Quantification of the Vulcanizing System of Rubber in Industrial Tire Rubber Production by Laser-Induced Breakdown Spectroscopy (LIBS).

The properties of natural and synthetic rubber critically depend on the concentration of the vulcanizing system, among others. Sulfur and zinc oxide are typically used as cross-linking and activating agents for the vulcanization reaction (0-3 wt %). We present an advanced spectroscopic method to chemically analyze the vulcanizing system in rubber under ambient conditions, and we demonstrate a novel application to measure the elements in-line of industrial rubber production. The laser-induced breakdown spectroscopy (LIBS) technique is optimized to ablate material from the surface of produced rubber sheets and to measure the optical emission of S and Zn from the rubber plasma in air. The sulfur lines in the near-infrared range are masked by molecular emission bands of the C-N radical and spectrally interfered by atomic lines of O. Plasma excitation in collinear double-pulse geometry and detection of plasma emission with time-gated detectors suppresses the spectroscopic overlays and enables to resolve the sulfur lines. For the determination of ZnO the weak Zn lines in the ultraviolet range are measured due to their superior intensity stability compared to the much stronger lines in the deeper UV. S and ZnO are quantified in three different rubber materials prepared from the most important polymers used in rubber production. The mean error of prediction of concentrations RMSEP is ≤0.07 wt % for S and ≤0.33 wt % for ZnO for all polymer types. Our results demonstrate that the vulcanizing system of rubber can be quantified under ambient conditions with LIBS. Other chemical elements could be analyzed also and the rubber production could be controlled employing this multielement detection technique as process analytical sensor.