New Values in Mercerizing Cotton

Charles F. Goldthwait.

Mercerization gives cotton a high luster, makes it easier to dye, and produces brighter and fuller shades. Although it tends also to increase strength, it is essentially a process to improve the appearance of cotton.

Mercerization under somewhat unusual and controlled conditions can bring out properties in cotton not yet developed to any extent commercially. Some of these greater retention of high strength under conditions of dryness, improved resistance to high temperature or long heating, increased flex life, and unusual elastic properties, for example will better adapt cotton for knitted and woven goods in which such properties are especially important.

Two types of cotton products, each developed far enough so that patents have been granted, exemplify the new possibilities of mercerization. One of them utilizes the added heat resistance of treated yarn; it is illustrated by its application in tire cord. The other utilizes the special elastic properties of treated cloth, illustrated by a semielastic surgical gauze.

The process of mercerization was discovered about a hundred years ago by John Mercer when he was filtering a caustic soda solution through cotton cloth. He observed that the cloth shrank and puckered and acquired a closed, or fulled, appearance. His idea that he had a fulled cotton came from a similarity to the hardening, or felting, of some wool goods by fulling. Mercer obtained a patent, but he could not follow up his invention because caustic soda had not then been made commercially. There have since been numerous efforts to make a woollike cotton by such processes; and, while there has been no great success, the possibility is still intriguing. About 40 years later the method of producing luster as an effect of tension was disclosed in a patent issued to Horace Lowe. Mercer never saw any of the lustrous cotton because it was not made until 25 years after his death.

While the cotton-finishing trade has associated the term "mercerization" almost exclusively with luster, the name has come into general use as meaning any kind of treatment of cotton with caustic strong enough to swell or shrink it a good enough usage, because that was the original meaning before luster was first observed.

The distinctive feature of ordinary mercerization is the application of tension to keep the cotton from shrinking. Caustic alkali causes cotton fibers to swell greatly if unrestrained and to shrink in length. If the alkali-swollen and shrunken fibers are constituent parts of yarn or cloth, the swelling causes the structures to shrink, too. But if the material is stretched back to its original dimensions by tension, the fibers become smooth; if the alkali is washed out while the material is still under tension, the fibers become lustrous.

The effect is also about the same if the yarn or cloth is held at its original length and prevented from shrinking during the treatment. The swelling of the restrained fibers develops high tension within the goods, and they are actually mercerized with tension, although no stretching force is applied.

Mercerization without tension--that is, with more or less shrinkage has been known longest. Although relatively little has been done with it in a practical way, it has possibilities that seem not to be appreciated fully. Cotton mercerized without tension differs from cotton as ordinarily mercerized (with tension) in that it has no appreciable added luster and undergoes no noticeable net change of strength, although it yields a much darker color than ordinary mercerized cotton when the two are dyed at the same time in the same bath. Yarn and cloth mercerized without tension may also assume new properties or may display old properties to a greater degree if the yarn and fabric are suitably selected for the treatment and the process is properly carried out.

THE MAIN CLUE leading to the study of mercerized cotton for greater heat resistance was the claim that rayon tire cord is more resistant to heat than cotton cord. Because the usual regenerated type of rayon consists of mercerized cellulose (the same type of cellulose that results from the mercerization of cotton) , it was natural to test the heat resistance of the latter. Two other discoveries had claimed heat resistance for mercerized cotton, but presented rather indefinite evidence because their data had to do with breaking while dry after a moderate amount of heating, rather than with severe tests to heat.

Because the main interest at the time was tire cord, most of the work was done on yarn and cord. It was found that several changes from the ordinary yarn-mercerizing procedure would improve the strength when the yarns, or the cords made from them, were evaluated by heat tests intended to predict the behavior of the cords in tire service. Specifically, low yarn twist favors a high degree of swelling and complete mercerization; for best results the yarn must be allowed to shrink while in the caustic soda. Finally, traces of acid must not be present.

In any mercerization it is difficult to remove the caustic soda by simple washing with water, so the last of the caustic is frequently removed by treating the goods with dilute sulfuric acid. The acid, in turn, is difficult to wash out; if ordinary raw (gray) cotton is being mercerized, the "souring" with acid means that some of the noncellulose constituents (usually considered impurities) are left in the cotton as insoluble, or difficultly soluble, acids. The acids apparently derive from the consecutive actions of the caustic and the sulfuric acid. Consequently, the use of acid is avoided, either by thorough washing or by using a small amount of sodium bicarbonate, which will neutralize either acid or alkali, so that there will be no acid residues in the yarn to cause degradation during tests or while in service.

The effects of mercerization and of variations in the process are shown by the results of tests for strength after heating for 15 days at 130 C. (266 F.) in an oven with air present, in comparison with similar unmercerized samples. Mercerized yarn neutralized with sulfuric acid retained 46 percent of its strength; that neutralized with sodium bicarbonate, 59 percent; and that neutralized with various alkalies, 58 to 70 percent. Untreated gray yarn retained 45 percent and 41 to 45 percent when small amounts of alkali - were present.

The specially mercerized yarn, if neutralized with sulfuric acid, had no greater heat resistance than the ordinary gray yarn. Various treatments with alkalies, which left small amounts in the yarn, made no improvement in the unmercerized, but caused appreciably better retention of strength in the mercerized.

Although the type of test made on yarn heating with full exposure to the atmosphere is of less interest for tire cord, which is used tightly enclosed in rubber with relatively little chance for oxidation by the air, similar tests were made on the tire cord, subject to interpretation for other uses. Two commercial-type unmercerized cotton tire cords retained 51 to 62 percent strength, while the special mercerized cord retained 70 percent, after the 15 days of heating. The mercerized cord still had half its strength after 32 days, while the regular-type cords were down to 40 percent. The mercerized cord thus had one-fourth to one-third greater strength after exposure to heat, an increase which might be very significant for some uses of cord, as for reinforcement for conveyor or driving belts subject to unusual exposure to heat.