Degrading to the degree it makes sense…

About degrading materials, its not very easy to understand with all these statements and brands coming out. This my short view on degradation of materials, in focus plastic materials. Let us talk about one aspect first; time, as in long time and short time. Most materials do degrade in natural environments like in the ocean or in the soil, however, the time scale of which this occur is very different for each material and in each environment. Looking at plastic materials, the word degrade should refer to the polymers in the plastics going from a long chain of monomers to individual monomers and/or their derivatives and then we can add that they can hopefully be metabolized by some species by natural inclution in digestive cycles and formation of let is say, sugar or something useful.

There are therefore a few things that can help most plastic materials to degrade i.e cut bonds in polymers; high temperatures, sunshine (UV-degradation), high or low pH (acid or alkali conditions), high salt content, microorganisms and funghi using enzymes to cut the polymer chains. For many natural polymers made in nature (such as cellulose in trees or starch in corn) water act as a facilitator of degradation and ”cutting” polymers, in the act of swelling for microorganisms to penetrate and/or allowing water to chemically attack bonds and ”cut them” apart, so called hydrolysis.

All these different parameters can commonly increase the degradation, but it also means that most polymer will act differently on each stimuli. The parameter that is most connected to degradation of plastics, is the bond that connect the monomers into polymers since it gives the polymers and therefore the plastics its bulk material properties.. these bonds are needed to be cut like with a scissor of a pearl necklace and the individual pearls or monomers. Needed if the polymer is about to degrade i.e. if the plastic should sufficiently degrade. The natural polymers made by nature has commonly bonds that by some species or chemical nature in some environment will be degraded, note not necessarily everywhere (compare the ocean and a dry soil… with very different chemical environment). The common bonds are ester, ether and amide bonds for enzyme degradation. These are found on many places in natural polymers. Ester is most common perhaps to use for the degradable man-made synthetic polymers such as poly(lactic acid) used in this renewable, “compostable” coffee bag, very nice for the purpose.

So man-made bonds, in synthetic polymers, that are too strong to degrade easy in nature hence plastic pollution, are bonds in polymers like poly(ethylene terephthalate) PET and poly(ethylene) (PE). These bonds where made strong to not degrade so easily to last and for example be used for water hoses and plastic cups and bottles etc, and they do not degrade easily. They are water repellant which helps a lot in this process to be protected against microorganisms and fungi, our “natural degraders”. There are reasons for the slow degradation being for example in PE’s case the bond to connect monomers, a strong carbon-carbon bond, unknown to most “natural degraders“. The ester bond in PET allows for more possibilities, why scientists have found PET degrading enzymes, see Nature paper from this year: Structure of the plastic-degrading Ideonella sakaiensisMHETase bound to a substrate (2019) GJ Palm et al. https://www.nature.com/articles/s41467-019-09326-3

So about the aspect of time when it comes to degrading materials, long and short time. Also to add the difference between biodegradable (“natural degraders” will be enough) and degradable (probably need chemical tough conditions). The easier and more degrading environment you place the plastic material the faster it degrades. And short should be within a couple of weeks and maximum 6 months period, so ask people and companies who claim degradable: “how long time”. Because many plastic materials will after tens or hundreds of years degrade i natural environment, like the oceans, but it would cause too much damage before that happens so it is not considered degradable during a reasonable time span.

To conclude, materials are to be produced with more reversed engineering, i.e 1. Choose product and user needs to know material demands, 2. create or pick the materials (renewable where applicable and exisiting) that degrade upon the fulfilled need and use they are targeted for and allow for degradation after usage. That would help so much in consumer materials use. And as consumer ask for more information about your materials from suppliers, degradable and compostable is important properties but not always what it sounds like. Should be put in perspective of time.

This particular coffee bag made with Ingeo PLA oxygen barrier film, since paper is not sufficient alone, actually note compostable according to an ASTM standard (very informative and good webpage https://www.astm.org/standardization-news/?q=features/standards-biodegradable-plastics-ma19.html). Learn about ASTM standard D6400, it is more and more used in labelling.