JPH1016047A - Manufacture of polyamide film and biaxially oriented polyamide film obtained by the manufacture - Google Patents

Abstract

PROBLEM TO BE SOLVED: To lessen an oblique difference in a hydrothermal shrinkage percentage which is a problem in the case when a polyamide film is manufactured sequentially by a biaxial stretching method. SOLUTION: On the occasion of manufacturing a biaxially stretched polyamide film sequentially by stretching longitudinally an unstretched film being amorphous and non-oriented substantially and then by stretching it laterally by a tenter type laterally stretching machine, stretching is conducted under the conditions of 2.7<=(multiplying rate of longitudinal stretching)<=3.0, 3.4<=(multiplying rate of lateral stretching)<=4.0 and (ratio between multiplying rates of longitudinal and lateral stretching)>=1.3, heat treatment is then conducted and relaxation treatment is conducted thereafter under the condition of 1.0<=(relaxation rate)<=4.0(%). The opposite ends of a stretched film thus obtained being released from clips of the tenter type laterally stretching machine, the film is relaxed in the lateral direction by reheat treatment under the conditions of 160<=(temperature of reheat treatment)<=200( deg.C) and 1.0<=(time of reheat treatment)<=4.0(s) by using floatation type heat treatment equipment and then it is cooled immediately and wound up.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a polyamide film and a biaxially oriented polyamide film obtained by the method. For more information,
The present invention relates to a method for producing a polyamide film by first stretching an unstretched polyamide film longitudinally, then transversely stretching it by a tenter method, and successively biaxially stretching it, and a biaxially oriented polyamide film obtained by the method.

[0002]

2. Description of the Related Art A biaxially oriented polyamide film is excellent in abrasion resistance, impact resistance, pinhole resistance, etc., including mechanical properties, optical properties, thermal properties, and barrier properties. It is widely used as a film for food and other packaging materials, and is particularly often used as a bag for retort food (bag for hot water treatment). Such a bag for retort food usually uses a biaxially oriented polyamide film as a base film, and various heat-sealing sealants (polyethylene, polypropylene, etc.) are used for the film.
Is laminated and then heat-sealed on three sides to form a so-called three-sided seal bag. In actual use,
After the bag is filled with food or the like, high-temperature boil processing is performed, so that thermal dimensional stability is required.

[0003] However, the angle of 45 ° with respect to the longitudinal direction of the base film (biaxially oriented polyamide film) is 1 °.
If the difference in the hot water shrinkage from the direction of 35 degrees (hereinafter referred to as “diagonal difference in hot water shrinkage”) is large, a phenomenon such as twisting or curling of the bag occurs and the appearance of the product is impaired. there were. In addition, such a phenomenon is noticeable in the so-called sequential biaxial stretching method in which the film is stretched in the longitudinal direction and then stretched in the transverse direction, and the influence increases as it goes to the end,
It was common knowledge that the polyamide film produced by this sequential biaxial stretching method could not be used for the above-mentioned applications.

In order to solve such a problem, as a method for improving the thermal dimensional stability of a thermoplastic resin film from a stenter, a method using an arc-shaped floating heat treatment has been proposed and is already known. For example, JP-A-4-29293
No. 4, in the film, immediately after biaxially stretching the film in the longitudinal direction and the transverse direction, the film is caused to levitate so as to draw an arc along the arcuate surface of the hot air blowing means having an arcuate vertical cross section. A method for performing heat treatment substantially simultaneously in the longitudinal and transverse directions of the film is disclosed.

[0005] However, in this method, although the thermal dimensional stability is improved by the subsequent heat treatment, it cannot be applied to a stretched film obtained under any conditions. According to the experiments of the present inventors, it is difficult to control the tension applied in the longitudinal direction of the film due to the characteristics of the device,
While the relaxation process in the vertical direction is insufficient, the relaxation process is sufficiently performed in the width direction (horizontal direction) because it is virtually free, so that the relaxation effect in each direction becomes uneven, It was found that the effect was not sufficient for the improvement of the oblique difference in the hot water shrinkage of the finally obtained film.

Further, JP-A-4-292937 discloses that immediately after a film is biaxially stretched in a machine direction and a transverse direction,
There is disclosed a method in which a buoyant steam having an arcuate vertical cross section is used to levitate and travel along an arcuate surface along an arcuate surface, and during the levitating travel, the reheat treatment is performed substantially simultaneously in the longitudinal and lateral directions of the film.

However, this method has a problem that when it is applied to a polyamide film which is liable to undergo dimensional change due to moisture, wrinkles are apt to occur during processing, roll formation is deteriorated, and commercial value is lost. There is.

[0008]

SUMMARY OF THE INVENTION An object of the present invention is to reduce the diagonal difference in shrinkage of hot water, which has been a problem when a polyamide film is produced by a successive biaxial stretching method, by devising production conditions. . More specifically, it is an object of the present invention to obtain a biaxially oriented polyamide film having excellent thermal dimensional stability, a small difference in shrinkage ratio of hot water, and practical strength.

[0009]

SUMMARY OF THE INVENTION In view of the above circumstances, the present inventors have developed a method for producing a biaxially oriented polyamide film capable of minimizing an oblique difference in hot water shrinkage in a sequential stretching method, and a method for producing a thermally dimensional stable film. We studied diligently on the development of a polyamide film having excellent heat resistance and a small difference in oblique hot water shrinkage.
As a result, if the stretching conditions and the heat treatment conditions are appropriately selected to form a film having a specific hot water shrinkage ratio, and if the conditions for the continuous reheat treatment are appropriately selected, not only the thermal dimensional stability but also the heat The present inventors have found that the difference in water shrinkage ratio can be improved, and that it is extremely effective in improving the product quality, and thus completed the present invention.

That is, the gist of the process for producing a polyamide film of the present invention is that a substantially amorphous, non-oriented, unstretched film is longitudinally stretched, and then transversely stretched by a tenter type transverse stretching machine to successively biaxially stretch. In producing a polyamide film, stretching is performed under the conditions of the following formulas (1) to (3), heat treatment is performed, and then relaxation treatment is performed under the condition of the following formula (4). Release from the clip of the tenter-type transverse stretching machine, re-heat treatment by the floating heat treatment device under the conditions of the following formulas (5) and (6) to relax in the horizontal direction, and then immediately cool and wind.

Longitudinal stretching ratio: X 2.7 ≦ X ≦ 3.0 (1) Lateral stretching ratio: Y 3.4 ≦ Y ≦ 4.0 (2) Ratio of longitudinal to transverse stretching ratio Y ≧ 1.3X (3) Relaxation Rate: Z 1.0 ≦ Z ≦ 4.0 (%) (4) Reheat treatment temperature: T 160 ≦ T ≦ 200 (° C.) (5) Reheat treatment time: θt 1.0 ≦ θt ≦ 4.0 (s) (6) In the method of the present invention, in the stage after the relaxation treatment and before the re-heat treatment, the film has a hot water shrinkage of 2.0% or less in a longitudinal direction and a direction perpendicular to the longitudinal direction. Hot water shrinkage ratio of 2.9% or less, (longitudinal hot water shrinkage ratio) <(hot water shrinkage ratio in a direction perpendicular to the vertical direction) The difference in the hot water shrinkage from the direction of the degree exceeds 1.8%.

The present invention also relates to a polyamide film obtained by the above method, wherein the hot water shrinkage in the longitudinal direction of the film is 2.0% or less, and the hot water shrinkage in a direction perpendicular to the longitudinal direction is 1%. 0% or less, the difference in hot water shrinkage between the 45 ° direction and the 135 ° direction with respect to the longitudinal direction of the film is 1.8% or less, and the breaking strength between the longitudinal direction of the film and its perpendicular direction is The gist of the present invention is a biaxially oriented polyamide film, both of which are not less than 25 kgf / mm ² .

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail.
The polyamide in the present invention is mainly a polyamide having oriented crystallinity, but is not particularly limited, and may be any linear polymer compound having an amide bond in the molecule. That is, in addition to the above-mentioned poly ε-capramide, polyhexamethylene adipamide (nylon 6
6), polyhexamethylene sebacamide (nylon 61
0), polyundecamide (nylon 11), polylauramide (nylon 12), polymethaxylylene adipamide (MXD6), and copolymers thereof, and nylon 6 which is excellent in cost performance is particularly preferably used.

[0014] These polyamides, if necessary,
Various additives such as a lubricant, an antistatic agent, an antiblocking agent, and inorganic fine particles can be added as long as the performance of the film is not adversely affected.

In the present invention, to obtain a substantially amorphous, non-oriented polyamide film (unstretched film),
For example, a polyamide is heated and melted by an extruder, extruded into a film from a T-die, and cooled and solidified on a cooling drum rotating by a known casting method such as an air knife casting method or an electrostatic application casting method to rapidly form a film. . When the unstretched film is oriented, stretchability may be reduced in a subsequent step.

Next, the unstretched film is preheated for film stretching by a roller-type longitudinal stretching machine composed of a group of heating rollers having different peripheral speeds, and then the unstretched film has a glass transition point or higher. At a temperature, the film is stretched longitudinally between a heated stretching roll and a cooling roll for cooling the film. At this time, in the present invention, it is necessary to set the longitudinal stretching ratio to the condition shown in the following formula (1).

Longitudinal stretching ratio: X 2.7 ≦ X ≦ 3.0 (1) The reason for setting the longitudinal stretching ratio in this range is as follows.
That is, if it is lower than 2.7 times, the hot water shrinkage rate becomes small, but the practical strength cannot be imparted, so that the object of the present invention cannot be achieved. If it is higher than 3.0 times, although high strength can be imparted, the hot water shrinkage in the vertical direction becomes larger than 2.0%, and the object of the present invention cannot be achieved. Further, the subsequent lateral stretchability is reduced, and the continuous productivity is deteriorated. Only when it is within the above range, the desired hot water shrinkage ratio can be obtained without impairing the strength and stretchability.

The longitudinally stretched polyamide film thus obtained is guided to a tenter type transverse stretching machine, gripped by clips, and preheated for stretching the film. Then, after that, the film is laterally stretched at an appropriate temperature in the range of 70 to 100 ° C., but in the present invention, it is necessary that the magnification conditions simultaneously satisfy the following expressions (2) and (3).

Lateral stretch ratio X: 3.4 ≦ Y ≦ 4.0 (2) Longitudinal stretch ratio ratio Y ≧ 1.3X (3) First, the transverse stretch ratio is set in the range of the above formula (3).
If it is lower than 3.4 times, it is difficult to obtain a film satisfying the above-mentioned hot water shrinkage ratio after leaving the tenter, and the oblique difference in the hot water shrinkage ratio cannot be improved even by re-heating at a later stage, which is not preferable. Conversely, if the ratio is higher than 4.0 times, the stretchability is lowered, and the film is easily broken, which is not preferable. It is preferably 3.6 to 4.0 times, and from the viewpoint of the balance of physical properties (strength, elongation, etc.) in each direction, stretchability, and reduction of the unstretched portion residual ratio at the end, the ratio is about 3.7 times. It is suitable.

In the present invention, the transverse stretching ratio is set within the range of the above formula (2), and the ratio of the longitudinal to transverse stretching ratio (ratio of the transverse stretching ratio to the longitudinal stretching ratio) satisfies the condition of the above formula (3). It is indispensable to keep this, but this is because the hot water shrinkage of the film after leaving the tenter is larger or smaller than the vertical hot water shrinkage (SMD) and the horizontal hot water shrinkage (STD). This is to make the relationship SMD <STD for the following reasons. That is, in general, the orientation and the heat shrinkage distribution take a shape inclined in the width direction particularly from the center to the edge of the film, but this phenomenon tends to be remarkable when the transverse stretching ratio is increased. For this reason, the difference in the hot water shrinkage ratio of the film coming out of the tenter becomes large. However, in the present invention, as will be described later, by re-relaxing mainly in the horizontal direction in the subsequent reheat treatment step, the final hot water shrinkage ratio in the vertical direction and the horizontal direction is balanced, and the Minimize the difference. If Y <1.3X, S
MD ≧ STD, and the hot water shrinkage in the lateral direction becomes too small in the heat treatment step at the subsequent stage, so that the difference in the hot water shrinkage obliquely increases.

Subsequently, in the tenter, a temperature of 200 ° C.
The heat treatment is performed at a temperature of 215 ° C, preferably around 210 ° C. Subsequently, relaxation heat treatment is performed at a temperature of 190 ° C. to 210 ° C. In the present invention, it is necessary to relax in the width direction under the condition of the following formula (4).

Relaxation ratio: Z 1.0 ≦ Z ≦ 4.0 (%) (4) This is the relationship between the hot water shrinkage (SMD) in the longitudinal direction and the hot water shrinkage (STD) in the perpendicular direction. As described above, this is because SMD <STD. When the relaxation rate is lower than 1.0%, the hot water shrinkage rate is in the relationship of SMD <STD, but the STD is higher than the above-mentioned 2.9%, In the latter stage, a high temperature must be set in the heat treatment step, which is not preferable because the flatness of the film is impaired. Conversely, when the relaxation rate is higher than 4.0%, SMD ≧ STD, and similarly to the above case, the oblique difference in the hot water shrinkage rate cannot be minimized even in the subsequent reheat treatment step. That is, only when the relaxation rate is set in the range of the above equation (4), it is possible to make the relations of STD ≦ 2.9 and SMD <STD. Note that this relaxation rate is calculated by the equation (4).
Is not particularly limited by the draw ratio within the range, but it is preferable to set a high draw ratio in the case of a high draw ratio in order to improve dimensional stability.

As described above, in the present invention, by setting the stretching ratio and the relaxation rate within the ranges of the above-described formulas, as described above, at the stage after the relaxation treatment and before the re-heat treatment, the SMD < It is necessary that STD and STD ≦ 2.9%. Further, in the present invention, at this stage, S
It is necessary that MD ≦ 2.0% and the difference in shrinkage ratio of hot water exceeds 1.8%. When the SMD is larger than 2.0%, the SMD hardly changes even by the re-heat treatment, and the re-heat treatment causes the smaller of the hot water shrinkage in the direction of 45 degrees and the hot water shrinkage in the direction of 135 degrees. When the hot water shrinkage ratio becomes further smaller, the shape of the heat yield distribution becomes a shape constricted in an oblique direction, which causes a problem that the hot water shrinkage ratio is not improved. If the difference in the hot water shrinkage ratio at this stage is 1.8% or less, for the same reason as described above,
The hot water shrinkage in the 45 ° direction and 1
There is a problem that the smaller one of the hot water shrinkage rates in the direction of 35 degrees becomes extremely small, and the oblique difference is not improved.

The stretched film thus obtained is
Once released from the clip, the remaining unstretched end is trimmed. After that, re-heat treatment is performed by continuously passing through the floating heat treatment apparatus. At this time, the processing conditions must be such that the reheating temperature: T160 ≦ T ≦ 200 (° C.) (5) The reheating time: θt1.0 ≦ θt ≦ 4.0 (s) (6) When the treatment temperature is lower than the range of the formula (5), the heat treatment effect is hardly obtained, and the difference in the shrinkage ratio of the hot water after the treatment is not improved. On the other hand, when the value is higher than the range of the expression (5), the film is excessively shrunk in the width direction to deteriorate the flatness, and it is not preferable because wrinkles at the time of winding cannot be eliminated. The processing time can be selected depending on the processing temperature within the range of the above formula (6), and when the processing temperature is high, it is short, and when the processing temperature is low, it is long. Preferably, re-heat treatment is performed at a temperature of 180 ° C. for 2 to 3 seconds.

In the present invention, it is necessary to immediately cool and wind the biaxially oriented polyamide film obtained under the above conditions. If not cooled, the film will be stretched in the longitudinal direction due to the tension and winding tension generated during film running, causing wrinkles and increasing the vertical hot water shrinkage of the obtained film. The purpose of the present invention cannot be achieved by rolling. Preferably the glass transition temperature (Tg)
The cooling is performed below, more preferably below room temperature. The cooling method for this is not particularly limited as long as it can cool the entire width of the film, and a known method such as a chill roll or a cool air blowing nozzle can be applied.

It is preferable that a device capable of cutting the tension be provided in addition to the film introduction / desorption portions before and after the arc-shaped floating type processing device. Examples of the device include a suction roll and an S-wrap roll.

[0027]

Next, the present invention will be described specifically with reference to examples. In addition, the measurement method used for evaluation of the following Examples and Comparative Examples, and the calculation method of the characteristic value are as follows.・ Hot water shrinkage ratio 100 mm with oil-based ink at a position 35% of the total width from the center to the left and right with respect to the entire width of the biaxially oriented polyamide film
A parallel line of the interval was marked, and this was slit to a width of 10 mm. The obtained sample was conditioned for 2 hours in an atmosphere at a temperature of 20 ° C. and a relative humidity of 65%, and the dimension between marks after the conditioning (hereinafter referred to as “dimension before processing”) was measured. This is 10
After boiling for 5 minutes in hot water of 0 ° C., and then humidifying again in an atmosphere of a temperature of 20 ° C. and a relative humidity of 65% for 2 hours, a dimension between marks (hereinafter, “dimension after processing”) is obtained. It was measured. Using these measured values, the hot water shrinkage was calculated by the following equation.

[0028]

(Equation 1)

Oblique difference in hot water shrinkage The absolute value of the difference between the hot water shrinkage in each direction of 45 ° and 135 ° with respect to the longitudinal direction of the film was defined as the hot water shrinkage oblique difference. Hot water shrinkage diagonal difference (%) = | Hot water shrinkage in the direction of 45 ° -Hot water shrinkage in the direction of 135 ° | Breaking strength A biaxially oriented polyamide film was prepared with a length of 150 mm and a width of 1
Sampling to 0mm, temperature 20 ℃, relative humidity 65%
Under the atmosphere for 2 hours. Then, using an autograph AG-100E manufactured by Shimadzu Corporation at a temperature of 20 ° C.
Under the condition of a relative humidity of 65%, the moisture-conditioned film is gripped at a distance between the chucks of 100 mm, and a pulling speed of 500 mm /
Then, it was broken by pulling in minutes, and the strength at that time was measured. -Lateral stretchability Continuous production was performed for 2 hours. At that time, those that hardly broke were evaluated as ○, and those that frequently broke and poor in continuous productivity were evaluated as x. Examples 1 to 6 Nylon 6 (A1030BRF manufactured by Unitika, melting point: 220 ° C.) having a relative viscosity of 3.0 (in 95% concentrated sulfuric acid at 25 ° C.)
Is melt-extruded in a sheet form from a T-die having a width of 630 mm at 260 ° C., and then closely cooled to a rotating drum at 15 ° C. by an air knife casting method, and rapidly cooled to a thickness of 150 μm.
As a result, an unstretched, substantially amorphous, unoriented polyamide film (hereinafter simply referred to as “unstretched film”) was obtained.

Then, the unstretched film was guided to a longitudinal stretching machine comprising a series of heating rollers having different peripheral speeds,
The film is stretched longitudinally under the various conditions shown in Table 1 at a temperature of
).

Subsequently, the longitudinally stretched film was guided to a tenter-type transverse stretching machine, gripped by clips, and preheated at 60 ° C. for film stretching.
The film was transversely stretched at various magnifications as shown in FIG. Thereafter, a constant width heat treatment is performed at 210 ° C. in the same tenter, and at a temperature of 200 ° C.
By relaxing under the conditions shown in Table 1, an intermediate-stage biaxially oriented polyamide film was obtained.

Subsequently, both ends of the biaxially oriented polyamide film in the intermediate stage are once released from the clips to trim the remaining unstretched end portions, and then subjected to a floating heat treatment apparatus at a temperature of 180 ° C. for 2.0 seconds. The sheet was reheated only for a short time, and immediately cooled by contact with a cooling roll adjusted to a surface temperature of 20 ° C. to obtain a final biaxially oriented polyamide film. Table 1 shows the characteristic values of the obtained film.
This is shown together with the reheat treatment manufacturing conditions.

[0033]

[Table 1]

Example 7 A biaxially oriented polyamide film was obtained in the same manner as in Example 1, except that the reheat treatment conditions were changed to 160 ° C. and 4.0 s. Example 8 A biaxially oriented polyamide film was obtained in the same manner as in Example 1, except that the conditions for the reheat treatment were set to 180 ° C. and 1.2 s. Example 9 A biaxially oriented polyamide film was obtained in the same manner as in Example 1, except that the reheat treatment was performed at 200 ° C. and 1.0 s.

In each of Examples 1 to 9, the longitudinal stretching ratio X was set to 2.7 ≦ X ≦ 3.0 as in the formula (1), and the lateral stretching ratio: Y was changed to Y as in the formula (2). 3.4 ≦ Y
≦ 4.0, the longitudinal / lateral stretching ratio is Y ≧ 1.3X as in the formula (3), and the relaxation ratio: Z is 1.0 ≦ Z ≦ 4.0 as in the formula (4). %), It was possible to make the hot water shrinkage ratio SMD <STD before the reheat treatment.

At the stage after the relaxation treatment and before the reheat treatment, the SMD is 2.0% or less, the STD is 2.9% or less, and the SMD is 2.0% or less.
<STD and hot water shrinkage diagonal difference of more than 1.8% do not cause extreme heat shrinkage after re-heat treatment, excellent thermal dimensional stability, and effective hot water shrinkage diagonal difference Was obtained.

After the reheat treatment, the SMD is 2.0% or less, the STD is 1.0% or less, and the difference in the hot water shrinkage is 1.
Since it was 8% or less, the thermal dimensional stability was good and the bag twist after bag making hardly occurred, so that it was practically used.

Further, the breaking strength in both the longitudinal direction and the perpendicular direction of the film was 25 kgf / mm ² or more, and sufficient strength in practical use was obtained. The film had good transverse stretchability, and hardly any breakage occurred even after continuous production for 2 hours.

That is, as is evident from Table 1, the dimensional stability was excellent and the difference in the hot water shrinkage ratio was minimized for the first time under the manufacturing conditions of the present invention without causing deterioration in operability and strength. A film was obtained. Comparative Examples 1 to 8 An unstretched film having a thickness of 150 μm was obtained in the same manner as in Example 1. Subsequently, stretching was performed under the condition that the stretching ratio did not satisfy the above formulas (1) to (3). Then, otherwise, in the same manner as in Example 1, a biaxially oriented polyamide film was obtained.

In Comparative Examples 1 and 2, although the diagonal difference in the hot water shrinkage ratio was small, the longitudinal stretching ratio was less than the range of the formula (1). Could not be obtained.

In Comparative Examples 3 to 6, the longitudinal / lateral stretching ratio does not satisfy the condition of the formula (3), and the hot water shrinkage ratio before re-heat treatment is SMD> STD. The hot water shrinkage diagonal difference of the film could not be minimized. Further, under the conditions of Comparative Example 5, the longitudinal stretching ratio was high, and frequent cutting occurred during transverse stretching.

In Comparative Examples 7 and 8, since the transverse stretching ratio was beyond the range of the formula (2), only those having a large transverse hot water shrinkage ratio STD after re-heat treatment were obtained, and the thermal dimensional stability was high. Sex was bad. Also often breaks during transverse stretching,
Continuous productivity was poor. Comparative Example 9 A biaxially oriented polyamide film was obtained in the same manner as in Example 1 except that the relaxation rate was 6.0%. However, since the hot water shrinkage ratios before the reheat treatment were in a relationship of SMD> STD, as in Comparative Examples 3 to 6, only those having a large difference in the hot water shrinkage ratio were obtained. Comparative Example 10 Example 1 was repeated except that the reheat treatment was performed at 150 ° C. for 10 seconds.
In the same manner as in the above, a biaxially oriented polyamide film was obtained. However, under these conditions, the temperature of the reheat treatment is too low,
Further, since the time was too long, there was no change in the oblique difference in the hot water shrinkage ratio before and after the reheat treatment, and the reheat treatment effect was not exhibited. Comparative Example 11 A biaxially oriented polyamide film was obtained in the same manner as in Example 1, except that the reheating condition was set to 210 ° C. and 2.0 s.
However, under these conditions, since the temperature of the reheat treatment was too high, the film was excessively shrunk in the lateral direction, and only those containing wrinkles were obtained. The oblique difference in the hot water shrinkage was also large. Comparative Example 12 A biaxially oriented polyamide film was obtained in the same manner as in Example 1, except that the reheat treatment time was 0.5 s. However, under these conditions, the reheating treatment time was too short to produce a treatment effect, and the hot water shrinkage ratio did not change before and after the reheating treatment. Comparative Example 13 A biaxially oriented polyamide film was obtained in the same manner as in Example 1 except that the relaxation rate was set to 0%. However, under these conditions, the transverse hot water shrinkage ratio STD before the reheat treatment becomes too large, and the transverse hot water shrinkage ratio STD even after the reheat treatment.
Was large, and the difference in hot water shrinkage ratio was large, and the object of the present invention could not be achieved. Comparative Example 14 A biaxially oriented polyamide film was obtained in the same manner as in Example 1, except that cooling was not performed after the reheat treatment by the floating heat treatment apparatus. However, the film was stretched in the vertical direction due to the tension and the winding tension generated during the running of the film, and as a result, the hot water shrinkage SMD in the vertical direction was increased, and the difference in the hot water shrinkage was also increased. In addition, wrinkles occurred in the vertical direction, and only poor appearance was obtained.

[0043]

According to the present invention, it is possible to produce a film having excellent dimensional stability and minimizing the difference in oblique hot water shrinkage without breaking, and a biaxially oriented polyamide by a sequential biaxial stretching method. It is extremely effective as a method for producing a film. In addition, since the obtained film has uniform physical properties in the width direction, the range of use of the biaxially oriented polyamide film, which has been limited so far, can be expanded.

Claims (3)

[Claims] 1. A substantially amorphous, non-oriented, unstretched film is longitudinally stretched, and then horizontally stretched by a tenter-type transverse stretching machine to produce a sequentially biaxially stretched polyamide film.
Stretching is performed under the conditions of the following formulas (1) to (3), then heat treatment is performed, and then relaxation treatment is performed under the condition of the following formula (4). After releasing from the clip, it is reheated by the floating heat treatment device under the conditions of the following formulas (5) and (6) to relax in the lateral direction.
Next, a method for producing a polyamide film, comprising immediately cooling and winding. Longitudinal stretching ratio: X 2.7 ≦ X ≦ 3.0 (1) Lateral stretching ratio: Y 3.4 ≦ Y ≦ 4.0 (2) Ratio of longitudinal to transverse stretching ratio Y ≧ 1.3X (3) Relaxation ratio: Z 1.0 ≦ Z ≦ 4.0 (%) (4) Reheat treatment temperature: T 160 ≦ T ≦ 200 (° C.) (5) Reheat treatment time: θt 1.0 ≦ θt ≦ 4.0 (s) (6) ) 2. After the relaxation treatment and before the re-heat treatment, the film has a hot water shrinkage of 2.0% or less in a longitudinal direction and a hot water shrinkage in a direction perpendicular to the longitudinal direction. 2.9% or less, (Hot water shrinkage in the vertical direction) <(Hot water shrinkage in the direction perpendicular to the vertical direction), and heat in directions of 45 degrees and 135 degrees with respect to the vertical direction of the film 2. The method for producing a polyamide film according to claim 1, wherein the difference in water shrinkage exceeds 1.8%. 3. The polyamide film obtained by the production method according to claim 1, wherein the hot water shrinkage in the longitudinal direction of the film is 2.0% or less, and the hot water shrinkage in a direction perpendicular to the longitudinal direction is less than 2.0%. 1.0% or less, the difference in hot water shrinkage ratio between the direction of 45 degrees and the direction of 135 degrees with respect to the longitudinal direction of the film is 1.8% or less,
A biaxially oriented polyamide film characterized in that the breaking strength in both the longitudinal direction and the perpendicular direction of the film is 25 kgf / mm ² or more.