Note: Descriptions are shown in the official language in which they were submitted.
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The present invention relates to a method of producing
a coated steel sheet, a coated steel sheet product and a coating
composition, more particularly, the invention comprises a method
in which a steel sheet is electrolytically t^eated in a suspen-
sion, and the resulting coating on the sheet is cured by heat
treatment.
Methods are known for producing a coated metal sheet,
by use of a polymer resin' such methods may be summarized as fol-
lows:
1. By the method of roller coating, knife coating,
spray coating and immersion coating, an organic solvent-type paint
is coated on the metal sheet and then is cured by heat treatment.
2. By the above-mentioned coating method, a mixture of
polymer resin powder and adhesive solution is coated on the steel
sheet, and then is cured by heat treatment.
3. By the method of injection coating, a polymer resin
powder is coated on the metal sheet.
4. By the method of electrophoresis coating, a metal
sheet is coated with an organic solvent containing a polymer resin
powder, is dried and is then cured by heat treatment.
5. By the method of electrophoresis coating, a metal
sheet is coated with a treatment solution containing a water-dis-
persible polymer resin and is then cured by heat treatment.
6. By the method of electrostatic coating, a polymer
resin powder is coated on a metal sheet, and is then cured by heat
treatment.
In the above coating methods 1, 2 and 3, it is diff'icult
to control the thickness or uniformity of the coating. Uniformity
of the coating thickness with regard to quality control is often
one of the most difficult factors to achieve. Also, for the ap-
plication of a thin coating thickness, the methods are very dis-
advantageous from an economical standpoint. Further, when powders
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of a polyrner resin and ino~ganic compound are used as coating mat-
erials, these methods become more and more difficult.
In the case of methods 1, 2 and 4, wherein an organic
solvent is used, a public nuisance is caused by the use of such
solvent.
Me-thod 5 requires that a strong ionic charge be produced
by dissociation of the polymer resin. Therefore, the polymer
resin used must be carefully chosen. For example, insoluble ny-
lon, polyethylene and others cannot be electrodeposited by the
methbd of 5.
Method 6 has the disadvantage that adhesion of the resin
powder is poor and the coated powder is often easily separated
from the base metal sheet by slight impact or contact after pro-
duction. In cases where the coating contains metal powder, the
coating adhesion deteriorates further.
On the other hand, according to Japanese patent laid-
open No. Sho 50-7S623,(June 20, 1975, Japan, K. Ariga et al.),
a metal sheet is cathodically electrot:reated in a suspension
consisting of al~nina sol of 1 to 500 m,u (particle diameter)
with a positive electric charge and an organic polymer resin~
By employing this method, one can easily control the coating
thickness by controlling the amount of electricity. It is not
necessary to use harmful organic solvents, therefore, a public
mlisance is not caused by the treatment solution. Also, the
above mentioned insoluble nylon, polyethylene, etc. and in-
organic powder can easily be electrodeposited in the suspension
as described above.
The present invention provides an improvement in the
coating adhesion in coated sheets of the kind described in the
aforementioned Japanese patent laid-open No. Sho 50-75623.
The present invention involves coating a steel plate
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with composite materials, which steel plate is electrotreated
in a suspension, which contains
i) at least one substance selected from the group
consisting of a water-soluble resin, and a water dispersible
resin;
ii) a water-dispersible sol of a metal compound; and
iii) an infusible organic polymer resin powder; and
further including in certain cases; and
iv) a metal powder, metal alloy powder or a difficult-
ly water-soluble or water-insoluble powdered metal compound.
It is an object of the present invention to produce a
steel sheet coated with a composite material containing
i) at least one substance selected from the group
consisting of a water-soluhle resin and a water-dispersible
resin;
ii) a water dispersible sol of a metal compound; and
iii) an infusible organic polymer resin powder; and
in some cases,
iv) a metal powder, metal alloy powder or a diffi-
cultly water-soluble or water-insoluble powdered metal compound;
which product has excellent corrosion resistance because of the
uniform coating with the composite material.
According to the invention, there is provided a method
of producing a composite coated steel sheet which comprises:
electrolytically treating a steel sheet in a suspen-
sion comprising:
i) at least one substance selected from the group
consisting of a water-soluble resin, and a water-dispersible
resin;
ii) a water dispersible sol of a metal compound; and
iii) an infusible organic polymer resin powder, re-
moving a coated steel sheet from the suspension, and
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curing the coated steel sheet by heat treatment.
According to another aspect of the invention, there
is provided a method of producing a composite coated steel
sheet which comprises electrolytically treating a steel sheet
in a suspension comprising:
i) at least one substance selected from the group
consisting of a water-soluble resin, and a water-dispersible
resin'
ii) a water-dispersible sol of a metal compound,
iii) an infusible organic polymer resin powder; and
iv) at least one member selected from the group con-
sisting of a metal powder, a metal alloy powder and a diffi-
cultly water-soluble or water-insoluble powdered compound of a
metal, being present in said suspension at a concentration not
less than 3 g/l, removing a coated steel sheet from the suspen-
sion and then curing the coated metal sheet by heat treatment.
According to a further aspect of the invention there
is provided a coated steel sheet, comprising a steel sheet sub-
strate having a corrosion resistant thermally cured coating
comprising:
i) at least one substance selected from the group
consisting of a water-soluble resin, and a water-dispersible
resin'
ii) a water-dispersible sol of a metal compound; and
iii~ an infusible organic polymer resin powder.
According to a still further aspect of the invention,
there is provided a coating composition for producing therm-
ally cured corrosion resistant coatings comprising in a liquid
medium
i) at least one substance selected from the group
consisting of a water-soluble resin, and a water-dispersible
resin,
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ii) a water-dispersible sol of a metal compound, and
iii) an infusible organic polymer resin powder.
The liquid medi~ is suitably an aqueous medium,
for example, water.
The water-soluble or water-dispersible organic polymer
resin which is one of the main components of the coating has
the effect of improving the coating adhesion. Such resins in-
clude, by way of example, phenol-type resins, alkyd-type resins,
epoxy-type resins, acrylic-type resins and butadiene-type res-
ins. The particle diameter of these resins is suitably under
lju. A particle diameter of more than lju is unfavourable because
it results in a deterioration in appearance of the deposited
surface.
The above polymer resin is suitably employed in a
concentration in the range of 2 to 50 g/l (as solid) in order
to improve the coating adhesion. A concentration of less than
2 g/l has little effect on the coating adhesion and on the
other hand, a concentration of more than 50 g/l has a remark-
ably bad influence on the throwing power.
The water-dispersible sol may suitably comprise an
oxide or hydroxide of a metal, Eor example, zinc, tin, chromium,
nickel, titanium, zirconium and aluminum. The diameter of the
sol particle is suitably 1 to 500 mJu, and the sol is suitably
employed in a concentration in the range of 1 to 100 g/l The
treatment suspension may contain one or more of such sols. If
the concentration of the water-dispersible sol is less than
1 g/l, the advantageous effects are not fully realized. At a
concentration of more than 100 g/l, the improvement in film-
forming is not in proportion to the increased concentration,
and in fact it disappears because if the viscosity of the sus-
pension becomes too high, the solution becomes difficult to
work with.
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Almost any infusible powder of an organic polymer
resin can be used for the suspension of the present invention,
but it is required that the above-mentioned powder not be sub-
stantially reactive in the electrolytic solution.
For example, there may be used nylon, polyethylene,
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polypropylene, polyamides, diacryl phthalate, ethylene-vinyl ace-
-tate copolymer resins, cellulose acetate butyrate, polyurethanes,
polystyrene, polycarbonakes, chlorovinyl resins, polyfluorovinyl
resins, epoxy resins and acrylic resins. The treatment solution
can contain one or more of these powder resins.
The particle size of -the infusible powder tends to in-
fluence the appearance of the coating layer and the coating ad-
hesion. Small size powder particles form a close deposit layer,
whereas, large size powder particles produce a porous deposit
layer and poor coating adhesion. However, these defects can be
overcome by treatment with a rolling press after electrodeposi-
tion or heat treatment. Therefore, although the powder size is
not specifically restricted, it is desirable that it be under
200~ in order to facilitate the production of a satisfactory coat-
ing.
The concentration of the infusible powder in the treat-
ment solution depends on the powder size, the kind of powder and
the combination of materials.
A concentration o~ less than 3 g/l tends to decrease
the current efficiency.
For the purpose of further improving corrosion resis-
tance, there may optionally be included in the suspension
at least one powder selected from metals, alloys and diff-
icultly water-soluble or water-insoluble compounds of the metals.
The metal powders include, by way of example, alumi-
num, zinc, chromium, cobalt, nickel, iron, tin, lead, copper, man
ganese, titanium, molybdenum, ~irconium, bismuth, antimony and
tungsten. Alloy powders, include, by way of example, the alloys
of the aforementioned metals. Difficultly water-soluble or water-
insoluble powder compounds include, by way of example, compoundsof the aforementioned metals, for example, the hydridel sulfide,
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chloride, oxide, sulfate, ni-trate, hydroxide and carbide.
The suspension can contain one or more of these
powders, suitably having a particle size (diameter) of un-
der 200~. A diameter of more than 200~ produces poor results
such as decrease in the formability and appearance of coating
layer and deterioration of the corrosion resistance~
The concentration of inorganic powder depends on
the particle size of the powder, the ~ind of powder and the
combination of materials, as in the case of the polymer resin
powder.
At a concentration of-less than 3 g/l, improvement in
corrosion resistance is not obtained.
The steel sheet is suitably treated with the suspen-
sion at a temperature of 10-70C. At a temperature higher
than 70C, it is difficult to keep the concentration of the bath
constant because of high evaporation.
The current density for the electrolytic treatment is
not especially restricted, but it must be increased for a short
period of time in order to obtain an increase in the coating
thickness.
On electroplating, the polarity of the metal sheet
treated depends upon the composition of the suspension.
In general, when the pH of the treatment solution is on the acid
side, the metal is cathodically treated. On the other hand, when
the pH of the solution is on the alkaline side, the metal sheet
is anodically treated.
Agitation during electrotreatment has a tendency to
decrease the deposit efficiency, but some agitation is desirable
in order to uniformly maintain the powder particles in the sus-
pensionu
Surface active agents or alcohol may also be added to
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the treatment solution in order -to uniformly disperse the powder
in the treatment solution. For example, nylon powder ~diameter:
50~) can uniformly and quickly be mixed by the addition of 1 g/l
of an aLkyl betaine-type surface active agent.
After preparation of the suspension by the above-men-
tioned method, the metal sheet is cathodically or anodically
treated in the suspension.
The electrodeposited layer formed on the metal sheet
can be rinsed with water and then dried.
The coated metal sheet formed by the above-mentioned
method is cured by heat treatment. The conditions of the heat
treatment depend on the kind of powder and coating thickness.
At any rate it is required that the resin powder be brought
into the molten state. Also before or after curing, a pressure
rolling treatment is desirable in order to improve the appear-
ance of the coated sheet.
The present invention is applicable to the surface
treatment of various metal sheets and other metal shapes. For
example, steel sheets, steel sheets electrotreated in a chrom-
ate solution, steel sheets plated with chromium, tin, zinc,nickel, aluminum and other metal or with alloys of these metals.
Also the present invention is applicable to those steel sheets
which have been subjected to a treatment such as with a chrom-
ate or phosphate.
The invention is further illustrated by reference to
the following examples which are not intended to be construed
as limiting.
EXAMPLE I
A low carbon steel sheet of 0.5 mm thickness, after an-
nealing and temper rolling, was cleaned in 70 g/l of sodium hydrox-
ide solution at a temperature of 70C for 10 seconds, rinsed with
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water and then placed in 70 g/l of sulfuric acid for 5 seconds at
20C and again rinsed with water. ~le pre-treated stee] sheet was
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immediately treated by the method of the present invention.
Treatment of the present invention:
Ba-th composition
hydroxide sol of chromium (average diameter
of particles:lOOm~)
epoxy powder (average
diameter:40~) 10 g/l
polyacrylamide (water-soluble
resin) 10 g/l
Temperature 20C
Current density (cathodic
treatment) 5 A/dm
P]ating time 15 seconds
The coated metal sheet thus ob-tained was blue in colour.
The coated sheet was heated for 2 minutes at a temperature of
250C and a light blue coated steel sheet having a coating thick-
ness of 1~1 was obtained. An Erichsen test indicated an 8 mm
depth. The coated sheet showed no evidence of adhesion loss of
the coated layer. The coated sheet was subjected to a corrosion
test (salk spray test) according to JIS (Japanese Indus-trial Stan-
dard) Z 2371 and the coated sheet did not show any red rust after
200 hours.
On the other hand, in a separate sample produced in the
same way but omitting the polyacrylamide, a red rust was observed
after 200 hours in the sal-t spray test.
By way of further comparison a steel sheet coated with
epoxy powder was produced by the conventional method of spray
coating; the interface adhesion between the coating and the base
metal was poor and the powder was separa-ted from -the base metal
by slight impact. The epoxy powder was baked carefully so as to
not separate from the base metal, but even so the coated sheet
obtained showed red rust after 10 hours in the aforementioned cor-
rosion test.
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EXAMPLE II
The same low carbon steel sheet was subjected to the
same pre-treatrnent as described in Example I. After that, the
pre-treated sheet was immediately treated by the method of the
present invention.
Treatment of the present invention:
~ ath composition
hydroxide sol of chromium (average diameter
of particles:l00m~)
35 g/l
epoxy powder (average
diameter:40~) 10 g/l
polyacrylamide (water-soluble
resin) 10 g/l
zinc powder (diameter:100~) 100 g/l
Temperature 20C
Current density (cathodic treatment) 5 A/dm2
Plating Time 15 seconds
The coated sheet thus obtained was gray in colour. The
coated sheet was heated for 2 minutes at a temperature of 250C
and a gray coated sheet having a coating thickness of 14~ was ob-
tained~
The Erichsen test showed an 8 mm depth and the coated
sheet showed no evidence of adhesion loss of the coating layer.
The coated sheet was evaluated by the salt spray test as described
in Example I. The results show that no red rust was observed
after 500 hours and that the addition of zinc powder was effect-
tive for improving the corrosion properties.
On the other hand, a steel sheet coated with epoxy pow-
der only by the conventional method of spray coating had inferiorinterface adhesion between the powder and base me~al. The epoxy
powder was baked carefully so as not to separate from the base
steel, but even so the coated sheet obtained showed a red rust
after 24 hours.
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EXAMPLE III
The same low carbon steel sheet as described in Example
I was electrogalvanized (weight of deposit:10 g/m2), and then
treated by a chromate method. It was then treated as follows:
Treatment o-f the present invention:
Bath composition
butadiene-type water soluble resin 20 g~l
epoxy powder (average diameter:~0~) 10 g/l
zinc powder (average diameter:100~ 100 g/l
Temperature 20C
Current density (anodic treatment) 5 A/dm
Plating time 15 seconds
The coated sheet thus obtained was gray in colour. The
coated sheet was baked for 30 seconds at a temperature of ~00C.
The results obtained showed that the gray coated sheet had a
coating thickness of 20~.
The coated sheet was evaluated by the coating adhesion
and corrosion resistance tests described in Example II. The re-
sults show excellent properties as described in Example II~
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