JPH04468B2 - - Google Patents

Description

[Detailed description of the invention]

(Field of Industrial Application) The present invention relates to a polyamide laminated film having excellent strength, heat resistance, color tone, etc., and this film is particularly suitable as a film material for food packaging. (Prior art) Among polyamide-based polymers, nylon 66 has a high melting point and therefore has excellent heat resistance.It also has various strength characteristics characteristic of polyamides, so nylon 66 biaxially stretched film is used as a packaging material, for example. It has the potential to become an excellent material. However, nylon 66 has the disadvantage that it deteriorates when heat treated in high-temperature air or moist heat, and its strength decreases significantly when high-temperature heat treatment is performed to increase heat resistance after biaxial stretching, and when nylon 66 is hydrated with high-temperature water during boiling or retorting. The problem was that it decomposed and its strength decreased significantly. This is one of the major reasons why it has not actually become a typical polyamide material for biaxially oriented films, despite its excellent properties. As a conventional technique to solve this problem, Japanese Patent Application Laid-Open No. 56-4632
The publication proposes a method of heat-treating in an inert gas after stretching to prevent thermal deterioration due to oxidation, and JP-A-58-158224 proposes a method of forming a film by mixing nylon 66 with nylon 6. A method of biaxial stretching and heat treatment to a temperature higher than the melting point of nylon 6 has been proposed.
By heat treatment at 235℃ with 100/25,
It has been shown that a biaxially stretched polyamide film with a good balance of strength and hot water shrinkage rate at 130°C can be obtained. (Problems to be solved by the invention) The method disclosed in JP-A-56-4632 attempts to suppress the oxidation reaction by lowering the oxygen concentration in contact with the film during heat treatment, and is an excellent method in principle. However, when biaxial stretching and heat treatment are performed using a tenter-type stretching machine, it is extremely difficult to seal the inside and outside of the machine due to the structure of the stretching machine, and the operating costs are high because it is necessary to constantly replenish the amount of leaked inert gas. This method has drawbacks and cannot be called an industrially practical method. The method disclosed in JP-A-58-158224 eliminates or relaxes the orientation of nylon 6 in the mixture by heat treatment at a temperature higher than the melting point of nylon 6 but sufficiently lower than the melting point of nylon 66. This method improves dimensional stability by making this part a resistor against heat shrinkage.
In this method, as the scrap film is recycled, the nylon 66 in the mixture is repeatedly heat-treated, and even if heat-treated at a relatively low temperature, thermal deterioration progresses gradually.As a result, as the recycling rate increases, the strength decreases. The color may deteriorate or the degree of coloring may become stronger. In addition, none of these methods discloses a solution to the problem that the strength is significantly reduced due to hydrolysis by high-temperature water during boiling or retorting. (Means for Solving the Problems) The present inventors have researched a method of forming a multilayer structure of nylon 66 film in order to heat-treat the film without contacting with high-temperature air or moist heat. When I searched for the material,
In view of the balance between ease of production, such as film formability and stretchability, and the quality of the obtained laminated film, we found that it is preferable to form a surface layer of nylon 6 and sandwich the inner nylon 66 layer with this layer. We have arrived at the present invention. Nylon 6 has excellent adhesion and similar stretching behavior to nylon 66, so lamination and stretching is easy.In terms of physical properties, it also has the characteristics of polyamide polymers, such as thrust strength and bending strength, over a wide temperature range. Because of its excellent properties, there are almost no defects caused by multi-layering. Another major advantage of combining nylon 6 and nylon 66 is that the compatibility between the two makes it easy to recycle the scrap film. The recycled polymer has a recycled layer (a mixture of nylon 66 and nylon 6) between the nylon 6 and nylon 66 layers.
It may be provided with a five-layer structure, or it may be blended with nylon 66 as an inner layer. In the case of a blend, if the thickness and recycling rate of each layer are selected so that the nylon 6 content is 10% or less, the inner layer will practically exhibit the same performance as nylon 66 alone. Nylon 6 forming the surface layer is a nylon 6 homopolymer (melting point 215℃) or a nylon 6 with a melting point of 5℃.
These are copolymerized with other polymers within a range that does not change over ℃, or blends with other polymers containing 90% or more of nylon 6, with small amounts of additives such as lubricants, antistatic agents, and antioxidants. Including added items. When a copolymer with nylon 6 having a melting point 5° C. or more lower than that of nylon 6 is used as the nylon forming the surface layer, the strength of the obtained film after boiling or retorting is greatly reduced, and the present invention can be achieved. I can't. Furthermore, if a copolymer with nylon 6 having a melting point 5° C. or more higher than that of nylon 6 is used, the bending strength of the resulting film will decrease. The surface layer is formed on both sides of the film, and the thickness is the same except for special purposes. The nylon 66 that forms the inner layer is a nylon 66 homopolymer or a copolymer of nylon 66 with other polymers within a range that does not change the melting point of nylon 66 by more than 10°C.
These include blends with other polymers that contain 90% or more of 66, to which small amounts of additives such as lubricants, antistatic agents, and antioxidants are added. The laminated structure is first formed by a coextrusion method, and the biaxial stretching is performed by a simultaneous biaxial stretching method. This combination allows laminated stretched films to be obtained without the use of adhesives in most cases. In particular, it is particularly preferable to laminate the materials by a feedblock type coextrusion method, since the interfaces are sufficiently compatible in a molten and pressurized state. The simultaneous biaxial stretching method is a stretching method suitable for multi-layer co-stretching since it is difficult to generate shearing force at the interface even when the stretching behavior of each layer is different. Therefore, by combining the two, a stretched film in which the interface between nylon 6 and nylon 66 is firmly adhered and integrated can be obtained. Suitable stretching ratios are 2.8 to 3.8 times in both length and width. The heat treatment after stretching is carried out at a temperature below or above the melting point of nylon 6, specifically at a temperature of 200 to 250°C for a period of 2 to 20 seconds. The configuration of each layer thickness can be arbitrarily selected depending on the intended quality of the application. Taking a three-layer structure of nylon 6/nylon 66/nylon 6 as an example, the inner layer nylon 66 has a thickness of 5 to 20μ, and the outer layer nylon 6 has a thickness of 1 to 10μ for both the front and back layers of the film. The thickness configuration and heat treatment temperature are selected depending on the purpose. When the heat treatment temperature increases, the nylon 6 on the surface layer oxidizes or decomposes to some extent, causing coloration and a decrease in strength, but because the thickness is thin, even if it is colored, it is not noticeable, and its contribution to the overall strength is small, so this is not a problem. Often not. When the heat treatment temperature is raised, a film with excellent heat-resistant dimensional stability can be obtained, and when the heat treatment temperature is lowered, a film with excellent strength and color tone can be obtained. (Function) The film of the present invention has both strength and dimensional stability and is particularly suitable for food packaging films subjected to boiling or retort sterilization processes. Nylon 6 surface layer
has a protective function not only as a protective layer during heat treatment during the manufacturing process, but also against hydrolysis caused by high-temperature water during boiling and retorting. The bending strength of nylon 66 is slightly inferior to that of nylon 6, but since it is an inner layer, the amount of deformation of the nylon 66 layer is small when the laminated film is bent, and the bending strength of the laminated film is improved. Moreover, the multilayer structure has the advantage of increasing the number of means for controlling various quality. For example, fine inorganic particles to impart slipperiness need only be added to the surface layer, so slipperiness can be improved without significantly impairing transparency, and even when antistatic agents or antioxidants are mixed in, they can be added only to the surface layer. Another reason is that a greater effect can be produced by increasing the concentration of additives in the surface layer. (Example and Comparative Example) Two 90mm extruders, feedblock type 3-layer T
A laminated biaxially stretched film was produced using a device whose main equipment was a die and a simultaneous biaxial stretching machine, in which the inner layer was made of nylon 66 with a relative viscosity of 3.8, and nylon 6 with a relative viscosity of 3.0 was laminated on both sides. A small amount of lubricant and antioxidant were added to nylon 6, and a small amount of antioxidant was added to nylon 66. Simultaneous biaxial stretching is 3.0 in the longitudinal direction using a tenter type stretching machine.
Stretched 3.5 times in the transverse direction, 3% in the transverse direction after stretching
Heat treatment was performed for 5 seconds while relaxing. Also, comparative example 1,
As Example 2, the same nylon 66 polymer was put into two extruders to form a single-layer film, and a single-layer nylon 66 stretched film was produced under the same conditions.
Furthermore, as Comparative Example 3, a laminated and simultaneously biaxially stretched film was obtained in the same manner as in the example except that a polymer (melting point 180° C.) obtained by copolymerizing nylon 6 with 15 mol % of nylon 66 was used as the nylon forming the surface layer. Table 1 summarizes the thickness structure, heat treatment temperature, and physical property values of the obtained film. It can be seen that by the method of the present invention, a film with well-balanced strength and hot water shrinkage rate can be obtained.

【table】

[Table] (Effects of the invention) As is clear from the above explanation, although the present invention has excellent properties, the thermal deterioration behavior during heat treatment was a major obstacle, and its applications could not be expanded in the past. This gives new possibilities to nylon 66 stretched film. i.e. nylon 6
The multi-layered structure of the film increases the means of controlling strength and dimensional stability, making the film suitable for a wider range of applications, and these films are especially useful as food packaging materials that are boiled or retort sterilized. It will be excellent.

Claims (1)

[Scope of Claims] 1 Consisting of at least three layers including an inner layer substantially composed of nylon 66 and a surface layer substantially composed of nylon 6, and has a strength retention rate of 70% or more after boiling treatment. A polyamide-based laminated biaxially stretched film. 2 After simultaneously biaxially stretching a laminated film of at least three layers, including an inner layer substantially composed of nylon 66 and a surface layer substantially composed of nylon 6, which are laminated by coextrusion method, short-time heat treatment is performed. The strength retention rate after boiling is 70%.
The method for producing a polyamide-based laminated biaxially stretched film as described above.