JP7470060B2 - Release film and method for producing release film - Google Patents

Description

The present invention relates to a release film and a method for producing a release film.

2. Description of the Related Art Release films are used in the manufacturing processes of printed wiring boards, flexible circuit boards, multilayer printed wiring boards, and the like.
In the manufacturing process of a flexible circuit board, a coverlay film is heat-pressed to a flexible circuit board body having a copper circuit formed thereon via a thermosetting adhesive or a thermosetting adhesive sheet. At this time, by disposing a release film between the coverlay film and the heat-press plate, it is possible to prevent the coverlay film and the heat-press plate from adhering to each other, and also to prevent problems such as the adhesive seeping out and interfering with the plating process of the electrode portion.

Release films are required to have releasability that allows them to be easily peeled off after heat press bonding. In order to improve the releasability, for example, the crystallinity of the release film has been adjusted. Patent Document 1 describes a release film that has a release layer containing a polyester resin on at least one side, and the crystallinity of the release layer is 10% or more and 50% or less.

JP 2016-2730 A

In recent years, as flexible circuit boards become thinner, further improvement in releasability is required for release films. In addition, in recent years, the manufacture of flexible circuit boards has also been automated using the roll-to-roll (RtoR) method and the like. In the RtoR method, the flexible circuit board body and the release film, etc., unwound from the roll are transported between heat press plates, heat press bonded, and then wound up on the roll again. In such an RtoR method, when the release film is peeled off from the flexible circuit board after heat press bonding, the peel angle tends to be low. Therefore, when a conventional release film is used, a larger force may have to be applied during peeling, which may lead to the occurrence of defects. Therefore, further improvement in releasability is required for the release film.
In addition, the release film may entrap air bubbles at the interface during press processing, causing wrinkles, which may lead to defective products. Conventionally, it has been difficult to realize a release film that has excellent releasability as described above and can suppress the occurrence of wrinkles (has wrinkle resistance).

The present invention aims to provide a release film that has better release properties and excellent wrinkle resistance than conventional release films, and a method for producing the release film.

A release film which is one embodiment of the present invention is a release film having at least one release layer, wherein the release layer has a degree of crystallinity of 50% or more as determined by oblique incidence wide-angle X-ray diffraction method with an incidence angle of 0.06°, and the arithmetic mean roughness Ra of the surface of the release layer is 2.0 μm or more.
The release film according to the present invention will be described in detail below.

The inventors have discovered that in a release film having at least one release layer, selectively increasing the crystallinity of only an extremely thin region (extreme surface) of the surface of the release layer, rather than the crystallinity of the entire release layer, can dramatically improve the release properties without impairing conformability to unevenness. They have also discovered that selectively increasing the crystallinity of only the extreme surface of the release layer and then performing an embossing treatment can achieve wrinkle resistance while expressing and maintaining excellent release properties, and have thus completed the present invention.

The release film of the present invention has at least one release layer.
The release layer has a crystallinity of 50% or more as determined by oblique incidence wide-angle X-ray diffraction method (In-Plane method) with an incidence angle of 0.06°.
By setting the incidence angle to 0.06° in the oblique incidence wide-angle X-ray diffraction method, the crystallinity of only the extreme surface of the release layer can be measured, rather than the crystallinity of the entire release layer. The extreme surface refers to an extremely thin region of the surface, more specifically, a region from the surface to a thickness of about 4 nm. When the crystallinity of the extreme surface of the release layer is 50% or more, the adhesive formed on the coverlay film during heat press bonding can be sufficiently suppressed from penetrating into the release layer. That is, the depth to which the adhesive penetrates into the release layer can be made shallow, and the anchor effect of the adhesive can be suppressed, so that the releasability of the release film is greatly improved. It is sufficient that at least one surface of the release layer has such a crystallinity of the extreme surface. The crystallinity of the extreme surface of the release layer is preferably 60% or more, more preferably 65% or more.

The upper limit of the degree of crystallinity of the extreme surface of the release layer is not particularly limited, but a preferred upper limit is 90%. If the degree of crystallinity of the extreme surface of the release layer is 90% or less, cracks are less likely to occur in the crystalline phase of the extreme surface of the release layer during heat press bonding, and deterioration of releasability can be suppressed. A more preferred upper limit of the degree of crystallinity of the extreme surface of the release layer is 80%.

The degree of crystallinity of the extreme surface of the release layer can be determined by drawing a baseline on the diffraction measurement plot obtained by analyzing the surface of the release layer using oblique incidence wide-angle X-ray diffraction with an X-ray incidence angle of 0.06°, fitting to the crystalline phase and the amorphous phase, respectively, and calculating the crystallinity from the obtained total peak area of the crystalline phase and the total peak area of the amorphous phase using the following formula (1).

As an X-ray diffraction apparatus for determining the degree of crystallinity of the outermost surface of the release layer, for example, a multi-function X-ray diffraction apparatus for surface structure evaluation (ATX-G type) manufactured by Rigaku Corporation set under the following conditions can be used.
X-ray source CuKα tube voltage-tube current 50kV-300mA
Incident optical system: Concentrated method Incident angle (ω): 0.06°
Measurement range: 5-70°
Measurement interval: 0.02°
Scanning speed: 1.0°/min
Scanning method: In-Plane method

The method of adjusting the crystallinity of the extreme surface of the release layer to the above range is not particularly limited, but it is preferable to adjust the arithmetic mean roughness Ra of the release layer before the surface treatment to be small and/or adjust the glossiness of the release layer before the surface treatment to be large, and then perform surface treatment on the surface of the release layer. By adjusting the arithmetic mean roughness Ra of the release layer before the surface treatment to be small and/or adjusting the glossiness of the release layer before the surface treatment to be large, the effect of increasing the crystallinity by performing surface treatment on the surface of the release layer is greatly improved, and the crystallinity of the extreme surface of the release layer can be adjusted to the above range. In particular, the present inventors have found that rubbing the surface after adjusting the surface of the original film of the release layer before the rubbing treatment to be sufficiently smooth is effective in reliably increasing the crystallinity of the extreme surface.
The method for adjusting the arithmetic mean roughness Ra of the release layer before the surface treatment to be small and/or the glossiness of the release layer before the surface treatment to be large is not particularly limited, but when melt-extruding the resin constituting the release layer and cooling the molten resin, it is preferable to adopt, for example, the following method. That is, it is preferable to use a cooling roll having a smoother surface and transfer the roll surface shape to the film, or to adjust the elongation stress applied to the molten resin during cooling to be large.

By reducing the arithmetic mean roughness Ra of the release layer before the surface treatment, the crystallinity of the extreme surface of the release layer can be adjusted to the above range. The arithmetic mean roughness Ra of the release layer before the surface treatment can be, for example, 0.50 μm or less.
In addition, the arithmetic mean roughness Ra of the release layer before the surface treatment needs only to be small during the surface treatment. Even if the arithmetic mean roughness Ra increases after the surface treatment as a result of embossing the surface of the release layer as described below, the crystallinity of the outermost surface itself is not significantly affected.

By increasing the glossiness of the release layer before the surface treatment, the crystallinity of the extreme surface of the release layer can be adjusted to the above range. The glossiness of the release layer before the surface treatment can be, for example, 100% or more.
In addition, the glossiness of the release layer before the surface treatment needs to be high at the time of the surface treatment. If the glossiness is reduced after the surface treatment as a result of embossing the surface of the release layer as described below, the crystallinity of the outermost surface itself is not significantly affected.

The reason why the effect of increasing the crystallinity by the surface treatment is improved by adjusting the arithmetic mean roughness Ra or the glossiness before the surface treatment is not clear, but it can be estimated as follows. When the arithmetic mean roughness Ra is relatively large, the influence of the surface treatment on the extreme surface may vary, but when the arithmetic mean roughness Ra is relatively small, the influence of the surface treatment on the extreme surface is uniform, and it is considered that the probability and amount of carbonyl groups on the extreme surface that penetrate into the surface are improved. For example, when performing surface treatment by friction treatment, if the arithmetic mean roughness Ra is relatively large, some physical property changes or uniform treatment may be hindered by the unevenness of the release layer surface. On the other hand, when the arithmetic mean roughness Ra is sufficiently small, it is considered that the physical property changes or uniform treatment hindered by the unevenness of the release layer surface are small, and the surface can be sufficiently changed by friction treatment.
In addition, since the glossiness is affected by the surface roughness of the object and the size of the crystal grains inside, a high glossiness means a low surface roughness and small crystal grains (a certain size or less). If the crystal grains before the surface treatment are relatively large, the multiple crystal grains become three-dimensional obstacles to each other, hindering the growth of crystals due to the surface treatment. On the other hand, if the crystal grains before the surface treatment are relatively small, the growth of crystals is promoted by the surface treatment without the above-mentioned three-dimensional obstacles, and as a result, it is considered that the effect of increasing the crystallinity of the extreme surface is improved. In addition, it can be considered that the size of the crystal grains is affected by the cooling and elongation stress during melt extrusion.

The arithmetic mean roughness Ra of the surface of the release layer is 2.0 μm or more. The lower limit of the arithmetic mean roughness Ra of the surface of the release layer is preferably 3.0 μm, more preferably 3.5 μm, and particularly preferably 5.0 μm. By having the arithmetic mean roughness Ra of the surface of the release layer in the above range, it is possible to obtain a release film with excellent wrinkle resistance. The upper limit of the arithmetic mean roughness Ra of the surface of the release layer is not particularly limited, but is preferably 10 μm, more preferably 8 μm.
The arithmetic mean roughness Ra of the surface of the release layer is the arithmetic mean roughness Ra in accordance with JIS B 0601:2013, and can be measured, for example, using a Surftest SJ-301 manufactured by Mitutoyo Corporation. The arithmetic mean roughness Ra of the surface of the release layer can be affected by the conditions during film formation, but if it is desired to actively change it, it is necessary to carry out an embossing process or a smoothing process separately. In addition, when a process such as a hot press (press annealing) is added, the unevenness of the surface is crushed, so that the value of the arithmetic mean roughness Ra generally tends to be smaller.

The surface glossiness of the release layer is not particularly limited, but the preferred upper limit is 100%, more preferably 50%, even more preferably 25%, and particularly preferably 10%. The surface glossiness of the release layer is within the above range, which makes it easier to improve the wrinkle resistance of the release film. The lower limit of the surface glossiness of the release layer is not particularly limited, but is substantially 5% or more.
The surface gloss of the release layer is measured at an incidence angle of 60° in accordance with JIS Z8741, and can be measured, for example, using a gloss meter VG-1D manufactured by Nippon Denshoku Industries Co., Ltd. The surface gloss of the release layer can be affected by the conditions during film formation, but if it is desired to actively change it, it is necessary to carry out an embossing process or a smoothing process separately. In addition, when a process such as a hot press (press annealing) is added, the unevenness of the surface is crushed, so that the gloss value generally tends to be large.

The surface treatment is not particularly limited, and examples thereof include friction treatment, heat treatment, uniaxial stretching or biaxial stretching, etc. These surface treatments may be used alone or in combination of two or more kinds.
The method of the friction treatment is not particularly limited, but a method of performing the friction treatment using a friction treatment device (for example, a polishing treatment device manufactured by Yamagata Kikai Co., Ltd., model YCM-150M) and using a woven fabric as the surface material of the friction treatment material is preferred.
The method of the heat treatment is not particularly limited, but a method in which the film is passed between rolls heated to a certain temperature, a method in which the film is heated by a heater, and the like are preferable.
The method for the uniaxial or biaxial stretching treatment is not particularly limited, but a method in which the formed film is stretched at a constant temperature is preferred.

The degree of crystallinity of the entire release layer is not particularly limited, but is preferably smaller than the degree of crystallinity of the extreme surface of the release layer. The preferred lower limit of the degree of crystallinity of the entire release layer is 25%, and the preferred upper limit is 50%.
If the crystallinity of the entire release layer is increased more than necessary, the flexibility of the entire release film is reduced, and the ability to follow unevenness is reduced, which may cause voids to occur during hot press bonding or increase the width of the adhesive seeping out. By adjusting the crystallinity of the release layer other than the extreme surface, the release layer can have a high crystallinity of 50% or more at the extreme surface while the release layer as a whole has a moderate crystallinity within the above range. With this configuration, the release film is further excellent in both releasability and ability to follow unevenness. If the crystallinity of the entire release layer is 25% or more, the heat resistance of the release film is improved. If the crystallinity of the entire release layer is 50% or less, the ability to follow unevenness of the release film is improved. The lower limit of the crystallinity of the entire release layer is more preferably 30%, and even more preferably 35%. The upper limit of the crystallinity of the entire release layer is more preferably 45%, even more preferably 40%, and particularly preferably 35%.

The crystallinity of the entire release layer can be determined by drawing a baseline on a diffraction measurement plot obtained by analyzing the entire release layer using wide-angle X-ray diffraction, fitting the crystalline phase and the amorphous phase, and calculating the crystallinity of the entire release layer from the total peak area of the crystalline phase and the total peak area of the amorphous phase using the following formula (2). When the release film is made of multiple layers, the layers of the release film can be peeled off and a sample consisting of only the release layer can be analyzed to evaluate the crystallinity of the entire release layer.

As an X-ray diffraction apparatus for determining the crystallinity of the entire release layer, for example, a horizontal sample X-ray diffraction apparatus for thin film evaluation (Smart Lab) manufactured by Rigaku Corporation set under the following conditions can be used.
X-ray source CuKα tube voltage-tube current 45kV-200mA
Incident optical system: Concentration method Measurement range: 5-80°
Measurement interval: 0.02°
Scanning speed: 5.0°/min
Scanning method: Out-of-Plane method

The method for adjusting the crystallinity of the entire release layer to the above range is not particularly limited, but when melt-extruding the resin constituting the release layer and cooling the molten resin, it is preferable to adopt, for example, the following method. That is, a method of adjusting the contact time between the molten resin and a cooling roll, a method of adjusting the cooling roll temperature, etc. are preferable. By adjusting the temperature gradient between the surface and the inside of the release layer in this way, the rate of crystallization on the surface and inside of the release layer can be adjusted, and the crystallinity of the entire release layer can be adjusted. The crystallinity of the entire release layer can also be adjusted to a value higher than the crystallinity of the extreme surface of the release layer.

The resin constituting the release layer is not particularly limited, but polyester, polyolefin or polystyrene is preferred because it improves the releasability of the release film.
The polyester preferably contains an aromatic polyester resin. The polyolefin preferably contains poly(4-methyl-1-pentene) or an alicyclic olefin resin. The polystyrene preferably contains a polystyrene resin having a syndiotactic structure. In particular, the release layer more preferably contains an aromatic polyester resin, since it has excellent conformability to irregularities and excellent resistance to bleeding of the adhesive formed on the coverlay film.

The aromatic polyester resin is not particularly limited, but crystalline aromatic polyester resin is preferred.Specific examples include polyethylene terephthalate resin, polybutylene terephthalate resin, polyhexamethylene terephthalate resin, polyethylene naphthalate resin, polybutylene naphthalate resin, terephthalic acid butanediol polytetramethylene glycol copolymer, etc.These aromatic polyester resins may be used alone or in combination of two or more.Among them, polybutylene terephthalate resin is preferred from the viewpoint of balance of heat resistance, releasability, conformability to unevenness, etc.
Also preferred is a mixed resin of a polybutylene terephthalate resin and a block copolymer of polybutylene terephthalate and an aliphatic polyether. The aliphatic polyether is not particularly limited, and examples thereof include polyethylene glycol, polydiethylene glycol, polypropylene glycol, and polytetramethylene glycol.

From the viewpoint of film formability, the aromatic polyester resin preferably has a melt volume flow rate of 30 cm ³ /10 min or less, more preferably 20 cm ³ /10 min or less. The melt volume flow rate can be measured in accordance with ISO 1133 at a measurement temperature of 250° C. and a load of 2.16 kg.

Among the above aromatic polyester resins, commercially available ones include, for example, "Pelprene (registered trademark)" (manufactured by Toyobo Co., Ltd.), "Hytrel (registered trademark)" (manufactured by DuPont-Toray Co., Ltd.), "Duranex (registered trademark)" (manufactured by Polyplastics Co., Ltd.), and "NovaDuran (registered trademark)" (manufactured by Mitsubishi Engineering Plastics Corporation).

The poly(4-methyl-1-pentene)-containing polyolefin preferably contains 90% by weight or more of poly(4-methyl-1-pentene) resin.
As the poly(4-methyl-1-pentene) resin, for example, a commercially available product such as TPX (registered trademark) manufactured by Mitsui Chemicals, Inc. can be used.

The alicyclic olefin resin is an olefin resin having a cyclic aliphatic hydrocarbon in the main chain or side chain, and from the standpoint of heat resistance, strength, etc., thermoplastic saturated norbornene resin is preferred.
Examples of the thermoplastic saturated norbornene resin include resins obtained by hydrogenating a ring-opening polymer or a ring-opening copolymer of a norbornene monomer (after performing modification such as addition of maleic acid or addition of cyclopentadiene, if necessary). Other examples include resins obtained by addition polymerization of a norbornene monomer, resins obtained by addition polymerization of a norbornene monomer and an olefin monomer such as ethylene or an α-olefin, and resins obtained by addition polymerization of a norbornene monomer and a cyclic olefin monomer such as cyclopentene, cyclooctene, or 5,6-dihydrodicyclopentadiene. Modified products of these resins are also included.

The polystyrene containing the polystyrene resin having a syndiotactic structure preferably contains 70% by weight or more and 90% by weight or less of the polystyrene resin having a syndiotactic structure.
The polystyrene resin having a syndiotactic structure is a resin having a stereoregular structure in which phenyl groups and substituted phenyl groups, which are side chains, are alternately positioned in opposite directions relative to the main chain formed from carbon-carbon sigma bonds.

The polystyrene resin having the syndiotactic structure is not particularly limited. For example, polystyrene having a syndiotacticity of 75% or more in racemic diads or 30% or more in racemic pentads, poly(alkylstyrene), poly(arylstyrene), poly(halogenated styrene), poly(halogenated alkylstyrene), poly(alkoxystyrene), poly(vinyl benzoate ester), etc. may be mentioned. In addition, hydrogenated polymers of these, mixtures of these, copolymers containing these as main components, etc. may be mentioned. The polystyrene resin having the syndiotactic structure may be, for example, a commercially available product such as XAREC (registered trademark) manufactured by Idemitsu Kosan Co., Ltd.

The release layer may contain a mixed resin containing polybutylene terephthalate resin and an elastomer. The elastomer is not particularly limited, and may be, for example, a block copolymer of polybutylene terephthalate and an aliphatic polyether. The aliphatic polyether is not particularly limited, and may be, for example, polyethylene glycol, polydiethylene glycol, polypropylene glycol, polytetramethylene glycol, etc.
The proportion of the polybutylene terephthalate resin in the resin constituting the release layer is not particularly limited, but is preferably 75% by weight or more. If the proportion of the polybutylene terephthalate resin is 75% by weight or more, the release property of the release film is improved. A more preferable lower limit of the proportion of the polybutylene terephthalate resin in the resin constituting the release layer is 80% by weight.

The release layer may contain a rubber component. When the release layer contains a rubber component, the ability to conform to the irregularities of the release film is improved.
The rubber component is not particularly limited, and examples thereof include natural rubber, styrene-butadiene copolymer, polybutadiene, polyisoprene, acrylonitrile-butadiene copolymer, ethylene-propylene copolymer (EPM, EPDM), polychloroprene, butyl rubber, acrylic rubber, silicone rubber, urethane rubber, etc. Examples of the rubber component include olefin-based thermoplastic elastomers, styrene-based thermoplastic elastomers, vinyl chloride-based thermoplastic elastomers, ester-based thermoplastic elastomers, amide-based thermoplastic elastomers, etc.

The release layer may contain a stabilizer.
The stabilizer is not particularly limited, and examples thereof include hindered phenol-based antioxidants and heat stabilizers.
The hindered phenol-based antioxidant is not particularly limited, and examples thereof include 1,3,5-trimethyl-2,4,6-tris(3,5-di-t-butyl-4-hydroxybenzyl)benzene, 3,9-bis{2-[3-(3-t-butyl-4-hydroxy-5-methylphenyl)-propionyloxy]-1,1-dimethylethyl}-2,4,8,10-tetraoxaspiro[5,5]undecane, and the like. The heat stabilizer is not particularly limited, and examples thereof include tris(2,4-di-t-butylphenyl)phosphite, trilauryl phosphite, 2-t-butyl-α-(3-t-butyl-4-hydroxyphenyl)-p-cumenylbis(p-nonylphenyl)phosphite, dimyristyl 3,3'-thiodipropionate, distearyl 3,3'-thiodipropionate, pentaerythryl tetrakis(3-laurylthiopropionate), ditridecyl 3,3'-thiodipropionate, and the like.

The release layer may further contain conventional additives such as fibers, inorganic fillers, flame retardants, ultraviolet absorbers, antistatic agents, inorganic substances, and salts of higher fatty acids.

The thickness of the release layer is not particularly limited, but the preferred lower limit is 10 μm, and the preferred upper limit is 40 μm. If the thickness of the release layer is 10 μm or more, the heat resistance of the release film is improved. If the thickness of the release layer is 40 μm or less, the conformability of the release film to the unevenness is improved. The more preferred lower limit of the thickness of the release layer is 15 μm, and the more preferred upper limit is 30 μm.
The thickness of the release layer may be 1 μm or less. By making the thickness of the release layer extremely thin, 1 μm or less, the flexibility of the release film can be increased and the followability can be improved. As described above, the release layer can exhibit high releasability by making the crystallinity determined by the oblique incidence wide-angle X-ray diffraction method with an incidence angle of 0.06° to be 50% or more, but regardless of the crystallinity of the release layer, the thickness of the release layer may be 1 μm or less.

The release film of the present invention may be a single-layer structure consisting only of the above-mentioned release layer, or it may be a multi-layer structure having layers other than the above-mentioned release layer.

The release film of the present invention preferably further comprises a cushion layer, which improves the ability of the release film to conform to unevenness.
When the release film of the present invention has the cushion layer, it is sufficient that the release film has at least one release layer and a cushion layer, and may have a two-layer structure or a three-layer structure or more. In particular, it is preferable that the release film has a structure in which a release layer is provided on both sides of the cushion layer. In this case, the release layers on both sides may have the above-mentioned crystallinity of the extreme surface, or only the release layer on one side may have the above-mentioned crystallinity of the extreme surface. In addition, the release layers on both sides may have the same resin composition or different resin compositions. In addition, the release layers on both sides may have the same thickness or different thicknesses.
Furthermore, the release film of the present invention may have a structure in which the release layer and the cushion layer are in direct contact with each other and integrated together, or may have a structure in which the release layer and the cushion layer are integrated together via an adhesive layer.

The resin constituting the cushion layer is not particularly limited, but it is preferable that the cushion layer contains the resin constituting the release layer.
The cushion layer contains the resin constituting the release layer, so that the adhesion between the release layer and the cushion layer is improved. The cushion layer more preferably contains the main component resin of the release layer, and further preferably contains the main component resin of the release layer and a polyolefin resin. Here, the main component resin of the release layer means the resin with the highest content among the resins contained in the release layer.

The content of the resin constituting the release layer in the cushion layer is not particularly limited, but the preferred lower limit is 10% by weight, and the preferred upper limit is 50% by weight. If the content of the resin constituting the release layer is 10% by weight or more, the adhesion between the release layer and the cushion layer is improved. If the content of the resin constituting the release layer is 50% by weight or less, the flexibility of the cushion layer is sufficient, and the ability to follow the unevenness of the release film is improved. A more preferred lower limit of the content of the resin constituting the release layer is 20% by weight, and an even more preferred lower limit is 25% by weight. A more preferred upper limit of the content of the resin constituting the release layer is 40% by weight, and an even more preferred upper limit is 35% by weight.

The polyolefin resin is not particularly limited, and examples thereof include polyethylene resins (e.g., high-density polyethylene, low-density polyethylene, linear low-density polyethylene), polypropylene resins, ethylene-vinyl acetate copolymers, etc. Also included are ethylene-acrylic monomer copolymers such as ethylene-methyl methacrylate copolymers, ethylene-ethyl acrylate copolymers, and ethylene-acrylic acid copolymers. These polyolefin resins may be used alone, or two or more types may be used in combination. Among these, polypropylene resins are preferred because they easily achieve both the ability to conform to uneven surfaces and heat resistance.

The content of the polyolefin resin in the cushion layer is not particularly limited, but the preferred lower limit is 50% by weight and the preferred upper limit is 90% by weight. If the content of the polyolefin resin is 50% by weight or more, the flexibility of the cushion layer is sufficient and the ability to follow the unevenness of the release film is improved. If the content of the polyolefin resin is 90% by weight or less, the adhesion between the release layer and the cushion layer is improved. A more preferred lower limit of the content of the polyolefin resin is 60% by weight, and an even more preferred lower limit is 65% by weight. A more preferred upper limit of the content of the polyolefin resin is 80% by weight, and an even more preferred upper limit is 75% by weight.

The cushion layer may further contain a resin such as polystyrene, polyvinyl chloride, polyamide, polycarbonate, polysulfone, or polyester.
The cushion layer may further contain additives such as fibers, inorganic fillers, flame retardants, ultraviolet absorbing agents, antistatic agents, inorganic substances, and salts of higher fatty acids.

The cushion layer may be a single-layer structure consisting of a single layer, or a multi-layer structure consisting of a laminate of multiple layers. When the cushion layer has a multi-layer structure, the multiple layers may be laminated together via an adhesive layer.

The thickness of the cushion layer is not particularly limited, but a preferred lower limit is 15 μm and a preferred upper limit is 200 μm. If the thickness of the cushion layer is 15 μm or more, the ability to conform to the unevenness of the release film is improved. If the thickness of the cushion layer is 200 μm or less, exudation of resin from the cushion layer at the end of the film during heat press bonding can be suppressed. A more preferred lower limit of the thickness of the cushion layer is 30 μm and a more preferred upper limit is 150 μm.

When the cushion layer is in direct contact with the release layer, it is preferable that the interface between the cushion layer and the release layer has unevenness. By having unevenness, the release property and the adhesion with the cushion layer can be improved, and the interlayer peeling between the release layer and the cushion layer can be prevented. That is, it is easy to handle when peeling from the flexible circuit board, and the load on the flexible circuit board can be reduced. The properties of the interface between the cushion layer and the release layer can be adjusted, for example, by reducing the thickness of the release layer and controlling the embossing conditions.
When the interface between the cushion layer and the release layer has projections and recesses, the arithmetic mean roughness Ra of the interface is preferably 3 μm or more.
The properties of the interface between the cushion layer and the release layer and the arithmetic mean roughness Ra of the interface can be confirmed by observing a cross section of the release film and measuring the roughness curve.

The method for producing the raw film of the release film of the present invention is not particularly limited, and examples thereof include a water-cooled or air-cooled coextrusion inflation method, a coextrusion T-die film formation method, a solvent casting method, a hot press molding method, and the like.
In the case of a structure having the release layers on both sides of the cushion layer, a method of preparing a film to be one of the release layers, laminating the cushion layer on this film by extrusion lamination, and then dry laminating the other release layer can be used. Also, a method of dry laminating the film to be one of the release layers, the film to be the cushion layer, and the film to be the other release layer can be used.
Among these, the co-extrusion T-die method is preferred because it provides excellent control over the thickness of each layer.

As described above, the present inventors have found that the degree of crystallinity of the extreme surface of the release layer can be adjusted to the above range by subjecting the release layer to a sufficiently smooth (or glossy) surface through friction treatment. A smooth surface means that the arithmetic mean roughness Ra is small. Having gloss means that the glossiness is high. More specifically, a release film having better releasability than conventional release films can be produced by having the arithmetic mean roughness Ra of the original film surface immediately before the friction process be 0.50 μm or less, or the glossiness of the original film surface immediately before the friction process be 100% or more. If the arithmetic mean roughness Ra is large or the glossiness is small due to the embossing treatment being performed before the friction treatment, the resulting release film may not have significant releasability.
The present invention also provides a method for producing a release film having at least one release layer, which includes a friction step of rubbing the surface of an original film of the release layer, and in which the arithmetic mean roughness Ra of the surface of the original film immediately before the friction step is 0.50 μm or less.
The present invention also provides a method for producing a release film having at least one release layer, which includes a friction step of rubbing the surface of the original film of the release layer, and in which the glossiness of the surface of the original film immediately before the friction step is 100% or more.

The upper limit of the arithmetic mean roughness Ra of the surface of the original film of the release layer immediately before the rubbing step is more preferably 0.30 μm, even more preferably 0.20 μm, and particularly preferably 0.10 μm. The lower limit of the arithmetic mean roughness Ra of the surface of the original film of the release layer is not particularly limited, but the substantially preferable lower limit is 0.01 μm.
In this specification, the arithmetic mean roughness Ra is an arithmetic mean roughness Ra in accordance with JIS B 0601:2013, and can be measured, for example, using a Surftest SJ-301 manufactured by Mitutoyo Corporation.

The more preferable lower limit of the glossiness of the surface of the original film of the release layer immediately before the rubbing step is 120%. The upper limit of the glossiness of the surface of the original film of the release layer is not particularly limited, but the substantially preferable upper limit is 200%.
In this specification, the glossiness is measured in accordance with JIS Z8741 at an incidence angle of 60°, and can be measured, for example, using a glossmeter VG-1D manufactured by Nippon Denshoku Industries Co., Ltd.

As a method for making the surface of the raw film sufficiently smooth (or glossy), for example, there is a method of using a cooling roll having a surface arithmetic mean roughness Ra of 0.50 μm or less when cooling the molten resin film during the production of the raw film.
The present invention also provides a method for producing a release film having at least one release layer, which includes a friction step of rubbing the surface of the original film of the release layer, and which uses a cooling roll having a surface arithmetic mean roughness Ra of 0.50 μm or less when cooling the molten resin film during the production of the original film of the release layer.

The method of the friction treatment is not particularly limited, but a method in which friction treatment is performed using a friction treatment device (e.g., a polishing treatment device manufactured by Yamagata Kikai Co., Ltd., model YCM-150M) and a woven fabric as the surface material of the friction treatment material is preferred.

In the above friction treatment, the tensile strength of the woven fiber used as the material of the surface of the friction treatment material is preferably 1.0 g/d at the lower limit and 5.0 g/d at the upper limit. If the tensile strength of the fiber is less than 1.0 g/d, the fiber may be stretched out during the above friction treatment, and the fiber may adhere to the resulting release film. If the tensile strength of the fiber is more than 5.0 g/d, the resulting release film may be damaged by wrinkles, scratches, etc., and the conformability to the substrate surface may decrease, and the adhesive may not be sufficiently suppressed from flowing out during hot press molding. The more preferred upper limit of the tensile strength of the fiber is 3.0 g/d.
In this specification, the tensile strength of a fiber means the tensile strength of a single fiber determined by a method in accordance with JIS-L-1095.

The elongation of the fibers is preferably 1% at the lower limit and 30% at the upper limit. If the elongation of the fibers is less than 1%, the fibers may be stretched out during the friction treatment, and the fibers may adhere to the resulting release film. If the elongation of the fibers is more than 30%, the resulting release film may be damaged by wrinkles, scratches, etc., and the conformability to the substrate surface may decrease, and the adhesive may not be sufficiently suppressed from flowing out during hot press molding. The more preferable upper limit of the elongation of the fibers is 29%.
In this specification, the elongation of a fiber means the tensile elongation of a single fiber determined by a method in accordance with JIS-L-1095.

Specific examples of the above-mentioned fibers include PET, rayon, cotton, wool, acetate, etc. Among these, rayon and wool are preferred because the friction treatment can further suppress damage such as wrinkles and scratches that occur in the release film, resulting in a release film with excellent conformability to the substrate surface, and can also reduce the amount of fibers that adhere to the release film.

In addition, the woven fabric used as the surface material of the friction treatment material has a friction coefficient of preferably 0.1 at the lower limit and 0.8 at the upper limit. If the friction coefficient of the woven fabric is less than 0.1, the releasability of the resulting release film may be reduced. If the friction coefficient of the woven fabric is more than 0.8, the resulting release film may be damaged by wrinkles, scratches, etc., and the conformability to the substrate surface may be reduced, and the adhesive may not be sufficiently suppressed from flowing out during hot press molding. The more preferred lower limit of the friction coefficient of the woven fabric is 0.3, and the more preferred upper limit is 0.7.
In this specification, the coefficient of friction of the woven fabric means the coefficient of friction of the woven fabric against a 2 mm polycarbonate plate determined by a method in accordance with JIS-K-7125.

The elastic modulus of the woven fabric is preferably 0.1 MPa at the lower limit and 4.0 MPa at the upper limit. If the elastic modulus of the woven fabric is less than 0.1 MPa, the releasability of the resulting release film may be reduced. If the elastic modulus of the woven fabric is more than 4.0 MPa, the resulting release film may be damaged by wrinkles, scratches, etc., and the conformability to the substrate surface may be reduced, and the adhesive may not be sufficiently suppressed from flowing out during hot press molding. The elastic modulus of the woven fabric is more preferably 0.7 MPa at the lower limit and 3.9 MPa at the upper limit.
In this specification, the elastic modulus of a woven fabric means the hardness of the woven fabric determined by a method in accordance with JIS-K-7127.

The tensile strength of the woven fabric is preferably 1.5 N/10 mm at the lower limit and 2.5 N/10 mm at the upper limit. If the tensile strength of the woven fabric is less than 1.5 N/10 mm, the fibers may be stretched out during the friction treatment, and the fibers may adhere to the resulting release film. If the tensile strength of the woven fabric is more than 2.5 N/10 mm, the resulting release film may be damaged by wrinkles, scratches, etc., and the conformability to the substrate surface may decrease, and the adhesive may not be sufficiently suppressed from flowing out during hot press molding. The tensile strength of the woven fabric is more preferably 1.6 N/10 mm at the lower limit, more preferably 2.3 N/10 mm at the upper limit, and even more preferably 1.8 N/10 mm at the upper limit.
In this specification, the tensile strength of a woven fabric means the tensile strength of a woven fabric determined by a method in accordance with JIS-K-7127.

The method for producing the release film preferably includes an embossing step of embossing the surface of the release layer. The method for producing the release film preferably includes the embossing step after the rubbing step. If the embossing step is performed after the rubbing step, it does not adversely affect the releasability and can impart wrinkle resistance.
As described above, the present inventors have found that the release property is significantly improved by sufficiently smoothing the surface of the original film of the release layer and then performing friction treatment. However, when the surface of the release layer of the release film is smoothed, air bubbles may be trapped at the interface when the release film is placed between the coverlay film and the heat press plate during press processing, and there is a concern that wrinkle resistance may be reduced. In other words, there is a trade-off between excellent release property and wrinkle resistance, and it has been difficult to achieve both in the past.
Here, the present inventors have found that embossing is effective for wrinkle resistance, but even if a film that has been embossed is subjected to a friction treatment, there are cases where the film does not have a significant releasability. The present inventors have found that a release film having excellent releasability and wrinkle resistance can be manufactured by sufficiently smoothing the surface of the release layer during the friction treatment, improving the releasability by the friction treatment, and then performing an embossing treatment again. In the past, since embossing was usually performed during extrusion molding, a film was once formed to be smooth, and then friction treatment and embossing treatment were not performed. In addition, since the process becomes complicated, it was usually difficult to imagine going through such a process.

The method of the embossing treatment is not particularly limited, and examples thereof include conventionally known methods such as an embossing roll method. Embossing the surface of the release film can improve the visibility and handleability (distinguishing between front and back and slipperiness) of the film, and can also suppress blocking between films. Embossing may also be performed on both sides of the release film. By embossing both sides of the release film, the visibility, handleability, and blocking resistance of the film can be further improved.

The embossing may be performed by heating. The heating method is not particularly limited, and examples thereof include a method of preheating the film and a method of heating the embossing roll.
As the embossing roll used in the embossing process, for example, an embossing roll having a surface arithmetic mean roughness Ra of 2.0 μm or more can be used. By using an embossing roll having a surface arithmetic mean roughness Ra of 2.0 μm or more, a release film that is less prone to wrinkles can be produced. As the embossing roll, an embossing roll having a surface arithmetic mean roughness Ra of 5.0 μm or more can be used.

The use of the release film of the present invention is not particularly limited, but it can be suitably used in the production processes of printed wiring boards, flexible circuit boards, multilayer printed wiring boards, and the like.
Specifically, for example, in the manufacturing process of a flexible circuit board, the release film of the present invention can be used when hot press bonding a coverlay film to a flexible circuit board main body having a copper circuit formed thereon via a thermosetting adhesive or a thermosetting adhesive sheet.
Since the release film of the present invention has extremely excellent releasability, it can also be suitably used in the production of flexible circuit boards by the roll-to-roll method, which requires high releasability.

According to the present invention, it is possible to provide a release film that has better release properties and excellent wrinkle resistance than conventional films.

The following examples further illustrate aspects of the present invention, but the present invention is not limited to these examples.

Example 1
(1) Production of release film Polybutylene terephthalate resin (PBT) was used as the resin constituting the release layer (release layer a and release layer b). 75 parts by weight of polypropylene resin (PP) and 25 parts by weight of polybutylene terephthalate resin (PBT) (main component resin of the release layer) were used as the resin constituting the cushion layer.
The resin constituting the release layer and the resin constituting the cushion layer were co-extruded in three layers at a T-die width of 400 mm using an extruder (GM Engineering, GM30-28 (screw diameter 30 mm, L/D 28)), and the extruded molten resin was cooled by a cooling roll (temperature 90°C). This resulted in a three-layer film having a release layer a (thickness 20 μm) and a release layer b (thickness 30 μm) on both sides of the cushion layer (thickness 50 μm). During cooling, the contact time between the molten resin and the cooling roll was 2.0 seconds, and the elongation stress when cooling the molten resin by the cooling roll was 400 kPa. As the cooling roll, a cooling roll with an arithmetic mean surface roughness Ra of 0.1 μm was used.

The tensile stress is expressed by the following formula (3).

また、ひずみ速度及び溶融樹脂の伸長粘度は、それぞれ下記式（４）、（５）で表される。

The strain rate and the elongational viscosity of the molten resin are expressed by the following formulas (4) and (5), respectively.

In equation (4), V is the roll speed (m/s), V0 is the flow velocity of the molten resin at the die exit (m/s), and L is the distance (m) from the die exit to the point where the molten resin contacts the roll.

While the obtained film was being fed by a roll, the surface of the release layer a was friction-treated using a friction treatment device (Yamagata Kikai Co., Ltd. polishing device, model YCM-150M) with a woven fabric as the surface material of the friction treatment material. During the friction treatment, a surface treatment roll was placed between the feed roll and the take-up roll, and a load was applied to the film by pressing the surface treatment roll against the film. The ratio of the rotation speed of the take-up roll to the rotation speed of the feed roll was adjusted to generate a tension of 400 N/m in the unwinding direction of the film. The amount of work energy applied during the friction treatment was 300 kJ. Furthermore, the arithmetic mean roughness Ra was 0.04 μm and the glossiness was 185% before the friction treatment.

The film after the friction treatment was fed with a roll and pressed with an embossing roll (surface roughness Ra 8 μm, temperature 160°C, linear pressure 50 kg/cm) to emboss the surface of the release layer a side, thereby obtaining a release film.

(2) Measurement of arithmetic mean roughness Ra The arithmetic mean roughness Ra of the surface of the release layer a after the friction treatment was measured in accordance with JIS B 0601:2013 using a Surftest SJ-301 manufactured by Mitutoyo Corp. The results are shown in Table 1.

(3) Measurement of Glossiness The glossiness of the surface of the release layer a after the rubbing treatment was measured in accordance with JIS Z8741 at an incident angle of 60° using a glossmeter VG-1D manufactured by Nippon Denshoku Industries Co., Ltd. The results are shown in Table 1.

(4) Measurement of the crystallinity of the extreme surface (crystallinity obtained by oblique incidence wide-angle X-ray diffraction with an incidence angle of 0.06°) The surface of the release layer a was analyzed by oblique incidence wide-angle X-ray diffraction with an incidence angle of 0.06°. A linear baseline was drawn in the range of 2θ = 9.5 to 35° on the obtained diffraction measurement plot. The crystalline phase and the amorphous phase were fitted as Gaussian functions, and the crystallinity of the extreme surface of the release layer was obtained from the obtained total peak area of the crystalline phase and the total peak area of the amorphous phase by the above formula (1). The results are shown in Table 1.

As the grazing incidence wide-angle X-ray diffraction device, a multi-function X-ray diffraction device for surface structure evaluation (ATX-G type) manufactured by Rigaku Corporation was used, which was set under the following conditions.
X-ray source CuKα tube voltage-tube current 50kV-300mA
Incident optical system: Concentrated method Incident angle (ω): 0.06°
Measurement range: 5-70°
Measurement interval: 0.02°
Scanning speed: 1.0°/min
Operation method: In-Plane method

(5) Measurement of crystallinity of entire release layer Each layer of the release film was peeled off to obtain a sample consisting of only release layer a. Release layer a was analyzed by wide-angle X-ray diffraction. A linear baseline was drawn in the range of 2θ = 12.0 to 28.18 ° on the obtained diffraction measurement plot. Fitting was performed as a Gaussian function for each of the crystalline phase and the amorphous phase, and the crystallinity of the entire release layer was calculated from the obtained total peak area of the crystalline phase and the total peak area of the amorphous phase by the above formula (2). The results are shown in Table 1.

As the wide-angle X-ray diffraction device, a horizontal type X-ray diffraction device for thin film evaluation (Smart Lab) manufactured by Rigaku Corporation was used, which was set under the following conditions.
X-ray source CuKα tube voltage 45kV-200mA
Incident optical system: Concentration method Measurement range: 5-80°
Measurement interval: 0.02°
Scanning speed: 5.0°/min
Scanning method: Out-of-Plane method

Example 2
A release film was obtained in the same manner as in Example 1, except that polybutylene terephthalate resin (PBT) and PBT-polytetramethylene glycol copolymer were used in a weight ratio of 80:20 as the resin constituting the release layers (release layer a and release layer b), and the contact time between the molten resin and the cooling roll, the cooling roll temperature and elongation stress, and the tension during surface treatment were changed as shown in Table 1. The physical properties of the obtained release film were determined in the same manner as in Example 1.

(Examples 3 to 6)
A release film was obtained in the same manner as in Example 1, except that the contact time between the molten resin and the cooling roll, the cooling roll temperature and elongation stress, and the tension during the surface treatment were changed as shown in Table 1. The physical properties of the obtained release film were determined in the same manner as in Example 1.

(Comparative Examples 1 to 6)
Except for not carrying out the embossing treatment, release films were obtained in the same manner as in Examples 1 to 6. The physical properties of the obtained release films were determined in the same manner as in Example 1.

(Comparative Examples 7 to 12)
Except for carrying out the embossing treatment before the rubbing treatment, release films were obtained in the same manner as in Examples 1 to 6. The physical properties of the obtained release films were determined in the same manner as in Example 1.

<Evaluation>
The release films obtained in the examples and comparative examples were evaluated as follows.

(1) Evaluation of releasability CVL1 (CISV2535, manufactured by Nikkan Kogyo Co., Ltd.) was used as an epoxy adhesive sheet, and a release film was placed on the epoxy adhesive sheet so that the release layer a was in contact with the epoxy adhesive sheet, and the sheet was heat-pressed for 5 minutes under conditions of 180°C and 30 kgf/ ^cm2 . After that, the release film naturally peeled off in some samples. For samples in which the release film did not naturally peel off even after 10 minutes had passed since the heat press, the samples were cut to a width of 30 mm, and a peel test was performed at a test speed of 500 mm/min and a peel angle of 30° to determine the 30° peel strength (30° peel value).
In addition, in order to compare the releasability under conditions more severe than those normally required, the epoxy adhesive sheet CVL1 was replaced with an epoxy adhesive sheet CVL2 (HXC2525, manufactured by DuPont) having stronger adhesive strength, and similar measurements were carried out.

Based on these measurement results, the releasability of the release film was evaluated as follows.
⊚: When the release film peeled naturally in the test using CVL1 and the 30° peel strength was 100 gf/cm or less in the test using CVL2 ◯: When the release film peeled naturally in the test using CVL1 and the 30° peel strength was greater than 100 gf/cm in the test using CVL2 ×: When the release film did not peel naturally in the test using CVL1 Note that a peel test performed at a peel angle of 30° is generally very difficult to peel, since the peel angle is lower than that in a test performed at a peel angle of 180°. In other words, a release film that gives a good result in a peel test at a peel angle of 30° has superior releasability compared to conventional release films.

(2) Evaluation of wrinkle resistance A CCL (20 cm x 20 cm, polyimide thickness 25 μm, copper foil 35 μm), a coverlay (20 cm x 20 cm, polyimide thickness 15 μm, epoxy resin adhesive layer 25 μm, Nikkan Kogyo Co., Ltd., CISV2535), and a release film were stacked in this order from the bottom. After positioning between the press dies preheated to 180 ° C. using a sliding vacuum heater press (MKP-3000V-WH-ST, Mikado Technos Co., Ltd.), pressing was started (about 10 seconds from placement to actual pressure application). A flexible circuit board (FPC) evaluation sample consisting of the CCL and the coverlay was produced by pressing for 2 minutes at 50 kg/cm2 under vacuum conditions.
Thereafter, the FPC evaluation sample and the release film were taken out, the release film was peeled off, and the number of wrinkles transferred onto the coverlay surface was measured. The number of wrinkles was evaluated as "◎" when there were 0 wrinkles, "◯" when there were 1 to 10 wrinkles, "△" when there were 11 to 20 wrinkles, and "×" when there were 21 or more wrinkles. Note that when there were 20 wrinkles or less, it can be said that the release film has sufficient wrinkle resistance for use in manufacturing flexible circuit boards. When there were 10 wrinkles or less, it can be said that the release film has excellent wrinkle resistance.

(3) Evaluation of conformability A coverlay film (12.5 cm x 12.5 cm, polyimide thickness 25 μm, epoxy adhesive layer thickness 35 μm) with a hole of φ = 1 mm was laminated on the copper foil surface of a copper clad laminate (CCL) (12.5 cm x 12.5 cm, polyimide thickness 25 μm, copper foil thickness 35 μm) so that the epoxy adhesive layer was in contact with the coverlay film. Furthermore, a release film was laminated so that the release layer a was in contact with the coverlay film. This laminate was hot pressed for 2 minutes under conditions of 180 ° C. and 30 kgf / cm ^2. Thereafter, the release film was peeled off, and the epoxy adhesive that flowed out onto the copper clad laminate (CCL) was observed with an optical microscope. The exudation width of the epoxy adhesive was measured at 12 points and the average value was calculated. Based on the measurement results, the conformability of the release film was evaluated as "○" when the average value of the exudation width of the epoxy adhesive was less than 55 μm, and "×" when it was 55 μm or more.

The present invention provides a release film having better release properties and excellent wrinkle resistance than conventional release films, and a method for producing the release film.

Claims (14)

A release film having at least one release layer,
The release layer has a crystallinity of 50% or more as determined by oblique incidence wide-angle X-ray diffraction method with an incidence angle of 0.06°,
A release film, wherein the release layer has a surface having an arithmetic mean roughness Ra of 2.0 μm or more. A release film as described in claim 1, characterized in that the crystallinity of the entire release layer is smaller than the crystallinity of the outermost surface of the release layer. A release film as described in claim 1 or 2, characterized in that the crystallinity of the entire release layer is 25 to 50%. A release film as described in claim 1, 2 or 3, characterized in that the release layer has a surface gloss of 50% or less. A release film as described in claim 1, 2, 3 or 4, further comprising a cushion layer and a release layer on both sides of the cushion layer. A release film as described in claim 1, 2, 3, 4 or 5, characterized in that the release layer contains an aromatic polyester resin. A release film as described in claim 6, characterized in that the aromatic polyester resin contains polybutylene terephthalate resin. A release film as described in claim 7, characterized in that the proportion of polybutylene terephthalate resin in the resin constituting the release layer is 75 weight % or more. A release film as described in claim 1, 2, 3, 4, 5, 6, 7 or 8, characterized in that it is used in the manufacture of flexible circuit boards by the RtoR method. A method for producing a release film having at least one release layer, comprising:
A method for producing a release film, comprising a rubbing step of rubbing the surface of an original film of the release layer, wherein the arithmetic mean roughness Ra of the surface of the original film immediately before the rubbing step is 0.50 μm or less. A method for producing a release film having at least one release layer, comprising:
A method for producing a release film, comprising a rubbing step of rubbing the surface of an original film of the release layer, wherein the glossiness of the surface of the original film immediately before the rubbing step is 100% or more. A method for producing a release film having at least one release layer, comprising:
A method for producing a release film, comprising a friction step of rubbing the surface of the original film of the release layer, and using a cooling roll having a surface arithmetic mean roughness Ra of 0.50 μm or less when cooling the molten resin film during the production of the original film of the release layer. A method for producing a release film as described in claim 10, 11 or 12, characterized in that it includes an embossing process for applying an embossing treatment to the surface of the release layer, and the embossing process is performed after the rubbing process. A method for producing a release film as described in claim 13, characterized in that in the embossing process, an embossing roll having a surface arithmetic mean roughness Ra of 2.0 μm or more is used.