Sampling


It is not possible or desirable to test all the raw material or all the final output from a production process because of time and cost constraints. Also many tests are destructive so that there would not be any material left after it had been tested. Because of this, representative samples of the material are tested. The amount of material that is actually tested can represent a very small proportion of the total output. It is therefore important that this small sample should be truly representative of the whole of the material. For instance if the test for cotton fibre length is considered, this requires a 20 mg sample which may have been taken from a bale weighing 250kg. The sample represents only about one eleven-millionth of the bulk but the quality of the whole bale is judged on the results from it.

The aim of sampling is to produce an unbiased sample in which the proportions of, for instance, the different fibre lengths in the sample are the same as those in the bulk. Or to put it another way, each fibre in the bale should have an equal chance of being chosen for the sample methods from the test lot.

• Test specimen: this is the one that is actually used for the individual measurement and is derived from the laboratory sample. Normally, measurements are made from several test specimens.

• Package: elementary units (which can be unwound) within each container in the consignment. They might be bump top, hanks, skeins, bobbins, cones or other support on to which have been wound tow, top, sliver, roving or yarn.

• Container or case: a shipping unit identified on the dispatch note, usually a carton, box, bale or other container which may or may not contain packages.

Fibre sampling from bulk

Zoning
Zoning is a method that is used for selecting samples from raw cotton or wool or other loose fibre where the properties may vary considerably from place to place. A handful of fibres is taken at random from each of at least 40 widely spaced places (zones) throughout the bulk of the consignment and is treated as follows. Each handful is divided into two parts and one half of it is discarded at random; the retained half is again divided into two and half of that discarded. This process is repeated until about nix fibres remain in the handful (where n is the total number of fibres required in the sample and x is the number of original handfuls). Each handful is treated in a similar manner and the fibres that remain are placed together to give a correctly sized test sample containing n fibres. The method is shown diagrammatically in Fig. 1. It is important that the whole of the final sample is tested.

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Fig:- Sampling by zoneing

Core sampling
Core sampling is a technique that is used for assessing the proportion of grease, vegetable matter and moisture in samples taken from unopened bales of raw wool. A tube with a sharpened tip is forced into the bale and a core of wool is withdrawn. The technique was first developed as core boring in which the tube was rotated by a portable electric drill. The method was then developed further to enable the cores to be cut by pressing the tube into the bale manually. This enables samples to be taken in areas remote from sources of power. The tubes for manual coring are 600mm long so that they can penetrate halfway into the bale, the whole bale being sampled by coring from both ends. A detachable cutting tip is used whose internal diameter is slightly smaller than that of the tube so that the cores will slide easily up the inside of the tube. The difference in diameter also helps retain the cores in the tube as it is withdrawn. To collect the sample the tube is entered in the direction of compression of the bale so that it is perpendicular to the layers of fleeces. A number of different sizes of nominal tube diameter are in use, 14, 15 and 18mm being the most common the weight of core extracted varying accordingly. The number of cores extracted is determined according to a sampling schedule and the cores are combined to give the required weight of sample. As the cores are removed they are placed immediately in an air-tight container to prevent any loss of moisture from them. The weight of the bale at the time of coring is recorded in order to calculate its total moisture content.

The method has been further developed to allow hydraulic coring by machine in warehouses where large numbers of bales are dealt with. Such machines compress the bale to 60% of its original length so as to allow the use of a tube which is long enough to core the full length of the bale.

Fibre sampling from combed slivers, rovings and yarn

One of the main difficulties in sampling fibres is that of obtaining a sample that is not biased. This is because unless special precautions are taken, the longer fibres in the material being sampled are more likely to be selected by the sampling procedures, leading to a length-biased sample. This is particularly likely to happen in sampling material such as sliver or yarn where the fibres are approximately parallel. Strictly speaking, it is the fibre extent as defined in Fig. 1.2 rather than the fibre length as such which determines the likelihood of selection. The obvious area where length bias must be avoided is in the measurement of fibre length, but any bias can also have effects when other properties such as fineness and strength are being measured since these properties often vary with the fibre length.

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Fig 2.:- The meaning of extenet

There are two ways of dealing with this problem:
1 Prepare a numerical sample (unbiased sample).
2 Prepare a length-biased sample in such a way that the bias can be allowed for in any calculation.

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Fig 3:- Selection of numerical sample

Numerical sample
In a numerical sample the percentage by number of fibres in each length group should be the same in the sample as it is in the bulk. In Fig.3, A and B represent two planes separated by a short distance in a sample consisting of parallel fibres. If all the fibres whose left-hand ends (shown as solid circles) lay between A and B were selected by some means they would constitute a numerical sample. The truth of this can be seen from the fact that if all the fibres that start to the left of A were removed then it would not alter the marked fibres. Similarly another pair of planes could be imagined to the right of B whose composition would be unaffected by the removal of the fibres starting between A and B. Therefore the whole length of the sample could be divided into such short lengths and there would be no means of distinguishing one length from another, provided the fibres
are uniformly distributed along the sliver. If the removal of one sample does not affect the composition of the remaining samples, then it can be considered to be a numerical sample and each segment is representative of the whole.

Length-biased sample
In a length-biased sample the percentage of fibres in any length group is proportional to the product of the length and the percentage of fibres of that length in the bulk. The removal of a length-biased sample changes the composition of the remaining material as a higher proportion of the longer fibres are removed from it.

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Fig4 :- selection of tuft sample

If the lines A and B in Fig. 3 represent planes through the sliver then the chance of a fibre crossing these lines is proportional to its length. If, therefore, the fibres crossing this area are selected in some way then the longer fibres will be preferentially selected. This can be achieved by gripping the sample along a narrow line of contact and then combing away any loose fibres from either side of the grips, so leaving a sample as depicted in Fig. 4 which is length-biased. This type of sample is also known as a tuft sample and a similar method is used to prepare cotton fibres for length measurement by the fibrograph. Figure  5 shows the fibre length histogram and mean fibre length from both a numerical sample and a length-biased sample prepared from the same material.

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Fig:5 Histogram of length based and numerical samples

By a similar line of reasoning if the sample is cut at the planes A and B the section between the planes will contain more pieces of the longer fibres because they are more likely to cross that section. If there are equal numbers of fibres in each length group, the total length of the group with the longest fibres will be greater than that of the other groups so that there will be a greater number of those fibres in the sample. Samples for the measurement of fibre diameter using the projection microscope are prepared in this manner by sectioning a bundle of fibres, thus giving a length-biased sample. The use of a length-biased sample is deliberate in this case so that the measured mean fibre diameter is then that of the total fibre length of the whole sample. If all the fibres in the sample are considered as being joined end to end the mean fibre diameter is then the average thickness of that fibre.

Random draw method
This method is used for sampling card sliver, ball sliver and top. The sliver to be sampled is parted carefully by hand so that the end to be used has no broken or cut fibres. The sliver is placed over two velvet boards with the parted end near the front of the first board. The opposite end of the sliver is weighed down with a glass plate to stop it moving as shown in Fig. 1.6. A wide grip which is capable of holding individual fibres is then used to remove and discard a 2mm fringe of fibres from the parted end. This procedure is repeated, removing and discarding 2mm draws of fibre until a distance equal to that of the longest fibre in the sliver has been removed. The sliver end has now been ‘normalised’ and any of the succeeding draws can be used to make up a sample as they will be representative of all fibre lengths. This is because they represent a numerical sample as described
above where all the fibres with ends between two lines are taken as the sample. When any measurements are made on such a sample all the fibres must be measured.

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Fig 6:- The random Draw method

Cut square method
This method is used for sampling the fibres in a yarn. A length of the yarn being tested is cut off and the end untwisted by hand. The end is laid on a small velvet board and covered with a glass plate. The untwisted end of the yarn is then cut about 5mm from the edge of the plate as shown in Fig. 7. All the fibres that project in front of the glass plate are removed one by one with a pair of forceps and discarded. By doing this all the cut fibres are removed, leaving only fibres with their natural length. The glass plate is then moved back a few millimetres, exposing more fibre ends. These are then removed one by one and measured. When these have all been measured the plate is moved back again until a total of 50 fibres have been measured. In each case once the plate has been moved all projecting fibre ends must be removed and measured. The whole process is then repeated on fresh lengths of yarn chosen at random from the bulk, until sufficient fibres have been measured.

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Fig7 :- The cut square method

Yarn sampling

When selecting yarn for testing it is suggested that ten packages are selected at random from the consignment. If the consignment contains more than five cases, five cases are selected at random from it. The test sample then consists of two packages selected at random from each case. If the consignment contains less than five cases, ten packages are selected at random from all the cases with approximately equal numbers from each case. The appropriate number of tests are then carried out on each package.

Fabric sampling

When taking fabric samples from a roll of fabric certain rules must be observed. Fabric samples are always taken from the warp and weft separately as the properties in each direction generally differ. The warp direction should be marked on each sample before it is cut out. No two specimens should contain the same set of warp or weft threads. This is shown diagrammatically in Fig. 8 where the incorrect layout shows two warp samples which contain the same set of warp threads so that their properties will be very similar. In the correct layout each sample contains a different set of warp threads so that their properties are potentially different depending on the degree of uniformity of the fabric. As it is the warp direction in this case that is being tested the use of the same weft threads is not important. Samples should not be taken from within 50mm of the selvedge as the fabric properties can change at the edge and they are no longer representative of the bulk.

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Fig 8:- Fabric Sampling

Wool


  • Merino wool samples that were sold by auction,...
    Image via Wikipedia

    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).

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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).

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  • 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).

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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).

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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).

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Disulphide linkage (Collier, 1974:27)

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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).

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The Burn Test to Identify Textile Fibers


The burn test is a simple, somewhat subjective test based on the knowledge of how particular fibers burn. Be prepared to note the following when testing your fibers:
• Do the fibers melt and/or burn?
• Do the fibers shrink from the flame?
• What type of odor do the fumes have?
• What is the characteristic(s) of any smoke?
• What does the residue of the burned fibers look like?

The burn test is normally made on a small sample of yarns or thread which are twisted together. Since the fiber content of yarns used in one direction of a fabric are not always made up of the same fibers used in the other direction, warp and filling yarns should be burned separately to determine the entire fiber content of the fabric. This test is very helpful in determining whether a fabric is made from synthetic or natural fibers, but it is not foolproof and the characteristics observed during the burning test can be affected by several things. If the fabric /yarn contains blends of fibers, identification of individual fibers can be difficult. Two or three different kinds of fibers burned together in one yarn may also be difficult to distinguish. The odor and burning characteristics exhibited may be that of several fibers, thus making your results difficult to analyze. Finishes used on the fabric can also change the observed characteristics.

  • Pull a small sample of at least six to eight yarns from your fabric about 4 inches long, and twist them together into a bundle about 1/8 inch in diameter. You can also use a small snippet of the fabric if you only need to determine whether it is a synthetic or natural fiber fabric and you are not seeking to determine the specific fiber(s) that make up the fabric.
  • Hold one end of the bundle with tweezers over a sink or a sheet of aluminum foil (about 10 to 12 inches square) to protect your working area. If the sample ignites it can be dropped into the sink or on the foil without damage. Use either a candle or a match (automatic lighters work well) as your flame.

Potential Test Results

Natural, Organic & Manmade Fibers

In general, if the ash is soft and the odor is of burning hair or paper, the fabric is a natural fiber. Cellulosic fibers (cotton, linen and rayon) burn rapidly with a yellow flame. When the flame is removed, there is an afterglow, then soft gray ash.

Cotton: Ignites on contact with flames; burns quickly and leaves a yellowish to orange afterglow when put out. Does not melt. It has the odor of burning paper, leaves, or wood. The residue is a fine, feathery, gray ash.
• Hemp: Same as cotton
• Linen: Same as cotton
• Ramie : Same as cotton
• Rayon : Same as cotton, but burns slowly without flame with slight melting; leaves soft black ash.
• Silk: Burns slowly, but does not melt. It shrinks from the flame. It has the odor of charred meat (some say like burned hair). The residue is a black, hollow irregular bead that can be easily to a gritty, grayish-black ash powder. It is self-extinguishing, i.e., it burns itself out.
Tencel : Same as Rayon
• Wool, and other Protein Fibers: Burns with an orange sputtery color, but does not melt. It shrinks from the flame. It has a strong odor of burning hair or feathers. The residue is a black, hollow irregular bead that can be easily crushed into a gritty black powder. It is self-extinguishing, i.e., it burns itself out.

Synthetic Fibers

Most synthetic fibers both burn and melt, and also tend to shrink away from the flame. Synthetics burn with an acrid, chemical or vinegar-like odor and leave a plastic bead.
Other identifying characteristics include:
• Acetate: Flames and burns quickly; has an odor similar to burning paper and hot vinegar. Its residue is a hard, dark, solid bead. If you suspect a fabric is acetate, double-check by placing a scrap of it in a small amount of fingernail polish remover-if you’re correct, the fabric will dissolve
• Acrylic: Flames and burns rapidly with hot, sputtering flame and a black smoke. Has an acrid, fishy odor. The residue is a hard irregularly-shaped black bead.
• Nylon: It will shrink from the flame and burn slowly. Has an odor likened to celery. Its residue is initially a hard, cream-colored bead that becomes darker gray.
• Olefin/Polyolefin: Has a chemical type odor. The residue id a hard, tancolored bead. The flames creates black smoke.
• Polyester: It will shrink from the flame and burn slowly giving off black smoke. Has a somewhat sweet chemical odor. The residue is initially a hard cream-colored bead that becomes darker tan.
Spandex: It burns and melts, but does not shrink from the flame. It has a chemical type odor. Its residue is a soft, sticky black ash.