Thermosets are polymers that harden and do not soften again when heated. The cross-links between molecules form a 3D structure, becoming highly resistant to mechanical effects and loads thanks to this structure.

They are relatively more brittle materials since their shapeshifting abilities are much lower than thermoplastics. This is the indication of their low fracture strength. They have very good resistance to high temperatures. As the temperature increases, the product neither softens nor flows, but burns directly. Resins, polyurethane, bakelite, cellulose and polybutadiene can be listed as the examples of thermoset plastics.