Aug. 06, 2024
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Some companies looking to move to Plastic-Free Packaging consider biodegradable plastic as one possible solution. However, bioplastic is plastic. This does not mean that certain types of bioplastic shouldn't be considered as part of an eco-friendly packaging strategy. However, it does mean that biodegradable plastic is not a silver bullet solution for you if you are choosing to go &#;Plastic-Free.&#;
In fact, some biodegradable plastic packaging solutions are actually extremely problematic, and are worse for the environment than traditional plastic.
Additionally, if you were hoping to go plastic free, but have found it impossible to avoid plastic in some element of your packaging, recycled, recyclable plastic is often ecologically superior to bioplastic.
In this article, we break down different types of biodegradable plastic and bioplastic, and summarize the upsides and downsides of the material as a sustainable packaging alternative. If you determine that a biodegradable plastic is best for you, we share guidance to help you select the right type of bioplastic material for the planet. We also share a few tips to help you market your bioplastic packaging accurately to your customers in ways that avoid greenwashing and support responsible disposal.
Bioplastic is plastic that is made (at least in part) by plants or fibers AND/OR is biodegradable in some way. The term "bioplastic" refers to a diverse set of materials, including:
Plant-Derived: Bioplastic can be plant-derived, and plants are a renewable resource. Today, most plant-based bioplastic comes from corn or sugarcane. It should be noted that many self-proclaimed plant-derived bioplastic solutions are only derived in-part from plants (the rest is petroleum based). Additionally, some bioplastics are actually derived entirely from fossil fuels with enzymes added to accelerate degradation. It is also important to remember that while corn and sugarcane are renewable resources, they do require fossil-fuel intensive agriculture that is polluting to water and soil. Bio-based plastic is also typically controlled by behemoth agricultural and chemical companies (often the very companies that are driving GMO seed development).
Compostability: Some bioplastic is compostable in industrial composting facilities, where temperature and timing are closely managed. While recycling is the preferred outcome for most packaging, composting is preferred to landfill or litter. For food product packaging in particular, compostable solutions are an an exciting innovation because this allows consumers to compost the package AND the food remnants in the packaging (allowing food to end up in a compost pile instead of the landfill).
A small set of bioplastic is home compostable. This is great because it means (1) the plastic can biodegrade in conditions that are not as strict and (2) it can be composted by households with a home compost setup even if they don&#;t have access to industrial compost. Note that most bioplastic labeled home compostable plastic is derived only in-part by plants (0-50% plant based) with the remainder made with petroleum-based plastic.
THE DOWNSIDES OF BIOPLASTIC
Does Not Address Marine Plastic Pollution: Most commercially available bioplastic does not rapidly biodegrade in the oceans! Typically, if bioplastic ends up in the ocean, it will behave like traditional plastic. People moving to bioplastic as a way to positively influence our marine plastic pollution crisis are often surprised to learn this. If a material is not marine biodegradable certified, assume it will not breakdown in the ocean.
Additionally, there are quite a few options out there labeled as "biodegradable plastic" that are actually oxo-biodegradable, meaning that they rapidly degrade into smaller bits of plastic when exposed to the sun. These oxo-biodegradable plastics are often considered more damaging to the oceans than traditional plastic, because they exacerbate the issue of micro-plastics. Read the Sustainable Packaging Coalitions view on this topic to learn more.
There is exciting emerging research and development of bioplastic formulations that are, in fact, certified to biodegrade in marine environments. We hope these are developed into materials used for straws, cup lids and grocery bags, which are the top items found in oceans. Even as this technology develops, we don&#;t think it should be used for ecommerce packaging, which brings us to our next point.
Recycling is preferred to composting: With the exception of food packaging, composting is not necessarily the ideal end of life solution for packaging.
Recycling is preferred to composting in the hierarchy of waste management (reduce -> reuse -> recycle -> compost -> landfill -> litter). If recycling is preferred to composting, then a packaging solution designed to be recycled is preferred to one that is designed for composting. The majority of bioplastic packaging options out there today are not recyclable.
Contamination: There's an abundance of confusion surrounding how to properly dispose of plastic as bioplastic almost always contaminates the waste stream.
Poor compost quality: Even when a package is truly and fully compostable, the quality of compost degrades with too much bioplastic. This isn&#;t a huge deal right now since compostable bioplastic is still relatively limited in use. But imagine a world if all plastic was replaced by compostable bioplastic and all of it were composted. The resulting compost would not be usable for certified organic or sustainable farming. It would lead to compost that isn't great for our soils, and composting facilities will have a much harder time finding buyers for their output (making the entire economics of the compost business more difficult for municipalities to manage). This is one reason why the vast majority of municipal industrial composting operations nationwide still do not accept compostable bioplastics.
Linear economy mindset: Most bioplastic solutions are designed with a linear economy thought process. They are made with virgin materials, used once, and then the material is meant to be disposed of (ideally composted if the bioplastic is compostable). Within the linear economy mindset, bioplastic is often superior to petroleum counterparts. EcoEnclose supports the circular economy, in which materials and packages are designed for recyclability and products are manufactured with as much recycled content as possible. This type of approach has a lower carbon footprint, maximizes the value of natural resources, and spurs innovation within recycling and the use of recycled materials in end products. All things equal, we support investments in improved recyclability (i.e. for example, R&D that allows us to create plastic film that uses 100% post-consumer waste) versus investments in finding new, more sustainable ways to make virgin plastic (though we recognize that both are important).
THE UGLY SIDE OF BIOPLASTIC
The biggest downside of bioplastic is the misleading marketing and messaging around it.
Bioplastic packaging is often accompanied by the messaging: "plant bottle" or "vegetable bag" or "this packaging is made of dirt." It is common to see bags shaded in green that claim to be both recyclable and biodegradable (which, if you see it, is likely inaccurate labeling).
Imagery and language like this, while generally well-intentioned, is often inaccurate and is very misleading to consumers.
For example, this bag claims to be recyclable #2 plastic as well as also biodegradable.
A quick glance at it would suggest it is made of plants and compostable. More than likely, neither of these things are true. In fact, it's more likely that the bag is a traditional plastic bag with additives to make it "oxo-biodegradable." Try to find truly recycled and recyclable paper shopping bags instead.
Messaging like "Plant Bottle" or "This Bag is Made of Dirt" would suggest that a package is 100% made of plants, which is often not the case. For example, Coca-Cola&#;s Plant Bottle is typically 30% sugarcane-based plastic (the remainder is traditional fossil fuel based plastic), but most people would assume a package called the "plant bottle" is entirely made of plants instead of the reality: the Coca-Cola Plant Bottle is only made partially from plants.
Sexy messaging around bioplastic is problematic. It paints bioplastics as a silver bullet solution. It suggests to consumers that their packaging is essentially a plant or dirt, and will dissolve back into the earth the same way a piece or paper or apple would.
But bioplastic is chemically different from the original &#;plant&#; that it was derived from. PLA (polylactic acid), for example, is made by adding enzymes to the endosperm of corn, which creates dextrose that is fermented into lactic acid, that is then converted to pellets of polylactic acid plastic. This is not a natural process, and the resulting material behaves nothing like the corn originally used to derive the lactic acid. This is why an organic farm can accept compost made with food waste but not compost that contains bioplastic.
This type of marketing makes people think that solving the "plastic problem" is easy, and it limit investments in the plastic recycling technology that is essential to a truly circular economy.
We believe bioplastic has an important place in the future of sustainable packaging, especially as it relates to food packaging. However, as with all aspects of sustainability, we also believe accurate, authentic, honest messaging is critical.
So, should I use bioplastic for my ecommerce packaging?
If you've made the decision that your company wants to be Plastic-Free, we strongly recommend packaging solutions that are made with naturally biodegradable materials such as paper.
If you have found the functionality of plastic to be essential to your shipping and operations, than we'd recommend you consider recycled, recyclable plastic instead. In fact, some forward thinking companies like Patagonia actually see bioplastic as so problematic that they restrict in their packaging (as based on their merchandising policy).
To date, we at EcoEnclose have made the decision not to offer bioplastic packaging solutions for the reasons outlined above. We recommend that companies seeking to go plastic-free look for packaging made with paper. If the characteristics of plastic are critical for your business, we continue to see recycled plastic as the best solution. We do, however, look forward to seeing legitimate advances made in bioplastic technology that result in a material that is truly a sustainable alternative to recycled plastic.
That said, we recognize that for some companies, the upsides of bioplastic is worth the downsides. If that is true for your brand, read on for some tips to help you select the right bioplastic solution packaging for your business.
Guidelines for using any bioplastic packaging solutions for ecommerce
For more biodegradable plastic film manufacturerinformation, please contact us. We will provide professional answers.
Avoid "biodegradable plastic" that is not certified as compostable or is not derived entirely from plants:
If you see a very inexpensive plastic packaging solution that claims to be biodegradable and recyclable, but is not certified as compostable, chances are that it is some version of oxo-biodegradable virgin plastic.Avoid these at all costs
! They contaminate the recycling stream, are extremely misleading to customers who may try to compost them, and - if they were to end up as ocean pollution - are actually more problematic than traditional plastic.Look for plant-based plastics that are recyclable (rather than compostable, or "biodegradable"):
A very small percentage of US households have access to industrial composting that will accept compostable plastic. Most of us have access to plastic recycling (either curbside or grocery drop offs). Bio-PET (the type of bioplastic that is in Coca-Cola's Plant Bottle) is one example of a recyclable plant-based plastic. Even though the bio-PET in the bottle is derived from sugarcane, it behaves exactly like petroleum-based PET. Because of this, the bottle can be recycled along with all other PET bottles. This type of recyclable bioplastic is a truly exciting alternative to virgin, petroleum-based plastic because it is designed for a circular economy and encourages continued investment in recycling infrastructure and technology.Be honest and transparent:
If your packaging is made of a percentage of plant-derived plastic, avoid marketing it as a "Plant Package" or a "Package that is Made of Dirt" or anything else that is misleading and may be perceived as green washing. Instead, be clear that X% of the plastic package is made from plants. Avoid language or imagery that implies that the package is harmless to the ocean or will dissolve in the landfill. Accurate, transparent messaging is a critical foundation to any sustainable packaging strategy.Be extremely clear on how to dispose of the package:
We strongly encourage you to avoid using the term "biodegradable" when it comes to plastic, as the term is often misused and misunderstood.If your packaging is recyclable, be very clear on how to recycle (i.e. along with other like plastics, in a grocery drop off, curbside, in a special stream, etc). If your packaging is industrial compostable, let them know how to find a facility. If your packaging is home compostable, let them know what that means (as even home compostable plastic requires fairly specific conditions). If your bioplastic packaging is neither recyclable nor certified compostable, advise your customers to landfill it when complete.
Have additional questions about plastic-free packaging? Please feel free to contact us and we'll gladly help you find the right solution for your brand.
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.
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 (Fig. 1).
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 remoulding 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 fibre 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%, )188, where an effective deposit system exists; Japan (83%, ), which has several EPR laws and fees in place189; and India (~90%, )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 (ref.192). Globally, PET bottle-to-bottle recycling was at only 7% before (refs2,193); the rest was downcycled into PET fibres (72%), sheets (10%) and tape (5%), which are generally non-recyclable19,194.
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 fibre polyesters can be depolymerized into monomers, which can then be polymerized into longer-chain polyesters that are required for bottles56,195. Impurities and colour 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 hydrolysed to 95% lactic acid without a catalyst at 160&#;180&#;°C for 2&#;h with an energy demand four times lower than that of virgin lactic acid production151 or depolymerized back into ~90% cyclic lactide monomers after 6&#;h 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 pyrolysed 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 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 discoloured 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 or ASTM D. 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.
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 , 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 D/DM-20 in panel a of the figure), 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 (panel b of the figure) 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 (panel c of the figure) 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 (panel d of the figure). 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 shown in panels e&#;g of the figure certify the bio-based carbon content in plastic products. The DIN biobased (panel e of the figure) and OK biobased (panel f of the figure) 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 (panel g of the figure) and, in Japan, labels are issued by the Japan BioPlastics Association (JBPA). All these labels follow standards such as EN (Europe), ISO (international) and ASTM D (USA).
Industrial compostability labels
The &#;OK compost&#; (panel h of the figure) and &#;seedling&#; (panel i of the figure) labels used in the EU and the &#;BPI compostable&#; (panel j of the figure) 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 and ASTM D: 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 (panel k of the figure) has seen increased use but is not based on a legislative standard. This label was proposed by TÜV Austria as a modification of EN , 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 (panels l&#;n of the figure). New bioplastic testing standards are under review, such as prEN () by the European Committee for Standardization (CEN), which focuses on tests aimed to inform home compostability specifically for plastic bags.
Panel a reprinted, with permission, from ASTM D/DM-20 Standard Practice for Coding Plastic Manufactured Articles for Resin Identification, copyright ASTM International, 100 Barr Harbour Drive, West Conshohocken, PA , USA. A copy of the complete standard may be obtained from ASTM International, www.astm.org. Panel b copyright Der Grüne Punkt &#; Duales System Deutschland GmbH. Panel c copyright OPRL Ltd. Panels d and e reprinted with permission from DIN CERTCO, www.dincertco.de. Panels f, h and k&#;n copyright TÜV AUSTRIA Group. Panel g copyright Department of Agriculture&#;s BioPreferred program based on third-party analysis. Panel i copyright European Bioplastics e.V. Panel j courtesy of the Biodegradable Products Institute.
Instead of complete biodegradation (composting), microorganisms and their hydrolysing 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 hydrolysed, 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 to form microplastic.
In the USA, ~20% of EOL plastic waste is incinerated ()3; in Europe, it is ~40% ()182. If only C/H/O-containing renewable material is combusted, CO2 emissions are net-zero and some of the resulting thermal energy can be recovered for energy production. However, combustion of N-containing, S-containing and Cl-containing polymers produces toxic NOx, SOx and HCl. Similarly, additives in polymers may release various toxic substances upon burning that require potentially costly capture and treatment interventions180,225. Furthermore, there are concerns of a &#;locking-in&#; effect, whereby the high investment cost for incineration plants and the need for constant waste influx may jeopardize the adoption of recycling technologies2.
In many countries, landfills are still the dominant waste disposal option: in the USA, 58% of waste ends up in landfills ()3, and in Europe, it is 27.3% ()182. Mismanaged and leaky landfills are considered a major source of environmental pollution. Biodegradable polymers should also be kept out of landfills as they can compost anaerobically to CH4, which has a GHG impact that is >20 times higher than that of CO2 (refs98,207). In the USA, the decomposition of organic material (such as paper and food scraps) in the ~1,500&#;2,000 operational landfills is the third largest CH4 emitter behind enteric fermentation (in farm animals) and natural gas systems226. Only 10% of CH4 produced in landfills was estimated to be captured globally in , which is an approach that offers potential for energy recovery while benefitting the climate and public health227,228. The UN has mentioned that landfilling fees could make recycling more cost-competitive16.
Controlled anaerobic digestion (which occurs in the absence of O2) in a methanization &#;biogas&#; facility produces CH4 from biodegradable polymer waste. The CH4 can then be captured and burned, which produces CO2 and H2O, and the heat and energy can be recovered for use. This process yields a net-zero carbon balance for the bioplastic waste while also producing energy229,230. The efficiency of anaerobic digestion can be increased by including elements such as a &#;bioreactor landfill&#;, in which H2O is circulated to enhance microbial activities for CH4 production227. Anaerobic digestion is feasible for several types of polymers, including thermoplastic starch, polycaprolactones and PHAs, as well as for PLA at elevated temperatures167.
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