Structure and Properties of Cotton Fiber: A literature Review
Presented to: Dr. Prof. Jiri Militky by Muhammad Mushtaq Ahmed Mangat Dec 14, 2009
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Structure and Properties of Cotton Fiber: A literature Review
1.1 Introduction Cotton fiber is one of the oldest natural fiber which is much familiar to human beings and widely used for numerous purposes. It is predominantly composed of cellulose, along with hemicellulose1. There are certain noncellulosic matters also attached and present in the cotton fiber. Such as sugars, starch, protein and some inorganic matters. Besides these matter lignin is found also in the cotton fiber, which is a complex organic compound. It woks as a binding force among various compounds and makes the whole structure firm and steady (Orwell et al. 2000).
It is work out that cotton was growing in ancient times and it was cultivated in India in 3000 BC (Militky, 2009). A drastic and incredible increase in the production of cotton is attributed to the invention of textile machinery and development of different cotton varieties, along with various chemicals to bring down the impact of pests and insects. Also availability of synthetic fertilizer accrued production by improving the fertility of the soil. In current times, production of cotton fiber is gauged 14-18 Million Tons per annum (Militky, 2009).
Cotton has a number of distinguishable characteristics and is graded as a fiber which is preferred by many people. Before the invention of man-made fibers, cotton was having the leading share. Although, there is a decreasing trend in the consumption of natural fibers, but still cotton has 40% share in total consumption of natural and man-made fibers.
This review is confined to describe the work of scientists and scholars about the structure of cotton fiber along with chemical and physical properties of A class of substances that occur as constituents of the cell walls of plants and are polysaccharides of simpler structure than cellulose (dictionary). 1
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cotton and its application in real life. However, a small portion of the review has been allocated to discourse the history and growth process of cotton. What is more, this literature survey aims to collect work of different people at one point so that reader could grasp the elementary and interested facts about cotton fiber.
1.2 History of Cotton Fiber Fiber refers to a fibrous material which is found in natural and man-made form. This term is used widely for a wide variety of fibers (Fritz, 2008). Hearle (2008) quoted the definition of fiber coined by The Textile Institute. It says that, “Fibers as units of matter characterized by flexibility, fineness, and high ratio of length to width”. There are many studies conducted to estimate the time when human being first time used cotton fiber. Literature provides many evidences about the use of cotton and to some extent agreed time is 3000 BC. Generally it is believed that history of cotton is as old as the history of human being. Nevertheless, many more fibers occurring naturally are also in use since centuries. However, cotton has a distinction among fibers due to numerous reasons. Particularly, its softness, absorbency, luster, strength and wearing comfort have contributed a lot to make it most liked and used fibers since ancient times. Even, today cotton has 40% apportion in total fiber consumption in the world (Hsieh, 2007).
Cotton growing started in several parts of the world. Literature provides evidence that Neil Valley, India and Peru are some of the areas, where people were cultivating cotton in ancient times (Myers, 1999). It is manifest from the fiber consumption pattern of the world, that cotton share has a downward trend. It is mainly due to the availability of synthetic and manmade fibers having many distinct qualities. Myers (1999) argues that cotton is more primal for economies of developing countries as compared to developed countries. Developed countries are more in man-made fibers as compared to under developing countries due to level of technology and volume of capital required for man-made fibers. Whereas, cotton cultivation
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demands a much labour, which is copiously available in the developing countries. There are more than 80 different countries, where cotton is cultivated today.
It is a fact that cotton is a natural fiber and it is true since it comes out from a seed and grows but the way people are managing the cultivation process has made it less natural and more dependent on fertilizers and pesticides. There is an increasing concern about the use of divergent chemicals which are not environmental friendly.Consequently, demand of organic cotton is increasing, which is grown in a naturally environment (Myers, 1999).
2.1 Composition of Cotton Fiber The major chemical component of a living tree is water, but on a dry weight sugar-based polymers (carbohydrates) becomes the main component of all plant cells, which are combined with lignin, along with smaller amount of extractives, protein, starch, and inorganics. There is diverse variation in the percentage of different chemical and it depends upon numerous factors, which include, plant to plant, and within different parts of the same plant, geographic locations, ages, climate, and soil conditions (Han and Rowell, 2009)
Sugar based polymers (carbohydrates) are the major constituent of naturally occurring agro-based plants. Lignin is one of the chemicals along with carbohydrates and responsible for the binding of the polymers. Additionally, other chemicals, like, protein, starch and a few inorganic chemicals are present. There are primary and secondary layers in the cell and chemical components are present throughout the cell but their concentration varies across the cell. Moreover, there is much variation in the composition of the chemicals from plant to plant. Even there is a marked variation within the plant (Orwell et al. 2000).
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Same is the case with cotton fiber. Cellulose is the main ingredient if cotton since it comes from a naturally growing plant. Cotton fiber has more than 90% cellulose along with some non-cellulosic matters. Nevertheless there is a slight variation in the chemical constituents of cotton, depending upon many factors e.g. the environment where it has been cultivated and after that type of seed along with the ginning process since damage of seed during ginning can add some protein matter to cotton fiber (Fan, 2005). There is an understood variation in the composition of the fiber since it is a natural product and there are many factors which can influence its composition.
Hearle (2007), ingeminate findings of Goldwaith and Guthrie (1954), that cotton fiber is mainly composed of α-cellulose, which is 88-96.5 % of the total mass of the fiber. This study further reveals that secondary wall of cotton fiber is of pure cellulose, whereas non-cellulosic material is present on the outer layers or inside the lumen2 of the fiber. Hsieh (2007) supports the general observation about the variation and points out that the chemical composition varies with the variation in varieties of cotton, environment where it is cultivated and maturity level of the fibers. With the increase in maturity level there is an increase in the cellulosic percentage. Table 01 indicates that there is a significant difference in the chemical composition between mature and immature cotton. Table 01 Ingredients of Cotton Fiber Ingredient
Mature Cotton (%) Immature Cotton (%)
Cellulose
96.41
92.44
Minerals
0.79
1.32
Wax
0.45
1.14
Protein
1.00
2.00
Source: Fan (2005) Militky (2009) reviews the components of cotton fiber in a more precise way and reports that cotton fiber has following composition: 2
It is the central cavity of a cotton fiber
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1. Cellulose: 88-96%. It is main part of the cotton fiber and secondary wall posses highest percentage of the total cellulose 2. Pectin’s: 0.9 -1.2% . These are ploygalacturonic acid, and its magnesium salts, methylester, xylose. These are mainly present in primary wall. 3. Proteins: 1.1-1.9%. These are protoplasm rest in lumen and aspartic, glutamic acid and proline, 0.2-0.3% of nitrogen are found in primary wall 4. Waxes: 0.3-1.00% . It is higher monovalent alcohol-tractional, palmitic, oleic acid, glycerine. Its melting point is 77 0C. It is found on surface and in primary wall. 5. Organic acids: 0.5-1.00%. Salts of citric and L-maleic acid 6. Mineral salts: 0.7-1.6%. These are hypochlorites, sulphates, phosphates, oxides of silicon, calcium, potassium, magnesium 7. Sugar: 0.3%. Glucose, galactose, fructose, pentose. Higher usage % indicate bacteria attack 8. Toxine: 0.9%. Endotoxine,evolved from bacterial cells (0.017-100 g per bale of mass 218 kgs) 9. Vitamins and pigment (flavone compounds).
All above discussion affirms that cellulose is composed of cellulosic and non-cellulosic material. Presence of all organic and inorganic matter varies a lot depending upon many factors. However, majority of the non-cellulosic material is taken away in the scouring process, which makes it absorbent so that it may pick and absorb dyes. All the same, this wax helps ginning, and spinning by providing a smooth surface which attributes a little friction. It is also crucial to note that after scouring and bleaching most of the impurities are vanished and it becomes pure cellulosic material. Nevertheless, it may still have insignificant percentage of added-impurities, which becomes part of the fiber during process or impurities which are not completely taken away.
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2.2 Chemistry of Cotton Cellulose Cellulose is an insoluble substance and mainly composed of polysaccharide, which holds chains of glucose monomers. It is only soluble in some specific solvents. It is the main constituent of plant cell walls and as well as of vegetable fibers. Cotton is one of the vegetable fiber which has the highest percentage of cellulose. There is a diverse structures and compositions of cellulose of cotton. Its structure makes it divergent from other naturally occurring matters. Cotton cellulose is unique in many ways and possesses a distinct characteristics which makes it highly useful for many purposes.
Das et al. (1954) has reviewed the work of Adams and Bishop (1953) and describes that it has been assumed that cotton cellulose structure is based on glucose unit only. However, it is also believed that there is modest amount of pentose is present, which is removed during scouring process. They further report that it has been notices from the chromatographic analysis that glucose, xylose, arabinose and a trace of rhamnose are also present in raw cotton. Whereas, Adams and Bishop (1953), as reported by Das et al. (1954) that there is no evidence of the presence of pentose. Das et al. (1954) strongly express their view about the presence of a part of pentose.
Cotton cellulose is highly crystalline in nature and well oriented and has a long and rigid molecular structure. The β-1,4-D glucopyranose are the principle building blocks of cotton cellulose chain and are linked by l,4glucodic bonds. Free rotation of the anhydrogluco-pyranose C-O-C link is stopped by steric effects. There are three hydroxyl groups attached to each anhydroglucose. One group is attached at C-6 and two at C-2 and C-3. Due to the presence of hydroxyl groups and the chain conformation, there are many more bonds possible (inter molecule and intramolecular). Such bonds make the fiber more rigid by increasing the rigidity of the structure of cotton cellulose (Hsieh, 2007).
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Carbon „6 –primary hydroxyl group; Carbon „2 and „3 –secondary hydroxyl group
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2.3 Structure of Cotton Fiber There are numerous studies accessible in literature which furnish evidence about the crystalline structure of the cotton fiber. Nevertheless, one can find a contradiction about the claims of structure, particularly concerning crystallinity (Hearle, 2007). Researcher have used different methods to evaluate the structure. The most common method is use of wide-angle X-ray diffraction and a multi-peak resolution.
In this method total scatter structure is resolved into peaks over a noncrystalline background. In this way crystallinity is determined which is the ratio of the summation of all resolved peaks to the total scatter. During structural investigation, seven peaks were found from the WAXD spectrum.
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They are between 5° and 40°. Nevertheless four peaks are near to 28 angles of 14.7°, 16.6°, 22.7° and 34.4°. These shows the characteristics of the 101, 10 T, 002 and 040 reflection of cellulose. These peaks are quite useful to have a better idea about the structure of cellulose. All this information confirms the crystalline structure of the cellulose (Hsieh, 2007). Considering all above discussion Hsieh (2007), holds the view that there is a strong evidence that cotton cellulose is composed of cellulose I crystalline structure and there is no II, III or IV. Hearle (2007) has also provided information about the structure of the cotton cellulose and agrees that there is a lattice which has linked molecules in a sheet and at the same time weak van der Walls forces present between the sheets. This sort of structure known as cellulose I, which has different structure and characteristics from cellulose II and III.
Crystallinity have a significant role in the performance of the fiber during forthcoming processes. Fiber strength is one of the characteristics, which are considered during spinning and onward process. Studies by Hindeleh (1980) as reported by Hearle (2007), on eight different varieties of Egyptian cotton arrives on the judgment that there is a positive correlation between the strength of a bundle of fibers by Stelometer and crystallinity. This issue has been further hashed out by many scholars and we find clearer picture of crystallinity and crystallized area and their relation with the strength. Study at different stages supports the idea that there is a significant, almost double changes in the degree of crystallinity from the formation of secondary wall. Crystallinity starts from 30% at 21 days past anthesis (dpa) and goes to 60% at 60 dpa (Hsieh et al., 1997).
Notwithstanding, there are certain reports which claims more than one structure. Furthermore, it is also easily perceivable that primary wall is composed of less crystalline structure as compared to secondary wall. There are few intriguing questions about the structure and correlation between structure and strength as well as elongation. Work of (Hsieh et al. 1997), put forward the conclusion that there is a positive correlation between single
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fiber breaking force and overall crystallinity of the fiber. It may differ from variety to variety. Thus we can say that variety has an impact on the strength. It is commonly known that strength of different varieties are different and they are graded based on many characteristics along with the strength of the fiber. Pima cotton is well known by its strength, thereby, Pima is preferred when we need high strength of fine counts.
Hsieh et al. (1997) points out the effect of crystallinity and crystallize area. These findings are crucial to understand the factor affecting the strength of the fiber and they come up with the view that crystallize size has higher correlation with the tenacities of the fiber than the crystallinity. It reveals that for a better strength, there is a need of a reasonable size of the crystalize area than of the crystallinity in a small area. Study further speaks that new cotton varieties are having better crystallized area. Consequently, there is a higher strength of the cotton fiber.
Each mature cotton fiber has a spiral fibrillar structure which can be observed underneath the primary wall. There are parallel grooves and ridges having an angle of 20-30° to the fiber axis. During scouring process these fibrils are exposed. However, these are not removed by slack mercerization or even on soaking (deGruy et al., 1973; Muller and Rollins, 1972; Tripp et al., 1957) as cited by the Hsieh (2007). This the reason, why we need sever scouring to make cotton fiber more absorbent. Since during scouring process fibrils are exposed and have ameliorate contact with the water molecules. In other words there is a rupture of primary wall and this rupture helps making cotton fiber to take more dyes during dyeing process. Stretching of yarn or fabric during mercerization makes residual ridges more parallel to the fiber axis of secondary walls. Due to this phenomenon stretching is mandatory during mercerization process for a better absorbency.
Hearle (2007) has sum up the studies of Warwicker et al. (1966); Rebenfeld (1977) and Balls (1928) which are related to the description of twist of
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cotton fiber. These studies has established that other than crystallinity, there is a twist in the fiber structure. Frequency and angle of convolution determine the parameters of fiber, particularly, strength. Number of twists in the fiber and convolution directions plays a critical role in the strength of the fiber. There is a 3.9 to 6.5 twist per mm and there is a spiral reversal changes one to three times per mm length. Along with the twist, convolution angle is quite important and it varies with the variation in the cotton variety, at different pasts of the fiber even between lint and fuzz.
There is an ample variation in the length and width of the cotton fiber. This variation is attributed to the genetic of the cotton. Superfine sea Island cotton has the highest length, up to 5 cm and 1 dtex linear density. Whereas Asiatic cotton has 1.5 cm length and 3 dtex. One to three dtex values correspond to 10-20 µm thickness of the cotton fiber. The above two values depicts the extreme values. Notwithstanding, majority of the cotton is in the middle of the above values (Hearle, 2007).
Cotton maturity is another significant factor and of high concern of the endusers. There is a regular growth of the cotton fiber, starting from the synthesis of cellulose, through primary wall formation and finally development of secondary wall. Maturity of the cotton is a ratio of the cell wall to the whole area of a circle, equivalent to the an urn-collapsed fiber. Its acceptable ratio is 0.85. If this value is less than 0.5, it indicates that the there is abundant immature cotton and not good to produce a premium products (Hearle, 2007). All talk favors the fact that, cotton fineness maturity level and length of cotton are some factors which are weighted at the time of grading of cotton. Since cotton is a natural fiber and variation are quite understood. There is a variation in the length, fineness and maturity level of the cotton even from the same variety or from the same plant. People accept variation in the properties of the cotton fiber and this range is decided by the endusers.
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Figure 01 Structure of Cotton Fiber
Molecular density in the fiber structure varies a lot. It is noticed that molecular packing is quite dense near the reversal as compared to other areas (Patel et al., 1990). They further found by examining fibrillar morphology 3 that reversal zones in cotton fiber has a reasonable difference in the seize of fibrillar aggregates on both sides of the structural reversal by getting it swollen with ZcCl, These observation are confirmed Kassenback (1970), as reported by Hearle (2007) that there is a deviation in the molecular packing density concerning bean. It is highly dense near the highly curved ends and thus have poor accessibility to reagent. Whereas, the convex structure is less dense and easily accessible by the reagent. Cotton fiber has a tapered shape. It is thin at the top as compared to the bottom. It has been observed that from the top 15% length has a smaller diameter as compared to the bottom.
3The
branch of biology that deals with the form of living organisms, and with relationships between their structures.
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As it is demonstrate from the Figure 01, cotton fiber is consist of primary and secondary layers, while lumen is present in the center. Primary layer holds up to 30% cellulose and non-cellulosic materials. This cellulose is of lower molecular weight with the degree of polymerization (DP) between 2,000 and 6,000. Secondary wall is rich with cellulose of higher weight with DP of 14,000. In addition its weight is distributed uniformly (Huwyler et al., 1979; Meinert and Delmer, 1977; Goring and Timell, 1962; Hessler et al., 1948, Figini, 1982 as cited by Hearle, 2007).
Figure 2 Cotton Flower
Figure 03 Convolution on Cotton Fiber
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Figure 04 Primary, Secondarily Walls and Lumen of Cotton Fiber
Figure 05 Dried Cotton Fiber
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Figure 06, Microscopic View of Cotton Fiber
Bean-shaped cross section through a cotton fiber 1. Wax layer 2. Primary layer 3. Secondary layer 4. Lumen wall 5. Lumen (cavity), air filled
Militky (2009) expands the cotton structure and exposit the clearer and understandable morphology of the cotton structure.
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Table 02 Cotton Fiber Structure (Militky, 2009) Cotton Fiber Structure Description
Chemical
Thicknes s
Surface of the Wax, pectin, protein fiber Primary wall
Fibrils bundles (transient lamesl)
0.1-0.2 nm
Secondary wall 95% of fiber mass First layer S 1 Fibrils bundles, spiral nm (transient lamesl)
0.1 nm
Second central 25-30 concentric fibrillar sheets (0.2 micron). 0.2 nm layer S 2 Sheets are spiral from fibrilar bundles. Direction changes in reversal point (weakest point, 20-30 points/cm). 45% breaks occur at these points. Third inner layer Border lumen
0.1 nm
S3 Fibrilar bundle
100-200 nm
Fibril
10-40 nm
Microfibril
Length 60 nm (30 chains)
3-6 nm
3.1
Cotton Fiber Development Stages Literature survey supports the idea of that study of growth of cotton plant and development of fiber is one of the favorite subject for researchers. Many, people from agriculture have done much research. It is imperative to have a brief explanation of the fiber development to understand cotton fiber,
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it would help a lot have a enrich our understanding about its performance in onward textile process.
Figure 07 Different Stages of Cotton Flower
Naithani et al., 1982 carve up the growth of cotton cellulose in four stages:
1. Initiation: beginning epidermis 4 cells from ovule5 surface 2. Elongation: primary walls are developed
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Outer layer of tissue in a plant, except where it is replaced by periderm.
Part of the ovary of seed plants that contains the female germ cell and after fertilization becomes the seed. 5
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3. Secondary wall thickening and maturation: synthesis starts (third stage) which takes 15-22 days and it continues for 30-40 days 4. Desiccation: removal of moisture takes place and resultantly fiber collapses In the following line there is little detail of the cotton development process.
3.2 Cellulosic Synthesis There is a synthesis process in the development of cellulose like in any polymerization process. Since cellulose is a natural polymer and there is a requirement of some initiating molecule. In case of cotton cellulose, it is synthesized by the condensation of glucose and the outcome is 30 cellulose molecules. They are in a straight and same direction and having a crystalline structure. In this way, a long microfibrils, which have about 7 nm width is produced. It is easily conceivable that cotton fiber has 100% crystalline structure. There are many ways to prove this fact and one is the X-ray diffraction, which provides a scientific evidence that cotton is 92.6 to 94.7% crystalline structure (Timpa and Ramey, 1994, as cited by Hearle, 2007).
3.3 Primary Wall Formation Cotton is longest single cell as compared to other agro-based fibers. Its growth initiates with the singe cell from the individual epidermal cells on the outer integument 6 of the ovules in the cotton fruit (Graves and Stewart, 1988). There are a number of cotton fibers grow simultaneously from the same cotton boll (cotton seed). Each fiber presents a single cell of plant. In first step this plant goes to its full length and during this period primary wall is formed. This primary wall holds the whole cell plant and keep it align and well oriented. During this period, when the fiber is totally consist of primary walls, fiber width is 2.98 mm and crystalline percentage is up to 30% (Hebert, 1993, as cited by Hearle, 2007).
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A tough outer protective layer of plant
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3.4 Secondary Wall Formation Secondary wall formation originates just after the completion of primary wall. Primary wall takes around 20-25 days and after that secondary wall synthesis commence which takes 15-22 days and it continues for 30-40 days. During this period fiber gains weigh of 130 ng/mm, as compared to 2 ng/mm while primary wall was under development (Meinert and Delmer, 1977). Hsieh et al. (1997) report that after completion of primary wall, secondary wall is formed. Secondary wall is in the shape of a ring inside the tube or cylinder. Notwithstanding, during this period a lumen in the center is also produced at the time of fiber maturity. Secondary wall is an example of pure cellulose. Study further reveals that secondary wall is 94% of the total mass of the fiber material. Thereby secondary wall is contemplated as the main responsible for all sorts of mechanical properties. There is a slight variation in the density of the cell wall before and after desiccation. It is 1.55 gm3 before drying and after dehydration it reduces to 1.52 gm3 under the 65% RH conditions. Note all these values are by including lumen into account. It shows that impelling density is less than the reported density (Hearle 2007).
At the last stage desiccation (removal of moisture) takes place and resultantly fiber collapses and it is converted from a cylindrical shape to a flat shape. Length to width ratio varies from 2000 to 3000. Exceptionally, some fibers attain this ratio up to 4000 (Cassman et al. 1990). During desiccation process fluids are taken away from the lumen along with intermolecular water from the cellulose. This ultimately becomes the reason of collapse of the cylindrical shape of the fiber. Thus the chances of intermolecular hydrogen-bonds increase many folds. The collapse of cylindrical shape gives birth to flat rib like structure, which is irreversible in nature. Some permanent changes occur, like, structural heterogeneity (irregular distribution of chemicals), decreasing porosity (minute holes), and reduction in sorption capacity. On the other side, there is an increase in the
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molecular strains and this ultimately reduces the chain mobility (Hsieh, 2007).
Militky (2009) comments on the outcome of the cotton development process and concludes that after complete drying following there three types of fibers are found: 5. Dead fibers: having only primary wall, not able to pick dyes 6. Partial mature fibers: thin secondary wall and small convolution 7. Mature fibers: thick secondary wall and regular convolution
Militky (2009) suggests that for microscopic evaluation, image analysis is used. For this analysis two parameters are required; cross-section circumference and secondary layer area. By using the following formula we can measure the maturity degree:
4*π A S= ----------P2 Where: S is degree of maturity A is secondary layer P is cross section circumference All above discussion is to have a finer idea about different properties of cotton fiber. It was noticed during literature survey that there a number of factors which can affect the properties of cotton fiber. Furthermore, data on properties were collected in different labs under different working conditions. Besides that samples were collected from different varieties. There is a diverse variation in the properties of cotton fiber even collected from same plant. Keeping all in view it is quite thorny to have an agreed set of quality parameters. However, there is a range of parameters, which has to
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be accepted and it is challenging to compare one data set with other Orwell et al. 2000). 4.1 Cotton Fiber Parameters Uster HVI spectrum, which is widely used to assess the quality parameters of cotton renders a list of parameters which is used to asses the quality level of cotton fiber. This list is used to measure following features of cotton fiber quantitatively:
1. Fiber strength 2. Fiber fineness 3. Staple diagram 4. Fiber maturity 5. Impurities 6. Color
Militky (2009) provides a list of quality characteristics, abbreviations and description, along with the unit:
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Table 03 Quality Parameters of Cotton Fiber and their Description Cotton Quality Parameters Quality Parameter Micronaire Upper half length UHML
Description Indicator of fineness
Unit of measure
Abbreviation Mic
Correspondence to the UHML calluses mm
mm
Measure for variation Uniformity index of fiber length, length uniformity
UI
%
Bundle tenacity Breaking tenacity
Strength
g/tex
Reflexion
Degree of reflexion of the cotton. The higher Rd this value, the better the cotton is rated
%
Yellowness
Assessment of color, +b degree of yellowness
%
Trash
Number of trash particles per defined area
CNT
-
Trash Area
Percentage of trash per defined area
Area
%
Source: Militky (2009)
Fiber quality parameters play an important role in the yarn quality. Scribe, one of the HVI manufactures provides the following information about the contribution of cotton parameters in yarn quality.
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Table 04 Fiber Quality Parameters and Their Role in Yarn Quality Fiber Quality Parameters and Their Role in Yarn Quality Parameter Contributing % Fineness 15 Elongation
5
Strength
20
Uniformity of length Length
20
Trash
3
Others
15
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Source: http://www.scribd.com/doc/7314282/HVI
5.1 Chemical Reactivity and Moisture Absorption Phenomenon of Cotton Cotton cellulose is not soluble in water. However, it can react with certain chemicals under specific conditions. This reaction starts from the amorphous region, which is more accessible and then it reaches to the surface of crystalline region. There are two main categories of reactions; etherification and esterification. Etherificaton is a process in which acetylation, phosporylation, and sulfating take place under the acidic conditions. Oxidizing agents readily attack cotton cellulose, like, peroxides, hypochlorites, dichromates (Hsieh, 2007). There is no priority in oxidation process to react with primary or secondary hydroxyl groups. Oxidation of cellulose is not a simple process. It has two stages and multiple outcome. Result of oxidation is reducing and acidic oxycellulose. In both ways reaction is different. In reducing cellulose hydroxyl (−OH) are converted into carbonyl groups (=C=O) or anhydrides {−C(O)OC(O)−}. In case of acidic oxycellulose hydroxyl group is converted into carboxyl groups (−COOH) or acids (Hsieh, 2007).
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5.2 Heating Effect on Cotton By increasing temperature there are convincing possibilities, which could happen in the substance. Dehydration and decomposition could be the first. Nevertheless, it depends upon the structure of the substance. In case of cotton cellulose, dehydration and decomposition takes place after one another. Acid works as catalyst in dehydration process. Whereas, alkali favors the decomposition process. In decomposition process, depolymerization takes place and ultimately, structure of cellulose is decomposed (Shafizadeh, 1975, as cited by Hsieh, 2007).
On heating cotton cellulose, dehydration process initiates and it remains till the 120°C. During this period moisture leaves the cotton cellulose and there is no adverse effect on the strength of the cellulose. However, further rise in temperature decreases the strength of the cellulose. There is a significant change occurs at 150° and above. This may be resulted as low molecular weight and consequently low tensile strength (Shafizadeh, 1975, as cited by Hsieh, 2007).
Till 250°C, heat effect is only in the amorphous region. However, at temperature above 250°C, changes becomes visible in crystalline regions. It starts by lowering the molecular weight and after 270°C, it disappears (Naithani et al. 1982). Shafizadeh further reports that between 200-300° C, levoglucosan evolves which are mainly 1,6-anhydro-β-D-glucopyranose. By further heating and reaching at 450°C only char left.
5.3 Mercerization of Cotton Cotton fiber becomes like a flat ribbon with many twists after desiccation. During this dehydration process, many change which occur and finally there is a transformation of a cylindrical shape into a flat shape. In mercerization process, where cotton is treated with caustic soda. There are many more other reagents, which can be used for mercerization. Nevertheless, caustic is
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the most common reagent. During the mercerization, swell and de-swelling of cotton fiber takes place which resulted in the cylindrical form of the fiber. Mercerization initiates from disruption of crystalline area cotton fiber. Notwithstanding, there is no exhaustive damage of the fiber integrity, so that there should be a chance of re-crystallization. Result is formation of a cellulose II, with anti-parallel chain (Hearle, 2007). There is a significant change in the degree of disorder before and after mercerization It is 49% after mercerization, whereas, it is only 29% in UN-mercerized cotton (Warwicker et al., 1966, as cited by Hearle, 2007). Contrary to mercerization in resin treatment process, there is permanent cross-linking between hydroxyl groups through resin and they develop covalent bonds, which are of permanent nature and help in improving the crease recovery tendency of the fabric.
5.4 Moisture Regain: A Critical Property of Cotton Fiber The terms, moisture regain or moisture absorbency of cotton fiber is used to describe the behavior of cotton fiber when it is put in contact of water or moisture in a certain environment and at a certain temperature, even for a certain time. Also in what shape it is put (in bundle or in isolation form). It is defined as a ratio of mass of water absorbed to the total mass of fiber (Denton and Daniels, 2002, as cited by Hearle, 2007).
Presence of water in a growing plant is one basic necessity which is required for synthesis and growth of the plant. During this period there is a plenty of water present in plants. Same is the case with cotton fiber. It has the highest procreating of water before the start of initiation period. There is a decrease in water quantity after the ball is opened and first phase (initiation) of fiber formation starts. During this period it develops an equilibrium with the ambient humidity and there is a constant change in the moisture percentage in try to have an equilibrium with the moisture present in the atmosphere. The change of moisture in an isolated fiber is fast as compared to assembly of fibers. In case of assembled fiber an interaction
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between the water diffusion and the transmission of heat sorption7 takes place (Hearle, 2007).
Under normal condition (65% RH and 20°C) cotton fiber can regain moisture up to 7-8%. There is a 0.9% increase in desorption due to hysteresis. Considering this phenomenon, recommended commercial value of cotton fiber regain is 8.5% (Hearle, 2007).
Cotton fiber density changes with the change in the moisture regain. There is no linear relationship between these two. This nonlinear relationship, which is understood from Figure 07, that up to 2% regain, density increases and after that there is a regular decrease in density and at 20% moisture regain it is equal to the water density, since water density is less than cotton fiber density (Hearle, 2007). Hearle further explicate that increase in the density is mainly due to empty spaces which was occupied by the water molecules. After that there is a decreasing trend. It is mainly since water has less density than cotton fiber.
Figure 07 Relationship between moisture regain and density
Absorption and adsorption are considered same process and here denoted by sorption 7
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Steady increase in the volume (swelling) of the cotton fiber is noted with increase of moisture regain. However, there is a significant difference between the increase in length and the width. Hearle (2007) summarizes many studies and concludes that swelling is predominantly in the transverse8 direction. There are some difficulties attached with the measurement of the increase in length and width. These are primarily due to the presence of lumen and cross-sectional shape of cotton fiber, twist angle, etc. However, Meredith (1953) as quoted by Hearle (2007) claims that there is a 1.2% increase in length and 14% increase in width starting from the dry and going to wet stage. There is a significant difference in the values given by different people, like, Preston and Nimkar (1949) (as cited by Hearle, 2007) report that there is a 7% increase in diameter and 21% increase in total volume. Whereas, Hsieh (2000) is of the view that there is a 10-15% in diameter and 0-0.5% contraction in length at 40% regain. In short, there are several discrepancies attached with the measurement of change in volume, but one thing is affirm that there is a more change in width than in length and overall volume increases with the increase of moisture regain. Furthermore, increase in the volume is also confirms from the Figure 08, which shows that there is a decrease in density with the increase in moisture regain. There is an intriguing questions about the relationship between density and absorbency. Density is ratio of mass and volume. As it is explained in previous paragraph, that initially there is an increase in the density but after reaching at a certain point it starts decreasing. This is only possible, if there is no change in volume. But in case when volume increases with the There is another possibility of increase in volume due to swelling. In such case density will also decrease since it is a ratio of mass and volume. It is presumed that decrease in density which is obvious from the graph, may be due to the increase in volume.
8
Extending across something
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Figure 08 Relationship between moisture regain and volume of cotton fiber
During wetting process of cotton fiber, there are certain structural modifications take place. It was first discussed by Peirce (1929), as cited by Hearle (2007). There are hydroxyl groups available and they are not linked with the crystalline structure. These are on the surface of the crystalline fibrils. These groups will be attacked by the water molecules and in this way water will be absorbed by cotton cellulose (direct absorption). After saturation with these free hydroxyl groups, water molecules will be attached with other hydroxyl groups (indirect absorption) or these water molecule will form a secondary layers with already attached water molecules (Hearle, 2007). Efficiency and heat of sorption in both cases is different. In case of direct absorption, there is more heat evolution. Besides that, initial absorption will increase the density, which is quite noticeable, however, further absorption will add only the volume equal to water, which is being absorbed. Peirce explains the direct and indirect absorption phenomenon and has reasonable evidence to assumes that there is an equal chances of direct and indirect absorption.
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Figure 09 Direct and indirect moisture regain and humidity
Peirce (1929) further explains that direct absorption is more firm but its quantity is less as compared to indirect absorption and indirect absorption plays a significant role in developing vapor pressure.
There is another theory to explain the absorption of cotton fiber. This theory is based on the analogy with some osmotic pressures and proposed by Barkas (1949) as cited by Hearle (2007). Barkas (1949) postulates that moisture taken by fiber depends upon the stresses generated by swelling of fiber. It will keep on regaining moisture till the time, it is prevented by the stresses generated by the swelling. Barkas used thermodynamics cycle to drive conclusion, which is based on directional swelling and directional stresses and moisture absorption.
6.1 Mechanical Properties of Cotton Fiber Cotton is comparatively a weak and less extensible fiber and is graded low in toughness, when it is compared with other fibers. However, longer fibers have better tenacity (measure of strength) and modulus ( measure of stiffness) found a positive change in tenacity with the increase in length array groups. (Hearle, 2007).
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Another study conducted in recent times by Foullc and McAlister (2002) finds a significant correlation between tenacity and birefringence with an increase from 0.2 N/tex at a birefringence of 0.04 to 0.37 N/tex at 0.05.
An intriguing feature of the strength is its coefficient of variation. Liu et al. (2005) conducted many tests from given samples and noted a significant variation in the results. It deviates from 37% to 45% for breaking force and 30 to 44% for breaking elongation. Hu and Hsieh (1998) conducted tests to measure the toughness (work of rupture) and reports coefficient of variation 51 to 56%. Results are skewed having a long tail towards high values, which indicates that there is a significant variation in the mechanical properties of fibers even taken from the sample. It is further describes that the such variation indicates lower bundle strength than single-fiber strength.
A significant variation is also reported by Hebert (1993), as cited by Hearle (2007). Hebert (1993) finds that strength of immature fiber with only primary wall is one-sixth as compared to a mature fiber. However, tenacity is just over half that of the mature fiber. This report confirm the basketweave orientation of fiber in the secondary wall.
Figure 10 Stress-strain curve for various cottons, from Sparrow (1973) as cited by Hearle (2007)
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Figure 11 Effect of humidity on stress-strain curves of cotton at 20 0C. From Meredith (1953).
6.2 Elongation and Elastic Recovery A big variation is found in the elongation properties of the fibers. It varies from a small number to very high. Industrial fibers have 1-2% value, whereas, textile fibers shows a high stretchability. It starts from 0.1 to 2% in fibers and yarns can go up to 5-70% (see Table 4 in appendix). Nonetheless, spandex has the highest value which ranges from 300-700% (Fritz, 2008). Maximum elongation is not achieved during difference processes. It is possible that fiber will not return to its original position. It may attain a stretched shape, which is not wanted in many cases. In is also important to note that there is a relationship between environmental and internal condition of the fiber and elongation. Like, moisture, temperature affects cotton elongation properties.
Elastic recovery is the ratio between recovered to the total extension and noted in percentage. Elasticity of different fiber depends upon its chemical and physical structurer and under what condition stress was applied. There are many values possible between stress and strain even for a particular cotton fiber under constant test conditions. Under higher strains, elongation curve is above the recovery curve and at zero stress urn-recovered extension increases with the increase in maximum stress and strain (Hearle, 2007).
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Figure 12 is about the correlation between stress (N/tex), elongation and elastic recovery.
Increase in stress has decreases the elongation, same is with the extension. By increasing extension, there is a decreasing trend in elongation. However, during cyclic tests between strains gives hysteresis effect (Hearle, 2007). There is a permanent extension possible after elongation under wetted out condition of fiber. Such phenomenon is related to glass transition temperature.
Figure 12 Elastic recovery of cottons plotted against stress and strain. From Meredith (1945b) as cited by Hearle (2007)
6.3 Stress and Time There is a significant relationship between stress and time and an approximate decrease up to 7% in stress for every tenfold increase in time has been found. Nevertheless there is a creep effect in the fiber, which is complementary (Hearle, 2007). Hearle (2007) summarizes the findings of Collins (1924) and Steinberger (1936) and put forward that there is an irregular change in the slope, however there is a linear relationship when parts of extension-time was plotted with log(time). During tensile tests, a rise in stress level has been detected at given strain with rate of extension. Hearle (2007) quotes that there is decrease in strength of cotton has been noticed by Meredith (1953) and this reduction is governed by the following equation:
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F1-F2 = 0.088log10(t2/t1) Where F I and F 2 are breaking loads after times t I and t 2 under load.
Figure 13 Stress relaxation of cotton from different extensions at 65% RH, 200C, from Meredith (1954).
6.4 Directional Effects Twisting and bending are some of the most remarkable properties of cotton but their exact explanation and description is quite complicated. Due the complexity attached with it, a limited data is available on this topic. It is mainly due to the structure of cotton which is like a flat ribbon. In contrary, in a linear-elastic rod, bending moment per unit curvature is proportional to the fourth power of the radius, and linear density T, specific modulus E and density p in consistent units: ηET2/4πp. Where η is a shape factor. It depends upon the distance of the point from a neutral plane. Its value for a circle is 1. Hearle (2007) proposes that to avoid the effect of linear density the equation can be used:
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[(ηE/4πρ) 10−3] N mm2/ tex2 By using a double pendulum method, Owen (1965), as cited Hearle (2007) by derived 0.53 mN mm2/tex2 specific flexural rigidity for cotton. Hearle (2007) expresses the following complications attached with the measurement of directional effect of cotton.
7. Due to the irregular shape of the cotton fiber 8. Ribbon-like structure and cotton fiber acquires a twist, which is perpendicular the width 9. It is difficult to calculate η for cotton, because of the irregular shape of the fiber. 10. Due to the ribbon-like form, cotton fibers will tend to twist in such a way as to bend perpendicularly to the greater width. 11. In case of tension there is al less information for cotton oriented molecular structure as compare to compression. Resultantly, the neutral plane turns towards the inside of a bend and stiffness of bending falls below that given by the linear-elastic case. 12. Loop and knot strengths are less as compared to straight fibers, it is 91% of the tensile strength.
By giving twist to the cotton fiber, the outer layer increases in length. It means that there will be a large influence of tensile modulus on torsional rigidity. Koch (1949) as described by Hearle (2007) observed that breaking twist angle of cotton is between 34 to 37°. Hearle (2007) views the findings of Dent and Hearle (1960) and Finlayson (1947) who have closely examined the effect of twist on strength of fiber. They express that by giving twist at constant length strength of cotton fiber fell from 0.34 N/tex at zero twist to near zero twist when twist factor is 110 tex1/2/cm. Meanwhile, there is a corresponding decrease in the length and this leads to reduction in breaking extension and conclude that shear strength of cotton fiber is about 35% of the tensile strength.
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6.5 Fracture and fatigue
Tensile fracture of cotton fiber count on not only the structure of fiber, rather it also depends upon the stage of fiber (Hearle and Sparrow, 1971, 1979d, as cited by Hearle, 2007). In figure 14, there is scanned picture of fiber (a) and other one is diagrammatically drawn (b). Hearle (2007), viewed the tensile fracture phenomenon closely and denotes that in case of mercerized fiber breaking points are next to the reversal. Nevertheless there is another view, which supports the idea that breaking of fiber takes place at the point of reversal. As shown in the figure, weak line, from where there are more chance of breakage is between the zone A and C. In case the fiber is under tension, untwisting will take place and ultimately, there will be a rise in shear stresses. In this way, a crack starts and covers whole lower bonding between the fibrils and finally reaches to the line of weakness and tears across the zone C.
Hearle (2007) has also examined the tensile fracture of resin-treated cotton at 65% RH and raw cotton at zero humidity. In this case breaking behavior is different from the mercerized cotton. Here, there is a granular (rough surface resulted) break, which is very common with natural textile fibers. Axial splitting is attached with the breaking phenomenon but in case of resin treated fiber, there is a resistant due to inter-fibrillar covalent bonds and hydrogen bonds in dry form. Axial splitting during twist breaks is obvious from the Figure 14 and sharp tear-off is present at the end.
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Figure 14 (a) Break of resin-treated cotton at 65% rho (b) Break of wet mercerized cotton. From Hearle et al. (1998).
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Figure 15 Scanning electron micrograph of break of cotton (a). Schematic of form of break of cotton. From Hearle et al. (1998).
Figure 16 (a) Twist break of raw cotton. (b) Twist break of mercerized cotton. (c) Axial splitting due to tensile fatigue. From Hearle et a/. (1998).
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6.6 Structural Mechanics Cotton fiber has a moderate strength and extensibility along with many more distinguishable features. These features are attributed to the mechanical structure, which is unparalleled in nature. Hearle (2007) summarizes the work of Hearle and Sparrow (1979b), Hearle and Sparrow (1979a) and Timoshenko (1957).
Hearle (2007) looks that by un-twisting the convolutions, extension will be the result. Timoshenko (1957), as cited by Hearle (2007) provides a mathematical expression by applying twist. Hearle (2007) quotes the findings of Hearle and Sparrow ( 1979a) who confirm its revers by using a nylon strip heat-set in a twisted form. They came up with the conclusion that extension of a ribbon of width b and twist φ is possible equal to φ2 b2/24 after complete untwist of the fiber. This can be expressed for convolution as (tan2 ω0)/6. Variation in cotton fiber is notice able as convolution angle along the fiber length. Furthermore, these are not equal to S or Z twist. Study further reveals that there is noticeable variation with in the fibers. There are certain fibers which are to a greater extent circular than others and some are like a wrapped-ribbon instead of twisted form.
Hearle (2007) further reports findings of Hearle and Sparrow(1979a) that extension of the fiber was observed under an electronic microscope and it was found that convolutions were gradually removed and after an extension of 3.8. % ribbon was partially urn-twisted and fully untwisted at 6.8% elongation and finally break at 7.4% elongation. Besides that helical (spiral) structure has its own role in the elongation process. It was observed that there is a reduction in strain of fibrils due to helix angle by cos2 θ and at the same time, it contributes to stress by cos2 θ.
Hearle (2007) concludes the whole discussion and put forward that due to complex nature of cotton fiber, variation in shape, internal structure mechanism, it is difficult to have a perfect mathematical model. However,
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model were developed to explain the changes up to a reasonable and reliable manner.
Figure 17 (a) Structural features of cotton, which determine the tensile properties. (b) Stress-strain plots. Line A is for extension of crystal lattice. Lowered to B due to helical structure, to C by untwisting of reversals and to 0 by pulling out convolutions. The dotted lines are for wet cotton. From Hearle (1991).
It is obvious from the Figure 17 that there is an increase in the strength after getting high moisture. This is mainly due to that fact that moisture reduces the internal stress by making hydrogen weak and the fiber structure becomes more oriented and well-defined and an increase in crystalline structure. All these factors ameliorate the strength of cotton fiber on increase of moisture.
6.7 Electrical Properties Static electrification and electric conductivity are two note worthy features of cotton. Cotton shows a remaarkable electric conductivity. However, it becomes very low, when moisture percentage become negligible. Due to
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high electric conductivity, cotton dissipate charge quickly. Hearle (2007) summarized the findings of Hearle (1953d). Hearle (1953d), views that in case of cotton, since it is a moisture-absorbing fiber, movement of charged ions are responsible for conductivity. This is further improved due to the presence of salt, which is a natural ingredient present in the cotton fiber. The salt increase the dissociation with increase in dielectric constant. This ultimately increase the charged ions to transport the electrons. Hearle (1953d) developed the following relation between electric resistance R and dielectric constant K:
log R = a/K + b
Where: a = proportional to the energy of dissociation with K and it is inversely proportional to temperature, b depends upon concentration and valency of available ions.
Hearle (2007) further reports that under normal condition measurement of dielectric properties are not easily doable without having air inside the fiber, which is present in the fiber assembly due to the empty sapce. Results of a cone having packing factor 45% show that there at lower frequencies, there is a rapid rise in dielectric constant, which is also named as relative permittivity, and power factor with humidity. There is linear decrease in log Rs with a RH from resistance of 1011 ohm g/cm2 at 10% RH to 106 ohm g/ cm2 at 80% RH.
As discussed above, presence of salts in one the reason of conductivity. Considering this observation, it is presumed that by removing the salts, there will be an increase of resistance. Hearle (2007) reports that there is a five times increase in resistance of cotton fiber after removal of salt.
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6.8 Optical Properties There is strong relationship between optical properties and electrical properties at very high frequencies. Refractive index, which is a measure of polarizability, depicts the optical properties. Polarizability is highly linked with the orientation of polarizable groups. In case of cotton, these are -OH groups (Hearle 2007).
Meredith (1946) as quoted by the Hearle (2007) calculated the difference between two refractive indexes; axial and transverse, which is called birefringence. These values prove that there is a decrease in birefringence, with the increase in spiral angle. Hearle (2007) submits the findings of Adderley (1924) and explains that luster of cotton is highly depended on the ellipticity (the degree of deviation from circularity), a/b,where a and b are equational and polar fiber x-section radii (plural form of radius) respectively. After examination of 10 different samples Adderley (1924) posit that no firm correlation was found for intermediate values. Nevertheless the highest value is for mercerized cotton which is 13.9, whereas arbitrary value is 5.7 for American cotton.
6.9 Friction Friction of cotton depicts the state of the cotton fiber surface. Wax is present during the growth of cotton. It is removed in most of the cases during wet processing. Nevertheless, there are softening agents available which are added during finishing process. These finishes improvers the smoothness and reduces the friction. The surface area of the cotton fiber is not like a plain surface. It is tapered and have some sort of roughens. Friction is different in different parts of the fiber and direction of the movement of the other object is also creates the difference. Thereby, normal laws of friction cannot be applied. In addition, there is a difference in friction with the speed and direction of rubbing (Hearle, 2007).
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Morrow (1931) reported (as cited by Hearle, 2007) that value of friction of cotton with steel changes from 0.24 at 0% moisture to 0.36 at 11% moisture. These results leads toward the theory that provides sufficient information of change in friction due to moisture, since moisture provides more intimacy. It was further concluded that friction force is proportional to shear strength * area of contact. Finally it can said that coefficient of friction of cotton is highly associated with the state of the fiber.
6.10 Fineness of Cotton Fiber Thickness of fiber stand for the distinct properties which fiber holds. There is a wide variation in the value of thickness. It varies from 1 to 100 µm. What is more, it is quite difficult to measure exact thickness of the fiber due to variation. Negligible variation in fiber uniformity, and cross-sectional area is sufficient to create hinders in the working of microscope (Fritz, 2008).
6.11 Strength of Cotton Fiber Exact measurement of cross-sectional of cotton fiber is challenging. It is primarily due to non-uniformity present in the structure of fiber. It becomes more crucial in case of natural fibers. To have a better idea about the strength of the fibers, it is not measured in giga Pascal (SI unit of pressure), whereas, it is expressed relative to fiber fineness. As a convention, maximum tensile force (force relating to tension) employed at break is taken centi newtons per tex (cN/tex). There are many more units are available. Nevertheless all depicts a relationship between tensile and fineness of the fiber (Fritz, 2008). It is further viewed by Fritz (2008, that textile and industrial fibers have tenacity ranging from 10 to 300 cN/tex. In case of cotton its range is 25-50 cN/tex. It is higher than wool, regenerated fibers, and jute. Nonetheless, it is less than majority of synthetic fibers and silk, flax (see Table 4 in appendix).
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Cotton fiber strength is embedded in its structure, which is composed of amorphous and crystalline regions. The main difference in amorphous and crystalline regions is the rigidity or packing of cellulose which are have inter molecule and intra-molecule bonds. Thanks to the latest instruments like, high volume instruments (HIV) and other instruments which helps a lot in measuring the fiber strength. There is a monumental change during the cotton fiber growth process. It starts from the primary wall and ends with rigid secondary wall and it becomes more dense when all fluids are taken away. It is obvious that strength will also keep on changing during this process. Here our concern is the strength of a mature cotton fiber and ways to measure it. We will further discuss the factors affecting its strength. This all will enrich our understanding about the cotton properties.
There is a steady increase in the Stelometer bundle strength of fiber between 30 and 70 dap. Kulshreshtha et al. (1973a) as cited by Hsieh (2007). There are a number of issues related to single fiber strength. Hsieh (2007) reviewed the findings of Hsieh (1994) and Hsieh et al.1995) who provide a detailed protocol for the testing of single fiber strength. It includes selection of fiber which should be able to represent population and testing procedures.
Hsieh (2007) prefers tensile tester (1122 TM) or Mantis single fiber tester to measure single fiber strength. There is a insignificant difference in the results from both instruments. Data shows that there is visible difference in the force required to break between 20 to 30 dap. This study also confirms that breaking elongation values are much higher for dried fiber as compared to hydrated fiber. This change is due to the change in the twist and convolution angles, which is result of dehydration. Nevertheless there is a decreasing trend in the strength this reported in later days; 50-70 dap (Hsieh, 2007).
Fibers from different parts of the cotton seed have also a variation in the strength. Hsieh (2007) divides the fiber source regarding cotton seed or ovule surface into three categories; chalazal, micropylar regions and the
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medial and measured different values related to strength and elongation. There is a remarkable difference in the strength of fibers from different places. This difference is attributed to the shape and density of the fiber. It is obvious from the Table 05.
Table 05 Properties of plant-matured fibers* from three seed locations of G. Hirsute (Maxxa) Properties/seed location Force to break (g) CV(%)
Chalazal
Medial
Micropylar
3.13 24
5.47 19
6.16 13
Breaking elongation 7.4 (%) CV(%) 16
7.6
7.6
10
10
Work to Break (µJ) 4.2
6.9
7.5
CV(%)
23
15
Linear Density (tex) 0.132
0.197
0.259
CV(%)
13
10
9
Tenacity (g/tex)
23.7
27.8
23.9
CV(%)
21
16
14
Ribbon Width
13.9
15.6
16.2
2
3
4
33
(µm) CV(%)
Considering all above results Hsieh (2007) concludes that there is a remarkable difference in the properties of fibers taken from different portions of the ovule. It indicates the difference in the maturity, density and inter molecular and intra-molecular bonding of the fibers.
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There are a number of cotton variety in the world and every year we find new varieties, which are better in performance and have more resistance to pest attacks. It is found that there is a significant variation in the breaking forces and elongation properties of different varieties. The pattern is highly depended upon the genotypes. It is further revealed from the analysis that with in each cultivar there is a lack of correlation between length of fiber and breaking and elongation forces. Nevertheless there is a significant difference across the five varieties under examination (Liu et al., 2005). It was also observed by Liu et al. (2005) that there is a positive correlation between single fiber breaking force and the length of the fiber in all five varieties (r=0.259 to 0.443). All above study concludes that primary cell wall is a major contributor (up to two-third) of fiber strength. It is also supported by the fact, which we observed during wet processing that by having severe scouring and bleaching with hypochlorites reduces the strength since it attacks the primary wall.
7.1 Cotton and Wearing Comfort It is an instinct demand of the end user of fabric to have a comfort. We can divide comfort into two categories; psychological and physiological. Comfort means, absence of discomfort. Starting from the fiber structure, yarn and fabric parameters, application of finishing chemicals and way it has been processes, all have a valid link wit the final comfort level of the fabric. At fiber stage, it depends upon the moisture absorbency, thermal capacity, bending rigidity, surface smoothness, strength of the fiber, change in high temperature etc (Fritz, 2008).
Keeping all above in view cotton posses properties which provides highest level of comfort to the end-users. This is one of the reasons that cotton is highly favored and much liked by the people. Smooth hand feel, high moisture absorbency, reasonable strength, ability to adapt the shape, thermal capacity have made cotton an outstanding fiber among the natural and manmade fibers.
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7.2 Utilization of Cotton Fiber Cotton has a diverse uses in all shapes; fiber, yarn, fabric and clothing. The major use of cotton fiber is to make yarn of different counts and different types. Notwithstanding, there are many uses of cotton as fiber. It is used to make surgical cotton, filters, pads, quilts, pillows, cushions, seats, etc. In summary, where we need some compressibility, moisture absorbency and comfort, we can use cotton fiber.
Cotton yarn is mainly used to make fabric. However, there are many uses of yarn as it is. For example, it is used to make ropes, nets, mesh, etc. Not all that, the major use of yarn is to make fabric. Fabric has a long list of applications. Major consumption of fabric is to make clothing and home textile. However, there are many technical products which are also made by using cotton fabric or we can say that cotton yarn is used to develop technical textiles.
Cotton is versatile in nature and could give required results. Its blending with polyester, wool, silk, and many more fibers. Yarn is made from these blends and is used for various purposes. The blended yarn serves multiple purposes.
8.1 Summary Plants are consist of cellulose and the ratio of cellulose is different from plant to plant. Cotton is unique in nature which has the highest percentage of cellulose. It varies from 90% to 95%. This concentration depends upon many factors, such that maturity level and variety of cotton, place on cotton seed from where it is drawn. There are always non-cellulosic matter attached with the cellulose. In cotton fiber there are certain non-cellulosic matter, which plays important role in growth and on forth coming processes. These are located either on the surface of the fiber or in center of fiber,
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means in side the lumen. Presence and percentage of different non-cellulosic material depends upon growing conditions. Being a natural fiber, there are many factors which affect the structure of cotton cellulose. These may be type of variety, environment where it is grown, level of fiber maturity, etc. There are primary and secondary walls in the cotton cellulose. Primary wall, or outer wall is less crystalline and has less percentage of cellulose. Whereas, secondary wall is around the lumen and consist of pure cellulose.
Cotton fiber is composed of the crystalline and amorphous structure. Both have their distinct nature of job during on going process. There are cellulose fibrils which spirals so that they have a decreasing angle directed towards the center. At the same time there is a spiral reverse which is found along the fiber axis. These spiral structures give twist during dehydration process and finally we see a twisted ribbon like fiber.
Β-1-4-D glucopyranose is the basic building block of the cotton cellulose. It is repeated thousands of times. This long chain is restricted or have limited rotational liberty at the anhydrogluco-pyranose C-O-C link. This link makes the fiber more rigid and less flexible. Rigidity is further attributed by the highly crystalline structure which is the result of unique structure of the cotton fiber.
There are many hydroxyl groups present at different points. These hydroxyl groups are prone to hydrogen bonds. These hydrogen bonds increase the structure stability moreover are the root cause of creasing effect on cotton fabric. Besides, these hydroxyl groups acts as reaction points for dyes and other chemicals. Reactive dyes one example which react with these -OH groups and form covalent bonds, which keeps dye attached with the cotton fiber. These -OH groups are also used to modify the cotton cellulose.
Three physical parameters of cotton fibers; fineness, length and strength are depended on the structure, crystallinity, cellulosic percentage, maturity level, secondary wall formation and genetic of the cotton seed. Besides,
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growing conditions and availability of different fertilizers have also an impact on the fiber parameters.
All above three properties have different values at early stages and there is a constant change, not linear all the time, in values during the growth process. Nevertheless in 30 dpa maximum strength is gained and during the rest of the period fiber development is mainly towards the wall thickness.
There is clear presence of cellulose I after 21 dpa and after that there is no change in forth coming period. There is regular increase in the crystallinity of the fibers during the fiber development stage. Also there is regular increase in the cellulosic material, which is less in start and become more in percentage after the fully development. However, non-cellulosic material, which is in high percentage during early days decreases in percentage. Apart from this, crystalline area goes on increasing during the fiber development process and it touches its peak after desiccation. A significant positive correlation is present is between the crystallinity and the single fiber breaking force.
Cotton fiber has a number of uses in shape of fiber, yarn and fabric. It is estimated that cotton will not lose its attractiveness even there is lot of development in man-made fibers which have great resemblance with cotton fiber.
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