Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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SPECIFICATION
ORGANIC COMPOSITE COATED STEEL SHEET HAVING A HIGH
CORROSION RESISTANCE IN A RUST-CONTAMINATED ENVIRONMENT
TECHNICAL FIELD:
This invention relates to an organic composite coated
steel sheet which can advantageously be used in making
automobile bodies or electric appliances.
BACKGROUND ART:
This corrosion of automobile bodies by the salts which
are sprinkled on road surfaces to prevent their freezing
during the wintertime has recently become a big social
problem in North America, northern Europe, and other
countries or regions having a cold winter. A high corrosion
resistance has, therefore, come to be required of the steel
sheets which are used for making automobile bodies, and there
has been a growing tendency to use coated steel sheets having
an improved corrosion resistance instead of conventional cold
rolled steel strips.
In this connection, there are known organic composite
coated steel sheets as disclosed in Japanese Patent
Publication (KOKOKU) No. Hei 4-48348 published on August
6, 1992 and Japanese Patent Application laid open (KOKAI)
on January 18, 1990 under No. Hei 2-15177. These steel
sheets comprise a steel sheet plated with zinc or a zinc
alloy, and having a surface coated with a chromate film
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as a first layer, and an organic resin film as a second layer
formed thereon. The organic resin film is composed of an
organic resin comprising a base resin obtained by adding one
or more basic nitrogen atoms and two or more primary hydroxyl
groups to the ends of molecules of an epoxy resin, a
polyisocyanate compound and a blocked isocyanate compound,
and further contains specific proportions of silica and a
sparingly soluble chromate. These sheets are excellent in
corrosion resistance, weldability, anti-powdering property
and paint adhesion. Japanese Patent Publication (KOKOKU) No.
Hei 4-76392 published on December 3, 1992, discloses a paint
composition for an organic resin film which is prepared by
reacting an epoxy resin with a polyisocyanate compound to form
a cured reaction product, and adding silica to it.
Attention has lately come to be drawn to corrosion
resistance in a corrosive environment in which iron rust is
present (hereinafter referred to as "rust-contaminated
environment") ["Current Advances in Materials and Processes",
vol. 5 (1992), p. 1693]. It has been pointed out that the
exposure of an organic composite coated steel sheet to such an
environment results in iron rust adhering ~o the surface of its
organic resin film, and causing a great reduction of its
excellent corrosion resistance to the extent that it is no
longer appreciably superior in corrosion resistance to any
ordinary zinc or zinc alloy plated steel sheet having no
organic resin film
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thereon. It has lately been found that none of the
conventional organic composite coated steel sheets as
disclosed in the Japanese publications referred to above is
always satisfactory in corrosion resistance in a rust-
contaminated environment.
GALVATECH '92, 2nd International Conference on Zinc
and Zinc Alloy Coated Steel Sheet, 1992, states that
there was obtained an organic composite coated steel
sheet having a lower corrosion resistance in a rust-
contaminated environment when the crosslinking density of
its organic resin film was lowered by reducing the amount
of a crosslinking agent added to an organic regin. It
however, fails to describe any specific rust-contaminated
environment, though the above statement may suggest that
an increase in the crosslinking density of an organic
resin film may be effective for achieving an improved
corrosion resistance.
Under there circumstances, it is an object of this
invention to provide an organic composite coated steel sheet
having an excellent corrosion resistance in a rust-
contaminated environment.
DISCLOSURE OF THE INVENTION:
We, the inventors of this invention, have found that
there are two conditions which are very effective for
improving the corrosion resistance of an organic composite
coated steel sheet in a rust-contaminated environment:
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(1) To form an organic resin film having a high
crosslinking density by using a polyfunctional
polyisocyanate compound as a curing agent; and
(2) To use a sparingly soluble chromate, preferably
both silica and a sparingly soluble chromate, as a
rust-preventing additive.
According to one aspect of the invention, there is
provided an organic composite coated steel sheet having
corrosion resistance in a corrosive environment in which
iron rust is present, which comprises a zinc or zinc
alloy plated steel sheet having a surface coated with a
chromate film having a coating weight of 10 to 200 mg/m2
in terms of metallic chromium, and an organic resin film
formed on the chromate film, having a thickness of 0.2 to
2.0 microns and comprising:
(i) a base resin obtained by adding at least one
basic nitrogen atom and at least two primary hydroxyl
groups to the ends of molecules of an epoxy resin;
(ii) 5 to 80 parts by weight of a polyfunctional
polyisocyanate compound containing at least six
isocyanate groups in each molecule for 100 parts by
weight of the base resin in solid form; and
(iii) at least one chromate selected from the group
consisting of barium chromate, strontium chromate,
calcium chromate, zinc chromate, potassium zinc chromate
and lead chromate;
wherein the base resin, the polyfunctional polyisocyanate
compound and the chromate are present in a weight ratio
defined by:
base resin + ~olyfunctional polyisocyanate compound
chromate
and equal to 90/10 to 40/60.
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According to another aspect of the invention, there
is provided an organic composite coated steel sheet
having corrosion resistance in a corrosive environment in
which iron rust is present, which comprises a zinc or
zinc alloy plated steel sheet having a surface coated
with a chromate film having a coating weight of 10 to
200 mg/m2 in terms of metallic chromium, and an organic
resin film formed on the chromate film, having a
thickness of 0.2 to 2.0 microns and comprising:
(i) a base resin obtained by adding at least one
basic nitrogen atom and at least two primary hydroxyl
groups to the ends of molecules of an expoxy resin;
(ii) 5 to 80 parts by weight of a polyfunctional
polyisocyanate compound containing at least six
isocyanate groups in each molecule for 100 parts by
weight of the base resin in solid form; and
(iii) at least one chromate and a silica, the
chromate being selected from the group consisting of
barium chromate, strontium chromate, calcium chromate,
zinc chromate, potassium zinc chromate and lead chromate;
wherein the base resin, the polyfunctional polyisocyanate
compound, the chromate and the silica are present in a
weight ratio defined by:
base resin + polyfunctional polyisocyanate compound
chromate + silica
and equal to 90/10 to 40/60; and
wherein the silica and chromate are present in a weight
ratio silica/chromate equal to 35/5 to 1/39.
According to a further aspect of the invention,
there is provided an organic composite coated steel sheet
having a corrosion resistance in a corrosive environment
in which iron rust is present, which comprises a zinc or
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zinc alloy plated steel sheet having a surface coated
with a chromate film having a coating weight of 10 to
200 mg/m2 in terms of metallic chromium, and an organic
resin film formed on the chromate film, having a
thickness of 0.2 to 2.0 microns and comprising:
(i) a base resin obtained by adding at least one
basic nitrogen atom and at least two primary hydroxyl
groups to the ends of molecules of an epoxy resin;
(ii) l0 to 50 parts by weight of a polyfunctional
polyisocyanate compound containing at least six
isocyanate groups in each molecule for 100 parts by
weight of the base resin in solid form; and
(iii) at least one chromate selected from the group
consisting of barium chromate, strontium chromate,
calcium chromate, zinc chromate, potassium zinc chromate
and lead chromate;
wherein the base resin, the polyfunctional polyisocyanate
compound and the chromate are present in a weight ratio
defined by:
base resin + polyfunctional polyisocyanate compound
chromate
and equal to 70/30 to 40/60.
According to a still further aspect of the invention
there is provided an organic composite coated steel sheet
having corrosion resistance in a corrosive environment
in which iron rust is present, which comprises a zinc or
zinc alloy plated steel sheet having a surface coated
with a chromate film having a coating weight of 10 to
200 mg/m2 in terms of metallic chromium, and an organic
resin film formed on the chromate film, having a
thickness of 0.2 to 2.0 microns and comprising:
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(i) a base resin obtained by adding at least one
basic nitrogen atom and at least two primary hydroxyl
groups to the ends of molecules of an epoxy resin;
(ii) 10 to 50 parts by weight of a polyfunctional
polyisocyanate compound containing at least six
isocyanate groups in each molecule for 100 parts by
weight of the base resin in solid form; and
(iii) at least one chromate and a silica, the
chromate being selected from the group consisting of
barium chromate, strontium chromate, calcium chromate,
zinc chromate, potassium zinc chromate and lead chromate;
wherein the base resin, the polyfunctional and
polyisocyanate compound, the chromate and the silica are
present in a weight ratio defined by:
base resin + ~olyfunctional polyisocyanate compound
chromate + silica
and equal to 70/30 to 40/60; and
wherein the silica and chromate are present in a weight
ratio silica/chromate equal to 20/20 to 1/39.
Preferably, the silica is hydrophobic.
A particularly high corrosion resistance in a rust-
contaminated environment can be achieved by employing
polyfunctional hexamethylene diisocyanate containing at
least six isocyanate groups in each molecule.
The sparingly soluble chromate can be selected from
among barium chromate, strontium chromate, calcium
chromate, zinc chromate, potassium zinc chromate and lead
chromate, and a mixture thereof.
BRIEF DESCRIPTION OF THE DRAWINGS:
Figure 1 is a graph showing the corrosion resistance
of coated steel sheets as determined by seven cycles of
corrosion tests in a rust-contaminated environment in
,D
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relation to the ratio of an organic resin comprising a
specific base resin and a polyfunctional polyisocyanate
compound (a hexafunctional polyisocyanate compound of the
isophorone diisocyanate series) to a sparingly soluble
chromate;
Figure 2 is a graph showing the corrosion resistance
of
_0.
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coated steel sheets as determined by even cycles of corrosion
tests in a rust-contaminated environment in relation to the
ratio of an organic resin containing a conventional
diisocyanate compound as a curing agent to a sparingly
soluble chromate;
Figure 3 is a graph showing the weldability of coated
steel sheets in relation to the thickness of an organic resin
film formed thereon;
Figure 4 is a graph showing the corrosion resistance of
coated steel sheets as determined by 15 cycles of corrosion
tests in a rust-contaminated environment in relation to the
ratio of an organic resin comprising a specific base rein and
a hexafunctional polyisocyanate compound of the hexamethylene
diisocyanate series to a sparingly soluble chromate;
Figure 5 is a graph showing the perforation corrosion
resistance of coated steel sheets and their corrosion
resistance as determined by seven cycles of corrosion tests
in a rust-contaminated environment in relation to the ratio
by weight of silica and a sparingly soluble chromate which
were added to an organic resin comprising a specific base
resin and a polyfunctional polyisocyanate compound (a
hexafunctional polyisocyanate compound of the isophorone
diisocyanate series);
Figure 6 is a graph showing the corrosion resistance of
coated steel sheets as determined in a rust-contaminated
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environment in relation to the ratio by weight of silica and
a sparingly soluble chromate which were added to an organic
resin containing a conventional diisocyanate compound as a
curing agent; and
Figure 7 is a graph showing the perforation corrosion
resistance of coated steel sheets and their corrosion
resistance as determined by 15 cycles of corrosion tests in
a rust-contaminated environment in relation to the ratio by
weight of silica and a sparingly soluble chromate which were
added to an organic resin comprising a specific base resin
and a hexafunctional polyisocyanate compound of the
hexamethylene diisocyanate series.
DETAILED DESCRIPTION OF THE INVENTION:
Description will now be made of the details of this
invention and the reasons for the limitations made for
defining it.
The organic composite coated steel sheet of this
invention has a chromate film formed on the surface of a zinc
or zinc alloy plated steel sheet, and an organic resin film
formed thereon from a specific composition. The organic
resin film inhibits the excessive dissolution of hexavalent
chromic acid ions from the chromate film into a corrosive
environment and thereby allows the chromate film to produce
a sustained effect of preventing corrosion. The organic
resin film is of a resin composition obtained by reacting a
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base resin with a polyfunctional polyisocyanate compound as
a curing agent, and having a high crosslinking density. The
base resin is obtained by adding one or more basic nitrogen
atoms and two or more primary hydroxyl groups to the ends of
molecules of an epoxy resin, and the polyisocyanate compound
has three or more isocyanate groups in each molecule. The
organic resin film further contains a specific proportion of
a rust-preventing additive which comprises a sparingly
soluble chromate, or both silica and a sparingly soluble
chromate. The resin composition and the rust-preventing
additive work synergically to realize a corrosion resistance
in a rust-contaminated environment which is outstandingly
higher than what can be expected from any organic composite
coated steel sheet known in the art.
The steel sheet which is used as base material may, for
example, be a steel sheet plated with zinc or a Zn-Ni, Zn-Fe,
Zn-Mn, Zn-Al, Zn-Cr, Zn-Co-Cr, Zn-Cr-Ni or Zn-Cr-Fe alloy, or
plated with a composite layer which contains one or more
additives, such as a metal oxide, sparingly soluble chromate,
or polymer, in a layer of zinc or a zinc alloy. It may also
be a steel sheet plated with a multi-layer consisting of
two or more layers of the same or different compositions.
The plating of the sheet can be effected by any method
selected from among electrodeposition, hot dipping and vapor-
phase deposition on a case to case basis, though
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electrodeposition may have an advantage over the other
methods for plating a cold rolled steel strip.
The chromate film formed on the surface of the zinc or
zinc alloy plated steel sheet inhibits the corrosion of the
steel sheet by its self-healing effect owing to hexavalent
chromic acid ions. If the chromate film has a coating weight
of less than 10 mg/m2 in terms of metallic chromium, it
cannot be expected to impart any satisfactory corrosion
resistance to the steel sheet, and if its coating weight
exceeds 200 mg/m2, it lowers the weldability of the steel
sheet. Therefore, the chromate film is so formed as to have
a coating weight of 10 to 200 mg/m2 in terms of metallic
chromium. Its coating weight is preferably from 20 to 100
mg/m2 in terms of metallic chromium to realize still higher
levels of corrosion resistance and weldability.
The chromate film may be of a reacted-in-place,
electrolytic, or dried-in-place type chromate coating. The
dried-in-place type chromate coating is, however, preferred
from the standpoint of corrosion resistance, since it can
form a chromate layer containing a large amount of hexavalent
chromic acid ions.
The dried-in-place type chromate coating can be formed
by coating the zinc or zinc alloy plated steel sheet with a
solution consisting mainly of a partially reduced aqueous
solution of chromic acid, and further containing one or more
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m
additives selected from among (1) to (7) below, if required,
and drying it without rinsing it with water:
(1) An organic resin such as a water-soluble or
-dispersible acrylic or polyester resin;
(2) a colloid and/or powder of oxide such as silica,
alumina, titania or zirconia;
(3) an acid such as molybdic, tungstic or vanadic
acid, and/or a salt thereof;
(4) a phosphoric acid such as phosphoric or
polyphosphoric acid;
(5) a fluoride such as zirconium fluoride,
silicofluoride or titanium fluoride;
(6) a metal ion such as a zinc ion; and
(7) an electrically conductive fine powder such as
iron phosphide or antimony-doped tin oxide.
A roll coater is usually employed for coating the strip
with the solution, though it is also possible to apply the
solution to the strip by dipping or spraying and regulate its
coating weight with an air knife, or by roll squeezing.
Referring now to the epoxy resin used to form the
organic resin coating, it is preferable to use mainly a
product of condensation of epichlorohydrin with bisphenol A.
Although there are also epoxy resins consisting solely of an
aliphatic or alicyclic structure, such as epoxidized oil and
epoxy polybutadiene, it is preferable to use an epoxy resin
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consisting mainly of the above product of condensation to
achieve excellent corrosion resistance. The preferred epoxy
resins which are commercially available include EPZCOAT* 828,
1001, 1004, 1007, 1009 and 1010, which are the products of
Shell Chemical Co., Ltd. It is possible to use one of these
products, or a mixture of two or more products. It is
desirable to use an epoxy resin having a number-average
molecular weight of at least 1500, if it has to be cured at
a low temperature.
It is possible to introduce basic nitrogen atoms and
primary hydroxyl groups into an epoxy resin by, for example,
adding alkanolamines and/or alkylalkanolamines to the oxirane
groups of the epoxy resin. The amines which can be employed
include monoethanolamine, diethanolamine, dimethylamino-
ethanol, monopropanolamine, dipropanolamine and dibutanol-
amine. One of these amines, or a mixture thereof may be
employed.
Explanation will now be made of the advantages which can
be expected from the use of the base resin as described
above. The epoxy resin obtained by the condensation of
epichlorohydrin with bisphenol A can form an organic resin
coating permitting the excellent adhesion of a cationic
electrodeposition paint which is usually employed for
preventing the rusting of automobile bodies. The following
is a summary of the advantages which can be expected from the
* Trade-mark
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introduction of basic nitrogen atoms and primary hydroxyl
groups into the epoxy resin:
(1) It is possible to prevent the destruction of the
organic resin coating by the action of an alkali
occurring from cationic electrodeposition and
stabilize its adhesion to the underlying chromate
coating and a coating formed by cationic
electrodeposition;
(2) The primary hydroxyl groups and an appropriately
selected organic solvent, which will hereinafter
be described, improve the reactivity of the epoxy
res in with a crosslinking agent at a low temperature; and
(3) The introduction of at least two mole of hydroxyl
groups per molecule of the epoxy resin enables the
formation of a film having a satisfactorily high
crosslinking density. No satisfactory
crosslinking can be expected from the introduction
of less than two mols of hydroxyl groups.
It is also effective to modify the epoxy resin partially
with another compound, though it is necessary to introduce an
average of at least two moles of primary hydroxyl groups per
molecule of the epoxy resin.
The partial modification of the epoxy resin can be
effected by, for example:
(1) Esterification by a monocarboxylic acid (e.g., a
21 48485
14
saturated or unsaturated fatty acid, such as
coconut, soybean or castor oil fatty acid; a low-
molecular aliphatic monocarboxylic acid, such as
acetic, propionic or butyric acid; or an aromatic
monocarboxylic acid, such as benzoic acid);
(2) Modification with an aliphatic or aromatic amine
(e. g., an aliphatic amine such as monomethylamine,
dimethylamine, monoethylamine, diethylamine or
isopropylamine; or an aromatic amine such as
aniline); or
(3) Modification with an oxo acid (e.g., lactic or y-
hydroxypropionic acid).
It is inappropriate for the purpose of this invention to
employ an epoxy resin modified with a dicarboxylic acid
(e.g., adipic or sebacic acid), since it has too high a
molecular weight. Moreover, the reaction is difficult to
control to maintain a uniform molecular weight distribution.
Furthermore, no improved corrosion resistance can be expected
from a layer of any such modified epoxy resin.
The organic resin coating is preferably cured by
utilizing a urethanation reaction between the hydroxyl groups
in the base resin and the isocyanate groups in the poly-
isocyanate compound.
Referring now to the polyisocyanate compound employed in
the organic resin coating, it is necessary to use a compound
21 48485
having at least three isocyanate groups in each molecule to
ensure an improved corrosion resistance in a rust-
contaminated environment. The isocyanate groups may or may
not be blocked. No satisfactorily improved corrosion
resistance can be expected from the use of any monoisocyanate
compound having one isocyanate group per molecule, or any
diisocyanate compound having two isocyanate groups per
molecule. We have found that the use of a polyfunctional
isocyanate compound having at least three, or preferably at
least four, or more preferably at least six, isocyanate
groups per molecule makes it possible to obtain a higher
corrosion resistance in a rust-contaminated environment than
what can be obtained when any monoisocyanate or diisocyanate
compound is employed.
Examples of the polyfunctional polyisocyanate compounds
having at least three isocyanate groups per molecule are a
compound having at least three isocyanate groups per
molecule, a compound obtained by reacting an isocyanate
compound having at least two isocyanate groups with a poly-
hydric alcohol, and a burette or isocyanuric ring type adduct
thereof. More specific examples are a polyisocyanate
compound having at least three isocyanate groups, such as
triphenylmethane-4,4',4"-triisocyanate, 1,3,5-
triisocyanatobenzene, 2,4,6-triisocyanatotoluene, 4,4'-
dimethyldiphenylmethane-2,2: 5,5'-tetraisocyanate; an
21 48485
16
adduct obtained by reacting a polyisocyanate compound in an
amount giving an excess of isocyanate groups with the
hydroxyl groups of a polyol, such as ethylene glycol,
propylene glycol, 1,4-butylene glycol, polyalkylene glycol,
trimethylolpropane or hexanetriol; and a burette or
isocyanuric ring type adduct thereof, such as hexamethylene
diisocyanate, isophorone diisocyanate, tolylene diisocyanate,
xylylene diisocyanate, 4,4'-diphenylmethane diisocyanate or
4,4'-methylenebis(cyclohexyl isocyanate).
Examples of the polyisocyanate compounds which can be
reacted with polyols to form adducts are a polyisocyanate
compound having at least three isocyanate groups; an
aliphatic diisocyanate compound such as hexamethylene
diisocyanate, 1,4-tetramethylene diisocyanate, dimer acid
diisocyanate or lysine diisocyanate; and alicyclic
diisocyanate compound, such as isophorone diisocyanate, 4,4'-
methylenebis(cyclohexyl isocyanate), methylcyclohexane-2,4-
(or -2,6-)diisocyanate or 1,3-(or 1,4-
di(isocyanatomethyl)cyclohexane; and an aromatic
diisocyanate compound such as xylylene diisocyanate, tolylene
diisocyanate, m-(or p-)phenylene diisocyanate,
diphenylmethane diisocyanate or bis(4-isocyanatophenyl)-
sulfone.
Polyfunctional hexamethylene diisocyanate is, among
others, preferred for achieving a high corrosion resistance
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m
in a rust-contaminated environment, though there are also
many other effective polyfunctional polyisocyanate compounds
having six isocyanate groups per molecule (hexafunctional
polyisocyanate compounds).
It is possible to use two or more polyfunctional
polyisocyanate compounds together, or a mixture of homologous
compounds having different numbers of isocyanate groups per
molecule.
It is necessary to protect the isocyanate groups in the
curing agent to form a stable coating. The isocyanate group
may be protected by a protective group (or a blocking agent)
which is dissociated upon heating for curing, so that the
isocyanate group may be regenerated.
Examples of the protective (or blocking) agents which
can be employed are:
(1) Aliphatic monoalcohols, such as methanol, ethanol,
propanol, butanol and octyl alcohol;
(2) Monoethers, such as monomethyl, monoethyl,
monopropyl (n- or iso), monobutyl (n-, iso or sec)
ethers, of ethylene glycol and/or diethylene
glycol;
(3) Aromatic alcohols, such as phenol and cresol, and
(4) Oximes, such as acetoxime and methyl ethyl ketone
oxime.
If one or more of these agents are reacted with the
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isocyanate compound, there is obtained an isocyanate compound
which is so protected as to be stable at least at ordinary
room temperature.
The polyfunctional polyisocyanate compound is preferably
employed in the amount of 5 to 80 parts, or more preferably
to 50 parts, by weight as the curing agent for 100 parts
by weight of the base resin (solid). If the amount of the
curing agent is less than 5 parts by weight, there is formed
only a coating having a crosslinking density which is too low
for achieving any satisfactorily improved corrosion
resistance in a rust-contaminated environment. If it exceeds
80 parts by weight, there is obtained only a coating which is
low not only in corrosion resistance in a rust-contaminated
environment, but also in perforation corrosion resistance and
adhesion, since the unreacted isocyanate absorbs water.
It is possible to use as a crossli,nking agent with the
isocyanate compound an alkyl-etherified amino resin obtained
by reacting with a monohydric alcohol having 1 to 5 carbon
atoms a part or all of a methylol compound obtained by
reacting at least one of melamine, urea and benzoguanamine
with formaldehyde.
Although the crosslinking agent as described above can
satisfactorily crosslink the resin, it is desirable to use
also a known curing catalyst to increase the crosslinkability
of the resin at a low temperature. It is possible to use,
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21 4r8485
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for example, N-ethylmorpholine, dibutyltin dilaurate, cobalt
naphthenate, stannous chloride, zinc naphthenate or bismuth
nitrate as the catalyst. The resin composition may further
contain, for example, a known acrylic, alkyd or polyester
resin to form a coating which is improved in adhesion and
some other physical properties.
The organic resin coating as described above is intended
for achieving a high level of perforation corrosion
resistance, a close adhesion to a multilayer film of paint
formed by two or more coats thereof, and a high level of
corrosion resistance in a corrosive environment including
iron rust. The following is a more specific description of
these features:
(1) The epoxy resin of which the resin composition
consists mainly enables the coating to adhere
closely to the steel surface and a film of paint
formed by cationic electrodeposition and exhibit a
high level of perforation corrosion resistance;
(2) The basic polarity of the resin prevents any
deterioration of its structure by an alkali
appearing on the interface during cationic
electrodeposition; and
(3) The composition can form a film having a high
crosslinking density and thereby a high corrosion
resistance in a rust-contaminated environment,
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since the polyfunctional polyisocyanate compound
having at least three isocyanate groups per
molecule is used as the curing agent for the epoxy
resin containing at least two hydroxyl groups per
molecule.
The composition can be used in the form of a water-
dispersible or -soluble composition obtained by neutralizing
the base in the epoxy resin with a low-molecular acid. It
is, however, advisable to use a composition dissolved in an
organic solvent without such neutralization for coating a
steel sheet which requires drying at a low steel temperature
not exceeding 250°C, and particularly, a BH steel sheet which
requires drying at a still lower temperature not exceeding
170°C.
A water-dispersible or -soluble composition is likely to
form a film which is somewhat low in corrosion resistance and
adhesion, since the acidic compound which is required for
making the composition water-soluble is likely to form a salt
which absorbs water in and under the film in a moist environ-
ment, while also disabling the formation of any
satisfactorily strong film by drying at a low temperature.
Although it is possible to use one or a mixture of
organic solvents which are usually employed in the
paint industry, it is advisable to avoid the use of any high-
boiling alcoholic solvent, since it inhibits the curing
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21
reaction of the film. Examples of the solvents which should
not be used are ethylene glycol, diethylene glycol, monoalkyl
ethers, and alcohols having five or more carbon atoms and
primary hydroxyl groups. The solvents which are recommended
include hydrocarbons, ketones, esters and ethers. It is also
appropriate to use alcohols having not more than four carbon
atoms and a low molecular weight, or having secondary or
tertiary hydroxyl groups.
The organic resin film or coating may contain a specific
proportion of a sparingly soluble chromate as the rust-
preventing additive. It cooperates with the resin
composition to achieve a high level of corrosion resistance
in a rust-contaminated environment.
The sparingly soluble chromate is considered to inhibit
the corrosion of the zinc or zinc alloy plated steel sheet by
discharging hexavalent chromic acid ions as a result of its
slight dissolution in a corrosive environment, as is the case
with the chromate in the underlying layer.
The sparingly soluble chromate which can be employed for
the purpose of this invention is a fine powder of, for
example, barium chromate (BaCr04), strontium chromate
( SrCr04 ) , calcium chromate ( CaCr04 ) , zinc chromate ( ZnCr04
4 Zn ( OH ) Z ) , potassium zinc chromate ( K20 ~ 4 Zn0 ~ 4Cr03 ~ 3H20 ) , or
lead chromate (PbCr04). It is also possible to use a mixture
of two or more such salts. It is, however, preferable from
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22
the standpoint of corrosion resistance to use barium or
strontium chromate, or both, since they can be expected to
exhibit the self-healing effect of chromic acid ions over a
long period of time. The use of barium chromate, which is
poorly soluble in water, is preferred to minimize the
dissociation of water-soluble chromium from the organic resin
coating during the treatment of automobile bodies prior to
paint application.
The sparingly soluble chromate is considered to rely
upon hexavalent chromic acid ions for healing any defect
formed in the organic resin coating by rust in a rust-
contaminated environment.
According to this invention, a high level of corrosion
resistance in a rust-contaminated environment can be achieved
by the combined effects of the resin composition and the
sparingly soluble chromate when the latter is employed in a
specific ratio by weight of nonvolatile matter to the former:
i.e.,
(Base resin + polyfunctional polyisocyanate compound/
sparingly soluble chromate = 90/10 to 40/60.
If the ratio exceeds 90/10, the resin composition and the
sparingly soluble chromate do not produce any satisfactorily
high combined effect, but form a film having only a low level
of corrosion resistance in a rust-contaminated environment.
I f the ratio i s lower than 4 0 / 6 0 , the amount o f the epoxy
21 48485
23
resin is too small to provide an effective binder for any
closely adhering layer of paint.
Figure 1 shows the corrosion resistance of coated steel
sheets as determined by seven cycles of corrosion tests in a
rust-contaminated environment in relation to the ratio by
weight of an' organic resin (No. 2 in Table 2), which
comprises a specific base resin and a hexafunctional
polyisocyanate compound of the isophorone diisocyanate
series, to a sparingly soluble chromate. As is obvious
therefrom, the ratio of the organic resin/sparingly soluble
chromate in excess of 90/10 resulted in an undesirably low
level of corrosion resistance in a rust-contaminated
environment, while the ratio falling short of 40/60 resulted
in an undesirably low paint adhesion. It is, thus, obvious
that the ratio of the organic resin/sparingly soluble
chromate is preferably from 90/10 to 40/60, and more
preferably, from 70/30 to 40/60. For the sake of comparison,
Figure 2 shows the corrosion resistance of coated steel
sheets as determined by seven cycles of corrosion tests in a
rust-contaminated environment in relation to the ratio by
weight of an organic resin containing a conventional
diisocyanate compound (HMDI) as the curing agent to a
sparingly soluble chromate. It is obvious from the results
shown in Figures 1 and 2 that the excellent corrosion
resistance of the products of this invention owes itself to
21 48485
24
the combined effects of the polyfunctional polyisocyanate
compound and the sparingly soluble chromate employed in the
specific proportions.
Figure 4 shows the corrosion resistance of coated steel
sheets as determined by 15 cycles of corrosion tests in a
rust-contaminated environment in relation to the ratio by
weight of an organic resin containing a hexafunctional
polyisocyanate compound of the hexamethylene diisocyanate
series as the curing agent to a sparingly soluble chromate.
The ratio of the organic resin/sparingly soluble chromate in
excess of 90/10 resulted in an undesirably low level of
corrosion resistance, while the ratio falling short of 40/60
resulted in an undesirably low paint adhesion. Thus, it is
obvious that the ratio is preferably from 90/10 to 40/60, and
more preferably from 70/30 to 40/60, as is the case with the
organic resin coating containing an isophorone diisocyanate
type compound as the curing agent.
The hexafunctional polyisocyanate compound of the
hexamethylene diisocyanate series, however, makes it possible
to achieve a higher level of corrosion resistance in a rust-
contaminated environment than what can be obtained when the
isophorone diisocyanate type compound is employed, if the
ratio of the organic resin/sparingly soluble chromate is the
same, as is obvious from the description of Examples which
will hereinafter appear (compare, for example, Nos. 51 and 55
25 21 4 8 4 8 5
in Example 1).
The organic resin coating may contain specific
proportions of both silica and a sparingly soluble chromate
as the rust-preventing additive to realize high levels of
corrosion resistance in a rust-contaminated environment and
perforation corrosion resistance.
Silica promotes the formation of basic zinc chloride,
which is an effective corrosion inhibitor, as one of the
corrosion products of a zinc or zinc alloy plated steel
sheet. Moreover, silica apparently can prevent corrosion
effectively by dissolving slightly in a corrosive environment
and forming a silicic acid ion which serves as a film-forming
corrosion inhibitor.
The silica which can be employed for the purpose of this
invention is, for example, fumed silica (e.g. products of
Nippon Aerozile Co., Ltd. known as AEROSIL~130, AEROSIL 200,
AEROSIL 300, AEROSIL 380, AEROSIL 8972, AEROSIL 8811 and
AEROSIL 8805), organosilica sol (e. g. products of Nissan
Chemical Industries, Ltd. known as MA-ST, IPA-ST, NBA-ST,
-* * * * -
IBA-ST, EG-ST, XBA-ST, ETC-ST and DMAC-ST), silica of the
precipitated type obtained by the reaction of sodium silicate
and mineral acids (e. g. products of Tokuyama Soda Co., Ltd.
known as T-32(S), K-41 and F-80), or silica of the gel type
obtained by the reaction of sodium silicate and mineral acids
(e. g. products of Fuji Davison Chemical Ltd. known as SYLOID
' Trade-mark
21 48485
26
244, SYLOID 150, SYLOID 72, SYLOID 65 and SHIELDEX). It is
also possible to use a mixture of two or more types of
silica.
There are hydrophilic and hydrophobic forms of silica.
Although hydrophilic silica may effectively be used for
achieving an improved corrosion resistance in a rust-
contaminated environment, hydrophobic silica is more
effective for that purpose.
Hydrophilic silica has a hydrophilic surface covered
I
with a hydroxyl group (silanol group-Si-OH). The silanol
I
group is so high in reactivity as to react easily with an
organic compound and give an organic surface to silica.
Hydrophobic silica has a hydrophobic surface formed by
substituting e.g. a methyl or alkyl group for a part or
substantially all of the silanol group on the surface of
hydrophilic silica.
While there are a variety of methods available for
preparing hydrophobic silica, it can typically be prepared by
the reaction of alcohols, ketones, esters or other organic
solvents and silanes, silazanes, polysiloxanes, etc. which
may be effected under pressure in an organic solvent, or
under heat in the presence of a catalyst.
Although silica as a whole is effective for preventing
corrosion, hydrophobic silica is particularly effective for
achieving an improved corrosion resistance in a rust-
* Trade-mark
21 48485
27
contaminated environment. Hydrophilic silica is less
effective for that purpose apparently because of its high
hydrophilic property which is very likely to cause the
penetration of iron ions, or oxides from iron rust. The use
of hydrophobic silica is, therefore, preferred for the
purpose of this invention.
According to this invention, it is possible to realize
high levels of corrosion resistance in a rust-contaminated
environment and perforation corrosion resistance by adding
specific proportions of silica and a sparingly soluble
chromate to the resin composition which comprises a specific
base resin and a polyfunctional polyisocyanate compound as
described above. More specifically, it is possible to
achieve a high level of corrosion resistance (corrosion
resistance in a rust-contaminated environment and perforation
corrosion resistance) by employing silica and a sparingly
soluble chromate in the ratios by weight of nonvolatile
matter as set forth below:
(1) (Base resin + polyfunctional polyisocyanate
compound)/(silica + sparingly soluble chromate) -
90/10 to 40/60; and
(2) Silica/sparingly soluble chromate = 35/5 to 1/39.
If the ratio of (base resin + polyfunctional polyisocyanate
compound)/(silica + sparingly soluble chromate) exceeds
90/10, the resin composition and the silica and sparingly
21 48485
28
soluble chromate do not produce any satisfactorily good
effect of preventing corrosion, but form a coating having
only a low level of corrosion resistance in a rust-
contaminated environment. If it falls short of 40/60, the
amount of the epoxy resin is too small to be an effective
binder for any closely adhering film of paint. If the ratio
of silica/sparingly soluble chromate exceeds 35/5, there is
formed only a coating having a low level of corrosion
resistance in a rust-contaminated environment, and if it
falls short of 1/39, there is formed only a coating having a
low level of perforation corrosion resistance.
Silica promotes the formation of stable corrosion
products and thereby restrains corrosion by rust in a rust-
contaminated environment, while the sparingly soluble
chromate heals by hexavalent chromic acid ions any defect
formed in the organic resin coating by rust in such an
environment. The combination of silica and sparingly soluble
chromate having different mechanisms for restraining
corrosion by rust as described above makes it possible to
achieve a high level of corrosion resistance in a rust-
contaminated environment. The combination also makes it
possible to obtain a high level of perforation corrosion
resistance.
Figure 5 shows the perforation corrosion resistance of
coated steel sheets as determined by 200 cycles of tests and
21 48485
29
their corrosion resistance in a rust-contaminated environment
as determined by seven cycles of tests in relation to the
ratio by weight of silica and a sparingly soluble chromate
which were added to an organic resin comprising a specific
base resin and a hexafunctional polyisocyanate compound of
the isophorone diisocyanate series (No. 2 in Table 2). As is
obvious therefrom, a low level of corrosion resistance in a
rust-contaminated environment results from any ratio by
weight of silica/sparingly soluble chromate exceeding 35/5,
and a low level of perforation corrosion resistance from any
ratio falling short of 1/39. It is, thus, obvious that the
ratio of silica/sparingly soluble chromate is preferably from
35/5 to 1/39, and more preferably from 20/20 to 1/39. For
the sake of comparison, Figure 6 shows the corrosion
resistance of coated steel sheets in a rust-contaminated
environment in relation to the ratio by weight of silica and
a sparingly soluble chromate which were added to an organic
resin containing a conventional diisocyanate compound (HMDI)
as the curing agent. It is obvious from the results shown in
Figures 5 and 6 that the excellent corrosion resistance of
the coated steel sheet according to this invention and its
excellent perforation corrosion resistance owe themselves to
the combined effects of the polyfunctional polyisocyanate
compound and the silica and sparingly soluble chromate
employed in the specific proportions.
21 48485
Figure 7 shows the perforation corrosion resistance of
coated steel sheets as determined by 200 cycles of tests and
their corrosion resistance in a rust-contaminated environment
as determined by 15 cycles of tests in relation to the ratio
by weight of silica and a sparingly soluble chromate which
were added to an organic resin containing a hexafunctional
polyisocyanate compound of the hexamethylene diisocyanate
series as the curing agent. It is obvious therefrom that the
ratio of silica/sparingly soluble chromate is preferably from
35/5 to 1/39, and more preferably from 20/20 to 1/39, as is
the case when the hexafunctional polyisocyanate compound is
of the isophorone diisocyanate type.
As is obvious from the description of Examples which
will hereinafter appear (compare e.g. Nos. 66 and 70 in
Example 2), it is possible to achieve a higher level of
corrosion resistance in a rust-contaminated environment by
employing a hexafunctional polyisocyanate compound of the
hexamethylene diisocyanate series than by employing a
compound of the isophorone diisocyanate series if the ratio
of silica/sparingly soluble chromate is the same.
Although silica and a sparingly soluble chromate may be
the principal additives to the resin composition, it may
further contain, for example, at least one of a silane
coupling agent, a color pigment (e.g. an organic pigment of
the condensed polycyclic type, or of the phthalocyanine
2148485
31
series), a color dye (e. g. an azo dye, or a dye in the form
of a complex salt of an azo dye and a metal ) , a lubricant
(e. g. polyethylene wax, teflon, graphite, or molybdenum
disulfide), a rust-inhibitive pigment (e. g. aluminum
dihydrogen tripolyphosphate, aluminum phosphomolybdate, or
zinc phosphate), an electrically conductive pigment (e. g.
iron phosphide, or antimony-doped tin oxide), and a surface
active agent.
The organic resin coating is formed on the chromate
coating so as to have a film thickness of 0.2 to 2.0 microns,
and a preferred film thickness of 0.5 to 1.5 microns. If its
thickness is smaller than 0.2 micron, it is impossible to
achieve any satisfactorily high corrosion resistance in a
rust-contaminated environment, and if its thickness exceeds
2.0 microns, it brings about an undesirable lowering of
weldability (particularly, continuous spot weldability).
Figure 3 shows the results obtained by comparing spot
weldability (continuous) with the thickness of the organic
resin coating. It is obvious therefrom that the coating
thickness exceeding 2.0 microns brings about an undesirable
lowering of spot weldability.
A roll coater is usually employed for coating the steel
sheet with the paint composition as described above, though
it is also possible to apply the composition by dipping or
spraying and regulate its coating weight with an air knife,
21 48485
32
or by roll squeezing. A hot-air, high-frequency induction,
or infrared heating oven can, for example, be employed for
heating the sheet coated with the composition. It is heated
to a temperature of from 80°C to 250°C, and preferably from
100°C to 200°C. If this invention is applied to a BH or bake
hardenable steel sheet, it is preferably heated to a
temperature not exceeding 150°C. It is a great advantage of
the coated steel sheet of this invention that it can be
manufactured by baking at such a low temperature.
No baking temperature lower than 80°C can promote the
crosslinking of the coating and achieve a satisfactorily high
level of corrosion resistance. Baking at a high temperature
over 250°C, however, results in a lower corrosion resistance.
This is probably due to the fact that baking at a high
temperature over 250°C promotes the volatilization of water
from the chromate layer and the condensation reaction of the
hydroxyl groups (-Cr-OH) and thereby causes the destruction
of the chromate layer by cracking and the reduction of
hexavalent chromium resulting in the loss of its passivating
action.
As cationic electrodeposition is usually employed for
coating automobile bodies with a paint, and as the coated
steel sheet of this invention is mainly used for making
automobile bodies, the chromate and organic resin coatings
are preferably so formed thereon as to have a total wet
21 48485
33
electrical resistance not exceeding 200 kiloohms per square
centimeter so that a satisfactory layer of paint may be
formed thereon by cationic electrodeposition.
The coated steel sheet of this invention may carry the
films, layers or coatings on one or both sides thereof.
Thus, the coated steel sheet of this invention may, for
example, have:
(1) a plating film, a chromate film and an organic
resin film on one side thereof, while the other
side is a steel surface;
(2) a plating film, a chromate film and an organic
resin film on one side thereof and a plating film
on the other side; or
(3) a plating film, a chromate film and an organic
resin film on both sides.
The organic composite coated steel sheet of this
invention can be used not only for making automobile bodies,
but also for making electric appliances, buildings, etc.
EXAMPLES:
Zinc or zinc alloy plated steel sheets were degreased
with an alkali, rinsed with water, dried, given chromating
treatment, and coated with paint compositions by a roll
coater, followed by their baking, to prepare organic
composite coated steel sheets for making automobile bodies.
The coated steel sheets were tested for corrosion resistance
21 48485
34
in a rust-contaminated environment, perforation corrosion
resistance, paint adhesion, anti-powdering property and
weldability. Tables 1 to 4 show the steel sheets, organic
resins, sparingly soluble chromates and silica which were
employed for preparing the coated steel sheets. Tables 5 to
12 show the results of Example 1, and Tables 13 to 22 show
the results of Example 2.
The following is a description of the materials, methods
and conditions employed in the preparation of the coated
steel sheets:
[1] Zinc or zinc alloy plated steel sheets:
Cold rolled steel sheets having a thickness of 0.8 mm
and a surface roughness (Ra) of 1.0 micron were plated with
zinc or zinc alloys to provide the starting materials (see
Table 1).
[2] Chromating treatment:
(1) Dried-in-place type chromate coating:
A chromating solution having the composition shown below
was applied by a roll coater, and dried without rinsing with
water. The coating weight of the chromate layer was
controlled by varying the ratio in peripheral speed of the
pickup and applicator rolls in the roll coater.
Chromic acid anhydride: 20 g/Q.
Phosphoric acid ion: 4 g/Q.
Zirconium fluoride ion: 1 g/Q.
21 48485
Zinc ion: 1 g/Q .
Hexavalent chromium/trivalent chromium: 3/3 (by
weight).
Chromic acid anhydride/zirconium fluoride ion: 20/1 (by
weight).
(2) Electrolytic chromate coating:
A bath containing 30 g of chromic acid anhydride and 0.2
g of sulfuric acid per liter and having a temperature of 40°C
was used for cathode electrolysis at a current density of 10
A/dm2 to form a chromate layer on the steel sheet and it was
rinsed with water, and dried. The coating weight of the
chromate layer was regulated by controlling the amount of the
electric current employed for the electrolysis.
(3) Reacted-in-place type chromate coating:
A solution containing 30 g of chromic acid anhydride, 10
g of phosphoric acid, 0.5 g of NaF and 4 g of KZTiF6 per liter
and having a temperature of 60°C was sprayed on the steel
sheet and it was rinsed with water, and dried. The coating
weight of the chromate layer was controlled by varying the
length of time spent for the treatment.
[3] Organic resin:
The organic resins which were employed are shown in
Table 2. The base resins and curing agents (polyisocyanates)
appearing in the table were prepared by the processes
described at (I) to (III) and (a) to (e) below.
21 48485
36
[Base resin]
( I ) A reaction vessel equipped with a reflux condenser,
a stirrer, a thermometer and a device for blowing nitrogen
gas was charged with 1600 g of Epicoat 1004 (an epoxy resin
of Shell Chemical Co., Ltd. having a molecular weight of
about 1600), 57 g of pelargonic acid (reagent) and 80 g of
xylene, and they were reacted at 170°C. Then, xylene was
removed under a reduced pressure, whereby an intermediate
reaction product [A] was obtained.
(II) A reaction vessel equipped with a stirrer, a
reflux condenser, a thermometer and a liquid dropping device
was charged with 1880 g (0.5 mol) of Epicoat 1009 (an epoxy
resin of Shell Chemical Co., Ltd. having a molecular weight
of about 3750) and 1000 g of a mixed solvent consisting of
methyl isobutyl ketone and xylene in a ratio of 1/1 (by
weight), and they were heated under stirring to form a
uniform solution at a temperature below the boiling point of
the solvent. Then, the solution was cooled to 70°C and 70 g
of di(n-propanol)amine was dropped into the solution in 30
minutes from the liquid dropping device. A reaction
temperature of 70°C was maintained until its dropping was
finished. Then, the solution was held at 120°C for two hours
to complete the reaction. A resin A was obtained as the
reaction product. The resin A contained 665 of effective
component.
21 48485
37
(III) The reaction vessel employed at (II) above was
charged with 1650 g of the intermediate reaction product [A]
and 1000 g of xylene, and after they had been heated to
100°C, 65 g of diethanolamine and 30 g of monoethanolamine
were dropped into the vessel in 30 minutes from the liquid
dropping device. Then, a temperature of 120°C was maintained
for two hours to complete the reaction. A resin B was
obtained as the reaction product. The resin B contained 63$
of effective component.
[Curing agent]
(a) Hexafunctional isocyanate:
A reaction vessel equipped with a thermometer, a stirrer
and a reflux condenser having a dropping funnel was charged
with 222 parts of isophorone diisocyanate and 34 parts of
methyl isobutyl ketone, and after a uniform solution had been
made, 87 parts of methyl ethyl ketone oxime was dropped in
two hours from the funnel into the isocyanate solution held
at 70°C under stirring. After the addition of 30.4 parts of
sorbitol, the solution was heated to 120°C to complete the
reaction. The examination of the reaction product by an
infrared analyzer confirmed that there was no absorption by
any isocyanate group at a wavelength of 2250 to 2270 cm'1.
The addition of 50.4 parts of butyl cellosolve to the
reaction product yielded a curing agent a. The curing agent
a contained 80~ of effective component.
21 48485
38
(b) Tetrafunctional isocyanate:
A reaction vessel equipped with a thermometer, a stirrer
and a reflux condenser having a dropping funnel was charged
with 222 parts of isophorone diisocyanate and 34 parts of
methyl isobutyl ketone, and after a uniform solution has been
made, 87 parts of methyl ethyl ketone oxime was dropped in
two hours from the funnel into the isocyanate solution held
at 70°C under stirring. After the addition of 34 parts of
pentaerythritol, the solution was heated to 120°C to complete
the reaction. The examination of the reaction product by an
infrared analyzer confirmed that there was no absorption by
any isocyanate group at a wavelength of 2250 to 2270 cm-1.
The addition of 52 parts of butyl cellosolve to the reaction
product yielded a curing agent b. The curing agent b
contained 80~ of effective component.
(c) Trifunctional isocyanate:
A reaction vessel equipped with a thermometer, a stirrer
and a reflux condenser having a dropping funnel was charged
with 550 parts of DURANATE* TPA-100 (product of Asahi Chemical
Industrial Co., Ltd., HMDI of the isocyanuric ring type) and
34 parts of methyl isobutyl ketone, and after a uniform
solution had been made, 270 parts of methyl ethyl ketone
oxime was dropped in two hours from the funnel into the
isocyanate solution held at 70°C under stirring. The
examination of the reaction product by an infrared analyzer
* Trade-mark
21 48485
39
confirmed that there was no absorption by any isocyanate
group at a wavelength of 2250 to 2270 ciril. The addition of
47 parts of butyl cellosolve to the reaction product yielded
a curing agent c. The curing agent c contained 90$ of
effective component.
(d) Difunctional isocyanate:
TAKENATE" B-870N (product of Takeda Chemical Industries,
Ltd.; a MEK oxime-blocked product of IPDI) was employed as a
curing agent d.
(e) D~AT$" MF-B80M (product of Asahi Chemical
Industrial Co., Ltd.; an oxime-blocked product of a
hexafunctional isocyanate compound of the hexamethylene
diisocyanate series) was employed as a curing agent e.
The following is a description of the method employed
for testing and evaluating the organic composite coated steel
sheets for various properties:
(a) Corrosion resistance (perforation):
After a sealing tape had been applied to the edges and
rear surface of each unpainted test specimen, a cross cut was
made in its lower half surface and it was given 200 cycles of
a cyclic corrosion test each consisting of:
Spraying a 5~ NaCl solution at 35°C (for 4 hours);
drying at 60°C (for 2 hours);
leaving at a RH of 95~ and 50°C (for 4 hours).
The results are shown by these symbols:
' Trade-mark
21 48485
~ : No red rust formed;
o+: Red rust covered only an area of less than 5~;
o : Red rust covered only an area of from 5~,
inclusive, to 10~, exclusive;
o-: Red rust covered an area of from 10$, inclusive,
to 20~, exclusive;
D . Red rust covered an area of from 20~, inclusive,
to 50~, exclusive;
x . Red rust covered an area of 50~ or more.
(b) Corrosion resistance in a rust-contaminated
environment:
After a sealing tape had been applied to the edges and
rear surface of each unpainted test specimen, it was given a
cyclic corrosion test in the presence of iron rust. Each of
specimens Nos. 1 to 50 in Example 1 and Nos. 1 to 65 in
Example 2 was examined after 7 cycles of the test for any
rust formed thereon, and each of Nos. 51 to 94 in Example 1
and Nos. 66 to 124 in Example 2 after 15 cycles. Each cycle
consisted of:
Dipping in a 5$ NaCl solution at 50°C in the presence of
iron rust (*) (for 18 hours);
leaving at a RH of 95~ and 50°C (for 3 hours); and
drying at 60°C (for 3 hours).
(*) Iron rust was supplied by dipping a cold rolled
steel sheet having an area of 10 cm2 per liter of
21 48485
41
salt solution.
The results are shown by these symbols:
~ : No red rust formed;
o : Red rust covered only an area of less than 10~;
o-: Red rust covered only an area of from 10~,
inclusive, to 20~, exclusive;
4 : Red rust covered an area of from 20~, inclusive,
to 50$, exclusive;
x : Red rust covered an area of 50$ or more.
(c) Paint adhesion:
Each test specimen was coated with an electrodeposited
layer of U-600; product of Nippon Paint Co., Ltd., having a
thickness of 25 microns and a top coat of LUGA BAKE B-531,
product of Kansai Paint Co., Ltd., having a thickness of 35
microns. It was immersed in ion-exchange water having a
temperature of 40°C, and removed from it after 240 hours.
After it had been left to stand at room temperature for 24
hours, 100 checkers each measuring 2 mm square were made in
the coating and an adhesive tape was attached to, and
detached from, the coating to see how ttfe coating would peel
off. The results are shown by these symbols:
m : No peeling occurred;
o . Only less than 3~ of the coating peeled off;
From 3$, inclusive, to 10~, exclusive, of the
coating peeled off;
* Trade-mark
21 48485
42
x : 10~ or more of the coating peeled off.
(d) Weldability:
A continuous spot welding test was made on each specimen
by employing a CF electrode, an electrode force of 200 kgf,
a weld time of 10 cycles/50 Hz and a welding current of 10
kA, and the number of the spots which could be made
continuously on the specimen was counted as a measure of its
weldability. The results are shown by these symbols:
~ : 5000 or more;
o : From 4000, inclusive, to 5000, exclusive;
D . From 3000, inclusive, to 4000, exclusive;
x . Less than 3000.
Example 1:
Tables 5 to 12 show the test results of the organic
composite coated steel sheets containing a sparingly soluble
chromate as the rust-preventing additive in the organic resin
coating.
Example 1 confirms the excellent corrosion resistance in
a rust-contaminated environment of the organic composite
coated steel sheets of this invention, and particularly,
those obtained by employing' as the curing agent poly-
functional polyisocyanates having at least four, and
preferably at least six, isocyanate groups per molecule.
Moreover, the results of Samples Nos. 51 to 53 and Nos. 55 to
57 in Table 9 confirm that a higher level of corrosion
21 48485
4.3
resistance in a rust-contaminated environment can be obtained
when the hexafunctional polyisocyanate compound employed as
the curing agent is of the hexamethylene diisocyanate type,
than when it is of the isophorone diisocyanate type.
Example 2:
Tables 13 to 22 show the test results of the organic
composite coated steel sheets containing a sparingly soluble
chromate and silica as the rust-preventing additive in the
organic resin coating.
Example 2 confirms the excellent corrosion resistance in
a rust-contaminated environment of the organic composite
coated steel sheets of this invention, and particularly,
those obtained by employing as the curing agent poly-
functional polyisocyanates having at least four, and
preferably at least six, isocyanate groups per molecule.
Moreover, the results of Samples Nos. 66 to 68 and Nos. 70 to
72 in Table 18 confirm that a higher level of corrosion
resistance in a rust-contaminated environment can be obtained
when the hexafunctional polyisocyanate compound employed as
the curing agent is of the hexamethylene diisocyanate type,
than when it is of the isophorone diisocyanate type.
The following is an explanation of what is meant by *1
to *7 in Tables 5 to 22:
*1 . "Inv." means a sample of this invention, while
"Com." means a comparative sample;
z~~s~~
44
*2 : The numbers correspond to those appearing in Table
1 as identifying the plated steel sheets;
*3 . Each number represents the coating weight of the
chromate layer in terms of metallic chromium;
*4 : The numbers correspond to those appearing in Table
2 as identifying the organic resins;
*5 : The numbers correspond to those appearing in Table
3 as identifying the sparingly soluble chromates;
*6 . The numbers correspond to those appearing in Table
4 as identifying the different types of silica;
and
*7 . The ratio by weight of nonvolatile matter.
INDUSTRIAL UTILITY:
The organic composite coated steel sheet of this
invention is useful as a material for automobile bodies,
electric appliances, etc.
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