensities () of UV-LED light and at different distances between the conveyor belt and the UV lamp (p) compared to untreated PET (¢). DMA measurements confirm the trend of increasing degree of cure with increasing curing intensity in relation to stiffness of the printed and cured photochromic material on PET fabric. The storage modulus as function of frequency between 1 and 12 Hz increases with increasing curing intensity as can be seen in Figure 26. Whereas the PET reference has a storage modulus around 300 MPa, PET printed with RR ink increases the storage modulus to ca. 1000 MPa for an ink with low crosslinking density and to 1400 MPa with an ink with high crosslinking density. 49 Color measurements with the LCAM Photochrom 3 spectrophotometer have shown that the first-order kinetic model for the coloration reaction (Equation 8) is only valid for photochromic prints with a high polymer crosslinking density, i.e. printed and cured at a belt speed of 50 mm s-1 and lamp intensity of 80%. For all other printing and curing parameters, the application of Equation 8 resulted in a poor and unacceptable fit (R2 < 0.45) of the coloration data, as seen in Figure 27. The deviation of the coloration data from the first-order kinetic model was found to be in line with the degree of polymer crosslinking in the following order, starting from the curing settings with the highest deviation: 300 mm s-1/ 1% > 300 mm s-1/ 25% > 50 mm s-1/ 1% > 50 mm s-1/ 25% > 300 mm s-1/ 80% > 50 mm s-1/ 80% Therefore, the development of an extended kinetic model for photochromic UV-curable prints with a low degree of polymer crosslinking was required in order to be able to characterize the material. The difference between the first-order kinetic model and the extended kinetic model is that the extended model also takes into account and corrects for a decay mechanism upon coloration of the photochromic prints. 50 a Colouration cycle 1 0.5 H7D7C '$.$'(%&,&(($!,!'& Fitted curve K/S 0.4 0.3 0.2 0.1 0 100 200 300 Time (s) Figure 27: Coloration data (n) and first-order curve fitting (–) according to Equation 8 of a photochromic print with 19 g m-2 deposited ink, cured at belt speed of 50 mm s-1 and 1% lamp intensity in cycle 1 with a fitting value R2 < 0.45. 7.3.1 Challenging Role of UV Light for Curing and Activation By combining photochromic and UV-curable material a challenging relationship exists between the mechanisms of color activation and polymerization, both initiated by molecular excitation via UV light. Upon UV excitation during color measurement of the photochromic prints, photochromic dyes are activated, as well as free radical polymerization of the UV-curable material can be initiated. As mentioned earlier, photochromic materials show fatigue, which degrades the photochromic color switching effect over time. Also, curing of a UV-curable resin requires sufficient light intensity and time in order for the free radical polymerization reaction to be terminated. While high UV radiation improves the cure speed of the resin 129, 130 and longer irradiance increases the degree of polymer crosslinking 131, photodegradation of photochromic dyes is commonly observed under prolonged UV irradiance 12, 69, 132. This means that if the polymer crosslinking density of photochromic prints is high, no further curing takes place while color measurement, which is the case for prints produced at 50 mm s-1 and 80%. Photochromic dyes are also better protected against fatigue when fixed in a fully cured matrix. However, a partially liquid resin has a lower viscosity, which facilitates oxygen diffusion 130. Hence dye degradation as a result of photo-oxidation is more likely to occur in a softer resin compared to when the dye is fixed in a solid matrix. Therefore, for prints with a low degree of polymer crosslinking, polymer crosslinking continues during color measurements and a simultaneous secondary decay mechanism is observed, which affects the color behavior of photochromic dyes. 51 "FIG?FKF:?IFD@:GI@EKJN@K?8CFNGFCPD<I:IFJJC@EB@E>;<EJ@KP8E<OK<E;<;B@E<K@: DF;<C @J @EKIF;L:<; N?@:? ;<=@E<J K?< B@E<K@:J F= 8 ;<:8P D<:?8E@JD FN@E> KF :FEK@ELFLJ :LI@E> N?@C< :FCFI D<8JLI<D<EK 8E; N?@:? I<:FEJKIL:KJ K?< 8:KL8C :FCFI8K@FE:LIM<J<<"@>LI< "FI12 :LI89C<G?FKF:?IFD@:@EBJPJK<DJN@K?K?< 8@DKF:?8I8:K<I@Q<G?FKF:?IFD@:D8K<I@8C8E;KFFGK@D@Q<@KJ:LI@E>:FE;@K@FEJ K?< =FCCFN@E><OK<E;<;B@E<K@:DF;<C@JGIFGFJ<; P ]U O W Y1@IML EZ N ^ :DA .:DA 2:DA 2:DA N ][ .;<: O 2;<: \ Y1@JKI EZ N 2;<: ^ N?<I< ' / :FC @J K?< @E@K@8C :FCFI M8CL< =FI K?< :FCFI8K@FE I<8:K@FE <%1. @J K?< D8O@DLD:FCFI8K@FEM8CL< &'%@JK?<@E@K@8CM8CL<F=K?<;<:8P8E;<&'%@JK?< =@E8CM8CL<F=K?<J<:FE;8IP;<:8P=FI<8:?8:K@M8K@FE:P:C< '@E<K@:I8K<:FEJK8EKJ=FI K?<:FCFI8K@FEI<8:K@FE8E;;<:8P8I<;<=@E<;8J-%1.8E;-&'% I<JG<:K@M<CP 1E;<I K?< 9FLE;8IP :FE;@K@FE K?8K K?< GIF:<JJ<J F= :FCFI8K@FE 8E; ;<:8P JK8IK J@DLCK8E<FLJCPLGFE12<O:@K8K@FE8E;8KA N?@:?D<8EJK?8K%1. 8E; &'% !HL8K@FEI<;L:<JKF P T O Y1@IML EZ V O T O Y1@JKI EZ V :DA 2:DA 2;<: JJ<<E@E"@>LI< K?<:FCFI@EK<EJ@KPF=G?FKF:?IFD@:GI@EKJGIF;L:<;8E;:LI<; 8K89<CKJG<<;F= DDJ 8E;8C8DG@EK<EJ@KPF=@J;<:I<8J<;K?IFL>?FLKK?< 8:K@M8K@FE:P:C<JKF@ M;LI@E>:FCFID<8JLI<D<EK "LIK?<IDFI< K?<J<:FE;8IP ;<:8P @J DFJK GIFEFLE:<; ;LI@E> K?< =@IJK 8:K@M8K@FE :P:C< 8E; ;<:I<8J<J >I8;L8CCP ;LI@E>:FEK@EL@E>:P:C<J "FIK?<GI@EK@E"@>LI<;<>I8;8K@FE9<:FD<JJK8K@JK@:8CCP @E<==<:K@M<@E8:K@M8K@FE:P:C<M !M<EKL8CCP K?<8GGC@:8K@FEF=!HL8K@FE8CCFNJ K?< ;@JK@E:K@FE F= 8 ;<:8P D<:?8E@JD 8E; K?< I<:FEJKIL:K@FE F= K?< 8:?@<M89C< :FCFI8K@FE:LIM<=FI>@M<E:LI@E>J<KK@E>J 2@:<M<IJ8 M@8K?<<OK<E;<;B@E<K@:DF;<C I<JG<:K@M<:LI@E>G8I8D<K<IJ:8E9<:?FJ<E=FI8;<J@I<;G?FKF:?IFD@:G<I=FID8E:<F= K?<D8K<I@8C H7E'$'**'*%&', 0,!$&+'*+ %EK?@JJ<:K@FE K?<:FCFIG<I=FID8E:<F=@EBA<K GI@EK<;G?FKF:?IFD@:K<OK@C<J8J8I<JLCK F=;@==<I<EKGIF;L:K@FEGIF:<JJG8I8D<K<IJ8E;=8JKE<JJK<JKJ@J;<J:I@9<; 8JN<CC8JK?< @DGC@:8K@FEJF=K?<J<=8:KFIJFEK?<;LI89@C@KP8E;K<OK@C<?8E;C<F=K?<G?FKF:?IFD@: GI@EKJ 8I< JLDD8I@Q<; "LIK?<IDFI< K?< :FCFI G<I=FID8E:< F= J:+ ;P<; G?FKF:?IFD@: K<OK@C<J 8E; K?< <==<:K F= N8J?@E> FE K?< LE@=FIDCP ;P<; =89I@:J @J ;<J:I@9<; H7E7B &#",9*!&, ',' *'%!0,!$+ 0?< G<I=FID8E:< F= @EBA<K GI@EK<; G?FKF:?IFD@: K<OK@C<J N8J JKL;@<; <OK<EJ@M<CP @E I<>8I;JKFJ<M<I8C=8:KFIJK?<8DFLEKF=;<GFJ@K<;@EBFEK?<K<OK@C< GIF;L:K@FEGIF:<JJ G8I8D<K<IJ ;LI89@C@KP8E;K<OK@C<GIFG<IK@<J 0(.(+(+ !"! Figure 28: Activation cycles 1-5 (i-v) of a print with 19 g m-2 deposited ink cured at 300 mm s-1 and 1%. Measured K/S (■) includes decay (– ·) and coloration (–). Calculated sum of decay and coloration (– –) validates the model. 133 53 %EG8G<I%% J@E>C< G8JJGI@EK@E>F=G?FKF:?IFD@:@EBN@K?..;P<N8JLJ<;KFJKL;PK?< <==<:K F= GI@EK@E> G8JJ<J 9<KN<<E 8E; @ < K?< 8DFLEK F= ;<GFJ@K<; @EB FE K?< K<OK@C< 8E; K?< <==<:K F= GI@EK@E> 8E; :LI@E> G8I8D<K<IJ FE K?< GI@EKd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igure 30: Schematic molecular rearrangement of photochromic dyes upon ring-opening and closing. c d The lower the crosslinking density of the UV-ink, the least rigid becomes the dyecarrying varnish. Less rigidity implies a higher degree of freedom for isomerization of the dye, which enables faster kinetic switching and higher color yields as can be seen in color performance values in Table 5. Eventually, prints produced with the highest _ belt speed of 300 mm s 1 combined with the lowest lamp intensity 1% achieve high color yields ΔK/S with 0.47 after printing and 0.25 after washing, fastest isomerization _ _ with kcol of 0.16 s 1 after printing and 0.11 s 1 after washing and good durability after washing with 54.1% of the initial ΔK/S, despite lowest polymer crosslinking density. Figure 29: (a) Color yield ΔK/S, (b) coloration rate constant kcol, (c) decoloration rate constant kdecol of printed (■) and washed (£) RR prints as function of printing passes and (d) Inactive (I), printed (P) and washed (W) RR prints with 1, 5 and 10 printing passes cured at 50 mm s-1 and 80%. 7.4.1.2 Production Process Parameters Tuning of dye kinetics as means of controlling the properties of the dye-carrying media is a topic of interest in many studies. The viscosity of the media, the chain length of polymers or the specific placement of the dye molecule in the polymer matrix can influence dye kinetics 134-141. In this thesis, control of dye kinetics as means of using production process parameters, i.e. conveyor belt speed and lamp intensity was investigated. The principle is based on enabling enough space for the photochromic dyes to undergo quick isomerization from the orthogonal ring-closed to the co-planar ring-opened form as visualized in Figure 30. 55 Figure 31: Change in kcol before and after washing of RR prints as function of curing intensity with low (,), intermediate (p,r) and strong (¢, £) curing. 56 (a) DK/S 0.12 0.20 0.06 0.03 0.05 50_80 0.00 300_1 0.25 50_80 0.10 0.06 0.03 0.05 Red 0.00 Green 0.25 Red 0.10 0.06 0.03 0.05 printed abraded 1 wash 10 washes 0.00 50_80 300_1 Red Green 0.012 0.008 0.016 0.09 0.15 0.008 0.004 Green 0.12 0.20 kdecol 0.016 0.09 0.15 (c) 0.012 0.004 300_1 0.12 0.20 0.00 0.016 kdecol (s-1) 0.10 0.00 kcol 0.09 0.15 0.00 (b) kdecol (s-1) 0.25 kdecol (s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kcol (s-1) kcol (s-1) kcol (s-1) ] ] ] ] ] ] Durability of photochromic prints for UV sensor applications is a measure of two factors – i) adhesion of the photochromic ink on PET and ii) performance of the photochromic effect over time. Whereas adhesion is influenced and determined by fastness tests as washing and abrasion and indirectly via melting peak temperatures, the performance over time is influenced by the materials’ fatigue properties. In paper III, exceeding the findings to controlling dye kinetics via production process parameters, we could also confirm that curing intensity influences the durability of photochromic prints. In general, it was observed that a reduced rigidity of the host matrix improves print durability. Wearing of textiles typically is caused mechanically as in abrasion or chemical-mechanically as in washing. As seen in the factor analysis (Figure 32), color performance as in ΔK/S, kcol and kdecol is less affected by treatments with mechanical action via abrasion with 20000 rubbing cycles than by chemicalmechanical action via one or 10 washing cycles. Washing has a harsher effect than abrasion. K/S (-) A! 7.4.1.3 Durability Curing intensity *1875**) 227 0)*(&= 7 ] ] ] ] ] *2&.3*) A! Dye 6.38*) 0(41 0)*(41 7 7 ] ] ] ] ] ] ] ] ] ] ] ] %&7-*) 0(41 0)*(41 7 7 ] ] ] ] ] ] ] ] ] ] ] ] A! ] ] ] ] ] ] K/S (-) &25 .38*37.8= &25 .38*37.8= K/S (-) *1875**) 227 kinetics of combined RR and SG prints with similar kcol of 0.05 – 0.08 s-1, RR should be preferably cured with a strong intensity (50 mm s-1 + 80%) and SG with a low intensity (300 mm s-1 and 1%). Eventually, production process parameters can be used to intensify or balance out differences in dye kinetics of multi-colored textiles, which may depend on the requirements of the desired application. Treatment $ F6 '$'* (*'*%& ' BA9(++ (*!&,+ /!, 3 85 #'$5 #'$ & #1 '* $$ -*!& +,,!&+,*(*!&,!&&/+ !&7 printed abraded 0.012 0.008 0.004 1 wash 10 washes printed abraded 1 wash 10 washes Figure 32: Effects of the factors curing intensity (¢), dye type () and treatment (p) on (a) ΔK/S, (b) kcol and (c) kdecol.142 58 The abrasion resistance of UV-curable inks is in general very good 99, 105, 143, 144. Comparing the color performance of SG and RR prints, SG prints are more durable as a result of abrasion cycles. Whereas, RR prints show a steady decrease of ΔK/S and kcol as function of abrasion cycles, SG prints are not as systematically affected (see Figure 33). Figure 33: Effect of abrasion cycles 5000, 10000, 15000 and 20000 on ΔK/S of RR (a) and SG (b) prints cured at 50 mm s-1 and 80% (50_80) and 300 mm s-1 and 1% (300_1) on and on kcol of RR (c) and SG (d) prints cured at 50 mm s-1 and 80% (50_80) and 300 mm s-1 and 1% (300_1).142 After a first washing cycle, photochromic prints are influenced noticeably. Prints with lower crosslinking density, however, are less affected both for RR and SG dyes. When washing the prints more extensively, prints with high crosslinking density show a steady decrease in ΔK/S, kcol and kdecol but prints with low crosslinking density dispose of stable values between 1 and 10 washes, which is also seen for multiple activations. It is important to know how the inkjet-printed material behaves after extensive washing and multiple activations in regard to maintenance and repeatability of potential textile UV sensors. Additionally, extensive washing is suggested as reduction process with respect to health concerns of uncured monomer, not just to remove potentially uncured ink of prints with low crosslinking density but also to achieve durable and reliable prints for long-term use. 59 Figure 34: Effect of 0, 1 and 10 washing cycles on ΔK/S, kcol and kdecol of (a) RR prints cured at 50 mm s-1 and 80% (█,■,□) and 300 mm s-1 and 1% (█,▲,∆) and of (b) SG prints cured at 50 mm s-1 and 80% (█,■,□) and 300 mm s-1 and 1% (█,▲,∆).142 A remarkable difference between RR and SG prints is the initial increase of ΔK/S of SG prints after initial washing or abrasion (as seen in Figure 33b and Figure 34b). This phenomenon has been observed in other prior studies, where the loosening effect of a washing procedure on the matrix believes to facilitate isomerization 7, 23. Eventually, when multi-colored and synchronized prints are desired for a specific application, the durability of RR and SG prints will be uneven. As stronger curing of RR prints is needed for synchronized rate constants with SG, which was mentioned earlier, the durability against abrasion and washing of RR dyes will be weakened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d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d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a) (b) 200 400 (a) SO-SG Unwashed (b) NP-RR Unwashed DK/S 0.08 0.06 0.04 0.02 0.00 0 600 800 1000 1200 1400 1600 Time (s) 0.12 (II) (a) SO-SG Washed (b) NP-RR Washed 0.10 DK/S H7F1+,'0,&, !,!%'& &#",9*!&,0,!$&+'* (b) 0.08 0.06 0.04 (a) 0.02 0.00 0 200 400 600 800 1000 1200 1400 1600 Time (s) Figure 36: Fitted curves of K/S values as function of time of the coloration and decoloration reaction of PET dyed with spirooxazine SG and naphthopyran RR (I) before and (II) after washing. Results obtained by scCO2 dyeing of PET with RR and SG dyes can be compared to UV-curable inkjet printing in terms of a bulk versus a surface treatment of the dyes on textile, but also in terms of how a polymer matrix influences isomerization and hence kinetics of the dyes. A general observation is that the color performance of RR and SG dyes, respectively, is opposite when applied by inkjet printing compared to scCO2 dyeing. Whereas RR prints show higher color yields, faster kinetics, but worse durability after abrasion and a first washing cycle, SG prints show lower color yields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igure 37: Color difference ΔE for stabilized samples with whey powder, wax and HALS before and after washing. Figure 39: Color difference ΔE of stabilized prints over multiple days of activation before washing. Figure 38: Color difference ΔE for impregnated samples with amylose before and after washing. Figure 40: Color difference ΔE of stabilized prints over multiple days of activation after washing. As seen in Figure 39 and Figure 40, where trend lines are used to guide the eye, both before and after washing photochromic prints treated with protein or wax exhibit higher color difference ΔE than prints containing HALS. However, over the course of 10 activation days washed prints containing HALS show an increasing trend of coloration as compared to protein and wax treatments, which decrease in coloration. The residual color difference, as ratio of the color difference ΔE before washing and the color difference ΔEwashed after washing, determines the fatigue of the photochromic prints and the durability of the physical stabilization layers after a washing procedure. As seen in Table 6, impregnation of RR prints with whey powder before curing achieved highest residual color difference with 64.5% for single activation. Upon multiple activations, i.e. after 10 days, wax coating applied after curing could protect 65 66 K?<G?FKF:?IFD@:GI@EKJ9<JKN@K? I<J@;L8C:FCFI;@==<I<E:< CJF N?<PGFN;E8K@FEJ 9<=FI< 8E; 8=K J?FN ?@>? 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( 6?8E> L8E;/ "L 1..1+&574(*;5+%1%*'/ 0) 52'%65 # ' /L:? . !M8EJ 8E; 0 , 8M@J #%41/1.'%7.'5 # /B<CK< %E;<G<E;<EKK?<J@J;M8E:<;C<M<C;<>I<<F=)8JK<I0NF5<8IJ /KL;<EKK?<J@J Paper I * I@OC8E; %E;<G<E;<EKK?<J@J;M8E:<;C<M<C;<>I<<F=)8JK<I0NF5<8IJ /KL;<EKK?<J@J Influence of physical parameters and temperature on ink jetting of UV-curable photochromic inks for smart textile applications S. Seipel, J. Yu and V. A. Nierstrasz Manuscript submitted Effect of physical parameters and temperature on the piezo-electric jetting behaviour of UV-curable photochromic inks Sina Seipel*, Junchun Yu and Vincent A. Nierstrasz Textile Materials Technology, Department of Textile Technology; Faculty of Textiles, Engineering and Business; University of Borås; 501 90 Borås, Sweden. *Author to whom correspondence should be addressed: E-Mail: sina.seipel@hb.se; Tel.: +46-33-435 4191. Abstract Although resource-efficient processes like inkjet printing have a large potential to foster the development of smart and functional textiles, one bottleneck still is the development of functional inks. To make inkjet printing and UV curing given production techniques for smart and functional specialty products, e.g. photochromic textiles, development and jet abililty of functional ink is needed. This paper focuses on the formulation and performance of UV-responsive and UV-curable inkjet inks, which are based on photochromic dyes and their application to produce UVresponsive textiles. Two commercial photochromic dyes – Reversacol® Ruby red and Sea green, which are especially feasible for textile applications in regards to their temperature-stability, have been used to develop the inks. Ruby red (RR) and Sea green (SG) represent dyes of the naphthopyran and spirooxazine class, respectively. The specially designed photochromic inks are characterized according to their physical-chemical and rheological properties in respect to temperature. The influence of temperature on the jetting performance and drop formation of the inks is analyzed using a high-speed camera and gives important information on the jettability of the ink. The printability of the RR and SG inks is framed and underpinned by theoretical calculations of the Z number. Discrepancies are observed and discussed between existing theory of ink jettability and visual evaluation of the photochromic ink. Keywords: Inkjet printing, UV curing, photochromic ink, jettability, drop formation 1 1. Introduction Digital inkjet technology offers a great opportunity for the textile industry not only through the conservation of resources, but also through its high potential to cultivate innovation. Through inkjet printing a flexible textile functionalization process for small batches with a reduced ecological footprint can be realized. Next to the minimized consumption of energy, water and chemicals, the functional chemistry can be applied locally and with complex designs while retaining the textile properties. Functional finishes on textiles offer an enriching possibility to create smart textile materials and find new fields of application. Until now, a major challenge having to be faced in fostering the technology is to formulate inks beyond the established colorants and instead with advanced and smart functions to cater to future trends and industrial development. According to Mendes-Felipe et al.1 the symbiosis of ink jetting technology and UV-curable materials has large potential as resource-efficient, sustainable and versatile production method for next generation devices and smart solutions. Drop-on-demand (DOD) inkjet printing is the most common type of inkjet printing, where tiny droplets with a volume of approx. 10 picolitres are precisely deposited on the surface of the printable substrate. As the name implies, in DOD printing a drop is formed when there is a demand for it according to the print pattern. The ink is fired upon an electric signal and a droplet with characteristic tail formation is ejected from the nozzle. Through polarization of the lead zirconium titanate the crystal undergoes distortion creating a pressure pulse in the ink chamber 2. The Starfire SA Dimatix print head used in this study is a typical industrial print head for textile printing, which features a total number of 1024 nozzles. Drop formation of an ink is influenced by the properties of the mixture of fluids – viscosity, surface tension, density – and by the velocity and size of the droplet 3. Electric voltage and pulse shape influence the drop formation related to the nozzle geometrics as well 2,3. As the ink reservoir is not pressurized, the surface tension in the printer prevents unwanted ink flow from the nozzles when in standby mode. Therefore, pressure initiated by the piezoelectric signal helps to overcome a certain surface tension in order for a drop to be formed at the orifice. The pressure difference, which has to be exceeded, is 4, !" Δ𝑃𝑃 = # (Equation 1) where 𝛾𝛾 is the surface tension and 𝑟𝑟 the radius of the nozzle. The theoretical printability of ink, which has been widely discussed in literature 5-9 can be calculated by a combination of dimensionless numbers, which depend on various physical-chemical properties of the printable fluid and dimensions of the printing orifice. The Reynolds number 𝑅𝑅𝑅𝑅 and the Weber number 𝑊𝑊𝑊𝑊 specify the relative magnitude of the fluid’s interfacial, viscous and inertial forces 3,6,7. 2 $%# (Equation 2) $! %# (Equation 3) 𝑅𝑅𝑅𝑅 = & 𝑊𝑊𝑊𝑊 = " where 𝜐𝜐 is the velocity, 𝜌𝜌 the density and 𝜂𝜂 the viscosity. The Reynolds number defines the fluid’s inertia to its viscosity, whereas the Weber number specifies the ratio of inertia to its surface tension. 2. Materials and Methods Fromm7 has developed a solution based on the Navier-Stokes equations 10 to express the limitations of drop ejection in regards to interfacial, viscous and inertial properties of the fluid 3. 𝑍𝑍 = ("%#)"/! & ) ,- = *+ = .- "/! formed 12. At flight a spherical droplet with a specific and constant volume will form before landing on the printable substrate. This paper focuses on the exploration of the jettability of UV-curable photochromic inks and their drop formation as function of temperature and its effect of changed physical fluid properties on the drop formation. The results entail empirical data, which is embedded in and confirmed using theoretical calculations based on the dimensionless numbers We, Re and Z. (Equation 4) 𝑍𝑍 is the inverse of the Ohnesorge number 𝑂𝑂ℎ and is defined as the ratio of the Reynolds number and the square root of the Weber number; also known as Laplace number 𝐿𝐿𝐿𝐿. The initial specification by Fromm7 that 𝑍𝑍 > 2 is required for the ejection of stable droplets was revised and updated by Derby11 to an acceptable range of 1 < Z < 10. The formation of stable droplets implies single droplets with tail formation (c) as seen in Figure 1. If these conditions cannot be met, so-called satellite droplets (b) will be formed, which impede print quality. Figure 1: Scheme of (a) continuous fluid flow, (b) satellite drops and (c) stable drop with tail. Looking closer into the effects on how a droplet with tail is formed, a cylindrical fluid shape flowing out of the nozzle is assumed in the beginning of the process. During the process a droplet develops from the cylindrically shaped fluid flow and forms a filament combining fluid cylinder and droplet. With continuous approach of the droplet towards the solid surface the filament eventually breaks and a flying droplet is 3 2.1 Ink formulation In the formulation of photochromic UV-curable inkjet inks two commercial heterocyclic spiro-compounds are used. Reversacol® Ruby Red (RR), a naphthopryan-type and Reversacol® Sea Green (SG), a spirooxazine-type dye from Vivimed Labs, UK are combined with a UV-curable varnish to fit print head specifications. The dye concentration in the designed inkjet inks is 4 g/l. The UVcurable varnish consists of dipropylene glycole diacrylate monomers (DPGDA), amine modified polyetheracrylate oligomers (Ebecryl® 81) supplied by Allnex SA/NV, Belgium and a UV-LED photo-initiator (Genocure® TPO-L) supplied by Rahn AG, Switzerland. The ratio of component parts for monomer/oligomer/photoinitiator is 21/1/1. For dissolution and homogeneous dye dispersion solvents are used in the ink formulation, which are removed after stirring by vacuum pumping. Ethyl acetate, 99.9% (Chromasolv® Plus) is used for RR and hexane, ≥ 97% (Chromasolv® HPLC) for SG, both purchased from Sigma-Aldrich. A mixture of isomers, dipropylene glycol methyl ether acetate (DPGMEA) purchased from Sigma-Aldrich, is used as standard cleaning fluid for the Starfire SA print head and serves as a reference fluid for the evaluation of the ink jettability of the photochromic inks. DPGMEA has a surface tension of 31.1 mN/m and a viscosity of 4.8 mPas at 20 °C. 2.2 Rheological and physical properties The viscosity was measured using a rheometer Physica MCR500 from Paar Physica with a double gap cylindrical cell. The viscosity was acquired at the maximum shear rate of the rheometer of 10000 1/s at a temperature sweep from 20 – 40 °C and 40 – 20 °C, which was repeated twice. A shear rate of 10000 1/s simulates the conditions upon jetting as the estimated shear rate at the nozzle tip of a Dimatix print head could reach 40000 1/s. The surface tension of the ink fluids was measured using an optical tensiometer Attension Theta from Biolin Scientific. Three individual measurements were made with the pendant drop method and drop size of 6 μl at 22 °C. 2.3 Visual evaluation of ink jettability The two photochromic inks and DPGMEA are jetted with a Starfire SA print head from Fujifilm Dimatix, which prints with a resolution of 400 dpi and in three different grey scales. The radius of the orifices featured in the print head is estimated to 10 μm and the printable range is specified with η = 8 – 20 mPas (recommended 10 – 14 4 mPas) and γ = 20 – 35 mN/m. Visual analysis of the drop formation is made with a Uridium drop watcher high-speed camera. Jetting results at a head voltage of 110 V, max. frequency of 14 kHz and with a waveform of three pulses with increasing amplitudes of 50/ 70/ 90 V resulting in three dots per drop (DPD). The photochromic inks’ behaviour as function of temperature was analysed in detail in regards to drop volume and drop velocity at a delay of 200 μs, which is optimal for picture processing. The temperature was increased in steps of 5 °C between 25 °C and 40 °C with a waiting time of 30 min for conditioning. For comparison of the visual analysis with the fluids’ theoretical jetting behavior, the printability of the fluids RR ink, SG ink and DPGMEA are calculated based on their physical properties using Equations 2 – 4. In Figure 3, the typical behavior of the inversely proportional relation between temperature and a substance’s viscosity is seen. The increase in temperature has a thinning effect on the different ink formulations. The viscosity of both the varnish (Figure 3a) and the photochromic inks (Figure 3b-d), irrespective of dye concentration and dye type, decreases with increasing temperature. The viscosity has been measured twice at a temperature sweep from 20 °C to 40 °C for the formulations after a cooling phase. It can be seen that for the second set of measurements the viscosity is lower at the starting temperature for all ink formulations, which is a characteristic of the inks’ inertia. On average a 20 °C increase in temperature lowers the viscosity from 0.014 Pas to 0.010 Pas, which approximates a decrease of 0.001 Pas per 5 °C. The change in viscosity as function of temperature seems to be characteristic for the UV-curable varnish, which is neither influenced by the type or concentration of photochromic dye. 3. Results and Discussion 3.1 Ink characterization UV-curable photochromic inks are specialty inks, which are designed for photochromic sensory applications on textile surfaces. The UV-curable photochromic inks RR and SG ink are characterized according to their absorbance for the expected photochromic color effects and to their physical properties in order to match print head requirements. Color effects of photochromic dyes are inherent to the respective dye class, influenced by solvent polarity and temperature with generally higher thermal conversion at higher temperatures 13,14. Naphthopryan dyes are inherently more stable and less temperature-sensitive than spirooxazine dyes 15-17. As can be seen in Figure 2, RR has stronger absorbance in polar solvents as acetonitrile, whereas SG has stronger absorbance in non-polar as hexane. Both these specific commercial photochromic dyes exhibit rather low temperature-dependence, which is desirable for the application at varying temperatures for wearable or other non-wearable smart applications. a b RR Hexane (472 nm) Toluene (485 nm) Absorbance Absorbance Toluene (623 nm) Acetonitrile (633 nm) 2 2 1 0 SG Hexane (635 nm) Acetonitrile (485 nm) 0 5 10 15 20 Temperature (°C) 25 30 Figure 3: Influence of temperature on the viscosity of UV-curable varnish (a) and photochromic inks with 2.5 g/l of RR (b), 4 g/l of RR (c) and 4 g/l SG (d) measured in two sweeps each (.1/.2) 1 In terms of physical properties as viscosity and surface tension, RR and SG ink are similar with a viscosity of ca. 14.5 mPas and a surface tension of 31 mN/m as listed in table 1. SG ink has a lower density with 1024 kg/m3 compared to RR ink with 1077 kg/m3. Despite differences in density, the resulting Z, which determines the ink printability, reaches similar values with 1.28 for RR ink and 1.24 for SG ink. In respect to drop velocity and calculated dimensionless numbers Re and We, RR and SG inks distinguish themselves more from one another. Whereas RR ink drops reach a velocity of 4.35 m/s at a delay of 200 μs, SG ink drops are slower with a velocity of 0 0 5 10 15 20 25 30 Temperature (°C) Figure 2: Temperature and solvent dependency of absorbance of (a) Ruby Red (RR) and (b) Sea Green (SG) photochromic dyes 5 6 2.39 m/s. This also has an impact on the Reynolds and Weber number, which both are higher for RR ink than SG ink. To compare the jetting behavior of the ink formulations with a standard fluid, DPGMEA is used. Compared with the photochromic inks, DPGMEA has a similar density of 980 kg/m3, a similar surface tension of 31.1 mN/m, but a three-fold lower viscosity with 4.8 mPas. The lower viscosity affects the dimensionless numbers Re, We and Z with increased values. Z of DPGMEA reaches a nearly three-fold higher value with 3.64 compared to the photochromic ink formulations. According to the original specification of ink jettability of Fromm7 that Z > 2, this would mean that DPGMEA is printable, but RR and SG inks are not. According to Reis and Derby’s18 further refinement, however, both photochromic inks and DPGMEA are classed as printable with stable drop formation. a b Table 1: Physical properties and dimensionless numbers of ink fluids at 22 °C. Ink fluid Density (kg/m3) Viscosity (Pas) Surface tension (N/m) Velocity at delay of 200μs (m/s) Reynolds number (Re) Weber number (We) RR 1077 ±31.8 1024 ±20.2 980 0.0143 ±0.001 0.0144 ±0.0004 0.0048 ±0.0001 0.031 ±0.0002 0.0309 ±0.0002 0.0311 ±0.0017 4.35 ±0.14 2.39 ±0.4 6.73 ±0.1 3.28 ±0.11 1.71 ±0.28 13.74 ±0.2 6.58 ±0.44 1.92 ±0.63 14.27 ±0.42 SG DPGMEA Inverse (Z) of Ohnesorge number (Oh) 1.28 1.24 c 3.64 3.2 Visual jetting performance The jetting behavior as function of temperature of the photochromic inks in relation to DPGMEA as a standard fluid is visually analyzed at a delay sweep between 50 and 200 μs. When ink is printed on a substrate, the substrate will be positioned at a distance of 2-3 mm from the nozzle plate, which means that according to the jetting sequences (Figure 4 – 6), UV-curable ink drops have travelled half way towards the substrate at a delay of 200 μs at 35 – 40 °C. Visual analysis is complemented by calculations using the dimensionless numbers Re, We and Z, which determine the printability of ink based on the physical properties of the fluid viscosity η, surface tension γ and drop velocity ν. As seen in the representative photo sequences, after firing of two ink drops of RR ink and SG ink (Figure 4 and 5, respectively), they initially are attached to the ligament, i.e. tail, until the tail dissipates in many small drops, i.e. satellite drops. For DPGMEA, unstable drop formation without definition of ligament and drop occurs from the start (Figure 6). 7 d Figure 4: Representative photo sequence of drop formation of RR ink at a delay between 50 and 200 μs at varying temperatures of (a) 25 °C, (b) 30 °C, (c) 35 °C and (d) 40 °C. 8 a a ¨ c b b ¨ c c ¨ c c ¨ c d ¨ c d ¨ c Figure 5: Representative photo sequence of drop formation of SG ink at a delay between 50 and 200 μs at varying temperatures of (a) 25 °C, (b) 30 °C, (c) 35 °C and (d) 40 °C. Figure 6: Representative photo sequence of drop formation of DPGMEA fluid at a delay between 50 and 200 μs at varying temperatures of (a) 25 °C, (b) 30 °C, (c) 35 °C and (d) 40 °C. 9 10 3.3 Effect of temperature Changes in velocity as a result of change in temperature have an impact on the drop formation, which is seen in Figures 4 – 6, but also in calculated Z numbers (Table 2). Changes in Z, eventually mean that also the expected print quality is affected, which however is not experimentally evaluated in this study. The impact of temperature on the surface tension of the fluids is neglected as of very small changes in the range of room temperature to 40 °C. Hence, γ is constant in the calculations of the Weber number We. When applying Equations 2 – 4 with available experimental data, i.e. temperaturedependent viscosity and velocity values, it can be understood that the jettability improves based on increasing Z values. For RR ink Z increases from 1.27 at room temperature to 1.98 at 35 °C. For higher temperatures than 35 °C the Z decreases again to 1.82, which suggests that 35 °C is an optimal printing temperature for the ink formulation. Table 2: Temperature dependence of physical properties and dimensionless numbers of photochromic inks. Ink fluid Temperature (°C) Viscosity (Pas) Volume (pL) RR ink 25 0.0121 ±0.0014 0.0114 ±0.0009 0.0092 ±0.0040 0.0100 ±0.0006 0.0133 ±0.0006 0.0123 ±0.0010 0.0111 ±0.0003 0.0105 ±0.0002 35.3 ±7 32.3 ±1.2 32.0 ±1.4 37.0 ±1.7 31 ±5 29.7 ±2.9 31.7 ±1.2 27.3 ±1.2 30 35 40 SG ink 25 30 35 40 Velocity at delay of 200μs (m/s) 4.54 ±0.02 5.27 ±0.12 6.04 ±0.06 6.60 ±0.06 2.43 ±0.13 4.17 ±0.12 4.95 ±0.1 5.54 ±0.13 Reynolds number (Re) Weber number (We) Inverse (Z) of Ohnesorge number (Oh) 4.04 ±0.01 5 ±0.11 7.09 ±0.07 7.09 ±0.06 1.87 ±0.1 3.48 ±0.1 4.58 ±0.09 5.42 ±0.12 7.17 ±0.05 9.66 ±0.42 12.63 ±0.24 15.12 ±0.27 1.96 ±0.2 5.76 ±0.33 8.11 ±0.32 10.19 ±0.47 1.51 1.61 1.99 1.82 1.34 1.45 1.61 1.7 Despite similar physical properties, i.e. surface tension and viscosity as presented in Table 1, of RR and SG inks, visual analysis reveals distinct jetting behavior of the photochromic inks. At room temperature (22 – 23 °C) and 25 °C droplets of SG ink have a remarkably lower velocity (ca. 2 m/s) with 2.4 m/s than RR ink droplets with 4.5 m/s. At higher temperatures the difference is smaller (ca. 1 m/s) with νSG = 5 – 5.5 m/s and νRR = 6 – 6.6 m/s. An explanation for the difference in jettability at lower temperatures can be due to the more bulky and rigid structure and higher molecular 11 weight of spirooxazines 19,20, which impedes jetting of the ink from the orifice. At temperatures from 35 °C the lower viscosity of the varnish enables more flexibility for the dye to move and hence smoother jetting flow. Another critical difference between the ink formulations RR and SG is the drop volume. Whereas, temperature does not have a significant and coherent effect on the drop volume, the type of dye has. SG ink exhibits generally smaller drops with 27 – 32 pL compared with RR ink, which jets larger drops of 32 – 37 pL. The reason for a smaller drop volume could also be owing to the difference in molecular structure of the dyes. As mentioned earlier, spirooxazines are more rigid than naphthopyrans and non-planar in their inactivated state, which is the case upon jetting. Spirooxazines also have a higher molecular weight than naphthopyrans, which however is not specified for the commercial dyes RR and SG. As the difference in molecular structure between the two dyes has shown to have a distinct effect on the color kinetics of photochromic prints using RR and SG 21, it is likely to affect drop formation and can cause a bottleneck at the orifice resulting in smaller drop volumes. 3.4 Non-Newtonian effects Most inkjet inks exhibit non-Newtonian behavior as they either contain particles, large molecules, surfactants or other additives 22 or as in the case of DPGMEA, RR and SG inks, consist of a mixture of isomers, monomers and oligomers. In Figure 4 and 5, it can be seen that despite the ink formulations RR and SG ink being classified as jettable fluids within the range 1 < Z < 10 according to Derby 23, satellite drops are formed. But according to the classification, a value of Z > 10 would result in the formation of satellite drops. The observed formation of satellite drops of the ink formulations RR and SG ink presumably is due to the more complex rheological effects of non-Newtonian fluids and their impact on drop formation 12,24,25. However, theoretical models are based on Newtonian fluids. For non-Newtonian fluids, specific adjustments in the waveform were successful in reducing satellite drops and improving print quality 26, which might be a promising investigation for UV-curable acrylate inks. Jo et al.27 found that increasing the viscosity of a fluid, targeting a decrease in Z, stabilizes the tail formation and reduces the risk for satellite drops for Newtonian fluids. Such an increase in viscosity must, of course, be within the possible range for the specific print head. For example, a photochromic RR ink with decreased monomer amount (with 7/1/1-parts of monomer/oligomer/photo-initiator) will result in a viscosity of 22 mPas and a surface tension of 31.9 mN/m. Whereas the surface tension of the ink is within the required range, the viscosity exceeds the specifications of the Starfire SA print head as stated earlier. Calculation of the Z value with an assumed similar drop velocity as for the RR ink with 4.35 m/s and similar density of 1080 kg/m3 results in a Z = 0.8, which predicts that the fluid is not printable. A Z < 1 would prevent drop ejection due to viscous dissipation 28. Despite the fact that this formulation is non-jettable, it shows that an increase in viscosity can decrease the Z value. According to print head specifications of the Starfire SA, the physical properties of DPGMEA are partially challenging. Whereas a surface tension of 31.1 mN/m is in the 12 jetting range, the three-fold lower viscosity than the photochromic inkjet inks of 4 – 5 mPas is below the recommended range for the Starfire SA print head. This would make DPGMEA not an ideal fluid for good print quality. However, according to calculations based on the physical properties of DPGMEA with Re = 13.74, We = 14.27 and a resulting Z = 3.64, the fluid is within the printable region of 1 < Z < 10. It is though obvious from the photo sequence (Figure 6) that due to the low viscosity of the fluid uncontrolled dripping results and satellite drops dominate. It can also be seen that temperature does not have a decisive effect in changing the jetting behavior as compared to photochromic inks, where it is obvious that with increasing temperature the tail becomes longer, a distinct drop forms earlier and the velocity increases (Figures 4 and 5). 3.5 Tail formation The length of the formed tail upon jetting as mentioned above varies as effect of temperature, but also between RR and SG inks as seen in Figure 4 and 5. SG ink tends to exhibit shorter tails than RR ink, with generally increasing length with higher temperature. This visual observation is contradictory to the improved calculated Z value as a result of temperature, as a longer tail is supportive of the formation of satellite drops. Vice versa, a short tail and an optimum viscoelastic behavior of the fluid will lead to that the tail is instantly pulled into the drop after ejection and jet breakup at the nozzle 29. If the formed tail separates from the drop head this is either caused by Rayleigh-Plateau instability or end pinching 30. The tail length is predominantly influenced by the printable fluid’s density, where a small increase in density may reduce the tail length significantly 31. But also, a decrease in viscosity can reduce the tail length non-linearly of polymer-loaded ink solutions. The decrease in viscosity of polymer-containing inks in the production of scaffolds to stabilize tail formation 31 suggests the opposite trend compared to an increase in viscosity of Newtonian fluids proposed by Jo et al 27. As an increase in viscosity of the photochromic ink negatively influenced the calculated Z value, change in density in the ink formulation may be help to stabilize drop formation by achieving shorter tail length. However, although SG ink has most stable drop formation with shortest tail at 25 °C, drop velocity at this temperature with 2.4 m/s is very low, which impeded printing on a textile substrate. Therefore, not just in theoretical calculations but also in practice, it could be shown that temperature is an important means of improving ink jettability and resulting print quality. For inkjet printing of photochromic inks on polyester fabric the print head temperature was set to 35 °C 21,32. However, these studied did not include the analysis of resolution and sharpness of the printed patterns, which possibly improves with more stable drop formation. 3.6 Jet breakup The formation of satellite drops, despite measured and calculated values for stable drop formation, necessitates the discussion in relation to breakup of liquid jets. Several factors influence the behavior of fluids upon breakup of liquid jets, which is decisive for how a drop is formed and the resulting print quality. Although the print 13 quality is not subject to this study, it is known that the instability and breakup of a liquid jet into fine secondary drops while travelling to the printable substrate, results in loss of print quality 33,34. Vastly studied phenomena in primary and secondary breakup of jets are distinguished with four modes of primary breakup, initially observed by Ohnesorge 35 and Reitz 36. Primary breakup occurs closer to the nozzle plate, where larger ligaments and drops detach from a liquid jet. With increasing perturbation effects in primary breakup, the Rayleigh, first and second wind induced and atomization mode are distinguished 37,38. According to the definitions, RR and SG inks mainly exhibit Rayleigh behavior, where a liquid jet is disrupted by capillary instability caused by axisymmetric perturbations. Drops, which are formed have similar or larger diameter as the jet itself. DPGMEA shows larger instability with agreement of what is defined as first wind induced breakup, where perturbations on the jet surface are present, formed drops are smaller, vary more in size distribution and satellite drops can occur in between main drops. In secondary breakup further deformation and breakup of the larger ligaments and drops into smaller ones continues until a stable drop is formed 37. Here, for fluids with a Weber number below the critical value of We ≈ 12, drops are impacted by oscillation, which cause deformation and eventually vibrational breakup 39,40. This is the case for SG and RR ink, although drops of SG ink are more stable with a lower We of 1.9 than for RR ink with We = 6.6. DPGMEA has a We of 14.3, which is classified as bag breakup (We > 12), where drops initially deform into a spherical cap and then to a flat disk 39-42. 4. Conclusion This paper points out the discrepancy between theory of jetting of ink, which is based on Newtonian fluids and its application for functional inkjet inks with non-Newtonian behavior. Although calculation of the dimensionless numbers We, Re and Z categorizes ink printability with stable drop formation of RR and SG inks, visual analysis displays flaws and the formation of satellite drops. Phenomena affecting the jettability of UV-curable photochromic ink as non-Newtonian fluid behavior, tail formation and jet breakup to explain the formation of satellite drops of different ink formulations are discussed. Temperature is a main factor in changing the jetting behavior of naphthopryan- and spirooxazine-based UV-curable photochromic inks. Higher temperature increases drop velocity and improves the theoretical printability of the ink expressed in the Z number. Eventually, a temperature of 35 °C is preferable for ink jetting of both RR and SG ink on a substrate. The combination of visual and theoretical analysis enables investigation of the special jetting behavior of functional inks. Therewith, the design of functional inks, exceeding photochromic inks, can be optimized and eventually facilitate the production of smart and functional textiles with inkjet printing and UV curing as given production techniques. Acknowledgements The authors gratefully acknowledge the support from Borås Stad, TEKO (The Swedish Textile and Clothing Industries Association) and Sparbanksstiftelsen 14 Sjuhärad for enabling this research. Author contributions S.S., J.Y. and V.A.N. established the conception, research questions and experimental design in this study. S.S. conducted all experiments, data acquisition, initial analysis and interpretation. Further analysis of the results, evaluation and discussion were equally done by all authors. S.S. prepared the manuscript draft and graphics, whereas improved versions were equally contributed to by all authors. 14 15 16 Competing interests The authors declare no competing interests. References 1 Mendes-Felipe, C., Oliveira, J., Etxebarria, I., Vilas-Vilela, J. L. & LancerosMendez, S. State-of-the-Art and Future Challenges of UV Curable PolymerBased Smart Materials for Printing Technologies. Advanced Materials Technologies 4, 1800618, doi:10.1002/admt.201800618 (2019). 2 Liou, T.-M., Chan, C.-Y. & Shih, K.-C. Effects of actuating waveform, ink property, and nozzle size on piezoelectrically driven inkjet droplets. Microfluid. Nanofluid. 8, 575-586, doi:10.1007/s10404-009-0488-4 (2010). 3 Liu, Y.-F., Tsai, M.-H., Pai, Y.-F. & Hwang, W.-S. Control of droplet formation by operating waveform for inks with various viscosities in piezoelectric inkjet printing. Appl. Phys. A 111, 509-516, doi:10.1007/s00339013-7569-7 (2013). 4 Glynne-Jones, P. 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Measurement of liquid droplet disintegration mechanisms. Int. J. Multiphase Flow 6, 227-239, doi:https://doi.org/10.1016/0301-9322(80)90013-0 (1980). 17 Paper II Inkjet printing and UV-LED curing of photochromic dyes for functional and smart textile applications S. Seipel, J. Yu, A. Periyasamy, M. Viková, M. Vik and V. A. Nierstrasz RSC Advances, 2018 RSC Advances RSC Advances PAPER Cite this: RSC Adv., 2018, 8, 28395 Inkjet printing and UV-LED curing of photochromic dyes for functional and smart textile applications† Sina Seipel, Michal Vik b *a Junchun Yu, a Aravin P. Periyasamy, and Vincent A. Nierstrasz a b Martina Viková, b Health concerns as a result of harmful UV-rays drive the development of UV-sensors of different kinds. In this research, a UV-responsive smart textile is produced by inkjet printing and UV-LED curing of a specifically designed photochromic ink on PET fabric. This paper focuses on tuning and characterizing the colour performance of a photochromic dye embedded in a UV-curable ink resin. The influence of industrial fabrication parameters on the crosslinking density of the UV-resin and hence on the colour kinetics is investigated. A lower crosslinking density of the UV-resin increases the kinetic switching speed Received 9th July 2018 Accepted 28th July 2018 DOI: 10.1039/c8ra05856c rsc.li/rsc-advances of the photochromic dye molecules upon isomerization. By introducing an extended kinetic model, which defines rate constants kcolouration, kdecay and kdecolouration, the colour performance of photochromic textiles can be predicted. Fabrication parameters present a flexible and fast alternative to polymer conjugation to control kinetics of photochromic dyes in a resin. In particular, industrial fabrication parameters during printing and curing of the photochromic ink are used to set the colour yield, colouration/decolouration rates and the durability, which are important characteristics towards the development of a UV-sensor for smart textile applications. Introduction The main drivers and motivation behind the sensor technology of smart textiles are the changing environment, an increasing need for protection mechanisms and a demand for lightweight products with highly integrated functions. Smart textiles help the user to sense and respond to external stimuli, and ideally adapt to them.1 The need for UV-sensors to detect UV rays from sunlight is demonstrated using a variety of materials like hydrated tungsten oxide nanosheets,2 conductive polymer and CNT composites,3 uorescent polyoxometalate and viologen,4 multifunctional ZnO-based biolms5 and various photochromic compounds6 in recent research studies. For smart textile applications, photochromic materials provide excellent properties to function as exible sensors, which display a reversible colour change and therewith warn the user of the presence of harmful UV-radiation in everyday situations where UV-light is not an obvious threat, i.e. windy and partially cloudy weather conditions. Upon UV-irradiation photochromic dyes such as organic spiro-compounds reversibly change colour, from colourless to a colour. Elevated photoelectric energy rearranges the a Textile Materials Technology, Department of Textile Technology, Faculty of Textiles, Engineering and Business, University of Borås, 501 90 Borås, Sweden. E-mail: sina. seipel@hb.se b Department of Material Engineering, Faculty of Textile Engineering, Technical University of Liberec, 461 17 Liberec, Czech Republic † Electronic supplementary 10.1039/c8ra05856c information (ESI) This journal is © The Royal Society of Chemistry 2018 available. See DOI: molecular structure of the dye, which is reversible either through visible light or heat. The isomers A and B of the dye have different absorption spectra.7 In recent literature, the development of UV-sensing textiles using photochromic dyes has been paid much attention. The integration of photochromic dyes into textile structures has been proven successful with traditional textile production techniques such as screen-printing,8–10 dyeing11,12 and mass dyeing.10,13,14 Moreover, the application of photochromic dyes using novel production techniques has emerged. Whereas, Aldib15 investigated the potential of digital inkjet printing of solvent-based ink to produce photochromic textiles, Fu et al.16 studied photonic curing to produce a photochromic crosslinked polymer. In other areas of textile colouration such as pigment printing, the advantages of inkjet printing in combination with photo-curing of an aqueous polyurethane acrylate system17 or by mini-emulsion encapsulation18 were explored. To the best of our knowledge, the combination of digital inkjet printing and UV-LED curing has not yet been studied in the industrially applicable fabrication of a UV-sensing smart textile using photochromic dye. UV-curable coatings distinguish themselves from thermally dried coating or binder systems, such as water-based acrylate binders typically used in screen-printing, by their high rigidity and three-dimensional cross-linked structure.19 According to Schwalm et al.20 and Agarwal,21 depending on the chemistry, i.e. the choice of oligomers and monomers in the formulation, the coating's exibility, adhesion and hardness can be controlled. RSC Adv., 2018, 8, 28395–28404 | 28395 Substrate-ink adhesion and exibility are particularly important when printing on exible substrates like textiles. In general, UVcurable inkjet inks exhibit good adhesion to porous substrates as compared to solvent-based alternatives.22 Inkjet printing and UV-curing also provide a controllable process, where the thickness of the printed ink and the cross-linked network can be controlled. Eventually, a high degree of crosslinking is desired to achieve good adhesion of dyes in a solid ink matrix onto the textile surface and to ensure a durable functional surface treatment. The degree of polymer crosslinking of the carrying ink also affects the rate of the switching reaction of photochromic dyes such as spirooxazines or naphthopyrans between their ring-closed colourless and ring-opened coloured state (Fig. 1(a)). A 90 rotation in the molecular structure between non-planar and planar isomers requires space.7,23,24 Hence, the structural molecular changes of the dyes upon isomerization are largely affected by the properties of the host matrix. Whereas, Ercole et al.25 and Malic et al.26 studied the placement of naphthopyran dye in host polymers, Mutoh et al.27 investigated block copolymers as host matrix. Other factors, which inuence the photochromic response are chain length, free volume and rigidity of the host matrix.24,27,28 In particular, the higher the crosslinking density of the ink, the lower the degree of freedom in the matrix and hence the slower the switching reaction of the photochromic dye is.25,26,29 In this paper, we explore and tune the colour performance of a novel UV-responsive textile using fabrication parameters of an industrially applicable process. Altering the curing intensity of the photochromic ink on the textile during fabrication, inuences the rigidity of the UV-resin, the dye-polymer matrix interaction and hence the degree of freedom of the dye to switch between isomers in the matrix. In particular, the photo-switching of the Paper photochromic textiles is inuenced by the combination of two challenging functions of electromagnetic radiation during production and characterization. The photochromic textile, which is fabricated by inkjet printing and UV-LED curing is activated by UV-light once in use. The impact of UV-rays during curing and activation requires the introduction of an extended kinetic model to describe the kinetics of UV-curable photochromic materials. The lowest curing intensity achieves highest colour yields and fastest switching behaviour between the coloured and colourless state of the photochromic dye Ruby Red in a UV-curable resin. The proposed kinetic model is an important step towards the development of a smart textile UV-sensor and allows the prediction of the colour performance based on fabrication parameters. Materials and methods Materials A commercial heterocyclic spiro-compound (Reversacol Ruby Red) from Vivimed Labs, UK was used as photochromic dye in a UVcurable inkjet ink formulation to produce a textile UV-sensor. The photochromic ink contains a dye concentration of 2.5 g L1. The UV-curable carrier consists of dipropylene glycole diacrylate monomers (DPGDA), amine modied polyetheracrylate oligomers (Ebecryl 81) supplied by Allnex SA/NV, Belgium and a UV-LED photo-initiator (Genocure TPO-L) supplied by Rahn AG, Switzerland. Ethyl acetate, 99.9% (Chromasolv Plus), purchased from Sigma-Aldrich was used as media to disperse the photochromic dye in the ink formulation, but was removed aerwards by vacuum pumping. Plain-woven polyester (PET) fabric with a weight of 147 g m2 and 20 and 23 threads cm1 in warp and we direction, respectively, received from FOV Fabrics, Sweden was used as the substrate for digital inkjet printing. Production of photochromic prints Fig. 1 (a) Isomerization of a photochromic dye between its colourless ring-closed and coloured ring-opened form. The planes indicate the remaining chemical groups of the heterocyclic organic compound that partially undergo a 90 rotation followed by cleavage of the spiro-carbonoxygen. (b) Production process scheme of photochromic prints. (1) Printing of various passes of the UV-curable photochromic ink on PET. (2) Substrate transportation with a belt speed of 50 mm s1 or 300 mm s1 and curing with 1, 25 or 80% of the maximum UV-LED lamp power. (c) Inactive (I) and active printed (P) and washed (W) photochromic prints with 1, 5 and 10 printing passes cured at 50 mm s1 and 80%. 28396 | RSC Adv., 2018, 8, 28395–28404 Photochromic prints were produced using a Sapphire QS 10 print head from Fujilm Dimatix, USA. Printing was carried out in multi-pass mode with a resolution of 300 dpi and at a print head temperature of 35 C. To determine the effect of the amount of ink deposition on the crosslinking density and colour behaviour, different amounts of ink were deposited during printing. 1, 3, 5, 7 and 10 passes of ink were printed on PET, which correspond to ink amounts of 3, 8, 10, 13 and 19 g m2, respectively. Subsequently, the prints were cured using a UV-LED lamp FireJet from Phoseon Technology, USA with emission wavelengths of 380 to 420 nm and a maximum emission power of 6 W cm2 (the UV-LED lamp is part of a continuous pilot-scale inkjet printing system with a conveyer belt for substrate transportation). To determine the optimal curing efficiency and colour performance, curing conditions were varied. Hence, two different conveyer belt speeds, 50 mm s1 and 300 mm s1, and three different lamp intensities, 1%, 25% and 80%, were used (Fig. 1(b)). To determine the effect of a cleaning step, one set of samples was washed once aer printing and curing. Washing was done using procedure 4 N and reference detergent 3 according to ISO 6330:2012. The samples were dried using drip at drying. This journal is © The Royal Society of Chemistry 2018 Paper RSC Advances UV-Vis spectroscopy UV-Vis spectra were collected using a Libra S60 double beam spectrophotometer from Biochrom, UK and a UV-3101PC from Shimadzu, Japan double beam spectrophotometer with integrated sphere accessory to affirm t of the photo-initiator for the UV-LED lamp of the pilot-scale inkjet printing system and to determine the inuence of UV-varnish on the absorption spectra of the dye. Colour analysis The colour behaviour of the photochromic prints was measured using an LCAM Photochrom 3 spectrophotometer, which allows continuous colour measurement upon cycles of UV-exposure and relaxation30 (Fig. 2). The activation light source was an Edixon UV-LED lamp (EDEV-3LA1) with a radiometric power FV of 350 mW and an emission peak with wavelengths between 395 nm and 410 nm. The measuring light source was a dual light source system of combined high power white LEDs with CCTs of 4000, 5000 and 7000 K. The measuring light source system illuminates samples with illuminance of 60 klx. This illuminance is a compromise between brightest sunlight intensity (120 klx) and daylight intensity without direct sunlight at noon (20 klx). In addition, the measuring light source is equipped with a set of high-pass lters preserving light transmission bellow 420 nm, which decrease possible activation of photochromic composition. Colour values K/S were measured from reectance values R using the Kubelka–Munk function: 2 K ð1 � RÞ ¼ 2R S (1) The kinetic model shown in eqn (2) follows rst-order kinetics and is generally used to describe photochromic colour behaviour for both the colouration and decolouration reaction as seen in Fig. 2.31 K K K K e�kt þ D � (2) ¼ S S 0 S N S N Based on the kinetic model, the sensor functionality was specied by its achieved colour intensity DK/S upon activation with UV-light, its rate constant of colour increase kcolouration to achieve maximum colouration K/SN, and its rate constant of colour decrease kdecolouration to revert to the initial colourless state K/S0. DSC analysis The inuence of deposited ink amount and curing conditions on the curing efficiency was determined indirectly by analysing the shi of the melting peak Tm of PET using a Q1000 DSC from TA Instruments, USA. The DSC samples sealed in aluminum pans were weighted. The balance (Kern) has 99.96% accuracy when calibrated against a standard weight of 100 mg. The heat ow of samples of about 5.0 mg was measured during a heating and cooling cycle from 25 � C to 290 � C and back to 25 � C with a rate of 10 � C min�1. The released energy DH of the endothermic reaction during melting of the material gives indirect information on the degree of polymer crosslinking at the interface between the PET fabric and the photochromic UVcured ink. The shi of the melting peak Tm towards higher temperatures compared to the Tm of pure PET fabric indicates that an insulating layer is built surrounding the textile yarn/ ber. Depending on the curing intensity, the insulation effect of the print varies. The adhesion of the ink on the PET surface is indicated by the difference in melting peak temperature DTwashed of the photochromic prints before and aer washing. Washing hence gives information about the durability of the photochromic, UV-cured surface treatment on PET. Statistical analysis Statistical analysis of colour measurement and DSC data was conducted in Origin 2017 from OriginLab Corporation, USA at a condence interval of 95%. One-way ANOVAs were used to determine the effect of washing on the crosslinking density of the ink, colour yields and rate constants of the dye. Two-way ANOVAs were used to determine the effect of the production parameters belt speed and lamp intensity, as well as the signicance of their interaction. RSC Advances ow into PET during DSC measurement. The DSC measurement therefore gives indirect information on the polymer crosslinking density of the photochromic ink by measuring the melting temperature (Tm) of the PET fabric with UV-cured photochromic prints. For prints with increased ink deposition, or cured with higher intensity, i.e. higher lamp intensity and/or lower belt speed, the ink has a higher crosslinking density. Hence, it forms a thicker or more distinct insulation layer on the PET fabric surface. This in return increases the Tm and differences in the melting peak temperature, i.e. DTm of the PET fabric with photochromic prints on its surface compared to the untreated PET fabric. Inkjet-printed samples with different printing passes cured at a belt speed of 50 mm s1 and 80% of the maximum lamp power (Fig. 3), showed a linear increase of Tm from 252.8 C for 1 pass to 255.4 C for ten passes. Prints with more amount of cross-linked polymer deviate more from the Tm of 252.9 C of untreated PET fabric as they provide progressively more insulation. Furthermore, the prints showed a more signicant shi of Tm with higher curing intensity (in combination with lamp intensity and belt speed). As shown in Fig. 4, an increase of lamp intensity, 1%, 25% to 80%, increases the Tm of the photochromic prints on PET. The prints produced at 300 mm s1 showed a progressive increment of Tm with 254.4 C, 255.0 C and 255.4 C, i.e. DTm of 1.5 C, 2.1 C and 2.5 C. A similarly increasing trend of Tm was detected for prints produced at 50 mm s1 with various curing intensities but with less significant variation in DTm, i.e. 2.3 C, 2.4 C and 2.5 C respectively. The saturation of Tm in prints produced at 50 mm s1 indicates that the increase of curing power cannot further increase the crosslinking density in the UV-curable ink. The inkjet-printed photochromic material accounts for ca. 10 wt% of the measured DSC sample. To ensure representative measurements, the Tm and the standard deviation were taken from three independently Results and discussion Design of the UV-curable ink UV-Vis spectra conrm the t of photo-initiator (absorption peak between 340 and 410 nm) used in the ink formulation with the UV-LED light source (emission band of 380 to 420 nm) to initiate polymer crosslinking. The normalized absorption peaks of the photochromic dye in solvent and UV-resin conform to each other. Ruby Red has an absorption peak of 500 nm in ethyl acetate and 490 nm in the UV-curable resin (Fig. S1 in ESI†). Analysis of the measured colour values, rate constants for colour development and reversion are hence made at a wavelength of 500 nm. Scheme of colour measurement cycle with 300 s of UVexposure for colouration and 900 s of relaxation for decolouration. Fig. 2 This journal is © The Royal Society of Chemistry 2018 RSC Adv., 2018, 8, 28395–28404 | 28397 Fig. 4 Comparison of melting peak temperatures Tm for 10-pass prints of printed samples transported at belt speed of 50 mm s1 ( ) and 300 mm s1 ( ) and washed samples transported at 50 mm s1 ( ) and 300 mm s1 ( ). Trend lines indicate the linear fit of Tm as function of lamp intensity. measured samples. The largest standard deviation in DSC measurements is 0.5 C, which is in general smaller than presented in Table 1. Furthermore, an indium sample was measured three times with a Tm of 156.9 0.05 C, which is comparable with literature, where indium is reported with a Tm of 156.6 C. Moreover, in order to exclude the effect of UV-radiation on Tm of the PET fabric, the Tm of UV-cured PET fabric only, i.e. non-printed, was measured by DSC with 254.0 0.1 C. In this case, the detected variation of DTm (Tm of measured samples subtracted by 254.0 C, Fig. 3(a)) which is reduced to 1 to 1.5 C, however, is still more signicant than the measurement error. It can be concluded that the detected variation of DTm is small in DSC in general, but as it shows systematic variation and higher signicance than the measurement error, it supports the validity of the experimental setup. Moreover, tting and extrapolation of the data (Fig. S2 in ESI†) suggested that the Tm increased exponentially as a function of printing passes, which saturates at around 50 printed passes and a Tm of 257.1 C. This indicates that measuring Tm with DSC can approximate the amount of cross-linked UV-ink at a certain curing condition on the PET fabric. Colour performance of the photochromic textile DSC analysis of crosslinks in the UV-curable ink Effect of deposited ink amount and curing settings on the crosslinking density. Thermal analysis of the interface between the printed photochromic ink and the PET bre was measured by DSC. The cross-linked UV-ink gives no endothermic behaviour, however, it behaves as heat insulation layer, which affects the heat Paper (a) Melting peak temperature difference DTm between UVcured prints and non-printed PET, after printing ( ) and washed ( ). The prints are cured at 50 mm s1 belt speed and 80% lamp intensity (b) linear increase of Tm for 1- to 10-pass prints cured at 50 mm s1 belt speed and 80% lamp intensity printed ( ) and washed ( ). Fig. 3 28398 | RSC Adv., 2018, 8, 28395–28404 Extended kinetic model for the colouration reaction. Colour measurements with the LCAM Photochrom 3 spectrophotometer have shown that the rst-order kinetic model for the colouration reaction (eqn (2)) is only valid for photochromic prints with a high polymer crosslinking density, i.e. a belt speed of 50 mm s1 and 80% lamp dosage (Fig. S3 in ESI†). For cured prints with a low polymer crosslinking density, hence lower Tm, polymer crosslinking continues during colour measurements and a simultaneous secondary decay mechanism occurs. The trend of deviation from normal photochromic colouration behaviour is supported by DSC measurements. The lower the speed of the transportation belt and the higher the lamp intensity for curing, the higher is the Tm and hence the insulation effect of the cured ink on PET (Fig. 4). When using This journal is © The Royal Society of Chemistry 2018 Paper RSC Advances Table 1 Melting peak temperatures Tm of 10-pass prints at printed and washed condition for all curing setting combinations. DTm denotes the difference in Tm between the printed and non-printed samples. DTwashed denotes the difference in Tm between the printed and washed samples and is a measure of print durability as a result of ink-fibre adhesion Belt speed (mm s�1) Untreated PET 300 50 Lamp intensity (%) Tm (� C) printed Twashed (� C) washed DTm (� C) Tm – UV-treated PET DTwashed (� C) 1 25 80 1 25 80 252.9 � 0.2 254.4 � 0.5 255.0 � 0.1 255.4 � 0.1 255.2 � 0.3 255.3 � 0.2 255.4 � 0.2 — 254.3 � 0.1 254.6 � 0.3 254.8 � 0.2 254.7 � 0.4 254.7 � 0.2 255.2 � 0.3 0.4 1.0 1.4 1.2 1.3 1.4 0.1 0.4 0.6 0.5 0.6 0.2 a conveyer belt speed of 50 mm s�1, the differences in polymer crosslinking density through curing at different radiation intensities, 1, 25 or 80%, become less crucial. At high belt speed, i.e. 300 mm s�1, increasing lamp power plays a decisive role in the crosslinking density that is achieved. Deviation from eqn (2) for the colouration reaction is in line with the degree of polymer crosslinking in the following order, starting from the curing settings with the highest deviation: 300 mm s�1/1% > 300 mm s�1/25% > 50 mm s�1/1% > 50 mm s�1/25% > 300 mm s�1/80% > 50 mm s�1/80% The combination of highest belt speed and lowest lamp intensity, 300 mm s�1 and 1%, has the highest deviation from the kinetic model in the rst colouration cycle as a result of lowest polymer crosslinking density. This conforms to the colour data, which shows that UV-light exposure during colour measurement of samples with low crosslinking density decreased DK/Scolouration in the subsequent colouration cycles 2, 3, 4 and 5 (Fig. 5). This can be a result of continuous curing and simultaneous dye degradation. High UV-radiation improves the cure speed of the resin32,33 and longer irradiance increases the degree of polymer crosslinking.21 Meanwhile, photodegradation of photochromic dyes is commonly observed under prolonged UV-irradiance.34–36 Therefore, for prints with lower polymer crosslinking density, we introduce a model that denes the kinetics of a decay mechanism as a result of continuous curing while colour measurement and reconstructs the actual colouration curve (Fig. 5). For ink systems, where UV-light is used to cure the photochromic prints with the aim to optimize curing conditions, the following extended kinetic model is proposed. K K K K D e�kcolouration t þ ¼ � S colouration S 0col S Ncol S Ncol Under the boundary condition that the processes of colouration and decay start simultaneously and at DK/S ¼ 0, which means that K/S0col ¼ 0 and K/S0dec ¼ 0, eqn (3) reduces to K K K D ¼ 1 � e�kcolouration t � S colouration S Ncol S Ndec 1 � e�kdecay t (4) Via continuous UV-exposure over 1500 s it could be excluded that the decolouration reaction interferes with the observed decay mechanism during the colouration reaction (Fig. S5 in ESI†). Rate constants of the decay mechanism kdecay were tted as exponential decay during UV-exposure, i.e. to the decrease in K/Scolouration throughout the ve UV-exposure cycles during the colouration reaction, which add up to 1500 s. The extended kinetic model, eqn (4), has been applied to the colour data of inkjet-printed samples with a deviation from eqn (2), i.e. R2 < 0.95, to reconstruct the K/Scolouration values and kcolouration, as well as to determine the extent of decay depending on the curing setting and applied amount of ink. An increasing amount of ink inuences the secondary decay of the dye throughout colour measurement. It is a common trend for all curing settings that the lower the amount of applied ink, the higher is kdecay. Prints cured at a belt speed of 50 mm s�1 and 1% lamp power show decreasing kdecay from 0.017 s�1 to 0.014 s�1 to 0.004 s�1 for 3, 5 and 10 printing passes, respectively. With increasing layer thickness oxygen diffusion is aggravated and the primarily affected surface area is proportionally reduced. As presented in Table 2, the secondary decay decreases with stronger curing conditions from kdecay ¼ 0.006 s�1 for 300 mm s�1 and 1% to 0.002 s�1 for 50 mm s�1 and 25%. Also, variation in kdecay is higher when transported at 300 mm s�1. Hence, the decreasing trend of variation in kdecay and rates with increasing crosslinking density of prints directly correlates with the trend of deviation from eqn (2). The intensity of the light source used for exposure in the LCAM Photochrom 3 spectrophotometer with 8 mW cm�2 at sample position is 750-times lower than the UV-LED lamp intensity, i.e. approx. 10-times lower than 1% intensity of the curing lamp. Also, exposure of the prints to UV-light stretches over 300 s during colour measurement instead of approx. 1 s during the curing process. Both low irradiation intensity and long exposure time of the photochromic dye in a liquid resin cause rapid degradation as a result of photo-oxidation. A partially liquid resin has a lower viscosity, which facilitates oxygen diffusion.33 It can be assumed that dye degradation as a result of photo-oxidation occurs in the so component of the resin, which is more prominent compared to when the dye is Belt speed (mm s�1) 300 50 Belt speed (mm s -1) (3) This journal is © The Royal Society of Chemistry 2018 Paper xed in a solid matrix. However, cut-off effects of the dyecarrying medium, i.e. shi in reectance properties as a result of changed degree of cure and media thickness have also been investigated.10,37 We therefore consider the observed decrease a secondary decay, which obviously reduces DK/S and colour reaction kinetics. Effect of amount of ink deposition on the colouration reaction. It was found that the achieved colour intensity DK/ Scolouration of printed and washed samples was linear towards the deposited amount of photochromic ink irrespective of curing conditions (Fig. 7), which agrees with the linearly increasing Tm as function of ink amount. Stronger colour yields with increasing ink amount are visible by eye for both printed and washed samples as can be seen in Fig. 1(c). According to Viková et al.13 and Periyasamy et al.,38 deeper shades are expected with increased layer thickness as a result of lowered surface-bulk ratio and hence, less colourless reection. Aldib15 also observed the trend of a continuous increase in colour yield from 1 to 10 inkjet printing passes of the same dye type but for solvent-based ink. This indicates that the crosslinking density and the adhesion of the UV-resin to the textile surface are not affected by the amount of ink deposition. The amount of printed photochromic ink on PET inuences the kinetics of the colouration reaction. Similar to the trend in decrease in kdecay, an increasing amount of applied material lowers kcolouration as seen in Fig. 6(b) for prints cured at a belt speed of 50 mm s�1 and 80% of lamp intensity. A decrease in kcolouration as function of printing passes can be explained by weakened activation of the photochromic dye with increased layer thickness.10 Washing has a signicant effect on the colouration behaviour of the photochromic prints. Both DK/ Scolouration and kcolouration signicantly decrease as a result of a washing cycle. The reduced colour yield DK/Scolouration approximates 40% of the initial value for all printing passes from 1 to 10. With a decrease of approx. 20% at 10 passes Table 2 Colour yields DK/Scolouration and rate constants kcolouration, kdecolouration and kdecay of 10-pass prints for all curing settings for printed and washed samples in activation cycle 1. Remained DK/S presents the percentage-wise residual colour yield DK/Scolouration after a washing process K K K � � e�kdecay t þ S 0dec S Ndec S Ndec where K/S0col is the initial colour value for the colouration reaction, K/SNcol is the maximum colouration value, K/S0dec is the initial value of the decay and K/SNdec is the nal value of the degrading reaction for each activation cycle. Kinetic rate constants for the colouration reaction and decay are dened as kcolouration and kdecay, respectively. RSC Advances Extended model for reconstruction of photochromic colouration throughout colouration cycles 1–5 (i–v) of a print with 19 g m�2 deposited ink cured at belt speed of 300 mm s�1 and 1% lamp intensity. Measured K/S data ( ) consists of reaction processes of ) and colouration ( ) during colour measurement. The decay ( ) validates the model. calculated sum of decay and colouration ( For this print degradation becomes statistically ineffective in activation cycle 5 (v). Printed Lamp intensity (%) DK/Scolouration kcolouration (s�1) kdecolouration (s�1) kdecay (s�1) 1 25 80 1 25 80 0.467 � 0.038 0.550 � 0.049 0.434 � 0.036 0.376 � 0.032 0.370 � 0.012 0.278 � 0.025 0.157 � 0.018 0.128 � 0.019 0.114 � 0.018 0.145 � 0.019 0.123 � 0.015 0.070 � 0.008 0.011 � 0.001 0.011 � 0.001 0.010 � 0.000 0.010 � 0.001 0.010 � 0.000 0.008 � 0.001 0.006 � 0.003 0.004 � 0.002 0.004 � 0.002 0.004 � 0.001 0.002 � 0.001 — Washed Lamp intensity (%) DK/Scolouration kcolouration (s�1) kdecolouration (s�1) Remained DK/S (%) 1 25 80 1 25 80 0.253 � 0.028 0.266 � 0.007 0.213 � 0.020 0.206 � 0.034 0.185 � 0.016 0.154 � 0.024 0.109 � 0.014 0.086 � 0.014 0.066 � 0.013 0.077 � 0.020 0.061 � 0.011 0.047 � 0.006 0.010 � 0.001 0.010 � 0.001 0.009 � 0.001 0.009 � 0.001 0.009 � 0.001 0.007 � 0.001 54.1 48.3 49.0 54.8 50.1 55.5 Fig. 5 RSC Adv., 2018, 8, 28395–28404 | 28399 300 50 28400 | RSC Adv., 2018, 8, 28395–28404 This journal is © The Royal Society of Chemistry 2018 Paper compared to 40% at 1 pass, kcolouration is less affected with increasing layer thickness. Effect of crosslinking density on the colouration reaction. The production parameters belt speed and lamp intensity have a signicant effect on DK/Scolouration and rate constants kcolouration and kdecay. In Fig. 7, curing parameters belt speed and lamp intensity, 300 mm s�1 + 1%, 50 mm s�1 + 1% and 50 mm s�1 + 80% represent settings, which result in low, intermediate and high crosslinking density, respectively. Prints with high crosslinking density have experienced strongest curing, which increases the rigidity of the UV-resin but may also cause dye degradation. As a result, DK/Scolouration is lowered. As seen in Table 2, highest DK/Scolouration are achieved with curing at belt speed of 300 mm s�1 and low lamp intensity, i.e. K/Scolouration ¼ 0.47 with 1% and K/Scolouration ¼ 0.55 with 25%. A high crosslinking density provides good ink-substrate adhesion, which is seen in high remaining DK/Scolouration values aer washing. The remaining DK/S of 10-pass prints aer a washing process is highest for strongest curing conditions, i.e. 55.5% for 50 mm s�1 and 80%. Washing decreases DK/ Scolouration signicantly for all curing conditions, with maximum remaining 55.5% to minimum 48.3% for 10-pass prints. It can be concluded that stronger curing conditions result in low absolute DK/Scolouration and low kcolouration and kdecolouration. A low polymer crosslinking density of the carrier matrix exhibits a higher amount of so component in the photopolymer, which enables more exibility and free volume for faster switching of the photochromic dye molecule.24,27 A comparison of colour intensities and colouration rates between the six combinations of curing settings reveals that the setting with the lowest curing intensity, i.e. belt speed of 300 mm s�1 and lamp intensity of 1%, is the most favourable. High DK/S and fast kinetic switching with kcolouration of 0.157 s�1 aer printing and 0.109 s�1 aer Trend lines indicate (a) linear increase of DK/Scolouration, (b) successive decrease in kcolouration, (c) linear increase of DK/Sdecolouration and (d) successive decrease in kdecolouration as function of printing passes from 1 to 10 cured at a belt speed of 50 mm s�1 and 80% of lamp intensity printed (-) and washed (,). Fig. 6 This journal is © The Royal Society of Chemistry 2018 RSC Advances Fig. 7 Linear increase of DK/Scolouration as a function of printing passes for low, intermediate and strong curing conditions, i.e. 300 mm s�1 + 1% ( , ), 50 mm s�1 + 1% ( , ) and 50 mm s�1 + 80% ( , ), respectively after printing (a) and washing (b). washing between the uncoloured and coloured state of the textile UV-sensor is achieved. A higher cross-linked resin results in lowered rates of colouration with i.e. kcolouration of 0.070 s�1 before and 0.047 s�1 aer washing for 10-pass prints cured at 50 mm s�1 and 80%. Viková10 applied naphthopyran dyes on textiles in a screenprinting paste and achieved colouration rates of 0.09 s�1 and 0.25 s�1 with a concentration of 0.25 wt% at lamp intensity of 800 mW cm�2. The same dyes applied with a concentration of 1 wt% in a spin dope, which was melt-blown to a non-woven achieved colouration rates between 0.01 s�1 and 0.03 s�1.10 With respect to different concentrations and activation powers it can be assumed that similar dye types to Reversacol Ruby Red applied via a UV-cured matrix obtain kcolouration, which are higher than in a bre structure but lower than in a water-based and thermally cured acrylate paste. Effect of amount of ink deposition on the decolouration reaction. The decolouration reaction as shown in Fig. 2 of naphthopyran dye Ruby Red is a thermal reaction, which takes place in the dark and therewith is independent from the colouration reaction.27,39 Decolouration yields DK/Sdecolouration, which correspond to DK/Scolouration, and independent decolouration rates kdecolouration, are calculated using eqn (2). A significant difference is seen for DK/Sdecolouration aer printing and RSC Adv., 2018, 8, 28395–28404 | 28401 RSC Advances aer a washing cycle. The same behaviour as for DK/Scolouration is seen for decolouration intensities. As seen in Fig. 6(c), DK/ Sdecolouration increase linearly as function of printing passes and exhibit approx. 60% of the initial yields aer washing. It is noted that for small amounts of printed ink, i.e. 1 and 3 printing passes, only 35% and 15% of the initial DK/Sdecolouration is lost during the washing process, respectively. With an increasing amount of printed ink, kdecolouration to revert the photochromic dye back to its uncoloured state are successively lowered by ca. 30% when cured with low speed and high lamp intensity, i.e. 50 mm s�1 and 80%. kdecolouration decreases from 0.012 s�1 for 1-pass prints to 0.008 s�1 for 10pass prints. A washing process has a signicant effect and lowers the decolouration rates by 10–15% for all printing passes (Fig. 6(d)). Same as for kcolouration, kdecolouration is inuenced by the thickness of the applied ink on the textile surface. An increase in layer thickness decreases the reaction rate. Effect of crosslinking density on the decolouration reaction. The crosslinking density has a statistically signicant effect on DK/Sdecolouration irrespective of the amount of printed ink, i.e. 1 to 10 passes, on PET before and aer washing. The higher the speed of the conveyer belt, the higher is DK/Sdecolouration. It is also valid, that the lower the UV-dosage during curing, the higher is the expected DK/S. Decolouration rates are most affected by strong curing conditions, that means 50 mm s�1 belt speed and 80% lamp intensity. kdecolouration follows the same trend as DK/Sdecolouration values as a result of change in belt speed and lamp power, i.e. crosslinking density. The lower the crosslinking density, the least rigid is the UV-resin and hence the higher is kdecolouration of the photochromic dye to its uncoloured state. For 10-pass prints transported with a belt speed of 300 mm s�1 and cured with a lamp intensity of 1% kdecolouration is nearly unchanged with a rate of 0.011 s�1 aer printing and 0.010 s�1 aer washing as listed in Table 2. Curing at this condition allows the production of photochromic prints with an expected DK/S ¼ 0.25 aer washing and switching on of the photochromic effect at a rate of 0.11 s�1. Impact on the change in kcolouration and kdecolouration achieved by differences in conveyer belt speeds and lamp intensities during the curing process is less compared to other means of polymer architecture. Ercole et al.25 and Malic et al.26 studied that mid placement of the photochromic dye in homo-polymers can optimize photochromic kinetics. A similar effect can be achieved by tuning the polymer length and rigidity of the polymer matrix. Whereas, via polymer conjugation and, of course with respect to the generic switching speed of the chosen dye molecule, decolouration rates of a naphthopyran dye can be doubled to increased up to a ve-fold from 0.01 to 0.05 s�1,26 by variation of fabrication parameters during curing approx. 1.4-times faster kdecolouration can be obtained. Durability and adhesion of the photochromic ink Effect of deposited ink amount and curing settings on the print durability. Washing, which is an essential feature in textile applications, gives information on the durability and adhesion of a functional treatment on the textile surface. Here, the 28402 | RSC Adv., 2018, 8, 28395–28404 Paper difference in Tm of the photochromic prints can be applied as a measure of durability in terms of adhesion of the ink on the PET surface. The durability of photochromic prints is assessed by measurement of Tm in DSC aer a standard washing cycle (ISO 6330), i.e. the difference between Tm and the melting peak temperature aer washing (Twashed), i.e. DTwashed. The more cross-linked UV-ink is le on the PET surface aer a washing cycle, the more durable is the functional surface, and hence, the smaller the DTwashed. The effect of washing (DTwashed) gives indirect information on the durability of the cross-linked photochromic ink and its adhesion to the surface of PET. Washed samples exhibit a statistically signicantly lowered melting temperature Twashed regardless of the curing condition, as shown in Fig. 4. The decreased Twashed, as listed in Table 1, is a measure for durability in terms of adhesion of the cured photochromic print to the PET fabric surface. While the DTwashed for prints cured at 300 mm s�1 with 1% lamp power remains nearly unchanged with a decrease of 0.1 � C, for prints transported at a belt speed of 50 mm s�1, the trend of the Tm as a function of lamp intensity changes aer washing. Photochromic prints that underwent strongest curing at 80% of the maximum lamp power had almost no change in DTwashed (0.2 � C within the measurement error). Prints that have been cured at both 25% and 1% lamp intensity have a Twashed of 254.7 � C. This gives a DTwashed of 0.6 � C and 0.5 � C for lamp powers 25% and 1%, respectively. The result for the samples processed with 50 mm s�1 suggested that higher lamp power could increase the crosslinking density in the printed UV-ink. Furthermore, the combination of Twashed and DTwashed assesses the durability of the functional layer with different crosslinking densities, e.g., the combination of high Twashed and small DTwashed gives an indication for a durable photochromic layer with high crosslinking density in the prints. Prints cured at 50 mm s�1 and 80% have the highest Twashed of 255.2 � C and a DTwashed of 0.2 � C. It implies that a higher polymer crosslinking density is achieved as the print creates a strong insulation layer on the PET surface. Prints cured with the lowest curing intensity, i.e. 300 mm s�1 and 1%, exhibit a lower polymer crosslinking density, but are durable and exible as the Twashed is the lowest with 254.3 � C (with DTwashed of 0.1 � C). As shown in Fig. 3(b), washing has an effect on the inkjetprinted samples cured at a belt speed of 50 mm s�1 and 80% lamp intensity from 1 to 10 printing passes. The slope of the linear t of Twashed as a function of printing passes is slightly lower than the t of Tm. However, changes in Twashed from 253.8 � C to 253.5 � C (DTwashed of 0.3 � C) for 3 passes and from 255.4 � C to 255.2 � C (DTwashed of 0.2 � C) for 10 passes are small (Fig. 3(b)). The durability of the functional layer (DTwashed) related to process parameters can be conrmed by the difference in DK/S from colour measurements aer washing. The samples produced at 300 mm s�1 and 1% exhibit 54.1% of the initial colour yield, whereas the sample produced at 50 mm s�1 and 80% preserves 55.5% of its initial colour aer one wash. Both curing conditions show decent durability assuming that the colour performance is in linear relationship with the amount of UV-ink printed (remained) on the PET fabric surface. This journal is © The Royal Society of Chemistry 2018 Paper Furthermore, the weight of printed and washed functional layers is complimentary to DSC and colour measurements (ESI†). As shown in Fig. S5,† the samples produced with various amounts of ink deposition cured at 50 mm s1 and 80% were weighed aer printing and washing. The weight of the functional layer, with the exception of 1-pass prints (measured weight loss of 100%), decreased by an average of 23 12%. The weight decrease is due to a loss of the photochromic print during washing, which correlates with the decreasing trends in Tm and DK/S. The result conrms that a belt speed of 50 mm s1 and lamp intensity of 80% are valid process parameters to obtain a durable functional layer with high crosslinking density in prints. In summary, both a belt speed of 300 mm s1 and lamp intensity of 1% and a belt speed of 50 mm s1 and lamp intensity of 80% can produce a durable functional layer but the latter results in a higher crosslinking density in prints. The Twashed of the photochromic prints on PET aer one washing cycle reaches a mean value of 254.7 C for all curing settings. Here, statistical analysis shows that differences in belt speed and lamp intensity are insignicant in regards to Tm and Twashed. Despite an increasing trend in crosslinking density with lower belt speeds and higher lamp powers, it can be assumed that stronger curing intensities than 300 mm s1 and 1% are not more benecial for the durability of the print. Conclusions The colour performance of a novel UV-curable and UVresponsive smart textile is optimized using the combination of two challenging functions of electromagnetic radiation, where high energy UV-rays both cure and activate UV-sensitive naphthopyran dye Ruby Red. Tuning of the crosslinking density of the photochromic ink and hence the dye kinetics using fabrication parameters is an important step towards the design of a textile UV-sensor. Irrespective of the deposited ink amount on the PET surface between 1 and 10 printing passes, curing with the highest belt speed of 300 mm s1 combined with the lowest lamp intensity 1% of the pilot-scale inkjet printing system achieves high colour yields DK/S with 0.47 aer printing and 0.25 aer washing, fastest isomerization with kcolouration of 0.16 s1 aer printing and 0.11 s1 aer washing and good durability aer washing with 54.1% of the initial DK/ S, despite lowest polymer crosslinking density. The effect of the solid photopolymer resin compared to waterborne or solventbased carriers for a photochromic dye, similar to Reversacol Ruby Red, applied on textiles results in slower switching rates between the coloured and uncoloured states, but faster than in mass-dyed and melt-spun fabrics. Our ndings add to the general trend of enhancing switching rates between the different isomers of photochromic dyes, as well as the overall colour yield as a result of reduced rigidity of the polymer matrix. Via a new kinetic model, which takes a decay mechanism of the dye upon colour measurement into account, the colour performance can be predicted and tuned based on fabrication parameters. Beyond tuning of colour kinetics, the novel and industrially applicable approach of UV-LED curing of This journal is © The Royal Society of Chemistry 2018 RSC Advances photochromic inkjet ink is favourable in regards to the exibility, resource-efficiency and cost-effectiveness of the production process. Inkjet printing combined with UV-LED curing has large potential to trigger innovative products and to promote the small-batch production of functional high-end and smart textiles. Conflicts of interest There are no conicts to declare. 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RSC Adv., 2018, 8, 28395–28404 | 28403 28404 | RSC Adv., 2018, 8, 28395–28404 This journal is © The Royal Society of Chemistry 2018 Paper III Colour performance, durability and handle of inkjet-printed and UV-cured photochromic textiles for multi-coloured applications S. Seipel, J. Yu, M. Viková, M. Vik, M. Koldinská, A. Havelka and V. A. Nierstrasz Fibers and Polymers, 2019 Fibers and Polymers 2019, Vol.20, No.7, 1424-1435 DOI 10.1007/s12221-019-1039-6 ISSN 1229-9197 (print version) ISSN 1875-0052 (electronic version) Inkjet-Printed and UV-Cured Photochromic Textiles Fibers and Polymers 2019, Vol.20, No.7 1425 Color Performance, Durability and Handle of Inkjet-Printed and UV-Cured Photochromic Textiles for Multi-Colored Applications Sina Seipel1*, Junchun Yu1, Martina Viková2, Michal Vik2, Marie Koldinská3, Antonín Havelka3, and Vincent A. Nierstrasz1 1 Textile Materials Technology, Department of Textile Technology, Faculty of Textiles, Engineering and Business, University of Borås, Borås 50190, Sweden 2 Department of Material Engineering, Faculty of Textile Engineering, Technical University of Liberec, Liberec 461 17, Czech Republic 3 Department of Clothing Technology, Faculty of Textile Engineering, Technical University of Liberec, Liberec 461 17, Czech Republic (Received October 31, 2018; Revised January 14, 2019; Accepted March 9, 2019) Figure 1. (a) Inkjet printing and UV-curing in a continuous production process using combined conveyor belt speeds (i) and lamp intensities (ii) of 50 mm/s and 80 % and 300 mm/s and 1 % and (b) partially activated multi-colored photochromic print on PET with RR and SG. This article is published with open access at Springerlink.com Abstract: The development and design of novel functional and smart textile materials such as textile sensors and multicolored systems based on photochromic dyes necessitate controls of color intensities, switching speeds, and material durability. Precise control and synchronization of dye kinetics are important for multi-colored photochromic applications especially. However, durability towards abrasion and washing should not be compromised on if we aim to design reliable future textile products. In this study, two different commercial photochromic dyes – a naphthopyran and a spirooxazine-based dye – have been applied on PET fabric by inkjet printing and UV-LED curing. The photochromic textiles’ color behavior, fastness to abrasion and washing, and handle are evaluated using spectrophotometry, scanning electron microscopy, and Kawabata evaluation system. Despite a decrease in color performance after washing, the photochromic inkjet print is effective and barely influences the textile structure. Reduced rigidity of the host matrix promoted higher color yields and faster dye kinetics, but also improved durability towards abrasion and washing. In order to synchronize kinetics of the different dye types for multi-colored applications, distinct curing conditions are preferable, which, however, result in varying print durability. In the design of multi-colored photochromic textiles, dye kinetics, and durability have to be balanced. Keywords: Inkjet printing, UV curing, Textile sensor, Photochromic, Durability ⓒThe Author(s) 2019, corrected publication 2019 handle of functional textile materials. In general, inkjet printing and UV-curing are ideal technologies for functional multi-colored patterns because of their flexibility, costeffectiveness, and technical precision. Still, when combining different photochromic dyes in one product, it is important to synchronize coloration effects and fastness properties in order to provide consistent product quality. Therefore, we investigate the color behavior, fastness, and handle of multicolored photochromic textiles as seen in Figure 1(b), which are used in the design towards smart textile UV-sensors. A naphthopyran dye, Reversacol Ruby Red (RR), and a spirooxazine dye, Reversacol Sea Green (SG) are representatives of a more durable, but slow switching dye and a faster switching, but more temperature-dependent dye, respectively [4,5]. Both dyes are T-type photochromic compounds, which can reversibly switch color as a result of the absorption of electromagnetic radiation and thermally triggered reversion. A common challenge in the application of photochromic dyes is their endurance in use, which is known as fatigue behavior. As a result of long-term UV-exposure and the influence of the predominant environmental factor oxygen, the color switching effect of photochromic compounds is weakened. Fatigue, which can also be referred to as photo- Introduction Although a limited number of products have reached the market, smart and functional textile high-end products have been an important topic for many years. In recent years, there has been an increasing interest in developing wearable, highly integrated technologies using resource-efficient processes [1]. Resource-efficiency, flexibility, and costeffectiveness play a key role in triggering innovation and promoting the production of smart and functional niche products like photochromic textiles among others. Combining inkjet printing and curing with UV-LED light facilitates a sustainable, flexible, and economic textile production and avoids the use of water, unnecessary amounts of chemicals, and energy while producing less waste. However, to the best of our knowledge, few studies on the resource-efficient production of photochromic textiles have been done. Aldib [2] explored inkjet printing of solvent-based photochromic inks and Fu et al. [3] investigated photonic curing. What often is neglected is that the usage of novel technologies in the application of smart materials presents new challenges in the general behavior, durability and *Corresponding author: sina.seipel@hb.se 1424 degradation, causes an irreversible chemical transformation where an undesired by-product is formed [6]. The formation of the fatigue product prevents the photochromic dye to revert to its original form, which prohibits further photoswitching [7]. Depending on the dye class and specific molecular modifications, photochromic dyes show varying photo-stability. Therefore, it is important to distinguish between fatigue of the dye and durability of the photochromic print as in its adhesion to the textile surface. This study focuses on the durability of the photochromic prints produced and its implication on the design of a textile UV-sensor. The photochromic prints have been printed in single-pass mode and cured under strong and weak curing conditions, which means at a conveyor belt speed of 50 mm/s combined with 80 % of the maximum lamp intensity and 300 mm/s combined with 1 % of the maximum lamp intensity in a continuous pilot-scale inkjet printing system (Figure 1(a)). Seipel et al. [8] studied the interaction of a photochromic dye and ink matrix on molecular level to tune the kinetics of the photochromic reaction based on chosen fabrication parameters using inkjet printing and UV-curing. In that study, photochromic prints based on a naphthopyran dye, which are produced with a belt speed of 300 mm/s and 1 % lamp intensity, as well as 50 mm/s combined with 80 %, indicated good wash fastness. Altering the viscosity and free volume of polymeric matrices to influence kinetics of photochromic dyes is an up-to-date topic. Dye kinetics in a host matrix can be influenced through different ways of polymer architecture by varying the type, dye placement, or Tg of soft polymers [9-14], by the use of block copolymers [15], through the control of photo-polymerization [8,16] and through pore dimensions in sol-gels [17]. In this study, however, we explore the effect of matrix rigidity on the durability of a spirooxazine and a naphthopyran-based print on textile via fastness tests, which generate mechanical action through abrasion and mechanical-chemical action at elevated temperature through washing. As textile sensors in use will be exposed to abrasion and washing, this investigation is necessary in regards to feasibility and design of future products for wearable or other textile sensor applications. Furthermore, the textile character and handle of the photochromic textiles are evaluated after intensive washing using a Kawabata evaluation system (KES) to determine the impact of the photochromic inkjet print on the textile structure. We conclude that the choice of fabrication parameters of an industrially applicable process can modify dye kinetics, but also increase the durability of photochromic textiles via tuning the rigidity of the host matrix. The photochromic surface functionalization is effective without influencing the textile character. For multi-colored photochromic textiles, synchronization of dye kinetics and durability towards abrasion and washing have to be weighed against each other. Experimental Materials Two types of photochromic dye were used in the formulation of UV-curable inkjet inks to produce UV-sensing textiles. The commercial heterocyclic spiro-compounds Reversacol Ruby Red and Reversacol Sea Green from Vivimed Labs, UK represent dyes of two different dye classes, naphthopyran and spirooxazine, respectively. The dye concentration in the designed inkjet inks was 4 g/l. The UV-curable resin consists of dipropylene glycole diacrylate monomers (DPGDA), amine modified polyetheracrylate oligomers (Ebecryl 81) supplied by Allnex SA/NV, Belgium, and a UV-LED photoinitiator (Genocure TPO-L) supplied by Rahn AG, Switzerland. Solvents were used to disperse the photochromic dyes in the UV-curable resin, which were removed after stirring by vacuum pumping. For Ruby Red, ethyl acetate, 99.9 % (Chromasolv Plus) and for Sea Green, hexane, ≥97 % (Chromasolv HPLC), both purchased from Sigma-Aldrich were used. The textile substrate for inkjet printing was a plain-woven polyester (PET) fabric with a weight of 147 g/m2 and 20 and 23 threads/cm in warp and weft direction, respectively, received from FOV Fabrics, Sweden. 1426 Fibers and Polymers 2019, Vol.20, No.7 Production of Photochromic Prints UV-responsive smart textiles were produced by inkjet printing and UV-curing in a continuous process using a pilot-scale inkjet printing system with a conveyer belt for substrate transportation. Printing was carried out with a Starfire SA print head from Fujifilm Dimatix, USA at a resolution of 400 dpi in single-pass mode. The Starfire SA print head is able to print 4 different color scales, i.e. ink amounts. In this study, the darkest shade, i.e. the highest amount of a single pass with an effective ink amount of 16 g/m2 was used. The print head temperature was set to 35 oC to facilitate ink flow upon printing. For curing, a UV-LED lamp FireJet from Phoseon Technology, USA with emission wavelength of 380 to 420 nm and a maximum emission power of 6 W/cm2 was used. The photochromic prints were cured at two distinct conditions, which represent a high and low crosslinking density of the ink on the textile. Printing and curing was carried out at conveyer belt speeds of 50 mm/s and 80 % of the maximum lamp intensity (50_80) and at 300 mm/s and 1 % of the maximum lamp power (300_1). Fastness Testing of Photochromic Prints The fastness of the photochromic prints in terms of mechanical and chemical action was evaluated as a result of abrasion and washing. The photochromic prints were abraded using a Martindale tester according to ISO 12947:2006 with a weight of 9 kPa during abrasion cycles of 5000, 10000, 15000, and 20000 rubs. To determine the effect of washing on the print durability, samples were washed once and ten times after printing and curing. Washing was conducted according to ISO 6330:2012 using procedure 4N and reference detergent 3. The samples were dried using drip flat drying. Color Analysis To evaluate the color behavior upon excitation (coloration) and relaxation (decoloration) of the photochromic prints an LCAM Photochrom 3 spectrophotometer was used. The specially designed device allows continuous color measurement Figure 2. Scheme of color measurement cycle with 300 s of UVexposure for coloration and 800 s of relaxation for decoloration. Sina Seipel et al. upon cycles of UV-exposure and relaxation [18] (Figure 2). As described previously, two distinct light sources were used for substrate activation and for color measurement [8]. An Edixon UV-LED lamp (EDEV-3LA1) with a radiometric power ΦV of 350 mW and an emission peak with wavelengths between 395 nm and 410 nm was used for activation. For measurement a dual light source system of combined high power white LEDs with CCTs of 4000, 5000, and 7000 K with sample illuminance of 60 klx was used. Measured reflectance values R are calculated to color values K/S using the Kubelka-Munk equation. 2 K 1 – R) ---- = (----------------S 2R (1) The kinetic model shown in equation (2) follows firstorder kinetics and is generally used to describe photochromic color behavior for both the coloration and decoloration reaction as seen in Figure 2 [19]. K K K K –kt Δ⎛⎝ ----⎞⎠ = ⎛⎝ ----⎞⎠ – ⎛ ----⎞ ⋅ e + ⎛⎝ ----⎞⎠ S 0 ⎝ S⎠∞ S ∞ S (2) In the case of UV-curable materials, equation (2) is used for prints with a high degree of polymer crosslinking. For prints with lower polymer crosslinking density, equation (3) is used, which defines the kinetics of a decay mechanism as a result of continuous curing while color measurement and reconstructs the actual coloration curve [8]. –k ⋅ t –k ⋅ t K K K Δ⎛⎝ ----⎞⎠ = ⎛⎝ ----⎞⎠ ⋅ ( 1 – e col ) – ⎛⎝ ----⎞⎠ ⋅ ( 1 – e dec ) S ∞dec S col S ∞col (3) Inkjet-Printed and UV-Cured Photochromic Textiles properties of textile fabrics and predicts the aesthetic quantities perceived by human touch. The primary hand value (HV) and the total hand value (THV) were calculated based on tensile, shear, bending, compression, and surface properties of the fabrics. To categorize the non-skin contact, wearable textile UV-sensor, a men’s summer suit was chosen as reference application for the KES-test. Here, the HV of the samples is based on evaluated stiffness (koshi), crispness (shari), fullness and softness (fukurami), and anti-drape stiffness (hari) of the fabrics on a scale between 1-10, where 1 stands for weak and 10 for strong properties. THV is evaluated using a scaling system between 0-5, where 0 stands for out of use and 5 for an excellent fit of the measured material for the application. Scanning Electron Microscopy Scanning electron microscopy (SEM) was performed using a JEOL JSM-6301F instrument. Images were taken at an acceleration voltage of 6 kV and a working distance of 39 mm. To improve sample conductivity prior to SEM analysis the substrates were sputtered with a 3 nm thick layer of gold. Results and Discussion Fitting of the Color Data of Prints Produced at 300 mm/s and 1 % The extended model according to equation (3) is a mathematical approach to fit the color data of UV-curable photochromic materials with a low polymer crosslinking Fibers and Polymers 2019, Vol.20, No.7 1427 density. Based on the measured K/S values of a photochromic dye, the coloration curve is fitted while taking a decay mechanism upon color measurement during UV-exposure and resulting activation into account. Equation (3) is a valid model for printed and abraded samples, which exhibit decay during color measurement. The fitted coloration curve of printed RR (Figure 3(a)) reaches a ΔK/S of 0.27, whereas after abrasion with 20000 rubs (Figure 3(b)), ΔK/S is lowered to 0.2. However, when fitting the color data of SG prints, ΔK/S of the printed sample (Figure 3(c)) with a value of 0.06 is lower than after abrasion with ΔK/S of 0.1 (Figure 3(d)). This repeated observation for all SG prints, which are cured at 300 mm/s belt speed and 1 % lamp intensity, shows that abrasion provides a loosening effect in the matrix and therewith less rigidity and more space for the dye molecule to undergo isomerization. A factor of influence on the distinct dye-matrix interaction of SG prints could be that spirooxazines have a large dipole moment [20]. A large dipole moment increases interactions of the dye molecule with the surrounding matrix and hence affects dye kinetics. Larkowska et al. [21] observed that the relaxation reaction appeared to be slower when the dipolar interactions were more pronounced. Therefore, the large dipole moment of the spirooxazine dye molecules and their consequent higher polarity during isomerization may influence their behavior in relation to the matrix more compared to RR prints. Factors of Influence on the Durability of Photochromic Prints Statistical analysis via a three-way ANOVA at a confidence where, under the boundary condition that the processes of coloration and decay start simultaneously and at ΔK/S=0, K/S∞col is the maximum coloration value and K/S∞dec is the final value of the decay reaction for each activation cycle. Kinetic rate constants for the coloration reaction and decay are defined as kcol and kdec, respectively. Based on the kinetic models, the performance of the textile UV-sensors was specified by its achieved color intensity ΔK/S upon activation with UV-light, its rate constant of color increase kcol to achieve maximum coloration K/S∞, and its rate constant of color decrease kdecol to revert to the initial colorless state K/S0. The color performance of the photochromic prints was analyzed at each dye’s reflectance minima. For prints containing naphthopyran dye Ruby red this refers to a wavelength of 500 nm and for those containing spirooxazine dye Sea green analysis was made at 620 nm. Kawabata Evaluation System The change in handle of the plain-woven PET compared to after printing with Ruby red ink and after washing for 10 cycles was objectively analyzed using a Kawabata evaluation system (KES). The samples with dimensions of 200×200 mm were printed and cured as three parallel stripes and measured both in warp and weft direction using a KES. KES measures Figure 3. Fitting of K/S data of prints cured at 300 mm/s+1 % (a) RR printed, (b) RR abraded with 20000 rubs, (c) SG printed, and (d) SG abraded with 20000 rubs. 1428 Fibers and Polymers 2019, Vol.20, No.7 Sina Seipel et al. Inkjet-Printed and UV-Cured Photochromic Textiles colored form of photochromic dyes as the dyes change structure and polarity upon isomerization. The different treatments, which are differentiated in mechanical and chemical-mechanical action, affect the color performance of both dyes in different extent. Generally, in the factor analysis it is observed that mechanical action as in abrasion influences the color behavior less than chemical-mechanical action as in washing. In a study of Kert and Gorjanc [28], where cotton, PET, and a blend fabric was functionalized with commercial photochromic microcapsules in a pad-drycure process, fastness to washing, wet and dry cleaning, and wet and dry rubbing were tested among others. According to their study, dry rubbing (crock fastness) affected the color difference value ΔE of the samples less than washing. They conclude fastness properties to be a measure of adhesion of the binder to the fiber, in which the photochromic microcapsules are entrapped. Hence, compared to dry rubbing, washing has a harsher effect on the padded samples due to elevated temperature and combined mechanical and chemical action. Figure 4. Effects of the factors curing intensity (■), dye (●), and treatment (▲) on (a) ΔK/S, (b) kcol, and (c) kdecol with connecting lines to guide the eyes. interval of 95 % showed that the factors, curing intensity, dye, and treatment have significant effects on the color performance of photochromic prints in terms of the color yield ΔK/S and the reaction rates kcol and kdecol. Curing intensity refers to a combined belt speed and lamp intensity of 50 mm/s and 80 % (50_80) and 300 mm/s and 1 % (300_1), which represent a high and a low curing intensity, respectively. Dye refers to naphthopyran dye Ruby red (RR) and spirooxazine dye Sea green (SG). Treatment includes printed, abraded with 20000 cycles, 1× washed, and 10× washed samples. As seen in Figure 4(a), prints cured with a low intensity (300_1) result in higher ΔK/S compared to curing with a high intensity (50_80) and red prints result in higher ΔK/S than green prints. Abrasion with 20000 cycles and 1 washing cycle have a similar effect, whereas 10 washing cycles reduce ΔK/S values by 50 %. The kinetics of the coloration reaction as seen in kcol (Figure 4(b)) is higher for a low curing intensity (300_1), as well as for RR compared to SG. Samples treated by abrasion, 1 wash, and 10 washes exhibit gradually decreasing kcol as compared to printed samples. The same trend in regards to the effect of curing intensity, dye, and treatment is seen for kdecol (Figure 4(c)). Higher kdecol is achieved with higher belt speed and lower lamp power (300_1) and for RR dye. A gradual decrease in the decoloration kinetics is seen for abraded, 1× washed, and 10× washed prints. Kinetics of photochromic dyes is affected by various properties of the surrounding media. Among these are the viscosity of the media, chain lengths of polymers, and specific placement of the dye molecules in the matrix [9,1113,15]. Seipel et al. [8] showed that adjusting the curing intensity of photochromic prints in an industrially applicable process changes the polymer crosslinking density of the UVresin. As a result of changed matrix rigidity, the dye-matrix interaction can be influenced and the kinetics of a photochromic textile controlled. Usually, spirooxazines are faster switching dyes than naphthopyrans [4]. Spirooxazines are more rigid with a higher molecular weight and more non-planar in structure in their colorless state [22]. However, upon isomerization spirooxazines become more or less planar and polar [23-25]. As can be seen in Figure 4 for the factor dye, RR disposes of stronger ΔK/S and higher switching rates kcol and kdecol. Consequently, the integration of photochromic dyes in a UV-cured, cross-linked matrix largely affects their kinetics upon isomerization between the non-colored ring-closed and colored ring-opened states. The UV-cured, cross-linked matrix disturbs the isomerization of photochromic dyes between the non-planar and relatively non-polar form, i.e. the colorless state, and more or less planar and polar form, i.e. the colored state. In general, the factors polarity, free volume, rigidity, and direct interactions influence the photochromic response of materials [26,27]. Dye-matrix interactions are most pronounced for the Color Performance as a Result of Abrasion Via abrasion tests using a Martindale instrument photochromic prints undergo mechanical wear. As observed after consequent color analysis, mechanical action by Fibers and Polymers 2019, Vol.20, No.7 1429 various rubbing cycles (5000 to 20000 in 5000 steps) influences the photochromic prints on the textile surface compared to printed, i.e. non-abraded substrates. As seen in Figure 5(a) and (b), ΔK/S of SG prints is lower than for RR prints. However, the difference between a high and low crosslinking density of the ink resin on ΔK/S plays less role for SG, the spirooxazine dye than for naphthopyran dye RR. SG prints with a low crosslinking density decrease in color yield from ΔK/S of 0.117 to 0.110 compared to the dye in a more rigid resin from ΔK/S of 0.085 to 0.063. SG as the generically more rigid dye molecule might not have enough space to switch between its different isomers in neither the low nor highly cross-linked structure of the UV-resin. RR, however, is more flexible and therewith, larger differences in color yield can be observed. The difference in color kinetics observed in kcol between the curing intensities 300_1 and 50_80 is larger for SG (5:1) than for RR (2:1) as seen in average values for 5000 to 20000 rubs in Tables 1 and 2. As a result, it can be concluded that the rigidity of the UV-resin as a result of the curing intensity affects SG more than RR. As previously mentioned, spirooxazines are generically faster switching dyes than naphthopyrans, but they also are more rigid in structure with a higher molecular weight and non-planarity in the colorless state than naphthopyrans [4,22,29]. If a dye that is generally fast-switching, but rigid Figure 5. Effect of abrasion cycles 5000, 10000, 15000, and 20000 on ΔK/S of RR (a) and SG (b) prints cured at 50 mm/s and 80 % (50_80) and 300 mm/s and 1 % (300_1) on and on kcol of RR (c) and SG (d) prints cured at 50 mm/s and 80 % (50_80) and 300 mm/s and 1 % (300_1). 1430 Fibers and Polymers 2019, Vol.20, No.7 Sina Seipel et al. Table 1. Color yields ΔK/S and rate constants kcol and kdecol of RR prints printed, 1× and 10× washed and abraded with 5000, 10000, 15000, and 20000 cycles Printed 1× washed 10× washed 5000 rubs 10000 rubs 15000 rubs 20000 rubs ΔK/S 0.103±0.009 0.062±0.002 0.039±0.003 0.086±0.008 0.077±0.013 0.079±0.002 0.075±0.004 50 mm/s+80 % kcol 0.089±0.007 0.059±0.008 0.048±0.009 0.060±0.000 0.056±0.002 0.056±0.002 0.053±0.001 kdecol 0.011±0.000 0.009±0.000 0.008±0.001 0.010±0.000 0.010±0.000 0.010±0.001 0.009±0.000 ΔK/S 0.235±0.068 0.154±0.044 0.136±0.005 0.266 0.262 0.242 0.225±0.012 300 mm/s+1 % kcol 0.114±0.027 0.102±0.007 0.101±0.005 0.120 0.123 0.108 0.113±0.006 kdecol 0.014±0.002 0.012±0.001 0.013±0.001 0.012 0.013 0.013 0.014±0.001 Table 2. Color yields ΔK/S and rate constants kcol and kdecol of SG prints printed, 1× and 10× washed and abraded with 5000, 10000, 15000, and 20000 cycles Printed 1× washed 10× washed 5000 rubs 10000 rubs 15000 rubs 20000 rubs ΔK/S 0.107±0.002 0.086±0.025 0.050±0.006 0.085±0.006 0.085±0.003 0.073±0.003 0.063±0.000 50 mm/s+80 % kcol 0.020±0.002 0.017±0.004 0.012±0.001 0.016±0.000 0.016±0.000 0.014±0.000 0.013±0.000 kdecol 0.008±0.001 0.007±0.004 0.005±0.000 0.007±0.000 0.007±0.000 0.006±0.000 0.007±0.004 in structure is entrapped in a matrix with high rigidity, its degree of freedom is limited and slow kinetic rates can be expected. However, when the rigidity of the matrix is decreased, a larger effect of relative increase in kcol can be observed for a fast-switching dye as a result of loosened dyematrix interaction. This can explain why the difference in photo-switching speed of SG prints is a 5-fold larger for 300_1 than for 50_80. The same change in matrix rigidity only affects RR prints with a 2-fold faster kcol for 300_1 compared to 50_80. kcol of RR prints are decreased by 50 % in a more rigid resin. For SG, an increased polymer crosslinking density of the matrix results in 20 % of the initial kcol compared to a softer matrix. SG prints seem to have a better abrasion resistance both observed in the color yield ΔK/S and the coloration rate kcol. Compared to RR prints, which show a steady decrease in ΔK/S and kcol as function of abrasion cycles, SG prints are more stable. What can also be seen in ΔK/S values, which are presented in Tables 1 and 2, is that the fastness properties are improved with increased matrix softness. Whereas ΔK/S of 300_1 RR prints are superior against abrasion compared to washing (Table 1), ΔK/S of 300_1 SG prints show generally stable values for washed and abraded samples compared to in a more rigid matrix (Table 2). UVcurable coatings are in general known for their good durability and abrasion resistance [30-32]. Guan et al. [33] observed ΔK/S 0.079±0.040 0.153±0.011 0.102±0.005 0.117±0.008 0.104 0.115 0.110±0.007 300 mm/s+1 % kcol 0.080±0.033 0.045±0.007 0.030±0.002 0.072±0.012 0.071 0.069 0.071±0.005 kdecol 0.013±0.003 0.014±0.001 0.010±0.001 0.019±0.010 0.012±0.000 0.013±0.002 0.013±0.001 Inkjet-Printed and UV-Cured Photochromic Textiles the naphthopyran dye is observed both when integrated in a matrix with high rigidity (50_80) and in low rigidity (300_1). However, the absolute ΔK/S with 0.103 (50_80) is lower for prints with high polymer crosslinking density compared to prints with low crosslinking density with ΔK/ S=0.235 (300_1) as seen in Table 1. Lower ΔK/S of prints with high matrix rigidity can be explained as a result of reduced space in the host matrix and hence limited degree of freedom for the dye upon isomerization. In terms of durability, 300_1 RR prints dispose of 58 % after 10 washing cycles compared to 50_80 RR prints, which only have 38 % remaining of the initial ΔK/S. Although the fastness to washing for 300_1 RR prints is better than for 50_80 RR prints, it should be noted that washing significantly affects the color yield of 300_1 prints as compared to abrasion. Abrasion after 20000 revolutions barely shows an effect with ΔK/S=0.225 compared to initial 0.235. Washing after 10 cycles, however, results in a significantly lower ΔK/S=0.136. Rate constants of the coloration reaction kcol differ between 300_1 and 50_80 RR prints. Prints with a low degree of polymer crosslinking (300_1) show higher switching speeds with kcol =0.114 1/s superior rubbing, wash, and color fastness of a UV-curable pigment ink compared to a commercial pigment ink, while the print softness was improved. After 20000 abrasion cycles, RR prints 50_80 and 300_1 dispose of similar remaining color yield ΔK/S with 70 % and 67 %, respectively. However, the speed of the coloration reaction is influenced more pronounced for 50_80 prints in regard to kcol with remaining 55 % of the initial rate constant compared to 300_1 prints with 76 % of the non-abraded value. SG prints, which have been abraded with 20000 rubs dispose of 58 % of the initial ΔK/S for a rigid ink resin (50_80) and of 140 % for a softer ink resin (300_1). After abrasion, the coloration rate kcol of 50_80 SG prints accounts for 70 % of the initial rate and 88 % for 300_1 SG prints. As previously mentioned, the increase in color yield ΔK/S for 300_1 SG prints is believed to result from a loosened matrix effect, which increases the mobility of the spirooxazine molecules and therewith the color intensity of the print. Color Performance as a Result of Washing The effect of washing on the photochromic prints is determined both after a single washing cycle and after extended washing of 10 cycles. Washing cycles have a significant effect on the color performance of RR and SG prints. The color yield ΔK/S of RR prints decreases gradually as a result of washing cycles. Decreasing ΔK/S of Figure 6. Effect of 0, 1, and 10 washing cycles on ΔK/S, kcol, and kdecol of (a) RR prints cured at 50 mm/s and 80% ( , ■, □) and 300 mm/s and 1 % ( , ▲, ∆) and of (b) SG prints cured at 50 mm/s and 80 % ( , ■, □) and 300 mm/s and 1 % ( ,▲, ∆). Fibers and Polymers 2019, Vol.20, No.7 1431 compared to prints with high matrix rigidity (50_80) with a kcol =0.089 1/s of printed samples. After washing, rate constants are lowered for all RR prints. Whereas washing with different cycles gradually reduces the rate of photoswitching of 50_80 prints from 0.089 to 0.054 to 0.048 1/s, 300_1 prints barely show a change between 1 and 10 washing cycles with kcol of 0.102 and 0.101 1/s (Figure 6(a)). Compared to the rate constant of printed samples, prints with low crosslinking density show superior fastness against washing with remaining 89 % of the printed kcol. 50_80 RR prints dispose of 54 % the initial kcol. The speed of the decoloration reaction is approximately one order of magnitude slower than kcol with kdecol =0.011 1/s and 0.014 1/s of printed RR 50_80 and 300_1 samples, respectively. The same trend as function of washing cycles as for kcol can be seen for rate constants under color reversion. Whereas kdecol of 50_80 prints decreases continuously after 1 to 10 washing cycles from 0.011 to 0.009 to 0.008 1/s, kdecol of 300_1 prints is stable after a single washing (Figure 6(a)). With kdecol=0.013 1/s after 10 washing cycles compared to the initial kdecol of 0.014 1/ s RR prints still dispose of 93 % of the reversion rate. RR prints with high rigidity show remaining 73 % after extensive washing. The degree of crosslinking of the UV-resin has a significant effect on the kinetics of the naphthopyran dye RR, but also its durability as a result of washing. It can be concluded that a softer and more flexible host matrix, which gives more space for the dye upon isomerization, also promotes the fastness properties as a result of chemicalmechanical action. The color performance of SG prints as a function of washing cycles differs from RR prints. In particular, color yield and kinetics of printed 300_1 samples deviate. As seen in Figure 6(b), ΔK/S of 300_1 printed, i.e. 0 washing cycles, with 0.079 is lower than after 1 washing cycle with 0.153 and also after 10 washing cycles with 0.102. Washing gradually decreases color yields of SG prints with a high degree of polymer crosslinking from ΔK/S=0.107 to 0.086 after 1 wash to 0.050 after 10 washes (Table 2). Whereas 50_80 prints show remaining 47 % of the initial color yield after extensive washing, 300_1 prints dispose of 129 % of the initial ΔK/S. Little and Christie [4] have observed that an initial washing process increased coloration for spirooxazine in screen-prints, but then decreased coloration for further washing cycles. For naphthopyran dye in screen-prints the coloration was decreased directly for initial washing, as we also observe in RR prints. The initial color increase for spirooxazine was argued to be a consequence of a loosened binder matrix and hence facilitated isomerization. Since, spirooxazines are more rigid in structure than naphthopyrans, a change in rigidity of the host matrix has a large effect. Little and Christie [4] concluded that much of the behavior they observed was due to binder, not dye effects. Similarly, Billah et al. [22] found that a mild washing with an aqueous 1432 Fibers and Polymers 2019, Vol.20, No.7 Sina Seipel et al. Fibers and Polymers 2019, Vol.20, No.7 1433 Table 3. HV including the single components and THV of the reference PET, printed and washed samples are tabulated. The functionalized samples were printed with 16 g/m of ink, cured at a belt speed of 300 mm/s and lamp intensity of 1 %, and then washed for 10 cycles alkaline surfactant treatment facilitated the formation of the merocyanine form of a spirooxazine and thereby increased photochromic intensity. Rate constants of the coloration reaction of prints with the spirooxazine dye differ depending on the curing intensity, i.e. the crosslinking density of the UV-resin. Same as for RR, 300_1 prints show faster photo-switching of printed SG with kcol =0.080 1/s than 50_80 prints with kcol =0.020 1/s. Washing affects the dye kinetics for both prints with high and low matrix rigidity. However, prints with low crosslinking density (300_1) are more affected with a loss in coloration rate of 62 % after 10 washing cycles. Prints with high crosslinking density affect kcol of SG with a 40 % decrease. The decoloration reaction, which has approx. 3 times slower kdecol compared kcol, is also significantly affected by washing. A continuous decrease in kdecol is seen for 50_80 SG prints from 0.008 1/s after printing to 0.007 1/ s after 1 wash and to 0.005 1/s after 10 washes. Decoloration kinetics of SG in prints with low matrix rigidity (300_1) are unchanged after 1 washing cycle, but then decrease from 0.014 to 0.010 1/s after 10 washing cycles. This effect can also be a result of a loosened structure of the UV-resin upon initial washing, but then decreases due to extensive chemical-mechanical action after 10 washing cycles. In terms of fastness to washing, SG 50_80 prints with 63 % remaining kdecol are inferior compared to 300_1 prints with 77 %. Maintenance and Repeatability of the Color Performance of Photochromic Prints Repeatability of the photochromic color switching is an essential characteristic toward the design of a textile UVsensor in use. Hence, the color performance of 1× and 10× washed prints is assessed throughout several cycles of color activation and deactivation. Color measurement of RR prints with an applied ink amount of 19 g/m2 cured at a belt speed of 300 mm/s and 1 % lamp intensity, show distinct behavior after 1 and 10 washing cycles. After a single washing process a continuous decrease in ΔK/S as function of activation and deactivation cycles is observed. However, prints after 10 washing cycles show stable and insignificantly different color yields ΔK/S and coloration, as well as decoloration rates over five and four activation cycles, respectively, compared to prints, which have been washed once (Figure 7). Periyasamy et al. [29] concluded that photo-degradation usually is higher during the initial cycles of UV-exposure due to a superficial layer of photochromic material, which degrades faster than material, which has penetrated further into fibers and fabrics. Our observation that more stable color performance is achieved after intensive washing, confirms the behavior. Therefore, we suggest that intensive washing should be considered as reduction process for UV-curable photochromic prints with low crosslinking density. Given that prerequisite, stable Inkjet-Printed and UV-Cured Photochromic Textiles 2 PET reference Printed 10× washed Koshi (stiffness) 8.14 8.37 7.95 Shari (crispness) 4.45 5.73 5.75 HV Fukurami (fullness and softness) Hari (anti-drape stiffness) 3.68 10.27 3.22 10.55 3.49 10.35 the inkjet-printed PET fabric via measurement of mechanical properties. The results of the objective evaluation of the fabric handle show that the chosen application for the fabric was not ideal with a THV of 1.60, which expresses a poor to below average fit. However, what can be seen is that after printing and washing the THV increases and therewith indicates a slightly better fit for the application. Important is the relative change of the fabric properties after printing and again after washing. According to the stiffness values (koshi) in Table 3, it can be seen that the stiffness of the fabric increases after printing from 8.14 to 8.37 and decreases to 7.95 after 10 washing cycles. This shows that the stiffness of the fabric is in general high and increases due to the application of an ink layer. 10 washing cycles smoothen the printed fabric. The fabric crispness (shari) also increases after printing, but does THV 1.60 1.84 2.01 not change after washing. The fabrics’ fullness and softness, however, decreases slightly after functionalization with the photochromic ink, which is almost reversed again by 10 washing cycles. All measured samples show high anti-drape stiffness (hari) with values above 10. The highest value with 10.55 is observed for the printed fabric and which is also slightly decreased again after washing. Another essential property of a fabric, which is related to fabric comfort (softness and rigidity), is the bending moment. It quantifies the feeling by hand when bending the product. When bending the fabric under low-stress mechanical action, the comfort of the finished fabrics is assessed. The printed fabric has a bending moment B=0.249 g·cm2/cm. After 10 washing cycles B is reduced to 0.221 g·cm2/cm. The larger the value of B is, the harder is the object. Therefore according to the definition the printed Figure 7. Comparison of ΔK/Scol (a), reaction rates kcol (b), and kdecol (c) of prints with 19 g/m of Ruby red ink cured at 300 mm/s and 1 % after 1 and 10 washes during 5 cycles of coloration and 4 cycles of decoloration. 2 color behavior is expected for long-term use. Effect of the Photochromic Print on the Fabric Structure and Handle KES analysis is made to objectively evaluate the handle of 2 Figure 8. Printed PET samples with 19 g/m of Ruby red ink after printing and curing at 100 mm/s belt speed and 25 % lamp intensity compared to reference PET; (a) printed weft multi-filament, (b) unprinted weft multi-filament, (c) printed warp multi-filament, and (d) unprinted warp multi-filament. 1434 Fibers and Polymers 2019, Vol.20, No.7 photochromic textile is less comfortable and the washed photochromic textile is more comfortable than the nonprinted reference PET with B=0.231 g·cm2/cm. In summary, it can be said that KES-measurements confirm a stiffening effect of the fabric after printing, which is more or less revoked after 10 washing cycles despite preserved photochromic activity, which is seen in color measurements. Also, observed changes of the fabric properties are low, which is attributed to the small amount of applied ink. In a research study of Karim et al. [32], where a UV-curable inkjet layer was applied on PLA fabric, the abrasion resistance of the fabric was improved while KES measurements showed that the ink layer did not significantly influence the handle of the fabric. Our study asserts their observation and hence that the photochromic surface functionalization of PET barely influences the textile handle of the plain-woven fabric. KES results are in line with the subjective handle when touching the fabrics, and are also supported by SEM analysis. As can be seen in Figure 8, the functional inkjetprinted material does not affect the fabric structure in terms of that the space between the fibers is not blocked. After printing with Ruby red ink (Figure 8(a) and (c)), the PET fabric does not show a difference compared to the reference fabric (Figure 8(b) and (d)). Therefore it can be expected that fabric properties like porosity and breathability are not affected. The pico-liter sized droplets, which are applied by ink jetting on the PET surface, are not detectable using SEM. Screen-printed fabrics typically show a change in surface structure, where the printing paste can clearly be detected on the fiber surface and pores between fibers are covered by the paste [4,5]. However, as is seen from the color data, inkjet-printed samples dispose of a photochromic color effect and hence prove that the ink adheres to the PET substrate without affecting the textile structure. Conclusion This study investigates important performance, durability and textile properties towards the production of a textile UVsensor using inkjet printing and UV-curing. With the aim to develop reliable textile sensors and multi-colored systems based on photochromic dyes, the controls of color intensities, switching speeds, and material durability in use are critical characteristics. In both cases, defined or even synchronized properties of different dyes are the pre-requisite for reliable future products. In this study, we showed that when combining the photochromic dyes Reversacol Ruby red and Sea green in one UV-cured smart textile, differences in kinetics and fastness towards abrasion and washing are to be expected. For spirooxazine-based UV-cured prints, color build-up will be slower and color yields lower than for naphthopyran-based dyes if prints are cured with the same intensity. Therefore, to synchronize dye kinetics with similar kcol between 0.05 and 0.08 1/s RR should preferably be cured Sina Seipel et al. with a strong intensity and SG with a weak intensity. However, synchronization of dye kinetics brings about compromises in terms of weakened durability of RR prints towards washing and abrasion. If synchronization of dye kinetics can be compromised on, the choice of suitable fabrication parameters of an industrially applicable process can increase the durability of photochromic textiles exceeding the control of kinetics. Reduced rigidity of the host matrix promotes higher color yields and faster dye kinetics, but also improved fastness towards abrasion and washing. Mechanical action through abrasion has less impact on the color performance of the photochromic prints than mechanical-chemical action at elevated temperature through washing. Although washing affects the material on the textile, the photochromic activity of the inkjet print is preserved and barely influences the textile character. KES analysis shows that the textile handle is stiffer as a result of ink jetting and curing of the photochromic ink on the fabric surface, which, however, after 10 washing cycles is revoked. 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Paper IV/ Book chapter Resource-efficient production of a smart textile UV sensor using photochromic dyes: Characterization and optimization S. Seipel, J. Yu, A. Periyasamy, M. Viková, M. Vik and V. A. Nierstrasz In: Narrow and Smart Textiles, 2018 Resource-Efficient Production of a Smart Textile UV Sensor Using Photochromic Dyes: Characterization and Optimization 252 1 Introduction Sina Seipel, Junchun Yu, Aravin P. Periyasamy, Martina Viková, Michal Vik and Vincent A. Nierstrasz Abstract Niche products like smart textiles and other technical high-end products require resource-efficient processes and small batches contrary to conventional textile processes that require larger batches and are water-, chemical- and energy-intensive. This study focuses on digital inkjet printing and UV light curing as a flexible and resource-efficient and therewith economic production process of a smart textile UV sensor. The UV sensor is based on a UV-curable inkjet ink and a commercial photochromic dye. The inkjet ink is cured via free radical polymerization initiated by a UV–LED lamp. This system contains two photoactive compounds for which UV light both cures and activates the prints. An important challenge is therefore polymer crosslinking of the resin and UV-sensing performance of the photochromic dye. In this paper, we present performance as a function of belt speed and lamp intensity during curing. Via wash tests, we investigate the durability of the photochromic prints. The UV-sensing textile is characterized by colour measurements, differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). S. Seipel (&) J. Yu V. A. Nierstrasz Faculty of Textiles, Engineering and Business, Department of Textile Technology, Textile Materials Technology Group, University of Borås, 501 90 Borås, Sweden e-mail: sina.seipel@hb.se J. Yu e-mail: junchun.yu@hb.se Today, smart textile UV sensors can typically be produced using screen printing [1–3], conventional dyeing [4, 5] and compounding and melt spinning [3, 6]. Successful UV-sensing textiles have been produced using these conventional technologies. However, due to the large production scale of these conventional processes and the fact that photochromic dyes are costly chemicals, this did not lead to an economically industrial-viable process. This paper therefore focuses on the potential of both digital inkjet printing and UV light curing in the production of a smart textile UV sensor using photochromic dyes. UV radiation triggers a reversible colour change in photochromic dyes as a result of a change in absorption spectra as shown in Fig. 1. The colour change is caused by ring-opening isomerization initiated by high photoelectric energy. Depending on the type of photochromic dye, the reverse reaction to the dye’s uncoloured state is triggered by visible light (P-type) or by temperature (T-type) [7, 8]. Both digital inkjet printing and UV-curing are resource-efficient technologies, which conserve unnecessary amounts of chemicals and energy, as well as waste that is produced. Furthermore, the combination of these technologies introduces process flexibility in regards to produced run lengths, change in pattern and ink [9–11]. Digital inkjet printing puts high physical requirements on the development of the printing ink to guarantee ink jet ability and storage. Properties like the ink’s stability, particle size, surface tension and viscosity have to be compatible with the chosen print head. In this paper, ink has been developed containing a commercial photochromic dye, mixture of acrylate monomer and oligomer and a UV–LED photo-initiator. The role of UV light as curing initiator and activator of the photochromic colour change and its influence on the performance of the UV-sensing prints is explored. The production of the textile UV sensor using digital inkjet printing and UV-curing Fig. 1 Scheme of change in absorption spectra between the colourless, ring-closed isomer A and the coloured, ring-opened isomer B initiated by UV light and reversed by either visible light or temperature V. A. Nierstrasz e-mail: vincent.nierstrasz@hb.se A. P. Periyasamy M. Viková M. Vik Faculty of Textile Engineering, Department of Material Engineering, Technical University of Liberec, Studentská 2, 461 17 Liberec, Czech Republic e-mail: aravinprince@gmail.com M. Viková e-mail: martina.vikova@tul.cz M. Vik e-mail: michal.vik@tul.cz © Springer International Publishing AG 2018 Y. Kyosev et al. (eds.), Narrow and Smart Textiles, https://doi.org/10.1007/978-3-319-69050-6_22 S. Seipel et al. 251 Resource-Efficient Production of a Smart Textile UV Sensor … 253 is optimized using different UV light intensities; 1, 25 and 80% of the maximum dosage, and two different conveyor belt speeds; 50 and 300 mm/s, during curing. The smart textile UV sensor is characterized and evaluated in terms of colour yield, colouration and decolouration rates and degree of polymer crosslinking of the ink on the textile surface. The mechanical properties of the UV-sensing textile are influenced by the different curing intensities. The change in stiffness of the photochromic prints is determined via DMA measurements. 2 Experimental Part As described previously [12], a photochromic inkjet ink, which consists of a UV-curable carrier and a commercial photochromic dye Reversacol Ruby Red (Vivimed Labs, UK), was used to produce photochromic prints on plain-woven polyester fabric of 147 g/m2 (FOV Fabrics, Sweden). The UV-curable carrier consists of dipropylene glycole diacrylate monomers (DPGDA), amine modified polyetheracrylate oligomers Ebecryl 81 (Allnex, Belgium) and a UV–LED photo-initiator Genocure TPO-L (Rahn AG, Switzerland). A digital inkjet printing system featuring a piezoelectric drop-on-demand print head Sapphire QS-256/10 AAA (Fujifilm Dimatix, USA) was used to produce prints in multi-pass mode with 3, 5 and 10 layers of photochromic ink (Fig. 2a). To cure the prints, a FireJet UV–LED lamp (Phoseon Technology, USA) with emission wavelengths of 380 to 420 nm was used (Fig. 2b). Ink characterization is done to ensure jetting of the ink formulation and desired drop placement on the textile. Essential ink properties and specifications according to the print head requirements are the fluid’s surface tension, viscosity, overall stability of the ink and the particle size. The surface tension of the ink is measured using a tensiometer One Attension (Biolin Scientific, Sweden) as pendant drop. The ink viscosity is measured at a shear rate of 10,000 s−1 at 20 °C using a rheometer Fig. 2 a Digital inkjet printing system featuring a piezoelectric print head for UV-curable ink and b curing of photochromic ink using a UV–LED lamp with emission wavelengths of 380–420 nm on plain-woven polyester fabric 254 S. Seipel et al. Physica MCR 500 (Anton Paar, Austria). Particle size assessment is done via filtration and ink stability is tested via storage tests. For process and sensor optimization, curing parameters were varied by (i) change in transportation speed of the print passing the UV source, i.e. conveyer belt speed, and (ii) intensity of the maximum UV–LED lamp power, i.e. lamp intensity. Conveyer belt speeds varied among 50 or 300 mm/s and were combined with 1, 25 or 80% of the maximum lamp intensity as the production process scheme shows in Fig. 3. The inkjet-printed and UV-cured photochromic ink on polyester fabric is heated from 25 to 290 °C at a scanning rate of 10 °C per min using a Q1000 DSC (TA Instruments, USA). The shift of the melting peak of polyester is used to analyse the degree of crosslinking of the ink for each curing setting; respective belt speed and lamp intensity. The photochromic print’s colour properties were measured using a custom-made LCAM Photochrome spectrophotometer (Technical University of Liberec, Czech Republic), which enables continuous colour measurements upon various cycles of UV activation and reversion [13]. The textile UV sensor’s performance is characterized by the achieved colour yield DK/S, colouration rate kcolouration and decolouration rate kdecolouration upon isomerization between the uncoloured and coloured state of the photochromic dye. The influence of the different curing settings on the mechanical properties of the photochromic prints on the polyester fabric is measured using a Q800 DMA (TA Instruments, USA). The stiffness of the textile UV sensors is determined via the storage modulus as a function of frequency. The storage modulus was measured at a frequency sweep between 1 and 12 Hz with a preload force of 0.01 N. Fig. 3 Production process scheme of a printing and b curing of photochromic prints on polyester fabric Resource-Efficient Production of a Smart Textile UV Sensor … 255 3 Results and Discussion The developed UV-curable photochromic ink fulfils the requirements of the Sapphire QS-256/10 AAA print head with the following specifications: • • • • Surface tension: ca. 35 mN/m Viscosity: 10–14 mPa s Particle size: <0.1 lm Ink stability. The different curing settings; combinations of 50 mm/s and 300 mm/s belt speeds and 1, 25 and 80% of maximum lamp intensity influence the crosslinking densities of the printed polyester fabrics and alter achievable colouration yields, as well as colouration and decolouration rates. High colour yields DK/S and fast reaction rates kcolouration and kdecolouration of the photochromic dye in the UV-curable matrix between the uncoloured and coloured states are favoured by high belt speed and low lamp intensity. As shown in Fig. 4a, the storage modulus of the inkjet-printed and UV-cured polyester fabric obtained higher storage modulus than the nonprinted polyester sample. The crosslinking of the UV-curable ink increased the hardness of the overall textile sensor. Furthermore, the sample cured at higher UV dosage, i.e. at 50 mm/s belt speed with 80% lamp intensity, had a higher storage modulus. At a frequency of 12 Hz, the polyester fabric without print has a storage modulus of 300 MPa. A photochromic print cured at low intensity, i.e. conveyor belt speed of 300 mm/s and lamp power of 1%, increases the storage modulus to ca. 1000 MPa. For prints, which undergo strong curing, i.e. 50 mm/s belt speed and 80% lamp intensity, the storage modulus reaches 1400 MPa. This suggests that there are more 256 S. Seipel et al. crosslinks of the sample cured under higher dosage of UV exposure. The same trend is confirmed by DSC measurements, which present an increase in melting peak as a function of curing intensity. However, the textile sensor is still bendable and rather flexible for the sample cured at 50 mm/s belt speed with 80% lamp intensity (Fig. 4b). 4 Challenges and Conclusion This paper demonstrates the potential of combined digital inkjet printing and UV– LED curing in a resource- and cost-efficient textile production process. We conclude that changes in UV light intensity and belt speed during the curing process achieve distinct results in terms of colouration yields and kinetics of the colouration and decolouration reaction of the photochromic dye. These changes also affect the degree of polymer crosslinking, which influences the durability of the smart textile UV sensor in use. The degree of polymer crosslinking also influences the stiffness of the UV-sensing textiles. The stronger the curing intensity, the higher is the storage modulus of the printed fabrics as a result of higher degree of polymer crosslinking. However, in use, the flexibility of the photochromic prints, irrespective of curing intensity, is suitable for wearable applications, such as smart textile sensors. In addition, we explore the effect of washing on the colour performance and switching reactions of the textile UV sensor. High belt speed and lower UV–LED light intensity enables faster colour changing and higher colour yield. The same effect is seen after washing despite a lower degree of polymer crosslinking. Acknowledgements The authors gratefully acknowledge the support of Borås Stad, Sparbankstifelsen Sjuhärad, TEKO (The Swedish Textile and Clothing Industries Association) and SST (Stiftelsen Svensk Textilforskning) for enabling this research. References Fig. 4 a Increased storage modulus of photochromic prints on polyester in comparison to the polyester fabric reference. Print cured at highest intensity, i.e. belt speed of 50 mm/s and lamp intensity of 80% has highest stiffness throughout a frequency sweep of 1–12 Hz. b Bendability demonstrates suitable flexibility of the textile UV sensor cured at 50 mm/s belt speed with 80% lamp intensity in use 1. Little, A. F., & Christie, R. M. (2010). Textile applications of photochromic dyes. Part 2: Factors affecting the photocoloration of textiles screen-printed with commercial photochromic dyes. Coloration Technology, 126, 164–170. 2. Feczkó, T., Samu, K., Wenzel, K., et al. (2013). Textiles screen-printed with photochromic ethyl cellulose–spirooxazine composite nanoparticles. Coloration Technology, 129, 18–23. 3. Viková, M. (2011). Photochromic Textiles. Edinburgh: Heriot-Watt University. 4. Billah, S. M. R., Christie, R. M., & Shamey, R. (2012). Direct coloration of textiles with photochromic dyes. Part 3: dyeing of wool with photochromic acid dyes. Coloration Technology, 128, 488–492. 5. Aldib, M., & Christie, R. M. (2011). Textile applications of photochromic dyes. Part 4: application of commercial photochromic dyes as disperse dyes to polyester by exhaust dyeing. Coloration Technology, 127, 282–287. Resource-Efficient Production of a Smart Textile UV Sensor … 257 6. Viková, M., Periyasamy, A. P., Vik, M., et al. (2016). Effect of drawing ratio on difference in optical density and mechanical properties of mass colored photochromic polypropylene filaments. Journal of The Textile Institute, 1, 1–6. 7. Bouas-Laurent, H., & Dürr, H. (2001). Organic photochromism (IUPAC Technical Report). Pure and Applied Chemistry, 73, 639. 8. Bamfield, P., & Hutchings, M. G. (2010). Chromic phenomena: Technological applications of colour chemistry. Place: Royal Society of Chemistry. 9. Hudd, A. (2011). Inkjet printing technologies. In S. Magdassi (Ed.), The chemistry of inkjet inks (pp. 3–18). Singapore: World Scientific. 10. Singh, M., Haverinen, H. M., Dhagat, P., et al. (2010). Inkjet Printing—Process and Its Applications. Advanced Materials, 22, 673–685. 11. Lin, A. H. L. (2004). Challenges of UV curable ink-jet printing inks—A formulator’s perspective. Pigment & Resin Technology, 33, 280–286. 12. Seipel, S., Yu, J., Nierstrasz, V., et al. (2017). Characterization and optimization of an inkjet-printed smart textile UV-sensor cured with UV-LED light. In AUTEX World Textile Conference. 13. Viková, M., Christie, R. M., & Vik, M. (2014). A unique device for measurement of photochromic textiles. Research Journal of Textile and Apparel, 18, 6–14. Paper V Digital inkjet functionalization of water-repellent textile for smart textile application J. Yu, S. Seipel and V.A. Nierstrasz Journal of Materials Science, 2018 J Mater Sci (2018) 53:13216–13229 J Mater Sci (2018) 53:13216–13229 13217 POLYMERS Polymers Digital inkjet functionalization of water-repellent textile for smart textile application Junchun Yu1,* 1 , Sina Seipel1 , and Vincent A. Nierstrasz1 Textile Materials Technology, Department of Textile Technology, Faculty of Textiles, Engineering and Business, University of Borås, 501 90 Borås, Sweden Received: 1 May 2018 ABSTRACT Accepted: 29 May 2018 Digital inkjet printing is a production technology with high potential in resource efficient processes, which features both flexibility and productivity. In this research, waterborne, fluorocarbon-free ink containing polysiloxane in the form of micro-emulsion is formulated for the application of water-repellent sportsand work wear. The physicochemical properties of the ink such as surface tension, rheological properties and particle size are characterized, and thereafter inkjet printed as solid square pattern (10 9 10 cm) on polyester and polyamide 66 fabrics. The water contact angle (WCA) of the functional surfaces is increased from \ 90 to ca. 140 after 10 inkjet printing passes. Moreover, the functional surface shows resistance to wash and abrasion. The WCA of functional surfaces is between 130 and 140 after 10 wash cycles, and is ca. 140 after 20000 revolutions of rubbing. The differences in construction of the textile as well as ink– filament interaction attribute to the different transportation behaviors of the ink on the textile, reflected in the durability of the functional layer on the textile. The functionalized textile preserves its key textile feature such as softness and breathability. Inkjet printing shows large potential in high-end applications such as customized functionalization of textiles in the domain of smart textiles. Published online: 14 June 2018 The Author(s) 2018 Introduction In the past decades, inkjet printing was recognized as an emerging production technology because of its manufacturing capabilities. Inkjet printing is applied in various applications such as micro-manufacturing, photovoltaics, electrochemical sensors and ceramic tile [1–4]. Gradual development of the inkjet printing technology [5] has now come to a stage where it is accurate and fast enough to compete as an alternative method for rapid printing, overall coating and periodic micro-patterning. Inkjet printing targeting various functional applications has been investigated intensively on non-absorbent substrates, such as glass, silicon wafer and polymer film [2]. Selective and mask-free deposition of functional materials is particularly important since the price of functional material is often high and the positioning of the material is critical in the field of microelectronics [6–10]. For example, inkjet printing Address correspondence to E-mail: Junchun.yu@hb.se https://doi.org/10.1007/s10853-018-2521-z was applied to manufacture conductive electrodes [6, 7], solar cell components [9], and to deposit precursor particles [10]. The selective deposition of functional fluids and the digitalized process made inkjet printing a very versatile and flexible fabrication technology driven by the application demands [10]. Furthermore, nanomaterials and/or functional materials that are compatible with the ink formulation can boost the performance of functional fluids with properties such as super-hydrophobicity/selfcleaning [11, 12], anti-bacterial and electrical conductivity [13]. Nanoparticles of SiO2 [14], TiO2, ZnO [15], and Ag [13, 16], functional materials such as polysiloxane [12, 14], polysilane [12] and poly(3,4ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) [17] are commonly involved. The efficient material usage, high precision and flexible production with high productivity made inkjet printing a potential method for resource efficient, high-end smart textile manufacturing. Moreover, inkjet printing could be beneficial to maintain the original characteristics of the carrying porous material, e.g., the softness in hand feeling and breathability of textile due to deposition of materials in pL range [18]. Water repellency, or sometimes extended to superhydrophobicity, is an important function on technical textile to obtain self-cleaning and stain-repellent properties [11, 14, 19], especially for sports- and work wear. This function can be achieved by applying low surface energy compounds and introducing surface roughness via dip-coating, padding, knife coating or spray coating, etc. Liu et al. [1] investigated a singleside super-hydrophobic finishing on cotton by bladecoater with fluoropolymer foam. A water contact angle (WCA) higher than 150 and hysteresis lower than 15 were achieved. Zeng et al. [20] reported a one-pot coating solution consisting of SU-8 (a negative photoresist), fluoroalkyl silane and silica nanoparticles to tune WCA of cotton from \ 90 to 160. The coating is cured by UV light and is durable to withstand 100 laundry cycles. In other researches, surface roughness is combined with hydrophobic substrates or with hydrophobic top-coatings to achieve hydrophobic/super-hydrophobic surfaces without fluorocarbon. Karim et al. [21] inkjet-printed hydroxyl functional polystyrene nanoparticles (emulsion) on polyester in order to introduce surface roughness, which boost the WCA of the surface from \ 90 to 143. Liu et al. [22] used polydopamine@octadecylamine (PDA@ODA) nanocapsules to functionalize polyester fabric. The functionalized surface has WCA of 145, and the water repellency can be healed by heating. Even though fluorocarbon (CxFy) in general shows excellent water and oil repellency among known chemical compounds, the bio-accumulative and persistence of fluorocarbon in man and everywhere else in nature make it unacceptable toward the environmental and sustainable development of modern society [23]. Perfluorooctanoic acid (PFOA), its salts and PFOArelated substances, one type of fluorocarbon (socalled C8), is restricted in EU [24]. The industry thereafter moved toward relatively short chain C6 and C4. However, the performance of C6 and C4 is worse than C8, and the bioaccumulation of persistence organic compound remains [25]. In this study, we investigated an alternative solution that stresses a resource efficient process, inkjet printing and environment-friendly polysiloxane ink, to replace conventional large run machineries with fluorocarbon chemistry. Polysiloxane used in this work is a linear-type polysiloxane (estimated as Mw & 70000 and n & 900, defined as middle-high molecular weight polysiloxane by Mojsiewicz-Pieńkowska et al. [26]) which does not pose threat to organisms via the respiratory system (when n [ 4) and therefore is favorable for textile application. Furthermore, middle-high molecular weight polysiloxane features low toxicity and does not bioaccumulate once it enters the environment according to existing studies [26]. Despite desired functionalities, the fastness properties of functionalized textile toward wash and abrasion are a fundamental challenge [27]. Zhou et al. [28] demonstrated a durable super-hydrophobic SiO2/PDMS nanocomposite coating on polyester fabrics by dip-coating. The WCA of the functional layer maintained almost the same (* 170) after 500 wash cycles or after abrasion testing (Martindale method) of 28000 cycles under 12-kPa load. Liu et al. [1] introduced a cross-linking agent, and therefore, the covalently bonded functional layer is capable to stand 20 wash cycles without significant changes in WCA (decreases from 153 to 148). In this research, we used inkjet printing to deposit water-repellent ink on polyester and polyamide 66, which is commonly used in the application of functional (technical) textile. There is a lack of publication concerning hydrophobic functionalization by inkjet 13218 printing on a porous and absorbent (textile) substrate. We focus on the inkjet formulation, the inkjet printing process and ink–substrate interaction for the application of functional textiles in sports- and work wear. Materials and methods Materials Raw polysiloxane micro-emulsion dispersion in water (Dow Corning Corporation) was kindly supplied by Univar Inc. The substrates used for inkjet printing were 100% plain woven polyester fabric (PET, FOV Fabrics AB, Sweden) and 100% plain woven polyamide 66 fabric (PA, FOV Fabrics AB, Sweden). The PET fabric has a zero twist two ply yarn warp (336 dtex) and weft of 368 dtex and a weight of 145 g m-2. The weft/warp count is 22/20 cm-1 for PET. The PA fabric has a warp and weft of 188 dtex and a weight of 118 g m-2. The weft/warp count is 27/39 cm-1 for PA. The fabrics were received as washed and preset from the producer and thereafter soaked and rinsed by deionized water and iron dried before inkjet printing. Reference PET and PA samples with the same construction but finished with fluorocarbon by exhaustion were provided by FOV Fabrics AB and used to compare with the sample functionalized by inkjet printing. The average pore size of PET and PA was 17.7 and 11.2 lm, respectively, which are measured by a liquid–liquid type porosimeter (PSM 165, Topas GmbH). The polysiloxane dispersion is chosen based on the molecular weight and architecture of polysiloxane, the carrying fluid and the rheological property of the dispersion. A waterborne dispersion containing linear polysiloxane with viscosity from a few to a few tens of mPa s which is close to the inkjet printing profile of print head is preferred to formulate the ink. Two types of polysiloxane dispersion, one contains amino functional dimethyl polysiloxane (Mw: 70000, CAS 71750-79-3) and the other contains methylamino siloxane with glycidyl trimethylammonium chloride (CAS 495403-02-6), are evaluated (Supplementary information, SI). It is convenient to formulate inkjet inks using existing polysiloxane dispersion which is available with industry-scale amount. The pre-examination showed that the inkjet-printed amino functional dimethyl polysiloxane dispersion had J Mater Sci (2018) 53:13216–13229 higher WCA than methylamino siloxane with glycidyl trimethylammonium chloride dispersion (SI, Fig. S1d). Therefore, amino functional dimethyl polysiloxane dispersion (so-called the functional ink later on) is chosen as the focus of hydrophobic component in this study. The amino functional dimethyl polysiloxane dispersion was used as it is after inkjet profile characterization. Methods The rheological properties of the functional ink were measured by a rheometer (Physica MCR500, Paar Physica) with a double gap cylindrical cell. The ink formulation was measured: (a) on heating from 15 to 40 C at a constant shear rate of 10000 s-1; and (b) at a shear rate increasing from 0 to 10000 s-1 at 25 C. The viscosity was acquired at highest measurable shear rate of the instrument at 10000 s-1. The estimated shear rate at the nozzle tip (e) of Dimatix print head could reach * 400000 s-1 by using e = v/D [29] (drop velocity, v, is 8 m s-1 and diameter of a nozzle, D, is 21 lm). The surface tension of the ink was measured using an optical tensiometer (Attension Theta, Biolin Scientific). The surface tension of the inks was measured by pendant drop method with an ink drop volume of 4 lL and reported in average of three independent measurements. The prepared ink formulation was filtrated through a nylon syringe filter with a pore size of 0.45 and 0.2 lm, respectively, to qualitatively evaluate the particle size. Still, in order to eliminate agglomeration of particles in the orifice or the nozzle channel, the functional ink was filtered through a nylon syringe filter with a pore size of 0.45 lm before loading into the print head. Inkjet printing was performed using a Xennia Carnelian 100 inkjet development and analysis system equipped with a piezoelectric type, Dimatix Sapphire QS-256/10 AAA print head. The print head features a fundamental printable drop size of 10 pL. Solid square pattern (10 9 10 cm) and custom-designed patterns were inkjet printed at a resolution of 300 dpi on PET and PA substrates by multi-pass. Thereafter, the inkjet-printed samples were heated in an oven at 150 C for 5 min to dry the functional ink. The dried samples that could still be hydrophilic at the functional surface were immersed in excessive amount of water to remove the water-soluble component in the ink. Eventually the samples were air- J Mater Sci (2018) 53:13216–13229 dried, and the functional surface became hydrophobic. WCA was measured by sessile drop method with the optical tensiometer (Attension Theta). Three to five random spots on a textile substrate were selected to represent the surface property and measured with a water drop volume of 3 lL. For a hydrophobic textile surface, the WCA was read 3 s after the water droplet was stabilized on the substrate. The observed WCA reduces with time for a hydrophilic textile material. Therefore, the WCA was read immediately once the water droplet landed on the substrate. All samples were air-dried in ambient condition with a temperature of 20 ± 2 C and a relative humidity of 20–30 ± 3% for 24 h and ironed prior to measurement. ASTM D5725-99 suggests a repeatability of 7% of WCA within a laboratory on absorbent substrates; therefore, we only distinguish the result when the WCA difference is larger than 10 [30]. The wash fastness was tested according to ISO 6330, with type A machine and procedure 4 N. The abrasion test was performed using a Martindale 2000 abrasion tester (Cromocol Scandinavia AB) at standard climate with a temperature of 20 ± 2 C and a relative humidity of 65 ± 2%. The tests were performed according to ISO 12947-2:1998/Cor.1:2002 but modified to investigate the hydrophobicity of the functional layer. The PET and PA samples were conditioned in standard climate for at least 18 h and subsequently rubbed at 3000, 5000, 10000 and 20000 revolutions, respectively, with a load of 12 kPa. The WCA of functional surface was measured at five random locations on each sample after wash or abrasion tests. Scanning electron microscope (SEM) was performed on JEOL JSM-6301F, and EDX analysis was conducted on a Zeiss Supra 60 VP SEM with EDX probe. Results and discussion Polysiloxane was chosen as fluorocarbon-free and water-repellent compound in the functional ink. As shown in Fig. 1a, the viscosity of the functional ink was compared with one type of Dimatix test ink. The viscosity of the functional ink shows a rather flat temperature dependence compared to Dimatix test ink. The viscosity of the functional ink decreases slightly from 11.7 to 9.5 mPa s from 15 to 40 C (comparing to 19.6 to 11.9 mPa s for Dimatix test 13219 ink), and has a viscosity of 9.9 mPa s (13.6 mPa s for Dimatix test ink) at chosen inkjet temperature of 35 C. Furthermore, as shown in Fig. 1b, both inks show a Newtonian behavior at a shear rate scan (0–10000 s-1). The functional ink has a viscosity of 9.8 mPa s at 100 s-1 (14.8 mPa s for Dimatix test ink) and 10.1 mPa s at 10000 s-1 (14.6 mPa s for Dimatix test ink) at 25 C. The Newtonian behavior of the fluid and weak temperature-dependent viscosity of the ink suggested that it is reasonable to present the viscosity of 9.9 mPa s of functional ink at 10000 s-1 (high shear rate) and 35 C (inkjet printing temperature) for ink characterization at inkjet printing condition. The presented viscosity of 9.9 mPa s of functional ink fits the inkjet printable range, i.e., from 8 to 14 mPa s. The surface tension and particle size were analyzed by pendant drop method and filtration test, respectively. The functional ink has a surface tension of 23.9 mN m-1, which is slightly below but still around the typically acceptable surface tension for inkjet printing between 25 and 35 mN m-1 [3]. The transparent functional ink containing polysiloxane in the form of a micro-emulsion should have a particle size distribution between 1 and 100 nm. The stated particle size (15 nm) is considerably below 200 nm, which is an experienced particle size limitation to avoid agglomeration of particles in the printing nozzle channel [31]. The functional ink was able to filter through nylon syringe filters with pore sizes of 0.45 and 0.2 lm, respectively, which confirmed that the functional particles are compatible with the inkjet printing process. The results obtained from measurements of viscosity, surface tension and particle size suggested that the functional ink fulfilled the print head specifications. The functional ink was inkjet printed as solid square (10 9 10 cm) or as other customized pattern at a resolution of 300 dpi by printing of 1, 3, 5 and 10 passes. The hydrophobicity of inkjet-printed samples was characterized by WCA measurements. As shown in Fig. 2a, untreated PET and PA woven fabrics showed overall hydrophilic surfaces with WCA \ 90, even though PET fiber is inherently hydrophobic but PA fiber is inherently hydrophilic. The inkjet functionalized surfaces showed significantly increased WCA. The one-pass printed PET (PET 9 1) and PA (PA 9 1) already show that WCA increased from \ 90 to more than 130. With increased amount of functional inks, the 10-pass printed PET (PET 9 J Mater Sci (2018) 53:13216–13229 (a) 25 (b)18 15 10 5 0 15 Formulation Dimatix test ink Functional ink 20 25 Surface tension (mN/m) 32.3 23.9 30 35 (a) (b) 4 160 140 15 Contact angle(θ) Viscosity (mPas) Viscosity (mPas) 20 13221 J Mater Sci (2018) 53:13216–13229 Printing deposition weight (gm2) 13220 12 9 120 100 80 60 40 o Temperature ( C) 6 0 2500 5000 7500 10000 40 0 -1 Shear rate (s ) Figure 1 Rheological properties and surface tension of functional and Dimatix test ink. a Viscosity of functional (blue squares) and Dimatix test (red circles) ink measured on heating from 15 to 40 C at a shear rate of 10000 s-1, inset: surface tension of functional and Dimatix test ink; b viscosity of functional (blue squares) and Dimatix test (red circles) ink measured at increasing shear rate from 100 to 10000 s-1 at 25 C. 10) and PA (PA 9 10) show a progressively improved WCA to 136 and 137, respectively. In comparison, the reference PET and PA with fluorocarbon finishing by exhaustion have a WCA of 143 and 140, respectively. The inkjet-printed functional surface has slightly lower WCA but close to the WCA of a fluorocarbon-finished surface. It is interesting to notice that the hydrophobic functionalized textile shows higher WCA than literature reported on smooth, solid form polysiloxane film (* 110) [32]. This indicates that the surface roughness of the textiles could be equivalent to a periodic pitch between 40 and 50 lm, in comparison with laser etched polysiloxane with periodic pitch pattern reported by Jin et al. [32]. Moreover, the inkjet-printed samples are all able to hold the water and other water-based droplets such as milk, coffee and blue-colored ink on the surface for more than 10 min without visually wetting the surface (Fig. 2d). A drop testing kit with various mixtures of isopropanol and water (0, 2, 5, 10, 20, 30 and 40 vol% of isopropanol/water mixture) supports the WCA result (SI, Table S2). SEM was applied to study the morphology of the functional layer. As shown in Fig. 5a, d the functional ink formed a thin layer wrapped around the textile filament. Besides, the functional ink formed uneven circular layers on the surface of PA filament (Fig. 5d), so-called coffee-stain effect after drying of the functional ink [3]. The formation of coffee-stain effect on the PA, which, however, is less obvious on PET and indicates that the inkjet-printed ink was spread more evenly on PET. Energy-dispersive X-ray spectroscopy (EDX) analysis was performed on PA 9 10 sample. As shown in EDX spectrum in Fig. 5h, O and Si were the main element detected which are the components of the functional ink (O could contribute from PA substrate as well but less likely due to detectable depth of EDX). Element mapping of Si by EDX was also performed locally on one filament of the PA 9 10 (SI, Fig. S5a). Accumulation of Si was detected as layer wrapped around the filament. The above results support that the hydrophobicity of the PET and PA surface is introduced by covering the surface with a thin layer of polysiloxane that is the active compound in the ink formulation. Inkjet printing can deposit functional fluid on demand. The SEM images in Fig. 3a characterized the inkjet printing boundary of the solid square of PET 9 10 sample. The side with smooth and dark appearance is the side with deposited functional ink which is clearly separated from the untreated PET side with rougher and brighter appearance. Moreover, logos of University of Borås and FOV fabrics (original pattern in SI, Fig. S4a, b) were inkjet printed and thereafter sprayed with blue-colored ink all over the surface to test the water repellency. As shown in Fig. 2c, the area without functional ink absorbed the blue-colored ink and therefore revealed the logo. The residual blue color on the functional surface could be rinsed away easily by running water (SI, Fig. S4c, d). Because of the advantage of digitalized communication, inkjet printing combined with functional ink could be applied as customized patterning tool with emphasis on both production capacity and flexibility for the textile sector. Moreover, inkjet printing is a resource efficient process where only materials 2 4 6 8 10 3 2 1 0 0 5 10 15 20 Printing passes (times) Printing passes (times) (c) (d) (e) Figure 2 a Water contact angle of PET (circle) and PA (triangle) untreated samples and after inkjet functionalization of hydrophobic ink. The PET (black circles) and PA (red triangles) samples were inkjet printed up to 10 passes (9 10); untreated PET (open circles) and PA (open red triangles), reference PET (open squares) and PA (open red diamonds) with fluorocarbon finishing were presented. The error bar is the standard deviation from three to five independent measurements. b The weight of the deposited functional material as function of printing passes on PA (red triangles). The red dashed line is the linear fit of the data. c Demonstration of water repellency of custom-designed pattern on PA 9 10 sample to blue-colored ink. The logos of University of Borås and FOV fabrics are inkjet printed (with original pattern in SI, Fig. S4a, b). Demonstration of water repellency of the functional surface to water, milk, coffee and blue-colored droplets. d PET 9 10 sample, and e PET 9 10_Wash 9 10 sample. The blue pen marked area is functionalized with water repellency. The scale bar in the figure is 1 cm. necessary are deposited. The samples before and after inkjet printing were weighed to calculate the amount of functional material. As shown in Fig. 2b, 3.4 g m-2 of functional material were deposited after 20 times or 1.6 g m-2 after 10 times inkjet printing. The thickness of the inkjet-printed 10-layer sample is calculated as 1.6 lm (density of 1 g cm-3). Inkjet printing has advantage to localize the deposition of materials on textile. The cross-sectional view of PET 9 10 sample (Fig. 3b, see arrow) suggested that most of the functional ink was kept at the top surface of the fabric and did not penetrate through the fabric (SEM in SI, Fig. S3a, b). The observation in microstructure was supported by WCA measurement that the printed front side of PET 9 1 after 10 washes showed WCA of * 11 higher than printed backside surfaces. It is good to note that the PET and PA used in this work are not pre-treated to promote surface absorption. Nechyporchuk et al. [13] found that the strike through of inkjet-printed pigment colorants on cotton was ca. 50% of the thickness of fabric and can be further constrained to the surface J Mater Sci (2018) 53:13216–13229 (a) 13223 J Mater Sci (2018) 53:13216–13229 (b) Contact angle (θ) 160 160 (a) 140 Contact angle (θ) 13222 140 120 100 ISO 6330 standard. During washing, the functional surface is exposed thoroughly to agitation by detergent, mechanical vibration and elevated temperature. As plotted in Fig. 4a, the PET 9 10 and PA 9 10 samples preserved similar level of WCA after 10 wash cycles. The WCA of PET 9 10 was 136 and became 133; the WCA of PA 9 10 was 137 and became 131 after 10 wash cycles. In comparison, the reference PET with fluorocarbon finishing has a decreased WCA from 143 to 126 whereas the reference PA with fluorocarbon finishing has a decreased WCA from 140 to 128 after 10 wash cycles. The results showed the functional layer demonstrated resistance to wash. The WCA almost stayed the same after 10 wash cycles. SEM analysis was performed on PET 9 10 and PA 9 10 samples after 1 and 10 wash cycles. After one wash, there are spherical-shaped particles and agglomerated clusters of such particles formed at the functional surface for both PET 9 10 and PA 9 10 samples (Fig. 5b, e). The formed particle has a diameter smaller than 5 lm after one wash but became progressively denser and tended to form clusters larger than 10 lm with various shapes and dimensions after 10 wash cycles (Fig. 5c, f). A tenable assumption is that the washing process abraded the functional surface and partially rubbed off some functional material to form the spherical particles. As shown in Fig. 5i, O, Si, Na and Ca were the main elements detected on PET 9 10_Wash 9 10 sample. The detection of O and Si elements suggested that the functional ink remains at the textile surface. The Na and Ca could be residuals from detergents added during wash tests (referring to surfactant, zeolites, 80 60 (c) 160 (b) Contact angle (θ) with a porous top coating. Inkjet printing has the advantage of efficient materials usage in comparison with other coating technologies. It is only possible to deposit the same amount of materials homogeneously all over the bulk textile by dip-coating, or other dipping methods. For single-side deposition technology such as knife coating, Liu et al. [1] demonstrated a fluorocarbon foam, which can typically apply 1.8 g m-2 on textile. This amount of material is slightly higher than the case for 10 passes of inkjet printing. However, other components such as rheology modifiers needed to be added in the formulation but was removed after deposition. For formulation with high solid content ([ ca. 50 wt.%), high viscosity (a few tens of Pa s) and low shear rate (* 2000 s-1) in knife coating, the materials applied (processing speed dependent) are often higher than a few tens of grams per square meter [33, 34]. Although dip-coating and knife coating hypothetically are flexible in production length, inkjet printing still has the advantage to efficiently deposit costly functional materials. Thanks to the precisely controllable deposition of ink volume in pL range, the key textile features such as tactile feeling and breathability are not affected after functionalization. As shown in the SEM pictures (Fig. 5), the inkjet-printed functional ink formed a thin layer that does not block the inter-yarn pores of the woven textiles, therefore maintaining the tactile feeling and breathability of the textile after inkjet functionalization. The durability of the functional layer toward wash and abrasion is assessed to simulate the usage of functional textiles in a daily environment. The functionalized textiles were washed at 40 C according to PET. b Cross-sectional view of inkjet-printed PET 9 10. The functional ink is deposited mostly on the top surface, as pointed by arrow. Contact angle (θ) Figure 3 SEM images of inkjet-printed ink on PET substrate (PET 9 10). a Top-view of inkjet-printed functional ink boundary. The left side with brighter appearance is untreated PET, whereas the right side with darker appearance is the inkjet functionalized 160 60 ʅm 100 40 80 2 mm 120 140 120 100 140 120 (d) 80 0 2 4 6 8 10 Wash cycles (times) 100 0 5000 10000 15000 20000 Revolutions (rubs) Figure 4 Water contact angle of inkjet functionalized PET and PA samples after various wash and abrasion cycles. a The inkjet functionalized PET 9 10 (black circles) and PA 9 10 (red triangles) samples were washed up to 10 cycles; reference PET (open squares) and PA (open red diamonds) with fluorocarbon finishing were presented. b The inkjet functionalized PET 9 1 (open circles) and PA 9 1 (open red triangles) samples were washed 10 and 2 cycles, respectively. c The inkjet functionalized PET 9 10 (black circles), PA 9 10 (red triangles), untreated PET (open circles), untreated PA (open triangles) samples, reference PET (open squares) and PA (open red diamonds) with fluorocarbon finishing; d The inkjet functionalized PET 9 1 (open circles), PET 9 3 (open diamonds), PA 9 1 (open red upward triangles) and PA 9 3 (open red downward triangles) were rubbed up to 20000 revolutions. The error bar is the standard deviation from five independent measurements, and the connection solid and dash lines are to guide the eyes. etc., as component in the washing detergent). The proportion of elements is not shown since EDX is a qualitative method. Furthermore, element mapping of Si confirmed that Si accumulated locally on a few aggregated spherical particles on one filament of PET 9 10_Wash 9 10 samples (SI, Fig. S5b). This suggested that the functional ink was mechanically abraded, chemically agitated and thereafter formed spherical particles, which stick to the textile substrate by physical interaction, or are partially removed (damaged) from the filament. As shown in Fig. 5g (see arrow), the washing process might attack the functional layer in a similar way like peeling an onion, i.e., layer by layer. The textile could preserve the hydrophobicity as long as there is some functional material wrapped around/sticking to the filament, regardless if it is a form of flat layer or in other spherical-like shapes. However, the change in microstructure indicated damage or disturbance of the inkjet-printed functional layer. Various liquids were placed on a PET 9 10_Wash 9 10 samples to verify the water repellency after washing. As shown in Fig. 2e, the inkjet-printed area of PET 9 10_ Wash 9 10 sample showed repellency to water, coffee and blue-colored ink. However, we noticed that the time for the textile to hold the water droplet after 10 washes is reduced. The WCA started to decrease after drop placement within 3–5 min. The deterioration of water repellency of the functional layer agreed with the damage of the functional layer in microstructure. The durability of the functional layers on textile is challenging and demanding. Similar comparison can be made with functional layers produced with other methods, and Vasiljevic et al. [35] produced a hierarchically roughened surface with SiO2 nanoparticles covered by fluoroalkyl functional oligosiloxane by pad-dry-cure method. During wash fastness testing, the functional layer starts to change after 10 domestic washes. Schwarz et al. [36] produced thin copper films on para-aramid yarns via 13224 J Mater Sci (2018) 53:13216–13229 (a) (b) 20 ʅm (d) (c) 70 ʅm (e) 20 ʅm 70 ʅm (f) 70 ʅm 70 ʅm (h) (i) (g) 10 ʅm Figure 5 SEM pictures of inkjet-printed PET and PA after various wash cycles and EDX spectra. PET 9 10 sample with a after inkjet printing, b after one wash cycle, c after 10 wash cycles; and PA 9 10 sample d after inkjet printing, e after one wash cycle, f after 10 wash cycles. g PET 9 10_Wash 9 5 sample showed peeling of functional materials and formation of particles/clusters (see arrow). h EDX analysis of PA 9 10 sample and i EDX analysis of PET 9 10_Wash 9 10 sample. wet chemistry route. The morphology of yarns shows damage after 25 washes. In both examples, the functional properties such as water and oil repellency or electrical conductivity deteriorated with damaged functional layers after washing. Introduction of a cross-linking mechanism in the ink formulation could reinforce the functional layer on textile as suggested by Liu et al. [1] and Zhou et al. [28]. Still, even though the polysiloxane used in this study could release to environment, it is a linear no-violate type material which is rather safe [26, 37]. The material is intended to be deposited on the outside surface of garment which has less direct contact with the human skin. The functionalized PET showed better wash fastness property due to (a) the stronger ink–filament interaction with PET, and (b) the deeper transportation of the functional ink in PET. As shown in Fig. 4b, the WCA of PET 9 1 was 132 and became 125 after 10 wash cycles. But the WCA of PA 9 1 became \ 90 after 2 wash cycles from an initial WCA of 134. The PET 9 1 shows better wash fastness than PA 9 1 sample. This could be a result of weak interaction between the functional ink and PA filament. As shown in liquid absorption tests of untreated PET and PA (SI, Table S1), the untreated PA absorbed slightly more high surface tension liquid, e.g., water [38] (72.09 mN m-1) but less low surface tension liquid, e.g., acetonitrile and ethyl acetate, than untreated PET. The surface tension of the functional ink (23.9 mN m-1) is very similar to acetonitrile [38] and ethyl acetate (27.76 mN m-1) (24.11 mN m-1) [39]. This means the PET might have stronger ink–filament interaction than PA due to better wettability with the functional ink. 13225 J Mater Sci (2018) 53:13216–13229 Furthermore, transportation of the functional ink is assessed theoretically and experimentally. The Washburn–Lucas equation [40] describes the transportation of liquid within a two-end open pore with external pressure opposing capillary flow. By assuming the opposition pressure is negligible, the equation became: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi � � ffi r � c � cos h �t ð1Þ l¼ 2�g where l is the distance penetrated by a liquid, r is the effective capillary radius of the pores, c is the surface tension of the applied liquid, h is the liquid/pore contact angle, g is the viscosity of the liquid and t is the time. The wicking property of the fabric was assessed by the absorption rate of the ink on the substrate (SI). As shown in Fig. S2, the absorption of the functional ink on PET is more or less linear in speed as in Eq. (1), and faster than the absorption on PA. Furthermore, the absorption of the functional ink on PA is a two-stage or exponential decay process. The ink was absorbed faster at the beginning but got slower with time. Moreover, in capillary height measurement in warp and weft direction of untreated PA and PET, the absorption rate of water in untreated PET is higher than PA (SI, Fig. S1). The results suggested that untreated PET could have larger pores, i.e., inter- and intra-yarn spaces r, than PA in Eq. (1); therefore, larger lPET was observed experimentally (assuming coshPET & coshPA). As a result, the ink spread more on the surface of PA rather than transporting it deeper. This could be deducted by experimentally measured smaller rPA (average pore size of PA of 11.2 lm \ PET of 17.7 lm) and lPA from Eq. (1). Finally, this assumption is supported by SEM picture of PA 9 10 sample, where the functional ink accumulated more on the PA top surface and therefore is more vulnerable to washing. Overall, the functional ink absorption measurement, capillary height measurement and pore size measurement suggested coherent evidences that the ink is transported deeper in PET. Liu et al. [1] studied the hydrophobicity and durability of fluorocarbon knifecoated cotton. The WCA of fluorocarbon-coated cotton is * 20 (WCA of * 153) higher than inkjetprinted polysiloxane on PET or PA. The WCA of noncrosslinked fluorocarbon coating decreased to * 144 after 10 wash cycles [1], similarly as decrease in WCA (137 to * 132) in our observation. Furthermore, polysiloxane padded [41] or grafted to cotton fabrics [42] can improve the WCA from \ 90 to 120–130. Polysiloxane combined inkjet printing shows the capability to functionalize a water-repellent surface as a more environmentally friendly alternative. The durability of the functional layer toward abrasion was tested by Martindale tester according to modified ISO 12947-2:1998/Cor.1:2002 standard. As shown in Fig. 4c, the PET 9 10 and PA 9 10 samples preserved the hydrophobicity after 20000 revolutions. The WCA of PET 9 10 was 136 before and became 140 after rubbing; the WCA of PA 9 10 was 137 and remained 143 after 20000 rubs. In comparison, the untreated PET and PA samples showed no improved WCA after abrasion tests. The SEM images (Fig. 6) suggested that the functional layer was not affected very much in both PET 9 10 and PA 9 10 samples after 20000 rubs. The microstructure is significantly different compared with inkjet functionalized textiles after wash fastness tests. Furthermore, the reference PET had a slightly decreased WCA from 143 to 127, whereas the reference PA remained the hydrophobicity from 140 to 133 after 20000 rubs. We noticed that the abrasion tests could produce a fluffy surface due to abrasion of textile substrate against the abrading material. At microscopic level, the inkjet functionalized textile became planar functionalized woven yarns with protruding filaments with functional wrapping. According to Cassie–Baxter equation [43, 44], cos hCB ¼ fs � ðcos hY þ 1Þ � 1 ð2Þ where hCB is the Cassie–Baxter contact angle on the rough surface, the fs is the fractions of the solid area divided by the projected surface area on the surface, and hY is the contact angle on smooth surface. Therefore, WCA of a surface is affected by the surface roughness. These small protruding filaments with hydrophobic functionalization on the top surface may boost the hydrophobicity by reducing fs (higher WCA hCB in Eq. 2). The resistance to abrasion of inkjet functionalized samples is comparable or slightly better than fluorocarbon-finished reference PET and PA. Abrasion tests were performed on PET 9 1, PET 9 3, PA 9 1 and PA 9 3 samples. As plotted in Fig. 4d, PET 9 1 and PET 9 3 samples have WCA of 130 and 121 after 20000 rubs. But 13226 Fig. 6 SEM images of PET 9 10 and PA 9 10 samples with 20000 rubs after abrasion tests. PET 9 10 sample with a 10009 and b 30009 magnification; and PA 9 10 sample with c 10009 and d 30009 magnification. J Mater Sci (2018) 53:13216–13229 20 ʅm (c) 7 ʅm (d) 20 ʅm PA 9 1 and PA 9 3 samples became hydrophilic after 10000 and 20000 rubs. The PET 9 1 and PET 9 3 samples showed better abrasion resistance compared to PA 9 1 and PA 9 3 samples. The results suggested that the functional ink adhered better/interacted stronger with PET. However, the capability of samples to hold water droplets decreased significantly, suggesting damage of the functional layer. After 3000 rubs, the droplets can wet PET 9 1, PET 9 3, PA 9 1 and PA 9 3 samples within 5 min, despite a high initial WCA. The ink transportation and ink–filament interaction are crucial to understand the durability of the functional layer in abrasion. Two factors, porosity/effective pore size of the textile substrate and ink–filament interaction played a key role. As shown experimentally in capillary height and functional ink absorption measures in Fig. S1 and S2, as well as theoretical deduction in Eq. (1), the functional ink penetrates deeper in PET fabrics. This can be the reason to obtain better abrasion resistance with PET. The inkjetprinted ink first covered the top layer of the textile surface and thereafter transported deeper through inter- and intra-yarn spaces into the bulky textiles. The ink transported deeper into the textile could offer Conclusion (b) (a) 7 ʅm better resistance, whereas functional ink accumulated on the top of the surface is more vulnerable to abrasion. Furthermore, PET indicates stronger ink–filament interaction with the functional ink. The functional ink adhered better on PET 9 1 and 9 3 samples than PA 9 1 and 9 3 samples, which agreed with previous wash fastness results. Moreover, the filaments wrapped around with a thicker layer of functional material are more resistant to abrasion. As shown in Fig. 4d, the sample with less functional material (PET 9 1, 9 3 and PA 9 1, 9 3, \ 1 g m-2) is more vulnerable to abrasion tests than PET 9 10 and PA 9 10 samples (1.6 g m-2). Overall, the inkjetprinted PET and PA surface with polysiloxane showed moderately high WCA, wash fastness and abrasive property. However, due to the nature of non-covalent interaction between the functional layer and textile substrate, the fastness properties are constrained. To improve water repellency, as well as fastness properties, it could be interesting to introduce covalent bonding between the functional layer and textile substrates in combination with nanoscale surface roughening. 13227 J Mater Sci (2018) 53:13216–13229 Waterborne, fluorocarbon-free and water-repellent ink with polysiloxane micro-emulsion was formulated and inkjet printed on plain woven PET and PA textile substrates, in order to functionalize the textile with water repellency. The PET and PA have a WCA from \ 90 to 140 after inkjet functionalization. The functional textile has a microstructure with polysiloxane wrapped around the filament which modified the surface property of the textiles. Furthermore, the functional surfaces show resistance to wash and abrasion. The 10-pass printed PET (PET 9 10) preserved a WCA of 133 after 10 washes and a WCA of 140 after 20000 rubs. We found that the ink transportation in the textile structure as well as the ink–filament interaction plays a key role in the durability of the functional layer. The functional ink was transported faster and deeper in PET, and with stronger non-covalent ink–filament interaction with PET than with PA. Therefore, PET showed better wash fastness and abrasion resistance. The microscopic images showed that inkjet printing did not or had little impact on the tactile feeling of the textile, i.e., the hand feeling and softness of textile is well preserved. Inkjet printing demonstrated selective and efficient deposition of functional material on textiles, as a promising resource-efficient process in the applications of functional and smart textiles. Acknowledgements The authors are grateful for the support from CROSSTEXNET, Digifun project, KK Stiftelsen (The Knowledge Foundation) with Diarienummer: 20150040, TEKO (The Swedish Textile and Clothing Industries Association), SST Stiftelsen (The foundation for Swedish Textile Research), Myfab, Borås stad and Sparbanksstiftelsen Sjuhärad for enabling this research. Compliance with ethical standards Conflict of interest The authors declare that they have no conflict of interest. Electronic supplementary material: The online version of this article (https://doi.org/10.1007/ s10853-018-2521-z) contains supplementary material, which is available to authorized users. 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The results showed that scCO2 dyed polyester fabrics exhibited reversible colour changing properties upon UV exposure and removal. The samples dyed with SO-SG demonstrated a comparable degree of photo-colouration, lower background colour, faster colouration and decolouration speeds, but inferior wash fastness compared with NP-RR dyed Molla Tadesse Abatea,b,c, Sina Seipela*, Junchun Yua, Martina Vikovád, Michal Vikd, samples. Particularly, the same class of dyes applied by scCO2 dyeing showed faster fading rates Ada Ferrib, Jinping Guanc, Guoqiang Chenc, and Vincent Nierstrasza compared with conventionally dyed samples. This study shows that scCO2 dyeing method is a potential alternative to develop uniformly coloured photochromic textiles providing excellent Textile Materials Technology, Department of Textile Technology, Faculty of Textiles, Engineering, and Business, University of Borås, 501 90 Borås, Sweden b Department of Applied Science and Technology, Politecnico di Torino, Corso Duca degli Abruzzi 24, 10129 Torino, Italy c College of Textile and Clothing Engineering, Soochow University, 215006 Suzhou, Jiangsu, China d Department of Material Engineering, Faculty of Textile Engineering, Technical University of Liberec, 461 17 Liberec, Czech Republic a photochromic performance with additional economic and environmental benefits. Keywords: Supercritical carbon dioxide (scCO2) dyeing, polyester, textile, photochromic dye, spirooxazine, naphthopyran 1. Introduction *Corresponding author: Sina Seipel Email: sina.seipel@hb.se Photochromic colourants change their colour from colourless to a coloured state when irradiated by UV light and revert to their original colourless state when exposed to visible light or increased temperature. Photochromic materials have been used in different application areas including Abstract ophthalmic lenses, optical data recording and storage, memories, light control filters, security printing, sensors and displays [1, 2], but their use in textiles has been limited. Nowadays, Photochromic molecules are well-established colourants in the manufacturing of niche products, however, the interest in using photochromic textiles has risen especially due to their important providing reversible colour change effects when irradiated with ultraviolet (UV) light. The high and potentially fun applications in smart and functional textiles [3]. Applications in textiles range material cost of such speciality dyes along with the general high carbon footprint and extensive from everyday clothing to high-tech smart and functional textiles such as in fashion and water consumption of textile products necessitates resource-efficient production processes. The intelligent design, security and brand protection, anti-counterfeit, camouflage, textile UV sensors, use of supercritical CO2 (scCO2) dyeing technique enables the economic production of textile and active protective clothes [4]. Photochromic textiles have the advantage of quickly changing high-end products, where a uniform through colouration is desired. This study investigates the colour in response to light while maintaining general textile properties. For example, UV sensing potential application of two commercial photochromic dyes based on spirooxazine (Sea Green – textiles show higher flexibility, are easily customizable and require low maintenance compared to SO-SG) and naphthopyran (Ruby Red – NP-RR) to polyester fabric using scCO2 dyeing technique and examines their photochromic behaviour. The dyeing was carried out at 120 °C and 25 MPa for one hour. The photochromic performance was evaluated using a specially designed conventional sensor systems [5]. Despite the availability of significant interest, commercial development of photochromic textiles is limited mainly due to technical issues associated with application methods and performance [1]. Several techniques have been reported for the online colour measurement system capable of simultaneous UV irradiation and continuous preparation of photochromic textiles, including screen-printing [6-11], digital inkjet printing [12- measurement of photochromic colour change even after the shutdown of the UV source. The 1 2 16], exhaust dyeing [15, 17-20], sol-gel coating [21, 22], pad-dry method [23, 24], electro- For uniformly through-coloured applications, the dyeing process in scCO2 is an attractive spinning [25, 26], and mass colouration [27, 28]. Details about the different methods of alternative to aqueous and solvent-based processes offering significant environmental and production of photochromic textiles, their properties, and applications in the textile field were economic benefits [32]. The primary advantage is that it avoids the use of freshwater and reviewed by Periyasami et al. [29]. Among the methods, screen-printing and conventional auxiliaries and the associated hazardous effluent. It has simpler dye formulation with a small exhaust dyeing are the most widely used methods. Screen-printing has been successfully used to amount of dyestuff particularly advantageous in the case of expensive materials such as apply photochromic dyes to textiles. While good photochromic effects have been obtained, the photochromic dyes. The dyeing process in scCO2 is generally faster due to its low viscosity and binder used usually imparts a relatively harsh handle to the fabrics, which affects their comfort high diffusivity properties. Briefly, the dye is dissolved in supercritical medium, transported to characteristics. It also has relatively poor fastness and photostability properties compared with the the fibre, and penetrates into the fibre relatively fast achieving uniform dyeing in a short time. In exhaust dyeing method [3]. Exhaust dyeing of photochromic dyes as disperse dyes to various addition, CO2 can swell and plasticize the polymer which also facilitates the diffusion of dye textile substrates has also been widely investigated [15, 18, 19]. It is commonly considered as the molecules. At the end of the process, CO2 and excess dye can be recycled and no drying is most convenient and easy technique compared with mass colouration and printing methods. required. Thus, it is a resource-efficient and economical production process offering an overall 50 However, exhaust dyeing has many inevitable problems such as the requirement of long dyeing % lower operating cost [33-35]. Furthermore, the use of scCO2 as a solvent avoids any time at high temperature, a large amount of water and auxiliary chemicals, incomplete exhaustion undesirable solvent effect on the photochromic properties as the CO2 can be easily removed from and fixation even with salts and alkalis, and the discharge of the contaminated wastewater, which the polymer during pressure release after completion of the dyeing procedure. makes it ecologically and environmentally less safe. During exhaust dyeing, approximately 5-10 % of the dyes are not used and ends up in wastewater [30]. In addition, the exhaustion and colour Photochromic dyes are suitable colourants to be applied in scCO2 medium owing to similar build-up properties of photochromic dyes were reported rather low [18, 19], which makes the molecular structure as the conventional disperse dyes in that they are small to medium-sized process even more costly given the high cost of photochromic colourants. Attempts to replace neutral molecules with a balance of hydrophilic and hydrophobic character [36]. They displayed water with organic solvents was also challenged by issues such as toxicity, recoverability, and the same dyeing behaviour as disperse dyes when applied to polyester fabric using exhaust cost [24]. Recently, application of photochromic colourants using digital inkjet printing has dyeing method [19]. Application of some photochromic dyes in polymeric matrixes using scCO2 presented promising results for one-sided patterned applications [12, 16, 31]. Despite its has been proven to be feasible. Glagolev and his research group [37-40] investigated the potential advantages over conventional screen-printing in regards to resource efficiency and process of scCO2 impregnation to produce different photochromic polymeric materials using various flexibility, the requirement of inks, which meet the stringent physical, chemical, and photoactive compounds. Lima et al. [41] also studied the impregnation of polycarbonate and environmental criteria along with the associated print qualities remain a challenge. Overall, most silica gel composite film into a photochromic dye Reversacol Graphite in scCO2 medium to of the conventional techniques currently used to produce photochromic textiles have common fabricate photochromic lenses. To the best of our knowledge, however, the application of issues such as higher cost of production, lower level of colour development, and insufficient photochromic dyes to polyester fabrics using scCO2 dyeing method has not yet been investigated. durability, that need to be addressed for successful commercial application [4]. Therefore, appropriate production techniques, which meet the necessary commercial requirements, are still This study investigates the potential application of selected commercial photochromic dyes, i.e. needed. spirooxazine and naphthopyran based dyes, to polyester fabric using the scCO2 dyeing technique to produce UV sensing smart textiles. The photo-colouration and fading properties of the photochromic dyed samples were performed using an online colour measurement system. The 3 4 colour yield and switching rates and their durability against washing were examined. Results Y Spirooxazine (SO) showed that both photochromic dyes were successfully incorporated into the polyester fabric Y R using scCO2 technique displaying comparable performance in terms of colour yield but a distinct N rate of colour build-up and wash fastness properties. This study provides an insight into the R" N behaviour of photochromic dyes applied directly as disperse dyes to the polyester fabric from O scCO2 media with a large potential to facilitate the development of uniformly coloured R photochromic textiles in an eco-friendly and resource-efficient way. X 2. Experimental (a) 2.1 Materials R" N R R UV light Heat and/or light N O R=alkyl R,R"=alkyl(usually methyl) X= H, amino, hetaryl, etc Y= H, halogen, etc X (b) The substrate used in this study was a heat-set plain-woven polyester fabric (147 g/m2) supplied by FOV Fabrics (Sweden). Two commercial photochromic dyes; Reversacol Ruby Red and Sea Naphthopyran (NP) Green (Vivimed Labs, UK) were selected (Table 1) and their generalized structures are X Y X illustrated in Scheme 1. The CO2 used in all experiments was supplied by AGA industrial gases (Sweden) and has 99.5 % purity. UV light Table 1. Commercial photochromic dyes used and their notations Notation Colour Chemical class SO-SG Sea Green Spirooxazine NP-RR Ruby Red Naphthopyran Heat and/or light O O Y R (a) X,Y= H, amino, alkoxy, etc R= H, amino, etc (b) R Scheme 1. General dye structure of spirooxazine (SO) and naphthopyran (NP) and their photochromic transformation from (a) ring-closed to (b) ring-opened forms. 2.2 Dyeing procedure Dyeing was performed using a batch type scCO2 dyeing apparatus ( Figure 1) equipped with a high-temperature oil (glycerine) bath, a rotary wheel where the vessels are mounted, a motor, temperature and time controller, a heater, and cooling element. The apparatus is a beam type dyeing machine, where the polyester fabric (21×30 cm, ca. 10 g) was 5 6 2.3 Dynamic colour measurement wrapped around a Teflon mesh, suspended inside the dyeing vessel (with internal volume = 290 mL, Pmax = 30 MPa, and Tmax = 130 °C) and the photochromic dye (0.027 % owf ~ 2.56 mg) was placed below the fabric at the bottom of the vessel. The amount of colourant was selected on the The colouration and decolouration properties of the photochromic textiles were measured using an LCAM Photochrom 3 spectrophotometer (Technical University of Liberec, Czech Republic), basis to compare results of photochromic textiles produced with digital inkjet printing [16]. which can measure the colour development and fading performance continuously over various cycles of UV- exposure and relaxation. The LCAM Photochrom 3 is equipped with LED Engine LZ1-00UV00 LED as an activation light source with a radiometric power 1680 mW and an emission peak with a wavelength of 365 nm. The measuring light source is a dual system of combined high-power white LEDs with CCTs of 4000, 5000 and 7000 K. Upon measurement, samples are illuminated with 60 klx, which constitutes an illuminance between brightest sunlight intensity (120 klx) and daylight intensity without direct sunlight at noon (20 klx). The temperature of the samples was controlled to around 21 °C and reflectance values were recorded in intervals of 2 s during continuous UV irradiation and shutdown. Colour analysis of each dye is made at their dominant wavelength, which is 500 nm for NP-RR and 620 nm for SO-SG. The colour values (K/S) were calculated from the reflectance spectra using the Kubelka-Munk function, Equation (1). Figure 1. Scheme of scCO2 lab dyeing procedure (1) CO2 tank, (2) Freezer, (3) Dyeing vessel, (4) High-temperature oil bath K (1 − R)2 = S 2R (1) where: R is reflectance percentage at maximum absorption wavelength, K is the absorption coefficient, and S is the scattering coefficient of the dyed samples. As illustrated in The colouration and fading kinetics, determined from the K/S-time datasets, were fitted to an Figure 1, the dyeing vessel (3) was pre-cooled in a separate freezer (2) and afterwards filled with extension of the first-order kinetic model, Equation (2). This model is generally used to describe the required amount of CO2. Then, the vessels were mounted on the shaft rotating inside the pre- the behaviour of photochromic dyes incorporated into the textile matrix during colouration and heated oil bath (4) and the temperature rose at a rate of 3 °C per minute to the working decolouration [42]. temperature. For all samples, dyeing was carried out using the same dyeing condition of 120 °C and 25 MPa for one hour. After dyeing, the vessels were removed from the oil bath and the CO2 was vented slowly and the samples were taken out for further analysis without reduction cleaning procedure. 7 ∆ 𝐾𝐾 K K K =. − 1 ∗ e(456) + 𝑆𝑆 S/ S0 S0 (2) 8 where: K/S0 and K/S∞ are the initial and final (saturated) colour intensity values respectively on the photochromic properties of samples before and after washing was performed to evaluate attained upon exposure or relaxation of UV light, k is the rate constant and t is the time of their performance against washing. The decrease in colour yield (K/S) after washing can be used exposure. The subscripts define the time relationships: (0) = time in the beginning and (∞) = time to express the washing fastness of the photochromic effect. The fastness is evaluated according to at infinity. From this equation, the halftime of colour change (t1/2) was calculated using Equation a relative loss in percentage using Equation (4), which measures the magnitude of the residual (3) [43]. photochromic performance after washing. t9/; = ln2 [𝑠𝑠] k (3) Fastness (%) = Thus, the photochromic performance of scCO2 dyed photochromic polyester fabrics were K/S(2) × 100 K/S(1) (4) where: K/S(1)and K/S(2) are the colour yield of the photochromic fabric before and after the evaluated in terms of the colour yield ΔK/S upon activation with UV-light, rate constants of standard washing respectively, upon UV exposure. colour increase (kcol) to achieve maximum colouration (K/S∞), and rate constant of colour decrease (kdecol) to revert to the ring-closed colourless state (K/S0), and half-lives (t1/2) of 2.6 Statistical analysis colouration and decolouration. Two replicate samples were produced, and three measurements were taken for each sample where each measurement contains three cycles of continuous colouration and two cycles of 2.4 Background colour measurement decolouration. The obtained data were fitted to the first-order kinetic model, Equation (2) using The conventional colour measurement system was conducted to evaluate the background colour Origin Lab software. The factor effects such as the type of the photochromic dye, washing of photochromic fabric samples in order to estimate the permanent shade developed in their inactive state due to the high-temperature dyeing process. The background colour is expressed as the colour difference ΔE (CMC) between the non-dyed polyester fabric and the photochromic condition, and the number of UV exposure and fading cycles on the colour yield and rate of colouration and decolouration were analysed using Minitab 17 software according to the criteria p≤0.05. dyed samples without UV irradiation. Datacolor Check Pro spectrophotometer (USA) was used under the condition of D65 illuminant and 10° standard observer. Three measurements were read at different sites of the same sample and average values are reported. All measurements were performed at room temperature (approx. 22 °C) without UV irradiation. 3. Results and discussion The photochromic behaviour of the scCO2 dyed polyester samples was evaluated by continuous measurement of the photo-colouration during UV irradiation and the decolouration after UV light 2.5 Durability towards washing is removed. For each sample, three cycles of colouration and two cycles of decolouration were measured continuously and the experimental data were fitted using the first-order kinetic model, To evaluate the durability property, samples were washed once according to ISO 6330:2012, Equation (2). All photochromic samples developed in scCO2 changed their colour when exposed domestic washing and drying procedure (at 40 ±3 °C, using 20 ±1 g reference detergent for 30 min) using a domestic laundry machine. Sufficient ballasts of 100 % polyester fabric were added in order to achieve the specified weight in the reference washing machine. A comparative study 9 to UV light and reverted to their original colour when UV light was removed. The samples produced using the two dye types generally showed comparable performance in terms of colour 10 yield but distinct kinetics and durability towards washing were obtained. The repeated UV exposure and fading cycles had no significant effect on the colour yield and kinetics of both dyes. This indicates good fatigue resistance although more cycles are required for a better understanding of the fatigue behaviour. 3.1 Data fitting The experimental data fitting procedure is presented here to show how the data was analysed. The experimental data sets contain reflectance values (converted into K/S values) collected at intervals of 2 s. For SO-SG dyed samples, the photochromic textile is activated upon 300 s (150 data points) and deactivated upon 400 s (200 data points). For NP-RR dyed samples 500 s Figure 2. Fitting of the experimental data of photochromic dyed samples measured upon UV exposure and removal. (a) SO-SG colouration, (b) SO-SG decolouration, (c) NP-RR colouration (b) NP-RR decolouration. Measured K/S data (■) and fitted curve using the first-order kinetic activation and 1000 s deactivation intervals were used, which account for 250 and 500 data points, respectively. The experimental data sets contain three replicates for each sample type and model (—). three cycles of colouration and two cycles of decolouration were collected for each replicate. Therefore, each sample has nine colouration and six decolouration data sets and these data sets The obtained R2 values suggest that the fit was satisfactory for analysis of the kinetics of the were fitted to the first-order kinetic model (Equation (3)) for comprehensive analysis. The photochromic dyes applied to polyester fabric. The model fits well for all samples and conditions experimental data of the colouration and fading of the second cycle measurement of the second except for NP-RR samples (unwashed samples only) seemingly underestimates the level of the replicate samples are shown as a representative sample as seen in Figure 2. photo colouration compared with the average values of the fitted data (Figure 2) based on qualitative observation of the graphs. However, the differences between the colour strength values of both dyes were insignificant suggesting similar colourability properties. 3.2 Colour yield (ΔK/S) The collected reflectance values during continuous UV exposure and relaxation were converted into K/S values using the Kubelka-Munk function. Generally, the K/S value corresponds to the intensity of the developed colour; the higher the value the deeper the developed colour. From the results, samples dyed with NP-RR exhibited a slightly lower colour yield with ΔK/S of 0.088 compared with SO-SG dyed samples with ΔK/S of 0.096, which is displayed in the fitted experimental data of the average colour yield of photochromic dyed samples (see Table 2). Thus, the obtained intensities of the developed colour by both dyes were similar. According to previous research, differences in the degree of colouration between dye classes were observed. However, there was no obvious correlation between the degree of photo colouration and the chemical class 11 12 of the dyes. The difference is mostly related to the particular chemical nature of individual dye showed a significantly higher rate of colour development and faded more rapidly than NP-RR structures [19]. Accordingly, it can be concluded that these specific dye structures used in this dyed samples. study have a similar degree of colouration when applied from the scCO2 solvent. In comparison to results of inkjet printing with UV-curable photochromic inks with the same types and amounts of dye deposited on polyester, colour yields of NP-RR dyes are similar both before and after washing incorporated in an ink resin with high crosslinking density. Ink-jetted and UV-cured prints showed an initial ΔK/S of 0.103, which is reduced to ΔK/S of 0.062 after one wash. SOSG prints with the same crosslinking density, display similar initial colour strength after printing with ΔK/S of 0.107. However, after one wash (ΔK/S = 0.086), not as much colour is lost during washing compared with scCO2-dyed samples [16]. Thus, the results of the colour yields in both methods showed that the dyes have similar colourability property but different washing performances. Table 2. Average colour yield (K/S) of photochromic dyed polyester samples upon UV exposure Sample Unwashed Washed SO-SG 0.096±0.016 0.026±0.013 NP-RR 0.088±0.004 0.068±0.007 3.3 The rates of colouration and decolouration Before the actual measurement, representative samples were exposed to UV light to estimate the time required to reach their saturation colouration and fading back to their uncoloured states. From the results, samples dyed with NP-RR took a relatively long time to reach their maximum colouration (500 s) and to revert to their uncoloured state (1000 s) while samples dyed with SOSG needed only 300 s and 400 s for complete colouration and decolouration, respectively. Thus, Figure 3. Average fitted plots for colouration (I) and decolouration ( II) curves of photochromic all further investigations and measurements were carried out based on these time intervals. dyed samples (a) SO-SG unwashed (b) SO-SG washed, (c) NP-RR unwashed, (d) NP-RR 3.3.1 Kinetic rate constants washed. The values are normalized to have the same initial and final values for the colouration Figure 3 shows the fitted average experimental data of photochromic dyed samples measured and decolouration curves, respectively. upon UV exposure and relaxation. As seen in the fitted curves, samples dyed with SO-SG 13 14 According to the rate constant values in Table 3, SO-SG samples have a kcol of 0.141 s-1 and NPRR samples have a kcol of 0.020 s-1, in which SO-SG had more than seven-fold faster colour development compared with NP-RR. This difference in rate kinetics between the two dyes could Table 3. Rate constants of scCO2 dyed photochromic samples before and after washing arise from the intrinsic difference between the two dye classes. Spirooxazines are inherently Rate constants (k in s-1) faster-switching dyes and more temperature-dependent dyes, whereas naphthopyrans are more kcol kdecol durable, but slow-switching [8, 44]. A faster colour development by spirooxazine dyes printed on Sample Before wash After wash Before wash After wash the polyester fabric was also reported compared with naphthopyrans [6]. Naphthopyran based SO-SG 0.141 ±0.038 0.022 ±0.003 0.072 ±0.004 0.016 ±0.001 NP-RR 0.020 ±0.003 0.011 ±0.001 0.007 ±0.001 0.005 ±0.000 dyes applied to polyester using the inkjet printing method also showed lower fading rates compared with spirooxazine based dyes [12]. Thus, the results obtained in the current study are consistent with previously reported ones. In addition to the inherent characteristics of the dyes, For both dyes and for all washing conditions, the fading rate is slower than the rate of colour the polymer matrix and the relative position of the dye molecules in the polymer matrix could affect the reaction kinetics. This observation was made, when inkjet printing SO-SG and NP-RR dyes in UV-curable ink onto polyester fabric [16]. Here, the inherently faster switching SO-SG build-up, which is a common behaviour of photochromic dyes. Unwashed samples of SO-SG have a two-fold slower and NP-RR an approx. three-fold slower decolouration reaction than colouration reaction. The fading curves of samples dyed with SO-SG and NP-RR are shown in dye showed a slower rate constant than NP-RR dyes. However, SO-SG was more affected by changes in the matrix rigidity of the dye-carrying ink, with a higher relative increase of reaction kinetics compared with NP-RR dyes. A less rigid matrix resulted in a five-fold increase of kcol of SO-SG prints compared to a two-fold increase of NP-RR prints. Furthermore, the weak fastness properties displayed by SO-SG dyed samples (Section 3.5) suggests limited dye penetration. This promotes fast response upon isomerization, as the dye concentration used was small and the dye Figure 3. As can be seen from the curves, SO-SG dyed samples demonstrated fast fading rate compared with NP-RR dyed samples. SO-SG and NP-RR samples have an average kdecol of 0.072 s-1 and 0.007 s-1, respectively (see Table 3). In both cases, the dyes start fading immediately upon the removal of the UV-light source. After washing, SO-SG dyed samples showed a dramatic decrease in fading rate while the washing effect was less significant for NP-RR dyed samples. molecules located near the surface provide free space, which facilitates faster ring opening and closing. In contrast, samples dyed with NP-RR displayed better wash fastness property, indicative 3.3.2 Half-lives of deeper dye penetration or the presence of more number of dye molecules locked within the The half-life (t1/2) of colouration/fading is a common measure to express the colour switching polymer matrix. Owing to the semi-crystalline and hydrophobic nature of polyester, these deeply penetrated NP-RR dye molecules are more constrained with limited space requiring a relatively long time for the photo transformation to occur. Matrix rigidity influences the behaviour of photochromic dyes significantly, which has been discussed in prior studies [16, 31, 45-48]. Thus, the assumed location of the dye in the fibre structure has an effect on dye kinetics and does not speeds of photochromic materials. Unlike the overall rate behaviour described above, the half-life values give information about the kinetic behaviour of dyes during their initial colouration and decolouration phases. Generally, the initial colouration and decolouration rates are faster compared with the rates near to saturation and towards the end of decolouration. This phenomenon is related to the half-life of the photochromic reaction which has important practical interfere with the inherent properties of the different photochromic dyes. As mentioned earlier, implications. The half-life of colouration (t1/2col) is the time required to reach half of the total spirooxazines are generally faster-switching dyes, but naphthopyrans are more durable. photochromic response upon exposure to UV irradiation and similarly, the half-life of fading (t1/2decol) is the time taken to revert half of the developed colour (K/S∞) [22]. This means that smaller numerical values of t1/2 indicate faster colouration and decolouration speeds and vice 15 16 versa. As can be seen from the results in Table 4, SO-SG dyed samples showed relatively higher Table 5. Half-lives of fading of photochromic dyes applied to the polyester fabric by using colouration and fading rates, expressed by lower half-lives of colouration and fading, compared different application methods with NP-RR dyed samples regardless of the washing condition. For both dyes, the half-lives of Application method Dye class t1/2decol (min) Reference colouration and decolouration increased after washing, which means slower colouration and Exhaust dyeing Spirooxazine 3.1* [19] Naphthopyran 18.1* Spirooxazine 3.1* Naphthopyran 11.5* Spirooxazine 1.0, 0.9* Naphthopyran 5.1, 0.8* Spirooxazine 0.15 Naphthopyran 1.8 fading. These results are expected and consistent with the kinetic results obtained in the previous section. The same trend has also been observed in previous studies of the same classes of photochromic dyes applied to polyester fabric using exhaust dyeing and screen printing method [6, 19]. In fact, a higher rate of colouration and fading by spirooxazine-based dyes compared with naphthopyrans was consistently reported in agreement with this study. Solvent dyeing Digital inkjet printing scCO2 dyeing Table 4. Half-lives (t1/2) of colouration and decolouration of dyes applied to the polyester fabric using scCO2 dyeing. [24] [12], [16] Current work *Minimum half-life values were taken to show the fastest achievable fading kinetics. t1/2 (s) Colouration Fading 3.4 Background colour Sample Unwashed Washed Unwashed Washed SO-SG 5.2 ±1.3 31.5 ±4.3 9.6 ±0.5 43.2 ±3.2 Background colour measurement was performed to evaluate the permanent shade of the NP-RR 35.6 ±4.9 65.3 ±4.2 100.2 ±6.2 155.3 ±8.6 photochromic textiles in their inactive state influenced by the dyeing process. During the dyeing process, high temperatures may thermally influence isomerization of photochromic dyes and hence the background colour. The background colour has shown to affect the degree of One notable observation was that the half-live of fading of the same class of dyes applied by photocolouration [12]. The colour strength expressed by K/S (Figure 4) and ΔE (CMC) values scCO2 dyeing are significantly lower than those obtained when applied by aqueous or solventbased dyeing methods [19, 24] and comparable with digital inkjet printing as presented in Table 5 [12, 16, 31]. The shortest time (t1/2decol = 0.15 min) was achieved for scCO2 dyed samples with spirooxazine (SO-SG) followed by inkjet printing of spirooxazine in a UV-curable ink and solvent-based ink. This is an additional advantage by scCO2 dyeing since a minimum half-life of fading is commercially useful. Faster reaction rates upon activation and deactivation can be preferable for improved product reliability depending on the application. The advantage of scCO2 dyeing towards conventional dyeing techniques includes the simplicity of the process and the (Table 6) were used to evaluate the background colour of dyed samples. The ΔE refers to the colour difference values between the dyed and non-dyed polyester fabrics without UV irradiation. From the background K/S values, it was evident that samples dyed with NP-RR have relatively higher K/S value (deeper colour) compared with samples dyed with SO-SG. A similar trend was also observed from the ΔE values as presented in Table 6. Higher ΔE values displayed by NPRR dyed samples with a ΔE value of 2.81 imply higher background colour compared with those dyed with SO-SG with ΔE = 1.47 before washing. absence of auxiliary chemicals. As only CO2 is used as dyeing medium and photochromic dyes are involved in the process, kinetic switching is not disturbed by the solvent or other material and hence is faster than with exhaust and solvent dyeing. 17 18 thermally to their coloured open (merocyanine) form and subsequently locked in this form within the structure of the textile fibre, resulting in a permanent colour [6]. Thus, the difference in the level of background colour between the two dyes could be due to a difference in their thermal sensitivity as well as the penetration level of the dyes in their ring-opened form. Generally, spirooxazines are more sensitive to temperature than naphthopyrans [1]. On the other hand, NPRR dye molecules seem to be deeper penetrated into the fibre bulk compared to SO-SG dyes as established from the wash fastness result. Deeper penetration into the bulk compared to if the dyes are predominantly situated on the fibre surface, results in more restricted space for the dyes to move and thus locks them in their coloured form. Table 6. Average CMC ∆E values of the background colour of photochromic dyed polyester fabrics and their washed counterparts. Treatment Samples Unwashed Washed SO-SG 1.47 ±0.12 1.35 ±0.13 NP-RR 2.81 ±0.15 2.69 ±0.08 3.5 Durability towards washing Washing was used as a measure of the durability of the photochromic behaviour and to gain knowledge about the dye penetration level and the affinity of the dye towards the polyester fabric. To assess the durability of photochromic dyed samples, a comparative study on the Figure 4. Background colour strength of (I) SO-SG and (II) NP-RR dyed samples before (■) and after (●) washing measured without UV-irradiation. photochromic colour build-up and fading properties of the samples before and after a standard washing cycle was used. According to the results discussed above, all samples generally displayed inferior photochromic performance after washing irrespective of the type of Generally, the ring-closed form of photochromic dyes is usually uncoloured. However, because of the textile application process, a characteristic background colour is often observed. The photochromic dye used. The mean ∆K/S of the two dyes before and after washing, expressed as fastness percentage, was significantly different. Compared with each other, samples dyed with NP-RR exhibited better fastness percentage (77 %) compared with SO-SG dyed samples with a presence of thermochromism and thermal instability of the dyes under application conditions were mentioned as the two main factors for the development of background colour. Among these factors, the degree of thermochromism is believed to be the dominant one [19, 24]. At a higher significantly lower fastness percentage of 27 %. This result is in line with the general behaviour of the two dye classes applied on textiles. The relatively better wash fastness behaviour by NP- treatment temperature, the ring-closed form of some photochromic dye molecules is converted 19 20 RR dyed samples suggests enhanced penetration or better affinity towards polyester compared to naphthopyrans and faster kinetics of spirooxazines. According to previous reports and which is in SO-SG. line with the current results, spirooxazines showed higher thermal sensitivity, which causes faster thermally driven decolouration than naphthopyrans [6, 49]. The wash fastness properties of the background colours of both dyed samples were also evaluated. Background colours of the photochromic samples with a generally deeper shade of 4. Conclusions NP-RR samples (see Figure 5b) decrease slightly after washing. According to ∆E values, the background colour of SO-SG samples reduces to 92 % from 1.47 to 1.35 compared to 96 % from This paper presents a novelty of water-free dyeing of UV sensing smart textiles using 2.81 to 2.69 for NP-RR samples (see Table 6). The higher decrease in the background colour of photochromic dyes with superior colour performance compared to conventional dyeing spirooxazine dyed polyester is likely to result from the removal of more dyestuff due to that SO- techniques. Two photochromic dyes belonging to the spirooxazine and naphthopyran dye classes SG dyes are located more on the fibre surface. were successfully applied to the polyester fabric using scCO2 dyeing. The uniformly coloured photochromic polyester fabrics prepared using scCO2 dyeing technique exhibit significant (a) (b) reversible colour changing properties when exposed to UV light. The two photochromic dyes on polyester fabrics showed comparable performance in terms of colour yield and distinct characteristics in terms of reaction kinetics, background colour, and wash fastness properties. Sea green from the spirooxazine family had faster reaction kinetics, lower background colour, and reduced wash fastness properties compared with the ruby red from the naphthopyran dye class. Notably, the photochromic fabric produced by scCO2 dyeing exhibited faster dye kinetics compared to photochromic textiles produced with conventional exhaust and solvent dyeing techniques. Improving the durability properties could be of interest in future research for effective utilization of this technique. Overall, the experimental findings suggest that scCO2 Figure 5. (a) Activation of the photochromic textiles using a UVA light torch and (b) Locally dyeing technique is a promising alternative to conventional processes for uniform colouration, activated SO-SG (left) and NP-RR (right) samples after washing. not only because of its economic importance but also due to its environmental advantages such as avoiding the use of water, organic solvents, auxiliary chemicals, and requiring small amounts of Furthermore, the washing has also a significant effect on the rate of colouration and dye and short dyeing time. The obtained photochromic textiles can be used for applications such decolouration of samples developed with both dye types. As presented in Table 3, for SO-SG as UV sensing smart textile. dyed samples, the washing lowers the colouration rate by 84% and decolouration rate by 77%. Similarly, for NP-RR dyed samples, kcol decreased by 46 %, and kdecol by 35 %. Nevertheless, Acknowledgement: This work was supported by the European Union under the framework of samples dyed with SO-SG still displayed a significantly higher rate of colouration (kcol = 0.022 s- the Erasmus Mundus program; Sustainable Management and Design for Textile (SMDTex) grant ) and fading (kdecol = 0.016 s-1) after washing compared with NP-RR dyed samples, which has a number 2016-1353/001-001-EMJD. All the Universities collaborating on the program are two-fold lower rate of colouration and three times lower decolouration speed. This result gratefully acknowledged. The authors are grateful for the support from Borås stad, TEKO (The 1 correlates with the inherent property of the specific dye classes with a more durable character of 21 22 References Swedish Textile and Clothing Industries Association) and Sparbanksstiftelsen Sjuhärad for enabling this research. [1] Nigel Corns S, Partington SM, Towns AD. Industrial organic photochromic dyes. Color Technol. 2009;125(5):249-61. 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