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Classification of Plastics


There exists terrific variety among the many hundreds of different types of plastics.  In order to simplify the organization of their similarites and differences, it is useful to classify plastics according to certain specific criteria:

          Polymerization Process
          Processibility
          Chemical Nature


Polymerization Process

Plastics are produced either by addition or condensation reactions.  Both processes involve the combination of two or more molecules to produce a polymer chain.  A table outlining the differences between these two processes is shown below.  

Plastics' Polymerization Processes

Addition Reactions

Condensation Reactions

By-Products

None

Water, Ammonia, Glycol

Polymer Chain Length

Specific

Varying

Molecular Weight of Products

Larger

Smaller

Examples

Polyolefins, Vinyls, Styrenes

Polyurethanes, Polyesters

Processibility

The processibility of a plastic is classified as either 'thermoplastic' or 'thermosetting'.

Thermoplastic

Thermoplastics generally have a linear or branched macromolecular structure.  A thermoplastic may be repeatedly heated and cooled.  Upon heating, the plastic softens.  Upon cooling, the plastic hardens.  It is the non-cross-linked molecular structure of these plastics which gives them their fusible properties.  Examples of thermoplastics include:

Thermosets

After these plastics have been formed, thermosets undergo a chemical reaction (by heat, catalyst, ultraviolet light) that results in a final cross-linking within the plastic.  As a result of this cross-linking, thermosets harden when they are heated; they cannot easily be molded after their initial formation.  Most phenolics are thermosets.

Chemical Nature

The chemical nature of a plastic is determined by its monomer.  There are many different plastics monomers which make up many different types of plastic.  To learn more about the monomers (as well as the properties and applications) of many different types of plastics, please visit our "Types of Plastics" page.

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