page_title: Forensic materials engineering
section_title: Elastomers
hierarchical_section_title: Forensic materials engineering / Elastomers
caption_attribution_description: English: Ozone cracking in natural rubber tubing
context_page_description: Forensic materials engineering, a branch of forensic engineering, focuses on the material evidence from crime or accident scenes, seeking defects in those materials which might explain why an accident occurred, or the source of a specific material to identify a criminal. Many analytical methods used for material identification may be used in investigations, the exact set being determined by the nature of the material in question, be it metal, glass, ceramic, polymer or composite. An important aspect is the analysis of trace evidence such as skid marks on exposed surfaces, where contact between dissimilar materials leaves material traces of one left on the other. Provided the traces can be analysed successfully, then an accident or crime can often be reconstructed. Another aim will be to determine the cause of a broken component using the technique of fractography.
context_section_description: Rubber products are often safety-critical parts of machines, so that failure can often cause accidents or loss of function. Failed products can be examined with many of the generic polymer methods, although it is more difficult if the sample is vulcanized or cross-linked. Attenuated total reflectance infra-red spectroscopy is useful because the product is usually flexible so can be pressed against the selenium crystal used for analysis. Simple swelling tests can also help to identify the specific elastomer used in a product. Often the best technique is ESEM using the X-ray elemental analysis facility on the microscope. Although the method only provides elemental analysis, it can provide clues as to the identity of the elastomer being examined. Thus the presence of substantial amounts of chlorine indicates polychloroprene while the presence of nitrogen indicates nitrile rubber. The method is also useful in confirming ozone cracking by the large amounts of oxygen present on cracked surfaces. Ozone attacks susceptible elastomers such as natural rubber, nitrile rubber and polybutadiene and associated copolymers. Such elastomers possess double bonds in their main chains, the group which is attacked during ozonolysis.
  The problem occurs when small concentrations of ozone gas are present near to exposed elastomer surfaces, such as O-rings and diaphragm seals. The product must be in tension, but only very low strains are sufficient to cause degradation.
caption: Ozone cracking in Natural rubber tubing
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