Due to the chemical stability and durability of industrial polymers, plastic waste does not easily degrade in landfills and is often burned, which produces carbon dioxide and other hazardous gases. In order to stop the growing flood of polymer waste and reduce carbon dioxide emissions, plastics have to be recycled or converted into new value-added products.
Currently, recycling of the vast majority of plastics is not economically feasible; their sorting and separation are time- and labor-intensive, while chemical processing and remanufacturing requires a significant energy input and toxic solvents. Re-processed polymers often show inferior performance to that of the freshly manufactured materials.
Scientists used processing by ball-milling to deconstruct commercial polystyrene in a single step at room temperature, in ambient atmosphere, and in the absence of harmful solvents. Ball-milling is a technique that places materials in a milling vial with metal ball bearings that is then agitated until a desired chemical reaction occurs. Called mechanochemistry, this experimental approach has numerous applications in new materials synthesis and attractive features where plastics recycling is concerned.
The deconstruction of polystyrene proceeds through a series of chemical events involving mechanical cutting apart of the macromolecules, which generates free radicals detectable in the milled material, even after its prolonged exposure to air. The metal bearings used for milling and the ambient oxygen act as co-catalysts that enable extraction of the monomeric styrene from the oligomeric radical-bearing species formed.
The experiments showed that the temperature rise in the material during milling is not responsible for the observed phenomenon, since the temperature inside the milled powder does not exceed 50 °C, while the thermal decomposition of polystyrene in air starts at about 325 °C. The team confirmed the comprehensive deconstruction of the original polymer into smaller fragments — oligomeric materials — suitable for further processing into new value-added products.
This method represents an important breakthrough that enables dismantling of a polymer simultaneously with its breakdown under ambient conditions; that is, ~300 °C below the thermal decomposition temperature of the pristine material. The discovery opens new avenues for low-temperature recovery of monomers from multicomponent polymer-based systems such as composites and laminates. Also, the technology will allow extracting the monomer from crosslinked materials containing styrene units in their structures.