Characterization of Cross-Linked Elastomer Networks by Thermal Desorption/Pyrolysis Direct Analysis in Real-Time Mass Spectrometry (TDPy-DART-MS)

Tires are complex polymeric materials composed of rubber elastomers (both natural and synthetic), fillers, steel wire, textiles, and a range of antioxidant and curing systems. These constituents are distributed differently among the various tire parts, which are classified based on their function and proximity to the rim. This study presents a rapid and sensitive approach for the characterization of tire components using mild thermal desorption/pyrolysis (TDPy) coupled to direct analysis in real-time mass spectrometry (DART-MS). This ambient technique was applied to five important tire parts: the tread, sidewall, inner liner, body plies, and bead chafer, to identify structural similarities and differences among their constituents through combined DART-MS and TDPy-DART-MS analysis, tandem mass spectrometry (MS/MS) fragmentation, and size/shape assessment via trapped ion mobility (IM) spectrometry of select ions produced in the DART source. The DART ion source was maintained at 300 °C, while the TDPy temperature was gradually raised from ambient to 600 °C. At low TDPy temperatures (100-250 °C), phthalate plasticizers such as hexadecyl octyl oxalate gave rise to the most abundant ions. At medium and high temperatures (250-550 °C), the dominant ions arose from the phenylenediamine antiozonants C-PPD and 6-PPD, as well as important additives and processing aides crucial to the tire's health and longevity such as antioxidants, dispersants, vulcanization accelerators, lubricants, sealants, and cross-linking agents. For identification of the tire backbone polymer, experiments were run at the highest TDPy temperature possible (550-600 °C) to promote thermal degradation of the cross-linked rubber network; all parts of the tire generated styrene butadiene oligomers except for the inner liner, where poly(isobutylene) chains were the dominant product.