(2) Atkins, D. C., Jr., Murphy, C. M., Saunders, C. E., Ibzd., p. 1395. (3) Dal Nogare, S., Juvet, R. S., Jr., “GasLiquid Chromatography,” p. 123, Interscience, New York, 1962. (4) Davison, W. H. T., Slaney, S., Wragg, A. L. Chem. I n d . (London) 44, 1356 (1954j. (5) Duswalt, A. A., Brandt, W. W., ANAL. CHEM.32, 272 (1960). (6) Guillet, J. E., Wooten, W. C Combs, R. L., J. A p p l . Polymer S k 3, 61 (1960).
(7) Janak, J., 3rd Conference on Analytical Chemistry, Prague, September 1959. (8) Kieser, M. E., Sissons, D. J., Nature 185, 529 (1960). (9) Luskina, B. M., Syavtsillo, S. V., Terent’ev, A. T., Turkel’taub, N. M., Doklady Akad. N a u k , S.S.S.R.141, 869 (1961). (10) Stein, K. C., Feenan, J. J., Thompson, G. P., Shultz, J. F., Hofer, L. J. E., Anderson, R. B., I n d . Eng. Chem. 52, 671 (1960).
(11) Sundberg, 0. E., Maresh, C., ANAL. CHEM. 32, 274 (1960).
ROBERT G. SCHOLZ JAMES BEDNARCZYH TERRY YAMAUCHI IIT Research Institute Technology Center Chicago, Ill. RECEIVEDfor review August 26, 1965. Accepted December 6, 1965.
Elastomer Identification by Ultraviolet Spectrometry SIR:There is considerable literature on the problems of qualitative and quantitative analyses of elastomers (4). A test method for elastomer identification which has replaced most of the lengthy or less exact chemical tests, infrared (IR) spectrometry, also has some shortcomings. The test samples require a certain amount of preparation before IR spectra can be made. Organic substances other than elastomers often interfere and have to be removed by extraction. The sample then has to be either dissolved in some solvent to separate carbon black and fillers, or has to be pyrolyzed to effect the separation. The pyrolysis step is simple but the I R spectra obtained often differ from those obtained by the dissolution method and are sometimes hard to interpret. The method described here uses the selective absorption of ultraviolet radiation by gaseous pyrolyzates from different elastomers. It was developed to permit rapid qualitative tests with a minimum amount of material and no sample preparation, and can be considered as an improvement of some previous attempts. Burchfield (I) has developed a rapid chemical test for elastomers utilizing their gaseous pyrolyzates and Hummel (2) describes I R methods using gaseous pyrolyzates of elastomers. This method alone and with I R and other methods has been used and tested by the writer for a considerable period of time and has proven to be quite accurate. Because of its simplicity, savings in analyses time have been considerable. Experience from tests performed has shown that ultraviolet spectrometry can be a valuable tool for elastomer identification. The method is not meant to replace I R and other reliable elastomer identification tests, but to supplement them. THEORETICAL CONSIDERATIONS
On pyrolysis elastomers produce large amounts of monomers from which they were made, some dimers, and higher molecular weight polymer break-down 334
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products. The gaseous pyrolyzates show UV absorptions which are characteristic of individual elastomers. For example, natural rubber pyrolyzate contains isoprene monomer fraction with its typical UV absorption spectrum. If an unknown elastomer pyrolyzate has a similar spectrum, then the elastomer is probably natural rubber, or polyisoprene. I n a similar manner other elastomers can be identified by comparing the UV spectra of their pyrolyzates with known elastomer spectra, made for reference. Because electronic absorptions (9) are selective materials which do not absorb in the test region do not interfere, but this selectivity also limits the method to substances which absorb UV radiation. Contrary to what is generally believed, the UV absorption spectra made by this method are quite specific to each elastomer, or groups of elastomers. The gaseous spectra of the pyrolyzates (monomers, etc.) show vibrational fine structure of diagnostic value which would tend to disappear when using solvents. By collecting the mainly monomeric gaseous fraction, the interference of the more complex liquid pyrolyzate is eliminated. Any COz produced and most hydrocarbons with no characteristic absorption groups such as methane or its derivatives have to be present in large amounts before they will show appreciable effect on the UV absorptions. EXPERIMENTAL
Apparatus. The Bausch & Lomb Spectronic 505 recording type UV spectrophotometer was used. The absorbance measuring logarithmic gears of the instrument produced absorption curves with better definition than the linear gears. For collecting the pyrolyzate a quartz (1 cm. path) cuvette was used. Procedure. The pyrolysis of an elastomer was accomplished by pressing the red hot tip of a triangular iron file or electric resistance wire against the elastomer sample. The cuvette was held about an inch above the sample and some of the pyrolyzate
was caught in the cuvette and retained there by sealing i t immediately with a finger of the hand holding it. The cuvette was turned upright and its cover placed on quickly. The UV absorption of the pyrolyzate was recorded by medium-fast scanning from 180 mp to about 260 mp. The absorption intensity of the spectrum recorded was proportional to the amount of the gaseous pyrolyzate collected in the cuvette. The optimum pyrolyzate concentration for each elastomer was determined by a few experiments. It was sometimes best to admit more than the required amount of pyrolyzate and then let some escape by removing the cover until the desired absorbance was obtained. The spectrum obtained from the unknown was compared with known elastomer spectra. After use the cuvette was cleaned with acetone or benzene to remove pyrolysis products which had condensed on its surface. RESULTS AND DISCUSSION
The spectra, obtained from several different elastomers and other organic materials, are described here and four characteristic spectra are shown. Natural Rubber. The pyrolyzates of natural rubber and polyisoprene have similar UV spectra (Figure 1) and therefore can not be distinguished. The most characteristic absorption band is in the 220-225 mp region. From this absorption the smallest amount of isoprene that can be identified in elastomer mixtures is less than 5%. In organic materials other than elastomers, the presence of less than 3% of isoprene can be detected. The absorptions a t 210 mp and 216 mp are common with two of the butadiene peaks, but show different intensities. Because these absorption peaks become modified, they can not beusedfor natural rubber identification in certain elastomer mixtures. Styrene-Butadiene Rubber. The SBR rubber pyrolyzates (Figure 2) show the typical styrene and butadiene absorption spectra combination. The styrene absorbs a t 195-200 mp and 235-240 mp, and the butadiene has absorptions in the 200-220 mp region,
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and these absorptions are quite different. In SBR pyrolyzates, the intensity of UV absorption by styrene is proportional to its amount. Therefore, the styrene content in an unknown SBR sample can be determined by comparing its pyrolysis spectrum with known styrene content SBR reference spectra. The accuracy obtainable is about =t5% and surpasses the rapid test methods used before. For estimation, comparisons can be made of the absorption intensities a t 200 mp, 210 mp, and 216 mp, or the differences between styrene absorptions a t 235 mp and at 225 mp or 260 mp can be compared. The presence of less than 3% of SBR has been detected in a sample of asphalt, resin and SBR mixture. SBR-NR Mixtures. It is possible to determine the SBR-NR ratio in mixtures with anywhere from 2 0 4 0 % of either component. I n this range about 5-10% accuracy is obtainable. If SBR content is less than 20%, only qualitative confirmation of its presence is possible. The reference curves should be made accurately and the absorption intensities of pyrolyzate from unknown should match the reference. Large amounts of filler other than carbon black interfere in the ratio determination. Unfilled elastomer mixtures should not be used as references for unknowns with filler. Absorptions can be read a t 210 mp and 216 mp. The SBR-NR ratio can be calculated by comparing these peak differences with those from reference spectra. The absorption a t 222 mp can give a rough estimate of NR content.
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Figure 2. Ultraviolet spectra of styrene-butadiene rubber pyrolyzates A. 28% bound styrene; B. styrene
Polysulfide Rubber. The UV spectrum of polysulfide (Figure 3) pyrolyzate is characteristic of materials which have the sulfur atom attached to carbon. The gas produced has the same spectrum as CS2, which must be the breakdown product, showing the typical absorption bands in the 195-20.5 mp region. All the sulfur bearing rubber accelerators produce similar spectra, with absorption intensit.y proportional to the sulfur content. This has served to distinguish MBT, Monex, and Tuex. I n pyrolyzates of elastomers with large amounts of these accelerators their absorptions show up indicating their presence. Nitrile-Butadiene and Butadiene Rubbers. The NBR rubber pyrolyzate (Figure 4) shows only butadiene absorption bands which are the same as for butadiene gas. Acrylonitrile does not show characteristic absorptions in the test region. I t s presence has to be determined by I R or nitrogen determination. Butadiene rubber has the same spectrum and can not be distinguished from NBR rubber by this method, but absence of nitrogen indicates polybutadiene. The ratio of NR to BR can be determined in their mixtures with an accuracy of about f 5 % , The minimum amount of each of the components in the mixture has to be over 10% for quantitative results.
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Butyl Rubber. Butyl rubber pyrolyzate has its maximum absorption a t 194 mp with a small secondary band in the 200 mp region. It is the same as for isobutylene gas. When sulfur is used for curing, its presence can be seen on the spectrum by its characteristic absorptions superimposed. This test method can be used to differentiate between sulfur and resin cured butyl rubbers. It is possible to detect butyl contamination in other rubber stocks. In mixtures of SBR with 2040% butyl, their rat,io has been determined in a fashion similar to SBR-NR ratio determination, but much lower accuracy has been obtained. Chlorosulfonated Polyethylene. The pyrolyzate of Hypalon exhibits absorption spectrum in 195-220 mp region, which has low intensity sulfur dioxide characteristics. Chloroprene Rubber. Because C R rubbers can be made unmodified, or copolymerized with other monomers, there is a variation in the UV spectra of their pyrolyzates which in general are not very characteristic. Chlorine can be determined readily by the Beilstein test. If isoprene is present i t can be detected and can help in differentiation between the different types. Acrylic Rubber. The acrylic rubber pyrolyzate has a single wide abVOL. 38, NO. 2, FEBRUARY 1966
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sorption peak a t 195-200 mp and is similar to acrylic plastic pyrolyzate. Ethylene-Propylene Rubber. The EP rubber pyrolyzate shows the ethylenic and propylenic absorption similar to the two plastics. Although they do not have markedly different UV absorptions spectra, an estimate can be made of their ratio in the copolymer. This is possible because polypropylene has branched chain, but polyethylene has straight chain structure, producing differences in their pyrolyzates. Polystyrene. The spectrum obtained from pyrolysis of polystyrenes is similar to the spectrum of styrene vapors. Because it is typical, styrene-containing plastics can be differentiated from other types. The butadiene :styrene ratio can be determined with good accuracy in ABS plastics from their UV spectra. The acrylonitrile content is determined from nitrogen analysis, or by IR. Sulfur. I n the procedure sulfur produces SO2gas, which shows intense absorptions in the 195-220 mp region. It has a secondary absorption band a t 295-320 mp. Other Polymers. The UV spectra of pyrolyzates from PVC and W A C are not very characteristic. They both are similar requiring a halogen
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or I R test t o distinguish between them. Polyurethanes and silicone rubber similarly show no typical UV spectra in the test region. Observations. Several hundred UV test determinations of elastomers and plastics of varied composition have shown that the method gives reproducible spectra. The same UV spectrum is obtained on repeat pyrolysis of the sample and is reproduced from different composition mixtures of the same elastomer. The spectra obtained from cured and uncured, filled or unfilled elastomers are the same. Large amounts of filler have an effect on spectra intensities only. Samples taken for analysis need not be extracted with acetone unless very large amounts of an interfering substance is present. This has occurred very seldom and even then qualitative identification has been possible. .Nost of the organic additives in concentrations usually used in rubber stocks do not contribute their UV absorptions. Any fumes produced change the spectra intensity only. Elastomers deposited in thin layers on substrates, such as paper or fabrics, can be pyrolyzed without any effect on their UV spectra. It is possible that pyrolysis probe temperatures have a bearing on the pyrolyzate composition, but no notice-
able changes have been observed from the usual temperature variations. No attempts have been made to see if there is an optimum pyrolysis temperature. The method is not limited to identification of elastomers and some plastics, and could be tried for characterization of other organic materials. Developed in conjunction with IR, gas chromatography, and other tests, it could serve in studies of polymer degradation by heat. LITERATURE CITED
(1) Burchfield, H. P., IND. ENQ.CHEM. ANAL. ED.16, 424 (1944). Ibid., 17,
806 (1945). (2) Hummel, D., Rubber Chem. Technol. 32, 854 (1959). (3) Gillan, A. E., Stern, E. S. “Electronic Absorption Spectroscopy.” Edward Arnold, Ltd., London, 1954. (4) Wake, W. C., “The Analysis of Rubber and Rubber Like Polymers,” Chap. 3, MacLaren & Sons Ltd., London, 1958.
G. J. ROZENTALS~ Research Laboratories Dominion Rubber Go. Guelph, Ontario, Canada 1 Present address, Research Laboratory, Ontario Hydro, Toronto, Ontario, Canada. RECEIVEDfor review June 9, 1965. Accepted December 13, 1965.
