Modifications Of Preceramic Polymers Suitable For Corrosion Resistant And High Temperature Coatings
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The chemical methodology was initially developed to synthesize, modify, or cure polymeric precursors to Si3 N4 , Si-C-N, Si2ON 2 , Si0 2 , Si-O-C, and SiC [4]. The methodology is based on dehydrocoupling reactions or more recently on a combination of sequential hydrosilylation and dehydrocoupling reactions performed in a one-pot reaction. Both reactions are catalyzed by transition metal compounds (Ru and Pt) that are extremely efficient and can reach rates of 100,000 turnovers per hour. The required level of catalyst is extremely low, in the range of a few to hundreds of ppm. The chemistry allows for (a) a generic synthesis approach for polysilazanes, polysiloxanes, and polysiloxazanes; (b) synthesis of novel polymers; (c) simple modification of polymers; and (d) processing of polymers including curing [5] and pyrolysis [6]. Figure 1 illustrates the generic nature of SRI's approach. The polymers can form polymeric materials with unique properties (especially high-temperature and chemical stability) and functional materials with tailored properties (hydrophobic, hydrophilic, or reactive coatings). The preceramic polymers can be deposited similarly to conventional paints as transparent or powder-filled coatings on metals, glasses, ceramics, and even organic polymeric materials. Some polymers are very inexpensive and are candidates for replacing organic polymeric materials in harsh environments. The polymers can be cured directly by the dehydrocoupling reaction with moisture, adding hydrosilylation reagents, or by incorporating latent reactive groups onto the polymer backbone, meaning that processing needs can be addressed and the final properties can be controlled [5]. The
281 Mat. Res. Soc. Symp. Proc. Vol. 576 © 1999 Materials Research Society
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Figure 1. Generic scheme of dehydrocoupling EXPERIMENTAL A commercially available PHMS was used as the base polymer for all the discussed modifications (PS 120, United Chemical Technologies, Inc.) The PHMS has a low molecular weight (MW = 2270) and very low viscosity (30 cps). The polymer is dried before reaction except when water is added as a reactant to form hydroxyl species or enhance crosslinking. Ruthenium carbonyl catalyst was supplied by Strem, Inc. The chemical procedure described below is typical for a modified PHMS with mixed alkoxy/hydroxy substitution. Synthesis of 20% Ethoxy-PHMS-OH PHMS (100 g) and 0.05 g Ru 3 (CO) 12 were added to 120 g of EtOH in a I-liter flask. The alk
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