Fluorine-containing paints are used as weather resistive paints for exterior and interior coating of medium- or low-storied buildings, and various techniques have been developed for preparation of the fluorine-containing paints. Though many of the fluorine-containing paints are in the form of organic solvent dispersions, there are problems in the use of organic solvents from safety and environmental protection points of view, and ones of aqueous dispersion type are preferable. As the methods of preparing an aqueous dispersion of a fluorine-containing polymer, there are known the methods mentioned below.
For example, JP-B-28669/1974 discloses that when vinyl fluoride is suspension-polymerized in preparation of paints, a nonionic surfactant is added in an amount of 0.05 to 5% by weight on the basis of the monomer to increase yield, to prevent deposition of the polymer onto an inside wall of a polymerization tank and to control particle size of the polymer.
Also JP-A-123646/1986 discloses that in preparing a fluorine-containing copolymer in an aqueous medium, a stable aqueous emulsion can be obtained by adding a dispersion stabilizing agent after polymerization of the fluorine-containing copolymer, but before or during the condensation process of the aqueous dispersion
Further JP-B-55441/1992 discloses that in preparing an aqueous fluorine-containing polymer dispersion for aqueous paints, 0.05 to 5.0% by weight of surfactants of various kinds including a nonionic surfactant is added when the seed-polymerization of a monomer having ethylenically unsaturated bond is carried out.
Also JP-A-225550/1990 discloses that a mixture of a fluorine-containing type surfactant and a nonionic type surfactant is used when copolymerizing a fluorine-containing olefin and a hydrophilic group-containing non-fluorine-containing olefin.
Also Koubunshi Ronbun Shu, Vol. 36, No. 11 (1979) discloses at pages 729 to 737 that when a large amount of surfactants of various kinds is used in emulsion polymerization, particles of a polymer latex become smaller, and also that by mixing an anionic surfactant and a nonanionic surfactant in polymerization of an acrylic monomer, there can be obtained the same effect as in case of adding an anionic surfactant solely.
However the technique in JP-B-28669/1974 does not relate to emulsion polymerization, but relates to suspension polymerization without using a fluorine-containing surfactant, and intends to obtain a particle size larger than that of the present invention. The particle size described in Example of this patent publication is as large as 3.4 to 4.5 .mu.m.
Also JP-A-123646/1986 discloses that a polymer latex having a concentration as high as not less than 30% by weight is obtained by using a fluorine-containing emulsifying agent in polymerization process. However there is no description as to the use of a fluorine-containing surfactant for controlling the particles at a size lower than a certain size. In addition, the invention is characterized by the use of a specific non-fluorine-containing dispersion stabilizing agent, and its amount to be used is as much as 2 to 8% by weight on the basis of a solid content. Usually the particle size of a latex tends to increase together with a polymer concentration, and when the fluorine-containing surfactant is used solely and if its amount is not more than 1% by weight, there cannot be obtained a particle size of not more than 200 nm if the solid content is assumed to be 30 to 50% by weight. Also there is no description as to controlling of the particle size of the aqueous latex. It is also described that if the nonionic surfactant is added in preparing an aqueous emulsion, there is an adverse effect such as coloring.
Further in the technique disclosed in JP-B-55441/1992, no surfactant is used in the polymerization of a fluorine-containing olefin, and also there is no disclosure as to the use of a fluorine-containing surfactant as the surfactant to be used in the seed-polymerization.
Also the technique specifically disclosed in JP-A-225550/1990 is one wherein chlorotrifluoroethylene is used as a fluoroolefin unit and the amount of the nonionic surfactant used is as relatively much as 3% by weight on the basis of water. However in copolymerization of VdF, there has been generally known that there occur problems such that when the nonionic surfactant is added, the reaction rate is lowered remarkably and the molecular weight is not increased.
Also in the techniques disclosed in the above-mentioned Koubunshi Ronbun Shu, the use of the surfactant in a large amount causes an adverse effect on water resistance when used for paints.
The present invention has been made to solve the above-mentioned problems, and it is an object of the present invention to provide an aqueous dispersion of a fluorine-containing polymer, which comprises a VdF polymer having a particle size as small as not more than 200 nm and contains solids in an amount as high as 30 to 50% by weight and a surfactant in an amount as low as not more than 1% by weight on the basis of water, and its preparation process.