Polymer degradation

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 (i.e. without air).[4] Reactions follow the general autoxidation mechanism, leading to the formation of organic peroxides and carbonyls. The addition of antioxidants may inhibit such processes.

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 ~250 °C (480 °F) onwards;[5] other polymers degrade at higher temperatures.[6]

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 (paraffin wax, wax esters, etc.) or metal stearates (i.e.zinc stearate).

Chlorine attack on an acetal resin plumbing joint

Drinking water which has been chlorinated to kill microbes may contain trace levels of chlorine. The World Health Organization recommends an upper limit of 5 ppm.[8] 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,[9][10] polybutylene [11] and acetal resin (polyoxymethylene)[12] pipework and fittings are all susceptible. Attack leads to hardening of pipework, which can leave it brittle and more susceptible to mechanical failure.

Plastics are used extensively in the manufacture of electrical items, such as circuit boards and electrical cables. These applications can be harsh, exposing the plastic to a mixture of thermal, chemical and electrochemical attack. Many electric items like transformers, microprocessors or high-voltage cables operate at elevated temperatures for years, or even decades, resulting in low-level but continuous thermal oxidation. This can be exacerbated by direct contact with metals, which can promote the formation of free-radicals, for instance, by the action of Fenton reactions on hydroperoxides.[13] High voltage loads can also damage insulating materials such as dielectrics, which degrade via electrical treeing caused by prolonged electrical field stress.[14][15]

Polymer degradation by galvanic action was first described in the technical literature in 1990 by Michael C. Faudree, an employee at General Dynamics, Fort Worth Division.[16][17] The phenomenon has been referred to as the "Faudree Effect",[18] and has had implications for preventing corrosion on the YF-22 (F-22 prototype) aircraft for safety such as changes in design.[19][20] When carbon-fiber-reinforced polymer is attached to a metal surface, the carbon fiber can act as a cathode if exposed to water or sufficient humidity, resulting in galvanic corrosion. This has been seen in engineering when carbon-fiber polymers have been used to reinforce weakened steel structures.[21][22] Reactions have also been seen in aluminium[23] and magnesium alloys,[24] polymers affected include bismaleimides (BMI), and polyimides. The mechanism of degradation is believed to involve the electrochemical generation of hydroxide ions, which then cleave the amide bonds.[25]

Photo-oxidation is the combined action of UV-light and oxygen and is the most significant factor in the weathering of plastics.[27] Although many polymers do not absorb UV-light, they often contain impurities like hydroperoxide and carbonyl groups introduced during thermal processing, which do. These act as photoinitiators to give complex free radical chain reactions where the mechanisms of autoxidation and photodegradation combine. Photo-oxidation can be held back by light stabilizers such as hindered amine light stabilizers (HALS).[30]

Polymers with an all-carbon backbone, such as polyolefins, are usually resistant to hydrolysis. Condensation polymers like polyesters,[31] polyamides, polyurethanes and polycarbonates can be degraded by hydrolysis of their carbonyl groups, to give lower molecular weight molecules. Such reactions are exceedingly slow at ambient temperatures, however, they remain a significant source of degradation for these materials, particularly in the marine environment.[32] Swelling caused by the absorption of minute amounts of water can also cause environmental stress cracking, which accelerates degradation.

Ozone cracking in Natural rubber tubing

Polymers, which are not fully saturated, are vulnerable to attack by ozone. This gas exists naturally in the atmosphere but is also formed by nitrogen oxides released in vehicle exhaust pollution. Many common elastomers (rubbers) are affected, with natural rubber, polybutadiene, styrene-butadiene rubber and NBR being most sensitive to degradation. The ozonolysis reaction results in immediate chain scission. Ozone cracks in products under tension are always oriented at right angles to the strain axis, so will form around the circumference in a rubber tube bent over. Such cracks are dangerous when they occur in fuel pipes because the cracks will grow from the outside exposed surfaces into the bore of the pipe, and fuel leakage and fire may follow. The problem of ozone cracking can be prevented by adding antiozonants.

The major appeal of biodegradation is that, in theory, the polymer will be completely consumed in the environment without needing complex waste management and that the products of this will be non-toxic. Most common plastics biodegrade very slowly, sometimes to the extent that they are considered non-biodegradable.[26][33] As polymers are ordinarily too large to be absorbed by microbes, biodegradation initially relies on secreted extracellular enzymes to reduce the polymers to manageable chain-lengths. This requires the polymers bare functional groups the enzymes can 'recognise', such as ester or amide groups. Long-chain polymers with all-carbon backbones like polyolefins, polystyrene and PVC will not degrade by biological action alone[34] and must first be oxidised to create chemical groups which the enzymes can attack.[35][36]

Oxidation can be caused by melt-processing or weathering in the environment. Oxidation may be intentionally accelerated by the addition of biodegradable additives. These are added to the polymer during compounding to improve the biodegradation of otherwise very resistant plastics. Similarly, biodegradable plastics have been designed which are intrinsically biodegradable, provided they are treated like compost and not just left in a landfill site where degradation is very difficult because of the lack of oxygen and moisture.[37]

Thermoplastic polymers like polyolefins can be remelted and reformed into new items. This approach is referred to as mechanical recycling and is usually the simplest and most economical form of recovery.[3] Post-consumer plastic will usually already bare a degree of degradation. Another round of melt-processing will exacerbate this, with the result being that mechanically recycled plastic will usually have poorer mechanical properties than virgin plastic.[40] Degradation can be enhanced by high concentrations of hydroperoxides, cross-contamination between different types of plastic and by additives present within the plastic. Technologies developed to enhance the biodegradation of plastic can also conflict with its recycling, with oxo-biodegradable additives, consisting of metallic salts of iron, magnesium, nickel, and cobalt, increasing the rate of thermal degradation.[41][42] Depending on the polymer in question, an amount of virgin material may be added to maintain the quality of the product.[43]

As polymers approach their ceiling temperature, thermal degradation gives way to complete decomposition. Certain polymers like PTFE, polystyrene and PMMA[44] undergo depolymerization to give their starting monomers, whereas others like polyethylene undergo pyrolysis, with random chain scission giving a mixture of volatile products. Where monomers are obtained, they can be converted back into new plastic (chemical or feedstock recycling),[45][46][47] whereas pyrolysis products are used as a type of synthetic fuel (energy recycling).[48] In practice, even very efficient depolymerisation to monomers tends to see some competitive pyrolysis. Thermoset polymers may also be converted in this way, for instance, in tyre recycling.

Condensation polymers baring cleavable groups such as esters and amides can also be completely depolymerised by hydrolysis or solvolysis. This can be a purely chemical process but may also be promoted by enzymes.[49] Such technologies are less well developed than those of thermal depolymerisation, but have the potential for lower energy costs. Thus far, polyethylene terephthalate has been the most heavily studied polymer.[50] Alternatively, waste plastic may be converted into other valuable chemicals (not necessarily monomers) by microbial action.[51][52]