Thermoplastics will melt before they start to chemically decompose. Whatever residue is in the plastic might be liberated by the heat (those residues will also migrate out at room temperature, so we tend to be somewhat careful about what residue ends up in products).

Thermoset plastics will not melt and will release all sorts of magnificent things when heated.

https://en.wikipedia.org/wiki/Thermoplastic

https://en.wikipedia.org/wiki/Thermosetting_polymer