Which description best characterizes thermosetting polymers?

• A. Polymers that have no cross-links.
• B. Materials that become ductile at high temperature.
• C. Plastics that chemically bond with cross-linking taking place and are then permanently hardened.
• D. Plastics that melt and flow upon heating.

Answer: (C)

Thermosetting polymers are defined by forming a chemical cross-linked network during curing, which bonds the chains together and locks the structure in place. This cross-linked network makes the material rigid and permanently hardened; after curing it does not melt or flow when heated. Heating a thermoset may lead to decomposition, not melting, because the network resists flow.

That’s why the description describing plastics that chemically bond with cross-linking taking place and are then permanently hardened is the best fit. The other statements describe behaviors of thermoplastics or incorrect behaviors: having no cross-links corresponds to thermoplastics, which soften or melt upon heating; becoming ductile at high temperature doesn’t capture the networked, non-melting nature; and melting and flowing upon heating is characteristic of thermoplastics, not thermosets.