| Polymer science |
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Polymer degradation is the lowering of apolymer, such as strength, caused by changes in its chemical composition. Polymers and particularlyplastics are subject to degradation at all stages of theirproduct life cycle, including during their initial processing, use, disposal into the environment and recycling.[1] 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 ofplastic waste by improving itsbiodegradability. Some forms ofplastic recycling can involve the complete degradation of a polymer back intomonomers 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 areoxidation andchain scission, leading to a reduction in themolecular weight anddegree of polymerization of the polymer. These changes affectphysical properties like strength,malleability,melt flow index, appearance and colour. The changes in properties are often termed "aging".
Plastics exist in huge variety, however several types ofcommodity polymer dominate global production:polyethylene (PE),polypropylene (PP),polyvinyl chloride (PVC),polyethylene terephthalate (PET, PETE),polystyrene (PS),polycarbonate (PC), andpoly(methyl methacrylate) (PMMA). The degradation of these materials is of primary importance as they account for mostplastic waste.
These plastics are allthermoplastics and are more susceptible to degradation than equivalentthermosets, as those are more thoroughlycross-linked. The majority (PP, PE, PVC, PS and PMMA) areaddition polymers with all-carbon backbones that are more resistant to most types of degradation. PET and PC arecondensation polymers which containcarbonyl groups more susceptible to hydrolysis andUV-attack.
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) depending on the polymer.[2] Polymers willoxidise 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 significantshear stress duringextrusion 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 ofhydroperoxides, 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 undergoescompounding to introduceadditives 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 (mechanical recycling) will usually display more degradation than fresh material and may have poorer properties as a result.[3]
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 generalautoxidation mechanism, leading to the formation oforganic peroxides and carbonyls. The addition ofantioxidants 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 withchain scission, generatingfree radicals, which primarily engage indisproportionation andcrosslinking.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 arenon-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 viashear 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).
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 advancedpolymer 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.[7]
Drinking water which has beenchlorinated to kill microbes may contain trace levels of chlorine. TheWorld 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] andacetal resin (polyoxymethylene)[12] pipework and fittings are all susceptible. Attack leads to hardening of pipework, which can leave it brittle and more susceptible tomechanical failure.
Plastics are used extensively in the manufacture of electrical items, such ascircuit boards andelectrical cables. These applications can be harsh, exposing the plastic to a mixture of thermal, chemical and electrochemical attack. Many electric items liketransformers,microprocessors orhigh-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 ofFenton reactions on hydroperoxides.[13] High voltage loads can also damage insulating materials such asdielectrics, which degrade viaelectrical treeing caused by prolonged electrical field stress.[14][15]
Polymer degradation bygalvanic 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 can possibly be used as a sustainable process to degrade non-recyclable thermoset plastics, and also has had implications for preventing corrosion on aircraft for safety such as changes in design.[19][20] Whencarbon-fiber-reinforced polymer is attached to a metal surface, thecarbon fiber can act as acathode if exposed to water or sufficient humidity, resulting ingalvanic 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 includebismaleimides (BMI), andpolyimides. The mechanism of degradation is believed to involve the electrochemical generation ofhydroxide ions, which then cleave theamide bonds.[25]
Most plastics do notbiodegrade readily,[26] however, they do still degrade in the environment because of the effects of UV-light, oxygen, water and pollutants. This combination is often generalised aspolymer weathering.[27] Chain breaking by weathering causes increasing embrittlement of plastic items, which eventually causes them to break apart.Fragmentation then continues until eventuallymicroplastics are formed. As the particle sizes get smaller, so their combined surface area increases. This facilitates theleaching of additives out of plastic and into the environment. Many controversies associated with plastics actually relate to these additives.[28][29]
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 asphotoinitiators to give complex free radical chain reactions where the mechanisms of autoxidation andphotodegradation combine. Photo-oxidation can be held back bylight stabilizers such ashindered amine light stabilizers (HALS).[30]
Polymers with an all-carbon backbone, such aspolyolefins, are usually resistant to hydrolysis. Condensation polymers likepolyesters,[31]polyamides,polyurethanes and polycarbonates can be degraded byhydrolysis 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 causeenvironmental stress cracking, which accelerates degradation.
Polymers, which are not fullysaturated, are vulnerable to attack byozone. This gas exists naturally in the atmosphere but is also formed bynitrogen oxides released in vehicle exhaust pollution. Many commonelastomers (rubbers) are affected, withnatural rubber,polybutadiene,styrene-butadiene rubber andNBR being most sensitive to degradation. Theozonolysis 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 ofozone cracking can be prevented by addingantiozonants.
The major appeal of biodegradation is that, in theory, the polymer will be consumed in the environment without needing complex waste management and that the products of this will be non-toxic. Mostcommon 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 cells, biodegradation initially relies on secretedextracellular enzymes to lower the chain-lengths. This requires the polymers bearfunctional groups the enzymes can 'recognise', such asester or amide groups. Long-chain polymers with all-carbon backbones like polyolefins, polystyrene, and PVC will not degrade by biological action alone.[34] and They must first be oxidised to create chemical groups which the enzymes can attack.[35][36]
Oxidation can be caused by melt-processing or weathering. Oxidation may be intentionally accelerated by the addition ofbiodegradable 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 likecompost and not just left in a landfill site where degradation is very difficult because of the lack of oxygen and moisture.[37]
The act of recycling plastic degrades its polymer chains, usually as a result of thermal damage similar to that seen during initial processing. In some cases, this is turned into an advantage by intentionally and completely depolymerising the plastic back into its startingmonomers, which can then be used to generate fresh, un-degraded plastic. In theory, this chemical (or feedstock) recycling offers infinite recyclability, but it is also more expensive and can have a highercarbon footprint because of its energy costs.[3] Mechanical recycling, where the plastic is simply remelted and reformed, is more common, although this usually results in a lower-quality product. Alternatively, plastic may simply be burnt as a fuel in awaste-to-energy process.[38][39]
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 bear 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, withoxo-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 theirceiling temperature, thermal degradation gives way to complete decomposition. Certain polymers likePTFE, polystyrene andPMMA[44] undergodepolymerization to give their starting monomers, whereas others like polyethylene undergopyrolysis, 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 ofsynthetic 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, intyre recycling.
Condensation polymers baring cleavable groups such as esters andamides can also be completely depolymerised by hydrolysis orsolvolysis. 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]
Hindered amine light stabilizers (HALS) stabilise against weathering by scavengingfree radicals that are produced by photo-oxidation of the polymer matrix.UV-absorbers stabilise against weathering by absorbing ultraviolet light and converting it into heat. Antioxidants stabilise the polymer by terminating the chain reaction because of the absorption of UV light from sunlight. The chain reaction initiated by photo-oxidation leads to cessation ofcrosslinking of the polymers and degradation of the property of polymers. Antioxidants are used to protect from thermal degradation.
Degradation can be detected before serious cracks are seen in a product usinginfrared spectroscopy.[53] In particular, peroxy-species andcarbonyl groups formed by photo-oxidation have distinct absorption bands.
