JP2014055283A - Stretched polypropylene film - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a stretched polypropylene film having high rigidity and a low shrinkage comparable to that of a polyethylene terephthalic acid ester at 150°C.SOLUTION: The stretched polypropylene film essentially comprises a polypropylene resin and has a thermal shrinkage of 9% or less in an MD direction and in a TD direction at 150°C, an impact strength of 0.6 J or more, and a haze of 5% or less. The polypropylene resin constituting the film has a lower limit of 96% of a meso-pentad fraction. The film has a lower limit of 0.0125 of a plane orientation coefficient. In the polypropylene resin, the upper limit of a copolymer amount is 0.1 mol%, and a xylene soluble content at normal temperature is 7 wt.% or less.

Description

  The present invention relates to a stretched polypropylene film. More specifically, the present invention relates to a biaxially stretched polypropylene film excellent in heat resistance and mechanical properties, which can be suitably used in various fields where dimensional stability at high temperature and high rigidity are required.

  Conventionally, by using polypropylene as a stretched film, it has been widely used in a wide range of applications such as packaging of foods and various products, electrical insulation, and surface protection films. However, the conventional polypropylene film has a shrinkage rate of several tens of percent at 150 ° C., and has low heat resistance and low rigidity as compared with PET and the like, and its application has been limited.

In order to solve these problems, there has been known a technique for producing a high-temperature rigid, heat-resistant film by forming a stretched film using polypropylene having high stereoregularity and a narrow molecular weight distribution (for example, Patent Document 1) reference).
In addition, a technology has been known that can be suitably used as a capacitor film having excellent electrical insulation, mechanical properties and the like by using a polypropylene film having high stereoregularity and having a wide molecular weight distribution as a stretched film ( For example, see Patent Document 2).

  Furthermore, a technology for forming a separator film using polypropylene having a low molecular weight and a soluble content of 0 ° C. by a temperature rising fractionation method in a specific range is known. This film has dimensions in a drying process and a printing process. It was also considered excellent in stability (for example, see Patent Document 3).

  However, Patent Documents 1 to 3 have difficulty in stretchability and inferior mechanical properties such as impact resistance.

  By adding a small amount of long-chain branched or cross-linked polypropylene to medium molecular weight products, it promotes the formation of child lamellae, improves stretchability, and excels in mechanical properties, heat resistance, voltage resistance properties, and uniformity in various physical properties. A technique for forming a film has been known (see, for example, Patent Document 4).

  Also known is a technology that balances rigidity and workability by using polypropylene with a film containing high molecular weight and medium molecular weight materials (substantially low molecular weight), wide molecular weight distribution, and low decalin soluble content. (See, for example, Patent Document 5).

In these patent documents 4 to 5, it cannot be said that the heat resistance at high temperature is sufficient, and a polypropylene film having high heat resistance and excellent impact resistance and transparency has not been known.
That is, they do not exceed the range of conventional polypropylene films, and their uses are limited. For example, no attention has been paid to heat resistance at high temperatures exceeding 150 ° C.

JP-A-8-325327 JP 2004-175932 A JP 2001-146536 A JP 2007-84813 A Special table 2008-540815

The present invention has been made against the background of such prior art problems. That is, an object of the present invention is to provide a stretched polypropylene film having a low shrinkage comparable to PET at 150 ° C. and having high rigidity.

As a result of intensive studies to achieve the above object, the present inventors have completed the present invention. That is, the present invention
A film composed mainly of a polypropylene resin, having a thermal shrinkage rate in the MD and TD directions at 150 ° C. of 9% or less, an impact strength of 0.6 J or more, and a haze of 6% or less. It is a stretched film characterized by this.
A stretched film is a film having an orientation stretched by a method such as uniaxial, simultaneous biaxial, sequential biaxial, etc. industrially. The degree of orientation is, for example, a plane orientation coefficient obtained from a refractive index. Etc.

  In this case, it is preferable that the lower limit of the mesopentad fraction of the polypropylene resin constituting the film is 96% and the lower limit of the plane orientation coefficient of the film is 0.0125.

  In this case, it is preferable that the upper limit of the amount of the copolymerization monomer of the polypropylene resin constituting the film is 0.1 mol%.

  Furthermore, in this case, it is preferable that the normal temperature xylene-soluble content of the polypropylene resin constituting the film is 7 wt% or less.

According to the present invention, a stretched polypropylene film can have a low shrinkage rate and high rigidity comparable to PET at 150 ° C., and thus can be thinned.
Furthermore, the polypropylene film of the present invention can maintain various physical properties even when exposed to an environment of 150 ° C. or higher, and can be used in a high-temperature environment that has not been considered with conventional polypropylene films.
For example, by setting the heat seal temperature high, it is possible to increase the line speed in the bag making process, and the productivity is improved. Further, the heat seal strength can be improved by increasing the heat seal temperature. Furthermore, the amount of deformation of the bag can also be suppressed when performing high temperature processing such as retort.

The present invention relates to a stretched polypropylene film excellent in dimensional stability at high temperatures and mechanical properties. The feature of the stretched polypropylene film of the present invention is the molecular weight distribution of the polypropylene resin used.
The present invention is a film mainly composed of a polypropylene resin, and has a thermal shrinkage rate in the MD direction and TD direction at 150 ° C. of 9% or less, an impact strength of 0.6 J or more, and a haze of 6 % Or less.

Here, the MD direction is the film flow direction, and the TD direction is the direction perpendicular to the film flow direction.

(Film characteristics)
The lower limit of the 150 ° C. heat shrinkage in the MD direction and the TD direction of the stretched film of the present invention is preferably 0.5%, more preferably 1%, still more preferably 1.5%, particularly preferably. 2%, most preferably 2.5%. If it is in the above range, realistic production may be easy in terms of cost or the like, and thickness unevenness may be reduced.
The upper limit of 150 ° C. heat shrinkage in the MD direction and TD direction is preferably 9%, more preferably 8%, still more preferably 7%, particularly preferably 6%, most preferably 5%. It is. When it is in the above range, it is easier to use in applications that may be exposed to a high temperature of about 150 ° C. If the heat shrinkage at 150 ° C. is up to about 2.5%, for example, it is possible to increase the low molecular weight component, adjust the stretching conditions and the fixing conditions, but below that, annealing treatment can be performed offline. preferable.
In the conventional stretched polypropylene film, the 150 ° C. heat shrinkage rate in the MD direction and the TD direction is 15% or more, and the 120 ° C. heat shrinkage rate is about 3%. By setting the heat shrinkage rate within the above range, a film having excellent heat resistance can be obtained.

The lower limit of the impact resistance (room temperature, 25 ° C.) of the polypropylene film of the present invention is preferably 0.6 J, more preferably 0.7 J. Within the above range, the film has sufficient toughness and does not break during handling.
The upper limit of impact resistance is preferably 2J from a practical aspect, more preferably 1.5J, still more preferably 1.2J, and particularly preferably 1J. For example, impact resistance tends to decrease when the total molecular weight is low when there are many low molecular weight components, when the total molecular weight is low, or when the molecular weight of the high molecular weight components is low. These components can be adjusted to be within the range.

The haze of the polypropylene film of the present invention is practically a lower limit of preferably 0.1%, more preferably 0.2%, still more preferably 0.3%, particularly preferably 0.4. %, And most preferably 0.5%.
The upper limit of haze is preferably 6%, more preferably 5%, still more preferably 4.5%, particularly preferably 4%, and most preferably 3.5%. If it is within the above range, it may be easy to use in applications requiring transparency. For example, when the stretching temperature and the heat setting temperature are too high, the haze tends to be worse when the cooling roll (CR) temperature is high and the cooling rate is slow, or when the low molecular weight is too much. I can do it.

(Polypropylene resin)
The polypropylene resin constituting the film may be a complete homopolypropylene obtained from only a propylene monomer or a copolymer with a copolymer monomer. Copolymeric monomer species can be ethylene, butene, hexene, octene, and the like.

If necessary, additives and other resins may be added to the film of the present invention, but it is preferably 30 wt% or less.
Examples of the additive include an antioxidant, an ultraviolet absorber, an antistatic agent, a lubricant, a nucleating agent, an adhesive, an antifogging agent, a flame retardant, an antiblocking agent, and an inorganic or organic filler. Examples of the other resin include polypropylene resins other than the polypropylene resin used in the present invention, random copolymers which are copolymers of α-olefin such as propylene and ethylene, butene, hexene and octene, and various elastomers.

(Molecular weight distribution of polypropylene resin)
The polypropylene resin used in the present invention is preferably mainly composed of a low molecular weight component having a molecular weight of, for example, about 100,000, and further containing a very high molecular weight component having a molecular weight of, for example, about 1,500,000. Crystallinity can be greatly increased by mainly using a low molecular weight component, and it is considered that a highly-stretched and heat-resistant stretched polypropylene film that has not been conventionally obtained is obtained. On the other hand, a low molecular weight polypropylene resin has a low melt tension when softened by heating, and it is generally difficult to obtain a stretched film. The high molecular weight component can be stretched by the presence of several percent to several tens of percent, and the high molecular weight component serves as a crystal nucleus, further increasing the crystallinity of the film, and the effect of the stretched film of the present invention. It is considered to have been achieved.
As an index representing such molecular weight distribution, (Mz + 1) / Mn, which is a ratio of Z + 1 average molecular weight (Mz + 1), which is an average molecular weight focusing on high molecular weight components, and number average molecular weight (Mn), is preferable.
The lower limit of Mz + 1 / Mn is preferably 50, more preferably 60, still more preferably 70, particularly preferably 80, and most preferably 90. If it is less than the above, it may be difficult to obtain the effects of the present application such as a low thermal shrinkage at high temperatures.
The upper limit of Mz + 1 / Mn is preferably 300, more preferably 200. If the above is exceeded, it may be difficult to produce a realistic resin.

The lower limit of Mz + 1 of the entire polypropylene resin constituting the film is preferably 2500,000, more preferably 3000,000, still more preferably 3300000, particularly preferably 3500000, and most preferably 3700000. Within the above range, the high molecular weight component is sufficient, and the effects of the present invention are easily obtained.
The upper limit of the total Mz + 1 is preferably 40000000, more preferably 35000000, and even more preferably 30000000. Within the above range, realistic resin production may be easy, stretching may be facilitated, and fish eyes in the film may be reduced.

The lower limit of Mn of the entire polypropylene resin constituting the film is preferably 20000, more preferably 22000, still more preferably 24000, particularly preferably 26000, and most preferably 27000. When it is in the above range, there are the advantages that stretching becomes easy, thickness unevenness is reduced, stretching temperature and heat setting temperature are easily raised, and thermal shrinkage rate is lowered.
The upper limit of the total Mn is preferably 65000, more preferably 60000, still more preferably 55000, particularly preferably 53000, and most preferably 52000. Within the above range, the effects of the present application such as a low heat shrinkage rate at high temperature, which is an effect of a low molecular weight substance, may be easily obtained, or stretching may be facilitated.
When the polypropylene resin having the above molecular weight distribution is represented by weight average molecular weight (Mw) / number average molecular weight (Mn), which is generally an index of the breadth of molecular weight distribution, the value is naturally large, but Mw / The lower limit of Mn is preferably 5.5, more preferably 6, still more preferably 6.5, particularly preferably 7, and most preferably 7.2.
The upper limit of Mw / Mn is preferably 30, more preferably 25, still more preferably 20, particularly preferably 15, and most preferably 13.

The lower limit of the weight average molecular weight (Mw) of the entire polypropylene resin constituting the film is preferably 250,000, more preferably 260000, still more preferably 270000, particularly preferably 280000, and most preferably 290000. . When it is in the above range, there are the advantages that stretching becomes easy, thickness unevenness is reduced, stretching temperature and heat setting temperature are easily raised, and thermal shrinkage rate is lowered.
The upper limit of the total Mw is preferably 500,000, more preferably 450,000, still more preferably 400,000, particularly preferably 380000, and most preferably 370000. Within the above range, the mechanical load may be small and stretching may be easy.

The lower limit of the melt flow rate (MFR) (230 ° C., 2.16 kgf) of the entire polypropylene resin constituting the film is preferably 1 g / 10 min, more preferably 1.2 g / 10 min, still more preferably 1.4 g. / 10 min, particularly preferably 1.5 g / 10 min, and most preferably 1.6 g / 10 min. Within the above range, the mechanical load may be small and stretching may be easy.
The upper limit of the total MFR is preferably 11 g / 10 min, more preferably 10 g / 10 min, still more preferably 9 g / 10 min, particularly preferably 8.5 g / 10 min, most preferably 8 g / 10 min. is there. When it is within the above range, stretching may be easy, thickness unevenness may be reduced, and the stretching temperature and heat setting temperature may be easily increased, resulting in a lower thermal shrinkage rate.

When the gel permeation chromatography (GPC) integration curve of the entire polypropylene resin constituting the film is measured, the lower limit of the amount of the component having a molecular weight of 10,000 or less is preferably 2% by mass, more preferably 2.5% by mass. More preferably, it is 3% by mass, particularly preferably 3.3% by mass, and most preferably 3.5% by mass. Within the above range, the effects of the present application such as a low heat shrinkage rate at a high temperature, which is an effect of a low molecular weight substance, may be more easily obtained, and stretching may be facilitated.
The upper limit of the amount of the component having a molecular weight of 10,000 or less in the GPC integration curve is preferably 20% by mass, more preferably 17% by mass, still more preferably 15% by mass, and particularly preferably 14% by mass. Most preferably, it is 13% by mass. When it is within the above range, stretching may be easy, thickness unevenness may be reduced, and the stretching temperature and heat setting temperature may be easily increased, resulting in a low thermal shrinkage rate.
Molecules with a molecular weight of about 10,000 or less do not contribute to the entanglement between the molecular chains, and have the effect of loosening the entanglement between the molecules as a plasticizer. The inclusion of a specific amount of a component having a molecular weight of 10,000 or less facilitates the entanglement of molecules during stretching, and enables stretching with low stretching stress, resulting in low residual stress and low shrinkage at high temperatures. It is thought that.

The lower limit of the amount of the component having a molecular weight of 100,000 or less in the GPC integration curve is preferably 35% by mass, more preferably 38% by mass, still more preferably 40% by mass, and particularly preferably 41% by mass. Most preferably, it is 42 mass%. Within the above range, the effects of the present application such as a low heat shrinkage rate at a high temperature, which is an effect of a low molecular weight substance, may be easily obtained, and stretching may be facilitated.
The upper limit of the amount of a component having a molecular weight of 100,000 or less in the GPC integration curve is preferably 65% by mass, more preferably 60% by mass, still more preferably 58% by mass, and particularly preferably 56% by mass. Most preferably, it is 55 mass%. If the above range is changed to a description that is a more preferable range, stretching may be facilitated, thickness unevenness may be reduced, and the stretching temperature and heat setting temperature may be easily increased, resulting in a low thermal shrinkage rate.

  A high molecular weight component and a low molecular weight component that are preferably used for obtaining a polypropylene resin having such characteristics of molecular weight distribution will be described.

(High molecular weight component)
The lower limit of MFR (230 ° C., 2.16 kgf) of the high molecular weight component is preferably 0.0001 g / 10 min, more preferably 0.0005 g / 10 min, still more preferably 0.001 g / 10 min, particularly preferably. Is 0.005 g / 10 min. If it is in the above range, the production of the resin is practically easy or the fish eye of the film may be reduced.
The MFR at 230 ° C. and 2.16 kgf of the high molecular weight component may be too small to make practical measurement difficult. In terms of high load MFR at a load of 10 times (21.6 kgf), the preferable lower limit is 0.1 g / 10 min, more preferably 0.5 g / 10 min, still more preferably 1 g / 10 min, Preferably, it is 5 g / 10 min.
The upper limit of the MFR of the high molecular weight component is preferably 0.5 g / 10 min, more preferably 0.35 g / 10 min, still more preferably 0.3 g / 10 min, and particularly preferably 0.2 g / 10 min. Most preferably, it is 0.1 g / 10 min. If the amount is in the above range, it is not necessary to use a large amount of polymer component in order to maintain the overall MFR, and the effects of the present application such as a low heat shrinkage rate at a high temperature, which is an effect of a low molecular weight product, can be more easily obtained. There is.

The lower limit of Mw of the high molecular weight component is preferably 500,000, more preferably 600,000, still more preferably 700,000, particularly preferably 800,000, and most preferably 1,000,000. If the amount is in the above range, it is not necessary to use a large amount of polymer component in order to maintain the overall MFR, and the effects of the present application such as a low heat shrinkage rate at a high temperature, which is an effect of a low molecular weight product, can be more easily obtained. There is.
The upper limit of Mw of the high molecular weight component is preferably 10000000, more preferably 8000000, still more preferably 6000000, and particularly preferably 5000000. If it is in the above range, the production of the resin is practically easy or the fish eye of the film may be reduced.

The lower limit of η of the high molecular weight component is preferably 3 dl / g, more preferably 3.2 dl / g, still more preferably 3.5 dl / g, and particularly preferably 4 dl / g. If the amount is in the above range, it is not necessary to use a large amount of polymer component in order to maintain the overall MFR, and the effects of the present application such as a low heat shrinkage rate at a high temperature, which is an effect of a low molecular weight product, can be more easily obtained. There is.
The upper limit of the intrinsic viscosity (η) of the high molecular weight component is preferably 15 dl / g, more preferably 12 dl / g, still more preferably 10 dl / g, and particularly preferably 9 dl / g. If it is in the above range, the production of the resin is practically easy or the fish eye of the film may be reduced.

The lower limit of the amount of the high molecular weight component is preferably 2% by mass, more preferably 3% by mass, still more preferably 4% by mass, and particularly preferably 5% by mass. Within the above range, it is not necessary to increase the molecular weight of the low molecular weight product in order to maintain the overall MFR, and the effects of the present application such as a low heat shrinkage rate at high temperatures may be more easily obtained.
The upper limit of the amount of the high molecular weight component is preferably 30% by mass, more preferably 25% by mass, still more preferably 22% by mass, and particularly preferably 20% by mass. Within the above range, the effects of the present application such as a low heat shrinkage rate at a high temperature, which is an effect of a low molecular weight substance, may be more easily obtained.
Here, as the high molecular weight component, a polypropylene resin having a long chain branching or a crosslinked structure can be used instead of the linear polypropylene resin, which is known as high melt tension polypropylene, manufactured by Borealis. There are Daploy WB130HMS and WB135HMS.

(Low molecular weight component)
The lower limit of the low molecular weight component MFR (230 ° C., 2.16 kgf) is preferably 70 g / 10 min, more preferably 80 g / 10 min, still more preferably 100 g / 10 min, particularly preferably 150 g / 10 min. Most preferably, it is 200 g / 10 min. Within the above range, the crystallinity is improved, and the effects of the present application such as a low thermal shrinkage rate at a high temperature may be more easily obtained.
The upper limit of the MFR of the low molecular weight component is preferably 2000 g / 10 min, more preferably 1800 g / 10 min, still more preferably 1600 g / 10 min, particularly preferably 1500 g / 10 min, most preferably 1500 g / 10 min. is there. Within the above range, the overall MFR can be easily maintained, and the film forming property may be excellent.

The lower limit of the Mw of the low molecular weight component is preferably 50000, more preferably 53000, still more preferably 55000, particularly preferably 60000, and most preferably 70000. Within the above range, the overall MFR can be easily maintained, and the film forming property may be excellent.
The upper limit of the Mw of the low molecular weight component is preferably 150,000, more preferably 140000, still more preferably 130,000, particularly preferably 120,000, and most preferably 110,000. Within the above range, the crystallinity is improved, and the effects of the present application such as a low thermal shrinkage rate at a high temperature may be more easily obtained.

The lower limit of η of the low molecular weight component is preferably 0.46 dl / g, more preferably 0.48 dl / g, still more preferably 0.5 dl / g, and particularly preferably 0.55 dl / g. Most preferably, it is 0.6 dl / g. Within the above range, the overall MFR can be easily maintained, and the film forming property may be excellent.
The upper limit of η of the low molecular weight component is preferably 1.1 dl / g, more preferably 1.05 dl / g, still more preferably 1 dl / g, particularly preferably 0.95 dl / g, Preferably it is 0.85 dl / g. Within the above range, the crystallinity is improved, and the effects of the present application such as a low thermal shrinkage rate at a high temperature may be more easily obtained.

The lower limit of the amount of the low molecular weight component is preferably 40% by mass, more preferably 50% by mass, still more preferably 55% by mass, and particularly preferably 60% by mass. Within the above range, the effects of the present application such as a low heat shrinkage rate at a high temperature, which is an effect of a low molecular weight substance, may be more easily obtained.
The upper limit of the amount of the low molecular weight component is preferably 98% by mass, more preferably 97% by mass, still more preferably 96% by mass, and particularly preferably 95% by mass. Within the above range, it is not necessary to increase the molecular weight of the low molecular weight product in order to maintain the overall MFR, and the effects of the present application such as a low heat shrinkage rate at high temperatures may be more easily obtained.

The lower limit of the MFR of the low molecular weight component / the MFR ratio of the high molecular weight component is preferably 500, more preferably 1000, still more preferably 2000, and particularly preferably 4000. Within the above range, the effects of the present application such as a low heat shrinkage at high temperatures may be more easily obtained.
The upper limit of the MFR ratio of the low molecular weight component / the MFR ratio of the high molecular weight component is preferably 1,000,000.

The high molecular weight component and the low molecular weight component may be a mixture of two or more resins corresponding to each component, and the blending amount in that case is a total amount.
In addition to the high molecular weight component and low molecular weight component in the above range, a component having a molecular weight other than the low molecular weight component and high molecular weight component of the present invention may be added in order to adjust the MFR as a whole polypropylene resin. In order to ease the entanglement of the molecular chains and to adjust the stretchability, a polypropylene resin having a low molecular weight component or less, particularly a molecular weight of about 30,000 or less, and further a molecular weight of about 10,000 or less may be added.

  In order to obtain a preferable molecular weight distribution state of a polypropylene resin using a high molecular weight component and a low molecular weight component, for example, when the molecular weight of the low molecular weight component to be used is low, the molecular weight of the high molecular weight component is increased. The distribution state can be adjusted, for example, by increasing, and the MFR can be adjusted to be easy to produce as a stretched film.

(Regularity of polypropylene resin)
The lower limit of the mesopentad fraction of the polypropylene resin constituting the film is preferably 96%, more preferably 96.5%, and still more preferably 97%. When it is within the above range, the crystallinity may be improved, and the thermal shrinkage rate at a high temperature may be lower.
The upper limit of the mesopentad fraction is preferably 99.5%, more preferably 99.3%, still more preferably 99%. In the above range, realistic production may be easy.

It is preferable that the heterogeneous bond of the polypropylene resin which comprises a film is not recognized. In addition, not being recognized here means that a peak is not seen by 500 MHz < ¹³ > C-NMR.

The lower limit of the xylene soluble content of the polypropylene resin constituting the film is preferably 0.1% by mass from a practical aspect.
The upper limit of the xylene-soluble content is preferably 7% by mass, more preferably 6% by mass, and further preferably 5% by mass. When it is in the above range, the crystallinity may be improved, and the thermal shrinkage at high temperatures may be reduced.

The lower limit of the isotactic chain length of the polypropylene resin constituting the film is preferably 100, more preferably 120, and still more preferably 130. When it is in the above range, the crystallinity may be improved, and the thermal shrinkage at high temperatures may be reduced.
The upper limit of the isotactic chain length is preferably 5000 from a practical aspect.

  The polypropylene resin constituting the film is most preferably a complete homopolypropylene obtained only from a propylene monomer, but may be a copolymer with a copolymerization monomer as long as it is in a trace amount. As the copolymerization monomer species, ethylene and butene are preferable.

  The upper limit of the amount of the comonomer is preferably 0.1 mol%, more preferably 0.05 mol%, still more preferably 0.01 mol%. When it is in the above range, the crystallinity may be improved, and the thermal shrinkage at high temperatures may be reduced.

  Conventionally, stretched polypropylene film is difficult to form a film because the range of conditions that can be stretched is extremely narrow, such as high crystallinity in industrially complete homopolypropylene and rapid drop in melt tension after melt softening. Added about 0.5% copolymerization component (mainly ethylene). However, the polypropylene resin having a molecular weight distribution as described above has a moderate decrease in tension after melt softening even if there is little or no copolymerization component, and industrial stretching is possible.

(Production method of polypropylene resin)
Propylene is obtained by polymerizing propylene as a raw material using a Ziegler-Natta catalyst, a metallocene catalyst, or the like. In particular, in order to eliminate the heterogeneous bond, it is preferable to use a catalyst capable of highly regular polymerization, such as a Ziegler-Natta catalyst.
As a polymerization method of propylene, a method of polymerizing in an inert solvent such as hexane, heptane, toluene, xylene, a method of polymerizing in liquid propylene or ethylene, a catalyst is added to propylene or ethylene as a gas, The method of superposing | polymerizing in a phase state or the method of superposing | polymerizing combining these is mentioned.
The high molecular weight component and the low molecular weight component may be mixed after polymerization separately, or may be produced in a single plant by a multistage reactor. In particular, a method is preferred in which a plant having a multi-stage reactor is used, and a high molecular weight component is first polymerized and then a low molecular weight component is polymerized in the presence thereof. The molecular weight can be adjusted by the amount of hydrogen mixed during the polymerization.

  If necessary, additives and other resins may be added to the film-forming resin composition of the present invention. Examples of the additive include an antioxidant, an ultraviolet absorber, an antistatic agent, a lubricant, a nucleating agent, an adhesive, an antifogging agent, a flame retardant, an antiblocking agent, and an inorganic or organic filler. Examples of other resins include polypropylene resins other than the polypropylene resin used in the present invention, random copolymers that are copolymers of ethylene and α-olefin, and various elastomers. These can be blended with polypropylene resin and a Henschel mixer, or master pellets prepared using a melt kneader in advance can be diluted with polypropylene to a predetermined concentration, or the total amount can be melt-kneaded in advance. Also good.

  By using a polypropylene resin with such a characteristic molecular weight distribution, it is possible to draw polypropylene mainly composed of low molecular weight, which was impossible to draw in the past, and adopts a high heat setting temperature. It is considered that the heat shrinkage rate at a high temperature can be lowered by a synergistic effect of high crystallinity and strong heat fixation.

(Film physical properties)

The lower limit of the plane orientation coefficient of the polypropylene film of the present invention is preferably 0.0125, more preferably 0.0126, still more preferably 0.0127, and particularly preferably 0.0128.
The upper limit of the plane orientation coefficient is preferably 0.0155 as a practical value, more preferably 0.015, still more preferably 0.0148, and particularly preferably 0.0145. The plane orientation coefficient can be set within the range by adjusting the draw ratio. Within this range, the film thickness unevenness is also good.

  The lower limit of the refractive index (Nx) in the MD direction of the stretched film of the present invention is preferably 1.502, more preferably 1.503, and still more preferably 1.504. The upper limit of Nx is preferably 1.52, more preferably 1.517, and even more preferably 1.515.

  The lower limit of the refractive index (Ny) in the TD direction is preferably 1.523, and more preferably 1.525. The upper limit of Ny is preferably 1.535, more preferably 1.532.

  The lower limit of the refractive index (Nz) in the thickness direction is preferably 1.48, more preferably 1.490, and still more preferably 1.501. The upper limit of Nz is preferably 1.51, more preferably 1.507, and even more preferably 1.505.

(Film crystallinity)
The stretched film of the present invention has the following highly crystalline characteristics.
The lower limit of the film crystallinity is preferably 55%, more preferably 56%, still more preferably 57%, particularly preferably 58%, and most preferably 59%. If it is less than the above, the thermal shrinkage at high temperatures may increase.
The upper limit of the film crystallinity is preferably 85%, more preferably 80%, still more preferably 79%, particularly preferably 78%, and most preferably 77%.
If the above is exceeded, realistic manufacturing may become difficult. The degree of crystallinity of the film can be set within the range by reducing or eliminating the copolymerization monomer, setting the stretching temperature, and the heat setting temperature to a high temperature.

The lower limit of the melting point is preferably 168 ° C, more preferably 169 ° C. Within the above range, the thermal shrinkage at high temperatures may be small.
The upper limit of the melting point is preferably 180 ° C, more preferably 177 ° C, and further preferably 175 ° C. In the above range, realistic production may be easy. The melting point can be kept within the range by reducing or eliminating the copolymerization monomer, increasing the low molecular weight component, and setting the stretching temperature and heat setting temperature to a high temperature.

Even when the conventional polypropylene film has a melting point peak around 170 ° C., when measured by DSC, a peak rise (beginning of melting) is recognized from around 140 ° C., and at 140 ° C. Although heat resistance could be expected, the heat shrinkage rate increased rapidly at 150 ° C. However, it is considered that the polypropylene film of the present invention has no peak rise even at 150 ° C. and low heat shrinkability at 150 ° C. is obtained.
The polypropylene film of the present invention can maintain various physical properties even when exposed to an environment of 150 ° C. or higher, and can be used in a high-temperature environment that has not been considered with conventional polypropylene films.
The start of melting can be determined from the DSC curve.
The DSC chart of an example of the stretched film of the present invention is shown in FIG.

By dividing the heat of fusion obtained as the endothermic peak area of 150 ° C. or higher by 209 J / g, the crystal fraction in all samples at 150 ° C. can be obtained. The lower limit of the crystal fraction at 150 ° C. is preferably 48%, more preferably 49%, still more preferably 50%, and particularly preferably 51%. Within the above range, the thermal shrinkage rate at high temperature may be smaller.
The upper limit of the 150 ° C. crystal fraction is preferably 85%, more preferably 80%, still more preferably 79%, and particularly preferably 78% from the practical viewpoint. The 150 ° C. crystal fraction can be kept within the range by techniques such as reducing or eliminating the copolymerization monomer, increasing the low molecular weight component, and setting the stretching temperature and heat setting temperature to a high temperature.

When the stretched film is a biaxially stretched film, the lower limit of the Young's modulus (25 ° C.) in the MD direction is preferably 2 GPa, more preferably 2.1 GPa, still more preferably 2.2 GPa, and particularly preferably 2.3 GPa, most preferably 2.4 GPa.
The upper limit of the Young's modulus in the MD direction is preferably 4 GPa, more preferably 3.7 GPa, even more preferably 3.5 GPa, particularly preferably 3.4 GPa, and most preferably 3.3 GPa. Within the above range, realistic manufacturing may be easy, and the MD-TD balance may be improved.

When the stretched film is a biaxially stretched film, the lower limit of the Young's modulus (25 ° C.) in the TD direction is preferably 3.8 GPa, more preferably 4 GPa, still more preferably 4.2 GPa, and particularly preferably 4.3 GPa.
The upper limit of the Young's modulus in the TD direction is preferably 8 GPa, more preferably 7.5 GPa, still more preferably 7 GPa, and particularly preferably 6.5 GPa. If it is in the above range, realistic production may be easy, and the MD-TD balance may be improved.
The Young's modulus can be increased by increasing the draw ratio. In the case of MD-TD stretching, the MD stretch ratio is set lower, and the Young's modulus in the TD direction is increased by increasing the TD stretch ratio. Can do.

The lower limit of the thickness uniformity of the stretched film of the present invention is preferably 0%, more preferably 0.1%, still more preferably 0.5%, and particularly preferably 1%.
The upper limit of thickness uniformity is preferably 20%, more preferably 17%, still more preferably 15%, particularly preferably 12%, and most preferably 10%. If it is in the above range, defects are unlikely to occur during post-processing such as coating and printing, and it is easy to use for applications that require precision.

The lower limit of the density of the stretched film of the present invention is preferably 0.91 g / cm ³ , more preferably 0.911 g / cm ³ , still more preferably 0.912 g / cm ³ , and particularly preferably 0.8. 913 g / cm ³ . Within the above range, the crystallinity is high and the thermal shrinkage rate may be small.
The upper limit of the film density is preferably 0.925 g / cm ^3, more preferably 0.922 g / cm ^3, more preferably from 0.92 g / cm ^3, particularly preferably at 0.918 g / cm ³ is there. Beyond the above, realistic manufacturing may be facilitated. The film density can be increased by increasing the draw ratio and temperature, increasing the heat setting temperature, and further performing offline annealing.

(Method for producing polypropylene film)
The stretched film of the present invention may be a uniaxially stretched film in the longitudinal direction (MD direction) or the transverse direction (TD direction), but is preferably a biaxially stretched film. In the case of biaxial stretching, sequential biaxial stretching or simultaneous biaxial stretching may be used.
By using a stretched film, a film having a low thermal shrinkage rate can be obtained even at 150 ° C., which could not be expected with a conventional polypropylene film.

A method for producing a film of sequential biaxial stretching of longitudinal stretching and transverse stretching, which is the most preferable example, will be described below.
First, a polypropylene resin is heated and melted with a monoaxial or biaxial extruder and extruded onto a chill roll to obtain an unstretched film.
The melt extrusion conditions are such that the resin temperature is 200 to 280 ° C., the sheet is extruded from a T-die and cooled and solidified with a cooling roll having a temperature of 10 to 100 ° C. Next, the film is stretched 3 to 7 times in the length (MD) direction with a stretching roll at 120 to 165 ° C., and subsequently at a temperature of 155 ° C. to 175 ° C., preferably 158 ° C. to 170 ° C. in the width (TD) direction. Stretch 6-12 times.
Further, heat treatment is performed at an ambient temperature of 165 to 175 ° C., preferably 166 to 173 ° C. while allowing relaxation of 1 to 15%.
A roll sample can be obtained by performing corona discharge treatment on at least one side of the polypropylene film thus obtained and then winding it with a winder.

The lower limit of the MD draw ratio is preferably 3 times, more preferably 3.5 times. If it is less than the above, film thickness unevenness may occur.
The upper limit of the MD draw ratio is preferably 8 times, more preferably 7 times. If the above is exceeded, it may be difficult to carry out TD stretching.

The lower limit of the MD stretching temperature is preferably 120 ° C, more preferably 125 ° C, and even more preferably 130 ° C. If it is less than the above, the mechanical load may be increased, the thickness unevenness may be increased, or the film may be roughened.
The upper limit of the MD stretching temperature is preferably 160 ° C, more preferably 155 ° C, and even more preferably 150 ° C. A higher temperature is preferable for lowering the thermal shrinkage, but may adhere to the roll and fail to stretch.

The lower limit of the stretching ratio of TD is preferably 4 times, more preferably 5 times, and further preferably 6 times. If it is less than the above, thickness unevenness may occur.
The upper limit of the TD stretch ratio is preferably 20 times, more preferably 17 times, and even more preferably 15 times. If the above is exceeded, the thermal shrinkage rate may increase or the film may break during stretching.

  The preheating temperature in TD stretching is preferably set to be higher by 10 to 15 ° C. than the stretching temperature in order to quickly raise the film temperature in the vicinity of the stretching temperature.

TD stretching is performed at a higher temperature than conventional polypropylene films.
The lower limit of the TD stretching temperature is preferably 157 ° C, more preferably 158 ° C. If it is less than the above, it may break without being sufficiently softened, or the thermal shrinkage rate may be increased.
The upper limit of the TD stretching temperature is preferably 170 ° C, more preferably 168 ° C. In order to lower the thermal shrinkage rate, the temperature is preferably higher. However, if the temperature is exceeded, the low molecular component may be melted and recrystallized, resulting in surface roughness or whitening of the film.

The stretched film is heat-set. The heat setting can be performed at a higher temperature than the conventional polypropylene film. The lower limit of the heat setting temperature is preferably 165 ° C, more preferably 166 ° C. If it is less than the above, the thermal shrinkage rate may increase. In addition, a long time is required to lower the heat shrinkage rate, and productivity may be inferior.
The upper limit of the heat setting temperature is preferably 175 ° C, more preferably 173 ° C. If the above is exceeded, the low molecular components may melt and recrystallize, resulting in surface roughness and whitening of the film.

It is preferable to relax at the time of heat setting. The lower limit of relaxation is preferably 2%, more preferably 3%. If it is less than the above, the thermal shrinkage rate may increase.
The upper limit of relaxation is preferably 10%, more preferably 8%. When the above is exceeded, the thickness unevenness may increase.

Further, in order to reduce the thermal shrinkage rate, the film produced in the above process can be once wound up into a roll and then annealed offline.
The lower limit of the offline annealing temperature is preferably 160 ° C., more preferably 162 ° C., and further preferably 163 ° C. If it is less than the above, the effect of annealing may not be obtained.
The upper limit of the offline annealing temperature is preferably 175 ° C., more preferably 174 ° C., and further preferably 173 ° C. When the above is exceeded, the transparency may be lowered, and the thickness unevenness may be increased.

The lower limit of the offline annealing time is preferably 0.1 minutes, more preferably 0.5 minutes, and even more preferably 1 minute. If it is less than the above, the effect of annealing may not be obtained.
The upper limit of the offline annealing time is preferably 30 minutes, more preferably 25 minutes, and further preferably 20 minutes. If the above is exceeded, productivity may be reduced.

  Although the thickness of a film is set according to each use, the minimum of film thickness becomes like this. Preferably it is 2 micrometers, More preferably, it is 3 micrometers, More preferably, it is 4 micrometers. The upper limit of the film thickness is preferably 300 μm, more preferably 250 μm, still more preferably 200 μm, particularly preferably 100 μm, and most preferably 50 μm.

  The polypropylene film thus obtained is usually formed as a roll having a width of 2000 to 12000 mm and a length of 1000 to 50000 m, and is wound up into a roll. Furthermore, it is slit according to each application and is provided as a slit roll having a width of about 300 to 2000 mm and a length of about 500 to 5000 m.

The polypropylene film of the present invention has excellent characteristics such as those described above which are not present in the prior art.
When used as a packaging film, it is highly rigid and can be thinned, thereby reducing costs and weight.
In addition, since it has high heat resistance, it can be dried at a high temperature when drying a coating or printing, and it is possible to use a coating agent, an ink, a laminating adhesive, or the like, which has been difficult to be used in production, or has been conventionally used.
Furthermore, it can also be used as an insulating film for capacitors and motors, a back sheet for solar cells, a barrier film for inorganic oxides, and a base film for transparent conductive films such as ITO.

  The present invention will be described in detail below based on examples, but the present invention is not limited to such examples. The measuring method of the physical property in an Example is as follows.

1) Melt flow rate (MFR, g / 10 min)
MFR was measured at a temperature of 230 ° C. in accordance with JIS K7210.

2) Molecular weight and molecular weight distribution The molecular weight and molecular weight distribution were determined on the basis of monodisperse polystyrene using gel permeation chromatography (GPC).
The columns and solvents used in the GPC measurement are as follows.
Solvent: 1,2,4-trichlorobenzene Column: TSKgel GMH _HR- H (20) HT × 3
Flow rate: 1.0 ml / min
Detector: RI
Measurement temperature: 140 ° C
The number average molecular weight (Mn), the mass average molecular weight (Mw), the Z average molecular weight (Mz), and the Z + 1 average molecular weight (Mz + 1) are molecular weights (Mi) at the respective elution positions of the GPC curve obtained through the molecular weight calibration curve. The number of molecules (Ni) is defined by the following formula.
Number average molecular weight: Mn = Σ (Ni · Mi) / ΣNi
Mass average molecular weight: Mw = Σ (Ni · Mi ² ) / Σ (Ni · Mi)
Z average molecular weight: Mz = Σ (Ni · Mi ³ ) / Σ (Ni · Mi ² )
Z + 1 average molecular weight: Mz + 1 = Σ (Ni · Mi ⁴ ) / Σ (Ni · Mi ³ )
Molecular weight distribution: Mw / Mn, Mz + 1 / Mn
The molecular weight at the peak position of the GPC curve was defined as Mp.
When the baseline is not clear, the baseline should be set in a range up to the lowest position on the high molecular weight side of the elution peak closest to the elution peak of the standard substance.
In order to quantify the low molecular weight component and the high molecular weight component contained in polypropylene, peak separation of the GPC molecular weight distribution curve was performed. Peak separation was performed as follows.
From the obtained GPC curve, two or more components having different molecular weights were subjected to peak separation. The molecular weight distribution of each component is assumed to be a Gaussian function, and Mw / Mn = 4 so as to be the same as the molecular weight distribution of ordinary polypropylene. Each average molecular weight was calculated from the obtained curve of each component.

3) Stereoregularity Measurement of isotactic mesopentad fraction and meso average chain length was performed using ¹³ C-NMR. The isotactic mesopentad fraction was determined according to the method described in Zambelli et al., Macromolecules, Vol. 6, page 925 (1973). C. It was calculated according to the method described in Randall, “Polymer Sequence Distribution”, Chapter 2 (1977) (Academic Press, New York).
¹³ C-NMR measurement was carried out at 110 ° C. using AVANCE 500 manufactured by BRUKER, and dissolving 200 mg of a sample in an 8: 2 mixture of o-dichlorobenzene and heavy benzene at 135 ° C.

4) Density (g / cm ³ )
The density of the film was measured by a density gradient tube method according to JIS K7112.

5) Melting point (Tmp, ° C)
Thermal measurement was performed using a DSC-60 differential scanning calorimeter manufactured by Shimadzu Corporation. About 5 mg of the sample was cut out from the film and sealed in an aluminum pan for measurement. The temperature was raised from room temperature to 230 ° C. at a rate of 20 ° C./min, and the melting endothermic peak temperature of the sample was defined as Tmp.

6) Crystallinity The melting peak area of the DSC melting profile measured at a heating rate of 20 ° C./min is ΔHm. Bu, S .; Z. D. Cheng, B.M. The crystallinity of the film is obtained by dividing ΔHm by 209 J / g using the heat of fusion of 209 J / g of the complete polypropylene crystal described in Makromolecular Chemie, Rapid Communication, Vol. 9, page 75 (1988) by Wunderlich et al. Got.
Similarly, the 150 ° C. crystal fraction was determined by dividing the DSC melting profile peak area of 150 ° C. or higher by 209 J / g.

6) Cold xylene soluble part (CXS, mass%)
Dissolve 1 g of polypropylene sample in 200 ml of boiling xylene, allow to cool, recrystallize in a constant temperature water bath at 20 ° C. for 1 hour, and calculate the ratio of the mass dissolved in the filtrate to the original sample amount by CXS (mass%) It was.

7) Thermal contraction rate (%)
Measurement was performed in accordance with JIS Z 1712.
(The stretched film was 20 mm wide and 200 mm long, cut in the MD and TD directions, suspended in a hot air oven at 150 ° C. and heated for 5 minutes. The length after heating was measured, and the shrinkage relative to the original length was measured. (The heat shrinkage rate was obtained at the ratio of the lengths obtained.)

9) Impact resistance It measured at 23 degreeC using the Toyo Seiki film impact tester.

10) Young's modulus (unit: GPa)
The tensile strength of MD and TD was measured at 23 ° C. in accordance with JIS K 7127.

11) Haze (Unit:%)
It measured according to JIS K7105.

12) Refractive index It measured using the Atago Abbe refractometer. The refractive indexes along the MD and TD directions were Nx and Ny, respectively, and the refractive index in the thickness direction was Nz.
13) Plane orientation coefficient It was set as Nx and Ny measured by said 12), and the refractive index of the thickness direction was computed from Nz using the formula of ((Nx + Ny) / 2] -Nz.

14) Thickness unevenness A square sample having a length of 1 m was cut out from the wound film roll, and was divided into 10 equal parts in the MD direction and TD direction to prepare 100 measurement samples. The thickness was measured with a contact-type film thickness meter at the approximate center of the measurement sample.
The average value of the 100 points of data obtained was obtained, the difference between the minimum value and the maximum value (absolute value) was obtained, and the value obtained by dividing the absolute value of the difference between the minimum value and the maximum value by the average value was obtained. It was.

15) Appearance of heat seal Heat-sealing is performed by stacking the produced film and PYLEN Film-CTP P1128 manufactured by Toyobo Co., Ltd., and using a test sealer manufactured by Seibu Machinery Co., Ltd. for 1 second at 170 ° C. and a load of 2 kg. It was. The degree of change in appearance due to shrinkage of the film after heat sealing was evaluated visually. A case where the deformation amount of the heat seal portion is small and does not affect the use is indicated by ○, and a case where the heat shrinkage is large and the deformation amount is large is indicated by ×.

Example 1
As a polypropylene resin, a polypropylene homopolymer PP-1 having a Mw / Mn = 7.7, Mz + 1 / Mn = 140, MFR = 5.0, [mmmm] = 97.3% (manufactured by Nippon Polypro Co., Ltd .: Novatec) PP SA4L) was used. Using a 65mm extruder, extrude into a sheet form from a T-die at 250 ° C, cool and solidify with a 30 ° C cooling roll, and stretch 4.5 times in the length direction at 135 ° C, and then sandwich both ends with clips. Then, it was introduced into a hot air oven, preheated at 170 ° C., stretched 8.2 times in the transverse direction at 160 ° C., and then heat treated at 168 ° C. while relaxing 6.7%. Thereafter, one side of the film was subjected to corona treatment and wound up with a winder. The thickness of the film thus obtained was 20 μm, and as shown in Tables 1, 2 and 3, films having a low thermal shrinkage and a high Young's modulus were obtained.

(Example 2)
10 parts by weight of a low molecular weight propylene homopolymer having a narrow molecular weight distribution and a molecular weight of 10,000 is added to 90 parts by weight of the above PP-1, and melt-kneaded in a 30 mm twin screw extruder to obtain a mixture PP- 2 pellets were obtained. A film was obtained from the pellets in the same manner as in Example 1. The physical properties of the obtained film are shown in Table 1, Table 2, and Table 3.

(Example 3)
A film was obtained in the same manner as in Example 1 except that PP-1 was used, the preheating temperature in the transverse stretching was 173 ° C., and the stretching temperature and the heat treatment temperature were 167 ° C. The physical properties of the obtained film are shown in Table 1, Table 2, and Table 3.

Example 4
A film was obtained in the same manner as in Example 2 except that the film was stretched 5.5 times in the length direction and 12 times in the transverse direction. The physical properties of the obtained film are shown in Table 1, Table 2, and Table 3.

(Example 5)
Using the film produced in Example 1, heat treatment was performed at 170 ° C. for 5 minutes in a tenter hot air oven.
The physical properties of the obtained film are shown in Table 1, Table 2, and Table 3.

(Example 6)
As a polypropylene resin, a polypropylene homopolymer having a Mw / Mn = 8.9, Mz + 1 / Mn = 110, MFR = 3.0 g / 10 min, and a mesopentad fraction [mmmm] = 97.1% (“HU300 manufactured by Samsung Central Co., Ltd.) ": The amount of copolymerization monomer was 0 mol%), and a stretched polypropylene film was obtained in the same manner as in Example 1 except that the preheating temperature for transverse stretching was 171 ° C and the heat treatment temperature after transverse stretching was 170 ° C. . The thickness of the obtained film was 20 μm, and the physical properties were as shown in Table 1, Table 2, and Table 3.

(Comparative Example 1)
As a polypropylene resin, Sumitomo Chemical Co., Ltd. Sumitomo Nobrene FS2011DG3 was used, and the same method as in Example 1 was used except that the preheating temperature in transverse stretching was 168 ° C, the stretching temperature was 155 ° C, and the heat treatment temperature was 163 ° C. A film was obtained. Mw / Mn = 4, Mz + 1 / Mn = 21, MFR = 2.5 g / 10 min. The physical properties of the obtained film are shown in Table 1, Table 2, and Table 3.

(Comparative Example 2)
A film was prepared in the same manner as in Comparative Example 1 except that the preheating temperature was 171 ° C, the stretching temperature was 160 ° C, and the heat treatment temperature was 165 ° C. The physical properties of the obtained film are shown in Table 1, Table 2, and Table 3.

(Comparative Example 3)
As the polypropylene resin, homopolypropylene having MFR = 0.5 g / 10 min, Mw / Mn = 4.5, Mz + 1 / Mn = 10 was used. A film was produced in the same manner as in Comparative Example 2. The physical properties of the obtained film are shown in Table 1, Table 2, and Table 3.

(Comparative Example 4)
As a polypropylene resin, Novatec PP SA03 manufactured by Nippon Polypro Co., Ltd., MFR = 30 g / 10 min was used, and biaxial stretching was attempted in the same manner as in the Examples, but it was not possible to obtain a film by breaking by transverse stretching. It was.

The polypropylene film of the present invention can be widely used for packaging applications and industrial applications. However, since it is particularly high in rigidity, it can be thinned and can be reduced in cost and weight.
In addition, since it has high heat resistance, it can be dried at a high temperature when drying a coating or printing, and it is possible to use a coating agent, an ink, a laminating adhesive, or the like, which has been difficult to be used in production, or has been conventionally used.
Furthermore, it is also suitable for insulating films such as capacitors and motors, solar cell backsheets, inorganic oxide barrier films, and transparent conductive film base films such as ITO.

DSC chart of the polypropylene film described in Example 1 and Comparative Example 1

Claims (4)

  A film composed mainly of a polypropylene resin, having a thermal shrinkage rate in the MD and TD directions at 150 ° C. of 9% or less, an impact strength of 0.6 J or more, and a haze of 6% or less. A stretched polypropylene film characterized by that.   The stretched polypropylene film according to claim 1, wherein the lower limit of the mesopentad fraction of the polypropylene resin constituting the film is 96%, and the lower limit of the plane orientation coefficient of the film is 0.0125.   The stretched polypropylene film according to claim 1 or 2, wherein the upper limit of the amount of copolymerization monomer of the polypropylene resin constituting the film is 0.1 mol%.   The stretched polypropylene film according to any one of claims 1 to 3, wherein the polypropylene resin constituting the film has a normal temperature xylene-soluble content of 7 wt% or less.