Note: Descriptions are shown in the official language in which they were submitted.
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DESCRIPTION
METHOD OF FORMING AN ELECTROCOATING FILM, ELECTROCOATING
FILM, AND ELECTRODEPOSITED ARTICLE
TECHNICAL FIELD
The present invention relates to a method of forming
an electroCOating film, an electrocoating film, and an
electrodeposited article.
BACKGROUND TECHNOLOGY
Recently attempts have been made to form an
insulating coating film from a coating on a metallic
surface.
Regarding the substrate of a mufti-layer printed
wiring board, for instance, a metallic substrate formed
with an epoxy resin insulating layer on which an electric
circuit is formed by electroless plating for improved heat
dissipation, called a metal core PWB, has so far been
developed (e.g. Itoh, Kinji: Introduction to Manufacture of
Printed Wiring Boards for Certification of Printed Wiring
Board Engineers, 1st Edition, July 6, 2001, pp.67-69).
However, the recent trend toward higher-integration
and higher-density substrate boards presents with the
problem of leaks between the metal substrate and the
circuit due to the minute pinkoles existing in the
insulating layer and, therefore, this insulating layer is
required to have high dielectric properties.
Electrocoating compositions have excellent throwing
powers and yield comparatively even coats regardless of the
shape of work as well as sufficient dielectric properties
so that attempts have been made to form insulating layers
using electrocoating compositions. Although such attempts
are rewarded with several advantages of electrocoating,
this electroCOating film is usually not so flat and smooth
as desired and tends to develop pinkoles owing to evolution
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of hydrogen gas in the electodepositing process, thus
making it difficult to implement a high-level dielectric
property using an electroCOating Composition.
SUWAR'.L ~F THE PRESENT INVENTION
The present invention has for its object to provide a
method of forming an electrocoating film having an
excellent smoothness and exceptionally high dielectric
properties with a drastically reduced incidence of pinhole
1~ formation, such. an electrocoating film, and such an
electrodeposited article.
The present invention is first directed to a method
of forming an electrocoating film comprising coating a work
with an electrocoating composition curable by heating and
irradiation with an activation energy beam
in which an eleCtrodepositing step, an aqueous
cleaning step, a pre-baking step, an activation energy beam
irradiation step, and a post-baking step are serially
carried out in the order mentioned.
2~ The activation energy beam irradiation step may be
carried out directly following said pre-baking step without
cooling the work.
The heating in the above post-baking step may be
continuous from the pre-baking step.
Preferably, the electrocoating composition comprises
a resin composition containing sulfonium and propargyl
groups.
Preferably, the electrocoating composition is a
cationic electrocoating composition.
The present invention is further directed to an
electrocoating film
which is formed by the above method of forming an
electrocoating film.
In addition, the present invention is directed to an
electrodeposited article having the electroCOating film.
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The present invention is further directed to a method
of forming a multilayer film
in which the above electroCOating film is further
coated with an overcoat.
The present invention is further directed to a
multilayer film
which is formed by the method of forming a multilayer
film.
Furthermore, the present invention is directed to an
article having the multilayer film.
DISCLOSURE OF INVENTION
The method of forming an electroCOating film
according to the present invention is a method of forming
an electrocoating film which comprises coating a work with
an electroCOating composition curable by heating and
irradiation with an activation energy beam, and comprises
an electrodepositing step, an aqueous cleaning step, a pre-
baking step, an activation energy beam irradiation step,
and a post-baking step.
The electrocoating composition mentioned above
contains a binder component curable by heating and
irradiation with an activation energy beam in addition to
,the ionic groups necessary for electrodeposition. The
binder component may be a component having a functional
group curable by heating and irradiation with an activation
energy beam or one having both a heat-curable functional
group and an activation energy beam-curable functional
group, and whichever of these can be employed.
The Curability by irradiation with an activation
energy beam, as referred to above, includes not only the
mode in which a Curing reaction is directly caused by an
activation energy beam itself but also the mode in which a
curing reaction is caused by the active species generated
by an activation energy beam. The activation energy beam-
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curable functional group may include an unsaturated bond,
such as a double bond or a triple bond, a combination of
the above unsaturated bond and a thiol group, an epoxy
group, a maleimide group, an oxetane group, an alkoxysilyl
group and so on, but in consideration of the stability of
coexistence with other functional groups, either an
unsaturated bond or a combination of the above unsaturated
bond and a thiol group is preferred. Moreover, said
unsaturated bond is not only curable by irradiation with an
activation energy beam but may be converted to a heat-
curable functional group by formulating a heat-sensitive
radical initiator, such as an dialkyl peroxide,
peroxycarboxylic acid, peroxycarbonate, peroxy-ester,
hydroperoxide, ketone peroxide, azodinitrile or
benzopinacol silyl ether, in the coating.
Further, as the heat-curable functional group, those
functional groups which are well known in the coating art
can be utilized. The functional group mentioned above is
not particularly restricted unless it directly reacts with
said ionic group necessary for electrodeposition or said
activation energy beam-curable functional group or
interferes with the electodepositing process or the curing
by an activation energy beam. Specifically, the preferred
one is hydroxyl group. As regards this functional group
capable of curing on exposure to heat, a curing agent
serving as a partner in the curing reaction is usually
incorporated in the coating composition. In the case where
the functional group capable of curing on exposure to heat
is hydroxyl, the curing agent which can be used may be any
of those well known in the art, such as an optionally
blocked polyisocyanate or melamine resin, for instance.
For example, Japanese Kokai Publication Hei-05-263026
discloses a UV-curable cationic elecrocoating composition
comprising 10 to 70 parts lay weight of a polyfunctional
acrylate having three or more acryloyl groups within the
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molecule and 30 to 90 parts by weight of a resin suitable
for cationic electrodeposition and having an average
molecular weight of 2000 to 30000 as active components.
Such electrocoating compositions containing a binder
5 component having an unsaturated bond as an activation
energy beam-curable functional group are known and,
therefore, by incorporating a heat-sensitive radical
initiator in such. a known coating, there can be provided an
electrocoating composition for use in the method of forming
an electrocoating film according to the present invention.
Furthermore, it is not difficult for those skilled in the
art to provide an electrocoating composition for use in the
method of forming an electrocoating film by introducing a
heat-curable functional group into a binder component
having said unsaturated bond as an activation energy beam-
curable functional group and selecting a curing agent
compatible therewith.
Furthermore, an aqueous dispersion comprising a
cathionic group-containing polyurethane (meth)acrylate
having an ethylenically unsaturated terminal (meth)acryloyl
double bond and a reactive diluent having at least two
ethylenically unsaturated (meth)acryloyl double bonds as a
binder component and a light-sensitive radical initiator
and/or a heat-sensitive radical initiator, which is
disclosed in Japanese Kohyo-Publication 2002-531676, can be
used as the electrocoating composition for use in the
method of forming an electrocoating film according to the
present invention. Here, the (meth)acryloyl double bond in
said aqueous dispersion has a bromine value equal to 20 to
150 g bromine/100 g solids and the terminal (meth)acryloyl
double bond of ethylenic unsaturation from said
polyurethane (meth)acrylate is bound to a cationic group-
containing polyurethane prepolymer through a urethane, urea,
amide or ester group.
Furthermore, a cationic electrocoating composition
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containing sulfonium and propargyl groups is disclosed in
WO 98/03595. This electrocoating composition, too, can be
used in the method of forming an electrocoating film
according to the present invention. The sulfonium group
content of this electrocoating composition per 100 g of
resin solids is 5 to 400 millimoles and the propargyl group
content on the same basis is 10 to 495 millimoles, the
total content of sulfonium and propargyl groups being not
greater than 500 millimoles. Furthermore, a Component
containing a double bond as unsaturation in addition to
propargyl group may also be used in combination.
Incidentally, it is known that said cationic electrocoating
composition containing sulfonium and propargyl groups
undergoes curing when heated even in the absence of the
heat-sensitive radical initiator. Moreover, because it
contains the propargyl group being an unsaturated bond, the
coating can be cured by irradiation with an activation
energy beam.
The electrocoating composition for use in the method
of forming an electrocoating film according to the present
invention may be whichever of an anionic electrocoating
composition and a cationic electrocoating composition but
is preferably a cationic electrocoating composition in view
of the release of ions from the work.
From the standpoint of dielectric property of the
formed electrocoating film, the electrocoating composition
for use in the method of forming an electrocoating film
according to the present invention is preferably said
cationic electrocoating composition containing sulfonium
and propargyl groups. As such cationic electroCOating
composition, there can be mentioned Insuleed Series
(electrolytically active electrocoatings, manufactured by
Nippon Paint Co.)
Where necessary, the electroCOating composition for
use in the method of forming an electroCOating film
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according to the present invention may contain a light-
sensitive radical initiator. As the light-sensitive
radical initiator which can be comprised in said
electrocoating composition, various compounds which are
well known to those skilled in the art can be employed.
Among such substances are benzoin compounds such as benzoin,
benzoin isopropyl ether, benzoin isobutyl ether, and so ona
benzophenone compounds such as benzophenone, 4,4'-
bis(dimethylamino)benzophenone (Michler's ketone), and so
on~ xanthone compounds such as xanthone, thioxanthone, and
so on; acetophenone compounds such as 2-phenyl-2-
hydroxyacetophenone, a,cx-dichloro-4-phenoxyacetophenone, 1-
hydroxycyclohexyl phenyl ketone, 2,2-diethoxyacetophenone,
2-hydroxy-2-methyl-1-phenylpropan-1-one, 2-hydroxy-2-
methylpropiophenone, 2,2-dimethoxy-2-phenylacetophenone
(benzyl dimethyl ketal), and so on; ethyl 4-
dimethylaminobenzoate, 4,4'-diazidostilbene-2,2'-disulfonic
acid, 1-phenyl-1,2-propanedione-2-(o-ethoxycarbonyl)oxime,
and so forth. These light-sensitive radical initiators are
usually formulated in a proportion of 0.1 to 10 o by weight
based on the resin solids of the coating composition.
The electrocoating composition for use in the method
of forming an electrocoating film according to the present
invention may contain those pigments and additives which
are conventionally formulated in electrocoating
compositions.
The work for use in the method of forming an
electrocoating film according to the present invention is
not particularly restricted provided that the area to be
coated is electrically conductive. There is no restriction
on the shape of the work.
The electrodepositing step in the method of forming
an electrocoating film according t~ the present invention
comprises electrodepositing an electrically conductive work
with the above-described electrocoating composition to give
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an uncured electrodeposited coat. The relevant
electrodepositing conditions such as electrodepositing bath
temperature, electrodepositing voltage and current time are
established according to the particular electrocoating
composition to be used but are preferably so set as to
yield a dry film thickness of 5 to 30 um.
The aqueous cleaning step in the method of forming an
electrocoating film according to the present invention,
which is carried out on completion of said
electrodepositing step, comprises washing off the
superfluous electrocoating composition remaining on the
work and electrodeposited coat. The solvent for use in
this aqueous cleaning step is preferably deionized water
from the standpoint of dielectric property of the resulting
electrocoating film but there may optionally be used a two-
stage method which comprises prewashing with a mixture
solvent of deionized water and a water-soluble carboxylic
acid compound, such as acetic acid, lactic acid or the like,
and postwashing with deionized water. The residual
electrocoating composition can be thereby removed more
effectively.
The mode of the above aqueous cleaning is not
particularly restricted but any of those techniques which
are well known in the art, such as dipping and spraying,
can be employed. The cleaning time is not particularly
restricted but may, for example, be 30 seconds to 2 minutes.
The pre-baking step which is carried out on
completion of said aqueous cleaning step in the method of
forming an electrocoating film according to the present
invention is not intended to cure the whole of said
electrodeposited coat but intended to melt and let flow the
uncured electrodeposited coat once formed to thereby
eliminate film defects such as pinholes for improved
evenness and surface smoothness of the coat. Therefore,
even in the case where the heating is further continued,
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this step is regarded as having been completed at the time-
point where the electrodeposited coat has been molten and
let flow. The melting and letting-flow of the uncured
electrodeposited coat in the above manner is advantageous
in that an improvement in the smoothness of the coat as
well as elimination of pinholes in the coat can be
accomplished to insure a high degree of dielectric property.
Furthermore, improving the smoothness of the coat in
this way enables the use of the obtained film in an
electric circuit utilizing high-frequency signal. This is
because a reduced coarseness of the coat leads to a
reduction in the disturbance of the magnetic field by a
current so that the product can be made adaptable to high-
frequency current applications which are apt to be
influenced by a magnetic field. Moreover, the improved
surface smoothness can reduces the risk for disconnection
of a conductor locating on an electrodeposited article and
enables fine-lined conversion of the electrodeposited
article on its patterning as well.
The heating temperature for use in the above pre-
baking step should, of necessity, be not lower than the
temperature causing the electrodeposited coat to melt and
it can be judiciously selected according to the kind of
electrocoating composition used but is generally within the
range of 60 to 130°C. If it is below 60°C, the flow of the
uncured electrodeposited coat on melting tends to be
insufficient so that pinholes may remain after this step
and the evenness or the surface smoothness of the coat may
be insufficient. On the other hand, if the temperature
exceeds 130°C, the uncured electrodeposited coat may begin
to cure before the coat has flown sufficiently, with the
consequence that, after this step, pinholes may remain, the
evenness of the coat be inadequate, or the surface
smoothness of the coat be insufficient. The preferred
range is 70 to 110°C. The heating time in said pre-baking
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step is not particularly restricted but from coat
meltability and industrial points of view, it may for
example be 2 to 30 minutes.
The activation energy beam irradiation step in the
5 method of forming an electrocoating film according to the
present invention, which precedes the post-baking step, is
intended to fix the coat surface which has been improved in
evenness and smoothness by said pre-baking step. By
carrying out this step, the reflow of the electrodeposited
10 coat is inhibited so that even when the electrodeposited
coat is further heated and caused to cure thoroughly, the
ultimate electrocoating film can be obtained with the high
surface smoothness achieved in this step being successfully
retained.
The activation energy beam which can be used includes
ultraviolet light, X-rays, an electron beam, near-infrared
light and visible light. The activation energy beam in the
context of the present invention does not include heat-
generating energy beams such as infrared light, high-
frequency waves and microwaves. However, although near-
infrared light is a heat-generating energy beam, it is
included in said activation energy beam because there exist
light-sensitive radical initiators showing initiating
functions in this wavelength region.
For irradiation with ultraviolet light, there can be
used a variety of light sources, such as a mercury arc lamp,
a xenon arc lamp, a fluorescent lamp, a carbon arc lamp, a
tungsten-halogen copier lamp, and so forth. On the other
hand, as emission sources of an electron beam, there can be
used electron beam generators such as Cockcroft type,
Cockcroft-Walton type, van de Graaff type, resonance
transformer type, transformer type, insulating core
transformer type, Dynamitron type, linear filament type,
high-frequency type, and other devices. It should be
understood that when an electron beam is used, it is not
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always necessary to use a light-sensitive radical initiator.
The conditions of said irradiation with an activation
energy beam in this step vary with the amount of
unsaturation in the resin and the molecular weight of the
resin in the electroCOating composition used but taking
ultraviolet light as an example of said activation energy
beam, its wavelength range may be 200 to 500 nm and the
integrating radiation dosage may for example be 100 to
10000 mJ/Cm2 when the coating used contains a light-
sensitive radical initiator or 1000 to 20000 mJ/Cm2 when
the coating does not contain a light-sensitive radical
initiator. When the integrating radiation dosage is
insufficient, fixing of the electrodeposited coat surface
is inadequate so that the ultimate electrocoating film
tends to be deficient in smoothness. An excessive
integrating radiation dosage would not cause any serious
trouble but lead to a waste of energy. On the other hand,
when an electron beam is used, it is advantageous to carry
out the irradiation using an electron beam generator with
an output energy of 50 to 500 ke~T for a predetermined time.
It is also recommendable to adjust the distance
between the electrodeposited coat and the light source
according to the shape of the work so as to provide for a
uniform irradiation with the activation energy beam and
bring said integrating radiation dosage and energy into a
predetermined range.
The above activation energy beam irradiation step may
be carried out directly following said pre-baking step
without cooling the work between the steps.
The post-baking step in the method of forming an
electroCOating film according to the present invention is
intended to thermally cure the electrodeposited coat having
a fixed surface following the activation energy beam
irradiation step. By carrying out this step, the entirety
of the electrodeposited Coat inclusive of its interior Can
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be caused to cure thoroughly. Although the
electrodeposited coat undergoes remelting to flow in this
step, the coat surface which has been hardened in the
preceding step does not melt so that all ultimately cured
electrocoating film can be produced with the high surface
smoothness obtained in the preceding step being fully
retained.
The heating conditions for use in this post-baking
step are not particularly restricted provided that the
electrocoating film having a fixed surface obtained after
said pre-baking step and subsequent activation energy beam
irradiation step can be thoroughly cured down to its
interior. The above heating conditions can be
appropriately established according to the kind of
electrocoating composition used and, for example, the
heating temperature may be 130 to X60°C. The heating time
in this post-baking step is not particularly restricted,
either, and may for example be 10 to 30 minutes.
The heating in this post-baking step may be a
continuation of the heating in said pre-baking step.
The method of forming an electrocoating film
according to the present invention is applicable to
electrically conductive substrates and can be used to form
electrocoating films on various metallic materials such as
copper, iron, galvanized steel sheet, aluminum and so forth.
The electrocoating film of the present invention is
formed by the above-described method of forming an
electrocoating film, and having such. a film, the
electrodeposited article of the present invention features
very satisfactory dielectric properties and surface
smoothness.
The method of forming a multilayer film according to
the present invention may comprise applying an overcoat in
superimposition on the cured electrocoating film described
above. The overcoat is intended to protect and impart an
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attractive appearance to said electrocoating film or adding
new functions to the multilayer film. The overcoat
mentioned just above is not particularly restricted but
includes, for example, those materials which can be caused
to undergo a curing reaction by heating and/or irradiation
with an activation energy beam. Among specific examples
are those binders, among the binder components mentioned in
the foregoing description of the electrocoating composition
of the present invention, which can be cured by heating
1~ and/or irradiation with an activation energy beam. In the
case of a non-aqueous one, the binder need not have an
ionic group. As the skeleton of the binder component,
there can be used an acrylic resin, polyester resin, epoxy
resin, urethane resin, or the like and where flexibility is
further required, these resins may have been modified by
introducing a butadiene skeleton, siloxane skeleton or
long-chain aliphatic skeleton.
In the case where the overcoat is heat-curable but
cannot be cured in the presence of said binder component
alone, a curing agent suited to the kind of said reactive
functional group can be used as an auxiliary binder
component. The curing agent mentioned just above includes
amino resins and optionally blocked polyisocyanates.
In addition to said binder component or components,
the above overcoat may contain a pigment, a resin particle,
and various additives. For imparting electrical
conductivity as a new function to the multilayer film, the
above overcoat may be supplemented with a metal particle,
carbon, metal oxide, and/or the like. For imparting high
3~ dielectric properties, glass fiber, ceramics, and so on may
be formulated.
The coating method is not particularly restricted but
includes the techniques well known to those skilled in the
art, such as bar coating, die coating, spray coating,
rotary atomizer coating, spin coating, and so forth. By
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curing the resulting film by heating and/or irradiation
with an activation energy beam, an overcoat can be
constructed. The heating conditions and the conditions of
activation energy beam irradiation can be judiciously
established according to the kind of overcoat used. The
cured thickness of the above overcoat is not particularly
restricted but may, for example, be 10 to 100 lam.
The application of the overcoat may be carried out in
a few divided cycles.
The multilayer film of the present invention is the
film obtained by the above-described method of forming a
multilayer film, and the article according to this
invention has said multilayer film and, therefore, features
a high degree of smoothness.
BEST MODE FOR CARRYING OUT THE PRESENT INVENTION
The following examples are intended to illustrate the
present invention in further detail without defining the
scope of the present invention. In the examples, "part(s)"
means "part(s) by weight" unless otherwise specified.
Example 1
Using Insuleed 1004 (product of Nippon Paint Co., an
electrolytically active electrocoating, melting temperature
90°C, curing temperature 180°C), a copper substrate (10 cm
x 10 cm x 700 um thick) was electrodeposited at an
electrodepositing voltage of 200 V for 1 minute in a dry
film thickness of 20 um to give an uncured electrodeposited
coat. The temperature of the electrodepositing bath was
30°C. After electrodepositing, the substrate was cleaned
by dipping it in deionized water for 1 minute to remove the
superfluous electrocoating composition on the substrate and
electrodeposited coat.
Then, the substrate carrying this uncured
electrodeposited coat was pre-baked in a heating oven at a
set temperature of 90°C for 10 minutes. After completion
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of this pre-baking, the substrate was taken out from the
oven and the electrodeposited coat on the substrate surface
was visually examined. It was found that the coat had
flown sufficiently to become smooth.
5 Then, using a high-pressure mercury-vapor lamp (peak
wavelength 365 nm, irradiation intensity 50 mJ/(cm2~s)),
the substrate was irradiated with ultraviolet light at an
integrating radiation dosage of 10000 mJ/cm'.
Thereafter, the substrate was post-baked in a heating
10 oven at a set temperature of 180°C for 20 minutes to give
an electrocoating film.
Example 2
A substrate having an uncured electrodeposited coat
15 as obtained in the same manner as in Example 1 was used as
the work. Meanwhile, a device capable of effecting both
heating and UV irradiation was set to a heating temperature
of 90°C and, for UV irradiation, to a peak wavelength of
365 nm, an irradiation intensity of 50 mJ/(cm~~s) and an
integrating radiation dosage of 10000 mJ/cm2. Using the
above device, heating alone was carried out for 9 minutes.
Then, with the heating being further continued under the
same conditions as above for 1 minute, the work was
irradiated with UV light for 1 minute.
Thereafter, the work was caused to cure by the post-
baking in a heating oven set to 180°C for 20 minutes to
give an electrocoating film.
Example 3
Using CCR-232GF (product of Asahi Chemical Research
Co., an epoxy resin overcoat), the electrocoating film
obtained in Example 2 was spray-coated in a cured thickness
of 25 um. Then the work was cured by heating at 150°C for
60 minutes to give a multilayer film.
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Example 4
Using a coating material prepared by adding 1 part of
Irgacure 651 (ben~yl dimethyl ketal, product of Chiba-Geigy,
a light-sensitive radical initiator) to 100 parts resin
solids of Insuleed 1004 in lieu of Insuleed 1004, the
procedure of Example 1 was otherwise faithfully followed to
give an uncured electrodeposited coat which had flown
sufficiently t~ present with a smooth surface.
Then, except that the integrating radiation dosage of
1~ a high-pressure mercury vapor lamp was set t~ 200 mJ/Cm~,
the work was post-baked in the same manner as in Example 1
to give an electrocoating film.
Comparative Example 1
Omitting the pre-baking and UV irradiation steps, the
procedure of Example 1 was otherwise faithfully followed to
give an electrocoating film.
Comparative Example 2
2~ Omitting the pre-baking step, the procedure of
Example 1 was otherwise faithfully followed to give an
electrocoating film.
Comparative Example 3
Omitting the UV irradiation step, the procedure of
Example 1 was otherwise faithfully followed to give an
electrocoating film.
<Evaluation Tests>
Smoothness
Using SJ-201 (the surface roughness tester
manufactured by Mitsutoyo Co.), the surface roughness Ra
values of the electroCOating films obtained in Examples 1,
2 and 4 and in Comparative Examples 1-3 and the
corresponding value of the multilayer film obtained in
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Example 3 were respectively measured. As measuring
conditions, a cutoff point of 0.8 mm was used. The results
are presented in Table 1.
Dielectric breakdown voltage
The dielectric breakdown voltage values of the
electrocoating films obtained in Examples 1, 2 and 4 and
Comparative Examples 1 to 3 and the Corresponding value of
the multilayer film obtained in Example 3 were respectively
measured with Auto Tester lTodel 8525 (the breakdown voltage
tester manufactured by Tsuruga Electric Co.). The results
are presented in Table 1.
Table 1
Example Comparative
Example
1 2 3 4 1 2 3
Smoothness(um) 0.15 0.22 0.20 0.16 0.35 0.67 0.35
Dielectric 4.6 4.9 7.5 4.4 2.5 0.8 2.7
property (KV)
It will be apparent from Table 1 that the
electrocoating films (Examples 1, 2 and 4) obtained by the
method of forming an electrocoating film according to the
present invention and the multilayer film (Example 3)
obtained by the method of forming a multilayer film
according to the present invention are outstanding in
smoothness and dielectric properties. However, the film
obtained by omitting the pre-baking step (Comparative
Example 2), the film obtained by omitting the pre-baking
and activation energy beam irradiation steps (Comparative
Example 1) and the film obtained by omitting the activation
energy beam irradiation steps (Comparative Example 3) were
inferior in surface smoothness and dielectric properties.
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INDUSTRIAL APPLICABILITY
The method of forming an electrocoating film
according to the present invention comprises a pre-baking
step and an activation energy beam irradiation step which
are carried out in that order prior to a post-laaking step,
with the result that the resulting electrocoating film is
outstanding in surface smoothness and dielectric properties.
Moreover, the method of forming a multilayer film
according to the present invention comprises applying an
overcoat in superimposition on the electrocoating film
obtained by said method of forming an electrocoating film
and the resulting multilayer film is also outstanding in
surface smoothness and dielectric properties.
The underlying principle of these methods is that the
surface roughness of the uncured coat formed by
electrodepositing and the pinholes existing in the coat can
be reduced or eliminated by the pre-baking step which
causes the uncured electrodeposited coat to melt and flow,
thus providing for evenness and smoothness, the film
surface is then fixed to preserve said evenness and
smoothness by irradiation with an activation energy beam
and, thereafter, the entirety of the coat is cured by post-
baking.
The method of forming an electrocoating film
according to the present invention and the method of
forming a multilayer film according to the present
invention provide an electrocoating film and a multilayer
film, both having every satisfactory surface smoothness and
dielectric properties and, thus, finding application in
various kinds of electronic and electric equipment.
