Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).
No data were used for the research described in this article.
Today, plastic materials are mostly made from fossil resources, and they are characterized by their long lifetime and pronounced persistence in the open environment. These attributes of plastics are one cause of the ubiquitous pollution we see in our environment. When plastics end up in the environment, most of this pollution can be attributed to a lack of infrastructure for appropriately collecting and recycling plastic waste, mainly due to mismanagement. Because of the huge production volumes of plastics, their merits of being cheap to produce and process and their recalcitrance have turned into a huge disadvantage, since plastic waste has become the end point of our linear economic usage model, and massive amounts have started to accumulate in the environment, leading to microplastics pollution and other detrimental effects. A possible solution to this is offered by “bioplastics”, which are materials that are either (partly) biobased and/or degradable under defined conditions. With the rise of bioplastics in the marketplace, several standards and test protocols have been developed to assess, certify, and advertise their properties in this respect. This article summarizes and critically discusses different views on bioplastics, mainly related to the properties of biodegradability and biobased carbon content; this shall allow us to find a common ground for clearly addressing and categorizing bioplastic materials, which could become an essential building block in a circular economy. Today, bioplastics account for only 1–2% of all plastics, while technically, they could replace up to 90% of all fossil-based plastics, particularly in short-lived goods and packaging, the single most important area of use for conventional plastics. Their replacement potential not only applies to thermoplastics but also to thermosets and elastomers. Bioplastics can be recycled through different means, and they can be made from renewable sources, with (bio)degradability being an option for the mismanaged fraction and special applications with an intended end of life in nature (such as in seed coatings and bite protection for trees). Bioplastics can be used in composites and differ in their properties, similarly to conventional plastics. Clear definitions for “biobased” and “biodegradable” are needed to allow stakeholders of (bio)plastics to make fact-based decisions regarding material selection, application, and end-of-life options; the same level of clarity is needed for terms like “renewable carbon” and “bio-attributed” carbon, definitions of which are summarized and discussed in this paper.
Keywords: biobased carbon content, biodegradability, aerobic, anaerobic, biopolymer, bioplastics, composite, composting, marine, litter, EN 13432, bio-attributed, renewable carbon, circular economy, degradation, renewable, circular
Plastics are a versatile group of materials with countless short-lived and durable applications. This versatility is a result of the possibility to produce polymers with different chain lengths and molecular weight distributions from various monomers, and from the ability to derive composites, e.g., through the addition of fillers that modify their mechanical properties over a wide range. Examples of articles with a short service lifetime are food and non-food packaging, while examples of articles with long lifetimes are plastic window frames and plastic pipes for drinking water, sewage, and natural gas. In some applications, alternatives already exist (e.g., glass or metal packaging), while for other cases, no practical alternatives or substitutes are in place, e.g., for electrical applications, where the unique properties of plastics (e.g., their high thermal and high electrical insulation, light weight, low cost per item, durability, and resistance against various media) necessitate their use. It is hard to imagine our daily life without plastics. The annual plastic production volume was 390.7 million tons in 2021 [1], 90.2% of which were virgin fossil-derived polymers, with only 8.3% being post-consumer recycled plastics and approx. 1.5% being renewable or “bio-based/bio-attributed” plastics [1]. Overall, recycled plastic materials accounted for 11.7% of all plastics in Europe [2], a figure that has gone up by only 1% during the last decade. Without drastic changes in our use and reuse patterns, the path to proclaimed “circularity” is out of reach. At a global level, no more than 9% of all plastics are recycled [3]. The major issue with plastics waste is the mismanagement of plastic items towards and at the end of their lifetime, which can be attributed to a lack of waste collection and recycling infrastructure in most developing and emerging countries and inadequate or inappropriate waste collection and sorting infrastructure even in the developed/industrialized countries, coupled with human behavioural aspects, and the leakage of micro- and nanoplastics particles that are hard-to-impossible to catch and retain with today’s technologies. Also, the broad range of plastic types and compounds, with different colors, fillers, and additives, aggravates the difficulties in attempts at (mechanical) recycling. Of note, the availability and price of raw fossil plastics are often more advantageous than those of recycled plastics, due to the low prices of fossil plastics (particularly commodities such as poly(ethene) (PE), poly(propene) (PP), poly(vinyl chloride) (PVC), and poly(ethene terephthalate) (PET)). Also, the demand for recycled plastics has increased strongly in some areas, with “green” products being more accepted by the customers, leading to comparatively higher prices and some shortages in supply. Figure 1 shows that globally, the amount of mismanaged materials is more than double the amount of recycled plastics.
Mismanagement of plastics is at the core of plastic-related environmental problems. Reproduced with permission from [3].
The use of plastics largely follows a linear system (the “take-use-dispose” paradigm). A very recent report by the UN states [4]:
Up to 80% of mismanaged plastics could be reduced by 2040. The annual costs from mismanaged plastics are 300–600 billion USD.Simply improving today’s recycling practices will not suffice. For instance, it was found that a single plastic waste recycling company in the UK emits up to 1.5 million kg of microplastics per year [5], which is half of the entire quantity of microplastics generated by this company; hence, half of the produced microplastics cannot be retained via filters or other means and enter the environment. In general, the formation and emission of microplastics has been undervalued and only recently has awareness on the topic emerged. Several companies are already developing solutions for capturing microplastics at the source, e.g., behind car tires or in washing machines; however, such attempts cannot catch the entire micro- and nanoplastics freight, and most points where microplastics occur, such as at sewage treatment plants, are not equipped with capturing technology at all. The European plastics industry has launched an effort called operation clean sweep (OCS) to reduce the emission (spills and losses) of plastics pellets, flakes and powder throughout the value chain, which constitute another major source of microplastics entering the environment. Also, technologies are being developed to capture bulk plastic items already littered into water bodies, e.g., via bubble curtains [6], yet all such attempts are at the very beginning and far from large-scale roll-out.
In order to reach recycling rates for plastics that are comparable to those of wood, paper, glass, and metals, paradigm changes are needed, with concerted action in legislation and technology development required, accompanied by changes in people’s attitude towards plastics and inadvertent plastics “dumping”.
Plastics are a virtually indispensable class of materials, and they have a bad reputation amongst consumers as numerous studies show, e.g., a recent one where consumers reported viewing plastics as the least environmentally friendly packaging material [7]. It seems that intrinsic material properties are mixed with plastics’ mismanagement and its consequences, but the root causes for that problem are hardly known and hence not addressed. To stick with that example, plastic materials can offer several advantages over other packaging materials, and a key question is how to make them circular, less carbon-intense, and, in general, more sustainable. According to the OECD, “sustainable plastics” can be defined as “plastics used in products that provide societal benefits while enhancing human and environmental health and safety across the entire product life cycle” [8].
This article focuses on the definitions of bioplastics alongside the dimensions of being “biobased” and/or being “biodegradable”. There is no plain “black and white” here, with many misconceptions. Therefore, this article intends to provide a detailed overview on bioplastics, and then delves into existing definitions. The key novelty and unique aspect of this review lies in offering an up-to-date summary of existing bioplastics definitions, with the aim of providing readers with a fast and handy reference.
A “plastic” by definition is a polymer-based formulation, which consists of one or more polymers (homopolymer, copolymers, blends) plus additives and fillers. In nature, several polymers can be found, e.g., starch, cellulose, lignocellulose, or proteins (so-called biopolymers and/or naturally occurring polymers). A bioplastic (bioplastics) can be defined as a biopolymer-derived formulation, e.g., starch + plasticizer, poly(lactic acid) (PLA) + additives for processing and coloration, or (natural) fiber-reinforced poly(3-hydroxybutyrate) (P3HB), to give three well-established examples. A plastic material derives its properties from the combination of polymer(s) and additives, which applies equally to fossil and to biobased plastics. Filled products are called “compounds” or “composite materials”.
A biopolymer is a macromolecule that is composed of biobased or “natural” building blocks. Plastics can be thermoplastics (the largest group), elastomers, or thermosets, and bioplastics can fall into any of these groups. Sometimes, the terms “biopolymer” and “bioplastics” are used synonymously; however, we prefer a delineation with the term “bioplastics” being used for the human-made product (formulation, compound) of biopolymer + other ingredients, for use in technical applications (processing and manufacturing of goods). “Bioplastics” are either biobased and/or biodegradable, at least to a certain degree and as per a given definition (standard, test method). Figure 2 summarizes the definition of bioplastics by the IfBB (Institute for Bioplastics and Biocomposites, Hannover, Germany) [9].
Bioplastics can be grouped into “old” and “new” economy. Reproduced with permission from [9].
The “old economy” bioplastics include rubber (used in tires), cellulose acetate (deployed in cigarette filters), and linoleum (found in floor systems). Vehicle tire abrasion, by the way, is one of the major sources of non-degradable microplastics, comparable in amount to fibers from polyester-based clothing [10] and plastics nurdles (pellets). While the sap of the rubber tree (Hevea brasiliensis) is biodegradable, the vulcanized natural rubber is cross-linked and persistent in the environment. The “new economy” bioplastics include “drop in” materials, which are essentially classic plastics made from a renewable resource, e.g., PE made from ethanol derived from sugar cane (“bio-PE”). They are biobased and have the advantage of behaving just in the same way as their fossil counterparts, so that converters do not need to change any settings in their manufacturing processes. Such materials are equally recyclable as fossil plastics; however, they undergo the same problematic end-of-life scenario as their fossil-based blueprints do: They are recalcitrant towards biodegradation, and a full life cycle assessment (LCA) is needed to describe and compare environmental impacts [11]. Most notably, they also generate persistent microplastics. The “chemical novel” bioplastics in Figure 2 are bioplastics that have no 1:1 correspondence among fossil plastics, with varying degrees of biobased carbon content and biodegradability. Examples are PLA or polyhydroxyalkanoates (PHAs) [12], which also require their own settings on processing equipment, as will be detailed below.
Figure 3 depicts bioplastics according to their two main characteristics “biodegradability” and “carbon source”.
Categories of biodegradable and non-biodegradable plastics. Reproduced with permission from European Bioplastics [13].
Bioplastics are found in the blue and green boxes of Figure 3 , not only in their overlapping region. They do not need to be biodegradable, and neither do they need to be biobased, as long as the other criterion is fulfilled. They can be produced by microorganisms or by chemical synthesis, from inorganic (e.g., CO2) or organic (e.g., CH4, sugar, starch) raw materials. Plants might be used as hosts, too. Apart from plant-based raw materials, animal-derived waste products (e.g., chitin) can be deployed.
Common bioplastics are summarized in the following Table 1 (note that in practice, blends are often used).
Overview of bioplastics. “x” stands for “yes” and “(x)” for “partly”. The number and alignment of arrows in the column “trend” give a qualitative indication on expected market volume development in the coming years (↑ to ↑↑↑↑: noticeable to rapid development; ↗: modest development; →: stagnation).
| Bioplastic Material | Short | Family (Class) | Biobased | Biodegradable | Applications | Fossil Counterparts | Market Volume [9], 2022, in kt/a | Trend | Comment |
|---|---|---|---|---|---|---|---|---|---|
| Poly(lactic acid) | PLA | Polyester | x | (x) | Packaging, 3D printing, consumer goods, medical fields, agriculture | PS | 430 | ↑↑ | [14] |
| Polyhydroxyalkanoates | PHA | Polyester | x | x | Packaging, 3D printing, biomedical use, bioremediation, commodity materials | PP and others | 93 | ↑↑↑↑ | [15] |
| Poly(butylene succinate) | PBS | Polyester | (x) | (x) | Packaging, disposable tableware, medical articles, agriculture (mulching films, release of pesticides, and fertilizers), fishery | 90 | → | [16] | |
| Poly(butylene adipate-co-terephthalate) | PBAT | Polyester | (x) | (x) | Packaging, antimicrobial foils, single-use catering items, horti- and agriculture, textile industry | LDPE | 310 | ↗ | [17] |
| Starch, thermoplastic starch | TPS | Polysaccharide | x | x | Injection-molded commodity materials, thermoformable flat films | - | 220 | → | [18] |
| Bio-poly(trimethene terephthalate) | Bio-PTT | Drop-in | (x) | Textile fibers (carpets, car floor mats) | PTT | 120 | → | [19] | |
| Bio-poly(propene) | Bio-PP | (x) | Automotive parts, electrical devices, concrete additive, textile fibers, plastic bank notes in tropical regions, packaging materials | PP | 120 | ↑↑↑ | [19] | ||
| Bio-polyamide | Bio-PA | (x) | Textile fibers, sailing, parachute, ropes, fishery, horticulture (grass trimmer lines), tennis rackets strings, musical instrument strings | PA | 205 | ↗ | [19] | ||
| Bio-poly(ethene) | Bio-PE | (x) | Packaging, agriculture, foils, injection-molded parts | PE | 300 | ↑↑ | [19] | ||
| Bio-poly(ethene terephthalate) | Bio-PET | (x) | Packaging, bottles, foils, textile fibers | PET | 100 | ↗ | [19] | ||
| Poly(ε-caprolactone) | PCL | Polyester | x | Biomedical use (release of pharmaceuticals, wound glues, tissue engineering), packaging | - | [20] | |||
| Cellulose acetate | CA | Polysaccharide (esterified) | x | (x) | Cigarette filters, artificial silk, eye glasses frames | - | [21] | ||
| Poly(ethene furanoate) | PEF | Polyester | x | (x) | Bottles, foils, fibers | PET | - | [22] |
PLA is one of the most commonly used and best-established bioplastic materials.
PHAs (polyhydroxyalkanoates) are a class of biopolymers, where the most common representatives are the homopolyesters poly(3-hydroxybutyrate) (P3HB), poly(4-hydroxybutyrate) (P4HB), and, to a lesser extent, poly(3-hydroxyvalerate) (PHV), along with their copolymers poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) and poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBHx), medium-chain-length PHAs like PHO (poly(3-hydroxyoctanoate)) homopolyesters, and their copolymers and blends [8]. An emerging field in PHA development concerns mcl-PHAs (medium-chain-length PHAs), which display properties of elastomers and bio-latexes [23].
Other bioplastics of lower volumes are, e.g., poly(glycolic acid) (PGA) and the copolymer of glycolic acid and lactic acid (PLGA), PPC (poly(propylene carbonate)) and PFA (poly(furfuryl alcohol)), chitosan, and protein-based (e.g., whey retentate-based) bioplastics. Most bioplastics are thermoplastics. An example of a degradable thermoset is the product made from citric acid + glycerol [24].
Bioplastics, analogous to conventional plastics, can also contain organic fillers, like wood chips or wood dust (WPC, wood—plastic composite), paper fibers, and natural fibers like kenaf, sisal, or hemp [25], which enable them to be fully biobased and biodegradable. Natural inorganic fillers, such as nanoclays [26], which trigger specific material properties like gas barrier behavior, are feasible, too.
Below, in Table 2 , several definitions from SAPEA (Science Advice for Policy by European Academies) [27] related to the field of biopolymers and bioplastics are provided.
Concise definitions related to bioplastics.
| Biobased plastic(s) | Plastic containing organic carbon of renewable origin from, plant, animal, or microbial sources | [27] |
| Biodegradable plastic(s) | Biodegradable plastic. A plastic that undergoes biodegradation involving the metabolic utilization of the plastic carbon by microorganisms such as bacteria, fungi, and algae, resulting in the conversion of plastic carbon to CO2 (and CH4) and microbial biomass | [27] |
| Biopolymer | A polymer produced by a living organism or isolated parts thereof (enzymes) | [27] |
| Degradable plastic | A plastic or matrix that can degrade under certain environmental conditions in specific time period, resulting in loss of properties as measured by standard test methods. Degradation of plastic can result either from hydrolysis (hydrolytic degradation), oxidation (oxidative degradation), light (photo degradation), or a combination of these effects (ASTM D883-20a) | [27] |
| Degradation | Chemical changes in a polymeric material that usually result in undesirable changes in the in-use properties of the material | [27] |
| Plastic biodegradation | The microbial conversion of all organic constituents in plastic to carbon dioxide, new microbial biomass, and mineral salts under oxic conditions, or to CO2, CH4, new microbial biomass, and mineral salts under anoxic conditions | [27] |
| Renewability | The ability of a resource or energy source to be naturally replenished or restored within a reasonable period, making it sustainable for long-term use without being depleted or exhausted. Renewable resources, such as solar energy, wind energy, hydropower, biomass, and geothermal energy, are considered environmentally friendly alternatives to non-renewable resources like fossil fuels, which have limited availability and contribute to environmental issues like climate change | |
| Biosynthesis | Polymers can be obtained via synthetic methods, e.g., under pressure or with catalysts, or be synthesized in nature, by, e.g., plants (starch) or bacteria (PHA) | |
| Biodegradability | Degradability can be brought about by irradiation or mechanical forces, whereas biodegradation is the cleavage of (in our case) polymers into smaller moieties, until complete mineralization to CO2 and H2O. Biodegradability is caused by enzymes from microorganisms | |
| Biocompatibility | The ability of a material or substance to safely and effectively interact with living tissues or biological systems without causing harm, adverse reactions, or immune responses. In the medical fields, biocompatible materials are essential for various applications, such as implants, medical devices, drug delivery systems, and tissue engineering. Not all bioplastics are biocompatible, and some fossil plastics also show biocompatibility |
The terms “biodegradable” and “biobased” will be revisited later in this manuscript in more depth. For a good primer on bioplastics, see, e.g., references [28,29].
With environmental problems becoming visible and a growing concern for the general public, organizations are starting to feel more pressure to justify their actions and to prove their good-doing, in an attempt to secure or enlarge their business. Consumers have become eco-anxious and spend money on more costly, supposedly more environmentally benign merchandise. Corporate social responsibility (CSR) has become a buzz term in this respect, where organizations address social and environmental concerns in their business operations on a voluntary basis. An organization or its individuals might become tempted to use marketing spins to present themselves as “eco”, “green”, or “good” to the outside world. The expression of “greenwashing” describes dishonest practices of organizations to appear “green” [30]. It purportedly was coined in 1986 by Jay Westervelt, an environmentalist, who observed hotels’ notices encouraging guests to reuse towels, while at the same time harming the environment in stronger ways, which he felt was obscured by directing peoples’ attention to a lesser point of concern. Greenwashing is being blamed, e.g., by environmental groups, but still very present. For a systematic review on concepts and forms of greenwashing, see, e.g., [31]. The trading of carbon emissions can also fall into the realm of greenwashing, when “free credits” are allocated to large emitters, the carbon price is low, carbon projects are only temporary or miscalculated, or consumers feel “clean” after having bought voluntary credits, while continuing with carbon-intense patterns. In addition, it needs to be stated that only a fraction of anthropogenic CO2, CH4, and other GHG (greenhouse gases) is covered by emissions trading and related schemes. The credibility of the various certificates for renewable energy and circular/sustainable materials can differ among the plentiful, sometimes non-accredited schemes. For a definition of circularity/circular economy, see [32]. One has to acknowledge that the industry is still developing, yet rigid and traceable standards are imperative right from the start, and realistic assumptions particularly for the mid and long term are vital. The expression “carbon footprint”, by the way, was invented by British Petroleum back in 2005 as a marketing sham [33]; they reframed the fossil fuel industry’s responsibility for CO2 emissions as consumers’ very own responsibility or “problem”, by asking them about “their carbon footprint”. The industry is not in the spotlight when this term is being used and applied, e.g., via various “CO2 footprint calculation” tools. Hence, we should avoid blindly repeating marketing speak with the term “carbon footprint”, and reframe that to the “fossil fuel footprint”. The responsibility of consumers with regards to environmental harm exists, yet we must not overemphasize it or put all of the blame/burden on their shoulders. It is the legal framework in which market incumbents operate and decide to place products on the market that is more the culprit. The consumers, in the end, can only chose among what they are being offered. Lately, a lot of products have been placed on the market with claims related to sustainability, which give the impression of being “eco-friendly”, yet we need true solutions to the plastic waste crisis that have to come from the materials side. It is obvious that “end of pipe” solutions of more waste collection, sorting, and recycling cannot completely solve the problem of persistent plastic waste in nature, as there will always be a certain rate of leakage, both of bulk items as well as of micro- and nanoplastics. Also, the use of additives in plastic formulations needs to be watched carefully, with full transparency and limitations on problematic ingredients.
Eventually, fossil plastics, with their stable carbon–carbon backbones, will degrade (in the order of up to hundreds of years), and all fossil carbon was once living matter (millions of years ago). Absolute statements have to be treated with caution, as with the degree to which different materials can be compared to one another, like in the case of, e.g., the toxicity of certain compounds. There is no such thing as a clear definition of “biodegradability” because that property is multifaceted. Let us draw an analogy to woody biomass: A large stem of a tree will take years, or even decades, to “disappear”, while leaves will be biodegraded within less than one year; the same is valid for the stem when undergoing crushing processes prior to biodegradation. [27] states: “We consider plastic biodegradation a system property, in that it results from the interplay of a specific material property of the plastic that makes it potentially biodegradable as well as the abiotic and biotic conditions in the specific receiving environment that leverage this potential and control the rates and extents of actual plastic biodegradation”.
The biodegradation of plastics is believed to progress in two steps, which can be preceded and accompanied by mechanical fragmentation (see also [34]):
Breakdown of the polymeric macromolecules into low-molecular-weight moieties.Uptake of these compounds by microorganisms and in metabolic consumption, to finally yield CO2, CH4, and H2O (complete mineralization).
The mere (bio)degradability of plastics is seen as insufficient to solve the plastic waste problem that the world is facing today because that property might tempt people to neglect the waste hierarchy and to use the materials in a linear fashion alone, with large quantities of plastic waste being littered/mismanaged. In 2020, the EU Group of Chief Scientific Advisors wrote regarding biodegradable plastics: “The 2018 EU Plastics Strategy sets out a cautious approach for the use of biodegradable plastics (BDP). While it acknowledges that targeted BDP applications have shown some benefits, it also identifies several challenges and points out that “It is important to ensure that consumers are provided with clear and correct information, and to make sure that biodegradable plastics are not put forward as a solution to littering” [35]. They hence recommend that we should “limit the use of BDPs in the open environment to specific applications for which reduction, reuse, and recycling are not feasible”. The nova-Institute has identified such applications in its study BioSinn [36], which are listed in an exemplary fashion here in Table 3 .
Examples of applications of plastics where biodegradability makes sense. Source: [36].
