Wednesday, 10 June 2009
Fibres made from polymers or copolymers of olefin hydrocarbons such as ethylene, propylene are called polyolefins.
Polyethylene: Of all the fibre forming polymers, polyethylene (made by addition polymerisation) Ch2==Ch2 has the simplest structure.
Manufacture: Ethylene is the principal raw material for producing polyethylene fibres. Ethylene gas is obtained by cracking petroleum.
Polymerisation: Ethylene is polymerised under severe conditions in autoclaves at 200 deg C and 1500 atmospheric pressure in the presence of traces (0.01%) of oxygen acting as a catalyst. The polymer resembles paraffin wax and is characterised by low density.
Spinning : Spinning of polyethylene is carried out by melt spinning. The polymer with a molecular weight of about 15,000 is spun from the melt at about 205 deg C and extended through a spinnerette of 0.1 mm diameter into a current of cooling gas. The filaments are cooled to 15 -60 deg C and stretched 4 to 10 times their original length. The drawn monofilaments are wound on spools.
Properties of polyethylene
a. Polyethylene fibre has a round cross section and has a smooth surface. Fibres made from low molecular weight polyethylene have a grease like handle.
b. Specific Gravity- 0.92
Tenacity - 1.0-1.5 gpd
Elongation at Break %- 45-50
Tensile Strength psi - 15000
Softening Range: deg C- 85-90
c The moisture regain of polyethylene is practically nil and hence moisture does not affect the mechanical properties of the fibres.
d. Polyethylene is insoluble in most of the common organic solvents at room temperature.
e. Polyethylene fibres have a high degree of resistance to acids and alkalies at all concentrations even at high temperature.
f. The fibre is generally inert and is resistant to wide range of chemicals at ordinary temperatures. They are attacked by oxidising agents.