The Effect of Filler Chemistry and Impurities on Stability

It is well documented that transition metals such as chromium, copper, iron and vanadium can catalyse the degradation of polymers [132, 139, 144]. These metals promote the decomposition of hydroperoxides, which are important in the degradation mechanism of most polymers. 

Antioxidants often contain functional groups that are capable of interaction with the filler surface. This can result in antioxidant adsorption depending upon the surface chemistry of the filler and the type of antioxidant. Once adsorbed, the antioxidant becomes ineffective because it is unable to diffuse to, and react with, the radicals that cause polymer degradation. The amount of deactivated antioxidant can he significant, and the usual response in industry is to add more antioxidant to attain the required level of stability. However, that approach raises the cost of the compound significantly. Another commercial approach is to use an epoxy additive that preferentially adsorbs onto the filler surface, physically blocking antioxidant adsorption. That helps to reduce cost, but the epoxy additive is itself still a relatively expensive chemical. 

Although thermoplastics and thermoplastic composites are potentially easy and economical to recycle, in practice there are some impediments to the implementation of widespread recycling. The main one is that the used materials must be collected, separated and cleaned economically. This is feasible in some instances but often it is not. In general, polymers are immiscible with one another, and, if melt processed as a mixture, the result is phase separation to give domains of one polymer in the other. This morphology leads to rather poor mechanical properties. Therefore, there are efforts to find better separation techniques in order to avoid the problem or to use compatibilisers [152] that lower the interfacial tension, improve the adhesion of the two phases, and encourage smaller domains of the disperse phase. 

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