JP2020033420A - Base material for surface protective film, production method of the base material, surface protective film using the base material, and optical film with surface productive film - Google Patents

Abstract

To provide a base material for a surface protective film, which has excellent flexibility or folding resistance and can well suppress light leakage, coloring and iridescent irregularity in an optical inspection.SOLUTION: The base material for a surface protective film in accordance with the present invention is constituted by a film comprising an acrylic resin and an elastomer, and shows an in-plane retardation Re(550) of 30 nm or less and 500 or more folding times until breakage occurs by an MIT test. A production method of the base material for a surface protective film in accordance with the present invention includes: forming a film-forming material comprising an acrylic resin and an elastomer into a film; and subjecting the formed film to sequential biaxial orientation or simultaneous biaxial orientation.SELECTED DRAWING: None

Description

  The present invention relates to a substrate for a surface protective film, a method for producing the substrate, a surface protective film using the substrate, and an optical film with a surface protective film.

  An optical film (for example, a polarizing plate or a laminate including a polarizing plate) includes the optical film (finally, the image display device) until the image display device to which the optical film is applied is actually used. In order to protect the surface protection film, a surface protection film is releasably attached. Practically, an optical display / surface protective film laminate is bonded to a display cell to produce an image display device, and the image display device is subjected to an optical test (for example, a lighting test) in a state where the laminate is bonded. ), And at an appropriate time thereafter, the surface protective film is peeled off. The surface protective film typically has a resin film as a substrate and an adhesive layer. According to the conventional surface protection film, light leakage, coloring, rainbow unevenness, and the like occur during an optical inspection, which may cause a decrease in the accuracy of the optical inspection. As a result, there is a case where the image display device itself is determined to be defective by an optical inspection before shipment even though there is no problem in the image display device itself, thereby lowering the efficiency from manufacture to shipment of the image display device. There is a problem. Furthermore, the surface protective film (substantially, the substrate) is required to have excellent flexibility in consideration of the operability at the time of bonding and peeling.

JP 2017-190406 A

  The present invention has been made in order to solve the above-mentioned conventional problems, and its main object is to have excellent flexibility or bending resistance, and to prevent light leakage, coloring and rainbow unevenness during optical inspection. An object of the present invention is to provide a surface protective film base material that can be favorably suppressed.

The substrate for a surface protection film according to the embodiment of the present invention is formed of a film containing an acrylic resin and an elastomer, has an in-plane retardation Re (550) of 30 nm or less, and has a number of bending times until breakage in an MIT test. 500 times or more, and the elastomer contains 6 parts by weight or more based on 100 parts by weight of the acrylic resin.
In one embodiment, the acrylic resin has at least one selected from the group consisting of glutarimide units, lactone ring units, maleic anhydride units, maleimide units, and glutaric anhydride units.
In one embodiment, the elastomer is at least one selected from a rubbery polymer, a polyamide-based elastomer, a polyethylene-based elastomer, a styrene-based elastomer, and a butadiene-based elastomer.
In one embodiment, the substrate for a surface protective film has a total light transmittance of 80% or more and a haze of 1.0% or less.
According to another aspect of the present invention, there is provided a method for producing the substrate for a surface protective film. This production method includes forming a film-forming material containing an acrylic resin and an elastomer into a film, and sequentially or biaxially stretching the formed film.
In one embodiment, in the above production method, the stretching temperature in the sequential biaxial stretching or the simultaneous biaxial stretching is Tg + 10 ° C. to Tg + 30 ° C.
According to still another aspect of the present invention, a surface protective film is provided. This surface protection film includes the above-mentioned substrate for a surface protection film and an adhesive layer.
According to still another aspect of the present invention, there is provided an optical film with a surface protection film. This optical film with a surface protective film includes an optical film and the above-mentioned surface protective film releasably bonded to the optical film.

  According to the embodiment of the present invention, by stretching a film containing an acrylic resin and an elastomer under predetermined stretching conditions (for example, stretching temperature and stretching speed), an extremely small in-plane retardation and excellent flexibility can be obtained. And a substrate for a surface protective film having both good heat resistance and bending resistance.

  Hereinafter, preferred embodiments of the present invention will be described, but the present invention is not limited to these embodiments.

A. Substrate for Surface Protective Film The substrate for surface protective film according to the embodiment of the present invention is composed of a film containing an acrylic resin and an elastomer.

  Any appropriate acrylic resin can be adopted as the acrylic resin. The acrylic resin typically contains an alkyl (meth) acrylate as a main component as a monomer unit. In the present specification, “(meth) acryl” means acryl and / or methacryl. Examples of the alkyl (meth) acrylate constituting the main skeleton of the acrylic resin include those having a linear or branched alkyl group having 1 to 18 carbon atoms. These can be used alone or in combination. Further, any appropriate copolymerizable monomer may be introduced into the acrylic resin by copolymerization. The type, number, copolymerization ratio and the like of such a copolymerized monomer can be appropriately set according to the purpose. The constituent components (monomer units) of the main skeleton of the acrylic resin will be described later with reference to the general formula (2).

  The acrylic resin preferably has a structural unit selected from a glutarimide unit, a lactone ring unit, a maleic anhydride unit, a maleimide unit and a glutaric anhydride unit. The acrylic resin may have only one of these structural units, or may have a plurality. The acrylic resin having a lactone ring unit is described in, for example, JP-A-2008-181078, and the description of the publication is incorporated herein by reference. The glutarimide unit is preferably represented by the following general formula (1):

In the general formula (1), R ¹ and R ² each independently represent hydrogen or an alkyl group having 1 to 8 carbon atoms, and R ³ represents an alkyl group having 1 to 18 carbon atoms and 3 to 12 carbon atoms. And an aryl group having 6 to 10 carbon atoms. In the general formula (1), preferably, R ¹ and R ² are each independently hydrogen or a methyl group, and R ³ is hydrogen, a methyl group, a butyl group or a cyclohexyl group. More preferably, R ¹ is a methyl group, R ² is hydrogen and R ³ is a methyl group.

  The alkyl (meth) acrylate is typically represented by the following general formula (2):

In the general formula (2), R ⁴ represents a hydrogen atom or a methyl group, and R ⁵ represents a hydrogen atom or an optionally substituted aliphatic or alicyclic hydrocarbon group having 1 to 6 carbon atoms. Show. Examples of the substituent include a halogen and a hydroxyl group. Specific examples of the alkyl (meth) acrylate include methyl (meth) acrylate, ethyl (meth) acrylate, n-propyl (meth) acrylate, n-butyl (meth) acrylate, and t- (meth) acrylate. Butyl, n-hexyl (meth) acrylate, cyclohexyl (meth) acrylate, chloromethyl (meth) acrylate, 2-chloroethyl (meth) acrylate, 2-hydroxyethyl (meth) acrylate, (meth) acrylic acid 3-hydroxypropyl, 2,3,4,5,6-pentahydroxyhexyl (meth) acrylate and 2,3,4,5-tetrahydroxypentyl (meth) acrylate. In the general formula (2), R ⁵ is preferably a hydrogen atom or a methyl group. Thus, a particularly preferred alkyl (meth) acrylate is methyl acrylate or methyl methacrylate.

The acrylic resin may include only a single glutarimide unit, or may include a plurality of glutarimide units having different R ¹ , R ² and R ³ in the general formula (1).

  The content ratio of the glutarimide unit in the acrylic resin is preferably 2 mol% to 50 mol%, more preferably 2 mol% to 45 mol%, further preferably 2 mol% to 40 mol%, and particularly preferably 2 mol%. % To 35 mol%, most preferably 3 mol% to 30 mol%. If the content is less than 2 mol%, the effects (for example, high optical properties, high mechanical strength, excellent adhesiveness with a polarizer, and thinness) exhibited from the glutarimide unit are sufficiently exhibited. It may not be done. If the content exceeds 50 mol%, for example, heat resistance and transparency may be insufficient. When the acrylic resin has a lactone ring unit, a maleic anhydride unit, a maleimide unit and / or a glutaric anhydride unit instead of or in addition to the glutarimide unit, the above content ratio is It may be the total content of these structural units.

The acrylic resin may include only a single alkyl (meth) acrylate unit, or may include a plurality of alkyl (meth) acrylate units in which R ⁴ and R ⁵ in the general formula (2) are different. Is also good.

  The content ratio of the alkyl (meth) acrylate unit in the acrylic resin is preferably 50 mol% to 98 mol%, more preferably 55 mol% to 98 mol%, further preferably 60 mol% to 98 mol%, and particularly preferably. Is from 65 mol% to 98 mol%, most preferably from 70 mol% to 97 mol%. If the content is less than 50 mol%, the effects (for example, high heat resistance and high transparency) expressed from the alkyl (meth) acrylate unit may not be sufficiently exhibited. If the content is more than 98 mol%, the resin becomes brittle and easily cracked, so that high mechanical strength cannot be sufficiently exhibited and productivity may be poor.

  The acrylic resin may contain an alkyl (meth) acrylate unit and a unit other than a glutarimide unit, a lactone ring unit, a maleic anhydride unit, a maleimide unit and / or a glutaric anhydride unit. For example, the acrylic resin may contain a copolymerizable vinyl monomer unit other than the above (other vinyl monomer unit). Other vinyl monomers include, for example, acrylonitrile, methacrylonitrile, ethacrylonitrile, allyl glycidyl ether, maleic anhydride, itaconic anhydride, N-methylmaleimide, N-ethylmaleimide, N-cyclohexylmaleimide, acrylic Aminoethyl acrylate, propylaminoethyl acrylate, dimethylaminoethyl methacrylate, ethylaminopropyl methacrylate, cyclohexylaminoethyl methacrylate, N-vinyldiethylamine, N-acetylvinylamine, allylamine, methallylamine, N-methylallylamine, -Isopropenyl-oxazoline, 2-vinyl-oxazoline, 2-acryloyl-oxazoline, N-phenylmaleimide, phenylaminoethyl methacrylate, styrene, α-methyl Styrene, p- glycidyl styrene, p- aminostyrene, 2-styryl - and oxazoline and the like. These may be used alone or in combination. Preferred are styrene monomers such as styrene and α-methylstyrene. The content ratio of the other vinyl monomer unit is preferably from 0 to 1% by weight, more preferably from 0 to 0.1% by weight. Within such a range, occurrence of an undesired retardation and reduction in transparency can be suppressed.

The imidation ratio in the acrylic resin is preferably 2.5% to 20.0%. When the imidation ratio is in such a range, a resin having excellent heat resistance, transparency, and moldability can be obtained, and generation of kogation and reduction in mechanical strength during film formation can be prevented. In the acrylic resin, the imidization ratio is represented by the ratio of glutarimide units to alkyl (meth) acrylate units. This ratio can be obtained, for example, from the NMR spectrum, IR spectrum, etc. of the acrylic resin. In the present embodiment, the imidation ratio can be determined by ¹ H-NMR measurement of the resin using ¹ H NMR BRUKE Avance III (400 MHz). More specifically, the peak area derived from the O-CH ₃ proton of the alkyl (meth) acrylate around 3.5 to 3.8 ppm is defined as A, and the glutarimide N-CH ₃ around 3.0 to 3.3 ppm is defined as A. Assuming that the area of the peak derived from the proton is B, the peak area can be obtained by the following equation.
Imidation ratio Im (%) = {B / (A + B)} × 100

  The acrylic resin preferably has an acid value of 0.10 mmol / g to 0.50 mmol / g. When the acid value is within such a range, a resin having an excellent balance between heat resistance, mechanical properties, and moldability can be obtained. If the acid value is too small, there may be problems such as an increase in cost due to the use of a denaturing agent for adjusting the acid value to a desired value and generation of a gel-like substance due to the remaining denaturant. If the acid value is too large, foaming tends to occur during film formation (for example, during melt extrusion), and the productivity of molded products tends to decrease. In the acrylic resin, the acid value is the content of a carboxylic acid unit and a carboxylic anhydride unit in the acrylic resin. In the present embodiment, the acid value can be calculated, for example, by the titration method described in WO2005 / 054311 or JP-A-2005-23272.

  The details of the acrylic resin, its production method and its properties are described in, for example, JP-A-2006-139027, JP-A-2007-316366 and JP-A-2008-181078. The description is incorporated herein by reference.

  Any appropriate elastomer can be adopted as the elastomer. Elastomers typically have hard segments that can act as pseudo-crosslinking points and soft segments that can primarily contribute to elasticity. Representative examples of the elastomer include a rubber-like polymer, a polyamide-based elastomer, a polyethylene-based elastomer, a styrene-based elastomer, and a butadiene-based elastomer. Elastomers may be used alone or in combination. Elastomers can be appropriately selected depending on the desired properties. For example, a styrene-based elastomer can be adopted from the viewpoint of easy design of optical characteristics. Typical examples of the styrene-based elastomer include those having polystyrene as a hard segment and polybutadiene and polyisoprene as soft segments, or a copolymer of polybutadiene and polyisoprene, and hydrogenated products thereof. The hydrogenated product may be a partially hydrogenated product such as polybutadiene or polyisoprene, or may be a completely hydrogenated product. Commercial products may be used as the styrene-based elastomer. Commercially available styrene elastomers include, for example, Tuftec, Tufprene (all made by Asahi Kasei Chemicals), Clayton (made by Clayton Polymer Japan), Dynaron, JSR TR, JSR SIS (all made by JSR), Septon (Kuraray) And Lavalon (manufactured by Mitsubishi Chemical Corporation). As the polyethylene-based elastomer, one having at least 50 parts of an ethylene block is preferable, and examples of commercially available products include Exelen FX (Sumitomo Chemical Co., Ltd.). The polyamide-based elastomer preferably has a polyether block, and commercially available products include Pebax (manufactured by Tokyo Material Co., Ltd.). As the rubber-like polymer, core-shell type particles are preferable, and W-300 (manufactured by Mitsubishi Chemical Corporation) as a commercial product is exemplified.

  The compounding amount of the elastomer is 6 parts by weight or more, preferably 6 parts by weight to 30 parts by weight, more preferably 8 parts by weight to 20 parts by weight based on 100 parts by weight of the acrylic resin. When the amount of the elastomer is in such a range, a base material for a surface protective film having both a very small in-plane retardation and excellent flexibility or bending resistance can be realized.

  In the embodiment of the present invention, the in-plane retardation Re (550) of the surface protective film substrate is 30 nm or less, preferably 10 nm or less, more preferably 5 nm or less, and still more preferably 3 nm or less. And particularly preferably 2.5 nm or less. The smaller the in-plane retardation Re (550), the better. The lower limit is ideally 0 nm, for example, 0.1 nm. When the in-plane retardation Re (550) is in such a range, when a surface protective film using the substrate for a surface protective film of the present invention is subjected to an optical inspection of an image display device, light leakage, coloring and Rainbow unevenness can be favorably suppressed. As a result, the accuracy of the optical inspection of the image display device can be remarkably improved, and the efficiency of the image display device from manufacture to shipment can be improved. Such an in-plane retardation Re (550) can be realized by stretching a film containing the specific acrylic resin and the specific elastomer as described above under predetermined stretching conditions as described later. In this specification, “Re (λ)” is an in-plane retardation measured with light having a wavelength of λ nm at 23 ° C. Re (λ) is determined by the formula: Re = (nx−ny) × d, where d (nm) is the thickness of the layer (film). Therefore, “Re (550)” is the in-plane retardation measured with light having a wavelength of 550 nm at 23 ° C. Here, “nx” is the refractive index in the direction in which the in-plane refractive index becomes maximum (ie, the slow axis direction), and “ny” is the direction perpendicular to the slow axis in the plane (ie, fast phase). (Axial direction).

  The thickness direction retardation Rth (550) of the surface protective film substrate is preferably 50 nm or less, more preferably 25 nm or less, further preferably 15 nm or less, and particularly preferably 10 nm or less. The thickness direction retardation Rth (550) is preferably as small as possible, and the lower limit is ideally 0 nm, for example, 0.5 nm. When the thickness direction retardation Rth (550) is within such a range, oblique light leakage, coloring, and rainbow unevenness can be favorably suppressed in the above-described optical inspection. As a result, accuracy can be significantly improved even in optical inspection of a large-sized image display device. The Nz coefficient of the substrate for a surface protective film is preferably from 1.1 to 20, more preferably from 1.5 to 5.4. Therefore, the refractive index characteristics of the substrate for a surface protective film may show, for example, a relationship of nx> ny> nz. When the Nz coefficient is in such a range, oblique light leakage, coloring, and rainbow unevenness can be more favorably suppressed in the optical inspection. “Rth (λ)” is a retardation in the thickness direction measured with light having a wavelength of λ nm at 23 ° C. Rth (λ) is determined by the formula: Rth = (nx−nz) × d, where d (nm) is the thickness of the layer (film). Therefore, “Rth (550)” is a retardation in the thickness direction measured with light having a wavelength of 550 nm at 23 ° C. Here, “nz” is the refractive index in the thickness direction. Further, the “Nz coefficient” is obtained by Nz = Rth (λ) / Re (λ).

  The substrate for a surface protective film may exhibit an inverse dispersion wavelength characteristic in which the in-plane retardation increases according to the wavelength of the measurement light, and the positive chromatic dispersion in which the in-plane retardation decreases according to the wavelength of the measurement light. A characteristic may be exhibited, and a flat wavelength dispersion characteristic in which the in-plane phase difference hardly changes even with the wavelength of the measurement light may be exhibited.

  The total light transmittance of the substrate for a surface protective film is preferably 80% or more, more preferably 85% or more, further preferably 90% or more, and particularly preferably 95% or more. Further, the haze of the surface protective film substrate is preferably 1.0% or less, more preferably 0.7% or less, further preferably 0.5% or less, and particularly preferably 0.3% or less. % Or less. According to the embodiment of the present invention, a substrate for a surface protective film having a very small in-plane retardation Re (550) as described above and having such excellent transparency is realized. be able to.

  In the embodiment of the present invention, the substrate for a surface protective film has a number of bending times to break in an MIT test of 500 times or more, preferably 1000 times or more, more preferably 1500 times or more, and still more preferably. Is 2000 times or more. That is, the substrate for a surface protection film can have very excellent flexibility or bending resistance. Due to such excellent flexibility or bending resistance of the substrate for a surface protective film, it is possible to obtain a surface protective film which is excellent in operability at the time of bonding and peeling and in which cracking is suppressed. it can. According to the embodiment of the present invention, such excellent flexibility or bending resistance and extremely small in-plane retardation Re (550) as described above can be compatible. Achieving such compatibility is one of the results of the present invention. The MIT test can be performed in accordance with JIS P8115.

  The elastic modulus of the substrate for a surface protective film is preferably 50 MPa to 350 MPa at a tensile speed of 100 mm / min. When the elastic modulus is in such a range, a surface protective film having excellent transportability and operability can be obtained. According to the embodiment of the present invention, it is possible to achieve both excellent elastic modulus (strength) and excellent flexibility or bending resistance (softness) as described above. The elastic modulus is measured according to JIS K 7127: 1999.

  The tensile elongation of the substrate for a surface protective film is preferably 70% to 200%. When the tensile elongation is in such a range, there is an advantage that it is difficult to break during transportation. The tensile elongation is measured according to JIS K6781.

  The thickness of the substrate for a surface protection film is typically from 10 μm to 100 μm, preferably from 20 μm to 70 μm.

B. Method for Producing Substrate for Surface Protective Film The method for producing the substrate for surface protective film according to the embodiment of the present invention comprises: And stretching the formed film.

  The film-forming material may contain another resin, an additive, or a solvent in addition to the acrylic resin and the elastomer. Any appropriate additive can be adopted as the additive depending on the purpose. Specific examples of additives include a reactive diluent, a plasticizer, a surfactant, a filler, an antioxidant, an antioxidant, an ultraviolet absorber, a leveling agent, a thixotropic agent, an antistatic agent, a conductive material, and a flame retardant. Is mentioned. The number, type, combination, addition amount, and the like of the additives can be appropriately set according to the purpose.

  As a method of forming a film from a film forming material, any appropriate forming method can be adopted. Specific examples include a compression molding method, a transfer molding method, an injection molding method, an extrusion molding method, a blow molding method, a powder molding method, an FRP molding method, a cast coating method (for example, a casting method), a calendar molding method, and a hot press. And the like. Extrusion molding or cast coating is preferred. This is because the smoothness of the obtained film can be improved and good optical uniformity can be obtained. The molding conditions can be set as appropriate according to the composition and type of the resin used, the characteristics desired for the substrate for the surface protective film, and the like.

  The stretching method of the film is typically biaxial stretching, and more specifically, sequential biaxial stretching or simultaneous biaxial stretching. This is because a substrate for a surface protective film having a small in-plane retardation Re (550) and excellent in flexibility or bending resistance can be obtained. The sequential biaxial stretching or simultaneous biaxial stretching is typically performed using a tenter stretching machine. Therefore, the stretching direction of the film is typically the length direction and the width direction of the film.

  The stretching temperature can vary depending on the in-plane retardation and thickness desired for the surface protective film substrate, the type of resin used, the thickness of the film used, the stretching ratio, and the like. Specifically, the stretching temperature is preferably Tg + 5 ° C. to Tg + 50 ° C., more preferably Tg + 10 ° C. to Tg + 30 ° C., with respect to the glass transition temperature (Tg) of the film. By stretching at such a temperature, a substrate for a surface protective film having appropriate characteristics in the embodiment of the present invention can be obtained.

  The stretching ratio can be changed according to the in-plane retardation and thickness desired for the surface protective film substrate, the type of resin used, the thickness of the film used, the stretching temperature, and the like. When employing biaxial stretching (for example, sequential biaxial stretching or simultaneous biaxial stretching), stretching ratio in the first direction (for example, length direction) and stretching in the second direction (for example, width direction) The magnification is preferably as small as possible, more preferably substantially equal. With such a configuration, a base material for a surface protective film having a small in-plane retardation Re (550) and excellent in flexibility or bending resistance can be obtained. When biaxial stretching (for example, sequential biaxial stretching or simultaneous biaxial stretching) is employed, the stretching ratio is determined in the first direction (for example, the length direction) and the second direction (for example, the width direction). For each, it can be, for example, 1.1-3.0 times.

  In the embodiment of the present invention, the stretching speed is preferably 10% / sec or less, more preferably 7% / sec or less, further preferably 5% / sec or less, and particularly preferably 2.5% / sec or less. % / Sec or less. By stretching the film containing the specific acrylic resin and the specific elastomer as described above at such a low stretching speed, the in-plane retardation Re (550) is small, and the film has flexibility or bending resistance. A substrate for a surface protection film having excellent resistance can be obtained. The lower limit of the stretching speed can be, for example, 1.2% / sec. If the stretching speed is too low, the productivity may not be practical. When biaxial stretching (for example, sequential biaxial stretching or simultaneous biaxial stretching) is employed, the stretching speed in the first direction (for example, the length direction) and the second direction (for example, the width direction) Is preferably as small as possible, and more preferably substantially equal. With such a configuration, the in-plane retardation Re (550) can be further reduced, and the flexibility or bending resistance can be further improved.

C. Surface protective film The substrate for a surface protective film described in the above sections A and B can be suitably used for a surface protective film. Therefore, embodiments of the present invention also include a surface protection film. The surface protective film according to the embodiment of the present invention includes the surface protective film substrate described in the above section A and B and an adhesive layer.

  Any appropriate pressure-sensitive adhesive can be adopted as the pressure-sensitive adhesive forming the pressure-sensitive adhesive layer. Examples of the base resin of the pressure-sensitive adhesive include an acrylic resin, a styrene resin, a silicone resin, a urethane resin, and a rubber resin. Such a base resin is described in, for example, JP-A-2015-120337 or JP-A-2011-201983. The descriptions in these publications are incorporated herein by reference. Acrylic resins are preferred from the viewpoints of chemical resistance, adhesion for preventing the intrusion of the treatment liquid during immersion, and the degree of freedom for the adherend. Examples of the crosslinking agent that can be included in the pressure-sensitive adhesive include an isocyanate compound, an epoxy compound, and an aziridine compound. The pressure-sensitive adhesive may include, for example, a silane coupling agent. The formulation of the pressure-sensitive adhesive can be appropriately set according to the purpose and desired properties.

The storage elastic modulus of the pressure-sensitive adhesive layer is preferably from 1.0 × 10 ⁴ Pa to 1.0 × 10 ⁷ Pa, and more preferably from 2.0 × 10 ⁴ Pa to 5.0 × 10 ⁶ Pa. When the storage elastic modulus of the pressure-sensitive adhesive layer is in such a range, blocking during roll formation can be suppressed. The storage elastic modulus can be determined, for example, from dynamic viscoelasticity measurement at a temperature of 23 ° C. and an angular velocity of 0.1 rad / s.

  The thickness of the pressure-sensitive adhesive layer is preferably 1 μm to 60 μm, and more preferably 3 μm to 30 μm. If the thickness is too small, the adhesiveness becomes insufficient, and air bubbles and the like may enter the adhesive interface. If the thickness is too large, problems such as sticking out of the pressure-sensitive adhesive tend to occur.

  Practically, the separator is temporarily attached to the surface of the pressure-sensitive adhesive layer in a releasable manner until the surface protective film is actually used (that is, bonded to an optical film or an image display device). By providing the separator, the pressure-sensitive adhesive layer can be protected, and the surface protection film can be wound into a roll. Examples of the separator include a plastic (for example, polyethylene terephthalate (PET), polyethylene, polypropylene) film, nonwoven fabric, or the like, which is surface-coated with a release agent such as a silicone-based release agent, a fluorine-based release agent, or a long-chain alkyl acrylate-based release agent. Paper and the like. Any appropriate thickness can be adopted for the thickness of the separator depending on the purpose. The thickness of the separator is, for example, 10 μm to 100 μm.

D. Optical Film with Surface Protection Film The surface protection film described in the above section C is used to protect the optical film until the optical film (finally, an image display device) is actually used. Therefore, the embodiment of the present invention also includes an optical film with a surface protection film. An optical film with a surface protective film according to an embodiment of the present invention includes an optical film and the surface protective film according to the above item C, which is releasably attached to the optical film.

  The optical film may be a single film or a laminate. Specific examples of the optical film include a polarizer, a retardation film, a polarizing plate (typically, a laminate of a polarizer and a protective film), a conductive film for a touch panel, a surface-treated film, and for these purposes. A laminate (for example, an anti-reflection circular polarizer, a polarizer with a conductive layer for a touch panel, a polarizer with a retardation layer, and a prism sheet-integrated polarizer) appropriately laminated according to the requirements is given.

  Hereinafter, the present invention will be described specifically with reference to Examples, but the present invention is not limited to these Examples. The measuring method of each characteristic in the example is as follows. Unless otherwise specified, “parts” and “%” in Examples are on a weight basis.

(1) In-plane retardation Re (550) and thickness direction retardation Rth (550)
The substrate for a surface protection film (biaxially stretched film) obtained in each of the examples and comparative examples was cut into a length of 4 cm and a width of 4 cm to obtain a measurement sample. The in-plane retardation and the thickness direction retardation of the measurement sample were measured using “Axoscan” manufactured by Axometrics. The measurement wavelength was 550 nm, and the measurement temperature was 23 ° C.
(2) Haze For the same measurement sample as in the above (1), the haze was measured using a haze meter (HM-150 type, manufactured by Murakami Color Research Laboratory). The measurement temperature was 23 ° C.
(3) MIT test The MIT test was performed according to JIS P8115. Specifically, the base material (biaxially stretched film) for a surface protective film obtained in each of the examples and comparative examples was cut into a length of 15 cm and a width of 1.5 cm to obtain a measurement sample. The measurement sample was attached to an MIT bending fatigue tester BE-202 type (manufactured by Tester Sangyo Co., Ltd.) (load: 1.0 kgf, R of the clamp: 0.38 mm), 2000 times at a test speed of 90 cpm and a bending angle of 90 °. The bending was repeatedly performed as an upper limit, and the number of times of bending when the measurement sample was broken was taken as a test value.
(4) Nijimura The surface protective film base material (biaxially stretched film) obtained in each of Examples and Comparative Examples was disposed between two polarizing plates in a crossed Nicols state. At this time, the surface protection film substrate was arranged such that the length direction of the surface protection film substrate was parallel to the transmission axis direction of one of the polarizing plates. In this state, light from a fluorescent lamp was irradiated from below the lower polarizing plate, and the presence or absence of rainbow unevenness was visually observed. Evaluation was made according to the following criteria.
：: no rainbow unevenness was observed
Δ: Slight rainbow unevenness was observed
×: Rainbow unevenness was remarkably observed. (5) Flexibility or bending resistance A 180 ° bending test was repeated 100 times for the surface protective film substrate (biaxially stretched film), and the presence or absence of breakage was confirmed. Evaluation was made according to the following criteria.
：: No break was observed
×: rupture was observed

<Example 1>
1-1. Polymerization of acrylic resin and blending of elastomer In a reaction vessel equipped with a stirrer, a temperature sensor, a cooling pipe and a nitrogen inlet pipe, 229.6 parts of methyl methacrylate (MMA) and 2- (hydroxymethyl) methyl acrylate (MHMA) were added. ) 33 parts, 248.6 parts of toluene, and 0.19 part of n-dodecyl mercaptan were charged, and the temperature was raised to 105 ° C while passing nitrogen through. At the start of the reflux due to the temperature rise, 0.28 parts of t-amyl peroxy isononanoate ("Lupelox (registered trademark) 570" manufactured by Arkema Yoshitomi) was added as a polymerization initiator, and t-amyl peroxy was further added. 0.56 parts of isononanoate and 12.4 parts of styrene are added dropwise over 2 hours to advance solution polymerization under reflux at about 105 ° C to 110 ° C. After the completion of the dropwise addition, aging is continued for 4 hours at the same temperature. Was done. Next, 0.21 part of stearyl phosphate (“Phoslex A-18” manufactured by Sakai Chemical Industry Co., Ltd.) was added as a catalyst (cyclization catalyst) for the cyclization condensation reaction to the obtained polymerization solution, and the mixture was added to about 90 to 110. The cyclization condensation reaction for forming a lactone ring structure was allowed to proceed for 2 hours under reflux at ° C. Next, the obtained polymerization solution was passed through a multitubular heat exchanger heated to 240 ° C. to complete the cyclocondensation reaction. Thus, an acrylic resin having a lactone ring unit was obtained.
Subsequently, continuously from the completion of the cyclization condensation reaction, the resin solution was heated at a barrel temperature of 250 ° C. and one rear vent and four for vents (first, second, third, and A vent-type screw twin-screw extruder (L) having a side feeder between the third vent and the fourth vent, and a leaf-disk-type polymer filter (filtration accuracy: 5 μm) at the tip. / D = 52) at a processing speed of 31.2 parts / hour (in terms of resin amount). At that time, ion-exchanged water was introduced from the back of the second vent at an introduction rate of 0.47 parts / hour, and 0.66 parts of an ultraviolet absorbent (“ADEKA STAB (registered trademark) LA-F70” manufactured by ADEKA) was dissolved in toluene. The solution dissolved in 1.23 parts was introduced from the back of the third vent at an introduction rate of 0.59 parts / hour, and ion-exchanged water was introduced from the back of the fourth vent at an introduction rate of 0.47 parts / hour. I put it in. Furthermore, pellets of a polyethylene elastomer (manufactured by Sumitomo Chemical Co., Ltd., product name “Excellen FX”) were fed from a side feeder at a feeding rate of 0.63 parts / hour. Thus, pellets containing the acrylic resin having a lactone ring unit and the polyethylene elastomer were produced. The obtained pellets contained 10 parts by weight of elastomer with respect to 100 parts by weight of acrylic resin.

1-2. Preparation of Film Containing Acrylic Resin and Elastomer The obtained pellets were vacuum-dried at 80 ° C. for 5 hours, and then subjected to a single-screw extruder (manufactured by Isuzu Kakoki Co., Ltd., screw diameter 25 mm, cylinder set temperature: 250 ° C.) Using a film forming apparatus equipped with a T die (width 200 mm, set temperature: 250 ° C.), chill roll (set temperature: 120 to 130 ° C.), and a winder, a film containing an acrylic resin and an elastomer having a thickness of 160 μm. Was prepared. The glass transition temperature (Tg) of the film containing the acrylic resin and the elastomer was 121 ° C.

1-3. Production of Surface Protective Film Substrate The film containing the acrylic resin and the elastomer obtained above was simultaneously biaxially stretched twice in the length and width directions. The stretching temperature was [Tg + 10 ° C.] (that is, 131 ° C.), and the stretching speed was 1.4% / sec in both the length direction and the width direction. Thus, a substrate for a surface protective film (thickness: 40 μm) was obtained. Re (550) of the obtained substrate for surface protective film was 0.3 nm, Rth (550) was 1.6 nm, haze was 0.6%, and MIT test value was 2,000 times of the upper limit. Exceeded. The obtained substrate for a surface protective film was subjected to the evaluations of the above (4) and (5). Table 1 shows the results.

<Example 2>
A substrate for a surface protective film was obtained in the same manner as in Example 1, except that the blending amount of the elastomer was 15 parts by weight and the stretching temperature was [Tg + 20 ° C.]. Re (550) of the obtained base material for surface protective film is 0.5 nm, Rth (550) is 9 nm, haze is 0.2%, and MIT test value exceeds the upper limit of 2000 times. Met. The obtained substrate for a surface protective film was subjected to the evaluations of the above (4) and (5). Table 1 shows the results.

<Example 3>
A substrate for a surface protective film was obtained in the same manner as in Example 1, except that the blending amount of the elastomer was 8 parts by weight and the stretching temperature was [Tg + 30 ° C.]. The Re (550) of the obtained base material for surface protective film was 2 nm, the Rth (550) was 20 nm, the haze was 0.3%, and the MIT test value was 1032 times. The obtained substrate for a surface protective film was subjected to the evaluations of the above (4) and (5). Table 1 shows the results.

<Example 4>
Example 1 was the same as Example 1 except that 10 parts by weight of a styrene-based elastomer (product name: "TUFTEC" manufactured by Asahi Kasei Corporation) was used instead of 10 parts by weight of the polyethylene-based elastomer, and the stretching temperature was set to [Tg + 20 ° C.]. Similarly, a substrate for a surface protective film was obtained. Re (550) of the obtained base material for surface protective film is 0.8 nm, Rth (550) is 3 nm, haze is 0.3%, and MIT test value exceeds the upper limit of 2000 times. Met. The obtained substrate for a surface protective film was subjected to the evaluations of the above (4) and (5). Table 1 shows the results.

<Example 5>
A substrate for a surface protective film was obtained in the same manner as in Example 4, except that the blending amount of the elastomer was 15 parts by weight and the stretching temperature was [Tg + 30 ° C.]. Re (550) of the obtained base material for surface protective film was 1.2 nm, Rth (550) was 5.1 nm, haze was 0.4%, and MIT test value was 2000 times of the upper limit. Exceeded. The obtained substrate for a surface protective film was subjected to the evaluations of the above (4) and (5). Table 1 shows the results.

<Example 6>
Example 1 was repeated except that 20 parts by weight of a polyamide-based elastomer (manufactured by Tokyo Material Co., Ltd., product name "Pebax") was used instead of 10 parts by weight of the polyethylene-based elastomer, and the stretching temperature was set to [Tg + 20 ° C.]. In the same manner as in the above, a substrate for a surface protective film was obtained. The Re (550) of the obtained substrate for surface protective film was 2.3 nm, the Rth (550) was 3.1 nm, the haze was 0.8%, and the MIT test value was 2,000 times at the upper limit. Exceeded. The obtained substrate for a surface protective film was subjected to the evaluations of the above (4) and (5). Table 1 shows the results.

<Example 7>
10 parts by weight of core-shell type rubber-like particles (manufactured by Mitsubishi Chemical Corporation, product name "W-300") were used instead of 10 parts by weight of a polyethylene elastomer, and the stretching temperature was set to [Tg + 20 ° C.] Except for the above, a substrate for a surface protective film was obtained in the same manner as in Example 1. Re (550) of the obtained base material for surface protective film was 0.3 nm, Rth (550) was 1 nm, haze was 0.3%, and MIT test value was 592 times. The obtained substrate for a surface protective film was subjected to the evaluations of the above (4) and (5). Table 1 shows the results.

<Comparative Example 1>
A commercially available norbornene-based resin film (manufactured by Zeon Corporation, trade name “Zeonor”, Tg: 150 ° C.) was used in place of the film containing the acrylic resin and the elastomer, and the stretching temperature was set to [Tg + 20 ° C.] A base material for a surface protective film was obtained in the same manner as in Example 1 except for performing the above. The Re (550) of the obtained base material for surface protective film was 2 nm, the Rth (550) was 8 nm, the haze was 0.1%, and the MIT test value was 150 times. The obtained substrate for a surface protective film was subjected to the evaluations of the above (4) and (5). Table 1 shows the results.

<Comparative Example 2>
An ultra-high retardation polyethylene terephthalate film (manufactured by Mitsubishi Chemical Corporation, trade name “Dia Foil”, Tg: 81 ° C.) was used in place of the film containing the acrylic resin and the elastomer, and the stretching temperature was set to [Tg + 20 ° C.] A substrate for a surface protective film was obtained in the same manner as in Example 1 except that Re (550) of the obtained substrate for surface protective film was 4500 nm, Rth (550) was 6000 nm, haze was 1.3%, and the MIT test value exceeded the upper limit of 2000 times. Was. The obtained substrate for a surface protective film was subjected to the evaluations of the above (4) and (5). Table 1 shows the results.

<Comparative Example 3>
A substrate for a surface protective film was obtained in the same manner as in Example 1 except that the blending amount of the elastomer was 5 parts by weight and the stretching temperature was [Tg + 20 ° C.]. Re (550) of the obtained base material for surface protective film was 0.6 nm, Rth (550) was 1.3 nm, haze was 0.2%, and MIT test value was 412 times. . The obtained substrate for a surface protective film was subjected to the evaluations of the above (4) and (5). Table 1 shows the results.

<Evaluation>
As is clear from Table 1, it can be seen that the substrate for a surface protective film according to the example of the present invention prevents rainbow unevenness and has excellent bending resistance (or flexibility). It is presumed that this can be realized by stretching a film containing a specific acrylic resin and a specific elastomer under predetermined stretching conditions. Furthermore, from the results of Comparative Example 3, it can be seen that there is a critical value where the bending resistance (or flexibility) falls below an allowable range near the amount of the elastomer exceeding 5 parts by weight. In addition, it was confirmed that the same result as that of the rainbow unevenness was obtained for light leakage and coloring.

The substrate for a surface protective film of the present invention is suitably used for a surface protective film. The surface protective film of the present invention is used to protect the optical film until the optical film (finally, the image display device) is actually used. By using the surface protective film substrate and the surface protective film of the present invention, the precision of the optical inspection of the image display device can be significantly improved.

Claims (8)

  It is composed of a film containing an acrylic resin and an elastomer, has an in-plane retardation Re (550) of 30 nm or less, has a number of bending times up to break of 500 or more in an MIT test, and has 100 parts by weight of the acrylic resin. A base material for a surface protective film, comprising 6 parts by weight or more of the elastomer.   The surface protective film group according to claim 1, wherein the acrylic resin has at least one selected from the group consisting of a glutarimide unit, a lactone ring unit, a maleic anhydride unit, a maleimide unit, and a glutaric anhydride unit. Wood.   The base material for a surface protective film according to claim 1, wherein the elastomer is at least one selected from a rubber-like polymer, a polyamide-based elastomer, a polyethylene-based elastomer, a styrene-based elastomer, and a butadiene-based elastomer.   The substrate for a surface protective film according to any one of claims 1 to 3, wherein the total light transmittance is 80% or more and the haze is 1.0% or less.   The method according to any one of claims 1 to 4, comprising forming a film-forming material containing an acrylic resin and an elastomer into a film, and sequentially or biaxially stretching the formed film. A method for producing a substrate for a surface protective film according to the above.   The method for producing a substrate for a surface protective film according to claim 5, wherein the stretching temperature in the sequential biaxial stretching or the simultaneous biaxial stretching is Tg + 10 ° C to Tg + 30 ° C.   A surface protective film comprising the substrate for a surface protective film according to claim 1 and a pressure-sensitive adhesive layer. An optical film with a surface protective film, comprising: an optical film; and the surface protective film according to claim 7, which is releasably attached to the optical film.