INDUSTRIAL A X D ENGINEERING CHEMISTRY
April, 1928
to a spiral condenser, which is built into a modified Perkin triangle, permitting the removal of distillate without breaking the vacuum. The triangle, which is equipped with a manometer, is attached to a block-tin condenser containing a calcium chloride hydrate freezing mixture to remove any solvent vapor. This condenser is connected with a Hyvac pump* Most of the residual solvent can be removed by slowly bubbling carbon dioxide through the heated oil, so that the pressure remains around 5 to 10 mm. For the removal of the last traces of solvent (below 0.5 per cent) it has been found advisable to allow the pressure to go as low as possible and then to admit a little gust of the gas. The temperature is gradually raised to 140' C. and the process repeated until no more solvent will come over. Properties of Extracted Oil
The oil obtained by this method is a clear, dark green oil, yellowish green in thin layers. It has an odor faintly reminiscent of oat meal. The constants of the oil thus prepared are given in Table I, column 1. It was essential for our purpose that the oat oil so obtained should not have undergone any oxidation or change in iodine value. I n order to test the above method, 20 pounds of fresh chopped oats were dried in a vacuum oven for 48 hours a t 35" C. and then extracted with anhydrous ether. After extraction the ether was removed by a current of carbon
427
dioxide a t 100" C. The iodine number of the oil thus prepared was 105.0, whereas the "technical" oil had a value of 107. These values show that the oil prepared in the extractor did not undergo oxidation during any part of the process. The acid value of the "technical" oil appeared rather high. The ether-extracted oil gave an acid value of 21.1. This agrees with Berry,2 who states that the free fatty acids are negligible, but that hydrolysis occurs, depending on the time between grinding and extraction. It is probable that hydrolysis has also taken place during the initial stages of extraction, since the cracked oats were undried and some water was found in the drum after several extractions. It is probable that hydrolysis can be inhibited by extracting immediately after cracking or chopping. The constants for oat oil prepared on a laboratory scale by different workers are given in the accompanying table. It will be seen that the acid values of all are considerably higher than that of the ether-extracted oil. Table I-Constants for Oat Oil "TECHNICAL" DUBOPAUL^ CON5TANT OIL VITZa (15/15O Specific gravity, 25/25' C. 0.9191 0.9110 0.925 Refractive index at 20' C. 1.4710 1.4706 1.4635 Iodine number (Wijs) 107.2 91.7 114.2 Saponification number 190 6 180.1 189.8 Unsaponifiable matter, per cent 2.26 1.61 1.30 Acid value 89.1 62.11 68.9 a Chcm.-Ztg., 42, 13 (1918). b Analyst, 46, 138 (1921).
c.)
2
STELLWAAGh
... .. . ...
192.4 2.65 70.26
J. Agr. Sci., 10, 366 (1920).
Direct Determination of Rubber in Soft Vulcanized Rubber' A. R. Kemp, W. S. Bishop, and T. J. Lackner BELLTELEPHONE LABORATORIES, INC., 463 WEST ST., NEW Y O R K , X. Y.
r;bber and g u t t a - p e r c h a parison. based upon the additive reaction of iodochloride (Wijs solution) with their hydrocarbons. It is the purpose of the present paper to describe a modification of this method for determining the rubber content of soft vulcanized rubbers. Although several direct methods3 are described in the literature, none of them have been found entirely satisfactory. A widely used method for determining the rubber content in vulcanized rubbers is that recommended by the Committee on Methods of Analysis of the Division of Rubber Chemistry of the AMERICANCHEMICALSOCIETY.~ This method is a complicated one and consists in determining the non-rubber constituents and taking rubber by difference. As mentioned in the committee report, the method is inaccurate in the presence of decomposable inorganic constituents, cellulose, or high percentages of mineral rubber. 1 Received
October 25, 1927. 2 Kemp, I n d . Eng. Chem., 19, 531 (1927). 8 Alexander, Gummi-Ztg., 18, 789 (1904); Axelrod, J . SOC. Chcm. I n d . , 26, 1058 (1907); Lewis and McAdams, J . I n d . Eng. Chem., 12, 673
(1920). 4 Ibid., 18, 397 (1924).
resins, free sulfur, oils, waxes, organic accelerators, age resistors, part of the mineral rubber, etc., followed by further removal of a portion of the mineral rubber with chloroform. A sample of the extracted residue is then dissolved in tetrachloroethane and the hydrocarbon present in the unsaturated state determined by the modified Wijs method.2s6 In order to determine the amount of hydrocarbon saturated with sulfur, a direct determination of sulfur combined with rubber is made on another portion of the extracted sample. The sum of the hydrocarbons in the unsaturated and combined states is then used to calculate the rubber content of the original sample. Experimental Procedure
The acetone and chloroform extractions are carried out on 2-gram samples according to standard p r ~ c e d u r e . ~The chloroform is removed by heating in a vacuum oven a t 70" C. to constant weight. An alcoholic alkali extraction is also conducted on one of the extracted samples to detect the 5
Fisher, India Rubber World, 76, 78 (1927).
428
presence of factice, for which, if present, correction is made as shown later. An accurately weighed portion of the acetone- and chloroform-extracted residue containing 0.07 to 0.10 gram of rubber which has been cut into very small pieces (20 mesh) is completely dispersed by refluxing in 50 cc. of tetrachloroethane for from 3 to 5 hours. (Eastman's technical grade fractionated between 146.5" and 147.5" C . (cor.) using a 24-inch Young column. After refluxing 5 hours 50 cc. should not liberate iodine from potassium iodide equivalent to more than 0.5 cc. of 0.1 N sodium thiosulfate.) For this purpose an Underwriter's flask' with glass condenser similar in design to the Underwriter's block tin condensere is used. It has been found that careful heating is essential to obtain concordant results. This is accomplished by the use of an oil bath kept a t 180-170" C . The flask containing the sample is so arranged that the level of the solvent is slightly above the level of the oil in the bath. "Scorching" of the sample is thus prevented. A blank determination is performed at the same time. After dispersion is complete, the flask is removed from the bath, allowed to cool, and the contents diluted with 25 cc. of carbon bisulfide (c. P. grade, purified by allowing to stand in contact with solid potassium hydroxide for 48 hours and finally distilled over same). Twenty-five cubic centimeters of 0.2 N Wijs solution are then added from a pipet, the flask is covered and immediately placed in ice water, and the reaction is allowed to proceed for 2 hours in diffused light under ordinary laboratory conditions. At the end of this period 25 cc. of 15 per cent potassium iodide solution and 50 cc. of distilled water are added. The liberated iodine is then titrated with 0.1 N sodium thiosulfate.' Before the brownish color of iodine has disappeared from the aqueous layer, 5 cc. of a 5 per cent soluble starch solution are added as an indicator. At this point the flask must be shaken vigorously to remove last traces of iodine from the solvent layer. Also care must be taken to allow dark suspended materials to settle after each shaking; otherwise, their coloration in suspension might obscure the end point. After the end point has been reached the aqueous layer should be colorless and remain so after shaking and standing. The difference between the blank and sample titration is used to calculate the iodine number in the usual manner. Procedure for. Sulfur Combined with Rubber
A 0.5-gram sample of the acetone- and chloroform-extracted residue is refluxed in 50 cc. of tetrachloroethane under the identical conditions mentioned above. After dispersion is complete, the contents of the flask are transferred to a 250-cc. volumetric flask with the aid of hot carbon tetrachloride. After the contents of the volumetric flask have cooled to 25" C., the solution is made up to the mark with carbon tetrachloride, the contents shaken thoroughly and the solids separated by centrifugation or allowed to settle overnight. One-hundred cubic centimeters of the upper portion of the solution are then pipetted and transferred to a 125cc. porcelain crucible. The solvent is evaporated on a steam bath and the sulfur in the residue determined according to the method of Waters and Tuttle.*** Calculation of Results
The theoretical iodine value for pure rubber hydrocarbon based on one double bond for every C6Hs group is 372.8; hence, the percentage of unsaturated hydrocarbon in the extracted residue is given by the ratio of the iodine value obtained for the sample to the theoretical iodine value. The a
L. Weber, "Chemistry of Rubbec Manufacture," p. 111.
I
Popoff and Wbitman, J . Am. Chcm. SOC.,47, 2259 (1826). Bur. Standards, Sci. Paim 174 (1911).
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I N D U S T R I A L AND ENGINEERING CHEMISTRY
hydrocarbon which is combined with sulfur is given by multiplying the per cent combined sulfur in the extracted residue by 2.13, which is the ratio of hydrocarbon to sulfur in CSH&. The hydrocarbon in the original sample is obtained by multiplying the hydrocarbon content found in the extracted residue by the difference between 100 and the combined percentages of acetone and chloroform extracts. For example: Per cent hydrocarbon in original sample = 100
A = iodine value of acetone- and chloroform-extracted
residue B = Per cent sulfur combined with rubber in acetone- and chloroform-extracted residue C = percentage of acetone and chloroform extracts Effect of Non-Rubber Constituents
Any method proposed for determining rubber in commercial rubber articles must take into account the large number of organic and inorganic compounding ingredients which may be present, as some of these materials may seriously affect the accuracy of a method. The presence of an unsaturated material other than rubber which will absorb iodochloride will obviously cause the results to be high. Resins, oils, waxes, accelerators, age resistors, etc., some of which have high iodine values, are removed by acetone and chloroform. Carbon black, glue, cellulose, and inorganic compounding ingredients do not absorb appreciable iodochloride. Mineral rubber is only partly extracted by acetone and chloroform, leaving a residue soluble in tetrachloroethane having a degree of unsaturation which will cause only a small error even when present in large quantities. Factice, on the other hand, is almost completely soluble in tetrachloroethane and has a high degree of unsaturation and a high combinedsulfur content, which introduce an error in the result for which correction must be made. The effect of carbon black, glue, cellulose, and a variety of inorganic compounding ingredients upon the results was tested by applying the foregoing method to 0.1-gram samples of these materials. The inorganic compounding ingredients tested were litharge, magnesium oxide, lithopone, zinc oxide, whiting, lime, Dixie clay, and talc. The results showed that the absorption of iodochloride by any of these materials except cellulose is practically nil. I n the case of a sample of extracted absorbent cotton the iodine value was found to be 7.8; therefore, 5 per cent of cellulose in a compound would be equivalent to only 0.1 per cent rubber hydrocarbon. The presence of cellulose as well as the other compounding materials mentioned above may therefore be neglected in this method of analysis. Table I-Analysis
of Factice BROWN
Acetone extract Chloroform extract Combined sulfur (extracted residue) Iodine value (extracted residue)
Per cent 13.5 0.10 9.65 115.0
WHITE
Per cent 4.4 0.03 7.84 80.0
Weber9 places the amount of mineral rubber insoluble in acetone and chloroform after vulcanization a t 45 per cent. The iodine value of a sample of mineral rubber as determined by the method outlined above wa0 65.7. I n a compound containing 10 per cent mineral rubber the insoluble portion, if equally unsaturated, would cause an error of less than 1 per cent in the hydrocarbon content on the basis of Weber's figure of 45 per cent unextracted material. In the case of compound I1 in Table I1 it was found that 70 per cent of the
* 09.
cit., p. 216.
INDUSTRIAL AND ENGINEERING CHEMISTRY
April, 1928
mineral rubber added was extracted by the acetone and chloroform. I n general, the effect of ordinary amounts of mineral rubber is small, though perhaps not entirely negligible in extreme cases. To illustrate the need for correcting for factice, analyses of samples of the bronm and white varieties are given in Table I. On the basis of these analyses 1 per cent of brown factice in a rubber compound is equivalent to 0.45 per cent of rubber hydrocarbon. In a similar manner 1 per cent of white factice is equivalent to 0.40 per cent of hydrocarbon. In analyzing rubber compounds containing factice, it would seem a t first that the analyses should be carried out on the alcoholic potash-extracted sample. It has been found, however, that in most cases this treatment, even after thorough removal of moisture and free alkali, changes the rubber t o a condition such that it is insoluble in the tetrachloroethane. Furthermore, as shown in Table 111, the alcoholic alkali extraction process removes only about 60 per cent of the factice added. When the alcoholic potash extract exceeds 1 per cent, indicating the presence of factice, the rubber hydrocarbon content is calculated in accordance with the formula already given, except that the value of C for total extracts should include the alcoholic potash extract. Results of Analyses
Several compounds the compositions of which are given in Table I1 were analyzed by the method outlined. The results of the analyses are given in Table 111. These compounds were chosen for the purpose of showing the effects of various compounding ingredients, and therefore do not conform to the best rubber-compounding practice. Because of their indefinite rubber content the analyses of reclaimed rubbers are given in a separate part of the paper. T a b l e 11-Composition
CONSTITUEKT Smoked sheet Sulfur Diphenylguanidine Tetramethylthiurammonosulfide Mineral rubber Mineral oil Stearic acid Factice brown) Factice [white) Zinc oxide Carbon black
of R u b b e r C o m p o u n d s I I1 I11 IV 92.5 83.3 80 80 7.5 3.4 3 3 2 2 0.8 8.3
1.0 4.0 0 3
_ _ - - - -
TOTAL 100.0 Table 111-Average
v 60.0 2.4
100.0
100
100
100.0
Analyses of C o m p o u n d s in T a b l e I1 I I1 I11 IV v 8.35 8.89 7.55 7.32 7.45 2.64 0.92 1.50 0.75 5.28 6.54
Acetone extract Chloroform extract Alcoholic potash extract Iodine value of extracted residue 334.8 317.8 310.5 301.3 216.7 Sulfur combined with rubber in extracted residue 2.37 1.28 2.84 2.19 1.54 Rubber hydrocarbon: Found 86.9 77.9 77.0 72.5 56.2 Calculated" 86.0 77.4 74.4 74.4 55.8 Difference +0.9 $0.5 4-2.6 -1.9 $0.4 a In calculating the hydrocarbon content 93.0 per cent was used a s the hvdrocarbon content of first-oualitv smoked sheets. This is in accordance Gth ichitby "Plantation~RuGber."p. 60
