CA1044642A - Electrocoating of metallic sheet or strip - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
This invention relates to the electrocoating of continuous metal strip or sheet wherein the high solids resinous material which remains on the sheet or strip surface after electrocoating is removed by a wet squeegee surface.

Description

11~J4'~ 2 This invention relates to an improved method of electrodepositing a water-soluble or water-dispersible coat-ing resin onto a conductive surface, and in particular, onto ~
5 a continuous metal strip or sheet. - -The electrodeposition of water-based paints, com-monly termed electrocoating, is a widely used process which has many advantages over other methods of coating, such as ., spraying, dipping, rolling and the like. The advantages of `~
10 electrocoating are well known. ffle process deposits a film of uniform thickness on essentially any conductive surface, even those which have sharp points and edges. me electro- ~ ;~
coated film is relatively water-free, and thus will not run `
or drip when taken out of the bath. Because little or no 15 organic solvents are used in the coating system, the process is essentially fumeless and requires no extensive fume col- i --lection and incineration equipment. mis latter point is important in view of the increased concern over environmental i~i pollution. An additional advantage is the fact that a ~ -20 second or top coat can be applied over the electrocoated film without curing the electrocoated film and then both coats can be cured in one baking operation. By eliminating the necessity of two furnaces, the cost of a two-coat ;I` -process can be considerably reduced. '~
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me electrocoating process generally comprises -immersing the article to be coated into the electrocoating bath, usually as an anode, and passing a current through the bath between the article and electrode. me process usually is self-arresting in that as the thickness of the ,. .: .

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10~42 1 resin coating increases, the resistance thereof also increases, thereby reducing the rate at which the resin is electrodeposited.
The overall anodic electrocoating process involves four separate processes, namely, electrophoresis, electro-coagulation, electroendosmosis and electrolysis. Electro-phoresis involves driving negatively charged resin particles to the positively charged anode which is the article to be coated. In electrocoagulation, the resin particle loses a negative charge in the close vicinity of the anode or in contact therewith which /auses the resin particles to lose their stability and coagulate on or about the anode.
Electroendosmosis occurs during and after electrocoagula-tion and involves driving water out of the coagulated `
15 resin, thus, in effect, drying out the electrocoated layer. -~
Electrolysis also occurs causing evolution of hydrogen at the cathode and oxygen at the anode. With aluminium and :
~;; other reactive metals, anodic oxida*ion usually occurs at - least initially. Most commercial~electrocoating systems 20 are anodic~in *hat the article to be coated is the anode in the;electrocoating cell as described above. However, ~; in certain situations, cathodic deposition, wherein the ...
article is the cathode and the resin carries a positive charge, has been found useful. ~ -:
~ - , ; The coating resin can be water-soluble in that it disassociàtes into macro-ions to form a true solution or it can be water-dispersible or water-emulsifiable.

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Usually, all three are present in commercially available `~
paint resin systems. At any rate, in the anodic electro-30 coating process, the resin particles are negatively charged ; - 3 -4ti~2 ~ -. . , ..... .. .
1 and under the application of an electrical field are driven toward the anode. Because of the negative charge, the par- ~
ticles tend to repel one another and thereby form a stable ~ -dispersed phase or solution. However, upon losing the negative charge at the anode, the resin particles coalesce and deposit as the insoluble acid, forming a tenacious film -~
on the conductive surface. It is not presently well known how this discharge occurs because after the initial forma-tion of the resin layer, which is essentially nonconducting, no direct contact with the metal surface for electrical dis- ~;
charge can occur. It is probable that after the initial -layer of resin is deposited, charge transfer occurs by ion migration. Specific anodic reactions can and probably do -vary from resin system to resin system.
Most commercially available coating resins for anodic electrocoating generally are polycarboxylic acid ;
based resins and frequently are acrylic acid or methacrylic ~ ~
acid based resins. To solubilize the resins, they are -usually completely or nearly completely neutralized by a i~ -~ .... --~ 20 base, such as an amine or KOH. With cathodic electrocoating, the resin generally is a basic polymer resin which has been neutrallzed with a soluble acid. During anodic electrocoat-ng, the amine takes on a hydrogen ion and is driven to the `
cathode where H2 is liberated. The amine or other neutral-25 izing agent is not deposited in the coating and will stay ;
in the bath except for small amounts which are lost through 1 dragout. To maintain a relatively constant level of amine, ;
it is preferred to treat the bath in an ultrafilter or -other suitable device to remove amines and other low -~, .

30 molecular weight contaminants from the bath. For an excel- ;
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lent discussion on the use of ultrafilters in ~ -: ' . ' . .
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1 purifying electrocoating baths, see the article "Ultrafil-tration of Electrocoating Systems", in Nonpolluting Coatings and Coating Processes, Plenum Press, (1973) Edited by -.
J. L. Gardon and J. W. Prane. Coupling agents which assist 5 in solubilizing the coating resin are frequently added. ~
The resin can be pigmented or clear as desired. , r ~ `
Most of the successful prior uses of electrocoat- -ing have been in the coating of discreet metal articles and little work has been done in electrocoating continuous or .~` -indeterminate lengths of metal sheet or strip. One of the major problems with the electrocoating continuous lengths of sheet or strip is handling the electrocoated strip or sheet after the electrocoated film has been deposited `
thereon. Although when the strip leaves the bath, the 15 electrocoated film itself is tenacious, essentially dry and capable of being handled in a reasonable fashion.
., However, the top surface of the film is frequently covered with a layer of relatively high solids resinous material (e.g., 20 to 80% solids) when the strip leaves the electro-20 coating bath. This high solids layer tends to build upand harden on subsequent material handling equipment, such as roIlers and the like, and thereafter deform the electro-coated surface or redeposit on the uncured surface which in both cases renders the sheet or strip unacceptable. The 2S prior art practice has been to either rinse the sheet or strip with copious amounts of water after the strip leaves the bath or cure the film before any significant contact occurs. In the former case, the rinse water usually must be treated before disposal and/or reuse, and this increases 30 considerably the cost of the operation. In the latter case, ~::~ 1~44ti4'~

one of the advantages of the process is lost, ie., applying two coats and curing both with one baking operation.
Against this background, the present invention was `~
developed. :
Figure 1 is a cross-sectional view of an electro- ~ -coating apparatus which is useful in the practice of the -~
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invention. Figure 2 is a block diagram illustrating a method -~ -~ .
of treating an electrocoating bath. Figures lA and lB show alternative embodiments of the invention.
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The invention as claimed herein is in the method of electrocoating a continuous sheet or strip of metal wherein ,~
squeegee rollers are employed to remove high solids, resinous ~ c material which remains on the electrocoated surface as the ~
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sheet or strip exits from the electrocoating both and wherein -the composition of the electrocoating bath is controlled at ~- `
least in part by passing the electrocoating bath through a -:
separating device utilizing a semipermeable membrane to remove ~
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from the electrocoating bath neutralizing agents, solubilizing agents and low molecular components as a liquid filtrate or permeate, the improvement comprising applying said filtrate to ~ at least one of the squeegee surfaces to prevent the buildup ~
;~ of resinous solids on the surface thereof. ,j ~ -In the method as claimed herein, the filtrate may be -applied in the upper nip of the squeegee rolls adjacent to the . ~ .
edge of the electrocoated strip. The squeegee rolls may have i~ -nonabsorbent elastomeric surfaces. At least one of the squeegee .. ~-.,~, -, : . .
rolls may be driven by suitable means with the same peripheral ~-velocity as the electrocoated strip. r-'' ' ' '' ..... .
The present invention relates to an improved method of electrodepositing a water-based resinous coating onto a ' continuous or indeterminate length of metal strip, and, in !,'.. .,'. , ~ 1 , . . .
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particular, is directed to a new method of treating the coated strip as it is removed from the aqueous electrocoating bath to remove therefrom the layer of high solids resinous material.
In accordance with the present invention, a wet non- `
absorbing (with reference to the bath) squeegee is employed to remove the layer of high solids resinous material from the strip surface as it emerges from the electrocoating bath. The squeegee member is maintained wet by bathing the surface thereof ~
with an aqueous liquid so as to prevent any building and drying -of the coating resin on the squeegee surfaces. In a preferred embodiment, the strip is rinsed with the electrocoating bath as the strip emerges from the bath before the strip contacts the squeegee surfaces.
The squeegee member generally can be formed of a nonabsorbent elastomeric material, such as natural rubber, polybutadiene, polyurethane, polyvinyl chloride or a - 6a -~ , ~

1 metallic member, such as chromium-plated steel and the like.
Moreover, the squeegee member can be a squeegee blade or a pair of squeegee rolls or other suitable squeegee devices.
If desired, the squeegee rolls can be power driven. By 5 bathing the operating surfaces of the squeegee with an aqueous liquid, the surface thereof can be maintained rela-tively clean with no resin buildup. If the squeegee member is not bathed in the manner of the present invention, the high solids material on the strip builds up and hardens on the squeegee member, thereby either defacing the uncured coating on the strip or redepositing the high solids material back onto the strip, both of which detrimentally affect the surfaces of the coated article. The buildup of resin is particularly noticeable on the squeegee surfaces 15 which contact the edge of the strip. By rinsing the strip with the low solids bath just as it emerges from the bath, significant amounts of the high solids material are removed from the strip, thereby reducing the amount of high solids material which must be removed by the squeegee.
The strip or sheet movement through the bath causes a pumping action within the bath carrying along a ,~
~ - ~ . . . -high concentration of resinous solids, impurities, undesir-. .: - , .
able foams, particulate matter and the like. In a preferred embodiment of the present invention, a weir overflow is 25 maintained at the surface of the bath on the painted side of 9', ! ' . - .
the strip to remove the particulates, impurities, foam and the like which are brought to the surface of the bath. :~
Preferably, weir overflows are positioned on both sides of the strip whether or not both sides of the sheet or strip `
30 are coated. The amine concentration in the bath generally .

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1 builds up, particularly between the strip and the cathodes and this can detrimentally affect the electrocoating opera-tion. By withdrawing the bath in the manner of the pre-ferred embodiment, the excess amine concentration can be effectively removed from the bath and treated in a suitable manner, such as by passing through an ultrafilter or other semipermeable membrane filter.
The pretreatment of the strip or sheet prior to electrocoating generally comprises (1) cleaning with an inhibited alkaline cleaner,

(2) rinsing, `~

(3) forming a chemical conversion coating on the surface of the strip, -

(4) rinsing,

(5) drying, and then

(6) electrocoating.
In some situations, it may be desirable to replace the cleaning in an inhibited alkaline cleaner by a cleaning -and etching step in a highly alkaline solution. However, 20 the etching is usually followed by desmutting in a 50%
nitric acid solution. The strip surface should be essen-tially dry before contacting the electrical contact roll to prevent any arcing therebetween which can detrimentally affect both the strip and the contact roll surfaces.
Reference is made to Figure 1 which illustrates a preferred embodiment of the present invention. The dry strip 10 is initially directed under guide roll 11 and up to the conductor roll 12 which supplies electrical power to the strip. The conductor roll 12 is provided with con-30 ventional slip rings (not shown) or other suitable means :1~4~tj4~ : ~
1 for the application of current thereto. The strip 10 should maintain an arc of contact with the conductor roll of at , least 90 to keep the current density within a reasonable i~
level and thereby prevent any arcing between the contact roll and the strip. The strip then passes into the bath 13 contained in the tank 14 adjacent to cathodes 15 wherein the electrodeposition of the resin primarily occurs. The strip passes under roller 16 which is journaled to the tank walls in the bottom thereof and then passes upward through 10 the bath. Weir overflow 17 is positioned on the coated side of the exiting strip to withdraw bath from the tank and drain by way of conduit 18 to a holding and mixing tank (not .- ~ ;
shown). Sprays 19 and 20 are disposed on both sides of the exiting strip and are adapted to spray the emerging strip 15 with the bath. Squeegee rolls 21 and 22 are disposed above sprays 19 and 20 to remove any excess resin remaining on the strip after the sprays. Sprays 23 and 24 are positioned ~-adjacent the squeegee rolls to direct an aqueous fluid onto r~
the squeegee rolls 21 and 22 to keep them wet and thereby :~,: :. .. .
20 prevent any coating buildup thereon. Alternate methods of ~ -applying the aqueous liquid to the squeegee roller surface is shown in Figures 3A and 3B. In Figure 3A, the liquid ". - .
is inked on, whereas in Figure 3B, the squeegee surfaces are maintained in the surface of the bath. After the strip , 25 10 passes through squeegee rolls 21 and 22, it is then . .
passed over roller 25 and then into blower 30 to remove any surface water which may remain on the strip. The strip can then pass to subsequent treatment steps, such as to a curing furnace if only one coat is to be applied or to 30 other coating facilities for the application of a top coat.

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1 As shown in Figure 1, the cathodes 15 are placed on one side of the strip to thereby electrodeposit the resin on only one side of the strip. There is a small over-lap or wraparound of electrodeposited film, but this is relatively small, particularly with wide strip. If it is desired to electrocoat on both sides, cathodes can be placed on both sides of the strip. It is preferred to maintain one or more cathodes on the upstream side to cor-rect any surface defects which may occur when the painted strip passes around the roller 12 at the bottom of the tank.
The capacity of the tank 11 shown in Figure 1 is relatively small in view of the amount of coating resin which is electrodeposited over a period of time. Thus, the 15 bath should be recycled to a holding tank wherein the con-centration of various bath components, bath temperature, impurities and the like are maintained at the desired levels. A suitable system for handling the bath is shown in Figure 2 wherein the bath is continuously recycled from 20 the electrocoating tank to a holding tank, passed through a coarse filtering device to remove any material greater than 75 microns in size, and then through an ultrafilter or other semipermeable membrane filtering media to remove amines, undesirable ions and the like. A heat exchanger 25 is usually maintained in the system to maintain the temper-ature of the bath within a particular desired level. The bath temperature can be maintained between about 60 and 130F, but in the present invention, it is preferred to maintain this temperature between about 80 and 120F to 30 accelerate the surface drying which occurs at the blower.
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1 In operating the electrocoating process of the present invention, it is preferred to maintain the rela-tively high voltage, i.e., about 80 to 225 volts, during the electrocoating process to accelerate electroendosmosis 5 which provides for a drier coating. Solids in the bath ; -can generally range from about 3 to 30%, preferably about 5 to 15%, by weight. -~
In accordance with the present invention, the squeegee surfaces should be kept wet to prevent any resin -~-10 buildup thereon. Any suitable means can be used. For ~ ~
example, the aqueous liquid can be sprayed as shown in -Figure 1, inked onto the squeegee surfaces as shown in Figure lA or the squeegee rolls could be disposed so that the lower surfaces thereof are in the electrocoating bath 15 as shown in Figure lB. The aqueous liquid employed to bathe the squeegee surfaces can be any aqueous liquid or solution which is compatible with the bath. The liquid can be the electrocoating bath, water, permeate from the ultrafilter or the ultrafiltered bath.
The following example is given to further illus-trate the advantages of the present invention. An experi-mental electrodepositing unit was employed in a commercial , coil coating line when two coats, a primer and a top coat, were applied to a continuous aluminium sheet. The electro-25 coat was the primer coat. In this coating line, the aluminium strip was first cleaned in an inhibited alkaline cleaner, rinsed, subjected to a treatment in an acidic chromate containing solution to form a conversion coat ' thereon, rinsed first in tap water and then deionized water 30 and then passed to the electrocoating operation. The 1~)44~2 l electrocoating unit was essentially that shown in Figure 1.
The ratio of the anode area to cathode area was 1.8 to 1Ø
The bath composition was essentially a 10% solid suspension hy"
of a water-based acrylic resin sold under the trade name of -Lectropon by DeSoto, Inc. The water-dispersible resin had been essentially completely neutralized or stabilized with dimethylethanol amine. Bath temperature was maintained at approximately 108F and strip speed was about 150 feet per minute. Cell voltage was maintained at about 100 volts DC
, 10 through the entire test period. Resin bath was sprayed against the sheet as it exits from the bath and also i sprayed onto the elastomeric squeegee rolls which were ;~ disposed above the previously described sprays (as shown in Figure l). The top coat was applied in a conventional 1 15 manner and then both coats were cured in a single baking ;l operation. These procedures were used to electrocoat j ~ aluminium strip continuously for over 24 hours with essen-tially no buildup of high density resin solids on the strip ; handling facilities and moreover with excellent quality . ~ .
20 products. With the same conditions prevailing and uaing ~ the~same equipment and resin, a second test was conducted ` --i~ whereby the sprays on the squeegee rolls and on the strip ~; were turned off. Within a short period of operation (less than one hour) resin buildup began to occur on the squeegee 25 rolls and on subsequent rolls beyond the squeegee rolls :~ .. ' ' i ~ which handled the strip, resulting in the defacing of the i;
electrocoated film and the scrapping of electrocoated sheet material.

, It is obvious that various improvements and modi-! ~
30 fications can be made to the present invention without i`

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4ti42 1 departing from the spirit thereof and the scope of the appended claims. For example, the process is fully appli- ~
cable to cathodic electrocoating processes wherein the , ~.
continuous sheet or strip is the cathode and wherein the 5 resin is a basic polymer resin neutralized with a soluble ~ :
acid. .
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Claims (4)

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

1. In the method of electrocoating a continuous sheet or strip of metal wherein squeegee rollers are employed to remove high solids, resinous material which remains on the electrocoated surface as the sheet or strip exits from the electrocoating bath and wherein the composition of the electro-coating bath is controlled at least in part by passing the electrocoating bath through a separating device utilizing a semipermeable membrane to remove from the electrocoating bath neutralizing agents, solubilizing agents and low molecular components as a liquid filtrate or permeate, the improvement comprising applying said filtrate to at least one of the squeegee surfaces to prevent the buildup of resinous solids on the surface thereof.

2. The method of claim 1 wherein the filtrate is applied in the upper nip of the squeegee rolls adjacent to the edge of the electrocoated strip.

3. The method of claim 1 wherein said squeegee rolls have nonabsorbent elastomeric surfaces.

4. The method of claim 1, 2 or 3 wherein at least one of the squeegee rolls is driven by suitable means with the same peripheral velocity as the electrocoated strip.