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Preparation of polymer/silica composite antiglare coatings on poly(ethylene terphathalate) (PET) substrates.

Abstract 3-(Trimethoxysilyl) propyl methacrylate and fluoroalkylsilane modified silica particles were UV-cured with the multifunctional crosslinking agent, dipentaerythritol hexaacrylate (DPHA), to prepare antiglare (AG) coatings that exhibit high hardness. Modified silica was used to impart surface roughness whereas DPHA was used to provide hardness and adhesiveness. The formed AG coatings were characterized in terms of topological, optical, and mechanical properties. It was found that with the increase of DPHA content in the coatings, the surface roughness and haze decreased while the gloss and clarity increased. For samples containing 40 and 50 wt% DPHA, the optical properties are comparable to commercial AG products for flat panel display and touch panels. These prepared coatings also demonstrate high hardness of 5-6H and 100% adhesion on poly(ethylene terphathalate) substrates.

Keywords Antiglare coating, Silica particle, Acrylate resin, UV-curing, Poly(ethylene terphathalate), Hardness

Introduction

Glare is a type of light distortion commonly encountered in our daily life. Discomfort glare reduces the contrast of visibility of the object being viewed, while intense glare can lead to temporary vision impairment. Smooth and glossy surfaces, such as those on headlamps, eyeglasses, monitor screens, etc., tend to cause glare by reflecting light coherently from external light source in the environments. Therefore, it is useful to apply antiglare (AG) treatments on these surfaces to lower the intensity of the glare. In fact, AG coatings are extensively applied on various consumer electronics products, such as liquid crystal display (LCD) monitors, mobile phones, touch panels, etc.

AG treatment on smooth (glossy) plastic surface is usually made by coating a thin AG film on top of the surface.1-25 Two approaches are often employed: (1) the AG coating is made to have a roughened surface such that it reflects undesirable light in different directions; (2) the AG coating contains large particles or phase domains, typically 1-10 [micro]m, which scatter reflected light. For example, Aegerter and Al-Dahoudi prepared AG conductive coatings on plastic substrates with adjustable gloss (60-80 @ 60[degrees]), low haze (<10%), and good clarity (75-90%). The coating surface consisted of 10-100 [micro]m features with an average roughness of 0.2 [micro]m. (3) Nakamura et al. developed an antireflection-antiglare bi-layer coating with haze 3-20% and water contact angle 90[degrees]-120[degrees]. (12) Kawahara et al. introduced inorganic particles, such as silica, alumna, titania, etc., into a UV-curable binder to form AG coatings on triacetate cellulose (TAC), which finds applications in LCD. (15)

Although many AG coating formulations have been developed, the mechanical strength, in particular, hardness, of the coating surface is seldom discussed. As AG coating is often applied on the outermost layer of various functional coatings, its hardness appears to be of significant concern when plastic substrates are used. (5), (7), (11), (13), (19), (21) Therefore, it is useful to develop AG coatings that are also hard enough to protect the plastic substrate. Such is made possible by spraying a coating containing an inorganic-organic sol on the plastic substrate followed by UV-curing. In this study, the inorganic component comprises silica particles that are surface modified by 3-(trimethoxysilyl) propyl methacrylate (MSMA) and fluoroalkylsilane. These particles aggregate to generate a roughened AG surface during the UV-curing process. The organic component is a hexa-functional monomer, which crosslinks with the MSMA moiety of silica to give a robust inorganic-organic hybrid film with good pencil hardness. The detailed preparation and characterization is presented in the sections below.

Experimental

Materials

Tetraethoxysilane (TEOS, >98%) and 2-propanol (IPA, [greater than or equal to]99.8%) were purchased from Fluka. 3-(Trimethoxysilyl) propyl methacrylate (MSMA, 98%) and dipentaerythritol hexaacrylate (DPHA) were purchased from Aldrich. The fluoroalkylsilane, 1H, 1H, 2H, 2H-perfluorooctyltriethoxysilane (reagent grade), was purchased from Degussa. The photoinitiator, 2-hydroxy-2-methyl-l-phenyl-propan-l-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.

Preparation of surface-modified silica sol

The synthetic procedure of surface-modified silica sol is shown in Scheme 1. 80 g TEOS was mixed with 92 g IPA at a molar ratio of 1:4 to form a homogeneous solution. Then, 27.7 g HCl aqueous solution (pH 3) with the molar ratio of [H.sub.2]O/TEOS equal to 4 was added to this solution under continuous agitation to initiate the hydrolysis and condensation reaction. The solution was stirred for 6 h, after which 4.6 g additional HCl aqueous solution (pH 3, [H.sub.2]O/TEOS = 1/1.5) was added into the formed silica sol, followed by 21.2 g of the coupling agent, MSMA, which was dropped in very slowly using a syringe pump. The molar ratio of TEOS/MSMA was set to be 4.5. After reacting for another 2 h, the surface-modified silica (called M[SiO.sub.2]) sol was obtained. The M[SiO.sub.2] sol was further modified by 0.2 g fluoroalkylsilane to improve the spread ability of the formed coating sol. Fluoroalkylsilane was added drop by drop into the M[SiO.sub.2] sol under vigorous agitation. The reaction was allowed to proceed for 1 h at room temperature, and the modified silica thus formed hereinafter will he referred to as MF[SiO.sub.2]. The theoretical solid content of the MF[SiO.sub.2], sol was 17 wt%.

Preparation of antiglare coatings

Multifunctional monomer DPHA and photoinitiator Darocure 1173 were added directly to the MF[SiO.sub.2] sol under agitation together with additional amount of solvent to yield coating cols with a fixed solid content (17 wt%). The formed coating sol was spray-coated on a poly(ethylene terphathalate) (PET) substrate, and subsequently predried in an oven maintained at 80[degrees]C for I min, followed by UV-irradiation at 12 mJ/[cm.sup.2]. The compositions of various species in the coating sol are listed in Table 1.
Table 1: Weight of various species for preparing AG coating sol

Sample  MF[SiO.sub.2]  DPHA (g)  1173   DPHA content in
name    sol (g)                   (g)     coating (wt%)

MO              30.03       0.00  0.14                0
M1              30.03       0.57  0.17               10
M2              30.03       1.28  0.20               20
M3              30.03       2.19  0.25               30
M4              30.03       3.41  0.31               40
M5              30.03       5.12  0.40               50
M6              30.03       7.68  0 52               60
M7              30.03      11.94  0.74               70


Characterization

Infrared absorption spectra of the prepared silica were taken using a Fourier transform infrared (FTIR) spectrophotometer (Magna-IR Spectrometer 550, Nicolet). Samples were prepared by dropping appropriate amount of silica sol on a KBr disc, and then the solvent was removed under vacuum. The synthesized MF[SiO.sub.2] particles were observed using a transmission electron microscope (TEM, H-7100, Hitachi). An appropriate amount (~2 [mirco]L) of sol sample was dropped on a standard 300 mesh copper grid and the solvent was evaporated at room temperature. The size and size distribution of silica particles were also determined by the dynamic light scattering (DLS) method, using a Zetasizer Malvern 3000 HS at 25[degrees]C. The surface and cross section morphologies of the AG film were observed using a scanning electron microscope (SEM, S-2600H, Hitachi) and a field emission scanning electron microscopy (FE-SEM, Leo 1530, LEO Elektronenmikroskopie, GmbH), respectively. For the surface images, the sample was vacuum-dried, cut into suitable size, and then attached to the holder by means of conductive copper tapes. The sample was sputtered with a thin layer of gold and observed with a tilt angle of 60[degrees]. For the cross section images, the samples were vacuum-dried and then fractured in liquid nitrogen. Surface morphology of the AG film was also examined by a surface profilometer ([alpha]-step 500, TENCOR, USA). The optical properties, including transmittance, gloss, haze, and clarity of the AG coatings were measured by UV/Vis spectrometer (UV500, Unlearn), gloss meter (Micro-TRI-Gloss Meter 4430, BYK-Gardner), and haze meter (Haze-Gard plus Meter 4725, BYK-Gardner), respectively. For gloss measurement, the 60[degrees] angle was used. A tape test (ASTM D3359), also called peel test, of the AG coating was carried out to examine the adhesion strength of the films 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 hardness of the AG coating was examined by the industrial pencil hardness test (ASTM D3363) using pencils of different hardness at the load of 500 g.

[ILLUSTRATION OMITTED]

Results and discussion

Chemical structure analysis by FTIR

In Fig. 1, the FTIR spectrum of the MF[SiO.sub.2] sample is shown. The broad band around 1075 [cm.sup.-1] corresponds to the stretching vibrations of the Si-O-Si bond, whereas the bending vibration of this bond is located at 440 [cm.sup.-1]. The peak at 931 [cm.sup.-1] is assigned to the Si-OH groups. The absorption bands of C=C and C=0 for the MSMA moieties are found at 1635 and 1700 [cm.sup.-1], respectively. (26-30) The signal associated with fluoroalkylsilane is not in evidence because of its low content and overlap with MSMA. Furthermore, since there are plenty of C=C on the surface of MF[SiO.sub.2] particles, the latter can be cross linked with the multifunctional monomer, DPHA, to form a strong inorganic-organic hybrid network during the UV-curing process.

[FIGURE 1 OMITTED]

TEM and SEM imaging

Figure 2 shows the DLS intensity distribution of the modified silica particles, MF[SiO.sub.2]. Two main particle sizes were found to be 16 and 131 nm, respectively. MF[SiO.sub.2] particles were also observed by means of TEM. Figure 3 shows a typical image. It appears that the particles (black dots) are separated with a size distributed largely over the range 100-150 nm, which is in agreement with the data determined by DLS measurement. The smaller particles measured by DSL cannot be observed due to the resolution of TEM. The modified silica particles were crosslinked with DPHA by UV irradiation to form AG coatings on PET substrates. Figure 4 shows the SEM micrographs of the surface morphology of four representative coating samples, M1, M3, M5, and M6, having different DPHA contents. Obviously, as the sample contains more DPHA, its surface becomes smoother and fewer features are observed. When the DPHA content reaches 60%, the surface is quite smooth. In other words, the MF[SiO.sub.2] particles are mostly embedded in the interior of the cured DPHA network for M6. Figure 5 shows the SEM micrograph of the cross section morphology of Ml. There is no significant aggregation of silica particle observed in the prepared hybrid films. The silica particles are well dispersed and their sizes are estimated to be less than 200 nm, thus avoiding the strong light scattering caused by phase separation. The above morphological evolution of the surface is confirmed by the [alpha]-step profilometry presented in Fig. 6. For sample Ml, the peak-to-trough variation is very intense over the scanning X-range of 1000 [micro]m. The average roughness ([R.sub.a]) is 0.54 [micro]m. Such severe topological variation is eased when the amount of DPHA is increased. For example, the surface profile for the sample M5 is smoother with a roughness equal to 0.44 [micro]m. As to the sample M6, the roughness is reduced to 0.19 [micro]m and the profile contains only three local minima. It is expected that at this high organic content, the influences of MF[SiO.sub.2] particles are covered by the continuous DPHA host, which tends to form an extremely smooth surface. When light is reflected on a rough surface, a direct correlation between the phase shift and the root mean square height (HRMS) can be found. For an ideal scattering surface, where the image projected onto the surface is lost in the reflected light, a phase shift of 2[pi] is required. An HRMS of about 0.4-0.8 [micro]m is necessary for the visible region from 400 to 800 nm. (1)

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]
         Diam. (nm)  %Intensity  Width (nm)

Peak 1:         131        57.8        12.3
Peak 1:        16.4        42.2        1.18
Peak 1:        0.00         0.0        0.00


Optical performance of the AG coatings

Haze, gloss, and clarity of the prepared AG coatings are shown in Table 2. Apparently, the haze decreases with increasing DPHA content in the coating. As is known, haze is related to the surface morphology and the size of the phase domains in the coatings; rougher surface or larger phase domain gives rise to higher haze. In the present study, the surface effect is considered to dominate the haze due to the micron-scale variation in the surface profile, as is compared with the size of the phase domains (~100 [micro]m) in the cured coatings (Fig. 5). The gloss, which represents the reflection intensity of the light at a specific angle, increases as the DPHA content is increased. This is consistent with the fact that a smoother surface scatters less and a stronger reflected intensity is detected. The clarity shows a trend opposite to that of the haze, as anticipated. The optical properties of some commercial products are given in Table 2 for comparison. In general, if the haze is too high, the surface looks hazy and the objects underneath the film become un-resolvable. Prepared coatings containing 40 and 50 wt% DPHA exhibit haze and gloss values that comply with commercial requirements. Specifically, the sample M5 whose optical properties, including transmittance, gloss, and haze, are close to the commercial AG film (by Cheeshin Technology Co. Ltd.) for touch panel applications.
Table 2: Optical and mechanical properties of AG coatings

Sample name    Transmittance  Gloss  Haze  Clarity
               (%)           (GU)

MO                      91.2   22.1  44.6  20.4

M1                      91.4   40.4  26.3    37

M2                      91.5   38.6  27.6  34.7

M3                      91.4   40.6  26.8  36.6

M4                      91.5   52.9  16.8  47.8

M5                      91.4   95.2  9.85    63

M6                      91.5    163  1.16  95.7

M7                      91.3    167  0.79  99.2

NITTO AGS2                --  31-44    12    --

NITTO AGT1                --     40    11    --

CHEESHIN                  90     95    10  65.5
CS-PE051A0100

Sample name    Hardness  Adhesion
                         (%)

MO                 2H         100

M1               2-3H         100

M2                 4H         100

M3                 4H         100

M4                 5H         100

M5               5-6H         100

M6                 6H         100

M7               6-7H         100

NITTO AGS2         3H          --

NITTO AGT1         3H          --

CHEESHIN           3H          --
CS-PE051A0100


Hardness and adhesion of the AG coatings

The hardness of AG coatings cured on PET was examined using the pencil test. The tested results are summarized in Table 2. It appears that the pencil hardness for all prepared samples is higher than 2H, and it increases with increasing DPHA content. The pencil hardness of UV-cured poly(DPHA) has been measured to be ca. 7-8H. Such high hardness was derived from the strong network structure built-up during crosslinking of the hexa-functional DPHA. As a result, higher DPHA content yields harder coatings. The sample M5 is of particular interest with optical properties satisfying AG standard and pencil hardness (5-6H) much higher than that of the commercial product (3H). As to the adhesiveness, all samples show 100% adhesion on tape test, regardless of their DPHA content.

Conclusions

AG coatings composed of surface-modified silica and DPHA were prepared by spraying and UV-curing, and their mechanical and optical properties were characterized. Modified silica was employed to engender surface roughness of the AG coating, whereas DPHA was used as a binder to provide photosensitivity, mechanical strength, and adhesiveness. As the DPHA content in the coatings was increased, surface roughness of the UV-cured coating was reduced, leading to the decrease of haze and increase of gloss and clarity. For samples containing 40 and 50 wt% DPHA, the optical properties are comparable to those of the commercial products, and their hardness reaches 5H, higher than the commercial ones. These samples also show 100% adhesion to the PET surface as determined by the peel test.

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C.-C. Chang, C.-M. Chen, F.-H. Hwang, L.-P. Cheng *

Department of Chemical and Materials Engineering, Tamkang University, 151, Yingzhuan Rd., Tamsui Dist., New Taipei City 25137, Taiwan, ROC

e-mail: Ipcheng@mail.tku.edu.tw

C.-C. Chang, C-C. Chen, L.-P. Cheng

Energy and Opto-Electronic Materials Research Center, Tamkang University, 151, Yingzhuan Rd., Tamsui Dist., New Taipei City 25137, Taiwan, ROC

[c] ACA and OCCA 2012

DOI 10.1007/s11998-011-9392-4
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Author:Chang, Chao-Ching; Chen, Chien-Ming; Hwang, Feng-His; Chen, Ching-Chung; Cheng, Liao-Ping
Publication:JCT Research
Date:Sep 1, 2012
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