Note: Descriptions are shown in the official language in which they were submitted.
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TITLE
ELECTRODEPOSITION BATHS CONTAINING A MIXTURE OF BORON-
CONTAINING COMPOUNDS AND CHLORHEXIDINE
FIELD OF THE INVENTION
The present invention relates to an improved electrodeposition process. More
particularly the present invention relates to an improved electrodeposition
bath
comprising a mixture of at least one boron-containing compound and
chlorhexidine.
BACKGROUND OF THE INVENTION
Electrodeposition as a coating method has become increasingly important in the
coatings industry. Globally, more than 80 percent of all motor vehicles
produced are
given a primer coating by cationic electrodeposition.
As compared with non-electrophoretic coating means, electrodeposition offers
the advantages of increased paint utilization, improved corrosion protection
and
relatively low environmental contamination. Electrodeposition typically offers
environmental advantages because (1) electrodepositable coating compositions
contain
very little organic solvent and, (2) downstream processes, such as closed loop
rinsing,
can minimize loss of coating components to the surrounding environment during
coating
application.
The electrodeposition process is well known, and involves immersing an
electroconductive substrate (that is, the work-piece) into a bath of an
aqueous
electrocoating composition. In the case of a cationic electrocoat composition,
the work-
piece serves as the cathode. After electrodeposition of a coating onto a
workpiece, the
electrocoated substrate is rinsed with an aqueous rinsing composition.
Typical rinsing operations have multiple stages which can include closed loop
spray and/or dip applications. Such rinsing processes are well known in
electrocoating
processes, but for clarity a closed loop spray process removes excess
electrocoat
material from the substrate by spray washing the surface with deionized or
reverse
osmosis water. A dip application removes excess electrocoat material from a
substrate
by submerging the substrate in a tank of dionized or reverse osmosis water.
The rinse
composition can be re-circulated and re-used. In a typical electrocoat
operation, the
electrodeposition bath is ultrafiltered to remove ionic contaminants and the
ultrafiltrate is
used in the rinsing operations.
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Recirculating the coating or rinse compositions is both economically and
environmentally desirable. However, recirculation an aqueous coating or rinse
composition can create an environment conducive to the growth of
microorganisms such
as algae, fungi and bacteria. Microorganisms can adversely affect the quality
and
appearance of an electrodeposited coating. Further, the presence of
microorganisms in
the electrocoating or rinsing composition can cause the formation of
precipitates in the
tanks, and variation in process parameters, for example, pH, conductivity,
film build,
throwpower (that is, the rate of film deposition relative to the position of
the anode) and
stability. Also, particulate "dirt" deposition and bio-fouling can occur,
thereby
detrimentally affecting the appearance of the applied coating and reducing
system
performance.
In early electrodeposition processes, the "ultrafiltrate" used in the rinse
stages
typically contained solvents, heavy metals, and other organic materials which
assisted in
the suppression of the aforementioned microorganism growth. However, as
environmentally undesirable components such as volatile organic compounds
(VOC),
hazardous air pollutants (HAPs), and heavy metals, such as lead and chrome
have been
reduced, increased bacterial infestation has occurred.
A number of compounds for controlling the growth of bacteria in heavy metal-
free, low organic solvent content-electrodeposition baths are known. For
example, silver
ion has been utilized, as well as oxidizing agents such as hydrogen peroxide
and
calcium hypochlorite. However, silver ion is costly and can contribute to dirt
formation
the electrodeposition bath. Oxidizing agents can oxidize organic components of
the
electrodepositable composition.
A microbiocide composition containing a mixture of 5-chloro-2-methyl-4-
isothiazolin-3-one and 2-methyl-4-isothiazolin-3-one can cause a rougher
appearance
than a coating composition without this microbiocide. Moreover, such
microbiocide
compositions can contain metal salts, for example, magnesium nitrate and
magnesium
chloride, which can cause coating defects due to gas generation at the
cathode. Use of
microbiocides may not be convenient, and they can lose their effectiveness
over time.
Moreover, some microbiocides can require special handling and disposal.
Halonitroalkanes can negatively affect the appearance of an applied coating
and
can contribute to corrosion of metallic parts.
U.S. Patent No. 4,732,905 discloses a composition used to control
microorganism growth in water systems. U.S. Patent No. 6,017,431 discloses the
use of
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sulfamic acid in electrodeposition baths. U.S. Patent Nos.: 3,937,679;
3,959,106;
3,975,346; and 4,001,101 disclose the use of boric acid as a solubilizing
agent for ionic
group-containing film-forming resins having onium salt groups, such as
quaternary
ammonium groups and ternary sulfonium groups. U.S. Patent No. 4,443,569
discloses
cathodically electrodepositable compositions based on a nitrogen base-
containing binder
containing tertiary amino groups and primary and/or secondary hydroxyl groups,
and a
metal compound. US2003/0033248 discloses an improved electrodeposition bath
containing boric acid.
In view of the foregoing, a need exists for a heavy metal-free, low or no VOC
electrodeposition bath that is resistant to biodegradation, while maintaining
excellent
coating application conditions, coating appearance and performance properties.
The
elimination of the necessity to handle toxic microbiocides that often are used
in
electrodeposition baths neutralized with organic acids is also desirable.
SUMMARY OF THE INVENTION
The invention provides an improved electrodeposition bath for microorganism
resistance. The improvement comprises the inclusion of both chlorhexidine and
an
effective amount of a boron-containing compound selected from at least one of
boric
acid, boric acid equivalents, and mixtures thereof in the electrodeposition
bath in an
amount sufficient to retard the growth of microorganisms in the
electrodeposition bath
relative to their growth in the absence of said components. The
electrodeposition bath
comprises an aqueous dispersion of an aqueous carrier and a film forming
binder. The
film forming binder comprises an epoxy-amine adduct and blocked isocyanates.
DETAILED DESCRIPTION OF THE INVENTION
Other than in the operating examples or where otherwise indicated, numerical
range limits or numerical parameters set forth herein are approximations.
Range
limitations used in the description of the invention and/or in the claims
should be
interpreted as if modified by the term "about". Slight variances above or
below stated
range limitations are not necessarily outside of the intended scope of
operation of the
present invention. Ranges provided herein are continuous, and understood to
incorporate any whole or fractional value within stated range limits, unless
said value is
specifically or specially excluded. As such, any value within a specified
range may be
relied upon as if that value is individually and specifically set out herein.
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It has been found that incorporating a mixture of chlorhexidine and boron-
containing compounds into an electrodeposition bath provides a level of
microbe
protection that is superior to using either compound individually at higher
concentrations.
Such results are unexpected, and it is further surprising that the use of
boron-containing
compounds and chlorhexidine in an effective amount to reduce microorganism
growth in
electrodeposition baths described herein can be accomplished without detriment
to
critical process parameters such as pH and conductivity of the bath.
Suitable boron-containing compounds include those selected from boric acid,
boric acid equivalents, and mixtures thereof. As used herein and in the
claims, by "boric
acid equivalents" is meant any of the numerous boron-containing compounds that
can
hydrolyze in aqueous media to form boric acid. Specific, but non-limiting
examples of
boric acid equivalents include boron oxides, for example, B203; boric acid
esters such as
those obtained by the reaction of boric acid with an alcohol or phenol, for
example,
trimethyl borate, triethyl borate, tri-n-propyl borate, tri-n-butyl borate,
triphenyl borate,
triisopropyl borate, tri-t-amyl borate, tri-2-cyclohexylcyclohexyl borate,
triethanolamine
borate, triisopropylamine borate, and triisopropanolamine borate.
Additionally, amino-
containing borates and tertiary amine salts of boric acid may be useful. Such
boron-
containing compounds include, but are not limited to, 2-(beta-
dimethylaminoisopropoxy)-
4,5-dimethyl-1,3,2-d- ioxaborolane, 2-(beta-diethylaminoethoxy)-4,4,6-
trimethyl-1,3,2-
dioxaborin- ane, 2-(beta-dimethylaminoethoxy)-4,4,6-trimethyl-1,3,2-
dioxaborinane, 2-
(betha-diisopropylaminoethoxy-1,3,2-dioxaborinane, 2-(beta-dibutylaminoethoxy)-
4-
m46hyl-1,3,2-dioxaborinane, 2-(gamma-dimethylaminopropoxy)-1,3,6,9-tetrapxa-2-
boracycloundecane, and 2-(beta-dimethylaminoethoxy)-4,4-(4-hydorxybutyl)-1,3,2-
dioxaborolane. Boric acid equivalents can also include metal salts of boric
acid (i.e.,
metal borates) provided that such metal borates can readily dissociate in
aqueous media
to form boric acid. Suitable examples of metal borates useful in the
electrodeposition
bath of the present invention include, for example, calcium borate, potassium
borates
such as potassium metaborate, potassium tetraborate, potassium pentaborate,
potassium hexaborate, and potassium octaborate, sodium borates such as sodium
metaborate, sodium diborate, sodium tetraborate, sodium pentaborate, sodium
perborate, sodium hexaborate, and sodium octaborate. Likewise, ammonium
borates
can be useful. Moreover, optional boron-containing compounds can be included,
for
example, bismuth borate and yttrium borate.
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Suitable boric acid equivalents can also include organic oligomeric and
polymeric
compounds comprising boron-containing moieties. Suitable examples include
polymeric
borate esters, such as those formed by reacting an active hydrogen-containing
polymer,
for example, a hydroxyl functional group-containing acrylic polymer or
polysiloxane
polymer, with boric acid and/or a borate ester to form a polymer having borate
ester
groups.
Polymers suitable for this purpose can include any of a variety of active
hydrogen-containing polymers such as those selected from at least one of
acrylic
polymers, polyepoxide polymers, polyester polymers, polyurethane polymers,
polyether
polymers and silicon-based polymers. By "silicon-based polymers" is meant a
polymer
comprising one or more --SiO-- units in the backbone. Such silicon-based
polymers can
include hybrid polymers, such as those comprising organic polymeric blocks
with one or
more --SiO-- units in the backbone. Preferably, boric acid is used in the
electrodeposition bath of the present invention.
Boric acid or boric acid equivalents of the present invention are present in
the
electrocoat bath at a level ranging from greater than 0.3% to less than 2.0%
of the total
weight of the bath. Preferably, boric acid or boric acid equivalents of the
present
invention are present in the electrocoat bath at a level ranging from 0.4% to
1.7% of the
total weight of the bath. Most preferably, boric acid or boric acid
equivalents of the
present invention are present in the electrocoat bath at a level ranging from
0.5% to
1.6% of the total weight of the bath.
Chlorhexidine is a known antiseptic compound. It is also known as 1,6-bis [5-
(p-
chlorophenyl) biguanidino] hexane and has the structural formula as shown in
Figure I.
ci a
NH NH NH NH
I I I
N N N `'6 ~ N N N
Chlorhexidine is present in the electrocoat bath at a level of from greater
than
0.01 % to 0.2% of the total weight of the bath. Preferably, chlorhexidine is
present in the
electrocoat bath at a level ranging from 0.02% to 0.18% of the total weight of
the bath.
Most preferably, chlorhexidine is present in the electrocoat bath at a level
ranging from
0.03% to 0.16% of the total weight of the bath.
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When chlorhexidine is present at its lowest level, 0.01 % of the total weight
of the
bath, it is desired to keep the level of boric acid at greater than 0.5% of
the total weight
of the bath, more preferably at a level of greater than or equal to 1.0% of
the total weight
of the bath.
In another embodiment, the present invention is an electrodepositable
composition suitable for use as an electrodeposition bath comprising film-
forming resins
having ionic salt groups wherein the electrodeposition bath includes boric
acid and
chlorhexidine. Examples of such film-forming resins are epoxy-based resins
having
amine salt groups and/or sulfonium salt groups.
In a preferred embodiment, the electrodepositable composition has a pH of 7 or
less. At a pH of greater than 7, such cationic compositions tend to adsorb
carbon
dioxide from the surrounding atmosphere and, consequently, can drift below pH
7 over
time. Therefore, compositions having a pH of 7 or less are more stable and
process
conditions are easier to control.
The term "principal emulsion" as used herein means an electrocoating
composition comprising an aqueous emulsion of a binder of an epoxy amine
adduct
blended with a crosslinking agent which has been neutralized with an acid to
form a
water-soluble product. The binder of the electrocoating composition typically
is a blend
of an epoxy amine adduct and a blocked polyisocyanate crosslinking agent.
While the
microbiocides are potentially usable with a variety of different cathodic
electrocoat
resins, the epoxy amine adduct resins are particularly preferred. These resins
are
generally disclosed in U.S. Patent No. 4,419,467 which is incorporated by
reference.
Preferred crosslinkers for the epoxy amine adduct resins are also well known
in
the prior art. These are aliphatic, cycloaliphatic and aromatic isocyanates
such as
hexamethylene diisocyanate, cyclohexamethylene diisocyanate, toluene
diisocyanate,
methylene diphenyl diisocyanate and the like. These isocyanates are pre-
reacted with a
blocking agent such as oximes, alcohols, or caprolactams which block the
isocyanate
functionality, i.e., the crosslinking functionality. Upon heating the blocking
agents
separate, thereby providing a reactive isocyanate group and crosslinking
occurs.
Isocyanate crosslinkers and blocking agents are well known in the prior art
and also are
disclosed in the aforementioned U.S. Patent No. 4,419,467.
The cathodic binder of the epoxy amine adduct and the blocked isocyanate are
the principal resinous ingredients in the electrocoating composition and are
usually
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present in amounts of about 30 to 50% by weight of solids of the composition.
To form
an electrocoating bath, the solids are generally reduced with an aqueous
medium.
Besides the binder resin described above, the electrocoating composition
usually
contains pigment which is incorporated into the composition in the form of a
pigment
paste. The pigment paste is prepared by grinding or dispersing a pigment into
a grinding
vehicle and optional ingredients such as wetting agents, surfactants, and
defoamers.
Any of the pigment grinding vehicles that are well known in the art can be
used or the
novel additive described above can be used. After grinding, the particle size
of the
pigment should be as small as practical; generally, the particle size is about
6-8 using a
Hegman grinding gauge.
Pigments which can be used in this invention include titanium dioxide, basic
lead
silicate, strontium chromate, carbon black, iron oxide, clay and the like.
Pigments with
high surface areas and oil absorbencies should be used judiciously because
these can
have an undesirable affect on coalescence and flow of the electrodeposited
coating.
The pigment to binder weight ratio is also important and should be preferably
less than 0.5:1, more preferably less than 0.4:1, and usually about 0.2:1 to
0.4:1. Higher
pigment to binder weight ratios have been found to adversely affect
coalescence and
flow.
The coating compositions of the invention can contain optional ingredients
such
as wetting agents, surfactants, defoamers and the like. Examples of
surfactants and
wetting agents include alkyl imidazolines such as those available from Ciba-
Geigy
Industrial Chemicals, Tarrytown, New York, as "Amine C", acetylenic alcohols
available
from Air Products and Chemicals, Allentown, Pennsylvania, as "Surfynol 104".
These
optional ingredients, when present, constitute from about 0.1 to 20 percent by
weight of
binder solids of the composition.
Optionally, plasticizers can be used to promote flow. Examples of useful
plasticizers are high boiling water immiscible materials such as ethylene or
propylene
oxide adducts of nonyl phenols or bisphenol A. Plasticizers are usually used
at levels of
about 0.1 to 15 percent by weight resin solids.
The electrocoating composition of this invention is an aqueous dispersion. The
term "dispersion" as used within the context of this invention is believed to
be a two-
phase translucent or opaque aqueous resinous binder system in which the binder
is in
the dispersed phase and water the continuous phase. The average particle size
diameter of the binder phase is about 0.1 to 10 microns, preferably, less than
5 microns.
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The concentration of the binder in the aqueous medium in general is not
critical, but
ordinarily the major portion of the aqueous dispersion is water. The aqueous
dispersion
usually contains from about 3 to 50 percent preferably 5 to 40 percent by
weight binder
solids. Aqueous binder concentrates which are to be further diluted with water
when
added to an electrocoating bath generally have a range of binder solids of 10
to 30
percent weight.
EXAMPLES
Preparation of antimicrobial additive containing electrocoat samples. Amounts
of
boric acid and/or chlorhexidine were added to 49m1 samples of CORMAX VI,
available
from DuPont, Wilmington, Delaware, according to Table 1. The amounts of boric
acid
and chlorhexidine listed in Table 1 are in percent by weight based on the
total weight of
the sample.
To evaluate the antimicrobial activity of the compounds, a "Challenge
Inoculum"
was prepared by isolating eleven unique bacteria from contaminated electrocoat
baths at
five different automotive assembly sites. Electrocoat samples were prepared by
adding
1 milliliter (ml) of the challenge inoculum to 49 ml of electrocoat sample
dispersion. This
bacterial inoculation produced a bacterial count ranging from 1.0 X 105 to 1.7
X 106
CFU/ml. These challenged samples were incubated at room temperature with
stirring
and sterile air agitation (air at < 0.1 Liters/minute) for 30 days. Samples
were tested for
the presence of bacteria by a standard plat count method after 1 hour, 24
hours, 1 week,
2 weeks, 3 weeks, and 30 days after inoculation. Tryptic Soy Agar (TSA) was
used for
enumeration of bacteria from the electrocoat samples using standard spread
plate
technique.
Table 1 shows the Bacterial Count data for the microbiocides tested and the
control data.
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TABLE 1
Sample Challenge 1 Hour 24 Hours 1 Week 2 Weeks 3 Weeks 30 days
Example Description Innoculum (CFU/ml) (CFU/ml) (CFU/ml) (CFU/ml) (CFU/ml)
(CFU/ml)
CFU/ml
A CorMaxO VI 0.00 N.D. N.D. N.D. N.D. N.D. N.D.
Control
B CorMax VI 1.70X1Ob 9.60X10 4.58X10 5.60X10 1.SOX10 1.80X10 2.70X10
Control
C Boric Acid 1.50X10 1.45X10 5.35X10 1.90X10 3.36X10 4.75X10 1.28X10
(1.0%)
D Chlorhexidine 170X10b 3.85X10 2.85X10 4.15X10 4.92X10 2.80X10 2.80X10
0.01 %
E Boric Acid 1.30X10 1.74X10 2.05X10 5.7OX1Ob 1.50X10 N.D. N.D.
(1.5%)
F Chlorhexidine 1.30X10 1.10X10 3.45X10 N.D. 2.25X10 8.OOX10 2.50X10
0.1 %
G Boric Acid
(1.0%) + 1.70X106 1.70X106 1.65X106 4.50X104 1.26X103 N.D. N.D.
Chlorhexidine
0.01 %
H Boric Acid
0
C1hlorhexidine 1.70X106 1.25X16 9.05X1' 6.85X102 N.D. N.D. N.D.
0.1 %
I Boric Acid
(0.5%) + 1.OOX105 1.75X102 3.50X10' 2.20X105 2.21X107 4.91X107 1.40X107
Chlorhexidine
0.01 %
J Boric Acid
(0.5%) + 1.OOX105 1.OOX102 4.50X10' N.D. 9.65X102 3.85X105 N.D.
Chlorhexidine
(0.05%)
K Boric Acid
C~ o0rhexidine 1.OOX105 2.00X10' 1.50X101 N.D. N.D. N.D. N.D.
(0.1%)
N.D. - No bacteria detected
The results of Table 1 show that the electrocoat baths containing only boric
acid
at 1.0% and 1.5% or chlorhexidine at 0.01% or 0.1% fail to give adequate
control of the
microbe population during the test period. Example E, containing 1.5% boric
acid
showed an increase in the microbe count before eventually controlling the
population
after 3 weeks. In contrast, electrocoat compositions containing 0.5% or
greater of boric
acid and greater than 0.01 % chlorhexidine showed decreasing microbe
populations.
Each of the electrocoat bath compositions above was tested to determine the
effect of the biocide additive on the electrocoat bath conductivity and film
build. The
results of the tests are shown in Table 2.
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TABLE 2
Example Conditions 25 F Film Build in mils 90 F Bath Temperature
Biocide pH Conductivity 175V 200V 225V 250V 275V 300V
S/cm
A CorMax Vi 6.09 1882 0.705 0.801 0.913 1.036 1.178 1.292
(control)
C Boric Acid (1.0%) 6.08 2069 0.653 0.768 0.906 1.048 1.301 rupture
D Chlorhexidine 6.05 2245 0.619 0.693 0.793 0.915 1.020 1.170
0.01 %
E Boric Acid 1.5% 5.71 1997 0.647 0.770 0.878 1.013 1.197 1.448
F Chlorhexidine 6.24 2222 0.658 0.729 0.811 0.888 0.999 1.150
(0.1%)
G Boric Acid (1.0%) +
Chlorhexidine 5.95 2008 0.672 0.738 0.861 0.984 1.172 1.381
0.01 %
H Boric Acid (1.0%) +
Chlorhexidine 6.08 2059 0.704 0.872 0.918 1.137 1.366 1.444
0.1 %
Boric Acid (0.5%) +
Chlorhexidine 5.94 2278 0.676 0.789 0.907 1.077 1.255 1.541
0.01 %
J Boric Acid (0.5%) +
Chlorhexidine 6.01 2354 0.689 0.789 0.899 1.089 1.246 1.461
(0.05%)
K Boric Acid (0.5%) +
Chlorhexidine 6.05 2368 0.854 0.955 1.072 1.245 1.369 1.576
(0.1%)
