JP7548346B2 - Method for producing biaxially oriented polypropylene film - Google Patents

Description

The present invention relates to a biaxially oriented polypropylene film. In particular, the present invention relates to a biaxially oriented polypropylene film that has excellent adhesion to printing ink and adhesives used for lamination with other film components.

Traditionally, biaxially oriented polypropylene films have been widely used as packaging materials for a variety of items, including food and textile products, due to their excellent transparency and mechanical properties. However, problems with polypropylene films include the low surface energy of polypropylene resins, which are non-polar, and therefore insufficient adhesion to printing inks or other materials during processing such as printing inks and lamination.

In particular, when biaxially oriented polypropylene films are used as packaging materials, they are generally laminated with other films using adhesives. However, if the lamination strength between these films is weak, the strength of the packaging material will be weakened and the film may tear, causing the contents to spill out. Furthermore, oxygen and water vapor may enter and exit through the torn parts of the bag, preventing it from functioning as a food packaging material.

Biaxially oriented polypropylene films are also commonly printed, and from the standpoint of color development and color fading of the prints, there is a growing demand for improved transfer of the printing ink from the printing roll to the surface of the biaxially oriented polypropylene film, as well as improved adhesion of the printing ink to the film surface.

Various methods have been proposed to address these problems. For example, a film has been disclosed in which a skin layer made of a composition in which organic polymer particles are blended with a propylene-ethylene random copolymer is laminated onto the surface of a biaxially oriented polypropylene film (see, for example, Patent Document 1). However, not only is the adhesion of the printing ink insufficient, but the process of providing a separate skin layer is required, resulting in poor productivity.

Patent document 1: JP 2000-127310 A

The object of the present invention is to provide a biaxially oriented polypropylene film that has high lamination strength with other film components without compromising the excellent transparency and mechanical properties inherent to biaxially oriented polypropylene films, and that has excellent printing ink transferability from the printing roll to the film and printing ink adhesion.

The present invention, which has solved the above problems, is a biaxially oriented polypropylene film having a base layer (A) mainly composed of a polypropylene resin and a surface layer (B) mainly composed of a polypropylene resin on at least one surface of the base layer (A), the surface roughness of the surface layer (B) opposite the base layer (A) is 0.027 μm or more and 0.040 μm or less, the surface resistivity of the surface layer (B) opposite the base layer (A) is 15 Log Ω or more, the wet tension of the surface layer (B) opposite the base layer (A) is 38 mN/m or more, the film thickness is 20 μm or more and 50 μm or less, and the haze (transparency) value of the film is 5% or less.

It is preferable that the central surface peak height SRp + central surface valley depth of the surface layer (B) on the side opposite the substrate layer (A) is 1.0 μm or more and 2.0 μm or less.

It is preferable that the thermal shrinkage rate of the biaxially oriented polypropylene film in the longitudinal and transverse directions at 150°C is 11% or less.

A laminate having a printed layer on the side opposite the substrate layer of the surface layer (B) of any of the biaxially oriented polypropylene films described above is preferred.

The biaxially oriented polypropylene film of the present invention has high lamination strength with other film components without impairing the excellent transparency and mechanical properties that are inherent to biaxially oriented polypropylene films, and also has excellent printing ink transferability from a printing roll to the film and adhesion of the printing ink, and can be produced efficiently.

The biaxially oriented polypropylene film of the present invention has a base layer (A) mainly composed of a polypropylene resin and a surface layer (B) mainly composed of a polypropylene resin on at least one surface of the base layer (A), and is characterized in that the arithmetic mean roughness of the surface of the surface layer (B) opposite the base layer (A) is 0.027 μm or more and 0.040 μm or less, the surface resistivity of the surface of the surface layer (B) opposite the base layer (A) is 15 Log Ω or more, the wet tension of the surface of the surface layer (B) opposite the base layer (A) is 38 mN/m or more, the film thickness is 20 μm or more and 50 μm or less, and the haze value of the film is 5% or less.
Here, the arithmetic surface roughness SRa of the surface of the surface layer (B) opposite to the base layer (A) is determined by measuring using a three-dimensional roughness meter with a stylus pressure of 20 mg, a measurement length of 1 mm in the X direction, a feed pitch of 2 μm in the Y direction, 99 recorded lines, a height direction magnification of 20,000 times, and a cutoff of 80 μm, in accordance with the definition of arithmetic average roughness described in JIS B 0601 (1994).

The arithmetic mean roughness SRa is less susceptible to the influence of one prominently large peak or valley, and is an index that represents the relatively small uneven undulations formed on the surface other than the relatively large peaks and valleys formed locally by the antiblocking agent or lubricant. Since most of the printing ink adheres to the surface other than the relatively large peaks and valleys formed by the antiblocking agent or lubricant, it is closely related to the adhesion of the printing ink. This is different from the central surface peak height SRp and central surface valley depth SRv described later.
In addition, the surface resistivity value of the surface of the surface layer (B) opposite the base layer (A) varies depending on the amount of antistatic agent present on the surface, and the smaller the amount of antistatic agent present on the surface, the larger the surface resistivity value.
Furthermore, the wetting tension of the surface layer (B) represents the numerical value of the surface tension (μN/cm) of the mixed liquid reagent judged to wet the film surface, and is related to the wettability of printing inks and adhesives.
Further details are provided below.

(1) Substrate layer (A)
The polypropylene resin used in the base layer (A) of the biaxially oriented polypropylene film of the present invention refers to a polymer of propylene or a polymer copolymerized with 0.5 mol% or less of propylene and ethylene and/or an α-olefin having 4 or more carbon atoms. The copolymerization component in the copolymer is preferably 0.3 mol% or less, more preferably 0.1 mol% or less, and a completely homopolypropylene containing no copolymerization component is most preferred.
When ethylene and/or an α-olefin having 4 or more carbon atoms is copolymerized in an amount of more than 0.5 mol %, the crystallinity and rigidity may be too low, and the thermal shrinkage rate at high temperatures may become large. Such resins may be blended and used.

The mesopentad fraction ([mmmm]%) measured by 13C-NMR, which is an index of stereoregularity of the polypropylene resin constituting the base layer (A) of the biaxially oriented polypropylene film of the present invention, is preferably 98 to 99.5%. More preferably, it is 98.1% or more, and even more preferably, it is 98.2% or more. If the mesopentad fraction of the polypropylene resin is small, the elastic modulus will be low and there is a risk of insufficient heat resistance. 99.5% is the practical upper limit.

The mass average molecular weight (Mw) of the polypropylene resin constituting the base layer (A) of the biaxially oriented polypropylene film of the present invention is preferably 180,000 to 500,000.
If the Mw is less than 180,000, the melt viscosity is low, so that the film-forming property may be deteriorated due to instability during casting. If the Mw exceeds 500,000, the amount of components having a molecular weight of 100,000 or less becomes 35% by mass, and the heat shrinkage rate at high temperatures is reduced.
A more preferred lower limit of Mw is 190,000, even more preferably 200,000, and a more preferred upper limit of Mw is 320,000, even more preferably 300,000, and particularly preferably 250,000.

The number average molecular weight (Mn) of the polypropylene resin constituting the base layer (A) of the biaxially oriented polypropylene film of the present invention is preferably 20,000 to 200,000.
If it is less than 20,000, the melt viscosity is low, so that the film-forming property may be deteriorated due to instability during casting. If it exceeds 200,000, the heat shrinkage rate at high temperatures is reduced.
A more preferable lower limit of Mn is 30,000, further preferably 40,000, particularly preferably 50,000, and a more preferable upper limit of Mn is 80,000, further preferably 70,000, particularly preferably 60,000.
In addition, when a high molecular weight component is added to the polypropylene, the high molecular weight component promotes the crystallization of the low molecular weight component, but the molecules become more entangled, and the heat shrinkage tends to increase even if the crystallinity is high. If the Mw/Mn becomes too large, the amount of high molecular weight components increases, and the heat shrinkage may increase, which is not preferable. Even if a high molecular weight component is added, it is preferable to keep the Mw/Mn in the range of 5.5 to 20.

Furthermore, the Mw/Mn, which is an index of molecular weight distribution, of the polypropylene resin of the base layer (A) is preferably 2.8 to 8. More preferably, it is 2.8 to 7, further preferably 2.8 to 6, and particularly preferably 2.8 to 5.4. The lower limit is preferably 3 or more, and more preferably 3.3 or more.
The molecular weight distribution of the polypropylene-based resin can be adjusted by polymerizing components having different molecular weights in multiple stages in a series of plants, blending components having different molecular weights offline in a kneader, blending catalysts having different performances and polymerizing the blended components, or using a catalyst that can realize a desired molecular weight distribution.

When the polypropylene resin of the base layer (A) of the biaxially oriented polypropylene film of the present invention has an Mw/Mn in the range of 2.8 to 5.4, it is preferable that the melt flow rate (MFR; 230°C, 2.16 kgf) is 4 g/10 min to 20 g/10 min.
The lower limit of the MFR of the polypropylene resin of the base layer (A) is more preferably 5 g/10 min, further preferably 6 g/10 min, and particularly preferably 7 g/10 min. The upper limit of the MFR of the polypropylene resin of the base layer (A) is more preferably 15 g/10 min, and further preferably 12 g/10 min.
When the Mw/Mn and MFR of the polypropylene resin of the base layer (A) are within the above ranges, the heat shrinkage rate at high temperatures can be kept small, and the adhesion to a cooling roll is good, resulting in excellent film formability.

(2) Surface layer (B)
The surface roughness of the surface layer (B) of the biaxially oriented polypropylene film of the present invention on the side opposite to the substrate layer (A) is preferably 0.027 μm or more and 0.040 μm or less. If it is less than 0.027 μm, the adhesion to printing ink and the lamination strength with other film members are insufficient, and if it exceeds 0.040 μm or more, problems such as increased haze and poor color development of printing occur.
The surface roughness of the surface layer (B) on the side opposite to the base layer (A) is more preferably 0.028 μm or more, further preferably 0.029 μm or more, and particularly preferably 0.030 μm or more.
In order to make the surface roughness of the surface layer (B) on the side opposite to the base layer (A) 0.027 μm or more and 0.040 μm or less, it is preferable to use a mixture of two or more polypropylene resins having different melt flow rates (MFR) as the polypropylene resin composition forming the surface layer (B). In this case, the difference in MFR is preferably 3 g/10 min or more, more preferably 3.5 g/10 min or more.
As described above, when the difference in melt flow rate (MFR) between two or more polypropylene resins in a mixture of polypropylene resins is different, the crystallization speed and crystallization degree of each polypropylene are different, so that it is presumed that the arithmetic mean roughness of the surface of the surface layer (B) opposite to the base layer (A) is 0.028 μm or more. Also, the arithmetic mean roughness of the surface of the surface layer (B) opposite to the base layer (A) is unlikely to exceed 0.040 μm.
As the polypropylene-based resin having a smaller MFR, a polymer obtained by copolymerizing propylene with ethylene and/or an α-olefin having 4 or more carbon atoms can be used. Examples of the α-olefin having 4 or more carbon atoms include 1-butene, 1-hexene, 4-methyl-1-pentene, and 1-octene. Furthermore, maleic acid having polarity may be used as another copolymerization component.
The total amount of ethylene and/or α-olefins having 4 or more carbon atoms and other copolymerization components is preferably 8.0 mol % or less. If copolymerized in an amount exceeding 8.0 mol %, the film may whiten and have a poor appearance, or may become sticky, making film formation difficult.
These resins may be used in combination of two or more kinds. When blended, each resin may be copolymerized in an amount exceeding 8.0 mol%, but the blend preferably contains 8.0 mol% or less of monomers other than propylene on a monomer unit basis.
As the polypropylene-based resin having a larger MFR, a copolymer of the above-mentioned propylene with ethylene and/or an α-olefin having 4 or more carbon atoms may be used, or a propylene homopolymer may be used. It is preferable to use a propylene homopolymer.

The polypropylene resin composition of the surface layer (B) of the biaxially oriented polypropylene film of the present invention preferably has an MFR of 1.0 g/10 min to 8 g/10 min. The lower limit of the MFR of the polypropylene resin composition of the surface layer (B) is more preferably 2 g/10 min, and even more preferably 3 g/10 min. The upper limit of the MFR of the polypropylene resin composition of the surface layer (B) is more preferably 7 g/10 min, and even more preferably 6.0 g/10 min. When the MFR is within this range, the film formability is good and the appearance is also excellent.
If the MFR of the polypropylene resin composition of the surface layer (B) is less than 1.0 g/10 min, when the MFR of the polypropylene resin of the base layer (A) is large, the viscosity difference between the base layer (A) and the surface layer (B) becomes large, so that unevenness (unevenness of the original roll) is likely to occur during film formation. If the MFR of the polypropylene resin composition of the surface layer (B) exceeds 8 g/10 min, the adhesion to the cooling roll becomes poor, air is entrained, the smoothness is poor, and there are many defects originating from this,
There is a risk that it will be difficult to achieve an appropriate surface roughness.

The surface resistivity of the surface layer (B) of the biaxially oriented polypropylene film of the present invention opposite the substrate layer (A) is preferably 15 Log Ω or more. If the surface resistivity is 15 Log Ω or more, the adhesion to printing ink and adhesives is improved. The surface resistivity is more preferably 16 Log Ω or more. In order to make the surface resistivity 15 Log Ω or more, it is possible to avoid using additives of low molecular weight compounds such as antistatic agents and antifogging agents as much as possible. If they are used, the additives contained in the substrate layer (A) may bleed to the surface of the surface layer (B) opposite the substrate layer (A), which makes it difficult to reduce the surface resistivity, so care must be taken.
In order to make the surface resistivity 15 Log Ω or more, it is preferable to carry out a physicochemical surface treatment such as a corona treatment or a flame treatment.
For example, in the corona treatment, it is preferable to use a preheat roll and a treatment roll and perform discharge in the air.

The surface of the biaxially oriented polypropylene film of the present invention, opposite to the substrate layer (A) of the surface layer (B), preferably has a wet tension of 38 mN/m or more. When the wet tension is 38 mN/m or more, the adhesion to printing ink or adhesives used for lamination with other film components is improved.
In order to achieve a wetting tension of 38 mN/m or more, additives such as antistatic agents and surfactants are usually used. However, these methods have the effect of lowering the surface resistivity, so it is preferable to carry out physicochemical surface treatments such as corona treatment and flame treatment.
For example, in the corona treatment, it is preferable to use a preheat roll and a treatment roll and perform discharge in the air.
Here, the surface resistivity is mainly related to the strength of the corona treatment, but the wetting tension is also related to the amount of bleed-out of the antistatic agent, so it is effective to set each within a suitable range.

The biaxially oriented polypropylene film of the present invention preferably has a center surface peak height SR+center surface valley depth SRv of 1.0 μm or more and 2.0 μm or less on the surface of the surface layer (B) opposite the base layer (A).
Here, the surface roughness central peak height SRp and central valley depth SRv of the surface of the surface layer (B) opposite to the base layer (A) are determined in accordance with the definition of arithmetic mean roughness described in JIS B 0601 (1994) by measuring using a three-dimensional roughness meter with a stylus pressure of 20 mg, a measurement length of 1 mm in the X direction, a feed pitch of 2 μm in the Y direction, 99 recorded lines, a height direction magnification of 20,000 times, and a cutoff of 80 μm.

The central surface peak height SRp + central surface valley depth SRv of the surface of the surface layer (B) of the biaxially oriented polypropylene film of the present invention opposite the base layer (A) is an index of the state of a relatively large uneven portion formed locally by an antiblocking agent or the like, and is closely related to the slipperiness between the surface layer (B) and the base layer (A) when they come into contact with each other when the biaxially oriented polypropylene film of the present invention having the surface layer (B) on at least one surface of the base layer (A) is wound into a roll.
When the central surface peak height SRp + central surface valley depth SRv of the surface layer (B) of the biaxially oriented polypropylene film of the present invention on the surface opposite to the base layer (A) is 1.0 μm or more, the unwinding property from the roll film is improved, and when it is 2.0 μm or less, transparency is maintained.
The sum of the central peak height SRp and the central valley depth SRv of the surface layer (B) on the side opposite to the base layer (A) is preferably 1.1 μm or more, more preferably 1.2 μm or more, and particularly preferably 1.3 μm or more.

In order to make the central surface peak height SRp + central surface valley depth SRv of the surface layer (B) of the biaxially oriented polypropylene film of the present invention opposite the base layer (A) 1.0 μm or more and 2.0 μm or less, it is a suitable method to blend an antiblocking agent in the polypropylene resin composition forming the surface layer (B).
The antiblocking agent can be appropriately selected from inorganic particles such as silica, calcium carbonate, kaolin, and zeolite, and organic particles such as acrylic, polymethacrylic, and polystyrene. Among these, it is particularly preferable to use polymethacrylic particles. The preferred average particle size of the antiblocking agent is 1.0 to 2.5 μm, and more preferably 1.0 to 2.0 μm. The average particle size is measured by taking a photograph with a scanning electron microscope, measuring the Feret's diameter in the horizontal direction using an image analyzer, and expressing the average value.
The antiblocking agent is preferably contained in an amount of 0.15% by mass based on the total weight of the polypropylene resin or the mixture thereof.

The polypropylene resin used in the present invention is obtained by polymerizing the raw material propylene alone or copolymerizing propylene with ethylene and/or α-olefins using a known catalyst such as a Ziegler-Natta catalyst or a metallocene catalyst. Among them, it is preferable to use a Ziegler-Natta catalyst in order to eliminate heterogeneous bonds and to use a catalyst capable of polymerization with high stereoregularity.
As the polymerization method, a known method may be adopted, and examples thereof include a method of polymerization in an inert solvent such as hexane, heptane, toluene, or xylene, a method of polymerization in a liquid monomer, a method of adding a catalyst to a gaseous monomer and polymerizing in a gas phase state, or a method of polymerization that combines these methods.

The substrate layer (A) of the biaxially oriented polypropylene film of the present invention may contain additives and other resins, such as antioxidants, ultraviolet absorbers, nucleating agents, adhesives, antifogging agents, flame retardants, inorganic or organic fillers, etc.
Examples of other resins include polypropylene resins other than the polypropylene resin used in the present invention, random copolymers which are copolymers of propylene and ethylene and/or α-olefins having 4 or more carbon atoms, various elastomers, etc. These may be sequentially polymerized using a multi-stage reactor, blended with polypropylene resin in a Henschel mixer, master pellets prepared in advance using a melt kneader may be diluted with polypropylene to a predetermined concentration, or the entire amount may be melt kneaded in advance before use.
The substrate layer (B) may contain additives and other resins, such as antioxidants, ultraviolet absorbers, nucleating agents, adhesives, antifogging agents, flame retardants, inorganic or organic fillers, etc.

(3) Biaxially oriented polypropylene film The biaxially oriented polypropylene film of the present invention may be a two-layer film having one base layer (A) and one surface layer (B), but may also be a film having three or more layers. A two-layer structure of base layer (A)/surface layer (B) is preferred, but a three-layer structure of surface layer (B)/layer A/surface layer (B),/base layer (A)/intermediate layer (C)/surface layer (B) or a multilayer structure having more than three layers may also be used.
When there are a plurality of base layers (A) or surface layers (B), the compositions of the layers may be different as long as each layer satisfies the required characteristics.
The total thickness of the biaxially oriented polypropylene film of the present invention is preferably 9 to 200 m, more preferably 10 to 150 μm, further preferably 12 to 100 μm, and particularly preferably 12 to 80 μm.

In the biaxially oriented polypropylene film of the present invention, the ratio of the total thickness of the surface layer (B) to the thickness of the base layer (A) is preferably 0.01 to 0.5, more preferably 0.02 to 0.4, and even more preferably 0.03 to 0.3. If the total surface layer (B)/total base layer (A) ratio exceeds 0.5, the shrinkage rate tends to increase. The thickness of the total base layer (A) relative to the total thickness of the film is preferably 50 to 99%, more preferably 60 to 97%, and particularly preferably 70 to 95%. The remainder is the surface layer (B) or the surface layer (B) and the intermediate layer (C). The effective thickness of the total surface layer (B) is preferably 0.5 to 4 μm, more preferably 1 to 3.5 μm, and even more preferably 1.5 to 3 μm.

The ink adhesion of the biaxially oriented polypropylene film of the present invention was evaluated by conducting a peeling test of gravure-printed printing ink and counting the number of peeled areas out of a total of 25 locations. Five or fewer peeled areas are preferable, three or fewer are more preferable, and zero is most preferable. If the number of peeled areas exceeds five, the extent to which the printing ink peels off becomes too great and is problematic. The method for evaluating ink adhesion is described below.

The longitudinal laminate strength after lamination to the biaxially oriented polypropylene film of the present invention is preferably 1.2 to 2.5 N/15 mm, more preferably 1.3 to 2.5 N/mm, and even more preferably 1.4 to 2.5 N/mm. The method for measuring the laminate strength will be described later.

The dynamic friction coefficient of the biaxially oriented polypropylene film of the present invention is preferably 0.5 or less, more preferably 0.48 or less, and particularly preferably 0.45 or less. If the dynamic friction coefficient is 0.5 or less, the film can be smoothly unwound from the roll film, and printing processing is easy. The method for measuring the dynamic friction coefficient will be described later.

The haze of the biaxially oriented polypropylene film of the present invention is preferably 5% or less, more preferably 0.2 to 5%, even more preferably 0.3 to 4.5%, and particularly preferably 0.4 to 4%. If it exceeds 5%, the transparency is poor and the printed display may become difficult to see. For example, the haze tends to deteriorate when the stretching temperature or heat setting temperature is too high, when the cooling roll temperature is high and the cooling rate of the unstretched (raw) sheet is slow, or when there is too much low molecular weight component, and by adjusting these, it is possible to keep the haze within the above range. The method for measuring the haze will be described later.

The heat shrinkage rate of the biaxially oriented polypropylene film of the present invention in the longitudinal and transverse directions at 150°C is preferably 11% or less, more preferably 10% or less, and particularly preferably 8% or less. By making the heat shrinkage rate 11% or less, it is possible to reduce pitch deviation during printing. The method for measuring heat shrinkage will be described later.

In the biaxially oriented polypropylene film of the present invention, the heat shrinkage in the longitudinal direction at 150°C is preferably 0.2 to 8%, more preferably 0.3 to 7%, and particularly preferably 0.5 to 6%. If the heat shrinkage is within the above range, the film can be said to have excellent heat resistance and can be used in applications where it may be exposed to high temperatures. The heat shrinkage at 150°C can be reduced to about 1.5% by, for example, increasing the amount of low molecular weight components or adjusting the stretching conditions and heat setting conditions, but to reduce it below that, it is preferable to perform an annealing treatment offline.

The tensile modulus in the longitudinal direction of the biaxially oriented polypropylene film of the present invention is preferably 1.8 to 4 GPa, more preferably 2.1 to 3.7 GPa, even more preferably 2.2 to 3.5 GPa, and particularly preferably 2.3 to 3.4 GPa. The tensile modulus in the transverse direction is preferably 3.8 to 8 GPa, more preferably 4 to 7.5 GPa, even more preferably 4.1 to 7 GPa, and particularly preferably 4.2 to 6.5 GPa. If the tensile modulus is within the above range, the film will be strong and can be used even with a small film thickness, making it possible to reduce the amount of film used. The method for measuring the tensile modulus will be described later.

The lower limit of the plane orientation coefficient of the biaxially oriented polypropylene film of the present invention is preferably 0.011, more preferably 0.012, and even more preferably 0.013. When in the above range, the heat resistance and rigidity of the film are likely to be increased.
Stretched laminated polypropylene films generally have a crystal orientation, and the direction and degree of the crystal orientation have a large effect on the film properties. The degree of crystal orientation tends to vary depending on the molecular structure of the polypropylene resin used and the process and conditions in film production, and can be adjusted to the above range. The method for measuring the plane orientation coefficient will be described later.

(4) Film Forming Method The biaxially oriented polypropylene film of the present invention can be obtained by melt-extruding the polypropylene resin composition for the base layer (A) and the polypropylene resin composition for the surface layer (B) using separate extruders, co-extruding them through a die, and cooling them with a cooling roll to form an unstretched sheet, stretching the unstretched sheet in the machine direction (MD) and the width direction (TD), and then subjecting it to a heat setting treatment.
The melt extrusion temperature is preferably about 200 to 280°C. In order to obtain a laminated film with good appearance without disturbing the layers within this temperature range, it is preferable that the viscosity difference (MFR difference) between the polypropylene raw material for the base layer (A) and the polypropylene raw material for the surface layer (B) is 6 g/10 min or less. If the viscosity difference is more than 6 g/10 min, the layers are likely to be disturbed, resulting in poor appearance. It is more preferably 5.5 g/10 min or less, and even more preferably 5 g/10 min or less.

The surface temperature of the cooling roll is preferably 25 to 35°C, more preferably 27 to 33°C. If the temperature exceeds 35°C, the film surface will become rough.

The lower limit of the stretching ratio in the machine direction (MD) is preferably 3 times, more preferably 3.5 times. If it is less than the above, it may cause unevenness in the film thickness. The upper limit of the stretching ratio in the MD is preferably 8 times, more preferably 7 times. If it exceeds the above, it may become difficult to perform subsequent 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 increase, the thickness unevenness may increase, and the surface of the film may become rough. The upper limit of the MD stretching temperature is preferably 150°C, more preferably 145°C, and even more preferably 140°C. A higher temperature is preferable for reducing the heat shrinkage rate, but it may adhere to the roll, making it impossible to stretch, or cause surface roughness.

The lower limit of the stretch ratio in the transverse direction (TD) is preferably 4 times, more preferably 5 times, and even more 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, even more preferably 15 times, and especially preferably 12 times. If it exceeds the above, the heat shrinkage rate may increase or the film may break during stretching. The preheating temperature for TD stretching is preferably set 5 to 15 ° C higher than the stretching temperature in order to quickly raise the film temperature to near the stretching temperature. TD stretching is performed at a higher temperature than conventional stretched polypropylene films. The lower limit of the TD stretching temperature is preferably 155 ° C, more preferably 157 ° C, even more preferably 158 ° C, and especially preferably 160 ° C. If it is less than the above, the film may break without being sufficiently softened, or the heat shrinkage rate may be high. The upper limit of the TD stretching temperature is preferably 170 ° C, more preferably 168 ° C, and even more preferably 163 ° C. A higher temperature is preferable to reduce the thermal shrinkage rate, but if the temperature exceeds the above range, the low molecular weight components will melt and recrystallize, causing not only a decrease in orientation but also surface roughness and whitening of the film.

The film after stretching is heat set. Heat setting can be performed at a higher temperature than conventional stretched polypropylene films. The lower limit of the heat setting temperature is preferably 165°C, more preferably 166°C. If the temperature is lower than the above, the heat shrinkage rate may be high. In addition, a long period of processing may be required to reduce the heat shrinkage rate, which may result in poor productivity. The upper limit of the heat setting temperature is preferably 176°C, more preferably 175°C. If the temperature exceeds the above range, low molecular weight components may melt and recrystallize, causing surface roughness and whitening of the film.

It is preferable to relax the film during heat setting. The lower limit of relaxation is preferably 2%, more preferably 3%. If it is less than this, the thermal shrinkage rate may be high. The upper limit of relaxation is preferably 10%, more preferably 8%. If it exceeds this, thickness unevenness may become large.

Furthermore, in order to reduce the thermal shrinkage rate, the film produced by the above process can be wound into a roll and then annealed offline. The lower limit of the temperature for offline annealing is preferably 160°C, more preferably 162°C, and even more preferably 163°C. If the temperature is lower 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 even more preferably 173°C. If the temperature exceeds the above, transparency may decrease and thickness unevenness may increase.

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 annealing effect may not be obtained. The upper limit of the offline annealing time is preferably 30 minutes, more preferably 25 minutes, and even more preferably 20 minutes. If it exceeds the above, productivity may decrease.

The biaxially oriented polypropylene film thus obtained can be subjected to corona discharge, plasma treatment, flame treatment, etc., as necessary, and then wound up with a winder to obtain the biaxially oriented polypropylene film roll of the present invention.

The biaxially oriented polypropylene film of the present invention can be widely used in applications where processing such as printing inks and lamination is required.

The present invention will be described in more detail below with reference to examples. However, the following examples do not limit the present invention, and modifications that do not deviate from the spirit of the present invention are included in the present invention.

(Measurement method)
The physical properties of the films obtained in the examples and comparative examples were measured as follows.

1) The stereoregular mesopentad fraction ([mmmm]%) was measured using 13C-NMR. The mesopentad fraction was calculated according to the method described in "Zambelli et al., Macromolecules, Vol. 6, p. 925 (1973)". 13C-NMR measurement was performed at 110°C using BRUKER's "AVANCE500" by dissolving 200 mg of a sample in a mixed solution of o-dichlorobenzene and deuterated benzene at a volume ratio of 8:2 at 135°C.

2) Melt flow rate (MFR; g/10 min) Measured in accordance with JIS K7210 at a temperature of 230° C. and a load of 2.16 kgf.
In the case of raw resin, the pellets (powder) were weighed out as required and used. In the case of film, the required amount was cut out and then cut into a sample of about 5 mm square.

3) Molecular weight and molecular weight distribution The molecular weight and molecular weight distribution of the raw material resin and the film were determined by gel permeation chromatography (GPC) using monodisperse polystyrene as a standard. The measurement conditions such as the column and solvent used in the GPC measurement are as follows:
Solvent: 1,2,4-trichlorobenzene Column: TSKgel GMHHR-H(20)HT x 3
Flow rate: 1.0ml/min
Detector: RI
Measurement temperature: 140°C

The number average molecular weight (Mn), weight average molecular weight (Mw) and molecular weight distribution are each defined by the number of molecules (N ⁱ ) of the molecular weight (M ⁱ ) at each elution position of the GPC curve obtained via a molecular weight calibration curve, as shown in the following equations.
Number average molecular weight: Mn = Σ (N ⁱ · M ⁱ ) / ΣN ⁱ Mass average molecular weight: Mw = Σ (N ⁱ · M ⁱ² ) / Σ (N ⁱ · M ⁱ )
Molecular weight distribution: Mw/Mn
When the baseline was unclear, the baseline was set in the range up to the lowest position of the high molecular weight side tail of the elution peak on the high molecular weight side closest to the elution peak of the standard substance.

4) Thickness The thickness of each of the base layer (A) and surface layer (B) was measured by cutting a cross section of a biaxially oriented laminated polypropylene film solidified with a modified urethane resin using a microtome and observing it under a differential interference microscope.

5) Heat shrinkage rate (%)
The thermal shrinkage was measured by the following method in accordance with JIS Z1712. The film was cut into a width of 20 mm and a length of 200 mm in each of the MD and TD directions, and was heated for 5 minutes by hanging it in a hot air oven at 150° C. The length before and after heating was measured, and the ratio (%) of the length before heating minus the length after heating to the length before heating was calculated to obtain the thermal shrinkage rate.

6) Tensile modulus (GPa)
The tensile modulus of elasticity in the MD and TD directions of the film was measured at 23° C. under the following conditions in accordance with JIS K7127.
Measuring equipment: Shimadzu Autograph ASS-100NJ
Sample size: width 15mm x length 200mm
Crosshead speed: 200 mm/min
Chuck distance: 100 mm
Strain range for elastic modulus measurement: 0.1 to 0.6%

7) Haze (unit: %)
Measurement was performed in accordance with JIS K7105.

8) Dynamic Friction Coefficient: According to JIS K7125, the surface layers (B) of two films were placed together and the dynamic friction coefficient was measured at 23°C.

9) Refractive index, plane orientation coefficient Measured using an Abbe refractometer manufactured by Atago Co., Ltd. according to JIS K7142-1996 5.1 (A method). The refractive indexes in the MD and TD directions were designated as Nx and Ny, respectively, and the refractive index in the thickness direction was designated as Nz. The plane orientation coefficient (ΔP) was calculated by (Nx+Ny)/2-Nz.

10) Surface Roughness The surface roughness of the obtained film was evaluated using a three-dimensional roughness meter (manufactured by Kosaka Laboratory, model number ET-30HK) with a stylus pressure of 20 mg, a measurement length in the X direction of 1 mm, a feed speed of 100 μm/sec, a feed pitch in the Y direction of 2 μm, 99 recorded lines, a height direction magnification of 20,000 times, and a cutoff of 80 μm, and was calculated in accordance with the definition of arithmetic average roughness described in JIS B 0601 (1994).
The arithmetic mean roughness (SRa), the central surface peak height (SRp), and the central surface valley depth (SRv) were each evaluated by performing three trials and averaging the results.

11) Surface resistivity (Log Ω)
According to JIS K6911, the film was aged at 23° C. for 24 hours, and then the surface layer (B) of the film was measured.

12) Wetting tension (mN/m)
In accordance with K 6768:1999, the film was aged at 23° C. and 50% relative humidity for 24 hours, and then the corona-treated surface of the film was measured according to the following procedure.
Step 1)
The measurements are carried out in a standard laboratory atmosphere (see JIS K 7100) with a temperature of 23°C and a relative humidity of 50%.
Step 2)
The test specimen is placed on the substrate of the hand coater (4.1) and a few drops of the test mixture are placed on the specimen and immediately spread out by pulling the wire bar.
If a cotton swab or brush is used to spread the test mixture, the liquid should be spread quickly over an area of at least 6 cm2, with just enough liquid to form a thin layer without pooling.
The wetting tension is judged by observing the liquid film of the test mixture in a bright place and the state of the liquid film after 3 seconds. If the liquid film does not break and the state at the time of application is maintained for 3 seconds or more, it is considered to be wet. If the wetness is maintained for 3 seconds or more, proceed to the mixed liquid with the next higher surface tension, and conversely, if the liquid film breaks in 3 seconds or less, proceed to the mixed liquid with the next lower surface tension.
This operation is repeated to select a mixture that can accurately wet the surface of the test piece within 3 seconds.
Step 3)
Use a new swab for each test. Brushes or wire burrs should be cleaned with methanol and dried after each use, as residual liquids will change composition and surface tension upon evaporation.
Step 4)
The procedure is repeated at least three times to select a mixture that can wet the surface of the test piece in 3 seconds, and the surface tension of the mixture thus selected is reported as the wetting tension of the film.

13) Ink adhesion On the surface layer (B) of the film, gravure printing (printing ink amount 2 g/m2) was performed at a speed of 50 m/min using a gravure printing machine (manufactured by Mitani Iron Works Co., Ltd.) The ink used was a water-based ink (product name Ecofine 709 White, manufactured by Dainippon Ink and Chemicals, Inc.).
(Registered trademark) This print sample was used to evaluate (in more detail) the print quality by grid peeling (25 x 2 mm squares, 90° peeling method using 18 mm wide Nichiban Cellophane Tape (Registered trademark)) and rank the print quality according to the practicality as follows:
Grid pattern peeling: 0 pieces. . . . : Excellent printing ink adhesion.
Same as above: 1 to 5 marks....○: Good printing ink adhesion.
Same rating: 6 to 15 points.... △: Poor printing ink adhesion.
Same as above: 1 or more....X: No adhesion of printing ink.

14) Laminate Strength The laminate strength was measured by the following procedure.
Procedure 1) Preparation of Laminate Film with Sealant Film This was carried out using a continuous dry laminating machine as follows.
The adhesive was gravure coated on the surface layer (B) of the biaxially oriented polypropylene film obtained in the examples and comparative examples so that the coating amount when dried was 3.0 g/m2, and then introduced into a drying zone and dried at 80°C for 5 seconds. Subsequently, the sealant film was laminated between rolls provided on the downstream side (roll pressure 0.2 MPa, roll temperature: 60°C). The obtained laminate film was aged at 40°C for 3 days in a wound state.
The adhesive used was an ether-based adhesive obtained by mixing 17.9% by mass of a base agent (TM329, manufactured by Toyo-Morton Ltd.), 17.9% by mass of a curing agent (CAT8B, manufactured by Toyo-Morton Ltd.), and 64.2% by mass of ethyl acetate, and the sealant film used was a non-oriented polypropylene-based film manufactured by Toyobo Co., Ltd. (Pylen (registered trademark) CT P1128, thickness 30 μm).
Step 2) Measurement of Laminate Strength The laminate film obtained above was cut into a rectangular shape (length 200 mm, width 15 mm) with the long side in the longitudinal direction of the biaxially oriented polypropylene film, and the peel strength (N/15 mm) was measured using a tensile tester (Tensilon, manufactured by Orientec Co., Ltd.) when T-peeling at a tensile speed of 200 mm/min in an environment of 23° C. The measurement was performed three times, and the average value was taken as the laminate strength.

(Raw resin)
Details of the polypropylene-based resin raw materials used in the following Examples and Comparative Examples are shown in Table 1.

Example 1
For the base layer (A), a polypropylene homopolymer PP-1 shown in Table 1 was used.
For the surface layer (B), a composition was used in which 49 weight % of polypropylene homopolymer PP-1 shown in Table 1 and 51 weight parts of ethylene copolymerized polypropylene polymer PP-3 shown in Table 1 were mixed, and commercially available polymethyl methacrylate (PMMA) particles (average particle size: 1.4 μm) were blended as an antiblocking agent in an amount equivalent to 0.15 mass % of the mixture. At this time, the melt flow rate (g/10 min) of the mixture of 49 weight % of polypropylene homopolymer PP-1 and 51 weight % of ethylene copolymerized polypropylene polymer PP-3 was 5.3.
The base layer (A) was made of a 60 mm extruder, and the surface layer (B) was made of a 65 mm extruder. The raw resins were melted at 250 ° C. and co-extruded from a T-die into two layers in a sheet shape, and the base layer (A) side was in contact with a cooling roll, and the film was cooled and solidified with a cooling roll at 30 ° C., and then stretched 4.5 times in the machine direction (MD) at 125 ° C. Then, in a tenter, both ends in the width direction of the film were clamped with clips, preheated at 170 ° C., stretched 8.2 times in the width direction (TD) at 158 ° C., and heat-set at 165 ° C. while relaxing 6.7% in the width direction (TD). The film-forming conditions at this time were film-forming conditions a.
In this way, a biaxially oriented polypropylene film was obtained in which one base layer (A) and one surface layer (B) were laminated.
The surface layer (B) side of the biaxially oriented polypropylene film was subjected to a corona treatment using a corona treatment machine manufactured by Softal Corona & Plasma GmbH under the condition of an applied current value of 0.75 A, and then wound up with a winder. The thickness of the obtained film was 20 μm.

Example 2
The resin used for the base layer (A) was changed to polypropylene resin PP-2, and the base layer (A) was made from a 60 mm extruder, and the surface layer (B) was made from a 65 mm extruder. The raw resins were melted at 250 ° C., co-extruded from a T-die into a sheet, cooled and solidified with a cooling roll at 30 ° C., and then stretched 4.5 times in the machine direction (MD) at 135 ° C. Then, in a tenter, both ends of the film width direction were clamped with clips, preheated at 175 ° C., stretched 8.2 times in the width direction (TD) at 160 ° C., and heat-set at 170 ° C. while relaxing 6.7% in the width direction (TD). The film formation conditions at this time were film formation conditions b.
In this way, a biaxially oriented polypropylene film was obtained in which one base layer (A) and one surface layer (B) were laminated.

Example 3
A biaxially oriented laminated polypropylene film was obtained in the same manner as in Example 1, except that the thickness of the base layer (A) was changed to 38 μm.

Example 4
A biaxially oriented laminated polypropylene film was obtained in the same manner as in Example 1, except that the thickness of the base layer (A) was changed to 18 μm.

(Comparative Example 1)
For the surface layer (B), a biaxially oriented laminated polypropylene film was obtained in the same manner as in Example 1, except that 0.15% by mass of polymethyl methacrylate (PMMA) particles (average particle size: 1.4 μm) was blended with the polypropylene homopolymer PP-1 as an antiblocking agent.

(Comparative Example 2)
A biaxially oriented laminated polypropylene film was obtained in the same manner as in Example 1, except that polypropylene homopolymers PP-1 and PP-4 were used for the surface layer (B).

(Comparative Example 3)
A biaxially oriented laminated polypropylene film was obtained in the same manner as in Example 1, except that no antiblocking agent was used in the surface layer (B).

(Comparative Example 4)
A biaxially stretched laminated polypropylene film was obtained in the same manner as in Example 1, except that the polypropylene homopolymer PP-1 was mixed with stearyldiethanolamine stearate (KYM-4K, manufactured by Matsumoto Oil Co., Ltd.) in an amount of 1.0% by mass relative to the polypropylene homopolymer PP-1 as an antistatic agent for the base layer (A). The physical properties of the obtained film are shown in Table 3.

(Comparative Example 5)
A biaxially oriented laminated polypropylene film was obtained in the same manner as in Example 1, except that the surface layer (B) side of the biaxially oriented polypropylene film was not subjected to corona treatment.

(Comparative Example 6)
After cooling and solidifying with a cooling roll at 40°C, the film was stretched 4.5 times in the machine direction (MD) at 135°C, then both ends in the width direction of the film were clipped in a tenter, preheated at 175°C, stretched 8.2 times in the width direction (TD) at 163°C, and heat-set at 1772°C while relaxing by 6.7% in the width direction (TD), in the same manner as in Example 1, except that no corona treatment was performed. The film-forming conditions at this time were designated as film-forming conditions c.

The raw materials, film-forming conditions, and physical properties of the obtained films used in the above Examples and Comparative Examples are shown in Tables 2, 3, and 4, respectively.

The biaxially oriented laminated polypropylene films obtained in Examples 1 to 4 had high lamination strength and excellent printing ink adhesion. Furthermore, they had low heat shrinkage and high Young's modulus.
In contrast, the films of Comparative Examples 1 to 5 all had poor printing ink adhesion.
Moreover, all of the films of Comparative Example 6 had high haze and poor transparency.

The biaxially oriented laminated polypropylene film of the present invention has good printing ink adhesion, so it can be used for food packaging such as sweets, as well as labels, etc., and the film can be produced at low cost, making it industrially useful.

Claims (2)

The present invention has a base layer (A) mainly composed of a polypropylene-based resin composed of a completely homogeneous polypropylene resin containing no copolymerization component and/or a polypropylene resin copolymerized with ethylene and/or an α-olefin having 4 or more carbon atoms, and a surface layer (B) on at least one surface of the base layer (A) mainly composed of a mixture of polypropylene-based resins composed of a completely homogeneous polypropylene resin containing no copolymerization component having two or more different melt flow rates (MFR) and/or a polypropylene resin copolymerized with ethylene and/or an α-olefin having 4 or more carbon atoms,
the arithmetic mean roughness of the surface of the surface layer (B) opposite to the base layer (A) is 0.027 μm or more and 0.040 μm or less ;
the surface resistivity of the surface layer (B) on the opposite side to the base layer (A) is 15 Log Ω or more and 16.5 Log Ω or less;
The surface of the surface layer (B) opposite to the base layer (A) has a wetting tension of 38 mN/m or more and 41 mN/m or less ;
and the central surface peak height SRp+central surface valley depth SRv of the surface of the surface layer (B) opposite to the base layer (A) is 1.1 μm or more and 1.57 μm or less,
The film thickness is 9 μm or more and 200 μm or less,
and a biaxially oriented polypropylene film having a haze value of 5% or less. A laminate having a printed layer on the surface of the surface layer (B) of the biaxially oriented polypropylene film according to claim 1 opposite to the substrate layer (A).