In the context of the anime , the "paper" referred to in the episode titled " Steam Gorilla " (Season 2, Episode 5) is Carbon Fiber Reinforced Plastic (CFRP) , or more specifically, hardened paper armor .
: Senku creates a primitive but powerful composite material by layering paper and coating it with liquid plastic (formulated from coal tar and other chemicals).
Watch the Science Kingdom build and test their technological marvel in this clip: