The synthesis, stabilization and sintering of high performance semicrystalline polymeric powders

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Virginia Tech


Aqueous dispersion prepregging, a relatively new method for forming graphite reinforced, polymer matrix composites, was investigated. This methodology could circumvent many of the environmental and processing problems prominent in the current state of the art. Aqueous dispersion prepregging requires that the matrix resin be in the form of a stable colIoidal dispersion, preferably of small uniform particles. Formation of submicron particles from poly(ether ether ketone) (PEEK) for use in aqueous dispersion prepregging was demonstrated. The procedure involved synthesis 4,4'-difluoro (N-benzohydroxylidene aniline) followed by the step polymerization of this monomer and hydroquinone via aromatic nucleophilic substitution to 'form an amorphous PEEK derivative, poly(ether ether ketimine). This monomer can also be statistically copolymerized with 4,4'-difluorobenzophenone to afford a semicrystalline, soluble PEEK derivative. Acid catalyzed hydrolysis of these derivatives to insoluble PEEK can be used to generate submicron particles.

A high performance stabilizer, which was used for suspending PEEK particles in water, has also been developed. This facilitated the development of processes for aqueous dispersion prepregging. The stabilizer is a poly(arylene ether) copolymer formed from 4,4'-difluoro- (N-benzohydroxylidene aniline), 2,6-dichloropyridine, and hydroquinone.

Poly{ ether ether ketone) has excellent mechanical properties as well as solvent resistance. However, it has been reported that under the recommended high temperature melt processing conditions the material may degrade via branching and crosslinking. An alternative to melt processing PEEK is to apply the powder metallurgy technique of sintering. This involves cold (room temperature) compaction of the polymeric powder, followed by pressure free sintering of the resultant green body. Sintering occurs due to a reduc1tion of surface free energy, and in this regard small particles (large surface area) have a large driving force for sintering. Pressure free sintering of PEEK par1icles with emphasis on the development of mechanical properties such as stiffness and strength as a function of sintering time, temperature, and particle size was examined. The data was analyzed using the two particle model developed by Frenkel and the crack healing theory developed by Wool. The latter is based on the reptation theory of de Gennes and Doi and Edwards. Sintering conditions were established which allowed for the attainment of comparable mechanical behavior to conventional processes.