134 SALT-“A THEORY OF LEATHER DYEING” [May 1928

had to propose the health of the guests present’ Amongst the guests was Mr. E. P. T. Elyard, Renter Warden of the Worshipful Company of Dyers of the City of London; Manchester was gratified that that ancient and worshipful com- pany was once again in close touch with the craft which it was originally formed to protect and to encourage, and he desired to express to the Warden their thanks for the interest which he and his Company took in the Society and in the craft.

They were also honoured by the presence of Sir Josiah Stamp, whose speech they had all greatly enjoyed, and Dr. R. H. Pickard, whom they were in the process of making one of them- selves, as he was now the head of the British Cotton Industry Research Association.

It was a rather curious coincidence that their speakers were representative of practically all the crafts covered by the Society. Mr. W. E. Thompson, as a merchant, was representative of the selling of their goods; there had been a dyer in Mr. Thorp Whitaker; the President and Mr. Hickson were colour manufacturers; Sir Josiah Stamp represented the transport of the goods; he himself happened to be a calico printer; and the educational side of the craft was to the fore in the person of Principal Mouat Jones. He had the pleasure of proposing the health and prosperity of their guests, and coupling with that toast the name of Principal Mouat Jones.

Principal B. Mouat Jones replied in a most humorous speech which was greatly enjoyed. He also referred to the resolution recommending the establishment of a Chair of Dyeing in the College of Technology, which, he said, speaking personally, unofficially, and without prejudice, had been in his mind for some time, and so far as he, as an individual, had anything to say to it, it would meet with his cordial support.

THE PRESIDENT. Professor I?. M. Rowe proposed the toast of

“The President,” who shortly responded, and the proceedings terminated.

WEST RIDING SECTION

Meeting held a t the Midland Hotel, Bradford, on 15th December 1927, Mr. H. JENNINGS in the chair.

A Theory of Leather Dyeing H. SALT

A clear knowledge of the processes involved in the manufacture of leather is essential to the good dyeing of leather.

Even a perfect skin, if such is to be found, will not be uniform in substance and will probably contain a certain amount of grease; and many defects are always found in hides and skins, both those occurring naturally and those

caused by flaying and preservation (or the lack of preservation). Clearly the leather dyer has to take account of difficulties not met with in textile dyeing, and in addition the difficulties of the textile dyer are all met with in leather dyeing.

Hides and skins are protein matter which bears a close resemblance to wool in chemical properties except that wool is not so reactive. The processes preliminary to tanning are such as will tend to open up the chemical structure; these include a treatment in alkaline liquors to loosen hair and epidermal matter, and a partial digestion to give fat-splitting and loosening of fibre. Hide substance ready for tanning is a re- active amphoteric matter, and this has a very distinct bearing on the tanning and subsequent dyeing processes.

The tanning methods ordinarily employed in making leather are of two types-mineral tannages and vegetable tannages. Mineral tanning employs basic salts of chromium or, sometimes, aluminium, and the basicity (or acidity) of such salts is about 50% of the normal. The basic chromium salts have long been understood to cause tanning, probably by occupation of the acid groupings of the hide. On the other hand, the vegetable tannins, which are all acids, cause the formation of leather by combination with the basic groups of the hide.

There has been no systematic investigation of the dyeing process on leather until the publication of some of the work now referred to. These investigations were begun with only one general assumption, viz., that colour bases would combine with acid groups and colour acids with basic groups.

The behaviour of chrome leather was first studied. It was found that acid and direct colours were readily taken up by chrome leather and basic colours were not. Apparently the acid groups are occupied in accordance with theory. Moreover, a chrome leather free from acid is capable of taking up very large amounts of acid and direct colours, indicating the possi- bility of all the basic groups being available for combination. The use of tannins on chrome leather before dyeing (a common practice in the leather trade) reduces the attraction of such leather for acid and direct colours, and the reduction corresponds very closely to the amount of tannin used. At the same time the amount of basic colour taken up is found to increase with the tannin fixed.

Small amounts of tannin (say 8%) repress the combining power of the surface of chrome leathep with respect t,o colour acids, but a t the same time penetration to the interior can take place. Clearly tannins and colour acids take up the basic groupings, and the chrome tannage occupies the acid groupings of the hide sub- stance.

Vegetable leather should be able to take up basic colour very readily for two reasons-the

May 19281 SALT--“A THEORY OH LEATHER DYEING” 135

large amount of tannin present and the free acid groups of the hide. This is found to be the case, and when retanned with basic chromium salts the absorption of basic colour is reduced. However, contrary to expectation the normal vegetable leathers can be dyed with acid colours, although they never exhibit the fullne3s of shade found on chrome leathers. Here was a difficulty which a t first seemed to upset the theory, since all the basic groups ought to be occupied by the tannin. It has been found that such dyeing only takes place when some of the tannin is loose and capable of ionising into the dyebath with the resultant exchange of acids between acid colour and hide. Apparently the leather-colour acid compound is insoluble, and a gradual exhaustion of the dyebath takes place. If before dyeing the tanning is fixed by drying thoroughly or by using tartar emetic, no dyeing takes place with acid colours.

This behaviour brings up some interesting problems of a chemical nature. If this exchange of acids takes place, and clearly it does, modi- fication of the dyebath should give little or much combination, according to the conditions. For instance, at, say, pH2, any free basic groups in a leather should be very reactive, and acid colours should be taken up rapidly and retained in the surface layer. This is the observation always made, with the additional observation that the colour is usually uneven. On the other hand, at, say, pH6, any free basic groups will be little activated, and even-dyeing and pene- tration to the interior should take place. In the case of vegetable leather, uneven dyeing is not so common as in the case of chrome leather dyed with acid or direct colours. On the assumptions above chrome leather a t pH6 should have little affinity for colour acids, result- ing in even-dyeing and penetration. This is found to be the case, and when a slow addition of acid is made the colour is developed evenly on the surface. This modification of a chrome leather dyebath is cheap, the use of sodium or ammonium acetate being quite satisfactory, subsequent addition of sulphuric acid being made. For penetration in making chrome suedes the buffer solution is allowed to run in for the greater part of the dyeing period, whereas for even surface development the acid is added soon after dyeing commences.

The chief point of theoretical interest is that free groupings in a leather behave exactly as one expects them to behave, even if the other groups were not occupied by a tannage; and the groups occupied by a tannage are completely blocked unless ionisation can take place. It is reasonable to draw the conclusion that leather dyeing can be explained on chemical lines, and that such dyeing is a chemical reaction.

DISCUSSION Mr. W. Y. Walker said i t was evident that

leather dyeing presented inherent difficulties A5

even more pronounced than those met with in textile dyeing. There was no doubt that the initial preparation of the leather and its subse- quent dyeing were closely connected.

Mr. E. N. Noble said the amphoteric com- position of wool and of leather probably accounted for the similarity between the dyeing of the two substances.

The Lecturer said that so far as the chemical theory of dyeing leather was concerned, he had published a certain number of results, but there was no such thing as a literature of leather dyeing. With regard to the chemical theory of the dyeing of wool and silk, it appeared to have been overlooked that in the modern sense chemistry was concerned with the union of elements or molecules by means of potential differences, which involved the electrical theory ; and colloidal matters were combined by chemical means or by potential differences, whilst adsorption again was entirely due to potential differences and was chemical in character. In other words, colloidal, electrical, adsorption, and chemical theories, in the modern view, were simply different ways of looking a t the same thing. He had done some work on the dyeing of leather with pigments and earths, dyeing them, similarly to water-soluble dyes, either with acids or alkalia. If a pigment was used with an opposite charge to the leather in alkaline solution, when the dyebath was made alkaline the pigment went on to the leather and would withstand ordinary washing in cold water. If the pigment had a different charge in acid solution from the leather, it would go on in acid solution. A dyebath could be exhausted in that way. Assuming that colloidal action, electrical action, adsorption or surface action, and chemical action were the same things under different names, the simplest general statement was to say that they were all potential differ- ences causing charged bodies to come together and to adhere by attraction. In that case there was sufficient justification for assuming that the chemical theory of leather dyeing was the one to adopt.

Dr. L. L. Lloyd said he thought there was a tendency to arrive a t conclusions too quickly in propounding theories of dyeing. It seemed to be assumed that all amphoteric bodies possessed definite basic groups and acidic groups, but the amphoteric ureides, according to their tauto- meric form, might be either acidic or basic compounds, one group being more active than another, according to the reaction under obser- vation. Barbituric acid could be transformed either into the anodic or the cathodic form, one being more basic and the other more acidic than the other. The theories put forward by Fort and himself (see this Jour., 1914, 30, 5, 73) assumed that the ketonic group acted as an acidic group, that the acidic carboxyl group also existed, and that amino-groups were present. It was now doubtful whether these groups actually exist,

136 SALT-“A THEORY OF LEATHER DYEING” [May I928

but they are possibly formed in certain reactions. As regards the electrical and adsorption theories, he thought they would get back to the older method of explanation by means of osmosis, in which one dealt with the dispersion of the substances used as dyestuffs. When sufficiently dispersed they could penetrate into dissimilar colloids ; by conversion into a lower dispersion they were then retained. He thought that was the only method of explaining the dyeing of sulphide colours on leather.

The Lecturer said an amphoteric compound behaved in certain suitable circumstances as an acid and in other circumstances as a base. In the case of hides, there were far more free groups of each type than in wool. It was possible, nevertheless, by treating the wool, to open up the groups a good deal. The chief point to remember was that in an acid solution the basic groups were active and able to combine; in an alkaline solution the acid groups were active and able to combine. With regard to the question of the fixation of a colloid, i t was easy to explain the dyestuff getting on to the material, but there must be some explanation of its fixation. His own idea with regard to leather was that the modern way of looking a t chemical theory gave the explanation. Chemical theory embraced not only purely chemical acticn as it used to be taught, but the colloidal theory, which itself involved adsorption effects and electrical‘ effects. Chemical action, according to the state of division of the material, included electrical, colloidal, the former so- called chemical, and adsorption effects. They were really all one, appearing in different forms and different ways, with one type of action more prominent than the other, but they were all a question of combination by some sort of mutual attraction. In the old days chemical action was looked upon as the sort of thing that happened between caustic soda and sulphuric acid, and adsorption as the sort of thing that happened between charcoal and acetic acid ; but the modern idea was that everything was due to electrical effects, and that electrical effects could show themselves in several different ways. Sulphide colours were used on wash- leather, i.e., oil-tanned leather in which the basic groups were taken up by fat acids. They were dyed as acid colours, and vat colours and the indigosols could be used also. Indigosol and Soledon colours dyed on leather in exactly the same way as acid colours. There was sufficient potential difference between sulphide and vat colours and leather in an alkaline solution to make the combination reasonably fast. That was the way the so-called washable gloves were being produced nowadays.

A member of the audience asked why the so-called chrome sole leathers were more impervi- ousito water than other leathers.

The Lecturer said chrome tannin was a chemical combination, and the maximum

amount of chromium in sole leather was 6-7% chromic acid on the dry weight. In vegetable tannings 50% could be fixed. The chrome sole leather manufacturer must therefore either sell his leather a t a much higher price than vegetable tanned leather or fill it up with p a r a h wax, stearine, resin, and so on, and so increase the weight, The filling-up of the chrome leather with certain waterproofing matters and resins was the real reason for the improved water- proofing properties. In chrome tanning the fibres of the leather were so separated that they very quickly absorbed water, so that a certain amount of wax or resin was necessary to make the leather waterproof.

Mr. E. N. Noble asked whether any success had been obtained with the azoic colours on leather.

The Lecturer said he had published a paper on the subject about twelve months ago (see abstract in this Jour., 1927, 43, 63). In raw untanned hides there were certain groups that would diazotise, and it was possible-he did not say it was practicable-to diazotise those amino- groups in the hide, couple them with phenols, and actually produce a leather. In the cases where the coupling gave a coloured product, dyeing and tanning were done simultaneously.

M i . C. Meadows asked why the use of dope pigment had developed so much in recent years ?

The Lecturer said it had been stated that the use of dopes was quite new. Actually they were quite old. So far back as the time of Edward 111. chrome-tanned leathers were finished with a certain amount of ochre and passed off as oil-tanned leathers, which were supposed to be preferred. He himself as a boy saw dope legging finishes used.

Mr.F. Dawson askedhowthevariousdifferences were obtained in dyeing leather with pigments. Could any pigment be used on any leather, and what kind of colours were generally used ? Was it usual to apply the colours in water solution, and if so, how were they fixed on the leather ?

The Lecturer said that pigment dyeing as an ordinary dyeing process was only satisfactory on a roughened surface like washleather. The spraying of leathers was ordinarily done with dopes containing finely-ground earths held in solution with casein or fish glue, or both; the solution being alkaline as a rule. They were sprayed on, and the leather being acid, tended to neutralise and fix the dope. The moisture would dry off, and there would be a sort of thin film left that would dry into a condition in which i t was not easily wetted again, and that would hold the pigment. There was also the cellulose acetate type of dope, which as soon as it dried was impervious to water.

Mr. W. P. Walker, referring to the neutral- ising of chrome leather for dyeing, asked whether the leather should be neutralised right

May 19281 SANDERSON-“THE COLOURING O F COLD-CURED RUBBER” 137

through, or should an unneutralised film be left in the centre ?r

The Lecturer said it depended on circum- stances. If the leather contained a middle film more than just weakly acid to litmus, the leather was too acid to give even dyeing. If the leather were thoroughly neutralised, a buffering agent like borax must be used, so that the solution was not too alkaline. Good neutralisation without using alkali could be obtained with ammonium acetate. If alkali was used the surface was probably over-neutral- ised, and the chromium compound became so basic that it split up. It depended on the degree of neutralisation. With a strong alkali, say soda, one dare not go right through the thick parts of the leather, because the outer surface would be over-neutralised. If borax was used, too alkaline a solution could not be produced, because of the presence of the boric acid ion, so that the leather could be neutralised right through. The neutralisation should be thoroughly done with borax and then a little ammonia added, which had itself a buffering action. Unless the leather was extremely thin, strong alkali should not be used.

The Chairman (Mi. H. Jennings) proposed a vote of thanks to Mi. Salt. The lecture had elicited an excellent discussion. It was quite evident from the lecture and from the discussion that the leather dyer had many special diffi- culties to contend with.

Mr. H. H. Bowen seconded the motion. ~~~~ ~~

MANCHESTER SECTION

Joint meeting with the Institution of the Rubber Industry, held in the rooms of the Manchester Literary and Philosophical Society, on 9th December 1927, Mr. J. R. HANNAY in the chair.

The Colouring of Cold-Cured Rubber W. E. SANDERSON, A.I.C.

During the past few years great advances in rubber technology have been made, due chiefly to the use of organic accelerators of vulcan- isation. This advance has resulted in brighter effects being obtained from the old pigments, but has also appreciably extended the available range of coloured products, and thus enabled brighter coloured rubber to be produced than has hitherto been possible..

The modern demand for bright and lively colours has introduced new difficulties for the rubber manufacturer. He is not in the more fortunate position of the textile colourist, who applies his colours to a practically finished article, but has to make his “mixing” in the case of heat-cured rubber, his “dough” or “solution” in the case of spread or dipped work, and hope for the best during the vulcanising

operation, knowing that it is impossible to modify the resulting colour subsequently in any way.

There is little in common between the colour- ing of textiles and the colouring of rubber. In the former case dym are applied from an aqueous solution, generally boiling, and the textile material is a network of fine fibres, giving a maximum surface exposed to the dye solution. On the other hand, rubber has an impervious surface, and whilst a superficial colouring is readily produced by aqueous solu- tions of suitable dyes, it is quite superficial, and comparatively easily removed, The method is used, however, to some extent for colouring toy balloons and cold-cured goods in general.

The dye solution may contain 1/10 02. to 14 ozs. per gall., according to the dyestuff used and the depth of colour required. The solution is heated to 180” F. (or 140” F. for yellow), and the goods stirred well in.the hot liquor for about one hour. They are then washed with cold water until- all loose dye is removed. Continuous stirring during dyeing, and thorough rinsing after dyeing, are essential to avoid streaky or spotty colours, which easily mark or rub off. The dyed goods are then dried by drumming in a current of warm air or by any other satisfactory method. Drumming with a little sawdust will give improved results by removing colour that is not firmly fixed.

A considerable variety of colours can be obtained by this method with a relatively small number of dyes ; e.g., violet is produced by mixing 14 parts Rhodamine with 1 part Victoria Blue B; blue with Victoria Blue B alone; green with a mixture of 95 parts Malachite Green and 5 parts Chrysoidine; yellow with Auramine alone ; orange with a mixture of 10 parts Chrys- oidine and 5 parts Rhodamine; and red with a mixture of 14 parts Rhodamine and 1 part Chryaoidine. These basic dyes are satisfactory for work where vivid colours are required, and the fastness to light is of secondary importance.

Another method of applying these and other water-soluble dyes is to use them in conjunction with an adhesive, e.g., a solution of casein in borax or other alkali, applying the solution to the rubber by dipping, brushing, or spraying. The field of application of this method is very limited, but i t is used extensively for toy balloons. When the balloons are turned outside- in the colours are perfectly fast to water and rubbing, and they are also faster to light, as the intervening film of rubber undoubtedly ha8 a protective influence against fading.

Another method of colouring finished rubber goods depends on the use of “oil colours,” viz., organic dyes, soluble in benzene or naphtha, with or without the addition of steanc or oleic acid. They are applied by dipping balloons or other articles in a solution of 1 part dye in 400 to 1,000 parts benzene or solvent naphtha, together with about Q part of stearic acid.