Progress in Organic Coatings 64 (2009) 39–46 Contents lists available at ScienceDirect Progress in Organic Coatings journal homepage: www.elsevier.com/locate/porgcoat Effect of nano-ZnO addition on the silicone-modiﬁed alkyd-based waterborne coatings on its mechanical and heat-resistance properties Shailesh K. Dhoke a,∗ , Rohit Bhandari b , A.S. Khanna a a b Indian Institute of Technology, Bombay, Maharashtra 400076, India University of Delhi, New Delhi 110007, India a r t i c l e i n f o Article history: Received 17 December 2007 Received in revised form 4 July 2008 Accepted 11 July 2008 Keywords: Waterborne coatings Nano-ZnO Heat-resistance a b s t r a c t A silicone-modiﬁed alkyd-based waterborne coating was developed using hexamethylmethoxymelamine (HMMM) as crosslinking agent and para-toluene sulphonic acid (p-TSA) as catalyst. The crosslinking ratio for resin and HMMM was ﬁxed to 70:30, based on FTIR and DSC studies. Nano-ZnO particles were added to this system in different concentrations. The coatings with nano-ZnO particles were characterized using FTIR and DSC. The nano-composite coatings were applied on mild steel panels and were cured at 130 ◦ C for 30 min. The coatings were evaluated for their mechanical and heat-resistance properties. They were exposed to 350 ◦ C for 10 min followed by water quenching. The process was repeated for 10 cycles. Heat-resistance property of the coatings was examined by TGA. Also, surface morphological changes were assessed using SEM and optical microscopy. It was found that the heat-resistance and mechanical properties of the coatings improved signiﬁcantly as a function of nano-ZnO addition. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Many high temperature industrial components suffer from severe degradation at higher temperatures. Their protection at these temperatures can be achieved by heat-resistant organic coatings. Silicone-based and silicone-modiﬁed coatings are extensively used for this purpose due to their good high temperature stability owing to the excellent bond strength of Si–O–Si  and good corrosion resistance. These coatings are also modiﬁed with commercial pigments like carbon black , titanium dioxide  to further improve their mechanical properties and thermal stability. However, the loading level required for these pigments is quite high and also optical transparency is not retained. With the rise in demand for eco-friendly, cost-effective and transparent coatings with improved heat-resistance and mechanical properties, their development is a major challenge. Fortunately, recent development in nanotechnology has eased the problems [4–6]. Use of nanoparticles as additives in coatings has been found to improve the thermal stability of the polymer [7,8], enhance scratch and abrasion resistance of the coatings without disturbing their optical and other properties [9,10]. The present paper deals with the investi- ∗ Corresponding author. E-mail address: [email protected] (S.K. Dhoke). 0300-9440/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.porgcoat.2008.07.007 gation on the effect of nano-ZnO additions on silicone-modiﬁed alkyd-based waterborne coatings. 2. Experimental 2.1. Raw materials Nano-ZnO with an average size of less then 40 nm and speciﬁc area 29 m2 g−1 was procured from Horsehead Corporation Company (USA). The waterborne silicone-modiﬁed alkyd resin (Worlee Sol 68A); few additives and crosslinking agent hexa(methoxymethyl)melamine (HMMM) were purchased from Worlee-Chemie GmbH, Germany. Flash rust inhibitor, long-term corrosion inhibitor and blocked acid catalyst para-toluene sulphonic acid (p-TSA) were purchased from King Industry, Germany, and were used in as received condition. Dimethylethanolamine (DMEA) was used as neutralizing medium. 2.2. Preparation of waterborne coatings Low molecular weight silicone-modiﬁed alkyd resin (acid value = 35–45 mgKOH/g) was ﬁrst neutralized with dimethylethylamine (DMEA) followed by continuous addition of deionized water with time to time stirring till a clear solution was obtained. The pH of resin solution was maintained between 8.2 and 8.5. 40 S.K. Dhoke et al. / Progress in Organic Coatings 64 (2009) 39–46 Fig. 1. FTIR spectra of resin/HMMM blend in different concentrations. Calculated amount of crosslinking agent HMMM and p-TSA catalyst was added to the system. In order to avoid formulation defects due to crucial properties of water, additives such as ﬂash rust inhibitors, wetting agents, biocides, etc. were added. All the contents were mixed properly in suitable proportion to form waterborne silicone-modiﬁed alkyd-based coating. Neutralization was done with due care so that the resin solution formed did not remain water sensitive after formation of coating on the substrate. 2.3. Preparation of nano-composite coatings Fig. 2. DSC thermogram of neat silicone-modiﬁed alkyd resin/HMMM blends in different concentrations as labeled. To form a nano-composite, a system with resin HMMM concentration of 70:30 was selected on the basis of FTIR and DSC studies and different concentrations of ZnO nano-particles (average particle size ≈40 nm) were added to it (0.05, 0.1, Fig. 3. FTIR spectra of neat silicone-modiﬁed alkyd-based waterborne coating with different concentrations of nano-ZnO. S.K. Dhoke et al. / Progress in Organic Coatings 64 (2009) 39–46 41 0.2 and 0.3% by weight). The nano-particle based coatings were prepared by dip-coating on pretreated cold rolled mild steel panels (76.2 mm × 152.4 mm × 0.8 mm). The panels after coating were cured in oven at 130 ◦ C for 30 min and then cooled to room temperature. The coating thickness measure was 9–10 m. 2.4. Characterization Fig. 4. DSC thermograms of neat silicone-modiﬁed alkyd-based waterborne coating with different concentrations of nano-ZnO as labeled. The resin/HMMM blend was characterized using FTIR (Nicolet Magna 550 FT-IR spectrometer) and DSC (nitrogen atmosphere 5–10 ml/min, heating rate of 10 ◦ C, temperature range of 25–250 ◦ C, using DSC Q10V9.8 Build 296 instrumentation). The nanocomposite coatings were characterized using FTIR, DSC and TGA (nitrogen atmosphere 5–10 ml/min, heating rate of 10 ◦ C, temperature range of 25–800 ◦ C using SDT Q600V8.3 Build 101 instrumentation). Mechanical properties were studied by Scratch (BS.3900) and Taber abrasion (ASTM D-3359-02) methods. Surface studies were done using optical microscopy (model no. GX51.233U, Fig. 5. SEM surface morphology of unexposed coatings: (a) silicone-modiﬁed alkyd without nano-particles, (b) with 0.05% nano-ZnO, (c) with 0.1% nano-ZnO, (d) with 0.2% nano-ZnO and (e) with 0.3% nano-ZnO. 42 S.K. Dhoke et al. / Progress in Organic Coatings 64 (2009) 39–46 OLYSIA.M3 software, OLYMPUS) and scanning electron microscopy (model no. S3400, Hitachi). 3. Result and discussion 3.1. Characterization of resin/HMMM blend 3.1.1. Fourier transform infrared spectroscopic analysis (FTIR) Fig. 1 shows the FTIR spectra of neat silicone-modiﬁed alkyd resin and HMMM blend in different concentrations of HMMM (50:50, 60:40, 70:30 and 80:20). It can be seen that a new peak at 910 cm−1 is also observed which was not initially present in neat silicone-modiﬁed alkyd resin and observed due to N–CH2 –O–R stretching corresponding to crosslinking reaction between resin and HMMM as shown in Eq. (1). It is clear that the hydroxyl group (–OH) on neat resin interacts with melamine alkoxy group (–N–CH2 –O–CH3 ) of HMMM as studied by Blank [11,12]: Table 1 Curing behavior during heating scan for silicone-modiﬁed alkyd resin and HMMM blends Resin:HMMM Tc (◦ C) Hc (J/g) 50:50 60:40 70:30 80:20 115.39 105.27 116.03 118.29 109.00 88.33 49.25 90.17 blend and also is an indicator of the content of reacted functional groups up to a deﬁnite time of reaction, at a certain temperature . A sharp exotherm for 70:30 ratio with enthalpy value (Hc = 49.25 J/g) less as compared to other mixing ratios indicates the effective crosslinking at this ratio. Hence, based on the FTIR and DSC studies the mixing ratio for resin and HMMM was ﬁxed to 70:30 at 130 ◦ C for 30 min. (1) 3.1.2. Differential scanning calorimetric analysis (DSC) The curing characteristic of silicone-modiﬁed waterborne alkyd resin with HMMM crosslinker was determined by DSC technique and the corresponding thermogram obtained is shown in Fig. 2. Table 1 shows the curing behavior during heating scan. For the resin/HMMM blend, as the mixing ratio increases, a slight shift in curing temperature (Tc ) is observed, whereas a remarkable decrease in the enthalpy values is noticed. The degree of curing increases with increasing ratio of melamine resin in the resin 3.2. Characterization of nano-composite resin 3.2.1. Fourier transform infrared spectroscopic analysis (FT–IR) Fig. 3 shows the overlay of FTIR spectra of silicone-modiﬁed alkyd-based waterborne coating with nano-ZnO particles in different concentrations. From this ﬁgure it can be noted that there is slight shift of FTIR band frequencies after incorporation of nanoparticles. It may be due to interaction of polymer matrix and Fig. 6. X-ray mapping of unexposed coating (a) with 0.05% nano-ZnO, (b) with 0.1% nano-ZnO, (c) with 0.2% nano-ZnO and (d) with 0.3% nano-ZnO. S.K. Dhoke et al. / Progress in Organic Coatings 64 (2009) 39–46 43 Table 2 Curing behavior during heating scan for nano-composite resin Nano-composite coatings Exotherm 1 ◦ Neat 0.05% nano-ZnO 0.01% nano-ZnO 0.02% nano-ZnO 0.03% nano-ZnO Exotherm 2 T ( C) H (J/g) T (◦ C) H (J/g) 84.21 85.18 83.56 80.76 71.43 73.47 482.3 258.1 637.1 840.4 93.70 119.8 105.75 124.28 122.79 329.9 111.6 216.5 29.40 10.77 nano-ZnO. The spectrum of silicone-modiﬁed alkyd with incorporated ZnO nano-particles shows the characteristic peaks (C O, C C, C–O) of base polymer matrix, indicating that the basic structure of base polymer matrix is undisturbed after functionalization. The peak at 1070.9 cm−1 in the functionalized nano-particles is attributed to Si–O–Si stretching, consistent with that reported in the standard FTIR spectra. The disappearance of the peak at 825.7 cm−1 characteristic of Si–O–CH3 and formation of new peaks at 897.8 cm−1 indicate the complete interaction between the hydrolyzed ZnO nano-particles surface and silicone-modiﬁed alkyd . Fig. 7. Thermogravimetric (TG) curve of nano-ZnO incorporated silicone-modiﬁed alkyd-based waterborne coatings. 3.2.2. Differential scanning calorimetric analysis (DSC) Fig. 4 shows the DSC thermogram of neat and zinc oxide based nano-composite coatings. Table 2 shows the curing behavior during heating scan. From ﬁgure it is clear that the curing temperature (Tcuring ) decreases as compared to the curing temperature of neat coating, also the heat of curing (Hcuring ) value increases (Exotherm 1). This indicates that the interaction Fig. 8. SEM surface morphology of heat-treated coatings: (a) silicone-modiﬁed alkyd without nano-particles, (b) with 0.05% nano-ZnO, (c) with 0.1% nano-ZnO, (d) with 0.2% nano-ZnO and (e) with 0.3% nano-ZnO. 44 S.K. Dhoke et al. / Progress in Organic Coatings 64 (2009) 39–46 3.3. Characterization of nano-composite coatings Fig. 9. Taber abrasion of coating as a function of nano-ZnO concentration. between nano-ZnO and polymer improves the curing of coatings. The curing reaction is followed by liberation of heat that corresponds to the post-curing reaction (Exotherm 2). This analytical parameter in DSC is called as residual crosslinking enthalpy, which decreases as the concentration of nano-ZnO increases suggesting improved crosslinking as compared to the neat coating . 3.3.1. Scanning electron microscopy and X-ray mapping Fig. 5 shows the surface morphology of the unexposed coatings. It clearly shows that the coating is free of pin-hole and is uniform without any surface heterogeneity. The surface morphologies of the coating (Fig. 5a) and its nano-composite with different concentrations of nano-ZnO particles (Fig. 5b–e) also look uniform and pore free. A few white spots uniformly distributed throughout the surface, probably indicate the location of encrusted nano-particles. Any volume defects or cavities present in silicone-modiﬁed alkyd coating disappear by the incorporation of ZnO nano-particles. This clearly indicates the establishment of new interactions between the system components, fact that can be supported by FTIR analysis. X-ray mapping studies (Fig. 6) also support the SEM study indicating uniform distribution of nanoZnO with varying loading level of nano-particles. The coating was then subjected to 350 ◦ C for 10 min followed by water quenching. The process was repeated for 10 cycles. After each cycle the Fig. 10. SEM surface morphology of abraded coating: (a) silicone-modiﬁed alkyd without nano-particles, (b) with 0.05% nano-ZnO, (c) with 0.1% nano-ZnO, (d) with 0.2% nano-ZnO and (e) with 0.3% nano-ZnO. S.K. Dhoke et al. / Progress in Organic Coatings 64 (2009) 39–46 Fig. 11. Scratch test of the coating as a function of nano-ZnO concentration. coating was visually observed. The observation showed that there was no char formation or delamination of the coatings from the substrate, only a color change was observed from clear coating to black. 45 3.3.2. Thermogravimetric analysis (TGA) Thermogravimetric analysis of the coating also conﬁrms the thermal stability of the coatings. Fig. 7 shows the TG curve obtained for coating system with and without nano-ZnO particles. It can be seen that all samples shows weight loss up to 100 ◦ C, this can be attributed to the loss of water from the system leading to 60–62% weight loss. Above 100 ◦ C there is no appreciable weight loss up to 350 ◦ C indicating stability of coatings. The cause for this stability is attributed to the interaction between the resin and large surface area of nano-ZnO particles, forming a stable nano-composite . This observation was further supported by SEM studies of coated panels heat treated at 350 ◦ C. Fig. 8 shows the surface morphology of heat-treated coatings without and with nano-ZnO particles. The coating without nano-ZnO (Fig. 8a) showed some deformation in coatings indicating poor response to the repeated heat and quenching cycle. The appearance of uniformly distributed rupture spots in the coating structure can be attributed to the initiation of oxidative Fig. 12. Optical microscopic photographs of scratched surface of the coatings (a) without nano-ZnO, (b) with 0.05% nano-ZnO, (c) with 0.1% nano-ZnO, (d) with 0.2% nano-ZnO and (e) with 0.3% nano-ZnO. 46 S.K. Dhoke et al. / Progress in Organic Coatings 64 (2009) 39–46 product formation, this is apparently due to the breakdown of the cohesive bond between the substrate and the coating followed by the oxidation of the mild steel substrate. With the incorporation of nano-ZnO particles no such deformation in coating was observed (Fig. 8b–e). This can be attributed to the combined effect of siliconemodiﬁed alkyd resin and the heat shielding effect of nano-ZnO, which because of its ceramic nature remains inert to subjected heat. 3.4. Mechanical properties 3.4.1. Abrasion resistance Taber abrasion test was carried out to determine the abrasion resistance of coating. The Taber wheel used was CS10, a resilient type wheel for mild abrading action. Depending upon the thickness of the coating, a speciﬁed number of revolutions were performed (1000 cycles) with all coated samples and weight loss is evaluated. This weight loss as a function of concentration of nano-ZnO particles is shown in Fig. 9. From the ﬁgure it is clear that with the increase in the concentration of ZnO nano-particles in coated sample, the weight loss is gradually being reduced. This can be explained on the basis of surface roughness. Silicone-modiﬁed alkyd coating without ZnO nano-particles have less dense connecting polymer network, creating a rougher surface. This rough surface is easy to abrade, but as ZnO nano-particles are introduced on the coating surface, the roughness of the surface decreases, as number of polymer connecting particles increases with increasing concentration of nano-ZnO. Fig. 10 shows the SEM photographs of abraded coatings. From ﬁgure the coating without nano-particles (Fig. 10a) show complete abraded surface morphology, where the substrate is clearly visible at the weared area. The incorporation of ZnO nano-particles into the coating changes the morphology of the coating due to enhanced interaction of ZnO nano-particles with resin structure, as a result of which coating seems to be more compact and less abraded as compared to the coating without nano-ZnO. This compactness of coating goes on increasing with an increase in concentration of nano-ZnO (Fig. 10b–e). The improvement in abrasion resistance is attributed to the combined effect of HMMM crosslinked resin and nano-zinc oxide. Also, it can be said that the catalytic action of zinc on the curing reaction helps in forming a hard and complex coating network. This indicates that the interfacial surface interaction between nano-ZnO and the base matrix is strong which is responsible for the improvement in the mechanical properties of the base matrix. 3.4.2. Scratch resistance Fig. 11 shows the variation of load (weight in grams) with different concentrations of ZnO nano-particles during scratch test. It was found that as the concentration of nano-ZnO in coating sample increases, the scratch resistance property of the sample also increases which means that the tendency of deformation of coated surface will be at higher weight than the reference polymer, hence an ascending curve is obtained for scratch study. This can be attributed to the more strong bonding network between silicone-modiﬁed alkyd coating and nano-ZnO particles which provides more resistance to scratch causing less deformation in the sample [9,16]. The shape of the scratch tracks serve as an indicator for severity of deformation as well as mechanism behind the scratching. The scratch interface is subjected to an extremely high shear strain and whenever shear strain exceeds the yield strain, the polymer starts deforming . Fig. 12 shows the optical microscopic photographs of the scratched surface. During scratch testing of coated materials, ridges are formed along the side of scratch. This is due to plastic deformation of the coating. The size of the ridges is directly proportional to the normal load used for scratching. From optical study, it is clear that as the concentration of ZnO nano-particles is increased in coating sample, scratch tracks are found to be less deformed than neat coating, indicating improvement in the scratch resistance property of the coating. 4. Conclusion The effect of nano-ZnO on a silicone-modiﬁed alkyd-based waterborne coating was studied. The coating obtained had enhanced heat stability and mechanical properties, which is quite large as compared to the conventional silicone-modiﬁed alkyd resin. A coating system with higher loading of nano-ZnO (0.3 wt.%) showed better performance. Optimization of nano-ZnO loading in polymer matrix or synergism with other nano-oxide particles can further improve these properties. 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