JPH10235730A - Manufacture of biaxially oriented polyamide film - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a biaxially oriented polyamide film which has good dimensional stability and can reduce bowing which causes curling during bag making. SOLUTION: In a method for producing a biaxially oriented polyamide film by a tenter type simultaneous biaxial orientation method through a drawing process, a heat treatment process, and a heat relaxation process, the temperature is increased gradually from the heat treatment process to the relaxation process to make a temperature gradient so that the maximum temperature is in the relaxation process.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a biaxially stretched polyamide film having a small bowing amount and a uniform heat shrinkage in all directions of the film using a tenter type simultaneous biaxial stretching method. .

[0002]

2. Description of the Related Art Biaxially stretched polyamide films are widely used mainly as packaging materials because of their excellent mechanical properties, optical properties, gas barrier properties, impact resistance and pinhole resistance. When a biaxially stretched polyamide film is manufactured by a tenter type stretching method, a bowing phenomenon occurs due to a stress generated in the film at a boundary portion between a stretched portion and a heat-treated portion. That is, when a biaxially stretched film is manufactured by a tenter stretching method, the film passes through a preheating section, a stretching section, a heat treatment section, a relaxation section, and a cooling section. The stretching stress is maximized at the position where it reaches. At this time, in the tenter, since both ends of the film are gripped by clips, a phenomenon occurs in which the central portion of the film having low stress is drawn to the end of the stretched portion. And since the film central part is drawn toward the stretching part from the heat treatment part in the process after the stretching part, when a straight line drawn at right angles to the traveling direction of the film before entering the stretching machine comes out of the stretching machine. In this case, a bowing phenomenon occurs in which the central portion of the film draws an arc protruding in the direction opposite to the traveling direction of the film.
Due to this phenomenon, the direction perpendicular to the film traveling direction is
The film ends are stretched in an oblique direction toward the end. When such a film is placed in hot water or steam, it causes a curl phenomenon. Therefore, when the film is used for packaging such as boil sterilization and retort sterilization, there is a problem in bag making.

In order to reduce bowing, it is desirable to raise the film temperature during stretching to lower the stretching stress as much as possible and to lower the heat treatment temperature as much as possible. However, if the stretching temperature is too high, the strength of the film is reduced. If the heat treatment temperature is lowered, the dimensional stability is lowered.

[0004]

SUMMARY OF THE INVENTION An object of the present invention is to provide a method for producing a biaxially stretched polyamide film which has excellent dimensional stability and can reduce bowing which causes curling during bag making. Things.

[0005]

Means for Solving the Problems The inventors of the present invention have made intensive studies to solve such a problem, and as a result, a temperature gradient is provided from the heat treatment section to the relaxation section so that the maximum temperature is obtained in the relaxation section. As a result, the inventors have found that the bowing amount can be reduced, and have reached the present invention.

That is, the gist of the present invention is to provide a method of manufacturing a biaxially stretched polyamide film through stretching, heat treatment, and thermal relaxation (relaxation) by a tenter-type simultaneous biaxial stretching method. In the method for producing a biaxially stretched polyamide film, the temperature is gradually raised during the step, and a temperature gradient is applied so as to reach the maximum temperature in the relaxing step.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail.
As the polyamide in the present invention, nylon 6, nylon 66 and polyamides mainly composed of these can be used.
Is preferred.

The polyamide may contain known additives such as a stabilizer, an antioxidant, a filler, a lubricant, an antistatic agent, an antiblocking agent and a coloring agent.

In carrying out the method of the present invention, first, a polyamide is formed into a film by a T-die method by a conventional method to obtain a substantially amorphous unstretched film. Next, the unstretched film is subjected to a moisture absorption treatment, and then subjected to a short-time pre-heat treatment in a preheating section, and subsequently, a longitudinal direction (MD) and a transverse direction (TD).
And then heat-treated, relaxed, and cooled to obtain a biaxially stretched polyamide film.

In the present invention, the heat treatment section preferably has two or more zones, preferably three or more zones.
The reason for this is that by performing the heat treatment in multiple stages, the stress distribution inside the film from the stretched portion to the heat-treated portion is made uniform, and bowing is reduced.

In the present invention, bowing can be reduced by providing a temperature gradient that gradually raises the temperature from the heat treatment section to the relaxation section, so that the maximum temperature is reached in the relaxation section. The temperature of the heat treatment section and the temperature of the relaxation section immediately before the relaxation section may be the same, but if the temperature of the relaxation section is lower than the temperature of the processing section, the effect of reducing the bowing is reduced, and the omnidirectional heat shrinkage rate is reduced. It is difficult to provide a proper balance.

[0012] The maximum temperature of the relaxing portion must be lower than the melting point of the polyamide, and is preferably in the range of 205 to 215 ° C.

In the present invention, the thickness of the biaxially stretched polyamide film is 5 to 100 μm, usually 10 to 50 μm.

[0014]

Although the reason why the bowing amount of the biaxially stretched polyamide film obtained by the method of the present invention is small is not clear, the temperature is gradually increased in the heat treatment and the relaxation process from the end of the stretched portion where the stretching stress is maximized. This seems to be due to dispersion of the stretching stress.

[0015]

Next, the present invention will be described specifically with reference to examples. In addition, the measuring method used for evaluation of an Example and a comparative example is as follows.

1. Boeing A straight line is drawn at right angles to the direction of film advance (the width direction of the film) at the entrance of the stretching machine using an oil-based felt pen. Through the stretching and heat treatment steps, the MD deformation (mm) at the center of the straight line of the film coming out of the tenter was measured, and the bowing amount was calculated by the following equation. Boeing amount (%) = (deformation amount at film center / film width) x 100

2. Hot water shrinkage The film at the center of the film and at 40% in the width direction on both sides from the center (assuming the total width of the film as 100%) is reduced to 10 mm in the width direction and 100 mm in the length direction. The film was cut, and the dimension (L ₀ ) between the marked lines was accurately measured by a scanning microscope in an atmosphere of 20 ° C. × 65% RH. Then, the film was immersed in boiling water for 5 minutes, and the film pulled out of the boiling water was heated at 20 ° C. After reaching equilibrium after being left in an atmosphere of 65% RH, the dimension (L ₁ ) between the marked lines was measured and determined by the following equation. Hot water shrinkage (%) = [(L ₀ −L ₁ ) / L ₀ ] × 100

3. Oblique Difference in Hot Water Shrinkage Rate Under a 20 ° C. × 65% RH atmosphere, the difference between the hot water shrinkage rates in the directions of 45 ° and 135 ° oblique to the width direction of the film was measured.
The measurement sample is 10mm wide x length along the above diagonal direction.
Cut to 100mm size, boiled in hot water at 100 ° C for 5 minutes, then left at 20 ° C x 65% RH for 2 hours to measure the size, calculate the shrinkage relative to the size before processing, and The absolute value of the difference in the shrinkage was defined as the oblique difference in the hot water shrinkage. The measurement sample was sampled such that the center of the measurement sample in the length direction was the above-described position at the center of the film and at a position 40% apart on both sides in the width direction from the center.

Example 1 Relative viscosity 3.0 (1 g / dl in 25 ° C., 95% concentrated sulfuric acid), melting point 2
Nylon 6 (A1030BRF manufactured by Unitika) at 20 ° C
After being extruded into a sheet from a T-die with a width of 630 mm at 260 ° C with a φ extruder, it is rapidly cooled by being in close contact with a cooling roll with a surface temperature of 20 ° C, and has a 150 µm thickness and is substantially amorphous and not oriented. An unstretched film was obtained. The obtained unstretched film was immersed in warm water at 50 ° C. to adjust the moisture content of the film to 4%, and then simultaneously biaxially stretched at 195 ° C. at a magnification of 3.0 times for MD and 3.3 times for TD. And the respective temperatures of the first to third zones
Heat treatment is performed at 202 ° C., 206 ° C., and 211 ° C., respectively, and then relaxation treatment is performed at 212 ° C. for 2 seconds.
A biaxially stretched polyamide film having a thickness of 15 μm and a width of 1420 mm was obtained. About the obtained biaxially stretched polyamide film,
Table 1 shows the results of measuring the bowing amount, the hot water shrinkage, and the oblique difference in the hot water shrinkage. A film having a small difference in the bowing amount and the hot water shrinkage was obtained.

Example 2 The heat treatment temperatures of the first to third zones were 202 ° C. and 20 ° C., respectively.
A biaxially stretched polyamide film was obtained in the same manner as in Example 1 except that the temperature was 8 ° C., 211 ° C., and the relaxation temperature was 211 ° C. About the obtained biaxially stretched polyamide film,
Table 1 shows the results of measuring the bowing amount, the hot water shrinkage, and the oblique difference in the hot water shrinkage.

Comparative Example 1 The heat treatment temperatures of the first to third zones were 202 ° C. and 21 ° C., respectively.
A biaxially stretched polyamide film was obtained in the same manner as in Example 1 except that the temperature was 2 ° C., 203 ° C., and the relaxation temperature was 200 ° C. About the obtained biaxially stretched polyamide film,
Table 1 shows the results of measuring the bowing amount, the hot water shrinkage, and the oblique difference in the hot water shrinkage. The obtained biaxially stretched polyamide film had a large bowing amount.

Comparative Example 2 The heat treatment temperatures of the first to third zones were 202 ° C. and 20 ° C., respectively.
A biaxially stretched polyamide film was obtained in the same manner as in Example 1 except that the temperature was 8 ° C. and 212 ° C., and the relaxation temperature was 200 ° C. About the obtained biaxially stretched polyamide film,
Table 1 shows the results of measuring the bowing amount, the hot water shrinkage, and the oblique difference in the hot water shrinkage. Although the bowing amount was good, the heat shrinkage of MD was larger than TD, and the balance of hot water shrinkage was poor.

[0023]

[Table 1]

[0024]

According to the present invention, a biaxially stretched polyamide film having a uniform performance balance in the width direction can be produced without reducing the mechanical properties and reducing bowing which causes curling during bag making. It is possible to do. In addition, since the cost is not required in terms of equipment, its industrial value is great.

Claims (1)

[Claims] 1. A method of manufacturing a biaxially stretched polyamide film through a tenter-type simultaneous biaxial stretching method through stretching, heat treatment, and thermal relaxation (relaxation) steps, wherein the temperature is gradually increased from the heat treatment step to the relaxation step. A method for producing a biaxially stretched polyamide film, wherein a temperature gradient is set so as to reach a maximum temperature in a relaxing step.