JP2000190387A - Manufacture of biaxially oriented polyamide film - Google Patents

Abstract

PROBLEM TO BE SOLVED: To efficiently produce films having superior characteristics of a bowing property, transparency or the like by separating a tubular biaxially oriented film into a specific sheets, and thereafter passing the film through a plurality of heat rolls step by step, then cooled by cooling rolls to be superimposed and heat-treated by a tenter. SOLUTION: A flat folded oriented film 9 is produced through tubular biaxial orientation. This film is cut in the folded part by a trimming device to be separated into two sheets. In the next place, the films are introduced in respective heat roller equipments 10 having the same structure, wherein these are provided with at least two steps or more heating rolls and cooling rolls immediately thereafter. In the equipment, after heat treatment is carried out in a state where the ends of the oriented film 9 are held between an endless belt and a heating roller, the film is cooled. This film is then superimposed again on another oriented film 9 having been heat-treated by the same manner, and in a tenter type hot air furnace 11, it is heat treated by hot air in a state where the ends are held by clips.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a process for producing a biaxially stretched polyamide film by a tubular method which is excellent in dimensional stability, transparency and processability under heating and excellent in dimensional stability of a bag-formed product. It is about the method.

[0002]

2. Description of the Related Art A biaxially stretched polyamide film represented by a nylon 6 film has excellent properties and is widely used as various packaging materials for foods, pharmaceuticals, sundries and the like. In particular, because of its excellent properties such as impact resistance, cold resistance, heat resistance, and pinhole resistance, bags made using a laminated film bonded with a sealant material such as polyethylene are used for freezing, refrigeration and retort foods. It is widely used for packaging materials. In such applications, in addition to mechanical properties and transparency, it is necessary to have excellent processing steps such as lamination and dimensional stability of the film under heating. Is small,
In addition, it is required that the heat shrinkage rate in the film plane is uniform.

[0003] As a method of stretching a polyamide film,
There is known a method of stretching an unstretched film obtained by quenching and solidifying a molten polyamide sheet by a flat simultaneous biaxial stretching method, a flat successive biaxial stretching method, a tubular simultaneous biaxial stretching method, or the like. . Since the film immediately after being stretched has a strong shrinkage, a problem occurs in a processing step such as printing or laminating as it is, and it is general to fix the stretched orientation structure by heat-treating the stretched film. is there. Therefore, by fixing the dimensions of the film and performing heat treatment at a temperature close to the melting point under tension, it is possible to obtain a film with excellent characteristics in which the dimensional change due to shrinkage is small and the molecular orientation structure is maintained by stretching. it can. A tenter-type hot blast stove is generally used in such a film heat treatment step.

In general, even when a biaxially stretched polyamide film is produced by a tubular method, a tenter-type hot blast stove is used in the heat treatment step. However, two major problems have been pointed out in the heat treatment step using a tenter-type hot blast stove in the tubular method. One of them is film blocking in the tenter heat treatment step. That is, since the biaxially stretched polyamide film has a relatively large hot water shrinkage, it is necessary to perform a heat treatment at a high temperature near the melting point to obtain a film with a small hot water shrinkage. On the other hand, in the stretching by the tubular method, after stretching, the tubular film is folded flat, and is transported to the heat treatment step as a flat film. When the stretched film in such a state is heat-treated by a tenter, fusion and blocking occur between the two folded films. For this reason, it has been pointed out that it is difficult to separate the two films after the heat treatment, and the film cannot be used as a product.

Another problem is the bowing deformation of the film in the tenter heat treatment step. Normal,
In the heat treatment step, when a film having a large shrinkage stress such as polyamide is heat-treated by a tenter, the center of the film is pulled in the direction opposite to the running direction due to the distribution of the film tension generated by the stress relaxation in the tenter, and the end of the film is pulled. There is a phenomenon that the running is delayed more than the part. When a straight line in the TD direction (film width direction) is drawn on the film before the heat treatment, a phenomenon is observed in the film after the heat treatment that a concave portion is concave in the center with respect to the advancing direction. Such a deformation is called a Boeing deformation. In general, films with large bowing deformation often have quality problems. For example, when a film having a large bowing deformation is formed into a bag, there is no problem with the film at the center in the TD direction, but when a film close to the end is used, the so-called S-curl of the bag is likely to be twisted. If the S-shaped curl is large, troubles are likely to occur in, for example, an automatic filling and packaging process or packing.

[0006] Such torsion of the bag is mainly related to the distribution of shrinkage of hot water in the plane of the film. That is, when the film is folded and bag-formed, the difference tends to be large when the difference in the film shrinkage behavior between the front and back surfaces is large. In general, the larger the bowing deformation of a film, the greater the anisotropy of the distribution of the hot water shrinkage in the film plane at the edge of the film, and the maximum and minimum directions of the hot water shrinkage are MD (length direction) or TD Greatly deviated from The greater the anisotropy of the hot water shrinkage ratio, the closer the inclination of the maximum and minimum directions from the MD or TD direction to 45 ° becomes, the greater the difference in shrinkage behavior between the front and back of the film when folded in bag making. , S-curl is likely to occur. As an evaluation index of the S-curl property based on such a difference in shrinkage behavior, MD
Alternatively, if the difference in the hot water shrinkage (%) in boiling water at 100 ° C. at angles of + 45 ° and + 135 ° from the TD direction is defined as the hot water shrinkage oblique difference Sd, Sd is 1.
If it is 0% or less, there is a tendency that the S-shaped curl hardly occurs. In particular, since the polyamide film has a relatively high shrinkage ratio with hot water, an S-shaped curl often causes a problem when the bag-made product is subjected to a retort treatment. This means that in the case of a polyamide film having a large bowing deformation, a film usable for a bag-made product is limited to a portion near the center in the width direction of the film, which is a major problem in industrial production.

Furthermore, if the bowing deformation is large, the film properties other than the heat shrinkage, for example, the quality such as the elastic modulus, elongation at break, and the strength at break at the edge and center of the film after heat treatment are different in the plane of the film. Problems arise. This is because the oriented structure that was uniform in the width direction of the film at the end of stretching is distorted toward the edge of the film due to the bowing deformation generated in the heat treatment process. When such a film is processed by applying a tension in the MD direction, for example, it often causes running failure and causes wrinkles. In addition, it often causes poor flatness of the film.

[0008] In order to solve the problem of film blocking, after stretching in a tube shape, the two-layer film folded flat is cut off at both ends and separated into two films, and then an endless rope-like material is formed. A method has been proposed in which a film is introduced into a tenter while maintaining a gap between two films and heat treatment is performed (Japanese Patent Publication No. 46-15439). In addition, after the film stretched in a tubular shape was folded flat, a preliminary low-temperature tenter heat treatment was performed at a relatively low temperature in a state where the two layers of film were overlapped, and the end of the film was cut off to separate into two pieces. After that, a method has been proposed in which two films are superimposed again with air interposed between the two films and heat-treated at a temperature near the melting point by a tenter (Japanese Patent Publication No. 5-35670). However, although the blocking of the two films can be prevented by any of the above methods, there is a problem that a strong thermal stress is generated at the time of the tenter heat treatment and a large bowing deformation occurs.

On the other hand, regarding the problem of bowing deformation,
There is known a method of substantially preventing bowing deformation by a tubular heat treatment method in which a film stretched into a tube shape is folded flat and then heat treatment is performed while forming a tube again. However, in this method, the internal pressure of the tube is extremely reduced during the heat treatment at a high temperature near the melting point, and the fluctuation of the tube diameter and the oscillation of the tube are likely to occur, which adversely affects the quality of the film and the stability of operation, Conversely, if heat treatment is performed at a relatively low temperature in order to minimize the effect on operability, sufficient heat treatment effect cannot be obtained, and dimensional stability under heating and high humidity of the film will decrease. There was a problem.

Further, after the film folded into a flat shape after stretching is once separated into two sheets, the film is held at a temperature range of 10 to 40 ° C. lower than the melting point of the film in a state where both end portions of the film are pressed against a roll surface by a belt-like material. There has been proposed a method of improving the bowing deformation by performing heat treatment by bringing the object into close contact with a heated object (Japanese Patent Application Laid-Open No. 2-10312).
No. 2). However, when the above method is used, although there is an effect of improving the bowing deformation, it is difficult to obtain a film having a small heat shrinkage, and heat treatment is performed at a higher temperature to improve the heat shrinkage. In such a case, a problem that the film is fused to the heated object occurs. Further, in the above-mentioned method, since the film is rapidly processed at a high temperature in the heat treatment step, there is a problem that the crystallization of the film proceeds and the transparency (haze) deteriorates. In addition, after stretching, the first
After performing the heat treatment of the stage at a relatively low temperature, the film is folded flat again, the both ends of the film are cut out, separated into two films, and then two layers are formed with air interposed between the two films. By superimposing and performing a heat treatment at a temperature near the melting point in the tenter, the crystallization of the stretched film proceeds to some extent in the first stage heat treatment, preventing the film from blocking in the second stage heat treatment process, and preventing bowing deformation. A method for improvement has been proposed (Japanese Patent Publication No. 5-85341).
No.). However, in the above method, when the heat treatment temperature by the first stage tubular method is increased in order to reduce the bowing deformation due to the second stage tenter heat treatment, the tube diameter fluctuates and stable operation becomes difficult. However, if the operability is prioritized by lowering the heat treatment temperature in the first stage, even if the tenter heat treatment in the second stage is performed, the effect of improving the bowing deformation becomes insufficient, and the quality of the film varies.

[0011]

As a result of detailed studies on the above-mentioned prior art, the present inventors have found that there is no problem in the manufacturing steps such as film fusion and the like in any of the methods. It was confirmed that there was no S-curl after retorting the bag produced using the resulting film, and that there was still room for improvement in order to produce a film excellent in bowing deformation.

[0012]

Means for Solving the Problems The present inventors have studied a method for solving the above problems with respect to a method for producing a biaxially stretched polyamide film by a tubular method. After separation into two pieces,
It has been found that a film having excellent properties such as bowing property and transparency can be industrially and efficiently produced by performing a stepwise heat treatment with a plurality of heat roll equipment having a specific structure.

That is, the gist of the present invention is as follows. A method for producing a tubular biaxially stretched polyamide film, which is produced by the following steps (1) to (5). (1) After tubular biaxial stretching, immediately after the stretched tubular film is folded into a plane, both ends of the film are cut and separated into two pieces. (2) A step of heat-treating each film separately by stepwise passing through two or more heat rolls having different surface temperatures in a state where both end portions of the two separated films are fixed on a heat roll by an endless belt. . (3) A step of cooling the film by a cooling roll after the heat treatment by the hot roll. (4) A step of superposing two cooled films. (5) A step of performing a heat treatment with a tenter while gripping both ends of the two superposed films.

The polyamide used in the present invention is:
A thermoplastic polymer having crystallinity, such as polycaproamide (nylon 6), polytetramethylene adipamide (nylon 46), polyhexamethylene adipamide (nylon 66), polyhexamethylene sebacamide ( Nylon 610), polyhexamethylene dodecamide (nylon 612), polyundecaneamide (nylon 11), polyundecamethylene adipamide (nylon 611), polydodecaneamide (nylon 12), polymethaxylylene polyamide (MXD6 nylon) And a homopolymer, a copolymer, or a mixture thereof of a polyamide containing, as a main component.

Next, a method for producing a polyamide film using the method of the present invention will be described with reference to FIG. In FIG. 1, a polyamide resin melt-extruded from an extruder 1 is extruded into a tube from a die 2. As a method of cooling and solidifying the molten tube as an unstretched film, there are an air cooling method, a method of bringing the molten tube into contact with a metal surface, a water cooling method, and the like, but a water cooling method is preferable. The outer surface and / or the inner surface of the molten tube is cooled by cold water by a water-cooled sizing bath 3 to obtain an unstretched film 4. After the unstretched film 4 is guided to the stretching tower 6, the unstretched film 4 is heated to a predetermined temperature in the stretching tower 6, and due to the peripheral speed difference between the pinch rolls 5 and 8 in the MD direction, and inside the tube in the TD direction. Each is stretched to a predetermined magnification by air pressure. The stretching ratio of the film is 2 times
It is desirable that the film be stretched 5 times, preferably 2.5 times to 4 times.

Next, a stretched film 9 is obtained by flattening the sheet through a guide plate 7 with a pinch roll 8. Further, the crease portion of the stretched film is cut off by the trimming device 12 before being introduced into the hot roll equipment 10 in FIG. 2, and the stretched film 9 is separated into two pieces (FIG. 2 shows a two-stage heating roll. FIG. 3 is a schematic view of a hot roll facility when having a hot roll).

Each of the two separated films is introduced into a separate hot roll installation 10 having the same structure.
The hot roll facility 10 has at least two or more heating rolls and a cooling roll immediately after the heating roll. In FIG. 2, one of the stretched films 9 separated by the trimming device 12 is introduced into the hot roll equipment 10 in a state in which the stretched film 9 is in close contact with the first-stage heating roll 14 by pressing the pinch roll 13. Without the pinch roll 13, contact with the stretched film 9 at the entrance of the heating roll 14 becomes nonuniform, so that the transparency of the stretched film 9 becomes uneven, and wrinkles and ripples occur due to shrinkage of the film. . As the pinch roll 13, for example, a rubber roll having a spiral air vent groove is preferably used to prevent accumulation of air between the heating roll 14 and the film.

In FIG. 2, the stretched film 9 introduced into the heating roll 14 is heat-treated while the end of the film is held between the endless belt 19 and the heating roll 14, as shown in FIG. You. FIG. 3 shows a cross-sectional shape of the heating roll 14 used in the present invention. The heating roll 14 has a stepped portion A at the end of the roll, and the end of the stretched film 9 has a stepped portion. At the falling part A, the endless belt 19 is gripped between the endless belt 19 and the surface of the heating roll 14 by the tension of the endless belt 19. By gripping the end of the film with the endless belt 19, shrinkage deformation of the film during heat treatment is prevented. Since the end of the stretched film 9 is sandwiched between the endless belt 19 and the step A of the heating roll 14, the gripping force of the film end is greatly increased. When a roll having no such step-down portion is used, the gripping force of the film is weak, for example, the belt surface is deteriorated after a long-term operation, and the grip of the film edge is unstable due to a decrease in frictional force. Becomes The height of the stepped portion A is preferably about 0.5 to 10 mm. If the height of the step is smaller than 0.5 mm, the effect of gripping the film is not sufficient, and if it is larger than 10 mm, the shape of the film at the stepped portion is undesirably damaged.

In the present invention, in the step of heat-treating the two separated stretched films 9 stepwise by the hot roll equipment 10, the first heat treatment temperature T1 (° C.) by the heat roll 14 satisfies the following equation. It is preferable to carry out under the following conditions. Tm-100 ≦ T1 ≦ Tm-60 where Tm indicates the melting point of the polyamide.

If the heat treatment temperature T1 is higher than (Tm−60) ° C., the transparency of the stretched film 9 is lowered, and the transparency tends to be uneven. If T1 is lower than (Tm-100) ° C., the effect of the heat treatment will be insufficient. The contact time of the heating roll 14 is not particularly limited, but is preferably 0.5 seconds or more. In FIG. 2, a pinch roll 15 is provided in a section from the heating roll 14 to the second-stage heating roll 16.

When there is no pressing by the pinch roll 15, the contact between the film and the heating roll 16 at the entrance of the heating roll 16 becomes uneven, causing unevenness in transparency, wrinkles and waves due to shrinkage of the film. House is easy to occur. The stretched film 9 introduced into the heating roll 16 is subjected to a heat treatment on the surface of the heating roll 16 in a state where the end thereof is gripped by the endless belt 19 and the step-down portion of the heating roll 16. As the heating roll 16, a roll having a stepped portion at an end is used like the heating roll 14.

The heat treatment temperature T2 (° C.) of the second stage by the heating roll 16 is preferably performed under the condition satisfying the following expression. Tm-60 ≦ T2 ≦ Tm-20 where Tm indicates the melting point of the polyamide. When the treatment temperature is higher than (Tm-20) ° C, the roll and the film are likely to adhere to each other, and when the treatment temperature is lower than (Tm-60) ° C, the heat treatment effect cannot be sufficiently obtained.

The heat treatment by the heating roll in the present invention needs to be performed using at least two or more heating rolls. At this time, it is preferable that the temperature Tn (° C.) of the heating roll at the n-th stage and the temperature T (n + 1) (° C.) of the heating roll at the (n + 1) -th stage are set under the conditions satisfying the following expressions. Tm-60 ≦ Tn ≦ Tm−20 Tn ≦ (Tn + 1) where Tm indicates the melting point of the polyamide.

In FIG. 2, the stretched film 9 that has been subjected to the final roll heat treatment by the heating roll 16 is immediately cooled by the cooling roll 18 to a temperature lower than the glass transition temperature of the polyamide. In the cooling roll 18,
Since almost no shrinkage of the film occurs, it is not necessary to restrain the end portion by the endless belt 19. However, in the section until the stretched film 9 and the endless belt 19 are separated, in order to prevent the endless belt 19 from meandering, the cooling roll 18 also uses a roll having a step-down structure similar to the heating roll 14. Is preferred.

The film cooled by the cooling roll 18 is again superimposed on the other film which has been subjected to the same hot roll treatment, and the two superimposed films are separated into two layers while keeping air between the layers. At the same time, as shown in FIG. 1, it is guided to a tenter-type hot blast stove 11 and is heat-treated with hot air while its ends are held by clips.

The heat treatment temperature (° C.) of the stretched film 9 in the tenter type hot blast stove 11 is preferably carried out under the following condition. Tm-20 ≦ Tf ≦ Tm-5 where Tm indicates the melting point of the polyamide. If the heat treatment temperature is lower than (Tm-20) ° C., the heat treatment effect becomes insufficient, and if it is higher than (Tm-5) ° C., the film is undesirably whitened or cut by melting.

After the heat treatment in the tenter type hot blast stove 11 is completed, the two films are separated by a roll or the like. Each of the separated films is wound on a separate bobbin after trimming the ends.

[0028]

According to the present invention, the film is heat-treated stepwise by two or more heat rolls, thereby suppressing abrupt and non-uniform crystallization of the stretched film in the roll heat treatment step, and improving the transparency and uniformity of the film. Can be improved. The mechanism is not clear, but is speculated as follows. That is, the crystallization of the film immediately after stretching proceeds very rapidly due to its oriented structure. When the film in such a state is rapidly and rapidly heated to a high temperature in one step by a heat source having a high transfer efficiency such as a hot roll, crystallization of the film excessively proceeds,
Since the crystallization is too fast, a slight difference in the thermal history, for example, a slight difference in the contact pressure to the hot roll tends to cause unevenness in the crystallization. As a result, it is considered that the transparency of the film deteriorates and the transparency tends to be uneven. Therefore, when the stretched film is heat-treated with a roll, heat treatment is performed stepwise in a predetermined temperature range, thereby suppressing excessive and rapid crystallization, and suppressing unevenness of crystallization due to a slight difference in heat history. It is thought that it can be prevented.

[0029]

Next, the present invention will be described in detail with reference to examples. In addition, the evaluation method of each characteristic of a film is as follows. 1) 100 ° C. hot water shrinkage ratio S The film was cut into strips having a length of 150 mm and a width of 10 mm, and marked lines were drawn at intervals of about 100 mm in the length direction. 2
After leaving for 1 day in an atmosphere of 0 ° C. and 65% RH, the length (L1) between the marked lines was measured. Next, the film is heated to 100 ° C.
In boiling water for 10 minutes. After wiping the surface moisture, leave it in an atmosphere of 20 ° C. and 65% RH for 1 day,
The length (L2) between the marked lines was measured. S (%) = [(L1−L2) / L1] × 100 as hot water shrinkage at 100 ° C.
I asked. 2) Maximum hot water shrinkage Smax in the film plane Central part (a) in the TD direction of the film, 75 m from the end
m The film at each position is centered on a portion (b) near the center, and an intermediate portion (c) between (a) and (b) at intervals of 5 °,
The above S was measured in the range of ゜ 175 °, and the hot water shrinkage at 100 ° C. in each direction within the film plane was measured (0 ° in the right hand direction with respect to the direction of movement of the film in the TD direction, and the + direction counterclockwise). And). From these measurement results, (a) to
The maximum value Smax of S in each part of (c) was obtained. 3) Oblique Hot Water Shrinkage Difference Sd At the film position (c), the absolute value of the difference between the hot water shrinkage rates in the 45 ° and 135 ° directions was determined as the hot water shrinkage oblique difference Sd. 4) Haze According to ASTM D1003, the haze of the central portion (a) of the film was measured to evaluate the transparency. The apparatus used was a haze meter TC-H.DPK (manufactured by Tokyo Denshoku Co., Ltd.). 5) Transparency spots The transparency spots were evaluated by observing the appearance. The evaluation criteria were as follows: those with no spots visually, を those with almost no spots, Δ with some spots,
Those with marked spots were marked as x. 6) S-curl of bag-made product by hot water The end of 400 mm of the laminated film obtained by dry lamination with LLDPE (sealant) using a urethane-based adhesive is slit, half-folded, and then three-side sealed. Length 250mm in MD direction, 200 width in TD direction
mm three-sided seal bags were prepared. Next, after filling the obtained bag with 150 cc of water, the opening was heat-sealed to prepare a packaging bag. Put this packaging bag in boiling water at 100 ° C.
After immersion for 0 minutes, the S-curl property of the packaging bag was evaluated. The evaluation was evaluated as ◎ when there was no twist by visual inspection, ○ when there was almost no twist, Δ when there was twist, and X when there was significant twist.

Example 1 Nylon 6 resin (melting point: 22) in which 800 ppm of amorphous silica particles having an average median diameter of 2 μm were dispersed as a slip agent at 800 ppm.
After melt-extruding 0 ° C., 98% sulfuric acid, a temperature of 25 ° C. and a relative viscosity of 3.5 g at a concentration of 1 g / dl from a circular die, the mixture was cooled and solidified in a water-cooled sizing bath at 15 ° C., and had a folding diameter of 450 mm and a thickness of 140 μm Was obtained. The unstretched film was introduced into a stretching tower, and the film was heated and stretched using hot air and far infrared rays together. The stretching ratio was 3 times in the MD direction due to the difference between the pinch roll peripheral speeds at the top and bottom of the tower, and 3.2 times in the TD direction due to expansion of the tube by air pressure. Next, the stretched film was introduced into a guide plate and folded flat to obtain a stretched film having a width of 1440 mm. Next, the fold portion at the end of the stretched film folded flat was cut open to separate it into two films, and heat-treated with a hot roll facility. Heat roll equipment having the same structure as that of FIG. 2 was used. As the heating roll and the cooling roll, those having a step-down structure at both ends of the roll as shown in FIG. 3 were used. The temperatures of the heating rolls 14 and 16 and the cooling roll 18 were 120 ° C., 160 ° C. and 15 ° C., respectively. In the roll heat treatment and roll cooling sections, the film was sandwiched between the endless belt 19 and the stepped portion at the end of the roll, and the endless belt 19 was conveyed while applying a tension of about 5 kg to restrain the end of the film. As the endless belt 19, a fiber reinforced belt having a width of 40 mm and a thickness of 2 mm was used. No shrinkage of the film or unevenness of transparency in the roll heat treatment step was observed. After the rolled heat-treated two stretched films were overlapped by a guide roll, the film was introduced into a tenter while holding both ends of the film with clips, and subjected to four stages of heat treatment at 210 ° C. for 5 seconds each. In the third stage heat treatment, a 5% relaxation treatment was performed in the width direction. After the tenter heat treatment, the film was separated into two sheets again, and about 30 mm of both ends of the film (the portion gripped by the clip) was trimmed. Finally, two stretched films having a width of 1300 mm were wound around a bobbin. Film was continuously produced for 48 hours from the start of operation, but the operation was stable and no problems were observed. Table 1 shows the results of evaluating various physical properties of the film thus obtained.

Examples 2 to 7 and Comparative Examples 1 to 5 Stretched films were produced in the same manner as in Example 1 except that the temperatures of the heating rolls 14 and 16 were changed. Table 1 shows the results of evaluating various physical properties of the obtained film. In Comparative Example 1, remarkable unevenness was observed in the transparency of the film in the second-stage heating roll 16. Also, the film shrinks slightly at the entrance of the second stage heating roll 16,
Intermittent wrinkles occurred. In Comparative Example 2, although the improvement was obtained as compared with Comparative Example 1, spots were generated in the transparency of the film by the second-stage heating roll 16 as in Comparative Example 1. In Comparative Example 3, the film and the roll tended to be fused to each other in the second-stage heating roll 16, and peeling from the roll was difficult. In Comparative Example 4, the first-stage heating roll 1
In No. 4, remarkable unevenness occurred in the transparency of the film. Also, the film shrinks at the entrance of the heating roll 14,
Wrinkles were easy to occur.

[0032]

[Table 1]

[0033]

According to the heat treatment method of the tubular biaxially stretched film of the present invention, the roll heat treatment is performed in a state where the film edge is restrained, and then the heat treatment is performed by a tenter-type hot blast stove to obtain a heat treatment in the tenter. Blocking and bowing deformation of the film are suppressed, and heat treatment at a high temperature near the melting point can be performed. As a result, hot water shrinkage is less than 2% in all directions in the plane of the film over the entire width of the film, and the hot water shrinkage diagonal difference Sd is 1.0% or less over the entire width of the film. Film can be produced stably. Also,
A bag-made product produced using the obtained stretched film has excellent S-curlability, including its retorted product. Also, in the roll heat treatment step, the gripping force of the film is increased by constraining the film edge between the stepped portion of the hot roll and the endless belt, and the film can be prevented from shrinking. The end is stably gripped and stable production is possible.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram of an example of a process for producing a biaxially stretched polyamide film by a tubular method of the present invention.

FIG. 2 In the present invention, the tubular stretched film is
It is explanatory drawing of an example of the process which cuts into sheets and heat-processes by a hot roll facility.

FIG. 3 is an explanatory view of an example of a heat roll having stepped shapes at both ends in the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Extruder 2 Die 3 Water-cooled sizing bath 4 Unstretched film 5 Pinch roll 6 Stretching tower 7 Guide plate 8 Pinch roll 9 Stretched film 10 Heat roll equipment 11 Tenter 12 Trimming device 13 Pinch roll 14 Heat roll 15 Pinch rubber roll 16 Heat roll 17 Pinch rubber roll 18 Cooling roll 19 Endless belt

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 4F201 AA29 AE01 AG01 AR06 BA07 BC01 BC03 BC13 BC15 BD10 BR14 BR15 4F210 AA29 AE01 AG01 AR06 QA05 QC07 QG02 QG18 QW14 QW15 QW21

Claims (4)

[Claims] 1. A method for producing a tubular biaxially stretched polyamide film, which is produced by the following steps (1) to (5). (1) After tubular biaxial stretching, immediately after the stretched tubular film is folded into a plane, both ends of the film are cut and separated into two pieces. (2) A step of heat-treating each film separately by stepwise passing through two or more heat rolls having different surface temperatures in a state where both end portions of the two separated films are fixed on a heat roll by an endless belt. . (3) A step of cooling the film by a cooling roll after the heat treatment by the hot roll. (4) A step of superposing two cooled films. (5) A step of performing a heat treatment with a tenter while gripping both ends of the two superposed films. 2. The step of heat-treating two separated films stepwise by a hot roll, wherein the temperature T1 (° C.) of the first hot roll is represented by the following formula (A), and the temperature of the n-th hot roll: The method for producing a biaxially stretched polyamide film according to claim 1, wherein Tn (° C) is represented by the following formulas (b) and (c). Tm-100 ≦ T1 ≦ Tm-60 (b) Tm−60 ≦ Tn ≦ Tm-20 (b) Tn ≦ T (n + 1) (c) where Tm indicates the melting point of the polyamide. 3. The method for producing a biaxially stretched polyamide film according to claim 1, wherein a heat treatment temperature Tf (° C.) of the tenter is represented by the following formula (d). Tm-20 ≦ Tf ≦ Tm-5 (d) where Tm indicates the melting point of the polyamide. 4. In a heat treatment step using a hot roll, each of the hot rolls and the subsequent cooling roll are provided with a pinch roll for bringing the film into close contact with the roll surface,
In addition, the heat roll and the cooling roll have a stepped shape at a portion where both ears of the film are in contact with each other, and an endless belt for fixing both ears of the film by pressing is arranged at the stepped part. The method for producing a biaxially stretched polyamide film according to any one of claims 1 to 3, wherein