Thermoset, or thermosetting, plastics are synthetic materials that strengthen during being heated, but cannot be successfully remolded or reheated after their initial heat-forming. This is in contrast to thermoplastics, which soften when heated and harden and strengthen after cooling. Thermoplastics can be heated, shaped and cooled as often as necessary without causing a chemical change, while thermosetting plastics will burn when heated after the initial molding. Additionally, thermoplastics tend to be easier to mold than thermosetting plastics, which also take a longer time to produce (due to the time it takes to cure the heated material).
Thermosetting plastics, however, have a number of advantages. Unlike thermoplastics, they retain their strength and shape even when heated. This makes thermosetting plastics well-suited to the production of permanent components and large, solid shapes. Additionally, these components have excellent strength attributes (although they are brittle), and will not become weaker when the temperature increases.
Thermoset plastic products are typically produced by heating liquid or powder within a mold, allowing the material to cure into its hardened form. These products can be removed from the mold even without allowing it to cool. The reaction used to produce thermosetting plastic products is not always the result of heating, and is sometimes performed by chemical interaction between specialized materials. Typical types of thermosetting plastics are epoxies, polyesters, silicones and phenolics. Vulcanized rubber is also an excellent example of a thermosetting plastic; anyone who has ever driven an automobile can attest to the properties of a superheated tire—it burns but does not mold into a new shape.
Each type of thermosetting plastic has a unique set of properties. Epoxies, for example, exhibit elasticity and exceptional chemical resistance, and are relatively easy to cure. Phenolics, while fairly simple to mold, are brittle, strong and hard. Because of their wide range of characteristics, thermosetting plastics find use in an extensive variety of applications, from electrical insulators to car bodies.