Evaluation of Rubber Products by Terahertz Time-domain Spectroscopy Carbon Black Dispersion and Vulcanization State

The quality of rubber products is strongly affected by the dispersion of the filler material and the vulcanization state; however, it is difficult to rapidly and nondestructively evaluate these conditions by conventional techniques. In this study, we investigated these properties with the use of THz radiation. Our examination of the THz spectroscopic characteristics of ingredients used in rubber products revealed that a typical filler material, carbon black (CB), has an extremely large apparent THz absorbance compared with those of other materials owing to the metal-like properties of CB. Through the use of this strong THz absorbance, the CB dispersion in rubber products was simply determined. We also evaluated a ruptured specimen from tensile tests. Through the use of the THz technique, we visualized a poorer CB dispersion at the ruptured section. We collected data on the crosslink densities in the rubber products created by vulcanization and were able to accurately evaluate their curing condition. We show that the THz absorbance was proportional to the crosslink density enabling rapid and nondestructive evaluation of the curing state.