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Addressing the need to create robust feedstocks of post-use plastics, researchers at the UK’s University of Bath have developed a rapid chemical recycling process that breaks down polycarbonates within 20 minutes at room temperature.
The process uses a zinc-based catalyst and methanol to break down commercial poly(bisphenol A carbonate) beads completely. From there, waste can be converted into bisphenol A (BPA) and dimethyl carbonate (DMC). DMC is a “green” solvent and component of additional industrial chemicals. BPA, a potential environmental pollutant, is isolated and prevented from leaking.
Polycarbonates feature prominently in construction and engineering applications. The catalyst employed in this process is useful with multiple commercial sources of BPA-PC and mixed waste feeds, according to the team at the university’s Centre for Sustainable and Circular Technologies (CSCT). For instance, the process can break down PLA and PET, albeit at higher temperatures.
Ultimately, the CSCT’s chemical recycling process leads to the recovery of high-quality constituent components, asserted Jack Payne, first author of a recently published paper, in an exclusive interview with PlasticsToday.
“Following reaction completion, crude BPA is easily recoverable via removal of the volatile components,” he explained. “Pure BPA can then be obtained by recrystallization from water followed by drying. DMC should be easily recoverable via fractional distillation of the volatile components; this is necessary to separate it from the solvent, 2-Me-THF, which has a comparable boiling point.
“Following this, we expect both BPA and DMC to be of virgin quality and, therefore, suitable for repolymerization. Since fractional distillation is a standard industrial technique and the operating conditions are quite mild — between 80 and 90°C — I would expect most costs to be associated with drying the BPA to remove H2O.”
Next on the CSCT’s agenda is scaling up the process. Payne told PlasticsToday how a material recovery facility (MRF) might employ the process.
“Whilst we have shown our catalyst is tolerant to feed impurities, including polycarbonate composites and other plastics, other prevalent contaminants — for example, food and debris — have not been investigated,” Payne noted. “We would expect MRFs to practice sorting for high-purity feeds in the interest of preserving process efficiency. This input feed would then be ideally directed to an onsite batch reactor. However, we have shown PET remains unreacted under our reaction conditions. Therefore, it would be possible to have an input feed of polycarbonate and PET. After the reaction, the unreacted PET could be filtered, dried, and directed elsewhere.”
So, how much could the process scale up?
“Chemical recycling plants typically require an input feed of between 50,000 to 100,000 tons a year to be economically viable,” Payne said, “so we would anticipate comparable scale for our process. However, this relies on obtaining a constant and reliable input feed of polycarbonate waste, which is dependent on sufficient collection and sorting infrastructure — requiring significant capital investment at both a regional and national level.”
Assessing the cost-effectiveness of the CSCT’s chemical recycling process vis-à-vis mechanical recycling will require a comprehensive techno-economic analysis (TEA) and life cycle analysis (LCA), Payne added.
“At all stages, we were conscious of transferability from the lab to industry,” he explained. “For catalyst design, we selected a cheap and abundant metal (zinc) in combination with simple and scalable ligands to reduce costs associated with making the catalyst. We expect this, coupled with reduced energy intensity under ambient conditions, to make this process economically competitive with mechanical recycling. [Editor's note: Mechanical recycling requires temperatures between 230° and 260°C.]
“Additionally, if the TEA did flag higher initial upfront costs relative to mechanical recycling, our method enables the long-term retention of material value within the plastic economy, so it potentially could be more economically attractive after a pay-back period.”
The results were published in the journal ChemSusChem.
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