Plastics Technology Practice - The Hong Kong Polytechnic University

2.2 Thermosets
Thermoset materials are heat-sensitive
synthetic materials which, when subject to
heat and usually pressure, will undergo
chemical change with their molecules crosslinked
together to become permanently
insoluble and infusible. Thermosets cannot
be remelted and reformed after cured and
the process is irreversible. This reaction is
somewhat like cooking an egg: once cooked, it is set permanently.
Plastics Technology Practice
2.3 Elastomers
Elastomers are natural or synthetic materials with rubbery properties that can be
stretched to at least 200 percent of their original length repeatedly (at room
temperature) and which will return with force to their approximate original length
when the applying stress is released. Natural rubber is an agricultural products
harvested mainly from Thailand, Indonesia and Malaysia in meeting the rapid
demand of automobile tyre industry, latex gloves, high pressure hydraulic hoses,
escalator handrails, rubber seals, rubber pad, elastic rubber thread and ribbed
rubber sheets. Thermoplastic elastomers (TPSs) is a kind of injection mouldable
plastics that are low modulus, flexible with both thermoplastic and elastomeric
properties in replacing traditional rubbers. The TPE is a class of copolymers based
on urethanes, polyesters, styranics and olefins. TPSs are found in products for the
consumers, medical, sports and leisure, automotive, lawn and personal care
market segments for their ease of processing and soft to touch texture.
2.4 Additives and Fillers
Additives and fillers are added to improve the performance or to reduce cost of
polymer during processing, or their servicing capabilities. The followings are some
common additives and fillers.
• Anti-microbial imparts protection against mould, mildew, fungi and
bacterial growth to materials. Without anti-microbials, polymers can
experience surface growths, causing allergic reactions, unpleasant odours,
staining, embrittlement, and premature product failure.
• Antioxidants are used in a variety of resins to prevent oxidative
degradation. Such degradation occurs by the initiation of free radicals,
which possess unpaired electrons and are highly reactive. These radicals
are created by heat, radiation, mechanical shear or metallic impurities.
Free radicals may also form during polymerization, processing of
fabrication. The function of antioxidants is to prevent the propagation
steps of oxidation.
• Antistatic agents are additives used in plastics to prevent the buildup of
excess electric charge. This electricity is formed during processing,
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