adhesive sheet

Abstract

To provide means capable of reducing the amount of a tackifier used on the whole adhesive layer while ensuring high adhesive performance to a low polarity adherend.SOLUTION: There is provided an adhesive sheet having a substrate, a first adhesive layer and a second adhesive layer in this order, wherein the first adhesive layer is formed of a first adhesive composition comprising a first acrylic polymer; the second adhesive layer is formed of a second adhesive composition comprising a second acrylic polymer and a tackifier; the content concentration (mass%) of the tackifier to the first acrylic polymer contained in the first adhesive composition is smaller than the content concentration (mass%) of the tackifier to the second acrylic polymer contained in the second adhesive composition; and the content mass of a constitutional unit derived from 2-ethylhexyl (meth)acrylate in the first acrylic polymer is smaller than the content mass of a constitutional unit derived from 2-ethylhexyl (meth)acrylate in the second acrylic polymer.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a pressure-sensitive adhesive sheet. [Background technology]

[0002] Acrylic pressure-sensitive adhesives are used in a wide range of applications due to their excellent heat resistance, weather resistance, etc. However, acrylic pressure-sensitive adhesives are known to have low adhesion to low-polarity adherends such as polyolefins. Therefore, techniques for combining tackifiers have been investigated to improve the adhesion of acrylic pressure-sensitive adhesives to low-polarity adherends. For example, Patent Document 1 discloses the use of a styrene polymer in combination with a hydrogenated petroleum resin / hydrogenated terpene resin as a tackifier. Patent Document 2 discloses the use of a rosin in combination with a hydrogenated petroleum resin as a tackifier. Furthermore, Patent Document 3 discloses that in a system containing an acrylic polymer and a tackifier, the acrylic polymer has a solubility parameter (SP value) difference of 0.2 (MPa) 1 / 2 It is disclosed that the acrylic polymer contains 15% by weight or more of each of the above two types of monomer units. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 1-60677 [Patent Document 2] Japanese Patent Application Publication No. 6-207151 [Patent Document 3] Japanese Patent Application Laid-Open No. 2009-256607 Summary of the Invention [Problem to be solved by the invention]

[0004] Tackifiers are expensive, so it is preferable to use a small amount of them. However, in order to ensure sufficient adhesion to low-polarity adherends, it is necessary to incorporate a large amount of tackifier into the adhesive layer.

[0005] Therefore, an object of the present invention is to provide a means for reducing the amount of tackifier used in the entire pressure-sensitive adhesive layer while ensuring high adhesive performance to low-polarity adherends. [Means for solving the problem]

[0006] The present invention is a pressure-sensitive adhesive sheet having, in this order, a substrate, a first pressure-sensitive adhesive layer, and a second pressure-sensitive adhesive layer, wherein the first pressure-sensitive adhesive layer is formed from a first pressure-sensitive adhesive composition containing a first acrylic polymer, and the second pressure-sensitive adhesive layer is formed from a second pressure-sensitive adhesive composition containing a second acrylic polymer and a tackifier, the concentration (% by mass) of the tackifier relative to the first acrylic polymer contained in the first pressure-sensitive adhesive composition is lower than the concentration (% by mass) of the tackifier relative to the second acrylic polymer contained in the second pressure-sensitive adhesive composition, and the mass content of structural units derived from 2-ethylhexyl (meth)acrylate in the first acrylic polymer is lower than the mass content of structural units derived from 2-ethylhexyl (meth)acrylate in the second acrylic polymer. [Effects of the Invention]

[0007] According to the pressure-sensitive adhesive sheet of the present invention, the amount of tackifier added to the entire pressure-sensitive adhesive sheet can be reduced while maintaining strong adhesive performance to adherends with low polarity. [Brief explanation of the drawings]

[0008] [Figure 1] 1 is a cross-sectional view showing one embodiment of the pressure-sensitive adhesive sheet of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0009] A first embodiment of the present invention is a pressure-sensitive adhesive sheet having, in this order, a substrate, a first pressure-sensitive adhesive layer, and a second pressure-sensitive adhesive layer, wherein the first pressure-sensitive adhesive layer is formed from a first pressure-sensitive adhesive composition containing a first acrylic polymer, and the second pressure-sensitive adhesive layer is formed from a second pressure-sensitive adhesive composition containing a second acrylic polymer and a tackifier, the concentration (% by mass) of the tackifier relative to the first acrylic polymer contained in the first pressure-sensitive adhesive composition is lower than the concentration (% by mass) of the tackifier relative to the second acrylic polymer contained in the second pressure-sensitive adhesive composition, and the mass content of structural units derived from 2-ethylhexyl (meth)acrylate in the first acrylic polymer is lower than the mass content of structural units derived from 2-ethylhexyl (meth)acrylate in the second acrylic polymer.

[0010] A second embodiment of the present invention is a pressure-sensitive adhesive sheet having, in this order, a substrate, a first pressure-sensitive adhesive layer, and a second pressure-sensitive adhesive layer, wherein the first pressure-sensitive adhesive layer is formed from a first pressure-sensitive adhesive composition containing a first acrylic polymer, and the second pressure-sensitive adhesive layer is formed from a second pressure-sensitive adhesive composition containing a second acrylic polymer and a tackifier, the concentration (% by mass) of the tackifier contained in the first pressure-sensitive adhesive layer is lower than the concentration (% by mass) of the tackifier contained in the second pressure-sensitive adhesive layer, and the mass content of structural units derived from 2-ethylhexyl (meth)acrylate in the first acrylic polymer is lower than the mass content of structural units derived from 2-ethylhexyl (meth)acrylate in the second acrylic polymer.

[0011] In this embodiment, a relatively large amount of tackifier is present in the second PSA layer (the adhesive surface) that contacts the adherend. Because a tackifier is incorporated to improve adhesion to low-polarity adherends, the technical idea is that the tackifier must be present on the PSA layer surface on the adherend side. This ensures high adhesion of the PSA sheet to low-polarity adherends. However, the present inventors have discovered that simply incorporating a relatively large amount of tackifier on the adherend side results in lower than expected adhesion to low-polarity adherends. The inventors speculate that this is because, even if a large amount of tackifier is incorporated into the PSA layer on the adherend side during manufacturing, the tackifier diffuses due to a concentration gradient within the PSA layer, resulting in a lower tackifier concentration (mass %) present on the PSA layer surface on the adherend side than the amount incorporated during manufacturing. Based on this speculation, the composition of the acrylic polymer that constitutes the PSA is varied in the thickness direction to prevent the tackifier from migrating to the PSA layer on the substrate side. Note that the above speculation does not limit the technical scope of the present invention.

[0012] The present embodiment will be described in detail below.

[0013] In this specification, the range "X to Y" means "X or more and Y or less." Unless otherwise specified, operations and measurements of physical properties are performed at room temperature (20 to 25°C) and a relative humidity of 45 to 55% RH. In this specification, "(meth)acrylic acid" refers to "acrylic acid or methacrylic acid," and "(meth)acrylate" refers to "acrylate or methacrylate."

[0014] The concept of a sheet includes things called tapes, labels, films, etc.

[0015] FIG. 1 is a cross-sectional schematic diagram showing one embodiment of the pressure-sensitive adhesive sheet of the present invention. The drawings are exaggerated for ease of explanation, and the dimensional proportions of the components in the drawings may differ from the actual figures. In FIG. 1, a pressure-sensitive adhesive sheet 10 is composed of a substrate 20, a first pressure-sensitive adhesive layer 30, a second pressure-sensitive adhesive layer 40, and a release liner 50. The first pressure-sensitive adhesive layer 30 is formed from a first pressure-sensitive adhesive composition containing a first acrylic polymer. The second pressure-sensitive adhesive layer 40 is formed from a second pressure-sensitive adhesive composition containing a second acrylic polymer. The release liner 50 is formed to prevent dust and other foreign matter from adhering to the second pressure-sensitive adhesive layer until the pressure-sensitive adhesive sheet 10 is attached to an adherend. Therefore, the release liner 50 is peeled off when the sheet is attached to an adherend.

[0016] The first pressure-sensitive adhesive layer and the second pressure-sensitive adhesive layer are disposed adjacent to each other. The second pressure-sensitive adhesive layer is a pressure-sensitive adhesive layer that exhibits adhesiveness to an adherend when attached to the adherend. Therefore, the second pressure-sensitive adhesive layer becomes the outermost layer when attached (before attachment, as described above, it is usually protected by a release liner).

[0017] In addition, in the embodiment of FIG. 1, the first pressure-sensitive adhesive layer is a pressure-sensitive adhesive layer that is directly attached to the substrate, but is not limited to a form in which it is arranged adjacent to the substrate, and other functional layers such as an easy-adhesion layer or other pressure-sensitive adhesive layers may be arranged between the substrate and the first pressure-sensitive adhesive layer.

[0018] The structure of each layer constituting the pressure-sensitive adhesive sheet will be described below.

[0019] (First adhesive layer and second adhesive layer) The first pressure-sensitive adhesive layer and the second pressure-sensitive adhesive layer are mainly composed of an acrylic polymer as a pressure-sensitive adhesive.

[0020] The mass content of the structural units derived from 2-ethylhexyl (meth)acrylate in the first acrylic polymer is less than the mass content of the structural units derived from 2-ethylhexyl (meth)acrylate in the second acrylic polymer. Compared to n-butyl acrylate, 2-ethylhexyl (meth)acrylate has a longer side chain alkyl group, which makes it more hydrophobic and highly compatible with tackifiers. Therefore, by adopting the above-mentioned configuration, migration of the tackifier to the pressure-sensitive adhesive layer (first pressure-sensitive adhesive layer) on the substrate side is suppressed, thereby ensuring adhesion of the pressure-sensitive adhesive layer (second pressure-sensitive adhesive layer) on the adherend (particularly a low-polarity adherend) to the adherend (particularly a low-polarity adherend).

[0021] Here, 2-ethylhexyl (meth)acrylate refers to 2-ethylhexyl acrylate and / or 2-ethylhexyl methacrylate, but it is preferable to use 2-ethylhexyl acrylate.

[0022] Taking into consideration the migration of the tackifier, the mass content of structural units derived from 2-ethylhexyl (meth)acrylate in the first acrylic polymer is preferably less than 50% by mass (lower limit: 0% by mass), more preferably 40% by mass or less (lower limit: 0% by mass), and even more preferably 30% by mass or less.

[0023] The first acrylic polymer preferably contains 50% by mass or more, more preferably more than 50% by mass, even more preferably 60% by mass or more (upper limit 100% by mass), particularly preferably 70% by mass or more (upper limit 100% by mass), and most preferably 80% by mass or more (upper limit 100% by mass) of structural units derived from n-butyl (meth)acrylate. This configuration makes it easier to ensure adhesion to the substrate.

[0024] Here, n-butyl (meth)acrylate refers to n-butyl acrylate and / or n-butyl methacrylate, but it is preferable to use n-butyl acrylate.

[0025] Furthermore, the content by mass of structural units derived from 2-ethylhexyl (meth)acrylate in the second acrylic polymer is preferably 50% by mass or more (upper limit 100% by mass), taking into consideration adhesion to the adherend and compatibility with the tackifier.

[0026] The mass content of the structural units derived from 2-ethylhexyl (meth)acrylate and the structural units derived from n-butyl (meth)acrylate in the acrylic polymer is usually the mass content of each monomer in the monomer mixture during production.

[0027] The acrylic polymer is formed by using a (meth)acrylic acid alkyl ester as the main monomer component, and optionally a monomer copolymerizable with the (meth)acrylic acid alkyl ester (copolymerizable monomer). Here, the main component refers to 50% by mass or more (up to 100% by mass) of the monomers, and the content of the (meth)acrylic acid alkyl ester relative to the total monomers is, in order of preference, 65% by mass or more, 85% by mass or more, 90% by mass or more, 95% by mass or more, 98% by mass or more, 99% by mass or more, and 99.5% by mass or more.

[0028] Examples of (meth)acrylic acid alkyl esters include the above-mentioned 2-ethylhexyl (meth)acrylate and n-butyl (meth)acrylate, as well as methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, isopropyl (meth)acrylate, isobutyl (meth)acrylate, n-pentyl (meth)acrylate, n-hexyl (meth)acrylate, cyclohexyl (meth)acrylate, isooctyl (meth)acrylate, decyl (meth)acrylate, dodecyl (meth)acrylate, myristyl (meth)acrylate, palmityl (meth)acrylate, and stearyl (meth)acrylate. These may be used alone or in combination of two or more.

[0029] Examples of copolymerizable monomers copolymerizable with (meth)acrylic acid alkyl esters include carboxyl group-containing monomers or anhydrides thereof, such as (meth)acrylic acid, itaconic acid, maleic acid, fumaric acid, crotonic acid, and isocrotonic acid; hydroxyl group-containing monomers, such as hydroxyalkyl (meth)acrylates (2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, and 4-hydroxybutyl (meth)acrylate, and glycerin dimethacrylate; amide group-containing monomers, such as acrylamide, methacrylamide, N-vinylpyrrolidone, and N,N-dimethylacrylamide; amino group-containing monomers, such as aminoethyl (meth)acrylate and (meth)acryloylmorpholine; aromatic vinyl compounds, such as styrene and substituted styrene; cyano group-containing monomers, such as acrylonitrile; and vinyl esters, such as vinyl acetate. These may be used alone or in combination of two or more. The amount of copolymerizable monomers copolymerizable with these alkyl (meth)acrylates is preferably 5% by mass or less based on the total amount of monomers.

[0030] In order to improve adhesiveness, it is preferable to include a carboxyl group-containing monomer as a copolymerizable monomer copolymerizable with a (meth)acrylic acid alkyl ester. Examples of the carboxyl group-containing monomer include (meth)acrylic acid, maleic acid, maleic anhydride, fumaric acid, fumaric anhydride, crotonic acid, itaconic acid, itaconic anhydride, myristoleic acid, palmitoleic acid, and oleic acid. The content of the carboxyl group-containing monomer in the monomer mixture used to produce the acrylic polymer is preferably 0.1 to 5% by mass, more preferably 0.3 to 3% by mass. Furthermore, the first acrylic polymer and / or the second acrylic polymer preferably contain 0.1 to 5% by mass, more preferably 0.3 to 3% by mass of structural units derived from the carboxyl group-containing monomer.

[0031] The weight average molecular weight of the acrylic polymer is not particularly limited, and is, for example, 100,000 to 10,000,000.

[0032] The content of the first acrylic polymer in the first pressure-sensitive adhesive layer is not particularly limited, but is preferably 70% by mass or more (upper limit 100% by mass), and may be 75% by mass or more, 80% by mass or more, 85% by mass or more, or 90% by mass or more.

[0033] The content of the second acrylic polymer in the second pressure-sensitive adhesive layer is not particularly limited, but is preferably 60 to 99.9% by mass, more preferably 70 to 99% by mass, even more preferably 75 to 98% by mass, and particularly preferably 80 to 95% by mass.

[0034] The method for producing the acrylic polymer is not particularly limited, and conventionally known methods such as solution polymerization using a polymerization initiator, emulsion polymerization, suspension polymerization, reverse-phase suspension polymerization, thin-film polymerization, and spray polymerization can be used. In addition to methods initiating polymerization using a polymerization initiator, methods initiating polymerization by irradiation with radiation, electron beams, ultraviolet rays, or the like can also be employed. Among these, emulsion polymerization is preferred because it further enhances the effects of the present invention. That is, in a preferred embodiment of the present invention, the first and second acrylic polymers are emulsion polymers.

[0035] An example of the emulsion polymerization method is a method in which an emulsifier and a polymerization initiator are added to a monomer mixture containing the above-mentioned monomers, and emulsion polymerization is carried out.

[0036] In emulsion polymerization, from the viewpoint of polymerization stability, it is preferable that the emulsifier (or a part of the emulsifier) ​​is dissolved in the monomer mixture or that the monomer mixture is made into an O / W emulsion in advance.

[0037] The procedure for carrying out emulsion polymerization may be, for example, the following methods (1) to (3). (1) The entire amount of the monomer mixture, emulsifier, water, etc. is charged, the temperature is raised, and the polymerization initiator dissolved in water is added dropwise or in portions to carry out polymerization. (2) Water, an emulsifier, and a portion of the monomer mixture are charged into a reaction vessel, and the temperature is raised. Then, a polymerization initiator dissolved in water is added dropwise or in portions to proceed with the polymerization reaction. Then, the remaining monomer mixture is added dropwise or in portions to continue the polymerization. (3) A polymerization initiator dissolved in water is placed in a reaction vessel, and after the temperature is raised, an emulsion consisting of a monomer mixture, an emulsifier, and water is added dropwise or in portions to carry out polymerization.

[0038] The emulsifier is not particularly limited, but from the viewpoint of improving the dispersion stability of the emulsion polymer, anionic emulsifiers or nonionic emulsifiers are preferred, and anionic emulsifiers are more preferred.

[0039] Examples of anionic emulsifiers include sodium lauryl sulfate, ammonium lauryl sulfate, sodium dodecylbenzenesulfonate, sodium polyoxyethylene alkyl ether sulfate, sodium polyoxyethylene alkyl phenyl ether sulfate, sodium polyoxyethylene alkyl ether sulfate, allyl alkyl sulfosuccinate salt, etc. Examples of nonionic emulsifiers include polyoxyethylene alkyl ether, polyoxyethylene alkyl phenyl ether, etc. These emulsifiers may be used alone or in combination of two or more.

[0040] The amount of emulsifier added is preferably 0.5 to 12 parts by mass, more preferably 0.8 to 8 parts by mass, and even more preferably 1 to 6 parts by mass, per 100 parts by mass of the monomer mixture, from the viewpoint of stability of the emulsion polymerization reaction and prevention of deterioration of physical properties due to remaining unreacted emulsifier.

[0041] The emulsifier may be added directly to a solution prepared by adding water to the monomer mixture, or may be added in advance to a polymerization vessel, or both may be used.

[0042] The polymerization initiator may be either water-soluble or oil-soluble. Examples include azo compounds such as 2,2'-azobis(2-methylpropionamidine) dihydrochloride and 2,2'-azobis(2-amidinopropane) dihydrochloride; persulfates such as potassium persulfate, sodium persulfate, and ammonium persulfate; and peroxides such as benzoyl peroxide, t-butyl hydroperoxide, and hydrogen peroxide. Redox initiators such as a combination of a persulfate and sodium bisulfite or a combination of a peracid and sodium ascorbate may also be used. The polymerization initiators may be used alone or in combination of two or more. Among these, persulfates or redox initiators are preferred due to their excellent polymerization stability.

[0043] The amount of the polymerization initiator added is preferably 0.01 to 6 parts by mass, more preferably 0.03 to 4 parts by mass, and even more preferably 0.05 to 2 parts by mass, relative to 100 parts by mass of the monomer mixture, from the viewpoint of accelerating the polymerization rate.

[0044] The polymerization initiator may be added to the reaction vessel in advance, may be added immediately before the start of polymerization, may be added in multiple batches after the start of polymerization, may be added to the monomer mixture in advance, or may be added to an emulsion of the monomer mixture after preparing the emulsion.

[0045] During emulsion polymerization, a known chain transfer agent or pH buffer may be further added.

[0046] The water used in emulsion polymerization is preferably ion-exchanged water, and the amount of water used is preferably 30 to 400 parts by mass, more preferably 35 to 200 parts by mass, and even more preferably 40 to 150 parts by mass, per 100 parts by mass of the monomer mixture.

[0047] It is preferable to further add aqueous ammonia, various water-soluble amines, or aqueous alkali solutions such as aqueous sodium hydroxide and aqueous potassium hydroxide to the emulsion polymer dispersion obtained by emulsion polymerization to adjust the pH to 5 to 9 (preferably pH 6 to 8.5).

[0048] The solid content concentration of the emulsion polymer dispersion is preferably 10 to 80 mass %, more preferably 25 to 70 mass %, and even more preferably 45 to 65 mass %.

[0049] The viscosity of the emulsion polymer dispersion at 25° C. is preferably 50 to 12,000 mPa·s, more preferably 100 to 10,000 mPa·s, and even more preferably 200 to 9,000 mPa·s. In this specification, the viscosity is a value measured using a B-type rotational viscometer.

[0050] The emulsion polymer has an emulsion shape (particulate shape) in which the emulsion polymer is dispersed. In this case, the average particle size of the emulsion polymer is preferably 50 to 500 nm, more preferably 100 to 300 nm. Here, the average particle size of the emulsion polymer is a volume-based median size measured by a laser diffraction dispersion method.

[0051] In the first embodiment, the tackifier concentration (% by mass) in the first pressure-sensitive adhesive composition is lower than the tackifier concentration (% by mass) in the second pressure-sensitive adhesive composition, and in the second embodiment, the tackifier concentration (% by mass) in the first pressure-sensitive adhesive layer is lower than the tackifier concentration (% by mass) in the second pressure-sensitive adhesive layer.

[0052] Since the amount of tackifier added to the PSA layer can be reduced, the concentration (mass %) of tackifier contained in the first PSA composition is preferably 5% by mass or less (lower limit 0% by mass) relative to the first acrylic polymer, more preferably 3% by mass or less (lower limit 0% by mass), even more preferably 1% by mass or less, and particularly preferably substantially free. Here, "substantially free of tackifier" means that the presence of tackifier at the level of an impurity is acceptable, and specifically, "substantially free of tackifier" refers to a content of 0.01% by mass or less, preferably 0.005% by mass or less (lower limit 0% by mass) relative to the first acrylic polymer.

[0053] Furthermore, since the amount of tackifier added to the PSA layer can be reduced, the concentration (% by mass) of the tackifier contained in the first PSA layer is preferably 10% by mass or less (lower limit: 0% by mass), more preferably 8% by mass or less (lower limit: 0% by mass), and even more preferably 5% by mass or less (lower limit: 0% by mass), relative to the first acrylic polymer. Even when no tackifier is added to the first PSA composition, it is difficult to avoid a small amount of tackifier migrating from the second PSA layer, and therefore the content of tackifier in the first PSA layer tends to be higher than the amount of tackifier added to the first PSA composition.

[0054] In consideration of adhesive performance to low-polarity adherends, the content (% by mass) of the tackifier in the second PSA composition is preferably 1% by mass or more, and more preferably 5% by mass or more, relative to the second acrylic polymer. In consideration of compatibility with the PSA, the content (% by mass) of the tackifier in the second PSA composition is preferably 40% by mass or less, more preferably 30% by mass or less, and even more preferably 20% by mass or less, relative to the second acrylic polymer.

[0055] Furthermore, in consideration of adhesive performance to low-polarity adherends, the content (% by mass) of the tackifier in the second pressure-sensitive adhesive layer is preferably 1% by mass or more, more preferably 5% by mass or more, relative to the second acrylic polymer. In consideration of compatibility with the pressure-sensitive adhesive, the content (% by mass) of the tackifier in the second pressure-sensitive adhesive layer is preferably 40% by mass or less, more preferably 30% by mass or less, relative to the second acrylic polymer.

[0056] The content concentration (mass %) of the tackifier in the pressure-sensitive adhesive layer can be measured by Fourier transform infrared spectroscopy (FT-IR).

[0057] The tackifier is not particularly limited, and examples thereof include alicyclic petroleum resins, terpene resins, rosin resins, and styrene resins. In a preferred embodiment, the tackifier is at least one selected from the group consisting of alicyclic petroleum resins, terpene resins, and rosin resins. In another preferred embodiment, from the viewpoint of adhesiveness, the tackifier is at least one selected from the group consisting of alicyclic petroleum resins and terpene resins.

[0058] Petroleum resins are obtained by polymerizing, by known methods, diolefins and monoolefins in the fraction of cracked oil, a by-product of the production of ethylene by steam cracking petroleum. C5 petroleum resins are made from C5 fractions such as isoprene, 1,3-pentadiene, cyclopentene, and cyclopentadiene, while C9 petroleum resins are made from C9 fractions such as styrene, vinyltoluene, α-methylstyrene, indene, alkylindene, and dicyclopentadiene.

[0059] Examples of alicyclic petroleum resins include alicyclic hydrocarbon resins obtained by cyclodimerizing C5 petroleum resins and then polymerizing them; polymers of cyclic diene compounds (cyclopentadiene, dicyclopentadiene, ethylidene norbornene, dipentene, ethylidene bicycloheptene, vinylcycloheptene, tetrahydroindene, vinylcyclohexene, limonene, etc.) or hydrogenated products thereof; and alicyclic hydrocarbon resins obtained by hydrogenating the aromatic rings of C9 petroleum resins or C5 / C9 resins.

[0060] Among these, the alicyclic petroleum resin is preferably an alicyclic petroleum resin obtained by hydrogenating a C5 petroleum resin / C9 petroleum resin.

[0061] The alicyclic petroleum resin may be a commercially available product, and examples of commercially available products include the Alcon (registered trademark) series (Alcon (registered trademark) P-90, P-100, P-115, P-125, P-140, M-90, M-100, M-115, M-135 (all manufactured by Arakawa Chemical Industries, Ltd.)), the Quintone (registered trademark) series (Quintone (registered trademark) 1105, 1325, 1340, TD-401, 1500, 1525L, 1920, 2940, etc. (all manufactured by Zeon Corporation), and the like.

[0062] The alicyclic petroleum resins may be used alone or in combination of two or more.

[0063] Examples of the terpene resin include terpene resin, terpene phenol resin, aromatic modified terpene resin, and hydrogenated terpene phenol resin obtained by hydrogenating these resins, among which aromatic modified terpene resin is preferred.

[0064] Commercially available terpene resins may be used. Examples of terpene resins include YS Resin (registered trademark) PX1250, PX1150, PX1000, PX800, PX1150N, and PX300N (all manufactured by Yasuhara Chemical Co., Ltd.). Examples of terpene phenol resins include YS Polystar (registered trademark) U130, U115, T160, T145, T130, T115, T100, T80, T30, S145, G150, G125, N125, K125, and TH130 (all manufactured by Yasuhara Chemical Co., Ltd.). Examples of aromatic modified terpene resins include YS Resin (registered trademark) TO125, TO115, TO105, and TO85 (all manufactured by Yasuhara Chemical Co., Ltd.). Examples of hydrogenated terpene phenol resins include YS Polystar (registered trademark) UH115 (all manufactured by Yasuhara Chemical Co., Ltd.).

[0065] The terpene resins may be used alone or in combination of two or more kinds.

[0066] Examples of rosin-based resins include natural rosin, rosin ester, hydrogenated rosin, hydrogenated rosin ester, polymerized rosin, polymerized rosin ester, and disproportionated rosin ester. Commercially available products may be used. The rosin-based resins may be used alone or in combination of two or more.

[0067] Examples of styrene-based resins include homopolymers of α-methylstyrene, homopolymers of styrene-based monomers, copolymers of α-methylstyrene and styrene-based monomers, and copolymers of α-methylstyrene, styrene-based monomers, and other monomers.

[0068] The method of adding the tackifier is not particularly limited. For example, in a preferred embodiment in which an acrylic polymer is obtained by emulsion polymerization, examples include: (1) a method in which a tackifier is added to a monomer mixture, an emulsifier, water, etc. before the start of polymerization, and then the monomer is polymerized; (2) a method in which a tackifier emulsion is prepared using a tackifier, an emulsifier, water, etc., and the like, and the emulsion is used to blend the tackifier into a pressure-sensitive adhesive composition; etc. The emulsifier used above can be any of the emulsifiers described in the section on emulsion polymerization of acrylic polymers.

[0069] The pressure-sensitive adhesive composition may contain a crosslinking agent. Known crosslinking agents can be used as the crosslinking agent. Examples include, but are not limited to, isocyanate-based crosslinking agents, epoxy-based crosslinking agents, metal chelate-based crosslinking agents, aziridine-based crosslinking agents, oxazoline-based crosslinking agents, and carbodiimide-based crosslinking agents. Among these, isocyanate-based crosslinking agents are preferred from the viewpoint of reactivity. A pressure-sensitive adhesive composition that does not contain a crosslinking agent is also suitable.

[0070] Isocyanate crosslinking agents include trimethylene diisocyanate, 1,2-propylene diisocyanate, tetramethylene diisocyanate, 1,2-butylene diisocyanate, 2,3-butylene diisocyanate, 1,3-butylene diisocyanate, hexamethylene diisocyanate (HDI), pentamethylene diisocyanate, 2,2,4-trimethylhexamethylene diisocyanate, 2,4,4-trimethylhexamethylene diisocyanate, 2,6-diisocyanate methylcaprate, lysine diisocyanate, lysine ester aliphatic diisocyanates such as 1,6,11-undecane triisocyanate, 1,3,6-hexamethylene triisocyanate, and decamethylene diisocyanate; aromatic diisocyanates such as tolylene diisocyanate and xylene diisocyanate; alicyclic diisocyanates such as isophorone diisocyanate; and isocyanate derivatives such as adducts of the above diisocyanate compounds and polyol compounds such as trimethylolpropane, and biurets and isocyanurates of the above diisocyanate compounds.

[0071] Examples of epoxy crosslinking agents include polyglycidylamine type epoxy resins, bisphenol A type epoxy resins, novolac type epoxy resins, alicyclic type epoxy resins, aliphatic epoxy resins, brominated epoxy resins, and alcohol type epoxy resins.

[0072] Examples of the metal chelate crosslinking agent include acetylacetonate complexes of metals such as aluminum, titanium, nickel, chromium, iron, zinc, cobalt, manganese, and zirconium.

[0073] The crosslinking agent may be used alone or in the form of a mixture of two or more kinds.

[0074] When a crosslinking agent is added, the amount of the crosslinking agent added is preferably 0.1 to 10 parts by mass, more preferably 0.5 to 5 parts by mass, per 100 parts by mass of each acrylic polymer.

[0075] The PSA composition may further contain other additives known in the art, such as fillers, pigments, UV absorbers, wetting agents, preservatives, etc. Examples of fillers include zinc oxide, silica, calcium carbonate, etc.

[0076] The method for forming the pressure-sensitive adhesive layer is not particularly limited, and the pressure-sensitive adhesive layer may be formed by directly applying the pressure-sensitive adhesive composition to the substrate, or the pressure-sensitive adhesive layer may be formed on a release liner and then attached to the substrate. Specifically, a method may be mentioned in which the pressure-sensitive adhesive composition is applied to a release liner and the pressure-sensitive adhesive layer made of the pressure-sensitive adhesive composition is transferred to the substrate.

[0077] The method for applying the PSA composition to the substrate or release liner is not particularly limited, and the composition can be applied using a known application device such as a roll coater, knife coater, air knife coater, bar coater, blade coater, slot die coater, lip coater, or gravure coater.

[0078] The thickness of each pressure-sensitive adhesive layer (film thickness after drying) is usually 5 to 100 μm, preferably 5 to 50 μm. The thicknesses of the pressure-sensitive adhesive layers may be different or the same. For example, by making the second pressure-sensitive adhesive layer thinner than the first pressure-sensitive adhesive layer, the amount of tackifier used can be reduced.

[0079] (base material) The substrate is not particularly limited, and various substrates used as support substrates for pressure-sensitive adhesive sheets can be used.The substrate can be a resin substrate; so-called synthetic paper made of resin; metal foil such as copper, aluminum, stainless steel, or the like; composite sheet substrate made by laminating a metal foil and a plastic film; vapor-deposited foil substrate made by vapor-depositing aluminum, silica, or the like; nonwoven fabric substrate; or paper substrate such as high-quality paper, kraft paper, glassine paper, art paper, coated paper, heat-sensitive coloring paper, or moisture-proof paper.Among these, the substrate is preferably a resin substrate, synthetic paper, or paper substrate. Examples of resins that constitute a suitable resin substrate include polyesters such as polyethylene terephthalate, polybutylene terephthalate, and polyethylene naphthalate, polyolefins such as polyethylene and polypropylene, polyvinyl chloride, polyvinylidene chloride, polyvinyl alcohol, ethylene-vinyl acetate copolymer, polystyrene, polycarbonate, polymethylpentene, polysulfone, polyether ether ketone, polyether sulfone, polyphenylene sulfide, polyetherimide, polyimide, fluororesin, polyamide, acrylic resin, norbornene-based resin, and cycloolefin resin.

[0080] In this embodiment, since the amount of tackifier in the pressure-sensitive adhesive layer on the substrate side is relatively low, the substrate is preferably a substrate other than a low-polarity substrate. Specific examples include polyesters such as polyethylene terephthalate, polybutylene terephthalate, and polyethylene naphthalate. Among these, the substrate is preferably polyethylene terephthalate because of its high mechanical strength and excellent heat resistance.

[0081] The thickness of the substrate is not particularly limited, but from the viewpoint of mechanical properties, it is preferably 10 μm or more, more preferably 20 μm or more, and even more preferably 40 μm or more, and from the viewpoint of thinning, it is preferably 200 μm or less, more preferably 150 μm or less, and even more preferably 100 μm or less.

[0082] (Release liner) The release liner is a component that protects the pressure-sensitive adhesive layer and prevents a decrease in adhesiveness. The release liner is peeled off from the pressure-sensitive adhesive sheet when the sheet is attached to an adherend. Therefore, the pressure-sensitive adhesive sheet of the present invention also includes a sheet that does not have a release liner.

[0083] The release liner is not particularly limited, but examples thereof include paper such as fine paper, glassine paper, clay-coated paper, and polyethylene-laminated paper; polyester films such as polyethylene terephthalate, polybutylene terephthalate, and polyethylene naphthalate; and plastic films such as polyolefin films such as polypropylene and polyethylene.

[0084] The thickness of the release liner is usually about 10 to 400 μm. The surface of the release liner may be provided with a layer of a release agent made of silicone or the like to improve the releasability of the pressure-sensitive adhesive layer. When such a layer is provided, the thickness of the layer is usually about 0.01 to 5 μm.

[0085] <Method of applying pressure-sensitive adhesive sheets to low-polarity substrates> The pressure-sensitive adhesive sheet of this embodiment is suitable for use on low-polarity adherends.

[0086] Therefore, another embodiment of the present invention is a method of applying the pressure-sensitive adhesive sheet of the first or second embodiment to an adherend with low polarity.

[0087] Here, examples of the low polarity adherend include polyolefin films such as polypropylene and polyethylene.

[0088] The adhesive performance to low-polarity adherends is preferably 7,000 mN / 25 mm or more. Here, the adhesive performance is the value measured for each adherend by the method described in the Examples. [Example]

[0089] The effects of the present invention will be explained using the following examples and comparative examples. In the examples, the units "parts" and "%" are sometimes used, but unless otherwise specified, they represent "parts by mass" or "% by mass." Unless otherwise specified, each operation is carried out at room temperature (25°C).

[0090] Example 1 <Preparation of Pressure-Sensitive Adhesive Composition> A reaction vessel equipped with a stirrer, a thermometer, a reflux condenser, and a dropping funnel was charged with 100 parts by mass of n-butyl acrylate and 1 part by mass of acrylic acid as raw material monomers, 2 parts by mass of polyoxyethylene alkyl ether sodium sulfate ("Latemul E-118B" manufactured by Kao Corporation) and 2 parts by mass of sodium allyl alkyl sulfosuccinate ("Eleminol JS-2" manufactured by Sanyo Chemical Industries, Ltd.) as emulsifiers, and 58 parts by mass of degassed ion-exchanged water, and the mixture was stirred at room temperature to prepare an emulsion.

[0091] Separately, 28 parts by mass of degassed ion-exchanged water was added to a reaction vessel equipped with a stirrer, thermometer, reflux condenser, and dropping funnel, and the temperature was raised to 60°C. Next, the emulsion was transferred to the dropping funnel and added dropwise over 4 hours. Concurrently, 2 parts by mass of a 5% by mass aqueous potassium persulfate solution was added dropwise as a polymerization initiator solution, and emulsion polymerization was carried out at a reaction temperature of 60°C. After completion of the addition, the mixture was aged at 60°C for 2 hours to obtain an emulsion polymerization composition. Thereafter, the mixture was cooled to room temperature, and the pH was adjusted to 8.0 with 25% by mass aqueous ammonia, to obtain a first pressure-sensitive adhesive composition.

[0092] In the method for obtaining the first adhesive described above, the mixing mass ratio of the raw material monomers 2-ethylhexyl acrylate and n-butyl acrylate was changed as shown in Table 1, and "Tamanol E-100" (terpene phenol type) manufactured by Arakawa Chemical Industries, Ltd. was added as a tackifier to obtain a second adhesive composition.

[0093] <Preparation of adhesive sheet> A polyethylene terephthalate film (thickness: 50 μm) was prepared as the substrate, and glassine paper (thickness: 70 μm) treated with silicone for release was prepared as the release liner.

[0094] The first and second pressure-sensitive adhesive compositions were simultaneously coated onto a release liner using a die coater, and then dried at 90°C for 1 minute to remove water, forming a first pressure-sensitive adhesive layer (thickness: 10µm) and a second pressure-sensitive adhesive layer (thickness: 10µm). Next, a substrate was attached to the first pressure-sensitive adhesive layer, producing a pressure-sensitive adhesive sheet in which the substrate, first pressure-sensitive adhesive layer, second pressure-sensitive adhesive layer, and release liner were laminated in this order.

[0095] (Examples 2 and 3, Comparative Examples 1 and 2) An adhesive sheet was obtained in the same manner as in Example 1, except that the mixing mass ratio of 2-ethylhexyl acrylate and n-butyl acrylate in the first acrylic polymer was changed as shown in Table 1.

[0096] Example 4 An adhesive sheet was obtained in the same manner as in Example 1, except that the mixing mass ratio of 2-ethylhexyl acrylate and n-butyl acrylate in the second acrylic polymer was changed as shown in Table 1.

[0097] Example 5 An adhesive sheet was obtained in the same manner as in Example 1, except that the tackifier was changed to "Super Ester E-720" (rosin ester type) manufactured by Arakawa Chemical Industries, Ltd.

[0098] Example 6 An adhesive sheet was obtained in the same manner as in Example 1, except that the tackifier was changed to an emulsified version of "Arcon P-140" (hydrogenated petroleum resin) manufactured by Arakawa Chemical Industries, Ltd.

[0099] (Comparative Example 3) An adhesive sheet was obtained in the same manner as in Example 1, except that the first adhesive layer was not formed and the thickness of the second adhesive layer was doubled in Example 1. In Comparative Example 3, the thickness of the second adhesive layer was doubled compared to Example 1, and therefore the amount of tackifier used in the entire adhesive sheet was double that of Example 1.

[0100] (Measurement method 1: Adhesive strength) The adhesive sheets obtained in the Examples and Comparative Examples were left standing under standard conditions (23°C, 50% RH) for one day, the release liner was peeled off, and the adhesive layer was attached to a polypropylene plate. The sheet was then left standing under standard conditions for 30 minutes, after which the adhesive strength was measured. Specifically, the sheet was peeled off in a 180° direction at a test speed of 300 mm / min using a tensile tester, and the adhesive strength was measured. The values ​​were converted to peel force per 25 mm of sheet width (mN / 25 mm). The results are shown in Table 1.

[0101] [Table 1]

[0102] From the above results, the pressure-sensitive adhesive sheets of the Examples had high adhesiveness to polypropylene, which is a low-polarity adherend. Furthermore, since Comparative Example 3 had a single pressure-sensitive adhesive layer, a large amount of tackifier was used to achieve adhesive strength similar to that of the Examples. [Explanation of symbols]

[0103] 10 adhesive sheets, 20 base material, 30 first adhesive layer, 40 second adhesive layer, 50 release liners.

Claims

1. A pressure-sensitive adhesive sheet having a substrate, a first pressure-sensitive adhesive layer, and a second pressure-sensitive adhesive layer in this order, the first adhesive layer is disposed adjacent to the substrate; the first PSA layer is formed from a first PSA composition containing a first acrylic polymer, and the second PSA layer is formed from a second PSA composition containing a second acrylic polymer and a tackifier; a content concentration (% by mass) of a tackifier relative to the first acrylic polymer contained in the first PSA composition is lower than a content concentration (% by mass) of a tackifier relative to the second acrylic polymer contained in the second PSA composition; the content by mass of structural units derived from 2-ethylhexyl (meth)acrylate in the first acrylic polymer is less than the content by mass of structural units derived from 2-ethylhexyl (meth)acrylate in the second acrylic polymer; the content by mass of structural units derived from 2-ethylhexyl (meth)acrylate in the first acrylic polymer is less than 50% by mass, the first acrylic polymer contains 50% by mass or more of structural units derived from n-butyl (meth)acrylate, and the content of the first acrylic polymer in the first pressure-sensitive adhesive layer is 70% by mass or more; the substrate is a resin substrate or a paper substrate, The resin constituting the resin substrate is at least one selected from the group consisting of polyester, polyvinyl chloride, polyvinylidene chloride, polyvinyl alcohol, ethylene-vinyl acetate copolymer, polystyrene, polycarbonate, polymethylpentene, polysulfone, polyether ether ketone, polyether sulfone, polyphenylene sulfide, polyetherimide, polyimide, fluororesin, polyamide, acrylic resin, norbornene resin, and cycloolefin resin.

2. The pressure-sensitive adhesive sheet according to claim 1 , wherein the first pressure-sensitive adhesive composition is substantially free of a tackifier.

3. A pressure-sensitive adhesive sheet having a substrate, a first pressure-sensitive adhesive layer, and a second pressure-sensitive adhesive layer in this order, the first adhesive layer is disposed adjacent to the substrate; the first PSA layer is formed from a first PSA composition containing a first acrylic polymer, and the second PSA layer is formed from a second PSA composition containing a second acrylic polymer and a tackifier; a content concentration (mass%) of a tackifier contained in the first pressure-sensitive adhesive layer is lower than a content concentration (mass%) of a tackifier contained in the second pressure-sensitive adhesive layer; the content by mass of structural units derived from 2-ethylhexyl (meth)acrylate in the first acrylic polymer is less than the content by mass of structural units derived from 2-ethylhexyl (meth)acrylate in the second acrylic polymer; the content by mass of structural units derived from 2-ethylhexyl (meth)acrylate in the first acrylic polymer is less than 50% by mass, the first acrylic polymer contains 50% by mass or more of structural units derived from n-butyl (meth)acrylate, and the content of the first acrylic polymer in the first pressure-sensitive adhesive layer is 70% by mass or more; the substrate is a resin substrate or a paper substrate, The resin constituting the resin substrate is at least one selected from the group consisting of polyester, polyvinyl chloride, polyvinylidene chloride, polyvinyl alcohol, ethylene-vinyl acetate copolymer, polystyrene, polycarbonate, polymethylpentene, polysulfone, polyether ether ketone, polyether sulfone, polyphenylene sulfide, polyetherimide, polyimide, fluororesin, polyamide, acrylic resin, norbornene resin, and cycloolefin resin.

4. The pressure-sensitive adhesive sheet according to any one of claims 1 to 3, wherein the second acrylic polymer contains 45 mass % or more of structural units derived from 2-ethylhexyl (meth)acrylate.

5. The pressure-sensitive adhesive sheet according to any one of claims 1 to 4, wherein the first acrylic polymer and the second acrylic polymer are emulsion polymers.

6. The pressure-sensitive adhesive sheet according to any one of claims 1 to 5, wherein the second pressure-sensitive adhesive layer is a non-photocrosslinkable pressure-sensitive adhesive layer.

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