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 composites. An 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.

8. Polypropylene tends to form beads during carding. The beads get deeply loaded on the cylinder wire and also on the needles. There has to be a regular programme for removing the beads from the wire and needle. Achievable production rates are therefore lower with polypropylene than polyester because of card loading. Card processing problems are more acute with lower deniers and recycled fibres with polypropylene.

9. PP is highly inert to chemicals and is suitable as fishing nets and geotextiles in alkaline and acidic soils. PES on the other hand loses its strength in alkaline soils and should not be used. Leaching also takes place with polyester in aqueous solution leading to loss in strength.

10. PES has higher creep resistance (retention of tensile properties over long duration of time) than PP and this is a distinct advantage in geotextiles used for anchoring of soils and similar applications. Because of its low glass transition value (Tg) PP has less creep resistance. Creep elongation increases from 10 - 20% within one week with PP.

11. Polyester/ cotton and Polyester/viscose blends are very popular as apparel material. This is because of the high strength and wear resistance of polyester and higher moisture regain and skin friendliness and comfort of cotton. Further Tg value of polyester enables manufacture of wrinkle free and 'minimum iron' blend fabrics. Polypropylene is on the other hand not usually blended with cotton or viscose.

About the author:

Dr. N. Balasubramanian, who is a B.Sc.(Tech)(Madras University), M.Sc.(Bombay University, Ph.D.(Leeds University), A.T.I., F.T.I. (Textile Institute, Manchester) has been conferred Honorary Fellowship of the Textile Institute, Manchester in 2005. This award is in recognition of creativity and advancement of knowledge as a result of ingenuity and application over many years. He has over 45 Years of experience in R&D and consultancy in spinning, nonwovens, testing, quality control, training and ISO 9000.

He retired as Joint Director of Bombay Textile Research Association, Mumbai, India. Has published over 120 papers in leading textile journals, including the journal of textile institute, Textile Research Journal, Indian Journal of fibre and textile research and Indian Textile journal. He has successfully guided many students to Masters and Doctorate degree of Bombay University and has presented over 100 papers in textile conferences, seminars and workshops. Presently he is a consultant to leading textile mills and nonwoven units in India. He conducts regular refresher and training courses for mill technicians and is a visiting professor at textile colleges.

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