Solvent-free acrylic pressure-sensitive adhesive composition and adhesive sheet

Abstract

To provide a solvent-free acrylic pressure-sensitive adhesive composition excellent in adhesive strength, holding power, constant-load peel strength, gasoline resistance, and alkali resistance.SOLUTION: The solvent-free acrylic pressure-sensitive adhesive composition comprises: an acrylic partial copolymer containing (a) 85 to 93 pts.mass of (meth)acrylic acid ester having an alkyl group with 4 to 8 carbon atoms and (b) 7 to 15 pts.mass of a carboxyl group-containing acrylic monomer; and, relative to 100 pts.mass of the acrylic partial copolymer, at least one of the following polymerizable monomers: (e) 0.10 to 1.5 pts.mass of a polymerizable monomer having two or more alkylene glycol groups and three or more (meth)acryloyl groups per molecule and (f) 0.03 to 0.40 pt.mass of a polymerizable monomer having one or more (meth)acryloyl groups and one or more functional groups reactive with active hydrogen atoms per molecule. The gel fraction of the adhesive layer formed from the solvent-free acrylic pressure-sensitive adhesive composition is 37% to 75%.SELECTED DRAWING: None

Description

[Technical Field]

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

[0002] Acrylic pressure-sensitive adhesives come in various forms, including solvent-based acrylic pressure-sensitive adhesives, water-dispersed (emulsion-type) acrylic pressure-sensitive adhesives, and solventless acrylic pressure-sensitive adhesives (acrylic syrup). Among these, solvent-based acrylic pressure-sensitive adhesives have been identified as having environmental hygiene and safety issues due to the solvent. Furthermore, water-dispersed (emulsion-type) acrylic pressure-sensitive adhesives have issues such as the need for significant drying energy to evaporate the water medium and reduced water resistance due to the inclusion of emulsifiers in the resulting coating film. On the other hand, solventless acrylic pressure-sensitive adhesives (acrylic syrup), particularly ultraviolet-curable pressure-sensitive adhesives, have attracted attention because they are free from the solvent-related problems and can provide excellent adhesive performance. For this reason, various proposals have been made regarding ultraviolet-curable pressure-sensitive adhesives and adhesive products using them.

[0003] In general, acrylic pressure-sensitive adhesives are widely used in applications such as tapes, labels, stickers, and seals, and are excellent in weather resistance, durability, and high transparency. In addition, since there are a large number of monomers that can be used for acrylic pressure-sensitive adhesives, acrylic pressure-sensitive adhesives are used in a wide variety of applications.

[0004] However, when acrylic adhesives are used for direct attachment to vehicles such as automobiles and motorcycles, detergents used in car washes and gasoline used in refueling can adhere to the acrylic adhesive, causing the adhesive to dissolve or swell, which can lead to problems such as peeling off of labels attached to vehicles with the acrylic adhesive.

[0005] For example, the use of a copolymer of a highly hydrophilic monomer such as an acrylic acid alkyl ester monomer and a carboxyl group-containing monomer can improve the gasoline resistance of an acrylic pressure-sensitive adhesive, but at the cost of poor alkali resistance. If the content of the carboxyl group-containing monomer is reduced to reduce hydrophilicity, the alkali resistance improves but the gasoline resistance deteriorates, and the cohesive strength decreases, resulting in poor holding power and constant-load peel strength.

[0006] When lipophilic acrylic acid alkyl ester monomers are blended into acrylic adhesives, alkali resistance improves, but gasoline resistance deteriorates, and the cohesive strength decreases as the glass transition temperature of the polymer itself decreases. Thus, it is difficult to achieve both gasoline resistance and alkali resistance by adjusting the lipophilicity and hydrophilicity through the blending balance of acrylic acid alkyl ester monomers and carboxyl group-containing monomers.

[0007] Therefore, Patent Documents 1 and 2 propose a solvent-based acrylic pressure-sensitive adhesive that uses a large amount of a carboxyl group-containing monomer and a hydroxyl group-containing monomer in order to achieve both the gasoline resistance and alkali resistance required of an acrylic pressure-sensitive adhesive.To address the issue of reduced liquid stability due to the generation of hydrogen bonds between the carboxyl group-containing monomer and the hydroxyl group-containing monomer, the liquid stability is achieved by using an alcohol solvent to inhibit the hydrogen bonds.

[0008] Patent Document 3 proposes an acrylic resin composition containing 40 to 90% by mass of an acrylate containing alkylene glycol and 10 to 20% by mass of units derived from a carboxylic acid-containing (meth)acrylic monomer. By increasing hydrophilicity, the oil resistance (resistance to oleic acid, gasoline, and toluene) is enhanced.

[0009] Patent Document 4 proposes a water-dispersible pressure-sensitive adhesive that contains a water-dispersible polymer and inorganic particles in order to improve oil resistance, and that is characterized in that the content of the inorganic particles is 3 to 90 parts by mass per 100 parts by mass of the water-dispersible polymer. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] Japanese Patent Application Laid-Open No. 2016-210863 [Patent Document 2] Japanese Patent Application Publication No. 2019-189876 [Patent Document 3] Japanese Patent Publication No. 2022-024067 [Patent Document 4] Japanese Patent Application Laid-Open No. 2012-117053 Summary of the Invention [Problem to be solved by the invention]

[0011] However, the manufacturing methods described in Patent Documents 1 and 2 pose environmental problems due to the discharge of solvents, and are restricted in that they do not allow the use of isocyanate-based crosslinking agents, which react with the hydroxyl groups of alcohols. As a result, there are few options for crosslinking agents that can be used with solvent-based acrylic pressure-sensitive adhesives, which leads to the problem of poor adhesion to substrates.

[0012] Furthermore, in Patent Document 3, due to the high hydrophilicity, alkaline water easily penetrates into the acrylic resin composition, causing the acrylic resin composition to swell and the coating film to peel off from the substrate or adherend. Therefore, the technology of Patent Document 3 cannot achieve sufficient alkali resistance.

[0013] Furthermore, Patent Document 4 has the problem that separation of the water-dispersible polymer and the inorganic particles occurs, reducing the solution stability of the pressure-sensitive adhesive.

[0014] Therefore, an object of the present invention is to provide a solventless acrylic pressure-sensitive adhesive composition that is excellent in adhesive strength, holding power, constant-load peel strength, gasoline resistance, and alkali resistance. [Means for solving the problem]

[0015] In order to achieve the object of the present invention, the present invention has the following configuration: Namely, a solventless acrylic pressure-sensitive adhesive composition comprising an acrylic partial copolymer containing 85 to 93 parts by mass of (a) a (meth)acrylic acid ester having 4 to 8 carbon atoms in the alkyl group and 7 to 15 parts by mass of (b) a carboxyl group-containing acrylic monomer, and at least one of 0.10 to 1.5 parts by mass of (e) a polymerizable monomer having two or more alkylene glycols and three or more (meth)acryloyl groups in one molecule and 0.03 to 0.40 parts by mass of (f) a polymerizable monomer having one or more functional groups reactive with active hydrogen groups and one or more (meth)acryloyl groups in one molecule, relative to 100 parts by mass of the acrylic partial copolymer, wherein a pressure-sensitive adhesive layer comprising the solventless acrylic pressure-sensitive adhesive composition has a gel fraction of 37 to 75%. [Effects of the Invention]

[0016] According to the present invention, it is possible to provide a solventless acrylic pressure-sensitive adhesive composition that is excellent in adhesive strength, holding strength, constant-load peel strength, gasoline resistance, and alkali resistance. DETAILED DESCRIPTION OF THE INVENTION

[0017] The following describes the embodiments in detail. Note that the following embodiments do not limit the scope of the invention as claimed, and not all combinations of features described in the embodiments are necessarily essential to the invention. Two or more features among the multiple features described in the embodiments may be combined in any desired manner.

[0018] In this specification, a numerical range specified using "to" is intended to include the numerical values ​​before and after "to" as the range's lower and upper limits.

[0019] In this specification, the term "(meth)acrylic" means to include both the terms "acrylic" and "methacrylic". Similarly, the term "(meth)acrylate" means to include both the terms "acrylate" and "methacrylate". Examples of (meth)acrylic monomers include (meth)acrylic acid esters and (meth)acrylic acid. Of these, it is more preferable to use (meth)acrylic acid esters and (meth)acrylic acid, and it is even more preferable to use (meth)acrylic acid esters.

[0020] <Solvent-free acrylic pressure-sensitive adhesive composition> The solvent-free acrylic pressure-sensitive adhesive composition contains an acrylic partial copolymer containing 85 to 93 parts by mass of (a) a (meth)acrylic acid ester having 4 to 8 carbon atoms in the alkyl group and 7 to 15 parts by mass of (b) a carboxyl group-containing acrylic monomer, and at least one of: 0.10 to 1.5 parts by mass of (e) a polymerizable monomer having two or more alkylene glycols and three or more (meth)acryloyl groups in one molecule; and 0.03 to 0.40 parts by mass of (f) a polymerizable monomer having one or more functional groups reactive with active hydrogen groups and one or more (meth)acryloyl groups in one molecule, relative to 100 parts by mass of the acrylic partial copolymer.

[0021] The solvent-free acrylic pressure-sensitive adhesive composition is substantially free of solvent. "Substantially free of solvent (e.g., organic solvent)" means that, except for cases where organic solvent is inevitably mixed in, organic solvent is not actively blended into the solvent-free acrylic pressure-sensitive adhesive composition. The solvent content of the solvent-free acrylic pressure-sensitive adhesive composition is preferably 2 wt% or less, more preferably 0 to 2 wt%, and even more preferably 0 to 1 wt%, relative to 100 wt% of the solvent-free acrylic pressure-sensitive adhesive composition. If the solvent content is too high, bubbles may form in the pressure-sensitive adhesive layer, which may reduce the durability of the pressure-sensitive adhesive sheet.

[0022] <Acrylic partial copolymer> An acrylic partial copolymer is a copolymer obtained by partially copolymerizing acrylic monomers, and is also referred to as "acrylic syrup" in this specification. The advantages of a solvent-free acrylic pressure-sensitive adhesive composition containing acrylic syrup are as follows. For example, since the acrylic pressure-sensitive adhesive composition does not contain solvent or water, there is no need to remove the solvent after application, making it possible to obtain a pressure-sensitive adhesive sheet using a work process that is environmentally friendly and highly safe for the human body. Furthermore, it is possible to save a large amount of drying energy required to evaporate water during the work process.

[0023] ((Meth)acrylic acid ester (a) having an alkyl group carbon number of 4 to 8) Examples of the (meth)acrylic acid ester (a) having an alkyl group with 4 to 8 carbon atoms include n-butyl (meth)acrylate, isobutyl (meth)acrylate, tert-butyl (meth)acrylate, sec-butyl (meth)acrylate, n-amyl (meth)acrylate, isoamyl (meth)acrylate, n-hexyl (meth)acrylate, heptyl (meth)acrylate, n-octyl (meth)acrylate, isooctyl (meth)acrylate, and 2-ethylhexyl (meth)acrylate. These monomers may be used alone or in combination of two or more. In particular, the (meth)acrylic acid ester (a) preferably contains at least one of butyl acrylate and 2-ethylhexyl acrylate.

[0024] When a (meth)acrylic ester with an alkyl group containing three or fewer carbon atoms is used, the glass transition temperature (Tg) of the polymer itself increases, increasing the cohesive strength, improving the holding power and constant-load peeling properties. However, the increased hydrophilicity reduces alkali resistance.

[0025] In addition, using a (meth)acrylic ester with an alkyl group containing 9 or more carbon atoms increases lipophilicity and improves alkali resistance. However, the glass transition temperature (Tg) of the polymer itself decreases, reducing cohesive strength, resulting in poor holding power and constant-load peeling properties.

[0026] The content of the (meth)acrylic acid ester (a) having an alkyl group with 4 or more and 8 or less carbon atoms is 85 to 93 parts by mass in 100 parts by mass of all the monomers constituting the acrylic partial copolymer.

[0027] (Carboxyl group-containing acrylic monomer (b)) The amount of the carboxyl group-containing acrylic monomer (b) can impart cohesive strength to the solventless acrylic adhesive composition, and can adjust the adhesive strength, holding power, and constant load peel properties. In addition, the incorporation of the carboxyl group-containing acrylic monomer (b) can increase the hydrophilicity of the acrylic adhesive composition, thereby improving its gasoline resistance.

[0028] Examples of the carboxyl group-containing acrylic monomer (b) include (meth)acrylic acid, monohydroxyethyl succinate (meth)acrylate, monohydroxyethyl phthalate (meth)acrylate, monohydroxyethyl hexahydrophthalate (meth)acrylate, monohydroxypropyl phthalate (meth)acrylate, β-carboxyethyl (meth)acrylate, and ω-carboxy-polycaprolactone mono(meth)acrylate. In the present invention, it is desirable to use at least one of these. Furthermore, among these, it is preferable that the carboxyl group-containing acrylic monomer (b) contains acrylic acid, taking into consideration copolymerizability with the (meth)acrylic acid ester (a) and reactivity with the polymerizable monomer (f) described below.

[0029] The content of the carboxyl group-containing acrylic monomer (b) is 7 to 15 parts by mass per 100 parts by mass of all monomers constituting the acrylic partial copolymer. If the content of the carboxyl group-containing acrylic monomer (b) is less than 7 parts by mass, the following problems arise. For example, the hydrophilicity of the acrylic pressure-sensitive adhesive composition decreases, resulting in poor gasoline resistance, and the adhesive strength of the pressure-sensitive adhesive layer decreases, resulting in poor holding power and constant-load peel properties. As such, the pressure-sensitive adhesive layer cannot achieve sufficient adhesive performance. On the other hand, if the content of the carboxyl group-containing acrylic monomer (b) is more than 15 parts by mass, the hydrophilicity of the acrylic pressure-sensitive adhesive composition improves, resulting in improved gasoline resistance, but poor alkali resistance. Therefore, to achieve both gasoline resistance and alkali resistance, the amount of the carboxyl group-containing acrylic monomer (b) used is preferably 7 to 15 parts by mass.

[0030] (Hydroxyl group-containing acrylic monomer (c)) The acrylic partial copolymer may further contain a hydroxyl group-containing acrylic monomer (c). The content of the hydroxyl group-containing acrylic monomer (c) is 7 parts by mass or less per 100 parts by mass of all monomers constituting the acrylic partial copolymer. When more than 7 parts by mass of the hydroxyl group-containing acrylic monomer (c) is used, the hydrophilicity of the acrylic pressure-sensitive adhesive composition increases, improving gasoline resistance, but decreasing alkali resistance.

[0031] Examples of the hydroxyl group-containing acrylic monomer (c) include (meth)acrylic acid esters (hydroxyalkyl (meth)acrylates) having a hydroxy group, such as 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, 2-hydroxybutyl (meth)acrylate, 3-hydroxybutyl (meth)acrylate, and 4-hydroxybutyl (meth)acrylate. Among these, in consideration of copolymerizability with the (meth)acrylic acid ester (a) and reactivity with the polymerizable monomer (f) described below, it is preferable that the hydroxyl group-containing acrylic monomer (c) contains 4-hydroxybutyl acrylate.

[0032] Using these monomers alone can improve gasoline resistance without significantly reducing alkali resistance. However, when the acrylic partial copolymer does not contain the carboxyl group-containing acrylic monomer (b) but contains the hydroxyl group-containing acrylic monomer (c), the adhesive strength and constant-load peel properties are poor, and the solventless acrylic pressure-sensitive adhesive composition cannot achieve sufficient adhesive properties. Therefore, when the acrylic partial copolymer contains the hydroxyl group-containing acrylic monomer (c), it is preferable to use a mixture of the carboxyl group-containing acrylic monomer (b) and the hydroxyl group-containing acrylic monomer (c). Using a mixture of the carboxyl group-containing acrylic monomer (b) and the hydroxyl group-containing acrylic monomer (c) can improve the hydrophilicity of the acrylic pressure-sensitive adhesive composition, thereby improving gasoline resistance. Furthermore, the cohesive strength of the acrylic pressure-sensitive adhesive composition can be improved by hydrogen bonding between the carboxyl group and the hydroxyl group, thereby improving the holding power and constant-load peel properties.

[0033] <Polymerization method> The acrylic partial copolymer can be produced by conventional solution polymerization, bulk polymerization, polymerization by active energy ray irradiation, etc., but bulk polymerization in which polymerization is performed without using a solvent and active energy ray polymerization are preferred. Here, active energy rays include ultraviolet rays. By bulk polymerization or active energy ray polymerization, the acrylic partial copolymer can be obtained without a step such as desolvation, and can be used as is in the production of the solventless acrylic pressure-sensitive adhesive composition of the present invention.

[0034] (thermal polymerization initiator) Examples of the thermal polymerization initiator used in producing the acrylic partial copolymer include oil-soluble organic peroxides such as 2,4-dichlorobenzoyl peroxide, t-butyl peroxypivalate, dibenzoyl peroxide, bis(2-methylbenzoyl) peroxide, bis(3,5,5-trimethylhexanoyl) peroxide, dioctanoyl peroxide, t-butylperoxy-2-ethylhexanoate, cyclohexanone peroxide, methyl ethyl ketone peroxide, dicumyl peroxide, dilauroyl peroxide, diisopropylbenzene hydroperoxide, t-butyl hydroperoxide, and di-t-butyl peroxide; and oil-soluble azo compounds such as 2,2'-azobisisobutyronitrile, 2,2'-azobis(2,4-dimethylvaleronitrile), and 2,2'-azobis(2,4-dimethyl-4-methoxyvaleronitrile). These may be used alone or in combination of two or more.

[0035] (Photopolymerization initiator (d)) In the present invention, a photopolymerization initiator (d) can be used when producing an acrylic partial copolymer and for the purpose of curing the polymerized acrylic adhesive composition with ultraviolet light. Representative photopolymerization initiators (d) include acetophenone-based, phosphine oxide-based, benzoin-based, benzophenone-based, and thioxanthone-based initiators. These can be used alone or in combination of two or more.

[0036] Examples of the acetophenone-based initiator include acetophenone, 2,2-diethoxyacetophenone, 2,2-dimethoxy-1,2-diphenylethan-1-one, 1-hydroxycyclohexyl phenyl ketone, and 2-hydroxy-2-methyl-1-phenylpropan-1-one.

[0037] Examples of the phosphine oxide initiator include bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide, bis(2,6-dimethoxybenzoyl)-2,4,4-trimethyl-pentylphosphine oxide, and 2,4,6-trimethylbenzoyl-diphenylphosphine oxide.

[0038] Examples of the benzoin initiator include benzoin, benzoin methyl ether, benzoin ethyl ether, benzoin isopropyl ether, and benzoin isobutyl ether.

[0039] Examples of the benzophenone initiator include benzophenone, p-methoxybenzophenone, o-benzoylbenzoic acid, methyl o-benzoylbenzoate, 4,4'-bis(dimethylamino)benzophenone, and 4,4'-bis(diethylamino)benzophenone.

[0040] Examples of the thioxanthone initiator include thioxanthone, 2-methylthioxanthone, 2-isopropylthioxanthone, 2,4-dimethylthioxanthone, 2,4-diethylthioxanthone, and 2-chlorothioxanthone.

[0041] These initiators can be used alone or in combination of two or more. In the present invention, it is preferable to use an acetophenone-based initiator and / or a phosphine oxide-based initiator because of their good curability.

[0042] The solventless acrylic pressure-sensitive adhesive composition contains 0.3 to 1.0 parts by mass of photopolymerization initiator (d) per 100 parts by mass of acrylic partial copolymer. If the content of photopolymerization initiator (d) is less than 0.3 parts by mass, unreacted monomers are likely to remain, causing odor problems in the acrylic pressure-sensitive adhesive composition. On the other hand, if the content of photopolymerization initiator (d) is more than 1.0 part by mass, the molecular weight of the polymer (acrylic copolymer) after curing decreases, resulting in poor constant-load peel properties.

[0043] (chain transfer agent) A chain transfer agent can be used as appropriate to adjust the molecular weight of the acrylic partial copolymer. Examples of the chain transfer agent include mercaptans such as octyl thioglycolate, methoxybutyl thioglycolate, octyl mercaptopropionate, methoxybutyl mercaptopropionate, stearyl mercaptan, lauryl mercaptan, and α-methylstyrene dimer. These can be used alone or in combination of two or more.

[0044] <Additives> The solvent-free acrylic pressure-sensitive adhesive composition can contain at least one of the polymerizable monomer (e) and the polymerizable monomer (f).

[0045] For example, the solventless acrylic pressure-sensitive adhesive composition contains, relative to 100 parts by mass of the acrylic partial copolymer, at least one of: 0.10 to 1.5 parts by mass of a polymerizable monomer (e) having two or more alkylene glycols and three or more (meth)acryloyl groups in one molecule; and 0.03 to 0.40 parts by mass of a polymerizable monomer (f) having one or more functional groups reactive with active hydrogen groups and one or more (meth)acryloyl groups in one molecule.

[0046] (Polymerizable monomer (e)) The polymerizable monomer (e) is a monomer obtained by modifying a trifunctional or higher polyfunctional (meth)acrylate with an alkylene glycol. Examples of alkylene oxides constituting the alkylene glycol include ethylene oxide, propylene oxide, butylene oxide, and isobutylene oxide, with ethylene oxide and propylene oxide being preferred. The number of alkylene glycols contained in the polymerizable monomer (e) (represented by "n" in this specification) is 2 or more, preferably 3 or more. The number of alkylene glycols contained in the polymerizable monomer (e) is 40 or less, preferably 35 or less. The range of the number of alkylene glycols contained in the polymerizable monomer (e) is any combination of the above upper and lower limits, and may be, for example, 2 to 40, 2 to 35, 3 to 40, or 3 to 35. The number of (meth)acryloyl groups contained in the polymerizable monomer (e) is preferably 3 to 5, more preferably 3 to 4. Examples of polymerizable monomers (e) include trimethylolpropane EO-modified (n≈3,20) tri(meth)acrylate, trimethylolpropane PO-modified tri(meth)acrylate, isocyanuric acid EO-modified triacrylate, and pentaerythritol EO-modified (n≈35) tri- and tetra(meth)acrylate. In the present invention, it is desirable to use at least one of these. Adding polymerizable monomer (e) together with photopolymerization initiator (d) to the solventless acrylic pressure-sensitive adhesive composition of the present invention and crosslinking the composition can impart cohesive strength. Furthermore, adding alkylene glycol to the crosslinked portion can improve hydrophilicity and gasoline resistance.

[0047] The content of the polymerizable monomer (e) is 0.10 to 1.5 parts by mass, preferably 0.15 to 1.5 parts by mass, per 100 parts by mass of the acrylic partial copolymer. If the content of the polymerizable monomer (e) is less than 0.10 parts by mass, the gel fraction of the pressure-sensitive adhesive layer decreases, making it impossible to obtain sufficient cohesive strength. This results in a deterioration in the holding power of the pressure-sensitive adhesive layer, and the gasoline resistance does not improve. On the other hand, if the content of the polymerizable monomer (e) is more than 1.5 parts by mass, the cohesive strength of the acrylic pressure-sensitive adhesive composition becomes too high, resulting in a loss of flexibility and a deterioration in constant-load peel properties.

[0048] (Polymerizable monomer (f)) The polymerizable monomer (f) is a monomer having one or more isocyanate groups and one or more (meth)acryloyl groups. Here, the number of isocyanate groups is 1 to 3, preferably 1. The number of (meth)acryloyl groups is 1 to 3, preferably 1 to 2. An example of the polymerizable monomer (f) having one isocyanate group and one (meth)acryloyl group is 2-isocyanatoethyl (meth)acrylate. From the viewpoint of further improving ultraviolet curability, an example of the polymerizable monomer (f) having one isocyanate group and two (meth)acryloyl groups is 1,1-(bis(meth)acryloyloxymethyl)ethyl isocyanate. Commercially available polymerizable monomers (f) include 2-isocyanatoethyl acrylate (trade name: Karenz AOI, manufactured by Resonac Co., Ltd.), 2-isocyanatoethyl methacrylate (trade name: Karenz MOI, manufactured by Resonac Co., Ltd.), and 1,1-bis(acryloyloxymethyl)ethyl isocyanate (trade name: Karenz BEI, manufactured by Resonac Co., Ltd.).

[0049] In addition, "Karenz MOI-BM" (manufactured by Resonac) in which the isocyanate group of 2-isocyanatoethyl methacrylate is blocked with an amine compound, "Karenz MOI-BP" (manufactured by Resonac) in which the isocyanate group is blocked with a pyrazole compound, a polymer of 2-hydroxyethyl acrylate (HEA) and 1,6-diisocyanatohexane (HDI) (trade name "Laromer PR9000" manufactured by BASF), glycidyl (meth)acrylate, 4-hydroxybutyl acrylate glycidyl ether, etc. can also be used as the polymerizable monomer (f).

[0050] In the present invention, in consideration of reactivity with active hydrogen groups, it is preferable that the polymerizable monomer (f) contains at least one of 2-isocyanatoethyl acrylate (trade name: Karenz AOI, manufactured by Resonac Co., Ltd.) and 2-isocyanatoethyl methacrylate (trade name: Karenz MOI, manufactured by Resonac Co., Ltd.).

[0051] In the present invention, the polymerizable monomer (f) imparts functional groups reactive with active hydrogen groups to the acrylic polymer that undergoes photopolymerization upon UV curing. Meanwhile, the constituent components of the acrylic partial copolymer of the present invention include active hydrogen group-containing monomers such as a carboxyl group-containing acrylic monomer (b) and a hydroxyl group-containing acrylic monomer (c). Therefore, active hydrogen groups such as carboxyl groups and hydroxyl groups are also imparted to the acrylic polymer in the acrylic partial copolymer and the acrylic polymer that undergoes photopolymerization upon UV curing. The functional groups reactive with the active hydrogen groups react with the active hydrogen groups such as carboxyl groups and hydroxyl groups during curing, thereby introducing a crosslinked structure into the acrylic polymer that contributes to the formation of the adhesive.

[0052] Here, the functional group that reacts with one or more active hydrogen groups in one molecule of the polymerizable monomer (f) is an isocyanate group. The number of functional groups that react with one or more active hydrogen groups in one molecule of the polymerizable monomer (f) is 1 to 3, and preferably 1.

[0053] An acrylic partial copolymer containing an acrylic polymer having a carbon-carbon double bond can also be obtained by pre-reacting a functional group reactive with one or more active hydrogen groups in the polymerizable monomer (f) with an active hydrogen group, such as a carboxyl group or a hydroxyl group, in the acrylic polymer in the acrylic partial copolymer. In this case, a heating step at about 40 to 80°C in a reaction vessel is required. To avoid the heating step, in the present invention, it is preferable to react the polymerizable monomer (f) with the acrylic polymer in the acrylic partial copolymer in the UV curing step and the curing step as described above, rather than pre-reacting the polymerizable monomer (f) with the acrylic polymer in the acrylic partial copolymer.

[0054] The content of the polymerizable monomer (f) is 0.03 to 0.40 parts by mass relative to 100 parts by mass of the acrylic partial copolymer. If the content of the polymerizable monomer (f) is less than 0.03 parts by mass, the gel fraction of the pressure-sensitive adhesive layer decreases, resulting in insufficient cohesive strength, and the pressure-sensitive adhesive layer therefore has poor holding power and constant-load peel properties. As a result, the pressure-sensitive adhesive layer does not exhibit sufficient adhesive performance. On the other hand, if the content of the polymerizable monomer (f) is more than 0.40 parts by mass, the cohesive strength of the acrylic pressure-sensitive adhesive composition increases too much, resulting in a loss of flexibility and poor constant-load peel properties.

[0055] <Other additives> The solventless acrylic pressure-sensitive adhesive composition may contain various additives as needed, provided that the additives do not impair the adhesive properties of the pressure-sensitive adhesive sheet. Examples of such additives include antioxidants, ultraviolet absorbers, fillers, pigments, thickeners, antistatic agents, silane coupling agents, flame retardants, and tackifying resins. These additives may be used alone or in combination of two or more.

[0056] <Application> The solventless acrylic pressure-sensitive adhesive composition of the present invention cures upon irradiation with active energy rays (e.g., ultraviolet rays), allowing for thick coating. The solventless acrylic pressure-sensitive adhesive composition of the present invention is used for marking films. Furthermore, pressure-sensitive adhesives using the solventless acrylic pressure-sensitive adhesive composition of the present invention are used, for example, as pressure-sensitive adhesives for marking films. A marking film is a sheet material with an adhesive. A typical method of using a marking film will be described. First, a user cuts a marking film (sheet material) to obtain a sheet with the desired shape of letters, logos, etc., in order to display, decorate, and advertise the name, business hours, etc., of a store owned by the user or another person. The user then attaches the marking film cut to the desired shape to, for example, a signboard or glass inside and / or outside the store. Alternatively, a user can directly attach a marking film of the user's desired shape to the painted surface of a vehicle, etc., in order to improve the design of a motorcycle, automobile, or other vehicle. This improves the aesthetic appearance of the vehicle, etc., and satisfies a variety of user preferences for vehicles.

[0057] The marking film has a substrate made of a synthetic resin film such as polyvinyl chloride (PVC), polyurethane, polyester, or polystyrene. One side of the substrate is colored or printed. An adhesive is applied to the other side of the substrate. From the viewpoints of weather resistance, colorability, and cost advantage, it is preferable to use PVC film as the substrate. When using PVC film, it may be plasticized with a polymer plasticizer or the like to prevent the plasticizer in the PVC film from migrating to the adhesive and softening it.

[0058] <Adhesive sheet> The pressure-sensitive adhesive sheet of the present invention comprises a substrate and a pressure-sensitive adhesive layer formed on at least one surface of the substrate and comprising a solventless acrylic pressure-sensitive adhesive composition. The pressure-sensitive adhesive layer is formed by curing the solventless acrylic pressure-sensitive adhesive composition using active energy rays. The pressure-sensitive adhesive sheet can be used in combination with the pressure-sensitive adhesive described above and can be used for the same purposes as the pressure-sensitive adhesive. The pressure-sensitive adhesive layer and the pressure-sensitive adhesive sheet can be formed by the methods described below.

[0059] Here, the pressure-sensitive adhesive layer has a gel fraction of 37 to 75%. The method for measuring the gel fraction will be explained in Test 7 below. For example, if the gel fraction is less than 37%, the acrylic pressure-sensitive adhesive composition does not have sufficient cohesive strength, resulting in a deterioration in the holding power of the pressure-sensitive adhesive layer and no improvement in gasoline resistance. On the other hand, if the gel fraction is more than 75%, the cohesive strength of the acrylic pressure-sensitive adhesive composition becomes too high, resulting in a loss of flexibility of the pressure-sensitive adhesive layer and a deterioration in constant-load peel properties.

[0060] <Method of manufacturing acrylic syrups A to J> Table 1 shows the raw material composition and physical properties of the acrylic syrup. Based on the raw material composition of acrylic syrup A in Table 1, a reactor equipped with a stirrer, thermometer, reflux condenser, and nitrogen inlet tube was charged with nitrogen gas. Then, 90 parts of butyl acrylate (BA), 10 parts of acrylic acid (AAC), and 0.005 parts of a thermal polymerization initiator (2,2'-azobisisobutyronitrile) were added. The mixture was stirred and reacted at 85°C for 5 hours in a nitrogen gas stream. After the reaction was completed, the mixture was diluted with the same monomer composition to obtain acrylic syrup A with an evaporation residue of 30.4% by mass. The viscosity of acrylic syrup A was measured using a viscometer (manufactured by Toki Sangyo Co., Ltd., model: B-type viscometer). Acrylic syrups B to J were obtained using the same process as acrylic syrup A based on the raw material composition in Table 1, and detailed explanations are omitted.

[0061] [Table 1] (remarks) BA: Butyl acrylate 2EHA: 2-Ethylhexyl acrylate AAC: Acrylic acid 4HBA: 4-hydroxybutyl acrylate

[0062] <Method for producing a solventless acrylic pressure-sensitive adhesive composition containing polymerizable monomer (e)> Table 2 shows the raw material blends of the solventless acrylic pressure-sensitive adhesive compositions of Examples 1 to 8 and Comparative Examples 1 to 6. Based on the raw material blend of Example 1 in Table 2, 0.5 parts of Aronix M-350 (trade name, manufactured by Toagosei Co., Ltd.) as the polymerizable monomer (e) and 1.0 part of Omnirad 651 (trade name, manufactured by IGM Resins) as the photopolymerization initiator (d) were added to 100 parts of acrylic syrup A and mixed thoroughly to obtain the solventless acrylic pressure-sensitive adhesive composition of Example 1. The solventless acrylic pressure-sensitive adhesive compositions of Examples 2 to 8 and Comparative Examples 1 to 6 were obtained based on the raw material blends in Table 2 using the same process as in Example 1, and detailed explanations thereof will be omitted.

[0063] [Table 2] (remarks) <Polymerizable monomer (e)> A: Trimethylolpropane EO-modified (n≒3) triacrylate (Aronix M-350 / Toagosei Co., Ltd.) B: Trimethylolpropane EO-modified (n≒20) triacrylate (NK Ester AT-20E / Shin-Nakamura Chemical Co., Ltd.) C: Pentaerythritol EO-modified (n≒35) tri- and tetraacrylate (NK Ester ATM-35E / Shin-Nakamura Chemical Co., Ltd.) D: Trimethylolpropane triacrylate (Aronix M-309 / Toagosei Co., Ltd.) The numerical value of "n" above is the number of polyalkylene glycols contained in the polymerizable monomer (e). <Photopolymerization initiator (d)> A: 2,2-dimethoxy-1,2-diphenylethan-1-one (Omnirad651 / IGM Resins) B: 1-hydroxycyclohexyl phenyl ketone (Omnirad184 / IGM Resins) C: Bis(2,4,6-trimethylbenzoyl)-phenylphosphineside (Omnirad819 / IGM Resins) D: 2,4,6-trimethylbenzoyl-diphenyl-phosphinoxide (Omnirad TPO / IGM Resins)

[0064] <Method for producing a solventless acrylic pressure-sensitive adhesive composition containing polymerizable monomer (f)> Table 3 shows the raw material blends of the solventless acrylic pressure-sensitive adhesive compositions of Examples 9 to 18 and Comparative Examples 7 to 12. Based on the raw material blend of Example 9 in Table 3, 0.40 parts of Karenz MOI (trade name, manufactured by Resonac) as the polymerizable monomer (f) and 1.0 part of Omnirad651 (trade name, manufactured by IGM Resins) as the photopolymerization initiator (d) were added to 100 parts of acrylic syrup A and mixed thoroughly to obtain the solventless acrylic pressure-sensitive adhesive composition of Example 9. The solventless acrylic pressure-sensitive adhesive compositions of Examples 10 to 18 and Comparative Examples 7 to 12 were obtained based on the raw material blends in Table 3 using the same process as in Example 9, and detailed explanations thereof will be omitted.

[0065] [Table 3] (remarks) <Polymerizable monomer (f)> A: 2-isocyanatoethyl methacrylate (Karenz MOI / Resonac) B: 2-isocyanatoethyl acrylate (Karenz AOI / Resonac Co., Ltd.) <Photopolymerization initiator (d)> A: 2,2-dimethoxy-1,2-diphenylethan-1-one (Omnirad651 / IGM Resins) B: 1-hydroxycyclohexyl phenyl ketone (Omnirad184 / IGM Resins) C: Bis(2,4,6-trimethylbenzoyl)-phenylphosphineside (Omnirad819 / IGM Resins) D: 2,4,6-trimethylbenzoyl-diphenyl-phosphinoxide (Omnirad TPO / IGM Resins)

[0066] <Method for producing a solventless acrylic pressure-sensitive adhesive composition containing polymerizable monomer (e) and polymerizable monomer (f)> Table 4 shows the raw material blends of the solventless acrylic pressure-sensitive adhesive compositions of Examples 19 to 27 and Comparative Examples 13 to 16. Based on the raw material blend of Example 19 in Table 4, 0.5 parts of NK Ester AT-20E (trade name, manufactured by Shin-Nakamura Chemical Co., Ltd.) as the polymerizable monomer (e), 0.08 parts of Karenz MOI (trade name, manufactured by Resonac Co., Ltd.) as the polymerizable monomer (f), and 0.3 parts of Omnirad 651 (trade name, manufactured by IGM Resins Co., Ltd.) as the photopolymerization initiator (d) were added to 100 parts of acrylic syrup B and thoroughly mixed to obtain the solventless acrylic pressure-sensitive adhesive composition of Example 19. The solventless acrylic pressure-sensitive adhesive compositions of Examples 20 to 27 and Comparative Examples 13 to 16 were obtained using the raw material blends in Table 4 and the same process as Example 19, and detailed explanations thereof will be omitted.

[0067] [Table 4] (remarks) The notes are the same as those in Tables 2 and 3 and are therefore omitted.

[0068] <Creating adhesive sheets for evaluation> The following evaluation pressure-sensitive adhesive sheets were prepared using the solvent-free acrylic pressure-sensitive adhesive compositions of Examples 1 to 27 and Comparative Examples 1 to 16. A 25 μm-thick pressure-sensitive adhesive layer was formed on a PVC film by the following method using the solvent-free acrylic pressure-sensitive adhesive composition of Examples 1 to 27 and Comparative Examples 1 to 16, to prepare a pressure-sensitive adhesive sheet. Each solvent-free acrylic pressure-sensitive adhesive composition was applied to a silicone resin-coated PET film (heavy release type releasable substrate), and then a PET film (light release type releasable substrate) was attached thereto. The composition was then cured by irradiating with ultraviolet light under the following ultraviolet curing conditions. After curing, the PET film (light release type releasable substrate) was peeled off, and the pressure-sensitive adhesive layer on the PET film (heavy release type releasable substrate) was transferred to a PVC film. The composition was then aged for 5 days in an environment of 23°C and 50% RH to obtain a pressure-sensitive adhesive sheet for evaluation. The ultraviolet curing conditions were a metal halide lamp with an illuminance of 172 mW / cm. 2 , cumulative light intensity 1550mJ / cm 2 The irradiation conditions at 1000 W or less were included. The illuminance and light intensity meter used was the "UV POWER PUCK" manufactured by EIT.

[0069] <Methods for evaluating the properties of adhesives and adhesive sheets> (Test 1: Measurement of adhesive strength) For each PSA sheet, the 180° peel adhesive strength to a test plate (melamine-coated plate) was measured in accordance with JIS Z0237 at 23°C and 50% RH. Specifically, the PSA sheet was cut to a width of 25 mm, attached to a melamine-coated plate, and pressed back and forth with a pressure roll under a 2 kg load to prepare a test piece for measuring adhesive strength to the melamine-coated plate. After being left for 48 hours at 23°C and 50% RH, the PSA sheet was peeled off the test piece at a peel speed of 300 mm / min, and the adhesive strength to the melamine-coated plate was measured. The adhesive strength of the PSA sheet was evaluated based on the following evaluation criteria. (Evaluation criteria) Pass: Adhesive strength is 13.0N / 25mm or more Fail: Adhesion strength is less than 13.0N / 25mm

[0070] (Test 2: Measurement of holding power) Each PSA sheet was cut to a size of 25 mm wide and 50 mm long to prepare test pieces for measuring holding power. This test piece was attached to a SUS plate so that the adhesive area was 25 mm x 25 mm, in accordance with JIS Z0237, and then pressed back and forth with a pressure roller under a 2 kg load. After 60 minutes at 23°C and 50% RH, the test piece was then left for a further 20 minutes at 40°C. A 1.0 kg weight was attached, including the weight attachment hook. After 24 hours, the distance the test piece shifted (shift length; mm) or the time it took for the test piece to completely peel off (fall time; minutes) was measured. The holding power of the PSA sheet was evaluated based on the following evaluation criteria. (Evaluation criteria) Pass: The deviation length is 0.5 mm or less Fail: The slippage exceeds 0.5 mm or the test piece falls within 24 hours of holding time.

[0071] (Test 3: Measurement of constant load peel strength (melamine coated board)) Each PSA sheet was cut to a size of 25 mm wide and 70 mm long to prepare test specimens for measuring constant-load peel strength. These test specimens were attached to melamine-coated panels with an adhesive area of ​​25 mm x 50 mm and pressed back and forth with a pressure roller under a 2 kg load. After leaving the test specimens for 60 minutes at 23°C and 50% RH, the pressed tape samples were placed in a dedicated jig, and a 0.20 kg weight, including a weight attachment hook, was attached to the edge of the tape sample to be evaluated so that a peel load was applied in a 90° direction relative to the adhesive surface. After 120 minutes, the peel distance (peel length; mm) of the test specimen or the time it took for the test specimen to completely peel off (drop time; minutes) was measured. The constant-load peel strength of the PSA sheets was evaluated based on the following evaluation criteria: (Evaluation criteria) Pass: Peel length is 40.0 mm or less Fail: Peel length exceeds 40.0 mm or the test piece falls within 120 minutes of holding time.

[0072] (Test 4: Measurement of constant load peel strength (Polypropylene (PP) plate)) Each PSA sheet was cut to a size of 25 mm wide and 70 mm long to prepare test specimens for measuring constant-load peel strength. These test specimens were attached to polypropylene (PP) plates with an adhesive area of ​​25 mm x 50 mm and pressed back and forth with a pressure roller under a 2 kg load. After leaving the test specimens in an environment of 23°C and 50% RH for 60 minutes, the pressed tape samples were placed in a dedicated jig at 40°C. A 0.05 kg weight, including a weight attachment hook, was attached to the edge of the tape sample to be evaluated so that a peel load was applied in a 90° direction relative to the adhesive surface. After 120 minutes, the peel distance (peel length; mm) of the test specimen or the time it took for the test specimen to completely peel off (drop time; minutes) was measured. The constant-load peel strength of the PSA sheets was evaluated based on the following evaluation criteria: (Evaluation criteria) Pass: Peel length is 40.0 mm or less Fail: Peel length exceeds 40.0 mm or the test piece falls within 120 minutes of holding time.

[0073] (Test 5: Evaluation of gasoline resistance) Each PSA sheet was cut to a size of 20mm x 50mm to prepare test pieces for gasoline resistance tests. Each test piece was attached to a melamine-coated plate and left for 24 hours at 23°C and 50% RH, then immersed in simulated gasoline (a 1:1 mixed solution of toluene and isooctane) for 30 minutes. After removal, the changes in the appearance of the samples attached to the melamine-coated plate after 24 hours were visually observed and evaluated according to the following criteria. ◯ indicates that there are no practical problems. △ and × indicate that the samples are not suitable for practical use. (Evaluation criteria) ○: No change, or wrinkles appear on the PVC film after removal, but it returns to its original state after 24 hours △: Wrinkles and slight peeling of the PVC film ×: The PVC film is deformed by wrinkles, swelling, shrinkage, etc., and the appearance is poor.

[0074] (Test 6: Evaluation of alkali resistance) Each PSA sheet was cut into a size of 20 mm x 50 mm to prepare test pieces for alkali resistance tests. Each test piece was attached to a melamine-coated plate and left for 24 hours at 23°C and 50% RH, then immersed in a 0.1 N aqueous sodium hydroxide solution for 7 hours. After removal, the sample attached to the melamine-coated plate was visually observed for changes in appearance after 24 hours and evaluated according to the following criteria. ○ indicates no practical problems, while △ and × indicate that the sample was not suitable for practical use. (Evaluation criteria) ○: No change, or the adhesive layer swells after removal, but returns to its original state after 24 hours △: The adhesive layer swells and does not return to its original state ×: The adhesive layer was changed by swelling, dissolution, etc.

[0075] (Test 7: Measurement of gel fraction) After UV curing, the adhesive film was cured for 5 days in an environment of 23°C and 50% RH. 0.1 g of the resulting adhesive film was accurately weighed (W1) and immersed in 50 ml of ethyl acetate for 1 day. The film was filtered using a weighed 200-mesh wire mesh (W2) to extract the soluble matter. The filtered 200-mesh wire mesh was then dried. The dried 200-mesh wire mesh was weighed (W3). The gel fraction (wt%) was calculated from these measurements using the following formula: Gel fraction (wt%) = ((W3 - W2) / W1) × 100 W1: Weight of adhesive coating W2: Weight of 200 mesh wire mesh W3: Weight of 200 mesh wire netting after drying

[0076] (Test results) Table 2 shows the test results for Examples 1 to 8 and Comparative Examples 1 to 6. Table 3 shows the test results for Examples 9 to 18 and Comparative Examples 7 to 12. Table 4 shows the test results for Examples 19 to 27 and Comparative Examples 13 to 16.

[0077] (Test results for Examples 1 to 27) According to Tables 2 to 4, Examples 1 to 27 have excellent properties in all of adhesive strength, holding strength, constant load peel strength, gasoline resistance, and alkali resistance.

[0078] (Test results for Comparative Examples 1 to 16) On the other hand, Comparative Examples 1 to 16 did not have excellent properties in any of adhesive strength, holding strength, constant-load peel strength, gasoline resistance, and alkali resistance. Hereinafter, adhesive strength, holding strength, constant-load peel strength, gasoline resistance, and alkali resistance will all be collectively referred to as "required performance."

[0079] (Consideration of Test Results of Comparative Examples 1 to 6) Comparative Example 1 did not satisfy the required performance because it contained a polymerizable monomer that did not contain alkylene glycol. Comparative Example 2 did not satisfy the required performance because it contained a polymerizable monomer (e) in an amount exceeding the blending amount specified in the present invention and a photopolymerization initiator (d) in an amount exceeding the blending amount specified in the present invention. Comparative Example 3 did not satisfy the required performance because it contained a polymerizable monomer (e) in an amount less than the blending amount specified in the present invention. Comparative Example 4 did not satisfy the required performance because it contained a carboxyl group-containing acrylic monomer (b) in an amount greater than the blending amount specified in the present invention. Comparative Example 5 did not satisfy the required performance because it contained a carboxyl group-containing acrylic monomer (b) in an amount less than the blending amount specified in the present invention and the gel fraction was below the range specified in the present invention. Comparative Example 6 did not satisfy the required performance because the gel fraction was below the range specified in the present invention.

[0080] (Consideration of Test Results of Comparative Examples 7 to 12) Comparative Example 7 does not satisfy the required performance because it contains less than the blending amount of polymerizable monomer (f) specified in the present invention. Comparative Example 8 does not satisfy the required performance because it contains more than the blending amount of polymerizable monomer (f) specified in the present invention. Comparative Example 9 does not satisfy the required performance because the gel fraction is below the range specified in the present invention. Comparative Example 10 does not satisfy the required performance because it contains less than the blending amount of carboxyl group-containing acrylic monomer (b) specified in the present invention. Comparative Example 11 does not satisfy the required performance because it contains more than the blending amount of carboxyl group-containing acrylic monomer (b) specified in the present invention. Comparative Example 12 does not satisfy the required performance because it contains more than the blending amount of hydroxyl group-containing acrylic monomer (c) specified in the present invention.

[0081] (Consideration of Test Results of Comparative Examples 13 to 16) Comparative Example 13 does not satisfy the required performance because it contains a polymerizable monomer (e) that does not have alkylene glycol. Comparative Example 14 does not satisfy the required performance because it contains a polymerizable monomer (e) in an amount less than the amount specified in the present invention. Comparative Example 15 does not satisfy the required performance because it contains a polymerizable monomer (e) in an amount exceeding the amount specified in the present invention. Comparative Example 16 does not satisfy the required performance because it contains a polymerizable monomer (f) and a photopolymerization initiator (d) in an amount exceeding the amount specified in the present invention.

[0082] As described above, the solventless acrylic pressure-sensitive adhesive composition of the present invention has remarkable effects such as excellent adhesive strength, holding power, constant-load peel strength, gasoline resistance, and alkali resistance.

[0083] Furthermore, the present invention can provide a solvent-free acrylic pressure-sensitive adhesive composition that is environmentally friendly and highly safe for the human body, which can contribute to the achievement of Goal 12 of the United Nations Sustainable Development Goals (SDGs), "Responsible Consumption and Production."

[0084] The invention is not limited to the above-described embodiment, and various modifications and variations are possible within the scope of the gist of the invention.

Claims

1. A solvent-free acrylic pressure-sensitive adhesive composition, an acrylic partial copolymer containing 85 to 93 parts by mass of a (meth)acrylic acid ester (a) having an alkyl group with 4 to 8 carbon atoms and 7 to 15 parts by mass of a carboxyl group-containing acrylic monomer (b); The acrylic partial copolymer contains at least one of: 0.10 to 1.5 parts by mass of a polymerizable monomer (e) having two or more alkylene glycols and three or more (meth)acryloyl groups in one molecule; and 0.03 to 0.40 parts by mass of a polymerizable monomer (f) having one or more functional groups reactive with active hydrogen groups and one or more (meth)acryloyl groups in one molecule, relative to 100 parts by mass of the acrylic partial copolymer; the pressure-sensitive adhesive layer comprising the solvent-free acrylic pressure-sensitive adhesive composition has a gel fraction of 37 to 75%. A solvent-free acrylic pressure-sensitive adhesive composition.

2. The acrylic partial copolymer further contains a hydroxyl group-containing acrylic monomer (c), the content of the hydroxyl group-containing acrylic monomer (c) is 7 parts by mass or less; The solvent-free acrylic pressure-sensitive adhesive composition according to claim 1 .

3. the functional group reactive with one or more active hydrogen groups in one molecule is an isocyanate group; The solvent-free acrylic pressure-sensitive adhesive composition according to claim 1 .

4. Further comprising a photopolymerization initiator (d), The photopolymerization initiator (d) is contained in an amount of 0.3 to 1.0 parts by mass relative to 100 parts by mass of the acrylic partial copolymer. The solvent-free acrylic pressure-sensitive adhesive composition according to claim 1 .

5. The hydroxyl group-containing acrylic monomer (c) includes 4-hydroxybutyl acrylate. The solvent-free acrylic pressure-sensitive adhesive composition according to claim 2 .

6. The (meth)acrylic acid ester (a) contains at least one of butyl acrylate and 2-ethylhexyl acrylate, The carboxyl group-containing acrylic monomer (b) contains acrylic acid. The solvent-free acrylic pressure-sensitive adhesive composition according to claim 1 .

7. The polymerizable monomer (f) includes at least one of 2-isocyanatoethyl acrylate and 2-isocyanatoethyl methacrylate. The solvent-free acrylic pressure-sensitive adhesive composition according to claim 1 .

8. For marking film, The solvent-free acrylic pressure-sensitive adhesive composition according to claim 1 .

9. A substrate; a pressure-sensitive adhesive layer formed on at least one surface of the substrate and comprising the solventless acrylic pressure-sensitive adhesive composition according to any one of claims 1 to 8, Adhesive sheet.

10. The pressure-sensitive adhesive layer is formed by curing the solvent-free acrylic pressure-sensitive adhesive composition using active energy rays. The pressure-sensitive adhesive sheet according to claim 9.

11. The active energy rays include ultraviolet rays. The pressure-sensitive adhesive sheet according to claim 10.

Images