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.
Rheological studies 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.
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.
Depending on the die used, extruders produce continuous cylinders of
polymer or fine filaments, as in nylon production.
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.