Journal of Sol-Gel Science and Technology 13, 409 413 (1998) c 1998 Kluwer Academic Publishers. Manufactured in The Netherlands. Coating of Tetraethylorthosilicate (TEOS)/Vinyltriethoxysilane (VTES) Hybrid Solution on Polymer Films YOUNG-JOO EO, DUK-JUN KIM AND BYEONG-SOO BAE Department of Materials Science & Engineering, Korea Advanced Institute of Science and Technology (KAIST), Taejon 305-701, Korea KI-CHANG SONG Department of Chemical Engineering, Keun-Yang University, Chungnam 320-800, Korea TAE-YOUNG LEE AND SEUNG-WON SONG Taedok Institute of Technology, Yukong Ltd., Taejon 305-370, Korea Abstract. Tetraethylorthosilicate (TEOS)/vinyltriethoxysilane (VTES) hybrid materials were prepared and the hydrolysis and condensation reactions during processing were investigated by means of 29 Si NMR solution spectroscopy. The variation of drying characteristics of the coating films was examined with respect to the tetraethylorthosilicate (TEOS)/vinyltriethoxysilane (VTES) ratio, as well as drying temperature, by FT-IR spectroscopy. It is shown that the TO mode of Si O Si stretching absorption was enhanced with increasing tetraethylorthosilicate (TEOS) content and drying temperature. Also, the wettability of the coating films on polymer films was independent of the solution composition but enhanced by the precoating of poly(4-hydroxystrene) (PHS) as a wetting agent. The adhesion between the coating and the films was also enhanced when the vinyltriethoxysilane (VTES) content in the coating solution was increased. Keywords: TEOS-VTES, inorganic-organic hybrid, sol-gel coating, wetting, adhesion 1. Introduction Inorganic-organic hybrid materials are a new type of composite material in which the inorganic and the organic components are combined at the molecular level [1, 2]. There has been much research on the application of these hybrid materials as functional coatings on glass and polymer substrates [3, 4]. In particular, coating such hybrid materials onto polymer films potentially produces a transparent, abrasion resistant coating. The organic part of the hybrid material can improve the adhesion between the coating and a polymer substrate, and the inorganic part can increase the hardness of the coating. The sol-gel method is now widely used for the preparation of hybrid material coatings, since it has the advantage of being a low temperature process and potentially giving highly homogeneous coatings. The TEOS (tetraethylorthosilicate) and VTES (vinyltriethoxysilane) system can be used to make protective, hard, crack-free coatings on PET (polyethyleneterephthalate) film substrates [4]. It has been shown that the hardness of the PET can be increased when coated with TEOS-VTES. However, the reaction mechanisms and coating characteristics of the TEOS-VTES system have not been investigated in detail before. Therefore, the present study aims to examine the hydrolysis and condensation reactions of the TEOS-VTES system. Also, the wetting and adhesion behavior, and drying characteristics at low temperature were investigated for coatings on PP (polypropylene), and on PET.
410 Eo et al. 2. Experimental VTES (Aldrich) and TEOS (Aldrich) were used asreceived with ethanol as the co-solvent, and HCl as the catalyst. Reactions were therefore conducted in the TEOS-VTES-ethanol-water-HCl system. For NMR measurements, tetramethylsilane (TMS) was used as the reference, D 2 O as the lock solvent, and chromium(iii)acetylacetonate (Cr(acac) 3 ) was added to reduce the 29 Si spin-lattice relaxation time. The composition of the solution which was analyzed by NMR spectroscopy was TEOS : VTES = 5 : 5, EtOH/(TEOS + VTES) = 20, (water + D 2 O)/(TEOS + VTES) = 10, and HCl/(TEOS + VTES) = 7 10 4. Infrared spectra of coatings coated as to a Si wafer were measured using an FT-IR spectrophotometer. The compositions of this solution were TEOS : VTES = 1: 9, 5: 5, and 9: 1 EtOH/(TEOS + VTES) = 2, water/(teos + VTES) = 4, and HCl/(TEOS + VTES) = 0.01. The solution was stirred for 1 h at room temperature and then spin coated onto a Si wafer at 1000 rpm for 30 s and dried at various temperatures for 30 min. In order to investigate the coating ability of the solutions onto polymer films, the contact angle between the solution and PP and PET (Yukong) films was examined using the different solution compositions. Also, poly(4-hydroxystrene) (Aldrich) was precoated on the films as a wetting agent. The cross-section of the coating films was observed by SEM in order to investigate the adhesion between coatings and polymer films. 3. Results and Discussion 3.1. NMR Spectroscopy The notation of Si environments in NMR spectroscopy is represented as follows. The silicon in VTES is represented by Tm n and the silicon in TEOS is represented by Q n m where n denotes the number of Si O Si bonds surrounding the silicon and m denotes the number of OH bonds. Figure 1 shows 29 Si NMR spectra for the TEOS- VTES system. According to the chemical shifts, the Figure 1. 29 Si NMR spectra of solutions of the TEOS-VTES system for reaction times: (a) 30 min; (b) 1hand(c)2h.
Coating of TEOS-VTES Hybrid Solution 411 peaks a, b, c, d are assigned to monomeric species of VTES: T3 0,T 2 0, T 1 0, T 0 0, respectively, and the peaks e, f, g, h, i are assigned to monomeric species of TEOS: Q 0 4, Q 0 3, Q0 2, Q0 1, Q0 0, respectively. In addition to the peaks of monomeric species, the peaks of dimeric species, peaks j and k in the T 1 region, and the peak m in the Q 1 region, are shown in the spectra for 30 min reaction. As the reaction proceeded, new strong dimeric species denoted peak l, in Q 1 region, appeared. Because these peaks are dependent on the structures of two silicons bonded to each other, they can provide assignment of the species produced by self-condensation and cocondensation. The assignments of dimeric species are based on the distribution of the monomeric species and the behaviour of signal intensities. Thus, the assignments are based on the assumption that two signals of dimeric species in the T 1 and Q 1 regions, having different environments of silicon, should have similar intensities in the spectra for the co-condensation reaction. One can expect two components of self-condensation and co-condensation in the T 1 region assuming perfect hydrolysis. One component for self-condensation, T2 1 in T 2 1 T 2 1, will be independent of the intensity of any other resonance in the spectrum, while the other component for co-condensation, T2 1 in T 2 1 Q1 3, will always have the same intensity of one component for co-condensation, Q 1 3 resonance in Q1 3 T 2 1, in the Q1 region. The other component for self-condensation in the Q 1 region is Q 1 3 in Q1 3 Q1 3. In the T 1 region, the intensity of peak k is stronger than that of peak j. Peak j has the same intensity as peak m, which is in the Q 1 region. Also, the T3 0 species has the strongest intensity in the T 0 regions. Thus, peak k is produced by self-condensation of T3 0 and can be assigned to T2 1 T 2 1. It is also expected that peak j may be assigned to T2 1 in T 2 1 Q1 3 and peak m may be assigned to the Q 1 3 resonance in Q1 3 T 2 1, both by co-condensation. As the reaction proceeded, peak l which is another component from self-condensation in the Q 1 region, Q 1 3 Q1 3, appeared and grew very rapidly. Therefore, self-condensation of VTES was dominant in the initial condensation stage because of more rapid hydrolysis of VTES compared to that of TEOS. However, the co-condensation and the self-condensation of TEOS became the dominant reaction mechanisms as the reaction proceeded further. 3.2. FT-IR Spectroscopy Figure 2 shows FT-IR spectra of the films coated on an Si wafer at various TEOS-VTES ratios as well as Figure 2. FT-IR spectra of the TEOS-VTES coating films dried at different temperatures for 30 mins. drying temperatures. The bonds around 1080 cm 1 and 1120 cm 1 are assigned to the asymmetric stretching absorption of the TO mode and LO mode of the Si O Si bond, respectively. The band at about 775 cm 1 is due to symmetric stretching of the Si O Si bond [5]. As the drying temperature and the TEOS content of the solution increased, the intensity of the absorption peak of the TO mode increased but that of the LO mode decreased. Thus, the more the Si O Si network was formed, the stronger the intensity of the band
P1: TPR Journal of Sol-Gel Science and Technology 412 KL648-69-JOOEO October 31, 1998 16:22 Eo et al. Table 1. Contact angles between the solution and polymer films (±4 ). Bare PP Bare PET PP on PHS PET on PHS TEOS : VTES = 0 : 10 39 20 TEOS : VTES = 1 : 9 39 19 TEOS : VTES = 5 : 5 42 20 TEOS : VTES = 9 : 1 38 18 TEOS : VTES = 10 : 0 41 21 due to the TO mode peak. The position of the TO mode band shifted to higher wavenumber and the intensity of the band at 910 cm 1, assigned to the Si OH bond, decreased with increasing drying temperature. This indicates that the polymerization of the Si O Si network by the condensation of Si OH groups was enhanced. On the other hand, the band at about 1400 cm 1 related to the Si C bond and that at 1600 cm 1 due to the C C, bond, which were more intense for a higher content of VTES in the solution, were diminished with increasing drying temperature. Also, the other bands around 3000 cm 1 related with C H bonds, which were more prominent with increasing VTES content, disappeared as the drying temperature was increased. This is due to the decomposition of these groups during the drying process. 3.3. Wetting and Adhesion on Polymer Films Wetting is a necessary condition for solution coating. Solution coating onto polymer films is difficult due to the film hydrophobicity. Table 1 presents the composition dependence of the contact angles between coating solutions and PP and PET films. It is shown that the contact angles of the solution and PP were 38 42 and for PET were 18 21 regardless of the composition of the TEOS-VTES solution. Thus, the inorganicorganic hybridization does not effect the wetting on polymer films. However, all the films precoated with PHS, which is commonly used as a wetting agent, were completely wetted by the TEOS-VTES hybrid solution. This means that PHS can be used as a wetting agent for TEOS-VTES solution coating on polymer films. Figure 3 shows the SEM photographs of crosssections of ormosil coatings on PET films. It is shown that the coating films were uniform and had a very smooth surface regardless of their solution composition. However, the coatings containing a low VTES content were not attached sufficiently, but the coatings containing a high VTES content were fully attached to the PET substrate. Thus, VTES acts as an effective adhesive joint material between PET films and a SiO2 network. Figure 3. SEM micrographs of the cross-section of coating films on PET (a) TEOS : VTES = 1 : 9, (b) TEOS : VTES = 5 : 5, (c) TEOS : VTES = 9 : 1.
Coating of TEOS-VTES Hybrid Solution 413 4. Conclusion 29 Si-NMR spectroscopy of the hydrolysis and initial condensation process of the TEOS-VTES hybrid solutions showed that TEOS and VTES are hybridized. In the FT-IR spectra, the intensity of the absorption band of the TO mode of Si O Si bond became dominant with an increasing degree of network connectivity; decomposition of the Si C, the C C and the C H bond was found to occur during the drying process. The wettability of the solution was independent of the solution composition. However, the solution underwent complete wetting when a precoating of PHS was placed on the polymer films as a wetting agent. Adhesion between the TEOS/VTES solution coating film and the PET substrate was enhanced with increasing VTES content of the hybrid solution. References 1. G. Phillips and H. Schmidt, J. Non-Cryst. Solids 63, 283 (1984). 2. G.L. Wilkes, B. Orler, and H. Huang, Poly. Prepr. 26, 300 (1985). 3. K. Tadanaga, K. Iwashita, and T. Minami, J. Sol-Gel Sci. Tech. 6, 107 (1996). 4. T. Iwamoto and J.D. Mackenzie, J. Mat. Sci. 30, 2566 (1995). 5. D.L. Ou and A.B. Seddon, J. Non-Cryst. Solids 210, 187 (1997).