Hot-melt adhesive for linerless labels

Abstract

The present invention addresses the problem of providing an acrylic hot-melt adhesive for linerless labels having good recyclability. A means for solving the problem is a hot-melt adhesive for linerless labels, which contains an acrylate ester polymer that does not have a functional group.

Description

Hot melt adhesive for linerless labels

[0001] The present invention relates to a hot-melt pressure-sensitive adhesive for linerless labels, a pressure-sensitive adhesive layer containing the pressure-sensitive adhesive, and a linerless label having the pressure-sensitive adhesive layer.

[0002] Conventionally, solvent-based adhesives and emulsion-based adhesives have been primarily used as adhesives for linerless labels that do not require release paper. Linerless thermosensitive recording labels that do not require release paper are used as recording media for so-called facsimiles, various computers, POS (point-of-sales) systems, and the like. For example, Patent Document 1 discloses a thermosensitive recording label that does not require release paper, which has a thermosensitive recording layer, an intermediate layer, and a release layer, sequentially, on one side of a support, and an adhesive layer on the other side of the support. Patent Document 1 also describes that a two-component crosslinked acrylic adhesive is preferred as the adhesive that is the main component of the adhesive layer. When a linerless thermosensitive recording label such as that disclosed in Patent Document 1 is rolled, the adhesive layer is positioned on the release layer, allowing it to be rolled without the need for a separate release paper. However, when the adhesive is a two-component crosslinked acrylic adhesive or an acrylic emulsion adhesive, the adhesive layer is often formed by transfer printing. Therefore, even for linerless labels, a liner (release paper) is often used in the manufacturing process.

[0003] In recent years, hot-melt adhesives have often been used for the adhesive layer of linerless labels. With hot-melt adhesives, the adhesive layer can be formed by directly coating the adhesive on a support. Unlike two-component crosslinkable acrylic adhesives and acrylic emulsion adhesives, there is no need to form the adhesive layer by transfer. This eliminates the need for a liner during the manufacturing process, making it easier to produce linerless labels.

[0004] Rubber-based hot-melt adhesives are commonly used as hot-melt adhesives. For example, Patent Document 2 discloses a thermosensitive recording adhesive label in which the adhesive layer is primarily composed of a hot-melt rubber-based adhesive. However, rubber-based adhesives tend to stretch more easily than acrylic-based adhesives. Therefore, when a linerless label using a rubber-based adhesive is cut with an automatic cutter attached to a label dispenser or automatic labeler, the adhesive may adhere to the cutter blade, making it difficult to cut the label, or the adhesive may adhere to the slitter blade during slitter finishing, making it difficult to cut the label.

[0005] Patent Document 3 discloses a linerless label in which the adhesive layer is an ultraviolet-crosslinkable acrylic hot-melt adhesive.

[0006] JP 11-24565, JP 10-268768, JP 2013-160957

[0007] When a UV-crosslinkable acrylic hot-melt adhesive is used in the adhesive layer of a linerless label, the adhesive can be applied directly to the support or substrate. In addition, compared to when a rubber-based hot-melt adhesive is used, the adhesive is less likely to adhere to the blade, which is expected to improve the cutting ability of the linerless label.

[0008] In recent years, growing environmental awareness has led to growing interest in the recycling of resin products. Therefore, the present inventors recovered the adhesive layer of a linerless label, melted it, and investigated the recyclability of the adhesive. As a result, they found that ultraviolet-crosslinkable acrylic hot-melt adhesives are not suitable for recycling because insoluble matter is generated from the crosslinking points during melt-kneading, and the insoluble matter causes defects and opacity. Furthermore, ultraviolet-crosslinkable acrylic hot-melt adhesives form an adhesive layer upon ultraviolet irradiation. However, if the base material of a linerless label is thermal paper, ultraviolet light may affect the thermal paper, such as discoloring the paper. Therefore, not only from the perspective of recycling but also from the perspective of reducing the impact on the thermal paper, there is a demand for adhesives that do not require ultraviolet irradiation.

[0009] SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide an acrylic hot-melt pressure-sensitive adhesive for linerless labels that has good recyclability.

[0010] The present inventors have conducted extensive research to solve the above problems and have arrived at the present invention, which provides the following preferred embodiments.

[0011] [1] A hot melt pressure-sensitive adhesive for linerless labels, comprising an acrylic acid ester polymer having no functional groups.

[0012] [2] The pressure-sensitive adhesive according to [1], which has a melt viscosity at 180°C measured with a Brookfield viscometer of 50,000 mPa·s or less, preferably 5,000 to 50,000 mPa·s, more preferably 8,000 to 45,000 mPa·s, and even more preferably 10,000 to 40,000 mPa·s, and which does not contain a plasticizer.

[0013] [3] The pressure-sensitive adhesive according to [1] or [2], wherein the acrylic ester polymer is an acrylic block copolymer.

[0014] [4] The pressure-sensitive adhesive according to [3], wherein the acrylic block copolymer has two polymer blocks (A1) and (A2) containing units derived from a methacrylic acid ester and one polymer block (B) containing units derived from an acrylic acid ester, and comprises an acrylic triblock copolymer (I) having an (A1)-(B)-(A2) block structure.

[0015] [5] The pressure-sensitive adhesive according to [4], wherein the acrylic block copolymer (I) has a weight average molecular weight (Mw) of 50,000 to 250,000, preferably 60,000 to 250,000, more preferably 100,000 to 230,000, and even more preferably 130,000 to 210,000; the total content of polymer blocks (A1) and (A2) is 35% by mass or less, preferably 4 to 35% by mass, more preferably 4 to 30% by mass, even more preferably 8 to 25% by mass, and particularly preferably 8 to 23% by mass; and the weight average molecular weight of each of polymer blocks (A1) and (A2) is 3,000 or more, preferably 3,000 to 30,000, more preferably 4,500 to 30,000, even more preferably 5,000 to 25,000, and even more preferably 6,000 to 20,000.

[0016] [6] The methacrylic acid esters that constitute the structural units of the polymer blocks (A1) and (A2) are CH ３ C(=CH ２ ) COOR １ (In the formula, R １ represents an alkyl group having 1 to 10 carbon atoms), and the acrylate ester serving as a structural unit of the polymer block (B) is a methacrylic acid ester represented by the formula: ２ =CH-COOR ２ (In the formula, R ２ [3] The adhesive according to [2] or [3], wherein the acrylate is an acrylic acid ester represented by the formula (I) wherein R represents an alkyl group having 1 to 10 carbon atoms.

[0017] [7] The pressure-sensitive adhesive according to any one of [4] to [6], wherein the acrylic block copolymer has two polymer blocks (C1) and (C2) containing units derived from a methacrylic acid ester and one polymer block (D) containing units derived from an acrylic acid ester, and further contains an acrylic triblock copolymer (II) having a (C1)-(D)-(C2) block structure, and at least one of the polymer blocks (C1) and (C2) has a weight average molecular weight of 300 or more and less than 3,000, more preferably 500 or more and less than 2,000.

[0018] [8] The pressure-sensitive adhesive according to [7], wherein the acrylic triblock copolymer (II) has a weight average molecular weight (Mw) of 10,000 to 120,000, preferably 20,000 to 110,000, more preferably 25,000 to 100,000, and even more preferably 30,000 to 90,000, and the total content of the polymer blocks (C1) and (C2) is 25% by mass or less, preferably 3 to 25% by mass, more preferably 3 to 20% by mass, and even more preferably 5 to 10% by mass.

[0019] [9] The pressure-sensitive adhesive according to any one of [1] to [8], further comprising a tackifier resin.

[0020]

[10] The pressure-sensitive adhesive according to [9], wherein the content of the tackifier resin is 1 to 190 parts by mass, preferably 10 to 180 parts by mass, more preferably 15 to 170 parts by mass, and even more preferably 35 to 100 parts by mass, per 100 parts by mass of the total of the acrylic triblock copolymer (I) and the acrylic triblock copolymer (II).

[0021]

[11] An adhesive layer comprising the hot melt adhesive for a linerless label according to any one of [1] to

[10] .

[0022]

[12] A linerless label having a release layer containing a silicone-based release agent as a main component on one side of a paper substrate, and the pressure-sensitive adhesive layer according to

[11] on the other side of the paper substrate that does not have the release layer.

[0023] According to the present invention, it is possible to provide an acrylic hot-melt pressure-sensitive adhesive for linerless labels that has good recyclability.

[0024] Hereinafter, embodiments of the present invention will be described in detail. Note that the scope of the present invention is not limited to the embodiments described here, and various modifications can be made without departing from the spirit of the present invention.

[0025] [Hot-melt adhesive for linerless labels] The hot-melt adhesive for linerless labels of the present invention (hereinafter also referred to as "acrylic hot-melt adhesive" or simply "adhesive") contains an acrylic ester polymer having no functional groups. Because the acrylic hot-melt adhesive of the present invention is a hot-melt adhesive, the adhesive can be directly applied to a substrate (or support). The acrylic ester polymer contained in the acrylic hot-melt adhesive of the present invention is a polymer having no functional groups, and an adhesive layer can be formed without UV irradiation. Furthermore, because the acrylic hot-melt adhesive of the present invention is not crosslinked, it does not have crosslinking points, and no insoluble matter is generated when the adhesive layer is melt-kneaded. Therefore, the acrylic hot-melt adhesive of the present invention has good recyclability. In this specification, the term "functional group" refers to a crosslinkable functional group that can form a crosslinked structure by a crosslinking reaction, for example, due to UV irradiation. In this specification, the term "acrylic ester polymer having no functional groups" refers to an acrylic ester polymer that is not crosslinked, i.e., cannot form a crosslinked structure, such as an acrylic ester polymer that is not UV-crosslinked.

[0026] The acrylic ester polymer is preferably an acrylic block copolymer from the viewpoints of excellent transparency, adhesive properties, and processability, and the acrylic block copolymer preferably contains an acrylic triblock copolymer from the viewpoints of maintaining the cohesive strength of the polymer and preventing the adhesive from leaving adhesive residue on the substrate.

[0027] <Acrylic Triblock Copolymer (I)> In one embodiment of the present invention, the acrylic block copolymer preferably includes an acrylic triblock copolymer (I) having two polymer blocks (A1) and (A2) containing units derived from a methacrylic acid ester and one polymer block (B) containing units derived from an acrylic acid ester, and having an (A1)-(B)-(A2) block structure.

[0028] (Polymer Blocks (A1) and (A2)) The acrylic triblock copolymer (I) has two polymer blocks containing units derived from a methacrylic acid ester, namely polymer blocks (A1) and (A2) containing units derived from a methacrylic acid ester.

[0029] Examples of methacrylate esters that serve as structural units of the polymer blocks (A1) and (A2) include methacrylates that do not have a functional group, such as methyl methacrylate, ethyl methacrylate, isopropyl methacrylate, n-propyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, sec-butyl methacrylate, tert-butyl methacrylate, n-hexyl methacrylate, cyclohexyl methacrylate, 2-ethylhexyl methacrylate, n-octyl methacrylate, lauryl methacrylate, tridecyl methacrylate, stearyl methacrylate, isobornyl methacrylate, phenyl methacrylate, and benzyl methacrylate.

[0030] Among these, from the viewpoint of improving the heat resistance and durability of the resulting polymer, methyl methacrylate, ethyl methacrylate, tert-butyl methacrylate, cyclohexyl methacrylate, isobornyl methacrylate, and phenyl methacrylate are preferred. ３ C(=CH ２ ) COOR １ (In the formula, R １ R represents an alkyl group having 1 to 10 carbon atoms). １ is preferably an alkyl group having 1 to 10 carbon atoms, more preferably an alkyl group having 1 to 6 carbon atoms, and even more preferably an alkyl group having 1 to 3 carbon atoms. １may be a linear alkyl group or a branched alkyl group. Furthermore, methyl methacrylate is more preferred from the viewpoint of achieving clearer phase separation between the polymer blocks (A1) and (A2) and the polymer block (B) and increasing the cohesive strength. The polymer blocks (A1) and (A2) may be composed of one or more of these methacrylic acid esters. The acrylic triblock copolymer (I) has two polymer blocks (A1) and (A2) as polymer blocks containing units derived from methacrylic acid esters, and the methacrylic acid esters constituting these polymer blocks (A1) and (A2) may be the same or different, but are preferably the same.

[0031] The proportion of units derived from methacrylic acid esters contained in polymer blocks (A1) and (A2) may be preferably 60 to 100% by mass, more preferably 80 to 100% by mass, even more preferably 90 to 100% by mass, and particularly preferably 95 to 100% by mass, based on 100% by mass of the total amount of units constituting each of polymer blocks (A1) and (A2). Furthermore, the proportion of units derived from methacrylic acid esters in polymer blocks (A1) and (A2) may be 100% by mass.

[0032] The stereoregularities of the units derived from methacrylic acid ester contained in each of the polymer blocks (A1) and (A2) containing units derived from methacrylic acid ester may be the same or different.

[0033] In one embodiment of the present invention, the total content of polymer blocks (A1) and (A2) in the acrylic triblock copolymer (I) is preferably 35% by mass or less, more preferably 30% by mass or less, even more preferably 25% by mass or less, and particularly preferably 23% by mass or less, based on 100% by mass of the total amount of structural units of the acrylic triblock copolymer (I). The total content of polymer blocks (A1) and (A2) is preferably 4% by mass or more, more preferably 8% by mass or more, based on 100% by mass of the total amount of structural units of the acrylic triblock copolymer (I). In one embodiment of the present invention, the total content of polymer blocks (A1) and (A2) in the acrylic triblock copolymer (I) may be preferably 4 to 35% by mass, more preferably 4 to 30% by mass, even more preferably 8 to 25% by mass, and particularly preferably 8 to 23% by mass, based on 100% by mass of the total amount of structural units of the acrylic triblock copolymer (I). When the total content of polymer blocks (A1) and (A2) is within the above range, when the acrylic triblock copolymer (I) is incorporated into a pressure-sensitive adhesive, it exhibits excellent cohesive strength, and the tack and adhesive strength are further improved. Furthermore, the heat-resistant adhesive and holding power of the pressure-sensitive adhesive tend to be superior. The contents of polymer blocks (A1) and (A2) contained in the acrylic triblock copolymer (I) may be the same or different. In this specification, "tack" refers to the stickiness and wettability of the pressure-sensitive adhesive. Furthermore, "adhesive strength" refers to the force and strength required to peel the linerless label from the adherend.

[0034] In one embodiment of the present invention, the weight-average molecular weight of each of the polymer blocks (A1) and (A2) is preferably 3,000 or more, more preferably 4,500 or more, and even more preferably 6,000 or more. The weight-average molecular weight of each of the polymer blocks (A1) and (A2) is preferably 30,000 or less, more preferably 25,000 or less. In one embodiment of the present invention, the weight-average molecular weight of each of the polymer blocks (A1) and (A2) is preferably 3,000 to 30,000, more preferably 4,500 to 30,000, even more preferably 5,000 to 25,000, and even more preferably 6,000 to 20,000. When the weight-average molecular weights of the polymer blocks (A1) and (A2) are within the above ranges, when the acrylic triblock copolymer (I) is incorporated into a pressure-sensitive adhesive, the pressure-sensitive adhesive exhibits excellent cohesive strength while maintaining excellent handleability during production, resulting in further improved tack and adhesive strength. Furthermore, the heat-resistant adhesiveness and holding power of the acrylic hot-melt pressure-sensitive adhesive of the present invention tend to be superior. The weight average molecular weights of the polymer blocks (A1) and (A2) contained in the acrylic triblock copolymer (I) may be the same or different.

[0035] The glass transition temperature of each of the polymer blocks (A1) and (A2) is preferably 60 to 140°C, more preferably 70 to 130°C, and even more preferably 80 to 130°C. When the glass transition temperature is within this range, the polymer blocks (A1) and (A2) act as physical pseudo-crosslinking points at the normal use temperature of the pressure-sensitive adhesive, thereby exhibiting cohesive strength, and when incorporated into the pressure-sensitive adhesive, the pressure-sensitive adhesive exhibits excellent adhesive properties, durability, heat resistance, etc. Note that the glass transition temperature in this specification is the extrapolated onset temperature of the curve obtained by DSC measurement.

[0036] The glass transition temperatures of the polymer blocks (A1) and (A2) may be the same or different. Even when the glass transition temperatures of the polymer blocks (A1) and (A2) are different, it is preferable that the glass transition temperatures of both polymer blocks are within the above range.

[0037] (Polymer Block (B)) The acrylic triblock copolymer (I) has one polymer block (B) containing units derived from an acrylic ester.

[0038] Examples of acrylic acid esters that serve as structural units of the polymer block (B) include acrylic acid esters that do not have a functional group, such as methyl acrylate, ethyl acrylate, isopropyl acrylate, n-propyl acrylate, n-butyl acrylate, isobutyl acrylate, sec-butyl acrylate, tert-butyl acrylate, amyl acrylate, isoamyl acrylate, n-hexyl acrylate, 2-ethylhexyl acrylate, n-octyl acrylate, isooctyl acrylate, decyl acrylate, isobornyl acrylate, lauryl acrylate, octadecyl acrylate, cyclohexyl acrylate, phenyl acrylate, and benzyl acrylate.

[0039] Among these, from the viewpoint of the transparency and flexibility of the adhesive, ２ =CH-COOR ２ (In the formula, R ２ represents an organic group having 1 to 10 carbon atoms). Examples of the organic group having 1 to 10 carbon atoms include an alkyl group, an alicyclic hydrocarbon group, and an aryl group having 1 to 10 carbon atoms. Among these, R ２is preferably an alkyl group having 1 to 10 carbon atoms, more preferably an alkyl group having 1 to 6 carbon atoms, and even more preferably an alkyl group having 1 to 3 carbon atoms. The alkyl group may be a linear alkyl group or a branched alkyl group. In terms of clear phase separation between the polymer blocks (A1) and (A2) and the polymer block (B) and the PSA exhibiting high cohesive strength, methyl acrylate, ethyl acrylate, isopropyl acrylate, n-propyl acrylate, n-butyl acrylate, isobutyl acrylate, sec-butyl acrylate, tert-butyl acrylate, amyl acrylate, isoamyl acrylate, n-hexyl acrylate, 2-ethylhexyl acrylate, n-octyl acrylate, isooctyl acrylate, decyl acrylate, isobornyl acrylate, cyclohexyl acrylate, phenyl acrylate, benzyl acrylate, and the like are more preferred. From the viewpoint of the adhesive having suitable adhesiveness at room temperature and exhibiting stable adhesive strength over a wide temperature range and wide peel speed conditions, at least one selected from methyl acrylate, n-butyl acrylate, 2-ethylhexyl acrylate, n-octyl acrylate and isooctyl acrylate is even more preferred, and n-butyl acrylate and 2-ethylhexyl acrylate are particularly preferred.

[0040] The polymer block (B) may be composed of one type of these acrylic acid esters, or may be composed of two or more types. The proportion of units derived from acrylic acid esters contained in the polymer block (B) is preferably 60 to 100 mass%, more preferably 80 to 100 mass%, even more preferably 90 to 100 mass%, and even more preferably 95 to 100 mass%, based on 100 mass% of the total amount of units constituting the polymer block (B). Furthermore, the proportion of units derived from acrylic acid esters in the polymer block (B) may be 100 mass%.

[0041] In the acrylic triblock copolymer (I), the content of the polymer block (B) is preferably 65% ​​by mass or more, more preferably 70% by mass or more, even more preferably 75% by mass or more, and particularly preferably 77% by mass or more, based on 100% by mass of the total amount of the constituent units of the acrylic triblock copolymer (I). The content of the polymer block (B) is preferably 96% by mass or less, more preferably 92% by mass or less, based on 100% by mass of the total amount of the constituent units of the acrylic triblock copolymer (I). The content of the polymer block (B) is preferably 65 to 96% by mass, more preferably 70 to 96% by mass, even more preferably 75 to 92% by mass, and particularly preferably 77 to 92% by mass, based on 100% by mass of the total amount of the constituent units of the acrylic triblock copolymer (I). When the content of the polymer block (B) is within the above range, the acrylic triblock copolymer (I) exhibits excellent cohesive strength when used as a pressure-sensitive adhesive, and the tack and adhesive strength are further improved.

[0042] The glass transition temperature of the polymer block (B) is preferably −100 to 30° C., more preferably −80 to 10° C., and even more preferably −70 to 0° C. If the glass transition temperature is within this range, excellent tack and adhesive strength can be exhibited when used in a pressure-sensitive adhesive.

[0043] In one embodiment of the present invention, the weight average molecular weight of the polymer block (B) may be preferably 30,000 to 200,000, more preferably 40,000 to 200,000, and even more preferably 50,000 to 200,000.

[0044] Polymer blocks (A1) and (A2) and polymer block (B) may contain the monomer components of each other to the extent that the effects of the present invention are not impaired. For example, a tapered structure may be formed at the boundary between polymer block (A1) or (A2) and polymer block (B). Furthermore, polymer block (A1) or (A2) and polymer block (B) may not contain the monomer components of each other.

[0045] Furthermore, polymer blocks (A1) and (A2) and polymer block (B) may contain other monomer units as necessary. Examples of such other monomers include aromatic vinyl monomers such as styrene, α-methylstyrene, p-methylstyrene, and m-methylstyrene; conjugated diene monomers such as butadiene and isoprene; olefin monomers such as ethylene, propylene, isobutene, and octene; and lactone monomers such as ε-caprolactone and valerolactone. When each polymer block contains such other monomer, the content thereof is preferably 0 to 20% by mass, more preferably 0 to 10% by mass or less, and even more preferably 0 to 5% by mass or less, based on the total mass of the monomers constituting each polymer block.

[0046] The acrylic triblock copolymer (I) has an (A1)-(B)-(A2) block structure in which the polymer block (A1), the polymer block (B), and the polymer block (A2) are bonded in this order. When the acrylic triblock copolymer (I) having such a structure is contained in a pressure-sensitive adhesive, the pressure-sensitive adhesive tends to have excellent adhesive strength, holding power, and cohesive strength.

[0047] The weight-average molecular weight of the acrylic triblock copolymer (I) may be preferably 50,000 to 250,000, and from the viewpoints of cohesive strength and ease of handling during production, it is preferably 60,000 to 250,000, more preferably 100,000 to 230,000, and even more preferably 130,000 to 210,000. When the weight-average molecular weight of the acrylic triblock copolymer (I) is equal to or greater than the lower limit, good cohesive strength is easily obtained. On the other hand, when the weight-average molecular weight is equal to or less than the upper limit, good handleability during production is easily achieved.

[0048] The molecular weight distribution (Mw / Mn) of the acrylic triblock copolymer (I) is preferably 1.0 to 1.5, more preferably 1.0 to 1.4, and even more preferably 1.0 to 1.3. In this specification, the weight average molecular weight (Mw) and number average molecular weight (Mn) are weight average molecular weights and number average molecular weights calculated in terms of standard polystyrene, determined by gel permeation chromatography (GPC).

[0049] <Acrylic Triblock Copolymer (II)> In one embodiment of the present invention, the acrylic block copolymer preferably includes, in addition to the acrylic triblock copolymer (I), an acrylic triblock copolymer (II) having two polymer blocks (C1) and (C2) containing units derived from a methacrylic acid ester and one polymer block (D) containing units derived from an acrylic acid ester, and having a (C1)-(D)-(C2) block structure. By further including the acrylic triblock copolymer (II), it is easier to maintain a balance between the adhesive strength, adhesive transfer suppression effect, and melt viscosity of the pressure-sensitive adhesive.

[0050] (Polymer Blocks (C1) and (C2)) The acrylic triblock copolymer (II) has two polymer blocks containing units derived from a methacrylic acid ester, namely polymer blocks (C1) and (C2) containing units derived from a methacrylic acid ester.

[0051] Specific examples of the methacrylic acid esters that serve as structural units of the polymer blocks (C1) and (C2) are the same as the methacrylic acid esters that serve as structural units of the polymer blocks (A1) and (A2) of the acrylic triblock copolymer (I).

[0052] Among these, from the viewpoint of improving the heat resistance and durability of the resulting polymer, methyl methacrylate, ethyl methacrylate, tert-butyl methacrylate, cyclohexyl methacrylate, isobornyl methacrylate, and phenyl methacrylate are more preferred. ３ C(=CH ２ ) COOR ３ (In the formula, R ３R represents an alkyl group having 1 to 10 carbon atoms). １ is preferably an alkyl group having 1 to 10 carbon atoms, more preferably an alkyl group having 1 to 6 carbon atoms, and even more preferably an alkyl group having 1 to 3 carbon atoms. ３ may be a linear alkyl group or a branched alkyl group. Furthermore, methyl methacrylate is more preferred because it results in clearer phase separation between the polymer blocks (C1) and (C2) and the polymer block (D) and increases the cohesive strength. The polymer blocks (C1) and (C2) may be composed of one or more of these methacrylic acid esters. The acrylic triblock copolymer (II) has two polymer blocks (C1) and (C2) as polymer blocks containing units derived from methacrylic acid esters, and the methacrylic acid esters constituting these polymer blocks (C1) and (C2) may be the same or different, but are preferably the same.

[0053] The proportion of units derived from methacrylic acid esters contained in polymer blocks (C1) and (C2) is preferably 60 to 100 mass%, more preferably 80 to 100 mass%, even more preferably 90 to 100 mass%, and particularly preferably 95 to 100 mass%, based on 100 mass% of the total amount of units constituting each of polymer blocks (C1) and (C2). The proportion of units derived from methacrylic acid esters in polymer blocks (C1) and (C2) may be 100 mass%.

[0054] The stereoregularities of the units derived from methacrylic acid ester contained in the polymer blocks (C1) and (C2) may be the same or different.

[0055] In one embodiment of the present invention, the total content of polymer blocks (C1) and (C2) in the acrylic triblock copolymer (II) may be preferably 25% by mass or less, more preferably 20% by mass or less, and even more preferably 10% by mass or less, based on 100% by mass of the total amount of structural units of the acrylic triblock copolymer (II). Furthermore, the content of polymer blocks (C1) and (C2) is preferably 3% by mass or more, and more preferably 5% by mass or more, based on 100% by mass of the total amount of structural units of the acrylic triblock copolymer (II). Furthermore, in one embodiment of the present invention, the total content of polymer blocks (C1) and (C2) in the acrylic triblock copolymer (II) may be preferably 3 to 25% by mass, more preferably 3 to 20% by mass, and even more preferably 5 to 10% by mass, based on 100% by mass of the total amount of structural units of the acrylic triblock copolymer (II). When the total content of the polymer blocks (C1) and (C2) is within the above range, when the acrylic triblock copolymer (II) is incorporated into a pressure-sensitive adhesive, the tack and processability tend to be further improved without impairing the cohesive force.

[0056] In one embodiment of the present invention, the acrylic triblock copolymer (II) preferably has polymer blocks (C1) and (C2) with different weight-average molecular weights. In this case, the weight-average molecular weight of at least one of the polymer blocks (C1) and (C2) is preferably 300 or more but less than 3,000, and more preferably 500 or more but less than 3,000. When the weight-average molecular weight of at least one of the polymer blocks (C1) and (C2) is 300 or more, the resulting acrylic triblock copolymer (II) tends to have excellent cohesive strength. Furthermore, when the weight-average molecular weight of at least one of the polymer blocks (C1) and (C2) is less than 3,000, the resulting acrylic triblock copolymer (II) tends to have a low melt viscosity, which tends to improve productivity in producing a pressure-sensitive adhesive.

[0057] In one embodiment of the present invention, the weight-average molecular weight of the polymer block (C2) is preferably 300 or more but less than 3,000, and more preferably 500 or more but less than 2,000. The weight-average molecular weight of the polymer block (C1) is preferably 3,000 to 10,000, and more preferably 3,000 to 7,000. When the weight-average molecular weight is within the above range, the viscosity, cohesive strength, and handleability during production are excellent.

[0058] The glass transition temperatures of the polymer blocks (C1) and (C2) are preferably 60 to 140° C., more preferably 70 to 130° C., and even more preferably 80 to 130° C. When the glass transition temperatures are within this range, the polymer blocks (C1) and (C2) act as physical pseudo-crosslinking points at temperatures at which the PSA is normally used, thereby exhibiting cohesive strength, and when incorporated into the PSA, the PSA exhibits excellent adhesive properties, durability, heat resistance, etc.

[0059] (Polymer Block (D)) The acrylic triblock copolymer (II) has one polymer block (D) containing units derived from an acrylic acid ester.

[0060] Specific examples of the acrylic acid ester that serves as a structural unit of the polymer block (D) are the same as the acrylic acid ester that serves as a structural unit of the polymer block (B) of the acrylic triblock copolymer (I).

[0061] Among these, from the viewpoint of transparency and flexibility when used as an adhesive, ２ =CH-COOR ４ (In the formula, R ４ represents an organic group having 1 to 10 carbon atoms). Examples of the organic group having 1 to 10 carbon atoms include an alkyl group, an alicyclic hydrocarbon group, and an aryl group having 1 to 10 carbon atoms. Among these, R ４is preferably an alkyl group having 1 to 10 carbon atoms, more preferably an alkyl group having 1 to 6 carbon atoms, and even more preferably an alkyl group having 1 to 3 carbon atoms. The alkyl group may be a linear alkyl group or a branched alkyl group. In terms of clearly phase-separating the polymer blocks (C1) and (C2) from the polymer block (D) and thereby enabling the PSA to exhibit high cohesive strength, acrylic esters having no functional group, such as methyl acrylate, ethyl acrylate, isopropyl acrylate, n-propyl acrylate, n-butyl acrylate, isobutyl acrylate, sec-butyl acrylate, tert-butyl acrylate, amyl acrylate, isoamyl acrylate, n-hexyl acrylate, 2-ethylhexyl acrylate, n-octyl acrylate, isooctyl acrylate, decyl acrylate, isobornyl acrylate, cyclohexyl acrylate, phenyl acrylate, and benzyl acrylate, are more preferred. In terms of the adhesive having suitable adhesiveness at room temperature and exhibiting stable adhesive strength over a wide temperature range and a wide peel speed range, at least one selected from methyl acrylate, n-butyl acrylate, 2-ethylhexyl acrylate, n-octyl acrylate and isooctyl acrylate is even more preferred, and n-butyl acrylate is particularly preferred.

[0062] The polymer block (D) may be composed of one type of these acrylic acid esters, or may be composed of two or more types. The proportion of units derived from acrylic acid esters contained in the polymer block (D) is preferably 60 to 100 mass%, more preferably 80 to 100 mass%, even more preferably 90 to 100 mass%, and even more preferably 95 to 100 mass%, based on 100 mass% of the total amount of units constituting the polymer block (D). Furthermore, the proportion of units derived from acrylic acid esters in the polymer block (D) may be 100 mass%.

[0063] In the acrylic triblock copolymer (II), the content of the polymer block (D) is preferably 75% by mass or more, more preferably 80% by mass or more, and even more preferably 90% by mass or more, based on 100% by mass of the total amount of the constituent units of the acrylic triblock copolymer (II). The content of the polymer block (D) is preferably 97% by mass or less, more preferably 95% by mass or less, based on 100% by mass of the total amount of the constituent units of the acrylic triblock copolymer (II). The content of the polymer block (D) in the acrylic triblock copolymer (II) is preferably 75 to 97% by mass, more preferably 80 to 97% by mass, and even more preferably 90 to 95% by mass, based on 100% by mass of the total amount of the constituent units of the acrylic triblock copolymer (II). When the content of the polymer block (D) is within the above range, when the acrylic triblock copolymer (II) is incorporated into a pressure-sensitive adhesive, the tack and processability are further improved without impairing the cohesive strength.

[0064] The glass transition temperature of the polymer block (D) is preferably −100 to 30° C., more preferably −80 to 10° C., and even more preferably −70 to 0° C. When the glass transition temperature is within this range, excellent tack and adhesive strength can be exhibited when used in a pressure-sensitive adhesive.

[0065] In one embodiment of the present invention, the weight average molecular weight of the polymer block (D) may be preferably 40,000 to 70,000, more preferably 40,000 to 65,000, and even more preferably 41,000 to 65,000.

[0066] Polymer blocks (C1) and (C2) and polymer block (D) may contain the monomer components of each other to the extent that the effects of the present invention are not impaired. For example, the boundary between polymer block (C1) or (C2) and polymer block (D) may have a tapered structure. Furthermore, polymer block (C1) or (C2) and polymer block (D) may not contain the monomer components of each other.

[0067] Furthermore, the polymer blocks (C1) and (C2) and polymer block (D) may contain other monomer units as needed. Specific examples of such other monomers are the same as the other monomers exemplified as being used for the other monomer units optionally contained in the polymer blocks (A1) and (A2) and polymer block (B) of the acrylic triblock copolymer (I). When each polymer block contains such other monomers, the content thereof is preferably 0 to 20% by mass, more preferably 0 to 10% by mass, and even more preferably 0 to 5% by mass, based on the total mass of the monomers constituting each polymer block.

[0068] The acrylic triblock copolymer (II) has a (C1)-(D)-(C2) block structure in which the polymer block (C1), the polymer block (D), and the polymer block (C2) are bonded in this order. When the acrylic triblock copolymer (II) having such a structure is contained in a pressure-sensitive adhesive, the tack and processability tend to be further improved.

[0069] The weight-average molecular weight of the acrylic triblock copolymer (II) may be preferably 10,000 to 120,000, and from the viewpoints of cohesive strength and ease of handling during production, it is preferably 20,000 to 110,000, more preferably 25,000 to 100,000, and even more preferably 30,000 to 90,000. When the weight-average molecular weight of the acrylic triblock copolymer (II) is equal to or greater than the lower limit, good cohesive strength is easily obtained. On the other hand, when the weight-average molecular weight is equal to or less than the upper limit, good handleability during production is easily achieved. In one embodiment of the present invention, by combining an acrylic triblock copolymer (I) having a relatively high molecular weight (e.g., a weight average molecular weight of 130,000 to 210,000) with an acrylic triblock copolymer (II) having a relatively low molecular weight (e.g., a weight average molecular weight of 10,000 to 120,000), the melt viscosity of the adhesive, particularly the melt viscosity at the coating temperature, can be easily reduced, and an adhesive having good coatability can be obtained, for example, without adding a plasticizer.

[0070] The molecular weight distribution of the acrylic triblock copolymer (II) is preferably 1.0 to 1.5, more preferably 1.0 to 1.4, and even more preferably 1.0 to 1.3.

[0071] In one embodiment of the present invention, the acrylic triblock copolymer (II) preferably has a melt viscosity of 100,000 mPa·s or less at 100°C, as measured with a Brookfield viscometer. The melt viscosity is preferably 1,000 to 100,000 mPa·s, and more preferably 5,000 to 100,000 mPa·s. By having such a melt viscosity, the acrylic triblock copolymer (II) is not solid at room temperature (approximately 25°C) but has low fluidity, which tends to improve workability when added to a pressure-sensitive adhesive. Therefore, in one embodiment of the present invention, by including an acrylic triblock copolymer (II) having a melt viscosity within the above range, the pressure-sensitive adhesive of the present invention can have a low melt viscosity even without the addition of a plasticizer, thereby achieving both coatability and adhesion to adherends. The melt viscosity of the acrylic triblock copolymer (II) can be adjusted by adjusting the weight-average molecular weight of the acrylic triblock copolymer (II), for example, to fall within the above-mentioned preferred range.

[0072] The mass ratio of the acrylic triblock copolymer (I) to the acrylic triblock copolymer (II) contained in the acrylic hot-melt pressure-sensitive adhesive of the present invention is not particularly limited, but from the viewpoint of achieving a good balance between adhesive strength, cohesive strength, and adhesive properties, the mass ratio of acrylic triblock copolymer (I):acrylic triblock copolymer (II) is preferably 100:1 to 100:500, more preferably 100:1 to 100:300, even more preferably 100:1 to 100:100, still more preferably 100:1 to 100:80, particularly preferably 100:1 to 100:60, and particularly more preferably 100:10 to 100:60.

[0073] The method for producing the acrylic triblock copolymer (I) and the acrylic triblock copolymer (II) is not particularly limited as long as it can produce a polymer that satisfies the conditions of the present invention regarding the chemical structure, and a method similar to a known technique can be adopted. Generally, a method for producing a block copolymer having a narrow molecular weight distribution is a method of living polymerization of monomers that are constituent units. Examples of such living polymerization techniques include a method of living polymerization using an organic rare earth metal complex as a polymerization initiator (see JP-A-06-93060), a method of living anionic polymerization using an organic alkali metal compound as a polymerization initiator in the presence of a mineral acid salt such as an alkali metal or alkaline earth metal salt (see JP-A-05-507737), a method of living anionic polymerization using an organic alkali metal compound as a polymerization initiator in the presence of an organoaluminum compound (see JP-A-11-335432), and atom transfer radical polymerization (ATRP) (see Macromolecular Chemistry and Physics, 2000, Vol. 201, pp. 1108-1114).

[0074] Among the above-mentioned production methods, the method of living anionic polymerization using an organic alkali metal compound as a polymerization initiator in the presence of an organoaluminum compound is preferred because the resulting block copolymer has high transparency, contains less residual monomer, has a reduced odor, and prevents the generation of bubbles after lamination when used as an adhesive. Furthermore, it is preferred because the molecular structure of the methacrylic acid ester polymer block is highly syndiotactic, which has the effect of enhancing the heat resistance of the adhesive, and because living polymerization can be carried out under relatively mild temperature conditions, resulting in a small environmental impact (mainly the power required for a refrigerator to control the polymerization temperature) when produced industrially.

[0075] The organoaluminum compound is, for example, a compound represented by the following general formula (1): AlR ５ R ６ R ７ (1) (In formula (1), R ５ , R ６ and R ７each independently represents an alkyl group which may have a substituent, a cycloalkyl group which may have a substituent, an aryl group which may have a substituent, an aralkyl group which may have a substituent, an alkoxy group which may have a substituent, an aryloxy group which may have a substituent, or an N,N-disubstituted amino group, or R ５ is any of the groups described above, and R ６ and R ７ and R 1 and R 2 are bonded together to form an arylenedioxy group which may have a substituent.

[0076] Preferred examples of the organoaluminum compound represented by the general formula (1) include isobutylbis(2,6-di-tert-butyl-4-methylphenoxy)aluminum, isobutylbis(2,6-di-tert-butylphenoxy)aluminum, and isobutyl[2,2'-methylenebis(4-methyl-6-tert-butylphenoxy)]aluminum, from the viewpoints of high living polymerization properties and ease of handling.

[0077] Examples of the organic alkali metal compound include alkyllithiums and alkyldilithiums such as n-butyllithium, sec-butyllithium, isobutyllithium, tert-butyllithium, n-pentyllithium, and tetramethylenedilithium; aryllithiums and aryldilithiums such as phenyllithium, p-tolyllithium, and lithium naphthalene; aralkyllithiums and aralkyldilithiums such as benzyllithium, diphenylmethyllithium, and dilithium produced by the reaction of diisopropenylbenzene with butyllithium; lithium amides such as lithium dimethylamide; and lithium alkoxides such as methoxylithium and ethoxylithium. These may be used alone or in combination of two or more. Among these, alkyllithiums are preferred due to their high polymerization initiation efficiency, with tert-butyllithium and sec-butyllithium being more preferred, and sec-butyllithium being even more preferred.

[0078] The living anionic polymerization is usually carried out in the presence of a solvent inert to the polymerization reaction, such as aromatic hydrocarbons (e.g., benzene, toluene, xylene, etc.), halogenated hydrocarbons (e.g., chloroform, methylene chloride, carbon tetrachloride, etc.), and ethers (e.g., tetrahydrofuran, diethyl ether, etc.), with toluene being preferred.

[0079] The block copolymer can be produced, for example, by repeating a process of forming a desired polymer block (polymer block (A1), polymer block (B), etc.) at the desired terminal of a living polymer obtained by polymerizing monomers a desired number of times, and then terminating the polymerization reaction. Specifically, for example, a multi-stage polymerization process including a first step of polymerizing a monomer that forms a first polymer block using a polymerization initiator composed of an organic alkali metal compound in the presence of an organoaluminum compound, a second step of polymerizing a monomer that forms a second polymer block, and a third step of polymerizing a monomer that forms a third polymer block can be carried out, and the active terminal of the resulting polymer can be reacted with an alcohol or the like to terminate the polymerization reaction, thereby producing the acrylic triblock copolymer (I) and the acrylic triblock copolymer (II). According to the above-described method, a triblock copolymer consisting of polymer block (A1), polymer block (B), and polymer block (A2), and a triblock copolymer consisting of polymer block (C1), polymer block (D), and polymer block (C2) can be produced.

[0080] The polymerization temperature is preferably 0 to 100°C when forming polymer blocks (A1), (A2), (C1), and (C2), and -50 to 50°C when forming polymer blocks (B) and (D). If the polymerization temperature is lower than the above range, the reaction proceeds slowly, and a long time is required to complete the reaction. On the other hand, if the polymerization temperature is higher than the above range, deactivation of living polymer ends increases, the molecular weight distribution becomes broad, or the desired block copolymer cannot be obtained. Furthermore, each polymer block can be polymerized within a range of 1 second to 20 hours.

[0081] <Tackifier Resin> In one embodiment of the present invention, the acrylic hot-melt pressure-sensitive adhesive of the present invention preferably further comprises a tackifier resin. When the acrylic hot-melt pressure-sensitive adhesive of the present invention comprises a tackifier resin, the adhesiveness, tackiness, and compatibility are further improved. Furthermore, when the acrylic hot-melt pressure-sensitive adhesive of the present invention comprises a tackifier resin, the melt viscosity can be further reduced even when no plasticizer is contained, making it easier to achieve both good coatability and high adhesiveness to adherends.

[0082] The tackifying resin that can be contained in the acrylic hot-melt pressure-sensitive adhesive of the present invention is not particularly limited and any of the tackifying resins that have conventionally been used in pressure-sensitive adhesives can be used. Examples of the tackifying resin that can be contained in the acrylic hot-melt pressure-sensitive adhesive of the present invention include rosin-based resins (rosin, rosin derivatives, hydrogenated rosin derivatives, etc.), terpene-based resins, terpene-phenol-based resins, (hydrogenated) petroleum-based resins, styrene-based resins, xylene-based resins, hydrogenated aromatic copolymers, phenol-based resins, and coumarone-indene-based resins.

[0083] In the present invention, rosin-based resin refers to an amber-colored, amorphous natural resin obtained from pine trees, whose main component is a mixture of abietic acid and its isomers. Rosin-based resins also include those modified by esterification, polymerization, hydrogenation, or other methods that utilize the reactivity of abietic acid or its isomers. Examples of rosin-based resins include unmodified rosins (e.g., tall rosin, gum rosin, and wood rosin), polymerized rosin, disproportionated rosin, hydrogenated rosin, maleic acid-modified rosin, fumaric acid-modified rosin, and their esters (e.g., glycerin esters, pentaerythritol esters, methyl esters, ethyl esters, butyl esters, and ethylene glycol esters), as well as those further modified by hydrogenation (hereinafter also referred to as hydrogenation). Hydrogenated rosin esters are particularly preferred from the standpoint of heat resistance and coloration resistance.

[0084] In the present invention, the terpene resin is an oligomer obtained by polymerizing a raw material containing a terpene monomer. The terpene resin also includes oligomers obtained by modifying the oligomer obtained in this way by hydrogenation or the like. Terpenes are generally isoprene (C５ H ８ ) polymers of monoterpenes (C １０ H １６ ), sesquiterpenes (C １５ H ２４ ), diterpenes (C ２０ H ３２ ) and the like. Terpene monomers are monomers having these as the basic skeleton, and examples thereof include α-pinene, β-pinene, dipentene, limonene, myrcene, alloocimene, ocimene, α-phellandrein, α-terpinene, γ-terpinene, terpinolene, 1,8-cineole, 1,4-cineole, α-terpineol, β-terpineol, γ-terpineol, sabinene, paramentadienes, and carenes. Furthermore, raw materials containing terpene monomers may contain other monomers copolymerizable with the terpene monomers, such as benzofuran (C ８ H ６ and coumarone-based monomers such as methylstyrene, methylstyrene, vinyltoluene, divinyltoluene, and 2-phenyl-2-butene; vinyl aromatic compounds such as styrene, α-methylstyrene, vinyltoluene, divinyltoluene, and 2-phenyl-2-butene; and phenol-based monomers such as phenol, cresol, xylenol, propylphenol, norylphenol, hydroquinone, resorcinol, methoxyphenol, bromophenol, bisphenol A, and bisphenol F. Examples of oligomers obtained by polymerizing raw materials containing other monomers copolymerizable with the above-mentioned terpene-based monomers include terpene-phenolic resins. Furthermore, oligomers obtained in this manner that have been modified by hydrogenation or the like are also included in the terpene-based resins.

[0085] In the present invention, the (hydrogenated) petroleum-based resin is C ５ Distillate, C ９ Distillate, C ５ Purified components of the fraction, C ９ It is an oligomer obtained by polymerizing a purified fraction or a raw material consisting of a mixture of these fractions or purified components. In addition, oligomers obtained in this manner that are modified by hydrogenation or the like are also included in the above (hydrogenated) petroleum-based resins. ５The fraction usually contains cyclopentadiene, dicyclopentadiene, isoprene, 1,3-pentadiene, 2-methyl-1-butene, 2-methyl-2-butene, 1-pentene, 2-pentene, cyclopentene, etc., and C ９ The fraction usually contains styrene, allylbenzene, α-methylstyrene, vinyltoluene, β-methylstyrene, indene, etc. ９ The fraction contains a small amount of C ８ Fraction and C １０ May contain fractions.

[0086] The (hydrogenated) petroleum-based resin is C ５ C made from fraction or its purified components ５ resin, C ９ C made from fraction or its purified components ９ resin, C ５ Fraction or its purified components and C ９ C, which is made from a fraction or a mixture of the fraction and its purified components ５ -C ９ Among these, hydrogenated petroleum-based resins are preferably used from the viewpoint of heat resistance and coloration resistance.

[0087] In the present invention, aromatic hydrocarbon resins such as styrene-based resins, xylene-based resins, hydrogenated aromatic copolymers, and phenol-based resins are oligomers obtained by polymerizing raw materials consisting of, for example, styrene, α-methylstyrene, vinyltoluene, β-methylstyrene, divinyltoluene, 2-phenyl-2-butene, methoxystyrene, t-butylstyrene, chlorostyrene, indene, methylindene, phenol, cresol, xylenol, propylphenol, nonylphenol, hydroquinone, resorcinol, methoxyphenol, bromophenol, bisphenol A, bisphenol F, or mixtures thereof. Oligomers obtained in this manner that have been modified by hydrogenation or other methods are also included. Other examples include coumarone-indene resins. Among these, styrene-based resins are particularly preferred from the viewpoints of compatibility, heat resistance, and coloration resistance.

[0088] These tackifier resins may be contained alone or in combination of two or more types, which is desirable in that the composition has an excellent balance of adhesive strength and tackiness.

[0089] In one embodiment of the present invention, the PSA of the present invention preferably contains two or more tackifying resins, more preferably two or more tackifying resins selected from the group consisting of rosin-based resins, terpene-based resins, and aromatic hydrocarbon-based resins, and even more preferably a rosin-based resin and a terpene-based resin. In some cases, by incorporating two or more of the above-mentioned specific tackifying resins into the PSA, adhesion to an adherend can be improved, and the melt viscosity of the PSA (particularly the melt viscosity at the coating temperature, e.g., at 180°C) can be sufficiently lowered without the use of a plasticizer, thereby improving coatability.

[0090] In one embodiment of the present invention, when the PSA of the present invention contains both a rosin resin and a terpene resin, the mass ratio of the rosin resin to the terpene resin (rosin resin / terpene resin) is preferably greater than 0.5 and not greater than 3, more preferably 0.55 to 2.5, and even more preferably 0.6 to 2. When the mass ratio is within the above range, the adhesiveness to the adherend is easily improved, and even when no plasticizer is contained, the melt viscosity of the PSA at 180°C is sufficiently reduced, making it easy to improve coatability.

[0091] The content of the tackifier resin may be preferably 1 to 190 parts by mass, more preferably 10 to 180 parts by mass, even more preferably 15 to 170 parts by mass, and particularly preferably 35 to 100 parts by mass, relative to 100 parts by mass of the total of the acrylic triblock copolymer (I) and the acrylic triblock copolymer (II). When the tackifier resin is a rosin-based resin, a terpene phenol-based resin, a styrene-based resin, a xylene-based resin, a hydrogenated aromatic copolymer, a phenol-based resin, a coumarone-indene-based resin, or the like, a tackifier resin content within the above range results in an excellent balance of adhesive strength, tack, and cohesive strength of the acrylic hot-melt pressure-sensitive adhesive.

[0092] <Plasticizer> In one embodiment of the present invention, the acrylic hot-melt pressure-sensitive adhesive of the present invention may contain a plasticizer. By containing a plasticizer in the acrylic hot-melt pressure-sensitive adhesive of the present invention, a pressure-sensitive adhesive having an excellent balance between adhesive strength and tackiness can be obtained, and further, the cost of the pressure-sensitive adhesive as a whole can generally be reduced.

[0093] Examples of such plasticizers include phthalates such as dibutyl phthalate, di-n-octyl phthalate, bis(2-ethylhexyl) phthalate, bis(2-ethylhexyl) terephthalate, di-n-decyl phthalate, and diisodecyl phthalate; adipates such as bis(2-ethylhexyl) adipate, diisodecyl adipate, and di-n-octyl adipate; sebacates such as bis(2-ethylhexyl) sebacate and di-n-butyl sebacate; azelaates such as bis(2-ethylhexyl) azelate; acetyl citrate tributary; citric acid esters such as ethyl acetate; paraffins such as chlorinated paraffin; glycols such as polypropylene glycol; epoxy-modified vegetable oils such as epoxidized soybean oil and epoxidized linseed oil; phosphate esters such as trioctyl phosphate and triphenyl phosphate; phosphites such as triphenyl phosphite; ester oligomers such as esters of adipic acid and 1,3-butylene glycol, esters of benzoic acid and dipropylene glycol; low molecular weight polymers such as low molecular weight polybutene, low molecular weight polyisobutylene, and low molecular weight polyisoprene; ARUFON Examples of suitable oils include acrylic oligomers such as UP1171 (manufactured by Toagosei Co., Ltd.), n-butyl polyacrylate, and 2-ethylhexyl polyacrylate; paraffinic oils such as Diana Process Oil PW series (manufactured by Idemitsu Kosan Co., Ltd.), SUNPUREL W70, and P series (manufactured by Japan Sun Oil Co., Ltd.); naphthenic oils such as SUNPUREN 90, NX90, and the SUNTHENE series (manufactured by Japan Sun Oil Co., Ltd.); and aromatic oils such as JSOAROMA 790 (manufactured by Japan Sun Oil Co., Ltd.) and Vivatec 500 (manufactured by H&R). These may be used alone or in combination of two or more.

[0094] In one embodiment of the present invention, the content of the plasticizer is preferably 0 to 80 parts by mass, more preferably 0 to 70 parts by mass, and even more preferably 0 to 60 parts by mass, relative to 100 parts by mass of the total of the acrylic triblock copolymer (I) and the acrylic triblock copolymer (II). By having the content of the plasticizer in the above range, an acrylic hot-melt pressure-sensitive adhesive that is free from bleeding and has excellent transparency can be obtained.

[0095] In another embodiment of the present invention, from the viewpoint of improving adhesiveness to adherends, the content of the plasticizer is preferably 0 to 50 parts by mass, more preferably 0 to 30 parts by mass, even more preferably 0 to 10 parts by mass, still more preferably 0 to 5 parts by mass, and particularly preferably 0 to 1 part by mass, per 100 parts by mass of the total of the acrylic triblock copolymer (I) and the acrylic triblock copolymer (II). It is particularly preferred that the acrylic hot-melt pressure-sensitive adhesive of the present invention does not contain a plasticizer. When the plasticizer content is within the above range, particularly when no plasticizer is contained, the adhesiveness of the present invention to adherends is likely to be improved.

[0096] The acrylic hot-melt pressure-sensitive adhesive of the present invention may contain other polymers; and additives such as softeners, heat stabilizers, light stabilizers, antistatic agents, flame retardants, foaming agents, colorants, dyes, refractive index modifiers, fillers, curing agents, and anti-sticking agents, as long as the effects of the present invention are not impaired. These other polymers and additives may be contained alone or in combination of two or more. The total content of the other polymers and additives is preferably 0 to 10 parts by mass, more preferably 0 to 5 parts by mass, and even more preferably 0 to 3 parts by mass, per 100 parts by mass of the total of the acrylic triblock copolymer (I) and the acrylic triblock copolymer (II).

[0097] Examples of other polymers include acrylic resins such as polymethyl methacrylate and (meth)acrylic acid ester copolymers; olefin resins such as polyethylene, ethylene-vinyl acetate copolymer, polypropylene, polybutene-1, poly-4-methylpentene-1, and polynorbornene; ethylene ionomers; styrene resins such as polystyrene, styrene-maleic anhydride copolymer, high impact polystyrene, AS resin, ABS resin, AES resin, AAS resin, ACS resin, and MBS resin; styrene-methyl methacrylate copolymer; polyethylene terephthalate, poly Examples of suitable adhesives include polyester resins such as butylene terephthalate and polylactic acid; polyamides such as nylon 6, nylon 66, and polyamide elastomers; polycarbonate; polyvinyl chloride; polyvinylidene chloride; polyvinyl alcohol; ethylene-vinyl alcohol copolymers; polyacetal; polyvinylidene fluoride; polyurethane; modified polyphenylene ether; polyphenylene sulfide; silicone rubber-modified resins; acrylic rubbers; silicone rubbers; styrene-based thermoplastic elastomers such as SEPS, SEBS, and SIS; and olefin-based rubbers such as IR, EPR, and EPDM. Among these, from the viewpoint of compatibility with the acrylic triblock copolymer (I) contained in the acrylic hot-melt pressure-sensitive adhesive of the present invention, acrylic resins, ethylene-vinyl acetate copolymers, AS resins, polylactic acid, polyvinylidene fluoride, and styrene-based thermoplastic elastomers are preferred, and (meth)acrylic acid ester copolymers are more preferred.

[0098] Examples of the filler include inorganic fibers such as glass fiber and carbon fiber, organic fibers, inorganic fillers such as calcium carbonate, talc, titanium oxide, silica, clay, barium sulfate and magnesium carbonate, carbon black, etc. When these fillers are contained, the resulting pressure-sensitive adhesive is endowed with durability, heat resistance and weather resistance.

[0099] When the acrylic hot-melt pressure-sensitive adhesive of the present invention contains an anti-sticking agent, it is expected to improve handling.As the anti-sticking agent, for example, fatty acids such as stearic acid, palmitic acid, etc.; fatty acid metal salts such as calcium stearate, zinc stearate, magnesium stearate, potassium palmitate, sodium palmitate, etc.; waxes such as polyethylene wax, polypropylene wax, montanic acid wax, etc.; low molecular weight polyolefins such as low molecular weight polyethylene and low molecular weight polypropylene, acrylic resin powder, polyorganosiloxane such as dimethylpolysiloxane, octadecylamine, alkyl phosphate, fatty acid ester, amide resin powder such as ethylenebisstearylamide, fluororesin powder such as tetrafluoroethylene resin, molybdenum disulfide powder, silicone resin powder, silicone rubber powder, silica, etc. can be mentioned.

[0100] <Hot melt pressure-sensitive adhesive for linerless labels> The acrylic acid ester polymer contained in the acrylic hot melt pressure-sensitive adhesive of the present invention is a polymer having no functional groups, and therefore a pressure-sensitive adhesive layer can be formed without irradiation with ultraviolet light. Furthermore, since the acrylic hot melt pressure-sensitive adhesive of the present invention is not cross-linked, it does not have cross-linking points, and therefore no insoluble matter is generated when the pressure-sensitive adhesive layer is melt-kneaded. Therefore, the acrylic hot melt pressure-sensitive adhesive of the present invention has good recyclability.

[0101] Furthermore, since the acrylic hot-melt pressure-sensitive adhesive of the present invention can form a pressure-sensitive adhesive layer without irradiating it with ultraviolet light, it is also suitable for use in linerless labels using thermal paper as a substrate.

[0102] Furthermore, since the acrylic hot-melt pressure-sensitive adhesive of the present invention is an acrylic hot-melt pressure-sensitive adhesive, it is less likely to adhere to the blades of an autocutter or slitter than rubber-based hot-melt pressure-sensitive adhesives, and has excellent cutting properties.

[0103] A unique challenge for hot-melt adhesives for linerless labels, particularly those that are not UV-crosslinkable, is the need to achieve both good coatability and high adhesion to the substrate. Coatability is related to the melt viscosity of the adhesive at the coating temperature; if the melt viscosity is too high, coatability decreases, making the adhesive unsuitable for linerless label applications. UV-crosslinkable hot-melt adhesives have a relatively low melt viscosity at the time of coating because they are not yet UV-crosslinked, making coatability a rare issue. The melt viscosity of an adhesive is generally reduced by adding a plasticizer. However, because plasticizers not only reduce the melt viscosity of the adhesive but also contribute to reducing the adhesive's adhesion to the substrate, adding an amount of plasticizer sufficient to achieve good coatability may result in insufficient adhesion for linerless label applications. Therefore, it is preferable to use an adhesive with a low melt viscosity without adding a plasticizer. Therefore, in one embodiment of the present invention, the acrylic hot-melt pressure-sensitive adhesive of the present invention preferably has a melt viscosity of 50,000 mPa·s or less at 180°C as measured with a Brookfield viscometer and does not contain a plasticizer. In this case, good coatability and high adhesion to the adherend can be achieved at the same time. The acrylic hot-melt pressure-sensitive adhesive of the present invention is also suitable for use as a pressure-sensitive adhesive.

[0104] In one embodiment of the present invention, the melt viscosity of the acrylic hot-melt pressure-sensitive adhesive of the present invention at 180°C, measured with a Brookfield viscometer, is preferably 50,000 mPa·s or less, more preferably 45,000 mPa·s or less, and even more preferably 40,000 mPa·s or less. Furthermore, the melt viscosity of the acrylic hot-melt pressure-sensitive adhesive of the present invention at 180°C, measured with a Brookfield viscometer, is preferably 5,000 mPa·s or more, more preferably 8,000 mPa·s or more, and even more preferably 10,000 mPa·s or more. Furthermore, in one embodiment of the present invention, the melt viscosity of the acrylic hot-melt pressure-sensitive adhesive of the present invention at 180°C, measured with a Brookfield viscometer, is preferably 5,000 to 50,000 mPa·s, more preferably 8,000 to 45,000 mPa·s, and even more preferably 10,000 to 40,000 mPa·s. When the melt viscosity of the acrylic hot-melt pressure-sensitive adhesive of the present invention is within the above range, the coatability of the pressure-sensitive adhesive can be improved.

[0105] The melt viscosity can be adjusted by appropriately adjusting the type of functional group-free acrylic ester polymer contained in the acrylic hot-melt pressure-sensitive adhesive of the present invention, as well as their composition, content, mass ratio, and weight-average molecular weight, for example, by adding acrylic block copolymer (II) in addition to acrylic block copolymer (I), the type of polymer blocks constituting acrylic block copolymer (I) and acrylic block copolymer (II), their composition, content, mass ratio, and weight-average molecular weight, and the type and content of a tackifier resin optionally contained, etc. For example, the melt viscosity can be adjusted to be equal to or less than the above upper limit by adjusting it to the range described as a preferred embodiment in this specification.

[0106] In one embodiment of the present invention, the breaking strength of the acrylic hot-melt pressure-sensitive adhesive of the present invention in a tensile test is preferably 1 to 10 MPa, more preferably 1.5 to 8 MPa, and even more preferably 2 to 5 MPa. The breaking elongation of the acrylic hot-melt pressure-sensitive adhesive of the present invention in a tensile test is preferably 100 to 2000%, more preferably 300 to 1800%, and even more preferably 500 to 1500%. When the breaking elongation is within the above range, particularly when it is equal to or less than the above upper limit, the cutting ability of the linerless label is easily improved. The breaking strength and breaking elongation can be measured by a tensile test, for example, by the method described in the Examples.

[0107] The method for producing the acrylic hot-melt pressure-sensitive adhesive of the present invention is not particularly limited. For example, it can be produced by mixing the components using a known mixing or kneading device such as a kneader-ruder, extruder, mixing roll, or Banbury mixer, usually at a temperature within the range of 100 to 250°C. Alternatively, the components may be dissolved and mixed in an organic solvent, followed by distilling off the organic solvent. The resulting acrylic hot-melt pressure-sensitive adhesive can be used by heating and melting. When the acrylic hot-melt pressure-sensitive adhesive of the present invention is used by heating and melting, a low melt viscosity is preferred from the viewpoints of processability and handling. On the other hand, a high melt viscosity is preferred from the viewpoint of achieving both the adhesive properties and high holding power (creep resistance) of the acrylic hot-melt pressure-sensitive adhesive.

[0108] [Adhesive Layer] The present invention encompasses an adhesive layer comprising the hot-melt adhesive for linerless labels of the present invention. The adhesive layer of the present invention can be formed by heating and melting the hot-melt adhesive for linerless labels of the present invention and applying it to a substrate. The coating method is not particularly limited, and examples include hot-melt coating, T-die coating, inflation coating, calendar molding, lamination, etc., with hot-melt coating being particularly preferred. For example, an adhesive layer can be formed by melt-coating the heated and melted acrylic hot-melt adhesive of the present invention onto a paper substrate using a hot-melt coater. For example, when an acrylic hot-melt adhesive is applied using this method, the hot-melt adhesive is less likely to penetrate into the release layer at the contact surface between the release layer and the adhesive layer, resulting in easier peeling between the release layer and the adhesive layer. The adhesive layer of the present invention contains an adhesive that adheres when pressure is applied, i.e., a pressure-sensitive adhesive.

[0109] The coating amount of the adhesive is not particularly limited, but from the viewpoint of the adhesiveness and handling of the linerless label having the adhesive layer of the present invention, the coating amount is preferably 5 to 50 g / m2 in terms of dry mass. ２ , more preferably 7 to 40 g / m ２ , more preferably 10 to 30 g / m ２ , and even more preferably 10 to 19 g / m ２ It is preferable that the coating amount of the adhesive be equal to or less than the above upper limit from the viewpoint of the cuttability of the linerless label having the adhesive layer of the present invention.

[0110] The adhesive strength of the adhesive layer, measured in accordance with JIS Z 0237, is preferably 1 to 30 N / 25 mm, more preferably 1.5 to 25 N / 25 mm, even more preferably 2 to 23 N / 25 mm, and particularly preferably 3 to 22 N / 25 mm. When the adhesive strength of the adhesive layer is equal to or greater than the lower limit, the linerless label is less likely to peel off when attached to an adherend. When the adhesive strength of the adhesive layer is equal to or less than the upper limit, peeling between the release layer and the adhesive layer in the linerless label is easier. The adhesive strength of the adhesive layer can be measured, for example, by the method described in the Examples.

[0111] [Linerless Label] The present invention also encompasses a linerless label having the adhesive layer of the present invention. The linerless label of the present invention has a release layer containing a silicone-based release agent as a main component on one side of a paper substrate, and the adhesive layer of the present invention on the other side of the paper substrate that does not have the release layer. That is, in one embodiment of the present invention, the linerless label may have at least a three-layer structure including (release layer) / (paper substrate) / (adhesive layer) in this order. The linerless label of the present invention is not limited to a three-layer structure of (release layer) / (paper substrate) / (adhesive layer), and may have other layers, for example, between the paper substrate and the adhesive layer. Examples of other layers include a protective layer and a barrier layer.

[0112] <Paper Base> The paper base is composed mainly of wood pulp, and may contain additives such as pigments and various auxiliaries as required.

[0113] Wood pulp may be either hardwood (L-wood) or softwood (N-wood), and various chemical pulps, semi-chemical pulps, mechanical pulps, recycled pulps, etc. may be used. Examples of chemical pulps include kraft pulp (KP), sulfite pulp (SP), and soda pulp (AP). Examples of semi-chemical pulps include semi-chemical pulp (SCP) and chemi-ground wood pulp (CGP). Examples of mechanical pulps include groundwood pulp (GP), thermomechanical pulp (TMP), bleached chemi-thermomechanical pulp (BCTMP), and refiner ground wood pulp (RGP). Examples of recycled pulp include deinked pulp (DIP) made from recycled paper. Non-wood pulps made from materials such as paper mulberry, mitsumata, hemp, and kenaf may also be used.

[0114] Furthermore, when a paper base material with high brightness is required, pulp bleached by a combination of various bleaching methods using chlorine, chlorine dioxide, oxygen, ozone, hydrogen peroxide, hypochlorous acid, etc. can be used. Among them, pulp obtained by chlorine-free bleaching (ECF) or totally chlorine-free bleaching (TCF), which are produced using a process that does not use chlorine, is preferably used because it is less likely to yellow. Pulps obtained by these bleaching methods may be used alone or in combination of two or more types.

[0115] Furthermore, the freeness (degree of beating) of the pulp can be adjusted as appropriate to adjust the papermaking suitability and the paper quality of the finished paper base material. The pulp used in the present invention preferably has a freeness after disintegration of 80 to 650 ml, more preferably 200 to 600 ml, based on the Canadian Standard Freeness (CSF) specified by JIS-P-8121. If the freeness is too high, not only will the surface of the paper base material become rough, but the paper strength of the paper base material may also decrease. If the freeness is too low, poor papermaking suitability may result, such as poor drainage and an inability to increase the papermaking speed. The pulp used in the present invention preferably contains 50% by mass or more, more preferably 80% by mass or more, of hardwood bleached kraft pulp (LBKP) with a freeness of 350 to 550 ml.

[0116] The pigment in the paper base material is blended for the purpose of imparting opacity to the linerless label or for the purpose of imparting writability and stampability. Examples of pigments that can be used include light calcium carbonate, calcined kaolin, white carbon, talc, and titanium oxide. Among these, talc, light calcium carbonate, and white carbon are preferred, with talc and light calcium carbonate being more preferred because they have favorable papermaking suitability (wire abrasion suitability) and are more likely to remain during papermaking. It is more preferred that the pigment be primarily light calcium carbonate, and it is particularly preferred that the light calcium carbonate account for 75% by mass or more of the total pigment content.

[0117] When a pigment is blended into the paper base material, the pigment content (ash content) in the paper base material is preferably 5 to 30% by mass, more preferably 15 to 25% by mass. If the ash content is too high, the paper strength may decrease. If the ash content is too low, the writability and printability may decrease.

[0118] Various auxiliary agents may also be added to the paper base material, such as internal sizing agents such as higher fatty acids and alkyl ketene dimers, fixing agents such as aluminum sulfate and cationic polyelectrolytes, paper strength agents such as starch, polyacrylamide and polyvinyl alcohol, cationizing agents, retention aids, dyes, fluorescent whitening agents, and mildew inhibitors.

[0119] As a means for making paper, known paper machines can be used, such as a Fourdrinier paper machine, a cylinder paper machine, a Yankee paper machine, a twin wire former, an inclined wire former, etc., or dryers such as a Yankee dryer, a multi-cylinder dryer, etc. The paper base material may also be one that has been subjected to known treatments such as calendering.

[0120] The paper substrate may also be one that has been subjected to sizing treatment using a size press or gate roll for the purpose of adjusting surface strength, sizing properties, etc. Examples of sizing solutions include treatment solutions containing appropriate blends of starches, polyvinyl alcohols, polyacrylamides, surface sizing agents, cationic resins, etc. Examples of surface sizing agents include styrene-maleic acid copolymers, olefin copolymers, alkyl ketene dimer compounds, alkenyl succinic anhydride compounds, styrene-acrylic copolymers, higher fatty acid compounds, petroleum resin sizing agents, and rosin sizing agents.

[0121] The coating amount of the sizing solution is 0.3 to 5.0 g / m2 in terms of dry mass. ２ It is preferable that the density is 0.3 g / m ２ If the sizing treatment is less than 5.0 g / m, the effect of the sizing treatment cannot be obtained. ２ If the amount exceeds this, the effect of the sizing treatment will reach a plateau, which is undesirable as it will lead to an increase in costs. A more preferable range is 0.5 to 3.0 g / m ２ When sizing is performed on both sides, it is preferable that the total amount of the sizing solution applied to both sides is within the above range.

[0122] The basis weight of the paper base material is 40 to 210 g / m ２ It is preferable that the range is 60 to 120 g / m ２ If the basis weight is too low, the paper strength will be insufficient when used as a linerless label, which may result in paper tearing. Conversely, if the basis weight is too high, the paper will be too thick and unsuitable.

[0123] The density of the paper substrate is 0.55 to 1.00 g / m ３ If the density is too low, problems such as paper breakage may occur. ３If the thickness exceeds 100 μm, the writing and stamping properties may be deteriorated, which is not preferable.

[0124] The paper substrate of the present invention is preferably uncoated paper or single-sided coated paper with a coating layer on one side. Examples of uncoated paper include fine paper, medium-quality paper, and single-sided glossy paper. Among these, fine paper is preferred in terms of printing ink peel strength. Single-sided coated paper may be any conventionally known paper, including general printing paper and thermal recording paper. Examples of the coating layer include a filling layer that prevents the adhesive from penetrating into the paper substrate, a polyethylene laminate layer, or a protective layer on which such a layer is provided on one side. These coating layers can be applied using a blade coater, knife coater, bar coater, air knife coater, die coater, slot die coater, gravure coater, or the like. When single-sided coated paper is used, it is preferable to provide a release layer on the side of the single-sided coated paper that does not have a coating layer. When a release layer is provided on a coating layer, a uniform release layer tends to be formed over the entire surface, which may result in the surface of the release layer becoming too smooth and resulting in poor writability and printability.

[0125] The adhesive layer in the linerless label of the present invention is made of a hot-melt adhesive for linerless labels containing an acrylate polymer having no functional groups, and does not require ultraviolet irradiation during the formation of the adhesive layer. Therefore, the linerless label of the present invention can also be suitably used for thermal labels that use thermal paper, such as thermal recording paper, as the paper substrate.

[0126] <Release Layer> The release layer preferably contains a silicone-based release agent as its main component. Examples of silicone-based release agents include heat-curable release agents such as addition-curable silicone-based release agents and condensation-curable silicone-based release agents; electron beam-curable silicone-based release agents such as radical-curable silicone-based release agents, cation-curable silicone-based release agents, and mercapto-curable silicone-based release agents; and ultraviolet-curable silicone-based release agents. Among these, ultraviolet-curable silicone-based release agents or electron beam-curable silicone-based release agents are preferred. Addition-curable silicone-based release agents or condensation-curable silicone-based release agents are cured by heat, but their curing rate is slow, and it takes time for the release agent to cure after application. As a result, the release agent may penetrate into the paper substrate before curing, making it difficult to peel between the release layer and the pressure-sensitive adhesive layer. On the other hand, curable silicone-based release agents or electron beam-curable silicone-based release agents have a fast curing rate, so by irradiating ultraviolet light or an electron beam immediately after application of the release agent, the release agent can be cured before it penetrates into the paper substrate. Furthermore, of the ultraviolet-curable silicone-based release agents and the electron beam-curable silicone-based release agents, ultraviolet-curable silicone-based release agents are more preferred because they require less capital investment, and cation-curable silicone-based release agents are particularly preferred.

[0127] Specific examples of these release agents include a mixed composition of a mercapto group-containing organopolysiloxane and a vinyl group-containing organopolysiloxane; an organopolysiloxane composition containing an acrylic group, a methacrylic group, or a cinnamoyl group; an organopolysiloxane composition containing a maleimide group or a phenylmaleimide group; a mixed composition of an azide group-containing organopolysiloxane and a vinyl group-containing organopolysiloxane; an organopolysiloxane composition containing a thioacrylic group, a thiomethacrylic group, or a thiocinnamoyl group; and an organopolysiloxane composition containing an acrylamide group, a methacrylamide group, or a cinnamoylamide group. As the cationically curable silicone release agent, a mixed composition of an epoxy group-containing organopolysiloxane and an onium salt photoinitiator is preferred. As the onium salt photoinitiator, known ones can be used.

[0128] The amount of onium salt photoinitiator used is not particularly limited and can be adjusted as needed. However, from the standpoint of economic efficiency and production operability, a use of 1 to 20 parts by mass per 100 parts by mass of the epoxy group-containing organopolysiloxane is desirable. The viscosity of the release agent is preferably 5,000 mPa·s or less, more preferably 1,000 mPa·s or less. If the viscosity is too high, poor writability and stampability may result. Furthermore, various auxiliary additives, such as other UV-curable resins, electron beam-curable resins, photopolymerization initiators, crosslinking agents, dyes, pigments, wetting agents, defoamers, dispersants, antistatic agents, leveling agents, and lubricants, may be added to the release agent as needed, as long as they do not impair the desired effects. The amount of such auxiliary additives is preferably 10% by mass or less based on the total mass of the release agent layer. It is particularly preferable to use no pigment (0% by mass). This is because adding a pigment to the release agent layer can make release difficult when used in a linerless label.

[0129] The method for applying the release agent is not particularly limited, and examples thereof include a bar coater, a multi-stage roll coater, an air knife coater, a gravure coater, and an offset gravure coater, with flexographic printing being preferred. Devices that can be used to harden the release agent include hot air dryers, far-infrared dryers, ultraviolet irradiators, and electron beam irradiators, with ultraviolet irradiators being more preferred. A particularly preferred method for providing a release layer is a method that combines ultraviolet-curing flexographic printing with an ultraviolet irradiator. This is because a flexographic printing unit and an ultraviolet irradiator can be installed on a printing press that prints the image and text for the linerless label, and if a hot-melt die coater that applies the adhesive layer is also installed on the printing press, linerless labels can be produced in a single pass. Examples of ultraviolet irradiators include low-pressure mercury lamps, medium-pressure mercury lamps, high-pressure mercury lamps, metal halide lamps, and electrodeless lamps. Ozone-less types that emit less ozone are also available, and any of these may be used. The coating amount of the release agent forming the release layer is 0.2 to 2.0 g / m2 in terms of dry mass. ２ is preferable, and more preferably 0.5 to 1.5 g / m ２If the coating amount of the release agent is too small, peeling between the release layer and the adhesive layer may be difficult, whereas if the coating amount is too large, writability and printability may be poor.

[0130] <Linerless Label> Applications of the linerless label of the present invention include cardboard labels, shrink packaging labels, mail bag labels, delicatessen labels, price labels, logistics labels, and suitcase labels.

[0131] The present invention will be described in more detail with reference to the following examples, but the present invention is not limited to these examples. In the production examples shown below, monomers and other compounds were dried and purified by conventional methods and degassed with nitrogen before use. Furthermore, the transfer and supply of monomers and other compounds to the reaction system were carried out under a nitrogen atmosphere.

[0132] [Measurement Method] <GPC Measurement> The weight average molecular weight (Mw), number average molecular weight (Mn), and molecular weight distribution (Mw / Mn) of the polymer (polymer forming the block) and the block copolymer were determined by GPC in terms of polystyrene equivalent molecular weight. The measurement device and conditions used for GPC are as follows. [GPC Measurement Device and Conditions] Device: GPC device "HLC-8020" manufactured by Tosoh Corporation Separation column: "TSKgel GMHXL", "G4000HXL", and "G5000HXL" manufactured by Tosoh Corporation connected in series Eluent: tetrahydrofuran Eluent flow rate: 1.0 ml / min Column temperature: 40°C Detection method: differential refractive index (RI)

[0133] < １ H-NMR Measurement> The content of each polymer block in the block copolymer is １ It was determined by H-NMR measurement. １ The measurement equipment and conditions used in the H-NMR measurement are as follows: １ H-NMR measurement apparatus and conditions] Apparatus: Nuclear magnetic resonance apparatus "JNM-LA400" manufactured by JEOL Ltd. Heavy solvent: deuterated chloroform １In the H-NMR spectrum, the signals at around 3.6 ppm and 4.0 ppm correspond to the ester groups (—O—CH ) of the methyl methacrylate units, respectively. ３ ) and the ester group of the acrylate ester unit (—O—CH ２ -CH ２ -CH ２ -CH ３ The ratio of the integrals of the polymer block (B) in the acrylic triblock copolymer (I-1) and the polymer block (D) in the acrylic triblock copolymer (II-1) was determined as follows: １ In the H-NMR spectrum, the ester group of n-butyl acrylate (—O—CH ２ -CH ２ -CH ２ -CH ３ From the signals derived from (B) and (D), the molar content of each monomer was determined based on the ratio of their integral values, and this was converted into a mass ratio based on the molecular weight of the monomer unit, which was used as the mass ratio of the monomers constituting the polymer blocks (B) and (D).

[0134] <Melt Viscosity> The melt viscosity of the hot melt adhesive was measured using a Brookfield viscometer. The measurement device and conditions were as follows: Device: BLOOKFIELD (DV2T (RV type)) Rotation speed: 6 to 50 rpm Measurement temperature: 160°C or 180°C Spindle: SC4-29

[0135] [Evaluation Methods] The evaluation methods used to evaluate the pressure-sensitive adhesives and linerless labels are described below.

[0136] <Adhesive Properties> (Test Method for Adhesion 1 (180° Peel Test)) A linerless label test piece was pressure-bonded to an adherend (low-density polyethylene) at 23°C using a laminating machine (manufactured by ChemInstruments; 2 kg rubber roller, 10 mm / sec). The test piece was then held at 23°C and 50% RH for 20 minutes to obtain a test piece. Subsequently, in accordance with JIS Z0237:2009, the force applied when peeling the film bonded to the test piece was measured under the following measurement environment and tensile conditions (tensile speed, peel angle, tensile tester), and the average value for five test pieces was calculated. The results are shown in Table 4. Tensile tester: Tabletop tensile tester (AGS-X: manufactured by Shimadzu Corporation) Tensile speed: 300 mm / min Peel angle: 180° Measurement environment: 23°C x 50% RH

[0137] (Test method for adhesive strength 2 (180° peel test)) The adhesive strength was measured in the same manner as in adhesive strength 1, except that the adherend was cardboard. The results are shown in Table 4.

[0138] (Test method for adhesive strength 3 (180° peel test)) The adhesive strength was measured in the same manner as in adhesive strength 1, except that the holding time was changed to 24 hours. The results are shown in Table 4.

[0139] (Test method for adhesive strength 4 (180° peel test)) The adhesive strength was measured in the same manner as in adhesive strength 2, except that the holding time was changed to 24 hours. The results are shown in Table 4.

[0140] (Loop Tack Test) A linerless label was looped in a 25 mm wide x 100 mm long loop in an atmosphere of 23°C and 50% RH, with the adhesive layer facing outward. A 25 mm wide test plate (low-density polyethylene, cardboard) was attached to a loop tack tester ("LT-1000" manufactured by ChemInstruments) in a direction intersecting the loop, and the maximum peel strength was measured according to the method described in PSTC-16. The loop tack value was calculated as the average of three tests. The results are shown in Table 4.

[0141] <Cutting ability> Cutting ability was evaluated in punching tests and tensile tests. In punching tests, adhesive residue on the cutter blade requires replacement, so it is important that there is no adhesive residue or stringiness. In tensile tests, the lower the elongation at which the film breaks, the easier it is to cut.

[0142] (Punching test) The obtained linerless label was cut into A4 size pieces and punched 10 times with a Thomson blade. The presence or absence of dirt on the cutter blade and the presence or absence of stringiness of the adhesive were visually observed and evaluated according to the following criteria. Cutter blade dirt / adhesive stringiness: None: OK Present: NG

[0143] (Tensile test) A 2 mm thick sheet of the obtained pressure-sensitive adhesive was prepared. This sheet was punched out into a dumbbell shape with a total length of 42 mm, a parallel portion width of 5 mm, and a parallel portion length of 3 mm, and tension was applied to a high-speed tensile tester HITS-TX (manufactured by Shimadzu Corporation) at 23°C and a tensile speed of 100 m / min to measure the breaking strength and breaking elongation. The results are shown in Table 3. From the results of the punching test and tensile test, the cuttability was evaluated according to the following criteria. The results are shown in Table 4. Cuttability evaluation criteria A: Punching test evaluation was OK, and the strength during the tensile test was 5.0 MPa or less and the elongation was 1500% or less B: Punching test evaluation was OK, and the strength during the tensile test was 5.0 MPa or more or the elongation was 1500% or more C: Punching test evaluation was NG, and the strength during the tensile test was 5.0 MPa or more or the elongation was 1500% or more

[0144] <Recyclability> After coating the adhesive on the substrate, the adhesive layer was peeled off from the substrate layer again, recovered, and melt-kneaded in a nitrogen atmosphere at 230°C for 30 minutes. A 100 μm-thick sheet was then prepared at 230°C using a heat press, and the appearance was visually inspected and evaluated according to the following criteria. The results are shown in Table 4. Transparent or translucent with no defects or opaque areas: OK (good recyclability) Defects or opaque areas: NG (not suitable for recycling)

[0145] <Acrylic Block Copolymer> (Production Example 1: Acrylic Triblock Copolymer (I-1)) (1) A three-way stopcock was attached to a 3-L three-neck flask, and the inside of the flask was purged with nitrogen. Then, 1,409 g of toluene and 32.7 g of 1,2-dimethoxyethane were added with stirring at room temperature. Subsequently, 48.6 g of a toluene solution containing 24.5 mmol of isobutylbis(2,6-di-t-butyl-4-methylphenoxy)aluminum was added, followed by 1.50 g of a cyclohexane solution of sec-butyllithium containing 2.55 mmol of sec-butyllithium. (2) Subsequently, 22.7 g of methyl methacrylate was added to the mixture. The reaction solution was initially yellow, but became colorless after stirring for 60 minutes at room temperature. A small amount was sampled, and the weight-average molecular weight of the resulting polymethyl methacrylate was determined by GPC measurement using the method described above. (3) Subsequently, the internal temperature of the polymerization solution was cooled to -30°C, and 434 g of n-butyl acrylate was added dropwise over 2 hours. After completion of the dropwise addition, the mixture was stirred at -30°C for 5 minutes. (4) Further, 29.4 g of methyl methacrylate was added thereto, and the mixture was stirred overnight at room temperature. (5) After 15 g of methanol was added to terminate the polymerization reaction, the resulting reaction solution was poured into 15 kg of methanol to precipitate a liquid precipitate. Thereafter, the liquid precipitate was collected and dried to obtain 440 g of an acrylic triblock copolymer (I-1). The weight-average molecular weight of the resulting acrylic triblock copolymer (I-1) was determined by GPC measurement using the method described above. In addition, １ The total content of polymer blocks composed of methyl methacrylate units in the acrylic triblock copolymer (I-1) was determined by H-NMR measurement.

[0146] (Production Example 2: Acrylic Triblock Copolymer (II-1)) (1) A three-way cock was attached to a 3 L three-neck flask, and the inside of the flask was replaced with nitrogen. Then, 1,409 g of toluene and 32.7 g of 1,2-dimethoxyethane were added with stirring at room temperature. Subsequently, 48.6 g of a toluene solution containing 24.5 mmol of isobutylbis(2,6-di-t-butyl-4-methylphenoxy)aluminum was added, followed by 4.50 g of a cyclohexane solution of sec-butyllithium containing 7.65 mmol of sec-butyllithium. (2) Subsequently, 30.1 g of methyl methacrylate was added to the mixture. The reaction solution was initially yellow, but became colorless after stirring for 60 minutes at room temperature. A small amount was sampled, and the weight-average molecular weight of the resulting polymethyl methacrylate was determined by GPC measurement using the method described above. (3) Subsequently, the internal temperature of the polymerization solution was cooled to -30°C, and 427 g of n-butyl acrylate was added dropwise over 2 hours. After completion of the dropwise addition, the mixture was stirred at -30°C for 5 minutes. (4) Further, 7.60 g of methyl methacrylate was added thereto, and the mixture was stirred overnight at room temperature. (5) 20 g of methanol was added to terminate the polymerization reaction, and the resulting reaction solution was poured into 15 kg of methanol to precipitate a liquid precipitate. The liquid precipitate was then recovered and dried, yielding 440 g of acrylic triblock copolymer (II-1). The weight-average molecular weight of the resulting acrylic triblock copolymer (II-1) was determined by GPC measurement using the method described above. Furthermore, the total content of polymer blocks composed of methyl methacrylate units in the acrylic triblock copolymer (II-1) was determined by the above-mentioned 1H-NMR measurement.

[0147] The block structures, weight-average molecular weights (Mw), molecular weight distributions (Mw / Mn), total content of PMMA polymer blocks (polymer blocks consisting of 100% by mass of methyl methacrylate units), structures of polymer blocks (B) and (D), weight-average molecular weights of polymer blocks (A1) or (C1), and weight-average molecular weights of polymer blocks (A2) or (C2) of the acrylic triblock copolymers (I-1) and (II-1) obtained in Production Examples 1 and 2 are shown in Table 1. In Table 1, PMMA means a polymer block consisting of 100% by mass of methyl methacrylate units, and PnBA means a block consisting of 100% by mass of n-butyl acrylate units.

[0148]

[0149] <Paper substrate> The paper substrates used in the examples and comparative examples are as follows: Paper substrate 1: Kanzan Paper "KLS46" manufactured by Kanzan Co., Ltd. Paper substrate 2: Jujo Paper "KMLZS2-75" manufactured by Jujo Thermal Co., Ltd. Paper substrate 3: Ricoh Paper "150LL" manufactured by Ricoh Thermal Co., Ltd.

[0150] <Tackifying resins and plasticizers> The tackifying resins and plasticizers used in the examples and comparative examples are as follows: (Tackifying resins) - Trade name "Dertophene T105" manufactured by DRT, terpene phenol resin - Trade name "Foral 85E" manufactured by Eastman, hydrogenated rosin ester - Trade name "Novares TM20AS" manufactured by Rain Carbon Inc., aromatic hydrocarbon resin - Trade name "YS resin SX100" manufactured by Yasuhara Chemical Co., Ltd., aromatic hydrocarbon resin - Trade name "Quintone DX396" manufactured by Zeon Corporation, aliphatic hydrocarbon resin

[0151] (Plasticizer) - Product name "ARUFON UP1171" manufactured by Toagosei Co., Ltd., acrylate oligomer - Product name "NS-100" manufactured by Idemitsu Kosan Co., Ltd., naphthenic oil

[0152] <Hot-melt adhesive> (Adhesive 1) In a nitrogen atmosphere, a stirring tank (1 m ３347 kg of the acrylic triblock copolymer (I-1) obtained in Production Example 1 was added to a molten mixture, and 2.0 kg of the antioxidant AntioxAHM-K was added and uniformly melted. The temperature was then set to 180°C, and 87 kg of the acrylic triblock copolymer (II-1) obtained in Production Example 2 was added, and the mixture was melt-kneaded to a uniform state. The temperature was then set to 170°C, and 104 kg of "Dertophene T105" was added as a tackifier resin. The temperature was then set to 160°C, and 174 kg of "Foral 85E" was added as a tackifier resin. The temperature was then set to 140°C, and 85 kg of "Novares™20AS" was added as a tackifier resin, to obtain PSA 1.

[0153] (Adhesive 2) In a nitrogen atmosphere, a stirring tank (1 m ３ 347 kg of the acrylic triblock copolymer (I-1) obtained in Production Example 1 was added to a mixture containing 100% acrylic triblock copolymer (I-1) and 2 kg of the antioxidant AntioxAHM-K, which were then uniformly melted. The temperature was then set to 180°C, and 174 kg of the acrylic triblock copolymer (II-1) obtained in Production Example 2 was added, followed by melt-kneading to form a uniform mixture. The temperature was then set to 170°C, and 174 kg of "Dertophene T105" was added as a tackifier resin. The temperature was then set to 160°C, and 104 kg of "Foral 85E" was added as a tackifier resin, to obtain Pressure-Sensitive Adhesive 2.

[0154] (Adhesive 3) In a nitrogen atmosphere, a stirring tank (1 m ３ 334 kg of the acrylic triblock copolymer (I-1) obtained in Production Example 1 was added to a 100-kJ / kg molten mixture, followed by 2 kg of the antioxidant AntioxAHM-K. The temperature was then set to 180°C, and 100 kg of the acrylic triblock copolymer (II-1) obtained in Production Example 2 was added, followed by melt-kneading to form a homogeneous mixture. The temperature was then set to 170°C, and 167 kg of "YSresinSX100" was added as a tackifier resin. The temperature was then set to 160°C, and 2,200 kg of "ARUFONUP1171" was added as a plasticizer, to obtain PSA 3.

[0155] (Adhesive 4) In a nitrogen atmosphere, a stirring tank (1 m３ 240 kg of QUINTONE R100 (manufactured by Zeon Corporation) was added to the mixture, followed by 2 kg of the antioxidant AntioxAHM-K and 120 kg of the plasticizer "NS-100," which were then stirred and melted uniformly. 200 kg of "Foral85E" and 240 kg of "QUINTONE DX396" were then added as tackifiers, and the mixture was melt-kneaded to form a uniform mixture, thereby obtaining PSA 4.

[0156] (Adhesive 5) As adhesive 5, an ultraviolet crosslinkable acrylic hot melt adhesive, "acResin DS3532X" manufactured by BASF Corporation, was used.

[0157] The formulations (parts by mass) of adhesives 1 to 4 are shown in Table 2.

[0158] Table 3 shows the breaking strength Tb (MPa) and breaking elongation Eb (%) of PSA 1 to 4 in the tensile test, as well as the melt viscosity (mPa·s) at 180° C. and 160° C.

[0159]

[0160] Example 1 A mixture of silicone release agents, TEGO RC Silicone RC730 / RC902 / A18, was applied to one side of a paper substrate 1 using a three-roll gravure coater, and the ultraviolet ray exposure was 90 mJ / cm. ２ The release agent was cured using a high-pressure mercury ultraviolet irradiation device at UV-C to form a release layer. The coating amount of this release agent was 1.0 g / m2 in terms of dry mass. ２ Next, a hot melt die coater was used to coat the backside of the paper substrate with PSA 1 to a thickness of 15 μm, providing an adhesive layer, which was then wound up to obtain a linerless label. The resulting linerless label was unwound and could be unwound without difficulty. The resulting linerless label was unwound and cut to a predetermined size, and the loop tack performance and 180° peel performance were evaluated as adhesive properties. In addition, cuttability was evaluated by a punching test and a tensile test. The recyclability of the adhesive was also evaluated.

[0161] Examples 2 to 7 Linerless labels were obtained in the same manner as in Example 1, except that the type and amount of adhesive applied, and the type of paper substrate were changed as shown in Table 4. Various evaluations were carried out in the same manner as in Example 1.

[0162] Comparative Example 1 A mixture of silicone release agents, TEGO RC Silicone RC730 / RC902 / A18, was applied to one side of a paper substrate 1 using a three-roll gravure coater, and the ultraviolet ray exposure was 90 mJ / cm. ２ The release agent was cured using a high-pressure mercury ultraviolet irradiation device at UV-C to form a release layer. The coating amount of this release agent was 1.0 g / m2 in terms of dry mass. ２ Next, a hot melt die coater was used to coat the backside of the paper substrate with adhesive 4 to a thickness of 18 μm to provide an adhesive layer, which was then wound up to obtain a linerless label. When the resulting linerless label was unwound, it could be unwound without difficulty. Various evaluations were carried out in the same manner as in Example 1.

[0163] Comparative Example 2 A mixture of silicone release agents, TEGO RC Silicone RC730 / RC902 / A18, was applied to one side of the paper substrate 1 using a three-roll gravure coater, and the ultraviolet ray exposure was 90 mJ / cm. ２ The release agent was cured using a high-pressure mercury ultraviolet irradiation device at UV-C to form a release layer. The coating amount of this release agent was 1.0 g / m2 in terms of dry mass. ２ Next, the adhesive 5 was applied to the back surface of the paper substrate in a thickness of 18 μm using a hot melt die coater, and the ultraviolet irradiation dose was 50 mJ / cm ２ The adhesive layer was formed by irradiating the adhesive layer with ultraviolet light so that the adhesive layer was in a state of 100%. The adhesive layer was then wound up to obtain a linerless label. Various evaluations were carried out in the same manner as in Example 1.

[0164] The evaluation results of Examples 1 to 7 and Comparative Examples 1 and 2 are shown in Table 4.

[0165]

[0166] The results in Table 4 confirm that the acrylic hot-melt adhesive of the present invention, which does not contain functional groups, is free of defects and opaque areas and has good recyclability. On the other hand, in the comparative example using a UV-curable acrylic hot-melt adhesive, insoluble matter originating from crosslinking points was generated in the recyclability evaluation, and defects and opaque areas occurred. Therefore, it was found that the UV-curable acrylic hot-melt adhesive is not suitable for recycling. Furthermore, while the linerless label using the acrylic hot-melt adhesive exhibited excellent cutting properties, the comparative example using a rubber-based hot-melt adhesive exhibited poor cutting properties. This clearly demonstrates that rubber-based hot-melt adhesives are prone to adhesion of the adhesive to the blade of a slitter or autocutter, resulting in poor finishes like a roll or poor cutting. Furthermore, the acrylic hot-melt adhesives of Examples 1 to 7 had low melt viscosities at 180°C and good coatability. Furthermore, it was confirmed that the PSA's of Examples 1 to 6, which had a melt viscosity at 180°C of 50,000 mPa·s or less and did not contain a plasticizer, had higher adhesive strength and higher adhesiveness than Example 7, which contained a plasticizer. Therefore, it was found that the PSA's of Examples 1 to 6 were PSA's that combined good coatability with high adhesiveness.

Claims

1. A hot melt adhesive for linerless labels containing an acrylate ester polymer that does not have functional groups.

2. The pressure-sensitive adhesive according to claim 1, which has a melt viscosity of 50,000 mPa·s or less at 180°C as measured with a Brookfield viscometer and does not contain a plasticizer.

3. The pressure-sensitive adhesive according to claim 1 or 2, wherein the acrylic ester polymer is an acrylic block copolymer.

4. The pressure-sensitive adhesive according to claim 3, wherein the acrylic block copolymer comprises two polymer blocks (A1) and (A2) containing units derived from a methacrylic acid ester and one polymer block (B) containing units derived from an acrylic acid ester, and comprises an acrylic triblock copolymer (I) having an (A1)-(B)-(A2) block structure.

5. The pressure-sensitive adhesive described in claim 4, wherein the acrylic block copolymer (I) has a weight average molecular weight (Mw) of 50,000 or more and 250,000 or less, the total content of polymer blocks (A1) and (A2) is 35 mass% or less, and the weight average molecular weight of each of polymer blocks (A1) and (A2) is 3,000 or more.

6. The methacrylic acid esters constituting the polymer blocks (A1) and (A2) are CH ３ C(=CH ２ ) COOR １ (In the formula, R １ represents an alkyl group having 1 to 10 carbon atoms), and the acrylate ester which is a structural unit of the polymer block (B) is represented by CH ２ =CH-COOR ２ (In the formula, R ２ The pressure-sensitive adhesive according to claim 4 or 5, which is an acrylic acid ester represented by the formula (I) wherein R represents an alkyl group having 1 to 10 carbon atoms.

7. The pressure-sensitive adhesive according to any one of claims 4 to 6, wherein the acrylic block copolymer has two polymer blocks (C1) and (C2) containing units derived from a methacrylic acid ester and one polymer block (D) containing units derived from an acrylic acid ester, and further comprises an acrylic triblock copolymer (II) having a (C1)-(D)-(C2) block structure, and at least one of the polymer blocks (C1) and (C2) has a weight average molecular weight of 300 or more and less than 3,000.

8. The pressure-sensitive adhesive according to claim 7, wherein the acrylic triblock copolymer (II) has a weight average molecular weight (Mw) of 10,000 to 120,000, and the total content of polymer blocks (C1) and (C2) is 25 mass% or less.

9. The pressure-sensitive adhesive according to any one of claims 1 to 8, further comprising a tackifier resin.

10. The pressure-sensitive adhesive according to claim 9, wherein the content of the tackifier resin is 1 to 190 parts by mass per 100 parts by mass of the total of the acrylic triblock copolymer (I) and the acrylic triblock copolymer (II).

11. An adhesive layer comprising the hot melt adhesive for linerless labels according to any one of claims 1 to 10.

12. A linerless label having a release layer mainly composed of a silicone-based release agent on one side of a paper substrate, and an adhesive layer according to claim 11 on the other side of the paper substrate not having the release layer.

Images