Polymer degradation
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".
File:Plastics market share en.svg|class=skin-invert-image|300px|thumb|alt=Pie chart showing 2013 European plastic demand by type|2013 European plastic demand by polymer type:
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, polypropylene, polyvinyl chloride, polyethylene terephthalate, polystyrene, polycarbonate, and poly. 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 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
Thermoplastic polymers must be heated until molten to be formed into their final shapes, with processing temperatures anywhere between 150-320 °C depending on the polymer. Polymers will oxidise under these conditions, but even in the absence of air, these temperatures are sufficient to cause thermal degradation in some materials. The molten polymer also experiences significant shear stress during extrusion and moulding, which is sufficient to snap the polymer chains. Unlike many other forms of degradation, the effects of melt-processing degrades the entire bulk of the polymer, rather than just the surface layers. This degradation introduces chemical weak points into the polymer, particularly in the form of hydroperoxides, which become initiation sites for further degradation during the object's lifetime.Polymers are often subject to more than one round of melt-processing, which can cumulatively advance degradation. Virgin plastic typically undergoes compounding to introduce additives such as dyes, pigments and stabilisers. Pelletised material prepared in this may also be pre-dried in an oven to remove trace moisture prior to its final melting and moulding into plastic items. Plastic which is recycled by simple re‑melting will usually display more degradation than fresh material and may have poorer properties as a result.
Thermal oxidation
Although oxygen levels inside processing equipment are usually low, it cannot be fully excluded and thermal-oxidation will usually take place more readily than degradation that is exclusively thermal. Reactions follow the general autoxidation mechanism, leading to the formation of organic peroxides and carbonyls. The addition of antioxidants may inhibit such processes.Thermal degradation
Heating polymers to a sufficiently high temperature can cause damaging chemical changes, even in the absence of oxygen. This usually starts with chain scission, generating free radicals, which primarily engage in disproportionation and crosslinking.PVC is the most thermally sensitive common polymer, with major degradation occurring from ~ onwards; other polymers degrade at higher temperatures.
Thermo-mechanical degradation
Molten polymers are non-Newtonian fluids with high viscosities, and the interaction between their thermal and mechanical degradation can be complex. At low temperatures, the polymer-melt is more viscous and more prone to mechanical degradation via shear stress. At higher temperatures, the viscosity is reduced, but thermal degradation is increased. Friction at points of high sheer can also cause localised heating, leading to additional thermal degradation.Mechanical degradation can be reduced by the addition of lubricants, also referred to as processing aids or flow aids. These can reduce friction against the processing machinery but also between polymer chains, resulting in a decrease in melt-viscosity. Common agents are high-molecular-weight waxes or metal stearates.
In-service degradation
Most plastic items, like packaging materials, are used briefly and only once. These rarely experience polymer degradation during their service-lives. Other items experience only gradual degradation from the natural environment. Some plastic items, however, can experience long service-lives in aggressive environments, particularly those where they are subject to prolonged heat or chemical attack. Polymer degradation can be significant in these cases and, in practice, is often only held back by the use of advanced polymer stabilizers. Degradation arising from the effects of heat, light, air and water is the most common, but other means of degradation exist.The in-service degradation of mechanical properties is an important aspect which limits the applications of these materials. Polymer degradation caused by in-service degradation can cause life threatening accidents. In 1996, a baby was fed via a Hickman line and suffered an infection, when new connectors were used by a hospital. The reason behind this infection was the cracking and erosion of the pipes from the inner side due to contact with liquid media.
Chlorine-induced cracking
which has been chlorinated to kill microbes may contain trace levels of chlorine. The World Health Organization recommends an upper limit of 5 ppm.Although low, 5 ppm is enough to slowly attack certain types of plastic, particularly when the water is heated, as it is for washing.
Polyethylene, polybutylene and acetal resin pipework and fittings are all susceptible. Attack leads to hardening of pipework, which can leave it brittle and more susceptible to mechanical failure.