Processing Amorphous and Semi-Crystalline Thermoplastics

6.1 Introduction

Thermoplastics can be subdivided into two distinct classes based upon differences in molecular structure. These differences can have a bearing on the performance of mouldings in service, and have a most significant effect on the behaviour of the material during processing. Materials such as polystyrene (PS), polycarbonate (PC), acrylics (PMMA), acrylonitrile-butadiene styrene (ABS) and polyvinyl chloride (PVC) are said to be amorphous thermoplastics. This signifies that in the solid state their molecular structure is random and disordered, the long chain molecules being

all entangled rather like solidified spaghetti.

Materials such as most of the nylons (PA), polyacetal (POM), polypropylene (PP), polyethylene (PE) and the thermoplastic polyesters (PET) have a much more ordered structure in the solid state, a considerable proportion of the long chain molecules being closely packed in regular alignment. These materials are known as semi-crystalline thermoplastics. It should be noted however, that with both the semi-crystalline and amorphous materials at sufficiently high temperature (this is when the material is in its melt state) the molecular structure is amorphous. Table 6.1 classifies some common materials into these two groups.

Most amorphous thermoplastics are transparent in their natural, unpigmented form, although ABS for example is an exception. Most semi-crystalline thermoplastics in their solid unpigmented form are translucent or an opaque white colour. It is interesting to observe (for example when purging injection moulding machines) that fully molten natural polypropylene or acetal are initially transparent, but as the

melt cools it clouds over becoming translucent in the case of polypropylene, and opaque white in the case of acetal. This clouding is due to the material’s molecular structure gradually rearranging itself from the tangled amorphous state in the melt to the more ordered semi-crystalline state in the solid.

The main differences in behaviour between the amorphous and semi-crystalline materials observed during injection moulding are:

a) Melting and solidification

Amorphous thermoplastics exhibit a progressive softening over a wide temperature span, whereas the semi-crystalline materials rapidly change from the solid melt condition over a quite narrow temperature band.

Conversely, when amorphous materials are cooled they slowly solidify over a wide range of temperature, as against the semi-crystalline plastics, which change from melt to solid over a narrow range of temperature.

b) Shrinkage

Amorphous thermoplastics display very low shrinkage when they solidify, typically between 0.5% and 1%. Semi-crystalline materials shrink very much more, usually between 1.5% and 5% depending upon the particular material.

The higher shrinkage with the semi-crystalline materials is due to the repeat units along the molecular chains being of such a form that they can pack very closely together in an ordered manner. By use of appropriate moulding conditions it is possible to vary the extent of the crystalline areas. For example, when semi-crystalline thermoplastics are moulded in hot moulds, cooling rates are slow allowing more time for the molecular chains to disentangle themselves and take up their crystalline formation. This results in a greater proportion of the material being in its crystalline state (higher crystallinity) giving a product with superior mechanical strength and dimensional stability, but with relatively high shrinkage.

If the same material is moulded in a cold mould, the more rapid cooling will inhibit the formation of crystalline areas. The resulting lower level of crystallinity will give the product inferior mechanical properties, and a lower shrinkage. This is accompanied by a tendency for dimensional instability and distortion during later service due to aftershrinkage.

The next section will describe the properties of some typical amorphous materials, semi-crystalline materials are discussed in Section 6.3.

6.2 Amorphous Plastics

This section gives

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