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  Cationic electrodeposition coating composition and coated article therewithUSPTO Application #: 20070029200Title: Cationic electrodeposition coating composition and coated article therewith Abstract: The present invention relates to a cationic electrodeposition coating composition comprising silica particles having pore volume of 0.44 to 1.8 ml/g and average particle size of no more than 10 μm, the composition showing square root of diffusion coefficient (√Tc) of no less than 2.5 upon diffusing a solution onto a coating film therewith; a cationic electrodeposition coating composition comprising silica particles having pore volume of 0.44 to 1.8 ml/g and average particle size of no more than 10 μm, wherein the composition shows minimum deposition pH of 11.90 to 12.00 during electrodeposition coating, and film resistance of 1,000 to 1,500 kΩ·cm2 at film thickness of 15 μm and applied voltage of 240 V; and an article coated with such cationic electrodeposition coating composition. Therefore, the present invention can also provide a coating film formed with the cationic electrodeposition coating composition which can ensure higher corrosion resistance and higher throwing power in addition to other excellent coating properties, even if the resulting coating has extremely thin film thickness (e.g., about 7 μm). (end of abstract) Agent: Wenderoth, Lind & Ponack, L.L.P. - Washington, DC, US Inventors: Toshiaki Sakamoto, Masaru Sakamoto, Yoshinori Hirahara, Tsutomu Shigenaga USPTO Applicaton #: 20070029200 - Class: 204471000 (USPTO) Related Patent Categories: Chemistry: Electrical And Wave Energy, Non-distilling Bottoms Treatment, Electrophoresis Or Electro-osmosis Processes And Electrolyte Compositions Therefor When Not Provided For Elsewhere, Coating Or Forming Of Object The Patent Description & Claims data below is from USPTO Patent Application 20070029200. Brief Patent Description - Full Patent Description - Patent Application Claims
TECHNICAL FIELD [0001] The present invention relates to a cationic electrodeposition coating composition, which especially provides a coating film having higher corrosion resistance even if the resulting coating is a considerably thin film, as well as an article coated with the composition. BACKGROUND OF THE INVENTION [0002] The electrodeposition coating is widely utilized as an under coating method for an article, which has a large and complicated shape and needs coating having higher corrosion resistance, such as vehicle bodies, since the coating can be formed to detailed portions, even if the article has a complicated shape, and the coating can be provided automatically and continuously. The electrodeposition coating is also commonly available as an industrial coating method, since the electrodeposition coating has economical advantages and extremely higher efficiency on applying a coating composition in comparison with other coating methods. The cationic electrodeposition coating is carried out by immersing an article to be coated into a cationic electrodeposition coating composition as a cathode, and applying voltage. [0003] Although the electrodeposition coating essentially has economical advantages and higher efficiency in applying a coating composition, as mentioned above, recently concepts based on resource conservation require much higher efficiency compared with that in the conventional methods even in the field of the electrodeposition coating. For example, thickness of the conventional coating film formed by the cationic electrodeposition coating method is approximately 20 .mu.m, however, recently it is desirable that thickness of the surface coating film on the outer panel is approximately 15 .mu.m. [0004] Since thinner film allows more reduced amount of the coating composition to be used, the resource conservation can be theoretically established. Conventionally, thinner film could be readily obtained by shortening the period of the electrodeposition coating and/or reducing the electrical quantity to be applied during the electrodeposition coating. However, it is difficult to ensure corrosion resistance of the resulting coating film, especially for back of the article panel. Because thickness of the resulting back, i.e., inner, coating film on the article is also reduced after subjection of the article to the electrodeposition coating to obtain no more than 15 .mu.m of outer, i.e., surface, film thickness. From this aspect, it can be seen that the key techniques providing a thinner outer surface coating and an adequate inner coating to the detailed portions, which coatings are both formed with the same composition having improved throwing power, and techniques improving corrosion resistance in the inner coated area (or thin layer area), wherein inner coated thin film has thickness of no more than 10 .mu.m, particularly no more than 7 .mu.m, are necessary. [0005] JP-A-2003-268315 discloses a leadless cationic electrodeposition coating composition including silica particles having pore volume of 0.44 to 1.8 ml/g and average particle size of no more than 10 .mu.m. The cationic electrodeposition coating composition is characterized in that the formed coating has a surface smoothing effect. However, it is not suggested in JP-A-2003-268315 that the coating, whose thickness is thin, for example, no more than 10 .mu.m, particularly approximately 7 .mu.m, has high corrosion resistance, and the composition has high throwing power. [0006] JP-A-2004-269627 discloses a leadless cationic electrodeposition coating composition comprising an aqueous medium, and a binder resin, a neutralizing acid, an organic solvent, a pigment and a metal catalyst, which are dispersed or dissolved in the aqueous medium, is characterized in that the film resistance of an electrodeposited coating film on an article in 20 .mu.m of thickness is 1,000 to 2,500 k.OMEGA.cm.sup.2, the electroconductivity of the coating composition is 1,500 to 2,000 .mu.S/cm, and the minimum deposition pH in the electrodeposition coating is 11.90 to 12.00. In addition, JP-A-2004-269627 discloses that square root of diffusion coefficient is 2.5 to 3.2 upon diffusing a solution onto the coating film therewith (see claim 3). However, extremely thin film (e.g., thickness is about 7 .mu.m), which is derived from the cationic electrodeposition coating composition, is not considered in JP-A-2004-269627. SUMMARY OF THE INVENTION [0007] The present invention consists in providing a cationic electrodeposition coating composition which can ensure higher corrosion resistance and higher throwing power in addition to other excellent coating properties, even if the resulting coating has extremely thin film thickness (e.g., about 7 .mu.m). [0008] Therefore, the present invention provides a cationic electrodeposition coating composition comprising silica particles having pore volume of 0.44 to 1.8 ml/g and average particle size of no more than 10 .mu.m, the composition showing square root of diffusion coefficient (i.e., {square root over ( )}Tc) of no less than 2.5 upon diffusing a solution onto a coating film therewith, and a cationic electrodeposition coating composition comprising silica particles having pore volume of 0.44 to 1.8 ml/g and average particle size of no more than 10 .mu.m, wherein the composition shows minimum deposition pH of 11.90 to 12.00 during electrodeposition coating, and film resistance of 1,000 to 1,500 k.OMEGA.cm.sup.2 at film thickness of 15 .mu.m and applied voltage of 240 V, in order to solve the above described problems. [0009] According to the preferable embodiment of the present invention, the cationic electrodeposition coating composition further comprises an amine modified epoxy resin (A) and a blocked isocyanate curing agent (B), wherein weight ratio of the amine modified epoxy resin (A) to the blocked isocyanate curing agent (B) is 50/50 to 90/10 (A/B), and the blocked isocyanate curing agent (B) comprises a blocked aliphatic polyisocyanate and a blocked aromatic polyisocyanate, wherein weight ratio of the blocked aliphatic polyisocyanate to the blocked aromatic polyisocyanate is 3/1 to 1/3 (the blocked aliphatic polyisocyanate/the blocked aromatic polyisocyanate). [0010] In more preferable embodiment, content of the silica particles in the composition is 1 to 30% by weight relative to total weight of pigments. [0011] Furthermore, the present invention provides an article coated with the cationic electrodeposition coating composition. BRIEF DESCRIPTION OF THE DRAWINGS [0012] FIG. 1 is a schematic cross sectional diagram illustrating an embodiment of measuring system determining diffusion coefficient of a coating film, which includes [0013] coated article plate (100), [0014] coating film (101), [0015] silicone rubber packings (102) and (102'), [0016] platinum ring electrode (103), [0017] Teflon .RTM. ring (104), [0018] article to be coated (105), [0019] electrometer (106), and [0020] recorder (107). [0021] FIG. 2 is a graph illustrating correlation between applied voltage and conducting period for determining minimum deposition pH. [0022] FIG. 3 is a schematic diagram illustrating an apparatus for measuring throwing power, which includes [0023] electrodeposition coating vessel (201), [0024] pipe (202), [0025] specimen plate (203), [0026] boundary (204), [0027] stirrer (205), [0028] power source (206), and [0029] electrodeposition coating composition (207). DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS [0030] The present invention is described as follows in detail. [0031] Generally, a cationic electrodeposition coating composition essentially comprises a cationic epoxy resin (particularly, an amine modified epoxy resin) and a curing agent for the resin (particularly, a blocked isocyanate curing agent), which are dispersed in an aqueous medium. In addition, pigments and/or other additives may be contained in the cationic electrodeposition coating composition in a dispersed state. Silica Particles [0032] As described above, the cationic electrodeposition coating composition according to the present invention is characterized in that the composition comprises certain silica particles. Silica particles are porous and have pore volume of 0.44 to 1.8 ml/g, preferably 0.8 to 1.6 ml/g. When silica particles have pore volume of less than 0.44 ml/g, it provides insufficient effect (i.e., poor corrosion resistance) of the coating film. When pore volume is further less than that value, this tendency is much more significant. When silica particles have pore volume of more than 1.6 ml/g, their dispersion stability is insufficient. When pore volume is further more than that value, their dispersion stability is significantly insufficient. Herein, "pore volume" means volume of pores in porous silica particles, i.e., porosity. Pore volume can be measured by mercury penetration method. Mercury penetration method includes allowing pores in a material (in a powder state) to be penetrated with mercury, and measuring pressure for the penetration and amount of penetrated mercury to determine specific surface and pore distribution. Mercury penetration type apparatus for pore distribution measurement can be employed to measure specific surface and pore distribution. [0033] Silica particles have average particle size of no more than 10 .mu.m, preferably 5 to 0.1 .mu.m. When average particle size is less than 0.1 .mu.m, it provides insufficient dispersion stability. When average particle size is more than 10 .mu.m, it provides poor corrosion resistance. Herein, "average particle size" is generally used to represent a grain size of a particle (or represent if the particle size is large or small). For example, median size corresponding to 50% by weight, arithmetical mean size, surface area mean size, volume-area mean size are used. Herein, "average particle size" is determined and represented by a laser method. Laser method includes dispersing particles in a solvent, irradiating laser beam on the dispersion, measuring the resulting scattering light, and computing to determine average particle size, particle size distribution, etc. Continue reading... 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