Inspection of internal filler alignment in visibly opaque carbon-black–rubber composites by terahertz polarization spectroscopy in reflection mode

As the industrial utilization of polymeric materials is increasing owing to recent improvements in material durability and functionality, product safety becomes an important issue. Therefore, various inspection methods for polymeric materials are being developed. In this work, the rapid and non-destructive inspection of the averaged internal orientation of conductive carbon black fillers in rubber composites is demonstrated using a handmade high-speed terahertz polarization spectroscopy system that employs reflection geometry. From the birefringent information obtained from the black rubber sample (with arbitrary stretching condition), the internal orientation of the conductive fillers, which plays an essential role in the properties of the rubber composite, was determined. In addition, since data acquisition with the reflection method is fast, the spatially resolved measurement of the filler orientation could also be performed. The information of the internal orientation of the carbon black fillers allows verification of the condition of the rubber product. •A novel methodology to reveal the internal filler condition of the black rubber with terahertz polarization reflection spectroscopy is provided.•The internal filler orientation is clearly determined through the birefringent properties.•By comparing the experiments with the model calculation, the validity of our inspection method is confirmed.•A developed high speed spectroscopy system enables to easily obtain the image.•Our novel method can measure most practical rubber products in-situ, nondestructively.