Cotton Fibers and its Properties

Cotton is the most important natural textile fiber, as well as cellulosic textile fiber, in the world, used to produce apparel, home furnishings, and industrial products. Worldwide about 40% of the fiber consumed in 2004 was cotton.

Cotton fibers are seed hairs from plants of the order Malvales, family Malvaceae, tribe Gossypieae, and genus Gossypium. Botanically, there are four principal domesticated species of cotton of commercial importance: hirsutum, barbadense, aboreum, and herbaceum. Thirty-three species are currently recognized; however, all but these four are wild shrubs of no commercial value. Each one of the commercially important species contains many different varieties developed through breeding programs to produce cotton with continually improving properties (e.g., faster maturing, increased yields, and improved insect and disease resistance) and fibers with greater length, strength, and uniformity.

The cotton fibers used in textile commerce are the dried cell walls of formerly living cells. Botanically, cotton fibers are trichomes or seed coat hairs that differentiate from epidermal cells of the developing cottonseed. The cotton flower blooms only for one day and quickly becomes senescent thereafter. On the day of full bloom, or anthesis, the flower petals are pure white in most hirsutum varieties. By the day after anthesis, the petals turn bright pink in color, and, usually by the second day after anthesis, the petals fall off the developing carpel (boll).

Each cotton fiber is composed of concentric layers and a hollow central core is known as the lumen. The outermost layer, known as the cuticle, is a thin layer of fats, proteins, and waxes.

Beneath the cuticle is the primary wall, composed mainly of cellulose in which fibrils are arranged in a criss-cross pattern. Further towards the center is the secondary wall composed of cellulose, which consists of the bulk of the fiber.

Composition Cotton

Classing Cotton

Cotton buyers judge cotton on the basis of samples cut from the bales. Skilled cotton classers grade or “class” the cotton according to standards established by the US Department of Agriculture such as cleanliness, the degree of whiteness, length of the fiber, and fiber strength.

The classes pull a sample. They discard most of the cotton until just a pinch of well-aligned fibers remains. They measure the length of the fibers, referred to as staple fibers. Longer staple fibers are higher-grade cotton and are sold at higher prices. Long staples range from 1.1 inches to 1.4 inches long.

The USTER® HVI classing is the standard classification system in the United States and also for the international cotton trade. USTER® HVI is used for measurement of the most important cotton fiber properties of micronaire, fiber length (UHML), uniformity, short fiber index, strength, elongation, color, trash content, and degree of maturity. HVI classing has been available to all growers since 1981.

Methods for Classifying Cotton Fiber

Measurements for fiber length, length uniformity, fiber strength, micronaire, color grade, trash, and leaf grade are performed by precise High Volume Instruments, in a process commonly referred to as “high volume instrument classification.” Only extraneous matter and special conditions are still officially classified by the traditional method of classer determination.

Fiber Length

Fiber length is the average length of the longer half of the fibers (upperhalf mean length). It is reported in both 100ths and 32nds of an inch. Fiber length is measured by passing a “beard” of parallel fibers through an optical sensing point. The beard is formed when fibers from a sample of cotton are automatically grasped by a clamp, then combed and brushed into parallel orientation.

Fiber length is largely influenced by variety,  but the cotton plant’s exposure to extreme temperatures, water stress, or nutrient
deficiencies may result in shorter fibers. Excessive cleaning or drying at the gin may also result in shorter fibers. Fiber 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 fibers also is influenced by fiber length.

Length Uniformity

Length uniformity is the ratio between the mean length and the upper-half mean length of the fibers, expressed as a percentage. If all of the fibers in the bale were the same length, the mean length, and the upper-half mean length would be the same, and the uniformity would be 100 percent. However, because of natural variation in the length of cotton fibers, length uniformity will always be less than 100 percent. The table below is a guide to interpreting length uniformity measurements.

Length uniformity affects yarn evenness and strength and the efficiency of the spinning process. It is also related to short-fiber content (the content of fibers shorter than 1/2 inch). Cotton with a low uniformity index is likely to have a high percentage of short fibers. Such cotton may be difficult to process and is likely to produce low-quality yarn.

Fiber Strength

Strength measurements are reported in grams per tex. A tex unit is equal to the weight in grams of 1,000 meters of fiber. Therefore, the strength reported is the force in grams required to break a bundle of fibers one tex unit in size. Strength measurements are made on the same beards of cotton that are used for measuring fiber length. The beard is clamped in two sets of jaws, 1/8 inch apart, and the amount of force required to break the fibers is determined. The table below is a guide to interpreting fiber strength measurements.

Fiber strength is largely determined by variety. However, it may be affected by plant nutrient deficiencies and weather. Fiber strength and yarn strength are highly correlated. Also, cotton with high fiber strength is more likely to withstand breakage during the manufacturing process.

Micronaire

Micronaire is a measure of fiber fineness and maturity. An airflow instrument is used to measure the air permeability of a constant mass of cotton fibers compressed to a fixed volume. The chart below is a guide to interpreting micronaire measurements.

Micronaire can be influenced during the growing period by environmental conditions such as moisture, temperature, sunlight, plant
nutrients, and extremes in plant or boll population. Fiber 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-fiber cotton require slower processing speeds to prevent damage Fiber length and strength measurements are made on the same “beard” of cotton.

Color Grade

Color grade is determined by the degree of reflectance (Rd) and yellowness (+b) as established by official standards and measured by the high volume instrument. Reflectance indicates how bright or dull a sample is, and yellowness indicates the degree of pigmentation.

The color of cotton fibers can be affected by rainfall, freezes, insects, fungi, and staining through contact with soil, grass, or cotton-plant leaf. Color can also be affected by excessive moisture and temperature levels during storage, both before and after ginning. Color deterioration because of environmental conditions affects the fibers’ ability to absorb and hold dyes and finishes and is likely to reduce processing efficiency.

Trash

Trash is a measure of the amount of non-lint materials in cotton, such as leaves and bark from the cotton plant. The surface of the cotton sample is scanned by a digital camera, and the digital image is analyzed. The percentage of the surface area occupied by trash particles (percent area) and the number of trash particles visible (particle count) are calculated and reported.

The ratio between the percent area of trash and trash particle count is a good indicator of the average particle size in a cotton sample. For instance, a low percent area combined with a high particle count indicates a smaller average particle size than does a high percent area with a low particle count.

A high percent area of trash results in greater textile mill processing waste and lower yarn quality. Small trash particles, or “pepper trash,” are highly undesirable, because they are more difficult for the mill to remove from the cotton lint than are larger trash particles.

Leaf Grade

Leaf grade is a measure of the leaf content in cotton. Recent extensive research and development work has resulted in acceptance of instrument leaf grade. Leaf grade is now determined by high volume instrument trash meter percent area and particle count (described above for trash). The leaf grade is calculated from these parameters based on the Universal Upland Grade Standards and American Pima Grade Standards.

Leaf content is affected by plant variety, harvesting methods, and harvesting conditions. The amount of leaf remaining in the lint after
ginning depends on the amount present in the cotton before ginning, the amount of cleaning, and the type of cleaning and drying equipment used. Even with the most careful harvesting and ginning methods, a small amount of leaf remains in the cotton lint. From the manufacturing standpoint, leaf content is all waste, and there is a cost factor associated with its removal. Also, small particles cannot always be successfully removed, and these particles may detract from the quality of the finished product.

Extraneous Matter

The extraneous matter is any substance in the cotton other than fiber or leaf. Examples of extraneous matter are the bark, grass, spindle twist, seed coat fragments, dust, oil, and plastic. The kind of extraneous matter and an indication of the amount (light or heavy) are noted by the classer as a remark on the classification document.

Another factor noted on the classification record under “extraneous matter” is abnormal preparation. “Preparation,” or “prep,” describes the degree of smoothness or roughness of the ginned cotton lint. Various methods of harvesting, handling, and ginning cotton produce differences in roughness or smoothness of preparation that sometimes are quite apparent. Abnormal preparation of Upland cotton has greatly decreased in recent years as a result of improved harvesting and ginning practices and now occurs in less than half of one percent of the crop.

Stickiness

As in the case of maturity, considerable effort has been directed over a number of years towards the HVI measurement of stickiness, a NIR-based measurement having been incorporated in HVI systems at one stage but then withdrawn thereafter (in 1995). On certain of the new generation high volume testing systems (e.g. Lintronic FQT Fibro-Lab), sticky spots on card rollers are measured directly.

Properties & Uses

The fibers are sent to a textile mill where carding machines turn the fibers into cotton yarn. The yarns are woven into cloth that is comfortable and easy to wash but does wrinkle easily. Cotton fabric will shrink about 3% when washed unless pre-treated to resist shrinking.

Cotton is prized for its comfort, easy-care, and affordability and is ideal for clothing, bedding, towels, and furnishings.

Characteristics of Cotton Fibers and Products

• Comfortable to wear
• Natural, cellulosic fiber
• Made from the cotton boll
• Absorbs water and “breathes”
• Slow to dry
• Resists static electricity build-up
• Wrinkles easily
• Can withstand heat, detergents, and bleach
• About 20% stronger when wet than dry
• Will shrink unless treated
• Can be damaged by mildew
• Can be damaged by prolonged exposure to sunlight
• Long-staple cotton (such as Supima, Pima, Egyptian, and Sea Island) can be woven into smooth, almost silky fabric

Fiber quality requirements

For any material to qualify as a textile fiber, it must possess certain essential properties. The primary requirements include a high length-to-width ratio, sufficient tenacity, flexibility, and cohesiveness (Dever, 1995).

In general cotton quality requirements Proceedings of the World Cotton Research Conference -2. Athens, Greece, September 6-12, 1998. pp.85-93. U. Kechagia and H. Harig of all spinning systems are summarized as follows (Kechagia, 1994).

• Identifiable, measured fiber properties
• Properly ginned cotton
• Contamination free
• Stickiness-free
• Even running lots

However, fiber properties are differently interpreted by the various spinning systems and the selection of the correct raw material for any of them is of utmost importance to the spinner.

The major quality parameters for the traditional ring system, the open-end or rotor spinning, and the recent innovations such as friction (DREF) and the air-jet spinning are given in table 1 in descending order of importance. In table 2 are given the acceptable limits for the same parameters in ring and rotor spinning.

The use-value of fibers depends mainly upon the above physical properties but as we mentioned these are not sufficient to describe the spinnability of any cotton. Others not included in this table are of equal or sometimes of greater importance. The estimation of short fiber content (SFC %) of neps and seed coat fragments as well as of color, stickiness, and foreign matter became a necessity.

Cotton fiber properties

An important consideration prior to any effort of improving quality is to obtain reliable information on the properties, likely to be improved as well as the technical specification of the input for various end uses.

The molecular arrangement within the fiber and the conditions of fiber formation, impact the properties that make cotton fiber readily distinguished from all other textile fibers. All significant fiber properties are listed below, classified in relevant groups (Steadman,
1997; Hunter, 1998).

• Length Related Properties
  • Staple Length Values
  • Span Length Values
  • Uniformity
  • Parameters for Length Distribution
  • Short Fiber Content
• Transverse Dimensions of Cotton
  • Micronaire
  • Fineness
  • Maturity
• Tensile Properties
  • Strength
  • Breaking Elongation.
• Non-Lint Content
  • Average Trash
  • Trash Particle
  • Size Distribution
  • Trash Type
• Dust
  • Level and Size
  • Seed Coat Fragments
  • Foreign Matter and Contaminants
  • Neps
• Cotton Colour
• Miscellaneous Fiber Properties
  • Fiber Friction
  • Cleanability
  • Microbial Attack
  • Cohesiveness
  • Compressibility and Resilience
  • Moisture Content.

Fiber elongation is an important property that has received little attention so far. The role of elongation has not been fully defined yet but there is evidence that it strongly influences processing efficiency (Kechagia, 1996). Recent research is focused on clarifying the
effect of elongation on yarn quality and improving or inventing methods to assess fiber elongation accurately (Uster, 1998).

The main objectives of cotton fiber improvement are summarized in:

• Development of marginally longer and finer cotton.
• Improvement (where possible) of fiber maturity, strength, and elongation.
• Reduction of short fiber content, neps, and other impurities.
• Maintaining existing fiber quality.
• Improving the evenness of all-fiber characters.
• Improvement of fiber quality and color range of colored cotton.

Commonly tested Cotton Fiber properties

• Neps
• Spinning Consistency Index (SCI)
• Micronaire
• Maturity Index
• Length (mm)
• Uniformity
• Short Fibre Index
• Strength (g/tex)
• Elongation (%)
• Moisture Content (%)
• Reflectance (Rd)
• Yellowness (+b)
• Trash Area (%)
• Trash Grade

Cotton fiber properties in different spinning systems

Spinnable limits for efficient spinning

Tensile Properties of Cotton Fibers

Tensile properties of yarns and fabrics depend on both complex fibers arrangements (includes length, diameter, friction, etc.) inside the yarn and fabric structure, and also on the tensile properties of fibers. That is, while information about the complex relationships between fiber arrangement parameters is necessary, having knowledge about the tensile properties of fibers is crucial for a better understanding of the mechanical behavior of yarns and fabrics.

Tensile properties of cotton fibers are influenced by the internal structure of the fibers. Cotton fiber is 98% cellulose. Cellulose molecule is synthesized from sucrose, the major type of sugar which can be found in the sap of the cotton plant. Sucrose will be converted within the cell into one molecule of glucose and one molecule of fructose; then, fructose will be converted into glucose.

Afterward, two glucose molecules will react together to form cellobiose; it will be then polymerized to form cellulose. It is reported that the rigidity of the cellulosic chains, the highly fibrillar and crystalline structure of cellulose macromolecules, and the extensive intermolecular and intramolecular hydrogen bonding are among the factors that affect most cotton fibers, tensile properties. Also, cotton fiber strength has been shown to be associated with the molecular weight of the cellulose, the crystalline regions in the fibers, and the reversals and convolutions of the fibers.

Viscoelasticity of Cotton Fibers

Viscoelasticity is the elastic response that occurs immediately after applying the load, and the viscous response which occurs smoothly and continuously when time goes on. The applied load, the cross-sectional area of fibers, the modulus of materials, are all affecting parameters for an elastic response. On the other side, the viscous response is a measure of a time-dependent deformation.

In some other references, the term “creep recovery” was also used, which corresponds to the behavior of fiber after the load removal. It has been stated that, after the load removal, recovery will take place immediately and will continue over a period of time.

Cotton Fiber Bundle Strength Measurements

High Volume Instrument (HVI) is used in this study to measure the fiber bundle tenacity. HVI performs the fiber bundle tenacity test on the same specimen as the one used for the length measurement. In fact, after the length measurement test, the specimen is repositioned; it is clamped between the two jaws (the gauge length is 0.3175 centimeters); then, the specimen is submitted to the tensile test with the inconstant rate of elongation. With the assumption that the linear density is constant across length groups, HVI estimates the mass of the specimen using an optical sensor and micronaire. The tenacity measurement is expressed as grams force (gf)  per tex.

Maturity and Perimeter Measurements

Fiber cross-sections were performed according to the protocol reported before (Hequet et al., 2006). Briefly, a small sliver of cotton fibers is mounted into a plastic tube. Fiber samples were embedded with a methacrylate polymer to behold in position. Fibers are
cross-sectioned (one-micron thick cross-sections). Then microscopic slides are prepared after dissolving the methacrylate polymer from the sample. The images are viewed with a microscope and analyzed by the FIAS software. More than eight thousand fibers were
tested for each sample.

Individual fiber tensile measurements

FAVIMAT, an individual fiber tensile tester, measures the tensile properties of textile fibers at a constant rate of elongation. One hundred and fifty cotton fibers were tested for each replication using a ten-millimeter gauge length. In total, three replications were done on each cotton sample. This gauge (10 mm) was used because this is the minimum gauge length in which FAVIMAT can measure the linear density using the vibroscope method.

Cotton Fiber Testing Definitions

• Maturity Index  – is expressed in theta (the degree of thickening) which is defined as the ratio of the area of the cell wall to the area of a circle having the same perimeter as the fiber cross-section.
• Elongation (%)  – is the extension of a fiber or a bundle of fibers during the tensile strength test. It is expressed as a percentage of the initial length.
• Force-to-break (or tensile force) – is the maximum tensile force to rupture an individual fiber or a bundle of parallel fibers. The normalized force-to-break by weight is called tenacity. The normalized force-to-break by cross-section is called tensile stress.
• Gravimetric fineness – it describes the linear density of fibers or yarns and is usually expressed in tex, mass per unit length.
• Elastic recovery – is the capacity of a stretched fiber to return to its original length when extended and then released.
• Stress-Strain curve – expresses the behavior of individual fiber under the gradually increasing applied force.
• Yield point – is a transition point between the initial steep slope in the stress-strain curve (first phase) and plasticity region (second phase) with rapid extension and permanent deformation.
• Initial Modulus – is equal to the slope of the stress-strain curve at the origin (after crimp removal)