For most plastics, the properties depend on the degree of polymerization, which explains to a large degree the wide range of properties available. For the group of plastics know as "thermosetting", a second type of polymerization takes. place in which cross-linking occurs between adjacent chains.
This thermosetting reaction frequently results in greatly increased rigidity. These two board groups pf plastics are based originally on their reaction to heat but more properly on the type of polymerization involved.
Thermoplastic Polymers
Plastics that are called "thermoplastics" have the degree of polymerization controlled in the initial manufacture of the plastic raw material or resin. These materials soften with increasing temperature and regain rigidity as the temperature is decreased. The process is essentially reversible, but in some cases, chemical changes that may cause some deterioration of properties are produced by heating.
Thermosetting Polymers
As noted before, the thermosetting plastics undergo a further cross-link type of polymerization. which for the early plastics was initiated by the application of heat, both which for many modern thermosetting plastics may be initiated by other means. In the fabrication by molding of thermosetting plastics, an initial thermoplastic stage is followed by the thermosetting reaction at higher temperatures or with prolonged heating.
Thermoplastics may be resoftened by reheating, but the thermosetting reaction is chemical in nature and irreversible so that once it has taken place, further heating results only in gradual charring and deterioration.
The origin of resin distinguishes a number of different types of plastics. Some true plastics are found in nature and uses essentially "as is". These include shellac, used most frequently as a finish for wood and as an adhesive constituent, and asphalt, used as a binder in road materials, as a constituent in some finishes, and, with fibrous filling materials, as molding compound.
A number of plastics are natural that have undergone some chemical modification but retain the general chemical characteristics of the natural material. Cellulose may be produced as paper with slight modification, as vulcanized fiber with a slightly greater modification, and as cellulose acetate with even more modification. Wood in its natural state has thermoplastic properties that are used in some manufacturing producers. Rubber latex, as found in nature, is a thermoplastic material but is generally modified by chemical additions to act as thermosetting material
The greatest number o plastics are most properly called "synthetic plastics". While many of them make use of some particular natural material, such as petroleum, as the principal constituent, the chemistry of the raw material and the chemistry of the finished plastic have no direct connection. The raw material may be thought of simply as the source of elements and compounds for the manufacture of the plastic.
The cellulose plastics among the thermoplastics and phenol formaldehyde (a phenolic) among thermosetting plastics where the first plastics to be developed and are still in wide use today.
the thermosetting plastics generally have better moisture and chemical resistance than the thermoplastics. The terms "high" and "low", when used for strengths, service temperatures and other characteristics are only relative and apply to plastics as a total group
None of the plastics has a useful service temperature as high as that of most metals, and the modulus of elasticity of all plastics is low compared with most metals.