1 N D U S T R I A L A N D ENGINEERING C H E M I S T R Y
Januarv. 1924
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Differences in Kind or Degree of Tannin Fixation'jz By Arthur W. Thomas and Margaret W. Kelly COLUMBIA UNIVERSITY, XEW YORK,N. Y.
I
T HAS been shown by Trunke13 that the water-insoluble compound formed by admixture of aqueous solutions of tannin and of gelatin can be practically completely resolved into its components by treatment with ethyl alcohol, provided the alcohol digestion is carried out before the precipitate has dried. The tannin is thereby dissolved out, leaving a residue of gelatin containing but a small portion of tannin resistant to alcohol treatment. This suggested the possibility that the treatment of vegetable-tanned collagen with alcohol might yield some interesting results, especially in the cases of collagen tanned a t differeni pH values where marked differences in rate of tanning have been found in this laboratory. Vegetable tannin solutions were made up at pH = 1, 3, 5, 7, and 9 by addition of the requisite amounts of hydrochloric acid or 3odium hydroxide, previously determined by electrometric titration, and 1-gram (dry basis) portions of hide powder (previously extracted with cbloroform to remove fats and lipoids) mere rotated for 24 hours in 50-cc. portions of the Ian liquors a t room temperature. They were then filtered and washed in Wilson and Kern4 extractors until the washings showed that all non-tans and uncombined tannins were reinoved as revealed by negative ferric chloride test. The wet tanned powders were extracted with 95 per cent ethyl alcohol in Thorn extractors in which the material is extracted by the hot vapor as well as by the condensed solvent. .4t various intervals the extracts were transferred to beakers, evaporated to dryness, followed by 4 hours' drying in vacuo at 100" C., and weighed. When it was decided to terminate the process of extraction, the extracted leathers were also dried i n vacuo and weighed, the loss of tannin due to alcohol treatment being calculated by comparison with a control series that had not been treated with alcohol. I
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HEMLOCK BARK
The 50-cc. portions of hemlock bark tanning solution contained 2.7 grams of total hemlock bark solids. The results of the alcohol extractions of the moist and completely waterextracted leathers are shown in Table I. The figures were obtained from the weight of the extracted material in the alcoholic, solutions for the 1 and 45-hour extractions. The data in Column A under 91-hour extraction were similarly obtained, but those under Column B were found from the loss of weight after drying and weighing the extracted leathers. TABLE I-ALCOHOL EXTRACTION O F MOISTHEMLOCK LEATHERS --Per Tanned ai p H 1 3 5 7 9
1 Hour 6.1 7 7 7 8 3 1 4 2
cent of Fixed Tannin Extracted-7 9 1 Hours-45 Hours (A) (B) 19 3 23 3 16 6 24 1 28 9 23 3 17 1 22 0 25 1 7 5 9 8 4 6 7.1 8.4 0.6
The lack of concordance between the data in Columns A and B demonstrates an interesting point-namely, that the Presented before the Division of Leather Chemistry a t the 64th Meeting of the American Chemical Society, Pittsburgh, P a , September 4 to 8 , 1922. Received May 9, 1923 Contribution No. 426 from the Chemical Laboratories o f Columbia University. a Bzochem. Z.,26, 468 (1910). 4 THIS JOURNAL 13, 772 (1921)
hot alcohol may be oxidized in contact with the tanned powders and result in an aldehyde tannage, thus counterbalancing to some extent the weight of tannin extracted and giving low values for the percentage of the latter, as shown under B. It is also probable that oxidation of t.he extracts upon drying may account to a certain extent for the higher figures in Column A. Since Trunkel3 showed that the dried gelatin-tannin precipitate was unaffected by alcoholic digestion, the effect of alcohol extraction was tried upon hemlock-tanned hide powder that had been dried at 100" C. before extraction. The results are given in Table 11. TABLE 11-ALCOHOLEXTRACTION O F DRIEDHEMLOCK LEATHERS ,-----Tanned a t pH---. 1 3 6 7 0 2 8 4.4 0.6
Per cent of tannin removed
9 0
It is apparent that the greatest amount of extraction is accomplished with leather tanned at the isoelectric point of collagen (pH = 5) and at pH = 3, and further, it is evident that, drying intensifies fixation since there is very little alcoholextractable tannin in the dried leathers.
WATTLEBARK Fifty-cubic centimeter portions of solutions were used which contained 2 grams of total wattle bark solids. The duration of the alcohol extraction was 250 hours. The results as epitomized in Table I11 were obtained from the weight of alcohol-extracted matter. TABLE111-ALCOHOL EXTRACTION O F MOIST WATTLE LEATHERS --Tanned a t pH1 3 5 7 9 Per cent of tannin removed 2 6 . 6 26.3 27.2 26.6 25.1
The behavior of these wattle leathers differs markedly from those which mere hemlock-tanned in that, about the same percentage of the fixed tannin was removed a t all pH values of initial tannage. Determination of the percentage extraction by means of drying and weighing the extracted leathers showed half as much extracted in the case of the leathers of pH = 1 and 3, the same amount for the leather of pH = 5, but a negative loss or rather a gain of 4 per cent and of 7 per cent for the leathers of pH = 7 and 9, respectively. This shows a pronounced oxidation of alcohol to aldehyde and fixation of the latter by the alkaline-tanned leathers. GAMBIER
A test was made with gambier-tanned hide powder, using 50-cc. portions of liquors containing 2 grams dry gambier solids. The alcohol extraction time was 45 hours. The results are tabulated in Table IV. The values given were calculated from the changes in weight of the extracted powders. TABLE IV-ALCOHOL EXTRACTION O F MorsT GAMBIER LEATHERS Tanned a t pH----------
7 -
1
1 Per cent of tannin removed
17 6
3 26.3
5 19 5
7 0.7
9 (gain of 13 8)
These results show greater percentage of alcohol-soluble tannin in the tanned leathers of pH = 3 and 5 as was the case with hemlock leathers, but in addition it is seen that the
INDUSTRIAL AND ENGINEERING CHEMISTRY
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leather of pH = 9 gained 13.8 per cent in weight even though it lost some tannin, showing a very pronounced oxidation of the alcohol followed by aldehyde tannage. CONCLUSIONS The data given show that tannin fixation at and upon the acid side of the isoelectric point of collagen is of lower degree of tenacity from that fixed on the alkaline side, or that certain substances which do not tan anionic collagen can combine with cationic collagen and that this combination becomes firmer upon desiccation of the leather. It appears to the authors that studies of alcohol extraction of various vegetable leathers may lead to interesting results concerning the difference in nature of tanning between cationic and anionic collagen, and help to distinguish between the different sorts of fixation of tannins by hide, which, without doubt, occur simultaneously-e. g., (1) collagen cations and tannin anions combine to form leather, possible
Vol. 16, No, 1
only on the acid side of the second isoelectric point of the collagen; but (2) since tanning is also accomplished on the alkaline side, a different sort of linkage from a simple ionic reaction must be accepted as possible. Powarnins believes that the keto form of tannins has the property of tanning pelt while the enol form has not. The keto form exists predominantly in acid solutions, the enol in alkaline. If this theory were the sole and correct explanation of the tanning property of vegetable tannins, then one would be forced to admit from the data given herein that in the presence of a predominance of the keto modification, the fixation is not so deep-seated as with the enol form, with the possible exception of wattle bark. ACKNOWLEDGMENT The authors are indebted to A. F. Gallun & Sons Company for grants in aid of this investigation. fi
Collegium, 1912, p. 105.
Reactions of Accelerators during Vulcanization' VI-Organic
Acids and Inorganic Accelerators
By C. W.Bedford and H. A. Winkelmann THE B.
HE effect of acids on
T
F. GOODRICH Co., AKRON, OHIO
since in a Pure gum Metallic oxides and organic acids, both of which haoe long been therateofvulcanizastock the Of the known US accelerators for the culcanization of rubber by surfur or tion of rubber has extract had Only a aromatic nitro compounds, are described as inactioc, one without the long occupied the attention effect On the rate Of vulother. The natural or added acids in rubber dissoloe the inorganic of the rubber chemist. acccIerators to form metaIIic soam. Solution of the metallic radical canization. EatonZand Stevens3 have in the rubber iS a prerequisite to its action as an-accelerator. THEORY OF EFFECTOF shown that strong mineral ORGANIC ACIDS acids cause a retardation of vulcanization, while acetic acid has only a slight effect. L. I n an attempt to corrolate and study the foregoing data, E. Weber4 states that the first organic accelerator of vul- an observation was made which explains the effect of organic canization, oleic acid, was discovered by C. 0. Weber in acids and throws further light on the mechanism of vulcaniza1904. Ostromuislenskii6 found that cinnamic acid aids in tion. An evolution of heat was observed on mixing litharge, the vulcanization of synthetic rubber, and this same acid lime, or magnesia into the dry acetone extract of pale crepe. was found as a constituent of Kickxia latex by Frank, On treatment with benzene a large amount of metallic soap Gnaedinger, and Marckwald.6 Dubosc' also described whale was found in solution, as shown by filtering and treatment oil as a vulcanization accelerator. Lead oleate, oleic, stearic, with hydrogen sulfide, which precipitated the metallic sulfides. benzoic, and many other organic acids have found industrial A working hypothesis was therefore formulated to the effect use as accelerators for low-grade rubbers since as early as that the litharge in Weber's experiments was not soluble in 1906. For example, it is reported that Camaron balls will the rubber mix owing to the removal of the natural acids in not vulcanize properly in a litharge-sulfur-rubber mix, while the rubber resins. It is now found that the deficiency caused the addition of the acetone extract from Jelutong rubber, of by removal of natural resin acids may be corrected by the stearic or of benzoic acid, will supply the natural deficiency addition of organic acids such as oleic acid. of Camaron rubber and give a satisfactory vulcanization. Further confirmation of.this theory is found in the vulL. E. Webers was the first to publish data on the use of canization of rubber by nitro compounds and litharge without litharge in an acetone-extracted plantation rubber. The the use of sulfur. A rubber mix comprising 100 parts rubber, low tensile strength which was obtained caused the writer to 10 parts sublimed litharge, and 2.5 parts by weight dinitroassume that the acetone-soluble constituents were necessary benzene gives a well-cured sheet in 50 minutes at 142' C. for vulcanization. Later, Stevensg proved that the acetone (287' F.) If acetone-extracted rubber is used, no vulcanizaextract was needed only when litharge was used in the rubber tion takes dace under the same conditions. The addition of 2 per cent of oleic acid t o this mix free from acetone extract 1 Presented before the Division of Rubber Chemistry a t t h e 65th Meetine - ofthe American Chemical Society, New Haven, Conn., April 2 to 7,1923. permits the mix to be vulcanized with the formation Of a 2 Agr. Bull. Federated Malay States, 4, 162 (1916). &ell-cured sheet. Bull. Rubber Growers' Assoc., 2 , 343 (1920). It is well known that lead oleate forms rapidly when 4 India Rubber J., 63, 793 (1922). J . SOC.Chem. Ind., 3 5 , 58 (1916);C. A . 10, 3176 (1916). litharge is stirred into oleic acid. For this reason lead a Grmmi-Zlg., 25, 840, 877 (1911). oleate instead of litharge and oleic acid was used in a m i x 7 Indza Rubber World, 56, 196 (1919). comprising acetone-extracted rubber, lead oleate, and dinitro8 8th Intern. Congress Aggl. Chem., 9,95 (1912). benzene, Vulcanization took place readily, although the 1 J . Soc Chem. I n d . , 36,874 (1916); Ibid.. 41,3261' (1922).
