Synthetic Tannins, Their Synthesis, Industrial Production and Application Part 16

In addition to formaldehyde, many other substances may, theoretically, induce condensation of phenolsulphonic acid; condensation takes place either with the elimination of water or, in addition to this, with the introduction of methane group.

So far, the following condensing agents have been investigated:--

(1) Heating _in vacuo_.

(2) Sulphur chloride.

(3) Phosphorus compounds.

(4) Aldehydes.

(5) Glycerol.

1. Condensation Induced by Heat

If phenolsulphonic acid is heated _in vacuo_ at 130 C. for twenty hours, condensation takes place [Footnote: Austr. Pat., 64,479.]

without the addition of any condensing agent, and an anhydride of the

^ __O__ ^ | | | | | | | | v v HSO_3 HSO_3

composition is formed. This product is a viscous liquid, possessing a very corrosive action. Added to a solution of gelatine, a light, fine flocculent precipitate is thrown down. a.n.a.lysed by the shake method of a.n.a.lysis, the tannin content of the product equals about 46 per cent. Its strongly acidic and hence swelling character does not express qualities consistent with the conception of suitability for tanning purposes: a sample of the product was therefore partly neutralised to the acidity of Neradol D, when the shake method of a.n.a.lysis yielded the following figures:--

Tanning matters 21.5 per cent.

Soluble non-tannins 48.3 "

Water 30.2 "

-------------- 100.0 per cent.

This partly neutralised sulphonic acid represents a white, pasty ma.s.s, which is not particularly easily soluble in water, yielding a solution of milky appearance. Treated with the usual tannin reagents, it exhibits the following characteristics:--

Gelatine Light Flocculent precipitate.

Bromine water Compete fixation.

Ferric chloride Cherry-red coloration.

Lead acetate Very slight Percipitate, insoluble HNO_3.

Aniline hydrochlonde Slight percipitate.

Solutions of this product in concentrations from 1-8 Be. exerted no tanning action whatever, whereas more concentrated solutions (15 Be.) converted pelt in eight days into a leather very similar to a Neradol D leather in colour and feel, but considerably harder.

In order to determine its phlobaphene-solubilising effects, samplesof the product were mixed with concentrated quebracho extract in the proportions 5,10, 20, and 30 per cent. on the weight of extract, and the following observations made:--5 and 10 per cent. were without effect, 20 and 30 per cent. showed some solubilising tendency, but on diluting the mixture with water the quebracho was completely thrown out of solution. Apparently this anhydride is, in this respect also, quite different from the partly neutralised diphenylmethanedisulphonic acid.

2. Condensation with Sulphur Chloride

When sulphur chloride is allowed to act upon phenolsulphonic acid whilst heat is applied, a yellowish-grey ma.s.s results, which dissolves in water, forming a reddish-yellow solution. Neutralised to acidity 10, it exhibits the following reactions:--

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

Ferric chloride---------Deep blue coloration.

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

Aniline hydrochloride---Precipitate.

Bromine water-----------No reaction.

The partly neutralised 2 Be. solution of this product yielded a reddish-grey coloured leather, the qualities of which were very similar to that yielded by Neradol D.

3. Condensation with Phosphorus Compounds

Schiff's well-known synthesis, [Footnote: Liebig's _Ann_., 178, 173.]

in which phosphorus oxychloride interacts with phenolsulphonic acid, yields a product which exhibits some tannin reactions, but which, when acting on pelt, converts the latter into a leather which, when dried, is very cracky. If, on the other hand, cresolsulphonic acid is condensed with phosphorus oxychloride by heating the two together, products eminently suitable for tanning purposes result. These products are non-crystalline bodies easily soluble in water, and are coloured bluish-violet by ferric chloride and precipitate gelatine. Solutions of the free acids and acidified solutions of the salts convert pelt into firm and white leathers possessing great softness and pliability.[Footnote: Austr. Pat, 66,895.]

4. Condensation with Aldehydes

By treating phenolsulphonic acid with acetaldehyde in the usual way, a viscous brown ma.s.s is obtained, which is very soluble in water, the solution being of a brown colour. When brought to acidity 10, the following reactions are exhibited by the product:--

Gelatine - - - Precipitate.

Ferric chloride - - Deep blue coloration.

Aqueous ammonia - - Cherry-red coloration.

Lead acetate - - - Yellowish precipitate, insoluble HNO_3.

Aniline hydrochloride - - Yellow precipitate, soluble excess aniline.

Bromine water- - - No reaction.

Tanning experiments with this substance yielded, even after extended tannage, an undertanned leather, the surfaces being coloured brown, the inner layers, however, white. Further neutralisation reduces the tanning intensity of the product; the addition of sodium sulphate to the original partly neutralised product hastened tannage, the leather, however, possessing dark colour and being undertanned. The following const.i.tution may be ascribed to this product:--

OH OH ^ ---CH_2---CH_2--- ^ | | | | | | | | v v HSO_3 HSO_3

If benzaldehyde is used in lieu of acetaldehyde for condensing phenolsulphonic acid, a water-soluble product results, exhibiting reactions similar to those of the acetaldehyde-condensation product. The former product is more suitable as a tanning agent and yields a reddish-brown rather firm and hard leather; it possesses the const.i.tution--

H OH || OH ^ ____C____ ^ | | ^ | | | | | | | | v | | v HSO_3 v HSO_3

For the purpose of condensing phenol with formaldehyde, it is not essential to first convert the phenol into the water-soluble phenolsulphonic acid, since it is possible to convert the condensation products of phenol and its derivatives, which are soluble in alkali, into water-soluble form by either heating the condensation products with concentrated solutions of formaldehyde and neutral sulphites, or by dissolving the condensation products in alkali and inducing reaction by means of formaldehyde bisulphite. [Footnote: _Collegium_, 1913, 518, 324.] Highly concentrated solutions result, which may be concentrated either as such or after the alkali present has been neutralised. The sulphurous acid formed prevents oxidation of the product on evaporation. A special advantage of this method of preparation is the fact that sulphuric acid, which is but difficultly removed from the end-product, is not employed at all.

The product thus obtained is a yellowish-white crumbly ma.s.s, which is very soluble in water, forming a clear solution. The latter exhibits the following reactions:--

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

Ferric chloride--------Deep blue coloration.

Aqueous ammonia--------Cherry-red coloration.

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

Aniline hydrochloride--Precipitate.

Bromine water----------No reaction.

The product brought to acidity 10, yielded on a.n.a.lysis the following figures:--

Tanning matters------------------ 25.2 per cent.

Soluble non-tannins-------------- 56.3 "

Insolubles----------------------- 0.0 "

Water---------------------------- 18.3 "

------------- 100.0 per cent.

Tanning experiments with this substance yielded white and soft leathers, which were indistinguishable from those tanned with Neradol D.

A characteristic feature of this synthetic tannin is its behaviour in concentrated form towards pelt, which is not attacked by it, but is readily tanned even at such high concentrations. An explanation of this is to be found in the large quant.i.ty of salts present in the product. A disadvantage of this synthetic tannin is its complete incapability of dissolving phlobaphenes, which is even so far extended as to precipitate otherwise easily soluble tannins when adding it to solutions of the latter in comparatively large proportions; here, again, the salts are responsible for this behaviour, their large quant.i.ties effecting a salting out of the natural tannins.