Bioplastics for a circular economy | Nature Reviews Materials

Leakage of plastic into the environment is a central issue of inappropriate EOL management3,22. Recycling of bioplastics is widely regarded as the most environmentally friendly EOL option and better than simple composting. However, bioplastics recycling streams are less established than those for traditional plastics98,99. Sorting of mixed plastic waste becomes even more demanding with novel (non-drop-in) bioplastics by increasing its heterogeneity, which raises concerns of higher rejection rates177,178. Spectroscopic techniques such as near-infrared scanners can be used to selectively identify bioplastics; for example, PLA can be identified with 98% accuracy179. Advanced sorting technologies include X-ray and UV spectroscopy, inert detectable markers in materials for ‘barcoding’ and using artificial-intelligence-based robotic sorting19,178.

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Plastic and bioplastic recycling is generally complicated by the presence of additives in almost every finished plastic product3. For example, typical PVC flooring can be composed of up to 80% fillers, plasticizers and pigments180. An ‘ingredients table’ (such as those found on food packaging or shampoo bottles) could detail the composition of a plastic product and, therefore, inform of its suitability for local recycling options. Furthermore, the complex and multimaterial design of plastic products typically prohibits recycling, which is why accounting for recyclability and simplicity in product design can greatly increase recycling rates. For example, achieving the necessary barrier properties for packaging through high-barrier monomaterials could improve recyclability by replacing non-recyclable multilayers2,128. Physical methods such as biaxial orientation can increase plastic film strength, clarity and barrier properties without the need for chemical additives180. Progressive extended producer responsibility (EPR) schemes, such as charging producers higher fees for less recyclable plastics, would help incentivize the design of easy-to-recycle products.

In this section, we discuss the EOL options for bioplastics, considering current and future recycling scenarios.

Mechanical Recycling

Mechanical recycling is the simplest, cheapest, and most common form of recycling181,182, and typically involves sorting the plastic waste by polymer type, removing labels, washing, mechanical shredding, melting, and remolding into new shapes. Mechanical recycling of bioplastics is generally not yet commercially available, but re-extrusion has been performed in the literature. The mechanical recycling of PLA and PHA is associated with the usual reduction in quality, such as loss of tensile strength and molecular weight125,151. Given the inability of mechanical recycling to effectively remove contaminants and additives from polymer waste, combined with the inherent thermal and mechanical stress, the products are generally ‘downcycled’ into goods of lower quality. Coloured or low-density materials (films, foams), as well as medical contaminants, are further complications and can render products non-recyclable21,59,181. Food-grade recycled materials are, therefore, hard to obtain183,184. Virgin polymers are often mixed with the recyclates to improve the quality of the recycled ones180,181. Nevertheless, mechanical recycling is often described as the most desirable EOL option, owing to its divergence from virgin resources. The environmental impact of mechanically recycled plastic is typically lower than that of virgin plastic. For example, the environmental impact (GHG emissions from transport and process energy use) of recycled PET (rPET) is two times lower than that of virgin PET, increasing to three times for recycled PE and PP (rPE and rPP, respectively) relative to their virgin materials185,186. The overall capacity of this form of recycling is, however, very limited: globally, ~10% of PET and high-density PE is recycled, whereas for polystyrene and PP, the numbers are closer to zero. Textiles and fiber products are also rarely recycled3.

Deposit-refund systems and EPR schemes can increase return and collection rates for post-consumer plastics and increase the quality of the plastic collected187. The plastic that is most commonly mechanically recycled is PET from beverage bottles. As a polycondensation polymer, its quality can be upgraded within existing recycling streams, wherein solid-state post-polymerization (effectively, heating of recycled flakes under vacuum to remove volatile polymerization by-products) increases the molecular weight of recyclates for commercial applications. Examples of countries with high recycling rates are Norway (97%, 2018)188, where an effective deposit system exists; Japan (83%, 2019), which has several EPR laws and fees in place189; and India (~90%, 2018)190, where informal collectors can make a living from returned bottles that recyclers pay for. In Germany, 99% of PET bottles under deposit schemes are recycled but only 65% of non-deposit bottles191. Recollection rates were roughly 30% in the USA in 2018 (ref.192). Globally, PET bottle-to-bottle recycling was at only 7% before 2016 (refs2,193); the rest was downcycled into PET fibers (72%), sheets (10%) and tape (5%), which are generally non-recyclable19,194.

Chemical Recycling

In contrast to mechanical recycling, chemical recycling offers the potential for making high-quality polymers from waste, termed ‘upcycling’. Plastic products are depolymerized into their monomeric subunits, which can then be repolymerized through controlled polymerization mechanisms into polymers of desired quality (such as with controlled molecular weight). For example, low-molecular-weight fiber polyesters can be depolymerized into monomers, which can then be polymerized into longer-chain polyesters that are required for bottles56,195. Impurities and color can also be removed. Chemical recycling is performed mainly through solvolysis or thermolysis.

In solvolysis, polymers with cleavable groups along their backbone, such as ester bonds in PET, PEF, and PLA, can be subjected to solvent-based depolymerization processes such as hydrolysis, glycolysis, or methanolysis56,181,196,197. Aliphatic polyesters, such as PLA, PBS or PHAs, are more hydrolysable than aromatic ones. For example, PLA can be hydrolyzed to 95% lactic acid without a catalyst at 160-180°C for 2 hours with an energy demand four times lower than that of virgin lactic acid production151 or depolymerized back into ~90% cyclic lactide monomers after 6 hours using Zn transesterification catalysts198. The resulting monomers present a useful feedstock for the production of high-quality plastics. However, the need for chemicals and more complex separation units make chemical recycling more expensive and, therefore, currently less economically competitive than mechanical recycling. Chemical recycling accounts for <1% of all recycled plastics. Several large chemical companies are developing processes to make ‘chemcycled’ products cost-competitive with virgin polymers57. As this approach provides monomers suitable for repolymerization into high-quality condensation polymers, such as polyesters and polyamides, the design and use of chemically recyclable polymers in plastic applications can solve persisting EOL issues and support a circular materials economy55,181.

In thermolysis, typically polyolefins, which do not possess hydrolysable functional groups, are pyrolyzed at temperatures of ~200-800°C (depending on the polymer and catalyst used) in the total or partial absence of O2. Under these conditions, the C-C bonds break, converting the polymer back into feedstock in the form of hydrocarbon oil or gas, or directly into olefin monomers. This feedstock can then be fed into traditional refineries and polymerization factories58,142,199. Thermolysis is most suitable for hydrocarbon polyolefin materials such as (bio)PE, (bio)PP and polystyrene. Thermolysis of polystyrene can recover >90% of liquid hydrocarbon oil58. One issue is the production of potentially toxic gases, as a result of the (often unknown) additives, that require appropriate capturing. Polyesters and other O-bearing, N-bearing and S-bearing polymers emit GHGs, such as CO, CO2, NOx and SOx, whereas halogenated polymers, such as PVC, produce HCl gas and chlorobenzene. The olefin monomer yield, selectivity and energy efficiency of thermolysis can be improved by incorporating advanced techniques, such as microwave pyrolysis, catalytic cracking, pressure and temperature profiling, and by adjusting the reactor configuration for surface maximization58,180,200.

Biodegradation and Composting

Biodegradation and composting describe the microbial digestion and metabolic conversion of polymeric material into CO2, H2O and other inorganic compounds by various known species111. This process is typically aided by physical processes, especially those that help with fragmentation and the reduction of particle size. For example, amorphization of crystalline structures in typically semi-crystalline plastics through micronization or extrusion can make them more susceptible to enzymatic degradation201,202. Hydrolysis cleaves susceptible bonds in accessible amorphous regions of a polymer, typically aliphatic esters, and microbial enzymes and acids or bases can enhance hydrolysis. Photodegradation using UV light breaks tertiary and aromatic C-C bonds, typically leaving a brittle and discolored material. This process can be enhanced by embedding metallic catalysts in the polymer203. Similarly, oxo-degradation (that is, decomposition by oxidation) can be triggered by metals; however, this can lead to fragmentation into microplastics and insufficient digestion. Thus, oxo-degradation has been restricted in the EU and Switzerland19,204.

Despite earlier hopes, biodegradation is non-trivial, as the rate of biodegradation is highly dependent on a polymer’s chemical structure, stabilizing additives, the surrounding conditions (such as the presence of H2O and O2) and any microorganisms used205. These conditions are often not met in home compost, open water or even in industrial composting facilities. Composters often reject biodegradable plastics, such as PLA shopping bags and utensils, as required decomposition times exceed typical composting process times of 6-8 weeks8,206. Typical biodegradation times for selected fossil-derived and bio-based polymers under industrial conditions and in ocean water are reported in Table 1.

Numerous certifications and labels are used to identify biodegradable materials (Box 2), typically related to industrial standards such as EN 13432 or ASTM D6400. However, revision and global harmonization of these guidelines are required, as the conditions mentioned in these standards may not necessarily be met in local disposal settings and, thus, may confuse consumers and converters39,179,207.

Box 2: Labelling Bioplastics

Plastic products are often labelled to indicate their chemical composition, whether they can be recycled, are bio-based and/or can be biodegraded and under which conditions. Consumers and converters are currently faced with various labels for bioplastics based on different industrial testing standards, some of which are referenced by major legislators, including the United Nations, the European Union (EU) or the US government. Some of these standards, particularly those certifying biodegradation, which were established around 2000, are currently under investigation, with the aim of revision and harmonization. It is important to understand the basis for these certifications and also who the agencies behind them are.

Identification Labels

The most commonly observed labels on plastic products are the plastic resin identification codes (examples from ASTM D7611/D7611M-20), which identify the polymer but provide no information on the recyclability. The older version of these labels — the ‘chasing arrows’ — still appears on products, and many consumers still falsely believe that products with these labels are recyclable, which may cause ‘wishcycling’ and lead to consumers placing non-recyclable items in recycling bins262. In the USA, only products labelled ‘1’ (polyethylene terephthalate (PETE)) or ‘2’ (high-density polyethylene) have a viable market and are, therefore, recycled262,263. Environmental organizations such as Greenpeace as well as some US states, such as California and New York, favour laws to prevent companies from using recycling symbols for non-recyclable products, and instead aim to use extended producer responsibility (EPR) laws to foster the design of recyclable materials262,264. Bioplastics such as polylactic acid are currently labelled as ‘7’ (other) and are typically not recycled.

Recycling-oriented Labels

The ‘green dot’ symbol used in the EU indicates that the producer has paid an EPR fee that is intended to fund collection and recycling programmes, but not that the product can be recycled. The on-pack recycling label (‘OPRL’) used in the UK recommends whether consumers should place individual plastic packaging components into trash or recycling bins, based on the nationwide probability that the component is successfully collected, sorted and reprocessed into a new product with a viable market. The German certification body DIN CERTCO has established new labels to certify the recyclability of a plastic product based on the polymer and existing infrastructure to recycle the latter. Similarly, new labels to certify the recycled content are being proposed. The US-based How2Recycle label aims to provide more information on the recyclability of individual packaging parts.

Bio-based Content Labels

The labels certify the bio-based carbon content in plastic products. The DIN biobased and OK biobased labels are granted by DIN CERTCO and the Austrian technical service company TÜV Austria, respectively. The US Department of Agriculture’s BioPreferred program issues a label based on third-party analysis, and in Japan, labels are issued by the Japan BioPlastics Association (JBPA). All these labels follow standards such as EN 16640 (Europe), ISO 16620 (international) and ASTM D6866 (USA).

Industrial Compostability Labels

The ‘OK compost’ and ‘seedling’ labels used in the EU and the ‘BPI compostable’ label used in the USA have become more prevalent in recent years, yet, consumers have to understand the need for industrial capacity to biodegrade. The ‘industrial’ sub-label is based on four tests specified in the standards EN 13432 and ASTM D6400: biodegradation (90% of material is converted into CO2 in inoculum derived from compost at 58°C after 6 months), disintegration (90% of material is smaller than 2 mm after 3 months at 40-70°C, depending on the standard), ecotoxicity (90% of regular plant growth in soil with plastic present) and the heavy metal content must not exceed a certain threshold265.

‘Custom’ Compostability/Biodegradability Labels

The ‘home’ compost label has seen increased use but is not based on a legislative standard. This label was proposed by TÜV Austria as a modification of EN 13432, with tests performed at 20-30°C over time frames that are twice as long as those in the original tests. Similarly, TÜV Austria has developed further labels and certification procedures for different environments in which plastics may end up. New bioplastic testing standards are under review, such as prEN 17427 (2020) by the European Committee for Standardization (CEN), which focuses on tests aimed to inform home compostability specifically for plastic bags.

Biological Recycling

Instead of complete biodegradation (composting), microorganisms and their hydrolyzing enzymes can be used to depolymerize condensation polymers into monomers, instead of CO2, similar to chemical recycling208. Such biological processes are still underexplored but hold promise as they could be cleaner than the chemical approach209. Aliphatic esters can be readily hydrolyzed, but aromatic polyesters are typically resistant to enzymatic hydrolysis. However, Ideonella sakaiensis 201-F6, a bacterium that was discovered in a Japanese recycling site, can depolymerize PET at ambient temperatures within 40 days201. Interestingly, its PETase enzyme is specific to aromatic polyester degradation and ineffective for aliphatic polyesters202. Leaf compost cutinase can be genetically modified to increase substrate specificity and thermal stability. The optimized enzyme can depolymerize 90% of micronized, amorphous PET into its monomers over 10 h at temperatures close to the glass transition of PET (~75°C)210. Near this temperature, the amorphous chain mobility increases, which increases the susceptibility to microbial degradation. The derived terephthalic acid monomer can be reused to synthesize bottle-grade PET210,211. This technology has also been used to depolymerize PEF212,213.

Compared with polyesters, polyurethanes are much less biodegradable, owing to the strength of the urethane bonds. However, fungi and various soil bacteria can help hydrolyse the ester groups within polyester-containing polyurethane214,215. Better understanding of enzymatic activity and gene editing to increase the specificity of microorganisms could potentially enhance the biorecycling of polyurethanes.

Biodegradation of polyolefin materials is even more challenging, as they lack cleavable functional groups along their backbones, are highly hydrophobic, have a high molecular weight and contain stabilizing additives216,217. Small fragments, <5,000 Da, are believed to be metabolized by some organisms; however, the molecular weight of most polyolefin plastics is millions of daltons. Partial biodegradation (5-20%) of PE films by waxworm bacteria as well as Pseudomonas strains has been observed, occurring over 1-2 months218,219,220,221.

Non-degradable polymers, such as PEF, can be made more degradable by copolymerization with more hydrolysable, more hydrophilic and less crystalline copolymers222,223. However, copolymerization can negatively affect the properties of the material. Polyolefins can also be blended with biodegradable polymers, such as starch, protein or natural fibre, to increase the material’s susceptibility to biodegradation224. However, it remains unclear whether such compounds decompose into sufficiently small particles or whether they are merely fragmented