How To Keep Dry Under Cotton

J. David Reid, Charles F. Goldthwait.

Cotton fabrics are the basis for many kinds of raincoats the once-popular oilskins, yellow slickers, and heavy rubber coats, and some of the newer plastic garments. Fabrics of those types resist water because of their construction; they are watertight but also airtight, so that the wearer may be almost as wet from sweat as he would be from rain without the coat. In attempts to improve cotton for water resistance, therefore, technicians have tried to develop fabrics that are porous to air but impermeable to water.

Cotton in the boll is not easily wet by the rain, because a tiny bit of wax on the surface of each fiber repels water and keeps the boll from becoming waterlogged and heavy, dragging on the ground, and meeting an untimely end through mildew and rot.

But this natural wax is not usually allowed to stay in the fiber after the cotton has been made into cloth. Without the wax, cotton becomes hydrophilic it shows a strong affinity for water and absorbs it readily. That trait is good in toweling, but far otherwise in products where resistance to water is essential. What is wanted is a means of making cotton hydrophobic that is, water-repellent.

Since 1870 or so, that has been possible through the application of water-repellent chemicals to the cotton fabrics. For a decade, Department of Agriculture chemists have been working on another possibility for developing water-resistant cotton by utilization of the swelling effect of wet cotton to prevent the passage of water. Cotton fabrics of this type are called self-sealing.

Water-repellent fabrics and self-sealing fabrics represent two classes of water-resistant cotton. We consider them in the pages that follow.

WATER-REPELLENT COTTON is fabric in which the individual fibers have been treated with a chemical which (like the natural wax in the growing fiber) repels water and causes it to collect in drops on the surface and run off instead of penetrating the yarns. Water-repellent fabrics are understood to be permeable to air because the spaces between the fibers have not been affected.

Chemical treatments that produce water repellency have been known since the late 1870's. Many, though, are not permanent; they are called re-treatable water repellents, for the fabric must be re-treated after each laundering or dry cleaning to be serviceable again. At first, the treatment was generally accomplished by dipping the cloth in a solution of soaplike emulsion, such as sodium ammonium stearate, drying it, and then passing it through a second solution of an aluminum salt, usually aluminum acetate, and washing. The metallic soap deposited on the cloth gave good water repellency. Later, a one-bath treatment was developed; the cloth was dipped in a single emulsion and dried. The effect was the same.

Much work has also been done to develop a durable treatment, one in which the water repellency is not removed by laundering or dry cleaning. To do that, two principal methods are theoretically available to the research worker. The first involves a reaction of the water-repellent material with the cellulose of the cotton fiber to yield a stable compound that will remain during laundering. The second method consists of allowing small molecules of a water-repellent material to penetrate the fiber and there polymerize to larger molecules too large to escape when the cloth is cleaned.

That a tremendous amount of research has been conducted on the problem is indicated by the fact that by 1950 scientists had published 332 reports on the strictly chemical methods of imparting water repellency.

UNTIL LATE in the 1930's, the commercial development of a durable water-repellent treatment seemed almost impossible. Around 1937, however, such compounds as the alkoxymethyl pyridinium chlorides used earlier to give special finishes to cloth were modified by a British concern to give water repellency and were patented. The success of the compounds initiated the investigation of many pyridinium and other quaternary nitrogen compounds in this connection. The compounds, applied to cloth at high temperatures, broke down and were assumed to be reacting either with themselves or with the cellulose to give the water-repellent finish. The finish is remarkably resistant to laundering and dry cleaning, in that the major portion of it is not removed from the cloth. But the detergents used in the cleaning tend to stay in the cloth and neutralize the water-repellent effect. Careful rinsing and hot pressing will restore the repellency, but such aftertreatment is sometimes so difficult that the durable treatment loses much of its advantage over other treatments.

Although this type of treatment is used commercially in several forms, the mechanism of reaction is not well known. F. V. Davis made a substantial contribution to our knowledge of the reaction of one of these compounds, stearamidomethyl pyridinium chloride, with cotton. Working with English commercial materials, he found evidence from which he tentatively concluded that, when the application was less than 1 percent, the compound reacted entirely with cellulose to form a stearamidomethyl ether of cellulose. For applications between 1 and 2 percent there was formed some methylol stearamide in loose association with the cloth in addition to the cellulose ether.

In contrast to the extensive practical work done on producing water-repellent compounds for textiles, surprisingly little has been done on the theoretical side. Workers in the Department of Agriculture have started investigating the theory of water repellency. For that they needed a method sensitive enough to determine slight differences in the effectiveness of different water repellents. Many instruments are available for determining the value of cloth for use in raincoats, but, because rain is a moving thing, most of the methods involve dynamic tests and thus are unsuitable for classifying slight differences between compounds or in methods of applying one compound to a standard cloth. H. A. Schuyten and others devised an apparatus for the more precise measurements. The method is simple. Wetting agents enable water to wet even a water-repellent cloth they lower the surface tension of water so that it no longer forms drops, which roll off the surface, but tends to sink in and wet the fabric. Starting with these facts, technicians devised a test in which the cloth is tilted at an angle of 45 and the solution is determined which will just wet it thoroughly instead of running off as pure water does. An electrical rather than a visual means is used to determine when the cloth is wet. The surface tensions of the various solutions are known, so the determination is called the surface-tension index of the fabric. The lower surface tension denotes the better repellency.

With the instrument, the investigators hope to determine the efficiency of applications, the comparative value of various compounds, and the effect of cleaning or other after treatments of treated fabrics to give eventually the ideal durable water-repellent fabric.

A COTTON FABRIC, suitably constructed, can approach watertightness through sealing itself by swelling as it becomes wet. The principle is the same as that of soaking up or swelling a dried-out wooden boat or bucket to stop leaking, except that instead of large cracks there are millions of minute spaces to be closed.

The idea of self-sealing of swelling-type fabrics is also conveniently illustrated by the linen fire hose, made without a rubber lining, long a common sight on shipboard and in the corridors of public buildings, where it is kept ready for emergency use. When the water is first turned on, the hose leaks badly for a few minutes; then the leakage all but stops, because the fibers have swollen to fill the minute air spaces in the cloth. The result is that the hose can transmit water effectively. The slight leakage that remains is desirable to keep the hose from burning.

The principle utilized in the linen hose has been applied also in a few other types of linen goods, such as tents, water bags, and canvas covers. But the principle was not successful with cotton until recently, when ways were found in England to make satisfactory unlined cotton hose and lightweight cotton fabrics. Work is in progress to apply the self-sealing idea to ordinary-weight cotton fabrics some intended for clothing and others for sleeping-bag covers, tarpaulins, tents, farm fabrics, and similar uses.

That method of approaching a high degree of waterproofness is obviously quite distinct from methods of imparting water repellency through chemicals, although treatments of the repellent type may sometimes be used in addition to the self-sealing effect. The property of resisting the passage of water by swelling ( with or without the aid of repellents) will be called for convenience water resistance, even though the opposite wetting is required initially for swelling to occur.

The first research by the Department of Agriculture on the water-resistant cotton fabrics of the swelling type was undertaken at the Southern Regional Research Laboratory during the Second World War to produce an unlined (rubberless) fire hose from cotton instead of linen to meet an expected shortage of linen.

At the time of the work, 1942, manufacturers of fire hose stated that many attempts had been made in all sorts of ways to manufacture such hose from cotton but without success.

You can get an idea of the problem by considering fibers and yarns of linen and cotton. Linen consists of relatively long, smooth, straight fibers of low elongation, readily laid parallel, and spun with low twist (small twist angle and little spirality) into strong yarn of low elongation. Such properties favor the tight packing of fibers in yarns as well as of the yarns in the hose structure and assure low stretchability of the hose walls, with a minimum tendency to expand and leak under pressure.

Untreated cotton fibers tend to make a less favorable type of yarn structure for the particular purpose. Cotton yarns are not so compact as linen yarns and tend to hold more air when made into a fabric, so that there is more space to fill. With more air space, there is necessarily less fiber within a given volume of fabric to swell and fill the space; and, to complicate matters, the natural cotton fiber has less swelling capacity than the linen fiber. Finally, the cotton yarns and fabric have more inclination to stretch with the result that the fabric expands under pressure, allowing the yarns to move apart slightly and cause even more leakage.

Briefly, then, the problem in making an unlined cotton fire hose or the swelling-type cotton cloth for other purposes is to make the fibers pack more tightly in the yarn and the yarns more tightly in the fabric, or to do anything else that will have the effect of reducing the space to be filled.