Identification of Antioxidants in Rubber Vulcanizates D. W. Carlson, M. W. Hayes, H. C. Ransaw, R. S . McFadden, a n d A. G . Altenau Central Research Laboratories, The Firesrone Tire & Rubber Company, Akron, Ohio 44317 THEPROBLEM of identifying small concentrations of antioxidants in polymers is a difficult task. Methods used previously such as dye formations ( I ) , paper and thin layer chromatography (2-4, and colorimetric (5, 6) and spectrophotometric procedures (7) were very time consuming and could not be utilized because of the interference of the extending oil present. Gas chromatographic methods have provided a means of quantitatively determining one or more antioxidants of similar structures (8, 9). Recently a spectrophotometric method was developed for identifying phenylenediamine type antioxidants (IO). The method is fast but it is limited t o phenylenediamine type antioxidants. However, there still exists a need for a general technique of identifying antioxidants in base and oil-extended rubber and rubber vulcanizates. This paper describes a very fdSt method for identifying a wide variety of rubber antioxidants by means of extraction followed by nuclear magnetic resonance (NMR), mass spectrometry, and infrared analysis. EXPERIMENTAL
Apparatus. A Beckman IR-4 infrared spectrophotometer was used to record the infrared spectra. A Varian HA-60-IL nuclear magnetic resonance spectrometer was used to record the N M R spectra. A MS-12 medium resolution mass spectrometer (Associated Electrical Industries, Manchester, England) was used to obtain the mass spectra. The operating conditions were a resolution of approximately 1000, source temperature of 200 "Cabove room temperature, and an accelerating voltage of 8 kV. The spectra were obtained using an ultraviolet galvanometer recorder with the instrument operating in a decreasing scan mode at a rate of 34 sec per decade of mass. The total ion current and source pressure varied with each sa m pl e. Antioxidants and Composition of Vulcanizates. Table I shows the sixteen antioxidants studied. The vulcanizates contained 100 parts of rubber, 50 parts of carbon black, 30 parts of a n aromatic extending oil, a total of several parts of zinc oxide, sulfur, stearic acid, and an accelerator for the vulcanization process, and one part antioxidant. The extending oil was a common type consisting of both paraffinic and aromatic fractions. In addition to one of the above antioxidants in each vulcanizate, di-teit-butylparacresol (DBPC) was initially added to the rubber to protect it during storage prior to being compounded.
(1) C. L. Hilton, Rubber Age ( N . Y.), 84, 263 (1958). ( 2 ) J. W. H. Zijp, R e d . T r m . Cliim. Pays-Bas, 76, 313 (1957); C A , 51, 13656 bd. (3) Zbid., 77, 129 (1958); C A , 52, 132991. (4) J. H. van der Newt and A. C. Maagdenberg, Plastics, 30, 66
(1966). ( 5 ) C. L. Hilton, ANAL.CHEM., 32, 383 (1960). (6) H. P. Burchfield and J. N. Judy, ihid., 19,786 (1947).
(7) A. Fiorenza, G. Bonomi, and R. Piocentini, Rubber. Chem. Techno/., 36, 1119 (1963). (8) R. W. Wise and A. B. Sullivan, Rubber. Age ( N . Y.), 91, 773 (1962). (9) L. J. Gaeta, E. W. Schlueter, and A. G. Altenau, ihid., 101 (3), 47 (1969). (10) R. H. Campbell and E. J. Young, ihid., 100 (3), 71-75 (1968). 1874
Table I. Trade name A02246 AgeRite Alba AgeRite Superlite AgeRite HP AgeRite Resin AgeRite Resin D AgeRite Superflex Stabilite Alba AgeRite Stalite S Santoflex 66 Santoflex DD PBNA Santoflex 13 DPPD Wingstay S DBPC
Antioxidants Studied Antioxidant 2,2'-Methylene-bis-(4-methyl-6-tert-butyl phenol) Hydroquinone monobenzyl ether A reaction product of isobutylene and bisphenol A Blend-65 % phenyl-P-naphthylamine and 35 % diphenyl p-phenylenediamine Aldol-or-naphthylamine Polymerized 1,2-dihydro-2,2,4-trimethylquinoline Reaction product of acetone and diphenylamine Di-o-tolylethylenediamine Mixture of alkylated diphenylamines N-phenyl-N'-cyclohexyl-p-phenylenediamine 6-Dodecyl-1,2-dihydro-2,2,4-trimethylquinoline N-phenyl-0-naphthylamine N-phenyl-N'-sec-hexyl-p-phenylenediamine N,N'-diphenyl-p-phenylenediamine Styrenated phenol 2,6-Di-rert-butylparacresol
Procedure. The vulcanizate is finely divided by passing it through a Wiley mill. Ten grams of the vulcanizate are shaken with about 50 ml of acetonitrile for 30 minutes. The mixture is filtered and the filtrate evaporated to about 20 ml on a 60 "C hot plate. The remaining solution is cooled to -20 "C for 2 to 3 hours. Most of the small amount of oil that was initially extracted with acetonitrile collects on the bottom of the vessel. The mixture is decanted and the filtrate evaporated to dryness on a 60 "C hot plate. The residue that remains after evaporation is very concentrated in antioxidant. The residue can be used directly for mass spectral and infrared analyses. For mass spectrometry, the sample is injected into the ion source using the insertion probe while for infrared, a film is prepared on a NaCl plate. RESULTS AND DISCUSSION
Figure 1 shows the N M R spectrum of Stabilite Alba as well as the N M R spectrum of the residue from the acetonitrile extraction of a vulcanizate containing Stabilite Alba. The signals from the antioxidant are readily seen in the spectrum of the residue. Even the signal from the rert-butyl group in DBPC, the antioxidant used to stabilize the rubber before compounding, is seen at 1.4 ppm (6). Therefore, the acetonitrile extraction removed both the compounding and polymer antioxidants. Even though the initial ratio of oil t o antioxidant is about 30 to 1 in the vulcanizates, the acetonitrile extract has approximately equal amounts of antioxidant and oil. This is the result of the antioxidant having a much greater solubility in the acetonitrile than the oil does. The relatively low concentration of oil in the acetonitrile extract is shown by the small signal at 1.2 ppm (6) which is due t o the protons of the methylene groups in the paraffinic fraction of the oil. Figure 2 shows the N M R spectrum of the same oil that was present in the vulcanizate.
ANALYTICAL CHEMISTRY, VOL. 43, NO. 13, NOVEMBER 1971
The choice of solvent for N M R analysis is important. Carbon disulfide and deuterated acetonitrile were used in this work. If the acetonitrile extract contained more than one antioxidant, the improper choice of solvent may lead t o selective dissolution of the antioxidants. Comparison of the unknown and reference N M R spectra should only be done if both spectra were obtained using the same solvent. The N M R spectra of antioxidants will change in different solvents. Films of the acetonitrile extracts were prepared o n sodium chloride plates for infrared analysis. The dried extract was usually greasy o r oily and made film preparation very easy. Figure 3 shows the infrared spectrum of Santoflex 13 as well as the infrared spectrum of the residue from the acetonitrile extraction of a vulcanizate containing Santoflex 13. The peaks of the antioxidant are easily seen in the infrared spectrum of the acetonitrile extract. Peaks from DBPC are masked by the compounding antioxidant but the 0-H band at 2.75 microns is apparent. Figure 4 shows a n infrared spectrum of the same oil that was present in the vulcanizate. Comparison of the infrared spectrum of the oil with the acetonitrile extract (Figure 3) shows that very little oil is seen in the infrared spectrum of the acetonitrile extract. The mass spectra of 70 eV of PBNA and the acetonitrile extract of a vulcanizate containing PBNA are shown in Figure 5 . The parent ion of PBNA at m/e 219 can be easily seen. The parent ion of DBPC a t m/e 220 and the 205 peak are also seen. Generally the small amount of oil in the acetonitrile extract causes more interference in the mass spectra than in the N M R and infrared spectra. This is the result of the fragmentation patterns of oils. Oils have large peaks every 14 mass units due t o the loss of methylene groups. Figure 6 shows the mass spectrum of the oil that was present in the vulcanizate. Even though there is some interference from the oil in the mass spectra of the acetonitrile extracts, all the antioxidants studied could be seen. Low voltage mass spectra are very useful in distinguishing between antioxidant and oil signals (11). Another technique, that is very fast, is t o insert the vulcanizate directly into the ion source of the mass spectrometer (11).
=PPMs
3
2
1
I
Figure 1. NMR spectra of Stabilite Alba (upper spectrum) and acetonitrile extract of a vulcanizate containing Stabilite Alba (lower spectrum)
I
(11) M. W. Hayes and A. G. Altenau, Riihber Age ( N . Y.), 102, (5), 59 (1970).
6
7
*
b
JPPM,
a
1
1
Figure 2. NMR spectrum of oil used in vulcanizates
/-
Figure 4. Infrared spectrum of oil used in vulcanizates
Figure 5. Mass spectra of PBNA (upper spectrum) and acetonitrile extract of a vulcanizate containing PBNA (lower spectrum)
0 /I,/!,I lllll ,,,,,,l/I,l, I/ l ' , ' " ~ I
1 0
200
Figure 6. Mass spectrum of oil used in vulcanizates Although the mass spectrum is due mostly t o the oil, peaks from the antioxidants can often be observed. It is useful t o run mass spectra at different ionizing voltages for better confirmation of the antioxidant signals. Spectra at low ionization tend t o enhance the parent or larger fragments of the antioxidants. Of course, with base polymers, there is no oil t o interfere with the antioxidant peaks. Acetone is a commonly used solvent in the rubber industry for extracting oil and antioxidants from vulcanizates. All of the oil and antioxidant is removed with acetone. The high concentration of oil relative t o the antioxidant in the acetone extracts masks most of the aromatic and aliphatic regions in the N M R spectra and all of the important infrared wavelengths used to identify the antioxidants. Mass spectrometry, being a very sensitive technique, is able t o detect the antioxi1876
dant o n many occasions in the acetone extract. The mass spectra of the acetone extract are almost entirely due t o the oil but with the use of different ionizing voltages, mass spectrometry does a reasonable j o b in picking out the fragments due to the antioxidant. All of the sixteen antioxidants studied were extracted by acetonitrile and detected by N M R , mass spectrometry, and infrared. The response seen in the N M R , infrared, and mass spectra would indicate that as low as 0.5 part of antioxidant per one hundred parts of rubber in a vulcanizate could be detected. Restated in terms of the entire compounding recipe, as low as 0.5 part of antioxidant per 250 parts of compounded stock can be detected. We have found that methyl alcohol behaves much in the same way as acetonitrile but tends to extract more oil which interferes with the identification of the antioxidants. Polar solvents selectively extract antioxidants relative t o the oil. It is difficult t o say which method, N M R , infrared, o r ma% spectrometry, is most valuable for identifying antioxidants. It is our experience that all three instruments are generally necessary t o identify a completely unknown antioxidant. NMR, infrared, and mass spectrometry provide structural information but each method emphasizes different structural features. N M R gives the ratio of aromatic to aliphatic protons, infrared supplies functional group analysis, and mass spectrometry provides molecular weight data. RECEIVED for review May 5, 1971. Accepted July 2, 1971. The authors thank The Firestone Tire and Rubber Company for permission t o publish this work.
ANALYTICAL CHEMISTRY, VOL. 43, NO. 13, NOVEMBER 1 9 7 1
