A biaxially stretched polyamide resin film using a polyamide such as nylon 6 and/or nylon 66 is excellent in the mechanical properties such as tensile strength, adhesive strength, pinhole strength and impact-resistant strength, and additionally, in gas-barrier property and heat resistance. Thus, laminated films, in which a biaxially stretched polyamide resin film is used as a front substrate and a sealant made of a polyolefin film is bonded to the front substrate by a method such as dry laminating or extrusion laminating, are used in wide fields including packaging materials for use in sterilization treatment such as boiling or retorting.
Such biaxially stretched polyamide resin films are usually used as front substrates, and are in many cases free from direct contact with contents. Accordingly, the behavior of the caprolactam monomer (hereinafter, abbreviated as “monomer” as the case may be) in the biaxially stretched polyamide resin films has not been much mentioned yet.
In these years, however, the issue of the deterioration of packaged articles and contents has undergone increasingly severe requests thereto, and the improvement of the issue has come to be demanded. In particular, in the medical applications or the like objecting to subtle compositional changes of the contents, the small molecular weight monomers contained in the polyamide resin film pass through the sealant to migrate into the contents, when heating, for example, for sterilization treatment is conducted, and hence it comes to be impossible to leave such an issue out of consideration.
For the purpose of coping with the issue, there have been proposed polyamide resins, in each of which the molecular weight of the constituent monomer unit is large, such as nylon 11 and nylon 12 or copolyamide resins mainly composed of nylon 11 and nylon 12 (JP 4-325159 A). Additionally, a copolymerized polyamide resin between 1,6-hexanediamine and sebacic acid has also been proposed (JP 2001-328681 A). However, these are specific polyamides, and are high in price and low in versatility. Consequently, strongly demanded are films in which highly versatile nylon 6 and/or nylon 66 is used and the amount of the monomer contained therein is low.
Even if the unreacted monomers and oligomers are removed from a polyamide resin at the stage of being chips prior to film molding, remelting of the polyamide resin chips with a melt extruder or the like regenerates monomers and oligomers, and consequently the monomers remain in the film to degrade the quality of the film. In particular, a polyamide in which caproamide is the main repeating unit thereof has a characteristic that the monomer tends to be more easily generated than in a polyamides formed of a dicarboxylic acid and a diamine.
In general, when the terminal group concentration of a polyamide resin is higher, the regeneration amount of the monomer at the time of remelting tends to be larger. Thus, there has been developed a polyamide in which the above-described problem is alleviated by adding a compound capable of reacting with the carboxyl terminals or the amino terminals of the polyamide. Specifically, there has been disclosed a method in which an organic glycidyl ester is reacted with the carboxyl groups and the amino groups of the polyamide (JP 10-219104 A). However, in this method, when the organic glycidyl ester and the polyamide chips are dry blended and melt-kneaded in an extruder, the organic glycidyl ester is allowed to react with the terminal groups of the polyamide. Therefore, in this method, it is difficult to perform uniform mixing in the dry blending step prior to film molding. Consequently, such non-uniform mixing offers a cause for the compositional variation. Thus, it is difficult to obtain a polyamide having a uniform terminal group concentration, and moreover, the dry blending step itself is unsuitable for films involving large melt extrusion amounts. Additionally, the amount of the monomer extracted after the melt molding remains to be as large as 0.35 to 0.5% by mass to show that the reduction amount of the monomer is insufficient.
On the other hand, there has been disclosed a method in which the terminal amino groups of a polyamide resin are blocked with a dicarboxylic acid anhydride (JP 2005-187665 A). However, the amount of the regenerated monomer at the time of melting remains to be as large as 0.27 to 0.75% by mass, revealing that it is difficult to sufficiently reduce the amount of the monomer extracted from the polyamide resin film.
On the other hand, in these years, recognized is a trend to regulate the discharge, from industrial plants and business institutions, of organic compound materials (generally abbreviated as “VOC”) which evaporate at normal temperature and pressure and easily volatilize into the air. For example, in Japan, on the basis of the revised Air Pollution Control Law, a government ordinance that specifies the type and the size of the institution as an object of regulation came into effect on Jun. 1, 2005. Additionally, the government and ministry ordinances for the discharge standard value, the notification items, the measurement methods and the like were proclaimed on Jun. 10, 2005 and came into effect on Apr. 1, 2006.
Further studies are needed as to whether or not the caprolactam monomer discharged in the air provides adverse effects. However, in the production of a polyamide resin film, in printing on the film, and in the steps of laminate processing, bag forming processing and the like using the film, it is the manufacturer's responsibility to reduce the amount of the caprolactam monomer discharged into the air from the film.
Accordingly, the reduction of the amount of the caprolactam monomer extracted from the film and the recovery of the caprolactam monomer at the time of production of the film are strongly demanded.