Synthetic Tannins, Their Synthesis, Industrial Production and Application Part 15

Whereas mixtures of Neradol D and vegetable tannins impart properties to the leather consistent with the proportions in which these materials are present, it is not possible to combine Neradol D with mineral tanning agents or fats (_e.g._, fish oils, etc.) in such a way that a leather characterised by the properties of either material is obtained. Experiments were carried out using (1) chrome salts plus Neradol D; (2) aluminium salts plus Neradol D; and (3) oils plus Neradol D, and the following conclusions were arrived at:--

1. CHROME-NERADOL D liquors, containing comparatively larger amounts of Neradol D, act too rapidly on the pelt and draw the grain; smaller amounts of Neradol D seem without influence on the finished leather, which possesses p.r.o.nounced characteristics of chrome leather. Another disagreeable factor is the following: the chrome salts must possess a certain degree of basicity in order to produce good leather; the Neradol D must, on the other hand, possess a certain acidity to produce the optimum results, and it is hence impossible to balance practically the basicity of the chrome salts and the acidity of the Neradol in order to justify the presence of both. If one of the two is used separately before the other, a leather always results possessing the characteristics of the material first employed, provided the time of action has been sufficiently extended. If insufficient time has been allowed, the characteristics imparted by the main tanning agent are not altered.

2. ALUMINIUM SALTS AND NERADOL require practically the same basicity and acidity respectively, and when combined always yield a leather possessing mainly the properties of one of the components. In addition to this fact, leathers tanned with aluminium salts possess great softness and stretch, those tanned with Neradol D greater firmness and less stretch, and these opposing qualities completely compensate one another and render _nil_ the value of such mixtures.

In addition to this, the presence of aluminium salts produces no better fixation on the leather fibre of basic coal-tar dyes, so that in this respect also a combination of aluminium salts and Neradol D is of no value.

3. FAT NERADOL D TANNAGE: Just as aluminium salts impart special characteristics to leather, this property is exhibited by fatty matters, especially so as regards stretchiness and softness. Both of the latter are not apparent to the same extent in an oil tannage into which Neradol D and oil enter as const.i.tuents. It is, however, not excluded that, in view of the fact that the combination of oils and Neradol D appear to produce the most promising results of the three from a technical point of view, such combination would yield products possessing less stretch and greater softness which, by occupying an intermediary position, might possess certain advantages and be useful for certain technical purposes.

9. a.n.a.lysis of Leather Containing Neradol D

Chemical examination of leathers tanned with Neradol D or with mixtures of natural tannins and Neradol D often involve a determination of the materials employed in tannage. In most leathers exclusively tanned with vegetable tanning materials, it is usually possible to determine at least the nature of the main tanning agent, whereas the attempts at determining those tannins which are only present in minor quant.i.ties rarely succeed. Since Neradol D usually is employed in comparatively small quant.i.ties, it has been imperative to find a method which also permits of the detection of smaller quant.i.ties of Neradol D. Provided the presence of not less than 5 per cent. (on the weight of the leather) of Neradol D, the following method yields reliable results:--20-30 gm. of the leather are ground or sliced as finely as possible and the powder (or the slices) treated in the cold with a sufficient volume of dilute ammonia solution (5 c.c. ammonia plus 95 c.c. of water) for eight to twelve hours. The object of this is to dissolve the tannins, but no protein should go into solution. The solution is filtered and the filtrate evaporated on the water bath till it occupies a volume of about 30 c.c. A few c.c. of aniline hydrochloride are now cautiously added, when it should be carefully noted if a precipitate is thrown down which might be either completely or only partly soluble in excess of aniline hydrochloride. A precipitate is always thrown down when Neradol D or wood pulp is present; only the Neradol D precipitate is soluble in excess of aniline hydrochloride. Partial solubility of the precipitate therefore indicates the presence of both wood pulp and Neradol D.

The quant.i.tative determination of sulphuric acid--the detection and estimation of which in leather is important--is considerably influenced by the presence of Neradol D. Practically all methods in vogue dealing with its determination were based on the estimation of the sulphur introduced into leather by sulphuric acid. The presence of Neradol D, the main const.i.tuent of which is dicresylmethanedisulphonic acid, renders it impossible by such methods to determine whether the combined sulphur owes its origin to sulphuric or sulphonic acid. It remains yet to be determined whether the sulphonic acid influences the leather substance to the extent that sulphuric acid does; it must, however, be borne in mind that Neradol D in addition to free sulphonic acid also contains sulphonates and sulphates, which may enter into the leather and thus increase the sulphur contents of the latter. A method must hence be devised which estimates the free acid only and provides the means of distinguishing this from all other acids of organic and inorganic acids. Paessler, [Footnote: _Collegium_, 1914, 527, 126; 531, 509; 532, 567.] by extracting the leather and dialysing the filtrate, has effected a separation of the acids and the tanning and colouring matters and quant.i.tatively estimated the sulphuric acid in the dialysate.

Immerheiser [Footnote 1:_Collegium_, 1918, 582, 293.] devised a method, based upon the property of sulphuric acid of combining with ether, for the purpose of determining free sulphuric acid in leathers:--10 gm. of the leather, cut into small pieces, are extracted three times with 200 c.c. distilled water at room temperature, the time of each extraction being ten to twelve hours, and the combined extracts evaporated to dryness on the water bath, 5 gm. of quart sand being added. The dry residue is now powdered, introduced into an Erlenmeyer flask provided with a gla.s.s stopper, and 200 c.c. of anhydrous ether [Footnote 2: To be tested for water by shaking with anhydrous copper sulphate.] added.

After about two hours, during which the flask is occasionally shaken, the ether is poured through a filter, the residue washed with a little ether, and the operation repeated twice with each 40 c.c. anhydrous ether, using the same filter. To the combined ether extracts (about 200 c.c.) HCl and [Greek: b]aCl_2 are added, the ether distilled off and the residue evaporated on the water bath, in order to decompose the ether-sulphuric acid compound. 50 c.c, of hot water acidified with HCl are now added, the precipitate allowed to settle, filtered, washed, dried, and weighed. The sulphuric acid thus estimated was present in the leather as _free sulphuric acid_. That present as sulphates soluble in water is estimated in the residue on the filter: the residue is extracted with hot water, the sand filtered off, the filtrate acidified with HCl, boiled for one quarter hour and filtered if necessary. The clear filtrate, which may be coloured, is brought to boil and [Greek: b]aCl_2 is added. The barium sulphate indicates the sulphuric acid present in the leather as water-soluble sulphates.

Whether the latter be sulphates or bisulphates may be indicated by the aqueous extract of the above residue, since neutral reaction would indicate the absence of bisulphates, acid reaction their presence in addition to possible normal sulphates; the quant.i.tative estimation of the metals would decide this point definitely.

10. Properties of Leathers Tanned with Neradol D

Whereas the colour of leathers tanned with Neradol D only is nearly a pure white, those tanned with mixtures of Neradol D and vegetable tanning materials are more or less light coloured according to the quant.i.ty of Neradol D present, as has been explained when discussing the phlobaphene-solubilising action of Neradol D. In any case, all leathers tanned with Neradol D possess fibre of remarkable length, which explains their increased tensile strength and elasticity. The tensile strength of a leather tanned with a mixture of Neradol D and vegetable tannins was 3.7 per cent, as compared to 3 per cent when no Neradol was used; the extension was 56 per cent, when tanning with Neradol D as against 36 per cent, without the latter.

The sensitiveness to light of leathers tanned with Neradol D may be mentioned. Exposed to direct sunlight, the surface of the leather a.s.sumes a yellowish colour after two days' exposure, and a.s.sumes a pure yellow colour after a further three days. A further fifteen days'

exposure only darkens the leather slightly, the final colour being very little different from the one obtaining after five days' exposure.

In pa.s.sing, it may be remarked that this yellow colour is observed on the surface only, the grain otherwise possessing that pure white colour characteristic of Neradol D tanned leather. Further, it may be noted that leathers tanned--with Neradol D fix basic coal-tar dyes excellently, whereas acid and substantive dyestuffs are fixed with other than their natural shades.

The author has a.n.a.lysed a leather exclusively tanned with Neradol D, and has obtained the following results:--[Footnote: _Collegium_, 1913, 521, 478.]

Moisture - - - - - 15.53 per cent.

Ash - - - - - - 0.93 per cent.

Fats- - - - - - 1.26 per cent.

Extraneous matters - - - 0.00 per cent.

Leather Substance |Tanning matters- 36.92 per cent.

Leather Substance |Hide substance - 45.36 per cent.

--------------- 100.00 per cent.

[Footnote: Sp. gr., 0.642.]

From these figures, those of "degree of tannage" and "yield"

(pelt-->leather) are calculated as 81.4 and 220 respectively.

These figures correspond closely to those obtained by the a.n.a.lysis of leathers tanned with vegetable tanning materials, and this proves the similarity between the Neradol D tannage and a vegetable tannage in their chemical aspects.

11. Neradol D Free From Sulphuric Acid

In order to prepare phenol and cresulphonic acids, such quant.i.ties of technical sulphuric acid are used as do not allow of the a.s.sumption of complete utilisation of the sulphuric acid; hence it was of theoretical interest to remove eventual traces of free sulphuric acid from the product. For this purpose, the author diluted crude Neradol to 20 Be. and gradually added small quant.i.ties of milk of lime; the precipitates were freed from the liquid by suction and washing, and a Neradol free from sulphuric acid resulted, which was then brought to the acidity of Neradol D with the calculated amount of alkali. From the calcium sulphate precipitate, the amount of sulphuric acid originally present was calculated, and was found to be only 4 per cent.

The acid-free sample of Neradol was tested with regard to its suitability as a tanning agent; leather tanned with this sample differed from one tanned with an untreated sample (Neradol D) by being harder and possessing a p.r.o.nouncedly greyish colour. This difference, however, may not be due to the absence of sulphuric acid but to the presence of the slightly soluble calcium sulphate in the sample treated with milk of lime. To prove this point, another way of preparing Neradol D free from sulphuric acid was looked out for. Sodium acetate was added to a solution of crude Neradol until the latter was no longer acid to congo-red; at this point no free sulphuric acid can be present in the solution. The product, partly neutralised till the acidity of Neradol D was reached (part of the acidity then being due to liberated acetic acid), yielded a leather which neither in colour nor in feel differed from the usual Neradol D tanned leather. This proves that the grey colour and the hardness of the leather described in the former experiment is due to the presence of calcium sulphate.

If the crude Neradol treated with sodium acetate is not partly neutralised, the a.n.a.lysis gives the following figures:--

Tanning matters 67.3 per cent.

Soluble non-tannins 8.6 "

Insolubles 0.0 "

Water 24.1 "

--------- 100.0 per cent.

Acidity: 1 gm. = 46 c.c. N/10 NaOH.

Compared to the a.n.a.lysis of crude Neradol containing sulphuric acid, the figures show that, on the one hand, the presence of the comparatively small quant.i.ty of sodium acetate but slightly influences the contents of non-tannins and water, but, on the other hand, reduces the contents of tannins and also the acidity. The tanning intensity of this product, however, is considerably increased, and using a 1 Be. solution a leather is obtained in a very short time very similar to that yielded by ordinary Neradol D, but considerably harder; the latter property is due to higher acidity and almost complete absence of salts in the product treated with sodium acetate.

The author finally attempted to partly neutralise crude Neradol with various hydroxides and carried out tanning tests with samples containing the different metals. Hardly any difference in the finished leathers could be observed as regards colour or quality; the tannage could by no means be described as that of a combination of Neradol D and the respective metals.

12. Neutral Neradol

Crude Neradol, completely neutralised with caustic soda, yields a product of the following composition:--

Tanning matters 19.8 per cent.

Soluble non-tannins 37.9 "

Insolubles 0.0 "

Water 42.3 "

------------ 100.0 per cent.

The qualitative reactions of this product differ from those of non-neutralised Neradol to the extent that gelatine is not precipitated and iron salts are not coloured blue, but dirty brown, by the aqueous solution of this product.

The completely neutralised product, diluted to various concentrations (of 1, 2, 3, and 5 Be.) and tested as to tanning properties, revealed the surprising fact that the pelts were not even surface tanned, and were coloured evenly blue throughout by indigotine.

It might have been antic.i.p.ated that sodium dicresylmethanedisulphonate would be as devoid of tanning powers as is a neutralised vegetable tannin, but it is difficult to explain the fact of the sodium salt being adsorbed by hide powder as "tanning matters" in the Official Method of a.n.a.lysis. Brought to a logical conclusion, the figure 19.8 per cent, should be deducted from 32.5 per cent, obtained in the a.n.a.lysis of a _partly_ neutralised Neradol D, which comparatively large quant.i.ties of the sodium sulphonate also adsorbed by hide powder, leaving the "tanning matters" of Neradol D at 13.5 per cent.

This diminished figure, however, does in no way reduce the value as a tanning agent of Neradol D; it merely shows how inadequate is the hide powder method of a.n.a.lysis when applied to substances of the composition of Neradol D. This is further confirmed by the Loewenthal permanganate method, which yields the following figures:--

Tanning matters 7.2 per cent.

Soluble non-tannins 59.1 [Footnote: Collegium, 1913, 521,487.]

If, on the other hand, completely neutralised Neradol is acidified with an organic acid, such as acetic acid, till the acidity, (1 gm.= 10 c.c. N/10 NaOH) is reached, the resulting product is in all respects similar to Neradol D and yields a corresponding leather.

It is permissible to a.s.sume that the irregularity exhibited by Neradol D as regards the a.n.a.lytical estimation of its tannin contents is connected with the low molecular weight of the tanning principle. Whereas all tannins so far isolated from the natural tanning materials possess rather high molecular weights, that of Neradol D deviates considerably from this rule, as is shown by the following table:--

Neradol D tannin Cl_5H_16S_2O_8 358 Mangrove " C_24H_40O_2l 670 Oak bark " C_28H_28O_23 840 Myrabolam " C_54H_48O_35 1256 Dividivi " C_54H_46O_35 1270 Malletto " (C_4lH_50O_20)_2 1724

This low molecular weight may mainly account for the figures obtained by the incorrect oxymetric estimation with permanganate; the apparent tannoid property of the tannoid-inactive neutral salt of dicresylmethanedisulphonic acid may be explained by a.s.suming that though it is, probably, in the colloidal state, and as such adsorbed by hide powder, it is still devoid of astringent properties.

G. Different Methods of Condensation as Applied to Phenolsulphonic Acid