Cotton, being a natural agricultural product, differs widely from growth to growth, crop to crop, lot to lot, bale to bale, within a bale and even fibre to fibre. In view of this and the important effect which variations in fibre properties have on processing performance, cost and product quality, it is of crucial importance that such variations in fibre properties be determined and quantified. Once cotton is ginned, and while it is being baled, a sample (minimum of 200 g) is taken from both sides of every bale and bulked together and sent to the classing facility for classification.
Originally, cotton was ‘classified’ by a classer’s subjective assessment of fibre length as well as colour and leaf using the United States Department of Agriculture (USDA) Universal Upland Grade Standards and American Pima Grade Standards (see Figure 2). These grade standards specify colour and leaf. There are twenty five official colour grades for Upland cotton and five categories of below grade colour. Universal Upland Grade Standards are valid for only one year, with the Pima Grade Standards valid for two years.
Cotton classers are skilled in visually determining the colour, trash and extraneous matter and then assigning such cotton to a certain established standard grade.
As the ‘Classer’ was not able to assess various important textile quality related fibre properties, such as strength, elongation and fineness, a number of instruments were developed which measure the required properties.
Due to the greater demand by modern spinning, the cost of raw material, and the increasingly competitive global market, there was a need to rapidly and accurately determine those cotton fibre quality parameters that affect processing performance and yarn quality in a cost effective way on large numbers of bales of cotton. This led to the development of high volume automatic testing systems. These systems, termed High Volume Instruments (HVI™), not only supplement, but are increasingly replacing the traditional ways of cotton fibre quality determination and classing. Testing by HVI™ provides the cotton spinner with valuable information regarding the fibre length, length uniformity, strength and micronaire of every bale of cotton purchased, thereby ensuring consistency in processing and yarn quality.
In Australia, colour is determined by both visual and HVI™, with leaf, extraneous matter (any substance other than fibre and leaf, such as bark, grass, seed coat fragments, oil etc.) and preparation (degree of smoothness or roughness of the cotton sample) still assessed by visual determination.
The quality of cotton can be expressed by several different measurements which are performed by cotton classers. These measurements are described in a wide range of grades (Figure 3, page 135), and affect the final price that is paid for a bale of cotton.
The price received for cotton is dependent on the quality of each bale of cotton. Cotton prices are quoted for ‘base grade’ 31-3-36, G5 (refer to Figure 3). Base grade refers to the grade of cotton that is used by cotton merchants as a basis for contracts, premiums and discounts Premiums and discounts apply for higher and lower grades, respectively.
These pricing adjustments reflect the change in suitability for the spinning and dyeing process (see Chapter 19, page 116, Table 1, ‘Consequences of poor fibre quality’ right column).
For this reason, variability in any quality characteristic may influence the price. Some of the key quality characteristics are outlined below: