Solutions of vinylidene fluoride polymers

Films of vinylidene fluoride polymers are cast from liquid compositions either (i) containing the polymer, having a particle size of 0.05 to 20 microns, dispersed in a liquid medium containing at least one high temperature organic "solvent" which imparts to the polymer a "crystalline melting temperature" ("Tm") between 90 and 160 DEG C. and has a boiling point above "Tm" and between 100 and 300 DEG C.; or (ii) wherein the polymer is dissolved in at least one solvent imparting a "Tm" between 50 and 90 DEG C. and boiling between 160 and 210 DEG C. "Tm" is defined in the Specification and is roughly the value to which the first order transition point of the polymer is lowered by the solvating effect of the particular solvent. Vinylidene polymers are defined as the homopolymer and copolymers containing more than 95% molar vinylidene fluoride. Specified comonomers are symmetrical dichlorodifluoroethylene, trifluorochloroethylene, tetrafluoroethylene and vinyl fluoride or chloride. The dispersion (i) may be diluted with water or an organic diluent e.g. xylene, acetone, dioxane, MEK, MiK, tetrahydrofuran or methyl amyl ketone. These organic diluents may also be used to reduce the viscosity of the solutions (ii). Specified organic "solvents" with their "Tm" and boiling points are: (for dispersions) diethylformamide 92, 178( DEG C.); triethylphosphate 97, 216; propylene carbonate 100, 242; dimethylsuccinate 120, 195; diethyloxalate 130, 185; diethylsuccinate 130, 218; tetraethylurea 135, 212; dimethylphthalate 135, 282; diethyladipate 138, 245; diethylphthalate 145, 299; (and for solutions) dimethylacetamide 52, 165; tetramethylurea 65, 177; diethylacetamide 78, 205; and dimethylsulphoxide 80, 177. In casting films from the compositions they are applied to a surface which is heated at a temperature between the Tm and boiling point of the "solvent" until about 90% of the liquid phase has evaporated and then at a higher temperature. When aqueous dispersions are used there is a preliminary stage at not more than 95 DEG C. until at least 90% of the water has evaporated. Aqueous dispersions may contain a surfactant, e.g. the reaction product of dodecyl or nonyl phenol or t-dodecyl mercaptans with ethylene oxide, and an azeotropic agent to expedite evaporation of the water e.g. butyl or amyl alcohol. Copolymer preparation Examples 24 and 25 describe the preparation of copolymers of vinylidene fluoride with trifluorochloroethylene and symmetrical dichlorodifluoroethylene