What is the difference between a thermoplastic and a thermoset?

• A. Thermoplastics can be heated and formed; thermosets cannot be reheated and reformed.
• B. Thermoplastics cannot be heated; thermosets can be reheated.
• C. Thermoplastics are always rigid; thermosets are flexible.
• D. Thermoplastics are recycled by melting; thermosets are not.

Answer: (A)

The key distinction is how heat affects the two types. Thermoplastics are not permanently set by their structure, so when heated they soften, flow, and can be reshaped; once cooled they harden again and can be reheated for reshaping. This makes recycling by melting feasible. Thermosets, in contrast, cure into a highly crosslinked network that locks the molecules in place, so they don’t melt when heated; they maintain their shape but will decompose or burn if heated too much, and they cannot be reformed by heating. So the defining difference is reheatable, reshaping behavior versus irreversible crosslinked rigidity.