Technical fibres

Mohsen Miraftab    Department of Textiles, Faculty of Technology, Bolton Institute, Deane Road, Bolton BL3 5AB, UK

2.1 Introduction

A number of definitions1–3 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.4 Specially developed fibres for use in technical textiles are often expensive to produce and have limited applications.

Historically, utilisation of fibres in technical capacities dates back to the early Egyptians and Chinese who used papyrus mats to reinforce and consolidate the foundations respectively of the pyramids and the Buddhist temples.5,6 However, their serious use in modern civil engineering projects only began after the floods of 1953 in The Netherlands in which many people lost their lives. The event initiated the famous Delta works project in which for the first time synthetic fibres were written into the vast construction programme.7 Since then, geotextiles in particular have matured into important and indispensable multifunctional materials.

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.

In light of extensive utilization of conventional fibres in the technical sector, this chapter initially attempts to discuss fibres under this category highlighting their importance and the scope of their versatility. The discussion covers concisely an outline of fibre backgrounds, chemical compositions and their salient characteristics. It then introduces other fibres which have been specially developed to perform under extreme stress and/or temperature; ultrafine and novel fibres are also discussed. Finally, the chapter concludes by identifying areas of application and the roles that selected fibres play in fulfilling their intended purpose.

Table 2.1 presents the complete range of fibres available to the end-user and some of their mechanical properties.

2.2 Conventional fibres

2.2.1 Natural fibres

Cotton accounts for half of the world's consumption of fibres and is likely to remain so owing to many of its innate properties and for economical reasons8 that will not be discussed here. Cotton is made of long chains of natural cellulose containing carbon, hydrogen and oxygen otherwise known as polysaccharides. The length of the chains determines the ultimate strength of the fibre. An average of 10000 cel- lulosic repeat or monomeric units make up the individual cellulose chains which are about 2 mm in length. The linear molecules combine into microfibrils and are held together by strong intermolecular forces to form the cotton 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. Chemical treatments such as Proban9 and Pyrovatex10 are two examples of the type of durable finishes that can be applied to make cotton fire retardant. 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. Geographical location, the breeding habits of the animals, and climatic conditions are some of the additional variables responsible for its properties. The overall high extensibility of wool, its natural waviness and ability to trap air has a coordinated effect of comfort and warmth, which also make it an ideal insulating material. The sophisticated dual morphology of wool produces the characteristic crimp which has also been an inspiration for the development of some highly technical synthetic fibres. Wool is inherently fire retardant, but further improvements can be achieved by a number of fire-retardant treatments. Zirconium- and titanium-treated wool is one such example which is now universally referred to as Zirpro (IWS) wool.11

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. Silk has been and will remain a luxury quality fibre with a special place in the fibre market. However, its properties of biocompatibility and gradual disintegration, as in sutures, have long been recognised in medical textiles.

2.2.2 Regenerated fibres

Viscose rayon was the result of the human race's first attempts to mimic nature in producing silk-like continuous fibres through an orifice. Cellulose from wood pulp is the main constituent of this novel system, started commercially in the early 1920s. 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. Further development and refinement of the manufacturing technique have created a whole range of fibres with improved properties. High tenacity and high wet modulus viscose compare in all but appearance to cotton in both dry and wet conditions. Chemically altered regenerated cellulose di- and triacetates do not burn like cotton and viscose to leave a fluffy...