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of the chemical constitution and structure. A glass melted from moist raw materials differs in working quality from a glass melted from the same materials dry. Chemically they do not differ. Two glasses otherwise identical, both without visible strain, will have different viscosities, depending on heat treatment. What structural differences exist among these glasses identical in chemical composition yet unlike in physical properties, we do not know. Nor has a systematic determination been made of equilibrium conditions among glass components. Investigation of these subjects and others as purely scientific problems undoubtedly would yield results of immediate practical application. A major task of the glass technologist today is the delivery to glass-working machines of material that is both homogeneous and uductuating as t o chemical composition, as t o temperature, and as t o physical properties. The hand worker can adapt his manipulation t o a varying material. For the machine the material should be constant. Present glass-melting methods are poorly adapted for delivering such a product. One reason why glass is not homogeneous is that it dissolves the clay walls of the tank. A long step toward meeting the crying need of the industry for a suitable refractory in which t o melt glass has been taken by Doctor Fulcher, inventor of cast refractories, which are aluminum silicates and refractory oxides handled as in iron foundry practice, the material being melted in an electric furnace and cast in molds for tank blocks and other shapes. The cast block is more resistant t o the corrosive attack
VOl. 21, XO. 2
of glass than the ordinary clay block, which lasts in exposed positions a t best a year and sometimes only a few months. Lack of homogeneity is caused also by temperature differences in the glass, and such differences, as well as an excessive waste of fuel, are t o be laid t o the conventional design of the glass-melting tank. I n the melting process, which is a large item of cost, only about 10 per cent of the coal used is actually required t o raise the raw materials t o temperature and to melt them. Ninety per cent of the heat value is lost. The excessive consumption of fuel is caused chiefly by the practice of maintaining a t or near melting temperature a mass of glass out of all proportion to the quantity actually being worked. For machines working, say, 50 pounds a minute we hold 6000 times that amount, or 160 tons a t white heat 24 hours a day, 7 days a week, and thus cause losses by radiation and otherwise amounting t o many times the total heat theoretically required. An abundance of interesting and fundamental work, then, remains t o be done in glass, both in pure science and in its application. One of my fellow students in Ostwald’s laboratory, in Leipzig, thirty years ago, who was laboring as we all were to give birth to something that might charitably be regarded as a contribution to science, was in the habit of lamenting bitterly that the easy things had all been done. In glass not even the easy things have all been done, and help is needed from those who like t o do the hard things. To workers in science we suggest the field of glass investigation as attractive and worthy.
CHANDLER LECTURE The Chandler Lecture for 1928 was delivered at Columbia University on December 7, by John Arthur Wilson, of Milwaukee, Wis., chief chemist of A. F. Gallun and Sons Company and consulting chemist and director of research for the Milwaukee Sewerage Commission. In presenting the medal t o Mr. Wilson, Dean George B. Pegram stated that the medalist was best known “for the way in which he has applied the most modern concepts of chemistry to one of the oldest industries, the making of leather,” and described his achievements as follows:
The Charles Frederick Chandler Foundation was established in 1910, when friends of Professor Chandler presented to the trustees of Columbia University a sum of money, and stipulated that the income was to be used to provide a lecture by an eminent chemist and also a medal t o be presented t o this lecturer in further recognition of his achievements in the chemical field. The previous lecturers, with the titles of their lectures, are as follows: 1914
L. H. Baekeland
Mr. Wilson’s published researches in physical chemistry, colloid chemistry, and the chemistry of proteins ; his application with great daring and acumen of wide and exact knowledge of the most modern advances in chemistry to the complex problems of leather chemistry, resulting in valuable improvements in processes; and his distinguished public service in introducing improvements in the process of sewage treatment that has not only made operable a sewage disposal plant for his own city of half a million people which is a valuable object lesson for all our cities, but has made it operable in such a way that it may soon be returning revenues to the city, are achievements that have placed him in the front rank of chemists.
1916
W. F. Hillebrand
1920
W. R. Whitney
1921
F. G. Hopkins
1922 1925
E. F. Smith R. E. Swain E. C. Kendall
1926
S. W. Parr
1927
Moses Gomberg
1923
Some Aspects of Industrial Chemistry [Vol. 6 769 (1914)l Ou; Analytical Chemistry and Its Future [Vol 9 170 (1917)I The Lktl;st Things in Chemistry [Vol. 12, 599 (1920)l Newer Aspects of the Nutrition Problem [Vol. 14, 64 (1922)l Samuel Latham Mitchill-A Father in American Chemistry [Vd. 14, 556 (1922) ] Atmospheric Pollution by Industrial Wastes [Vol. 15 296 (1923)l Influence hf the Thyroid Gland on Oxidation in the Animal Organism [Vol. 17,525 (1925) The Constitution of Coal-Having Speciall Reference to the Problems of Cirbonization [Vol. 18, 640 (1926)l Radicals in Chemistry, Past and Present [Vol. 20, 159 (1928)l
.......
Chemistry and Leather John A r t h u r Wilson A. F. GALLUN& SONSCOMPANY, MILWAUKEE, WIS.
S
INCE the dawn of civilization, leather has been one of the world’s most important commodities. It has become so much a part of our everyday life that we should find ourselves in a quandary if it were suddenly taken from us. And yet, after thousands of years of daily use, its properties remain but poorly defined. Leather is not a simple and homogeneous material of definite properties. On the contrary, it is of very
variable chemical composition; it has an exceedingly complex and variable physical structure; and every variation in composition or structure causes some corresponding change in properties and in serviceability. Unfortunately, the relations involved are not yet well understood. Occasionally we find glaring examples of the far-reaching effects of our ignorance in this respect. It will be sufficient t o cite one.
February, 1929
INDUSTRIAL AND ENGINEERING CHEMISTRY
181
heat from the body, using the evaporation of water to accelerate The results of one of our recent investigations have indicated that the great majority of people now suffer unnecessary foot discom- the heat escape, when necessary, and supplying an oily material fort because of the methods employed in tanning the leather used t o the surface of the skin t o retard the loss of heat when the exin making their shoes. The discomfort arises from a n excessive ternal temperature falls. The skin is also an organ of sense, shrinkage and expansion of the leather with changing atmospheric equipped with nerves sensitive to touch, pain, heat, and cold. It conditions, which can be overcome t o a very considerable extent is an organ of secretion and excretion and is supplied with glands, by changing the method of tanning the leather. Very few people, ducts, muscles, and blood vessels. It is a covering protecting if any, previously suspected that the discomfort was in any way the body against bacterial infection and acting as a buffer against shocks and blows. I n strong sunlight it is capable of developing related to the composition of the leather. Because of the important part which leather plays in the daily color filters to protect the underlying tissues from the destructive life of nearly every civilized human being, it is apparent that a action of the ultra-violet rays of the sun. The many intricate great service can be rendered to mankind by the development functions of the skin are associated with a structure and chemical of a scientific control of all the important properties of leather. composition that are exceedingly complex and variable. The skin is divided sharply into two layers, Chemists have appreciated this fact for more distinct both in structure and origin: a relathan a century, but the task has been too tively very thin outer layer of epithelial great for their limited facilities. It involves tissue, the epidermis; and a much thicker studies of the materials used in making layer of connective and other tissues, the leather and of their chemical reactions, as derma or corium. Raw skin, as an article well as measurements of properties of leather of commerce, has also a third layer, the which are not well defined. A study of the superficial fascia, known t o the tanner simply raw skin alone offers seemingly infinite diffias “flesh” and containing both adipose and culties. Its structure is very complex and areolar tissues. I n life the areolar connecvaries according t o the kind of animal, its tive tissues connect the skin proper very age, food, habits, and climatic conditions under which it lived, and also according to loosely t o the underlying tissues of the body. the particular location in the skin. Animal The derma lies between the superficial fascia and the epidermis. In the preparation of skin contains a number of ‘different proteins, fats, and other materials in variable proporskin for tanning, except in special cases, such tions. Few materials known t o the chemist as the tanning of fur skins, the flesh and are so complex as the proteins and the tannins the entire epidermal system must be rew h i c h a r e e m p l o y e d t o convert protein moved, leaving the purified and unharmed matter into leather. In the manufacture derma t o be converted into leather. of leather, one also encounters bacteria, Figure 1 is a photomicrograph of a cross section of cowhide cut from the butt. It molds, enzymes, complex inorganic salts, emulsions of various kinds of oils, dyestuffs, will serve t o show the general structure of John Arthur Wilson and finishing materials, including waxes, skin. The epidermis appears as a thin, dark gums, resins, soluble proteins, and lacquer and varnish materials. line forming the upper boundary of the section and occupying To solve the basic problems of leather chemistry actually requires barely 0.5 per cent of the total thickness, the rest being the derma, the elucidation of the basic problems of most other branches of the areolar tissue having been removed from this portion of the chemistry. hide in flaying. The epidermis is made up of cellular strata However, the outlook is far from being hopeless. During the originating from the ectoderm, the outer layer of the young empast decade leather chemistry has kept pace with the unprece- bryo; and the derma is derived from the mesoderm, or middle dented speed of development of other branches of the science. layer. These two layers grow independently throughout life and In the task of producing a more serviceable leather under scientifi- differ materially in both chemical and physical properties. The cally controlled conditions, a few definite results have already tanner makes good use of this difference in the preparation of skin been obtained and we may confidently look forward t o further for tanning. important developments in the near future. The top fifth of the section shown in Figure 1 has a structure I n this lecture a n attempt will be made to portray the present very different from t h a t of the rest of the section and has been status of leather chemistry in such manner as t o be of interest t o called the “thermostat layer,” which indicates its primary funcchemists in general. This involves descriptions of the structure, tion. The lower portion is known as the reticular layer because composition and chemical reactions of animal skin, the basic of the network appearance of the fibers of connective tissue. It principles underlying the major processes of leather manufacture, is very advantageous t o deal with these two layers separately, and the effects of variations in operations upon the finished because the structure of the reticular layer determines many of leather. For accounts of the development of each theory and the the physical properties of the leather, such as tensile strength, objections raised against it, reference must be had to the litera- solidity, resilience, temper, etc., while the structure of the thermoture, for time will not permit us to do more than t o state the stat layer determines more particularly the appearance of the theory that appears most plausible a t present. Any individual leather. In making the finer grades of leather, a great deal of theory presented must be accepted only with reservations. The attention must be paid to the thermostat layer. The reticular object here is merely to present a picture of leather Chemistry as layer is made up chiefly of interlacing bundles of fibers of white a whole. connective tissue, composed of the protein collagen. Collagen is the leather-forming substance of the skin. It is insoluble in cold Animal S k i n water, but hot water converts it into gelatin and dissolves it. Many of the important properties of leather depend upon the The reticular layer also contains blood vessels and nerves and a structure of the skin from which it was made. A knowledge of very small amount of yellow connective tissue made up of the the functions of skin will enable one to understand this structure protein elastin. It also contains blood and lymph. In some more clearly. One of the most important functions of the skin animals part, or even all, of the fibrous portion may be replaced is t o keep the body temperature constant. It is supplied with a by adipose tissue containing an abundance of fat cells. The wonderfully delicate mechanism which controls the escape of fatty tissue may even extend up into the thermostat layer. Since
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INDUSTRIAL AND ENCZNEERING CHEAUSTRY
this fatty tissue has no value for leather-making, the value of the skin decreascs in proportion to the amount of fatty tissue present. Figure 2 shows a portion of t h e thermostat layer of Figure 1 a t higher magnification. The individual cella of the epidermis can be differentiated. The epidermis may be likened t o layers of bacteria clinging to the surface of t h e true skin. The portion of epidermis in contact with the true skin is a layer of living epithelial cells, rather elongated in shape. In reproducing, each cell increases in height and then subdivides, forming two cells, one above t h e other. I I This p~ocessis going on continuously. the food being supplied by derma, the epid e r m i s h a v i n g no blood vessels of its own. As the older cells are pushed outw a r d , f o o d i s no longer available and the cells die, dry up a n d b e c o m e scaly, and are g r a d u a l l y w o r n a w a y . This scaling is often very n o t i c e a b l e on the scalp in the form of dandruff. T h e independent growth of the epidermis and derma involves a number of important a p p e n d ages of the skin. In the epidermal system tlie reproduction of epithelial cells p r o duces, not only the e p i d c r m i s , but also the hair and the oil and sweat glands. 4 glance a t Figure 1 will show t h a t t h e skin contains many pockets. A t the botFigure I-Verrlcal Secflon of Cowhide tom of each pocket, Taken from Butt. 16 x or hair follick. thcre is available a supply of nourishment from the true skin, which is used by the epithelial cells in the pocket and causes them t o reproduce very rapidly. The cells become so numerous that many are pushed up out oi the pocket. As they are forced out of the region containing b o d , they die and become glued together farming a hair. The hair is molded into the shape of the pocket and is either straight or curly, according t o the curvature of the pocket. TT7hen the tiny blood vessels supplying a hair follicle become hardened with age, they do not permit the passage of the larger particles containing pigmenting substances and so cells are reproduced containing no pigments and the hair becomes gray. When all flow into the follicle is Cut off, all the cells die and baldness results. When a hair is pulled out, another soon forms t o replace it. Attachcd t o the bottom of each hair follicle and extending obliquely upward through the thermostat layer, almost t o the surface of the skin, is a bundle of "on-striated muscle tissue, known as the erector pili muscle. This is clearly shown in Figure 2. Just above this mosfle and emptying into the hair follicle a t about its midpoint is a group of sebaceous 01oil glands. Just below the bottom end of the muscle there is a n open space
Vol. 21, No. 2
containing a collapsed sac, which consists of sudoriferous or sweat glands. These glands have ducts leading t o the surface of the skin and they supply water, the evaporation of which carries away the excess heat developed by the body reactions and prevents overheating of the body. When the outside temperature falls, the pilo-motor nerves communicate with the erector pili muscles, causing thcm t o contract. The appearance t o the naked eye is the phenomenon known as gooseflesh. The muscle is actually putting pressure upon the sebaceous glands. The oil cells are broken down and the oily material is discharged into the hair follicle and from thence to the surface of the skin, where it retards the evaporation of water and conserves some of t h e heat of the body. The proper balance between the operations of the two kinds of glands keeps the body temperature constant. The sweat glands assist also in the discharge of waste materials from the blood. When the sebaceous ducts become clogged with dirt, the oil is trapped and the pressure behind it causes the appearance of blackheads. Various kinds of pimples result from improper functioning of the oil glands due t o improper dirt. The high development of these glands in thc sheep and some other animals crowds and distorts the hair follicles so that they are curved and cause the production of curly hair. The thermostat layer contains also many fibers of yellow connective tissue, made up of the protein elastin. The names elastin, collagen, keratin, etc., arc used as though they indicated individual chemical substances, but they probably cover whole classes of closely related protein substances. Very few elastin fibers are found in the reticular layer. They seem to play some part in the operation of the thermostat mechanism. Collagen is the most abundant constituent of the skin and the &ost important to the tanner, because i t is the material finally converted into leather. The fibers farming the upper boundary of the derma arc composed of protein matter resembling collagen, but diffrring from collagen in resistance t o hydrolysis under C ~ I tain conditions. Keratin is the chief protein constituent of the epidermis and hair; it is removed as far as possible from the skin before tanning, except in the case of furs. The elastin appears t o be of no valuc in leather-making, but the fibers of elastin arc too tinyaiid too fewin numbertohave mucheffectupouthe properties of the leather if not rcmovcd. The albumins, globulins,and mucins are usuallyleached out of the skin in the operations of soaking, liming, and bating, prior t a taming. Thc sugars and inorganic salts arc not present in sufficient quantities t o have much effect. In a fewtypes of skins the fats, lecithins, and cholestrrols are sufficiently abundant t o cause trouble, if not removed prior to tanning. S t r u c t u r e of Proteins Lcathcr chemistry is very greatly concerned with the molecular structure of the proteins, particularly collagen. Unfortunately the structure of the proteins is not definitely known. Pischer demonstrated that condensation products of amino acids have properties that would class thcm as proteins, provided there is a sufficient number of amino acid residues in the molecule. He prepared one containing 15 glycine and 3 leucine residues, which had a molecular weight of 1213. It gave the biuret test for protein, piecipitated tannin from solution, and would have been classed as a protein had i t been found in nature. Abderhalden and Fodor later succeeded in preparing a polypeptide containing 15 glycine and 4 leucine residues and having a molecular weight of 1326. Fischer pointed out that this polypeptide was redly only one of 3876 possible isomers, without considering the tautomerism of the peptide linking. When more than two kinds of amino acids arc involved, the possible number of isomers increases very rapidly. If a protein be imagined made up of 30 molecules of 18 different amino acids, one taken twice, one three times, another three, one four, one five times, and thirteen taken once
INDUSTRIAL AND ENGINEERJNG CHEMISTRY
February, 1929
each, there would be loz7isomers, even if there were no tautomerism of the peptide group and if the linking took place only in the simple way as with monoamino-monocarboxylicacids. Holleman has pointed out that it is possible for each of the different kinds of living material to have its own individual protein and that the infinite variety of forms found in organic nature is partly the result of isomerism in the protein molecule. As the work on protein structure developed, Abderhalden and others were led to the view that the proteins contain dioxopiperazine nuclei capable of keto-enol tautomerism, thus
NH
/\
0:F
NH
HC.R
/\
€10.:
5.R
v NH
IH
where R represents an amino acid or polypeptide group. Herzog and k n e l l examined collagen with the x-ray spectroscope and concluded that it is much more simply constructed than had previously been supposed, being composed of a material having a molecular weight of about 700. This value is interesting in view of the work done in our own laboratorics which indicates a combining weight of 750 for collagen. In proteins like collagen, which have a jelly structure, it seems highly probable that the polypeptide or dioxopiperazine groups form continuous networks throughout the entire mass, there being no individual molecules in the orthodox sense of the term, just as we now know that a crystal of sodium chloride contains sodium and chlorine atoms, but no discrete molecules of sodium chloride‘. The protein structure is pictured as a three-dimensional network of atoms with interstices of such magnitude as to permit the free passage of water molecules and of simple ionogens.
183
true solution in the water present in the interstices of the protein network. This solution will contain hydrogen and chlorine ions in equal number derived from the ionization of the acid and, in addition, i t will have the chlorine ions from the substituted ammonium chloride, which we may call gelatin chloride. In other words, the solution in the interstices of the protein network wiU have 8 greater concentration of chlorine ions than of hydrogen ions. In the outer solution, free from protein, hydrogen and chlorine ions ate present in equal concentrations. This is essentially the condition described in Donnan’s now famous theory of membrane equilibria. It is obvious, from Donnsn’s reasoning, that the total concentration of chlorine plus hydrogen ions is p r a t e r in the jelly solution than in the external solution at equilibrium. This difference is the mcasuxe of a force tending t o cause ions t o pass from the jelly t o the external solution. The conditions here are differat from those pictured in Donnan’s theory in that there is no membrane. The ions in actual solution in the jelly find no mechanical obstruction t o their passage into the external solution, but are held back by the attraction of the anions for the positive charges on the protein network. The effect is that of a force pulling on the network and t a d i n g t o drag i t out through the external solution. Gelatin jellies follow Hooke’s law up to an elastic limit and the increase in volume is directly proportional to the excess in concentration of all ions of the jelly solution over that of the external solution. For any special case the quantitative relation involves two coilstants, the hydrolysis constant of the substituted ammonium chloride and the constant corresponding t o the bulk modulus of elasticity of the protein. Given these constants, all the relations can be calculated by simple ihermodynamic reasoning. Thi3 was done by Wilson and Wilson for the gelatin-IIC1 cquilibrium ovcr
Protein Equilibria If a skin is immersed in water and then acid is added very graduslly, the collagen fibers begin to swell by absorbing the aqueous solution. As more acid is added, the degree of swelling increases t o a maximum and then decreases as still more acid is added. The addition of salt causes a decrease in swelling. This effect was frequently noticcd by tanners of the nineteenth century. hut chemists were unable t o explain it. Toward the end of the last century the late Professor Proctcr, known as the “iather of leather chemistry,” concluded that any hope of developing a science of leather chemistry depended upon finding a solution of the phenomenon of protein swelling; unless one understood the comparatively simple phenomena observed in pickling, one could hardly hope t o understand the much more complex phenomena observed in tanning. He devoted the remainder of his life primarily to this problem. He chose what he considered the simplest possible case, the swelling of a strip of purified gelatin in contact with a solution of varying concentrations of hydrochloric acid and sodium chloride. The work finally culminated in the Procter-Wilson theory of swelling. It would be going too far afield to give the development of the theory here. We shall, rather. attempt t o draw a picture of the molecular mechanism of swelling without considering the mathematical relations involved. Consider a strip of gelatin immersed in a solution of hydrochloric acid. Water and acid molecules penetrate into the interstices of the protein network. It has been demonstrated that polypeptide and dioxopiperazine groups do not lose their reactivities when built up into protein-like structures. The protein contains trivalent nitrogen, which reacts with hydrochloric acid, farming a substituted ammonium chloride. The remarkable thing about this salt is that the cation is insoluble and the anion very soluble. The cation is part of the protein network and is not in true solution. The effect is to confer upon the network a positive electrical charge. The anion is in
Figure &-Vertical Section of Thermostat Layer of Cowhide. Taken from Same Section as Thar of Figure 1. 75 x
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INDUSTRIAL AhW ENGINEERING CHEMISTRY
the entire swelling range studied by Procter, and the agreement between calculated and observed results for degree of swelling and distribution of ions between the two phases was absolute, nitkin the limits of experimental error. This work not only furnished a new basis for leather chemistry, but i t started a new trend of thought in other branches of chemistry, a n outstanding example being the work of the late Jacques Loeb on colloidal behavior. It also opened up a new field of investigation on the nature of aqueous solutions in the region of electrically charged surfaces. When a charged particle is suspended in a n aqueous solution of some ionogen, the ions tend t o distribute themselves between the bulk of solution and the thin film of solution, wetting the particle in a manner analogous t o the distribution of ions between a n aqueous solution and a gelatin jelly. It has not yet been found possible t o measure concentrations directly in such thin films of solution, but by studying the relations involved in the case of the jellies mu& has been learned of the relations involved in the case of colloidal dispersions.
Vol. 21, No. 2 Curing
After flaying, it is customary t o cure the skin by packing it in sodium chloride. This protects i t against bacterial damage until it is ready t o be worked in the tannery. The salt diffuses into the skin, gradually saturating the water remaining in it. During this process the skin gives up some of its water, which flows away as brine. The fresh skin contains about 62 per cent of water and the cured skin, about 40 per cent. When the skinsare washed to remove blood and soluble proteins before salting, the curing is much more effective against bacterial damage. I n general, the greater the amount of soluble protein matter present, the greater is the concentration of sodium chloride required to prevent appreciable bacterial action. Sodium chloride and a number of other halide salts catalyze the hydrolysis of collagen. Pfeiffer found that halide salts form definite and fairly stable compounds with both amino acids and dioxopiperazines. Thomas found that those salts which most readily form such compounds are most active in increasing the hydrolysis of collagen. While halides catalyze the hydrolysis, sulfates inhibit it. This has raised the question as to the relative merits of sodium chloride and sodium sulfate as CUIing agents. I n practice the hydrolysis produced by sodium chloride must be very small. On the other hand, the bacterial damage resulting from the use of insufficient salt may be v e ~ y great. At low temperatures the solubility of sodium sulfate is very much less than that of sodium chloride. In comparing the two salts in a practical way, it is not easy t o ditrerentiate between the effects of the salts directly upon the collagen and the effects of the salts upon bacterial activity. On the other hand, the hydrolytic activity of the chlorides of calcium and magnesium is so great that skins would be destroycd if saturated with either. Even fully tanned leather will shrivel up when soaked in saturated solutions of calcium or magncsium chlorides. Soaking a n d Fleshing
Figure 3-Verticsl Section of Caifskin from Buff. Taken after 40 Hours In Lime ~ i ~ ~ 25 o rx .
The apparent attraction of the gelatin for the aqueous solution is the result of the acquisition by the solution of electrons belonging t o the atoms in the protein network. A similar condition obtains in the case of an aqueous dispersion of electrically charged particles: the charging endows the barticles with a n attraction for the water that increases the stability of the dispersion. The principles learned in this work on leather chemistry were applied also to the filtering and drying ofsewage sludge and made p~ssihle the successful operation of the great sewage disposal plant a t Milwaukee, conceded by many t o be the most efficient sewage dig. posal plant in the world. These few examples emphasize the direct bearing of the fundamental problems of leather chemistrr upon the fundamentql problems of many other branches of chemistry.
When the awed skin reaches the tannery, it is washed free from blood and dirt end then soaked in cold water to leach out the soluble protein matter and t o allow the collagen fibers t o absorb water. A uniform distribution of water throughout the skin is essential to permit satisfactory fleshing. After a t least a preliminary soaking, the areolar tissue, or flesh, is removed by placing the skin in a machine which forces the flesh side of the skin against a revolving roller set with sharp blades. After fleshing, the skins are sometimes soaked again in order the more completely t o remove soluble proteins. If soaking is unduly prolonged in warm water, the skins are certain t o sufferfrom bacterial damage, unless protected by some antiseptic. Bacterial activity can be greatly retarded by the application of about 100 parts of chlorine per million of water. It is the proteolytic enzyme secreted by the bacteria that does the real damage. When water colder than about 12" C. is used for soaking, in the ordinary way, antiseptics are unnecessary. because enzyme activity a t this low temperature is so slow that the soaking operation is completed long before any appreciable damage is done. It is probable, also, that much less enzyme is produced a t this temperature than at temperatures 10 t o 20 degrees higher. Unhairing Only the conncctive tissues of the skin are convertedintolcather. In separating the hair and epidermis from the derma, use is made of the diflrrence in reactivity towards acids and alkalies of kerntin, the chief protein of the epidermal system, and collagen, the protein of the white connective tissues. I n contact with alkaline mlutions, keratin is much more rapidly hydrolyzed than collagen; in contact with acid solutions, collagen is much more rapidly attacked than keratin. This was known t o the ancient tanners. who used limewater to looscn the hair. If a calfskin is soaked in
INDUSTRlAL AND EhrQIhrEERINQCEIEMISTRY
February, 1929
saturated limewater at 20" C. for abo& 5 days, the hair and epidermis will become loosened from the derma so that they can be rubbed off with a blunt blade. For many years tanners have known also that this action can he greatly accelerated by the addition of a small quantity of soluble sulfide. Figure 3 shows a cross section of a calfskin after soaking in saturatcd limewater containing 0.7 gram per liter of sodium sulfide for 2 days a t 25" C. The alkali has destroyed the epidermal cells which rest upon the true skin'. The older cells, forming the corneous layer of the epidermis, still cling together and appear as a continuous mass somewhat separated from the der-. Thc skin, after liming, is unhaired by machine. I t is thrown over a rubber slab and forced against a roller Set with blunt knife blades which rub off the loose hair and epidermis. A somewhat similar machine then scuds out the glands and epithelial cells still lying in the hair follicles. The role played by sulfide in accelerating hydrolysis of keratin has long puzzled chemists. Merrill showed that keratinremovec, sodium sulfide from solution, but that collagen does not. Nor does sulfide accelerate the hydrolysis of collagen by alkalies. The sulfide apparently reacts with keratin, producing a product which is much more rapidly hydrulyzrd by alkali than the original keratin. Many lines of investigation have brought forth the suggestion that the sulfide reduces the cystine residues of the+keratin molecule, effectinga break between the adjoining sulfur atoms. This view led Merrill t o predict that any strong reducing agent, such as stannous chloride, should accelerate the hydrolysis of keratin hy alkaline solutions, and his predictions were confirmed by experiment. Stannous chloride acts like sodium sulfide in the unhairing process. Bating Perhaps the most curious of all the processes involved in making leather is that of bating. For centuries this was one of the mysterious processes of the taiinery and, until comparatively recently, i t was casiiy the most disgusting. It has been so completely enshrouded in secrecy, jealously guarded, that it has not been possible to trace its origin. However, the process as handed down by the generations past is still a n unpleasant memory t o many tanners living today. It consisted in digesting the limed and unhaired skins in a warm infusion of the dung of dogs or fowls until all plumpness had disappeared and the skins had hecome so soft as to retain the kprcssion of thumb and finger when pinched and suffxciently porous to permit the passage of air under pressure. When this process was omitted, the leather always lacked its otherwise fineness of appearance. Wood demonstrated that the active principle of the dung was a tryptic enzyme. The tanning world was very quick to abandon the use of oiIensive dungs and t o replace them by pancreatic enzymes. However, it was not until very recently that chemists discovered the part played by the enzyme. Leather made from unbated skins has a certain characteristic, but undesirable, grainy appearance. Wilson and Daub studied the action of trypsin on limed calfskin under the microscope and found that it hydrolyzes the tiny elastin fibers in the thermostat layer. In practice bating is not carried this far. Wilson and Merrill demonstrated that the value of bating comes from the hydrolysis by the trypsin of keratose, a degradation product of keratin. Keratose is soluble in neutral or alkaline solutions, but is precipitated at its isoelectric point, p H = 4.1. It is formed in the thermostat layer during liming. If not removedpriorto taming, itbecomes precipitated in the thermostat layer by the acid tan liquors and thus causes the roughness of appearance characteristic of unbated skins. This discovery made possible the scientific control of a Process that had given the tanners more trouble than any other involved in the making of leather. The bating value of any enzyme preparation can now he determined easily by measuring its activity upon purified keratose.
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Vegetable Tanning After the skin has been bated and rinsed, it is ready to be tanned. Thousands of years ago the discovery was made that the properties of skin substance change completely when the wet skin is brought into contact with the aqueous extract of those f o r m s o f p l a n t life which have since come to be classed as veget a b l e t a n n i n g materials. A t a n l i q u o r may be made by grinding the bark of a tree and leaching it in a coffee percolator, just as one would prepare coffee. When a piece of bated skin is suspended in suchaliquor, it becomes c o l o r e d a tan shade. The color substances diffuse into the protein fibers and render them imputrcscible. The active principle of the extxact is called tannin. Collagen and tannin combine to form a new substance, leather, which is very much less readily hydrolyzed t h a n collagen. I n practice the batedskins are kept suspended in tan liquors until all of the collagen has been converted into leather. Because of the very low rate of d i f f u s i o n o f tannin into the intcrior of the skin, this usually
for very heavy hides. A voluminous literature now describes the attcmpts t o isolate and identify the active tanning principle af various lorms of plant life. Fischer isolated the tannin from Chincse nutgalls and then succeeded in synthesizing it, He prepared penta-mdigalloyl-6-glucose, which proved t o he an isomer of the tannin from nutgalls. Its formula is
F--1 HC-C-C-C-C-CH
where R is the radical OH
0 -.
HO OH O
O
H
O-OC Fischer's success spurred on studies of tannins from-different Sources, and it was found that they differ considerably in:composition and in properties. In spite of the voluminous literature on tannins which has been built up, the compositions:of the tannins
186
INDUSTRIAL A N D ENGINEERING CHEMISTRY
most widely used in making leather have escaped detection, The chemistry of the tannins is evidently still in its infancy, Even though the compositions of both collagen and the tannins are not definitely known, one may speculatQ from generalities, as to the nature of their combination. Tanning appears t o be analogous to the combination of a weak base with a weak acid. Collagen is an ampholyte with a n isoelectric point a t p H = 5. I n practice, tan liquors have a p H value less than 5 . I n contact with solutions more acid than p H = 5, collagen acts as a base. Tannin, on the other hand, acts like a weak acid; in an electrical field it migrates t o the anode. Collagen may be classed as a weak nitrogen base and the tannins as phenolic substances. Baeyer and Villiger studied the combination of phenolic substances with weak bases. One molecule of diethylenediamine will combine with one molecule of phenol. Two molecules of quinoline will combine with one ‘molecule of resorcin. This type of combination persists through all degrees of complexity of the nitrogen bases and phenols from aniline and phenol to protein and tannin, lending great support t o the simple chemical theory of vegetable tanning. Phenols combine similarly with oxygen bases. This has led Freudenberg t o suggest that the tannins may become attached to both oxygen and nitrogen atoms in the collagen molecule. If the combination of collagen and tannin is really that of a weak base with a weak acid, it follows that the tanning of collagen will reduce its capacity for combination with acid. Wilson and Bear demonstrated that the capacity of collagen to combine with sulfuric acid decreases as it becomes more heavily tanned. It also follows from the theory that an increasing degree of tannage must lower the isoelectric point of collagen. Gustavson tested this, using Loeb’s dye technic to measure the isoelectric point of the collagen-tannin compound. Tanning collagen with an extract obtained from quebracho wood caused a reduction of its isoelectric point from a pH value of 5.0 to 4.0; tanning with hemlock bark extract caused a reduction to 3.9. The theory was confirmed in another way by Thomas, who showed that deaminization of collagen greatly reduces its capacity t o combine with tannin. Because of the probability that the various tannins present in a tan liquor have very different molecular weights, attempts a t calculating combining ratios for collagen and tannin involve much speculation. In its combination with hydrochloric acid, collagen exhibits a combining weight of about 750. If each digalloyl radical in pentadigalloyl glucose is capable of combining with collagen, we arrive a t a combining ratio of 340 parts of this tannin t o 750 parts of collagen, or 45 per 100 parts of collagen. The great majority of analyses of vegetable-tanned leathers at our disposal show a ratio of combined tannin t o collagen lying between 45 and 90, suggestive of monotannates and ditannates of collagen. C h r o m e Tanning Although vegetable tanning is of ancient origin, chrome tanning is a development of only the last forty years. Most of the world’s supply of light leather is now chrome-tanned. A chrome-tan liquor usually consists of a solution of basic chromium sulfate. It can easily be prepared for study in the laboratory by bubbling sulfur dioxide through a solution of sodium dichromate until the reduction is complete. Usually the skins are pickled before tanning in order t o make them uniform in composition. Bated skin may contain variable amounts of calcium carbonate, which would disturb the tanning process. A common method of pickling consists in soaking the skins in a 12 per cent solution of sodium chloride t o which sulfuric acid is added to bring the solution t o a definite final equilibrium concentration. The skins are then transferred t o a revolving drum and tumbled in a chrome liquor until tanned t o the desired degree, which may require only a few hours or 2 or 3 days. The chromium salt penetrates
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the skin very rapidly as compared with vegetable tannin, colors it green, and renders it capable of withstanding the action of boiling water. The compound of collagen and chromium is extremely resistant t o hydrolysis. It was but natural t o suppose t h a t chromium forms salts with collagen analogous to the chromium salts of amino acids or of dioxopiperazines. The equivalent weight of chromic oxide is 25.3. Taking the combining weight of collagen as 750, a combining ratio was cilculated of 3.4parts of chromic oxide per 100 parts of collagen. Leathers have been made with 3.4, 6.8, 13.5, and 27.2 grams of chromic oxide combined with each 100 grams of collagen under conditions which favored the suggestion that they represented what might be termed monochrome, dichrome, tetrachrome, and octachrome collagen, respectively. During tanning the p H value of the chrome liquor usually lies between 3 and 4. At first sight it might be wondered how collagen could act as an acid in contact with a chrome liquor having an acid reaction much greater than the isoelectric point of collagen. The thought involved in the theory is that the ionization of collagen as an acid never becomes reduced to zero, even though it may become extremely small with increasing acidity. This means that, even if the electrical charge on the protein structure is predominantly positive, there still remains a very small, but finite, number of negatively charged groups scattered throughout this structure. Chromic ions diffuse into the collagen jelly and combine with these negatively charged groups wherever encountered. The ion which first combines with the protein may have only a single positive charge and might be indicated by the simplified formula, Cr(0H):. Having neutralized the electrical charges on each other, both the collagen and chromium compounds become capable of ionizing further, the chromium group giving off another hydroxide ion and the collagen another hydrogen ion. With a repetition of this process, all three bonds of the chromium become united directly with the collagen structure. The fundamental assumption underlying this view is that, however small may be the concentration of negatively charged groups in the collagen structure under the conditions of tanning, it is very much larger than would result from the dissociation of the chromium compound of collagen. The remarkable resistance of chrome leather to hydrolysis is in line with this view. Recent work has made it quite clear that this simple theory of the combination of chromium and collagen does not represent the whole fact. Many phenomena have been observed which could be explained only by introducing Werner’s coordination theory, which views a chromium ion, not as a chromium atom with three positive charges, but as an electrically charged micell or nucleus in which a central chromium atom is surrounded by six coordinatively bound groups. Gustavson has shown that it is such a micell as a whole that combines with the protein. Some atoms are able to combine with others, not only by means of their recognized primaryvalency forces, but also by means of additional forces called auxiliary valencies. According to Werner’s theory certain atoms tend to draw to themselves, in the form of surrounding shells and by forces other than primary valency, a number of other atoms or coordinated groups. The central atom with its coordinated groups constitutes a nucleus outside of which are located the atoms or radicals which are held to the rest of the molecule by primary valency forces. The coordination number of a n element indicates the number of groups which an atom can hold in this surrounding shell. Most metals have a coordination number of six, while some of the non-metallic elements have a coordination number of four. Three different forms of chromic chloride are known. The first, called the a form, is a violet salt of the formula CrC13.6Ht0. All of its chlorine atoms are precipitated from solution by the addition of silver nitrate. The second salt, called the B form, is a green salt of the formula CrCla.5H20. Only two-thirds of its chlorine is DreciDitated from solution by silver nitrate. The .~~~
-~
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INDUSTRIAL A N D ENGINEERING CHE.MIISTRY
third salt, callrd the y farm, is a green salt of the formula CrCIH.4H,O. Only one-third of its chlorine can be precipitated from solution by silver nitrate. The structural formulas for these three salts, according to Werner's theory, are as follows:
The nudeus of the a form h a s three loose electrons which go to complete the octet in each of the outer shells of electrons of the three chlorine atoms, converting them into chloride ions capable of precipitation by silver nitrate. Under certain conditions one chloride ion will penetratc into the nucleus, carrying its extra electron with it and displacing one of the six cwrdinatively bound water molecules. The number of coordinative groups is kept at six, but now there arc only two positive charges on the nucleus, one of the three loose electrons having been returned t o the nucleus by the incoming chloride ion. When the chloride ion enters the nucleus it ckases t o be a separate ion, but forms part of the complex which constitutes the nucleus. Since only two chloride ions are left, only two-thirds of thc total chlorine can now be precipitated by silver nitrate. When the second chloride ion entcrs the nucleus, replacing a second water molecule, only one chloride ion is left capable of precipitation by silver nitrate. The nucleus then has only a single positive chargc. Reasoning by analogy t o other compounds, we can picture four more chromium chlorides. I n the presence of hydrochloric acid and sodium chloride it is possible for all coordinativcly bound water molecules t o be displaced by chloride ions in the following order:
Sodium pentachlore-aquo chromiatc
Sodium hexiiehioro chromiite
The trichloro-triaquo-chromium is not known, but a corresponding alcoholo compound has been prepared. The chromiates exist in the prescnce of a large excess of hydrochloric acid, although they do not form nearly so readily as the corresponding oxalato or similar compounds. An interesting and important fact is that a solution of chromium salts may contain chromium nuclci of variable electrical charge from three positive t o thrce negative chargcs per atom of chromium. In the electrophoresis of ordinary chrome tanning liquors it is usual to find bath anodic and cathodic migration of the chromium a t the same time. When alkali is added slowly t o a solution of a chromic salt, very complex nuclei are formed. owing t o the tendency for chromium atoms t o share hydroxo-groups, thus:
187
building up complex chromium nuclei has been called olhication. It may be extended indefinitely by displacing aquo-groups by hydroxo-groups. By adding 2.5 equivalents of sodium hydroxide per mol of chromic chloride very slowly, allowing sufficient intervals between additions for olification to take place, Bjerrum succeeded in preparing an ol-compourid with a nucleus containing 12 chromium atoms. Gustavson has devised a very ingenious method of studying the composition of positively charged chromium nuclei in which the solution is allowed t o react with sodium permutite and the insoluble chrornium compound of permutite which is formed is washed free from the adhering solution and analyzed. His investigations with this method have thrown much light on the mechanism of chrome tanning. It is evident from his work that it is the entire chromium iiucleus which combines with collagen in tanning. Werner's theory presents threc diffcrent types of possible combinations of chromium and collagen: ( I ) the acidic groups of the protein may combine with a positively charged chromium nucleus; (2) the basic groups of the protein may combine with a negatively charged chromium nucleus; ( 3 ) certain groups of the protein may penetratc into the chromium nucleus, becoming coordinativcly bound and replacing other groups from the nucleus. By c o m b i n i n g types (2) and ( 3 ) one can conceive a combinatimi. in which the chromium is bouiidto the protein by nine bonds per chromium atom. When six negative protein groups occupy all six co5rdinative positions about the central chromium atom, the nucleus acquires three negativc charges, p e r m i t t i n g further c o m b i n a t i o n with three basic protein groups. Gustavson t a n n P d c o l l a g e n w i t h both cationic and a n i o n i c c h r o m i u m nuclei. W h e n a n i o n i c chromium was used, the isoe l e c t r i c point of the collagen droppcd from pH = 5 to p H = 4, just as in the case of vcgelablc tanning, ind i c a t i n~- combination Fisure §-Vertical Seftlon of Chromewith the basic protein Tanned Calf Leather from B u f f . x g r o u p s . Whencolla- Skin a8Thsf S h o w n i n F i ~ u r e 4 . 75Ssme gen was tanned with cationic chromium, the isoelectric point rose from 5 to 6, indicating combination with the acidic protein groups. Tlie isoelectric points were determined by the dye technic. A mass of experimental data accumulated during the past few years indicates that the chief reaction occurring in ordinary chrome tanning is the combination ol cationic chromium nuclei with collagen acting as an acid, but that all threc types of cambination occur to some extent. I
The cohrdinativcly bound groups about a central chromium atom may be likened t o the electrons in the outer shell of a n atom and the central stom to the nucleus of an atom. The two hydroxogroups may be likened to the two valence electrons that hold two chlorine atoms together as a molecule. This process of
Miscellaneous T s n n a g e s Many salts of heavy metals are capable of tanning collagen, but none are quite so satisfactory as those of chromium. For this
INDUSTRIAL A N D ENGINEERING CHEMISTRY
188
reason chromium salts are the only ones used very extensively. Studies of the tanning action of ferric and aluminum salts by Thomas indicate a mechanism similar to that of tanning with cationic chromium. The tanning action of fish oils has been known for many centuries. In the modern manufacture of chamois leather the tanning agent is cod-liver oil. The leather is made from the reticular layer of sheep skin, which is split from the grain layer so that the two layers may be tanned separately by different methods. The flesh splits are pickled and then swabbed with cod oil, after which they are pommeled in a specially built machine. They are spread out t o cool, re-oiled and pommeled, alternately. During the process acrylic aldehyde and other pungent products are evolved. By the time the tanning action is completed, practically all of the water has been replaced by oil. The leather is washed in a warm solution of sodium carbonate t o remove the free oil and any free acid left from the pickle. The leather is then washed in running water, dried, bleached in strong sunlight, staked, and buffed t o make it smooth. Chambard and Michallet showed that chamoising consists of a chemical reaction involving collagen, the free fatty acids of a n easily oxidizable oil, oxygen, and water, in which collagen is converted into a different protein more resistant t o hydrolysis than the original collagen. Payne and Pullman patented the use of formaldehyde as a tanning agent in 1898. Since then, many studies have been made of its combination with collagen and with gelatin. Formaldehyde combines with amino acids and dioxopiperazines, the points of attachment being the nitrogen atoms. In proteins it is evident that the formaldehyde becomes attached t o the basic groups, as indicated by the shift in the isoelectric point of collagen t o a more acid region and the fact that a formaldehyde tannage reduces the capacity of collagen t o combine with acid, tannin, or chromium. It will be remembered that deaminization of collagen has a similar effect. Quinone has been found t o be a very powerful tanning agent, rendering collagen quite inert in boiling water. Meunier has suggested that the mechanism of quinone tanning is indicated by the following equation : 0
RNHz Collagen
+ 2CsH402 = R.N(0)C~H~ + Ce.Hk(OH)% Quinone
Quinone leather
Hydroquinone
This was confirmed by Thomas and Kelly, who showed that quinone actually does combine with the basic groups of the collagen. In 1913 Stiasny presented to the world a new kind of tanning material consisting of sulfonated aromatic compounds condensed with aldehyde in such manner as to form soluble products. Wolesensky has suggested that these compounds, called syntans, enter into chemical combination with collagen through the interaction of the sulfonic groups with the amino groups of the protein. These compounds are now used widely in admixture with vegetable tanning materials, whose action they modify in a desirable way. The waste sulfite liquors from paper mills furnishes a valuable tanning material called sulfite cellulose, which resembles syntans in certain properties. It is apparent, from the wide variety and chemical nature of the materials used t o tan animal skin and the differences in properties of the leathers produced, that no one chemical equation can be given which will cover all tanning reactions. It is possible, however, to generalize. Collagen and gelatin exhibit marked attraction for water and are readily hydrolyzed. When they undergo chemical changes which markedly decrease their attraction for water and tendency to hydrolyze under a variety of conditions, they are considered to have been tanned. There are probably a number of definite points in the protein molecule where hydrolytic splitting takes place or where water or highly ionized molecules may become attached. When a substance
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combines with the protein a t these points and becomes so firmly attached as to prevent any further combination with water or highly ionized molecules, the effect is one of tanning and the substance is listed as a tanning material. It is possible that the same effect may also be obtained through some rearrangement in the protein molecule not involving actual combination with some other material.
Fat-Liquoring Although the tanning of animal skin lessens the tendency for the fibers to stick together upon drying, it does not lubricate them so that they slip easily over one another. In fact, when leather is dried immediately after tanning, without further treatment, it is usually very stiff and will crack upon bending sharply. In order to give it the desirable softness and pliability and to increase its tensile strength and resistance t o tearing, oils and greases are incorporated into it to lubricate the fibers. For the finer leathers, such as calf and kid, this is done by tumbling the wet leather in a n emulsion of oil in water, a process called fat-liquoring. A fat-liquor emulsion usually consists of an oil, such as neat’s-foot or cod-liver oil; an emulsifying agent, such as soap, sulfonated oil, moellon degras, or egg yolk, or combinations thereof; a material t o adjust the p H value, such as borax or sodium carbonate; and water. When any of the more common types of leather are brought into contact with water, they acquire a positive electrical charge with respect to the water. On the other hand, the oil globules suspended in the fat-liquor possess a negative charge. When leather and fat-liquor are brought together, the positively charged leather structure and negatively charged oil globules tend to neutralize each other and the emulsion is broken, not by the globules coalescing with each other, but by their condensing on the surface of the leather fibers. In fat-liquoring, the oil globules do not completely penetrate the leather, but become lodged in the outer layers. When neutral oils are used, they tend to penetrate more deeply into the leather during the subsequent drying, replacing the water lost through evaporation. This type of penetration also occurs where wet leather has been swabbed with oil or grease instead of being fatliquored. The extent of penetration of the oil into the leather has a marked influence upon its physical properties. In chrome calf leather, much used for shoe uppers, we have found the most desirable physical properties t o result from a rather high concentration of fat in the outer layers, the amount decreasing to zero as the middle of the leather is approached. One of the most common fat-liquors for chrome calf leather is a mixture of sulfonated oil and egg yolk. Unlike neutral oils, sulfonated oils do not diffuse towards the middle of the leather during drying. The fixation of sulfonated oil by chrome leather resembles the fixation of tannin by raw skin in vegetable tanning. Apparently chemical combination takes place between the leather and the oil, which explains the failure of the oil to shift its position during drying. The nature of this fixation has received much attention, It now appears probable that the sulfo-fatty acid radical penetrates into the chromium nucleus, becoming coordinatively bound and displacing other groups. It has frequently been observed that chrome leather loses some of its resistance to boiling water upon being fat-liquored with sulfonated oil. This can be explained by the theory of chrome tanning just presented, which assumes that the force holding chromium and collagen together is the attraction of the chromium nucleus for its electrons which are held by the protein structure. Each sulfo-fatty acid radical which penetrates the chromium nucleus carries an extra electron with it and the attraction of chromium nucleus for the collagen structure is lowered correspondingly. This view has been tested in a number of ways. Various anions can be placed in a sort of electromotive series according to their ability to displace or be displaced by others from the
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INDUSTRIAL AND ENGINEERING CHEMISTRY
chromium nucleus. For example, a tartrate ion will displace a phosphate ion and a phosphate ion will displace a sulfate ion, etc. When chrome leather is treated with sodium phosphate, the phosphate ions displace sulfate ion from the nucleus, whereupon the leather appears t o lose its power t o combine with sulfonated oil. The inference t o be drawn is that the sulfo-fatty acid radical penetrates into the nucleus more rapidly than sulfate ion, but less readily than phosphate ion. The tartrate ion penetrates the nucleus so vigorosuly as t o effect complete detannization of the leather. While the foregoing reasoning is admittedly extremely speculative, it has proved very useful in explaining observed variations in fat-liquoring in actual practice. Since the value of a theory lies in its usefulness, the reasoning, however speculative, seems t o be justified. Coloring The shade of color desired in a given piece of leather is produced by a combination of dyeing and finishing operations. Sometimes leather is given the desired color in the dye bath and then finished in such manner as not to change this color, but often the finishing processes alter the color of leather, making it necessary to correlate the processes of dyeing and finishing in order to get the desired effect in the finished leather. Some leathers are colored before and others after fat-liquoring, while still others are given a bottom color before and a top color after fat-liquoring. It is possible to color some leathers satisfactorily by the application of only a simple solution of dyestuffs, but others require a series of baths of mordants, strikers, bottom colors, and top colors. Tanners find it convenient to divide the dyestuffs used in coloring leather into three classes-acid, basic, and direct. It has been shown that collagen combines with acid dyes only at pH values less than 5 and with basic dyes only at p H values greater than 5. This is in line with the amphoteric nature of collagen, which acts as a base at p H values less than 5 and as an acid at p H values greater than 5. The purely chemical theory of dyeing has been given very strong support by the work of Chapman, Greenberg, and Schmidt on the combination of acid dyes with gelatin. At p H = 2 they found that 1 gram of gelatin combined with 0.00104 gram equivalent of any one of six different acid dyes. This enabled them t o calculate a combining weight of 962 for gelatin. Deaminization of the gelatin caused it to lose its capacity to combine with acid dyes by an amount equivalent to the nitrogen removed by deaminization. Both chrome- and vegetable-tanned leathers have a n acid reaction a t the time of dyeing. Since the protein of leather is never completely saturated with tanning material in practice, we should expect either kind of leather to combine vigorously with acid dyes; and this is the case. A t the low p H values obtaining in leather-dyeing we should not expect combination t o any appreciable extent between collagen and basic dyestuffs. However, combination may conceivably be brought about through the agency of some other material capable of forming stable compounds with both collagen and the basic dyestuff. Tannin is such a material. It precipitates basic dyestuffs so completely as to be used as a test for the presence of basic dyes in solution, and it forms extremely stable compounds with collagen. If the active valencies of the tannin are not completely saturated in the combination with collagen, then a further combination with basic dyes is possible. The fact is that basic dyes combine vigorously with the collagen-tannin compound a t a p H value of about 4,whereas they do not combine with collagen alone t o any appreciable extent a t this p H value. Nor do they combine with chrome leather to any appreciable extent a t this p H value unless the leather has first been treated with tannin or some other mordant. The assumption is thus warranted that the combination of basic dyes with leather is between tannin or similar groups acting as acids and the dyes acting as bases.
189
The direct dyes are extremely weak color acids or their salts. Whether because of the extremely low power of dissociation of the color acid or some other cause, direct dyes do not combine readily with the protein of vegetable-tanned leather as is the case with acid dyes. But when skin is chrome-tanned, it takes up direct dyes from solution with such vigor that nearly all of the fixation takes place a t the surface and very little dye penetrates into the leather. Here the chromium is evidently acting as a mordant. It seems highly probable that direct dyes have a powerful tendency to penetrate into the chromium nucleus, becoming coordinatively bound. Varo found that pretreatment of chrome leather with tartrate ion, which enters the chromium nucleus with very great avidity, causes the leather to take up direct dyes less readily, permitting them to diffuse into the leather. Similarly, Gustavson showed that tannin readily enters the chromium nucleus, which explains why chrome leather loses some of its affinity for direct dyes when retanned with tannin. It seems logical to assume that acid dyes combine with chrome leather in two ways-by direct union with the protein and by entering the chromium nucleus as pictured for direct dyes. Finishing Leathers which have been colored, fat-liquored, and dried rarely meet the demands of the ultimate consumer without further treatment. They may not have the desired temper; the grain surface may not be sufficiently lustrous and the color may lack the desired uniformity, tone and depth; defects in the grain may be too pronounced; and the leather may absorb water too readily. In finishing, the tanner tries to develop those finer qualities of the leather which are most appreciated by the consumer. The finishing operations may be divided into two classes. One involves the application to the leather surface of such materials as casein, albumin and other protein materials, gums, mucilages, resins, waxes, pigments, dyes, and lacquer materials. The other includes mechanical operations, such as staking, rolling, brushing, glazing, buffing, graining, embossing, trimming, ironing, and plating. In the preceding operations the skins are treated in lots of several hundred, but in finishing each skin receives individual attention and its own special series of operations. Finishing is really a fine art requiring great skill. Much of the value of the leather depends upon the ability of the finisher. Properties of Leather Users of leather know surprisingly little about its properties. Their choice is usually determined by appearance rather than by the possibilities of service. Makers of leather, on the other hand, merely strive to produce what users will be likely to choose. As a result relatively little effort has been made to discover and control those properties which really mean most to the ultimate consumer. This unsatisfactory condition is aggravated by the fact that studies of the properties of leather are exceedingly difficult and time-consuming. In many cases it is not apparent just what properties are important for certain purposes. In other cases the properties may be recognized by experts, who have difficulty in describing them to laymen, the properties in question never having been named. Methods for measuring many properties have yet to be developed and each property will have to be measured as a function of many different variable factors before any clear picture can be drawn. When the important properties of leather have been defined, there will still remain the task of educating the people who use it. There is a ray of hope in the recent organization of a national committee for the purpose of recognizing, naming, measuring, and cataloging the important properties of leather. The chief aim of this committee is to render leather of the greatest possible service to mankind. There have already been discovered several quantitative relations between important properties of leather
'
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INDUSTRIAL AND ENGINEERING CHEMISTRY
and controllable variable factors and the prospects are bright that the ultimate consumer will find a steady improvement in quality of leather. When used for footwear, leather must have great strength and yet it must stretch sufficiently t o permit the shoe t o conform perfectly to the shape of the foot. It has been shown that the greatest variable factor iduencing strength and stretch is the extent to which the thickness of the leather is reduced by splitting. When a sample of chrome calf leather was split into two layers of equal thickness, the grain layer was found to be only 26 and the flesh layer only 16 per cent as strong as the unsplit leather, per unit width. The resistance of the leather to stretch was found to vary directly with the strength. Increasing the oil content of leather caused a n increase in strength with a decrease in resistance to stretch. Increasing the water content, which occurs naturally with increasing relative humidity of the atmosphere, has a similar effect. There is a very great difference in both strength and stretch over the area of a skin. Quantitative relations have been plotted for both chrome- and vegetabletanned calf leathers, but the task of utilizing the information for the benefit of the consumer is difficult. I n the first place changes made to alter strength and stretch values also change other important properties of the leather. Secondly, while increasing ease of stretch enables the shoe more readily to conform t o the shape of the foot, it also causes the shoe t o lose its shape and fineness of appearance more quickly. This is very marked in comparing shoes of calf leather with those of kid leather, which stretches much more easily. Another important property of leather is its power to ventilate the foot. I n this respect it is a very remarkable material. It can be made water-repellent from the outside while still retaining the power to pass water from the foot to the outside air. Tests have shown that a good shoe-upper leather will transmit water from a moist to a dry atmosphere about 70 per cent as fast as though the two atmospheres had direct contact over the same area. This power is decreased in proportion to the kind and amount of finishing material, such as casein, wax, lacquer, etc., applied to the surface. A small amount of finishing material very greatly increases the resistance of the leather to wetting. With the first application of finishing material, the water-repellence of the leather is increased out of all proportion to the decrease in ventilating power. Quantitative studies of these relations have
Quantitative Relations of the Countercurrent Washing Process
Vol. 21, No. 2
made it possible to increase the serviceability of a shoe very greatly. The comfort of a shoe is also largely determined by the temper, elasticity, flexibility, and resilience of the leather. These properties are greatly influenced not only by the amount of oils incorporated in the leather, but also by their distribution throughout the thickness of the leather. Investigations in this field are complicated by the necessity of analyzing the leather a t different depths and a t different locations over the area of the skins. In the early part of this lecture a property was mentioned which greatly affects the wearer of shoes-namely, the tendency for the leather to suffer dimensional changes with the relative humidity of the atmosphere. Many people have attributed to their toes the power to foretell changes in the weather, little realizing that the pain was merely a n indication that the leather in their shoes was shrinking. In going from a dry to a moist atmosphere, chrome leathers increase in area by a n average of about 18 per cent. They shrink in area correspondingly when the relative humidity falls. Vegetable-tanned leather, on the other hand, undergoes changes in area only one-third as great as this. For reasons that had nothing to do with the comfort and happiness of the ultimate consumer, it became desirable for the tanner and shoe manufacturer to have about 95 per cent of all the sole leather vegetable-tanned and about 95 per cent of all the shoe-upper leather chrome-tanned. When this was brought about, no one suspected the differences of shrinkage and expansion of the two kinds of leather. The shoe upper is thinner and tends to reach equilibrium with the air much more quickly than the very thick sole, and so the changing size is much more effective when the upper is chrome-tamed. Nearly everybody wears shoes with chrome-tanned uppers, which are subject t o these great size changes with changing atmospheric conditions. For this reason most people suffer unnecessary discomfort. The tanner will use any method of tanning which the consumer demands, but, like most people, is slow t o make a change until the demand is urgent. The interesting scientific fact is that the kind of tannage so greatly influences the power of leather to take up water and t o change in size. It has not been possible, in this lecture, t o do more than give just a glimpse of leather chemistry as a whole and t o show what the leather chemist is doing t o make the footsteps of his fellow man a little less weary.
ANfl - A [AA'+l - 1 1 weight of solute recovered weight of solute fed number of tanks werght liquor transferred as liquid weight liquor transferred adhering t o solid
=
where L =
W = N = A =
Editor of Industrial and Engineering Chemistry: In the article by Ludwik Silberstein under this title, IND. At the end of his paper the author gives an illustrative example, ENG.CHEM.,20, 899 (1928), the author's mathematical analysis which will now be solved by the above equation. For the case leads to equations which are correct, but the work is needlessly where N = 4 tanks, we have
involved. The consideration of limits gone through to find conditions in the final steady state is unnecessary, as these relations can be derived very easily by simple material balances. Moreover, the author has apparently entirely overlooked certain similarities in the equations he develops for systems of different numbers of tanks, and has therefore failed t o discover that a single equation can be written for a system of any number of tanks. This has led him t o an enormous amount of work in developing a new equation for each system, as the process is quite laborious for systems of three or more tanks. Some time ago the writer had occasion to attack this problem, and developed and proved an equation applicable to a system of N tanks.
700 = 1.868 375 W = (0.40) (375) = 150 pounds L = 150 [1.8686 = 144 pounds recovered 1.86S5 - 1 150 - 144 = 6 pounds lost -(6) = ('O0) 1.6 per cent concentration, agreeing with the 375 author's figure. SMITH D. TURNER
A
=
-
~
HUMBLE OIL & REFINING COMPANY BAYTOWN, TEXAS October 13, 1928
