Which type of plastic becomes permanently set and cannot be remelted after curing?

Which type of plastic becomes permanently set and cannot be remelted after curing?

• A. Thermoplastic
• B. Elastomer
• C. Resin
• D. Thermoset

Answer: (D)

Permanently setting a plastic comes from chemical crosslinking during curing. When crosslinks form, the polymer chains become interconnected in a three‑dimensional network that locks the structure in place. Because of these bonds, the material can’t flow or melt when heated; instead, it resists deformation and may decompose if overheated. This is what makes thermoset plastics stay set after curing, such as epoxies, phenolics, and many urethanes. In contrast, thermoplastics don’t develop this extensive crosslinking, so heating softens and allows melting and reshaping. The term resin is broad and can refer to materials that cure into thermosets or remain thermoplastic, so it isn’t a precise descriptor here. Elastomers are rubbery polymers that can be either thermoplastic or thermoset; the important point for this question is the permanent, non-melting state that comes specifically from a crosslinked thermoset network.

Permanently setting a plastic comes from chemical crosslinking during curing. When crosslinks form, the polymer chains become interconnected in a three‑dimensional network that locks the structure in place. Because of these bonds, the material can’t flow or melt when heated; instead, it resists deformation and may decompose if overheated. This is what makes thermoset plastics stay set after curing, such as epoxies, phenolics, and many urethanes.

In contrast, thermoplastics don’t develop this extensive crosslinking, so heating softens and allows melting and reshaping. The term resin is broad and can refer to materials that cure into thermosets or remain thermoplastic, so it isn’t a precise descriptor here. Elastomers are rubbery polymers that can be either thermoplastic or thermoset; the important point for this question is the permanent, non-melting state that comes specifically from a crosslinked thermoset network.