. "Other useful resins are fluorocarbons such as polytetrafluoroethylene, poly(vinylidene fluoride), mixtures of these, and copolymers of vinylidene fluoride and tetrafluoroethylene; see, for example, U.S. Pat. Nos. 4,545,060; 4,478,925; 4,076,857; and 3,970,571, whose disclosures are incorporated herein by reference." . . .