Introduction to Polymers and Plastics

Plastics are an important part of everyday life; products made from plastics range from sophisticated products, such as prosthetic hip and knee joints, to disposable food utensils. One of the reasons for the great popularity of plastics in a wide variety of industrial applications is the tremendous range of properties exhibited by plastics and their ease of processing. Plastic properties can be tailored to meet specific needs by varying the atomic composition of the repeat structure, by varying molecular weight and molecular weight distribution. The flexibility can also be varied through the presence of side chain branching, via the lengths and polarities of the side chains. The degree of crystalline can be controlled through the amount of orientation imparted to the plastic during processing, through co-polymerization, blending with other plastics, and through the incorporation of an enormous range of additives (fillers, fibers, plasticizers, stabilizers). Given all of the avenues available for tailoring any given polymer, it is not surprising that the variety of choices available to us today exist.
Polymeric materials have been used since early times even though their exact nature was unknown. In the 1400s, Christopher Columbus found natives of Haiti playing with balls made from material obtained from a tree. This was natural rubber, which became an important product after Charles Goodyear discovered that the addition of sulfur dramatically improved the properties; however, the use of polymeric materials was still limited to natural-based materials. The first true synthetic polymers were prepared in the early 1900s using phenol and formaldehyde to form resins—Baekeland’s Bakelite. Even with the development of synthetic polymers, scientists were still unaware of the true nature of the materials they had prepared. For many years, scientists believed they were colloids—a substance that is an aggregate of molecules. It was not until the 1920s that Herman Staudinger showed that polymers were giant molecules or macromolecules. In 1928, Carothers developed linear polyesters and then polyamides, now known as nylon. In the 1950s, Ziegler and Natta’s work on anionic coordination catalysts led to the development of polypropylene, high-density, linear polyethylene, and other stereospecific polymers. More recent developments include Metallocene catalysts for preparation of stereospecific polymers and the use of polymers in nanotechnology applications.

Properties of Selected Materials

Materials are often classified as either metals, ceramics, or polymers. Polymers differ from the other materials in a variety of ways but generally exhibit lower densities, thermal conductivities, and moduli. Table 1.1 compares the properties of polymers to some representative ceramic and metallic materials. The lower densities of polymeric materials offer an advantage in applications where lighter weight is desired. The use of additives allows the compounder to develop a host of materials for specific application. For example, the addition of conducting fillers generates materials from insulating to conducting. As a result, polymers may find application in EMI shielding and antistatic protection.
Polymeric materials are used in a vast array of products. In the automotive area, they are used for interior parts and in under-the-hood applications. Packaging applications are a large area for thermoplastics, from carbonated beverage bottles to plastic wrap. Application requirements vary widely but, luckily, plastic materials can be synthesized to meet these varied service conditions. It remains the job of the part designer to select from the array of thermoplastic materials vailable to meet the required demands.

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