Polysilazane thermosets

Polysilazane Thermosets as Precursors for Silicon Carbide and Silicon Nitride... 

None of the above described ring opening polymerization methods has, as yet, proved useful for the formation of polysilazane preceramic polymers. However, Si-N bond cleavage and reformation, as it occurs in reaction (13), is probably responsible in part for the curing or thermoset step in transition metal catalyzed dehydrocoupling polymerization of hydridosilazanes (31), as described below. 

This chapter discusses the development of thermosetting preceramic polymers, emphasizing peroxide-curable polysilazanes. These polymers are excellent precursors for both silicon nitride and silicon carbide. Vinyl-substituted polysilazanes may be readily thermoset with peroxide initiators. In addition, a new class of polysilazanes which contain peroxide substituents directly boxmd to the polymer has been developed The utility of peroxide-cured polysilazane precursors for the formation of silicon nitride articles has been demonstrated. 

Likewise, vinyl-substituted polysilazanes may be thermally crosslinked at elevated temperatures. A liquid oligovinylsilazane, (CH2=CHSiHNH)x, prepared by the ammonolysis of vinyldichlorosilane, thermoset to an infusible, insoluble solid after heating for 2 hours at 110°C (23,24). This solid had a char yield of over 85 wt% when pyrolyzed to 1200°C. For this system, infrared analysis showed that hydrosilylation, and not vinyl polymerization, was the predominant crosslinking mechanism. 

PolysUazanes with Vinyl and SiH Substituents. In contrast to polysilazanes containing only Si-CH=CH2 substituents, liquid polysilazanes containing both Si-H and Si-CH=CH2 substituents thermoset very rapidly (14,16,18,19). For example, an isocyanate-modified polysilazane (2) was prepared by the reaction of 0.5 wt%... 

Solid state NMR spectroscopy showed complete consumption of the vinyl moieties in the thermoset polymer 2. Pyrolysis of the cured polysilazane under NH3 from ambient to 1000 C, followed by heating under Ar to 1600°C gave a mixture of a- and P-Si3N4 19). In contrast, pyrolysis under Ar fh)m ambient to 1600°C gave P-SiC. Indeed, this thermoset polysilazane is an excellent precursor for silicon carbide as well as silicon nitride 19). 

A further extension of the concept of thermosetting polysilazanes with peroxide initiators has been the preparation of new polysilazanes with peroxide groups bound directly to the polymer 20). Potential advantages of a peroxide-substituted polysilazane over systems in which the peroxide is simply admixed with the polymer include 1) segregation of the peroxide upon storage cannot occur, 2) dissolution or dispersion of a peroxide in the polysilazane is not necessary, and 3) homogeneous distribution of e peroxide in solid, as well as liquid, polysilazanes is possible. We have prepared a new class of peroxide-substituted polysilazanes by the reaction of a hydroperoxide with a poly(methylvinyl)silazane. The liquid polymers may be thermoset, even with extremely low levels of peroxide substitution. This chemistry provides access to a class of polysilazanes previously unknown as ceramic precursors. 

The level of silylperoxide substitution required to cure 4 and produce a solid with a good char yield was investigated. The data in Table I show the maximum exotherm reached during cure and TGA results obtained from the vinyl-substituted polysilazane 4 reacted in hexane at room temperature with the indicated weight percentage of BuOOH. Accurate dispensing of the hydroperoxide was possible because it was supplied as a 3.0 M solution in isooctane. In each reaction, a 1-2°C exotherm and gas evolution (ammonia) accompanied the hydroperoxide addition. Following in vacuo removal of the hexane, each liquid polymer was thermoset in a... 

Thus, even very low levels of peroxide substituents on the polysilazane were effective in the thermoset cure. The TGA yields compare quite favorably with that obtained when [(MeSiHNH)o.8(CH2=CHSiMeNH)o.2]x W is mixed with 0.5 wt% dicumyl peroxide and heated under the same conditions. In that case, a char yield of about 75 wt% is observed. 

Despite the expectation that the silylperoxide substituents might be inefficient initiators, the vinyl-substituted polysilazanes containing such moieties thermoset readily producing highly crosslinked solids which had good pyrolysis yields. 

Preceramic polymers have the potential to improve traditional ceramic processing methods (2). In particular, ceramic injection molding may be enhanced by the replacement of organic binders with thermosetting polysilazanes. Table II compares the attributes of the two binder systems. 

The peroxide may be added to the polysilazane or may be bound directly to the polymer through a simple synthetic procedure. In either case, a system is produced which thermosets very rapidly at less than 200°C. Because they cure so... 

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