Fully Recyclable Polycarbonates
Plastics are unavoidable. The majority of products are made with plastic. Plastics remarkable versatility, strength, and permanence has led to incredible breakthroughs in medical devices, food packaging, and more. But its properties come as a double-edged sword. Plastics can now be found in every part of the earth, even in the bloodstream of every human. Microplastics litter our fields, rivers, and oceans with indeterminate consequence.
Plastic was first hailed as a miracle product given its unique properties, especially it’s resiliency to biodegradation. There are a variety of different plastics, but they are commonly lightweight, durable, flexible, and cheap to produce. It can be formed into any shape or size to fit our needs. This is how plastic came to proliferate in our world. The overlooked issue with plastics was their resiliency. Ironically, what made them so enticing to use has become their downfall.
Plastics do not readily degrade in our environment. The world’s bacteria and fungus did not abundantly evolve to break apart these polymeric bonds. The few processes that do break down plastics are mostly mechanical, resulting in smaller and smaller bits of plastic. Each small bit of plastic grinds away until we are left with microplastics that can be transported into animals and people.
The concept of recycling has long been hailed as a steadfast way to alleviate these issues. Recycling can reduce production of new plastics and remove it from our rubbish. However, recycling cannot solve the issues of existing microplastics and leakage of plastics from recycling streams. Alternatively, plastics could be designed from renewable feedstocks with improved recyclability and lower toxicity.
Planning for plastics to end up in the environment can help us reduce the long-term impacts of plastic accumulation by designing for degradation. Selective scission of a polymer’s backbone can be accomplished via pH changes, UV light, or temperature changes. Biorenewable polymers can be designed to outperform petroleum-based products on degradation, but must also compete on cost and performance. Without these properties, it is unlikely that widespread usage will occur.
Biorenewable Polycarbonate Plastics
There are a variety of mechanisms for depolymerization in the environment. One new potential explored by teams at Los Alamos and The George Washington University is acetalization and ketalization of a diol and carbonyl .
The goal of this research was to produce plastics that could be easily deconstructed for recycling and/or have a route for decomposition in the environment. The base materials used were glycerol derivatives, epichlorohydrin and hydroxyacetone. Epichlorohydrin can be cost competitively produced via glycerol-to-epichlorohydrin (GTE) pathways instead of propylene as a base material. It is cost competitive given the abundance of affordable glycerol from growing biodiesel production. The monomer tested was made from reaction of hydroxyacetone and 1-benzyloxy-2,3-propanediol with p-toluenesulfonic acid to produce a ketal compound. This was subsequently debenzylated over palladium on carbon with hydrogen. If you are like me though it is much easier to look at the reaction scheme below.
Monomer 2 shown in Figure 2 was condensed with a mixture of diphenylcarbonate and 1 mol% potassium hydride while heating to 220 °C. A vacuum was slowly pulled during the experiment to remove phenol produced. The end product was a biorenewable polycarbonate with a molecular weight ranging from 2,000 up to 16,000 g/mol. Thermal properties, glass transition, and resistance to degradation via acids, room temperature polar protic solvents (MeOH/EtOH), and thermal decomposition were all tested. The polycarbonate did have a weak spot which is heating (50 – 100 °C) while mixed with methanol, ethanol, or neat phenol. This resulted in complete depolymerization.
To analyze the depolymerization products, see Figure 3, a GC-Polyarc-FID/MS was used. The Polyarc reactor flow path was located between the FID inlet and column splitter. It converted all analytes to methane, effectively normalizing the FID response per mole of carbon for each analyte. This benefited the analysis of products from transesterification and transketalization during depolymerization, since there could be a variety of methyl or ethyl groups. Not all variations of this can be readily calibrated to account for variability in the FID response. The conversion of the analytes to methane via the Polyarc reactor removes the FID variability to oxygen content.
Even though there was depolymerization, the core functional groups remained. The recovered monomers were able to be separated and reacted back into the same polycarbonate material. There appeared to be no difference in the polymer material and 95% of the monomer was recovered during depolymerization. No small molecules were detected. Small-oxygenated hydrocarbons (C3-) can have low sensitivity with FID alone relative to a Polyarc-FID. Without significant loss of the monomer 2, the base material can be perpetually converted to polymer 3 and recycled in a closed loop. A simulated mixed plastic feedstock was used with a yield of 60% pure monomer after a 65 °C methanol bath, filtering of insoluble plastics, and an ethyl acetate wash to remove the dissolved plasticizers. Further work was completed to determine health and environmental impacts. These mostly resulted in predicted properties that lacked health concern and bioaccumulation.
Glycerol based poly(carbonate acetal) can be readily synthesized at a 95% yield with existing infrastructure to produce a renewable plastic with similar physical properties and significantly reduced environmental impact. The polymer was designed to be easily recycled back to the base monomer, which was achieved with a >95% yield via heating. Recycled monomers and virgin monomers showed no significant difference in polymer thermal properties. Alternatively, if accidentally (or purposely littered), the polymer can be hydrolyzed into the initial building blocks with a low predicted chance of toxicity. Further research is required, but this is a strong proof-of-concept that we can produce a competitive biorenewable plastic. Petroleum products have long been overused for consumer products. It is long overdue to discover and implement new solutions for a greener tomorrow.
By Connor Beach, Technical Sales Engineer, Activated Research Company
Fully Recyclable Polycarbonates from Simple, Bio-Derived Building Blocks. Christopher D. Roland, Cameron M. Moore, Juan H. Leal, Troy A. Semelsberger, Charlotte Snyder, Jakub Kostal, and Andrew D. Sutton. ACS Applied Polymer Materials 2021 3 (2), 730-736. DOI: 10.1021/acsapm.0c01028
Image Adaptation by Connor Beach, Technical Sales Engineer, Activated Research Company