Carbon dioxide-based polymers: Turning carbon emissions into plastic

Carbon dioxide as a problem:

The soaring carbon dioxide (CO₂) concentration in the atmosphere as a result of fossil fuel (coal, gas and oil) consumption is the principal reason behind climate change; or the climate crisis. Since CO₂ is effective at blocking heat from escaping the planet, high CO₂ concentrations, currently (as of January 2020) above 410 parts per million, a historic high, have led to a global temperature increase with devastating effects on the environment and human well-being. 

Although CO₂ can be removed through natural processes, including uptake by water and land masses, the current rate of CO₂ emission outpaces that of natural elimination. Increasing public dissatisfaction and institutional pressure are forcing governments around the world to take action in curbing CO₂ emissions and harnessing its abundance.   

Carbon dioxide as resource:

Despite the Kyoto and, more recently, Paris agreements to combat climate change, the growing demand for energy makes the complete eradication of fossil fuels as an energy source unlikely in the foreseeable future. Alternatively, CO₂ can be captured, stored and converted into a precursor for value-added products.

As illustrated in Figure 1, a broad range of attractive applications could utilize CO₂; in fact, today, approximately 230 million tonnes of CO₂ are used annually, primarily for fertilizer production and oil recovery wherein it is used directly. While encouraging,  such consumption constitutes only approximately 0.7 % of present-day CO₂ emissions.  

Due  to the inert nature of CO₂, its conversion routes are typically energy-intense and rather inefficient. 

Carbon dioxide as a source for polymers:

However, as more effective conversion processes continue to emerge, the interest in using CO₂ for the production of chemicals and polymers increases. Polymers, including plastics such as low-density polyethylene and polypropylene, are one of the most widely used materials, and their main ingredients are traditionally obtained from petroleum, itself a finite resource. Furthermore, oil refinement – a necessary process for obtaining the principal compounds for polymers – is highly energy-intense, and thus, greatly contributes to CO₂ emissions.

Alternatively, CO₂ is abundant and can be either used for obtaining chemical intermediaries, such as methanol, for polymer production or directly by partially replacing fossil fuel derived ingredients. In the former case, the high demand for electricity makes the conversion process rather expensive; however, as the price for the production of electrical energy from renewable sources decreases, this route is gaining greater traction. Conversely, direct CO₂ utilization renders the production process more commercially viable, and a number of companies have started to capitalize on this business opportunity.

Carbon dioxide-based polymers:

Direct combination of CO₂ with oxygen-containing, ring-like molecules called cyclic ethers yields linear chain polycarbonates (L-PCs), a polymer family distinguished by some outstanding properties. Although mechanically inferior to and less thermally stable than conventional polycarbonates, L-PCs are biodegradable and exhibit excellent gas barrier properties, thus becoming attractive for packing applications.

The mediocre properties of L-PCs can be improved through blending them with biopolymers, such as starch, or synthetic polymers, such as poly(methyl methacrylate), wherein L-PC imparts biodegradability or acts as an important additive, respectively. In addition, L-PCs can be processed using traditional processing techniques, such as injection molding or melt extrusion, thereby making industrial-scale production possible. Furthermore, L-PCs can be used to produce traditional, superior, bisphenol A-based polycarbonates via ‘greener’, toxic phosgene-free, routes. Lastly, up to half of the mass of L-PC can be comprised of CO₂.

As of now, however, L-PCs are mainly used for the production of polyols, chemical compounds for poly(urethane) manufacturing. Other possible end use applications of L-PC include electronics, mulch films, foams, and in the biomedical and healthcare sectors. A few examples of commercially available CO₂-based polymeric products are given in Table 1.

As mentioned previously, CO₂ can also be used to yield chemical compounds for polymer production. Accordingly, this opens up the possibility of obtaining a range of thermosetting polymers, such as urea-formaldehyde (UF) and melamine-formaldehyde (MF) resins, as well as engineering plastics, such as poly(oxymethylene) or poly(methyl methacrylate). In the former case, the UF resin can be obtained from urea, which is directly produced from reacting CO₂ with ammonia, and formaldehyde, which is obtained from CO₂-derived methanol; similarly, the ingredients for MF resins are melamine, obtained from urea, and formaldehyde. In the latter case, POM may be produced from CO₂ via formic acid, and PMMA – from methyl acrylate obtained from CO₂-derived methanol.  

Table 1. Some of the companies, CO₂-based polymers and their main uses

Summary

Due to environmental concerns, abundance and desirable properties, CO₂ is emerging as an alternative for fossil fuel-based chemical compounds for polymer manufacturing. It can either be utilized directly to yield CO₂-based polymers, most notably biodegradable linear chain polycarbonates, or indirectly, through the production of chemical precursors, such as methanol, for polymerization reactions. Since CO₂ conversion technologies and their applications are in their early stages, the full potential of harnessing CO₂ for polymerization or as precursor for polymer manufacturing has yet to be unlocked and presents an exciting business opportunity. 

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