- Polymer degradation
Polymer degradation is a change in the properties -
tensile strength ,colour , shape, etc - of apolymer or polymer based product under the influence of one or more environmental factors such asheat ,light orchemical s. These changes are usually undesirable, such as changes during use, cracking and depolymerisation of products or, more rarely, desirable, as inbiodegradation or deliberately lowering themolecular weight of a polymer forrecycling .In a finished product such a change is to be prevented or delayed. However degradation can be useful for
recycling /reusing the polymer waste to prevent or reduce environmentalpollution . Degradation can also be induced deliberately to assiststructure determination .Polymeric
molecule s are very large (on the molecular scale), and their unique and useful properties are mainly a result of their size. Any loss in chain length lowers tensile strength and is a primary cause of premature cracking.Commodity polymers
Today there are primarily six commodity polymers in use, namely
polyethylene ,polypropylene ,polyvinyl chloride ,polyethylene terephthalate or PET,polystyrene andpolycarbonate . These make up nearly 98% of all polymers and plastics encountered in daily life. Each of these polymers has its own characteristic modes of degradation and resistances to heat, light and chemicals. Polyethylene and polypropylene are sensitive tooxidation andUV radiation , while PVC may discolour at high temperatures due to loss ofhydrogen chloride gas, and become very brittle. PET is sensitive tohydrolysis and attack by strongacid s, while polycarbonate depolymerizes rapidly when exposed to strongalkali s.For example, polyethylene usually degrades by "random scission" - that is by a random breakage of the linkages (bonds) that hold the
atoms of the polymer together. When this polymer is heated above 450Celsius it becomes a complex mixture of molecules of various sizes which resemble gasoline. Other polymers - like polyalphamethylstyrene - undergo 'unspecific' chain scission with breakage occurring only at the ends; they literally unzip or depolymerize to become the constituentmonomers .Examples
Many polymers, especially step-growth polymers, are degraded by specific chemicals such as strong
acids and strongalkalis . They are made bycondensation polymerization , so degradation is a reversal of the synthesis reaction. Other degradation routes involve interaction with strongoxidising agents and interaction with UV radiation.Hydrolysis
Nylon is sensitive to degradation by acids, a process known ashydrolysis , and nylon mouldings will crack when attacked by strong acids. A fuel pipe fractured when a small drip of 40%sulphuric acid from a nearbylead-acid battery fell onto anylon 6,6 moulded connector in the diesel line. The crack grew with time until it penetrated the interior, so initiating a slow leak of diesel. The crack continued to grow until final separation occurred, anddiesel fuel poured into the road. Diesel is especially hazardous when it is present on road surfaces because it forms an extremely slippery surface which cannot be seen easily by road users (just likeblack ice ).The leak caused several accidents to other cars, one of which caused serious injuries to the driver. The fracture surface of the connector showed the progressive growth of the crack from the initial acid attack (Ch) to the final cusp (C) of polymer. The problem is known as
stress corrosion cracking , and in this case was caused byhydrolysis of the polymer. It was the reverse reaction of the synthesis of the polymer:The owner of the vehicle on which the fuel pipe leak should have spotted the leak well before the final accident, and the injured driver was awarded compensation by the insurers.Ozonolysis
Cracks can be formed in many different
elastomers byozone attack. Tiny traces of the gas in the air will attack double bonds in rubber chains, withNatural rubber ,Styrene-butadiene rubber andNBR being most sensitive to degradation. Ozone cracks form in products under tension, but the critical strain is very small. The cracks 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 very dangerous when they occur in fuel pipes because the cracks will grow from the outside exposed surfaces into the bore of the pipe, so fuel leakage and fire may follow. The problem ofozone cracking can be prevented by adding anti-ozonants to the rubber beforevulcanization . Ozone cracks were commonly seen in automobiletire sidewalls, but are now seen rarely thanks to the use of these additives. On the other hand, the problem does recur in unprotected products such as rubber tubing and seals.Oxidation
Polymers are susceptible to attack by atmospheric
oxygen , especially at elevated temperatures encountered during processing to shape. Many process methods such asextrusion andinjection moulding involve pumping molten polymer into tools, and the high temperatures needed for melting may result in oxidation unless precautions are taken. For example, a forearmcrutch suddenly snapped and the user was severely injured in the resulting fall. The crutch had fractured across apolypropylene insert within the aluminium tube of the device, andinfra-red spectroscopy of the material showed that it had oxidised, possible as a result of poor moulding.Oxidation is usually relatively easy to detect owing to the strong absorption by the
carbonyl group in the spectrum ofpolyolefins .Polypropylene has a relatively simple spectrum with few peaks at the carbonyl position (likepolyethylene ). Oxidation tends to start attertiary carbon atoms becausefree radicals here at more stable, so last longer and are attacked byoxygen . The carbonyl group can be further oxidised to break the chain, so weakening the material by lowering themolecular weight , and cracks start to grow in the regions affected.Chlorine-induced cracking
Another highly reactive gas is
chlorine , which will attack susceptible polymers such asacetal resin andpolybutylene pipework. There have been many examples of such pipes and acetal fittings failing in properties in the USA as a result of chlorine-induced cracking. Essentially the gas attacks sensitive parts of the chain molecules (especially secondary , tertiary orallylic carbon atoms), oxidising the chains and ultimately causing chain cleavage. The root cause is traces of chlorine in the water supply, added for its anti-bacterial action, attack occurring even atparts per million traces of the dissolved gas. The chlorine attacks weak parts of a product, and in the case of anacetal resin junction in a water supply system, it was the thread roots which were attacked first, causing a brittle crack to grow. The discolouration on the fracture surface was caused by deposition ofcarbonates from thehard water supply, so the joint had been in a critical state for many months. When it finally failed, it did so at the worst possible time, at the weekend when no-one was around to sort the problem. The leak flooded computer labs below, and caused substantial damage. The problems in the USA also occurred topolybutylene pipework, and led to the material being removed from that market, although it is still used elsewhere in the world.tabilisers
Hindered-amine light stabilisers(HALS) stabilise against weathering by scavenging
free radicals that are produced by photo-oxidation of the polymer matrix. UV-absorbers stabilises against weathering by absorbing ultraviolet light and converting it into heat.Antioxidant s stabilizes the polymer by terminating the chain reaction due to the adsorption of UV light from sunlight. The chain reaction initiated by photo-oxidation leads to cessation ofcrosslinking of the polymers and degradation the property of polymers.ee also
*
Applied spectroscopy
*Forensic engineering
*Forensic materials engineering
*Forensic polymer engineering
*Environmental stress fracture
*Ozone cracking
*Oxidation
*Polymer engineering
*Polymer
*Stress corrosion cracking
*Thermal degradation of polymers
*UV degradation
*Environmental stress cracking
*Synthetic biodegradable polymer
*Weather testing of polymers
*Chemically Assisted Degradation of Polymers References
* Lewis, Peter Rhys, Reynolds, K and Gagg, C, "Forensic Materials Engineering: Case studies", CRC Press (2004)
* Ezrin, Meyer, "Plastics Failure Guide: Cause and Prevention", Hanser-SPE (1996).
* Wright, David C., "Environmental Stress Cracking of Plastics" RAPRA (2001).External links
* [http://www.elsevier.com/wps/find/journaldescription.cws_home/30190/description#description The journal Engineering Failure Analysis]
* [http://www.forensic-courses.com/wordpress/?p=42; Forensic science and engineering]
* [http://www.open2.net/forensicengineering/modern_methods.html Methods of analysis]
* [http://openlearn.open.ac.uk/file.php/2980/formats/print.htm New course]
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