Treatment of metal surfaces with aqueous solution of melamine-formaldehyde composition
United States Patent 1 Guhde 1 TREATMENT OF METAL SURFACES WITH AQUEOUS SOLUTION OF MELAMINE-FORMALDEHYDE COMPOSITION [52] U.S. Cl. 148/6.l5; 148/614; 148/617;  
  117/132; 260/294 R [51] Int. Cl. G23f 7/00 [58] Field of Search 148/615, 6.14, 6.17;  
 [ 51 Apr. 15, 1975 [56] References Cited UNITED STATES PATENTS 2,383,589 8/1945 Bunting et al 117/132 BF 3,397,077 8/1968 Boller et al 148/6. 15 Z 3,519,495 7/1970 Plaxton 148/616 3,586,543 6/1971 lijima et a1. 148/6.l4 R  
 Primary Examiner-Mayer Weinblatt Attorney, Agent, or Firm-James W. Adams, Jr.; William H. Pittman [57] ABSTRACT Metal surfaces are treated, preferably after phosphating, with an aqueous solution (preferably acidic) of a melamine-formaldehyde composition and are subsequently dried, preferably by heating. The treatment is a suitable substitute for the conventional chromate rmse.  
 10 Claims, N0 Drawings TREATMENT OF METAL SURFACES WITH AQUEOUS SOLUTION OF MELAMINE-FORMALDEHYDE COMPOSITION This invention relates to methods of treating metal surfaces and compositions for use therein. More particularly, it relates to a method of treating a metal object which comprises contacting the surface of said object with an aqueous solution comprising a melamineformaldehyde composition and subsequently drying said object.  
  It has been the practice for many years to form conversion coatings on metal surfaces by treatment with aqueous solutions of various chemicals. These chemicals react with the metal surface to form a coating which protects the metal against corrosion and also serves as a paint base. The most commonly used con version coating compositions are phosphate and chromate compositions. and many of these are known in the art. Frequently, a phosphate conversion coating is formed on the metal surface and is subsequently treated with a chromate solution as a post-rinse. which improves the corrosion resistance and paint adhesion of the coated metal.  
  Recent emphasis on water pollution problems has drawn attention to the fact that chromate-containing effluents are serious pollutants. To meet water-quality standards. it is frequently necessary to go through a multi-stage purification sequence in order to remove chromates from the process effluent. Typical steps in this sequence include reduction of the hexavalent chromium to trivalent chromium and precipitation with lime or some similar chemical. The result is that the chromium content of the effluent water is substantially decreased. but the expense of the treatment sequence to the user is quite high.  
  To alleviate this problem. interest has turned to the development of metal treatment methods which do not involve chromium chemicals. A number of these have been tried but none has come into substantial use because they do not give the desired level of protection of the metal surface. 7  
  A principal object of the present invention, therefore, is to develop new methods and compositions for treatment of metal surfaces to increase corrosion resistance and paint adhesion.  
  A further object is to provide a metal surface treatment system which can be used as a post-rinse following the deposition of a phosphate coating on the metal.  
  Still another object is to provide a chromium-free metal treatment system.  
  A still further object is to develop a system for metal treatment which will not result in substantial water pollution.  
  Other objects will in part be obvious and will in part appear hereinafter.  
  The present invention is based on the discovery that aqueous solutions of melamine-formaldehyde composi tions serve as excellent treatment compositions for metal surfaces. These compositions are known in the art and many are commercially available. They are typically formed by the reaction of melamine with formaldehyde, often in the presence of a lower alkanol (preferably methanol). The principal constituents of these compositions are polymethylolmelamines, especially tetramethylolmelamine, pentamethylolmelamine and hexamethylolmelamine; lower alkyl ethers thereof such as hexamethoxymethylmelamine; and condensation products of these materials. The substances are often referred to in the art as melamineformaldehyde or alkylated melamine-formaldehyde resins.&#34; but it should be noted that the compositions contemplated herein are not always resinous. and they are water-soluble whether monomeric or polymeric.  
  The preparation of melamine-formaldehyde compositions useful in the method of this invention is illustrated by the following examples.  
 EXAMPLE 1 A mixture of 504 grams (4 moles) of melamine. 1312 grams of &#34;Methyl Formcel&#34; (a solution of formaldehyde in a mixture of methanol and water. said solution containing about 55% formaldehyde) (containing 24 moles of formaldehyde) and 944 grams (29.5 moles) of methanol is heated under reflux for 1 /2 hours. Acetic acid. 8 ml.. is added and the mixture is refluxed for an additional 3 hours; a further 4 ml. of acetic acid is then added and refluxing is continued for an additional onehalf hour. The solution is cooled to room temperature and adjusted by the addition of 10% alcoholic potassium hydroxide to a pH of 10.5. lt is then concentrated by heating at 65C. under vacuum to yield an solution. in a mixture of methanol and water. of the desired melamine-formaldehyde composition.  
 EXAMPLE 2 Following the procedure of Example I, a product is prepared from 126 grams (1 mole) of melamine. 273 grams of Methyl Formcel (containing 5 moles of formaldehyde) and 261 grams (8.15 moles) of methanol.  
 EXAMPLE 3 Following the procedure of Example 1, a product is prepared from 63 grams (0.5 mole) of melamine. 168.5 grams of Methyl Formcel (containing 3 moles of formaldehyde) and 274.6 grams (3.7 moles) of n-butyl alchol.  
 EXAMPLE 4 Following the procedure of Example I, a product is prepared from 126 grams (1 mole) of melamine, 437 grams of Methyl Formcel (containing 8 moles of formaldehyde) and 187 grams (5.84 moles) of methanol.  
 EXAMPLE 5 A commercially available hexamethoxymethylmelamine composition, Cymel 303&#34; (50 parts by weight), is diluted with 25 parts of water and 25 parts of isopropanol to produce a clear solution.  
 EXAMPLE 6 A commercially available methylated melamineformaldehyde *resin&#34;, Cymel 481&#34;, is diluted with a methanol-water (1:4 by weight) mixture to 60% solids. A clear solution is obtained.  
  The method of this invention may be used for treating ferrous metal, galvanized and aluminum surfaces, with steel and galvanized steel surfaces being preferred. In general, the use of solutions containing very small amounts of the active ingredient is contemplated; typically, the concentration of the melamine-formaldehyde composition in the metal treatment solution is about ODS-0.75% by weight, preferably about 0.05-0.57c. The pH of the metal treatment solution is ordinarily within the acidic range, typically about 3-6 and preferably about 3-5, with the lower pH values being especially preferred for the treatment of galvanized metal. The acid used to adjust the pH may be a mineral acid such as sulfuric, hydrochloric or phosphoric acid. or an organic acid such as acetic acid. Phosphoric acid is preferred.  
  The metal treatment solution is applied to the surface of the metal article by any of the usual techniques such as brushing. dipping. spraying. roller coating and the like. Following such application, the metal surface may be rinsed again with water although omission of this rinse is preferred, and is then dried by conventional means, usually by air-drying at ambient temperature and/or heating to a temperature high enough to remove volatile materials (including water and alcohol used as solvent). Temperatures of about l50200C. are preferred during at least part of the drying operation. It is believed that the melamine-formaldehyde composition on the metal surface cures when dried to form a very thin resin coating.  
  In a preferred embodiment of the method of this invention. the metal object is initially cleaned and a phosphate conversion coating is deposited thereon. The phosphate coating may be any of the known types such as iron phosphate, zinc phosphate, manganese phosphate, calciumand/or magnesium-modified zinc phosphate, etc.. and its application is accomplished by any of the methods known to those skilled in the art. Following the phosphating treatment, the metal object is usually rinsed again and is then treated as described hereinabove.  
  The method of this invention is illustrated by a series of tests in which cold-rolled steel panels are cleaned and treated with the composition described in Example 6. diluted to about 0.5% solids (by weight) to provide a coating weight of 20 mg. per square foot. The panels are air-dried at room temperature and finally dried at 176C, after which they are coated with a white alkyd baking enamel. The paint film on each panel is ruptured down to the bare metal by scoring a 6-inch line on the surface of the panel and scored panel is placed in a cabinet containing a aqueous sodium chloride solution at 95F. Air is bubbled through the solution to produce a corrosive salt atmoshpere which acts on the surface of the test panels, suspended above the level of the salt solution. The panels remain in this atmosphere for 168 hours after which they are removed, washed with water and dried with a cloth. A pressure-sensitive tape is then applied to each panel and removed suddenly. This procedure is repeated until no more paint can be removed in this manner. The loss of adhesion caused by corrosion from the scribed line is measured in thirty-seconds of an inch.  
  When tested in this way, a panel treated with the composition of Example 6 had a loss of adhesion of 0-2 thirty-seconds of an inch. A control panel, which was merely cleaned and rinsed with water prior to painting, showed complete loss of adhesion.  
 What is claimed is:  
  1. A method of treating a phosphated ferrous metal, galvanized or aluminum object without the application of chromium chemicals thereto which comprises contacting the surface of said object with an aqueous acidic solution consisting essentially of at least one melamine-formaldehyde composition. and subsequently drying said object.  
  2. A method according to claim 1 wherein said solution contains about 0.05-0.757c by weight of melamine-formaldehyde composition.  
 3. A metal object which has been treated according to the method of claim 1.  
  4. A method according to claim 2 wherein the metal object is a steel object.  
  5. A method according to claim 4 wherein the melamine-formaldehyde composition contains polymethylolmelamines, lower alkyl ethers thereof, and condensation products of the same.  
  6. A method according to claim 5 wherein the drying is effected at least partially at a temperature of about l50-200C.  
  7. A method according to claim 5 wherein the lower alkyl ethers are methyl ethers.  
  8. A steel object which has been treated according to the method of 5.  
  9. A method according to claim 7 wherein the drying is effected at least partially at a temperature of about l50-200C.  
 10. A steel object which has been treated according to the method of claim 7.