1. Field of the Invention
This invention relates to a manufacturing method of a laminated metal plate in which a plastic film (sheet) is laminated on a surface of a metal plate, and to an improved laminated metal plate manufactured by the method.
2. Description of the Prior Art
As a metal container (made of a steel plate, for example) for containing kerosene, animal and vegetable oils, solvents, chemicals, adhesive agents, etc., a zinc-lined or tin-lined container is used. However, such a lining is not enough in many cases depending on a contained substance. Therefore, lining by a baked coating material like epoxy resin, or lining using an adhesive agent of polyethylene, polypropylene, polyester film, etc. is performed, in addition to the afore-mentioned lining.
Bonding of the lining material to the metal plate is usually carried out by a thermo-fusion method in addition to the method using the above-mentioned adhesive agents. In the thermal adhesion method, the metal plate and a synthetic resin film are heated by making them go through a heating furnace, by Joule heat by eddy current induced in the metal plate (magnetic material: a steel plate or the like), by an electromagnetic induction coil, or by flame treatment as suggested by the inventor of this invention in Japanese Patent Publication No. 7-100,350 (No. 7-100,350).
According to the flame treatment method, the metal plate and the synthetic resin film are continuously joined together while treating each of the surfaces by flame immediately before joining. According to this method, sufficient joint strength can be obtained without pretreatment on the metal plate or the synthetic resin film for improvement of adhesion performance, a harmful substance like an adhesive agent is eliminated, and use of a large-scale heating furnace or an electromagnetic induction device is eliminated, wherefore there is an advantage that the economical impact is extremely large.
When a steel plate is coated or lined with resin, a predetermined pretreatment (cleaning of the metal plate surface or the synthetic resin film: sanding, degreasing, washing, drying, anchor coating, etc.) for improving the adhesion performance has to be carried out upon adhesion. In the film lining like polyethylene, the steel-side needs the pretreatment and a primer coat, and the film-side needs a corona treatment, a use of a double-layer structure film in which an adhesive resin is laid, or coating of the primer on the film. On the other hand, according to this flame treatment method, these processes are eliminated.
In the meantime, as the synthetic resin film, an inert fluorine resin film is suitable for use as a lining material in addition to the above-mentioned polyethylene film, polypropylene film and polyester film that have been conventionally used as lining materials. The fluorine resin film has excellent properties in corrosion resistance, contamination resistance, adhesion resistance, chemical resistance, heat resistance, incombustibility, etc. Therefore, the fluorine resin film is one of the best lining materials for lining a metal container for containing kerosene, animal and vegetable oils, various solvents, chemicals, adhesive agents, etc. In particular, the fluorine resin film as a lining material is more effective when used in cooking equipments and food processing equipments, cooking heaters like an inner plate of a microwave oven, an inner pan of a rice cooker or a top panel of a gas cooking stove, kitchen material like a material for a range hood, medical apparatuses, outdoor construction materials like an external facing panel of a road, a roof and exterior wall panel of a building, etc.
The fluorine resin film has the problems that the surface tension is small and securing of adhesiveness by thermo-fusion is difficult due to its non-adhesiveness. Thermo-fusion of the fluorine resin film with the metal surface is particularly difficult when one molecular unit contains two or more fluorine atoms, like tetrafluoroethylene-perfluoroalkylvinylether copolymer resin (PFA), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), or tetrafluoroethylene-tetrafluoroethylene copolymer resin (ETFE), in particular.
A method for solving the problems in the thermo-fusion of the fluorine resin film on the metal plate surface is, as suggested in Japanese Patent Laid-Open Publication No. 2000-280,402, that a colored coating film layer comprising a polyvinylidene fluoride resin and acrylic resin, is laid on the metal surface in advance for the thermo-fusion, which is then baked and dried. After that, a transparent fluoroplastic film comprising the polyvinylidene fluoride resin and the acrylic resin, is thermally fused.
However, in the above-mentioned method of thermo-fusion, the metal plate needs pretreatment before the thermo-fusion, and the fluorine resin film needs the primer. As suggested earlier in No. 7-100,350, the inventor studied whether lining by thermally fusing the fluorine resin film is possible by adopting the flame treatment method which is a simple means needing no pretreatment. Based on the method disclosed in No. 7-100,350, various experiments were conducted by using tetrafluoroethylene-perfluoroalkylvinylether copolymer resin (PFA), tetrafluoroethylene-hexafluoropropylene copolymer resin (FEP), or tetrafluoroethylene-tetrafluoroethylene copolymer resin (ETFE), however, excellent adhesiveness was not obtained.
Possible reasons why excellent adhesiveness was obtained in polyethylene, polypropylene, or polyester film without any problems, but not in the fluorine resin film, were studied and analyzed. As a result, it is speculated that the conventional flame treatment on the metal plate is insufficient because the material has a very small surface tension and has a characteristic of non-adhesiveness.