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ASSOClATlON CHEMISTRY TEACBEBS
Leather Technology in Wartime' KENNETH E. BELL A. C . Lawrence Leather Company, Peabody, Massachusetts
A
N OUTLINE of modern tannery practice was presented a t the 1940 Summer conference. This article is intended to cover advances and changes in leather technology in the intervening years. Most of the thought and energy of the industry has been devoted to the development and production of military leathers. One of the most interesting was the development of self-sealing gasoline tanks for our planes to render them capable of remaining air borne in modern combat, in which my company was active in collahoration with the United States Army Air Forces. As a result, our ships were protected during the nerveracking months of 1940 and 1941. Later many of them gave a good account of themselves in combat in theaters of war around the world. Metal gasoline tanks offer absolutely no protection against bullets which readily pass in one side and out the other, leaving permanent holes through which gasoline can pour. As planes must continue to fight under these conditions, metal has been eliminated from the gasoline tank. Since individual leather fibers have an ultimate strength of 80,000 pounds as against 35,000 pounds for aluminum, it was possible to substitute chrome-tanned cowhide for metal. While it is not possible to publish details of the construction employed, suffice it to say that tanks resemble gigantic punching hags suspended in the wings. As can he imagined, many nice problems of hydrodynamics were involved in their design. With this type of tank planes continue to fight without gasoline leakage, even though 50-caliber machine gun bullets pierce the wings and pass through the tank. A modified construction is now used, as the supply of cattle hide would have proved inadequate and the amount of handwork involved in the requisite quantity of these tanks would have made them a production bottleneck. However, many authorities consider the leather type of tank best and it has given an excellent account of itself in combat. Brightly colored shearlingssheepskin tanned with the wool on-were displayed a t the 1940 meeting.
Since that time the Army Air Forces and the Naval Aeronautical Bureau have frozen the supply of this material for their exclusive use in clothmg for highaltitude flying. I t is light in weight and acts as a windbreaker and insulator, whiie a polyacrylate plastic coating renders it oil, gasoline, and water resistant. A U. S. Department of Agriculture program helped to expand the supply of raw material to an extraordinary volume. The tanning industry increased its output many fold and virtually without new constrnction. As a result, adequate supplies of flying suits, boots, and helmets were available for the use of our flying forces. A multitude of news photographs of admirals and generals wearing shearling jackets show how popular and comfortable they are considered. These two instances illustrate the important position leather has assumed in this war. In addition to these new uses, enormous quantities of cattle hide have been employed in footwear both for our own Army and those of our allies. The Signal Corps has used leather field telephone cases, and the Navy huge quantities of strap and rigging leather. Chamois has been employed for gasoline strainers, helmet liners, and many uses which are still a military secret. As a result of military requirements, the civilian .supply of leather has been drastically curtailed. Indeed, our Army has eliminated the use of leather in many items solely to conserve the supply for more essential uses. The German and Russian armies, however, have used much more leather per soldier than our own. It is not the purpose of this paper to extol the merits of leather but rather to paint a picture of changes in its technology. Significant of the change is shift in technical personnel in the industry. Dr. John Arthur Wilson has died. His books are still authoritative and ontstanding in their wealth of technical details. Dr. Wilson, like Proctor, McLaughlin, and others, was trained primarily as a chemist. There has been a marked trend in the last few years for biologists and physicists to enter our f i e l d ~ perr haps I should say, for the leather industry to recognize Abstract of apaper presented a t the Sixth Summer Conference their contribution and to cooperate with them. Also. of the New England Association of Chemistry Teachers a t the research workers in the industry have given increasing Connecticut College for Women, New London, Connecticut, attention to the biological approach. Thus, Dr. BergAugust 26, 1944.
A. C. Lowrenre Lealhrr Co.
mann of the Rockefeller Institute, Dr. Highberger of the University of Cincinnati, Dr. Dorothy Jordan Lloyd and Dr. Astbury of England, and more recently Dr. Schmitt of the Massachusetts Institute of Technology have directed intensive researches on the structure of collagen. The broad background of physics, chemistry, and biology of these workers has resulted in valuable and interesting conclusions. The earlier investigators in the field developed the technology then available in a skillful way. The work of Proctor, McLaughlin, Wilson, and others showed that leather was a protein material which hydrolyzed into amino acids. I t should he recalled these are amphoteric, the carboxyl group reacting with alkalies and the amino group with acids. The conclusion was that collagen and the other proteins making up leather were long-chain linkages of the different amino acids. The exact composition was then, and is still, unknown. Whenever leather chemists met there was healthy debate on the structure of leather proteins and the manner in which mineral and vegetable compounds combined with collagen to form leather. These earlier workers made excellent leather sections which they examined under the microscope. These showed that the fibrous nature of leather, with which every tanner was acquainted, extended to the micro-
scopic structure. Figure 1, which is a product of this era, shows a section cut through vegetable-tanned cowhide. The hair or grain surface is made up of the protein keratin, while the body of the leather is collagen. It can he readily seen that leather is fibrous and the three-dimensional mixture of the fiber bundles explains the unique properties of leather. No matter from what direction stress is applied, there are fiber bundles in position to take it up. Many synthetic substitut6s have laminar structure which results in separation into the component layers on flexing, while leather retains its structure unimpaired under such treatment. I t should he noted that the magnification of Figure 1 as printed is approximately 33 diameters. Figure 2 shows that fiber bundles are made of parallel fibers of collagen which are held together a t intervals by the threadlike protein reticulin. The
FIGURE 3.-SHORT SPACING X-RAYDIFFRACTION PATTERN OF BEEF TENDON COLLAOEN.FIBER AXIS IS VERTICAL
bottom view shows how fiber bundles can be teased into finer fibers. Grossly visible leather fibers range from to l/,m of an inch in thickness. These can be teased apart with needles and separated into finer fibers having a thickness of '/am to '/zooo of an inch. Such are, of course, visible with a good microscope. Further teasing breaks them into still smaller fibers of thickness of about of an inch, while these primitive fibers in turn can be separated into fibrils l/l~~,oao to '/,,m,oao of an inch thick and which cannot he seen by the best light FIGURE 2.-FIBER BUNDLESMADE UP OF PARALLEL FIBERS microscope. oa COLLAGEN HELDTOGETHER B Y RETICULIN. BOTTOM\'Ic\YNOTEHow FIBER BUNDLES CANR E TEASED INTO FINER FIBERS In the last few years Drs. Dorothy Jordan Lloyd and
Astbury in England, and Bergmann, Clark, Bear, and others in the United States have investigated such fibers by the X-ray diffraction method (Figure 3). More recently, Professor Schmitt of the Massachusetts Institute of Technology and his collaborators have used not only the X-ray diffraction method but the electron microscope as well as the polarizing and ultra-microscope in an intensive study of this subject. While their initial attacks were made from the medical angle, it is interesting to note that they find collagen to be much more widely distributed in animal structures than. formerly believed possible. Further, and more important, the structure of collagen is practically identical in all organs and creatures in which i t occurs. Through the courtesy of Professor Schmitt Figures 4 and 5 are published. These electron micrographs of collagen as kangaroo tendon and in beef are magnified 21,000 diameters, as printed. This is in contrast with the magnification of 33 diameters for the slides shown as Figure 1. It will be noted that as we approach the molecular size the fiber structure still persists. The lines of cleavage do not run across the fiber as might be expected but longitudinally, that is, further separation is into even finer fibers. In explaining the molecular architecture, Professor Schmitt and his coworkers believe that the denser portions are where the molecule is folded or convoluted. They find that the repeat pattern occurs a t intervals of 640 A. U. or roughly 1 / 6 ~ , W 0 of an inch. This fundamental spacing is corroborated by the X-ray diffraction work of Bear and others. Larger angle X-ray diffraction readings lead to the conclusion that there is a fundamental intermediate repeat pattern, of alanine, proline, and hydroxyproline going to make up the chain-that is, these individual proteins are believed to occur regularly in the same order. Cross linkages are superimposed on this long-chain structure so that several chains in parallel with cross linkages go to make up the molecule. It is the present theory that tanning consists in strengthening or stiffeningthese cross linkages by means of chromium complexes or combinations with other tanning materials. References are given for those who are interested in further details of this complex subject. A much less fundamental concept but one which has interesting possibilities of revolutionizing vegetable tanning is the use of acetone solutions of vegetable tannins. Initial work on this subject was conducted by the Tanners' Council Research Laboratory. Since time immemorial hides and skins have been tanned with water infusions of vegetable tannins. A dilute solution is used first to avoid astringent action. Penetration of the tannins into the hides takes place slowly arid as this occurs the strength of the solution or "tan" is gradually increased. The use of strong tan or too rapid increase in the strength results in over-tannage of the surface and inhibition of penetration to the interior. After the hides have taken up all the tan possible from solution they are "laid away" with ground-up solid tannins in direct contact which is slowly taken up by the hide. A period of four or five
OP COLLAGEN FIBRILS PROM FIGURE8.-ELECTRON MICROGRAPH KANGAROO TAILTENDON. MAG.21,000 X
months or even longer is involved for the whole process. The disadvantages are obvious. Large floor space is required, and carrying charges on the necessary inventory are big. Further, the value of the hide which is the main item of cost may drop sharply during the process and ruin the tanner. Tannins are soluble in acetone and acetone is miscible with water in all proportions. Thus, it is possible to introduce the requisite tannin into hides from acetone solution in a very few days rather than months. So far as is known this development has not been carried into full-scale production as yet but i t has possibilities which cannot be ignored. The recovery of inflammable and volatile acetone and a balanced tannin takeup are necessary if the process is to prove economical.
Morrochurc11r Instilulc of Tachnology
FIGURE 5.-ELECTRONMICROGRAPH OF COLLAGEN FIBRILS FROM BEEFTAILTENDON. MAG.21,000 X
Leather finishing was stabilized for a period of nearly 20 years on formulations of casein, shellac, and pigments, plasticized with oils. True, there was a brief period during which nitrocellulose finishes were popular, but unsuitable plasticizer which migrated into the leather leaving the nitrocellulose film brittle and tending to peel made them lose popularity rapidly. Gradually, modern synthetic resins have been adapted to leather work and are now well established. Polyvinylchloride is oil-, water-, and gasoline proof and was adopted by the h y Air Forces as a coating for flying suits. The toxic and inflammable solvents used, the restriction of application to a bristle brush, and the need for heating to a temperature of 150°F. made it a production bottleneck. As a result, water dispersions of polyacrylate resin have displaced it. These can be applied by .. - spray . .m - n at room temperature and without vapor hazards. The trend toward high-altitude flying and fighting exDoses eauinment to temoeratures as low as -65°F. and present-day specifications require flexibility at this temperature. Cellulose nitrate and acetate finishes have come into their own with the advent of suitable plasticizers, and many attractive effects are obtained by the use of these materials. Urea-formaldehyde, phenol-formaldehyde, and many other families of resins are now in use as leather finish components. It is anticipated that many &
other new materials will be adopted as they become available. Prediction is always hazardous. The rosy dreams of advertising copywriters have led us to believe that Utopia will be realized after the war when all the new developments are released. We have been told that plastics will prove ubiquitous and that they will replace everything from metals to leather. While we cannot shut our eyes to progress, ~ o ~ ; e ' s lines: "Be not the first by whom the new is tried, Nor yet the last t o lay the old aside"
still hold. In the case of leather, it is believed that the unique structure with which nature has endowed it still renders it. superior for many uses. Thus, on the basis of comfort, appearance, and service it is predicted that leather will long have a place in shoe uppers. After experience with unrationed shoes many people will gladly return to leather as a material for shoe soles.
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BIBLIOGRAPHY
Asrsmu. W. T.. "The molecular structure of the fibres of the collagen group" (first Proctor memorial lecture), J. Intern. Leather Tradts Chem., 24, 69 (1940). BEAR, Rrca~noS., "X-ray diffraction studier. on protein fibers." J. A m . Chem. Soc., 66, 1297 (1944). SCHMITT, F. 0.. "Electron microscope technique on collagen," J. Cellular Comp. Phyriol., 20, 11 (1912). WILSON,JOHN A,, "Modern Practice in Leather Manufacture," Reinhold Publishing Corp., New York. 1911.
