Use of a water-soluble composition for water-repellent treatments of zinc and zinc alloy and method for water-repellent treatment

Abstract: A composition for water-repellent treatment of zinc and zinc alloys comprises a non-ionic polymeric compound having both hydrophilic and hydrophobic groups in the molecule, and a balance of water. The composition can be used in a method for water-repellent treatment of zinc and zinc alloys which comprises the steps of immersing zinc or zinc alloys which has been subjected to a conversion treatment, then washing it with water and drying the same. The use of the water-soluble composition for water-repellent treatments permits considerable reduction of the contact area between the repellent-treated metal material and moisture which becomes a cause of corrosion under the corrosive environment in which water and oxygen coexist and this accordingly results in the improvement in the corrosion-resistance of the metal material.

Description: The present invention relates to a water-soluble composition for water-repellent treatment capable of imparting water repellency to zinc and zinc alloys, in particular, zinc and zinc alloys which are subjected to a conversion treatment as well as a method for water-repellent treatment of these materials. There have widely been used the chromate treatments which make use of hexavalent chromium in order to further improve the characteristic properties of zinc and zinc alloys, in particular, zinc and zinc alloy films plated on the surface of metals such as steel. The chromate treatments permit considerable improvement of the zinc and zinc alloy-plated films in their durability, but they should in general be further subjected to a finishing treatment using chromic acid and/or application of a variety of coating compositions in order to further improve the corrosion resistance thereof. Moreover, the chromate treatment and the finishing treatment using chromic acid have a bad influence upon environment because these treatments require the use harmful hexavalent chromium. In addition, the application of a coating composition suffers from such problems that the treating processes are quite complicated and that the coating composition per se is quite expensive. For this reason, there has been desired for the development of a novel method for treating the surface of zinc and zinc alloy-plated metals without using harmful hexavalent chromium. JP-A-56069379 discloses a surface treatment in which a plated steel plate is coated with a solution obtained by adding a silica solution to a chromic acid solution, followed by coating with a polyvinyl alcohol solution. GB-1027548 discloses an aqueous acidic solution for use in the treatment of surfaces of metal consisting wholly or predominantly of iron, zinc or aluminum which comprises the product of the reaction of an aqueous acid solution of hexavalent chromium with formaldehyde, and one or more of polyacrylic acid, polyvinyl alcohol and/or hydroxyethyl ethers of cellulose. Chemical Abstracts, Vol.83 (No. 10), 8 September 1975, Columbus, Ohio, US, Abstract No. 83444 K discloses an aqueous solution comprising chromium and polyvinyl alcohol, polyvinylpyrollidinone, hydroxyethyl cellulose or polyvinyl butyral. Accordingly, it is an object of the present invention to provide the use of a water-soluble composition for water-repellent treatment capable of imparting water repellency to zinc and zinc alloys through a simple method. Another object of the present invention is to provide a method for efficiently imparting water repellency to zinc and zinc alloys. More particularly, another object of the present invention is to improve the corrosion resistance of conversion-treated films without using hexavalent chromium. These and other objects of the present invention will be apparent from the following description and Examples. The present invention has been completed on the basis of such a finding that the foregoing drawbacks associated with the conventional techniques can effectively be eliminated and water repellency can be imparted to zinc and zinc alloys, by simply immersing them in an aqueous solution comprising a specific polymer compound. According to an aspect of the present invention, there is provided the use of a composition for the water- repellent treatment of zinc and zinc alloys, the composition comprising a water-soluble polyvinyl alcohol, a water soluble polyacrylic acid salt and water, wherein the composition is free of hexavalent chromium. According to another aspect of the present invention, there is provided a method for the water-repellent treatment of zinc and zinc alloys, the method comprising the steps of immersing zinc or a zinc alloy, which has been subjected to a conversion treatment, into a composition as herein described, then washing the zinc or zinc alloy with water and drying the same. The present invention will hereinafter be explained in more detail. The molecular weights of the water-soluble polyvinyl alcohol is not restricted to a specific range insofar as it is soluble in water, but it is desirable to use those having a weight average molecular weight of the order of 10,000 to 160,000. In this respect, the water-soluble polyvinyl alcohol preferably has a degree of saponification of not more than 82 mole %, in particular ranging from 70 to 80 mole %. The concentration of the water-soluble polyvinyl alcohol in the aqueous solution is also not restricted to any particular range, but preferably ranges from 0.1 to 10% by weight on the basis of the total weight of the composition. The water-soluble polyacrylic acid salt is preferably an alkali metal salt of the acid into the water-soluble composition for water-repellent treatments according to the present invention. The water-soluble polyacrylic acid salt may have any molecular weight in so far as they are soluble in water, but it is desirable to use those having a weight average molecular weight of the order of 250,000 to 7,000,000. The water-soluble polyacrylic acid salt content of the composition is not also limited to any specific range, but desirably ranges from 0.01 to 10% by weight based on the total weight of the composition. In addition, the water-soluble composition as herein described used in the present invention may further comprise, for instance, water-soluble melamine resins and/or water-soluble acrylic resins as optional components. Moreover, the pH value of the water-soluble composition may be arbitrarily be selected, but preferably ranges from 6 to 8. Zinc and zinc alloys to be treated according to the present invention may be zinc and zinc alloys per se as well as products obtained by forming, preferably through plating, zinc or zinc alloy films on substrates such as steel and copper substrates. Examples of the zinc alloys include those of zinc with, for instance, iron, chromium, tin, nickel, cobalt and manganese. The thickness of the zinc or zinc alloy films may arbitrarily be selected, but preferably ranges from about 1 to 25 µm. In the present invention, it is preferred to use, as materials to be treated, zinc and zinc alloys, in particular, zinc and zinc alloy films which are subjected to conversion treatments. Examples of such conversion treatments preferably include those which use conversion treatment solutions free of hexavalent chromium such as rust proof film-forming treatments as disclosed in Japanese Patent Application Serial No. Hei 7-211585; and phosphoric acid salt-treatments as disclosed in J.P. KOKAI No. Sho 59-116383. In this respect, it is a matter of course that the present invention is also applicable to chromate-treated substances. According to the water-repellent treatment method of the invention, zinc and zinc alloys subjected to conversion treatments are immersed in the water-soluble composition for water-repellent treatments of the present invention after washing with water or without water-washing, followed by washing with water and drying. More specifically, they are desirably immersed in the water-soluble composition at a temperature ranging from 15 to 35°C for about 5 to 120 seconds, then washed with water at a temperature ranging from 10 to 30'C for about 5 to 60 seconds and dried at a temperature ranging from 40 to 60 °C for about 3 to 20 minutes. As has been discussed above in detail, the use of the water-soluble composition for water-repellent treatments according to the present invention permits considerable reduction of the contact area between the water-repellent metal material treated and moisture which becomes a cause of corrosion under the corrosive environment in which water and oxygen coexist and this accordingly results in the improvement in the corrosion-resistance of the metal material. The use of the composition and method according to the present invention will hereinafter be described in more detail with reference to the following non-limitative working Examples and Comparative Examples. Example 1 (not within the scope of the present invention) A Zn component which comprised an SPCC steel plate provided thereon with a Zn-plated film was subjected to a rust proof film-forming treatment at 25 °C for 60 seconds using a treating solution as disclosed in Japanese Patent Application Serial No. Hei 7-211585, i.e., an aqueous solution which comprised 50 g/l of 35% hydrogen peroxide, 10 g/l of potassium silicate and 10 g/l of a 20% titanium chloride solution and whose pH was adjusted to 1.6 with sulfuric acid. The Zn component subjected to the rust proof film-forming treatment was then immersed, at 25 °C for one minute, in an aqueous solution for water-repellent treatments which had been prepared by dissolving 4 g/l of a polyvinyl alcohol having a degree of saponification ranging from 71.0 to 82.0 mole% and a degree of polymerization ranging from 1000 to 1500, then washed with water and dried at 50°C to give a water-repellent treated Zn component. Example 2 (not within the scope of the present invention) The same procedures used in Example 1 except that the Zn component was subjected to a chromate treatment instead of the rust proof film-forming treatment to thus give a water repellent-treated Zn component. In this regard, the chromate treatment solution used was "z-496" (principal components: anhydrous chromic acid, nitric acid) available from Dipsole Company and the chromate treatment was carried out at 25°C for 20 seconds. Example 3 (not within the scope of the present invention) The same procedures used in Example 1 except that the Zn component was subjected to a phosphoric acid salt treatment instead of the rust proof film-forming treatment to thus give a water repellent-treated Zn component. In this regard, the solution used for the phosphoric acid salt treatment was "P-670" (principal components: phosphoric acid, nitric acid) available from Dipsole Company and the treatment was carried out at 50°C for 20 seconds. Example 4 The same procedures used in Example 1 except for using, as the solution for the water-repellent treatment, an aqueous solution which was prepared by dissolving 4 g/l of a polyvinyl alcohol having a degree of saponification ranging from 71.0 to 82.0 mole% and degree of polymerization ranging from 1000 to 1500 and to which 10 g/l of sodium polyacrylate was supplemented to thus give a water repellent-treated Zn component. Example 5 The same procedures used in Example 4 except that the Zn component was subjected to a chromate treatment instead of the rust proof film-forming treatment to thus give a water repellent-treated Zn component. Example 6 The same procedures used in Example 4 except that the Zn component was subjected to a phosphoric acid salt treatment instead of the rust proof film-forming treatment to thus give a water repellent-treated Zn component. Comaparative Example 1 The Zn component of Example 1 subjected to the rust proof film-forming treatment was used as a comparative sample without subjecting it to any water-repellent treatment. Comparative Example 2 The Zn component of Example 2 subjected to the chromate treatment was used as a comparative sample without subjecting it to any water-repellent treatment. Comparative Example 3 The Zn component of Example 3 subjected to the phosphoric acid salt treatment was used as a comparative sample without subjecting it to any water-repellent treatment. The Zn components prepared in the foregoing Examples and Comparative Examples were inspected for their corrosion resistance by the following method. Corrosion Resistance Evaluation Test Each sample was subjected to the salt spray test in which a 5% saline was used according to JIS Z2371 to determine the time (white rust-forming time: W.R.T.) required till the white rust-gathering surface area reached 5% of the total surface area of the sample. The results thus obtained are summarized in the following Table 1. Ex.No. Conversion Treatment Water Repellnt Treatment W.R.T.(hr.) 1 rust proof film polyvinyl alcohol 408 2 chromate polyvinyl alcohol 384 3 phosphoric acid salt polyvinyl alcohol 48 4 rust proof film polyvinyl alcohol+ sodium polyacrylate 480 5 chromate polyvinyl alcohol+ sodium polyacrylate 432 6 phosphoric acid salt polyvinyl alcohol+ sodium polyacrylate 72 1* rust proof film None 144 2* chromate None 240 3* phosphoric acid salt None not more than 12 1*∼ 3*: comparative examples Examples 1 to 3 do not fall within the scope of the present invention.

Claims: Use of a composition for the water-repellent treatment of zinc alloys, the composition comprising a water-soluble polyvinyl alcohol, a water-soluble polyacrylic acid salt and water, wherein the composition is free of hexavalent chromium. Use of a composition as claimed in claim 1, wherein the water-soluble polyvinyl alcohol has a degree of saponification of not more than 82 mole %. Use of a composition as claimed in claim 2, wherein the water-soluble polyvinyl alcohol has a degree of saponification ranging from 70 to 80 mole %. Use of a composition as claimed in any one of claims 1 to 3, wherein the water-soluble polyvinyl alcohol is present in the composition in an amount of from 0.1 to 10% by weight. Use of a composition as claimed in any one of claims 1 to 4 having a pH of from 6 to 8. Use of a composition as claimed in any one of claims 1 to 5, wherein the water-soluble polyacrylic acid salt is present in the composition in an amount of from 0.01 to 10% by weight. A method for the water-repellent treatment of zinc and zinc alloys, the method comprising the steps of immersing zinc or a zinc alloy, which has been subjected to a conversion treatment, into a composition as defined in any one of claims 1 to 6, then washing the zinc or zinc alloy with water and drying the same. A method as claimed in claim 7, wherein the conversion-treated zinc or zinc alloy is one which has been subjected to a conversion treatment using a conversion treatment solution free of hexavalent chromium. A method as claimed in claim 7 or claim 8, wherein the zinc or zinc alloy is immersed in the composition at a temperature of from 15 to 30°C for from 5 to 120 seconds, the resultant is washed with water at a temperature of from 10 to 30°C for from 5 to 60 seconds and dried at a temperature of from 40 to 60°C for from 3 to 20 minutes.

Inventors: INOUE MANABU, OHNUMA TADAHIRO, SATO GO, YAMAMOTO TOMITAKA, INOUE, MANABU, OHNUMA, TADAHIRO, SATO, GO, YAMAMOTO, TOMITAKA, Inoue, Manabu, c/o Dipsol Chemicals Co., Ltd., Ohnuma, Tadahiro, c/o Dipsol Chemicals Co., Ltd., Sato, Go, c/o Dipsol Chemicals Co., Ltd., Yamamoto, Tomitaka, c/o Dipsol Chemicals Co., Ltd.
Applicants: DIPSOL CHEM, DIPSOL CHEMICALS CO., LTD.
Application year: 1996
Main classification: C23C22
IPC codes: B05D7, B05D5, C23C22, C09K3