for additives that will not significantly change you products, try EPI or Willow Ridge (search for the name adding "degradable" and you'll find them.
This may not help, but I remember stumbling into a web site that does exactly that, with tips and considerations. I think you'll find it by searching for "gear design", ar maybe add "plastic" or your specific material. There may even be a "design wizard" - you enter...
PP is not biodegradable. The addition of starch does not cause the PP to degrade. The starch, being biodegradable, degrades, and the remains are PP dust. For some aplications this is acceptable, but it does not comply with the definition of "biodegradable".
Yori
yoramc@tama.co.il