In 2014 the School of Packaging, Michigan State University, evaluated the performance of 5 biodegradation-promoting additives to plastic drink bottles (PE & PET).
Additives for polymers are increasingly being used around the world with the claim that they effectively render commercial polymers biodegradable. The additives are meant to chemically cut the chain like molecules and increase their water solubility. None tested significantly increased biodegradability.
Under landfill or composting conditions the plastics may fracture and not be visible but it does not mean the compounds are biodegradable.
AS 4736-2006/Amdt 1-2009 Biodegradable Plastics – Biodegradable plastics suitable for composting and other microbial treatment
AS 5810-2010 Biodegradable Plastics – Biodegradable plastics suitable for home composting.
Michigan State University – School of Packaging
Compostable vs Biodegradable