Plastics and elastomers are important engineering materials primarily because of their wide
range of properties, relative ease of forming into a desired shape, and relatively low cost.
Plastic materials can be conveniently divided into two classes: thermoplastics and thermosetting plastics (thermosets). Thermoplastics require heat to make them formable, and
after cooling they retain the shape they were formed into. These materials can be reheated and reused repeatedly. Thermosetting plastics are usually formed into a permanent shape by heat and pressure during which time a chemical reaction takes place that bonds the atoms together to form a rigid solid. However, some thermosetting reactions take place at room temperature without the use of heat and pressure. Thermosetting plastics cannot be remelted after they are “set” or “cured,” and upon heating to a high temperature, they
degrade or decompose.
The chemicals required for producing plastics are derived mainly from petroleum, natural gas, and coal. Plastic materials are produced by the polymerizing of many small molecules called monomers into very large molecules called polymers. Thermoplastics are composed of long-molecular-chain polymers, with the bonding forces between the chains being of the secondary permanent dipole type. Thermosetting plastics are covalently bonded throughout with strong covalent bonding between all the atoms.
The most commonly used processing methods for thermoplastics are injection molding, extrusion, and blow molding, whereas the most commonly used methods for thermosetting plastics are compression and transfer molding and casting.
There are many families of thermoplastics and thermosetting plastics. Examples of some general-purpose thermoplastics are polyethylene, polyvinyl chloride, polypropylene, and polystyrene. Examples of engineering plastics are polyamides (nylons), polyacetal,
polycarbonate, saturated polyesters, polyphenylene oxide, and polysulfone. (Note that the
separation of thermoplastics into general-purpose and engineering plastics is arbitrary.)
Examples of thermosetting plastics are phenolics, unsaturated polyesters, melamines, and
epoxies.
Elastomers or rubbers form a large subdivision of polymeric materials and are of great engineering importance. Natural rubber is obtained from tree plantations and is still much in demand (about 30 percent of the world’s rubber supply) because of its superior elastic properties. Synthetic rubbers account for about 70 percent of the world’s rubber supply, with styrene-butadiene being the most commonly used type. Other synthetic rubbers such as nitrile and polychloroprene (neoprene) are used for applications where special properties such as resistance to oils and solvents are required.
Thermoplastics have a glass transition temperature above which these materials behave as viscous or rubbery solids and below which they behave as brittle, glasslike solids. Above the glass transition temperature, permanent deformation occurs by molecular chains sliding past each other, breaking and remaking secondary bonds. Thermoplastics used above the glass transition temperature can be strengthened by intermolecular bonding forces by using polar pendant atoms such as chlorine in polyvinyl chloride or by hydrogen bonding as in the case of the nylons. Thermosetting plastics, because they are covalently bonded throughout, allow little deformation before fracture.
The application of polymeric materials to the biomedical field has increased significantly. Polymers are used for cardiovascular, opthalmic, drug delivery, and orthopedic applications. Polymers are also the principal material used as biodegradable scaffolding in the tissue engineering field.
