Thermoplastics vs. Thermosets

Plastics fall into two families based on their molecular architecture. Thermoplastics like polyethylene and PET are held together by weak intermolecular forces, so they can be melted and reshaped over and over — and recycled again and again.

Thermosets like epoxy and vulcanized rubber form permanent cross-links during curing. Once set, they cannot be melted: heating them causes them to decompose instead of soften. That makes them stronger, but impossible to recycle conventionally — so they end up in landfills.