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UV-Cured Polysiloxane Epoxy Coatings
Containing Titanium Dioxide as
Photosensitive Semiconductor
Marco Sangermano,* Paola Palmero, Laura Montanaro
UV-cured polysiloxane epoxy coatings containing titanium dioxide were prepared by means of
a cationic photopolymerization process. A good distribution of the inorganic filler was
achieved within the polymeric network with an average size dimension of around
500 nm. UV-vis analysis performed on organic
dye (methylene blue) stained coatings showed a
high efficiency of the titania photocatalytic
activity: a complete degradation of the dye on
the coating surface is reached after 60 min of UV
irradiation without affecting the matrix photodegradation.
Introduction
There is a growing interest in the application of
nanotechnology for cleaning and detoxification of surfaces. It is commonly known that photosensitive semiconductors are able to generate active oxygen upon UV
exposure. The generated oxygen oxidizes and decomposes
organic substances. Therefore coating materials including
photosensitive semiconductors can decompose organicbased stains that adhere on its surface resulting in a selfcleaning coating.
Titanium dioxide, which is a metal oxide semiconductor,
is the most preferred material thanks to its high
photocatalytic activity, chemical/photocorrosion stability
and non-toxicity; although it does have a large band gap
(3.2 eV) and only absorbs in the UV region.[1]
M. Sangermano, P. Palmero, L. Montanaro
Politecnico di Torino, Dipartimento di Scienza dei Materiali e
Ingegneria Chimica, C.so Duca degli Abruzzi 24, 10129 Torino, Italy
E-mail: [email protected]
M. Sangermano, P. Palmero, L. Montanaro
INSTM, Research Unit PoliTO – LINCE Laboratory, Politecnico di
Torino, C.so Duca degli Abruzzi 24, 10129 Torino, Italy
Macromol. Mater. Eng. 2009, 294, 323–329
ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
The efficiency of TiO2 as a catalyst of photooxidation
processes has been fully evaluated.[2–6]
Irradiation of TiO2 with a energy source higher than its
band gap produces electrons and holes in the conduction
band and valence band, respectively. These photogenerated holes and electrons can combine with the
surface adsorbed species (e.g., water and oxygen) to form
highly reactive radical species such as hydroxyl radicals
and superoxide anion. These reactive species have strong
oxidizing power and can oxidize most organic compounds
and some inorganic compounds (such as the NOx
derivatives), yielding carbon dioxide and dilute mineral
acids as the final products.[7]
Very often TiO2 photocatalyst is used in the form of a
thin film, mostly coated on inorganic substrates such as
ceramics and glass. In recent years, with the progress of
coating techniques, application of TiO2 photocatalyst to
organic substrates has also become possible.[8,9]
One of the key subjects that has to be solved in the case
of photoactive titania dispersion into an organic coating is
DOI: 10.1002/mame.200800374
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M. Sangermano, P. Palmero, L. Montanaro
how to protect the substrate from degradation due to the
strong oxidizing power of the TiO2 photocatalyst. Different
approaches have already been proposed in the scientific
literature and patents.
Nano-TiO2 particles were coated with silica and then
dispersed in an acrylated matrix, achieving polymeric
coatings characterized by a good photocatalytic effect
towards organic dye decoloration.[10] As an alternative,
appropriate intermediate layers between TiO2 and the
substrate have been often used: multi-intermediate layers
have been employed in practice,[11] which makes the
coating processes complicated and expensive.
Utilization of an inorganic-organic hybrid intermediate
layer with a gradient change of the two components
between the substrate and TiO2 (inorganic rich at the TiO2
side and organic rich near the substrate) was also
proposed.[12] However, achievement of such a gradient
hybrid intermediate layer greatly depends on the synthetic conditions, which have to be controlled carefully.
From these examples it is evident that a more simple,
convenient and more economic alternative route should be
proposed, and this is part of the aim of this paper.
In this paper, we report the photocatalytic study of a
polysiloxane epoxy-based UV-cured coating containing
nano-TiO2 in the anatase form. The selection of a
polysiloxane resin is based on the fact that the main
Si O Si chain is resistant to attack from TiO2 photocatalysis, as was already previously reported in the
literature.[13]
The UV curing technique was chosen because of its
peculiarity and also because of its increasing importance
in coatings applications; by UV irradiation it is possible to
induce the polymer formation with a fast transformation
of the liquid monomer into a solid film with tailored
physico-chemical and mechanical properties. UV curing
can be considered an environmental friendly technique,
due to the solvent free process, and it is usually carried out
at room temperature, therefore guarantees the saving of
energy.[14] Furthermore, the cationic photopolymerization
process of epoxy systems present some advantages
compared to the radical one:[15] lack of inhibition by
oxygen, low shrinkage, good adhesion and mechanical
properties of cured films.
Therefore, in this paper a new strategy for achieving the
synthesis and design of new and highly efficient polymeric
coating materials containing nanto-TiO2 for photocatalysis applications is proposed.
epoxy resin. The iodonium salt photoinitiator 4(1-methylethyl)phenyl(4-methylphenyl)iodoniumtetrakis(pentafluorophenyl)
borate (Rhodorsil 2074, PI) was obtained from Rhodia (France) and
was used at 2 wt.-% with respect to the epoxy resin in each
formulation. Titania nanoparticles were supplied by Degussa
(TiO2, P25, Degussa, Germany; average primary particle size
21 nm, specific surface area 50 m2 g 1). Methylene blue (Aldrich)
was used as an organic dye for photodegradation study. The
chemical structures of the resin and photoinitiator are shown in
Scheme 1.
Sample Preparation
Ethanolic solutions of methylene blue were prepared containing
TiO2 at different contents. Photocatalytic experiments were
performed in an open beaker, immersed into an ice bath in order
to maintain the temperature. The suspensions in the reactor were
left for 10 min in the dark in order to achieve the maximum
absorption of the dye on the particle surface. Afterwards,
irradiation was carried out with a medium pressure Hg arc lamp
(Hamamatsu) equipped with an optical guide. The average light
intensity on the reaction vessel at a distance of 15 cm from the
lamp was found to be around 50 mW cm 2. In all the studies,
solutions containing an appropriate amount of photocatalyst
were magnetically stirred, before and during illumination.
Samples were collected each minute, filtered and analyzed by
means of UV-vis spectrophotometry. Changes in the concentration
of methylene blue were observed from its characteristic absorption at 650 cm 1.
For the photocatalytic study on UV-cured coating, ethanolic
titania solution was added to the epoxy resin in order to prepare
hybrid materials with titania contents in the range between 1 and
2 g l 1 with respect to the epoxy resin. The alcoholic titania
solution were stable and well dispersed, with particles average
size around 1 mm (determined by laser granulometry analysis).
The photoinitiator was added at 2 wt.-% in each formulation,
ultrasonicated for 15 min, coated on the glass substrate and UV
irradiated by means of a Fusion lamp, with a light intensity on the
surface of the sample of about 500 mW cm 2. White-colored UVcured coatings of about 25 mm were achieved.
The obtained coatings were stained with an alcoholic solution
of methylene blue on the sample surface. Photocatalytic experi-
Experimental Part
Materials
Poly{dimethylsiloxane-co-[2-(3,4-epoxycyclohexyl)ethyl]methylsiloxane} (EPOX), purchased from Aldrich, was selected as the
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ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Scheme 1. Chemical structures of the siloxane epoxy-based resin
(top) and the iodonium salt photoinitiator (bottom).
DOI: 10.1002/mame.200800374
UV-Cured Polysiloxane Epoxy Coatings Containing Titanium Dioxide . . .
ments were performed by irradiation with a medium-pressure Hg
arc lamp (Hamamatsu) equipped with an optical guide. The
average light intensity on the reaction vessel at a distance of 15 cm
from the lamp was found to be of around 50 mW cm 2. The cured
samples were analyzed by means of UV-vis spectrophotometry
following the methylene blue discoloration.
Sample Characterization
As-received titania powder was characterized by means of laser
granulometry (Fritsch model Analysette 22 Compact) in order to
evaluate the agglomerate size distribution. Then, different
alcoholic (absolute ethanol) powder suspensions, with solid loads
ranging from 20 to 40 wt.-%, were prepared and maintained under
magnetic stirring up to 48–96 h, depending on the powder
content. The de-agglomeration degree of the dispersions was
monitored by laser granulometry as a function of the stirring time
to achieve stable and well dispersed suspensions.
UV-vis spectra were collected either on alcoholic solutions or on
UV-cured coatings by means of a Perkin Elmer spectrophotometer.
The kinetics of the photopolymerization were determined by
Real-Time Fourier-transform infrared (FT-IR) spectroscopy,
employing a Thermo-Nicolet 5700 instrument. The formulations
were coated onto a silicon wafer. The sample was exposed
simultaneously to the UV beam, which induces the polymerization, and to the IR beam, which analyzes in situ the extent of the
reaction. Epoxy conversion was followed by monitoring the
decrease in the absorbance of the epoxy ring centred at 790 cm 1.
A medium-pressure mercury lamp (Hamamatsu) equipped with
an optical guide was used to induce the photopolymerization
(light intensity on the surface of the sample of about
30 mW cm 2).
The gel content was determined on the cured films by
measuring the weight loss after 24 h extraction with chloroform
at room temperature, according to the standard test method ASTM
D2765-84.
Results and Discussion
In this paper, the photocatalytic effect of titanium dioxide
was evaluated dispersing the catalyst into an epoxy
polymer network. In Figure 1 (solid line), the agglomerate
size distribution of the as-received titania is presented,
showing a certain agglomeration of the commercial nanopowder. SEM micrographs for the commercial powder are
reported in Figure 2 showing a very fine primary particle
size but high agglomeration.
In order to decrease the particle agglomeration dimension, alcoholic suspensions of titanium dioxide with
different solid contents were prepared and dispersed
under magnetic stirring. The agglomerate size distribution
of the powder alcoholic dispersion is also reported in
Figure 1 (dashed line). To better comparing the agglomerate size distributions of the as-received and dispersed
powder, the diameters corresponding to 10 (d10), 50 (d50)
Macromol. Mater. Eng. 2009, 294, 323–329
ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Figure 1. Distribution as a function of the agglomerate size for asreceived (solid line) and in alcoholic dispersion (dashed line)
titania powder.
Figure 2. SEM micrograph of commercial TiO2 nanopowder.
Table 1. Agglomerate size corresponding to 10 (d10), 50 (d50) and
90% (d90) of the cumulative distribution of as-received and
dispersed titania powder.
Sample
d10
d50
d90
mm
mm
mm
as-received TiO2
1.5
3.0
8.2
dispersed TiO2
0.4
1.2
3.9
and 90% (d90) from the cumulative distribution are
collected in Table 1. The photocatalysis of methylene blue
in solution was firstly evaluated. Afterwards the TiO2
alcoholic dispersion was added to a UV curable epoxy resin
and the cured network stained with methylene blue. The
titania photocatalytic effect was also followed for the
epoxy coatings.
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M. Sangermano, P. Palmero, L. Montanaro
Methylene Blue Photodegradation in Alcoholic
Titania Solution
In order to understand the titania concentration effect on
the rate of photocatalysis, UV-vis spectra were collected for
alcoholic solutions of methylene blue at 5 ppm with
different concentrations of catalyst.
In Figure 3, the absorption spectra of the 5 ppm alcoholic
solution of the methylene blue, containing 1 g L 1 of TiO2
is reported for different irradiation times. It is evident that
there is a decrease in intensity of the absorption spectrum
by increasing the irradiation time. The rapid decrease in
intensity of the band centered around 650 nm is attributed
to the photodegradation of methylene blue.
In Figure 4, the percentage of degradation as a function
of irradiation time are reported for the different titania
contents investigated. An increase in the initial photodegradation rate by increasing the amount of TiO2 from 1
to 1.5 g L 1 has been observed. By further increasing the
catalyst content up to 2 g L 1, the photodegradation rate
remains almost constant.
The data are in accordance with previously reported
investigations on the titania photodegradation of methylene blue in solution. This behavior can be explained
taking into account that by increasing the catalyst content
the dye molecules adsorbed are increased and thus the rate
gets enhanced. Above a certain TiO2 content, the dye
molecules available are not sufficient for adsorption by
the increased number of catalyst molecules. Therefore, the
additional catalyst powder is not involved in the photocatalytic activity and the rate does not increase further.
These preliminary investigations allow the definition of
the upper limit of catalyst to add to the epoxy resin for the
preparation of photo-oxidative coatings.
UV Curing and Characterization of Cured Films
Figure 3. Change in the UV-vis absorption spectra of the 5 ppm
alcoholic solution of the MB, containing 1 g L 1 of TiO2 irradiated
with UV light.
Figure 4. Methylene blue photodegradation rate as a function of
irradiation time for alcoholic solutions containing different titania content, in the range between 1 to 2 g L 1.
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ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
After adding the alcoholic titania dispersion into the epoxy
resin, in order to achieve a TiO2 content between 1 and
2 g L 1, the formulations were cured by means of UV light
in the presence of a iodonium salt as a cationic
photoinitiator. The upper limit of titania content was
established on the basis of the previous photodegradation
investigations performed in solution.
The main concern with the use of the UV curing
technique was the well known UV absorption property of
anatase. It was therefore necessary to understand the
possibility to cure formulations containing titania content
up to 2 g L 1 by UV light.
The conversion curves as a function of irradiation time
for the pristine epoxy resin and in the presence of
increasing amounts of TiO2 are reported in Figure 5.
It is clear that by increasing the amount of TiO2 catalyst
in the photocurable formulations, a decrease of initial
photopolymerization rate, which is evident from a
decrease of the slope of the curve, and on epoxy group
final conversion is induced. When the TiO2 content reaches
2 g L 1 in the formulation, the epoxy group conversion
decrease from about 95 to 75% after 120 s of UV irradiation,
together with a slight decrease in photocuring rate.
The relatively lower rate of polymerization and epoxy
group conversion may be due to the UV light shielding
effect of TiO2 nanoparticles. This competitive effect of UV
absorption with the photoinitiator will generate a lower
amount of reactive species with a decrease of the epoxy
group conversion. In order to avoid this competitive effect,
a sensitized system could be employed with a shift of the
absorption by the photoinitiator to longer wavelengths
where the nanoparticles are transparent. This approach
was not followed in this specific investigation but it can be
taken into consideration.
DOI: 10.1002/mame.200800374
UV-Cured Polysiloxane Epoxy Coatings Containing Titanium Dioxide . . .
The good dispersion and the low degree of agglomeration of the inorganic nanoparticles into the cured coatings
is a key point for its catalytic activity.
Methylene Blue Photocatalytic Degradation on Epoxy
UV-Cured Coatings
Figure 5. Conversion curves as a function of irradiation time
obtained by RT-FTIR, following the decrease of the epoxy peak
cantered at 790 cm 1, for the pure epoxy resin (curve a) and for its
formulation containing 1 wt.-% (curve b) and 2 wt.-% (curve c) of
TiO2.
In any case, even by using a UV light source, quite high
epoxy conversions are achieved for longer UV irradiation
times and furthermore high gel content values (always
above 98%) were obtained for all UV-cured coatings,
indicating the formation of a highly crosslinked polymer
network.
Morphological analysis of the achieved UV-cured coatings containing titania was performed by SEM. In Figure 6,
the SEM micrograph for the epoxy coating containing
1 wt.-% of TiO2 is reported; a homogeneous distribution of
the inorganic particles is evident with an average size of
about 500 nm. This result shows a further decrease of the
particle agglomerations, probably reached because of
ultrasonication during photocurable formulation preparation.
Figure 6. SEM micrograph of UV-cured epoxy coating containing
1 wt.-% of TiO2.
Macromol. Mater. Eng. 2009, 294, 323–329
ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
The UV-cured coatings were stained with an alcoholic
solution of methylene blue at 5 ppm concentration. The
photocatalytic activity of titania dispersed within the
polymeric coating was evaluated following the methylene
blue degradation during UV irradiation. The dye decomposition was evaluated by measuring the UV-vis absorbance of the stained coatings at regular intervals of
irradiation time.
In Figure 7, the change in absorption spectra during UV
irradiation are reported for the cured epoxy coating
containing 1 g L 1 of TiO2. As previously observed for
the photocatalytic investigation in solution, a decrease in
intensity by increasing irradiation time is well evident.
After 90 min of irradiation, the peak centered at 650 nm
due to the methylene blue chromophore, completely
disappears, indicating the complete decomposition of
the organic target molecule.
In Figure 8, the percentage of methylene blue degradation as a function of irradiation time are reported for the
epoxy coatings containing increasing titania contents up
to 2 g L 1. The results are in agreement with the data
collected for the photodegradation investigation performed in solution: an increase in the initial photodegradation rate is observable by increasing the amount of
TiO2 from 1 to 1.5 g L 1 to level off when 2 g L 1 of TiO2 is
added. This data indicates, therefore, the limit of titania
content above which the rate of photodegradation does
not increase further.
It is also evident that longer irradiation times are needed
in order to achieve a complete degradation process: while
in solution, in the presence of 1.5 g L 1 of TiO2, a complete
Figure 7. Change in absorption spectrum during UV irradiation for
the cured epoxy coating containing 1 g L 1 of TiO2.
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M. Sangermano, P. Palmero, L. Montanaro
stability of the cured films containing TiO2 under normal
sun-weathering conditions.
Conclusion
Figure 8. Methylene blue photodegradation rate as a function of
irradiation time for UV-cured coatings containing different titania content, in the range between 1 to 2 g L 1.
degradation of methylene blue is achieved after 10 min of
irradiation, in the cured epoxy coating the complete
degradation of methylene blue on the surface is reached
only after 60 min of irradiation, always in the presence of
1.5 g L 1 of TiO2. This time difference is due to the fact that
the photodegradation is a surface process and when
titania are dispersed within the polymeric matrix a lower
surface availability is present, with a lower organic
molecule-inorganic catalyst surface-surface contact.
It is anyway quite interesting to realize that after 1 h of
UV irradiation, the coating is clean with a complete
organic molecule degradation.
To evaluate the photodegradation effect of titania
towards the cured epoxy substrate, the change of the IR
spectra of the pristine epoxy network was compared with
the change of the IR spectra of the coatings containing TiO2
under UV irradiation.
In the polysiloxane network, the methyl group Si–CH3 is
relatively easily oxidized to the silanol group Si–OH. An
increase in hydroxyl groups can therefore be evidenced
during the photooxidation process, together with a slight
reduction of alkyl groups centered at around 2 800 cm 1.
In our investigations, it was found that the photodegradation rate of the pristine epoxy coating was negligible
for an irradiation time of 60 min, and comparable behavior
was found for the photodegradation rate of the coating
containing titania. We can therefore assume that, as
expected, the Si O Si chain is resistant to attack from
TiO2 photocatalysis, during this short irradiation time.
Therefore, by choosing a polysiloxane matrix it is possible
to take advantage of the photocatalytic effect of titania
towards organic pollutants on the polymeric coatings
without affecting the photostability of the matrix.
Longer irradiation times are in progress (in accelerator
weathering chamber) in order to evaluate the shelf-life
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ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Polysiloxane coatings containing titania nanopowder as a
photocatalyst were prepared by means of UV induced
polymerization. The filler content was selected to be in the
range between 1 to 2 wt.-%, on the basis of the methylene
blue photodegradation study performed in alcoholic
solution. A good distribution of the inorganic filler was
achieved within the polymer network with an average size
dimension of around 500 nm. UV-vis analysis performed
on organic dye stained coatings showed a high efficiency
of the titania photocatalytic activity: complete degradation on the coating surface is reached after 60 min of UV
irradiation. FT-IR analysis showed that the polymeric
network keeps its stability during the same UV irradiation
time. It was therefore possible to take advantage of the
photocatalytic effect of titania towards organic pollutants
on the polymeric coatings without affecting the matrix
photo-degradation thanks to the high stability of the
silicone matrix.
Acknowledgements: This research is partially supported in the
frame of the HITEX project.
Received: December 23, 2008; Revised: February 27, 2009;
Accepted: March 2, 2009; DOI: 10.1002/mame.200800374
Keywords: coatings; curing of polymers; degradation; epoxy;
photochemistry
[1] S. K. Lee, S. McIntrye, A. Mills, J. Photochem. Photobiol. A:
Chem. 2004, 162, 203.
[2] C. Han Kwon, H. Shin, J. Hun Kim, W. Suk Choi, K. Hyun Yoon,
Mater. Chem. Phys. 2004, 86, 78.
[3] S. Senthilkumaar, K. Porlodi, R. Gomathi, A. Geetha Maheswari, N. Manonmani, Dyes and Pigments 2006, 69, 22.
[4] F. Sayilkan, M. Asilturk, S. Erdemoglu, M. Akarsu, H. Sayilkan,
M. Erdemoglu, E. Arpac, Mater. Lett. 2006, 60, 230.
[5] P. S. Awati, S. V. Awate, P. P. Shah, V. Ramaswamy, Catalysis
Comm. 2003, 4, 393.
[6] A. Syoufian, O. H. Satriya, K. Nakashima, Catalysis Comm.
2007, 8, 755.
[7] N. Serpone, E. Pelizzetti, ‘‘Photocatalysis Fundamentals and
Applications’’, Wiley Interscience, Amsterdam 1989.
[8] S. P. Yew, H. J. Tang, K. Sudesh, Polym. Degrad. Stabil. 2006, 91,
1800.
[9] B. Su, X. Liu, X. Peng, T. Xia, Z. Su, Mater. Sci. Eng. 2003, A349,
59.
[10] S. T. Hwang, Y. B. Hahn, K. S. Nahm, Y. S. Lee, Colloid Surf. 2005,
259, 63.
[11] WO97/00134 (1997), PCT Int. Appl., invs: T. Watanabe, et al.
[12] WO00/23523 (2000), PCT Int. Appl., invs.: T. Watanabe, et al.
DOI: 10.1002/mame.200800374
UV-Cured Polysiloxane Epoxy Coatings Containing Titanium Dioxide . . .
[13] K. Iketani, R. D. Sun, M. Toki, K. Hirota, O. Yamaguchi, J. Phys.
Chem. Solids 2003, 64, 507.
[14] R. S. Davidson, ‘‘Exploring the science, technology and applications of U.V and E.B. curing’’, SITA Technology Ltd., London
1998.
Macromol. Mater. Eng. 2009, 294, 323–329
ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
[15] M. Sangermano, R. Bongiovanni, G. Malucelli, A. Priola,
‘‘New developments in cationic photopolymerization: process and properties’’, in: Horizons in Polymer Research, R. K.
Bregg, (Ed., Nova Science Publishers, New York 2006,
p. 61.
www.mme-journal.de
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