A variety of alkali-resistant filmand fiber-forming polymeric materials are known which may be used in this connection, including for example a nylon (polyhexamethylene adipamide, polycaprolactam, polyhexamethylene sebacamide or the like), a hydrocarbon polymer such as polypropylene, an ester such as polyethylene terephthalate, a nitrile polymer such as polyacrylonitrile, and so forth.