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BACKGROUND OF THE INVENTION
(a) Field of the Invention
This invention relates to a method for coating on a substrate an aqueous
paint comprising a dispersion of a fluoroethylene homopolymer or copolymer
resin alone or together with an inorganic filler in an aqueous medium, a
method for coating on the above-mentioned coating layer (used as a primer
layer) a fluoroethylene resin, and a structure of the resulting coating
film.
(B) Description of the Prior Art
Fluoroethylene resins, i.e., homopolymers of a fluoroethylene or copolymers
comprising a major amount of a fluoroethylene such as tetrafluoroethylene,
trifluoroethylene, vinylidene fluoride and trifluoromonochloroethylene
have excellent chemical resistances, as well-known, to most acids and
alkalis, halogen gases, oxidative chemicals, organic solvents such as
halohydrocarbons, and the like. The resins are mechanically strong in that
they have large tensile strength and impact strength, and excellent creep
resistance and abrasion resistance. Furthermore, they are thermally stable
in that fusion temperature is in the range of 150.degree.-320.degree. C.
and decomposition temperature is in the range of 200.degree.-340.degree.
C. In other words, it is expected from these properties that
fluoroethylene resins are very preferable materials for the anti-corrosive
coating onto chemical process apparatus.
As to the methods for coating with fluoroethylene resins, there are known
the method for coating with the resin dispersed in organic solvents such
as an organosol, the method for coating with the aqueous latexes and the
method for powder coating with the dry resin powder.
The organosol method which employs organic solvents has the defects in that
workers may be exposed to harmful vapors of the solvents upon coating
operations and fire is apt to occur. The aqueous latex becomes unstable at
a higher concentration of the resins. It is required to use a large amount
of emulsifiers in order to increase emulsion stability of the latex. The
problems reside in that a general emulsifier is apt to decompose at a
temperature for coating with fluoroethylene resins and a perfluororesin
acid type emulsifier having good thermal stability is very expensive. A
latex of lower concentration is generally employed. When such a dilute
latex is applied onto a perpendicular wall, a very thin coating film is
obtained by each coating operation since such latex is of low viscosity
and easily flows down the wall. For example, in order to obtain a coating
film as thick as 1 mm, the operation of coating--drying--baking must be
repeated for several tens of times.
The method for powder coating is most suitable for obtaining relatively
thick coating film by one coating operation. The powder coating method has
also some defects given below.
The apparatus which is to be used at a relatively low temperature and a dry
state may be coated with fluoroethylene resins alone. But, the chemical
process apparatus which is to be used under corrosive circumstances of
high temperature and high humidity suffers from the following troubles,
even if the coated resin film does not undergo corrosion or swelling.
One of the troubles is that the coating film on a substrate having concave
sufaces peels off at its concave surfaces and the peeling spreads outside.
Another trouble is that a lot of blisters are produced and the coating
peels off from the surface of the substrate as the blisters grow. These
defects cause a decrease in the durable life of the coated apparatus.
Various efforts to depress the formation of blisters have been made. For
example, a primer coating layer has been placed between an anti-corrosive
coating layer and a substrate. In this case, the primer coating layer
should have a good bonding property to the anti-corrosive surface coating
layer. In the primer coating layer is generally employed the resin of the
same kind as the surface coating resin or the resin having good
compatibility therewith. The powder coating method may sometimes be
applied to a primer coating layer. It is generally difficult to form a
uniform coating film on a curved or uneven surface with the powder coating
method. Therefore, the method is generally not suitable for forming a
relatively thin primer coating.
Such peeling off is apt to occur when the coating film is thicker and when
the substrate has a curved surface of a large curvature. The coefficient
of thermal expansion of thermoplastic resins is generally about 10 times
as large as that of metals. Therefore, a large strain owing to the
difference in thermal expansion takes place at the bonding surface between
the metal substrate and the coating resin layer when temperature changes.
Thus, the coating resin layer peels off. In this case, the thicker the
coating film is, the larger the strain on the bonding surface. The strain
is concentrated into the place having a large curvature. Accordingly, a
relatively thick coating film is formed by the powder coating method, and
peeling off is most liable to occur.
The intensive efforts to enhance bonding strength between the resin and a
substrate have hitherto been made for the purpose of eliminating formation
of blisters. But, it seems that such enhancement of bonding strength
scarcely serves to prevent blisters. In other words, peeling between a
substrate and a coating film due to formation of blisters often depends on
cracks in the resin film itself which takes place in the vicinity of the
bending portion therebetween. Therefore, the enhancement of bonding
strength by itself would not solve the problems.
The present inventors developed the following hypothesis. Small cracks are
at first produced at the bonding surface between a substrate and a coating
layer or at the resin layer in the vicinity thereof owing to the
difference in thermal expansion between a fluorinated thermoplastic resin
and a metal substrate. Water or a chemical liquid may more or less
permeate into a synthetic resin. Since a very small amount of a chemical
liquid or water can permeate into a fluoroethylene thermoplastic resin
having no pinholes, the chemical liquid penetrates into the pores in the
resin layer for a long period of time, which were formed by cracks in the
resin layer. The chemical liquid which has penetrated into the pores
expands the pores due to liquid expansion on elevation of its vapor
pressure at an elevated temperature. When temperature falls, the resulting
reduced volume of pressure facilitates penetration of the chemical liquid
into the expanded pores. When the vaporization and condensation of the
chemical liquid are repeated, there occurs a very large difference in
pressure. Owing to the repeated vaporization and condensation, blisters
may grow for a long period of time to result in peeling of a coating, or
blistered portions may produce holes owing to repeated strains at the
blistered portions.
From the above mentioned points of view, sufficient voids were given to the
primer layer of a coating film. Thus, the difference in thermal expansion
can be absorbed by the voids, and air contained in the voids can moderate
the elevation of vapor pressure caused by the penetrated chemical liquid.
Formation of blisters can be regulated by these synergistic effects. It
has some effects on the regulation of blisters to incorporate an inorganic
powder having a small coefficient of thermal expansion into a primer layer
and decrease the average coefficient of thermal expansion of the mixed
composition comprising the inorganic powder and resin. In this case, the
bonding strength to a substrate is naturally decreased when the porous
primer layer is employed. It is considered, however, that the coating film
is substantially prevented from peeling by regulating formation of
blisters.
SUMMARY OF THE INVENTION
An object of the invention is to provide a novel method for coating with a
slurry or paste type aqueous dispersion paint of fluoroethylene resins in
which no organic solvent is employed.
Another object of the invention is to provide a novel method for a
corrosion-resistant coating of fluoroethylene resins which are firmly
bonded even to a metal substrate having concave surfaces and do not
produce blisters even when the coating is exposed to a high temperature
and high moisture for a long period of time, said method comprising
applying the above-mentioned aqueous dispersion paint to a substrate as a
primer coating and then applying a surface coating thereonto with a
fluoroethylene resin powder.
Still another object of the invention is to provide a corrosion-resistant
coating of fluoroethylene resins which can readily and safely be applied
to a substrate.
Further object of the invention is to provide the structure of a
fluoroethylene resin coating on which blisters scarcely come out, said
structure comprising a porous primer coating layer and a surface coating
layer having no substantial void.
These objects of the invention have been fulfilled by the present
invention, which comprises:
(i)
(a) coating with an aqueous fluoroethylene resin dispersion paint the
surface of a substrate; said dispersion paint comprising the particulates,
having a particle size of about 0.03-2.mu., of a fluoroethylene resin
selected from a fluoroethylene polymer and a copolymer of a fluoroethylene
as major comonomeric component and a copolymerizable monomer therewith,
inorganic solid powder which is substantially water-insoluble and has a
geometric average particle size of about 0.03-100.mu., and an aqueous
medium; said particulates and said powder being employed in a volume ratio
of about 40-100/60-0, and being dispersed in an aqueous solution
containing about 0.1-10% by weight of a water-soluble high-molecular
suspension agent, and the solid content therein being about 10-60% by
weight;
(b) drying the coated paint; and
(c) forming a coating film by heating the coated surface of the substrate
to a temperature of not lower than the fusion temperature of the
fluoroethylene resin: or
(ii)
(a) forming a primer layer about 10-50.mu. thick on a substrate according
to the procedure mentioned above in (i), the volume ratio of
fluoroethylene resin to inorganic solid powder being about 40-80/60-20;
(b) coating a primer layer which is in a molten state with the powder
having a particle size of about 10-300.mu. of a fluoroethylene polymer
and/or copolymer which can undergo deposition on the primer layer and may
be identical with or different from the fluoroethylene resin employed in
the primer layer; and
(c) heating the coated resin powder to a temperature of not lower than the
fusion temperature of the resin powder to form a surface coating layer
about 50-3,000.mu. thick;
(d) if desired, the procedures of coating with the fluoroethylene resin
powder and heating may be repeated to obtain the surface coating layer.
In accordance with an embodiment of the present invention there is provided
a fluoroethylene resin coating film on a substrate, which comprises a
porous primer coating layer having a void volume of about 10-40% which
consists substantially of about 20-60 parts by volume of an inorganic
filler and about 80-40 parts by volume of a fluoroethylene resin, and a
surface coating layer having no substantial void and consisting
substantially of a fluoroethylene resin which may be identical with or
different from the resin employed in the primer coating layer and has a
thermal bonding property to the resin.
Incidentally, the term "volume ratio" appearing above in the procedure (i)
(a) means the ratio of calculated volumes, and the calculated volume is a
quotient of the weight of the resin divided by its true specific gravity.
DESCRIPTION OF THE DRAWINGS
FIG. 1 is a graph showing the relation between concentration of filler and
void volume (%) in the present invention.
FIG. 2 is a graph showing the relation between concentration of filler and
coefficient of linear thermal expansion in the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The fluoroethylene resin aqueous dispersion paint to be employed in the
present invention is an aqueous dispersion paint having a solid content of
about 20-60% by weight, which can be prepared by dispersing about 40-100
parts by volume of a fluoroethylene resin having a particle size of about
0.03-2.mu. prepared via emulsion polymerization and about 60-0 parts by
volume of inorganic solid powder which has an average particle size of
about 0.03-100.mu. and is substantially water-insoluble onto an aqueous
solution containing about 0.1-10% by weight of a water-soluble
high-molecular suspension agent.
The inorganic solid powder to be employed should be substantially insoluble
in water since it is to be suspended in water. Furthermore, inorganic
powder is preferably such as does not impair the thermal stability of the
fluoroethylene resin to a large extent. The inorganic solid materials
include fillers or pigments which can generally be employed for plastics,
such as talc, clay, diatomaceous earth, Sirasu earth (a volcanic earth),
silica sand, mica, vermiculite, lime stone, quartz, feldspar, gypsum,
other natural minerals or the ground powder thereof; metallic powders of
aluminium, copper, brass, tin and the like; alumina, titanium oxide,
graphite, and carbon black. When the paint is to be stored for a long
period of time, the particle size of the inorganic solid powder is
preferably as fine as possible, since that having a large particle size
impairs its dispersibility into water. The solid powder which has a
particle size of about 0.03-100.mu. and is readily available is generally
employed, although there is no lower limit on the particle size. The
preferred particle size is generally in the range of about 0.1-50.mu..
When such inorganic solid powder is used in a primer paint, its particle
size is preferably a little larger and the preferred range thereof is
about 0.3-100.mu..
Incidentally, the powder of mica, vermiculite, aluminium, copper or natural
graphite may be in the form of lamina of a polygon, and such laminae can
readily be dispersed into water even when a side of the polygon of the
lamina is more than 200.mu. in length. Therefore, the particle size should
be considered based on the geometric average of its longitudinal and
transverse lengths when such laminae are employed. In the present
invention, the particle size of the inorganic powder is especially defined
by its geometric average size. In case of the resin particles, the
geometric average size is not employed, unless the resin particles are
especially of flat or needle-like shapes.
The water-soluble high-molecular suspension agent has the role of the
suspension stabilizer of the resin powder and inorganic powder. The agent
also serves to increase the viscosity of a dispersed liquid and to
facilitate coating operation, as well as to form a stable film when
moisture is removed by drying and to prevent the resulting unbaked film
from peeling off by means of friction. Such high-molecular suspension
agents include those which may be generally employed as a suspension agent
in suspension polymerization or as a water-soluble sizing agent, such as
water-soluble cellulosic derivatives, e.g. methoxycellulose,
ethoxycellulose, hydroxymethoxycellulose, hydroxyethoxycellulose, sodium
carboxymethylcellulose; polyvinyl alcohol, polyacrylates or
polymethacrylates, e.g. especially the alkali metal salts thereof such as
sodium salts, and ammonium salts; polyvinyl pyrrolidone, polyvinyl methyl
ether, starch, sodium alginate, gelatin, mannan, pectin, and the like, and
mixtures thereof. When the aqueous dispersion paint is stirred upon
application, it is not necessarily required for the paint to have a long
dispersion stability. The more preferable suspension agent for obtaining
the dispersion paint which is stable for a long period of time is an agent
of cellulosic derivative type, polyvinyl alcohol and the like.
The concentration of the water-soluble suspension agents is in the range of
about 0.1-10% by weight and preferably about 0.2-5% by weight. When the
concentration is too low, the viscosity of the solution is decreased, its
dispersibility deteriorates, and the film-forming ability of the dried
coating film is also decreased. When the concentration is too high, the
viscosity of the solution becomes too high, and powder is added to the
solution, it is difficult to disperse the powder therein. Even if the
powder is dispersed, the viscosity remarkably increases by the addition of
the powder and the workability of coating worsens.
The viscosity of a solution of suspension agent varies widely depending on
the species of suspension agents, the molecular weight thereof and the
like. For example, methoxycellulose may give to a 2% aqueous solution
thereof a viscosity of more than several thousands to 10 thousands
centipoise. Such agent is preferably employed in a concentration that
gives the resulting solution a viscosity of not more than about 5,000
centipoise. The agent such as polyvinyl alcohol gives a lower viscosity as
low as several hundreds centipoise to its 10% or more solution, in which
powder can readily be dispersed homogeneously. In such case, the
concentration is preferably not more than 10%, since the strength of the
coating film is lowered when the ratio of the suspension agent to the
resin becomes too large. In general, the lower limit of the viscosity of a
solution is preferably not less than 10 centipoise from the viewpoint of
dispersion stability and film-forming ability. The more preferable
viscosity thereof is about 10-1,000 centipoise.
The aqueous dispersion paint can readily be prepared by homogeneously
mixing the predetermined amounts of a fluoroethylene resin, inorganic
solid powder and an aqueous solution of suspension agent by means of a
ball mill, a colloid mill or a high-speed mixer.
The viscosity of the resulting paint dispersion is also an important
factor. The viscosity is generally required to be in the range of about
300-5,000 centipoise and preferably about 500-2,500 centipoise, which is
measured by a Brookfield spindle No. 3 viscometer at the spindle turning
speed of 60 R.P.M.
When the viscosity is too low, the paint which was applied on a steep slope
or a perpendicular plane may flow down before drying and a uniform coating
cannot be obtained. When the viscosity is too high, coating operation gets
into difficulty not only by spraying but also by brushing.
The aqueous dispersion paint is coated by a conventional means such as
brush or spray. When it is coated on a metal surface, the metal is
preferably subjected, before being coated, to a surface treatment such as
a roughing treatment by means of sand-blast or shot-blast to remove rusts
therefrom. Then, the coating is dried to remove moisture by exposing it to
the outdoor temperature or heating it to a temperature of not higher than
100.degree. C. When the drying is there is formed a primary coating film
wherein the resin powder and inorganic solid powder are deposited on the
surface of the substrate by the aid of the resulting water-soluble high
polymer film binder. The primary film is then baked by heating it to a
temperature of not lower than the melting point of the resin and not
higher than the decomposition point of the resin by means of a heating
oven or hot air. The preferred temperature of heating ranges from the
melting point of the resin to about 340.degree. C. Through this baking
procedure, the polymer in the coating layer is fused and flows to form a
uniform molten film and cover the substrate layer. Incidentally, the
water-soluble high polymer may not decrease the waterproof property of the
resulting film since it is thermally decomposed and carbonized to lose its
hydrophilic property by the baking.
The coating film which is secondarily formed is preferably not less than
about 10.mu. in thickness. When the thickness is less than about 10.mu.,
the film may sometimes lack its continuous property. The upper limit of
the thickness is determined mainly from the economical point of view, and
the coating of more than 3000.mu. thick is not generally needed. When the
coating is too thick, a curved portion of the coated surface may easily
peel off owing to thermal strain between the substrate and the coating
film. When the coating is carried out with the aqueous dispersion paint
alone, the thickness of the film is preferably about 30-2,000.mu..
The thickness of the coating film formed by means of one brushing or
spraying operation is generally about several to several tens .mu.. When a
thicker coating is needed, the coating is effected through several
operations. The primary film formed by 2 or 3 coating operations may be
subjected to baking and such procedures may be repeated. But, it is
preferable that formation of the primary film and baking is repeated for
each coating operation, since a thicker primary film is apt to cause
mud-cracks. If desired, the coating can be carried out by employing two or
more aqueous dispersion paints. For example, a dispersion paint containing
inorganic solid powder can be used for a primer layer and a paint prepared
by dispersing a fluoroethylene resin alone into the aqueous solution of a
suspension agent can be applied thereon to make a surface coating.
The coating with an aqueous dispersion coating is a very excellent method
in that no organic solvent is employed and its bonding strength to a
substrate is superior. But, the method requires a drying step for each
coating operation, to remove moisture, since an aqueous dispersion is
used. The coating operation is a little more complicated than a powder
coating operation, owing to formation of a thinner film per coating and
the like. Furthermore, the repeated heating may sometimes cause
decomposition of a fluoroethylene resin since there is generally a small
difference between the fusion temperature and decomposition temperature of
the fluoroethylene resins.
In accordance with another embodiment of the present invention there is
provided a novel method for surface coating, which comprises employing the
above-mentioned aqueous dispersion paint for primer coating and carrying
out a surface coating with the relatively coarse powder of fluoroethylene
resins which has been prepared by suspension polymerization, precipitation
polymerization employing solvents, bulk polymerization, grinding of molded
articles of the resins, and the like.
The method for surface coating with the coarse resin powder is given in the
following. A primer layer of about 10-500.mu. thick is formed by employing
an aqueous fluoroethylene resin dispersion paint. The primer film in a
molten state is then coated with the coarse powder of a fluoroethylene
resin having a particle size of about 10-300.mu., which is then subjected
to fusion by heating to a temperature of not lower than the melting point
of the coarse resin powder and not higher than the decomposition point
thereof. Thus, a surface coating layer of about 50-3,000.mu. thick is
formed.
In the above-mentioned method, the aqueous dispersion paint to be employed
for the primer coating should contain about 80-40 parts by volume of a
fluoroethylene resin and about 20-60 parts by volume of inorganic solid
powder which is insoluble in water. The primer layer coated with an
aqueous dispersion paint having the above-mentioned composition forms a
porous film having a void volume of about 10-40%, and an anti-corrosive
surface film coated thereon satisfies both anti-blister property and
bonding property. In case that the amount of the inorganic solid powder
employed in the aqueous dispersion paint is less than about 20 parts by
volume, the resulting primer layer has a smaller void volume and the
surface-coated anti-corrosive film is apt to produce blisters when the
film is exposed to corrosive circumstances of high temperature and high
humidity. In case that the amount of the inorganic solid powder is more
than about 60 parts by weight, the resulting anti-corrosive coating film
exhibits a weak bonding strength to a metal substrate. In these cases,
both the aqueous dispersion paints fail to exhibit the excellent effects
on primer coating. The bonding strength to a metal substrate varies
depending on the species of the inorganic powder. The most preferable
inorganic powder which gives a good bonding property and prevents the
resulting film from producing blisters includes the powder of lamina
particles of talc, mica, vermiculite, aluminium or the like.
The thickness of the primer layer is preferably about 10-500.mu.. The
primer of less than 10.mu. may sometimes lack its continuous property and
the resulting anti-corrosive coating is apt to peel off. As described
above, the coating with an aqueous dispersion paint gives a thinner film
than the film obtained by powder coating per one coating operation.
Therefore, when a surface coating is carried out by means of powder
coating, the primer coating does not need to be as thick as about 500.mu..
The more preferable thickness of the primer layer is in the range of about
20-300.mu.. The primer coating can be carried out in the same way as in
the coating with an aqueous dispersion paint alone which was mentioned
above.
The primer coating is preferably carried out by dividing the procedure into
generally two or more operations. In this case, the secondary coating is
preferably effected after baking the primary coating film. In other words,
when a thick primer layer is needed, a primary coating film which was
thickly coated by one operation and dried to remove moisture is apt to
produce mud-cracks. In case of a thin primer coating, a more uniform
coating film can be obtained by the divided coating operations.
Onto the primer coating in a molten state following the final baking, a
coarse fluoroethylene resin powder can be applied by means of a flock
coating, a fluidized bed coating, and other known powder coating
techniques. The fluoroethylene resin powder applied thereto immediately
begins to sinter and form film. A completely continuous surface coating
layer is formed by baking the film in a heating oven at a temperature of
about 200.degree.-300.degree. C. The thickness of the surface coating
layer should be in the range of about 50-3,000.mu.. When the thickness of
the surface coating layer is less than about 50.mu. the resulting
anti-corrosive coating is apt to have pinholes. When the thickness is more
than about 3,000.mu. the resulting coating is liable to peel off owing to
thermal strain between the substrate and the primer coating. The
preferable thickness of the surface coating layer is in the range of about
100-2,000.mu..
Application of the resin powder is preferably divided into two or more
operations in order to obtain a surface coating layer of more than about
300.mu. thick. Otherwise, bubbles may be contained in the surface coating
layer which is subjected to baking treatment. When the resin powder is
applied by the divided operations, the resin powder applied by each
operation is subjected to baking and the application of the resin powder
is repeated when the baked resin is substantially in a molten state. The
coated substrate is taken out from a heating oven after the final baking
of the surface coating is completed, and is cooled to outdoor temperature.
Thus, the anti-corrosive coating of the present invention is completed.
The most preferable structure of the anti-corrosive coating film comprises
a porous primer coating layer having a void volume of about 10-40% which
comprises about 20-60 parts by volume of an inorganic filler and about
80-40 parts by volume of a fluoroethylene thermoplastic resin and is
bonded to a substrate, and a surface coating layer having no substantial
void which comprises a fluoroethylene thermoplastic resin having the
thermal bonding property to the primer coating layer, the resin being
identical with or different from the resin employed in the primer coating
layer.
The anti-corrosive coating described above does not produce blisters even
when the coating surface is exposed to 100.degree. C. steam for 2,000
hours or longer. Whereas, the coating with the fluoroethylene resin powder
alone without employing the primer layer of the present invention produces
blisters within 10 hours under the same condition. The anti-corrosive
coating comprising a primer coating and a surface coating according to the
present invention does not produce peeling at the concave surfaces of the
substrate and does not deteriorate its bonding strength even with water
molecules which permeate through coating film, although its initial
bonding strength is rather smaller than that of the coating without primer
coating. For example, in the accelerated test in which a coating having
slits reaching the surface of metal substrate from the coating surface is
immersed in 100.degree. C. water, the anti-corrosive coating according to
the present invention maintains its initial peeling strength. From these
experimental facts, it is expected that a chemical process apparatus to
which the anti-corrosive coating according to the present invention is
applied will be durable in use for a long period of time under severe
corrosive circumstances of high temperature.
By the fluoroethylene resins to be employed in the present invention is
generally meant a homopolymer of a fluoroethylene, a copolymer of two or
more fluoroethylenes, a copolymer comprising a major amount (by weight) of
one or more fluoroethylenes and a minor amount (by weight) of other
monomer or monomers copolymerizable therewith or mixtures of the polymers.
The "fluoroethylene" means a derivative of ethylene in which at least one
hydrogen atom has been replaced by a fluorine atom: the remaining
hydrogens may or may not be further replaced by another halogen such as
chlorine. Such fluoroethylenes include trifluoroethylene, vinylidene
fluoride, vinyl fluoride, tetrafluoroethylene,
trifluoromonochloroethylene, difluoromonochloroethylene,
monofluoromonochlorovinylidene and the like. The monomers copolymerizable
with a fluoroethylene include a fluoroolefin (except ethylene) such as
hexafluoropropylene, heptafluoropropylene, and perfluoroacrylic acid, and
an olefin such as ethylene, propylene and butylene. The especially
preferable resins are these homopolymers or copolymers having a melting
temperature of about 200.degree.-320.degree. C.
The present invention is further explained by way of the following
examples. It is to be understood that these examples should not limit the
scope of the invention.
EXAMPLE 1
Each of aqueous dispersion paints A, B, C, D, E, F and G was prepared by
mixing the following copolymer with the following mica powder in the
volume ratio, calculated on the basis of true specific gravity, of 100/0,
90/10, 80/20, 70/30, 60/40, 50/50, and 45/55, respectively, and milling
the resulting mixture with the same proportion by weight of a 2% aqueous
solution of methoxy cellulose (tradename: Metlose SM-100, supplied by
Shinetsu Chemical Industry Co., Ltd., Japan) for 24 hours in a ball mill;
copolymer: ethylene-tetrafluoroethylene equimolar copolymer (containing 78%
by weight of tetrafluoroethylene component) having a particle size of
about 0.1.mu., melting point of 265.degree. C. and true specific gravity
of 1.69, which was obtained by emulsion polymerization;
mica powder: mica powder consisting of plate-like particles 0.1-1.mu.
thick, 2-150.mu. in diameter and 2.96 in true specific gravity.
The viscosities of the paints A, B, C, D, E, F and G measured by means of a
Brookfiels spindle No. 3 viscometer operated at the spindle turning speed
of 60 r.p.m. were 1020, 1060, 980, 940, 1020, 620 and 740 C.P.,
respectively.
By using the above aqueous disperson paints, a film was formed on one
surface each of 100.times.100.times.4.5 mm mild steel plates, the surfaces
of which had been polished by means of sand-blast, according to the
following procedure. An aqueous paint prepared in the above was sprayed
onto one surface of the steel plate in such an amount as to form a film
about 50.mu. thick per spraying operation (200-250 g/m.sup.2 per
operation). The coated plate was air-dried, baked for 40 minutes in a
heating oven maintained at 300.degree. C. and then cooled. The above film
forming procedure was repeated five times in total when the paint A was
employed. On the other hand, when the paint B, C, D, E, F or G was
employed, the procedure was repeated three times, followed by repeating
the procedure twice by using the paint A in each case.
The uncoated back surface of the coated steel plate sample was connected to
one electrode of an ohmmeter. A testing electrode 15 mm in diameter was
connected to another electrode of the ohmmeter and applied with the
voltage of 1 KV. The surface of the testing electrode, which had been
wetted with | | |
