Quantitative Determination of Natural Rubber, Styrene-Butadiene Rubber, and Ethylene-Propylene-Terpolymer Rubber in Compounded Cured Stocks by Pyrolysis-Gas Chromatography Anoop Krishen The Goodyear Tire and Rubber Company, Research Division, Akron, Ohio 44316 Quantitative estimation of the polymeric constituents of a compounded cured sample has so far been possible only after extensive chemical treatment of the material. Pyrolytic degradation of the polymeric materials in an inert atmosphere followed by gas chromatography has been extensively used for qualitative identification and some work has been done on quantitation by this technique as well. However analysis of complex mixtures is troublesome and no data have been published for mixtures containing ethylene-propylene-terpolymer rubber. Pyrolysis under carefully controlled conditions was found to produce products which can be used for quantitative estimation of three common polymeric materials. The gaseous pyrolysis products from natural rubber, styrene-butadiene rubber, and ethylene-propyleneterpolymer rubber were analyzed by gas chromatography. Variables such as sample size, pyrolysis temperature, and time allowed for pyrolysis were critically examined to obtain optimum conditions for the production of characteristic products for each of the polymers. These conditions were then used to obtain standard curves from the analysis of mixtures containing these polymers. A sample size of about 500 fig was used. Carbon black and other similar components did not constitute any interference.
RAPID QUANTITATIVE ESTIMATION of the polymeric constituents in a compounded cured sample is hampered by the presence of carbon black which interferes with infrared analysis and necessitates laborious chemical degradation procedures ( I ) . The ease and convenience of a combined pyrolysis-gas chromatographic procedure has prompted a large number of attempts to use this method. An excellent comprehensive review of the earlier literature has been presented by Audebert ( 2 ) ; and some of the more recent work (3-7) has incorporated the commercial developments in pyrolysis apparatus. When dealing with polymers which produce almost quantitative amounts of their monomeric constituents-polymethylmethacrylate and polystyrene-the identification and quantitation is relatively simple; however, in other cases complications arise. An application of the pyrolysis-gas chromatographic technique for quantitation of rubber constituents was recently published by Ney and Heath (6). They have used this technique for the determination of (1) J. K. Clark and R. A. Scott, J. Appl. Polym. Sci., 14, 1 (1970). (2) R. Audebert, Ann. Chim. (Paris), 3, 49 (1968); Chem. Abstr. 68, 96403 (1968). (3) T. Okumoto, T. Takeuchi, and S. Tsuge, Bunseki Kagaku, 19, 1093 (1970); Post-J, 8 , 526 (1970); Chem. Abstr., 73, 99801d (1970). (4) G. L. Coulter and W. C. Thompson, Column, 3, 6 (1970); Post-J., 7, 3698 (1970); Chem. Abstr., 73, 36267p (1970). ( 5 ) K. V. Alekseeva, L. P. Khramova and I. A. Strol' Nikova, Zauod. Lab., 36, 1304 (1970); Post-J., 8 , 3109 (1971); Chem. Abstr., 74, 32503k (1971). (6) E. A. Ney and A. B. Heath, J. Inst. Rub. Ziid., 2, 276 (1968). (7) E. Cianetti and G. Pecci, Ind. Gomma, 13,47 (1969). 494
ANALYTICAL CHEMISTRY, VOL. 44, NO. 3, MARCH 1972
polyisoprene, polybutadiene and styrene-butadiene rubber in tire tread samples. Their analysis is based on the estimation of the dimers: dipentene and vinylcyclohexene, and the monomer styrene. An extension of a similar technique to include ethylenepropolylene-terpolymer rubber is of considerable interest since the major monomer units, ethene and propene do not represent unique products and are produced on pyrolysis of almost all rubbers. EXPERIMENTAL Apparatus. MICROBALANCE. A Cahn Electrobalance Model G (Cahn Division, Ventron Instrument Corp., Paramount, Calif.) was used for weighing the samples precisely. The weighings were accurate to 1 0 . 2 pg at the 1-mg range used. PYROLYSIS UNIT. The pyrolyzer was obtained from Loenco Division, Infotronics Corp., Altadena, Calif. This unit, Model 260, uses a 3-in. x I/&. quartz U-tube as the pyrolysis chamber. It is heated by sliding a thermostatically controlled heated furnace over it for a fixed time. A sliding gas valve arrangement in the unit allows uninterrupted carrier gas flow through the gas chromatographic column while the sample chamber is being reloaded or purged with helium. STRIPPING AND BACKFLUSHING VALVE. A Carle MicroVolume Eight Port Gas Chromatography Valve No. 2012 (Carle Instruments, Fullerton, Calif.) was used to avoid the introduction of high boiling pyrolysis products onto the chromatographic column. It was connected to the columns and the gas chromatograph as shown in Figure 1. The pyrolysis products were allowed to go onto the main column for the first 15 minutes only. GAS CHROMATOGRAPH. The pyrolysis products were analyzed with an F & M 1609 gas chromatograph equipped with a flame ionization detector. The chromatographic peaks were recorded with a 1-mV Brown Electronik recorder (Minneapolis-Honeywell Company, Brown Instrument Division, Philadelphia, Pa.). The peaks were recorded at electrometer range of 100 and a chart speed of '/*-inch per minute. The area measurements were made by the triangulation method (Peak height X width at half peak height). The main column was aluminum tubing 40 ft X a/16-in.o.d., 0.032-in. wall thickness, It was packed with 10% tricresylphosphate on 60-80 mesh Chromosorb P. The stripper and restrictor columns were IO-ft lengths of the a/18-in.aluminum tubing packed as the main column. The columns were operated at 35 OC. The injection port and the detector block were maintained at 175 "C. The helium, hydrogen, and air pressures were 30 psi (Flowrator at 4.0), 15 psi (Flowrator at 7.5), and 20 psi (Flowrator 9.0), respectively. Reagents. Tricresylphosphate used as the stationary phase in the gas chromatographic column was obtained from Eastman Organic Chemicals (Catalog No. 1483). Chromosorb P, 60-80 mesh, the column substrate was supplied by Johns-Manville Products Corp., Celite Division. Natural
Clockwise Position
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Figure 3. Gas chromatogram of pyrolysis products from a mixture of 4 0 z natural rubber, 30z styrene-butadiene rubber, and 30 ethylene-propylene-terpolymer rubber
Figure 1. Stripping and backflushing valve connections M.C. Main column, 40 ft X 3/1~-in.10 % tricresylphosphate on 6080 mesh ChromosorbP S Stripping column 10 ft X 3 / ~ ~ - i10% n . tricresylphosphate on 60-80 mesh Chromosorb P R Restrictor column 10 ft X 3/16-in.10 tricresylphosphate on 60-80 mesh Chromosorb P
Pyrolysis Temperature PO
Figure 4. Effect of pyrolysis temperature on isoprene peak area
Minutes
Figure 2.
Gas chromatography of pyrolysis products
+
1, Methane; 2, Ethane Ethene; 3, Propane; 4, Propene; 5, 2-Methylpropane; 6, Propadiene f Butane; 7, 1-Butene 2Methylpropene; 8, truns-2-Butene; 9, cir-2-Butene; 10, 1,3-Butadiene; 11, 3-Methyl-1-butene; 12, 1-Pentene; 13, 2-Methyl-lbutene; 14, trans-2-Pentene; 15, cis-2-Pentene; 16, 2-Methyl-2butene; 17, Isoprene
+
rubber, standard styrene-butadiene rubber (23.5 styrene, 76.5 butadiene, Goodyear's Plioflex 1006) and ethylenepropylene-terpolymer rubber (Royalene 301 T from Uniroyal) were used to prepare various binary and ternary standard mixtures. These were then compounded and cured using standard rubber compounding recipes and operational techniques. The ternary blends all contained 3 0 x styrenebutadiene rubber while the percentages of ethylene-propyleneterpolymer rubber and natural rubber were varied from 10 to 50%. Procedure. The cured rubber samples were subdivided into small pieces with a pair of scissors. A 0.1- to 1-gram sample of these pieces was extracted with methanol in Underwriters' extraction apparatus for 8-12 hours. The solvent was completely removed from the rubber by placing the sample in a vacuum oven at 60 "C for 30 minutes. A 500to 800-pg sample of the dried rubber was weighed on the microbalance and carefully placed inside the quartz pyrolysis tube. Small borosilicate glass wool plugs were put on each
z
end of the U-tube. This tube was then installed in the pyrolyzer unit, purged with a helium flow of 30 ml per minute for two minutes, and then heated for 30 seconds at 700 "C by positioning the furnace over the tube. At the end of 30 seconds, the furnace was moved away; the pyrolysis tube was connected in series with the column carrier flow using the appropriate valve on the pyrolysis unit; and the start of the chromatogram was marked on the recorder chart. At the end of about 60 seconds, the carrier gas flow through the pyrolysis tube was terminated by sliding up the valve on the pyrolysis unit, The tube was removed from the pyrolysis unit, glass wool plugs were removed and the tube cleaned by heating it thoroughly in a burner flame to burn off all combustible material. Another weighed sample was then placed in the tube and it was installed in the pyrolysis unit as before ready for the next run. After 15 minutes, the flow of the carrier gas through the stripper column was terminated and the restrictor column was placed in series with the main column by turning the Carle valve knob to the counter clockwise position (Figure 1). The stripper was backflushed while the chromatogram was being recorded. At the end of the chromatographic run, the Carle valve knob was turned back to the clockwise position and the apparatus was ready to accept the next run. RESULTS AND DISCUSSION Peak Identification. The column arrangement and the operating conditions were specifically designed to give a suitable separation of most of the peaks of interest through ANALYTICAL CHEM1STRY;VOL.
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Figure 7. Estimation of natural rubber by 2-methyl-2-butene peak area
Figure 5. Effect of pyrolysis time on isoprene peak area
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Figure 8. Estimation of styrene-butadiene rubber by 1,3butadiene peak area
Figure 6. Estimation of natural rubber by isoprene peak area isoprene. Compounds eluting after isoprene were deliberately eliminated by the column switching arrangement. The relative retention data given in Table I were obtained on the tricresylphosphate column by injecting known materials. The column dead volume was determined from the elution time of air when the column was operated in a gas chromatograph equipped with a thermal conductivity detector. The chromatogram obtained with a blend of known compounds is shown in Figure 2. While the pyrolysis products from the three rubbers studied did correspond to these compounds in relative retention (Figure 3), an exhaustive study to eliminate or identify other possible products with relative retentions identical to those of the compounds listed, was not undertaken. Sample Size. The upper limit on the sample size was governed by the heating capacity of the furnace, rate of heat transfer, and desirability of relatively "instantaneous" pyrolysis. The lower limit was dependent on sample homogeneity and weighing accuracy. Experimentally a sample size of 500 to 800 pg was found to be most suitable. Pyrolysis Temperature. Using the 500- to 800-pg samples, pyrolyses were conducted at 500, 600, 700, 800, and 900 "C (Figure 4). Pyrolysis at the two lower temperatures, 500 and 600 "C failed to produce enough material in the CI to CS range while pyrolysis at the two higher temperatures, 800 and 900 "C, severely decreased the quantitative duplicability of the products. It was found that 700 "C was the optimum 496
ANALYTICAL CHEMISTRY, VOL. 44, NO. 3, M A R C H 1972
Table I. Peak Identification and Relative Retention Data Tricresylphosphate Column at 35 "C Peak No. Relative retention, in Figures nonane = 1.oo00 Compound 2 and 3 o.ooo9 Methane 1 0.0082 Ethane Ethene 2 0.0268 Propane 3 0,0347 Propene 4 0.0849 2-Methylpropane 5 0.0948 Propadiene Butane 6 0.1048 I-Butene 2-Methylpropene I 0.1409 trans-2-Butene 8 0.1649 cis-2-Butene 9 0.1769 10 1,3-Butadiene 0.2074 11 3-Methyl-1-butene 0.3038 1-Pentene 12 0.3367 2-Methyl-1-butene 13 0.3848 trans-2-Pentene 14 0,3993 15 cis-2-Pentene 0.4476 2-Methyl-2-butene 16 0.5391 11 Isoprene
+
+
+
temperature for pyrolysis under the experimental conditions employed. Pyrolysis Interval. The peak areas increased with the time allowed for pyrolysis up to 20 seconds. There was no significant change when the time was increased from 20 to 50 seconds (Figure 5 ) . On the basis of these data, 30 seconds was chosen as the optimum time for pyrolysis.
0.6 I
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Figure 9. Estimation of ethylene-propylene-terpolymer rubber by 1-pentene peak area
Peak Selection. After a complete series of known mixtures had been pyrolyzed and the products examined by gas chromatography, a critical evaluation of all 17 peaks (Figure 3) was conducted in order to detect those suitable for quantitative determination. The isoprene peak area was shown to be linear with natural rubber concentration (Figure 6) as had been found by many other workers previously (3-7). However small amounts of isoprene were produced by styrene-butadiene rubber and ethylene-propylene-terpolymer rubber as well. Although these small amounts did not constitute a serious interference, it was discovered that smaller amounts of natural rubber could be determined more conveniently from the 2-methyl-2-butene peak area (Figure 7). Deviations from linearity were noted at the higher concentrations of natural rubber. Styrene-butadiene rubber was determined from the peak area of the 1,3-butadiene peak (Figure 8). This determination was satisfactory when the butadiene fraction of the styrene-butadiene rubber in the sample being analyzed was known. Since the chromatographic conditions used did not allow the determination of styrene, it was not possible to determine the styrene to butadiene ratio. The long straight chain structure in ethylene-propyleneterpolymer rubber was found to be suitable for the production of straight chain alkenes. The important
pyrolysis products were ethene, propene, 1-butene, and 1pentene. Ethene, propene, and 1-butene were also produced in large amounts from the other two rubbers and were unsuitable for the determination of ethylene-propyleneterpolymer rubber. However 1-pentene was appropriate for this purpose (Figure 9). Interference from other rubbers was negligible. Compounding Ingredients. The technique of using peak area per microgram helped in giving the absolute amounts of individual rubbers in the compounded cured stocks. This method was found to be equally useful when uncured and uncompounded stocks were used if calibration curves were prepared from identical mixtures. This observation has been reported earlier for natural rubber, and styrene-butadiene rubber (6). Precision. Replicate measurement using known blends produced relative standard deviations of 3.2, 3.3, and 10.27& respectively, for styrene-butadiene rubber, natural rubber, and ethylene-propylene-terpolymer rubber. Interferences. While no serious mutual interferences were observed in the analysis of the three rubbers, the presence of other components like neoprene, polybutadiene, chlorobutyl, and butyl rubbers interfered with the quantitative estimation. Limitations. The standard mixtures used in this study were prepared from standard styrene-butadiene rubber, natural rubber, and ethylene-propylene-terpolymer rubber, Royalene 301T. While structural variations in the standard styrene-butadiene rubber did not affect the results, a styrenebutadiene rubber with a different styrene to butadiene ratio will require the use of a different standard curve. Similarly any ethylene-propylene-terpolymer rubber other than Royalene 301T cannot be determined without standard curves prepared specifically from known mixtures containing that particular polymer. RECEIVED for review August 9, 1971. Accepted November 4, 1971. Presented at the 99th meeting of the Division of Rubber Chemistry, American Chemical Society, Miami Beach, Fla., April 27, 1971. Permission by The Goodyear Tire and Rubber Company to publish is gratefully acknowledged.
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