silica hybrid hard coatings with enhanced hydrophobicity on plastic substrates

Journal of Non-Crystalline Solids 358 (2012) 72–76

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Preparation of polymer/silica hybrid hard coatings with enhanced hydrophobicity on plastic substrates Chao-Ching Chang a, b, Tai-Yueh Oyang a, Feng-Hsi Hwang a, Ching-Chung Chen b, Liao-Ping Cheng a, b,⁎ a b

Department of Chemical and Materials Engineering, Tamkang University, No. 151, Yingzhuan Rd., Danshui Dist., New Taipei City 25137, Taiwan Energy and Opto-Electronic Materials Research Center, Tamkang University, No. 151, Yingzhuan Rd., Danshui Dist., New Taipei City 25137, Taiwan

a r t i c l e

i n f o

Article history: Received 21 June 2011 Received in revised form 25 August 2011 Available online 21 September 2011 Keywords: Colloidal silica; Hard coating; UV-curing; Plastic substrate; Hydrophobicity

a b s t r a c t In this study, an easy method to increase hydrophobicity of the polymer/silica hybrid coating was demonstrated. UV-curable nano-sized colloidal silica was synthesized and surface-modified both by a coupling agent, 3-(trimethoxysilyl)propyl methacrylate (MSMA), and a capping agent, trimethyethoxysiliane (TMES). The formed particles were introduced into the poly(2-hydroxyethyl methacrylate) (PHEMA) matrix to yield PHEMA/silica hybrid hard coatings on plastic substrates via a UV-curing process. Differential scanning calorimetric (DSC) analyses of the hybrids indicated increases of the glass transition temperature (Tg) with increasing silica content in the hybrids; in general, an increase of 23 °C could be achieved for hybrids doped with 15 wt.% silica. Thermal decomposition temperature (Td), as measured by the thermal gravimetric analyzer (TGA), was found to depend on the silica content in a trend similar to that on Tg. Specifically, a large increase of 25 °C was observed when the sample contained 15 wt.% silica. The pencil hardness of the PHEMA/silica hybrids coated on poly(methyl methacrylate) (PMMA) substrates can reach 5H, in comparison with 2H for pure PHEMA coating. Abrasion resistance was enhanced when silica nanoparticles were incorporated. Furthermore, due to the incorporation of TMES, hydrophobicity of the hybrid coating increased considerably as the TMES content was increased. In the extreme case, a hard surface with a water contact angle (92°) has been obtained. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Plastic materials are used in a wide variety of industries to produce commodities for daily life as well as high technology products for specific applications, thank to their unique properties, such as low density, water-resistance, corrosion resistance, low electrical and thermal conductivities, processability, etc. Most plastics, however, suffer from the drawback of insufficient mechanical strength; in particular, plastic surfaces are relatively soft and subjected to damages from scratching or abrasion, which not only causes esthetic defect but also may shorten the service life of the product. A layer of hard coating can be applied on the plastic surface for protection. In fact, a hard coating on the plastic surface are extensively employed on various optic/electric products, such as lens, LCD monitors, mobile phones, touch panels, etc. Conventionally, hard coatings on plastics are often made by curing multi-functional organic monomers, such as trimethylolpropane triacrylate (TMPTA), on the plastic substrate to form a highly cross-

⁎ Corresponding author at: Department of Chemical and Materials Engineering, Tamkang University, No. 151, Yingzhuan Rd., Danshui Dist., New Taipei City 25137, Taiwan. Tel.: + 886 2 26215656x2725; fax: + 886 2 26209887. E-mail address: [email protected] (L.-P. Cheng). 0022-3093/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2011.08.024

linked dense layer that exhibits medium–high hardness, depending on the functionality of the monomer and the process conditions. Hard coatings of organic–inorganic hybrids emerge and quickly become the target of extensive research since 1990s [1–19]. It is found that in most cases the incorporation of inorganic nanoparticles improves the mechanical and thermal properties of plain organic coating considerably. More importantly, because the inorganic domains in the hybrids are usually in the nano-sized range, the coatings are optically transparent and suitable for optical applications. The first approach to prepare polyacrylate–silica hybrid materials is incorporating silica domains into organic resin or polymer matrices using TEOS as the precursor via an in-situ synthesis. However, the reaction stages and solution viscosity cannot be exactly controlled throughout the continuous sol–gel process. Therefore, thickness and properties are not easy to control when the hybrids are applied to optical coatings. Besides, a standard operating procedure cannot be built. The second approach to prepare polyacrylate–silica hybrid materials is incorporating silica nanoparticles into organic resins or polymers. This ex-situ synthesis provides synthetic control over the particle size, size distribution, and surface properties of silica nanoparticles. It also provides operating control over the concentration, reaction stages, and solution viscosity. Via ex-situ synthesis one can build a fast and standard operating procedure that is more suitable for industrial applications, especially for optical coatings.

C.-C. Chang et al. / Journal of Non-Crystalline Solids 358 (2012) 72–76

Silica nanoparticles can be surface-modified to possess certain functional groups to satisfy specific needs. For example, in previous works, silica nanoparticles containing methacrylate groups have been prepared by reacting with a silane, 3-(trimethoxysilyl)propyl methacrylate (MSMA), in a sol–gel process [8,19–25]. The modified silica nanoparticles would undergo co-polymerization with the binder to form a strong organic–inorganic hybrid via a UV-curing process. In the current research, the MSMA-modified silica nanoparticles were further modified by a capping agent, trimethyethoxylsilane (TMES), which served to reduce particle–particle aggregation by consuming Si–OH groups on the particle surface. Scheme 1 shows the syntheses route for the surface-modified silica nanoparticles. Hybrid hard coatings on the plastic substrate were made based on the acrylic monomer and the modified nanoparticles via a UV-curing process. These coatings exhibits improved thermal stability and hardness with respect to those without using inorganic particles. Moreover, the hydrophobicity of the coatings increases considerably with increasing TMES content, and in many cases, coatings with a water contact angle of N90° may be obtained. Detailed preparation and characterization are presented and discussed below.

73

Table 1 Weight ratio of various chemical species for preparing MTSiO2 sols. MTSiO2 sol

b

R=0 R = 0.1 R = 0.2 R = 0.3 R = 0.4

Weight ratio MSiO2 sola

TMES

H2O

IPA

75 75 75 75 75

– 2.6 5.3 9.1 14.3

– 0.4 0.8 1.4 2.2

– 1.3 5.3 9.3 14.5

a

MSiO2 was prepared with the molar ratio of TEOS:MSMA:H2O:IPA = 4.5:1:21:5. R value stands for the mole ratio of TMES/(TMES + TEOS). R = 0 means unmodified MSiO2 sol. b

yield a film of about 15 μm in thickness and with N90% transmittance over the visible range. Monolith samples were also prepared for thermal analyses, for which the coating was poured in an aluminum mold and then UV-cured and post-baked at 100 °C for 3 h to ensure total remove of residual solvent traces. The samples designated as TxSy stand for prepared hybrid coatings, where T0, T1, T2, T3 and T4 denote the used MTSiO2 sol with R value = 0, 0.1, 0.2, 0.3 and 0.4; S1, S2 and S3 indicate nominal loadings of 5, 10, and 15 wt.% SiO2.

2. Experimental 2.4. Characterization 2.1. Materials 2-Hydroxyethyl methacrylate (2-HEMA, regent grade), 3-(trimethoxysilyl)propyl methacrylate (MSMA, 98%), and trimethylethoxysilane (TMES, 97%) were purchased from Aldrich. Tetraethoxysilane (TEOS, N98%) was purchased from Fluka. 2-Propanol (IPA, 99.5%) purchased from Fluka was used as a solvent. The photo-initiator, 2-hydroxy-2methyl-1-phenyl-propan-1-one (Darocure 1173), was purchased from Ciba-Geigy. Aqueous hydrochloric acid (37 wt.%) was purchased from J.T. Baker. All materials were used as received. 2.2. Preparation of surface-modified silica nanoparticles Colloidal silica nanoparticles were prepared by a modified sol–gel process shown in our previous publications [20,26]. Briefly, TEOS was mixed with IPA at a molar ratio of 1.0 to form a homogeneous solution. Then, HCl aqueous solution (pH 1.2) was added to this solution under continuous agitation for 3 h to form the silica sol. The molar ratio of H2O/TEOS was set to be equal to four. After that, MSMA and additional HCl aqueous solution (pH 1.2) with a molar ratio of H2O/MSMA = 3 and TEOS/MSMA = 4.5, was slowly dropped into the silica sol. The reaction was allowed to proceed for another 3 h to modify the silica surface. The MSMA-modified silica was called MSiO2. To prevent particle–particle aggregation, the residual –OH groups on MSiO2 colloidal particles were further capped by reacting with the capping agent, TMES. Appropriate amounts of TMES, IPA, and HCl(aq) at pH 0.8 were added into the MSiO2 sol under vigorous agitation. After reaction for 3 h at room temperature, TMES-capped silica (called MTSiO2) was obtained. The compositions of various chemical species for this reaction are summarized in Table 1. R values in the table stand for the mole ratio TMES/(TMES + TEOS). The solid content of each MTSiO2 sol based on complete hydrolysis and condensation of alkoxy groups was about 26 wt.%. The particle sizes of MTSiO2 particles were about 5–10 nm [26]. 2.3. Preparation of transparent PHEMA/silica hybrid coatings To prepare a photosensitive coating sol, appropriate amounts of 2HEMA and photo-initiator were added into the synthesized MSiO2 or MTSiO2 sol under mild agitation. The prepared coating sol was uniformly spread on a poly(methyl methacrylate) (PMMA) or poly(ethylene terephthalate) (PET) substrate, and then pre-baked at 80 °C for 5 min, followed by UV irradiation at 72 mJ/cm 2 (broadband) to

Fourier transform infrared (FTIR) spectra of the samples were performed on the Nicolet Magna-IR 550 spectrometer. For sample preparation, the coating sol was dropped onto a KBr disk, pre-baked (at 80 °C for 5 min) then UV cured and post-baked at 100 °C. For all scans, the spectra were collected over the wavenumber range of 400–4000 cm −1 with a resolution of 4 cm −1. Ultraviolet–visible (UV–visible) spectra of the samples were performed on the UNICAM UV 500 spectrometer. Morphology of the cured coating was observed using a field emission scanning electron microscope (FE-SEM, Leo 1530, Carl Zeiss). The samples were vacuum-dried and then fractured in liquid nitrogen to expose the cross section. The lateral sides of the samples were wrapped with conductive copper tape and clamped in a sample holder. Silver paste was applied at the edges of the sample to enhance electronic conductivity. It was then sputtered with a thin layer (ca. 1.0 nm) of a Pt–Pd alloy and imaged at high magnifications by means of an in-lens detector. Thermal gravimetric analyzer (TGA, model 2950, TA instruments) was used to measure the thermal decomposition temperature (Td) of the cured samples. In order to remove the adsorptive water, the samples for TGA measurement were maintained at 100 °C for 10 min before starting the tests. Samples were heated to 600 °C with a heating rate of 20 °C/min under nitrogen flow. The glass transition temperature (Tg) of the cured samples were obtained by differential scanning calorimeter (DSC, model 2010, TA Instruments) according to heat–cool–heat cycles. The temperature was raised from 25 °C to 250 °C at a constant rate of 10 °C/min under nitrogen flow. Tg of the sample was determined from the thermogram of the second heating cycle. The glass transition temperature (Tg) of the cured samples was obtained by differential scanning calorimeter (DSC, model 2010, TA Instruments). The temperature was raised from 25 °C to 250 °C and then dropped to 25 °C for 1 cycle at a constant rate of 10 °C/min under nitrogen flow. Tg of the sample was determined from the thermogram of the second heating cycle. Tape test (ASTM D3359), also called peel test, was carried out to evaluate the adhesion of the coating on the substrate. The degree of adhesion between the film and the substrate was counted as the percentage of the residual film on the substrate after peeling by tapes (3M-610). The contact angle was measured by a contact angle/surface tension analyzer (FTA 125, USA) at room temperature (ca. 25 °C). Water (2 μl) was dropped onto the surface of the coating. The static contact angle was measured at a contact time of 30 s. Each sample was measured at five separate points for the same contact time, and the average value was reported. The hardness of the

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a

3.1. Chemical structure analysis by FTIR The FTIR analyses for the synthesis of MTSiO2 sols by hydrolysis and condensation reactions can be found in the literatures [20,26]. The FTIR spectra of the hybrid coating T4S3 before and after UV-curing are shown in Fig. 1. The absorption bands of the Si–C, Si–O–Si, C O, and C–H bonds are located at 847, 1078, 1730 and 2960 cm−1, respectively. The absorption band at 1639 cm−1 assigned to the stretching vibration of the C C bond for 2-HEMA or MSMA approaches zero, which suggests that most of the vinyl groups have already undergone free radical polymerization. A broad band around 3472 cm−1 assigned to the stretching vibration of various –OH groups (including those on MTSiO2, HEMA, and adsorptive water) can be seen. However, the absorption bands at 945 cm−1 for Si–OH approaches zero. 3.2. Thermal analyses TGA was utilized to measure the thermal decomposition of various cured samples. Fig. 2a shows the TGA and DTG (differential thermogravimetry) thermograms of poly(2-hydroxyethyl methacrylate) (PHEMA) and the hybrid T4S3. For pure PHEMA, thermal decomposition follows a two-stage pattern. The maximum decomposition rate (Td,max) for the 1st stage is at 311 °C whereas that for the 2nd stage is at 426 °C, consistent with the result shown in the literature [27]. The decomposition temperature (Td) defined as the temperature at 5 wt.% loss, is 251 °C. The decomposition is quite complete with a char yield approaching zero. The effect of silica content on thermal decomposition behavior of various PHEMA/silica hybrids has been investigated. For illustration, the TGA and DTA thermograms of the hybrid T4S3 are demonstrated in Fig. 2b. Apparently, this hybrid decomposes at higher temperatures than pure PHEMA, with an increase of 16 °C for Td and 26 °C for Td,max1. Measured decomposition temperatures and char yields of various hybrids are summarized in Table 2. Typically, Td, Td,max1, and Td,max2 are found to increase with increasing silica content in the hybrids. However, Td,max2 is lower in the hybrids than in the pure PHEMA in every case where the silica content is 5 or 10 wt.%. Moreover, the DTA thermograms indicate a new decomposition stage around 470 °C, which may be associated with

-0.6 60

0 100

Abs.

200

300

400

0.0 600

500

Temperature (°C)

b

-1.0 100 -0.8 80 -0.6 60 -0.4

40

-0.2

20 0 100

200

300

400

0.0 600

500

Temperature (°C) Fig. 2. TGA and DTA profiles of a pure PHEMA and b T4S3 hybrid.

the decomposition of organic moieties (from MSMA or TMES) on the silica surface. Char yields of various samples, as shown in the table, follow the trend consistent with the SiO2 content in the hybrids. The glass transition temperatures (Tg) of various cured samples were measured using DSC. The Tg of PHEMA, as determined by the midpoint method, is 90 °C. Incorporation of modified silica nanoparticles raises the Tg considerably. Fig. 3 shows the thermograms of PHEMA/ silica hybrids with R = 0.2. As can be seen in Fig. 3, Tg increases progressively with silica content in the hybrid. Similar trend is observed for other hybrids (R = 0.1–0.4), as shown in Table 2. Furthermore, Table 3 indicates that Tg decreases slightly when R value is raised.

Sample

(b)

3500

-0.2

20

Table 2 Thermal properties of PHEMA and PHEMA/silica hybrids.

(a)

4000

-0.4

40

Deriv. Weight (%/°C)

-0.8 80

Deriv. Weight (%/°C)

3. Results and discussion

-1.0 100

Weight (%)

coating was examined by the industrial pencil hardness test (ASTM D3363). Taber abrasion was carried out using a Taber abrasion tester. Abrading wheels (CS-17) loaded for 500 g were on the sample, and the sample was rotated at 500 cycles. Transmittance over the visible range of the coating after abrasion was measured.

Weight (%)

74

3000

2500

2000

1500

1000

500

Wavenumbers (cm-1) Fig. 1. FTIR spectra of the hybrid coating T4S3 before (a) and after (b) UV-curing.

PHEMA T0S1 T0S2 T0S3 T1S1 T1S2 T1S3 T2S1 T2S2 T2S3 T3S1 T3S2 T3S3 T4S1 T4S2 T4S3

Td (°C) 251 253 259 256 253 255 251 250 262 268 263 259 276 256 264 267

Td,

max1

Td,

max2

Td,

max3

(°C) 311 322 322 335 322 320 329 330 328 338 325 321 345 321 322 337

426 405 421 427 404 418 430 403 421 429 412 421 437 403 418 428

– 463 475 475 464 473 480 – 476 476 467 470 – 464 465 476

Char yield (%)

Tg (°C)

0.2 5.6 9.1 15.6 5.6 9.1 15.7 6.9 10.2 15.6 5.6 10.2 15.8 5.9 10.2 15.6

90 112 116 122 110 118 126 100 107 118 105 109 117 104 110 113

C.-C. Chang et al. / Journal of Non-Crystalline Solids 358 (2012) 72–76

75

Heat Flow (W/g)

T2S1

T2S2

T2S3

60

70

80

90

100

110

120

130

140

150

Temperature (°C) Fig. 3. DSC thermograms of PHEMA/silica hybrids with R = 0.2.

3.3. Surface properties The hardness of samples cured on PMMA or PET was examined using the industrial pencil test. All tested results are summarized in Table 3. As is expected, the hardness of PHEMA/silica hybrid coatings is higher than that of pure PHEMA. The pencil hardness of the hybrids coated on PMMA and PET substrates can reach 5H and 4H respectively, in comparison with 2H for pure PHEMA coatings. As regards the adhesion strength, all samples show 100% adhesion on tape test, regardless of whether silica has been added. This suggests that a fine dispersion of silica nanoparticles in the polymer host has been achieved. Otherwise, if phase separation occurred, the large separated inorganic phases may deteriorate the adhesion [27]. Fig. 4 compares the SEM images of the cross-section of PHEMA coating and T4S3 coating, which contains 15% silica. Apparently, the latter exhibits a much smoother structure with little evidence of microscopic cracks that are prevalent in the former. The addition of silica nanoparticles has produced a coating that exhibits pure brittle failure, as opposed to the PHEMA coating, where the roughness indicates at least some small amount of ductility in the system. This explains in part why the hybrids are harder than pure PHEMA. Also, no silica aggregates are observed at a resolvable scale of ca. 40 nm in Fig. 4b, which confirms the good compatibility between polymer and modified silica nanoparticles. Taber abrasion was carried out using a Taber abrasion tester. Transmittance at 550 and 633 nm of PHEMA and hybrid coatings are summarized in Table 3. Transmittance of hybrid coatings loaded with 30 wt.% silica Table 3 Hardness, abrasion test, and contact angle of PHEMA and PHEMA/silica hybrid coatings. Samples

Hardness

PMMA

PET

550 nm

633 nm

PHEMA T0S1 T0S2 T0S3 T1S1 T1S2 T1S3 T2S1 T2S2 T2S3 T3S1 T3S2 T3S3 T4S1 T4S2 T4S3

2H 5H 5H 5H 5H 5H 5H 5H 5H 5H 5H 5H 5H 5H 5H 5H

2H 4H 4H 4H 4H 4H 4H 4H 4H 4H 4H 4H 4H 4H 4H 4H

62 66 65 66 64 67 65 65 66 66 62 66 66 65 64 65

66 69 69 70 67 70 69 69 70 70 66 70 69 69 68 69

a

On PET substrate.

Transmittance after Tabera (%)

Contact anglea (degree)

60 64 70 74 70 77 80 78 82 85 83 86 87 87 90 92

Fig. 4. SEM image of the cross-section of a PHEMA coating and b T4S3 coating.

was 69–70% at 633 nm, in comparison with 66% for the pure PHEMA coating. The hybrid coatings show higher transmittance than that of PHEMA coating, which means abrasion resistance is enhanced when silica nanoparticles are incorporated. Effect of R on abrasion resistance is not obvious. The contact angles of water on various hybrids coated on PET are shown in Table 3. It appears that TMES has effectively raised the contact angle of the prepared coatings. Specifically, a hydrophobic surface (H4S3, R = 0.4) with a 92° water contact angle has been obtained. 4. Discussion In Fig. 2, the hybrid T4S3 displays more low-temperature mass loss than PHEMA. This implies that small molecules, such as unreactive TMES, exist in the hybrid. As listed in Table 2, Td, Td,max1, and Td,max2 of the hybrids with R = 2, 3, and 4 are higher than those of hybrids with R = 0 and 1 in the same silica content. The lower Td, Td,max1, and Td,max2 of T4S3 compared with those of T3S3 may result from the excess and unreactive TMES. The unreactive TMES moieties may also locally plasticize the polymer, resulting in less of an increase in the glass transition temperature. The pencil hardness for all hybrid coatings is identical regardless of silica content or surface modification on silica particles. As hardness would be expected to change as a function of silica content, considering increase in crosslink density, a possible reason is that the pencil hardness is subject to the substrate effect when the coating is thin enough. A coating surface with water contact angle of N90° may be considered as an easy-to-clean (ETC) surface [28]. PHEMA is as a polymer of low-intermediate hydrophobicity with water contact angle near 60°. In this research, we use TMES not only to stabilize the silica particles in the sol, but also to increase the hydrophobicity of the formed coating. Hydrophobicity of the hybrid coating increased considerably as the

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TMES content was increased. A hard surface with a water contact angle (92°) has been obtained. This contact angle is higher than that of the transparent untreated polystyrene (PS) thin film [29]. In other words, an ETC hard coating has been developed in the present research through the employment of TMES-modified silica nanoparticles. 5. Conclusions Hybrid coatings composed of surface-modified silica nanoparticles embedded in PHEMA matrix were prepared, and their thermal and surface properties were examined. It was found that Tg and Td for the coatings increased with increasing silica content in the hybrid. At the maximum silica content (15%), a significant increase over 36 °C on Tg were observed. On the other hand, Td may be raised by 25 °C at this silica dosage. The enhancement of hardness by incorporation of silica was manifested by the pencil test. The hardness of PHEMA/silica hybrids coated on PMMA reaches 5H. Abrasion resistance is enhanced when silica nanoparticles are incorporated. In addition, because silica nanoparticles have been modified by the hydrophobic TMES, hydrophobicity of the hybrid coatings increased considerably. In the extreme case (R = 0.4), the coatings with a water contact angle of 92° may be obtained, which is 32° higher than that of pure PHEMA. Acknowledgment The authors would like to thank anonymous reviewers and the Editor for their comments. References [1] R. Nass, E. Arpac, W. Glaubitt, H. Schmidt, Modelling of ORMOCER coatings by processing, J. Non-Crystal. Solids 121 (1990) 370–374. [2] H. Schmidt, H. Wolter, Organically modified ceramics and their applications, J. Non-Crystal. Solids 121 (1990) 428–435. [3] J. Gilberts, A.H.A. Tinnemans, M.P. Hogerheide, T.P.M. Koster, UV curable hard transparent hybrid coating materials on polycarbonate prepared by the sol–gel method, J. Sol-Gel Sci. Tech. 11 (1998) 153–159. [4] S. Sepeur, N. Kunze, B. Werner, H. Schmidt, UV curable hard coatings on plastics, Thin Solid Film 351 (1999) 216–219. [5] M.S. Lee, N.J. Jo, Coating of methyltriethoxysilane — modified colloidal silica on polymer substrates for abrasion resistance, J. Sol-Gel Sci. Tech. 24 (2002) 175–180. [6] T.P. Chou, G. Cao, Adhesion of sol–gel-derived organic–inorganic hybrid coatings on polyester, J. Sol-Gel Sci. Tech. 27 (2003) 31–41. [7] G. Schottner, K. Rose, U. Posset, Scratch and abrasion resistant coatings on plastic lenses — state of the art, current developments and perspectives, J. Sol-Gel Sci. Tech. 27 (2003) 71–79. [8] F. Bauer, R. Flyunt, K. Czihal, M.R. Buchmeiser, H. Langguth, R. Mehnert, Nano/micro particle hybrid composites for scratch and abrasion resistant polyacrylate coatings, Macromol. Mater. Eng. 291 (2006) 493–498.

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