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6.11.1 PLASTICS MANUFACTURE
The raw materials for virtually all plastics are natural gas, petroleum, and liquified petroleum gases.
Simple hydrocarbon monomers serve as the building blocks for conventional plastics. These
monomers are linked together to form long chains of repeating molecules called polymers. In the
simplest case, gaseous ethylene monomers (–CH –) are concatenated to produce a solid polymer
2
measuring tens of thousands of carbons in length (Figure 6.11). Hundreds of these high-molecular-
weight polymers are in use for plastics manufacture. Each polymer possesses unique properties
such that it will meet the requirements of industry and the consumer. About 80% of plastic used in
consumer products is either polyethylene terepthalate (PET), also known as #1 plastic, or high-den-
sity polyethylene (HDPE) or # 2 plastic. As we shall see, these are the most commonly recycled
polymers as well. The plastics numbering system appears in Figure 6.12.
There are two main categories of synthetic polymers, thermoplastics and thermosets. A ther-
moplastic is one that consists of individual (nonlinked) chains of the polymer. They can be melted
and reformed into the same polymer repeatedly. In contrast, thermosets consist of polymer chains
linked to each other by cross-bonding (Figure 6.13). Once a product made of thermoset plastic is
melted, it cannot be reformed. Thermoplastics make up about 90% of all plastic products.
The main manufacturing processes used to transform newly formed polymers into a useful form
are extrusion, blow molding, and injection molding. Most of these processes begin with plastic resins
as pellets. These are subsequently subjected to heat and pressure and melted before processing.

6.11.2 EXTRUSION

Extrusion molding is employed to convert plastics into continuous sheeting, film, tubes, rods, and
filaments, and to coat wire and cable. In extrusion, dry plastic beads are loaded into a hopper and
then fed into a long heating chamber through which it is moved by the action of a continuously
revolving screw. At the end of the heating chamber, the molten plastic is forced out through a small

FIGURE 6.11 Ethylene monomers joining to form a polyethylene polymer. (From McMurry, J., Organic
Chemistry, 3rd ed., 1992. Reproduced with kind permission of Brooks/Cole, a division of Thomson Learning:
www.thomsonrights.com.)

FIGURE 6.12 Plastics numbering system.