Plastics

Plastics are relatively tough substances with high molecular weight that can be molded with (or without) the application of heat. In general, plastics are subclassified into thermoplastics, polymers that can be resoftened by heat, and thermosets, which cannot be resoftened by heat. Thermoplastics have moderate crystallinity. They can undergo large elongation, but this elongation is not as reversible as it is for elastomers.

Examples of thermoplastics are polyethylene and polypropylene. Thermosetting plastics are usually rigid due to high crosslinking between the polymer chains. Examples of this type are phenolfomaldehyde and polyurethanes. Crosslinking may also be promoted by using chemical agents such as sulfur or by heat treatment or irradiation with gamma rays, ultraviolet light, or energetic electrons. Recently, high energy ion beams were found to increase the hardness of the treated polymer drastically.