Polymer degradation is the lowering of a polymer, such as strength, caused by changes in its chemical composition. Polymers and particularly plastics are subject to degradation at all stages of their product life cycle, including during their initial processing, use, disposal into the environment and recycling. The rate of this degradation varies significantly; biodegradation can take decades, whereas some industrial processes can completely decompose a polymer in hours.
Technologies have been developed to both inhibit or promote degradation. For instance, polymer stabilizers ensure plastic items are produced with the desired properties, extend their useful lifespans, and facilitate their recycling. Conversely, biodegradable additives accelerate the degradation of plastic waste by improving its biodegradability. Some forms of plastic recycling can involve the complete degradation of a polymer back into monomers or other chemicals.
In general, the effects of heat, light, air and water are the most significant factors in the degradation of plastic polymers. The major chemical changes are oxidation and chain scission, leading to a reduction in the molecular weight and degree of polymerization of the polymer. These changes affect physical properties like strength, malleability, melt flow index, appearance and colour. The changes in properties are often termed "aging".
class=skin-invert-image|300px|thumb|alt=Pie chart showing 2013 European plastic demand by type|2013 European plastic demand by polymer type:<br />PP: [[polypropylene, PE: polyethylene, PVC: Polyvinyl chloride, PS: Polystyrene, PET: Polyethylene terephthalate ]]
Susceptibility
Plastics exist in huge variety, however several types of commodity polymer dominate global production: polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), polyethylene terephthalate (PET, PETE), polystyrene (PS), polycarbonate (PC), and poly(methyl methacrylate) (PMMA). The degradation of these materials is of primary importance as they account for most plastic waste.
These plastics are all thermoplastics and are more susceptible to degradation than equivalent thermosets, as those are more thoroughly cross-linked. The majority (PP, PE, PVC, PS and PMMA) are addition polymers with all-carbon backbones that are more resistant to most types of degradation. PET and PC are condensation polymers which contain carbonyl groups more susceptible to hydrolysis and UV-attack.
Degradation during processing
thumb|right|475px|alt=See caption|Plastic compounding scheme
thumb|right|200px|alt=See caption|Short video on injection molding (9 min 37 s)
Thermoplastic polymers (be they virgin or recycled) must be heated until molten to be formed into their final shapes, with processing temperatures anywhere between 150-320 °C (300–600 °F)<!-- leave this please,