Introduction
Plastics are polymers. What is a polymer? The simplest definition of a polymer is something made of many units. Think of a polymer as a chain. Each link of the chain is the "mer" or basic unit that is made of carbon, hydrogen, oxygen, and/or silicon. To make the chain, many links or "mers" are hooked or polymerized together. Polymerization can be demonstrated by linking strips of construction paper together to make paper garlands or hooking together hundreds of paper clips to form chains.
Polymers constitute a wide range of materials which are derived at least in part from organic, usually petroleum-based, raw materials; they consist of repeating molecular units and have special properties obtained by engineering the form of the molecular structures. The term polymer is derived from Greek roots and means “having many parts,” a term which aptly describes the infinite number of compounds which can be synthesized from a relatively limited number of monomer units. The term plastic is often used in describing polymers, although this term is not in current usage since it is a general descriptive
which refers to the forming rheology of many polymers but is too general to accurately describe this group of materials.
Polymers are used as engineering materials in the neat form, i.e., as the pure material, or in combination with a large diversity of additives, both organic and inorganic. These additives may be, among others, plasticizers which reduce the rigidity or brittleness of the material, fillers which increase strength andload deflection behavior under load, or stabilizers which protect the polymer against ultraviolet radiation.
The following discussion will separate polymers into two groups, thermoplastic and thermosetting, based on the distinctly different thermal processing behavior of these two broad classes of polymers.
Thermoplastic polymers soften when heated and can be reshaped, the new shape being retained oncooling. The process can be repeated many times by alternate heating and cooling with minimal degradation of the polymer structure. Thermosetting polymers (or thermosets) cannot be softened and reshaped by heating. They are plastic and moldable at some state of processing, but finally set to a rigid solid and cannot be resoftened. Thermosets are generally stronger and stiffer than thermoplastic.
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