When selecting a plastic packaging material for use, it is important to distinguish between the two basic categories, 1)
thermoplastic and 2) thermoset.
Thermoplastics
can be repeatedly softened by heating and hardened by cooling in order to reprocess the plastic in a moulding or extrusion operation, Conversely, thermosets, which have been formed and cured by heat, remain substantially infusible and insoluble; they stiffen with the application of heat. Thus, thermoplastics can be readily recycled back into the original form, and thermosets cannot.
Although
thermosets
cannot be recycled, they often can be used for other applications. For example, flexible polyurethane foams are commonly shredded into a small particle size before being compressed into rebounded polyurethane foam for carpet underpayment. That kind of material can be reused for packaging material or sound absorption, but it is a heavy material.
Both PP and PE are thermoplastic, PU is a thermoset.