Adhesive composition and protective sheet

Abstract

Provided is an adhesive composition that reduces the generation of cutting scraps and adhesive layer damage during cutting, and with which an adhesive layer having suitable peel strength and strength can be obtained. The adhesive composition according to the present invention contains a polyurethane (A), an ethylenically unsaturated group-containing monomer (B), and a photopolymerization initiator (C). The polyurethane (A) has a skeleton including a polyol-derived structure and a polyisocyanate structure, and has an ethylenically unsaturated group at a terminal end. The weight-average molecular weight of the polyurethane (A) in the adhesive composition is 3000-30,000.

Description

Pressure-sensitive adhesive composition and protective sheet

[0001] The present invention relates to a pressure-sensitive adhesive composition and a protective sheet. This application claims priority to Japanese Patent Application No. 2022-110520, filed on July 8, 2022, the contents of which are incorporated herein by reference.

[0002] Various optical films are used in optical components such as liquid crystal displays and touch panels of smartphones, personal computers, televisions, etc. A protective sheet (surface protective sheet) is generally laminated onto the surface of these optical films to prevent contamination and scratches during transportation, manufacturing, and inspection processes. This protective sheet is peeled off in a subsequent process such as an assembly process. Various urethane adhesives have been proposed as adhesives for such protective sheets.

[0003] For example, Patent Document 1 discloses an ultraviolet-curable pressure-sensitive adhesive composition for a protective film, which contains a polyurethane (A), an alkyl mono(meth)acrylate (B) having an alkyl group with 6 to 18 carbon atoms, a polyfunctional (meth)acrylate (C), and a plasticizer (D). The polyurethane (A) is a reaction product of a polyol (a1), a polyisocyanate (a2), a hydroxyl group-containing (meth)acrylate (a3), and a hydroxyl group-containing photopolymerization initiator (a4).

[0004] Japanese Patent Application Laid-Open No. 2020-164840

[0005] These protective sheets may be cut into a desired shape using a cutter or the like before or after application to an adherend. At this time, cutting debris from the adhesive layer may be generated on the cross section of the protective sheet, which may lead to contamination of the adherend surface. Furthermore, when the protective sheet is repeatedly laminated or peeled or subjected to external impact, the adhesive layer may be damaged, resulting in contamination of the adherend, or the adhesive layer may be insufficiently strong, causing the impact to be transmitted to the adherend, resulting in damage. In recent years, with the increasing size of display screens and the thinning of panel components, etc., optical components themselves have become more susceptible to cracking. Therefore, protective sheets are required to have appropriate adhesiveness (peel strength) while also being able to be peeled off with less force (light releasability).

[0006] The present invention has been made to solve the above-mentioned problems, and an object of the present invention is to provide a pressure-sensitive adhesive composition that reduces the generation of cutting debris and damage to the pressure-sensitive adhesive layer when cut, and that can provide a pressure-sensitive adhesive layer with appropriate peel strength and strength. Another object of the present invention is to provide a protective sheet that reduces the generation of cutting debris and damage to the pressure-sensitive adhesive layer when cut, and that has an appropriate peel strength and strength.

[0007] The present invention includes the following aspects. [1] A pressure-sensitive adhesive composition comprising a polyurethane (A), an ethylenically unsaturated group-containing monomer (B), and a photopolymerization initiator (C), wherein the polyurethane (A) is a polyurethane having a skeleton containing a polyol-derived structure and a polyisocyanate-derived structure, and has an ethylenically unsaturated group at a terminal, and wherein the weight-average molecular weight of the polyurethane (A) is 3,000 to 30,000. [2] The pressure-sensitive adhesive composition according to [1], wherein the polyol is a polyoxyalkylene polyol having a number-average molecular weight of 300 to 1,800. [3] The pressure-sensitive adhesive composition according to [1] or [2], wherein the ethylenically unsaturated group equivalent of the polyurethane (A) is 900 to 3,000 g / mol. [4] The pressure-sensitive adhesive composition according to any of [1] to [3], wherein the concentration of urethane bonds in the polyurethane (A) is 1.0 to 5 mol / kg. [5] The pressure-sensitive adhesive composition according to any one of [1] to [4], wherein the polyurethane (A) has an ethylenically unsaturated group at a site other than the terminal. [6] The pressure-sensitive adhesive composition according to any one of [1] to [5], wherein the polyisocyanate contains an ethylenically unsaturated group-containing polyisocyanate and an ethylenically unsaturated group-free polyisocyanate. [7] The pressure-sensitive adhesive composition according to any one of [1] to [6], wherein the ethylenically unsaturated group-containing monomer (B) contains a monofunctional (meth)acrylate (B1), and the monofunctional (meth)acrylate (B1) is a chain alkyl (meth)acrylate having 6 to 18 carbon atoms. [8] The pressure-sensitive adhesive composition according to any one of [1] to [7], wherein the ethylenically unsaturated group-containing monomer (B) contains a monofunctional (meth)acrylate (B1) and a polyfunctional (meth)acrylate (B2). [9] The pressure-sensitive adhesive composition according to [8], which contains 5 to 89 mass% of a monofunctional (meth)acrylate (B1) and 0.1 to 30 mass% of a polyfunctional (meth)acrylate (B2) relative to a total of 100 mass% of the polyurethane (A) and the ethylenically unsaturated group-containing monomer (B).

[10] The pressure-sensitive adhesive composition according to claim 1, comprising 10 to 90% by mass of polyurethane (A) and 10 to 90% by mass of ethylenically unsaturated group-containing monomer (B), relative to a total of 100% by mass of the polyurethane (A) and the ethylenically unsaturated group-containing monomer (B).

[11] The pressure-sensitive adhesive composition according to any one of [1] to [9], further comprising a plasticizer (D).

[12] The pressure-sensitive adhesive composition according to

[11] , comprising 0.05 to 5 parts by mass of photopolymerization initiator (C) and 1 to 30 parts by mass of plasticizer (D), relative to a total of 100 parts by mass of the polyurethane (A) and the ethylenically unsaturated group-containing monomer (B).

[13] A cured product of the pressure-sensitive adhesive composition according to [1] or

[11] .

[14] An elongation at break of 92 to 300% and a breaking strength of 300 to 800 g / mm. ２

[15] A protective sheet having a substrate and a pressure-sensitive adhesive layer formed on one surface of the substrate, the pressure-sensitive adhesive layer comprising the cured product according to

[13] .

[0008] According to the present invention, it is possible to provide a pressure-sensitive adhesive composition that reduces the generation of cutting debris and damage to the pressure-sensitive adhesive layer when cut, and that allows a pressure-sensitive adhesive layer to be obtained that has appropriate peel strength and strength. It is also possible to provide a protective sheet that reduces the generation of cutting debris and damage to the pressure-sensitive adhesive layer when cut, and that has an appropriate peel strength and strength.

[0009] 1 is a schematic diagram showing the dimensions of the test piece used in the tensile test of the examples. It is a cross-sectional micrograph of a pressure-sensitive adhesive sheet obtained in an evaluation test for cutting chips and cracks in the adhesive layer for the protective sheet of Example 1 (using A4300) (upper layer: adhesive layer, lower layer: substrate). It is a cross-sectional micrograph of a pressure-sensitive adhesive sheet obtained in an evaluation test for cutting chips and cracks in the adhesive layer for the protective sheet of Comparative Example 1 (using A4300) (upper layer: adhesive layer, lower layer: substrate). It is a cross-sectional micrograph of a pressure-sensitive adhesive sheet obtained in an evaluation test for cutting chips and cracks in the adhesive layer for the protective sheet of Comparative Example 4 (using A4300) (upper layer: adhesive layer, lower layer: substrate).

[0010] Hereinafter, embodiments of the present invention will be described in detail. However, the present invention is not limited to the embodiments shown below. Herein, the (meth)acryloyl group is a group represented by the chemical formula CH２ =CH-CO- and the group represented by the chemical formula CH ２ =C(CH ３ The (meth)acryloyloxy group refers to one or more functional groups selected from the group consisting of those represented by the chemical formula CH ２ =CH-CO-O-, and a group represented by the chemical formula CH ２ =C(CH ３ ) —CO—O—.

[0011] <Pressure-Sensitive Adhesive Composition> The pressure-sensitive adhesive composition of the present embodiment contains polyurethane (A), an ethylenically unsaturated group-containing monomer (B), and a photopolymerization initiator (C). If necessary, a plasticizer (D) may also be contained. By using the pressure-sensitive adhesive composition of the present embodiment as the pressure-sensitive adhesive layer of a protective sheet, the generation of cutting debris and damage to the pressure-sensitive adhesive layer during cutting can be reduced. The generation of cutting debris and damage to the pressure-sensitive adhesive layer is thought to be a complex combination of multiple factors. Examples of factors include the type and thickness of the substrate and pressure-sensitive adhesive layer, the type and conditions of the process during use of the protective sheet, the type of adherend, and the type of external impact, as well as various other external factors. Among these factors, the use of a combination of components that significantly reduces the generation of cutting debris and damage to the pressure-sensitive adhesive layer can effectively reduce cutting debris and damage to the pressure-sensitive adhesive layer.

[0012] [Polyurethane (A)] The polyurethane (A) used in this embodiment is a polyurethane having a skeleton containing a polyol-derived structure and a polyisocyanate-derived structure, and has an ethylenically unsaturated group at its terminal. The "polyol-derived structure" refers to a structure introduced by using a polyol, which is a compound having two or more hydroxy groups, as a polyurethane raw material. The "polyisocyanate-derived structure" refers to a structure introduced by using a polyisocyanate, which is a compound having two or more isocyanato groups, as a polyurethane raw material. Methods for introducing an ethylenically unsaturated group at the terminal include adding a hydroxyl-containing ethylenically unsaturated compound to an isocyanato-terminated urethane prepolymer and adding an isocyanato-containing ethylenically unsaturated compound to a hydroxyl-terminated urethane prepolymer. The polyurethane (A) may also have an ethylenically unsaturated group at a site other than the terminal, as necessary, from the viewpoint of reducing the peel force when peeling the protective sheet from the adherend. Examples of a method for introducing an ethylenically unsaturated group into a site other than the terminal include a method using an ethylenically unsaturated group-containing polyol or an ethylenically unsaturated group-containing polyisocyanate as a raw material for synthesizing the polyurethane (A).

[0013] "Polyol" The polyol used in this embodiment is a compound having no isocyanato groups and two or more hydroxy groups. The number of hydroxy groups in the polyol is preferably two, from the viewpoints of suppressing gelation during synthesis and enabling uniform elongation of the polymer.

[0014] A preferred polyol is a polyoxyalkylene polyol having a polyoxyalkylene chain, from the viewpoint of improving the elongation of the pressure-sensitive adhesive layer and more effectively suppressing the generation of cutting debris during cutting. In particular, when the substrate constituting the protective sheet is hard or thick, the pressure-sensitive adhesive layer is prone to generate cutting debris or breakage during cutting or when subjected to external impact due to its brittleness. The alkylene chain constituting the polyoxyalkylene chain preferably has 2 to 8 carbon atoms, more preferably 2 to 6 carbon atoms, and even more preferably 2 to 4 carbon atoms. Specific examples include polyoxyethylene polyol, polyoxypropylene polyol, polyoxybutylene polyol, etc. Among polyoxyalkylene polyols, polyoxyalkylene glycols are preferably used, with polypropylene glycol and polytetramethylene glycol being particularly preferred, and polypropylene glycol being even more preferred.

[0015] The polyoxyalkylene polyol may contain two or more types of alkylene chains. The polyurethane (A) may have a structure in which structures derived from two or more different polyoxyalkylene polyols are bonded to each other with a polyisocyanate sandwiched therebetween.

[0016] The number-average molecular weight of the polyoxyalkylene polyol is preferably 300 to 1800, more preferably 330 to 1500, and even more preferably 360 to 1200. When the number-average molecular weight of the polyoxyalkylene polyol is 300 or more, the peel force required to peel the protective sheet from the adherend can be reduced. When the number-average molecular weight of the polyoxyalkylene polyol is 1800 or less, the weight-average molecular weight of the polyurethane (A) can be reduced, resulting in a pressure-sensitive adhesive layer with good hardness. This reduces the generation of cutting debris and breakage when the protective sheet is subjected to external impact or when subjected to forces applied during cutting, application, and peeling operations.

[0017] The hydroxyl value of the polyoxyalkylene polyol is preferably 30 to 400 mgKOH / g, more preferably 50 to 350 mgKOH / g, and even more preferably 80 to 300 mgKOH / g. A hydroxyl value of 30 mgKOH / g or more has the advantage of reducing the generation of cutting debris during cutting. A hydroxyl value of 300 mgKOH / g or less has the advantage of obtaining a cured product with appropriate adhesive strength. The hydroxyl value is measured by a method conforming to Method B of JIS K1557-1.

[0018] Another preferred example of the polyol is an ethylenically unsaturated group-containing polyol, from the viewpoint of reducing the peel strength when peeling the protective sheet from the adherend. In particular, it is preferable to use an ethylenically unsaturated group-containing polyol as the polyol under any of the following conditions. The above conditions are conditions where a lower peel strength is required from the viewpoint of the strength of the adherend, such as miniaturization or thinning of the adherend, or conditions where cutting debris or breakage is likely to occur due to high peel strength and insufficient crosslinking or insufficient cohesive strength of the pressure-sensitive adhesive layer. Examples of ethylenically unsaturated groups contained in the ethylenically unsaturated group-containing polyol include (meth)acryloyloxy groups, vinyl groups, and allyl groups. From the viewpoint of reducing peel strength, (meth)acryloyloxy groups are preferred. By using an ethylenically unsaturated group-containing polyol, ethylenically unsaturated groups can be introduced into positions (within the molecular chain) other than the terminals of the polyurethane (A). This reduces the peel strength of the pressure-sensitive adhesive layer, particularly during high-speed peeling. Furthermore, improved crosslink density and cohesive strength of the pressure-sensitive adhesive layer can suppress cutting debris and breakage.

[0019] (Meth)acryloyloxy group-containing polyols can be obtained, for example, by synthesizing a compound having two hydroxy groups by subjecting the epoxy group of a (meth)acryloyloxy group-containing epoxy compound to an acid-catalyzed cleavage reaction. Examples of (meth)acryloyloxy group-containing epoxy compounds include glycidyl (meth)acrylate, 3,4-epoxycyclohexylmethyl (meth)acrylate, (meth)acrylates having alicyclic epoxy groups and lactone adducts thereof, 3,4-epoxycyclohexylmethyl-3',4'-epoxycyclohexanecarboxylate, epoxidized products of dicyclopentenyl (meth)acrylate, and epoxidized products of dicyclopentenyloxyethyl (meth)acrylate. Furthermore, examples of (meth)acryloyloxy group-containing polyols include compounds obtained by adding a carboxy group-containing ethylenically unsaturated compound such as (meth)acrylic acid to a polyepoxy compound such as 1,6-hexanediol diglycidyl ether.

[0020] The (meth)acryloyloxy group-containing polyol may be a commercially available product, such as glycerin monomethacrylate (Blemmer (registered trademark) GLM, manufactured by NOF Corp.) obtained by cleaving the epoxy group of glycidyl methacrylate, or an acrylic acid adduct of 1,6-hexanediol diglycidyl ether (Lipoxy (trademark) SP-16LDA, manufactured by Showa Denko K.K.).

[0021] These polyols may be used alone or in combination of two or more. From the viewpoint of balancing multiple performances, such as suppressing the generation of cutting debris during cutting and reducing peeling force, it is preferable to use an ethylenically unsaturated group-containing polyol and a polyoxyalkylene polyol in combination. The ratio (molar ratio) of the ethylenically unsaturated group-containing polyol to the polyoxyalkylene polyol is preferably 0.03 to 0.8, more preferably 0.08 to 0.7, even more preferably 0.1 to 0.6, and particularly preferably 0.1 to 0.4.

[0022] "Polyisocyanate" The polyisocyanate used in the present embodiment is not particularly limited as long as it is a compound having no hydroxy groups and two or more isocyanato groups. The number of isocyanato groups in the polyisocyanate is preferably two, from the viewpoints of suppressing gelation during synthesis and enabling the polymer to be uniformly elongated.

[0023] Examples of polyisocyanates include ethylenically unsaturated group-free polyisocyanates such as tolylene diisocyanate and hydrogenated products thereof, xylylene diisocyanate and hydrogenated products thereof, diphenylmethane diisocyanate and hydrogenated products thereof, 1,5-naphthylene diisocyanate and hydrogenated products thereof, hexamethylene diisocyanate, trimethylhexamethylene diisocyanate, tetramethylxylylene diisocyanate, isophorone diisocyanate, 4,4'-dicyclohexyl diisocyanate, 1,3-bis(isocyanatomethyl)cyclohexane, and norbornane diisocyanate. Among these, from the viewpoint of easy reaction control during synthesis of the polyurethane (A) and imparting rigidity to the PSA, polyisocyanates having a cyclic hydrocarbon structure are preferred, and one or more selected from isophorone diisocyanate, 4,4'-dicyclohexyl diisocyanate, 1,3-bis(isocyanatomethyl)cyclohexane, and norbornane diisocyanate are more preferred, with isophorone diisocyanate being even more preferred.

[0024] The polyisocyanate preferably contains an ethylenically unsaturated group-containing polyisocyanate from the viewpoint of reducing the peel strength when peeling the protective sheet from the adherend. In particular, it is preferable to contain an ethylenically unsaturated group-containing polyol as the polyol under any of the following conditions. These conditions include conditions where low peel strength is required from the viewpoint of adherend strength, such as miniaturization or thinning of the adherend, and conditions where high peel strength, insufficient crosslinking of the pressure-sensitive adhesive layer, insufficient cohesive strength, etc., lead to the generation of cutting debris or breakage. Examples of ethylenically unsaturated groups contained in the ethylenically unsaturated group-containing polyisocyanate include (meth)acryloyloxy groups, vinyl groups, allyl groups, etc., and (meth)acryloyloxy groups are preferred from the viewpoint of reducing peel strength. By using an ethylenically unsaturated group-containing polyisocyanate, ethylenically unsaturated groups can be introduced into positions (in the molecular chain) other than the terminals of the polyurethane (A). This reduces the peel strength of the pressure-sensitive adhesive layer, particularly during high-speed peeling. Furthermore, by improving the crosslink density and cohesive strength of the pressure-sensitive adhesive layer, cutting debris and breakage can be suppressed.

[0025] Examples of (meth)acryloyloxy group-containing polyisocyanates include reaction products of hydroxy group-containing (meth)acrylates and polyisocyanates. They can be obtained by synthesizing a compound having an allophanate bond by carrying out urethanization and allophanatization according to the methods described in JP-A 2002-533542 (JP-T) and JP-A 2012-111851 (JP-A). Specific examples of hydroxy group-containing (meth)acrylates include those similar to the hydroxy group-containing (meth)acrylates exemplified as the hydroxy group-containing ethylenically unsaturated compound that is the raw material for polyurethane (A) described below. Examples of polyisocyanates include those similar to the aforementioned compounds. Among these, allophanate bond-containing compounds obtained by reacting a hydroxy group-containing (meth)acrylate with a diisocyanate in excess of the diisocyanate are preferred. Allophanate bond-containing compounds obtained by reacting a hydroxyalkyl (meth)acrylate with an alkylene diisocyanate in excess of the alkylene diisocyanate are more preferred. Further, a reaction product obtained by allophanating one or more selected from 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, and 2-hydroxybutyl (meth)acrylate with one or more selected from hexamethylene diisocyanate and trimethylhexamethylene diisocyanate is more preferred.

[0026] The (meth)acryloyloxy group-containing polyisocyanate may be a commercially available product, such as Laromer (registered trademark) PR9000 manufactured by BASF, which is a reaction product of 2-hydroxyethyl acrylate and hexamethylene diisocyanate and contains an allophanate bond.

[0027] These polyisocyanates may be used alone or in combination of two or more. From the viewpoint of balancing multiple performances, such as suppressing the generation of cutting debris during cutting and reducing peeling force, it is preferable to use an ethylenically unsaturated group-containing polyisocyanate in combination with an ethylenically unsaturated group-free polyisocyanate. The ratio (molar ratio) of the ethylenically unsaturated group-containing polyisocyanate to the ethylenically unsaturated group-free polyisocyanate is preferably 0.03 to 0.8, more preferably 0.08 to 0.7, even more preferably 0.1 to 0.6, and particularly preferably 0.1 to 0.4.

[0028] When an isocyanato group-terminated urethane prepolymer is obtained, the total amount of isocyanato groups in the polyisocyanate is preferably 1.05 to 1.8 mol, more preferably 1.1 to 1.7, and even more preferably 1.15 to 1.6, per mol of the total amount of hydroxy groups in the polyol. When the total amount of isocyanato groups is 1.05 mol or more, the weight average molecular weight of the polyurethane (A) falls within the appropriate range.

[0029] When a hydroxy-terminated urethane prepolymer is obtained, the total amount of hydroxy groups in the polyol is preferably 1.05 to 1.8 mol, more preferably 1.1 to 1.7, and even more preferably 1.15 to 1.6, per mol of the total amount of isocyanato groups in the polyisocyanate. When the total amount of hydroxy groups is 1.05 mol or more, the weight average molecular weight of the polyurethane (A) falls within the appropriate range.

[0030] [Hydroxy group-containing ethylenically unsaturated compound] The hydroxy group-containing ethylenically unsaturated compound is not particularly limited as long as it does not have an isocyanato group and has a hydroxy group and an ethylenically unsaturated group. From the viewpoint of curability, the ethylenically unsaturated group is preferably at least one selected from the group consisting of a vinyl group, an allyl group, and a (meth)acryloyloxy group, and is preferably a (meth)acryloyloxy group.

[0031] "Hydroxy Group-Containing (Meth)acrylate" The hydroxy group-containing ethylenically unsaturated compound of this embodiment is preferably a hydroxy group-containing (meth)acrylate. The hydroxy group-containing (meth)acrylate is not particularly limited as long as it is a compound that does not have an isocyanato group and has a hydroxy group and a (meth)acryloyloxy group. Examples include hydroxyalkyl (meth)acrylates such as 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, and 4-hydroxybutyl (meth)acrylate; and monools having a (meth)acryloyl group derived from various polyols such as 1,3-butanediol mono(meth)acrylate, 1,6-hexanediol mono(meth)acrylate, and 3-methylpentanediol mono(meth)acrylate. From the viewpoints of reactivity with the isocyanato group of polyisocyanate and photocurability as a pressure-sensitive adhesive composition, hydroxyalkyl (meth)acrylates are preferred, hydroxyalkyl (meth)acrylates having an alkyl group with 2 to 6 carbon atoms are more preferred, and 2-hydroxyethyl (meth)acrylate and 4-hydroxybutyl (meth)acrylate are more preferred. These may be used alone or in combination of two or more.

[0032] "Compound Having a Hydroxy Group and a Vinyl Group" The hydroxyl group-containing ethylenically unsaturated compound of this embodiment may be a compound having a hydroxyl group and a vinyl group without having an isocyanato group. Examples of the compound having a hydroxyl group and a vinyl group include polyalkylene glycol monovinyl ether, hydroxyalkyl vinyl ether, hydroxycarboxylic acid vinyl ester, and hydroxyalkyl vinyl ester.

[0033] Examples of polyalkylene glycol monovinyl ethers include diethylene glycol monovinyl ether and triethylene glycol monovinyl ether. Examples of hydroxyalkyl vinyl ethers include 4-hydroxybutyl vinyl ether and 4-hydroxycyclohexyl vinyl ether. Examples of hydroxycarboxylic acid vinyl esters include vinyl hydroxyacetate, vinyl hydroxypropanoate, vinyl hydroxybutanoate, vinyl hydroxyhexanoate, and 4-hydroxycyclohexyl vinyl acetate. Examples of hydroxyalkyl vinyl esters include vinyl hydroxycyclohexylcarboxylate.

[0034] "Compound Having a Hydroxy Group and an Allyl Group" The hydroxyl group-containing ethylenically unsaturated compound of this embodiment may be a compound having a hydroxyl group and an allyl group without having an isocyanato group. Examples of the compound having a hydroxyl group and an allyl group include hydroxyalkyl allyl ethers, hydroxycarboxylic acid allyl esters, and hydroxyalkyl allyl esters.

[0035] Examples of hydroxyalkyl allyl ethers include 2-hydroxyethyl allyl ether, 4-hydroxybutyl allyl ether, glycerol monoallyl ether, 4-hydroxycyclohexyl allyl ether, etc. Examples of hydroxycarboxylic acid allyl esters include allyl hydroxyacetate, allyl hydroxypropanoate, allyl hydroxybutanoate, allyl hydroxyhexanoate, 4-hydroxycyclohexyl allyl acetate, etc. Examples of hydroxyalkyl allyl esters include hydroxyethyl allyl ester, hydroxypropyl allyl ester, hydroxybutyl allyl ester, hydroxyisobutyl allyl ester, hydroxycyclohexyl allyl ester, etc.

[0036] The ratio of the hydroxy group-containing ethylenically unsaturated compound is preferably set so that the total amount of hydroxy groups in the polyol and the hydroxy group-containing ethylenically unsaturated compound is equivalent to the total amount of isocyanato groups in the polyisocyanate. "Equivalent" means that the difference in the number of moles between the two is preferably 0.05 moles or less, more preferably 0.01 moles or less. If the difference in the number of moles between the two is 0.05 moles or less, the amount of unreacted monomer can be sufficiently reduced, and the amount of unreacted hydroxy groups and isocyanato groups can also be sufficiently reduced, so that even when the cured product is obtained, the change in gel fraction over time can be suppressed.

[0037] [Isocyanato Group-Containing Ethylenically Unsaturated Compound] The isocyanato group-containing ethylenically unsaturated compound is not particularly limited as long as it does not have a hydroxy group and has an isocyanato group and an ethylenically unsaturated group. The isocyanato group-containing ethylenically unsaturated compound is preferably an isocyanato group-containing (meth)acrylate. Examples include isocyanatoalkyl (meth)acrylates such as 2-isocyanatoethyl (meth)acrylate, 2-isocyanatopropyl (meth)acrylate, 4-isocyanatobutyl (meth)acrylate, and 6-isocyanatohexyl (meth)acrylate. From the viewpoints of reactivity with the hydroxy group of the polyoxyalkylene polyol (a1) and photocurability as a pressure-sensitive adhesive composition, an isocyanatoalkyl (meth)acrylate having an alkyl group having 2 to 6 carbon atoms is preferred, and 2-isocyanatoethyl (meth)acrylate and 4-isocyanatobutyl (meth)acrylate are more preferred. These may be used alone or in combination of two or more.

[0038] The proportion of the isocyanato group-containing ethylenically unsaturated compound is preferably set so that the total amount of isocyanato groups in the polyisocyanate and the isocyanato groups in the isocyanato group-containing ethylenically unsaturated compound is equivalent to the total amount of hydroxy groups in the polyol. "Equivalent" means that the difference in the number of moles between the two is preferably 0.05 moles or less, more preferably 0.01 moles or less. If the difference in the number of moles between the two is 0.05 moles or less, the amount of unreacted monomer can be sufficiently reduced, and the amount of unreacted hydroxy groups and isocyanato groups can also be sufficiently reduced, so that even when the cured product is obtained, the change in gel fraction over time can be suppressed.

[0039] The weight-average molecular weight of the polyurethane (A) is 3,000 to 30,000, preferably 5,000 to 28,000, and more preferably 6,000 to 25,000. When the weight-average molecular weight is 3,000 or more, the cured product of the pressure-sensitive adhesive composition has sufficient flexibility, and a protective sheet having this cured product as a pressure-sensitive adhesive layer has sufficient lamination properties. When the weight-average molecular weight is 30,000 or less, the pressure-sensitive adhesive layer has appropriate hardness, suppressing the generation of cutting debris during cutting and reducing damage to the pressure-sensitive adhesive layer and damage and contamination of the adherend upon application of external impact. The weight-average molecular weight of the polyurethane (A) may be in the range of 3,000 to less than 10,000. In this case, the proportion of polyurethane (A) in the pressure-sensitive adhesive composition can be increased while maintaining the coatability of the pressure-sensitive adhesive composition, resulting in relatively high coating surface strength.

[0040] The urethane bond concentration of polyurethane (A) is preferably 1.0 to 5 mol / kg, more preferably 1.5 to 5 mol / kg, and even more preferably 1.85 to 3.5 mol / kg. When the urethane bond concentration is 1.0 mol / kg or higher, the resulting PSA layer has a moderate elongation percentage and good hardness and tensile strength. When the urethane bond concentration is 5 mol / kg or lower, the protective sheet has good low peelability, preventing damage to the adherend when peeled from the adherend.

[0041] The ethylenically unsaturated group equivalent of the polyurethane (A) is preferably 900 to 3000 g / mol, more preferably 950 to 2700 g / mol, even more preferably 1000 to 2500 g / mol, and even more preferably 1100 to 2300 g / mol. When the ethylenically unsaturated group equivalent is 900 g / mol or more, the flexibility and elongation of the pressure-sensitive adhesive layer are good. When the ethylenically unsaturated group equivalent is 3000 g / mol or less, the hardness and tensile strength of the pressure-sensitive adhesive layer are good.

[0042] The lower limit of the ethylenically unsaturated group equivalent of polyurethane (A) is preferably 900 g / mol or more, more preferably 950 g / mol or more, even more preferably 1000 g / mol or more, and even more preferably 1100 g / mol or more. When the ethylenically unsaturated group equivalent is 900 g / mol or more, the flexibility and elongation of the pressure-sensitive adhesive layer are good. The upper limit of the ethylenically unsaturated group equivalent of polyurethane (A) is preferably 3000 g / mol or less, more preferably 2700 g / mol or less, even more preferably 2500 g / mol or less, and even more preferably 2300 g / mol. When the ethylenically unsaturated group equivalent is 3000 g / mol or less, the hardness and tensile strength of the pressure-sensitive adhesive layer are good. The above upper limit and lower limit can be combined arbitrarily.

[0043] The content of polyurethane (A) is preferably 10 to 90% by mass, more preferably 25 to 80% by mass, even more preferably 35 to 75% by mass, and even more preferably 40 to 70% by mass, relative to 100% by mass of the total of polyurethane (A) and ethylenically unsaturated group-containing monomer (B). When the content of polyurethane (A) is 10% by mass or more, the elongation and tensile strength of the pressure-sensitive adhesive layer are good. When the content of polyurethane (A) is 90% by mass or less, the low peelability of the protective sheet is good.

[0044] The lower limit of the content of polyurethane (A) is preferably 10% by mass or more, more preferably 25% by mass or more, even more preferably 35% by mass or more, and even more preferably 40% by mass or more, relative to 100% by mass of the total of polyurethane (A) and ethylenically unsaturated group-containing monomer (B). When the content of polyurethane (A) is 10% by mass or more, the elongation and tensile strength of the pressure-sensitive adhesive layer are good. The upper limit of the content of polyurethane (A) is preferably 90% by mass or less, more preferably 80% by mass or less, even more preferably 75% by mass or less, and even more preferably 70% by mass or less, relative to 100% by mass of the total of polyurethane (A) and ethylenically unsaturated group-containing monomer (B). When the content of polyurethane (A) is 90% by mass or less, the protective sheet has good low peelability. The above upper and lower limits can be combined arbitrarily.

[0045] [Ethylenically Unsaturated Group-Containing Monomer (B)] The ethylenically unsaturated group-containing monomer (B) is not particularly limited as long as it is a monomer having an ethylenically unsaturated group. From the viewpoint of curability, it preferably has a vinyl group or a (meth)acryloyl group, and more preferably has a (meth)acryloyl group. In particular, from the viewpoints of dilution of the pressure-sensitive adhesive composition, punching processability of the protective sheet, and reduction of cutting debris during cutting, it is preferable that the ethylenically unsaturated group-containing monomer (B) contains a monofunctional (meth)acrylate (B1). Furthermore, from the viewpoint of improving the low peelability, hardness, and tensile strength of the protective sheet, the ethylenically unsaturated group-containing monomer (B) may contain a polyfunctional (meth)acrylate (B2) as necessary. "Monofunctional" means having only one (meth)acryloyloxy group, and "polyfunctional" means having multiple (meth)acryloyloxy groups.

[0046] Examples of the monofunctional (meth)acrylate (B1) include alkyl (meth)acrylates, cyclic alkyl (meth)acrylates, alkoxyalkyl (meth)acrylates, alkoxy(poly)alkylene glycol (meth)acrylates, hydroxy group-containing (meth)acrylates, carboxy group-containing (meth)acrylates, fluorinated alkyl (meth)acrylates, dialkylaminoalkyl (meth)acrylates, (meth)acrylamides, and epoxy group-containing (meth)acrylates.

[0047] Examples of alkyl (meth)acrylates include methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, n-butyl (meth)acrylate, tert-butyl (meth)acrylate, isobutyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, isodecyl (meth)acrylate, n-hexyl (meth)acrylate, isooctyl (meth)acrylate, isostearyl (meth)acrylate, lauryl (meth)acrylate, and tridecyl (meth)acrylate.

[0048] Examples of cyclic alkyl (meth)acrylates include cyclohexyl (meth)acrylate, norbornyl (meth)acrylate, isobornyl (meth)acrylate, norbornanyl (meth)acrylate, dicyclopentenyl (meth)acrylate, dicyclopentenyloxyethyl (meth)acrylate, dicyclopentanyl (meth)acrylate, dicyclopentanyloxyethyl (meth)acrylate, and tricyclodecanedimethylol di(meth)acrylate.

[0049] Examples of alkoxyalkyl (meth)acrylates include ethoxyethyl (meth)acrylate, methoxyethyl (meth)acrylate, butoxyethyl (meth)acrylate, 2-methoxyethoxyethyl (meth)acrylate, and 2-ethoxyethoxyethyl (meth)acrylate.

[0050] Examples of the alkoxy(poly)alkylene glycol (meth)acrylate include methoxydiethylene glycol (meth)acrylate, ethoxydiethylene glycol (meth)acrylate, and methoxydipropylene glycol (meth)acrylate.

[0051] Examples of hydroxy group-containing (meth)acrylates include 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, 1,3-butanediol (meth)acrylate, 1,4-butanediol (meth)acrylate, 1,6-hexanediol (meth)acrylate, and 3-methylpentanediol (meth)acrylate.

[0052] Examples of the carboxy group-containing (meth)acrylate include (meth)acrylic acid and β-carboxyethyl (meth)acrylate.

[0053] Examples of fluorinated alkyl (meth)acrylates include octafluoropentyl (meth)acrylate.

[0054] Examples of dialkylaminoalkyl (meth)acrylates include N,N-dimethylaminoethyl (meth)acrylate and N,N-diethylaminoethyl (meth)acrylate.

[0055] Examples of (meth)acrylamides include (meth)acrylamide, N-methyl(meth)acrylamide, N-ethyl(meth)acrylamide, N-propyl(meth)acrylamide, N-isopropylacrylamide, N-hexyl(meth)acrylamide, N,N-dimethyl(meth)acrylamide, N,N-diethyl(meth)acrylamide, (meth)acryloylmorpholine, and diacetone acrylamide.

[0056] Examples of epoxy group-containing (meth)acrylates include glycidyl (meth)acrylate.

[0057] Of these, from the viewpoint of reducing cutting debris when the protective sheet is cut, alkyl(meth)acrylates are preferred, and chain alkyl(meth)acrylates having 6 to 18 carbon atoms are more preferred. More specifically, 2-ethylhexyl(meth)acrylate, lauryl(meth)acrylate, isooctyl(meth)acrylate, isostearyl(meth)acrylate, and isobornyl(meth)acrylate are preferred, and 2-ethylhexyl(meth)acrylate and lauryl(meth)acrylate are more preferred. Furthermore, the monofunctional (meth)acrylate (B1) may be composed of one type of compound, or may be composed of two or more types of compounds.

[0058] The polyfunctional (meth)acrylate (B2) is preferably an ester compound of a polyol compound and (meth)acrylic acid, and more preferably a tri- to hexa-functional (meth)acrylate. Examples include polyethylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, neopentyl glycol di(meth)acrylate, triethylene glycol di(meth)acrylate, tripropylene glycol di(meth)acrylate, hydroxypivalic acid ester neopentyl glycol di(meth)acrylate, 1,3-bis(hydroxyethyl)-5,5-dimethylhydantoin di(meth)acrylate, α,ω-di(meth)acrylbisdiethylene glycol phthalate, and trimethylolpropanediol. Examples of the diacrylate include panthenyl tri(meth)acrylate, trimethylolpropane ethoxylate tri(meth)acrylate, ethylene glycol di(meth)acrylate, tetraethylene glycol di(meth)acrylate, polyethylene glycol di(meth)acrylate, 1,4-butanediol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, diacryloxyethyl phosphate, dipentaerythritol trihydroxy(meth)acrylate, and pentaerythritol tetra(meth)acrylate.

[0059] Among these, from the viewpoint of keeping the peel strength of the protective sheet low, 1,6-hexanediol di(meth)acrylate, trimethylolpropane tri(meth)acrylate, trimethylolpropane ethoxylate tri(meth)acrylate, dipentaerythritol trihydroxy(meth)acrylate, and pentaerythritol tetra(meth)acrylate are more preferred, and trimethylolpropane tri(meth)acrylate and trimethylolpropane ethoxylate tri(meth)acrylate are even more preferred. Furthermore, the polyfunctional (meth)acrylate (B2) may be composed of one type of compound, or two or more types of compounds.

[0060] The ethylenically unsaturated group-containing monomer (B) may contain an ethylenically unsaturated group-containing monomer (B3) other than the components (B1) and (B2). Examples of the ethylenically unsaturated group-containing monomer (B3) other than the components (B1) and (B2) include acrylonitrile, methacrylonitrile, styrene, α-methylstyrene, vinyl acetate, vinyl propionate, vinyl stearate, vinyl chloride, vinylidene chloride, alkyl vinyl ethers, vinyl toluene, N-vinylpyridine, N-vinylpyrrolidone, dialkyl itaconates, dialkyl fumarate esters, allyl alcohol, hydroxybutyl vinyl ether, hydroxyethyl vinyl ether, 4-hydroxymethylcyclohexylmethyl vinyl ether, triethylene glycol monovinyl ether or diethylene glycol monovinyl ether, methyl vinyl ketone, N-acrylamidomethyltrimethylammonium chloride, allyltrimethylammonium chloride, and dimethylallyl vinyl ketone.

[0061] The content of the ethylenically unsaturated group-containing monomer (B) is preferably 10 to 90% by mass, more preferably 20 to 75% by mass, even more preferably 25 to 65% by mass, and even more preferably 30 to 60% by mass, relative to 100% by mass of the total of the polyurethane (A) and the ethylenically unsaturated group-containing monomer (B). When the content of the ethylenically unsaturated group-containing monomer (B) is 10% by mass or more, the low peelability of the protective sheet is good. When the content of the ethylenically unsaturated group-containing monomer (B) is 90% by mass or less, the elongation and tensile strength of the pressure-sensitive adhesive layer are good.

[0062] The lower limit of the content of the ethylenically unsaturated group-containing monomer (B) is preferably 10% by mass or more, more preferably 20% by mass or more, even more preferably 25% by mass or more, and even more preferably 30% by mass or more, relative to 100% by mass of the total of the polyurethane (A) and the ethylenically unsaturated group-containing monomer (B). When the content of the ethylenically unsaturated group-containing monomer (B) is 10% by mass or more, the protective sheet has good low peelability. The upper limit of the content of the ethylenically unsaturated group-containing monomer (B) is preferably 90% by mass or less, more preferably 75% by mass or less, even more preferably 65% ​​by mass or less, and even more preferably 60% by mass or less, relative to 100% by mass of the total of the polyurethane (A) and the ethylenically unsaturated group-containing monomer (B). When the content of the ethylenically unsaturated group-containing monomer (B) is 90% by mass or less, the protective sheet has good low peelability. The above upper and lower limits can be combined arbitrarily.

[0063] When the ethylenically unsaturated group-containing monomer (B) contains a monofunctional (meth)acrylate (B1) and a polyfunctional (meth)acrylate (B2), the content of the monofunctional (meth)acrylate (B1) is preferably 5 to 89% by mass, more preferably 15 to 74% by mass, and even more preferably 25 to 57% by mass, relative to 100% by mass of the total of the polyurethane (A) and the ethylenically unsaturated group-containing monomer (B). When the content of the monofunctional (meth)acrylate (B1) is 5% by mass or more, the protective sheet exhibits good low peelability. When the content of the monofunctional (meth)acrylate (B1) is 89% by mass or less, the pressure-sensitive adhesive has an appropriate viscosity, facilitating application when preparing a pressure-sensitive adhesive sheet. The content of the polyfunctional (meth)acrylate (B2) is preferably 0.1 to 30% by mass, more preferably 1 to 20% by mass, and even more preferably 3 to 15% by mass, relative to 100% by mass of the total of the polyurethane (A) and the ethylenically unsaturated group-containing monomer (B). When the content of the monofunctional (meth)acrylate (B1) is 0.1% by mass or more, the generation of cutting debris and damage to the pressure-sensitive adhesive layer are suppressed, and the low peelability as a protective sheet is good. When the content of the monofunctional (meth)acrylate (B1) is 30% by mass or less, the elongation of the pressure-sensitive adhesive layer is good.

[0064] [Photopolymerization initiator (C)] The photopolymerization initiator (C) is not particularly limited as long as it is a polymerization initiator that generates radicals upon light irradiation, and examples thereof include carbonyl-based photopolymerization initiators, sulfide-based photopolymerization initiators, acylphosphine oxides, quinone-based photopolymerization initiators, sulfochloride-based photopolymerization initiators, and thioxanthone-based photopolymerization initiators.

[0065] Examples of the carbonyl-based photopolymerization initiator include benzophenone, benzil, benzoin, ω-bromoacetophenone, chloroacetone, acetophenone, 2,2-diethoxyacetophenone, 2,2-dimethoxy-2-phenylacetophenone, p-dimethylaminoacetophenone, p-dimethylaminopropiophenone, 2-chlorobenzophenone, 4,4'-dichlorobenzophenone, 4,4'-bisdiethylaminobenzophenone, Michler's ketone, benzoin methyl ether, and benzoin isobutyl ether. methyl ether, benzoin-n-butyl ether, benzyl methyl ketal, 1-hydroxycyclohexyl phenyl ketone, 2-hydroxy-2-methyl-1-phenyl-propan-1-one, 1-(4-isopropylphenyl)-2-hydroxy-2-methylpropan-1-one, methyl benzoyl formate, 2,2-diethoxyacetophenone, 4-N,N'-dimethylacetophenone, 2-methyl-1-[4-(methylthio)phenyl]-2-morpholinopropan-1-one, and the like.

[0066] Examples of sulfide-based photopolymerization initiators include diphenyl disulfide, dibenzyl disulfide, tetraethyl thiuram disulfide, and tetramethyl ammonium monosulfide.

[0067] Examples of acylphosphine oxides include 2,4,6-trimethylbenzoyldiphenylphosphine oxide and 2,4,6-trimethylbenzoylphenylethoxyphosphine oxide.

[0068] Examples of the quinone-based photopolymerization initiator include quinone-based photopolymerization initiators such as benzoquinone and anthraquinone.

[0069] Examples of sulfochloride-based photopolymerization initiators include 2-naphthalenesulfonyl chloride.

[0070] Examples of the thioxanthone-based photopolymerization initiator include thioxanthone, 2-chlorothioxanthone, and 2-methylthioxanthone.

[0071] Among these exemplified compounds, from the viewpoint of the transparency of the pressure-sensitive adhesive layer obtained by curing the pressure-sensitive adhesive composition, carbonyl-based photopolymerization initiators and acylphosphine oxides are preferred, and 1-hydroxycyclohexyl phenyl ketone and 2,4,6-trimethylbenzoyldiphenylphosphine oxide are more preferred. Furthermore, the photopolymerization initiator (C) may consist of one type of compound or two or more types of compounds.

[0072] The content of the photopolymerization initiator (C) is preferably 0.05 to 5 parts by mass, more preferably 0.1 to 3 parts by mass, and even more preferably 0.3 to 1 part by mass, relative to 100 parts by mass of the total of the polyurethane (A) and the ethylenically unsaturated group-containing monomer (B). When the content of the photopolymerization initiator (C) is 0.05 parts by mass or more, the pressure-sensitive adhesive composition has sufficient photocurability. When the content of the photopolymerization initiator (C) is 5 parts by mass or less, contamination of the adherend when the protective sheet is peeled off can be suppressed.

[0073] [Plasticizer (D)] Any known plasticizer can be used as the plasticizer (D) without any particular restrictions. For example, fatty acid esters are preferably used. The plasticizer (D) can improve the lamination properties (wettability) and bubble release properties (ease of release of air bubbles trapped during lamination) of the protective sheet.

[0074] Examples of fatty acid esters include esters of monobasic acids or polybasic acids having 8 to 18 carbon atoms and branched alcohols having 18 or less carbon atoms, esters of unsaturated fatty acids or branched acids having 14 to 18 carbon atoms and tetrahydric alcohols, etc. A specific preferred example of the fatty acid ester is ethylhexyl stearate.

[0075] When a plasticizer (D) is used, its content is preferably 1 to 30 parts by mass, more preferably 5 to 25 parts by mass, and even more preferably 10 to 23 parts by mass, relative to 100 parts by mass of the combined total of the polyurethane (A) and the ethylenically unsaturated group-containing monomer (B). When the content of the plasticizer (D) is 1 part by mass or more, the effects of adding the plasticizer (D) can be fully expected, such as improved lamination properties (wettability) of the protective sheet and improved bubble release properties (ease of release of air bubbles trapped during lamination). When the content of the plasticizer (D) is 30 parts by mass or less, contamination of the adherend when the protective sheet is peeled off can be fully suppressed.

[0076] [Solvent] The PSA composition of this embodiment contains the ethylenically unsaturated group-containing monomer (B) as a low-molecular-weight component, allowing for adjustment of viscosity to a coatable level without the addition of a solvent. That is, the PSA composition need not substantially contain a solvent. In this case, the step of heat-drying the solvent can be omitted when producing a protective sheet, thereby increasing productivity. In particular, when producing a protective sheet with a thickness exceeding 50 μm, it is preferable that the PSA composition be substantially free of the solvent. In the present invention, "substantially free" means that the content of the solvent in the PSA composition of the present invention is 0 to 1% by mass, preferably 0 to 0.5% by mass or less, and more preferably 0 to 0.1% by mass or less. A solvent may be added to the PSA composition to adjust the viscosity during application. The solvent can be selected appropriately depending on the other components contained in the PSA composition, but an organic solvent is preferred. Examples of organic solvents that can be used include, but are not limited to, methyl ethyl ketone, acetone, ethyl acetate, tetrahydrofuran, dioxane, cyclohexanone, n-hexane, toluene, xylene, n-propanol, and isopropanol. These organic solvents may be used alone or in combination of two or more. After the pressure-sensitive adhesive composition is applied to a substrate or the like, the solvent is preferably removed by drying, and then photocuring is performed.

[0077] Furthermore, other additives may be added to the pressure-sensitive adhesive composition as needed, provided that the transparency is not impaired. Examples of the additives include surface lubricants, leveling agents, softeners, antioxidants, antiaging agents, light stabilizers, ultraviolet absorbers, polymerization inhibitors, benzotriazole-based light stabilizers, phosphate ester-based and other flame retardants, antistatic agents such as surfactants, and dyes.

[0078] Here, an example of a method for synthesizing the ethylenically unsaturated group-containing polyurethane (A) will be described, but the ethylenically unsaturated group-containing monomer (B) and other components contained in the pressure-sensitive adhesive composition vary depending on the type of compound used, and commercially available products can also be used, so description of the synthesis method will be omitted.

[0079] <Method for Synthesizing Polyurethane (A)> An example of a preferred method for synthesizing the polyurethane (A) contained in the pressure-sensitive adhesive composition of the present embodiment will be described below. However, the method for synthesizing the polyurethane (A) is not limited to this and can be appropriately modified depending on the conditions of the raw materials and equipment used in the synthesis. In this example, the reaction of the hydroxy group with the isocyanato group is carried out in each step using a urethanization catalyst such as dibutyltin dilaurate, dibutyltin diethylhexoate, or dioctyltin dilaurate in the presence of an organic solvent inert to the isocyanato group. The reaction is preferably carried out at 30 to 100°C for 1 to 5 hours. The amount of the urethanization catalyst used is preferably 50 to 500 ppm by mass based on the total mass of the reactants.

[0080] First, a polyol and a polyisocyanate are charged in a ratio such that the amount of isocyanato groups (by number, the same applies hereinafter) is greater than the amount of hydroxyl groups (by number, the same applies hereinafter), and are reacted to synthesize a urethane prepolymer having isocyanato groups at its terminals. Specific examples of the polyol and polyisocyanate are as exemplified in the section on polyurethane (A).

[0081] In this case, when an ethylenically unsaturated group-containing polyisocyanate and an ethylenically unsaturated group-free polyisocyanate are used in combination as the polyisocyanate, they may be added simultaneously or separately, and the order of addition may be any. The position of the ethylenically unsaturated group introduced into the main chain of polyurethane (A) can also be adjusted by the order of adding the ethylenically unsaturated group-containing polyisocyanate and the ethylenically unsaturated group-free polyisocyanate. From the viewpoint of shortening the distance between crosslinking points of polyurethane (A) and reducing the peeling property of the pressure-sensitive adhesive layer, it is preferable that an ethylenically unsaturated group is also introduced near the center of the main chain. That is, it is preferable to add the ethylenically unsaturated group-containing polyisocyanate first, or to add the ethylenically unsaturated group-containing polyisocyanate and the ethylenically unsaturated group-free polyisocyanate simultaneously, and it is more preferable to add the ethylenically unsaturated group-containing polyisocyanate first.

[0082] Similarly, when a polyoxyalkylene polyol and an ethylenically unsaturated group-containing polyol are used in combination as the polyol, they may be added simultaneously or separately, and the order of addition is not important. From the viewpoint of reducing the peeling property of the pressure-sensitive adhesive layer, it is preferable to add the ethylenically unsaturated group-containing polyol first, or to add the ethylenically unsaturated group-containing polyol and the polyoxyalkylene polyol simultaneously, and it is more preferable to add the ethylenically unsaturated group-containing polyol first.

[0083] Next, the isocyanato group-containing urethane prepolymer is reacted with a hydroxy group-containing ethylenically unsaturated compound to synthesize a polyurethane (A) having ethylenically unsaturated groups introduced at molecular chain terminals. Ethylenically unsaturated groups are preferably introduced at 90 to 100% of the polyurethane terminals contained in the ethylenically unsaturated group-containing polyurethane (A) by number, more preferably 95 to 100%, and even more preferably 100%. When the amount of ethylenically unsaturated groups introduced is 90% or more by number relative to the isocyanato groups, the adhesive layer obtained by curing the adhesive composition can achieve sufficient cohesive strength. The ratio of the number of terminals having ethylenically unsaturated groups introduced to the number of terminals of all polyurethane molecular chains can be measured by IR, NMR, etc.

[0084] <Second synthesis method> First, a polyol and a polyisocyanate are charged in a ratio such that the amount of hydroxy groups (by number, the same applies hereinafter) is greater than the amount of isocyanato groups (by number, the same applies hereinafter), and these are reacted to synthesize a urethane prepolymer having hydroxy groups at its terminals. Specific examples of polyoxyalkylene polyols and polyisocyanates are as exemplified in the section on polyurethane (A).

[0085] Next, a hydroxy group-containing urethane prepolymer is reacted with an isocyanato group-containing ethylenically unsaturated compound to synthesize a polyurethane (A) having ethylenically unsaturated groups introduced at molecular chain terminals. Ethylenically unsaturated groups are preferably introduced at 90 to 100% of the polyurethane terminals contained in the polyurethane (A) by number, more preferably 95 to 100%, and even more preferably 100%. When the amount of ethylenically unsaturated groups introduced is 90% or more by number relative to the number of hydroxy groups, the adhesive layer obtained by curing the adhesive composition can have sufficient cohesive strength. The ratio of the number of terminals having ethylenically unsaturated groups introduced to the number of terminals of all polyurethane molecular chains can be measured by IR, NMR, etc.

[0086] <Method for producing pressure-sensitive adhesive composition> The pressure-sensitive adhesive composition is produced by mixing an ethylenically unsaturated group-containing polyurethane (A), an ethylenically unsaturated group-containing monomer (B), a photopolymerization initiator (C), and, if necessary, a plasticizer (D), other additives, and an organic solvent. The mixing method is not particularly limited, and can be carried out, for example, using a stirring device equipped with stirring blades such as a homodisper or paddle blade.

[0087] All the components may be added at once and mixed, or each component may be added in several batches and mixed repeatedly. If there is a component that is solid at room temperature, the component can be easily mixed with high uniformity in the PSA composition by adding the component dissolved in a solvent or dispersed in a dispersion medium, or by adding the component melted by heating.

[0088] <Protective Sheet> <Configuration of Protective Sheet> The protective sheet according to this embodiment includes a substrate and a pressure-sensitive adhesive layer, with a pressure-sensitive adhesive layer formed on one side of the substrate, the pressure-sensitive adhesive layer being made of a cured product of the pressure-sensitive adhesive composition. The thickness of the pressure-sensitive adhesive layer is preferably 3 to 150 μm, more preferably 5 to 130 μm, and even more preferably 10 to 100 μm. When the thickness of the pressure-sensitive adhesive layer is 3 μm or more, the strength of the pressure-sensitive adhesive layer is sufficient, and when the thickness is 150 μm or less, the thickness of the pressure-sensitive adhesive layer is easily controlled. Furthermore, when it is desired to impart to the protective sheet the function of protecting the adherend from impact (impact resistance), the thickness of the pressure-sensitive adhesive layer is preferably 50 μm or more.

[0089] The material of the substrate can be appropriately selected depending on the application of the protective sheet, and examples thereof include resin films. When the protective sheet is used, for example, as a protective sheet for a display surface in the manufacturing process of optical components, etc., and when inspecting the adherend, i.e., the product, for scratches or foreign matter, the protective sheet is laminated on the substrate, the substrate is preferably transparent. Examples of transparent substrates include polyethylene terephthalate, polyethylene, polypropylene, polystyrene, polyimide, polyvinyl alcohol, polyvinyl chloride, cellulose, etc.

[0090] The thickness of the substrate can be appropriately selected depending on the application of the protective sheet and is not particularly limited, but when the substrate is a resin film, the thickness of the substrate is preferably 5 μm or more, more preferably 10 μm or more, and even more preferably 20 μm or more, from the viewpoints of handleability and strength. Furthermore, in consideration of the flexibility of the resin film, the thickness of the substrate is preferably 200 μm or less, more preferably 150 μm or less, and even more preferably 100 μm or less.

[0091] In addition, the substrate is preferably antistatically treated. The antistatic treatment applied to the substrate is not particularly limited, but a method of providing an antistatic layer on at least one surface of the substrate, a method of kneading an antistatic agent into the substrate, etc. can be used. Furthermore, the surface of the substrate on which the pressure-sensitive adhesive layer is formed may be subjected to an easy-adhesion treatment such as an acid treatment, an alkali treatment, a primer treatment, a corona treatment, a plasma treatment, an ultraviolet treatment, or an ozone treatment, as necessary.

[0092] In the protective sheet, a release film can be laminated on the surface of the pressure-sensitive adhesive layer for the purpose of protecting the pressure-sensitive adhesive layer. As the material for the release film, for example, paper, plastic film, etc. can be used, but plastic film is preferred from the viewpoint of excellent surface smoothness. The plastic film used as the release film is not particularly limited as long as it can protect the above-mentioned pressure-sensitive adhesive layer, and examples thereof include polyethylene, polypropylene, polyethylene terephthalate, polybutene, etc. In addition, it is preferable that the surface in contact with the pressure-sensitive adhesive layer is subjected to easy-release treatment.

[0093] <Method for producing protective sheet> A protective sheet according to this embodiment can be produced, for example, by applying a pressure-sensitive adhesive composition to a substrate, laminating a release film as needed, and then irradiating the pressure-sensitive adhesive composition with ultraviolet light to photocure it. The ultraviolet light may be irradiated through a transparent release film, or if the substrate is transparent, it may be irradiated from the substrate side.

[0094] The method for applying the pressure-sensitive adhesive composition to the substrate is not particularly limited and can be appropriately selected. Examples of the method for applying the pressure-sensitive adhesive composition to the substrate include methods using various coaters such as a gravure roll coater, a reverse roll coater, a kiss roll coater, a dip roll coater, a bar coater, a knife coater, a spray coater, a comma coater, and a direct coater, and a screen printing method.

[0095] Furthermore, examples of light sources for photocuring the pressure-sensitive adhesive composition include black light, low-pressure mercury lamp, high-pressure mercury lamp, ultra-high-pressure mercury lamp, metal halide lamp, xenon lamp, etc. The light irradiation intensity may be any intensity that can sufficiently cure the pressure-sensitive adhesive composition, and may be, for example, 50 to 3000 mW / cm. ２ If the irradiation intensity of light is too weak, it takes a long time to cure, which reduces productivity.

[0096] <Uses and Required Performance of Protective Sheet> In the manufacturing process of optical components, etc., the protective sheet may be cut to fit the shape of the product or component before laminating it on the product or component. Alternatively, the product or component may be punched while the protective sheet is laminated on it. Protective sheets may also be suitably used for protecting the surface of plastic films used as optical components. Examples of the optical components include polarizing plates, wavelength plates, retardation plates, optical compensation films, reflective sheets, brightness enhancement films, etc., used in liquid crystal displays of smartphones, personal computers, televisions, etc.

[0097] When the protective sheet according to this embodiment is used as such a protective sheet, it is required that the protective sheet does not generate cutting debris when cut to prevent contamination of the adherend. Furthermore, it is also required that the protective sheet have sufficient hardness and tensile strength to prevent damage to the adhesive layer or the adherend when an external impact or peeling force is applied. In this case, it is preferable that the adhesive layer constituting the protective sheet has a certain level of hardness or higher and a moderate elongation and tensile strength. From these viewpoints, the pencil hardness (coating surface strength) of the adhesive layer is preferably 4B or higher. The elongation at break of the adhesive layer is preferably 40 to 300%, more preferably 50 to 300%, more preferably 92 to 300%, and even more preferably 92 to 250%. The breaking strength (tensile strength) of the adhesive layer is 100 to 800 g / mm ２ is preferably 200 to 800 g / mm ２ More preferably, it is 300 to 800 g / mm ２ More preferably, it is 330 to 600 g / mm ２ Specific methods for measuring these physical properties of the pressure-sensitive adhesive layer of the protective sheet will be described later in the Examples.

[0098] Furthermore, when the protective sheet according to this embodiment is used as the protective sheet described above, a minimum peel strength is required to prevent the protective sheet from peeling off from a product or component during handling, such as transportation. On the other hand, when the protective sheet is to be peeled off from a product or component, a low peel strength is required to facilitate the peeling operation or to prevent deformation or damage to the product or component during peeling. From these perspectives, when the peeling speed is 0.3 m / min, the peel strength of the protective sheet is preferably 0.1 to 10 gf / 25 mm, more preferably 0.2 to 8 gf / 25 mm, even more preferably 0.2 to 3 gf / 25 mm, and even more preferably 0.2 to 1 gf / 25 mm, depending on the thickness of the substrate and the pressure-sensitive adhesive layer. When the peel speed is 2.4 m / min, the peel strength is preferably 1 to 50 gf / 25 mm, more preferably 1.5 to 40 gf / 25 mm, even more preferably 1.5 to 10 gf / 25 mm, and even more preferably 1.5 to 4.5 gf / 25 mm, depending on the thickness of the substrate and pressure-sensitive adhesive layer. Specific methods for measuring the peel strength of the protective sheet are described below in the examples.

[0099] As described above, a protective sheet is required to have multiple properties, and the optimum range of the required properties may change depending on the thickness and type of the substrate and adhesive used in the protective sheet. For example, if the thickness of the substrate is increased or the pencil hardness of the substrate is changed to a higher one, the impact when the protective sheet is cut is more likely to be transmitted to the adhesive layer, making it more likely to generate cutting debris. From this perspective, physical properties of the adhesive layer that further reduce the generation of cutting debris include an elongation of 92% or more and a tensile strength of 300 gf / mm ２ or more. For example, when an alkyl(meth)acrylate, which is a monofunctional (meth)acrylate (B1), is used as the ethylenically unsaturated group-containing monomer (B), the elongation tends to improve when the alkyl chain contained in the alkyl(meth)acrylate is made longer. From this viewpoint, it is preferable to use a chain alkyl(meth)acrylate having 8 to 18 carbon atoms, more preferably a chain alkyl(meth)acrylate having 12 to 18 carbon atoms, and even more preferably lauryl(meth)acrylate.

[0100] The elongation of the PSA layer can also be improved by using a polyoxyalkylene polyol as the polyol used in the polyurethane (A), lowering the number average molecular weight of the polyoxyalkylene polyol, and increasing the urethane bond concentration in the polyurethane (A). From this viewpoint, it is preferable to use a polyoxyalkylene polyol having a number average molecular weight of 360 to 1200, and the urethane bond concentration of the polyurethane (A) is preferably 1.9 to 5 mol / kg, more preferably 2 to 5 mol / kg, and even more preferably 2.3 to 5 mol / kg.

[0101] On the other hand, increasing the elongation of the PSA layer can increase the peel strength, making the adherend more susceptible to damage, or decrease the tensile strength, resulting in damage to the PSA layer, or contaminating the adherend when the protective sheet is peeled off. From this perspective, using a polyfunctional (meth)acrylate (B2) as the ethylenically unsaturated group-containing monomer (B) can reduce the peel strength of the PSA layer while improving its hardness and tensile strength. In this case, since the use of the polyfunctional (meth)acrylate (B2) reduces the elongation of the protective sheet, it is necessary to adjust the amount of the polyfunctional (meth)acrylate (B2) so that the elongation of the protective sheet does not fall below 92%, in order to balance the performance of both. In this case, the content of the polyfunctional (meth)acrylate (B2) is preferably 0.1 to 15% by mass, more preferably 1 to 12% by mass, and even more preferably 3 to 10% by mass, relative to 100% by mass of the combined total of the polyurethane (A) and the ethylenically unsaturated group-containing monomer (B). Furthermore, various properties can also be adjusted by changing the blending ratio of the polyurethane (A) and the monofunctional (meth)acrylate (B1).

[0102] From the viewpoint of further reducing the generation of cutting debris while balancing various performance characteristics, it is preferable that the polyurethane (A) be contained in an amount of 50% by mass or more relative to 100% by mass of the total of the polyurethane (A) and the ethylenically unsaturated group-containing monomer (B). Within this range, increasing the amount of monofunctional (meth)acrylate (B1) tends to increase the elongation of the pressure-sensitive adhesive layer and reduce the peel strength, while increasing the amount of polyurethane (A) tends to increase the hardness and strength of the pressure-sensitive adhesive layer. From the viewpoint of balancing various performance characteristics and suppressing the generation of cutting debris regardless of the type of substrate, it is preferable that the content of polyurethane (A) be 50 to 70% by mass and the content of monofunctional (meth)acrylate (B1) be 20 to 50% by mass.

[0103] The present invention will be explained in more detail below with reference to examples and comparative examples, but the present invention is not limited to the following examples.

[0104] Synthesis of Polyurethane (A) Synthesis Example 1 A four-neck flask equipped with a thermometer, a stirrer, a dropping funnel, and a condenser with a drying tube was charged with 2 mol of polypropylene glycol D-1000 (manufactured by Mitsui Chemicals, number average molecular weight 1000) having a hydroxyl group at the end and a hydroxyl value of 111 mg KOH / g, and 3 mol of isophorone diisocyanate (Desmodur I, manufactured by Sumika Covestro Urethane Co., Ltd.), and the temperature was raised to 80°C, allowing the reaction to proceed for 7 hours. 2 mol of 2-hydroxyethyl acrylate was added to the resulting urethane prepolymer. The reaction was then continued for 2 hours at 80°C, yielding a polyurethane (A-i) having an acryloyloxy group at the end. This polyurethane (A-i) was analyzed by IR, and it was confirmed that the peak derived from the isocyanato group had disappeared. The weight average molecular weight of the resulting polyurethane (A-i) was 7,000. The weight average molecular weight, urethane bond concentration, and ethylenically unsaturated group equivalent were evaluated by the following methods, and the results are shown in Tables 1 and 2.

[0105] (Measurement of Weight Average Molecular Weight) In the Synthesis Examples and Comparative Synthesis Examples, the weight average molecular weight of the obtained polyurethane (A) is a value measured in terms of polystyrene by gel permeation chromatography (Shodex (registered trademark) GPC-101 manufactured by Showa Denko K.K., hereinafter referred to as GPC). The measurement conditions for GPC are as follows: Column: LF-804 manufactured by Showa Denko K.K. Column temperature: 40°C Sample: 0.2 mass% tetrahydrofuran solution of polyurethane (A) Flow rate: 1 ml / min Eluent: tetrahydrofuran Detector: RI detector (differential refractive index detector)

[0106] (Urethane bond concentration) This is a theoretical value calculated from the amount of each raw material charged.

[0107] (Ethylenically Unsaturated Group Equivalent) This is a theoretical value calculated from the amounts of each raw material charged.

[0108] (Synthesis Examples 2 to 9, Comparative Synthesis Examples 1 to 6) Polyurethanes (A-ii) to (A-ix) and (cA-i) to (cA-vi) were obtained in the same manner as in Synthesis Example 1, except that the polyols and polyisocyanates shown in Tables 1 and 2 were used in the amounts shown in Tables 1 and 2. Each polyurethane was evaluated for weight average molecular weight, urethane bond concentration, and ethylenically unsaturated group equivalent weight in the same manner as in Synthesis Example 1. The results are shown in Tables 1 and 2.

[0109]

[0110]

[0111] In Tables 1 and 2, "PR9000" is a compound represented by the following formula (1).

[0112] The following compounds were used as described in Tables 1 and 2. Polypropylene glycol D-2000 (manufactured by Mitsui Chemicals, number average molecular weight 2000) Polypropylene glycol D-1000 (manufactured by Mitsui Chemicals, number average molecular weight 1000) Polypropylene glycol D-400 (manufactured by Mitsui Chemicals, number average molecular weight 400) Epoxy aquaacrylate (manufactured by Showa Denko K.K., Lipoxy (trademark) SP-16LDA) PR9000: Laromer (registered trademark) PR9000 (manufactured by BASF) Isophorone diisocyanate (Desmodur I, manufactured by Sumika Covestro Urethane Co., Ltd.) Coronate HX: Isocyanurate of hexamethylene diisocyanate (manufactured by Tosoh) Hydrogenated diphenylmethane diisocyanate (Desmodur W, manufactured by Sumika Covestro Urethane Co., Ltd.) Hexamethylene diisocyanate (manufactured by Asahi Kasei Co., Ltd.) 2-Hydroxyethyl acrylate (manufactured by Osaka Organic Chemical Industry Co., Ltd.)

[0113] Example 1 Preparation of Pressure-Sensitive Adhesive Composition PU1 50 parts by mass of the polyurethane (A-i) obtained in Synthesis Example 1 as the polyurethane (A), 50 parts by mass of EHA, which is a monofunctional (meth)acrylate, as the ethylenically unsaturated group-containing monomer (B), 0.6 parts by mass of Irg-184 as the photopolymerization initiator (C), and 20 parts by mass of IPM as the plasticizer (D) were blended and mixed at 25°C using a disper, to prepare Pressure-Sensitive Adhesive Composition PU1 according to Example 1.

[0114] <Preparation 1 of Protective Sheet PUS1> A protective sheet having a pressure-sensitive adhesive layer on one side of an optical PET film substrate was prepared. First, using an applicator, the pressure-sensitive adhesive composition PU1 prepared above was applied to a 75 μm thick optical PET film (A4300 manufactured by Toyobo Co., Ltd.) or a 75 μm thick PET film (HY-S10 manufactured by Higashiyama Film Co., Ltd.) as a substrate. The applied pressure-sensitive adhesive composition was covered with a 75 μm thick release PET film (E7006 manufactured by Toyobo Co., Ltd.). Next, using an ultraviolet irradiation device (3 kW UV irradiation device, high-pressure mercury lamp manufactured by iGraphics Co., Ltd.), ultraviolet light was irradiated from the release PET film side to photocure the pressure-sensitive adhesive composition, thereby preparing a pressure-sensitive adhesive layer. The ultraviolet irradiation distance was 25 cm, the lamp movement speed was 1.0 m / min, and the irradiation dose was 1000 mJ / cm. ２ The thickness of the adhesive layer after curing was calculated by measuring the thickness of the protective sheet using a dial gauge and then subtracting the 75 μm thickness of the optical PET film and the 75 μm thickness of the release PET film from this measurement. The measurement surface of the dial gauge was a circular flat surface with a diameter of 5 mm, and the measuring force was 0.8 N. A protective sheet PUS1 of Example 1 was obtained with an adhesive layer thickness of 71 μm. Protective sheet PUS1 was evaluated for the coating surface strength (pencil hardness) of the adhesive layer, the peel strength of the protective sheet, cutting debris, and cracking of the adhesive layer using the evaluation methods below. The results are shown in Table 3.

[0115] <Preparation 2 of sample PUS1 for tensile test> Sample PUS1 for tensile test, in which both sides of the pressure-sensitive adhesive layer were release PET films, was prepared in the same manner as in Preparation 1 of protective sheet, except that a 75 μm thick release PET film (E7006 manufactured by Toyobo Co., Ltd.) was used as the substrate. Tensile tests (elongation and tensile strength) were evaluated using the following evaluation methods. The results are shown in Table 3.

[0116] (Examples 2 to 21, Comparative Examples 1 to 15) <Preparation of Pressure-Sensitive Adhesive Compositions> Pressure-sensitive adhesive compositions PUS2 to 21 and cPS1 to 15 according to Examples 2 to 21 and Comparative Examples 1 to 15 were prepared in the same manner as in Example 1, except that the components and their blending amounts shown in Tables 3 to 7 were used.

[0117] <Preparation of Protective Sheet 1> Protective sheets PUS2 to 21 and cPUS1 to 15 of Examples 2 to 21 and Comparative Examples 1 to 15 were obtained in the same manner as in Example 1, except that the pressure-sensitive adhesive compositions PUS2 to 21 and cPS1 to 15 obtained above were used. The protective sheets were evaluated for the coating surface strength (pencil hardness) of the pressure-sensitive adhesive layer and the peel strength of the protective sheet using the same evaluation methods as in Example 1. The results are shown in Tables 3 to 7.

[0118] <Preparation of Samples for Tensile Test 2> Samples for tensile test PUS2 to 21 and cPUS1 to 15 of Examples 2 to 21 and Comparative Examples 1 to 15 were obtained in the same manner as in Example 1, except that the pressure-sensitive adhesive compositions PUS2 to 21 and cPUS1 to 15 obtained above were used. The samples for tensile test were evaluated for tensile test (elongation, tensile strength) using the same evaluation method as in Example 1. The results are shown in Tables 3 to 7.

[0119]

[0120]

[0121]

[0122]

[0123]

[0124]

[0125] The compounds used in the table are as follows: EHA: 2-ethylhexyl acrylate (manufactured by Toagosei Co., Ltd.) LA: lauryl acrylate (manufactured by Osaka Organic Chemical Industry Ltd.) TMPTA: trimethylolpropane triacrylate (M-309, manufactured by Toagosei Co., Ltd.) Irg-184: 1-hydroxycyclohexyl phenyl ketone (manufactured by Tokyo Chemical Industry Co., Ltd.) IPM: Exepar IPM (manufactured by Kao Corporation) HY-S10: manufactured by Higashiyama Film Co., Ltd. A4300: manufactured by Toyobo Co., Ltd.

[0126] <Coating film surface strength (pencil hardness)> The prepared protective sheet was cut into a size of 100 mm x 100 mm, and the release PET film was peeled off. An electric pencil scratch hardness tester (manufactured by Yasuda Seiki Seisakusho, product name "No. 553-M") was used. The pencil used was a "Uni" (product name) pencil manufactured by Mitsubishi Pencil Co., Ltd. Then, the pencil hardness of the pressure-sensitive adhesive layer surface of the protective sheet was measured under a load of 100 g in accordance with JIS K 5600-5-4.

[0127] <Peel Strength of Protective Sheet> The prepared protective sheet was cut into a size of 25 mm × 150 mm, and the release PET film was peeled off. Next, the exposed adhesive surface was attached to a glass plate, and a 2 kg rubber roller (width: approximately 50 mm) was rolled back and forth once to prepare a measurement sample. 30 minutes after pressure bonding, a tensile test was performed in the 180° direction at peeling speeds of 0.3 m / min and 2.4 m / min to measure the peel strength (g / 25 mm) of the protective sheet to the glass plate in accordance with JIS Z0237.

[0128] <Tensile Test (Elongation, Tensile Strength)> As shown in Figure 1, a sample prepared for the tensile test was cut into a dumbbell shape with a width of 5 mm and a length of 50 mm, and the release PET films on both sides were peeled off to expose the pressure-sensitive adhesive layer. The elongation at break and the breaking strength (tensile strength) were measured according to the method of JIS K 7161-1:2014. A tensile tester (manufactured by Stable MicroSystems) was used. The tensile test of the pressure-sensitive adhesive layer was performed under conditions of a gauge length of 20 mm and a pulling speed of 100 mm / min. The elongation was calculated according to the following formula (A).

[0129] Test piece dimensions: Total length: 50 mm Parallel length: 25 mm Gauge distance: 20 mm Parallel width: 5 mm See Tables 3 to 7 for thickness. Grip width: 20 mm Grip sensing distance: 30 mm

[0130] Elongation = {(gauge length at break) - (initial gauge length 20 mm)} / (initial gauge length 20 mm) × 100 (A)

[0131] <Cutting debris and cracks in adhesive layer> The prepared protective sheet was cut with a pinnacle blade (blade height: 0.80 mm, blade angle: 50° (manufactured by Tsukatani Cutlery Manufacturing Co., Ltd.)), and the release PET film was peeled off. The edge surface of the adhesive layer after cutting was observed with a microscope (magnification: 800x (manufactured by Hirox Co., Ltd. RH-2000)) with the adhesive layer on the upper side and the base layer on the lower side, and evaluated according to the following criteria.

[0132] "Evaluation criteria for cutting debris and cracks in the adhesive layer" 1: No cutting debris or cracks are observed in the adhesive layer (for example, the cross-sectional view shown in Figure 2) 2: Cutting debris is observed in part of the adhesive layer, but no cracks are observed (for example, the cross-sectional view shown in Figure 3) 3: Cracks are observed in part of the adhesive layer, but no cutting debris is observed 4: Cutting debris or cracks are observed throughout the adhesive layer (for example, the cross-sectional view shown in Figure 4)

[0133] According to the present invention, it is possible to provide a pressure-sensitive adhesive composition that reduces the generation of cutting debris and damage to the pressure-sensitive adhesive layer when cutting, and that allows the formation of a pressure-sensitive adhesive layer with appropriate peel strength and strength. Therefore, it is possible to provide a protective sheet that has a pressure-sensitive adhesive layer that is a cured product of the pressure-sensitive adhesive composition, that reduces the generation of cutting debris and damage to the pressure-sensitive adhesive layer when cutting, and that has an appropriate peel strength and strength.

Claims

1. Polyurethane (A); an ethylenically unsaturated group-containing monomer (B); A photopolymerization initiator (C); A plasticizer (D), The polyurethane (A) is a polyurethane having a skeleton including a structure derived from a polyol and a structure derived from a polyisocyanate, and has an ethylenically unsaturated group at a terminal thereof, The ethylenically unsaturated group-containing monomer (B) contains a monofunctional (meth)acrylate (B1) and a polyfunctional (meth)acrylate (B2). The polyol is a polyoxyalkylene polyol having a number average molecular weight of 300 to 1800. The pressure-sensitive adhesive composition, wherein the polyurethane (A) has a weight average molecular weight of 3,000 to 30,000.

2. The pressure-sensitive adhesive composition according to claim 1, wherein the polyurethane (A) has an ethylenically unsaturated group equivalent of 900 to 3000 g / mol.

3. 3. The pressure-sensitive adhesive composition according to claim 1, wherein the polyurethane (A) has a urethane bond concentration of 1.0 to 5 mol / kg.

4. The pressure-sensitive adhesive composition according to claim 1 or 2, wherein the polyurethane (A) has an ethylenically unsaturated group at a site other than a terminal.

5. The pressure-sensitive adhesive composition according to claim 1 or 2, wherein the polyisocyanate comprises an ethylenically unsaturated group-containing polyisocyanate and an ethylenically unsaturated group-free polyisocyanate.

6. 3. The pressure-sensitive adhesive composition according to claim 1, wherein the monofunctional (meth)acrylate (B1) of the ethylenically unsaturated group-containing monomer (B) is a chain alkyl (meth)acrylate having 6 to 18 carbon atoms.

7. 3. The pressure-sensitive adhesive composition according to claim 1, wherein the monofunctional (meth)acrylate (B1) is contained in an amount of 5 to 89 mass% and the polyfunctional (meth)acrylate (B2) is contained in an amount of 0.1 to 30 mass% relative to a total of 100 mass% of the polyurethane (A) and the ethylenically unsaturated group-containing monomer (B).

8. 3. The pressure-sensitive adhesive composition according to claim 1, wherein the polyurethane (A) is contained in an amount of 10 to 90 mass%, and the ethylenically unsaturated group-containing monomer (B) is contained in an amount of 10 to 90 mass%, relative to a total of 100 mass% of the polyurethane (A) and the ethylenically unsaturated group-containing monomer (B).

9. 3. The pressure-sensitive adhesive composition according to claim 1, further comprising 0.05 to 5 parts by mass of a photopolymerization initiator (C) and 1 to 30 parts by mass of the plasticizer (D) relative to a total of 100 parts by mass of the polyurethane (A) and the ethylenically unsaturated group-containing monomer (B).

10. A cured product of the pressure-sensitive adhesive composition according to claim 1 or 2.

11. Elongation at break is 92-300% and breaking strength is 300-800g / mm ２ The cured product according to claim 10 .

12. A protective sheet comprising a substrate and a pressure-sensitive adhesive layer formed on one surface of the substrate, the pressure-sensitive adhesive layer comprising the cured product according to claim 10.