The binder is preferably a thermoplastic resin, and examples thereof include homopolymers and copolymers of acrylic monomers such as acrylic acid, methacrylic acid, acrylates, methacrylates and the like; cellulose polymers such as methyl cellulose, ethyl cellulose and cellulose acetate; homopolymers and copolymers of vinyl monomers such as polystyrene, polyvinyl pyrrolidone, polyvinyl butyral, pol