3 Synthetic fibres

B. A. Muralidhar

1. INTRODUCTION

Textile fibres both natural and manufactured have been used to make clothing for several thousand years. Manufactured fibres are fibres produced from chemicals and can have superior properties to natural fibres that are grown or developed from plant and animals. The production of manufactured fibres is an example of how industrial process has contributed to modern life. Manufactured fibres are those materials that are made by human beings by drawing and orientation of polymers that are commonly called fibre-forming polymers. A manufactured fibre also referred to as synthetic fibre is a chain of small units (a chemical substance) joined together to form a large single unit called a polymer. Polymer is a Greek word; Poly meaning many and mer meaning unit/part. Polymers were accidentally synthesised, however as more and more applications were discovered polymers have penetrated into all fields of applications in large volumes. Polymers may be broadly classified as Natural and Synthetic, various other ways of classification of polymers is given in Figure

Synthetic fibres are prepared from the respective monomers by a process called polymerization. Polymerization is a chemical process that prepares and combines the components for fibre formation. Polymers are macromolecules built up by linking-up large number of smaller molecules called monomers. Polymerization can be accomplished by the following ways.

Step growth polymerization (condensation polymerization)

Chain growth polymerization (addition polymerization)

Step growth or condensation polymerization generally proceeds in distinguishable steps with the possibility to carry forward the chain propagation indefinitely. In synthesis of polymers, bi-functional or poly-functional reacting molecules are essential. Further, in this type of polymerization a small by-product like water or other low molecular weight products are eliminated.

Chain growth or addition polymerization processes are commonly characterized by a chain reaction mechanism and the chain propagation may take place by a free radical mechanism or ionic mechanism. Here the reactive double bond of the monomer is activated and opened up to form new bonds. The chain growth polymerization is characterised by three steps: initiation, growth and termination reactions.

Polymers are classified in the following ways:

1. Based on Structure,
2. Polymerization methods,
3. Response technology,
4. Chemical constituents and
5. Catalyst

Based on structure the polymers are further classified as

Physical

•  Crystalline
• Semi-crystalline
• Amorphous

Chemical

1. Linear
2. Branched
3. Cross-linked
4. Interpenetrating
5. Stero polymers

• Isotatic
• Atactic
• Syndiotactic

Based on Polymerization Methods

• Addition
• Condensation
• Special

Based on Response Technology

• Thermoplastic
• Thermosetting

Based on Chemical Constituent

• Homo polymer
• Copolymer               – Alternating- Random – Block – Graft

Composite

– Polyblend

– Plastic alloy

Based on Catalyst

• Ziegler-Natta Catalysed
• Metallocene Catalysed

Metathesis

In natural polymers, the fibre forming substance i.e. the relevant long chain polymer molecules, has been made by nature in a ready-made fibrous form, such as cotton, flax, wool and silk etc.

Synthetic fibres, on the other hand, are those in which man has generated a suitable fibrous form for him. The performance properties of synthetic fibres are determined by fibre structure, which in turn depend on the processing techniques and physical and chemical structure of the polymer.

Synthetic fibres are classified as follows:

• Polyamide fibres
• Polyester fibres
• Polyolefin fibres
• Polyurethane fibres
• Polyvinyl derivatives and other
• Miscellaneous synthetic fibres

2.  POLYAMIDE FIBERS

Polyamide  fibres  are  characterised  by  the  recurring  presence  of  -NHCO-  amide linkages in the chain backbone. Synthetic polyamide fibres are derived by two methods:

1. through the interaction of a diamine and dibasic acid by a condensation reaction e.g. hexamethylene diamine and adipic acid and

   2. the ring opening polymerization of a lactam e.g. self-condensation of caprolactam After the end of World War II, two varieties of fibre nylon 6 and nylon 66 have established as the most important synthetic fibres.

Nylon 6.6 is spun by condensation of hexamethylene diamine and adipic acid whereas, Nylon 6 is spun from the self-condensation of caprolactam. Both these fibres are produced as monofilament, multifilament, tow and staple form to virtually suit all textile requirements. Nylon are smooth surfaced fibres produced by melt spinning normally in a round cross-section. The fibres are available in dull, semi-dull and bright lustres. They posse exceptional mechanical properties include high breaking elongation, high strength-to-weight ratio, excellent recovery from deformation and good flex and abrasion resistance. Its specific gravity is about 1014 g/cm3. In recent years, a number of new polyamide fibres have assumed commercial importance. Nylon was the first synthetic fibre to be commercialised in the year 1939.

End-Uses of Nylon fibres:

1. Carpets and upholstery

2. Apparel including lingerie, swimwear, sportswear, hosiery, socks, gloves etc.

3. Industrial applications include tyre cord, hose, conveyor belt, fishing net, twines rope, tents reinforced plastics etc.

3.    POLYESTER FIBERS

Polyester fibres are long chain, linear polymers made by the condensation reaction between an acid and an alcohol in which the linkage occurs through the formation of ester groups. Synthetic polyester fibres are derived by the interaction of a dibasic acid with a dihydric alcohol. First commercial polyester were spun from polyethylene terephthalate (PET) e.g. Terylene. Polyethylene terephthalate is made by the condensation reaction of ethylene glycol with dimethyl terephthalate. Pet fibres are produced as tow, staple fibres, mono and multifilament fibres generally in circular cross-section in a range of brightness and lustre, with wide range of properties to suit the specific requirements. Polyester is a medium weight fabric with its specific gravity about 1.38g/cm3 and it absorbs a very small amount of moisture about 0.4%. It has good resistance to acids, dilute alkalis, oxidising and reducing agents. Polyester fibres are smooth rod-like, having circular, trilobal or multilobal cross-section. They show good all round elastic recovery under compression, tension and shear. Have high modulus and are dimensionally stable, with low moisture regain, low creep.

End-Uses of Polyester fibres:

Polyester fibres have made their way into almost every type of apparel end use suiting’s, dress materials, floor coverings, industrial applications such as conveyor belts, fire hose, sail cloth, ropes fillings, tyre cords, electrical insulations, sewing threads etc.

4.   POLYOLEFIN FIBERS

Polyolefin are fibres manufactured by long chain polymer composed of at least 85% by weight of propylene, ethylene or other olefin units. They are spun from polymers (or) copolymers of olefin hydrocarbons, namely ethylene and propylene (monomers) products of naphtha cracking. The two most important polyolefin fibres are polyethylene and polypropylene. Olefins undergo addition polymerization

4.1 POLYETHYLENE FIBRES

The polyethylene molecules are generally branched and the degree of branching depends upon the conditions of polymerization. The polymerization of double bonded ethylene proved a difficult task. However, during the World War II, the low density polyethylene (LDPE) were developed and put to some use, nonetheless its low melting temperature and low strength were the limitations. This paved the way for the development of high density polyethylene employing the Ziegler-Natta catalyst. High density polyethylene (HDPE) is by large a linear polymer, with a high degree of crystallinity (80-98%). Both types of polyethylene may be spun into fine dinear multifilament or monofilaments in a range of diameters in round, flat, oval cross sections. Polyethylene fibres cannot be dyed effectively using the regular dyeing methods as such it is dope dyed. The specific gravity of low density polyethylene is in the region of 0.92 and for the high density polyethylene it is about 0.95 to 0.96. Polyethylenes are highly resistant to acids, alkalis and to most common organic solvents.

End-Uses of Polyethylene fibre:

The properties of polyethylene are largely influenced by the spinning and subsequent stretching. Major applications include ropes, tows, twines, nettings, upholstery, filtration fabrics, blinds, awnings, woven sacks for packaging and bags for carrying. Low cost, non-toxic lightness, rot resistance, flexibility, easy handling, resiliency are some of the advantages of polyethylene.

4.2 POLYPROPYLENE

In the early 1960s, the polyolefin were regarded as fibres with immense potential importance comparable to polyamides and polyesters. Polypropylene fibres can be melt spun like other synthetic fibres. Polypropylene molecules consists of zig-zag three dimensional chain structure with long chain of carbon atoms attached with methyl side groups. Polypropylene fibres offer higher temperature resistance, high strength, stiffness and optimum crystallinity. The spinning and processing conditions have a great influence on fibre properties. The fibres are produced in the forms of staple fibre, tow, mono and multi filament yarns and slit films. Polypropylene is inert to a wide range of chemicals and has excellent resistance to acids and alkalis.

End-Uses of Polypropylene fibres:

Polypropylene is the lightest fibre with its specific gravity about 0.90 g/cm3, both polypropylene and polyethylene are lighter than water. Its applications include, blankets, sweaters, upholstery, knitwear, conveyor belts, tyre cord, tufted carpets, fishing nets, twines, ropes, sewing threads, woven sacks, packaging etc.

5.   POLYURETHANE FIBERS

Segmented polyurethanes (Elastomers) are man-made fibres in which the fibre forming substance is a long chain synthetic polymer composed of at least 85% of segmented polyurethane. Linear polyurethane are polymers with an inter unit linkage of (–HN-COO-) urethane, made by the reaction of a diisocynate with a diol or glycol. Elastomeric polyurethane have the structural feature of block copolymer and are known by the generic name “spandex”. In the block copolymers, the long flexible segment of the molecules are joined by urethane linkages to the short stiffer segment. They are formed by the chain extension reaction of low molecular weight hydroxyl-terminated polyether and a diisocyanate. Elastomeric fibres are those which display elasticity characteristics, i.e. they stretch to several times to their original length and will snap back quickly to recover their original length. Segmented polyurethanes may be spun in the form of mono or multi filaments. Its specific gravity is in the range of 1.2 to 1.25 g/cm3. Its resistance to alkalis, solvents and common chemicals is generally good but its resistance to acids varies depending on the type of spandex.

End-Uses of Polyurethane fibres:

Segmented polyurethane filaments could be used as a bare filament, covered yarns, core-spun yarn or core-twisted yarns. Major applications include garments, swimwear, hosiery, power nets, lace, core spun yarn etc.

6.  POLYVINYL DERIVATIVES

Polymers containing the vinyl group (CH2=CH-) commonly undergo addition polymerization, without elimination of water of other materials. Modern PAN based fibres are basically co-polymers of acrylonitrile with suitable co-monomer. They are now produced under a variety of trade names. PAN fibres were subdivided into two classes based on the proportion of acrylonitrile as follows:

1.Acrylic fibres are manufactured from polymers comprising of at least 85% by weight of acrylonitrile repeating units and

2. Modacrylic fibres are manufactured from polymers comprising of at least 35-85% by weight of acrylonitrile repeating units.

The poly acrylonitrile fibres were found to be strong resistant polymers, difficult to dye. 100% PAN fibres are usually highly crystalline.

End-Uses of Polyvinyl fibres:

Acrylic have good photo stability and are stable in dilute alkali/ acid. They are used as filament or spun yarns. Its specific gravity is about 1.16 to 1.18 g/cm3. Acrylic fibres are resistant to mild acids, alkalis and most common organic solvents. Strong acids and alkalis attack the fibres. Acrylic fibres used in making carpets, knitwear, sportswear, blankets, dress materials, draperies, furnishings, non-woven and industrial fabrics

Modacrylic fibres have excellent resistance to acids, organic solvents and alkalis. Its specific gravity is relatively high at 1.37 g/cm3. Its applications include pile fabrics. Knitted goods, industrial fabrics, carpets, drapery and upholstery etc.

7.  MISCELLANEOUS SYNTHETIC FIBERS 7.1 Carbon fibres

The development of carbon filament began as early as 1850, and in 1880 carbon filament was patented for use in incandescent lamps. One of the popular techniques used in carbon fibre production is the controlled oxidation of polyacrylonitrile (PAN) precursor fibre followed by high temperature carbonization in an inert atmosphere to completely remove the nitrogen and hydrogen atoms from the polyacrylonitrile chain leaving behind carbon fibres. This carbonized carbon fibre is further heated at an elevated temperature of the range of 1000 to 3000oC to axially organize the crystalline structure. This final heat treatment is very important in producing high modulus carbon fibres.

Carbon fibres are black in colour, highly inert to chemicals, solvents and oxidising agents. They are smooth surfaced with circular cross-section and have lustre. Their density is in the range of 1.75-1.90 g/cm3. They are characterised by high strength and stiffness.

Applications include specialised composites for space vehicles, aircrafts, automobile, sports goods, marine applications, submarines, pressure vessels protective clothing etc.

   7.2 Glass fibres

Glass is essentially manufacture from sodium calcium silicate and other ingredients namely magnesia, alumina, potash, soda, boric acid in different percentages. The ingredients are charged in a furnace where they are fused at high temperature to from glass filaments directly or formed into marbles. Glass is manufactured in a large variety of compositions, two main types being ‘E’ and ‘C’ glass. ‘E’ glass has good electrical and high heat resistance. Whereas ‘C’ glass has good corrosion resistance to a wide range of acids, alkalis and chemicals.

The specific gravity of glass is about 2.54 g/cm3. Glass fibres are resistant to most solvents, acids and alkalis. Glass is not a good conductor of heat as such used for thermal and electrical insulation. Glass fibres are produced in continuous filament form or in the short staple form.

Applications of glass fibres include reinforcements in plastics, industrial filter fabrics, tyre cords, belting fabrics etc. Glass fibre in the form of chopped strand of roving’s are extensively used as reinforcing fibres in constructing aircrafts components, ship hulls, automobiles, structural sheets, roofing’s, filter cloth, optic fibres, fire proof fabrics.

7.3 Aluminium silicate fibres

Aluminium silicate fibres are manufactured by the fusion of aluminium oxide and silicon di-oxide. These fibres are used for insulation in the temperature range of 450 to 1300oC, where the regular glass, asbestos and mineral fibres become ineffective. They are typically produced as short or long staple fibres and have good resistance to most acids, and dilute alkalis.

The specific gravity of aluminium silicate fibres is 2.72 g/cm3 and are predominantly used in high temperature insulations, including electrical, thermal insulation gaskets, filters conveyors, engine blankets joint packing in furnaces, engine silencers, gas filters, castables, pressure vessels, fire resistant products, missiles, rockets, filtration of radioactive particles.

7.4 Metallic fibres

A manufacturedfibre consisting of metal filament, plastic coated metal or metal coated plastic (or) core metal filament.Metallic filaments are produced by drawing ductile metal filaments of strands.Metallic fibres have a long history and have been produced since the dawn of civilization for ornamental applications in brocades, damasks and carpets. Today metallic yarns primarily consists of aluminium filaments covered with thermoplastics or polyester films. These yarns may not be very strong but their strength is sufficient.

They are mostly used for decorative purposes in upholstery, carpets, industrial applications, medical application, fibre reinforcement, dress materials, table linen, footwear, vests, jackets curtains, packaging material etc.

8. Conclusion:

In this module, we have touched upon some of the most important synthetic fibres use in the textile industry. Here we introduce the viewer’s, to synthetic fibres such as the polyester, polyamide, polyolefin, polyethylene, polypropylene, polyurethane, poly vinyl derivatives and other miscellaneous fibres such as the carbon fibre,aluminium silicate,glass, and metallic fibres.The classification, developments, features, properties and applications are briefly discussed.

References

1. Gordon Cook J. 2001. Handbook of Textile Fibres – Man-made fibres-II. Woodhead Publishing Limited, Cambridge England.
2. Premamoy Ghosh.Fibre Science and Technology. Tata McGraw-Hill Publishing Company Limited, New Delhi.