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Q&A: Comparing bioplastic degradation rates

by Green Plastics

 

Young Ju Do wrote in with a question:

Dear Green Plastics,
Do you have any data or researches about that how long bioplastic takes to be degraded in different environments/conditions?
I made bio plastic then put in compost bin. It degraded. But I also want to know different cases. Also want to know how long it will last in air.

 

Thank you for your great question! As I am sure you know, different types of bioplastic biodegrade at different rates, depending on what polymer they use, what additives they include in the formulation, and how it is processed.  Additionally, of course, the rate of biodegradation will depend on details of the environment as well: the temperature, exposure to moisture and microorganisms, the amount of initial physical degradation (“breaking apart”) of the product, and so on, all play a role in determining the final timeline.

Unfortunately, to the best of my knowledge there is no comprehensive comparative study or meta-study out there: something that compares the rate of decomposition of different bioplastics in one Scenario A, then compares the rate of decomposition of those same bioplastic materials in another Scenatio B, and so on.  This kind of comparison grid would be very interesting, and very valuable. But I do not believe it exists.

To of our readers: If you know of such a study or report that provides such a comparison, please let us know so that we can post it for people here.

 

The closest thing available that I know about is the website bpiworld.org. You should take a look at that excellent website.  They have a list of products the have passed the ASTM standard tests for compostability and biodegradability; i.e., whether the products are compostable in municipal and industrial composting sites (ASTM D6400 and ASTM D6868).

The ASTM standards, like the international standards, simply answer the question: will the material degrade to the required extent in the specified period of time. It’s a simple yes or no (pass or fail). The exact rates of degradation need not be displayed as long as they pass.  If they pass, manufacturers can then display a logo to show they passed.

So, you can’t tell from this whether one product degrades adequately in one month compared to another that degrades adequately in four months. That kind of detail is not considered important, and precise timelines aren’t available. People just don’t want companies to advertise products as biodegradable or compostable if the products won’t even degrade after two years, or twenty years. Being certified is then an advantage, because the companies can legally claim their product is biodegradable and compostable (within the established and accepted time periods.)
There are some scientific studies on individual materials where the actual timeline of degradation is available, but they are few and far between.
Another thing to consider, when looking at these statistics, is that the tests mentioned above are limited to measuring degradation in a composting environment. There are now other ASTM standard tests to measure degradability in other environments (e.g., ocean water, etc.). There is no certification process for advertisers who might want to say, for example, that their products biodegrade in the ocean.
Materials that readily biodegrade in a composting environment may be very stable in air. Biodegradation requires microorganisms, moisture, and an adequate temperature.  A homemade starch or gelatin item can last years sitting on a shelf, for example. This is why, when people are asking about their home-made projects, I always tell them not to worry about it decomposing for a long time, unless it is going to be in a moist environment, or outdoors, or is going to be handled frequently, exposing it to microorganisms.


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