Chaptar(2.10.2)-Cotton Fiber and Its Properties(Part-1)

1. INTRODUCTION

Cotton today is the most used textile fiber in the world. Its current market share is 56 percent for all fibers used for apparel and home furnishings and sold in the U.S. [1]. Another contribution is attributed to nonwoven textiles and personal care items. It is generally recognized that most consumers prefer cotton personal care items to those containing synthetic fibers. World textile fiber consumption in 1998 was approximately 45 million tons. Of this total, cotton represented approximately 20 million tons. [2]. The earliest evidence of using cotton is from India and the date assigned to this fabric is 3000 B.C. There were also excavations of cotton fabrics of comparable age in Southern America. Cotton cultivation first spread from India to Egypt, China and the South Pacific. Even though cotton fiber had been known already in Southern America, the large-scale cotton cultivation in Northern America began in the 16th century with the arrival of colonists to southern parts of today's United States. [3] .The largest rise in cotton production is connected with the invention of the saw-tooth cotton gin by Eli Whitney in 1793. [4] With this new technology, it was possible to produce more cotton fiber, which resulted in big changes in the spinning and weaving industry, especially in England.

2. Cotton Consumption And Production in Million Tons in Year 2002

The graph bellow shows Production and consumption of leading cotton producing countries in Millions of tones in year 2002 [5].

Countries	Production	Consumption
US	3.8	1.7
India	2.5	3
Pakistan	1.8	1.9
Turkey	0.9	1.4
Brazil	0.7	0.9
Indonesia	0.4	0.6
China	4.8	5.9

Today, cotton is grown in more than 80 countries worldwide. The Distribution of cotton is shown in the bellow Map:

3. CHARATERISTICS OF COTTON

Cotton, as a natural cellulosic fiber, has a lot of characteristics, such as;

• Comfortable Soft hand
• Good absorbency
• Color retention
• Prints well
• Machine-washable
• Dry-cleanable
• Good strength
• Drapes well
• Easy to handle and sew

4. End Uses of Cotton:

• Apparel - Wide range of wearing apparel: blouses, shirts, dresses, childrenswear, active wear, separates, swimwear, suits, jackets, skirts, pants, sweaters, hosiery, neckwear.

• Home Fashion - curtains, draperies, bedspreads, comforters, throws, sheets, towels, table cloths, table mats, napkins

5. STRUCTURE AND PROPERTIES OF COTTON FIBERS

5.1 FIBER STRUCTURE AND FORMATION

The botanical name of American Upland cotton is Gossypium Hirsutum and has been developed from cottons of Central America. Upland varieties represent approximately 97% of U.S. production [4].

Each cotton fiber is composed of concentric layers. The cuticle layer on the fiber itself is separable from the fiber and consists of wax and pectin materials. The primary wall, the most peripheral layer of the fiber, is composed of cellulosic crystalline fibrils. [9] The secondary wall of the fiber consists of three distinct layers. All three layers of the secondary wall include closely packed parallel fibrils with spiral winding of 25-35^o and represent the majority of cellulose within the fiber. The innermost part of cotton fiber- the lumen- is composed of the remains of the cell contents. Before boll opening, the lumen is filled with liquid containing the cell nucleus and protoplasm. The twists and convolutions of the dried fiber are due to the removal of this liquid. The cross section of the fiber is bean-shaped, swelling almost round when moisture absorption takes place.

The overall contents are broken down into the following components.

5.2 Raw cotton components:

80-90%	Cellulose
6-8%	Water
0.5 - 1%	Waxes and fats
0 - 1.5%	Proteins
4 - 6%	Hemicelluloses and pectin’s
1 - 1.8%	Ash

During scouring (treatment of the fiber with caustic soda), natural waxes and fats in the fiber are saponified and pectin’s and other non-cellulose materials are released, so that the impurities can be removed by just rinsing away. After scouring, a bleaching solution (consisting of a stabilized oxidizing agent) interacts with the fiber and the natural color is removed. Bleaching takes place at elevated temperature for a fixed period of time [1]. Mercerization is another process of improving sorption properties of cotton. Cotton fiber is immersed into 18- 25% solution of sodium hydroxide often under tension [9]. The fiber obtains better luster and sorption during mercerization.

After scouring and bleaching, the fiber is 99% cellulose. Cellulose is a polymer consisting of anhydroglucose units connected with 1,4 oxygen bridges in the beta position. The hydroxyl groups on the cellulose units enable hydrogen bonding between two adjacent polymer chains. The degree of polymerization of cotton is 9,000-15,000 [1]. Cellulose shows approximately 66% crystallinity, which can be determined by X-ray diffraction, infrared spectroscopy and density methods.

Each crystal unit consists of five chains of anhydroglucose units, parallel to the fibril axis. One chain is located at each of the corners of the cell and one runs through the center of the cell. The dimensions of the cell are a = 0.835nm, b = 1.03 nm and c = 0.79 nm. The angle between ab and BC planes is 84º for normal cellulose, i.e., Cellulose I [8].

5.3 Repeat unit of cellulose

The current consensus regarding cellulose crystallinity (X-ray diffraction) is that fibers are essentially 100% crystalline and that very small crystalline units imperfectly packed together cause the observed disorder.

The density method used to determine cellulose crystallinity is based on the density gradient column, where two solvents of different densities are partially mixed. Degree of Crystallinity is, then, determined from the density of the sample, while densities of crystalline and amorphous cellulose forms are known (1.505 and 1.556 respectively). Orientation of untreated cotton fiber is poor because the crystallites are contained in the micro fibrils of the secondary wall, oriented in the steep spiral (25-30^o) to the fiber axis.

6. PHYSICAL PROPERTIES OF COTTON

6.1  FIBER LENGTH

Fiber length is described [7] as "the average length of the longer one-half of the fibers (upper half mean length)" This measure is taken by scanning a "beard " of parallel fibers through a sensing region. The beard is formed from the fibers taken from the sample, clasped in a holding clamp and combed to align the fibers. Typical lengths of Upland cottons might range from 0.79 to 1.36in.

Cottons come from the cotton plant; the longer strand types such as Pima or Sea Island produce the finest types of cotton fabrics [18].

6.2  LENGTH UNIFORMITY

Length uniformity or uniformity ratio is determined as " a ratio between the mean length and the upper half mean length of the fibers and is expressed as a percentage"[7]. Typical comparisons are illustrated below.

LENGTH UNIFORMITY	UNIFORMITY INDEX [%]
Very High	>85
High	83-85
Intermediate	80-82
Low	77-79
Very Low	<77

Low uniformity index shows that there might be a high content of short fibers, which lowers the quality of the future textile product.

6.3 FIBER STRENGTH

Fiber strength is measured in grams per denier. It is determined as the force necessary to break the beard of fibers, clamped in two sets of jaws, (1/8 inch apart) [7]. Typical tensile levels are illustrated. The breaking strength of cotton is about 3.0~4.9 g/denier, and the breaking elongation is about 8~10%. [20]

DEGREE OF STRENGTH	FIBER STRENGTH [g/tex]
Very Strong	>31
Strong	29-30
Average	26-28
Intermediate	24-25
Weak	<23

6.3  MICRONAIRE

Micronaire measurements reflect fiber fineness and maturity. A constant mass (2.34 grams) of cotton fibers is compressed into a space of known volume and air permeability measurements of this compressed sample are taken. These, when converted to appropriate number, denote Micronaire values.

COTTON RANGE	MICRONAIRE READING
Premium	3.7-4.2
Base Range	4.3-4.9
Discount Range	>5.0

6.4  COLOR

The color of cotton samples is determined from two parameters: degree of reflectance (Rd) and yellowness (+b). Degree of reflectance shows the brightness of the sample and yellowness depicts the degree of cotton pigmentation. A defined area located in a Nickerson-Hunter cotton colorimeter diagram represents each color code. The color of the fibers is affected by climatic conditions, impact of insects and fungi, type of soil, storage conditions etc. There is five recognized groups of color: white, gray, spotted, tinged, and yellow stained. As the color of cotton deteriorates, the process ability of the fibers decreases.

Work at the University of Tennessee has led to color measurement using both a spectrometer CIE-based average color measurement and a color uniformity measurement using image analysis to improve the accuracy and provide additional measurement for color grading [19]. Later the investigators developed two color grading systems using expert system and neural networks.

6.5  TRASH

A trash measurement describes the amount of non-lint materials (such as parts of cotton plant) in the fiber. Trash content is assessed from scanning the cotton sample surface with a video camera and calculating the percentage of the surface area occupied by trash particles. The values of trash content should be within the range from 0 to 1.6%. Trash content is highly correlated to leaf grade of the sample.

6.6  LEAF GRADE

Leaf grade is provided visually as the amount of cotton plant particles within the sample. There are seven leaf grades (#1-#7) and one below grade (#8).

6.7  PREPARATION

Preparation is the classer's interpretation of fiber process ability in terms of degree of roughness or smoothness of ginned cotton.

6.8  EXTRANEOUS MATTER

Extraneous matter is all the material in the sample other than fiber and leaf. The classer either as “light” or “heavy” determines the degree of extraneous matter.

6.9  NEPS

A nep is a small tangled fiber knot often caused by processing. Neps can be measured by the AFIS nep tester and reported as the total number of neps per 0.5 grams of the fiber and average size in millimeters. Nep formation reflects the mechanical processing stage, especially from the point of view of the quality and condition of the machinery used.