Pressure-sensitive adhesive compositions and adhesive films

A pressure-sensitive adhesive composition comprising an acrylic polymer as a base polymer and a crosslinking agent, characterized in that said acrylicpolymerh as a weight-average molecular weight of 1,500,000 or above and the proportion of materials having a molecular weight of 100,000 or below in said acrylic polymeris 10% or less, and that the storage modulus at 80° C. of said composition after forming a cross-linked structure is from 5×10 4 Pa to 5×10 5 Pa inclusive,which composition is capable of forming an adhesive layer having excellent weather resistance.

DESCRIPTION OF THE PREFERRED EMBODIMENTS An acrylic polymer used as a base polymer in a pressure-sensitive adhesive composition of the present invention is a homopolymer or copolymer of a monomer or monomers comprising alkyl (meth)acrylate as a principal component. As used herein, “alkyl (meth)acrylate” means alkyl acrylate and/or alkyl methacrylate (hereinafter the term “(meth)” has the same similar meaning anywhere it occurs). The alkyl group may be straight or branched, and has usually about 1 to 18 carbon atoms. Specific examples of alkyl (meth)acrylates include, but not limited to, methyl (meth) acrylate, ethyl (meth) acrylate, propyl (meth) acrylate, butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, octyl (meth)acrylate, nonyl (meth)acrylate, and lauryl (meth)acrylate. In addition to, and in combination with, the above-described alkyl (meth)acrylate(s), other monomers copolymerizable therewith may also be used in the acrylic polymers. As such other copolymerizable monomers, various monomers known as monomers for modifying acrylic polymers used in acrylic pressure-sensitive adhesives may be used, including vinyl acetate, styrene, (meth)acrylonitrile, (meth)acrylamide, (meth)acrylic acid, 2-hydroxyethyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, 6-hydroxyhexyl (meth)acrylate, glycidyl (meth)acrylate, methoxyethyl (meth) acrylate, and N,N-dimethylaminoethyl (meth) acrylate. Such other copolymerizable monomers are usually used in the proportion of 50% by weight or less based on the total amount of the monomers constituting the acrylic polymer. For adjusting the storage modulus to the above-described range, an optional monomer used is preferably a monomer reactive with the crosslinking agent. For example, when an isocyanate compound is used as the crosslinking agent, it is preferred to use a hydroxyalkyl (meth)acrylate containing a hydroxyl group, preferably in an amount of about 0.01 to 5% by weight, and more preferably of 0.01 to 1% by weight, based on the total amount of the monomers constituting the acrylic polymer. For preparing an acrylic polymer of the present invention, various methods maybe adopted without particular restrictions. For example, according to the usual method, a monomer or monomers comprising as a principal component an alkyl (meth) acrylate described above may be polymerized, for example, by solution polymerization, emulsion polymerization or bulk polymerization using a polymerization initiator such as an azo compound or a peroxide, by photopolymerization using a photoinitiator, or by a polymerization method involving irradiation, and an organic solvent which can dissolve only the low molecular weight component may be then added to the resulting polymer in order to fractionally remove only a low molecular weight component. Other methods for preparing acrylic polymers of the present invention include, but not limited to, a method in which a solvent having a small chain transfer constant is used as a polymerization solvent in solution polymerization, a method in which polymerization is conducted with a high monomer concentration at as low temperature as possible, and a photopolymerization using a photoinitiator in which process conditions during polymerization are appropriately selected. In preparation of acrylic polymers, any selected polymerization process as described above may also be combined as appropriate with the above process for removing the low molecular weight component after polymerization. In a pressure-sensitive adhesive composition of the present invention, any crosslinking agent conventionally known may be used without particular restrictions; it is particularly preferred to use a polyfunctional isocyanate compound. Examples of polyfunctional isocyanate compounds include, for example, tolylene diisocyanate, hexamethylene diisocyanate, polymethylene polyphenyl diisocyanate, diphenylmethane diisocyanate, trimethylolpropane tolylene diisocyanate, polyether polyisocyanate, and polyester polyisocyanate. These is isocyanate compounds are suitably used in an amount of about 0.01 to 20 parts by weight, and preferably in an amount in the range of 0.05 to 15 parts by weight, per 100 parts by weight of the above acrylic polymer. Other crosslinking agents,which may be preferably used, are polyfunctional melamine compounds and polyfunctional epoxy compounds. Specific examples of polyfunctional melamine compounds are methylated methylol melamine and butylated hexamethylol melamine, and specific examples of polyfunctional epoxy compounds are diglycidyl aniline and glycerin diglycidyl ether. These crosslinking agents are suitably used in an amount of about 0.001 to 10 parts by weight, and preferably in an amount in the range of 0.001 to 5 parts by weight, per 100 parts by weight of the acrylic polymer. In addition to the acrylic polymer and the crosslinking agent described above, a pressure-sensitive adhesive composition of the present invention may also comprise solvents or various additives such as tackifier, plasticizer, filler, coloring agent, or UV absorber, to the extent that they are not adverse to the purpose of the present invention. An adhesive film of the present invention is a film in which the above pressure-sensitive adhesive composition is provided as a cross-linked structure layer on a base film. The cross-linked structure layer is formed by coating and drying (curing) a pressure-sensitive adhesive composition on a base film. The adhesive film may be in the form in which the cross-linked structure layer (pressure-sensitive adhesive layer) is provided on a single side or both sides of the base film, as in so-called pressure-sensitive adhesive sheets (self-adhesive sheets) or pressure-sensitive adhesive tapes (self-adhesive tapes). Alternatively, the adhesive film may also be in the form to be used without base materials wherein the cross-linked structure layer is provided on a release liner (base film) of which surface has been release-treated (mold release-treated) to form a configuration similar to that described above. Any base film conventionally used may be employed without particular restrictions. For example, various plastic films such as polyethylene terephthalate film, polycarbonate film, and triacetylcellulose film may be used. The thickness of the base film is usually about 10 to 1000 &mgr;m. When the base film is one of various functional films such as conductive film, antiglare film, heat ray shielding film, and antireflective film, the thickness of such functional film is usually preferred to be about 50 to 200 &mgr;m. The thickness of the cross-linked structure layer of pressure-sensitive adhesive composition formed on the adhesive film is usually about 5 to 500 &mgr;m, and preferably about 10 to 100 &mgr;m. Drying (curing) of pressure-sensitive adhesive composition is usually conducted at about 80 to 150° C. for about 2 to 5 minutes. Furthermore, a display device of the present invention is produced by sticking a functional film in the form of an adhesive film of the present invention on the display area of the display device. 
 EXAMPLES The present invention is specifically described below with reference to the following examples, but the present invention is not so restricted to such examples. In the following examples, the number of parts and percents are all on a weight basis. Preparation 1 (Preparation of acrylic polymer) (1) Synthesis of Acrylic Polymer In a three-necked flask, 60 parts of ethyl acrylate, 35 parts of butyl acrylate, 5 parts of acrylic acid, 0.06 parts of 3-hydroxypropyl acrylate, and 45 parts of ethyl acetate as a polymerization solvent were placed, and stirred for 2 hours while introducing nitrogen gas. After removing oxygen in the polymerization system in this way, 0.2 parts of benzoyl peroxide was added. The temperature was raised to 70° C. and the reaction was allowed to proceed for 10 hours. Then, ethyl acetate was added to the reaction liquid to yield an acrylic polymer solution (A-1) having a solid content of 30%. The weight-average molecular weight of the obtained acrylic polymer (A-1) was 2,000,000, and the proportion of materials having a molecular weight of 100,000 or below in the polymer was 14%. (2) Adjustment of Molecular Weight and Other Properties To 100 parts of the acrylic polymer solution (A-1), 120 parts of n-heptane was added dropwise with stirring over 6 hours. The mixture was allowed to stand for 24 hours and the supernatant was then removed. Next, to the polymer solution from which the supernatant had been removed, toluene was added to dissolve the polymer, and a treatment in which 120 parts of n-heptane was added and the supernatant was removed as described above was further repeated five times. To the final polymer solution thus obtained, 80 parts of toluene was added and the mixture was concentrated to yield an acrylic polymer solution (A-2) having apolymer concentration of 25% by weight. The weight-average molecular weight of the obtained acrylic polymer (A-2) was 2,600,000, and the proportion of materials having a molecular weight of 100,000 or below in the polymer was 5%. Preparation 2 (Preparation of Acrylic Polymer) An acrylic polymer solution (B-1) was obtained using the same polymerization method as in Preparation 1 (1) with the exception that the kinds and amounts of the monomers used were 45 parts of methyl acrylate, 52 parts of 2-ethylhexyl acrylate, 3 parts of acrylic acid, and 0.06 parts of 3-hydroxypropyl acrylate. The weight-average molecular weight of the obtained acrylic polymer (B-1) was 1,900,000, and the proportion of materials having a molecular weight of 100,000 or below in the polymer was 12%. The acrylic polymer solution (B-1) was then subjected to the same treatment as in Preparation 1 (2) to yield an acrylic polymer solution (B-2). The weight-average molecular weight of the acrylic polymer (B-2) was 3,200,000, and the proportion of materials having a molecular weight of 100,000 or below in the polymer was 4%. Preparation 3 (Preparation of Acrylic Polymer) An acrylic polymer solution (C-1) was obtained using the same polymerization method as in Preparation 1 (1) with the exception that the kinds and amounts of the monomers used were 70 parts of methyl acrylate, 10 parts of butyl acrylate, 10 parts of acrylic acid, 10 parts of acrylonitrile, and 0.06 parts of 3-hydroxypropyl acrylate. The weight-average molecular weight of the obtained acrylic polymer (C-1) was 2,000,000, and the proportion of materials having a molecular weight of 100,000 or below in the polymer was 12%. The acrylic polymer solution (C-1) was then subjected to the same treatment as in Preparation 1 (2) to yield an acrylic polymer solution (C-2). The weight-average molecular weight of the acrylic polymer (C-2) was 3,900,000, and the proportion of materials having a molecular weight of 100,000 or below in the polymer was 4%. Preparation 4 (Preparation of Acrylic Polymer) An acrylic polymer solution (D-1) was obtained using the same polymerization method as in Preparation 1 (1) with the exception that the kinds and amounts of the monomers used were 70 parts of 2-ethylhexyl acrylate, 20 parts of isobutyl acrylate, 3 parts of acrylic acid, 10 parts of methoxyethyl acrylate, and 0.05 parts of 3-hydroxypropyl acrylate. The weight-average molecular weight of the obtained acrylic polymer (D-1) was 1,500,000, and the proportion of materials having a molecular weight of 100,000 or below in the polymer was 17%. The acrylic polymer solution (D-1) was then subjected to the same treatment as in Preparation 1 (2) to yield an acrylic polymer solution (D-2). The weight-average molecular weight of the acrylic polymer (D-2) was 2,200,000, and the proportion of materials having a molecular weight of 100,000 or below in the polymer was 4%. Preparation 5 (Preparation of Acrylic Polymer) (1) Synthesis of Acrylic Polymer In a three-necked flask, 60 parts of ethyl acrylate, 35 parts of butyl acrylate, 5 parts of acrylic acid, 0.06 parts of 3-hydroxypropyl acrylate, and 100 parts of ethyl acetate as a polymerization solvent were placed, and stirred for 2 hours while introducing nitrogen gas. After removing oxygen in the polymerization system in this way, 0.2 parts of benzoyl peroxide was added. The temperature was raised to 75° C. and the reaction was allowed to proceed for 7 hours. Then, ethyl acetate was added to the reaction liquid to yield an acrylic polymer solution (E-1) having a solid content of 30% by weight. The weight-average molecular weight of the acrylic polymer (E-1) was 900,000, and the proportion of materials having a molecular weight of 100,000 or below in the polymer was 15%. (2) Adjustment of molecular weight and other properties To 100 parts of the acrylic polymer solution (E-1), 120 parts of n-heptane was added dropwise with stirring over 6 hours. The mixture was allowed to stand for 24 hours and the supernatant was then removed. Next, to the polymer solution from which the supernatant had been removed, toluene was added to dissolve the polymer, and the treatment in which 120 parts of n-heptane was added and the supernatant was removed as described above was further repeated five times. To the final polymer solution thus obtained, 80 parts of toluene was added and the mixture was concentrated to yield an acrylic polymer solution (E-2) having apolymer concentration of 25% by weight. The weight-average molecular weight of the obtained acrylic polymer (E-2) was 1,200,000, and the proportion of materials having a molecular weight of 100,000 or below in the polymer was 5%. 
 Example 1 To 100 parts of acrylic polymer solution (A-2) , 1.5 parts of tolylene duisocyanate was added to prepare a pressure-sensitive adhesive composition (solution). This solution was applied by cast coating on a 50-aim-thick polyethylene terephthalate film of which surface had been mold release-treated (release liner) so that the thickness after drying would be about 25 aim, heat-dried at 130IC for 3 minutes, and further aged at 50° C. for 72 hours to make a pressure-sensitive adhesive sheet that comprised a pressure-sensitive adhesive layer having cross-linked structure. 
 Example 2 A pressure-sensitive adhesive sheet was obtained in the same manner as in Example 1 with the exceptions that the acrylic polymer solution (B-2) was used in place of the acrylic polymer solution (A-2) in Example 1 and that 1.0 part of diphenylmethane diisocyanate was used in place of 1.5 parts of tolylene diisocyanate in Example 1. 
 Comparative Example 1 A pressure-sensitive adhesive sheet was obtained in the same manner as in Example 1 with the exceptions that the acrylic polymer solution (C-2) was used in place of the acrylic polymer solution (A-2) in Example 1 and that 0.8 parts of trimethylolpropane tolylene duisocyanate was used in place of 1.5 parts of tolylene dulsocyanate in Example 1. 
 Comparative Example 2 A pressure-sensitive adhesive sheet was obtained in the same manner as in Example 1 with the exceptions that the acrylic polymer solution (D-2) was used in place of the acrylic polymer solution (A-2) in Example 1 and that 0.01 parts of glycerine diglycidyl ether was used in place of 1.5 parts of tolylene diisocyanate in Example 1. 
 Comparative Example 3 A pressure-sensitive adhesive sheet was obtained in the same manner as in Example 1 with the exceptions that the acrylic polymer solution (E-2) was used in place of the acrylic polymer solution (A-2) in Example 1 and that 0.015 parts of glycerine diglycidyl ether was used in place of 1.5 parts of tolylene diisocyanate in Example 1. 
 Comparative Example 4 A pressure-sensitive adhesive sheet was obtained in the same manner as in Example 1 with the exceptions that the acrylic polymer solution (A-1) was used in place of the acrylic polymer solution (A-2) in Example 1 and that 0.6 parts of trimethylolpropane tolylene diisocyanate was used in place of 1.5 parts of tolylene diisocyanate in Example 1. The pressure-sensitive adhesive sheets obtained in the above Examples and Comparative Examples were measured for their storage modulus, and their adhesive strength and weather resistance were also evaluated in the following tests. The evaluation results are shown in Table 1. For reference, the weight-average molecular weight of polymer used and the proportions of materials having molecular weight of 100,000 or below in the polymer determined by GPC method are also included in Table 1. Adhesive Strength Test Each pressure-sensitive adhesive sheet was laminated onto a polycarbonate sheet (100 &mgr;m thick), and test pieces 20 mm in width and 120 mm in length were then prepared. After removing the release liner, the test pieces were each stuck on a polyethylene terephthalate sheet, an acrylic resin plate (polymethyl methacrylate), or a polycarbonate sheet, and bonded together using a roller having a weight of 19.6 N reciprocating once on a glass plate under atmosphere at 23° C. After leaving at 50° C. for one day and standing to cool to 23° C., the peel adhesive strength was measured using a TENSILON-type tester at a peeling rate of 300 mm/min. Weather Resistance Test Each pressure-sensitive adhesive sheet was laminated onto a polyethylene terephthalate film (188 Am thick), and after removing the release liner, stuck and fixed on an acrylic resin plate (2 mm thick) to prepare test pieces. These test pieces were then placed under the conditions at 80° C. or 60° C. and 95% R. H. for 500 hours to conduct a weather resistance test. After completion of this weather resistance test, the test pieces were evaluated by visual inspection, wherein test pieces showing no appearance defect such as bubble or lifting at the adhesive interface were recorded with “O”, and any test pieces showing any such bubble or lifting were recorded with “X” regardless of the extent of the defects. 1 TABLE 1 Example Example Comparative Comparative Comparative Comparative 1 2 Example 1 Example 2 Example 3 Example 4 Acrylic polymer A-2 B-2 C-2 D-2 E-2 A-1 used Weight—average 260 320 390 220 120 200 molecular weight (×10 4 ) Proportion of 5 4 4 4 5 14 materials having molecular weight of 100,000 or below (%) Storage modulus 1 3.5 5.5 0.3 0.8 1 (×10 5 Pa) Adhesive strength (N/20 mm) to PET 14 12 10 12 13 13 to Acrylic resin 18 14 11 13 16 17 to PC 19 14.5 11 15 16 18 Weather &xcirc; &xcirc; X X X X resistance