(12) Kolthoff, I. M., Stenger, V. il., IND. EXG.CHEM.AKAL.ED. 7, 79 (1935). (13) Li, C. H., White, C. F., J . Am. Chem. SOC.65, 335 (1943). (14) Leibhafsky, H. A., Makower, B., J . Physzchem. 37, 1037 (1933). (15) Messinger, J., Ber. 21, 3366 (1888). (16) Palmen, J., Fiska Kemistsamfundets Medd. 42,46 (1933). (17) Zbid., 43, 15 (1934). (18) Prutton, C. F., Maron, S. H., J . Am. Ckem. SOC.57, 1652 (1935).
(19) Skrabal, A., Monatsh 32, 167, 185 (1911). (20) Zbid., 71, 251 (1938). (21) Skrabal, A., 2. Elektrochem. 40, 232 (1934). (22) Zbid., 48, 314 (1942). (23) Skrabal, A., Weberitsch, S. R., Monatsh 36, 237 (1915). (24) Tomicek, O., Filipovic, P., Collection Czech. Chem. Commun. 10, 340, 415 (1938).
(25) Tornicek, O., Jasek, M., Zbid., 10, 353 (1938).
M. H. HASHMI A. A. AYAZ ABDURRASHID EHSANALI West Regional Laboratories Pakistan Council of Scientific & Industrial Research Lahore, Pakistan
Rapid Determination of Relative Proportions of Natural and SBR Rubber in Manufactured Articles SIR: The Tariff Schedules of The United States Annotated (1963) provide that certain footwear is subject to rates of duty which depend on whether or not the rubber portion of the footwear is over 50% by weight of natural rubber. -in effective approach to the problem of determining natural rubber in natural rubber-SBR vulcanizates was made by Tryon, Horowitz, and Nandel ( I ) . They pyrolyzed acetone-extracted, vacuum-dried samples a t 550" C. and analyzed the pyrolyzates by infrared absorption spectroscopy. An empirical relationship was developed between the ratio of the band intensities a t 11.02 and 11.25 microns and the natural rubber content. This laboratory was able to duplicate the results of Tryon, Horowitz, and Mandel but the calibration curve thus obtained was applicable only to a
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Figure 1. Calibration curve obtained from two vulcanization recipes pyrolyzed at 550" C. Results from three mechanical mixtures of Composition A are also indicated
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ANALYTICAL CHEMISTRY
Figure 2. Composite calibration curve for six different vulcanization recipes pyrolyzed at 950" C.
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particular vulcanization recipe. R e also found that mechanical mixtures of natural rubber vulcanizates and SBR vulcanizates gave different infrared absorbance ratios after pyrolysis than covulcanizates of these polymers using the same vulcanization recipe. Figure 1 shows the calibration curves obtained from two vulcanization recipes (given in Table I), as well as the results obtained from mechanical mixtures. b n attempt was made to eliminate the apparent matrix effect by treatment of the rubbers with dilute acids; by treatment of the rubbers with dilute bases, by mixing the rubbers with carbon black; and by mixing the rubbers with calcium carbonate. None of these measures was successful. Tryon, Horowitz, and Mandel had found only a small temperature effect over the range of pyrolysis temperatures they had tried-i.e., 400" to 550' C. At higher temperatures, a definite temperature effect was found which caused the different calibration curves, including those based on mechanical mixtures, to move closer together and to become indistinguishable when the rubber samples were pyrolyzed between 850" and 950" C. X composite curve, based on pyrolysis at 950" C., is shown in Figure 2. This curve was found applicable to the six different vulcanization recipes tested over the entire range of zero to one hundred percent
Table I.
Vulcanization Recipes
Ingredient Polymer Carbon black Zinc oxide Sulfur Stearic acid Calcium carbonate Accelerators
Parts by weight Conwo- ComDosition sition A €3 100 100 40 50 5 5 2.5 2.5 2 2 0
.iO ..
1.5 1.3 151.0 210.8
natural rubber. Thirty-seven samples of vulcanized natural rubber and 26 samples of vulcanized SBR rubber were obtained from rubber companies throughout the United States. From these 63 samples, 20 randomly selected mechanical mixtures of 50y0 natural rubber and 50% SBR rubber were pyrolyzed. The greatest deviation of the infrared absorbance ratios from the composite calibration curve was 1.3% (based on the average of six determinations). LITERATURE CITED
(1) Tryon, M., Horowitz, E., Mandel, J. J . Res. ,Vat. Bur. Std. 55, 219 (1955).
MELVINLERNER RALPHC. GILBERT U. S. Customs Laboratory Baltimore, Md.
