SPECIFICATIONMethod for production of metal sheet covered with polyester resin filmThe present invention relates to a method for the production of a metal sheet covered with polyester resin film produced by laminating a crystalline and oriented polyester resin film on the surface of a metal sheet heated to a temperature above the melting point of polyester resin film and then rapidly quenching the laminate.
At present, organic resin film laminated metal sheets are widely used in various fields such as electrical components, furniture and building materials. In general, there are two well-known methods for continuous lamination of organic resin film on the surface of the metal sheet. The first method is one in which an adhesive coated metal sheet is used. Namely, at first the adhesive in which thermosetting resin is mainly dissolved in a solvent is coated on the surface of the metal sheet. After curing the adhesive, the organic resin film is laminated on the surface of the metal sheet and then heated in an oven with a large capacity for several minutes. After that, the organic resin film laminated metal sheet is cooled.
The second method is one in which an adhesive coated organic resin film is used. In this case, the organic resin film is laminated on the surface of the metal sheet and then is also heated in the same type of oven as in the first method.
However, these methods have some problems because a long time is required for curing the adhesive and heating after lamination of the organic resin film. For example, the production speed is low, e.g. 10 to 30 m/min. In some cases, the characteristics of the organic resin film such as the corrosion resistance are deteriorated by heating for a long time and the laminated organic resin film may be peeled off from the surface of the metal sheet by severe forming because the formability of thermosetting resin used for the adhesive is not good.
Furthermore, in Laid-Open Japanese Patent Application No. Sho 53-81 530, a modified polyester resin film is used for lamination of the organic resin film on the metal sheet without an adhesive such as thermosetting resin.
In the can producing industry, continuous lacquer coating at high speed on the metal sheet has been investigated in order to decrease the production cost. However, such process is very difficult to practice industrially, because a lacquer which can be cured by heating for a few seconds has not yet been developed.
Furthermore, the corrosion resistance after forming of the lacquer coated metal sheet is poor as compared with that of the organic resin film laminated metal sheet.
It is the first object of the present invention to provide a metal sheet covered with polyester resin film having an excellent corrosion resistance after forming as compared with that of the lacquer coated metal sheet and other organic resin film laminated metal sheets. It is the second object of the present invention to provide a method for the continuous lamination of polyester resin film on the surface of the metal sheet at high speeds such as 100 to 600 m/min.
The reason why the metal sheet having an excellent corrosion resistance after forming is obtained according to the method of the present invention is as follows:In general, polyester resin film having the crystalline and oriented structure has an excellent nonpermeability to water and steam, but it is not adhered to the metal sheet without a thermosetting resin adhesive. On the contrary, polyester resin film having the amorphous structure which is obtained by rapidly cooling the crystalline and oriented polyester resin film heated to a temperature above the melting point of polyester resin for a long time strongly bonds to the metal sheet as shown in JapanesePatent Publication No. Sho 49-34180, but it is poor in corrosion resistance because water and vapor easily permeates the film.
As described above, both polyester resin films have different properties.
Thus, it is considered that one reason why the metal sheet according to the present invention has excellent corrosion resistance after forming is that the thin amorphous layer of polyester resin film having excellent bonding strength to the metal sheet is formed between the upper layer of crystalline and oriented polyester resin film and the surface of the metal sheet by rapidly quenching after laminating the crystalline and oriented polyester resin film.
The second reason is that the thicker layer of the crystalline and oriented polyester resin film having the excellent non-permeability to water and steam remains after laminating the crystalline and oriented polyester resin film to the surface of the metal sheet.
Therefore, the method of the present invention is characterized by the lamination of the crystalline and oriented polyester resin film on the surface of the metal sheet heated above the temperature of the melting point of the used polyester resin film and then rapidly quenching.
The metal sheet according to the present invention can be used in applications wherein excellent corrosion resistance after severe forming is required, such as drawn cans, drawn and redrawn cans (DR can) in addition to can ends, instead of lacquered electrotinplate and tin free steel having a duplex layer consisting of an upper layer of hydrated chromium oxide and a lower layer of metallic chromium.
Furthermore, it is also used for the can body adhered by polyester resin film instead of the lacquered tin free steel can body seamed by nylon adhesive.
In the present invention, the employed polyester resin film is prepared by processing polyester resin which is produced by the esterification of at least one saturated polycarboxylic acid by 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, 1,4-cyclohexane dicarboxylic acid and trimellitic acid anhydride.
Saturated polyalcohols are selected from ethylene glycol, 1 ,4-butane diol, 1 ,5-pentane diol, 1,6hexane diol, propylene glycol, 1,4-dimethanol cyclohexane, 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 polyester resin film used for the present invention.
In the present invention, the use of polyethylene terephthalate film having biaxial oriented structure is especially desirable from the viewpoint of corrosion resistance and economy.
The thickness of the used polyester resin film should be 5-100 ym, preferably 5-50 ym, although it is not limited to such range. However, if the thickness of the employed polyester resin film is below 5 ym, the excellent corrosion resistance after severe forming in the metal sheet according to the present invention is not obtained.
Moreover, the use of the polyester resin film having a thickness above 100 ym is not economically suitable for the film to be laminated to the metal sheet, because the polyester resin film used for the present invention is expensive as compared with epoxy phenolic lacquers widely used in the can industry and other organic resin films such as polypropylene film.
The metal sheet used in the present invention is steel sheet or aluminum sheet with or without a surface treatment.
Particularly, tin free steel having an upper layer of hydrated chromium oxide and a lower layer of metallic chromium, lightly tin coated steel sheet, Ni plated steel sheet, Cr plated steel sheet, Cu plated steel sheet, steel sheet covered with hydrated chromium oxide and aluminum sheet with chromate treatment and phosphate treatment are suitable as the metal sheet for the present invention, because these metal sheets have excellent bonding strength with respect to the polyester resin film.
Furthermore, the following duplex, triple, composite and alloy plated steel sheets can also be used as the metal sheet in the present invention.
Duplex and triple coated steel sheets are selected from the group consisting of lightly tin coated steel sheet with Ni plating, chromate treatment, phosphate treatment or tin free steel treatment, Ni plated steel sheet with Cr plating, light Sn plating, chromate treatment, phosphate treatment of tin free steel treatment, Cr plated steel sheet with chromate treatment, Cu plated steel sheet with light tin plating, Ni plating, Cr plating, chromate treatment or tin free steel treatment, and Zn plated steel sheet with chromate treatment, phosphate treatment or silicate treatment.
Composite and alloy coated steel sheets are selected from the group consisting of Ni-Sn plated steel sheet with or without chromate treatment and Zn plated steel sheet in which a small amount of at least one metal, hydroxide or oxide of Ni, Co, Fe, Cr and Mo is contained in the plated Zn layer.
In the continuous production of the metal sheet according to the present invention at high speed, tin free steel having below 0.050 g/m2 as chromium in hydrated chromium oxide and below 0.2 g"m2 of metallic chromium, lightly tin coated steel sheet having below 1.0 g/m2 of Sn, Cr plated steel sheet having below 0.2 g/m2 of Cr and Ni plated steel sheet having below 3.0 g/m2 of Ni are suitable for the metal sheet according to the present invention.
The temperature of the metal sheet heated just before the lamination of the polyester resin film which is an important factor in the present invention should be maintained in the range of Tm-Tm +1 600C, in which Tm is the melting point of the polyester resin film which exhibits an endothermic peak in an ordinary differential thermal analysis run at a heating rate of 1 OoC/min., in order to get the excellent bonding strength between the polyester resin film and the metal sheet.
If the temperature of the metal sheet heated just before the lamination of the polyester resin film is above Tm + 1 600C, the corrosion resistance of the polyester resin film laminated metal sheet deteriorates, because the ratio of the non-oriented part to the oriented part in the laminated polyester resin film will perhaps increase. With a temperature below Tm, the polyester resin film laminated metal sheet having excellent bonding strength is not obtained, because the lower side of the employed polyester resin film in contact with the metal sheet is not sufficiently melted.
Furthermore, the quenching conditions after laminating the polyester resin film on the metal sheet is also one of the important factors in the present invention. Namely, the maximum temperature on the surface, which does not contact the surface of the metal sheet, of the laminated polyester resin film should be below TsOC, at which the melting of crystals in the polyester resin film starts, preferably below Ts-200C.
More specifically, Ts represents the temperature at which the endothermic reaction of polyester resin film starts, as determined by ordinary differential thermal analysis which is run at a heating rate of100C/min.
If the temperature on the surface of the laminated polyester resin film is above Ts, the characteristics of the metal sheet according to the present invention become remarkably poor.
For example, the appearance of the laminated polyester resin film changes from clear to milky and the corrosion resistance after forming becomes poor.
Furthermore, the quenching time for cooling the metal sheet to below Ts after the lamination of the polyester resin film is also an important factor in the present invention. It should be below 10 seconds. In the case where the quenching time is longer, the corrosion resistance of the polyester resin film laminated metal sheet also becomes remarkably poor, because it is considered that the greater part of the crystalline and oriented structure changes to the non-oriented structure in the laminated polyester resin film.
In the present invention, the method for heating the metal sheet to which the polyester resin film is laminated is not limited. However, from the standpoint of continuous and stable production of the polyester resin film laminated metal sheet at high speed, induction heating and/or resistance heating which are used for reflowing tinplate in the conventional process for production of electrotinplate is suitable as the method for heating the metal sheet to be laminated, because the metal sheet to be laminated is rapidly heated and the temperature of the heated metal sheet is easily controlled. Namely, it is desirable in the present invention that the metal sheet to be laminated is heated in the range of from 1-20 seconds.
Furthermore, in the present invention, various methods were considered for quenching the metal sheet heated above the melting point of the polyester resin film after the lamination of the polyester resin film. However, quenching by water spray, water immersion, liquid nitrogen or employing a roller cooled by water or liquid nitrogen are industrially suitable as the method for quenching the heated metal sheet after the lamination of polyester resin film. The temperature of water used for quenching the heated metal sheet should be kept below 900C in consideration of the continuous production of the polyester resin film laminated metal sheet according to the present invention, although it should be kept as low as possible.
The present invention is explained in further detail by reference to the following examples.
EXAMPLE 1A coid rolled steel sheet having a thickness of 0.23 mm was electrolytically degreased 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 cathodically treated by using an electrolyte consisting of 30 g/l of CrO3 and 1.5 g/l of NaF in water under 20 A/dm2 of cathodic current density at an electrolyte temperature of 300 C. The thus treated steel sheet was rinsed with hot water having a temperature of 800C and dried. After that, a crystalline and oriented polyester resin film (Trade name: Melinex S made by ICI Co., Ltd.) having a thickness of 20 Hm was laminated on the thus treated steel sheet under the following conditions and was quenched.
Conditions of the lamination of the polyester resin film:Method for heating the treated steel sheet Resistance heatingTemperature of the treated steel sheet just before the lamination 285"C Maximum temperature on the surface of the laminated polyesterresin film between laminating and quenching 1 500C Quenching time to 1 500C on the surface of the laminatedpolyester resin film 2 sec.
EXAMPLE 2The steel sheet was electroplated with 0.3 g/m2 of Sn by using an electrolyte consisting of 25 g/l of stannous sulfate, 1 5 g/l of phenolsulfonic acid (60% aqueous solution) and 2 g/l of ethoxylated anaphthol sulfonic acid in water under 20 A/dm2 of cathodic current density at an electrolyte temperature of 400C after the pretreatment as in Example 1. The tin plated steel sheet was rinsed with water and dried.
After that, a crystalline and oriented polyester resin film (Trade name: Lumirror F made by ToreCo., Ltd.) having a thickness of 50 Mm was laminated on the thus tin plated steel sheet under the following conditions and quenched.
Conditions for the lamination of the polyester resin film:Method for heating the tin plated steel sheet Resistance heatingTemperature of the tin plated steel sheet just before the lamination 2700CMaximum temperature on the surface of the laminated polyesterresin film between laminating and quenching 1 700C Quenching time to 1 700C on the surface of the laminatedpolyester resin film 3 sec.
EXAMPLE 3The steel sheet was electroplated with 0.6 g/m2 of Ni by using a Watt's bath consisting of 40 g/l of NICK, 6H20, 250 girl of NiSO4. 6H20 and 40 g/l of H3BO3 in water under 10 A/dm2 of cathodic current density at a bath temperature of 400 C. The Ni plated steel sheet was rinsed with water after chromate treatment by using 30 g/l of sodium dichromate solution under 10 A/dm2 of cathodic current density at an electrolyte temperature of 450C and dried.
After that, a crystalline and oriented polyester resin film (Trade name: Melinex 377 made by ICICo., Ltd.) having a thickness of 50 um was laminated on the treated Ni plated steel sheet under the following conditions and quenched.
Conditions for the lamination of the polyester resin film:Method for heating the Ni plated steel sheet Induction heatingTemperature of the Ni plated steel sheet just before the lamination 3200CMaximum temperature on the surface of the laminated polyesterresin film between laminating and quenching 1 850C Quenching time to 1 850C on the surface of the laminatedpolyester resin film 4 sec.
EXAMPLE 4The steel sheet was electroplated with 0.7 g/m2 of Ni containing 0.04 g/m2 of Sn by using aWatt's bath used in Example 3 added 5 g/l of stannous sulfate under the same conditions as in Example 3 after the pretreatment as in Example 1.
The Ni-Sn plated steel sheet was rinsed with water and dried. After that, a crystalline and oriented polyester resin film (Trade name: W 3030 made by Teijin Co., Ltd.) having a thickness of 30 ym was laminated on the thus Ni-Sn plated steel sheet under the following conditions.
Conditions for the lamination of the polyester resin film:Method for heating the Ni-Sn plated steel sheet Infrared heatingTemperature of the Ni-Sn plated steel sheet just before the lamination 2800CMaximum temperature on the surface of the laminated polyesterresin film between laminating and quenching 1 6O0C Quenching time to 1 600C on the surface of the laminatedpolyester resin film 2 sec.
EXAMPLE 5The steel sheet was electroplated with 0.3 g/m2 of Ni by using a Watt's bath used in Example 3.
After rinsing the Ni plated steel sheet was treated by using an electrolyte consisting of 50 g/l of CrO3 and 0.5 g/l of sulfuric acid in water under 10 A/dm2 of cathodic current density at an electrolyte temperature of 550C and then rinsed with water and dried.
After that, a crystalline and oriented polyester resin film (Trade name: E-5000 made by ToyoboCo., Ltd.) having a thickness of 40 Mm, was laminated on the Ni plated steel sheet having metallic chromium and hydrated chromium oxide under the following conditions and was quenched.
Conditions for the lamination of the polyester resin film:Method for heating the Ni plated steel sheet Induction heatingTemperature af the Ni plated steel sheet just before the iamination 3000CMaximum temperature on the surface of the laminated polyesterresin film between laminating and quenching 190 C Quenching time to 1 SO0C on the surface of the laminatedpolyester resin film 6 sec.
EXAMPLE 6An aluminum sheet (JIS 3004) having a thickness of 0.23 mm was cathodically degreased in a solution of 30 g/l sodium carbonate and then treated in a solution containing 30 g/l sodium dichromate at a solution temperature of 650C. The thus chromate treated aluminum sheet was rinsed with water and dried.
After that, a crystalline and oriented polyester resin film (Trade name: Flu mirror S10 made by ToreCo., Ltd.) having a thickness of 20 Mm was laminated on the treated aluminum sheet under the following conditions and quenched.
Conditions for the lamination of the polyester resin film:Method-for heating the treated aluminum sheet Resistance heatingTemperature of the treated aluminum sheet just before the lamination 2900CMaximum temperature on the surface of the laminated polyesterresin film between laminating and quenching 1 800C Quenching time to 1 8O0C on the surface of the laminatedpolyester resin film 3 sec.
COMPARATIVE EXAMPLE 1The same polyester resin film and the same treated steel sheet used for Example 1 were prepared.
This polyester resin film was laminated to the treated steel sheet under the same conditions as inExample 1.
After lamination, the laminated steel sheet was gradually cooled without water quenching.
COMPARATIVE EXAMPLE 2The same polyester resin film and the same treated steel sheet used for Example 3 were prepared.
This polyester resin film was laminated to the treated steel sheet under the same conditions asianExample 3 except for the quenching time to 1850C on the surface of the laminated polyester resin film.
Quenching time to 185PC 45 sec.
COMPARATIVE EXAMPLE 3An aluminum sheet (JIS 3004) having a thickness of 0.23 mm was treated under the same conditions as in Example 6.
After that, a crystalline and oriented polyester resin film (Trade name: Lumirror S10 made by ToreCo., Ltd.) having a thickness of 4 ym, was laminated to the treated aluminum sheet under the same conditions as in Example 6.
The characteristics of the resultant metal sheet were evaluated by the following test methods, after the measurement of the coating weight on the resultant metal sheet by the X-ray fluorescent method, the results of which are shown in the Table.
(1) Bonding strength between the metal sheet and the polyester resin film:The polyester resin film laminated sample was cut to a size of 8 cm x 8 cm, and the polyester resin film laminated side was cut crosswise with a razor. After 6 mm of the sample was extruded by using a conventional Erichsen testing machine, the laminated polyester resin film of the formed part was peeled off by a pincette. Bonding strength was divided into 5 ranks, namely, 5 was excellent, 4 was good, 3 was fair, 2 was poor and 1 was bad.
(2) Corrosion resistance against an acidic solution after forming:The polyester resin film laminated sample was cut to a circular blank having a diameter of 80 mm by a punch press, and the blank was deeply drawn to form a cup in which the polyester resin film laminated side was inside at a drawing ratio of 2.0.
50 ml of citric acid adjusted to pH 2.2 was filled into the drawn cup, and the iron or aluminum pick up was measured after aging for 30 days at 550C.
(3) Discoloration of the laminated polyester resin film after the retort treatment:The drawn cup prepared by the method described in (2) above was set in a retort into which steam, heated to 1 25-1 300C under a pressure of 1.6-1.7 kg/cm2, was blown for 6 hours. After that, the discoloration of the laminated polyester resin film was evaluated by the naked eye.
TABLE
Comp. Comp. Comp.
Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 Ex. 1 Ex. 2 Ex. 3 Base Steel Steel Steel Steel Steel Aluminium Steel Steel Aluminium Ni Metal Coated *1 *2 0.3 Sheet layer Cr *1 ni Ni Cr Cr Ni (g/m) 0.11 Sn 0.6 0.7 0.023 Crox 0.12 Crox Crox 0.3 Crox Sn Crox 0.017 Crox Crox 0.016 0.015 0.004 0.04 0.012 0.015 0.004 Thickness of polyester film 20 m 50 m 50 m 30 m 40 m 20 m 20 m 50 m 4 m Bonding strength 5 4 5 5 5 4 5 5 4 Fe or Al pick up (ppm) 0.05 0.05 0.10 0.23 0.18 0.43 13.5 8.3 3.8 Discoloration No No No No No No Lightly Heavily No of polyester film change change change change change change milky milky change Remarks: *1 The greater parts of plated Sn changes to iron-tin alloy by heating.
*2 The greater parts of plated Ni and Sn may change to Ni-Sn alloy by heating.
*3 Cr is metallic Cr and Crox is Cr in the formed hydrated Cr oxide.