My understanding is that the C of
PVC is of course Chloride, (PolyVinyl Chloride). The Chloride
part that lots of people above say will will just colour the coins green, can react with mositure in the air, and the copper in the bronze, to form Cuprous Chloride. Further moisture in the air can make this Cuprous Chloride into Hydrochloric acid, and Cupric Cloride, at which point the acid will attack the coin further and form more Cuprous Chloride. This can continue until there is no metal left.
This is otherwise known as
Bronze Disease, or Green powdery death for bronze coins!
regards
Mark