Willowflex got certificate and since EU standards are a bit more strict, sure, it is composable. It is possibly the only filament at the moment.
I don’t think Willowflex is PLA though or at most only some of it is PLA. Quite possibly a TPS-PLA blend!? A polymer is just consist the same chemical unit(s) stitched together in a chain, sort of like Lego. How they are arranged make all the differences.
There are quite a few differences between buzzwords so I would like to simplify a few first:
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Bioplastics: Meaning they are made from organics materials. As opposed to oil-based or synthesized. It does not mean that they are all biodegradable. You can modify bio-based units until you get oil-based unit. Oil was once organics after all.
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Biodegradable: Microorganism can “eat” or “break” it over time in (at least) some conditions. As opposed to ABS, PET for example would “break down” instead (due to environment: heat, wind, UV, etc…).
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Composable: You can throw it in compost bin and expect it to break quite fast.
PLA is a bioplastics and biodegradable. However, in ideal environment (60°C, Oxygen rich, selected compost, and a bunch of microorganisms) it still take months. In nature, PLA still last 400-800 year to biodegrade so please follow local recycle requirements.
It is a heck load faster than ABS (1000-3000 years) and PS (more than 5000 years, nobody know for sure yet) but nowhere near compost level.
Other common biodegradable filaments available now are: PVA, PBS, PES, PHA (usually used to reinforce PLA). Actual data on biodegrade time, if it exist, is difficult to find.
Please also remember that filament don’t just contain thermoplastic(s), manufacturers usually use additive to boost production, make filament stick to glass, better extrusion, reduce temperature, increase/reduce crystallization, filler/structure modifier, colouring, bleaching, fire resistance, etc… Even if you are sure that biodegrade time is short, additives can still not environmental friendly.
