By: N. Balasubramanian

Retired Jt. Director (BTRA) and Consultant

Polypropylene (PP) and Polyester(PES) are the two major fibres mainly used in traditional spinning and weaving, Nonwovens, Industrial yarns and compositesAn understanding of their relative merits and limitations will be useful to those in textile business. Polyester is made from Dimethyl terepthalate (DMT) and Mono Ethylene Glycol. Modern processes use pure Terepthalic acid (PTA) in place of DMT. Polypropylene is a polyolefin made from a polypropylene monomer obtained from naphtha. Both fibres are available as virgin and bottle grade (from regenerated material). Virgin fibre is used for apparel purposes and regenerated fibre is used in nonwovens for making carpets, floor coverings, blankets and filters. Regenerated bottle grade fibre is cheaper, has lower strength and higher variability in fibre characteristics. Further it contains more fused fibres and molten chips and requires accurate control over humidity and temperature in carding room.

1. PES is available in higher tenacity grades compared to polypropylene. For industrial fabrics with higher stipulated strength, PES will be able to meet the requirements and specifications more easily

2. Polyester fibres for producing sewing threads of super tenacity are also available. Polypropylene is not normally used for sewing threads because of its low melting point.

3 Elongation is much higher in PP. This gives better elasticity for material and improved moulding in moulded automobile carpets.

4. Density of polypropylene (0.91g/cc) is much lower than that of polyester (1.38 g/cc). Diameter of polypropylene fibre is therefore proportionately higher than polyester fibre of the same denier. As a result thicker, bulkier yarns and loftier fabrics and more comfortable carpets are made with the former for a given count of yarn and area density of fabric.

5. Polypropylene is dope dyed and is available in an extensive range of colours and shades. It is therefore much easier to achieve colour and shade matching by mixing a minimum number of shades of fibres. Dope dyed polyester, on the other hand, is available only in a limited number of colours and shades. The required shade has often to be developed through R&D work or fibre has to be often dyed. With dyed fibre fastness to rubbing and washing will not be as good as dope dyed fibre.

6. Melting point of polypropylene (165oC) is much lower than that of polyester (260oC). Material made from this fibre is therefore not suitable in fire fighting and similar clothing where temperatures are high. Heating time, temperature and pressing time are therefore more critical in moulding with polypropylene. Flame retardancy by burning rate is inferior with polypropylene than with polyester. A flame retardant compound has to be added to the binder to meet the flammability requirements in Exports with polypropylene. This adds to the costs.

7. Resistance to UV light is inferior with PP compared to PES. Photo degradation takes place upon exposure to sunlight with polypropylene. UV stabiliser has to be added during manufacture of polypropylene to improve its resistance to UV light. Carbon black is the usually added UV stabiliser. With geotextiles and upholstery material continuously exposed to sunlight, PES is more suitable than PP.