In 1936 the British company ICI succeeded in making the first aliphatic polyolefin plastic, polyethylene. Olefins are hydrocarbons which have one or more double bonds, and ethylene, (CH2=CH2) is the simplest member of the group. It is polymerized at about 390° F (200° C) and a pressure of more than 1000 times atmospheric pressure to give molecules of polyethylene which are simply chains of -CH2- groups.
The polyethylene produced in this way is called low-density polyethylene, which is a tough, flexible material. High-density polyethylene is more rigid than the low-density type and is made by polymerizing ethylene at much lower pressure and in the presence of a CATALYST. The difference between the two types is that the molecules of high-density polyethylene are straight whereas the molecules of low-density polyethylene are branched. By combining the use of catalysts and comonomers, a linear low-density polyethylene is produced. It gives the softness of low-density polyethylene but with much greater strength, so the thickness of film can be reduced considerably before it punctures easily. Another important polyolefin plastic is polypropylene, made by polymerizing propylene (CH2=CHCH3). Polypropylene can be represented as follows:
Polyethylene and polypropylene are used to make plastic bottles and other containers, films for packaging, pipes for plumbing, and in many other applications.
Another plastic which is related to the polyolefin plastics is polytetrafluoroethylene (PTFE), made by polymerizing tetrafluoroethylene (CF2=CF2). It consists of chains of -CF2- groups linked together, and is used where heat resistance and low surface friction are important. Nonstick coatings on domestic cooking utensils are generally made of PTFE.