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What is the difference between a thermoset and a thermoplastic?


The terms thermoset and thermoplastic have been traditionally used to describe the different types of plastic materials.

A thermoset allows only one chance to liquefy and shape it. With plastic materials in this group, shortly after the viscous melt stage is reached a further chemical reaction occurs which results in adjoining molecules linking together to form a three dimensional network. This process is known as cross-linking. The molecules linked in this manner will no longer flow again under the influence of heat and pressure, even though they are heated to the point where the material chars. These materials can be cured or polymerised using heat and pressure, or as with epoxies, a chemical reaction started by a chemical inhibitor. These types of plastics are generally rigid and are usually brittle and are usually mixed with reinforcing fillers to render them less brittle. Common members of the group are phenol formaldehyde (phenolics); urea formaldehyde (ureas); melamine formaldehyde (Melamines); polyester resins (polyesters); epoxy resins (epoxies) and polyimides.

A thermoplastic can be melted and shaped several times.  When plastics in this group are heated to the viscous melt stage, no cross-linking takes place between adjacent molecules, thus these materials are capable of being softened by heating many times. Thermoplastics range from soft, flexible materials to those which are hard and rigid, depending on the type of polymer and possibly the degree of modification it has received. The thermoplastic materials are either crystalline or amorphous but advances in chemistry have been made to the point where the distinction between them is less clear.

Again the advances in chemistry make it possible for a chemist to construct a material to be either thermoset or thermoplastic. The main difference between the two classes of materials is whether the polymer chains remain linear and separate after moulding or whether they undergo a chemical change and form a network by cross-linking. Due to recent advances in polymer chemistry, the exceptions to the rule are continually growing. These materials are actually cross-linked thermoplastics with the cross-linking occurring either during the processing or during the annealing cycle. The linear materials are thermoplastics and are chemically unchanged during moulding (except for possible degradation) and can be reshaped again and again. Cross-linking can be initiated by heat, chemical agents, irradiation or a combination of these factors. Theoretically, any linear plastic can be made into a cross-linked plastic with some modification to the molecule so that cross-links form in orderly positions to maximize properties.  The formulation of a material, cross-linked or linear, will determine the processes that can be used to successfully shape the material. Generally, cross-linked materials (thermosets) demonstrate better properties, such as improved resistance to heat, less creep and better chemical resistance than their linear counterpart; however, they will require a more complex process to produce a part, rod, sheet or tube.