Polymer Identification and Quantitative Determination of Additives by Photolysis-Gas Chromatography Richard S. Juvet, Jr.,’ John L. S. Smith,2 and Kuang-Pang Lil Department of Chemistry, Unioersity of Illinois, Urbana, Ill., and Department of Chemistry, Arizona State Unicersity, Tempe, Ariz. 85281 Photolytic degradation and gas chromatography have been combined for the identification of polymeric materials and for the qualitative and quantitative analysis of trace amounts of both volatile and nonvolatile polymer additives-antioxidants and plasticizers-without prior separation from the polymer matrix. The polymers studied gave simple, distinctive degradation patterns, and the identification and rapid quantitative determination of common types of polymer additives is demonstrated. Since degradation of these materials results from absorption of a narrow range of energies, reproducibility among laboratories is anticipated to prove superior to that obtained for the various thermal degradation methods now in common use. A useful mathematical model has been developed which allows complete interpretation of working curve characteristics.
THECOMBINATION of photolytic degradation and gas chromatography has been shown ( I , 2) to be a powerful analytical technique for the structural identification of volatile organic compounds. This paper describes the extension of this technique to the identification of polymeric materials and the quantitative determination of common polymer additives. An alternate technique, pyrolysis-GLC has been widely employed since the early days of gas chromatography in the identification of nonvolatile substances, primarily by empirical “fingerprint” comparison of the chromatographic peaks of the degradation products. Sufficient difficulty is associated with obtaining reproducible degradation patterns by this earlier technique, however, to hamper exchange of data between laboratories. In fact, the history of pyrolysis-GLC has been characterized (3) as a continual search for the perfect pyrolysis unit capable of producing reproducible results from one laboratory to another. The critical effects of pyrolysis temperature, sample size or thickness, residua! solvent trapped in the polymer matrix, heating technique, filament history, and other experimental variables which can drastically alter a pyrolysis degradation pattern have been previously enumerated (2). Although the Curie-point pyrolyzer ( 4 ) appears to have significant advantages over the more common filament pyrolysis systems, FarrC-Rius and Guiochon (5) have shown the critical parameter in polymer pyrolysis to be the rate of heat transfer to the sample and not the equilibrium pyrolysis temperature. Thus variations in sample size, method of application, entrapped solvent, and the thermal conductivity Present address, Department of Chemistry, Arizona State University, Tempe, Ariz 85281 Present address, Union Carbide Corp., Tarrytown, N.Y. 10591 (1) R. S. Juvet and L. P. Turner, ANAL.CHEM., 37, 1464 (1965). (2) R. S . Juvet, R. L. Tanner, and J. C. Y. Tsao, J. Gas Chromatogr., 5, 15 (1967). (3) M. P. Stevens, “Characterization and Analysis of Polymers by
Gas Chromatography,” Marcel Dekker, New York, N.Y.. 1969, p 65. (4) W. Simon and H. Giacobbo, Chem. Ing. Tech., 37,709 (1965). (5) F. Farr6-Rius and G. Guiochon, ANAL.CHEM., 40,998 (1968).
of the polymer are still probable sources of error. Extremely rapid heating of polymer samples has been investigated using both a pulsed ruby laser (6, 7) and a neodymium-doped glass laser (8). Both laser systems depend on thermal degradation and require mixing of the samples with powdered carbon for rapid absorption of the laser thermal energy. With the neodymium-glass system, slight changes in beam focus produced substantially different pyrolysis patterns, and the fragmentation pattern was strongly dependent upon the laser beam energy and the concentration of added carbon with the pulsed ruby laser system. Although information is available detailing primary thermal degradation mechanisms for polymeric materials (9), and recent instrumental advances have provided somewhat improved reproducibility, the persistent experimental problems and fundamental uncertainties associated with thermal degradation methods warrent the investigation of other degradation schemes and have prompted this study. Photolytic degradation has been shown to yield considerably more simple and reproducible decomposition patterns due to greater control of input energy and the more predictable manner in which compounds decompose photolytically. Retention data have been tabulated for the irradiation products of volatile aliphatic esters and alcohols (1) and for aldehydes, ketones, and ethers (2). These data demonstrate that products are obtained in a predictable fashion for members of a homologous series and, further, that identification of the products is not necessary to establish the structure of the compound under investigation. In addition, extensive reviews of photochemical processes, such as the one by Calvert and Pitts (IO), may often be used to prwide structural information for compounds not yet studied by photolysis-gas chromatography . Fox (11) has published an excellent review of the literature through 1966 covering the photolytic degradation of a number of common polymers. Although a wealth of experimental data has been published, considerable disagreement is found in the polymer literature concerning mechanisms, quantum efficiencies, and even the products formed from a given polymer. Since polymer photolysis is generally carried out in the solid state, variations in surface state, impurities, and crystallinity, which have long plagued the investigation of solid phase photochemistry, may occur unless suitable precautions are taken to prevent these problems. In addition, differences in composition, residual monomer, and catalyst content occurring from batch to batch, both in the research laboratory and (6) 0. F. Folmer, Jr., and L. V. Azarraga, J . Chromatogr. Sci., 7, 665 (1969). (7) 0. F. Folmer, Jr., ANAL.CHEM., 43, 1057 (1971). (8) B. T. Guren, R. J. O’Brien, and D. H. Anderson, ibid., 42, 115 (1970). (9) S. L. Madorsky, “Thermal Degradation of Organic Polymers,” Interscience, New York, N.Y., 1964. (10). J. G. Calvert and J. N. Pitts, Jr., “Photochemistry,” Interscience, New York, N.Y., 1966. (11) R. B. Fox, Progr. Polym. Sci., 1,45 (1967).
ANALYTICAL CHEMISTRY, VOL. 44, NO. 1, JANUARY 1972
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9-13 I
2 v2”
Figure 1. Heated compressing unit for rapid preparation of polymer films prior to photolytic degradation and GC analysis Compression time, 5 sec. Temperature maintained above polymer glass transition temperature (usually 150 “C) in commercial production, are likely to introduce variation into fundamental studies of polymeric materials. In spite of these potential problems, certain volatile decomposition products are consistently formed which may be used for identification of the basic polymer type. Quantitative analysis of additives such as antioxidants and plasticizers in polymers by chemical techniques has been widely employed, usually for lack of alternative techniques, but this approach suffers from difficulties in the quantitative separation of the additives from the sample. Ericksen and Brown (12) have listed common techniques for separation by precipitation or by grinding and solvent extraction, but these measures are often time consuming, requiring up to 96 hours for some methods given. Infrared analysis has been successful in some cases but is usually not acceptable for trace concentrations of additives where absorption bands overlap those of the polymer. Zulaica and Guiochon (13) were able to obtain linear working curves for some plasticizers by partial pyrolysis of samples in an injection port heated to ca. 600 “C, but the procedure requires that the additive be sufficiently volatile to be eluted from a chromatographic column. Majors (14) has reported excellent separations of phthalate ester plasticizers and certain antioxidants by liquid chromatography, but the technique requires Soxhlet extraction for two days to remove the additives from the polymer samples. It is shown below that characteristic reaction products occur during photolytic degradation, and polymer additives (12) P. H. Ericksen and B. F. Brown in “Analytical Chemistry of Polymers,” Part I, G. M. Kline, Ed., Interscience, New York, N.Y., 1959, p 97. (13) J. Zulaica and G. Guiochon, ANAL.CHEM., 35, 1724 (1963). (14) R. E. Majors in “Advances in Chromatography, 1970,” A. Zlatkis, Ed., Chromatography Symposium, University of Houston Publishers, 1970, pp 406-15. 50
may be identified without separation from the polymer samples. The excellent reproducibility reported for photolytic degradation of liquid samples indicated the possibility of quantitative determination of polymer additives, and typical examples are presented which demonstrate the applicability of photolysis gas chromatography to the quantitative determination of antioxidants and plasticizers in polymers. Because of the large number of polymeric materials produced today and the almost limitless polymer/additive combinations, no effort has been made here to catalog the products for all possible materials. Instead, several representative polymers and polymer/additive combinations have been investigated, experimental procedures with which other compounds may be studied are described, and equations are developed to predict the shape of calibration curves expected for both trace level additives and those present at higher concentrations. Since photolytic analysis does not require separative steps and can be performed with samples weighing less than a milligram, it provides a rapid, practical approach for the determination of additives in polymeric materials. EXPERIMENTAL
Materials. Well-characterized polyethylene samples, designated PE-80, PE-82, and PE-85, and “dinonyl” phthalate, actually 3,3,5-trimethylhexyl phthalate, were obtained from the Plastics Department, E. I. du Pont de Nemours and Co., Wilmington, Del, High density polyethylene and propylene was furnished by Amoco Chemical Corp., Naperville, Ill. Polystyrene, Monsanto HF-77 ; Plastanox LTDP (dilaurylthiodipropionate) ; and Plastanox STDP (distearylthiodipropionate) were supplied by the Intermediates Department, American Cyanamid Co., Bound Brook, N. J. Poly(methy1 methacrylate) was obtained from Rohm and Haas Co., Philadelphia, Pa., in the form of Plexiglas A-100 Molding Powder. All polymer samples were analyzed by gas chro-
ANALYTICAL CHEMISTRY, VOL. 44, NO. 1, JANUARY 1972
HYDRAULIC PRESS
i I
I
'
;+/CARTRIDGE I HEATER
I
e---,--
Figure 2. Heated compressing unit for use with IR-laboratory KBr pellet press for preparation of polymer mixtures of known composition
Figure 3. Elliptocylindrical ultraviolet light source geometry used for maximum intensity and uniform polymer irradiations
matography under conditions used for separation of photolysis products t o estimate the amount of residual volatile materials prior to irradiation. This amount was subtracted from the total amount present following irradiation Sample Preparation. Pure polymer samples are prepared for photolysis with the apparatus shown in Figure 1. A sample of up to 50 mg is placed between two plane glass microscope slides which are then inserted between the two large aluminum blocks. The temperature of the blocks is maintained slightly above the glass transition temperature of the polymer (usually ca. 150 " C ) by controlling the voltage to the 65-watt cartridge heaters with a variable transformer. The 3 / ~ - i n ~machine h screw is tightened and the sample flattened into a thin film rapidly (about 5 seconds) t o preclude thermal degradation of the polymer. The resulting film is placed between two glass plates with one edge slightly protruding and sliced into strips ca. 0.8 mm wide with a scalpel. These strips are weighed to the nearest microgram with a Cahn Model M-10 Electrobalance and transferred t o 1.0-mm i.d. quartz capillaries, previously sealed at one end. The capillary is evacuated for 5 minutes to remove air and is then sealed, care being taken t o prevent thermal degradation in this operation. In preparing samples of known composition for calibration purposes, weighed amounts of additives are mixed with pure polymer samples prior to casting into thin films with the apparatus diagrammed in Figure 2. A stainless steel cylinder, kept at a temperature slightly above the glass transition temperature of the polymer, is placed under the top stop of a n ordinary infrared-laboratory KBr pellet press. A Model C-30 30-ton hydraulic press manufactured by Research and Industrial Instruments, London, England, was used in the work described here. The polymer, together with a weighed amount of additive, is placed on the cylinder and compressed until it forms a disk, the diameter of which equals that of the cylinder. The pressure is released, the cylinder withdrawn, and the sample is further mixed by folding and compressing several times with a small spatula, after which it is recompressed with the hydraulic press. Small portions of the disk thus formed are cast into thin films between glass slides as above and a supply of standard mixtures sufficient for several months of analyses can be prepared in less than one day. The homogeneity of the sample thus prepared was determined spectrophotometrically by mixing 2-hydroxy-4-methoxybenzophenone with polyethylene in the amount of 1.0% w/w and
treating the sample five times by the above mixing procedure. The film was attached t o the burner mounting of a Heath flame photometer module, the position of which can be varied by a micrometer adjustment system. Heath light source, photomultiplier tube, and monochromator modules were used to isolate light at 360.0 nm. The light beam from the exit slit was focused t o dimensions of ca. 0.2 mm wide and 1.0 mm high. Absorbance measurements were made a t 1.8-mm intervals across the full 18-mm diameter of the disk. Absorbance measurements on a disk 0.100 m m in thickness yielded a constant value of 0.394 with an average relative deviation of 0.5 %, thus indicating the homogeneity expected for known calibration samples prepared using this technique. The mixing of additives and polymers by solution in a solvent followed by solvent evaporation was avoided since residual amounts of certain solvents mechanically trapped in the polymer matrix might show photolytic degradation as well as the polymer. Sample Irradiation. To provide maximum ultraviolet intensity for photolysis in the solid phase, a n irradiation apparatus (Figure 3) was constructed using a n Engelhard Hanovia Type 612C medium-pressure mercury source. The irradiation cavity is of a n elliptocylindrical shape in which the source and sample are positioned at the foci. The cavity was formed by placing a cylinder of spring brass, 13 cm high and 0.22-mm wall thickness within four upright posts arranged in a rectangular pattern of 8 X 13 cm. The brass cylinder assumes the shape of an ellipse having major and minor axes of 18 cm and 14 cm, respectively. A lining of highly reflecting aluminum foil concentrates a large percentage of the emitted radiation on the sample. The water jacketed sample compartment consists of an inner 2-mm i.d. quartz tube surrounded by a 25-mm i.d. quartz tube with sidearms for cooling water circulation. Directional irregularities in light intensity are minimized by rotating the sample with a 1 rpm synchronous motor. Product Separation and Identification. After irradiation, the samples are introduced onto the chromatographic column with a capillary crusher similar in design t o the F and M (Hewlett-Packard) Model SI-4 solid sampler but constructed to a larger scale to accommodate 1.0-mm i.d. quartz tubing. A MicroTek DSS 172 D P F F gas chromatograph with flame ionization detector was used to obtain all chromatographic data. Most data were obtained by using a 6-foot, lI8-inch
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0.d. stainless steel column packed with 10% w/w SE-30 on 80/100-mesh Chromport S. The injection port temperature was maintained at about 150 “C. A temperature programming rate of ca. 6”/min and a helium carrier gas flow rate of ca. 30 ml/min were employed. In some cases, products were also separated, either isothermally or under temperature programmed conditions, on a &foot, 1/4-inch 0.d. stainless steel column containing 15 w/w Carbowax 20M on silane-treated 60/80-mesh Chromosorb P. Retention data were reported as Kovats retention indices, and some tentative peak assignments were made using the compilation of McReynolds (15). Peak area measurements were made with a Keuffel and Esser Type 4242 planimeter reproducible to 1 0 . 1 cm2. To aid in the determination of the retention indices of the photolysis products, a device was constructed which automatically indicates column oven temperature on the recorder chart during temperature programming. The device consists of an iron-constantan thermocouple placed within the column coils in the oven, a calibrating potentiometer, and a microswitch actuated by a cam on a 1-rpm synchronous motor. Once each minute the microswitch disconnects the recorder input from the electrometer for 2-3 seconds and connects the output from the thermocouple. With proper calibration, the column oven temperature may then be indicated on the chart of any 1-mV to 10-mV recorder and can be read directly in degrees Centigrade. A mixture of nhydrocarbons from n-pentane to n-pentacosane is chroniatographed daily from which a Calibration curve of column oven temperature 6s. Kovats retention index is prepared. The retention indices of eluted compounds are obtained by graphical comparison of the retention temperature with the calibration curve.
Table I. Photolysis Products of Common Polymers Retention Index, I on SE-30 Polymer Rel. peak areaa 1.0 Polyethylene > 1
Equation 1then simplifies to:
This expression may be interpreted as indicating that a nonlinear calibration curve, concave downward, would be predicted at higher concentrations, and at sufficiently high concentrations one would expect that the chromatographic peak area of the irradiation product, B, would eventually become constant, independent of the original concentration of the additive and dependent only upon the irradiation time, sample
56
thickness, quantum yield, and the UV intensity. This explains the nonlinear behavior of the calibration curve for LTDP at concentrations greater than about 1 %. Of course, a nonlinear working curve at high concentrations, in itself, does not influence appreciably the accuracy expected for analytical purposes. It should be noted that in the region of the calibration curve where the linear relationship expressed by Equation 3 holds, the radiation product peak area should be independent of the sample thickness and dependent only upon the concentration of the additive in the polymer, the irradiation time and intensity, and the molar absorptivity of the additive in the polymer matrix. At higher concentrations, where the curve begins to deviate from linear behavior, some minor dependence of peak area on sample thickness would be predicted from Equation 4, increasing in importance as the product adCAo becomes larger so that the relationship expressed in Equation 2 no longer holds. The linear portion of the working curve may be extended to higher concentrations, however, and any dependence on sample thickness eliminated by decreasing sample thickness to the point where adCAo < -0.1. Under these conditions, Equation 3 holds and the irradiation product peak area becomes proportional to additive concentration at constant irradiation time and intensity. It is tempting to attribute the nonlinear working curve obtained for the Z = 658 (on Carbowax 20M liquid phase) peak of dinonyl phthalate in poly(methy1 methacrylate) to this phenomena. A retention index of 658 agrees closely with that calculated for 2,4-dimethylpentane, a probable product of photolysis of "dinonyl" phthalate [di(3,3,5-trimethylhexy1)phthalatel. Dioctylphthalate in poly(viny1 chloride), however, shows linear dependence as high as 19 weight per cent for the decomposition product peak appearing at I = 796 on SE-30, probably n-octane, and other phenomena such as differences in matrix absorbance or radical scavenger effect may be involved as well. Further work will be required to resolve this question.
RECEIVED for review July 28, 1971. Accepted August 30, 1971. The financial support of the Nationdl Science Foundation under Grants GP-8630 and GP-25553 is gratefully acknowledged,
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