阅读说明：本技术 表面保护薄膜用基材、该基材的制造方法、使用该基材的表面保护薄膜 (Base material for surface protection film, method for producing the base material, and surface protection film using the base material ) 是由 中原步梦 清水享 于 2019-08-28 设计创作，主要内容包括：本发明提供表面保护薄膜用基材、该基材的制造方法、使用该基材的表面保护薄膜。提供具有优异的挠性或耐弯折性、并且可良好地抑制光学检查时的漏光、着色及彩虹状不均匀的表面保护薄膜用基材。本发明的表面保护薄膜用基材由包含丙烯酸类树脂和弹性体的薄膜构成,面内相位差Re(550)为30nm以下、MIT试验中的直至断裂为止的弯折次数为500次以上。本发明的表面保护薄膜用基材的制造方法包括：将包含丙烯酸类树脂和弹性体的薄膜形成材料成形为薄膜状；及对成形得到的薄膜进行逐次双轴拉伸或同时双轴拉伸。(The invention provides a substrate for a surface protection film, a method for producing the substrate, and a surface protection film using the substrate. Provided is a substrate for a surface protective film, which has excellent flexibility and bending resistance and can well inhibit light leakage, coloring and rainbow unevenness during optical inspection. The substrate for a surface protection film of the present invention is composed of a film comprising an acrylic resin and an elastomer, and has an in-plane retardation Re (550) of 30nm or less and a number of times of bending until fracture in an MIT test of 500 or more. The method for producing a substrate for a surface protective film of the present invention comprises: forming a film-forming material comprising an acrylic resin and an elastomer into a film shape; and subjecting the film obtained by the molding to sequential biaxial stretching or simultaneous biaxial stretching.)

1. A substrate for a surface protective film, which comprises a film comprising an acrylic resin and an elastomer, wherein the in-plane retardation Re (550) is 30nm or less, the number of times of bending until fracture in an MIT test is 500 or more, and the elastomer is contained in an amount of 6 parts by weight or more per 100 parts by weight of the acrylic resin.

2. The substrate for a surface protection film according to claim 1, wherein the acrylic resin has at least 1 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.

3. The substrate for a surface protection film according to claim 1 or 2, wherein the elastomer is at least 1 selected from the group consisting of a rubbery polymer, a polyamide-based elastomer, a polyethylene-based elastomer, a styrene-based elastomer, and a butadiene-based elastomer.

4. The substrate for a surface protection 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.

5. A method for producing the substrate for a surface protective film according to any one of claims 1 to 4, comprising: forming a film-forming material comprising an acrylic resin and an elastomer into a film shape; and subjecting the film obtained by the molding to sequential biaxial stretching or simultaneous biaxial stretching.

6. The method for producing a substrate for a surface protection film according to claim 5, wherein a stretching temperature in the sequential biaxial stretching or simultaneous biaxial stretching is Tg +10 ℃ to Tg +30 ℃.

7. A surface protective film comprising the substrate for a surface protective film according to any one of claims 1 to 4 and an adhesive layer.

8. An optical film with a surface protective film, comprising: an optical film, and the surface protective film according to claim 7 releasably attached to the optical film.

Technical Field

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.

Background

An optical film (for example, a polarizing plate or a laminate including a polarizing plate) is bonded with a surface protective film in a peelable manner so as to protect the optical film (eventually, an image display device) until the image display device to which the optical film is applied is actually used. In practical use, an image display device is manufactured by bonding a laminate of an optical film and a surface protective film to a display unit, the image display device is subjected to an optical test (for example, a lighting test) in a state where the laminate is bonded, and the surface protective film is peeled off and removed at an appropriate time thereafter. The surface protective film typically has a resin film as a substrate and an adhesive layer. With conventional surface protective films, light leakage, coloration, rainbow unevenness, and the like may occur during optical inspection, which may cause a reduction in the accuracy of optical inspection. As a result, the image display device itself may be determined to be defective even in the optical inspection before shipment, and thus the efficiency from the manufacture of the image display device to shipment may be reduced. Further, the surface protective film (substantially, the base material) is required to have excellent flexibility in consideration of workability at the time of bonding and peeling.

Disclosure of Invention

Problems to be solved by the invention

The present invention has been made to solve the above-mentioned conventional problems, and a main object thereof is to provide a substrate for a surface protective film which has excellent flexibility and bending resistance and can satisfactorily suppress light leakage, coloring and rainbow unevenness in optical inspection.

Means for solving the problems

The substrate for a surface protection film according to an embodiment of the present invention is composed of a film including an acrylic resin and an elastomer, the in-plane retardation Re (550) is 30nm or less, the number of times of bending until fracture in MIT test is 500 or more, and the elastomer is included by 6 parts by weight or more per 100 parts by weight of the acrylic resin.

In 1 embodiment, the acrylic resin has at least 1 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.

In 1 embodiment, the elastomer is at least 1 selected from the group consisting of a rubbery polymer, a polyamide-based elastomer, a polyethylene-based elastomer, a styrene-based elastomer, and a butadiene-based elastomer.

In 1 embodiment, the surface-protecting-film substrate 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. The manufacturing method comprises the following steps: forming a film-forming material comprising an acrylic resin and an elastomer into a film shape; and subjecting the film obtained by the molding to sequential biaxial stretching or simultaneous biaxial stretching.

In 1 embodiment, in the above production method, the stretching temperature in the sequential biaxial stretching or simultaneous biaxial stretching is Tg +10 to Tg +30 ℃.

According to still another aspect of the present invention, a surface protective film is provided. The surface-protecting film comprises the substrate for a surface-protecting film and an adhesive layer.

According to still another aspect of the present invention, there is provided an optical film with a surface protective film. The optical film with a surface protective film comprises: an optical film and the surface protective film releasably attached to the optical film.

ADVANTAGEOUS EFFECTS OF INVENTION

According to the embodiment of the present invention, a substrate for a surface protective film that can achieve both a very small in-plane retardation and excellent flexibility and bending resistance can be realized by stretching a film comprising an acrylic resin and an elastomer under predetermined stretching conditions (for example, a stretching temperature and a stretching speed).

Detailed Description

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

A. Base material for surface protective film

The surface-protecting-film substrate according to the embodiment of the present invention is composed of a film containing an acrylic resin and an elastomer.

As the acrylic resin, any suitable acrylic resin can be used. The acrylic resin typically contains an alkyl (meth) acrylate as a main component as a monomer unit. In the present specification, "(meth) acrylic acid" means acrylic acid and/or methacrylic acid. Examples of the alkyl (meth) acrylate constituting the main skeleton of the acrylic resin include alkyl (meth) acrylates having 1 to 18 carbon atoms and having a linear or branched alkyl group. They may be used alone or in combination. Further, any suitable comonomer may be introduced into the acrylic resin by copolymerization. The kind, amount, copolymerization ratio and the like of such comonomers 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 the group consisting of 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 1 of these structural units, or may have a plurality of types. Acrylic resins having a lactone ring unit are described in, for example, Japanese patent laid-open No. 2008-181078, the description of which is incorporated herein by reference. The glutarimide unit is preferably represented by the following general formula (1),

in the general formula (1), R¹And R²Each independently represents hydrogen or C1-C8 alkyl, R³Represents an alkyl group having 1 to 18 carbon atoms, a cycloalkyl group having 3 to 12 carbon atoms, or an aryl group having 6 to 10 carbon atoms. In the general formula (1), R is preferably¹And R²Each independently is hydrogen or methyl, R³Is hydrogen, methyl, butyl or cyclohexyl. More preferably R¹Is methyl, R²Is hydrogen, R³Is methyl.

The above-mentioned alkyl (meth) acrylate is typically represented by the following general formula (2),

in the general formula (2), R⁴Represents a hydrogen atom or a methyl group, R⁵Represents a hydrogen atom or an optionally substituted aliphatic or alicyclic hydrocarbon group having 1 to 6 carbon atoms. Examples of the substituent include halogen and hydroxyl. Specific examples of the alkyl (meth) acrylate include methyl (meth) acrylate, ethyl (meth) acrylate, n-propyl (meth) acrylate, n-butyl (meth) acrylate, t-butyl (meth) acrylate, n-hexyl (meth) acrylate, cyclohexyl (meth) acrylate, chloromethyl (meth) acrylate, 2-chloroethyl (meth) acrylate, 2-hydroxyethyl (meth) acrylate, 3-hydroxypropyl (meth) acrylate, 2,3,4,5, 6-pentahydroxyhexyl (meth) acrylate, and 2,3,4, 5-tetrahydroxypentyl (meth) acrylate. In the general formula (2), R⁵Preferably a hydrogen atom or a methyl group. Thus, a particularly preferred alkyl (meth) acrylate is methyl acrylate or methyl methacrylate.

The acrylic resin may contain only a single glutarimide unit, or may contain R in the general formula (1)¹、R²And R³Different glutarimide units.

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%, even more preferably 2 mol% to 40 mol%, particularly preferably 2 mol% to 35 mol%, and most preferably 3 mol% to 30 mol%. If the content ratio is less than 2 mol%, the effects derived from the glutarimide unit (for example, high optical properties, high mechanical strength, excellent adhesion to a polarizer, and thinning) may not be sufficiently exhibited. If the content exceeds 50 mol%, for example, heat resistance and transparency may become insufficient. When the acrylic resin has a lactone ring unit, a maleic anhydride unit, a maleimide unit and/or a glutaric anhydride unit in addition to or instead of a glutarimide unit, the content ratio may be the total content ratio of these constituent units.

The acrylic resin may contain only a single alkyl (meth) acrylate unit, or may contain R in the general formula (2)⁴And R⁵Different plural alkyl (meth) acrylate units.

The content ratio of the alkyl (meth) acrylate unit in the acrylic resin is preferably 50 to 98 mol%, more preferably 55 to 98 mol%, still more preferably 60 to 98 mol%, particularly preferably 65 to 98 mol%, and most preferably 70 to 97 mol%. If the content ratio is less than 50 mol%, the effects (e.g., high heat resistance and high transparency) derived from the alkyl (meth) acrylate unit may not be sufficiently exhibited. If the content ratio is more than 98 mol%, the resin may be brittle and easily broken, and high mechanical strength may not be sufficiently exhibited, resulting in poor productivity.

The acrylic resin may contain a copolymerizable vinyl monomer unit (other vinyl monomer unit) other than the above-described one, and examples of the other vinyl monomer include acrylonitrile, methacrylonitrile, ethacrylonitrile, allyl glycidyl ether, maleic anhydride, itaconic anhydride, N-methylmaleimide, N-ethylmaleimide, N-cyclohexylmaleimide, aminoethyl acrylate, propylaminoethyl acrylate, dimethylaminoethyl methacrylate, ethylaminopropyl methacrylate, cyclohexylaminoethyl methacrylate, N-vinyldiethylamine, N-acetylvinylamine, allylamine, methallylamine, N-methylallylamine, 2-isopropenyloxazoline, 2-vinyloxazoline, 2-acryloyloxazoline, N-phenylmaleimide, phenylaminoethyl methacrylate, styrene, α -methylstyrene, p-glyceryl styrene, p-styrene, 2-vinyloxazoline, 2-acryloyloxazoline, and the like, and preferably, when these monomers are used alone or in combination, the ratio of these monomers is preferably 0 to 1% by weight, and when these monomers are used alone, the ratio of styrene is preferably 0 to 1% by weight.

The imidization ratio of the acrylic resin is preferably 2.5% to 20.0%. When the imidization ratio is in such a range, a resin excellent in heat resistance, transparency and molding processability can be obtained, and occurrence of scale formation and reduction in mechanical strength at the time of film molding 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 imidization ratio can be used¹Passing HNMR BRUKER AvanceIII (400MHz) through the resin¹H-NMR measurement. More specifically, about 3.5 to 3.8ppm of O-CH derived from an alkyl (meth) acrylate₃A represents the peak area of proton, and the N-CH derived from glutarimide in the vicinity of 3.0 to 3.3ppm₃The area of the peak of proton is B, and is obtained by the following equation.

Imidization ratio Im (%) { B/(A + B) } × 100

The acid value of the acrylic resin is preferably 0.10mmol/g to 0.50 mmol/g. When the acid value is within such a range, a resin having an excellent balance among heat resistance, mechanical properties, and moldability can be obtained. If the acid value is too small, the following problems may occur: an increase in cost due to the use of a modifier for adjusting the acid value to a desired value, generation of a gel-like material due to the residue of the modifier, and the like. If the acid value is too large, foaming tends to occur during film molding (e.g., during melt extrusion), and the productivity of the molded article tends to be lowered. The acid value of the acrylic resin is the content of carboxylic acid units and carboxylic acid anhydride units in the acrylic resin. In the present embodiment, the acid value can be calculated by the titration method described in, for example, WO2005/054311 or Japanese patent application laid-open No. 2005-23272.

Details of the acrylic resin, its production method, its properties, and the like are described in, for example, Japanese patent laid-open Nos. 2016-139027, 2007-316366, and 2008-181078, the descriptions of which are incorporated herein by reference.

As the elastic body, any suitable elastic body can be used. Elastomers typically have hard segments that can function as pseudo-crosslinks and soft segments that can contribute primarily to elasticity. Typical examples of the elastomer include rubbery polymers, polyamide elastomers, polyethylene elastomers, styrene elastomers, and butadiene elastomers. The elastomers may be used alone or in combination. The elastic body may be appropriately selected according to the desired characteristics. For example, a styrene-based elastomer can be used from the viewpoint of ease of designing optical properties. As a typical example of the styrene-based elastomer, there can be mentioned a styrene-based elastomer having polystyrene as a hard segment and polybutadiene, polyisoprene or a copolymer of polybutadiene and polyisoprene as a soft segment, and hydrogenated products thereof. The hydride may be one obtained by hydrogenating a part of polybutadiene, polyisoprene, or the like, or may be one obtained by hydrogenating all of them. As the styrene-based elastomer, commercially available products can be used. Examples of commercially available styrene-based elastomers include Tuftec, Tufprene (manufactured by Asahi Kasei Chemicals Corporation, supra), Kraton (manufactured by Kraton Corporation), DYNARON, JSR TR, JSR SIS (manufactured by JSR Corporation, supra), SEPTON (KURARAAY CO., LTD, supra), and RABALON (manufactured by Mitsubishi Chemical Corporation). The polyethylene elastomer is preferably a polyethylene elastomer having 50 parts or more of an ethylene block, and examples of commercially available products include EXCELLEN FX (sumitomo chemical co., ltd.) and the like. The polyamide elastomer preferably has a polyether block, and commercially available products include Pebax (manufactured by tokyo materials co., ltd.). The rubber-like polymer is preferably a core-shell type particle, and a commercially available product such as W-300 (manufactured by Mitsubishi chemical corporation) can be mentioned.

The amount of the elastomer is 6 parts by weight or more, preferably 6 to 30 parts by weight, and more preferably 8 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 surface-protecting film substrate having both a very small in-plane retardation and excellent flexibility and bending resistance can be realized.

In the embodiment of the present invention, the in-plane retardation Re (550) of the substrate for a surface protective film is 30nm or less, preferably 10nm or less, more preferably 5nm or less, still more preferably 3nm or less, and particularly preferably 2.5nm or less. The smaller the in-plane retardation Re (550) is, the more preferable the lower limit thereof is 0nm, and for example, it may be 0.1 nm. When the in-plane retardation Re (550) is in such a range, light leakage, coloration, and rainbow unevenness can be favorably suppressed when the surface protection film using the substrate for a surface protection film of the present invention is subjected to optical inspection of an image display device. As a result, the accuracy of optical inspection of the image display device can be significantly improved, and the efficiency from the manufacture to the shipment of the image display device can be improved. Such an in-plane retardation Re (550) can be achieved by stretching a film comprising the above-mentioned specific acrylic resin and the specific elastomer under predetermined stretching conditions as described later. In the present specification, "Re (λ)" is an in-plane retardation measured at 23 ℃ with light having a wavelength of λ nm. For Re (λ), assuming that the thickness of the layer (film) is d (nm), according to the formula: re ═ x-ny) × d. Thus, "Re (550)" is an in-plane retardation measured at 23 ℃ with light having a wavelength of 550 nm. Here, "nx" is a refractive index in a direction in which the in-plane refractive index is maximum (i.e., the slow axis direction), and "ny" is a refractive index in a direction orthogonal to the slow axis (i.e., the fast axis direction) in the plane.

The retardation Rth (550) in the thickness direction of the substrate for a surface protective film is preferably 50nm or less, more preferably 25nm or less, further preferably 15nm or less, and particularly preferably 10nm or less. The lower the thickness direction retardation Rth (550) is, the more preferable, the lower limit thereof is preferably 0nm, and for example, it may be 0.5 nm. When the thickness direction retardation Rth (550) is in such a range, the light leakage in the oblique direction, coloring, and rainbow unevenness in the optical inspection can be favorably suppressed. As a result, the accuracy can be significantly improved even in the optical inspection of a large-sized image display device. The surface-protecting film substrate preferably has an Nz coefficient of 1.1 to 20, more preferably 1.5 to 5.4. Therefore, the refractive index characteristics of the substrate for a surface protection film can show a relationship of, for example, nx > ny > nz. When the Nz coefficient is in such a range, light leakage in an oblique direction, coloring, and rainbow unevenness in optical inspection can be further favorably suppressed. "Rth (λ)" is a phase difference in the thickness direction measured at 23 ℃ with light having a wavelength of λ nm. With respect to Rth (λ), when the thickness of the layer (film) is d (nm), according to the formula: and Rth ═ x-nz) × d. Thus, "Rth (550)" is a retardation in the thickness direction measured at 23 ℃ with light having a wavelength of 550 nm. Here, "nz" is a refractive index in the thickness direction. The "Nz coefficient" is obtained by Nz ═ Rth (λ)/Re (λ).

The surface protective film substrate can exhibit reverse dispersion wavelength characteristics in which the in-plane retardation increases according to the wavelength of the measurement light, can exhibit positive wavelength dispersion characteristics in which the in-plane retardation decreases according to the wavelength of the measurement light, and can exhibit flat wavelength dispersion characteristics in which the in-plane retardation does not substantially change depending on the wavelength of the measurement light.

The total light transmittance of the surface-protecting-film substrate 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 substrate for a surface protective film 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. According to the embodiment of the present invention, a substrate for a surface protective film having such a very excellent transparency as having the very small in-plane retardation Re (550) as described above can be realized.

In the embodiment of the present invention, the number of times of bending until breakage in the MIT test is 500 or more, preferably 1000 or more, more preferably 1500 or more, and further preferably 2000 or more times with respect to the substrate for a surface protective thin film. That is, the substrate for a surface protective film can have very excellent flexibility or bending resistance. Due to such excellent flexibility and bending resistance of the substrate for a surface protection film, a surface protection film which is excellent in handling property at the time of bonding and peeling and is suppressed in breakage can be obtained. According to the embodiment of the present invention, such excellent flexibility and bending resistance can be achieved at the same time as well as the very small in-plane retardation Re (550) described above. The achievement of such a compromise is one of the achievements of the present invention. The MIT test can be performed according to JIS P8115.

The elastic modulus of the surface-protecting-film substrate is preferably 50MPa to 350MPa at a stretching speed of 100 mm/min. When the elastic modulus is in such a range, a surface protection film excellent in transportability and handling properties can be obtained. According to the embodiments of the present invention, it is possible to achieve both of excellent elastic modulus (strength) and excellent flexibility or bending resistance (flexibility) as described above. The elastic modulus was measured in accordance with JIS K7127: 1999.

The tensile elongation of the surface-protecting film substrate is preferably 70% to 200%. When the tensile elongation is in such a range, there is an advantage that the sheet is not easily broken during conveyance. The tensile elongation is measured according to JIS K6781.

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

B. Method for producing substrate for surface protective film

The method for producing a substrate for a surface protection film according to an embodiment of the present invention includes: the method for producing a film of the present invention comprises the steps of forming the film-forming material (resin composition) comprising the acrylic resin and the elastomer as described in the above item A into a film, and stretching the film obtained by the forming.

The film-forming material may contain other resins, additives, and solvents in addition to the acrylic resin and the elastomer. As the additive, any suitable additive may be used according to the purpose. Specific examples of the additives include reactive diluents, plasticizers, surfactants, fillers, antioxidants, anti-aging agents, ultraviolet absorbers, leveling agents, thixotropic agents, antistatic agents, conductive materials, and flame retardants. The amount, kind, combination, addition amount, and the like of the additives may be appropriately set according to the purpose.

As a method of forming a thin film from the thin film forming material, any appropriate forming process may be employed. Specific examples thereof include compression molding, transfer molding, injection molding, extrusion molding, blow molding, powder molding, FRP molding, casting coating (for example, casting), calendering, and hot pressing. 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 may be appropriately set depending on the composition and type of the resin used, the desired properties of the substrate for a 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 flexibility and bending resistance can be obtained. The sequential biaxial stretching or simultaneous biaxial stretching is typically performed using a tenter. Therefore, the stretching direction of the film is typically the longitudinal direction and the width direction of the film.

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

The stretch ratio may vary depending on the in-plane retardation and thickness desired for the substrate for a surface protective film, the type of resin used, the thickness of the film used, the stretching temperature, and the like. In the case of biaxial stretching (for example, sequential biaxial stretching or simultaneous biaxial stretching), the difference between the stretch ratio in the 1 st direction (for example, the longitudinal direction) and the stretch ratio in the 2 nd direction (for example, the width direction) is preferably as small as possible, and more preferably substantially equal. With such a configuration, a substrate for a surface protective film having a small in-plane retardation Re (550) and excellent flexibility and bending resistance can be obtained. In the case of biaxial stretching (for example, sequential biaxial stretching or simultaneous biaxial stretching), the stretching ratio may be, for example, 1.1 to 3.0 times in each of the 1 st direction (for example, the longitudinal direction) and the 2 nd direction (for example, the width direction).

In the embodiment of the present invention, the stretching speed is preferably 10%/second or less, more preferably 7%/second or less, further preferably 5%/second or less, and particularly preferably 2.5%/second or less. By stretching a film comprising the above-mentioned specific acrylic resin and the specific elastomer at such a low stretching speed, a substrate for a surface protective film having a small in-plane retardation Re (550) and excellent flexibility and bending resistance can be obtained. The lower limit of the drawing speed may be, for example, 1.2%/second. If the stretching speed is too low, the productivity may become impractical. In the case of biaxial stretching (for example, sequential biaxial stretching or simultaneous biaxial stretching), the difference between the stretching speed in the 1 st direction (for example, the longitudinal direction) and the stretching speed in the 2 nd 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 and the bending resistance can be further improved.

C. Surface protective film

The surface-protecting film substrate described in the above items A and B can be suitably used for a surface-protecting film. Accordingly, embodiments of the present invention also include surface protection films. A surface-protecting film according to an embodiment of the present invention comprises the substrate for a surface-protecting film described in the above items A and B and a pressure-sensitive adhesive layer.

As the adhesive for forming the adhesive layer, any suitable adhesive can be used. Examples of the base resin of the binder include acrylic resins, styrene resins, silicone resins, urethane resins, and rubber resins. Such a base resin is described in, for example, Japanese patent laid-open Nos. 2015-120337 and 2011-201983. The descriptions of these publications are incorporated herein by reference. From the viewpoints of chemical resistance, adhesion for preventing the entry of a treatment liquid during immersion, freedom in adhesion to an adherend, and the like, an acrylic resin is preferred. Examples of the crosslinking agent that can be contained in the adhesive include isocyanate compounds, epoxy compounds, and aziridine compounds. The binder may, for example, comprise a silane coupling agent. The compounding recipe of the adhesive can be appropriately set according to the purpose and the desired characteristics.

The storage modulus of the adhesive layer is preferably 1.0X 10⁴Pa～1.0×10⁷Pa, more preferably 2.0X 10⁴Pa～5.0×10⁶Pa. When the storage modulus of the adhesive layer is in such a range, blocking at the time of roll formation can be suppressed. The storage modulus can be determined by, for example, dynamic viscoelasticity measurement at a temperature of 23 ℃ and an angular velocity of 0.1 rad/s.

The thickness of the pressure-sensitive adhesive layer is preferably 1 to 60 μm, more preferably 3 to 30 μm. If the thickness is too thin, the adhesiveness may become insufficient, and air bubbles may be mixed in the adhesion interface. If the thickness is too large, problems such as bleeding of the adhesive tend to occur.

In practice, the surface protective film is temporarily attached to a separator in a releasable manner on the surface of the pressure-sensitive adhesive layer before the film is actually used (i.e., attached to an optical film or an image display device). By providing the separator, the adhesive layer can be protected and the surface protection film can be wound up in a roll shape. Examples of the separator include a plastic (for example, polyethylene terephthalate (PET), polyethylene, and polypropylene) film, a nonwoven fabric, and paper, which are surface-coated with a release agent such as a silicone release agent, a fluorine release agent, and a long-chain alkyl acrylate release agent. As for the thickness of the separator, any appropriate thickness may be adopted according to the purpose. The thickness of the separator is, for example, 10 μm to 100 μm.

D. Optical film with surface protective film

The surface protective film described in the above item C is used for protecting an optical film (ultimately, an image display device) before the optical film is actually used. Accordingly, embodiments of the present invention also include optical films with surface protective films. 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 described in the above item C 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 a laminate obtained by appropriately laminating them according to the purpose (for example, a circular polarizing plate for antireflection, a conductive layer-equipped polarizing plate for a touch panel, a polarizing plate with a retardation layer, and a prism sheet-integrated polarizing plate).