Note: Descriptions are shown in the official language in which they were submitted.
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SPECIFICATION
TITLE OF THE INVENTION
TIN-FREE STEEL SHEETS WAVING IMPROVED LACQUER ADHESION
TECHNICAL FIELD
This invention relates to tin-free steel sheets having
improved lacquer adhesion.
TECHNICAL BACKGROUND
Surface treated steel sheets having double coatings,
metallic chromium and chromium oxide coatings are designated
tin-free steel of chromium type to be simply referred to as
TFS, hereinafter). TFS is regarded as a substitute for tin
plates because of its improved properties as can-forming
material, and the demand for TFS is increasing in these years.
Since TFS has metallic chromium and chromium oxide coatings on
the surface, it does not possess sufflcient weldability. In
manufacturing cans from TFS, a can barrel is formed by applying
an epoxy-phenol resin to a blank and bonding the blank with a
nylon adhesive.
Recently, the extent of application of TFS cans has
been further spread. That ls, TFS cans are not only used for
so-called cold packs prepared by filling cans with contents
such as carbonated beverage and beer at low temperatures, but
also used for so-called hot packs prepared by filling cans
with contents such as fruit juice and coffee at high temperatures
for sterilization as well as retort packs requiring a high
temperature retorting treatment for sterilization at the end
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of packing. In the latter applications, there often occurred
accidents of rupture of can barrels.
This can barrel rupture occurs in bonded TFS cans during
hot packing and retorting treatment because hot water penetrates
through the lacquer film at the barrel junction to deteriorate
the interfacial adhesion between the lacquer film and the
TFS sheet to eventually separate the lacquer film from the
TFS sheet. Extensive investigations have been made to develop
TFS sheets which are not susceptible to deterioration of
the adhesion between the lacquer film and the TFS sheet.
~ISCLOS~RE OF THE INVENTION
Particularly, the inventors have made investigations
how lacquer adhesion varies in relation to the structure
of the TFS surface coating to find that lacquer adhesion
is closely correlated to the degree of olation of the chromium
oxide coating as analyzed by Fourier transform infrared
spectroscopy (to be simply referred to as FIIR, hereinafter),
and the present invention is based on this discovery.
According to the present invenion, there is provided
a tin-free steel sheet having metallic chromium and chromium
oxide layers on a steel sheet, characteri7ed in that for
improved lacquer adhesion, I5r0/(I5r0 +`ICr0) has a value
of 0.5 or more provided that ICr0 is the percent trasmission
corresponding to ol linkage and ICr0 is the percent trans-
mission corresponding to oxo linkage in the infrared absorption
spectrum of said chromium oxide layer as measured by Fourier
transform infrared spectroscopy (FIIR).
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BRIEF DESCRIPTION OF TMF, DRAWING
Fig. 1 is a chart of the infrared absorption spectrum of
a Cr coating on a TFS sheet as measured by F'TIR.
Fig. 2 is an enlarged chart illustratiny the infrared
absorption spectra near 600 cm 1 of the surface of TFS sample
A having superior lacquer adhesion, sample B having intermediate
lacquer adhesion, and sample C having inferior lacquer adhesion.
Fig. 3 illustrates the infrared absorption spectra of
TFS before and after heating for analysis of CrX coating
structure.
Fig. 4 is a diagram illustrating lacquer adhesion in
relation to degree of olation.
Fig. 5 illustrates how to determine the intensity of
transmission in an infrared absorption spectrum as measured
by FTIR.
Fig. 6 illustrates how to evaluate the adhesion of
lacquer to TFS sheets.
DETAILED DESCRIPTION OF THE INVENTION
The present lnvention is directed to TFS sheets having
a metallic chromium coating (to be referred to as CrM coating)
deposited to 50 to 200 mg/m2 and a chromium oxide coating
(to be referred to as CrX coating, herèinafter) deposited
to 10 to 30 mg/m of metallic chromium on each surface of
a cold-rolled steel sheet. TFS usually has a CrM layer
deposited to 50 to 200 mg/m2 because corrosion resistance
is poor for CrM layers of less than 50 mg/m2 while no
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further irnprovement in corrosion resistance is expectable
for CrM layers exceeding 200 mg/m2. On the other hand,
the desired lacquer adhesion is not achievable with Cr layers
of less than 10 mg/m , while Cr layers exceeding 30 mg/m
appear poor, losing commercial acceptance.
The adhesion of lacquer to TFS is very important because
TFS is generally coated with an epoxy-phenol lacquer prior
to use. Presuming that it is the structure of the CrX
coating that controls the lacquer adheslon the inventors
continued research works.
There were found rnany reports which used electron spectro-
scopy for chemical analysis (ESCA) and Auger electron spectroscopy
(AES) as measures for structural analvsis of Cr coatings.
The inventors also made initial research~we~ using these
analytical techniques, but failed to get as good results
as needed by the inventors thcmsclv~s. The CrX coating
has a giant molecule structure composed essentially of Cr-OH
linkages (oi-linkages) and Cr-O linkages (oxo-linkages), in
which many molecules of H2O (bound water) or anions such as
So2 , F , etc. are contained. Direct analysis of spectra
obtained by ESCA and AES cannot distinguish those O elements
assigned to Cr-OH linkage, Cr-O linkage H2O, and the like
from each other.
Lo e, ~2c~
Then, the inventors - en to apply the infrared
spectroscopy sensitive to atomic bond structures to the analysis
of the CrX coating structure. As ordinary dispersive lnfrared
spectrometers are less sensitive for the measurement of the
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coating surEace vicinity, Fourier transform infrared
spectroscopy (FTIR) is employed herein. Fig. 1 is an exemplary
infrared absorption spectrum of a Cr coating on TFS as
measured by FTIR. Peaks at 1589 cm , at 972 cm , and
near 600 cm~1 are assigned to OH group, So2 group, and
chromium oxide, respectively. The inventors have found
that the profile near 600 cm has a significant influence
on the adhesion of lacquerto TFS. Fig. 2 illustrates in
an enlarged fashion those portions near 600 cm of the
superficial infrared absorption spectra of a TFS sheet having
superior lacquer adhesion (Sample A), a TFS sheet having inter-
mediate lacquer adhesion (Sample B), and a TFS sheet having
inferior lacquer adhesion (Sample C). A deep absorption peak
is found at 580 cm 1 for Sample A, absorption peaks at
580 cm and 660 cm are of substantially the same intensity
for Sample B, and a deeper absorption peak appears at 660 cm
and the absorption peak at 580 cm 1 becomes more vague for
Sample C.
In order to determine the linkages in the CrX coating
structure to which the absorption peaks appearing at 580 cm
and 660 cm 1 are assigned, the following experiment was carried
out. A TFS sample was heated at 700C for removal of water,
and the infrared ahsorption spectrum of the sample was
measured before and after heating for comparison (see Fig.
3). The absorption peak at 1580 cm assigned to OH group
completely disappeared after the heat treatment, and at
the same time, the absorption peak at 580 cm 1 disappeared
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and the absorption peak at 660 cm becarne more intense.
When the fact that Cr(OH)3 is completely converted to Cr2O3
through the heat treatment at 700C is taken into account,
it is presumed that the absorption peaks appearing at 580
cm and 660 cm are assigned to Cr-OH linkage (ol-linkage)
and Cr-O linkage (oxo-linkage), respectively. As evident
from this result, Fig. 2 indicates that more oxo-linkages
and less ol-linkages are present in the CrX coating of
TFS having inferior lacquer adhesion whereas more ol-linkages
and less oxo-linkages are present in the CrX coating of
TFS having superior lacquer adhesion.
In order to quantitatively describe these results
of analysis of the Cr coating structure on TFS, the inventors
y ye 7/c~
ha_ sot up clegree of olation. That is, it is presumed that
in the infrared absorption spectrum of the coating as measured by
FTIR, the proportion of ol~linkages present in the Cr
coating is expressed by ICr0/(Icr0 + I5r0) provided that
ICr and ICr represent percent transmissions at 580 cm
and 660 cm in the infrared absorption spectrum as measured
on the TFS surface by FTIR, and this ratio is defined as
degree of olation.
The adhesion of lacquer to TFS was determined in relation
to the degree of olation as defined above, with the results
shown in E`ig. 4. Lacquer adhesion is poor in those TFS sheets
having CrX coatings with a low degree of olation while
lacquer adhesion is good in those TFS sheets having CrX
coatings with a high degree of olation. It was found that
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lacquer adhesion is greatly improved particularly when the CrX
coating has a degree of olation of 0.5 or more. This correla-
tion of lacquer adhesion to degree of olation is explained as
follows. The adhesive force between a lacquer film (OH group-
containing epoxy-phenol resin) and a TFS sheet is attributable
to the bond formed between OH groups in the lacquer film and
Cr-OH linkages in the CrX coating (neither H2O nor Cr-O
linkages are pertinent). Since more OH groups (resulting
from Cr-OH linkages) are present on the surface of a TFS sheet
having a CrX coating with a higher degree of olation, the
form more bonds with OH groups in the fader film, For this
reason TFS sheets having a higher degree of olation show
improved lacquer adhesion thereto. The bonds between OH
groups in the lacquer film and OH groups in the CrX coating
may include simple hydrogen bonds and bonds resulting from
certain reactions between such OH groups (for example,
dehydration-condensation reaction), but the exact mechanism is
unclear.
The FTIR spectrometer used in the present invention is
2~ FTIR model JIR 100 manufactured by Nihon Electronics K.K. and
the measurement conditions are: resolving power 8 cm 1,
reflection angle 75 degrees, and integration number 200 times,
with the reference being tin plate lT~CA). Measurement
direction was fixed to the L direction, that is, the rolling
direction of steel sheets. Of course, the measurement direction
is not limited to the L direction because directional
difference is eliminated by converting measurements to an
~3~q)'75
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inten~i~y ratio as measure of the degree of olation. The only
reason why measurement was made in the fixed I, direction of
steel sheets in the present invention is that the intensity
itself varies with mea6llrement direction. Intensity values
given as I5C80 and I6C60 are calculated by drawing a base wine in
a chart plotted for percent transmission as shown in Fig. 5.
The lacquer adhesion to steel sheets was evaluated as
follows. Referring to Fig. 6a, a phenol-epoxy lacquer was
applied to the surface of a steel sheet 1 to a build-up of 60
mgJm and baked at 210C for 1'' minutes to form a thick film
lA. The same lacquer was applied to the surface of another
sheet 2 to a build-up of 25 mg/m2 and baked under the same
conditions as above to form a thin film 2A. These sheets were
cut to pieces of 70 mm wide by 60 mm long, and the longitudinal
end portions of the two pieces having lacquer films of different
thickness were overlapped a distance of 3 mm with a nylon film 3
of loom thick interposed therebetween. Using a hot press, the
overlapped pieces were pre-heated to 200 C for 120 seconds and
press bonded under a pressure of 3 kg/cm2 at 200 C for 30
seconds. Ten samples 4 were prepared in each example, mounted
in a jig 5 as shown in Fig. 6b, and placed in a retort ket-tle at
130C to determine the number of separated samples after 150 and
300 minutes. Evaluation was made according to the following
criterion in Examples and Comparative Examples. Samples marked
with a circle "0" were judged superior in paint adhesion.
3~ P9'S
0: 0 - 1 sample separated after 150 min., and
O - 5 samples separated after 300 min.,
X: O - l sample separated after 150 min., and
6 or more samples separated after 300 min.
X: 2 or more samples separated after 150 min.
BEST MODE FOR CARRYING OUT THE INVENTION
The following examples and comparative examples are
given to further illustrate the present invention.
Cold rolled steel sheets (T4CA) having a thickness of
0.22 mm were electrolytically degreased in 5% Homezarine (a
degreasing agent comprising 75~ NaOH and a surface active agent
produced by Kao Kabushiki Kaisha) at 80C at a current density of
5A~dm , rinsed with water, pickled by immersing them in 10%
H~S04 for 5 seconds, rinsed with water, and then subjected to
e~.ectroplating treatments as described below such that the
built-ups of CrM and CrX might fall in the ranges of
50 - 200 mg/m2 and 10 - 30 mg/m2, respectively.
Example 1
In an electrolytic solution containing 150 grams of
CrO3, 5 grams of Na2SiF6, 0.8 grams of H2S04, and 2
grams of Cr3+ per liter of the solution at 50C, a steel sheet
was first made the cathode and subjected to a chromium
* denotes a trade mark
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electroplating treatment at 50 A/dm2 for 1.5 seconds, and the
sheet was then made the anode and subjected to a reverse
electrolytic treatment in the same solution at 5 A/dm2 for 0.2
seconds. After rinsing with water, the treated steel sheet was
made the
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cathode and subjected to an electrolytic chromate treatment
in an electrolytic solution containing 50 grams of CrO3 and
0.05 grams of l-l2SO4 per liter of the solution at 40C at 15 A/dm2
for 3 seconds. Water rinsing and drying resulted in a TFS
5 sheet.
Comparative Example 1
A TFS sheet was manufactured by the same procedure as
used in Example 1 except that the reverse electrolytic treatment
was omitted and the conditions for the electrolytic chromate
treatment were changed to 15 A~dm2 for 1 second in order to
adjust the build-up of CrX to 10 to 30 mg/m2.
Example 2
In an electrolytic solution containing 90 grams of
CrO3, 0.1 grams of H2SO4, and 2.0 grams of HBF4 per llter of
the solution at 55C, a steel sheet was made the cathode and
electrolyzed at 60 A/dm for 1 second, and then dried.
Example 3
In an electrolytic solution containing 80 grams of CrO3,
4 grams of Na2SiF6, and 1 gram of NaF per liter of the solution
at 50C, a steel sheet was made the cathode and electrolyzed
at 40 A/dm2 for 1.5 seconds, and then dried,
Comparatlve Example 2
In an electrolytic solution containing 50 grams of CrO3,
2 grams of NaF, and 0.005 grams of H2SO4 per liter of the
solution at 45C, a steel sheet was made the cathode and
electrolyzed at 30A/dm2 for 2 seconds, and then rinsed with
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water and dried.
The results of the adhesion of lacquer to the TES sheets
manufactured in Examples 1 - 3 and Comparative Examples 1 - 2
are shown in Table 1 along with the degree oE olation of the
corresponding Cr coatings as measured by FTIR. As seen from
Table 1, the sheets of Examples 1 - 3 having CrX coatings with
a degree of olation of more than 0.5 exhibit superior lacquer
adhesion whereas the sheets of Comparative Examples 1 - 2
having CrX coatings with a degree oE olation of less than 0.5
exhibit inferior lacquer adhesion.
Table 1
Degree Samples separated Samples separated
of after after l,acquer
olation 150 min. at 130C 300 min. at 130C adhesion
Example 1 0.80 0 0
Example 2 0.69 0 2 O
Example 3 0.54 1 4 Q
Comparative
Example 1 0.25 10 - X
Comparative
Example 2 0.~5 6 8 X
