Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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FIELD OF THE lNv~NllON
The present invention relates to a copolyester resin film
laminated steel sheet having excellent corrosion resistance after
severe forming, and a method for its production. The present
invention relates to a copolyester resin film laminated steel sheet
having double layers consisting of a lower layer of metallic
chromium and an upper layer of hydrated chromium oxide (TFS film)
on a tin plated steel sheet which is characterized by the state of
plated tin, and a method for production of this copolyester resin
film laminated steel sheet which is characterized by an
electroplating with tin under restricted conditions, by the
formation of the above TFS film onto the plated tin as well as the
exposed surface of steel sheet and by the lamination of said
copolyester resin film on one or both sides of the surface treated
steel sheet which has been heated to above the melting temperature
of tin just before the lamination of the copolyester resin film.
BACRGROUND AND OBJECTIVE
At present, metal sheets such as electrotinplate, tin free
steel and aluminum sheet are widely used for can stock after being
coated, at least once, with lacquer. This lacquer coating is
disadvantageous from an energy standpoint as significant time is
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required for curing the lacquer and large volumes of solvent
discharged during the lacquer curing process must be burned in
another furnace in order to prevent air pollution.
Recently, lamination of thermoplastic resin film on a metal
sheet was attempted in order to avoid these problems. For example,
the methods shown in Japanese Patent Publication Nos. Sho 61-3676
and Sho 60-47103, Laid-Open Japanese Patent Application Nos. Sho
61-149341 and Hei 1-249331 are already known.
Japanese Patent Publication No. Sho 60-3676 relates to a
process for lamination of a polyester resin film onto a tin plated
steel sheet which comprises preliminarily laminating the polyester
resin film to a tin plated steel sheet at below the melting
temperature of tin, and then reheating the laminate at above the
melting temperature of tin in order to completely bond the film to
the tin plated steel sheet.
Japanese Patent Publication No. Sho 60-47103 relates to a
process for lamination of a crystalline polyester resin film to a
metal sheet such as tin free steel and electrotinplate by heating
the sheet above the melting point of said polyester resin film and
thereafter immediately quenching the laminate.
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Laid-Open Japanese Patent Application Nos. Sho 61-149341 and
Hei 1-249331 relate to a process for lamination of a polyester
resin film precoated with a special adhesive, such as an epoxy
resin containing a curing agent to a metal sheet such as tin free
steel and electrotinplate which has been heated below or above the
melting temperature of said polyester resin film.
The electrotinplate covered with the polyester resin film
obtained by these patents above is not suitable for applications
wherein excellent corrosion resistance after severe forming is
required because the adhesion of the polyester resin film after
severe forming is inferior to that in the polyester resin film
laminated tin-free steel. Particularly, in the electrotinplate in
which the polyester resin film is laminated at above the melting
temperature of tin, the adhesion of the polyester resin film after
severe forming is noticeably poor, because it is considered that
the adhesion between the tin layer and metallic chromium layer,
which is present on the tin layer, becomes poor by melting of a tin
or an iron-tin alloy layer which forms by heating during the
lamination of the polyester resin film is destroyed by severe
forming. In the electrotinplate in which the polyester resin film
is laminated at below the melting temperature of tin, the laminated
polyester resin film is easily peeled off by severe forming, even
if said TFS film is formed on a plated tin layer, because the
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adhesion of the tin layer to the metallic chromium layer formed on
the plated tin layer is poor compared with the adhesion of the
metallic chromium layer to the steel surface in tin-free steel.
On the other hand, in the polyester resin film laminated tin
free steel obtained by these patents described above, the laminated
polyester resin film is not peeled off by severe forming, if the
polyester resin film having excellent formability is laminated.
However, in the process wherein the polyester resin film is
laminated on tin-free steel or the polyester resin film laminated
tin-free steel is formed, the impurities such as dust and steel
powder may be mixed in the interface between the laminated
polyester resin film and the surface of the tin free steel or may
stick on the surface of the polyester resin film laminated tin free
steel. These impurities may act as the starting point for the
birth of many cracks in the laminated polyester resin film during
severe forming. These cracks deteriorate the corrosion resistance
of the polyester resin film laminated tin free steel. For example,
when some corrosive drinks such as a carbonated beverage or a
sports drink is packed in the deeply drawn can made by the
polyester resin film laminated tin free steel and then are stored
for about one month at room temperature, perforations may arise
from many cracks in the laminated polyester resin film of the
formed part in the drawn can, because the TFS film does not prevent
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the corrosion of base steel exposed by severe forming which occurs
electrochemically.
Therefore, the tin free steel covered with the polyester resin
film is not suitable for industrial applications where excellent
corrosion resistance after severe forming is required. If the
polyester resin film laminated tin free steel is used for
applications described above, the environment of the processes for
laminating the polyester resin film on the tin free steel and
forming the polyester resin film laminated tin free steel must be
sufficiently controlled in order to prevent the mixture of these
impurities into the polyester resin film laminated tin free steel.
BRIEF DESCRIPTION OF THB INVENTION
The first objective of the present invention can be
accomplished by the lamination of a copolyester resin film on the
surface treated steel sheet having double layers consisting of a
lower layer of metallic chromium and an upper layer of hydrated
chromium oxide on a tin plated steel sheet in which 5 to 40% of the
surface of the steel sheet is covered with plated tin and a space
between plated tin particles is 0.5 to 50 ~m.
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The second objective of the present invention can be
accomplished by the lamination of the copolyester resin film at
above the melting temperature of tin on the surface treated steel
sheet in which tin is electroplated under restricted conditions
which is characterized by a lower amount of additives in the
tinplating electrolyte compared with those in the conventional
tinplating electrolyte and then said double layer is uniformly
formed on the plated tin and the exposed steel surface.
It is a very important point and an inventive feature in the
present invention that the plated tin lies scattered after
tinplating and metallic chromium is positively deposited on the
surfaces of plated tin and the remaining exposed steel surface
which was not plated with tin, and furthermore that the surface of
the metallic chromium is uniformly covered with hydrated chromium
oxide. It is considered that the surface treated steel sheet used
as the base for the lamination of the polyester resin film in the
present invention is a hybrid surface of a tin free steel and a tin
plated steel sheet, in which demerits or problems are removed while
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merits and improvements are retained and made in
both surface treated steel sheets.
The copolyester resin film laminated steel
sheet according to the present invention can be used
in applications wherein excellent corrosion
resistance after severe forming, particularly
excellent pitting corrosion resistance is required,
such as deeply drawn cans, drawn and redrawn cans,
drawn and stretch formed cans, drawn and partially
ironed cans having high can height and a high
drawing ratio and can ends where a tab for easy
opening is attached. Particularly, this copolyester
resin film laminated steel sheet is suitable as a
,~an material wherein corrosive contents such as
carbonated beverages, juice and sports drinks are
packed. Furthermore, the copolyester resin film
laminated steel sheet according to the present
invention can be used for screw caps and crown caps
in contact with some corrosive contents.
DESCRIPTION OF THE FIGURES
Figure 1 depicts a magnified schematic diagram
which is observed from the surface in the surface
treated steel sheet used
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as the base for the lamination. A shows a part plated with tin,
B shows a part not plated with tin, and the part of A and B is
covered with double layers consisting of a lower layer of metallic
chromium and an upper layer of hydrated chromium oxide.
Figure 2 depicts the relationship between the amount of
additives in tinplating electrolyte and the covering ratio of the
steel surface by the plated tin.
DETAILED DESCRIPTION OF THE INVENTION
It is indispensable in the present invention that the surface
treated steel sheet having all of the following factors is used as
the base for the lamination of the copolyester resin film from the
viewpoint of the adhesion to the laminated copolyester resin film
and the corrosion resistance after severe forming. These factors
are:
(1) 5 to 40% of the surface of steel sheet is covered with
plated tin;
(2) A space between plated tin particles is 0.5 to 50 ~m;
(3) An amount of plated tin is 200 to 4300 mg/m2;
(4) A surface of tin plated steel sheet having features of
(1) to (3) described above is uniformly covered with TFS
film consisting of a lower layer of metallic chromium and
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an upper layer of hydrated chromium oxide; and
(5) An amount of metallic chromium and hydrated chromium
oxide as chromium is 30 to 300 mg/m2 and 5 to 30 mg/m2,
respectively, in said TFS film.
If the covering ratio of the steel surface by plated tin is
below 5% which results when there is above 50 ~m of space between
plated tin particles, i.e. it is covered almost with only TFS film,
then the corrosion of steel sheet is accelerated from cracks in the
laminated copolyester resin film arising from severe forming,
-because the area plated with tin which effectively prevents the
corrosion of steel sheet decreases. It is also not suitable in the
present invention that the amount of plated tin is below 200 mg/m2,
because the corrosion of steel sheet is not prevented effectively
by plated tin, even if the excellent adhesion of the laminated
copolyester resin film is obtained. On the other hand, if the
covering ratio of the steel surface by plated tin is above 40% and
the space between plated tin particles is below 0.5 ~m, the
laminated copolyester resin film is easily peeled off by severe
forming, although the corrosion of steel sheet is prevented
effectively by- an increase in the area plated with tin.
Particularly, in above 4300 mg/m2 of plated tin, the adhesion of
the laminated copolyester resin film becomes noticeably poor.
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For the reasons described above, it is preferable in the
present invention that the state of tin plated steel sheet before
the formation of TFS film is satisfied with factors (1) to (3)
described above. More preferably, the covering ratio of the steel
surface by plated tin is 10 to 30~, the space between plated tin
particles is 2 to 20 ~m and the amount of plated tin is 1000 to
3000 mg/m2.
In the present invention, it is indispensable that the tin
plated steel sheet having features described above is uniformly
covered with TFS film consisting of metallic chromium and hydrated
chromium oxide.
It is preferable in the present invention that the amount of
metallic chromium and hydrated chromium oxide as chromium is in the
range of 30 to 300 mg/m2 and 5 to 30 mg/m2, more preferably 5 to 200
mg/m2 and 7 to 20 mg/m2, respectively, in the TFS film formed on tin
plated steel sheet.
If the amount of hydrated chromium oxide is above 30 mg/m2 or
below 5 mg/m2 as chromium, the bonding strength to the copolyester
resin film becomes noticeably poor in severely formed areas.
Particularly, with below 5 mg/m2 as chromium of the hydrated
chromium oxide, the corrosion resistance in the area covered with
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only TFS film becomes noticeably poor.
In the case of below 30 mg/m2 of metallic chromium, the
corrosion resistance in the area covered with only TFS film becomes
poor, even if the amount of hydrated chromium oxide as chromium is
5 to 30 mg/m2. The deposition of metallic chromium above 300 mg/m2
is not suitable in the continuous production of the copolyester
resin film laminated steel sheet according to the present invention
at high speed, although the adhesion of said copolyester resin film
does not become remarkably poor.
The surface treated steel sheet used as the base for the
lamination of the copolyester resin film in the present invention
is easily produced by the following process: degreasing with an
alkali and pickling with an acid, followed by water rinsing, which
is followed by electrotinplating by using a special electrolyte
followed by water rinsing, which is followed by formation of TFS
film, which is followed by water rinsing, and then followed by
drying.
In order to obtain the tin plated steel having features
described above, the steel sheet is plated with tin by using an
electrolyte having the following composition under a cathodic
current density of 15 to 40 A/dm2 at an electrolyte temperature of
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40 to 60C used for conventional electrotinplating.
Composition of tinplating electrolyte:
Concentration of stannous sulfate: 20 to 100 g/l
Concentration of phenolsulfonic acid: 20 to 80 g/l
Concentration of additives: 0.05 to 0.12 g/1
Ethoxylated ~-naphthol or ethoxylated ~-naphtholsulfonic acid
are used as additives for tinplating electrolyte described above.
In the composition of tinplating electrolyte described above,
the concentration of additives is very important in order to obtain
a tin plated steel sheet having features described above.
As shown in Figure 2, the covering ratio of the surface of the
steel sheet by plated tin increases with an increase in the
concentration of additives. With below 0.12 g/l of additives, the
covering ratio of the steel surface by plated tin is below 40%.
The concentration of additives below 0.05 g/l is not suitable in
the tinplating electrolyte used in the present invention, because
the adhesion of plating tin to the steel sheet becomes poor and
plated tin is easily peeled off from the surface of the steel sheet
by contact with a conductor roll or a sink roll in an
electrotinplating section. At above 0.12 g/l in the concentration
of additives, excellent adhesion of copolyester resin film to the
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201 98~61
surface treated steel sheet is not obtained because
the greater part of the steel surface is uniformly
covered with plated tin, even if TFS film is
uniformly formed on the tin plated steel sheet.
It has been known by USP 4,816,348 that a low
tin plated steel sheet wherein 30 to 80% of the
surface of the steel sheet is covered with plated
tin and an effective diameter of an irregularly
shaped plated area, which is defined as the diameter
of a circle having the identical area, is controlled
between 0.5 and 20 ~m is obtained by using an
electrolyte containiing 30 to 80 g/l of stannous
sulfate, 15 to 60 g/l of acid as sulfuric acid and
0.2 to 2 g/l of ethoxylated a-naphthol additive
under a cathodic current density of 2 to 10 A/dm2 at
an electrolyte temperature of 40 to 60C. However,
the tin plated steel sheet obtained by this method
described above is poor in the adhesion of the
laminated copolyester resin film, even if TFS film
is uniformly formed, because the plated tin is
present in a large lump and the formed TFS film is
destroyed by melting of the plated tin. Furthermore,
this method is poor in productivity because the low
current density is indispensable.
The tin plated steel sheet produced under the
conditions described above is covered with TFS film.
For the formation of said TFS film on the tin plated
steel sheet, the following two
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methods which are used for the production of TFS are utilized:
1) The one is a two-step process in which metallic chromium
is plated and then hydrated chromium oxide is formed on the
metallic chromium layer.
2) The other is one-step process in which said TFS film is
simultaneously formed on the tin plated steel sheet. However, the
one step process has merit with a view to use of less equipment.
In the present invention, the TFS film is formed on the tin
plated steel sheet by cathodic electrolysis in an electrolyte
containing 50 to lO0 g/l of chromic acid and an optimum amount of
additives such as fluorine compounds and/or sulfuric acid under a
cathodic current density of 40 to 80 A/dm2 at an electrolyte
temperature of 40 to 60C. It is preferable to use higher current
density in order to form the TFS film uniformly.
It is preferable that the amount of sulfuric acid and/or a
fluorine compound added to the chromic acid solution is 1 to 5% of
chromic acid. The fluorine compound is selected from the group
consisting of hydrofluoric acid, fluoboric acid, fluosilicic acid,
ammonium bifluoride, an alkali metal bifluoride, ammonium fluoride,
an alkali metal fluoride, ammonium fluoborate, and alkali metal
fluoborate, ammonium fluosilicate and an alkali metal fluosilicate.
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In the present invention, the copolyester resin film to be
used should be selected from the copolyester resin film prepared
by processing according to known methods and comprising a
copolyester resin consisting of 75 to 99 mole % of polyethylene
terephthalate and 1 to 25 mole % of a polyester resin which is
produced by the esterification of at least one saturated
polycarboxylic acid with at least one saturated polyalcohol
selected from the following polycarboxylic acids and polyalcohols.
Saturated polycarboxylic acids are selected from phthalic
acid, isophthalic acid, terephthalic acid, succinic acid, azelaic
acid, adipic acid, sebacic acid, diphenylcarboxylic acid, 2,6-
naphthalene dicarboxylic acid, 1,4-cyclohexane dicarboxylic
acid and trimellitic acid anhydride.
- Saturated polyalcohols are selected from ethylene glycol, 1,4-
butanediol, 1,5-pentanediol, 1,6-hexanediol, propylene glycol,
polytetoramethylene glycol, trimethylene glycol, triethylene
glycol, neopenthyl glycol, 1,4-cyclohexane dimethanol, trimethylol
propane and pentaerythritol.
Furthermore, the use of the biaxially oriented copolyester
resin film is preferable from the viewpoints of the formability and
anti-permeability of the copolyester resin film.
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In some cases, additives such as antioxidants, stabilizer,
pigments, anti-static agent, lubricants and corrosion inhibitors
are added during the manufacturing process of the copolyester resin
film used for the present invention.
The thickness of the copolyester resin film used in the
present invention should be 5 to 50 ~m. If the thickness of the
employed copolyester resin film is below 5 ~m, good corrosion
resistance after formation of the copolyester resin film laminated
steel sheet according to the present invention is not obtained and
the continuous lamination of thin copolyester resin film to the
surface treated steel sheet becomes noticeably difficult. The use
of copolyester resin film having a thickness above 50~m is not
economically suitable for the film to be laminated to the surface
treated steel sheet, because the copolyester resin film used for
the present invention is expensive as compared with epoxy phenolic
lacquers widely used in the can industry.
Furthermore, it is preferable from the viewpoint of filiform
corrosion resistance of the copolyester resin film laminated steel
sheet according to the present invention, that the one side of the
employed copolyester resin film which is contact with the surface
treated steel sheet is precoated with 0.1 to 5.0 g/m2 of a resin
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composite containing at least one radical selected from the group
consisting of an epoxy radical, a hydroxyl radical, an amide
radical, an ester radical, a carboxyl radical, a urethane radical
an acrylic radical and an amino radical. Epoxy resin, nylon resin,
polyester resin, modified vinyl resin, urethane resin, acrylic
resin and urea resin are examples of such resin composites.
It is desirable that the resin composite be coated on one side
of the employed copolyester resin film as uniformly and thinly as
possible. This is because the bonding strength of resin composite
layer to the surface treated steel sheet and the copolyester resin
film becomes gradually poorer with an increase in the thickness of
the precoated resin composite. However, it is very difficult to
uniformly coat in amounts below 0.1 g/m2 of the resin composite on
the copolyester resin film. Furthermore when the amount of the
resin composite is below or above 5.0 g/m2, the bonding strength of
the resin composite layer to the surface treated steel sheet and
the copolyester resin film becomes noticeably poor in severely
formed areas.
- It is preferable that the resin composite is diluted by a
solvent and then coated by a roller or spray method in order to
form a uniform and thin resin composite layer on the copolyester
resin film. If the temperature for drying a resin composite
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diluted by a solvent which is coated on one side of the copolyester
resin film is below 60C, a long time is required for the removal
of solvent and the formed resin composite layer becomes tacky.
When the drying temperature is above 150C, the chemical reaction
of the resin composite coated on the copolyester resin film is
accelerated, and the bonding strength of the resin composite to the
surface treated steel sheet becomes noticeably poor. It is
preferable that the drying time of the resin composite solution
coated on the copolyester resin film is 5 to 30 seconds at a
temperature of 60 to 150C. If the drying time is less than 5
seconds, the solvent is not sufficiently removed. On the other
hand, a drying time of more than 30 seconds results in poor
productivity.
It is usually impossible to laminate the polyester resin film
to an electrotinplate which is heated to above the melting
temperature of tin, because the plated tin is melted. However, the
lamination of polyester resin film to the surface treated steel
sheet used in the present invention which is heated to above the
melting temperature of tin can be carried out, because the plated
tin which lies scattered does not flow continuously by heating to
above the melting temperature of tin. Therefore, the copolyester
resin film having higher melting temperature than that of tin can
be used in the present invention. However, if the copolyester
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resin film is laminated to the surface treated steel sheet which
is heated to above the melting temperature (Tm)+50C, the
copolyester resin film laminated steel sheet having excellent
corrosion resistance after forming is not obtained, because the
large amount of noncrystalline copolyester resin layer is formed
in the interface between a biaxially oriented copolyester resin
layer and the surface treated steel sheet. On the other hand, if
the laminating temperature is below Tm, the laminated copolyester
resin film is easily peeled off by severe forming. Therefore, it
is preferable in the present invention that the copolyester resin
film is laminated to the surface treated steel sheet which is
heated to a temperature which is Tm to Tm+50C and above the
melting temperature of tin.
In the present invention, the method for heating the surface
treated steel sheet to which the copolyester resin film is
laminated is not especially limited, However, from the continuous
and stable production of the copolyester resin film laminated steel
sheet according to the present invention at high speed, conduction
heating by a roller heated by induction heating, induction heating
and/or resistance heating which are used for reflowing
electrotinplate in the production process of electrotinplate is
suitable as the method for heating the surface treated steel sheet,
because the surface treated steel sheet can be rapidly heated and
the temperature of the heated steel sheet can be easily controlled.
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Furthermore, it is also preferable in the present invention that
heating with a roller heated by hot steam or heating in an electric
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oven can be used as an auxiliary method for preheating the surface
treated steel sheet to be laminated.
A method for cooling after the lamination of copolyester resin
film to the surface treated steel sheet is not limited to rapidly
cooling or gradually cooling, because the slight amount of the
noncrystalline copolyester resin layer is only formed at the
interface between biaxially oriented copolyester resin film and the
surface treated steel sheet and then the obtained characteristics
do not change by both methods, if the laminating temperature is not
remarkably higher than Tm.
The present invention is explained in further detail by
reference to the following examples. It will be understood that
the specification and examples are illustrative but not limitative
of the present invention and that other embodiments within the
spirit and scope of the invention will suggest themselves to those
skilled in the art.
EXAMPLE 1
A cold rolled steel sheet having a thickness of 0.21 mm was
electrolytically decreased in a solution of 70 g/l sodium hydroxide
and then pickled in a solution of 100 g/l sulfuric acid. The steel
sheet, after being rinsed with water, was electroplated with tin
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under the following conditions (A). After rinsing
with water, the tin plated steel sheet was
cathodically treated under the following conditions
(B) for the formation of TFS film and then was
rinsed with water and dried.
After that, a biaxially oriented copolyester
resin film having characteristics shown in (C) was
laminated on both surfaces of the thus treated steel
sheet under the following conditions (D).
(A) Conditions for tin plating
Composition of tin plating electrolyte
SnS04:80 g/l
Phenolsulfonic acid (65% solution):60 g/l
Ethoxylated a-naphthol:0.06 g/l
Temperature of electrolyte:45C
Cathodic current density:20 A/dm2
Flow speed of electrolyte:300 m/minutes
Amount of plated tin:1300 mg/m2
(B) Conditions for the formation of TFS film
Composition of electrolyte:
CrO3:50 g/l
H2So4:0.5 g/l
Temperature of electrolyte:50C
Cathodic current density:40 A/dm2
Amount of metallic chromium:95 mg/m2
Amount of hydrated chromium oxide:ll mg/m2
as Cr
`~S
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(C) Characteristics of the emploYed copolyester resin film:
Thickness:25 ~m
Melting temperature:240C
Refractive indexes in all planar dimensions:l.6598
Refractive index in thickness direction:1.5175
Type of the added lubricant:SiO2
Average particle size of added lubricant:l.5 ~m
Amount of added lubricant:0.07 weight % relative to the
weight of the employed copolyester resin
Composition of precoated resin composite:
Epoxy resin 80 parts
Resole product from paracresol: 20 parts
Drying temperature of precoated resin composite:120C
Amount of precoated resin composite after drying:0.25
g/m2
(D) Conditions for lamination of copolyester resin film
Method for heating the treated steel sheet:
Roller heated by induction heating
Temperature of the treated steel sheet just before
lamination:260C
Method for cooling after lamination:Rapid cooling by
water
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EXAMPLE 2
The same steel sheet pretreated as in Example 1
was electroplated with 2700 mg/m2 of tin under the
same conditions (A) as in Example 1 except for the
amount of additive in the tin plating electrolyte.
After rinsing with water, TFS film consisting of 83
mg/m2 of metallic chromium and 14 mg/m2 of hydrated
chromium oxide as chromium was formed on the tin
plated steel sheet by using the same conditions (B)
as in Example 1. The thus treated steel sheet was
rinsed with water and dried.
After that, a biaxially oriented copolyester
resin film having the same characteristics (C) as in
Example 1 was laminated on both surfaces of the thus
treated steel sheet, heated to 240C and then the
laminate was gradually cooled.
Ethoxylated a-naphthol: 0.10 g/l
EXAMPLE 3
The same steel sheet pretreated as in Example 1
was electroplated with tin under the following
conditions (A). After rinsing with water, the tin
plated steel sheet was treated under the following
conditions (B). The thus treated steel sheet was
rinsed with water and dried.
After that, a biaxially oriented copolyester
resin film having characteristics shown in (C) was
laminated on both surfaces of the thus treated steel
sheet under the following conditions (D).
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(A) Conditions for tin Platinq
Compositions of tin plating electrolyte:
SnSO4:60 g/l
Phenolsulfonic acid (65% solution):40 g/l
Ethoxylated ~-naphthol sulfonic acid:0.08 g/l
Temperature of electrolyte:45C
Cathodic current density:25 A/dm2
Flow speed of electrolyte:300 m/minutes
Amount of plated tin:1300 mg/m2
(B) Conditions for formation of TFS film
Composition of electrolyte:
CrO3:80 g/l
H2So4:0-5 g/l
NaF:0.5 g/l
Temperature of electrolyte:55C
Cathodic current density:40 A/dm2
Amount of metallic chromium:120 mg/m2
Amount of hydrated chromium oxide:17 mg/m2 as chromium
(C) Characteristics of the employed copolyester resin film:
Thickness:25 ~m
Melting temperature:230C
Refractive indexes in all planar dimensions:l.6475
Refractive index in thickness direction:l.5264
Type of added lubricant:S ioz
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Average particle size of added lubricant:l.5 ~m
Amount of added lubricant:O.o7 weight % relative to the
weight of the employed copolyester resin
(D) Conditions for lamination of copolyester resin film:
Temperature of the treated steel sheet just before
lamination:260C
Method for cooling after lamination:Rapid cooling by
water
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BXAMPLE 4
The same steel sheet pretreated as in Example 1 was
electroplated with 3200 mg/m2 of tin under the same conditions (A)
as in Example 3. After rinsing with water, TFS film consisting of
50 mg/m2 of metallic chromium and 8 mg/m2 of hydrated chromium oxide
as chromium was formed on the tin plated steel sheet under the same
conditions tB) as in Example 3. The thus treated steel sheet was
rinsed with water and dried.
After that, a biaxially oriented copolyester resin film having
the same characteristics (C) as in Example 3 was laminated on both
surfaces of the thus treated steel sheet heated to 260C and then
immersed into hot water having a temperature of 80C, and after
that, was gradually cooled.
COMPARATIVB EXAMPLB 1
The same steel sheet pretreated as in example 1 was
electroplated with tin under the following conditions (A). After
rinsing with water, TFS film consisting of 90 mg/m2 of metallic
chromium and 13 mg/m2 of hydrated chromium oxide as chromium under
the same conditions (B) as in Example l was formed on the tin
plated steel sheet. The thus treated steel sheet was rinsed with
water and dried.
After that, a biaxially oriented copolyester resin film having
the same characteristics (C) used in Example 1 was laminated on
- 28 -
;~0~986~
both surfaces of the thus treated steel sheet under the same
conditions (D) as in Example 1
(A) Conditions for tin Platinq
Composition of tin plating electrolyte
SnSO4:80 g/l
Phenolsulfonic acid (65% solution): 60 g/l
Ethoxylated ~-naphthol:7 g/l
Temperature of electrolyte: 45C
Cathodic current density: 20 A/dm2
Flow speed of electrolyte:300 m/minutes
Amount of plated tin:2700 mg/m2
COMPARATIVE EXAMPL~ 2
The same steel sheet pretreated as in Example 1 was
electroplated with 1800 mg/m2 tin under the same conditions (A) as
in Example 1. After rinsing with water, TFS film consisting of 11
mg/m2 of metallic chromium and 4 mg/m2 of hydrated chromium oxide
as chromium was formed on the tin plated steel sheet under the same
conditions (B) as in Example 1. The thus treated sheet was rinsed
with water and dried.
After that, a biaxially oriented copolyester resin film having
the same characteristics (C) as in Example 1 was laminated on both
surfaces of the thus treated steel sheet under the same conditions
as in Example 1.
- 29 -
2o19861
COMPARATIVE EXAMPLE 3
The same steel sheet pretreated as in Example 1
was electroplated with tin under the following
conditions (A). After rinsing with water, TFS film
consisting of 56 mg/m2 of metallic chromium and 14
mg/m2 of hydrated chromium oxide as chromium was
formed on the tin plated steel sheet. The thus
treated steel sheet was rinsed with water and dried.
After that, a biaxially oriented copolyester
resin film having the same characteristics (C) used
in Example 3 was laminated on both surfaces of the
thus treated steel sheet under the same conditions
(D) as in Example 3.
(A) Conditions for tin plating
Composition of tin plating electrolyte
SnS04:80 g/l
Phenolsulfonic acid (65% solution):60 g/l
Ethoxylated a-naphthol:0.3 g/l
Temperature of electrolyte:45C
Cathodic current density:5 A/dm2
Flow speed of electrolyte:300 m/minutes
Amount of plated tin:2800 mg/m2
COMPARATIVE EXAMPLE 4
The same steel sheet pretreated as in Example 1
was treated under the following conditions (A) for
the formation of TFS film. The thus treated steel
sheet was rinsed with water and dried.
- 30 -
After that, a biaxially oriented copolyester resin film having
the same characteristics (C) used in Example 1 was laminated on
both surfaces of the thus treated steel sheet under the same
conditions (D) as in Example 4.
(A) Conditions for formation of TFS film
Composition of electrolyte:
CrO3:100 g/l
H2S04:0 8 g/l
NaF:2.0 g/l
Temperature of electrolyte:50C
Cathodic current density:40 A/dm2
Flow speed of electrolyte:300 m/minutes
Amount of metallic chromium:102 mg/m2
Amount of hydrated chromium oxide:17 mg/mZ as chromium
The characteristics of the resultant steel sheet were
evaluated by the following testing methods, after measurement of
the coating weight on the resultant steel sheet by the X-ray
fluorescent method. The results are shown in the Table.
(1) Covering ratio of steel surface by plated tin and
space between plated tin particle:
The scanning electromicrograph of the surface treated steel
sheet obtained by 400 magnification was divided in to a white part
in which the steel surface is plated with tin and a black part in
which the steel surface is covered with only TFS film by using the
~Olg~
-
image analyzer. After that, the covering ratio of the steel
surface by plated tin is determined by the ratio of the area in the
white part to that in the black part.
The space between plated tin particles was determined by the
value measured by using slide calipers divided by the 400
magnification in the same scanning electromicrograph. These values
are an average value of 10 range of vision measured by the methods
described above, respectively.
(2) Formability by deep drawing:
The resultant steel sheet was cut by a punch press to a
circular blank having a diameter of 158 mm. The blank was deeply
drawn to form a cylindrical cup at a drawing ratio of 2.92. The
formability of the resultant steel sheet was evaluated by the
degree of cracks in the copolyester resin film and the degree in
the peeling off of the copolyester resin film in the formed area
and then divided into the following 3 ranks:
EvaluationDegrees of cracks and Peelinq off of film
Excellent o%
FairLess than 20%
PoorMore than 20%
- 32 -
201 9861
(3) Corrosion resistance after cup drawing:
50 pieces of the cylindrical cup obtained by
the method described above in method (2) were filled
with carbonate beverage (Coca Cola) and stored at
37.5C. After 3 months, the corrosion resistance of
the resultant steel sheet was evaluated by the ratio
of perforated can.
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- 33 -
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