EPS (390311) consists of polystyrene beads that contain a blowing agent—typically pentane—and expand when heated to form foam. Standard non-expandable polystyrene (390319) is supplied as pellets, powder, or granules without a blowing agent. The physical form and the presence of a blowing agent are the distinguishing criteria; a product marketed as 'general-purpose polystyrene' in pellet form without blowing agent goes to 390319.

ABS resins from specific origins have been subject to antidumping investigations and orders in the past. Importers should check the CBP ADD/CVD portal for active orders covering the country of origin of their ABS supply, as duty orders can be initiated, modified, or revoked through periodic reviews.