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Forensic polymer engineering is the study of failure inpolymeric products. The topic includes thefracture of plastic products, or any other reason why such a product fails in service, or fails to meet itsspecification. The subject focuses on the material evidence from crime or accident scenes, seeking defects in those materials that might explain why an accident occurred, or the source of a specific material to identify a criminal. Many analytical methods used for polymer identification may be used in investigations, the exact set being determined by the nature of the polymer in question, be itthermoset,thermoplastic,elastomeric orcomposite in nature.
One aspect is the analysis oftrace evidence such asskid marks on exposed surfaces, where contact between dissimilar materials leaves material traces of one left on the other. Provided the traces can be analyzed successfully, then an accident or crime can often be reconstructed.
Thermoplastics can be analysed usinginfra-red spectroscopy,ultraviolet–visible spectroscopy,nuclear magnetic resonance spectroscopy and theenvironmental scanning electron microscope. Failed samples can either be dissolved in a suitable solvent and examined directly (UV, IR and NMR spectroscopy) or be a thin film cast from solvent or cut usingmicrotomy from the solid product. Infra-red spectroscopy is especially useful for assessing oxidation of polymers, such as thepolymer degradation caused by faultyinjection moulding. The spectrum shows the characteristiccarbonyl group produced by oxidation ofpolypropylene, which made the productbrittle. It was a critical part of a crutch, and when it failed, the user fell and injured herself very seriously. The spectrum was obtained from a thin film cast from a solution of a sample of the plastic taken from the failed forearmcrutch.
Microtomy is preferable since there are no complications from solvent absorption, and the integrity of the sample is partly preserved.Thermosets,composites andelastomers can often be examined using only microtomy owing to the insoluble nature of these materials.
Fractured products can be examined usingfractography, an especially useful method for all broken components usingmacrophotography andoptical microscopy. Although polymers usually possess quite different properties tometals,ceramics andglasses, they are just as susceptible to failure frommechanical overload,fatigue andstress corrosion cracking if products are poorly designed or manufactured.
Scanning electron microscopy orESEM is especially useful for examining fracture surfaces and can also provide elemental analysis of viewed parts of the sample being investigated. It is effectively a technique ofmicroanalysis and valuable for examination oftrace evidence. On the other hand, colour rendition is absent inESEM, and there is no information provided about the way in which those elements are bonded to one another. Specimens will be exposed to a partial vacuum, so any volatiles may be removed, and surfaces may be contaminated by substances used to attach the sample to the mount.
Many polymers are attacked by specific chemicals in the environment, and serious problems can arise, includingroad accidents andpersonal injury.Polymer degradation leads to sample embrittlement, and fracture under low applied loads.
Polymers for example, can be attacked by aggressive chemicals, and if under load, then cracks will grow by the mechanism ofstress corrosion cracking. Perhaps the oldest known example is theozone cracking ofrubbers, where traces of ozone in the atmosphere attackdouble bonds in the chains of the materials.Elastomers with double bonds in their chains includenatural rubber,nitrile rubber andstyrene-butadiene rubber. They are all highly susceptible to ozone attack, and can cause problems like vehicle fires (from rubber fuel lines) andtyre blow-outs. Nowadays, anti-ozonants are widely added to these polymers, so the incidence of cracking has dropped. However, not all safety-critical rubber products are protected, and, since onlyppb of ozone will start attack, failures are still occurring.
Another highly reactive gas ischlorine, 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 US 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 is the thread roots that were attacked first, causing a brittle crack to grow. The discoloration 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.
Most step-growth polymers can sufferhydrolysis in the presence of water, often a reaction catalysed byacid oralkali.Nylon for example, will degrade and crack rapidly if exposed to strong acids, a phenomenon well known to people who accidentally spill acid onto their tights.
The broken fuel pipe caused a serious accident when diesel fuel poured out from a van onto the road. A following car skidded and the driver was seriously injured when she collided with an oncoming lorry.Scanning electron microscopy or SEM showed that thenylon connector had fractured bystress corrosion cracking due to a small leak of battery acid. Nylon is susceptible tohydrolysis in contact withsulfuric acid, and only a small leak of acid would have sufficed to start a brittle crack in theinjection moulded connector by a mechanism known asstress corrosion cracking, or SCC.
The crack took about 7 days to grow across the diameter of the tube, hence the van driver should have seen the leak well before the crack grew to a critical size. He did not, therefore resulting in the accident. The fracture surface showed a mainly brittle surface withstriations indicating progressive growth of the crack across the diameter of the pipe. Once the crack had penetrated the inner bore, fuel started leaking onto the road. Diesel is especially hazardous on road surfaces because it forms a thin oily film that cannot be seen easily by drivers. It is akin toblack ice in lubricity, so skids are common when diesel leaks occur. The insurers of the van driver admitted liability and the injured driver was compensated.
Polycarbonate is susceptible to alkali hydrolysis, the reaction simply depolymerising the material.Polyesters are prone to degrade when treated with strong acids, and in all these cases, care must be taken to dry the raw materials for processing at high temperatures to prevent the problem occurring.
Many polymers are also attacked byUV radiation at vulnerable points in their chain structures. Thuspolypropylene suffers severe cracking insunlight unlessanti-oxidants are added. The point of attack occurs at thetertiary carbon atom present in every repeat unit, causing oxidation and finally chain breakage.Polyethylene is also susceptible to UV degradation, especially those variants that are branched polymers such asLDPE. The branch points aretertiary carbon atoms, sopolymer degradation starts there and results in chain cleavage, and embrittlement. In the example shown at right,carbonyl groups were easily detected byIR spectroscopy from a cast thin film. The product was aroad cone that had cracked in service, and many similar cones also failed because an anti-UV additive had not been used.
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