Determination of Polymer Composition of Rubber Vulcanizates D. W. Carlson, H. C. Ransaw, and A. G. Altenau The Firestone Tire & Rubber Company, Central Research Laboratories, Akron, Ohio 44317 VARIOUS METHODS exist for the quantitative determination of the polymer composition in vulcanized stocks. These methods consist of dissolving the vulcanizates in boiling o-dichlorobenzene, removal of the carbon black by filtration, and then infrared determination of the polymer (I). Pyrolysis of the stock followed by infrared analysis of the products has also been used (2). This technique had the disadvantages of requiring an analysis of products other than the original polymer or polymers. The infrared calibration based on polymers would no longer be valid. There may also be some uncertainty about changes in the relationship between the original microstructure and the pyrolysis products. Other infrared studies have been made but no quantitative measurements were reported ( 3 6 ) . Recently an N M R method was published for vulcanizate analysis (7). The method consisted of dissolving the vulcanizate in hexachlorobutadiene, recording the N M R spectrum of the solution, and determining the total amount of butadiene, styrene, and natural rubber from the spectrum. Good results were obtained on a variety of vulcanizates. The one disadvantage of the method was that only a limited amount of microstructure data could be obtained. This was due to the lack of resolution of the 60-megacycle NMR. We have now developed a technique which allows enough of the rubber to dissolve in carbon disulfide for infrared analysis. This solution is free of carbon black. Infrared analysis provides microstructure data on the butadiene and/or isoprene portions as well as the total styrene content. EXPERIMENTAL Apparatus. A Beckman IR-4 infrared spectrophotometer was used. Sample solutions were run in liquid cells, A Varian HA-60-IL nuclear magnetic resonance spectrometer was used to record the spectra. Procedure. Samples of the vulcanized rubber were cut into small pieces and extracted with acetone overnight in a Soxhlet apparatus to remove the oil, antioxidant, and any other extractable compounds. The extracted vulcanizate was dried to remove any residual acetone. About 1 gram was placed in a test tube which was capped with a glass wood plug and heated at 200 "C for 10 minutes. The vulcanizate was allowed to cool to room temperature. Carbon disulfide was then added and the mixture was shaken for 30 minutes. After this time, enough polymer was in solution for suitable infrared and NMR spectra. These solutions, which were free of carbon black, were filtered through filter paper to remove any undissolved vulcanizate and then run on the infrared and NMR instruments. For vulcanizates containing only polyisoprene as the elastomer, 150 "C. rather than 200 'C. was used. If this were not done, the carbon disulfide solution was not free of carbon black. (1) H. L. Dinsmore and D. C. Smith, Rubber Chem. Technol., 22, 572 (1949). (2) D. A. MacKillop, ANAL.CHEM., 40, 607 (1968). (3) D. L. Harms, ibid., 25, 1140 (1953). (4) D. Hummel, Rubber Chem. Technol., 32, 854 (1959). (5) M. Lerner and R. C. GiIbert, ANAL.CHEM., 36, 1382 (1964)i (6) M. Tyron, E. Horowitz, and J. J. Mandel, J. Res. Nut. Bur. Std., 55, 219 (1955). (7) D. W. Carlson and A. G. Altenau, ANAL.CHEM., 41,969 (1969).
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Infrared Measurement. The following are the wavelengths that were used to measure the amounts of the different microstructures: cis-l,4-butadiene, 7.6 p ; trans-l,4-butadiene, 10.35 1.1; 1,2-butadiene, 10.98 p ; styrene, 14.3 p ; 1,4isoprene, 11.9 p ; 3,4-isoprene, 11.25 p. Base-line absorbances were determined from the spectra at each wavelength. The absorptivities of the different microstructures were previously determined (8-10). The latter references also describe the procedure used for the infrared measurement, Since the absorptivities and cell thicknesses are known and the absorbance is measured, the concentration of each microstructure can be calculated. The material that is soluble in carbon disulfide after heating the vulcanizate is rubbery and still polymeric. This permits the use of the same infrared calibration data that is used with base polymers. RESULTS AND DISCUSSION
Table I shows the analyses of vulcanizates containing either polybutadiene or styrene-butadiene copolymer (SBR), or a blend of these two rubbers. The relative amounts of polybutadiene and SBR varied in these blends as well as the types of polybutadiene and SBR. These differences are reflected in the calculated per cents for the samples. The found styrene values were slightly higher than the calculated amounts and the found trans-1,Cbutadiene values were slightly lower than the calculated amounts. The exact reason for these differences is not known. The high styrene values could be the result of the products formed during the heating of the vulcanizate which absorb at the styrene wavelength. It could also be simply that the carbon disulfide solution of the vulcanizate after heating is richer in styrene compared to a solution of the original blend of polymers. This could be the result of SBR devulcanizing faster than polybutadiene. The lower trans values are probably the result of the measured styrene absorbance being slightly high. The styrene absorbance is part of the equation used to calculate the microstructure of the butadiene portion. Consequently the butadiene microstructure will be affected by the higher styrene absorbance value. This would be reflected mostly in the trans value because it is the predominant microstructure of the butadiene. The infrared spectra of a carbon disulfide solution of uncompounded SBR and the soluble portion of a vulcanizate containing SBR after the vulcanizate was heated at 200 "C for 10 minutes are very similar. However it should be noted that the spectra of the heated vulcanizate showed a very small carbonyl absorption, at 5.9 p, probably indicating some oxidation of the polymers. Carbon disulfide absorbs between 6 to 7 microns. Consequently no polymer absorption is seen in this region. This solvent absorption causes no problem because the 6- to 7-micron region is not used for the determination of microstructure of the rubbers involved in this work. Table I1 shows the analyses of vulcanizates containing natural rubber, polybutadiene, and SBR. The infrared re(8) J. L. Binder, ANAL.CHEM., 26, 1877 (1954). (9) J. L. Binder, J. Polym Sci., Part A , 1,47 (1963). (10) Ibid., 3, 1587 (1965).
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Styrene 15 21 26 17
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Styrene 19 29 14 24 13
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Table I. Determination of Polymer Composition Found compounded formula Calculated compounded formula Butadiene Butadiene cis trans 192 Styrene cis trans 55 8 35 55 37 14 30 56 16 27 16 21 51 20 53 12 23 15 49 13 52 14 28 12 48 8 53 14 43 23 17 22 17 39 Table 11. Determination of Polymer Composition Calculated compounded formula Found compounded formula Butadiene Natural Butadiene cis trans 192 rubber Styrene cis trans 1.2 19 28 4 50 13 18 3 ... 37 54 10 37 55 8 4 22 20 35 14 4 20 12 6 35 30 ... 31 11 39 19 5 32 9 40 10 5 30 6 ... 30 10 49 11 8 54 15 4 27 7 50 8 6 22 6 7 53 14 ... 32 9 49 10
suits indicate that the concentration of natural rubber in the carbon disulfide solution is considerably higher than that present in the vulcanizate. This results from the tendency of natural rubber to devulcanize or depolymerize faster than polybutadiene and SBR. Other conditions were investigated such as higher and lower temperatures and different heating times but there was no improvement. The first line of analyses of each sample in Table I1 indicates the data on the basis of total polymer composition. The second line shows the data on the basis of 100 % polybutadiene, if there were not any styrene present, or on the basis of 100% SBR if styrene were present. The data were split up in this way in order to observe whether the ratio of styrene to butadiene and the microstructure of the butadiene portion changed compared to the calculated values. Table I1 shows that there were changes, greater than observed in vulcanizates containing only SBR and polybutadiene (Table I). This is probably the result of the exceptionally fast breakdown of natural rubber upon heating at 200 "C. However the data that are obtained provide a good insight into the microstructure of unknown vulcanizates containing natural rubber. Therefore infrared analysis of the polymer solution obtained after heating a vulcanizate containing natural rubber is only useful for determining the relative amounts of cis- and truns1,4-butadiene and 1,2-butadiene. Although vulcanizates containing synthetic polyisoprene were not studied, it would be expected that these vulcanizates would behave similarly to vulcanizates containing natural rubber. The infrared spectra of a carbon disulfide solution of an uncompounded styrene-butadiene copolymer and natural rubber and the carbon disulfide soluble portion of a vulcanizate containing a blend of styrene-butadiene copolymer and natural rubber after the vulcanizate was heated at 200 "C for 10 minutes showed no significant difference. The recent paper ( 7 ) demonstrating the technique of dissolving the vulcanizate in hexachlorobutadiene and determining the relative amounts of butadiene, styrene, and natural rubber from the N M R spectrum can be used in conjunction with this heating technique. This especially true for a vulcanizate containing natural rubber. The relative amounts of butadiene, styrene, and natural rubber can be determined from the N M R spectrum of the hexachlorobutadiene solution
1,2 10 57 12
13 12 22
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Natural rubber 66 ... 50
... 49
... 59
..
of the vulcanizate ( 7 ) . The microstructure of the butadiene portion can be determined by infrared analysis on the carbon disulfide solution obtained after heating this vulcanizate at 200 "Cfor 10 minutes. Thereby using the hexachlorobutadiene method and this heating technique, the polymer composition as well as the microstructure of the butadiene can be determined in a vulcanizate containing natural rubber. If synthetic polyisoprene rather than natural rubber were used, infrared analysis would also be able to determine the total 1,4-polyisoprene and 3,4-polyisoprene contents. Natural rubber is essentially 100 cis-1 ,4-polyisoprene. N M R spectra were run on the same carbon disulfide solutions of the vulcanizates that were used for infrared analysis. The N M R spectra were the same as those of uncompounded polymers. The resolution and response are excellent. N M R analysis is only capable of determining the relative amounts of 1,2-butadiene and 1,4-butadiene. The individual amounts of cis- and truns-l,4-butadiene could not be determined because of complete overlapping of two signals as is also the case with uncompounded polymers ( 7 , l l ) . In the case of a blend of polybutadiene and/or SBR with natural rubber, it is very difficult to determine the amounts of any of the different microstructures because the signals from the olefinic protons severely overlap one another. Therefore N M R analysis is only useful in determining the relative amounts of each monomer in the vulcanizate (7, 11). This heating technique has been used successfully for qualitative analysis of vulcanizates containing butyl or ethylene propylene (EPDM) rubber. Neoprene vulcanizates also responded to this heating technique but the polymer solution obtained is much less concentrated in neoprene compared t o the other rubbers studied. The concentration of the polymer solution can be improved if the temperature is increased from 200 to 300 "C. ACKNOWLEDGMENT
The authors thank the Firestone Tire and Rubber Company for permission to publish this work.
RECEIVED for review March 13,1970. Accepted June 15, 1970. (11) V. D. Mochel, Rubber Chem. Techrtol., 40, 1200 (1967).
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