Polymer processing polysiloxane polymers

The efficiency of this process is usually less than one cross-link per peroxide molecule decomposed. To increase the cross-linking efficiency, small amounts of unsaturation are introduced into the polymer structure. We have already discussed EPDM polymers, which are essentially diene monomers copolymerized with ethylene-propylene (EPR) polymers. For polysiloxanes, copolymerization of small amounts of vinyl-methylsilanol greatly enhances cross-linkability (Equation 5.7). The unsaturation introduced into an otherwise saturated structure provides additional sites for cross-linking through chain reaction. 

The most widely used methods of preparing organo-polysiloxane polymers are based either on a reequilibration, on a hydrosilylation, or on a dehydrogenation process. Polydimethylsiloxanes with functional groups F may be classified in four categories [2] (Fig. 1). 

Problems concerning the hydrosilylation processes in polymer chemistry and material science were discussed in most of the abovementioned reviews as well as concentrate only on polymers in the others (28). On account of their practical importance, some classes of functionalized silicones, such as fiuorosiloxanes (29), liquid crystalline polysiloxanes (30), and silicone polyethers (31), were discussed separately. 

Figure 9.7 Suggested mechanism of compatibilization of polysiloxane/PVDFHFP blend using silyl peroxide as dual-function coupling agent. From H. Zhou et al.. Proceeding of the 15th annual meeting of The Polymer Processing Society, May 31-June 4, 1999.

Maxillofacial polymers include the chlorinated polyethylenes, polyethemrethanes, polysiloxanes (see Elastomers), and conventional acrylic polymers. These are all deficient in a number of critical performance and processing characteristics. It is generally agreed that there is a need for improved maxillofacial polymers that can be conveniently fabricated into a variety of prostheses (218,227,228). 

A useful and detailed comparison between specific examples of a polyether, a cationic polysiloxane and a polyquaternary compound is available [301]. This review includes details of practical application via various processing routes available for loose stock, tops, yarn, knitted garments and woven or knitted piece goods. As mentioned earlier no single polymer fulfils all requirements and combinations of different types are sometimes used. Some indication of this is given in Table 10.33. 

The presence of even small amounts of cyclic oligomers can be detrimental for the utilization of a polymer if the cyclics migrate out of the product during its use. Many commercial processes remove cyclic by extraction (e.g., with steam in polyamide production) or thermal devolatilization (polysiloxane). 

The crosslinking efficiency of the peroxide process can be increased for some systems by incorporating small amounts of a comonomer containing vinyl groups into the polymer. This approach is used for polysiloxanes by copolymerization with small amounts of vinyltrimethylsilanol... 

Peroxide crosslinking of the copolymer is more efficient than that of the homopolymer (Table 9-1). The process becomes a chain reaction (but with short kinetic chain length) involving polymerization of the pendant vinyl groups on the polysiloxane chains in combination with coupling of polymeric radicals. The crosslinking of EPDM rubbers is similarly more efficient when compared to EPM rubbers since the former contain double bonds in the polymer chain. 

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