Plastic Materials –An Overview
Plastics are synthetic materials called polymers, which are long-chain molecules made up of repeating units joined together. These units contain various combinations of oxygen, hydrogen, nitrogen, carbon, silicon, chlorine, fluorine, and sulfur. Although plastics are soft and mold-able and approach a liquid condition during manufacture, they are solid in their finished state. As more repeating units are added, the plastic’s molecular weight increases. Addition of more repeating units to the chain makes the molecule heavier. For example, Figure 1 shows the simple compound, methane (CH4), a gas. As molecular weight increases, typical materials are pentane (a liquid), paraffin wax (a solid), and finally properties can chain length and composition. Plastic properties can also be modified both by alloying and blending with various substances and reinforcements.
The classification of plastics can be extensive and confusing, as illustrated in Figure 1.02. However, two major groups can be identified: thermoplastics, which are the main focus of this manual, and thermosets, which are discussed only in general terms here. In addition to the broad categories of thermoplastics and thermosets, polymers can be classified in terms of their structure, i.d., crystalline, amorphous, and liquid crystalline. Other classed of plastics commonly referred to in the literature are copolymers, alloys, and elastomers. Finally, additives, reinforcements, and fillers play a major role in modifying properties. Each of these is discussed briefly.
Thermoplastics are resins that repeatedly soften when heated and harden when cooled. Most thermoplastics are soluble in specific solvents and can burn to some degree. Softening temperatures vary with