Thermoset materials

Cross-linking is a chemical process in which chemical bonds form among molecules of thermosetting materials, resulting in an interconnected network. This cross-linking process is the principal difference between thermoplastics and thermosets.

Prior to molding, the chain-like structure of thermosets is similar to thermoplastics. During processing, with the activation of heat and/or chemical means, thermosets polymerize (react or cure) into a cross-linked microstructure. Once the reaction is completed, the polymer chains are bonded (cross-linked) together to form a three-dimensional network. These cross bonds among molecules prohibit the slippage of individual molecular chains.

Consequently, a thermoset becomes an infusible and insoluble solid that cannot be re-softened and reprocessed by the application of heat, without degrading some linkages. Thermosets are analogous to hard-boiled eggs, where the yolk has turned from a liquid to a solid and cannot be converted back to a liquid.

The table below compares the structure and properties of thermosets and thermoplastics.

Material	Thermoplastics	Thermosets
Microstructure	Linear or branch molecules. No chemical bonds among the molecules.	Cross-linking network with chemical bonds among molecules after the chemical reaction.
Reaction to Heat	Can be re-softened (physical phase change).	Cannot be re-softened after cross-linking without degradation.
General Properties	Higher impact strength. Easier processing. Better adaptability to complex designs.	Greater mechanical strength. Greater dimensional stability. Better heat and moisture resistance.