What is PBAT?

Our comPOST Packs are 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 …

22 Comments

  • Shweta

    Hi Kate,

    very well explained and easy too understand article. I am exploring to use these materials for a snack packing where the moisture barriers are very high. Would I have to combine these materials with something else so as to keep the product fresh and crunchy? Important is that the entire combination is compostable.
    Thanks and looking forward to hearing your inputs.

    • kate

      Hi Shweta, yes you would. These materials, while waterproof, do ‘breathe’ somewhat, so won’t keep your snacks crispy on their own. Kate

  • Daisy

    Hello Kate, Nice article and indeed contains very useful information> My question is whether PLA and be combined with PBT instead of PBAT? If yes, what percentage is recommended? will the PLA/PBT bag eventually degrade into microplastics, since PBT is petroleum-based?

    • kate

      Hi Daisy, to my knowledge PBT is Polybutylene terephthalate. It behaves like a traditional plastic ie. it is engineered to last for hundreds of years, so there would be no point combining PBT with PLA.

  • Daniel

    Hi Kate,

    Very interesting and informative article! Per my understanding, PLA itself is also a biodegradable chemical which is made from wasted corn. That means it’s 100% plant base. Is the only reason to add PBAT is to make it degrades faster? In fact, are you able to make the bags in 100% PLA?

    • kate

      Hi Daniel, You’ve probably come across 100% PLA things before – compostable clear smoothie cups, coffee cups lids, cutlery … They are all very rigid. We need to add the PBAT to our material to give it flex and a little bit of stretch.

  • Claire

    Hi Kate,

    Great article thank you so much. Does a bag that is made out of PLA/ PBAT have to be a certain micron to decompose or will it decompose regardless of how thick it is?

    • kate

      Hi Claire, YES it does. Every time a material is certified to be either home or industrially compostable, a maximum thickness is also specified. There’s a heap more info and a full explanation of exactly this in another blog post of ours here. Happy to answer any further questions! Kate for TBPCo.

  • Patty

    What is the functioning purpose of the cornstarch within the three components (PLA + PBAT + cornstarch)? In other words, what job does it do if PLA is the plastic resin itself and the PBAT is the component that’s added aid compostability?

    • kate

      Hi Patty, that’s a great question. The cornstarch in our packaging is really just a filler to give the PLA and PBAT something to bind to.

  • Anushe Everest

    Hi Kate and Everyone,

    You may want to check out a company called Avani. They have succeeded in making non-petroleum based plastic made of cassava plant. The plastic bags and containers they make are edible and once use it is best that it is thrown in the garden to naturally compost. This company is based in Indonesia and they were recently one of the winners of the most innovative technology in eco products. Please check out their website. https://www.avanieco.com/

    I have also heard that a few other companies have achieved similar effects using seaweed. I personally haven’t checked them out.

    Cheers,
    Anushe

    • kate

      Thanks Anushe, we are following their progress but aren’t confident yet in the performance of the bags they make. Hopefully one day soon!

  • Hui

    Just wondering whether the decomposed byproducts of PBAT can cause microplastic pollution?

    • kate

      Hi Hui, they absolutely do not create microplastics. The material breaks down completely into water, CO2 and carbon matter.

  • Keith

    Hi Kate,
    Are you aware that starch makes methane as a biodegradation product? I’m sure you know methane is 17x more powerful of a greenhouse gas than carbon dioxide.

    Keith

    • kate

      Hi Keith, we’re very, very conscious that any organic matter creates methane if it degrades under anaerobic (without oxygen) conditions as found in most landfills. Under composting conditions however, methane is not generated – only carbon dioxide – hence why we’re trying to create a composting movement at the same time as making packaging. We want ALL organic matter to be composted properly and not sent to landfill, including our packaging. Kate

  • Rensso

    Hello! Just got a sampler, would you be using GMO-corn?

    • kate

      Hi Rensso, our products are made from PLA (made in America from corn) which is certified not to contain any Genentically Modified material. The other ingredient is cornstarch which is made from non GMO corn stockpiled in case of famine but no longer edible.

      • Wendy AUSTIN

        Saw an add for plastic wrap on Insta.Great Wrap is the company,claiming their product is fully compostible.I think not..how about you?

        • kate

          Hi Wendy, we’re not sure, but best to ask to see their certifications. Any certification should be accompanied by a number. If not, it’s not valid – it’s like driving a car without a number plate! That number can be checked to ensure it’s legit. Have a look at our other post ‘How to know if your compostable packaging is legit’ for more info and links to the certifying bodies.

  • Aroa

    Hello! Very interesting, thank you for the information. Could you share what’s the % composition of each of the three materials, per bag? Ex. ComPOST packs, black. Thanks a lot!

    • kate

      Hi Aroa, the percentages vary for our different products, but for the comPOST Packs, it’s 5-10% PLA (from corn), 20-25% cornstarch, and the remainder PBAT.

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