JP2023109131A - Polyolefin film and release film - Google Patents

Abstract

An object of the present invention is to provide a polyolefin film excellent in releasability and surface uniformity (quality). SOLUTION: The peak density Spd of the peak is 100 (1/mm2) or more and 1000 (1/mm2) or less, and the arithmetic mean curve Spc of the peak point is -200 (1/mm) or more -10 (1/mm). ) A polyolefin film, wherein at least one of the following surfaces is the A surface. [Selection figure] None

Description

TECHNICAL FIELD The present invention relates to a polyolefin film excellent in releasability and quality, and a release film using the same.

Polyolefin films are excellent in various properties such as transparency, mechanical properties, and electrical properties, so they are used in various applications such as packaging, mold release, tape, cable wrapping, and electrical applications such as capacitors. there is In particular, because of its excellent surface release properties and mechanical properties, it is suitably used as a release film or process film for various members such as plastic products, building materials, and optical members.

In recent years, polyolefin films are sometimes used as cover films for adhesive resin layers such as photosensitive resin layers. When used as such a cover film, if the releasability is poor, the polyolefin film cannot be peeled cleanly from the adherend, and the shape of the resin layer of the adherend may change. Peeling marks may remain on the layer. In order to solve such problems, a technique is used in which the releasability is improved by roughening the film surface to reduce the contact area with the resin layer. On the other hand, when the surface roughness of the cover film is increased, coarse protrusions are likely to be formed, and the surface unevenness of the cover film due to the coarse protrusions may be transferred to the resin layer of the adherend and affect the visibility of the product. There is

From the above, in order to use a polyolefin film as a release film in the field of optical members, etc., which have high requirements for peelability and surface uniformity, it is necessary to have a uniform and finely roughened surface with few coarse protrusions. are required to have As means for obtaining such a polyolefin film having a dense and rough surface, for example, the following methods are known.

Patent Literatures 1 and 2 disclose a method of making the surface of a polypropylene film, which is a type of polyolefin film, dense and rough. Specifically, an example is described in which spherulites of β crystals, which is one of the crystal forms of polypropylene, are formed and stretched to form craters on the film surface to roughen the film surface, thereby improving process transportability. ing. Further, Patent Document 3 describes an example in which particles are added to a film and the film is uniaxially stretched to roughen the surface, thereby enhancing process transportability. Further, Patent Document 4 describes an example in which the surface is roughened by adding particles to the inner layer of the film and stretching the film.

International Publication No. 2016/006578 JP 2017-125184 A JP 2005-138386 A JP 2017-077752 A

However, the methods described in Patent Documents 1 to 3 have the problem of insufficient surface roughness. In addition, in the method described in Patent Document 4, coarse protrusions are formed by particles with high hardness, and when used as a release film, unevenness caused by the coarse protrusions is formed on the resin layer of the optical member that is the adherend. It was sometimes transcribed.

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to solve the above-described problems. That is, an object of the present invention is to provide a polyolefin film excellent in releasability and surface uniformity (quality).

In order to solve the above problems, the polyolefin film of the present invention has the following constitution. That is, the polyolefin film of the present invention has a peak density Spd of 100 (1/mm ² ) or more and 1000 (1/mm ² ) or less, and an arithmetic mean curve Spc of the peak points of −200 (1/mm). The polyolefin film is characterized in that at least one side is the A side when the surface having a thickness of -10 (1/mm) or less is the A side.

Further, the polyolefin film of the present invention can have the following aspects, and can also be used as a release film as described below.
(1) A peak density Spd of 100 (1/mm ² ) or more and 1000 (1/mm ² ) or less, and an arithmetic mean curve Spc of the peak points of -200 (1/mm) or more -10 (1/mm) mm) or less, wherein at least one of the surfaces is the A surface.
(2) The polyolefin film according to (1), wherein at least one of the A surfaces has an average roughness Sa of 200 nm or more and 600 nm or less.
(3) The film thickness is t (μm), the peel force is N (N/19 mm) when a polyester adhesive tape is laminated on the A side, and the peel speed is 300 mm / min and the peel angle is 180 °. The polyolefin film according to (1) or (2), wherein N/t is 0.06 or more and 0.20 or less.
(4) The polyolefin film according to any one of (1) to (3), having a laminate structure of at least two layers.
(5) A release film using the polyolefin film according to any one of (1) to (4).

INDUSTRIAL APPLICABILITY According to the present invention, it is possible to provide a polyolefin film that is excellent in releasability and quality and can be suitably used as a release film.

The polyolefin film of the present invention has a peak density Spd of 100 (1/mm ² ) or more and 1000 (1/mm ² ) or less, and an arithmetic mean curve Spc of the peak points of −200 (1/mm) or more − It is characterized in that at least one of the surfaces having a thickness of 10 (1/mm) or less is defined as the A surface. The polyolefin film of the present invention will be specifically described below.

A polyolefin film refers to a sheet-like molded body containing polyolefin resin as a main component. The main component refers to a component containing more than 50% by mass and 100% by mass or less when all constituent components of an object (here, a film) are 100% by mass. Hereinafter, the main component is interpreted in the same way. be able to. When a plurality of components corresponding to polyolefin resin are included, the polyolefin resin is regarded as the main component if the total amount is more than 50% by mass. A polyolefin resin is a resin in which the main structural unit is an olefin unit, and the main structural unit is a structural unit containing more than 50 mol% and 100 mol% or less when the total structural units are 100 mol%.

From the viewpoint of releasability, the polyolefin film of the present invention has a peak density Spd of 100 (1/mm ² ) or more and 1000 (1/mm ² ) or less, and an arithmetic mean curve Spc of the peak points of -200. (1/mm) or more and −10 (1/mm) or less is defined as the A surface, it is important that at least one of the surfaces is the A surface. The crest vertex density Spd is one of the indices representing surface roughness, and represents the number of crest vertices per unit area. The arithmetic mean curve Spc of the peak of the mountain is the average value of the radius of curvature of the peak of the mountain. In the following description, the ``mountain peak density Spd'' may be simply referred to as ``Spd'', and the ``arithmetic mean curve of peak points Spc'' may be simply referred to as ``Spc''. By adopting such a mode, excellent peelability can be realized when the A side is bonded to the adherend so as to be in contact with the adherend.

Spd, Spc, and average roughness Sa, which will be described later, can be measured using a known layer cross-sectional shape measuring device. "VertScan"® 2.0 can be used. A specific method for measuring Spd, Spc, and average roughness Sa (described later) using the same apparatus will be described later.

When a polyolefin film that does not have an A surface is used as a release film, various problems arise in terms of releasability, as described below. When the Spd of the surface to be adhered to the adherend is less than 100 (1/mm ² ), the contact area between the adherend and the polyolefin film becomes excessively large, and if the adherend has strong adhesiveness, the two do not adhere to each other. become excessively rigid. As a result, the polyolefin film cannot be peeled cleanly from the adherend, and the shape of the adherend surface may change, or peeling marks may remain on the adherend surface. On the other hand, when the Spd of the surface to be adhered to the adherend exceeds 1000 (1/mm ² ), the contact area between the adherend and the polyolefin film is insufficient, and the adhesion between the two becomes weak. The polyolefin film may peel off from the adherend.

In addition, when the Spc of the surface to be bonded to the adherend is less than -200 (1/mm), the curvature of the protrusion is insufficient and the contact area between the adherend and the polyolefin film increases, resulting in adhesion between the two. becomes excessively strong, and the peelability of the polyolefin film may deteriorate. On the other hand, if the Spc of the surface to be adhered to the adherend exceeds −10 (1/mm), the protrusions on the surface of the polyolefin film become excessively sharp and bite into the adherend, which may deteriorate the releasability of the two. be.

That is, the polyolefin film of the present invention has an A surface in which both Spd and Spc have values within the above ranges, so that when the A surface is attached to an adherend, projections do not bite into or strike the adherend. Since the marks can be reduced, the release property and quality are excellent, and the film can be suitably used as a release film.

From the above viewpoint, the Spd of the A side is preferably 200 (1/mm ² ) or more and 800 (1/mm ² ) or less, more preferably 400 (1/mm ² ) or more and 600 (1/mm ² ) or less. From the same point of view, the Spc of the A side is preferably −100 (1/mm) or more and −10 (1/mm) or less, more preferably −80 (1/mm) or more and −10 (1/mm ² ) below.

In order to make at least one surface the A surface, it is effective to set the raw material composition of the polyolefin film to the range described below and the film-forming conditions to the range described below. More specifically, a resin composition containing a preferable amount of branched polypropylene resin described later is used to form the surface layer (the film itself in the case of a single layer film), and the surface temperature of the casting drum is set to the preferable amount described later. Spd and Spc can be controlled within preferable ranges by setting the film temperature (preheating temperature) in stretching in the longitudinal direction within the range described later. Here, the longitudinal direction refers to the direction in which the film travels in the manufacturing process of the film, and refers to the winding direction when the film roll is formed. A direction orthogonal to the longitudinal direction in the plane of the film is referred to as a width direction.

The polyolefin film of the present invention is not particularly limited as long as at least one surface is the A surface, and the other surface is not particularly limited. Both sides of the film are preferably A-sides. In addition, in a polyolefin film in which both surfaces are A surfaces, the values of Spd and Spc may be the same or different on both surfaces.

The polyolefin film of the present invention preferably has an average roughness Sa of 200 nm or more and 600 nm or less on at least one side A from the viewpoint of suppressing the transfer of dents and improving the transportability and windability of the film, and more preferably. is 300 nm or more and 500 nm or less. Here, "at least one A side" refers to the side when only one side is the A side, and when both sides are the A side, it refers to both sides or one side. In addition, hereinafter, "average roughness Sa" may be simply referred to as "Sa". When Sa is 200 nm or more on the A side, the contact area between the adherend and the polyolefin film (A side) does not become excessively large, and the adhesion to the adherend can be moderately suppressed. Therefore, the polyolefin film can be peeled cleanly from the adherend, and the change in shape of the surface of the adherend and the generation of peel marks on the surface of the adherend can be reduced. On the other hand, since the Sa of the A side is 600 nm or less, the contact area between the adherend and the polyolefin film is not insufficient, and peeling of the polyolefin film from the adherend during transportation in the bonding process can be suppressed.

In order to set the Sa of the A side in the range of 200 nm or more and 600 nm or less, it is effective to set the raw material composition of the film to the range described later and the film forming conditions to the range described later. More specifically, the same method as the method for controlling Spd and Spc within the preferred range can be used. Moreover, from the viewpoints of suppressing the transfer of dents on the back surface and transporting and winding the film, Sa is preferably 200 nm or more and 600 nm or less, more preferably 300 nm or more and 500 nm or less, on both sides of the polyolefin film.

From the viewpoint of realizing appropriate releasability, the polyolefin film of the present invention has a film thickness of t (μm), laminates a polyester adhesive tape on the A side, and peels under the conditions of a peeling speed of 300 mm/min and a peeling angle of 180°. It is preferable that N/t is 0.06 or more and 0.20 or less, where N (N/19 mm) is the peeling force at the time of peeling. From the above viewpoint, N/t is more preferably 0.08 or more and 0.18 or less, more preferably 0.10 or more and 0.16 or less. When both sides are A sides, the above requirement is satisfied if N/t is 0.06 or more and 0.20 or less on at least one side. Hereinafter, the "peeling force when a polyester adhesive tape is laminated on side A and peeled at a peeling speed of 300 mm/min and at a peeling angle of 180°" may be simply referred to as "peeling force N". The film thickness t and the peel force N can be measured with a known electronic micrometer and a known tensile tester, respectively, and the details thereof will be described later.

When used as a release film so that the adherend is attached to the surface of the polyolefin film, if the N / t of the A surface is 0.06 or more, the film thickness is excessive with respect to the adhesive strength of the adherend. Since the stiffness of the polyolefin film can be suppressed without increasing the thickness, the risk of unintended peeling is reduced. On the other hand, when the N/t of the A side is 0.20 or less, the film thickness is sufficiently secured against the adhesive force of the adherend, and the stiffness to the extent that it can be easily peeled can be secured, so clean peeling is possible. As a result, shape change and peeling traces on the surface of the adherend are reduced.

The film thickness t of the polyolefin film of the present invention is not particularly limited, but is preferably 1 μm or more and 100 μm or less from the viewpoint of handling, more preferably 5 μm or more and 50 μm or less, and 10 μm or more and 30 μm or less. More preferred. The film thickness t can be adjusted by the number of screw rotations of the extruder, the width of the unstretched sheet, the film-forming speed, the stretching ratio, and the like.

Although the peel force N is not particularly limited, it is preferably 0.01 N/19 mm or more and 9.0 N/19 mm or less from the viewpoint of handling, and more preferably 0.1 N/19 mm or more and 5.0 N/19 mm or less. and more preferably 1.2 N/19 mm or more and 2.7/19 mm or less. The peeling force N can be adjusted by the surface free energy of the release film or the like, and the surface free energy here means the surface tension acting on the substance to reduce the surface area. The surface free energy can be increased by, for example, applying a surface treatment such as corona discharge treatment.

In addition, the polyolefin film of the present invention preferably has a coefficient of friction of 0.20 or more and 1.00 or less from the viewpoint of reducing problems such as meandering and wrinkles during transport, winding, and unwinding of the film. It is preferably 0.40 or more and 0.60 or less. The coefficient of friction here means that the film is stacked so that different surfaces are in contact under an environment where the temperature is 23 ± 3 ° C and the humidity is 65 ± 5%, a load of 200 g is applied from above, and the upper film The value calculated by the following formula from the resistance value when sliding When the coefficient of friction is 0.20 or more, a certain amount of frictional force is ensured between the film and the roll, so that slippage of the film during transportation and winding is reduced. As a result, it is possible to reduce the occurrence of scratches, wrinkles, winding misalignment, meandering, etc. caused by slippage. On the other hand, when the coefficient of friction is 1.00 or less, the frictional force does not become excessive, and it is possible to reduce the occurrence of running problems and scratches due to friction.
Friction coefficient = resistance value (g)/load (g).

The coefficient of friction can be adjusted, for example, by adjusting the Sa of the A surface. More specifically, the coefficient of friction can be increased by lowering the Sa of the A surface. The method for adjusting Sa on the A side is as described above.

In addition, if the coefficient of friction of the polyolefin film of the present invention is within the preferred range, the film surface is less likely to be scratched because it has appropriate frictional force and slipperiness. Usually, when a stretching method using a peripheral speed difference between stretching rolls is used, the film is in contact with a preheating roll that preheats the film, a stretching roll that stretches the film, and a cooling roll that cools the film. There is a problem that scratches occur on In addition, regardless of the stretching process, the same problem occurs when the film is conveyed by rolls. Since a film with a smooth surface is inferior in lubricity, the runnability is not stable, and scratches are likely to occur due to friction between the film surface and the roll. Therefore, by setting the coefficient of friction within the above preferable range, moderate lubricity is ensured, and the film surface is less likely to be scratched.

Next, the layer structure and raw materials of the polyolefin film of the present invention will be described, but the layer structure and raw materials of the polyolefin film of the present invention are not necessarily limited to these.

From the viewpoint of transparency and heat resistance, the polyolefin film of the present invention preferably contains a polypropylene resin as a main component, that is, is a polypropylene film. Here, the polypropylene resin refers to a resin in which more than 50 mol% and 100 mol% or less is propylene units when all structural units constituting the resin are 100 mol% (hereinafter, the same applies to other polyolefin resins such as polyethylene can be interpreted as ). When the polyolefin film of the present invention is a polypropylene film, the content of the polypropylene resin is preferably 95% by mass or more and 100% by mass or less, more preferably 100% by mass of all components constituting the film. 96% by mass or more and 100% by mass or less, more preferably 97% by mass or more and 100% by mass or less, and particularly preferably 98% by mass or more and 100% by mass or less.

From the viewpoint of achieving both heat resistance and releasability, the polyolefin film of the present invention preferably has a laminated structure of at least two layers. By adopting such an aspect, a layer that plays a role of ensuring thermal dimensional stability as a polyolefin film (hereinafter sometimes referred to as a base layer (A)) and an adherend are excellent when bonded together. Since a layer that plays a role in realizing releasability (hereinafter sometimes referred to as surface layer (B)) can be provided, it becomes easy to form a polyolefin film having both heat resistance and releasability. Moreover, "at least two or more layered structure" means a state in which at least two or more types of layers are laminated in total two or more layers. Specific examples of such an aspect include a two-kind, two-layer structure of base layer (A)/surface layer (B), a two-kind, three-layer structure of surface layer (B)/base layer (A)/surface layer (B), and the like. .

The main component of the base layer (A) of the polyolefin film of the present invention is preferably polyolefin resin. The polyolefin resin that is the main component of the base layer (A) of the polyolefin film is sometimes referred to as polyolefin resin I hereinafter. The polyolefin resin I of the base layer (A) is more preferably 90% by mass or more and 100% by mass or less, still more preferably 95% by mass or more and 100% by mass or less, still more preferably 96% by mass or more and 100% by mass or less, and particularly preferably 97% by mass or more and 100% by mass or less, most preferably 98% by mass or more and 100% by mass or less.

The polyolefin resin I in the polyolefin film of the present invention is preferably polypropylene resin, preferably homopolypropylene, from the viewpoint of strength and heat resistance. Homopolypropylene is a polypropylene resin in which 99 mol % or more and 100 mol % or less are propylene units when the total structural units constituting the resin are taken as 100 mol %.

The polyolefin resin I preferably has a melting point of 155° C. or higher, more preferably 160° C. or higher. A melting point of 155° C. or higher improves the heat resistance of the polyolefin film. Therefore, when the polyolefin film is used, for example, as a release film, the softening of the polyolefin film in the heat-applying process after lamination with the adherend and the elongation in the direction of tension due to softening are suppressed. Reduces body deformation. Also, the melting point is preferably 200° C. or lower, more preferably 190° C. or lower, and even more preferably 180° C. or lower. Since the melting point of the polyolefin resin I is 200° C. or less, restrictions on equipment during melt extrusion are less likely to occur.

When the polyolefin resin I is a polypropylene resin, its mesopentad fraction is preferably 0.90 or more, more preferably 0.93 or more, and still more preferably 0.94 or more. The mesopentad fraction is an index indicating the stereoregularity of the crystalline phase of polypropylene measured by nuclear magnetic resonance (NMR), and generally, the higher the value, the higher the crystallinity and melting point. Therefore, by using a polypropylene having a high mesopentad fraction as the polyolefin resin I, the dimensional stability at high temperatures becomes high when the film is formed. Although the upper limit of the mesopentad fraction is not particularly specified, it is 0.99 from the viewpoint of use in film formation.

In order to obtain polypropylene having such a high mesopentad fraction, there are methods such as washing the obtained resin powder with a solvent such as n-heptane, selecting a catalyst and/or co-catalyst, and selecting a composition as appropriate. preferably employed. Generally, a polymerization catalyst containing a metallocene compound having a cyclopentadienyl skeleton in its molecule is preferably used as the catalyst.

Further, when the polyolefin resin I is a polypropylene resin, the melt flow rate (MFR) thereof is preferably 1 to 10 g/10 min, more preferably 1, from the viewpoint of film formability and strength when formed into a film. ~8 g/10 min, more preferably 2 to 5 g/10 min. The MFR referred to herein is the MFR measured under conditions of a temperature of 230° C. and a load of 21.18 N, and the same applies to the MFR below. In order to set the melt flow rate (MFR) of the polypropylene resin to the above value, methods such as controlling the average molecular weight and molecular weight distribution are employed. More specifically, the MFR can be increased by reducing the number-average or weight-average molecular weight or molecular weight distribution.

The polyolefin resin I may contain a copolymerization component of an unsaturated hydrocarbon other than the main structural unit, etc., within the range not impairing the object of the present invention. Examples of monomers constituting such copolymer components include ethylene, propylene, 1-butene, 1-pentene, 3-methylpentene-1, 3-methylbutene-1, 1-hexene, 4-methylpentene-1. , 5-ethylhexene-1, 1-octene, 1-decene, 1-dodecene, vinylcyclohexene, styrene, allylbenzene, cyclopentene, norbornene, 5-methyl-2-norbornene and the like. The amount of copolymerization is preferably less than 1 mol % from the viewpoint of dimensional stability when made into a film.

Further, the base layer (A) may contain a polyolefin resin other than the polyolefin resin I which is the main component. The term "polyolefin resin other than polyolefin resin I" as used herein refers to polyolefin resins different from polyolefin resin I in the main structural units. Main structural units of polyolefin resins other than polyolefin resin I include ethylene, propylene, 1-butene, 1-pentene, 3-methylpentene-1, 3-methylbutene-1, 1-hexene, 4-methylpentene-1, 5 -ethylhexene-1, 1-octene, 1-decene, 1-dodecene, vinylcyclohexene, styrene, allylbenzene, cyclopentene, norbornene, 5-methyl-2-norbornene and the like. The content of the polyolefin resin other than polyolefin resin I is preferably less than 10% by mass when all the components constituting the base layer (A) are taken as 100% by mass.

When the polyolefin resin I is a polypropylene resin and contains polyethylene as the polyolefin resin other than the polyolefin resin I, the polyethylene resin contained in the base layer (A) is preferably 10% by mass or less, more preferably 5% by mass. 3% by mass or less, more preferably 3% by mass or less. As the content of the polyethylene resin increases, the crystallinity of the resulting film decreases, so the transparency tends to be improved. On the other hand, by setting the content of the polyethylene resin to 10% by mass or less, it is possible to reduce the deterioration of the strength and heat resistance when the film is made, and the deterioration of the resin during the extrusion process is reduced, and the film is made. The occurrence of fish eyes can also be suppressed.

Next, the polyolefin resin used for the surface layer (B) of the polyolefin film of the present invention will be described. For the surface layer (B) of the present invention, it is preferable to use a resin composition in which the polyolefin resin I contains a branched polypropylene resin. The branched polypropylene resin in the surface layer (B) is preferably 5.0% by mass or more and 30.0% by mass or less, more preferably 10.0% by mass or more and 27.0% by mass or less, of all the components constituting the layer. , more preferably 15.0% by mass or more and 24.0% by mass or less.

Since the branched-chain polypropylene resin functions as a crystal nucleating agent, when the content of the branched-chain polypropylene resin in the surface layer (B) is 5.0% by mass or more, crystals of the polyolefin resin are easily formed on the surface. On the other hand, when the content of the branched-chain polypropylene resin is 30.0% by mass or less, excessive crystal formation on the surface can be suppressed. That is, when the surface layer (B) contains 5.0% by mass or more and 30.0% by mass or less of the branched chain polypropylene resin, the density of the protrusions due to crystal formation on the surface is in an appropriate range, so the Spd of the surface can be a suitable range.

As the branched polypropylene resin, it is preferable to use a polypropylene resin having a branched structure in the molecular chain. The term "polypropylene resin having a branched structure in the molecular chain" refers to a polypropylene resin having 5 or less internal trisubstituted olefins in 10,000 carbon atoms in the molecular chain. can be confirmed by the proton ratio of the ¹ H-NMR spectrum. Moreover, the MFR of the branched polypropylene resin is preferably 1.0 g/10 min or more and 10.0 g/10 min or less from the viewpoint of film forming stability.

Branched polypropylene resins that can be suitably used for the surface layer (B) of the polyolefin film of the present invention include, for example, "Profax" (registered trademark) (PF-814, etc.) manufactured by Lyondell Basell, "Daploy" manufactured by Borealis (trademark) (WB130HMS, WB135HMS, etc.), Japan Polypropylene Corporation "WAYMAX" (registered trademark) (MFX3, MFX6, MFX8, EX6000, EX8000, etc.). The branched-chain polypropylene resin itself has the effect of a crystal nucleating agent for α crystals or β crystals, but other α crystal nucleating agents (e.g., dibenzylidene sorbitols, dibenzylidene sorbitols, sodium benzoate, etc.), β crystal nucleating agents (e.g., potassium 1,2-hydroxystearate, magnesium benzoate, amide compounds such as N,N'-dicyclohexyl-2,6-naphthalene dicarboxamide, quinacridone compounds, etc. ) may be added. These crystal nucleating agents may be added to either the base layer (A) or the surface layer (B), or both.

However, excessive addition of the other types of nucleating agents described above may cause deterioration in transparency and strength due to deterioration in stretchability of the film and formation of voids. , it is usually 0.5% by mass or less, preferably 0.1% by mass or less, and more preferably 0.05% by mass or less.

The polyolefin film of the present invention may contain various additives, such as antioxidants, heat stabilizers, slip agents, antistatic agents, antiblocking agents, fillers, viscosity modifiers, and coloring agents, as long as they do not impair the purpose of the present invention. Inhibitors and the like may also be included. Among these, selection of the type and amount of antioxidant to be added is important from the viewpoint of bleed-out of the antioxidant. Phenol-based antioxidants having steric hindrance are preferred as such antioxidants, and when multiple types of antioxidants are used in combination, at least one of them is preferably of a high molecular weight type with a molecular weight of 500 or more. Various specific examples thereof include 2,6-di-t-butyl-p-cresol (BHT: molecular weight 220.4) and 1,3,5-trimethyl-2,4,6 -tris(3,5-di-t-butyl-4-hydroxybenzyl)benzene (eg BASF "Irganox" ® 1330: molecular weight 775.2) or tetrakis[methylene-3(3,5-di It is preferable to use t-butyl-4-hydroxyphenyl)propionate]methane (for example, “Irganox” (registered trademark) 1010 manufactured by BASF, molecular weight 1177.7) in combination.

The total content of these antioxidants is 0 when the entire raw material for obtaining a polyolefin film is 100% by mass, from the viewpoint of reducing coloring due to deterioration of the polymer and decrease in transparency due to bleeding out of the antioxidant. A range of 0.03 to 1.0% by weight is preferred. When the content of the antioxidant is 0.03% by mass or more, it is possible to reduce coloration of the film due to deterioration of the polymer during the extrusion process and decrease in long-term heat resistance. On the other hand, when the antioxidant content is 1.0% by mass or less, deterioration in transparency due to bleeding out of the antioxidant can be suppressed. From the above viewpoint, the content of the antioxidant is more preferably 0.05 to 0.9% by mass, more preferably 0.1 to 0.8% by mass. These antioxidants may be added to either the base layer (A) or the surface layer (B), or both.

Also, the polyolefin film of the present invention preferably does not contain organic particles and inorganic particles. For example, the polypropylene resin that can be suitably used for the polyolefin film of the present invention has a low affinity for organic particles and inorganic particles. Coarse protrusions formed by particles with a high particle diameter may be transferred as unevenness to the resin layer of the adherend. Therefore, it is preferable that the polyolefin film does not contain a lubricant such as organic particles or inorganic particles when used as a protective film or a substrate film for manufacturing optical members used in products that require high quality such as display members.

The polyolefin film of the present invention is preferably a biaxially oriented film. A biaxially oriented film is a film in which molecules are oriented in two orthogonal directions, and is usually obtained by stretching in two orthogonal directions (for example, the longitudinal direction and the width direction). As the biaxial stretching method, any of the inflation simultaneous biaxial stretching method, the stenter simultaneous biaxial stretching method, and the stenter sequential biaxial stretching method may be employed. It is preferable to adopt the stenter sequential biaxial stretching method from the viewpoint of controlling the high rigidity and dimensional stability of the film. Here, the sequential biaxial stretching method using a stenter refers to a method in which stretching in the longitudinal direction and in the width direction are performed in separate steps, and at least the stretching in the width direction is performed using a stenter.

Next, one aspect of the method for producing a polyolefin film of the present invention will be described using a polypropylene film as a specific example, but it is not necessarily limited to this. First, a polypropylene resin is supplied to a single-screw extruder for the base layer (A), and a mixture of polypropylene resin and branched-chain polypropylene resin is supplied to a single-screw extruder for the surface layer (B). , more preferably 220 to 280°C, more preferably 240 to 270°C. Then, after removing foreign matter, modified polymer, etc. with a filter installed in the middle of the polymer tube, a multi-manifold type composite T die is used to form a structure of surface layer (B) / base layer (A) / surface layer (B). , discharged onto a casting drum, and solidified by cooling to obtain an unstretched sheet having a layer structure of surface layer (B)/base layer (A)/surface layer (B).

At this time, the casting drum has a surface temperature of 40 to 100°C, preferably 50 to 100°C, more preferably 60 to 100°C. As a method of adhering to the casting drum, any of the electrostatic application method, the adhering method using the surface tension of water, the air knife method, the press roll method, and the underwater casting method may be used. The air knife method is preferable from the viewpoint of improving the surface roughness and facilitating the control of the surface roughness. The air temperature of the air knife is preferably 40-80° C., and the blown air velocity is preferably 130-150 m/s. Also, in order to prevent the sheet from vibrating, it is preferable to appropriately adjust the position of the air knife so that the air flows to the downstream side of the film formation. The casting speed is preferably 5 m/min or more and 16 m/min or less, more preferably 8 m/min or more and 14 m/min or less. When the casting speed is 5 m/min or more, the production speed of the polyolefin film can be ensured. On the other hand, since the casting speed is 16 m / min or less, the casting time is sufficiently secured, so that the formation of coarse crystals of the polyolefin resin and the accompanying decrease in Spd and Spc are reduced, so the peel resistance is increased. can be suppressed.

Next, the obtained unstretched sheet is biaxially stretched and biaxially oriented. As specific stretching conditions, first, the unstretched sheet is preheated to a temperature for stretching in the longitudinal direction. As the preheating method, a method using a temperature-controlled rotating roll, a method using a hot air oven, or the like can be employed alone or in combination as appropriate. The film temperature (preheating temperature) when stretching in the longitudinal direction is 130° C. or more and 160° C. or less, preferably 140° C. or more and 160° C., considering the viewpoint of controlling protrusions on the film surface to a preferable range by high temperature stretching. 145° C. or more and 154° C. or less is more preferable. The draw ratio is preferably 3.0 times or more and 6.0 times or less, more preferably 4.0 times or more and 5.7 times or less, in order to control the mechanical properties and thermal properties within a preferable range. It is preferably 4.5 times or more and 5.5 times or less. When the draw ratio is 3.0 times or more, the film can be drawn more uniformly, and thickness unevenness can be suppressed. On the other hand, when the draw ratio is 6.0 times or less, film breakage in the longitudinal drawing step and the subsequent transverse drawing step is reduced.

Next, the longitudinally uniaxially stretched film obtained by stretching in the longitudinal direction is once cooled to 10°C or higher and 70°C or lower. When the cooling temperature is 10° C. or higher, curling of the film can be suppressed. On the other hand, when the cooling temperature is 70° C. or less, promotion of thermal crystallization can be suppressed. After that, the film is introduced into a tenter, both ends in the width direction thereof are gripped with clips, preheated, and then transversely stretched 7.0 to 13 times in the width direction. Considering the viewpoint of controlling the protrusions on the film surface to a preferable range by high-temperature stretching, the preheating and stretching temperatures are 156 to 166°C, more preferably 159 to 165°C, and still more preferably 160 to 164°C. Further, the heat treatment may be performed in the tenter as it is. At this time, in order to control the heat shrinkage rate in the width direction, the heat treatment temperature is preferably 110 ° C. or higher and lower than 170 ° C., and 150 ° C. or higher and 165 ° C. or lower. It is more preferable to have In addition, the heat treatment may be performed while relaxing the film in the width direction. By doing so, the heat shrinkage rate in the width direction can be set to an appropriate range, and the balance of dimensional stability can be optimized.

The polyolefin film obtained as described above can be used in various applications such as packaging films, surface protection films, process films, sanitary products, agricultural products, construction products, and medical products. Since it is excellent, it can be preferably used as a surface protective film, a process film, and a release film.

The polyolefin film of the present invention will be described in more detail below with reference to examples, but the polyolefin film is not limited to such embodiments. The properties were measured and evaluated by the following methods.

(1) Film Thickness and Thickness Variation A measurement sample was prepared by stacking five polyolefin films, and the thickness of the measurement sample was measured using an electronic micrometer (TT80 manufactured by TESA). Then, the obtained value was divided by 5 to calculate the thickness per sheet of polyolefin film. Further, the same measurement was performed using a position shifted by 50 mm in the width direction as a measurement point, and the same measurement was repeated a total of 5 times. From the obtained measurement results, the maximum value, minimum value, and average value of the thickness per sheet of polyolefin film were determined, and the average value was defined as the film thickness (t: μm), and the thickness unevenness (%) was determined from the following formula.
Thickness unevenness (%)=((maximum thickness value−minimum thickness value)/average thickness value)×100.

(2) Mountain peak density (Spd), arithmetic mean curvature of peak points (Spc), average roughness (Sa)
Using a non-contact surface/layer cross-sectional shape measuring system “VertScan” (registered trademark) 2.0 (model: R3300GL-Lite-AC) manufactured by Ryoka Systems Co., Ltd., measurements were made under the following procedures and conditions. First, a polyolefin film is unwound from a film roll, and measurement samples are taken so that 10 points are measured at random along a straight line passing through the center in the width direction and parallel to the longitudinal direction. Spd, Spc, Sa were measured. The average value of each measurement value obtained was calculated and used as Spd, Spc, and Sa of the polyolefin film. In one measurement, one field of view (viewing area: vertical 939 μm×horizontal 1,252 μm=1,175,628 μm ² ) was measured.

A. Measurement conditions CCD camera: SONY HR-57 1/2
Objective lens: 10X
Lens barrel: 0.5X BODY
Wavelength filter: 530 white
Measurement mode: Wave
Field of view size: 640 x 480
Scan range: (start) 5 μm, (stop) −5 μm.

B. Fixing method of the measurement sample A dedicated sample holder was used to fix the measurement sample. The sample holder is two detachable metal plates with a circular hole in the center, and the measurement sample is clamped and fixed without wrinkles between them, and the measurement sample positioned in the circular hole is measured.

C. Analysis Method The data obtained by the above measurements were analyzed with the image analysis software VS-Viewer of "VertScan" (registered trademark) 2.0. First, noise was removed by a median filter (5×5), and undulation components were removed by a Gaussian filter with a cutoff value of 250 μm. Next, Spd, Spc, and Sa defined in ISO25178 (2012) were measured using the "ISOPara" function. In the "ISOPara" function, the S-Filter was set to 6.0 μm.

(3) Evaluation of Releasability from Adhesive Tape Polyester Adhesive Tape NO. 31B was pasted with a roller and cut into a width of 19 mm to prepare a sample. The sample was peeled using a tensile tester under the conditions of a peel speed of 300 mm/min and a peel angle of 180°, and the peel force N (N/19 mm) was measured and evaluated according to the following criteria (○, △ 1 and Δ2, it can be used as a release film without any problem, and these are accepted.). The polyester adhesive tape was attached to the inner surface of the film roll.
◯: No delamination occurred between the surface layer and the base layer, and delamination was possible at a constant rate.
Δ1: No delamination occurred between the surface layer and the base layer, but the peeling resistance was slightly strong and the peeling was unstable.
Δ2: The adhesion between the adhesive tape and the surface layer was insufficient, and although delamination did not occur, the peeling resistance was slightly weak and the peeling was unstable.
x: Delamination occurred between the surface layer and the base layer, or the delamination was very severe, and a delamination mark remained on the surface of the adherend.

(4) Friction coefficient Under the specified environment (temperature: 23 ± 3 ° C, humidity: 65 ± 5%), the temperature and humidity of the test piece (measurement reference length: width 75 mm × length 100 mm) They were superimposed so as to be in contact with each other and set in a measuring device (slip tester). A load of 200 g was placed on the two superimposed test pieces, and the resistance value was measured by sliding the upper test piece with a measuring instrument, and the friction coefficient was calculated by the following equation.
Friction coefficient = resistance value (g)/load (g).

(5) Product Roll Winding Appearance After winding the product roll, wrinkles and winding misalignment in the product roll were visually observed and evaluated according to the following criteria. In addition, when the evaluation result was ○ or △, it was regarded as a pass.
◯: No wrinkles or winding misalignment occurred in the product roll.
Δ: Wrinkles and winding misalignment occurred in the product roll, but they were at a minor level with no actual damage in processing.
x: Wrinkles and winding misalignment occurred in the product roll, which was at an excessive level causing actual damage in processing.

(6) Scratches on Film Surface First, a film sample sampled in a square shape of 1 m (longitudinal direction)×1 m (width direction) was prepared. Then, in a darkroom, one side (arbitrary) of the film sample was irradiated with light from an LED light (EB-10KM manufactured by OHM Co., Ltd.) of 2000 lx while changing the incident angle in the range of 30° to 90°. While irradiating with LED light, the film sample was observed from the irradiated surface side, and visually confirmed flaws were sampled. After that, the sampled scratches were observed with a microscope, and scratches having a length of 1 mm or more and a width of 0.1 mm or less were counted. The same measurement was repeated five times with different film samples, and the average number of scratches obtained was determined and evaluated according to the following criteria.
◯: Number of scratches≦10/m ² .
Δ: 10/m ² <number of scratches≦30/m ² .
x: Number of scratches >30/m ² .

(7) N/t
It was calculated by dividing the peel force N (N/19 mm) measured by the method described in (3) by the film thickness t (μm) measured by the method described in (1).

[material]
The following raw materials were used to produce the polyolefin films of the Examples and Comparative Examples.

(1) Resin Polyolefin resin I (homopolypropylene resin): manufactured by Prime Polymer Co., Ltd., a highly stereoregular homopolymer having an MFR of 2.9 g/10 min, a melting point of 164°C, and a mesopentad fraction of 0.94. Polypropylene resin.
Branched chain polypropylene resin: "WAYMAX" (registered trademark) (MFX3) manufactured by Nippon Polypropylene Co., Ltd. A branched chain polypropylene resin having an MFR of 9.0 g/10 minutes.
Polymethylpentene resin (PMP): "TPX" (registered trademark) MX004 manufactured by Mitsui Chemicals, Inc.
Ethylene/propylene random copolymer: ethylene/propylene random copolymer manufactured by Sumitomo Chemical Co., Ltd., having an MFR of 4.0 g/10 min and an ethylene content of 1.1% by mass.

(2) Antioxidant Antioxidant 1: "Irganox" (registered trademark) 1010 manufactured by BASF Japan.
Antioxidant 2: 2,6-di-t-butyl-p-cresol (BHT) manufactured by ADEKA.

(Example 1)
As a raw material for the base layer (A), a polyolefin resin I is supplied to a single screw extruder, and as a raw material for the surface layer (B), 80 parts by mass of the polyolefin resin I and 20 parts by mass of a branched chain polypropylene resin are dry-blended. It was fed to another single screw extruder and melt extruded at 260° C. respectively. Next, after removing foreign matter with a 20 μm cut sintered filter, the surface layer (B) / base layer (A) / surface layer (B) was adjusted to a thickness ratio of 1/8/1 with a feed block type composite T die. Each raw material was laminated and formed into a sheet, discharged to a casting drum whose surface temperature was controlled at 90° C., and brought into close contact with the casting drum by an air knife. After that, the non-cooled drum surface of the sheet on the casting drum was cooled by jetting compressed air to obtain an unstretched sheet. Subsequently, the unstretched sheet was preheated to 155°C using transport rolls, stretched 5.0 times in the longitudinal direction between rolls provided with a peripheral speed difference, and after stretching, the uniaxially stretched film was cooled with rolls at 40°C. . Next, the uniaxially stretched film was gripped with clips at both ends in the width direction, introduced into a tenter type stretching machine, preheated at 162 ° C. for 3 seconds, stretched 8.8 times at 162 ° C., and 10.5% in the width direction. A heat treatment was performed at 163° C. while giving a relaxation of . After that, the film was passed through a cooling process at 135° C., led to the outside of the tenter-type stretching machine, the clips at the ends in the width direction were released, and the inner surface of the roll was subjected to a corona discharge treatment. Subsequently, both ends in the width direction (portions gripped by clips) were cut off with a slitter, and the film was wound up with a winder to obtain a biaxially oriented polypropylene film with a thickness of 15 μm. Table 1 shows the physical properties and evaluation results of the obtained biaxially oriented polypropylene film.

(Examples 2-8, Comparative Examples 1-5)
A biaxially oriented polypropylene film having a thickness shown in Table 1 was obtained in the same manner as in Example 1 except that the layer structure, the composition of each layer, and the film forming conditions were as shown in Table 1. In the example of a single film, one single screw extruder is used, and the thickness is adjusted by controlling the resin discharge amount by the number of rotations of the extruder, and the casting speed, longitudinal draw ratio, and transverse draw ratio. was performed by controlling the Table 1 shows the physical properties of the obtained film and the evaluation results for each item.

Since Comparative Example 5 has a single-layer structure, the layer composition is described assuming that only a base layer exists without a surface layer.

As described above, the polyolefin film of the present invention is excellent in releasability and quality, so it can be suitably used as a release film or a process film, and is particularly suitable as a release film such as a cover film for an adhesive resin layer. be. Furthermore, since the polyolefin film of the present invention is excellent in surface smoothness, it can be preferably used as a release film and a process film for applications requiring surface smoothness of products. The polyolefin film of the present invention having the above properties can also be used in various applications such as packaging films, process films, sanitary goods, agricultural goods, construction goods, and medical goods.

Claims (5)

A peak density Spd of 100 (1/mm ² ) or more and 1000 (1/mm ² ) or less, and an arithmetic mean curve Spc of the peak points of −200 (1/mm) or more and −10 (1/mm) or less A polyolefin film, wherein at least one of the surfaces is the A surface when the surface of the polyolefin film is the A surface. 2. The polyolefin film according to claim 1, wherein the average roughness Sa of at least one A surface is 200 nm or more and 600 nm or less. When the film thickness is t (μm), the peel force when a polyester adhesive tape is laminated on the A side and peeled at a peel speed of 300 mm / min and a peel angle of 180 ° is N (N / 19 mm). , N/t is 0.06 or more and 0.20 or less. 3. The polyolefin film according to claim 1, which has a laminated structure of at least two layers. A release film comprising the polyolefin film according to claim 1 or 2.