Note: Descriptions are shown in the official language in which they were submitted.
FIELD OF THE INVENTION
The present i~vention relates to a polyester resin film ~A I 337042
laminated steel sheet for a drawn and ironed can ~DI can).
In detail, the polyester resin film laminated steel sheet
comprises laminati~g a copolyester resin film on the one
side of the steel ~heet to be employed for the inside of the
DI can and plating a ductile metal on the other side of the
steel sheet to be employed for the outside of the DI can.
~ACKGROUND AND OBJECTIVE
At present, tinplated steel sheet, namely tinplate and
aluminum sheet is widely used as a material for DI cans for
carbonated beverages and beer. These DI cans are produced
by the following p~ocess: cutting to a circular blank -~
drawing -~ redrawing -~ ironing several times -~ washing the
coolant oil used f~r forming -~ surface treatment of the
formed can by phosphate or zirconium salt -~ rinsing with
water -~ drying -~ spray coating of lacquer on the inside of
the formed can -~ çolor printing the outside of the formed
can.
The production cost of the DI can is expensive because
the production process of DI can is complex as described
above.
Recently, a p~ecoated material was investigated as a
cheaper material for DI cans. For example, the steel sheet
coated with polyvinyl chloride organosol (Laid-Open Japanese
Patent Application No. Sho. 61-92850), the metal sheet
coated with a lacquer of thermosetting resin containing a
wax of hydrocarbon type as a lubricant (Laid-Open Japanese
Patent Application No. Sho. 62-275172 and polyes.er resin
film laminated steel sheet (Laid-Open Japanese Patent
Application No. Sho. 60-168643) have been employed.
These precoated metal sheets for use in DI cans reduce
-2-
costs because the production process of DI cans is
simplified. However, the quality of DI cans produced from ~ A 1 3 3 ~ 0 4 2
these precoated metal sheets is inferior to that of the DI
cans produced by the present process. For exampie, the
lacquered metal sheets shown in Laid-Open Japanese Patent
Application No. ~ho. 61-92850 and Laid-Open Japanese Patent
Application No. ~ho. 62-275172 are not used for DI cans in
which corrosive b~everages are packed without a spray coating
of lacquer on the inside of the formed DI cans because many
small cracks are observed in the lacquer film coated on the
inside of the formed DI cans even if these precoated metal
sheets can be easily formed into DI cans.
The characteristics of DI cans produced from a
polyethylene terephthalate film laminated steel sheet shown
in Laid-Open Japanese Patent Application No. Sho, 60-168643
deteriorate remarkably by reheating upon curing the color
printing ink applied on the outside of the forme~ DI can.
Namely, much filiform corrosion arises from the edge of DI
cans reheated for curing the color printing ink during long
storage in the atmosphere having high humidity and high
temperature.
It is assumed that the cause of filiform corrosion is
due to the deterioration of the adhesion of polyethylene
terephthalate film to the steel sheet by recrystallization
of polyethylene terephthalate film during reheating at above
160C, although the structure of polyethylene terephthalate
film may change to the monoaxial oriented state from the
amorphous non-oriented state by ironing.
Accordingly, it is the first objective of the present
invention to provide a copolyester ~e~irl film laminated
steel sheet or strip as a material for DI cans having excel-
lent characteristics in the adhesion of copolyester resin
film to the steel sheet after forming into DI cans, filiform
co~rosion resistance in the formed part after reheating for
curing the color printing ink subjected to the outside of ~ A 1 3 3 7 0 4 2
the formed DI cans at the temperature of 160 to 2Q0C and
corrosion resistance to the packed contents such as
carbonated beverages and fruit juices.
It is the second objective of the present invention to
provide a production method of a material for DI cans having
an excellent corra~ion resistance which can be used without
an inner lacquer coating after forming of DI cans.
BRIEF DESCRIPTION OF THE INVENTION
The first objective of the present invention can be
accomplished by the continuous lamination of a copolyester
resin film produced from 75 to 95 mole ~ of polyethylene
terephthalate and 5 to 25 mole % of other polyester resin
having the restricted chemical and physical properties on
the one side of the steel sheet having at least hydrated
chromium oxide and the deposition of a ductile metal on the
other side of the steel sheet.
The second objective of the present invention can be
accomplished by the following two methods. The first method
is one in which the copolyester resin film precoated with a
small amount of resin composite is laminated on the one side
of the steel sheet heated to a melting temperature of
copolyester resin film + 50C. The second method is one in
which said copolyester resin film is directly laminated on
the one side of the steel sheet heated to above melting
temperature of said copolyester resin film.
The present invention is characterized by the use of
the special copolyester resin film having an exce~lent
formability and excellent corrosion resistance described
above, in addition to the use of the steel sheet in which
the one side is covered with at least hydrated chromium
oxlde and the other side is plated with a ductile metal. In
the present invention, the presence of a hydrated chromium ~ A 1 3 3 7 C 4 2
oxide and a ductile metal layer on each side of the steel
sheet are indispehsable in order to obtain excellent adhesion
to the copolyeste~ resin film and excellent formability to
DI can.
The copolyester resin film laminated steel ~heet
according to the present invention can be also used as a
material for drawn cans, drawn and redrawn cans, drawn and
thin redrawn cans and can ends.
, .
DETAILED DESCRIPTION OF THE INVENTION
In the present invention, the copolyester resin film
applied on the inside of the DI can is prepared by processing
according to a known method, a copolyester resin which is
composed of 75 to 95 mole % of polyethylene terephthalate
and 5 to 25 mole % of a polyester resin 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, diphenyl
carboxylic 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, polytetramethylene glycol, trimethylene
glycol, triethylene glycol, 1,4-cyclohexane dimethanol,
trimethylol propane and pentaerythritol.
In some cases, additives such as antioxidants, stabilizers,
pigments, antistatic agents and corrosion inhibitors are
added during the manufacturing process of the copolyester
--5--
resi~ film used for the present invention.
In the present invention, the use of copolyester resin ~ A 1 3 3 7 0 4 2
film having a biaxial oriented structure is especially
desirable from the viewpoint of corrosion resistance,
although non-oriented copolyester resin film can be also
used.
The thickness of the copolyester resin film used in the
present invention should be 10 to 50 ~m, preferably 10 to 30
~m. If the thickness of the employed copolyester resin film
is below 10 ~m, many cracks are observed in the copolyester
resin film laminated on the steel sheet according to the
present invention after forming into the DI can and the
continuous lamination of thin copolyester resin film to the
steel sheet at high speed becomes remarkably difficult.
Moreover, use of a copolyester resin film above 50 ~m is not
economically suitable for the film to be laminated to the
steel sheet, because the copolyester resin film used for the
present invention is expensive as compared with lacquers
widely used in the can industry.
In the present invention, the softening temperature and
the melting temperatu~e of the employed copolyester resin
film are also importa~t factors. The softening temperature
is defined as the temperature at which the insertion of the
needle into the copolyester resin film starts at a heating
rate of 10C/min. in the thermal mechanical analyzer. (TMA
100 made by Seiko Den~hi Kogyo Co.) The melting temperature
is defined as the temperature at which the endothermic peak
is obtained at a heat1ng rate of 10C/min. in the differential
scanning calorimeter. (SS10 made by Seiko Denshi Kogyo Co.)
In the present ipvention, the copolyester resin ~ilm
having a 170 to 235C softening temperature and a 190 to
250C melting temperature should be used. The copolyester
resin film having the softening temperature of above
235C becomes poor in formability and bonding strength to the ~ A 1 3 3 7 0 4 2
steel sheet because the copolyester resin film is easily
crystallized by reheating to cure the color printing ink
subjected to the outside of the DI can. On the other hand, if
the copolyester resin film having a softening temperature
below 170C is used, the efficiency in the production process
of the DI can becomes remarkably poor because the copolyester
film becomes soft by reheating to cure the color printing ink
applied to the outside of the DI can at a higher temperature
l0 than the softening temperature of the copolyester resin film.
The use of the copolyester resin film having a melting
temperature above 250C is not suitable in the present
invention because this copolyester film is rigid and is poor
in formability.
If the copolyester resin film having a melting
temperature below 190C is applied to the steel sheet for the
DI can according to the present invention, many cracks may be
observed in the laminated copolyester resin film after
flanging and necking the DI can because the mechanical
20 strength of this copolyester resin film becomes remarkably
poor by reheating to cure the color printing ink applied to
the outside of the DI can. Therefore, the use of the
copolyester resin film having a melting temperature below
190C is not also suitable in the present invention.
Furthermore, the orientation and mechanical properties
of the copolyester resin film are also very important factors
from the viewpoint of the formability of the copolyester
resin film.
Namely, in the copolyester resin film used in the
30 present invention, the orientation coefficient which is
defined as the degree of the orientation of the copolyester
- 7 -
- 1 337042
resin should be in the range of 0 to 0.100. The orientation
coefficient defined above is determined by a refraction meter
and is shown by the following equation in the present
invention.
A=(B+C)/2-D
where, A represents the orientation coefficient of the
copolyester resin film,
B represents the index of refraction in the
lengthwise direction of the copolyester film,
C represents the index of refraction in the
widthwise direction of the copolyester resin film,
D represents the index of refraction in the
thickness direction of the copolyester resin film.
If the copolyester resin film having above 0.100 of the
orientation coefficient is applied to the steel sheet
according to the present invention, many cracks arise in the
copolyester resin film laminated to the sheel sheet after
forming to DI can, because the formability of this
copolyester resin film becomes remarkably poor.
In the present invention, an elongation at break and a
strength at break of the employed copolyester resin film,
which are determined at the speed of 100 mm/min. at 25C in
an ordinary tensile testing machine, should be in the range
of 150 to 500 % and 3 to 18 kg/mm2, respectively. If the
copolyester resin film having below 150 ~ of elongation at
break is used in the present invention, many cracks arise in
the copolyester resin film after forming into DI cans,
because the formability of this copolyester resin film
becomes remarkably poor. On the other hand, if the
copolyester resin film having above 500 % of elongation at
break is used in the present invention, this film is easily
.
- ~Al 337042
damaged by severe forming because the thickness of this
copolyester resin film becomes non-uniform during production
of this film from the extruder.
- 8a -
~ The copolyester resin film having above 18 kg/mm2 of ~ A 1 3 3 7 0 4 2
strength at break is poor in formability and the bonding
strength to the steel sheet covered with hydrated chromium
oxide. Therefore, if this copolyester resin film is used in
the present invention, this film is easily peeled off from
the surface of the steel sheet with many cracks. On the
other hand, if the cQpolyester resin film having below 3
kg/mm2 of the strength at break is used in the present
invention, this copolyester resin film is easily damaged by
scratches in the process for making of DI can, because this
film has poor toughness.
In the present invention, the copolyester resin film
selected by various restrictions described above is laminated
on the steel sheet by the following two methods. The first
method comprises laminating a copolyester resin film which
has been precoated with a small amount of resin composite to
a steel sheet. The second method comprises laminating a
copolyester resin film directly to a steel sheet which is
heated to above a melting temperature of said copolyester
resin film.
In the first method, the copolyester resin film which
has been precoated with 0.1 to 5 gJm2 of a resin composite
containing in its molecular structure at least one radical
consisting of epoxy radical, hydroxyl radical, amide radical,
ester radical, carboxyl radical, urethane radical, acryl
radical and amino radical is laminated to a steel sheet
which is heated to a melting temperature of said copolyester
resïn film + 50C.
At below 0.1 g/m2 of the resin composite, the bonding
strength of he copolyester resin fiim to the steel sheet in
the body wall of the formed DI can becomes unstable because
the resin composlte is not precoated uniformly and thinly to
said copolyester resin film.
_g_
At above 5.0 g/m2 of the resin composite, the copolyester ~A 1 337042
resin film in the body wall of the formed DI can is easily
peeled off from the surface of the steel sheet.
Furthermore, if the heating temperature of the steel
sheet is below the melting temperature -50C, said polyester
resin film is easily peeled off from the surface of the
steel sheet or an interface between copolyester resin film
and resin composite layer after forming into DI cans.
In the case of the melting temperature of said
copolyester resin film +50C in the heating temperature of
the steel sheet, the body wall of the obtained DI can is
remarkably corroded because said copolyester resin film
deterlorates by heating at higher temperature.
In the second method, the copolyester resin film is
directly laminated on the steel sheet which is heated to the
melting temperature +50C. If the heating temperature of
the steel sheet is below the melting temperature of said
copolyester resin film, said copolyester resin film laminated
on the steel sheet is easily peeled off after forming to DI
can. If the temperature of the heated steel sheet is above
the melting temperature of said copolyester resin film
+50C, the body wall of the obtained DI can is easily
corroded because said copolyester resin film deteriorates by
heating at higher temperature as in the first method.
In the first method and the second method of the
present invention, it is desirable that the copolyester
resin film laminated steel sheet be rapidly cooled compared
with gradual cooling, because said copolyester resin film is
slightly recrystallized in the cooling stage from the higher
temperature than the melting temperature of said copolyester
resin film.
Especially, the presence of the resin composite between
said copolyester resin film and the steel sheet prevents the
--10--
growth of filiform corrosion at a severely formed part, ~ A 1 3 3 7 0 4 2
while the formed DI can is kept at an atmosphere having
higher temperature and higher humidity for long time before
the contents such as carbonated beverage is packed into the
formed DI can. Therefore, the copolyester resin film
laminated steel sheet by the first method i5 preferable to
that by the second method.
In the present invention, a surface treated steel sheet
having at least hydrated chromium oxide is used.
Especially, the presence of an optimum amount of hydrated
chromium oxide in the one side of the steel sheet wherein
the copolyester resin film is laminated is indispensable in
order to obtain an excellent adhesion of the steel sheet to
the copolyester resin film or the resin composite. The
optimum range for the amount of hydrated chromium oxide as
chromium is 0.005 to 0.050 g/m2, preferably 0.010 to 0.030
g/m2 on said metal sheet.
If the amount of hydrated chromium oxide as chromium is
below 0.005 g/m2 or above 0.050 g/m2, the adhesion of the
copolyester resin film may become poor in a severely formed
part.
Furthermore, where excellent corrosion resistance is
required inside the obtained DI can, the steel sheet covered
with at least one metal selected from the group consisting
of chromium, nickel, tin, zinc and aluminum under the
hydrated chromium oxide layer should be used for the
copolyester resin film laminated steel sheet according to
the present invention.
The optimum range for the amount of plated chromium,
nickel, tin, zinc and alum~num ic '!.01 ~o '! 30 g~m2, U.U~ fn
1.0 g/m2, J.Ol .o 10.0 g/m2, 0.5 to 2.0 g/m2 and 0.1 to
0.7 g/m2, respectively.
If the amount of each plated metal is below the lower
--11--
1 337042
limit, the effect of the plated metal on the corrosion
resistance in the inside of the DI can is very small,
despite further plating. On the other hand, the deposition
of each metal above the upper limit is not suitable from the
viewpoint of economy, as the corrosion resistance in the
inside of the DI can is not remarkably improved.
On the other hand, one side of the steel sheet for the
outside of the DI can should be plated with at least one
ductile metal selected from the group consisting of tin,
nickel, zinc and aluminum.
The optimum range for the amount of a plated ductile
metal should be controlled at 0.5 to 11.2 g/m2 in tin, 0.5
to 5.0 g/m2 in nickel, 1.0 to 10.0 g/m2 in zinc and 1.0 to
5.0 g/m2 in aluminum, respectively. If the amount of a
ductile metal is below the lower limit, the formability to
DI cans becomes remarkably poor. The deposition of each
ductile metal of above the upper limit is not suitable from
the viewpoint of economy, although the formability does not
change.
The surface af the plated ductile metal may be
chemically treated by chromate or phosphate solution by
electrolytical methods, immersion or spray methods, if these
treatments have no bad effects on the formability of the DI -
can.
The present invention is explained in further detail by
reference to the following examples.
EXAMPLE 1
TFS film con~isting of a lower layer of 0.12 g/m2 of
metallic chromium and an upper layer of 0.015 g/m2 of
hydrated chromium oxide as chromium was formed on the one
side of the steel strip having a thickness of 0.30 mm and a
temper of T-2.5 by a known electrolytic chromic acid treatment.
After rinsing with water, 5.6 g/m2 of tin was electroplated
-12-
~- 1 337042
on the other side of said steel strip by using a known
tinplating electrolyte and was rinsed with water and dried.
The obtained surface treated steel strip was heated to
220C by using a roll heater and then a biaxially oriented
copolyester resin film produced from a condensation of
ethylene glycol and polycarboxylic acid consisting of 80
mole % of terephthalic acid and 20 mole % of isophthalic
acid having a thickness of 25 ~m, softening temperature of
176C, melting temperature of 215C, elongation at break of
330 %, strength at break of 8.2 kg/mm2 and orientation
coefficient of 0.024 was laminated on one side having a TFS
film on said steel strip. After that, said copolyester
resin film laminated steel strip was rapidly quenched and
then dried.
The thus copolyester resin film laminated steel strip
was formed into a DI can in which the copolyester resin film
was laminated to the inside of the can under the following
conditions:
~ Forming conqitions of a DI can)
l.Diameter of sample blank: 123.5 mm
2.Drawing ratio in the first drawing: 1.82
3.Drawing ratio in the second redrawing: 1.29
4.Diameter of an ironing punch: 52.64 mm
5.Total ironing ratio: 64 %
EXAMPLE 2
2.8 g/m2 of tin was electroplated on both sides of the
same steel strip as in Example 1 by using a known tinplating
electrolyte. After rinsing with water, 0.006 g/m2 of
hydrated chromium oxide film as chromium was formed on said
tin plated steel strip by a known electrolytic chromic acid
treatment and then rinsed with water and dried.
The same copolyester resin film as in Example 1 was
- ~ ~337042
laminated on the one side of said tin plated steel strip
under the same laminating conditions as in Example 1.
The thus copolyester resin film laminated steel strip
was formed into a DI can under the same forming conditions
as in Example 1.
EXAMPLE 3
A copolyester resin film produced from a condensation
of ethylene glycol and polycarboxylic acid consisting of 85
mole % of terephthalic acid and 15 mole % of isophthalic
acid having a thickness of 30 ~Im~ softening temperature of
192C melting temperature of 239C, elongation at break of
210 %, strength at break of 12.3 kg/mmZ and orientation
coefficient of 0.065 was laminated on the one side having
TFS film of the same steel strip as in Example 1 which is
heated to 240C by using a roll heater and then was rapidly
quenched and dried.
The thus copolyester resin film laminated steel strip
was formed into a DI can under the same conditions as in
Example 1.
EXAMPLE 4
The same copolyester resin film as in Example 3, which
is precoated with 0.2 g/m2 ~drying weight) of a resin
composite consisting of 80 parts of epoxy resin having an
epoxy equivalent of 3000 and 20 parts of resol of paracresol
type, was laminated on the one side of the same tin plated
steel strip as in Example 2 which was heated to 220C and
then was rapidly quenched and dried.
The thus copolyester resin film laminated steel strip
was formed into a DI can under the same conditions as in
Example 1.
-14-
1 33704~
COMPARATIVE EXAMPLE 1
A biaxially oriented polyethylene terephthalate film
having a thickness of 25 ~m, softening temperature of 242C,
melting temperature of 260C, elongation at break of 131 %,
strength at break of 23.2 kg/mm2 and orientation coefficient
of 0.147 was laminated on the one side having a TFS film of
the same steel strip as in Example 1 which was heated to
300C by using a roll heater and then was rapidly quenched
and dried.
The thus polyethylene terephthalate film laminated
steel strip was formed into a DI can under the same conditions
as in Example 1.
COMPARATIVE EXAMPLE 2
A copolyester resin film produced from a condensation
of ethylene glycol and polycarboxylic acid consisting of 85
mole % of terephthalic acid and 15 mole % of isophthalic
acid having a thickness of 30 ~m, softening temperature of
194C, melting temperature of 241C, elongation at break of
190 %, strength at break of 13.6 kg/mm2 and orientation
coefficient of 0.129 was laminated on the one side having
TFS film of the same steel strip as in Example 1 which was
heated to 240C by using a roll heater. After that, said
copolyester resin film laminated steel strip was rapidly
quenched and dried.
The thus copolyester resin film laminated steel strip
was formed to a DI can under the same conditions as in
Example 1.
COMPARATIVE EXAMPLE 3
A copolyester resin film produced from a condensation
of ethylene glycol and polycarboxylic acid consisting of 96
mole % of terephthalic acid and 4 mole % of isophthalic acid
-15-
1 3370~
having a thickness of 30 ~m, softening temperature of 235C,
melting temperature of 250C, elongation at break of 155 %,
strength at break of 20.6 kg/mm2 and orientation coefficient
of 0.131, which was precoated with the same resin composite
as in Example 4, was laminated on the one side of the same
steel strip as in Example 2 which was heated to 220C by
using a roll heater. After that, said copolyester resin
film laminated steel strip was rapidly quenched and dried.
The thus copolyester resin film laminated steel strip
was formed into a DI can under the same conditions as in
Example 1.
The characteristics of the resultant drawn and ironed
can was evaluated by the following methods, after washing
the coolant oil for forming a DI can, drying, heating at
190C for 15 minutes which corresponds to conditions for
curing the lacquer and printing ink subjected to the outside
of the DI can and flange forming.
The results are shown in the Table.
(1) Degree of the exposed metal in the inside of a DI
can.
After 1 % sodium chloride solution was filled in the DI
can, the degree of the exposed metal was evaluated by a
current value which was flown between an anode of said DI
can and a cathode of a stainless steel rod inserted in said
DI can at constant voltage of 6.3 volts.
(2) Filiform corrosion during storage.
The filiform corrosion which grows near the flange
parts in the inside of the DI can was evaluated after
storage for 3 months at a relative humidity of 92 % at 27C.
The obtained filiform corrosion resistance was divided into
5 ranks, namely 5 was excellent, 4 was good, 3 was fair, 2
was poor and 1 was bad.
(3) Degree of cracks of polyester resin film in the
flange part after seaming.
-16-
1 337042
A lacquered alllm;nllm lid was double-seamed to the DI
can. The degree of cracks of polyester resin film in the
flange part near the seaming part was observed after removing
the seamed aluminum lid.
(4) Corrosion resistance by pack test.
A lacquered aluminum lid was double-seamed after Coca
Cola was filled in the DI can. After the storage for 3
months at 37C, the degree of corrosion was observed by
naked eye with measurement of iron pick up.
T~LE
Comp Comp Comp
EX. 1 EX. 2 EX. 3 EX. 4 ex. 1 ex. 2 ex. 3
Coating weight in outsid2e Sn 5.6 Sn 2.8 Sn 5.6 ~n 2.8 Sn 5.6 Sn 5.6 Sn 2.8
of the formed ean (g/m ) Cr~0.006 Cr~0.006 CrXO.
Coating weight in inside- Cr 0.12 Sn 2.8 Cr 0.12 Sn 2.8 Cr 0.12 Cr 0.12 Sn 2.8
of the formed can (g/m2) Cr~0.015 CrX0.006 Cr~0.015 Cr~0.006 CrX0.015 Cr~0.015 Cr~0~00
So tening temp.(~) 7 :7. :9 5: 4~ :9~ ~3
Characte- Me_ting temp. (C _1 1 3 3 6 _4: ~5
ristics ~~r entation coeff_cient . 24 ~. 24 ~.~65 ~. 65 . 47 ._29 . 31
of employed longation at break ~%) 3 3 1 ~1 _3_ 9 -5
copolyester trength at break (kg/mm2) .~ .~ _2.3 _2.3 ~3.2 _3.6 ,0.6
resin film
Meta: Exposure (mA) ~.03 .01 ~.10 .50 .88 302 Peeloff
~ili orm Corr. resistanee ~ ~ . 2 -----
Characte- ~raccs by seaming .o crack :o crack i-o crack o crack ~any micro cracks -----
ristics of ron pick up (ppm) .05 .02 .13 .42 1.20 14.8 -----
the formed ~orr. by pack test ~-ood ~-ood ~-ood ~-ood Pitting Pitting -----
DI can
Remarks: *Cr represents metallic chromium and Cr~represents chromium in hydrated chromium oxide.
~*Peel off represents peeling off of the laminated copolyester resin film.
~,
