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Polymeric materials are characterized by long chains of repeated molecule units known as "mers". These long chains intertwine to
form the bulk of the plastic. The nature by which the chains intertwine determine the plastic's macroscopic properties.
Typically, the polymer chain orientations are random and give the plastic an amorphous structure. Amorphous plastics have good
impact strength and toughness. Examples include acrylonitrile-butadiene-styrene (ABS), styrene-acrylonitrile copolymer (SAN),
polyvinyl chloride (PVC), polycarbonate (PC), and polystyrene (PS).
If instead the polymer chains take an orderly, densely packed arrangement, the plastic is said to be crystalline. Such plastics
share many properties with crystals, and typically will have lower elongation and flexibility than amorphous plastics. Examples of
crystalline plastics include acetal, polyamide (PA; nylon), polyethylene (PE), polypropylene (PP), polyester (PET, PBT), and
polyphenylene sulfide (PPS).
Most plastics can be classified as either thermoplastic or thermoset, a label which describes the strength of the bonds between
adjacent polymer chains within the structure. In thermoplastics, the polymer chains are only weakly bonded (van der Waals forces).
The chains are free to slide past one another when sufficient thermal energy is supplied, making the plastic formable and
recyclable.
In thermosets, adjacent polymer chains form strong cross links. When heated, these cross links prevent the polymer chains from
slipping past one another. As such, thermosets cannot be reflowed once they are cured (i.e. once the cross links form). Instead,
thermosets can suffer chemical degradation (denaturing) if reheated excessively.
Parts and products made from polymers tend to have a much lower production cost than when they were fabricated from traditional
materials such as steel or other metals. Reasons for this include the net-shape and high-throughput molding processes and the fact
that polymers are primarily a by-product of the extremely high volume oil industry.
Additives help tailor a material to a particular application, enhance quality, and improve production. They must be used with care
and measure with standard laboratory tests since enhancing one property may degrade another. For example siffeners can cause
brittleness.
The main additives used are:
UV stabilisers, which reduce sunlight-caused brittleness.
Anti-microbials, which prevent or limit bacteria, fungus and other organic growth.
Anti-stats, which prevent build-up of static electric charges.
Flame-retardants, which control burn characteristics.
Fillers, which can reduce costs, lighten and stiffen.
Colorants, which customise appearance.
Polymers can be grouped in a few major categories.
THERMOPLASTICS: one part, re-softenable w/ heat alone, crystalline or amorphous.
| Amorphous characteristics | Crystalline characteristics |
THERMOSETS: two or more parts react, amorphous.
ELASTOMERS: Soft, pliable polymers such as rubber and neoprene.
- THERMOPLASTIC ELASTOMERS (e.g. Santoprene): injection-moldable so applicable to mass-production.
- POLYMER BLENDS: Also known as alloys, polymer blends are "mixtures" of various polymer chains to form a distinct polymer substance. Polymer blends can also include additives, reinforcements, and fillers.
- SYNTHETIC POLYMER SUBSTANCES
- MODIFIED NATURAL POLYMER SUBSTANCES
- HOMOPOLYMER: All mers along a polymer chain are of the same type.
- COPOLYMER: Mers along a polymer chain are of two or more types.
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