U.S. patent number 3,616,396 [Application Number 04/860,131] was granted by the patent office on 1971-10-26 for electrophoretic coating process.
This patent grant is currently assigned to E. I. du Pont de Nemours and Company. Invention is credited to Ralph G. Swanson.
United States Patent |
3,616,396 |
Swanson |
October 26, 1971 |
ELECTROPHORETIC COATING PROCESS
Abstract
An improved process for the electrophoretic deposition of a
coating on a metal article in which the bath contains an
electrically charged film-forming material and a porous partition
which separates the article being coated (e.g., the anode) from
electrode (e.g., the cathode) and in which a portion of the bath
which is deficient in film-forming material is removed and
replenished with film-forming material and recycled into the bath
in the proximity of the article being coated. An apparatus having
the required novel features necessary for the improved
electrophoretic process is also disclosed.
Inventors: |
Swanson; Ralph G. (Flint,
MI) |
Assignee: |
E. I. du Pont de Nemours and
Company (Wilmington, DE)
|
Family
ID: |
25332547 |
Appl.
No.: |
04/860,131 |
Filed: |
July 18, 1969 |
Current U.S.
Class: |
204/482 |
Current CPC
Class: |
C25D
13/22 (20130101); B01D 61/46 (20130101) |
Current International
Class: |
C25D
13/22 (20060101); B01D 61/46 (20060101); B01D
61/42 (20060101); B01k 005/02 (); C23b
013/00 () |
Field of
Search: |
;204/181 |
References Cited
[Referenced By]
U.S. Patent Documents
|
|
|
3200057 |
August 1965 |
Burnside et al. |
|
Primary Examiner: Williams; Howard S.
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a continuation of my copending application Ser.
No. 457,551 filed May 21, 1965, now abandoned.
Claims
What is claimed is:
1. In a method for coating an electrically conductive object in an
aqueous bath with a paint comprising an organic resin having
ionized sites thereon and intimately dispersed within said bath
with an ionized dispersal assistant, said method comprising
immersing said object within said bath, utilizing said bath as the
aqueous electrolyte and said object as a first electrode of an
electrical circuit comprising said bath, said first electrode, and
a second electrode in contact with said bath and spaced apart from
said object, providing a difference of electrical potential between
said first electrode and said second electrode sufficient to cause
a direct current of electrical energy through said bath and between
said first electrode and said second electrode having direction and
sufficiency to effect electrodeposition of a coating of said paint
upon said object from said bath, the improvement which comprises
interposing a diffusion barrier admitting of liquid flow
therethrough between said first electrode and said second electrode
forming a bath-withdrawal zone, withdrawing from said zone aqueous
electrolyte of reduced paint concentration and increased
concentration of dispersal assistant relative to the corresponding
concentrations of said bath where said bath is in contact with said
first electrode, admixing said electrolyte with replenishment paint
for said bath and returning the resultant replenishment feed to
said bath.
2. The method of claim 1 wherein said electrically conductive
objects are continuously introduced into said bath and said
replenishment feed is introduced into said bath at a rate
sufficient to maintain the concentration of paint and dispersal
assistant in said bath at substantially constant levels.
3. In a method for coating an electrically conductive object in an
aqueous bath with a paint comprising a polycarboxylic acid resin
having ionized sites thereon and intimately dispersed within said
bath with ionized amine, said method comprising immersing said
object within said bath, utilizing said bath as the aqueous
electrolyte and said object as an anode of an electrical circuit
comprising said bath, said anode and a cathode in contact with said
bath and spaced apart from said anode, providing a difference of
electrical potential between said anode and said cathode sufficient
to cause a direct current of electrical energy through said bath
and between said anode and said cathode having direction and
sufficiency to effect electrodeposition of a coating of said paint
upon said anode from said bath, the improvement which comprises
interposing a diffusion barrier admitting of liquid flow
therethrough between said anode and said cathode forming a cathode
zone otherwise separated from the bath in an anode zone wherein
said anode is in contact with said bath, withdrawing from said
cathode zone aqueous catholyte of reduced paint concentration and
increased concentration of said amine relative to the corresponding
concentrations of said paint and said amine in said bath in said
anode zone, admixing said catholyte with replenishment paint for
said bath and returning the resultant replenishment feed to said
bath in said anode zone.
4. The method of claim 3 wherein said replenishment feed contains a
lower concentration of said amine than said bath in said anode
zone.
Description
BACKGROUND OF THE INVENTION
This invention concerns application of aqueous coating compositions
to metal substrates and more particularly to an improved process
for electrophoretic deposition of such compositions and apparatus
useful therefor.
Corrosion of automobile bodies and other metal articles ordinarily
exposed to exterior atmospheric conditions has been a bothersome
problem for many decades. Apart from the task of developing
suitable primers and paints for protecting these structures from
corrosion, difficulty has also been encountered in adequately
coating all surfaces of these articles with such primers and paints
in order to provide the protection which such compositions can
afford. It is well known that incomplete coverage of metal
structures with protective coatings provides sites for initial
corrosion which then accelerates deterioration of the coatings in
other portions of the structure.
Electrophoretic deposition of coating compositions on metal
substrates overcomes many of the deficiencies attending
conventional coating procedures such as spraying, dipping, and the
like. By means of electrophoretic deposition, recessed areas of
automobile frames and bodies and the interiors of tubular
structures are readily coated with a protective finish where
electric current can reach these areas. Understandably, therefore,
the process of applying protective coatings to metal articles by
electrophoretic deposition has attracted the attention of
manufacturers interested in prolonging the useful life of their
metal products.
Two major problems have beset the process of electrophoretic
deposition of water borne paints as heretofore practiced and
because of these problems continuous surveillance and control has
been required to render the process useful in commercial
installations operating at high work load. Thus prior art processes
of this type have been plagued by a progressively decreasing
throwing power and concentration of paint solids in the
electrophoretic deposition tank. Also, undesirable variations in
paint thickness, film smoothness and corrosion resistance on the
coated products have resulted from changes in pH of the bath
composition in the vicinity of the anode. In operating prior art
processes this pH level has had to be carefully watched and
adjusted periodically in order to insure effective deposition of
paint. Nevertheless undesirable wide variations in pH do occur and
these affect the coatings adversely. Each electrophoretic paint
composition has its own optimum pH level which must be closely
maintained.
In view of the above shortcomings of prior art electrophoretic
deposition processes it is not surprising that coatings produced by
such a process have had quality defects and the process has not
achieved general use. Obviously the most uniform coatings are
achieved if the concentration of paint solids and the critical pH
level surrounding the work piece being coated can be maintained
close to optimum values. The process and apparatus of the instant
invention achieve this objective.
SUMMARY OF THE INVENTION
The process of this invention involves coating a metal article by
electrophoretic deposition by passing an electric current between
an electrode and said article through an electrolyte bath
consisting essentially of an aqueous coating composition containing
an electrically charged film-former, said electrode (e.g., cathode)
and said metal article (e.g., anode) being at least partially
immersed in the coating composition bath and electrically
communicative through the electrolyte, but partially isolated from
each other by a porous partition, removing at the cathode a portion
of the coating bath (e.g., catholyte) including pigments, and
simultaneously adding to the portion of the bath in the vicinity of
the anode, for uniform distribution therein, aqueous coating
composition of a pH and composition suitable for maintaining the pH
and solids concentration of the coating bath at predetermined
levels, usually those optimum for electrophoretic deposition of the
coating bath being utilized.
DESCRIPTION OF THE INVENTION
The metal workpiece being coated can be the anode or the cathode in
operating this invention, depending upon the type of electrolyte
utilized but preferably it is the anode and for convenience the
system described particularly herein will refer to the workpiece as
the anode, the electrolyte in the vicinity of the work piece as
anolyte; the pole opposite in polarity to the workpiece as the
cathode and the electrolyte in the immediate vicinity of the
cathode as catholyte.
The process and apparatus of this invention are not only more
effective than prior art procedures of this type but also permits
continuous coating without substantial change of composition or pH
of the coating bath. Also, coatings applied to metal articles
according to this invention are more uniform and adhesive than
heretofore.
Another advantage of this invention is the greater ease of
dispersion of the acidic "makeup" primer into the bath contents.
This advantage is obtained through the concentration of alkalinity
in the confined catholyte which more effectively reacts with the
"makeup" primer than would analyte of lower pH.
In accordance with a preferred embodiment catholyte withdrawn from
the tank is admixed with a make up coating composition having a pH
and solids composition adjusted to provide a mixture suitable for
addition to the coating bath as aforesaid to maintain its pH and
composition practically constant.
The present invention can be readily understood by reference to the
drawings:
FIG. I is a schematic side view of a preferred embodiment of the
apparatus of this invention showing the flow of coating bath.
FIG. II is a schematic top view of the apparatus of FIG. I with the
flow of coating bath also shown.
FIG. III is a schematic cross-sectional view of the apparatus
showing the location of the cathodes and the porous partitions
positioned around the cathodes and separating them from the main
body of the coating bath. Catholyte flow is also indicated by
arrows.
FIG. IV is a schematic cross-sectional view of another embodiment
of the apparatus showing a different arrangement at the cathodes
and a different catholyte flow pattern.
Referring to FIGS. I and II, the apparatus comprises a tank 1
having a plurality of compartments 2 along its walls and enclosed
by porous partitions 3. The inner surface of tank 1 is electrically
insulated from its liquid contents by insulating layer 4. Provision
is made to withdraw catholyte from each compartment at outlets 5,
preferably continuously, pass it through a common conduit 6 to
either a mixing tank 7 or in-line mixer 7 where it is uniformly
mixed with make up coating composition from tank 8 to form a
concentrated anolyte which is conducted back to tank 1 by conduit 9
entering the tank at inlet 10.
The flow of the catholyte can be actuated by gravity or by pump 11
depending upon the rate of flow desired and the location of outlets
5 and inlets 9. The latter are located to provide the greatest
uniformity of coating bath composition possible.
Make up composition is preferably added continuously to tank 7 (or
in-line mixer 7) from tank 8 and has a pH and solids composition
such that when mixed with catholyte entering tank 7 via conduit 6,
and transmitted back to tank 1 the pH and solids composition of
tank 1 will be maintained substantially at their original level. In
this way the electrophoretic deposition process continues to
operate continuously at the same high level of efficiency and
effectiveness as at the start up and without the wide variations in
solids content and pH which has characterized prior art
processes.
In FIG. III the cathode 12 is shown in close proximity to tank wall
4 and with porous or semipermeable membrane partitions 3 partially
isolating the cathode and separating it from the coating bath,
thereby permitting isolation and removal of catholyte. Each
partition is topped by a weir 17 over which coating bath passes
when the coating bath level in the tank becomes high enough.
Coating bath including solids dispersed therein, can also pass
through porous partition 3 and it is important for purposes of this
invention that the pore or mesh size of the porous partition be no
larger than essential to permit passage of the coating bath and its
dispersed solids, including pigment, through the partitions to the
cathode. Failure of the partition to so function will detract
substantially from the effectiveness of this process by permitting
excessive back diffusion of alkalinity into the anolyte. Back
diffusion should be as low as possible. Back diffusion can be held
to a minimum by using a semipermeable membrane as the
partition.
It will be noted from FIG. III that catholyte leaving the tank at
outlet has a higher pH (indicated to be about 10 for exemplary
purposes) than catholyte near the weir. The flow cycle prevents
catholyte having a high pH from mixing with the main body of the
coating bath so that the latter is maintained at its optimum pH,
while catholyte with a pH higher than this optimum value is
continuously removed and replaced with the new coating bath to
maintain the optimum pH. Coating bath solids are simultaneously
replenished by additions to mixing tank 7 (or in-line mixer 7).
The arrangement of porous partitions 3 in FIG. IV is slightly
different from those in FIG. III in that they are topped by an
impervious member which extends from slightly below to slightly
above (for example, one inch below to one inch above) the surface
of the coating bath. The latter can pass through the partitions, if
porous, as in FIG. III but can pass even more readily under the
partitions at 14 to reach the cathode. In this arrangement outlet 5
is fitted with standpipe 15 the top 16 of which is just below the
surface of the coating bath so that coating flows upward along the
cathode in order to enter the standpipe for withdrawal through
outlet 5. Naturally the pH of catholyte near the bottom of the
cathode approximates that of the main coating bath and pH of the
catholyte increases as catholyte approaches the top of the
standpipe. The porous partition is selected so that it provides
sufficient impedance to flow of coating bath to prevent channeling
and short-circuiting of the bath and favors sweeping much of the
catholyte past the full length of the cathode before entering the
standpipes.
The materials of construction of the apparatus of this invention
need not be other than conventional. The tank 1 can be fabricated
from iron, steel, plastic or any other strong and durable material.
The tank lining 4 can be any known nonconducting material suitable
for use in conventional electroplating processes. A mixture of
epoxy resin and tar with catalyst, applied to the interior surfaces
of the tank at a thickness of about one-eighth inch and then air
dried, has been found to be satisfactory but other durable
insulation materials can be used. It will be readily apparent to
anyone skilled in the art that the cathode elements, transmission
lines, pumps, mixing tanks, etc. should be appropriate to the
materials being handled. The walls 3 of cathode compartments 2 can
be any porous fabric or semiporous sheeting which will permit
electrolytic conduction of current between cathode and anode and
which has pores sufficiently small to prevent substantial
intermixing of catholyte (bath in the immediate vicinity of the
cathodes) with anolyte (bath outside of the cathode compartments
2). The sidewalls 17 of compartments 2 can be the same material as
the front walls 3 but need not be porous and are preferably a
plastic sheet material of polyvinyl chloride or polymethyl
methacrylate or the like which is strong and rigid enough to impart
sufficient structural rigidity to the compartment to prevent
collapse.
The following examples illustrate the operation of the
apparatus.
EXAMPLE 1
This describes a laboratory size dip tank, isolated cathode
compartment in the tank, catholyte flow rate, electrical conditions
for deposition, the anode area, and the electrophoretic paint
compositions used.
A metal dip tank measuring inside 12 inches long, 4 inches wide and
14 inches tall is electrically insulated from ground. Inside the
tank a single cathode compartment is installed on the end wall. The
compartment measures overall 4 inches wide, one-half inch deep and
14 inches tall with one face exposed and the top and bottom ends
open. The cathode compartment is positioned with its bottom open
end about one-half inch from the bottom of the tank so that
dispersion can enter the compartment at a rate at least as great as
its withdrawal rate but not substantially greater, thus intermixing
of catholyte and anolyte is avoided. It is lined overall with an
electrically insulating paint coating except for a bare inside area
of 2 inches .times. 10 inches which serves as the cathode. A porous
fabric or semiporous plastic membrane is used to separate the
cathode surface from the main section of the tank and form a
cathode compartment. A syphon tube of 1 mm. inside diameter leads
from the top of the cathode compartment to a separate vessel. A
screw clamp on the syphon controlled the catholyte flow rate which
is adjusted to yield 10 cc. per minute. The dip tank is charged
with 11,100 grams water diluted material consisting of 1,110 grams
electrophoretic primer solids, neutralized "charging primer," and
the balance is deionized water to yield a 10 percent solids bath.
The material is maintained in uniform dispersion by means of
constant agitation with a propeller agitator.
Another primer used, called "makeup primer," has the same
composition of solids as the neutralized "charging primer" except
that it does not contain water and is acidic. This "makeup" primer
is added to the catholyte, as it is removed, at a controlled flow
rate and the mixture added to the main or anolyte section of the
tank so as to maintain the anolyte pH and solids at optimum value.
The composition and preparation of the primers follow:
---------------------------------------------------------------------------
Alkyd Resin
__________________________________________________________________________
Tall Oil Fatty Acid 825 grams Neopentylglycol 918 Trimellitic
Anhydride 818 Methylisobutylketone 129 2,690
Less water of esterification 160 Total 2,530 grams
__________________________________________________________________________
The above ingredients are charged to a reactor and cooked at reflux
temperature which rises to 404.degree. F. while removing 160 grams
of water of esterification. Methylethylketone is added to produce
an acid number of 45.2 which represents 45.2 milligrams of KOH per
gram of alkyd solids. The viscosity is Z2 plus one-half bubble
Gardner Holt viscosity.
---------------------------------------------------------------------------
Final temperature is 228.degree. F.
acidic Vehicle A
__________________________________________________________________________
Alkyd Resin 2,530 grams Hexamethoxymethylmelamine 329
Methylethylketone 351 Total 3,210
__________________________________________________________________________
The alkyd resin temperature is raised to 148.degree. F. and then
the amine and ketone are admixed while cooling to 114.degree. F.
---------------------------------------------------------------------------
neutralized Vehicle B
__________________________________________________________________________
Acidic Vehicle A 3,210 grams Triethylamine 146 Deionized water
3,466 Total 6,822
__________________________________________________________________________
Acid Vehicle A is heated to 114.degree. F. and then the amine and
water are mixed until clear. Final viscosity is T plus
one-half.
The metal panels to be electrocoated are 4 inches .times. 12 inches
size and are electrically connected to the positive pole of a DC
rectifier capable of yielding 150 DV volts. The cathode face of the
cathode compartment is electrically connected to the negative pole
of the rectifier. To electrocoat the panel, the system is energized
at 150 DC volts for 90 seconds during which time the amperage falls
from an initial high value to a final low value. The initial and
final amperes are characteristic with the type of metal used and
its prior surface conditioning. Clean ferrous and zinc metals show
an initial value of about 6 amperes and a final value of about 0.3
amperes. Clean aluminum foil used in the following examples showed
an initial value of about 3 amperes and a final value of about 0.6
amperes during a 90 second deposition time.
---------------------------------------------------------------------------
Neutralized Charging Primer
Neutralized Vehicle B 7.72 % Iron Oxide No. 5.55 Bentone 34 0.62
Methylethylketone 1.61
__________________________________________________________________________
Mill above to 0.25 mil pigment fineness, then add the following mix
to yield uniform dispersion.
Neutralized Vehicle B 76.9 Creosole 0.3 Deionized Water 7.3 Total
100.0 %
Acidic Makeup Primer
__________________________________________________________________________
Acidic Vehicle A 6.38 % Iron Oxide No. 9.75 Bentone 34 1.08
Methylethylketone 18.63
Mill above to 0.25 mil pigment fineness, then add the following and
mix to yield uniform dispersion.
Acidic Vehicle A 63.63 Creosole 0.53 Total 100.00 %
EXAMPLE 2
This example illustrates the objectional pH rise that takes place
in a conventional tank operating at the same deposition rate per
unit volume of primer in the tank as in the following examples. The
equipment in example 1 is used except that the cathode compartment
is excluded. Instead a separate bare steel cathode of the same
dimension is used. Thirty aluminum foil panels are coated in the
course of 60 minutes. The pH of the tank contents is measured
initially and after 10, 20 and 30 panels. The pH is measured as
7.25, 7.35, 7.40 and 7.45, respectively, illustrating the rapid pH
rise for conventional coating processes. In another trial 90
phosphated steel panels are coated. The initial and final pH values
were respectively 7.2 and 8.3 and the corresponding film
thicknesses were 0.85 mils and 0.6 mils showing a rapid
deterioration of coating efficiency due to the rapid pH rise for
this short period. By doubling the number of panels, coating
efficiency and throwing power progressively deteriorate and film
thickness quickly approaches zero.
EXAMPLE 3
This example illustrates the negligible pH rise of the anolyte when
the catholyte is substantially isolated from the anolyte and
withdrawn from the cathode compartment. The equipment of example 1
is used including the cathode compartment. A porous fabric
separating the cathode from the anode consists of loosely woven
fiber glass of 60 thread-count per inch; two layers of this fabric
are used. As panels are coated, the catholyte is syphoned into a
collecting vessel at the rate of 10 cc. per panel per 2 minutes.
The pH of the initial anolyte and catholyte is identical at 7.25.
After the tenth, twentieth and thirtieth panel the pH of the
anolyte and catholyte are:
---------------------------------------------------------------------------
Panel Anolyte Catholyte
__________________________________________________________________________
Initial 7.25 7.25 Tenth 7.25 8.05 Twentieth 7.30 8.25 Thirtieth
7.35 8.35
__________________________________________________________________________
In this example the small rise in pH of the anolyte is due to some
back diffusion of catholyte alkalinity through the porous fabric of
the cathode compartment. This back diffusion of alkalinity
decreases as the porosity of the fabric decreases. The coating on
all panels is smooth and of uniform thickness (0.7 mils).
EXAMPLE 4
This example illustrates that resistance to back diffusion is
increased with a major decrease in porosity of the cathode wall
construction by using a semipermeable membrane. In this instance
the procedure is the same as that in example 3 except that the
fiber glass wall of the cathode compartment is replaced with a
semipermeable membrane coded 61 AZL-066 as supplied by Ionics, Inc.
of Cambridge, Massachusetts. This membrane contains a cationic
exchange resin. Thirty panels are coated to yield a total of 48
grams of dry film. Determinations of pH produced the following
values:
---------------------------------------------------------------------------
Anolyte Catholyte
__________________________________________________________________________
Initial 7.20 7.20 After 30 panels 7.30 9.00
__________________________________________________________________________
The addition of 48 grams of makeup primer solids to the catholyte
and addition of this mixture to the anolyte restored the anolyte to
its initial pH value of 7.2. The panels' coatings are smooth and
are uniformly 0.7 mils thick.
EXAMPLE 5
This example illustrates the complete controlled catholyte process.
The equipment covered in example 3 is used. The catholyte flow rate
is adjusted to 20 cc. per 4 inches .times. 12 inches panel per 2
minutes. The panels are coated at the rate of 1.6 grams of dry film
per panel. The catholyte is added to a collecting vessel which
contains an agitator and admixed with acidic makeup primer of the
composition given in example 1. This mixture is admixed with the
anolyte of the dip tank. After completing depositions on 30 panels
over a period of one hour, 600 cc. of catholyte has flowed and been
simultaneously mixed with 48 grams of "makeup primer" solids. The
pH of the neutralized catholyte is 7.1, just 0.15 less than the
anolyte. In this instance the neutralized catholyte offsets the
slight rise in pH of the anolyte caused by back diffusion of
alkalinity as mentioned in example 3. All panels have a smooth
uniform coating of film former 0.7 mils thick.
* * * * *