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  • Length and length uniformity

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

    Length, length uniformity and length distribution, including short fibre content, are probably the most important cotton fibre properties, although their importance does depend somewhat on the spinning system used. Fibre length characteristics are determined by genetic (cotton variety) factors as well as by ginning and textile processing conditions, the latter in terms of fibre breakage. Cotton fibre length characteristics are probably the best criterion for ring spinning performance and spinning limits and often also of yarn strength. An increase of 1 mm in fibre length increases yarn strength by some 0.4 cN/tex or more (Frey, M). The staple length, upper half mean length (UHML) and 2.5% span length all provide similar, but not identical, measures of the length of the bulk of the long fibres in a sample, and approximate the length of the fibres when carefully detached from the seed by hand. They are measured by HVI and other similar systems, as well as by slower single fibre measurement systems, such as AFIS. These measures are useful for setting drafting roller distances, with UHML increasingly being measured by high volume systems and adopted for trading purposes. A length above 28 mm is desirable in most cases, although this depends upon the spinning system and yarn count. The mean length (ML) or 50% span length is generally regarded as providing a better measure of spinning performance and yarn quality. Longer cottons, which are often also finer, are generally more prone to forming neps during carding and are therefore often carded at lower speeds and also combed to remove neps and to even better align the fibres. Longer fibres also enable lower roving and yarn twists and higher ring spinning speeds to be employed, also producing finer, stronger, more even and less hairy yarns, as well as stronger fabrics with better appearance.

    Excessive fibre length variation (e.g. CV of fibre length, uniformity ratio or uniformity index) tends to increase manufacturing waste and to adversely affect processing performance, including spinning performance and yarn quality. The inverse of length uniformity also provides a measure of floating fibres within the drafting zone, although the short fibre content (SFC) is a better indicator of the floating fibres. SFC is generally defined as the percentage, by weight, of fibres shorter than ½" (12.7 mm). SFC by number is, however, considered a more sensitive measure of processing conditions. Although the uniformity index is typically, and accurately, measured on high volume systems, on its own it does not provide an accurate measure of SFC. A uniformity index of above 83% and uniformity ratio above 48% are desirable, although it depends upon the spinning system and yarn count.

    An increase in SFC increases spinning end breaks, processing waste (including comber noils), fly and optimum roving twist, and causes deterioration in yarn and fabric properties, notably yarn strength and evenness. An increase of 1% (absolute) in SFC can decrease ring spun yarn strength by 1% or more. Fabric strength and abrasion resistance also tend to deteriorate with an increase in SFC. Different instruments will find differences in SFC and other measures of fibre distribution. An SFC below 8% (by weight) is desirable, although the SFC level is generally a function of the staple length (UHML).