REFERENCES.
Payne and Pullman, English Patent 1898, 2872.
Bennett, "Manufacture of Leather," pp. 250 and 379.
SECTION V.—SYNTHETIC TANNING MATERIALS
In spite of much valuable work on the constitution of the vegetable tannins and the compounds usually associated with them, such as that of E. Fischer, K. Freudenberg and their collaborators on gallo-tannic acid, and that of A.G. Perkin on ellagic acid and catechin, we are still in the dark with respect to the constitution of the tannins which are of commercial importance, and any synthetic production of these materials is thus out of the question as yet. Attempts, however, have been made to produce artificially substances which possess similar properties to the tannins and which may be used for converting pelt into leather. Into this category fall some of the earlier attempts to synthesize gallo-tannic acid by heating gallic acid with condensing reagents.
The first commercial success in this direction was attained by Stiasny, who produced condensation products of the phenolsulphonic acids, to which products he gave the general name of "syntans" (synthetic tannins). The Badische Co. placed one of these products on the market as "Neradol D," and later took out subsidiary patents for the manufacture of similar products by slightly differing methods of productions. Since the outbreak of the European War such patent rights have been suspended, and several British firms have been manufacturing synthetic tanning materials by similar methods, but doubtless with developments and improvements of their own discovery. These products (e.g. Cresyntan, Maxyntan, Paradol, Syntan, etc.) are now in use in many factories, and assist rather than substitute the vegetable tannins in producing leather of the desired colour and quality.
These synthetic tanning materials resemble the vegetable tannins in the following respects. They are organic acids containing phenolic groups. They are semi-colloidal, passing slowly through semipermeable membranes. They precipitate gelatin, basic dyestuffs and lead acetate, give a violet-blue colour with ferric salts, and convert hide into an undoubted leather. They differ from the vegetable tannins in that they contain sulphur and sulphonic acid groups, but they agree in that both are aromatic derivatives. In each case the tanning effect is diminished by alkalies, but the synthetic materials are the more sensitive.
Methods of Manufacture.—There are, broadly speaking, three types of method by which these condensation products are produced, viz., condensation by formaldehyde, condensation by phosphorus trichloride or similar reagents, and condensation by heat alone. Illustrative methods will now be given.
Condensation by formaldehyde was the first method used. The procedure is given by the Austrian patent 58,405. A phenol, e.g. crude cresylic acid, is heated with the equivalent amount of sulphuric acid for a few hours to 100°-210° C., cooled, and formaldehyde added slowly whilst cooling and stirring, in the proportion of one molecule of formaldehyde to 2 molecules of phenol. The free mineral acid is neutralized, and the resulting product is the syntan "Neradol." By this procedure only water-soluble products are obtained, but an alternative process is to heat the phenols in slightly acid solution, and then to render soluble the resinous products obtained by treating with sulphuric acid. The proportion of formaldehyde to phenol used led Steasny to conclude that the resulting products were diphenyl-methane derivatives which polymerize to form molecules of considerable size. The formaldehyde supplies the "carbon bridge." This view was criticized by A. G. Green as too simple, and he suggested the alternative theory that polymerization does not take place at all, but that more advanced or higher condensation products are formed; he thought that o-hydroxy-benzyl alcohols were first produced, that these condensed with another molecule, and afterwards the process was repeated. The result was a "colourless dyestuff." This view receives some support from the other types of method of manufacture.
With the use of other condensing reagents the procedure may be as in the process of the B.A.S.F. (Fr. pat. 451,875-6), thus: 225 parts of o-cresol-sulphonic acid are heated to 60° C. for 4 hours with 262.5 parts of phosphorus oxychloride. The excess of oxychloride is removed by distillation under reduced pressure and the residue washed with dilute hydrochloric acid.
Condensation by heat alone is illustrated by the method given in the same patents, thus: phenol-p-sulphonic acid is heated to 130° C. for 24 hours under a pressure of 20 mm. or in a current of dry air at atmospheric pressure. The product may be used direct or may be purified by dissolving in water, neutralizing with caustic soda, filtering and evaporating to dryness. A white powder is obtained which tans when its solution is acidified. An alternative is to mix phenol with sulphuric acid and heat the mixture to 140° C. for 72 hours under 20 mm. pressure and purify as before.