JPH0458373B2 - - Google Patents

Description

[Detailed description of the invention]

"Industrial Application Field" The present invention relates to a method for producing a biaxially oriented polyamide film. More specifically, the present invention relates to an improvement in the method of manufacturing a biaxially stretched polyamide film by first stretching it in the longitudinal direction using a roll-type longitudinal stretching method, then holding it with tenter clips and stretching it in the transverse direction, and suppressing the bowing phenomenon. , relates to a method for producing a film having uniform physical properties in the width direction of the film. ``Prior art'' Biaxially stretched polyamide film has excellent toughness, heat resistance, cold resistance, transparency, printability, chemical resistance, etc., and has characteristics such as being resistant to pinholes. It is widely used as a base film for food and other packaging. When biaxially stretched polyamide film is used for food packaging, printing, lamination, bag making, food filling, and heat sealing are usually performed.
The quality of the base film has a great influence on the production speed, product yield, etc. in printing, laminating, bag making processes, etc. For example, if a base film with poor flatness or uneven physical properties is used, problems such as printing pitch misalignment, wrinkles, curling, and meandering may occur during printing, laminating, bag making, etc. However, there are disadvantages in that good quality products cannot be obtained and production speed cannot be increased. In addition, the flatness and uniformity of physical properties of the base film are related to the presence or absence of bowing distortion, and a base film with improved bowing distortion has better flatness and more uniform physical properties. The request is extremely strong. "Boeing distortion" is caused by the Boeing phenomenon. For example, the Boeing phenomenon is
As explained in Japanese Patent Application Laid-open No. 55221 and Japanese Patent Application Laid-Open No. 58-147322, if a straight line is marked on an unstretched film at right angles to the transport direction,
After completing biaxial stretching in the longitudinal and transverse directions and heat setting, the straight line is distorted into an arched shape, a phenomenon that lags closer to the center of the film. Furthermore, if a large number of small circles are drawn instead of a straight line, the circle remains enlarged in diameter at the center of the film, but becomes a slanted ellipse at the edges in the width direction of the film. As a method for suppressing the bowing phenomenon that occurs during the production of such biaxially stretched films, Japanese Patent Application Laid-Open No. 1986-
The techniques described in Japanese Patent Application Laid-open No. 13706 and JP-A-51-80372 have been proposed, but the techniques proposed in these publications are techniques applied to the simultaneous biaxial stretching method, and the method of the present invention is There is no effect even if it is applied to a sequential biaxial stretching method such as Additionally, Japanese Patent Publication No. 43-5557 proposes a method of providing a buffer zone between the lateral stretching zone and the heat treatment zone, but according to experiments conducted by the present inventors, the method proposed here is It was found that applying this technique to polyamide film was ineffective. Furthermore, JP-A-50-73978 proposes a method of providing a group of nip rolls in the transverse stretching zone and the heat treatment zone, but this method tends to cause scratches on the film surface due to the nip rolls, making it impractical for practical use. I can't offer it. Furthermore, JP-A-57-87331 discloses that as a measure to suppress the bowing phenomenon in the sequential biaxial stretching method, a film that has been sequentially biaxially stretched in the longitudinal and transverse directions is immediately cooled to below the glass transition temperature. After that, the authors propose a method in which the grip on the film side edge is released, and the film side edge is gripped again to perform heat treatment. Although it is effective for films, it is completely ineffective for films such as polyamides that are stretched at relatively low temperatures, and it has not been possible to suppress the bowing phenomenon. The present inventors have previously completed a method for manufacturing a sequentially biaxially stretched polyamide film (for example,
59-171626, Japanese Patent Application No. 59-30101, etc.), it has been found that when a film is manufactured by this method, the bowing phenomenon of the film occurs during the process of heat-setting the film. "Problem to be Solved by the Invention" The present invention suppresses the bowing phenomenon that occurs when heat-setting a biaxially stretched polyamide film according to the sequential biaxial stretching method, and has uniform physical properties in the width direction. The object of the present invention is to provide a method for producing a film with excellent flatness. "Means for Solving the Problems" However, the gist of the present invention is to stretch a substantially amorphous and unoriented polyamide film using a roll longitudinal stretching method at a temperature of 45 to 60°C. Depending on the deformation speed of 10000%/min or more, 2.7~
The film was stretched 3.5 times in the longitudinal direction, and then both ends of the film were held with tenter clips, the film temperature was kept below 100°C, and the average deformation rate was 2000 to 10000.
%/min in the transverse direction, and then the first heat treatment is performed at a temperature of 100 to 170°C while holding both ends of the film. After the film is cooled to below the glass transition temperature of the raw material polyamide, the grips at both ends of the film are released, and then transferred to the next heat treatment zone, where the film is gripped at both ends with another tenter clip and transferred. , from the melting point of the raw material polyamide
The present invention resides in a method for producing a biaxially oriented polyamide film, characterized in that a second stage heat treatment is performed at a temperature that is 10° C. lower as an upper limit. The present invention will be explained in detail below. The polyamide used as a raw material in the present invention is a homopolymer of ε-caprolactam, which has ε-caprolactam as its main component,
2 to 10 mol% of copolymers with other compounds copolymerizable with these homopolymers and/or copolymers, and 5 to 20% by weight of polymers compatible with these homopolymers and/or copolymers. It means a mixture. Examples of compounds copolymerizable with ε-caprolactam include aliphatic or aromatic diamines and condensates with aliphatic or aromatic dicarboxylic acids. Specific examples of diamines include ethylenediamine, tetramethylenediamine, pentamethylenediamine, hexamethylenediamine, octamethylenediamine, decamethylenediamine, metaxylenediamine, paraxylenediamine, and the like. Dicarboxylic acids include adipic acid, sebacic acid, corkic acid, glutaric acid, azelaic acid,
Examples include β-methyladipic acid, terephthalic acid, isophthalic acid, decamethylene dicarboxylic acid, and pimelic acid. Examples of polymers that are compatible with the homopolymer and/or copolymer include copolymers of the diamines and dicarboxylic acids. These raw material polyamides include lubricants, antistatic agents, antiblocking agents, stabilizers, dyes, pigments,
Various resin additives such as inorganic fine particles can be added to the extent that they do not adversely affect the properties of the film. When using the method of the present invention, a substantially amorphous and unoriented polyamide film (hereinafter referred to as "unstretched film") is used. An unstretched film can be produced by, for example, heating and melting polyamide in an extruder, extruding it into a film through a T-die, and casting it by a known casting method such as an air knife casting method, an electrostatic application method, or a vacium chamber method. It can be produced by rapid cooling on a casting roll maintained at a temperature not higher than 35°C, more preferably not higher than the dew condensation temperature. When using the method of the present invention, the unstretched film is
First, the film is stretched in the longitudinal direction (hereinafter simply referred to as "longitudinal stretching") by a roll longitudinal stretching method. Stretching by a roll type longitudinal stretching method refers to a method of longitudinal stretching using a roll type longitudinal stretching machine, and in the present invention, a conventionally known roll type high speed longitudinal stretching machine can be used. To longitudinally stretch an unstretched film, it is preferable to first adjust the temperature of the unstretched film to 45 to 65°C using a temperature-controlled preheating roll. If the temperature of the unstretched film is lower than 45°C, the film tends to have longitudinal stretching irregularities, and if it is higher than 65°C, the film tends to stick to the roll surface, which also causes the film to develop after longitudinal stretching. It is easy to cause longitudinal stretching unevenness, and furthermore, directional hydrogen bonds occur in the direction of stretching, and during the next stretching in the transverse direction (hereinafter simply referred to as "lateral stretching"), transverse stretching unevenness occurs in the film. This is not preferable because it may cause unstretched residues or unstretched portions, and the film may be easily torn. In the longitudinal stretching process, the deformation speed is set to 10000%/
It is necessary to adopt stretching conditions of 2.7 to 3.5 times or more and a stretching ratio of 2.7 to 3.5 times. Here, the deformation speed is a value calculated by the formula (), and the stretching ratio is a value calculated by the formula (). V _MD = (X-l) _/ L×(U _L +U _H )/Z×100… ₍ ) has the meaning of V _MD : Film longitudinal deformation speed (%/min) X: Film longitudinal stretching ratio L: Length of longitudinal stretching section (m) U _L : Linear speed of low speed roll (m/min) U _H : High speed roll linear velocity (m/min)] If the deformation velocity (V _MD ) is lower than 10000%/min,
Even if the longitudinal stretching is performed well, the film tends to develop unevenness in the transverse stretching during the subsequent transverse stretching, which is not preferable. The upper limit of the deformation rate can be selected depending on the structure and performance of the equipment used, the temperature of the film at the start of stretching, etc., but 50000%/
It is best to keep it under 1 minute. Note that when the film temperature at the start of stretching is low, the deformation rate is preferably reduced within the above range, and when the film temperature is high, it is preferably increased within the above range. When the longitudinal stretching ratio of the film is smaller than 2.7 times, the desired orientation effect cannot be imparted to the final film, and when it is larger than 3.5 times, transverse stretching unevenness or unevenness may occur during the next transverse stretching. This is not preferable because it tends to cause unstretched residue and also tends to tear. When using the method of the present invention, the film longitudinally stretched under the above conditions is immediately adjusted to a temperature range of 45 to 60°C, and the time expressed by the following formula (), t=e ^{(3.9-0.053T1)} ... ...() [In the formula (), t means the time (seconds) from the end of longitudinal stretching to the start of transverse stretching, and _T1 is the temperature of the film during this time (°C), which is 45 to 60°C.
Selected from the range of °C. e means the base of the natural logarithm], it is preferable to transport the tenter rail to the next lateral stretching start position (the position where the tenter rail starts widening). After finishing the longitudinal stretching, this film is
The reason for adjusting the temperature range to 60°C is as follows.
That is, if the temperature of the film is lower than 45°C, the temperature will be too low during transverse stretching and the film will easily tear, which is undesirable; if it is higher than 60°C,
This is because the transport time from the end of longitudinal stretching to the starting position of transverse stretching becomes extremely short, which causes problems in the design, arrangement, and operability of the apparatus, which is undesirable. After the longitudinal stretching, the film is transferred to the next transverse stretching step. Since polyamide has a fast crystallization rate, hydrogen bonds in the film become stronger over time after the longitudinal stretching. For this reason, it is preferable to transfer the material in a short time at a temperature as low as possible to allow lateral stretching. If the time calculated by the above formula () is exceeded, the film is likely to have lateral stretching irregularities during lateral stretching, or unstretched residues are likely to be formed at the edges of the film in the width direction. Undesirable. When using the method of the present invention, when transversely stretching by the tenter type transverse stretching method, until the mechanically set magnification between the tenter clips reaches 1.4 times or more of the original value,
It is preferable to widen the film at an angle of 6 degrees or less with respect to the center line in the width direction of the film, and maintain the temperature of the tenter clip during this time to be lower than the temperature of the film. Setting the conditions immediately after the start of lateral stretching at the lateral stretching start position as above suppresses the occurrence of necks near the tenter clips and randomizes the position of occurrence of neck stretching in the center of the width direction of the film.
This is to avoid breakage of the film and to make the physical properties uniform along the width direction of the film. When performing transverse stretching using a tenter, the temperature of the film is raised stepwise from the transverse stretching position, and at the transverse stretching end position, the film temperature is 100°C or lower, preferably 70°C or lower.
The temperature conditions must be within the range of 100°C, particularly preferably within the range of 75 to 90°C. When using the method of the present invention, the temperature of the film that has been transferred to the transverse stretching start position after finishing the longitudinal stretching, that is, 45 to 60°C, is too low for the transverse stretching of the film. If this is done, the film is likely to break at the tenter clips, making stable transverse stretching difficult. In order to perform stable transverse stretching and to obtain a film with relatively balanced longitudinal orientation, as mentioned above, the widening angle of the tenter clips at the initial stage of the transverse stretching process is set to a specific value. In addition to randomizing the starting point of the net stretching that occurs at the center in the width direction of the film, it is necessary to transversely stretch the film while raising the temperature in stages. If the temperature rises rapidly when horizontally stretching the film,
The portions of the film where net stretching has not yet begun, that is, the portions of the film that have not yet been horizontally stretched, are exposed to strong heat, which strengthens the directional hydrogen bonds that occurred during the longitudinal stretching process, and the directional hydrogen bonds formed during the longitudinal stretching process become stronger. This is undesirable because it causes uneven stretching in the lateral direction and unstretched portions, resulting in a film with significantly unbalanced orientation in the longitudinal and lateral directions. According to experiments conducted by the present inventors, when a film is laterally stretched using a tenter, the film temperature is raised stepwise from the lateral stretching start position, and at the lateral stretching end position, the film temperature is 100°C or less. ,Preferably
Temperature conditions in the range of 70 to 100°C, particularly preferably in the range of 75 to 90°C, suppress hydrogen bonds from becoming strong and set the net stretching vanishing point early in the transverse stretching process. It was found that it was possible to stably produce a film with good orientation balance and good thickness accuracy. In order to raise the temperature of the film stepwise in the transverse stretching process, at least two or more sections are provided on the upper and/or lower surface of the film in a direction perpendicular to the film traveling direction, and within each section, Any method such as blowing hot air, installing an infrared heater, or a combination of these methods may be used. The film temperature at the end of horizontal stretching is 70 to 100.
℃ is preferable, but if the film deformation speed and stretching ratio are high, the film temperature should be set higher within the above range, and if the deformation speed and drawing ratio are low, the film temperature should be set lower within the above range. It is preferable to choose. In the lateral stretching process, the average deformation speed is set at 2000~
It is necessary to adopt stretching conditions of 10,000%/min and a stretching ratio of 3 to 5 times, more preferably 3.5 to 4.5 times. Here, the average deformation speed refers to a value calculated by the following equation (). V _TD = (Y-l)/L _T ×U _T ×100...() [In formula (), each symbol has the following meaning. V _TD : Average deformation speed of the film (%/min) Y: Mechanically set stretching ratio (times) of the film,
Value obtained by dividing the width between tenters at the end position of lateral stretching by the width between tenters at the start position of lateral stretching U _T : Speed of tenter (m/min) L _T : Length of lateral stretching section (m)] Average When the deformation speed (V _TD ) is lower than 2000%/min, horizontal stretching spots are likely to occur in the film, and when the deformation speed is lower than 10000%/min,
If it is larger than 1 minute, the film is likely to break,
Undesirable. When the transverse stretching ratio of the film is less than 3 times, unstretched portions tend to be formed, and when it exceeds 5 times, the transversely stretched film tends to break, which is not preferable. A polyamide film stretched in the longitudinal direction and then in the transverse direction at a relatively low temperature while suppressing crystallization according to the above method hardly exhibits the bowing phenomenon at the end of the transverse stretching. However, if the film that has been laterally stretched is used for various purposes as it is, it has the drawback of shrinkage and poor dimensional stability. In order to impart dimensional stability to the biaxially stretched film, a method of heat treating the biaxially stretched film is employed. It has been found that when a biaxially stretched film produced according to the method of the present invention is heat treated in the same tenter after transverse stretching, a significantly large bowing phenomenon occurs in the final film. . In order to suppress the bowing phenomenon that occurs in this heat treatment process, the present inventors conducted examination experiments by changing the temperature increase pattern during heat treatment, the rail pattern of the tenter, etc., but as long as heat treatment is performed with one tenter, It has been found that there is a limit to suppressing the bowing phenomenon that appears in the final film, and that it is extremely difficult to reduce the bowing distortion within the target range. As a result of further experiments, it was found that the bowing phenomenon that appears in the final film can be significantly suppressed by performing the heat treatment in two stages. The two-step heat treatment method is to heat the film at a temperature of 100 to 170°C, preferably 110 to 170°C, while holding both ends of the transversely stretched film with tenter clips.
The first stage of heat treatment is carried out within a range of 150°C, and then the film is cooled to below the glass transition temperature of the raw material polyamide, and then the grips at both ends of the film are released, and then the film is transferred to the next heat treatment zone. Here, while holding both ends of the film with separate tenter clips and transporting it, the temperature is lowered from the melting point of the raw polyamide
This is a method in which the second stage heat treatment is performed at a temperature 10°C or more lower. In this case, as will be described later, by selecting the temperature conditions for the second stage heat treatment within a range that satisfies the above requirements, it is possible to produce a non-heat shrinkable film used for general packaging and a film used for shrink packaging. Any of the following heat-shrinkable films can be obtained. In the first stage heat treatment, the heat treatment temperature was 170
If the temperature is higher than 0.degree. C., a large bowing phenomenon will occur in the film during the heat treatment step under these conditions, which is undesirable. Also, if the heat treatment temperature is lower than 100℃,
After cooling, the film whose both end grips are released undergoes significant shrinkage in the transfer direction and width direction, or irregular shrinkage occurs, which is undesirable since the flatness of the final film is impaired. The tenter clip spacing during the first stage heat treatment may be the same as that at the end of the lateral stretching and the tenter clips may be in a tensioned state, or the tenter clips may be performed in a relaxed state by narrowing the spacing. After the first heat treatment, it is necessary to cool the film to a temperature below the glass transition temperature of the raw material polyamide while holding both ends of the film with tenter clips. If the grips on both sides of the film are immediately released after the first heat treatment without cooling it below the transition temperature of the glass of the raw material polyamide, the film will have irregularities while being transferred to the next heat treatment zone. Shrinkage occurs, and since the film comes into contact with the guide rolls etc. in a hot state, the film may stick to the guide rolls etc. or be cooled unevenly, significantly impairing the flatness of the final film. . After cooling the film, which has been cooled to below the glass transition temperature of the raw material polyamide, it is necessary to temporarily release both ends of the film from being held by the tenter clips. As a result of this release, stress fields in the transport direction that cause the bowing phenomenon, such as relaxation stress due to longitudinal stretching and machine direction stress based on Poisson's ratio during transverse stretching, are released, and in the subsequent heat treatment zone, Bowing distortion caused by these stresses can be avoided. The cooled film released from the grip of the tenter clips is transferred to the succeeding second stage tenter for heat treatment, and both ends of the film are gripped again. To transfer between the first stage heat treatment tenter and the second stage heat treatment tenter, use a guide roll, an expander, a dancer roll for tension control, a tension pick-up roll, a nip roll, etc. It can be installed and used as needed. To carry out the second stage heat treatment, the film is transferred to the second stage heat treatment tenter, and while the film is being transferred, it is heated at 10
The second stage heat treatment is performed under temperature conditions where the upper limit is a temperature lower than 0.degree. If the heat treatment is performed at a temperature higher than this upper limit temperature, the surface of the film may become white or the film may break, which is not preferable. When using the method of the present invention, the temperature conditions for the second stage heat treatment can be freely selected from the following conditions depending on the properties to be imparted to the film to be finally obtained. In other words, the lower limit of the heat treatment temperature is 190°C when obtaining a non-heat-shrinkable film that has a shrinkage rate of 4% or less when immersed in boiling water for 5 minutes.
The temperature should be selected within a temperature range whose upper limit is 10°C lower than the melting point of the raw material polyamide, preferably within a temperature range of 195 to 205°C. If the heat treatment is performed at a temperature lower than 190°C, the shrinkage rate of the final film will increase, and a non-heat-shrinkable film with a small shrinkage rate will not be obtained. In addition, the heat treatment temperature for obtaining a heat-shrinkable film for use in shrink packaging that has a shrinkage rate of 15% or more when immersed in boiling water for 5 minutes is as follows:
The temperature should be selected from 100 to 170°C, preferably from 120 to 150°C. Films that have been heat-treated at temperatures below 100℃ will naturally shrink during storage indoors due to insufficient heat treatment, and films that have been heat-treated at temperatures below 170℃ will naturally shrink when stored indoors.
The heat shrinkage rate becomes small, and a heat-shrinkable film with a large shrinkage rate cannot be obtained. In addition, when performing the second stage heat treatment,
If necessary, the stretching balance can be adjusted by making the rail spacing between the left and right tenters a little narrower, making it a constant width, or making it a little wider. Particularly when obtaining a non-heat-shrinkable film, it is effective to suppress the bowing phenomenon by narrowing the tenter rail spacing in the initial stage to give relaxation in the width direction, and then making it a constant width. When obtaining a film, it is more effective to suppress the bowing phenomenon if the width is consistently constant. The film that has undergone the second stage heat treatment is always cooled and wound up. The biaxially stretched polyamide film produced in this manner has a low bowing amount (defined later) of 5% or less, has uniform physical properties in the width direction of the film, and has good flatness. "Effects of the Invention" When the method of the present invention is used, it is possible to suppress the bowing phenomenon that occurs when heat-setting a polyamide film that has been biaxially stretched by the sequential stretching method, and therefore, the film has uniform physical properties in the width direction. It is possible to easily produce a non-heat-shrinkable film or a heat-shrinkable film that has excellent flatness and is sufficiently heat-set. "Examples" Next, the present invention will be described in more detail based on Examples, but the present invention is not limited to the following examples unless it exceeds the gist thereof. In the following examples, the physical properties of the resulting films were evaluated by the methods described below. (1) Film thickness (μm) Measurements were made at 30 points along the width of the film, and the average value was used. (2) Film thickness unevenness (%) Calculated using the following formula. Thickness unevenness = d _nax − d _nio / d _av × 100 where d _nax = maximum thickness in the width direction d _nio = minimum thickness in the width direction d _av = average thickness in the width direction (3) Bowing amount (% ) On the surface of the unstretched film, draw a straight line with a felt pen at right angles to the film transport (flow) direction.
This is the rate at which the straight line is deformed on the product film obtained through a series of processes such as longitudinal stretching, transverse stretching, and heat treatment, and is the length by which the center of the film lags behind the film side edges, divided by the film width. to the value
The value multiplied by 100 was used. (4) Heat shrinkage rate and heat shrinkage rate difference (%) First, the product film was heated at a temperature of 25℃ and a relative humidity of 45%.
After conditioning the film in an atmosphere of
The square was drawn so that each side was parallel to the vertical and horizontal directions of the film. Next, this sample was immersed in boiling water for 5 minutes, taken out, and left again in an atmosphere at a temperature of 25° C. and a relative humidity of 45% for 24 hours. The dimensions of the square before and after immersion in boiling water were measured and calculated using the following formula. Longitudinal heat shrinkage rate = l ₁ − l ₁ ′/l ₁ ×100 Transverse heat shrinkage rate = l ₂ −l ₂ ′/l ₂ ×100 Diagonal heat shrinkage rate difference = | (l ₃ − l ₃ ′ /l ₃ −l ₄ −l ₄ ′/l ₄ ×100｜ Here, l ₁ and l ₁ ′ are the lengths of the sides along the longitudinal direction of the film before and after immersion, and l ₂ and l ₂ ′ are the lengths of the sides along the film length. l ₃ , l ₃ ′ are the lengths of one diagonal of the square before and after immersion, and l ₄ , l ₄ ′ are the lengths of the other diagonal of the square before and after immersion. Each length means the length of the film.The shrinkage percentage in the longitudinal direction and the shrinkage percentage in the transverse direction are preferably 3% or less, and the difference in shrinkage percentage in the diagonal direction is preferably 1% or less. (5) Flatness of the film (mm) As shown in Figure 1, draw straight lines 2 and 3 parallel to each other 2 meters apart on a flat table 1, and attach one end of the 3 meter length (product film cut in the length direction) to an adhesive. Fix it on the table 1 with tape 4. Apply a load of 20 grams per 1 cm in the width direction to the other end of the film to tension the film, and draw a marked line 6 on the film corresponding to the base line 3. Then, apply the load to the other end of the film. The film is cut into strips at 3 cm intervals along the length of the film.Then, a load of 60 grams is applied to the free end of each strip, and the base line 3 on the table and the marked line 6 on the strip are cut. The difference between the maximum and minimum measured values is used as an index of flatness. If this index is less than 2 mm, there is no practical problem with the product film, but More preferably,
1.5mm or less is better. Example 1 Poly-ε-caproamide (manufactured by Mitsubishi Chemical Industries, Ltd., Novamitsudo 1020CA) with a relative viscosity of 3.5 was
Melt and knead with a φ extruder at a cylinder temperature of 280℃, extrude into a film from a T-die, and rapidly cool on a 600mmφ casting roll kept at 30℃ to a thickness of approximately 140μ and width. A substantially amorphous, unoriented (unstretched) film of approximately 350 mm was obtained. In order to measure the amount of bowing on the surface of this unstretched film, a straight line was drawn with a red felt tip pen in a direction perpendicular to the film transport direction.
This unstretched film was introduced into a longitudinal stretching machine consisting of multiple rolls with a diameter of 150 mm and a width of 700 mm at a transfer speed of 8 m/min, heated and adjusted to 50°C, and then the deformation speed was adjusted between the rolls with different circumferential speeds. Longitudinal stretching was carried out at 14,700%/min and a stretching ratio of 3.1 times, and the longitudinally stretched film was heated to 45°C by a group of rolls following the longitudinal stretching zone, and then stretched into a 1.5 m wide and 20 m long film. It was transferred to the horizontal stretching start position of a tenter type horizontal stretching machine. Next, both ends of the film were held with tenter clips, and while the temperature was raised from 60°C to 80°C, transverse stretching was performed at a deformation rate of about 3000%/min and a stretching ratio of 4.5 times. The film that has been horizontally stretched is then subjected to the first stage heat treatment at 130°C for 2 seconds while gripping both sides of the film, then cooled to 30°C, and once the film is gripped at both ends. Released. The released film passes through a guide roll and an expander roll until it has a width of 1.5 mm.
The film was then guided to another tenter with a length of 15 m, gripping both ends of the film again with tenter clips, and a second stage of heating at 200°C for 10 seconds with 10% relaxation in the width direction. After the heat treatment, the film was cooled to 30° C., released from the tenter clip, trimmed at both ends, and wound into a film approximately 15 μm thick and 1 m wide. Various properties of the thus obtained film were measured based on the methods described above. Measurement results first
Shown in the table. The obtained film had a very small bowing amount of 2.5%, and its physical properties were uniform along the width direction. Comparative Example 1 An unstretched film of the same type as used in Example 1 was biaxially stretched in the same manner as described in that example. After completing the horizontal stretching, the film was heat-treated at 200℃ for 10 seconds in the same tenter, then cooled to 30℃, released from the tenter clip, and trimmed on both sides. Winding ivy. Regarding the film thus obtained, Example 1
Various characteristics were measured in the same manner. The results are shown in Table 1. The obtained film has a bowing amount of 8.6%
The film was very large, and the center of the film was more flat than the edges. The physical properties along the width direction were also more non-uniform than the film obtained in Example 1. Although the method described in Comparative Example 1 omits the first heat treatment, cooling, and releasing the film from the tenter clips after cooling in the example described in Example 1, a good product can be obtained. It can be seen that these steps cannot be omitted in order to achieve this goal. Examples 2 and 3 In the example described in Example 1, the second step was carried out using the same procedure as in the same example, except that the heat treatment temperatures of the first and second stages were changed as shown in Table 1. A film was obtained which was axially stretched and heat treated. Table 1 shows the results of measuring the bowing amount of the obtained film. All of the obtained films had a small amount of bowing and were good. Comparative Examples 2 to 4 In the example described in Example 1, the second stage was carried out by the same procedure as in the same example, except that the heat treatment temperatures of the first stage and the second stage were changed as described in Table 1. A film was obtained which was axially stretched and heat treated. Table 1 shows the measurement results of the bowing amount of the obtained film and the observation results of the film appearance. The obtained film had poor appearance, film breakage during the second stage heat treatment, and a large amount of bowing, and it was not possible to produce a good product in any case. Example 4 An unstretched film of the same type as used in Example 1 was biaxially stretched in the same manner as described in that example. After completing the horizontal stretching, the film was subjected to the first heat treatment at 100°C for 2 seconds in the same tenter with 5% relaxation in the width direction, and then heated to 30°C. After cooling, both ends of the film were released. The released film passes through a guide roll and an expander roll, and the width of the film is 1.5 mm.
After guiding the film to another tenter with a length of 15 m and holding both ends of the film with tenter clips and performing a second heat treatment at 120°C for 2 seconds at a constant width, The film was cooled to 30°C, released from the tenter clip, trimmed at both ends, and wound into a film approximately 15μ thick and 1m wide. Various physical properties of the thus obtained film were measured based on the methods described above. Measurement results first
Shown in the table. The obtained film had a very small bowing amount of 2.4%, and its physical properties were uniform along the width direction. Further, the heat shrinkage rate was 29%, which is suitable for a heat-shrinkable film, and the difference in heat shrinkage rate in the diagonal direction was sufficiently small. Examples 5 and 6 In the example described in Example 4, the second heat treatment was carried out in the same manner as in the same example, except that the heat treatment temperatures of the first and second stages were changed as shown in Table 1. A film was obtained which was axially stretched and heat treated. Table 1 shows the first and second heat treatment temperatures as well as the results of measuring the amount of bowing of the obtained film. All of the obtained films had a small amount of bowing and were good films.

[Table] (Note) The - mark in the table indicates that no measurements were taken.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing a method for evaluating the flatness of a film obtained by the method of the present invention. In the figure, 1 is a table, 2 and 3 are base lines, 4 is an adhesive tape for fixing the film, 5 is a cutting line of the sample, and 6 is a marked line.

Claims (1)

[Claims] 1. A substantially amorphous and unoriented polyamide film is subjected to a roll-type longitudinal stretching method at a temperature of 45 to 65°C at a deformation rate of 10,000%/min or more. Stretch the film by ~3.5 times in the longitudinal direction, then hold both ends of the film with tenter clips.
The film temperature is 100℃ or less, and the average deformation rate is
The film is stretched 3 to 5 times in the transverse direction at a rate of 2,000 to 10,000%/min, and then the first heat treatment is performed at a temperature of 100 to 170°C while holding both ends of the film. Then, after the film was cooled to below the glass transition temperature of the raw material polyamide, the grips at both ends of the film were released, and then transferred to the next heat treatment zone, where both ends of the film were gripped with another tenter clip. A method for producing a biaxially oriented polyamide film, which comprises carrying out a second heat treatment under a temperature condition whose upper limit is 10° C. lower than the melting point of the raw material polyamide while being transported. 2. The biaxially oriented polyamide according to claim 1, wherein the second stage heat treatment is carried out in a temperature range with a lower limit of 190°C and an upper limit of a temperature 10°C lower than the melting point of the raw material polyamide. Film manufacturing method. 3. The method for producing a biaxially stretched polyamide film according to claim 1, characterized in that the second stage heat treatment is carried out within a temperature range of 100°C to 170°C.