CHF Polymers & People
COMMERCIAL GIANTS
"Just remember one word... plastics."
From the motion picture The Graduate (1967)
Describing his first polymer laboratory in the 1920s, Herman Mark called his staff "synthesizers, characterizers, and appliers." At the huge German chemical firm of I.G. Farben his laboratory group experimented with reactants and catalysts until polymerization occurred. Only then would they try to analyze the product to discover what had happened in the polymerization. If they were in luck, they might be able to transform the substance into a useful material.
By 1960 polymer chemists had succeeded in turning this scenario on its head. Continued improvement in instruments for polymer study (like the electron microscope and new forms of spectroscopy), detailed investigations of the behavior of polymeric reactions, and accumulated industry know-how made possible the building of molecules by design. Control of properties at the molecular level led to a panoply of polymer uses and shapes at the workplace and at home. Production of synthetic plastics and resins had grown from 15,000 tons to over 400,000 tons between 1930 and 1945. Fifteen years later production had skyrocketed to over three million tons. The new commercial giants had arrived.
A major advance in polymer synthesis was the development of catalysts that exactly controlled the positioning of atoms attached to polymer chains. In 1953 a research team led by German chemist Karl Ziegler (1898-1973) discovered that organic compounds of metals, such as aluminum alkyl, enabled the gas ethylene to polymerize at room temperature and normal atmospheric pressure. Polyethylene had been a commercially successful plastic for a decade before Ziegler's findings, but no one had ever produced it without exposing the ethylene to high pressures and temperatures. These conditions were expensive to sustain in commercial operation. Ziegler also found that his polyethylene did not consist of ethylene units randomly attached in a branched pattern, as was the case with the polymers produced by the older methods. The compounds he and his coworkers had created contained very ordered, very long, straight-chain molecules.
Shortly after Ziegler's group obtained their new polyethylene, Giulio Natta (1903-1979) announced that his laboratory in Milan had used a Ziegler catalyst to obtain a stereoregular polypropylene. In this polymer the atoms of the propylene units that were not part of the main carbon chain--the "backbone" of the substance--occurred in a regular pattern either above or below the chain. In his polymerization of propylene, Natta obtained a mixture of a rubberlike substance and a plastic material. The only molecular difference between these two polypropylenes was the position of the side groups on the chain. When units in the chain had the same side group placement, they were plastic. When the side groups were randomly placed, they created a rubber. Changing the catalyst altered the placement of these side groups. They were either isotactic (all on the same side of the chain), syndiotactic (alternately above and below the chain), or atactic (randomly above or below).
By using Ziegler-Natta catalysts chemists could duplicate the special arrangement of side groups in naturally occurring polymers. The commercial market for such processes was tremendous, as attested soon after their discovery by the frenzied competition among corporations over proprietary rights to the new methods. In 1954 researchers achieved a long-awaited goal: they duplicated the exact structure of natural rubber, cis-polyisoprene. Our investigation of molecular giants had come full circle.
Chemical engineers produce the great variety of polymer products through two basic processes, molding and extrusion. Compression molding, in which polymer resin is placed in a heated mold and compacted, shaped the earliest plastics. Heating in such a mold creates cross-linkings between polymer chains, links that give thermosetting polymers their extreme rigidity and durability. The heated plates of a press, or platens, may seal the mold and its polymer tight, forming a positive mold; or they may allow for the flow of excess plastic, or flash, between the two halves of the form. This flash is later removed by grinding or tooling in a finishing process.
Rheology, the study of the conditions in which thermoplastic polymers flow, has enabled many improvements in another technology, injection molding. In this process polymer granules are fed from a hopper into a steel cylinder. As they enter the cylinder, the granules are forced to the heated outer edges of the vessel, where they are softened. A ram then forces the softened polymer through a narrow opening, or sprue, into a cooled mold which shapes the finished article.
The use of plastics as replacements for glass containers is aided by a process combining traditions in both polymer and glass fabrication, blow molding. In this process a sealed tube of polymer, or parison, is placed inside a hollow mold. Air is forced into the heated parison, expanding the polymer to fill the shape of the mold. Here the glassblower's craft has its modern counterpart in polymer manufacturing machinery.
Extrusion.
Depending on the die used, extruders produce continuous cylinders of polymer or fine filaments, as in nylon production. Photo courtesy of Union Carbide Corporation.
Nearly half of all polymer plastics are processed by extrusion. In this technique granular polymer is melted under heat and pressure, then forced through specially designed dies to yield a desired cross sectional shape. The extrusion process works much like the pastry bag used in decorating a cake. In order for the strand of polymer to retain its shape, it is quickly cooled by air or water upon emergence from the die.
The variety of cross-sectional shapes that can be obtained by extrusion is limited only by the imagination of the polymer engineer. Thus slot dies produce continuous polymer sheets, ring-shaped dies form tubes for wire insulation and piping, and multiholed spinneret dies yield filaments for fabric production.
Many substances are used in plastics manufacture. Plasticizers are materials added to polymers to increase their workability; common plasticizers are phthalate esters, phosphates, and glycerols. While plasticizers enable polymers to be more readily shaped, stabilizers are used to maintain the physical and chemical properties of a polymer throughout its intended period of use. Stabilizers include lead and other metal compounds and organometallic compounds. Polyvinyl chloride and polyethylene are two examples of plastics requiring stabilizers. Finally the use of plastic colorants, such as dyes and metal pigments, has introduced a spectrum of color into goods and materials where color never before existed. It was this proliferation of color that put much of the "pop" in the pop culture of the 1960s and proclaimed proudly the "art" in artificial.
Artificial polymers have certainly altered our concepts of beauty, from cellulose acetate hairsprays and casein jewelry to synthetic fabrics and polyurethane furniture. But what has been the cost of this new material world? Must the birth of polymers by artifice spell the end of natural beauty? Edwin Slosson once boasted that the greatest difficulty chemists faced in creating synthetic materials was duplicating the imperfections of nature. In the late 1960s and early 1970s, however, many people began to fear that artificial polymers might be completely incompatible with nature. By 1973 plastics made up nearly ten percent of municipal garbage; when disposed of in a sanitary landfill, chemically inert plastics were no more troublesome than other solid wastes.
Yet their resistance to attack from microorganisms threatened to make improperly handled plastics a form of permanent litter. Manufacturers of polymer products, many of which contained the company name molded into their forms, were easy targets for criticism as polluters. The issue was new to the plastics industry, which had been concerned from its earliest days primarily with the creation of products--with users at the consumer level, not with end locations in the environment. Two solutions to the plastic litter problem have received sustained attention from polymer scientists and engineers.
Recycling, a popular choice in a resource-conscious culture, proved not to be a quick remedy. Plastics made up such a small component of the total amount of waste that there was no practical way of separating them from other solids. Thermosetting plastics could not be recycled or reshaped at all. Additionally, the low cost of plastics per pound made recycled plastic uneconomical. In the 1980s, however, recycling is again receiving notice. The growth of polymer packaging, such as two-liter polyethylene terephthalate soft drink bottles, is increasing the proportion of plastics in solid wastes. While this plastic packaging cannot be reused for food storage, it may be processed for other uses, from fibrous fill material to paintbrush bristles.
Incineration has been the one proven method of complete plastics disposal. However, air quality is threatened by incomplete incineration of metal pigments and stabilizers in plastics or by the residue of thermal decomposition of polymers. The technology of the early 1970s has undergone substantial refinement. New high temperature incineration facilities have been built, allowing for "clean burns" of plastics and solid wastes with minimal release of particulates.
Other techniques have also been developed. Researchers have discovered that most plastics, because of their high molecular weight and low surface area, are resistant to microorganisms. Fungi and bacteria attack the ends of large molecules, yet the number of end groups, which is inversely proportional to the molecular weight, is relatively small in polymers. Polymer chemists have since added to the structure of many common plastics chemical groups that, though inert in visible light, absorb ultraviolet radiation in sunlight and cause polymers to break at the site of the group throughout the polymer chain. Molecular weight is reduced, and the surface area available for attack by microorganisms is increased.
The struggle to make the products of our ingenuity fully compatible with the natural environment is not unique to polymer science and engineering. Challenges lie ahead in completing the cycle of earth and air into plastics, of plastics into earth and air.
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