These materials have many different chemical and physical forms, such as cross-linked versus non-cross-linked, crystalline versus amorphous, and rubbery versus glassy. More recently, structural polymers having liquid crystalline order have become important. Structural polymers are rarely used in the pure form but often contain additives in small quantities, such as antioxidants, stabilizers, lubricants, processing aids, nucleating agents, colorants, and antistatic agents or, in larger quantities, plasticizers or fillers. There is rapid growth in the areas of blends and composites. In composites, a material of fixed shape, such as particles (filler) or fibers, is dispersed in a polymer matrix. The filler or fiber may be an inorganic material or another organic polymer. Blends (or alloys) on the other hand consist of two or more polymers mixed together and differ from composites in that the geometry of the phases is not predetermined prior to processing. Some polymers are used for many different purposes. A good example is poly(ethylene terephthalate), or PET, which was originally developed as a textile fiber. It is now used in film and tape (virtually all magnetic recording tape is based on PET), as a molding material, and as the matrix for glass-filled composites. One of its largest uses is for making bottles, especially for soft drinks. PET is also used in blends with other polymers, such as polycarbonate.
The word "plastic" is frequently used loosely as a synonym for "polymer," but the meaning of "polymer'' is based on molecular size while "plastic" is defined in terms of deformability. Plastics are polymeric materials that are formed into a variety of three-dimensional shapes, often by molding or melt extrusion processes. They retain their shape when the deforming forces are removed, unlike some other polymers such as the elastomers, which return to their original shape. Plastics are usually categorized as thermoplastics or thermosets, depending on their thermal processing behavior.
Thermoplastics are polymers that soften and flow upon heating and become hard again when cooled. This cycle can be repeated many times, which makes reprocessing during manufacturing or recycling after consumer use possible using heat fabrication techniques such as extrusion or molding. The polymer chains in thermoplastics are linear or branched and do not become cross-linked as in the case of thermosets. While there are many different chemical types of thermoplastics, those made from only four monomers (ethylene, propylene, styrene, and vinyl chloride) account for about 90 percent of all thermoplastics produced in the United States (Figure 3.1). Of these four types, polypropylene has grown most rapidly in recent years—production has increased eightfold over the past two decades. Thermoplastic polyesters, primarily PET, are growing even more rapidly at the present time (driven mainly by