Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
FA-0478-A lITLE
NONAQU~OUS CATIIODIC ELECTROCOAT PIGMENT PASTE
CROSS REFERENCE TO RELATED APPLICATION
This application is a continuation-in-part
of Serial No. 07/599,2~5 filed October 18, 1990.
ECHNICAL FIELD
The field of art to which this invention
pertains is electrodepositable cationic baths
con-taining a nonaqueous pigment paste.
BACKGROUND
The coating of electrically conductive
substrates by electrodeposition is a well known and
important industrial process. (For instance,
electrodeposition is widely used in the automotive
industry to apply primers to automotive substrates).
In this process, a conductive article is immersed as
one electrode in a bath of a coating composition of an
aqueous emulsion of film-forming polymer. An electric
current is passed between the article and a
counter-electrode in electrical contact with the
aqueous emulsion, until a desired coating is produced
on the article. In a cathodic electrocoating process
the article to be coated is the cathode and the
co~ ter-electrode is the anode.
~ esin compositions used in the bath of
catho~ic electrodeposition process are also well known
in the art. These resins are typically made from
polyepoxide resins which have been chain extended and
then an adduct i5 formed to include a nitrogen in the
resin. Nitrogen is typically lntroduced through
reaction with an amine compound. Typically these
resins are blended with a crosslinking agent and then
2 ,
neutralized with an acid to form a water emulsion
which is usually referred to as a principal emulsion.
The principal emulsion is combined with a
pigment paste, coalescent solvents, water, and other
5 aclditives (usually at the coating site) to form the
electrodeposition bath. The electrodeposition bath is
placed in an insulated tank containing the anode. The
article to be coated is made the cathode and is passed
through the tank containing the electrodeposition
10 bath. The thickness of the coating is a function of
the bath characteristics, the electrical operating
characteristics, the immersion time, and the like.
The coated object is removed from the bath
after a given amount of time. The object is rinsed
15 with deionized water and the coating is cured
typically in an oven at sufficient temperature to
produce crosslinked coating.
The prior art of cathodic electrodepositable
resin compositions, coating baths, and cathodic
20 electrodeposition processes are disclosed in U.S.
Patents 3,922,253; 4,419,467; 4,137,140; and
4,468,307 which are incorporated herein by reference.
The pic3ment dispersant is an important part
o~ all electrocoat primer composition. The dispersion
25 process involves ttle separation of the primary pigment
particles from their agglomerates or aggregates, the
disp]acement ol` occluded air and absorbed water, and
the wetting and coating of the pigment surfaces with
t:he clispersion resin. Ideally, each primary particle,
30 hav in~ been mechanically separated during dispersion,
is also stabilized against flocculation. If the
pigment particles are not properly dispersed and
stabilized in the paint, the advantages built into the
plgment by the manufacturer may be lost. For
35 instance, the pigment may settle in the
3 ~ G
~lectr-o(~e}~osition bath which can result in loss of
orroC:iorl protection oI the substrate. In addition,
surf.lce appearance, operating characteristics and the
~ e may be adversely impacted by inadequate pigment
dispel-sion.
~ 11 previous pigment pastes for cathodic
electrocoat processes contain water to reduce the
viscosity and to make the paste easier to grind.
(U.S. Patent No. 4,110,292 discloses using a solvent
based pigmen-t paste which is incorporated into a
non-electrocoat latex paint). ~lowever, pigment pastes
containing water can cause foaming in the pigment
paste and/or flocculation of the pigment under certain
circumstances. Foaming can be a serious problem for
the following reasons: (1) foaming makes it more
difficult to grind pigment; (2) foaming makes it
difficult to check the pigment particle size; (3)
foaming makes it more difficult to unload the pigment
paste from the grinding mill; and (4) foaming can
introduce surface defects on the film or coating
deposited by the electrocoating process.
~ n additive can be used in an attempt to
solve a foaming problem. However, there are no
satisfactory defoamers for use in making water based
pigment pastes. Furthermore, the defoamers that are
available can cause corrosion problems and adhesion
loss of the topcoat to the electrocoated film.
What is needed is to eliminate or
substantially reduce foaming during the process for
making pigment pastes which are used in a cathodic
electrocoating bath.
Summary of the Invention
It has been discovered that foaming can be
eliminated or substantially reduced in making the
r~ic;lnellt ~)aste used in a cathoclic electrocoating bath
O r a coating composition containing an epoxy-amine
adduct and a blocked polyisocyanate by replacing the
water in the pigment paste with an organic solvent.
-
Detailed Descri~tion of the Invention
This invention relates to a novel nonaqueouspiyment paste ior use in cathodic electrocoat.
As previously mentioned, it is well known
the emulsion in ca-thodic electrodeposition baths has a
binder resin which is an epoxy amine adduct blended
with a cross-linking agent and neutralized with an
acid to provide a water soluble cationic product. The
novel pigment dispersant is potentially usable with a
variety of different cathodic electrocoat binder
resins. A typical binder resin is the epoxy amine
adduct of the prior art. These resins are disclosed in
U.S. Patent No. 4,419,467 which is incorporated herein
by reference.
Likewise our preferred crosslinkers for the
above-mentioned binder resins are also well known in
the prior art. They are aliphatic and aromatic
isocyanates such as hexamethylene diisocyanate,
toluene diisocyanate, methylene diphenyl diisocyanate
and the like. These isocyanates are pre-reacted with
a blocking agent to form a blocked isocyanate.
Typical blocking agents are oximes and alcohols which
block the isocyanate functionality ~i.e. the
crosslinking functionality of the isocyanate). ~pon
I~eating the oximes or alcohols unblock and free
isocyanate is formed which in turn reacts with the
hydroxy functionality of the epoxy-amine adduct after
application to a substrate to give a crosslinked
finish. These crosslinking agents are also disclosed
in U.S. Patent No. 4,419,467.
The neutralization of the epoxy-amine adduct
~ the binder resin with an acid to attain its
cationic character is likewise well known in the art.
The resulting binder resin is combined with pigment
paste, deionized water and additives (e.g.
anti-cratering agents, plasticizers and the like) to
~orm the electrodeposition bath.
The main ingredients in the principal
elllulsion are the epoxy amine adduct and the blocked
lo i.ocy~ln~te, anc~ are usually present in amounts of
al~out 30 ~o 50 percent by weight of solids.
ncsides the resinous ingredients described
above, the electrocoating composition of this
inven-tion contains a pigment which is incorporated
into the composition in the form of a paste. The
pigment paste is prepared by mixings the pigments with
a pigment dispersant resin and organic solvents along
with optional additives such as wetting agents,
surfactants, and defoamers and grinding the resulting
mixture in a conventional grinding mill.
Pigment dispersant resins and the optional
additives discussed above are well known in the art.
Potentially any pigment dispersant resin which is
water soluble can be used. Water solubility of the
pigment dispersant resin is necessary because it will
be incorporated into the water based cathodic
electrocoat bath. Pigment dispersant resins well
known in the art include quaternary ammonium salts,
epoxy-amine adducts and imidazoline based resins. Our
preferred pigment dispersant resin is a nonionic
dispersant (ethoxylated styrenated phenoll which is
disclosed in copending application Serial No.
07/483,838 filed February 23, 1990. The weight ratio
of pigment to dispersant resin ranges from 0.2:1.0 to
5.0:1Ø
6 j'~ C 3 ~ ?
The organic solvents used to replace water .
in the pigment paste can be either alcoholic or
nonalcoholic. The preferred solvents are alcoholic.
Solvents whi.ch can be used include N-methyl
~)yrroli.dnne, methyl i.sohutyl ketone, methyl ethyl
k~tone, 2-~utoxy ethanol and the like. Of this group
of so:lven~-;, th~ most preerred is 2-butoxy ethanol.
One particularly preferred solvent is 2,4
p~n~allediolle. When 2,4 pentanedione is used as the
:l0 sol.vent to form the pigment paste, a lesser amount can
l)e used in compar:i.son to the abo~e solvents.
.';~lrprisi.ng]y, 2,~ pentanedione reduces the viscosity
ol the piyment paste and less can be used, thereby
reducing the VOC (volatile organic compound) of
resulting composition. Also, dibutyl tin oxide (DBTO)
which is a catalyst that is commonly used in
electrocoating compositions can be dissolved in 2,4
pentanedione and substantially reduces the time
required to form a pigment paste which is generally
formed by grinding using conventional techniques such
as sand grinding or ball mill grinding. The
aforementioned solvents do not dissolve DBTO which is
difficult to disperse and hence longer grinding times
wexe needed to form a pigment paste containing DBTO.
The amount of 2,4 pentanedione used can be
from 20 to 100% of the solvent used to form the
pigment paste. Preferably, about 30-70% 2,4
pentanedione is used and the remainder is one or a
mixture of the aforementioned alcoholic or
,30 non-alcoholic solvents. One preferred solvent blend
is a 50:50 blend of 2,4 pentanedione and 2-butoxy
ethanol.
The pigment paste is prepared by adding
pigments to the pigment dispersant under sufficient
agitation and then adding sufficient amount of the
organic solvent to give a viscosity of 500 to 3000
(cps) centipoise. The preferred viscosity is between
900 and 1500 cps and most preferred is about 2200
centipoise. This mixture is ground in any grinding
mill, such as a sand grinder or ball mill.
After grinding, the particle size of the
pigment should be as small as practical, generally, a
Hegman grinding gauge of about 6 to 8 is usually
employed.
In one preferred method for forming a
pigment paste, all orgnaic solvents, grinding resins
and pigments are premixed preferably by a high speed
a~itator an~ then the ingredients are charged into a
sand mill and ground to form the pigment paste. If
I)BT0 is used, it is mixed with 2,4 pentanedione in the
premix. The DE3To is readily dissolved in 2,4
pentanedione with high speed agitation and then the
remaining ingre~ients are added and mixed and ground
in a sancl mi~l. The grinding time is greatly reduced
2() in comparison to pigment pastes prepared without 2,4
pentaneclione and a good pigment paste can be made with
only one pass through a sand mill whereas it normally
takes 2 to 3 passes to disperse DBT0 without the
presence of 2,4 pentanedione. Total weight solids of
these pigment pastes are about 70-90% and the
viscosity of the pigment paste is about 700-2000 cps.
A low VoC electrocoating composition can be prepared
using pigment pastes made with 2,4 pentanedione.
Pigments which can be employed in the
practice of this invention include titanium dioxide,
basic lead silicate, strontium chromate, carbon black,
iron oxide, clay and the like or mixtures thereof.
These are the pigments typically used in automotive
primers.
~5
';~ b~ `' r,
The pigment paste is then added to the
principal emulsion containing the binder resin and
other additives. The pigment-to-resin weight ratio in
the electrocoat paint bath is very important and
should be preferably less than 50:100, more preferably
less than 40:100, and usually about 20 to 40:100.
~ligher pigment-to-resin solids weight ratios have been
found to adversely affect coalescence and flow.
The coating compositions of the invention
can contain optional additives such as wetting agents,
~ rfactants, defoamers and so forth. Examples of
suractants and wetting agents include alkyl
imidazolines such as those available from Ciba-Geigy
Industrial Chemicals as Amine C~, acetylenic alcohols
available from ~ir Products and Chemicals as
Surfynol 104 ~. These optional ingredients, when
present, constitute from about 0.1 to 20 percent by
weight of resin solids. Plastici7.ers which promote
flow of the co~ting on curing are optional
ingredients. Examples are high boiling water
immiscible materials such as ethylene or propylene
oxide adducts of nonyl phenols or bisphenol A.
Plasticizers can be used at levels of about 0.1 to 15
percent by weight resin solids.
Curing catalysts such as tin catalysts are
usually present in the composition. Examples are
di~utyl tin dilaurate and dibutyl tin oxide. When
used, they are typically present in amounts of about
0.05 to 2 percent by weight tin based on weight of
, 30 total resin solids.
The electrodepositable coating compositions
of the present invention are dispersed in aqueous
medium. The term "dispersionn as used within the
context of the present invention is believed to be a
two-phase translucent or opaque aqueous resinous
-~J-~ ti~
system in which the resin is in the dispersed phase
alld water the continuous phase. The average particle
size diameter of the resinous phase is about 0.1 to 10
microns, preferably less than 5 microns. The
concentration of the resinous products in the aqueous
medium is, in general, not critical, but ordinarily
the major portion of the aqueous dispersion is water.
The aqueous dispersion usually contains from about 3
to 50 percent preferrably 5 to ~0 percent by weight
resin solids. Aqueous resin concentrates which are to
be further diluted with water, generally range from 10
to 30 percent by total weight solids.
Besides water, the aqueous medium may also
contain a coalescing solvent. Useful coalescing
solvents include hydrocarbons, alcohols, esters,
ethers and ketones. The preferred coalescing solvents
include alcohols, polyols and ketones. Specific
coalescing solvents include monobutyl and monohexyl
ethers of ethylene glycol, and phenyl ether of
propylene glycol. The amount of coalescing solvent is
not unduly critical and is generally between about 0
to 15 percent by weight, preferably about 0.5 to 5
percent by weight based on total weight of the resin
solids.
The following examples illustrate the
invention. All parts and percentages are on a weight
basis unless indicated otherwise.
EXAMPLE 1
3() EpoxY-Amine Adduct
The following components were charged into a
suitable reactor vessel: 1,478 parts Epon 828~ (a
cliglycidyl ether of Bisphenol A from Shell Chemical
Company having an epoxy equivalent weight of 188); 533
I~arts of ethoxylated Bisphenol A having a hydroxyl
~ 3
equivalent weight of 247 (Synfac 8009~ from Milliken
Co.); 427 parts Bisphenol A; and 121 parts xylene.
rhe charge is ~leated to 145C under a nitrogen
bkanket. 2.5 parts of benzyl dimethylamine are added.
'rhe reaction mixture was further heated to 160C, and
he],d for 1 hours. An additional 5.1 parts oE benzyl
dimethylamine were added, and the mixture held at
147C until a 1,0~0 weight per epoxide (WPE) was
obtained. The mixture was cooled to 98C and 168
parts diketimine (from diethylenetriamine and methyl
isobutyl ketone at 72.7% solids) and 143 parts methyl
ethanol amine were added. The mixture was held at
120C for 1 hours, then 727 parts methyl isobutyl
ketone (MIBK) were added. The resulting epoxy-amine
adduct resin solution has a nonvolatile of 75%.
Crosslinkinq Resin
A blocked polyisocyanate was prepared by
charging 522.0 parts of toluene diisocyanate (~Mondur"
lD 80 from Mobay Chemical Company) into a suitable
reaction vessel. 0.15 parts of dibutyl tin dilaurate
and 385.2 parts anhydrous methyl isobutyl ketone were
added and the resulting reaction mixture was blanketed
with nitrogen. 390.0 parts of 2-ethyl hexanol was
2S added to the mixture and the mixture was held at 60-C.
133. n parts of trimethylol propane were added and the
m;xt~lre was held at 120C for one hour until
essentially all free isocyanate was consumed. Then
63.0 parts butanol was added and the mixture was
,30 oooled to room temperature. The resulting blocked
polyisocyanate mixture had a 70.0% non-volatile
o~ontent. An emulsion was prepared as follows:
~0
Weiqht Solids
l,poxy-~mine Adduct Resin Solution 563.33 422.50
(prepared above)
Crosslinking Resin Solution 325.00 227.50
~, (prepared above)
I.actic acid 23.73 --
Deionized water 949.65
Total 1861.71 650.00
The epoxy-amine adduct, the crosslinking
resin solution and lactic acid were thoroughly mixed.
M;xing was continued until a majority of the organic
ketone evaporated. The resulting emulsion had a
solids content of 36%.
A pigment paste was prepared as follows:
Weiqht Solids
Synfac 8334~ 236 236
(Ethyloxated styrenated phenols
from Milliken Chemical Company)
2-butoxy ethanol 175 --
Titanium dioxide pigment410 410
Aluminum silicate pigment76 76
Carbon black pigment ~ 12 12
Lead silicate pigment 47 47
Dibutyl tin oxide 44 44
Total 1000 825
The ethoxylated styrenated phenol and
2-butoxy ethanol were charged into a steel container
and mixed with a Cowles Blade Mixer at medium speed.
The pigments were then added and the mixture passed
,30 through a sand grinder until a Hegman grinding gauge
of about 7 was achieved.
An electrocoat bath was prepared as follows:
,7 !1
Wei~ht Solids
l,mulsion (prepared above)1693.33609.60
}'igmellt paste (prepared above)230.79 190.40
Deionized water 2075.88 -- _
Total4000.00 800.00
An electrocoating bath was prepated by
blending together the above ingredients. The bath had
a pH of 6.08 and a conductivity of 200 microseman.
Zinc phosphate cold-rolled steel panels were
cathodically electrocoated in the bath at 200 volts
and 2~C for 2 minutes. The wet films were cured at
]~ODC for 15 minutes and had 32 micron film thickness
and were smooth and even.
EXAMPLE 2
A pigment paste was prepared as follows:
Weiqht Solids
Synfac 8334~ 236 236
(Ethyloxated styrenated phenols
from Milliken Chemical Company)
2-Butoxy ethanol 73 --
2,4 Pentanedione 73 --
Titanium dioxide pigment410 410
Aluminum silicate pigment76 76
25 Carbon black pigment 12 12
Lead silicate pigment 47 47
Dibutyl tin oxide 44_ 44
Total971 825
Dibutyl tin oxide and 2,4 pentanedione were
. 30 charged into a steel container and mixed with a Cowles
~lade Mixer at medium speed until all dibutyl tin
oxide is dissolved. Synfac 8334~, 2-butoxy ethanol,
and the pigments were charged and mixed. The mixture
Lassed through a sand grinder once, and a Hegman
yrinding gauge of about 7 was achieved.
13 ; ~
An electrocoat bath was prepared as follows:
Weiqht Solids
Emulsion (prepared in Example 1) 1693.33 609.60
Pigment paste (prepared224.10 190.40
in Example 1)
Deionized water 2082.57 ~-
Total 4000.00 800.00
An electrocoating bath was prepared by
hlending together the above ingredients. The bath had
.I pll of 6.0 and a conductivity of 210 microseman.
7,inc phosphate cold-rolled steel panels were
c~t11odically electrocoated in the bath at 200 volts
and at 28C for 2 minutes. The wet films were cured at
l80C for 15 minutes and had 34 micron film thickness
and were smooth and even.
