Identification of the Compounds Used in Waste Bottle Caps Using Terahertz Radiation for Sustainable Resource Recycling to Benefit International Cooperation

In Japan, when people take their waste bottle caps to designated recycling companies, JPY 5/kg is donated to developing countries for polio vaccine dissemination activities. The waste volume of plastic bottles and caps is increasing every year, and there is not a day that goes by that we do not see more bottles and caps. PET bottles are more easily recycled, as the bottle body is made of a single material. However, bottle caps are made of two different materials (PP and PE), which are difficult to identify perfectly when utilising the existing NIR technology in the recycling field. This is because the designs and colours of PP and PE bottle caps are becoming more diverse, with black and dark-coloured caps being difficult to sort using NIR technology. In addition, their specific gravity is almost the same; therefore, it is difficult to separate them by their water-based specific gravity, which is an approach commonly used by recyclers. Unidentified caps are sent for thermal recycling, which runs counter to the need for decarbonisation. They may also be mixed with identified caps and sold as recycled raw material, resulting in a reduction in the purity of the recycled plastic. If PP and PE caps can be identified using simple technology, it is expected to promote high resource recycling. Therefore, in order to develop such a sorting process, this study proposes a new method for the material identification of waste caps using terahertz waves. Therefore, this research aims to identify a new identification method for waste plastic caps, which has been a challenge both domestically and internationally. Specifically, we describe the limitations of the conventional methods for identifying PP and PE bottle caps according to their weight, specific gravity, and melting point and propose an identification method using terahertz waves, demonstrating its measurement-related merits and high identification accuracy.