The Modern Era of Technical Textiles
Classification, application, methods of processing, and finishing of technical textiles
The technical textiles supply chain is a long and complex one, stretching from the manufacturers of polymers for technical fibers, coating, and specialty membranes through to the converters and fabricators who incorporate technical textiles into finished products or use them as an essential part of their industrial operations.
Developments In Technical Fibers
A number of definitions have been used to describe the term ‘technical textiles’ with respect to their intended use, functional ability and their non-aesthetic or decorative requirements. However, none of these carefully chosen words include the fundamental fibre elements, technical or otherwise, which make up the technical textile structures. The omission of the word ‘fibre’ may indeed be deliberate as most technical textile products are made from conventional fibres that are already well established. In fact over 90% of all fibres used in the technical sector are of the conventional type.
Specially developed fibres for use in technical textiles are often expensive to produce and have limited applications. Use of silk in semitechnical applications also goes back a long way to the lightweight warriors of the Mongolian armies, who did not only wear silk next to their skin for comfort but also to reduce penetration of incoming arrows and enable their subsequent removal with minimal injury. Use of silk in wound dressing and open cuts in web and fabric form also dates back to the early Chinese and Egyptians.
Developments in Technical fibers
Cotton accounts for half of the world’s consumption of fibres and is likely to remain so owing to many of its innate properties. The length of the chains determines the ultimate strength of the fibre. The unique physical and aesthetic properties of the fibre, combined with its natural generation and biodegradability, are reasons for its universal appeal and popularity. High moisture absorbency, high wet modulus and good handle are some of the more important properties of cotton fibre.
Wool, despite its limited availability and high cost, is the second most important natural fibre. It is made of protein: a mixture of chemically linked amino acids which are also the natural constituents of all living organisms. Keratin or the protein in the wool fibre has a helical rather than folded chain structure with strong inter- and intrachain hydrogen bonding which are believed to be responsible for many of its unique characteristics.
Flax, jute, hemp and ramie, to name but a few of the best fibres, have traditionally taken a secondary role in terms of consumption and functional requirements. They are relatively coarse and durable, and flax has traditionally been used for linen making. Jute, ramie and to a lesser extent other fibres have received attention within the geotextile sector of the fibre markets which seeks to combine the need for temporary to short-term usage with biodegradability, taking into account the regional availability of the fibres. Silk is another protein-based fibre produced naturally by the silkworm, Bombyx Mori or other varieties of moth.
Silk is structurally similar to wool with a slightly different combination of amino acids which make up the protein or the fibroin, as it is more appropriately known. Silk is the only naturally and commercially produced continuous filament fibre which has high tenacity, high lustre and good dimensional stability.
Viscose rayon was the result of the human race’s first attempts to mimic nature in producing silk-like continuous fibres through an orifice. Thin sheets of cellulose are treated with sodium hydroxide and aged to allow molecular chain breakage. Further treatment with carbon disulphide, dissolution in dilute sodium hydroxide and ageing produces a viscous liquid, the viscose dope, which is then extruded into an acid bath. The continuous filaments that finally emerge are washed, dried and can be cut to staple lengths. The shorter cellulose molecules in viscose and their partial crystallisation accounts for its rather inferior physical properties relative to cotton.
Lyocell, is the latest addition to this series of fibres, commercially known as Tencel (Acordis), has all the conventional properties of viscose in addition to its much praised environmentally friendly production method. The solvent used is based on non-toxic N-methyl morpholine oxide used in a recyclable closed loop system, which unlike the viscose process avoids discharge of waste. Highly absorbent derivatives of Tencel, known as Hydrocell are establishing a foothold in wound dressing and other medical-related areas of textiles.
The first synthetic fibre that appeared on the world market in 1939 was nylon 6.6. It was produced by DuPont and gained rapid public approval. A series of nylons commonly referred to as polyamides now exists in which the amide linkage is the common factor.
Nylon 6.6 and nylon 6 are most popular in fibre form. They are melt extruded in a variety of cross-sectiona shapes and drawn to achieve the desired tenacity.They are well known for their high extensibility, good recovery, dimensional stability and relatively low moisture absorbency Nylon was later surpassed by the even more popular fibre known as polyester, first introduced as Dacron by DuPont in 1951.
Polyester is today the second most used fibre after cotton and far ahead of other synthetics both in terms of production and consumption. Polyethylene terephthalate or polyester is made by condensation polymerisation of ethylene glycol and terephthalic acid followed by melt extrusion and drawing. It can be used in either continuous form or as short staple of varying lengths.The popularity of polyester largely stems from its easycare characteristics, durability and compatibility with cotton in blends.
Its very low moisture absorbency, resilience and good dimensional stability are additional qualities Wool- like properties are shown by polyacrylic fibres which are produced by the polymerisation of acrylonitrile using the addition route into polyacrylonitrile. They can then be spun into fibres by dry or wet spinning methods. Orlon14 was produced by DuPont. It had a distinctive dumbbell shaped cross-section and was extruded by the dry process in which the solvent is evaporated off.
Polyolefin fibres include both polyethylene and polypropylene made by addition polymerisation of ethylene and propylene and subsequent melt extrusion, respectively. Polyethylene has moderate physical properties with a low melting temperature of about 110 °C for its low density form and about 140 °C for its high density form which severely restricts its application in low temperature applications.
Polypropylene has better mechanical properties and can withstand temperatures of up to 140 °C before melting at about 170°C.
High performance Inorganic fibres
Any fibre that consists of organic chemical units, where carbon is linked to hydrogen and possibly also to other elements, will decompose below about 500°C and cease to have long-term stability at considerably lower temperatures. For use at high temperatures it is therefore necessary to turn to inorganic fibres and fibres that consist essentially of carbon.
Glass, asbestos and more recently carbon are three well-known inorganic fibres that have been extensively used for many of their unique characteristics.Use of glass as a fibre apparently dates back to the ancient Syrian and Egyptian civilizations which used them for making clothes and dresses. Their good resistance to heat and very high melting points has also enabled them to be used as effective insulating materials.
Ultra-fine and novelty fibres
Ultra-fine or microfibres were developed partly because of improved precision in engineering techniques and better production controls, and partly because of the need for lightweight, soft waterproof fabrics that eliminate the more conventional coating or lamination processes. As yet there are no universal definitions of microfibres.
Textile Terms and Definitions simply describes them as fibres or filaments with linear densities of approximately 1.0 dtex or less. Others have used such terms as fine, extra-fine and micro-fine corresponding to linear densities ranging from 3.0 dtex to less than 0.1 dtex. They are usually made from polyester and nylon polymers, but other polymers are now being made into microfibres.
The Japanese first introduced microfibres in an attempt to reproduce silk-like properties with the addition of enhanced durability. They are produced by at least three established methods including island-in-sea, split process and melt spinning techniques and appear under brand names such as Mitrelle, Setila, Micrell, Tactel and so on.
Once in woven fabric form their fine diameter and tight weave allows up to 30000 filaments cm-2, making them impermeable to water droplets whilst allowing air and moisture vapour circulation. They can be further processed to enhance other characteristics such as peach-skin and leather-like appearances. The split technique of production imparts sharp-angled edges within the fibre surface, which act as gentle abraders when made into wiping cloths that are used in the optical and precision microelectronic industries. Microfibres are also used to make bacteria barrier fabrics in the medical industries. Their combined effect of low diameter and compact packing also allows efficient and more economical dyeing and finishing.
Finally, constant pressure to achieve and develop even more novel applications of fibres has led to a number of other and, as yet, niche fibrous products. In principle, the new ideas usually strive to combine basic functional properties of a textile material with special needs or attractive effects.