Note: Descriptions are shown in the official language in which they were submitted.
1(~6~09~ .
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
.. ,. ,.. ,,. , , - : .
. . ~ . ~ . . . .
~.n~
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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,',Z()9~ ~
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.
6'ZO~
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.
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: . - . : . .
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
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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.
30 MEK RESISTANCE
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.
_g_
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BF.ND ADHESION
... . .
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.
EXAMPLE 1
. ~
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
--10--
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
!
EXAMPLE 2
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.
EXAMPLE 3
A treating solution was prepared to contain:
* Trade Mark
12
~ 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.
EXAMPLE 4
~n aqueous composition was prepared to contain
approximately:
Trade Mark
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'' ~ ' '' ' ' - ' ' ' ' ' ~!. ' '' ::
.
~06Z~)~8 :
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
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