阅读说明：本技术 表面保护薄膜及带有保护薄膜的光学构件 (Surface protective film and optical member with protective film ) 是由 山形真人 小川圭太 片冈贤一 于 2018-12-05 设计创作，主要内容包括：表面保护薄膜(10)具备固着层叠于薄膜基材(1)的主表面上的粘合剂层(2)。薄膜基材(1)的正面延迟为100nm以下。表面保护薄膜(10)在温度60℃相对湿度90％下的透湿度为300g/m<Sup>2</Sup>·24h以下,对亚克力板的拉伸速度30m/分钟下的180°剥离力为3N/25mm以下。通过将表面保护薄膜贴合于光学构件的表面,可得到带有保护薄膜的光学构件。(The surface protection film (10) is provided with a pressure-sensitive adhesive layer (2) which is fixedly laminated on the main surface of a film base material (1). The front retardation of the film base material (1) is 100nm or less. The surface protective film (10) has a moisture permeability of 300g/m at a temperature of 60 ℃ and a relative humidity of 90% 2 24 hours or less, and a 180 DEG peel force at a stretching speed of 30 m/min to an acrylic sheet of 3N/25mm or less. By bonding a surface protective film to the surface of an optical member, an optical member with a protective film can be obtainedA component.)

1. A surface-protecting film comprising a film base and an adhesive layer fixedly laminated on a first main surface of the film base,

the front retardation of the film base material is 100nm or less,

the surface protection film has a moisture permeability of 300g/m at a temperature of 60 ℃ and a relative humidity of 90%²24 hours or less, and a 180 DEG peel force at a stretching speed of 30 m/min to an acrylic sheet of 3N/25mm or less.

2. The surface protection film according to claim 1, wherein the film substrate has a thickness direction retardation of 100nm or less.

3. The surface protection film according to claim 1 or 2, which has a tensile breaking strength of 120MPa or less.

4. The surface-protecting film according to any one of claims 1 to 3, wherein the film substrate is a cycloolefin-based film or an acrylic-based film.

5. The surface protection film according to any one of claims 1 to 4, wherein the adhesive layer has a thickness of 1 to 50 μm.

6. The surface protection film according to any one of claims 1 to 5, wherein the adhesive layer comprises an acrylic base polymer having a crosslinked structure.

7. The surface protection film according to claim 6, wherein a crosslinked structure is introduced into the base polymer.

8. The surface protection film according to any one of claims 1 to 7, wherein an antistatic layer is provided on the second main surface of the film base material.

9. The surface protection film of claim 8, wherein the antistatic layer has a surface resistivity of 1.0 × 10¹²Omega/□ or less.

10. An optical member having a protective film, wherein the surface protective film according to any one of claims 1 to 9 is bonded to a surface of the optical member.

Technical Field

The present invention relates to a surface protection film having a pressure-sensitive adhesive layer on a film base material, and an optical member having the surface protection film bonded thereto.

Background

A surface-protecting film is provided on the surface of optical devices such as displays, electronic devices, and films and glass materials that are components of these devices for the purpose of surface protection, impact resistance, and the like. The surface protection film includes a pressure-sensitive adhesive layer on a main surface of a film base material, and is bonded to a surface of an adherend to be protected via the pressure-sensitive adhesive layer.

The biaxially stretched polyester film and the biaxially stretched polypropylene film have a large retardation, and therefore, in inspection using polarized light, light leakage, coloration, rainbow unevenness (rainbow uneveness) and the like due to the retardation of the film base material occur.

In order to solve such a problem, patent document 1 proposes to use a low-phase difference film such as a cellulose-based resin film, a cycloolefin-based resin film, or a non-stretched polypropylene film as a film base.

Disclosure of Invention

Problems to be solved by the invention

As the low retardation film, a film having no stretch or a low stretch ratio or a film made of a low birefringent material is generally used. However, polyester films, polypropylene films, and the like generally have improved mechanical strength by orienting molecules in the in-plane direction of the film and crystallizing the molecules by biaxial stretching, and therefore unstretched films have low mechanical strength. Further, even when a film made of a low birefringent material such as a cyclic olefin or an acrylic is a stretched film, the mechanical strength is often lower than that of a biaxially stretched polyester film. Therefore, when the surface protective film is peeled and removed from an adherend, the film base material is likely to be cracked or broken, and peeling becomes difficult in some cases, and such a tendency tends to be remarkable particularly when peeling is performed at a high speed.

The cellulose-based film has a property that molecules are easily oriented in the in-plane direction even in a non-stretched state, and has higher mechanical strength than a cycloolefin-based film. However, a surface protection film using a cellulose-based film base material has a problem that peeling or floating from an adherend is likely to occur when exposed to a high-temperature and high-humidity environment in a state of being bonded to the adherend.

In view of the above, an object of the present invention is to provide a surface protection film which does not inhibit optical inspection in a state of being bonded to an adherend, has excellent moisture resistance, and is easily peeled from the adherend.

Means for solving the problems

The present invention relates to a surface protection film including an adhesive layer fixedly laminated on a first main surface of a film base. The film substrate has a front retardation of 100nm or less. The retardation in the thickness direction of the film substrate is preferably 100nm or less. The tensile breaking strength of the film base is, for example, 200MPa or less, and may be 120MPa or less or 100MPa or less. Examples of the film substrate include a cycloolefin film and an acrylic film.

The surface protection film has a moisture permeability of 300g/m at a temperature of 60 ℃ and a relative humidity of 90%²24h or less. In order to set the moisture permeability of the surface protective film to the above range,the moisture permeability of the film base is preferably in the above range.

The 180 DEG peel force of the surface protective film to an acrylic plate at a stretching speed of 30 m/min is 3N/25mm or less, preferably 2.5N/25mm or less. In particular, when the film base material has a low breaking strength (for example, tensile breaking strength of 120MPa or less), the 180 DEG peel force at a stretching speed of 30 m/min of the surface protective film is preferably 2N/25mm or less in order to prevent cracking or breaking of the film at the time of high-speed peeling.

The thickness of the adhesive layer is preferably 1 to 50 μm. In particular, the thickness of the pressure-sensitive adhesive layer is preferably 25 μm or less in order to suppress white turbidity of the pressure-sensitive adhesive layer in a high-humidity environment and maintain transparency. As the adhesive constituting the adhesive layer, an acrylic adhesive containing an acrylic base polymer is preferable. A crosslinked structure may be introduced into the base polymer.

The surface protective film may be provided with an antistatic layer on the second main surface of the film base material the surface resistivity of the antistatic layer is preferably 1.0 × 10¹²Omega/□ or less.

ADVANTAGEOUS EFFECTS OF INVENTION

The surface protection film of the present invention has a low phase difference, and therefore can prevent light leakage, coloration, iridescence, and the like in optical inspection of an adherend to which the protection film is bonded. Therefore, the protective film of the present invention is suitable as a protective film for various optical members. The surface protection film of the present invention has high humidification reliability of adhesive force, and is less likely to cause floating and peeling of the surface protection film even when an adherend with the surface protection film is exposed to a high humidity environment. Further, since the surface protection film of the present invention has a small peeling force from an adherend, cracking and breaking are less likely to occur at the time of high-speed peeling even when the film base material has a low strength.

Drawings

Fig. 1 is a sectional view showing a laminated structure of surface protection films.

Fig. 2 is a sectional view showing a laminated structure of the surface protective film to which the separator is temporarily bonded.

Detailed Description

[ constitution of surface protective film ]

Fig. 1 is a sectional view showing an embodiment of a surface protective film. The surface protection film 10 includes an adhesive layer 2 on a first main surface of a film base 1. The adhesive layer 2 is fixedly laminated on the first main surface of the film base 1. An antistatic layer (not shown) may be provided on the second main surface of the film base 1.

As shown in fig. 2, a separator 5 may be temporarily bonded to the adhesive layer 2 of the surface protective film 10. The separator 5 temporarily bonded to the surface of the pressure-sensitive adhesive layer 2 is peeled off and removed, and the exposed surface of the pressure-sensitive adhesive layer 2 is bonded to an adherend, whereby the surface of the adherend can be protected. "fixed" means that 2 layers stacked are firmly bonded and peeling at the interface between the two layers is impossible or difficult. The "temporary bonding" is a state in which the adhesion between the stacked 2 layers is small and the two layers can be easily peeled off from each other at the interface.

The 180 DEG peel force (stretching speed: 30 m/min) of the surface protective film 10 to an acrylic plate was 3N/25mm or less. Hereinafter, unless otherwise specified, the peeling force in the 180 ° peeling test at a stretching speed of 30 m/min of the acrylic sheet is simply referred to as "peeling force". When the peeling force is 3N/25mm or less, the protective film is easily peeled from the adherend at a high speed, and the workability is excellent.

The peel force of the surface protective film 10 is preferably 2.5N/25mm or less. In particular, when the strength of the surface protective film 10 is small (for example, when the tensile breaking strength of the film base is 120MPa or less), the peeling force is preferably 2N/25mm or less, more preferably 1.7N/25mm or less, and still more preferably 1.5N/25mm or less. When the peeling force is in the above range, even when the strength of the film base material is small, the film can be prevented from being cracked or broken at the time of high-speed peeling. The peel force is preferably 0.1N/25mm or more, more preferably 0.2N/25mm or more, and still more preferably 0.3N/25mm or more, from the viewpoint of securing adhesiveness to an adherend. The peel force of the surface protective film 10 is mainly affected by the characteristics of the adhesive layer 2.

The surface protective film 10 has a moisture permeability of 300g/m²24h or less. Moisture permeability according to JIS Z0208: 1976 the value measured by the Water vapor Permeability test (cup method) is a value of a permeation area of 1m per 24 hours in an environment of 60 ℃ and 90% relative humidity²The weight of water vapor of the sample (c). Hereinafter, the moisture permeability measured under the above conditions is simply referred to as "moisture permeability" unless otherwise specified. The moisture permeability of JIS Z0208 tends to be a value larger than the value measured at a temperature of 25 ℃ or 40 ℃ and a relative humidity of 90%, the value measured at a higher temperature of 60 ℃.

The moisture permeability at a temperature of 60 ℃ and a relative humidity of 90% is 300g/m²When the time is 24 hours or less, the adherend to which the surface protection film 10 is bonded can maintain adhesiveness even when exposed to a high-temperature and high-humidity environment, and the surface protection film 10 can be prevented from floating or peeling from the adherend. The surface protective film 10 preferably has a moisture permeability of 200g/m²24h or less, more preferably 150g/m²24h or less, more preferably 100g/m²24h or less.

The moisture permeability of the surface protection film 10 is influenced by both the film base 1 and the pressure-sensitive adhesive layer 2, but is mainly influenced by the moisture permeability of a layer having a low moisture permeability. The moisture permeability of the surface protective film 10 is substantially equal to the moisture permeability of the film substrate 1, because the film substrate 1 generally has a lower moisture permeability than the adhesive layer 2, depending on the material, thickness, and the like of the film substrate 1 and the adhesive layer.

[ film base ]

As the film substrate 1, a plastic film is used. The film base material has a thickness of about 5 to 500 μm, for example. The film base 1 preferably has a thickness of 10 to 300 μm, more preferably 15 to 200 μm, and still more preferably 20 to 150 μm, from the viewpoint of satisfying both the protective performance against an adherend and the flexibility.

In the case where the optical inspection of the adherend is performed in a state where the surface protective film 10 and the adherend are bonded, the film base material 1 is preferably transparent. The total light transmittance of the film base 1 is preferably 80% or more, more preferably 85% or more, and further preferably 90% or more. The haze of the film base 1 is preferably 10% or less, more preferably 5% or less, and further preferably 2% or less.

The film base material 1 has a front retardation Re of 100nm or less. By using the film base material 1 having a small front retardation Re, the front retardation of the surface protective film 10 is reduced, and therefore, light leakage, coloration, iridescence, and the like in optical inspection of an adherend to which the protective film is bonded can be prevented. The front retardation Re of the film substrate 1 is preferably 50nm or less, more preferably 30nm or less, and still more preferably 20nm or less.

The retardation Rth in the thickness direction of the film substrate 1 is preferably 100nm or less, more preferably 50nm or less, and further preferably 30nm or less. By making the retardation Rth in the thickness direction of the film base material 1 small, light leakage, coloration, iridescence and the like can be prevented even when viewed from an oblique direction. Therefore, in a wide-range captured image by a fixed camera, the difference in color and brightness between the vicinity of the center (front direction) and the peripheral portion (oblique direction) is small, and data processing is easy.

The front retardation Re and the thickness direction retardation Rth are defined below, and unless otherwise specified, all values are measured at a wavelength of 590 nm.

Re＝(nx-ny)×d

Rth＝(nx-nz)×d

nx is a refractive index in an in-plane slow axis direction, ny is a refractive index in an in-plane fast axis direction, nz is a refractive index in a thickness direction, and d is a thickness.

Generally, the adhesive layer has a smaller phase difference than the film base material. Therefore, the front retardation Re and the thickness direction retardation Rth of the surface protective film 10 having the pressure-sensitive adhesive layer 2 provided on the film base 1 are substantially equal to the front retardation Re and the thickness direction retardation Rth of the film base 1, respectively. Therefore, by using a film base material having the front surface retardation Re and the thickness direction retardation in the above ranges, the front surface retardation Re and the thickness direction retardation Rth of the surface protective film 10 are also in the above ranges.

As for the moisture permeability of the film base 1, the moisture permeability is preferably 300g/m²24h or less, more preferably 200g/m²24h or less, more preferably 150g/m²24h or less, particularly preferably 100g/m²24h or less. When the moisture permeability of the film base 1 is in the above range, even when the moisture permeability of the pressure-sensitive adhesive layer 2 is high, the moisture content can be suppressed from spreading to the pressure-sensitive adhesive layer 2 and stickingThe adhesive layer 2 penetrates into the bonding interface with the adherend. Therefore, even when the adherend to which the surface protection film 10 is bonded is exposed to a high-temperature and high-humidity environment, the surface protection film 10 can be prevented from floating and peeling from the adherend.

The resin material constituting the film base is not particularly limited, and examples of the material that can satisfy the requirements of high transparency, low phase difference, and low moisture permeability include a cycloolefin resin and an acrylic resin.

Examples of the cycloolefin resin include polynorbornene, and commercially available products of the cycloolefin resin include ZEONOR and ZEONEX manufactured by Zeon Corporation, ARTON manufactured by JSR, APE L manufactured by mitsui chemical, TOPAS manufactured by TOPAS ADVANCED PO L YMERS.

Examples of the acrylic resin include poly (meth) acrylates such as polymethyl methacrylate, methyl methacrylate- (meth) acrylic acid copolymers, methyl methacrylate- (meth) acrylic acid ester copolymers, methyl methacrylate-acrylic acid ester- (meth) acrylic acid copolymers, methyl (meth) acrylate-styrene copolymers (such as MS resins), and polymers having an alicyclic hydrocarbon group (for example, methyl methacrylate-cyclohexyl methacrylate copolymers, methyl methacrylate- (meth) acrylic acid norbornyl ester copolymers, and the like). As a commercially available acrylic resin, Acryset manufactured by Mitsubishi chemical corporation is exemplified. A (meth) acrylic resin having a lactone ring structure, a (meth) acrylic resin having an unsaturated carboxylic acid alkyl ester unit and a glutarimide unit, and the like can also be suitably used as a constituent material of the film substrate 1. The acrylic film preferably contains 50% by weight or more of an acrylic resin.

The film base may contain antioxidants, ultraviolet absorbers, light stabilizers, nucleating agents, fillers, pigments, surfactants, antistatic agents, and the like.

When the front retardation Re of the film substrate 1 is in the above range, it may be a stretched film or a non-stretched film. For example, by using a low refractive index material, a low retardation film having a front retardation in the above range can be obtained even in the case of a stretched film. Further, when the longitudinal and lateral stretching ratios are appropriately controlled, the front retardation can be reduced even in the case of a material having a large birefringence.

As described above, by using a cycloolefin film or an acrylic film as the film substrate 1, transparency, low moisture permeability, and low phase difference can be achieved. However, these materials have low mechanical strength, and the tensile breaking strength of both the stretched film and the unstretched film is usually 120MPa or less. When a non-stretched film or a low stretch ratio film is used for the purpose of reducing the phase difference, the tensile breaking strength is often as low as 100MPa or less. Since the pressure-sensitive adhesive layer 2 has high viscosity and is easily deformed, the surface protecting film 10 having the pressure-sensitive adhesive layer 2 provided on the film base 1 is cracked or broken depending mainly on the strength of the film base. Therefore, the tensile break strength of the surface protective film 10 is substantially equal to the tensile break strength of the film base 1.

The surface protection film 10 using such a low-strength film base material 1 is likely to be cracked or broken when peeled from an adherend. In the present invention, the peeling force is reduced by controlling the properties of the pressure-sensitive adhesive layer, and thus, even when a film base material having low strength is used, cracking or breaking at the time of peeling the surface protective film from an adherend can be prevented.

The film base material 1 may be subjected to surface treatment such as corona treatment or easy adhesion treatment, an antistatic layer may be provided on the main surface of the film base material 1 on the side opposite to the surface on which the pressure-sensitive adhesive layer is provided, and in the case where the film base material is provided with an antistatic layer, the surface resistivity of the antistatic layer-formed surface is preferably 1.0 × 10¹²Omega/□ or less, more preferably 1.0 × 10¹¹Omega/□ or less, more preferably 1.0 × 10¹⁰Omega/□ or less.

The film base material 1 is provided with the antistatic layer, whereby adhesion of dust due to static electricity of the surface protective film 10 and a reduction in workability can be suppressed, and the film base material 1 is provided with the antistatic layer, whereby defects of a panel due to static electricity and the like at the time of a lighting test of a liquid crystal panel, an organic E L panel and the like as an adherend can be prevented, and further, the film base material 1 is provided with the antistatic layer, whereby generation of static electricity at the time of peeling the surface protective film 10 from the adherend can be suppressed, and defects such as adhesion of dust due to static electricity to the adherend, defects of the adherend, and a reduction in workability can be prevented.

Examples of the antistatic layer include layers formed by adding various resins with an antistatic component, and the resins include various types of resins such as thermosetting resins, ultraviolet-curable resins, electron beam-curable resins, and two-component mixing resins. Examples of the antistatic component include organic or inorganic conductive substances, various antistatic agents, and the like. As the organic conductive substance, various conductive polymers can be preferably used. Examples of such a conductive polymer include polyaniline, polypyrrole, polythiophene, polyethyleneimine, and allylamine polymers. Examples of the inorganic conductive material include various metals, alloys, and conductive metal oxides. The inorganic conductive substance is preferably contained in the antistatic layer in the form of fine particles having a particle diameter of 0.1 μm or less (typically 0.01 μm to 0.1 μm). The antistatic component can be a cationic antistatic agent, an anionic antistatic agent, a zwitterionic antistatic agent, a nonionic antistatic agent, and the like.

[ adhesive layer ]

The adhesive layer 2 is preferably a transparent adhesive layer. In particular, when an adherend (object to be protected) of the surface protective film is an optical device such as a display or a component thereof, a highly transparent pressure-sensitive adhesive is used. In addition, when the optical inspection is performed in a state where the surface protection film 10 is attached to the adherend, a pressure-sensitive adhesive having high transparency is also preferably used. The total light transmittance of the pressure-sensitive adhesive layer 2 is preferably 80% or more, more preferably 85% or more, and further preferably 90% or more. The haze of the pressure-sensitive adhesive layer 2 is preferably 10% or less, more preferably 5% or less, further preferably 2% or less, and particularly preferably 1% or less.

The composition of the pressure-sensitive adhesive constituting the pressure-sensitive adhesive layer 2 is not particularly limited, and a pressure-sensitive adhesive using a polymer such as a rubber-based polymer such as an acrylic polymer, a silicone polymer, a polyester, a polyurethane, a polyamide, a polyvinyl ether, a vinyl acetate/vinyl chloride copolymer, a modified polyolefin, an epoxy-based, a fluorine-based, a natural rubber, or a synthetic rubber as a base polymer can be suitably selected and used. In particular, acrylic adhesives based on acrylic polymers are preferably used because of their excellent optical transparency.

As the acrylic base polymer, those having a main skeleton of a monomer unit of an alkyl (meth) acrylate can be suitably used. In the present specification, "(meth) acrylic acid" means acrylic acid and/or methacrylic acid.

As the alkyl (meth) acrylate, an alkyl (meth) acrylate in which the number of carbon atoms in the alkyl group is 1 to 20 can be suitably used. Examples thereof include methyl (meth) acrylate, ethyl (meth) acrylate, butyl (meth) acrylate, isobutyl (meth) acrylate, sec-butyl (meth) acrylate, tert-butyl (meth) acrylate, pentyl (meth) acrylate, isopentyl (meth) acrylate, neopentyl (meth) acrylate, hexyl (meth) acrylate, heptyl (meth) acrylate, 2-ethylhexyl (meth) acrylate, octyl (meth) acrylate, isooctyl (meth) acrylate, nonyl (meth) acrylate, isononyl (meth) acrylate, decyl (meth) acrylate, isodecyl (meth) acrylate, undecyl (meth) acrylate, dodecyl (meth) acrylate, isotridecyl (meth) acrylate, tetradecyl (meth) acrylate, isotetradecyl (meth) acrylate, and mixtures thereof, Pentadecyl (meth) acrylate, cetyl (meth) acrylate, heptadecyl (meth) acrylate, octadecyl (meth) acrylate, isostearyl (meth) acrylate, nonadecyl (meth) acrylate, aralkyl (meth) acrylate, and the like.

The content of the alkyl (meth) acrylate is preferably 40% by weight or more, more preferably 50% by weight or more, and further preferably 60% by weight or more, based on the total amount of the monomer components constituting the base polymer. The acrylic polymer may be a copolymer of a plurality of alkyl (meth) acrylates. The arrangement of the constituent monomer units may be random or block.

The acrylic polymer preferably contains a monomer component having a crosslinkable functional group as a copolymerization component. Examples of the monomer having a crosslinkable functional group include a hydroxyl group-containing monomer and a carboxyl group-containing monomer. Among these, the copolymerization component of the base polymer preferably contains a hydroxyl group-containing monomer. The hydroxyl group and the carboxyl group of the base polymer serve as reaction sites with a crosslinking agent described later. By introducing a crosslinked structure into the base polymer, the cohesive force of the pressure-sensitive adhesive is increased, a proper adhesive force to an adherend is exhibited, and the peel force when the surface protective film is peeled from the adherend tends to be reduced. Therefore, when the surface protection film is peeled, the tensile stress and strain applied to the film are reduced, and cracking and breaking can be prevented.

Examples of the hydroxyl group-containing monomer include 2-hydroxyethyl (meth) acrylate, 2-hydroxypropyl (meth) acrylate, 4-hydroxybutyl (meth) acrylate, 6-hydroxyhexyl (meth) acrylate, 8-hydroxyoctyl (meth) acrylate, 10-hydroxydecyl (meth) acrylate, 12-hydroxylauryl (meth) acrylate, and (4-hydroxymethylcyclohexyl) -methyl acrylate. Examples of the carboxyl group-containing monomer include (meth) acrylic acid, carboxyethyl (meth) acrylate, carboxypentyl (meth) acrylate, itaconic acid, maleic acid, fumaric acid, and crotonic acid.

The acrylic polymer may contain, as other comonomer components, acid anhydride group-containing monomers, caprolactone adducts of acrylic acid, sulfonic acid group-containing monomers, and phosphoric acid group-containing monomers, and the like, in addition to the above, vinyl monomers such as vinyl acetate, vinyl propionate, N-vinylpyrrolidone, methyl-vinylpyrrolidone, vinylpyridine, vinylpiperidone, vinylpyrimidine, vinylpiperazine, vinylpyrazine, vinylpyrrole, vinylimidazole, vinyloxazole, vinylmorpholine, N-vinylcarboxylic acid amides, styrene, α -methylstyrene, and N-vinylcaprolactam, epoxy group-containing acrylic monomers such as acrylonitrile and methacrylonitrile, epoxy group-containing acrylic monomers such as glycidyl (meth) acrylate, glycol group-containing acrylic monomers such as polyethylene glycol (meth) acrylate, polypropylene glycol (meth) acrylate, methoxyethylene glycol (meth) acrylate, methoxypolypropylene glycol (meth) acrylate, tetrahydrofurfuryl (meth) acrylate, fluorine (meth) acrylate, silicone (meth) acrylate, and 2-methoxyethyl acrylate, and the like, as modifying monomers.

The ratio of the comonomer component in the acrylic polymer is not particularly limited, and for example, when a hydroxyl group-containing monomer and a carboxyl group-containing monomer are used as the comonomer components for the purpose of introducing a crosslinking point, the total content of the hydroxyl group-containing monomer and the carboxyl group-containing monomer is preferably about 1 to 20%, more preferably about 2 to 15%, based on the total amount of the monomer components constituting the base polymer.

The acrylic polymer as a base polymer is obtained by polymerizing the above monomer components by various known methods such as solution polymerization, emulsion polymerization, bulk polymerization, and suspension polymerization. The solution polymerization method is preferable from the viewpoint of balance of properties such as adhesive strength and holding power of the adhesive, cost, and the like. As a solvent for the solution polymerization, ethyl acetate, toluene, or the like can be used. The concentration of the solution is usually about 20 to 80 wt%. As the polymerization initiator, various known polymerization initiators such as azo-based polymerization initiators and peroxide-based polymerization initiators can be used. For adjusting the molecular weight, a chain transfer agent may be used. The reaction temperature is usually about 50 to 80 ℃ and the reaction time is usually about 1 to 8 hours.

The molecular weight of the base polymer may be appropriately adjusted so that the pressure-sensitive adhesive layer 2 has a desired adhesive strength, and for example, the weight average molecular weight in terms of polystyrene is about 5 to 200 ten thousand, preferably about 7 to 180 ten thousand, more preferably about 10 to 150 ten thousand, and still more preferably about 20 to 100 ten thousand. When a crosslinked structure is introduced into the base polymer, the molecular weight of the base polymer before introduction of the crosslinked structure is preferably in the above range.

In order to obtain the pressure-sensitive adhesive layer 2 having suitable adhesiveness to an adherend in a normal temperature environment, the glass transition temperature (Tg) of the base polymer in terms of the Fox equation is preferably 0 ℃ or lower. The Tg of the base polymer is preferably-10 to-80 ℃, more preferably-15 to-75 ℃, and still more preferably-20 to-70 ℃.

For the purpose of adjusting the adhesive strength of the pressure-sensitive adhesive layer 2, etc., a crosslinked structure may be introduced into the base polymer. For example, a crosslinking agent is added to a solution of the base polymer after polymerization, and the solution is heated as necessary to introduce a crosslinked structure. Examples of the crosslinking agent include isocyanate crosslinking agents, epoxy crosslinking agents, oxazoline crosslinking agents, aziridine crosslinking agents, carbodiimide crosslinking agents, and metal chelate crosslinking agents. Among them, isocyanate-based crosslinking agents and epoxy-based crosslinking agents are preferable because they have high reactivity with hydroxyl groups and carboxyl groups of the base polymer and facilitate introduction of a crosslinked structure. These crosslinking agents react with functional groups such as hydroxyl groups and carboxyl groups introduced into the base polymer to form a crosslinked structure.

Examples of the isocyanate crosslinking agent include lower aliphatic polyisocyanates such as butylene diisocyanate and hexamethylene diisocyanate, alicyclic isocyanates such as cyclopentylene diisocyanate, cyclohexylene diisocyanate and isophorone diisocyanate, aromatic isocyanates such as 2, 4-tolylene diisocyanate, 4' -diphenylmethane diisocyanate and xylylene diisocyanate, trimethylolpropane/tolylene diisocyanate trimer adducts (for example, "CORONATE L" manufactured by tokyo co., ltd.), trimethylolpropane/hexamethylene diisocyanate trimer adducts (for example, "CORONATE H L" manufactured by tokyo co., ltd.), trimethylolpropane adducts of xylylene diisocyanate (for example, "TAKENATE D110N" manufactured by mithon chemical corporation), isocyanurate compounds of hexamethylene diisocyanate (for example, "CORONATE H" manufactured by tokyo co., ltd.), and the like.

As the epoxy-based crosslinking agent, a polyfunctional epoxy compound having 2 or more epoxy groups in 1 molecule can be used. The epoxy group of the epoxy-based crosslinking agent may be a glycidyl group. Examples of the epoxy-based crosslinking agent include N, N, N ', N' -tetraglycidyl m-xylylenediamine, diglycidylaniline, 1, 3-bis (N, N-diglycidylaminomethyl) cyclohexane, 1, 6-hexanediol diglycidyl ether, neopentyl glycol diglycidyl ether, ethylene glycol diglycidyl ether, propylene glycol diglycidyl ether, polyethylene glycol diglycidyl ether, polypropylene glycol diglycidyl ether, sorbitol polyglycidyl ether, glycerol polyglycidyl ether, pentaerythritol polyglycidyl ether, polyglycerol polyglycidyl ether, sorbitan polyglycidyl ether, trimethylolpropane polyglycidyl ether, adipic acid diglycidyl ester, phthalic acid diglycidyl ester, triglycidyl-tris (2-hydroxyethyl) isocyanurate, and mixtures thereof, Resorcinol diglycidyl ether, bisphenol-S-diglycidyl ether, and the like. As the epoxy crosslinking agent, commercially available products such as "Denacol" manufactured by Nagase ChemteX Corporation, "TETRAD X" and "TETRAD C" manufactured by Mitsubishi gas chemical Corporation can be used.

A crosslinking agent is added to the base polymer after polymerization, thereby introducing a crosslinked structure into the base polymer. The amount of the crosslinking agent to be used may be appropriately adjusted depending on the composition of the base polymer, the molecular weight, the intended adhesion characteristics, and the like. The amount of the crosslinking agent to be used is preferably 1.5 parts by weight or more, more preferably 2 parts by weight or more, and further preferably 2.5 parts by weight or more per 100 parts by weight of the base polymer, in order to provide the pressure-sensitive adhesive with an appropriate cohesive force and to adjust the peel strength when the protective film is peeled from the adherend to an appropriate range. The amount of the crosslinking agent to be used is preferably 20 parts by weight or less, more preferably 15 parts by weight or less, and still more preferably 10 parts by weight or less, based on 100 parts by weight of the base polymer, in order to provide adequate adhesiveness to an adherend.

When a crosslinked structure is introduced into the base polymer, the gel fraction of the pressure-sensitive adhesive increases, and the peeling force when the protective film is peeled from the adherend tends to decrease as the viscosity behavior decreases. The gel fraction of the pressure-sensitive adhesive layer 2 is preferably 70.0% or more, more preferably 80.0% or more, and still more preferably 90.0% or more. If the gel fraction of the pressure-sensitive adhesive layer 2 is too large, wettability to an adherend may be reduced, and the adhesive strength may be insufficient. Therefore, the gel fraction of the pressure-sensitive adhesive layer 2 is preferably 99.5% or less, more preferably 98.5% or less, and still more preferably 97.5% or less. The gel fraction can be determined as an insoluble component with respect to a solvent such as ethyl acetate, and specifically, as a weight fraction (unit: weight%) of an insoluble component after the pressure-sensitive adhesive layer is immersed in ethyl acetate at 23 ℃ for 7 days with respect to a sample before immersion. Generally, the gel fraction of a polymer is equal to the degree of crosslinking, and the more crosslinked portions in the polymer, the greater the gel fraction becomes.

The storage modulus G' of the pressure-sensitive adhesive layer 2 at 23 ℃ is preferably 5.0 × 10 from the viewpoint of making the pressure-sensitive adhesive layer 2 have an appropriate hardness to reduce the peeling force when peeling the protective film from the adherend⁴Pa or more, more preferably 7.5 × 10⁴Pa or more, and more preferably 1.0 × 10⁵Pa or more, when the storage modulus of the pressure-sensitive adhesive layer 2 is too large, the wettability to an adherend may decrease and the adhesive strength may become insufficient, and therefore, the storage modulus G' at 23 ℃ of the pressure-sensitive adhesive layer 2 is preferably 5.0 × 10⁶Pa or less, more preferably 2.5 × 10⁶Pa or less, more preferably 1.0 × 10⁶Pa or less. The storage modulus G' is determined by measuring the viscoelasticity in the shear mode under the conditions of a frequency of 1Hz, a temperature range of-70 ℃ to 150 ℃ and a temperature rise rate of 5 ℃/min using a dynamic viscoelasticity measuring apparatus (for example, "ARES" manufactured by Rheometrics).

The storage modulus of the adhesive layer can be adjusted by the composition of the adhesive. For example, when a base polymer having a high glass transition temperature is used, the storage modulus tends to be large. When the gel fraction is increased by introducing a crosslinked structure into the base polymer, the storage modulus tends to be increased.

The pressure-sensitive adhesive composition may contain additives such as a silane coupling agent, a tackifier, a plasticizer, a softening agent, a deterioration preventing agent, a filler, a colorant, an ultraviolet absorber, an antioxidant, a surfactant, and an antistatic agent in addition to the above-mentioned components within a range not to impair the characteristics of the present invention.

The surface protection film 10 is obtained by laminating the pressure-sensitive adhesive layer 2 on the film base 1. The pressure-sensitive adhesive layer 2 may be formed directly on the film base 1, or may be formed in a sheet form on another base and transferred onto the film base 1.

The adhesive composition is applied to a substrate by roll coating, kiss roll coating, gravure coating, reverse coating, roll brushing, spray coating, dip roll coating, bar coating, blade coating, air knife coating, curtain coating, lip coating (lip coating), die coating, or the like, and the solvent is dried and removed as necessary, thereby forming an adhesive layer. As the drying method, an appropriate method can be suitably employed. The heating and drying temperature is preferably 40 to 200 ℃, more preferably 50 to 180 ℃, and still more preferably 70 to 170 ℃. The drying time is preferably 5 seconds to 20 minutes, more preferably 5 seconds to 15 minutes, still more preferably 10 seconds to 10 minutes, and particularly preferably 10 seconds to 5 minutes.

When the pressure-sensitive adhesive composition contains a crosslinking agent, it is preferable to crosslink the pressure-sensitive adhesive composition by heating or curing at the same time as or after drying the solvent. The heating temperature and the heating time are appropriately set depending on the kind of the crosslinking agent used, and usually, the crosslinking is carried out by heating at 20 to 160 ℃ for about 1 minute to 7 days. The heat for drying off the solvent may be used as the heat for crosslinking.

The thickness of the pressure-sensitive adhesive layer 2 is not particularly limited, and from the viewpoint of satisfying both the adhesion to an adherend and the releasability from the adherend, the thickness of the pressure-sensitive adhesive layer 2 is preferably 1 to 50 μm, more preferably 2 to 40 μm, and still more preferably 3 to 35 μm. The smaller the thickness of the pressure-sensitive adhesive layer 2, the more the releasability from the adherend tends to be improved. In particular, when the film base 1 has a low breaking strength, the thickness of the pressure-sensitive adhesive layer 2 is preferably 25 μm or less, more preferably 20 μm or less, from the viewpoint of preventing cracking or breaking of the film when the surface protection film 10 is peeled off from an adherend at a high speed. From the viewpoint of suppressing white turbidity of the pressure-sensitive adhesive layer 2 in a high-humidity environment and maintaining transparency, the thickness of the pressure-sensitive adhesive layer is preferably 25 μm or less, more preferably 20 μm or less.

In the case where the pressure-sensitive adhesive layer 2 is formed on a substrate other than the film substrate 1, the pressure-sensitive adhesive layer 2 is transferred onto the film substrate 1 after the solvent is dried on the substrate, thereby obtaining the surface protection film 10. The substrate used in the formation of the adhesive layer may be used directly as the separator 5.

As the separator 5, a plastic film such as polyethylene, polypropylene, polyethylene terephthalate, or polyester film is preferably used. The thickness of the separator is usually 3 to 200 μm, preferably about 10 to 100 μm, and more preferably about 15 to 50 μm. The surface of the separator 5 in contact with the pressure-sensitive adhesive layer 2 is preferably subjected to a mold release treatment using a mold release agent such as a silicone-based, fluorine-based, long-chain alkyl-based, or fatty acid amide-based agent, or silica powder. The plastic film constituting the separator 5 may contain an antistatic agent.

[ Properties of surface protective film ]

As described above, the 180 DEG peel force of the surface protective film 10 to an acrylic sheet at a stretching speed of 30 m/min is 3N/25mm or less, preferably 2.5N/25mm or less. In particular, when the film base material 1 having a tensile breaking strength of 120MPa or less is used, the peeling force is preferably 2N/25mm or less, more preferably 1.7N/25mm or less, and further preferably 1.5N/25mm or less, from the viewpoint of preventing cracking or breaking of the surface protective film 10 at the time of peeling from an adherend to improve the peeling workability.

The peel force of the surface protection film 10 can be set to the above range by adjusting the adhesive force of the pressure-sensitive adhesive layer 2. As described above, the peeling force tends to be smaller as the thickness of the pressure-sensitive adhesive layer 2 is smaller. Further, by increasing the amount of the crosslinked structure introduced into the base polymer constituting the pressure-sensitive adhesive layer 2, the gel fraction tends to increase and the peeling force tends to decrease.

From the viewpoint of suppressing the peeling of the surface protection film 10 from the adherend under a high-temperature and high-humidity environment, the surface protection film 10 preferably has a large peeling force (adhesive force). On the other hand, when the peeling force is large as described above, when the surface protective film is peeled from the adherend, cracking or breaking is likely to occur. The moisture permeability of the surface protective film is preferably 300g/m from the viewpoints of improving the adhesion reliability in a humidified environment without excessively increasing the peeling force and suppressing peeling and lifting of the surface protective film from an adherend²24h or less, more preferably 200g/m²24h or less, further preferredSelecting 150g/m²24h or less, particularly preferably 100g/m²24h or less.

As described above, the moisture permeability of the surface protective film 10 is substantially equal to the moisture permeability of the film base 1. Therefore, in order to set the moisture permeability of the surface protective film 10 within the above range, it is preferable to use a film base 1 having a low moisture permeability such as a cycloolefin film or an acrylic film.

When the optical inspection is performed in a state where the surface protection film 10 is bonded to an adherend, the phase difference of the surface protection film 10 causes coloring of the inspection light. The front retardation Re of the surface protection film 10 is preferably 100nm or less, more preferably 50nm or less, further preferably 30nm or less, and particularly preferably 20nm or less, from the viewpoint of preventing light leakage, coloration, iridescence, and the like in optical inspection. From the same viewpoint, the retardation Rth in the thickness direction of the surface protective film 10 is preferably 100nm or less, more preferably 50nm or less, and further preferably 30nm or less.

As described above, the front retardation Re and the thickness direction retardation Rth of the surface protective film 10 in which the pressure-sensitive adhesive layer 2 is provided on the film base 1 are substantially equal to the front retardation Re and the thickness direction retardation Rth of the film base 1, respectively. Therefore, in order to set Re and Rth of the surface protective film 10 within the above ranges, a low birefringent material film, a non-stretched film or a low stretch ratio film is preferably used as the film substrate 1. In the present invention, since the peeling force of the surface protective film 10 is appropriately adjusted, even when a film having low strength (for example, tensile breaking strength of 120MPa or less) is used as the film base 1, the film can be prevented from being cracked or broken at the time of high-speed peeling.

The peeling static voltage when peeling the surface protection film from the adherend is preferably 2.5kV or less, more preferably 2kV or less. In particular, when the surface protective film is bonded to an adherend susceptible to static electricity, the peeling static voltage is preferably 1.5kV or less, more preferably 1kV or less. For example, by providing an antistatic layer on the pressure-sensitive adhesive layer non-application surface of the film base 1, the peeling electrostatic voltage of the surface protection film 10 can be reduced.

The adherend to which the surface protection film of the present invention is bonded can be prevented from being damaged by damage or impact on the surface. Since the surface protection film has low moisture permeability and high reliability of adhesion in humidification, the surface protection film is less likely to float or peel even when an adherend to which the surface protection film is attached is exposed to a high humidity environment.

The surface protective film of the present invention can be used as a surface protective film for various optical members, examples of the optical member include optical films such as a polarizing plate, a retardation plate, an optical compensation film, a viewing angle expanding film, a viewing angle control film, a brightness enhancement film, an antireflection film, a reflective sheet, a transparent conductive film, a prism sheet, and a light guide plate, image display panels such as a liquid crystal panel and an organic E L panel, and image display devices incorporating the image display panels.

The surface protection film of the present invention has a low phase difference, and therefore, even when optical inspection is performed in a state where the surface protection film is bonded to an optical member, the surface protection film does not hinder the optical inspection, and contributes to improvement of efficiency and accuracy of the inspection. Further, the surface protection film of the present invention has a low peeling force, and therefore, even when the film base material has a low strength, the film is less likely to crack or break at the time of high-speed peeling, and is excellent in handling properties.