Thermosets and Thermoplastics
This distinction is probably the most fundamental way of classifying plastics.Using this system the family of plastics materials is divided into two major branches, the thermosets (e.g. Bakelite) and the thermoplastics (e.g. Nylon, polythene or ABS).
The main difference between the products made from thermosets and those made from thermoplastics, apart from a few exceptions, is that thermoplastics components will always soften and eventually melt if subject to high temperatures, whereas the thermosets
will not. There is also a major difference in the techniques used to process these two classes of plastics materials. Thermoplastic materials have to be heated until they soften sufficiently to be moulded into the desired shape, after which they must be cooled in order to solidify and retain the required shape. If a thermoplastic product is reheated to its original processing temperature it will soften again.
The thermoset materials also soften when first heated,
which enables them to be moulded into shape, but instead of cooling, the material is then subject to additional heating. This additional heating causes a chemical reaction to take place within the material, which converts the hot, softened mass into an equally hot permanent solid. Further heating, even to a higher temperature, will not re-soften the thermoset product. The reason for the difference in behaviour between the thermoset and thermoplastic lies in the chemical composition of the molecular chains.
With the thermosets, the polymer in the moulding materials has roughly linear chain structure; this enables the material to behave rather like a thermoplastic when first heated, allowing it to soften and flow under pressure in order to take up the shape of the mould.
However, the structure of the thermoset polymer contains chemically reactive positions at intervals along the molecular chains, which facilitate joining or cross-linking between adjacent molecules provided that sufficient heat is applied. The cross-linking (sometimes called curing) converts the linear structure into an infusible three-dimensional network.
With thermoplastics the long-chain molecules in the moulding compound do not have reactive positions within the chain, so under normal moulding conditions no crosslinking can occur. The material can only be solidified by cooling and of course it will soften and
melt again if reheated.
Walsh's Plastic consulting. 