FIG. 4.—TRIPLE-EFFECT EVAPORATOR
(Blair, Campbell & McLean, Ltd.)
Synthetic Tannins
The discovery of a synthetic tannin, in 1911, by Dr. Stiasny, who was then an assistant in the Leather Industries Department of Leeds University, created a great deal of interest in the leather trade, and can certainly be regarded as a triumph of the application of chemistry to industry. It was thought at first that these tannins might play as important a part in the leather trade as the synthetic dye-stuffs have, but it is now generally believed that this will not be the case. Their use is likely to be as an aid to tanning, rather than as a complete tanning agent, although it has been found practicable to use them for one or two classes of light skins, where it is essential that the colour of the leather should be nearly white.
The original patent was taken out in September, 1911, in Austria, the native country of the inventor, but the patent rights have also been protected in other countries, while the manufacture and sale of the product passed into the hands of a large German dye firm, who have since taken out several patents for other synthetic tannins. The original tannin is produced by treating a sulphonated phenol with formaldehyde. A patent had been taken out several years before for the use of formaldehyde in tanning, but this expired early in 1911; many patents had also been granted for sulphonated phenol preparations, chiefly as disinfectants, but it was left to Dr. Stiasny to discover the value of combining the two chemicals by the process of condensation. The preparation, first known as "neradol," is now made in England under licence, but, since the war, its cost has been more than doubled, so that it is unlikely to replace natural tanning materials to any great extent.
Formaldehyde itself has certain properties which are harmful to leather, and it must be used with extreme caution and in small quantities to avoid these defects. It is used in the leather trade chiefly for keeping up the substance of hides during the tanning process, and thus causing a rapid absorption of tannin. At the beginning of the tanning process, the liquors are acidified, preferably with a weak organic acid, which causes the fibres of the hides to distend. It is at this stage that the formaldehyde is used. This method, which cannot improve, but may easily damage, the leather, is almost entirely confined to the Continent. Formaldehyde has a hardening and tanning effect on animal tissues, and leather treated by this process often has inferior wearing qualities.
In the case of the artificial tannin, this property of formaldehyde is to a large extent modified by the chemical reaction with a sulphonated phenol, the addition of which also increases its tanning effect; but, while the tannage is very rapid, especially in the case of light skins, leather produced solely by means of artificial tannin has a slight tendency to dryness after being in stock for some time.
No doubt, however, methods will be found to overcome initial difficulties, and various liquors will be prepared to meet different requirements. The question of price remains the deciding factor as to its use on an extensive scale, for, while it is so high, natural tanning materials will be preferred, except in a few special cases. According to the hide-powder method of tannin analysis, Neradol contains about 30 per cent. of tannin, but there are many natural tannin extracts containing nearly double the amount of tannin at nearly half the price of the artificial tannin; the latter, however, has a more rapid action and also produces a nearly white leather. Another advantage of Neradol is that it prevents drawn grain, so that, if raw hides be treated with a small quantity before tanning, stronger vegetable tan liquors may be safely used to hasten the process. In the manufacture of sole leather, for example, limed hides, after washing in water, may be suspended in a solution of Neradol containing 3 to 5 lb. per 100 gal. of water for twelve to twenty-four hours. This quantity produces a slight tanning effect, the hides are thoroughly delimed, and strong vegetable tan liquors may then be used to complete the tannage without the grain of the hide being drawn or dark in colour, as would be the case if the hides were not first treated with the artificial tannin.
Neradol is also said to be an effective bleaching agent for tanned leather in a 5 per cent. solution for a few hours, without any loss of weight.
This property may also be utilised in the production of chrome leather, where a whiter colour is required than that produced by the ordinary chrome tannage. For this purpose it may be used in a pickling liquor before the one-bath process, or in the second bath of the two-bath tannage.
For dressing wool and fur skins, the synthetic tannins are much superior to the alum and salt process.