Engineering and Environmental Technology, Research Triangle Institute
The first totally man-made polymer to be synthesized was the phenol formaldehyde resin (called Bakelite at the time) made by Leo Baekeland in his garage in Yonkers, New York, back in 1907.1 It was an immediate success not only as a replacement for shellac in electrical wiring (the primary reason for its invention) but also in numerous consumer uses including the body of the old black dial telephones and in early electrical fittings. Since that time, plastics have grown rapidly and have now become an indispensable part of everyday life. The exponential growth of plastics and rubber use, essentially over a short period of half a century, is a testimony to the versatility, high performance, and cost effectiveness of polymers as a class of materials.
Polymers derive their exceptional properties from an unusual molecular architecture that is unique to polymeric materials, consisting of long chainlike macro-molecules. While both plastics as well as elastomers (rubber-like materials) are included in polymers, discussions on environment-related issues have mostly centered around plastics because of their high visibility in packaging and building applications. But elastomers are used in high-volume applications as well, particularly in automobile tires where the postconsumer product has to be effectively disposed of or recycled. With the rapid growth in automobiles, there is a consistent high demand for tires, which have a relatively short lifetime (compared to that of the automobile itself). The environmental concerns on elastomers are outside the scope of this discussion but are certainly serious enough to warrant a comprehensive study. These include the health impacts of rubber
1 The first plastic material ever made was probably cellulose nitrate synthesized in 1862 by Alexander Parkes. A second cellulose-based polymer, cellophane, was invented in early 1900s by a Swiss chemist (Edwin Brandenberger). Cellophane found many applications and is still manufactured in the U.S.
Plastics and the Environment, Edited by Anthony L. Andrady. ISBN 0-471-09520-6 © 2003 John Wiley & Sons, Inc.
compounding chemicals, the inhalation of particulate rubber that enters the atmosphere through breakdown of tires in use, and problems related to disposal of postconsumer tires.
Many of the common thermoplastics2 used today, however, were developed after the 1930s; and a few of these even emerged after World War II. Among the first to be synthesized were the vinyl plastics derived from ethylene. The Du Pont Company and I.G. Farben (Germany) filed the key patents on the synthesis of vinyl chloride copolymers back in 1928. The homopolymer poly(vinyl chloride) (PVC) was already known but not recognized as being particularly useful at the time, although Semon in B.F Goodrich found the plasticized resin to be somewhat useful in the waterproofing of fabrics. Vinyl polymers and their plasticized compounds were being commercially produced in the United States even prior to World War II. But the now common rigid PVC used in building was a postwar development that rapidly grew in volume to a point that by the early 1970s the demand for vinyl resin was close to that for polyethylene!
Polyethylene, the plastic used in highest volume worldwide, was discovered at Imperial Chemical Industries (ICI) research laboratories in 1933, but the company filed the patent on the relevant polymerization process only in 1936. This high-pressure polymerization route was exclusively used to commercially produce low-density polyethylene (LDPE) for nearly two decades until the low-pressure processes for high-density polyethylene (HDPE) were developed in 1954. Linear low-density copolymers of ethylene (LLDP), intermediate in structure and properties between the HDPE and LDPE, followed even more recently in the 1970s. In the last decade yet another new class of polyethylene based on novel metallocene catalysts has been developed. Polypropylene manufacture started relatively late in the 1950s only after the stereospecific Ziegler-Natta catalysts that yielded high-molecular-weight propylene polymers became available. While a range of copolymers of ethylene is also commercially available, the homopolymer of propylene enjoys the highest volume of use. Polyethylene, polypropylene (and their common copolymers) are together referred to as polyolefins.
Several other common thermoplastics emerged about the same time as LDPE in 1930s. Polystyrene, for instance, was first produced in 1930 and by 1934 plants were in operation producing the commercial resin in both Germany and the United States. Poly(methylmethacrylate) (PMMA) was developed by ICI about the same period. Carothers's discovery of nylons (introduced in 1939 at the World's Fair in New York) yielded a material that particularly served the allied war effort. Nylon was used extensively in tire reinforcement, parachute fabric, as well as in everyday products such as toothbrushes and women's stockings. Engineering thermoplastics such as polycarbonate by comparison are a more recent development, with commercialization by General Electric Company around 1958.
2 The term 'thermoplastics' refers to plastic materials that can be formed into different shapes by the application of heat and pressure, over and over again. These are therefore easily recyclable into other products by remelting and processing into a different shape. The other group of plastics, the thermosets, (including epoxy and polyurethane material) are crosslinked on curing and will not soften on heating to allow these to be formed into a different shape.
The millions of metric tons of polymer resins manufactured annually worldwide are predominantly derived from petroleum and natural gas feedstock, but other raw materials such as coal or even biomass might also be used for the purpose. In regions of the world where natural gas is not readily available, petroleum or coal tar is in fact used exclusively as feedstock. About half the polyolefins produced in the United States today is based on petroleum, the remainder being derived from natural gas. The crude oil is distilled to separate out the lighter components such as gases, gasoline, and kerosene fractions. Cracking is the process of catalytically converting the heavier components (or "residues" from this distillation) of crude oil into lighter more useful components. About 45% of the crude oil reaching a refinery is converted to gasoline.
Ethylene from cracking of the alkane gas mixtures or the naphtha fraction can be directly polymerized or converted into useful monomers. (Alternatively, the ethane fraction in natural gas can also be converted to ethylene for that purpose). These include ethylene oxide (which in turn can be used to make ethylene glycol), vinyl acetate, and vinyl chloride. The same is true of the propylene fraction, which can be converted into vinyl chloride and to ethyl benzene (used to make styrene). The catalytic reformate has a high aromatic fraction, usually referred to as BTX because it is rich in benzene, toluene, and xylene, that provides key raw materials for the synthesis of aromatic polymers. These include ^-xylene for polyesters, o-xylene for phthalic anhydride, and benzene for the manufacture of styrene and polystyrene. When coal is used as the feedstock, it can be converted into water gas (carbon monoxide and hydrogen), which can in turn be used as a raw material in monomer synthesis. Alternatively, acetylene derived from the coal via the carbide route can also be used to synthesize the monomers. Commonly used feedstock and a simplified diagram of the possible conversion routes to the common plastics are shown in Figure 2.1.
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