Pressure-sensitive adhesive composition for protective film and protective film

Abstract

To provide a pressure-sensitive adhesive composition for a protective film, capable of forming a pressure-sensitive adhesive layer having moderate adhesive force required for the protective film and excellent in conformability to an adherend, and to provide the protective film.SOLUTION: A constitutional unit (a1) derived from an alkyl (meth) acrylate monomer and a constitutional unit (a2) derived from a monomer having a hydroxyl group, in which a content of the constitutional unit (a2) is 1.0% to 10.0% by mass with respect to the entire constitutional unit, and a (meth) acrylic copolymer having a Mw of 50000 to 2 million, an isocyanate-based crosslinking agent, and a plasticizer which is an adipic acid diester compound having a molecular weight of 350 to 450, in which a ratio of the number of moles of isocyanate groups in the isocyanate-based crosslinking agent to the number of moles of reactive functional groups in the (meth) acrylic copolymer is 0.18 to 3.36, and a content of the plasticizer is 15 to 35 parts by mass with respect to 100 parts by mass of the (meth) acrylic copolymer.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present disclosure relates to a pressure-sensitive adhesive composition for a protective film and a protective film. [Background technology]

[0002] As the use modes and environments of PSA films become more diverse, various PSA compositions have been developed to accommodate various shapes and environments (see, for example, Patent Document 1).

[0003] One known use of PSA films is to apply them to articles to protect their surfaces in order to prevent damage and contamination when processing, inspecting, or transporting such articles, such as metal plates, coated steel plates, synthetic resin plates, optical components, electronic components, etc. PSA films used in this manner are also called protective films, and typically have a structure in which a PSA layer is provided on one side of a film-like substrate. A protective film is peeled off from an adherend once it has completed its protective role. If the adhesive strength of the adhesive layer is too high when peeling the protective film from an adherend, handling may be impaired, or if the adhesion between the substrate and the adhesive layer is poor, adhesive residue may remain on the adherend. On the other hand, if the adhesive strength of the adhesive layer is too low, the protective film may peel off unintentionally from the adherend. Therefore, the adhesive layer provided in the protective film is required to have an adhesive strength that is neither too low nor too high (i.e., an appropriate adhesive strength) so that the protective film does not peel off unintentionally from the adherend while protection of the adherend is required, and can be easily peeled off from the adherend without leaving any adhesive residue when peeling it from the adherend. For this reason, acrylic adhesive compositions containing a (meth)acrylic resin as a main component are often used as adhesive compositions for protective films because their adhesive properties, such as adhesive strength and cohesive strength, can be easily adjusted.

[0004] For example, Patent Document 2 discloses an ionizing radiation-curable removable pressure-sensitive adhesive composition containing an acrylic polymer (A) and a plasticizer as essential ingredients, wherein the pressure-sensitive adhesive composition contains 40% by mass to 250% by mass of the plasticizer relative to 100% by mass of the acrylic polymer (A), and wherein an adhesive film obtained by curing the pressure-sensitive adhesive composition takes 5 seconds or less from the start of wetting until the entire surface is wetted when the composition is placed on an adherend having a surface with a height difference of 0.1 μm to 10 μm, and wherein the adhesive strength after being attached to the adherend and maintained at 23°C for 20 minutes is 0.01 N / 25 mm to 0.2 N / 25 mm, and the ratio of the adhesive strength after being attached to the adherend and maintained at 110°C for 20 hours to the above adhesive strength is 0.5 to 5. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2007-327036 [Patent Document 2] Japanese Patent Application Laid-Open No. 2010-235856 Summary of the Invention [Problem to be solved by the invention]

[0006] In addition to having the above-described appropriate adhesive strength, the pressure-sensitive adhesive layer of the protective film may also be required to have the property of quickly wetting the adherend under its own weight (so-called conformability). If the pressure-sensitive adhesive layer has excellent conformability, for example, even if the edge of the protective film is curled up due to an external factor, the protective film can re-adhere to the adherend under its own weight and naturally return to its original state.

[0007] The present disclosure has been made in light of the above-mentioned circumstances. The problem to be solved by one embodiment of the present disclosure is to provide a pressure-sensitive adhesive composition for a protective film that can form a pressure-sensitive adhesive layer that has the appropriate adhesive strength required for a protective film and has excellent compatibility with an adherend. Another problem to be solved by another embodiment of the present disclosure is to provide a protective film including a pressure-sensitive adhesive layer formed from the pressure-sensitive adhesive composition for a protective film. [Means for solving the problem]

[0008] Specific means for solving the problems include the following aspects. <1> a (meth)acrylic copolymer comprising a structural unit (a1) derived from a (meth)acrylic acid alkyl ester monomer and a structural unit (a2) derived from a monomer having a hydroxyl group, wherein the content of the structural unit (a2) is 1.0% by mass to 10.0% by mass with respect to all structural units and the weight average molecular weight is 50,000 to 2,000,000; an isocyanate-based crosslinking agent; a plasticizer that is an adipic acid diester compound having a molecular weight of 350 to 450; Including, the ratio of the number of moles of isocyanate groups in the isocyanate crosslinking agent to the number of moles of reactive functional groups in the (meth)acrylic copolymer [number of moles of isocyanate groups in the isocyanate crosslinking agent / number of moles of reactive functional groups in the (meth)acrylic copolymer] is 0.18 to 3.36; The pressure-sensitive adhesive composition for protective films, wherein the content of the plasticizer is 15 to 35 parts by mass relative to 100 parts by mass of the (meth)acrylic copolymer. <2> the (meth)acrylic copolymer further contains a structural unit (a3) ​​derived from a monomer having a carboxy group; <1> The pressure-sensitive adhesive composition for a protective film according to claim 1. <3> The plasticizer includes at least one of dioctyl adipate and diisodecyl adipate. <1> or <2> The pressure-sensitive adhesive composition for a protective film according to claim 1. <4> The structural unit (a2) includes a structural unit derived from 4-hydroxybutyl acrylate. <1> ~ <3> 10. The pressure-sensitive adhesive composition for a protective film according to claim 9. <5> The isocyanate-based crosslinking agent is a hexamethylene diisocyanate-based compound. <1> ~ <4> 10. The pressure-sensitive adhesive composition for a protective film according to claim 9. <6> The hexamethylene diisocyanate compound is an isocyanurate of a hexamethylene diisocyanate compound. <5> The pressure-sensitive adhesive composition for a protective film according to claim 1. <7> A substrate; provided on the substrate, and <1> ~ <6> a pressure-sensitive adhesive layer formed from the pressure-sensitive adhesive composition for a protective film according to any one of the above items; A protective film is provided. [Effects of the Invention]

[0009] According to one embodiment of the present disclosure, there is provided a pressure-sensitive adhesive composition for a protective film that can form a pressure-sensitive adhesive layer that has the appropriate adhesive strength required for a protective film and excellent compatibility with an adherend. According to another embodiment of the present disclosure, there is provided a protective film including a pressure-sensitive adhesive layer formed from the pressure-sensitive adhesive composition for a protective film. DETAILED DESCRIPTION OF THE INVENTION

[0010] The pressure-sensitive adhesive composition for a protective film and the protective film of the present disclosure will be described in detail below. The following description of the requirements may be based on a representative embodiment of the present disclosure, but the present disclosure is not limited to such an embodiment and can be implemented with appropriate modifications within the scope of the purpose of the present disclosure.

[0011] In the present disclosure, a numerical range indicated using "to" means a range that includes the numerical values ​​before and after "to" as the lower and upper limits, respectively. In the numerical ranges described in stages in the present disclosure, the upper or lower limit value described in a certain numerical range may be replaced with the upper or lower limit value of another numerical range described in stages. Furthermore, in the numerical ranges described in the present disclosure, the upper or lower limit value described in a certain numerical range may be replaced with a value shown in the examples.

[0012] In the present disclosure, a combination of two or more preferred embodiments is a more preferred embodiment.

[0013] In the present disclosure, the amount of each component in a pressure-sensitive adhesive composition for a protective film means the total amount of the above-mentioned multiple substances present in the pressure-sensitive adhesive composition for a protective film, unless otherwise specified, when multiple substances corresponding to each component are present in the pressure-sensitive adhesive composition for a protective film.

[0014] In the present disclosure, unless otherwise specified, "solid content" means components other than the solvent contained in the composition, and "solvent" means water and organic solvents. For example, when the only solvent contained in the composition is water, the solid content refers to the components other than water contained in the composition; when the only solvent contained in the composition is an organic solvent, the solid content refers to the components other than the organic solvent contained in the composition; and when the solvents contained in the composition are water and an organic solvent, the solid content refers to the components other than water and the organic solvent contained in the composition.

[0015] In the present disclosure, the term "(meth)acrylic copolymer" refers to a copolymer that contains structural units derived from (meth)acrylic monomers and in which the proportion of structural units derived from (meth)acrylic monomers is 50 mass % or more. In the present disclosure, the term "(meth)acrylic monomer" means a monomer having a (meth)acryloyl group.

[0016] In the present disclosure, "(meth)acrylic" is a term that encompasses both "acrylic" and "methacrylic," "(meth)acrylate" is a term that encompasses both "acrylate" and "methacrylate," and "(meth)acryloyl" is a term that encompasses both "acryloyl" and "methacryloyl."

[0017] In this disclosure, "n-" means normal, "i-" means iso, "s-" means secondary, and "t-" means tertiary.

[0018] In the present disclosure, the terms "monomer" and "monomer compound" are synonymous, and the terms "polymer" and "polymeric compound" are synonymous.

[0019] In the present disclosure, "% by mass" and "% by weight" are synonymous, and "parts by mass" and "parts by weight" are synonymous.

[0020] In the present disclosure, the term "structural unit derived from a monomer" refers to a structural unit formed by addition polymerization of a monomer.

[0021] In the present disclosure, the term "process" includes not only an independent process but also a process that cannot be clearly distinguished from other processes, as long as the intended purpose of the process is achieved.

[0022] [Adhesive composition for protective film] The pressure-sensitive adhesive composition for a protective film of the present disclosure (hereinafter also simply referred to as the "pressure-sensitive adhesive composition") comprises a (meth)acrylic copolymer containing a structural unit (a1) derived from a (meth)acrylic acid alkyl ester monomer and a structural unit (a2) derived from a monomer having a hydroxyl group, the content of the structural unit (a2) being 1.0 mass % to 10.0 mass % of all structural units, and having a weight-average molecular weight of 50,000 to 2,000,000; an isocyanate-based crosslinking agent; and a hydroxyl group-containing monomer having a molecular weight of 350 to 450. and a plasticizer which is an adipic acid diester compound, wherein the ratio of the number of moles of isocyanate groups in the isocyanate crosslinking agent to the number of moles of reactive functional groups in the (meth)acrylic copolymer [number of moles of isocyanate groups in the isocyanate crosslinking agent / number of moles of reactive functional groups in the (meth)acrylic copolymer] is 0.18 to 3.36, and the content of the plasticizer is 15 to 35 parts by mass per 100 parts by mass of the (meth)acrylic copolymer.

[0023] The pressure-sensitive adhesive composition of the present disclosure can form a pressure-sensitive adhesive layer that has the appropriate adhesive strength required for a protective film and also has excellent compatibility with an adherend. The reason why the PSA composition of the present disclosure can exhibit such effects is unclear, but the present inventors speculate as follows: However, the following speculation is not intended to limit the PSA composition of the present disclosure, but is provided as an example.

[0024] In the present disclosure, "a (meth)acrylic copolymer containing a structural unit (a1) derived from a (meth)acrylic acid alkyl ester monomer and a structural unit (a2) derived from a monomer having a hydroxyl group, wherein the content of the structural unit (a2) is 1.0% by mass to 10.0% by mass of all structural units and wherein the weight average molecular weight is 50,000 to 2,000,000" is also referred to as "a specific (meth)acrylic copolymer." In addition, in the present disclosure, "a plasticizer that is an adipic acid diester compound having a molecular weight of 350 to 450" is also referred to as "a specific plasticizer."

[0025] The pressure-sensitive adhesive composition of the present disclosure contains an adipic acid diester compound as a plasticizer, and when a pressure-sensitive adhesive layer is formed, the adipic acid diester compound segregates on the surface of the pressure-sensitive adhesive layer, improving the wettability of the interface between the pressure-sensitive adhesive layer and the adherend. This improves the compatibility of the pressure-sensitive adhesive layer with the adherend. Furthermore, adipic acid diester compounds with a molecular weight of 350 to 450 have relatively short molecular chains, making them less likely to become entangled with the specific (meth)acrylic copolymer. Therefore, the phenomenon of the adipic acid diester compound being incorporated into the polymer chain of the specific (meth)acrylic copolymer and therefore less likely to segregate on the surface of the pressure-sensitive adhesive layer is less likely to occur. Furthermore, since the PSA composition of the present disclosure contains the specific plasticizer in a specific range relative to the specific (meth)acrylic copolymer, the PSA layer formed has an appropriate hardness that is neither too soft nor too hard, and therefore the appropriate cohesive strength developed in the PSA layer by crosslinking between the specific (meth)acrylic copolymer and the isocyanate crosslinking agent is maintained without being impaired. Furthermore, in the pressure-sensitive adhesive composition of the present disclosure, the ratio of the number of moles of isocyanate groups in the isocyanate-based crosslinking agent to the number of moles of reactive functional groups in the specific (meth)acrylic copolymer is within a specific range, and therefore, when a pressure-sensitive adhesive layer is formed, the balance between the cohesive strength of the entire pressure-sensitive adhesive layer and the wettability of the pressure-sensitive adhesive layer to the interface with the adherend is improved. From the above, it is presumed that the pressure-sensitive adhesive composition of the present disclosure can form a pressure-sensitive adhesive layer that has the appropriate adhesive strength required for a protective film and excellent compatibility with the adherend.

[0026] In contrast to the pressure-sensitive adhesive composition for protective films of the present disclosure, the pressure-sensitive adhesive composition described in Patent Document 1 (JP 2007-327036 A; the same applies hereinafter) does not focus on the conformability of the pressure-sensitive adhesive layer to the adherend. The pressure-sensitive adhesive composition described in Patent Document 1 contains an adipic acid-based plasticizer as a plasticizer, but because it is an adipic acid polyester compound with a long molecular chain rather than an adipic acid diester compound, entanglement between the plasticizer and the acrylic resin is likely to occur. When entanglement occurs between the plasticizer and the acrylic resin, the plasticizer is incorporated into the polymer chain, making it difficult for the plasticizer to segregate to the surface of the pressure-sensitive adhesive layer. Therefore, it is presumed that the pressure-sensitive adhesive layer formed from the pressure-sensitive adhesive composition described in Patent Document 1 has insufficient wettability at the interface with the adherend, resulting in poor conformability to the adherend. Furthermore, it is presumed that the long-chain adipic acid polyester compound is strongly entangled with the acrylic resin, resulting in poor flexibility in the pressure-sensitive adhesive layer formed. The adhesive composition described in Patent Document 2 (JP 2010-235856 A) achieves both adhesive strength and conformability, but differs from the adhesive composition for protective films of the present disclosure in that it contains a large amount of plasticizer, thereby imparting conformability to the adhesive layer with respect to the adherend.

[0027] [Specific (meth)acrylic copolymer] The pressure-sensitive adhesive composition of the present disclosure includes a (meth)acrylic copolymer (i.e., a specific (meth)acrylic copolymer) that includes a structural unit (a1) derived from a (meth)acrylic acid alkyl ester monomer and a structural unit (a2) derived from a monomer having a hydroxyl group, wherein the content of the structural unit (a2) is 1.0 mass % to 10.0 mass % of all structural units and the weight-average molecular weight is 50,000 to 2,000,000. The pressure-sensitive adhesive composition of the present disclosure may contain one type of specific (meth)acrylic copolymer alone, or may contain two or more types.

[0028] <Constituent unit (a1)> The specific (meth)acrylic copolymer contains a structural unit (a1) derived from a (meth)acrylic acid alkyl ester monomer. The type of (meth)acrylic acid alkyl ester monomer is not particularly limited. The (meth)acrylic acid alkyl ester monomer may be an acrylic acid alkyl ester monomer or a methacrylic acid alkyl ester monomer. The alkyl group contained in the (meth)acrylic acid alkyl ester monomer may be unsubstituted or may have a substituent (excluding a hydroxyl group and a carboxyl group), but is preferably unsubstituted. The alkyl group contained in the (meth)acrylic acid alkyl ester monomer may be linear, branched, or cyclic. The alkyl moiety of the (meth)acrylic acid alkyl ester monomer preferably has 1 to 18 carbon atoms, more preferably 1 to 12 carbon atoms, and even more preferably 1 to 8 carbon atoms, for example.

[0029] Specific examples of the (meth)acrylic acid alkyl ester monomer include methyl (meth)acrylate, ethyl (meth)acrylate, n-butyl (meth)acrylate, i-butyl (meth)acrylate, s-butyl (meth)acrylate, t-butyl (meth)acrylate, n-octyl (meth)acrylate, i-octyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, n-nonyl (meth)acrylate, i-nonyl (meth)acrylate, n-decyl (meth)acrylate, stearyl (meth)acrylate, lauryl (meth)acrylate, cyclohexyl (meth)acrylate, and isobornyl (meth)acrylate.

[0030] The structural unit (a1) preferably contains at least one of a structural unit derived from 2-ethylhexyl acrylate and a structural unit derived from n-butyl acrylate, more preferably contains a structural unit derived from 2-ethylhexyl acrylate, and even more preferably is a structural unit derived from 2-ethylhexyl acrylate.

[0031] The specific (meth)acrylic copolymer may contain one type of structural unit (a1) alone, or may contain two or more types.

[0032] The content of the structural unit (a1) in the specific (meth)acrylic copolymer is not particularly limited, but for example, it is preferably 50.0 mass% to 99.0 mass%, more preferably 60.0 mass% to 98.5 mass%, even more preferably 70.0 mass% to 97.0 mass%, and particularly preferably 88.5 mass% to 97.0 mass%, relative to all structural units of the specific (meth)acrylic copolymer. The content of the structural unit (a1) in the specific (meth)acrylic copolymer being 50.0 mass% or more relative to all structural units of the specific (meth)acrylic copolymer means that the structural unit (a1) is contained as a main component of the structural units of the specific (meth)acrylic copolymer.

[0033] <Constituent unit (a2)> The specific (meth)acrylic copolymer contains a structural unit (a2) derived from a monomer having a hydroxyl group, and the content of the structural unit (a2) is 1.0% by mass to 10.0% by mass with respect to all structural units. The hydroxyl group of the structural unit (a2) contributes to a crosslinking reaction with the isocyanate group of the isocyanate-based crosslinking agent described below.

[0034] The type of the hydroxyl group-containing monomer is not particularly limited. Examples of the monomer having a hydroxyl group include a monomer having at least one hydroxyl group and an ethylenically unsaturated group in one molecule. Examples of the ethylenically unsaturated group include a vinyl group, an allyl group, a vinylphenyl group, a (meth)acrylamide group, and a (meth)acryloyl group.

[0035] Specific examples of the monomer having a hydroxyl group include 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, 6-hydroxyhexyl (meth)acrylate, 8-hydroxyoctyl (meth)acrylate, 10-hydroxydecyl (meth)acrylate, 12-hydroxylauryl (meth)acrylate, 3-methyl-3-hydroxybutyl (meth)acrylate, 1,1-dimethyl-3-hydroxybutyl (meth)acrylate, butyl (meth)acrylate, 1,3-dimethyl-3-hydroxybutyl (meth)acrylate, 2,2,4-trimethyl-3-hydroxypentyl (meth)acrylate, 2-ethyl-3-hydroxyhexyl (meth)acrylate, N-hydroxyethyl (meth)acrylamide, glycerin mono(meth)acrylate, polypropylene glycol mono(meth)acrylate, polyethylene glycol mono(meth)acrylate, and poly(ethylene glycol-propylene glycol) mono(meth)acrylate.

[0036] The structural unit (a2) preferably contains a structural unit derived from 4-hydroxybutyl acrylate, and more preferably is a structural unit derived from 4-hydroxybutyl acrylate, from the viewpoint of facilitating the formation of a pressure-sensitive adhesive layer having the appropriate adhesive strength required for a protective film, for example. When the structural unit (a2) contains a structural unit derived from 4-hydroxybutyl acrylate, the distance between crosslinking points can be more sufficiently ensured compared to when the structural unit (a2) contains a structural unit derived from 2-hydroxyethyl acrylate, and therefore the pressure-sensitive adhesive layer formed tends to have a suppressed increase in cohesive strength and not have an excessively high adhesive strength.

[0037] The specific (meth)acrylic copolymer may contain one type of structural unit (a2) alone, or may contain two or more types.

[0038] The content of the structural unit (a2) in the specific (meth)acrylic copolymer is 1.0% by mass to 10.0% by mass based on the total amount of all structural units in the specific (meth)acrylic copolymer. When the content of the structural unit (a2) in the specific (meth)acrylic copolymer is 1.0 mass% or more based on the total structural units of the specific (meth)acrylic copolymer, the pressure-sensitive adhesive layer formed does not exhibit excessively high adhesive strength, and tends to achieve the appropriate adhesive strength required for a protective film. In addition, the pressure-sensitive adhesive layer formed tends to have excellent compatibility with the adherend. The content of the structural unit (a2) in the specific (meth)acrylic copolymer is preferably at least 2.0 mass %, and more preferably at least 3.0 mass %, based on all structural units in the specific (meth)acrylic copolymer. When the content of the structural unit (a2) in the specific (meth)acrylic copolymer is 10.0 mass% or less based on all structural units of the specific (meth)acrylic copolymer, the pressure-sensitive adhesive layer formed does not exhibit excessively low adhesive strength, and tends to be able to achieve the appropriate adhesive strength required for a protective film. The content of the structural unit (a2) in the specific (meth)acrylic copolymer is preferably 8.0 mass % or less, and more preferably 5.0 mass % or less, based on all structural units in the specific (meth)acrylic copolymer. In one embodiment, the content of the structural unit (a2) in the specific (meth)acrylic copolymer may be 2.0% by mass to 8.0% by mass, 2.0% by mass to 5.0% by mass, or 3.0% by mass to 5.0% by mass, relative to all structural units in the specific (meth)acrylic copolymer.

[0039] <Constituent unit (a3)> The specific (meth)acrylic copolymer preferably further contains a structural unit (a3) ​​derived from a monomer having a carboxy group. When the specific (meth)acrylic copolymer further contains the structural unit (a3), the pressure-sensitive adhesive layer formed therefrom tends to exhibit adhesive strength more suitable for the protective film, and the exhibited adhesive strength tends to be well maintained over time. The carboxy group of the structural unit (a3) ​​contributes to a crosslinking reaction with the isocyanate group of the isocyanate-based crosslinking agent described below.

[0040] The type of the monomer having a carboxy group is not particularly limited. Examples of the monomer having a carboxy group include a monomer having at least one carboxy group and an ethylenically unsaturated group in one molecule. Examples of the ethylenically unsaturated group include a vinyl group, an allyl group, a vinylphenyl group, a (meth)acrylamide group, and a (meth)acryloyl group. Specific examples of the monomer having a carboxy group include (meth)acrylic acid, crotonic acid, maleic acid, fumaric acid, itaconic acid, glutaconic acid, citraconic acid, ω-carboxy-polycaprolactone mono(meth)acrylate [e.g., ω-carboxy-polycaprolactone (n≒2) monoacrylate], and succinic acid derivatives (e.g., 2-acryloyloxyethyl-succinic acid).

[0041] The structural unit (a3) ​​preferably includes a structural unit derived from acrylic acid, and is more preferably a structural unit derived from acrylic acid.

[0042] When the specific (meth)acrylic copolymer contains the structural unit (a3), it may contain either one type of structural unit (a3) ​​alone, or two or more types of structural unit (a3).

[0043] When the specific (meth)acrylic copolymer contains the structural unit (a3), the content of the structural unit (a3) ​​in the specific (meth)acrylic copolymer is not particularly limited, but for example, it is preferably less than 5.0 mass%, more preferably 0.5 mass% or more and less than 5.0 mass%, and even more preferably 0.5 mass% or more and 2.0 mass% or less, relative to all structural units of the specific (meth)acrylic copolymer. When the content of the structural unit (a3) ​​in the specific (meth)acrylic copolymer is less than 5.0 mass% of all structural units in the specific (meth)acrylic copolymer, the adhesive strength of the pressure-sensitive adhesive layer formed tends not to be excessively high, and the compatibility of the pressure-sensitive adhesive layer formed with the adherend tends to be improved. When the content of the structural unit (a3) ​​in the specific (meth)acrylic copolymer is 0.5 mass% or more relative to all structural units of the specific (meth)acrylic copolymer, the resulting pressure-sensitive adhesive layer tends to have improved compatibility with the adherend.

[0044] <Other structural units> The specific (meth)acrylic copolymer may, if necessary, contain a structural unit that does not fall under any of the structural units (a1), (a2), and (a3) ​​(so-called other structural units), as long as the effects of the pressure-sensitive adhesive composition of the present disclosure are not impaired.

[0045] Other structural units include, for example, structural units derived from monomers having an amino group, such as 2-dimethylaminoethyl (meth)acrylate, 2-diethylaminoethyl (meth)acrylate, 2-diisopropylaminoethyl (meth)acrylate, and dimethylaminopropyl (meth)acrylamide; structural units derived from (meth)acrylates having an aromatic ring, such as benzyl (meth)acrylate and phenoxyethyl (meth)acrylate; structural units derived from alkoxyalkyl (meth)acrylates, such as methoxyethyl (meth)acrylate and ethoxyethyl (meth)acrylate; structural units derived from aromatic monovinyls, such as styrene, α-methylstyrene, t-butylstyrene, p-chlorostyrene, chloromethylstyrene, and vinyltoluene; structural units derived from vinyl cyanide, such as acrylonitrile and methacrylonitrile; and structural units derived from vinyl esters, such as vinyl formate, vinyl acetate, vinyl propionate, and vinyl versatate.

[0046] When the specific (meth)acrylic copolymer contains other structural units, it may contain one type of other structural unit alone, or two or more types of other structural units.

[0047] When the specific (meth)acrylic copolymer contains other structural units, the content of the other structural units in the specific (meth)acrylic copolymer can be set appropriately within a range that does not impair the effects of the pressure-sensitive adhesive composition of the present disclosure.

[0048] <<Weight-average molecular weight of specific (meth)acrylic copolymer>> The weight average molecular weight (also referred to as "Mw") of the specific (meth)acrylic copolymer is 50,000 to 2,000,000. When the weight average molecular weight of the specific (meth)acrylic copolymer is 50,000 or more, there is a tendency that a lack of cohesive force when formed into a pressure-sensitive adhesive layer can be suppressed. When the weight average molecular weight of the specific (meth)acrylic copolymer is 2,000,000 or less, the specific (meth)acrylic copolymer tends to be easier to produce. The weight average molecular weight of the specific (meth)acrylic copolymer is, for example, preferably 100,000 to 1,500,000, and more preferably 200,000 to 1,000,000.

[0049] The weight average molecular weight of the specific (meth)acrylic copolymer is a value measured by the following method, specifically, according to the following (1) to (3). (1) A solution of the specific (meth)acrylic copolymer is applied to a release paper and then dried at 100° C. for 1 minute to obtain a film of the specific (meth)acrylic copolymer. (2) Using the film-like specific (meth)acrylic copolymer obtained in (1) above and tetrahydrofuran, a sample solution having a solids concentration of 0.2% by mass is obtained. Note that the "solids concentration" here refers to the mass proportion of the specific (meth)acrylic copolymer in the sample solution. (3) The weight average molecular weight of the specific (meth)acrylic copolymer is determined as a standard polystyrene equivalent value by gel permeation chromatography (GPC) under the following conditions.

[0050] ~Conditions~ Measurement equipment: High-speed GPC [Model: HLC-8420 GPC, manufactured by Tosoh Corporation] Detector: Differential refractometer (RI) [built into HLC-8420, manufactured by Tosoh Corporation] Column: TSKgel GMH XL Two Tosoh products are used. Column temperature: 40℃ Eluent: tetrahydrofuran Sample solution injection volume: 100 μL Flow rate: 0.8mL / min

[0051] The weight average molecular weight of the specific (meth)acrylic copolymer can be adjusted to a desired value by adjusting the polymerization temperature, polymerization time, amount of organic solvent used, type of polymerization initiator, amount of polymerization initiator used, etc. when polymerizing the monomers.

[0052] <<Specific (meth)acrylic copolymer content>> The content of the specific (meth)acrylic copolymer in the pressure-sensitive adhesive composition of the present disclosure is not particularly limited, but is, for example, preferably 66.2 mass% to 86.4 mass%, more preferably 70.4 mass% to 82.5 mass%, and even more preferably 73.0 mass% to 82.0 mass%, relative to the total solid content in the pressure-sensitive adhesive composition.

[0053] In the present disclosure, the "total solid content in the PSA composition" means the total mass of the PSA composition when the PSA composition does not contain a solvent, and means the mass of the residue remaining after removing the solvent from the PSA composition when the PSA composition contains a solvent.

[0054] [Method for producing specific (meth)acrylic copolymer] The method for producing the specific (meth)acrylic copolymer is not particularly limited. The specific (meth)acrylic copolymer can be produced by polymerizing the above-mentioned monomers by a known polymerization method, such as solution polymerization, emulsion polymerization, suspension polymerization, or bulk polymerization. As the polymerization method, solution polymerization is preferred because the processing steps are relatively simple and can be completed in a short time when preparing the pressure-sensitive adhesive composition of the present disclosure after production.

[0055] In the solution polymerization method, a predetermined organic solvent, a monomer, a polymerization initiator, and an optional chain transfer agent are generally charged into a polymerization vessel and reacted by heating for several hours with stirring, for example, at the reflux temperature of the organic solvent. In this case, at least a portion of the organic solvent, the monomer, the polymerization initiator, and the optional chain transfer agent may be added sequentially. Alternatively, the reaction may be carried out in a nitrogen gas stream.

[0056] Examples of the organic solvent used in the polymerization reaction include aromatic hydrocarbon compounds, aliphatic hydrocarbon compounds, alicyclic hydrocarbon compounds, ester compounds, ketone compounds, glycol ether compounds, and alcohol compounds. More specifically, examples of the organic solvent used in the polymerization reaction include aromatic hydrocarbon compounds such as benzene, toluene, ethylbenzene, n-propylbenzene, t-butylbenzene, o-xylene, m-xylene, p-xylene, tetralin, decalin, and aromatic naphtha; aliphatic or alicyclic hydrocarbon compounds such as n-hexane, n-heptane, n-octane, i-octane, n-decane, dipentene, petroleum spirit, petroleum naphtha, and turpentine; ester compounds such as methyl acetate, ethyl acetate, n-butyl acetate, n-amyl acetate, 2-hydroxyethyl acetate, 2-butoxyethyl acetate, 3-methoxybutyl acetate, and methyl benzoate; acetone; Examples of the alcohol compounds include ketone compounds typified by ethyl ketone, methyl-i-butyl ketone, isophorone, cyclohexanone, and methylcyclohexanone; glycol ether compounds typified by ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, and diethylene glycol monobutyl ether; and alcohol compounds typified by methyl alcohol, ethyl alcohol, n-propyl alcohol, i-propyl alcohol, n-butyl alcohol, i-butyl alcohol, s-butyl alcohol, and t-butyl alcohol.

[0057] In producing the specific (meth)acrylic copolymer, it is preferable to use an organic solvent that is unlikely to cause chain transfer during the polymerization reaction, such as an aromatic hydrocarbon compound, an ester compound, or a ketone compound. In particular, it is preferable to use ethyl acetate and toluene from the viewpoints of the solubility of the specific (meth)acrylic copolymer, ease of the polymerization reaction, etc.

[0058] During the polymerization reaction, one type of organic solvent may be used alone, or two or more types may be used.

[0059] Examples of the polymerization initiator include organic peroxides and azo compounds that are used in ordinary solution polymerization methods. Specific examples of organic peroxides include t-butyl peroxy-2-ethylhexanoate, t-butyl hydroperoxide, cumene hydroperoxide, dicumyl peroxide, benzoyl peroxide, lauroyl peroxide, caproyl peroxide, di-i-propyl peroxydicarbonate, di-2-ethylhexyl peroxydicarbonate, t-butyl peroxypivalate, 2,2-bis(4,4-di-t-butylperoxysilane), peroxycyclohexyl)propane, 2,2-bis(4,4-di-t-amylperoxycyclohexyl)propane, 2,2-bis(4,4-di-t-octylperoxycyclohexyl)propane, 2,2-bis(4,4-di-α-cumylperoxycyclohexyl)propane, 2,2-bis(4,4-di-t-butylperoxycyclohexyl)butane, and 2,2-bis(4,4-di-t-octylperoxycyclohexyl)butane. Specific examples of azo compounds include 2,2'-azobisisobutyronitrile [AIBN], 2,2'-azobis(2,4-dimethylvaleronitrile) [ABVN], 2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile), 1,1'-azobis(cyclohexane-1-carbonitrile), and 2,2'-azobis(isobutyrate) dimethyl.

[0060] During the polymerization reaction, one type of polymerization initiator may be used alone, or two or more types may be used.

[0061] The amount of the polymerization initiator used is not particularly limited, and can be appropriately set depending on, for example, the molecular weight of the target specific (meth)acrylic copolymer.

[0062] In producing the specific (meth)acrylic copolymer, a chain transfer agent may be used as needed. Examples of the chain transfer agent include cyanoacetic acid, alkyl ester compounds of cyanoacetic acid having 1 to 8 carbon atoms, bromoacetic acid, alkyl ester compounds of bromoacetic acid having 1 to 8 carbon atoms, aromatic compounds such as α-methylstyrene, anthracene, phenanthrene, fluorene, and 9-phenylfluorene, aromatic nitro compounds such as p-nitroaniline, nitrobenzene, dinitrobenzene, p-nitrobenzoic acid, p-nitrophenol, and p-nitrotoluene, benzoquinone derivatives such as benzoquinone and 2,3,5,6-tetramethyl-p-benzoquinone, borane derivatives such as tributylborane, carbon tetrabromide, tetrabromide, tetrachloromethane, benzoquinone derivatives such as benzoquinone ... Examples of such compounds include halogenated hydrocarbon compounds such as carbon chloride, 1,1,2,2-tetrabromoethane, tribromoethylene, trichloroethylene, bromotrichloromethane, tribromomethane, and 3-chloro-1-propene, aldehyde compounds such as chloral and furaldehyde, alkyl mercaptan compounds having 1 to 18 carbon atoms, aromatic mercaptan compounds such as thiophenol and toluene mercaptan, mercaptoacetic acid, alkyl ester compounds of mercaptoacetic acid having 1 to 10 carbon atoms, hydroxyalkyl mercaptan compounds having 1 to 12 carbon atoms, and terpene compounds such as pinene and terpinolene.

[0063] When a chain transfer agent is used in producing the specific (meth)acrylic copolymer, the amount of the chain transfer agent used is not particularly limited and can be appropriately set depending on, for example, the molecular weight of the target specific (meth)acrylic copolymer.

[0064] The polymerization temperature is not particularly limited and can be appropriately set depending on, for example, the molecular weight of the target specific (meth)acrylic copolymer.

[0065] [Isocyanate-based crosslinking agent] The pressure-sensitive adhesive composition of the present disclosure contains an isocyanate-based crosslinking agent. The isocyanate group of the isocyanate crosslinking agent can undergo a crosslinking reaction with the hydroxyl group derived from the structural unit (a2) contained in the specific (meth)acrylic copolymer.

[0066] In the present disclosure, the term "isocyanate-based crosslinking agent" refers to a compound having two or more isocyanate groups in one molecule (so-called polyisocyanate-based compound).

[0067] Examples of the isocyanate crosslinking agent include aliphatic polyisocyanate compounds, alicyclic polyisocyanate compounds, and aromatic polyisocyanate compounds.

[0068] The "aliphatic polyisocyanate compound" includes, for example, an aliphatic polyisocyanate compound, a polymer of an aliphatic polyisocyanate compound, an adduct of an aliphatic polyisocyanate compound and a polyol compound [e.g., trimethylolpropane (TMP); the same applies hereinafter], and a biuret of an aliphatic polyisocyanate compound. Specific examples of the aliphatic polyisocyanate compound include hexamethylene diisocyanate (HDI), pentamethylene diisocyanate (PDI), tetramethylene diisocyanate, trimethylhexamethylene diisocyanate, and lysine diisocyanate. The "alicyclic polyisocyanate compound" includes, for example, an alicyclic polyisocyanate compound, a polymer of an alicyclic polyisocyanate compound, an adduct of an alicyclic polyisocyanate compound and a polyol compound, and a biuret of an alicyclic polyisocyanate compound. Specific examples of the alicyclic polyisocyanate compound include isophorone diisocyanate (IPDI), hydrogenated tolylene diisocyanate, hydrogenated xylene diisocyanate, hydrogenated 4,4'-diphenylmethane diisocyanate, and 4,4'-dicyclohexylmethane diisocyanate. The term "aromatic polyisocyanate compound" includes, for example, aromatic polyisocyanate compounds, polymers of aromatic polyisocyanate compounds, adducts of aromatic polyisocyanate compounds and polyol compounds, and biuret compounds of aromatic polyisocyanate compounds. Specific examples of aromatic polyisocyanate compounds include tolylene diisocyanate (TDI), xylylene diisocyanate (XDI), and 4,4'-diphenylmethane diisocyanate.

[0069] As the isocyanate-based crosslinking agent, an aliphatic polyisocyanate-based compound is preferred, and a hexamethylene diisocyanate-based compound is more preferred. The "hexamethylene diisocyanate-based compounds" include, for example, HDI, biuret forms of HDI, polymers of HDI (for example, isocyanurates (so-called trimers)), and adducts of HDI and polyol-based compounds. As the hexamethylene diisocyanate compound, an isocyanurate of HDI is preferred.

[0070] As the isocyanate-based crosslinking agent, commercially available products can be used. Examples of commercially available isocyanate crosslinking agents (including those that were previously on the market) include "Coronate HX," "Coronate HK," "Coronate HL," "Coronate HL-S," "Coronate L," "Coronate L-45E," "Coronate 2031," "Coronate 2037," "Coronate 2234," "Coronate 2770," "Coronate 2785," "Coronate 2793," "Aquanate 200," and "Aquanate 210" (all manufactured by Tosoh Corporation), "Sumidur N-75," "Sumidur N3300," "Desmodur N75 MPA / X," "Desmodur N100," "Desmodur N3200," and "Desmodur N3400" (all manufactured by Sumika Covestro Urethane Co., Ltd.), "Duranate D201," "Duranate E405-70B," "Duranate Examples of such adhesives include "Takenate E405-80T," "Duranate AE700-100," "Duranate 24A-100," "Duranate TSE-100," and "Duranate TMA-100" (all manufactured by Asahi Kasei Corporation), as well as "Takenate D-110N," "Takenate D-101E," "Takenate D-120N," "Takenate D-140N," "Takenate D-160N," "Takenate D-172N," "Takenate M-631N," "MT-Olestar NP1200," and "Stabio XD-340N" (all manufactured by Mitsui Chemicals, Inc.). The above-mentioned "Coronate," "Aquanate," "Sumidur," "Desmodur," "Duranate," "Takenate," "Olestar," and "Stabio" are all registered trademarks.

[0071] The pressure-sensitive adhesive composition of the present disclosure may contain one type of isocyanate crosslinking agent alone, or may contain two or more types.

[0072] The content of the isocyanate crosslinking agent in the pressure-sensitive adhesive composition of the present disclosure is not particularly limited, as long as the ratio of the number of moles of isocyanate groups in the isocyanate crosslinking agent to the number of moles of reactive functional groups in the specific (meth)acrylic copolymer [number of moles of isocyanate groups in the isocyanate crosslinking agent / number of moles of reactive functional groups in the (meth)acrylic copolymer] is 0.18 to 3.36. The content of the isocyanate-based crosslinking agent in the pressure-sensitive adhesive composition of the present disclosure is, for example, preferably 0.7 parts by mass to 16.0 parts by mass, more preferably 1.0 parts by mass to 12.0 parts by mass, and even more preferably 2.0 parts by mass to 8.0 parts by mass, relative to 100 parts by mass of the specific (meth)acrylic copolymer.

[0073] <<Ratio of the number of moles of isocyanate groups in an isocyanate-based crosslinking agent to the number of moles of reactive functional groups in a specific (meth)acrylic copolymer>> In the pressure-sensitive adhesive composition of the present disclosure, the ratio of the number of moles of isocyanate groups in the isocyanate crosslinking agent to the number of moles of reactive functional groups in the specific (meth)acrylic copolymer [number of moles of isocyanate groups in the isocyanate crosslinking agent / number of moles of reactive functional groups in the (meth)acrylic copolymer] is 0.18 to 3.36. In the present disclosure, the term "reactive functional group" refers to a functional group that can react with an isocyanate group in an isocyanate-based crosslinking agent to form a crosslinked structure. Specific examples of reactive functional groups include a hydroxyl group, a carboxyl group, and an amino group. In the present disclosure, the term "amino group" encompasses primary amino groups, secondary amino groups, and tertiary amino groups.

[0074] In the pressure-sensitive adhesive composition of the present disclosure, the ratio of the number of moles of isocyanate groups in the isocyanate-based crosslinking agent to the number of moles of reactive functional groups in the specific (meth)acrylic copolymer is within the above range, so that when a pressure-sensitive adhesive layer is formed, the balance between the cohesive strength of the entire pressure-sensitive adhesive layer and the wettability of the pressure-sensitive adhesive layer to the interface with the adherend is improved, and the pressure-sensitive adhesive layer can exhibit the appropriate adhesive strength and excellent compatibility with the adherend required for a protective film. In the pressure-sensitive adhesive composition of the present disclosure, when the ratio of the number of moles of isocyanate groups in the isocyanate-based crosslinking agent to the number of moles of reactive functional groups in the specific (meth)acrylic copolymer is 0.18 or more, the amount of unreacted specific (meth)acrylic copolymer can be reduced, thereby preventing transfer to the adherend to an extent that is not visible to the naked eye. Furthermore, because the pressure-sensitive adhesive composition has an appropriate crosslink density, the pressure-sensitive adhesive layer formed does not exhibit excessively high adhesive strength, and tends to be able to achieve the appropriate adhesive strength required for a protective film. In the pressure-sensitive adhesive composition of the present disclosure, when the ratio of the number of moles of isocyanate groups in the isocyanate-based crosslinking agent to the number of moles of reactive functional groups in the specific (meth)acrylic copolymer is 3.36 or less, an excessive increase in cohesive strength due to the hardness of the isocyanate-based crosslinking agent itself can be prevented, and the pressure-sensitive adhesive layer formed does not exhibit excessively high adhesive strength, and tends to be able to achieve the appropriate adhesive strength required for the protective film. In the pressure-sensitive adhesive composition of the present disclosure, the ratio of the number of moles of isocyanate groups in the isocyanate crosslinking agent to the number of moles of reactive functional groups in the specific (meth)acrylic copolymer [number of moles of isocyanate groups in the isocyanate crosslinking agent / number of moles of reactive functional groups in the (meth)acrylic copolymer] is preferably 0.25 to 3.00, more preferably 0.35 to 2.30.

[0075] In the pressure-sensitive adhesive composition of the present disclosure, the ratio of the number of moles of isocyanate groups in the isocyanate-based crosslinking agent to the number of moles of reactive functional groups in the specific (meth)acrylic copolymer is calculated using the following formulas (1) to (3): When the specific (meth)acrylic copolymer contains multiple types of structural units derived from monomers having reactive functional groups, calculations are performed for each monomer, and the resulting values ​​are then summed.

[0076] Number of moles of isocyanate groups in the isocyanate crosslinking agent (unit: mmol) = [Isocyanate group content in isocyanate crosslinking agent (unit: mass%) / Solid content of isocyanate crosslinking agent (unit: mass%) × Amount of isocyanate crosslinking agent (amount as solid content) (unit: g)] / Molecular weight of isocyanate group (unit: g / mol) × 1000 (1)

[0077] Number of moles of reactive functional groups in a specific (meth)acrylic copolymer (unit: mmol) = [Content of structural units derived from monomers having reactive functional groups in specific (meth)acrylic copolymer (unit: mass%) / 100 × blend amount of specific (meth)acrylic copolymer (unit: g) / molecular weight of structural units derived from monomers having reactive functional groups (unit: g / mol) × number (valence) of reactive functional groups in structural units derived from monomers having reactive functional groups × 1000] (2)

[0078] The ratio of the number of moles of isocyanate groups in the isocyanate crosslinking agent to the number of moles of reactive functional groups in the specific (meth)acrylic copolymer = Value calculated using formula (1) / Value calculated using formula (2) (3)

[0079] For example, in Example 1 shown later, "N3300" (trade name: Sumidur (registered trademark) N3300, isocyanurate of hexamethylene diisocyanate (HDI), solid content: 100% by mass, manufactured by Sumika Covestro Urethane Co., Ltd.) is used as the isocyanate crosslinking agent, and the content of isocyanate groups is 21.8% by mass. Therefore, the number of moles (unit: mmol) of isocyanate groups in the isocyanate crosslinking agent can be calculated from formula (1) as follows: (21.8 / 100 × 2.0) / 42 × 1000 = 10.380... ≒ 10.38. Furthermore, the specific (meth)acrylic copolymer contains, as constituent units derived from monomers having reactive functional groups, 3.0% by mass of constituent units derived from 4-hydroxybutyl acrylate [molecular weight: 144 g / mol], a monomer having a hydroxyl group, and 0.5% by mass of constituent units derived from acrylic acid [molecular weight: 72 g / mol], a monomer having a carboxyl group. Therefore, the number of moles of reactive functional groups in the specific (meth)acrylic copolymer [unit: mmol] can be calculated from formula (2) as follows: 3.0 / 100×100 / 144×1000×1 + 0.5 / 100×100 / 72×1000 = 27.777... ≒ 27.78. The ratio of the number of moles of isocyanate groups in the isocyanate-based crosslinking agent to the number of moles of reactive functional groups in the specific (meth)acrylic copolymer is calculated from the values ​​calculated by formula (1) and formula (2) based on formula (3), resulting in 10.38 / 27.78 = 0.373 ≒ 0.37.

[0080] [Specific plasticizers] The pressure-sensitive adhesive composition of the present disclosure contains a plasticizer (i.e., a specific plasticizer) that is an adipic acid diester compound having a molecular weight of 350 to 450, and the content of the specific plasticizer is 15 parts by mass to 35 parts by mass per 100 parts by mass of the specific (meth)acrylic copolymer. The pressure-sensitive adhesive composition of the present disclosure contains an adipic acid diester compound as a plasticizer, and when a pressure-sensitive adhesive layer is formed, the adipic acid diester compound segregates on the surface of the pressure-sensitive adhesive layer, improving the wettability of the interface between the pressure-sensitive adhesive layer and the adherend, which tends to improve the compatibility of the pressure-sensitive adhesive layer with the adherend.

[0081] For example, plasticizers with long molecular chains, such as adipic acid polyester compounds, are prone to entanglement with (meth)acrylic copolymers. When entanglement occurs between the plasticizer and the (meth)acrylic copolymer, the plasticizer is incorporated into the polymer chain, making it difficult for the plasticizer to segregate on the surface of the adhesive layer. This results in insufficient wettability at the interface between the adhesive layer and the adherend, and poor conformability of the adhesive layer to the adherend. Furthermore, when entanglement between the plasticizer and the (meth)acrylic copolymer becomes strong, the flexibility of the adhesive layer decreases, and the conformability of the adhesive layer to the adherend deteriorates. In contrast, adipic acid diester compounds with a molecular weight of 450 or less have relatively short molecular chains, making them less likely to become entangled with the specific (meth)acrylic copolymer. Therefore, the adipic acid diester compound is less likely to be incorporated into the polymer chain and more likely to segregate on the surface of the pressure-sensitive adhesive layer, improving the wettability of the interface between the pressure-sensitive adhesive layer and the adherend. Furthermore, because the adipic acid diester compound is less likely to become entangled with the specific (meth)acrylic copolymer, the flexibility of the pressure-sensitive adhesive layer is less likely to decrease. This improves the compatibility of the pressure-sensitive adhesive layer with the adherend. The molecular weight of the adipic acid diester compound being 450 or less is a rule for distinguishing between adipic acid diester compounds and adipic acid polyester compounds. The molecular weight of the adipic acid diester compound is 350 or more from the viewpoint of availability.

[0082] Examples of the specific plasticizer include dioctyl adipate, diisononyl adipate, and diisodecyl adipate. The specific plasticizer preferably contains at least one of dioctyl adipate and diisodecyl adipate, and more preferably at least one of dioctyl adipate and diisodecyl adipate.

[0083] The pressure-sensitive adhesive composition of the present disclosure may contain one type of specific plasticizer alone, or may contain two or more types of specific plasticizers.

[0084] The content of the specific plasticizer in the pressure-sensitive adhesive composition of the present disclosure is 15 to 35 parts by mass relative to 100 parts by mass of the specific (meth)acrylic copolymer. When the content of the specific plasticizer in the pressure-sensitive adhesive composition of the present disclosure is 15 parts by mass or more relative to 100 parts by mass of the specific (meth)acrylic copolymer, the pressure-sensitive adhesive layer formed tends to have the appropriate adhesive strength required for a protective film without exhibiting excessively high adhesive strength. In addition, the wettability of the interface between the pressure-sensitive adhesive layer and the adherend is improved, so the pressure-sensitive adhesive layer tends to have excellent compatibility with the adherend. The content of the specific plasticizer in the pressure-sensitive adhesive composition of the present disclosure is preferably 20 parts by mass or more, and more preferably 25 parts by mass or more, per 100 parts by mass of the specific (meth)acrylic copolymer. When the content of the specific plasticizer in the pressure-sensitive adhesive composition of the present disclosure is 35 parts by mass or less per 100 parts by mass of the specific (meth)acrylic copolymer, the pressure-sensitive adhesive layer formed does not become too soft, and the cohesive strength of the entire pressure-sensitive adhesive layer is less likely to decrease, and therefore the pressure-sensitive adhesive layer formed tends to have the appropriate adhesive strength required for a protective film without exhibiting excessively low adhesive strength. The content of the specific plasticizer in the pressure-sensitive adhesive composition of the present disclosure is preferably 30 parts by mass or less relative to 100 parts by mass of the specific (meth)acrylic copolymer. In one embodiment, the content of the specific plasticizer in the pressure-sensitive adhesive composition of the present disclosure may be 20 parts by mass to 30 parts by mass, or may be 25 parts by mass to 30 parts by mass, per 100 parts by mass of the specific (meth)acrylic copolymer.

[0085] [Organic solvent] The pressure-sensitive adhesive composition of the present disclosure may contain an organic solvent. When the pressure-sensitive adhesive composition of the present disclosure contains an organic solvent, the coating properties can be improved. Examples of the organic solvent include the same organic solvents as those used in the polymerization reaction of the above-mentioned specific (meth)acrylic copolymer.

[0086] When the pressure-sensitive adhesive composition of the present disclosure contains an organic solvent, it may contain only one type of organic solvent, or may contain two or more types of organic solvents.

[0087] When the pressure-sensitive adhesive composition of the present disclosure contains an organic solvent, the content of the organic solvent is not particularly limited and can be set appropriately depending on the purpose.

[0088] [Other ingredients] The pressure-sensitive adhesive composition of the present disclosure may contain components other than the above-mentioned components (so-called other components) as needed, within the range that does not impair the effects of the composition. Examples of other components include various additives such as polymers other than the specific (meth)acrylic copolymer, crosslinking catalysts, antioxidants, colorants (e.g., dyes and pigments), light stabilizers (e.g., ultraviolet absorbers), and antistatic agents.

[0089] When the pressure-sensitive adhesive composition of the present disclosure contains other components, the content of the other components can be set appropriately within a range that does not impair the effects of the pressure-sensitive adhesive composition of the present disclosure.

[0090] <<Application>> The pressure-sensitive adhesive composition of the present disclosure is a pressure-sensitive adhesive composition used for a protective film. The pressure-sensitive adhesive composition of the present disclosure is suitable as a pressure-sensitive adhesive composition for forming a pressure-sensitive adhesive layer included in a protective film, since it has the appropriate adhesive strength required for a protective film and can form a pressure-sensitive adhesive layer that has excellent compatibility with the substrate.

[0091] [Protection film] The protective film of the present disclosure includes a substrate and a pressure-sensitive adhesive layer provided on the substrate and formed from the pressure-sensitive adhesive composition of the present disclosure. The pressure-sensitive adhesive layer provided in the protective film of the present disclosure is a pressure-sensitive adhesive layer formed from the pressure-sensitive adhesive composition of the present disclosure, and therefore has the appropriate adhesive strength required of a protective film and excellent compatibility with the adherend. Examples of the substrate to which the protective film of the present disclosure can be applied include optical components and electronic components.

[0092] The substrate is not particularly limited as long as it allows the pressure-sensitive adhesive layer to be formed thereon. The substrate is preferably in the form of a film. The material of the substrate is not particularly limited, but is preferably, for example, a resin. Specific examples of the substrate include resin films containing polyolefin resins (e.g., polyethylene (PE) and polypropylene (PP)), polyester resins (e.g., polyethylene terephthalate (PET)), acetate resins (e.g., triacetyl cellulose (TAC)), polyethersulfone resins, polycarbonate resins, polyamide resins, polyimide resins, polyurethane resins, (meth)acrylic resins, vinyl chloride resins, ABS (Acrylonitrile Butadiene Styrene) resins, fluorine-based resins, etc.

[0093] The surface of the substrate on which the adhesive layer is provided may be subjected to a surface treatment such as corona discharge treatment or plasma discharge treatment (so-called easy-adhesion treatment) in order to improve the adhesion between the substrate and the adhesive layer.

[0094] The substrate may contain various additives such as plasticizers, colorants (eg, dyes and pigments), heat stabilizers, light stabilizers, antistatic agents, flame retardants, antioxidants, fillers, and the like. The substrate may be partially or entirely patterned.

[0095] The thickness of the substrate is not particularly limited, but is preferably from 10 μm to 250 μm, more preferably from 13 μm to 150 μm, and even more preferably from 25 μm to 100 μm.

[0096] In this disclosure, "thickness of the substrate" means the average thickness of the substrate. The average thickness of the substrate is a value determined by the following method. The thickness of the substrate is measured at 10 randomly selected locations in the thickness direction using a film thickness meter. The arithmetic mean of the measured values ​​is calculated and this value is taken as the average thickness of the substrate.

[0097] The pressure-sensitive adhesive layer is provided on a substrate. An intermediate layer may be provided between the substrate and the pressure-sensitive adhesive layer. The pressure-sensitive adhesive layer is preferably provided directly on one surface of the substrate.

[0098] The pressure-sensitive adhesive layer is formed from the pressure-sensitive adhesive composition of the present disclosure and includes a cured product of the pressure-sensitive adhesive composition of the present disclosure, including, for example, a crosslinked product of a specific (meth)acrylic copolymer obtained by crosslinking and curing with an isocyanate-based crosslinking agent.

[0099] The thickness of the pressure-sensitive adhesive layer is not particularly limited, but is, for example, preferably 5 μm to 50 μm, more preferably 5 μm to 30 μm, and even more preferably 5 μm to 20 μm. When the thickness of the pressure-sensitive adhesive layer is 5 μm or more, the tackiness of the pressure-sensitive adhesive layer improves, and the protective film tends to be less likely to peel off easily from the adherend. When the thickness of the adhesive layer is 50 μm or less, the adhesive strength of the adhesive layer does not become excessively high, and the protective film tends to be easily peeled off from the adherend.

[0100] In the present disclosure, the "thickness of the pressure-sensitive adhesive layer" refers to the average thickness of the pressure-sensitive adhesive layer. The average thickness of the pressure-sensitive adhesive layer is a value determined by the following method. The thickness of the adhesive layer is measured at 10 randomly selected locations in the thickness direction using a film thickness meter. The arithmetic mean of the measured values ​​is calculated and this value is taken as the average thickness of the adhesive layer.

[0101] The adhesive strength of the adhesive layer included in the protective film of the present disclosure is preferably 0.020 N / 25 mm to 0.040 N / 25 mm, more preferably 0.020 N / 25 mm to 0.035 N / 25 mm, and even more preferably 0.020 N / 25 mm to 0.030 N / 25 mm. If the adhesive strength of the adhesive layer is 0.020 N / 25 mm or more, the protective film tends to be less likely to peel off easily from the adherend. If the adhesive strength of the adhesive layer is 0.040 N / 25 mm or less, the protective film tends to be easily peeled off from the adherend.

[0102] In the present disclosure, the adhesive strength of the pressure-sensitive adhesive layer is a value measured by the following method. A 10 μm thick adhesive layer formed on a substrate is used as the adhesive film for evaluation. The adhesive film for evaluation is cut to a size of 25 mm × 75 mm (long side) to obtain an adhesive film piece for evaluation. The adhesive layer of the adhesive film piece for evaluation is placed on a glass plate, and then a 2 kg roller is rolled back and forth once to press it together, preparing a sample for adhesive strength evaluation. The prepared adhesive strength evaluation sample is left to stand for 30 minutes in an environment with an ambient temperature of 23°C and 50% RH to obtain a test piece. The adhesive strength (unit: N / 25 mm) of this test piece when the adhesive film piece for evaluation is peeled 180° in the long side (75 mm) direction from the glass plate is measured using a single-column materials testing machine as the measuring device, at an ambient temperature of 23°C and 50% RH, and a peel speed of 300 mm / min. As a measuring device, for example, a single column type material testing machine (model number: STA-1225) manufactured by A&D Co., Ltd. can be suitably used, but the measuring device is not limited to this.

[0103] The gel fraction of the pressure-sensitive adhesive layer provided in the pressure-sensitive adhesive film of the present disclosure is not particularly limited, but from the viewpoint of controlling the cohesive strength of the pressure-sensitive adhesive layer by the crosslink density, it is preferably 70% by mass to 98% by mass, more preferably 75% by mass to 90% by mass, and even more preferably 78% by mass to 85% by mass.

[0104] In the present disclosure, the gel fraction of the pressure-sensitive adhesive layer is measured according to the following [1] to [4]. [1] Approximately 0.15 g of adhesive layer is attached to a 250 mesh wire mesh (100 mm x 100 mm) whose mass has been accurately measured using a precision balance, and the wire mesh is folded five times with the attached adhesive layer facing inward to prevent leakage of the gel. The mass is then accurately measured using a precision balance. [2] The obtained sample is immersed in 80 mL of ethyl acetate for 3 days. [3] Remove the sample, wash it with a small amount of ethyl acetate, and dry it at 100°C for 3 hours. Then, accurately measure the mass using a precision balance. [4] Calculate the gel fraction using the following formula: Gel fraction (unit: mass%) = (ZX) / (YX) × ​​100 where X is the mass of the wire mesh (unit: g), Y is the mass of the wire mesh with the adhesive layer attached (unit: g) before immersion, and Z is the mass of the wire mesh with the adhesive layer attached after immersion and drying (unit: g).

[0105] The protective film of the present disclosure may be provided with a release sheet from the viewpoint of protecting the exposed surface of the pressure-sensitive adhesive layer. Generally, the release sheet protects the surface of the pressure-sensitive adhesive layer until the protective film is put to practical use, and is peeled off at the time of use.

[0106] The release sheet is not particularly limited as long as it can be easily peeled off from the pressure-sensitive adhesive layer. Examples of release sheets include resin films, paper, synthetic paper, and composite sheets made by laminating two or more of these, each of which has been surface-treated with a release agent on one or both sides (so-called easy-release treatment). In the present disclosure, a release sheet in an embodiment in which one or both sides of a resin film have been surface-treated with a release treatment agent (so-called easy-release treatment) is also referred to as a "release film." Examples of release agents include silicone-based release agents (such as silicone), wax-based release agents (such as paraffin wax), and fluorine-based release agents (such as fluorine-based resins). Examples of resin films include polyester films such as polyethylene terephthalate (PET) films. Examples of paper include fine paper and coated paper. The thickness of the release sheet is not particularly limited, and is generally 20 μm to 180 μm.

[0107] [How to make protective film] The method for producing the protective film of the present disclosure is not particularly limited. The protective film of the present disclosure can be produced by a known method. Examples of methods for producing the protective film of the present disclosure include the following methods.

[0108] The pressure-sensitive adhesive composition of the present disclosure is applied to one surface of a substrate (preferably the surface treated for easy adhesion) to form a coating film on the substrate. The formed coating film is then dried to form a pressure-sensitive adhesive film on the substrate. The exposed surface of the formed pressure-sensitive adhesive film is then laminated to the easy-release treated surface of a release sheet, and the resulting laminate is then cured to produce a protective film of the present disclosure having a laminate structure of substrate / pressure-sensitive adhesive layer / release sheet.

[0109] Another method is, for example, the following method. The pressure-sensitive adhesive composition of the present disclosure is applied to the easy-release treated surface of a release sheet to form a coating film on the release sheet. The formed coating film is then dried to form a pressure-sensitive adhesive film on the release sheet. The exposed surface of the formed pressure-sensitive adhesive film is then laminated to one surface of a substrate (preferably the easy-adhesion treated surface), and then cured to produce a protective film of the present disclosure having a laminate structure of substrate / pressure-sensitive adhesive layer / release sheet.

[0110] The method for applying the pressure-sensitive adhesive composition is not particularly limited. Examples of methods for applying the pressure-sensitive adhesive composition include known methods using a gravure roll coater, reverse roll coater, kiss roll coater, dip roll coater, knife coater, spray coater, bar coater, applicator, etc. The amount of the pressure-sensitive adhesive composition to be applied is not particularly limited, and is set appropriately depending on, for example, the thickness of the pressure-sensitive adhesive layer to be formed.

[0111] The method for drying the coating film is not particularly limited. Examples of methods for drying the coating film include natural drying, heat drying, hot air drying, and vacuum drying. The drying temperature and drying time of the coating film are not particularly limited and are set appropriately depending on the thickness of the coating film, the amount of organic solvent in the coating film, and the like. Drying conditions include, for example, drying using a hot air circulation dryer at 60°C to 120°C for 30 to 180 seconds.

[0112] As a method for curing, for example, a method of leaving the product to stand for 2 to 7 days in an environment with an atmospheric temperature of 20°C to 35°C and a relative humidity of 45% to 55% can be mentioned. [Example]

[0113] The pressure-sensitive adhesive composition and protective film of the present disclosure will be described in more detail below using examples. The present disclosure is not limited to the following examples as long as they do not depart from the gist of the disclosure.

[0114] [Production of (meth)acrylic copolymer] [Production Example A-1: ​​(Meth)acrylic Copolymer A-1] 1,158.0 parts by mass of 2-ethylhexyl acrylate [2EHA; acrylic acid alkyl ester monomer], 36.0 parts by mass of 4-hydroxybutyl acrylate [4HBA; monomer having a hydroxyl group], and 6.0 parts by mass of acrylic acid [AA; monomer having a carboxy group] were placed in a container and mixed to prepare 1,200.0 parts by mass of a monomer mixture. In a reactor equipped with a thermostat, a stirrer, and a reflux condenser, 300 parts by mass of the prepared monomer mixture (an amount corresponding to 25% by mass of the prepared monomer mixture), 390.0 parts by mass of ethyl acetate [organic solvent], and 0.15 parts by mass of 1,1′-azobis(cyclohexane-1-carbonitrile) [V-40; polymerization initiator] were placed and mixed to obtain a mixture. Next, the mixture in the reactor was heated to 90°C while stirring, and then the remaining 900.0 parts by mass of the prepared monomer mixture (corresponding to 75% by mass of the prepared monomer mixture), 2.4 parts by mass of V-40, and 300 parts by mass of ethyl acetate were sequentially added to the mixture in the reactor, and the mixture was maintained for 2 hours. After that, 3.0 parts by mass of 2,2'-azobisisobutyronitrile (AIBN; polymerization initiator) and 300.0 parts by mass of toluene (organic solvent) were sequentially added, and the mixture was maintained for another 1.5 hours to polymerize the mixture, obtaining a polymerization reaction product. The resulting polymerization reaction product was diluted with 380 parts by mass of ethyl acetate to a solids concentration of 36% by mass, and then cooled to obtain a solution of (meth)acrylic copolymer A-1.

[0115] The "solid content concentration" here means the mass proportion of the (meth)acrylic copolymer A-1 in the solution of the (meth)acrylic copolymer A-1. The same applies to the solutions of the (meth)acrylic copolymers A-2 to A-15 produced below.

[0116] [Manufacturing example A-2] 386.0 parts by mass of 2-ethylhexyl acrylate [2EHA; acrylic acid alkyl ester monomer], 12.0 parts by mass of 4-hydroxybutyl acrylate [4HBA; monomer having a hydroxyl group], and 2.0 parts by mass of acrylic acid [AA; monomer having a carboxy group] were placed in a container and mixed to prepare 400.0 parts by mass of a monomer mixture. A reactor equipped with a thermostat, a stirrer, and a reflux condenser was charged with 130.0 parts by weight of ethyl acetate (organic solvent). The ethyl acetate in the reactor was then heated to 80°C while stirring. Then, 400.0 parts by weight of the prepared monomer mixture, 4.45 parts by weight of 2,2'-azobis(2-methylpropionate)dimethyl (V-601; polymerization initiator), and 100 parts by weight of toluene (organic solvent) were sequentially added to the ethyl acetate in the reactor. The mixture was maintained for 3 hours to allow the polymerization reaction to proceed, yielding a polymerization product. The resulting polymerization product was diluted with 480 parts by weight of ethyl acetate to a solids concentration of 36% by weight and then cooled to yield a solution of (meth)acrylic copolymer A-2.

[0117] [Manufacturing example A-3] 193.0 parts by mass of 2-ethylhexyl acrylate [2EHA; acrylic acid alkyl ester monomer], 6.0 parts by mass of 4-hydroxybutyl acrylate [4HBA; monomer having a hydroxyl group], and 1.0 part by mass of acrylic acid [AA; monomer having a carboxy group] were placed in a container and mixed to prepare 200.0 parts by mass of a monomer mixture. A reactor equipped with a thermostat, a stirrer, and a reflux condenser was charged with 200 parts by weight of the prepared monomer mixture and 120.0 parts by weight of ethyl acetate (organic solvent) to obtain a mixture. The mixture in the reactor was then heated to 70°C while stirring, and 10.0 parts by weight of ethyl acetate (organic solvent) and 0.004 parts by weight of 2,2'-azobis(2,4dimethylvaleronitrile) (ABVN; polymerization initiator) were sequentially added to the mixture in the reactor. After holding for 55 minutes, 0.006 parts by weight of 2,2'-azobis(2,4dimethylvaleronitrile) and 225.3 parts by weight of ethyl acetate were sequentially added. The mixture was then held for an additional 1.5 hours to allow the polymerization reaction to proceed, obtaining a polymerization reaction product. The resulting polymerization reaction product was diluted with 769 parts by weight of ethyl acetate to a solids concentration of 15% by weight and then cooled to obtain a solution of (meth)acrylic copolymer A-3.

[0118] [Manufacturing examples A-4 to A-14] In Production Examples A-4 to A-14, the monomer composition of the (meth)acrylic copolymer was changed to the monomer composition shown in Table 1, and at least one of the amount of organic solvent used and the amount of polymerization initiator used was adjusted to produce (meth)acrylic copolymers having the monomer composition and weight average molecular weight shown in Table 1. Except for this, the same operation as in Production Example A-1 was carried out to obtain solutions of (meth)acrylic copolymers A-4 to A-14 each having a solid content concentration of about 36 mass%.

[0119] [Manufacturing example A-15] In Production Example A-15, the monomer composition of the (meth)acrylic copolymer was changed to the monomer composition shown in Table 1, and at least one of the amount of organic solvent used and the amount of polymerization initiator used was adjusted to produce a (meth)acrylic copolymer having the monomer composition and weight average molecular weight shown in Table 1. Except for this, the same operation as in Production Example A-3 was performed to obtain solutions of (meth)acrylic copolymer A-15 having a solid content concentration of about 15 mass%.

[0120] Table 1 shows the monomer compositions (unit: mass %) and weight average molecular weights (denoted as "Mw") of the (meth)acrylic copolymers A-1 to A-15.

[0121] The weight average molecular weights of the (meth)acrylic copolymers A-1 to A-15 were measured by the same method as the method for measuring the weight average molecular weight of the specific (meth)acrylic copolymer described above.

[0122] Among the (meth)acrylic copolymers A-1 to A-15, the (meth)acrylic copolymers A-1 to A-4, A-9 to A-11, A-14, and A-15 correspond to the specific (meth)acrylic copolymers of the present disclosure.

[0123] [Table 1]

[0124] In Table 1, "(a1)", "(a2)", and "(a3)" indicate the monomers of "structural unit (a1)", "structural unit (a2)", and "structural unit (a3)", respectively.

[0125] Details of each monomer listed in Table 1 are as follows: (a1) (Meth)acrylic acid alkyl ester monomer "2EHA": 2-ethylhexyl acrylate "n-BA": n-butyl acrylate (a2) Monomer having a hydroxyl group> "4HBA": 4-hydroxybutyl acrylate (a3) Monomer having a carboxy group "AA": acrylic acid

[0126] In Table 1, "-" in the column of monomer composition means that the monomer in that column was not used.

[0127] [Preparation of Pressure-Sensitive Adhesive Composition] Example 1 A pressure-sensitive adhesive composition of Example 1 was obtained by thoroughly mixing 277.8 parts by mass (100 parts by mass in terms of solid content) of the solution of (meth)acrylic copolymer A-1, 2.0 parts by mass (2.0 parts by mass in terms of solid content) of Sumidur (registered trademark) N3300 [trade name, isocyanurate of hexamethylene diisocyanate (HDI), solid content: 100% by mass, manufactured by Sumika Covestro Urethane Co., Ltd.] as an isocyanate crosslinking agent, 20 parts by mass (20 parts by mass in terms of solid content) of DIDA [trade name: DIDA(NB), diisodecyl adipate, manufactured by Taoka Chemical Co., Ltd.] as a plasticizer, and an appropriate amount of ethyl acetate [organic solvent].

[0128] [Examples 2 to 13 and Comparative Examples 1 to 3] In Examples 2 to 13 and Comparative Examples 1 to 3, the same procedure as in Example 1 was carried out, except that the composition of the adhesive composition was changed to the composition shown in Table 2, to obtain each of the adhesive compositions of Examples 2 to 13 and Comparative Examples 1 to 3.

[0129] [Examples 14 to 22 and Comparative Examples 4 to 13] In Examples 14 to 22 and Comparative Examples 4 to 13, the same procedure as in Example 1 was carried out, except that the composition of the adhesive composition was changed to the composition shown in Table 3, to obtain each of the adhesive compositions of Examples 14 to 22 and Comparative Examples 4 to 13.

[0130] [Examples 23 to 30 and Comparative Examples 14 to 21] In Examples 23 to 30 and Comparative Examples 14 to 21, the pressure-sensitive adhesive compositions of Examples 23 to 30 and Comparative Examples 14 to 21 were obtained in the same manner as in Example 1, except that the formulation of the pressure-sensitive adhesive composition was changed to the formulation shown in Table 4. Note that in Example 25, Comparative Example 14, and Comparative Example 15, in which DOTDL was used as the crosslinking accelerator, DOTDL was diluted with 1 part by mass of acetylacetone before being added.

[0131] [Preparation of adhesive film for evaluation] The adhesive composition prepared above was applied to the adhesive-treated surface of a polyethylene terephthalate (PET) film (trade name: Toyobo Ester (registered trademark) Film, model number: HPE, surface-treated for easy adhesion, thickness: 50 μm, manufactured by Toyobo Co., Ltd.) as a substrate to form a coating film. The amount of adhesive composition applied was such that the thickness of the adhesive film described below would be 10 μm. The formed coating film was then dried using a hot air circulation dryer at a drying temperature of 100°C for a drying time of 60 seconds to form an adhesive film on the PET film. Next, the exposed surface of the adhesive film formed on the PET film was laminated onto the easily peelable surface of a release film (trade name: Film Byna (registered trademark) 100E-0010 No. 23, thickness: 100 μm, manufactured by Fujimori Kogyo Co., Ltd.) that had been surface-treated with a silicone-based release agent (so-called easy peel treatment). The laminate obtained by lamination was left standing in an environment of an atmospheric temperature of 23° C. and 50% RH for 7 days (so-called curing period) to cure the adhesive film. In this manner, an adhesive film for evaluation having a structure of substrate (PET) / adhesive layer / release film was prepared.

[0132] [Measurement and Evaluation] 1. Adhesive strength (1) Preparation of samples for adhesive strength evaluation The evaluation adhesive film prepared above was cut into a size of 25 mm × 75 mm (long side) to obtain evaluation adhesive film piece X1. Next, the release film was peeled off from the obtained evaluation adhesive film piece X1 (configuration: substrate / adhesive layer / release film) to obtain evaluation adhesive film piece X2 (configuration: substrate / adhesive layer). Next, the exposed adhesive layer surface of evaluation adhesive film piece X2 was placed on one side of a soda glass plate (manufactured by Matsunami Glass Industry Co., Ltd.) as an adherend, and then pressure-bonded by moving a 2 kg roller back and forth once. In this manner, a sample for evaluating adhesive strength was prepared, which had a structure of substrate (PET) / adhesive layer / adherend (soda glass plate).

[0133] (2) Measurement of adhesive strength -Initial adhesive strength- The adhesive strength evaluation sample prepared above was left to stand for 30 minutes in an environment with an ambient temperature of 23°C and 50% RH to obtain a test piece. For this test piece, the adhesive film piece X2 for evaluation was peeled 180° in the long side (75 mm) direction from the adherend (soda glass plate), and the adhesive strength (unit: N / 25 mm) was measured using a single-column materials testing machine (model number: STA-1225, manufactured by A&D Co., Ltd.) at an ambient temperature of 23°C and 50% RH at a peel rate of 300 mm / min. The initial adhesive strength was then evaluated according to the following evaluation criteria. The measured values ​​of adhesive strength and the evaluation results are shown in Tables 2 to 4. In the following evaluation criteria, "A" and "B" are levels that are practically acceptable, with "A" being the most preferable.

[0134] -Evaluation criteria- A: The adhesive strength was within the range of 0.020 N / 25 mm or more and less than 0.030 N / 25 mm. B: The adhesive strength was within the range of 0.030 N / 25 mm or more and less than 0.040 N / 25 mm. C: The adhesive strength was less than 0.020 N / 25 mm. D: The adhesive strength was 0.040 N / 25 mm or more. E: There was adhesive residue on the adherend (adhesive strength is not important).

[0135] -Adhesive strength over time- The adhesive strength evaluation sample prepared above was left to stand for 7 days in an environment with an ambient temperature of 23°C and 50% RH to obtain a test piece. For this test piece, the adhesive film piece X2 for evaluation was peeled off from the adherend (soda glass plate) at an angle of 180° along the long side (75 mm) to measure the adhesive strength (unit: N / 25 mm) using a single-column materials testing machine (model number: STA-1225, manufactured by A&D Co., Ltd.) at an ambient temperature of 23°C and 50% RH at a peel speed of 300 mm / min. The adhesive strength after aging was then evaluated according to the following evaluation criteria. The measured values ​​and evaluation results are shown in Tables 2 to 4. In the following evaluation criteria, "A" and "B" are levels that are practically acceptable, with "A" being the most preferable.

[0136] -Evaluation criteria- A: The adhesive strength was within the range of 0.020 N / 25 mm or more and less than 0.030 N / 25 mm. B: The adhesive strength was within the range of 0.030 N / 25 mm or more and less than 0.040 N / 25 mm. C: The adhesive strength was less than 0.020 N / 25 mm. D: The adhesive strength was 0.040 N / 25 mm or more. E: There was adhesive residue on the adherend (adhesive strength is not important).

[0137] 2. Familiarity (1) Preparation of samples for conformability evaluation The pressure-sensitive adhesive film for evaluation prepared above was cut into a size of 50 mm x 230 mm to obtain a pressure-sensitive adhesive film piece for evaluation, which was used as a sample for conformability evaluation.

[0138] (2) Evaluation test The release film of the conformability evaluation sample prepared above was peeled off in an environment with an ambient temperature of 23°C and 50% RH to obtain conformability evaluation sample Y (configuration: substrate / adhesive layer). Next, conformability evaluation sample Y was gently placed on a soda glass plate (manufactured by Matsunami Glass Industry Co., Ltd.) as an adherend so that only the center portion of the exposed adhesive layer of conformability evaluation sample Y was in contact with one side of the soda glass plate, and the time until the entire adhesive layer of conformability evaluation sample Y had wetted and spread across the soda glass plate was measured. The conformability to the adherend was then evaluated according to the following evaluation criteria. The measured values ​​and evaluation results are shown in Tables 2 to 4. In the following evaluation criteria, "A" and "B" are levels that are practically acceptable, with "A" being the most preferable.

[0139] -Evaluation criteria- A: The time it took to wet and spread was less than 7.0 seconds. B: The time it took for the sample to spread was within the range of 7.0 seconds or more and less than 10.0 seconds. C: Time until wetting and spreading was 10.0 seconds or more.

[0140] 3. Gel fraction (1) Preparation of adhesive film for gel fraction measurement The pressure-sensitive adhesive composition prepared above was applied to the easily peelable surface of a release film X (trade name: Film Byna (registered trademark) 100E-0010 No. 23, thickness: 100 μm, manufactured by Fujimori Kogyo Co., Ltd.) that had been surface-treated (so-called easily peelable) with a silicone-based release treatment agent to form a coating film. The amount of the pressure-sensitive adhesive composition applied was such that the thickness of the adhesive film described below would be 10 μm. The formed coating film was then dried using a hot air circulation dryer at a drying temperature of 100°C for a drying time of 60 seconds to form an adhesive film on the release film X. Next, the exposed surface of the adhesive film formed on the release film X was laminated onto the easily peelable surface of a release film Y (trade name: Film Byna (registered trademark) 100E-0010 No. 23, thickness: 100 μm, manufactured by Fujimori Kogyo Co., Ltd.) that had been surface-treated (so-called easily peelable) with a separately prepared silicone-based release treatment agent. The laminate obtained by lamination was left standing in an environment of an atmospheric temperature of 23° C. and 50% RH for 7 days (so-called curing period) to cure the adhesive film. In this manner, a pressure-sensitive adhesive film for measuring gel fraction having a structure of release film X / pressure-sensitive adhesive layer / release film Y was prepared.

[0141] (2) Measurement of gel fraction The pressure-sensitive adhesive layer peeled from the pressure-sensitive adhesive film for gel fraction measurement prepared above was used to measure the gel fraction according to the following [1] to [4]. The measured values ​​are shown in Tables 2 to 4. [1] Approximately 0.15 g of adhesive layer was attached to a 250 mesh wire mesh (100 mm x 100 mm) whose mass was accurately measured using a precision balance, and the wire mesh was folded five times with the attached adhesive layer facing inward to prevent leakage of the gel. The mass was then accurately measured using a precision balance. [2] The obtained sample was immersed in 80 mL of ethyl acetate for 3 days. [3] The sample was taken out, washed with a small amount of ethyl acetate, and dried at 100°C for 3 hours. Then, the mass was accurately measured using a precision balance. [4] The gel fraction was calculated using the following formula: Gel fraction (unit: mass%) = (ZX) / (YX) × ​​100 where X is the mass of the wire mesh (unit: g), Y is the mass of the wire mesh with the adhesive layer attached (unit: g) before immersion, and Z is the mass of the wire mesh with the adhesive layer attached after immersion and drying (unit: g).

[0142] [Table 2]

[0143] [Table 3]

[0144] [Table 4]

[0145] Details of the components listed in Tables 2 to 4 are as follows. <Isocyanate-based crosslinking agent> "N3300" (product name: Sumidur (registered trademark) N3300, isocyanurate of hexamethylene diisocyanate (HDI), solid content concentration: 100% by mass, manufactured by Sumika Covestro Urethane Co., Ltd.) "L-45E" (trade name: Coronate (registered trademark) L-45E, adduct of tolylene diisocyanate (TDI) and trimethylolpropane (TMP), solid content concentration: 45% by mass, manufactured by Tosoh Corporation) "N-75" (trade name: Sumidur (registered trademark) N-75, hexamethylene diisocyanate (HDI) trimethylolpropane (TMP) adduct, solid content: 75% by mass, manufactured by Sumika Covestro Urethane Co., Ltd.)

[0146] <Plasticizer> -Adipic acid diester compound- "DIDA" [trade name: DIDA (NB), diisodecyl adipate, molecular weight: 428, manufactured by Taoka Chemical Co., Ltd.] "DOA" (trade name, dioctyl adipate, molecular weight: 371, manufactured by Taoka Chemical Co., Ltd.) -Other plasticizers- "DOP" (trade name, dioctyl phthalate, molecular weight: 391, manufactured by J-Plus Co., Ltd.) "PN-446" (trade name: Adeka Cizer (registered trademark) PN-446, adipic acid polyester, molecular weight: 2000, manufactured by ADEKA Corporation) "PN-1430" (trade name: Adeka Cizer (registered trademark) PN-1430, adipic acid polyester, molecular weight: 1800, manufactured by ADEKA Corporation) "DGB" (trade name, diethylene glycol benzoate, molecular weight: 314, manufactured by Tokyo Chemical Industry Co., Ltd.)

[0147] <Crosslinking accelerator> "DOTDL" (product name: dioctyltin dilaurate, solids concentration: 0.33% by mass, manufactured by ADEKA Corporation)

[0148] In Tables 2 to 4, the values ​​shown in the "blending amount" column are all solid content converted values. In Tables 2 to 4, "-" in the column for the composition of the adhesive composition means that the component in that column was not blended. In Tables 3 and 4, the "-" in the column for adhesive strength after aging indicates that adhesive residue was confirmed on the adhesive adherend in the initial adhesive strength evaluation test, so an evaluation test for adhesive strength after aging was not performed.

[0149] As shown in Tables 2 to 4, it was confirmed that the pressure-sensitive adhesive layers formed using the pressure-sensitive adhesive compositions of Examples 1 to 30 had the appropriate adhesive strength required for a protective film and had excellent compatibility with the adherend. It was also confirmed that the pressure-sensitive adhesive layers formed using the pressure-sensitive adhesive compositions of Examples 1 to 30 could maintain the appropriate adhesive strength required for a protective film over time. On the other hand, it was confirmed that the adhesive layers formed using the adhesive compositions of Comparative Examples 1 to 21 were inferior in at least one of initial adhesive strength and conformability to the adherend when used as a protective film compared to the adhesive layers formed using the adhesive compositions of the Examples.

Claims

1. a (meth)acrylic copolymer comprising a structural unit (a1) derived from a (meth)acrylic acid alkyl ester monomer and a structural unit (a2) derived from a monomer having a hydroxyl group, wherein the content of the structural unit (a2) is 1.0% by mass to 10.0% by mass with respect to all structural units, and the weight average molecular weight is 50,000 to 2,000,000; an isocyanate-based crosslinking agent; a plasticizer that is an adipic acid diester compound having a molecular weight of 350 to 450; Including, the ratio of the number of moles of isocyanate groups in the isocyanate crosslinking agent to the number of moles of reactive functional groups in the (meth)acrylic copolymer is 0.18 to 3.36; The pressure-sensitive adhesive composition for protective films, wherein the content of the plasticizer is 15 to 35 parts by mass per 100 parts by mass of the (meth)acrylic copolymer.

2. The pressure-sensitive adhesive composition for protective films according to claim 1 , wherein the (meth)acrylic copolymer further comprises a structural unit (a3) ​​derived from a monomer having a carboxy group.

3. The pressure-sensitive adhesive composition for protective films according to claim 1 , wherein the plasticizer comprises at least one of dioctyl adipate and diisodecyl adipate.

4. The pressure-sensitive adhesive composition for protective films according to claim 1, wherein the structural unit (a2) includes a structural unit derived from 4-hydroxybutyl acrylate.

5. The pressure-sensitive adhesive composition for a protective film according to claim 1 , wherein the isocyanate-based crosslinking agent is a hexamethylene diisocyanate-based compound.

6. The pressure-sensitive adhesive composition for protective films according to claim 5 , wherein the hexamethylene diisocyanate compound is an isocyanurate of hexamethylene diisocyanate.

7. A substrate; a pressure-sensitive adhesive layer provided on the substrate and formed from the pressure-sensitive adhesive composition for a protective film according to any one of claims 1 to 6; A protective film is provided.

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