Specific examples of the synthetic polymers include vinyl polymers such as polyvinyl alcohol, polyvinyl pyrrolidone, and polyvinyl methyl ether; acrylic polymers such as non-crosslinked polyacrylamide, polyacrylic acid, metal salts of polyacrylic acid, and water-soluble styrene-acrylic resins; water-soluble styrene-maleic resins, water-soluble vinylnaphthalene-acrylic resins, water-soluble vinylna