Polyethylene Concept
Polyethylene is a thermoplastic synthetic polymer obtained by polymerization of ethylene. It is a partially crystalline and partially amorphous material, whitish in color and translucent. The different types of polyethylene found in the market are the result of the different operating conditions carried out in the polymerization reaction.
Polyethylene structure
The chemical structure of polyethylene is -(CH2-CH2-)n. This molecule is composed in its structural unit by two carbon atoms and 4 hydrogen atoms joined together by covalent bonds. The strength of the C-C and C-H bonds is 347 and 414 Kj/mol respectively. This basic unit can be repeated indefinitely to form Polyethylene.
The number of times this basic unit is repeated depends on the type of catalyst used in the chemical reaction, temperature and pressure.
Polyethylene Classification
In general, three different types of polyethylene can be classified according to their density, which is a good indication of the type of structure of the polymer.
- Low density polyethylene
- Medium density polyethylene
- High density polyethylene
Low density polyethylene is a branched chain polymer. It is obtained by polymerization of ethylene at high pressures by the free radical mechanism.
It contains alkyl substituents, or small branches in the chain structure, which are produced during the synthesis process. It is a polymer with a density between 0.910 - 0.925 g/cm3; it is colorless, odorless and non-toxic.
Low density polyethylene is divided into: Low Density Polyethylene, Linear Low Density Polyethylene, Very Low Density Polyethylene and Ethyl - Vinyl - Acetate.
Medium density polyethylene is a polymer with a density between 0.930 - 0.940 gr/cm3, which is used especially in the manufacture of pipes.
High density polyethylene is a polymer with a linear structure and very few branches. It is obtained by polymerization of ethylene at relatively low pressures using Ziegler-Natta or Phillips Process catalysts, although there is a third one used; the Metallocene catalysts, used only to obtain Ultra High Molecular Mass Polyethylene (UHMWPE).
It is a polymer with a density between 0.941 - 0.954 gr/cm3, colorless, odorless, non-toxic and resistant to both stress and chemical agents.
Source:
UNIVERSIDAD DE SAN CARLOS DE GUATEMALA FACULTY OF ENGINEERING SCHOOL OF CHEMICAL ENGINEERING
ERNESTO ROCA GIRÓN