Chaptar(2.10.2)-Cotton Fiber and Its Properties-(Part-2)

7. CHEMICAL PROPERTIES OF COTTON

Cotton swells in a high humidity environment, in water and in concentrated solutions of certain acids, salts and bases. The swelling effect is usually attributed to the sorption of highly hydrated ions. The moisture regain for cotton is about 7.1~8.5% and the moisture absorption is 7~8%. [20]

Cotton is attacked by hot dilute or cold concentrated acid solutions. Acid hydrolysis of cellulose produces hydro-celluloses. Cold weak acids do not affect it. The fibers show excellent resistance to alkalis. There are a few other solvents that will dissolve cotton completely. One of them is a copper complex of cupramonium hydroxide and cupriethylene diamine (Schweitzer's reagent [11])

Cotton degradation is usually attributed to oxidation, hydrolysis or both. Oxidation of cellulose can lead to two types of so-called oxy-cellulose [12], depending on the environment, in which the oxidation takes place.

7.1 Insert formula or equation: Oxy-cellulose

Also, cotton can degrade by exposure to visible and ultraviolet light, especially in the presence of high temperatures around 250~397° C [20] and humidity. Cotton fibers are extremely susceptible to any biological degradation (microorganisms, fungi etc.)

7.2 OPTICAL PROPERTIES OF COTTON

Cotton fibers show double refraction when observed in polarized light. Even though various effects can be observed, second order yellow and second order blue is characteristic colors of cellulosic fibers. [10] A typical birefringence value as shown in the table of physical properties, is 0.047.

7.3 COTTON CLASSIFICATION

Cotton classification is used to determine the quality of the cotton fiber in terms of grade, length and Micronaire [1]. USDA [7] classification specifically identifies the characteristics of fiber length, length uniformity, strength, Micronaire, color, preparation, leaf and extraneous matter. In the past, these qualities were classified just by hand-and-eye of an experienced classer. Since 1991, all classification has been carried out with a set of up-to-date instruments, called "HVI"(High Volume Instrumentation) classification [1]. However, measuring techniques of other qualities of cotton fiber, such as fiber maturity and short fiber content, are also being developed. 

7.4. COTTON IN NON-WOVENS

Cotton is the most important apparel fiber throughout the world. It is a fiber that was used fairly extensively during the early, developmental period of the Nonwovens business primarily because the emerging dry-laid producers came from the textile industry and had an intimate knowledge of cotton and its processing characteristics [25].It was in the early part of 20th Century that a few cotton mills in the US wanted to find ways to upgrade the waste cotton fibers into saleable products. The first method used was bonding the short cotton fibers (fiber waste) with latex and resin. These products were used mainly as industrial wipes. After World War II, products like draperies, tablecloths, napkins and wiping towels were developed. It was realized that woven fabrics have much better properties than Nonwovens; so, the approach was to claim the market where superior qualities of woven or knit fabrics were not essential but where qualities better than those of paper were needed. As the quality requirements for nonwoven fabrics increased and particularly as the need for white, clean fabric emerged; the use of raw cotton became unacceptable and was abandoned by the industry except for a few isolated product areas. Within the last decade, bleached cotton fiber suitable for processing on conventional nonwoven equipment has become available and has substantially increased interest in this fiber. This is particularly true in medical and healthcare applications, wiping and wiper markets, and some apparel markets. The raw cotton consists of about 96% cellulose and 4% of waxes, pectin, and other pertinacious and plant material. These minor constituents that must be removed in the scouring and bleaching process to give the soft, clean, white, absorbent fiber that is satisfactory for the nonwovens industry after the application of an appropriate finishing oil. The fiber length of cotton is important, particularly as to its process ability. Longer staple cotton (0.75 in. to 1.25 in.) is satisfactory for nonwoven production. The fiber has excellent absorbency and feels comfortable against the skin. It has fairly good strength both wet and dry, and has moderate dimensional stability and elastic recovery. But the resilience of cotton is relatively low, unless it is cross-linked by a chemical treatment. In nonwoven applications, the purity and absorbency of bleached cotton are utilized in growing medical and healthcare applications. The spun lace process usually produces such fabrics. For similar reasons, cotton spun lace fabrics are well accepted in personal and related wipes, especially in Japan and the ASIAN region. In a sense, bleached cotton fiber for nonwoven application is a relatively new fiber. It is a comparatively expensive fiber and available from only a few sources. Consequently, its use still is restricted to specialized applications. This situation is likely to change in the future as the price is further reduced and availability increased.

8. FIBER PROCESSING

About 30% of world cotton machines harvest production. Australia, Israel and USA are the only countries where all cottons are picked by machines. Fifteen percent of world cotton production is ginned on roller gins and almost all rest of cotton is saw ginned in most countries [14].Cotton fibers in non-wovens are generally used in their bleached form. A lot of research and development has taken place for the efficient production of bleached fibers. The Kier bleaching process produces most of the bleached cotton fibers. Since cotton of lesser grades is useful for non-wovens, a conventional cleaning system does not suffice. This might include a coarse wire carding, called Cotton Master Cleaners, for cleaning the cotton.

• The conventional bleaching method for cottons meant for non-wovens is a 9 step process are:

a) Fiber opening and cleaning
b) Alkali scouring application
c) Alkali reaction stage
d) Rinsing
e) Bleach application
f) Bleach reaction stage
g) Rinsing
h) Finish application
i) Drying

A continuous textile processing system and method have been disclosed recently for producing a nonwoven web containing bleached cotton fibers in a single line system which includes a supply of fibers such as a bale opening device, The final nonwoven web consisting of bleached cotton fibers may be made into highly purified and absorbent wipes, pads, and other articles for medical, industrial, or domestic use [17].

Finally, there is opening and bale formation.

·         Cotton Incorporated patented a processing line, which promised better productivity and quality. It consists of:

a) Fiber opening and Cleaning
b) Formation of web
c) Steam purging and Alkali impregnation onto the sandwiched cotton web between 2 porous conveyors.

d) After reaction, a pressure squeezing operation.
e) Similar processes for bleaching and then finishing.

·         The recent system for scouring a bleaching of cotton fiber is the Continuous Wet Finishing Technique' patented by Lawrence Girard and Walter E Meyer and assigned to Greenville Machinery Corporation. It consists of:

1. Opening and Cleaning
2. Conversion of fibers into a bat, weighing 10-30 ounces/sq. yard, by Needle punching or Air-lay technique.
3. Scouring
4. Bleaching
5. Finishing
6. Washing
7. Drying
8. Fiber opening

Advantages of Continuous Finishing Techniques are:

a) Uniformity of scouring and bleaching
b) Uniformity of finish application
c) Shorter time in process for the materials
d) Lower water consumption and less effluent for treatment
e) The ability to provide additional chemical treatments to the cotton.

8.1. COST OF PRODUCING COTTON

The international cotton advisory committee (ICAC) undertakes a survey of the cost of the production of cotton every three years based on the data from 31 countries. [16] Several factors are considered, such as land rent, fertilizers, insect control, irrigation, harvesting and ginning. The cost of seed cotton is more than $500 in USA to produce one hectare of seed cotton. The net cost of producing lint from one hectare (the value of seed and land rent were excluded from the total cost) is highest in Australia (US$1,056) followed by the USA (US$889), Pakistan (US$814), Zimbabwe (US$426) and China (US$416). It is most expensive to produce a kilogram of lint in the USA (US$1.20), Australia (US$0.75) and china (US$0.48).

8.2. WEB PROCESSING WITH COTTON

Cotton fibers are used in the manufacture of nonwovens either alone or in a blend. The various processes for the manufacture of non-wovens are:

8.3. HYDROENTANGLEMENT:

This method of bonding provides strength to the Nonwovens, comparable to woven fabric of the same basis weight. This method yields high strength without interfering with the absorbency, tensile strength and aesthetic properties of cotton. This type of nonwovens can be wet processed like the conventional woven textiles for bleaching, dyeing and finishing. To manufacture soft loose nonwovens, partially entangled webs are produced by subjecting cotton webs to low water jet pressures (approx. 300-500 psi). These types of webs can be wet processed in a pad/batch state. The limitations of this process are that production has been limited to fiber blends because of problems in recycling water and the quality of bleached cotton.

8.4. NEEDLE PUNCHING:

Needle punched cotton provides highly efficient filter media based on the irregular fiber shape and absorption properties. Increased tenacity in the wet condition can be an important advantage for cotton filters. To build strength, scrim materials can be used as in bed blankets and industrial fabrics. Needles of 36-42 gauges have been found appropriate for the production of cotton needle punched nonwovens. For very heavy fabrics, use is made of gauge 32 and for finer fabrics 40-42 gauge needles are being used.

8.5. THERMAL BONDING:

In this process cotton webs with blends of thermoplastic fibers are passed between 2 hot rollers (Calendar rollers). The thermoplastic fiber softens/melts and bonds the web. The initial work was done with polyester as the thermoplastic fiber. Later polypropylene was extended for the study because of economics, density and melting temperature considerations. This was mainly to study the application as a diaper lining material. Substantial work is still being done to develop this type of nonwovens.

8.6. OTHER BONDING SYSTEMS:

1. Impregnating the web with a resin or other adhesive material.
2. Stripping off of the web with adhesive, which bonds the fibers together at regular intervals.
3. Stitch bonding: cotton web is stitched like in sewing and the product performance depends on web weight, stitch/inch and type of sewing thread.

9. APPLICATIONS AND MANUFACTURERS OF COTTON NON-WOVENS

Cotton nonwovens are used as swabs, puffs, wipes, filters, weddings, personal care products like in diapers & feminine hygiene products, semi-durable segments like bedding, household furnishing, pillow fillers, etc.

9.1.MANUFACTURERS OF COTTON

• BARHARDT MANUFACTURING
• BBA NONWOVENS VERATEC
• BRANNOC FIBERS Ltd.
• COTTON INCORPORATED
• IHSAN SONS (PVT) LIMITED
• LEIGH FIBERS

• TEXTILES AND NONWOVENS DIRECTORY

10. RECENT RESEARCH

• New instrumentation to measure cotton contamination [21].
• Cotton linters to replace the traditional 100% wood pulp fibers for producing absorbent cores for disposable diapers and famine pads [22]
• New quality measurements of small sample cotton are being developed [26
• Cotton is being blended with kenaf fibers to improve the softness and hand [27]
• Buckeye Technologies has developed 100% natural cotton for tampon manufacture [29
• Clustering analysis is developed for cotton trash classification [30]
• New method to improve the dyeability of cotton with reactive dyes. [31]

10.1RECENT DOVELOPMENTS IN COTTON

10.1.1 COLORED COTTON

Cotton fiber is dyed with chemical dyes in order to get wide range of colors. These chemical dyes and their finishing demands large amount of water in turn when these water is disposed they cause soil and water pollution. Many dyes are of chemical origin; particularly the azure ones and these are not environment friendly. Hence many countries, including India, have prohibited use of these dyes.

The negative effects of dyeing can be reduced by naturally colored cotton. This colored cotton is developed by gene transplantation. Crossing the genes from wild cotton varieties with the cultivated white ones develops this colored cotton. The research is being conducted at The University of Agricultural Sciences (UAS), Dharwad Karnataka India, to promote the cultivation of natural colored cotton. The colors that have been developed are White, Orange, Red, Yellow, Green, Purple, Brown, Blue, And Black. These negative effects of dyeing can be avoided by extensive research and growth of colored cotton. (33).

10.1.2. BT COTTON

Cotton requires severe pesticide in order to combat numerous pests after some years of use of pesticide by farmers these pests develop resistance to Particular pesticide. This resistance force farmers to use more amounts of pesticides. BT Cotton is developed by transgenic technique of implanting Bacillus Thuringiens bacterial gene in to cottonseeds, which makes the cotton plant and seeds resistant to majority of pests including bollworm (A. Lepidoptera), Tobacco budworm (Heliothis virescens). Bt cotton is now one of the most widely used transgenic crops. It is currently grown throughout the United States. More than 2 million acres of Bt cotton are grown in the United States alone. Other countries include China, India, and Australia. (34) According to Dept. of Agricultural and Resource Economics, University of Arizona. Bt cotton planted from 1996 to 1998 is estimated to have yielded 5% more on average than if traditional and decreased the quantity of foliar spray [35].

10.1.3 COTTON’S FUTURE TRENDS

The world's cotton fiber production is approximately 89 million bales [6]. In 1997, a production forecast [6] shows that the U.S. is the largest cotton producer (18.4 million bales), followed by China (17.5 million bales), India (12.8 million bales), Pakistan (8.0 million bales) and the former U. S. S. R. republics (7.7 million bales). Other important cotton producers are Australia, Egypt, Turkey, Brazil, Argentina, Paraguay, Greece and Mexico. The highest cotton consumption is attributed to China (21.2 million bales), India (12.9 million bales) and U.S. (11.3 million bales).

Supplies: The world production will increase a little bit. The 1998 U.S cotton crop is best described as a disaster due to cool wet spring in the west and inadequate rainfall in the southeast [24].

Consumption: World cotton consumption is lagging a bit behind production. After a surge in the mid-1980s, world cotton consumption has been rather flat. But the long term potential for cotton demand remains large [23].

All cotton plantings for 1999 are expected to total 14.6 million acres, 9 percent above 1998, and 5 percent greater than 1997. Upland cotton is expected to total 14.2 million acres, up 9 percent from last year. Growers planted 318,200 acres of American-Pima cotton. This is a 3% decrease from last year's number, but 27% higher than the acreage of 2 years ago. Planting in Georgia started extremely slow due to a severely dry spring, but by June 1 was nearly on pace with average. Conversely, Texas experienced a near normal planting season although some replanting was necessary due to wind and hail damage [15].

11.Graph of World cotton area/World cotton yields/World cotton production/World cotton consumption[11] Graph of Cotton Prices

12. CONCLUSION

Cotton nonwovens can be recycled, re-used or disposed off by natural degradation conditions. Cotton is a readily renewable resource with long-term supply assurance. Extensive research works is improving bleached fiber quality and quantity. Nonwoven industries are producing various types of nonwovens with different manufacturing techniques, for better production. Cotton share of the textile fiber market has been steadily increasing and will continue to increase as cotton-containing items is preferred by the consumers.

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COTTON FIBERS

Updated: April, 2004- Raghavendra R. Hegde, Atul Dahiya, M. G. Kamath

(Xiao Gao and Praveen Kumar Jangala)