The widely used plastic polyethylene terephthalate (PET) can be depolymerized by reacting it with sodium hydroxide (NaOH) under strong mechanical impact from metal spheres in a ball mill. This type of mechanochemical route to recycling could alleviate some of the challenges with current PET depolymerization, including solubility and separations difficulties, as well as the need for homogeneous catalysts. But the complexity of the reaction environments has hindered optimization of mechanochemical recycling inside ball mills.
Research by a team at the Georgia Institute of Technology (GT; Atlanta, Ga.; www.gatech.edu) has revealed insights about the mechanisms of the impact-driven depolymerization and has provided a path for optimizing large-scale depolymerization processes using mechanical impacts.
To uncover information about the precise effects of the impacts on the polymer, the GT team examined controlled single impacts using three spatially resolved analytical measurements, and then carried out material-point-method (MPM) simulations.
Single impacts of a ball on a polymer film were measured with focused ion-beam microscopy, Raman spectro-microscopy and small-angle X-ray scattering. “These measurements highlight the contributions of plastic deformation, amorphization and depolymerization during the transfer of kinetic energy in collisions relevant to ball mills, and will enable reactor models based on fundamental kinetics,” the researchers, led by professor Carsten Sievers and postdoctoral scientist Kinga Golabek, write.
Each collision creates a tiny crater in the polymer, with the center absorbing the most energy. In this zone, the plastic stretched, cracked and even softened slightly, the researchers say, creating ideal conditions for chemical reactions with sodium hydroxide.
The GT team also used MPM, a numerical method useful for modeling events like impacts, to map how the energy from collisions distributes across the plastic and triggers chemical and structural transformations. These types of simulations “enable bottom-up modeling of these processes across different scales,” the researchers say.
The suite of experiments showed changes in structure and chemistry of PET in tiny zones that experience different pressures and heat. By mapping these transformations, the team gained new insights into how mechanical energy can trigger rapid, efficient chemical reactions.
The study also showed the importance of the energy threshold for impacts strong enough to cause cracks and plastic deformation, and to expose new surfaces to sodium hydroxide for rapid chemical breakdown. “Understanding this energy threshold allows engineers to maximize efficiency while minimizing unnecessary energy use,” Sievers explained.