Material Science of Polymers for Engineers

As the word itself suggests, polymers¹ are materials composed of many molecules or parts that result in long chains. These large molecules are generally referred to as macromolecules. The unique material properties of polymers and the versatility of their processing methods are attributed to their molecular structure. For many applications, the ease with which polymers and plastics² are processed makes them the most sought-after material today. Because of their relatively low density and their ability to be shaped and molded at relatively low temperatures compared to traditional materials such as metals, plastics, and polymers are the material of choice when integrating several parts into a single component – a design aspect usually called part consolidation. In fact, parts and components that have traditionally been made of wood, metal, ceramic or glass are redesigned for plastics on a daily basis.

Most topics are best introduced from a historical perspective. Although synthetic polymers and the plastics industry is a product of the 20^th Century, the history of polymers goes back several centuries. This section presents some of the key materials and people involved in making the polymer industry into what it is known today.

Polymers are macromolecular structures that are generated either synthetically or through natural processes. Cotton, silk, natural rubber, ivory, amber, and wood are a few materials that occur naturally with an organic macromolecular structure, whereas natural inorganic materials include quartz and glass. The other class of organic materials with a macromolecular structure is represented by synthetic polymers, which are generated through addition or chain growth polymerization, and condensation or radical initiated polymerization.

The heat flow through a material can be defined by Fourier’s law of heat conduction.

Rheology is the field of science that studies fluid behavior during flow-induced deformation. Of the variety of materials that rheologists study, polymers have been found to be the most interesting and complex. Polymer melts are shear-thinning, viscoelastic, and their flow properties are temperature dependent. Viscosity is the most widely used material parameter when determining the behavior of polymers during processing. Because the majority of polymer processes are shear rate dominated, the viscosity of the melt is commonly measured using shear deformation measurement devices. However, some polymer processes, such as blow molding, thermoforming, and fiber spinning, are dominated by either elongational deformation or by a combination of shear and elongational deformation. In addition, some polymer melts exhibit significant elastic effects during deformation. This chapter will concentrate on shear deformation models but, for completeness, elongational flows, concentrated suspensions, and viscoelastic fluids will also be covered. Modeling and simulation of polymer flows will be briefly discussed. For further reading on rheology of polymer melts, the reader should consult the literature [1–6]. For more detail on polymer flow and processing simulation the literature [7, 8] should also be reviewed.

The mechanical properties and the performance of a finished product are always the result of a sequence of events. Manufacturing of a plastic part begins with material choice in the early stages of part design. Then follows processing, which determines the properties of the final part and freezes them into place. During design and manufacturing of any plastic product one must always be aware that material, processing, and design properties all go hand-in-hand and cannot be decoupled. This approach is often referred to as the six P’s: polymer, processing, product, performance, post-consumer life, and profit. This chapter presents the most important polymer processing techniques available today¹. Extrusions² is covered first, followed by mixing processes and injection molding³. Secondary shaping operations are discussed next. At the end of the chapter other processes, such as calendering, coating, compression molding, and rotational molding are presented.

The mechanical properties and dimensional stability of a molded polymer part are strongly dependent upon the anisotropy of the finished part¹. The structure of the final part, in turn, is influenced by the design of the mold cavity, e. g., the type and position of the gate, and by the various processing conditions, such as injection speed, melt or compound temperatures, mold cooling or heating rates, and others. The amount and type of filler or reinforcing material also has a great influence on the quality of the final part.

Solidification is the process during which a material undergoes a phase change and hardens. The phase change occurs as a result of either a reduction in material temperature or a chemical curing reaction. As discussed in previous chapters, a thermoplastic polymer hardens as the temperature of the material is lowered below either the melting temperature for a semi-crystalline polymer or the glass transition temperature for an amorphous thermoplastic. A thermoplastic has the ability to soften again as the temperature of the material is raised above the solidification temperature. On the other hand, the solidification of a thermosetting polymer leads to crosslinking of molecules. The effects of crosslinking are irreversible and lead to a network that hinders the free movement of the polymer chains, independent of the material temperature.

Polymeric materials are implemented into various designs because of their low cost, processability, and desirable material properties. Of interest to the design engineer are the short and long-term responses of a loaded component. Properties for short-term responses are usually acquired through short-term tensile tests and impact tests, whereas long-term responses depend on properties measured using techniques such as the creep and the dynamic test.

Failure of a component is a designer’s and an engineer’s biggest headache. The field of study that analyzes failed products or predicts failure is very large and complex. With polymers, failure is caused by mechanical, thermal, chemical, or other environmental influences. This chapter begins with a small overview of fracture mechanics. Here, simple models are presented that can be used to quantify the strength of polymers. The subsequent sections cover short-term behavior, impact strength, creep rupture, fatigue, wear, and environmental effects.

In contrast to metals, common polymers are poor electron conductors. Similar to mechanical properties, their electric properties depend to a great extent on the flexibility of the polymer’s molecular blocks. The intent of this chapter is to familiarize the reader with electrical properties of polymers by discussing their dielectric, conductive, and magnetic properties.

Because some polymers have excellent optical properties and are easy to mold and form into any shape, they are often used to replace transparent materials such as inorganic glass. Polymers have been introduced into a variety of applications, such as automotive headlights, signal light covers, optical fibers, fashion jewelry, chandeliers, toys, and home appliances. Organic materials, such as polymers, are also an excellent choice for high-impact applications where inorganic materials, such as glass, would easily shatter. However, due to the difficulties encountered in maintaining dimensional stability, they are not suitable for precision optical applications. Other drawbacks include lower scratch resistance, when compared to inorganic glasses, making them still impractical for applications such as automotive windshields.

Because of their low density, polymers are relatively permeable to gases and liquids. A more in-depth knowledge of permeability is necessary when dealing with packaging applications and with protection coatings for corrosive environments. The material transport of gases and liquids through polymers consists of various steps.

Sound waves, similar to light waves and electromagnetic waves, can be reflected, absorbed, and transmitted when they strike the surface of a body. The transmission of sound waves through polymeric parts is of particular interest to the design engineer. Of importance is the absorption of sound and the speed at which acoustic waves travel through a body, for example in a pipe, in the form of longitudinal, transversal, and bending modes of deformation.