JPH11166057A - Polypropylene-based film for retorting use - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a polypropylene-based film simultaneously satisfying requirements as a sealant film of laminates for retorting use such as low- temperature impact resistance, blocking resistance and heat resistance. SOLUTION: This film consists of a propylene-ethylene block copolymer, having the characteristics specified in (a) to (c) described below: (a) melting point is 157-164 deg.C; (b) xylene soluble content is 16-25 wt.%; (c) birefringence falls within the following range: (3.0-0.017G)×10<-3> <=>=n<=(4.0-0.017G)×10<-3> wherein >=n: birefringence; G: film thickness (μm) falling within the range of 50-150 μm.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a polypropylene film for retort. More specifically, the present invention relates to a film used for a heat seal layer of a retort packaging bag, and more particularly to a retort packaging film having excellent resistance to bag breakage at a low temperature, excellent blocking resistance, and good openability.

[0002]

2. Description of the Related Art A conventional film made of a propylene / ethylene block copolymer has heat resistance and is relatively excellent in heat sealability.
(High-temperature sterilization at 135 ° C) Widely used as a sealant film for packaging bags.

However, due to insufficient impact resistance at low temperatures, standard laminated bags (stretched polyethylene terephthalate film 12 μm / stretched nylon film 15 μm)
/ Aluminum foil 9 μm / Propylene / ethylene block copolymer film 70 μm) is generally used for small bags having an internal capacity of 1 kg or less.

[0004] In order to improve the low-temperature impact resistance, attempts have been made to incorporate low-crystalline ethylene / propylene rubber or the like, but the impact resistance and blocking resistance are insufficient. Therefore, there has been a demand for the development of a polypropylene film having good low-temperature impact resistance and good blocking resistance.

JP-B-58-10414, JP-A-5
Films using a propylene / ethylene block copolymer prepared by a solvent polymerization method disclosed in JP-A-6-84712, JP-A-1-225648, etc., all have insufficient impact resistance at low temperatures.

JP-A-6-93061 discloses a propylene / ethylene block copolymer obtained by polymerization in the absence of a solvent. The film using the propylene / ethylene block copolymer has excellent impact resistance at low temperatures, but has a large blocking and is practically useful.
The opening of the bag was at a problematic level.

[0007]

SUMMARY OF THE INVENTION The present invention solves the above-mentioned drawbacks of a film using a conventionally known propylene / ethylene block copolymer, and provides a low-temperature film which is a characteristic required as a sealant film for a laminate for retort. It is an object of the present invention to provide a film that simultaneously satisfies impact resistance, blocking resistance, heat resistance, and the like.

[0008]

As a result of intensive studies by the present inventors, the above object of the present invention has been industrially advantageously achieved by the present invention having the following constitution.

[1] A polypropylene film for a retort, comprising a propylene / ethylene block copolymer, wherein the film has the properties specified in the following (a) to (c). (A) The melting point is 157 to 164 ° C. (B) The xylene-soluble content is 16 to 25%. (C) Birefringence is in the following range. (3.0-0.017G) × 10 ⁻³ ≦ Δn ≦ (4.0−
0.017G) × 10 ⁻³ Δn: birefringence G: film thickness (unit: μm), but in the range of 50 to 1
It is 50 μm.

[2] Impact resistance measured at 0 ° C. is 5.8 J
The polypropylene film for retort according to the above [1], characterized in that:

The most important feature of the present invention is that in the step of melt-extrusion film formation using the above-mentioned specific propylene / ethylene block copolymer, specific film formation conditions in which birefringence falls within the following range are applied. Thus, a polypropylene film for retort having excellent low-temperature impact resistance and blocking resistance is provided by an industrially easy method. (3.0-0.017G) × 10 ⁻³ ≦ Δn ≦ (4.0−
0.017G) × 10 ⁻³ Δn: birefringence G: film thickness (unit: μm), but in the range of 50 to 1
It is 50 μm.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the configuration will be described in more detail.

The propylene / ethylene block copolymer used in the present invention can be obtained by a polymerization method disclosed in JP-A-6-93061.

However, in order to simultaneously satisfy the desired low-temperature impact resistance, blocking resistance, heat resistance, and the like, the melting point, xylene-soluble content, and MFR must be limited to certain ranges.

The melting point of the propylene / ethylene block copolymer in the present invention must be in the range of 157 to 164 ° C. When the melting point exceeds 164 ° C., the propylene / ethylene block copolymer has high crystallinity, so that crystallization of the heat seal portion is accelerated and brittle fracture occurs at the low temperature of the seal portion. Getting worse. If the melting point is less than 157 ° C, the high retort temperature is 135 ° C.
Too close, the risk of heat resistance such as fusion during retort increases, which is not preferable.

The xylene-soluble content of the propylene / ethylene block copolymer in the present invention must be in the range of 16 to 25%. If the xylene-soluble content is too small, sufficient low-temperature impact resistance cannot be obtained.
% Or more is necessary, and 18% or more is more preferable. If the amount of the xylene-soluble component is too large, the film becomes sticky, so that the content is required to be 25% or less, more preferably 22% or less.

The MFR of the propylene / ethylene block copolymer in the present invention needs to be in the range of 1.0 to 3.0 g / 10 minutes. If the MFR is less than 1.0 g / 10 minutes, the motor load of the extruder increases, and the productivity decreases. If the MFR is large, the surface roughness (Ra) becomes small, and the blocking resistance is lowered.
0 minutes or less is required.

The propylene / ethylene block copolymer according to the present invention has a range in which the effects of the present invention are not impaired.
An antioxidant, a slip agent, a neutralizing agent, an anti-blocking agent and the like can be included as necessary.

The birefringence of the polypropylene film for retort according to the present invention must be within the following range. (3.0-0.017G) × 10 ⁻³ ≦ Δn ≦ (4.0−
0.017G) × 10 ⁻³ Δn: birefringence G: film thickness (unit: μm), but in the range of 50 to 1
It is 50 μm.

When the birefringence is less than the above range, the blocking becomes large and the opening property of the bag deteriorates. If the birefringence exceeds the above range, the impact resistance decreases.

The thickness of the polypropylene film for retort of the present invention must be 50 to 150 μm. If the thickness is less than 50 μm, the thickness is too thin, and problems such as generation of pinholes and insufficient bag drop strength occur. If the thickness exceeds 150 μm, the thickness becomes too large, the amount of heat at the time of bag making becomes insufficient, and the variation in heat sealing strength becomes large, so that the JAS standard cannot be guaranteed.

The method of forming the polypropylene film for retort of the present invention is not particularly limited, but a known film forming apparatus such as an inflation method and a T-die method can be used, and the T-die method is particularly preferable. .

An example of the method for producing the polypropylene film for retort of the present invention by the T-die method will be described below.

[A] Using a specific propylene / ethylene block copolymer, the pellets are fed to an extruder and melted, and then extruded from a T-die set at the same temperature as the extruder.

Then, the specific extruded propylene / ethylene block copolymer melt-extruded is cooled by pressure on a cooling roll through which cooling water is passed to form a film having a desired thickness, and then wound up to a predetermined length through a take-off machine. A heat treatment apparatus may be provided in the take-off machine or separately and re-heated.

[B] The extrusion temperature is preferably lower,
If the temperature is too low, the motor load of the extruder increases, and the productivity decreases.

[C] The temperature of the cooling roll is preferably higher, but if the temperature is too high, the birefringence becomes too large.

[D] The heat treatment temperature in the heat treatment apparatus is 60 to
100 ° C. is preferred.

The polypropylene film for retort of the present invention can be subjected to a surface treatment such as a corona discharge treatment, a flame treatment, a plasma treatment, an ozone treatment, etc. by a method usually employed in industry.

The polypropylene film for retort of the present invention has excellent properties such as impact resistance, blocking resistance and heat resistance at low temperatures, and is therefore suitably used as a heat seal layer of a laminate for retort. As a configuration of the laminate for retort, a configuration of 12 μm of stretched polyethylene terephthalate film / 15 μm of stretched nylon film / 9 μm of aluminum foil / 100 μm of polypropylene film for retort according to the present invention is preferable.

The polypropylene film for retort of the present invention is also suitably used as a sealing layer of a laminate for heavy-weight packaging at a low temperature.

The characteristic values used in the present invention are based on the following measuring methods.

(1) Melting point (Tm) Differential scanning calorimeter (DS) at a rate of temperature rise of 10 ° C./min.
The peak temperature of the maximum peak at the time of crystal melting of the chart measured using C) was defined as the melting point.

(2) Xylene-soluble content FDA 177.1720. (D) The amount of xylene solubles at 25 ° C. was measured by the method described in (4). In addition, the regulation value of this item when used for food packaging is 30% or less.

(3) Birefringence The retardation was calculated from the Berk compensator method using a polarizing microscope.

(4) MFR Measured according to JIS-K-7210 under the condition of -14.

(5) Impact resistance The sample was kept at 0 ° C. and 5351 g from a height of 2 m
Drop speed when dropping the iron ball and breaking through the sample,
The impact energy was determined from the difference in speed between the drop speed of the iron ball alone without the sample.

The calculation is based on the following equation.

[0039] ^{E = 1 / 2ml 2 (1} / t 0 2 -1 / t 1 2) E: impact resistance (J) m: weight of the ball [0.5351Kg] l: distance between two phototube [0 .1m] t ₀ : Iron ball passage time without film [S] t ₁ : Iron ball passage time when breaking and penetrating the film [S] In this measurement method, 5.8 J or more was judged to be good. .

(6) Blocking Resistance Using a film of 30 mm × 40 mm, the sealing surfaces of the films are superposed on each other, a load of 500 g / 12 cm ² is applied, and the film is heated and pressed in an oven at 50 ° C. for 24 hours. After leaving for 30 minutes or more in an atmosphere of 65 ° C. and a humidity of 65%, the shear peeling force was measured at a speed of 300 mm / min using Tensilon manufactured by Toyo Seiki.

According to the present measurement method, 1000 g / 12 cm ² or less was judged to be good.

[0042]

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

Example 1 A propylene / ethylene block copolymer having a melting point of 163 ° C., a xylene-soluble content of 20.1%, and an MFR of 1.5 g / 10 minutes was supplied to an extruder.
It is melt-extruded from a T-die at a temperature of 240 ° C.
Was formed into a film.

Table 1 shows the evaluation results of the obtained films. Both the impact resistance and the blocking resistance were at a sufficient level.

Comparative Example 1 The same polymer as in Example 1 was used and extruded at the same extrusion temperature as in Example 1;
Film formation of a 100 μm film was evaluated at 5 ° C. Table 1 shows the evaluation results of the obtained films.

In Comparative Example 1, the birefringence was too small and the blocking was poor.

Example 2 A propylene / ethylene block copolymer having a melting point of 161 ° C., a xylene-soluble content of 22.0%, and an MFR of 2.3 g / 10 minutes was supplied to an extruder at 250 ° C. And extruded from a T-die at a temperature of 50 ° C. to form a 70 μm film at a CD temperature of 50 ° C.

Table 1 shows the evaluation results of the obtained films. Both the impact resistance and the blocking resistance were at a sufficient level.

[Comparative Example 2] The same polymer as in Example 2 was extruded at the same extrusion temperature as in Example 2;
Film formation of a 70 μm film was evaluated at 5 ° C. Table 1 shows the evaluation results of the obtained films.

In Comparative Example 2, the birefringence was too large.
Insufficient impact resistance.

Example 3 A propylene / ethylene block copolymer having a melting point of 159 ° C., a xylene-soluble content of 19.3% and an MFR of 1.9 g / 10 minutes was fed to an extruder at 250 ° C. And extruded from a T-die at a temperature of 40 ° C. to form a 100 μm film at a CD temperature of 43 ° C.

Table 1 shows the evaluation results of the obtained films. Both the impact resistance and the blocking resistance were at a sufficient level.

Comparative Example 3 A propylene / ethylene block copolymer having a melting point of 155 ° C., a xylene-soluble content of 27.1% and an MFR of 2.1 g / 10 minutes was supplied to an extruder at 230 ° C. And extruded from a T-die at a temperature of 45 ° C. to form a 100 μm film at a CD temperature of 45 ° C. Table 1 shows the evaluation results of the obtained films.

In Comparative Example 3, blocking was poor due to too much xylene solubles.

Comparative Example 4 A propylene / ethylene block copolymer having a melting point of 164 ° C., a xylene-soluble content of 13.9%, and an MFR of 2.4 g / 10 minutes was supplied to an extruder at 250 ° C. And extruded from a T-die at a temperature of 30 ° C. to form a 100 μm film at a CD temperature of 30 ° C. Table 1 shows the evaluation results of the obtained films.

In Comparative Example 4, the impact resistance was insufficient because the xylene-soluble component was too small.

[0057]

[Table 1]

[0058]

According to the present invention, by using a specific propylene / ethylene block copolymer and forming a film under the condition that a specific birefringence can be obtained, excellent impact resistance at a low temperature, which has not been achieved conventionally, In addition, a retort polypropylene film having excellent blocking resistance can be obtained.

Claims (2)

[Claims] 1. A film comprising a propylene / ethylene block copolymer, wherein the film comprises:
A polypropylene film for retort, having the characteristics defined in (c). (A) The melting point is 157 to 164 ° C. (B) The xylene-soluble content is 16 to 25%. (C) Birefringence is in the following range. (3.0-0.017G) × 10 ⁻³ ≦ Δn ≦ (4.0−
0.017G) × 10 ⁻³ Δn: birefringence G: film thickness (unit: μm), but in the range of 50 to 1
It is 50 μm. 2. The polypropylene film for retort according to claim 1, wherein the impact resistance measured at 0 ° C. is 5.8 J or more.