what are the two basic categories of synthetic plastics?
['Low-density polyethylene (LDPE)', 'High-density polyethylene (HDPE)', 'Polypropylene (PP)', 'Polyvinyl chloride (PVC)', 'Polystyrene (PS)', 'Nylon, nylon 6, nylon 6,6.', 'Teflon (Polytetrafluoroethylene)', 'Thermoplastic polyurethanes (TPU)']