Introduction

        Engineering thermoplastics were originally identified as such for their ability to replace metallic parts in applications such as automobiles, appliances, and housewares, and other natural materials in which strength, stiffness, or some other mechanical property was critical for use. Although this criterion still applies, these resins have been used in many applications beyond those originally associated with structural applications. Therefore, a more useful definition focuses on the properties of the resins. Most of the resins identified as engineering thermoplastics possess the following key property characteristics:

• High strength and stiffness, comparable to most metals and other natural, structural materials when adjusted for the difference in weights. Some of the engineering thermoplastics may require reinforcements to match the stronger metals in strength or stiffness.
  
• Retention of mechanical properties over a wide range of temperatures, especially high temperatures. Most engineering thermoplastics have continuous-use temperatures, suggesting a substantial retention of properties in excess of 175°F (80°C).
  
• Toughness that is sufficient to withstand incidental impacts that accompany applications in which these plastics may be used. Some are as tough as the ductile metals.
  
• Dimensional stability throughout the temperature range of normal use. This is especially important when plastic parts are substituted for metals. Dimensional stability can be determined by measuring the creep and the coefficient of thermal expansion (CTE) or the coefficient of linear expansion (CLE). (Creep is the tendency of materials to change dimensions under load while at normal-use temperatures and is a particular problem when the material is used in a structural application. CTE and CLE are measures of the expansion of a material when heated. Low values of creep and expansion are preferred so that the fit of the part into an assembly can remain within tight tolerances.)
  
• The ability of the material to withstand environmental factors such as water, solvents and other chemicals, UV light, and oxygen. Engineering thermoplastics and metals are resistant to most of these factors, but not all. Environmental resistance is, therefore, an important consideration for each intended use. Resistance to all environmental factors is seldom required, although the broader the spectrum of resistance, the better.
  
• As easy to shape and finish as metals because they are often substituted directly for a metal. Plastics are normally superior to metals in this property. The opportunity to combine several metal parts into a single plastic molded part is one of the most important considerations in the substitution of the plastic for a metal.

          Other properties can be important for specific applications. Some of the most common are abrasion resistance, extended fatigue life, lubricity, electrical properties (generally high dielectric strength, high dissipation factor, and low dielectric constant), flammability, and overall cost (which may include manufacturing time and complexity, equipment investment, possibilities for parts consideration, and raw material cost). Some applications require a level of performance that can only be met by an engineering thermoplastic. In these cases, the plastic is not replacing a metal but is creating a new application capability. Examples are the electrical resistance and lubricity of fluoropolymers and the clarity and shatter resistance of polycarbonate bulletproof windows. 

        The engineering plastics usually are sold as molding resins. They can be molded into near-net-shape parts that require little finishing work after molding and into stock shapes (such as rods, tubes, and blocks) that can be machined and finished into the final part shape. Most engineering resins can also be made into fibers, films, and coatings, which is beyond the capability of most metals. 

        The commodity resins discussed in the previous chapter can meet some of the properly expectations of engineering thermoplastics but will typically have a major deficiency that prohibits their use in an engineering application. The use of commodity resins is, therefore, chiefly dependent upon their cost and processibility rather than on their structural properties. 

        The engineering resins considered in this chapter can be divided into families, each of which is discussed separately. The general family characteristics are covered, usually with comparisons to other engineering thermoplastic families and to metals or other natural materials, and the most common specific resin types within each of the families are described and the differences between types pointed out. The differences between the members of a family are generally less pronounced than the differences between one family and another or other types of materials. 

         The following families of thermoplastic (TP) engineering plastics are considered: cellulosics, polyamides or nylons, acetals or polyoxymethylenes, TP polyesters, polycarbonate, acrylics, fluoropolymers, and high-performance thermosplastics (including polyphenylenes, polysulfones, TP polyimides, and polyaryletherketones). Appendix 1 presents a table comparing the most important properties of representative members of each of these families. The reader may want to consult that table while reading this chapter to assist in distinguishing the differences between the engineering thermoplastics families. 

         The names of the engineering plastics families are based on the material of origin of the plastic or on the most important or distinguishing functional group or type of bond in the polymer. For instance, cellulosics are derived from cellulose, the amide functional group characterizes the polyamides, and the acetal bond characterizes the acetals. The distinguishing bond or functional group is often the bond or group that is formed in the polymerization reaction. A prior knowledge of these functional groups or types of bonds is useful but is not required for a clear understanding of the properties of each of the families. The polymer properties are based upon the principles previously used to understand and predict polymeric properties, namely, functional group size, electronegativities within the functional group, chemistry of the group, and intermolecular interactions. 

        In general, the names of individual polymers, that is, specific polymers within the various families, are based on two systems that depend upon the polymerization mechanism. If the polymer is made by addition polymerization, the name of the monomer is the basis of the polymer name. For instance, polytetrafluoroethylene is made by addition polymerization from the monomer tetrafluoroethylene. (The commodity polymers polyethylene, polypropylene, polystyrene, and polyvinyl chloride are all made by addition polymerization, so they are also named from their respective monomers.) On the other hand, if the polymer is made by condensation polymerization, the polymer is named according to the distinguishing functional group or bond, just as the engineering thermoplastic families are named. Examples are polycarbonate, which is distinguished by the carbonate bond, and polyetheretherketone, which is named by describing all of the key bonds in the polymer. Engineering thermoplastics are made by both the addition and condensation polymerization reactions. The key to their performance is not the method of polymerization, but the structure and chemistry of the polymer and its interactions.