Regarding a film comprising a crystalline thermoplastic resin, an easily-formable polyester film such as typically those in Patent References 1 to 3 and Non-Patent Reference 1 is used in various fields of wrapping materials, electronic/electric parts forming materials, automobile parts forming materials, surface forming materials for large-sized shaped members and others, and is specifically noted in those fields because of its excellent low-temperature/high-temperature formability, heat resistance, chemical resistance, etc.
As a method for producing a film by the use of such a crystalline thermoplastic resin having excellent formability, for example, Patent Reference 2 and Non-Patent Reference 1 disclose a production method comprising an essential step of preheating an unstretched film at a predetermined temperature for a predetermined period of time followed by a subsequent biaxial stretching step. However, the method comprises three steps of a preheating step before stretching, a longitudinal stretching step and a lateral stretching step, and therefore requires planning a complicated process, and in addition, the facilities cost for producing a special longitudinal stretcher is high. Therefore, it is desired to improve the method in point of the cost and the simplicity thereof. In case where the easily-formable polyester film produced according to the references is laminated on a metal plate, it requires adhesiveness between the metal plate and the film, and therefore it is also desired to further improve the property of the film itself. Patent Reference 3 discloses a method for producing an easily-formable polyester film that may be adhered to a metal plate under thermal pressure though having a high degree of crystallinity, in which is used a copolyester resin of PET and PBT. The production method in the reference requires an indispensable step of previously mixing and pre-kneading a specific phosphorus compound with the PET resin and controlling the interesterification by the action of the phosphorus compound serving as a catalyst for the purpose of severely controlling the copolymerization ratio of the copolyester resin, and further requires an additional indispensable step of melting the phosphorus-containing PET resin pellets and PBT resin pellets in an extruder after controlling their size to fall within a specific range. Accordingly, the process itself is complicated and, in addition, the technique of producing the resins is also complicated, and the cost of the resin materials is high. For these reasons, it is still desired to develop the method in point of the cost and the simplicity thereof.
On the other hand, various film production methods have been developed. For example, Patent Reference 4 discloses a film production apparatus for touch roll film formation from a PET resin melt between two metal rolls. The reference suggests a film-finishing technique of smoothing both surfaces of the formed film in some degree by nip-pressing both the surface and the back of the film between the metal surfaces of the rolls. However, regarding the special effect in using the production apparatus for PET resin, the reference discloses only the fact that, since the film can be rapidly cooled by the use of the touch rolls each having a thin-wall metal sheath, the produced film could attain the transparency on the same level as that of ordinary PET films. Specifically, in the reference, there is made neither investigation nor discussion relating to the formability of the produced film and even relating to the relationship between the nip-pressing pressure and the film formability. Accordingly, the reference makes neither suggestion nor disclosure of an efficient production method for a crystalline thermoplastic resin having excellent film formability.    Patent Reference 1: Japanese Patent No. 3873334    Patent Reference 2: JP-A 2002-347109    Patent Reference 3: JP-A 2002-173541    Patent Reference 4: Japanese Patent No. 3194904    Non-Patent Reference 1: Journal of Seikei-Kakou, Vol. 19, No. 8 (2007)
As in the above, heretofore no one knows a simple and inexpensive production method for producing a crystalline thermoplastic film that has good adhesiveness to a metal plate and is excellent in formability and strength.
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