This blog post is translated from German with Deepl..
Many of us have already done this: in the garden you lay out a large area of black mulch film to smother unwelcome herbage. The film is only a few µm thick and, according to the seller, should have rotted away after 4-6 months. Now, after 18 months, the film is still in the garden and has not decomposed. A colleague at work deposited his biodegradable coffee capsules, another colleague deposited the biodegradable plastic bags in the compost, but neither have degraded. Not a single corner of the capsule or plastic bag has decomposed. And what about the PLA filament that we at Gimelli Engineering, like many of our customers, use in one of the 3D printers? Is the idea of the biodegradable polymer just a nice wish or does the principle really work?
To be blunt, it exists and it works, but it has to be done “right”. However, let’s first differentiate the both seductive and fashionably diffuse syllable “BIO.”
- There are polymers made from renewable sources such as corn or sugar cane. They are called → bio-based plastics (see green oval in the graphic). The counterpart to these are the classic, mineral oil-based polymers (fossil-based).
- Just because a polymer is made from corn or sugar cane does not mean that it can be degraded by microorganisms. Only so-called biodegradable plastics can be decomposed → called bio-degradable plastics (see blue oval). There are also mineral oil-based polymers that can be biodegraded!
The following is an overview in terms of degradability and starting material:
The degradation process is a biochemical process carried out by microorganisms. They decompose the polymer to CO2, water and biomass. Due to printing on the plastics, there may be residual substances such as heavy metals and toxins, but these originate from the printing and not from the plastic material. The packaging standard EN13432, for example, describes test procedures for testing the residual amount of heavy metals and toxins in the degradation products. The decomposition process is influenced by PH, temperature, moisture, aeration/air exclusion, preparation of the material to be decomposed, and ultimately by the “feeding desire” or “performance” of the microorganisms.
A private compost is not exposed to controlled conditions due to its characteristics and is also relatively small, which is why the degradation process is rather poor when a biodegradable plastic is added. In industrial composting plants, the parameters are monitored and specifically controlled to ensure that the degradation process is controlled and efficient. This begins with the preparation of the material to be degraded, which is shredded to provide sufficient surface area for the microorganisms to attack. The rearrangement of the compost mass also takes place at the right time and is correspondingly easier than with home compost. According to EN13432 (requirements for the recovery of packaging by composting and biodegradation), compostable polymer must be degraded within 3-6 months to the extent that more than 90% of the polymer has been converted to CO2, water and biomass.
Picture: Possibilities of plastic recycling in general
Back to the examples mentioned at the beginning to clear them up. The mulch film purchased is a PLA blend. Biodegradable mulch films according to EN 17033 decompose after 24 months if they are plowed under. This has not (yet) been done in our example in the home garden. The basic principle of these mulch films: they should cover the soil and not decompose as long as they lie there on the surface. So that they do not have to be gathered up and disposed of in the trash after use, they are plowed under and only then rot as intended. Ergo, it would be time to get the spade and dig the film under in the garden to make the test successful. More information can be found here.
The coffee capsules and plastic bags are basically intended for industrial composting, we further learned. Exceptions are plastic products explicitly designed and certified for garden composting. More information on this can be read here.
PLA filament from the Gimelli 3D printer is biodegradable when actively recycled, according to the graphic above. This means that this material is also not suitable for domestic compost, because there are not the appropriate necessary conditions for degradation.
Image: Homecomposting – increasingly popular… but done right?
When dealing with biodegradable plastics at home, there is therefore a danger of equating “biodegradable plastics” with “garden composting”. This is not the case in every case. Biodegradable plastics are a reality, but in most cases require appropriate industrial processing and controlled composting for successful decomposition. It is possible that home composting will be possible in the future when more advanced plastics appear on the market.
Further interesting correlations can be read at European Bioplastics, and the EN 14995 standard Evaluation of the compostability of plastics might also be of further help if interested.
Now, logically, the next question arises: so where to put biodegradable plastic products such as coffee capsules or plastic bags if the local municipality does not offer an appropriate collection? In Milan, for example, it has been mandatory for a few years now to collect biological household waste, including biodegradable plastic bags, in a “brown garbage can”. That’s where you’re set up. And what does that look like in the community where you live?
The Federal Office for the Environment (FOEN) has put a small guide on the web about this: Bioplastics – all degradable?
Recyclable plastics are a reality and they work. But: plastic recycling is not equal to home composting. Biodegradable plastics require appropriate treatment in order to be recycled, which means that private individuals can and should collect, but must then return this waste to the recycling loop via, for example, recycling centers. Then the system works.
We humans must be more careful with our resources and strengthen recycling in general. Surprisingly, nature itself is helping us to do this. PET plastics (thermoplastic polyesters) have been around since about the 1950s. In 2016, the PETase enzyme was detected for the first time in the bacterium Ideonella sakaiensis, which allows the bacterium to feed on the carbon produced by the decomposition of PET. There are many interesting and further articles and technical reports on the Internet.
Image: Evolution continues: bacteria specializing in PET effectively decompose PET bottles