I have worked with a biodegradable plastic used in the manufacture of garbage bags. The plastic itself (HDPE/LDPE) is not biodegradable. The plastic will be affected If you add a small amount of a degradable substance (starch) however. When the starches degrade, the crosslinking is disrupted within the plastic structure, allowing the plastic film to crumble. This of course won't happen within a day, and will even be slower if landfilled as to not be exposed to the power of UV.
One of the great strengths of plastics is it's resistance to degredation.
Which leads to a compromise, or more specifically, responsible use. Plastics contain a large amount of energy within thier state. Not only the energy derived from it's base product, but from the fact that it retains the energies applied in processing it to it's current state. This is why plastics have such a wonderous ability to be 'recycled'. Plastics should not be allowed to degrade.
In my opinion, what needs to be done, is for used plastics to be stored in large landfills, for future mining when we develop the technology to either sort, or combine the various formulations. The purer the store, the higher the value. Dispersing plastics into fine amounts, over the land is ensuring a non renewable resource.
We are looking to make a new casing for paintballs. Now the casings are made of a soft gel product. We are looking for something that is less expensive to make than soft gel, but share most of it properties. The soft gel desolves in water and can be eaten.