Gary Thomas Burns

Alkyl- and arylsilsesquiazanes: effect of the R group on polymer degradation and ceramic char composition

A series of alkyl- and arylsilsesquiazanes [RSi(NH)3/2] was prepared by the ammonolysis of the corresponding RSiCl3. Each polymer was pyrolysed to 1200° C in an inert atmosphere to give amorphous Si-C-N ceramic chars. The major volatile products in the pyrolysis effluent were identified and quantified by a combination of TGA-GCMS, TGA-GCFTIR and pyrolysis-capillary GC techniques. Mechanistic aspects correlating the alkyl/aryl group on the polymer structure, decomposition and ceramic char composition were studied. Oxidative stability of the ceramic chars fits percolation theory. The oxygen contents of ceramic chars containing ⩽ 25 wt% carbon increased 1 to 2% after 12 h at 1200° C. At higher levels of carbon the chars were completely oxidized to SiO2.

Synthesis and use of colloidal silica for reinforcement in silicone elastomers

Abstract Aqueous suspensions of colloidal silicas are readily silylated with either chlorosilanes or disiloxanes in the presence of acid and isopropyl alcohol without aggregation of the silica particle. By using a mixture of chlorosilanes or disiloxanes, spherical nanoparticles with controlled functionality can be made and transferred to an organic phase to provide stable, water free suspensions. The hydrophobic silica particles readily disperse into silicone polymers. At sufficient loading levels, they provide mechanical reinforcement comparable to traditional fillers but with improved clarity and lower viscosities. Modulus and durometer control in the cured elastomer is possible by varying the ratio of the vinyl concentration on the filler particle to the vinyl concentration in the polymer phase.

Reinforcement of Silicone Elastomers with Treated Silica Xerogels: Silica—Silicone IPNs

Abstract The use of silylated xerogels as reinforcing fillers in silicone elastomers was investigated. The silylated xerogels used in this study were mesoporous solids with a high degree of surface treatment and an open, interconnected high porosity. Some macropores were also present in the original xerogel particle. During compounding the original size of the xerogel particles is reduced substantially by fracture through the macroporous regions. The mesoporous regions of the xerogel are retained and exist as a uniform dispersion of filler particles in the siloxane polymer phase. The absence of a bound rubber phase, and the presence of an open porosity in the xerogel suggests reinforcement occurs via a silica—silicone interpenetrating network. This creates additional chain restrictions which lead to increases in modulus and other mechanical properties.

Polysilacyclobutasilazanes: pre-ceramic polymers for the preparation of sintered silicon carbide monoliths

A family of pre-ceramic polymers based upon silacyclobutasilazanes was prepared. Upon heating to 200–250 °C the polymers crosslink to intractable resins through a ring-opening polymerization of the silacyclobutyl group. In an inert atmosphere the polymers convert to Si-C-N-O chars upon pyrolysis to 1200 °C. At higher temperatures (> 1400 °C) the Si-C-N-O chars loose nitrogen and carbon monoxide to give stable chars containing only silicon carbide and carbon. The polymers were used as binders in press- and-sinter and transfer-moulding applications to give silicon carbide monoliths with sintered densities above 3.13 g cm−3 and four-point flexural strengths above 70 kpsi (483 MPa).

Silicon Carbide Preceramic Polymers as Binders for Ceramic Powders

While a variety of organosilicon polymers have been prepared over the years as precursors to silicon carbide ceramics, only a few have been used with silicon carbide powders in traditional ceramic processing. Of these polymer systems none have the design flexibility to allow for their use with ceramic powders over a range of polymer levels. For example, polycarbosilane has both a fixed polymer rheology and ceramic composition. Some of the focus within the Dow Corning Advanced Ceramics Program has been to develop a family of preceramic polymers that in combination with ceramic powders are useful for the preparation of monolithic ceramics. This paper describes the development of polymers that provide specific ceramic compositions and controlled rheologies making them useful as polymeric ceramic binders. A discussion of the polymer families developed, their physical and ceramic properties, the ceramic processing employed (die pressing, isopressing, injection molding, etc.) and some of the properties of the resulting ceramics is presented.