SECTION III
TANNING EFFECTS OF MIXTURES AND NATURAL PRODUCTS
1. Mixture of Phenolsulphonic Acid and Formaldehyde
The most important invention relatively to the search for new tanning materials was that of Weinschenk,[Footnote: Ger. Pat., 184,449.] who first showed that pelt may be converted into leather by the action upon it of mixtures of naphthols and formaldehyde. This process consists of two steps: the pelt is first immersed in a 0.25-0.50 per cent, formaldehyde solution, and secondly in an aqueous solution of -[Greek: a] or -[Greek: b] naphthol; this order may be reversed. If, on the other hand, a pasty mixture is made of formaldehyde and naphthol, and this is allowed to act upon the pelt, the latter is rapidly converted into leather, but the mixture must be administered very gradually or otherwise the insoluble methylenedinaphthol is formed outside the pelt and hinders any tanning effect.
Leather obtained through the action of [Greek: a]-naphthol is, when freshly tanned, pure white and sufficiently soft and firm, but quickly assumes a brown colour on storing; if, however, [Greek: b]-naphthol is employed, a cream-coloured leather results, the colour of which turns only slightly more yellowish even when exposed to the direct rays of the sun.
A similar process has recently (25, xii., 1915) been protected by Ger. Pat, 305,516, granted to the Deutsch-Koloniale Gerb—und Farbstofif Gesellschaft, in Karlsruhe. According to this patent, pelt is treated in separate solutions, one of which is formaldehyde, the other being that of such aromatic compounds or their salts which yield water-soluble condensation products with formaldehyde; for example, pelt is immersed in 2-5 per cent, solution of formaldehyde for a few days, and is subsequently treated with 1-2 per cent neutral or faintly acidified solutions of [Greek: a]-naphthylamine hydrochloride, resorcinol or sodium phenate or cresylate, for several days. The resultant leather is claimed to be soft and full and to possess good tensile strength.
The tanning properties of mixtures of phenolsulphonic acid and formaldehyde have been examined by the author with the following results:—
I. II. III.
Grammes formaldehyde 10 20 40 " phenolsulphonic acid 20 50 100 " caustic soda (sol, 40 per cent.) 10 20 40 " water 500 500 500
The above solutions were made up and allowed to act upon pelt pieces weighing 15 gm.; whereas Solution I. remained clear throughout the experiment, Solution II. became somewhat clouded, and Solution III. assumed a milky appearance. The pelts were tanned through in seven days and yielded leathers which, after drying and finishing, possessed yellow colour, long fibre, and good tensile strength, but a rather empty feel.
To prevent separation of insoluble matter during tannage, another experiment was carried out, in which the pelts were first submitted to the action of formaldehyde (10, 20, and 40 gm. in 500 c.c. water) for three days, being subsequently removed to fresh solutions of partly neutralised phenolsulphonic acid (cf. above). Similar results were obtained, but the leather felt even more empty than those obtained by the former experiment.
Attempts at converting pelt into leather by first immersing the pelt in a partly neutralised solution of phenolsulphonic acid, and subsequently transferring it to fresh solutions of formaldehyde, gave merely negative results; the phenolsulphonic acid effected pickling action upon the pelt, but was subsequently quickly replaced by the formaldehyde, before the latter had penetrated the pelt in sufficient quantity to induce condensation, thereby exerting tanning action.
To explain the tanning effects of these mixtures, the author analysed the leathers resulting from the effects of the latter, and was able to show, that in these cases also, condensation of phenolsulphonic acid and formaldehyde takes place inside the pelt, since on the one hand the analyses left no doubt but that true tannage had been effected, and on the other hand an ammoniacal extract of the leathers gave the typical reaction for condensation products of phenolsulphonic acid, with aniline hydrochloride. [Footnote: Collegium 1913, 516, 142.]
The leather analyses gave the following figures:—
Moisture - - - 18.30 per cent.
Fats - - - - 0.47 "
_ Ash - - - - 0.98 "
Leather { Tannin - - - 26.37 "
substance { Hide substance - - 53.88 "
A characteristic feature is the low value of tannin, which is considerably higher [Footnote: Ibid., 1913, 521, 478.] where condensation products of phenolsulphonic acids are used as tanning agents; the action effected by the separate constituents, therefore, is more that of pickling.
2. Mixture of Phenolsulphonic Acid and Natural Tannins
A piece of pelt was immersed in a half-neutralised solution, measuring 6° Bé., of phenolsulphonic acid, and left sixteen hours in the solution, which completely penetrated the pelt during this time; it was then transferred to a 12° Bé. solution of a mixture of quebracho and chestnut, which in two days converted the pelt into a light coloured leather possessing good tensile strength.
By using a bath composed of half-neutralised phenolsulphonic acid and quebracho extract in 7° Bé. solution, another piece of pelt was completely tanned in two days. The same result was obtained by first half neutralising the phenolsulphonic acid and then adding sulphited quebracho extract till a 5° Bé. solution was obtained.
A piece of pelt received a 2º Bé. liquor composed of 3 parts of phenolsulphonic acid and 1 part of formaldehyde for sixteen hours, and was then completely penetrated; it was subsequently transferred to a 10º Bé. liquor composed of chestnut and quebracho, being completely tanned in two days. The same result was obtained on adding sufficient sodium sulphate to the above mixture of phenolsulphonic acid and formaldehyde to raise the density from 2º-3º Bé.
Sixty grammes of phenolsulphonic acid were partly neutralised with 100 c.c. of a 10 per cent solution of caustic soda, and 10 c.c. formaldehyde added to 400 c.c. of the mixture (2º Bé.): a piece of pelt was completely penetrated by the solution in sixteen hours, and was subsequently tanned in two days, using an extract of 10º Bé. Similarly, by treating a pelt with 400 c.c. of a half-neutralised solution of phenolsulphonic acid (3º Bé.) plus 8 c.c. formaldehyde, and adding after eighteen hours sulphited quebracho extract to the same bath, strengthening the latter to 6º Bé., the pelt was converted into leather in two days; in this case, however, much of the tannin was precipitated by the formaldehyde present in the solution. If, on the other hand, a mixture of 80 gm. dilute phenolsulphonic acid (1:1 aq) and 14 gm. of formaldehyde were cooled for several hours and subsequently strengthened with sulphited quebracho extract to 7º Bé., no tannin was precipitated in the liquor, and a piece of pelt immersed in the latter was completely tanned in sixteen hours.
To prevent the precipitation of tannin caused by the formaldehyde, sulphite cellulose extract (wood pulp) was substituted for sulphited quebracho extract, and the following experiments carried out:—
To 200 c.c. of a 6º Bé. sulphite cellulose extract plus 200 c.c. of half-neutralised phenolsulphonic acid solution was added 15 c.c. formaldehyde, and this solution tanned pelt in four days; the resultant leather was light brown, firm, and possessed good tensile strength and long fibre.
Another piece of pelt was immersed in a solution of 400 c.c. phenolsulphonic acid of 3ºBé. plus 15 c.c. formaldehyde for eighteen hours, and was then tanned in a 6º Bé. solution of sulphite cellulose extract. The resultant leather was extremely light coloured, and possessed qualities similar to those described in the former experiment. Finally, pelt was immersed in a 6° Bé. solution composed of 140 gm. of a 15° Bé. sulphite cellulose extract, 10 gm. of formaldehyde, 400 gm. water, 15 gm. phenolsulphonic acid, and 30 gm. of a 10 per cent caustic soda solution, and was tanned in four days. This leather also was coloured light brown, of good tensile strength, and rather firm.
These experiments prove that when pelt is treated with formaldehyde, phenolsulphonic acid, and vegetable tannins, the two former components effect, more or less, actual tannage; it is admittedly a matter of some difficulty to establish whether the effect is one of pickling or pseudo-tannage, or whether the tannage may be considered a true one. The final effect, however, is nearly always that of a true tannage, i.e., by varying the composition of the tanning solutions leather is obtained with properties identical with those tanned with true tannins of vegetable origin. The only difficulty encountered in these combinations is the property of formaldehyde, of precipitating the natural tannins, and it is hence essential, for practical purposes, to so arrange the combination that their value is not reduced by the property referred to. The fact that not only compounds already existing may convert pelt into leather, but that a similar effect is obtained inside the pelt, by their components, is indeed of theoretical interest.
3. Tanning Effects of Different Natural Substances
In addition to the vegetable tannins, Nature has also provided other substances of vegetable origin, which, admittedly, do not effect tannage in their original state, but which may, by suitable treatment, acquire this property. The oldest information on this point is supplied by Resch, [Footnote: Scherer's Jour., 1801, 6, 495.] who carried out tanning experiments, using three parts of peat and one part of oak bark.
By the action of nitric acid on substances of vegetable and animal origin, Hatchett, [Footnote: Gehlen's Jour., 1805, 1, 545.] Chevreul, [Footnote: Ann. Chim., 1810, 73, 36.] and Vogel [Footnote: Jour. Chem. Phys., 1812, 6, 101.] claim to have obtained tanning materials, whilst later, Buff [Footnote: Ibid., 1827, 51, 38.] obtained a material suitable for tanning purposes from indigo.
By subsequent treatment with lime and soot, or tar, Ashmore [Footnote: Dingier's Jour., 1833, 48, 67.] claims to have converted pelt into leather.
By treating peat with nitric acid, Jennings [Footnote: Jahresber. d. Chem., 1858, 666.] and Payne [Footnote: Chem. Centralbl., 1908, ii. 554; Ger. Pat., 200, 539.] have produced artificial tanning materials.
Skey [Footnote: Chem. News, 1866, 206; Zeits. f. Chem., 1866, 753.] obtained a dark brown extract, soluble in water and precipitating gelatine, by treating bituminous coal or lignite with nitric acid; by extracting coal with alkalies, Reinsch [Footnote: Pharm. Centralh., 1887, 141.] isolated a substance (pyrofuscine) which, when partly neutralised with carbon dioxide, was capable of converting pelt into leather.
In addition to these tanning materials the recovery of a substance possessing tanning properties from the so-called acid rosins has been made the subject of a patent; [Footnote: Ger. Pat., 36,019.] this rosin is formed when crude oil is treated with concentrated sulphuric acid in the oil refineries. The greasy substance is partly neutralised with alkali and is claimed to produce a very springy leather.
The waste liquors obtained in the manufacture of cellulose, the so-called sulphite and sodium cellulose waste, have, however, been the subject of numerous investigations, and several hundred publications have appeared and a great number of patents [Footnote: "Literatur überiSulfitablauge" 1910-13. (Reprint from WocheWochenblPapiePapierfabrikation)] taken out, the first one being that of Mitscherlich [Footnote: Jahresber. d. Chem., 1893, 890; Ger. Pat., 72,161.] and Hönig [Footnote: Chem. Centralbl., 1902, ii. 174; Ger. Pat., 132,224.]
The waste liquors contain large quantities of acids and lime, and in order to utilise the liquors for tanning purposes, the excessive sulphuric and sulphurous acids as well as the lime must be removed. The active tannin is no doubt the ligninsulphonic acid, and those cellulose extracts containing the largest amounts of free ligninsulphonic acid may also be considered the most efficient.
According to the author,[Footnote: Technikum, 1912, 20, 156.] such sulphitecellulose extracts precipitate gelatine, aniline hydrochloride, ammoniacal zinc acetate, and basic coal-tar dyes, and give a greenish-black coloration with ferric chloride. These reactions indicate the presence of tanning matters in cellulose extracts.
The official shake method of analysis gives the following results:—[Footnote: Ibid.]
Tanning matters 23.0 per cent. Non-tannins 30.3 " Insoluble matters 0.7 " Water 46.0 " ———————- 100.0 per cent.
Ash 4.3 "
Sulphurous acid 0.6 "
Many other substances have been used for tanning experiments, a number of them precipitating gelatine. Zacharias [Footnote: Zeits. f. Ang. Chem., 1907, 1645.] obtained leather by the action of many coal-tar dyes on pelt, similarly Herzog and Adler, by using Prussian blue, Neufuchsin, patent blue V, crystal violet, and colloidal gold.
Most inorganic substances possess tanning properties when in the colloidal state, e.g., sulphur, halogens, chromium salts, iron salts, silver oxide, and the salts of mercury, copper, bismuth, zinc, lead, platinum, cesium, vanadium, and the rare earths (salts of cerium, lanthanum, didymium, neodymium, thorium, and zerconium).
For practical purposes, however, only sulphur, chrome, and alum salts are used, the latter two being of the greatest importance.