A research group led by Project Professor Haruro Ishitani from the Graduate School of Science and Project Professor Shu Kobayashi from the Organization for Interdisciplinary Research Projects at the University of Tokyo has demonstrated that polyethylene terephthalate (PET) resin processed by the bead mill method can be converted back into its starting materials, dimethyl terephthalate and ethylene glycol, at the low temperature of 90℃. Their findings suggest the possibility of fundamentally transforming conventional PET recycling methods that require high temperatures and large amounts of energy. The results were published in Chemical Science.
Plastic production continues to rise and is projected to reach one gigaton per year globally within 30 years, making the disposal of used plastics a serious social problem. In addition to material recycling, which recycles the material itself, there is also chemical recycling, a process that converts plastic back into its original raw materials or into other high value-added chemicals. Chemical recycling requires a depolymerization reaction to break down the polymer chains, but most plastics, including PET, do not dissolve in solvents, making it difficult to engage them in reactions as they are. Methods such as melting at high temperatures are therefore required to initiate chemical reactions, and this has been a barrier to the widespread adoption of chemical recycling.
Reducing plastic to submicron-sized particles is expected to result in increased surface area and enhance reactivity. However, most general-purpose plastics soften and melt due to heat during the physical pulverization process, then re-solidify upon cooling, making it virtually impossible to recover them as stable fine particles and rendering micronization impractical.
Most physical grinding methods place the target substance in a grinder and use the shear force and impact energy of cutters, hammers, or balls to pulverize it. The research group reasoned that grinding in a solvent—which could be expected to dissipate heat generated during the process and stabilize the fine particles—would be effective, and focused on the bead mill method, in which approximately two-millimeter-diameter beads are agitated at high speed in a solvent to pulverize the material.
After extensive research, the group found that performing the bead milling step prior to the depolymerization reaction dramatically accelerated the subsequent decomposition reaction (a methanolysis reaction) in the presence of an appropriate catalyst, enabling PET to decompose into its starting materials at temperatures as low as approximately 90℃. Notably, it was found that during the bead milling process, PET not only decreased in physical size but also underwent molecular chain scission, meaning that its chemical bonds were broken, resulting in smaller-molecular-weight polymers. Although researchers were aware of the phenomenon of chemical bond cleavage and molecular weight reduction in polymers during physical grinding, this study revealed that the phenomenon is closely linked to the presence of a solvent—in other words, it is a characteristic feature of wet pulverization using the bead mill.
The research group also developed a raw material recovery method that does not even require heating to 90℃, by using a circulation-type bead mill apparatus to carry out the pulverization and decomposition reaction simultaneously in a single step. The circulation-type approach is a processing method unique to solvent-based milling, in which the throughput is not limited by the size of the grinder. This method demonstrated the possibility of simultaneously resolving the challenge of scaling up, which had previously been a major obstacle.
Furthermore, the research group used the dimethyl terephthalate obtained through this raw material recovery process to synthesize tamibarotene, a pharmaceutical drug used in the treatment of acute promyelocytic leukemia, thereby presenting a vision of a circular society in which plastic used for beverages can also serve as a raw material for pharmaceuticals.
In Japan, PET recycling is well established and achieves a high recycling rate. However, the majority of this recycling is still energy recovery, and further innovation is needed to achieve a truly circular society. The technology developed in this study has potential applicability not only to PET but to many other types of plastic as well and is expected to contribute to the conversion of plastic waste into valuable resources.
Journal Information
Publication: Chemical Science
Title: Bead mill-driven acceleration in catalytic methanolysis reaction of poly(ethylene terephthalate) toward low-energy chemical recycling of polymers
DOI: 10.1039/d5sc06930k
This article has been translated by JST with permission from The Science News Ltd. (https://sci-news.co.jp/). Unauthorized reproduction of the article and photographs is prohibited.