Plastic is a material unique to modern times. Plastic is made from crude oil
and is characterized by a wide variety of qualities. Some plastics, such as
the types used to make plastic bags, are thin and flexible. Other plastics are
hard and rigid, such as the kind used to make computers. Plastics, literally
unknown 100 years ago, are now an integral part of our modern world.
Plastic is a general term that can refer to a variety of polymers, or chains of chemical units. There are six common types of polymers used for packaging materials. Most of these polymers are chains of a hydrocarbon called ethylene. Generally, plastic bags are made from low-density polyethylene. Milk jugs are made from high-density polyethylene. Plastic soda bottles are made from polyethylene terephthalate.
To obtain ethylene, crude oil is heated in a fractioning tower that separates the oil into its components according to their boiling point. Naptha, a component that boils about 110 degrees Celsius, is used to produce ethylene. The ethylene is polymerized and formed into small pellets. Pellets of ethylene, or any other desired polymer, are sold to manufacturers.
Manufacturers of soda bottles use ethylene to make PETE. This polymer is desirable because of its light weight and durability. Hot PETE, in the shape of a tube, is pushed into a bottle mold where a jet of air forces the tube to expand against the walls of the mold. The mold is then opened and the PETE bottle is allowed to cool.
It is now possible to recycle PETE into the new soda bottles. First, the PETE plastic must be separated from other types of plastics. This can be done manually by using the numbers on the bottom of most containers (PETE = 1). Automated systems, which separate plastics according to differences in their density, are being developed. At this time, however, these systems are too expensive for commercial applications.
Once separated from other plastics, the PETE is shredded, cleaned and granulated. The PETE is then reduced to its monomer form (ethylene) in a process called methanolysis. The resulting monomers are then repolymerized, repelletized and resold to manufacturers.
PETE plastic has been recycled into carpet, sleeping bags and park benches for a number of years. "Closed loop" plastic recycling (i.e. making new plastic soda bottles from old plastic soda bottles) is a relatively new technology. For many years it was prohibited due to health concerns. In 1991, however the methanolysis process was approved by the Food and Drug Administration for PETE soda bottles. At this time, it is unclear whether recycling reduces the amount of energy of pollution involved in the manufacturing of PETE.
Take a closer look at the types of plastics.
"Small Planet" Monterey Regional Waste Management District School Education Program.