Thermoplastics can be subdivided into two distinct classes based upon differences in theirmolecular structure. These differences can have a significant bearing on the performanceof mouldings in service, and have a marked effect on the behaviour of the materialduring processing.Materials such as polystyrene, polycarbonate, acrylics, SAN, ABS and PVC are said to beamorphous thermoplastics. This signifies that in the solid state their molecular structureis random and nonordered, the long-chain molecules being all entangled rather likesolidified strands of spaghetti.Materials such as most of the Nylons, acetal, polypropylene, polythene and thethermoplastic polyesters, have a much more ordered structure in the solid state thanamorphous materials. They have a considerable proportion of the long-chain moleculespacked closely together in regular alignment. These materials are known assemicrystalline thermoplastics. It should be noted, however, that with both types ofmaterial the molecular structure is amorphous at melt temperature.
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