Page 22 - Plastics Engineering

General Properties of Plastics 5
Amorphous (continued) Crystalline (continued)

arrangement of molecules amorphous state the polymers
produces little volume change take up a closely packed,
and hence low shrinkage. highly aligned structure. This
Low chemical resistance - the produces a significant volume
more open random structure change manifested as high
enables chemicals to penetrate shrinkage.
deep into the material and to High chemical resistance - the
destroy many of the secondary tightly packed structure
bonds. prevents chemical attack deep
Poor fatigue and wear within the material.
resistance - the random structure a Good fatigue and wear
contributes little to fatigue or resistance - the uniform
wear properties. structure is responsible for
good fatigue and wear
properties.
Examples of amorphous and crystalline thermoplastics
Amorphous Crystalline
Polyvinyl Chloride (PVC) Polyethylene (PE)
Polystyrene (PS) Polypropylene (PP)
Polycarbonate (PC) Polyamide (PA)
Acrylic (PMMA) Acetal (POM)
Acrylonitrile-butadiene-styrene (ABS) Polyester (PEW, PBTF’)
Polyphenylene (PPO) Fluorocarbons (PTFE,
PFA, FEP and ETFE)

(b) Thermosetting Plastics
A thermosetting plastic is produced by a chemical reaction which has two
stages. The first stage results in the formation of long chain-like molecules similar
to those present in thermoplastics, but still capable of further reaction. The second
stage of the reaction (cross-linking of chains) takes place during moulding,
usually under the application of heat and pressure. The resultant moulding will
be rigid when cooled but a close network structure has been set up within the
material. During the second stage the long molecular chains have been interlinked
by strong bonds so that the material cannot be softened again by the application
of heat. If excess heat is applied to these materials they will char and degrade.
This type of behaviour is analogous to boiling an egg. Once the egg has cooled
and is hard, it cannot be softened again by the application of heat.
Since the cross-linking of molecules is by strong chemical bonds,
thermosetting materials are characteristically quite rigid materials and their