Mass Spectrometric Identification of 2-Mercaptobenzothiazole Sulfenamide Accelerators in Rubber Vulcanizates

Abstract

The identification of accelerators in rubber vulcanizates has traditionally been a difficult and tedious task. The mass spectrometric method described in this paper offers a simple and expedient approach to the identification of 2-mercaptobenzothiazole sulfenamides. Neither complicated sample preparation nor solvent extraction of the vulcanizates is required as in the more common methods employed. Only about one-half gram of vulcanizate is needed for the analysis. The sample tube is attached to the mass spectrometer, heated for 15–20 min at 150–200° C and then the mass spectrum of the volatiles is recorded. The characteristic amine fragment ions are utilized for identification of the amine portion while the presence of benzothiazole is indicated by a peak at m/e=135. Spectra obtained from analysis of seven vulcanizates containing different accelerators are presented and the results discussed. The identification of the accelerator in a competitor's tread is also discussed.

Identification of organic additives in rubber vulcanizates using mass spectrometry

“…The (M + H)+ ion (m/z 88) for morpholine (from OBTS) was readily observed by CI-MS from vulcanízate B (Figure 5), and ( + H)+ for dimethylamine (m/z 46) was observed by CI-MS from vulcanízate C (Figure 14). Ions due to benzothiazole are sometimes detected from sulfenamide accelerators in vulcanizates (13), but these ions were not prominent in our analyses of compounds A and B, which contained OBTS. Finally, we note that fragments from the secondary accelerator diphenylguanidine were not detected in mass spectra from vulcanízate B.…”

“…An interesting feature is the m/z 88 ion, which is ( + H)+ for morpholine, a decomposition product from the accelerator OBTS. Sulfenamide accelerators decompose during vulcanization to yield, among other products, an amine plus benzothiazole (13). Accelerator fragments are volatile and are thus more intense in earlier mass scans (for which the sample temperature is low).…”

“…Although the oligomers desorb quite well by FD analysis (Figure 15) (17), their intensities in spectra from vulcanizates are generally low compared to other organic components (particularly stabilizers, oil, and wax). Since accelerators and other curatives decompose during the cross-linking process (13), one does not usually find the original (intact) accelerator in the vulcanízate. Another general problem in detecting curatives in vulcanizates is that accelerator fragments (and most aromatic amines) can become bound to the rubber during processing and aging (18)(19)(20).…”

“…Another approach is to heat the vulcanízate in a closed vessel at constant temperature, then to introduce the evolved vapors into the mass spectrometer ion source via a heated molecular leak. Hilton and Altenau detected sulfenamide accelerator fragments from rubber vulcanizates in this manner (13). While all of the above studies used El ionization, softer ionization methods (chemical ionization, CI-MS, and field ionization, FI-MS) can be used to obtain spectra that are less complex, and thus easier to interpret (2).…”

Rubber

“…Other peaks are characteristic of the amine part of the accelerator. Extender oils do not interfere (153).…”