There are two polyolefin polymers used to make synthetic fibres, polypropylene and polyethylene, with polypropylene being by far the most important. The 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 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. Recently 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.
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 °C for polyethylene and 160 °C for polypropylene. They have a high resistance to chemical attack and modern polypropylene fibres have a high resistance to UV degradation.
Polypropylene (PP) and polyethylene (PE) polymers are the starting point of polyolefin (PO) textiles production. They are plastic resins, polymerised from propylene in the case of PP and from ethylene in the case of PE. They are supplied to customers most commonly in the form of pellets in 25 kg bags on pallets or in 25 tonne bulk loads.
Textiles are only one of many uses for PP and PE polymers. They are also used in a wide range of applications in packaging, automotive, electric & electronics, and household appliances, building & construction and industrial applications.
For further information on PP and PE polymers, and on plastics in general, see the website of PlasticsEurope www.plasticseurope.org
Staple fibre can be produced from either polypropylene (PP) or polyethylene (PE) but in practice the vast majority is made from PP.
It is manufactured by an extrusion process, melting PP or PE polymer, and by pumping the molten polymer, with the addition of additives to improve its properties, through a spinneret – a metal plate with a large number of holes. In most cases, pigments are added at this stage to give colour to the fibres. As it leaves the spinneret, the molten polymer is cooled, and forms a large number of continuous very fine fibres, called tow. These are then put through drawing and crimping processes to increase their strength and bulk, and finally cut into short lengths of fibre.
Staple fibre has a wide variety of uses, either by using the fibre directly or by putting it through a traditional textile spinning process to convert it into yarn. The biggest uses are in carpets and in nonwovens. Staple fibre is also widely used in making automotive textiles and a wide variety of technical textiles.
It is manufactured by an extrusion process melting PP or PE polymer, and by pumping the molten polymer, with the addition of additives to improve its properties, through a spinneret – a metal plate with a large number of holes. In most cases, pigments are added at this stage to give colour to the multifilament. As it leaves the spinneret, the molten polymer is cooled, and forms continuous lengths of very fine fibres. These are twisted together to form a multifilament yarn, and put through a series of drawing process to increase their strength and stability. The fineness of each individual filament of fibre, and the number of filaments combined to form the multifilament yarn, determine the thickness of the yarn. Bulked continuous filament (BCF) yarn for carpets is treated in ways which increase its bulk to make it ideal for carpet manufacture. Other processes can be carried out to modify the characteristics for specific end-uses, for example texturing to provide additional bulk and elasticity.
Multifilament yarn is very widely used in carpet pile, and also in a wide range of technical products such as geotextiles, agrotextiles and ropes. Artificial grass is another rapidly growing application. It is also used in home furnishings, and increasingly in performance sportswear and other apparel.
Both polypropylene (PP) and polyethylene (PE) are widely used in monofilaments.
Monofilament is manufactured by an extrusion process melting PP or PE polymer and by pumping the molten polymer, with the addition of additives to improve its properties, through a spinneret – a metal plate with holes to allow the polymer mixture to exit. As it leaves, the molten polymer is cooled, and each forms a single filament (much thicker than the individual filaments in multifilament yarn). This monofilament goes through a series of drawing processes to increase its strength before being wound up on spools.
Monofilament has a wide variety of industrial applications, including technical fabrics and in packaging.
TAPE AND SLIT FILM
Tapes and slit film are manufactured by an extrusion process melting polypropylene (PP) or polyethylene (PE) polymer and by pumping the molten polymer, with the addition of additives to improve its properties, as film. The film is slit into flat tapes, which are then drawn to increase strength and wound on to spools for subsequent processing. The tape is often fibrillated to improve its properties.
Tape and slit film have a huge variety of industrial and technical uses, including sacks, FIBCs, carpet backing, geotextiles, agrotextiles, and artificial grass.
Strapping is manufactured by melting polypropylene (PP) or polyethylene (PE) polymer and by extruding the molten polymer, with the addition of additives to improve its properties, as long lengths of flat strapping. Special drawing techniques give the high strength and tenacity needed by the product.
Strapping is used for packaging industrial products, often compressing them in order to reduce their bulk.
SPUNBOND AND MELTBLOWN NONWOVENS
Spunbonding is a process in which extruded molten polypropylene (PP) or polyethylene (PE) filaments are collected on to a moving belt, which are then immediately transformed into a nonwoven fleece by thermal treatment.
Meltblown nonwovens are produced by blowing high velocity air through molten polymer, spraying discontinuous very fine fibres on to a moving belt. The fibres are immediately transformed into a nonwoven fleece by thermal treatment.
Spunbond and meltblown nonwovens have a wide range of uses, including hygiene and medical products, geotextiles, agrotextiles and consumer products.
You will find more information on these and other nonwovens on www.edana.org.
Flexible Intermediate Bulk Containers (FIBCs) are one of the most widely used ways of transporting industrial and construction materials in bulk. FIBCs are normally produced by weaving polypropylene (PP) tapes into fabric, and sewing them into large bags of different formats – one loop, two loop, four loop etc. In some cases, where powders such as chemicals or fertilisers are being transported, the FIBC contains a polyethylene (PE) liner, and has valves for ease of filling and unloading. The FIBCs transport large weights of materials, and are designed to be handled mechanically by cranes and forklift trucks.
FIBCs offer a cost-effective, efficient and environmentally friendly way of transporting bulk materials.
Woven polypropylene (PP) and polyethylene (PE) sacks are used for transporting small quantities of industrial materials. They are normally produced by weaving PP tape into fabrics, followed by sewing into sacks.
Sacks are ideal for transporting any material needed in small quantities, without the requirement for mechanical handling techniques. Chemical, fertilisers and dyestuffs are often transported in sacks.
Polypropylene (PP) carpets have won a large and growing part of the European market for carpets, carpet tiles and rugs.
Several manufacturing techniques are used. Carpets can be woven or tufted, using PP BCF multifilament yarns or spun yarns using PP staple fibre, or manufactured direct from staple fibres by a needlepunching technique. BCF yarn can be given additional processing such as heatsetting to improve its characteristics even more.
PP carpets have attractive appearance and performance advantages such as stain resistance and good resilience, as well as favourable economics resulting from the high floor coverage given by each kilogramme of fibre. The fact that PP fibres and yarn are already coloured at the time of their production avoids costly and environmentally sensitive dyeing techniques.
Polypropylene (PP) is the leading material used for carpet backing – the layers underneath a carpet which provide the base for the pile surface and provide stability, comfort and cushioning.
Primary backing – the layer into which the pile of a tufted carpet is inserted – is normally in fabric woven from PP spun yarn or tape, but in some cases is made from nonwoven PP.
Secondary backing – the layer in contact with the floor – makes a large contribution to the comfort of a carpet, and is normally made from woven PP fabric, although nonwoven fabrics are also sometimes used.
ROPES, TWINES, NETS
Polypropylene (PP) and polyethylene (PE) are both used in the manufacture of ropes, twines and nets, using multifilament, monofilament or tape. Heavy duty ropes are braided together to give strength, while agricultural baler twine is made directly from fibrillated tape. Nets – for fishing or for packaging – are normally produced from monofilament.
Some ultra-high strength ropes are made from a special form of PE.
PP and PE are widely used because of their high strength, their resistance to chemicals and – an important point for marine uses – their ability to float on water. Further information on ropes, twines and nets is available on www.eurocord.com
Polypropylene ( PP) and polyethylene (PE) are used on an increasing scale in artificial sports surfaces and other forms of artificial grass. The new generation of yarns, mainly from PE but sometimes from PP, are produced from multifilament or fibrillated tape, and provide a playing surface which is soft and resilient, but allows intensive use of the sports facility throughout the year. The PP and PE yarn forms part of a construction which also includes rubber and sand foundations for the playing surface. Artificial grass surfaces are now breaking through into top-level professional sport.
Geotextiles are fabric structures used in construction and civil engineering projects, to stabilise soil, to separate layers of different materials or to assist drainage. They are a valuable means of improving effectiveness of a project, speeding up the work and reducing costs. Most geotextiles are made from spunbonded Polypropylene (PP) or polyethylene (PE), or from woven or knitted fabric (mostly using tape).
Agrotextiles are increasingly used in agriculture for a wide variety of purposes, including ground cover to insulate against cold and prevent weed growth, nets to repel birds from consuming fruit and awnings to provide shade in hot countries. Most are made from spunbonded Polypropylene (PP) or polyethylene (PE), or from woven tape.
HYGIENE AND MEDICAL
Polypropylene (PP) and polyethylene (PE) textiles are widely used for hygiene and medical purposes. Uses include disposable diapers for babies, incontinence pads for older people, wipes for cleaning, disposable garments for medical staff and much else. PP and PE are ideal for these purposes, given their resistance to unhygienic staining and their ability to contain liquids without any absorption. Bioactive additives can improve the hygiene properties of PP and PE even more.
Most of these products are made from spunbonded nonwovens, or from other nonwoven techniques using staple fibre.
FURNISHING AND AUTOMOTIVE
Polypropylene (PP) and polyethylene (PE) are widely used in the home to provide colourful and practical furnishings. Upholstery fabric and mattress covers are often made with fabrics woven from PP multifilament. Many inner and decorative parts of upholstered furniture also use polyolefin textiles.
PP and PE are also increasingly widely used in motor cars, where their light weight contributes to fuel economy. PP automotive applications include fibre for noise insulation, interior fabrics and carpets and lining for the luggage compartment. Composite moulded structures incorporating PP fibres are also penetrating the market.
Polypropylene (PP) usage in apparel is currently concentrated in socks and performance sportswear, where PP woven or knitted fabric has the capability to transport perspiration from the skin to the outer surface, where it evaporates. The garment itself remains dry and comfortable, and retains its insulation properties in cold climates. PP is also now penetrating other apparel markets, using very fine multifilament with attractive appearance and touch.
Europe is a world leader in machinery for polyolefin textiles.
European companies produce world-beating equipment for extrusion, drawing and winding, texturing, weaving, knitting, braiding, spunbonding and FIBC production.
Chemical additives are important to the market success of polyolefin fibres and textiles.
They contribute to the processability of the product, for example with lubricants and spin finishes, and provide stability to light, softness, flame retardancy and much else.
The colours and dyestuffs used also have an important role.
The success of polyolefins in textile and fibre applications is explained by a particularly positive combination of properties that meet many end-user requirements. Polyolefins are naturally hydrophobic and they don’t decompose in a wet environment. Polyolefin fibres have the ability to evacuate moisture by an unusual and unique method of transferring it towards the outside allowing it to evaporate more easily, a property that is taken most advantage of in sportswear, in baby diapers and adult incontinence products. Their chemical resistance is also of great interest and makes them resistant to soiling and to acids and alkali. Finally, the processing technologies are easily accessible and extremely versatile allowing the raw material to be processed with very low energy consumption; Environmentally, polyolefins are remarkably well tolerated and from a health and safety point of view, they are particularly harmless.
The end of life considerations for polyolefin fibres and textiles are very favourable and can offer excellent opportunities as a source of energy. Material recycling could in theory be envisaged technically, but economically it makes no sense, and the environmental benefits would not justify the effort either. The operations needed to isolate clean polyolefin fibres from other fibres and contaminants could only be justified in very limited quantities and for very specific applications. The energy recovery option is therefore much more appropriate and offers excellent possibilities. Polyolefin textile waste has a very high calorific value and is an ideal source for the production of non-hazardous “solid recovered fuels” (SRF). SRFs are becoming more and more popular in industrial processes such as cement kilns, blast furnaces or thermo-electric power plants. In some regions, communal centralised distant heating is able to combust these new fuels under safe environmental conditions to make additional savings in non-recoverable fossil fuels.