THE OILISG OF LEATHER* BY T . G. ROCHOW
The use of oils on leather is one of the most important processes in leather manufacturing. Oil seenis to be the cure-all of the tanner. Schlosstcinl lists the following functions of oils and fats: leather-building, protecting, waterproofing, body-giving, weight-increasing, softening, lubricating, and preserving. Surely a theory explaining all these functions of an oil mould be expected to be very complicated, and a review of the literature certainly does not disappoint t,he investigator on this point. It has long been known that to give a satisfactory product the presence of water during oiling was necessary, except in the “burning-in” process whereby tanners found a method of stuffing heavy leathers by applying the hot grease or oil to the dry, hot leather. But in all the other processes for currying, water is one of the conutituerits. It mas the explanation of the function of the water that was particularly bothersome to early investigators. Fractical books on tanning still describe the “drawing-in” of the oil as the water evaporates. Fortunately, in the past ten or twelve yews considerahle work concerning the oiling of leather has been published. I t is the purpose of this paper to correlate these in>&igations and the theories concluded from them, demonstrating wherein they conflict or coincide with the findings in our experimental work or that of other investigators.
Chemical Combination Theory Quite recently eonsidcrable work has been dorie to explain the oil tannage of leather, Most of the theories seem to agree with that of Fahrion? which is substantiated by L. Meunier3. Briefly, this theory involves the oxidation of the unsaturated bonds in fish oils: (-CH=CH-) Oz=-CH-CH
+
l
i
0-0 These bi-peroxides (the oils contain two or more double bonds) act upon the proteins, one peroxide attacking the amino groups which are oxidized by loss of hydrogen: the hydrogen thus liberated transforms the oxygen of the second peroxide group into two hydroxy groups; finally one of the hydrosygroups thus formed lactonizes the carboxyl of the fatty acid. Another portion of the peroxide acids undergoes molecular rearrangement, causing the appearance of hydroxy groups which form lactones with the carboxyl * This work was done as part of the senior thesis under Professor Bancroft. J. Am. Leather Ass., 14, 41 (1919).
* 2. angew. Chem., 1903, 665, 1911, 361. 3
J. Am. Leather Chem. Ass., 13, 530 (1918).
THE O I L I S G O F LEATHER
1.529
groups. These lactones are retained by the fibers, preferably because they are insensitive to alkaline washing. It was only natural that this theory be applied to the oiling of leather in general because marine oils for a long time have been the most used by tanners. Schlosstein’ in an article advocating the use of fish oils for currying leather, states that “to a certain extent the “active principle” of fish oils is always formed when oil is applied to leather and that, therefore, currying cannot be considered as a purely physical lubricating and softening but as a combined physical and chemical process in the nature of a second tannage.” lloeller?uses the oxidation theory more completely as a theory explaining the waterproofing and other properties given leather by fish oils as well as a method of explaining the old question of the functions of water during the process. K a t e r hydrolyzes the peroxides formed on oxidation of unsaturated fish oils yielding a n atom of oxygen:
[
-CH-CH-
]
+H?O
=
[-TH-YH-]+ OH OH
0
This oxygen oxidizes the polyphenols of the free tannins in the leather to give quinone compounds similar to the phlabophenes. These products are good tanning agents just as t,he phlabophenes are. They increase the water resistance of the leather since they are in themselves insoluble. As long as there is hide substance to be tanned, fish oils are going to tan it whether they are introduced primarily to tan the leather or to oil it. But as a general theory explaining the functions of an oil in leather currying, this theory is very limited in its applications, since it applies only to vegetableand oil-tanned leather and not to chrome- or aluni-tanned leather. I t cannot apply to animal oils and vegetable oils containing little oxidizable substance or to mineral oils. These classes are largely used in practice and work very well. Schlosstein himself, in the article referred to, says that hydrogenized oils are coming into use. As for the fish oils themselves, when used on vegetable-tanned leather, it is difficult to see how these oxidized tannins are very active in the softening, lubricating, or any other of the functions of a leather oil as listed by Schlosstein, except, as Moeller states, that of waterproofing. As a method of explaining the use of water in the process the theory is equally weak. Moeller says that “evaporation of the water is very improbable on account of the isolating fatty layer.” The vapor pressure of two immiscible liquids is equal to the sum of the vapor pressures of both, so that this assumption is not valid, though the rate of evaporation will be decreased if the oil layer is outside. Moeller then goes on to state that the water is not necessary as a means of “drawing” the oil into the leather but merely acts as a reacting substance converting the C = C group into oxyacids. As proof he uses the well-known fact that dry leather will adsorb train or any other oil -
J. Am. Leather Chem. .h., 14, 41 (1919)
* Der Gerber, 45, 277
(1919).
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T. G . ROCHOTV
but very slowly and leaves the surface slowly. As a matter of fact, even if every oil used on leather were a drying oil, and if every drying oil classed as such, were composed entirely of unsaturated compounds, the amount of water used on the leather is many times that necessary to hydrolyze the peroxides when one considers that most leathers contain only 5 - 1 5 7 ~of their weight of oil and that the molecular weight of the oil so high that the combining weight of u-ater would be extremely small. Resides air-dried leather contains about j or 6Yc of moisture, more than enough to hydrolyze the peroxide of the oils. Rloeller says that the oxidation of the oil is strongly influenced by the phenolic compounds of tanning agents as confirmed by the presence of oxyacids in the dried train oil. K h y one mould not attribute this to ordinary oxidation is not known. I n a later article Xloeller' tried to determine whether train oil was most greatly changed by tannin, leather, hide, or simply by the air. His theory was not well substantiated by his results, since pure tannin was next to the lowest in making train oil insoluble and the least effective in reducing the iodine number of the train oil. The chemical combination theory has also been dragged into use to explain why solvents mill not extract as much oil, after some time, as they did soon after the oil was applied to the leather. Lauffmann? states that all ordinary fat solvents (ethyl ether, petroleum ether, carbon tetrachloride) except chloroform and carbon disulfide extracted less oil after 1 2 0 days than after z days standing. He says that the low results were probably caused by combination of fat with the hide substance. Turkey Red Oil (sulfonated castor) gave the same results as did degras, so the oxidation products of the oil cannot be the combining substances. In the article referred to, Xloeller3 says that oil not removed by solvents is not necessarily combined with the collagen because it, is difficult and sometimes impossible to remove oil even from emulsions. He goes on to state that the retention of fatty acid and glycerides in leather is intelligible on the basis of colloid-chemical theories of emulsions. It was to disprove the oil-collagen combination theory that he attempted to show that train oil reacted with tannin and therefore was insoluble in petroleum ether. When he found that train oil was only zjyc soluble in petroleum ether when mixed with equal amounts of various tanning extracts and kept three days, he immediately assumed that chemical combination had taken place, after stating that oils were difficult to remove from any emulsion. His attempts to prove combination between the oil and tannin seemed to have failed, He mixed 100 grams of each of various tanning agents with 40 gram3 of train oil and compared results with a 45% solution of pure tannin mixed with 40 grams of train oil. The tannin was next to the least active in making the oil insoluble. I t was next to the best in increasing the acid value, but it' Z.Leder und Gerberei Chem., 1, 20 (1921).
* Ledertechn.
Rundschau, 19, 63, Der Gerber, 45, 277 (1919).
jI
(1927).
THE OILISG OF LEATHER
153'
was least effective in saturating the desirable bonds as shown by the highest iodine number and lowest values for oxy-fatty acids. There is a great difference of opinion as to the desirability of the use of mineral oils on leather. h brief discussion i s brought in here to disprove the theory that an unsaturated oil is necessary for the proper currying of leather. Gi-asser' says that mineral oils are widely used for leather and that vaseline oils are the most important. The paraffine and ceresines are also excellent preservatives. He recommends sulfonated castor oil merely as an emulsifying agent.? Hart3 states that mineral oils are widely used by the leather industry. Bunckei gives a blow to the oxidation theory when he states that spotting is caused by an excess of oxidized fatty acids or lack of sufficient alkali to neutralize the acids liberated by sulfonated oils. He recommends sulfonated oils merely as emulsifiers for neutral oils. This is in accordance with 1Iezey'ej work in which it was found that' the greater the proportion of unsulfonated neatsfoot oil, or mineral oil, in mixtures of these with sulfonated neatsfoot oil or train oil, the greater the adsorption of the oil by chrome leather, unless the sulfonated oil was in too little quantity to emulsify the unsulfonated neatsfoot or the mineral oil. Schindler6 also says that free fatty acids and oxy-fatty acids should be as low as possible and that sulfonated oils act only as emulsifying agents. Whitmore, Hart, and Bach' showed that petroleum plus paraffin gave nearly as great an increase in tensile strength, with strap and harness leather as did a mixture of tallow, cod oil, and wool grease. They state that' the increased tensile strength is due to lubrication and strengthening of the leather fibers. Very few investigators deny that mineral oil can be used with satisfactory results t o oil leather. I , 8zsJ9 5 5 , and absolute alcohol, and xylene. Sample Yo. 3 was removed and dried to be used as a “blank.” Sample KO. 1 was immersed in a I j?; solution, by volumc, of the cod liver oil in xylene. It was removed after one day and dried for two days in the air. After all samples had dried at least two days they mere compared. Sample KO. z of course was softer than S o . I . Sample No. 3 was softer than both; so prolonged immersion in the solutions did have a softening effect, due to the extraction of water solubles and substances used to increase the weight of the cow hide. But Sample S o . .r, was very much softer than No. 3. It was the most pliable of any cow hide oiled in these experiments. Conc1z~sion:A very soft and pliable leather is obtained by displacing the water completely from thoroughly-wetted cow hide leather by alcohol solutions and introducing cod oil by means of a solution in xylene. Part of the flexibility was due to extraction of solids by the solutions but the oil made a very flexible leather when introduced in t,his way. Using the same procedure, mineral oil and castor oil also gave a very flevible leather though not quite so soft as the one oiled with cod liver oil. I n tanned leather, the fibers are isolated somewhat from each other by the tannin. I n raw hide, the fibers have no such isolating material and consequently the fibers cohere as soon as they become dry, and a hard, bony mass results. I n pure water the fibers swell and become separated. If we could get a lubricant between the fibers while the hide is in this condition, the skin would remain soft and pliable. This was successfully accomplished experimentally. Pickled sheepskins as received by the tanner, ready for tanning, were used. They were pickled in sulphuric acid and salt. Since they were beamed, bated, and limed, removing the fat, epidermis, and elnstin, we may consider the sheepskins, ready for tanning, as nearly pure collagen. The pickled sheepskins were ciit into pieces about two inches square. They were soaked in distilled water for 24 hours. The water was changed and the skins allowed t o soak for z 3 hours more. Then the gradient solutions of the histologist were again brought into play. The skins were soaked 2 3 hours or more in each of the following, and in the order given; soycalcohol, 80%~ 9 jVc, sbsolute alcohol, and xylene. One piece was removed and dried to be used as a blank. The others were cut in two and marked in pairs. One of each pair by volume, of the folwas immersed for two days in xylene solutions, 107~ lowing oils; codliver oil (Squibb), white medicinal mineral oil, and pure
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T. G. ROCHOW
castor oil. The other pieces were dried three days and each immersed in the oil corresponding to that used for the other sample of each pair. I t is well-known that alcohol gives a pseudo-tanning to animal skins, but that the treated skin swells in water and is converted to gelatine on boiling just as the untreated raw skin is. For some reason not understood, the fibers do not stick or glue together on drying but remain partially separated. Thus, the blank made in this set of experiments looked more or less like leather, but it was stiff, hard, and somewhat shrunken. On boiling with water i t was converted quickly to gelatine. The samples dehydrated and oiled by solution without intermediate drying were all remarkably soft and flexible. The sample immersed in mineral oil was less flexible and had shrunk more on drying. I t was, however, considerably softer than the blank. The other samples of the pairs, which had been dried before being placed in the oil solution were no softer than the blank and of course had shrunken just as much since they received no more swelling by water after drying. The one dried and then oiled by cod oil was slightly more flexible than the blank but far stiffer than the sample oiled by cod oil without previous drying. Both samples oiled with mineral oil went into colloidal solution on boiling, the oil going partly into emulsion and the rest floating on the water. Most of the skins oiled by castor oil formed a sol on boiling, although a few quite small gelatinous pieces about a millimeter in diamater resisted the action of boiling water. Both samples of the pair, however, reacted the same way towards boiling, so the difference in flexibility between the one oiled while swollen, and the one oiled after drying was not due to the fact that the first was tanned and the ot,her was not. Little of the cod-oiled skins went into colloidal solution on boiling. Appreciable tanning by the cod liver oil was apparent, but here again both reacted the same way toward boiling water. 'I he raw skins dehydrated by gradient alcohol solutions and oiled by xylene solutions were more flexible and softer, than the ones that were dried previous to oiling, because the fibers of the first samples were kept continually swollen and separated during oiling while those of t,he second series of samples were allowed to stick together before oiling. Pieces of thc pickled skin washed, and swollen in distilled water, dipped in oil (one in cod liver, the other in castor oil) became hard and brittle as soon as the water had evaporated. Another sample was dehydrated by the gradient alcohol solutions, soaked in xylene, dried five days, and then immersed in the cast.or oil-xylene solution for a day. I t was as stiff and hard as the blank, as shown before. I t was hoped that, by soaking it in water, the fibers would swell and become separated without displacing the water. But water is as easily adsorbed by collagen as by gelatin and the adsorbed oil was therefore displaced by the water and was seen floating on top of the water. Consequently, the skin dried in to a horny mass as though untreated. It is, therefore, necessary to remove the water before oiling untanned skin, but the fibers of collagen must not be allowed to cohere.
T H E OILING OF LEATHER
I539
It is interesting to note that the above reactions are entirely reversible,
h sample treated similarly to the one just described above likewise became a horny mass when dried, after soaking in distilled water for z or 3 hours, again running the sample through the gradient solutions and the castor oil-xylene solution, this time without intermediate drying, rather a soft flexible product was again obtained. The reactions thus are truly reversible, Conclusions: Untanned skin swollen by pure water and kept swollen through the gradient solutions, which removed all the water, when immersed in a xylene solution of the oil remains soft and flexible on drying; while skin treated the same way but allowed to dry before immersion in the oil solution remains as hard and stiff as an moiled, but otherwise similarly treated, piece of skin. I t may be concluded then that the function of water in the oiling processes is to swell and separate the fibers so that the oil may penetrate between the smallest fibers and fibrils, lubricating them and keeping them from cohering. Preferential V e t t i n g of Leather by Oil: (2.) The question naturally arises as to how oil can displace iyater from leather fibers, since tanned leather exhibits the ability to adsorb water and swell just as untanned collagen does, but to a much less extent than collagen, 11-ater was seen to displace castor oil from untanned skin, making things look dark for advocators of a theory of preferential wetting of leather by oil, But the theory of preferential wetting of a solid emulsifying agent by the external phase of an emulsion, helped us out of our difficulty. If a solid emulsifies water in oil, it has been shown’ that the solid is wetted partially by both liquids but preferentially by the oil, since the solid lies on this side of the interface. The oak-tanned cow hide previously described was ground up and extracted eight hours by petroleum ether in a Soxhlet extractor. Twenty C . C . of the medicinal cod liver oil was added to 2 5 C.C. of distilled water in each of three glass stoppered bottles. To one no emulsifying agent was added. In the second bottle the ground leather was soaked in the oil a few minutes and then the water added. I n the third bottle the leather dust was soaked in water first and the oil added. Each bottle heretofore not agitated, was shaken one minute. The first emulsion settled in I second, the second in 2 3 seconds, and the third in j seconds. On repeated intermittent shaking the second emulsion became very stable, lasting for hours, while the third emulsion continued to settle in 5 or I O seconds during the first day. The mixture of oil and water continued t o settle out in I or z seconds throughout these experiments. The mixtures were allowed to stand the next day without agitation. During all the third day the emulsions showed little change in the relative stability although mixtures Kos. 2 and 3 seemed both a little more stable than on the first day. The two emulsions were proudly displayed as a peculiar example where in the adsorbed water on the leather dust in the third sample was not displaced by the oil which is sample S o . 2 made a stable Bancroft: “Applied Colloid Chemistry,” 352 (1926).
1540
T. G . ROCHOW
emulsion by wetting the leather. By noon, however, sample S o . 3 refused to work as it was scheduled, but remained an emulsion as long as S o . 2 . Repeated shaking during the morning had wetted the leather with oil. These experiments were repeated using a finer leather dust (about j 5 to IOO mesh). This time the oil displaced the water on the leather dust much quicker (in an hour of intermittent shaking). I n this run and in the previous one, the emulsion was definitely proved to be water in oil by adding a drop of oil to a small portion of the emulsion. The drop of oil disappeared immediately, while a drop of water when added to the emulsion remained in a separate globule. Similar emulsions were not obtained for castor oil or mineral oil using the same proportions. The usual precautions in preparing emulsions were not followed, however. The leather dust n-as simply wetted by the desired phase and the oil and water poured together and shaken. More work should be done on this to determine whether by slow additions of the water with intermittent shaking or whether changing the proportions will not produce an emulsion in the presence of leather dust. Conclusions: Cod oil, at least, wets leather in preference to water but the displacement of water by oil is quite slow, proving that leather is wetted almost as easily by water. This is why an under-oiled piece of leather may be saturated with water, when immersed in water. (3.) T h e Interjacial T e n s i o n between W a t e r and Oil: I t is apparent that, to get greater penetration of an oil into leather, the oil must present a greater surface to the leather fibers while they are swollen and separated. Since water is necessary in oiling, to swell the fibers, the interfacial tension between water and oil is very important. From du Youy’s table we find that with castor, olive, and oleic acid the surface tension in air is 2.4, or more, times as great as the surface tension in water. This explains why these oils work so well by simply swabbing them on wet leather. The mineral oils, however, show a surface tension in air I . 4 j to 1.48times less than that in water. This explains why we obtained such a stiff lenther when wet COR hide was oiled with white medicinal oil. In practice mineral oil is always mixed with soap, sulphonated oils, or an animal oil to reduce its surface tension in water. blicroscopical examination verifies this. Droplets of cod, neatsfoot, castor, and olive oil all increased their surfaces when droplets of water were introduced to them, while a mineral oil droplet showed a decided decrease in size when water was introduced to it. Small (one square centimeter) pieces, of the cow hide described, were dehydrated by gradient solutions of water and alcohol. The alcohol was displaced by xylene, and the xylene displaced by xylene-paraffin solution. The time in each solution was at least one day. The samples were then immersed in molten paraffin and then imbedded in paraffin. Sections were cut on a microtone, the paraffin dissolved out by xylene, the xylene by alcohol, and the sections were dried. The oils to be used were dyed with Sudan 3
T H E OILING OF LEATHER
1541
to make them visible in water. Results in later experiments (using oblique illumination) were the same so the dye did not change the surface tension of the oils enough to affect the results. The dried sections mere wetted at their edges by the oils mentioned in the preliminary droplet experiments. But penetration was very slow, it taking hours for the oils to penetrate visibly even the very thin sections used. [This does not exclude that some oil was adsorbed within the sections.] All the vegetable and animal oils used, when introduced at the edge of the v e t t e d section of the leather spread out to conform with the surface of the water. When this surface retreated across the section the oil followed it. As soon as the oil came in contact with the fibers they soon became transparent showing they had adsorbed the oil. With mineral oil the appearance of the film in the presence of water was quite different. Instead of spreading out the oil droplet contracted in the presence of water, and as the surface of the water changed the oil droplet remained fixed in position. After all the water had evaporated the oil droplet was still stranded from the surface of the leather section. It had to be mechanically pushed to the surface of the leather and then its penetration into the fibers was so slow that it could not be seen after an hour of patient waiting. After a day or so it had penetrated into the section as did the droplet of mineral oil introduced on a dry section. Thus, while water separates the fibers, this advantage is lost because the interfacial tension between water and oil is too great to allow penetration while the fibers are isolated in water. I t is very interesting to note that Shereshefsky' found that two immiscible liquids in a capillary tubing will move in the direction of the one having the greater surface tension. This will explain the action of the animal and vegetable oils in the microscopic sections. Attempts t o duplicate the microscopic results in larger pieces of leather have failed. Pieces of the cow hide weighing 30 grams were soaked in water one hour and one end dipped in oil. When the exposed end was dry, the pieces were removed and cut in three sections parallel to the end dipped in oil. The sections wert: then ground up and extracted with petroleum ether. S o differencein oil content between the middle and top sections was found. The trouble was that the sections were rectangular and the short side exposed to the oil. Thus the oil had to rise too high to be detected in the middle section. Experiments should be repeated dipping the long end in the oil. The decrease in surface tension of an oil on heating explains why the "burning in" process is successful in currying heavy non-flexible leathers. In this process the hot oil is applied to the hot dry leather. I t has been shown2 that the grease or oil penetrates very well in this,method. Kater is not used and therefore as soft and pliable a leather is not obtained as though Nature, 122, 312 (1928). Balderston: J. Am. Leather Chem. Ass., 17,4 o j (1922).
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T. G . ROCHOTV
the fibers had been separated. But “burning in” is used to give strength and water proofness to heavy leathers like belting and harness leather, where softness is not so desirable. Conclusions: To give greater penetration more surface of the oil must be offered to the leather fibers. For most vegetable and animal oils the water alone, which is already present to swell the fibers, is sufficient to reduce the surface tension of the oil. These oils are therefore used alone with water in hand or drum stuffing. If mineral oil is used, substances must be added to reduce its surface tension. In fat-liquoring the surface tension of the oil is already reduced to form the emulsion. In “burning in” without water, heat lowers the surface tension sufficiently to give good penetration. As soft a product is not obtained because the fibers were not previously separated by water. Further investigation should be done with the microscope to determine whether mineral oil acts like the animal and vegetable oils on microscopical sections of leather wetted by water. h method of photographing the penetration of oils in sections wetted by water would be very convincing. The difficulty a t present is that oil wets the glass slide and cover-glass, in preference to, or as well as, leather. Great enough depth of focus cannot be obtained for photography to allow the use of uncovered specimens. Cow hide cannot be used for sections for photography since it is too tough to be cut into thin enough sections to make them sufficiently transparent or light enough in color. Cornel2 Uniaersity.
