Polyethylene Concept
Polyethylene is a synthetic thermoplastic polymer obtained through the polymerization of ethylene. It is a partially crystalline and partially amorphous material that is whitish and translucent. The various types of polyethylene available on the market result from different operating conditions during the polymerization reaction.
Structure of Polyethylene
The chemical structure of polyethylene is –(CH₂–CH₂)–n. Each structural unit of this molecule consists of two carbon atoms and four hydrogen atoms, all joined by covalent bonds. The bond energies of the C-C and C-H bonds are 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, as well as the temperature and pressure.
Classification of Polyethylene
In general, polyethylene can be classified into three different types based on its density, since density is a good indicator of the polymer's structure.
– Low-density polyethylene
– Medium-density polyethylene
– High-density polyethylene
Low-density polyethylene is a branched-chain polymer. It is produced by the polymerization of ethylene under high pressure via a free-radical mechanism.
It contains alkyl substituents, or small branches in the chain structure; these branches form during the synthesis process. It is a polymer with a density ranging from 0.910 to 0.925 g/cm³; it is colorless, odorless, and non-toxic.
Low-density polyethylene is classified into the following types: low-density polyethylene, linear low-density polyethylene, ultra-low-density polyethylene, and ethyl vinyl acetate.
Medium-density polyethylene is a polymer with a density ranging from 0.930 to 0.940 g/cm³, which is used primarily in the manufacture of pipes.
High-density polyethylene is a polymer with a linear structure and very few branches. It is produced by the polymerization of ethylene at relatively low pressures using Ziegler-Natta catalysts or the Phillips process, although a third method is also used: metallocene catalysts, which are used exclusively to produce ultra-high-molecular-weight polyethylene (UHMWPE).
It is a polymer with a density ranging from 0.941 to 0.954 g/cm³; it is colorless, odorless, non-toxic, and resistant to both mechanical stress and chemical agents.
Source:
UNIVERSIDAD DE SAN CARLOS DE GUATEMALA FACULTY OF ENGINEERING SCHOOL OF CHEMICAL ENGINEERING
ERNESTO ROCA GIRÓN