Polymer degradation 530817 218835689 2008-06-12T11:17:41Z Inwind 2901026 cat: '''Polymer degradation''' is a change in the properties - [[tensile strength]], [[colour]], shape, etc - of a [[polymer]] or polymer based product under the influence of one or more environmental factors such as [[heat]], [[light]] or [[chemical]]s. These changes are usually undesirable, such as changes during use, cracking and depolymerisation of products or, more rarely, desirable, as in [[biodegradation]] or deliberately lowering the [[molecular weight]] of a polymer for [[recycling]]. 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 environmental [[pollution]]. Degradation can also be induced deliberately to assist [[structure 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]] and [[polycarbonate]]. 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 to [[oxidation]] and [[UV radiation]], while PVC may discolour at high temperatures due to loss of [[hydrogen chloride]] gas, and become very brittle. PET is sensitive to [[hydrolysis]] and attack by strong [[acid]]s, while polycarbonate depolymerizes rapidly when exposed to strong [[alkali]]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 450 [[Celsius]] 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 constituent [[monomers]]. ==Examples== [[image: Plate4new.jpg|thumb|200px|Close-up of broken fuel pipe from road traffic accident]] [[Image:Broken fuel pipe.jpg|thumb|200px|Close-up of broken fuel pipe connector]] Many polymers, especially step-growth polymers, are degraded by specific chemicals such as strong [[acids]] and strong [[alkalis]]. They are made by [[condensation polymerization]], so degradation is a reversal of the synthesis reaction. Other degradation routes involve interaction with strong [[oxidising agents]] and interaction with UV radiation. ===Hydrolysis=== [[Nylon]] is sensitive to degradation by acids, a process known as [[hydrolysis]], and nylon mouldings will crack when attacked by strong acids. A fuel pipe fractured when a small drip of 40% [[sulphuric acid]] from a nearby [[lead-acid battery]] fell onto a [[nylon 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, and [[diesel 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 like [[black 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 by [[hydrolysis]] of the polymer. It was the reverse reaction of the synthesis of the polymer: [[Image:Condensation polymerization diacid diamine.svg|400px]]{{clear}} 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=== [[Image:Ozone cracks in tube1.jpg|thumb|left|200px|[[Ozone cracking]] in [[Natural rubber]] tubing]] Cracks can be formed in many different [[elastomers]] by [[ozone]] attack. Tiny traces of the gas in the air will attack double bonds in rubber chains, with [[Natural rubber]], [[Styrene-butadiene]] rubber and [[NBR]] 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 of [[ozone cracking]] can be prevented by adding anti-ozonants to the rubber before [[vulcanization]]. Ozone cracks were commonly seen in automobile [[tire]] 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=== [[Image:Irspec1.jpg|thumb|right|200px|IR spectrum showing [[carbonyl]] absorption due to oxidative degradation of [[polypropylene]] [[crutch]] moulding]] Polymers are susceptible to attack by atmospheric [[oxygen]], especially at elevated temperatures encountered during processing to shape. Many process methods such as [[extrusion]] and [[injection 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 forearm [[crutch]] suddenly snapped and the user was severely injured in the resulting fall. The crutch had fractured across a [[polypropylene]] insert within the aluminium tube of the device, and [[infra-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 of [[polyolefins]]. [[Polypropylene]] has a relatively simple spectrum with few peaks at the carbonyl position (like [[polyethylene]]). Oxidation tends to start at [[tertiary carbon]] atoms because [[free radicals]] here at more stable, so last longer and are attacked by [[oxygen]]. The carbonyl group can be further oxidised to break the chain, so weakening the material by lowering the [[molecular weight]], and cracks start to grow in the regions affected. ===Chlorine-induced cracking=== [[Image:Chlorine attack1.jpg|thumb|right|200px|chlorine attack of acetal resin plumbing joint]] Another highly reactive gas is [[chlorine]], which will attack susceptible polymers such as [[acetal resin]] and [[polybutylene]] 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 or [[allylic]] 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 at [[parts per million]] traces of the dissolved gas. The chlorine attacks weak parts of a product, and in the case of an [[acetal 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 of [[carbonates]] from the [[hard 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 to [[polybutylene]] pipework, and led to the material being removed from that market, although it is still used elsewhere in the world. ==Stabilisers== 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 of [[crosslinking]] of the polymers and degradation the property of polymers. ==See 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] [[Category:Forensics]] [[Category:Mechanical failure modes]] [[Category:Materials science]] [[Category:Polymer chemistry]] [[Category:corrosion]]