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Polymers Expertise

by Dr Peter Rhys Lewis
BSc PhD FIMMM CEng MFSS
Forensic Engineer
(More about Dr Lewis)

Products made wholly of polymers (plastics, rubbers and composites) frequently fail. If this failure causes someone to be injured or damages property, or if infringement of intellectual property rights arise, then disputes can occur. Experts will be needed because the parties to the dispute require detailed knowledge of the properties of the polymer-based products.

Polymer products are, comparatively speaking, of much more recent origin than conventional materials like metals, ceramics and glasses. So experience is more limited, and often unpublished.

Plumbing products

Some of the most serious problems have arisen with plumbing products, extruded pipes and moulded fittings, where internal attack by chlorinated water has caused brittle cracks to develop and eventually catastrophic flooding. One of the longest class actions has taken place in the USA, where acetal fittings and polybutylene pipe manufacturers are still paying out on failures in domestic houses which started in the late 1970s.

Cracks in gas pipes are far more serious because of the possibility of explosion of the developing gas cloud. There have been numerous examples where property has been destroyed and lives lost. Failure starts at the weakest parts of pipe system, such as junctions, often overloaded by ground movement after installation. Further problems are likely simply from the great length of polyethylene gas lines already in place.

Injection moulding

One of the big advantages of thermoplastics is their ease of manufacture, often by injection moulding. This process allows very complex shapes to be made quickly and easily. But often these parts fail suddenly because designers have not considered or allowed for the problem of fatigue from stress concentrations in the products, or the challenges posed by the environments in which they work. Allowance must also be made for the grade of polymer used because product strength falls rapidly with lower molecular weight grades.

Premature product failure

  • Premature failure of plastic chairs is not unusual.
  • Ladder accidents are also common, and plastic parts such as tips and feet are safety critical. When they fail, serious injuries can occur to the user.
  • Stepladders too are frequent causes of accidents, sometimes by failure of plastic connections.
  • Handles on power tools such as angle grinders are usually injection moulded plastic, and sudden fracture during use can cause severe injury.

Environmental cracking

Stress corrosion cracking and environmental stress cracking are two serious problems with all polymer products. They result from processes such as oxidation, ultra-violet degradation, hydrolysis and ozone cracking. Attack by active agents is severe and cumulative; failure is usually sudden and often catastrophic.

Critical components such as seals can cause widespread failures, especially in the pneumatic systems used for controlling industrial processes and automotive systems such as brakes.

Polymer analysis

All such problems require analysis of the broken parts using methods such as:

  • optical microscopy
  • scanning electron microscopy and
  • X-ray elemental analysis.

They are known collectively as fractography.

Chemical analysis using infra-red spectroscopy and scanning calorimetry are key checks for degradation by comparing the failed polymer with a standard.

Access to Experts

Many failures could be prevented by contacting appropriate experts in the area to perform research before introducing new polymer products into the market. The Institute of Materials, Mining and Minerals processing (IOMMM) has a list of experts, and most universities have expertise available.

Experts in this area should have relevant qualifications include an engineering, physics or chemistry degree, plus experience of investigation of polymer failures. Whilst making the initial enquiry, it is also worth asking if the expert has direct access to the necessary testing equipment.

by Dr Peter Rhys Lewis
BSc PhD FIMMM CEng MFSS
Forensic Engineer
(More about Dr Lewis)

 

 
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