Synthetic Tannins, Their Synthesis, Industrial Production and Application Part 10

The volume of liquid in the flask must, after cooling, be accurately made up to 1 litre.

6. _Filtration_.--The infusion shall be filtered till optically clear (_vide_ 2). No correction for absorption is needed for the Berkefeld candle, or for S. and S. 590 paper [Footnote: Schleicher and Schull, Duren (Rheinland), Germany.] if a sufficient quant.i.ty (250-300 c.c.) is rejected before measuring the quant.i.ty for evaporation, and the solution may be pa.s.sed through repeatedly to obtain a clear filtrate.

If other methods of filtration are employed, the average correction necessary must be determined in the following manner:--About 500 c.c. of the same or a similar tanning solution is filtered perfectly clear, and after thorough mixing 50 c.c. is evaporated to determine "Total Soluble A." A further portion is now filtered in the exact method for which the correction is required (time of contact and volume rejected being kept as constant as possible), and 50 c.c. is evaporated to determine "Total Soluble B." The difference between "A" and "B" is the correction sought, which must be added to the weight of the total solubles found in a.n.a.lysis. An alternative method of determining correction, which is equally accurate and often more convenient, is to filter a portion of the tanning solution through the Berkefeld candle till optically clear, which can be generally accomplished by rejecting 300 or 400 c.c., and returning the remaining filtrate repeatedly; and at the same time to evaporate 50 c.c. of the clear filtrate obtained by the method for which correction is required, when the difference between the residues will be the correction sought. An average correction must be obtained from at least five determinations. It will be found that this is approximately constant for all materials, and amounts in the case of S. and S. 605, 150 c.c. being rejected, to about 0.005 gm., and where 2 gm. of kaolin are employed in addition to 0.0075 gm. The kaolin must be previously washed with 75 c.c. of the same liquor, which is allowed to stand fifteen minutes and then poured off. Paper 605 has a special absorption for a yellow colouring matter often contained in sulphited extracts.

7. Hide powder shall be of a woolly texture, thoroughly delimed, preferably with hydrochloric acid. It shall not require more than 5 c.c. or less than 2.5 c.c. of decinormal NaOH or KOH to produce a permanent pink colour with phenolphthalein on 6.5 gm. of the dry powder suspended in water. If the acidity does not fall within these limits it must be corrected by soaking the powder before chroming for twenty minutes in ten to twelve times its weight of water, to which the requisite calculated quant.i.ty of standard alkali or acid has been added. The hide powder must not swell in chroming to such an extent as to render difficult the necessary squeezing to 70-75 per cent. of water, and must be sufficiently free from soluble organic matter to render it possible in the ordinary washing to reduce the total solubles in a blank experiment with distilled water below 0.005 gm per 100 c.c. The powder, when sent out from the maker, shall not contain more than 12 per cent. of moisture, and shall be sent out in air-tight tins.

The detannisation shall be carried out in the following manner:--

The moisture in the air-dried powder is determined, and the quant.i.ty equal to 6.5 gm. actual dry powder is calculated, which will be practically constant if the powder be kept in an air-tight vessel. Any multiple of this quant.i.ty is taken according to the number of a.n.a.lyses to be made, and wet back with approximately ten times its weight of distilled water. Two grammes per 100 of dry powder of crystallised chromic chloride, CrCl_3.6aq., is now dissolved in water and made basic with 0.6 gm. of Na_2CO_3 by the gradual addition of 11.25 c.c. of normal Na_2CO_3, thus making the salt correspond to the formula Cr_2Cl_3(OH)_3. In laboratories where a.n.a.lyses are continually being made, it is more convenient to employ a 10 per cent stock solution, made by dissolving 100 gm. of Cr_2Cl_6.6aq. in a little distilled water in a litre flask and very slowly adding a solution containing 30 gm. of anhydrous sodium carbonate, with constant stirring, finally making up to the mark with distilled water and well mixing. Of this solution 20 c.c. per 100 gm., or 1.3 c.c. per 6.5 gm. of dry powder, should be used. This solution is added to the powder, and the whole churned for one hour. At the end of the one hour the powder is squeezed in linen to free it as far as possible from the residual liquor, and washed and squeezed repeatedly with distilled water, until, on adding to 50 c.c. of the filtrate one drop of 10 per cent. K_2CrO_4 and four drops of decinormal silver nitrate, a brick-red colour appears. Four or five squeezings are usually sufficient. Such a filtrate cannot contain more than 0.001 gm. of NaCl in 50 c.c.

The powder is then squeezed to contain 70-75 per cent, of water, and the whole weighed. The quant.i.ty Q containing 6.5 gm. dry hide is thus found, weighed out, and added immediately to 100 c.c. of the unfiltered tannin infusion along with (26.5-Q) of distilled water. The whole is corked up and agitated for fifteen minutes in a rotating bottle at not less than 60 revs. per minute. It is then squeezed through linen, the fitrate stirred and filtered through a folded filter of sufficient size to hold the entire filtrate, returning till clear. Sixty c.c. of the filtrate is then evaporated and calculated as 50 c.c., or the residue of 50 c.c. multiplied by 6/5. The non-tannin filtrate must give no turbidity with a drop of a solution of 1 per cent, gelatine and 10 per cent, common salt. [Footnote: It is convenient for technical purposes to employ the commercially obtainable chromed hide powder as prepared, for instance, by the German Experimental Station at Freiberg, Saxony.]

One gramme of kaolin, freed from all soluble matter, may be added to the filtrate, or it may be used by mixing it with the hide powder in the shaking bottle.

The a.n.a.lysis of used liquors and spent tans shall be made by the same methods as are employed for fresh tanning materials; the liquors being diluted, are concentrated by boiling _in vacuo_, or in a vessel so closed as to restrict access of air, until the tanning matter is if possible between 3.5 and 4.5 gm. per litre, but in no case beyond a concentration of 10 gm. per litre of total solids, and the weight of hide powder used shall not be varied from 6.5 gm.

The results shall be reported as shown by the direct estimation, but it is desirable that in addition efforts shall be made, by determination of acids in the original solution and in the non-tannin residue, to ascertain the amount of lactic and other non-volatile acids absorbed by the hide powder, and hence returned as "tanning matters."

In the case of tanning materials it must be clearly stated in the report whether the calculation is on the sample with moisture as received, or upon some arbitrarily a.s.sumed percentage of water; and in that of liquors whether the percentage given refers to weight or to grammes per 100 c.c., and in both cases the specific gravity shall be reported.

All a.n.a.lyses reported must be the average result of duplicate determinations, which must agree in the case of liquid extracts within 0.6 per cent, and of solid extracts within 1.5 per cent, or the a.n.a.lysis shall be repeated until such agreement is obtained.

All reports shall be marked: a.n.a.lysed in accordance with the rules of the S.L.T.C. (I.A.L.T.C.)--when the a.n.a.lyses have been carried out according to the method described above.

As has been repeatedly emphasised in this treatise, the synthetic tannins form a special cla.s.s of substances, and the results obtained by either of the two hide-powder methods do not give figures which are always comparable to those of the natural tannins. An example of the inapplicability of the methods where synthetic tannins are concerned is ill.u.s.trated by the behaviour towards hide powder of them when partly neutralised to varying degrees: commercial Neradol D of acidity 1 gm.= 10 c.c. N/10 NaOH contains 33 per cent. tanning matters, completely neutralised Neradol D, which exerts no true tanning action on pelt, still contains 20 per cent tanning matter when a.n.a.lysed according to the Official Method; a difference hence exists regarding the adsorption by hide powder of a tannin and the adsorption of the latter by hide. As, however, we are unable to make a distinction between these two different properties by using hide powder only, we are also unable to draw the factor into account.

Another source of error is the swelling influence on hide powder by acids; for instance, an acid extract of vegetable tannins would show higher tannin contents in the a.n.a.lysis than would the same extract when less acid. The free sulphonic acid, however, is the active principle in synthetic tannins, and since the latter always contain other acids (of organic and inorganic origin) devoid of tannoid character, a source of error is thus introduced, which we cannot eliminate by the present method of a.n.a.lysis.

Of other methods of estimating the quality of a tanning material or tanning extract the _determination of solubility_, _ash_, _colour_, and _weight-giving properties_ in addition to the _firmness imparted to the leather_ by the particular material are of importance. As regards the synthetic tannins they are as a rule very soluble and it will generally be found sufficient to subject them to the ordinary qualitative examination. The ash determination in synthetic tannins, on the other hand, is not of such value as in the case of natural tanning extracts. From their composition we know that synthetic tannins contain considerable quant.i.ties of mineral salts, the presence of some of which on the one hand emphasises their pickling effect, and that on the other hand the property of dissolving phlobaphenes exhibited by the synthetic tannins is closely connected with their salt contents.

A colour determination of synthetic tannins is not of much importance, since synthetic tannins nearly always impart a white or light brown colour to the hide. In those cases only where coloured decomposition products appear as a result of intermediary reactions, may the former impart greyish or dirty colorations of little beauty to the hide. This is easily ascertained by lightly tanning a pelt.

The determination of the weight and solidity-giving properties is important both for leathers tanned with vegetable tanning extracts and for those treated with synthetic tannins, but the results obtained when using animalised cotton are not directly convertible into figures required for practical purposes. Comparative figures are better obtained by actually tanning pieces of pelt on as practical a scale as is possible, and testing the weights and tensile strengths of the pieces as against those of the original pelts, whereby in the former case the yield (pelt --> leather) is obtained.

Its capability as a tanning agent may be ascertained by submitting the synthetic tannin to an actual test tannage. The latter is carried out by introducing the dilute extract into open gla.s.s jars, holding about 400 c.c. at a width of about 8 cm. [Footnote: Acc.u.mulator jars are excellent for the purpose.--_Transl_.] The concentration of the solution is chosen according to acidity and salt contents of the synthetic tannin, the most suitable being 1.5-2.5 Be. A piece of bated pelt is suspended in the liquor in such a way that the pelt is completely surrounded by liquor, without, however, being creased or touching the bottom. If the pelt were creased during tannage, the wrinkles would become fixed and would show in the finished leather. Thus an unfair judgment of the extract would be delivered, since similar results are produced by liquors which are either too concentrated or are not properly composed, and naturally this property of an extract would be greatly to its disadvantage.

The various stages of tannage may be judged from various standpoints when examining the pelt as tannage proceeds. On the one hand, the surface of the but slightly porous pelt is altered so as to present a more porous appearance, which is now rendered more capable of absorbing liquids. On the other hand, a similar alteration takes place _within_ the pelt, to the extent to which the tanning matter has penetrated it.

How far the penetration has proceeded is easily determined by utilising the different adsorption of coal-tar dyes by untanned and tanned pelt (see p. 121). An indicator for those synthetic tannins, which are derived from the phenols, is ferric chloride, which only colours those parts of the pelt which have been penetrated by the synthetic tannins; clearer and better results are, however, obtained when the dyestuffs referred to above are employed.

As soon as the tanning matter has completely penetrated the pelt, the total time of tannage is noted, and the velocity with which the tanning matter converts the pelt into leather at that particular concentration is thus obtained. The tannage completed, the leather must be well washed in running water to remove excess of synthetic tannin and then dried. On examining the dry leathers, the colour may then be observed, and a cut will give an idea of the tensile strength and the length of fibre of the leather. The tensile strength is, however, not of much value in such a barely tanned leather and cannot be compared with that obtained in leathers tanned on a practical scale. The length of fibre is, however, of some importance, since a special feature of finished leathers tanned with synthetic tannins is the beautifully long fibre--a property which manifests itself when the leather is torn and in which an expression of the quality of the synthetic tannin may be found.

Similarly, tanning experiments combining synthetic and natural tannins may be carried out, the most interesting features of these being the different proportions in which the two products are mixed. Such experiments may be done, for instance, by preparing 2 Be. solutions of each extract and then mixing them in proportions of, say, 10:90, 20:80, 30:70, etc. Here it is again possible to infer the _tanning intensity_ of the synthetic tannin from the concentration and the time used for tannage.

A further determination of the quality of a synthetic tannin is the capability of the latter of dissolving or precipitating the natural tannins. As is well known, synthetic tannins frequently possess the practically important property of rendering natural tannins easily soluble in water. In some cases, however, synthetic tannins appear to solubilise natural tannins in concentrated solutions; when, however, the latter are diluted, the natural tannin is precipitated with varying completeness, the reason of which is often the presence of excessive acid or the presence of such salts as have no phlobaphene-solubilising properties.

For practical purposes this determination may be carried out by mixing, in different proportions, concentrated tannin solutions and the synthetic tannin; heating the mixture on the water bath for a short time, cooling and finally diluting 10, 20, and 30 gm. of the mixture to 100 c.c., which are then left in measuring cylinders for twelve to twenty-four hours; the amount deposited will then be an indication of the solubilising or precipitating effect exhibited by the synthetic tannin.

Other properties of the synthetic tannins connected with their practical application will be discussed in Part II. of this treatise.

PART II

SYNTHETIC TANNINS: THEIR INDUSTRIAL PRODUCTION AND APPLICATION

With regard to their _industrial production_, but few synthetic tannins are, to-day, of practical and commercial interest. In addition to simplicity in the method of manufacture a certain degree of purity of the raw materials const.i.tutes the criterion of their suitability. The methods of manufacture, of which nearly all are the property of the B.A.S.F., have been so worked out that the production of synthetic tannins presents no difficulties on a practical scale. Cresols, naphthalenes, and higher hydrocarbons are used as starting materials in the production of synthetic tannins; the former substances or their oxidation products are sulphonated by means of concentrated sulphuric acid, and the tanning matter produced by condensing the sulphonic acids with formaldehyde. The crude synthetic tannin thus obtained has yet to be diluted and partly neutralised before it can be applied in practice, and this is carried out by mixing the crude product with strong caustic lye. By these means the high acidity is reduced to a suitable degree learned from experience on the one hand; on the other hand, the salts of the sulphonic acids form valuable components of the commercial synthetic tannins.

The first product placed on the market was named _Neradol D_; this represents the condensation product of cresolsulphonic acid. The second synthetic tannin was _Neradol N_, which represents the condensation product of naphthalenesulphonic acid; when diluted and neutralised to the same extent as is done in the case of Neradol D, the product is named _Neradol N D_. The latest synthetic tannin has been called _Ordoval G_, the starting material of which is a still higher hydrocarbon.

The tannoid-chemical properties of these synthetic tannins have been exhaustively determined by the author, who employed Neradol D, which is most suitable for such a purpose, and the investigations relating to it will now be treated fully in the following chapters. The two other synthetic tannins exhibit very similar properties, but their few characteristics shall be shortly dealt with.

The condensation product obtained by the method described on p. 55 forms a viscous, dark coloured ma.s.s, the a.n.a.lysis of which by the shake method gives the following figures:-

Tanning matters 62.6 per cent.

Non tannins 6.4 "

Insolubles 0.0 "

Water 31.0 "

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

Acidity: 1 gm. = 40 c.c. N/10 NaOH.

According to its chemical const.i.tution, this product may be considered to be dinaphthylmethanedisulphonic acid.

Samples of this crude, strongly acid material were partly neutralised, and the following figures obtained on a.n.a.lysis:--

Acidity. Tanning Soluble Water.

Matters. Non-tans.

Per Cent. Per Cent. Per Cent.

1 gm. = 35 c.c. N/10 NaOH 61.8 7.0 31.2 1 " = 30 " " 58.9 7.1 34.0 1 " = 25 " " 50.1 7.9 42.0 1 " = 20 " " 42.2 8.9 48.9 1 " = 15 " " 37.4 10.4 52.2 1 " = 10 " " 31.6 13.6 54.8 1 " = 5 " " 26.3 16.6 57.1

Experimental tanning tests which were carried out with the various partly neutralised samples yielded leathers which, on an average, were nearly white, but which in comparison with a leather tanned with Neradol D appeared rather more greyish and were much harder.

A solution of the half-neutralised substance (1gm. = 20 c.c.

N/10 NaOH) gives the following reactions:---

Gelatine--Precipitate, partly soluble in excess tannin solution.

Ferric chloride-----No coloration.

Barium chloride-----Precipitate, insoluble HNO_3.

Bromine water-----No reaction.

Silver nitrate-----No reaction.

Aniline hydrochloride----Precipitate, dissolves when solution is heated.

This condensation product is very soluble in water, but insoluble in most solvents, excepting methyl and ethyl alcohols. The above reactions show the similarity of this dinaphthyl derivative to the dicresyl derivative, and the absence in the former of characteristic reactions with iron salts is mainly accounted for by its lack of phenolic groups.

The absence of this reaction does not, of course, influence the tannoid character of dinaphthylmethanedisulphonic acid in the least, and is of no importance in practice, since the various stages of tannage may be demonstrated by means of a solution of indigotine.

From a technical point of view the absence of this reaction is advantageous to this extent, that it eliminates the exceedingly great care to avoid the contact of tan liquors and tanned pelt with iron particles which has to be observed when tannins of phenolic character are employed.