In "Imprisoned," the earthbenders bend coal. This means they can bend carbon - and specifically hydrocarbons at that.

This probably means they can bend plastic. Now you know.

to be honest, i don't find this to be satisfying evidence of the question "can earthbenders bend plastic" because there is no logical connection between being able to bend coal and this fact necessitating that earthbenders can thus bend any organic compound. the reason that earthbenders can bend coal is because coal is a type of rock, not because it contains hydrocarbons. unlike waterbending, which is specifically the ability to manipulate the H2O molecule, it seems like earthbenders, rather, can bend anything that either is or contains minerals. that said, i can see a basis for arguing for plastic bending in this fact, since plastic often contains minerals. i plan on making a longer post unpacking the physical stipulations of bending mechanics someday, so stay tuned for that.