U.S. patent application number 10/237929 was filed with the patent office on 2003-01-23 for car body coating process.
Invention is credited to Hiraki, Tadayoshi, Tominaga, Akira, Watanabe, Tadashi, Yawata, Takeshi.
Application Number | 20030015427 10/237929 |
Document ID | / |
Family ID | 18546682 |
Filed Date | 2003-01-23 |
United States Patent
Application |
20030015427 |
Kind Code |
A1 |
Watanabe, Tadashi ; et
al. |
January 23, 2003 |
Car body coating process
Abstract
This invention provides a car body coating process wherein the
metal-exposed portion of a shell body, a part or whole of which has
been made of plastics-covered metal plates, is electrocoated with
use of an electrodeposition paint which contains no coloring
pigment and has a bath solid content of at most 10% by weight and
which is capable of forming a clear coating film.
Inventors: |
Watanabe, Tadashi;
(Hiratsuka-shi, JP) ; Hiraki, Tadayoshi;
(Odawara-shi, JP) ; Tominaga, Akira;
(Chigasaki-shi, JP) ; Yawata, Takeshi;
(Hiratsuka-shi, JP) |
Correspondence
Address: |
WENDEROTH, LIND & PONACK, L.L.P.
2033 K STREET N. W.
SUITE 800
WASHINGTON
DC
20006-1021
US
|
Family ID: |
18546682 |
Appl. No.: |
10/237929 |
Filed: |
September 10, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10237929 |
Sep 10, 2002 |
|
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09770209 |
Jan 29, 2001 |
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6475365 |
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Current U.S.
Class: |
204/471 ;
204/489; 204/499; 204/502 |
Current CPC
Class: |
Y10T 29/49888 20150115;
C25D 13/12 20130101; C09D 5/44 20130101; C25D 13/22 20130101 |
Class at
Publication: |
204/471 ;
204/489; 204/499; 204/502 |
International
Class: |
C08F 002/58; C25D
001/12; G01L 001/20; C07K 001/26; C02F 001/469; G01F 001/64; C25B
007/00; G01L 009/18; C25D 013/00; C25B 015/00; C25D 015/00; G01N
027/26; B01D 057/02; B01D 059/42; B01D 059/50; B01D 061/42; B01D
061/58; C09D 005/44 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 28, 2000 |
JP |
2000-20065 |
Claims
1. A car body coating process which is characterized in that the
main exterior of a car body is produced with use of
plastics-covered metal plates to form a shell body, and that the
metal-exposed portion of the shell body is electrocoated with an
electrodeposition paint which contains no coloring pigment and has
a bath solid content of at most 10% by weight and which is capable
of forming a clear coating film.
2. A car body coating process which is characterized in that car
parts which are made by cutting, molding and joining
plastics-covered metal plates are attached to the main exterior of
main body which has previously been assembled, and that thus formed
shell body has its metal-exposed portions electrocoated with an
electrodeposition paint which contains no coloring pigment and has
a bath solid content of at most 10% by weight and which is capable
of forming a clear coating film.
3. A process of claim 1 or claim 2 wherein the plastics-covered
metal plate is prepared by sticking a film-like or sheet-like
plastic onto a metal plate.
4. A process of claim 3 wherein the film-like or sheet-like plastic
is stuck onto a metal plate via an adhesive
5. A process of claim 1 or claim 2 wherein the thickness of plastic
layer on the plastics-covered metal plate is in the range of 3 to
75 .mu.m.
6. A process of claim 1 or claim 2 wherein the electrodeposition
paint does not substantially contains extender pigment either.
7. A process of claim 1 or claim 2 wherein the electrodeposition
paint has a bath solid content of 2 to 7% by weight.
8. A process of claim 1 or claim 2 wherein the electrodeposition
paint has a bath solid content of 3 to 5% by weight.
9. A process of claim 1 or claim 2 wherein the electrodeposition
paint is of cationic type.
10. A process of claim 9 wherein the electrodeposition paint
comprises a base resin (a) having hydroxyl group(s) and
cationizable group(s) and a blocked polyisocyanate compound (b) as
a crosslinking agent, and, as circumstances might demand,
gelatinizing polymer particles as well.
11. A process of claim 10 wherein the base resin (a) is a resin
which is obtained by further making a cationizing agent react with
epoxy resin which has been prepared by reaction between polyphenol
compound and epichlorohydrin.
12. A process of claim 10 wherein the proportion of base resin (a)
is within a range of 40 to 90% on the basis of the weight of total
solid contents of base resin (a) and blocked polyisocyanate
compound (b).
13. A process of claim 10 wherein the proportion of gelatinizing
polymer particles is within a range of 1 to 20 parts by weight per
100 parts by weight of total solid contents of base resin (a) and
blocked polyisocyanate compound (b).
14. A process of claim 1 or claim 2 wherein the coating film which
is formed by electrodeposition coating has a thickness in the range
of 5 to 40 .mu.m.
15. A process of claim 1 or claim 2 wherein electrocoating film
after the electrodeposition is subjected to water washing in such a
manner that the amount of drainage of washing water is at most 500
ml per square meter of electrocoated area.
16. A process of claim 15 wherein coating film after the
electrodeposition is washed either with ultrafiltration filtrate
alone or firstly with ultrafiltration filtrate and subsequently
with pure water for the adjustment of electrobath liquid
surface.
17. A process of claim 1 or claim 2 wherein the step of water
washing of coating film after the electrodeposition is omitted.
18. A process of claim 1 or claim 2 wherein coating film after the
electrodeposition is cured by heating at a temperature of about
100.degree. C. to about 200.degree. C.
19. A car body which is coated by the process of claim 1 or claim
2.
Description
[0001] The present invention relates to a car body coating process
wherein the metal-exposed portion of shell body of a car body, a
part or whole of which has been made of plastics-covered metal
plates, is electrocoated with use of an electrodeposition paint
which contains no coloring pigment and has a bath solid content of
at most 10% by weight and which is capable of forming a clear
coating film.
[0002] In the body of passenger car or of small passenger car, the
portion which is mainly constituted of sheet metal and which is
free of riggings such as engine or wheels is called shell body.
Shell body is usually constituted of a main body, which consists of
underbody, side member, roof, cowl, upper back, lower back, etc.,
and outer cover parts such as hood, front balance, front fender,
cowl louver, door, luggage (back door), etc.
[0003] Up to the present, metal plates which have been cut and
molded in the size and shape of each component part have been
assembled to form main body and outer cover parts, which in turn
are combined to make a shell body, which is dipped into a cationic
electrodeposition paint bath so that the surface, backside, edge
surface portion, etc., of the metal plates may be electrocoated,
and, thus, the shell body is coated with a primer film.
[0004] Usually, after conducting this electrodeposition,
water-washing is carried out several times with use of
ultrafiltration filtrate, deionized water, tap water, etc., so as
to remove superfluous electropaint which has adhered to the
electrocoated surface or electropaint which has collected in the
interior of bag-like portion. Then, heating is conducted so that
thus formed coating film may be cured. Outside portions of shell
body are coated with intermediate paint and topcoat paint.
[0005] In car body coating, however, cutting down of effluent from
the process of water-washing of electrocoating film has recently
been strongly desired for the sake of step saving, energy saving
and CO.sub.2 reduction in the coating line, and for the
preservation of environment. Moreover, there has further been
desired improvement not only in throwing power of electrodeposition
paint to edge surface portion, but also in chipping resistance,
corrosion resistance, etc., of total coating film including
intermediate coating and top coating.
[0006] In the coating of a car body, on the other hand, there is
proposed an inverse method (for example, Japanese Pre-grant Patent
Publication No. 41317/1980), in which a substrate is powder-coated
and the resulting coating film is cured, and, thereafter, the
uncoated portion is electrocoated, for the purpose of the reduction
of organic solvent discharged and the improvement of edge surface
coatability of electrodeposition paint. This method has, however,
such problems that powder coating facilities need to be newly
provided, and that the corrosion resistance of the boundary portion
between powder coating film and electrodeposition coating film is
not sufficient.
[0007] The chief objective of the present invention is to provide a
car body coating method which satisfies the above-mentioned
requirements in car body coating and which is free of the
above-mentioned defects.
[0008] [Means to Attain the Objective]
[0009] As a result of assiduous study, the inventors of this
invention have found that the above-mentioned objective can be
attained by making a part or whole of shell body by using
previously prepared metal plates covered with plastics layer, and
then electrocoating the metal exposed portion of thus made shell
body with an electrodeposition paint which contains no coloring
pigment and has a bath solid content of at most 10% by weight and
which is capable of forming a clear coating film, and, thus, have
completed this invention.
[0010] Thus, according to the present invention, there is provided
a car body coating process (hereinafter referred to as Process I)
which is characterized in that main outer parts of a car body is
made of plastics-covered metal plates to form a shell body, and
that the metal exposed portion of the shell body is electrocoated
with an electrodeposition paint which contains no coloring pigment
and has a bath solid content of at most 10% by weight and which is
capable of forming a clear coating film.
[0011] According to the present invention, there is also provided a
car body coating process (hereinafter referred to as Process II)
which is characterized in that car parts which have been made by
cutting, molding and joining plastics-covered metal plates are
attached to the main outer portions of main body of a car body
which has previously been assembled, and that that the metal
exposed portion of thus formed shell body is electrocoated with an
electrodeposition paint which contains no coloring pigment and has
a bath solid content of at most 10% by weight and which is capable
of forming a clear coating film.
[0012] The car body coating processes of the present invention are
described in more detail in the following.
[0013] The processes of the present invention can be applied mainly
to normal and small passenger car, but can also be applied to
truck, bus, motorcycle, and specially equipped vehicles in the same
manner.
[0014] As a metal plate to be covered with plastics, metals which
have heretofore been used for car body can be similarly used. As
the material of metal, there may be mentioned, for example, iron,
steel, stainless steel, aluminium, copper, and alloys containing
these metals, and, furthermore, plate of these metals whose surface
is plated with zinc, zinc/nickel, iron, etc. For use, they can be
processed into coils or cut plates. The thickness of these metal
plates is suitably in the range of 0.3 to 2.0 mm, particularly 0.5
to 1.0 mm. The surface of these metal plates is preferably
subjected to suitable treatment such as grinding treatment,
degreasing treatment or phosphate treatment with a view to
improvement in adhesivity with plastics layer, corrosion
resistance, etc.
[0015] As plastic materials to cover the above-mentioned metal
plates, there can be used known thermoplastic resins, for example,
polyolefin resin such as polyethylene and polypropylene; polyester
resin such as polyethylene terephthalate (PET); polycarbonate
resin; epoxy resin; vinyl acetate resin; vinyl chloride resin;
fluorine-containing resin; polyvinyl acetal resin; polyvinyl
alcohol resin; polyamide resin; polystyrene resin; acrylic resin;
polyurethane resin; phenolic resin; polyether resin; cellulose type
resin; etc. These plastic materials may contain color pigment,
extender pigment, etc.
[0016] Covering of a metal plate with these plastics materials can
be performed by known methods, which include sticking, onto metal
plate, film- or sheet-like plastics formed by such a method as
extrusion molding, injection molding, calender molding or
compression molding; sticking with pressure, onto metal plate,
molten plastics extruded into film or sheet; adhering plastics in
powder form onto metal plate by methods such as fluidized
immersion, electrostatic coating, etc. and then melting by heating.
Particularly preferable is the process of sticking plastics in film
or sheet form onto metal plate. Covering of a metal plate with
plastics is performed at least on the surface of a metal plate
located at outer side of a car body. However, it is possible to
cover both surfaces, if desired. The thickness of the plastics
layer covering a metal plate is preferably in the range of usually
1 to 100 .mu.m, particularly 3 to 75 .mu.m, more desirably 5 to 50
.mu.m. Moreover, it is possible to treat the surface of these
plastics by corona discharge, plasma, flame, etc., before or after
the covering onto the metal plate.
[0017] When a metal plate is to be covered with plastics by means
of sticking film-like or sheet-like plastics onto the metal plate,
it is preferable to previously apply an adhesive to the metal plate
and/or to plastics film or sheet in order to increase adhesivity
between the two. As such an adhesive there may be mentioned
thermosetting or thermoplastic adhesives containing one or more
kinds of resins selected from, for example, bisphenol type epoxy
resin, resol type epoxy resin, acrylic resin, aminoplast resin,
polyester resin, polysiloxane resin, etc., and optionally further
containing a curing agent. Further, there can preferably be used as
adhesive triazinethiol type compounds such as
2,4,6-trimercapto-S-triazine,
2-dibutylamino-4,6-dimercapto-S-triazine,
2,4,6-trimercapto-S-triazine-monosodium salt,
2,4,6-trimercapto-S-triazin- e-trisodium salt, etc.
[0018] In the Process I according to the present invention,
plastics-covered metal plates which are prepared by the
above-mentioned manner are cut, molded or jointed to assemble a
shell body with. Specifically, from the plastics-covered metal
plates, there are produced main body and outer cover parts, which
are then assembled to form a shell body.
[0019] Shell body is such a portion in a car body as is constituted
mainly of sheet metal and is free from riggings such as engine or
wheels. Its main body is constituted mainly with parts such as
underbody, side member, roof, cowl, upper back, lower back etc. and
the outer cover parts consists mainly of parts such as hood, front
balance, front fender, cowl louver, door, luggage (back door), etc.
Parts of outer cover parts are called car parts.
[0020] Underbody here means the floor portion of the cabin, trunk
room, etc., and is a generic term for front underbody, front floor,
rear floor, etc. Side member forms the side of a cabin joining with
a front body, roof panel, underbody, etc., and prevents the car
from bending and/or twisting. Cowl is a panel combining left,
right, front and rear pillars. Upper back is a panel combining left
and right quarter panels (rear fender) at the back portion of a car
body and forming outer surface of the car body.
[0021] In the Process I of the present invention, in order to form
the above-mentioned parts constituting a shell body, a
plastics-covered metal plate, which has been prepared as mentioned
above, is cut to the suitable shape and size, pressed and molded by
a press etc., and, if necessary, joined by adhering with adhesive,
welding, bolted etc. to prepare parts of the main body such as
underbody, side member, roof, cowl, upper back, lower back, etc.;
and parts (car parts) of outer cover parts such as hood, front
balance, front fender, cowl louver, door, luggage, etc. These
cutting, molding and joining can be performed by known methods.
Then the parts of the main body thus formed by using a
plastics-covered metal plate are assembled, and joined to form a
main body, to which parts of outer cover parts (car parts) such as
hood, front balance, front fender, cowl louver, door, luggage, etc.
are attached.
[0022] While at least the outer surface of the shell body which has
thus been assembled with using plastics-covered metal plates is
covered with a plastics layer, at least the edge surface portion of
cut plastics-covered metal plates has its metal portion exposed.
Moreover, the back side is preferably covered with plastics layer
although metal portion may be exposed. In the Process I of the
present invention, these exposed metal portions are subsequently
coated by electrodeposition.
[0023] In Process II according to the present invention, on the
other hand, parts of outer cover parts (car parts) such as hood,
front balance, front fender, cowl louver, door, luggage, etc. are
made by cutting, molding and joining plastics-covered metal plates
which have been prepared in the above-mentioned manner, and, then,
these car parts are attached to the previously assembled main body
of a car body, and, thus, a shell body is formed. Each part outer
cover parts (car parts) which are made of plastics-covered metal
plates may have the same structure as in the above-mentioned
Process I.
[0024] In Process II of the present invention, most or the whole of
the car parts, which constitute outer cover parts, are prepared by
using the above-mentioned plastics-covered metal plate. For
example, in order to form each part constituting outer cover parts
such as hood, front balance, front fender, cowl louver, door,
luggage (back door), etc., a plastics-covered metal plate is cut to
the suitable shape and size, pressed and molded by a press etc.,
and, if necessary, joined by adhering with adhesive, welding,
bolting etc. to prepare parts (car parts) such as hood, front
balance, etc. These cutting, molding and joining can be performed
by known methods. At least the outer surface of thus formed outer
cover parts (car parts) is covered with a plastics layer, while the
edge surface portion of a cut steel plate has an exposed metal
portion. The back side may be uncovered and have exposed metal, or
may be covered with plastics.
[0025] In Process II of the present invention, the main body
constituted by underbody, side member, roof, cowl, upper back,
lower back, etc., to which these car parts are attached, are
prepared usually without using a plastics-covered metal plate.
Instead, an uncovered metal plate is cut, molded and processed, and
joined by known methods to form main body. A shell body is formed
by attaching outer cover parts (car parts) which has been prepared
from plastics-covered metal plate to a main body which has been
prepared from such an uncovered metal plate. In Process II of the
present invention, the whole surface of the main body and the
exposed metal portion of outer cover parts (car parts) in thus
assembled shell body are coated by electrodeposition.
[0026] In Processes I and II of the present invention, metal
exposed portions of the assembled shell body is electrocoated with
an electrodeposition paint which contains no coloring pigment, has
a bath solid content of at most 10% by weight and which is capable
of forming a clear coating film. The use of such a specific
electrodeposition paint makes it possible to cut down effluent from
water-washing process by which to remove superfluous electropaint
which has adhered to electrocoated surface.
[0027] In detail, the use of electropaint which contains no
coloring pigment and is capable of forming a clear coating film
produces the following effects:
[0028] 1) Thermal fluidity of coating film improves with the result
that it becomes possible to form a coating film which is excellent
in smoothness and corrosion resistance even though thin. Based on
this merit, it becomes possible to shorten the current application
time.
[0029] 2) Although electropaint which has permeated into joint
portions of coated article spouts out in baking process and adheres
to other portions, it causes no popping owing to good thermal
fluidity, and, thus, it is possible to simplify a process of
water-washing permeated portions.
[0030] 3) Even though electrodeposition paint adheres to
plastic-covered surfaces, there occur no uneven drying nor
secondary sag since formed coating film is clear, and, thus,
appearance is not damaged.
[0031] 4) In electrocoating bath, pigment or the like does not
deposit at horizontal portions of article to be coated, and, thus,
water-washing process can be simplified.
[0032] When there is used an electrodeposition paint which has a
bath solid content of at most 10% by weight, uneven drying hardly
occurs even though unelectrodeposited material remains on the
coated surface after water washing, and, thus, decrease of
appearance can be inhibited. Moreover, although electropaint which
has permeated into joint portions spouts out during baking, it
hardly causes sag or popping.
[0033] In order to remove unelectrodeposited material which has
adhered electrocoated surfaces, there have usually been employed,
in combination, primary water-washing with use of a filtrate which
has been prepared by UF (ultrafiltration) of a part of electropaint
in electrobath, and secondary water-washing with use of pure water.
In particular, water after secondary water-washing is usually
discharged outside after suitable waste disposal treatment. The
amount of this waste water is said to be 5 to 10 tons per hour.
Owing to the above-mentioned effects produced by Processes I and II
of this invention, it becomes possible to omit said secondary
water-washing, which shortens water-washing process, and is
preferable both in view of environment preservation and
economy.
[0034] Incidentally, when the solid content of electrodeposition
paint at the time of application exceeds 10% by weight, both the
paint which has penetrated into, and collected at, joint portions
in mold-processed plastics-covered metal plates and the
non-electrodeposited paint which has adhered both to the surface of
plastics-covered metal plates and to the surface of
electrodeposition coating which has deposited on the metal-exposed
portion come to have such a high concentration of solid contents
that the above-mentioned primary water-washing becomes insufficient
to fully remove said non-electrodeposited paint, with the result
that the appearance is undesirably damaged.
[0035] There is no particular restriction on the electrodeposition
paints to be used in Processes I and II of the present invention,
and both anionic and cationic type ones will do so long as the
electrodeposition paints satisfy the condition that they contain no
coloring pigment and desirably no extender pigment, and that they
have a bath solid content of at most 10% by weight, preferably 2 to
7% by weight, further desirably 3 to 5% by weight, and, moreover,
that the electrodeposition paints are capable of forming a clear
coating film. Especially preferable is a cationic type
electrodeposition paint which is capable of forming a coating film
with excellent corrosion resistance.
[0036] As a cationic electrodeposition paint, there can be
mentioned an electrodeposition paint which is prepared by mixing
and dispersing, in aqueous medium, a base resin (a) having hydroxyl
group(s) and cationizable group(s), and a blocked polyisocyanate
compound (b), and, as circumstances might demand, gelatinizing
polymer particles as well.
[0037] As a base resin (a) having hydroxyl group(s) and
cationizable group(s), the following can be mentioned.
[0038] 1) reaction product of epoxy resin and cationizing agent; 2)
acid-protonized product of polycondensate of polycarboxylic acid
and polyamine (cf. U.S. Pat. No. 2,450,940 Specification); 3)
acid-protonized product of polyadduct of polyisocyanate compound,
polyol and mono- or polyamine; 4) acid-protonized product of
copolymer of acryl type or vinyl type monomers having hydroxyl
group and amino group [cf. Japanese Laid-Open Patent Publications
No. 12395/ 1970 (=U.S. Pat. No. 3,455,806) and No. 12396/1970)
(=U.S. Pat. No. 3,454,482)]; 5) acid-protonized product polyadduct
of polycarboxylic acid resin and alkyleneimine (cf. U.S. Pat. No.
3,403,088 Specification).
[0039] Preferable among the above is a resin included in reaction
products of the above 1) which is obtained by further making a
cationizing agent react with epoxy resin which has been prepared by
reaction between polyphenol compound and epichlorohydrin, since
said resin forms a coating film which is excellent in corrosion
resistance.
[0040] Particularly suitable one among the above-mentioned epoxy
resins has at least two epoxy groups in the molecule, a number
average molecular weight of at least 200, preferably 800 to 2000,
and an epoxy equivalent in the range of 190 to 2000, preferably 400
to 1000. Such epoxy resins include polyglycidyl ether of polyphenol
compound. As said polyphenol compound there can be mentioned, for
example, bis(4-hydroxyphenyl)-2,2-pr- opane,
4,4'-dihydroxybenzo-phenone, bis(4-hydroxyphenyl)-1,1-ethane,
bis(4-hydroxyphenyl)-1,1-isobutane,
bis(4-hydroxy-tert-butylphenyl)-2,2-p- ropane,
bis(2-hydroxybutyl)methane, 1,5-dihydroxynaphthalene,
bis(2,4-dihydroxyphenyl)methane,
tetra(4-hydroxyphenyl)-1,1,2,2-ethane, 4,4'-dihydroxydiphenyl
ether, 4,4'-dihydroxydiphenyl sulfone, phenol novolac, cresol
novolac, etc.
[0041] These epoxy resins may be further modified by subjecting to
reaction with polyol, polyehter polyol, polyester polyol,
polyamidoamine, polycarboxylic acid, polyisocyanate compound, etc.,
and further may be graft-polymerized by .epsilon.-caprolactone,
acrylic monomer, etc.
[0042] Cationizing agent in the above-mentioned 1) is made to react
with most or whole of the epoxy groups existing in the epoxy resin,
by which to introduce cationizable group such as secondary amino
group, tertiary amino group, quaternary ammonium base, etc. into
the resin, and, thus, there is given a base resin (a) having a
hydroxyl group(s) and a cationazable group(s) by reacting.
[0043] As such a cationazing agent, there can be mentioned amine
compound, for example, primary amine, secondary amine, tertiary
amine, polyamine, etc. Here, there can be mentioned as a primary
amine compound, for example, methylamine, ethylamine,
n-pro-pylamine, isopropylamine, monoethanolamine, n-propanolamine,
isopropanolamine, etc.; as a secondary amine compound, for example,
diethylamine, diethanolamine, di-n-propanolamine,
diisopropanolamine, N-methylethanolamine, N-ethylethanolamine,
etc.; and as a tertiary amine compound, for example, triethylamine,
triethanolamine, N,N-dimethylethanolamine, N-methyldiethanolamine,
N,N-di-ethylethanolamine, N-ethyldiethanolamine, etc. As a
polyamine there can be mentioned, for example, ethylenediamine,
diethylenetriamine, hydroxyethylaminoethylamine,
ethylaminoethylamine, methylaminopropylamine,
dimethylaminoethylamine, dimethylaminopropylamine- , etc.
[0044] As a hydroxyl group in the base resin (a), there may be
mentioned a primary hydroxyl group which is introduced by, for
example, a reaction with an alkanol amine in the above-mentioned
cationizing agent, or by a ring-opening reaction with caprolactone
which may be introduced in the epoxy resin, or by a reaction with a
polyol, etc.; a secondary hydroxyl group in the epoxy resin etc.
Among these, a primary hydroxyl group introduced by a reaction with
an alkanolamine is preferable due to its excellent crosslinking
reactivity with a blocked polyisocyanate compound (crosslinking
agent). The amount of hydroxyl groups in the base resin (a) is
preferably in the range of generally 20 to 5000 mgKOH/g,
particularly 100 to 1000 mgKOH/g. Particularly, it is preferable
that the primary hydroxyl group equivalent be in the range of 200
to 1000 mgKOH/g. The amount of a cationizable group is preferably
at least the minimum limit which is necessary for the dispersion of
the base resin (a) in water, preferably in the range of 3 to 200,
particularly 5 to 150, and especially 10 to 80, calculated as KOH
(mg/g solid content) (amine value).
[0045] It is desirable that the base resin (a) does not
substantially contain a free epoxy group.
[0046] Blocked polyisocyanate compound (b) as a crosslinking agent
is a compound in which substantially all isocyanate groups in the
polyisocyanate compound are blocked by a volatile blocking agent.
When heated over the predetermined temperature (usually 100 to
200.degree. C.), the blocking agent is dissociated to regenerate an
isocyanate group which takes part in the crosslinking reaction with
hydroxyl group of the base resin (a).
[0047] Polyisocyanate compound is a compound having at least two
isocyanate groups in the molecule, examples of which include
aliphatic diisocyanate such as hexamethylene diisocyanate,
trimethylene diisocyanate, tetramethylene diisocyanate, dimer acid
diisocyanate, lysine diisocyanate, etc.; alicyclic diisocyanate
such as isophorone diisocyanate,
methylenebis(cyclohexylisocyanate), methylcyclohexane diisocyanate,
cyclohexane diisocyanate, cyclopentane diisocyanate, etc.; aromatic
diisocyanate such as xylylene diisocyanate, tolylene diisocyanate,
diphenylmethane diisocyanate, naphthalene diisocyanate, toluidine
diisocyanate, etc.; urethanation adducts, biuret type adducts,
isocyanuric ring type adducts of these polyisocyanate compounds,
etc.
[0048] As a blocking agent to block a free isocyanate group of
these polyisocyanate compounds, there can be used known blocking
agents of phenol type, alcohol type, active methylene type,
mercaptan type, acid amide type, imide type, amine type, imidazole
type, urea type, carbamic acid type, imine type, oxime type,
sulfurous acid type, lactam type, etc.
[0049] As for the constitutional proportion of the base resin (a)
and the blocked polyisocyanate compound (b), the former preferably
accounts for 40 to 90%, particularly 50 to 80%, and the latter
accounts for 60 to 10%, particularly 50 to 20%, each based upon the
total solid content weight of both components (a) and (b).
[0050] A cationic electrodeposition paint which is usable in
Processes I and II of this invention contains no coloring pigment
and desirably no extender pigment, and can be prepared, for
example, by neutralizing the cationizable group in a base resin (a)
with an acid compound such as acetic acid, formic acid, lactic
acid, phosphoric acid, etc., and then mixing and dispersing in
water together with a blocked polyisocyanate compound (b) as a
crosslinking agent. The pH at the time of its electrocoating is
suitably in the range of generally 3-9, particularly 5-7.
[0051] To a cationic electrodeposition paint, with a view to
improvement in corrosion resistance and curability of
electrocoating film, there can be suitably blended, as
circumstances might demand, organic acid salt or inorganic acid
salt of a metal selected from aluminium, nickel, zinc, zirconium,
molybdenum, tin, antimony, lanthanum, tungsten, bismuth, etc., in
an amount of 0.1 to 10 parts by weight, particularly 0.5 to 5 parts
by weight, per 100 parts by weight of total solid contents of base
resin (a) and blocked polyisocyanate compound (b).
[0052] Cationic electrodeposition paint may further contain
gelatinizing polymer particles, which give rise to effects that
excessive thermal fluidity can be inhibited, resulting in improved
corrosion resistance (edge covering property) of edge surface
portion of metal exposed portions.
[0053] Gelatinizing polymer particles are prepared by three
dimensionally crosslink-polymerizing polymerizable monomers. There
are suitably usable known ones such as those which are disclosed in
Japanese Patent KOKAI Publication No. Hei 2 (1990)-47173 (=U.S.
Pat. No. 5,021,530) and Japanese Patent KOKAI Publication No. Hei 2
(1990)-64169. Concretely, there can be mentioned gelatinizing
polymer which is produced by emulsion-polymerizing monomer (1)
which has both polymerizable double bond and hydrolyzable
alkoxysilane group, monomer (2) which has at least two
polymerizable double bonds in a molecule, monomer (3) which has
both polymerizable double bond and hydroxyl group and another
monomer (4) which has polymerizable double bond, in the presence of
cationically reactive emulsifier which has allyl group in its
molecule.
[0054] Monomer (1) is a compound which has at least one
polymerizable double bond and at least one hydrolyzable
alkoxysilane group in a molecule. Examples of monomer (1) include
vinyltrimethoxysilane, vinyltriethoxysilane,
vinyltris(2-methoxyethoxy)silane,
.gamma.-methacryloxypropyltrimethoxysilane and
vinyltriacetoxysilane, among which
.gamma.-methacryloxypropyltrimethoxysilane is suitable.
[0055] Monomer (2) is a compound which has at least two
polymerizable double bonds in a molecule. Examples of monomer (2)
include polymerizable unsaturated monocarboxylic acid ester of
polyhydric alcohol, polymerizable unsaturated alcohol ester of
polybasic acid and aromatic compound which is substituted by at
least two vinyl groups. Concretely, there can be mentioned
ethyleneglycol di(meth)acrylate; 1,6-hexanediol di(meth)acrylate;
diallylphthalate; and divinylbenzene.
[0056] Monomer (3) is a compound which has at least one
polymerizable double bond and at least one hydroxyl group in a
molecule, and which introduces hydroxyl group into gelatinizing
polymer particles, imparts hydrophilicity to said particles and
functions as crosslinking functional group between particles.
Concretely, preferable examples include 2-hydroxy(meth)acrylate and
hydroxypropyl(meth)acrylate.
[0057] Monomer (4) is a compound which has polymerizable double
bond other than the above-mentioned monomers (1) to (3). Examples
include alkyl (having 1 to 20 carbon atoms) ester of acrylic or
methacrylic acid such as methyl(meth)acrylate, ethyl(meth)acrylate,
propyl(meth)acrylate, hexyl(meth)acrylate, lauryl(meth)acrylate and
cyclohexyl(meth)acrylate; vinylaromatic monomer such as styrene,
.alpha.-methylstyrene and vinyltoluene; amides of acrylic or
methacrylic acid; and (meth)acrylonitrile.
[0058] Constitutional proportion of these monomers is not in
particular restricted, and can be selected optionally according to
objective. Preferably, however, monomer (1) is in a range of 1 to
30% by weight, in particular 3 to 20% by weight; monomer (2) is in
a range of 1 to 30% by weight, in particular 3 to 20% by weight;
monomer (3) is in a range of 1 to 30% by weight, in particular 3 to
20% by weight; and monomer (4) is in a range of 10 to 97% by
weight, in particular 40 to 91% by weight; on the basis of the
weight of total solid contents of these monomers.
[0059] As an example of cationically reactive emulsifier which has
allyl group in its molecule, there can be mentioned a reactive
emulsifier having quaternary ammonium salt which has the following
formula:
X.sup..crclbar..multidot.R.sub.1N.sup..sym.(R.sub.2)(R.sub.3)--CH.sub.2--C-
H(OH)--CH.sub.2--O--CH.sub.2--C(R.sub.4).dbd.CH.sub.2
[0060] wherein R.sub.1 denotes a hydrocarbon having 8 to 22 carbon
atoms which may be substituted; R.sub.2 and R.sub.3 each denote an
alkyl group having 1 to 3 carbon atoms; R.sub.4 denotes a hydrogen
atom or a methyl group; and X.sup..crclbar. denotes a monovalent
anion.
[0061] As said cationically reactive emulsifier, there can be used
one which has already been known [from, for instance, Japanese
KOKAI Patent Publication No. Sho 60 (1985)-78947], such as
"Ratemuru K-180" (trade name; Kao Corporation) which has been put
on the market. Such an emulsifier is used in an amount of 0.1 to 30
parts by weight, in particular 0.5 to 5 parts by weight per 100
parts by weight of solid content of gelatinizing polymer
particles.
[0062] The polymerization reaction of the above-mentioned monomers
is conducted by emulsion polymerization which is known as a method
for the production of acrylic polymer. Concretely, a mixture of
monomers is subjected to reaction in an aqueous medium in the
presence of the above-mentioned emulsifier and a polymerization
initiator such as water-soluble azoamide compound, usually at a
reaction temperature of about 50 to about 100.degree. C. for about
1 to about 20 hours. Aqueous dispersion of thus obtained
gelatinizing polymer particles have usually a resin solid content
of 10 to 40% by weight. Said particles have a particle size of at
most 500 nm, in particular 10 to 300 nm, and most desirably 50 to
100 nm. Particle size can easily be adjusted by varying the amount
of the above-mentioned emulsifier. The amount of gelatinizing
polymer particles blended is preferably within the range of 1 to 20
parts by weight, in particular 5 to 15 parts by weight, per 100
parts by weight of total solid content of base resin (a) and
blocked polyisocyanate compound (b) in the cationic
electrodeposition paint.
[0063] When applied, the cationic electrodeposition paint is
adjusted so that it may have a pH of 3 to 9, preferably 5 to 7, and
may have a solid content of at most 10% by weight, preferably 2 to
7% by weight, most desirably 3 to 5% by weight, and a bath
temperature within a range of 25 to 35.degree. C. In the bath of
said electrodeposition paint, there is dipped a shell body of a car
which is produced with use of plastics-covered metal plates
prepared by Processes I and II. Then, by means of current
application, metal exposed portions such as edge surface portions
of the shell body is coated with electrodeoposition coating film.
As for current application condition, voltage is preferably 100 to
400 V, and current application time is preferably 1 to 10 minutes.
When electrodeposition is conducted in this manner, an
electrodeposition coating film is formed on metal exposed portions
such as edge surface portion which appears by the cutting of
plastics-covered metal plate and the backside or right side portion
of said metal plate which is free of plastics cover. The thickness
of an electrodeposition coating film is preferably in the range of
about 5 to about 40 .mu.m, particularly 10 to 20 .mu.m, as a cured
film, on flat.
[0064] After the electrocoating is over, the article to be coated
(shell body) is drawn up from the electrobath, and, is then
preferably washed with water so that undeposited electropaint which
has adhered to electrocoated surface or electropaint which has
collected in the interior or bottom of bag-like portions may be
removed.
[0065] Electrocoating film after the electrodeposition may be
washed only with a filtrate which has been prepared by
ultrafiltration of a part of electropaint which had been taken from
electrobath. Otherwise, said electrocoaint film may be first washed
with said filtrate, and then with water (e.g., pure water) which is
supplied to keep the liquid surface of electrobath constant. Water
which has been used for these water washing processes is recovered
into electrobath, and, therefore, effluent is not drained outside,
which manner is to be called closed system. Or, otherwise,
electrocoating film after the electrodeposition may be subjected to
water washing in such a manner that the amount of drainage of
wasihing water may fall within the range of at most 500 ml,
preferably at most 300 ml, per square meter of electrocoated area
(in which case, drain disposal is carried out by open system). Or
again, these water washing processes may be dispensed with
with.
[0066] After water washing, electrocoated film can be cured by
heating at a temperature of about 100 to about 200.degree. C.,
preferably about 120 to about 180.degree. C., for about 10 to 40
minutes.
[0067] Through the aforementioned Processes I and II of the present
invention, the following effects can be achieved.
[0068] (1) Since the electrodeposition paint deposits easily at the
boundary portion between the plastics covering film and exposed
metal, the corrosion resistance of this portion improves
remarkably.
[0069] (2) Since metal plates which have previously been covered
with plastics are used for the production not only of outer cover
parts such as hood panel, fender panel, door panel, luggage door
panel etc. of the car body, but also of main body consisting of
underbody, side member, roof, cowl, upper back, lower back, etc.,
it is possible to largely reduce the amount of the
electrodeposition paint to be used at the next step.
[0070] (3) Since at least the outer surface of outer cover parts is
covered with a layer of plastics having a high volume specific
resistance and, since, under circumstances, at least the outer
surface of the main body is also covered with a layer of plastics
having a high volume specific resistance, the area of the portion
of shell body to be coated by electrodeposition (portion with
exposed metal) is small, and, consequently, the throwing power of
the paint increases, and particularly the corrosion resistance of
the edge surface portion is improved.
[0071] (4) It is possible to give the properties, which the
covering plastics have, such as chipping resistance, corrosion
resistance etc., to a car body.
[0072] (5) The use of electropaint which contains no coloring
pigment and is capable of forming a clear coating film produces the
following effects:
[0073] 1) Thermal fluidity of coating film improves with the result
that it becomes possible to form a coating film which is excellent
in smoothness and corrosion resistance even though thin. Based on
this merit, it becomes possible to shorten the current application
time.
[0074] 2) Although electropaint which has permeated into joint
portions of coated article spouts out in baking process and adheres
to other portions, it causes no popping owing to good thermal
fluidity, and, thus, it is possible to shorten a process of
water-washing permeated portions.
[0075] 3) Even though electrodeposition paint adheres to
plastic-covered surfaces, there occur no uneven drying nor
secondary sag marks since formed coating film is clear, and, thus,
appearance is not damaged.
[0076] 4) In electrocoating bath, pigment or the like does not
deposit at horizontal portions of article to be coated, and, thus,
water-washing process can be simplified.
[0077] (6) When there is used an electrodeposition paint which has
a bath solid content of at most 10% by weight, uneven drying hardly
occurs even though unelectrodeposited material remains on the
coated surface after water washing, and, thus, decrease of
appearance can be inhibited. Moreover, although electropaint which
has permeated into joint portions spouts out during baking, it
hardly causes sag or popping.
[0078] (7) Owing to the use of such an electropaint, undeposited
material which has adhered to the elctrocoated surface needs no
water-washing for removal, or, even though it is to be
water-washed, primary water washing is sufficient, with the result
that water washing process is shortened, which is preferable in
view of the preservation of environment and economy.
[0079] The present invention will be described more specifically by
Examples and Comparative Examples. Parts and % are by weight and
the film thickness is that of the cured film.
[0080] 1. Preparation of Plastics-Covered Metal Plate
[0081] (a) Both sides of a polyester film with a film thickness of
16 .mu.m were treated by corona discharge, and, then, one side was
coated with a thermocurable polyester resin type adhesive to a film
thickness of 7 .mu.m, which was dried by heating at 120.degree. C.
for 30 seconds, and wound up. Next, both sides of a cold rolled
steel plate of 0.8 mm thickness were plated with alloyed molten
zinc so that the plated amount may be 45 g/m.sup.2, and, then, were
degreased and chemically treated with zinc phosphate ("PB #3080
Treatment"; tradename of a product of Nihon Parkerrizing Co.,
Ltd.). One side of this metal plate was covered with the
above-mentioned polyester film by adhering with heat and pressure
through the intermediary of adhesive, and, thus, plastics-covered
metal plate was obtained.
[0082] 2. Examples and Comparative Examples
EXAMPLE 1
[0083] An (about {fraction (1/25)} scale) model of a main body
consisting of underbody, side member, roof, cowl, upper back and
lower back was previously prepared by cutting, molding and joining
the plastics-covered metal plate (a). Furthermore (about {fraction
(1/25)} scale) models of outer cover parts (car parts) such as
hood, fender, door, luggage door, etc. were prepared by cutting,
molding and joining the plastics-covered metal plate (a).
[0084] Subsequently, outer cover parts were attached to the main
body to form a shell body, which was then dipped into a cationic
electrodeposition paint bath (NOTE 1), and, thus, metal exposed
portions of the shell body were electrocoated under the conditions
of bath temperature of 28.degree. C., voltage of 200 V, and totally
dipped current passing time of 2 minutes. Then, the shell body was
inclined up and down twice in the lengthwise direction at an angle
of about 30 degrees so that sagging electropaint might be drained
off. Next, the shell body was washed with mist spray of an filtrate
which had been prepared by ultrafiltration of a part of
electrodeposition paint bath (NOTE 1), and, thereafter, was set for
five minutes with an inclination of about 30 degrees, and, then,
the electrodeposition coating film was heated at 170.degree. C. for
30 minutes to be cured. The thickness of the flat portion of the
electrodeposition coating film was 20 .mu.m.
[0085] (NOTE 1) Cationic electrodeposition paint bath:
[0086] There were blended 70 parts (solid content) of amine added
polyamide modified epoxy resin, 30 parts (solid content) of blocked
polyisocyanate compound (4,4-diphenylmethane diisocyanate blocked
with 2-ethylhexylalcohol) and 1 part of dibutyltin laurate. The
resultant mixture was neutralized with 15 parts of 10% acetic acid.
Then, the solid content of this mixture was adjusted to 5% with use
of deionized water.
EXAMPLE 2
[0087] A shell body similar to the one mentioned in Example 1 was
dipped into a cationic electrodeposition paint bath (NOTE 2), and,
thus, metal exposed portions of the shell body were electrocoated
under the conditions of bath temperature of 28.degree. C., voltage
of 200 V, and totally dipped current passing time of 2 minutes.
Then, the shell body was inclined up and down twice in the
lengthwise direction at an angle of about 30 degrees so that
sagging electropaint might be drained off. Next, without water
washing, the electrodeposition coating film was heated at
170.degree. C. for 30 minutes to be cured. The thickness of the
flat portion of the electrodeposition coating film was 20
.mu.m.
[0088] (NOTE 2) Cationic electrodeposition paint bath:
[0089] There were blended 70 parts (solid content) of amine added
polyamide modified epoxy resin, 30 parts (solid content) of blocked
polyisocyanate compound (4,4-diphenylmethane diisocyanate blocked
with 2-ethylhexylalcohol) and 1 part of dibutyltin laurate. The
resultant mixture was neutralized with 15 parts of 10% acetic acid.
Then, the solid content of this mixture was adjusted to 3% with use
of deionized water.
EXAMPLE 3
[0090] An (about {fraction (1/25)} scale) model of a main body
consisting of underbody, side member, roof, cowl, upper back and
lower back was previously prepared by cutting, molding and joining
a metal plate plated with alloyed molten zinc whose both surfaces
were uncovered. On the other hand, (about {fraction (1/25)} scale)
models of outer cover parts (car parts) such as hood, fender, door,
luggage door, etc. were prepared by cutting, molding and joining
the plastics-covered metal plate (a).
[0091] Subsequently, outer cover parts were attached to the main
body to form a shell body, which was then dipped into a cationic
electrodeposition paint bath (NOTE 1), and, thus, metal exposed
portions of the shell body were electrocoated under the conditions
of bath temperature of 28.degree. C., voltage of 200 V, and totally
dipped current passing time of 2 minutes. Then, the shell body was
inclined up and down twice in the lengthwise direction at an angle
of about 30 degrees so that sagging electropaint might be drained
off. Next, the shell body was washed with mist spray of an filtrate
which had been prepared by ultrafiltration of a part of
electrodeposition paint bath (NOTE 1), and, thereafter, was set for
five minutes with an inclination of about 30 degrees, and, then,
the electrodeposition coating film was heated at 170.degree. C. for
30 minutes to be cured. The thickness of the flat portion of the
electrodeposition coating film was 20 .mu.m.
EXAMPLE 4
[0092] A shell body similar to the one mentioned in Example 1 was
dipped into a cationic electrodeposition paint bath (NOTE 3), and,
thus, metal exposed portions of the shell body were electrocoated
under the conditions of bath temperature of 28.degree. C., voltage
of 200 V, and totally dipped current passing time of 2 minutes.
Then, the shell body was inclined up and down twice in the
lengthwise direction at an angle of about 30 degrees so that
sagging electropaint might be drained off. Next, the shell body was
washed with mist spray of an filtrate which had been prepared by
ultrafiltration of a part of electrodeposition paint bath (NOTE 3),
and, thereafter, was set for five minutes with an inclination of
about 30 degrees, and, then, the electrodeposition coating film was
heated at 170.degree. C. for 30 minutes to be cured. The thickness
of the flat portion of the electrodeposition coating film was 20
.mu.m.
[0093] (NOTE 3) Cationic electrodeposition paint bath:
[0094] There were blended 70 parts (solid content) of amine added
polyamide modified epoxy resin, 30 parts (solid content) of blocked
polyisocyanate compound (4,4-diphenylmethane diisocyanate blocked
with 2-ethylhexylalcohol), 10 parts (solid content) of gelatinizing
resin fine particles and 1 part of dibutyltin laurate. The
resultant mixture was neutralized with 15 parts of 10% acetic acid.
Then, the solid content of this mixture was adjusted to 5% with use
of deionized water.
EXAMPLE 5
[0095] A shell body similar to the one mentioned in Example 1 was
dipped into a cationic electrodeposition paint bath (NOTE 3), and,
thus, metal exposed portions of the shell body were electrocoated
under the conditions of bath temperature of 28.degree. C., voltage
of 200 V, and totally dipped current passing time of 2 minutes.
Then, the shell body was inclined up and down twice in the
lengthwise direction at an angle of about 30 degrees so that
sagging electropaint might be drained off. Next, the shell body was
washed with mist spray of an filtrate which had been prepared by
ultrafiltration of a part of electrodeposition paint bath (NOTE 3),
and further washed with mist of pure water for the adjustment of
electrobath liquid surface, and, thereafter, was set for five
minutes with an inclination of about 30 degrees, and, then, the
electrodeposition coating film was heated at 170.degree. C. for 30
minutes to be cured. The thickness of the flat portion of the
electrodeposition coating film was 20 .mu.m.
Comparative Example 1
[0096] A shell body similar to the one mentioned in Example 1 was
dipped into a cationic electrodeposition paint bath which had been
prepared by adjusting the solid content of the above-mentioned
cationic electrodeposition paint bath (NOTE 1) to 15%, and, thus,
metal exposed portions of the shell body were electrocoated under
the conditions of bath temperature of 28.degree. C., voltage of 200
V, and totally dipped current passing time of 2 minutes. Then, the
shell body was inclined up and down twice in the lengthwise
direction at an angle of about 30 degrees so that sagging
electropaint might be drained off. Next, the shell body was washed
with mist spray of an filtrate which had been prepared by
ultrafiltration of a part of electrodeposition paint bath, and,
then, the electrodeposition coating film was heated at 170.degree.
C. for 30 minutes to be cured. The thickness of the flat portion of
the electrodeposition coating film was 20 .mu.m.
Comparative Example 2
[0097] A shell body similar to the one mentioned in Example 1 was
dipped into a cationic electrodeposition paint bath ("Elecron
#9600", tradename of a product of Kansai Paint Co., Ltd; Gray
color) having a solid content of 20%, and, thus, metal exposed
portions of the shell body were electrocoated under the conditions
of bath temperature of 28.degree. C., voltage of 200 V, and totally
dipped current passing time of 2 minutes. Then, the shell body was
inclined up and down twice in the lengthwise direction at an angle
of about 30 degrees so that sagging electropaint might be drained
off. Next, the shell body was washed with mist spray of an filtrate
which had been prepared by ultrafiltration of a part of
electrodeposition paint bath ("Elecron #9600"), and, then, the
electrodeposition coating film was heated at 170.degree. C. for 30
minutes to be cured. The thickness of the flat portion of the
electrodeposition coating film was 20 .mu.m.
[0098] 3. Performance Test Results
[0099] The electrocoating films which were formed in Examples and
Comparative Examples were visually observed with respect to stains
of uneven drying, marks of secondary sagging and of popping caused
by electropaint which had spouted out from joints, and edge
corrosion resistance. The results are shown in Table 1.
1 TABLE 1 Comparative Examples Examples 1 2 3 4 5 1 2 Uneven drying
.largecircle. .largecircle. .largecircle. .largecircle.
.largecircle. .DELTA. X Marks of secondary sagging .DELTA. .DELTA.
.DELTA. .DELTA. .largecircle. X X and of popping Edge corrosion
resistance .largecircle. .largecircle. .largecircle.
.circleincircle. .circleincircle. .largecircle.
.circleincircle.
[0100] Test Methods:
[0101] Uneven drying:
[0102] .largecircle. shows that there was observed no stain of
uneven drying of adhered electropaint liquid; .DELTA. shows that
stains of uneven drying of adhered electropaint liquid was seen a
bit; and X shows that stains of uneven drying of adhered
electropaint liquid were seen remarkably all over.
[0103] Marks of secondary sagging and of popping:
[0104] .largecircle. shows that there was observed no marks of
sagging and of popping caused by electropaint which had spouted out
from joints, and that the appearance was good; .DELTA. shows that
marks of sagging (unevenness) and of popping were seen a bit, and
that the appearance was no good; and X shows that marks of sagging
(unevenness) and of popping were seen remarkably, and that the
appearance was no good.
[0105] Edge corrosion resistance:
[0106] The models which had been electrocoated in Examples and
Comparative Examples were placed in a salt spray resistance test
machine (35.degree. C.), and, after 240 hours, the corrosion
resistance at the acute angle edge of cut portion of
plastics-covered steel plate of the shell body was observed.
[0107] .circleincircle. shows that no rust was observed at edge
surface portion; .largecircle. shows that the occurrence of rust
was slightly seen at the edge surface portion; is observed at all,
.DELTA. shows that the occurrence of rust and blisters was observed
a bit at the edge surface portion; and X shows that the occurrence
of rust and blisters was remarkably seen at the edge surface
portion.
* * * * *