Chemically Modified Cotton

J. David Reid, James D. Dean.

For all its usefulness, cotton cannot compete with other fibers in certain respects. It is not so flame-resistant, soil-resistant, or resilient as wool. It resists mildewing and rotting less well than cellulose acetate rayon. It is weaker and less elastic than silk or nylon. It is less water-absorbent than flax or ramie.

Plant breeders and growers have made some changes in the qualities of the cotton fiber, but its essential composition remains unchanged. It is the chemist's task to modify the cotton to make it more suitable for a particular use. Like many synthetic fibers, cotton fiber must be tailor-made for its job.

Chemical modification of cotton is sometimes difficult. Cotton is so sensitive to many chemical treatments that the fiber may lose strength. Occasionally, fiber characteristics are markedly changed by modifications so slight as to be almost undetected by analytical methods; again, the modification required to change qualities to the desired extent is great.

By chemical modification of cotton is meant the transformation of all or part of its cellulosic material into another chemical compound, which retains the fibrous character of the cotton. This modification differs from the chemical modification of purified cellulose made from cotton linters, wood, or fibrous agricultural wastes to obtain a chemical compound that can be dissolved and then spun or cast into materials like rayon or transparent sheeting. Chemical modification of cotton is also in contrast to that type of cloth finishing in which an inert finishing agent is added physically, rather than chemically, to a fabric.

We have to understand the nature of cellulose in order to bring about the precise degree of modification demanded for a special use.

Cellulose resembles alcohols and sugars in chemical structure indeed, it is composed of long chains of glucose residues layered together. Much of the fundamental chemistry of cellulose remains to be explored, however. A great deal of chemical work has been done in practical research (to obtain cotton with certain desirable characteristics) and in fundamental research (to determine the nature of the changes). Often the fundamental work precedes the practical. Fundamental research involves a search for truth, often with no immediate practical application in mind. It is concerned with such things as molecular structure, methods of carrying out reactions, and instruments or methods for complex measurements in general, with problems not necessarily of monetary value. Fortunately, once such truths become evident, practical use may be made of many of them. Practical research alone, without insight into reaction mechanisms, is likely to be extremely limited in scope almost "trial and error" work. Fundamental work points the way to a wider scope; it is generally true that fundamental research leads to broader applications of a method or to entirely new uses.

THE REACTION OF CELLULOSE with formaldehyde is interesting because the addition of amounts as small as a few tenths of 1 percent changes the fibers greatly. The reaction has limited practical application now, but it encourages the chemist to continue his search for reactions that may yield fibers of greater usefulness.

The formaldehyde-treated fibers become more brittle, will not dye with many of the direct cotton dyes, and will not dissolve in cuprammonium hydroxide solution. Those properties indicate the possible formation of cross-linked methylene ethers of the cellulose apparently the formaldehyde has linked together the long chains that make up the cotton fiber. If the fiber is visualized as being made up of tiny glucose residues and the relative sizes of building units to fiber are calculated, only about 0.05 percent of the units are on the surface of an ideally uniform fiber. A. C. Walker, of the Bell Telephone Laboratories, however, has estimated that to cover the many internal surfaces between the fibrils making,up the fiber about 1 percent of water would be required to form a mono-molecular film. Apparently, in the case of formaldehyde, it is only necessary to change those readily available surface units to change greatly the characteristics of the fiber.

Formaldehyde alone is not of great practical importance as a treating agent for textiles, but resins formed by condensing melamine or urea with formaldehyde have been widely used in cloth finishing; an estimated 40 million pounds was used in 1949. The process involves the application of a water solution or emulsion of the material to the fabric, followed by polymerization, or insolubilization, in place, with heat. This type of finish, originated by an English concern, is used to make cloth creaseproof or wrinkle-proof. Another commercial concern has modified this method to give a "permanently glazed" chintz by using the resin and following it by friction calendering, or pressing, to obtain the glaze.

For apparel fabrics, the treatments have so far proved more applicable to rayon and wool than to cotton; cotton loses some tensile strength after treatment. The technique of resin application to fabrics, however, is being steadily improved. The simple melamine, for example, is being replaced by a more effective methylated methylol compound, and a thermoplastic vinyl resin is being combined with the formaldehyde condensates to give stronger creaseproof cotton products. The first objective of such research is the production of cotton apparel fabrics that resist creasing and wrinkling. Present progress indicates the near availability of resin-finished goods having a resilience that will not be removed by laundering.

Chemists differ as to whether the changes in fabric properties caused by the action of the resins are physical or chemical. Filling the vacant spaces in the fiber with the insoluble resin undoubtedly makes a great deal of difference in its properties, and many maintain that that is the whole story. On the other hand, because formaldehyde alone has such a great effect, it is possible that the formaldehyde component of the resin reacts with the cellulose to change its character. In this field of study, the practical developments have far outdistanced the fundamental theoretical work. Full understanding of the physical and chemical effects of such resin applications to cotton should be followed by the appearance of cool summer-wear cotton suiting that is the equal of wool in nonsoiling and wrinkle-free properties.

W. O. KENYON, of the Eastman Kodak Research Laboratories, in 1936, observed that gaseous nitrogen dioxide would oxidize cotton to a product that retained its fibrous form and a reasonable amount of tensile strength, although . it was soluble in weak alkaline solutions. It would even disintegrate in 1 percent sodium bicarbonate solution because of the presence of carboxyl groups in the cellulose chain. This apparent fault was turned to advantage by other workers, who found that the material could be absorbed by the body and could be implanted n body tissues to avoid adhesions. As a dressing for wounds it could carry thrombin, a natural blood-coagulating agent, and was itself able to prevent the flow of blood. The material now is manufactured and sold as a medical specialty. It is often packed into a wound and left there. The body absorbs it in 7 to 20 days.

The characteristic solubility of the oxidized cellulose fiber in alkalies has b( en utilized in studies on the structure of the cotton fiber by microscopists of the Department of Agriculture, who found that the gaseous oxidation made the fiber capable of solution but that it did not destroy the physical form. As the fiber swelled and successive layers dissolved under the microscope, it was possible to study the details of its structure.

FLAMEPROOFED CLOTH is generally understood to mean a cloth that will not transmit flame across its surface after the igniting source has been removed. Flameproofed cloth is important in wartime. It has been less used in peacetime, although serious accidents to children, fire in public gathering places, and similar accidents draw attention to the dangers of such materials as flammable play clothes and draperies. Restrictive legislation is increasing with regard to flammable fabrics, particularly those used as draperies in public places.

Flameproofing is often done by home methods, but such treatment is generally temporary in that it is removed by laundering. The cloth is saturated with solutions of certain water-soluble salts and then dried. One recommended method uses 7 parts of borax to 3 parts of boric acid, the whole diluted with 64 parts of water. In 1946 a more permanent type was developed at the Department of Agriculture by two chemists, Kenneth S. Campbell and Jack E. Sands. The fabric is padded with an emulsion incorporating a chlorinated hydrocarbon wax, a water-soluble urea-formaldehyde resin, and antimony oxide, and cured at a high temperature. Because of the technique and equipment necessary, the treatment is suitable for commercial application alone.

Only two of the known methods of commercial flameproofing are considered to be chemical modifications of the cellulose. One of the methods uses a complex mixture of titanium and antimony chloride compounds.

When it is soaked in a solution of the mixture and then made alkaline and washed, cloth becomes resistant to burning. The resistance remains through numerous launderings. In this case, practical application has preceded theoretical knowledge. It remains for fundamental research to determine whether a compound of cellulose has been formed and, if so, what further variations and applications of the method may be made.

The second method of commercial flameproofing is known to modify chemically the cellulose of the cotton fiber. The process involves esterification a process of combination of the cellulose with phosphoric acid. Although treatment with strong acids generally causes a great loss of strength in cotton cloth, the industry has developed a method that ingeniously avoids much of this degradation. Cloth is impregnated with a solution of the urea salt of phosphoric acid, dried, then heated for a few minutes between 300 and 350 F. The high temperature causes esterification and at the same time breaks down the urea to give ammonia, which counteracts the degradative effect of the acid and simultaneously forms the ammonium salt of the newly formed cellulose phosphate. An average substitution of about 1 phosphate group in each 6 of the glucose anhydride groups of which the cotton fiber is composed gives a satisfactory protective action when the material is in the form of the ammonium salt.