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  • 2.2-COTTON VALUE ADDITION-THE IMPACT OF COTTON FIBRE PROPERTIES ON TEXTILE PROCESSING PERFORMANCE, QUALITY AND COSTS

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  • The impact of cotton fibre properties on textile processing performance, quality and costs

    Chapter 2 - Cotton value addition - The impact of cotton fibre properties on textile

     
     

    Cotton fibre is increasingly facing competition from artificial fibres, notably polyester. Cotton, being a natural product, varies widely in its fibre characteristics, both physical and chemical (mainly physical), because of genetic, environmental, harvesting and ginning factors. There are essentially four commercially grown cotton species: medium staple length and medium fine Gossypium hirsutum, American Upland (which accounts for over 90% of global cotton production); long staple and fine Gossypium barbadense; and the short staple coarse Gossypium arboreum and G. herbaceum (together known as Desi cottons). The physical, chemical and related characteristics of cotton lint, including the type and amount of non-fibrous matter present and ‘fibre configuration’ (preparation, neps etc.), determine its textile processing performance and behaviour, in terms of processing waste and efficiency (including machine stoppages and spinning breaks) and yarn and fabric quality (see figures 2.10 and 2.11). Ultimately these characteristics also determine both conversion costs and product end-use, price and quality.

     

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    The fibre represents 50%–70% of the yarn manufacturing costs. Ideally, therefore, the price of cotton should be linked to fibre characteristics. The relationship between cotton fibre price and properties has been investigated by Chakraborty et al (see figure 2.12); Deussen and Neuhaus have also advanced tables suggesting a link between cotton price and fibre properties.

     

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    Source: Chakraborty et al.

    Increasing quality and performance demands are being placed on the entire textile pipeline, from raw material to end-product. For example, some 20 years ago 15 non-reparable faults per 100 metres of cotton fabric were permitted, today it is 5, and thismay become 3 in future. Seconds have also come down from 3% to 0.5%, with 0.3% possible in future (Weissenberger and Legler). Weaving machine stops have decreased by 50% over the same period, some 20%–30% of such stops being due to yarn defects, the repair of each end-break costing about 70 cents. It is known that thin yarn places having extension and strength below certain minimum limits cause weaving end-breaks, such thin places and other defects in the yarn being influenced by fibre properties and spinning mill conditions.

    In view of this, it is understandable that efforts are continuously being directed towards improving the desirable properties of cotton and eliminating or minimizing any undesirable properties. Such efforts are aimed at breeding, farming and ginning practices as well as at textile processing systems and conditions. Furthermore, it is hardly surprising that for over a century so much effort has gone into developing instrument methods for accurately measuring cotton fibre properties (preferably testing each bale of cotton), and quantitatively relating the measured properties to processing performance and yarn and fabric properties, so as to improve and optimize quality all-round (see box below). Tremendous progress has been made in this regard, a very good example being the development of the systems for high volume testing of cotton, commonly referred to as HVI systems. In 2006 there were some 2,000 such systems in place in over 70 countries, which, in theory, were capable of annually testing the entire global cotton crop of some 25 million tons.

     

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    In spite of the extensive research (experimental and theoretical) carried out to relate the measured characteristics of cotton to processing performance and yarn quality, no ‘generic’ relationships, or other empirical or theoretical means are as yet available to accurately relate cotton fibre properties to subsequent textile performance. The reasons for this include the tremendous variations in cotton fibre properties and their interrelationships, as well as variations in processing conditions, and the interactions between processing conditions and fibre properties. The relative importance of the fibre properties also depends on the spinning system (see table 2.1), on whether or not the cotton is combed, and on the fineness of the yarn being spun.

     

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    This chapter briefly discusses the measurement of fibre properties and the impact of changes in fibre properties on textile processing performance, quality and costs. Nevertheless, the cost implications of changes in fibre properties are complex, being not only highly mill and product dependent but also difficult to isolate and quantify, even within a mill. For example, how does one calculate the cost implications of a deterioration in yarn evenness due to a decrease in cotton fibre length or an increase in short fibre content? Another example is the cost implications of an increase in cotton waste due to an increase in short fibre content, taking into consideration the recycling and/or selling of waste. It has been estimated that an increase of 1% in carding waste and in blowroom waste increases yarn costs by about 1%, while an increase of 1% each in blowroom, carding, combing and spinning waste can increase yarn costs by over 3%. Because of these complexities, the cost implications of changes in fibre properties will only be touched upon. The measurement and effect of cotton fibre properties.

    The initial cotton fibre instrument measurement (laboratory) systems, developed during the first half of the twentieth century (e.g. Pressley tester in the early 1940s and the Stelometer and Colorimeter in the early 1950s), tended to be time consuming and to be fairly operator-dependent, and it was increasingly realized systems were needed, preferably automatic or even online, in which all relevant fibre characteristics could be measured accurately, rapidly and cost effectively, with little operator involvement. Nevertheless, it took many decades before this goal was reached. A major step in this direction was the development of the High Volume Instrument (HVI). From its development in the late 1960s, commercial introduction in the late 1970s and first use for cotton classing in the early 1980s, the high volume testing of cotton has made enormous strides and has become widely accepted globally. Despite certain shortcomings, it remains the only method for the wide-scale and cost-effective testing and classing of the global cotton crop.

    The latest generations of high volume testing systems can test for all the traditional HVI measured properties, plus short fibre content, neps, seed coat neps, stickiness, maturity and moisture content as well as additional colour parameters (separately from those of trash and other contaminants). In some cases, however, such detailed testing is accompanied by a loss in testing speed and further improvements are still required, particularly in terms of the measurement and characterisation of trash. It can safely be said that the lint characteristics routinely measured by high volume systems today account for the bulk, but not all, of the variations in the textile processing behaviour and yarn quality of cotton. Nevertheless, the accuracy and reproducibility of the test results for some of the properties described above have not as yet achieved the levels required by industry. Under the auspices of the International Cotton Advisory Committee (ICAC), Washington, DC, a Task Force was installed in 2003 to facilitate standardized and harmonized test results for commercial use of high volume testing: the ICAC Task Force on Commercial Standardization of Instrument Testing of Cotton (CSITC). One main objective of the CSITC is
    the installation of a CSITC Round Trial system. With this system three aims
    can be achieved:

    • Evaluation of the high volume test methods and the test result variability
      • Inter-laboratory variability;
      • In-laboratory variability.
       
    • Evaluation/rating of the participating laboratories based on the trueness of the results.
    • Detailed analysis of laboratory results to achieve more accurate results based on trueness and precision.

    The Round Trial system was installed in 2007, and every testing facility is invited to participate.

    The first aim will help to assess the suitability of the properties tested with high volume systems. With the second aim a certification scheme for laboratories is given, although there are no “pass/fail” criteria, but a grading of the overall results. Each testing facility will be able to proof its qualitification for testing with the certificate based on the rating. The third aim will help the laboratories to achieve more reliable results.

    ICAC is hosting the CSITC Round Trials, which are conducted in cooperation between the United States Department of Agriculture (USDA-AMS) and the Bremen Fibre Institute (FIBRE). Information is given and registration is possible on the ICAC web pages (www.icac.org) or by e-mail ( csitcsecretariat@icac.orgThis e-mail address is being protected from spambots. You need JavaScript enabled to view it).

    The ultimate aim is to be able to measure once only, in an accurate, routine, rapid and cost-effective manner, all those cotton characteristics (see box on page 44) that play a role, however small, in determining processing route and performance, product quality, utilisation and application and ultimately the commercial value, and then to be able to quantitatively relate these properties to the subsequent textile processing performance, utilization and quality, on a mill-specific basis. The results so generated should accompany the bale until it reaches its final destination.

    Another important and popular development relates to rapid, individualized fibre measurement systems for cotton (e.g. electro-optical systems, such as AFIS – Advanced Fibre Information System), enabling the accurate and detailed laboratory measurement of properties such as length (including short fibre content), neps (fibrous as well as seed coat), trash, dust, fineness and maturity (also immature fibre content, <0.25), and their respective distributions. In 2006 there were some 800 AFIS systems in place worldwide. The advantage of such systems is that they supply more detailed information, even down to the individual fibre level, also covering properties presently not measured by high volume systems. The main drawback of these systems, in terms of the routine high volume testing of cotton for classing and trading purposes, is their relative slowness, although higher speed systems are slowly making their appearance.

    The application of NIR (near infra-red), and other parts of the electromagnetic spectrum, for measuring certain cotton properties (e.g. maturity, stickiness and moisture content) also represents a potentially promising field of research. Such measurement systems, being non-contact and non-destructive, lend themselves to online applications, as well as to being extremely rapid and versatile.