Wool

The production of wool:

The word wool is restricted to the description of the curly hairs that form the fleece produced by sheep (Rogers, 2006:931). The sheep’s fleece is removed once a year by power-operated clippers. The soiled wool at the edges is removed before the fleeces are graded and baled. The price of raw wool is influenced by fineness and length. This is representative of the yarn into which it can be spun. The average fibre length will also determine the type of fabric for which it will be used (Collier, 1974:24).

Newly removed wool is known as raw wool and contains impurities such as sand, dirt, grease and dried sweat. Altogether, these can represent between 30 and 70% of the wool’s weight (Kadolph, 2002:51). The wool is sorted by skilled workers who are experts in distinguishing quality by touch and sight. The grade is determined by type, length, fineness, elasticity and strength (Corbman, 1983:271).

Long wool fibres will be combed and made into worsteds, while short wools are described as carding, or clothing wools. When the quality has been determined, the wool is offered for sale as complete fleeces or as separate sections (Collier, 1974:24).

When the wool arrives at the mill it is dirty and contain many impurities that must be removed before processing. The raw wool is scoured with a warm alkaline solution containing warm water, soap and a mild solution of alkali, before being squeezed between rollers (Corbman, 1983:272). This procedure is repeated three to four times, after which the wool is rinsed in clean water and dried.

The quality and characteristics of the fibre and fabric depend on a number of factors, such as the kind of sheep, its physical condition, the part of the sheep from which the wool is taken, as well as the manufacturing and finishing processes (Corbman, 1983:273).

• The chemical composition of wool:

The protein of the wool fibre is keratin (Azoulay, 2006:26), which contains carbon, hydrogen, oxygen and nitrogen, but in addition wool also contains sulphur. These are combined as amino acids in long polypeptide chains (Kadolph, 2002:54). Wool contains 18 amino acids, of which 17 are present in measurable amounts (Joseph, 1986:48). These are glycine, alanine, valine, leucine, isoleucine, phenylalanine, proline, serine, threonine, tyrosine, aspartic, glutamic, arginine, lysine, histidine, tryptophan, cystine and methionine (Stout, 1970:107). In addition to the long-chain polyamide structure, wool has cross-linkages called cystine or sulphur linkages, plus ion-to-ion bonds called salt bridges and hydrogen bonds (Tortora, 1978:74).

The cross-linkages in the chains permit the ends to move up and down, which provides the resiliency of the fibre (Labarthe, 1975:51). Keratin reacts with both acids and bases, which makes it an amphoretic substance (Hollen and Saddler, 1973:17).

When keratin is in a relaxed state it has a helical, or spiral structure called alpha-keratin (Gohl and Vilensky, 1983:75), which is responsible for wool’s high elongation property (Kadolph, 2002:54). When the fibre is stretched it tends to unfold its polymers and this unfolded configuration is known as beta-keratin (Gohl and Vilensky, 1983:78).

Helical arrangement of the wool molecule (Wool Bureau, Inc. as cited in Kadolph, 2002:56).

The tenacity of wool is improved by the presence of the hydrogen bonding between the oxygen and hydrogen atoms of alternate spirals of the helix. This strengthens the structure and a greater force is required to stretch the molecules (Smith and Block, 1982:91).

• The physical structure of wool:

The fibre consists of three layers – an outer layer of scales called the cuticle, a middle layer called the cortex and an inner core, called the medulla Joseph, 1986:49).

The wool fibre is a cylinder, tapered from root to tip and covered with scales (Ito et al, 1994:440). The scales are irregular in shape and overlap each other towards to the tip of the fibre. These then have a directional effect that influences the frictional behaviour of wool because of its resistance to deteriorating influences (Joseph, 1986; Hall, 1969:15). These scales are responsible for wool textile’s tendency to undergo felting and shrinking as a consequence of the difference of friction in the ‘with-scale’ and ‘against scale’ directions (Silva et al, 2006:634; Cortez et al, 2004:64). Each cuticle cell contains an inner region of low sulphur content, known as the endocuticle, plus a central sulphur rich band, known as the exocuticle. Around the scales is a shield, a membrane called the epicuticle (Maxwell and Hudson, 2005:127), which acts as a diffusive barrier and can also affect the surface properties of the fibre. The epicuticle is present as an envelope that bounds the entire inner surface of the cell (Swift and Smith, 2001:204). The sub-cuticle membrane is a thin layer between the cuticle and the cortex (Morton and Hearle, 1975:59).

fig:-Physical structure of a wool fibre (Gohl and Vilensky, 1983:73).

The cortex is the bulk of the fibre and the hollow core at the centre is called the medulla. The cortex consists of millions of long and narrow cells, held together by a strong binding material. These cells consist of fibrils, which are constructed from small units and lie parallel to the long axis of the long narrow cells. The wool fibre gets its strength and elasticity from the arrangement of the material composing the cortex (Collier, 1974:25). The medulla resembles a honeycomb, i.e. contains empty space that increases the insulating power of the fibre (Hollen and Saddler, 1973:19).

Wool appears to be divided longitudinally into halves because of its bilateral structure, with one side called the paracortex and the other the orthocortex. The chemical composition of the cells of the ortho- and paracortex is different, i.e. the paracortex contains more cystine groups that cross-link the chain molecules and is therefore more stable. It is this difference between the ortho- and paracortex that brings about the spiral form of the fibre and explains why the paracortex is always found on the inside of the curve as the fibre spirals around in its crimped form. In addition, these two parts react differently to changes in the environment, which leads to the spontaneous curling and twisting of wool (Gohl and Vilensky, 1983:74).

fig:-Three-dimensional crimp of the wool fibre (Gohl and Vilensky, 1983:75).

The fibres have a natural crimp, i.e. a built in waviness, which increases the elasticity and resiliency of the fibre. The spiral formed by the crimp is three-dimensional and does not only move above and below the central axis, but also to its left and right (Joseph, 1986:49).

The cross-section of the wool fibre is nearly circular and in some cases even oval in shape (Joseph, 1986:49). The longitudinal view shows both the scale structure, plus the striations on the epicuticle that can occur on the original undamaged fibres. These arise from an interaction in the follicle with the cuticle of the inner root sheath. When the fatty acids are stripped from the surface, the striations have been shown to reflect a corresponding irregularity of the epicuticle’s surface (Swift and Smith, 2001:203).

Wool fibres vary in length between 2cm to 38cm, depending on various factors such as the breed of the sheep and the part of the animal from where it was removed (Joseph, 1986:50; Smith and Block, 1982:92). The diameters of the wool also vary. Fine fibres have a diameter of 15 to 17μm, medium fibres have a diameter 24 to 34μm and coarse wool has a diameter of about 40μm (Joseph, 1986:50). Hollen and Saddler (1973) differ in as much that they claim the diameter of a wool fibre varies from 15 to 50μm, with Merino lamb’s wool averaging 15 μm in diameter.

The colour of the natural wool depends on the breed of sheep, but most wool is an ivory colour, although it can also be grey, black, tan and brown (Joseph, 1986:50).

• Physical properties of wool:

lustre

The lustre of a fibre depends on the amount and pattern of light reflected from the fibre (Hopkins, 1950:593).

The lustre of wool varies, but it is not generally considered to be a lustrous fibre. Nevertheless, lustre also depends on factors such as the specific breed of sheep, conditions of living and the part of the animal from which it was taken (Tortora, 1978:76). Fine and medium wools have more lustre than coarse fibres (Joseph, 1986:50) because lustre is due to the nature and transparency of the scale structure (Stout, 1970:113).

Strength:

Wool is a weak natural textile fibre (Corbman, 1983:280). It has a large amorphous area containing bulky molecules that can’t be packed close enough together to allow strong hydrogen bonding. Thus wool has many weak bonds and a few strong cystine linkages. Moisture weakens the hydrogen bonds, which makes the fibre even weaker when wet (Hollen and Saddler, 1973:20). When a garment is wet, the weight of the water puts strain on the weakened fibre and the shape can be distorted (Hollen and Saddler, 1973:21).

The strength of a fibre is dependent on the cross-sectional area of the fibre being tested. The smallest fibre diameter and the rate of change in diameter are important determinants of strength. There are a variety of environmental and physiological factors that influence the strength of wool fibres. The nutrient supply has a great influence as it provides amino acids, trace elements and vitamins. The fibre strength is also influenced by pregnancy and lactation through competition for essential nutrients (Reis, 1992:1337). During wear, however, resistance to abrasion is more important than tensile strength. The scale structure of the wool fibre gives excellent abrasion resistance, which makes wool fabric very durable (Smith and Block, 1982:93).

Elasticity:

Wool fibres are very elastic and, when stretched, they quickly return to their original size (Smith and Block, 1982:92). This is due to the crimp, or waviness, of the fibre which enables it to be stretched out and then relaxed to the crimp form, like a spring (Collier, 1974:26). The molecules are in a folded state, but become straightened when stretched. The cross-linkages between the molecules, plus the disulphide and salt linkages tend to resist any permanent alteration in shape (Collier, 1974:27).

Disulphide linkage (Collier, 1974:27)

Salt linkage (Collier, 1974:27)

Wool fibres can be stretched from 25 to 30% of their original length before breaking, which also reduces the chances of tearing under tension (Corbman,1983:280).

Wool’s recovery is excellent and after a 2% extension the fibre has an immediate regain in length of 99% (Joseph, 1986:50). Elasticity is a valuable characteristic because it leads to the easy shedding of wrinkles. Wrinkles will easily hang out of wool garments, especially when hung in a damp atmosphere (Tortora, 1978:76).

Resilience:

The molecules in the wool fibre are arranged in long parallel chains, which are held together by cross-linkages. When the fibres are stretched or distorted, these cross-linkages will force the fibre back to shape (Cowan and Jungerman, 1969:9). This shows that the fibres will recover quickly from creasing (Thiry, 2005:19; Azoulay, 2006:26), but through the application of heat, moisture and pressure, pleats and creases can be put into the fabric. This is a result of the molecular adjustment and the formation of new crosslinkages in the polymer.

The resilience of the wool fibre also contributes to the fabrics’ loft, which can either produce open porous fabrics with good covering power, or thick and warm fabrics that are also light in weight (Joseph, 1986:50).

Wool is classified as a resilient fibre. Therefore a bunch of irregular fibres should: a) offer moderate resistance to compression, and; b) spring back vigorously upon relaxation (Demiruren and Burns, 1955:666).

Wool and silk have the ability to resist the formation of wrinkles (Buck and McCord, 1949).

Absorption and moisture regain:

Water is usually shed by the wool fibres because of a combination of factors that include, for instance, the protection by the scales and the membrane, interfacial surface tension, uniform distribution of pores and low bulk density (Joseph, 1986:51).

However, once the moisture seeps between the scales, the high degree of capillarity within the fibre will cause ready absorption (Ito et al., 1994:440). Wool can absorb 20% of its own weight in water without feeling wet (Corbman, 1983:282). According to Cowan and Jungerman (1969:9) wool is a hygroscopic fibre because it absorbs water vapour. Most of the moisture is absorbed into the spongy matrix, which then causes the rupture of hydrogen bonds and leads to the swelling of the fibre. The absorbent nature is due to the polarity of the peptide groups, salt linkages and amorphous polymer system (Cook and Fleischfresser, 1990:43). Wool dries very slowly (Corbman, 1983:282).

Hydrogen bonds are broken by moisture and heat, so the wool structure can be reshaped by mechanical action like that of an iron. While the heat dries the wool, new hydrogen bonds are formed in the structure as the water escapes in the form of steam. The new hydrogen bonds maintain the new shape while humidity is low. When the wool is dampened or in a high humidity atmosphere, the new bonds are broken and the structure returns to its original shape.

This is why garments shaped with ironing lose their creases and flatness, and show relaxation shrinkage on wetting (Hollen and Saddler, 1973: 21) Wool produces heat as part of the absorption function (Azouly, 2005:25), which is known as heat of wetting. This is due to the energy generated by the collision of water molecules and the polar groups in the wool polymers. The polymer system will continue to give off heat until it becomes saturated. As wool begins to dry, the evaporation causes the heat to be absorbed by the fibre and a chill may be experienced (Joseph, 1986:51).

The behaviour of wool in relation to moisture can be summarized by saying that wool is water repellent, but with prolonged exposure to moisture the fibre does absorb large quantities of water. Since the moisture is held inside the fibre, the surface still feels dry (Tortora, 1978:77; Etters, 1999). Wool is hydrophilic and contains various amounts of absorbed water depending on the conditions (Cook and Fleischfresser, 1990:43). The standard moisture regain of wool is set at 16 to 30% (Hollen and Saddler, 1973:22), but according to Lyle (1976:29) and Hunter (1978:46) it is only 15%. Cowan and Jungerman (1969:9) and Joseph (1986) report a regain of 13 to 16%.

Dimensional stability:

The structure of wool fibres contributes to its non stability (Joseph, 1986:51). All fabrics made of wool are subject to shrinkage (Corbman, 1983:282). Two kinds of shrinkage occur: felting shrinkage and relaxation shrinkage. Felting shrinkage occurs as a result of combined agitation, heat and moisture (Lenting et al., 2006:711; Cortez et al., 2004:64).

When wet, untreated wool fabric is agitated, the fibres will tend to move in a root ward direction and the root curls upon itself (Gohl and Vilensky, 1983:71). The scales interlock and hook together, causing the fibres to become entangled (Silva et al., 2006:634).

When the felting is not properly controlled, the fabric will become stiff and thick, and it will shrink considerably (Joseph, 1986:52). Felting is enhanced by heat, which causes the fibre to become more elastic and thus more likely to move. This, in turn, will make it distort and entangle itself with other fibres. Heat also causes the fibre to swell, a condition that is enhanced by acid or alkaline conditions. Swelling leads to more inter-fibre contact and inter-fibre friction (Gohl and Vilensky, 1983:71).

Relaxation shrinkage occurs as a result of the elasticity of the fibre. Fibres are stretched and extended during the construction of fabrics, and when the fibre is exposed to  moisture, the yarns return to their original length that causes the fabric to shrink (Joseph, 1986:52; Garcia et al., 1994:466). This also includes exposure to steam, which causes shrinkage (Lyle, 1976:103). The felting shrinkage of wool is progressive. Wool will continue to shrink if it is not washed in cold water with a neutral pH and minimum handling to
minimize felting (Tortora, 1978:77).

Warmth:

The warmth of wool is due to its spongy structure and scales that incorporate many extremely small pockets of air (Miller, 1992:26).

Stationary air is a bad conductor of heat and therefore wool is a good heat insulator and feels warm (Corbman, 1983:281).

cool absorbs atmospheric moisture and through the heat of absorption makes the wearer feel warmer (Cowan and Jungerman, 1969:9), and the fibre is protein and therefore doesn’t transmit heat quickly (Miller, 1992:29).

Thermal properties of wool:

Wool is not a very flammable fibre. Dry wool will burn slowly with a sputtering smoky flame, and will self-extinguish when removed from the source of flame (Smith and Block, 1982:94). Wool fibres scorch at 204°C and will eventually turn to char at 300°C. During combustion it will give off a smell similar to burning feathers. When removed from the flame each fibre will form a charred black knob (Cook, 1984:90).

• Chemical properties of wool:

Effect of alkalis:

Wool is easily attacked by alkalis. Weak alkalis like soap, sodium phosphate, ammonia, borax and sodium silicate will not damage wool if the temperatures are low (Labarthe, 1975:63). Alkaline solutions can open the disulphide cross-links of wool, while hot alkalis may even dissolve it (Chapman, 1974:56). Wool dissolves when boiled in a 5% solution of
sodium hydroxide (Labarthe, 1975:63). Caustic soda will completely destroy wool. Wool turns yellow as it disintegrates, then it become slick and turn into a jelly-like mass, and goes into solution (Hollen and Saddler, 1973:22).

Weak solutions of sodium carbonate can damage wool when used hot, or for a long period (Hall, 1969:17).
Concentrated alkalis below 31°C gives wool increased lustre and strength, by fusing the scales together; it is called mercerized wool (Labarthe, 1975:63).

Effect of acids:

Wool is more resistant to acids. This is because they hydrolyse the peptide groups but leave the disulfide bonds intact, which cross-link the polymers. Although this weakens the polymer system, it doesn’t dissolve the fibre (Gohl and Vilensky, 1984:81).

Wool is only damaged by hot sulphuric acid (Corbman, 1983:282) and nitric acid (Joseph, 1986). Acids are used to activate the salt linkages in the wool fibre, making it available to the dye (Hollen and Saddler, 1973:22). Concentrated mineral acids will destroy wool if the fabric is soaked in it for more than a few minutes. It will also destroy wool when it dries on the fabric (Labarthe, 1975:63).

Effect of bleach:

Bleaches that contain chlorine compounds will damage wool. Products with hypochlorite will cause wool to become yellow and dissolve it at room temperature. Various forms of chlorine are used to make ‘unshrinkable wool’, by destroying the scales. This wool is weaker, less elastic and has no felting properties (Labarthe, 1975:63).

Bleaches containing hydrogen peroxide, sodium perborate, sodium peroxide (Corbman, 1983:282) and potassium permanganate won’t harm wool and are safe to use for stain removal (Wingate and Mohler, 1984:308).

Effect of sunlight:

Wool will weaken when exposed to sunlight for long periods (Schmidt and Wortmann, 1994). The ultraviolet rays will cause the disulfide bonds of cystine to break, which leads to photochemical oxidation. This will cause fibre degradation and eventual destruction (Joseph, 1986:53). Wet fabrics exposed to ultraviolet light are more severely faded and weakened than dry fabrics (Labarthe, 1975:62).

Effect of perspiration:

As already stated, wool is easily deteriorated by alkalis and therefore perspiration which is alkaline will weaken wool as a result of hydrolysis of peptide bonds and amide side chains (Maclaren and Milligan, 1981:89). Perspiration in general will lead to discoloration (Corbman, 1983:283).

Effect of water:

Wool loses 10 to 25% of its strength when wet, although it is regained upon drying (Stout, 1970:113). Prolonged boiling will dissolve and decompose small amounts of the fibre. Boiling water will reduce lustre and promote felting (Labarthe, 1975:63). The heat makes the fibre more elastic and plastic which makes it easier to move and entangle itself with other fibres (Gohl and Vilenski, 1983:72).

• Biological properties of wool:

Wool is vulnerable to the larvae of moths and carpet beetles (Corbman, 1983:282), as they are attracted by the chemical structure of the cystine cross-linkages in wool (Tortora, 1978:78).
Raw wool may contain inactive spores, which becomes active when wet. Mildew will develop when wool is left in a damp condition for a long period (Labarthe, 1975:59).

• Care:

Dry cleaning is the recommended care method for wool items (Kadolph, 2002:56), because the solvents do not harm wool and create less wrinkling, fuzzing and shrinkage (Hollen and Saddler, 1973:22). Wool fabrics should not be tumble dried, because the tumbling of the damp fabrics may cause excessive felting shrinkage. The dryer will provide all of the conditions necessary for felting, namely heat, moisture and friction (Tortora, 1978:78). Wool items should be dried flat to prevent strain on any part of the garment. Heat has a negative effect on wool fibres and therefore it is necessary to keep ironing temperatures low, and to use a press cloth (Tortora, 1978:78). Steaming will partially shrink and condition the fabric, so should be done with care (Wingate and Mohler, 1984:319).

Posted in: Fibres,Wool