JPH08197620A - Manufacture of biaxially oriented polyamide film - Google Patents

Abstract

PURPOSE: To reduce irregularity of physical property in the cross direction of a film to be generated in a lateral stretch process by a sequential biaxial stretch method of polyamide film wherein longitudinal stretching is performed at a specified temperature by dividing into two steps of before and after steps, and longitudinal stretching at the latter step is performed by sticking and stretching a film on a roll. CONSTITUTION: When a biaxially oriented polyamide film is to be manufactured, a polyamide material is dried, melted and extruded by an extruder, cast on a rotary drum from a mouthpiece, quenched and solidified so as to obtain a polyamide sheet. In a manner that the maximum value of heat contractive stress in the longitudinal direction at the central part of a film after longitudinal stretching becomes 0.6kg/mm<2> or below, a before step longitudinal stretching of a sheet is firstly performed at a temperature of (glass transition temperature of material polyamide +20 deg.C) or higher and a temperature of (low temperature crystallization temperature of material polyamide +20 deg.C) or lower. Next, an after step longitudinal stretching is performed by using a heat roll. In the after step longitudinal stretching, stretching is performed by sticking the film to a stretching roll for 0.01-0.70sec.

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 a longitudinal and transverse sequential biaxial stretching method.
More specifically, it relates to a method for producing a biaxially oriented polyamide film having uniform physical, chemical and physicochemical properties in the width direction.

[0002]

2. Description of the Related Art Biaxially oriented polyamide films have hitherto been excellent in various properties such as toughness, high gas barrier property, anti-pinhole property, transparency, and easy printability.
It is widely used as a bag packaging material for liquid foods such as konjac, hamburger steak, miso, and ham, aquatic foods, frozen foods, retort foods, pasty foods, livestock meat and seafood. In general, a longitudinal and transverse sequential biaxial stretching method is known as a method for producing a biaxially oriented film, and this is also used for a polyamide film.

[0003]

However, it is known that the longitudinal-transverse sequential biaxial stretching method tends to cause variations in the physical properties in the film width direction. The reason for this is that in the tenter in the transverse stretching step, longitudinal stress due to transverse stretching and contraction stress in the longitudinal direction are generated by heat. By the way, while both ends of the film are gripped and constrained by clips, the central part of the film is weak in the influence of the gripping means and the constraining force is weak. Therefore, due to the influence of the above-mentioned stress, the traveling speed of the central portion of the film is delayed with respect to the both end portions gripped by the clip as compared with the both end portions, so that the physical properties vary in the width direction. Polyamide film used for bag packaging material has physical properties in the width direction, for example, if there is a large variation such as an oblique difference in boiling water shrinkage ratio, it causes a twist phenomenon in heat treatment after bag making and causes a serious trouble. Become.

In order to avoid this problem, it is effective to reduce the longitudinal stretching ratio to reduce the above-mentioned stress, but not only another problem of impairing the strength in the machine direction occurs, but also the production. This leads to a decrease in speed, which is not preferable in industrial production.

An object of the present invention is to provide a method for producing a biaxially oriented polyamide film, which reduces the variation in the physical properties in the width direction of the film which occurs in the transverse stretching step in the successive biaxial stretching method. More specifically, it is an object of the present invention to provide a stretching method that reduces variations in physical properties such as an oblique difference in boiling water shrinkage in the film width direction without reducing the production rate and the longitudinal stretching ratio.

[0006]

In view of the above problems, the present inventor
As a result of their earnest research, they finally reached the present invention.
Was. That is, the present invention is a substantially unoriented polyamide sheet.
Obtained by stretching the sheet in the machine direction and then in the transverse direction.
In the sequential biaxial stretching method of a polyamide film
Maximum value of heat shrinkage stress in the longitudinal direction of the center of the stretched film
Is 0.6 kg / mm ²Longitudinal stretching is
Liamide glass transition temperature (Tg) +20] ° C. or higher,
[Low temperature crystallization temperature (Tc) of polyamide +20] ° C or less
At the temperature of, the total longitudinal stretching ratio is divided into two stages, the first stage and the second stage.
3.0 times or more and 4.0 times or less and longitudinal stretching in the latter stage
Roll the film for 0.01 seconds or more and 0.70 seconds or less
Biaxially oriented polyamide characterized by being stretched in close contact
It relates to a method for producing a film.

The method of the present invention is applied to the production of a polyamide film, and is particularly suitable for producing a biaxially oriented film from a polyamide containing nylon 6 as a main component. As the polyamide, for example, nylon 6, a copolymer obtained by copolymerizing nylon 6 with a small amount of nylon salt, nylon 6
And a blend of nylon salt and the like. Examples of the nylon salt include a nylon salt of hexamethylenediamine and adipic acid or isophthalic acid, a nylon salt of metaxylylenediamine and adipic acid, and the like.
The polyamide may contain a small amount of known additives such as various anti-blocking agents, antistatic agents, and stabilizers, as long as the properties thereof are not impaired.

According to the present invention, a biaxially oriented polyamide film is obtained by vertically stretching a substantially unoriented polyamide sheet in two stages, subsequently stretching it in the transverse direction, and further thermally fixing it. More specifically, in longitudinally stretching a substantially unoriented polyamide sheet, the longitudinal stretching is divided into two stages,
The stretching ratio and stretching temperature of the first and second stages of the longitudinal stretching are adjusted so that the maximum value of the heat shrinkage stress in the longitudinal direction of the central portion of the film after the longitudinal stretching is 0.6 kg / mm ² or less. The film is stretched by adhering the film to a stretching roll for longitudinal stretching in the second stage for a certain period of time. Then, it is laterally stretched and further heat set.

The method for producing the biaxially oriented polyamide film according to the present invention will be described in detail below. After drying the above polyamide raw material, it is melt extruded by an extruder, cast on a rotary drum from a die and rapidly cooled and solidified to obtain a polyamide sheet. This polyamide sheet is substantially unoriented.

This sheet is first longitudinally stretched at a temperature of [glass transition temperature (Tg) +20] ° C. of the starting polyamide or more and low temperature crystallization temperature (Tc) +20] ° C. of the starting polyamide or less. . Here, the low temperature crystallization temperature (T
c) is a crystallization temperature caused by heating from a glass state. The longitudinal stretching is (Tg of raw material polyamide + 20) ° C.
If the temperature is lower than the range, necking is likely to occur and thickness unevenness tends to increase. On the other hand, (raw material polyamide Tc + 20)
If stretching is carried out at a temperature above 0 ° C, thermal crystallization proceeds,
It is not preferable because it is easily broken by transverse stretching. A more preferable stretching temperature is (Tg + 30 of raw polyamide) to (Tc + 10) of raw polyamide. If the stretching ratio (running speed of the film after stretching / running speed of the film before stretching) in the first longitudinal stretching is too low, the stretching effect cannot be obtained. Conversely, if it is too high, oriented crystallization proceeds. However, the thickness unevenness is likely to increase, and the stretching stress in the second-stage stretching, which will be described later, becomes too high, leading to breakage, or it leads to fracture in transverse stretching. Further, the maximum value of the heat shrinkage stress in the longitudinal direction of the central portion of the film after longitudinal stretching, which will be described later, is 0.6 kg / mm.
^When it exceeds ² , the variation in the physical properties of the biaxially oriented film in the width direction becomes large. From this viewpoint, the stretching ratio in the first longitudinal stretching is preferably 1.1 to 2.9 times. A more preferable stretching ratio is 1.5 to 2.5 times. For the first-stage longitudinal stretching, known longitudinal stretching methods such as hot roll stretching and infrared radiation stretching can be used.

After the first stage stretching, the second stage stretching is continued. Hot roll stretching is used for the second-stage longitudinal stretching. The film is stretched to a stretching roll by 0.0 in the second longitudinal stretching.
It is one of the features of the present invention that the film is adhered and stretched for 1 second or more and 0.70 seconds or less. In the second stage stretching, the time for bringing the film into close contact with the stretching roll is 0.70.
If they are brought into close contact with each other for longer than a second, thermal crystallization proceeds, transverse stretching stress increases, and breakage frequently occurs. Further, the thickness unevenness in the transverse direction is also deteriorated, and the maximum value of the heat shrinkage stress in the longitudinal direction of the central portion of the film after longitudinal stretching exceeds 0.6 kg / mm ² , and 2
The variation in the physical properties of the axially oriented film in the width direction becomes large. On the other hand, when the adhesion time is shorter than 0.01 seconds, oriented crystallization proceeds, lateral stretching stress increases, and breakage frequently occurs. Further stretching starting point becomes unstable in vertical and horizontal directions, vertical and horizontal directions of the thickness unevenness even worse, and the maximum value of the central portion longitudinal thermal shrinkage stress of the film after longitudinal stretching exceeds the 0.6 kg / mm ^2, 2 The variation in the physical properties of the axially oriented film in the width direction becomes large. The preferred adhesion time is 0.10 seconds or more and 0.30
It is less than a second. The contact stretching is performed by considering geometrical roll arrangement, roll diameter, material, and roughness parameters. For example, by using a roll material having a small releasability from the film, the roll surface roughness is reduced, the distance between the rolls is shortened, and the roll diameter is increased, thereby facilitating close contact stretching.

The stretching ratio of the second longitudinal stretching is adjusted so that the total longitudinal stretching ratio is 3.0 times or more and 4.0 times or less. When it is less than 3.0 times, the variation in the physical properties of the biaxially oriented film in the width direction becomes small, but the strength in the longitudinal direction becomes small. If the longitudinal stretching ratio exceeds 4.0 times, the effect of reducing the variation in the physical properties of the biaxially oriented film in the width direction may not be exhibited. Furthermore, the maximum value of the heat shrinkage stress in the longitudinal direction of the central portion of the film after longitudinal stretching is 0.6 kg.
/ Mm ² , the variation in the physical properties of the biaxially oriented film in the width direction tends to increase. Taking this into consideration, the preferable total longitudinal stretching ratio is 3.0 to 3.8 times, and more preferably 3.3 to 3.6 times.

The stretching temperature in the second-stage longitudinal stretching is also (Tg + 20 of raw polyamide) to (Tc + of raw polyamide).
20) ° C. The stretching temperature is (Tg of the raw material polyamide
If it is less than +20) ° C., the stretching stress becomes high and the transverse stretching tends to cause breakage. On the other hand (Tc of raw polyamide + 20) ° C
When it exceeds, the thickness unevenness becomes large. More preferably,
(Tg of raw polyamide + 25) ° C. to (Tc of raw polyamide + 10) ° C.

The maximum value of the heat shrinkage stress in the longitudinal direction of the central portion of the uniaxially oriented film thus obtained is 0.6 kg /
mm ² or less. The heat shrinkage stress is the stress per unit cross-sectional area at the time of shrinkage generated by heating. When the maximum value of the heat shrinkage stress exceeds 0.6 kg / mm ² , the variation in the physical properties of the biaxially oriented film in the width direction becomes large. It is preferably 0.40 kg / mm ² or less. In this case, sampling is performed using a winder immediately after uniaxial stretching and immediately before the stenter.

The resulting uniaxially oriented film is then stretched in the transverse direction using a stenter. Here, if the transverse stretching temperature is too low, the transverse stretching property may be deteriorated (breakage may occur), while if it is too high, unevenness in thickness tends to increase. From such a point, the transverse stretching temperature is preferably 100 ° C to a temperature lower than the melting point, and more preferably 100 ° C to 180 ° C. Further, from the viewpoint of securing the strength in the transverse direction, the transverse stretching ratio is 3.
It is preferably 0 times or more, more preferably 3.5 times or more.
The biaxially oriented film thus obtained is heat set and wound.

As described above, according to the method of the present invention,
The longitudinal stretching is performed in two stages, and in the second stage stretching,
The film is brought into close contact with a roll for 0.01 to 0.70 seconds and stretched, and the maximum value of the heat shrinkage stress in the longitudinal direction of the central portion of the film after the longitudinal stretching is set to 0.6 kg / mm ² or less to obtain a width. It is possible to obtain a biaxially oriented polyamide film with little variation in the physical properties in the direction. The reason is not only the effect of reducing the stretching stress by dividing the longitudinal stretching into two stages, but also the stretching stress generated in the first stage stretching by contacting the roll for a specific time in the second stage stretching. This is because it has the effect of alleviating. Furthermore, by reducing the maximum value of the heat shrinkage stress in the longitudinal direction of the central part of the film longitudinally stretched under specific conditions to 0.6 kg / mm ² or less, the heat shrinkage stress in the longitudinal direction generated during transverse stretching is reduced. This is because it becomes possible to facilitate the formation of orientation in the lateral direction during transverse stretching, and the developed lateral orientation can be treated without distortion even during heat setting. Moreover, it is considered that the stretchability is improved by reducing the transverse stretching stress.

[0017]

EXAMPLES Hereinafter, the present invention will be described in detail based on examples, but it goes without saying that the present invention is not limited to the following examples. The film temperature, physical properties and characteristics used in Examples and Comparative Examples are measured and defined as follows.

Glass transition temperature (Tg) and low temperature crystallization temperature (Tc) An unoriented polyamide sheet was frozen in liquid nitrogen, thawed under reduced pressure, and then using a DSC manufactured by Seiko Denshi, a heating rate of 10 ° C. /
Measured in minutes.

Heat shrinkage stress A uniaxially oriented film after two-stage lengthwise stretching is treated at 30 ° C. × 1.0 m
Dry in a vacuum dryer adjusted to mHg or less for 6 hours or more, take it out, and immediately use a desiccator for 20 hours.
After adjusting for 24 hours or more under the environment of ℃ × 30% RH, it was measured at a temperature rising rate of 10 ℃ / min using SSC 5020 of TMA manufactured by Seiko Denshi Kogyo.

Film temperature (stretching temperature) The temperature in the longitudinal stretching is a radiation thermometer I manufactured by Minolta Co., Ltd.
The temperature of the film was measured using R-004. Regarding the temperature in the transverse stretching, the temperature of the film was measured using a radiation thermometer RHP3 manufactured by Raytec Japan Co., Ltd.

Thickness variation A 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. Calculate thickness unevenness per 1m by the following formula, repeat this 5 times,
The average value was defined as thickness unevenness.

[0022]

[Equation 1]

Diagonal Difference in Boiling Water Shrinkage Percentage A biaxially oriented polyamide film is cut out from the center of the entire width to the left and right at a position (edge) of 40% of the entire width into 21 cm squares, and used as samples. Draw a circle with a diameter of 20 cm around the center of each sample, draw a straight line through the center of the circle in the 45 ° and 135 ° directions when the vertical direction is 0 °, measure the diameter in each direction, and process. Let it be the previous length. This sample is heat-treated in boiling water for 30 minutes and then taken out to remove water adhering to the surface and air-dry. After air drying,
The diameter in each direction is measured and used as the length after treatment. The boiling water shrinkage is calculated using the following formula.

[0024]

[Equation 2]

45 ° and 135 when the vertical direction is 0 °
The absolute value of the difference in boiling water shrinkage in the ° direction was determined, and the average value at both ends was used as the diagonal difference in boiling water shrinkage.

Film-forming condition The film was sequentially biaxially stretched for 2 hours under the same conditions as described in Example 1. Immediately after the film was broken during that time, the film was formed and stretched, and the number of breaks was examined.

Example 1 Nylon 6 pellets [relative viscosity (RV) = 2.8] were vacuum dried, then fed to an extruder and melted at 260 ° C., extruded into a sheet form from a T-type die, and a high DC voltage was applied. A voltage was applied to electrostatically adhere to a cooling roll, and the mixture was cooled and solidified 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 is first preheated at a temperature of 50 ° C., then at a stretching temperature of 77 ° C., the first longitudinal stretching is performed 1.7 times, and then the second stretching is performed.
Adhere to the second drawing roll for 0.20 seconds and draw temperature 72 ° C
The longitudinal stretching of the second step was performed so that the total stretching ratio was 3.4 times, and subsequently this sheet was continuously introduced into a stenter, horizontally stretched at 4 times at 130 ° C, and heat set at 210 ° C. After subjecting to 5% lateral relaxation treatment, it was cooled and 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.

Example 2 A biaxially oriented polyamide film was obtained in the same manner as in Example 1 except that the total stretching ratio in longitudinal stretching was 3.1.

Example 3 A biaxially oriented polyamide film was obtained in the same manner as in Example 1, except that the total stretching ratio in longitudinal stretching was 3.7.

Example 4 The same procedure as in Example 1 was carried out except that the arrangement of the second-stage stretching roll in the longitudinal stretching and the cooling roll after that was changed, and the second-stage stretching roll was brought into close contact for 0.05 seconds. As a result, a biaxially oriented polyamide film was obtained.

Example 5 The same procedure as in Example 1 was performed except that the arrangement of the second stretching roll in the longitudinal stretching and the subsequent cooling roll was changed, and the second stretching roll was brought into close contact with the second stretching roll for 0.50 seconds. As a result, a biaxially oriented polyamide film was obtained.

Comparative Example 1 A biaxially oriented polyamide film was obtained in the same manner as in Example 1, except that the total stretching ratio in longitudinal stretching was 4.1.

Comparative Example 2 A biaxially oriented polyamide film was obtained in the same manner as in Example 1, except that the first-stage magnification in longitudinal stretching was 3.2.

Comparative Example 3 A biaxially oriented polyamide film was obtained in the same manner as in Example 1 except that stretching was carried out without being brought into close contact with the roll in the second stage of longitudinal stretching.

Comparative Example 4 The same procedure as in Example 1 was carried out except that the arrangement of the second stretching roll in the longitudinal stretching and the cooling rolls thereafter were changed and the second stretching roll was closely contacted for 0.80 seconds. As a result, a biaxially oriented polyamide film was obtained.

Table 1 shows film forming conditions and film evaluation results in Examples and Comparative Examples.

[0037]

[Table 1]

[0038]

EFFECTS OF THE INVENTION According to the production method of the present invention, the thickness unevenness is small without breakage, and the oblique difference in the boiling water shrinkage can be reduced. It is extremely effective.

Claims (1)

[Claims] 1. A sequential biaxial stretching method for a polyamide film obtained by stretching a substantially unoriented polyamide sheet in the longitudinal direction and then in the transverse direction, wherein the longitudinal center portion of the film after longitudinal stretching Maximum value of heat shrinkage stress is 0.6
kg / mm ² to be less than, the longitudinal stretching [glass transition temperature of the polyamide (Tg) +20] ℃ above, [cold crystallization temperature (Tc) +20 polyamide] ℃ temperature below,
The total longitudinal stretching ratio of 3.0 times or more and 4.0 times or less is divided into two stages of the former stage and the latter stage, and the longitudinal stretching of the latter stage is performed by closely adhering the film to the roll for 0.01 seconds or more and 0.70 seconds or less. A method for producing a biaxially oriented polyamide film, which comprises stretching.