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  • 2.5.4-COTTON VALUE ADDITION-USE OF INSTRUMENTATION IN COTTON CLASSIFICATION

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  • Use of instrumentation in cotton classification

    Chapter 2 - Cotton value addition - Classing and grading 

     
     

    Cotton testing instruments have been under development and in small-scale use since the 1960s. However, it was not until the early 1990s that the cotton industry began to accept instrument classification on a wide-scale basis. In 1991, USDA implemented instrument classification on 100% of the United States crop. Current instrument classification in USDA includes measurements for upper-half-mean length, length uniformity index, strength, micronaire, colour Rd/+b, and per cent area trash. Classing operations throughout the world have also adopted instrument classification. When using cotton testing instruments for classification, there are several critical elements necessary to maintain precise and accurate results. They include calibration standards, lab conditions, sample conditioning and instrument verification procedures.

    Instrument standards

    In order to maintain the integrity of a classification system, official standards and standardized procedures should be used throughout system. Instrument standards refer to cottons used for instrument calibration and/or test level verification. The internationally accepted standards for instrument classification are the Universal HVI Cotton Standards which are produced and distributed by USDA. These standards include calibration cottons for the measurements of micronaire, upper-half-mean length, length uniformity index, and strength. USDA also provides colour calibration tiles for colour calibration and Universal HVI Colour Standards for the verification of actual cotton colour testing levels. Like the Universal Upland grade standards, USDA’s instrument standards are universal standards and are recognized under the Universal Cotton Standards Agreement.

    Cotton selected for use in instrument calibration must pass rigorous screening procedures. As a first step, USDA conducts an extensive search across the United States crop by reviewing instrument classification data. Uniform lots of cotton bales with fibre properties within the desired ranges are targeted. Candidate bales are purchased from growers or merchants and then shipped to USDA. Candidate bales undergo rigorous instrument testing to verify bale uniformity and to establish standard values. Bales meeting USDA’s high standards for acceptance are established for use as calibration cotton.

    Establishing values for calibration cotton

    In addition to a high degree of within-bale fibre uniformity, bales selected as calibration standards must meet the length and strength criteria for their intended use. For example, a typical Upland long/strong calibration cotton bale will have a length and strength of approximately 1.17" (29.7 mm) and 33 g/tex. An Upland short/weak calibration cotton bale will have a length and strength of approximately 1.00" (25.4 mm) and 23 g/tex.

    Currently, six laboratories work together to establish values for Universal HVI Calibration Cottons. These consist of four USDA laboratories, one independent laboratory within the United States and one independent international laboratory. The independent United States and international labs are required to operate under the same rigid specifications as USDA in order to participate in the value establishment process. Combining all laboratory tests, a minimum of 120 tests are required on each bale in order to establish values. Laboratory results are compiled and analysed to confirm each bale’s uniformity and to determine the standard values for establishing the bale as calibration cotton. For reference purposes, samples previously established as benchmark calibration cottons are included in the testing along with the samples from the candidate bales. The benchmark cottons provide the required reference level for accurate value establishment. If the test results within a bale are not within prescribed limits, the bale is rejected. If all testing criteria are met on a bale, the bale is accepted and packaged into 5 lb (2.27 kg) units for distribution as a Universal HVI Calibration Cotton.

    Calibration of instruments

    Instruments are calibrated for upper-half-mean length, length uniformity index, micronaire and strength by using Universal HVI Calibration Cottons. USDA established tiles are used to calibrate colour and trash measurements. Calibration should be performed at regular intervals for each factor. USDA recommends the following calibration tolerances for instrument testing:
     

    Instrument determination calibration tolerance 

    Micronaire (units)  ±0.100 
    Colour Rd (units)  ±0.400 
    Colour +b (units)  ±0.400 
    Trash (% area)  ±0.050 
    Length (inches)  ±0.007 
    Uniformity (%)  ±0.700 
    Strength (g/tex)  ±0.500 

     

    Laboratory conditioning

    Atmospheric conditions influence the measurement of cotton fibre properties. Therefore, the temperature and relative humidity of classing laboratories must be tightly controlled. Temperature should be maintained at 21 degrees Celsius (70 degrees Fahrenheit), plus or minus 1 degree Celsius (plus or minus 2 degrees Fahrenheit), and relative humidity should be maintained at 65%, plus or minus 2%. For ASTM International Lab Conditioning Standards for Cotton Classification, see the Standard Practice for Conditioning and Testing Textiles, ASTM D1776-04.

    Sample conditioning

    Prior to instrument testing, cotton samples should be conditioned to bring the moisture content to equilibrium with the approved atmospheric conditions. Properly conditioned samples will have moisture content of 6.75%–8.25% (dry weight basis). There are two methods for conditioning samples: passive conditioning and active conditioning.

    In passive conditioning, cotton samples are placed in single layers in trays with perforated bottoms to allow free circulation of air. The samples must be exposed to standard atmosphere conditions for 48 hours in order to assure thorough conditioning.

    In active conditioning, a rapid conditioning device is used in which air at standard atmospheric conditions is drawn through the sample until the required moisture content for instrument testing is attained. Depending on the type of rapid conditioning device used, the time required to condition samples properly can be reduced to as little as 10 minutes.

    The moisture content of conditioned samples should be monitored by checking sample moisture prior to instrument testing to verify that the appropriate moisture content has been reached.

    Instrument verification procedures

    Minimum performance requirements for classing instruments should be an integral part of any cotton classification system. Newly purchased instruments should be evaluated with a series of thorough tests before they are accepted and put into operation. Instruments should also be re-evaluated annually, typically before each cotton grading season begins. Testing should be done to verify both precision and accuracy of instrument measurements. The term ‘precision’ refers to the ability of an instrument to produce the same measurement result time after time. The term ‘accuracy’ refers to how well an instrument measures a certain property in relation to its true value.

    Fibre length

    Upper-half-mean length is the average length of the longer half of the fibres. It is measured in both hundredths of an inch and millimetres. For trade purposes, the instrument length is often converted into staple length. Table 2.4 gives the conversions from inches to staple length. Instrument-based length is performed by passing a small tuft of parallel fibres, commonly referred to as a ‘beard’, through a sensing point. The beard is formed when fibres from a sample of cotton are grasped by a clamp, then combed and brushed to straighten and parallel the fibres.


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    Fibre length is largely determined by variety, but the cotton plant’s exposure to extreme temperatures, water stress, or nutrient deficiencies may shorten the length. Excessive cleaning and/or drying at the gin may also result in shorter fibre length. Fibre length affects yarn strength, yarn evenness, and the efficiency of the spinning process. The fineness of the yarn that can be successfully produced from given fibres is also influenced by the length of the fibre.

    Length uniformity index

    Length uniformity index (LUI) is the ratio between the mean length and the upper half mean length of the fibres and is expressed as a percentage. If all of the fibres in the bale were of the same length, the mean length and the upper half mean length would be the same, and the LUI would be 100%. However, there is a natural variation in the length of cotton fibres, so LUI will always be less than 100%. Table 2.5 provides a general guide for interpreting LUI.


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    LUI affects yarn evenness, yarn strength and the efficiency of the spinning process. It is also strongly related to short fibre content (fibres shorter than 1/2" or 12.5 mm). Cotton with a low LUI is likely to have a high percentage of short fibres. Short fibres are largely removed as waste while those remaining tend to aggregate during drafting (grasping and pulling with increasing speed) and cause thick places in yarn. Yarns with thick places are not uniform and cannot be used in high quality products. Short fibres reduce the strength of ring-spun yarns and the thick places are frequently points of weakness in yarns. The aggregates of short fibres can cause processing disruptions known as ends-down.

    During the ginning process, fibre breakage can reduce LUI by adding to the short fibre content. When fibres are removed from the seed during ginning, some fibres break at a point other than near the seedcoat and must be removed in two pieces. Fibre breakage is also caused by lint cleaners. Immature fibres have less resistance to breakage than mature fibres. Cotton with low micronaire has comparatively lower LUI than high micronaire cotton. Fibre strength also affects resistance to breakage. Stronger cotton generally has higher LUI than weaker cottons.

    Fibre strength

    Strength measurements are made on the same specimen or beard of cotton as used for measuring fibre length. The beard is clamped in two sets of jaws, spaced 1/8" apart, and the amount of force required to break the fibres is measured. Strength is measured in terms of grams per tex (g/tex). A tex unit is equal to the weight in grams of 1000 metres of fibre. Therefore, the strength reported is the force in grams required to break a bundle of fibres 1 tex unit in size. Table 2.6 can be used as a guide in interpreting fibre strength measurements.


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    There is a high correlation between fibre strength and yarn strength. Also, cotton with high fibre strength is more likely to withstand breakage during the manufacturing process. Fibre strength is largely determined by variety. It may be affected by plant nutrient deficiencies and weather; however, it is less influenced by adverse growing conditions than are length and micronaire.

    Micronaire

    Micronaire is a measure of fibre fineness and maturity. An airflow instrument is used to measure the air permeability of a constant mass of cotton fibres compressed to a fixed volume. The volume of airflow through the specimen of cotton fibres is expressed as the micronaire.

    Cottons with micronaire measurements between 3.7 and 4.2 are considered in the premium range of micronaire. Cottons within the micronaire ranges of 3.5–3.6 or 4.3–4.9 are considered base quality, while cottons above 4.9 or below 3.5 are in the discount ranges.

    Micronaire measurements can be influenced during the growing period by environmental conditions such as moisture, temperature, sunlight, plant nutrients, and extremes in plant or boll population. Favourable growing conditions result in fully mature fibres with premium range micronaire readings. Unfavourable conditions, such as lack of moisture, early freeze, or any other conditions that interrupt plant processes, will result in immature fibres and low micronaire measurements. High micronaire cotton is caused by such things as abnormally warm temperatures during boll maturation, or poor boll set leading to excessive availability of carbohydrates and over-maturing of fibres.

    Fibre fineness affects processing performance and the quality of the end-product in several ways. In the opening, cleaning, and carding processes, low micronaire or fine-fibre cottons require slower processing speeds to prevent damage to the fibres. Yarns made from finer fibre result in more fibres per cross section, which in turn produces stronger yarns. High micronaire or coarse fibres are not suitable for fine yarns since the result would be fewer fibres per cross section, which would reduce the yarn strength. Dye absorbency and retention varies with the maturity of the fibres. Low maturity fibres have poor dye absorbency and retention while higher micronaire fibres have good absorbency and retention.

    Instrument colour

    Instrument colour in cotton classification is measured in units of reflectance (Rd) and yellowness (+b). Rd indicates how bright or dull a cotton sample is and +b indicates the degree of yellow colour pigmentation. Low Rd levels indicate dullness or greyness while high Rd levels indicate brightness or lack of grey. High +b levels indicate a high degree of yellowness while low +b levels indicate a low level of yellowness. Figures 2.27 and 2.28 relate Rd and +b levels to Upland and Pima colour grades. The vertical axis in these figures indicates Rd and the horizontal axis indicates +b.


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    Figure 2.27: HVI colour chart for American Upland cotton

     

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    Figure 2.28: HVI colour chart for American Pima cotton

    The colour grade as established by the Universal Upland Cotton Grade Standards is determined by the degree of reflectance (Rd) and yellowness (+ b) as shown in the Upland colour chart in figure 2.27. The colour grade as established by the USDA American Pima Standards is determined by Rd and +b as shown in the Pima colour chart in figure 2.28. Colour grades can be interpreted from Rd and +b measurements by locating the point at which the Rd and +b values intersect on the colour charts. Since 1999, USDA has utilized the instrument instead of the human classer for determining colour grade for official Upland cotton classification.

    The colour of cotton fibres can be affected by rainfall, freezes, insects and fungi, and by staining through contact with soil, grass, or the cotton plant’s leaf. Colour can also be affected by excessive moisture and temperature levels while cotton is being stored, both before and after ginning. As the colour of cotton deteriorates because of environmental conditions, the probability of reduced processing efficiency is increased. Colour deterioration also affects the ability of fibres to absorb and hold dyes and finishes.

    Trash

    Trash is a measure of the amount of non-lint materials in cotton, such as leaf and bark from the cotton plant. The surface of the cotton sample is scanned by a digital camera and digitized for image analysis. The percentage of the surface area occupied by trash particles and the number of trash particles visible are calculated and reported. A high percentage area of trash results in greater textile mill processing waste and lower yarn quality. The ratio between percentage area of trash and trash particle count is a good indicator of the average particle size in a cotton sample. For instance, a low percentage trash area combined with a high trash particle count indicates a smaller average particle size than a high percentage trash area combined with a low trash particle count. Small trash particles or ‘pepper trash’ are very undesirable because they are more difficult for the textile mill to remove from the cotton lint than larger trash particles.