Styrene Acrylonitrile (SAN) - Styrene and acrylonitrile monomers can be copolymerized to form a random, amorphous copolymer that has improved weatherability, stress crack resistance, and barrier properties. The copolymer is called styrene acrylonitrile or SAN. The SAN copolymer generally contains 70 to 80% styrene and 20 to 30% acrylonitrile. This combination provides higher strength, rigidity, and chemical resistance than polystyrene, but it is not quite as clear as crystal polystyrene and its appearance tends to yellow more quickly.