US 3634146 A
A method for preparing metal surfaces for receipt of a coating such as a paint or adhesive (whereby increased coating adhesion and corrosion resistance is achieved) and the treated metal per se, are disclosed. The method comprises contacting the metal surface with various phosphinic and phosphonic acids.
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Description (OCR text may contain errors)
United States Patent Vernon Paul Wystrach Wilton;
Francis Clyde Rauch, Stamford, both 01 Conn. 855,397
Sept. 4, 1969 Jan. 1 1, 1972 American Cyanamid Company Stamford, Conn.
Inventors Appl. No. Filed Patented Assignee CHEMICAL TREATMENT OF METAL  References Cited UNITED STATES PATENTS 2,230,371 2/1941 Keiser 143/615 2,311,306 2/1943 148/615 3,224,908 12/1965 148/615 R 3,293,088 12/1966 143/6. 15 R 3,468,725 9/1969 Uhlig 148/65 R FOREIGN PATENTS 1,049,191 H1959 Germany 148/615 R Primary Examiner Ralph S. Kendall Attorney- Frank M. Van Riet ABSTRACT: A method for preparing metal surfaces for receipt of a coating such as a paint or adhesive (whereby increased coating adhesion and corrosion resistance is achieved) and the treated metal per se, are disclosed. The method comprises contacting the metal surface with various phosphinic and phosphonic acids.
CHEMICALTREATMENT OF METAL BACKGROUND OF THE INVENTION The use of various chemical materials in the treatment of metal surfaces to thereby render them corrosion resistant is well known to those skilled in the art. For example, U.S. Pat. No. 1,798,2l8 describes a method whereby certain molybdenum compounds are utilized whereas U.S. Pat. No. l,9ll,537, discloses the use of dicarboxylic and hydroxydicarboxylic acids for the same purpose. Phosphoric acid salts, (U.S. Pat. Nos. 1,936,533; l,936,534; 2,952,699) phosphates, (U.S. Pat. Nos. 2,224,695; 2,472,099; 2,769,737) and orthophosphoric acid-chlorinated hydrocarbon solutions, (U.S. Pat. No. 2,789,070) have also been disclosed for similar purposes.
While these prior art techniques generally provide acceptable corrosion resistance, they usually fail in regard to the adhesion of surface coatings such as paints, varnishes, enamels, adhesives etc. thereto. Additionally, many of these antiquated systems are severely polluted by water, i.e., when contacted with water they tend to peel, blister etc.
SUMMARY We have found that the adhesion of coatings to metals can be materially increased or strengthened by first treating the metal with a chemical material which is chemisorbed, i.e., chemically reacted with or absorbed via strong bonds. In this manner, a foundation (integral chemical or chemically bound coating) is formed on the metal surface via reaction with the metal, which foundation is then more susceptible to an ultimate or surface coating, such as a paint or adhesive, than materials utilized in the past. Our method results in coatings which are more securely bonded or adhered to the foundation layer because the coating is chemically bonded to the foundation and the foundation is chemically bonded to the metal surface. That is to say, upon treating the metal according to our novel method, a reaction, as mentioned above, causes a strong bonding of the acid layer to the metal. There then remains free, for further reaction with a surface coating, a second reactive group in the acid layer. This second reactive group then chemically combines with any surface coating applied thereto to produce a metal having a coating tightly bonded thereto. Additionally, the corrosion resistance of the metal treated according to the present invention, with or without an extraneous coating on its surface, is at least as effective as known corrosion resistant systems.
DESCRIPTION OF THE INVENTION INCLUDING PREFERRED EMBODIMENTS Our novel process comprises treating a metal surface, such as that of aluminum, steel, iron, copper, titanium etc. with a reactive phosphinic or phosphonic acid. The useful phosphinic acids have the formula and the useful phosphonic acids have the formula /R2 HO OH tyl) phosphonic acid, bis(aminomethyl) phosphinic acid, bis(a-aminobenzyl phosphinic acid, (p-vinylphenyl) phosphonic acid (2 hydroxynaphthyl) phosphonic acid, bis (l-aminonaphthyl) phosphinic acid, (p-acryloylphenyl) phosphonic acid, (mercaptomethyl) phosphonic acid, bis( 2- mercaptoethyl) phosphonic acid, (diallyl) phosphinic acid, (2-butylene) phosphonic acid, (aminomethyl) phosphinic acid, (mercaptomethyl) phosphinic acid, (2-hydroxyethyl) phosphinic acid and the like. These specific compounds are mentioned herein for purposes of illustration only and the list is not meant to be inclusive of all possible useful compounds.
The phosphinic acids and phosphonic acids used herein are well known to those skilled in the art as are methods for their production, such as, for example, those taught in U.S. Pat. Nos. 2,678,940; 2,717,906; 2,874,] 84; 3,032,500 and 3,322,716, which patents are hereby incorportated herein by reference.
While not wishing to be bound by any particular theory, we believe that the =0 and OH groups of the phosphinic acids (or the OH groups of the phosphonic acids) react with the oxide layer formed on the surface of the metal to be treated. The existence of such oxide layers is well recognized by a cogent workers in the art. Such layers form upon exposure of the metal to the atmosphere. After the acid-oxide bonding is complete, the reactive groups of the acid foundation layer are then free to react with the surface coating, i.e., paint, adhesive etc., thereby chemically bonding the coating to the metal. The acid-oxide bonding is pseudochemical in nature and can be more accurately described as a chemisorption of chelation of the acid by the metal.
The acid foundation layer may be applied to the metal surface, the metal first being thoroughly cleaned such as by degreasing with trichlorethylene etc. or other common techniques, by immersing, dipping, painting, brushing, wiping, spraying etc. the metal article to be treated with solutions of one or more of said phosphinic or phosphonic acids, for a length of time such that the metal surface absorbs or reacts with a sufficient amount of acid. The metal is then merely removed from the solution and allowed to dry.
Additionally, the phosphinic or phosphonic acid can be applied to the metal by first incorporating the acid into the surface coating material, e.g., the paint or adhesive, and then applying the surface coating. In this manner, the acid can be added, for example, to the paint vehicle, and the paint then can be sprayed etc. onto the metal. Similarly, the acid can be added to one part of a two-part adhesive system and the adhesive can then be applied to the metal. In each instance, the reactive groups of the surface coating material will react with the appropriate groups of the acid while the acid itself reacts with the oxide on the metal surface, as more specifically described above.
The acid solution can comprise from about one part to about 50 parts of the acid per L000 parts of solvent, e.g., ethanol, methanol, water etc. The treatment is preferably conducted at room temperature although higher or lower temperatures may be utilized, if desired. Complete chemisorption of the acid onto the metal surface is generally achieved in from about 10 to about 20 minutes, the lower the acid concentration, the longer the reaction time necessary.
As mentioned above, the foundation layer of phosphinic or phosphonic acid affords a chemically available site whereby the reactive groups of a paint layer may chemically react to thereby form a tightly adhering coating layer. Examples of paints, adhesives or other coatings which may be used include epoxy paints and adhesives, i.e., those containing chemically available CH--OH- groups, urethane paints and adhesives, i.e., those containing chemically available NCO groups, acrylic paints and adhesives, i.e., those containing chemically available groups, vinyl paints and adhesives i.e., those containing dipped in a solution of the appropriate phosphinic or phosphonic acid for about l minutes.
After treatment, the aluminum electrode is placed in the cell and the appropriate electrical connections to the potenchemically available (,:H grouPs and h like AS is tiostat made. The electrolyte is a citrate buffer of pH 4.6. The clear from the enuimmmnPfthe chemfcany avallable groups initial potential of the aluminum electrode is then set at -l.0 of the palms the free f volts, with respect to the reference electrode. The voltage the ayallable f of the Phosphlmc or Phosphomc scan is then turned on. A sweep rate of 0.2 volts/minute in the F' layer prevlously appl'ed to the metal Surface- Thesefeac' positive direction is used and the current developed is meagroups are represented RI and R2 fibove and m the sured on the recorder. The resulting polarization curve, voltcase of epoxy and urethane adheswes would i age on the X-axis and the current on the Y-axis may then by NH OH or SH groups, while in the case of acrylic or analyzed to give the corrosion ram vinyl paints and adhesives, the group would be the unsaturated The method of analysis is that used by Evans et 3|. L E]ec SUbStltUEnt, li'lClUdll'lg vinyl, etc. trochem. Soc.
TABLE I Corrosion Breakdown rate (mg./ potential Example Acid Solvent Concentration dm day) (volts) Control.. No treatment 6.43 69 1 Bis(hydroxymethyl) phosphinic acid Ethanol -.01% (10- M) 3.33 -.38 2 a-Aminobenzyl) phosphonic acid.. er -.01% (10- M) .35 01 3 (2-aminoethyl) phosphonic acid-.. d0. 01% (10* M) 24 +.16 4 (l-aminoethyl) phosphonic ac1d d0 01% (10- M) .90 +.60 5 (Aminomethyl) phosphonic acid. .do -.0l% (10- M) 1.11 +.96 6 (l-aminobutyi) phosphonic acid ..d0 -.01% (10- M) .90
The paints can be applied in a condition such that the reaction concurs while the paint vehicle evaporates or in a condition such that the paint must more completely polymerize before it forms a useful coating. in the latter case, if the functional group of the phosphinic or phosphonic acid is such that it initiates polymerization of the paint, the paint may be applied in a prepolymer or semipolymer condition. An example of such a treatment is illustrated by the use of an hydroxy group containing phosphonic or phosphinic acid and an epoxy prepolymer. In this case, the hydroxy group both reacts with and cures (polymerizes) the epoxy prepolymer.
Additionally, we have found that our novel processing procedure can be utilized in conjunction with known corrosion resistance enhancing procedures to obtain a pseudosynergistic effect. For example, we can improve the corrosion resistance of metals treated according to our invention by first treating the metal with an inorganic chromate in a manner known in the art. Alternatively, the chromate (e.g. potassium dichromate; chromic acid solution, etc.) may be incorporated into the phosphinic or phosphonic acid solution of our novel method before treating the metal according to our invention. in this manner the corrosion resistance of the metal is increased without loss of the enhanced surface coating adherence mentioned above.
The pretreating technique and products of the instant invention find utility in military, industrial and consumer fields such as in the treatment of aircraft and ship surfaces, cooling towers, heat exchangers, window screens, siding etc.
The following examples are set forth for purposes of illustration only and are not meant to be construed as limitations on the present invention except as set forth in the appended claims. All parts and percentages are by weight unless otherwise specified.
EXAMPLE I in determining the corrosion rate according to table I, a modified polarographic apparatus is used. The apparatus includes an appropriate experimental cell, a potentiostat and recorder. A three electrode system is used: (I) an aluminum electrode; (2) a calomel reference electrode and (3) a platinum counter electrode. The aluminum electrode is 0.030 inch-diameter wire which is potted with a commercially available epoxy resin which in then sanded off so that the actual electrode area is the circular cross section ofthe wire. For the control, the samples are merely untreated electrodes. For examples 1 6, the freshly polished aluminum electrode is EXAMPLE 7 Aluminum panels, 3X5 inch, are degreased by dipping in benzene and further cleaned by dipping in hot l0 percent solution of a commercially available aluminum cleaner. The panels are then allowed to dry in air. One panel is then immersed in a solution of bis(hydroxymethyl) phosphinic acid in ethanol. After 15 minutes the panel is removed and allowed to dry.
The panel is then spray painted with a commercially available epoxy paint and allowed to dry and cure for 5 days at room temperature.
The painted panel is then subjected to a modified version of the Cross-Hatch Tape Test" formulated by the National Coil Coaters Association. in the test, the painted surface is cut l0 times vertically and I0 times horizontally with a razor blade, the scratch lines being approximately 5 mm. apart. Scotch cellophane tape No. 600 is applied over the test area and rubbed with sufficient pressure to remove all air bubbles. The panel is allowed to set for 10 minutes and the tape is then removed sharply with a pull at right angles to the test surface. A visual examination allows a reasonably accurate estimation of the percent finish remaining on the panel in the test area.
The average results of tests conducted on panels treated according to example 7, in addition to the results recorded utilizing different acids according to the process of the present invention, are set forth in table II, below.
Following the procedure of example 1 except that (pacryloylphenyl) phosphonic acid is utilized in place of the acid used therein, a similar corrosion resistant panel is produced.
EXAMPLE 13 Again following the procedure of example I a corrosion-re sistant panel is produced utilizing (8-aminooctyl) phosphonic acid.
EXAMPLE l4 Utilizing the procedure of example 4 except that bis(mercaptomethyl) phosphinic acid is employed, a corrosion resistant panel is produced.
EXAMPLE 15 The procedure of example 7 is followed except that bis(pvinylphenyl) phosphinic acid is utilized as the foundation layer and the surface coating is commercially available white acrylic paint. A panel similar in surface coating retention to that of said example is obtained.
EXAMPLE 16 The procedure of example 7 is again followed except that after cleaning the surface of the metal panel with a degreasing agent and an alkali cleaning agent, 3 parts of bis(hydroxymethyl) phosphinic acid are added to 100 parts of the catalyst-curing agent package of a commercially available, two-package polyurethane adhesive composition. After blending the contents of the two packages together, the resultant mixture is applied to the clean aluminum panel and cured under the recommended conditions. The adhesive is bonded more tightly to the metal panel than it is on a control specimen formed without the added phosphinic acid.
EXAMPLE 17 The procedure of example 7 is again followed except that in place of the epoxy paint used therein, a commercially available epoxy adhesive is used. The bonding of the adhesive to the metal is similar to that of the paint of said example.
EXAMPLE 18 The procedure of example 1 is again followed except that (2-hydroxynaphthyl) phosphinic acid is used. Similar results are observed.
EXAMPLE 19 The procedure of example I6 is followed except that (diallyl) phosphinic acid is utilized in place of the acid used therein and the surface coating applied is in the form of an acrylic paint. The adhesion of the paint to the metal is superior to that of a test sample wherein no phosphinic acid is present.
EXAMPLE 20 A panel identical to that of example 14 is produced and coated with a commercially available epoxy paint. The adhesion of the paint to the metal panel is excellent.
EXAMPLE 21 The procedure of example 15 is again followed except that a commercially available vinyl paint is used in place of that paint of said example and the metal used is steel. The adhesion of the paint to the steel panel is superior to that of a panel coated without the foundation layer of phosphinic acid.
EXAMPLE 22 The procedure of example I is again followed except that the aluminum is replaced by stainless steel. Similar results are recorded.
EXAMPLE 23 Following the procedure of example 5 except that the metal utilized is carbon steel, effective corrosion inhibition results.
EXAMPLE 24 The procedure of example 7 is again followed with replacement of the aluminum panels with similar sized sections of titanium sheet. The adherence of the epoxy paint to the phosphinic acid layer is excellent.
EXAMPLE 25 The use of nickel sheet for aluminum of example l0 results in 45 percent of the epoxy paint finish remaining after eight tests.
EXAMPLE 26 P O OH or a compound having the formula O\ /R2 H O OH wherein R 10R are, individually, aromatic (C -C or aliphatic (C -Cg) radicals both containing an NH OH or SH group and R is hydrogen or R,, and thereafter coating the resultant treated surface with a surface coating containing groups chemically reactive with at least one of the groups of said compound.
2. A method according to claim 1 wherein said compound is added as a mixture with said surface coating reactive with said compound.
3. A method according to claim 1 wherein said compound is bis(hydroxymethyl) phosphinic acid.
4. A method according to claim 1 wherein said compound is (a-aminobenzyl) phosphonic acid.
5. A method according to claim 1 wherein said surface coating is a paint.
6. A method according to claim 2 wherein said surface coating is a paint.
7. A method according to claim 1 wherein said surface coating is an adhesive.
8. An article according to claim 1 wherein said metal is aluminum.