Note: Descriptions are shown in the official language in which they were submitted.
590
BACKGROUND OF THE INVENTION
The present invention relates to a method of
cathodically depositing polymeric coatings. More speci-
fically, it relates to a method of depositing coatings
of acrylic polymers containing amine functionality with
minimum retained water and acid in the coatings.
It is known that organic coatings can be electro-
deposited either on an anodically-charged conducting
substrate or on a cathodically-charged substrate. Although
most of the earlier work in electrodeposition was done with
anodic deposition, that type of process has certain
disadvantages. Anodic electrodeposition is normally done
in a coating bath having a basic pH. The pH decreases
at the surface being coated, creating conditicns which,
when combined with the electrolytic action of the coating
bath, can cause the dissolution of substrate metal ions
and their subsequent deposition in the coatings being
formed. This can be a source of staining and diminished
corrosion resistance. Also, electrolysis tends to attack
preformed phosphate coatings. Furthermore, oxygen formed
at the anodic substrate being coated can cause a variety of
difficulties such as degradation of coatings by oxidation.
Electro-endoosmosis tends to expel water from
anodic coatings being formed, leading to low water re-
tention with about 85 to 95% solids in the coatings
This is an advantage over cathodic coating in which this
phenomenon would not be expected to be helpful. (Parts
and percentages herein are by weight except where indi-
cated otherwise, and the expression of a range as "X to Y"
or as "X-Y", wherein X and Y are numbers, is meant to be
~111590
inclusive of both X and Y.)
Cathodic electrodeposition has developed more
slowly, due in part to the acidic pH needed for the bath.
Also, water tends to be drawn into the coatings and held
there, along with acid residues from the bath. It is
apparent that this can lead to difficulties in the coatings.
In contrast to the oxygen formed at anodes in anodic elec-
trodeposition, hydrogen is formed at the cathode in cathodic
electrodeposition. Even though this hydrogen can cause
pinholes in coatings, it, of course, does not cause oxida-
tive film degradation.
Prior to coating with protective organic coatings,
metal surfaces, particularly iron and steel, are normally
given a pretreatment such as phosphatizing. U.S. Patent
870,937 - Coslett (1907) describes a method of treating
iron or steel surfaces with phosphoric acid solutions
which may include iron powder or iron phosphates. In the
evolution of phosphatizing coatings for metals, particularly
ferrous metals, several chemical modifications of the
phosphate coating have been found desirable, including the
incorporation of calcium and molybdenum into the coating
and post-rinsing with chromate solutions.
Processes and compositions for the cathodic
electrodeposition of paints are described in U.S. Patent
2 ,345,543-Wohnsiedler, et al., (1944), which uses a
cationic melamine-formaldehyde resin, and in U.S. Patent
3,922,212-Gilchrist (1975~, among others. Gilchrist is
directed to a process for supplen-enting the bath compo-
sition with a make-up mixture of materials containing
an ionizing acid that is not consumed at as fast a rate
~15~0
as the resin. The acid is present in the make-up at lower
concentrations than are used in the bath, so as not to
build up the concentration of the acid in the bath.
Gilchrist uses particular aminoalcohol esters of polycar-
boxylic acids and discloses that acrylic polymers can be
codeposited with zinc phosphate from solution on a cathodic
substrate at low pH's such as 2.7 with phosphoric acid as
the ionizing acid.
Two U.S. patents dealing with nitrogen-based
copolymers and their cathodic electrodeposition are 3,455,806
and 3,458,420, both to Spoor, et al., (1969). Cathodic
sulfonium systems are described by Wessling et al. on
pages 110-127 of "Electrodeposition of Coatings," Ed. E. F.
Brewer, American Chemical Society tl973)-
Electrodeposition processes have been frequently
described in the literature. Two useful reviews of the
technology are: "Electro-painting Principles and Process
Variables,l' Brower, Metal Finishing, September, 1976, p. 58;
and "Coatings Update: Electrocoating",Americus, Pigment and
Resin Technology, August, 1976, p. 17. However, neither
of these articles nor any of the patents mentioned above
suggest means for obtaining cathodically electrodeposited
resin coatings with optimally low levels of water and acid
retention and high corrosion resistance.
SUMMARY OF $HE INVENTION
$he present invention provides a process for
electrocoating with a coating composition a negatively-
charged substrate immersed in a coating bath containing an
aqueous dispersion of said coating composition, said bath
~0 having a cathode zone containing said substrate and an
590
anode zone containing a charged anode, said substrate and
said anode constituting oppositely-charged electrodes, the
charged electrodes being maintained in electrical contact
with each other by means of said bath, wherein said bath
comprises a cationic film-forming polymer, an acidic
ionizing agent, and a crosslinking agent, the improvement
which comprises:
employing phosphoric acid as an acidic ionizing agent;
employing as a cationic film-forming polymer a graft
copolymer having a backbone portion containing
secondary and/or tertiary amine functionality,
said graft copolymer being stabilized in the aque-
ous dispersion by a phosphate salt of the amine
functionality, said backbone portion being graft
polymerized with hydrophobic copolymer derived
. from epoxy esters, said hydrophobic copolymer
having a high enough concentration in the graft
copolymer that the coating deposited on the
substrate has at least about 83% solids content
and so that the phosphoric acid concentration in
the deposited coating composition is no more than
about 17.5% of the phosphoric acid concentration
in the bath; and
employing as the crosslinking agent a composition
which is nonreactive in the bath but reactive with
said film-forming polymer at elevated temperatures.
The invention also comprises acrylic resins
particularly suited for use in coating compositions for the
processes of the invention, including both a resin suitable
for use in primer compositions and a resin suitable for
~illS~O
top-coat compositions which can be used either as a single
coat or applied cathodically over an electrically-conductive
primer.
The primer resin is a graft copolymer of an
epoxide grafted onto an acrylic backbone and consisting
essentially of, by weight based on the graft copolymer,
about:
a. in the acrylic backbone portion: 15 to 25% of a
polymer or copolymer of at least one unit
selected from alkyl, aminoalkyl, and hydroxyalkyl
acrylates and methacrylates, said copolymer con-
taining 0.02 to 0.1 (preferably 0.02 to 0.06)
, equivalent of secondary and/or tertiary amine
- functionality and optionally 0.01 to 0.05
(preferably 0.01 to 0.02) equivalent of quater-
nary ammonium functionality; and
b. in the graft: 75 to 85% of a copolymer contrib-
uting:
3 to 7~ of a glycidyl ester of a tertiary
carboxylic acid containing 7 to 9 carbon
atoms, and
72 to 80% of a blend of a 55 to 60%
condensation polymer of epichlorohydrin
and bisphenol-A with 15 to 20% tall oil
fatty acids.
(Equivalents herein means equivalents of functionality per
100 grams of graft copolymer.)
One preferred embodiment is a graft copolymer
consisting essentially of, by weight based on the graft
copolymer, about:
sso
a. 17 to 21%, preferably 19%, of a copolymer
contributing:
3 to 5%, preferably 4~, methyl methacrylate,
4 to 6%, preferably 5~, butyl acrylate,
1 to 4%, preferably 3%, hydroxyethyl
methacrylate,
1 to 3%, preferably 2%, dimethylamino ethyl
methacrylate, and
: 4 to 6%, preferably 5%, t-butylamino ethyl
methacrylate, graft polymerized with
b. 83 to 79%, preferably 81~, of mixture of about
5% of
Rl O ~ 0~
R2-C--C-O-CH2-CH--CH2
R3
wherein the Rl, R2 and R3 groups are saturated
aliphatic chains which contain a total of 7 to
9 carbon atoms, and at least one of ~, R2 and
R3 is a methyl group, and
74 to 78%, preferably 76%, of a blend of
57 to 60%, preferably 58.5~, of a condensation
polymer of
27 to 31~, preferably 29.25%, epichlorohydrin
and 27 to 31%, preferably 29.25%, bisphenol-A
with 16 to 19~, preferably 17.5~, tall oil
fatty acids~
The top-coat resin is a copolymer of an epoxide
grafted onto an acrylic backbone which consists essentially
of, by weight based on the graft copolymer, about:
X
llllS90
a. in the acrylic backbone portion: 80 to 92% of a
polymer or copolymer of at least one unit
selected from alkyl, aminoalkyl, and hydroxyalkyl
acrylates and methacrylates, said copolymer
containing 0.02 to 0.1 (preferably 0.02 to 0.08)
equivalent of secondary and/or tertiary amine
functionality and optionally 0.01 to 0.05
(preferably 0.01 to 0.03) equivalent of quater-
nary ammonium functionality; and
b. in the graft: 20 to 8% of a comonomer which is
a glycidyl ester of a tertiary carboxylic acid
containing 7 to 9 atoms.
One preferred embodiment is a graft copolymer
consisting essentially of, by weight based on the graft
copolymer, about:
a. 85 to 91%, preferably 89%, of a copolymer contrib-
uting:
9 to 11%, preferably 10~, methyl methacrylate,
42 to 46%, preferably 44%, 2-ethyl hexyl
acrylate,
6.5 to 8.5%, preferably 7.5%, t-butylamino-
ethyl methacrylate,
2 to 3%, preferably 2.5~, dimethylaminoethyl
methacrylate, and
23 to 27%, preferably 25~, hydroxyethyl
methacrylate,
graft polymerized with
b. 15 to 9~, preferably 11~, of
~1590
R O O
,1 " / \
R2 -C~-O-CH2 -CH~H2
R3
wherein the ~ , R2 and R3 groups are saturated
aliphatic chains which contain a total of 7 to
9 carbon atoms, and at least one of ~ , R2 and
R3 is methyl group.
DETAILED DESCRIPTICN OF THE INVENTICN
The present invention provides a practical means
for cathodically electrodepositing first a passivating
phosphate salt coating, preferably as at least a monQ~ayer
and then electrodepositing over the phosphate a protective
resin coating having low water and acid retention and high
resulting durability and corrosion resistance. The process
of the invention can be used either on pretreated metal
such as phosphatized steel or on bare metal such as steel
which has been cleaned but not phosphatized. It can also
be used on other metal substrates containing zinc, such
as galvanized steel, as well as on aluminum and various
alloys. Since the process of the invention deposits a
phosphate coating underneath the polymer coating, it is
less sensitive than other electrocoating processes to
variations in the substrate and its pretreatment. If the
process of the invention is applied to ~aterial which has
already been phosphatized, this versatility of the invention
would enhance the phosphate coating. However, phosphate
pretreatment is not necessary.
The invention preferably provides in the bath
a water-soluble dihydrogen phosphate salt, M(H2PO4)2
(M=Fe,Zn,Ca,Mg or Al). With cathodic electrodeposition of
y
1~1590
the invention, the pH rises at the cathode, creating a
boundary layer perhaps 0.01 to 0.1 cm thick, in which the
soluble phosphate salt converts to an insoluble phosphate
salt, M(HPO4) or M3(PO4)2, which is deposited onto the
substrate surface. Thus, the driving force for the salt
deposition is a pH change and precipitation at the sub-
strate surface. The same type of pH change phenomenon
also causes deposition of dense polymer coatings with the
present invention. This leads to denser coatings than
electrophoresis of quaternary ammonium salts wherein a
concentration gradient is the main driving force.
In a specific embodiment of the invention, with
the coating voltage applied while a ferrous metal substrate
is being immersed into the coating bath, the phosphoric
acid in the bath dissolves small amounts of ferrous ion
from the substrate. The Fe(H2PO4)2 salt so formed is
soluble in the bath at pH levels of 3.0 or below. However,
a boundary layer of increased pH quickly develops at the
substrate, leading to the formation and deposition on the
substrate of the insoluble salts Fe(HPO4) and Fe3(PO4)2.
These salts give effective corrosion protection which is
enhanced by the fact that they generally form a continuous
layer on the substrate, usually at least a monolayer, rather
than being entirely dispersed up into the overlying sub-
sequently-formed resin coating. In a preferred embodiment
of the invention, water-soluble zinc salts are included in
the bath, and they too deposit as insoluble phosphates at
the substrate. The dihydrogen phosphate of zinc, Zn(H2PO4)2
is stable in the bath at a pH of 2.5 to 3.5. The invention
is useful at pH values in the range of 2.0 to 4.0,
preferably 2.5 to 3.0~ At a pH below about 3.0, free
-- 10 --
. _
1~15590
phosphoric acid in the bath will react with ferrous metal
substrates to generate the soluble dihydrogen phosphate of
iron, Fe(H2PO4)2. Since the solubility of the dihydrogen
metal phosphates decreases with increasing temperature, the
pxocess of the invention is best operated with a bath
temperature of 20 to 25C. When zinc salts are dissolved in
the bath, the phosphate coating formed on the steel may likely
include two minerals as the principal constituents. These
are phosphophyllite, Zn2Fe(PO4)2~4H2O, and hopeite,
Zn3(Po4)2 4H2
m e lack of practical success of several previous
cathodic electrodeposition painting processes is due at
least in part to the amount of water t~at is held in the
resin coating and the acids and salts that are dissolved
in that water, not readily removable from the coating. The
water can lead to coating failure by various mechanisms, and
the acid residues can encourage subsequent corrosion, either
directly or by providing a hygroscopic material in the coating
which encourages penetration of water and other corrosive
agents.
In contrast to the useful effect of electroendo-
osmosis at the anode in anodic electrodeposition of paint
which tends to expel water from an anodic coating, water is
not electrically expelled from a cathodic coating and may
actually be drawn into the coating by electrical forces.
However, water held in a cathodic coating can be particularly
undesirable. To minimize such effects, the present invention
provides resins with a degxee of hydrophobicity and
hardness or denseness of the coating which combine to expel
water from the coating as the ooating is formed.
The desirable effects of the invention are
-- 11 --
. ~r
~11590
achieved by using certain hydrophobic graft copolymers
containing in their backbone portions secondary and/or
tertiary amine functionality. Such functionality aids in
adhesion of the resin coating to the substrate even after
heating the deposited coatings to cause them to crosslink.
This is an advantage over cathodic sulfonium systems in
which hydrophobicity is only developed after thermal
decomposition of the sulfonium groups. Thermal decomposi-
tion of sulfonium groups during crosslinking of the film
would make them unavailable for enhancing adhesion of the
resin coating to the substrate. Also, although quaternary
ammonium salts can be present in film-forming polymers of
the invention, they cannot replace the secondary or tertiary
amine groups. The quaternary ammonium salts would decompose
to some extent when the film is heated to cause crosslinking,
thereby losing their effectiveness in promoting adhesion to
the substrate. The polymer compositions of the invention
are discussed in more detail below.
In the process of the invention, although there
are advantages in using live entry, in which the coating
voltage is ~pplied while the articles to be coated are
being immersed into the bath, it will be apparent that
reduced voltage can be applied upon entry if desired for
certain special effects. However, the additional electrical
apparatus required for reduced voltage entry is not nor-
mally necessary or desirable. It is desirable for the
coated substrate to be removed from the bath with the
coating voltage still applied or soon after it is turned
off.
For operating electrocoating ~aths of the
- 12 -
1111590
invention, the tank can be lined with an organic coating
resistant to the acidic pH of the bath, and stainless steel
or plastic piping and pump parts can be used to minimize
corrosion. However, carbon steel parts often can be used,
and the ferrous ions added to the bath by gradual dissolu-
tion of the equipment could be helpful rather than harmful
to the coating process. Due to its autopassivating effects,
phosphoric acid is less corrosive to steel than some other
mineral acids at the pH levels used, so that more expensive
materials of construction often are not necessary.
It has been found that common bacteria do not
grow in the aqueous coating compositions of the invention.
Therefore, ordinary ultrafiltration can be used in recir-
culating the bath components to rinse contaminants from
the coated parts. Furthermore, membranes and ordinary
flushed anodes may be desirable but are unnecessary.
As an alternative to flushed anodes, excess phosphoric
acid build-up in the bath can be consumed by additions
of 2inc, ZnO, Zn(OH)2, or other metals or compounds which
form the dihydrogen metal phosphates in solution.
Although an uncoated tank can be used as the
anode, in commercial practice one would normally use
stainless steel anodes having a surface area smaller than
that of the cathodic substrate which is to be coated. This
gives a favourable current density distribution~
In the novel electrocoating process, the metal
article providing the substrate to be coated is immersed
in a bath of an electrocoating cell. The ~ath is an aqueous
dispersion of about 2-3~ by weight of a cationic film-
forming polymer at least partially neutralized with an
5~0
acidic material. Preferably phosphoric acid is used in an
amount of from 60~ of that required for stoichiometric
reaction of the first hydrogen of the trivalent acid with
all of the available amine group bonds in the polymer to an
excess of 120~ of stoichiometric. m e use of less than about
6Q~ of the stoichiometric amount of phosphoric acid can lead
to instability in the bath. More than 120%, even as high as
270% or higher, can sometimes be tolerated, even though a
low pH limit of 2.2 to 2.5 is approached as more free phos-
phoric acid is present. In the presence of the acid, the
film-forming polymer forms cations in the bath.
The metal article is connected to the negative
side of a direct current (D.C.) power source to become the
cathode of the cell. A voltage of about 1 to 500 volts is
passed through the cell for the full dwell time of the
article in the bath, about 0.01 to 5 minutes, and a coating
of the cationic polymer is deposited. When the coating
reaches the desired thickness, the article is removed from
the bath. Preferably, the article is rinsed with water or
with filtrate taken from the process to remove excess
coating. Then the article is dried at ambient temperatures
or baked for about 5 to 40 minutes at about 100 to 300C.
to give a finished coating about 0.1 to 5 mils thick.
Typical efficiencies of about 30 mg film solids deposited
per coulomb of electricity are obtained.
The current density used in the electrocoating
cell generally does not exceed 1.85 amperes~cm2 (0.3
amperes/in2) of anode surface which i5 immersed in the
bath, and it is preferable to use lower current densities.
In the deposition of the cationic film-forming polymer,
- 14 -
590
voltages of 5 to 400 for 0.25 to 2 minutes are preferred
to form a high quality finish.
Coating compositions of the present invention
can contain pigments. The pigments are normally added to
the composition in the usual manner by forming a mill
base or pigment dispersion with the pigment and the
aforementioned cationic film-forming polymer or another
water-dispersible polymer or surfactant. This mill base
is then blended with additional film-forming constituents
and the organic solvents. When the mill base is subse-
quently acidified and dispersed in water, the polymers
tend to wrap themselves around the pigments. This has the
effect of preventing destabilization of the dispersion or
other undesirable effects that could come from using a
basic pigment such as Tio2 in an acidic dispersion. Pig-
ments stable in acidic media can be used, such as the
surface-treated TiO2 pigments of U.S. Patent 3,941,603 -
Schmidt (1976). Other pigments that could be used include
metallic oxides such as zinc oxides, iron oxides, and the
like, metal flakes such as aluminum flake, metal powders,
mica flakes with and without surface treatment such as
with titania and carbon black, chromates such as lead
chromates, sulfates, carbon black, silica, talc, aluminum
silicates including china clay and finely divided kaolin,
organic pigments and soluble organic dyes.
Aside from cathoaic electrodeposition, the novel
coating compositions of the present invention can also be
applied by any conventional method such as spraying,
electrostatic spraying, dipping, brushing, flowcoating
and the like. ~eaction of the amine groups of the polymer
- 15 -
X
15~0
with phosphoric acid is generally not necessary when the
coating composition is to be used for purposes other than
electrodeposition. Organic thermally decomposable acids,
such as formic acid, can be used to obtain water solubility
for such purposes. The coating would then be baked for
about 5 to 40 minutes at about 175 to 200C to give coatings
of about 0.1-5 mils thickness. When applied by cathodic
electrodeposition, coating compositions of the invention
are preferably applied to give dried thicknesses of about
0.8-1.2 mils.
A crosslinking agent which can be water dispersed
along with the film-forming constituent is used in the novel
composition. Based on the proportions of solids in the
bath, which are roughly equal to the proportions of solids
in the film, about 60 to 95%, preferably about 70~, of
cationic film-forming polymer are used along with about
5 to 40~, preferably about 30~, of crosslinking agent.
Typical crosslinking agents that can be used
with the invention are melamine formaldehyde, alkylated
melamine-formaldehyde resins such as hexakis-(methoxymethyl)
melamine and partially-methylated melamine formaldehyde
resins, butylated melamine formaldehyde resins, methylated
urea-formaldehyde resins, urea-formaldehyde resins, phenol-
formaldehyde and the like. One particularly useful cross-
linking agent which forms a high quality product with the
cationic polymers is a benzoguanamine-formaldehyde resin.
A preferred benzoguanamine formaldehyde resin ls XM 1125*
produced by American Cyanamid Co., an acidic self-catalyzed
crosslinking agent with an acid number of 25 to 32.
When the novel compositions of this invention are
* denotes trade mark
- 16 -
~llS90
used as primers over metals including treated and untreated
steel, aluminum and other metals, conventional acrylic
enamels, acrylic dispersion enamels and other coating com-
positions can be applied directly as topcoats over such
primers. Acrylic lacquers, acrylic dispersion lacquers,
and acrylic powder coatings can be applied over the novel
compositions, but a suitable intermediate coat such as a
sealer can be used to improve adhesion of the lacquer or
powder topcoat to the primer.
The glycidyl ester used in both the primer and
topcoat compositions and the optional epoxy-fatty acid
constituents used in the primer composition contribute
sufficient hydrophobicity to the polymer so that the elec-
trodeposited film contains at least about 83~ solids, and
preferably 85 to 95% solids. Although such high solids
levels are not uncommon for anodically deposited coatings,
they are not readily achieved in cathodic electrodeposition
because of the amount of water usually entrapped. The
phosphoric acid concentration of the electrodeposited film
is in the range of 10 to 15% of the concentration of phos-
phoric acid in the bath. This is on the order of about
0.05% of the electrodeposited film itself. These figures
apply to the film as electrodeposited, before drying and
baking. The amine functionality in the film causes some
small phosphate concentration in the film, but retained
water will deleteriously increase the phosphoric acid
content. Empirical tests have shown that 20 to 25~ of the
concentration of phosphoric acid in the bath being present
in the film is an undesirable level, causing diminished
corrosion resistance, blistering, and other undesirable
- 17 -
X
l~llS9~
effects.
In the process of the invention, the critical
concentration of phosphoric acid in the dry film is speci-
fied in terms of a percentage of the concentration of
phosphoric acid in the bath. Equivalents of phosphoric
acid, in the form of phosphates reacted with amine groups
of the polymer and metal phosphate salts, are included in
the term ~'phosphoric acid" for these purposes. Relative
to the entire electrodeposited coating, the metal phosphate
layer directly on the substrate will contribute negligible
amounts of phosphate. Most of the phosphoric acid equiv-
alents will be present as free phosphoric acid or as amine
salts along with the water entrapped in the film. The
amount of phosphate ionically bound in the polymer will
vary depending on the amount of amine in the polymer.
Larger amounts of amines will lead to larger amounts of
bound phosphate reacted with them. Most of the phosphate
is released from the polymer as phosphate ion as a result
of pH change and electrical phenomena when the polymer is
deposited on the substrate, but a variable amount remains
in the film. The most reliable measurement of the concen-
tration of phosphate to be deposited is as a proportion of
the concentration in the bath. This is a dynamic value
which depends upon coating speed, dragout and flushing
rates. It is best averaged over a period of time as
phosphoric acid is added to the bath and partially removed
in the coatings.
Although the present invention uses phosphoric
acid for several reasons, most importantly to allow the
production of a phosphate coating on the sbustrate in the
- 18 -
590
same process that produces the paint coating, other acids
could be used in addition to the phosphoric acid for
similar results. Acids which form water soluble salts of
the desirable metals at low pH, especially such salts of
zinc, iron, calcium, magnesium and aluminum, and which then
convert to insoluble salts in the boundary layer at
increased pH, can be useful. Oxalic, chromic, sulfamic,
benzoic and boric acids can have such effects. However,
the deposited salts of such acids in the absence of phos-
phates may not have the passivating or corrosion-inhibiting
effects of phosphates.
Compositions of the invention can include addi-
tional adjuvants that do not materially change the basic
and novel characteristics of the invention and thus are
within the scope of "consisting essentially" terminology.
Some such adjuvants are thickeners, defoamers, pigments,
microgels, pigments dispersants, polymeric powders, micro-
biocides, and coalescing solvents. Typical coalescing
solvents which might be used at a concentration of about
0.5~ of the total bath volume are ethylene glycol monobutyl
ether, diethylene glycol monobutyl ether, and cyclohexanol.
m e graft copolymers of the invention can have
backbone portions of a variety of types so long as they
contain the requisite amine functionality and are made
adequately hydrophobic by grafting with epoxy copolymers.
The preferred backbone portions are acrylics, including
alkyl acrylates such as me~hacrylics, and polymers derived
from acrylics and methacrylics. Other useful backbone
portions include polyamines of maleinized oils, polyesters,
maleinized polybutadiene, and epoxidized oils.
-- 19 --
5~0
Secondary amines in the backbone portion of the
graft copolymer can function similarly to tertiary amines.
Secondary amines can be provided, for instance, by reacting
glycidyl methacrylate with ammonia to form a primary amine
which is converted to a secondary amine on grafting with
appropriate amounts of epoxies. It should be kept in mind
that graft copolymerization to produce compositions of the
invention changes secondary amines in the reactants to
tertiary amines and likewise changes primary amines to
secondary amines and tertiary amines to quaternary ammonium
salts.
Quaternary ammonium salts would be coated onto
the substrates mainly by concentration gradient effects
rather than by pH changes in the narrow boundary zone which
cause the deposition of secondary and tertiary amines. The
concentration gradient effect is more gradual than the bound-
ary zone effect, leading to softer, less dense coatings in
the absence of the secondary and tertiary amine groups.
Such softer coatings would be bulkier and more porous and,
therefore, more conductive. This means that they would
continue to build up in thickness with further electro-
deposition. In contrast, the self-limiting effect of less
conductive films gives coatings of more uniform thickness.
In addition to increasing the adhesion of the film to the
substrate after baking, secondary and tertiary amines in
the backbone portion also enhance stability of the polymer
in water dispersions.
For enhanced adhesion to substrates and dispersion
stability in water, the film-forming polymer of the inven-
tion preferably contains 0.04-0.8 equivalent of tertiary
- 20 -
590
amine functionality. The preferred primer contains about
0.04 equivalent, and the preferred topcoat contains about
0.05 equivalent of tertiary amine functionality. The
preferred secondary amine before grafting is t-butyl amino
ethyl methacrylate, and the preferred tertiary amine is
dimethylaminoethyl methacrylate.
Tertiary amines in the acrylic backbone portion
before graft copolymerization which are converted to
quaternary ammonium salts upon grafting serve the useful
purpose of enhancing the graft copolymerization. Therefore,
graft copolymers of the invention preferably contain about
0.01 to O.OS (more preferably about 0.01 to 0.~2) equivalent
of quaternary ammonium functionality. However, the quater-
nary ammonium functionality need not be built into the
backbone portion but can be provided as an external catalyst
to enhance the graft polymerization. In such a case, the
somewhat delecterious effects of quaternary ammonium func-
tionality in the backbone portion of electrodeposited
coatings are avoided.
Although it is difficult to meaningfully quantify
the softness or hardness of the resin, it is known that
certain resins of the invention have a degree of hardness
which is useful in combination with the hydrophobicity
characteristics of the resins in forcing water out of films
to obtain the indicated levels of retained water and acid.
The molecular weights of polymers of the invention
are generally not critical. However, typical average
molecular weights determined by gel permeation chromato-
graphy are: for the backbone portion - 12,000; for the
primer graft copolymer - 11,000 to 12,000; and for the
~,
l~llS~O
topcoat graft copolymer - 15,000. These figures show that
typically 80 to 85% of the epoxide is grafted onto the
backbone portion.
Although thoughts are expressed herein on why
and how the advantages of the invention are obtained, the
invention is described by the claims and does not depend
upon theories.
Specific examples will now be given of the
preparation of graft copolymers of the invention and their
use in cathodic electrodeposition processes of the invention.
E ~PLE I
A black primer coating composition is prepared
and used as follows:
Part I and Part II describe the two resin
compounds that are graft polymerized and used with the
pigment dispersions of Part III in the paint of Part IV.
Part I
This part descri~es the preparation of an epoxy
ester for graft copolymerization.
The following ingredients are charged into a
reaction vessel equipped with a stirrer, thermometer,
reflux condenser and a heating mantle to form an epoxy
ester resin solution:
Portion I Parts by Weight
Epoxy resin (EPON* 10~1) 1677.00
(EPON 1001 is an epoxy resin of the formula
* denotes trade mark
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A
1~15~'~
CH2 -CH-CH2~ o-CH2-CH-CH23,~
3 /O
~ --CH2--CH--CH2 '
CH3
where m is an integer sufficiently large to provide a
Gardner-Holdt viscosity at 25C of D-G measured in a
40% weight solids polymer solution using ethylene
glycol monobutyl ether solvent, and the resin has an
epoxide equivalent of 450-550).
Portion 2
Tall oil fatty acids 503.10
Benzyl trimethylamonium hydroxide 1.70
Portion 3
Ethylene glycol monoethyl ether419.30
Portion 1 is charged into the reaction vessel,
blanketed with nitrogen and heated to about 128 to 140C
to melt the resin. Portion 2 is then added, and the ingre-
dients are heated to about 150 to 160C for about 3 hours
with constant agitation until the reaction mixture has an
acid number of 0.01. Portion 3 is added, and the ingre-
dients are cooled and filtered.
The resulting epoxy ester resin solution has a
solids content of about 84%, an acid number no higher than
0.01, an epoxide equivalent of 1300-1900, and a Gardner-
Holdt viscosity of D-F at 25C in a 40% solids polymer
solution using ethylene glycol monoethyl ether solvent.
Part II
This part describes the preparation of an
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5~
acrylic resin and the graft polymerization of the epoxy
ester described above onto it.
Portion 1 Parts of Weight
Isopropanol 400-00
Portion 2 Parts by Weight
Methyl methacrylate monomer100.00
Butyl acrylate 125.00
Tertbutylaminoethylene methacrylate 140.00
Dimethylaminoethyl methacrylate 40.00
Hydroxyethyl methacrylate75.00
Portion 3
Isopropanol 100.00
Methylethyl ketone 25.00
Axobisisobutyronitrile 10.00
Portion 4
Methylethyl ketone 8.00
Axobisisobutyronitrile 1.00
Portion 5
Ethylene glycol monoethyl ether 350.00
Portion 6
Epoxy ester prepared in Part I 2300.00
Ethylene glycol monoethyl ether 350.00
CARDURA* E-10 125.00
(glycidyl ester of epichlorohydrin
reacted with versatic acid 911
produced by Shell Oil Co.)
Dioni~ed water 50.00
Portion 1 is charged into a reaction vessel,
equipped as described above, and is heated to its reflux
temperature. The reaction mixture is held under nitrogen
during the entire reaction. Portions 2 and 3 are separately
* denotes trade mark
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A
llllS90
premixed and added slowly simultaneously over a 90-minute
period while maintaining the reaction mixture at its reflux
temperature. The reaction is continued for an additional
60 minutes. The Portion 4 is added, and the reaction
mixture is held at its reflux temperature for an additional
30 minutes. Stripping of the reaction solvent is conducted
simultaneously with the addition of Portion 5 which is to
replace the reaction solvent. When 533.00 parts of solvent
are stripped and all of Portion 5 is added to the reaction
vessel, Portion 6 is added and the temperature is brought
at 115C to 117C and maintained for 4 hours with continuous
agitation. At the end of that period the epoxy number is
determined. When the epoxy equivalent is zero or less than
1 epoxy unit per 500,000 gm, the reaction is finished. The
solids content is 70%, and the Garner-Holdt viscosity at
25~ reduction of solids with ethylene glycol monoethylether
is U to X.
Part III
A black pigment dispersion is prepared as follows:
Parts by Weight
Solution polymer prepared in Part II 318.00
Ethylene glycol monoethylether 84.00
Carbon black pigment 31.80
The above ingredients are premixed and charged into a
conventional sand mill and ground at a rate of 30 gallons
per minute while controlling the temperature of the mixture
below 70C. The resulting carbon black dispersion has
about 58% solids content.
An extender pigment dispersion using Al-silicate
as the extender pigment is prepared as follows:
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1~11590
Parts by Weight
Solution polymer prepared in Part II 193.00
Ethylene glycol monoethylether 142.00
Aluminum silicate 206.00
The above ingredients are premixed and charged
into a conventional sand mill and ground at a rate of 30
gallons per minute while controlling the temperature of
the mixture below 70C. The resulting aluminum silicate
dispersion has about 63~ solids.
A water soluble phosphate salt of zinc, zinc
dihydrogen phosphate, that is added to the coating com-
position of above polymer to improve its corrosion resis-
tance when cathodically electrodeposited, is prepared as
follows:
Parts by Weight
ZnO(zinc oxide) 4.00
Phosphoric acid (85~) 14.00
Deionized Water 500.00
The above ingredients are mixed for 5 to 8 hours
at room temperature until complete solubility of the zinc
oxide takes place. The pH of the solution is 2.6 to 3.0
Part IV
The eIectrocoating composition of a flated black
paint is prepared as follows:
Portlon 1 Parts by Weight
Resin solution of Part II320.00
Black pigment dispersion
of Part III 97.00
Aluminum silicate pigment
dispersion of Part III440.00
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X
~11590
Benzoguanamine formaldehyde
solution (~1 1125 produced by
~nerican Cyanamid Co., 85% in
ethylene glycol monobutyl ether) 190.00
Portion 2
Deionized water 632.00
Phosphoric acid (85%) 22.00
Portion 3
Zinc dihydrogen phosphate 510.00
Portion 1 is added into a mixing vessel, heated
to 150F and mixed for 3 hours, maintaining a temperature
of 150F. Portion 2 is added into another mixing vessel
mixed for 10 minutes, and Portion 1 is added into Portion
2 with continuous agitation. The pigmented water
dispersion is mixed for 2 hours and diluted to about
15% solids with deionized water and Portion 3 so that
the concentration of zinc dihydrogen phosphate salt in the
paint dispersion will be 450 ppm based on the total weight
of the electrocoating composition.
The electrocoating composition, having a pH of
2.7 and a conductivity of 1700 micromhos, is charged into
a stainless steel tank for electrodeposition. An untreated
cold rolled steel panel or a phosphatized steel panel is
positioned in the center of the tank, electrically connected
to the negative side of a DC power source, and forms the
cathode of the electrocoating cell. The tank is connected
to the positive side of a DC power source and forms the
~node of the cell.~ A direct current of 150 volts is applied
to the cell for 2 minutes at an ambient temperature of
20-25C, and a paint film of about 0.6 mils is deposited
on the panel. The coated metal panel is removed from the
electrocoating cell, washed and baked at about 160C for
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y
l~llS~O
30 minutes. The resulting primer film has excellent
adhesion to the metal substrate, is hard and has very good
corrosion and saponification resistance over bare cold
rolled steel and phosphatized steel. An acrylic enamel
adheres to the primer film, and conventional acrylic
lacquers can be applied with a conventional sealer coat
over the primer to form a high quality finish.
~ ypical deposited films contain 90 to 95% solids
and 10 to 12~ of the phosphoric acid present in the bath.
This coating composition is particularly useful
for priming automobile and truck bodies by electrodeposi-
tion for maximum corrosion protection over all parts of
the car including areas of poor phosphate pretreatment or
no pretreatment at all.
EXAMPLE II
A white pigment dispersion is prepared as follows:
Parts by Weight
Solution polymer of Part II of Ex. I 314.00
Ethyleneglycol monoethylether 137.00
Titanium Oxide 549.00
The above ingredients are premixed and charged
into a conventional sand mill and ground at a rate of 30
gallons per minute while controlling the temperature of the
mixture below 70C. The resulting titanium oxide dispersion
has about 76% solids content.
A white coating compositior. is prepared as follows:
Portion 1
Parts by Weight
Resin solution of Part II of Ex. 1 560.00
Benzoguanamine formaldehyde resin solu-
tion (85% in ethylene glycol monobutyl
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~ ,,
1111590
ether) 245.00
Titanium dioxide pigment dispersion 700.00
Portion 2
Phosphoric acid (85%) 30.00
Deionized water 1400.00
An electrocoating composition of 15~ solids and
pH of 2.8 is prepared using Portions 1 and 2 and electro-
coated following the procedure described in Example 1.
This coating composition is useful either as a
primer or as a single coat directly on metal for appliances
or industrial equipment. It has good corrosion resistance
and detergent resistance over bare cold rolled steel or
phosphatized steel.
Coatings prepared as in Example I give similar
results.
EXAMPLE III
Portion 1 Parts by Weight
Isopropanol 1200.00
Portion 2
Methylene methacrylate 300.00
2-Ethylhexyl acrylate 1000.00
Tert-butylamino methyl methacrylate 180.00
Dimethylamino ethyl methacrylate 60.00
Hydroxy ethyl methacrylate600.00
Azobis-isobutyronitrite 40.00
Portion 3 Parts by Weight
Azobis isobutyronitrile 2.00
Acetone 15.00
11~1590
Portion 4
Ethylene glycol monoethyl ether 900.00
Portion 5
CARDURA E-10 (Shell Oil Co. product) 260.00
Portion 1 is charged into a reaction vessel
equipped as described above and is heated to its reflux
temperature. The reaction mixture is held under nitrogen
during the entire reaction.
Portion 2 is separately premixed and added slowly
simultaneously over a 90-minute period while maintaining the
reaction mixture at its reflux temperature. The reaction
is continued for an additional 60 minutes. Then Portion 3
is added, and the reaction mixture is held at its reflux
temperature for an additional 30 minutes. Stripping of the
reaction solvent takes place simultaneously with addition
of Portion 4 which is to replace the reaction solvent.
1200.00 parts of solvent are stripped, and all of Portion 4
is added to the reaction vessel. Then Portion 5 is added,
and the temperature is brought to 115 to 117C where it is
maintained for four hours with continuous agitation. At
the end of that period, the epoxy number is determined.
When it is zero or less than 1 epoxy unit per 500,000 gm,
the reaction is finished. The solids content is 70%, and
the product has a Gardner-Holdt viscosity of Z2-~4
Coatings are prepared as in Example I. Typically
the coatings contain 85 to 90% solids and 15% of the phosphoric
acid present in the bath.
EXAMPLE IV
A white pigment dispersion is prepared as follows:
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A
llllS90
Parts by Weight
Resin solution of Example III 320.00
Ethylene ~lycol monoethyl ether 140.00
Titanium oxide 550.00
The above ingredients are premixed and charged
into a conventional sand mill, and ground at a rate
of 30 gallons per minute while controlling the temperature
of the mixture below 70C. The resulting titanium oxide
pigment dispersion has about 76% solids content.
A white coating composition is prepared as follows:
Portion 1
Resin solution of Example III560.00
Benzoguanamine formaldehyde resin
solution (85~ in ethylene glycol
monobutyl ether) 250.00
Titanium oxide pigment dispersion 700.00
Portion 2
Phosphoric acid (85%) 30.00
Deionized water 1400.00
Portion 3
Zinc dihydrogen phosphate 510.00
Using Portions 1, 2 and 3, and electrocoating com-
position of 15% solids and a pH of 2.7 is prepared and elec-
trocoated following the procedure described in Example I.
This coating composition is particularly useful
as a single coat directly on metal finishes that require
good gloss and gloss retention after UV exposure, and good
corrosion resistance regardless of the type and quality of
pretreatment. It also enables one to obtain a white finish
without the typical discoloration characteristics of an
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1~11590
electrocoating finish.
Coatings prepared as in Example I give similar
results.
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