Organically Modified Silicates by the Sol-Gel Process
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ORGANICALLY MODIFIED SILICATES BY THE SOL-GEL PROCESS
H.SCHMIDT Fraunhofer-Institut ffr Silicatforschung, Neunerplatz 2 D-8700 Wfirzburg, F.R.G.
ABSTRACT The introduction of organic groups into inorganic networks by the sol-gel process opens the possibility for the preparation of new materials, and typical properties resulting from inorganic as well as from organic components may be combined. Some general aspects and different examples of material developments are reviewed.
1. INTRODUCTION Generally solid materials are prepared by the sol-gel process by hydrolysis and condensation of metal-organic network forming and network modifying monomers 11-1] . In order to get pure inorganic polymers it is necessary to remove every organic residue, as hydrolyzing groupings or organic solvents. Therefore generally only starting compounds with totally hydrolyzable organic groups are used. In the case of silicon the orthoesters are used, since the organic residues are =Si-O- bonded and the eSi-O-C• bond easily can be hydrolyzed. The use of unhydrolyzable =Si-Cbonded ligands leads to organic modification of the aSi-O-Si= network. If -CH3 or -C6H5 are used as modifying components, silicones are obtained [11-14] . The introduction of organic groups may change the material properties strongly: Substitution of one =Si-O- bridge (e.g. in fused silica) by one =Si-CH3 group (monomethylpolys~loxane resin) increasys the thermal to about 100.10 .K . The question expansion coefficient from 0.5.10 arises, whether it will be a general possibility to develop new materials by introducing other organofunctional groupings into siliceous or homologous networks. It was shown by Gulledge and Andrianov that basically it is possible to obtain solids by cocondensation of organo silicon with other network forming components like Ti, Al, P and others [15, 16 . Organically modified or additional network modified silicates are called heteropolysiloxanes in this paper. The preparation of multicomponent systems is strongly affected by the reactivity of the starting monomers and reaction conditions, since this may influence the condensation, the way of crosslinking and the material properties [17-20, 1] . The strong influence of the type of catalyst (HCI or NH 3) could be demonstrated in [21]. In connection with questions of application, technical aspects of the synthesis of materials and the manufacturing of products from these have to be taken into account. Thus, sometimes a complete line, starting with the synthesis of special monomers and ending up with the manufacturing procedure has to be built up, if new materials for practical use should be developed. In the following chapters some examples will be given to demonstrate how heteropolysiloxanes may be used to fulfilldemands for very special materials.
Hat. Reg. Soc. syrup. Proc. Vol.
32 (1984) Published by Elsevier Science Publishing OD., Inc.
328
2.
RESULTS
2.1. General aspects of the introduction of organic groups The aSi-C- bond opens the possibility to introduce any organic group Y into a
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