Synthetic Tannins, Their Synthesis, Industrial Production and Application Part 17

The cla.s.s of aldehyde condensations also comprises that of inducing condensation by means of sugars; if phenolsulphonic acid is heated with glucose, a reddish-brown liquid results, which is soluble in water. The solution exhibits reactions similar to those of Neradol D. It is, however, not possible, by this method of condensation, to prepare as highly concentrated products as is possible in the case of Neradol D, since employing sugars as condensation agents means liberation of a large volume of water. a.n.a.lysis of this product, using the shake method, gives a tannin content of 16.2 per cent; tanning experiments demonstrated that the time of tannage, using a 2 Be. solution, was the same as that required by Neradol D, and yielded a leather, the surface of which was reddish-grey, the inner layers being white, but which is otherwise very similar to Neradol D tanned leather. [Footnote: Austr. Pat, 69,375, 69,376, 69,377.]

Relatively to its capability of solubilising phlobaphenes, this product exhibits similar properties to that obtained by merely heating phenolsulphonic acid, to a slight extent only solubilising quebracho extract, which, on diluting the mixture, is completely thrown out of solution.

5. Condensation with Glycerol

Phenolsulphonic acid, when heated with glycerol, undergoes the process of condensation, and forms a brown fluid, which, when brought to acidity 10, exhibits the following reactions:--

Gelatine-----------------Precipitate.

Ferric chloride----------Brown-black coloration.

Lead acetate-------------White precipitate, insoluble in HNO_3.

Aniline hydrochloride----Slight precipitate.

Tanning experiments with this partly neutralised product resulted in a very gradual conversion of the pelt into a greenish-grey coloured leather; the colour, however, does not penetrate the pelt and is hence caused by colloidally suspended impurities. If the solution is filtered through a filter candle, a somewhat clearer solution results, but the latter also tans very slowly and yields a brown coloured leather.

a.n.a.lysis of the partly neutralised product reveals a tannin content of 17.6 per cent. A 2 Be. solution of the non-neutralised product showed a rapid tanning effect at first, when brought into contact with pelt, on which it had a strong swelling effect, and to which it imparted a greenish colour; the tanning effect, however, slowed down considerably, after a few days, and the solution penetrated the pelt only very gradually; this is probably due to the presence of large quant.i.ties of colloidally suspended impurities, which, when the substance is partly neutralised with the formation of salts of the sulphonic acids, are brought into true solution and hence penetrate the pelt with greater rapidity.