There are two polyolefin polymers used to make synthetic fibres, polypropylene and polyethylene, with polypropylene being by far the most important. The BISFA definition for polyethylene fibres is "fibre composed of linear macromolecules of unsubstituted saturated aliphatic hydrocarbons" and for polypropylene fibres "fibre composed of linear macromolecules made up of saturated aliphatic carbon units in which one carbon atom in two carries a methyl side group...". Polyethylene was first produced in the UK in 1933 by polymerising ethylene under pressure. In 1938 in Germany polyethylene was made by polymerising ethylene in an emulsion. Polypropylene was commercialised in 1956 by polymerising propylene using catalysts. Both of these polyolefins are very important in plastic moulding and for making plastic sheet but both are also spun into synthetic fibres on a large scale.
Polyolefin fibres are made by melt spinning. Usually polymer granules - made by specialist producers rather than fibre companies - are fed to an extruder which melts the polymer which is then pumped through a spinneret. The filaments are cooled in an air stream before being wound on a package or collected in cans as a tow. Because the fibres are difficult to dye, coloured pigments are often added to the polymer stream before extrusion.
An alternative process is to produce a film, cut the film into strips and then fibrillate the individual strips before winding onto a package. A new family of catalysts to make polypropylene has been developed called metallocene catalysts. It is claimed that the polymers made from these catalysts can be spun to finer counts and drawn to give higher tenacities than existing polymers.
Properties and End-Uses
Both polyolefin fibres have a density less than 1.0 and therefore, at a given decitex, are thicker than other man-made fibres and give more cover. They do not absorb moisture, which is an advantage in many end- uses, but without modification, they cannot be dyed. Their melting points are around 130 degrees C for polyethylene and 160 degrees C for polypropylene. They have a high resistance to chemical attack and modern polypropylene fibres have a high resistance to UV degradation.
Polypropylene fibre consumption has grown rapidly during the past decade. This is due largely to its acceptance as a carpet fibre and the growth in the nonwoven end-uses, especially disposables and geotextiles where polypropylene is now the dominant fibre.
In addition to these products the properties of the polyolefins make them ideal for end uses such as ropes, tapes, twines, fishing nets and FIBCs.