Sol-gel synthesis and low-temperature deposition of thin SiO2 layers as scratch resistant coatings for functional films

In recent years, functional window films have gained much interest. These windows films can be used for a wide variety of applications. Some common examples of these applications are UV-blocking, window tinting, insulation and shatter resistance. Most of the window films are functionalized by thin layers on a flexible substrate. It is also known that these thin layers are not stable in air and prone to deterioration caused by moisture and abrasion. This drawback can be overcome by the deposition of an additional scratch resistant coating.\n\nA thin silica coating was deposited on a Polyethyleneterephtalate (PET) film with a thickness of 60µm, functionalized with a thin metal oxide layer. This oxide layer was deposited by sputtering and is often used for functional window films. It is shown by contact angle measurements that the silica precursor and the metal oxide have a good chemical compatibility. The precursor composition was modified to be applicable in industrial settings by keeping chlorides out of the reaction mixture. The effect of organic acids compared to the effect of hydrochloric acid on the sol-gel chemistry1,2 was inquired by studying the kinetics of the condensation reactions.\n\nSecondly, the drying behavior of the precursor solution was optimized for coating. This was done by introduction of a precondensation step and a final quenching step. These additional steps were monitored by 29Si-NMR3,4. It is shown that these steps lead to a better condensation of the precursor after deposition and a decrease in drying time. Subsequently, the shelf life of the precursor was looked into. It was shown that the precursor solution was sufficiently stable to assure continuous deposition during at least one working shift of eight hours.\n\nThe precursor solution was deposited onto the substrate by reverse gravure roll-coating5. The ratio between web speed and application speed was optimized to obtain sufficient wetting of the film. A temperature program for in-line drying of the coating is set up. The temperature program is important to transfer the results to industrial setups where the time available for drying is limited. One must keep in mind that the temperature should be high enough to obtain complete drying of the precursor solution while it should be low enough to avoid melting or breakdown of the organic substrate.\nThe deposited coatings were subject to application tests, which consisted of cross hatch (ASTM D3359), Pencil hardness (ASTM D3363) and visual light transmittance tests. The results showed that the deposited coatings had a good adhesion to the substrate. The pencil hardness turned out to be 3H and a drop in visual light transmittance of maximal 3% was recorded.\n\nThe determination of the thickness of a transparent coating on a transparent substrate is very difficult. Secondary ion mass spectrometry (SIMS) combined with optical profilometry has shown to be an ideal approach to tackle this problem. These methods have been used to correlate coating thickness with pencil hardness and adhesion onto the substrate.\n\nPatent pending\n\n[1] J. Zarzycki, Journal of Sol-Gel Science and Technology, 8, 17–22. \n[2] C. Brinker and G. Scherer, Journal of Non-Crystalline Solids, 1985, 70, 301–322. \n[3] T. Iwamoto, K. Morita and J. D. Mackenzie, Journal of Non-Crystalline Solids, 1993, 159, 65 – 72.\n[4] A. Depla, D. Lesthaeghe, T. S. van Erp, A. Aerts, K. Houthoofd, F. Fan, C. Li, V. V. Speybroeck, M. Waroquier, C. E. A. Kirschhock and J. A. Martens, The Journal of Physical Chemistry C, 2011, 115, 3562–3571.\n[5] F. C. Krebs, Solar Energy Materials and Solar Cells, 2009, 93, 394–412.\n