JPH07290565A - Production of biaxially oriented polyamide film - Google Patents

Abstract

PURPOSE:To stretch a polyamide film reduced in thickness irregularity and lowered in the oblique difference of a boiling water shrinkage factor without losing strength without generating breakage in the production of a biaxially oriented polyamide film. CONSTITUTION:In a method for producing a biaxially oriented polyamide film by the sequential biaxial stretching of a polyamide film obtained by longitudinally stretching a substantially non-oriented polyamide sheet based on polyamide and subsequently laterally stretching the same by 3 times or more, longitudinal stretching is performed within a temp. range from glass transition temp. (Tg)+20 deg.C to low temp. crystallization temp. (Tc)+20 deg.C in two front and rear stages so that synthetic stretching magnification becomes 3.1-3.8 times.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a method for producing a biaxially oriented polyamide film by successive longitudinal and transverse stretching.

[0002]

2. Description of the Related Art Conventionally, biaxially oriented polyamide films are
Toughness, high gas barrier property, anti-pinhole property, transparency,
From various characteristics such as easy printability, it is widely used as a bag packaging material for liquid foods such as soup, konjac, hamburger steak, miso, and ham, seafood foods, frozen foods, retort foods, pasty foods, meat and seafood products. ing. Generally, as a method for producing a biaxially oriented film, a longitudinal-transverse sequential biaxial stretching method is known, and it is also used for polyamide. However, it is known that in the longitudinal and transverse sequential biaxial stretching method, physical property distribution is likely to occur in the film width direction. Polyamide film used as a packaging material has physical properties in the width direction, for example, if the distribution of the oblique difference in boiling water shrinkage is large, it causes a twisting phenomenon in heat treatment after bag making, which causes a serious trouble. There was a problem. In order to avoid this problem, the longitudinal stretching ratio may be reduced, but another problem of impairing the strength in the machine direction occurs, and it also leads to a decrease in production speed, which is not preferable in industrial production.

[0003]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a method for producing a biaxially oriented polyamide film, which reduces the difference in the physical properties of the film in the width direction in the successive biaxial stretching method. Specifically, it is an object of the present invention to provide a longitudinal stretching method that reduces the distribution in the film width direction of physical properties such as an oblique difference in boiling water shrinkage ratio without reducing the longitudinal stretching ratio.

[0004]

In view of the above problems, the present inventors finally arrived at the present invention as a result of earnest research. That is, in a sequential biaxial stretching method of a polyamide film obtained by stretching a substantially unoriented polyamide sheet containing polyamide as a main component in the machine direction, and subsequently stretching it in the transverse direction by 3 times or more, Glass transition temperature (Tg) +
At a temperature of 20 ° C. or higher and a low temperature crystallization temperature (Tc) + 20 ° C. or lower, the total draw ratio is 3.1 in two stages, the former stage and the latter stage.
It is a method for producing a biaxially oriented polyamide film, characterized in that the longitudinal stretching is carried out so as to obtain a stretch ratio of up to 3.8 times, and preferably between the front stage and the rear stage without cooling to Tg or lower.

The polyamide used in the present invention is preferably nylon 6 having a relative viscosity of 2 to 3.5, and nylon 6 containing hexamethylenediamine and adipic acid or isophthalic acid or a metaxylylenediamine and adipine. Examples thereof include a copolymer obtained by copolymerizing a small amount of a nylon salt with an acid and / or a blend. Further, these polyamides may contain a small amount of various substances such as anti-blocking agents, antistatic agents, stabilizers, etc. within a range that does not impair their properties.

The present invention is a method for producing a biaxially oriented polyamide film, which comprises stretching a substantially non-oriented polyamide sheet in two stages in the longitudinal direction, subsequently stretching in the transverse direction, and further heat fixing. That is, when the substantially unoriented polyamide sheet is longitudinally stretched, the first stage is stretched, and the second stage is continuously stretched without cooling to Tg or less, and then 3 times or more, preferably Is a method for producing a biaxially oriented polyamide film, which comprises laterally stretching at a draw ratio of 3.5 times or more and further heat fixing. For these longitudinal stretching,
Known longitudinal stretching methods such as hot roll stretching and infrared radiation stretching may be used. Hereinafter, the method for producing the biaxially oriented polyamide film according to the present invention will be described in detail. First, the polyamide raw material is dried, then melt-extruded by an extruder, cast on a rotary drum from a die and rapidly solidified to obtain a polyamide sheet. This polyamide sheet is substantially unoriented.

This sheet is first subjected to Tg + 20 ° C. or above, Tc
The first stage drawing is performed 1.1 to 3.0 times at a temperature of + 20 ° C. or less. If it is 1.1 or less, the stretching effect does not appear, and 3.
If it exceeds 0, oriented crystallization proceeds, and the stretching stress in the second-stage stretching, which will be described later, increases, causing breakage, or it leads to fracture in transverse stretching, which is not preferable. More preferably,
It is 1.5 to 2.5 times. If the stretching temperature is less than Tg + 20 ° C., necking is likely to occur, and thickness unevenness tends to increase,
If it exceeds Tc + 20 ° C., thermal crystallization proceeds, and breakage is likely to occur in transverse stretching, which is not preferable. More preferably, Tg +
It is 30-Tc + 10 degreeC. Here, the low temperature crystallization temperature is a temperature at which a substantially amorphous polyamide sheet is crystallized in the ascending process.

After the first stage stretching, the second stage stretching is continued, and one of the features of the present invention is how the sheet temperature during that period is set. That is, the temperature is not kept forcibly cooled but kept warm, and is also used for preheating of the second stretching or heating for stretching. If it is forcibly cooled and then reheated for the second stretching, thermal crystallization remarkably progresses, the transverse stretching stress increases, and breakage frequently occurs, which is not preferable. Thermal crystallization also progresses in this heating and heat retaining section, but it is much slower than the above-mentioned forced cooling and reheating and does not pose a practical problem. Next, this sheet is second-stage stretched so that the total longitudinal stretching ratio is 3.1 to 3.8 times.
When it is less than 3.1 times, the physical property distribution in the width direction of the biaxially oriented film becomes small, but the longitudinal strength becomes small,
When it exceeds 3.8 times, the effect of reducing the physical property distribution in the width direction of the biaxially oriented film is not exhibited, which is not preferable. More preferably,
It is 3.3 to 3.6 times. The stretching temperature at this time is Tg.
It is + 20 ° C to Tc + 20 ° C. If it is less than Tg + 20 ° C., the stretching stress becomes high, and it tends to be broken by transverse stretching, and Tc +
If the temperature exceeds 20 ° C, the thickness unevenness becomes large, which is not preferable. More preferably, it is Tg + 25 ° C to Tc + 10 ° C. The uniaxially oriented film thus obtained is heated at a temperature of 100 ° C. to a temperature below the melting point using a stenter, preferably 100+
It is transversely stretched at 180 ° C. three times or more, preferably 3.5 times or more, then heat set and wound. If the stretching temperature is too low, the transverse stretchability deteriorates (breakage occurs), and if it is too high, the thickness unevenness deteriorates. If the stretching ratio for transverse stretching is not more than 3 times, the strength in the transverse direction will be low.

In this way, the longitudinal stretching is divided into two stages, the first
After stretching, without cooling to below Tg, the second
The reason why a biaxially oriented polyamide film having a small difference in physical properties in the width direction can be obtained by performing stretching, then transverse stretching and heat setting is that the stretching stress can be reduced by dividing the longitudinal stretching into two stages. First, by heating and maintaining the temperature between the first stretching and the second stretching, the crystallization promoting action due to the hydrogen bond peculiar to polyamide which occurs at the time of forced cooling to reheating is prevented, and further the sheet orientation relaxation action after the first stage stretching is performed. It is considered that the structure of the uniaxially oriented film before drawing and transverse stretching was made gradual, which facilitated the formation of transverse orientation that develops during transverse stretching, and further improved the stretchability by reducing the transverse stretching stress.

Examples [0010] Hereinafter, the present invention will be described in detail based on examples, but it goes without saying that the present patent is not limited to this method. The film temperature, physical properties and characteristics used in Examples and Comparative Examples are measured and defined as follows. (1) Glass transition temperature (Tg) and low temperature crystallization temperature (T
c) Freezing the unoriented polyamide sheet in liquid nitrogen, decompressing it under reduced pressure, and using a DSC manufactured by Seiko Denshi, a temperature rising rate of 10 ° C. /
Measured in minutes.

(2) Film temperature The temperature in the longitudinal stretching is the radiation thermometer I manufactured by Minolta Co., Ltd.
The temperature of the film was measured using R-004.

(3) Thickness unevenness The biaxially oriented polyamide film is cut into strips of 1 m × 5 cm each in the longitudinal direction and the transverse direction, and the thickness shape is measured using a thickness meter K306C manufactured by Anritsu Electric Co., Ltd. 1 according to the following formula
Thickness unevenness per m was calculated, and this was repeated 5 times to obtain a thickness unevenness. Thickness unevenness = (maximum thickness-minimum thickness) / average thickness x 100
(%)

(4) Boiling water shrinkage ratio and oblique difference of boiling water shrinkage ratio A biaxially oriented polyamide film is cut out into 21 cm squares from the central portion of the entire width and the position (end portion) of 40% of the entire width from the center to the left and right to make samples. . Draw a circle with a diameter of 20 cm centered on the center of each sample and draw a straight line through the center of the circle in the directions of 0 °, 45 °, 90 ° and 135 ° when the vertical direction is 0 °. Measure the diameter and use it as the length before treatment. 3 this sample in boiling water
After heat treatment for 0 minutes, it is taken out, the water adhering to the surface is removed, and air-dried. After air drying, measure the diameter in each direction,
It shall be the length after processing. The boiling water shrinkage is calculated using the following formula. Boiling water shrinkage = (length before treatment−length after treatment) / length before treatment × 100 (%) The difference between the boiling water shrinkages at 45 ° and 135 ° when the longitudinal direction is 0 °. The absolute value was obtained, and the average value at both ends was taken as the boiling water shrinkage diagonal difference.

(5) Breaking strength (Kg / mm ² ) Tensiron UMT-II-500 manufactured by Toyo Baldwin was used and measured at a temperature of 23 ° C. and a relative humidity of 65%. The shape of the sample is 15 cm in length and 1 cm in width, the distance between chucks is 10 cm, and the pulling speed is 10 c.
m / min.

(6) Situation of film formation: Biaxial stretching was successively conducted under the same conditions for 2 hours, and the number of breaks was examined.

Example 1 Nylon 6 pellets (RV = 2.8) were vacuum dried, fed to an extruder, melted at 260 ° C., extruded into a sheet form from a T-type die, and a high DC voltage was applied. And electrostatically adhered onto a cooling roll and cooled to solidify to obtain an unoriented sheet having a thickness of 200 μm. This sheet had Tg of 40 ° C and Tc of 68 ° C. This sheet was first stretched 1.7 times at a stretching temperature of 75 ° C. and then second warm stretched at a stretching temperature of 70 ° C. while keeping the temperature at 70 ° C. so that the total stretching ratio was 3.4 times. Continuously, this sheet is continuously introduced into a stenter, transversely stretched 4 times at 130 ° C, and 210 ° C.
After heat setting at 5% and transverse relaxation treatment at 5%, cool down,
Both edges were cut and removed to obtain a biaxially oriented polyamide film. Table 1 shows the film-forming conditions, physical properties and characteristics of the film at this time.

Comparative Example 1 A biaxially oriented polyamide film was obtained in the same manner as in Example 1, except that the longitudinal stretching was carried out in one step at a temperature of 65 ° C. in a ratio of 3.4.

Comparative Example 2 The same procedure as in Example 1 was carried out except that the first longitudinal stretching was followed by rapid cooling to 35 ° C. and reheating for the second stretching.

Examples 2 to 5 and Comparative Examples 3 to 6 Biaxially oriented polyamide films were obtained in the same manner as in Example 1 except that the longitudinal stretching ratio was adjusted as shown in Table 2.

Comparative Examples 7 to 10 Biaxially oriented polyamide films were obtained in the same manner as in Example 1 except that the longitudinal stretching temperature was set as shown in Table 3.

[0021]

[Table 1]

[0022]

[Table 2]

[0023]

[Table 3]

[0024]

According to the manufacturing method of the present invention, without breakage,
The thickness unevenness is small, and the oblique difference in the boiling water shrinkage can be reduced without impairing the strength, which is extremely effective for a method for producing a biaxially oriented polyamide film by sequential longitudinal and transverse stretching.

Claims (4)

[Claims] 1. A method for sequentially biaxially stretching a polyamide film obtained by stretching a substantially unoriented polyamide sheet containing polyamide as a main component in a longitudinal direction, and subsequently stretching it in a transverse direction by 3 times or more, Longitudinal stretching: glass transition temperature (Tg) + 20 ° C. or higher, low temperature crystallization temperature (Tc) +20
A method for producing a biaxially oriented polyamide film, which comprises performing longitudinal stretching at a temperature of not more than 0 ° C. in two stages of a front stage and a rear stage so as to obtain a total stretching ratio of 3.1 to 3.8 times. 2. A method for producing a biaxially oriented polyamide film, wherein the polyamide according to claim 1 is nylon 6 having a relative viscosity of 2 to 3.5. 3. The longitudinal stretching ratio according to claim 1, which is 3.
It is 0 or less, The manufacturing method of the biaxially oriented polyamide film characterized by the above-mentioned. 4. A method for producing a biaxially oriented polyamide film, wherein the space between the front stage and the rear stage in the longitudinal stretching according to claim 1 is not cooled below Tg.