Why Plastics Need to be Sorted

We have already seen that thermosets cannot be reprocessed in the same manner as thermoplastics, because they do not re-melt. Therefore separation is needed of thermosets and thermoplastics. But what about thermoplastics do they need to be separated from each other  

Can we simply just mix all the plastics together and reprocess them  

if we could do this, plastics recycling would certainly be much easier. In some cases it may be possible to simply mix and reprocess but generally the properties of mixed plastics are not very good. Plastics tend not to be very compatible with each other, due to differences in their chemical compositions. The polymer molecules repel each other rather than mixing, much like a mixture of oil and water. 

Polystyrene on its own is very brittle, whilst polybutadiene, as would be expected for a rubbery material, is a very good energy absorber and has the ability to bend instead of break. When these materials are blended, the result is a plastic which is much tougher than polystyrene alone. 

One important point the strength of these systems depend on how well one polymer is dispersed in the other. The better the materials can be dispersed, the better the properties of the blend will be. An additive called a compatibiliser can be used to assist one material to become very finely dispersed in the other. This will be discussed further in Chapter 7. 

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