Mercerization, cotton fibers

An important chemical finishing process for cotton fabrics is that of mercerization, which improves strength, luster, and dye receptivity. Mercerization iavolves brief exposure of the fabric under tension to concentrated (20—25 wt %) NaOH solution (14). In this treatment, the cotton fibers become more circular ia cross-section and smoother ia surface appearance, which iacreases their luster. At the molecular level, mercerization causes a decrease ia the degree of crystallinity and a transformation of the cellulose crystal form. These fine stmctural changes iacrease the moisture and dye absorption properties of the fiber. Biopolishing is a relatively new treatment of cotton fabrics, involving ceUulase enzymes, to produce special surface effects (15). 

An older method of cellulose fiber modification is mercerization [22,33-36], which has been widely used on cotton textiles. Mercerization is an alkali treatment of cellulose fibers. It depends on the type and concentration of the alkalic solution, its temperature, time of treatment, tension of the material, and the additives used [33,36]. At present there is a tendency to use mercerization for natural fibers as well. Optimal conditions of mercerization ensure the improvement of the tensile properties [33-35,37] and absorption characteristics [33-35], which are important in the composing process. 

Cotton treated in this way is almost pure cellulose, and su Is in greal demand by those industries using cellulose. The pure cellulose of the fiber may be dissolved and then precipitated in sheets, giving the familiar thin transparent cellophane. Or the dissolved cellulose may be pressed through fine holes and solidified, giving rayon. If treated wilh concentrated causlic soda, cotton fibers lake on a high degree of luslcr. The product of this process is called mercerized collon, after John Mercer, its discoverer. 

As far as we know, apart from a preliminary report about mercerization of cotton fibers (3) and a recent paper about chemically etched PETP filaments (4), high modulus aramide fibers and carbon fibers are practically the only systems with detailed (recently) published work on electron diffraction and dark field of ultrathin sections of textile or paratextile fibers (5, 6,... 

Cotton is seed hairs that protect the seeds in their pods from desiccation in the dry season and from moisture in the rainy season. Under the microscope, cotton appears as a collapsed, tubelike fiber twisted at irregular intervals. In its natural state it repels water because of a waxy substance covering the fiber. Once the wax has been removed, cotton becomes the most absorbent fiber known to man. Cotton is cellulose it withstands moderate alkaline conditions but is easily affected by acids. Because of twist in the fiber, cotton lacks glossiness. Mercerization, a chemical process, gives luster to cotton fiber by straightening the twist. 

Treatments of cotton fiber often alter the structure at more than one level. Therefore, it is not always obvious that the effect of the treatment should correlate with a change at a particular level of structure. For example, cotton is often treated with NaOH in a process called mercerization. This treatment can alter the crystal structure, but it also reduces the density of the whole fiber and increases its luster. For example, the change in crystal structure is probably not directly responsible for the change in fiber luster. 

During mercerization, the swelling induced by the caustic is inhibited from outward expansion by the presence of the primary wall of the cotton fiber. The changes observed in fiber morphology by mercerization include deconvolution, decrease in the size of the lumen, and a more circular cross section. 

The structure has primary and secondary alcohol groups uniformly interspersed throughout the length of the polymer chain. These hydroxy units impart high water absorption characteristics to the fiber and can act as reactive sites. The cotton fibers are hydrophilic and swell in water. It is hydrolyzed by hot acid and swollen by concentrated alkali. The cotton is treated with caustic soda solution (12 to 25 percent) under tension to develop a silk-luster and stop longitudinal shrinkage. This process is called mercerization. Mercerized cotton exhibits increased moisture and dye absorption. 

Figure 4. The effect ofNaOH mercerization on cotton fiber of the acala variety.

Use In the manufacture of paper, vulcanized fiber, mercerized cotton, viscose rayon. 

Mercerization. The purified cotton fibers were mercerized in 20 sodium hydroxide aqueous solution for 5b at 20 C. Then the samples were washed with distilled water, immersed in 0.1 acetic acid, washed with distilled water until acetic acid was throughly removed and then air-dried. 

Celluloses can be converted to other useful products by reorientation of their fiber structure. Paper, parchment paper, vulcan fiber, mercerized cotton, and hydrocelluloses belong in this class. 

For over a decade NIR quality control has been used in the textile and fiber industries. Perhaps the widest use of NIR has been in the cotton industry. Cotton blending, mercerization, and fiber maturity measurement have been used offline for rapid process control. The wool industry has also employed NIR to measure the residual grease after scouring. Calibrations have been developed for measuring moisture and heat set temperature in nylon yam. Online NIR analysis is being developed for the quality control of the dyeing procedure for carpet yam and for measuring yarn diameter. 

Mercerized cotton has a different structure. With the use of a strong alkaline process, cotton fibers swell. The cross-section of mercerized cotton is round, like the premature cotton, and untwisted. 

E.T.N. Bisanda, and M.P. Ansell, The effect of silane treatment on the mechanical and physical properties of sisal-epoxy composites. Compos. Sci. Technol. 41,165-178(1991). S.H. Zeronian, H. Kawabata, and K.W. Alger, Factors affecting the tensile properties of nonmercerized and mercerized cotton fibers. Text. Res. Inst. 60,179-183 (1990). 

Typical one-step commercial mercerization of cotton yam with caustic or liquid ammonia causes only partial conversion to cellulose II or cellulose III. Cotton cellulose is partially converted to cellulose II by repeated mercerization, the swelling of cellulose in strong alkali (eg, 23% NaOH), followed by rinsing and drying. Cellulose III results from treatment of cellulose with liquid ammonia (ammonia mercerization) or amines. Cellulose III can be made from either cellulose I or II. When treated with water, cellulose III can revert to its parent stmcture. Cellulose IV can be prepared by treating cellulose I, II, or III in glycerol at temperatures 260°C. Conversion of the crystal form in cotton fibers to cellulose IV can be effected by heat treatment of ethylamine-treated cotton cellulose in either saturated steam or formamide with minimal fiber degradation (86). Like cellulose III, cellulose IV preparations can revert to their parent stmctures. 

Comparative properties of cellulose nano-ciystals from native and mercerized cotton fibers. Cellulose, 19,1173-1187. 

It was later determined that the crystallographic form cellulose I was irreversibly chang to the form cellulose II. The unit cell of cellulose II also contained two cellobiose units with axes a = 0.814 nm, b = 1.03 nm, c = 0.914 nm, and fi = 62° (angle between a and c axes). The fibrillar cellulose II is less ordered than fibrillar cellulose I and is about 60% crystalline. This 1840s development, now called mercerization, is probably the most important process for the modification of natural cellulose, particularly cotton cellulosic textile fibers, to current date. Lowe later confirmed these results and showed the effects of mercerization on cotton fibers while under tension. 

The largest use of zinc chloride in the United States is in wood preservation, fluxes, and batteries (see Batteries). Zinc chloride solution dissolves vegetable fiber and is widely used in mercerizing cotton (qv), swelling fibers, as a mordant in dyeing, parchmentizing paper, etc (see Fibers, vegetable ... 

Viscose Also known as the Cross-Bevan-Beadle process. A process for making regenerated cellulose fibers. The product has been known by the generic name rayon since 1924. Cellulose, from cotton or wood, is first reacted with sodium hydroxide ( mercerization), yielding alkali cellulose. This is dissolved in carbon disulfide, yielding cellulose xanthate, which is dissolved in sodium hydroxide solution. Injection of this solution (known as viscose... 

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