Editing Polymer (section)

==Degradation==
{{Main|Polymer degradation}}
[[File:PlasticDamage.JPG|right|thumb|A plastic item with thirty years of exposure to heat and cold, brake fluid, and sunlight. Notice the discoloration and [[crazing]] of the material (compared with replacement item in foreground).]]
Polymer degradation is a change in the properties—tensile strength, [[color]], shape, or molecular weight—of a polymer or polymer-based product under the influence of one or more environmental factors, such as [[heat]], [[light]], and the presence of certain [[chemical]]s, oxygen, and [[enzymes]]. This change in properties is often the result of bond breaking in the polymer backbone ([[chain scission]]) which may occur at the chain ends or at random positions in the chain.

Although such changes are frequently undesirable, in some cases, such as [[biodegradation]] and [[recycling]], they may be intended to prevent environmental [[pollution]]. Degradation can also be useful in biomedical settings. For example, a copolymer of [[polylactic acid]] and [[polyglycolic acid]] is employed in hydrolysable stitches that slowly degrade after they are applied to a wound.

The susceptibility of a polymer to degradation depends on its structure. Epoxies and chains containing aromatic functionalities are especially susceptible to [[UV degradation]] while polyesters are susceptible to degradation by [[hydrolysis]]. Polymers containing an [[Saturated and unsaturated compounds|unsaturated]] backbone degrade via [[ozone cracking]]. Carbon based polymers are more susceptible to thermal degradation than [[inorganic polymer]]s such as [[polydimethylsiloxane]] and are therefore not ideal for most high-temperature applications.{{citation needed|date=February 2024}}

The degradation of polyethylene occurs by random scission—a random breakage of the bonds that hold the [[atom]]s of the polymer together. When heated above 450&nbsp;°C, polyethylene degrades to form a mixture of hydrocarbons. In the case of chain-end scission, monomers are released and this process is referred to as unzipping or [[depolymerization]]. Which mechanism dominates will depend on the type of polymer and temperature; in general, polymers with no or a single small substituent in the repeat unit will decompose via random-chain scission.

The sorting of polymer waste for recycling purposes may be facilitated by the use of the [[resin identification code]]s developed by the [[Society of the Plastics Industry]] to identify the type of plastic.

===Product failure===
[[File:Chlorine attack1.jpg|thumb|left|Chlorine attack of acetal resin plumbing joint]]
Failure of [[safety-critical]] polymer components can cause serious accidents, such as fire in the case of cracked and degraded polymer [[fuel line]]s. Chlorine-induced cracking of [[acetal resin]] plumbing joints and [[polybutylene]] pipes has caused many serious floods in domestic properties, especially in the US in the 1990s. Traces of [[chlorine]] in the water supply attacked polymers present in the plumbing, a problem which occurs faster if any of the parts have been poorly [[extruded]] or [[Injection molding|injection molded]]. Attack of the acetal joint occurred because of faulty molding, leading to cracking along the threads of the fitting where there is [[stress concentration]].
[[File:Ozone_cracks_in_tube1.jpg|thumb|Ozone-induced cracking in natural rubber tubing]]
Polymer oxidation has caused accidents involving [[medical device]]s. One of the oldest known failure modes is ozone cracking caused by chain scission when [[ozone]] gas attacks susceptible [[elastomer]]s, such as [[natural rubber]] and [[nitrile rubber]]. They possess double bonds in their repeat units which are cleaved during [[ozonolysis]]. Cracks in fuel lines can penetrate the bore of the tube and cause fuel leakage. If cracking occurs in the engine compartment, electric sparks can ignite the [[gasoline]] and can cause a serious fire. In medical use degradation of polymers can lead to changes of physical and chemical characteristics of implantable devices.<ref>{{cite journal|last1=Iakovlev|first1=V.|last2=Guelcher|first2=S.|last3=Bendavid|first3=R.|date=28 August 2015|title=Degradation of polypropylene in vivo: A microscopic analysis of meshes explanted from patients|journal=Journal of Biomedical Materials Research Part B: Applied Biomaterials|volume=105|issue=2|pages=237–248|doi=10.1002/jbm.b.33502|pmid=26315946}}</ref>

[[Nylon 6,6|Nylon 66]] is susceptible to [[acid hydrolysis]], and in one accident, a fractured fuel line led to a spillage of diesel into the road. If [[diesel fuel]] leaks onto the road, accidents to following cars can be caused by the slippery nature of the deposit, which is like [[black ice]]. Furthermore, the [[asphalt concrete]] road surface will suffer damage as a result of the diesel fuel dissolving the [[asphaltene]]s from the composite material, this resulting in the degradation of the asphalt surface and structural integrity of the road.