JPH08300470A - Biaxially stretched polypropylene film - Google Patents

Abstract

PURPOSE: To provide a biaxially stretched polypropylene film capable of being suitably used as a general laminate packing film, a fiber packing film and a retort food packing film because excellent in rigidity and transparency and reduced in dimensional change at a time of heating. CONSTITUTION: A biaxially stretched polypropylene film is characterized in that the tensile modulus of elasticity in a film flow direction (MD) and the direction (TD) right-angled to the film flow direction is 180kg/mm<2> or more, a heat shrinkage factor at 150 deg.C is 5% or less and transparency satisfies Hz<=(2/3)d<1/2> [Hz is a haize value % and d is film thickness (μm)] and obtained by stretching a sheet composed of specific crystalline polypropylene by 3-6 times in MD and 8-12 times in TD.

Description

Detailed Description of the Invention

[0001]

The present invention has excellent rigidity and transparency,
Further, the present invention provides a biaxially oriented polypropylene film which can be suitably used as a film for general laminate packaging, a film for fiber packaging, and a film for retort foods, since the dimension change upon heating is small.

[0002]

2. Description of the Related Art Biaxially oriented polypropylene films are widely used as packaging materials because of their excellent transparency and rigidity. However, the thermal shrinkage is large due to the residual strain during stretching,
Not suitable for retort packaging applications where the temperature rises during secondary processing. Also, in general laminate packaging applications and fiber packaging applications, if the sealing temperature is too high, the appearance will be poor due to heat shrinkage, which is a barrier to rationalization by high-speed packaging.

Conventionally, as a method for relaxing the residual strain of a stretched polypropylene film and reducing the heat shrinkage rate, for example, JP-A-3-39227 and JP-A-5-116 are known.
As disclosed in Japanese Patent Publication No. 214, etc., a method is known in which a stretched film is introduced into a heating roll and subjected to relaxation heat treatment.

[0004]

In these methods, the relaxation caused by the relaxation heat treatment is increased by 15
Although it is possible to reduce the heat shrinkage at 0 ° C. to about 5% or less, the film surface is apt to be rough due to the heat at the time of contact with a heating roll, resulting in a poor appearance, and the obtained stretched polypropylene film. Transparency tends to decrease.

Further, since the orientation of polypropylene constituting the stretched polypropylene film becomes small, there is a problem that the high rigidity, which is a feature of the conventional biaxially stretched polypropylene film, is lost.

Therefore, the object of the present invention is that it can be suitably used for general laminate packaging film, fiber packaging film, and retort food film, which has a small heat shrinkage rate and is excellent in rigidity and transparency. An object is to provide a biaxially oriented polypropylene film.

[0007]

Means for Solving the Problems As a result of intensive studies to solve the above problems, the present inventors have found that highly crystalline polypropylene having a specific melt index and composition is processed under specific conditions. As a result, it was found that a biaxially stretched polypropylene film having a small heat shrinkage, excellent rigidity and transparency can be obtained, and the present invention has been completed.

That is, according to the present invention, the tensile elastic modulus is 180 kg / mm ² or more and 150 ° C. in the film flow direction (hereinafter, also referred to as MD) and the direction perpendicular to the film flow direction (hereinafter, also referred to as TD). The biaxially stretched polypropylene film has a heat shrinkage ratio of 5% or less and a haze value of transparency satisfying the following formula (1).

Hz ≦ (2/3) d ^1/2 (1) (where, Hz is haze value (%), d is film thickness (μ
m). ) Further, the present invention, as a suitable production method for obtaining the above-mentioned biaxially oriented polypropylene film, has a melt index of 2 to 10 g / 10 minutes and a high α-olefin content other than propylene of 0.4 mol% or less. A sheet made of crystalline polypropylene is placed under a temperature condition that suppresses the orientation of the highly crystalline polypropylene by 3 to 6 times in the film flow direction and 8 to 12 in the direction perpendicular to the film flow direction.
The present invention also provides a method for producing a biaxially stretched polypropylene film, which comprises stretching at a draw ratio of 2 times.

The biaxially oriented polypropylene film of the present invention has a tensile elastic modulus of 180 kg / m in MD and TD.
m ² or more, and the heat shrinkage ratio at 150 ° C. is 5
% Or less. Furthermore, in the biaxially oriented polypropylene film of the present invention, the haze value (%) showing transparency in addition to the above properties satisfies the following formula (1).

Hz ≦ (2/3) d ^1/2 (1) (where Hz is haze value (%), d is film thickness (μ
m). The haze value is 3.3% or less in the case of a film having a thickness of 25 μm, and it can be said that the haze value is extremely high.

As described above, the heat shrinkage at 150 ° C. is suppressed to a low shrinkage of 5% or less,
Moreover, a biaxially oriented polypropylene film that exhibits rigidity and transparency, which are excellent properties of the biaxially oriented polypropylene film, was first proposed by the present invention.

The biaxially oriented polypropylene film of the present invention is suitable for use as a packaging film because it has such characteristics. That is, since the tensile elastic modulus in MD and TD is as high as 180 kg / mm ² or more, the generation of wrinkles after bag making is effectively prevented. The above tensile elastic modulus is particularly 200 kg / mm ² in order to more effectively prevent the generation of wrinkles after bag making.
It is preferable to select and use the above.

Since the biaxially stretched polypropylene film of the present invention has a heat shrinkage rate of 5% or less at 150 ° C., it has a problem of heat sealing, which has been a problem in the conventional stretched polypropylene film not subjected to heat relaxation treatment. It is possible to effectively prevent the phenomenon that the film is wrinkled by the bag making and the appearance is poor, and in particular, at the time of high-speed bag making, there is an effect of preventing troubles due to the wrinkling and improving working efficiency. The heat shrinkage at 150 ° C. is particularly preferably 4% or less for use.

Furthermore, since the biaxially oriented polypropylene film of the present invention also has excellent transparency as described above, when it is used as a packaging film, it is excellent in the transparency of the packaged article and is extremely ideal. Can be used as a packaging material.

In the biaxially stretched film of the present invention,
In particular, a preferable film for packaging has a tensile breaking strength of 12 kg / mm ² or more in MD and 25 kg in TD.
/ Mm ² or more.

The biaxially oriented polypropylene film of the present invention may be a single layer or multiple layers. As an example of a typical embodiment having a multi-layer structure, a base material layer uses a crystalline polypropylene described below, and a surface layer forms a layer composed of the crystalline polypropylene having a different composition. Layer made of a resin having excellent heat-sealing property, for example, polyethylene, propylene-ethylene copolymer, ethylene-1-butene copolymer, ethylene-propylene-1-butene copolymer,
A layer comprising an ethylene-propylene-butadiene copolymer, an ethylene-vinyl acetate copolymer, and a blended product containing these polyolefins as a main component is provided within a range capable of exhibiting the characteristics of the biaxially stretched polyolefin film of the present invention. An example is a mode in which the layers are laminated in thickness.

The biaxially oriented polypropylene film of the present invention is selected to have a thickness of 10 to 60 μ depending on the application.
In the laminated film, when the surface layer resin is other than the above-mentioned highly crystalline polypropylene, the surface layer thickness is usually 20% of the total film thickness in view of the balance of elastic modulus and heat shrinkage rate of the film.
% Or less, particularly preferably 10% or less.

The method for producing the biaxially oriented polypropylene film of the present invention is not particularly limited, but it is preferable to use crystalline polypropylene having a melt index of 2 to 10 g / 10 minutes and an α-olefin content other than propylene of 0.4 mol% or less. Under a temperature condition that suppresses the orientation of the crystalline polypropylene, by stretching at a stretching ratio of 3 to 6 times in the film flow direction and 8 to 12 times in the direction perpendicular to the film flow direction. It is possible to obtain.

The crystalline polypropylene used in the present invention is
Α-olefin content other than propylene 0.4 mol%
The following crystalline polypropylene, specifically, homopolypropylene or α-olefin content other than propylene is 0.4 mol% or less, preferably 0.3 mol% or less propylene-α-olefin copolymer, Or a mixture of these.

Examples of the above α-olefin include ethylene, 1-butene, 1-pentene, 1-hexene, 1-heptene, 1-octene, 1-nonene, 1-decene and 4-methyl-1-pentene. Etc. can be mentioned. When the content of these α-olefins is more than 0.4 mol%, the heat shrinkage rate of the obtained biaxially oriented polypropylene film becomes too large and the polypropylene film of the present invention cannot be obtained.

The crystalline polypropylene used in the present invention is
Those having an isotacticity (hereinafter abbreviated as II) of 97% or more, particularly 98% or more, which is a guideline for crystallinity, are preferably used.

The crystalline polypropylene used in the present invention has a melt index (hereinafter abbreviated as MI) of 2 to 10.
g / 10 minutes, preferably 3 to 8 g / 10 minutes. That is, when the MI is less than 2 g / 10 minutes, the heat shrinkage rate becomes large, and when it is more than 10 g / 10 minutes, the mechanical strength decreases, so that it is not suitable for the present invention.

An antistatic agent, a lubricant, an anti-blocking agent and the like can be added to the crystalline polypropylene used in the present invention, if necessary. Also, other resins may be added within a range that does not deteriorate the performance. Examples of other resins include homopolymers of the above α-olefins, copolymers of the above α-olefins, petroleum resins, acid-modified polyethylene or polypropylene at the terminals, hydrocarbon polymers having hydroxyl groups at the terminals, and the like. You can

The biaxially stretched polypropylene film of the present invention can be produced by melt-extruding the above-mentioned crystalline polypropylene powder or pellets into a sheet or the like, and biaxially stretching it. In the case of a laminated film, the resin forming each layer is coextruded and then biaxially stretched, or one layer of resin is melt extruded and uniaxially stretched, and the resin of the other layer is melt extruded thereon. Then, a method of stretching in a direction substantially perpendicular to the uniaxial stretching direction is adopted. The stretching ratio is preferably 3 to 6 times in MD and 8 to 12 times in TD. If the stretching ratio is less than 3 times in MD and less than 8 times in TD, the tensile elastic modulus of the film becomes small, and if the stretching ratio is more than 6 times in MD and more than 12 times in TD, the heat shrinkage ratio becomes large.

Further, it is important to adopt a temperature condition for suppressing the orientation of the above-mentioned highly crystalline polypropylene for stretching in MD and TD, and generally, the stretching temperature is set to a high temperature. The stretching temperature varies depending on the characteristics of the film forming machine, but is 150 ° C. to 165 ° C., preferably 156 ° C. to 160 ° C. for MD. The stretching temperature is 1
When the temperature is lower than 50 ° C, the heat shrinkage rate becomes large, and it becomes difficult to obtain the biaxially oriented polypropylene of the present invention.
If the temperature is higher than 65 ° C, the MD sheet may stick to the roll.

On the other hand, the stretching temperature of TD is 160 ° C to 190 ° C.
C, preferably 170 to 180 ° C. Even when the stretching temperature is lower than 160 ° C., the heat shrinkage ratio becomes large, and it becomes difficult to obtain the biaxially stretched polypropylene of the present invention.
On the other hand, if the temperature is higher than 190 ° C., whitening of the film will occur and the transparency will decrease, making it difficult to obtain the biaxially oriented polypropylene of the present invention.

The TD relaxation rate is 0 to 8%, preferably 0.
~ 3%. If it exceeds 8%, the orientation in the TD direction becomes too small, and as a result, the heat shrinkage rate in the MD direction increases.

Further, the biaxially stretched polypropylene film of the present invention can be subjected to a relaxation heat treatment during secondary processing after film formation to further reduce the heat shrinkage rate. Since the rough appearance becomes poor, the transparency is likely to be lowered, and the rigidity expressed by the tensile elastic modulus is also likely to be lowered, it is preferable to carry out under the condition of not deteriorating these performances.

Therefore, in the method of the present invention, it is most preferable that heat relaxation treatment is not performed after film formation.

[0031]

As described above, the biaxially stretched polypropylene film of the present invention is excellent in rigidity and transparency by stretching the resin having a specific composition under specific conditions, and has a dimension when heated. It has an excellent property that there is little change, which cannot be realized by the conventional biaxially oriented polypropylene film.

Further, in the method for producing a biaxially stretched polypropylene film of the present invention, by using polypropylene having high crystallinity as a raw material, rigidity can be exhibited even with a small number of orientations, and the MI is relatively high.
That is, it is presumed that by using a highly crystalline polypropylene having a short molecular chain, the strain generated during stretching can be reduced and the heat shrinkage rate can be reduced.

Since the biaxially stretched polypropylene film of the present invention has the above-mentioned characteristics, it is suitable as a packaging film for general laminate packaging film, fiber packaging film, retort food film, etc., which is excellent in high-speed bag-making property. Can be used for

[0034]

EXAMPLES The present invention will be described below with reference to examples and comparative examples, but the present invention is not limited to these examples.

The measuring method used in the following examples will be described.

(1) Melt Index (MI) It was measured according to JIS-K7210.

(2) Isotacticity (II) 5 g of a sample was extracted with n-heptane at 120 ° C. for 12 hours to remove low crystalline components, and the residue was vacuum dried at 80 ° C. for 2 hours. Let The isotacticity (I.
I) was calculated.

I. I (%) = (b / a) × 100 a: weight of sample before extraction (g) b: weight of residue after extraction (g) (3) Transparency According to JIS-K6714, haze value of film Was measured.

(4) Tensile Modulus According to JIS-K7113, the tensile modulus was measured by the following method.

Width 10 mm, length 100 mm from film
The sample was cut out, and both ends of the sample were fixed with a chuck of a tensile strength measuring machine (Autograph; Shimadzu).
In this case, the chuck gap in the length direction of the sample is 20m.
It was adjusted to be m. A tensile test was performed at a tensile speed of 20 mm / min to create a tensile stress-strain curve.

The tensile modulus was calculated by the following equation using the first linear portion of the tensile stress-strain curve.

Em = Δδ / Δε Em: Tensile modulus Δδ: Difference in stress between two points on the straight line due to the original average cross-sectional area of the sample Δε: Difference in strain between the same two points. MD and TD of film during processing
Was measured.

(5) Heat Shrinkage According to JIS-C2318, the heat shrinkage of MD and TD at 150 ° C. was measured.

(6) Appearance of Film After Thermal Processing A 10 cm × 10 cm film piece was cut out, and the film piece was squeezed with a heat sealer to compare changes in appearance. In addition, it measured about MD and TD.

Sealing temperature: 170 ° C. Evaluation criteria ○: Almost no wrinkles Sealing pressure: 2 kg / cm ² △: Slightly wrinkles occur Sealing time: 1 second ×: Very wrinkles occur (7) Tensile breaking strength JIS-Z1521 According to the above, the measurement was performed in the MD and TD directions. Examples 1 to 7, Comparative Examples 1 to 4 100 parts by weight of the crystalline homopolypropylene or propylene-α-olefin copolymer shown in Table 1 was added to 0.03 parts by weight of erucic acid amide and 0.04 parts of calcium stearate.
Parts by weight and 0.1 parts by weight of spherical silica having a particle size of 1.5 μ were added and melt-kneaded. The above resin was extruded from a T-die and stretched at a stretching temperature shown in Table 1 using a tenter method biaxial stretching machine.
It was stretched 5 times in MD and 10 times in TD to obtain a stretched film having a thickness of 25 μm. The heat shrinkage rate, the tensile elastic modulus, the tensile breaking strength, the transparency (haze), and the appearance of the film after heat processing were measured for this film, and the results are shown in Table 1.

[0046]

[Table 1]

Examples 8 to 10, Comparative Examples 5 and 6 100 parts by weight of crystalline homopolypropylene, 0.03 part by weight of erucic acid amide, 0.04 part by weight of calcium stearate, and spherical silica 0.1 having a particle size of 1.5 μm. Parts by weight were added and melt-kneaded. The above resin was extruded through a T-die and formed into a film having a thickness of 25 μm by using a tenter biaxial stretching machine at a stretching ratio and a stretching temperature shown in Table 2. With respect to this film, the heat shrinkage rate, tensile elastic modulus, transparency (haze), and film appearance after heat processing were measured, and the results are shown in Table 2.

[0048]

[Table 2]

Examples 11 to 13, Comparative Examples 7 and 8 MI = 5 g / 10 minutes, I.V. I = 98.9% of crystalline homopolypropylene 100 parts by weight of erucic acid amide 0.03
Parts by weight and 0.04 parts by weight of calcium stearate were added, and this was used as a resin for the base material layer. On the other hand, the crystalline homopolypropylene or propylene-α shown in Table 3
-Using 100 parts by weight of an olefin copolymer, 0.03 part by weight of erucic acid amide, 0.04 part by weight of calcium stearate and 0.1 part by weight of spherical silica having a particle size of 1.5μ as a resin for the surface layer. Then, these resins were melted and kneaded to obtain a three-layer laminated film in which the surface layers were laminated on both sides of the base material layer by a three-layer die and a tenter biaxial stretching machine. The film forming temperature was MD158 ° C. and TD172 ° C., and the stretching ratio was MD5 times and TD10 times. About this film, heat shrinkage, tensile modulus, transparency (haze),
The appearance of the film after thermal processing was measured, and the results are shown in Table 3.

[0050]

[Table 3]

Claims (2)

[Claims] 1. A tensile elastic modulus of 180 kg in a film flow direction and a direction perpendicular to the film flow direction.
/ Mm ² or more, the heat shrinkage rate at 150 ° C. is 5% or less, and the transparency satisfies the following formula (1) in terms of haze value, a biaxially oriented polypropylene film. Hz ≦ (2/3) d ^1/2 (1) (where, Hz is haze value (%), d is film thickness (μ
m). ) 2. A melt index of 2 to 10 g / 10.
Min, a sheet made of crystalline polypropylene having an α-olefin content other than propylene of 0.4 mol% or less, under a temperature condition for suppressing the orientation of the crystalline polypropylene,
The method for producing a biaxially stretched polypropylene film according to claim 1, wherein the film is stretched at a draw ratio of 3 to 6 times in the film flow direction and 8 to 12 times in a direction perpendicular to the film flow direction.