Bioplastics for a circular economy

Abstract

Bioplastics - typically plastics manufactured from bio-based polymers - stand to contribute to more sustainable commercial plastic life cycles as part of a circular economy, in which virgin polymers are made from renewable or recycled raw materials. Carbon-neutral energy is used for production and products are reused or recycled at their end of life (EOL). In this Review, we assess the advantages and challenges of bioplastics in transitioning towards a circular economy. Compared with fossil-based plastics, bio-based plastics can have a lower carbon footprint and exhibit advantageous materials properties; moreover, they can be compatible with existing recycling streams and some offer biodegradation as an EOL scenario if performed in controlled or predictable environments. However, these benefits can have trade-offs, including negative agricultural impacts, competition with food production, unclear EOL management and higher costs. Emerging chemical and biological methods can enable the 'upcycling' of increasing volumes of heterogeneous plastic and bioplastic waste into higher-quality materials. To guide converters and consumers in their purchasing choices, existing (bio)plastic identification standards and life cycle assessment guidelines need revision and homogenization. Furthermore, clear regulation and financial incentives remain essential to scale from niche polymers to large-scale bioplastic market applications with truly sustainable impact.

Keywords: Polymer chemistry; Polymers; Sustainability.

Fig. 1. The circular plastic economy.

The plastics industry has traditionally been based on linear life cycles (grey arrows): crude oil is cracked and refined into monomers and polymer products using fossil energy, which, at their end of life, are either disposed of (~80%) with potential environmental leakage, incinerated (~10%) or, in the minority of cases (10% globally), mechanically recycled into lower-grade products, which also end up landfilled^,,,,. In a ‘circular plastic economy’ (green arrows), plastic waste becomes raw material for a recycling process at its end of life, and all production and recycling processes are supplied with renewable energy^,,. Renewable resources (lignocellulosic biomass and pyrolysis oils) are the starting materials for polymer products, which all have a defined circular end-of-life scenario. CO₂ generated through bioplastic incineration (blue arrow), aerobic composting or incineration of CH₄ from anaerobic composting is a net-zero addition to the carbon cycle, as it is captured by photosynthesis into new biomass. Advanced recycling routes enable upcycling of plastic waste: polymers with functional backbones (such as polyesters or polyamides) can be depolymerized biologically or chemically, and the subsequent monomers are polymerized into tailored high-quality or virgin-quality products^,,,. Polymers with non-functional backbones such as polyolefins (including polyethylene (PE), bio-based PE, polypropylene (PP) and polystyrene) are better suited for cracking into hydrocarbon oil and gas by thermolysis and can then follow a similar upcycling path^,,,. PEF, polyethylene furanoate; PET, polyethylene terephthalate; PHA, polyhydroxyalkanoate; PLA, polylactic acid.

Fig. 2. Routes for synthesizing polymers from fossil-based and bio-based resources.

Petrochemical feedstocks (bottom) are the traditional resources for most commercial monomers and polymers for durable and single-use applications (such as polyethylene (PE), polypropylene (PP), polyvinylchloride (PVC) and polyethylene terephthalate (PET)), as well as for several fossil-based biodegradable polymers (such as polybutylene adipate-co-terephthalate (PBAT) and polyvinyl alcohol (PVA))^,. Several plastic families, such as polyamides, are not included here for reasons of space and complexity. Using renewable raw materials (top), biorefineries upgrade first-generation and second-generation biomass (that is, edible plant products and non-edible biowastes, respectively) into the same building blocks as those derived from petroleum, as well as others^,,. These monomers can be polymerized into several durable drop-in polymers (such as bioPE and bioPET), new durable polymers (such as polyethylene furanoate (PEF))^,,,,, as well as biodegradable ones (such as polylactic acid (PLA) and bio-polybutylene succinate (bioPBS))^,. Polyhydroxyalkanoates (PHAs) are biosynthesized in microorganisms from various feedstocks^,,,. Advanced catalysis unlocks captured CO₂, which, together with plant-oil-derived terpenes and epoxides, can be used to synthesize polycarbonates (PCs)^,. Bio-based non-isocyanate polyurethanes (PUs) can be made from plant-oil-based polyols. Separated lignin is often incinerated for energy recovery but its phenolic network can also be converted into useful chemicals^,. Polysaccharides can be extracted from plant biomass and converted chemically into plasticized starch and cellulose-based products^,,. BPA, bisphenol A; EG, ethylene glycol; FDCA, 2,5-furandicarboxylic acid; HMF, 5-hydroxymethylfurfural; PS, polystyrene; TA, terephthalic acid.

Fig. 3. Implementation framework for companies switching to sustainable materials.

Existing products in company portfolios have a set of properties that enable their business case. When switching to a more sustainable plastic replacement in the same application, companies are faced with a balancing act between conserving the same functionality, ideally at similar cost, and the requirements of a life cycle assessment to prove that the alternative is more environmentally friendly than the incumbent material^,. Some aspects are visible to the consumer, but most aspects that affect the sustainability of a product remain invisible. Yet, all aspects must be addressed to comply with upcoming regulations, extended producer responsibility (EPR) schemes and certification rules^,. EOL, end of life.