Note: Descriptions are shown in the official language in which they were submitted.
CA 02277830 1999-OS-07
MULTILAYER COATING FILM FORMATION PROCESS
Technical Field
The present invention relates to a process of
forming a multilayer coating film with excellent smooth-
ness and with thinner intermediate coating film without
deteriorating the coating film properties by using an
intermediate paint with a specific pigment composition
and by coating a topcoat paint after curing of the
coating film of said intermediate paint, when a multi-
layer coating film is formed by successive coatings by
an undercoat paint, an intermediate paint and a topcoat
pa i nt.
Background technology
A process of forming a multilayer coating film
by successively coating with an undercoat paint (such as
an electrodeposition paint), an intermediate paint and a
topcoat paint is known. Concerning the intermediate
paint, however, it is necessary to coat to a thick
coating film of usually more than 30 ~,m (as a cured
coating film) in order to hide the undercoat layer and
to maintain the coating film properties. Therefore, it
is desired to lower the coating cost of the whole multi-
layer coating film by making the intermediate coating
film thinner without deteriorating the hiding properties
and the coating film properties.
The present inventors had been conducting an
intensive research to solve these problems and as a
result they found that in a process of successive coat-
ings by an undercoat paint, an intermediate paint and a
topcoat paint it is possible to make the intermediate
coating film thinner with improved undercoat hiding
properties and chipping resistance of the intermediate
coating film and, in addition, with improved smoothness
CA 02277830 1999-OS-07
2
of the topcoat surface by using a thermocurable paint
containing both components of aluminium powder and
titanium oxide pigment as the intermediate paint and by
coating a topcoat paint after curing of the intermediate
coating film, and completed the present invention.
Disclosure of the invention
Thus the present invention provides a process
of forming a multilayer coating film characterized by
that in a process of forming a multilayer coating film
by successively coating a substrate with an undercoat
pa i nt (A) , an i ntermed i ate pa i nt (B) and a topcoat pa i nt
(C) ,
(1) to use a liquid thermocurable paint con-
taining 0.1-30 parts by weight of aluminium powder and
1-200 parts by weight of titanium oxide pigment per 100
parts by weight of a thermosetting resin composition and
the hiding film thickness of its coating film being less
than 25 ~,m as sa i d i ntermed i ate pa i nt (B) ,
(2) to use a solid color paint, a metallic
paint or an interference pattern paint as said topcoat
pa i nt (C) , and
(3) to coat sa i d topcoat pa i nt (C) after
curing by heating of the coating film of said intermedi-
ate pa i nt (B) .
The process of forming a multilayer coating
film of the present invention (hereinafter referred to
as ~~the process~~) is described more specifically here-
i nbe I ow.
Undercoat paint A
Undercoat paint (A) is used to give anticorro-
sivity, adhesivity etc. by directly coating a substrate
made of metal or plastics. In the process any usual
undercoat paint can be used without special restriction,
only if it suits to these purposes. As a substrate
applicable to said undercoat paint outer panels of
CA 02277830 1999-OS-07
3
automobile are particularly preferable. It is usually
desirable to suitably conduct rust removal, washing and
chemical treatments of the substrate previously.
If a substrate is metalbacked or has an
electroconductive surface, a cationic electrodeposition
paint is preferable as an undercoat paint. As a catio-
nic electrodeposition paint there can be used a per se
known one comprising an aqueous solution or aqueous
dispersion of a salt of cationic high polymer compound,
compounded, as necessary, with crossliking agent, pig-
ment and various additives and its sort is not specially
restricted. As a cationic high polymer compound there
can be mentioned, for example, acrylic resin or epoxy
resin having crosslinking functional groups in which
cationic groups such as amino groups are introduced.
They can be made water-soluble or water-dispersible
through neutralization with an organic acid or an inor-
ganic acid. As a crosslinking agent to cure these high
polymer compounds block polyisocyanate compounds, ali-
cyclic epoxy resins etc. can be preferably used.
Electrodeposition coating can be conducted by
dipping metallic substrates such as outer panels of
automobile or bumpers as a negative electrode in a bath
of said cationic electrodeposition paint and by deposit-
ing the paint on said substrate by sending an electric
current between it and a positive electrode under the
usual conditions. The thickness of the formed electro-
deposition coating film is preferable usually in a range
of 10-40 ~.m based upon a cured coating film, and a
coating film can be cured by crosslinking through heat-
ing at about 140 to about 220°C for about 10 to about 40
minutes. In the process it is preferable to coat an
intermediate paint after curing said electrodeposition
coating film. Optionally, however, an intermediate
paint can be coated during the latter is in the uncured
state.
CA 02277830 1999-OS-07
4
Intermediate paint (B~:
In the process a liquid thermocurable paint
containing 0.1-30 parts by weight of aluminium powder
and 1-200 parts by weight of titanium oxide pigment per
100 parts by weight of a thermosetting resin composition
and the hiding film thickness of its coating film being
less than 25 ~,m as the intermediate paint (B),
Using both aluminium powder and titanium oxide
pigment in the intermediate paint (B) increases the
hiding power of the coating film and make it possible to
sufficiently hide the undercoat surface with a cured
coating film of less than 25 ~,m, especially with a thin
film of 10-25 ~,m and thus can achieve making the inter-
mediate coating film thinner.
Thermosetting resin composition which is used
as a vehicle component in such an intermediate paint (B)
consists fundamentally of a base resin and a cross-
linking agent or a self-crosslinking type resin. As a
base resin there can be mentioned, for example, acrylic
resin, polyester resin, alkyd resin etc. having more
than 2 crosslinking functional groups such as hydroxyl
group, epoxy group, isocyanate group, carboxyl group
etc. in the molecule. As a crosslinking agent there can
be used, for example, amino resins such as melamine
resin, urea resin etc., polyisocyanate compounds which
may be blocked, compounds containing carboxyl groups
etc. As the above-mentioned self-crosslinking type
resin there can be mentioned, for example, resins con-
taining more than 2 alkoxysilane groups in the molecule,
resins containing a carboxyl groups) and a hydroxyl
group (s) in the molecule, resins containing a hydroxyl
group (s) and an i socyanate group (s) wh i ch may be b I ock-
ed etc. These resins are based upon, for example, vinyl
resin, acrylic resin, polyester resin, urethane resin
etc.
As aluminium powder to be compounded in the
CA 02277830 1999-OS-07
intermediate paint (B) aluminium powder whose average
particle diameter is generally less than 40 Vim, prefera-
bly less than 10 Vim, and more preferably in a range of
3-7 ~m is suitable. Particularly, if fine powder of an
5 average particle diameter of less than 10 ~m is used,
the formed intermediate coating film itself has no
brilliance. ~~Average diameter~~ here means a median
diameter measured by laser diffraction scattering
(LA-500). The main component of this aluminium powder
is metallic aluninium, whose surface may be treated with
a silane coupling agent or the like.
As titanium oxide pigment to be compounded in
the intermediate paint (B) according to the process
those which are per se known as pigments for paint can
be used. Their average particle diameter is preferable
to be generally 5 Vim. Moreover, the surface of said
titanium oxide pigment may be treated with alumina,
silica etc.
Concerning compounding amount of aluminium
powder and titanium oxide pigment per 100 parts by
weight of a thermosetting resin composition (as solid
content), aluminium powder can be in a range of 0.1-30
parts by weight, preferably 0.5-20 parts by weight and
more preferably 1-7 parts by weight and titanium oxide
pigment can be in a range of 1-200 parts by weight,
preferably 40-160 parts by weight and more preferably
80-120 parts by weight. Furthermore, aluminium powder
is preferably used in a range of 1-15 parts by weight,
preferably 1.5-10 parts by weight and more preferably
2-7 parts by weight per 100 parts by weight of titanium
oxide pigment.
It is indispensable that the intermediate
paint (B) which is used in the process contains both
aluminium powder and titanium oxide pigment. Total
compounding amount of both pigments can be selected to
be an amount which enables to make the hiding film
CA 02277830 1999-OS-07
6
thickness of the coating film to be formed by using said
paint (B) less than 25 ~,m, particularly less than 10-25
~,m (as cured coat i ng f i I m) . ~~H i d i ng f i I m th i ckness~~
here means the minimum film thickness of a coating film
through which the color of the surface to be coated
cannot be recognized and specifically means the minimum
film thickness of a coating film coated on a plate with
black and white checkered pattern through which black
and white cannot be discriminated by the naked eye. In
the process compounding both aluminium powder and tita-
nium oxide pigment in combination at specified amounts
enables to make the hiding film thickness of a coating
film a thin film of less than 25 ~,m. In other words, a
thin film of even less than 25 B.m can sufficiently hide
the color of the ground. Without any of these both
components it is difficult to hide with such a thin
f i I m.
Intermediate paint (B) can be prepared by
mixing and dispersing the above-mentioned thermosetting
resin composition, aluminium powder and titanium oxide
pigment in a solvent such as an organic solvent and/or
water. Furthermore other color pigments than the above-
mentioned aluminium powder and titanium oxide pigment,
extender pigment, antisettle agent etc. can be suitably
compounded, as necessary.
Said intermediate paint (B) is preferably
coated in a film thickness of less than 25 B.m, particu-
larly in a range of 10-25 ~.m based upon a cured coating
film on the cured or uncured undercoat surface by means
of electrostatic coating, air spray, airless spray etc.
In the process a topcoat paint (C) mentioned
below is coated after the coating film of the intermedi-
ate paint (B) has been cured by heating. Curing by
heating of the coating film of the intermediate paint
(B) can be conducted, for example, by heating said
coating film at temperatures of about 140 to about 200°C
CA 02277830 1999-OS-07
7
for about 10 to about 40 minutes.
Topcoat pa i nt (C)
According to the present invention solid color
paint (C-1), metallic paint (C-2) or interference pat-
s tern paint (C-3) is coated as a topcoat paint on the
cured coating surface of the intermediate paint(B). All
of these topcoat paints are desirably of thermocurable
type.
First of all, as a solid color paint (C-1)
there is used preferably a liquid thermocurable paint
containing a thermosetting resin composition and a color
pigment as main components and substantially not con-
taining metallic pigment or interference color pigment.
Thermosetting resin composition which is used
in a color paint (C-1) consists fundamentally of a base
resin and a crosslinking agent or a self-crosslinking
type resin. As a base resin there can be mentioned, for
example, acrylic resin, polyester resin, alkyd resin
etc. having more than 2 crosslinking functional groups
such as hydroxyl group, epoxy group, isocyanate group,
carboxyl group etc, in the molecule. As a crosslinking
agent there can be mentioned, for example, amino resins
such as melamine resin, urea resin etc., polyisocyanate
compounds which may be blocked, compounds containing
carboxyl groups etc. Further, as the above-mentioned
self-crosslinking type resin there can be mentioned, for
example, resins containing more than 2 alkoxysilane
groups in the molecule, resins containing a carboxyl
group (s) and a hydroxy I group (s) i n the mo I ecu I e,
resins containing a hydroxyl group (s) and an isocyanate
groups) which may be blocked etc. These resins are
based upon, for examp I e, v i ny I res i n, acry I i c res i n,
polyester resin, urethane resin etc.
Color pigment which can be compounded in a
sold color (C-1) does not substantially contain metallic
pigment or interference pigment but is a component to
CA 02277830 1999-OS-07
8
give solid color to the multilayer coating film which is
formed according to the process of the present invention
and usual organic or inorganic color pigments for paint
can be used. Specifically there can be mentioned, for
example, inorganic pigments such as titanium oxide, zinc
oxide, carbon black, cadmium red, molybdenum red, chrome
yellow, chrome oxide, Prussian Blue, Cobalt Blue; or-
ganic pigments such as azo pigment, phthalocyanine
pigment, quinacridone pigment, isoindoline pigment,
threne type pigment, perylene pigment etc. These pig-
ments desirably have an average particle diameter of
generally less than 5 Vim.
Compounding amount of these color pigments can
be freely selected according to the coloring power of
the pigment itself and the purpose. It can be in a
range of generally 0.5-200 parts by weight, preferably
1-150 parts by weight per 100 parts by weight of the
thermosetting resin composition and an amount which
allows the hiding film thickness of the coating film to
be formed to be less than 50 Vim, particularly less than
40 ~m as a cured coating film.
Solid color paint (C-1) can be prepared by
mixing and dispersing the above-mentioned components in
a solvent such as an organic solvent and/or water. In
said paint, as necessary, extender pigment, antisettle
agent etc. can be further compounded suitably.
As a metallic paint (C-2) there can be used
preferably a liquid thermocurable paint containing a
thermosetting resin composition, about which is men-
tinned above in the item of a solid color paint (C-1),
and a metallic pigment as main components. Metallic
pigment which is compounded in this metallic paint is a
scale-like particle pigment of metal or metal oxide
having a glittering brilliancy and specifically there
can be mentioned, for example, aluminium flake, mica-
like iron oxide etc. These scale-like pigment particles
CA 02277830 1999-OS-07
9
can have an average particle diameter of generally more
than 10 ~.m, preferably in a range of 10-50 ~,m and more
preferably in a range of 15-40 ~,m. Compounding amount
of these metallic pigments can be in a range of gener-
ally 0.1-20 parts by weight, preferably 3-10 parts by
weight per 100 parts by weight of the thermosetting
resin composition and an amount which allows the hiding
film thickness of the coating film to be formed to be
less than 50 ~,m, particularly less than 30 ~.m as a cured
coat i ng f i I m.
Metallic paint (C-2) can be prepared by mixing
and dispersing the above-mentioned components in a
solvent such as an organic solvent and/or water. In
said paint, as necessary, extender pigment, color pig-
ment, antisettle agent etc. can be compounded suitably.
As a metallic paint (C-2) contains metallic pigment with
relatively large particle diameter, the coating film of
said metallic paint itself shows a glittering bril-
I i ancy.
As an interference pattern paint (C-3) there
is used preferably a liquid thermocurable paint contain-
ing a thermosetting resin composition, about which is
mentioned above in the item of a solid color paint
(C-1), and an interference pigment as main components.
As an interference pigment which is compounded in this
interference pattern paint, scale-like mica, whose
surface is covered with metal oxide such as titanium
oxide, iron oxide etc., so-called interference mica, is
particularly preferable. Covering thickness of metal
oxide on this interference mica is desirably more than
200 nm based upon an optical thickness and more than 80
nm based upon a geometrical thickness. If said covering
thickness is less than mentioned above, it is not pre-
ferable because the interference action by light gener-
ally lowers. Said interference pigment can have an
average particle diameter of generally more than 10 ~,m,
CA 02277830 1999-OS-07
preferably in a range of 10-50 ~,m and more preferably in
a range of 15-40 ~,m.
Compounding amount of said interference pig-
ment can be in a range of generally 1-100 parts by
5 weight, preferably 5-50 parts by weight per 100 parts by
weight of the thermosetting resin composition.
Interference pattern paint (C-3) can be pre-
pared by mixing and dispersing the above-mentioned
components in a solvent such as an organic solvent and/
10 or water. In said paint, as necessary, color pigment,
metallic pigment, extender pigment, antisettle agent
etc. can be compounded suitably.
The above-mentioned topcoat paints (C) are
preferably coated in a film thickness in a range of
10-60 ~,m, particularly in a range of 20-35 ~,m based upon
a cured coating film on the cured intermediate coating
surface by means of electrostatic coating, air spray,
airless spray etc.
The coating film of the above-mentioned top-
coat pa i nts (C) can be cured, for examp I e, by heat i ng at
temperatures of about 120 to about 180°C for 10-40
m i nutes.
Clear paint
I n the process, A c I ear pa i nt (D) may be coat-
ed, as necessary, on the coating surface of the topcoat
paint (C) of the multilayer coating film formed as
mentioned above. The clear paint (D) can be coated on
the coating surface of the topcoat paint (C) formed as
mentioned above in the cured or uncured state.
As a clear paint (D), there can be preferably
used a liquid paint comprising a thermosetting resin
composition and a solvent as main components, and, as
necessary, color pigment, metallic pigment, interference
pigment, ultraviolet absorber and other additives for
paint to such an extent as not to deteriorate the trans-
parent feeling of the coating film.
CA 02277830 1999-OS-07
The above-mentioned thermosetting resin compo-
sition consists fundamentally of a base resin and a
crosslinking agent, or a self-crosslinking type resin.
As a base resin there can be mentioned, for example,
acrylic resin, polyester resin, alkyd resin, urethane
resin etc. having more than 2 crosslinking functional
groups such as hydroxyl group, epoxy group, isocyanate
group, carboxyl group etc. in the molecule. As a cross-
linking agent there can be mentioned, for example,
melamine resin, urea resin, polyisocyanate compounds
which may be blocked, compounds containing carboxyl
groups etc. Further, as a self-crosslinking type resin
there can be mentioned, for example, resins containing
more than 2 alkoxysilane groups in the molecule, resins
containing a carboxyl groups) and a hydroxyl group (s)
in the molecule, resins containing a hydroxyl group (s)
and an isocyanate groups) which may be blocked, etc.
These resins are based upon, for example, vinyl resin,
acrylic resin, polyester resin, urethane resin etc.
As a solvent, an organic solvent and/or water
can be used. By dissolving or dispersing the above-men-
tioned thermosetting resin composition and other compo-
nents in such a solvent, a clear paint (D) can be pre-
pared.
The clear paint (D) can be coated on the un-
cured or cured coating surface of the topcoat paint (C)
formed as mentioned above by means of electrostatic
coating, air spray, airless spray etc. Its film thick-
ness is preferably in a range of 10-60 ~,m and particu-
larly 20-50 ~,m based upon a cured coating film. The
coating film itself of said clear paint (D) can be cured
by crosslinking at temperatures of about 120 to about
1 80°C for 10-40 m i nutes.
According to the above-mentioned process of
forming a multilayer coating film of the present inven-
tion, for example, there are obtained the effects men-
CA 02277830 1999-OS-07
12
tinned below:
(1) The cost of the whole multilayer coating
film can be lowereed, because the film thickness of the
intermediate coating film can be made thinner (less than
25 H,m, preferab I y 10-20 ~,m) than before (usua I I y more
than 30 H,m) .
(2) The smoothness of the topcoat coating
film is excellent, because the topcoat paint is coated
after the intermediate coating film has been cured by
1 0 heat i ng.
(3) As the intermediate paint has an excel-
lent hiding properties of the ground, the color stabil-
ity of the topcoat coating film is good even when coated
with a thin coating film and the color design of the
topcoat coating film can be freely changed according to
the purposes.
(4) Formed multilayer coating film has an
excellent chipping resistance.
EXAMPLE
The process of the present invention is de-
scribed more specifically by means of examples and
comparative examples as follows:
I . Samp I a
(1) Cationic electrodeposition paint (A)
~~ELECTRON9400HB~~ (made by Kansai Paint; trade
name; epoxy resin polyamine block isocyanate compound
type)
(2) I ntermed i ate pa i nt (B)
Organic solvent type paints comprising polyes-
ter resin, melamine resin, fine aluminium powder and
titanium oxide pigment in the ratios shown in the fol-
lowing Table 1. Compounding amount of each component in
Table 1 is the solid content ratio by weight.
CA 02277830 1999-OS-07
13
Table 1
intermediate
paint
(B)
B-1 B-2 B-3 B-4 B-5
Polyester resin (*1) 65 70 75 70 70
Melamine resin (*2) 35 30 25 30 30
Fine aluminium powder (*3) 5 2 2 - 2
Titanium oxide pigment (*4) 120 100 80 80 -
I ron ox i de p i gment (red)2 2 2 2 2
(*5)
H i d i ng f i I m th i ckness11 13 15 100 150
(B,m) (*6)
(*1): Polyester resin of phthalic anhydride-hexahydro-
phthalic anhydride type (number average molecular
weight: about 4000, hydroxyl group value: 82, acid
va I ue : 7)
(*2): U-Van28-60 (made by Mitsui-Toatsu Chemicals;
trade name)
(*3) : K-9800 (made by Asah i Chem i ca I ; trade name) ,
average paraticle diameter: 5-6 ~.m
(*4): TITANJR701 (made by Teikoku Kako; trade name),
average parat i c I a d i ameter : 0. 3-0. 6 ~.m
(*5): KNO-W iron oxide (made by Toda Kogyo; trade
name) , average parat i c I a d i ameter : 0. 2-0. 5 ~,m (red
solid color pigment)
(*6) : The m i n i mum f i I m th i ckness (~.m) of a coat i ng f i I m
coated on a black and white plate with checkered pattern
through which black and white cannot be discriminated by
the naked eye was measured.
(3) Topcoat pa i nt (C)
Organic solvent type paints comprising acrylic
resin, melamine resin, solid color pigment or metallic
pigment in the ratios shown in the following Table 2.
CA 02277830 1999-OS-07
14
Compounding amount of each component in Table 2 is the
solid content ratio by weight.
Table 2
Topcoat
pa i
nt (C)
C-1 C-2 C-3
Acrylic resin (*7) 65 70 75
Melamine resin (*8) 35 30 25
Titanium white pigment (*g) 80 - -
Carbon b I ack (*10) 0. 2 - -
Interference pigment (*11) - g g
H i d i ng f i I m th i ckness100< 100< 100<
(~,m) (*6)
(*7): Acrylic resin of methyl methacrylate type with
number average molecular weight of about 2000, hydroxyl
group value of 70 and acid value of 8
(*8): U-Van28-60 (made by Mitsui-Toatsu Chemicals;
trade name)
(*g): TITANCR93 (made by Ishihara Sangyo; trade name)
(*10) : CarbonFW200 (made by DEGUSSA; trade name)
(*11): Exterior Highlight Blue (made by Mahl; trade
name; average paraticle diameter: 14-18 ~,m)
(5) C I ea r Pa i nt (D)
~~Magicron Clear~~ (made by Kansai Paint; trade
name; acrylic resin-melamine resin type; organic solvent
type)
I I . Exam~l es and com~arat i ve examp I es
Multilayer coating films were formed by coat
ing, using the above-mentioned samples and according to
the coating procedures shown in Table 3, followed by
curing by heating. In Table 3 the results of the per-
CA 02277830 1999-OS-07
15
formance tests of the multilayer coating films are
ment i oned, too.
CA 02277830 1999-OS-07
16
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CA 02277830 1999-OS-07
17
The cationic electrodeposition paint (A) was
painted by electrodeposition on a steel plate, which had
been degreased and treated with zinc phosphate, to the
film thickness of 20 ~,m according to the usual method
and the coating film was cured by heating at 170°C for
30 minutes. On said electrodeposition coating surface
an i ntermed i ate pa i nt (B-1 ) - (B-5) was coated so that the
film thickness would be 25 ~,m and the intermediate
coating film was cured by heating at 140°C for 30 min-
utes in Examples 1-3 and in Comparative example 1 and 2,
while it was kept standing at room temperature for 5
minutes in Comparative example 3. Then on the interme-
diate coating surface a topcoat paint (C-1)-(C-3) was
coated using a minibell type rotary electrostatic coater
under the conditions of output 150 cc, rotation number
50000 rpm, shaping pressure 1kg/cm2, gun distance 30 cm,
booth temperature 20°C, booth humidity 75%. Coating
film thickness was 15-25 ~,m. After said topcoat paint
had been kept standing in the booth for 5 minutes, the
coating film of the topcoat paint (C) was cured by
heating at 140°C for 30 minutes in Example 1 and in
Comparative example 1 and 3. On the other hand, in
Examples 2 and 3 and Comparative example 2, a clear
paint (D) was coated on the uncured coating surface of
the topcoat paint (C) using a minibell type rotary
electrostatic coater under the conditions of output 300
cc, rotation number 40000 rpm, shaping pressure 5kg/cm2,
gun distance 30 cm, booth temperature 20°C, booth humid-
ity 75%. Coating film thickness was 45-50 ~,m. After
being kept standing at room temperature for 3 minutes
after coating, the double layer coating film consisting
of the above-mentioned topcoat paint (C) and clear paint
(D) was simultaneously cured by heating at 140°C for 30
minutes using a hot air circulation type drying furnace.
Coating film performance test methods and
evaluation standards are as follows:
CA 02277830 1999-OS-07
18
Smoothness: Visual evaluation.
O : good, O : a I i tt I a face roughen i ng,
X : remarkable face roughening.
Finishing appearance: Color floating and
hiding properties are visually evaluated.
O : good, D : fa i r I y good, X : no good.
Metallic feeling: Visual evaluation about
metallic mottling etc..
O : good, D : fa i r I y good, X : no good.
Chipping resistance: Using Gravelometer (Made
by Q Panel) as a testing machine, a shock is given to a
coating film by blowing 500 g of No.7 crushed stones by
an air pressure of 3kg/cm2 at 20°C onto the coating
surface at an angle of 45°. Then an adhesive tape is
stuck on said coating surface, and the state of peel-
ing-off of the coating film around the crack caused by
the shock is examined, after rapidly peeling-off the
adhesive tape.
O : No or little peeling-off of the coating
film around the crack is observed.
D : Peeling-off of the coating film around
the crack is clearly observed.
X : Peeling-off of the coating film around
the crack is remarkably observed.
