Inorganic−Organic Nanocomposite Based Hard Coatings on Plastics

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Inorganic-Organic Nanocomposite Based Hard Coatings on Plastics Using In Situ Generated Nano-SiO2 Bonded with tSisOsSisPEO Hybrid Network Samar Kumar Medda and Goutam De* Sol-Gel DiVision, Central Glass and Ceramic Research Institute, 196, Raja S. C. Mullick Road, JadaVpur, Kolkata 700 032, India (Council of Scientific and Industrial Research)

Inorganic-organic hybrid nanocomposite sols were prepared using tetraethyl orthosilicate (TEOS), 3-(glycidoxypropyl)trimethoxysilane (GLYMO), n-butanol, water, methanol, and catalytic amounts of HCl and Al(acac)3. Hydrolysis-condensation reactions of TEOS at pH ≈ 1.3 generates silica nanoparticles in the sol that remain bonded with the -Si-O-Si- network and protected by the organic functionality of GLYMO. The pH of the final sol was adjusted to close to the isoelectric point of silica (pH ∼ 2) to increase the shelf life of the sol. The resulting sol (obtained from the best optimized composition TEOS:GLYMO ) 2.33:1) when deposited on CR-39 or related plastics yielded optically transparent and spot-free hard coatings after thermal curing at 95 °C. About 1.5-2 µm thick coatings serve all international specifications required for hard coatings. Thermal curing in the presence of Al(acac)3 ensured polymerization of GLYMO originated epoxy groups to polyethylene oxide (PEO). BET surface area measurement confirms that the cured coatings are nonporous (surface area 0.6-0.8 m2 g-1) in nature. The density of the coating was measured by the X-ray reflectivity technique (XRR) and found to be 1.70 g cm-3. TEM shows flaky plastic-like characteristics of the coatings, and small-angle X-ray scattering (SAXS) study reveals the presence SiO2 nanoparticles of average size 5.4 nm inside the coatings. The pencil hardness value of the coatings (thickness 1.5-2 µm) was >6H. The high hardness of these nanocomposite coatings is mainly due to the in situ generated silica nanoparticles chemically bonded with the highly cross-linked silica-PEO network. 1. Introduction Present published literature shows that sol-gel hybrid materials exhibit abrasion resistance in addition to optical clarity.1-20 Thermal1-7 as well as UV curable1,2,8-11,16,19 hard coatings on transparent plastic substrates (e.g., CR-39; trademark of PPG, U.S.A., polycarbonate, polymethylmethacrylate, etc.) having refractive index values ranging from 1.48 to 1.75 are known.3,9-11,19 Most of these films were prepared by a combination of silicon alkoxide like tetraethyl orthosilicate (TEOS) and epoxy/methacryloxy functional group containing organic chain bonded with silicon alkoxides like 3-glycidoxypropyl-trimethoxysilane (GLYMO) and 3-methacryloxy-propyl-trimethoxysilane (MEMO). The hardness and abrasion resistance of organic polymer coatings is improved by mixing an inorganic oxide, ex situ, such as silica, boehmite nanoparticles, and so forth, with the composition that is used to form the coating. Polymer compositions that include colloidal silica/boehmite are disclosed in many patents.21-27 Preformed colloidal silica particles are very porous and have a density that usually is in the range of 1.0-1.5 g cm-3 depending on the process used to form the particles whereas fused silica has a density of 2.2 g cm-3. Because of the low density and porous nature of colloidal silica particles, thin film coatings formed from such compositions are not hard enough. In addition to this, most of the published/patented work remained silent about the shelf life of the sol, and a relatively high thickness (>5 µm) of coatings is necessary to achieve the required hardness. For the above reasons, it would be desirable to have a film forming composition where a dense glass-like silica component is self-generated in situ within the solution during preparation of the composition and is covalently bonded with the silica-organic polymer network on a molecular level to provide * To whom correspondence should be addressed. Tel.: +91 33 24838086. Fax: +91 33 24730957. E-mail: [email protected].

an essentially single phase state that has no interface problems. We observed generation of glass-like silica particles in solution by hydrolysis and condensation reactions of tetraethyl orthosilicate (TEOS) in presence of acid at room temperature.28,29 We also found that about 1/3 mol % of total TEOS was transformed into dense silica glass nanoparticles,29 and the rest remained as soluble silica. Such dense silica nanoparticles bonded with the silica network in combination with the organic polymer-silica network should produce very hard and dense coatings on plastics after curing.30 In the present work we describe a simple yet much improved technology for the development of a relatively stable inorganicorganic hybrid sol applicable for depositing hard and abrasion resistant coatings having relatively low thickness values (e2 µm) with improved quality on CR-39 grade plastic ophthalmic lenses, sheets, and other shapes. The synthesis combines hydrolysis of silicon alkoxides, generation of in situ glass-like dense nano-SiO2 particles at the sol stages, and polymerization of 3-glycidoxypropyl trimethoxysilane (GLYMO) originated epoxy functionalities, which enables formation of the highly interpenetrating cross-linked -Si-O-Si-PEO- network bonded with nano-SiO2 particles.30 The detailed synthetic process is discussed as a function of sol processing and thermal curing (e95 °C) steps supported by systematic analysis of FTIR, optical absorption, light scattering, BET surface area, XRR, SAXS, and TEM studies as well as adhesion and abrasion characteristics of the coatings. 2. Experimental Section All chemicals were used as received. Tetraethyl orthosilicate (TEOS) and (3-glycidoxypropyl)-trimethoxysilane (GLYMO) were supplied by Sigma-Aldrich while HAuCl4 · 3H2O, HCl (35.4%), n-butanol, and methanol were obtained from SD Fine-

10.1021/ie801632k CCC: $40.75  2009 American Chemical Society Published on Web 03/26/2009

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Chem Ltd. Aluminum acetylacetonate (Al(acac)3) was supplied by Lancaster. Double distilled water was prepared in our laboratory. 2.1. Preparation of Sols. The detailed preparation technique of the inorganic-organic hybrid nanocomposite sols using TEOS and GLYMO was described in ref 30. In this work hybrid sols were prepared with slight modifications of our method.30 TEOS and GLYMO in the molar ratio of 2.33:1 were first dissolved in n-butanol (65% by weight of total). To this, a mixture of acid and water (pH ≈ 1.2-1.3) mixed with methanol (∼0.8 mol with respect to GLYMO) was added to the above solution with stirring. A water/alkoxy group mole ratio in the range of 0.6-0.7 was maintained. The stirring was continued for 20 min, and the resultant mixture was refluxed at 79-80 °C for 90 min and allowed to cool at room temperature. Al(acac)3 (0.02 mol per mol of GLYMO) was then added at this stage, stirring until it dissolved. The remaining amount of n-butanol (35%) was then added, and the stirring was continued for another 30 min. The resulting clear sol was then subjected to pH adjustment at ∼2.0 using the HCl (35.4%)-n-butanol mixture (1:1 in weight ratio). The sol was aged at room temperature in closed conditions for 1 d prior to coating deposition on plastic surfaces. In the case of Au doping a calculated amount of HAuCl4 · 3H2O (∼0.5 wt % of the final sol) was dissolved in small amount of ethanol-water and added to the final sol with stirring. In the final coating sol about 23.0 equivalent wt % of (SiO2 + PEO) was maintained. To obtain different loadings of inorganic (SiO2)-organic (PEO) components in the coatings, several other sols with different TEOS:GLYMO molar ratios of 4:1, 3:1, 1.86:1, and 1.5:1 were also prepared. 2.2. Preparation of Coatings. Prior to coatings deposition, CR-39 ophthalmic lenses and sheets were cleaned with neutral detergent, followed by washing with tap water and rinsing with distilled water and ethanol and finally 5 min in warm isopropanol. The coatings were prepared using the dipping technique (Dip-master 200, Chemat Corporation) with a withdrawal velocity of 6 in. min-1. The as-prepared films were first dried at 60 °C for 1 h followed by cured at 95 °C for 2 h. Similar coatings were deposited on silicon wafers (both side polished, intrinsic, IR transparent), silica, and soda-lime glass substrates for the FTIR and UV-visible spectral studies and thickness measurements, respectively. The thickness of the coatings was measured by a Surfcorder SE-2300 profilometer (Kosaka Laboratory Ltd., Japan). The FTIR of the coatings deposited on both side polished intrinsic silicon wafers were recorded by FTIR spectrometry (Nicolet, model 5PC). In the case of sols, FTIR spectra were obtained by placing the liquid samples in between two KBr windows separated by an annular Teflon ring spacer of thickness 0.05 mm. The UV-visible spectra were obtained using a Cary 50 scan spectrophotometer. The refractive index of the hard coatings was measured using a Gaertner Ellipsometer (model L116B) at 632.8 nm. Multi-point BET surface area of the scratched off coatings was measured at -196 °C using an Autosorb 1 (Quantachrome) surface area analyzer. Transmission electron microscopic (TEM) measurements were carried out with a JEOL 2010 transmission electron microscope equipped with an EDS (energy dispersive X-ray scattering) facility. TEM samples were prepared by scratching a low thickness (∼100-150 nm) film and placing the scratched off films on a carbon coated copper grid. Particle size distribution of the sols was measured using a Malvern particle size analyzer (Zetasizer 1000 HS). X-ray reflectivity (XRR) and small-angle X-ray scattering

(SAXS) measurements of the hard coating were performed with a Rigaku SmartLab X-ray diffractometer operating at 45 kV and 200 mA, and the results were analyzed using Rigaku’s Nanosolver software. For the XRR/SAXS studies coating was deposited on soda-lime-silica glass substrate. The adhesion,16,30,31 abrasion,30,31 chemical durability,30-32 and thermal tests30,31 of the coated plastics were performed following international specifications. Details of these tests are given as Supporting Information. 3. Results and Discussion It is expected that with increase of inorganic (SiO2) component in the coatings the hardness would increase; however, at the same time it developed brittleness. On the contrary, increase of organic (PEO) component yielded relatively soft coatings. So the sols and coatings were optimized by changing the inorganic (TEOS) and organic-inorganic hybrid (GLYMO) precursors considering the hardness as well as flexibility. Out of the different compositions the TEOS:GLYMO ) 2.33:1 came out as the best optimized hybrid composition and yielded the best quality coatings considering the adhesion, abrasion, hardness, and cosmetic appeal as well as chemical durability and thermal tests. So the detailed characterization of sol obtained from the composition TEOS:GLYMO ) 2.33:1 and coatings prepared from this sol are presented in this work. The nominal composition of this coating after curing should be ∼77 equivalent mol % silica/23 mol % PEO or ∼62.54 equivalent wt % silica/37.46% PEO. The main point in this work is to hydrolyze the TEOSGLYMO mixture using acidulated water of pH value close to 1.3.28,29 To ensure near completion of the hydrolysis and condensation reactions the mixture was refluxed for 90 min. After these hydrolysis and condensation reactions, it is expected that both the precursors would link through uniform -Si-O-Silinkages. At the same time silica nanoparticles would also generate in the solution (see the schematic diagram presented as Figure 1).28,29 Earlier we reported the synthesis of glass-like dense spherical silica particles by hydrolyzing TEOS with acidulated water having pH values of 1.3-2.2 at room temperature.28,29 We deliberately maintained the similar hydrolysis conditions29 in this work so that glass-like silica particles would generate from TEOS in the sol. Thus, the hydrolysis-condensation reactions of alkoxy groups link the TEOS and GLYMO originated Si atoms through the -Si-OSi- network, and the generated silica nanoparticles remain bonded with this network. The solution generated silica particles would have plenty of surface hydroxyl groups, and they would easily condense with the hydroxyls groups attached with -Si-O-Si-. The hydrolysis-condensation reactions leading to the formation of such a homogeneous network at the sol stage has been illustrated schematically in Figure 1. The presence of silica nanoparticles in the refluxed and final pH adjusted sols has been checked with the dynamic light scattering method, and the results are presented in Figure 2. In both the cases nanoparticles of average size distributions 〈D〉 ≈ 7.0 nm have been observed. In our previous work28,29 we observed generation of silica nanoparticles from the TEOS, and with time these particles grew in size. However, in the present system we noticed formation of stable and much smaller nanoparticles probably because the hydrolysis of TEOS was carried out in the presence of GLYMO. It is expected that the GLYMO originated organic chain would protect the generated silica nanoparticles for further growth.

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Figure 1. Schematic representation of the formation of silica nanoparticles and Si-O-Si network at the sol stage via hydrolysis-condensation reactions.

Figure 2. Particle size distribution showing the mean diameter of the silica nanoparticles generated in the sol: (a) after reflux and (b) after addition of Al(acac)3 (0.02 mol/mol of GLYMO) to the sol and subsequent aging for 24 h.

FTIR studies (see Supporting Information; Figure S1) at different stages of the hydrolysis/condensation reactions show gradual depletion of alkoxy peaks and generation of silica related and alcohol (ethanol/methanol) peaks. After refluxing the mixture at 79 ( 1 °C for 90 min the FTIR spectrum shows almost a disappearance of alkoxide originated peaks and appearance of peaks due to Si-O-Si (1088 cm-1) and Si-OH

(950 cm-1).28,29,33-35 After 2 h of the Al(acac)3 addition the spectrum shows complete absence of alkoxide peaks and the presence of peaks at 1166 (longitudinal optic (LO) component of Si-O-Si asym. stretching), 1088 (Si-O-Si asym. stretching), 950 (Si-OH stretching), and 798 cm-1 (Si-O-Si sym. stretching)28,36 along with GLYMO originated epoxide vibrations36,37 at 910 and 853 cm-1 (which remains almost unaffected except for the slight decrease in intensity after Al(acac)3 addition) and alcohol peaks. Although Al(acac)3 has been used as epoxy polymerization initiator, the FTIR result indicates that it also helps the hydrolysis-condensation reaction as also pointed out by other researchers.38,39 The sol after addition of Al(acac)3 showed a pH of 2.5-2.6 which was then adjusted to pH∼2 using a HCl-n-butanol mixture. This sol after aging for about 24 h at room temperature can be used for the deposition of coatings on plastic surfaces. The thickness of the coatings could be adjusted from 1-5 µm by a single dip-coating technique. The suitability of the sol for such coating deposition depends on the viscosity of the sol. We maintained the sol pH close to the isoelectric point (IEP) of silica keeping in view increasing the shelf life of the sol. It is known that colloidal silica sols are found to flocculate in medium pH but are surprisingly stable at the IEP (pH ∼ 2).40-42 Very strong hydration ability of the silica makes the colloids stable at the IEP.40,41 This approach gives partial success, and the sol remains fairly suitable for coating deposition up to a reasonable period of time (about 20 days) even if stored at 27 ( 1 °C. However, the viscosity of the sol tends to increase with time and temperature of storing. We observed up to a viscosity value of 30 cP that the sol can be useful for the deposition of spotfree transparent hard coatings on plastics. So stability of sols at certain storing temperature means the time period when the viscosity of sol reaches a value > 30 cP. The viscosity of the sol at 27 ( 1 °C with respect to time has been monitored, and we observed that the viscosity value reaches from about 6 to 30 cP after about 20 days. However, if the sol is stored in a normal refrigerator (4 ( 1 °C) the stability of the sol can be increased to more than 3 months, and viscosity remains within 20 cP in this period of time. It is known that the coating thickness and viscosity of the sol are directly proportional at a constant withdrawal velocity of the substrate from the sol. So it is expected that the thickness of the coatings obtained at a fixed withdrawal velocity should increase with increasing the viscosity sol. Figure 3 shows a plot of thickness of cured coatings prepared at a withdrawal velocity of 6 in. min-1 with respect to the viscosity of the sol. As expected the thickness of coating increases with the viscosity of sol. In this case a near linear relationship of the thickness and viscosity of the sol has been observed. It may be mentioned here that a minimum thickness of 1.5 µm is necessary for the required hardness and other properties of the coatings when applied on CR-39 grade plastics. However, thickness can be increased up to 5 µm without affecting the quality, clarity, and other properties. The higher thickness (∼3-5 µm) is more suitable for polycarbonate (PC) substrates because the surface hardness of PC is much lower (pencil hardness ∼HB) than CR-39 (∼3H). Thermal curing of the coatings was monitored by FTIR and UV-visible spectroscopy, and the results are presented in Figures 4 and 5, respectively. The FTIR of the as-deposited coatings (Figure 4; curve a) on intrinsic Si wafer shows peaks at ∼1200, 1088, 946, 795, and 446 cm-1, all attributed to the silica related vibrations excepting the peak at ∼1200 which is due to Si-CH2 vibrations.34,36 Appearance of GLYMO originated epoxide ring stretchings at 910 and 853 cm-1 indicates

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Figure 3. Variation of coating thickness (µm) with respect to the viscosity of the sol. The withdrawal velocity was maintained at 6 in. min-1 in all cases. Figure 5. UV-visible spectra of the hard coating deposited on silica glass with respect to different processing conditions as indicated in the body of the figure. In the inset, the spectrum of Al(acac)3 in n-butanol is shown as reference.

Figure 4. FTIR spectra of the as-prepared and cured (95 °C/2 h) hard coatings. For such measurement the coating was deposited on a two-sided polished intrinsic Si wafer.

that the epoxide rings are still present in the as-deposited coating. The hardness of the film has been developed after thermal curing at 95 °C for about 2 h. At this stage FTIR shows (Figure 4; curve b) an absence of epoxide bands confirming the polymerization of epoxides to polyethylene oxides (PEO). The presence of the peak at ∼1200 cm-1 confirms that the Si-CH2linkage remains unaffected after curing. Thermal curing also shows a decrease of Si-OH peak intensity indicating some silanol condensations (tSisOH + HOsSit f tSisOsSit + H2O) which helps further strengthen the silica network. We have used Al(acac)3 mainly as an epoxy polymerization initiator. The UV-visible spectral evolution of the film deposited on the silica glass substrate with respect to the drying and curing

conditions is shown in Figure 5a-c. The as-prepared film shows (Figure 5; curve a) a strong band at 286 nm due to Al-acac chelates43-45 (the spectrum of Al(acac)3 in n-butanol is given in the inset as reference). It may also be mentioned here that coatings prepared from sol without Al(acac)3 show no such peak in the UV-visible spectrum (see curve d of Figure 5). The intensity of the 286 nm band is gradually decreased (Figure 5b) and almost vanished (Figure 5c) after curing of the coating at 95 °C when epoxy polymerization takes place with hardening of the coating. Although the use of Al(acac)3 is known as one of the epoxy ring-opening initiators, its exact functioning mechanism might be complicated. Different types of epoxy ringopening mechanism using Al(acac)3 are suggested by several workers.46-49 On the basis of our observations we also suggest a possible mechanism which has been illustrated schematically in Figure 6. At the first instance it is obvious that the epoxy oxygen will interact with the Al atom which is Lewis acidic in nature resulting in opening of acac chelates. This would make the neighboring carbon atom +ve in character which could then interact with the oxygen atom of the other epoxy group, and polymerization continues with the formation of poly(ethelene oxide) (PEO) chains. The gradual disappearance of the 286 nm peak indicates that Al(acac)3 is gradually decomposed during curing. The decomposed product is expected to be acetylacetone and escaped from the matrix as shown in the schematic Figure 6. Thus, the epoxy polymerization after thermal curing leads to the formation of an interpenetrating homogeneous -Si-OSi-PEO- hybrid network with covalently bonded nano-silica particles. The above structure of the hard coating can be represented schematically as Figure 7. We have characterized the hard coatings by XRR, SAXS, and TEM studies. The main aim of these studies was to evaluate the density and identify the silica nanoparticles and overall structure of the coatings. Figure 8 represents the experimental XRR profile along with the simulated curve for the density calculation. The simulated curve is in good agreement with the

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Figure 6. Schematic representation of the possible mechanism of epoxy polymerization in the presence of Al(acac)3 as a curing agent.

Figure 7. Schematic representation of the formation of an interpenetrating homogeneous -Si-O-Si-PEO- hybrid network with covalently bonded nano-silica particles after thermal curing of the coatings.

Figure 8. Experimental and simulated X-ray reflectivity profiles of the hard coating.

experiment, and such XRR analysis reveals that the coating has a density of 1.70 g cm-3. This density data is comparable with the expected approximate density value using the nominal composition of the cured coatings (62.54 wt % silica-37.46% PEO). The density calculation based on this nominal composi-

tion of the cured hard coatings is given as Supporting Information.50 The reflected SAXS analysis (Figure 9) of the hard coating clearly shows the existence of nanoparticles of average size 〈D〉 ) 5.4 nm. Figure 9a shows the reflected SAXS profile of the coating along with the simulated curve. The simulated curve using a spherical particle model is in good agreement with the measurement. Figure 9b shows the size distributions of the nanoparticles simulated from the SAXS patterns. From this analysis, it turns out that nanoparticles of average diameter of 5.4 nm with a dispersion of about 30% are embedded in the coatings. Figure 10 shows the TEM images of scraped films with different magnifications (a-c) along with the EDS spectrum (d). The selected area diffraction pattern presented in the inset of Figure 10b indicates the amorphous nature of the films, and the TEM-EDS pattern (Figure 10d) shows the presence of elements C, O, and Si. The low magnification image (Figure 10a) shows that the scraped films are flaky and folded like plastic materials. This indicates that the film has some elastic charactersitics supporting the inorganic-organic nature of the material. We could not locate any distinct silica nanoparticles in the high magnification images (Figure 10b,c) most probably due to the formation of uniform hybrid structure and very similar nature of the matrix (-Si-O-Si-PEO-) and the silica nanoparticles. It can be argued whether the SAXS result is due to the presence of nanoparticles or pores. Although TEM studies do not show any porous structure, we have investigated the surface area/ porosity of the coatings to confirm its nonporous nature. The surface area/porosity of the scratched off hard coatings (for this purpose the coatings were deposited on glass substrates and cured at 95 °C for 2 h) was evaluated using N2 adsorptiondesorption analyses at -196 °C. About 1 g of coating sample was taken for such analysis, and the sample was degassed at 70 °C under vacuum (3.8 × 10-2 T) for 12 h prior to surface area measurements. The multi-point BET surface area of the coating is found to be extremely low (typically 0.6-0.8 m2 g-1) indicating nonporous characteristics of the coatings. It may also be noted here that the coating remains unaffected even after 10 cycles of boiling in salt solution test (see Supporting Information), and we observed that even sub-nanosized dye molecules could not penetrate into the coating when the coated plastics were immersed in a dye solution. Furthermore, high hardness of the coatings is contrary to the porous characteristics of the

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Figure 10. TEM images of the hard coating flakes in different magnifications (a-c) along with the EDS spectrum (d).

Figure 9. (a) Reflected SAXS profile of the hard coating along with the simulated curve using a spherical particle model and (b) size distributions of the generated silica nanoparticles estimated from SAXS patterns.

coatings.20 All these results confirm that the coating is practically nonporous and the SAXS profile appears due to the presence of silica nanoparticles in the coatings. Figure 11 shows photograph of some CR-39 lenses with a hard coating (a) along with transmission spectra of coated and uncoated CR-39 (b) lenses. The thickness of these coatings is within the range of 1.5-2 µm. It may be noted here that relatively low thickness (1.5-2 µm) coatings are sufficient to meet all the international specifications required for hard coatings. As shown in the figure, the coatings are optically transparent and spot-free, having a refractive index value of 1.480 ( 0.005, and showed no deterioration of transparency of CR-39 after application of such coatings (see the transmission spectra shown in Figure 11b). It can also be seen from the figure that the coating can be applied on any kind of lenses, that is, mono- and bifocal (both “O” and “D” types), high power, and previously tinted lenses (see the hard coated tinted lens shown at the extreme right top of Figure 11a). Several tests of the hardcoated CR-39 lenses were undertaken to verify their applicability for commercial use. Results of such tests are given in Supporting

Figure 11. Photograph showing the plastic (CR-39) ophthalmic lenses (mono-, bifocal, and tinted) with hard coating and (b) transmission spectra of hard-coated and uncoated CR-39 lenses.

Information (see Table S1). The coated CR-39 lenses were subjected to the adhesion test following ASTM and DIN procedures (cross-cut adhesive tape test; see Supporting Information), and after the test the coated surface was examined under optical microscope using 100× magnification. It shows no peeling off of the coatings from the substrate and can be classified as ASTM 5B (highest standard). The coated CR-39 lenses could resist more than 100 abrasion cycles following the U.S. federal specifications without any noticeable damage,

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the isoelectric point of silica (pH ∼2). Thermal curing of the coatings at 95 °C for 2 h using Al(acac)3 as a curing agent ensures polymerization of GLYMO originated epoxy groups to poly(ethylene oxide) (PEO). Coatings of 1.5-2 µm thickness show high transparency, excellent abrasion, hardness (6H plus), and adhesion (ASTM Class 5B) properties as well as thermal and chemical resistance characteristics. Dynamic light scattering of sols and SAXS analysis of the coatings reveal the presence of silica nanoparticles. An extremely low surface area of the coatings confirms its nonporous characteristics. The high hardness of the nanocomposite coatings is mainly due to the in situ generated dense silica nanoparticles in the sol which remain chemically bonded with the highly interpenetrated cross-linked SiO2-PEO network of the cured coating. These hard coatings can also be made colorful by in situ generating metal nanoparticles in it. Acknowledgment Figure 12. (a) Photograph of Au-nanoparticle-doped hard-coated CR-39 lenses and (b) UV-visible spectrum of a representative lens showing AuSPR absorption.

whereas the corresponding uncoated lens suffers from damage after 20 abrasion cycles. The coated CR-39 lenses could resist up to 10 cycles of boiling in salt solution test (see Supporting Information)30,31 without any damage, indicating its compatibility with the substrate and chemical inertness as well as thermal stability. The coatings also passed the thermal tests (see Supporting Information, Table S1). Another interesting aspect of this work is that many dopants, for example, organic dye, metal salts (which will subsequently decompose to metal nanoparticles during curing at 95 °C), and so forth, can be introduced easily into the sol. As a representative example we have prepared a Au nanoparticle-doped hard coating on CR-39 ophthalmic lenses to make the coating colorful. For this purpose the coating has been prepared on CR-39 lenses after incorporating HAuCl4 · 3H2O (0.5 wt %) in the sol. The curing (at 95 °C for 2 h) of the coating causes also in situ generation of Au nanoparticles which makes the coatings a nice light pinkish color with all other specifications related to the adhesion, abrasion resistance, and so forth, as mentioned earlier. Figure 12 shows photograph of Au-nanoparticle-doped hardcoated CR-39 lenses (Figure 12a) along with a representative UV-visible spectrum (Figure 12b) taken from it. The spectrum clearly shows absorption near 520 nm due to the surface plasmon resonance (SPR) of Au nanoparticles.16 It is also possible to prepare yellow colored hard coatings using Ag nanoparticle as a coloring source. It may be mentioned here that the colors obtained by dye doping are not stable enough and easily deteriorate in sunlight. Moreover, dyes are usually poisonous. However, colors originated from the plasmon resonance of metal nanoparticles can be useful for a prolonged time. 4. Conclusions In this work we have described the synthesis of a relatively stable inorganic-organic hybrid nanocomposite sol suitable for the deposition of thermal (∼95 °C) curable hard and abrasion resistant coatings on CR-39 and related plastic substrates. The synthetic protocol enables in situ generation of dense silica nanoparticles in the hybrid sol derived from TEOS and epoxy functionalized silicon alkoxide (GLYMO). The shelf life of the sol increases as the final pH of the sol is adjusted close to

Financial support from the Department of Science and Technology (DST), Govt. of India, under National Nano Mission is gratefully acknowledged. National Research and Development Corporation (NRDC), Govt. of India, is gratefully acknowledged for awarding the authors a “NRDC Meritorious Invention Award-2007” for this work. The authors thank Director, CG&CRI, for his kind permission to publish the paper. Supporting Information Available: FTIR spectra recorded at different stages of sol preparation (Figure S1), approximate density calculation based on the nominal composition of the cured hard coatings, standard testing procedures of hard coatings on plastics, and the test results of the thermally cured nanocomposite coatings deposited on CR-39 grade plastic lens substrates having coating thickness 1.5-2 µm (Table S1) are provided. This material is available free of charge via the Internet at http://pubs.acs.org. Literature Cited (1) Sanchez, C.; Julian, B.; Belleville, P.; Popall, M. Applications of Hybrid Organic-Inorganic Nanocomposites. J. Mater. Chem. 2005, 15, 3559. (2) Schottner, G.; Rose, K.; Posset, U. Scratch and Abrasion Resistant Coatings on Plastic LensessState of the Art, Current Developments and Perspectives. J. Sol-Gel Sci. Technol. 2003, 27, 71. (3) Schmidt, H. K. Organically Modified Silicates and Ceramics as TwoPhasic Systems: Synthesis and Processing. J. Sol-Gel Sci. Technol. 1997, 8, 557. (4) Hwang, D. K.; Moon, J. H.; Shul, Y. G. Scratch Resistant and Transparent UV-Proctive Coating on Polycarnone. J. Sol-Gel Sci. Technol. 2003, 26, 773. (5) Li, C.; Jordens, K.; Wilkers, G. L. Abrasion Resistant Coatings for Plastics and Soft Metallic Substrates by Sol-Gel Reactions of a Triethoxysilylated Diethylenetriamine and Tetramethoxysilane. Wear 2000, 242, 152. (6) Chou, T. P.; Cao, G. Adhesion of Sol-Gel-Derived Organic-Inorganic Hybrid Coatings on Polyester. J. Sol-Gel Sci. Technol. 2003, 27, 31. (7) Lee, H. T.; Kang, E. S.; Bae, S. B. Catalytic Effects of Aluminum Butoxyethoxide in Sol-Gel Hybrid Hard Coatings. J. Sol-Gel Sci. Technol. 2003, 27, 23. (8) Sepeur, S.; Kunze, N.; Werner, B.; Schmidt, H. UV Curable Hard Coatings on Plastics. Thin Solid Films 1999, 351, 216. (9) De, G.; Kundu, D.; Medda, S. K. A process of making inorganicorganic hybrid sol useful as a scratch resistant coatings on polycarbonate sheets and lenses and other related plastics. Indian Patent 202349, 2003. (10) Medda, S. K.; De, G. A process of making UV curable methacrylate-silica nanocomposite based sol useful for the deposition of anti-scratch coatings on plastics. Indian Patent application no. 1416Del, 2007. (11) Medda, S. K.; Kundu, D.; De, G. Inorganic-Organic Hybrid Coatings on Polycarbonate. Spectroscopic Studies on the Simultaneous Polymerizations of Meythacrylate and Silica Networks. J. Non-Cryst. Solids. 2003, 318, 149.

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ReceiVed for reView May 8, 2008 ReVised manuscript receiVed March 10, 2009 Accepted March 10, 2009 IE801632K