JPH04125126A - Production of polyamide biaxially stretched film - Google Patents

Abstract

PURPOSE:To enhance flat properties by stretching and thermally-fixing a film and thereafter controlling temp. and moisture content in a slow cooling stage. CONSTITUTION:After a polyamide film is biaxially stretched, it is thermally fixed so that the shrinkage factor in hot water at 100 deg.C is regulated to <=3% in the longitudinal direction and the lateral direction. Thereafter while continuously blowing humidified air at <=150 deg.C, this film is slowly cooled at the rate of <=80 deg.C/sec. Simultaneously humidity conditioning is performed for the film and the moisture content of the film is regulated to 0.5-2%. Thereafter the polyamide biaxially stretched film is produced by winding the said film. In such a way, generation of a curl, a sagging and a wrinkle, etc., due to moisture absorption is reduced in the humudity-conditioned film.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a method for producing a biaxially stretched polyamide film, and provides a method for producing a film with excellent flatness.

(Prior Art) Taking a biaxially stretched polyester film as an example, it is known that the flatness of the stretched film can be improved by slowly cooling it after stretching and heat setting.

For example, Japanese Patent Publication No. 44-20000 discloses that when a heat-set biaxially oriented polyethylene terephthalate film is cooled from the heat-setting temperature, the temperature is 80 to 90°C and the temperature difference in the width direction is within an atmosphere of 10"C. , a method of cooling at a cooling rate slower than 70° C./sec while preventing shrinkage has been proposed.

The present inventors believe that the reason why the flatness is improved by slow cooling is as follows.

That is, even if the film is held at both ends with tenter clips after being stretched and heat-set and is kept flat in appearance, there are latent physical property differences in the width direction due to, for example, the bowing phenomenon. When the heat-set film is rapidly cooled, it separates from the clip without sufficiently relieving these latent physical property differences, especially shrinkage stress, resulting in a difference in the amount of shrinkage in the width direction, which impairs the flatness of the film. On the other hand, at the heat setting level, the shrinkage stress in the direction where the potential shrinkage is large is preferentially relaxed, so the flatness of the film after cooling is improved compared to the case of rapid cooling.

In other words, the effect of slowing down the cooling rate is considered to be the effect of extending these relaxation times.

(Problems to be Solved by the Invention) The present inventors have found that the same principle holds true in the case of biaxially stretched polyamide films. However, it has been found that control is more difficult than with polyester film, and reproducibility is more difficult to obtain than with polyester film. This is thought to be due to the fact that the polyamide film is highly sensitive to heat, is susceptible to the influence of ambient temperature, and is also affected by moisture. For example, even if the thermal history in the stretching machine is strictly controlled, the flatness may change due to the humidity in the cooling process and winding process. Moreover, this instability becomes more pronounced in films whose heat shrinkage rate is reduced by strengthening the heat setting conditions after stretching.

When we investigated the dimensional change due to moisture absorption in a biaxially stretched polyamide film, we found that the dimensional change is an energetic behavior, that is, elongation due to swelling, and an entropic behavior, that is, the structure relaxes due to moisture absorption and attempts to return to the state before stretching. I learned that it is a composite of the tendency to shrink due to moisture absorption.

The former is a reversible change, and the latter is an irreversible change.

A film with a weak heat setting level after stretching has a strong tendency to shrink due to moisture absorption, so the elongation due to swelling and the shrinkage due to relaxation of moisture absorption are offset, and dimensional changes due to moisture absorption may not be a problem in practice. However, films that have been strongly heat-set after being stretched (for example, films that have been subjected to relaxation heat treatment after being set at a high temperature), specifically, should have a shrinkage rate of 3% or less in both the vertical and horizontal directions when immersed in hot water at 100"C. The tendency of a film to shrink due to moisture absorption is weakened, so the elongation due to moisture absorption appears as it is.Moreover, moisture absorption elongation has directionality and tends to increase in the direction along the orientation axis.Therefore, due to the bowing phenomenon during stretching, In a film in which the main orientation direction changes in the film width direction, the flatness of the stretched film deteriorates because the amount and direction of moisture absorption elongation differ in the film width direction.

Biaxially stretched polyamide film has excellent heat resistance, strength, and gas barrier properties, so it is widely used as a food packaging material, but in order to take advantage of the properties of polyamide more effectively, it needs to be heat-treated after stretching. It is necessary.

As mentioned above, films with a shrinkage rate of 3% or less in 100°C hot water are packaging materials that can be boiled and retort sterilized after packaging food, so it is important to improve the flatness of such films. It was strongly desired.

An object of the present invention is to provide a method for producing a biaxially stretched polyamide film that is highly heat treated and has excellent flatness.

(Means for Solving the Problems) As a result of intensive studies to solve these problems, the present inventors have found that flatness can be improved by controlling the temperature and moisture content in the slow cooling process after stretching and heat setting. The present invention was developed based on the knowledge that an excellent polyamide biaxially stretched film can be obtained.

That is, in the present invention, after biaxially stretching a polyamide film, it is heat-set in hot water at 100°C so that the shrinkage ratio is 3% or less in both the vertical and horizontal directions, and then continuously stretched at a temperature of 150°C or less. 80°C/ while blowing humidified air.
This method of producing a biaxially stretched polyamide film is characterized in that the film is slowly cooled at a speed of less than a second, and at the same time the film is subjected to tAfW to bring the moisture content of the film to 0.5 to 2% before being wound up.

It is known that polyamide films are manufactured by simultaneous two-wheel stretching method and sequential two-wheel stretching method, but the method of the present invention is different from the method in which both ends of the film are gripped with heat-setting and cooling clips after stretching. can be applied.

Furthermore, when a treatment involving relaxation is performed in a part of the heat fixing section or the cooling section, the effect of the method of the present invention becomes greater. The heat treatment accompanied by relaxation is a method in which the crimp pitch in the opposing clip interval (horizontal direction) or the machine direction (vertical direction) is set slightly back from the maximum stretching ratio, and the film is heat treated while being loosened from the tensioned state. Since the relaxation heat treatment operation can lower the thermal (water) shrinkage rate of the stretched film without increasing the heat treatment temperature so much, it is possible to prevent the film from whitening or decreasing its strength during heat treatment. The method of the present invention is even more useful when the relaxation treatment in the horizontal direction, which is simple in terms of equipment, is effective, or when the relaxation heat treatment is performed on both longitudinal and horizontal wheels. The amount of relaxation is 2-1
0% is appropriate.

The film after heat treatment needs to be cooled to at least 100°C or less before being released from the clips, so the stretching machine is equipped with a cooling zone after the heat treatment zone, and air is blown at a temperature lower than that of the heat treatment zone. In the present invention, the cooling air in this cooling zone is humidified, and the film is simultaneously cooled and humidity-controlled in this zone.

In order to adjust the humidity of a film in a short time, it is necessary to devise ways to optimize the conditions for the temperature of the film and the humidity of the air in contact with the film.

The water absorption rate of a film is proportional to the product of the water adsorption concentration on the film surface, which is determined mostly by the relative humidity of the air in contact with the film, and the diffusion coefficient, which determines the speed at which water molecules move in the thickness direction of the film. Here, the diffusion coefficient increases as the film temperature increases. On the other hand, when trying to increase the relative humidity of the air, the lower the air temperature, the easier it is.

This is because the cooling zone is located at the exit of the drawing machine, and air tends to enter and exit from the outside air, so in order to maintain a high relative humidity of the air in this area, it is necessary to supply a large amount of moisture to the air. In addition, if such air is locally cooled, it may become frosty, causing water droplets to adhere to the film or cause rust to occur in the device. Therefore, the air temperature during the cooling and humidity conditioning process after heat fixation is at least 150°C.
Hereinafter, the temperature needs to be more preferably 130°C or less. Humidity control speed is hardly affected by air blowing speed, but cooling speed is affected by blowing air speed.

Therefore, in order to actively control the cooling curve and obtain uniformity in the width direction, a moderate wind speed, e.g.
It is desirable to blow air at a wind speed of 30 m/sec from both sides of the film.

The cooling rate of the film must be 80° C./second or less over the entire range from heat setting to winding.

For this purpose, for example, the cooling and humidity control zone can be divided into two parts, and the first half zone is cooled with air at 100 to 150"C, and the second half zone is blown with air at 50 to 100"C. The entire range of the cooling zone is efficiently passed through to achieve a favorable slow cooling curve.

The simultaneous use of the cooling zone as a humidity control zone is the most important feature of the process according to the invention, which improves the quality of the biaxially oriented polyamide film. For this purpose, the air blown onto the film in this zone is humidified.

The air humidification method can be carried out using any suitable method, such as blowing steam into the air distribution system, inserting a steam or mist generator into the system, or a combination of steam blowing and adiabatic cooling using a water shower. You can choose. Air humidification conditions are not particularly limited and need to be controlled according to film thickness, speed, and temperature. In this case, first select the air temperature in the cooling zone so that the film temperature cooling pattern is 80°C/second or less over the entire range of the cooling zone, then adjust the air temperature, film temperature, cooling time, and film thickness to The air humidification conditions are selected so that the moisture content of the film is in the range of 0.5 to 2% during winding. For example, when humidifying by steam blowing, the flow rate of the steam is controlled. By controlling the steam flow rate in feedback while actually measuring the film moisture content using, for example, an infrared absorption type non-contact moisture meter, it becomes possible to always control the film moisture content within a desired range regardless of the atmospheric humidity.

A suitable film moisture content is in the range of 0.5 to 2%. 0
．． If it is less than 5%, there is no effect of promoting relaxation due to moisture absorption or slowing down of subsequent dimensional changes due to moisture absorption, and 2
% or more is not only uneconomical because the time required for moisture absorption is too long, but also the slip property decreases due to moisture absorption, uneven moisture absorption occurs, and the uniformity of the film deteriorates.

The polyamides to which the method of the present invention is applied are ordinary film-forming polyamide resins, but the method of the present invention is particularly effective on poly-ε-capramide.

In addition, the method of the present invention involves immersing in hot water at 100°C for 15 minutes, and measuring the gauge line dimensions in the vertical and horizontal directions before and after immersion by 20°C.
It is applied to films that have been subjected to intense heat treatment so that the contraction rate due to immersion is 3% or less in both the vertical and horizontal directions when measured in an atmosphere of 165% RH at °C.

As a heat treatment method, the shrinkage rate in hot water is particularly reduced by performing a heat treatment that involves relaxation in the vertical, horizontal, and both axial directions, and in this case, the effect of the method of the present invention is particularly remarkable.

(Function) First of all, the film produced by the method of the present invention has improved flatness immediately after winding, and this effect is more stable than before, and is not affected by the weather or season, for example. This is thought to be because the moisture content of the film is actively controlled in the slow cooling step after heat setting, and the influence of atmospheric humidity is alleviated by the synergistic effect of the so-called thermal setting effect in the slow cooling step.

Next, the film produced by the method of the present invention, namely 0
．． A film that is moistened to 5 to 2% tljl and subjected to a thermal setting effect in the slow cooling process will be less susceptible to dimensional changes due to changes in atmospheric humidity under subsequent usage conditions. When moisture is gradually added to an absolutely dry biaxially stretched polyamide film, a large dimensional change often occurs at the initial stage of moisture absorption (0.5 to 2%). Therefore, if moisture within this range is applied in advance, subsequent dimensional changes will be alleviated.

For example, if a film roll wound in an absolutely dry state is left in a normal atmosphere, wrinkles will occur in the film roll due to moisture absorption and swelling, but the extent of wrinkles will be reduced in the film roll wound by the method of the present invention.

In addition, polyamide biaxially stretched film is generally used as a packaging material after undergoing post-processing such as printing, laminating, and bag making, but even during this process, dimensional changes due to moisture absorption tend to cause various problems. . for example,
These include curling of film during printing and laminating processes, curling of bag-made products, and problems when filling contents. Since the film produced by the method of the present invention has already been humidity-controlled and is less sensitive to changes in atmospheric humidity and film moisture content, problems caused by moisture absorption and dimensional changes during post-processing and final use are avoided. significantly reduced.

The present invention will be specifically explained below using comparative examples and examples.

In addition, the measurement was performed by the following method.

Moisture content: A sample of the film was taken from the roll immediately after winding, and the moisture content was measured using an infrared heating rapid moisture needle (Mo-1 model manufactured by Shimadzu Corporation).

Kei Aimoto: Cut out a strip of sample with marked lines in both the vertical direction (MD) and the horizontal direction (TD), and soak it in boiling water at 100"C for 10 minutes.
20℃, 65%R
)! , measured in an equilibrium state, and expressed as a percentage of the amount of shrinkage due to treatment with respect to the size of the treated area.

Sagging: Place the film between rolls arranged in parallel at a distance twice the width of the film, and apply a tension of 0.5 kg/ra uniformly across the entire width. , set a stainless steel scale that is longer than the full width of the film parallel to the rolls and slightly higher than the film surface in the center between the two rolls, and measure the distance from the scale to the film surface in the width direction IO.
The distance was measured, and the difference between the maximum and minimum distances was defined as the amount of walnuts.

Comparative Examples 1 to 5 In the latter half of the heat treatment section, there is a simultaneous relaxation heat treatment section of 5% in both MD and TD, and each length is 2 following the heat treatment section.
．． A poly-ε-capramide film having a thickness of 15 μm was produced at a speed of 130 m/win using a simultaneous two-wheel stretching machine equipped with a first cooling section and a second cooling section of 5 m and 1.5 m. In order to reduce the concentration of oligomers that evaporate from the film, fresh air of 30% of the circulating air volume is supplied and exhausted to the first cooling section and the second cooling section, and after the temperature of this clean air is controlled, it is passed through nozzles from the top and bottom of the film to the film. Being sprayed.

While changing the temperature of the cooling section sequentially and monitoring the film cooling rate with thermometers installed at the inlet and outlet of each cooling section, 4000 m of each film was manufactured.The moisture content and hot water shrinkage rate of the film immediately after winding were measured. , Kurji was measured. The results are shown in Table 1.

Comparative Example 6 From the results of Comparative Examples 1 to 5, the conditions of Comparative Example 3 were selected and continuous production was carried out for about one month under the same conditions. Films were sampled every 2 or 3 days, and the range of variation in quality was investigated, and the following results were obtained.

Moisture content 0.4% or less Hot water shrinkage MD 2.3-2.5% TD 2.4
~2.6% Curzi 15-40 mm Examples 1 to 3 Films were manufactured under the same conditions as Comparative Example 3, except that steam was blown into the fan suction parts of the first cooling section and the second cooling section. 4 different types of films with different steam flow rates
000s+ was produced and the film quality was measured immediately after winding. The results are shown in Table 1.

Example 4 Continuous production was carried out for about one month under the same conditions as in Example 2, and the film was sampled every 2 to 3 days to evaluate its quality 5 times.

Moisture content 0.9-1.2% Hot water shrinkage MD 2.0-2.2% TD 2.0
~2.2% Curzi 10-20 mm (Effects of the Invention) As is clear from the results of Comparative Examples and Examples, the method of the present invention improves the hot water shrinkage rate and Curzi, and also stabilizes its quality.

In addition, the appearance of the film controlled by the method of the present invention is improved because small wrinkles are less likely to occur on the surface of the roll, and it also has an improved appearance, which has previously been a problem in post-processing processes such as printing, laminating, bag making, and filling. Curls and curls due to moisture absorption,
It is possible to reduce the occurrence of wrinkles and the like.

Patent applicant: Unitika Co., Ltd.

Claims (2)

[Claims] (1) After biaxially stretching the polyamide film, at 100°C
The film is heat-set so that the shrinkage rate in hot water is 3% or less in both the vertical and horizontal directions, and then the film is removed at a speed of 80°C/sec or less while continuously blowing humidified air at a temperature of 150°C or less. A method for producing a biaxially stretched polyamide film, which comprises cooling and simultaneously controlling the humidity of the film to bring the moisture content of the film to 0.5 to 2% before winding it up. (2) A claim characterized in that the heat setting after biaxial stretching is a heat treatment accompanied by relaxation in both the horizontal and vertical directions
1) Method for producing the film described above.