UNDERSTANDING TEXTILEFIBRES AND THEIRPROPERTIES: WHAT IS ATEXTILE FIBRE?1.1 INTRODUCTIONFibres are the foundation for all textile products and can either be natural (natural fibres) or man-made (manufacturedor man-made rengenerated). Within these two types or groups, there are two main kinds of fibres:• Fibres of indefinite (very great) length, called filaments• Fibres of much shorter length, called staple fibresFilaments are generally combined and twisted to form yarns, whilst staple fibres are spun to createyarns. Yarns are then typically woven or knitted into fabrics. A piece of a fabric contains a huge numberof fibres. For example, a small piece of lightweight fabric may contain over 100 million fibres (Morton& Hearle, 2001). Individual types of fibres can be used on their own or combined with other types offibres to enhance the quality of the end-product. The process for combining fibres is known as blending.There are many well-known blended fibres on the market, such as Viyella, whichis made of a blend ofcotton and wool (Corbman, 1983).A fibre is defined as a small threadlike structure (Hearle, 2009). The American Society for Testingand Materials (ASTM) defines a ‘fibre’ (spelt ‘fiber’ in countries using American English) as ‘a genericterm for any one of the various types of matter that form the basic element ofa textile, and it is characterisedby having a length at least 100 times its diameter’ (Anonymous, 2001). The Textile Institutedefines a fibre as a ‘textile raw material, generally characterised by flexibility, fineness and high ratioof length to thickness’ (Anonymous, 2002). A similar industry definition is a ‘unit matter with a length at least 100 times its diameter, a structure of long chain molecules having a definite preferred orientation,a diameter of 10–200 microns (micrometres), and flexibility’ (Landi, 1998). All fibres have amolecular structure that contributes to their specific attributes and properties.

The common characteristics of fibres from these definitions are:• The diameter of a fibre is small relative to its length• Properties of ‘flexibility’ and ‘fineness’ (a way of describing the thickness of a fibre)It is these key characteristics and qualities that make it possible to manipulate fibres to create the muchlarger structures that we are familiar with such as yarns and fabrics (Wilson,2001). Key characteristics,such as fineness and flexibility, as well as length and diameter, have a profound effect on the properties ofany textile product. Many of today’s textile products are made up of blends ormixtures of different typesof fibres which give a particular mix of properties that best fit how the product will ultimately be used.There are several reasons why different fibres may be blended or mixed (Bunsell, 2009; Erberle, 2004):• To compensate for weaker attributes or properties of one type of fibre• To improve the performance of the resulting yarn or fabric• To improve or provide a different appearance• To improve the efficiency of processing, especially of spinning, weaving andknitting• To reduce costsThe advantages and disadvantages of blending and mixing fibres are discussed in Part 1 and Part 2 ofthis book, and include environmental issues such as recycling blended yarns (Fletcher, 2008).

1.2 TYPES OF TEXTILE FIBRESTextile production up until the seventeenth century was predominantly a specialised domestic productionsystem (cottage industry) mainly done by women (Gordon, 2011). The key fibres used were wool, cotton,silk, hemp and flax (for linen). The advent of the Industrial Revolution meantmechanisation of the productionprocess, allowing new and much faster methods of manufacturing to emerge. Overthe next 300 years, itwould be both developments in processing and advances in engineered fibres that would change the textilelandscape. Whilst the first man-made or manufactured fibres, namely regenerated cellulose fibres, weredeveloped in the late nineteenth century, industrial production of these fibres only really started in the earlytwentieth century. Synthetic fibres were developed in the late 1930s and production took off after the SecondWorld War. The quest in the 21st century is now to create fibres that are bothfunctional and sustainable, along

with smart fibres that can be adapted precisely to the changing needs of today’s users (Aldrich, 2007; Clarke-Braddock & Harris, 2012). Designers, whether in fashion or textiles, seek alsoto push the boundaries ofdesign, by exploiting the key characteristics of fibres and their properties, such as Japanese textiles companyNUNO, or fashion designer Issey Miyake, as characterised in his formidable ‘Pleats Please’ fashion range.There are three basic types of fibre groups:• Natural fibres• Regenerated fibres• Synthetic fibresRegenerated and synthetic fibres are collectively known as man-made or manufactured fibres. The varioustypes of textile fibres are summarised in Figure 1.1. Images of some of these fibres are shown inFigure 1.2. Natural fibres are, as the name suggests, those which occur in nature, such as wool from

sheep or cotton from cotton plants (Kozlowski, 2012a, 2012b). Natural fibres are discussed in Chapters2 and 3 of this book. Regenerated fibres are made from natural polymers that are not useable in theiroriginal form but can be regenerated (i.e. reformed) to create useful fibres (Woodings, 2001). One offirst regenerated fibres was rayon, also referred to as viscose or viscose rayon, regenerated from woodpulp. Regenerated fibres are discussed in Chapter 4. The structure and properties of both natural andregenerated fibres are discussed in Eichhorn, Hearle, Jaffe, and Kikutani (2009b). In contrast, syntheticfibres are made by polymerising smaller molecules into larger ones in an industrial process (McIntyre,2004). Synthetic fibres are discussed in Chapters 5–7.Natural fibres can be divided into two main types:

• Vegetable or cellulosic fibres (discussed in Chapter 2)• Animal or protein fibres (discussed in Chapter 3)There is also one natural mineral fibre, asbestos, though this is no longer used because it has been foundto be carcinogenic. Natural rubber is sometimes included in classifications ofnatural fibres. Otherproducts, such as leather, are also sometimes classified as natural fibres, but fall outside the standarddefinitions of fibres given above.Animal (protein)-based fibres can be divided into the following categories:• Wool (from sheep)• Hair (e.g. from goats, such as mohair and cashmere; or from rabbits, such asangora)• Silk (from silkworms)Wool fibres are discussed in Simpson and Crawshaw (2002) and Johnson and Russell (2008); silk isreviewed in Babu (2013) and Sonwalker (1993); whilst other animal fibres are covered in Franck (2001).Based on which part of the plant they come from, vegetable fibres can be divided into:• Seed (e.g. cotton)• Bast (fibres derived from the outer, or bast, layers of plant stems, e.g. flax, hemp and jute)• Leaf (e.g. sisal)Cotton is reviewed in Gordon and Hsieh (2006) and Hallett and Johnston (2010, 2014), whilst bast andother plant fibres are discussed in Franck (2005).

FIGURE 1.2Images of a range of fibres: (a) longitudinal view of cotton fibres showing their characteristic twist (Source:Gordon and Hsieh (2006).); (b) view of cross-sections of cotton fibres showing their characteristic dog-boneshape (Source: Kozlowski (2012a).); (c) wool fibres showing scales on the fibre surface (Source: Eichhornet al. (2009b).); (d) silk fibres showing a trilobal cross-section (Source: Eichhorn et al. (2009b).); (e) nylonfibre with round cross-section (Source: Hearle, Lomas, and Cooke (1998).); (f) nylon fibres withhollowcross-sections in a woven fabric (Source: McIntyre (2004).); (g) viscose rayon fibre with a multilobalcross-section (Source: Hearle et al. (1998).); (h) acrylic fibre showing a dog-bone shaped cross-section.Source: Hearle et al. (1998).

Manufactured, or man-made, fibres can be classified as:• Synthetic polymers, e.g. polyester, nylon (polyamide), acrylic, lycra• Regenerated, e.g. viscose, modal, acetate• Inorganic, e.g. carbon, glass, ceramic and metallic fibers

Non-polymer fibres are discussed in Chapter 7 and are also reviewed in Eichhorn et al. (2009b). Syntheticpolymer fibres can be classified in a number of ways. One classification is asfollows (includedwith their technical definitions):• Polyesters: Defined as any long-chain synthetic polymer composed of at least85% by weight ofan ester of a substituted aromatic carboxylic acid, including, but not restricted to, substitutedterephthalate units and parasubstituted hydroxybenzoate units (e.g. PET, PTT, PBT, PEN, PLA,high-modulus high-tenacity (HM-HT) fibres).• Polyamides: Defined as polymers having in the chain recurring amide groups, at least 85% ofwhich are attached to aliphatic or cyclo-aliphatic groups (e.g. nylon, PVA, PVC).• Aramids: These are defined as polyamides, where each amide group is formed by the reaction ofan amino group of one molecule with a carboxyl group of another (e.g. Kevlar, Nomex).• Olefins: Defined as manufactured fibres in which the basic unit is any long-chain syntheticpolymer composed of at least 85% by weight of ethylene, propylene or other olefin units (e.g.polypropylene, polyethylene).• Elastomers: Defined as materials that, at room temperature, can be stretchedrepeatedly to at leasttwice their original length, and upon immediate release will return to approximately the originallength (e.g. polyurethane, Lycra, Spandex).• Acrylics: Defined as manufactured fibres in which the basic substance is a long-chain syntheticpolymer composed of at least 85% by weight of acrylonitrile units.These groups are discussed in Chapters 5 and 6 and are also reviewed in McIntyre (2004), Deopora,Alagirusamy, Joshi, and Gupta (2008), Ugbolue (2009) and Eichhorn, Hearle, Jaffe, and Kikutani (2009a).1.3 FIBRES, YARNS AND FABRICSFibres are commonly classified in the following way:• Staple fibres• Filaments• Tow

As noted earlier, a staple fibre is a fibre of relatively short length, as is the case with most natural fibres,which range from a few millimetres (e.g. the shortest cotton fibres, known as linters) to around a metre(e.g. fibres from bast plants). Staple fibres are typically between 3 and 20 cm in length. Given the differencesin average fibre length, cotton fibres (2–3 cm) and wool fibres (5 cm or more)are, for example,sometimes referred to as ‘short staple’ and ‘long staple’ fibres, respectively.A filament is a fibre of indefinite length. The various silks are the only natural filament fibres. Mostregenerated and synthetic fibres are produced as filaments. These can be used in single or multifilamentform. Some of these are also assembled to produce a ‘tow’ which is then cut orbroken into required shortlengths to produce staple fibres suitable for blending with other fibres, in particular with cotton or wool.A tow can mean two different things:• In the synthetic fibre industry, a tow is a large assembly of filaments thatis destined to be cut intoshorter (staple) fibres.• In the processing of natural fibres (flax), tow is the shorter fibre produced when the stalks areprocessed to extract the fibres (the long fibres are called line flax).

A yarn has been defined as ‘a product of substantial length with a relatively small cross-section, consistingof fibres and/or filaments with or without twist’ (Anonymous, 2002; Elsasser, 2011). Another definitionof yarn is ‘groupings of fibres to form a continuous strand’ (Cohen, 1997). Most staple fibres aremade into yarn through a process of drawing, spinning and twisting that allowsan assembly of fibres tohold together in a continuous strand (Briggs-Goode, 2011). There are differentmethods of spinning,depending on the fibre being spun, which are discussed in Chapters 9 and 10 and further identified anddefined by El Mohaghzy (2009) and Cohen and Johnson (2010). Fibres can also beassembled into largerstructures in other ways, e.g. felt and nonwoven fabrics. Nonwoven fabrics arediscussed in Chapter 13.Yarns can be categorised in various ways. Basic yarn types are:• Monofilament• Multifilament• Staple or spun

These types are illustrated in Figure 1.3. As their name suggests, monofilament yarns contain a singlefilament. More commonly, many filaments are twisted together to form multifilament yarns.As noted earlier, staple or spun yarns consist of staple fibres combined by spinning into a long,continuous strand of yarn. The key elements of a staple yarn are content, fineness and length, yarn plyand twist. There are many ways of creating a staple yarn from groups of fibres. Typical yarn formationsinclude:• Single (fibres combined into a single yarn)• Ply/plied (two or more yarns twisted together)• Cabled/corded (several plied yarns twisted together)• Blended/compound (different fibre types combined in a yarn)• Core spun (a yarn with one type of fibre, usually a filament, in the centre (core) of the yarn, whichis usually covered (wrapped) by staple fibres)• Fancy or effect yarns (yarns with special effects or deliberate irregularities, e.g. slubs (thickerportions) or loops occurring regularly or randomly along the length of the yarn)Some of these structures and the process of spinning yarns are discussed in Chapters 9 and 10. Thecombination of different fibres and yarn structures can be used to engineer a particular set of properties(Gong, 2011; Lawrence, 2010; Lord, 2003). Sewing threads are an example of a yarn that is specificallyengineered for a specific purpose. Additional finishes are often added to yarns to ensure they are fit forpurpose.

FIGURE 1.3Types of yarn: (a) monofilament; (b) multifilament; (c) staple/spun.

Yarn fineness, thickness or size is characterised by two main types of numbering systems:• Direct yarn numbering system: Based on weight or mass of a fixed length of yarn (mass per unitlength).• Indirect yarn numbering system: Based on the length of yarn of a fixed weight (length per unitmass).Both systems provide a measure of the fineness (or thickness) of the yarn, which is important for itsappropriate application in fabric construction techniques such as knitting andweaving.Once yarns are processed, they then need to be assembled in some way to produce a fabric. A fabricis defined as ‘a manufactured assembly of fibres and/or yarns that has substantial surface area inrelation to its thickness, and sufficient cohesion to give the assembly usefulmechanical strength’(Anonymous, 2002). There are many ways of combining yarns to create a fabric. Some of the mostimportant are:• Weaving (discussed in Chapter 11)• Knitting (discussed in Chapter 12)• Nonwoven fabric production (discussed in Chapter 13)Some more specialised fabric types include lace, nets, braids and felts.The different ways of constructing fabrics result in wide variations in texture, appearance, drape(the way a fabric hangs) and hand/handle (the feel of a fabric); as well as performance characteristicssuch as strength, durability (Schwartz, 2008), comfort and protection. The combination of differentfibre types and yarn and fabric structures results in a huge range of products, with widely differingproperties (Briggs-Goode, 2011). Weaving is discussed in more depth in varioustexts, such as Beheraand Hari (2010) and Gandhi (2012), Cohen and Johnson (2010) and O’Mahoney (2011); knitting isreviewed in Au (2011) and Spencer (2001); whilst nonwoven production is covered in various textssuch as Russell (2006) and Chapman (2010). The use of finishing techniques forfabric allows furtherrefinement of these properties (Fan & Hunter, 2009; Gulrajani, 2013; Rouette, 2001; Wei, 2009). Finishingtechniques are discussed in Chapters 17–19. Both fashion and textile designerswill exploit theinherent properties and performance characteristics of different types of fabrics when designing and

making products.1.4 FIBRE PROPERTIESFibres have a wide range of physical, mechanical and chemical properties (Howes and Laughlin, 2012).Physical and mechanical properties include:• Length, shape and diameter (fineness)• Colour and lustre• Strength and flexibility• Abrasion resistance• Handle (or feel), e.g. soft (cashmere), harsh (coir), crisp (linen), elastic(Lycra)• Moisture absorbency• Electrical properties

• Important chemical properties, including fire resistance• Chemical reactivity and resistance (e.g. resistance to acids, alkalis, solvents, light, etc.)• Antimicrobial propertiesThese are discussed in the following sections. Figure 1.4 suggests a way of categorising fibre propertiesrelated to performance (El Mogahzy, 2009).

1.5 FIBRE LENGTH, SHAPE AND DIAMETERThis chapter has already discussed the length of different fibre types. Fibre length affects many of theproperties of a staple yarn, including strength, evenness and hairiness. Because they are continuous,filament yarns can be made into yarn with little or no twist, producing a smooth, bright appearance,particularly when no crimp (see below) is present. Staple fibres need to be twisted together to form alength of yarn with fibre ends protruding from the surface of the yarn. This produces a duller appearanceand more uneven texture. This can be an advantage since it gives some fabrics a softer ‘hand’ orfeel (Behery, 2005).A characteristic feature of some fibres is ‘crimp’, which refers to the waviness of a fibre along itslength. The only natural fibres that have significant crimp are animal fibres (wool and hairs). Crimp is often imparted to synthetic filament fibres, in order to make them more bulky and comfortable,and to man-made staple fibres for blending with wool or cotton and to facilitate processing. Crimpcan be measured by counting the number of crimps or waves per unit length or the percentageincrease in fibre length on removal of the crimp. Crimped fibres tend to be more bulky and cohere(cling) together more effectively when being spun into staple fibre yarn. Fibre ‘cohesiveness’ is animportant factor in the successful spinning of staple fibres, and can produce stronger yarns. Crimpedfibres, such as wool, have more bulk and better insulating properties, the latter being due to moreentrapped air.Natural fibres come in a range of shapes, whilst synthetic fibres can be manufactured in almost anyshape required. Fibre shape can essentially be analysed in two ways:• By looking at the cross-section of a fibre (i.e. cross-sectionally)• By looking at the fibre lengthways (i.e. longitudinally)Some common types of fibre cross-section are:• Round• Dog-bone shaped• Trilobal• Multilobal• Serrated• HollowSome common types of cross-section are illustrated in Figure 1.5. Amongst natural fibres, cotton fibres

have a characteristic dog-bone (bean or kidney) shape, whilst silk fibres havea more rounded, trilobalshape. Whilst there is greater flexibility in selecting and producing a particular cross-section for syntheticfibres, some tend to have the same basic shape (mostly round or serrated).Looking at fibres longitudinally, some have a smooth surface whilst others arerough and uneven.Wool fibres have scales similar to human hair, whilst cotton fibres have a characteristic twist. Theshape and surface characteristics of a fibre can have an important effect on properties such as cohesion,wetting, wicking, how easily fibres can be cleaned, as well as fabric cover (discussed below)and lustre.Synthetic filament yarns can be texturised (textured) to alter their surface and other properties. Thepurpose of texturising is to increase the bulkiness of the yarns and the comfort of the fabrics. Texturedyarns can be knitted or woven into fabrics that have the appearance, drape andalmost the handle ofwool, cotton or silk fabrics, whilst retaining the synthetic fabrics advantages of better washability, easeof care and lower cost compared to natural fibre fabrics.An important property of a fabric is ‘cover’, which describes the degree to which the yarn and fibrescover the space or area occupied by the fabric. The more open the fabric, withmore open spaces orgaps, the lower the cover. Good cover in a fabric is important because it determines certain comfortrelatedand other properties such as degree of protection and transparency. The ‘coverfactor’ of awoven fabric is a number indicating the area covered by the fibres and yarns in the fabric, relative tothat covered by the fabric.The diameter of a fibre is the distance across its cross-section. Because theyare irregular, the diameterof natural fibres usually varies over their length, so an average is used. Fibre diameter is usuallymeasured in millionths of a metre, known as ‘microns’ or ‘micrometres’ (using the symbol μm).

FIGURE 1.5Fibre shapes.

Typical textile fibres have a diameter of between 10 and 20 μm, though some can reach 50 μm. Naturalfibres range in diameter from silk (10–13 μm) to wool (up to 40 μm). Syntheticfibres can be manufacturedin diameters from as small as 6 μm (known as microfibers) up to heavy-duty carpet fibres (over

40 μm). Nanofibres, with a diameter below 100 nm (nanometres), are also produced (Houck, 2009). Asmall diameter produces ‘finer’ fibres with a greater pliability, flexibility and softness. This results in afabric with better or softer hand (i.e. feel) and drape (the way a fabric hangs). The fineness of fibres isdiscussed in more detail below. Apparel is typically manufactured from small-diameter fibres, whereaslarger-diameter fibres are often used for heavy-duty applications such as carpets.

1.6 FIBRE COLOUR AND LUSTREThe colours of natural fibres vary. Wool fibres can, for example, vary in colour from black to white,usually being creamish in colour. Filaments are usually white when manufactured but can be colouredto almost any colour, either prior to manufacturing or subsequently. When synthetic fibres are massdyed (also called dope dyeing, mass-pigmentation or mass-colouring), the colouring matter is incorporatedin the polymer before the filaments are formed (i.e. extruded). The colour andsurface characteristics of different fibres have a major effect on fabric appearance, including lustre (the amountand nature of light reflected by a fibre, yarn or fabric). A smooth surface and more regular cross-sectionalshape (e.g. the smooth, trilobal shape of silk fibres) will reflect light morestrongly and evenly,creating a high lustre. A fibre like cotton, with a rough surface and irregular, twisted cross-sectionalshape, has a lower lustre. As well as reflecting light more strongly, smooth, round fibres tend to showsoiling more easily than, e.g. multilobal fibres, which are preferred for products such as carpets wheredirt and wear may be a significant problem.Another method of colouring fibres is to dye them in fibre form, before they are spun into yarns.This is called stock dyeing, and is to all intents and purposes confined to wool and hair fibres. Stockdyeing permits the mixing of differently coloured fibres before spinning, and this in turn permits theproduction of yarns containing several intimately mixed colours. This enables designers to achievecolour effects that cannot be obtained in any other way. Harris tweeds are an example of these coloureffects. Dyeing can also be carried out in yarn, garment or fabric form, the latter being the mostcommon.

1.7 FIBRE FINENESSIn textiles, fineness refers to the thinness of the fibre. Thinner fibres havegreater surface-to-weight ratiosand are more flexible, giving a softer drape (flexibility) and handle than thicker fibres. Generally speaking,the thinner the fibre, yarn or fabric, the better its quality and the higher its price. Fineness is measuredin textiles in several ways, depending, for example, on whether it is in fibre, yarn or fabric form.1.7.1 FIBRESThere are two usual ways of measuring the fineness of textile fibres. The one usually used for wool andanimal hairs is to measure the diameters of a sample of the fibres and expressthe result as the averageof these diameters. The most usual unit used, bearing in mind the smallness ofthese diameters, is themicron (μm), which is one millionth of a metre. As an example, fine Merino wool would have a diameterof between 18 and 20 μm.The second common way of expressing the fineness of textile fibres, which applies particularly tofilaments such as silk and certain synthetic fibres such as polyester and nylon, is in terms of weight perunit length, such as denier or tex. A denier is the weight in grams of 9000 m of filament or fibre. Anexample is ladies 15 denier nylon stockings, which vary from about 1.3 denier to 2.5 denier. The texsystem, expressed in grams per 1000 m (see below), is also sometimes applied to fibres, especially inscientific and technical circles. Since fibres are usually very fine, millitexis more often used(1000 millitex = 1 tex).1.7.2 YARNSThe systems of yarn fineness used in the textile industry vary according to the fibres used and the traditionsof the areas where the yarns are produced. Yarn fineness measurement systems can be either director indirect.In the direct system, the weight of a given length of yarn is used, the denierand tex systems being the most commonly used. The denier system is based on the weight in grams of 9000 m of filament or yarn. This is the traditional system used for silk and often for synthetic filaments. Therefore, 9000 m ofa 20 denier silk yarn will weigh 20 g. The same length of 40 denier yarn will weigh 40 g. In the tex system(discussed in more detail below), 1000 m of a 3 tex yarn will weigh 3 g, and 1000 m of a 30 tex yarn

will weigh 30 g. In direct systems, the greater the number of the yarn, the thicker the yarn. Direct systemsare sometimes called linear density systems, since they represent the mass perunit length of yarn.Over the last 50 years or so, as textiles became a truly global business, (Diamond, 2002) there wasa growing need to have common systems of reference and measurement, including yarn fineness. Thisresulted in the development of the tex direct system of yarn fineness or linear density measurement,which has now attained worldwide recognition. The unit in this system is called the ‘tex’ and it correspondsto the number of grams per 1000 m (1 km) of yarn. Sometimes, for yarns finer than 1 tex, toavoid using fractions or decimals, the yarn number is expressed in decitex, where 10 decitex equals1 tex (to convert from tex to decitex, multiply by 10).Indirect systems are based on the length of yarn per unit mass. Different indirect measurement systemshave developed over time. A traditional British name for these indirect measurement systems, alsotermed counts, is ‘grist’ (see Figure 1.6).

1.7.3 FABRICSThe usual way of measuring the ‘fineness’ or ‘lightness’ of fabric (normally one would use the phrase ‘theweight of the fabric’) is in grams per square metre, although sometimes it is expressed in terms of gramsper running metre. A running metre is 1 m of the full width of the fabric, which is often, but not always,about 150 cm, depending, for woven fabrics, on the type and width of the loom used to weave the cloth.

1.8 FIBRE STRENGTH, FLEXIBILITY AND ABRASION RESISTANCEThis section reviews a number of mechanical properties: tensile strength, extension (elongation), flexibility(stiffness), elasticity, resiliency and abrasion resistance.

1.8.1 TENSILE STRENGTH AND EXTENSION (ELONGATION)Tensile strength and extension are important characteristics of fibres since they influence the performancecharacteristics of the yarns and fabrics produced from them. As an example, ifyou are making climbingropes, you will need a rope that stretches if you fall during a climb, as the stretch (extension) of the rope

will absorb the force exerted (impact) when the rope reaches its full length at the end of your fall, thuspreventing the sudden shock that might otherwise cause serious injury. You would use an extensible fibre,such as nylon filaments, for such a rope. You would not use flax, hemp or sisal (traditional rope fibres),which have practically no extensibility. These fibres, on the other hand, makeexcellent marine ropes,where extensibility would be a disadvantage. In the case of the example of a climbing rope, you would alsoneed a rope that will not break when subjected to your full weight (plus that of your back-pack). Nylon, forexample, has both high extensibility and high strength, making it a good fibrefor climbing purposes.The resistance to breaking, when a fibre is stretched, is called its ‘tensile strength’. It is measuredby exerting force, in the form of weights or springs, on the fibre or yarn, and noting both the extensionof the fibre when it reaches breaking point (this is called ‘elongation at break’ or ‘breaking elongation’)and the force required to achieve this. The elongation at break of different fibres is shown in Figure 1.7.

FIGURE 1.7Elongation of various fibres.

FIGURE 1.8Tenacity of various fibres.

In terms of tensile strength, a complicating factor is the thickness of the fibre or yarn. It is obviousthat if you compare the tensile strength of a nylon filament of 15 denier withthat of a similar filamentof 30 denier, the latter will have a higher (close on double) tensile strength. To overcome this problem,the ‘tenacity’ is calculated, by dividing the tensile strength by the linear density (fineness, usually measuredin tex), or cross-sectional area of the fibre or yarn. Tenacity is expressed in Newtons per tex1 orsometimes in grams of force per denier or tex. In addition, because tenacity is influenced by the amountof moisture in the fibre, tenacity values are often measured in both dry (or conditioned) and wet conditions.The tenacity of various fibres is shown in Figure 1.8.

1.8.2 FLEXIBILITY OR STIFFNESSFlexibility (stiffness) in fibres is important because it affects the processing of the fibre into yarn, aswell as the drape and handle of fabrics. Fibre flexibility is a function of the chemical and physicalstructure of the fibre, and in particular the thickness and cross-sectional shape of the fibre. For example,bast fibres (e.g. flax) are composed principally of highly oriented crystalline cellulose molecules andare therefore fairly rigid when compared to cotton, where the cellulose is much more amorphous. Cottonfibres are also thinner than flax fibres, and the net result is that cotton fibres are more flexible than

FIGURE 1.9Modulus of various fibres.