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The use of pyrolysis gas-liquid chromatography for the determination of submicrogram quantities of polymer
MICROCHEMICAL JOURNAL
9, 500-509 (1965)
The Use of Pyrolysis Gas-Liquid Chromatography for the Determination of Submicrogram Quantities of Polymer' 1\1.
DIMBAT AND
F. T.
EGGERTSEN
Shell Development Company, Emeryville, California Received May 19, 1965
INTRODUCTIO:\T
Polymers are not amenable to direct analysis by gas-liquid chromatography (GC) because of their low volatility. However, they can be characterized by analyzing their pyrolysis products. This was demonstrated even before the advent of gas-liquid chromatography by Zemany (11), who employed mass spectrometry to analyze the pyrolyzates, Harms (4), and by Bentley and Rappaport (1), who used infrared spectrometry. More recently, pyrolysis-mass spectrometric methods have been developed also by Bua and Maneressi (2). Gas-liquid chromatographic analysis of pyrolysis products was reported as early as 1954 by Davison et al, (3) The pyrolysis products were first trapped then analyzed by GC in a separate step. In 1959, Lehrle and Robb (8) combined the pyrolysis and GC steps by decomposing the sample in the inlet of the separating column. Since that time a large number of papers have appeared on the application of the Lehrle and Robb technique to various samples and its usefulness for polymer identification and potentialities for quantitative analysis are now well established. In order to fully realize those potentialities for sensitive, reproducible, and meaningful pyrolysis analysis, the proper equipment must be used in the best possible manner. Some of the advances made in instrumentation and technology in these laboratories will be discussed in this paper. I
Paper presented at the International Symposium on Microchemical Techniques
-1965, held at The Pennsylvania State University, University Park, Pennsylvania, U.S.A., August 22-27, 1965. 500
501
DETERMINATION OF POLYMER
Apparatus Figure 1 is a schematic diagram of the pyrolysis-GC instrument. Each segment of the apparatus will be discussed separately with major stress Pyrolyzer Pro~_.,-+--...,
Carrier Gas
I
~To
Trap
Program-Heated Oven
FIG. 1. Schematic diagram of pyrolysis-GC instrument.
being placed on those aspects that are most important in the quantitative analysis of polymers by pyrolysis. Columns and conditions. The selection of the column packing best suited for a particular analysis depends upon the nature of the polymer being pyrolyzed. Polymers such as polymethylmethacrylate which produce polar fragments will require a polar column liquid such as a polyglycol. Polyethylene and other purely hydrocarbon polymers that produce only olefins can best be analyzed on a less polar column like a methyl silicone oil. The double bonds produced may shift under the conditions of pyrolysis and cause 2 or 3 peaks for each basic structure. Hydrogenation of these fragments in the chromatographic column produces a single paraffin from all of the olefins with a similar carbon skeleton. This simplifies the pyrogram and makes quantitative analysis simpler and more accurate. Kolb and Kaiser (6) used a separate hydrogenation section always maintained at a high temperature. We have chosen to make the hydrogenation catalyst an integral part of the column packing. Platinum dioxide (3% by wt.) is physically mixed with the column packing just prior to filling the column. The platinum dioxide adheres to the stationary liquid phase to form a uniform coating and is reduced to platinum metal catalyst by the hydrogen carrier gas after the column is packed. The pyrolysis products then are hydrogenated in the column at the lowest possible temperature and chances for rearrangements and degradation are minimized. The presence of the platinum metal in the column does not appear to affect the efficiency of
502
],I .
DIM BAT AND F. T. EGGERTSEN
the column or the order of emergence of the components . The hydrogenation of olefinic bonds is complete in the column but aromatics appear to be untouched. The length of the column required depends on the type of sample being pyrolyzed and on the amount and kind of information required from the chromatogram. When extremely small samples are being analyzed it is important to elute the components of interest from the column in the shortest possible time to avoid spreading in the column and thus maximize peak to noise ratio. Very short columns operated at relatively high temperatures will accomplish this if the sample contains only one type of polymer. In case of mixtures, longer columns may be needed to assure separation of the characteristic peak of each polymer. For the determination of polystyrene in very dilute solutions, a l-It SE 30 silicone column operated at IOQoe gave a peak height of 38 mm for 0.1 flg of polymer, while a 20-ft column, also at 100°C , gave a peak height of only 8 mm . The areas under the two peaks were equal but the accuracy of measurement of the taller peak was much greater because of the greater signal to noise ratio . Pyrolysis produces fragments of widely varying boiling points. Therefore, in order to achieve adequate separation of the lower boiling components and still have the higher boiling components emerge in a reasonable time, it is essential that programmed temperature operation of the column be available. Rapidly stirred air baths are generally used for temperature programming because they can be heated and cooled quickly. Detectors. The hydrogen flame detector is generally the detector of choice because of its greater sensitivity, and because it is compatible with programmed temperature operation of the column. The response of the flame detector is dependent upon many factors which must be carefully controlled , if good repeatability is to be obtained. Sternberg et al, (JO) have dealt at length with these factors and therefore they will not be discussed in this paper. We have used detectors from several manufacturers as well as some developed in these laboratories with nearly equal success. For very small samples one may wish to use % -inch columns to reduce the lateral diffusion. The low flow rates used in these columns makes it important that the detector as well as the sample injector have small volumes. This is not the case with all commercial detector systems. It should be emphasized that frequent use of standard samples to check the detector response is good insurance for quantitative work.
503
DETERMINATION OJo' POLYMER
A disadvantage of the flame detector is that it does not respond to certain compounds, such as water and the oxides of carbon. When these components are to be determined, another detector, such as thermal conductivity, should be employed; however, with thermal conductivity the sample size must be increased to approximately 0.1 mg. Where this is not a limitation, good reproducibility can be obtained with thermal conductivity detectors. Dual column operation makes them suitable for temperature programmed columns as well as isothermal operation. Pyrolyzers. Two types of pyrolyzers are in common use. They are the electrically heated wire and the furnace and boat. The choice here is not clear cut. Each type has its proponents and there are advantages and disadvantages to each type that must be weighed. In a furnace, the temperature of pyrolysis can be accurately controlled. However, the pyrolysis products are in contact with the high temperature of the furnace where cyclization, dehydrogenation, and rearrangements can take place. In the pyrolysis of polyethylene the non-normal paraffins produced in the furnace pyrolyzer are three times as great as on a filament pyrolyzer at the same temperature. These non-normal products are largely due to cyclization and dehydrogenation, although some of them do arise from the branching that exists even in linear polyethylene. The products in the polyethylene are not particularly temperature labile. With polymers where one of the products is sensitive to heat, the effect of contact with the furnace wall is even more serious. The need for temperature control of the pyrolysis was recognized and the filament pyrolyzer shown in Fig. 2 was devised to answer that need. 6.2 mm Soda Glass
.orz" Plattnurn , z" In Length jSpot Weld Thermocouple t o Coil NICkel Sleeve
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- .005 11 Platinum
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Fro. 2. Filament pyrolyzer.
._- ---1
504
M. DIM BAT AND F . T . EGGERTSEN
This type of filament was first used by Hunter (5) in another application. The platinum filament has a resistance of 0.1 ohm and requires a current of about 3 amp to heat it to 600°C in nitrogen. The glass tube is mounted in standard fittings so that it can be attached easily to the sample inlet of a gas-liquid chromatograph. A %6 to Y.i -inch Swagelok reducer (Crawford Fitting Company , Cleveland , Ohio) is drilled to allow the glass tube to fit inside with the filament protruding. The glass is sealed into position in the reducer by use of a ~ -i nch Nylon nut with Teflon ferrules. Special Leeds and Northrup thermocouple hook-up wire is used on the platinum/ platinum-lO% rhodium thermocouple, copper-nickel alloy no. 11 to the platinum wire and copper to the platinum-rhodium. This special wire is required to reduce the extraneous potential produced at the lead junctions that may become heated during the pyrolysis. The platinum filament is very soft and can be easily bent during the insertion of samples. This may cause shorting between the loops of the filament and cause hot spots in the pyrolyzer. To prevent this, the filament is coated with glass by lightly sprinkling soft glass micro beads (170- to 200-mesh pavement marking beads) on the red-hot helix. This treatment not only strengthens the filament but makes the temperature more uniform across the helix and improves the reproducibility of the decomposition temperature. These glass coated filaments have a further advantage in that they hold solutions as well as finely divided powders much better than do the uncoated wires. Residual material from pyrolysis tends to become imbedded in the glass coating and should be removed after each pyrolysis by heating the filament in air at a bright-red heat for a few seconds . A filament blank pyrogram should be obtained occasionally to ensure that the filament has been fired sufficiently to clean it. Concern has been expressed that the material in contact with the sample during the pyrolysis will affect the pyrolysis pattern obtained. In thi s connection, we have found that the glass coated platinum filaments produce the same pyrolysis patterns as the uncoated wires when they are both employed at relatively low heating rates. However, large differences in the pyrolysis patterns can be produced in the same platinum wire by merely heating the wire at two different rates . These differences are believed to he caused by the difference in temperature at which the pyrolysis actually takes place at the two heating rates. By using the glass coated platinum helix, a study has been made of the temperature at which decomposition actually takes place with polystyrene as the sample. The results of these tests are shown in Table 1. It is noteworthy that at very slow heating
50S
DETERMINATION OF POLYMER
rates the polystyrene decomposes at 366°e, whereas at the very high rate of 300 0 e per second, the pyrolysis is not complete until the wire reaches 620 o e. With larger samples and with polymers that decompose more slowly, the wire temperature may reach 900 0 e before pyrolysis is complete at these TABLE 1 EFt'ECT OF HEATING RATE ON PYROLYSIS TEMPERATURE
(Sample 0.09 rng polystyrene added to the filament as 5 ul of 2% solution in benzene)
Heating rate
Decomposition temperature (OC)
1.6°C/min 3.6 5.6
366 384 390 393 393 426 450 620
7.1 13.4 19.8
22.2°C/sec 300·C/sec
high heating rates. At the low heating rates the decomposition is so slow that the peaks in the chromatogram are too broad to be useful. In order to produce narrow peaks it is necessary to heat the sample at the rate of at least 20 e per second. At this rate the pyrolysis takes place between 450 and 475°C. The rate of heating is not extremely important in polystyrene pyrolysis since the primary product is the monomer, and presumably the mechanism for degradation is a depolymerization, and the temperature at which the pyrolysis occurs does not seriously affect the product distribution. However, the mechanism for the thermal degradation of saturated polymers like polyethylene is affected by the temperature of pyrolysis. At high temperature the amount of beta scission of the primary free radical increases thereby increasing the amount of ethylene formed. At lower temperatures, there is less beta scission and the predominant reaction is the transfer of the free radical down the chain by backbiting to pull off a hydrogen atom and form a secondary free radical further down the chain, which then undergoes beta scission to form the higher molecular weight fragments (9). The pyrolysis pattern is thus changed by the rate at which the filament is heated. At very high heating rates the wire reaches a temperature at which dehydrogenation can occur and benzene and toluene are formed. At very high temperatures the primary products are toluene, benzene, and ethylene. This is not desirable if one wishes to study the structure of the original 0
506
M. DIM BAT AND F. T. EGGERTSEN
polymer, but if the primary purpose is to determine the amount of sample present, the high temperature pyrolysis may be preferable. Lack of proper temperature control is probably the cause of the nonreproducibility reported by other users of filament pyrolyzers. We have found the computor integration technique of Johnson and Stross (7) extremely valuable in the interpretation of complex pyrolysis patterns. A valve has been provided between the exit of the column and the detector to allow for trapping of peaks for mass spectrometric or infrared identification. The following analytical application has been selected to illustrate many of the principles discussed in the previous sections.
Thickness of a Polymer Coating on Paperboard The thickness of a polymer film, coated on paperboard as a waterproofing treatment, can be measured conveniently and precisely by pyrolysis-GC. For example, a method has been developed for determining the coating thickness of an ethylene-propylene copolymer by using a very small punched disc specimen of the paper. By use of such small specimens it is possible to test uniformity of the coating on the paper. Attempts to apply hot-wire pyrolysis directly to the coated paper were not successful because the pyrolysis products from the polymer are obscured by the relatively much larger amount of pyrolysis products from the paper. But the analysis can be readily done by an extraction technique in which the polymer is dissolved from the specimen with hot toluene. An aliquot of the solution is then evaporated on the filament and the residue is determined by pyrolysis-GC. In the procedure employed a punched disc of the coated paperboard, 6 mm in diameter, is placed in a 2-ml volumetric flask with toluene and heated at 100°C for one-half hour. The flask, still containing the paper disc, is cooled, filled to volume with additional toluene and mixed well. The pyrolysis filament is heated to 100°C, and 20 fil of the solution is injected into the warm filament in three portions. After each addition of solution the solvent is evaporated and the filament temperature allowed to rise again to 100°. Condensation of solvent vapors in the body of the pyrolysis filament unit is minimized by drawing them off through a funnel connected to house vacuum. Other volatiles present with the polymer can also be removed, if desired, by adjusting the filament temperature to a higher level.
507
DETERMINATION OF POLYMER
After the solvent has been evaporated , which requires only a few minutes , the filament unit is placed in the pyrolysis inlet, which is maintained at lZ0 D C. When a steady base line has been attained , requiring about 5 minutes, the sample is pyrolyzed rapidly at 900 D e (maximum temperature). The pyrograrn is obtained with a 15-ft silicone column at lOO DC, in which nit rogen is used as the carrier gas at 30 cc per minute. The hydrogen flame ionization detector is employed at a sensitivity setting to give a response for a Co hydrocarbon, which is employed as the test peak, of about 0.1 mv per microgram of polymer pyrolyzed. This method was developed before the technique of hydrogenation in the column was adopted. It is expected that greater sensitivity would be achieved by use of hydrogenation. Typical pyrograms for coated and uncoated paperboard are shown in Fig . 3. It is noted that the material extracted from the paper produced a 0 .4 ~--------------------,
E thylene I P ropylene Po lymer ~ o n Pape r, 0.25 mg/ cm' O.} f-
o.z >--
c, 0 .11mv
~ I
I
.....
I
I
I
0. 1 . . . - - - - - - -- -- - - - - - - - - -------, Paper Blank
T i me. mi nutes
FI G. 3 .
Pyrograms of eth ylene /propylene polymer extracted from coated paper.
508
M. DIM BAT AND F. T. EGGERTSEN
pattern, necessitating a small blank correction. A calibration curve, constructed from data for known amounts of polymer on paperboard, is shown in Fig. 4. The height of the Ca test peak varies linearly with polymer concentration, and thus is a direct measure of the amount of polymer on the
O.Z
>
e
O~
o
-.l..
.l...-
5 10 Concentration In Solvent, o;ow x 10)
....I
15
FIG. 4. Calibration curve for ethylene/propylene polymer coated on paper.
specimen. Concentrations down to about 0.00170 weight can be measured, which corresponds to only 0.2 f.lg of polymer in the 20 f.ll of solution. Precise temperature control during the pyrolysis, which is essential for good reproducibility, is achieved by means of the thermocouple on the filament. The thermocouple output during each pyrolysis was monitored routinely by using a strip chart recorder at a fast chart speed. Adjustments of the voltage to the filament were made as necessary to obtain a heating rate of 150°C per second measured at 400 to 500°C. With this technique the standard deviation for duplicate analyses was about 0.03 mg per square centimeter, over the range 0.2 to 1 mg of polymer per square centimeter of paper. In this method a relatively high heating rate was employed in order to effect rapid injection of the pyrolysis products into the GC column. This is desirable in order to obtain a sharp C6 test peak. CONCLUSIONS
Pyrolysis-GC analysis has become an indispensable analytical tool in these laboratories. The most important single requirement for good pyrolysis analysis is proper control of the pyrolysis conditions. The most satis-
DETERMINATION 01" POLYMER
509
factory means for control of these conditions is the glass coated platinum filament with a platinum/platinum-I oro rhodium thermocouple attached. It allows good control over a wide range of decomposition rates. Further automation of the wire heating rate and temperature should prove useful in obtaining even greater reproducibility. ACKNOWLEDGMENTS The authors are grateful to W. M. Sawyer for his ~uidance and counsel in the work with the polymer-coated paper; to E. Meyer and H. E. Melling for construction of the filament-thermocouple pyrolyzer; and to R. F. Tremoureux and Miss Edna Dean for skilled assistance in the experimental work. REFERENCES 1.
2.
3. 4. .'i.
6. 7. 8. 9. 10.
11.
BENTLEY, F. F., AND RAPPAPORT, G., Semiquantitative analysis of buna N phenolic blends by the infrared spectra of their pyrolyzates. Anal. Chern. 26, 1980-1985 (1954) . BUA, E., AND MANERESSI, P., Quantitative analysis of ethylene-propylene copolymers by the mass spectra of their pyrolyzates, Anal. Chern. 31, 2022-2024 (1959). DAVISON, W. H. T., SLANEY, S., AND WRAGG, A. L., A novel method of identification of polymers. Chern. Ind. (London) 1954, 1356. HARMS, D. L., Identification of complex organic materials. Anal. Chern. 25, 11401155 (1953) . HUNTER, L., AND FORBES, J. W., Structural investigation of polyacrolein by fractional dehydration. 148th National ACS Meeting, Chicago, III., September 1964, abstract 72. KOLB, B., AND KAISER, K. H., Hydrogenation as a technique of identification in pyrolysis gas chromatography. J. Gas Chromatog, 2, 233-234 (1964). JOHNSON, H. W., Storage and complete automatic computation of gas chromatographic data. Anal. Chern. 35, 521 (l96.n. LEIlRLE, R. S., AND ROBB, J. C., Direct examination of the degradation of high polymers by gas chromatography. Nature 183, 1671 (1959). SIMHA, R., WALL, L. A. AND BLATZ, P. J., "Depolymerization as a Chain Reaction," J. Polymer Sci. 5, 615-632 (1950). STERNBERG, J. C., GALLAWAY, W. S., AND JONES, D. T. L., The mechanism of response of flame ionization detectors. In "Gas Chromatography" (N. Brenner, ed.), pp. 231-267, Academic Press, New York, 1962. ZEMANY, P. D., Identification of complex organic materials by mass spectrometric analysis of their pyrolysis products. Anal. Chern. 24,1709-1713 (1952).
9, 500-509 (1965)
The Use of Pyrolysis Gas-Liquid Chromatography for the Determination of Submicrogram Quantities of Polymer' 1\1.
DIMBAT AND
F. T.
EGGERTSEN
Shell Development Company, Emeryville, California Received May 19, 1965
INTRODUCTIO:\T
Polymers are not amenable to direct analysis by gas-liquid chromatography (GC) because of their low volatility. However, they can be characterized by analyzing their pyrolysis products. This was demonstrated even before the advent of gas-liquid chromatography by Zemany (11), who employed mass spectrometry to analyze the pyrolyzates, Harms (4), and by Bentley and Rappaport (1), who used infrared spectrometry. More recently, pyrolysis-mass spectrometric methods have been developed also by Bua and Maneressi (2). Gas-liquid chromatographic analysis of pyrolysis products was reported as early as 1954 by Davison et al, (3) The pyrolysis products were first trapped then analyzed by GC in a separate step. In 1959, Lehrle and Robb (8) combined the pyrolysis and GC steps by decomposing the sample in the inlet of the separating column. Since that time a large number of papers have appeared on the application of the Lehrle and Robb technique to various samples and its usefulness for polymer identification and potentialities for quantitative analysis are now well established. In order to fully realize those potentialities for sensitive, reproducible, and meaningful pyrolysis analysis, the proper equipment must be used in the best possible manner. Some of the advances made in instrumentation and technology in these laboratories will be discussed in this paper. I
Paper presented at the International Symposium on Microchemical Techniques
-1965, held at The Pennsylvania State University, University Park, Pennsylvania, U.S.A., August 22-27, 1965. 500
501
DETERMINATION OF POLYMER
Apparatus Figure 1 is a schematic diagram of the pyrolysis-GC instrument. Each segment of the apparatus will be discussed separately with major stress Pyrolyzer Pro~_.,-+--...,
Carrier Gas
I
~To
Trap
Program-Heated Oven
FIG. 1. Schematic diagram of pyrolysis-GC instrument.
being placed on those aspects that are most important in the quantitative analysis of polymers by pyrolysis. Columns and conditions. The selection of the column packing best suited for a particular analysis depends upon the nature of the polymer being pyrolyzed. Polymers such as polymethylmethacrylate which produce polar fragments will require a polar column liquid such as a polyglycol. Polyethylene and other purely hydrocarbon polymers that produce only olefins can best be analyzed on a less polar column like a methyl silicone oil. The double bonds produced may shift under the conditions of pyrolysis and cause 2 or 3 peaks for each basic structure. Hydrogenation of these fragments in the chromatographic column produces a single paraffin from all of the olefins with a similar carbon skeleton. This simplifies the pyrogram and makes quantitative analysis simpler and more accurate. Kolb and Kaiser (6) used a separate hydrogenation section always maintained at a high temperature. We have chosen to make the hydrogenation catalyst an integral part of the column packing. Platinum dioxide (3% by wt.) is physically mixed with the column packing just prior to filling the column. The platinum dioxide adheres to the stationary liquid phase to form a uniform coating and is reduced to platinum metal catalyst by the hydrogen carrier gas after the column is packed. The pyrolysis products then are hydrogenated in the column at the lowest possible temperature and chances for rearrangements and degradation are minimized. The presence of the platinum metal in the column does not appear to affect the efficiency of
502
],I .
DIM BAT AND F. T. EGGERTSEN
the column or the order of emergence of the components . The hydrogenation of olefinic bonds is complete in the column but aromatics appear to be untouched. The length of the column required depends on the type of sample being pyrolyzed and on the amount and kind of information required from the chromatogram. When extremely small samples are being analyzed it is important to elute the components of interest from the column in the shortest possible time to avoid spreading in the column and thus maximize peak to noise ratio. Very short columns operated at relatively high temperatures will accomplish this if the sample contains only one type of polymer. In case of mixtures, longer columns may be needed to assure separation of the characteristic peak of each polymer. For the determination of polystyrene in very dilute solutions, a l-It SE 30 silicone column operated at IOQoe gave a peak height of 38 mm for 0.1 flg of polymer, while a 20-ft column, also at 100°C , gave a peak height of only 8 mm . The areas under the two peaks were equal but the accuracy of measurement of the taller peak was much greater because of the greater signal to noise ratio . Pyrolysis produces fragments of widely varying boiling points. Therefore, in order to achieve adequate separation of the lower boiling components and still have the higher boiling components emerge in a reasonable time, it is essential that programmed temperature operation of the column be available. Rapidly stirred air baths are generally used for temperature programming because they can be heated and cooled quickly. Detectors. The hydrogen flame detector is generally the detector of choice because of its greater sensitivity, and because it is compatible with programmed temperature operation of the column. The response of the flame detector is dependent upon many factors which must be carefully controlled , if good repeatability is to be obtained. Sternberg et al, (JO) have dealt at length with these factors and therefore they will not be discussed in this paper. We have used detectors from several manufacturers as well as some developed in these laboratories with nearly equal success. For very small samples one may wish to use % -inch columns to reduce the lateral diffusion. The low flow rates used in these columns makes it important that the detector as well as the sample injector have small volumes. This is not the case with all commercial detector systems. It should be emphasized that frequent use of standard samples to check the detector response is good insurance for quantitative work.
503
DETERMINATION OJo' POLYMER
A disadvantage of the flame detector is that it does not respond to certain compounds, such as water and the oxides of carbon. When these components are to be determined, another detector, such as thermal conductivity, should be employed; however, with thermal conductivity the sample size must be increased to approximately 0.1 mg. Where this is not a limitation, good reproducibility can be obtained with thermal conductivity detectors. Dual column operation makes them suitable for temperature programmed columns as well as isothermal operation. Pyrolyzers. Two types of pyrolyzers are in common use. They are the electrically heated wire and the furnace and boat. The choice here is not clear cut. Each type has its proponents and there are advantages and disadvantages to each type that must be weighed. In a furnace, the temperature of pyrolysis can be accurately controlled. However, the pyrolysis products are in contact with the high temperature of the furnace where cyclization, dehydrogenation, and rearrangements can take place. In the pyrolysis of polyethylene the non-normal paraffins produced in the furnace pyrolyzer are three times as great as on a filament pyrolyzer at the same temperature. These non-normal products are largely due to cyclization and dehydrogenation, although some of them do arise from the branching that exists even in linear polyethylene. The products in the polyethylene are not particularly temperature labile. With polymers where one of the products is sensitive to heat, the effect of contact with the furnace wall is even more serious. The need for temperature control of the pyrolysis was recognized and the filament pyrolyzer shown in Fig. 2 was devised to answer that need. 6.2 mm Soda Glass
.orz" Plattnurn , z" In Length jSpot Weld Thermocouple t o Coil NICkel Sleeve
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/
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.005" Plal1num with 10% Rhodium
- .005 11 Platinum
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Fro. 2. Filament pyrolyzer.
._- ---1
504
M. DIM BAT AND F . T . EGGERTSEN
This type of filament was first used by Hunter (5) in another application. The platinum filament has a resistance of 0.1 ohm and requires a current of about 3 amp to heat it to 600°C in nitrogen. The glass tube is mounted in standard fittings so that it can be attached easily to the sample inlet of a gas-liquid chromatograph. A %6 to Y.i -inch Swagelok reducer (Crawford Fitting Company , Cleveland , Ohio) is drilled to allow the glass tube to fit inside with the filament protruding. The glass is sealed into position in the reducer by use of a ~ -i nch Nylon nut with Teflon ferrules. Special Leeds and Northrup thermocouple hook-up wire is used on the platinum/ platinum-lO% rhodium thermocouple, copper-nickel alloy no. 11 to the platinum wire and copper to the platinum-rhodium. This special wire is required to reduce the extraneous potential produced at the lead junctions that may become heated during the pyrolysis. The platinum filament is very soft and can be easily bent during the insertion of samples. This may cause shorting between the loops of the filament and cause hot spots in the pyrolyzer. To prevent this, the filament is coated with glass by lightly sprinkling soft glass micro beads (170- to 200-mesh pavement marking beads) on the red-hot helix. This treatment not only strengthens the filament but makes the temperature more uniform across the helix and improves the reproducibility of the decomposition temperature. These glass coated filaments have a further advantage in that they hold solutions as well as finely divided powders much better than do the uncoated wires. Residual material from pyrolysis tends to become imbedded in the glass coating and should be removed after each pyrolysis by heating the filament in air at a bright-red heat for a few seconds . A filament blank pyrogram should be obtained occasionally to ensure that the filament has been fired sufficiently to clean it. Concern has been expressed that the material in contact with the sample during the pyrolysis will affect the pyrolysis pattern obtained. In thi s connection, we have found that the glass coated platinum filaments produce the same pyrolysis patterns as the uncoated wires when they are both employed at relatively low heating rates. However, large differences in the pyrolysis patterns can be produced in the same platinum wire by merely heating the wire at two different rates . These differences are believed to he caused by the difference in temperature at which the pyrolysis actually takes place at the two heating rates. By using the glass coated platinum helix, a study has been made of the temperature at which decomposition actually takes place with polystyrene as the sample. The results of these tests are shown in Table 1. It is noteworthy that at very slow heating
50S
DETERMINATION OF POLYMER
rates the polystyrene decomposes at 366°e, whereas at the very high rate of 300 0 e per second, the pyrolysis is not complete until the wire reaches 620 o e. With larger samples and with polymers that decompose more slowly, the wire temperature may reach 900 0 e before pyrolysis is complete at these TABLE 1 EFt'ECT OF HEATING RATE ON PYROLYSIS TEMPERATURE
(Sample 0.09 rng polystyrene added to the filament as 5 ul of 2% solution in benzene)
Heating rate
Decomposition temperature (OC)
1.6°C/min 3.6 5.6
366 384 390 393 393 426 450 620
7.1 13.4 19.8
22.2°C/sec 300·C/sec
high heating rates. At the low heating rates the decomposition is so slow that the peaks in the chromatogram are too broad to be useful. In order to produce narrow peaks it is necessary to heat the sample at the rate of at least 20 e per second. At this rate the pyrolysis takes place between 450 and 475°C. The rate of heating is not extremely important in polystyrene pyrolysis since the primary product is the monomer, and presumably the mechanism for degradation is a depolymerization, and the temperature at which the pyrolysis occurs does not seriously affect the product distribution. However, the mechanism for the thermal degradation of saturated polymers like polyethylene is affected by the temperature of pyrolysis. At high temperature the amount of beta scission of the primary free radical increases thereby increasing the amount of ethylene formed. At lower temperatures, there is less beta scission and the predominant reaction is the transfer of the free radical down the chain by backbiting to pull off a hydrogen atom and form a secondary free radical further down the chain, which then undergoes beta scission to form the higher molecular weight fragments (9). The pyrolysis pattern is thus changed by the rate at which the filament is heated. At very high heating rates the wire reaches a temperature at which dehydrogenation can occur and benzene and toluene are formed. At very high temperatures the primary products are toluene, benzene, and ethylene. This is not desirable if one wishes to study the structure of the original 0
506
M. DIM BAT AND F. T. EGGERTSEN
polymer, but if the primary purpose is to determine the amount of sample present, the high temperature pyrolysis may be preferable. Lack of proper temperature control is probably the cause of the nonreproducibility reported by other users of filament pyrolyzers. We have found the computor integration technique of Johnson and Stross (7) extremely valuable in the interpretation of complex pyrolysis patterns. A valve has been provided between the exit of the column and the detector to allow for trapping of peaks for mass spectrometric or infrared identification. The following analytical application has been selected to illustrate many of the principles discussed in the previous sections.
Thickness of a Polymer Coating on Paperboard The thickness of a polymer film, coated on paperboard as a waterproofing treatment, can be measured conveniently and precisely by pyrolysis-GC. For example, a method has been developed for determining the coating thickness of an ethylene-propylene copolymer by using a very small punched disc specimen of the paper. By use of such small specimens it is possible to test uniformity of the coating on the paper. Attempts to apply hot-wire pyrolysis directly to the coated paper were not successful because the pyrolysis products from the polymer are obscured by the relatively much larger amount of pyrolysis products from the paper. But the analysis can be readily done by an extraction technique in which the polymer is dissolved from the specimen with hot toluene. An aliquot of the solution is then evaporated on the filament and the residue is determined by pyrolysis-GC. In the procedure employed a punched disc of the coated paperboard, 6 mm in diameter, is placed in a 2-ml volumetric flask with toluene and heated at 100°C for one-half hour. The flask, still containing the paper disc, is cooled, filled to volume with additional toluene and mixed well. The pyrolysis filament is heated to 100°C, and 20 fil of the solution is injected into the warm filament in three portions. After each addition of solution the solvent is evaporated and the filament temperature allowed to rise again to 100°. Condensation of solvent vapors in the body of the pyrolysis filament unit is minimized by drawing them off through a funnel connected to house vacuum. Other volatiles present with the polymer can also be removed, if desired, by adjusting the filament temperature to a higher level.
507
DETERMINATION OF POLYMER
After the solvent has been evaporated , which requires only a few minutes , the filament unit is placed in the pyrolysis inlet, which is maintained at lZ0 D C. When a steady base line has been attained , requiring about 5 minutes, the sample is pyrolyzed rapidly at 900 D e (maximum temperature). The pyrograrn is obtained with a 15-ft silicone column at lOO DC, in which nit rogen is used as the carrier gas at 30 cc per minute. The hydrogen flame ionization detector is employed at a sensitivity setting to give a response for a Co hydrocarbon, which is employed as the test peak, of about 0.1 mv per microgram of polymer pyrolyzed. This method was developed before the technique of hydrogenation in the column was adopted. It is expected that greater sensitivity would be achieved by use of hydrogenation. Typical pyrograms for coated and uncoated paperboard are shown in Fig . 3. It is noted that the material extracted from the paper produced a 0 .4 ~--------------------,
E thylene I P ropylene Po lymer ~ o n Pape r, 0.25 mg/ cm' O.} f-
o.z >--
c, 0 .11mv
~ I
I
.....
I
I
I
0. 1 . . . - - - - - - -- -- - - - - - - - - -------, Paper Blank
T i me. mi nutes
FI G. 3 .
Pyrograms of eth ylene /propylene polymer extracted from coated paper.
508
M. DIM BAT AND F. T. EGGERTSEN
pattern, necessitating a small blank correction. A calibration curve, constructed from data for known amounts of polymer on paperboard, is shown in Fig. 4. The height of the Ca test peak varies linearly with polymer concentration, and thus is a direct measure of the amount of polymer on the
O.Z
>
e
O~
o
-.l..
.l...-
5 10 Concentration In Solvent, o;ow x 10)
....I
15
FIG. 4. Calibration curve for ethylene/propylene polymer coated on paper.
specimen. Concentrations down to about 0.00170 weight can be measured, which corresponds to only 0.2 f.lg of polymer in the 20 f.ll of solution. Precise temperature control during the pyrolysis, which is essential for good reproducibility, is achieved by means of the thermocouple on the filament. The thermocouple output during each pyrolysis was monitored routinely by using a strip chart recorder at a fast chart speed. Adjustments of the voltage to the filament were made as necessary to obtain a heating rate of 150°C per second measured at 400 to 500°C. With this technique the standard deviation for duplicate analyses was about 0.03 mg per square centimeter, over the range 0.2 to 1 mg of polymer per square centimeter of paper. In this method a relatively high heating rate was employed in order to effect rapid injection of the pyrolysis products into the GC column. This is desirable in order to obtain a sharp C6 test peak. CONCLUSIONS
Pyrolysis-GC analysis has become an indispensable analytical tool in these laboratories. The most important single requirement for good pyrolysis analysis is proper control of the pyrolysis conditions. The most satis-
DETERMINATION 01" POLYMER
509
factory means for control of these conditions is the glass coated platinum filament with a platinum/platinum-I oro rhodium thermocouple attached. It allows good control over a wide range of decomposition rates. Further automation of the wire heating rate and temperature should prove useful in obtaining even greater reproducibility. ACKNOWLEDGMENTS The authors are grateful to W. M. Sawyer for his ~uidance and counsel in the work with the polymer-coated paper; to E. Meyer and H. E. Melling for construction of the filament-thermocouple pyrolyzer; and to R. F. Tremoureux and Miss Edna Dean for skilled assistance in the experimental work. REFERENCES 1.
2.
3. 4. .'i.
6. 7. 8. 9. 10.
11.
BENTLEY, F. F., AND RAPPAPORT, G., Semiquantitative analysis of buna N phenolic blends by the infrared spectra of their pyrolyzates. Anal. Chern. 26, 1980-1985 (1954) . BUA, E., AND MANERESSI, P., Quantitative analysis of ethylene-propylene copolymers by the mass spectra of their pyrolyzates, Anal. Chern. 31, 2022-2024 (1959). DAVISON, W. H. T., SLANEY, S., AND WRAGG, A. L., A novel method of identification of polymers. Chern. Ind. (London) 1954, 1356. HARMS, D. L., Identification of complex organic materials. Anal. Chern. 25, 11401155 (1953) . HUNTER, L., AND FORBES, J. W., Structural investigation of polyacrolein by fractional dehydration. 148th National ACS Meeting, Chicago, III., September 1964, abstract 72. KOLB, B., AND KAISER, K. H., Hydrogenation as a technique of identification in pyrolysis gas chromatography. J. Gas Chromatog, 2, 233-234 (1964). JOHNSON, H. W., Storage and complete automatic computation of gas chromatographic data. Anal. Chern. 35, 521 (l96.n. LEIlRLE, R. S., AND ROBB, J. C., Direct examination of the degradation of high polymers by gas chromatography. Nature 183, 1671 (1959). SIMHA, R., WALL, L. A. AND BLATZ, P. J., "Depolymerization as a Chain Reaction," J. Polymer Sci. 5, 615-632 (1950). STERNBERG, J. C., GALLAWAY, W. S., AND JONES, D. T. L., The mechanism of response of flame ionization detectors. In "Gas Chromatography" (N. Brenner, ed.), pp. 231-267, Academic Press, New York, 1962. ZEMANY, P. D., Identification of complex organic materials by mass spectrometric analysis of their pyrolysis products. Anal. Chern. 24,1709-1713 (1952).