- Importance of certain greasy wool characteristics
|Last update: April 3, 2012 08:30:17 AM|
IMPORTANCE OF CERTAIN GREASY WOOL CHARACTERISTICS
Dr J.J. Venter
Head: Wool and Mohair Research
Agricultural Research Institute, Grootfontein
IN the valuation and purchasing of wool the importance of an intimate knowledge on the part of the producer and buyer of the most important physical characteristics of greasy wool can hardly be overemphasized. This enables the buyer and manufacturer to forecast with a high degree of accuracy the processing results which could be obtained from every purchase.
It should then be evident that clip preparation and a sound classing technique must similarly depend on an appreciation of the same criteria the buyer uses to select and value his wool. This infers that within any clip good classing relies on an appreciation of the extent to which various wool characteristics are important in determining the price.
It may be worthwhile for woolgrowers to consider some of the more important characteristics of greasy wool that are of importance in processing and the quality of the end product.
The wool processor is interested in the quantity of pure wool fibre in the raw wool he buys and the greater the yield of fibre which he can obtain from the fleece, the more will be its economic advantage as far as he is concerned.
Yield can be determined in two ways, that is by estimating the yield subjectively (traditional method) or by laboratory analyses. Estimating the yield is as old as the wool trade itself. It is a subjective method and calls for a lot of experience.
Today buyers are making more and more use of the actual yield, that is a laboratory determined yield which is part and parcel of objective measurement implemented in July 1972.
Raw wool consists of pure wool keratin and natural impurities such as wool wax (grease) and suint as well as acquired impurities such as vegetable matter, dirt, dust and sand. These different impurities may vary in different proportions and thus influence the clean yield of the wool.
The yolk (wool wax and suint) is, however, considered necessary to coat and also to bind the fibres together into staples preventing the fibres from falling open and being damaged deeper into the staple by the sun. Wools deficient in yolk normally also feel harsh.
It was, however, established by Truter & Woodford (1955) that the normal quantity of wool wax on the wool fibres, generally varying from below 15 percent to over 40 percent, had to increase more than tenfold before any real protection could be observed. An increase in the iodine number (indicative of a fluid wool wax) was considered of more value.
According to Bonsma & Starke (1934) production rations either high or low in protein do not seem to have any influence upon the secretion of yolk. The percentage of wool wax in the wool as stated by Venter (1966), remained practically constant up to the age of 3½ years of age. From 3½ years to 6½ years a gradual increase was noted in the percentage of wool wax in the wool from wethers and ewes, but a sharp increase in the wool of rams. This increase in wool wax consequently caused a sharp decline in the yield percentage of the wool of rams.
Generally, the weight of vegetable matter is small and has little influence on yield. The greater percentage of vegetable matter is removed mechanically in the carding and combing processes. In considering clean values of wools which in the greasy form contain vegetable material, account must be taken of the additional loss of fibres which cling to vegetable material, such as burr, during carding and also the additional fibre breakage when beating the burr from the wool.
Wool that cannot be cleansed mechanically has to be carbonised. In the carbonising process the wool is treated with diluted sulphuric acid. Wool treated in this way becomes inferior because of the damage sustained by the fibres.
The presence of vegetable matter is a problem in the processing of wool and high degrees of contamination with burr and fine grass seed should be avoided as far as possible.
Dirt, dust and sand - anyone or all together - varies most of all foreign matter in the fleece.
The length of the wool fibre largely determines whether it will be manufactured on the woollen or the worsted system. Wools up to about 35 mm in length are manufactured on the woollen system and are referred to as clothing wools, whereas wools longer than 35 mm are processed on the worsted system. The latter wools are known as combing wools.
It must be remembered that there is staple length as well as straight fibre length to consider. The evidence is overwhelming that there is a close relationship between the length of staples and the mean straight length of the fibres within the staples. Due to this close relationship between mean fibre length and staple length a wool clip can be successfully classed for length if there are staple length differences between fleeces.
Some variability of fibre length is important because it enables the spinner to spin more evenly and to enable him to blend properly. However, a great variation in length results in weaker and less extensible yarns (Walls, 1974). As a rule the longer the wool, the longer and also stronger and more uniform yarn can be spun from a given weight of wool.
During the various processes in manufacture, especially in carding, the fibres are subjected to a high degree of strain. The carding operation is a very severe one and a large percentage of fibres is broken. This results in a decrease of average fibre length and an increase in the percentage of short and very short fibres.
The term 'tearage' refers to the removal of the very short fibres in the combing process. It is an important subject both from the standpoint of costing and from its effect upon the spinning property of the wool. The short fibres which are removed during combing are known as Noil and the remaining longer fibres are known as the Top.
As noil has less than half the value of top, the percentage of wool removed as noil has considerable commercial significance and therefore in estimating the value of any lot of wool, it is necessary for the buyer to examine the wool for this feature in addition to those previously mentioned.
Walls (1974) stated that when there is no break in the wool, the finer and hence weaker fibres tend to give more noil than the coarser fibres. A break in the staple results in increased fibre breakage and a higher percentage of noil.
Seasonal changes cause a thinning of the fibres that is enhanced by poor nutrition, but the final stimulus causing the break is usually a stress condition brought on by disease or sudden exposure to wet cold conditions, etc.
Weathering also has a marked adverse effect on the soundness of wool. Degradation of wool, especially in the tip portion of the staple, takes place on the back of the sheep and to a lesser extent on the other parts of the body of the sheep due to exposure to sunlight, dust and heat.
It was found that the wool component in the card losses come mainly from the staple tips which had been weathered during growth. In an open fleece the fibres maybe more deeply damaged than in the case of a dense fleece. A loss in length of up to 30% is not unusual. Walls (1974) postulated that as the tip content can vary from almost zero for rugged wools to over 5%, it has considerable commercial significance.
In order to produce a sound root to tip wool the weathering of the wool on the sheep's back should be limited as far as possible. A high degree of weathering can be ascribed to a lack of substance and under-crimpiness (Veldsman & Kritzinger, 1960). Consequently, the penetration of dirt into the wool staples promotes damage (Le Roux, 1958). The adaptability of the animal to a certain environment also proved to have an effect on the degree of weathering (Snyman, 1960). Steenkamp, Venter & Edwards (1970) found that the degree of weathering depends on the season and time of shearing. Severe weathering of the extreme tip usually occurs during the hot summer months. During this period long wool is liable to be more deeply weathered than shorter wool.
Deeper weathering of the wool in the staple can be limited by selecting and breeding for better substance and a better staple formation. A more fluid wool yolk and also apparently more suint with a higher pH proved to restrict deeper weathering (Le Roux, 1958; Louw, Swart & Mellet, 1963; Venter, 1976).
From a management point of view the degree of weathering deeper into the staple can be reduced by changing the shearing time: so as to have a short wool over the hot summer period (Steenkamp, et aI, 1970; Venter, 1976) or protecting the sheep all the time or only over the three hottest months of the year. The availability of shade in camps especially trees or open sheds is recommended as the most practical method (Venter, 1976; Venter, Nel and Edwards, 1977).
It is thus clear that a sound, well nourished wool with a good tensile strength from base to tip is the ideal from the manufacturer's point of view as it will give the least fibre waste and cause the least problems during manufacture and will result in a high quality end-product.
The fineness of wool determines to a great extent for what purpose the wool will be used. Superfine wool is used for the manufacturing of very light materials, while strong wool is used for coarser materials such as blankets and carpets. Wiggens (1976) stated that mean fibre diameter determines the spinning performance of wool for fine yarns and in fabrics it is associated with softness, warmth and flexibility. It is also known that finer fibres result in smoother and softer handling fabrics. Finer wools are also more valuable than thicker wools as stated by Whan (1968). The impact of fibre fineness on wool values is much more pronounced in the superior styles and between finer counts. However, there are realistic differences in wool values, even in the poorer styles, which stem directly from fibre fineness effects.
The relationship between spinning count, fibre thickness and number of crimps per inch (25,4 mm) (crimp frequency) as compiled by Duerden (1929) led to the use of crimps per inch (25,4 mm) as an indication of the fineness of wool. Whitely (1972) pointed out that in practice the classer is forced to rely on the relationship between staple crimp frequency and diameter.
A large variation in the nature of crimping in relation to fibre thickness generally occurs. As far as South African Merino wool is concerned, Bosman (1934) showed that the Duerden relationship only holds in 28 percent of the cases while 36 percent proved to be over-crimped and 36 percent under-crimped. Corresponding deviations were noted by Lang (1974) in Australian Merino wool and appeared to vary within flocks and different strains. The marked deviation from the former relationship can, according to Whitely & Charlton (1975), be attributed to genetic variations among flocks although district and seasonal variations in nutrition are also important, for diameter is more sensitive to climatic changes than is crimp frequency.
Lately, various studies showed that the South African wool clip is to a large extent undercrimped. The commercial wool producer and stud breeder have for years emphasised the production of a good quality wool (kind handle). The logical consequence was that the wool clip has become more and more under-crimped (Kruger, 1964; 1971; Uys, 1966). In the majority of cases the wool is actually much finer than indicated by crimps. As fibre diameter is accepted as the most important factor in processing it is clear that the traditional appraisal system for fineness has become insufficient. Objective measurement of fibre diameter is thus the only means of ensuring that a fair price is paid for greasy wool sale lots.
To base the breeding policy on fibre thickness alone will not be sufficient. Venter & Steenkamp (1971) pointed out that breeding should be directed towards certain combinations of fibre thickness and crimps. Van Wyk (1946) concluded "assuming that the experienced sorter may be able to estimate fibre thickness without being misled by crimping, he has to choose between the two. In any case, variations in the relation between fibre thickness and crimping will result in a lack of uniformity in the sorted lot".
Research has shown that wools of which the crimps and fibre thickness comply with Duerden standards have certain desirable properties. Duerden-true wool still possesses the so-called kind handle, have a greater resilience, a lower felting property and suffer less weathering damage than under-crimped wools which, apart from being flabby, suffer from many other defects.
As far as the manufacturing is concerned, Duerden-true wools proved to have a better processing performance together with better properties in the end commodities. This is of so much importance, that the production of Duerden-true wool through selection and breeding is strongly recommended by Veldsman (1970).
Crimps in wool are characteristic of most wool types. The crimp greatly enhances some excellent properties of wool as textile fibres. As in the case of fibre fineness, the nature as well as the number of crimps per unit length of fibre, vary with different breeds, between individuals within the same breed and on different parts of the body and even along the length of the same fibre.
There is also a wide variation in the shape of the crimps in the fibres. The depth of crimp as given by the ratio of the straight fibre length to the crimped or staple length may vary from shallow to deep. The resistance to bending of the fibre is very sensitive to a change in the crimp ration. The difference in the handle of wool can mainly be ascribed to the crimp ration and the number of crimps per unit length in relation to the diameter of the wool fibre (Van Wyk & Venter, 1954). A kind handle is considered a prerequisite for a wool of good quality.
The quality of the wool is largely based on a well-defined crimp in the staple. Uniformity of crimp over the entire fibre length is also indicative of a uniform fibre diameter and consequently of good tensile strength.
Crimps have a marked effect on the compressibility and resilience as well as the feltability of wool. Properties like crease-resistance and drapability in the end commodities as well as resilience; elasticity and bulk in carpets are mainly attributed to the crimps in the fibre. Kerr (1976) pointed out that fresh opportunities for wool are opening up by imparting intermediate crimps in flat less crimped wool so as to give greater bulk and elasticity in carpet wools.
Reports on the importance of crimps in affecting the processing performance of wool are rather conflicting, depending on the system of manufacturing (Chapman, 1964). Menkart & Detenbeck (1957) however, found that crimps in wool enhance the cohesion between fibres in spinning and also gives rise to a bulkier yarn and greater air permeability in knitwear. Lipson (1972) noted a less significant effect namely that the knitted fabric made from a higher crimped wool had a somewhat bulkier and more consolidated appearance than a fabric made from a wool of low crimps. Another unexpected finding was in the rate of felting which was lower for high crimped wool.
Though crimp frequency at present is considered of less importance and not incorporated in the Objective Measurement Testing System, its effect cannot completely be ignored. Venter (1976), after studying the difference between wool of good and of poor quality in respect to various fibre properties, stated that any difference in quality can mainly be ascribed to the nature of crimping in relation to the fibre diameter in wool.
Irrespective of any change of the crimp-fineness relationship due to climate, feeding and age, Venter & Steenkamp (1971), in support to Whan (1968, stated that it might be worthwhile to class wool according to different combinations of crimp-fineness ratios, that is "Duerden-true", "undercrimped" and "over-crimped" for various fibre thickness groupings.
Such a system of classing will enable the buyer to select and value more accurately the specific type of wool for his requirements.
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