As polymers approach their ceiling temperature, thermal degradation gives way to complete decomposition. Certain polymers like PTFE, polystyrene and PMMA  undergo depolymerization to give their starting monomers, whereas others like polyethylene undergo pyrolysis, with random chain scission giving a mixture of volatile products. Where monomers are obtained, they can be converted back into new plastic (chemical or feedstock recycling),  whereas pyrolysis products are used as a type of synthetic fuel (energy recycling).  In practice, even very efficient depolymerisation to monomers tends to see some competitive pyrolysis. Thermoset polymers may also be converted in this way, for instance, in tyre recycling.