Steffen Hofacker

Sol–gel: a new tool for coatings chemistry

Abstract Improved mechanical and chemical resistance is in an increasing demand in a variety of coating applications. Since today’s high performance coating systems are always a compromise between elasticity on one hand and good abrasion resistance on the other hand, there is a strong need for new types of coating raw materials. Organic–inorganic hybrid materials offer the opportunity to combine the desirable properties of organic polymers (toughness, elasticity) with those of inorganic solids (hardness, chemical resistance). Many different attempts have been made to obtain intimate mixtures of organic and inorganic materials. Sol–gel processing, the hydrolysis and condensation of low molecular weight alkoxides, is one possible route to produce such desired molecular or nanoscopic mixtures. The commonly used sol–gel precursors are commercially available organofunctional or (non-functional) organoalkoxysilanes, which are widely used as coupling agents in fiber-reinforced polymers. Hybrid materials with promising mechanical properties were synthesised on various routes, but weathering data have only been published very rarely. Post-cure of sol–gel films produced from commonly used trialkoxysilanes in the presence of water or photochemical degradation of certain organofunctional alkoxysilanes under UV irradiation are possible mechanisms which may finally result in the failure of such a hybrid coating upon weathering. Due to this, new precursors were sought, which would be able to chemically link the inorganic and organic moieties in a hybrid material, and which would be preferably UV transparent, resulting in polymers with inherent UV stability suitable for outdoor coating applications.