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Thermosetting plastics, or thermosets, include amino, epoxy, phenolic, and unsaturated polyesters. These materials undergo a chemical change during processing and become hard solids. Unlike the linear molecules in a thermoplastic, adjacent molecules in a thermosetting plastic become cross-linked during processing, resulting in the production of complex networks that restrain the movement of chains past each other at any temperature.

Typical thermosets are phenolics, urea-formaldehyde resins, epoxies, cross-linked polyesters, and most polyurethanes. Elastomers may also be thermosetting. Examples include both natural and synthetic rubbers.

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Science EncyclopediaScience & Philosophy: Planck mass to PositPlastics - History, Chemistry, Polymerization, Manufacture And Processing, Thermoplastics, Crystalline And Noncrystalline Thermoplastics, Thermosets - Molecular weight