poly-oxepanone biodegradability
(TLDR: It's comparable to vegetation; A little slower than cellulose, a little faster than lignin.)
disposal environments
Our biodegradable cable ("zip") ties have been tested in a variety of different disposal conditions, and the following images show what happened. These tests were conducted using our original 8mm design (4.5g) and left at ambient temperatures, typically 5-30°C depending on the season. The slimmer versions of our ties use considerably less material and can therefore be expected to biodegrade somewhat more rapidly than those shown.
Results in other environments and climates will, of course, depend on your local conditions.
in garden soil
Garden soils are quite variable in their composition and microbial diversity. They can also be subjected to fertilizers, herbicides, insecticides and other chemicals. Rich organic soils containing a lot of plant matter are generally more favourable, but as seen here, our ties still biodegraded even in a poor quality mineral soil containing a lot of clay and sand.
After 12 months, some biodegradation had clearly occurred, but the ties were still mostly intact and recognisable. However, after being buried in the soil for 24 months, only a few short pieces could be recovered. These were very soft and too weak clean up
in home compost
Home compost generally does not get very hot, especially in small bins. Low temperatures are not a problem for poly-oxepanone biodegradation (see below), but the ties still have to compete with the surrounding plant material for microbial attention. Experiments have shown that poly-oxepanone is not quite as degradable as cellulose, and in this test, the leaves certainly decomposed more quickly than the ties did.
Consequently, substantial tie fragments were still remaining after 12 months, although they were clearly being attacked by the bacteria & fungi present. After 24 months, however, very little was left other than the woody twigs & stems. (These are much tougher to decompose due to their lignin content.)
in commercial compost
Because commercial compost has already been substantially degraded, it contains lots of active microbes and little residual plant matter. When poly-oxepanone is introduced, it gets quickly consumed because there are few remaining plants and lots of hungry fungi. Nevertheless, this medium was not perfect, and some fragments of our ties were still remaining after the first year.
After an additional year, however, the residual ties had almost completely disappeared, and no solid pieces could be found. Once they had disintegrated into powder like this, microbes will slowly convert it back into water and CO2.
in fine potting compost
This finer grade of compost was given an extra boost with the addition of some dextrose. This should act as an accelerator and help the microbes multiply. (Like brewing with yeast.) Because there was very little plant matter in this medium, the ties had no competition, and once the sugar ran out, they were the only food source available. Suprisingly, the poly-oxepanone ties had been completely destroyed when we checked them after 12 months, and nothing could be found digging through it.
However, such disintegration does not necessarily mean the degraded residues have been fully converted to CO2 & water just yet. Bioassimilation generally takes longer than biodisintegration, and disintegrated poly-oxepanone can take a couple of years to achieve >90% conversion at ambient temperatures.
in shallow oceans
Poly-oxepanone (PCL) is one of the few thermoplastics known to biodegrade in aquatic environments. Due to the diluting effect of the water, the process is much slower than in terrestrial conditions, and larger items such as cable ties are expected to take several years to fully decompose. It must also be considered exactly where in the ocean the ties end up: Warm tidal conditions are preferable to cold deep sea environments.
in anaerobic conditions
Poly-oxepanone can degrade both aerobically and anaerobically. Anaerobic biodegradation is generally slower, and also leads to the release of methane. In liquids, the erosion profile can also be quite different from that seen with aerobic breakdown in soil & compost. In this case, because the ties were swimming in decomposition juices, they were attacked from all sides. This led to a more uniform dissolution rather than localised rotting.
compared to cellulose
Poly-oxepanone has been extensively studied for many decades and there are numerous published papers detailing its biodegradability. (It is historically better known as poly-caprolactone, or PCL). Many compare this polymer against a cellulose reference, an approach followed by formal international standards. These generally consider a material to be biodegradable if it can degrade 90% relative to cellulose within a stipulated test period.
The following charts show how poly-oxepanone (PCL) performed against cellulose under simulated home and industrial composting conditions. In both cases, the cellulose initially degraded about twice as quickly as the poly-oxepanone, but then started to slow down. In the lower temperature test (28°C), the overall time needed for 90% biodegradation was about the same for both polymers.
low temperature
high temperature
under the microscope
It is interesting to see the resulting biodegradation at a closer scale, and how it compares to natural organic decomposition. The first image is a piece of wood, with the horizontal grain striations clearly visible. As well as a noticeable red staining (possibly bacterial), there is some obvious surface pitting and edge erosion.
Compare this with the image in the middle, which shows a piece of poly-oxepanone exposed to the same ambient composting conditions for 12 months. Although neither surface shows any conspicuous fungal growth, the rough erosion pattern is characteristic of localised fungal attack.
The final image shows the remaining surface of a poly-oxepanone (PCL) cable tie after 12 months in liquid compost. By contrast, the erosion seen here is completely smooth. This happens because the tie is fully immersed in a liquid into which microbes (mostly bacteria) are releasing hydrolysing enzymes. These disolved enzymes then get uniformly dispersed, which means they affect the entire surface very evenly, rather than just at sites of concentrated activity.
Biodegradation of wood (damp aerobic).
Biodegradation of poly-oxepanone (damp aerobic).
Biodegradation of poly-oxepanone (wet anaerobic).
temperature effects
Our poly-oxepanone polymer has an unusually low melting point of just 60°C, which, unlike other materials, helps it biodegrade is the natural environment.
In the comparison charts shown above, it can be seen that both poly-oxepanone and cellulose decompose more rapidly in hotter conditions. This is to be expected, but as shown below, the effect is not actually linear because different microbial species are active in different temperature bands.
Experiments have shown that the speed of poly-oxepanone biodegradation does not vary significantly between (essentially) 0°C and around 40°C, and the primary degrading species that thrive in these cooler conditions (such as fusarium and penicillium) perform about as well regardless of temperature. However, they are not adapted to higher temperatures and are unable to exploit the full potential of hotter conditions. Their capacity for biodegradation is actually thermally constrained.
In hot compost heaps, thermophillic species (such as thermomyces) come to dominate, and this change in biodiversity is a significant factor in the more rapid decomposition seen above 40°C. Biodegradation in this higher temperature range can be over 3x faster:
poly-oxepanone in compost (lab)
poly-oxepanone in soil (lab)
These charts are adapted from the tests published in 2019 by Asma Al Hosni at the University of Manchester. For full details see "Microbial degradation of four biodegradable polymers in soil and compost demonstrating polycaprolactone as an ideal compostable plastic." (Al Hosni et al, 2019. DOI: 10.1016/j.wasman.2019.07.042)
Al Hosni also conducted outdoor soil tests which yielded very similar results (about 50% in 300 days, regardless of temperature variations).
A far more significant factor appears to be the richness of the disposal medium and the microbes it contains. As evidenced by our photographs at the top of the page, biodegradation is relatively slow in poor quality mineral soils but very much faster in highly organic conditions.
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