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Effect of plasma pretreatment on adhesion and mechanical properties of UV-curable coatings on plastics
Applied Surface Science 257 (2011) 4360–4364
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Effect of plasma pretreatment on adhesion and mechanical properties of UV-curable coatings on plastics T. Gururaj ∗ , R. Subasri, K.R.C. Soma Raju, G. Padmanabham International Advanced Research Centre for Powder Metallurgy and New Materials (ARCI), Balapur, Hyderabad-500005, AP, India
a r t i c l e
i n f o
Article history: Received 18 August 2010 Received in revised form 9 December 2010 Accepted 12 December 2010 Available online 17 December 2010 Keywords: Plasma activation Plastics Adhesion Scratch resistance Abrasion resistance
a b s t r a c t An attempt was made to study the effect of plasma surface activation on the adhesion of UV-curable sol–gel coatings on polycarbonate (PC) and polymethylmethacrylate (PMMA) substrates. The sol was synthesized by the hydrolysis and condensation of a UV-curable silane in combination with Zr-n-propoxide. Coatings deposited by dip coating were cured using UV-radiation followed by thermal curing between 80 ◦ C and 130 ◦ C. The effect of plasma surface treatment on the wettability of the polymer surface prior to coating deposition was followed up by measuring the water contact angle. The water contact angle on the surface of as-cleaned substrates was 80◦ ± 2◦ and that after plasma treatment was 43◦ ± 1◦ and 50◦ ± 2◦ for PC and PMMA respectively. Adhesion as well as mechanical properties like scratch resistance and taber abrasion resistance were evaluated for coatings deposited over plasma treated and untreated surfaces. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Scratch and abrasion resistance are the primary requirements for any coatings of industrial significance especially for plastic materials. Abrasion and scratch resistant hybrid coatings from sol–gel route for plastic substrates are popular and widely published [1–13]. Most of the plastic substrates become soft at temperatures below 150 ◦ C and hence many of the reported hybrid sol compositions were either only UV curable or a combination of UV and low temperature thermal curable coatings. In case of plastic substrates like polyvinylchloride, polypropylene, polycarbonate and polymethylmethacrylate, the adhesion between the sol–gel coating and substrate plays a major role to improve the properties of the coating. Surface activation methods such as polymer base inter layer [6,14,15] and in some studies, plasma activation in vacuum chamber, oxygen and inert gas plasma were used to activate the surface [9–11] and hence, improved the wettability and bonding between the coating and the substrate. Most of the literature reads their utility in enhancing the scratch and wear resistance; scarce information exists in the literature on the effect of atmospheric air plasma treatment as a surface activation treatment of plastic surface prior to the application of sol–gel coatings. There are only very few studies on the mechanical proper-
∗ Corresponding author. Tel.: +91 40 24443567; fax: +91 40 24442699. E-mail address: [email protected] (T. Gururaj). 0169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.12.060
ties of a coating composition where two metal alkoxides on plastics have been used. Hence the overall objective of the present study was to investigate the effect of atmospheric air plasma surface activation on the coating adhesion and other mechanical properties on polycarbonate (PC) and polymethylmethacrylate (PMMA) substrate materials by using a multi-metal alkoxide UV-curable precursor system for the synthesis of the sol. 2. Experimental 2.1. Materials and synthesis The hybrid sol for coating was synthesized by hydrolysis and condensation of a UV-curable silane in combination with zirconium-n-propoxide. 3-methacryloxypropyltrimethoxysilane (MPTS, 98%) and zirconium-n-propoxide (70%) from Gelest Inc., USA were used as precursors. IRGACURE 184 as photoinitiator from Ciba Geigy, Methacrylic acid (MAA, 99%) from ABCR, Germany and hydrochloric acid (HCl) were used as starting materials. MPTS was hydrolysed with deionized water using 0.1 N HCl as the catalyst. Zirconium-n-propoxide was independently complexed with methacrylic acid using a molar ratio of 1:0.7 under vigorous stirring. The zirconium-methacrylic acid complex was added to the hydrolyzed MPTS under stirring. 1 wt% IRGACURE 184 was added followed by stirring for 15 min. The resulting sol was then filtered through a 0.8 m pore size filter paper and the clear sol was used for coating.
T. Gururaj et al. / Applied Surface Science 257 (2011) 4360–4364
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Fig. 1. Schematic sketch depicting the plasma process and processing parameters.
2.2. Plasma treatment and coating deposition PC and PMMA substrates of 100 mm × 100 mm × 3 mm dimensions were selected for the coating experimental work. The as received substrates has a temporary film to protect against scratch during transportation and handling and the same was removed and cleaned with isopropanol prior to surface activation using atmospheric air plasma (M/s Plasmatreat GmbH, Germany) treatment. Plasma was created with a compressed air at a supply pressure of 3 bar and 100 l/h flow rate. Plasma power was fixed at 5 kW. The stationary plasma head has two nozzles as presented in Fig. 1, separated apart by 80 mm. While the treatment was in progress, the head rotates and generated a plasma zone of 80 mm dia. The substrate is picked by a vacuum pad held by a MOTOMAN robot and moved over the plasma jet at a speed of 100 cm/min to the activate surface. The substrate to plasma nozzle, working distance was fixed at 10 mm throughout the experiments. A laboratory dip coater designed and fabricated by M/s EPG AG Germany was employed to generate coatings on the substrates. All coatings were generated at 1 mm/s withdrawal speed. The films were UV cured using a three-medium-pressure-mercury lamp (120 W/cm with 12 kW wattage/lamp) conveyorized UV curing unit. The belt speed was maintained at 2 m/min during curing. The light dose measured by UV radiometer (EIT Inc., USA) was 871 mJ/cm2 in the UV-C region. Coated samples were UV cured within 10 min of coating and no cracks or wrinkles were observed after UV curing. The coatings were then thermally densified at 80 ◦ C in case of PMMA and 130 ◦ C in case of PC for 1 h in an air circulating oven. 2.3. Characterization Viscosity of the sol was measured using a Rheometer (Anton Paar Physica MCR-51). Morphology of the coated surfaces was evaluated using an optical microscope (Olympus) and a scanning electron microscope (Hitachi S-4300SE/N) equipped with a field emission gun. Coating thickness was measured using Filmetrics Inc. F20 thin film measuring instrument and surface roughness was measured using Perthometer precision surface profilometer (Mahr, Germany). Water contact angle was measured using Krüss drop shape analyzer to validate for the hydrophobocity/hydrophilicity of the plasma treated surface. Hardness of the coatings was validated using pencil scratch as per ASTM D 3363-05 method. Adhesion of the coatings to the substrate was estimated using cross hatch cut and a tape peel test according to ASTM D335902 method. Abrasion test was carried out on a Taber tester at 2 × 500 g load, CS10 wheels and for 500 revolutions as per ASTM D4060-01 method. Haze of the specimens was measured using Hazeguard Dual (BYK Gardner GmbH) employing a method ASTM D1044.
Fig. 2. Change in contact angle before and after plasma surface treatment.
3. Results and discussion Contact angle of water droplet or a solution close to the viscosity of the depositing sol is a measure of the wettability of the treated surface. It is now well established that plasma treatment removes organic contaminants on the surface of the work piece by the bombardment of reactive species of plasma on the surface thereby causing simultaneous surface oxidation [16]. Subsequently, these free radicals couple with active species from the plasma environment to form polar groups such as –(C–O)–, –(C O)– and–(C O)–O– on the surface of work piece. [17]. Water molecule has a property to form hydrogen bond with polar functional groups created on the polymer surface leading to the decrease in contact angle measurements which is a reflection of increase of surface free energy. Higher the surface free energy, lower the water contact angle and hence the adhesive strength between polymer and coating is higher [18,19]. While PC and PMMA substrates cleaned using IPA show water contact angles of 80◦ and 65◦ respectively, the plasma activated surfaces exhibited water contact angles of 40◦ and 50◦ respectively as shown Fig. 2. It is evident from the decrease in contact angle that the surface free energy has increased, and the extent of increase in surface free energy depends on the substrate. However, the activated surface undergoes substantial changes in their surface chemical composition during aging. Aging can be explained based on post-plasma oxidation and surface restructuring processes [20]. Post-plasma oxidation is initiated by reaction between remaining radicals and oxygen from surrounding atmosphere. It was found that the repetitive motions of the chains/segments of polymer from surface to bulk will lead to change in the interfacial chemistry of polymer surface known as surface restructuring or surface reorientation. These processes lead to substantial decrease in the surface density of polar groups resulting in increase in contact angle over a period of time [21,22]. The contact angle of the plasma treated samples was followed up for a period of 120 hours to assess the maximum permissible time lag between surface activation and coating deposition. It can be seen from the Fig. 3 that the best option would be to deposit coatings within 10 min of plasma treatment. However one can still expect considerable wettability after up to 1 h of treatment, after which advantage of plasma treatment may be lost. The hybrid sol synthesized was homogeneous, transparent and the viscosity measured was 10.6 cP. Coatings were deposited with the above sol at a fixed withdrawal speed of 1 mm/s using a dip coater. Coated wet samples were room dried and UV cured and
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Fig. 3. Variation of contact angle with time on polycarbonate substrate after plasma treatment.
Fig. 4. SEM micrograph of coated PC.
subsequently thermally densified in an electric oven resulting in densified hybrid coating embedded with silica and zirconia inorganic network. An average surface roughness of the coating applied on untreated PC and PMMA was 0.02 m (Ra ) and 0.05 m (Ra ) respectively. The coated surfaces of plasma activated PC and PMMA
Fig. 6. Bar graph showing change in haze of bare with that of coated PC and PMMA samples after 500 cycles of taber testing.
have exhibited an average surface roughness of 0.3 m (Ra ) and 0.03 m (Ra ) respectively. Pencil scratch hardness of the coating was 2H for PC and 1H in case of PMMA and was unaffected by the surface activation. The pencil scratch hardness is more dependent on the extent of densification or network formation in the coating (hardness) rather than bonding or adhesion with substrate and since coating on PC was densified at higher temperature scratch hardness of coated PC is higher than that of PMMA. Coated PC and PMMA substrates were subjected to scanning electron microscopy to investigate for possible defects. The coatings are amorphous and featureless. Continuous, transparent and crack-free coatings were obtained as seen from the micrograph shown in Fig. 4. The thickness of the coatings was measured to be 3 ± 0.3 m. Fig. 5a and 5b shows photographs of half-coated PC and PMMA substrates respectively after taber testing. As can be seen clearly the coated areas in both substrates exhibited very little loss of transparency while the uncoated area shows significant wear pattern and hence substantial loss of transparency. Haze of the bare, as coated and worn surfaces were measured for every 100 cycles of the taber test on both PC and PMMA till completion of 500 cycles. Fig. 6 shows the percentage (%) haze change on plasma treated and untreated samples after completion of 500 cycles of the taber testing on both PC and PMMA substrate materials. It has to be noted here that coated PC was thermally treated at 130 ◦ C while PMMA
Fig. 5. Photographs of samples after 500 cycles of taber abrasion test (a) PC substrate lower-half coated (b) PMMA substrate lower-half coated.
T. Gururaj et al. / Applied Surface Science 257 (2011) 4360–4364
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Fig. 7. Pictures showing results of adhesion test for PC (a) with plasma treatment and (b) without plasma treatment and for PMMA (c) with plasma treatment and (d) without plasma treatment.
was at 80 ◦ C because of their softening nature beyond these temperatures. Fig. 7 presents the micrograph of the cross hatch cut and tape peel tested PC and PMMA samples with and without plasma surface activation prior to coating. As can be seen clearly the coating adhesion to the substrate was better in case of PC which was ranked as 4B with less than 5% material removal after tape test than that of PMMA substrate which could be ranked as 3B with 5–15% material removal, when plasma surface activation was employed. Similar trend could be observed in case of samples that were not plasma treated prior to coating. While PC substrate exhibited 15–35% material removal equalent to rank 2B, PMMA had more than 65% removal of coating after tape test, ranked as equivalent to 0B. These results imply that the adhesion of coating to the substrate was better in case of plasma treated substrates than untreated substrate irrespective of the type of the substrate material. The reason for higher rank of adhesion in case of PC when compared to PMMA, may be due to two reasons; one can be due to higher densification temperature employed for PC, and hence more cross-linking of coating in addition to adhesion to the substrate. The other could be due to higher wettability induced after plasma treatment in case of PC compared to PMMA. 4. Conclusions The effect of the plasma surface activation on adhesion and mechanical properties of hard coatings on plastics like PC and PMMA was investigated. The effect of surface activation was retained for nearly an hour and thereafter decreased slowly with time. Pencil scratch hardness of the coating was 2H for PC and 1H for PMMA. The abrasion resistance as estimated from the change in haze was nearly 3 fold improved when compared to the bare in case
of plasma treated PMMA. Similar trend was evident in case of PC also. Adhesion measurements from the cross hatch cut and tape test indicated that the plasma surface activation definitely enhanced the adhesion irrespective of the type of plastic selected though the rank varied depending on the substrate material. Acknowledgement The authors gratefully acknowledge the support of Dr. G. Sundararajan, Director ARCI throughout the course of this investigation. References [1] H.K. Schimdt, E. Geiter, M. Mannig, H. Krug, C. Becker, R.-P. Winkler, J. Sol–Gel Sci. Technol. 13 (1998) 397–404. [2] H.G. Floch, P.F. Belleville, J. Sol–Gel Sci. Technol. 1 (1994) 293–304. [3] P. Etienne, J. Phalippou, R. Sempere, J. Sol–Gel Sci. Technol. 2 (1994) 171–173. [4] P. Etienne, J. Denape, J.Y. Paris, J. Phalippou, R. Sempere, J. Sol–Gel Sci. Technol. 6 (1996) 287–297. [5] F. Del Monte, P. Cheben, C.P. Grover, J.D. Mackenzie, J. Sol–Gel Sci. Technol. 15 (1999) 73–85. [6] Li Chenghong, Kurt Jordens, L. Garth, Wilkes Wear 242 (2000) 152–159. [7] D. Blanc, A. Last, J. Franc, S. Pavan, J.L. Loubet, Thin Solid Films 515 (2006) 942–946. [8] S. Etienne-calas, A. Duri, P. Etinne, J. Non-Cryst. Solids 344 (2004) 60–65. [9] L.Y.L. Wu, E. Chwa, Z. Chen, X.T. Zeng, Thin Solid Films 516 (2008) 1056–1062. [10] K.T. Jung, D.H. Kim, D.W. Lee, J. Sol–Gel Sci. Technol. 26 (2003) 783–787. [11] Z. Chen, L.Y.L. Wu, E. Chwa, O. Tham, Mater. Sci. Eng. A 493 (2008) 292–298. [12] R. Kasemann, H. Schimidt, New J. Chem. 18 (1994) 1117–1123. [13] S. Sepeur, N. Kunze, B. Werner, H. Schmidt, Thin Solid Films 351 (1999) 216–219. [14] Young Jae Shin, Mee Hye Oh, Yeo Seong Yoon, Jae Sup Shin, Polym. Eng. Sci. 48 (2008) 1289–1295. [15] A. Hozumi, Y. Kato, O. Takai, Surf. Coat. Technol. 82 (1996) 16–22. [16] C.H. Yi, Y.H. Lee, G.Y. Yeom, Surf. Coat. Technol. 171 (2003) 237–240.
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[17] S.K. Koh, J.S. Cho, K.H. Kim, S. Han, Y.W. Beag, J. Adhes. Sci. Technol. 16 (2002) 129. [18] C.-S. Ren, K. Wang, Q.-Y. Nie, D.-Z. Wang, S.-H. Guo, Appl. Surf. Sci. 255 (2008) 3421–3425. [19] G. Borcia, C.A. Anderson, N.M.D. Brown, Appl. Surf. Sci. 221 (2004) 203–214.
[20] K.S. Siow, L. Britcher, S. Kumar, H.J. Griesser, Plasma Process. Polym. 3 (2006) 392–418. [21] R.C. Chatelier, X. Xie, T.R. Gengenbach, H.J. Griesser, Langmuir 11 (1995) 2576–2584. [22] K.S. Kim, C.M. Ryu, C.S. Park, G.S. Sur, C.E. Park, Polymer 44 (2003) 6287–6295.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Effect of plasma pretreatment on adhesion and mechanical properties of UV-curable coatings on plastics T. Gururaj ∗ , R. Subasri, K.R.C. Soma Raju, G. Padmanabham International Advanced Research Centre for Powder Metallurgy and New Materials (ARCI), Balapur, Hyderabad-500005, AP, India
a r t i c l e
i n f o
Article history: Received 18 August 2010 Received in revised form 9 December 2010 Accepted 12 December 2010 Available online 17 December 2010 Keywords: Plasma activation Plastics Adhesion Scratch resistance Abrasion resistance
a b s t r a c t An attempt was made to study the effect of plasma surface activation on the adhesion of UV-curable sol–gel coatings on polycarbonate (PC) and polymethylmethacrylate (PMMA) substrates. The sol was synthesized by the hydrolysis and condensation of a UV-curable silane in combination with Zr-n-propoxide. Coatings deposited by dip coating were cured using UV-radiation followed by thermal curing between 80 ◦ C and 130 ◦ C. The effect of plasma surface treatment on the wettability of the polymer surface prior to coating deposition was followed up by measuring the water contact angle. The water contact angle on the surface of as-cleaned substrates was 80◦ ± 2◦ and that after plasma treatment was 43◦ ± 1◦ and 50◦ ± 2◦ for PC and PMMA respectively. Adhesion as well as mechanical properties like scratch resistance and taber abrasion resistance were evaluated for coatings deposited over plasma treated and untreated surfaces. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Scratch and abrasion resistance are the primary requirements for any coatings of industrial significance especially for plastic materials. Abrasion and scratch resistant hybrid coatings from sol–gel route for plastic substrates are popular and widely published [1–13]. Most of the plastic substrates become soft at temperatures below 150 ◦ C and hence many of the reported hybrid sol compositions were either only UV curable or a combination of UV and low temperature thermal curable coatings. In case of plastic substrates like polyvinylchloride, polypropylene, polycarbonate and polymethylmethacrylate, the adhesion between the sol–gel coating and substrate plays a major role to improve the properties of the coating. Surface activation methods such as polymer base inter layer [6,14,15] and in some studies, plasma activation in vacuum chamber, oxygen and inert gas plasma were used to activate the surface [9–11] and hence, improved the wettability and bonding between the coating and the substrate. Most of the literature reads their utility in enhancing the scratch and wear resistance; scarce information exists in the literature on the effect of atmospheric air plasma treatment as a surface activation treatment of plastic surface prior to the application of sol–gel coatings. There are only very few studies on the mechanical proper-
∗ Corresponding author. Tel.: +91 40 24443567; fax: +91 40 24442699. E-mail address: [email protected] (T. Gururaj). 0169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.12.060
ties of a coating composition where two metal alkoxides on plastics have been used. Hence the overall objective of the present study was to investigate the effect of atmospheric air plasma surface activation on the coating adhesion and other mechanical properties on polycarbonate (PC) and polymethylmethacrylate (PMMA) substrate materials by using a multi-metal alkoxide UV-curable precursor system for the synthesis of the sol. 2. Experimental 2.1. Materials and synthesis The hybrid sol for coating was synthesized by hydrolysis and condensation of a UV-curable silane in combination with zirconium-n-propoxide. 3-methacryloxypropyltrimethoxysilane (MPTS, 98%) and zirconium-n-propoxide (70%) from Gelest Inc., USA were used as precursors. IRGACURE 184 as photoinitiator from Ciba Geigy, Methacrylic acid (MAA, 99%) from ABCR, Germany and hydrochloric acid (HCl) were used as starting materials. MPTS was hydrolysed with deionized water using 0.1 N HCl as the catalyst. Zirconium-n-propoxide was independently complexed with methacrylic acid using a molar ratio of 1:0.7 under vigorous stirring. The zirconium-methacrylic acid complex was added to the hydrolyzed MPTS under stirring. 1 wt% IRGACURE 184 was added followed by stirring for 15 min. The resulting sol was then filtered through a 0.8 m pore size filter paper and the clear sol was used for coating.
T. Gururaj et al. / Applied Surface Science 257 (2011) 4360–4364
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Fig. 1. Schematic sketch depicting the plasma process and processing parameters.
2.2. Plasma treatment and coating deposition PC and PMMA substrates of 100 mm × 100 mm × 3 mm dimensions were selected for the coating experimental work. The as received substrates has a temporary film to protect against scratch during transportation and handling and the same was removed and cleaned with isopropanol prior to surface activation using atmospheric air plasma (M/s Plasmatreat GmbH, Germany) treatment. Plasma was created with a compressed air at a supply pressure of 3 bar and 100 l/h flow rate. Plasma power was fixed at 5 kW. The stationary plasma head has two nozzles as presented in Fig. 1, separated apart by 80 mm. While the treatment was in progress, the head rotates and generated a plasma zone of 80 mm dia. The substrate is picked by a vacuum pad held by a MOTOMAN robot and moved over the plasma jet at a speed of 100 cm/min to the activate surface. The substrate to plasma nozzle, working distance was fixed at 10 mm throughout the experiments. A laboratory dip coater designed and fabricated by M/s EPG AG Germany was employed to generate coatings on the substrates. All coatings were generated at 1 mm/s withdrawal speed. The films were UV cured using a three-medium-pressure-mercury lamp (120 W/cm with 12 kW wattage/lamp) conveyorized UV curing unit. The belt speed was maintained at 2 m/min during curing. The light dose measured by UV radiometer (EIT Inc., USA) was 871 mJ/cm2 in the UV-C region. Coated samples were UV cured within 10 min of coating and no cracks or wrinkles were observed after UV curing. The coatings were then thermally densified at 80 ◦ C in case of PMMA and 130 ◦ C in case of PC for 1 h in an air circulating oven. 2.3. Characterization Viscosity of the sol was measured using a Rheometer (Anton Paar Physica MCR-51). Morphology of the coated surfaces was evaluated using an optical microscope (Olympus) and a scanning electron microscope (Hitachi S-4300SE/N) equipped with a field emission gun. Coating thickness was measured using Filmetrics Inc. F20 thin film measuring instrument and surface roughness was measured using Perthometer precision surface profilometer (Mahr, Germany). Water contact angle was measured using Krüss drop shape analyzer to validate for the hydrophobocity/hydrophilicity of the plasma treated surface. Hardness of the coatings was validated using pencil scratch as per ASTM D 3363-05 method. Adhesion of the coatings to the substrate was estimated using cross hatch cut and a tape peel test according to ASTM D335902 method. Abrasion test was carried out on a Taber tester at 2 × 500 g load, CS10 wheels and for 500 revolutions as per ASTM D4060-01 method. Haze of the specimens was measured using Hazeguard Dual (BYK Gardner GmbH) employing a method ASTM D1044.
Fig. 2. Change in contact angle before and after plasma surface treatment.
3. Results and discussion Contact angle of water droplet or a solution close to the viscosity of the depositing sol is a measure of the wettability of the treated surface. It is now well established that plasma treatment removes organic contaminants on the surface of the work piece by the bombardment of reactive species of plasma on the surface thereby causing simultaneous surface oxidation [16]. Subsequently, these free radicals couple with active species from the plasma environment to form polar groups such as –(C–O)–, –(C O)– and–(C O)–O– on the surface of work piece. [17]. Water molecule has a property to form hydrogen bond with polar functional groups created on the polymer surface leading to the decrease in contact angle measurements which is a reflection of increase of surface free energy. Higher the surface free energy, lower the water contact angle and hence the adhesive strength between polymer and coating is higher [18,19]. While PC and PMMA substrates cleaned using IPA show water contact angles of 80◦ and 65◦ respectively, the plasma activated surfaces exhibited water contact angles of 40◦ and 50◦ respectively as shown Fig. 2. It is evident from the decrease in contact angle that the surface free energy has increased, and the extent of increase in surface free energy depends on the substrate. However, the activated surface undergoes substantial changes in their surface chemical composition during aging. Aging can be explained based on post-plasma oxidation and surface restructuring processes [20]. Post-plasma oxidation is initiated by reaction between remaining radicals and oxygen from surrounding atmosphere. It was found that the repetitive motions of the chains/segments of polymer from surface to bulk will lead to change in the interfacial chemistry of polymer surface known as surface restructuring or surface reorientation. These processes lead to substantial decrease in the surface density of polar groups resulting in increase in contact angle over a period of time [21,22]. The contact angle of the plasma treated samples was followed up for a period of 120 hours to assess the maximum permissible time lag between surface activation and coating deposition. It can be seen from the Fig. 3 that the best option would be to deposit coatings within 10 min of plasma treatment. However one can still expect considerable wettability after up to 1 h of treatment, after which advantage of plasma treatment may be lost. The hybrid sol synthesized was homogeneous, transparent and the viscosity measured was 10.6 cP. Coatings were deposited with the above sol at a fixed withdrawal speed of 1 mm/s using a dip coater. Coated wet samples were room dried and UV cured and
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Fig. 3. Variation of contact angle with time on polycarbonate substrate after plasma treatment.
Fig. 4. SEM micrograph of coated PC.
subsequently thermally densified in an electric oven resulting in densified hybrid coating embedded with silica and zirconia inorganic network. An average surface roughness of the coating applied on untreated PC and PMMA was 0.02 m (Ra ) and 0.05 m (Ra ) respectively. The coated surfaces of plasma activated PC and PMMA
Fig. 6. Bar graph showing change in haze of bare with that of coated PC and PMMA samples after 500 cycles of taber testing.
have exhibited an average surface roughness of 0.3 m (Ra ) and 0.03 m (Ra ) respectively. Pencil scratch hardness of the coating was 2H for PC and 1H in case of PMMA and was unaffected by the surface activation. The pencil scratch hardness is more dependent on the extent of densification or network formation in the coating (hardness) rather than bonding or adhesion with substrate and since coating on PC was densified at higher temperature scratch hardness of coated PC is higher than that of PMMA. Coated PC and PMMA substrates were subjected to scanning electron microscopy to investigate for possible defects. The coatings are amorphous and featureless. Continuous, transparent and crack-free coatings were obtained as seen from the micrograph shown in Fig. 4. The thickness of the coatings was measured to be 3 ± 0.3 m. Fig. 5a and 5b shows photographs of half-coated PC and PMMA substrates respectively after taber testing. As can be seen clearly the coated areas in both substrates exhibited very little loss of transparency while the uncoated area shows significant wear pattern and hence substantial loss of transparency. Haze of the bare, as coated and worn surfaces were measured for every 100 cycles of the taber test on both PC and PMMA till completion of 500 cycles. Fig. 6 shows the percentage (%) haze change on plasma treated and untreated samples after completion of 500 cycles of the taber testing on both PC and PMMA substrate materials. It has to be noted here that coated PC was thermally treated at 130 ◦ C while PMMA
Fig. 5. Photographs of samples after 500 cycles of taber abrasion test (a) PC substrate lower-half coated (b) PMMA substrate lower-half coated.
T. Gururaj et al. / Applied Surface Science 257 (2011) 4360–4364
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Fig. 7. Pictures showing results of adhesion test for PC (a) with plasma treatment and (b) without plasma treatment and for PMMA (c) with plasma treatment and (d) without plasma treatment.
was at 80 ◦ C because of their softening nature beyond these temperatures. Fig. 7 presents the micrograph of the cross hatch cut and tape peel tested PC and PMMA samples with and without plasma surface activation prior to coating. As can be seen clearly the coating adhesion to the substrate was better in case of PC which was ranked as 4B with less than 5% material removal after tape test than that of PMMA substrate which could be ranked as 3B with 5–15% material removal, when plasma surface activation was employed. Similar trend could be observed in case of samples that were not plasma treated prior to coating. While PC substrate exhibited 15–35% material removal equalent to rank 2B, PMMA had more than 65% removal of coating after tape test, ranked as equivalent to 0B. These results imply that the adhesion of coating to the substrate was better in case of plasma treated substrates than untreated substrate irrespective of the type of the substrate material. The reason for higher rank of adhesion in case of PC when compared to PMMA, may be due to two reasons; one can be due to higher densification temperature employed for PC, and hence more cross-linking of coating in addition to adhesion to the substrate. The other could be due to higher wettability induced after plasma treatment in case of PC compared to PMMA. 4. Conclusions The effect of the plasma surface activation on adhesion and mechanical properties of hard coatings on plastics like PC and PMMA was investigated. The effect of surface activation was retained for nearly an hour and thereafter decreased slowly with time. Pencil scratch hardness of the coating was 2H for PC and 1H for PMMA. The abrasion resistance as estimated from the change in haze was nearly 3 fold improved when compared to the bare in case
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