CA1062098A - Melamine-formaldehyde and tannin treatment of metal surfaces - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
Disclosed is a single-application non-electrolytic method for the treatment of bare metal surfaces to form a corrosion-resistant coating. The clean metal surfaces are treated with an aqueous composition containing a melamine-formaldehyde resin and a vegetable tannin. The resulting dried coating exhibits superior qualities for a single-application coating process when used as a paint base and the process does not require the use of environ-mentally objectionable chromium compounds. Application to ferrous zinc or aluminum surfaces may be by any known technique designed to provide a deposited layer of desired uniformity.

Description

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Thi8 lnvcntion rol~t~ to ~n improved proceRs for co~tlng metal surfaces and more particularly relates to improvements in the process for applying A protective paint base coating to metallic surfaces such as zinc, iron and aluminum. The need for applying protective coatingSto metal surfaces has long been recognized in the art. These coatings are normally referred to as "conversion coat-ings" because the metal surface is converted from a chemically active one readily susceptible to oxidation to a surface which is relatively inactive and resistant to oxldation. One of the most important common uses Or these conversion coatings is as a base for subsequent painting. Under these circumstances, the user will look to the ad-hesion of the paint to the metallic surface, as well as the resist-ance of the painted surface to humidity, salt-spray and similar tests ln order to deter~ine the corrosion resistance of the painted article.
In th~ past, difficulties have often been encountered in developing a method for treating metal surfaces which produces an end product which, when painted, exhibits satisfactory adhesion and corrosion characteristics in all respects. In some instances, it has been possible to obtain satis~actory results by sub~ecting the metal surface to a series of chemical treatments. The most commonly used process of this type involves a first treatment of~the cleaned surface with a conversion coatin~ solution which, for example, may produce zinc or iron phosphate coatin~s on ~teel, zinc phosphates or complex oxides on galvanized steel or chromic oxide or phosphate coating~ on aluminum followed by a second treatment with a dilute hexavalent chromiu~-containing composition with an intervenlng water rlnse. However, multlple-stage treating involves additional personnel~ inter-stage contamination problems, and extended line length when compared to a single-application process as in the present invention~ Because of the potential for inter-stage conta-106'~098 ~lnation due to dra~_out, rinsing steps must be added which increasethe consumption of water and add to disposal and pollution problems.
Furthermore, the use of composition~ containing hexavalent chromium compounds is environmentally ob~ectionable and should be avoided if po~sible~
The present invention provide6 a single-appllcation method for the treàtment of bare metal surfaces for the purposes of forming a corro~ion-resistant coating for paint base appllcations thereon.
It has been found that, if the metal is contacted with an aqueous ~or~ing composition containing a melamine-formaldehyde resin and a vegetable tannin, and the thus-treated surface is subsequently dried, a coating is obtained which exhibits characteristics comparable to a con~entional converslon coating. If either of the components is omitted from the composition, the conYersion coating i8 deficient in one or more respects, A~ mentioned~ this sinele-application process has obvious advantages o~er prlor art methods and employlng two or more staGe6 to obtain a converslon coating having comparable properties.
U.S. Patent 3,397,077 teaches the use of an aqueous system containing an inorganic oxyacid (such as phosphoric acid) and an organic resin-rorming material (e.g., based upon a melamine-formal-dehyde reaction product) but makes no suggestion of the advantages of including a ~egetable tannln in the compo~ltion.
It has now been discovered that an aqueous coating compo-sition contalning a melamine-formaldehyde resin and a vegetable tannin will produce a satisfactory paint base coating on ferrous, zinc and aluminum surfaces.
The exact composition of the melamine-~ormaldehyde resin ~uitable for use in the present invention i~ not critical. Monomerlc, ~0 dimeric, and higher order re~ins are ~uitable. As i8 well-known, formaldehyde methylolates the amine groups of the melamine hetero-

-2-,, . . , . - - . ~ - - . .

cyclo ~ftor whloh cro~-link~6o botwoun th~ moloculo~ t~ pl~co via condensation polymerization. The degree of methylolation is not critical although at least one mole of formaldehyde per mole of melamine i8 preferred and at least 1-1/2 mole6 of formaldehyde per mole of melamlne i8 more preferred. The commercially available melamine-formaldehyde resin~ also include variou~ degrees of short chain alkyl substltution or etherification with for example, methyl, ethyl, propyl or butyl groups. Table I lists a number of the com-mercially available melamine-formaldehyde re61ns along uith thelr suppliers. While these resins are suitable, the lnvention is not limited thereto. The use of tannins for the treatment Or metal sur-~aces has been described in U.S. Patent Nos. 1,798,218; Re.24,604;
566~037; 759,986; 1,Q79,453; 1,501,425; 1,817,174; 2,311,563;
2,ô54,368; 3,547,710; and 3,578,508. Very small quantlties of the tannin material, when included ln comblnation with a melamine-formal-dehydo resln, have now been found Yer~ effective in increasing the anti-corrosion properties imparted by the treatine 601ution of the present invention. It is desirable to include at least .01 g/l of the vegetable tannin in the solution. Most preferably, the ~eight ratlo Or the re~in to the tannin i8 at lea~t 1:1 wlth a resin con-ccntration of at least 0.01 g/l. More preferably, the Yrelght ratio of the resin to the tannin 18 at least about 3.75:1 and most prefer-ably at least about 7.5:1. Table II lists names of Yarious tannins which may be employed in the present invention together with the supplles and identification codes. ABain~ these are only exemplary and the invention is not limited thereto.
Suitable additional components YJhich may also be included comprise the commonly employed metal di- and trivalent cations such as zinc, manganese, cobalt, nickel and iron; nitrate; inert coloring ~0 agents designed to provide a specific color to the protective coating;
silicon compounds; a conductive material to improve weldability such .. ,. ,.. ,,. , , - : .
. . ~ . ~ . . . .

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as pul~erulent metal as disclosed in U.S. Patent No. 3,671,331 (zinc) or a conducti~e carbon; and ~tabilizlns agents necessary to keep the organlc components from separating.
The aqueous composltions may be used wlthout any need for pH ad~ustment but can be 80 ad~usted if desired. pH values of from 2 to 10 are sultable.
Aluminum, zinc and rerrous surfaces and alloys containing a predominant poition o~ these metals may be successfully treated wlth the composition ln accordance with the invention.
~he final film thicknes6 or coating weight is a function o~ the total concentration of components in the treating composition -and the thickness to ~hich the composition is applied to the surrace.
As the surrace i8 then dried, the composition becomes concentrated and a reaction will begin to take place between the components of the compo~ition to form the coating Or the invention.

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T A B L E _I
MEL,AMINE~FORMALDEHYDE RESINS
NAME SUPPIIER
CR2024 Clark Chemical Corporation Resin G-3 Jersey State Chemical Co.
Mel-Tron A Crown Metro, Inc.
Schercomel Scher Bros. Inc.
X-3387 Cargill, Inc.
Uformite MM-83 Rhom & Haas Company Resydrol WM 501 American Hoechst Corporation Resimene X 712 Monsanto Company Resimene X 714 Monsanto Company Resimene X 720 Monsanto Company Resimene X 730 Monsanto Company Reslmene X 735 Monsanto Company Resimene X 740 Monsanto Company Cymel 370 American Cyanamid Company Aerotex MW American Cyanamld Company Aerotex 92 American Cyanamid Company Tanak M3 American Cyanamid Company Aerotex P225 American Cyanamid Company Tanak MRX American Cyanamid Company Cymel 7273-7 American Cyanamid Company All these compounds are trade marks.

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T A B L E ~II
ANNINS
NAME SUPPLIER
Tannic Acid Merck & Company, Inc.
Tannic Acid (NFXll) S.B. Penick and Company Tannic Acid (Tech. 3C) The Harsha~ Chemical Co.
Tannic Acid (Tech. XXX) The Harshaw Chemical Co~
Tannic Acld (Tech 7c) The Harshaw Chemical Co.
Chestnut Extract The Mead Corporatlon Spray Dried Chestnut Arthur C. Trask Corp. -~
Bisulfited Quebracho Extract Arthur C. Trask Corp.
Non-Bisulfited Quebracho Extract Arthur C. Tra k Corp. -Wattle Extract Arthur C. Tra~k Corp.
Cutch Extract Arthur C. Tra~k Corp.

: . - . : . .

It is preferred to maintain the temperature of the working composition at fairly low levels in order to avoid any premature re-action between the actlve components of the composition. Normal ambient temperatures are sultable for the worklng bath. As tempera-turos increase~ sub~tantial reactions begin to occur ln the bath itselr.
The metal surface itself may be preheated ln order to hasten this drying process. Metal temperatures of up to 200F or hlgher may be employed for roll-on applications ~ithout degrading the bath. Much higher temperatures may be employed in connection with mist-on application as dlsclosed in U.S. Patent No. 3,578,510. The mann¢r of drying is not critical 80 long as the liquid film is not untuly disrupted, e.e., but hot air currents or physical contact during the drying proces~. If ti~e i8 not critical, the surraces may even be per~ltted to dry at room temperature. ~owe~er, under normal operations~ lt is desirable to use ele~ated o~en temperatures and ~arm air streams of ~elocity insurficient to dlsturb the wet film. From a practlcal standpoint, the o~en temperature should re-sult in a metal temperature Or bet7een about 125 and 350F and pre~erably between about 150 and 250F~
Coating wei8hts may vary from as little as one milligram per square foot to as much as 400 mill~grams per square ~foot or higher. Normally, the coating weight will be between 5 and 100 milligrams per square foot. Coating ~eights for aluminum surfaces will typically be between 5 and 30 mg/ft2, while for zinc and ferrous surfaces the coatlng weight will typically be at least 20 mg/rt2.
Application Or the aqueous composition to the metal surface may be accomplished in any Or the conventional manners (spray, immersion~ roll-on, flooding) 80 long as sufficient care i8 taken to obtain a reasonably uniform thickne~s of the aqueous fllm. For n at surfaces such as sheet or strip, this control may be accomplished most readily through the use of rollers, or squeegees. Required - `:

contact times from initial application to dry-off can be less than 30 ~econds and are usually less than 20 seconds. Mist_on techniques may be employed on preformed articles. The paint is applied to the drled coated surface by conventional means. While the particular ~aint employed will affect the over-all corrosion resistance and adhesion~ with mo~t commerclal paints tested the present 61ngle ap- ~ ;
plication process gave results comparable to those obtained by con-ventional two or three-stage proces6es.
SAL1~ SPRAY CORROSION RESISTANCE
Salt spray corrosion reslstance was measured in accordance with the procedure of ASTM B117-61. The panels were rated in terms of the amount Or paint 10~8 from a scribe in 1/16 inch increments ~N for no loas of paint at any point). The principal numbers re-present the general ranee of the creepage from the scribe along its longth wheroas the superscripts represent spot or non-representative creepage at the paint of maximum creepage along the length of the scribe~ Thus, 2-7l8 means representative creepaee varied from 2/16 to 7/16" with a maxlmum of 10/16" at one or two spots. Where corro~ion was extensive, the results were expressed as % peel over the entire panel surface~ e.g., 6~/oP~
HUMIDITY CORROSION RESISTANCE
Humidity corrosion resistance was mea~ured in accordance wlth the procedure o~ ASTM 2247-64T~ The panels were rated in term~
of the number and size of the blisters, F for few, M for medium and D for dense~ and from 9 for very small si~e to 1 for very large. 10 represent6 no blisters. Where the rating is preceeded by a G or C, the panel gave a 10 rating except for blisters due to handling (G) or concentration effects (C) such as those vvhich would result from solution run down.

A cloth soaked with methyl ethyl ketone i8 rubbed back and ~QG;~(~9~

forth at con~tant pressure across the cured painted surface to remove the paint over approximately a 10 mm length at the polnt of contact.
The number oS back and forth rub6 is recorded.
ACETIC ACID SALT SPRAY RESISTANCE
Acetic acid salt spray resistance was measured ln accordance v~th ASTM B287. Conditions are similar to ordinary ~alt spr~y testing except the salt solution is ad~usted to pH 3.2 with acetic acid and the chamber is maintained at 95F. Ratings are given as in the Salt Spray Test.
WELDABILITY
In the following examples~ the "2000 spot weld" test is employed to eYaluate the weldabillty of a coated surrace. The test measures the ability of a single pair of electrodes to perform at least 2000 succes6i~e Rpot welds Or acceptab}e ~uality. Unless the conductivity of the coating i8 ~ufflciently high, the welds produced will be unsatisfactory. The test ls performed uslng RWMA
Class lI copper electrodes ln a~ air-operated, sin~le point press, low inertia weldin~ ~ystem~ For a mlnimum metal thickness of .036 inches~ the pre~cribed sy~tem parameters are an electrode force of 650 pound6~ a weld time of 13 cycles~ a secondary current Or 14,000 amps and an electrode rsCe configuration of 0.25" x 45. For the test, pairs of treated 1 x 4~ coupons and pairs Or 4 x 12" panels are placed with thelr untreated ~ides touching. 2000 successive welds are performed. Coupons are welded in two ~pots and then pulled apart for the initial welds and after each serles o~ 250 welds. One coupon then has a hole at the spot weld and the~ther ha~ a ~'button" Or metal. The button ls then measured across its narrowest apparent diameter. The test is a failure if the diameter of weld buttons is less than 0.22". If results are acceptable, the treated metAl i8 considered hi~hly suitable for resi~tAnce welding.

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BF.ND ADHESION
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The test for paint adhesion is the 180 O-T bend test. In this test~ the painted panel is bent 180 . The radius of the bend may be controlled by bending the test panel around a mandrel of pre-:::
determined thickness, usually one or more panels of the same thick-neaB as the test panel. The mo~t severe test i8 where no mandrel i8 employed and the panel is bent so that the untreated surfaces are touching. This i5 the so-called O-T (zero mandrel thlckness) bend.
8endin~ around one panel thickness would be a l-T bend, etc. After bendlng, the panel is tested for paint adhesion by the application and removal of a standard transparent tape (Scotch #?10*). The ex-tent of paint removed by the tape is rated 10 for essentially no removal to O for complete removal. Values of 9 through 1 are assigned for intermediate adherence ratings in proportion to % paint ~dherence to the sub~trate.
OLSEN DRAW ADHESION
A 1" diameter tool is used to deform the panel 0.3" by ~ormln~ a depres~lon ln the unpainted slde. ~710 Scotch tape must ~-6how no paint removal and the paint must exhibit little red rust a~ter 240 hour salt spray (A "3" or less rating on a scale of O to 8 according to plctoral standards of Ford Motor Company).
In the followlng examples, all concentrations are net solids unless otherwise specifically noted. The examples are illus-trative only and not intended as limitat~ons of the invention.

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An aqueous concentrate solution ~as prepared to contain: -ÇomPonent Weight Melamine-formaldehyde Resin (supplied as Resimene X714*)24.5 Quebracho extract (non-bisulfited) 1.5 NaOH 0.25 Water Balance * Trade Mark 9~

The quebracho was added as an aqueous solution containing a small amount of NaOH for solubilizing. The resin and tannin supplies are g~ven in Tables I and II.
A treatlng solution was prepared of the above in Detroit tap water at a concentrat1on of 16.0 g/l corresponding to approxi-mately 4 g/l resin and 0.25 g/l tannin. The pH was adjusted to 3.0 Yrith ~hosphoric acid (25%). SAE 1010 cold rolled steel panels were then processed according to the sequence.
(1) Alkaline cleaner - 1 oz/gal - 150F
1 minute spray.
(2) Warm Water Rinse 1/2 minute spray

(3) Treating solution - ambient temperature 1/2 minute spray

(4) Dry-off oven - 5 minutes at 350F. ;
The panels were painted ~ith Dulux 704-6731~ white alkyd based paint supplied by DuPont de Nemours, Inc. and sub~ected to the Salt Spray and Humidity tests for 336 hours. As controls, identical panels were treated with a conventional iron phosphatin~ bath con-taining approximately 1% P04 and 0.5% chlorate, water rinsed, and post-treated with a conventional dilute (0.1% CrO3) hexavalent chromium rinse. The results were:
Conventional Resin-Tannin Treatment Treatment Humidity 10 10 Salt Spray 2-3 3-5 The results show that the corrosion resistance for resin-tannin treated surfaces as measured by the Salt Spray and Humidity tests is as good as or better than that of conventionally treated surfaces.
* Trade Mark !

An aqueous treating solution was prepared to contain: ~
ComPonent ~/1 i.
t`
Mclamine-formaldehyde resin (~uppl1ed as Cymel 7273-7*) 12.6 Quebracho Extract (non-bisulfited) 1.7 NaOH 0.16 1;
Triton CF 54* 0.4 Triton CF 54* is a modified polyethoxy adduct supplied by Rohm & Haas Co. The above room-temperature solution v~as roll-coated onto aluminum (3003 alloy) panels after the panels had been cleaned with the alkaline cleaner of Example 1. The p~nels were then oven dried at 400F for 20 ~econds (metal temperature 150-180F). The coating wei~ht was about 10 mg/ft2. Groups of the panels were then separately painted with polyester, acrylic and vinyl_based paints. Thereafter, the panels were subJected to the Salt Spray, Acetic Acid Salt Spray MEK Re6istance and Bend Adhesion tests.
The result~ were:
Paint, Polyester Acrylic, Vinyl Salt Spray (336 Hr.) N N N

Acetic Acid Salt Spray (336 Hr.) 0-1 0-1 N
MEK Resistance 200~ 200~
Bend Adhe~ion (O-T) 9.8 10 10 These results shoY~ that the resin-tannin treatment provides a paint base of acceptable quality.

A treating solution was prepared to contain:
* Trade Mark ~ 1o6~og~

Com~onent _~L

Melamine-formaldehyde resln (supplied as Tanak MRX*) 10.0 Quebracho Extract (non-bisulfited) 0.6 ~aOH 0.02 pH 8.5 In this solution, the Neight ratio of resin to tannin is 16:1. 3003 alumiJIum panels were cleaned for 15 seconds in a con-~rentional alkaline cleaner~ hot water rln6ed for 15 seconds~ dlpped 10 into the melamine-formaldehyde resln/tannin solution for 5-10 seconds at room te~perature~ squeegeed, and then oven dried for 5 minutes at 350F.
The panels llere painted with an acryllc paint and sub~ected to conventional impact, butter ~ormlng, knife blade adheslon and O-T
physical tests~ and were tested ln salt spray for 1008 hrs. Control panels whlch ~vere cleaned~ ~ater rlnsed, and dried only were also painted and tested. ~-The results o~ the test were as rollows:

Salt Spray Treatment O-T Bend 1008 hrs.
Clean only O 0-1 Clean and treated with melamine-formaldehyde/tannin solution 9.5 N
Far superior paint adhesion i8 observed vrith panels treated with the new 601ution as compared to cleaned only panels, as indicated by the O-T test. Both sets of panels rated excellent ln the remaining tests.

~n aqueous composition was prepared to contain approximately:

Trade Mark '' ~ ' '' ' ' - ' ' ' ' ' ~!. ' '' ::
.

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Componont e/l Cy~el 7273-7* 17.0 Non-bi~ulfited quebracho extract 2.5 NaOH 0.2 Triton CF 54* 0.6 Zinc dust 400 Biopolymer XB-23* (anionic hetero-polysaccharide produced by the fermentation of carbo-hydrate by the bacterium xanthomonas compestris) 2.0 Water Balance to 1000g. `
The zinc dust was supplied by ASARC0, Inc. as Federated L-15*, and the Biopolymer by General Mills. The above composition was applied to a number Or clean cold rolled steel tSEA 1010) panels and dried to give a dry film thizkness of about 0.1 ~il.
A zinc-rich epoxy-ba~ed weldable pri~er paint ~W.C. Rlchards S-7859-1*) was then applied to a dry film thickness of 0.5 ~118.
Separate panels were then sub~ected to the previously described tests for Weldabllity, Bend Adhesion and 01son Draw Adhesion. Excellent results of 10 were obtained ~or t,he Bend and 01son Draw adhesions and Weldability was acceptable after 2000 spot welds.
* Trade Mark -14_

Claims (8)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:-

1. A process for treating a metal surface to impart improved corrosion resistance and adhesion of subsequently-applied paint comprising contacting the surface with a chromium-free aqueous composition containing a melamine-formaldehyde resin and a vegetable tannin.

2. The process Or Claim 1, wherein the composition contains at least 0.01 g/l or the resin and at least 0.01 g/l of the tannin.

3. The process of Claim 1, wherein the pH value of the composition is from 2 to 10.

4. The process of Claim 1, wherein the weight ratio of resin:tannin is at least 1:1.

5. The process Or Claim 4, wherein said ratio is at least about 3.75:1.

6. The process of Claim 1, wherein the surface is sub-sequently dried at elevated temperature.

7. The process of Claim 1, wherein the concentrations of the resin and tannin are selected to yield a coating weight of at least 1 mg/ft2.

8. The process of Claim 1, wherein the composition additionally contains a conductive component selected from pul-verulent metals and conductive carbon compounds.