Degradation
The public’s perception is that plastics are bad because they don’t degrade. Countless websites, including the WWF, state that it takes 450, 500, or even 1000 years for plastics to degrade. Some even claim that they never degrade; they merely crumble into smaller and smaller pieces. This is from the book that may have started the popular perception of plastics degradation.
“Many plastics take as long as 500 years to decompose. Their very strength and durability make them a persistent pollution problem.”
M. Gorman in Environmental Hazards: Marine Pollution, ABC-Clio Inc. USA, 1993
That statement was simply made up, without proof of any kind. Nevertheless, it has been repeated over and over again by groups seeking to demonise plastics. In this chapter, we will look at the current perception and compare it to the scientific evidence. There are thousands of peer-reviewed articles on the topic of plastics degradation. What do they tell us? Is the popular narrative true?
There have been millions of experiments on the degradation of plastics. The reason for that is simple — when a plastic car part, piece of garden furniture, or medical device is made, the manufacturer must be certain that it will last the intended amount of time. What use is a bulletproof Kevlar vest if it crumbles to dust after a week? Plastic pipes bring us clean water. Can you imagine the cost of digging up and replacing those water pipes if they failed after a year or two? Brands want to make high-quality products, and because the cost of premature failure is so high, huge amounts of time and money have been spent researching the degradation of plastics.
Every day we see plastics degrading with our own eyes. Think of the polypropylene garden chairs that become white and brittle until the legs snap off when you sit on them. Think of the polycarbonate car headlamp covers that become yellow and foggy over time.
We are told by the WWF and others that plastic shopping bags take hundreds of years to degrade, but scientists have studied the degradation rate of polyethylene shopping bags, and all peer-reviewed studies found they disintegrate very rapidly, meaning less than one year left outdoors in the open.
“After 9 months exposure in the open-air, all bag materials had disintegrated into fragments.”
I. E. Napper & R. C. Thompson, Environmental Deterioration of Biodegradable, Oxo-biodegradable, Compostable, and Conventional Plastic Carrier Bags in the Sea, Soil, and Open-Air Over a 3-Year Period, Environmental Science & Technology, 53 (9), pp 53 (9), pp 4775–4783, 2019
“This study shows that the real durability of olefin polymers may be much shorter than centuries, as in less than one year the mechanical properties of all samples decreased virtually to zero, as a consequence of severe oxidative degradation…”
T. Ojeda et al., Degradability of linear polyolefins under natural weathering, Polymer Degradation and Stability, 96, pp. 703–707, 2011
O. Olaosebikan et al., Environmental Effect on Biodegradability of Plastic and Paper Bags, IOSR Journal of Environmental Science, Toxicology and Food Technology, 8 (1), pp. 22–29, 2014
Unstabilised low density polyethylene (LDPE) lost more than half of its strength in just 30 days when left exposed outdoors and lost over 70% strength in 90 days. The film was seen to crack and tear. Even with stabiliser added, the bags degraded rather rapidly because such items contain low amounts of stabiliser that are rapidly used up. Again, shopping bags are made from LDPE, and NGOs tell us, without evidence, that they take hundreds of years to degrade when science says just the opposite.
M. A. Tuasikal, Influence of Natural and Accelerated Weathering on the Mechanical Properties of Low-Density Polyethylene Films, International Journal of Polymer Analysis & Characterization, 19, pp. 189–203, 2014
Once again, we have been lied to by NGOs who make a living from demonising plastics.
Why do they degrade? Plastics are held together by the same chemical bonds as natural polymers like cellulose, silk, collagen, enzymes, and even the DNA that holds the program responsible for life. Since the chemistry is similar, the degradation rate and final degradation products are similar. All the materials just mentioned degrade to smaller and smaller particles, then to molecules until, eventually, they form carbon dioxide and water. They are attacked by oxygen, heat, and light, and despite what you may have been told, they biodegrade too.
“The ultimate products of degradation are CO2, H2O, and biomass under aerobic conditions. Anaerobic microorganisms can also degrade these polymers under anoxic conditions.”
J. Arutchelvi et al., Biodegradation of polyethylene and polypropylene, Indian Journal of Biotechnology, 7, pp. 9–22, 2008
Museum curators experience the deterioration of plastic items firsthand. They witness plastic and rubber exhibits becoming brittle and crumbling in real time, and they go to great lengths to preserve the fragile plastic items that reveal our past. I know that because a good friend of mine, Dr. Edward Then, was a plastics conservator at the Victoria & Albert Museum in London, England. As early as 1992, he was charged with working out what plastic each item was made of and how best to preserve it. That is not a simple task because conservators must analyse the exhibits without altering or destroying them, so the techniques they can use are limited to non-invasive types like infrared spectroscopy.
http://www.vam.ac.uk/content/journals/conservation-journal/issue-21/plastics-not-in-my-collection/
Books and thousands of peer-reviewed journal articles find that plastics do degrade. That is a scientific certainty — a fact. There is zero doubt. Here are the different ways that plastics are degraded by natural forces.
W. L. Hawkins, Polymer Degradation & Stabilization, Springer Berlin / Heidelberg, 1984
Inamuddin et al. (Eds.), Degradation of Plastic Materials, Materials Research Forum, 2021
Y. Shashoua, Conservation of Plastics: Materials science, degradation and preservation, Routledge, 2008
S. Balasubramanian, Degradation of plastics by Microbes, Lambert Academic Publishing, 2018
M. Srikanth et al., Biodegradation of plastic polymers by fungi: a brief review, Bioresources & Bioprocessing, 9 (42), 2022
G. Weber, U. T. Bornscheuer, R. Wei (Eds.), Enzymatic Plastic Degradation (Methods in Enzymology, Volume 648), AP, 2021
We have firmly established that plastics degrade, rather rapidly in many cases, but do we want them to? Looking at life cycle analyses, the answer is clear — products that are more durable tend to be greener. That being the case, what can we do to make plastics last longer? The answer is to copy Mother Nature. Just like natural nuts and oils contain vitamin E as an antioxidant, the plastics we use contain similar antioxidants and stabilisers. These are added in tiny amounts, usually in the 0–1000 parts per million concentration range, and yet they can greatly extend the useful life of the plastic materials we use. The useful life might be extended from years to decades. You may not realise it, but billions of dollars are spent each year on stabilisers to make plastics last longer and thereby make them greener. Companies would not spend billions on stabilisers for plastics if they really were stable like the NGOs claim.
Polymer Stabilizer Market by Type (Antioxidant, Light Stabilizer, Heat Stabilizer), End-use Industry (Packaging, Automotive, Building & Construction, Consumer Goods), and Region – Global Forecast to 2022 — Markets and Markets Report CH 5459, July 2017
Adding the right stabilisers also helps with recycling. Without any stabiliser, the plastic degrades rapidly and cannot be reused or recycled. Experiments show that an unstabilised polypropylene film degrades and becomes useless in less than a year at room temperature indoors. In fact, PP, one of our greenest and most widely used plastics, would not be of any use at all without a dash of stabiliser.
“Without stabilizers, the degradation of PP is so fast as to make this polymer unsuitable for most purposes. Even at room temperature unstabilized PP loses its mechanical strength within a year.”
P. Gjisman, J. Hennekens, J. Vincent, Polymer Degradation and Stability, 39, pp. 271–277, 1993
PVC is another common, versatile, inexpensive, and low environmental impact plastic that requires stabilisers to protect it from degradation when it is melted and processed. However, once properly stabilised, it can remain stable in service for decades.
There are many books filled with studies on the degradation of plastics under all kinds of conditions. Here is one study on the degradation of polyethylene (PE), polypropylene (PP), and polystyrene (PS) plastics outdoors. Degradation is obvious from left to right, even to the untrained eye, as the surface becomes pitted and rougher.
“The results suggest that the degradation of plastic debris proceeds relatively quickly in salt marshes and that surface delamination is the primary mechanism by which microplastic particles are produced in the early stages of degradation.”
J. E. Weinstein et al., From Macroplastic to Microplastic: Degradation of High-Density Polyethylene, Polypropylene, and Polystyrene in Salt Marsh Habitat, Environmental Toxicology & Chemistry, 35 (7), pp. 1632–1640, 2016
A Real-World Example
You may have seen me on CBS’s 60 Minutes TV show with Scott Pelley talking about PP medical mesh implanted into people. Polypropylene mesh is used for vaginal repair and for hernias. A class-action lawsuit started when 100,000 women reported problems, and similar lawsuits sprang up about men with hernia mesh. A key topic was the stability of the polypropylene plastic. Such mesh needs around 60 years of stability, but calculations showed it would only last 2–4 years before degrading. The prosecution presented evidence that there was not enough stabiliser added, and the wrong kinds of stabiliser were used.
The defence claimed that polypropylene is inert and does not degrade even though massive amounts of peer-reviewed science show the opposite. For example, here is just one study showing that polypropylene degrades through oxidation even at near room temperature.
L. Achimsky et al., On a transition at 80 °C in polypropylene oxidation kinetics, Polymer Degradation and Stability, 58, pp. 283–289, 1997
That was a real-world example of how plastics degrade rapidly and the consequences. We were able to get financial settlements for thousands of women. Note that my role was to show the truth about plastics because, as a professional, independent scientist, my goal is not to promote plastics but rather to expose the facts. My appearance on 60 Minutes was unpaid, whereas others accepted payment for their work on the show. I worked for free, as I believed it was important for those women to understand the truth about what had been done to them. I later appeared on the BBC and Sky News then assisted in a UK government inquiry, all for free and in the name of justice.
Why are common plastics so sensitive to attack by oxygen, heat, and light? The long molecules that make up plastic materials give strength to the material by tangling together. Only long chains can tangle well, in the same way that only long hair gets tangled. When the polymer molecules are attacked, it only requires the cutting of a few chains for the structure to unravel, leaving the material weak and crumbling. Think of a knitted sweater made of one long piece of yarn. As soon as the yarn is cut, the whole garment can unravel. The same concept applies to the polymer chains that form plastic materials.
Degradation of Other Plastics
Polyethylene and polypropylene are chemically similar, and both degrade rapidly. It is only the addition of stabilisers that produces the illusion of stability so that, to the layperson, they appear to be immune to degradation. Together, those two types of thermoplastics make up over 50% of the market, but what about other common plastics? Do they degrade as well?
Another common plastic is PET. Ioakeimidis et al. found that PET bottles degraded, with clear changes in the chemistry found by infrared spectroscopy. After 15 years in the sea, the characteristic chemical bonds were almost gone, indicating severe degradation.
C. Ioakeimidis et al., The degradation potential of PET bottles in the marine environment: An ATR-FTIR based approach, Scientific Reports, 6 (3501), 2016
A more recent study revealed that PET degrades more rapidly than previously thought in ocean water due to the presence of metal ions in the water. 50% degradation (depolymerisation back to the starting materials) was said to occur in 4.5 years and 100% degradation in 72 years.
“According to our research, the time of reaction for a PET conversion of 50% at 35 °C is only 4.5 years in any tropical zone of the Atlantic, Pacific and Indian Oceans or the Caribbean Sea. Also, total PET depolymerization, at a temperature of 30 °C needs only 162 years in any marine water on the globe. All these calculated data provide precise information about the period of depolymerization of waste PET floating in marine waters and correct old estimations of more than 400 years for the total degradation of waste PET.”
D. Stanica‐Ezeanu & D. Matei, Natural depolymerization of waste poly(ethylene terephthalate) by neutral hydrolysis in marine water, Nature Scientific Reports, 11, 4431, 2021
These numbers do not include the added degradation from ultraviolet light and marine organisms, so actual degradation is likely much faster still.
Although the chemistry of PET degradation is completely different compared to PE and PP, we still see that the plastic degrades over a period of years or decades, not centuries or millennia.
Even polystyrene, usually thought of as very resistant, was found to degrade much more rapidly than previously thought when exposed to sunlight.
“In the current study, we report the first direct evidence of complete oxidation of PS to CO2 by solar wavebands. All five PS samples were converted to CO2 by sunlight. For example, when exposing PS to increasing durations of simulated sunlight (up to 72 h), DIC increased, indicating that PS was completely photo-oxidized to CO2.”
C. P. Ward et al., Sunlight Converts Polystyrene to Carbon Dioxide and Dissolved Organic Carbon, Environmental Science & Technology Letters, 6, 11, pp. 669–674, 2019
PVC was also found to be attacked and biodegraded by larvae, thus dispelling the myth that it is impervious.
“The discovery in this study demonstrates that PVC can be depolymerized and biodegraded in Tenebrio Molitor Larvae, which extends observations of PS and PE biodegradation to another major polymer PVC.”
B.-Y. Peng et al., Biodegradation of Polyvinyl Chloride (PVC) in Tenebrio molitor (Coleoptera: Tenebrionidae) larvae, Environmental International, 145, 106106, 2020
New York proposed a ban on laundry and detergent pods because they claim such pods do not dissolve or degrade and instead form microplastics. However, the peer-reviewed science shows the opposite.
“In conclusion, PVOH used in liquid detergent capsule films does not meet any of the definitions of microplastic:(1) it is not micro- or nano-sized; (2) it is highly water-soluble; and (3) it is biodegradable in the environmental conditions where it is discharged.”
D. Byrne et al., Biodegradability of Polyvinyl Alcohol Based Film Used for Liquid Detergent Capsules, Tenside Surfactants Detergents, 58 (2), pp. 88–96, 2021
Why are people so keen to propose action before checking the facts first? It is unprofessional and counterproductive.
Biodegradation of Common Plastics
When people first realise that common plastics like PE, PP, PVC, and PET degrade, instead of being satisfied and relieved, they instead look for some other reason to cling to their negative attitude. They will say, “Well, perhaps they degrade, but they don’t biodegrade.” — Or words to that effect. However, they are wrong there too. Conventional plastics do biodegrade. There are many studies from research groups all over the world reporting and measuring the biodegradation of the plastics we use. As this idea is so contrary to the public perception, I will provide plenty of evidence below.
“This review discusses the literature on biodegradation of PE and PP. Most of the examples deal with fungi and bacterial degradation. Pre-treated polymers degrade more easily than the untreated polymers.”
J. Arutchelvi et al., Biodegradation of polyethylene and polypropylene, Indian Journal of Biotechnology, 7, pp. 9–22, 2008
“In this study, Lysinibacillus sp., isolated and identified as a novel strain, was investigated to decompose polyethylene and polypropylene. In the microbial cultivation medium without any physicochemical pretreatment, the Lysinibacillus sp. reduced the weight of polypropylene and polyethylene by approximately 4 and 9%, respectively, over 26 days.”
J.-M. Jeon et al., Biodegradation of polyethylene and polypropylene by Lysinibacillus species JJY0216 isolated from soil grove, Polymer Degradation and Stability, 191, 109662, 2021
“For LDPE, however, remarkable whitening of the film which was directly in contact with soil was observed. A lot of small holes which are passing through the film was observed around the whitened part. The degradation was more remarkable for samples which were buried in shallow places where the activity of aerobes is high.”
The rate of degradation is slower if the plastic is buried but faster if it is first exposed to sunlight to start the degradation process.
“The results show that high-molecular-weight polyethylene can really biodegrade under bioactive circumstances if the test period is long enough.”
J.-M. Jeon et al., Biodegradation of low-density polyethylene, polystyrene, polyvinyl chloride, and urea formaldehyde resin buried under soil for over 32 years, Journal of Applied Polymer Science, 56, pp. 1789–1796, 1995
“The Pseudomonas alcaligenes was found to be more effective than Desulfotomaculum nigrificans in degradation of polythene bag at 30 days.”
M. Ariba Begum et al., Biodegradation of Polythene Bag using Bacteria Isolated from Soil, International Journal of Current Microbiology and Applied Sciences, 4 (11), pp. 674–680, 2015
Polyethylenes and PVC were also found to biodegrade under marine conditions.
“The mineralization of plastic film was found to be maximum in LDPE followed by HDPE and PVC. Bacterial interaction had increased roughness and deteriorated the surface of plastics which is revealed by the scanning electron microscope and atomic force microscope.”
“The results of the present study revealed the ability of marine bacterial strain for instigating their colonization over plastic films and deteriorating the polymeric structure.”
A. Kumari et al., Destabilization of polyethylene and polyvinylchloride structure by marine bacterial strain, Environmental Science and Pollution Research, 26, pp. 1507–1516, 2018
“At least parts of the vast amounts of plastic litter in the ocean may thus serve as a carbon source for fungi and possibly other microbes, too.”
A. Vaksmaa et al., Polyethylene degradation & assimilation by the marine yeast Rhodotorula mucilaginosa, ISME Communications, 3 (68), 2023
“This study revealed that the active biodegradation of LDPE film by marine bacteria and these bacteria could reduce plastic pollution in the marine environment.”
S. D. Khandare et al., Marine bacterial biodegradation of low-density polyethylene (LDPE) plastic, Biodegradation, 32, pp. 127–143, 2021
People often criticise plastics for not degrading in a landfill, which is unjust because even paper and food degrade slowly in a landfill due to low oxygen levels. Scientists recovered decades-old newspapers that could still be read, which is how they knew how old they were.
W. Rathje & C. Murphy, Rubbish! The Archaeology of Garbage: What Our Garbage Tells Us About Ourselves, Harper Collins, New York, NY, USA 1992
Landfills are designed to slow down degradation because converting solids into carbon dioxide is what most people are trying to avoid. Even so, studies show that PE and PP degrade in a landfill, just like paper and other organic matter do.
“This research analyzed the degradability/biodegradability of polypropylene films (PP) and Bioriented polypropylene (BOPP) polymers after 11 months interred in the São Giácomo landfill in Caxias do Sul.”
L. Canopoli et al., Degradation of excavated polyethylene and polypropylene waste from landfill, Science of the Total Environment, 698, 134125, 2020
“SEM and OM revealed the start of degradation/biodegradation processes of the polymeric film in the landfill typified by microorganism colonies on the polymer surface, chromatic alteration and formation of cracks.”
C. Longo et al., Degradation Study of Polypropylene (PP) and Bioriented Polypropylene (BOPP) in the Environment, Materials Research, 14(4), pp. 442–448, 2011
“The evidence that biodegradation occurs comes from the increasing concentrations of the methylene chloride extraction products of the incubated polypropylene, together with the contemporary weight loss of the sample. Spectral analysis revealed that the extraction products were mainly hydrocarbons.”
“Hence, we suggest that the well-known metabolic flexibility and adaptability of microorganisms and mycelia can result in the biodegradation of isotactic polypropylene and polyethylene, two macromolecules that supposedly are highly recalcitrant to biological metabolism.”
I. Cacciari et al., Isotactic polypropylene biodegradation by a microbial community: physicochemical characterization of metabolites produced, Applied and Environmental Microbiology, 59 (11), pp. 3695–3700, 1993
PET was found to degrade in sunlight and even more quickly when moisture and soil were present as well. Polymer chain scissions means breaking the long molecules into shorter ones. Such degradation weakens the plastic material.
N. Allen et al., Physicochemical aspects of the environmental degradation of poly(ethylene terephthalate), Polymer Degradation and Stability, 43, pp. 229–237, 1994
“FTIR analysis implies structural changes in biodegraded PET samples unlike the control. The biodegradation is further substantiated by SEM which manifested the development of fissures and a sign of significant erosions which were progressive with the incubation time.”
M. G. H., Zaidi, Comparative in situ PET biodegradation assay using indigenously developed consortia, International Journal of Environment and Waste Management, 13 (4), pp. 348–361, 2014
“We eventually found a unique microbial consortium, named No. 46, in a landfill. This consortium is able to grow on low-crystallinity PET film; it assembles on the film and utilizes PET as a major carbon and energy source, degrading it into CO2 and water.”
K. Hiraga et al., Biodegradation of waste PET, Science & Society, 20, e49365, 2019
We have now seen a robust array of studies illustrating that PE, PP, and PET biodegrade, but what about polystyrene? Most people believe it to be non-degradable.
“Fed with Styrofoam as the sole diet, the larvae lived as well as those fed with a normal diet (bran) over a period of 1 month.”
“Within a 16 day test period, 47.7% of the ingested Styrofoam carbon was converted into CO2.”
“The discovery of the rapid biodegradation of PS in the larval gut reveals a new fate for plastic waste in the environment.”
Y. Yang et al., Biodegradation and Mineralization of Polystyrene by Plastic-Eating Mealworms: Part 1. Chemical and Physical Characterization and Isotopic Tests, Environmental Science & Technology, 49, 20, pp. 12080–12086, 2015
You read that correctly — mealworms fed only with polystyrene foam survived perfectly for a month and converted the plastic fully into carbon dioxide. I was surprised too. In fact, I was so surprised that I checked to make sure this was real and replicated by other research groups.
“Academics researchers and “citizen scientists” from 22 countries confirmed that yellow mealworms, the larvae of Tenebrio molitor Linnaeus, can survive by eating polystyrene (PS) foam.”
“The results indicate that mealworms from diverse locations eat and metabolize PS and support the hypothesis that this capacity is independent of the geographic origin of the mealworms, and is likely ubiquitous to members of this species.”
S.-S. Yang et al., Ubiquity of polystyrene digestion and biodegradation within yellow mealworms, larvae of Tenebrio molitor Linnaeus (Coleoptera: Tenebrionidae), Chemosphere, 212, pp. 262–271, 2018
The same mealworms could also eat other plastics, including polyethylene and polyurethane.
“Microbial degradation in environmental conditions in vitro is extremely slow for major plastics at degradation rates on the basis of a month or even a year time, but recent discoveries show that the fast biodegradation of specific plastics, such as PS, PE, and PUR, in some invertebrates, especially insects, could be enhanced at rates on basis of hours.”
X.-G. Yang et al., Plastic biodegradation by in vitro environmental microorganisms and in vivo gut microorganisms of insects, Frontiers in Microbiology, 13, 1001750, 2023
It is not only one type of mealworm that can perform this amazing feat; other larvae and also snails can do the same.
“For the first time, this study reveals that land snails Achatina fulica has the capacity to depolymerize and biodegrade polystyrene. Mass balance, GPC, FTIR and 1H NMR analyses confirmed the limited extent de-polymerization and oxidation of PS polymers, which supported the occurrence of biodegradation.”
“Concerning land snail was one of the mostly popular and rapidly proliferated terrestrial animals, these findings are significant in regards to the fate of plastic litter and its biodegradation in soil environments.”
Y. Song et al., Biodegradation and disintegration of expanded polystyrene by land snails Achatina fulica, Science of the Total Environment 746, 141289, 2020
So, insects and snails can biodegrade plastic, and it turns out that bacteria can degrade a wide range of plastics as well.
“This review has discussed the microorganisms and enzymes reported to biodegrade these synthetic polymers. Many strains of Pseudomonas and Bacillus have been observed to degrade complex, recalcitrant compounds such as polyaromatic hydrocarbons, and have been associated with the partial degradation of a wide-range of petro-plastics, including PE, PS, PP, PVC, PET and ester-based PU. The gut microbes in insects have also been found to depolymerize PE, PS and PVC polymers. Enzymes specifically associated with depolymerization of PET and ester-based PU have been identified and intensively studied, while enzymes that effectively depolymerize PE, PP, PS, and PVC have not yet been identified and characterized.”
N. Mohanan et al., Microbial and Enzymatic Degradation of Synthetic Plastics, Frontiers in Microbiology, 11, 580709, 2020
“After considering the above results of the present study, it is to be concluded that PET and PS can be degraded by micro-organisms (biodegradation) like Pseudomonas aeruginosa, Bacillus subtilis, Staphylococcus aureus, Streptococcus pyogenes, and Aspergillus niger, present in different types of soils.”
K. Asmita et al., Isolation of Plastic Degrading Micro-organisms from Soil Samples Collected at Various Locations in Mumbai, India, International Research Journal of Environment Sciences, 4 (3), pp. 77–85, 2015
Not only do plastics degrade by heat, light, and oxygen and biodegrade via bacteria and insects, but fungi are also proven to contribute to plastics biodegradation.
“The oxidation or hydrolysis by the enzyme creates functional groups that improve the hydrophilicity of polymers, and consequently degrade the high molecular weight polymer into low molecular weight. This leads to the degradation of plastics within a few days. Some well-known species which show effective degradation on plastics are Aspergillus nidulans, Aspergillus flavus, Aspergillus glaucus, Aspergillus oryzae, Aspergillus nomius, Penicillium griseofulvum, Bjerkandera adusta, Phanerochaete chrysosporium, Cladosporium cladosporioides, etc., and some other saprotrophic fungi, such as Pleurotus abalones, Pleurotus ostreatus, Agaricus bisporus and Pleurotus eryngii which also helps in degradation of plastics by growing on them.”
PE: Phanerochaete chrysosporium, Aspergillus, Cladosporium, Fusarium, Penicillium, Phanerochaete, Pencillium. Simplicissimum, Aspergillus niger, Aspergillus japonicas and Fusarium. sp., Penicillium chrysogenum NS10
PP: Bjerkandera adusta, Lasiodiplodia theobromae, Coriolus versicolor
PS: Cephalosporium spp., Mucor spp. Gloeophyllum striatum, Gloeophyllum trabeum DSM 1398, Pleurotus ostreatus, Phanerochaete chrysosporium
PUR: Gliocladium roseum, Aspergillus spp., Emericella spp., Fusarium spp., Penicillium spp., Trichoderma spp., Gliocladium pannorum, Nectria gliocladiodes, Penicillium ochrochloron, Aureobasidium pullulans, Rhodotorula aurantiaca, Kluyvermyces spp.
PC: Phanerochaete chrysosporium NCIM 1170, Geotrichum spp., Fusarium, Ulocladium, Chrysosporium, Penicillium
PET: Fusarium, Humicola, Candida antarctica, Aspergillus sp., Penicillium sp., Fusarium sp.
PVC: Cochliobolus sp., Phanerochaete chrysosporium, Aspergillus niger, Penicillium funiculosum ATCC 9644, Trichoderma viride ATCC 13631, Paecilomyces variotii CBS 62866, Aureobasidium pullulans, Chaetomium globosum, Rhodotorula aurantiaca, Kluyveromyces spp.
M. Srikanth et al., Biodegradation of plastic polymers by fungi: a brief review, Bioresources & Bioprocessing, 9 (42), 2022
The scientific evidence is clear — plastics degrade and biodegrade.
Biodegradable Plastics
Biodegradable plastics are designed to degrade, but do they make sense? They seem superficially attractive because we could throw our litter on the ground or in the ocean and then “abracadabra,” it would vanish all by itself. That sounds marvellous, doesn’t it? However, life cycle analysis (LCA) studies show that biodegradable plastics have a greater impact than normal plastics like PE and PP. Of course, they are more expensive too and they have worse properties. Plus, when they degrade, they rapidly release carbon dioxide, which is just what people are campaigning against because it is a greenhouse gas.
What about the particles formed when conventional plastics degrade? One common argument proposed for biodegradable plastics is to prevent microplastics. Some people believe plastics degrade to small particles and then degradation stops, but that is not the case. In fact, the smaller the plastic pieces are, the faster they degrade because oxygen and bacteria can attack them more readily. The reason is simple — degradation occurs mainly at the surface and the smaller the particles become, the greater the surface area exposed. More about that later.
Oxo-degradable plastics are where a catalyst (usually iron, nickel, manganese, or cobalt stearate) is added to a plastic like PE or PP to make it break down more rapidly. They are sold as green products, but the green claims do not stand up to scrutiny. Firstly, we know that durable products create less impact, so speeding up failure is unwise. Degradation means converting solids into greenhouse gas, which is the opposite of what most people consider desirable. In addition, the catalysts can contaminate the recycling stream, destabilising the rest of the PE and PP plastic and ruining their recyclability. We know PE and PP degrade rather rapidly in the environment anyway, so if we wanted those plastics to degrade faster, there is no need to add a catalyst. Instead, it would be cheaper to just remove the stabiliser. So, for good reason, oxo-degradables have been banned in the EU, and other regions are likely to follow.
Perspective & Context
We are led to believe that plastics are intrinsically evil because they last forever, whereas other materials do not. Is that really the case? No, it is not, because other common materials like ceramics, metals, stone, and glass all take longer to degrade than plastics do. Even paper can take longer to degrade than common plastics like PE, PP, and PET, depending on the conditions. The oldest paper documents known are over 1000 years old and still readable. In fact, it has been estimated that paper takes 2700 years to degrade at room temperature when dry. Compare that to polypropylene film, which has been shown to disintegrate in under one year. Why does paper take such a long time to fragment and decay? The answer is that paper contains a large amount of natural stabiliser called “lignin,” which is very effective at protecting against oxidation.
As a rule of thumb, a piece of common plastic like PE or PP will degrade at about the same rate as another piece of organic matter of the same size and shape. So, a PE or PP film will degrade similarly to a leaf or a piece of paper. Why is that? It is because PE and PP are organic materials made of carbon-carbon bonds, just like other substances such as cellulose, lignin, cotton, and so on.
When the object is thicker, degradation takes far longer. Fallen sequoia trees have remained intact for at least 500 years with hardly any degradation (Scott, 1999) in the same way that a gigantic piece of plastic, metal, or glass would take much longer to degrade.
Gerald Scott, Polymers and the Environment, RSC Paperbacks, p. 97, 1999
The exact degradation rate depends on temperature, the size of the object, the amount of sunlight, and so on, but the fact remains that common plastics degrade as quickly or even more quickly than the other materials we encounter.
Clearly, claiming plastics are bad because they take longer to degrade than other materials is not a valid argument, as it is not true.
It has been claimed that plastics create a problem because they eventually release CO2 when they degrade. Would that be a fair criticism of plastics relative to other materials? The answer is no because all organic matter does that too — leaves, wood, cotton, jute, hemp, and paper all degrade in the same way.
Plastic Prejudice
When we discover a 400-year-old wooden ship in the ocean, we celebrate, build a museum, and sell tickets to look at this “treasure.” The same applies when we find 2000-year-old Roman coins made of metal. Stonehenge, a bunch of 5000-year-old rocks, attracts a million visitors per year while 15 million flock to see the pyramids.
Whether it is glass, clay, stone, animal remains, wood, or metal, we are filled with joy to find it, and the older it is, the better. A recent scientific paper even hailed the discovery and analysis of 2700-year-old human excrement.
F. Maixner et al., Hallstatt miners consumed blue cheese and beer during the Iron Age and retained a non-Westernized gut microbiome until the Baroque period, Current Biology 31, pp. 1–14, 2021
There is a clear plastics prejudice at work, whereby it is implied that plastics are evil if they take a long time to degrade when every other material is celebrated when it does not degrade. How unjust.
Summary
Let us summarise what we have discovered and what policies might make sense based on the evidence. We have seen that the notion that plastics don’t degrade is false and is, therefore, not a fair or valid criticism.
We know from life cycle analyses that the degradation of plastics is not desirable because it makes products less green. Durable products usually minimise environmental impact. Therefore, in most cases, we want to increase the life of plastics, and we do that by adding the appropriate type and amount of stabiliser.
It follows that biodegradable plastics make little sense. They increase harm to the environment, according to LCA studies, in part because they rapidly release carbon dioxide as they degrade. They also cost more and have worse properties than standard plastics we are familiar with.
Biodegradable plastics are not a solution to litter either. Quite the opposite: degradables exacerbate the problem because they encourage people to litter more.
It turns out that the greenest path is to continue using the plastics that cause the least impact, such as PE, PP, PVC, and PET. Stabilisers should be added to adjust the degradation rate and to ensure that the material is in a fit condition to be recycled into new objects.
Thin PE shopping bags contain minimal amounts of stabiliser and disintegrate in under one year outdoors, which is a similar rate to paper bags. At the other end of the scale, we have durable products like water pipes, which are thicker with more and better stabilisers added to ensure that they last a hundred years or more.
This is the ideal situation in which we can control the plastics degradation rate to be optimal for each different use case.
Knowing all of this, it becomes clear that people are not really against plastics because they do not degrade rapidly enough for them; after all, they do not care that other materials like concrete, metal, glass, and ceramics all degrade slower than plastic. Nor do people criticise paper and wood, even though they degrade at a similar speed to a similarly sized piece of PE or PP plastic. No, the real reason that people want plastic to degrade faster is so that they can drop it on the floor and have it magically vanish. This is the only explanation that makes sense. It is the driving force behind sales of biodegradable and compostable plastics.
This is a rare instance where scientists know the answers, but it might be better not to communicate them too widely to the public because when the customer thinks that the litter will degrade, then they litter more.