1. Introduction

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).

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.

Common bioplastics are summarized in the following Table 1 (note that in practice, blends are often used).

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.

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]):

(1) 

Breakdown of the polymeric macromolecules into low-molecular-weight moieties.

(2)

 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 are plastic components in fireworks, fruit and vegetable stickers, floral foam, dolly ropes, and wet wipes, where reuse and recycling of the materials is hardly feasible. While many packaging and other applications allow up- and downcycling of the materials, there are use cases like those presented in Table 3 above where a substantial fraction of a product will end up in the open environment and cannot be collected. Still, degradability makes sense for many more products, when one considers the considerable fraction of leaked plastics (bulk items and microplastics [37]), e.g., wrappings of sweets and small snacks, golf tees, and various single-use items (where no suitable alternative exists, e.g., a wound dressing). Primary microplastics stem from textiles (which are often made from PTT, PET, or poly(tetrafluoroethylene) (PTFE)—Gore-Tex™) and tire abrasion, which is considered one of the key sources of secondary microplastic [38], and even abrasion of shoe soles [39]. Paints and coatings are another important source of plastic-containing microparticles. Such microplastics are also generated without any littering, both in the use phase and during recycling, so degradability of plastics will, in any case, be beneficial to avoid accumulation and build-up of such materials in the environment. Degradable plastics can mitigate the consequences of both primary and secondary microplastics. Microplastics from degradable polymer materials can, however, also have detrimental effects on the environment, e.g., when that additional carbon freight is brought to an ecosystem or the particles act as carriers for absorbed toxins [40]. A combined approach is needed, where the freight of plastics ending up in the open environment is significantly reduced, and where that material will mineralize as soon as possible. Depending on the particles’ size and type of biodegradable plastics, as well as the ecosystem, the degradation can last from days to years, but not hundreds of years as in the reference case with the xenobiotic fossil plastics.