As the polymer, there may be mentioned a polymer or copolymer such as polystyrene, polymethylmethacrylate, polyvinylidene chloride, polyacrylonitrile or polyvinyl acetate; a cellulose derivative such as cellulose diacetate, cellulose triacetate, cellulose nitrate, ethyl cellulose or cellulose propionate; an acetal such as polyvinyl formal, polyvinyl acetal or polyvinyl benzal; and the like.