An aluminum alloy sheet laminated with a thermoplastic resin, for example polyester resin, has already been used for the manufacture of can lids and so on. However, when adhesion of the laminated thermoplastic resin layer to the aluminum alloy sheet is not sufficient, there are some cases where the thermoplastic resin layer peels off during forming and corrosion is generated at the portion where adhesion of the laminated thermoplastic resin layer is not sufficient. The adhesion is affected by formability of the aluminum alloy sheet and characteristics of the thermoplastic resin to be laminated thereto, and it is particularly affected by the surface roughness of the aluminum alloy sheet. For this reason, several surface pre-treatments mentioned below have been so far given to an aluminum alloy sheet in order to improve the adhesion of a thermoplastic resin layer or a coating layer to the aluminum alloy sheet.
(1) Chemical treatment of an aluminum alloy sheet in a phosphate solution or in a chromate solution PA1 (2) Coating a thermosetting resin primer on one surface of a thermoplastic resin film or on an aluminum alloy sheet PA1 (3) Forming an anodic oxide film having 5 to 60% of an occupied area rate by micro pores of 2000 angstrom or more of diameter and 5 .mu.m or less of depth on an aluminum alloy sheet using a solution including chromic acid (Laid Open Japanese Patent No. Hei 3-44496) PA1 (4) Forming an oxide film having 20 angstrom or more of thickness by heating an aluminum alloy sheet for 2 hours or more at the temperature of 250 to 650.degree. C. in the atmosphere after cleansing (Laid Open Japanese Patent No. Hei 6-272015) PA1 (5) Forming an oxide film having 500 to 5000 angstrom of thickness and branched micro pores by electrolytically treating in an alkali solution using alternate wave-form current after cleansing (Laid Open Japanese Patent No. Hei 6-267638) PA1 i) The ratio (specific area) of the imaginary area of the sample having no unevenness (projected area), that was the standard, to the later measured area of the sample of the present invention (actual area) was calculated. PA1 ii) The increasing rate of the ratio was shown in percentage.
In the aforementioned chemical treatment (1) in a phosphate solution or in a chromate solution, phosphate, chromate or fluoride is mainly used in the treating solution, and the formed film by the chemical treatment has an effect on the improvement of adhesion of coating film. Therefore, it is generally used. But when the thus chemically treated aluminum alloy sheet covered with a coating or laminated with a thermoplastic resin is formed by a more severe than usual forming, which is the objective of the present invention, the coating or the laminated thermoplastic resin tends to peel off, which is not favorable. Coating a primer for adhesion according to treatment (2) is not favorable from the viewpoint of an increase in cost for coating of primer as well as the productivity because it requires another process for baking. It also requires an exhauster for organic solvent. Forming an anodic oxide film (3) having specified micro pores using chromate solution is not favorable from the viewpoint of productivity because it takes long time to form the anodic oxide film and there is a tendency of peeling off of the thermoplastic resin layer when the laminate is heavily formed. Forming an oxide film (4) by heating in the atmosphere for a long time is not favorable since the thermoplastic resin layer peels off when the laminate is heavily formed as with the case of (3), and this prior technique is also unfavorable from the viewpoint of productivity because it takes a long time to form the oxide film. Forming an oxide film (5) having 500 to 5000 of thickness by electrolytically treating in an alkali solution using alternate wave-form current can be practiced continuously in a short duration of electrolysis and has some effect on the adhesion after forming of the laminated resin film; however, it does not give sufficient adhesion after forming since the laminated resin film peels off when the laminate is heavily formed as drawn, stretch-formed and further pressed.
As mentioned above, the lamination of a thermoplastic resin on aluminum alloy sheet that is surface-treated using conventional surface-treating manner can not be successfully used for adhesion therebetween after heavy forming, which is the objective of the present invention, since the laminated thermoplastic resin layer peels off when the laminate is heavily formed as drawn, stretch-formed and further pressed.
The technical problems to be solved by the present invention is to produce a thermoplastic resin laminated aluminum alloy sheet having excellent adhesion after forming in such a manner that the laminated thermoplastic resin does not peel off even when the laminate is deep-drawn, stretch-formed and further pressed to shape, that is superior to a laminate in which resin film is laminated on the conventionally surface-treated aluminum alloy sheet, and to obtain excellent adhesion after sterilization. The technical problems to be solved by the present invention are is also to produce a manufacturing method and a manufacturing apparatus thereof which can produce the laminate at low cost and high production speed.