- Polymer degradation
Polymer degradation is a change in the properties -
tensile strength, colour, shape, etc - of a polymeror polymer based product under the influence of one or more environmental factors such as heat, lightor chemicals. These changes are usually undesirable, such as changes during use, cracking and depolymerisation of products or, more rarely, desirable, as in biodegradationor deliberately lowering the molecular weightof 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.
molecules 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.
Today there are primarily six commodity polymers in use, namely
polyethylene, polypropylene, polyvinyl chloride, polyethylene terephthalateor PET, polystyreneand 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 oxidationand UV radiation, while PVC may discolour at high temperatures due to loss of hydrogen chloridegas, and become very brittle. PET is sensitive to hydrolysisand attack by strong acids, while polycarbonate depolymerizes rapidly when exposed to strong alkalis.
For example, polyethylene usually degrades by "random scission" - that is by a random breakage of the linkages (bonds) that hold the
atomsof the polymer together. When this polymer is heated above 450 Celsiusit 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.
Many polymers, especially step-growth polymers, are degraded by specific chemicals such as strong
acidsand 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 agentsand interaction with UV radiation.
Nylonis 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 acidfrom a nearby lead-acid batteryfell onto a nylon 6,6moulded 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 fuelpoured 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 hydrolysisof 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.
Cracks can be formed in many different
elastomersby ozoneattack. Tiny traces of the gas in the air will attack double bonds in rubber chains, with Natural rubber, Styrene-butadienerubber and NBRbeing 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 crackingcan be prevented by adding anti-ozonants to the rubber before vulcanization. Ozone cracks were commonly seen in automobile tiresidewalls, 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.
Polymers are susceptible to attack by atmospheric
oxygen, especially at elevated temperatures encountered during processing to shape. Many process methods such as extrusionand injection mouldinginvolve pumping molten polymer into tools, and the high temperatures needed for melting may result in oxidation unless precautions are taken. For example, a forearm crutchsuddenly snapped and the user was severely injured in the resulting fall. The crutch had fractured across a polypropyleneinsert within the aluminium tube of the device, and infra-red spectroscopyof 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 groupin the spectrum of polyolefins. Polypropylenehas a relatively simple spectrum with few peaks at the carbonyl position (like polyethylene). Oxidation tends to start at tertiary carbonatoms because free radicalshere 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.
Another highly reactive gas is
chlorine, which will attack susceptible polymers such as acetal resinand polybutylenepipework. 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 allyliccarbon 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 milliontraces of the dissolved gas. The chlorine attacks weak parts of a product, and in the case of an acetal resinjunction 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 carbonatesfrom the hard watersupply, 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 polybutylenepipework, and led to the material being removed from that market, although it is still used elsewhere in the world.
Hindered-amine light stabilisers(HALS) stabilise against weathering by scavenging
free radicalsthat are produced by photo-oxidation of the polymer matrix. UV-absorbers stabilises against weathering by absorbing ultraviolet light and converting it into heat. Antioxidants 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 crosslinkingof the polymers and degradation the property of polymers.
Forensic materials engineering
Forensic polymer engineering
Environmental stress fracture
Stress corrosion cracking
Thermal degradation of polymers
Environmental stress cracking
Synthetic biodegradable polymer
Weather testing of polymers
Chemically Assisted Degradation of Polymers
* 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).
* [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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