What is Cellulosic Fibers? Physical-Chemical-Structural Properties of Cotton ?

Cellulosic Fibers:

Cellulose is a fibrous material of plant and the basis of all natural and man-made cellulosic fibers. The natural cellulosic fibers include cotton, flax, hemp, jute, and ramie. The major man-made cellulosic fiber is rayon, a fiber produced by regeneration of dissolved forms of cellulose. The cellulose acetates are organic esters of cellulose.
Cellulose is a polymeric sugar (polysaccharide) made up of repeating 1,4-8-anhydroglucose units connected to each other by 8-ether linkages. The number of repeating units in cellulosic fibers can vary from less than 1000 to as many as 18,000, depending on the fiber source. Cellulose is ,a hemiacetal and hydrolyzes in dilute acid solutions to form glucose, a simple sugar.

The long 1inear chains of cellulose permit the hydroxyl functional groups on each anhydrogl ucose unit to interact with hydroxyl groups on adjacent chains through hydrogen bonding and van der Waal s forces. These strong intermolecular forces between chains, coupled with the high linear• ity of the cellulose molecule, account for the crystalline nature of cellu• losic fibers. It is believed that a gradual transition from alternating areas of greater molecular alignment or crystallinity to more disordered or amorphous areas occurs in cellulose. The number, size, and arrangement of crystalline regions within celluloses determine the ultimate properties of a particular fiber.

CELLULOSE:

The predominant reactive groups within cellulose are the primary and secondary hydroxyl functional groups. Each repeating an hydroglucose unit contains one primary and two secondary hydroxyl functional groups which are capable of undergoing characteristic chemical reactions of hydroxyl groups. The primary hydroxyls are more accessible and reactive than secondary hydroxyls; nevertheless, both types enter into many of the chemical reactions characteristic of cellulose.

COTTON:

Cotton is the most important of the natural cellulosic fibers. It still accounts for about 50 % of the total fiber production of the world, although man-made fibers have made significant inroads into cotton's share during the last three decades. Cotton fibers grow in the seed hair pod (boll) of cotton plants grown and cultivated in warm climates.

Structural Properties :

Cotton is very nearly pure cellulose. As many as 10,000 repeating an• hydroglucose units are found in the polymeric cellulosic chains of cotton. Studies have shown that all of the hydroxyl hydrogens in cotton are hydro• gen bonded. These hydrogen bonds will hold several adjacent cellulose chains in close alignment to form crystalline areas called microfibrils. These micro fibrils in turn align themselves with each other to form larger crystalline units called fibrils, which are visible under the electron microscope. In cotton the fibrils are laid down in a spiral fashion within the fiber. Modern fiber theory suggests that each cellulose molecule is present within two or more crystalline regions of cellulose will be held together. Between the crys ta 11 i ne regi ons in cotton, amorphous unordered regions are found. Voids, spaces, and irregularities in structure will occur in these amorphous areas, whereas the cellulose chains in crystalline regions will be tightly packed. Penetration of dyestuffs and chemicals occurs more readily in these amorphous regions. Approximately 70% of the cotton fiber is crystalline. Individual cotton fibers are ribbonlike structures of somewhat irregular diameter with periodic twists or convolu• tions along the length of the fiber (Figure 3-1). These characteristic convolutions, as well as the cross-sectional shape of cotton are caused by collapse of the mature fiber on drying.
Three basic areas exist within the cross section of a cotton fiber. The primary outer wall or cuticle of cotton is a protective tough shell for the fiber, whi Ie the secondary wall beneath the outer shell makes up the bulk of the fiber. The fibrils within the secondary wall are packed along• side each other al igned as spiral s running the length of the fiber. The lumen in the center of the fiber is a narrow canal-like structure running the length of the fiber. The lumen carries nutrients to the fiber during growth, but on maturity the fiber dries and the lumen collapses.

Physical Properties:

The high crystallinity and associative forces between chains in cotton result in a moderately strong fiber having a tenacity of 2-5 g/d (18-45 g/tex). The hydrophilic (water-attracting) nature of cotton and the effect of absorbed water on the hydrogen bonding within cotton cause the tensile strength of cotton to change significantly with changes in moisture con• tent. As a result, wet cotton is about 20% stronger than dry cotton. Cot• ton breaks at elongations of less than 10%, and the elastic recovery of cotton is only 75% after only 2% elongation.
Cotton is a relatively stiff fiber; however, wetting of the fiber with water plasticizes the cellulose structure, and the cotton becomes more pliable and soft. The resi 1iency of dry and wet cotton is poor, and many finishes have been developed to improve the wrinkle recovery characteristics of cotton.
Cotton is one of the more dense fibers and has a specific gravity of 1.54.

The hydroxyl groups of cotton possess great affinity for water, and the moisture regain of cotton is 7%-9% under standard conditions. At 100% relative humidity, cotton has moisture absorbency.

The heat conductivity of cotton i s high, and cotton fabrics feel cool to the touch. Cotton has excellent heat characteristics, and its physical properties are unchanged by heating at 120°C for moderate periods. The electrical resistivity of cotton is low at moderate relative humidities, and the fiber has low static electricity buildup characteristics.

Cotton is not dissolved by common organic solvents. Cotton is swollen slightly by water because of its hydrophilic nature, but it is soluble only in solvents capable of breaking down the associative forces within the crystalline areas of cotton . Aqueous cuprammonium hydroxide and cupriethylenediamine are such solvents.

Chemical Properties:

Cotton is hydrolyzed by hot dilute or cold concentrated acids to form hydrocellulose but is not affected by dilute acids near room temperature . Cotton has excellent resi stance to alkalies. Concentrated alkali solutions swell cotton, but the fiber is not damaged.

The swell ing of cotton by concentrated sodium hydro xide solution is used to chemically finish cotton by a technique called mercerization . In aqueous alkali, the cotton swells to form a more circular cross section and at the same time loses convolutions. If the cotton is held fast during swelling to prevent shrinkage, the cellulose fibers deform to give a fiber of smoother surface. After washing to remove a"lkali, followed by drying, the cotton fiber retain s a more cyl indrical shape and circular cross section. Although little chemical difference exists between mercerized and unmercerized cotton, mercerization does give a more reactive fiber with a higher regain and better dyeability.

Dilute solutions of oxidizing and reducing agents have little effect on cotton; however, appreciable attack by concentrated solutions of hydro• gen peroxide, sodium chlorite, and sodium hypochlorite is found.

Most insects do not attack cotton; however, silverfish will attack cotton in the presence of starch. A major problem with cotton results from fungi and bacteria being able to grow on cotton. Mildews feed on hot moist cotton fibers, causing rotting and weakening of the fibers. Characteristic odor and pigment staining of the cotton occurs when mildews attack. Addi• tives capable of protecting cotton are available and commercially applied to cotton fabrics used outdoors. These materials are often metal salts of organic compounds which are capable of inhibiting growth of mildews and similar organisms.



Cotton is only slowly attacked by sunlight, since cellulose lacks for the most part groups which absorb ultraviolet light between 300 and 400 nm. Over long periods sunlight degrades cotton, causing it to lose strength and to turn yellow. Certain vat dyes tend to accelerate the rate of cotton photodegradation through sensitization reaction called "phototendering."
Although cotton has excellent heat resistance, degradation due to oxi• dation becomes noticeable when cotton is heated in the air at I50°C for long periods. Spontaneous ignition and burning of cotton occurs at 390°C. At low humidities in the absence of heat and light, cotton will not deter• iorate over long periods of storage.

End-Use Properties:

The properties of cotton fiber are such that it serves as nature's utility fiber. Although cotton has some properties which are undesirable from the viewpoint of the consumer, the superior properties of cotton, coupled with its low cost, nevertheless make it a valuable fiber in many applications. Different species of cotton produce fibers of various average lengths. In the United States lengths of cotton staples are designated as follows:

1. Extra long staple 
2. Long staple 
3. Medium staple
4. Short staple

The two major types of American cotton are American-Upland (including deltapine and acala varieties) and American-Egyptian (including pima).Cotton has excellent hand, and the drapabil ity of cotton fabrics is qu ite acceptable. Fabri cs of cotton are of sat is factory appea ranee and have a low luster unless mercerized or resin finished.
The superior absorbency of cotton, coupled with its ability to desorb moisture, makes it a very comfortable fiber to wear. This absorbency per• mits cotton to be used in applications where moisture absorption is impor• tant, such as in sheets and towels. Mildewing of cotton under hot moist conditions and its slowness of drying are undesirable properties associated with its high affinity for water.
The cotton fiber has sufficient strength in the dry and wet states to make it suitab1e for most consumer textile appl ications. The increased strength of cotton on wetting adds to its long useful I ife. Cotton wears wel1 without undue abrasion, and pills do not tend to form as it wears. Cotton's low resil iency and poor recovery from deformation means that it wrink1es easily in both the dry and wet states and exhibits inferior crease retent ion. Sta rchi ng of cotton improves these properties, but the effect is only temporary, and it is necessary to renew this finish after each laundering.

The resistance of cotton to common household chemicals, sunlight, and heat makes it durable in most textile applications. Cotton can be dyed successfully by a wide variety of dyes, and the colorfastness of properly dyed cotton is satisfactory.

Fabrics of cotton are maintained with a moderate degree of care. Cot• ton fabrics launder readily, and its wet strength and alkali resistance mean that cotton is resistant to repeated washings. Stresses which occur during the spinning and weaving process will cause cotton fabrics to under• go relaxation shrinkage during initial launderings. Relaxation shrinkage can be controlled through resin stabilization or through the well known compression shrinkage process called Sanforization.

Cotton can be drip dried or tumbled dry, but in both cases the dry cotton will be severely wrinkled, and ironing will be necessary. Cotton can be ironed safely at temperatures as high as 205 °C. Cotton, as well as all cellulosic fibers, is highly flammable and continues to burn after removal from a flame. After extinguishing of the flame the cotton will continue to glow and oxidize by a smoldering process called afterglow. The Limiting Oxygen Index (LO!) of cotton is 18. A number of topical treat• men ts ha ve been deve loped to lower the fl ammab il ity of cot ton and other cellulosics.

The undesirable properties of cotton can be corrected to varying de• grees through treatment of the fiber with special finishes; however, the abrasion resistance of cotton is adversely affected . As a result, blends of cotton with the stronger man-made fibers have become important. Al• though man-made fibers have made inroads into appl ications previously re• served for cotton, cotton continues to be the major textile fiber due to its great versatility, availability, and cost.