Aqueous Resistant Metal Pigment-Containing Paste and Method for Making

The present invention is related to paste compositions containing metal pigment particles suitable for forming coating compositions, particularly aqueous coating systems. Increasingly stringent environmental regulations have required that coating systems dramatically reduce volatile organic solvent levels. One way to comply with such regulations is to use water in place of the volatile organic solvents previously used.

Within this application several publications are referenced by arabic numerals within parentheses. Full citations for these references may be found at the end of the specification immediately preceding the claims. The disclosures of these publications in their entireties are hereby expressly incorporated by reference into this application.

In the area of coating systems utilizing metal pigment particles, aqueous systems present rather formidable difficulties. This is particularly true with respect to aluminum and zinc pigments. Thus, the metal pigment can readily react with water to generate hydrogen gas. The amount of gas generated can produce a safety hazard, creating high pressures within the composition containers. Also, the water reaction substantially diminishes the aesthetic value of metal pigments. The reaction of aluminum pigments with water can be depicted as follows:

Due to the increasing demand for aqueous systems, a number of techniques have been proposed for inhibiting the attack on the pigment particles by water. Unfortunately, most of these techniques have not provided sufficient protection.

One technique that provides inhibiting properties is the passivation of the metal pigment particles with an ionic organic phosphate as disclosed by Williams et al., U.S. Pat. No. 4,565,716, the disclosure of which is incorporated herein by reference. Another technique involves the use of compounds containing hexavalent chromium or pentavalent vanadium compounds as disclosed in Kondis U.S. Pat. No. 4,693,754, the disclosure of which is incorporated herein by reference. Other techniques include the use of organic phosphites as disclosed in Kondis et al. U.S. Pat. No. 4,808,231, the disclosure of which is incorporated herein by reference, or the use of nitroparaffin solvents. Still another technique includes the use of either (a) an ionic organic phosphate compound, for example as taught in Williams et al., U.S. Pat. No. 4,565,716, or (b) a pentavalent vanadium compound, for example as taught in Kondis, U.S. Pat. No. 4,693,754, or (c) an organic phosphite compound, for example as taught in Kondis et al. U.S. Pat. No. 4,808,231, in combination with a nitro-containing solvent such as a nitroparaffin as disclosed in Keemer et al., U.S. Pat. No. 5,215,579, the disclosure of which is incorporated herein by reference.

The treated metal pigment particles can be used to form a metal pigment paste. The treatment produces a metal pigment paste which has improved gassing stability over conventionally treated products.

The present invention is directed to metal particles which have been treated with at least one heteropoly anion. The metal particles, (e.g. aluminum flakes), which have been treated with at least one heteropoly anion compound are stabilized to a degree suitable for use in aqueous coating systems.

A principle object of the invention is to provide a metal flake which can be used in aqueous systems and which is resistant to hydrogen evolution.

Another object of the invention is to provide a metal flake which maintains acceptable aesthetic values and intercoat and intracoat adhesion in the paint film.

Other objects, advantages and features of the present invention will be more readily appreciated and understood when considered in conjunction with the following detailed description.

All the disclosed embodiments can be made using conventional compounds and procedures without undue experimentation.

As indicated above, the present invention is based on the discovery that certain types of heteropoly anion compounds effectively stabilize metal pigments from reacting with water, rendering the metal pigment suitable for use in water-based coatings without significant evolution of hydrogen without loss of adhesion, or degradation of optical properties. The present invention is especially useful for zinc, aluminum and bronze pigments.

Heteropoly anions are polymeric oxoanions which are formed by the acidification of solutions containing the requisite simple anions or by introduction of the hetero element ion after first acidifying the molybdate or tungstate anions. Heteropoly anions are described in Table 22C-2, at page 857 of Advanced Inorganic Chemistry by Cotton and Wilkenson.sup.(1), the entire disclosure of which is incorporated herein by reference. The largest and best known group of heteropoly anions is composed of those with the hetero atom(s) enshrouded by a cage of MO.sub.6 octahedra. One of the most common structures of heteropoly anions is the Keggin structure which is represented by [X.sup.+n M.sub.12 O.sub.40 ].sup.(8-n)- where M represents molybdenum or tungsten and X represents silicon, germanium, phosphorus, arsenic, titanium, zirconium, etc.. Preferred heteropoly anion groups are the heteropoly molybdates and the heteropoly tungstates. Preferred heteropoly anion compounds are silicomolybdic acid (SiMoA), phosphotungstic acid (PWA) and silicotungstic acid (SiWA). An especially preferred compound is phosphomolybdic acid (PMoA). The concentration of the heteropoly anion compound should be from 0.1% to 30%, preferably from 1% to 10%, based on the metal particle weight.

Solvents for use with the heteropoly anions may include glycol ethers, glycol ether acetates, alcohols, water and nitroparaffins, or any other solvent compatible with coating systems, in which the heteropoly anion is soluble. Among the nitroparaffins for use with heteropoly anions, the lower members of the nitroparaffin series, i.e., nitromethane, nitroethane and 1-nitropropane, are preferred on the basis of toxicological properties and availability. The solvent for use with heteropoly anions should be present at 5% to 100%, but preferably 20% or more, most preferably 35% or more, of the total weight of solvent in the final metal pigment paste. The solvent for use with heteropoly anions is generally about 28% to 50% by weight of the paste. The heteropoly anion solvent may also include surface active agents such as surfactants or anti-foaming agents.

A preferred method to incorporate the heteropoly anion compound is a variation of the slurry method taught in Kondis U.S. Pat. No. 4,693,754. A metal pigment particle filter cake or paste, typically containing 50% to 95%, preferably 60% to 85%, of metal pigment in a solvent, is added to a mixture composed of 15% to 94.5%, preferably 65% to 89%, of a solvent in which the heteropoly anion is soluble. The desired amount of the heteropoly anion compound, typically 0.1 to 30%, preferably 1.0-10%, based on the weight of the metal particles, is added to form a reaction mixture. Surfactants, dispersants, anti-foaming agents, etc., may also be added to the reaction mixture. The reaction mixture is agitated at a temperature of from 20.degree. C. to 100.degree. C., preferably ambient to 80.degree. C., for a period of time ranging from 0.5 to 30 hours, preferably 2 to 8 hours. Solvent is then removed to obtain the desired final metal pigment particle content, typically 40% to 90%.

In addition to the treatment method described above, another technique is to introduce the heteropoly anion compound into a ball mill, along with atomized powder or foil, lubricants and solvents etc. used to produce aluminum pigments. Thus, the metal pigment surfaces are stabilized as they are being generated in the ball mill.

The preferred treatment processes provides excellent stability, regardless of the lubricant used in milling.

The pigment paste obtained can be used in a variety of known coating systems, as a direct replacement for currently used pastes. Examples include maintenance, general industrial, roof coating, and automotive coating systems. Thus, the paste may be used, for example, with acrylic polymer emulsions, water reducible alkyd resin systems, water reducible alkyd/melamine cross-linked systems, waterborne epoxy coatings, polyester emulsions and water reducible polyester melamine coatings.

The heteropoly anion treatment can be combined with other treatment methods including the use of nitroparaffins, ionic organic phosphates, organic phosphites, phosphosilicates and vanadium compounds. The other treatment methods can be carried out before, during or after the heteropoly anion treatment.

Specific embodiments of the invention will now be further described by the following, non-limiting examples.

124.3 grams of a non-leafing aluminum paste feed TUFFLAKE.RTM. 5843 (non-volatile 70.8%) is slurried with 514 grams of glycol ether PM to yield a slurry concentration of 13.8% aluminum. To this slurry 10% by weight phosphomolybdic acid is added, based on the weight of aluminum in the paste feed. The material is agitated for 5 hours, at 80.degree. C. The slurry is then filter pressed to obtain a finished paste of 64% non-volatile content and then tested for aqueous stability as described below.

124.3 grams of a non-leafing aluminum paste feed TUFFLAKE.RTM. 5843 is slurried with 514 grams of glycol ether PM to yield a slurry concentration of 13.8% aluminum. To this slurry 10% by weight silicomolybdic acid is added, based on the weight of aluminum paste in the feed. The material is agitated for 5 hours, at 80.degree. C. The slurry is then filter pressed to obtain a finished paste of 69.2% non-volatile content and then tested for aqueous stability as described below.

124.3 grams of a non-leafing aluminum paste feed TUFFLAKE.RTM. 5843 is slurried with 514 grams of glycol ether PM to yield a slurry concentration of 13.8% aluminum. To this slurry 10% by weight phosphotungstic acid is added, based on the weight of aluminum in the paste feed. The material is agitated for 5 hours, at 80.degree. C. The slurry is then filter pressed to obtain a finished paste of 65.2% non-volatile content and then tested for aqueous stability as described below.

130 grams of a non-leafing aluminum paste feed TUFFLAKE.RTM. 5843 is slurried with 443 grams of glycol ether PM to yield a slurry concentration of 16.1% aluminum. In order to provide a comparative example, no heteropoly anion is added to this slurry. The slurry is then filter pressed to obtain a finished paste of 70% non-volatile content and then tested for aqueous stability as described below.

Each of the finished pastes obtained from the above examples is incorporated into a typical aqueous general industrial coating formulation prepared according to the following procedure. Enough of each paste to yield 20.5 g of aluminum is weighed out. The paste, 41.2 g glycol ether EB, 5.1 g Texanol (supplier--Eastman), 1.03 g Patcote 519 (supplier--Patcote), 0.62 g Dow Corning 14 (supplier--Dow), 73.5 g deionized water, and 313.7 g Joncryl 537 Resin, an acrylic emulsion (supplier--Johnson Wax) are blended together to form a uniform coating.

The formulations are placed in a constant temperature bath at 52.degree. C. and the gas evolved is collected in an inverted water-filled buret for 168 hours. The data are summarized in Table 1.

The results clearly show that the use of a heteropoly anion as an inhibitor reduces undesirable gassing and that the type of heteropoly anion used affects the amount of observed gassing.

The finished aluminum paste of Example 1 was incorporated into an aqueous automotive basecoat formulation. The formulation obtained was sprayed onto electrocoated steel panels, and then clear coated with a solvent-borne automotive formulation. This panel was placed in a enclosed chamber maintained at 100.degree. F. (38.degree. C.) and 100% relative humidity, in accordance with ASTM D2247-87. After 10 days of exposure, the panel was removed from the chamber and dried. No blistering or visual degradation was noted. The panel was then tested for adhesion, in accordance with ASTM D3359-87, Test Method B. No loss of adhesion occurred.

112.2 grams of a non-leafing aluminum paste feed SPARKLE SILVER.RTM. 5245-AR (non-volatile of 78.1%) is slurried with 531.4 grams of glycol ether PM to yield a slurry concentration of 13.6% aluminum. To this slurry 1.6% by weight phosphomolybdic acid is added, based on the weight of aluminum in the paste feed. The material is agitated for two hours, at 80.degree. C. The slurry is then filter pressed to obtain a finished paste of 66% non-volatile content and then tested for aqueous stability as described below.

41.3 grams of a non-leafing aluminum paste feed SPARKLE SILVER.RTM. 5245-AR is slurried with 602.4 grams of glycol ether PM to yield a slurry concentration of 5.0% aluminum. To this slurry 4.0% by weight phosphomolybdic acid is added, based on the weight of aluminum in the paste feed. The material is agitated for five hours, at 80.degree. C. The slurry is then filter pressed to obtain a finished paste of 68.1% non-volatile content and then tested for aqueous stability as described below.

76.8 grams of a non-leafing aluminum paste feed SPARKLE SILVER.RTM. 5245-AR is slurried with 562.2 grams of glycol ether PM to yield a slurry concentration of 9.4% aluminum. To this slurry 10.0% by weight phosphomolybdic acid is added, based on the weight of aluminum in the paste feed. The material is agitated for 0.5 hours, at 80.degree. C. The slurry is then filter pressed to obtain a finished paste of 68.8% non-volatile content and then tested for aqueous stability as described below.

41.3 grams of a non-leafing aluminum paste feed SPARKLE SILVER.RTM. 5245-AR is slurried with 600.5 grams of glycol ether PM to yield a slurry concentration of 5.0% aluminum. To this slurry 10.0% by weight phosphomolybdic acid is added, based on the weight of aluminum in the paste feed. The material is agitated for 2 hours, at 55.degree. C. The slurry is then filter pressed to obtain a finished paste of 68.2% non-volatile content and then tested for aqueous stability as described below.

112.2 grams of a non-leafing aluminum paste feed SPARKLE SILVER.RTM. 5245-AR is slurried with 524 grams of glycol ether PM to yield a slurry concentration of 13.8% aluminum. To this slurry 10.0% by weight phosphomolybdic acid is added, based on the weight of aluminum in the paste feed. The material is agitated for 5 hours, at 30.degree. C. The slurry is then filter pressed to obtain a finished paste of 65.2% non-volatile content and then tested for aqueous stability as described below.

112.7 grams of a non-leafing aluminum paste feed SPARKLE SILVER.RTM. 5245-AR is slurried with 474 grams of glycol ether PM to yield a slurry concentration of 15% aluminum. In order to provide a comparative example, no heteropoly anion is added to this slurry. The slurry is then filter pressed to obtain a finished paste of 70% non-volatile content and then tested for aqueous stability as described below.

Each of the finished aluminum pastes obtained from the above examples is incorporated into a general industrial aqueous coating formulation. The formulations are placed in a constant temperature bath at 52.degree. C., and the gas evolved is collected in an inverted water-filled buret for 168 hours. The data are summarized in Table 2.

The data clearly shows that the use of a heteropoly anion reduces the amount of undesirable gassing compared to an untreated sample.

While there is shown and described herein certain specific examples embodying this invention for the purpose of clarity of understanding, the same is to be considered as illustrative in character, it being understood that only preferred embodiments have been shown and described. It will be manifest to those skilled in the art that certain changes, various modifications and rearrangements of the parts may be made without departing from the spirit and scope of the underlying inventive concept and that the same is not limited to the particular forms herein shown and described except insofar as indicated in the scope of the appended claims.

The entirety of everything cited above or below is expressly incorporated herein by reference.

1. Cotton, F. Albert, and Wilkinson, Geoffrey, Advanced Inorganic Chemistry, 4th Ed., published by Wiley-Interscience.