The easy answer is that a thermoset plastic cannot be remoulded after being cured, while a thermoplastic one can be reheated and remoulded.
As many questions related to materials engineering, to answer that we must zoom in a little bit and understand a bit more about chemistry at a molecular level.
- In a thermoplastic, strong bonds link monomers into polymer chains, however, these long monomers are joined to one another by weak bonds! These bonds can easily break apart when the plastic is heated and quickly reform again as it cools.
- Thermosetting plastics, on the other hand, have monomers that are cross-linked, thus, have extremely strong bonds!
Polymer matrix composites can have either thermoplastic or thermoset matrices. We will talk more about their differences in the future!