LITERATURE CITED (1) E. L. Kargen, L. R . Snyder, and Csaba Howath, "An Introduction to Separation Science", John Wiley and Sons, New York, N.Y., 1973. (2) W. A. Dark and R . J. Limpert, J. Chromatogr. Sci., 11, 114 (1973). (3) J. H. Knox, Chem. Ind., (London),29 (1975). (4) J. J. DeStefano and J. J. Kirkland, Anal. Chem., 47, 1193A (1975). (5) R . M. Baldwin, J. 0. Golden, J. H. Gary, R. L. Bain, and R . J. Long, Chem. Eng. Progr., 71, 70 (1975). (6) E. L. Huffman, 1975 Frontiers of Power Technology Conference, Stillwater, Okla., October 1975. (7) K. H. Altgelt and J. C. Moore, "Polymer Fractionation", Manfred J. K. Cantow. Ed., Academic Press, New York, N.Y., 1967. (8) W. A. Dark, Am. Lab., 50 (1975). (9) K. D. Bartle, T. G. Martin, and D. F. Williams, Fuel, 54, 226 (1975). (10) Y. Maekawa, S. Veda, Y. Hasegawa, Y. Nakata, S. Yokoyama, and Y. Yoshida, Division of Fuel Chemistry, 170th National Meeting, American Chemical Society, Chicago, Ill., August 1975. (1 1j J. Y. Sun and E. H. Burk, Workshopon the Fundamental Organic Chemistry of Coal, July 1975, Knoxville, Tenn. (12) K. H. Altgelt and T. H. Gouw, Adv. Chromatogr., 13, 71 (1975). (13) K. H. Altgelt, Macromol. Chem., 88 75 (1965). (14) K. H. Altgelt, J. Appl. Polym. Sci., 9, 3389 (1965). (15) K. H. Altgelt, "Gel Permeation Chromatography", K. H. Altgelt and L. Segal, Ed., Marcel Dekker, New York, N.Y., 1971, pp 193, 623.
(16) W. M. Coleman, D. L. Wooton, H. C. Dorn, and L. T. Taylor, J. Chromatogr., 123, 419 (1976). (17) D. L. Wooton, W. M. Coleman, L. T. Taylor, and H. C. Dorn, Fuel, 55, 224 (1976). (18) J. C. Moore, "Gel Permeation Chromatography", K. H. Altgeit and L. Segal, Ed., Marcel Dekker Inc., New York, 1971, pp 159, 623. (19) H. H. Oelert and J. H. Webber, ErdblKohle, 23, 484 (1974). (20) H. Reerink and J. Lijzenga, Anal. Chem., 47, 216 (1975). (21) H. W. Sternburg, R . Raymond, and F. K. Schweighardt, Science, 188, 49 (1975). (22) G. A. Haley, Anal. Chem., 43, 371 (1971). (23) R . Epton, Aldrichim. Acta, 9, 23 (1976). (24) E. J. Mair, P. T. R. Huang, and R. G. Ruberto, Anal. Chem., 39, 838 (1967). (25) P. C. Talarico, E. W. Albaugh, and R. E. Snyder, Anal. Chem., 40, 2192 (1968). (26) D. L. Wooton, W. C. Coleman, L. T. Taylor, and H. C. Dorn, submitted for publication in Fuel.
RECEIVEDfor review October 11,1976.Accepted December 21, 1976. The financial support of the Virginia Polytechnic Institute and State University Research Division and the Commonwealth of Virginia is gratefully appreciated.
Pyrolysis Gas Chromatography Analysis of Rubbers and Other High Polymers John Chih-An Hu Quality Assurance Laboratories, Boeing Aerospace Company, Seattle, Wash. 98 724
A simple, reproducible, rapid, and inexpensive method for quality assurance testing of rubbers and other high polymers is developed. It is a new approach in pyrolysis gas chromatography by whlch the components of a compounded rubber sample are divided in the injection port into three groups: volatlle, polymeric, and inorganic, each of which Is analyzed sequentially. A chromatogram which characterizesthe volatile additives is followed by a pyrogram which Identifies the polymers. The inorganic group is isolated in pyrolysis probe for subsequent analysis. Conventional analyses of additives and polymers were done in multistep, off-line processes with large sample size and extensive sample preparations. This new method does the same analyses in a one-step, on-line process with minute sample size without sample preparation. it is suitable for controlling batch uniformity and monitoring formulation changes.
Since 1954 considerable studies on pyrolysis gas chromatography (PGC) have been done ( I ) and a number of review articles on this subject have been published (2-9). Despite the efforts during the last two decades, PGC has not, as yet, reached maturity ( I O ) . The impetus to develop this technique seems to be retarded by the problems of interlaboratory reproducibility and standardization (11). Several approaches were taken in an effort to overcome these difficulties, such as the use of the Curie point pyrolyzer (12, 13), sealed sample holders ( I 4 ) ,and multicolumn systems (12).Each of these approaches did improve the performance in a certain area, but the overall problem has not been completely resolved. This necessitated exploration of new approaches. As a result of this investigation, a new approach has been found promising. This report describes some preliminary results obtained from this new approach. The primary goal of this investigation was to find analytical methods for testing commercial compounded polymeric materials such as rubbers and other high polymers to meet the standards required by Aerospace Quality Assur-
ance provisions. For this purpose the analytical methods must be 1) reproducible, 2) rapid, 3) simple, and 4) inexpensive. Among the available instrumental methods, PGC appears to be the most promising in meeting these criteria if the reproducibility can be further improved while the overall analytical process can be further simplified. The variations of PGC data were attributed to several factors such as instrument design, pyrolyzer geometry, and temperature rise time (15-17). However, there are still other considerations which deserve special attention. When the sample is a pure polymer or a compounded polymeric material with volatile constituents removed by extraction ( I 2 ) ,reproducibility is relatively easy to achieve. But complications usually arise when a compounded polymeric material in its original form is analyzed ( I , 13, 18, 19). I t is worthwhile to point out that the pyrolysis chromatogram of a compounded polymeric material in its original form is not a simple pyrogram, but rather a compound one. This compound pyrogram results from the inconsistent superposition of a number of component chromatograms. During pyrolysis both vaporization and thermal degradation occur a t the same time (20).If the injection port temperature approaches the boiling points of some of the volatile constituents, they will be vaporized as soon as the sample is introduced and develop a chromatogram. The remaining volatile constituents will be vaporized by the initial heating of the pyrolysis probe heater when the power of the probe heater is applied. These vapors will develop a different chromatogram. Part of the vapors may undergo vapor phase pyrolysis to produce a pyrogram. Finally, the polymer is pyrolyzed to develop a different pyrogram. All of the chromatograms and pyrograms, except the pyrogram of the polymer itself, are subjected to variations depending on injection port temperature and post injection waiting period (the time interval between sample insertion and pyrolysis). It is not surprising that variable results are obtained from the superposition of variable components in an inconsistent manner. A logical solution would be to eliminate the variable components ANALYTICAL CHEMISTRY, VOL. 49,
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Figure 1. New silicone cavity seal (A) vs. used one (B) Column 10% SE-30. '/*-in. X 6-ft. S. Steel. Pyrolysis with 10 A for 15 s. Attenuation: left X1; right X2 J
before pyrolysis. This was done by solvent extraction in the literature (12).But extraction, which is time consuming and requires large sample size, does not meet the quality assurance criteria of simplicity, rapidity, and inexpensiveness. To meet these criteria and to remedy the reproducibility problem, this new approach is developed. In this new approach the chemical components of a compounded polymeric material such as the vulcanized rubber are divided into three distinctive groups: 1)volatile components, 2) polymers, and 3) inorganic components. This division is accomplished within the injection port of gas chromatograph in a single sample introduction step. Each of these three groups is analyzed separately and sequentially. This is a one-step/two-shot process. The first shot drives out the volatile components and the second shot drives out the polymers. The remains are the inorganic components. The injection port is preheated to and maintained at 270 O C . As soon as the pyrolysis probe which has been loaded with sample is inserted into the hot injection port, the volatile components are removed from the rest of the sample by vaporization and immediately enter the column as a slug. The sample insertion is considered as the first shot and this first shot produces a characteristic, reproducible chromatogram which is a fingerprint of the chemical composition of the first group of components of the sample. Chromatograms of the same material from repeated runs are essentially identical. The chromatogram has been found to be the same as that obtained from the liquid mixture which was extracted from the same material with a solvent. For quality assurance purposes it is not necessary to identify the chemical structure of each peak on the chromatogram, but merely recognize the chromatogram pattern obtained. Material identification is done empirically by comparing the unknown fingerprint with a standard. The methods for chemical structure determination of each peak on the chromatogram are available such as gas chromatography by means of retention parameters (21), gas chromatography-mass spectrometry, or vapor phase pyrolysis gas chromatography (22). After the chromatogram has been completely developed, the second shot is fired simply by pushing the pyrolysis button. The second shot produces a reproducible, characteristic pyrogram. This pyrogram is a 538
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Figure 2. Good batch (A) vs. bad batch (B) of MIL-1-7444 Column 5 % SE-30, %-in.X 6-ft. S. Steel. Pyrolysis with 14 A for 10 s. Attenuation: left X16; right X4
fingerprint of the chemical composition of the pyrolysates which are derived specifically from the polymers. By comparing the unknown pyrogram with a standard, the polymer can be quickly identified. After the pyrogram has been completely developed, the pyrolysis probe which holds the sample remains is withdrawn from injection port. A positive conclusion can be drawn without further analysis of the third group of components of the sample. The inorganic components are still available for complete analysis if desired.
EXPERIMENTAL Apparatus. A FM 810 gas chromatograph and FM 80 pyrolysis unit (both manufactured by Hewlett-Packard) were used for this work. Platinum spiral coil pyrolysis probe was used for the sliver sample, For the micro sample, powder sample, or conductive sample, a glass capillary tube served as a sample holder. The dimensions of the spiral coil are approximately 8 mm long, 3-mm o.d., 2-mm i.d. and the coil will accommodate the glass capillary tube. For odd shaped samples, a platinum ribbon pyrolysis probe was used. The sample size is not limited by the detectability of the apparatus but by the toughness of a sample to be cut and the ability to place a tiny sample firmly into the probe. The convenient range of sample weight is between 2 and 8 mg. The pyrolysis unit did not have a temperature monitoring system. Only the current reading (amperage) was monitored as a n indication of temperature. The temperature of ribbon probes was calibrated by the manufacturer. It is known that the resistance of platinum coil will change with age due to dissolution of carbon in platinum. Jennings and Dimick (23)reported that heater temperature decreased as the coil was aged in the presence of phosphoric acid. However, this temperature drop was not apparent in this work. At least, the temperature change was so small that it did not affect the pyrogram pattern. There was a small drop in amperage (approximately 2-3 A) during a fraction of a second when the current was first turned on, but after that moment the current reading was stable throughout the pyrolysis. SE-30 columns were used. The variation in column size and percentage loading of the liquid phase did not alter the pyrogram pattern, but did change retention time to a certain degree. The column tem-
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Figure 4. Identification of butyl rubber (A) A M s 3237. (6)Polysar Butyl 101. Column 10% SE-30, '/&in. X 6 4 . S.Steel. Pyrolysis with 12 A for 15 s. Attenuation: (A) X2, left (6)X1, right (6)X4
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Figure 3. Identification of nitrile rubber (A) MIL-R-2765. (6)MIL-R-6855, Class 1, Grade 60. (C) Paracril C. Column 10% SE-30, 'h-in. X 6-ft. S.Steel. Pyrolysis with 14 A for 5 s. Attenuation: (A) and (6)X16, left (C) X1, right (C) X4
perature was programmed. Helium was used as the carrier gas and flow rates were 30 mL per minute for a 0.318-cm ('h-inch) column and 60 mL per minute for a 0.635-cm (Yd-inch)column. A flame ionization detector (FID) was used (range: 10'). The selection of FID was not only due to its high sensitivity t o organic molecules but also due to its insensitivity to air, water, and carbon dioxide. This insensitivity was utilized to mask the disturbance of the steady gas flow a t the beginning of each run. The injection port temperature was 270 " C . Materials. Polymeric materials of four different types were analyzed. All these materials were commercial products which were purchased for Aerospace applications and subjected to Receiving and Inspection tests in the Quality Assurance Laboratories. The two silicone rubber seals (one new part and one used part) were made by the same manufacturer. T h e plasticized poly(viny1 chloride) tubings, which were used as electrical insulation sleevings, were purchased according to government specification MIL-1-7444. The two nitrile rubbers were supplied by two different manufacturers and procured according to two different specifications: MIL-R-2765 and MIL-R6855. The butyl rubber was purchased according to AMS 3237 (Aerospace Materials Specification published by the Society of Automotive Engineers, Inc.). Paracril C was from Uniroyal and Butyl 101 was from Polysar, Inc. Procedure. A thin sliver of sample (for spiral probe) or a flat sample (for ribbon probe) was cut and loaded into the pyrolysis probe. The loaded probe was quickly inserted into the preheated injection port as soon as the septum retainer was removed. The time when the injection port was re-closed was noted as the zero time of the chromatogram. T h e column temperature was programmed from 100 to 300 "C a t 20 " C per minute and was then held a t 300 "C until the chromatogram was completely developed. After the completion of the chromatogram of the volatile components, the column was cooled to 30 "C and the nonvolatile polymers were pyrolyzed a t a pre-determined amperage and duration. The time when the pyrolysis button was pushed was noted as the zero time of the pyrogram. The column temperature was programmed from 30 t o 300 "C a t 20 "C per minute and was then held a t 300 "C until the pyrogram was completely developed. The column was then cooled to 100 " C . The pyrolysis probe which held the sample residue was withdrawn from the injection port and the sample residue was collected for subsequent analysis. T h e
probe was cleaned simply by pushing the pyrolysis button for a second while it was held in the air.
RESULTS The results are illustrated in four figures. The curves on the left side of each figure are the chromatograms of the volatile components, which resulted from the first shot. The right side curves are the pyrograms of high polymers, which resulted from the second shot. The Y-axis of these figures represents recorder response. Figure 1 shows the comparison of a new silicone cavity seal and a used one which had cracks in it. The two chromatograms a t the left are completely different from each other, while the two pyrograms at the right are essentially identical. These results indicate that the volatile components of the cavity seal were lost during the service life while the polymer components remained unchanged. Figure 2 illustrates the comparison of two different batches of a flexible electrical insulation sleeving material (MIL-1-7444) which were manufactured by the same supplier according to the same formulation. Batch A met the specification requirements but Batch B failed the flammability test. A comparison of the curves in Figure 2 reveals that the high polymers are the same in both batches but the volatile components are different. Furthermore, the differences are not due to the incorporation of new components, but merely due to the different distribution of the same components. The similarity of pyrograms has eliminated the doubt that a different polymer might be accidentally used in Batch B. This clearly shows that during the manufacturing of Batch B either the degree of mixing was not sufficient to produce a homogeneous product or weighing errors were made. Figures 3 and 4 demonstrate how the polymer systems can be quickly identified. Figure 3 shows the comparison of two different commercial rubbers (A and B) and a standard nitrile gum (C). Note that the chromatograms a t the left side of Figure 3 are different from one another, but the pyrograms of the three at the right side are the same. Since material C is the known nitrile gum, Paracril C, the polymer system used in rubber A and rubber B is identified as a nitrile rubber. In a similar manner a commercial rubber which met the requirements of AMS 3237 was analyzed and, by using a known butyl rubber gum, Polysar Butyl 101,it was identified as a butyl rubber as shown in Figure 4. ANALYTICAL CHEMISTRY, VOL. 49, NO. 4, APRIL 1977
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DISCUSSION In general gas chromatography practice, the injection port temperature is usually kept below 300 "C for a longer rubber septum life. In this investigation, a similar limitation is imposed because of the thermal stabilities of polymeric samples and rubber gaskets in the pyrolysis probe assembly. A lower temperature is also desirable because of the frequent removal of septum retainer for sample introduction. The polymers used in most rubber compounding are stable below 300 "C. The thermal stability of polymers can be easily determined by TGA (Thermal Gravimetric Analysis), which is available in this laboratory and also widely available in many other laboratories. Our TGA data of silicones, neoprene, nitrile, butyl, poly(viny1 chloride), and ethylene-propylene-terpolymer showed their stability below 300 "C. Deur-Siftar and Mitrovic reported that poly(viny1 chloride) underwent very slow reactions at 190 "C during a period of 60 minutes to yield benzene (24). In Figure 2, benzene, which should be eluted within the first minute, was not shown in the chromatogram. The small sample size and short time made this possible slow reaction negligible. On the other hand, higher temperature is desirable for the removal of the volatile components from the sample. A literature search revealed that high-boiling additives could be eluted a t a substantially lower temperature. For example, heavy esters boiling up to 400 "C under atmospheric pressure could be eluted a t 200-240 "C (20). The fluoroesters of camphoric acid with a molecular weight of 1228, bp 460 "C, were eluted a t 325 "C within 14 minutes (25).Ionox 330 (a phenol type antioxidant, C54H7803) was eluted within 5 minutes after reaching 300 "C (26).Figures 3 and 4 show that the pyrogram patterns of the compounded materials (A and B in Figure 3; A in Figure 4) are the same as that of the gum polymers (C in Figure 3; B in Figure 4). These are evidences that the volatile additives were removed completely a t 270 "C. Because, otherwise, the remaining high-boiling additives, if any, would contribute different peaks and change the pyrogram patterns of the compounded materials. For the present purpose, 270 "C for the injection port has been satisfactory for the types of materials that were analyzed. However, the injection port temperature may have to be lowered in special cases where polymers are not stable a t 270 "C. No attempt was made to establish the relationship between injection port temperature and completeness of removal of the volatile additives in all compounded polymeric materials. The pyrolysis amperage and duration have been optimized a t 12 amperes (approximately 1000 "C) and 15 seconds. Figures 1to 3 are preliminary results and could be done under the optimum conditions. Variable pyrolysis conditions are not necessary to obtain good results for different samples. The pyrogram patterns of the four polymer types as shown in the four figures are quite different and each has its own specificity. In recognizing specific patterns, it is not necessary that the peak heights of one pyrogram are exactly the same as the corresponding ones of another. Peak heights are affected by sample size and column efficiency. Minor peak height variations are tolerable and they do not seriously interfere with pattern recognition. For example, in the pyrogram of Figure 1B the minor peak immediately after the first major peak is lower, while the other minor peaks are higher than the corresponding ones in 1A. But essentially both A and B have the same pattern. Similarly, there are also slight differences in the peak heights in the other three figures. The differences of pyrograms between different polymer types are so distinctive that one can identify the polymer type with little difficulty even though such minor peak height variations exist.
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In the literature, the analyses of volatile additives and of polymers were done in separate processes. These off-line processes were time-consuming and require large sample size. This new approach provides a way to integrate these two processes into one on-line process. In this single on-line process, no component of the original sample is lost and no foreign materials such as solvents and contaminants are added. The results represent complete information from the whole sample. This new approach is particularly suitable for quality assurance in the manufacturing of compounded materials such as rubbers, flexible plastics, and sealants. The performance of these materials depends not only on polymers but also on the proper proportions of plasticizers and the exact dosage of the minor additives. The concentration of the minor additives is critical to the properties of these products. The variations of additives in different batches are demonstrated in Figure 2 and they do alter the properties of the products. The batch uniformity can be easily checked by this new method without interruption of the manufacturing process. This new method is also useful for the purchaser quality assurance in monitoring formulation changes. Despite the provision of prohibiting formulation change in most materials specifications, occasionally the formulations of the compounded materials are changed without notifying the purchaser. This commonly occurs because there are no detection methods provided in the specifications and very often the changed material still can meet the specification requirements of mechanical properties. In Figure 3, A and B are compounded from the same polymer with different formulations by different manufacturers. The chromatograms of A and B on the left side of Figure 3 are distinctively different. Each represents the specific formulation.
ACKNOWLEDGMENT Thanks are due to R. H. LeDoux, Jr., and A. B. Hunter for their encouragement. LITERATURE CITED (1) W. H. T. Davison, S. Slaney, and A. L. Wragg, Chem. lnd. (London), 1954, 1356. (2)S.G. Perry, J. Gas Chromatogr., 2,54 (1964). (3) R. W. McKinney, J. Gas Chromatogr., 2, 432 (1964). (4)G. M. Brauer, J. Polymer Sci., Part C, 8,3 (1965). (5) R. L. Levy, Chromatogr. Rev., 8, 48 (1966). (6) R. Audebert, Ann. Chim. (Paris), 3, 49 (1968). (7)E. Cianetti and G. Pecci, lnd. Gomma (Milan), 13, 47 (1969). (8) S.P. Cram and R. S. Juvet, Jr., Anal. Chem., 48, IOlR (1974). (9)C. W. Wadelin and M. C. Morris, Anal. Chem., 47, 327R (1975). (IO) C. E. R. Jones, "Gas Chromatography, 1968",C. L. A. Harbourn, Ed., Elsevier, Amsterdam, 1969,406 pp. (11) R. S.Levy (Panel Chairman), L. S. Ettre, W. E. Harris, C. E. R. Jones, R. S. Juvet, S. G. Perry, M. V. Stack, A. 0. Wist, C. J. Wolf, and N. B. Coupe, "Standardization of Pyrolysis Gas Chromatography Techniques" in "Gas Chromatography, 1968", C. L. A. Harbourn, Ed., Elsevier, Amsterdam, 1969,pp 410-416. (12)A. Krishen and R. G. Tucker, Anal. Chem., 46, 29 (1974). (13)G. L. Coulter and W. C. Thompson, Column, 3, 6 (1970). (14)C. Karr, Jr., J. R. Comberiati, and W. C. Warner, Anal. Chem., 35, 1441 (1963). (15) R. S. Juvet, Jr., and L. P. Turner, Anal. Chem., 37, 1464 (1965). (16) R. S.Juvet, Jr., R. L. Tanner, and J. C. Y. Tsao, J. Gas Chromatogr., 5, 15 (1967). (17)R. S.Juvet, Jr., J. L. S. Smith, and K. P. Li, Anal. Chem., 44, 49 (1972). (18) J. Janak, Nature (London), 185, 684 (1960). (19)B. B. Coldwell and M. Smith, J. Forensic Sci., 11 (l),28 (1966). (20)J. Zulaica and G. Guiochon, Anal. Chem., 35, 1724 (1963). (21)G. Schomburgand G. Dielmann, J. Chromatogr. Sci., 11, 151 (1973). (22)J. V. Derby and R . W. Freedman, Am. Lab., May 1974,pp 10-16. (23)E. C. Jennings, Jr., and K. P. Dimick, Anal. Chem., 34, 1543 (1962). (24)D. Deur-Siftar and V. Mitrovic, Chromatographia, 5, 573 (1972). (25) H. R . Felton, "A Novel High Temperature Gas Chromatography Unit" in "Gas Chromatography, 1957", V. J. Coates, H. J. Noebels, and I. S. Fagerson, Ed., Academic Press, New York, N.Y., 1958,Chapter 15,p 131. (26)H. S.Knight and H. Siegel, Anal. Chem., 38, 1221 (1966).
RECEIVEDfor review October 12,1976. Accepted December 10, 1976.
