CA1230075A - Tin-free steel sheets having improved lacquer adhesion - Google Patents

Abstract

ABSTRACT

This invention provides a tin-free steel sheet, having superior lacquer adhesion, comprising metallic chromium and chromium oxide coatings on a steel sheet, characterized in that the chromium oxide coating is controlled such that has a value of 0.5 or more provided that is the percent transmission corresponding to ol linkage and

Description

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 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).

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 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.

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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 ~lZ3~
-s-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 .~23~ 5 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 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 - ,3 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 ~31)~7~i - 9a -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 - l o -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 ~;23~ 7~i 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

Claims

The embodiments of the invention in which an exclusive property or privileye is claimed are defined as follows:

1. A tin-free steel sheet having metallic chromium and chromium oxide coatings on a steel sheet, characterized in that for improved laccuer adhesion, has a value of 0.5 or more provided that is the percent transmission correspollding to ol linkage and is the percent transmission corresponding to oxo linkage in the infrared absorption spectrum of said chromium oxide coating as measured by Fourier transform infra-red spectroscopy (FTIR).