Putting 3D Prints in a home compost

There are a ton of filaments for 3D printing on the market that advertise themselves as being biodegradable or compostable. If you do more research, you’ll find that most of them, especially PLA, will only biodegrade on industrial composts, but what happens if you put some of these materials in a regular garden compost pile that many of us have at home? There have been other videos on that topic by Hobbyhoarder and recently Angus from Makers Muse, but I think my real-life test and the materials I used will add quite a bit to the discussion!

Last summer, I went through my pile of filament and selected four different biomaterials for this test. At first, regular PLA from the German filament maker DasFialment. They state on their website – “made from renewable sources, hence biodegradable”. Let’s talk about this in a bit. Then I chose colorFabbs PLA/PHA, a blend of two different biopolymers to reduce the inherent brittleness of PLA. Both components should be biodegradable. Next, I used GreenTEC Pro from Extrudr, another local filament manufacturer. GreenTEC Pro is made from PLA, CoPolyester, and Additives and is biodegradable per DIN EN ISO 14855. Finally, I bought a roll of Filamentums NonOilen material that made some headlines last year because it’s advertised to decompose three times faster than PLA. NonOilen is made from a blend of PLA and PHB. As a wildcard contestant, I also put some of my own Coffee PLA into the compost to see if the coffee grains help during the decomposition process. My Coffee PLA is made from NatureWorks 4043D PLA and fine coffee grounds. I didn’t have a lot of this material, so I was only able to print one sample.

I made two identical sets of test samples for every material consisting of a 3DBenchy, a test hook, and four standing as well as 4 lying tensile test samples. Additionally, I designed a stepped composting test sample consisting of different material thicknesses from 1 to 6 printed layers, which equals 0.2 to 1.2 mm. That’s quite important because it might tell us how fast the decomposition is going. Biodegradation is a process of hydrolyzation where water in the material breaks down the polymer chains into shorter junks and then the work of fungi and bacteria with their enzymes that slowly chew away the material, which can only happen on the surface. That’s why thicker materials take longer to decompose than thinner ones. I stringed one set of samples on stainless steel wire to avoid getting lost in the pile. The other set will remain in my office as a reference. I then secured a piece of telephone wire to everything that I’ll attach to the outside of the compost so I could backtrack everything easily.

The experiment started in mid-August 2021. I removed some fresh material on the top of our compost, put all the samples inside, and covered everything again. Our compost pile is nothing special, and I’m aware that it’s far from “industrial” composting conditions. It is a thermo-composter which means that it’s a closed container with a double wall for insulation to prevent it from cooling off too much. We fill it with our kitchen waste, coffee grounds, garden leftovers, chopped grass from time to time, and chicken and bunny droppings. Some might call it too wet; some too dry some might say that it’s not aerated enough. The only thing I can say is that it doesn’t smell rotten, which is a good sign for aerobic composting. It is full of worms, fungi, and larva and gives us great compost that we use to grow our vegetables on once a year. It’s definitely not perfect, but a good starting point if we want to investigate which materials decompose under good conditions in the environment.

But let’s quickly talk about some important terminology that often gets mixed up and misused. The term biodegradable is defined in standards. Here in Europe, it’s in EN 13432. In the US, it’s defined in the ASTM D6400. The definitions vary slightly, but they generally say that a polymer is biodegradable if most of it can be decomposed by living organisms within a certain time limit into CO2, water, minerals, and biomass. The leftovers need to be non-toxic and beyond a certain size limit. Compostable materials are a subset of biodegradable materials that only decompose under very strict conditions involving elevated temperatures, specific moisture levels, and organisms usually only found in industrial composting plants and not home compost piles. PLA, for example, is one of these materials. This means that PLA will not biodegrade to CO2, water, minerals, and biomass if the right conditions aren’t met. PLA might decompose and break apart over time in outside conditions, but this will leave you with PLA microplastics that will pollute the environment for many years.

The next misunderstanding starts with discussing bio-based polymers and if all bio-based polymers are automatically biodegradable or compostable. Let’s look at this graph. This sorts polymers into four different groups: Bio-based and bio-degradable polymers, fossil-based and bio-degradable polymers, bio-based but not bio-degradable materials, and our conventional polymers that are fossil-based and not-biodegradable. This nicely shows that all combinations are possible, and the source on which a material is based does not define if it’s biodegradable or not. There are materials like Bio-PET used for bottles that are plant-based but are just as bad when thrown away as normal PET. On the other hand, there are petroleum-based materials like PVA, that we use for water soluble supports, that are biodegradable. The source of a material has an impact on its carbon footprint but does not define if it’s biodegradable or not.

Source: https://www.european-bioplastics.org/bioplastics/

Back to our composting test. At the end of November last year, after coincidentally 100 days, I thought that I’d aerate our compost and check in on the parts. I opened our Thermo-composter, which revealed a nice, layered view of the pile with the fresh material on top and the more and more composted materials towards the bottom with a lot of life in it. After carefully shoveling out the material, I first found the subscribed sign I put into the compost when the test started, which looked entirely untouched by the environment. Following the telephone wire, I also quickly found the rings of samples, which, at least at first glance, didn’t show huge signs of degradation. I washed them with water and let them properly dry for a good week.

The results were exciting, and without a detailed inspection, I could see that the parts significantly changed in color. The blue, pink, and green prints looked really pale, and the formerly white NonOilen parts got a yellow-brownish color. I wasn’t able to spot any signs of degradation on the regular PLA parts. The prints still felt very solid, only some of them were really deformed due to the pressure in the compost together with the raised temperatures. The neon-pink PLA/PHA parts lost quite some color, and besides being slightly deformed, they didn’t show any signs of degeneration. GreenTEC Pro didn’t show any deformation, probably because it’s the most temperature resistant of the bunch, but you could interpret some degradation into the missing fine hairs on the 3DBenchy.

Filamentums NonOilen was the most interesting, and I’m happy that I included it. At first, two tensile samples already broke in the compost because the material became so weak. On the degradation card, all of the membranes were still present, but the material became really fluffy and looked as if it slowly fell apart. On the 3DBenchy, it was most apparent because some chunks of material were missing, which clearly shows that some of the material degraded. Lastly, there was the test card from my own Coffee PLA that I put into the mix. Unfortunately, I don’t have a reference part to directly compare it to because I had so many problems printing the material. Still, this sample also showed some interesting changes. Most obvious, it’s deformed but also became very opaque, which might be a sign of annealing due to the elevated temperatures in the compost. Then the thinnest section became fluffy, and there were a ton of holes in the surface where the coffee grains once were. Again, this material didn’t magically fall apart, but such a bio-composite might help the degeneration process.

The initial idea was to only check in on the parts, put them back into the compost, and then check in on them after the winter but not test samples for strength. Because there were so few signs of degradation on all of the parts besides the NonOilen, I decided to call this first investigation done and test the samples for their strength if anything happened. I’m planning to get myself one of these drum composters in spring that help with aeration and should bring me closer to industrial composting, but maybe one of you can give me some input here.

Anyways, let’s now test the strength of the samples that were in the compost and compare them against our reference samples. I had four samples for each orientation which makes 16 samples for every material, plus one of my test hooks just for the fun of it. I always measured the section under test and then loaded them one after the other into my DIY universal test machine.

Regular PLA from dasFilament was the strongest, with 66 MPa for the lying and 46 MPa for the standing samples. After composting for 100 days, the lying specimens lost around 10% strength. Layer adhesion was still good, and the low values in the graph are partly due to the deformed specimens. The hooks were also impressively strong, with 65 kg of failure load and a whopping 72 kg after composting. The last sample seems stronger because it deformed and annealed, making the lever arm smaller. Overall, we can clearly say that plain PLA doesn’t significantly weaken in a home compost.

Next came Colorfabbs PLA/PHA, which failed at 59 MPa on average for the lying samples and also decreased around 10% in strength to 53 MPa after composting. Layer adhesion was okay on the reference parts with 30 MPa, and even though it seems on the graph that it got weaker, it stayed quite similarly strong, and only the deformed parts flawed the results. Similar results on the hooks which failed at 58 kg pre and 59 kg post composting. Therefore, I can again say that I’m pretty sure that not a lot of degradation took place during the 100 days.

Please take the next values with a grain of salt. I didn’t tune the setting for GreenTEC Pro and simply used the presets in PrusaSlicer, but these samples had really bad layer adhesion even before composting. On average, the lying samples broke at 58 MPa, the standing ones at only 14. Similar to the last two materials we tested, the strength after composting for three months only minimally decreased. Since the samples did not deform due to their temperature resistance, we can even see that the layer adhesion stayed on the same level. The hooks also failed on a similar level, and the strength decreased only from 48 to 46 kg, though the mode in which they failed also shows that there is something wrong with layer adhesion.

Finally, the most interesting contestant which is Filamentums NonOilen. This material was the weakest of the selection, even in the reference state with only okay layer adhesion. 43 MPa in the lying printing orientation and 15 MPa standing. After composting, the strength of the lying samples stayed basically the same, only layer adhesion almost didn’t exist anymore, which is also the reason why two samples already broke within the compost. The hooks, also because they were printed lying, failed on a very similar level, and the strength only decreased from 33 to 31kg. Though interestingly, the mode of failure changed. The reference hook nicely snapped, whereas the one exposed to the composting environment cracked between the layers. I think these results show how much the degree of decomposition is dependent on the material thickness. The laying samples are 3 mm thick, and they were only attacked at the surface, which has minimum impact on the properties. The standing samples on the other side might have a ton of minimal voids and cracks between the layers where bacteria and fungi can migrate and weaken the material.

In the end, I think only the NonOilen showed significant degradation in my garden compost though if only the PHB got dissolved and left us with PLA microplastics is something I just can’t say with my tests and equipment. PLA alone and even in Colorfabs and Extrudrs bioplastic compounds seems to be basically untouched because the conditions of an industrial composting plant are not reached, which shows that even though PLA is biobased and compostable, it shouldn’t be disposed of in nature. Hence, many bio-based and biodegradable polymers are not a great solution for tackling the waste and microplastic problem. If the right conditions aren’t met, they equally pollute like conventional polymers. There are barely any composting plants that even accept biodegradable plastics, so they still need to go into the regular trash if you dispose of them. And you could even argue that buring old prints in waste plants makes more sense because that heat energy gets then used instead of just dissipating in a compost pile. However, I’m happy that the awareness of these problems grows, which fuels research and demand in well usable, easily biodegradable polymers for packaging but also 3D printing. Personally, I’m still really excited to find out if we can achieve better results in one of these barrel composts I plan to buy. If you have any input on the methods and procedures, please leave them in the comments!

Putting 3D Prints in a home compost

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