Thermosetting

Thermosetting polymers, or thermosets, are polymers that irreversibly cure to form a rigid, three-dimensional network. After curing, they do not melt or flow with heat, unlike thermoplastics, and only degrade or char at high temperatures.

Curing and crosslinking occur when liquid or flexible resin systems are exposed to heat, catalysts, or other triggering conditions. During curing, reactive groups form bonds that link polymer chains into a network, creating a hard, solvent-resistant material. Curing can follow addition mechanisms (for example epoxies, phenolics, polyurethanes) or condensation mechanisms (for example melamine-formaldehyde, urea-formaldehyde).

Common thermosetting resins include epoxy, phenolic (novolac, resole), unsaturated polyester, silicone, melamine/formaldehyde, and urea/formaldehyde. Some polyurethane systems are thermosetting, as are certain polyimides and other specialty resins. Properties such as high heat resistance, chemical resistance, and dimensional stability arise from the crosslinked network, along with good electrical insulation. However, thermosets are typically more brittle than many thermoplastics and are not easily reprocessed after cure.

Processing methods encompass various molding and casting techniques, including compression molding, transfer molding, reaction injection molding, and resin transfer molding, as well as composite fabrication methods like lamination and filament winding. Curing temperatures and times vary widely, from ambient to several hundred degrees Celsius, depending on formulation and application.

Applications are broad, including coatings, adhesives, electrical insulation, printed circuit boards, laminates in composites, automotive and aerospace parts, and dental and other medical resins. End-of-life handling is challenging due to the crosslinked structure; recycling is often limited to mechanical grinding for fillers, energy recovery, or chemical recycling under severe conditions, with ongoing research into reworkable or recyclable thermosets. The discovery of Bakelite in 1909 marks the early development of this class.