1
Biodegradable Polymers - A Tutorial for a Circular Plastics Economy

Jean-Paul Lange1,2, Michiel Dusselier3, and Stefaan De Wildeman4

1Shell Global Solutions International B.V., Grasweg 31, Amsterdam, 1031 HW, Netherlands

2University of Twente, Sustainable Process Technology, Faculty of Science and Technology, Drienerlolaan 5, P.O. Box 217, Enschede, 7522 NB, Netherlands

3KU Leuven, Center for Sustainable Catalysis and Engineering (CSCE), Faculty of Bioscience Engineering, Celestijnenlaan 200F, 3001 Leuven, Belgium

4B4Plastics R&D, IQ-Parklaan 2A, Dilsen-Stokkem, 3650, Belgium

1.1 Context

From the appearance of the first bakelite object in the early twentieth century to the ban on certain single-use plastics in 2021 in the EU, the modern world has changed drastically due to the advent of synthetic plastics. Although the mass production of most plastics - notably polypropylene and polyethylene - only truly started in the 1950s, the cumulative volumes produced since have grown rapidly [1-3]. This growth comes from their utility on our comfort, convenience, and health that plastics have delivered so far and, arguably, will deliver in the future - the age of 3D printing at home has just begun. However, these benefits also come at a price. There is a growing accumulation of plastic litter on land, in rivers and seas all over the world, and even inside living organisms along the whole food chain, from mussels to birds and mammals. Best exemplifying the scale of the issue are the plastic soups, i.e. concentrated floating plastic debris in all major subtropical ocean gyres. Sources estimate that more than 67% of the plastic marine load originates from 20 major rivers [3].

This pollution problem has two main root causes. The first and most important one is us, the consumers, that are discarding waste plastic without consideration for the consequences and/or, by lack of proper waste collection infrastructure. Corporations also cause spillage and ill-disposal. Plastics were never meant to end up in the environment, although little consideration has been given to their end-of-life in the first 50?years since their mass production. As a matter of fact, nowadays 30% of the production is littered, notwithstanding an additional 40% that ends up in landfill and waste dumps [1]. The second root cause is the high chemical stability of the materials: plastics should indeed be considered as persistent pollutants that were never present together with fauna and flora during their almost four billion years of evolution.

The first root cause would be subject of a book on social and political science and will be kept out of scope here. But the second is a consequence of technology, leaning close to the purpose of this book. Is there a reason to run the plastics economy further with almost completely inert materials, or do we have alternatives that lead us to a new plastics economy, as advocated by the Ellen McArthur Foundation? [1]. Of course, as the challenges of the current plastics industry are rapidly urging us to take actions, we foresee interwoven debate and regulations to align both root causes.

Diving to the heart of technological issue, the book aims at presenting a comprehensive view on biodegradable plastics, encompassing a range of aspects from synthesis, e.g. based on renewable resources, to properties and applications, recycling, biodegradation, and environmental impacts as well as business challenges. The following sections will summarize and analyze for you the 13 chapters of the book with a critical look, extract their major learnings, and formulate an outlook for the future of biodegradable plastics. The structure of the book is visually outlined in Figure 1.1.

Figure 1.1 Visual outline of the book with its 14 chapters [4-16] divided thematically.

Source: Lilly Jenisch is thanked for making this figure.

But the plastic litter is not the sole problem to solve to develop a truly circular economy. The chemical industry also needs to disconnect from fossil hydrocarbons as feedstock and energy sources and minimize all its emissions, particularly eliminating those greenhouse gas emissions that contribute to climate change [2]. Accordingly, the industry must eventually switch to recycled and renewable carbon as feedstock for chemicals and plastics [2]. A major role is therefore laid down for the use of atmospheric CO2, via sustainable biomass or direct air capture, to produce sustainable plastics. These sustainable plastics represent the nexus of plastic litter and climate change. They will therefore be an inevitable subject of discussion in this book on biodegradable plastics.

The book kicks off by sketching the generation of plastic litter and its environmental impact (Chapter 3, [4]) and analyzing the mechanisms of biodegradation (Chapter 2, [5]) as potential remediation to the problem (Chapter 3). Before diving into the different types of (semi-)degradable plastics out there, the book reviews today's polymers, both fossil- and bio-based (Chapter 4, [6]), their manufacture, properties, and applications. Among the various families of potentially biodegradable polymers, polyesters (Chapters 5-7, [7-9]) and polysaccharide-based plastics (Chapters 8 and 9 [10,11]) feature prominently, but lignin-derived materials (Chapter 10, [12]) and more specialized recyclable thermosets - vitrimers - (Chapter 11, [13]) are also covered. Going beyond these technical options, degradable plastics are discussed in relation to plastic waste (end-of-life) management (Chapter 12, [14]) and from a life-cycle (analytical) perspective (Chapter 13, [15]). Finally, business challenges and marketing strategies for launching new polymers are outlined (Chapter 14, [16]).

Before diving into the heart of the subject, we wish to clarify a few points of confusion: the difference between biodegradable, compostable, bio-based, and bioplastics. These labels are often assumed to cover the same plastics and to oppose them to the traditional fossil-based plastics. But bio-based origin and biodegradable function are in fact unrelated; the former refers to its source while the latter to its end-of-life [17]. Bio-based plastics are literally derived from biological feedstocks, being from animal, vegetal, or other living organisms (e.g. fungi and bacteria). Many of them show significant biodegradability, but others are equally environmentally persistent as today's fossil plastics. For instance, biological feedstocks are sometimes the source for molecular structures that are identical to or resemble those derived from fossil feedstock and, therefore, show a similar lack of biodegradability. An obvious example is the inert bio-based polyethylene that is produced by converting sugars to ethanol, ethylene, and finally polyethylene. Along the same line, however, a limited number of fossil-based plastics show chemical structures that are readily biodegradable, as illustrated by oil-based polycaprolactone (PCL) (Chapter 7). We also need to differentiate compostable from biodegradable plastics. Compostable plastics are a sub-family of biodegradable plastics; they degrade most swiftly to make industrial composting affordable, as discussed below. Finally, we would recommend to avoid using the label "bioplastics" that is often confusingly used to cover all bio-based plastics (biodegradable and non-degradable) as well as the fossil-based biodegradable plastics. One could reserve the label of "bioplastics" to "bio-based and bio-made plastics," such as polyhydroxyalkanoates (PHAs) or biopolymers such as silk, cellulose, or starch (see Chapter 4). Such distinction should be limited to the scientific arena however, for it adds little value for the larger public, beyond confusion on source and end-of-life. An appendix in the tutorial (Chapter 4) further clarifies the spectrum of these alternative plastics.

1.2 Plastics in the Environment - Biodegradation and Impact of Litter

The biodegradation of polymers is a complex process that depends on many factors. Some are determined by the polymer itself, but others are determined by the environment such as T, pH, air, light, and density in microorganisms [1]. The suitability of conditions ranges from industrial composting as most favorable environment to home composting, soil and sludges, to fresh water and, finally, sea water and landfill as least favorable ones.

Rao and coworkers (Chapter 2, [5]) elegantly showed that biodegradation proceeds in three major steps: (i) formation of a biofilm - a colony of various wild microorganisms - on the plastic, (ii) depolymerization to low-Mw components after excretion of various enzymes onto the plastic, and (iii) ingestion and conversion of the low-Mw components as "feed" for the microorganisms. The polyolefins and aromatics polymers that form most today's polymers are generally recalcitrant to the first two...