TW201540477A - Biaxially oriented polyamide film and method for producing the same - Google Patents

Abstract

Provided is a biaxially oriented polyamide film characterized by, after a boiling treatment in 100 DEG C boiling water for 5 minutes, the detected shrinkage rate on a longitudinal direction (MD) and a lateral direction (TD) at the time of film-production are both 2.0% to 5.0%, the difference between the shrinkage rate on MD and the shrinkage rate on TD is 0.5% or less, the difference between the shrinkage rate on 45 DEG and 135 DEG direction with respected to TD is 0.5% or less.

Description

Biaxially extended polyamine film and method of producing the same

The present invention relates to a biaxially stretched polyamide membrane which is excellent in balance of shrinkage.

A container package of a beverage, a dessert, a cooked food, or the like, which is packaged in a synthetic resin container or a composite container, is filled with a content, and then sealed and sealed using a cover material containing a laminate film. Such a container package may be stored in a low-temperature storage state, but is generally included in a room temperature circulatory person, and is subjected to a heat sterilization treatment such as boiling treatment or retort treatment after sealing.

In the heat sterilization treatment of the container package, since the contents, air and the like in the container are expanded, the internal pressure is applied to the cover material which is thinner and lower in strength than the container body, so that the cover material is stretched, so returning to the normal temperature It is unavoidable to loosen or leave wrinkles in the closure material.

On the other hand, in order to suppress deformation of the cover material due to expansion of air, the container package may be sealed and sealed in a state in which the contents are filled in the container. At this time, from the viewpoint of aesthetics, it is preferable that the cover material has elasticity. It is also preferred that the closure flap is not crimped, for example, It is preferable to make it curl downward even if it is curled.

Patent Documents 1 and 2 disclose a cover material formed by laminating a film obtained by bonding a film having a specific heat shrinkage ratio, thereby improving the elasticity of the cover material and improving the appearance of the cover material tongue.

[Previous Technical Literature] [Patent Literature]

[Patent Document 1] Japanese Patent Laid-Open Publication No. 2007-203532

[Patent Document 2] Japanese Patent Laid-Open No. Hei 11-34258

However, the films having a specific heat shrinkage ratio described in Patent Documents 1 and 2 are not uniform in heat shrinkage in all directions, and are, for example, contracted in a state in which the longitudinal direction and the lateral direction are not balanced. If such a film is applied for printing and used as a capping material, the printing pattern of the capping material may be skewed after the heat sterilization process, and the problem is particularly noticeable when the opening is a circular container. The printed pattern of the cover material becomes rounded after heat sterilization.

An object of the present invention is to provide a biaxially stretched polyamide membrane which is excellent in balance of shrinkage ratio.

The inventors of the present invention have continually studied in order to solve the above problems, and as a result, have achieved the present invention.

That is, the gist of the present invention is as follows.

(1) A biaxially-stretched polyimide film which is subjected to boiling treatment in boiling water at 100 ° C for 5 minutes, and the measured shrinkage ratio is 2.0 in both the machine direction (MD) and the transverse direction (TD) at the time of film formation. 5.0%, the difference between the shrinkage ratio of MD and the shrinkage ratio of TD is 0.5% or less, and the difference between the shrinkage ratio of the 45° direction and the 135° direction with respect to TD is 0.5% or less.

(2) A method for producing a biaxially-stretched polyimide film, which is a method for producing the biaxially-stretched polyimide film according to (1) above, which is subjected to biaxial stretching by simultaneous biaxial stretching After the biaxial stretching, the relaxation rate of either of MD and TD is 1 to 10%, and the ratio of relaxation rate (MD/TD) is 0.66 to 1.50, and reluxation is performed.

(3) The method for producing a biaxially stretched polyamide film according to (2), wherein the heat treatment is performed at 180 to 210 ° C after the biaxial stretching, before the relaxation treatment, or simultaneously with the relaxation treatment.

(4) The method for producing a biaxially stretched polyamide film according to (2) or (3), wherein the simultaneous biaxial stretching method is a tenter type simultaneous biaxial stretching method.

(5) The method for producing a biaxially stretched polyamide film according to (4), wherein the tenter is driven by a linear motor.

(6) A laminate which uses the biaxially oriented polyimide film described in the above (1).

(7) A cover material for a container, which is obtained from the laminate according to the above (6).

According to the present invention, a biaxially stretched polyamide film which is excellent in balance of shrinkage ratio and can be uniformly shrunk in all directions can be obtained. The second invention The axially extending polyimide film can reduce the positioning deviation during printing, and when used as a container cover material, not only the elasticity after the heat sterilization treatment is good, but also the deformation of the cover sheet tongue can be suppressed, and the printed pattern can be reduced. Skewed to improve design.

The invention is described in detail below.

The polyamine resin constituting the biaxially-stretched polyamide film of the present invention may, for example, be nylon 6 mainly composed of ε-caprolactam. Further, examples of the other polyamide resin include a polyamine resin obtained by polycondensation of a meglumine, an ω-amino acid, a diproton acid, and a diamine having a ring of 3 or more members.

Specifically, examples of the indoleamines include, in addition to the ε-caprolactam previously described, heptanoin, octaneamine, and laurylamine.

Examples of the ω-amino acid include 6-aminohexanoic acid, 7-aminoheptanoic acid, 9-aminodecanoic acid, and 11-aminoundecanoic acid.

Examples of the diprotonic acid include adipic acid, glutaric acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, undecanedioic acid, dodecanedioic acid, hexadecandioic acid, and twentieth. Alkanoic acid, eicosadienoic acid, 2,2,4-trimethyladipate, p-nonanoic acid, isodecanoic acid, 2,6-naphthalene dicarboxylic acid, benzenedicarboxylic acid Wait.

Examples of the diamines include ethylenediamine, trimethylenediamine, tetramethylenediamine, hexamethylenediamine, pentamethylenediamine, undecyldiamine, 2,2, 4 (or 2,4,4)-trimethylhexamethylenediamine, cyclohexanediamine, double -(4,4'-Aminocyclohexyl)methane, m-xylylenediamine, decanediamine, decanediamine, and the like.

Further, by using the polymer obtained by the polycondensation or the copolymer, for example, nylon 6, 7, 11, 12, 6.6, 6.9, 6.11, 6.12, 6T, 9T, 10T, 6I, MXD6 can be used. (meta-xylene jipan amide 6), 6/6.6, 6/12, 6/6T, 6/6I, 6/MXD6, and the like. Among them, nylon 6 which is excellent in balance between heat resistance and mechanical properties is preferable.

The biaxially-expanded polyamidamide film of the present invention may be formed of the above-mentioned polyamidamide resin alone or may be obtained by mixing or laminating two or more kinds.

The relative viscosity of the polyamide resin used in the present invention is not particularly limited, and is preferably from 1.5 to 5.0, more preferably from 2.0 to 4.0. If the relative viscosity of the polyamide resin is less than 1.5, the mechanical properties of the resulting film are liable to be remarkably lowered. Further, if the relative viscosity of the polyamide resin exceeds 5.0, the film formability of the film tends to cause problems. In addition, the relative viscosity was measured by using a Ubbelohde viscometer to measure a sample solution (liquid temperature: 25 ° C) in which 96% sulfuric acid was dissolved so that the concentration of the polyamide resin was 1.0 g/dl.

The biaxially stretched polyamide membrane of the present invention comprises a film of the above polyamidamide resin and is a biaxially stretched one.

The shrinkage ratio measured by the biaxially stretched polyamidamide film of the present invention after boiling for 5 minutes in boiling water at 100 ° C is required to be 2.0 to 5.0% in both the machine direction (MD) and the transverse direction (TD) at the time of film formation. Good is 3.0 to 4.0%. If the shrinkage ratio of MD and TD of the biaxially stretched polyamide film after boiling treatment is less than 2.0%, the obtained capping material loses elasticity after heat sterilization treatment. another On the other hand, if the shrinkage ratio of MD and/or TD after boiling treatment exceeds 5.0%, the container may be deformed due to shrinkage of the obtained closure material, and the sealing material tongue may be curled upward.

Further, the difference between the shrinkage ratio of MD measured by the biaxially stretched polyamidamide film of the present invention after boiling for 5 minutes in boiling water at 100 ° C and the shrinkage ratio of TD, and 45 ° direction and 135 ° with respect to TD The difference in the heat shrinkage ratio in the direction is required to be 0.5% or less, preferably 0.4% or less, more preferably 0.3% or less, still more preferably 0.1% or less. If the difference between the shrinkage ratios of the biaxially stretched polyimide film after boiling treatment exceeds 0.5%, the resulting cover material shrinks in a single direction after the heat sterilization treatment, and wrinkles and capping materials are generated. The printed pattern is skewed to cause damage and aesthetics.

When the biaxially stretched polyamide film which satisfies the above-described shrinkage ratio and satisfies the difference in shrinkage ratio and improves the balance of shrinkage is used for a container closure material or the like, the cover material is sufficiently cured after the heat sterilization treatment. The elasticity is not slack, wrinkle, etc., and the printed pattern is not skewed, and the container is not deformed, and the cover sheet tongue is not curled upward, and a container package excellent in appearance can be obtained.

The thickness of the biaxially stretched polyimide film of the present invention is not particularly limited, and is preferably from 6 to 50 μm, more preferably from 10 to 30 μm, when used for packaging purposes. If the thickness of the biaxially stretched polyimide film is less than 6 μm, sufficient physical strength cannot be obtained. Further, when the thickness of the biaxially stretched polyamide film exceeds 50 μm, the stress after stretching becomes high, and it becomes difficult to relax, and it becomes difficult to sufficiently reduce the shrinkage ratio.

The biaxially stretched polyamidamine film of the invention does not damage the hair The range of characteristics can also include pigments, thermal stabilizers, antioxidants, weathering agents, flame retardants, plasticizers, mold release agents, and strengthening agents. For example, examples of the heat stabilizer and the antioxidant include hindered phenols, phosphorus compounds, hindered amines, sulfur compounds, copper compounds, and alkali metal halides.

Further, the biaxially stretched polyamide film of the present invention may contain various inorganic lubricants or organic lubricants in order to improve the slip property of the film. Specific examples of the lubricant include clay, talc, calcium carbonate, zinc carbonate, ash stone, cerium oxide, aluminum oxide, magnesium oxide, calcium citrate, sodium aluminate, calcium aluminate, magnesium alumininate, and glass balloons ( Glass balloon), carbon black, zinc oxide, antimony trioxide, zeolite, hydrotalcite, layered niobate, ethylenebisstearylamine, and the like.

Further, the biaxially stretched polyamide film of the present invention may be subjected to surface treatment such as corona discharge treatment or easy adhesion treatment as needed in the range which does not impair the effects of the present invention.

The biaxially stretched polyamine film of the present invention can be produced in the following manner.

First, after melting the polyamide resin in an extruder, the molten sheet is extruded from a T-shaped mold, and is adhered to a cooling cylinder adjusted to a surface temperature of 0 to 25 ° C for rapid cooling to obtain a continuous unstretched film. .

The obtained unstretched film was sent to a warm water bath adjusted to a temperature of 20 to 80 ° C before the biaxial stretching, and subjected to a water absorption treatment for 10 minutes or less. By this water absorbing treatment, the unstretched film is moderately plasticized, and the crystallization of the polyamide resin is suppressed, whereby the film can be prevented from being broken at the stretching step.

The unstretched film which has been absorbed by the above treatment has a moisture content of preferably 3.0 to 7.0% by mass, more preferably 4.0 to 6.0% by mass. When the moisture content of the unstretched film is less than 3.0% by mass, the elongation stress in the stretching step is increased to cause breakage and the like, and workability is lowered. On the other hand, when the moisture content of the unstretched film exceeds 7.0% by mass, crease wrinkles occur in the water absorbing treatment, and problems such as serpentine of the film tend to occur, and the obtained biaxially stretched polyamide film The strength will decrease and the thickness variation of the TD film will increase.

The unstretched film subjected to the above water absorbing treatment is preheated at 200 to 230 ° C, more preferably at 220 to 230 ° C, and then biaxially stretched at 190 to 210 ° C, more preferably at 195 to 200 ° C. The stretching ratio is preferably from 2.5 to 4.0 times, more preferably from 3.0 to 3.5 times, in the machine direction (MD) and the transverse direction (TD), respectively. Moreover, the ratio of the stretching ratio of MD to the stretching ratio of TD (MD/TD) is preferably from 0.80 to 1.25, more preferably from 0.9 to 1.1. If the stretch ratio of MD and the stretch ratio of TD are less than 2.5 times, the obtained cover material cannot sufficiently express the mechanical strength as a packaging material. On the other hand, when the MD stretching ratio and the stretching ratio of TD exceed 4.0 times, the shrinkage stress of the film after stretching becomes high, and handling of stress relaxation becomes difficult.

The biaxial stretching method for producing the biaxially stretched polyimide film of the present invention requires a simultaneous biaxial stretching method. By extending in the simultaneous biaxial stretching method, a biaxially oriented polyimide film having a relatively uniform balance in the plane direction can be obtained as compared with the sequential biaxial stretching method.

Moreover, the simultaneous biaxial stretching method is preferably a tenter type simultaneous biaxial stretching method. By extending in the tenter type simultaneous biaxial stretching method, compared with the tubular simultaneous biaxial stretching method, a biaxially stretched polyamide film which is less likely to undergo positioning displacement during multicolor printing can be obtained.

The tenter type simultaneous two-axis stretching method can be performed using a zoom tenter, a spiral tenter, a linear motor tenter, and the like. In the present invention, it is preferable to use a linear motor type tenter before the above shrinkage ratio is satisfied and the difference in shrinkage ratio is satisfied and the shrinkage ratio is balanced.

The extension device has a linear motor type tenter, and each clip is separately driven by a linear motor, and has flexibility for arbitrarily regulating the change of the MD extension ratio by adjusting the variable frequency drive. Further, the stretching magnification trajectory is a change in the stretching magnification from the starting point of elongation to the point at which the maximum stretching magnification is reached, and the variation can be defined, for example, by a table or the like. In addition, the extension device of the linear motor type tenter can set the details of the MD extension magnification trajectory and the TD extension magnification trajectory, and can be correctly and smoothly adjusted, so that the extended biaxially extended polyamide film can be extended. The relaxation rate described later is subjected to relaxation treatment, and the shrinkage ratio of the biaxially stretched polyamide film and the difference therebetween are made to be easy to balance the shrinkage ratio.

On the other hand, the zoom tenter and the helical tenter are structurally difficult to perform relaxation treatment on the extended biaxially stretched polyimide film of the MD. For example, in the case of simultaneous biaxial stretching, the zoom tenter generally used can maintain both the folded state when the unstretched film is grasped and the stretched state when stretched, but it is difficult The third type in which a plurality of tortuous states are applied to relax the biaxially stretched polyimide film is maintained.

At the same time, the biaxially stretched polyimide film is preferably subjected to a heat treatment at a temperature of 180 to 210 ° C before the relaxation treatment or at the same time as the relaxation treatment, and the heat treatment temperature is preferably 180 to 200 ° C, and more preferably 80 to 190. °C. The relaxation of the extension stress of the film, although it is generally known that the high temperature heat treatment is Effective, if the heat treatment temperature exceeds 210 ° C, the mechanical strength of the obtained biaxially stretched polyamide film will decrease, the function as a packaging material such as a cover material may be lowered, and the shrinkage rate during boiling treatment There will be cases where it becomes difficult to reach 2.0% or more. On the other hand, when the heat treatment temperature is lower than 180 ° C, even if the relaxation treatment described later is performed, it becomes quite time consuming until the film is sufficiently slack, and the production is poor, and the shrinkage rate upon boiling treatment It becomes difficult to reach the situation of 5.0% or less.

In the present invention, after the simultaneous biaxial stretching, a relaxation treatment of the biaxially stretched polyamide membrane is required. By performing the relaxation treatment, the mechanical strength of the biaxially stretched polyamide film can be prevented, and the shrinkage rate during the boiling treatment can be controlled.

The relaxation rate needs to be 1 to 10%, preferably 5 to 8%, in either MD or TD.

When the relaxation rate exceeds 10%, the polyimide film is loosened (flexed) or the like and comes into contact with the production machine to cause scratches and breakage, which causes a problem that the production efficiency is remarkably lowered.

On the other hand, if the relaxation rate is less than 1%, in order to sufficiently reduce the shrinkage ratio of the biaxially stretched polyamide film, it is necessary to carry out the treatment at a high temperature, which causes a decrease in the mechanical properties of the film.

Moreover, the ratio of the relaxation rate of MD and the relaxation rate of TD (MD/TD) needs to be 0.66 to 1.50, preferably 0.80 to 1.25. If the ratio of the relaxation rate (MD/TD) is less than 0.66 or exceeds 1.50, the balance between the MD and TD shrinkage of the biaxially stretched polyamide film is deteriorated. If the heat treatment is performed after printing, the printing will be skewed and damaged. And aesthetics.

The temperature of the relaxation treatment is preferably from 180 to 210 ° C, more preferably from 185 to 200 ° C. The treatment time is preferably from 1 to 10 seconds. The relaxation treatment can be performed simultaneously with MD and TD, and can also be performed before and after.

The biaxially-stretched polyimide film of the present invention can be, for example, a sealing film such as a polyolefin film or the like, and a plastic film such as a PET film or an EVOH film, and the like, and a known method such as a dry double layer method or an extrusion coating method. A barrier film of an aluminum foil or a transparent vapor deposition film is laminated to form a laminate.

A laminate having a sealing film laminated thereon, for example, can be used as a cover material for a container, heat-sealing the flange of the container, and used for sealing the packaging container.

(Example)

The invention is further illustrated by the examples, but the invention is not limited to the examples.

1. Determination method (1) Shrinkage rate

For the reels of the biaxially stretched polyamide film obtained in the examples and the comparative examples, the surface layer portion of the reel was removed, the inside of the reel was sampled, and the sample was conditioned for 2 hours in a gas atmosphere of 23 ° C × 50% RH.

Next, an elongated test piece (150 mm × width 10 mm in each direction) in the MD direction, the TD direction, the 45° direction with respect to TD, and the TD direction with respect to TD was cut out from the sample. A pair of punctuation marks were provided at intervals of about 100 mm along the longitudinal direction of the strip-shaped test piece, and the distance between the punctuation points (L ₀ (mm)) was measured.

The strip test piece is boiled (5 minutes of boiling water at 100 ° C) After the treatment, the humidity was adjusted again in a gas atmosphere of 23 ° C × 50% RH for 2 hours or more, and then the distance between the punctuation points (L (mm)) was measured, and the shrinkage ratio was calculated by the following formula. Further, the measurement system was carried out for each of the three test pieces in each direction, and the average value was used as the shrinkage ratio.

Shrinkage ratio (%) = {(L _{0 -} L) / L ₀ } × 100

(2) Model test for printing skew

It is assumed that the cover material which is actually subjected to printing is subjected to heat sterilization treatment, and the cover material is skewed, and the evaluation test is performed by the following model.

Water was filled in a circular polypropylene container (110 mm φ) with a flange formed of an extrusion-molded sheet having a thickness of 600 μm, and a laminate film obtained in each of the examples or the comparative examples was used as a capping material to seal The cup machine is sealed and sealed to make a sealed packaging container.

The sealed packaging container is placed in a gas environment of 20 ° C × 65% RH for 24 hours, and after conditioning, the direction of any one of the sealing materials is 0°, and the direction of each direction is 12 directions in 15 directions. , a pair of punctuation points are set at intervals of 100 mm.

The sealed packaging container is heat-sterilized (100 ° C × 30 minutes), placed in a gas atmosphere of 20 ° C × 65% RH for 24 hours, after humidity adjustment, the distance X (mm) between each punctuation is measured, by the following formula The calculation of the shrinkage rate (%) is performed.

Shrinkage ratio (%) = (distance between punctuation points before processing - distance between punctuation points after processing) / (distance between punctuation points before processing) × 100 = {(100 - X) / 100} × 100

The difference between the maximum value and the minimum value of the shrinkage ratio is obtained from the shrinkage ratio in 12 directions, and if the difference is 1.0% or less, the cover material is judged to have a small print skew. It is evaluated as "○" to the extent that it does not cause practical problems when printing is actually performed. If it exceeds 1.0%, it is judged that the printing of the cover material is large and there is a problem in practical use. "."

(3) The elasticity of the cover material

After the heat-sterilized sealed packaging container obtained in the above (2) was cooled and dried, the elasticity of the sealing material was visually evaluated from the state of wrinkles, the elasticity, and the deformation of the container. The person who has no slack/container deformation is evaluated as "○", and the one with slack/container deformation is evaluated as "x".

(4) Curl curling

In the sealed packaging container after heat sterilization, the cover sheet member was not curled or the downward curled person was evaluated as "○", and the cover sheet member curled upward was evaluated as "x".

Example 1 (Manufacture of biaxially oriented polyimide film)

Polyacetamide 6 resin (A1030BRF manufactured by UNITIKA Co., Ltd.) having a relative viscosity of 3.0 was used as a polyamide resin, melt-extruded from a T-shaped mold at a temperature of 260 ° C, and cooled on a cylinder at 15 ° C to obtain a substantially non-oriented thickness of 150 μm. Unstretched film.

The obtained unstretched film was immersed in a warm water bath at 40 ° C for 10 seconds, and then immersed in a warm water bath at 60 ° C for 100 seconds to carry out water absorption treatment so that the moisture content of the unstretched film was 4.0%.

The water-absorbed unstretched film is guided to a linear motor-driven tenter type simultaneous biaxial stretching machine, and after preheating at 212 ° C, the elongation temperature is 196 ° C, the MD stretching ratio is 3.3 times, and the TD stretching ratio is 3.3 times. At the same time, the two axes extend.

Next, the film which was simultaneously biaxially stretched was heat-treated in a heat treatment zone in which the temperature of the first half was set to 190 ° C, and the temperature of the latter half was set to 185 ° C for 4 seconds, and 6.0% relaxation treatment was performed on the MD and TD of the film, respectively. A biaxially stretched polyimide film having a thickness of 15 μm was obtained, and was taken up in a reel shape.

The manufacturing conditions and shrinkage ratios of film formation and elongation are shown in Table 1.

(manufacturing of laminated body)

A coating agent obtained by combining a main component (polyurethane resin)/hardener (polyisocyanate resin) after the corona treatment of the obtained biaxially stretched polyimide film on one side (Dic Dry LX401A, manufactured by DIC Graphics Co., Ltd.) SP60) was applied to the corona surface by a dry laminator so that the dried film thickness was 3 μm to form a multi-layered adhesive layer. Further, on the surface of the adhesive layer, a sealing film (TUX-MCS manufactured by Mitsui Chemicals Tohcello Co., Ltd., non-stretched polyethylene film, thickness: 50 μm) was pressure-bonded to the corona surface of the sealing film by means of an adhesive. The laminate was bonded at a temperature of 50 ° C, and aged at 40 ° C for 3 days to cure the adhesive layer, thereby obtaining a laminate.

(Production of sealed packaging container)

A round polypropylene container (110 mm φ) with a flange was prepared from an extruded sheet having a thickness of 600 μm, and water was filled therein. The above-mentioned laminated body was used as a capping material, and sealed by a cup sealing machine at a sealing temperature of 160 ° C, a sealing pressure of 2.5 kg/mm ² , and a sealing time of 2 seconds. container. The obtained sealed packaging container was subjected to heat sterilization treatment, and a sealing test of the sealing material of the sealed packaging container was performed to evaluate the elasticity and the curl of the tongue piece. The results are shown in Table 1.

Examples 2 to 9 and Comparative Examples 1 to 10

A biaxially stretched polyamide film was obtained in the same manner as in Example 1 except that the stretching ratio, the heat treatment temperature, and the relaxation rate were changed as shown in Tables 1 and 2. The evaluation results of the film production conditions, shrinkage ratio, and capping material are shown in Tables 1 and 2.

Contraction of the two-axis extended polyamine film of Examples 1 to 9 The difference between the rate and the shrinkage rate is within the range specified by the present invention, and the obtained cover material is excellent in the appearance of the print after the heat sterilization treatment.

The shrinkage ratio of the biaxially stretched polyamide film of Comparative Example 1 was too low, so that the obtained capping material lacked elasticity, wrinkles, slack, and damage and its appearance. On the other hand, the shrinkage ratio of the biaxially stretched polyamide film of Comparative Example 2 was too high, so that the container used as the capping material was deformed after the heat sterilization treatment, and the cover sheet tongue was curled upward, and Damage and its aesthetics.

The TD shrinkage ratio of the biaxially stretched polyamide film of Comparative Example 3 was too low, so that the obtained capping material was slack, and the MD shrinkage ratio of the biaxially stretched polyamide film of Comparative Example 4 was too high, so the film was used. The container as a cover material is deformed. Further, in Comparative Examples 3 and 4, the two-axis extended polyamidamide film had a poor balance of shrinkage ratio in each direction, so that the obtained cover material was printed with a large skew and was damaged in appearance.

The shrinkage ratio of the biaxially stretched polyamide film of Comparative Example 5 was too low, so that the obtained capping material lacked elasticity, wrinkles, slack, and damage and its appearance.

The ratio of the relaxation rate (MD/TD) of the biaxially stretched polyamide film of Comparative Example 6 was out of the range, so the difference between the MD shrinkage ratio and the TD shrinkage ratio exceeded 0.5%, and the balance of the shrinkage ratio was poor, so the obtained closure was obtained. The printing of the material is large and the damage is beautiful.

The TD relaxation ratio of the biaxially stretched polyamide membrane of Comparative Examples 7 and 8 was outside the range, and the MD relaxation ratio of the biaxially stretched polyamide membrane of Comparative Examples 9 and 10 was outside the range, and the MD of either one was The difference between the shrinkage ratio and the TD shrinkage ratio is more than 0.5%, and the balance of the shrinkage ratio is poor, so that the obtained cover material has a large print skew and is damaged in its appearance.

Claims (7)

A biaxially stretched polyamide membrane which is subjected to boiling treatment in boiling water at 100 ° C for 5 minutes, and has a shrinkage ratio of 2.0 to 5.0% in both the machine direction (MD) and the transverse direction (TD). The difference between the shrinkage ratio of MD and the shrinkage ratio of TD is 0.5% or less, and the difference in shrinkage ratio between the 45° direction and the 135° direction with respect to TD is 0.5% or less. A method for producing a biaxially stretched polyimide film, which is a method for producing a biaxially stretched polyamide film according to claim 1 of the patent application, which is subjected to biaxial stretching by simultaneous biaxial stretching method, and After the biaxial stretching, the relaxation rate of any of MD and TD is 1 to 10%, and the ratio of the relaxation rate (MD/TD) is 0.66 to 1.50, and the relaxation treatment is performed. The method for producing a biaxially stretched polyamide film according to the second aspect of the invention, wherein the heat treatment is performed at 180 to 210 ° C after the biaxial stretching, before the relaxation treatment, or simultaneously with the relaxation treatment. The method for producing a biaxially stretched polyamide film according to the second or third aspect of the invention, wherein the simultaneous biaxial stretching method is a tenter type simultaneous biaxial stretching method. The method for producing a biaxially stretched polyamide film according to claim 4, wherein the tenter is driven by a linear motor. A laminate comprising the biaxially oriented polyimide film described in claim 1 of the patent application. A closure material for a container, which is formed by the laminate according to claim 6 of the patent application.