polymer degradation
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Polymer degradation is the reduction in the physical properties of a
polymer A polymer (; Greek '' poly-'', "many" + ''-mer'', "part") is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic a ...
, such as strength, caused by changes in its chemical composition. Polymers and particularly
plastic Plastics are a wide range of synthetic or semi-synthetic materials that use polymers as a main ingredient. Their plasticity makes it possible for plastics to be moulded, extruded or pressed into solid objects of various shapes. This adaptab ...
s are subject to degradation at all stages of their
product life cycle In industry, Product Lifecycle Management (PLM) is the process of managing the entire lifecycle of a product from its inception through the engineering, design and manufacture, as well as the service and disposal of manufactured products. PLM ...
, including during their initial processing, use, disposal into the environment and recycling. The rate of this degradation varies significantly;
biodegradation Biodegradation is the breakdown of organic matter by microorganisms, such as bacteria and fungi. It is generally assumed to be a natural process, which differentiates it from composting. Composting is a human-driven process in which biodegrada ...
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 Polymer stabilizers (British: polymer stabilisers) are chemical additives which may be added to polymeric materials, such as plastics and rubbers, to inhibit or retard their degradation. Common polymer degradation processes include oxidation, UV- ...
ensure plastic items are produced with the desired properties, extend their useful lifespans, and facilitate their recycling. Conversely,
biodegradable additives Biodegradable additives are additives that enhance the biodegradation of polymers by allowing microorganisms to utilize the carbon within the polymer chain as a source of energy. Biodegradable additives attract microorganisms to the polymer throug ...
accelerate the degradation of
plastic waste Plastic pollution is the accumulation of plastic objects and particles (e.g. plastic bottles, bags and microbeads) in the Earth's environment that adversely affects humans, wildlife and their habitat. Plastics that act as pollutants are catego ...
by improving its
biodegradability Biodegradation is the breakdown of organic matter by microorganisms, such as bacteria and fungi. It is generally assumed to be a natural process, which differentiates it from composting. Composting is a human-driven process in which biodegradati ...
. Some forms of
plastic recycling Plastic recycling is the reprocessing of plastic waste into new products. When performed correctly, this can reduce dependence on landfill, conserve resources and protect the environment from plastic pollution and greenhouse gas emissions. A ...
can involve the complete degradation of a polymer back into
monomer In chemistry, a monomer ( ; '' mono-'', "one" + ''-mer'', "part") is a molecule that can react together with other monomer molecules to form a larger polymer chain or three-dimensional network in a process called polymerization. Classification ...
s 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 Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a ...
and
chain scission Chain scission is a term used in polymer chemistry describing the Fracture in polymers, degradation of a polymer main chain. It is often caused by Thermal degradation of polymers, thermal stress (heat) or ionizing radiation (e.g. light, UV radiatio ...
, leading to a reduction in the
molecular weight A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bioch ...
and
degree of polymerization The degree of polymerization, or DP, is the number of monomeric units in a macromolecule or polymer or oligomer molecule. For a homopolymer, there is only one type of monomeric unit and the ''number-average'' degree of polymerization is given b ...
of the polymer. These changes affect physical properties like strength, malleability,
melt flow index The Melt Flow Index (MFI) is a measure of the ease of flow of the melt of a thermoplastic polymer. It is defined as the mass of polymer, in grams, flowing in ten minutes through a capillary of a specific diameter and length by a pressure applied ...
, appearance and colour. The changes in properties are often termed "aging".


Susceptibility

Plastics exist in huge variety, however several types of commodity polymer dominate global production:
polyethylene Polyethylene or polythene (abbreviated PE; IUPAC name polyethene or poly(methylene)) is the most commonly produced plastic. It is a polymer, primarily used for packaging ( plastic bags, plastic films, geomembranes and containers including b ...
(PE),
polypropylene Polypropylene (PP), also known as polypropene, is a thermoplastic polymer used in a wide variety of applications. It is produced via chain-growth polymerization from the monomer propylene. Polypropylene belongs to the group of polyolefins a ...
(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 Plastic pollution is the accumulation of plastic objects and particles (e.g. plastic bottles, bags and microbeads) in the Earth's environment that adversely affects humans, wildlife and their habitat. Plastics that act as pollutants are catego ...
. These plastics are all
thermoplastic A thermoplastic, or thermosoft plastic, is any plastic polymer material that becomes pliable or moldable at a certain elevated temperature and solidifies upon cooling. Most thermoplastics have a high molecular weight. The polymer chains associate ...
s and are more susceptible to degradation than equivalent
thermoset In materials science, a thermosetting polymer, often called a thermoset, is a polymer that is obtained by irreversibly hardening (" curing") a soft solid or viscous liquid prepolymer ( resin). Curing is induced by heat or suitable radiation ...
s, as those are more thoroughly
cross-linked In chemistry and biology a cross-link is a bond or a short sequence of bonds that links one polymer chain to another. These links may take the form of covalent bonds or ionic bonds and the polymers can be either synthetic polymers or natural ...
. The majority (PP, PE, PVC, PS and PMMA) are
addition polymer In polymer chemistry, an addition polymer is a polymer that forms by simple linking of monomers ''without'' the co-generation of other products. Addition polymerization differs from condensation polymerization, which ''does'' co-generate a product ...
s with all-carbon backbones that are more resistant to most types of degradation. PET and PC are condensation polymers which contain
carbonyl In organic chemistry, a carbonyl group is a functional group composed of a carbon atom double-bonded to an oxygen atom: C=O. It is common to several classes of organic compounds, as part of many larger functional groups. A compound containi ...
groups more susceptible to hydrolysis and UV-attack.


Degradation during processing

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. Polymers will
oxidise Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a d ...
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 Shear stress, often denoted by (Greek: tau), is the component of stress coplanar with a material cross section. It arises from the shear force, the component of force vector parallel to the material cross section. ''Normal stress'', on the ...
during
extrusion Extrusion is a process used to create objects of a fixed cross-sectional profile by pushing material through a die of the desired cross-section. Its two main advantages over other manufacturing processes are its ability to create very complex ...
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
hydroperoxide Hydroperoxides or peroxols are compounds containing the hydroperoxide functional group (ROOH). If the R is organic, the compounds are called organic hydroperoxides. Such compounds are a subset of organic peroxides, which have the formula ROOR. ...
s, 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 In the field of pharmacy, compounding (performed in compounding pharmacies) is preparation of a custom formulation of a medication to fit a unique need of a patient that cannot be met with commercially available products. This may be done for me ...
to introduce
additives Additive may refer to: Mathematics * Additive function, a function in number theory * Additive map, a function that preserves the addition operation * Additive set-functionn see Sigma additivity * Additive category, a preadditive category with fi ...
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.{{cite journal , last1=Schyns , first1=Zoé O. G. , last2=Shaver , first2=Michael P. , title=Mechanical Recycling of Packaging Plastics: A Review , journal=
Macromolecular Rapid Communications ''Macromolecular Rapid Communications'' is a biweekly peer-reviewed scientific journal covering polymer science. It publishes Communications, Feature Articles and Reviews on general polymer science, from chemistry and physics of polymers to pol ...
, date=February 2021 , volume=42 , issue=3 , pages=2000415 , doi=10.1002/marc.202000415, pmid=33000883 , doi-access=free


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 (i.e. without air). Reactions follow the general
autoxidation Autoxidation (sometimes auto-oxidation) refers to oxidations brought about by reactions with oxygen at normal temperatures, without the intervention of flame or electric spark. The term is usually used to describe the gradual degradation of organi ...
mechanism, leading to the formation of organic peroxides and carbonyls. The addition of
antioxidant Antioxidants are compounds that inhibit oxidation, a chemical reaction that can produce free radicals. This can lead to polymerization and other chain reactions. They are frequently added to industrial products, such as fuels and lubricant ...
s may inhibit such processes.


Thermal degradation

{{main, Thermal degradation of polymers Heating polymers to a sufficiently high temperature can cause damaging chemical changes, even in the absence of oxygen. This usually starts with
chain scission Chain scission is a term used in polymer chemistry describing the Fracture in polymers, degradation of a polymer main chain. It is often caused by Thermal degradation of polymers, thermal stress (heat) or ionizing radiation (e.g. light, UV radiatio ...
, generating
free radical A daughter category of ''Ageing'', this category deals only with the biological aspects of ageing. Ageing Ailments of unknown cause Biogerontology Biological processes Causes of death Cellular processes Gerontology Life extension Metabo ...
s, which primarily engage in
disproportionation In chemistry, disproportionation, sometimes called dismutation, is a redox reaction in which one compound of intermediate oxidation state converts to two compounds, one of higher and one of lower oxidation states. More generally, the term can ...
and
crosslink In chemistry and biology a cross-link is a bond or a short sequence of bonds that links one polymer chain to another. These links may take the form of covalent bonds or ionic bonds and the polymers can be either synthetic polymers or natural ...
ing. PVC is the most thermally sensitive common polymer, with major degradation occurring from ~{{convert, 250, C, round=5 onwards; other polymers degrade at higher temperatures.


Thermo-mechanical degradation

Molten polymers are
non-Newtonian fluid A non-Newtonian fluid is a fluid that does not follow Newton's law of viscosity, i.e., constant viscosity independent of stress. In non-Newtonian fluids, viscosity can change when under force to either more liquid or more solid. Ketchup, for ex ...
s 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 Shear stress, often denoted by (Greek: tau), is the component of stress coplanar with a material cross section. It arises from the shear force, the component of force vector parallel to the material cross section. ''Normal stress'', on the ...
. 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 Paraffin wax (or petroleum wax) is a soft colorless solid derived from petroleum, coal, or oil shale that consists of a mixture of hydrocarbon molecules containing between 20 and 40 carbon atoms. It is solid at room temperature and begins to ...
,
wax ester A wax ester (WE) is an ester of a fatty acid and a fatty alcohol. Wax esters comprise the main components of three commercially important waxes: carnauba wax, candelilla wax, and beeswax.. Wax esters are formed by combining one fatty acid with on ...
s, etc.) or metal stearates (i.e.
zinc stearate Zinc stearate is a "zinc soap" that is widely used industrially. In this context, soap is used in its formal sense, a metal salt of a fatty acid: in this case stearic acid. It is a white solid that repels water. It is insoluble in polar solvents ...
).


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 Polymer stabilizers (British: polymer stabilisers) are chemical additives which may be added to polymeric materials, such as plastics and rubbers, to inhibit or retard their degradation. Common polymer degradation processes include oxidation, UV- ...
. 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

Drinking water Drinking water is water that is used in drink or food preparation; potable water is water that is safe to be used as drinking water. The amount of drinking water required to maintain good health varies, and depends on physical activity level, a ...
which has been
chlorinated In chemistry, halogenation is a chemical reaction that entails the introduction of one or more halogens into a compound. Halide-containing compounds are pervasive, making this type of transformation important, e.g. in the production of polyme ...
to kill microbes may contain trace levels of chlorine. The
World Health Organization The World Health Organization (WHO) is a specialized agency of the United Nations responsible for international public health. The WHO Constitution states its main objective as "the attainment by all peoples of the highest possible level of h ...
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 Polybutylene (polybutene-1, poly(1-butene), PB-1) is a polyolefin or saturated polymer with the chemical formula (CH2CH(Et))n. Not be confused with polybutene, PB-1 is mainly used in piping. Production Polybutylene is produced by polymerisatio ...
and
acetal resin Polyoxymethylene (POM), also known as acetal, polyacetal, and polyformaldehyde, is an engineering thermoplastic used in precision parts requiring high stiffness, low friction, and excellent dimensional stability. As with many other synthetic pol ...
(polyoxymethylene) pipework and fittings are all susceptible. Attack leads to hardening of pipework, which can leave it brittle and more susceptible to
mechanical failure Structural integrity and failure is an aspect of engineering that deals with the ability of a structure to support a designed structural load (weight, force, etc.) without breaking and includes the study of past structural failures in order to ...
.


Electronics

Plastics are used extensively in the manufacture of electrical items, such as
circuit board A printed circuit board (PCB; also printed wiring board or PWB) is a medium used in electrical and electronic engineering to connect electronic components to one another in a controlled manner. It takes the form of a laminated sandwich struc ...
s and
electrical cable An electrical cable is an assembly of one or more wires running side by side or bundled, which is used to carry electric current. One or more electrical cables and their corresponding connectors may be formed into a ''cable assembly'', whic ...
s. These applications can be harsh, exposing the plastic to a mixture of thermal, chemical and electrochemical attack. Many electric items like
transformer A transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple circuits. A varying current in any coil of the transformer produces a varying magnetic flux in the transformer' ...
s,
microprocessor A microprocessor is a computer processor where the data processing logic and control is included on a single integrated circuit, or a small number of integrated circuits. The microprocessor contains the arithmetic, logic, and control circ ...
s or
high-voltage cable A high-voltage cable (HV cable) is a cable used for electric power transmission at high voltage. A cable includes a conductor and insulation. Cables are considered to be fully insulated. This means that they have a fully rated insulation system t ...
s 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. High voltage loads can also damage insulating materials such as
dielectric In electromagnetism, a dielectric (or dielectric medium) is an electrical insulator that can be polarised by an applied electric field. When a dielectric material is placed in an electric field, electric charges do not flow through the mate ...
s, which degrade via
electrical treeing In electrical engineering, treeing is an electrical pre-breakdown phenomenon in solid insulation. It is a damaging process due to partial discharges and progresses through the stressed dielectric insulation, in a path resembling the branches of ...
caused by prolonged electrical field stress.


Galvanic action

Polymer degradation by galvanic action was first described in the technical literature in 1990. When
carbon-fiber-reinforced polymer Carbon fiber-reinforced polymers (American English), carbon-fibre-reinforced polymers (Commonwealth English), carbon-fiber-reinforced plastics, carbon-fiber reinforced-thermoplastic (CFRP, CRP, CFRTP), also known as carbon fiber, carbon compo ...
is attached to a metal surface, the
carbon fiber Carbon fiber-reinforced polymers (American English), carbon-fibre-reinforced polymers (Commonwealth English), carbon-fiber-reinforced plastics, carbon-fiber reinforced-thermoplastic (CFRP, CRP, CFRTP), also known as carbon fiber, carbon compo ...
can act as a
cathode A cathode is the electrode from which a conventional current leaves a polarized electrical device. This definition can be recalled by using the mnemonic ''CCD'' for ''Cathode Current Departs''. A conventional current describes the direction in wh ...
if exposed to water or sufficient humidity, resulting in
galvanic corrosion Galvanic corrosion (also called bimetallic corrosion or dissimilar metal corrosion) is an electrochemical process in which one metal corrodes preferentially when it is in electrical contact with another, in the presence of an electrolyte. A sim ...
. This has been seen in engineering when carbon-fiber polymers have been used to reinforce weakened steel structures. Reactions have also been seen in aluminium and magnesium alloys, polymers affected include
bismaleimide Maleimide is a chemical compound with the formula H2C2(CO)2NH (see diagram). This unsaturated imide is an important building block in organic synthesis. The name is a contraction of maleic acid and imide, the -C(O)NHC(O)- functional group. Malei ...
s (BMI), and polyimides. The mechanism of degradation is believed to involve the electrochemical generation of
hydroxide Hydroxide is a diatomic anion with chemical formula OH−. It consists of an oxygen and hydrogen atom held together by a single covalent bond, and carries a negative electric charge. It is an important but usually minor constituent of water. I ...
ions, which then cleave the
amide In organic chemistry, an amide, also known as an organic amide or a carboxamide, is a compound with the general formula , where R, R', and R″ represent organic groups or hydrogen atoms. The amide group is called a peptide bond when it i ...
bonds.


Degradation in the environment

Most plastics do not
biodegrade Biodegradation is the breakdown of organic matter by microorganisms, such as bacteria and fungi. It is generally assumed to be a natural process, which differentiates it from composting. Composting is a human-driven process in which biodegradati ...
readily,{{cite journal , last1=Andrady , first1=Anthony L. , title=Assessment of Environmental Biodegradation of Synthetic Polymers , journal=Journal of Macromolecular Science, Part C: Polymer Reviews , date=February 1994 , volume=34 , issue=1 , pages=25–76 , doi= 10.1080/15321799408009632 however, they do still degrade in the environment because of the effects of UV-light, oxygen, water and pollutants. This combination is often generalised as polymer weathering.{{cite journal , last1=Feldman , first1=D. , title=Polymer Weathering: Photo-Oxidation , journal=Journal of Polymers and the Environment , date=1 October 2002 , volume=10 , issue=4 , pages=163–173 , doi=10.1023/A:1021148205366, s2cid=92300829 Chain breaking by weathering causes increasing embrittlement of plastic items, which eventually causes them to break apart. Fragmentation then continues until eventually
microplastics Microplastics are fragments of any type of plastic less than in length, according to the U.S. National Oceanic and Atmospheric Administration (NOAA) and the European Chemicals Agency. They cause pollution by entering natural ecosystems from a v ...
are formed. As the particle sizes get smaller, so their combined surface area increases. This facilitates the
leaching Leaching is the loss or extraction of certain materials from a carrier into a liquid (usually, but not always a solvent). and may refer to: * Leaching (agriculture), the loss of water-soluble plant nutrients from the soil; or applying a small amou ...
of additives out of plastic and into the environment. Many controversies associated with plastics actually relate to these additives.{{cite journal , last1=Hahladakis , first1=John N. , last2=Velis , first2=Costas A. , last3=Weber , first3=Roland , last4=Iacovidou , first4=Eleni , last5=Purnell , first5=Phil , title=An overview of chemical additives present in plastics: Migration, release, fate and environmental impact during their use, disposal and recycling , journal= Journal of Hazardous Materials , date=February 2018 , volume=344 , pages=179–199 , doi=10.1016/j.jhazmat.2017.10.014, pmid=29035713 , doi-access=free


Photo-oxidation

{{main, Photo-oxidation of polymers Photo-oxidation is the combined action of UV-light and oxygen and is the most significant factor in the weathering of plastics. 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
photoinitiator A photoinitiator is a molecule that creates reactive species ( free radicals, cations or anions) when exposed to radiation (UV or visible). Synthetic photoinitiators are key components in photopolymers (for example, photo-curable coatings, adhesiv ...
s to give complex free radical chain reactions where the mechanisms of autoxidation and
photodegradation Photodegradation is the alteration of materials by light. Commonly, the term is used loosely to refer to the combined action of sunlight and air, which cause oxidation and hydrolysis. Often photodegradation is intentionally avoided, since it destro ...
combine. Photo-oxidation can be held back by light stabilizers such as
hindered amine light stabilizers Hindered amine light stabilizers (HALS) are chemical compounds containing an amine functional group that are used as stabilizers in plastics and polymers. These compounds are typically derivatives of tetramethylpiperidine and are primarily used t ...
(HALS).


Hydrolysis

Polymers with an all-carbon backbone, such as polyolefins, are usually resistant to hydrolysis. Condensation polymers like polyesters,{{cite journal , last1=Allen , first1=Norman S. , last2=Edge , first2=Michael , last3=Mohammadian , first3=Mehrdad , last4=Jones , first4=Ken , title=Hydrolytic degradation of poly(ethylene terephthalate): Importance of chain scission versus crystallinity , journal= European Polymer Journal , date=January 1991 , volume=27 , issue=12 , pages=1373–1378 , doi=10.1016/0014-3057(91)90237-I
polyamide A polyamide is a polymer with repeating units linked by amide bonds. Polyamides occur both naturally and artificially. Examples of naturally occurring polyamides are proteins, such as wool and silk. Artificially made polyamides can be made through ...
s,
polyurethane Polyurethane (; often abbreviated PUR and PU) refers to a class of polymers composed of organic units joined by carbamate (urethane) links. In contrast to other common polymers such as polyethylene and polystyrene, polyurethane is produced from ...
s and polycarbonates can be degraded by
hydrolysis Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution, elimination, and solvation reactions in which water is the nucleophile. Biological hydrolys ...
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. Swelling caused by the absorption of minute amounts of water can also cause environmental stress cracking, which accelerates degradation.


Ozonolysis of rubbers

{{main, Ozonolysis, Ozone cracking Polymers, which are not fully
saturated Saturation, saturated, unsaturation or unsaturated may refer to: Chemistry * Saturation, a property of organic compounds referring to carbon-carbon bonds ** Saturated and unsaturated compounds **Degree of unsaturation ** Saturated fat or fatty ac ...
, are vulnerable to attack by
ozone Ozone (), or trioxygen, is an inorganic molecule with the chemical formula . It is a pale blue gas with a distinctively pungent smell. It is an allotrope of oxygen that is much less stable than the diatomic allotrope , breaking down in the lo ...
. This gas exists naturally in the atmosphere but is also formed by
nitrogen oxides Nitrogen oxide may refer to a binary compound of oxygen and nitrogen, or a mixture of such compounds: Charge-neutral *Nitric oxide (NO), nitrogen(II) oxide, or nitrogen monoxide *Nitrogen dioxide (), nitrogen(IV) oxide * Nitrogen trioxide (), or ...
released in vehicle exhaust pollution. Many common
elastomers An elastomer is a polymer with viscoelasticity (i.e. both viscosity and elasticity) and with weak intermolecular forces, generally low Young's modulus and high failure strain compared with other materials. The term, a portmanteau of ''elastic p ...
(rubbers) are affected, with
natural rubber Rubber, also called India rubber, latex, Amazonian rubber, ''caucho'', or ''caoutchouc'', as initially produced, consists of polymers of the organic compound isoprene, with minor impurities of other organic compounds. Thailand, Malaysia, and ...
, polybutadiene,
styrene-butadiene Styrene-butadiene or styrene-butadiene rubber (SBR) describe families of synthetic rubbers derived from styrene and butadiene (the version developed by Goodyear is called Neolite). These materials have good abrasion resistance and good aging st ...
rubber and NBR being most sensitive to degradation. The
ozonolysis In organic chemistry, ozonolysis is an organic reaction where the unsaturated bonds of alkenes (), alkynes (), or azo compounds () are cleaved with ozone (). Alkenes and alkynes form organic compounds in which the multiple carbon–carbon b ...
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 Cracks can be formed in many different elastomers by ozone attack, and the characteristic form of attack of vulnerable rubbers is known as ozone cracking. The problem was formerly very common, especially in tires, but is now rarely seen in those ...
can be prevented by adding
antiozonant An antiozonant, also known as anti-ozonant, is an organic compound that prevents or retards damage caused by ozone. The most important antiozonants are those which prevent degradation of elastomers like rubber. A number of research projects study ...
s.


Biological degradation

{{main, Synthetic biodegradable polymer, Biodegradable plastic, Biodegradable polymer, Plastic degradation by marine bacteria 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. 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 group In organic chemistry, a functional group is a substituent or moiety in a molecule that causes the molecule's characteristic chemical reactions. The same functional group will undergo the same or similar chemical reactions regardless of the re ...
s the enzymes can 'recognise', such as
ester In chemistry, an ester is a compound derived from an oxoacid (organic or inorganic) in which at least one hydroxyl group () is replaced by an alkoxy group (), as in the substitution reaction of a carboxylic acid and an alcohol. Glycerides ar ...
or amide groups. Long-chain polymers with all-carbon backbones like polyolefins, polystyrene and PVC will not degrade by biological action alone and must first be oxidised to create chemical groups which the enzymes can attack. Oxidation can be caused by melt-processing or weathering in the environment. Oxidation may be intentionally accelerated by the addition of
biodegradable additives Biodegradable additives are additives that enhance the biodegradation of polymers by allowing microorganisms to utilize the carbon within the polymer chain as a source of energy. Biodegradable additives attract microorganisms to the polymer throug ...
. 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.


Degradation during recycling

{{Main, Plastic recycling 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 starting
monomers In chemistry, a monomer ( ; ''mono-'', "one" + '' -mer'', "part") is a molecule that can react together with other monomer molecules to form a larger polymer chain or three-dimensional network in a process called polymerization. Classification M ...
, 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 higher carbon footprint because of its energy costs.{{cite journal , last1=Schyns , first1=Zoé O. G. , last2=Shaver , first2=Michael P. , title=Mechanical Recycling of Packaging Plastics: A Review , journal=
Macromolecular Rapid Communications ''Macromolecular Rapid Communications'' is a biweekly peer-reviewed scientific journal covering polymer science. It publishes Communications, Feature Articles and Reviews on general polymer science, from chemistry and physics of polymers to pol ...
, date=February 2021 , volume=42 , issue=3 , pages=2000415 , doi=10.1002/marc.202000415, pmid=33000883 , doi-access=free
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 a
waste-to-energy Waste-to-energy (WtE) or energy-from-waste (EfW) is the process of generating energy in the form of electricity and/or heat from the primary treatment of waste, or the processing of waste into a fuel source. WtE is a form of energy recovery. Mo ...
process.


Remelting

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. 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. 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 OXO-degradation is the degradation of polymers such as plastic by an oxidative process. Such degradation breaks down plastic into small pieces that are easier to biodegrade (but not necessarily enough to be called "biodegradable"). When biodegrad ...
additives, consisting of metallic salts of iron, magnesium, nickel, and cobalt, increasing the rate of thermal degradation. Depending on the polymer in question, an amount of virgin material may be added to maintain the quality of the product.


Thermal depolymerisation & pyrolysis

{{main, Thermal depolymerization As polymers approach their
ceiling temperature Ceiling temperature (T_c) is a measure of the tendency of a polymer to revert to its constituent monomers. When a polymer is at its ceiling temperature, the rate of polymerization and depolymerization of the polymer are equal. Generally, the cei ...
, thermal degradation gives way to complete decomposition. Certain polymers like PTFE, polystyrene and
PMMA PMMA may refer to: * para-Methoxymethamphetamine, a stimulant drug * Philippine Merchant Marine Academy The Philippine Merchant Marine Academy ( fil, Akademiya sa Bapor Pangkalakalan ng Pilipinas) also referred to by its acronym PMMA) is a ma ...
undergo
depolymerization Depolymerization (or depolymerisation) is the process of converting a polymer into a monomer or a mixture of monomers. This process is driven by an increase in entropy. Ceiling temperature The tendency of polymers to depolymerize is indicated by ...
to give their starting monomers, whereas others like polyethylene undergo
pyrolysis The pyrolysis (or devolatilization) process is the thermal decomposition of materials at elevated temperatures, often in an inert atmosphere. It involves a change of chemical composition. The word is coined from the Greek-derived elements ''py ...
, 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), whereas pyrolysis products are used as a type of
synthetic fuel Synthetic fuel or synfuel is a liquid fuel, or sometimes gaseous fuel, obtained from syngas, a mixture of carbon monoxide and hydrogen, in which the syngas was derived from gasification of solid feedstocks such as coal or biomass or by refo ...
(energy recycling). 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.


Chemical depolymerisation

Condensation polymers baring cleavable groups such as esters and
amides In organic chemistry, an amide, also known as an organic amide or a carboxamide, is a compound with the general formula , where R, R', and R″ represent organic groups or hydrogen atoms. The amide group is called a peptide bond when it is ...
can also be completely depolymerised by hydrolysis or
solvolysis In chemistry, solvolysis is a type of nucleophilic substitution (S1/S2) or elimination reaction, elimination where the nucleophile is a solvent molecule. Characteristic of S1 reactions, solvolysis of a chirality (chemistry), chiral reactant affor ...
. This can be a purely chemical process but may also be promoted by enzymes. 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. Alternatively, waste plastic may be converted into other valuable chemicals (not necessarily monomers) by microbial action.


Stabilisers

{{main, Polymer stabilizers Hindered amine light stabilizers (HALS) stabilise against weathering by scavenging free 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 of
crosslinking Cross-linking may refer to *Cross-link In chemistry and biology a cross-link is a bond or a short sequence of bonds that links one polymer chain to another. These links may take the form of covalent bonds or ionic bonds and the polymers ca ...
of the polymers and degradation of the property of polymers. Antioxidants are used to protect from thermal degradation.


Detection

Degradation can be detected before serious cracks are seen in a product using
infrared spectroscopy Infrared spectroscopy (IR spectroscopy or vibrational spectroscopy) is the measurement of the interaction of infrared radiation with matter by absorption, emission, or reflection. It is used to study and identify chemical substances or function ...
.{{cite journal , last1=Celina , first1=Mathew C. , last2=Linde , first2=Erik , last3=Martinez , first3=Estevan , title=Carbonyl Identification and Quantification Uncertainties for Oxidative Polymer Degradation , journal=Polymer Degradation and Stability , date=March 2021 , volume=188 , pages=109550 , doi=10.1016/j.polymdegradstab.2021.109550, osti=1772948 , s2cid=233639741 In particular, peroxy-species and
carbonyl group In organic chemistry, a carbonyl group is a functional group composed of a carbon atom double-bonded to an oxygen atom: C=O. It is common to several classes of organic compounds, as part of many larger functional groups. A compound containi ...
s formed by photo-oxidation have distinct absorption bands.


See also

* Applied spectroscopy *
Forensic polymer engineering Forensic polymer engineering is the study of failure in polymeric products. The topic includes the fracture of plastic products, or any other reason why such a product fails in service, or fails to meet its specification. The subject focuses on t ...
* Environmental stress fracture *
Polymer engineering Polymer engineering is generally an engineering field that designs, analyses, and modifies polymer materials. Polymer engineering covers aspects of the petrochemical industry, polymerization, structure and characterization of polymers, properties of ...
*
Weather testing of polymers Accelerated photo-ageing of polymers in SEPAP units is the controlled polymer degradation and polymer coating degradation under lab or natural conditions. The prediction of the ageing of plastic materials is an important subject that concerns ...


Bibliography

* Lewis, Peter Rhys, Reynolds, K and Gagg, C, ''Forensic Materials Engineering: Case studies'',
CRC Press The CRC Press, LLC is an American publishing group that specializes in producing technical books. Many of their books relate to engineering, science and mathematics. Their scope also includes books on business, forensics and information tec ...
(2004) * Ezrin, Meyer, ''Plastics Failure Guide: Cause and Prevention'', Hanser-SPE (1996). * Wright, David C., ''Environmental Stress Cracking of Plastics'' RAPRA (2001). * Lewis, Peter Rhys, and Gagg, C, ''Forensic Polymer Engineering: Why polymer products fail in service'', Woodhead/CRC Press (2010).


References

{{Reflist {{plastics {{DEFAULTSORT:Polymer Degradation Mechanical failure modes Polymer chemistry Corrosion Forensic phenomena Materials degradation