Opinion What’s in a
Opinion What’s in a name? – Definitions It is generally known that one should not ask people for the definition of a word or term when they have a business interest in the outcome. After all, turkeys don’t vote for Christmas. One should also distinguish between what it is and what it does when making definitions of words and never combine these matters in just one description. It leads to a lot of confusion when one does. Words like plastic or biodegradation, and several others have one thing in common: ask ten people for the definition and you get at least eight different answers. Even from academics, there are many different answers, although one expects they would be fully familiar with the matter. And believe it or not, but many people say that polymer is just another word for plastic. All this does not contribute to much clarity about plastic let alone to explain the benefits of such materials. It also makes it impossible for legislators to come up with a proper legal text in their efforts to ban plastic waste since any text only leads to infinite discussions on definitions rather than tackling the real problem. So, what can be done about these definitions? Before going into that, one should consider some common practices of how we sometimes contradict ourselves: The English Dictionary of Oxford Languages describes plastic as: “A synthetic material made from a wide range of organic polymers, such as polyethylene, PVC, nylon etc., that can be moulded into shape while soft, and then set into a rigid or slightly elastic form”. On the other hand, Merriam-Webster describes plastic as: “Any of numerous organic synthetic or processed materials that are mostly thermoplastic or thermosetting polymers of high molecular weight and that can be made into objects, films or filaments”. There should be no surprise that people are confused when reading these two definitions, which are not based on a systematic scientific approach, but more on a general understanding or impression of materials that are around in society. If one uses the definition of “plastic” from the English Dictionary the term “micro-plastic” immediately becomes an impossibility, something like a “square circle”, since several studies have demonstrated that “micro-plastic” consists for about 40 % of thermoset materials from coatings, paints, and rubber tires, while the other 60 % comes from thermoplastic materials. Of course, most contain different types of additives as well. Although Merriam-Webster rightfully includes thermoplastics and thermoset materials in its definition, it should replace the term “molecular weight” with “molecular mass”, and it should also allow for substances of low molecular mass being present in thermoplastic as well as in thermoset “plastics”. IUPAC (International Union of Pure and Applied Chemistry) uses a different approach to the definitions and comes with a somewhat surprising recommendation for the word plastic although allowing for both, thermoplastic and thermoset materials: “Generic term used in the case of polymeric material that may contain other substances to improve performance and/or reduce costs”. At the same time, IUPAC makes two interesting notes: Note 1: The use of this term instead of polymer is a source of confusion and thus is not recommended. Note 2: This term is used in polymer engineering for materials often compounded that can be processed by flow. So, if the word plastic already creates so much confusion, how on earth are we ever going to have a good common understanding of words like bioplastic or microplastic? It’s good to take a step-by-step approach before going into that by addressing a number of definitions: Molecule: “The smallest identifiable unit into which a pure substance can be divided and still retain the composition and chemical properties of that substance”. Monomer: “A molecule that can be chemically bonded to identical molecules to form a macromolecule”. Macromolecule: “Molecule of relative high molar mass, the structure of which essentially comprises the multiple repetitions of units derived, actually or conceptually, from molecules of relative low molar mass”. This definition of macromolecule indicates that it can be derived directly from monomers by a chemical reaction (actually) or that the repeating units in a macromolecule can be formed in a different way, like by enzymatically controlled biochemical conversions of fatty acids or sugars for instance (conceptually). In the first case, we talk about thermal or chemo-catalytic conversion of monomers to macromolecules, like PE, PP, PVC, PET, PLA, PBAT, PBS, etc. In the second case, the macromolecules are usually made by bacteria or yeasts in and by nature, like polysaccharides, polypeptides, natural polyhydroxyalkanoates (PHAs), lignin, cellulose, etc. Polymer: “Substance composed of macromolecules”. In this case, a polymer consists of a large number of macromolecules, which is always the case in thermoplastic material, or a large number of macromolecules that are crosslinked to form a polymeric network which is called a thermoset material. 46 bioplastics MAGAZINE [03/22] Vol. 17
By: Jan Ravenstijn Biomaterials Consulting Basics Meerssen, the Netherlands Note: The crosslinking in thermoset materials can be due to the nature of the monomer(s) used to make the polymer or can be achieved by adding so-called crosslinking agents to the polymer, depending on the thermoset material of choice. Of course, many people then use a definition of plastic by saying that plastics consist of a combination of one or more polymers plus one or more additives. But if one follows this thinking, bearing in mind the abovementioned definitions of macromolecule and polymer, one can safely say that paper is plastic, since depending on the type of paper, it always contains cellulose and additives, but could also include lignin and even some synthetic polymer like PE. Not only the word plastic is causing a lot of confusion depending on the definition that people chose to use, because it fits them best. The word bioplastic is even worse since too many people believe that all bioplastics biodegrade (we know better of course), but some of us also use a definition combining what it is with what it does, whilst adding to the belief that all bioplastics biodegrade. Perhaps one should refrain from using the term bioplastic by following the IUPAC Note 1 under its definition of plastic. Most people say that polymers are made by polymerization. Although the general understanding is that polymerization is done thermally or chemo-catalytic in a large chemical plant, some proposed legislative texts even use the phrase “polymerization in nature”. And it is very clear that the current definitions are not all consistent with each other: Polymerization: Cambridge dictionary: “a chemical reaction in which many small molecules join to make a polymer”. Of course, this reaction does not make a polymer, instead, it makes a macromolecule. Merriam-Webster dictionary: 1) “a chemical reaction in which two or more molecules combine to form larger molecules that contain repeating structural units”. 2) “reduplication of parts in an organism”. Indeed, the macromolecules made in and by nature fit with the second description here. Finally, the definition of biodegradation: Biodegradation: OECD: “Biodegradation is the process by which organic substances are decomposed by micro-organisms into simpler substances such as carbon dioxide, water, and ammonia”. Medical Dictionary: “The breakdown of any material (e.g. an organic chemical), to a simpler nontoxic form, usually understood to be by bacterial action”. Bruno DeWilde (OWS): “In the case of polymers, biodegradation is the mineralization of polymeric substances to minerals and biomass through microorganisms. So the products of biodegradation results in CO 2 , H 2 O, humus, and biomass”. Although the definitions use different words they pretty much describe what it is in the same way, albeit that perhaps a few points need to be added for clarification: • Disintegration of a polymer is not the same as biodegradation, since this usually leads to other molecules than those mentioned in the OWS or OECD definition. Complete biodegradation usually has to follow the disintegration of a part. • So-called oxo-biodegradation makes use of chemical additives to mainly cause disintegration resulting in many different chemical moieties ending up in the environment. • Claiming biodegradability is not a very wise thing to do. After all, man is biodegradable but most people hope to last 100 years before that happens. • When claiming biodegradability, one should be very specific about the nature of the subject (geometry, construction material) and the biodegradation conditions (humidity, acidity, type of micro-organisms, temperature, aerobic/anaerobic). Usually, wood (50 % lignin) and several types of paper (low percentage lignin) biodegrade well in compost or soil, but not in freshwater, marine, or landfill, because lignin needs fungi for biodegradation, and these are not present in those environments. • Biodegradable polymers are no solution for littering, so should never be promoted as such. However, if they accidentally or on purpose end up in the environment it would be best that they fully biodegrade in every thinkable environment. It can be concluded that global unanimous acceptance of definitions from the examples mentioned in this article could be very hard to come by because environmental, legislative, and industrial parties often have a desired outcome regarding those definitions. However, if the world wants to do something about challenges like the plastic soup, perhaps the focus should not be on using the right definitions (i.e. what it is), but on what we need to change in order to meet our target, like get rid of the plastic soup” (i.e. what it does). The best way to accomplish this is to use materials that appear in, and that are made by nature. After all, also nature makes many different macromolecular materials that can be used as polymers for many construction applications. bioplastics MAGAZINE [03/22] Vol. 17 47
