The amount of waste tires is increasing every year, and their reuse as recycled materials is an important issue from the perspective of environmental problems and resource conservation. This article summarizes the technology for recycling waste tires using supercritical carbon dioxide (ScCO₂ ) as the reaction medium. This technology allows diphenyl disulfide, the cleavage reagent, to efficiently penetrate into the network of rubber molecules and selectively cleave the cross-linking points formed by sulfur. The recycled materials, which were blend of waste truck tire rubber and virgin rubber, exhibited relatively good properties, although not as good as the completely virgin material. This technology was developed for natural rubber, and then applied to other synthetic rubbers. This technique was further developed as a method to crush tires by injecting a jet of ScCO₂ and then cleaving the cross-link points.

Continuous devulcanization technology for vulcanized rubber has the potential to greatly contribute to reducing environmental impact and realizing a circular economy. The continuous devulcanization process using a twin-screw extruder effciently devulcanizes vulcanized rubber by applying high internal pressure and shear strain to the vulcanized rubber, and the properties of the devulcanized rubber can be controlled by the reaction temperature, screw rotation speed, and the addition of devulcanizing agents and process oils. Devulcanized rubber can be re-vulcanized and has mechanical properties almost equivalent to those of virgin vulcanized rubber, making it possible to manufacture high-quality recycled rubber products. [br]In 2023, automotive rubber parts using this technology obtained the "ISCC PLUS Certifcation." This certifcation certifes the use of sustainable raw materials, making it possible to offer them to the market as products with high environmental value.

Several physical phenomena called “mysteries of physical properties of rubber” are observed in real cross-linked rubber, which are quite difficult to interpret reasonably using the conventional rubber elasticity theory and molecular network structure model, such as,
①Why the frictional coefficient of cross-linked rubber is very high (µ=1 ~ 3) compared with other materials (μ=0.2 ~ 0.3) ?
②Why Poisson’s ratio of cross-linked rubber is almost 0.5 ?
③Why hysteresis loss of cross-linked rubber becomes larger significantly at a large extension ?
④Why fracture properties of cross-linked rubber drops down extremely by swelling with solvents ?
⑤Why the edge of notch of cross-linked rubber is not sharp but round in shape under large extension ?
In this final discussion, the gel-network structure model could answer to these mysterious phenomena in cross-linked rubber almost reasonably depending on its characteristic structure and properties. Thus, now we might be able to understand the relationship between structure and the properties of cross-linked rubber in a comprehensive and systematic fashion.