Sol-gel preparation of non-hygroscopic siliceous thin films enriched with alkaline-earth ions

Thin film layers of silica with calcium or magnesium ions were prepared by the sol-gel method in order to obtain protective coatings with low water affinity for historical glasses. The films were synthesised by pre-polymerised solutions of tetraethoxysilane [Si(OC2H5)(4)] (TEOS) and deposited by dipping soda-time glasses. Then they were immersed in saturated solutions of Ca(OH)(2) or Mg(OH)(2) for time intervals of 1, 24 and 36 h to adsorb calcium or magnesium ions. No heat treatment was done, to apply this method to cover real historical glasses, which could be irreversibly damaged by high temperature. The X-ray photoelectron spectroscopy and secondary ions mass spectrometry analyses show that the films present an almost homogeneous in depth distribution of calcium or magnesium. Magnesium adsorbed films also present a decrease of free non-bridging oxygen (SiOH groups). By atomic force microscopy it was observed that the average roughness is low (around 10 Angstrom). Contact angle measurements reveal a degree increase starting from soda-lime uncoated glasses, through silica coated glasses up to calcium or magnesium adsorbed silica coated glasses. The data obtained revealed a real decrease of water affinity, strictly important to improve the protective performances of the films. (C) 2003 Elsevier B.V. All rights reserved.