Use of Variable Frequency Microwave Heating to Chemically Recycle Polymer Composites

Microwave heating is advantageous for the chemical recycling of plastics due to its rapid, selective, and volumetric heating capabilities that can help reduce process times and improve energy efficiency. Unfortunately, the use of fixed frequency microwave heating leads to strict operational limitations that promote two popular approaches for processing post-consumer plastics: 1- Solid state pyrolysis with strong microwave absorbers to recover oils, gases, and carbon, and 2- solvolysis of the polymer to recover monomeric derivatives. While solvolysis recovers high-value monomers, the use of solvents reduces its energy efficiency compared to pyrolysis. Our group investigated the use of variable frequency microwave (VFM) heating to achieve rapid thermal depolymerization of polymer composites in their solid states back to their corresponding monomers, including polyphthalaldehyde, polyhydroxyalkanoates, poly(propylene carbonate), and nylon-6. The degradation of the polymers with the additives has been achieved and confirmed by nuclear magnetic resonance and gel permeation chromatography. A wide range of additives was also investigated to select the best performers for microwave heating at low energy. Carbon nanotubes, carbon nanofibers, carbon black, graphene, and reduced graphene oxide were down-selected for further investigation by incorporating the materials into different polymer matrices. Overall, the use of VFM energy aids in the volumetric heating and selective depolymerization of the polymer composites at fast heating rates. The results of these VFM studies can be broadly applied to the chemical recycling and monomer recovery of polymers.