JP7852247B2 - Biaxially oriented polypropylene film - Google Patents

Description

This invention relates to a biaxially oriented polypropylene film that exhibits excellent rigidity and heat resistance. More specifically, it relates to a biaxially oriented polypropylene film that is suitable for use in packaging bags because it easily maintains the shape of the bag when used as a packaging bag and produces fewer wrinkles at the sealed portion when heat-sealed.

Biaxially oriented polypropylene films are used in packaging and industrial applications because they possess moisture resistance, as well as the necessary rigidity and heat resistance. In recent years, as their applications have expanded, there has been a demand for higher performance, particularly improved rigidity. Furthermore, due to environmental considerations, maintaining strength even when reducing volume (film thickness) is required, which necessitates a significant improvement in rigidity. While improvements in catalysts and process technology during the polymerization of polypropylene resin are known to improve its crystallinity and melting point, despite these improvements, a biaxially oriented polypropylene film with sufficient rigidity has not yet been available.

In the manufacturing process of biaxially oriented polypropylene films, methods have been proposed in which, after stretching in the width direction, a first-stage heat treatment is performed while relaxing the film at a temperature below that of the width-direction stretching, and a second-stage heat treatment is performed at a temperature between the first-stage temperature and the width-direction stretching temperature (see, for example, Patent Document 1), or a method in which, after stretching in the width direction, further stretching is performed in the longitudinal direction (see, for example, Patent Document 2). However, although the film described in Patent Document 2 has excellent rigidity, it is prone to wrinkles in the sealed area after heat sealing and has poor heat resistance. Furthermore, the orientation of the film described in Patent Document 1 is low and its rigidity is insufficient.

International Gazette WO2016/182003 Japanese Patent Publication No. 2013-177645

The object of the present invention is to solve the above-mentioned problems. Specifically, it relates to a biaxially oriented polypropylene film that has excellent rigidity and heat resistance at high temperatures of 150°C. More specifically, it aims to provide a biaxially oriented polypropylene film that easily maintains the shape of a packaging bag and has few wrinkles in and around the sealed area when heat-sealed.

As a result of diligent research conducted by the present inventors to achieve the above objective, they have found that by using a biaxially oriented polypropylene film that satisfies the following conditions (1) to (3), a biaxially oriented polypropylene film with excellent rigidity and heat resistance at high temperatures of 150°C can be obtained.
(1) When the film is restrained on all four sides and wide-angle X-ray diffraction measurements are taken by incidenting X-rays perpendicular to the film surface while heating it from 40°C to 180°C at a rate of 10°C/min, the temperature dependence of the maximum intensity of the azimuthal profile obtained by scanning the scattering originating from the (110) plane of the α-type crystal of polypropylene in the circumferential direction is 115% or more.
(2) The degree of orientation in the width direction is 0.85 or higher.
(3) The thermal shrinkage rate at 150°C is 10% or less in the longitudinal direction and 30% or less in the width direction.

In this case, it is preferable that the ratio (E''h/E'' g) of the maximum value of the loss modulus (E'') at 100°C or above (E'' _h ) to the maximum value of the loss modulus (E'') between -25°C and 25°C (E'' _g ) in the temperature dependence of the width direction loss modulus (E'_' ) obtained by dynamic viscoelastic measurement ( _DMA ) of the biaxially oriented polypropylene film is 70% or more.

Furthermore, in this case, it is preferable that the tensile elongation at break of the biaxially oriented polypropylene film is 25% or more in the longitudinal direction and 195% or more in the width direction at 23°C.

Furthermore, in this case, it is preferable that the heat shrinkage rate of the biaxially oriented polypropylene film at 120°C is 2.0% or less in the longitudinal direction and 5.0% or less in the width direction, and that the heat shrinkage rate at 120°C in the longitudinal direction is smaller than the heat shrinkage rate at 120°C in the width direction.

Furthermore, in this case, it is preferable that the refractive index Ny in the longitudinal direction of the biaxially oriented polypropylene film is 1.5230 or higher, and that ΔNy is 0.0220 or higher.

Furthermore, in this case, it is preferable that the haze of the biaxially oriented polypropylene film is 5.0% or less.

Furthermore, in this case, it is preferable that the mesopentade fraction of the polypropylene resin constituting the biaxially oriented polypropylene film is 97.0% or more.

Furthermore, in this case, it is preferable that the crystallization temperature of the polypropylene resin constituting the biaxially oriented polypropylene film is 105°C or higher, and the melting point is 161°C or higher.

Furthermore, in this case, it is preferable that the melt flow rate of the polypropylene resin constituting the biaxially oriented polypropylene film is 4.0 g/10 min or more.

Furthermore, in this case, it is preferable that the amount of polypropylene resin components with a molecular weight of 100,000 or less constituting the biaxially oriented polypropylene film is 35% by mass or more.

The biaxially oriented polypropylene film of the present invention has high rigidity and excellent heat resistance even at high temperatures of 150°C, making it easy to maintain the shape of the bag when used as a packaging bag, and also producing fewer wrinkles at the sealed area when heat-sealed, thus providing a biaxially oriented polypropylene film that can be suitably used as a packaging bag. Furthermore, because the biaxially oriented polypropylene film has excellent rigidity, it can maintain strength even when the film thickness is reduced, and can also be suitably used in applications where even higher rigidity is required.

The diffraction intensity ratio-temperature curves for the films obtained in Example 1, Comparative Example 1, and Comparative Example 2 are shown, based on the maximum intensity at 40°C, at the azimuthal angle profile of the diffraction intensity derived from the polypropylene α-type crystal (110) plane. The loss modulus-temperature curves in the width direction for the films obtained in Example 1, Comparative Example 1, and Comparative Example 2 are shown.

The biaxially oriented polypropylene film of the present invention will be described in more detail below.
The biaxially oriented polypropylene film of the present invention consists of a polypropylene resin composition mainly composed of polypropylene resin. "Main component" means that the proportion of polypropylene resin in the polypropylene resin composition is 90% by mass or more, more preferably 93% by mass or more, even more preferably 95% by mass or more, and particularly preferably 97% by mass or more.

(Polypropylene resin)
The polypropylene resin used in the present invention can be a polypropylene homopolymer or a copolymer with ethylene and/or an α-olefin having 4 or more carbon atoms. A propylene homopolymer that is substantially free of ethylene and/or an α-olefin having 4 or more carbon atoms is preferred, and even when it contains an ethylene and/or α-olefin component having 4 or more carbon atoms, the amount of the ethylene and/or α-olefin component having 4 or more carbon atoms is preferably 1 mol% or less, more preferably 0.5 mol% or less, even more preferably 0.3 mol% or less, and particularly preferably 0.1 mol or less. Crystallinity tends to improve within the above range. Examples of α-olefin components having 4 or more carbon atoms that constitute such a copolymer include 1-butene, 1-pentene, 3-methylpentene-1, 3-methylbutene-1, 1-hexene, 4-methylpentene-1, 5-ethylhexene-1, 1-octene, 1-decene, 1-dodecene, 1-tetradecene, 1-hexadecene, 1-heptadecene, 1-octadecene, and 1-eicosene.
Polypropylene resins can be made from two or more different polypropylene homopolymers, copolymers with ethylene and/or α-olefins having four or more carbon atoms, or mixtures thereof.

(stereoregularity)
The mesopentad fraction ([mmmm]%), which is an indicator of the stereoregularity of the polypropylene resin used in the present invention, is preferably in the range of 97.0 to 99.9%, more preferably in the range of 97.5 to 99.7%, even more preferably in the range of 98.0 to 99.5%, and particularly preferably in the range of 98.5 to 99.3%.
A mesopentad fraction of 97.0% or higher increases the crystallinity of the polypropylene resin, improving the melting point, degree of crystallinity, and degree of crystal orientation of the crystals in the film, making it easier to obtain rigidity and heat resistance at high temperatures. A fraction of 99.9% or lower makes it easier to reduce costs in polypropylene manufacturing and makes the film less prone to breakage during film formation. The mesopentad fraction is measured by nuclear magnetic resonance (NMR) spectroscopy.
A concentration of 99.5% or less is more preferable. The mesopentade fraction is measured by nuclear magnetic resonance (NMR) spectroscopy.
In order to keep the mesopentade fraction of the polypropylene resin within the above-mentioned range, methods such as washing the obtained polypropylene resin powder with a solvent such as n-heptane, selecting a catalyst and/or co-catalyst, and appropriately selecting the components of the polypropylene resin composition are preferably employed.

(Melting temperature)
The lower limit of the melting temperature (Tm) of the polypropylene resin constituting the biaxially oriented polypropylene film of the present invention, as measured by DSC, is preferably 160°C, more preferably 161°C, even more preferably 162°C, even more preferably 163°C, and still more preferably 164°C. When Tm is 160°C or higher, rigidity and heat resistance at high temperatures are easily obtained. The upper limit of Tm is preferably 170°C, more preferably 169°C, even more preferably 168°C, even more preferably 167°C, and particularly preferably 166°C. When Tm is 170°C or lower, it is easier to suppress cost increases in terms of polypropylene manufacturing and the film is less likely to break during film formation. The melting temperature can also be further increased by blending a crystal nucleating agent into the aforementioned polypropylene resin.
Tm is the main peak temperature of the endothermic peak observed during melting when a 1-10 mg sample is placed in an aluminum pan, set in a differential scanning calorimeter (DSC), melted at 230°C for 5 minutes under a nitrogen atmosphere, cooled to 30°C at a scanning rate of -10°C/min, held for 5 minutes, and then heated again at a scanning rate of 10°C/min.

(crystallization temperature)
The lower limit of the crystallization temperature (Tc) of the polypropylene resin constituting the biaxially oriented polypropylene film of the present invention, as measured by DSC, is 105°C, preferably 108°C, and more preferably 110°C. When Tc is 105°C or higher, crystallization proceeds easily during the widthwise stretching and subsequent cooling process, making it easier to obtain rigidity and heat resistance at high temperatures. The upper limit of Tc is preferably 135°C, more preferably 133°C, even more preferably 132°C, even more preferably 130°C, particularly preferably 128°C, and most preferably 127°C. When Tc is 135°C or lower, it is less likely to increase the cost of polypropylene production and the film is less likely to break during film formation. The crystallization temperature can also be further increased by blending a crystal nucleating agent into the aforementioned polypropylene resin.
Tc is the primary peak temperature of the exothermic peak observed when a 1-10 mg sample is packed into an aluminum pan, set in a DSC, melted at 230°C for 5 minutes under a nitrogen atmosphere, and then cooled to 30°C at a scanning rate of -10°C/min.

(Melt flow rate)
The melt flow rate (MFR) of the polypropylene resin constituting the biaxially oriented polypropylene film of the present invention is preferably 4.0 to 30 g/10 min, more preferably 4.5 to 25 g/10 min, even more preferably 4.8 to 22 g/10 min, particularly preferably 5.0 to 20 g/10 min, and most preferably 6.0 to 20 g/10 min, when measured in accordance with the conditions M (230°C, 2.16 kgf) of JIS K 7210 (1995).
When the melt flow rate (MFR) of the polypropylene resin is 4.0 g/10 min or higher, it is easier to obtain a biaxially oriented polypropylene film with a low thermal shrinkage rate.
Furthermore, if the melt flow rate (MFR) of the polypropylene resin is 30 g/10 min or less, it is easier to maintain the film-forming properties.

From the viewpoint of film properties, it is preferable to set the lower limit of the melt flow rate (MFR) (230°C, 2.16 kgf) of the polypropylene resin constituting the film to preferably 5.0 g/10 min, more preferably 5.5 g/10 min, even more preferably 6.0 g/10 min, particularly preferably 6.3 g/10 min, and most preferably 6.5 g/10 min.
When the melt flow rate (MFR) of the polypropylene resin is 5.0 g/10 min or higher, the amount of low molecular weight components in the polypropylene resin constituting the film increases. Therefore, by employing the widthwise stretching process in the film manufacturing process described later, the orientation and crystallization of the polypropylene resin are further promoted, and the degree of crystallinity in the film tends to increase. In addition, the entanglement between polypropylene molecular chains in the amorphous portion is reduced, making it easier to improve heat resistance.
In order to keep the melt flow rate (MFR) of the polypropylene resin within the above range, it is preferable to employ methods to control the average molecular weight and molecular weight distribution of the polypropylene resin.

In other words, the lower limit of the amount of components with a molecular weight of 100,000 or less in the GPC integrated curve of the polypropylene resin constituting the film of the present invention is 35% by mass, preferably 38% by mass, more preferably 40% by mass, even more preferably 41% by mass, and particularly preferably 42% by mass.
The upper limit of the amount of components with a molecular weight of 100,000 or less in the GPC integrated curve is preferably 65% by mass, more preferably 60% by mass, and even more preferably 58% by mass. When the amount of components with a molecular weight of 100,000 or less in the GPC integrated curve is 65% by mass or less, the film strength is less likely to decrease.
In this case, including high molecular weight components or long-chain branched components with long relaxation times makes it easier to adjust the amount of components with a molecular weight of 100,000 or less in the polypropylene resin without significantly changing the overall viscosity, thus improving film-forming properties without greatly affecting rigidity or thermal shrinkage.

(molecular weight distribution)
The polypropylene resin used in the present invention has a lower limit of mass-average molecular weight (Mw)/number-average molecular weight (Mn), which is an indicator of the breadth of the molecular weight distribution, preferably 3.5, more preferably 4, even more preferably 4.5, and particularly preferably 5. The upper limit of Mw/Mn is preferably 30, more preferably 25, even more preferably 23, particularly preferably 21, and most preferably 20.
Mw/Mn can be obtained using gel permeation chromatography (GPC). When Mw/Mn is within the above range, it is easy to increase the amount of components with a molecular weight of 100,000 or less.

Furthermore, the molecular weight distribution of polypropylene resin can be adjusted by polymerizing components of different molecular weights in a multi-stage process in a series of plants, blending components of different molecular weights offline in a kneader, polymerizing with a blend of catalysts having different properties, or using a catalyst capable of achieving the desired molecular weight distribution. The shape of the molecular weight distribution obtained by GPC can be a smooth molecular weight distribution with a single peak in a GPC chart with the logarithm of molecular weight (M) (logM) on the horizontal axis and the differential distribution value (weight fraction per logM) on the vertical axis, or it can be a molecular weight distribution with multiple peaks or shoulders.

(Method for producing biaxially oriented polypropylene film)
The biaxially oriented polypropylene film of the present invention is preferably obtained by preparing an unstretched sheet made from a polypropylene resin composition mainly composed of the polypropylene resin described above, and then biaxially stretching it. The biaxial stretching can be performed by any of the following methods: simultaneous inflation biaxial stretching, simultaneous tenter biaxial stretching, or sequential tenter biaxial stretching. However, sequential tenter biaxial stretching is preferred from the viewpoint of film formation stability and thickness uniformity. In particular, it is preferable to stretch in the longitudinal direction first, followed by stretching in the width direction, but a method of stretching in the width direction first, followed by stretching in the longitudinal direction is also acceptable.

Next, the method for manufacturing the biaxially oriented polypropylene film of the present invention will be described below, but it is not necessarily limited thereto. The biaxially oriented polypropylene film of the present invention may have layers having other functions laminated on at least one side. Lamination may be performed on one side or both sides. In that case, the resin composition of the other layer, or the central layer, may be the polypropylene resin composition described above. Alternatively, it may be a different polypropylene resin composition from the one described above. The number of layers to be laminated may be one, two, or three or more layers per side, but from a manufacturing viewpoint, one or two layers are preferred. As for the lamination method, for example, co-extrusion by a feed block method or a multi-manifold method is preferred. In particular, to improve the processability of the biaxially oriented polypropylene film, a heat-sealable resin layer can be laminated to the extent that it does not degrade the properties. Furthermore, corona treatment may be applied to one or both sides to impart printability.

The following describes an example for a single layer, specifically focusing on the use of the Tenter successive biaxial stretching method.
First, a resin composition containing polypropylene resin is heated and melted in a single-screw or twin-screw extruder, extruded in a sheet form from a T-die, and cooled and solidified on a cooling roll. To promote solidification, it is preferable to further cool the sheet cooled on the cooling roll by immersing it in a water tank or the like.

Next, the sheet is stretched longitudinally by increasing the rotation speed of the rear stretching roll using two heated stretching rolls, thereby obtaining a uniaxially oriented film.

Next, after preheating the uniaxially oriented film, the film is stretched in the width direction at a specific temperature using a tenter-type stretcher while gripping the film edges, thereby obtaining a biaxially oriented film. This width-direction stretching process will be described in detail later.

After the widthwise stretching process is completed, the biaxially oriented film is heat-treated at a specific temperature to obtain a biaxially oriented film. During the heat treatment process, the film may be relaxed in the widthwise direction.

The biaxially oriented polypropylene film thus obtained can be subjected to corona discharge treatment on, for example, at least one side, as needed, and then wound up with a winder to obtain a film roll.

The following sections will explain each step in detail.
(Extrusion process)
First, a polypropylene resin composition, mainly composed of polypropylene resin, is heated and melted in a single-screw or twin-screw extruder at a temperature of 200°C to 300°C. The sheet-like molten polypropylene resin composition is then extruded from the T-die and cooled and solidified in contact with a metal cooling roll. Preferably, the resulting unstretched sheet is then placed in a water tank.
The temperature of the cooling roll, or the temperature of the cooling roll and water bath, is preferably in the range of 10°C to Tc. If the transparency of the film is to be increased, it is preferable to cool and solidify with a cooling roll at a temperature in the range of 10 to 50°C. Lowering the cooling temperature to 50°C or below tends to increase the transparency of the unstretched sheet, so it is preferably 40°C or below, and more preferably 30°C or below. In order to increase the degree of crystal orientation after sequential biaxial stretching, it may be preferable to set the cooling temperature to 40°C or below when using a propylene homopolymer with a mesopentat fraction of 97.0% or more, as described above, in order to facilitate the stretching in the next step and reduce thickness variations, it is preferable to set the cooling temperature to 40°C or below, and more preferably 30°C or below.
The thickness of the unstretched sheet is preferably 3500 μm or less for optimal cooling efficiency, and more preferably 3000 μm or less. This can be adjusted as appropriate depending on the film thickness after sequential biaxial stretching. The thickness of the unstretched sheet can be controlled by the extrusion speed of the polypropylene resin composition and the lip width of the T-die, etc.

(Longitudinal stretching process)
The lower limit of the longitudinal stretching ratio is preferably 3 times, more preferably 3.5 times, and particularly preferably 3.8 times. Within this range, it is easier to increase strength and reduce film thickness unevenness. The upper limit of the longitudinal stretching ratio is preferably 8 times, more preferably 7.5 times, and particularly preferably 7 times. Within this range, it is easier to perform the widthwise stretching process, and productivity is improved.
The lower limit of the longitudinal stretching temperature is preferably Tm-40°C, more preferably Tm-37°C, and even more preferably Tm-35°C. Within this range, the subsequent widthwise stretching becomes easier and thickness unevenness is reduced. The upper limit of the longitudinal stretching temperature is preferably Tm-7°C, more preferably Tm-10°C, and even more preferably Tm-12°C. Within this range, the thermal shrinkage rate is easily reduced, and there is less chance of the material adhering to the stretching roll and becoming difficult to stretch, or of the quality degrading due to increased surface roughness.
Furthermore, longitudinal stretching may be performed in two or more stages using three or more stretching rolls.

(Preheating process)
Before the widthwise stretching process, the uniaxially oriented film after longitudinal stretching must be heated to a temperature range of Tm to Tm+25°C to soften the polypropylene resin composition. A temperature above Tm promotes softening, facilitating widthwise stretching. A temperature below Tm+25°C promotes orientation during transverse stretching, making it easier to achieve rigidity. More preferably, the temperature is Tm+2 to Tm+22°C, and particularly preferably Tm+3 to Tm+20°C. Here, the highest temperature reached during the preheating process is defined as the preheating temperature.

(Width direction stretching process)
In the widthwise stretching process after the preheating process, the preferred method is as follows:

In the widthwise stretching process, it is preferable to stretch at a temperature of Tm -10°C or higher and below the preheating temperature. At this time, the widthwise stretching may start when the preheating temperature is reached, or it may start when the temperature is lowered after reaching the preheating temperature and reaches a temperature lower than the preheating temperature.
The lower limit of the temperature in the widthwise stretching process is more preferably Tm-9°C, even more preferably Tm-7°C, and particularly preferably Tm-5°C. When the widthwise stretching temperature is within this range, it is easier to improve the rigidity of the resulting biaxially oriented film.
The upper limit of the temperature during the widthwise stretching process is preferably Tm + 10°C, more preferably Tm + 7°C, and particularly preferably Tm + 5°C. When the widthwise stretching temperature is within this range, uneven stretching is less likely to occur.
The film is cooled immediately after the end of stretching in the width direction, that is, when the final stretching ratio in the width direction is reached. The cooling temperature at this time is preferably below the temperature of stretching in the width direction, and is between Tm-80°C and Tm-15°C, more preferably between Tm-80°C and Tm-20°C, even more preferably between Tm-80°C and Tm-30°C, and particularly preferably between Tm-70°C and Tm-40°C.
The temperature can be gradually lowered from the end of widthwise stretching to the cooling temperature, but it can also be lowered in steps or in a single step. Lowering the temperature in steps or in a single step is preferable because it makes it easier to increase the crystal orientation in the film.

The lower limit of the final widthwise stretching ratio in the widthwise stretching process is preferably 10 times, more preferably 11 times. A ratio of 12 times or higher makes it easier to increase rigidity and reduce film thickness unevenness. The upper limit of the widthwise stretching ratio is preferably 20 times, more preferably 17 times, and even more preferably 15 times. A ratio of 20 times or lower makes it easier to reduce the thermal shrinkage rate and makes it less likely to break during stretching.

Thus, by using a highly crystalline polypropylene resin with high stereoregularity and a high melting point, and employing the widthwise stretching process described above, the molecules of the polypropylene resin are highly aligned in the principal orientation direction (which corresponds to the widthwise direction in the widthwise stretching process described above). As a result, the resulting biaxially oriented film exhibits very strong crystal orientation, and crystals with high melting points are easily formed.
Furthermore, the orientation of the amorphous regions between crystals increases in the main orientation direction (which corresponds to the width direction in the width direction stretching process described above). Because there are many crystals with high melting points around the amorphous regions, the elongated polypropylene molecules in the amorphous regions do not easily relax at temperatures below the melting point of the crystals, and tend to maintain their tensioned state. Therefore, the entire biaxially oriented film can maintain high rigidity even at high temperatures.
Furthermore, it is noteworthy that by employing this widthwise stretching process, the thermal shrinkage rate at high temperatures of 150°C is also more easily reduced. The reason for this is that there are more crystals with high melting points around the amorphous region, so at temperatures below the melting point of the crystals, the stretched polypropylene resin molecules in the amorphous region do not relax easily, and there is less entanglement between the molecules.

Furthermore, it is noteworthy that increasing the low molecular weight components of the polypropylene resin makes it easier to achieve a higher degree of crystallinity in the film, and also reduces the entanglement of polypropylene resin molecular chains in the amorphous portion, thereby weakening the thermal shrinkage stress and further lowering the thermal shrinkage rate. Considering that conventionally, improving either strength or thermal shrinkage rate tends to decrease the other property, this can be considered a groundbreaking development.

(Heat treatment process)
Biaxially oriented films can be heat-treated as needed to further reduce their thermal shrinkage. The upper limit of the heat treatment temperature is preferably Tm + 10°C, more preferably Tm + 7°C, and particularly preferably Tm + 5°C. Keeping the temperature below Tm + 10°C facilitates the development of rigidity, prevents excessive roughness on the film surface, and reduces the likelihood of the film whitening. The lower limit of the heat treatment temperature is preferably Tm - 5°C, more preferably Tm - 2°C, and particularly preferably Tm°C. Below Tm - 5°C, the thermal shrinkage may be high.
By employing the widthwise stretching process described above, even when heat treatment is performed at temperatures between Tm-5°C and Tm+10°C, the highly oriented crystals generated in the stretching process are less likely to melt, and the thermal shrinkage rate can be reduced without decreasing the rigidity of the resulting film. To adjust the thermal shrinkage rate, the film may be relaxed (relaxed) in the widthwise direction during heat treatment, but the upper limit of the relaxation rate is preferably 4%. Within this range, the film strength is less likely to decrease, and the variation in film thickness tends to be small. More preferably, it is 3%, even more preferably 2%, even more preferably 1%, and particularly preferably 0%.

(Film thickness)
The thickness of the biaxially oriented polypropylene film of the present invention is set according to each application, but in order to obtain the strength of the film, the lower limit of the film thickness is preferably 2 μm, more preferably 3 μm, even more preferably 4 μm, particularly preferably 8 μm, and most preferably 10 μm. When the film thickness is 2 μm or more, it is easier to obtain the rigidity of the film. The upper limit of the film thickness is preferably 100 μm, more preferably 80 μm, even more preferably 60 μm, particularly preferably 50 μm, and most preferably 40 μm. When the film thickness is 100 μm or less, the cooling rate of the unstretched sheet during the extrusion process does not tend to decrease.
The biaxially oriented polypropylene film of the present invention is typically manufactured as a roll with a width of 2,000 to 12,000 mm and a length of 1,000 to 50,000 m, and then wound into a film roll. Furthermore, it is slit according to various applications and supplied as slit rolls with a width of 300 to 2,000 mm and a length of 500 to 5,000 m. The biaxially oriented polypropylene film of the present invention makes it possible to obtain longer film rolls.

(Uniformity of thickness)
The lower limit of the thickness uniformity of the biaxially oriented polypropylene film of the present invention is preferably 0%, more preferably 0.1%, even more preferably 0.5%, and particularly preferably 1%. The upper limit of the thickness uniformity is preferably 20%, more preferably 17%, even more preferably 15%, particularly preferably 12%, and most preferably 10%. Within the above range, defects are less likely to occur during post-processing such as coating and printing, making it easy to use in applications requiring precision.
The measurement method was as follows: A 40 mm wide test piece was cut from the steady-state region where the film properties were stable in the length direction of the film. The film thickness was continuously measured over 20,000 mm using a film feeding device manufactured by Micron Measuring Instruments Co., Ltd. (product number: A90172) and a continuous film thickness measuring instrument manufactured by Anritsu Corporation (product name: K-313A wide-range high-sensitivity electronic micrometer), and the thickness uniformity was calculated from the following formula.
Thickness uniformity (%) = [(Maximum thickness - Minimum thickness) / Average thickness] × 100

When wide-angle X-ray diffraction measurements were performed by constraining the film on all four sides and increasing the temperature from 40°C to 180°C at a rate of 10°C/min while incident X-rays perpendicular to the film surface, the temperature dependence of the maximum intensity of the azimuthal profile obtained by scanning the diffraction originating from the (110) plane of the α-type crystal of polypropylene in the circumferential direction is characterized by the ratio of the maximum value of the maximum intensity in the range of 40°C to 130°C to the maximum intensity at 40°C ((110) intensity ratio) being 115% or more. The case of the film of Example 1 is shown in Figure 1.
(110) The strength ratio is preferably 120% or more, more preferably 123% or more, even more preferably 125% or more, and particularly preferably 126% or more.
When a biaxially oriented film, in which polypropylene molecules are highly aligned in the principal orientation direction (corresponding to the width direction in the width direction stretching process described above), is constrained on all four sides and heated from near room temperature, secondary crystallization easily progresses along the principal orientation direction, increasing the degree of crystallinity. This is due to the presence of many low-molecular-weight, highly stereoregular polypropylene molecules that are easily mobile and crystallize. The degree of crystallinity is maximized at around 100°C, and thereafter, the diffraction intensity decreases as the crystals melt. Furthermore, from around 140°C and above, the diffraction intensity increases again with recrystallization. As a result, the biaxially oriented polypropylene film of the present invention has desirable characteristics such as excellent rigidity and low thermal shrinkage.

(degree of orientation)
The lower limit of the degree of orientation calculated from Wh of the biaxially oriented polypropylene film of the present invention using the following formula is preferably 0.850, more preferably 0.855, and even more preferably 0.861. Setting it to 0.850 or higher makes it easier to increase rigidity.
Orientation degree = (180-Wh)/180
The upper limit of the degree of orientation is preferably 0.928, more preferably 0.922, and even more preferably 0.917. Setting it to 0.928 or lower makes film formation more stable.

(Film characteristics)
The biaxially oriented polypropylene film of the present invention is characterized by the following properties. Here, the "longitudinal direction" in the biaxially oriented polypropylene film of the present invention refers to the direction corresponding to the flow direction in the film manufacturing process, and the "width direction" refers to the direction perpendicular to the flow direction in the film manufacturing process. For polypropylene films in which the flow direction in the film manufacturing process is unknown, wide-angle X-rays are incident perpendicular to the film surface, and the scattering peaks originating from the (110) plane of the α-type crystal are scanned in the circumferential direction. The direction with the greatest diffraction intensity in the obtained diffraction intensity distribution is defined as the "longitudinal direction," and the direction perpendicular to it is defined as the "width direction."

(Heat shrinkage rate at 150°C)
The upper limit of the longitudinal heat shrinkage rate of the biaxially oriented polypropylene film of the present invention at 150°C is preferably 10%, more preferably 7.0%, even more preferably 6.0%, even more preferably 5.0%, and particularly preferably 4.0% or less. The upper limit of the widthwise heat shrinkage rate at 150°C is preferably 30%, more preferably 24%, even more preferably 21%, and particularly preferably 18% or less.
When the thermal shrinkage rate in the longitudinal direction is 10% or less and the thermal shrinkage rate in the width direction is 30% or less, wrinkles are less likely to occur during heat sealing. In particular, when the thermal shrinkage rate in the longitudinal direction at 150°C is 8.0% or less and the thermal shrinkage rate in the width direction at 150°C is 20% or less, distortion is small when the chuck portion is fused to the opening portion, which is preferable. To reduce the thermal shrinkage rate at 150°C, it is effective to set the lower limit of the amount of components with a molecular weight of 100,000 or less, when the gel permeation chromatography (GPC) integrated curve of the polypropylene resin constituting the film is measured, to 35% by mass, and adjust the stretching ratio, stretching temperature, and heat fixing temperature.
(Loss modulus of elasticity)
The biaxially oriented polypropylene film of the present invention uses highly stereoregular polypropylene and employs the above-described widthwise stretching process. This increases the degree of crystallinity and crystal alignment within the film, which contribute significantly to its rigidity. This reduces relaxation associated with crystal melting near the glass transition temperature of the polypropylene resin, and consequently suppresses relaxation of the amorphous regions. Only at high temperatures does the melting of the highly oriented crystals finally end, and the previously suppressed relaxation of the amorphous regions also progresses, resulting in significant relaxation at high temperatures.
In other words, the biaxially oriented polypropylene film of the present invention has a low loss modulus near the glass transition temperature, and conversely, a high loss modulus at high temperatures (especially above 100°C).
Therefore, in the temperature dependence of the widthwise loss modulus (E'') obtained by dynamic viscoelasticity measurement (DMA) of the biaxially oriented polypropylene film of the present invention, the ratio (E''h/ _E''g ) of the maximum value of the loss modulus (E'') at 100°C or above ( _E''h ) to the maximum value of the loss modulus (E'') between -25° _C and 25° _C is preferably 70% or more, more preferably 73% or more, even more preferably 75% or more, particularly preferably 77% or more, and most preferably 78% or more. When the ratio ( _E''h / _E''g ) is 70% or more, the rigidity and heat resistance in the practical temperature range close to room temperature tend to improve further. To increase the ratio ( _E''h / _E''g ), it is effective to set the lower limit of the amount of components with a molecular weight of 100,000 or less, when the gel permeation chromatography (GPC) integrated curve of the polypropylene resin constituting the film is measured, to 35% by mass, and adjust the stretching ratio, stretching temperature, and heat fixing temperature.

(Tensile elongation at 23°C)
The lower limit of the longitudinal tensile elongation at break of the biaxially oriented polypropylene film of the present invention at 23°C is preferably 195%, more preferably 200%, even more preferably 210%, and particularly preferably 220% or more. An elongation of 195% or more tends to reduce film breakage and packaging bag tearing. The upper limit of the longitudinal tensile elongation at break at 23°C is preferably 300% and more preferably 280% as a practical value.

The lower limit of the tensile elongation at break in the width direction of the biaxially oriented polypropylene film of the present invention at 23°C is preferably 25%, more preferably 30%, even more preferably 32%, and particularly preferably 35%. If it is 25% or higher, film breakage and packaging bag tearing tend to be less likely. The upper limit of the tensile elongation at break in the width direction at 23°C is preferably 60%, more preferably 55%, and even more preferably 50%. If it is 60% or lower, printing pitch misalignment when transferring printing ink is less likely to occur, and the durability of the packaging bag tends to be excellent.
The tensile elongation at break can be kept within a specified range by adjusting the stretch ratio, stretching temperature, and heat-fixing temperature.

(Tensile breaking strength at 23°C)
The lower limit of the longitudinal tensile breaking strength of the biaxially oriented polypropylene film of the present invention at 23°C is preferably 90 MPa, more preferably 100 MPa, even more preferably 110 MPa, and particularly preferably 120 MPa. A strength of 90 MPa or higher reduces the likelihood of misalignment of the printing pitch when transferring printing ink, and also improves the durability of the packaging bag. The upper limit of the longitudinal tensile breaking strength at 23°C is preferably 200 MPa as a practical value, more preferably 180 MPa, and even more preferably 160 MPa. A strength of 200 MPa or lower tends to reduce film breakage and packaging bag tearing.
The lower limit of the tensile breaking strength in the width direction of the biaxially oriented polypropylene film of the present invention at 23°C is preferably 380 MPa, more preferably 400 MPa, even more preferably 430 MPa, and particularly preferably 450 MPa. A strength of 380 MPa or higher reduces the likelihood of misalignment of the printing pitch when transferring printing ink, and also improves the durability of the packaging bag. The upper limit of the tensile breaking strength in the width direction at 23°C is preferably 550 MPa as a practical value, more preferably 520 MPa, and even more preferably 500 MPa. A strength of 550 MPa or lower tends to reduce film breakage and packaging bag rupture. To increase the tensile breaking elongation, it is effective to set the lower limit of the amount of components with a molecular weight of 100,000 or less, as measured by gel permeation chromatography (GPC) integration curve of the polypropylene resin constituting the film, to 35% by mass, and then adjust the stretching ratio, stretching temperature, and heat-fixing temperature.

(Stress at 23°C and 5% elongation)
The lower limit of the stress (F5) of the biaxially oriented polypropylene film of the present invention at 23°C with 5% elongation in the longitudinal direction is preferably 40 MPa, more preferably 42 MPa, even more preferably 44 MPa, even more preferably 46 MPa, and particularly preferably 48 MPa. At 40 MPa or higher, the rigidity is high, making it easier to maintain the shape of the bag when used as a packaging bag, and preventing deformation of the film during processing such as printing.
The upper limit of F5 in the longitudinal direction at 23°C is preferably 70 MPa, more preferably 65 MPa, even more preferably 62 MPa, and particularly preferably 60 MPa. Below 70 MPa, practical manufacturing becomes easier and the balance of the vertical width tends to improve.
The lower limit of F5 in the width direction at 23°C for the biaxially oriented polypropylene film of the present invention is preferably 160 MPa, more preferably 170 MPa, even more preferably 180 MPa, and particularly preferably 190 MPa. Above 160 MPa, the rigidity is high, making it easier to maintain the shape of the bag when used as a packaging bag, and preventing deformation of the film during processing such as printing. The upper limit of F5 in the width direction at 23°C is preferably 250 MPa, more preferably 230 MPa, and even more preferably 220 MPa. Below 250 MPa, practical manufacturing is easier, and the vertical-to-width balance is easier to achieve.
F5 can be kept within the range by adjusting the stretching ratio, relaxation rate, and temperature during film formation.

(Heat shrinkage rate at 120°C)
The upper limit of the longitudinal heat shrinkage rate of the biaxially oriented polypropylene film of the present invention at 120°C is preferably 2.0%, more preferably 1.5%, even more preferably 1.2%, and particularly preferably 1.0%. If it is 2.0% or less, misalignment of the printing pitch when transferring printing ink becomes less likely. The upper limit of the widthwise heat shrinkage rate at 120°C is 5.0%, preferably 4.0%, more preferably 3.5%, and particularly preferably 2.5%. If it is 5.0% or less, wrinkles are less likely to occur during heat sealing.
If the longitudinal heat shrinkage rate at 120°C is smaller than the widthwise heat shrinkage rate at 120°C, misalignment of the print pitch during ink transfer becomes less likely. The balance between the longitudinal and widthwise heat shrinkage rates at 120°C can be kept within a certain range by adjusting the stretching ratio, stretching temperature, and heat-fixing temperature.

(Refractive index)
The lower limit of the longitudinal refractive index (Nx) of the biaxially oriented polypropylene film of the present invention is preferably 1.4950, more preferably 1.4970, and even more preferably 1.4980. A refractive index of 1.4950 or higher tends to increase the rigidity of the film. The upper limit of the longitudinal refractive index (Nx) is preferably 1.5100, more preferably 1.5070, and even more preferably 1.5050. A refractive index of 1.5100 or lower tends to provide a good balance of longitudinal-width properties of the film.

The lower limit of the refractive index (Ny) in the width direction of the biaxially oriented polypropylene film of the present invention is 1.5230, preferably 1.5235, and more preferably 1.5240. A refractive index of 1.5230 or higher makes it easier to increase the rigidity of the film. The upper limit of the refractive index (Ny) in the width direction is preferably 1.5280, more preferably 1.5275, and even more preferably 1.5270. A refractive index of 1.5280 or lower makes it easier to achieve a good balance of longitudinal-width properties of the film.

The lower limit of the refractive index (Nz) in the thickness direction of the biaxially oriented polypropylene film of the present invention is preferably 1.4960, more preferably 14965, and even more preferably 1.4970. A refractive index of 1.4960 or higher makes it easier to increase the rigidity of the film. The upper limit of the refractive index (Nz) in the thickness direction is preferably 1.5020, more preferably 1.5015, and even more preferably 1.5010. A refractive index of 1.5020 or lower makes it easier to increase the heat resistance of the film.
The refractive index can be kept within a specified range by adjusting the stretching ratio, stretching temperature, and heat-fixing temperature.

(△Ny)
The lower limit of △Ny in the biaxially oriented polypropylene film of the present invention is 0.0220, preferably 0.0225, more preferably 0.0228, and even more preferably 0.0230. A value of 0.0220 or higher tends to increase the rigidity of the film. The upper limit of △Ny is preferably 0.0270 as a practical value, more preferably 0.0265, even more preferably 0.0262, and particularly preferably 0.0260. A value of 0.0270 or lower tends to result in good thickness uniformity. △Ny can be brought within the range by adjusting the stretching ratio, stretching temperature, and heat-fixing temperature of the film.
△Ny is calculated using the following formula, where Nx, Ny, and Nz are the refractive indices along the longitudinal, width, and thickness directions of the film, respectively. It represents the degree of orientation in the width direction within the overall orientation of the film in the longitudinal, width, and thickness directions.
△Ny=Ny-[(Nx+Nz)/2]

(Coefficient of surface orientation)
The lower limit of the surface orientation coefficient (ΔP) of the biaxially oriented polypropylene film of the present invention is preferably 0.0135, more preferably 0.0138, and even more preferably 0.0140. A value of 0.0135 or higher indicates good balance in the planar direction of the film and good thickness uniformity. The upper limit of the surface orientation coefficient (ΔP) is preferably 0.0155, more preferably 0.0152, and even more preferably 0.0150 as a practical value. A value of 0.0155 or lower tends to result in excellent heat resistance at high temperatures. The surface orientation coefficient (ΔP) can be brought within the range by adjusting the stretching ratio, stretching temperature, and heat-fixing temperature.
Furthermore, the surface orientation coefficient (ΔP) was calculated using the formula [(Nx + Ny)/2] - Nz.

(Full width at half maximum of diffraction peaks originating from oriented crystals)
In the azimuthal angle dependence of the diffraction peak of the (110) plane of polypropylene α-type crystals obtained by wide-angle X-ray measurement perpendicular to the film surface of the biaxially oriented polypropylene film of the present invention, the upper limit of the full width at half maximum (Wh) of the diffraction peak originating from the orientation crystals in the width direction of the film is 27°, preferably 26°, more preferably 25°, particularly preferably 24°, and most preferably 23°. A full width at half maximum (Wh) of 27° or less makes it easier to increase the rigidity of the film. The lower limit of Wh is preferably 13°, more preferably 14°, and even more preferably 15°.

(Hayes)
The upper limit of the haze of the biaxially oriented polypropylene film of the present invention is preferably 5.0%, more preferably 4.5%, even more preferably 4.0%, particularly preferably 3.5%, and most preferably 3.0%. A haze of 5.0% or less makes it easy to use in applications where transparency is required. The lower limit of the haze is preferably 0.1%, more preferably 0.2%, even more preferably 0.3%, and particularly preferably 0.4% as a practical value. A haze of 0.1% or more makes it easy to manufacture. The haze can be kept within the range by adjusting the cooling roll (CR) temperature, the widthwise stretching temperature, the tenter preheating temperature before widthwise stretching, the widthwise stretching temperature, or the heat setting temperature, or the amount of components with a molecular weight of 100,000 or less in the polypropylene resin, but it may increase with the addition of anti-blocking agents or the application of a sealing layer.

(Practical characteristics of the film)
The practical properties of the biaxially oriented polypropylene film of the present invention will be described below.

(Wrinkles from heat sealing)
To form food packaging bags, the contents are filled into pre-made bags, and the bags are heated to melt and fuse the film together for sealing. This process is often carried out similarly when filling bags while they are being made. Typically, a sealant film made of polyethylene or polypropylene is laminated onto a base film, and these sealant film surfaces are fused together. The heating method involves applying pressure from a heating plate on the base film side to press and seal the film, with a seal width of approximately 10 mm being common. During this process, the base film is also heated, causing shrinkage and resulting wrinkles. Fewer wrinkles are desirable for bag durability and to increase purchasing intent. While the sealing temperature may be around 120°C, higher temperatures are required to increase the bag-making speed, and even then, smaller shrinkage is preferable. When a zipper is fused to the opening of the bag, an even higher sealing temperature is required.

(Print pitch misalignment)
Packaging films typically consist of a laminated film made of a printed base film and a sealant film. Bags are manufactured using bag-making machines, and various types of bags are used, including three-side seal bags, standing bags, and gusseted bags. Print pitch misalignment is thought to occur because the film's base material expands and contracts due to tension and heat applied during the printing process. Eliminating defective products due to print pitch misalignment is important for the efficient use of resources and for increasing consumer purchasing intent.

(Film processing)
The biaxially oriented polypropylene film of the present invention can be printed using letterpress printing, lithographic printing, intaglio printing, stencil printing, or transfer printing methods, depending on the application.
Furthermore, unstretched sheets, uniaxially oriented films, and biaxially oriented films made of low-density polyethylene, linear low-density polyethylene, ethylene-vinyl acetate copolymer, polypropylene, and polyester can be laminated together as a sealant film to provide heat-sealing properties. To further enhance gas barrier properties and heat resistance, unstretched sheets, uniaxially oriented films, and biaxially oriented films made of aluminum foil, polyvinylidene chloride, nylon, ethylene-vinyl alcohol copolymer, and polyvinyl alcohol can be provided as intermediate layers between the biaxially oriented polypropylene film and the sealant film. Adhesives applied by dry lamination or hot-melt lamination can be used to laminate the sealant films.
To enhance gas barrier properties, aluminum or inorganic oxides can be deposited onto biaxially oriented polypropylene films, intermediate films, or sealant films. Vacuum deposition, sputtering, and ion plating can be used as deposition methods, but vacuum deposition of silica, aluminum, or mixtures thereof is particularly preferred.

The biaxially oriented polypropylene film of the present invention can be made suitable for packaging fresh produce such as vegetables, fruits, and flowers, which require high freshness, by adjusting the amount of antifogging agents in the film, such as polyhydric alcohol fatty acid esters, higher fatty acid amines, higher fatty acid amides, and ethylene oxide adducts of higher fatty acid amines or amides, to a range of 0.2 to 5% by mass.

Furthermore, within limits that do not impair the effects of the present invention, various additives for improving quality such as lubricity and antistatic properties may be incorporated. For example, lubricants such as waxes and metal soaps, plasticizers, processing aids, heat stabilizers, antioxidants, antistatic agents, and ultraviolet absorbers may be added to improve productivity.

(Industrial applicability)
Because the biaxially oriented polypropylene film of the present invention has the above-mentioned superior properties not found in conventional films, it can be preferably used in packaging bags, and it is also possible to make the film thinner than conventional films.

Furthermore, it is suitable for applications that use high temperatures, such as insulating films for capacitors and motors, backsheets for solar cells, barrier films for inorganic oxides, and base films for transparent conductive films such as ITO, as well as applications requiring rigidity, such as separator films. In addition, it enables high-temperature coating and printing processes using coating agents, inks, and laminating adhesives that were previously difficult to use, leading to improved production efficiency.

The present invention will be described in detail below with reference to examples. The characteristics were measured and evaluated by the following methods.
(1) Melt flow rate The melt flow rate (MFR) was measured in accordance with JIS K7210 at a temperature of 230°C and a load of 2.16 kgf.

(2) Mesopentad fraction The mesopentad fraction ([mmmm]%) of polypropylene resin was measured using 13C-NMR. The mesopentad fraction was calculated according to the method described by Zamberli et al., Macromolecules, Vol. 6, p. 925 (1973). 13C-NMR measurement was performed using a BRUKER AVANCE 500, dissolving 200 mg of the sample in an 8:2 mixture of o-dichlorobenzene and deuterated benzene at 135°C and then at 110°C.

(3) Number average molecular weight, weight average molecular weight, amount of components with a molecular weight of 100,000 or less, and molecular weight distribution of polypropylene resin. These were determined using gel permeation chromatography (GPC) as the molecular weight equivalent to PP, based on monodisperse polystyrene. When the baseline was not clear, the baseline was set within the range from the lowest point of the high molecular weight side of the elution peak closest to the elution peak of the standard substance to the lowest point of the high molecular weight side of the tail.
The GPC measurement conditions are as follows:
Equipment: HLC-8321PC/HT (manufactured by Tosoh Corporation)
Detector: RI
Solvent: 1,2,4-Trichlorobenzene + Dibutylhydroxytoluene (0.05%)
Columns: TSKgelguardcolumnHHR(30)HT (7.5mm I.D. × 7.5cm) × 1 + TSKgelGMHHR-H(20)HT (7.8mm I.D. × 30cm) × 3 Columns Flow rate: 1.0 mL/min
Injection volume: 0.3mL
Measurement temperature: 140℃
The number-average molecular weight (Mn) and mass-average molecular weight (Mw) are defined by the following equations, based on the number of molecules (Ni) of molecular weight (Mi) at each elution position of the GPC curve obtained via the molecular weight calibration curve.
Number average molecular weight: Mn=Σ(Ni・Mi)/ΣNi
Mass average molecular weight: Mw=Σ(Ni・Mi ² )/Σ(Ni・Mi)
Here, the molecular weight distribution can be obtained as Mw/Mn.
Furthermore, the proportion of components with a molecular weight of 100,000 or less was determined from the integral curve of the molecular weight distribution obtained by GPC.

(4) Crystallization temperature (Tc), melting temperature (Tm)
Thermal measurements were performed under a nitrogen atmosphere using a Q1000 differential scanning calorimeter manufactured by T.A. Instruments. Approximately 5 mg was cut from a polypropylene resin pellet and sealed in an aluminum pan for measurement. The temperature was raised to 230°C and held for 5 minutes, then cooled to 30°C at a rate of -10°C/min, and the exothermic peak temperature was defined as the crystallization temperature (Tc). The heat of crystallization (ΔHc) was determined by setting a baseline so that the area of the exothermic peak was smoothly connected from the start to the end of the peak. The temperature was then held at 30°C for 5 minutes, and the temperature was raised to 230°C at a rate of 10°C/min, and the main endothermic peak temperature was defined as the melting temperature (Tm).

(5) Film thickness The film thickness was measured using a Seiko EM Millitron 1202D.

(6) Haze: Measured at 23°C in accordance with JIS K7105 using NDH5000 manufactured by Nippon Denshoku Industries Co., Ltd.

(7) Tensile Test The tensile strength in the longitudinal and width directions of the film was measured at 23°C in accordance with JIS K 7127. The sample was cut from the film to a size of 15 mm x 200 mm, and the chuck width was 100 mm. It was set on a tensile testing machine (Instron 5965 dual-column benchtop testing machine manufactured by Instron Japan Company Limited). The tensile test was performed at a tensile speed of 200 mm/min. From the obtained strain-stress curve, the stress at 5% elongation was defined as F5. The tensile breaking strength and tensile breaking elongation were defined as the strength and elongation at the time the sample broke, respectively.
The F5 value at 80°C was determined by performing the measurement in a constant temperature bath at 80°C. The measurement was performed by setting the chuck in a constant temperature bath pre-set to 80°C, attaching the sample, and holding it in place for one minute until measurement.

(8) Heat shrinkage rate The following method was used in accordance with JIS Z 1712. The film was cut to a length of 20 mm with a width of 20 mm, and the film was cut in both the longitudinal and width directions. The cuts were suspended in a hot air oven at 120°C or 150°C and heated for 5 minutes. The length after heating was measured, and the heat shrinkage rate was determined as the ratio of the length shrunk to the original length.

(9) Refractive index, ΔNy, and surface orientation coefficient The refractive index was measured using an Abbe refractometer manufactured by Atago Corporation at a wavelength of 589.3 nm and a temperature of 23°C. The refractive index along the longitudinal and width directions of the film was denoted as Nx and Ny, respectively, and the refractive index in the thickness direction was denoted as Nz. ΔNy was calculated using Nx, Ny, and Nz with the formula Ny - [(Nx + Nz)/2]. The surface orientation coefficient (ΔP) was calculated using the formula [(Nx + Ny)/2] - Nz.

(10) Diffraction intensity ratio originating from the (110) plane of the α-type crystal. Wide-angle X-ray (WAXS) measurements of the film were performed in the second hutch of beamline BL03XU, owned by the Frontier Soft Matter Development Industry-Academia Alliance (FSBL) within the large synchrotron radiation facility SPring-8. The X-ray wavelength was set to 0.1 nm, and a two-dimensional detector SOPHIAS was used as the detector. Transmittance was calculated from the values of ion chambers set before and after the sample.
The film was sandwiched between two non-slip stainless steel plates, each with a 4 mm diameter hole in the center, and placed in a custom-made holder, restrained on all four sides. Temperature-controlled air was blown onto the film, increasing its temperature from 40°C to 180°C at a rate of 10°C/min. X-rays were incident perpendicular to the film surface, and wide-angle X-ray diffraction patterns were obtained at each temperature.
The obtained two-dimensional images were corrected for air scattering considering transmittance. Cerium oxide ( _CeO₂ ) was used to measure the camera length, and the azimuthal angle profile of the (110) plane of the α-type crystal of polypropylene was calculated using Fit2D (software from European Synchrotron Radiation Facility [http://www.esrf.eu/computer/scientific/FIT2D/]). The temperature dependence of the maximum diffraction intensity originating from the (110) plane of the α-type crystal oriented in the width direction was calculated as the ratio (diffraction intensity ratio) (%) of the maximum maximum intensity in the range from 40°C to 130°C to the maximum intensity at 40°C.

(11) X-ray full width at half maximum and degree of orientation The measurements were taken using the transmission method with an X-ray diffractometer (RINT2500, manufactured by Rigaku Corporation). X-rays with a wavelength of 0.15418 nm were used, and a scintillation counter was used as the detector. A sample was prepared by stacking films to a thickness of 500 μm. The sample stage was placed at the diffraction peak position of the (110) plane of the α-type crystal of polypropylene resin (diffraction angle 2θ = 14.1°), and the sample was rotated 360° around the thickness direction of the film to obtain the azimuthal angle dependence of the diffraction intensity of the (110) plane. From this azimuthal angle dependence, the full width at half maximum Wh of the diffraction peak originating from the oriented crystal in the width direction of the film was determined.
Furthermore, the degree of orientation was calculated using Wh from the following formula.

(12) Dynamic viscoelasticity measurement (DMA)
Dynamic viscoelasticity measurements were performed using an RSA-G2 instrument manufactured by T.A. Instrument Japan. A 4 mm wide film sample was set in the instrument with a 10 mm gap between the chucks, and under a load of 10 g, the temperature was increased from -60°C to 160°C at a rate of 5°C/min in a nitrogen atmosphere. Measurements were taken at a frequency of 10 Hz to obtain the temperature dependence of the loss modulus (E'') (Pa). From the loss modulus-temperature curve, the maximum value of the loss modulus (E'') between -25°C and 25°C (E'' _g ) and the maximum value of the loss modulus (E'') above 100°C (E'' _h ) were obtained.

(Example 1)
As the polypropylene resin, 80 parts by weight of PP-1 (Sumitomo Noblen FLX80E4, manufactured by Sumitomo Chemical Co., Ltd.), which has an MFR of 7.5 g/10 min, [mmmm] of 98.9%, Tc of 116.2°C, and Tm of 162.5°C, was blended with 20 parts by weight of PP-2 (EL80F5, manufactured by Sumitomo Chemical Co., Ltd.), which has an MFR of 11 g/10 min, [mmmm] of 98.8%, Tc of 116.5°C, and Tm of 161.5°C.
The material was extruded into a sheet from a T-die at 250°C, brought into contact with a cooling roll at 20°C, and then immediately placed in a 20°C water bath. Subsequently, it was stretched 4.5 times in the longitudinal direction using two pairs of rolls at 142°C, then clipped at both ends and guided into a hot air oven. After preheating to 170°C, it was stretched 12 times in the width direction at 167°C. Immediately after the widthwise stretching, it was cooled at 100°C while still held by the clips, and then heat-treated at 165°C without any relaxation in the width direction. The thickness of the resulting film was 20.3 μm.
Table 1 shows the structure of the polypropylene resin, and Table 2 shows the film-forming conditions. As shown in Table 3, the resulting film had high rigidity and low thermal shrinkage at high temperatures.

(Comparative Example 1)
As the polypropylene resin, PP-3 (FL203D, manufactured by Nippon Polypropylene Co., Ltd.) was used, with MFR = 3.0 g/10 min, [mmmm] = 94.8%, Tc = 117.2°C, and Tm = 160.6°C. It was extruded into a sheet from a T-die at 250°C, brought into contact with a cooling roll at 20°C, and then immediately placed in a 20°C water bath. After that, it was stretched 4.5 times in the longitudinal direction at 134°C, then the film was heated in a tenter with a preheating temperature of 170°C, followed by stretching 8.2 times in the width direction at a stretching temperature of 161°C, and then heat-set at 165°C while applying a 6.6% relaxation. The thickness of the obtained film was 20.3 μm. Table 1 shows the structure of the polypropylene resin, Table 2 shows the film-forming conditions, and Table 3 shows the physical properties. As shown in Table 3, the physical properties were poor in terms of rigidity and thermal shrinkage rate at 150°C.

(Comparative Example 2)
As the polypropylene resin, 70 parts by weight of PP-1 and 30 parts by weight of PP-4 (manufactured by Sumitomo Chemical Co., Ltd., FS2012), with an MFR of 2.7 g/10 min, [mmmm] = 98.7%, Tc = 114.7°C, and Tm = 163.0°C, were blended and used. The mixture was extruded into a sheet from a T-die at 250°C, brought into contact with a cooling roll at 20°C, and then immediately placed in a 20°C water bath. Subsequently, the film was stretched 4.5 times in the longitudinal direction at 135°C, then heated in a tenter with a preheating temperature of 173°C, followed by stretching 8.2 times in the width direction at a stretching temperature of 165°C, and then heat-set at 171°C while applying a 6.6% relaxation. The thickness of the obtained film was 20.9 μm. Table 1 shows the structure of the polypropylene resin, Table 2 shows the film-forming conditions, and Table 3 shows the physical properties. As shown in Table 3, its physical properties indicated poor rigidity.

Claims (8)

A biaxially oriented polypropylene film that satisfies the following conditions (1) to (7) .
(1) When the film is restrained on all four sides and wide-angle X-ray diffraction measurements are taken by incidenting X-rays perpendicular to the film surface while heating it from 40°C to 180°C at a rate of 10°C/min, the temperature dependence of the maximum intensity of the azimuthal profile obtained by scanning the scattering originating from the (110) plane of the α-type crystal of polypropylene in the circumferential direction is 115% or more.
(2) The degree of orientation in the width direction obtained by wide-angle X-ray measurement is 0.85 or higher.
(3) The thermal shrinkage rate at 150°C is 10% or less in the longitudinal direction and 30% or less in the width direction.
(4) The longitudinal elongation at break at 23°C is 195% or more.
(5) The biaxially oriented polypropylene film has a tensile breaking strength in the width direction of 400 MPa or more at 23°C.
(6) The melt flow rate of the polypropylene resin constituting the biaxially oriented polypropylene film is 4.0 g/10 min or more.
(7) The amount of polypropylene resin components with a molecular weight of 100,000 or less that constitute the biaxially oriented polypropylene film is 35% by mass or more. The biaxially oriented polypropylene film according to claim 1, wherein, in the temperature dependence of the loss modulus (E'') in the width direction obtained by dynamic viscoelastic measurement (DMA) of the biaxially oriented polypropylene film, the ratio (E''h/E''g) of the maximum value of the loss modulus (E'') at 100°C or above (E''h) to the maximum value of the loss modulus (E'') between -25°C and 25°C is 70% or more. The biaxially oriented polypropylene film according to claim 1 or 2, wherein the tensile elongation at break of the biaxially oriented polypropylene film is 25% or more in the longitudinal direction and 195% or more in the width direction at 23°C. The biaxially oriented polypropylene film according to any one of claims 1 to 3, wherein the heat shrinkage rate at 120°C is 2.0% or less in the longitudinal direction and 5.0% or less in the width direction, and the heat shrinkage rate at 120°C in the longitudinal direction is smaller than the heat shrinkage rate at 120°C in the width direction. The biaxially oriented polypropylene film according to any one of claims 1 to 4, wherein the refractive index Ny in the width direction of the biaxially oriented polypropylene film is 1.5230 or more, and ΔNy is 0.0220 or more. The biaxially oriented polypropylene film according to any one of claims 1 to 5, wherein the haze of the biaxially oriented polypropylene film is 5.0% or less. The biaxially oriented polypropylene film according to any one of claims 1 to 6, wherein the mesopentade fraction of the polypropylene resin constituting the biaxially oriented polypropylene film is 97.0% or more. The biaxially oriented polypropylene film according to any one of claims 1 to 7, wherein the crystallization temperature of the polypropylene resin constituting the biaxially oriented polypropylene film is 105°C or higher, and the melting point is 160°C or higher.