Biaxially oriented polypropylene film

Abstract

To provide a biaxially oriented polypropylene film having high lamination strength with other film members without impairing the excellent transparency and mechanical properties inherent to biaxially oriented polypropylene films, and having excellent printing ink transferability from a printing roll to the film and excellent adhesion of the printing ink. A biaxially oriented polypropylene film having a base layer (A) primarily composed of a polypropylene resin and a surface layer (B) primarily composed of a polypropylene resin on at least one surface of the base layer (A), wherein 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 9 μm or more and 200 μm or less, and the haze (transparency) value of the film is 5% or less.

Description

[Technical field]

[0001] The present invention relates to a biaxially oriented polypropylene film, and more particularly to a biaxially oriented polypropylene film having excellent adhesion to printing ink and adhesives used for lamination with other film members. [Background technology]

[0002] Conventionally, biaxially oriented polypropylene films have been widely used as packaging materials for various items, including food, textile products, etc., because of their excellent transparency and mechanical properties. However, problems with polypropylene films include, for example, the low surface energy of polypropylene resins, which are non-polar, and therefore insufficient adhesion to printing inks or other materials in processes such as printing inks and lamination.

[0003] In particular, when biaxially oriented polypropylene films are used as packaging materials, they are generally laminated to other films using an adhesive. However, if the strength of the laminate between these films is weak, the strength of the film as a packaging material will be weakened and it 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.

[0004] In addition, biaxially oriented polypropylene-based films are generally printed, and from the viewpoint of color development and color fading of the prints, there is a growing demand for improvement in the transferability of the printing ink from the printing roll to the surface of the biaxially oriented polypropylene-based film and the adhesion of the printing ink to the film surface.

[0005] 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 on the surface of a biaxially oriented polypropylene film (see, for example, Patent Document 1, etc.). However, not only is the adhesion of the printing ink insufficient, but a separate process for providing the skin layer is required, resulting in poor productivity. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] JP 2000-127310 A Summary of the Invention [Problem to be solved by the invention]

[0007] The present invention aims to provide a biaxially oriented polypropylene film that has high lamination strength with other film components without impairing the excellent transparency and mechanical properties inherent to biaxially oriented polypropylene films, and that has excellent printing ink transferability from a printing roll to the film and adhesion of the printing ink. [Means for solving the problem]

[0008] The present invention, which has solved the above-mentioned problems, is a biaxially oriented polypropylene-based film having a base layer (A) mainly composed of a polypropylene-based resin and a surface layer (B) mainly composed of a polypropylene-based resin on at least one surface of the base layer (A), wherein 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 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.

[0009] It is preferable that the central plane peak height SRp+central plane valley depth of the surface of the surface layer (B) opposite to the base layer (A) is 1.0 μm or more and 2.0 μm or less.

[0010] The biaxially oriented polypropylene film preferably has a heat shrinkage rate of 11% or less at 150° C. in both the longitudinal and transverse directions.

[0011] A laminate having a printed layer on the side of the surface layer (B) of any of the biaxially oriented polypropylene films described above opposite to the base layer is suitable. Effect of the Invention

[0012] 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. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] 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 the base layer (A) is measured 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 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).

[0014] The arithmetic mean roughness SRa is less affected by 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 anti-blocking agents and lubricants. Since most of the printing ink adheres to the surface other than the relatively large peaks and valleys formed by anti-blocking agents and lubricants, 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, which will be described later. In addition, the surface resistivity 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. Furthermore, the wetting tension of the surface layer (B) represents the surface tension (μN / cm) of the mixed liquid reagent determined to wet the film surface, and is related to the wettability of printing inks and adhesives. Further details are provided below.

[0015] (1) Base material 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 propylene and ethylene and / or an α-olefin having 4 or more carbon atoms at 0.5 mol % or less. 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 exceeding 0.5 mol %, the crystallinity and rigidity are too low, and the thermal shrinkage rate at high temperatures may become large. Such resins may be blended and used.

[0016] 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 may be low and the heat resistance may be insufficient. 99.5% is the practical upper limit.

[0017] 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 the resin is not stable when cast, and film-forming properties may be poor. If the Mw is more than 500,000, the amount of components with a molecular weight of 100,000 or less becomes 35% by mass, and the heat shrinkage rate at high temperatures decreases. 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.

[0018] 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, whereas if it exceeds 200,000, the heat shrinkage rate at high temperatures is reduced. A more preferable lower limit of Mn is 30,000, even more preferably 40,000, and particularly preferably 50,000, and a more preferable upper limit of Mn is 80,000, even more preferably 70,000, and 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 rate tends to increase even if the crystallinity is high. If the Mw / Mn is too large, the amount of high molecular weight components increases, which is not preferable because the heat shrinkage rate may increase. 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.

[0019] 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 2.8 to 7, still more 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 a series of plants in multiple stages, 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.

[0020] 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 this range, the heat shrinkage rate at high temperatures can be kept small, and the adhesion to the cooling roll is good, resulting in excellent film formability.

[0021] (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, particularly preferably 0.030 μm or more. In order to make the surface roughness of the surface layer (B) 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 with 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 polypropylene resin mixture is different, the crystallization speed and degree of crystallization of each polypropylene differ, so that it is presumed that the arithmetic mean roughness of the surface of the surface layer (B) opposite the base layer (A) is 0.028 μm or more. Also, the arithmetic mean roughness of the surface of the surface layer (B) opposite the base layer (A) is unlikely to exceed 0.040 μm. As polypropylene-based resins with a smaller MFR, copolymers of propylene and ethylene and / or α-olefins with 4 or more carbon atoms can also be used. Examples of α-olefins with 4 or more carbon atoms include 1-butene, 1-hexene, 4-methyl-1-pentene, and 1-octene. Polar maleic acid and the like can also be used as other copolymerization components. 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 excess of 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 in monomer units. 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.

[0022] 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. Within this range, the film formability is good and the appearance is excellent. If the MFR of the polypropylene resin composition of the surface layer (B) is less than 1.0 g / 10 min, the viscosity difference between the base layer (A) and the surface layer (B) will be large when the MFR of the polypropylene resin of the base layer (A) is large, and unevenness (unevenness of the original roll) will easily occur during film formation. If the MFR of the polypropylene resin composition of the surface layer (B) is more than 8 g / 10 min, the adhesion to the cooling roll will be poor, air will be entrained, and smoothness will be poor, resulting in many defects starting from these, and there is a risk that it will be difficult to achieve an appropriate surface roughness.

[0023] 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 inks and adhesives is improved. The surface resistivity is more preferably 16 Log Ω or more. In order to achieve a surface resistivity of 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.

[0024] 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 wetting tension of 38 mN / m or more. When the wetting tension is 38 mN / m or more, the adhesion to printing inks and 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 use 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.

[0025] 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 cut-off of 80 μm.

[0026] The central plane peak height SRp+central plane valley depth SRv of the surface opposite the base layer (A) of the surface layer (B) of the biaxially oriented polypropylene film of the present invention is an index of the state of relatively large uneven parts formed locally by an antiblocking agent or the like, and is closely related to the sliding properties 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 opposite 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 center plane peak height SRp+center plane valley depth SRv of the surface of the surface layer (B) opposite to the base layer (A) is preferably 1.1 μm or more, more preferably 1.2 μm or more, particularly preferably 1.3 μm or more.

[0027] In order to set the central plane peak height SRp+central plane valley depth SRv of the surface layer (B) of the biaxially oriented polypropylene film of the present invention opposite the base layer (A) to 1.0 μm or more and 2.0 μm or less, it is a suitable method to incorporate an antiblocking agent into 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 entire polypropylene resin or the mixture thereof.

[0028] 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.

[0029] 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.

[0030] (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 one layer 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 from 9 to 200 μm, more preferably from 10 to 150 μm, further preferably from 12 to 100 μm, and particularly preferably from 12 to 80 μm.

[0031] In the biaxially oriented polypropylene film of the present invention, the ratio of the thickness of the entire 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 ratio of the entire surface layer (B) to the entire base layer (A) exceeds 0.5, the shrinkage rate tends to increase. The thickness of the entire base layer (A) relative to the thickness of the entire 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 substantial thickness of the entire 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.

[0032] The ink adhesion of the biaxially oriented polypropylene film of the present invention was evaluated by carrying out a peeling test of gravure-printed printing ink, and counting the number of peeled off portions out of a total of 25 portions. The number of peeled off portions is preferably 5 or less, more preferably 3 or less, and most preferably 0. If the number of peeled off portions exceeds 5, the extent of peeling of the printing ink becomes large, which is problematic. The method of evaluating the ink adhesion will be described later.

[0033] The laminate strength in the longitudinal direction 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.

[0034] 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. When 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.

[0035] 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 become worse when the stretching temperature or heat setting temperature is too high, the cooling roll temperature is high and the cooling rate of the unstretched (raw) sheet is slow, or there is too much low molecular weight component, and it can be adjusted to be within the above range. The method for measuring the haze will be described later.

[0036] The thermal shrinkage 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 thermal shrinkage rate 11% or less, it is possible to reduce pitch deviation during printing. The method for measuring thermal shrinkage will be described later.

[0037] 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. Note that 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.

[0038] 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 have a strong stiffness 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.

[0039] 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 it is within the above range, the heat resistance and rigidity of the film tend 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.

[0040] (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 transverse 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 and the appearance is likely to be poor. It is more preferably 5.5 g / 10 min or less, and even more preferably 5 g / 10 min or less.

[0041] 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 is likely to become rough.

[0042] 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, the film thickness may become uneven. The upper limit of the stretching ratio in the MD is preferably 8 times, more preferably 7 times. If it exceeds the above, the subsequent TD stretching may become difficult. 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 become large, the thickness unevenness may become large, 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 the surface may become rough.

[0043] The lower limit of the stretching 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 stretching ratio is preferably 20 times, more preferably 17 times, even more preferably 15 times, and particularly preferably 12 times. If it exceeds the above, the heat shrinkage rate may increase or the film may break during stretching. The preheating temperature in TD stretching is preferably set to be 5 to 15°C higher than the stretching temperature in order to quickly raise the film temperature to the vicinity of the stretching temperature. TD stretching is performed at a higher temperature than conventional stretched polypropylene-based 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 increase. The upper limit of the TD stretching temperature is preferably 170°C, more preferably 168°C, and even more preferably 163°C. To reduce the heat shrinkage rate, a higher temperature is preferable, 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.

[0044] The stretched film 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 treatment 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.

[0045] It is preferable to relax the film during heat setting. The lower limit of the relaxation is preferably 2%, more preferably 3%. If it is less than the above, the thermal shrinkage rate may be high. The upper limit of the relaxation is preferably 10%, more preferably 8%. If it exceeds the above, the thickness unevenness may become large.

[0046] 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 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 even more preferably 173°C. If it exceeds the above, the transparency may decrease or the thickness unevenness may increase.

[0047] 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 even more preferably 20 minutes. If it exceeds the above, the productivity may decrease.

[0048] The biaxially oriented polypropylene film thus obtained can be subjected to corona discharge, plasma treatment, flame treatment, etc., as required, and then wound up with a winder to obtain the biaxially oriented polypropylene film roll of the present invention.

[0049] 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. EXAMPLES

[0050] 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 any modifications and variations that do not depart from the spirit of the present invention are included in the present invention.

[0051] (Measurement method) The physical properties of the films obtained in the examples and comparative examples were measured as follows.

[0052] 1) Stereoregularity The 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 measurements were performed at 110°C using BRUKER's "AVANCE500" by dissolving 200 mg of a sample in a mixture of o-dichlorobenzene and heavy benzene at a volume ratio of 8:2 at 135°C.

[0053] 2) Melt flow rate (MFR; g / 10 min) The measurements were performed 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 and used as required. In the case of film, the required amount was cut out and then cut into samples of about 5 mm square.

[0054] 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℃

[0055] The number average molecular weight (Mn), mass average molecular weight (Mw), and molecular weight distribution are respectively calculated by the molecular weight (M i ) number of molecules (N i ) is defined by the following formula: Number average molecular weight: Mn=Σ(N i M i ) / ΣN i Mass average molecular weight: Mw=Σ(N i M i 2 ) / Σ(Ni 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.

[0056] 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 with a differential interference microscope.

[0057] 5) Heat shrinkage rate (%) The measurement was performed according to the following method in accordance with JIS Z1712. The film was cut to a width of 20 mm and a length of 200 mm in each of the MD and TD directions, and was hung in a hot air oven at 150°C and heated for 5 minutes. 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 determine the thermal shrinkage rate.

[0058] 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: 200mm / min Chuck distance: 100mm Strain range for elastic modulus measurement: 0.1-0.6%

[0059] 7) Haze (unit: %) Measurements were performed in accordance with JIS K7105.

[0060] 8) Coefficient of kinetic friction In accordance with JIS K7125, the surface layers (B) of two films were placed together and the measurement was carried out at 23°C.

[0061] 9) Refractive index, plane orientation coefficient Measurements were performed using an Atago Abbe refractometer according to JIS K7142-1996 5.1 (Method A). The refractive indices along the MD and TD directions were designated as Nx and Ny, respectively, and the refractive index along the thickness direction was designated as Nz. The plane orientation coefficient (ΔP) was calculated by (Nx+Ny) / 2-Nz.

[0062] 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 magnification of 20,000 times, and a cutoff of 80 μm, and was calculated in accordance with the definition of arithmetic mean roughness described in JIS B 0601 (1994). The arithmetic mean roughness (SRa), central surface peak height (SRp), and central surface valley depth (SRv) were each evaluated by performing three trials and averaging the results.

[0063] 11) Surface resistivity (LogΩ) 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.

[0064] 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 using the following procedure. Step 1) The measurements shall be 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), a few drops of the test mixture are placed on the specimen, and immediately spread 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. Wetting tension is judged by observing the liquid film of the test mixed liquid in a bright place and the state of the liquid film after 3 seconds. If the liquid film does not break and remains in the same state as when it was applied 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 process 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 burs 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 the 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.

[0065] 13) Ink adhesion The surface layer (B) of the film was gravure-printed at a speed of 50 m / min using a gravure printing machine (Mitani Iron Works Co., Ltd.) with a printing ink amount of 2 g / m 2 The ink used was a water-based ink (manufactured by Dainippon Ink and Chemicals, Inc.: trade name Ecofine 709 White). (Registered trademark) This printed sample was used to evaluate (in more detail) the product using a grid peel method (25 x 2 mm squares, 90° peel method using 18 mm wide Nichiban Cellophane Tape (Registered trademark)) and rank it as follows based on its practicality. Grid pattern peeling: 0 pieces. ◎: Excellent printing ink adhesion. Same grade: 1 to 5 pieces...○: Good printing ink adhesion. Same grade: 6 to 15 pieces...△: Poor printing ink adhesion. Same as above: 1 or more...×: No adhesion of printing ink.

[0066] 14) Laminate strength The laminate strength was measured by the following procedure. Step 1) Creating a laminate film with a sealant film The lamination was carried out using a continuous dry laminating machine as follows. The adhesive was applied to the surface layer (B) of the biaxially oriented polypropylene film obtained in the Examples and Comparative Examples in an amount of 3.0 g / m2 when dried. 2 After gravure coating so that the film was coated, it was introduced into a drying zone and dried at 80°C for 5 seconds. Then, it was laminated with a sealant film between rolls installed downstream (roll pressure: 0.2MP, roll temperature: 60°C). The obtained laminate film was aged at 40°C for 3 days in a rolled 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), 17.9% by mass of a hardener (CAT8B, manufactured by Toyo Morton), 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) Measuring laminate strength The laminate film obtained above was cut into a rectangular shape (200 mm long, 15 mm wide) 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-shaped peeling was performed 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.

[0067] (raw resin) Details of the polypropylene resin raw materials used in the following Examples and Comparative Examples are shown in Table 1.

[0068] [Table 1]

[0069] 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 the polypropylene homopolymer PP-1 shown in Table 1 and 51 weight % of the 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 the polypropylene homopolymer PP-1 and 51 weight % of the ethylene copolymerized polypropylene polymer PP-3 was 5.3. The base layer (A) was made with a 60 mm extruder, and the surface layer (B) was made with a 65 mm extruder, and the raw material resins were melted at 250 ° C. and co-extruded from a T-die into two layers in the form of a sheet, and the base layer (A) side was in contact with a cooling roll, and after cooling and solidifying with a cooling roll at 30 ° C., it was stretched 4.5 times in the machine direction (MD) at 125 ° C. Next, 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 by 6.7% in the width direction (TD). The film formation conditions at this time were designated as film formation 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) of the biaxially oriented polypropylene film was subjected to corona treatment using a corona treatment machine manufactured by Softal Corona & Plasma GmbH at an applied current value of 0.75 A, and then wound up with a winder. The thickness of the obtained film was 20 μm.

[0070] Example 2 The resin used for the base layer (A) was changed to polypropylene resin PP-2, and the base layer (A) was made using a 60 mm extruder, and the surface layer (B) was made using 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 transverse direction (TD) at 160 ° C., and heat-set at 170 ° C. while relaxing 6.7% in the transverse direction (TD). The film-forming conditions at this time were film-forming 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.

[0071] 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.

[0072] 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.

[0073] Comparative Example 1 A biaxially oriented laminated polypropylene film was obtained in the same manner as in Example 1, except that the surface layer (B) was made by blending 0.15 mass% of polymethyl methacrylate (PMMA) particles (average particle size: 1.4 μm) with the polypropylene homopolymer PP-1 as an antiblocking agent.

[0074] 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).

[0075] 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).

[0076] Comparative Example 4 A biaxially oriented laminated polypropylene film was obtained in the same manner as in Example 1, except that the polypropylene homopolymer PP-1 was mixed with stearyl diethanolamine 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.

[0077] Comparative Example 5 A biaxially oriented laminated polypropylene film was obtained in the same manner as in Example 1, except that the corona treatment was not performed on the surface layer (B) side of the biaxially oriented polypropylene film.

[0078] 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, and then in a tenter, both ends in the width direction of the film were clamped with clips, preheated at 175°C, and stretched 8.2 times in the transverse direction (TD) at 163°C. The film was then heat-set at 1772°C while relaxing by 6.7% in the transverse direction (TD). The same procedure was followed as in Example 1, except that no corona treatment was performed. The film formation conditions at this time were designated as film formation conditions c.

[0079] 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.

[0080] [Table 2]

[0081] [Table 3]

[0082] [Table 4]

[0083] The biaxially oriented laminated polypropylene films obtained in Examples 1 to 4 had high lamination strength and excellent printing ink adhesion, and further had low heat shrinkage and high Young's modulus. In contrast, all of the films of Comparative Examples 1 to 5 were poor in printing ink adhesion. Moreover, all of the films of Comparative Example 6 had high haze and poor transparency. [Industrial Applicability]

[0084] The biaxially oriented laminated polypropylene film of the present invention has good printing ink adhesion and can be used not only for food packaging such as for confectionery, but also for labels, etc., and is industrially useful because the film can be produced at low cost.

Claims

1. The present invention has a base layer (A) mainly composed of a polypropylene-based resin made 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) mainly composed of a polypropylene-based resin made 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 has a structure of either base layer (A) / surface layer (B) or surface layer (B) / base layer (A) / surface layer (B), and the surface layer (B) is opposite to the base layer (A). a surface resistivity value of the surface of the surface layer (B) opposite to the base layer (A) being 15 Log Ω or more and 16.5 Log Ω or less; a wet tension of the surface of the surface layer (B) opposite to the base layer (A) being 38 mN / m or more; and a central plane peak height SRp + central plane valley depth SRv of the surface of the surface layer (B) opposite to the base layer (A) being 1.1 μm or more and 1.57 μm or less; a film thickness of 9 μm or more and 200 μm or less; and a haze value of the film being 5% or less.

2. 2. The biaxially oriented polypropylene film according to claim 1, wherein the surface layer (B) has a surface wetting tension of 38 mN / m or more and 41 mN / m or less.

3. 3. The biaxially oriented polypropylene film according to claim 1, wherein the heat shrinkage rate of the film in the longitudinal and transverse directions at 150° C. is 11% or less.

4. A laminate having a printed layer on the surface of the surface layer (B) of the biaxially oriented polypropylene film according to any one of claims 1 to 3, opposite to the base layer (A).

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