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I N D U S T R I A L A N D ENGINEERING CHEMISTRY
Vol. 16, No. 3
Chemistry and Comfort' Relation between the Chemical Composition of Leather and the Comfort of Shoes Made Therefrom By John Arthur Wilson and Albert F. Gallun, Jr. A. F. GALLUNSONS & Co., MILWAUKEE, WIS.
however, essentially the HIS is the first of a Shoes made from vegetable-fanned calf leather hold their shape same difference was found series of investigabetter and are very much more comfortable to wear than shoes of the between chrome and veget,ions being made to same size and shape made from chrome calf leather. Because table leathers. It,will suffice determine how the comfort of the much higher contents of protein and sulfuric acid in chrome here to give one represenof shoes is affected by difleather, i f absorbs water to a much greater extent than vegetable tative comparison between ferences in chemical comleather, increasing in volume and area correspondingly. With two samples of the finest position of the leather used increasing relative humidify of the atmosphere, from 0 to 100 per cent grades of calf upper leather, in making them. The work chrome leather increases about 18 per cent in area against only 6 one chrome and the other described herein is limited to per cent for vegetable leather, a corresponding shrinkage occurring v ege t a ble-t a n n e d . The a comparison of vegetablewhen the relative humidify is again lowered. leathers were entirelv fintanned leather with chrome ished and ready for hanuleather. It seemed logical to start with such a comparison, not only because nearly all facture into shoe uppers. Their analyses are given below. leather is made either by chrome or vegetable tanning, but TABLEI PERCENT OF DRYLEATHER also because there is a much greater difference between vegeChrome Vegetable Calf Calf table and chrome leathers generally than is found between Protein (N X 5.62) 75.0 48.6 different samples of either kind to be found on the market. 0.5 Sulfuric acid 5.0 10.7 F a t (chloroform extract) 6.1 The comfort of a shoe depends very largely upon how well Water-soluble organic matter 0.0 12.6 it fits the foot. In comparing the comfort of vegetable and 27.3 Insoluble organic matter 4.0 Chromic oxide 6.8 0.0 chrome leathers it is therefore necessary that the shoes Aluminium oxide 1.4 0.2 0.1 Ferric oxide 0.5 be made exactly the same size and shape. It has been Sodium sulfate 0.5 0.0 the writers' experience, when this is done, that the shoes 0.0 Sodium chloride 0.7 made from vegetable-tanned leather are invariably much more The insoluble organic matter is usually considered as comcomfortable to wear and also hold their shape much better. bined tannin, but really includes dyestuffs and other organic The difference in comfort is very marked when taking long matters. The sulfuric acid is not free, but is combined with walks or when remaining long in a warm, dry atmosphere. the protein matter; when the leather is brought into contact In all similar tests by others of which the writers are aware, with water, the protein-acid compound hydrolyzes, giving up the results have invariably favored vegetable leather as a small amount of acid to the water. against chrome. Greeves2 quotes an officer of long experience in the late war, and one who knew a great deal about PROCEDURE leather, to the effect that chrome shoes were most uncomThe advantages of sulfuric acid solutions for humidity fortable to sleep in, many soldiers risking trench feet and frostbite rather than lying down in chrome shoes. Sportsmen control have been described by R. E. Wilson13whose compilations were used in making up solutions to give definite relawho have made the test say that a t the end of a day's tramp- tive humidities. Airtight desiccators proved to be very ing for shooting the feet ache less in vegetable than in chrome- convenient receptacles for the work. Atmospheres of 100 tanned leathers. It is also reported that there was always a per cent relative humidity were created by covering the botrush on the part of soldiers who knew the difference in com- toms of the desiccators with water, 0 per cent by using pure fort to get vegetable-tanned shoes. sulfuric acid, and intermediate degrees of relative humidity The preliminary work of this investigation indicated pretty by using sulfuric acid of various concentrations taken acclearly that the discomfort experienced in wearing chrome cording to Wilson's tables. shoes was due mainly to their marked tendency to change in Strips of leather were cut 50.8 cm. (20 inches) long and 15 size with changing relative humidity of the atmosphere, mm. (0.6 inch) in width, the length running parallel to the which, in turn, is a function of the tendency of the leather to line of the backbone but several centimeters away from it. take on or give up water to thc atmosphere, a property possessed by most materials to a greater or less degree. There- The cutting was done so as to make all strips from one skin practically identical in composition. The strips were carefore, experiments were made to determine the extent to which fully weighed and their areas determined accurately by each kind of leather takes up water and increases in area means of instruments. Each strip was then coiled loosely with changing relative humidity of the atmosphere. and suspended above the acid solution in the desiccator upon a cross of copper wire. Each day the strips were reLEATHER SAMPLES EXAMINED moved from the desiccators for a fraction of a minute to Several samples of each kind of leather were used in making permit making measurements of area and weight. After the tests. I n one experiment a fresh calfskin was cut into 32 days the strips were put into the oven a t 100' C. and left halves along the line of backbone and one-half was chrome- until their weights were constant, thus giving the dry weight tanned and the other vegetable-tanned. In all experiments, of each strip. This and the weight of the strip from the 1 Presented before the New York Section of the American Chemical desiccator made it possible to calculate the water content Society a t Rumford Hail, New York, N. Y., January 4, 1924, as part of dn of the leather a t any time during the experiment.
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address by J. A. Wilson. 2 Leather W o v l d , 16, 1077 (1923).
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THIS J O U R N A L , 13, 326 (1921).
I N D UXTRIAL A N D ENGINEERING CHEMISTRY
March, 1924
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RESULTS The water contents of the leathers after 32 days’ contact with atmospheres of constant relative humidity are shown in Fig. 1. At zero reIative humidity both leathers still contained d +r
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Time of contact of leather and atmosphere of constant relative 32 days. Temp. 2 5 O C . humidity
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Time o f contact of leather and atmosphere of constant relative humidity = 32 daya. Temp. 25.C.
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PI
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Per C e n t Relatire EumidLty F I G . I-SHOWINQ EFFECT OF RBLATIVE HUMIDITY UPON THE WATERCONTENT OF CHROME AND VEGETABLE-TANNED LEATHERS
3.5 parts of water per 100 of dry leather; in the case of the chrome leather this increased with relative humidity up to 53.2, against only 35.4 for the vegetable leather. Apparently, the systems had not reached absolute equilibrium, but the daily changes had become almost immeasurably small. Fig. 2 shows the effect of water absorption upon the areas of the kitrips. The area a t zero relative humidity was taken as unity and the percentage increase due to water absorption calculated from this. The curve for chrome leather is of the same general shape as that for water absorption, with a point of inflection a t about 50 per cent relative humidity, but that for the vegetable leather has two points of inflection. This might have been attributed to experimental error were it not for the fact that it occurred also with the other vegetable leathem examined. The magnitude of the changes in area is really startling. The increase of 6.2 per cent in the case of the vegetable leather is large, but the increase of 18.2 per cent for the chrome leather is sensational. The processes were found to be entirely reversible and the rate of approach towards equilibrium similar for the two kinds of leather. When the dry leather is put into a desiccator a t 100 per cent relative humidity, in the first day it takes up 50 per cent and in 2 days 60 per cent as much water as it will take up in a total time of 1 month, the same being true for either chrome or vegetable leather. Leathers kept over water for a month and then placed over pure sulfuric acid gave up 70 per cent of their water the first day and 85 per cent by the end of the second day. The changes in area corresponded to the changes in water content.
DISCUSSION Collagen, the leather-forming protein of skin and the parent substance of gelatin, is noted for its power to increase
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Per Cent Relative Humidity FIG.%-SHOWING HOW LEATHERS CHANGES
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AREA OF CHROME A N D VEGETABLE-TANNED RELATIVE HUMIDITY OF THE ATMOSPHERE
It is estimated that a total change in area of shoe leather of about 4 per cent, or rather plus or minus 2 per cent from the normal value, is about all the average foot can stand without some discomfort. Both leathers show changes exceeding this, but the wonder regarding chrome leather is that the discomfort is not even greater than that actually found. Probably the pinching effect of chrome leather upon shrinking, in actual wear, is lessened by the ease with which the leather stretches and becomes distorted, which explains the greater tendency of chrome shoes to lose their shape. The problem of making chrome leather more comfortable is not simply one of neutralizing the sulfuric acid. This acid appears to be necessary to give the leather certain desirable properties, and when its amount is reduced below a certain value the leather becomes brittle. Lowering the protein content without altering the desirable properties of chrome leather presents an even greater problem. Vegetable-tanned leathers have low protein contents naturally and do not require mineral acid to give them the properties desirable in shoe leather. The writers believe that this series of experiments relating shoe comfort to the chemical composition of the leather will result in compelling tanners to add shoe comfort to the many properties they are now required to furnish in shoe leather. A grant of $400 from the Bache Fund of the National Academy of Arts and Sciences has been made to Professor Harold Hibbert, Yale University, for the purchase of apparatus to be used for the determination of ultra-violet absorption spectra of compounds of interest in connection with the “constitution and properties of carbohydrates and polysaccharides.” An additional grant of $300 from the Cyrus M. Warren Fund has also been made for the purchase of apparatus and supplies in furtherance of this work.
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