Protective film

Abstract

A protective film contains a base material and a pressure-sensitive adhesive layer formed thereon, wherein the surface of the pressure-sensitive adhesive layer has a contact angle with methylene iodide as measured just after contact, &thgr;1, of 70° or smaller and a change in contact angle with methylene iodide through 30-second standing, &Dgr;&thgr;, of 8% or less.

Description

FIELD OF THE INVENTION

[0001] The present invention relates to a protective film with which the metal layers or metal oxide layers used as electromagnetic shielding materials or the like can be kept in a satisfactory state without undergoing fouling or the like.

BACKGROUND OF THE INVENTION

[0002] In plasma display panels (PDPs), CRTs, and the like, electromagnetic shielding is highly important for preventing electromagnetic noises. In one technique for electromagnetic shielding, a metal such as gold or silver or a metal oxide such as ITO is used as an electromagnetic shielding material for these apparatus to form a thin conductive layer or mesh pattern by vapor deposition or the like. Such metal layers or metal oxide layers are generally kept being protected with a protective film until use.

[0003] However, when general protective films are used, the surfaces of those metal layers or metal oxide layers are apt to be fouled probably because these surfaces are especially active. There have been cases where this fouling impairs the function of electromagnetic shields.

SUMMARY OF THE INVENTION

[0004] Accordingly, an object of the invention is to provide a protective film with which the metal layers and metal oxide layers used as electromagnetic shields or the like can be kept in a satisfactory state without undergoing fouling or the like.

[0005] The present inventors have found through intensive investigations that the fouling which occurs on the surfaces of metal layers or metal oxide layers is considerably influenced by the surfaces of the pressure-sensitive adhesive layers. Specifically, it has been found that when a protective film comprising a base material and formed thereon a pressure-sensitive adhesive layer whose surface has a contact angle with methylene iodide (CH2I2) as measured just after contact, &thgr;1, of 70° or smaller, preferably from 50 to 70°, and a change in contact angle with methylene iodide through 30-second standing, &Dgr;&thgr;, is 8% or less, preferably 6% or less, is used, then the surface of the metal layer or metal oxide layer can be prevented from being fouled and can retain a satisfactory state. The invention has thus been achieved.

[0006] The invention provides a protective film comprising a base material and a pressure-sensitive adhesive layer formed thereon, wherein the surface of the pressure-sensitive adhesive layer has a contact angle with methylene iodide as measured just after contact, &thgr;1, of 70° or smaller and a change in contact angle with methylene iodide through 30-second standing, &Dgr;&thgr;, of 8% or less.

[0007] The change in contact angle through 30-second standing, &Dgr;&thgr;, can be determined using the following equation. Values of contact angle are obtained through measurements with a commercial contact angle meter at 23° C. by the &thgr;/2 method.

&Dgr;&thgr;=|&thgr;1-&thgr;2|×100/&thgr;1

[0008] &Dgr;&thgr;: change in contact angle through 30-second standing (%)

[0009] &thgr;1: contact angle with methylene iodide as measured just after contact (°)

[0010] &thgr;2: contact angle with methylene iodide as measured at 30 seconds after contact (°)

[0011] In case where the value of &thgr;1 exceeds 70° or &Dgr;&thgr; exceeds 8%, fouling is apt to occur probably because such a pressure-sensitive adhesive layer surface shows enhanced bonding to the metal layer or metal oxide layer surface.

[0012] In this protective film, the main polymer of the pressure-sensitive adhesive layer preferably is a (meth)acrylic polymer. The protective film of the invention is preferably used for the surface protection of an electromagnetic shielding material.

DETAILED DESCRIPTION OF THE INVENTION

[0013] The protective film of the invention comprises a base material and a pressure-sensitive adhesive layer formed thereon. The protective film may have an interlayer such as, e.g., a primer layer between the base material and the adhesive layer. The pressure-sensitive adhesive layer may be made of any material as long as the requirements concerning contact angle and change in contact angle are satisfied. Namely, the contact angle between the surface of the pressure-sensitive adhesive layer in the protective film of the invention and methylene iodide as measured just after contact, &thgr;1, is 70° or smaller, preferably from 50 to 70°, and the change in contact angle between the surface and methylene iodide through 30-second standing, &Dgr;&thgr;, is 8% or less, preferably 6% or less.

[0014] The main polymer of the pressure-sensitive adhesive layer preferably is a (meth)acrylic polymer from the standpoint of obtaining a balance between adhesive properties and suitability for peeling. The (meth)acrylic polymer used here is a polymer obtained by the homopolymerization or copolymerization of one or more monomers such as, e.g., (meth)acrylic esters having an alkyl group having 1 to 18 carbon atoms optionally together with one or more copolymerizable modifying monomers by an appropriate technique such as, e.g., solution polymerization or emulsion polymerization.

[0015] Examples of the (meth)acrylic esters and copolymerizable modifying monomers include (meth)acrylic esters having an alkyl group such as butyl, 2-ethylhexyl, isooctyl, isononyl, ethyl, or methyl and modifying monomers such as (meth)acrylonitrile, vinyl acetate, styrene, (meth)acrylic acid, maleic anhydride, vinylpyrrolidone, (meth)acrylic esters having a glycidyl group, dimethylaminoethyl group, or hydroxyl group, (meth)acrylamide, vinylamine, allylamine, and ethyleneimine. The term “(meth)acrylic” as used herein refers to “acrylic and/or methacrylic”; the term “(meth)acrylonitrile” as used herein refers to “acrylonitrile and/or methacrylonitrile”; and the term “(meth)acrylate” as used herein refers to “acrylate and/or methacrylate”.

[0016] Although the (meth)acrylic polymer by itself may be used as the base polymer of the pressure-sensitive adhesive, a crosslinking agent is usually incorporated into the polymer before use for the purpose of improving the cohesive force of the pressure-sensitive adhesive. A crosslinked structure can be imparted to an acrylic polymer by adding a polyfunctional (meth)acrylate or the like as an internal crosslinking agent when the acrylic polymer is synthesized or by adding a polyfunctional epoxy or isocyanate compound or the like as an external crosslinking agent after the acrylic polymer has been synthesized. Besides these, a crosslinking treatment with exposure to a radiation may be used. Preferred of these techniques for forming a crosslinked structure is the method in which a polyfunctional epoxy compound or a polyfunctional isocyanate compound is incorporated as an external crosslinking agent.

[0017] The term “polyfunctional” as used herein means to have a functionality of 2 or higher. Examples of the polyfunctional epoxy compound include various compounds having two or more epoxy groups in the molecule. Typical examples thereof include sorbitol tetraglycidyl ether, trimethylolpropane glycidyl ether, tetraglycidyl-1,3-bisaminomethylcyclohexane, tetraglycidyl-m-xylenediamine, and triglycidyl-p-aminophenol. Examples of the polyfunctional isocyanate compound include various compounds having two or more isocyanate groups in the molecule. Typical examples thereof include diphenylmethane diisocyanate, tolylene diisocyanate, and hexamethylene diisocyanate.

[0018] Those crosslinking agents can be used alone or in combination of two or more thereof. The amount of the crosslinking agents to be used can be suitably selected according to the composition and molecular weight of the (meth)acrylic polymer, etc. A crosslinking catalyst in general use for pressure-sensitive adhesives, such as, e.g., dibutyltin laurate, may be added in order to accelerate the reaction. Furthermore, ordinary additives may be incorporated into the pressure-sensitive adhesive according to need.

[0019] It is preferred to minimize the incorporation of additives in forming a pressure-sensitive adhesive layer. However, suitable additives can be .incorporated according to need for the purpose of, e.g., regulating pressure-sensitive adhesive properties. Examples thereof include softeners, silicone polymers, acrylic copolymers, tackifiers, antioxidants, hindered-amine light stabilizers, ultraviolet absorbers, and fillers or pigments such as calcium oxide, magnesium oxide, silica, zinc oxide, and titanium oxide. However, such additives preferably are ones which cause no damage to the adherend.

[0020] In the invention, it is preferred to take the following measures in order to satisfy the above-described requirements concerning contact angel and change thereof. For attaining a reduced contact angle, it is preferred to minimize the amount of units derived from short-side-chain acrylic monomers (e.g., acrylic monomers having a side chain having not more than 2 carbon atoms) in the (meth)acrylic polymer as long as the properties of the polymer are not impaired thereby. For attaining a reduced change in contact angle, it is preferred to minimize the amount of additives such as low-molecular plasticizers.

[0021] The base material preferably is a thermoplastic film made of a single polyolefin such as, e.g., polyethylene or polypropylene, a mixture of two or more such polyolefins, or another polymer such as a nylon, PET, polyimide, polycarbonate, or PTFE. From the standpoints of transparency, fish-eye diminution, etc., a PET film is preferred. Although the thickness of the base material is generally from 5 to 200 &mgr;m, preferably from 20 to 100 &mgr;m, it is not limited to these values.

[0022] The protective film can be formed by known techniques for adhesive-sheet formation. For example, use may be made of: a method in which either a solution of a material for pressure-sensitive adhesive layer formation in a solvent or a melt of the material is applied to a base material; a method in which a pressure-sensitive adhesive layer formed on a release liner by applying the solution or melt is transferred to a base material; a method in which a material for pressure-sensitive adhesive layer formation is applied to a base material by extrusion coating; or a method in which a pressure-sensitive adhesive layer is laminated to a base material. Although the thickness of the pressure-sensitive adhesive layer to be formed may be suitably determined, it is generally 100 &mgr;m or smaller, preferably from 1 to 50 &mgr;m, especially preferably from 1 to 40 &mgr;m. Until the protective film is put to practical use, the pressure-sensitive adhesive is kept being protective according to need by provisionally applying a release liner or the like thereto.

[0023] The protective film of the invention is suitable for use in the surface protection of the metal layer or metal oxide layer of an electromagnetic shielding material, transparent conductive film, sheet coated with a vapor-deposited metal, or the like. Especially preferably, it is used for the surface protection of an electromagnetic shielding material.

[0024] The constitution and effect of the invention will be explained below by reference to Examples thereof, etc.

EXAMPLE 1

[0025] A 20% toluene solution of an acrylic polymer formed from 2-ethylhexyl acrylate (70 parts (parts by weight; the same applies hereinafter)), N-acryloylmorpholine (30 parts), and acrylic acid (3 parts) was prepared by an ordinary method. To this solution were added 2 parts of an epoxy crosslinking agent (Tetrad C, manufactured by Mitsubishi Gas Chemical Co., Inc.) and 2 parts of an isocyanate crosslinking agent (Coronate L, manufactured by Nippon Polyurethane Co., Ltd.) per 100 parts of the acrylic polymer. The resultant composition was applied to a 38-&mgr;m PET film in a thickness of 5 &mgr;m on a dry basis, and the coating was dried. Thus, a protective film was obtained, which had a &thgr;1 of 56.5° and a &Dgr;&thgr; of 1.7%.

EXAMPLE 2

[0026] A 20% toluene solution of an acrylic polymer formed from 2-ethylhexyl acrylate (55 parts), vinyl acetate (45 parts), and acrylic acid (3 parts) was prepared by an ordinary method. To this solution was added 2 parts of an epoxy crosslinking agent (Tetrad C, manufactured by Mitsubishi Gas Chemical Co., Inc.) per 100 parts of the acrylic polymer. The resultant composition was applied to a 38-&mgr;m PET film in a thickness of 5 &mgr;m on a dry basis, and the coating was dried. Thus, a protective film was obtained, which had a &thgr;1 of 58.3° and a &Dgr;&thgr; of 1.2%.

EXAMPLE 3

[0027] A 20% toluene solution of an acrylic polymer formed from 2-ethylhexyl acrylate (100 parts) and hydroxyethyl acrylate (4 parts) was prepared by an ordinary method. To this solution were added 3.5 parts of an isocyanate crosslinking agent (Coronate L, manufactured by Nippon Polyurethane Co., Ltd.) and 0.05 part of a reaction catalyst (OL-1, manufactured by Tokyo Fine Chemical Co., Ltd.) per 100 parts of the acrylic polymer. The resultant composition was applied to a 38-&mgr;m PET film in a thickness of 20 &mgr;m on a dry basis, and the coating was dried. Thus, a protective film was obtained, which had a &thgr;1 of 62.8° and a &Dgr;&thgr; of 3.9%.

EXAMPLE 4

[0028] A 20% toluene solution of an acrylic polymer formed from 2-ethylhexyl acrylate (50 parts), butyl methacrylate (50 parts), acrylic acid (0.5 part), and hydroxyethyl acrylate (2 parts) was prepared by an ordinary method. To this solution were added 3 parts of an isocyanate crosslinking agent (Coronate L, manufactured by Nippon Polyurethane Co., Ltd.) and 0.1 part of a reaction catalyst (OL-1, manufactured by Tokyo Fine Chemical Co., Ltd.) per 100 parts of the acrylic polymer. The resultant composition was applied to a 38-&mgr;m PET film in a thickness of 5 &mgr;m on a dry basis, and the coating was dried. Thus, a protective film was obtained, which had a &thgr;1 of 59.1° and a &Dgr;&thgr; of 5.1%.

COMPARATIVE EXAMPLE 1

[0029] A 20% toluene solution of an acrylic polymer formed from 2-ethylhexyl acrylate (50 parts), methyl acrylate (50 parts), and acrylic acid (2 parts) was prepared by an ordinary method. To this solution were added 3.5 parts of an isocyanate crosslinking agent (Coronate L, manufactured by Nippon Polyurethane Co., Ltd.), 0.05 part of a reaction catalyst (OL-1, manufactured by Tokyo Fine Chemical Co., Ltd.), and 15 parts of a plasticizer (dibutoxyethoxyethyl adipate) per 100 parts of the acrylic polymer. The resultant composition was applied to a 38-&mgr;m PET film in a thickness of 20 &mgr;m on a dry basis, and the coating was dried. Thus, a protective film was obtained, which had a &thgr;1 of 72.9° and a &Dgr;&thgr; of 10.0%.

COMPARATIVE EXAMPLE 2

[0030] The same procedure as in Comparative Example 1 was conducted, except that the plasticizer was omitted. Thus, a protective film was obtained, which had a &thgr;1 of 73.5° and a &Dgr;&thgr; of 3.8%.

COMPARATIVE EXAMPLE 3

[0031] The same procedure as in Example 3 was conducted, except that 15 parts of a plasticizer (dibutoxyethoxyethyl adipate) was added. Thus, a protective film was obtained, which had a &thgr;1 of 61.5° and a &Dgr;&thgr; of 8.8%.

[0032] The protective films obtained in the Examples and Comparative Examples were aged at 40° C. for 3 days.

[0033] Evaluation Method

[0034] Each of the protective films obtained in the Examples and Comparative Examples was applied to an ITO-coated film by press-bonding the protective film to the ITO side of the adherend with a laminator at a linear pressure of 78 N/cm. The protective films applied were allowed to stand at 80° C. for 72 hours and then peeled off. Thereafter, the ITO-coated films were held to the light from a green lamp to visually examine the films for fouling. The results of this evaluation are shown in the following tables. 1 TABLE 1 Example 1 Example 2 Example 3 Example 4 &thgr;1 (°) 56.5 58.3 62.8 59.1 &Dgr;&thgr; (%) 1.7 1.2 3.9 5.1 Fouling not occurred not occurred not occurred not occurred

[0035] 2 TABLE 2 Comparative Comparative Comparative Example 1 Example 2 Example 3 &thgr;1 (°) 72.9 73.5 61.5 &Dgr;&thgr; (%) 10.0  3.8  8.8 Fouling occurred occurred occurred

[0036] The results given in Tables 1 and 2 show that the protective films of the Examples caused no fouling and were highly satisfactory, whereas the protective films of the Comparative Examples caused fouling.

[0037] While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof.

Claims

1. A protective film comprising a base material and a pressure-sensitive adhesive layer formed thereon, wherein the surface of the pressure-sensitive adhesive layer has a contact angle with methylene iodide as measured just after contact, &thgr;1, of 70° or smaller and a change in contact angle with methylene iodide through 30-second standing, &Dgr;&thgr;, of 8% or less.

2. The protective film of claim 1, wherein the pressure-sensitive adhesive layer comprises a (meth)acrylic polymer as a main polymer.

3. The protective film of claim 1, which is for use in the surface protection of an electromagnetic shielding material.

4. The protective film of claim 2, which is for use in the surface protection of an electromagnetic shielding material.

5. The protective film of claim 2, wherein the (meth)acrylic polymer is a polymer obtained by the homopolymerization or copolymerization of one or more (meth)acrylic esters having an alkyl group having 1 to 18 carbon atoms by solution polymerization or emulsion polymerization.

6. The protective film of claim 2, wherein the (meth)acrylic polymer is a polymer obtained by the copolymerization of one or more (meth)acrylic esters having an alkyl group having 1 to 18 carbon atoms and one or more copolymerizable modifying monomers by solution polymerization or emulsion polymerization.

7. The protective film of claim 5, wherein the (meth)acrylic ester is selected from the group consisting of (meth)acrylic esters having a butyl group, a 2-ethylhexyl group, an isooctyl group, an isononyl group, an ethyl group, or a methyl group.

8. The protective film of claim 6, wherein the (meth)acrylic ester is selected from the group consisting of (meth)acrylic esters having a butyl group, a 2-ethylhexyl group, an isooctyl group, an isononyl group, an ethyl group, or a methyl group.

9. The protective film of claim 6, wherein the copolymerizable modifying monomer is selected from the group consisting of (meth)acrylonitrile, vinyl acetate, styrene, (meth)acrylic acid, maleic anhydride, vinylpyrrolidone, (meth)acrylic esters having a glycidyl group, a dimethylaminoethyl group, or a hydroxyl group, (meth)acrylamide, vinylamine, allylamine, and ethyleneimine.