eFunda: Overview of Polymers (Plastics)
engineering fundamentals Polymers: Overview
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Molecular Structure

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.

More about Polymers

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
Growth in polymer use has been driven by this economy, utility, design flexibility, part consolidation, and resistance to corrosion.
Discovery of polymer materials has often been accidental. First volume production plastic: Celluloid, 1868. First fully synthetic product: Bakelite, volume production started in 1909. Acetylene, obtained from coal, was originally the source of raw material for polymer production. Today, most raw materials for polymer manufacture are obtained from the cracking of naphtha in oil refineries.
A leading area in polymer development has been that of synthetic rubber (SBR w/ carbon black filler) for auto tires. Research in this area was catalyzed by the cut-off of natural rubber supply centers before WWII. The current leading area in high-volume polymer usage and development is still the auto industry.
Computers and software have become important for design and testing due to the myriad of polymer varieties.

Major Grouping Methods

Major industrial plastics groups include:

THERMOPLASTICS: one part, re-softenable w/ heat alone, crystalline or amorphous.
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.
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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