Pakistan Textile Journal. march index

Critical characteristics of cotton that affect yarn quality in Ring Spinning
by Tanveer Hussain and Muhammad Bilal.

Abstract: Effects of critical cotton fiber characteristics on the yarn quality in ring spinning have been comprehensively reviewed. Properties of cotton fiber not only affect its mill price but also have a profound effect on the yarn quality. The effect of some cotton fiber characteristics may be as far reaching as to influence the quality of the fabric even after dyeing and finishing. This review will hopefully help the cotton spinners in better selection of cotton for ring spinning. Furthermore, it will also help them troubleshooting yarn defects resulting due to poor cotton fiber properties.
Keywords: Cotton, Fiber Properties, Yarn Properties.

Cotton is a natural vegetable fiber with many desirable characteristics such as strength, absorbency, comfort and dyeability, which make it ideally suitable for making a wide range of textile products. Around 56% of apparel and home furnishing market share is captured by cotton fiber alone. Additionally cotton also finds extensive use in non-woven textiles and personal care items, which are preferred by consumers to items containing synthetic fibers.

World cotton production is estimated at a record of 117.05 million bales in 2006-7 [1], while its consumption during 2006-7 is estimated at 26.5 million tones [2]. Cotton is grown in about 80 countries, China, USA, India, Pakistan, Uzbekistan and Turkey being the leading producers of cotton as shown in Table 1.

Cotton grows best in tropical and warm subtropical latitude. The cotton flower-bud blossoms and develops into an oval boll that splits open at maturity, revealing a mass of long white seed-hair, called ‘cotton lint’. When fully mature and dry, each of cotton fiber is a thin flattened tubular cell, with a pronounced spiral twist, attached to the cotton seed.

Although some physical properties of cotton can be improved after harvesting through mercerization or liquid ammonia treatment [3], length, strength, fineness and colour are the inherent properties of cotton fiber which are affected, among other factors, by genetics, agronomy and time of harvest [4-5]. Cotton fiber properties not only determine its selling price but also the quality of yarn produced from it in spinning [6]. This article reviews the effect of cotton fiber properties on the properties of spun yarn.

The quality of cotton yarn in ring spinning is determined, among other factors, by the following cotton fiber characteristics:

Fiber length; length uniformity; short fiber content; fiber strength; micronaire; fiber maturity; elongation; colour; wax content; sugar content; moisture content; neps; trash content; and microdust. The effect of these cotton characteristics on the subsequent yarn properties is described as following.

v Upper Quartile Length (UQL): The length that is exceeded by 25% of the fibers in the cotton sample is called Upper Quartile Length.

v Upper Half Mean Length: The average of mean length of the longest one-half of the fibers in the sample is known as Upper Half Mean Length [7].

v 2.5% Span Length: It is defined as “the length of at least 2.5% of total fibers having length exceeding this value”.

v 50% Span Length: It is defined as “the length of at least 50% of total fibers having length exceeding this value”.

v Mean Length: Mean length of cotton fibers is defined in three ways [8-9]:

(i) Mean length based on number of fibers.
(ii) Mean length based on fiber cross-section, and
(iii)Mean length based on fiber weight.

v Spinnability: Longer fibers are easier to be spun as compared to short fibers.

v Yarn Evenness: Yarns produced from longer fiber lengths are more even as there are less fiber ends in a given yarn length.

v Yarn Strength: Higher yarn strengths can be achieved from the longer fibers for the same level of twist.

v Yarn Handle: Yarns of same strength can be produced by using longer fibers with a lower level of twist, so giving a softer handle.

v Yarn Tenacity: Longer the cotton fiber, higher will be the yarn tenacity [10].

v Yarn Hairiness: Longer the fibers less will be the yarn hairiness [11] and the length of the protruding ends [12].

v Spinning Process Efficiency: Higher the fiber length higher will be the efficiency of the spinning process.

The ratio between the mean length and upper half mean length of the fibers expressed as percentage is called ‘length uniformity index’ [13]. If in a cotton sample, all the fibers have same length then the mean length and upper half mean length would be the same and uniformity index would be 100. However, cotton length varies naturally so length uniformity is usually less than 100.

v Yarn Hairiness: Higher the fiber length uniformity index, lower will be the yarn hairiness [11].

v Yarn Strength: Higher the fiber length uniformity index better will be the yarn strength [13].

v Yarn Evenness: Higher the fiber length uniformity index better will be the yarn evenness [13].

v Spinning Process Efficiency: Higher the fiber length uniformity index better will be the efficiency of the spinning process [13].

The fibers having length less than 12.7 mm are counted as ‘short fibers’ [14]. Short fiber content is defined in two ways [15]:

The short fiber content has been found to decrease as the calcium content in cotton decreases and other non-cellulosic content increases [16-17]. Short fiber content affects the yarn properties as follows:

v Yarn End Breakage: Lower the short fiber content less will be the occurrence of yarn end breakages during spinning process [18].

v Yarn Quality: Lower the short fiber content, better will be the yarn quality in terms of yarn strength and absence of thick places as well as hairiness [19].

Fiber strength is usually expressed in terms of ‘grams per Tex’. A ‘Tex’ is defined as “the weight in grams of 1000 meters of fibers”. Cotton fiber strength has been found to increase as non-cellulosic content in cotton fiber increases and the calcium content decreases [16]. Fiber strength affects the yarn properties as follows:

v Yarn Strength: Higher the fiber strength higher will be the yarn strength [20-21].

v Fiber Breakage: Higher the fiber strength less will be the fiber breakage during spinning process.

Fineness is one of the most important properties of the cotton fiber. It is defined as “weight per unit length” or “the product of fiber cross-sectional area and fiber density” [22]. A number of factors have to be taken into account during measurement of cotton fiber fineness, because neither its cross-section is circular nor its shape uniform along the fiber length.

Micronaire value is a commonly used measure of cotton fiber fineness [23] and maturity [24-25]. Greater the fiber thickness and/or higher the fiber maturity, higher will be the micronaire value and vice versa [26-29].

Micronaire value of cotton influences not only its processing performance but also the appearance and quality of the end-product [30]. The effects of cotton fiber fineness and/or micronaire value on the spinning process and the resulting yarn and fabric quality are given as follows:

v Yarn Evenness: Use of finer fibers for a particular yarn count will enable the production of a more even yarn.

v Spinning Limit: The point at which the fibers can no longer be twisted into a yarn is called ‘spinning limit’ and this limit is reached earlier with a coarser fibers.

v Yarn Twist: The twist provides the force that holds the individual fibers together when they are twisted to form a yarn. For the finer fibers, less twist is required to make a yarn of a given strength as the greater surface area of finer fibers provides more cohesion.

v Yarn Stiffness: Coarser fibers are stiffer and will result in stiffer yarns.

v Processing Speed: Finer fibers require slower processing speeds to prevent fiber damage in opening, cleaning and carding processes.

v Yarn Tenacity: For a given yarn count, cotton with low micronaire value will result in higher yarn tenacity than cotton with a high micronaire value [31].

v Yarn Strength: Stronger yarns are produced from finer fibers as there are more fibers per cross-section.

v Yarn Hairiness: As micronaire value increases, hairiness and length of protruding ends decreases [28].

v Yarn End Breakage: Low micronaire (3.5 and below) causes excessive nepping during carding and high micronaire (4.9 and above) results in high end breakage as there are fewer fibers per cross-section area of yarn due to which yarn strength is reduced [32].

v Fiber Damage: Fiber fineness is positively correlated with fiber damage during lint cleaning [33].

v Dyeability: A fabric produced from 5.0 micronaire cotton may dye up to 25 % darker than that produced from the fibers having micronaire value of 3.4 [30].

v Fabric Barré: Micronaire variation should be kept at minimum possible level to avoid fabric barré [24].

v White Specks: Cotton of lower micronaire value is more likely to produce white specks in dyed fabrics than that of higher micronaire value.

The degree of cotton fiber cell-wall thickening is denoted by its maturity. All cotton samples contain a certain amount of immature fibers [34-35], which can be defined in the following terms:

Maturity Ratio: It is the ratio of the amount of fibers with 0.5 or more circularity and the amount of fibers with 0.25 or less circularity.

Immature Fiber Content (IFC %): It is the percentage of fibers with less than 0.25 circularity.

The effects of cotton fiber maturity on the spinning process and the resulting yarn and fabric quality are given as follows:

v Fiber Breakage: Lower the fiber maturity more will be the fiber breakage during processing [36].

v Nep Formation: Lower the fiber maturity more will be the tendency of nep formation.

v Tendency of Entanglement: Lower the fiber maturity more will be its tendency to become entangled around leaf and trash particles.

v Yarn Strength: Lower the fiber maturity, weaker will be the yarn strength [7].

v Yarn Quality and Production: Fiber maturity plays an important role in yarn quality and yarn production as immature cotton leads to high end breakage in spinning and high pneumafil waste [37].

v Fabric Barré: The variation of immature fiber content in the individual bale laydown will cause fabric barré in the finished fabric; the difference between the average values of IFC of laydowns should not be more than 0.5 % [24, 38].

v Fabric Appearance: Lower the fiber maturity, poor will be the fabric appearance.

v Poor Dyeability: Immature fibers dye to a lighter shade than the mature fibers. Immature fibers are more amorphous so they absorb more dye but while washing after dyeing, dye from immature fiber will be washed away which leads to dye wastage and also cause shade variation in the cloth [37].

v White Specks: Since immature fibers dye lighter as compared to fully mature fibers, if both are present in a deep dyed fabric, the immature fiber may appear as light/white specks [39].

Elongation is the increase in length of the fiber from its starting length expressed in units of the length. Cotton which has more crimp in the fibers show more elongation, force to break, linear density, tenacity, and work to rupture. Fiber elongation has the following affects on the yarn:

v Yarn Production: The crimped fibers with higher elongation need less twist to form yarns and so help to optimize production [40].

v Yarn Extension: Higher the fiber elongation, higher will be the yarn extension.

v Yarn End Breakage: If the value of elongation is better then it will help in reducing end breakages in spinning resulting in higher productivity with low wastage of raw material.

Cotton colour is defined in terms of degree of reflectance (Rd) and yellowness (+b) [7]. From reflectance we know how bright or dull a sample is and yellowness indicates the degree of colour pigmentation [41]. Yellower cotton with higher trash content has been found to have higher length, length uniformity and strength but less short fiber content [42].

v Barré: Variation in fiber colour can cause barré problem in the fabric.

v Dye Absorption: The ability of fibers to absorb and hold dyes and finishes is also affected by colour deterioration.

The weight of solvent extractables per unit mass of fibers is known as wax content. Wax content has the following affects:

v Yarn Strength: There may be up to 30% increase in strength by the removal of wax from yarns spun from normal cottons21.

v Yarn Elongation: There may be up to 4% decrease in elongation by the removal of wax from yarns spun from normal cottons [21].

v Ends Down: With increasing levels of wax, ends down rate in ring spinning increases [41].

Higher the sugar content more will be the stickiness of fibers that may create problems in processing such cottons.

Moisture content is the weight of water in fiber expressed as percentage of the total fiber weight[43]. It has the following affects:

v Invisible Loss: If the moisture content in cotton is more than 8.5% then there will be more invisible loss during spinning.

v Yarn Breakage: If moisture content is less than 8.5% then there will be a tendency of brittleness of fibers resulting in frequent yarn breakages.

Neps are the entanglements of fibers. Nep-forming potential during lint cleaning is inversely proportional to fiber maturity and directly proportional to non-lint content (trash nuclei nep) [33]. One of the reasons of nep formation is the involvement of certain fiber stress effects during mechanical harvesting and ginning [43]. The neps may also be formed during spinning because of aggressive cleaning and may not necessarily be composed of immature fibers [33]. However, main determinants of neps, cut seeds and motes are genetic, agronomic and ginning style [5].

There are two distinguished types of neps, viz. seed coat neps and fiber neps. The characterization of neps is done as follows:

Seed Coat Nep Count per Gram (SCN Cnt/g): These are actually the broken parts of cotton seed with which the fibers are still attached. Seed coat neps are created during ginning but some cotton varieties tend to create more seed coat neps than others. These are very difficult to clean and in spinning process are believed to create stickiness. Seed coated fragments in the cotton lint account for the imperfections found in yarns and about 13 % of total neps comprise seed coated neps [44-46]. On the plain fabric surface, seed coat neps are easily visible as black spots. Before dyeing process such fabric has to be bleached for uniform dyeing.

Seed Coat Nep (SCN) Size: This value gives the mean size of seed coat neps.

Nep Count per Gram (Nep Cnt/g): It is the total nep count normalized per gram. Both seed coated and fiber neps are included in this value.

Nep Size: This value shows the mean size of all the neps including both seed coat and fiber neps. From this value we can assess mean size of neps in raw cotton and, afterwards at each step of spinning process, check the efficient performance of each step.

v Yarn Imperfections: In yarn manufacturing one of the most critical quality problems is yarn imperfections caused by fiber neps. The nep number in yarn depends upon both the nep number in raw material and changes in number and size occurred during spinning process [47].

v Ends Down: The ends down rate at the spinning frame can be increased due to neps having direct influence on the spinning efficiency.

v White Spots: In dyeing process, neps have less dye affinity and may appear as white spots in a dyed fabric [38].

v Regardless of the size, all the particles that are removed by the AFIS (Advanced Fiber Information System) fiber individualizer are counted as trash.

v It is easier to remove larger trash than small sized dust particles which stick to cotton fiber material. It is more difficult to remove finer trash as fiber damage may occur due to aggressive cleaning methods [43].

v Trash has Ends Down affect, as the trash content in cotton increases, the rate of ends down on ring frame decreases [41].

v Fiber fragments, vegetable trash particles and micro-particles of less than 150 microns are called ‘microdust’. Most of the microdust comes from the leaf, the vein of the leaf, the bract and the fiber fragments [48]. The higher the microdust, the higher will be the chances of yarn breakage particularly in rotor spinning.

v Cotton producers, ginners, traders and spinners specify, evaluate and select cotton using two types of machines, viz. Uster HVI (high volume instrument) and Uster AFIS (advanced fiber information system) [49-55]. Following parameters of cotton are measured by HVI: fiber length; length uniformity; short fiber content; fiber strength; micronaire; elongation; and colour.

v Following properties can be measured on USTER AFIS PRO: Nep Cnt/g – (Nep Count per Gram); nep size; SCN Cnt/g (Seed Coat Nep Count per Gram); SCN Size (Seed coat Nep Size); L(n) [Length by number (inch or mm)]; L(n) % CV; SFC(n) [Short Fiber Content by Number]; L(w)[Length by weight]; L(w) % CV; SFC(w) [Short fiber content by weight]; UQL(w) [Upper quartile length by weight]; Fineness; Maturity Ratio; IFC (Immature fiber content); Total Trash [Cnt/g] (Total trash Count per Gram); Trash [Cnt/g] (Trash Count per gram); Dust [Cnt/g] (Dust Count per Gram); Mean size; and VFM [%] (Visible Foreign Matter Percent by Weight).

v The adage of “garbage in, garbage out” holds absolutely true in case of yarn manufacturing. The quality of cotton yarn and the resulting fabric is greatly determined by the quality of cotton fibers used in yarn manufacturing.

v Key yarn parameters which are affected by cotton fiber properties include: yarn strength, tenacity, evenness, neps, hairiness, handle and yarn imperfections.

v Cotton fiber properties not only affect the quality of yarn but fabric defects such as barré, dyeability and shade variation can also be attributed to poor cotton fiber properties.

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