Plastic was an element for a while (in beta, about v42). I recompiled the last commit before it was removed.
Look, Plastic really wasn't THAT useful when they added back then, that's why they removed it. If you come up with alot of uses for plastic THEN it might get added. It's not all about all the cool explosions and reactions in TPT, it's about creativity and the complicated contraptions people build.
Please don't use all caps... And they removed it because it was breaking saves with oil, as it was made from oil.