JPH08267569A - Biaxially oriented polyamide resin film - Google Patents

Abstract

PURPOSE: To certainly reduce the S-shaped curl phenomenon of a three-side sealed bag used as a film for packing retort food after heat sterilization treatment by setting the direction of the orientation main axis of an α-crystal to a specific angle with respect to the longitudinal or lateral direction of a film. CONSTITUTION: In the constitution of a biaxially oriented polyamide resin film, the direction of the orientation main axis of an α-crystal is set to an angle of 14 deg. or less with respect to the longitudinal or lateral direction of the film. This film can be easily obtained by longitudinally stretching a substantially non-oriented polyamide resin sheet based on nylon 6 of which the relative viscosity is pref. 2-3.5 over two stages and biaxially stretching this stretched film in area magnification of 9 times or more before thermally fixing the same. There is no special limit in the longitudinal and lateral stretching magnifications of the film but, in order to obtain a more uniform film free from thickness irregularity, it is desirable that the longitudinal stretching magnification is set to 1.5-6 times and lateral stretching magnification is set to 1.5-7 times.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention, when applied to packaging of retort foods, produces S generated by heat sterilization.
The present invention relates to a biaxially oriented polyamide resin film in which the character curl phenomenon is minimized.

[0002]

2. Description of the Related Art Biaxially oriented polyamide-based resin films, particularly biaxially oriented polyamide-based resin films containing nylon 6 as a main component, have toughness, high gas barrier properties, pinhole resistance, transparency and easy printability. Since it has various excellent properties such as, it is widely used as a bag packaging material for various liquid foods, aquatic foods, frozen foods, retort foods, pasty foods, livestock meat and marine foods, etc. In recent years, it has been widely used as a bag packaging material for retort foods.

The above polyamide resin film used as a bag packaging material is usually printed on its surface and then laminated with a polyolefin resin film such as polyethylene (PE) or polypropylene (PP) to obtain a polyamide resin. The film is placed outside and folded in two in parallel to the flow direction, and the three sides are heat-sealed and cut out to obtain a three-side sealed bag having one side opened. Then, the bag is filled with the above-mentioned contents, sealed, and usually heat-sterilized in boiling water before being put on the market.

2 used for such heat sterilization treatment applications
As a method for producing an axially oriented polyamide resin film,
The horizontal sequential biaxial stretching method has become the mainstream.
One of the drawbacks pointed out for the laterally biaxially oriented polyamide resin film is that the physical properties tend to vary in the film width direction. The reason for this is that in the tenter treatment step for transverse stretching, longitudinal contraction stress due to heat is generated, and both ends of the film are held by clips while being restrained, while the central part of the film is held by the holding means. It is considered that since the influence force is small and the restraining force is weak, the shrinkage of the central portion of the film is delayed with respect to both end portions held by the clip due to the influence of the shrinkage stress.

That is, the biaxially oriented polyamide resin film for bag packaging obtained by the conventional method has an oblique difference in physical properties (for example, boiling water shrinkage) in the width direction, and warps at the corners of the bag after heat treatment. There is a problem that the two sides of the bag are curled in an S shape and the appearance of the product is significantly deteriorated. This is based on the non-uniformity of the physical properties in the width direction, and as a countermeasure therefor, for example, Japanese Patent Laid-Open No.
It has been proposed to define the molecular orientation angle difference measured by microwave, as shown in 103335 and the like.

However, the molecular orientation measured by microwaves is information that combines the amorphous and crystalline orientations of the polyamide resin film, and the relationship between the amorphous and crystalline states and the respective orientation states are also clarified. However, it cannot be said that the structure of the film is clearly defined. Actually,
Satisfying the condition of the molecular orientation angle difference measured by microwaves as defined in the above publication does not necessarily suppress the S-curl phenomenon. That is, the definition of the molecular orientation angle difference measured by microwave is
It cannot be said that the characteristics for reducing the S-curl are sufficient.

[0007]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and its object is to heat sterilize a three-side sealed bag used as a film for packaging retort foods and the like. An object of the present invention is to provide a biaxially oriented polyamide resin film capable of surely reducing the S-curl phenomenon after treatment and providing a packaged product with high commercial value.

[0008]

The biaxially oriented polyamide resin film according to the present invention, which has been able to achieve the above object, has a structure in which the orientation main axis of the α-type crystal is in the machine direction (MD) or in the transverse direction of the film. The gist is that it is 14 degrees or less with respect to the (TD) direction. The polyamide-based resin film having the above-mentioned properties is preferably a substantially unoriented polyamide-based resin sheet mainly composed of nylon 6 having a relative viscosity of 2 to 3.5 and stretched in two or more stages in the longitudinal direction. In addition, it can be easily obtained by biaxially stretching at an area ratio of 9 times or more and heat fixing.

[0009]

As the polyamide resin used as the film material in the present invention, all polyamides that form α-type crystals can be used. For example, nylon 6, nylon 66, nylon 610, nylon 6/66/610. Copolymer, Nylon 11, Nylon 12, Polyethylene isophthalamide, Polymethaxylylene adipamide, Poly (hexamethylene isophthalamide / terephthalamide),
Poly (hexamethylene isophthalamide / monomethyl terephthalamide), a copolymer of hexamethylene isophthalamide / terephthalamide and epsilon caprolactam, a copolymer of hexamethylene terephthalamide and hexamethylene adipamide, etc. can be used. However, polyamides containing nylon 6 as a main component are particularly preferable. Specifically, nylon 6 alone or nylon 6 with hexamethylenediamine and adipic acid or isophthalic acid as a nylon salt or metaxylylenediamine is used. A copolymer obtained by copolymerizing a small amount of a nylon salt with adipic acid and /
Alternatively, a blended product thereof may be used. Further, these polyamide-based resins may be blended with known additives such as small amounts of various anti-blocking agents, antistatic agents, stabilizers, etc. within a range that does not impair their properties.

The above polyamide-based resin has a relative viscosity (R
V) is preferably in the range of 2 to 3.5, and when the relative viscosity is less than 2, breakage easily occurs during longitudinal and transverse stretching, which makes it difficult to form a film, and exceeds 3.5. In the case of a product, the load at the time of extrusion becomes too high and it becomes unsuitable for production.

When a film or sheet made of the polyamide resin is longitudinally / transversely stretched to obtain a biaxially oriented film, the longitudinal / transverse stretching ratio is preferably set to 9 times or more in terms of area ratio. With a stretching ratio of 1, it becomes difficult to obtain a biaxially stretched film without unevenness in thickness. The longitudinal and transverse stretching ratios are not particularly limited, but in order to obtain a more uniform film without uneven thickness, the longitudinal stretching ratio is 1.5 to 6 times and the transverse stretching ratio is in the range of 1.5 to 7, respectively. It is desirable to set.

Next, the most important constitutional factor in the present invention is the direction of the orientation main axis of the α-type crystal in the biaxially stretched polyamide resin film. ) Or transverse (TD) direction is 14 degrees or less, more preferably 11 degrees or less. This is an S-shaped curl generated during heat treatment by suppressing the orientation state of crystals inside the film, particularly the azimuth angle of the orientation main axis of α-type crystals, with respect to the longitudinal (MD) direction or the transverse (TD) direction of the film. This is to reduce the phenomenon, and this idea is to suppress the contraction of the central part of the film from being delayed later than the contraction of the both ends of the film held by the clip, reduce the diagonal difference in the boiling water contraction ratio, and eventually S. This is to reduce the character curl phenomenon.

As described above, the conventional molecular orientation measured by microwaves is merely synthetic information of the amorphous and crystalline orientations, and the relationship between the amorphous and crystalline orientations of the polyamide resin constituting the film and the respective orientations. It did not reveal even the state, nor did it give accurate orientation information in a strict sense. Therefore, it cannot be said that the molecular orientation information measured by microwaves alone is sufficient to define the characteristics for reducing the S-curl phenomenon.

However, as described above, as a stricter control item of the orientation direction, if the crystal structure which plays a role of the skeleton of the biaxially stretched film, especially the most stable α-type crystal structure and the orientation state are properly controlled. It was confirmed that the S-curl phenomenon can be reduced more reliably and effectively.
Therefore, in the present invention, about 20% or less, more preferably 1% or less of crystals other than α-type occupy in all the polyamide crystals.
It is desirable to use those of 0% or less.

Here, the crystal orientation main axis of the α-type crystal is a wide angle X
It can be determined by the line scattering method. That is, when nylon 6 is used as the polyamide, the azimuth angle distribution of the (200) plane signal in the α-type crystal in the film plane is measured by the transmission method, and the reflection from the (200) plane at this time is the maximum. The incident angle of X-rays on the sample is optimized so that Then, the angle that gives the maximum intensity of the azimuth angle dependence of the obtained (200) plane reflection is determined as the crystal orientation main axis, and the direction of the crystal orientation main axis and the longitudinal direction of the film (M
It can be determined by measuring the angle formed by the D) direction or the transverse (TD) direction.

When the direction of the crystal orientation main axis of the α-type crystal exceeds 14 degrees with respect to the machine direction (MD) direction or the transverse (TD) direction of the film, the oblique difference in boiling water shrinkage becomes too large, and In the heat treatment after the bag, the corners of the bag are curled after the processing, so that the four sides of the bag are curled into an S shape and the appearance of the product is deteriorated, but this angle is 14 degrees or less, more preferably It was confirmed that the S-curl phenomenon as described above can be remarkably suppressed if the angle is 11 degrees or less.

The method for obtaining a biaxially stretched polyamide resin film satisfying the above requirements is not particularly limited, but if it is illustrated as the most general method, the longitudinal stretching is divided into two or more stages and a plurality of films are formed under low stress. This is a method in which longitudinal stretching of steps is performed to facilitate the orientation formed by transverse stretching, followed by heat setting and, if necessary, relaxation treatment.

The biaxially oriented polyamide resin film satisfying the above physical properties is processed into a three-way sealed bag while having the toughness and pinhole resistance which the biaxially oriented polyamide film originally has, Even after filling and heat sterilization, the S-curl phenomenon is unlikely to occur, and the appearance as a packaged product can be improved and the quality can be improved.

The biaxially oriented polyamide resin film of the present invention is usually used by laminating it with other materials as described above. Other materials to be laminated are most common polyethylene and polypropylene. In addition to the polyolefin film, various resin films made of polyester resin, acrylic resin, polycarbonate resin, polyvinyl resin, urethane resin, etc. can be used, and if necessary, metal foil or metal vapor deposition film. It is also possible to laminate with etc.

[0020]

EXAMPLES Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited to the following examples, and may be appropriately applied within a range compatible with the gist of the preceding and the following. Modifications can be made, and all of them are included in the technical scope of the present invention. The methods for determining physical properties and characteristic values used in Examples and Comparative Examples are as follows. In addition, the biaxially oriented polyamide resin film used for each measurement and judgment / evaluation of S-curl has a one-third portion of the entire width of the film immediately after film formation before trimming the end portion, which is closer to the end portion. It carried out about what was collected from the central part.

(1) Crystal Orientation Main Axis In the case of nylon 6, the crystal orientation main axis of α type crystal is wide angle X
Obtained by the line scattering method. Transmission method measurement is performed to measure the azimuth angle distribution of the (200) plane signal in the α-type crystal in the film plane. At this time, the incident angle of X-rays on the sample was optimized so that the reflection from the (200) plane was maximized, and the angle giving the maximum intensity of the azimuth angle dependence of the obtained (200) plane was crystallized. The main axis of orientation was used. The direction of the principal axis of this crystal orientation and the longitudinal (MD) direction of the film or the transverse (T
The angle formed with the D) direction was determined.

(2) Diagonal difference in boiling water shrinkage (BS) (BS)
a) A biaxially oriented polyamide-based resin film is cut into a 21 cm square to form a sample, and the sample is left to stand in an atmosphere of 23 ° C. and 65 RH% for 2 hours or more. A circle with a diameter of 20 cm is drawn centered on the center of this sample, and when the vertical direction is 0 °, 15 °
A straight line passing through the center of the circle in the clockwise direction at intervals of 0 to 165 ° is drawn, the diameter in each direction is measured, and the length before treatment is taken as the length.

This sample was heat-treated in boiling water for 30 minutes and then taken out. After water adhering to the surface was removed and air-dried, the sample was left to stand at 23 ° C. and 65 RH% for 2 hours or more. After that, the length of the straight line drawn in the diameter direction of the circle is measured and taken as the length after treatment, and the shrinkage rate of boiling water is calculated by the following formula. BS = [(length before treatment-length after treatment) / (length before treatment)] x 100 (%) BSa; absolute difference in shrinkage in 45 ° and 135 ° (-45 °) directions value

(3) Judgment of S-shaped curl of a three-sided sealed bag The laminated films obtained in each of Examples and Comparative Examples described later were parallel to the winding length direction using a test sealer manufactured by Seibu Machine Co., Ltd. Each side is continuously heat-sealed at 150 ° C by 20 mm in the longitudinal direction while being folded in two, and is also heat-sealed intermittently at a width of 10 mm at intervals of 150 mm in the direction perpendicular to it, and a semi-finished bag with a width of about 200 mm is obtained. obtain.

After cutting both edges of the semi-finished product bag in the winding length direction so that the sealing portion becomes 10 mm, the semi-finished product bag is cut at the boundary of the sealing portion in a direction perpendicular to this, and a three-way sealing bag (sealing width) 10 mm) was prepared, 10 bags were heat-treated in boiling water for 30 minutes, and then air-dried overnight at 23 ° C. and 65 RH%. Further, these 10 bags were stacked, a load of 1 kg was applied to the entire surface of the bag from above, and the bag was stored for one day and night. Then, the load was removed and the degree of curling (S-curl) of the bag was evaluated according to the following criteria. ○… There is no warp. ×: Clearly warped. XX: Warping is remarkable.

Example 1 Nylon 6 pellets (RV = 2.8) containing 4% of MXD6 (polymeta-xylylene adipamide) were vacuum dried and then fed to an extruder to melt at 260 ° C. It was extruded in a sheet form from a die and applied with a high DC voltage to electrostatically adhere to a cooling roll for cooling and solidification to obtain a substantially unoriented sheet having a thickness of 200 μm (500 mm in total width). This sheet is first preheated at a temperature of 50 ° C., then first stretched 1.7 times at a stretching temperature of 75 ° C., and then kept at 70 ° C. at a stretching temperature of 70 ° C. and a total stretching ratio of 3.4. The second longitudinal stretching is performed so that the number of sheets is doubled, and subsequently this sheet is continuously introduced into a tenter, and 130 ° C.
Was transversely stretched to 4 times, and heat-set at 210 ° C. and 5% transverse relaxation treatment, and then cooled. Then, both edges were cut and removed to obtain a biaxially oriented polyamide resin film.

Example 2 An unstretched sheet produced in the same manner as in Example 1 was heated to 50 ° C.
After pre-heat treatment at the temperature of 1, the first longitudinal stretching was performed 1.2 times at the stretching temperature of 75 ° C, and then the stretching temperature of 75 ° C while keeping the temperature at 75 ° C.
A biaxially oriented polyamide resin film was obtained in the same manner as in Example 1 except that the second longitudinal stretching was performed so that the total stretching ratio was 3.4 times at 0 ° C.

Comparative Example 1 The same nylon 6 pellets used in Example 1 were fed to an extruder, melted at 275 ° C., extruded in a sheet form from a T-type die, and a DC high voltage was applied to the cooling roll. It was electrostatically adhered to the top and cooled and solidified to obtain a substantially unoriented polyamide resin sheet having a thickness of 200 μm (total width 1100 mm).

This sheet was first preheated at a temperature of 40 ° C., and then longitudinally stretched at a stretching temperature of 65 ° C. by a factor of 3.4, and subsequently, this sheet was continuously introduced into a tenter and a factor of 4 at 120 ° C. Transversely stretched, heat set at 210 ° C and 5
% Lateral relaxation treatment was performed and then cooled. Then, both edges were cut and removed to obtain a biaxially oriented polyamide resin film.

Comparative Example 2 A biaxially oriented polyamide resin film was obtained in the same manner as Comparative Example 1 except that the transverse stretching was carried out in a single step at a temperature of 130 ° C. and 4.3 times. Comparative Example 3 A biaxially oriented polyamide resin film was obtained in the same manner as in Example 1 except that the longitudinal stretching was performed in one step at a temperature of 65 ° C. and 3.0 times. Table 1 collectively shows the properties and performances of the biaxially oriented polyamide resin films obtained in the above Examples and Comparative Examples.

[0031]

[Table 1]

Reference Example 1 Nylon 6 having an RV of 1.5 is used as a polyamide,
An attempt was made to form a film in the same manner as in Example 1, but it was difficult to obtain a biaxially oriented film because the film was easily broken during longitudinal stretching.

Reference Example 2 Nylon 6 having an RV of 4.5 is used as a polyamide,
An attempt was made to form a film in the same manner as in Example 1, but it was difficult to melt-extrude the film itself.

Reference Example 3 A biaxially oriented film was obtained in exactly the same manner as in Example 1 except that the transverse stretching ratio was 2.0 (total longitudinal / transverse stretching ratio: 6.8). This film has remarkable thickness unevenness,
It was not suitable as a packaging film.

[0035]

The present invention is configured as described above.
Without sacrificing the toughness, pinhole resistance, etc. inherent in the axially oriented polyamide resin film, an S-shaped curl may occur in the bag even after heat sterilization after processing into a three-side sealed bag and filling the contents. In addition, the appearance as a packaged product can be improved and the quality can be improved. Moreover, this film has excellent adhesiveness to various laminated base materials and also has excellent delamination resistance.

Claims (2)

[Claims] 1. A biaxially oriented polyamide resin film, wherein the direction of the orientation main axis of the α-type crystal is 14 degrees or less with respect to the longitudinal direction or the transverse direction of the film. 2. A polyamide resin having a relative viscosity of 2-3.
The biaxially oriented polyamide-based resin film according to claim 1, which contains 5 of nylon 6 as a main component.