Sol-gel

Progress in Organic Coatings 45 (2002) 159–164

Sol–gel: a new tool for coatings chemistry
Steffen Hofacker a,∗ , Markus Mechtel a , Michael Mager b , Harald Kraus b
a

Bayer AG, Coatings, Colorants and Special Raw Materials, Building Q 1, D-51368 Leverkusen, Germany b Bayer AG, Central Research, Building Q 18, D-51368 Leverkusen, Germany Received 1 September 2001; received in revised form19 September 2001; accepted 20 September 2001

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–inorganichybrid 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 toproduce 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-cureof 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 moietiesin a hybrid material, and which would be preferably UV transparent, resulting in polymers with inherent UV stability suitable for outdoor coating applications. © 2002 Elsevier Science B.V. All rights reserved.
Keywords: Sol–gel; Organic–inorganic hybrid; Abrasion resistance; UV stability; Outdoor applications; Polyfunctional precursors

1. Introduction The sol–gel process [1] can be used toprepare inorganic as well as organically modified inorganic materials via hydrolysis and condensation of silicon or other metal alkoxides and organoalkoxysilanes, respectively. By using tetraalkoxysilanes like tetraethoxysilane (TEOS), only very thin ( 1 m) but “pure” inorganic sol–gel coatings can be obtained, whereas the cohydrolysis and cocondensation with organoalkoxysilanes leads to organicallymodified hybrid coatings with reduced brittleness and coating thicknesses significantly above 1 m can be realised. Those organically modified sol–gel glasses can roughly be divided into two major groups based on the type of organoalkoxysilane used: (a) non-functional organoalkoxysilanes, and (b) organofunctional alkoxysilanes. Typical examples of group (a) are organotri- and diorganodialkoxysilanessuch as CH3 –Si(OR)3 and (CH3 )2 Si(OR)2 (R = CH3 , C2 H5 ), respectively. Organofunctional alkoxysilanes of group (b) are mostly acrylic, methacrylic or
∗ Corresponding author. E-mail address: steffen.hofacker.sh@bayer-ag.de (S. Hofacker).

epoxy-functional organosilanes. These organofunctional alkoxysilanes usually undergo an additional crosslinking, before, during or after the formation of thesilica network by photochemical or thermal-induced polymerisation. The additional crosslinking of the organic moieties is expected to result in a higher crosslink density and better mechanical properties in comparison to sol–gel materials based on non-functional organosilanes. Many different combinations of alkoxides and organoalkoxysilanes have been prepared to produce coatings with improved...