(41a) Production of Renewable Styrene Using Catalytic Microwave Depolymerization

According to the EPA [1], less than 10 % of the plastic produced in the USA is recycled. The principal reason is the incapacity to reprocess the plastic waste in such way that the output can be reused into virgin plastic applications, such as production of food and beverage containers and other consumer goods. Although technologies exist for polyethylene terephthalate (PET) where it can be reintroduced into some FDA compliant applications [2], it is not the case for most of other post-consumer plastics such as polystyrene, especially when they are contaminated, e.g., organic materials and paints. The present paper shows how catalytic microwave depolymerization (CMD) can be applied at commercial scale for recycling post-consumer contaminated polystyrene and reuse them in food packaging applications. The mechanism of action is simple: the use of CMD breaks down the polystyrene into monomers that can then be reintroduced in the existing ecosystem of refining and polymerization industries. One of the differences of CMD from other conventional pyrolysis processes is the heating rate which is faster because energy is transferred on a volumetric basis, whereas other conventional approaches are slower because the heating occurs on a conduction/convection basis. The decomposition step also allows handling of a broader amount of contaminants [3] compared to mechanical/physical technologies.

This paper discusses the change in the heating mechanism inferred by the use of microwaves and how it affects the heating profile and the kinetic of decomposition. Results show that the Arrhenius ratio E_­A­/RT in a microwave environment is lower by a factor of 4 to 6 compared to the same ratio obtained by conventional conductive/convective heating. Where there are no explanations that can support a change in activation energy E_­A, which is the normal way to approach kinetic analysis, this paper investigates the possibility that the local temperature might be significantly higher to explain a change in this ratio.

The phenomenon is approached by considering resonance between the microwave electrical field and the molecular structure of the polymers being decomposed. This resonance is believed to increase the molecular kinetic energy of the polymer molecules and to increase its molecular energy. This increased level of energy is believed to make it more susceptible to break chemical bonds and generate smaller sets of molecules. These newly formed smaller molecules would also absorb some of the energy through resonance with the microwave field and break apart into even smaller molecules. The production of rather high levels of monomers, less amounts of dimers and trimers, and almost no chains of monomers longer than three molecules also suggest that the absorption of the microwave energy might have something to do with the molecular weight. The long polymer chains would absorb microwave energy to break into smaller monomer chains up to a point that the small chains no longer resonate with the electrical field and remain only subject to conduction and convection heating.

To support this assumption, the absorption of microwave energy was investigated through the analysis of dielectric properties, which is a measure of how much of the absorbed microwave energy is converted into heat. The dielectric properties of the material were investigated as a function of temperature and molecular weight. The results show that the dielectric properties of the polymer material change as a function of both the temperature and the molecular weight and that the dielectric loss tangent can vary up to a factor 20. This supports the claim that at higher temperature and depending on the polymer chains length, the material absorbs more energy over a wider range of modes which increases its kinetic energy (local temperature) and initiates a rapid decomposition. This is considered as a theory to support the variation in product yields obtained with CMD over polystyrene feedstock versus the yields obtained using conductive/convective heating. CMD produces small amounts of trimers, more dimers than trimers and predominantly styrene monomer, whereas conventional heating produces predominantly ethylbenzene and other aromatics.

1. U.S. Environmental Protection Agency (EPA). Advancing Sustainable Materials Management: Facts and Figures Report. 2015

2. U.S. Food and Drug Administration (FDA). Guidance for Industry: Use of Recycled Plastics in Food Packaging: Chemistry Considerations. 2006

3. J. Doucet, J.-P. Laviolette, S. Farag and J. Chaouki. Distributed Microwave Pyrolysis of Domestic Waste. Waste and Biomass Valorization. 2013;5(1):1-10