Polyethylene is manufactured by the low pressure polymerisation of ethylene. The structure of ethylene is given in figure 2 below.

Polyethylene molecules are branched, with the degree of branching in the molecular chains and the length of side chains having a considerable effect on the properties of polyethylene. When molten polyethylene cools, it crystallises. Different degrees of crystallisation produce different densities, due to the crystallisation regions having a higher density than amorphous regions. The degree of crystallinity ranges between 35% and 80% according to polyethylene grade. High density polyethylene (HDPE) attains a 60-80% degree of crystallinity at a density of 0.940 g/cm 3 to 0.965 g/cm 3; medium density polyethylene (MDPE) attains a 50-60% degree of crystallinity at a density of 0.930 g/cm 3.

Figure 2

Structure of Ethylene

The properties of polyethylene are primarily determined by density, molar mass and molar mass distribution.

With increasing density (higher crystallinity), the following properties are increased: tensile strength; stiffness; hardness; solvent resistance and impermeability to gases and vapours. On the other hand impact strength, transparency and stress cracking resistance decrease with increasing density.

The cylinders of Chem Resist vessels are extrusion wound in High Density Polyethylene.

This Black material exhibits excellent corrosion resistance to a wide range of inorganic and organic acids and bases.

Properties of High Density Polyethylene

Property

Test Method

Units

Value

Nominal Density

DIN 53479

g/cm3

0.95

Breaking Strength

DIN 53455

N/mm2

20…32

E-Modulus through bending (60s – value)

DIN 53457

N/mm2

650

Impact Strength

DIN 53453

kJ/m2

13

Hardness

DIN 53505

Shore D

61

Linear Coefficient of Thermal Expansion

ASTM D696-44

1/K

1.7…2.0 x 10^-4

Thermal Conductivity

DIN 52612

W/m K

0.37…0.43

Working Temperature without Mechanical Pressure

DIN 53446

ºC

-40 TO +80