Chapter 2

Synthetic Tannins: Their Industrial Production and Application

A. Condensation of Free Phenolsulphonic Acid B. Condensation of Partly Neutralised Phenolsulphonic Acid C. Condensation of Completely Neutralised Phenolsulphonic Acid D. Condensation of Cresolsulphonic Acid E. Relative Behaviour of an Alkaline Solution of Bakelite and Natural Tannins F. Dicresylmethanedisulphonic Acid (Neradol D) 1. Neradol D Reactions 2. Electro-Chemical Behaviour of Neradol D 3. The Influence of Salts and Acid Contents on the Tanning Effect of Neradol D 4. Phlobaphene Solubilising Action of Neradols 5. Effect of Neradol D on Pelt 6. Reactions of Neradol D with Iron and Alkalies 7. Reagents suitable for Demonstrating the Various Stages of Neradol D Tannage 8. Combination Tannages with Neradol D (1) Chrome Neradol D Liquors (2) Aluminum Salts and Neradol (3) Fat Neradol D Tannage 9. Analysis of Leather containing Neradol D 10. Properties of Leather Tanned with Neradol D 11. Neradol D, Free from Sulphuric Acid 12. Neutral Neradol G. Different Methods of Condensation as Applied to Phenolsulphonic Acid 1. Condensation Induced by Heat 2. Condensation with Sulphur Chloride 3. Condensation with Phosphorus Compounds 4. Condensation with Aldehydes 5. Condensation with Glycerol

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INDEX

INTRODUCTION

CLASSIFICATION OF SYNTHETIC TANNINS

In laying down a definition of "Synthetic Tannins," it is first of all necessary to clearly define the conception of "tannin." Primarily, tannins may be considered those substances of vegetable origin which may be found, as water-soluble bodies, in many plants, exhibiting certain chemical behaviour, possessing astringent properties and being capable of converting animal hide into leather. This latter property of the tannins, that of converting the easily decomposable protein of animal hide into a permanently conserved substance and imparting to this well-defined and technically valuable properties, has become the criterion of the practical consideration of a tannin. It appears that different substances certainly show the chemical reactions peculiar to the tannins, and to a certain extent also exhibit astringent character without, however, possessing the important property peculiar to the tannins of converting hide into leather. Such substances, in our present-day terminology, are termed pseudo-tannins (_e.g._, the "tannin" contained in coffee-beans). Decomposition products of the natural tannins, to which belong, for instance, gallic acid and the dihydroxybenzenes, exhibit the well-known reactions of the tannins (coloration with iron salts), but they cannot be regarded as tannins from either a technical or a physiological standpoint.

As regards their chemical constitution, the natural (true) tannins probably belong to different groups of organic compounds, and with our present-day scant knowledge of their chemistry, it is impossible to classify them. One is, however, justified in assuming that both the natural tannins and the related humic acids are ester-derivatives of hydroxybenzoic acids. [Footnote: E. Fischer, _Ber._, 1913, 46, 3253.]

The production of synthetic tannins employs two quite distinct methods; one is to synthesise the most simple tannin, viz., the tannic acid contained in galls (tannin), or to build up substances similar in character to the tannins, from hydroxybenzoic acids. The other, entirely new way, is to produce chemical substances, which certainly have nothing in common with the constitution of the natural tannins, but which behave like true tannins in contact with animal pelt, and in addition, since they can be manufactured on a commercial scale, are of practical value.

Owing to the fact that, until recently, the constitution of tannin has remained unknown, it is easy to comprehend that the efforts to synthesise the latter substance, or compounds similar to it, have been mainly attempted on similar lines. The oldest investigation in this direction dates from H. Schiff,[Footnote: Liebig's _Ann._, 1873, 43, 170.] who prepared substances similar to tannin by dehydrating hydroxybenzoic acids. By allowing phosphorus oxychloride to interact with phenolsulphonic acid, he obtained a well-defined substance possessing tanning properties, which he considered an esterified phenolsulphonic acid anhydride, the composition of which he determined as HO.C_6H_4.SO_2.O.C_6H_4HSO_3. It is, however, probable that this substance is not homogeneous, but consists of a mixture of higher condensation products.

Klepl [Footnote: _Jour. pr. Chem._, 1883, 28, 208.] obtained--by simply heating _p_-hydroxybenzoic acid--a so-called di- and tridepside, but this simple method is not applicable to many other hydroxybenzoic acids, since these are decomposed by the high temperature required to induce reaction.

Amongst other attempts to produce condensation products with characteristics similar to those possessed by the tannins, those by Gerhardt [Footnote: Liebig's _Ann_, 1853, 87, 159.] and Loewe [Footnote: _Jahresh. f. Chem._, 1868, 559.] must be especially noted; they treated gallic acid with phosphorus oxychloride or arsenic acid, and thereby obtained amorphous compounds, exhibiting the reactions characteristic of tanning substances. E. Fischer and Freudenberg, [Footnote: Liebig's _Ann._, 372, 45.] by treating _p_-hydroxybenzoic acid in the same way, succeeded in obtaining a didepside, and during the last years practically only these two investigators have demonstrated the syntheses of these depsides and produced high-molecular polydepsides.

At the same time researches were instituted with the object of determining the constitution of tannin, and E. Fischer succeeded in demonstrating its probable composition as being that of a glucoside containing 5 molecules of digallic acid per 1 molecule of glucose.

This last-named class of synthetic tannins--which may be properly termed "tanning matters" in contradistinction to the true tannins--exhibit very distinct tanning character when brought in contact with animal hide, but from the point of view of chemical constitution have nothing in common with the natural tannins. Not only are they of interest to the industry from a practical point of view; they have also been examined very closely from a chemical standpoint.

It is, however, necessary to differentiate with great exactitude between the conception of _true tanning effect_ and _pickling effect_ when considering the action of chemical substances on pelt (i.e., animal hide, treated with lime, depilated, and the surplus flesh removed). Whereas any _true tannage_ is characterised by the complete penetration of the substance and its subsequent fixation by the pelt in such a way that a thorough soaking and washing will not bring about a reconversion (of the leather) to the pelt state; _pickling_, on the other hand, is only characterised by the penetration of the substance in the pelt and fixation to such an extent that a subsequent washing of the pickled pelt will bring back the latter to a state closely approximating that of a true pelt. Simple as such a differentiation appears, there are still a number of cases occupying a position between the two referred to, and which we may term _pseudo-tannage_. An example of the latter is formaldehyde tannage; formaldehyde has for a long time been employed in histological work for the purpose of hardening animal hide, by which it is readily absorbed from solution whereby it hardens the hide without, however, swelling it. A hide which has thus been treated with formaldehyde absorbs the natural tannins with greater ease; this, on the one hand, argues the probability of formaldehyde acting as a pickling agent; on the other hand, it is also one of its characteristics that it will either in neutral acid, [Footnote: R. Combret, Ger. Pat, 112, 183.] or, still better, in alkaline [Footnote: J. Pullman, Ger. Pat, 111,408; Griffith, _Lea. Tr. Rev._, 1908.] solution, convert pelt into leather. In a formaldehyde-tanned leather, however, no trace of tannin can be detected; and the yield (of leather, based on the pelt employed), which, from a practical standpoint, is so important, is so very low that it is hardly possible to speak of it as a tannin in the ordinary sense of the word. Formaldehyde must, therefore, be termed a pseudo-tannin.

The tanning effect of formaldehyde is, according to Thuau, [Footnote: _Collegium_, 1909, 363, 211.] increased by those salts which bring about colloidal polymerisation of the formaldehyde, the resultant compounds being absorbed by the hide fibre. Fahrion considers this to be a true tannage, and is supported by Nierenstein [Footnote: _Ibid._, 1905, 157, 159.]:--

R.NH_2 R.NH-| +O.C.H. = CH_2 + H_2O R.NH_2 | R.NH-| (Hide.) H (Leather.)

A peculiar combination between true tannage and pickling is to be found in the tawing process (tannage with potash, alum, and salt), whereby, firstly, the salt and the acid character of the alum produce a pickling effect, and secondly, the alum at the same time is hydrolysed, and its dissociation components partly adsorbed by the hide, thereby effecting true tannage. This double effect is still more pronounced in the synthetic tannins which contain colloidal bodies of pronounced tanning intensity on the one hand, inorganic and organic salts on the other, which then act as described above. Their real mode of action can only be explained with the aid of experimental data. The following chapters will deal with the different behaviour of the various groups of synthetic tannins.