U.S. patent application number 09/754140 was filed with the patent office on 2001-07-19 for cationic electrodeposition coating composition.
Invention is credited to Shirakawa, Shinsuke, Tsutsui, Keisuke, Yamada, Mitsuo.
Application Number | 20010008692 09/754140 |
Document ID | / |
Family ID | 18530671 |
Filed Date | 2001-07-19 |
United States Patent
Application |
20010008692 |
Kind Code |
A1 |
Shirakawa, Shinsuke ; et
al. |
July 19, 2001 |
Cationic electrodeposition coating composition
Abstract
A cationic electrodeposition coating composition is provided
which contains blocked isocyanate curing agent which is blocked
with a substance not recognized as a HAPs (hazardous atmospheric
pollutant). The cationic electrodeposition coating composition of
the present invention contains a cationic group-containing epoxy
modified base resin and a blocked isocyanate curing agent, wherein
said blocked isocyanate curing agent is obtained by reacting a
polyisocyanate compound with, as a blocking agent, a terminal
primary OH-containing propylene glycol monoalkyl ether and
expressed by the formula RO(CH(CH.sub.3)CH.sub.2O).su- b.nH (where
R is an alkyl group having 1 to 8 carbons, which may be branched,
and n is 1 to 3).
Inventors: |
Shirakawa, Shinsuke;
(Osaka-fu, JP) ; Tsutsui, Keisuke; (Hyogo-ken,
JP) ; Yamada, Mitsuo; (Osaka-fu, JP) |
Correspondence
Address: |
WENDEROTH, LIND & PONACK, L.L.P.
2033 K STREET N. W.
SUITE 800
WASHINGTON
DC
20006-1021
US
|
Family ID: |
18530671 |
Appl. No.: |
09/754140 |
Filed: |
January 5, 2001 |
Current U.S.
Class: |
428/413 ;
523/404 |
Current CPC
Class: |
C08G 18/283 20130101;
C08G 18/581 20130101; C08G 18/8074 20130101; Y10T 428/31511
20150401; C08G 18/8064 20130101; C09D 5/4453 20130101; Y10T
428/31529 20150401 |
Class at
Publication: |
428/413 ;
523/404 |
International
Class: |
B32B 027/38; C08K
003/20 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 7, 2000 |
JP |
001435/2000 |
Claims
What is claimed is:
1. A cationic electrodeposition coating composition comprising a
cationic group-containing epoxy modified base resin and a blocked
isocyanate curing agent, wherein the blocked isocyanate curing
agent is obtained by reacting a polyisocyanate compound with a
terminal primary OH-containing propylene glycol monoalkyl ether as
a blocking agent, expressed by a
formulaRO(CH(CH.sub.3)CH.sub.2O).sub.nH(where R is an alkyl group
having 1 to 8 carbons which may be branched and n is 1 to 3).
2. The cationic electrodeposition coating composition according to
claim 1, in which said polyisocyanate compound is diphenylmethane
diisocyanate, and R in the formula is a n-butyl group and n is 1 to
2.
3. An electrodeposition-coated article, which is coated using the
cationic electrodeposition coating composition according to anyone
of claims 1 or 2.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a cationic
electrodeposition coating composition. In particular, it relates to
a cationic electrodeposition coating composition which contains a
curing agent that has been blocked with a terminal primary
OH-containing propylene glycol monoalkyl ether.
[0003] 2. Description of the Related Art
[0004] Blocked isocyanate curing agents are generally used in
cationic electrodeposition coatings. The blocked isocyanate curing
agents are obtained by reacting a polyisocyanate compound with a
blocking agent which is reacted with the isocyanate groups and
stable at ambient temperature, but can regenerate free isocyanate
groups when heated to a dissociation temperature or higher. The
blocking agents contain an active hydrogen and can be suitably
selected according to the type of polyisocyanate compound to be
employed.
[0005] However, the increasing level of awareness of environmental
issues of late have been accompanied in developed countries by
efforts to regulate the amounts of hazardous atmospheric pollutants
(HAPs). Since the blocked isocyanate curing agents release blocking
agents into the atmosphere when heated, the blocked isocyanate
curing agents also need to be considered as a substance under HAPs
as blocked by a substance which is considered as a HAPs. For
example, conventionally used cationic electrodeposition coating
compositions contain diphenyl methane diisocyanates (MDI) which are
blocked with .epsilon.-caprolactam and butyl cellosolve. Since both
of the blocking agents are HAPs substances, there is the concern
that their use is banned through enforcement of the environmental
regulatory standards.
SUMMARY OF THE INVENTION
[0006] It is an object of the present invention to provide a
cationic electrodeposition coating composition which contains a
blocked isocyanate curing agent that has been blocked with a
substance not recognised as a HAPs.
[0007] The cationic electrodeposition coating composition of the
present invention contains an epoxy-modified base resin having a
cationic group and a blocked isocyanate curing agent, wherein the
blocked isocyanate curing agent is obtained by reacting a
polyisocyanate compound with a terminal primary OH-containing
propylene glycol monoalkyl ether as a blocking agent, as expressed
by the formula RO(CH(CH.sub.3)CH.sub.2O).sub- .nH (where R is an
alkyl group having 1 to 8 carbons, which may be branched, and n is
1 to 3). The polyisocyanate compound described above is e.g.
diphenyl methane diisocyanate, and R in the formula for the
propylene glycol monoalkyl ether is an n-butyl group and n is 1 to
2.
[0008] In addition, an article is coated using the cationic
electrodeposition coating composition.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0009] The cationic electrodeposition coating composition of the
present invention contains an epoxy modified base resin having a
cationic group and a blocked isocyanate curing agent.
[0010] The blocked isocyanate curing agent contained in the
cationic electrodeposition coating composition of the present
invention is obtained by reacting a polyisocyanate compound with a
terminal primary OH-containing propylene glycol monoalkyl ether as
a blocking agent, as expressed by the formula
RO(CH(CH.sub.3)CH.sub.2O).sub.nH (where R is an alkyl group having
1 to 8 carbons, which may be branched, and n is 1 to 3).
[0011] Examples of the polyisocyanate compound include alkylene
diisocyanate, such as trimethylene diisocyanate, trimethyl
hexamethylene diisocyanate, tetramethylene diisocyanate, and
hexamethylene diisocyanate; cycloalkylene diisocyanate, such as
bis(isocyanatomethyl)cy- clohexane, cyclopentane diisocyanate,
cyclohexane diisocyanate, and isophorone diisocyanate; aromatic
diisocyanate, such as tolylene diisocyanate, phenylene
diisocyanate, diphenylmethane diisocyanate, and diphenylether
diisocyanate; aromatic/aliphatic diisocyanate, such as xylylene
diisocyanate, and diisocyanate diethylbenzene; triisocyanate, such
as triphenylmethane triisocyanate, triisocyanate benzene, and
triisocyanate toluene; tetraisocyanate, such as diphenyl dimethyl
methane tetraisocyanate; polymerized polyisocyanate, such as dimer
or trimer of tolylene diisocyanate; and terminal
isocyanate-containing compounds which are obtained by reacting the
above polyisocyanate compounds with a low molecular active
hydrogen-containing organic compound such as ethylene glycol,
propylene glycol, diethylene glycol, trimethylol propane,
hydrogenated bisphenol A, hexanetriol, glycerine, pentaerythritol,
castor oil and triethanolamine; and the like.
[0012] On the other hand, the terminal primary OH-containing
propylene glycol monoalkyl ether is a compound expressed by
RO(CH(CH.sub.3)CH.sub.2- .sub.2O).sub.nH. In the formula, R is an
alkyl group having 1 to 8 carbons, which may be branched. Specific
examples of alkyl groups include methyl groups, ethyl groups,
n-propyl groups, isopropyl groups, n-butyl groups, isobutyl groups,
t-butyl groups, amyl groups, hexyl groups, octyl groups and
2-ethylhexyl groups. The number n is 1 to 3, but does not have to
be an integer. A preferable formula for the propylene glycol
monoalkyl ether has R as an n-butyl group and n being a number
between 1 and 2.
[0013] The reaction between the polyisocyanate compound and the
terminal primary OH-containing propylene glycol monoalkyl ether can
be conducted using a well-known method. For example, the
polyisocyanate compound is dissolved in a solvent which does not
contain active hydrogen, then adding thereto a terminal primary
OH-containing propylene glycol monoalkyl ether in an amount
corresponding to the NCO equivalent in the polyisocyanate compound,
in the presence of a urethanizing catalyst such as a tin compound,
then heating the mixture and causing the reaction to occur. The
reaction can be confirmed as having finished when the isocyanate
group absorption spectrum has disappeared in an IR absorption
spectrum.
[0014] The cationic group-containing epoxy modified base resin,
which is another component contained in the cationic
electrodeposition coating composition of the present invention, is
manufactured by opening the epoxy rings in the starting material
epoxy resin by bringing about a reaction with a mixture of a
primary amine, secondary amine, tertiary amine acid salt or other
amine, a sulfide and an acid. The term "cationic group" in the
present specification shall refer to a group which is cationic in
itself or a group rendered cationic by an addition of an acid. A
typical example of the starting raw material resin is a polyphenol
polyglycidyl ether epoxy resin formed from a reaction between
bisphenol A, bisphenol F, bisphenol S, phenol novolac, cresol
novolac or other polycyclic phenol compound and epichlorohydrin.
Another example of the starting raw material resin is an
oxazolidone ring-containing epoxy resin as taught in Japanese
Patent Application Laid-open No. 5-306327. This epoxy resin is
obtained by a reaction between a diisocyanate compound or a
bisurethane compound obtained by blocking the NCO groups in a
diisocyanate compound with methanol, ethanol or other lower
alcohol, and epoxy groups.
[0015] The epoxy resin which is the starting raw material can be
used after employing a bifunctional polyester polyol, polyether
polyol, bisphenol or dibasic carboxylic acid for chain extension,
prior to the epoxy ring-opening reaction brought about by the amine
or sulfide. Similarly, in order to adjust the molecular weight or
amine equivalent, or to improve the heat flow property, some epoxy
rings of the epoxy resin may be reacted with 2-ethyl hexanol, nonyl
phenol, ethylene glycol mono-2-ethyl hexyl ether, propylene glycol
mono-2-ethyl hexyl ether or other monohydroxy compound, prior to
the epoxy ring-opening reaction.
[0016] Examples of amines which can be used when opening the epoxy
rings and introducing the amino groups include butylamine,
octylamine, diethylamine, dibutylamine, methylbutylamine,
monoethanolamine, diethanolamine, N-methylethanolamine,
triethylamine acid salt, and N,N-dimethylethanolamine acid salt or
other primary amine, secondary amine or tertiary amine acid salt. A
ketimine blocked primary amino group-containing secondary amine
such as amino ethyl ethanol amine methyl isobutyl ketimine may also
be used. It is necessary for at least an equivalent amount of these
amines to be reacted with the epoxy rings in order to open all of
the epoxy rings.
[0017] Examples of sulfides include diethyl sulfide, dipropyl
sulfide, dibutyl sulfide, dihexyl sulfide, diphenyl sulfide, ethyl
phenyl sulfide, tetramethylene sulfide, pentamethylene sulfide,
thiodiethanol, thiodipropanol, thiodibutanol,
1-(2-hydroxyethylthio)-2-propanol,
1-(2-hydroxyethylthio)-2-butanol, and
1-(2-hydroxyethylthio)-3-butoxy-1-p- ropanol. Examples of acids
include formic acid, acetic acid, lactic acid, propionic acid,
boric acid, butyric acid, dimethylolpropionic acid, hydrochloric
acid, sulphuric acid, phosphoric acid, N-acetylglycine,
N-acetyl-.beta.-alanine and others.
[0018] If the starting material epoxy resin contains a hydroxyl
group, then a self-crosslinkable epoxy modified based resin can be
obtained by an addition reaction between the hydroxyl group and an
isocyanate which has been half-blocked with the terminal primary
OH-containing propylene glycol monoalkyl ether. The half-blocked
isocyanate can be obtained by using the propylene glycol monoalkyl
ether in an amount which corresponds to half of the NCO equivalent
in the polyisocyanate compound in the manufacture of the blocked
isocyanate curing agent.
[0019] It is preferable that a number average molecular weight of
the cationic group-containing epoxy modified base resin is in the
range of 600 to 4,000. A number average molecular weight of less
than 600 decreases solvent resistance, corrosion resistance and
other properties in the resulting coating film. Conversely, a
number average molecular weight in excess of 4,000 not only makes
the synthesis process difficult owing to the limited control over
the resin solution viscosity, but also makes difficult a handling
of the resulting resin during such procedures as emulsification
dispersion. In addiction, since it has a high viscosity, the flow
property during heating and curing would be adversely affected,
which leads to markedly worse external appearance of the coating
film. It is preferable that an amino value or sulfonium value of
the cationic group-containing epoxy modified base resin is 30 to
150, and more preferably 45 to 120. Should the amino or sulfonium
value fall below 30, it is more difficult for a stable emulsion to
be obtained, while if the values exceed 150, drawbacks arise with
Coulomb efficiency, redissolution and other electrodeposition
coating-related operational considerations.
[0020] In the cationic electrodeposition coating composition of the
present invention, it is preferred that a solid content weight
ratio of the cationic group-containing epoxy modified base
resin/the blocked isocyanate curing agent is 50/50 to 90/10, and
more preferably 60/40 to 80/20. If the ratio falls outside these
ranges, curing ability may be adversely affected.
[0021] The cationic electrodeposition coating composition of the
present invention further contains a neutralizing acid in order to
disperse the components in an aqueous medium. Examples of the
neutralizing acids include formic acid, acetic acid, lactic acid,
propionic acid, boric acid, butyric acid, dimethylolpropionic acid,
hydrochloric acid, sulphuric acid, phosphoric acid,
N-acetylglycine, N-acetyl-.beta.-alanine and others. An amount of
acid can vary with the amino group or sulfonium group content in
the cationic electrodeposition coating composition, but it is
preferable for the amount thereof to be sufficient to allow water
dispersion.
[0022] The cationic electrodeposition coating composition of the
present invention may additionally contain a pigment and a pigment
dispersing resin. There is no particular limitation on the pigment,
as long as it is a known pigment. Examples of the pigments include
coloring pigment, such as titanium dioxide, carbon black and red
iron oxide; extender pigment, such as kaolin, talc, aluminum
silicate, calcium carbonate, mica, clay, and silica; corrosion
resistant pigment, such as zinc phosphate, iron phosphate, aluminum
phosphate, calcium phosphate, zinc phosphite, zinc cyanide, zinc
oxide, aluminum tripolyphosphate, zinc molybdate, aluminum
molybdate, calcium molybdate and aluminum phosphomolybdate. A
cationic or non-ionic low molecular weight surfactant and modified
epoxy resins which generally contain quaternary ammonium groups
and/or tertiary sulfonium groups can be used as the pigment
dispersing resin.
[0023] The pigment dispersing resin and pigment are mixed in a
prescribed amount by using a ball mill, sand grinding mill or other
known dispersing device until predetermined particle sizes have
attained uniformly to obtain a paste in which the pigment has been
dispersed. The pigment-dispersed paste can be used as long as the
pigment in the cationic electrodeposition coating composition
constitutes 0-50 wt % of the solid content.
[0024] The cationic electrodeposition coating composition of the
present invention can be prepared by adding a neutralizing acid to
a mixture of an epoxy modified base resin having a cationic group
and a blocked isocyanate curing agent to disperse in an aqueous
medium, and then adding a pigment dispersed paste thereto. An
additive, such as surfactant, antioxidant, UV absorbing agent,
curing accelerator may be added to the system as needed, at the
desired stages.
[0025] In the present invention, the cationic electrodeposition
coating composition is coated on an article. The article can be one
that is subjected to electrodeposition. The cationic
electrodeposition coating can be performed according to a known
method. Typically, the cationic electrodeposition coating
composition is diluted with deionized water to a solid content of 5
to 40 wt % and preferably 15 to 25 wt %, to form an
electrodeposition bath containing the cationic electrodeposition
coating composition having a pH range of 5.5 to 8.5.
Electrodeposition can be conducted at a temperature of 20 to
35.degree. C. and a voltage of 100 to 450 V.
[0026] A thickness of a film produced by electrodeposition coating
can preferably be 5 to 40 .mu.m when dried, and more preferably 10
to 30 .mu.m. It is preferable to control conditions for
electrodeposition coating so as to obtain the above mentioned
thickness range. It is appropriate for the coating film to be baked
at 100 to 220.degree. C., and preferably at 140 to 200.degree. C.
for 10 to 30 minutes.
[0027] The electrocoated article may be further coated with an
intermediate coat or a top coat. The intermediate coat and top coat
can be applied by art known methods from paint and coating
conditions as used for a surface of automobiles.
EXAMPLES
[0028] "Parts" as referred to in the following shall denote "weight
parts".
Manufacturing Example 1
Manufacturing the Cationic Group-Containing Epoxy Modified Base
Resin
[0029] In to a flask equipped with a stirrer, a cooling tube, a
nitrogen introduction tube, a thermometer, and a dropping funnel,
92 Parts of 2,4-/2,6-tolylene diisocyanate (weight ratio=8/2), 95
parts of methyl isobutyl ketone ("MIBK" below) and 0.5 parts of
dibutyltin laurate were introduced, to which 21 parts of methanol
was added dropwise under stirring. The reaction temperature began
at room temperature and then increased to 60.degree. C. by
generation of heat. The reaction was then continued for 30 minutes,
to which 57 parts of ethylene glycol mono-2-ethyl hexyl ether was
added dropwise via a dropping funnel. Further, 42 parts of 5-mol
bisphenol A-propylene oxide adduct was added to the reaction
mixture. The reaction was primarily conducted at 60 to 65.degree.
C. and was continued until IR spectrographic assessment revealed
the absorption due to the isocyanate groups had disappeared.
[0030] Next, 365 parts of epoxy resin having an epoxy equivalent of
188, which had been synthesised from bisphenol A and
epichlorohydrin using a known method, was added in to the blocked
isocyanate so obtained, and the temperature was raised to
125.degree. C. Then, 1.0 part of benzyldimethylamine was added
thereto and the reaction carried out at 130.degree. C. until an
epoxy equivalent of 410 parts of had been attained. Thereafter, 87
parts of bisphenol A was added and the reaction carried out at
120.degree. C., whereupon an epoxy equivalent of 1190 had been
attained. The reaction mixture was then cooled, after which 11
parts of diethanol amine, 24 parts of N-ethyl ethanol amine and 25
parts of ketimined aminoethylethanolamine (79 wt % MIBK solution)
were added thereto and the reaction carried out at 110.degree. C.
for two hours. Diluting the mixture with MIBK to bring a
non-volatile content to 80% resulted in a cationic group-containing
epoxy modified base resin having a glass transition point of
22.degree. C.
Manufacturing Example 2
Manufacturing the Pigment-Dispersed Paste
[0031] Into a flask fitted with a stirrer, a cooling tube, a
nitrogen introduction tube, a thermometer and a dropping funnel,
222.0 parts of isophorone diisocyanate ("IPDI" below) was
introduced and diluted with 39.1 parts of MIBK, to which 0.2 parts
of dibutyltin dilaurate was added. The temperature of the mixture
was raised to 50.degree. C., to which 131.5 parts of 2-ethyl
hexanol was added dropwise over two hours in a dry nitrogen
atmosphere under stirring. By cooling the mixture appropriately,
the reaction temperature was maintained at 50.degree. C. 2-ethyl
hexanol-half-blocked IPDI was thereby obtained (90.0% solid resin
content).
[0032] Next, 87.2 parts of dimethylethanolamine, 117.6 parts of 75%
aqueous lactic acid solution and 39.2 parts of ethylene glycol
monobutyl ether were successively added into a suitable reaction
vessel and stirred together for approximately 30 minutes at
65.degree. C. to yield a quaternising agent.
[0033] Next, 710.0 parts of Epon 829 (bisphenol A-type epoxy resin;
epoxy equivalent: 193 to 203; Shell Chemical Company) and 289.6
bisphenol A were introduced into a suitable reaction vessel and
heated in a nitrogen atmosphere at 150 to 160.degree. C. to cause
an initial exothermic reaction. The reaction was carried out in the
reaction mixture for approximately one hour at 150 to 160.degree.
C., after which the reaction mixture was cooled to 120.degree. C.,
to which 498.8 parts of the 2-ethyl hexanol half-blocked IPDI (MIBK
solution) preliminary prepared was added.
[0034] The reaction mixture was maintained at 110 to 120.degree. C.
for approximately 1 hour, to which 1390.2 parts of ethylene glycol
monobutyl ether was added. The mixture was then cooled to 85 to
95.degree. C. and once it had achieved a uniform state, 196.7 parts
of the quaternising agent prepared above was added thereto. The
reaction mixture was kept at 85 to 95.degree. C. until the acid
value had reached 1, and then 37.0 parts of deionized water was
added thereto, and once the quaternarization in the epoxy-bisphenol
A resin had been stopped, a pigment dispersing resin varnish which
contained a quaternary ammonium salt moiety was obtained (50% solid
resin content).
[0035] 60.0 Parts of pigment dispersing resin varnish (epoxy-based
quaternary ammonium salt pigment dispersing resin) in a solid
state, 2.0 parts of carbon black, 100.0 parts of kaolin, 80.0 parts
of titanium dioxide, 18.0 parts of aluminum phosphomolybdate and
deionized water in an amount sufficient to bring the pigment paste
solid content to 48% were introduced into a sand grinding mill and
allowed to disperse therein until the particle sizes were no
greater than 10 .mu.m. A pigment-dispersed paste was thereby
obtained.
Manufacturing Example 3
Manufacturing the Blocking Agent Which Has Been Blocked With a
Terminal Primary OH-Containing Propylene Glycol Monoalkyl Ether
[0036] Into a reaction vessel, 1,000 parts of diphenylmethane
diisocyanate and 288.5 parts of MIBK were introduced, and heated to
80.degree. C., followed by adding 1.0 part of dibutyltin dilaurate
thereto. Next, 1,596.8 parts of NBP-10 (Sanyo Kasei Co., Ltd.;
terminal primary OH-containing propylene glycol monoalkyl ether)
was added dropwise thereto over two hours at 80.degree. C. The
mixture was heated for one hour at 100.degree. C., and it was
confirmed that absorption due to the isocyanate groups had
disappeared. The mixture was cooled to yield a blocked isocyanate
curing agent.
Manufacturing Example 4
Manufacturing the Blocked Isocyanate Curing Agent
[0037] Into a reaction vessel, 1,250 parts of diphenylmethane
diisocyanate and 266.4 parts of MIBK were introduced and were
heated to 80.degree. C., to which 2.5 parts of dibutyltin dilaurate
was added. A solution containing 226 parts of .epsilon.-caprolactam
dissolved in 944 parts of butyl cellosolve was then added dropwise
thereto at 80.degree. C. for 2 hours. After heating the mixture for
a further 4 hours at 100.degree. C., IR spectrographic assessment
was used to confirm that absorption due to the isocyanate groups
had disappeared. The mixture was allowed to cool, after which 336.1
parts of MIBK was added thereto to yield a blocked isocyanate
curing agent.
Example
Manufacturing the Cationic Electrodeposition Coating
Composition
[0038] The cationic group-containing epoxy modified base resin
obtained in Manufacturing Example 1 and the blocked isocyanate
curing agent obtained in Manufacturing Example 3 were mixed
together until uniform, with a solid content ratio of 70/30. Next,
ethylene glycol-2-ethyl hexyl ether was added so as to obtain a
solid content of 3 wt %. Glacial acetic acid was added thereto
until a 45% neutralization had been attained, and the mixture was
further diluted by the gradual addition of deionized water. An
emulsion with a solid content of 36% was obtained by removing the
MIBK under a reduced pressure.
[0039] 1,697 Parts of emulsion, 393.9 parts of pigment-dispersed
paste as obtained in Manufacturing Example 3, 1,889.6 parts of
ion-exchange water, and 19.5 parts of dibutyltin oxide were mixed
together, resulting in a cationic electrodeposition coating
composition having a 20 wt % solid content. The ratio of pigment to
solid resin content in the cationic electrodeposition coating
composition was 1/4.5.
Comparative Example
[0040] A cationic electrodeposition coating composition was
obtained as described in Example, with the exception that the
blocked isocyanate curing agent obtained in Manufacturing Example 4
was used instead of the blocked isocyanate curing agent obtained in
Manufacturing Example 3.
[0041] The cationic electrodeposition coating compositions obtained
in the Example and Comparative Example were baked to form cationic
electrodeposition coating films, which were subjected to the
evaluation tests below. The results of the tests are displayed in
Table 1.
[0042] Corrosion Resistance During Immersion in Salt Water
[0043] The cationic electrodeposition coating compositions were
electrodeposited on zinc phosphate-treated plates made from cold
rolled steel such that the dried films were 20 .mu.m thick.
Cationic electrodeposition coating films obtained by baking these
for 25 minutes at 170.degree. C. were immersed for 240 hours at
55.degree. C. in 5% salt water, and thereafter a portion of each
was cut and peeled off as a tape. The peeling widths of both sides
of the cut portions were assessed against the following
criteria:
1 .largecircle.: <3 mm .DELTA.: 3 to 6 mm "X" mark: >6 mm
[0044] Gel Content
[0045] Electrodeposition coating compositions were electrodeposited
on tin plates which had been weighed beforehand to a thickness of
20 .mu.m, and these were baked under prescribed conditions. The
resulting coated plates were weighed, then immersed in acetone for
6 hours under reflux, and then dried for 20 minutes at 105 to
110.degree. C. After drying, a weight of the coated plates was
measured. The gel content was calculated according to the formula
below:
Gel content (%)=(W2-W0)/(W1-W0).times.100
[0046] where:
2 W0: a weight of tin plate W1: a weight of coated plates after
baking W2: a weight of coated plates after immersion in
acetone.
[0047]
3 TABLE 1 Working Comparative Example Example Corrosion resistance
during .largecircle. .largecircle. immersion in salt water (mm) Gel
Baking 160.degree. C. .times. 20 min 95 93 content conditions
170.degree. C. .times. 20 min 93 94 (%) 180.degree. C. .times. 20
min 91 92
[0048] The above results confirm that blocked isocyanate curing
agents which are obtained through a reaction with a terminal
primary OH-containing propylene glycol monoalkyl ether as a
blocking agent exhibit performance which is not inferior to
conventional curing agents.
[0049] Since the cationic electrodeposition coating composition of
the present invention uses a terminal primary OH-containing
propylene glycol monoalkyl ether as a blocking agent in a blocked
isocyanate curing agent, coating films can be obtained in which
curing reactivity is improved, and which possess similar properties
to conventional films.
[0050] Moreover, the blocking agent which is dispersed in the
cationic electrodeposition coating composition of the present
invention is not recognised as an HAPs, and adverse effects on the
environment can therefore be minimised.
* * * * *