Plastics Engineering Department
University of Massachusetts,Lowell
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 due to the tremendous range of properties exhibited by plastics and their ease of processing. Plastic properties can be tailored to meet spe- cific needs by varying the atomic makeup of the repeat structure; by varying molecular weight and molecular weight distribution; by vary- ing flexibility as governed by presence of side chain branching, as well as the lengths and polarities of the side chains; and by tailoring the degree of crystallinity, the amount of orientation imparted to the plas- tic during processing and through copolymerization, blending with other plastics, and through modification with an enormous range of additives (fillers, fibers, plasticizers, stabilizers). Given all of the avenues available to pursue tailoring any given polymer, it is not sur- prising that such a 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—aggregates of molecules with a particle size of 10- to 1000-nm diameter. 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 stere- ospecific polymers.
Polymers come in many forms including plastics, rubber, and fibers. Plastics are stiffer than rubber, yet have reduced low-temperature properties. Generally, a plastic differs from a rubbery material due to the location of its glass transition temperature (Tg). A plastic has a Tg above room temperature, while a rubber will have a Tg below room temperature. Tg is most clearly defined by evaluating the classic rela- tionship of elastic modulus to temperature for polymers as presented in Fig. 1.1. At low temperatures, the material can best be described as a glassy solid. It has a high modulus and behavior in this state is char- acterized ideally as a purely elastic solid. In this temperature regime, materials most closely obey Hooke’s law:
o = Eε
where o is the stress being applied and ε is the strain. Young’s modu- lus, E, is the proportionality constant relating stress and strain.
In the leathery region, the modulus is reduced by up to three orders of magnitude for amorphous polymers. The temperature at which the polymer behavior changes from glassy to leathery is known as the glass transition temperature, Tg. The rubbery plateau has a relatively stable modulus until as the temperature is further increased, a rub- bery flow begins. Motion at this point does not involve entire mole- cules, but in this region deformations begin to become nonrecoverable as permanent set takes place. As temperature is further increased, eventually the onset of liquid flow takes place. There is little elastic recovery in this region, and the flow involves entire molecules slipping past each other. Ideally, this region is modeled as representing viscous materials which obey Newton’s law :
o = y ε
Figure 1.1 Relationship between elastic modulus and temperature.
Plastics can also be separated into thermoplastics and thermosets. A thermoplastic material is a high molecular weight polymer that is not cross-linked. A thermoplastic material can exist in a linear or branched structure. Upon heating a thermoplastic, a highly viscous liquid is formed that can be shaped using plastics processing equip- ment. A thermoset has all of the chains tied together with covalent bonds in a network (cross-linked). A thermoset cannot be reprocessed once cross-linked, but a thermoplastic material can be reprocessed by heating to the appropriate temperature. The different types of struc- tures are shown in Fig. 1.2.
A polymer is prepared by stringing together a series of low molecu- lar weight species (such as ethylene) into an extremely long chain (polyethylene) much as one would string together a series of beads to make a necklace. The chemical characteristics of the starting low molecular weight species will determine the properties of the final polymer. When two different low molecular weight species are poly- merized, the resulting polymer is termed a copolymer such as ethylene vinylacetate.
The properties of different polymers can vary widely, for example, the modulus can vary from 1 MN/m2 to 50 GN/m2. Properties can be varied for each individual plastic material as well, simply by varying the microstructure of the material.
In its solid form a polymer can take up different structures depend- ing on the structure of the polymer chain as well as the processing con- ditions. The polymer may exist in a random unordered structure termed an amorphous polymer. An example of an amorphous polymer
Figure 1.2 Linear, branched, cross-linked polymer structures.
is polystyrene. If the structure of the polymer backbone is a regular, ordered structure, then the polymer can tightly pack into an ordered crystalline structure, although the material will generally be only semicrystalline. Examples are polyethylene and polypropylene. The exact makeup and details of the polymer backbone will determine whether or not the polymer is capable of crystallizing. This microstruc- ture can be controlled by different synthetic methods. As mentioned previously, the Ziegler-Natta catalysts are capable of controlling the microstructure to produce stereospecific polymers. The types of microstructure that can be obtained for a vinyl polymer are shown in Fig. 1.3. The isotactic and syndiotactic structures are capable of crys- tallizing because of their highly regular backbone. The atactic form would produce an amorphous material.