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non-hydrolytic wet chemical routes on PMMA substrates
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CERAMICS INTERNATIONAL
Ceramics International 40 (2014) 10615–10619 www.elsevier.com/locate/ceramint
Investigations on coatings generated from silica–zirconia hybrid sols synthesized through hydrolytic/non-hydrolytic wet chemical routes on PMMA substrates K. Mamatha, R. Subasrin Centre for Sol-Gel Coatings, International Advanced Research Centre for Powder Metallurgy and New Materials (ARCI), Balapur, Hyderabad 500005, Andhra Pradesh, India Received 18 February 2014; accepted 10 March 2014 Available online 19 March 2014
Abstract Organic–inorganic silica–zirconia hybrid sols were synthesized by a wet chemical process using an organically modified silane (methacryloxypropyltrimethoxysilane) and zirconium-n-propoxide. One sol was synthesized through a hydrolytic route using isopropanol as the solvent, whereas another was synthesized through a non-hydrolytic route using tetrahydrofuran as the solvent. These sols were independently dip coated on flat polymethylmethacrylate (PMMA) substrates. The coated substrates were subjected to ultraviolet curing followed by thermal curing at 80 1C for 6 h. The cured coatings were characterized and tested for their optical (UV–vis transmittance, haze), mechanical (abrasion resistance) and weather resistance properties according to ASTM standard testing methods. The FTIR spectra were also acquired and analyzed to study the bonding characteristics under the two synthesis conditions. The properties of the coatings obtained from the hybrid sols synthesized through hydrolytic/non-hydrolytic routes were compared. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: Abrasion resistance; Hydrolytic/non-hydrolytic route; Organic–inorganic hybrid coatings; Transmittance; Weather resistance
1. Introduction Transparent plastics like polycarbonate (PC) and polymethylmethacrylate (PMMA) are nowadays being used as optical materials as a replacement to glass mainly due to their light weight property, ease of mass production and low cost. These have potential applications in lenses for ophthalmic/ophthalmoscopic applications, as windshields in aircrafts and automobiles, etc. [1]. But due to their poor scratch resistance, the usage has been limited. In recent years, there has been considerable interest in use of organic–inorganic hybrid coatings on transparent plastics for improving their scratch resistance [1–10]. Organic– inorganic hybrid coatings blend the combined effect of polymers prepared with inorganic back bone along with organic bridges which exhibit better mechanical and optical properties than the n
Corresponding author. Tel.: þ91 40 24452465; fax: þ 91 40 24442699. E-mail address: [email protected] (R. Subasri).
http://dx.doi.org/10.1016/j.ceramint.2014.03.042 0272-8842/& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
individual moieties. Organic polymers have the advantages like ease of processing, lightweight and good impact resistance. Inorganic polymers have the properties like hardness, thermal and chemical stabilities. The main advantage of the hybrid coatings is that they can be cured using alternate curing techniques like ultraviolet (UV) curing and can be thermally cured at low temperatures especially when used on temperature sensitive substrates like plastics. These hybrids are based mainly on the use of metal alkoxides like zirconium or titanium tetraalkoxides along with organosilanes and synthesized via sol–gel processing to produce a multi-metal oxide based coating, that are expected to have superior mechanical properties like scratch and abrasion resistance. The hydrolysis rates are however very much different for the metal alkoxides and the organically modified silanes [11]. They are higher for the metal alkoxides. Due to this mismatch in the hydrolysis and condensation rates of the precursors, there is a possibility of a phase separation occurring when such multi-metal oxide systems are being
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K. Mamatha, R. Subasri / Ceramics International 40 (2014) 10615–10619
processed using the sol–gel technique, which is based on hydrolysis and polycondensation reactions. This phase separation is a concern when such systems are targeted for use as transparent, defect-free, scratch resistant and optical quality coatings. The phase separation can be controlled by using a non-hydrolytic synthesis route. Though there are investigations reported on synthesis of hybrid materials in a powder form through a non-hydrolytic route [12,13] there are no reports on evaluation of multi-metal oxide hybrid coatings derived from non-hydrolytic route. Hence, the objective of this investigation was to synthesize hybrid sols comprising silica–zirconia through hydrolytic and non-hydrolytic routes, generate coatings from these sols; evaluate and compare the optical, mechanical and weather resistance properties of the coatings. 2. Experimental 2.1. Materials 3-Methacryloxypropyl trimethoxy silane (97% Alfa Aesar, England), zirconium n-propoxide 70% in Propanol purchased from Gelest Inc. USA, and Methacrylic acid (MAA) 99% stabilized (ABCR GmbH, Germany) were used as-received without further purification. 0.1 N hydrochloric acid, tetrahydrofuran (THF, 99.8% unstabilized) and dry propan-2-ol procured from Qualigens Fine Chemicals, Mumbai, India; Alfa Aesar, England and S.D. Fine Chemicals Ltd., Mumbai, India respectively were used for the sol synthesis. 2.2. Synthesis 2.2.1. Silica–zirconia hybrid sol using the hydrolytic route Part A: MPTMS (0.138 mol) was taken in a 150 ml reaction vessel to which 0.03 mol of hydrochloric acid was added dropwise with stirring. After 5 min of stirring, 0.138 mol of de-ionized water was added drop-wise under vigorous stirring. Since the reaction was found to be exothermic, the vessel containing the reaction mixture was kept in an ice bath during the addition of water. Part B: 0.04 mol of zirconium n-propoxide (Zr(O-npr)4) was independently complexed with 0.03 mol of MAA under vigorous stirring for 30 min. Part B was added dropwise to part A under vigorous stirring. 0.04 mol of de-ionized water was added to hydrolyze the zirconium n-propoxide. The total mixture was stirred for 3–4 h, after which it was diluted to 160% w/w with IPA (isopropyl alcohol or propan-2-ol). 2.2.2. Synthesis of silica–zirconia hybrid sol using a non-hydrolytic route 0.343 mol of THF and 0.04 mol of zirconium n-propoxide (Zr(O-npr)4 ) were taken in a reaction vessel to which 0.03 mol of MAA was added. The reaction mixture was stirred for 15–20 min and to this 0.138 mol of MPTMS was added dropwise under continuous stirring. This reaction was determined to be endothermic in nature. Hence, the Zr–MAA complex was heated to 50–60 1C and then MPTMS was added
by simultaneously maintaining the temperature to 50–60 1C. 0.343 mol THF was then added to mixture to achieve a dilution of 100% (w/w) with THF as the solvent. In some cases, the sols were sonicated for 15 min using a 750 W Vibra-Cell™ ultrasonic liquid processor (Sonics and Materials Inc., USA) operating at 20 kHz. The sol prepared using the hydrolytic route will henceforth be referred to as MZH-sol, and the sol prepared using a non-hydrolytic route will henceforth be referred to as MZNH-sol. 2.3. Substrate cleaning, coating deposition and curing The PMMA substrates (100 mm 100 mm 3 mm) were cleaned with IPA (isopropyl alcohol), followed by rinsing with de-ionized water and finally drying with moisture-free compressed air/dry nitrogen. The MZH and MZNH sols were separately dip coated on cleaned PMMA substrates by employing a withdrawal speed of 3 mm/s. Irgacure 184s (1 wt%) was added as a photoinitiator to the MZH and MZNH sols prior to coating. The coated substrates were UV cured on both sides using a three-medium-pressure-mercury lamp (120 W/cm with total wattage/lamp=12 kW) conveyorized UV curing unit. The belt speed was maintained at 2 m/min during the curing. The light dose as measured by a UV radiometer (EIT Inc., USA) was 871 mJ/cm2 in the UV-C region. The UV curing was followed by thermal curing at 80 1C for 6 h in a drying oven. 2.4. Characterization The visible light transmittance was measured using a Cary Varian 500 UV–vis–NIR spectrophotometer in the range of 400–800 nm. Haze of the coatings was measured using a haze meter (BYK-Gardner GmbH, Germany) according to ASTMD1003. Abrasion resistance of the coatings was measured using Taber Rotary Platform Abraser Model 5135/5155 using CS-10F wheels with a load of 2 250 g 1000 cycles. After every 100 cycles, the abraded surface was cleaned with compressed air to remove the debris following which, the haze was measured. After every 500 cycles, the wheels were refaced for 50 cycles. Fourier transform infrared spectroscopic (FTIR) analysis on the heat treated powders derived from the two sols was carried using a FTIR Spectrometer (Vertex 70, Bruker Optik GmbH, Germany). Powders of heat-treated sols were prepared by exposing the sol to the same heat treatment schedules as the coatings. The powder was ground well using an agate mortar and pestle. 1 mg of this powder along with 99 mg of KBr (potassium bromide) was mixed well and compacted as a transparent pellet. FT-IR spectroscopy was carried out on the pellet. The weathering resistance of the coated specimens was measured using a Q-SUN Xenon test chamber (model XE-3-HBS) supplied by Q-Lab Corporation, USA, according to ASTM G-155. The chamber had three xenon lamps to provide the irradiation. The weather test comprised a light cycle of 1 h and 42 min with an irradiance intensity of 0.35 W/m2 per lamp at 30% RH and a light spray cycle of 18 min with an irradiance intensity of 0.35 W/m2 at 30% RH, for a total duration of 120 h. The specimens were visually observed and
K. Mamatha, R. Subasri / Ceramics International 40 (2014) 10615–10619
the transmittance was measured before and after the weathering exposure test.
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In case of the MZH sol, the average % transmittance slightly increased after sonication, i.e. from 89.2% to 89.9%. The reason could be the complete network formation of MZH sol under hydrolytic conditions.
3. Results and discussion 3.1. Optical property measurements 3.1.1. Transmittance measurements The average percentage transmission of the coatings derived from MZNH and MZH sols was measured over the visible wavelength range. The effect of sonication was also studied in case of both the sols. An important point to be noted here is that in case of the MZNH sol, the average transmittance was found to increase from 89.9% to 91.4% after sonication. Based on this initial observation and subsequent temperature measurement during synthesis trials, it was concluded that the sol synthesized through the non-hydrolytic route was endothermic in nature and during sonication, due to the heat generated, the polycondensation reaction proceeded in the silica–zirconia hybrid network leading to the increase in the transmittance. Since the MZNH-sol was prepared through a non-hydrolytic route using THF as solvent, it could be possible that during synthesis, the hybrid silica–zirconia network was not formed completely, due to which, there could be a phase separation and the transmittance decreased. After the addition of zirconium n-propoxide–MAA complex to MPTMS, heat was supplied to the reaction mixture through ultrasonication and when the coatings were generated using this sol, there was an increase in the transmittance, possibly due to the complete formation of the silica–zirconia network. Fig. 1 indicates the difference between the two cases. From this, it could be supposed that the reaction during the formation of the silica– zirconia hybrid network carried out under non-hydrolytic conditions is an endothermic reaction. Hence, the subsequent sol synthesis for MZNH was carried out by heating the reaction mixture in the temperature range of 50–60 1C. The coatings generated using this sol when coated on PMMA substrates and cured consistently yielded an average visible light transmittance of 91.0%.
3.1.2. Haze measurement The percentage haze and transmittance measured for the bare and coated PMMA substrates at 550 nm are shown in Table 1. It can be seen that the haze of all coated substrates whether derived from MZH or MZNH sols is lower than that of the bare substrates, which implies that the coatings are able Table 1 The data of % haze and % transmittance of coated samples. Substrates
% T (at 550 nm)
% haze
Bare PMMA MZNH-before sonication MZNH-after sonication MZH-before sonication MZH-after sonication
91.80 92.4 92.1 91.1 91.2
1.8 0.33 0.16 0.74 0.70
Fig. 2. Comparison of change in haze after abrasion testing for coated and bare substrates; test conditions: CS-10F wheels, 2 250 g for 1000 cycles.
Fig. 1. The effect of sonication on transmittance of coatings derived from MZNH and MZH sols.
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K. Mamatha, R. Subasri / Ceramics International 40 (2014) 10615–10619
Table 2 Average visible weathering tests. Serial no
1 2 3
light
transmission
Substrate
Bare PMMA MZH-sol coated PMMA MZNH-sol coated PMMA
of
substrates
before
and
after
Average percentage visible light transmission Before test
After test
90.3 89.9 90.9
89.5 89.6 90.0
to even conceal certain scratches on the substrate surface. Between the MZH and MZNH, the MZNH exhibits lower haze and higher transmittance. Upon sonication, the haze values are reduced for both MZH and MZNH sols, implying the breaking down of soft agglomerates, if any. Fig. 3. FT-IR data of MZH and MZNH coated PMMA substrates.
3.2. Mechanical properties 3.2.1. Taber abrasion test The PMMA substrates coated with MZH and MZNH sols, followed by UV-curing and thermal curing, were subjected to taber abrasion and measurement of haze. The percentage haze change after 1000 cycles is 22.5, 9.5 and 1.2, for the bare PMMA, MZNH and MZH-sol coated PMMA respectively, as shown in Fig. 2. The above results indicate that the coatings derived from the sol synthesized under hydrolytic conditions show a complete network formation and exhibit better abrasion resistance than the coatings derived from sol synthesized using non-hydrolytic conditions. Generally, for automotive applications, a haze change o 2% under the testing conditions as mentioned here (CS-10F wheels, 2 250 g load, 1000 cycles) would suffice for application on components like rear windshields, where the abrasive action is due to the use of wiper blades. 3.3. Weathering resistance measurements The bare PMMA and coated PMMA substrates were subjected to weathering resistance tests. The transmittance of all coupons was measured before and after exposure to weather tests and the measured data are presented in Table 2. The MZH sol coated substrate exhibits higher weather resistance since the transmission before and after the weathering tests did not change substantially, when compared to the MZNH sol coated substrate. 3.4. Fourier transform infrared spectroscopy The FT-IR transmittance data of the heat-treated sols of MZH and MZNH are depicted in Fig. 3 over the wavenumber range of 400–4000 cm 1. The peaks at 3500 cm 1 and 2900 cm 1 represent the Si–OH stretch and sp2 C–H stretch respectively. The Si–OH peak in MZNH-sol is broader (3300– 3500 cm 1) than MZH-sol, which is characteristic of an incomplete hydrolysis. The CQO stretch and CQC stretch are observed at 1720 and 1640 cm 1 respectively. The Si–O–Si
stretch and Si–O–Zr stretch are observed at 1100 cm 1 and 965 cm 1 respectively. And Zr–O–C stretch and Zr–O–Zr stretch are observed at 1556 cm 1 and 489 cm 1 respectively. The Zr–O–Zr peak is sharp and intense in MZNH-sol than the MZH-sol. 4. Conclusion The sol prepared using the hydrolytic route was seen to possess better mechanical properties than the sol synthesized using a non-hydrolytic route. The reason for good mechanical properties of hydrolytic sol is that the hydrolysis and condensation processes are completed, leading to a complete network formation. However, the sol synthesized using a nonhydrolytic route was seen to exhibit better optical properties than the sol synthesized through a hydrolytic route. The mechanical properties of coatings derived from sol synthesized through a non-hydrolytic route are comparatively poor than the MZH sol possibly due to incomplete hydrolysis and polycondensation process. The present investigation also confirmed the endothermic nature of sol synthesis using the non-hydrolytic route. Depending on the applications envisaged, one can adopt a hydrolytic or non-hydrolytic route for the sol synthesis. Acknowledgment The authors acknowledge the support and encouragement provided by Dr. G. Sundararajan and Dr. G. Padmanabham, ARCI, Hyderabad throughout the course of this investigation. References [1] G. Schottner, Hybrid sol–gel derived polymers: applications of multifunctional materials, Chem. Mater. 13 (2001) 3422–3435. [2] D.K. Hwang, J.H. Moon, Y.G. Shul, K.T. Jung, D.H. Kim, D.W. Lee, Scratch resistant and transparent UV-protective coating on polycarbonate, J. Sol–Gel Sci. Technol. 26 (2003) 783–787.
K. Mamatha, R. Subasri / Ceramics International 40 (2014) 10615–10619 [3] P. Fabbri, M. Messori, M. Toselli, P. Veronesi, J. Rocha, F. Pilati, Enhancing the scratch resistance of polycarbonate with poly(ethylene oxide)–silica hybrid coatings, Adv. Polym. Technol. 27 (2008) 117–126. [4] R. Chantarachindawong, W. Luangtip, P. Chindaudom, T. Ostochan, T. Srikhirin, Development of the scratch resistance on acrylic sheet with basic colloidal silica (SiO2)–methyltrimethoxysilane (MTMS) nanocomposite films by sol–gel technique, Can. J. Chem. Eng. 90 (2012) 888–896. [5] L.Y.L. Wu, G.H. Tan, X.T. Zeng, T.H. Li, Z. Chen, Synthesis and characterization of transparent hydrophobic sol–gel hard coatings, J. Sol–Gel Sci. Technol. 38 (2006) 85–89. [6] D. Blanc, A. Last, J. Franc, S. Pavan, J.L. Loubet, Thin Solid Films 515 (2006) 942–946. [7] J.D. Mackenzie, E.P. Bescher, Physical properties of sol–gel coatings, J. Sol–Gel Sci. Technol. 19 (2000) 23–29. [8] W. Tanglumlert, P. Prasassarakich, P. Supaphol, S. Wongkasemjit, Hardcoating materials for poly(methyl methacrylate) from glycidoxypropyltrimethoxysilane-modified silatrane via a sol–gel process, Surf. Coat. Technol. 200 (2006) 2784–2790.
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[9] S.K. Medda, D. Kundu, G. De, Inorganic–organic hybrid coatings on polycarbonate. Spectroscopic studies on the simultaneous polymerizations of metacrylate and silica networks, J. Non-Cryst. Solids 318 (2003) 149–156. [10] P. Fabbri, B. Singh, Y. Leterrier, J.-A.E. Månson, M. Messori, F. Pilati, Cohesive and adhesive properties of polycaprolactone/silica hybrid coatings on poly(methyl methacrylate) substrates, Surf. Coat. Technol. 200 (2006) 6706–6712. [11] C.J. Brinker, G.W. Scherer, in: Sol–Gel Science—The Physics and Chemistry of Sol–Gel Processing, Academic Press, New York, 1990. [12] R. Di Maggio, S. Dire, E. Callone, F. Girardi, G. Kickelbick, Hybrid organic–inorganic materials using zirconium based NBBs and vinyl trimethoxysilane: effect of pre-hydrolysis of silane, Polymer 51 (2010) 832–841. [13] S. Ivanovici, G. Kickelbick, Synthesis of hybrid polysiloxane-MO2 (M¼ Si, Ti, Zr) nanoparticles through a sol–gel route, J. Sol–Gel Sci. Technol. 46 (2008) 273–280.
CERAMICS INTERNATIONAL
Ceramics International 40 (2014) 10615–10619 www.elsevier.com/locate/ceramint
Investigations on coatings generated from silica–zirconia hybrid sols synthesized through hydrolytic/non-hydrolytic wet chemical routes on PMMA substrates K. Mamatha, R. Subasrin Centre for Sol-Gel Coatings, International Advanced Research Centre for Powder Metallurgy and New Materials (ARCI), Balapur, Hyderabad 500005, Andhra Pradesh, India Received 18 February 2014; accepted 10 March 2014 Available online 19 March 2014
Abstract Organic–inorganic silica–zirconia hybrid sols were synthesized by a wet chemical process using an organically modified silane (methacryloxypropyltrimethoxysilane) and zirconium-n-propoxide. One sol was synthesized through a hydrolytic route using isopropanol as the solvent, whereas another was synthesized through a non-hydrolytic route using tetrahydrofuran as the solvent. These sols were independently dip coated on flat polymethylmethacrylate (PMMA) substrates. The coated substrates were subjected to ultraviolet curing followed by thermal curing at 80 1C for 6 h. The cured coatings were characterized and tested for their optical (UV–vis transmittance, haze), mechanical (abrasion resistance) and weather resistance properties according to ASTM standard testing methods. The FTIR spectra were also acquired and analyzed to study the bonding characteristics under the two synthesis conditions. The properties of the coatings obtained from the hybrid sols synthesized through hydrolytic/non-hydrolytic routes were compared. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: Abrasion resistance; Hydrolytic/non-hydrolytic route; Organic–inorganic hybrid coatings; Transmittance; Weather resistance
1. Introduction Transparent plastics like polycarbonate (PC) and polymethylmethacrylate (PMMA) are nowadays being used as optical materials as a replacement to glass mainly due to their light weight property, ease of mass production and low cost. These have potential applications in lenses for ophthalmic/ophthalmoscopic applications, as windshields in aircrafts and automobiles, etc. [1]. But due to their poor scratch resistance, the usage has been limited. In recent years, there has been considerable interest in use of organic–inorganic hybrid coatings on transparent plastics for improving their scratch resistance [1–10]. Organic– inorganic hybrid coatings blend the combined effect of polymers prepared with inorganic back bone along with organic bridges which exhibit better mechanical and optical properties than the n
Corresponding author. Tel.: þ91 40 24452465; fax: þ 91 40 24442699. E-mail address: [email protected] (R. Subasri).
http://dx.doi.org/10.1016/j.ceramint.2014.03.042 0272-8842/& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
individual moieties. Organic polymers have the advantages like ease of processing, lightweight and good impact resistance. Inorganic polymers have the properties like hardness, thermal and chemical stabilities. The main advantage of the hybrid coatings is that they can be cured using alternate curing techniques like ultraviolet (UV) curing and can be thermally cured at low temperatures especially when used on temperature sensitive substrates like plastics. These hybrids are based mainly on the use of metal alkoxides like zirconium or titanium tetraalkoxides along with organosilanes and synthesized via sol–gel processing to produce a multi-metal oxide based coating, that are expected to have superior mechanical properties like scratch and abrasion resistance. The hydrolysis rates are however very much different for the metal alkoxides and the organically modified silanes [11]. They are higher for the metal alkoxides. Due to this mismatch in the hydrolysis and condensation rates of the precursors, there is a possibility of a phase separation occurring when such multi-metal oxide systems are being
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K. Mamatha, R. Subasri / Ceramics International 40 (2014) 10615–10619
processed using the sol–gel technique, which is based on hydrolysis and polycondensation reactions. This phase separation is a concern when such systems are targeted for use as transparent, defect-free, scratch resistant and optical quality coatings. The phase separation can be controlled by using a non-hydrolytic synthesis route. Though there are investigations reported on synthesis of hybrid materials in a powder form through a non-hydrolytic route [12,13] there are no reports on evaluation of multi-metal oxide hybrid coatings derived from non-hydrolytic route. Hence, the objective of this investigation was to synthesize hybrid sols comprising silica–zirconia through hydrolytic and non-hydrolytic routes, generate coatings from these sols; evaluate and compare the optical, mechanical and weather resistance properties of the coatings. 2. Experimental 2.1. Materials 3-Methacryloxypropyl trimethoxy silane (97% Alfa Aesar, England), zirconium n-propoxide 70% in Propanol purchased from Gelest Inc. USA, and Methacrylic acid (MAA) 99% stabilized (ABCR GmbH, Germany) were used as-received without further purification. 0.1 N hydrochloric acid, tetrahydrofuran (THF, 99.8% unstabilized) and dry propan-2-ol procured from Qualigens Fine Chemicals, Mumbai, India; Alfa Aesar, England and S.D. Fine Chemicals Ltd., Mumbai, India respectively were used for the sol synthesis. 2.2. Synthesis 2.2.1. Silica–zirconia hybrid sol using the hydrolytic route Part A: MPTMS (0.138 mol) was taken in a 150 ml reaction vessel to which 0.03 mol of hydrochloric acid was added dropwise with stirring. After 5 min of stirring, 0.138 mol of de-ionized water was added drop-wise under vigorous stirring. Since the reaction was found to be exothermic, the vessel containing the reaction mixture was kept in an ice bath during the addition of water. Part B: 0.04 mol of zirconium n-propoxide (Zr(O-npr)4) was independently complexed with 0.03 mol of MAA under vigorous stirring for 30 min. Part B was added dropwise to part A under vigorous stirring. 0.04 mol of de-ionized water was added to hydrolyze the zirconium n-propoxide. The total mixture was stirred for 3–4 h, after which it was diluted to 160% w/w with IPA (isopropyl alcohol or propan-2-ol). 2.2.2. Synthesis of silica–zirconia hybrid sol using a non-hydrolytic route 0.343 mol of THF and 0.04 mol of zirconium n-propoxide (Zr(O-npr)4 ) were taken in a reaction vessel to which 0.03 mol of MAA was added. The reaction mixture was stirred for 15–20 min and to this 0.138 mol of MPTMS was added dropwise under continuous stirring. This reaction was determined to be endothermic in nature. Hence, the Zr–MAA complex was heated to 50–60 1C and then MPTMS was added
by simultaneously maintaining the temperature to 50–60 1C. 0.343 mol THF was then added to mixture to achieve a dilution of 100% (w/w) with THF as the solvent. In some cases, the sols were sonicated for 15 min using a 750 W Vibra-Cell™ ultrasonic liquid processor (Sonics and Materials Inc., USA) operating at 20 kHz. The sol prepared using the hydrolytic route will henceforth be referred to as MZH-sol, and the sol prepared using a non-hydrolytic route will henceforth be referred to as MZNH-sol. 2.3. Substrate cleaning, coating deposition and curing The PMMA substrates (100 mm 100 mm 3 mm) were cleaned with IPA (isopropyl alcohol), followed by rinsing with de-ionized water and finally drying with moisture-free compressed air/dry nitrogen. The MZH and MZNH sols were separately dip coated on cleaned PMMA substrates by employing a withdrawal speed of 3 mm/s. Irgacure 184s (1 wt%) was added as a photoinitiator to the MZH and MZNH sols prior to coating. The coated substrates were UV cured on both sides using a three-medium-pressure-mercury lamp (120 W/cm with total wattage/lamp=12 kW) conveyorized UV curing unit. The belt speed was maintained at 2 m/min during the curing. The light dose as measured by a UV radiometer (EIT Inc., USA) was 871 mJ/cm2 in the UV-C region. The UV curing was followed by thermal curing at 80 1C for 6 h in a drying oven. 2.4. Characterization The visible light transmittance was measured using a Cary Varian 500 UV–vis–NIR spectrophotometer in the range of 400–800 nm. Haze of the coatings was measured using a haze meter (BYK-Gardner GmbH, Germany) according to ASTMD1003. Abrasion resistance of the coatings was measured using Taber Rotary Platform Abraser Model 5135/5155 using CS-10F wheels with a load of 2 250 g 1000 cycles. After every 100 cycles, the abraded surface was cleaned with compressed air to remove the debris following which, the haze was measured. After every 500 cycles, the wheels were refaced for 50 cycles. Fourier transform infrared spectroscopic (FTIR) analysis on the heat treated powders derived from the two sols was carried using a FTIR Spectrometer (Vertex 70, Bruker Optik GmbH, Germany). Powders of heat-treated sols were prepared by exposing the sol to the same heat treatment schedules as the coatings. The powder was ground well using an agate mortar and pestle. 1 mg of this powder along with 99 mg of KBr (potassium bromide) was mixed well and compacted as a transparent pellet. FT-IR spectroscopy was carried out on the pellet. The weathering resistance of the coated specimens was measured using a Q-SUN Xenon test chamber (model XE-3-HBS) supplied by Q-Lab Corporation, USA, according to ASTM G-155. The chamber had three xenon lamps to provide the irradiation. The weather test comprised a light cycle of 1 h and 42 min with an irradiance intensity of 0.35 W/m2 per lamp at 30% RH and a light spray cycle of 18 min with an irradiance intensity of 0.35 W/m2 at 30% RH, for a total duration of 120 h. The specimens were visually observed and
K. Mamatha, R. Subasri / Ceramics International 40 (2014) 10615–10619
the transmittance was measured before and after the weathering exposure test.
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In case of the MZH sol, the average % transmittance slightly increased after sonication, i.e. from 89.2% to 89.9%. The reason could be the complete network formation of MZH sol under hydrolytic conditions.
3. Results and discussion 3.1. Optical property measurements 3.1.1. Transmittance measurements The average percentage transmission of the coatings derived from MZNH and MZH sols was measured over the visible wavelength range. The effect of sonication was also studied in case of both the sols. An important point to be noted here is that in case of the MZNH sol, the average transmittance was found to increase from 89.9% to 91.4% after sonication. Based on this initial observation and subsequent temperature measurement during synthesis trials, it was concluded that the sol synthesized through the non-hydrolytic route was endothermic in nature and during sonication, due to the heat generated, the polycondensation reaction proceeded in the silica–zirconia hybrid network leading to the increase in the transmittance. Since the MZNH-sol was prepared through a non-hydrolytic route using THF as solvent, it could be possible that during synthesis, the hybrid silica–zirconia network was not formed completely, due to which, there could be a phase separation and the transmittance decreased. After the addition of zirconium n-propoxide–MAA complex to MPTMS, heat was supplied to the reaction mixture through ultrasonication and when the coatings were generated using this sol, there was an increase in the transmittance, possibly due to the complete formation of the silica–zirconia network. Fig. 1 indicates the difference between the two cases. From this, it could be supposed that the reaction during the formation of the silica– zirconia hybrid network carried out under non-hydrolytic conditions is an endothermic reaction. Hence, the subsequent sol synthesis for MZNH was carried out by heating the reaction mixture in the temperature range of 50–60 1C. The coatings generated using this sol when coated on PMMA substrates and cured consistently yielded an average visible light transmittance of 91.0%.
3.1.2. Haze measurement The percentage haze and transmittance measured for the bare and coated PMMA substrates at 550 nm are shown in Table 1. It can be seen that the haze of all coated substrates whether derived from MZH or MZNH sols is lower than that of the bare substrates, which implies that the coatings are able Table 1 The data of % haze and % transmittance of coated samples. Substrates
% T (at 550 nm)
% haze
Bare PMMA MZNH-before sonication MZNH-after sonication MZH-before sonication MZH-after sonication
91.80 92.4 92.1 91.1 91.2
1.8 0.33 0.16 0.74 0.70
Fig. 2. Comparison of change in haze after abrasion testing for coated and bare substrates; test conditions: CS-10F wheels, 2 250 g for 1000 cycles.
Fig. 1. The effect of sonication on transmittance of coatings derived from MZNH and MZH sols.
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Table 2 Average visible weathering tests. Serial no
1 2 3
light
transmission
Substrate
Bare PMMA MZH-sol coated PMMA MZNH-sol coated PMMA
of
substrates
before
and
after
Average percentage visible light transmission Before test
After test
90.3 89.9 90.9
89.5 89.6 90.0
to even conceal certain scratches on the substrate surface. Between the MZH and MZNH, the MZNH exhibits lower haze and higher transmittance. Upon sonication, the haze values are reduced for both MZH and MZNH sols, implying the breaking down of soft agglomerates, if any. Fig. 3. FT-IR data of MZH and MZNH coated PMMA substrates.
3.2. Mechanical properties 3.2.1. Taber abrasion test The PMMA substrates coated with MZH and MZNH sols, followed by UV-curing and thermal curing, were subjected to taber abrasion and measurement of haze. The percentage haze change after 1000 cycles is 22.5, 9.5 and 1.2, for the bare PMMA, MZNH and MZH-sol coated PMMA respectively, as shown in Fig. 2. The above results indicate that the coatings derived from the sol synthesized under hydrolytic conditions show a complete network formation and exhibit better abrasion resistance than the coatings derived from sol synthesized using non-hydrolytic conditions. Generally, for automotive applications, a haze change o 2% under the testing conditions as mentioned here (CS-10F wheels, 2 250 g load, 1000 cycles) would suffice for application on components like rear windshields, where the abrasive action is due to the use of wiper blades. 3.3. Weathering resistance measurements The bare PMMA and coated PMMA substrates were subjected to weathering resistance tests. The transmittance of all coupons was measured before and after exposure to weather tests and the measured data are presented in Table 2. The MZH sol coated substrate exhibits higher weather resistance since the transmission before and after the weathering tests did not change substantially, when compared to the MZNH sol coated substrate. 3.4. Fourier transform infrared spectroscopy The FT-IR transmittance data of the heat-treated sols of MZH and MZNH are depicted in Fig. 3 over the wavenumber range of 400–4000 cm 1. The peaks at 3500 cm 1 and 2900 cm 1 represent the Si–OH stretch and sp2 C–H stretch respectively. The Si–OH peak in MZNH-sol is broader (3300– 3500 cm 1) than MZH-sol, which is characteristic of an incomplete hydrolysis. The CQO stretch and CQC stretch are observed at 1720 and 1640 cm 1 respectively. The Si–O–Si
stretch and Si–O–Zr stretch are observed at 1100 cm 1 and 965 cm 1 respectively. And Zr–O–C stretch and Zr–O–Zr stretch are observed at 1556 cm 1 and 489 cm 1 respectively. The Zr–O–Zr peak is sharp and intense in MZNH-sol than the MZH-sol. 4. Conclusion The sol prepared using the hydrolytic route was seen to possess better mechanical properties than the sol synthesized using a non-hydrolytic route. The reason for good mechanical properties of hydrolytic sol is that the hydrolysis and condensation processes are completed, leading to a complete network formation. However, the sol synthesized using a nonhydrolytic route was seen to exhibit better optical properties than the sol synthesized through a hydrolytic route. The mechanical properties of coatings derived from sol synthesized through a non-hydrolytic route are comparatively poor than the MZH sol possibly due to incomplete hydrolysis and polycondensation process. The present investigation also confirmed the endothermic nature of sol synthesis using the non-hydrolytic route. Depending on the applications envisaged, one can adopt a hydrolytic or non-hydrolytic route for the sol synthesis. Acknowledgment The authors acknowledge the support and encouragement provided by Dr. G. Sundararajan and Dr. G. Padmanabham, ARCI, Hyderabad throughout the course of this investigation. References [1] G. Schottner, Hybrid sol–gel derived polymers: applications of multifunctional materials, Chem. Mater. 13 (2001) 3422–3435. [2] D.K. Hwang, J.H. Moon, Y.G. Shul, K.T. Jung, D.H. Kim, D.W. Lee, Scratch resistant and transparent UV-protective coating on polycarbonate, J. Sol–Gel Sci. Technol. 26 (2003) 783–787.
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