JP7470502B2 - Food packaging film and food packaging body - Google Patents

Description

The present invention relates to food packaging films and food packages.

Oriented polypropylene film has an excellent balance of performance, including processability, water vapor barrier properties, transparency, mechanical strength, and rigidity, and is used as a packaging film for packaging food.

Technologies relating to food packaging films using such stretched polypropylene films include those described in Patent Document 1 (JP 2008-73926 A) and Patent Document 2 (JP 2004-82499 A), for example.

Patent Document 1 describes a biaxially oriented multilayer polypropylene film comprising a biaxially oriented film made of a propylene polymer composition containing 75 to 90% by weight of a propylene homopolymer (A) and 25 to 10% by weight of a tackifier (D), the biaxially oriented film having a layer made of a propylene-α-olefin random copolymer (C) having a melting point in the range of 125 to 145° C., via a layer made of a propylene polymer (B) having a melting point of 155° C. or higher, and the biaxially oriented film having a layer made of a propylene polymer (E) on the other side thereof.
Patent Document 1 describes that the biaxially oriented multilayer polypropylene film having the above-mentioned structure can suppress the seepage of petroleum resins and the like onto the film surface, and has excellent laminate strength and moisture resistance.

Patent Document 2 describes a multilayer resin film having a biaxially oriented polypropylene resin layer containing 10 to 40% by weight of a highly crystallized resin and 6 to 15% by weight of a petroleum resin, and further having a polyvinyl alcohol resin layer via an adhesive layer on at least one surface of the biaxially oriented polypropylene resin layer, the multilayer resin film being characterized in that the oxygen transmission rate at a relative humidity of 85% RH and a temperature of 23°C is 600 mL/ ^m2 ·day·MPa or less, and the water vapor transmission rate at a relative humidity of 90% RH and a temperature of 40°C is 3.5 g/ ^m2 ·day·20 μm or less.
Patent Document 2 describes that the multilayer resin film having the above-mentioned structure has excellent oxygen gas barrier properties and moisture resistance.

JP 2008-73926 A JP 2004-82499 A

From the viewpoint of reducing the environmental impact, it is required to further improve the water vapor barrier property of the stretched polypropylene film. If the water vapor barrier property is improved, the thickness of the stretched polypropylene film can be reduced, so that the amount of the propylene-based polymer used can be reduced, and the environmental impact can be reduced.
According to the study by the present inventors, it was found that the water vapor barrier property of the stretched polypropylene film can be improved by using a highly crystalline propylene polymer as the propylene polymer constituting the stretched polypropylene film. However, it was revealed that the use of such a highly crystalline propylene polymer is likely to cause stretching unevenness, and the thickness unevenness of the stretched polypropylene film obtained after the stretching step may become large.
Thus, the present inventors have found that there is a trade-off between the water vapor barrier property and thickness unevenness in a stretched polypropylene film. In other words, the present inventors have found that there is room for improvement in stretched polypropylene films from the viewpoint of improving both the water vapor barrier property and the suppression of thickness unevenness in a well-balanced manner.

The present invention was made in consideration of the above circumstances, and aims to provide a food packaging film with minimal thickness variation and improved water vapor barrier properties.

The present inventors have conducted extensive research to solve the above problems, and have found that a measure of the calorific value of endothermic peak A observed in the range of 150° C. to 180° C. in the DSC curve of a stretched film layer in the second run of differential scanning calorimetry is effective as a design guideline for improving the balance between improving water vapor barrier property and suppressing thickness unevenness.
As a result of further intensive investigations based on the above findings, the inventors have found that a food packaging film with reduced thickness unevenness and improved water vapor barrier property can be obtained by using a uniaxially or biaxially stretched film layer in which the heat quantity of endothermic peak A observed in the range of 150°C or higher and 180°C or lower is within a specific range and which is composed of a propylene polymer composition containing a propylene-based polymer and a tackifier, and have arrived at the present invention.

That is, the present invention provides the food packaging film and food packaging body shown below.

[1]
A film for packaging food, comprising:
The present invention comprises a uniaxially or biaxially stretched film layer composed of a propylene polymer composition containing a propylene-based polymer and a tackifier,
The stretched film layer was measured using a differential scanning calorimeter.
A first differential scanning calorimetry (1st Run) consisting of a process of increasing the temperature from -50°C to 250°C at a temperature increase rate of 10°C/min, an isothermal process of maintaining the temperature at 250°C for 10 minutes, and a process of decreasing the temperature from 250°C to -50°C at a temperature decrease rate of 10°C/min;
A second differential scanning calorimetry (2nd Run) consisting of a process of heating from -50°C to 250°C at a heating rate of 10°C/min;
When I continued
In the DSC curve 2 obtained by the second differential scanning calorimetry, an endothermic peak A is observed in the range of 150° C. or higher and 180° C. or lower,
The heat quantity of the endothermic peak A is 80 J/g or more and 120 J/g or less.
[2]
In the food packaging film described in the above [1],
In the DSC curve 1 obtained by the first differential scanning calorimetry, an endothermic peak B is observed in the range of 150°C or higher and 165°C or lower, and an endothermic peak C is observed in the range of more than 165°C and 180°C or lower.
[3]
In the food packaging film described in [2] above,
A food packaging film in which the ratio (C/B) of the peak height C of the endothermic peak C to the peak height B of the endothermic peak B is 1.0 or more and 2.5 or less.
[4]
In the food packaging film according to any one of the above [1] to [3],
A food packaging film in which an exothermic peak D is observed in the range of 100°C or higher and 130°C or lower in a DSC curve 1 obtained by the first differential scanning calorimetry.
[5]
In the food packaging film described in [4] above,
The half-width of the exothermic peak D is 2.0°C or more and 7.0°C or less.
[6]
In the food packaging film according to any one of the above [1] to [5],
The food packaging film, wherein the softening temperature of the tackifier is 100°C or higher and 150°C or lower.
[7]
In the food packaging film according to any one of the above [1] to [6],
The food packaging film, wherein the tackifier comprises at least one selected from petroleum-based hydrocarbon resins and hydrogenated petroleum-based hydrocarbon resins.
[8]
In the food packaging film according to the above [7],
The food packaging film, wherein the tackifier comprises a hydrogenated petroleum-based hydrocarbon resin.
[9]
In the food packaging film according to any one of the above [1] to [8],
The food packaging film, wherein the content of a tackifier contained in the propylene-based polymer composition is 0.5% by mass or more and 30% by mass or less, when the entire propylene-based polymer composition is taken as 100% by mass.
[10]
In the food packaging film according to any one of the above [1] to [9],
The food packaging film has a tensile modulus T1 in the MD direction of 1000 MPa or more and 8000 MPa or less, as measured using a tensile tester in accordance with JIS K7127 (1999) at a measurement temperature of 23±2°C, ₅₀ ±5% RH, and a tensile speed of 5 mm/min.
[11]
In the food packaging film according to any one of the above [1] to [10],
The food packaging film has a total (T1 + T2) of the tensile modulus T1 in the MD direction and the tensile modulus _T2 in the TD direction of the food packaging film, which is measured in accordance with JIS K7127 (1999) using a tensile tester at a measurement temperature of 23± ₂ ° _C , 50±5% RH, and a tensile speed of 5 mm _/ min, of 5,000 MPa or more and 10,000 MPa or less.
[12]
In the food packaging film according to any one of the above [1] to [11],
The food packaging film further comprises a heat seal layer on at least one surface of the stretched film layer.
[13]
In the food packaging film according to the above [12],
The heat seal layer is provided so as to be in direct contact with the one surface of the stretched film layer.
[14]
In the food packaging film according to the above [12] or [13],
The heat seal layer of the food packaging film comprises one or more members selected from the group consisting of homopolypropylene and random copolymers of propylene and an α-olefin having from 2 to 10 carbon atoms.
[15]
In the food packaging film according to any one of the above [1] to [14],
The food packaging film further comprises a surface layer on one side of the stretched film layer.
[16]
In the food packaging film according to the above [15],
The surface layer of the food packaging film comprises one or more materials selected from the group consisting of homopolypropylene and random copolymers of propylene and an α-olefin having from 2 to 10 carbon atoms.
[17]
In the food packaging film according to any one of the above [1] to [16],
Food packaging film used for outer packaging bags.
[18]
A food packaging product using the food packaging film according to any one of [1] to [17] above.

The present invention provides a food packaging film with minimal thickness variation and improved water vapor barrier properties.

1 is a cross-sectional view showing a schematic example of the structure of a food packaging film according to an embodiment of the present invention. 1 is a cross-sectional view showing a schematic example of the structure of a food packaging film according to an embodiment of the present invention.

Below, an embodiment of the present invention will be described with reference to the drawings. Note that the drawings are schematic and do not correspond to the actual dimensional ratio. Note that "~" between numbers in the text indicates "above" to "below" unless otherwise specified.

<Food packaging film>
1 and 2 are cross-sectional views that diagrammatically show an example of the structure of a food packaging film 100 according to an embodiment of the present invention.
The food packaging film 100 according to this embodiment is a film for packaging food, and includes a uniaxially or biaxially stretched film layer (hereinafter also referred to as stretched film layer 101) made of a propylene polymer composition containing a propylene-based polymer and a tackifier. The stretched film layer is subjected to a process of increasing the temperature from -50°C to 250°C at a heating rate of 10°C/min, an isothermal process of maintaining the temperature at 250°C for 10 minutes, and a process of decreasing the temperature at a rate of 10°C/min. When a first differential scanning calorimetry (1st Run) consisting of a process of lowering the temperature from 250° C. to −50° C. at a heating rate of 10° C./min and a second differential scanning calorimetry (2nd Run) consisting of a process of raising the temperature from −50° C. to 250° C. at a heating rate of 10° C./min are successively performed, in DSC curve 2 obtained by the second differential scanning calorimetry, an endothermic peak A is observed in the range of 150° C. or more and 180° C. or less, and the heat quantity of the endothermic peak A is 80 J/g or more and 120 J/g or less.
This makes it possible to realize a food packaging film with less unevenness in thickness and improved water vapor barrier properties.
The lower limit of the heat quantity of the endothermic peak A is 80 J/g or more, but from the viewpoint of further improving the water vapor barrier property of the food packaging film 100, it is preferably 85 J/g or more, more preferably 90 J/g or more, and even more preferably 93 J/g or more.
The upper limit of the heat quantity of the endothermic peak A is 120 J/g or less, but from the viewpoint of further suppressing thickness unevenness, it is preferably 110 J/g or less, more preferably 108 J/g or less, and even more preferably 105 J/g or less.
Here, the heat quantity of the endothermic peak A is calculated by determining the area enclosed by the melting endothermic curve including the endothermic peak A and the baseline. The baseline is a line obtained by differentiating the Heat Flow with respect to time before and after the endothermic peak A to display the Deriv. Heat Flow, and connecting the point where the change in the Deriv. Heat Flow starts (i.e., the point where the flat region of the Deriv. Heat Flow ends) and the point where the change in the Deriv. Heat Flow ends (i.e., the point where the Deriv. Heat Flow enters the flat region).
When a plurality of endothermic peaks are observed in the range of 150° C. or higher and 180° C. or lower, the maximum peak is regarded as endothermic peak A.

As described above, from the viewpoint of reducing the environmental load, it is required to further improve the water vapor barrier property of the stretched polypropylene film. If the water vapor barrier property is improved, the thickness of the stretched polypropylene film can be reduced, so that the amount of the propylene-based polymer used can be reduced, and the environmental load can be reduced.
According to the study by the present inventors, it was found that the water vapor barrier property of the stretched polypropylene film can be improved by using a highly crystalline propylene polymer as the propylene polymer constituting the stretched polypropylene film. However, it was revealed that the use of such a highly crystalline propylene polymer is likely to cause stretching unevenness, and the thickness unevenness of the stretched polypropylene film obtained after the stretching step may become large.
Thus, the present inventors have found that there is a trade-off between the water vapor barrier property and thickness unevenness in a stretched polypropylene film. In other words, the present inventors have found that there is room for improvement in stretched polypropylene films from the viewpoint of improving both the water vapor barrier property and the suppression of thickness unevenness in a well-balanced manner.

The present inventors have conducted extensive research to solve the above problems, and have found that a measure of the calorific value of endothermic peak A observed in the range of 150° C. to 180° C. in the DSC curve of a stretched film layer in the second run of differential scanning calorimetry is effective as a design guideline for improving the balance between improving water vapor barrier property and suppressing thickness unevenness.
As a result of further intensive investigations based on the above findings, the inventors have found that a food packaging film with reduced thickness unevenness and improved water vapor barrier property can be obtained by using a uniaxially or biaxially stretched film layer in which the heat quantity of endothermic peak A observed in the range of 150° C. or higher and 180° C. is 80 J/g or higher and 120 J/g or lower and which is composed of a propylene polymer composition containing a propylene-based polymer and a tackifier.
That is, the food packaging film 100 according to this embodiment can provide a food package with less unevenness in thickness and improved water vapor barrier properties.
Furthermore, the food packaging film 100 according to this embodiment can improve the water vapor barrier property, so that sufficient water vapor barrier property can be obtained even if the thickness of the stretched film layer 101 is made thinner. Therefore, the food packaging film 100 according to this embodiment can reduce the amount of propylene-based polymer used in the food packaging film or package, thereby reducing the environmental load.
As described above, according to this embodiment, it is possible to realize an environmentally friendly food packaging body having sufficient water vapor barrier properties, and it is also possible to provide a food packaging film 100 with little thickness unevenness and excellent appearance, packaging suitability, and bag-making properties.

The stretched film layer 101 containing a propylene-based polymer and a tackifier according to this embodiment is controlled so that the calorific value of endothermic peak A observed in the range of 150° C. to 180° C. in the DSC curve 2 obtained by the second differential scanning calorimetry is 80 J/g to 120 J/g. This allows for a balanced improvement in both water vapor barrier property and suppression of thickness unevenness. The reason for this is unclear, but the following is thought to be the reason.
First, it is considered that the heat quantity of endothermic peak A is equal to or greater than the lower limit, thereby increasing the crystallinity of the stretched film layer 101, and thus improving the water vapor barrier property of the food packaging film 100 according to this embodiment. In addition, it is considered that the heat quantity of endothermic peak A is equal to or less than the upper limit, thereby reducing stretching unevenness due to crystallization of the stretched film layer 101, and thus reducing thickness unevenness that occurs during stretching of the stretched film layer 101. Furthermore, it is considered that the inclusion of a tackifier in the stretched film layer 101 makes it possible to widen the appropriate range of the heat quantity of endothermic peak A, and thus improves the degree of freedom in design.
The calorific value of the endothermic peak A of the stretched film layer 101 can be achieved, for example, by adjusting the content ratio of the propylene-based polymer contained in the stretched film layer 101 and various conditions during stretching. More specifically, the calorific value can be adjusted by using two or more kinds of propylene-based polymers having different melting points, crystallinity, stereoregularity, etc. in combination as the propylene-based polymers constituting the stretched film layer 101 and adjusting the ratio of these polymers, or by appropriately adjusting the stretch ratio during stretching, the temperature during stretching, the temperature and time of heat treatment, etc.

In the DSC curve 1 obtained by the first differential scanning calorimetry of the food packaging film 100 according to this embodiment, it is preferable that, for example, an endothermic peak B is observed in the range of 150°C or higher and 165°C or lower, and an endothermic peak C is observed in the range of more than 165°C and 180°C or lower.
By having endothermic peak B in the range of 150° C. or more and 165° C. or less, it is possible to further reduce unevenness in the stretched film layer 101, and as a result, it is possible to further reduce unevenness in thickness that occurs during stretching of the stretched film layer 101. In addition, by having endothermic peak C in the range of more than 165° C. and 180° C. or less, it is possible to further increase the crystallinity of the stretched film layer 101, and as a result, it is possible to further improve the water vapor barrier property of the food packaging film 100 according to this embodiment.

In the food packaging film 100 according to this embodiment, the ratio (C/B) of the peak height C of the endothermic peak C to the peak height B of the endothermic peak B is preferably 1.0 or more, more preferably 1.2 or more, and even more preferably 1.4 or more, from the viewpoint of further improving the water vapor barrier property of the food packaging film 100.
In addition, in the food packaging film 100 of this embodiment, the ratio (C/B) of the peak height C of the endothermic peak C to the peak height B of the endothermic peak B is preferably 2.5 or less, more preferably 2.0 or less, and even more preferably 1.8 or less, from the viewpoint of further suppressing thickness unevenness of the food packaging film 100.
Here, the peak height B of endothermic peak B and the peak height C of endothermic peak C are the Heat Flow values of a straight line P-Q, where Q is the intersection point between the baseline and a line drawn perpendicular to the X-axis from the peak top P. The baseline is a line obtained by differentiating the Heat Flow with respect to time before and after endothermic peak B and endothermic peak C to display the Deriv. Heat Flow, and connecting the point where the change in the Deriv. Heat Flow begins (i.e., the point where the flat region of the Deriv. Heat Flow ends) and the point where the change in the Deriv. Heat Flow ends (i.e., the point where the Deriv. Heat Flow enters the flat region).

In the DSC curve 1 obtained by the first differential scanning calorimetry of the food packaging film 100 according to this embodiment, it is preferable that an exothermic peak D is observed, for example, in the range of 100°C to 130°C, preferably in the range of 100°C to 120°C. This can further increase the crystallinity of the stretched film layer 101, and as a result, the balance between improving the water vapor barrier property and suppressing thickness unevenness of the food packaging film 100 according to this embodiment can be further improved.

The half width of the exothermic peak D is, for example, 2.5°C or more and 7.0°C or less, preferably 3.0°C or more and 6.5°C or less, and more preferably 3.5°C or more and 6.0°C or less.
By setting the half-width of the heat generation peak D to be equal to or greater than the lower limit, the internal strain of the stretched film layer 101 can be further reduced, and as a result, the thickness unevenness that occurs during stretching of the stretched film layer 101 can be further reduced.
Furthermore, by setting the half-width of the exothermic peak D to be equal to or less than the upper limit, the crystallinity of the stretched film layer 101 can be further increased, and as a result, the water vapor barrier property of the food packaging film 100 in this embodiment can be further improved.
Here, the half-width of the exothermic peak D can be calculated, for example, by the following method. First, the intersection point between the baseline and a line drawn perpendicularly from the peak top R of the exothermic peak D toward the X-axis is defined as S. Next, when a straight line is drawn that passes through the midpoint of the straight line R-S and is parallel to the X-axis, the points at which the straight line intersects with the exothermic peak D are defined as _T1 and _T2 , respectively. The length of the straight line _T1 - _T2 is defined as the half-width. The baseline is a line that connects the point at which the change in the Deriv. Heat Flow begins (i.e., the point at which the flat region of the Deriv. Heat Flow ends) and the point at which the change in the Deriv. Heat Flow ends (i.e., the point at which the Deriv. Heat Flow enters the flat region) before and after the exothermic peak D by differentiating the Heat Flow with respect to time.

In the food packaging film 100 according to this embodiment, the sum (T1 + T2) of the tensile modulus T1 in the MD direction and the tensile modulus _T2 in the TD direction, measured in accordance with JIS K7127 (1999) using a tensile tester at a measurement temperature of ₂₃ ±2°C, 50± ₅ % RH, and a tensile speed of 5 mm/min, is preferably 5,000 MPa or more, more preferably 6,000 _MPa or more, even more preferably 7,000 MPa or more, and particularly preferably 7,500 MPa or more, and is preferably 10,000 MPa or less, more preferably 9,000 MPa or less, and even more preferably 8,500 MPa or less.
When the sum ( _T1 + _T2 ) of the tensile modulus _T1 in the MD direction and the tensile modulus _T2 in the TD direction is equal to or greater than the lower limit, the food packaging film 100 according to this embodiment can have a good balance of heat sealability, water vapor barrier property, and transparency. Furthermore, the food packaging film 100 according to this embodiment can have good stiffness, which can prevent the film from shifting position during heat sealing, and prevent the occurrence of sealing defects.
In other words, when the sum of the tensile modulus _T1 in the MD direction and the tensile modulus _T2 in the TD direction ( _T1 + _T2 ) is equal to or greater than the above-mentioned lower limit, the food packaging film 100 of this embodiment can achieve a good balance of heat sealability, water vapor barrier property, transparency, and packaging suitability.
In addition, when the sum ( _T1 + _T2 ) of the tensile modulus _T1 in the MD direction and the tensile modulus _T2 in the TD direction is equal to or less than the upper limit, problems such as breakage during film formation are unlikely to occur, continuous stretching of the film is facilitated, and industrial continuous productivity can be improved.
Such a tensile modulus is a substitute value for quantitatively measuring the stiffness of a film, and can be achieved, for example, by adjusting the contents of the first propylene-based polymer and the second propylene-based polymer contained in the stretched film layer 101 and various conditions during stretching. More specifically, the elastic modulus can be adjusted by using the first propylene-based polymer and the second propylene-based polymer in combination as the propylene-based polymer constituting the stretched film layer 101, or by appropriately adjusting the stretch ratio during stretching, the temperature during stretching, the temperature and time of heat treatment, etc., to adjust the elastic modulus and thus the tensile modulus of the food packaging film 100.

In addition, in the food packaging film 100 according to this embodiment, the lower limit of the tensile modulus _T1 in the MD direction is preferably 1000 MPa, more preferably 1500 MPa or more, even more preferably 2000 MPa or more, and particularly preferably 2400 MPa or more.
stomach.
When the tensile modulus _T1 in the MD direction is equal to or greater than the lower limit, the food packaging film 100 according to this embodiment can have a better balance of heat sealability, water vapor barrier property, and transparency. Furthermore, the stiffness of the food packaging film 100 according to this embodiment can be improved, which in turn can prevent the film from shifting position during heat sealing, and can further prevent the occurrence of sealing defects.
In other words, when the tensile modulus _T1 in the MD direction is equal to or greater than the above lower limit, the food packaging film 100 according to this embodiment can have a better balance of heat sealability, water vapor barrier property, transparency, and packaging suitability.
In addition, in the food packaging film 100 according to this embodiment, the upper limit of the tensile modulus _T1 in the MD direction is preferably 8000 MPa or less, more preferably 6000 MPa or less, even more preferably 4000 MPa or less, and particularly preferably 3000 MPa or less.
When the tensile modulus _T1 in the MD direction is equal to or less than the upper limit, the stretching unevenness due to crystallization of the stretched film layer 101 can be further reduced, and as a result, the thickness unevenness occurring during stretching of the stretched film layer 101 can be further reduced.

Here, the food packaging produced using the food packaging film 100 according to this embodiment exhibits sufficient performance in terms of water vapor barrier properties. Therefore, it can be particularly suitably used as a film constituting a food packaging for packaging foods (e.g., dried foods) that require water vapor barrier properties but do not require much oxygen barrier properties.

Food packages produced using the food packaging film 100 according to this embodiment have sufficient water vapor barrier properties. From the viewpoint of stably obtaining food packages with excellent water vapor barrier properties, the food packaging film 100 preferably has a water vapor permeability measured by the following method of 5.0 g/( ^m2 ·24 h) or less, more preferably 4.5 g/( ^m2 ·24 h) or less, and even more preferably 4.0 g/( ^m2 ·24 h) or less.
(Measuring method)
The food packaging film 100 is folded back so that the heat seal layer 103 is on the inside, and the two sides are heat sealed to form a bag. Calcium chloride is then placed inside as the content. The other side is then heat sealed to create a bag with a surface area of 0.01 ^m2 . The resulting bag is then stored for 72 hours under conditions of 40°C and 90% RH. The weight of the calcium chloride before and after storage is measured, and the water vapor permeability (g/( ^m2 ·24h)) is calculated from the difference.
Such a water vapor permeability can be achieved, for example, by adjusting the content ratio of the propylene-based polymer and tackifier contained in the stretched film layer 101, the thickness of the stretched film layer 101, and the like.

The thickness of the food packaging film 100 in this embodiment is not particularly limited, but can be set arbitrarily depending on the desired objectives such as water vapor barrier properties, cost, mechanical strength, transparency, etc., and is, for example, not particularly limited, from 5 μm to 100 μm, preferably from 10 μm to 50 μm, and more preferably from 15 μm to 40 μm.
When the thickness of the food packaging film 100 is within the above range, the balance of bag formability, mechanical properties, handleability, appearance, transparency, moldability, lightweight property, and the like is excellent.

Below, we will explain each layer that makes up the food packaging film 100.

[Stretched film layer]
The stretched film layer 101 according to this embodiment is formed, for example, by uniaxially or biaxially stretching a film constituted of a propylene polymer composition containing a propylene-based polymer and a tackifier.

The stretched film layer 101 according to this embodiment may be a single layer, or may be a laminate of multiple layers made of a propylene-based polymer composition, but it must be uniaxially or biaxially stretched.

In addition, in the food packaging film 100, the ratio of the thickness of the stretched film layer 101 to the total thickness of the food packaging film 100 is preferably 50% or more and 100% or less, more preferably 60% or more and 99% or less, even more preferably 70% or more and 97% or less, and particularly preferably 75% or more and 95% or less.

(Propylene-Based Polymer Composition)
The propylene-based polymer composition according to the present embodiment contains a propylene-based polymer and a tackifier.
The total content of the propylene polymer and the tackifier in the propylene polymer composition according to the present embodiment, i.e., the stretched film layer 101, is preferably 50% by mass or more and 100% by mass or less, more preferably 70% by mass or more and 100% by mass or less, still more preferably 90% by mass or more and 100% by mass or less, and particularly preferably 95% by mass or more and 100% by mass or less, when the entire propylene polymer composition is taken as 100% by mass. This makes it possible to achieve a better balance of stiffness, water vapor barrier property, mechanical properties, handleability, appearance, moldability, etc. of the film.

(Propylene-based polymer)
Examples of the propylene-based polymer according to the present embodiment include propylene homopolymers and copolymers of propylene and ethylene or α-olefins having 4 to 20 carbon atoms. Examples of the α-olefins having 4 to 20 carbon atoms include 1-butene, 1-pentene, 1-hexene, 4-methyl-1-pentene, 1-octene, 1-decene, 1-dodecene, 1-tetradecene, 1-hexadecene, 1-octadecene, and 1-eicosene. Among these, ethylene or α-olefins having 4 to 10 carbon atoms are preferred, and ethylene is more preferred. These α-olefins may form a random copolymer or a block copolymer with propylene. The content of the structural unit derived from ethylene or α-olefins having 4 to 20 carbon atoms is preferably 5 mol % or less, and more preferably 2 mol % or less, when the entire propylene-based polymer is taken as 100 mol %. The propylene-based polymer in the stretched film layer 101 may be used alone or in combination of two or more kinds.
Among these, from the viewpoint of obtaining a stretched film layer 101 having a better balance of properties such as heat resistance, water vapor barrier property, mechanical properties, and rigidity, a propylene homopolymer is preferred as the propylene-based polymer.

Here, in order to satisfy the DSC characteristics such as the endothermic peak characteristics and exothermic peak characteristics of the stretched film layer 101 described above, it is important to select an appropriate propylene polymer.
More specifically, in the stretched film layer 101 according to this embodiment, for example, two or more kinds of propylene-based polymers having different melting points, crystallinity, stereoregularity, etc. are used in combination, and the ratio of these polymers is adjusted, thereby making it possible to adjust the DSC characteristics such as the endothermic peak characteristics and exothermic peak characteristics described above.

For example, the propylene-based polymer contained in the stretched film layer 101 according to this embodiment may include a first propylene-based polymer having a melting point measured by DSC in the range of 130° C. or more and 162° C. or less, preferably 133° C. or more and 162° C. or less, and a second propylene-based polymer having a melting point measured by DSC in the range of more than 162° C. and 180° C. or less, preferably 163° C. or more and 175° C. or less.
When the total amount of the first propylene-based polymer and the second propylene-based polymer contained in the stretched film layer 101 is taken as 100% by mass, from the viewpoint of improving the water vapor barrier property of the food packaging film 100, the content of the second propylene-based polymer is preferably 1% by mass or more, more preferably 10% by mass or more, even more preferably 25% by mass or more, even more preferably 35% by mass or more, and particularly preferably 50% by mass or more.
Furthermore, when the total amount of the first propylene-based polymer and the second propylene-based polymer contained in the stretched film layer 101 is taken as 100% by mass, the content of the second propylene-based polymer is preferably 85% by mass or less, more preferably 75% by mass or less, and even more preferably 70% by mass or less, from the viewpoint of further suppressing unevenness in the thickness of the food packaging film 100.

In the present embodiment, the second propylene polymer is preferably a highly stereoregular propylene polymer. Here, the highly stereoregular propylene polymer refers to a propylene polymer having an isotactic mesopentad fraction (mmmm), which is an index of stereoregularity, of 96.0% or more.
The isotactic mesopentad fraction (mmmm) of the highly stereoregular propylene polymer according to the present embodiment is preferably 96.5% or more, more preferably 97.0% or more. The upper limit of the isotactic mesopentad fraction (mmmm) of the highly stereoregular propylene polymer is not particularly limited, but from the viewpoint of ease of production, it is 99.5% or less, more preferably 99.3% or less, and even more preferably 99.0% or less.
The isotactic mesopentad fraction (mmmm) can be determined from ¹³ C-nuclear magnetic resonance (NMR) spectrum by a known method.

The propylene-based polymer according to this embodiment can be produced by various methods. For example, it can be produced using a known catalyst such as a Ziegler-Natta catalyst or a metallocene catalyst.

The melt flow rate (MFR) of the propylene-based polymer according to this embodiment, measured in accordance with ASTM D1238 at 230°C under a load of 2.16 kg, is preferably 0.5 g/10 min or more, more preferably 1 g/10 min or more, and even more preferably 2 g/10 min or more, from the viewpoints of fluidity and moldability, and is preferably 20 g/10 min or less, more preferably 10 g/10 min or less, and even more preferably 7 g/10 min or less, from the viewpoint of further stabilizing moldability.

(Tackifier)
As the tackifier according to the present embodiment, a resinous substance having the property of imparting tackiness, which is generally manufactured and sold as a tackifier, can be used.
Examples of such tackifiers include chroman-based resins such as chroman-indene resins; phenol-based resins such as phenol-formaldehyde resins and xylene-formaldehyde resins; terpene-based resins such as terpene-phenolic resins, terpene resins (α,β-pinene resins), aromatic modified terpene resins, and hydrogenated terpene resins; petroleum-based hydrocarbon resins such as synthetic polyterpene resins, aromatic hydrocarbon resins, aliphatic hydrocarbon resins, aliphatic cyclic hydrocarbon resins, aliphatic-alicyclic petroleum resins, aliphatic-aromatic petroleum resins, unsaturated hydrocarbon polymers, and hydrocarbon tackified resins; hydrogenated products of the above petroleum-based hydrocarbon resins (also called hydrogenated petroleum hydrocarbon resins); and rosin-based resins such as pentaerythritol esters of rosin, glycerin esters of rosin, hydrogenated rosin, hydrogenated rosin esters, special rosin esters, and rosin-based tackifiers.
Among these, at least one selected from petroleum-based hydrocarbon resins and hydrogenated petroleum-based hydrocarbon resins is preferred, from the viewpoint of being compatible with propylene-based polymers and being able to more effectively improve the water vapor barrier properties while further suppressing unevenness in the thickness of the food packaging film 100, and hydrogenated petroleum-based hydrocarbon resins are more preferred.
Here, the hydrogenation rate of the hydrogenated petroleum-based hydrocarbon resin is not particularly limited, but from the viewpoint of superior compatibility with the propylene-based polymer, it is preferably 90% or more, more preferably 95% or more, and even more preferably 99% or more.

The lower limit of the softening temperature of the tackifier according to the present embodiment, measured in accordance with JIS K2207, is not particularly limited, but is preferably 100° C. or higher, more preferably 110° C. or higher, even more preferably 125° C. or higher, and particularly preferably 130° C. or higher. When the softening temperature is equal to or higher than the above lower limit, the generation of smoke or the like during molding of the stretched film layer 101 can be suppressed, and as a result, the dirt of the molding machine can be suppressed. Furthermore, when the softening temperature is equal to or higher than the above lower limit, the orientation of the propylene-based polymer in the stretched film layer 101 can be improved, so that the water vapor barrier property of the stretched film layer 101 can be further improved and the thickness unevenness of the stretched film layer 101 can be further suppressed.
The upper limit of the softening temperature is not particularly limited, but is preferably not more than 150° C., more preferably not more than 145° C., and even more preferably not more than 140° C. When the softening temperature is not more than the upper limit, compatibility with the propylene-based polymer is improved and the orientation of the propylene-based polymer in the stretched film layer 101 can be improved, so that thickness unevenness of the stretched film layer 101 can be further suppressed.

From the viewpoint of further improving the water vapor barrier property of the food packaging film 100 and further suppressing thickness unevenness, the lower limit of the content of the tackifier in the propylene-based polymer composition according to this embodiment, i.e., the stretched film layer 101, is preferably 0.5% by mass or more, more preferably 1% by mass or more, even more preferably 3% by mass or more, and particularly preferably 5% by mass or more, when the entire propylene-based polymer composition is taken as 100% by mass.
Furthermore, the upper limit of the content of the tackifier contained in the propylene-based polymer composition according to the present embodiment, i.e., the stretched film layer 101, is preferably 30% by mass or less, more preferably 25% by mass or less, even more preferably 20% by mass or less, and particularly preferably 15% by mass or less, when the entire propylene-based polymer composition is taken as 100% by mass, from the viewpoint of improving the processability, dimensional stability, transparency, etc. of the food packaging film 100. When the content of the tackifier is equal to or less than the upper limit, bleeding out of the tackifier onto the film surface is suppressed, thereby improving the blocking resistance and lamination strength of the food packaging film 100 and suppressing contamination of the molding machine.

(Other ingredients)
To the propylene polymer composition according to the present embodiment, various additives such as a heat stabilizer, a weather stabilizer, an antioxidant, an ultraviolet absorber, a lubricant, a slipping agent, a nucleating agent, an antiblocking agent, an antistatic agent, an antifogging agent, a pigment, a dye, and an inorganic or organic filler may be added as necessary within a range that does not impair the object of the present embodiment.

(Method for preparing propylene polymer composition)
The propylene polymer composition according to the present embodiment can be prepared by mixing or melt-kneading the components using a dry blend, a tumbler mixer, a Banbury mixer, a single-screw extruder, a twin-screw extruder, a high-speed twin-screw extruder, a heat roll, or the like.

[Heat seal layer]
In order to impart heat sealability, the food packaging film 100 according to this embodiment preferably includes a heat seal layer 103 on at least one surface of the stretched film layer 101. The heat seal layer 103 may be provided on both surfaces of the stretched film layer 101.
In addition, from the viewpoint of improving the heat sealability of the food packaging film 100, the heat seal layer 103 is preferably provided as the outermost layer of the food packaging film 100 according to this embodiment.

The heat seal layer 103 is preferably provided so as to be in direct contact with the surface of the stretched film layer 101. This simplifies the manufacturing process of the food packaging film 100.

In the food packaging film 100, the thickness of the heat seal layer 103 is preferably 0.1 μm to 10 μm, more preferably 0.2 μm to 9 μm, even more preferably 0.5 μm to 8 μm, and particularly preferably 1 μm to 8 μm. Here, the thickness of the heat seal layer 103 refers to the thickness of the heat seal layer 103 provided on one side of the stretched film layer 101.
When the thickness of the heat seal layer 103 is equal to or greater than the above lower limit, the heat sealability of the food packaging film 100 can be further improved.
In addition, by making the thickness of the heat seal layer 103 equal to or less than the above upper limit, the blocking properties and slip properties required for a food packaging film can be further improved.
That is, by providing the heat seal layer 103 so as to be in direct contact with the surface of the stretched film layer 101, the manufacturing process of the food packaging film 100 can be simplified.
In this embodiment, when the heat seal layer 103 is provided on both sides of the stretched film layer 101 , the above thickness of the heat seal layer 103 indicates the thickness of the heat seal layer 103 provided on one side of the stretched film layer 101 .

In the food packaging film 100, it is preferable that the heat seal layer 103 provided on one side is a single layer. This makes it possible to further simplify the manufacturing process of the food packaging film 100.

The heat seal layer 103 is preferably formed by stretching simultaneously with the unstretched film of the stretched film layer 101. This allows the food packaging film 100 to be produced using a molding method such as co-extrusion, i.e., a laminated film produced in a single molding operation, further simplifying the manufacturing process of the food packaging film 100. Therefore, it is preferable that the heat seal layer 103 is uniaxially or biaxially stretched.

(Polyolefin)
The heat seal layer 103 according to the present embodiment is composed of, for example, a polyolefin-based resin composition (A) containing a polyolefin. Examples of the polyolefin constituting the heat seal layer 103 include homopolymers or copolymers of α-olefins such as ethylene, propylene, butene-1, hexene-1, 4-methyl-pentene-1, and octene-1; high-pressure low-density polyethylene; linear low-density polyethylene (LLDPE); high-density polyethylene; polypropylene; random copolymers of propylene and α-olefins having 2 to 10 carbon atoms; ethylene-vinyl acetate copolymers (EVA); and ionomer resins.
Among these, the polyolefin constituting the heat seal layer 103 is preferably at least one selected from homopolypropylene and a random copolymer of propylene and an α-olefin having 2 to 10 carbon atoms, in view of an excellent balance of adhesion to the stretched film layer 101, heat sealability, etc.
From the viewpoint of heat sealability and stability of heat seal strength, the heat seal layer 103 preferably contains an olefin-based elastomer among the above polyolefins.

The propylene-α-olefin random copolymer according to the present embodiment is a random copolymer of propylene and an α-olefin (however, the α-olefin excludes propylene), and examples of the α-olefin include ethylene, 1-butene, 1-pentene, 1-hexene, 4-methyl-1-pentene, 1-octene, etc. These copolymers may be used alone or in combination of two or more.
Among the propylene/α-olefin random copolymers, propylene/ethylene random copolymers, propylene/ethylene/1-butene random copolymers, and propylene/1-butene random copolymers are preferred.

The melting point of the polyolefin constituting the heat seal layer 103 according to this embodiment is preferably in the range of 60° C. to 175° C., more preferably 65° C. to 170° C., and even more preferably 70° C. to 167° C. When the melting point of the polyolefin is equal to or higher than the lower limit, stickiness of the surface of the heat seal layer 103 can be suppressed, and the blocking resistance of the food packaging film 100 can be improved.
Furthermore, when the melting point of the polyolefin is equal to or lower than the above upper limit, the heat sealability of the food packaging film 100 can be improved.

Examples of the olefin-based elastomer include an α-olefin polymer having 2 to 20 carbon atoms or a copolymer of ethylene and an α-olefin, each of which has a melting point of preferably 110° C. or less, more preferably 100° C. or less, and even more preferably 80° C. or less, or which has no observable melting point; a copolymer of ethylene and an unsaturated carboxylic acid or an unsaturated carboxylic acid ester; and the like.
Specifically, ethylene-propylene copolymer, ethylene-1-butene copolymer, ethylene-1-hexene copolymer, ethylene-4-methylpentene-1 copolymer, ethylene-1-octene copolymer, propylene homopolymer, propylene-ethylene copolymer, propylene-ethylene-1-butene copolymer, 1-butene homopolymer, 1-butene-ethylene copolymer, 1-butene-propylene copolymer, 4-methylpentene-1 homopolymer, 4-methylpentene-1-propylene copolymer, 4-methylpentene-1-1-butene copolymer, 4-methylpentene-1-propylene-1-butene copolymer, propylene-1-butene copolymer, ethylene-vinyl acetate copolymer, ethylene-methacrylic acid copolymer, ethylene-methyl methacrylate copolymer, and the like can be mentioned.
From the viewpoint of heat sealability and stability of heat seal strength, a propylene-1-butene copolymer is particularly preferred.

The melt flow rate (MFR) of the polyolefin constituting the heat seal layer 103 according to this embodiment, measured in accordance with ASTM D1238 at 230°C and a load of 2.16 kg, is preferably 0.5 g/10 min or more, more preferably 1 g/10 min or more, and even more preferably 2 g/10 min or more, from the viewpoint of fluidity and moldability, and is preferably 20 g/10 min or less, more preferably 10 g/10 min or less, and even more preferably 7 g/10 min or less, from the viewpoint of further stabilizing moldability.

The content of polyolefin in the polyolefin resin composition (A) according to the present embodiment, i.e., the heat seal layer 103, is preferably 50% by mass or more and 100% by mass or less, more preferably 70% by mass or more and 100% by mass or less, still more preferably 90% by mass or more and 100% by mass or less, and particularly preferably 95% by mass or more and 100% by mass or less, when the entire polyolefin resin composition (A) is taken as 100% by mass. This makes it possible to achieve a better balance between adhesion to the stretched film layer 101, heat sealability, and the like.
In addition, the content of the olefin-based elastomer in the polyolefin resin composition (A) according to this embodiment, i.e., the heat seal layer 103, is preferably 10% by mass or more and 50% by mass or less, more preferably 15% by mass or more and 45% by mass or less, and even more preferably 20% by mass or more and 40% by mass or less, when the content of the polyolefin contained in the stretched film layer 101 is taken as 100% by mass.

(Other ingredients)
The polyolefin resin composition (A) constituting the heat seal layer 103 according to this embodiment may contain various additives such as a tackifier, a heat stabilizer, a weather stabilizer, an antioxidant, an ultraviolet absorber, a lubricant, a slip agent, a nucleating agent, an antiblocking agent, an antistatic agent, an antifogging agent, a pigment, a dye, an inorganic or organic filler, etc., as necessary, within a range that does not impair the object of this embodiment. In particular, the heat seal layer 103 according to this embodiment preferably contains an antiblocking agent from the viewpoint of improving the blocking resistance of the food packaging film 100 according to this embodiment.
Examples of the anti-blocking agent include talc, silica, clay, calcium carbonate, synthetic zeolite, starch, aluminum oxide, acrylic resin, methacrylic resin, silicone resin, polytetrafluoroethylene resin, and the like.

Moreover, from the viewpoint of improving the heat sealability of the heat seal layer 103, it is preferable that the heat seal layer 103 does not substantially contain a tackifier. More specifically, the content of the tackifier in the heat seal layer 103 is preferably 0.5% by mass or less, more preferably 0.1% by mass or less, even more preferably 0.01% by mass or less, and particularly preferably 0% by mass.
Here, the tackifier is a resinous substance having the property of imparting adhesiveness, which is generally manufactured and sold as a tackifier.
Examples of such tackifiers include chroman-based resins such as chroman-indene resins; phenol-based resins such as phenol-formaldehyde resins and xylene-formaldehyde resins; terpene-based resins such as terpene-phenolic resins, terpene resins (α,β-pinene resins), aromatic modified terpene resins, and hydrogenated terpene resins; petroleum-based hydrocarbon resins such as synthetic polyterpene resins, aromatic hydrocarbon resins, aliphatic hydrocarbon resins, aliphatic cyclic hydrocarbon resins, aliphatic-alicyclic petroleum resins, aliphatic-aromatic petroleum resins, unsaturated hydrocarbon polymers, and hydrocarbon tackified resins; hydrogenated products of the above petroleum-based hydrocarbon resins (also called hydrogenated petroleum hydrocarbon resins); and rosin-based resins such as pentaerythritol esters of rosin, glycerin esters of rosin, hydrogenated rosin, hydrogenated rosin esters, special rosin esters, and rosin-based tackifiers.

(Method for preparing polyolefin resin composition (A))
The polyolefin resin composition (A) according to the present embodiment can be prepared, for example, by mixing or melt-kneading the components using a dry blend, a tumbler mixer, a Banbury mixer, a single-screw extruder, a twin-screw extruder, a high-speed twin-screw extruder, a heated roll, or the like.

[Surface layer]
In order to improve the printability of the surface, the food packaging film 100 of this embodiment preferably further comprises a surface layer 105 on one side of the stretched film layer 101 (on the side opposite to the side on which the heat seal layer 103 is provided, in the case where the film has a heat seal layer 103) as shown in FIG. 2.
In addition, from the viewpoint of improving the printability of the food packaging film 100, the surface layer 105 is preferably provided as the outermost layer of the food packaging film 100 according to this embodiment.

The surface layer 105 is preferably provided so as to be in direct contact with the surface of the stretched film layer 101. This simplifies the manufacturing process of the food packaging film 100.

In the food packaging film 100, the thickness of the surface layer 105 is preferably 0.1 μm or more and 10 μm or less, more preferably 0.2 μm or more and 9 μm or less, even more preferably 0.5 μm or more and 8 μm or less, and particularly preferably 1 μm or more and 8 μm or less. Here, the thickness of the surface layer 105 refers to the thickness of the surface layer 105 provided on one side of the stretched film layer 101.
By making the thickness of the surface layer 105 equal to or greater than the above lower limit, the printability of the food packaging film 100 can be further improved.
Furthermore, by having the thickness of the surface layer 105 be equal to or less than the above upper limit, the blocking properties and slip properties required during printing can be further improved.
That is, by providing the surface layer 105 so as to be in direct contact with the surface of the stretched film layer 101, the manufacturing process of the food packaging film 100 can be simplified.

In the food packaging film 100, it is preferable that the surface layer 105 is a single layer. This can further simplify the manufacturing process of the food packaging film 100.

In addition, the surface layer 105 is preferably formed by stretching simultaneously with the unstretched film of the stretched film layer 101. This allows the food packaging film 100 to be produced using a molding method such as co-extrusion, i.e., a laminated film produced in a single molding operation, thereby further simplifying the manufacturing process of the food packaging film 100. Therefore, it is preferable that the surface layer 105 is uniaxially or biaxially stretched.

The surface layer 105 may be subjected to a surface treatment in order to improve the printability of the food packaging film 100. Specifically, a surface activation treatment such as corona treatment, flame treatment, plasma treatment, primer coating treatment, or ozone treatment may be performed.

(Polyolefin)
The surface layer 105 according to the present embodiment is made of, for example, a polyolefin-based resin composition (B) containing a polyolefin. Examples of the polyolefin constituting the surface layer 105 include homopolymers or copolymers of α-olefins such as ethylene, propylene, butene-1, hexene-1, 4-methyl-pentene-1, and octene-1; high-pressure low-density polyethylene; linear low-density polyethylene (LLDPE); high-density polyethylene; polypropylene; random copolymers of propylene and α-olefins having 2 to 10 carbon atoms; ethylene-vinyl acetate copolymers (EVA); and ionomer resins.
Among these, the polyolefin constituting the surface layer 105 is preferably at least one selected from homopolypropylene and a random copolymer of propylene and an α-olefin having 2 to 10 carbon atoms, because it has an excellent balance of adhesion to the stretched film layer 101, printability, and the like.

The propylene-α-olefin random copolymer according to the present embodiment is a random copolymer of propylene and an α-olefin (however, the α-olefin excludes propylene), and examples of the α-olefin include ethylene, 1-butene, 1-pentene, 1-hexene, 4-methyl-1-pentene, 1-octene, etc. These copolymers may be used alone or in combination of two or more.
Among the propylene/α-olefin random copolymers, propylene/ethylene random copolymers, propylene/ethylene/1-butene random copolymers, and propylene/1-butene random copolymers are preferred.

The melting point of the polyolefin constituting the surface layer 105 according to this embodiment is preferably in the range of 90°C to 175°C, more preferably 95°C to 170°C, and even more preferably 100°C to 167°C. If the melting point of the polyolefin is equal to or higher than the lower limit, the stickiness of the surface of the surface layer 105 can be suppressed, and the blocking resistance of the food packaging film 100 can be improved.

The melt flow rate (MFR) of the polyolefin constituting the surface layer 105 according to this embodiment, measured in accordance with ASTM D1238 at 230°C under a load of 2.16 kg, is preferably 0.5 g/10 min or more, more preferably 1 g/10 min or more, and even more preferably 2 g/10 min or more, from the viewpoint of fluidity and moldability, and is preferably 20 g/10 min or less, more preferably 10 g/10 min or less, and even more preferably 7 g/10 min or less, from the viewpoint of further stabilizing moldability.

The content of polyolefin in the polyolefin resin composition (B) according to this embodiment, i.e., the surface layer 105, is preferably 50% by mass or more and 100% by mass or less, more preferably 70% by mass or more and 100% by mass or less, even more preferably 90% by mass or more and 100% by mass or less, and particularly preferably 95% by mass or more and 100% by mass or less, when the entire polyolefin resin composition (B) is taken as 100% by mass. This allows for a better balance between adhesion to the stretched film layer 101, printability, etc.

(Other ingredients)
If necessary, various additives such as tackifiers, heat stabilizers, weather stabilizers, antioxidants, UV absorbers, lubricants, slip agents, nucleating agents, antiblocking agents, antistatic agents, antifogging agents, pigments, dyes, and inorganic or organic fillers may be added to the polyolefin resin composition (B) constituting the surface layer 105 in this embodiment, within a range that does not impair the object of this embodiment.
In particular, the surface layer 105 according to this embodiment preferably contains an anti-blocking agent from the viewpoint of improving the blocking resistance of the food packaging film 100 according to this embodiment.
As the anti-blocking agent, for example, the same anti-blocking agent as that used in the heat seal layer 103 described above can be used.

(Method for preparing polyolefin resin composition (B))
The polyolefin resin composition (B) according to the present embodiment can be prepared, for example, by mixing or melt-kneading the respective components using a dry blend, a tumbler mixer, a Banbury mixer, a single-screw extruder, a twin-screw extruder, a high-speed twin-screw extruder, a heated roll, or the like.

<Method of manufacturing food packaging film>
The food packaging film 100 according to this embodiment can be obtained, for example, by co-extruding a propylene-based polymer composition for forming the stretched film layer 101, and, if necessary, a polyolefin-based resin composition (A) for forming the heat seal layer 103, and a polyolefin-based resin composition (B) for forming the surface layer 105 into a film, and then uniaxially or biaxially stretching the film obtained by the co-extruding process using a known stretched film production method.
The molding device and molding conditions are not particularly limited, and conventionally known molding devices and molding conditions can be used. As the molding device, a T-die extruder, a multi-layer T-die extruder, an inflation molding machine, a multi-layer inflation molding machine, or the like can be used. As the stretching conditions, for example, known manufacturing conditions for stretched polypropylene films can be used. More specifically, in the sequential biaxial stretching method, for example, the longitudinal stretching temperature may be set to 100°C to 145°C, the longitudinal stretching ratio may be set to 4.5 to 6 times, the transverse stretching temperature may be set to 130°C to 190°C, and the transverse stretching ratio may be set to 9 to 11 times.
In addition, the food packaging film 100 of this embodiment can also be obtained by separately molding the stretched film layer 101 and, if necessary, the heat seal layer 103 and the surface layer 105, and then laminating and heat molding these.

<Applications of food packaging film>
The food packaging film 100 according to this embodiment can also be suitably used as a film constituting a food package. The food package according to this embodiment is, for example, a packaging bag itself used for the purpose of containing food, or a bag containing food. Furthermore, the food package according to this embodiment may use the food packaging film 100 in a part thereof, or the food packaging film 100 may be used for the entire food package, depending on the application.

The food packaging film 100 according to this embodiment is preferably used for outer packaging bags that require water vapor barrier properties.
Furthermore, when the food packaging film 100 according to the present embodiment is used in an integrated package consisting of food, individual packaging bags for individually packaging the food, and an outer packaging bag for packaging a plurality of the individual packaging bags, the food packaging film 100 is preferably used in the outer packaging bag for which water vapor barrier properties are required in the integrated package, thereby making it possible to obtain an integrated package having sufficient water vapor barrier properties.

The above describes the embodiments of the present invention with reference to the drawings, but these are merely examples of the present invention, and various configurations other than those described above can also be adopted.

The present embodiment will be described in detail below with reference to examples and comparative examples. Note that the present embodiment is not limited in any way to the descriptions of these examples.

1. Raw Materials Raw materials used in the Examples and Comparative Examples are shown below.
(1) Propylene polymer PP1: High stereoregular propylene homopolymer (MFR: 3 g/10 min, melting point: 167° C., isotactic mesopentad fraction (mmmm): 98.5%)
PP2: Propylene homopolymer (MFR: 3 g/10 min, melting point: 161° C.)
(2) Tackifier TF1: Hydrogenated petroleum hydrocarbon resin (softening point: 137°C)

2. Measurement and Evaluation Methods (1) Isotactic mesopentad fraction (mmmm) of propylene polymer
The isotactic mesopentad fraction (mesopentad fraction, (mmmm)) was measured by ¹³ C-NMR. The isotactic mesopentad fraction was calculated according to the method described in Zambelli et al., Macromolecules, Vol. 6, p. 925 (1973), and the isotactic meso average sequence length was calculated according to the method described in J. C. Randall, "Polymer Sequence Distribution", Chapter 2 (1977) (Academic Press, New York).

(2) MFR of propylene polymer
The measurement was performed in accordance with ASTM D1238 at 230° C. and under a load of 2.16 kg.

(3) Differential Scanning Calorimetry A test piece of about 5.0 mg was cut out from the food packaging film obtained in the Examples and Comparative Examples. Next, a differential scanning calorimeter (product name: Q200DSC, manufactured by TA Instruments) was used to perform a first differential scanning calorimeter (1st Run) consisting of a process of heating from -50°C to 250°C at a heating rate of 10°C/min, an isothermal process of keeping at 250°C for 10 minutes, and a process of cooling from 250°C to -50°C at a heating rate of 10°C/min under nitrogen gas flow, and a second differential scanning calorimeter (2nd Run) consisting of a process of heating from -50°C to 250°C at a heating rate of 10°C/min.
From the obtained DSC curve, the heat quantity (J/g) of endothermic peak A, the presence or absence of endothermic peaks B and C, the ratio of the peak height C of endothermic peak C to the peak height B of endothermic peak B (C/B), the presence or absence of exothermic peak D, and the half-width of exothermic peak D were each determined.

(4) Tensile elastic modulus Test pieces of 15 mm x 15 cm were cut out from the food packaging films obtained in the Examples and Comparative Examples. Then, the tensile elastic modulus in the MD direction T1 and the tensile elastic modulus in the TD direction T2 of the test pieces were measured using a tensile tester manufactured by Orientec Co., Ltd. in accordance with JIS K7127 (1999) under the conditions of a measurement temperature of ₂₃ ±2°C, 50±5% RH, and a pulling speed of ₅ mm/min.

(5) Evaluation of thickness unevenness Using a sequential biaxial stretching machine, a food packaging film with a width of about 1 m in the direction perpendicular to the machine direction was produced by the method described below. Ten sheets of film were stacked, and the thickness was measured at 10 points excluding both ends in the 1.1 m width direction at 10 cm intervals. The measured values were applied to the following formula to calculate X. The smaller X is, the better the thickness unevenness is.
X = (maximum thickness - minimum thickness) / (maximum thickness + minimum thickness)
Next, the thickness unevenness of the food packaging film was evaluated according to the following criteria.
◎: X is less than 2%. ◯: X is 2% or more and less than 3%. △: X is 3% or more and less than 4%. ×: X is 4% or more.

(6) Evaluation of stretching unevenness The appearance of the stretched film was checked, and visually confirmed whether or not there were any obviously thick parts remaining in the film except for the 125 mm areas at both ends. If there were any thick parts remaining, this meant that stretching unevenness had occurred, and the physical properties of the film were not stable.
◎: No uneven stretching ×: Uneven stretching

(7) Water vapor barrier property The food packaging film was folded back so that the heat seal layer was on the inner side, and the two sides were heat sealed to form a bag. Then, calcium chloride was placed in the bag as the content. Next, the other side was heat sealed to make a bag with a surface area of 0.01 ^m2 . The obtained bag was then stored for 72 hours under conditions of 40°C and 90% RH. The weight of the calcium chloride was measured before and after storage, and the water vapor transmission rate (g/( ^m2 ·24h)) was calculated from the difference.
Here, a heat seal layer was formed on one side of each of the food packaging films obtained in the Examples and Comparative Examples.
Next, the water vapor barrier property of the food packaging film was evaluated according to the following criteria.
◎◎: Water vapor permeability is 4.0g/( ^m2 ·24h) or less ◎: Water vapor permeability is between 4.0g/( ^m2 ·24h) and 4.5g/( ^m2 ·24h) or less 〇: Water vapor permeability is between 4.5g/( ^m2 ·24h) and 5.0g/( ^m2 ·24h) or less △: Water vapor permeability is between 5.0g/( ^m2 ·24h) and 5.5g/( ^m2 ·24h) or less ×: Water vapor permeability is over 5.5g/( ^m2 ·24h)

[Examples 1 to 20 and Comparative Examples 1 to 9]
The propylene-based polymer compositions shown in Table 1 were extruded and then biaxially stretched to produce food packaging films, which were then evaluated. The extrusion conditions and biaxial stretching conditions were as follows.
Extrusion molding machine: 60 mmφ multi-layer T-die extrusion molding machine (screw: L/D=27, manufactured by Screw Seiki Co., Ltd.)
Extrusion temperature setting: 230 to 250°C, processing speed: 20 m/min (winding speed)
Longitudinal stretching temperature: 115 to 130°C
Longitudinal stretching ratio: 5 times Transverse stretching temperature: 140 to 175°C
Transverse stretching ratio: 10 times

100 Food packaging film 101 Stretched film layer 103 Heat seal layer 105 Surface layer

Claims (18)

A film for packaging food, comprising:
The present invention comprises a uniaxially or biaxially stretched film layer made of a propylene - based polymer composition containing a propylene-based polymer and a tackifier,
The stretched film layer was subjected to a differential scanning calorimeter.
A first differential scanning calorimetry (1st Run) consisting of a process of increasing the temperature from -50°C to 250°C at a temperature increase rate of 10°C/min, an isothermal process of maintaining the temperature at 250°C for 10 minutes, and a process of decreasing the temperature from 250°C to -50°C at a temperature decrease rate of 10°C/min;
A second differential scanning calorimetry (2nd Run) consisting of a process of heating from -50°C to 250°C at a heating rate of 10°C/min;
When I continued
In the DSC curve 2 obtained by the second differential scanning calorimetry, an endothermic peak A is observed in the range of 150° C. or higher and 180° C. or lower,
The heat quantity of the endothermic peak A is 80 J/g or more and 120 J/g or less,
The propylene polymer includes a first propylene polymer having a melting point in the range of 130° C. or more and 162° C. or less as measured by DSC, and a second propylene polymer having a melting point in the range of more than 162° C. and 180° C. or less as measured by DSC,
A food packaging film, wherein the ratio of the thickness of the stretched film layer to the total thickness of the food packaging film is 50% or more and 100% or less. The food packaging film according to claim 1,
In the DSC curve 1 obtained by the first differential scanning calorimetry, an endothermic peak B is observed in the range of 150°C or higher and 165°C or lower, and an endothermic peak C is observed in the range of more than 165°C and 180°C or lower. The food packaging film according to claim 2,
A food packaging film, wherein the ratio (C/B) of the peak height C of the endothermic peak C to the peak height B of the endothermic peak B is 1.0 or more and 2.5 or less. The food packaging film according to any one of claims 1 to 3,
A food packaging film in which an exothermic peak D is observed in the range of 100°C or higher and 130°C or lower in a DSC curve 1 obtained by the first differential scanning calorimetry. The food packaging film according to claim 4,
The half-width of the exothermic peak D is 2.0°C or more and 7.0°C or less. The food packaging film according to any one of claims 1 to 5,
The softening temperature of the tackifier is 100°C or higher and 150°C or lower. The food packaging film according to any one of claims 1 to 6,
The food packaging film, wherein the tackifier comprises at least one selected from petroleum-based hydrocarbon resins and hydrogenated petroleum-based hydrocarbon resins. The food packaging film according to claim 7,
The food packaging film, wherein the tackifier comprises a hydrogenated petroleum-based hydrocarbon resin. The food packaging film according to any one of claims 1 to 8,
The food packaging film, wherein the content of a tackifier contained in the propylene-based polymer composition is 0.5% by mass or more and 30% by mass or less, when the entire propylene-based polymer composition is taken as 100% by mass. The food packaging film according to any one of claims 1 to 9,
The food packaging film has a tensile modulus of elasticity T1 in the MD direction of 1000 MPa or more and 8000 MPa or less, as measured using a tensile tester in accordance with JIS K7127 (1999) at a measurement temperature of 23±2°C, ₅₀ ±5% RH, and a tensile speed of 5 mm/min. The food packaging film according to any one of claims 1 to 10,
The food packaging film has a total (T1 + T2) of the tensile modulus T1 in the MD direction and the tensile modulus _T2 in the TD direction of the food packaging film, which is measured in accordance with JIS K7127 (1999) using a tensile tester at a measurement temperature of 23± ₂ ° _C , 50±5% RH, and a tensile speed of 5 mm _/ min, of 5,000 MPa or more and 10,000 MPa or less. The food packaging film according to any one of claims 1 to 11,
The food packaging film further comprises a heat seal layer on at least one surface of the stretched film layer. The food packaging film according to claim 12,
The heat seal layer is provided so as to be in direct contact with the one surface of the stretched film layer. The food packaging film according to claim 12 or 13,
The heat seal layer comprises one or more materials selected from the group consisting of homopolypropylene and random copolymers of propylene and an α-olefin having from 2 to 10 carbon atoms. The food packaging film according to any one of claims 1 to 14,
The food packaging film further comprises a surface layer on one side of the stretched film layer. The food packaging film according to claim 15,
The surface layer of the food packaging film comprises one or more materials selected from the group consisting of homopolypropylene and random copolymers of propylene and an α-olefin having from 2 to 10 carbon atoms. The food packaging film according to any one of claims 1 to 16,
Food packaging film used for outer packaging bags. A food package using the food packaging film according to any one of claims 1 to 17.

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