The term “polymer” describes classes of molecules with large numbers of repeating structural units connected through covalent bonds. The major identifying feature distinguishing polymers from other molecules is the repetition of many similar, identical or complementary subunits.
Most manufacturers decide upon polyethylene as the polymer of choice for the plastic moulding process because of its availability, ease of use and suitable properties. According to recent reports, 80–90% of all polymers used in the plastic moulding industry are polyethylene compounds (HDPE, LDPE and LLDPE), although PVC, nylons and polypropylene compounds are also used.
The Four Main Families
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Polyethylene
PE, LDPE, HDPE, MDPE, LLDPE. Tough, weatherproof, low cost — the default choice for rotomoulding.
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Polypropylene
PP. Properties between LDPE and HDPE, with good fatigue and heat resistance.
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PVC
Polyvinyl chloride. Highly versatile and compatible with many additives; rigid or plasticised.
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Nylons
Polyamides. Tough, with strong thermal and chemical resistance up to 185 °C.
Further Reading
A paper written by J.D. Ratzlaff of Chevron Phillips Chemical Company LP in 2004, entitled “Polyethylene: Process Sensitivity in Rotational Moulding”, presents the results of a study of the impact sensitivity of polyethylene to processing conditions and discusses methods to maintain high impact standards.