JP4097823B2 - Method for producing biaxially stretched polyamide film - Google Patents

Description

【００３３】
【発明の効果】
本発明におけるチューブラー法二軸延伸フィルムの熱処理法において、フィルム端部を拘束した状態でロール熱処理を行った後、テンター式熱風炉により熱処理を行うことにより、テンター内でのフィルムのブロッキングやボーイング変形が抑制され、融点近傍の高温での熱処理を行うことが可能となる。
この結果、フィルムの全幅にわたって、フィルム面内のすべての方向で熱水収縮率が２％未満で、フィルムの全幅にわたって熱水収縮率斜め差Ｓｄが１．０％以下の透明性に優れた延伸フィルムを安定して生産することができる。
また、得られた延伸フィルムを用いて作製した製袋品はそのレトルト処理品も含めてＳ字カール性の優れた性能を有するものである。
また、ロール熱処理工程において、熱ロールの段落ち部分とエンドレスベルト間にフィルム端部を拘束することによりフィルムの把持力が増大し、フィルムの収縮を防ぐことができ、長時間の生産においてもフィルム端部が安定的に把持され安定生産が可能となる。
【図面の簡単な説明】
【図１】本発明のチューブラー法による二軸延伸ポリアミドフィルムを製造する工程の一例の説明図である。
【図２】本発明において、チューブ状延伸フィルムを２枚に切開した後、熱ロール設備により熱処理する工程の一例の説明図である。
【図３】本発明における両端部に段落ち形状を有する熱ロールの一例の説明図である。
【符号の説明】
１ 押出機
２ ダイ
３ 水冷サイジングバス
４ 未延伸フィルム
５ ピンチロール
６ 延伸塔
７ 案内板
８ ピンチロール
９ 延伸フィルム
１０ 熱ロール設備
１１ テンター
１２ トリミング装置
１３ ピンチロール
１４ 加熱ロール
１５ ピンチゴムロール
１６ 加熱ロール
１７ ピンチゴムロール
１８ 冷却ロール
１９ エンドレスベルト[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method 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-made product.
[0002]
[Prior art]
Biaxially stretched polyamide film represented by nylon 6 film has excellent characteristics and is widely used as various packaging materials mainly for foods, pharmaceuticals, miscellaneous goods and the like. In particular, because it has excellent performance 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 frozen, refrigerated, retort foods, etc. It is actively used for packaging materials.
In such applications, in addition to mechanical properties and transparency, it is necessary to have excellent dimensional stability of the film under processing steps such as bonding and heating. Is small and the heat shrinkage rate in the film plane is required to be uniform.
[0003]
As a method for stretching a polyamide film, an unstretched film obtained by quenching and solidifying a melted polyamide sheet is obtained by a flat simultaneous biaxial stretching method, a flat sequential biaxial stretching method, a tubular simultaneous biaxial stretching method, or the like. A method of stretching is known.
Since the film immediately after being stretched has a strong shrinkage, it causes problems in processing steps such as printing and laminating as it is, so it is common to heat the stretched film to fix the stretched orientation structure. is there.
Therefore, by fixing the dimensions of the film and performing heat treatment at a temperature near the melting point under tension, it is possible to obtain a film having excellent characteristics in which the dimensional change due to shrinkage is small and the molecular orientation structure is maintained due to stretching. it can. A tenter type hot stove is generally used for the heat treatment process of such a film.
[0004]
Generally, even when a biaxially stretched polyamide film is produced by a tubular method, a tenter type hot air furnace is used in the heat treatment step. However, two major problems have been pointed out in the heat treatment process using a tenter type hot air furnace 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 rate, it is necessary to perform heat treatment at a high temperature near the melting point in order to obtain a film having a small hot water shrinkage rate. On the other hand, in the stretching by the tubular method, after stretching, the tubular film is folded flat and conveyed to the heat treatment step as a flat film. When the stretched film in such a state is heat-treated with a tenter, the two folded films are fused or blocked. For this reason, it has become difficult to separate the two films after the heat treatment, and it has been pointed out that the film cannot be used as a product.
[0005]
Another problem is the problem of bowing deformation of the film in the tenter heat treatment process. Normally, in the heat treatment process, when a film with high shrinkage stress such as polyamide is heat treated with a tenter, the center of the film is pulled in the opposite direction to the running direction due to the film tension distribution caused by stress relaxation in the tenter. There is a phenomenon that the running is delayed from the end. When a straight line in the TD direction (film width direction) is drawn on the film before heat treatment, in the film after heat treatment, a phenomenon is observed in which an arcuate curve with a concave center in the traveling direction is observed. Such deformation is called Boeing deformation.
In general, films with large bowing deformation often have quality problems. For example, when a bag having a large bowing deformation is formed into a bag, there is no problem with the film in the center portion in the TD direction, but if a film close to the end portion is used, twisting of the bag called a so-called S-curl tends to occur. If the S-curl is large, troubles are likely to occur during, for example, an automatic filling and packaging process or packaging.
[0006]
Such twisting of the bag is mainly related to the hot water shrinkage distribution in the film plane. That is, it is likely to occur when there is a large difference in film shrinkage behavior between the front and back when the film is folded to make a bag.
In general, the larger the bowing deformation, the greater the anisotropy in the film surface of the hot water shrinkage distribution at the film edge, and the maximum and minimum directions of the hot water shrinkage are MD (length direction) or TD direction. It is greatly deviated from. The greater the anisotropy of the hot water shrinkage and the closer the inclination from the maximum or minimum MD or TD direction is to 45 °, 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 the difference in shrinkage behavior, the difference in hot water shrinkage (%) in boiling water at 100 ° C. when the angle from the MD or TD direction is + 45 ° and + 135 °. When the water shrinkage rate oblique difference Sd is defined, if Sd is 1.0% or less, S-curl tends to hardly occur.
In particular, since the polyamide film has a relatively large hot water shrinkage rate, S-curl often becomes a problem when a bag-making product is retorted. This means that in a polyamide film having a large bowing deformation, a film that can be used for a bag-making product is limited to a portion close to the center in the width direction of the film, which is a serious problem in industrial production.
[0007]
Furthermore, if the bowing deformation is large, there is a problem that film properties other than the heat shrinkage rate, such as elastic modulus, elongation at break, and strength at break, differ in the film plane at the film edge and center after heat treatment. To do. This is because the orientation structure that was uniform in the film width direction at the end of stretching is distorted toward the film edge due to bowing deformation that occurs in the heat treatment step. When such a film is processed, for example, by applying tension in the MD direction, it often causes poor running or causes wrinkles. Moreover, it often causes poor flatness of the film.
[0008]
In order to solve the problem of blocking the film, it is stretched into a tube shape, then cut off at both ends of the two-layer film folded flat and separated into two films, and then two sheets with endless ropes A method of introducing heat into a tenter while maintaining a gap between the films is subjected to heat treatment (Japanese Patent Publication No. Sho 46-15439).
In addition, after the film stretched into a tube shape is folded flat, a preliminary tenter heat treatment at a relatively low temperature is performed in a state where the two layers of the films are overlapped, and the film ends are cut into two pieces. Thereafter, there has been proposed a method in which two films are overlapped again with air interposed between the two films and heat-treated at a temperature near the melting point with a tenter (Japanese Patent Publication No. 5-35670).
However, although any of the above methods can prevent blocking of the two films, there is a problem in that a large thermal deformation occurs during the tenter heat treatment and a large bowing deformation occurs.
[0009]
On the other hand, with respect to the problem of bowing deformation, a method of substantially preventing bowing deformation by a tubular heat treatment method in which a film stretched in a tube shape is folded flat and then heat-treated while forming a tube again is known. ing. However, in this method, the internal pressure of the tube is extremely reduced during heat treatment at a high temperature near the melting point, the tube diameter is likely to fluctuate and the tube is oscillated, adversely affecting film quality and operational stability, Conversely, if heat treatment is performed at a relatively low temperature in order to minimize the influence on operability, sufficient heat treatment effect cannot be obtained, and the dimensional stability under heating or high humidity of the film is reduced. There was a problem.
[0010]
In addition, after the film folded into a flat shape after stretching is once separated into two sheets, the both ends of the film are pressed against the roll surface by a belt-like material, and the object is heated in a temperature range lower by 10 to 40 ° C. than the melting point of the film. There has been proposed a method for improving the bowing deformation by performing heat treatment by adhesion (JP-A-2-103122).
However, when the above method is used, although there is an effect of improving bowing deformation, it is difficult to obtain a film having a low wet heat shrinkage rate, and heat treatment is performed at a higher temperature in order to improve the wet heat shrinkage rate. In such a case, there arises a problem that the film is fused to the heated object. Moreover, in said method, since the film was processed at high temperature rapidly in the heat processing process, there existed a problem that crystallization of a film advanced and transparency (haze) worsened.
Further, after stretching, the first stage heat treatment is performed at a relatively low temperature by a tubular method, and then the film is folded again flatly, both ends of the film are cut open, separated into two sheets, and then between the two films. By superposing two layers in the presence of air and performing 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, and in the second stage heat treatment process. A method for preventing the blocking of the film and improving the bowing deformation has been proposed (Japanese Patent Publication No. 5-85341).
However, in the above method, in order to reduce the bowing deformation by the second stage tenter heat treatment, if the heat treatment temperature by the first stage tubular method is increased, the tube diameter fluctuates and stable operation becomes difficult. If the heat treatment temperature in the first stage is lowered and the operability is given priority, even if the tenter heat treatment in the second stage is performed, the improvement effect of the bowing deformation becomes insufficient and the film quality varies.
[0011]
[Problems to be solved by the invention]
As a result of conducting detailed studies on the above-described prior art, the present inventors have found that there is no problem in the manufacturing process such as film fusion in any of the methods, and a bag produced using the obtained film is used. In order to produce a film having no S-curl after retorting and excellent in bowing deformation, it was confirmed that there was still room for improvement.
[0012]
[Means for Solving the Problems]
The inventors of the present invention have studied a method for solving the above-mentioned problem regarding a method for producing a biaxially stretched polyamide film by a tubular method. As a result, after separating a film stretched by the tubular method into two sheets, a specific structure is obtained. It has been found that a film excellent in characteristics such as bowing and transparency can be produced industrially and efficiently by performing heat treatment step by step with a plurality of heat roll equipment having the above.
[0013]
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, a step of incising both ends of the film immediately after folding the stretched tubular film into a flat shape and separating it into two.
(2) A process of heat-treating each film separately through two or more heat rolls having different surface temperatures separately in a state where both ends of the two separated films are fixed on the heat roll by an endless belt. .Here, the temperature T1 (° C.) of the first heat roll is represented by the following equation (A), and the temperature Tn (° C.) of the nth heat roll is represented by the following equations (B) and (C).
Tm-100 ≦ T1 ≦ Tm-60 (I)
Tm- 60 ≦ Tn ≦ Tm−20 (B)
Tn ≦ T (n + 1) (C)
However, Tm shows the melting point of polyamide.
(3) The process of cooling a film with a cooling roll after heat processing with a hot roll.
(4) A step of superposing two cooled films.
(5) A step of heat-treating with a tenter while gripping both ends of the two overlapped films.Here, the heat treatment temperature Tf (° C.) by the tenter is represented by the following formula (d).
Tm-20 ≦ Tf ≦ Tm-5 (D)
However, Tm shows the melting point of polyamide.
[0014]
The polyamide used in the present invention is a thermoplastic polymer having crystallinity, which is polycaproamide (nylon 6), polytetramethylene adipamide (nylon 46), polyhexamethylene adipamide (nylon 66). , Polyhexamethylene sebamide (nylon 610), polyhexamethylene dodecane (nylon 612), polyundecanamide (nylon 11), polyundecane adipamide (nylon 611), polydodecanamide (nylon 12), Examples thereof include a homopolymer, a copolymer, or a mixture of polyamides mainly composed of polymetaxylylene polyamide (MXD6 nylon).
[0015]
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, the polyamide resin melt-extruded from the extruder 1 is extruded from the die 2 into a tube shape. As a method for cooling and solidifying the molten tube as an unstretched film, there are an air cooling method, a method of bringing it into contact with a metal surface, a water cooling method, and the like, and a water cooling method is desirable. The outer surface and / or the inner surface of the melting tube is cooled with cold water by a water-cooled sizing bath 3, and an unstretched film 4 is obtained.
The unstretched film 4 is guided to the stretching tower 6 and then heated to a predetermined temperature in the stretching tower 6. Due to the difference in peripheral speed between the pinch roll 5 and the pinch roll 8 in the MD direction, Each is stretched to a predetermined magnification by air pressure.
The stretching ratio of the film is desirably stretched 2 to 5 times, preferably 2.5 to 4 times, both vertically and horizontally.
[0016]
Next, the stretched film 9 is obtained by being folded flatly by the pinch roll 8 through the guide plate 7.
Further, the fold portion of the stretched film is cut by a trimming device 12 in front of the hot roll facility 10 in FIG. 2, and the stretched film 9 is separated into two sheets (FIG. 2 shows a two-stage heating roll). It is the schematic of the heat roll equipment in the case of having).
[0017]
The two separated films are introduced into a separate hot roll facility 10 having the same structure. The hot roll facility 10 has at least two stages of 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 heat roll facility 10 in a state of being in close contact with the first stage heat roll 14 by pressing of the pinch roll 13. Without the pinch roll 13, contact with the stretched film 9 becomes uneven at the entrance of the heating roll 14, so that the transparency of the stretched film 9 is generated, and wrinkles and waviness are generated due to film shrinkage. .
As the pinch roll 13, in order to prevent accumulation of air between the heating roll 14 and the film, it is desirable to use, for example, a rubber roll in which a spiral air vent groove is cut.
[0018]
In FIG. 2, the stretched film 9 introduced into the heating roll 14 is heat-treated with the end of the film held between the endless belt 19 and the heating roll 14, as shown in FIG.
FIG. 3 shows the 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 is stepped. The falling portion A 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 portion of the stretched film 9 is sandwiched between the endless belt 19 and the stepped portion A of the heating roll 14, the film end portion gripping force is dramatically increased. When a roll having no stepped portion is used, the gripping force of the film is weak. For example, the belt surface deteriorates after long-term operation, and the gripping of the film edge is unstable due to the reduction of frictional force. It becomes.
The height of the stepped portion A is preferably about 0.5 to 10 mm. If the step height is smaller than 0.5 mm, the film gripping effect is not sufficient, and if it is larger than 10 mm, the shape of the film is damaged at the stepped portion, which is not preferable.
[0019]
  In the present invention, in the step of heat-treating the two separated stretched films 9 by the hot roll equipment 10 in a stepwise manner, the heat treatment temperature T1 (° C.) of the first step by the heating roll 14 is a condition satisfying the following formula. To dois necessary.
                    Tm-100 ≦ T1 ≦ Tm-60
  However, Tm shows the melting point of polyamide.
[0020]
If the heat treatment temperature T1 is higher than (Tm-60) ° C., the transparency of the stretched film 9 is lowered, and spots are easily generated in the transparency. On the other hand, if T1 is lower than (Tm-100) ° C., the effect of the heat treatment becomes insufficient.
Although the contact time in the heating roll 14 is not specifically limited, 0.5 second or more is desirable. In FIG. 2, a pinch roll 15 is provided in the section from the heating roll 14 to the heating roll 16 in the second stage.
[0021]
When there is no pressing by the pinch roll 15, contact between the film and the heating roll 16 becomes uneven at the entrance portion of the heating roll 16, so that spots are generated in transparency, and wrinkles and waviness are generated due to shrinkage of the film. It's easy to do.
The stretched film 9 introduced into the heating roll 16 is heat-treated on the surface of the heating roll 16 in a state where the end portions are gripped by the stepped portions of the endless belt 19 and the heating roll 16. As the heating roll 16, a roll having a stepped portion at the end is used similarly to the heating roll 14.
[0022]
The second stage heat treatment temperature T2 (° C.) by the heating roll 16 is preferably performed under the condition satisfying the following formula.
Tm-60 ≦ T2 ≦ Tm-20
However, Tm shows the melting point of polyamide.
When the treatment temperature is higher than (Tm-20) ° C., the roll and the film are easily adhered, and when the treatment temperature is lower than (Tm-60) ° C., the heat treatment effect cannot be sufficiently obtained.
[0023]
  The heat treatment by the heating roll in the present invention needs to be performed using at least two heating rolls.
  At this time, the temperature Tn (° C.) of the n-th stage heating roll and the temperature T (n + 1) (° C.) of the (n + 1) -th stage heating roll should be performed under the conditions satisfying the following expression.is necessary.
                      Tm-60 ≦ Tn ≦ Tm-20
                            Tn ≦T ( n + 1)
However, Tm shows the melting point of polyamide.
[0024]
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 the film hardly shrinks, it is not necessary to constrain the end by the endless belt 19. However, in order to prevent the endless belt 19 from meandering in the section until the stretched film 9 and the endless belt 19 are separated, the cooling roll 18 is also a roll having a step-down structure similar to the heating roll 14. Is preferred.
[0025]
The film cooled by the cooling roll 18 is overlaid again with the other film that has been subjected to the same heat roll treatment, and the two laminated films simultaneously hold the air between the two sheets. As shown in FIG. 1, it is guided to the tenter type hot air furnace 11 and heat-treated with hot air in a state where the end portion is held by a clip.
[0026]
  The heat treatment temperature (° C.) of the stretched film 9 in the tenter type hot stove 11 should be performed under the conditions satisfying the following formula.is necessary.
                      Tm-20 ≦ Tf ≦ Tm-5
  However, Tm shows the melting point of polyamide.
  When the heat treatment temperature is lower than (Tm-20) ° C., the heat treatment effect is insufficient, and when it is higher than (Tm-5) ° C., the film is undesirably cut or whitened or melted.
[0027]
After the heat treatment by the tenter type hot stove 11 is completed, the two films are separated by a roll or the like. Each separated film is trimmed at the end and then wound on a separate bobbin.
[0028]
[Action]
In the present invention, by heat treating the film stepwise with two or more stages of hot rolls, rapid and non-uniform crystallization of the stretched film in the roll heat treatment step is suppressed, and the transparency and uniformity of the film are improved. be able to.
The mechanism is not clear, but is presumed as follows.
That is, the film immediately after stretching is crystallized very rapidly because of its orientation structure. When a film in such a state is heated rapidly and at a high temperature in one step by a heat source having a good transfer efficiency such as a heat roll, the film is excessively crystallized or the crystallization is too fast, so that a little heat is generated. Due to the difference in history, for example, a slight difference in contact pressure to the hot roll, crystallization is likely to occur. As a result, it is considered that the transparency of the film is deteriorated or spots are easily formed on the transparency. Therefore, when heat-treating the stretched film with a roll, by carrying out heat treatment stepwise in a predetermined temperature range, while suppressing excessive and rapid crystallization, crystallization spots due to slight thermal history differences are observed. It can be prevented.
[0029]
【Example】
Next, the present invention will be specifically described with reference to examples. In addition, the evaluation method of each characteristic of a film is as showing next.
1) 100 ° C. hot water shrinkage S
The film was cut into strips having a length of 150 mm and a width of 10 mm, and marked lines at intervals of about 100 mm were written in the length direction. After being left for 1 day in an atmosphere of 20 ° C. and 65% RH, the length (L1) between the marked lines was measured.
Next, the film was immersed in boiling water at 100 ° C. for 10 minutes. After wiping the surface moisture, the sample was left in an atmosphere of 20 ° C. and 65% RH for 1 day, and then the length (L2) between the marked lines was measured.
S (%) = [(L1-L2) / L1] × 100 was determined as the 100 ° C. hot water shrinkage.
2) Maximum hot water shrinkage Smax in the film plane
Centered in the center part (a) in the TD direction of the film, the part (b) closer to the center part 75 mm from the end part, and the intermediate part (c) between (a) and (b), the film at each position is spaced by 5 °, The above S was measured in the range of 0 to 175 °, and the 100 ° C. hot water shrinkage in each direction within the film plane was measured (the right hand direction was 0 ° and the counterclockwise direction was + Direction).
From these measurement results, the maximum value Smax of S in each part of (a) to (c) was obtained.
3) Hot water shrinkage rate diagonal difference Sd
At the film position (c), the absolute value of the difference between the hot water shrinkage rates in the 45 ° direction and the 135 ° direction was determined as the hot water shrinkage rate oblique difference Sd.
4) Haze
According to ASTM D1003, the haze of the central part (a) of the film was measured to evaluate transparency. The apparatus used was a haze meter TC-H • DPK (manufactured by Tokyo Denshoku Co., Ltd.).
5) Transparent spots
Transparency spots were evaluated by observing the appearance.
The evaluation criteria were as follows: ◎ when there was no spots visually, ◯ when there was almost no spots, △ when there was some spots, and x where there was marked spots.
6) S-curl property of bag-making products with hot water
After slitting the end 400mm of the laminate film obtained by dry lamination with LLDPE (sealant) using a urethane adhesive, and half-folding, it is 250mm in length in the MD direction and 200mm in width in the TD direction by a three-side seal A three-sided seal bag was created.
Next, after filling the obtained bag with 150 cc of water, the opening was heat-sealed to prepare a packaging bag. This packaging bag was immersed in boiling water at 100 ° C. for 10 minutes, and the S-curl property of the packaging bag was evaluated.
The evaluation was ◎ when there was no twist by visual observation, ◯ when there was almost no twist, Δ when there was twist, and × when the twist was significant.
[0030]
Example 1
Nylon 6 resin (melting point 220 ° C., 98% sulfuric acid, temperature 25 ° C., relative viscosity 3.5 at a concentration of 1 g / dl) in which 800 ppm of amorphous silica particles having an average median diameter of 2 μm are dispersed as a slip agent is melted from a circular die. After extrusion, it was cooled and solidified in a water-cooled sizing bath at 15 ° C. to obtain a tubular unstretched film having a folding diameter of 450 mm and a thickness of 140 μm.
This unstretched film was introduced into a stretching tower, and the film was heated and stretched using hot air and far infrared rays in combination. The draw ratio was 3 times in the MD direction due to the difference in peripheral speed between the top and the bottom of the tower, and 3.2 times in the TD direction due to the expansion of the tube by air pressure. Next, the stretched film having a width of 1440 mm was obtained by introducing the stretched film into the guide plate and folding it flat.
Next, the crease | fold part of the edge part of the stretched film folded flat was cut open, it isolate | separated into two films, and it heat-processed with the hot roll equipment. A hot roll facility having the same structure as that shown in FIG. 2 was used. Further, 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 temperature of the heating rolls 14 and 16 and the cooling roll 18 was 120 degreeC, 160 degreeC, and 15 degreeC, respectively. In the roll heat treatment and roll cooling section, a film was sandwiched between the endless belt 19 and the stepped portion of the roll end, and the endless belt 19 was transported in a state where the end of the film was restrained by applying a tension of about 5 kg. As the endless belt 19, a fiber reinforced belt having a width of 40 mm and a thickness of 2 mm was used.
There was no film shrinkage or transparency spots in the roll heat treatment process.
The two stretched films that had undergone the roll heat treatment were overlapped by a guide roll, and then 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. In the third stage heat treatment, 5% relaxation treatment was performed in the width direction.
After the tenter heat treatment, the film was again separated into two sheets, and about 30 mm of both end portions of the film (gripping portions by clips) were trimmed, and finally two stretched films having a width of 1300 mm were wound on a bobbin.
Films were produced continuously for 48 hours from the start of operation, but the operation was stable and no problems were found.
Table 1 shows the results of evaluating various physical properties of the film thus obtained.
[0031]
Examples 2-7, Comparative Examples 1-5
A stretched film was produced in the same manner as in Example 1 except that the temperature of the heating rolls 14 and 16 was changed. Table 1 shows the results of evaluating various physical properties of the obtained film.
In Comparative Example 1, noticeable spots were observed in the transparency of the film in the second-stage heating roll 16. In addition, the film slightly contracted at the inlet of the second stage heating roll 16, and wrinkles were generated intermittently.
In Comparative Example 2, although improved compared to 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, there was a tendency for the film and the roll to be fused in the second stage heating roll 16, and peeling from the roll was difficult.
In Comparative Example 4, noticeable spots occurred in the transparency of the film in the first-stage heating roll 14. Further, the film contracted at the entrance of the heating roll 14 and wrinkles were easily generated.
[0032]
[Table 1]

[0033]
【The invention's effect】
In the heat treatment method of the tubular method biaxially stretched film in the present invention, the film heat treatment is performed in a tenter type hot air furnace after performing the roll heat treatment in a state in which the film end portion is restrained, thereby blocking or bowing the film in the tenter. Deformation is suppressed, and heat treatment can be performed at a high temperature near the melting point.
As a result, the film has a hot water shrinkage rate of less than 2% in all directions within the film surface over the entire width of the film, and has excellent transparency with a hot water shrinkage slant difference Sd of 1.0% or less over the entire width of the film. A film can be produced stably.
Moreover, the bag-making goods produced using the obtained stretched film have the performance of the S-curl property excellent also including the retort processing goods.
Also, in the roll heat treatment process, the film gripping force is increased by restraining the film end between the stepped portion of the heat roll and the endless belt, and the film can be prevented from shrinking. The end portion is stably held and stable production is possible.
[Brief description of the drawings]
FIG. 1 is an explanatory diagram showing an example of a process for producing a biaxially stretched polyamide film by the tubular method of the present invention.
FIG. 2 is an explanatory diagram showing an example of a step of heat-treating with a hot roll facility after cutting a tubular stretched film into two sheets in the present invention.
FIG. 3 is an explanatory diagram of an example of a heat roll having a stepped shape at both ends in the present invention.
[Explanation of symbols]
1 Extruder
2 die
3 Water-cooled sizing bath
4 Unstretched film
5 Pinch roll
6 Stretching tower
7 Information board
8 Pinch roll
9 Stretched film
10 Heat roll equipment
11 Tenter
12 Trimming device
13 Pinch roll
14 Heating roll
15 Pinch rubber roll
16 Heating roll
17 Pinch rubber roll
18 Cooling roll
19 Endless belt

Claims (2)

A method for producing a tubular biaxially stretched polyamide film, which is produced by the following steps (1) to (5).
(1) A step of incising both ends of the film after tubular biaxial stretching and then immediately folding the stretched tubular film into two pieces.
(2) A process of heat-treating each film separately through two or more heat rolls having different surface temperatures separately in a state where both ends of the two separated films are fixed on the heat roll by an endless belt. . Here, the temperature T1 (° C.) of the first heat roll is represented by the following equation (A), and the temperature Tn (° C.) of the nth heat roll is represented by the following equations (B) and (C).
Tm-100 ≦ T1 ≦ Tm-60 (I)
Tm- 60 ≦ Tn ≦ Tm−20 (B)
Tn ≦ T (n + 1) (C)
However, Tm shows the melting point of polyamide.
(3) The process of cooling a film with a cooling roll after heat processing with a hot roll.
(4) A step of superposing two cooled films.
(5) A step of heat-treating with a tenter while gripping both ends of the two overlapped films. Here, the heat treatment temperature Tf (° C.) by the tenter is represented by the following formula (d).
Tm-20 ≦ Tf ≦ Tm-5 (D)
However, Tm shows the melting point of polyamide. In the heat treatment process using a hot roll, each hot roll and the subsequent cooling roll are provided with a pinch roll for bringing the film into close contact with the roll surface, and the hot roll and the cooling roll are in contact with both ears of the film. 2. The biaxial stretching according to claim 1, wherein the portion to be cut has a stepped shape, and an endless belt for fixing both ear portions of the film by pressing is disposed at the stepped portion. A method for producing a polyamide film.

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