A plastique is not a UK derivation of plastic which may be similar to color/colour, theater/theatre or gener/genre. Don’t mind the last set. Anyway, it’s an explosive wrapped in a plastic sheath. Here is what it looks like:
Once I remember this destructive weapon, I immediately recall the fight scene between the Dark Knight (Batman) and Amazo in the movie, Under the Red Hood.
I also thought of “plastic” because here in Lipa, Batangas, they promote a non-plastic policy. When I bought something at the mall, they didn’t use plastic. The government implemented a thousand pesos as a strict penalty to those who use them. Instead, they wrapped the shopping item in a paper sheath. That’s when “plastique” came into my mind.
Just a random thought.