Some Industrially Important Polymers - Fundamentals of Industrial Chemistry: Pharmaceuticals, Polymers, and Business - page 112

be used to increase M w /M n . The use of multiple reactors in a polymerization
process also allows for changing the polydispersity by varying the conditions
in each reactor.
Polyethylene is made in a continuous fashion by solution, slurry, and
gas-phase processes. In a solution process, the reaction is run at high tem-
peratures (maybe 200 - 300 ∘ C) in the presence of an organic solvent such as
hexane. At the reaction temperature, the ethylene and the polyethylene remain
in solution during the polymerization. After the reaction, the pressure is
released and the solvent removed. In a slurry process, the temperature is lower
and the polyethylene forms as a slurry in the organic solvent. In a gas-phase
process, solid polyethylene seeds are suspended in a gaseous stream of
ethylene and the ethylene polymerizes onto these suspended seeds. These
descriptions are gross oversimplifications and much process research has
been, and continues to be, done in this area. Details of each of these processes
and recent improvements can be found by consulting the patent literature.
Polyethylene can be processed by a variety of techniques. It can be blown
into film which is then used to make items such as shrink wrap and the ubiqui-
tous grocery bags. It can be injection molded into a variety of shapes or blow
molded into milk bottles and detergent bottles. It can be rotomolded into large
parts such as toys, agricultural tanks, or kayaks.
Polyethylene has limited adhesion to paints and inks. This is because it is a
non-polar hydrocarbon incapable of forming hydrogen bonds. Adhesion can
be improved by copolymerizing ethylene with polar monomers such as ethyl
acrylate or vinyl acetate to give ethylene ethyl acrylate (EEA) copolymers or
ethylene vinyl acetate (EVA) copolymers. EVA is often used for shoe soles.

O

CO 2 CH 2 CH 3 O CH 3

Ethyl acrylate Vinyl acetate

8.2 POLYPROPYLENE

Polypropylene has many of the same desirable properties of polyethylene,
but has a higher heat capability and excellent fatigue properties. Most com-
mercial polypropylene is made with Ziegler - Natta catalysts in a gas-phase
process and is isotactic. The T m is about 160 ∘ C, about 50 ∘ C higher than
HDPE. Because of this it can be used for applications requiring somewhat
higher temperatures such as those that need sterilization by autoclave.
Unlike ethylene, propylene cannot be polymerized to a high molecular
weight by a radical process [1]. Competing with the propagation step is