Gas Barrier Performance of Surface-Modified Silica Films with Grafted

Surface-modified polymeric substrates with excellent gas barrier performances have been fabricated by coating the substrates with a silica film by mea...
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Langmuir 2003, 19, 8331-8334

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Gas Barrier Performance of Surface-Modified Silica Films with Grafted Organosilane Molecules Katsuya Teshima,*,†,‡ Hiroyuki Sugimura,† Yasushi Inoue,§ and Osamu Takai| Department of Materials Processing Engineering, Graduate School of Engineering, Research Center for Nuclear Materials Recycle, and Center for Integrated Research in Science and Engineering, Nagoya University, Nagoya 464-8603, Japan, and Advanced Technology Laboratory, Research & Development Center, Dai Nippon Printing Co., Ltd., Kashiwa 277-0871, Japan Received January 30, 2003. In Final Form: May 26, 2003 Surface-modified polymeric substrates with excellent gas barrier performances have been fabricated by coating the substrates with a silica film by means of plasma-enhanced chemical vapor deposition (PECVD) and the subsequent deposition of an organosilane layer on the silica film by a low-temperature CVD method. The hydrophobic organosilane layer on the silica-deposited substrates prevented adsorption of oxygen and water molecules on the substrate surfaces. Consequently, oxygen and water vapor transmission rates were markedly decreased down to 60% as compared to the substrates without the hydrophobic layers. In particular, the fluoroalkylsilane-coated substrate showed a water vapor transmission rate of 0.32 g/(m2‚ day), which was drastically low compared with that of the alkylsilane-coated substrate.

Introduction Gas barrier coatings on flexible substrates have received much attention in industries for food and medical packaging as well as flat-panel display devices.1-11 In the food packaging field, aluminum films deposited by vacuum evaporation or sputtering are frequently used for gas barriers against oxygen and/or water vapor permeation, although such films possess some defects.2,3,12-15 Besides aluminum, silica coatings have highly eligible properties, * Corresponding author: e-mail [email protected]. † Department of Materials Processing Engineering, Graduate School of Engineering, Nagoya University. ‡ Advanced Technology Laboratory, Research & Development Center, Dai Nippon Printing Co. § Research Center for Nuclear Materials Recycle, Nagoya University. | Center for Integrated Research in Science and Engineering, Nagoya University. (1) Chatham, H. Surf. Coat. Technol. 1996, 78, 1. (2) Benmalek, M.; Dunlop, H. M. Surf. Coat. Technol. 1996, 76, 821. (3) Hertlein, J.; Singh, R. P.; Weisser, H. J. Food Eng. 1995, 24, 543. (4) Tropsha, Y. G.; Harvey, N. G. J. Phys. Chem. B 1997, 101, 2259. (5) Brennan, D. J.; White, J. E.; Brown, C. N. Macromolecules 1998, 31, 8281. (6) da Silva Sobrinho, A. S.; Latre`che, M.; Czeremuszkin, G.; Klemberg-Sapieha, J. E.; Wertheimer, M. R. J. Vac. Sci. Technol. A 1998, 16, 3190. (7) Erlat, A. G.; Spontak, R. J.; Clarke, R. P.; Robinson, T. C.; Haaland, P. D.; Tropsha, Y.; Harvey, N. G.; Vogler, E. A. J. Phys. Chem. B 1999, 103, 6047. (8) da Silva Sobrinho, A. S.; Czeremuszkin, G.; Latre`che, M.; Wertheimer, M. R. J. Vac. Sci. Technol. A 2000, 18, 149. (9) Dennier, G.; Houdayer, A.; Se´gui, Y.; Wertheimer, M. R. J. Vac. Sci. Technol. A 2001, 19, 2320. (10) Teshima, K.; Inoue, Y.; Sugimura, H.; Takai, O. Vacuum 2002, 66, 353. (11) Teshima, K.; Inoue, Y.; Sugimura, H.; Takai, O. Thin Solid Films 2002, 420, 318. (12) Barker, C. P.; Kochem, K.-H.; Revell, K. M.; Kelly, R. S. A.; Badyal, J. P. S. Thin Solid Films 1995, 259, 46. (13) Garcı´a-Ayuso, G.; Va´zquez, L.; Martı´nez-Duart, J. M. Surf. Coat. Technol. 1996, 80, 203. (14) Henry, B. M.; Dinelli, F.; Zhao, K.-Y.; Grovenor, C. M. R.; Kolosov, O. V.; Briggs, G. A. D.; Roberts, A. P.; Kumar, R. S.; Howson, R. P. Thin Solid Films 1999, 355, 500. (15) Erlat, A. G.; Henry, B. M.; Ingram, J. J.; Mountain, D. B.; McGuigan, A.; Howson, R. P.; Grovenor, C. R. M.; Briggs, G. A. D.; Tsukahara, Y. Thin Solid Films 2001, 388, 78.

such as transparency, recyclability, and microwavability, and accordingly are more useful than metal coatings.1-4,6-11 In addition, silica is ecologically benign since it is abundant in nature and harmless to human beings and the environment. Therefore, silica is a promising candidate for an alternative to the metal gas barrier coatings. Polymeric materials, such as poly(ethylene terephthalate) (PET), polypropylene, nylon, and polyethylene, are used for food packaging applications. Since these materials are not heat-resistant, the silica coatings must be prepared at a low temperature. Sputtering, vacuum evaporation, ion plating, and plasma-enhanced chemical vapor deposition (PECVD) have been successfully utilized to deposit silica films on polymeric substrates below the glass transition temperature of the polymers.1-4,6-11,16,17 Among these methods, PECVD has advantages in adhesion and step coverage of silica coating and, thus, is promising for effective gas barrier coatings. In addition, the silica films have excellent flexibility and thus durability for cracks in converting processes, e.g., laminating, printing, and making pouch. The gas barrier performance of silica-deposited substrates is known to be governed by chemical compositions18,19 of the silica layers, both at their surface and inside, besides structural defects such as pinhole, crack, and grain boundary.6-9 The surface and inner compositions of the silica layers correspond, respectively, to gas adsorption on the surfaces and molecular diffusion into the Si-O-Si networks, which greatly govern gas permeation rates. As we have reported,10,11 some amounts of impurities as forms of Si-CH3, Si-H, and Si-OH were present in silica films prepared by PECVD. These impurities terminate the Si-O-Si network of such a film and, consequently, seriously degrade the film’s effectiveness as an oxygen gas barrier. The most important finding was that a loosely packed and poorly linked Si-O-Si (16) Mercea, P. V.; Baˆr_an, M. J. Membr. Sci. 1991, 59, 353. (17) Felts, J. T.; Grubb, A. D. J. Vac. Sci. Technol. A 1992, 10, 1675. (18) Joly, C.; Smaihi, M.; Porcar, L.; Noble, R. D. Chem. Mater. 1999, 11, 2331. (19) Toy, L. G.; Nagai, K.; Freeman, B. D.; Pinnau, I.; He, Z.; Matsuda, T.; Teraguchi, M.; Yampolskii, Y. P. Macromolecules 2000, 33, 2516.

10.1021/la034164f CCC: $25.00 © 2003 American Chemical Society Published on Web 08/30/2003

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network in the film enhanced oxygen permeation. To remove the impurities from the film and to form a densely packed silica film, PECVD silica deposition must be carried out under an oxygen-rich atmosphere. However under this condition, the as-prepared surface is covered with hydrophilic functional groups, that is, OH groups. Since such a silica film shows high hydrophilicity, a large amount of oxygen and water molecules adsorb on the surface and then permeate into the film. In this paper, we report surface modification of silica films in order to achieve higher performances as gas barriers for applications such as packaging and industrial parts. The PECVD silica surfaces were coated with a hydrophobic layer in order to prevent adsorption of oxygen and/or water molecules on the surfaces and permeation through the films. The hydrophobic layer, which has been widely applied to control physical and chemical properties of silica surfaces on silicon wafers, is formed on the hydroxyl-bearing silica surfaces from organosilane molecules through the silane coupling chemistry.20-23

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Figure 1. Chemical structures of organosilane precursors.

Experimental Section Silica layers were prepared by use of a capacitively coupled RF PECVD system. Details of the apparatus used for the experiments have been described elsewhere.10,11 Biaxially oriented PET sheets (PTM-12, Unitika Co. Ltd.) of 12 µm thickness were used as polymeric substrates. Prior to silica deposition, the substrates were treated with oxygen plasma (RF power 100 W, total pressure 10 Pa, and treatment time 10 min) in order to remove any contaminants from the surface and to ensure that the deposited films would adhere strongly to the substrates. Organosilane, that is, tetramethoxysilane [TMOS, Shin-etsu Chemical, Si(OCH3)4], and oxygen are used as PECVD sources. Partial pressures of TMOS and O2 were fixed at 5 and 5 Pa, respectively. A silica layer was deposited on the substrate by generating plasma for 10 min at an RF power of 200 W. During the deposition, the substrate temperature was kept below 70 °C. Next, a hydrophobic layer was prepared on each of the PECVDsilica film surfaces by a low-temperature CVD method. Two types of organosilane precursors were employed: n-octadecyltrimethoxysilane[ODS,TokyoKaseiOrganicChemical,H3C(CH2)17Si(OCH3)3] and heptadecafluoro-1,1,2,2-tetrahydrodecyl-1-trimethoxysilane (a type of fluoroalkylsilane) [FAS, Shin-etsu Chemical, F3C(CF2)7(CH2)2Si(OCH3)3]. The chemical structures of these precursors are shown in Figure 1. The silica-deposited substrates were placed together with glass cup filled with 0.4 cm3 of organosilane liquid into a 130 cm3 Teflon container. The container was sealed with a cup and placed for 5 h in an oven maintained at 100 °C. The chemical compositions and bonding states of the silica and hydrophobic layers were studied by Fourier transform infrared spectroscopy (FTIR, Bio-Rad, FTS-175) and X-ray photoelectron spectroscopy (XPS, Escalab, 220I-XL). The surface morphologies of the modified substrates were observed by scanning electron microscope (SEM, Hitachi, S-5000H). Water contact angles of the modified substrates were measured with a contact angle meter (Kyowa Interface Science, CA-D) based on the sessile water drop measuring method, with a water drop of 2 mm in diameter. Oxygen transmission rates (OTRs) and water vapor transmission rates (WVTRs) of the modified and unmodified PET substrates were measured by an isopiestic method (OTR, Modern Controls Inc., OX-TRAN 2/20; WVTR, Modern Controls Inc., Perma-Tran 3/31). A test cell was divided into two halves with the PET sheet sample. The temperature of the test cell was kept at 23 °C (OTRs) or 37.8 °C (WVTRs). The edges of the test (20) Plueddemann, E. P. Silane Coupling Reagents; Plenum Press: New York, 1991. (21) Sagiv, J. J. Am. Chem. Soc. 1980, 102, 92. (22) Wasserman, S. R.; Tao, Y.-T.; Whitesides, G. M. Langmuir 1989, 5, 1074. (23) Hoffmann, P. W.; Stelzle, M.; Rabolt, J. F. Langmuir 1997, 13, 1877.

Figure 2. FTIR spectrum of silica layer prepared by plasmaenhanced CVD. cell were tightly sealed to prevent air from leaking into the cell from the outside. Oxygen (90%, RH) or water vapor (100%, RH) was sequently admitted to the outer half of the test cell and allowed to exit through an exhaust port. A carrier gas, nitrogen (90%, RH) was continuously admitted to the inner half of the test cell. The permeated gas (oxygen or water vapor) through the sample was picked up by a carrier gas (nitrogen) admitted into the outer half cell so as to be carried to an oxygen sensor, which consisted of a coulometric fuel cell that produces an electrical current when exposed to oxygen.

Results and Discussion From FTIR results, silica layers deposited by PECVD were found to consist of Si-O-Si networks and silanol groups,24-30 as shown in Figure 2 and summarized in Table 1. Absorption bands corresponding to carbon impurities were not observed at all. In addition, depth profiles of the concentrations of Si, O, and C atoms in the silica layer were measured by XPS. The concentrations of Si and O (24) Socrates, G. Infrared Characteristic Group Frequencies, 2nd ed.; Wiley: Chichester, U.K., 1994. (25) Wro´bel, A. M.; Walkiewicz-Pietrzykowska, A.; Hatanaka, Y.; Wickramanayaka, S.; Nakanishi, Y. Chem. Mater. 2001, 13, 1884. (26) Theil, J. A.; Brace, J. G.; Knoll, R. W. J. Vac. Sci. Technol. A 1994, 12, 1365. (27) Lucovsky, G.; Manitini, M. J.; Srivastava, J. K.; Irene, E. A. J. Vac. Sci. Technol. B 1987, 5, 530. (28) He, L.; Inokuma, T.; Kurata, Y.; Hasegawa, S. J. Non-Cryst. Solids 1995, 185, 249. (29) Bogart, K. H. A.; Dalleska, N. F.; Bogart, G. R.; Fisher, E. R. J. Vac. Sci. Technol. A 1995, 13, 476. (30) Deshmukh, S. C.; Aydil, E. S. J. Vac. Sci. Technol. A 1995, 13, 2355.

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Figure 3. Cross-sectional image of the silica layer deposited by plasma-enhanced CVD on the PET substrate. Figure 5. Photographs of a water droplet on the silica-deposited PET substrates coated with (a) ODS, (b) FAS, and (c) uncoated with an organosilane.

Figure 4. XPS C1s spectra obtained from the silica-deposited PET substrates coated with (a) ODS and (b) FAS. Table 1 peak position (cm-1)

band assignment

3200-3700 higher wavenumber lower wavenumber 1060-1100 925 810-845

OH stretching isolated surface OH group H-bonded OH group Si-O-Si asymmetric stretching Si-OH stretching Si-O-Si deformation

were almost constant through the entire inner region at about 35, 65, and 0 at. %, respectively. The O/Si ratio was approximately 1.85. Figure 3 shows a cross-sectional SEM image of the silica layer deposited on the PET substrate. As clearly indicated in this figure, a densely packed silica layer without apparent boundaries, pinholes, and cracks was formed on the PET substrate. Onto the PECVD silica films, a hydrophobic layer was formed in order to improve their gas barrier performance. Figure 4 shows XPS C1s spectra obtained from the silicadeposited substrates coated with (a) ODS and (b) FAS. The C1s spectrum of the ODS layer (Figure 4a) consists of a main peak centered at a binding energy of 285.0 eV beside minimal peaks originated from Si-C bonds at 283.8 eV and C-O bonds at 286.5 eV.34 On the other hand, the C1s spectrum of the FAS layer (Figure 4b) could be deconvolved into seven components centered at 283.8, 285.0, 286.5, 288.9, 290.5, 292.2, and 294.5 eV. These components correspond to Si-C, C-C, C-O, -COO-, -CF2-CH2-, -CF2-CF2-, and CF3-CF2- groups, respectively.31-33 Figure 5 shows photographs of a water droplet on the silica-deposited PET substrates covered (31) Sugimura, H.; Hozumi, A.; Kameyama, T.; Takai, O. Surf. Interface Anal. 2002, 34, 550. (32) Gerenser, L. J. J. Vac. Sci. Technol. A 1990, 8, 3682. (33) Hozumi, A.; Ushiyama, K.; Sugimura, H.; Takai, O. Langmuir 1999, 15, 5, 7600. (34) Sugimura, H.; Ushiyama, K.; Hozumi, A.; Takai, O. J. Vac. Sci. Technol. B 2002, 20, 393.

Figure 6. Oxygen transmission and water vapor transmission rates of the modified PET substrates (silica deposition and hydrophobic layer coating).

with (a) ODS and (b) FAS. The water contact angle of the silica-coated substrate changed from approximately 10° before hydrophobic layer coatings (Figure 5c) to larger than 100° afterward. Since a hydrophilic surface has chemical affinity for organosilane molecules, ODS and FAS molecules chemisorb onto the silica-deposited substrates when the substrates are exposed to the organosilane vapors. The hydrophobic layers are confirmed to be certainly formed on the PECVD silica films through lowtemperature CVD, although the ODS and FAS layers showed smaller water contact angles than self-assembled monolayers (SAMs) formed on flat silica substrates and the CF2/CF2 and CF2/CH2 ratios of our FAS layer were quite different from those of the precursor molecules and the reported FAS SAM.31,34 Therefore, the coverages of the ODS and FAS layers are thought to be not complete compared with the SAMs formed on flat surfaces. Figure 6 shows the gas barrier behavior of the modified PET substrates. OTR and WVTR of the uncoated PET substrate were greater than 200 cm3/(m2‚day‚atm) and 50 g/(m2‚day), respectively. OTR and WVTR of the PET substrate coated with the PECVD silica layer of 250 nm in thickness were approximately 1.3 cm3/(m2‚day‚atm) and 1.5 g/(m2‚day), respectively. These values satisfy the requirements for industrial products in the field of food packaging. Meanwhile, gas permeation rates of the silicadeposited PET substrates covered with the organosilane layers could be decreased to less than 60% of the rates before the organosilane layer coatings. In particular,

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molecules are thought to permeate more readily through a silica layer more loosely packed and formed with SiO-Si networks fragmented through the termination by impurities. Finally, the molecules desorb from the “exit” surface. Consequently, the silica layer, which has a hydrophobic surface, consisting of tightly packed Si-OSi networks, showed excellent gas barrier performance. Conclusions

Figure 7. Gas permeation models of the modified PET substrates.

WVTR of the FAS layer-coated substrate decreased markedly down to less than 25% of that before the FAS coating. Here, we discuss oxygen and water vapor permeation mechanisms of the modified PET substrates. Figure 7 illustrates gas permeation models of the modified PET substrates. The gas permeation mechanisms are quite different between microporous substrates (pore diameter smaller than 2 nm) and nonporous substrates. Gas permeation through the microporous substrates is generally described as microporous diffusion following LennardJones potential and cylindrical pore geometry, that is, the capillary flow.18 On the other hand, gas permeation through the nonporous substrates is described as the dissolution-diffusion flow.18 Since micropores such as pinhole, crack, and grain boundary are not present in our silica layers, their gas permeations are governed by the dissolution-diffusion flow process. In this process, first, oxygen and water vapor molecules adsorb and condense on the “entry” surface. Subsequently, the condensed molecules dissolve into the substrate to some degree. If an entry surface is hydrophobic, adsorption and condensation of the molecules are restrained. The dissolved molecules diffuse within the Si-O-Si networks. This diffusion rate is, in general, slow and is governed by the density and of completeness the Si-O-Si networks. The

We have succeeded in providing high gas barrier performances to the PET substrates by coating the substrate with silica and subsequent hydrophobic layers. First, the silica layers, with tightly packed Si-O-Si networks, were deposited on the plasma-treated PET substrates by PECVD via a TMOS/O2 mixture. Subsequently, the hydrophobic layer was deposited on each of the silica surfaces by low-temperature CVD, with ODS or FAS in order to prevent adsorption of oxygen and/or water vapor. Since the silica layers prepared by PECVD were tightly packed and did not include any structural defects, they showed sufficient gas barrier performances, which were congruous with the requirements for industrial food packaging products. Furthermore, OTR and WVTR of the silica-deposited substrates coated with the hydrophobic layers could decrease to 60% or less than those before the hydrophobic coatings. In the case of the FAS layer coatings, particularly, WVTR of the modified substrate was decreased down to as small as 25% of that of the substrate without the hydrophobic layer. This study clearly demonstrates that gas barrier performances of the modified substrates are improved by restraining gas adsorption and condensation on the surfaces and molecular diffusion in the substrates. Through this approach, we can reduce the thickness of the silica layer for attaining a sufficient gas barrier performance, so that such gas barrier films will become more flexible. Furthermore, our doublelayered coatings will contribute to further lowering of gas transmission rates for gas barrier films made of other polymeric materials. LA034164F