Mechanocatalytic Upcycling of Waste Plastics

Mechanochemical recycling uses collisions in ball mills to create reactive environments within solid plastic feedstocks. Carsten Sievers examines how these environments enable depolymerization and where they impose kinetic and thermodynamic limits.

For PET milled with sodium hydroxide, reaction rates depend on interfaces between reactants. Raman microscopy, focused-ion-beam transmission electron microscopy, small-angle X-ray scattering and material-point simulations characterize individual impact craters. Amorphous regions react above a minimum impact force, whereas crystalline regions require impacts exceeding PET’s yield point.

Milling polystyrene produces styrene at steady rate and selectivity alongside minor products, including oxygenates. Removing monomer continuously prevents repolymerization, while thermodynamic analysis identifies limits on olefin release from chain-end radicals and motivates more energetic, longer-lived reaction environments. Coupling chain cleavage to hydrogenation or oxidation provides another driving force. An example uses hydrogen peroxide and an iron-oxide catalyst during polyethylene milling, producing oxidation at positions along the backbone followed by carbon-carbon bond cleavage.