Our comPOST Range is made of;
- corn starch (from corn not fit for consumption)
- PLA (Polylactide, which is made from waste corn too and other plants)
- and this other stuff called PBAT (Polybutyrate Adipate Terephthalate).
While PBAT is incredibly biodegradable and will decompose in home compost leaving no toxic residues, it is currently partly derived from petrochemicals, yip, oil. This means it's not renewable (because the earth's oil stocks are finite and becoming depleted) and this is why we're working super-hard to research and test some of the emerging resins which have a higher bio-base (ie. are more made from plants).
Interestingly, it is PBAT that is added to make the product degrade quickly enough to meet the home compostability criteria. To our knowledge there are no bio-based plastics suitable for making courier bags that do not have a binding agent like PBAT in them. There is a lot of research currently to find an alternative, and there has been some success.
We are currently supporting our supplier to trial a higher percentage bio-based PBAT. The irony of this is it can impact the rate of compostability. Unfortunately, when it comes to plant-based inputs there is a trade-off between renewability and compostability – the higher the % renewable, plant-based components the slower it is to compost!
As an aside corn cobs, banana skins and avocado skins would not pass the AS5810 home compost test - go figure. So people understandably are cautious about putting something in their compost that is derived from oil but PBAT is 100% ok.
Let's "break it down"
Petroleum is actually a natural substance formed when large quantities of dead organisms, mostly zooplankton and algae, are buried underneath sedimentary rock and subjected to both intense heat and pressure.
Petroleum is separated using a technique called fractional distillation, i.e. separation of a liquid mixture into fractions differing in boiling point by means of distillation.
Some fractions are taken off and formed into plastics, tyres etc. and others are used to make PBAT. Here's the crucial bit - it is what is done to them at this point that determines how they then behave ie. whether or not they will break down quickly or take an age - like plastic.
Traditional plastic is engineered to last as long as possible, but PBAT is engineered to be fully biodegradable when composted. This is due to the presence of butylene adipate groups.
In short, just because PBAT is derived from petroleum, doesn’t mean it biodegrades the same way as traditional plastics and synthetics, in fact quite the opposite! It actually biodegrades quicker and better than a corn cob or avocado skin!
We have, as yet, not found a 100% bio-based, ie. non-oil derived binding agent for our purposes and would love to hear from anyone who has ... or purports to have ...