In the first post I tried to define the different types of bioplastics and the biodegradation and composting processes.
Here’s the graphic to refresh the different bioplastics:
Source: European Bioplastics
In this second post about biodegradable plastics, I’ will summarize the main arguments for and against the main two types of “biodegradable” polymers.
The case for Bio-based plastics
So as I mentioned before, I will leave out the non-biodegradable bio-based plastics and focus on bio-based and biodegradable/compostable plastics:
That leaves us with the most important bio-based biodegradable plastics are:
We could also add polybutylene succinate (PBS). The most important are PLA and starch based polymers, here are the main suppliers:
Bio-based biodegradable advantages
The main benefit for these plastics is their renewable origin (crops, microorganisms), followed by their compostability.
Let’s not consider non-biodegradable bioplastics like bio-based PET. They would be treated as their fossil based alternative.
There’s no separate recycling stream for bio-based biodegradable plastics, so they will not be part of any mechanical recycling will end with the rest of non-sorted plastics and used as feedstock, landfill or as energy (incineration).
Of course, the best option to manage the waste of these types of plastics would be composting. I’ve commented some about this option in section, but let’s see a diagram of industrial composting:
Source: Bio-based and biodegradable plastics – Facts and Figures Focus on food packaging in the Netherlands, Martien van den Oever, Karin Molenveld, Maarten van der Zee, Harriëtte Bos, April 2017. http://edepot.wur.nl/408350
Other benefits of these plastics are their reduced carbon footprint (because of their reduced dependency on fossil fuels, reduced greenhouse gases emissions, and less energy demand during production), help conserve petroleum supplies, and increase resources efficiency.
Of course, composting would reduce the volume of municipal waste (if collected together with food scraps or yard waste).
Against bio-based biodegradable polymers
There’s not a consensus advocating the advantage of bioplastics during their end of life. Most of the arguments against must be taken with a grain of salt, since they come from their competition, the producer of oxo-biodegration additives to be used in normal polymers.
Among these arguments most of the appearing in the document “Rethinking the Future of Plastics” are:
- The term ‘biodegradable plastic’ should not be used, as it immediately begs the question whether you mean oxo-biodegradable or hydro-biodegradable”.
- The designed end-of-life for these materials is composting, whether in an industrial plant or home compost. In other environments they biodegrade but very slowly, and generate CO2 in landfills
- They have a different origin than regular plastic and they must be kept separate when recycled, lest it contaminate the recycling stream.
They also contend the reduced carbon footprint claim due to the energy and materials used in growing the crops.
Image from Rethinking the Future of Plastics
- Fertile land should not be used to produce plastics.
- Performance processability issues.
- The (in my opinion) unlikely possibility that pesticides sprayed on the crops may contaminate the final product is also mentioned 9. On the other hand, the use of biodegradable polymers to release pesticides to the crops 10 is also being studied.
- Plant-based biodegradable plastics come for genetically modified crops (specially PLA from GM corn). Also, Genetically modified (micro)organisms (GMOs) are sometimes used in the production of crop-based biodegradable polymers:
The case for additivated biodegradable polymers
The second type of biodegradable polymers, are common polymers that became biodegradable by adding substances (sold as masterbatches, with a loading rate of 1-10%) that makes them biodegradable (although it seem that the jury’s still out I about that).
There are two main types of additives (two types of degradation mechanisms enables according to the nature of the additives): oxo-biodegradable and enzyme-mediated plastics (also sold as “organic” or “bio” additives).
Conventional plastics be enriched with organic additives, resulting in so called “enzyme-mediated degradable” plastics.
According to the producers of “enzyme-mediated degradable” additives, the organic additive, together with its carrier material (in most cases ethylene vinyl acetate), is consumed by the micro-organisms which later excrete acids and enzymes that should break down the plastic into materials that are easily consumed by microbes.
However, the report Review of information on enzyme-mediated degradable plastics (Deconinck, S. and De Wilde, B. 2013) concludes that there could find no scientific articles on the behavior of this plastics nor was the principle of the degradation scientifically explained or the available data showed complete biodegradation.
In this post we are going to focus on the second type of additives:
They are based on conventional plastics, like polyethylene (PE), polypropylene (PP), polystyrene (PS) and polyethylene terephthalate (PET), to which additives are added.
The additives should cause the plastic to degrade by a process initiated by oxygen and accelerated by light and/or heat.
To summarize the process, the catalytic effect of the additive breaks the carbon=carbon bond releasing free radicals. These free radicals react with the oxygen atoms in the environment and generate low molecular weight subproducts (aldehydes, ketones, alcohols). These subproducts are hydrophilic and can be colonized by microorganisms and fungi. Light, heat and mechanical stress accelerate the process.
Source:SlideShare d2w: Tecnología oxo-biodegradable para plásticos
The additives are typically metal salts of carboxylic acids or dithiocarbamates based on cobalt (Co), iron (Fe), manganese (Mn) or nickel (Ni), with Co being used more for packaging and Fe and Ni more for mulch film. Other transition metals like Cerium (Ce) have also been reported to exhibit strong pro-oxidative effects.
Oxo-biodegradable plastics (OBDs) advantages
Their main advantage consists on turning normal plastics (PP, PE, PS, and even PET, but no so clearly) into biodegradable polymers. The main arguments in favor also come from suppliers. Most of them are based on the fact of continuing to use the same polymers we are currently using (HDPE, LDPE, PP, PET) either fossil based or from renewable sources, with the same features and uses. Then, at the end of their after their life, they will disintegrate in landfill.
These polymers are much cheaper than the bio-based ones. The amount of additive can be as small as 1% and polyolefins would be destroyed in 5 years or less.
Also the suppliers claim that they can be sorted and recycled as conventional plastics, with the advantage that they will degrade in the environment by the action of microorganisms.
Again, the main literature for these advantages comes from the suppliers 11, or form the Oxo-Biodegradable Plastics Association 12). Most of the independent sources recommend ox-biodegradable plastics for limited applications such as plastic shopper bags 13 or the aforementioned Review of information on enzyme-mediated degradable plastics.
Against Oxo-biodegradadble plastics
The bio-based polymers community (European Bioplastics) is – as expected – virally against the use of these plastics, but there are several articles and reports that question their biodegradability, and saying that there’s only a microfragmentation that can be even more harmful to the environment (14, 15, 16).
OBD polymers are being heavily criticized by several public and private institutions and companies.
Over 150 organizations (FMCG companies, technology institutes, and (not surprisingly) bio-based polymer supplier) have backed a statement recommending the banning of oxo-degradable plastics.
The controversy has already led the European Commission to issue a report against OBDs. This report announces a process to restrict the use of oxo-plastics in the EU will be started. At the same time The Netherlands is looking to ban oxo-degradable plastics, and in France a group of have called for a similar ban for plastic bags.
However, it seems the fight is not over 17.
Other less important arguments against the use of OBDs are:
- The additives used come from fossil fuels (the counterargument is that they’re a by-product of oil refining, and that the amount t used is very mall ).
- They contain heavy metals (again, the amounts don’t see to be of concern).
After a neither thorough not superficial look at the different argumentations for both types of biodegradable polymers, I’ll try to summarize some key points, and some personal opinions:
Bio-based biodegradable polymers (BBPs):
- Bio-based biodegradable polymers are best disposed of by composting, where they will generate biomass and close the circle. These plastics may cause problems when recycled together with fossil based plastics or in landfills. Their sustainability increases when used in for disposable tableware or as food packaging, were they can be composted together with the food scraps.
- The claim that OBPs can be recycled as conventional plastics seem to be contested by UK recyclers 33 .
- The concern about using crops for plastic production is in my opinion justified, as well as the use of genetically modified crops. My personal concerns about them are twofold: There’s no data about their long-term effects, and the creation of a de facto monopoly of crops for food.
- When BBDs are thrown into the trash, they may be ended up in landfills, where even PLA manufacturers admit and it may take very long time over there to compost due to lack of appropriate conditions. Moreover, there’s evidence that PLA products may generate methane in landfills.
- It’s no easy, even in developed countries to implant adequate sorting processes and facilities for industrial composting. Also, not all lifestyles make home composting easy (people live in flats and have no gardens, and there’s little space and no use for the composted biomass)
- It doesn’t seem that oxo-biodegradable plastics are really biodegradable, but it is beyond my knowledge to make an affirmation one way or the other. I think that the weight of evidence regarding the biodegradability of plastics with OBD additives is in the manufacturers. Therefore I completely understand any cautionary measure to avoid increasing the amount of microplastics in the environment.
- There’s also concern about the effects of polymer microfragments when they reach microscopic levels since they can be easily transported anywhere and be ingested by the animals (mainly by marine animals in the ocean).
- The degraded plastic fragments can also attract and retain hydrophobic elements such as PCG (polychlorinated biphenyls) y DDT (dichlorodiphenyltrichloroethane). Japanese researchers have reported high concentrations PCBs (polychlorinated biphenyl), DDE (dichlorodiphenyldichloroethylene) and NPs (nonylphenols) in biodegraded PP.
July 2018, Bruno Rey – The Packaging Blog –