Thermoplastics vs. Thermoset Elastomers

Elastomers are commonly grouped into two major categories:

    Thermoplastic
    Thermoset

Structure

Thermoplastic Elastomers are materials that repeatedly soften/melt when heated and harden when cooled. Most thermoplastics are soluble in specific solvents and can burn to some degree. Softening/melt temperatures vary with polymer type and grade. Because of the heat/shear sensitivity of thermoplastics, care must be taken to avoid degrading, decomposing or igniting the material.



Most thermoplastic molecular chains can be thought of as independent, intertwined strings resembling spaghetti (see the figure above). When heated (for example, for molding), the individual chains slip, causing plastic flow. When cooled, the chains of atoms and molecules are once again held firmly. When subsequently heated, the chains slip again. There are practical limitations to the number of heating/ cooling cycles to which thermoplastics can be subjected before appearance and mechanical properties are affected.

Thermoset Elastomers undergo a chemical change during processing to become permanently insoluble and infusible. It is this chemical cross-linking that is the principal difference between thermoset and thermoplastic systems. Natural and synthetic rubbers such as latex, nitrile, millable polyurethane, silicone, butyl and neoprene, which attain their properties through a process known as vulcanization, are typical thermoset elastomers.

As can be seen by the diagram below, when thermosets are cured or hardened, cross-links are formed between adjacent molecules, resulting in a complex, interconnected network. These cross bonds prevent the individual chains from slipping, thus preventing plastic flow when heat is added. If excessive heat is added to the thermoset elastomer after the cross-linking is complete, the polymer is degraded rather than melted. This behavior is somewhat similar to an egg when it is cooked: further heating does not return the egg to its liquid state, it only burns.



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