Adhesive tape

Abstract

To provide an adhesive tape excellent in adhesion to an adherend, and excellent in color tone retention even when exposed to a high temperature and high humidity environment.SOLUTION: An adhesive tape includes an adhesive layer formed with an adhesive composition, wherein the adhesive composition contains a (meth)acrylic copolymer and a tackifier resin, the adhesive layer has a glass transition temperature of -10°C or higher and 20°C or lower, and an amount of change in b* measured in color difference measurement before and after heating is 2.00 or less when the adhesive tape is heated for 120 hours under an environment of 60°C, 90% RH.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to an adhesive tape. [Background technology]

[0002] Adhesive tapes are used for assembly of portable electronic devices such as mobile phones and personal digital assistants (PDAs) (for example, Patent Documents 1 and 2). Adhesive tapes are also used for bonding optical components (for example, Patent Document 3). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2009-242541 [Patent Document 2] Japanese Patent Application Laid-Open No. 2009-258274 [Patent Document 3] Japanese Patent Application Laid-Open No. 2012-214544 Summary of the Invention [Problem to be solved by the invention]

[0004] In recent years, with the miniaturization and thinning of electronic devices, there has been a demand for thinner adhesive tapes. However, because thinner adhesive tapes can have reduced adhesive strength, adhesive tapes are required to have excellent adhesion to adherends even when they are thin. In order to improve the adhesion of adhesive tapes to adherends, it has been considered to incorporate a tackifier resin into the adhesive composition used to form the adhesive layer of the adhesive tape. However, incorporating a tackifier resin has presented a problem in that the tape may turn yellow when exposed to a high-temperature, high-humidity environment after being attached to the adherend. In particular, adhesive tapes used to fix LED backlights and the like in image display devices have been plagued with problems such as reduced light output and poor visibility of displayed images when the color tone changes due to yellowing.

[0005] An object of the present invention is to provide a pressure-sensitive adhesive tape that has excellent adhesion to an adherend and excellent color tone retention even when exposed to a high-temperature, high-humidity environment. [Means for solving the problem]

[0006] Disclosure 1 relates to a pressure-sensitive adhesive tape having a pressure-sensitive adhesive layer formed using a pressure-sensitive adhesive composition, the pressure-sensitive adhesive composition containing a (meth)acrylic copolymer and a tackifier resin, the pressure-sensitive adhesive layer having a glass transition temperature of -10°C or higher and 20°C or lower, and when the pressure-sensitive adhesive tape is heated in an environment of 60°C and 90% RH for 120 hours, the change in b* measured in color difference measurement before and after heating is 2.00 or lower. The present disclosure 2 is the pressure-sensitive adhesive tape of the present disclosure 1, wherein the total content of the tackifier resin in the pressure-sensitive adhesive composition is 20 parts by mass or less per 100 parts by mass of the (meth)acrylic copolymer. Disclosure 3 is the pressure-sensitive adhesive tape of Disclosure 1 or 2, wherein the (meth)acrylic copolymer has a structural unit derived from 1-methylheptyl (meth)acrylate. Disclosure 4 is the pressure-sensitive adhesive tape of Disclosure 3, wherein the (meth)acrylic copolymer further comprises a structural unit derived from a monomer having a crosslinkable functional group. A fifth aspect of the present disclosure is the pressure-sensitive adhesive tape of the fourth aspect of the present disclosure, wherein the monomer having a crosslinkable functional group includes at least one selected from the group consisting of a carboxy group-containing monomer and a hydroxy group-containing monomer. The present disclosure 6 is the pressure-sensitive adhesive tape of the present disclosure 4 or 5, wherein the (meth)acrylic copolymer contains 0.01 mass % or more and less than 20 mass % of the structural unit derived from the monomer having a crosslinkable functional group. Disclosure 7 is the pressure-sensitive adhesive tape of Disclosure 3, 4, 5, or 6, wherein the (meth)acrylic copolymer further has a structural unit derived from n-heptyl (meth)acrylate. Disclosure 8 is the pressure-sensitive adhesive tape of Disclosures 3, 4, 5, 6, or 7, wherein the (meth)acrylic copolymer further comprises a structural unit derived from at least one monomer having a non-crosslinkable ether structure selected from the group consisting of monomers having a cyclic ether structure other than an epoxy group or an oxetanyl group, and monomers having an acyclic ether structure. Disclosure 9 is the pressure-sensitive adhesive tape of Disclosure 8, wherein the (meth)acrylic copolymer contains more than 0 mass % and 50 mass % or less of the structural unit derived from the monomer having a non-crosslinkable ether structure. A tenth aspect of the present disclosure is the pressure-sensitive adhesive tape of the first, second, third, fourth, fifth, sixth, seventh, eighth, or nineth aspect of the present disclosure, wherein the (meth)acrylic copolymer has a weight-average molecular weight of 700,000 or more and 1,500,000 or less. Disclosure 11 is the pressure-sensitive adhesive tape of Disclosures 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10, wherein the pressure-sensitive adhesive composition further contains a crosslinking agent. Disclosure 12 is the pressure-sensitive adhesive tape of Disclosure 11, wherein the crosslinking agent includes at least one selected from the group consisting of an isocyanate-based crosslinking agent and an epoxy-based crosslinking agent. Disclosure 13 is the pressure-sensitive adhesive tape of Disclosure 12, wherein the crosslinking agent includes the isocyanate-based crosslinking agent and the epoxy-based crosslinking agent. Disclosure 14 is the adhesive tape of Disclosures 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, or 13, which has a 180° peel adhesive strength to a polycarbonate substrate of 10 N / 25 mm or more. Disclosure 15 is the pressure-sensitive adhesive tape of Disclosures 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, or 14 used for fixing electronic equipment components or vehicle-mounted components. The present invention will be described in detail below.

[0007] The present inventors investigated how to adjust the glass transition temperature of the adhesive layer of an adhesive tape to a specific range and how to make the change in b* measured in color difference measurement before and after heating the adhesive tape to a specific value or less when the adhesive tape is heated in an environment of 60°C and 90% RH for 120 hours. As a result, they found that an adhesive tape can be obtained that has excellent adhesion to an adherend and excellent color retention even when exposed to a high-temperature, high-humidity environment, and have completed the present invention.

[0008] When the pressure-sensitive adhesive tape of the present invention is heated for 120 hours in an environment of 60°C and 90% RH, the change in b* (hereinafter also referred to as "Δb*") measured by measuring the color difference before and after heating is 2.00 or less. When the Δb* is 2.00 or less, the pressure-sensitive adhesive tape of the present invention exhibits excellent color tone retention even when exposed to a high-temperature, high-humidity environment. The upper limit of the Δb* is preferably 1.50, and more preferably 1.00. Moreover, it is most preferable that the above Δb* is 0, that is, there is no change in b* before and after heating. The above Δb* can be measured by the following method. Specifically, a glass plate measuring 52 mm wide, 76 mm long, and 1 mm thick was prepared. An adhesive tape cut to the same width and length as the glass plate was attached to one side of the glass plate, and the release PET film on the other side was peeled off to prepare a test piece. When the adhesive tape had an adhesive layer on only one side of the substrate, a glass plate was attached to the adhesive layer side of the adhesive tape to prepare a test piece. The b* (b* before heating) of the obtained test piece was then measured using a spectrophotometer at room temperature (20°C to 25°C). Furthermore, adhesive tape cut to the same width and length was placed in a constant temperature and humidity oven at 60°C and 90% RH and heated for 120 hours. A test piece was similarly prepared using the heated adhesive tape, and the b* (b* after heating) was measured using a spectrophotometer at room temperature (20°C to 25°C). The Δb* value can be calculated as the absolute difference between the b* value before heating and the b* value after heating. An example of the spectrophotometer is the CM-3700D (manufactured by Konica Minolta).

[0009] Furthermore, when the pressure-sensitive adhesive tape of the present invention is heated for 120 hours in an environment of 60°C and 90% RH, it is preferable that the b* before heating and the b* after heating are both 2.50 or less. When the b* before heating and the b* after heating are both 2.50 or less, the pressure-sensitive adhesive tape of the present invention can be more suitably used for bonding LED backlights and the like. It is more preferable that the b* before heating and the b* after heating are both 2.00 or less. There is no particular preferred lower limit for the b* before heating and the b* after heating, but the substantial lower limit is −1.00. The b* before heating and the b* after heating can be measured by the method described above.

[0010] The pressure-sensitive adhesive tape of the present invention has a pressure-sensitive adhesive layer formed using a pressure-sensitive adhesive composition. The pressure-sensitive adhesive layer has a glass transition temperature that is −10°C at minimum and 20°C at maximum. Having a glass transition temperature of −10°C or higher results in high heat resistance and high bulk cohesive strength, resulting in excellent adhesive strength retention and high-temperature repulsion resistance. Having a glass transition temperature of 20°C or lower results in excellent adhesion to interfaces. That is, having a glass transition temperature of the pressure-sensitive adhesive layer within the above range results in the pressure-sensitive adhesive tape of the present invention having excellent adhesion to adherends. The pressure-sensitive adhesive layer preferably has a lower limit of −8°C and an upper limit of 17°C, a more preferred lower limit of −5°C and an even more preferred upper limit of 15°C. In this specification, the "glass transition temperature" refers to the temperature at which a maximum due to micro-Brownian motion appears among the maximum loss tangents (tan δ) obtained by dynamic viscoelasticity measurement. For the dynamic viscoelasticity measurement carried out to measure the glass transition temperature of the pressure-sensitive adhesive layer, for example, the following method can be employed. That is, first, the pressure-sensitive adhesive layers are stacked to prepare a laminate of approximately 1 mm in thickness, which is then cut into a 6 mm wide and 10 mm long specimen. The specimen is then subjected to dynamic viscoelasticity measurement in shear mode using a dynamic viscoelasticity measuring device under a nitrogen atmosphere at a temperature of -40°C to 140°C, a heating rate of 5°C / min, a frequency of 1 Hz, and a strain of 0.08%. Examples of the dynamic viscoelasticity measuring device include the DVA-200 (manufactured by IT Instrumentation & Control Co., Ltd.).

[0011] The Δb* of the pressure-sensitive adhesive tape and the glass transition temperature of the pressure-sensitive adhesive layer can be adjusted to the above-mentioned values ​​by adjusting the type and content of each component constituting the pressure-sensitive adhesive composition. Examples of methods for forming the pressure-sensitive adhesive layer using the pressure-sensitive adhesive composition include a method in which the pressure-sensitive adhesive composition is applied to a release film or the like, and then the pressure-sensitive adhesive composition is dried by heating, etc. The pressure-sensitive adhesive layer may contain the uncrosslinked pressure-sensitive adhesive composition, or may contain a crosslinked product of the pressure-sensitive adhesive composition.

[0012] The pressure-sensitive adhesive composition contains a (meth)acrylic copolymer. The (meth)acrylic copolymer preferably has a structural unit derived from 1-methylheptyl (meth)acrylate. By using the (meth)acrylic copolymer having a structural unit derived from 1-methylheptyl (meth)acrylate, the glass transition temperature of the resulting pressure-sensitive adhesive layer can be easily adjusted to the above range without incorporating a large amount of a tackifier resin into the pressure-sensitive adhesive composition. In this specification, the term "(meth)acrylic" means acrylic or methacrylic, and the term "(meth)acrylate" means acrylate or methacrylate.

[0013] The content of the structural units derived from 1-methylheptyl (meth)acrylate in the (meth)acrylic copolymer is preferably 50% by mass at its lower limit and 99% by mass at its upper limit. When the content of the structural units derived from 1-methylheptyl (meth)acrylate is within this range, it becomes easier to adjust the glass transition temperature of the resulting pressure-sensitive adhesive layer to the aforementioned range. The content of the structural units derived from 1-methylheptyl (meth)acrylate is more preferably 60% by mass at its lower limit, even more preferably 70% by mass at its lower limit, particularly preferably 85% by mass at its lower limit, and even more preferably 97% by mass at its upper limit. Furthermore, as long as the Δb* of the pressure-sensitive adhesive tape and the glass transition temperature of the pressure-sensitive adhesive layer can be adjusted to the above-mentioned values, the lower limit of the content of the structural units derived from 1-methylheptyl (meth)acrylate in the (meth)acrylic copolymer may be set to a value smaller than 50% by mass, specifically, for example, 20% by mass, 30% by mass, 35% by mass, 45% by mass, etc. The content of the structural unit derived from 1-methylheptyl (meth)acrylate in the (meth)acrylic copolymer can be determined by measuring the (meth)acrylic copolymer by mass spectrometry and / or nuclear magnetic resonance spectroscopy ( 1 H-NMR measurement, 13 C-NMR measurement, etc., and the value can be calculated from the integrated intensity ratio of the hydrogen peak derived from the above-mentioned 1-methylheptyl (meth)acrylate. The content of the structural unit derived from 1-methylheptyl (meth)acrylate in the acrylic copolymer can also be calculated by pyrolysis GC-MS measurement. More specifically, a standard sample with a known content of structural units derived from 1-methylheptyl(meth)acrylate is subjected to pyrolysis GC-MS measurement, a calibration curve is created from the measured peak area of ​​2-octene, and the content of structural units derived from 1-methylheptyl(meth)acrylate can be calculated using the created calibration curve. More specifically, a calibration curve can be created as follows: First, an acrylic copolymer (standard sample) with a known content of structural units derived from 1-methylheptyl (meth)acrylate is prepared by adjusting the blending ratio of n-heptyl (meth)acrylate and 1-methylheptyl (meth)acrylate is prepared. The standard sample is weighed out and subjected to pyrolysis GC-MS measurement under the conditions below to obtain each total ion current chromatogram. A calibration curve can be created by plotting the content of structural units derived from 1-methylheptyl (meth)acrylate on the horizontal axis and the peak area of ​​2-octene in the total ion current chromatogram on the vertical axis. Next, the same amount of the acrylic copolymer to be measured as the standard sample used to create the calibration curve is weighed out and subjected to pyrolysis GC-MS measurement under the conditions below. The content of structural units derived from 1-methylheptyl (meth)acrylate is calculated from the peak area of ​​2-octene in the obtained total ion current chromatogram and the created calibration curve. Using a similar method, the content of structural units derived from n-heptyl (meth)acrylate, which will be described later, can be calculated from the peak area of ​​1-heptene. <Pyrolysis GC-MS measurement conditions> Equipment PY-3030D (manufactured by FRONTIER LAB) Thermal decomposition temperature 550℃ GC-MS equipment Agilent 7890B (manufactured by Agilent Technologies) & JMS-Q1500 (manufactured by JEOL Ltd.) Inlet temperature 300℃ Sample amount: 0.2 mg Column: Ultra-ALLOY-1 (non-polar) 0.25 mm diameter x 30 m x 0.25 μm He flow rate 1.0 mL / min (split ratio 1:50) Column temperature: 40°C (3 min) → 10°C / min → 300°C (5 min) MS temperature: Ion source: 230°C, interface: 250°C MS measurement range: 35-600 Ionization method EI method Measurement Mode Scan Ionization voltage 70eV

[0014] The (meth)acrylic copolymer preferably further contains a structural unit derived from a monomer having a crosslinkable functional group. When the (meth)acrylic copolymer has a structural unit derived from the monomer having a crosslinkable functional group, the cohesive strength of the pressure-sensitive adhesive layer is improved, and the resulting pressure-sensitive adhesive tape has better adhesion to the adherend.

[0015] Examples of the monomer having a crosslinkable functional group include a carboxy group-containing monomer, a hydroxy group-containing monomer, a glycidyl group-containing monomer, an amide group-containing monomer, a nitrile group-containing monomer, etc. Among these, the monomer having a crosslinkable functional group preferably includes at least one selected from the group consisting of a carboxy group-containing monomer and a hydroxy group-containing monomer, since this facilitates adjustment of the gel fraction of the pressure-sensitive adhesive layer, which will be described later. The monomer having a crosslinkable functional group preferably has a (meth)acryloyl group. In this specification, the term "(meth)acryloyl" means acryloyl or methacryloyl.

[0016] The carboxy group-containing monomer may, for example, be (meth)acrylic acid. Examples of the hydroxyl group-containing monomer include 2-hydroxyethyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, 3-hydroxybutyl (meth)acrylate, 1-methyl-3-hydroxypropyl (meth)acrylate, 1-methyl-2-hydroxyethyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, 1-methyl-2-hydroxypropyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 2-hydroxybutyl (meth)acrylate, 1-ethyl-2-hydroxyethyl (meth)acrylate, 6-hydroxyhexyl (meth)acrylate, 5-hydroxypentyl (meth)acrylate, 7-hydroxyheptyl (meth)acrylate, 8-hydroxyoctyl (meth)acrylate, 9-hydroxynonyl (meth)acrylate, and polypropylene glycol mono(meth)acrylate. The glycidyl group-containing monomer may, for example, be glycidyl (meth)acrylate. Examples of the amide group-containing monomer include isopropyl(meth)acrylamide and dimethylaminopropyl(meth)acrylamide. Examples of the nitrile group-containing monomer include (meth)acrylonitrile.

[0017] The (meth)acrylic copolymer preferably has a lower limit of 0.01% by mass and an upper limit of 20% by mass for the content of the structural unit derived from the monomer having a crosslinkable functional group. When the content of the structural unit derived from the monomer having a crosslinkable functional group is within this range, the resulting pressure-sensitive adhesive tape has superior adhesion to the adherend. The more preferred lower limit of the content of the structural unit derived from the monomer having a crosslinkable functional group is 0.1% by mass and an even more preferred upper limit is 10% by mass. The content of the structural unit derived from the monomer having the crosslinkable functional group in the (meth)acrylic copolymer can be determined by measuring the (meth)acrylic copolymer by mass spectrometry and / or nuclear magnetic resonance spectroscopy ( 1 H-NMR measurement, 13C-NMR measurement or the like) and calculation can be performed from the integrated intensity ratio of the hydrogen peak derived from the monomer having the crosslinkable functional group.

[0018] The (meth)acrylic copolymer may contain structural units derived from other monomers other than the structural units derived from 1-methylheptyl (meth)acrylate and the structural units derived from the monomer having a crosslinkable functional group, as long as the object of the present invention is not impaired. Examples of the other monomers include alkyl (meth)acrylates other than the 1-methylheptyl (meth)acrylate, monomers having a cyclic ether structure other than an epoxy group or an oxetanyl group, and monomers having an acyclic ether structure.

[0019] Examples of the other (meth)acrylic acid alkyl esters include methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, n-butyl (meth)acrylate, tert-butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, n-hexyl (meth)acrylate, 2-ethylbutyl (meth)acrylate, 2-methylpentyl (meth)acrylate, 4-methyl-2-pentyl (meth)acrylate, n-heptyl (meth)acrylate, and n-octyl (meth)acrylate. Examples of suitable (meth)acrylic acid esters include acrylate, isooctyl (meth)acrylate, n-nonyl (meth)acrylate, isononyl (meth)acrylate, myristyl (meth)acrylate, cetyl (meth)acrylate, stearyl (meth)acrylate, esters of 5,7,7-trimethyl-2-(1,3,3-trimethylbutyl)-1-octanol and (meth)acrylic acid, esters of alcohols having a total of 18 carbon atoms and one or two methyl groups in the linear main chain and (meth)acrylic acid, behenyl (meth)acrylate, and arachidyl (meth)acrylate. Among these, n-heptyl (meth)acrylate is preferred. That is, when the (meth)acrylic copolymer contains the above-mentioned other (meth)acrylic acid alkyl ester-derived structural units, the (meth)acrylic acid copolymer preferably contains n-heptyl (meth)acrylate-derived structural units as the other (meth)acrylic acid alkyl ester-derived structural units.

[0020] When the (meth)acrylic copolymer has the structural units derived from 1-methylheptyl (meth)acrylate and the structural units derived from the other (meth)acrylic acid alkyl ester, the content of the structural units derived from 1-methylheptyl (meth)acrylate in the total of the structural units derived from 1-methylheptyl (meth)acrylate and the structural units derived from the other (meth)acrylic acid alkyl ester may be 15% by mass or more, 25% by mass or more, 40% by mass or more, 50% by mass or more, 60% by mass or more, or 70% by mass or more. Furthermore, the content of the structural units derived from 1-methylheptyl (meth)acrylate in the total of the structural units derived from 1-methylheptyl (meth)acrylate and the structural units derived from the other (meth)acrylic acid alkyl ester may be 99% by mass or less, 97% by mass or less, 95% by mass or less, or 92% by mass or less.

[0021] When the (meth)acrylic copolymer has a structural unit derived from at least one monomer having a non-crosslinkable ether structure selected from the group consisting of the monomer having a cyclic ether structure other than the epoxy group and the oxetanyl group, and the monomer having an acyclic ether structure (hereinafter, also simply referred to as a "structural unit derived from a monomer having a non-crosslinkable ether structure"), the resulting pressure-sensitive adhesive tape has superior adhesion to an adherend.

[0022] Examples of the monomer having a cyclic ether structure other than the epoxy group and the oxetanyl group include tetrahydrofurfuryl (meth)acrylate. Examples of the monomer having an acyclic ether structure include 2-methoxyethyl (meth)acrylate, 2-butoxyethyl (meth)acrylate, 2-phenoxyethyl (meth)acrylate, and ethyl carbitol (meth)acrylate. Among these, 2-methoxyethyl (meth)acrylate is preferred. That is, when the (meth)acrylic copolymer has a structural unit derived from the monomer having an acyclic ether structure, the structural unit derived from the monomer having an acyclic ether structure is preferably a structural unit derived from 2-methoxyethyl (meth)acrylate.

[0023] When the (meth)acrylic copolymer has the structural units derived from the 1-methylheptyl (meth)acrylate and the structural units derived from the 2-methoxyethyl (meth)acrylate, the content of the structural units derived from the 1-methylheptyl (meth)acrylate in the total of the structural units derived from the 1-methylheptyl (meth)acrylate and the structural units derived from the 2-methoxyethyl (meth)acrylate may be 25% by mass or more, 40% by mass or more, 50% by mass or more, 60% by mass or more, 70% by mass or more, or 80% by mass or more. Furthermore, the content of the structural units derived from the 1-methylheptyl (meth)acrylate in the total of the structural units derived from the 1-methylheptyl (meth)acrylate and the structural units derived from the 2-methoxyethyl (meth)acrylate may be 99% by mass or less, 97% by mass or less, or 95% by mass or less.

[0024] When the content of the structural units derived from the monomer having a non-crosslinkable ether structure in the (meth)acrylic copolymer exceeds 0% by mass, the preferred upper limit of the content of the structural units derived from the monomer having a non-crosslinkable ether structure is 50% by mass. By having the content of the structural units derived from the monomer having a non-crosslinkable ether structure exceed 0% by mass and not more than 50% by mass, the resulting pressure-sensitive adhesive tape will have superior adhesiveness. The preferred lower limit of the content of the structural units derived from the monomer having a non-crosslinkable ether structure is 0.01% by mass, and more preferably 0.1% by mass. Furthermore, the more preferred upper limit of the content of the structural units derived from the monomer having a non-crosslinkable ether structure is 30% by mass, and even more preferably 10% by mass. The content of the structural unit derived from the monomer having a non-crosslinkable ether structure in the (meth)acrylic copolymer can be determined by measuring the (meth)acrylic copolymer by mass spectrometry and / or nuclear magnetic resonance spectroscopy ( 1 H-NMR measurement, 13 C-NMR measurement or the like) and calculation can be performed from the integrated intensity ratio of the hydrogen peak derived from the monomer having the non-crosslinkable ether structure.

[0025] Examples of the other monomers include cyclohexyl (meth)acrylate, benzyl (meth)acrylate, etc. Furthermore, examples of the other monomers that can be used include various monomers that are commonly used as raw materials for (meth)acrylic copolymers, such as vinyl acetate and styrene.

[0026] The weight-average molecular weight of the (meth)acrylic copolymer preferably has a lower limit of 700,000 and an upper limit of 1,500,000. When the weight-average molecular weight of the (meth)acrylic copolymer is within this range, the resulting pressure-sensitive adhesive tape has better adhesion to the adherend. The weight-average molecular weight of the (meth)acrylic copolymer is more preferably 800,000 and an upper limit of 1,200,000. In this specification, the weight-average molecular weight is a weight-average molecular weight measured by GPC (gel permeation chromatography) in terms of standard polystyrene. Specifically, a (meth)acrylic copolymer is diluted 50 times with tetrahydrofuran (THF) and the resulting diluted solution is filtered through a filter (material: polytetrafluoroethylene, pore diameter: 0.2 μm) to prepare a measurement sample. Next, this measurement sample is supplied to a gel permeation chromatograph, and GPC measurement is performed under conditions of a sample flow rate of 1 mL / min and a column temperature of 40°C. The polystyrene-equivalent molecular weight of the (meth)acrylic copolymer is measured, and this value is taken as the weight-average molecular weight of the (meth)acrylic copolymer. An example of the gel permeation chromatograph is the 2690 Separations Module (manufactured by Waters). Examples of methods for adjusting the weight average molecular weight of the (meth)acrylic copolymer include a method of changing the type or amount of a polymerization initiator or the monomer concentration during the polymerization reaction, a method of adding a small amount of a chain transfer agent such as dodecyl mercaptan, a method of changing the type of polymerization reaction solvent to control chain transfer to the solvent, and a method of changing the temperature and time during the reaction.

[0027] The (meth)acrylic copolymer can be obtained by polymerizing a mixture of raw material monomers through a radical reaction in the presence of a polymerization initiator. Examples of the radical reaction method include living radical polymerization, free radical polymerization, etc. Living radical polymerization can provide a copolymer having a more uniform molecular weight and composition compared to free radical polymerization, and can suppress the generation of low molecular weight components, etc., thereby increasing the cohesive strength of the resulting pressure-sensitive adhesive layer and providing a pressure-sensitive adhesive tape with superior adhesion to an adherend. The method for polymerizing the monomer mixture can be a conventionally known method, such as solution polymerization (boiling point polymerization or constant temperature polymerization), UV polymerization, emulsion polymerization, suspension polymerization, bulk polymerization, etc. Among these, solution polymerization and UV polymerization are preferred because the resulting pressure-sensitive adhesive tape has better adhesion to the adherend. When solution polymerization is used as the method for polymerizing the monomer mixture, examples of the reaction solvent include ethyl acetate, toluene, methyl ethyl ketone, dimethyl sulfoxide, ethanol, acetone, and diethyl ether.

[0028] Examples of the polymerization initiator include organic peroxides and azo compounds. Examples of the organic peroxides include 1,1-bis(t-hexylperoxy)-3,3,5-trimethylcyclohexane, t-hexylperoxypivalate, t-butylperoxypivalate, 2,5-dimethyl-2,5-bis(2-ethylhexanoylperoxy)hexane, t-hexylperoxy-2-ethylhexanoate, t-butylperoxy-2-ethylhexanoate, t-butylperoxyisobutyrate, t-butylperoxy-3,5,5-trimethylhexanoate, and t-butylperoxylaurate. Examples of the azo compound include azobisisobutyronitrile and azobiscyclohexanecarbonitrile. Furthermore, when the radical reaction method is the living radical polymerization, the polymerization initiator may be, for example, an organic tellurium polymerization initiator. The organic tellurium polymerization initiator is not particularly limited as long as it is one that is generally used in living radical polymerization, and examples thereof include organic tellurium compounds, organic telluride compounds, etc. Note that, in the living radical polymerization, in addition to the organic tellurium polymerization initiator, an azo compound may also be used as the polymerization initiator for the purpose of accelerating the polymerization rate.

[0029] The preferred lower limit of the content of the (meth)acrylic copolymer in the pressure-sensitive adhesive composition is 50% by mass, and the preferred upper limit is 98% by mass. By ensuring that the content of the (meth)acrylic copolymer is within this range, the resulting pressure-sensitive adhesive tape will be more excellent in terms of both adhesion to the adherend and transparency. The more preferred lower limit of the content of the (meth)acrylic copolymer is 70% by mass, and the more preferred upper limit is 96% by mass.

[0030] The pressure-sensitive adhesive composition contains a tackifier resin. By containing the tackifier resin, the pressure-sensitive adhesive tape of the present invention has excellent adhesion to an adherend.

[0031] The tackifier resin preferably contains at least one selected from the group consisting of rosin ester resins and terpene resins. By containing at least one selected from the group consisting of rosin ester resins and terpene resins as the tackifier resin, the resulting pressure-sensitive adhesive tape has better adhesion to the adherend. The tackifier resin more preferably contains a rosin ester resin and a terpene resin. The terpene resin includes a terpene phenol resin.

[0032] Examples of the rosin ester resin include Pencel D-135, Pine Crystal KE-359, Ester Gum AA-V, and Ester Gum H (all manufactured by Arakawa Chemical Industries, Ltd.). Examples of the terpene resin include YS Resin PX1250 (manufactured by Yasuhara Chemical Co., Ltd.) and YS Polystar G150 (manufactured by Yasuhara Chemical Co., Ltd.).

[0033] The tackifier resin preferably contains a tackifier resin having a softening temperature of 80°C or higher. By containing the tackifier resin having a softening temperature of 80°C or higher, the PSA layer does not become too soft, and a decrease in the adhesive strength of the resulting PSA tape can be suppressed. The softening temperature of the tackifier resin having a softening temperature of 80°C or higher is preferably 90°C or higher, and more preferably 100°C or higher. Furthermore, from the viewpoint of improving the wettability of the interface of the pressure-sensitive adhesive layer, the softening temperature of the tackifier resin having a softening temperature of 80°C or higher is preferably 170°C or lower, and more preferably 160°C or lower. In this specification, the "softening temperature" refers to the softening temperature measured by a method in accordance with JIS K 2207 (ring and ball method).

[0034] In the pressure-sensitive adhesive composition, the preferred upper limit of the total content of the tackifier resin relative to 100 parts by mass of the (meth)acrylic copolymer is 40 parts by mass. By keeping the total content of the tackifier resin at 40 parts by mass or less, the resulting pressure-sensitive adhesive tape will have better color tone retention. The more preferred upper limit of the total content of the tackifier resin is 30 parts by mass, and even more preferred upper limit is 20 parts by mass. Furthermore, from the viewpoint of making the resulting pressure-sensitive adhesive tape have better adhesion to the adherend, the lower limit of the total content of the tackifier resin is preferably 3 parts by mass, more preferably 5 parts by mass, and even more preferably 10 parts by mass.

[0035] The pressure-sensitive adhesive composition preferably further contains a crosslinking agent. Examples of the crosslinking agent include an isocyanate-based crosslinking agent, an epoxy-based crosslinking agent, an aziridine-based crosslinking agent, a metal chelate-based crosslinking agent, etc. Among these, since the resulting pressure-sensitive adhesive tape will have better adhesion to an adherend and transparency, the crosslinking agent preferably contains at least one selected from the group consisting of the isocyanate-based crosslinking agent and the epoxy-based crosslinking agent, more preferably contains the isocyanate-based crosslinking agent, and even more preferably contains the isocyanate-based crosslinking agent and the epoxy-based crosslinking agent.

[0036] In the pressure-sensitive adhesive composition, the content of the crosslinking agent relative to 100 parts by mass of the (meth)acrylic copolymer is preferably 0.1 parts by mass at the lower limit and 10 parts by mass at the upper limit. By ensuring that the content of the crosslinking agent is within this range, the resulting pressure-sensitive adhesive tape will have better adhesion to the adherend. The content of the crosslinking agent is more preferably 0.2 parts by mass at the lower limit and 5 parts by mass at the upper limit. In this specification, the "content of crosslinking agent" means the content of the solid content of the crosslinking agent.

[0037] The pressure-sensitive adhesive composition may further contain a crosslinking catalyst for accelerating crosslinking by the crosslinking agent. Examples of the crosslinking catalyst include dibutyltin dilaurate, dibutyltin diacetate, and dioctyltin dilaurate as crosslinking catalysts for the isocyanate-based crosslinking agents.

[0038] The pressure-sensitive adhesive composition may contain additives such as a silane coupling agent, a plasticizer, a softener, a filler, a pigment, a dye, etc., as needed, within the scope of not impairing the object of the present invention.

[0039] The preferred lower limit of the content of bio-derived carbon in the pressure-sensitive adhesive layer is 10%. When the content of bio-derived carbon in the pressure-sensitive adhesive layer is 10% or more, the pressure-sensitive adhesive tape of the present invention is excellent in terms of saving petroleum resources and reducing carbon dioxide emissions, and can reduce the environmental load. The content of bio-derived carbon in the pressure-sensitive adhesive layer is preferably 40% or more, and more preferably 60% or more. The upper limit of the content of the biologically derived carbon is not particularly limited and may be 100%. Furthermore, the content of the biologically derived carbon may be 95% or less, or 90% or less. While carbon derived from living organisms contains a certain percentage of the radioactive isotope (C-14), petroleum-derived carbon contains almost no C-14. Therefore, the content of carbon derived from living organisms can be calculated by measuring the concentration of C-14 contained in the adhesive layer. Specifically, this can be measured in accordance with ASTM D6866-22, a standard used in many bioplastic industries.

[0040] The preferred lower limit of the gel fraction of the pressure-sensitive adhesive layer is 10% by mass, and the preferred upper limit is 70% by mass. When the gel fraction of the pressure-sensitive adhesive layer is in this range, the resulting pressure-sensitive adhesive tape has better adhesion to the adherend. The more preferred lower limit of the gel fraction of the pressure-sensitive adhesive layer is 20% by mass, and the more preferred upper limit is 50% by mass. The gel fraction of the pressure-sensitive adhesive layer is measured by the following method. That is, first, a pressure-sensitive adhesive tape having the pressure-sensitive adhesive layer is cut into a flat rectangular shape of 20 mm wide and 40 mm long to prepare a test piece, and the test piece is immersed in ethyl acetate at 23°C for 24 hours, then removed from the ethyl acetate and dried at 110°C for 1 hour. The mass of the test piece after drying is measured, and the gel fraction is calculated using the following formula (I). Note that no release film for protecting the pressure-sensitive adhesive layer is laminated on the test piece. If the pressure-sensitive adhesive tape is a non-support tape without a substrate, the measurement is performed using a test piece obtained by adhering the tape to a substrate and cutting it, or the calculation is performed without using a substrate, with W0 in the following formula (I) set to 0. Gel fraction (mass%) = 100 × (W2 - W0) / (W1 - W0) (I) (W0: Mass of the substrate, W1: Mass of the test piece before immersion, W2: Mass of the test piece after immersion and drying)

[0041] The pressure-sensitive adhesive layer in the pressure-sensitive adhesive tape of the present invention preferably has a lower limit of 3 μm and an upper limit of 300 μm. By ensuring that the pressure-sensitive adhesive layer has a thickness within this range, the resulting pressure-sensitive adhesive tape will have superior adhesion to the adherend and transparency. The pressure-sensitive adhesive layer more preferably has a lower limit of 5 μm and an upper limit of 200 μm.

[0042] The pressure-sensitive adhesive tape of the present invention may be a non-support type tape having no substrate, or a supported type tape having the above-mentioned pressure-sensitive adhesive layer on one or both sides of a substrate.

[0043] Examples of the substrate include films and nonwoven fabrics. Furthermore, from the viewpoint of increasing the content of bio-derived carbon in the entire pressure-sensitive adhesive tape, the substrate is preferably a substrate made of a bio-derived material. Examples of the above-mentioned biologically-derived materials include polyesters (PES) such as polyethylene terephthalate (PET), polyethylene furanoate (PEF), polylactic acid (PLA), polytrimethylene terephthalate (PTT), polybutylene terephthalate (PBT), and polybutylene succinate (PBS), polyethylene (PE), polypropylene (PP), polyurethane (PU), triacetyl cellulose (TAC), cellulose, and polyamide (PA), which are derived from plants.

[0044] The substrate is preferably a film containing PES or a film containing PA from the viewpoint of substrate strength, and is preferably a film containing PA from the viewpoint of heat resistance and oil resistance. Examples of the PA include nylon 11, nylon 1010, nylon 610, nylon 510, nylon 410, etc., which are made from castor oil, and nylon 56, etc., which are made from cellulose.

[0045] Furthermore, from the perspective of reducing the use of new petroleum resources and reducing the environmental burden by suppressing carbon dioxide emissions, substrates made from recycled resources may be used. Examples of resource recycling methods include collecting waste materials such as packaging containers, home appliances, automobiles, construction materials, and food, as well as waste generated during manufacturing processes, and then cleaning, decontaminating, or decomposing the extracted materials by heating or fermentation to reuse them as raw materials. Examples of substrates made from recycled resources include films and nonwoven fabrics made from PET, PBT, PE, PP, PA, etc., which are made from recycled plastics that have been re-resinized. Furthermore, collected waste materials may be burned and used as thermal energy for the production of substrates and their raw materials, or the oils and fats contained in the collected waste materials may be mixed with petroleum, fractionated, and purified, and then used as raw materials.

[0046] The substrate may be a foam substrate from the viewpoint of improving compression characteristics. The foam substrate is preferably a foam substrate containing at least one selected from the group consisting of PE, PP, and PU, and from the viewpoint of achieving a high degree of both flexibility and strength, a foam substrate containing PE is more preferred. Examples of the constituents of the foam substrate containing PE include PE made from sugarcane.

[0047] A preferred method for producing the foam base material is, for example, to prepare a foamable resin composition containing a PE resin containing sugarcane-derived PE and a foaming agent, and then foam the foaming agent when extruding the foamable resin composition into a sheet using an extruder, and optionally crosslink the resulting polyolefin foam.

[0048] The preferred lower limit of the thickness of the foam substrate is 50 μm, and the preferred upper limit is 5000 μm. By having the thickness of the foam substrate within this range, it is possible to exhibit high impact resistance while also exhibiting high flexibility that allows it to be adhered to the shape of the adherend. The more preferred upper limit of the thickness of the foam substrate is 1000 μm, and even more preferred upper limit is 300 μm.

[0049] The adhesive tape of the present invention has a total thickness (total thickness of the substrate and adhesive layer) of preferably 3 μm at the lower limit and 6000 μm at the upper limit. Having a total thickness within this range of the adhesive tape enhances adhesive strength. The adhesive tape's total thickness is more preferably 1200 μm at the upper limit and even more preferably 500 μm at the upper limit.

[0050] The pressure-sensitive adhesive tape of the present invention has a preferred lower limit of 10 N / 25 mm for the 180° peel adhesive strength against a polycarbonate substrate. When the 180° peel adhesive strength against the polycarbonate substrate is 10 N / 25 mm or more, the tape can exhibit sufficient adhesion to the adherend, making it suitable for use in, for example, fixing electronic device components and in-vehicle components. A more preferred lower limit of the 180° peel adhesive strength against the polycarbonate substrate is 15 N / 25 mm. Furthermore, although there is no particular upper limit to the 180° peel adhesive strength to the polycarbonate substrate, the practical upper limit is 30 N / 25 mm. The 180° peel adhesive strength to the polycarbonate substrate can be measured by the following method. Specifically, adhesive tape cut into 25 mm wide strips is first bonded to a polycarbonate substrate by rolling a 2 kg rubber roller back and forth at a speed of 300 mm / min. The tape is then left to stand at 23°C and 50% RH for 20 minutes to obtain a test piece. The resulting test piece is subjected to a tensile test using a tensile tester in accordance with JIS Z 0237 at 23°C, a peel speed of 300 mm / min, and a peel angle of 180°, to measure the 180° peel adhesive strength.

[0051] The method for producing the pressure-sensitive adhesive tape of the present invention is not particularly limited, and the tape can be produced by a conventionally known production method. For example, in the case of a double-sided pressure-sensitive adhesive tape, the following method can be mentioned. First, a solvent is added to a (meth)acrylic copolymer, a tackifying resin, and an optional crosslinking agent, etc. to prepare a pressure-sensitive adhesive composition A. The resulting pressure-sensitive adhesive composition A is applied to the surface of a substrate, and the solvent in the composition is completely dried and removed by heating to form a pressure-sensitive adhesive layer A. Next, a release film is superimposed on the formed pressure-sensitive adhesive layer A with its release-treated surface facing the pressure-sensitive adhesive layer A. Next, a release film separate from the above release film is prepared, and PSA composition B, prepared in the same manner as PSA composition A, is applied to the release-treated surface of this release film. The solvent in the composition is then completely dried and removed to produce a laminate film in which PSA layer B is formed on the surface of the release film. The obtained laminate film is overlaid on the back surface of the substrate on which PSA layer A is formed, with PSA layer B facing the back surface of the substrate to produce a laminate. The laminate is then pressed with a rubber roller or the like to produce a double-sided PSA tape in which PSA layers are on both sides of the substrate and the surfaces of the PSA layers are covered with release films.

[0052] Alternatively, two sets of laminate films may be prepared in a similar manner, and these laminate films may be superimposed on each of both surfaces of a substrate with the pressure-sensitive adhesive layer of the laminate film facing the substrate to prepare a laminate. This laminate may then be pressed with a rubber roller or the like to obtain a double-sided pressure-sensitive adhesive tape having pressure-sensitive adhesive layers on both surfaces of the substrate, the surfaces of the pressure-sensitive adhesive layers being covered with release films.

[0053] The use of the pressure-sensitive adhesive tape of the present invention is not particularly limited, but it is preferably used for fixing electronic device components or vehicle-mounted components. Specifically, the pressure-sensitive adhesive tape of the present invention can be suitably used for adhesively fixing electronic device components in large portable electronic devices, adhesively fixing vehicle-mounted components (for example, vehicle-mounted panels), etc. [Effects of the Invention]

[0054] According to the present invention, it is possible to provide a pressure-sensitive adhesive tape that has excellent adhesion to an adherend and excellent color tone retention even when exposed to a high-temperature, high-humidity environment. DETAILED DESCRIPTION OF THE INVENTION

[0055] The following examples further illustrate aspects of the present invention, but the present invention is not limited to these examples. The materials used in the examples and comparative examples are as follows.

[0056] <1-Methylheptyl acrylate containing bio-derived carbon> Ricinoleic acid derived from castor oil was fused in an alkali to obtain a mixture containing sepacic acid and 1-methylheptyl alcohol. The sepacic acid was then separated from the resulting mixture by distillation to obtain 1-methylheptyl alcohol containing bio-derived carbon. The resulting 1-methylheptyl alcohol containing bio-derived carbon was esterified with acrylic acid (manufactured by Nippon Shokubai Co., Ltd.) to prepare 1-methylheptyl acrylate containing bio-derived carbon.

[0057] <n-Hexyl acrylate containing bio-derived carbon> Linoleic acid derived from castor oil was converted to linoleic acid hydroperoxide using lipoxygenase, and then a mixture containing n-hexylaldehyde was obtained using isomerase. The resulting mixture was distilled to obtain n-hexylaldehyde containing bio-derived carbon. The obtained n-hexylaldehyde containing bio-derived carbon was then hydrogenated to obtain n-hexyl alcohol containing bio-derived carbon. The obtained n-hexyl alcohol containing bio-derived carbon was esterified with acrylic acid (manufactured by Nippon Shokubai Co., Ltd.) to prepare n-hexyl acrylate containing bio-derived carbon.

[0058] <n-heptyl acrylate containing bio-derived carbon> Ricinoleic acid derived from castor oil was cracked to obtain a mixture containing undecylenic acid and n-heptyl alcohol. The undecylenic acid was then separated from the resulting mixture by distillation to obtain n-heptyl alcohol containing bio-derived carbon. The resulting n-heptyl alcohol containing bio-derived carbon was esterified with acrylic acid (manufactured by Nippon Shokubai Co., Ltd.) to prepare n-heptyl acrylate containing bio-derived carbon.

[0059] <2-hydroxyethyl acrylate containing bio-derived carbon> Ethanol containing bio-derived carbon was obtained by fermenting sugars contained in sugarcane. The obtained bio-derived ethanol was dehydrated to obtain ethylene, which was then oxidized to obtain ethylene oxide, and water was added to obtain ethylene glycol containing bio-derived carbon. The obtained bio-derived ethylene glycol was esterified with acrylic acid (manufactured by Nippon Shokubai Co., Ltd.) to prepare 2-hydroxyethyl acrylate containing bio-derived carbon.

[0060] <Bio-derived carbon-free monomer> n-Butyl acrylate (Tokyo Chemical Industry Co., Ltd.) 2-Ethylhexyl acrylate (Tokyo Chemical Industry Co., Ltd.) 2-Methoxyethyl acrylate (Tokyo Chemical Industry Co., Ltd.) Acrylic acid (Nippon Shokubai Co., Ltd.)

[0061] <Tackifying resin> Tackifying resin A: Rosin ester tackifying resin (Arakawa Chemical Industries, Ltd., "Pensel D-135", softening temperature: 130°C to 140°C) Tackifying resin B: Rosin ester tackifying resin (Arakawa Chemical Industries, Ltd., "Pine Crystal KE-359", softening temperature: 94°C to 104°C) Tackifying resin C: Terpene-based tackifying resin (terpene phenol resin) (Yasuhara Chemical Co., Ltd., "YS Polystar G150", softening temperature: 145°C to 155°C)

[0062] <Crosslinking agent> Isocyanate crosslinking agent (Covestro, "Desmodur L-75") Epoxy crosslinking agent (Mitsubishi Gas Chemical Company, "Tetrad C")

[0063] Example 1 (1) Production of acrylic copolymer Ethyl acetate was added as a polymerization solvent to a reaction vessel, and after bubbling with nitrogen, the reaction vessel was heated while nitrogen was flowing in to initiate reflux. Next, a polymerization initiator solution prepared by diluting 0.1 parts by mass of azobisisobutyronitrile 10 times with ethyl acetate was added to the reaction vessel, and 94.9 parts by mass of 1-methylheptyl acrylate containing bio-derived carbon, 0.1 parts by mass of 2-hydroxyethyl acrylate containing bio-derived carbon, and 5.0 parts by mass of acrylic acid were added dropwise over 2 hours. After the dropwise addition was completed, a polymerization initiator solution prepared by diluting 0.1 parts by mass of azobisisobutyronitrile 10 times with ethyl acetate was added again to the reaction vessel, and the polymerization reaction was carried out for 4 hours to obtain an acrylic copolymer-containing solution. The resulting acrylic copolymer was diluted 50-fold with tetrahydrofuran (THF), and the resulting diluted solution was filtered through a filter (material: polytetrafluoroethylene, pore size: 0.2 μm) to prepare a measurement sample. This measurement sample was fed to a gel permeation chromatograph (Waters, "2690 Separations Module") and subjected to GPC measurement at a sample flow rate of 1 mL / min and a column temperature of 40°C. The polystyrene-equivalent molecular weight of the acrylic copolymer was measured, and the weight-average molecular weight was calculated. The results are shown in Table 1.

[0064] (2) Manufacturing of adhesive tapes To the obtained acrylic copolymer-containing solution, 10.0 parts by mass of tackifier resin B and an isocyanate-based crosslinking agent were added so that the solid content was 1.5 parts by mass per 100 parts by mass of the acrylic copolymer in the acrylic copolymer-containing solution, thereby preparing a pressure-sensitive adhesive composition. The obtained pressure-sensitive adhesive composition was applied to the release-treated surface of a 75 μm-thick release PET film so that the thickness of the pressure-sensitive adhesive layer after drying would be 50 μm, and then dried at 110°C for 5 minutes to obtain a pressure-sensitive adhesive layer. The obtained pressure-sensitive adhesive layer was placed on the release-treated surface of a 75 μm-thick release PET film and aged at 40°C for 48 hours to obtain a pressure-sensitive adhesive tape (non-support type).

[0065] (3) Measurement of the glass transition temperature of the adhesive layer Both release PET films of the resulting adhesive tape were peeled off, and the adhesive layers were stacked to form a laminate approximately 1 mm thick. This was then cut into a 6 mm wide and 10 mm long specimen. Dynamic viscoelasticity measurements were performed on the resulting specimens using a dynamic viscoelasticity measuring device (IT Measurement & Control Co., Ltd., "DVA-200") in shear mode under a nitrogen atmosphere at a temperature range of -80°C to 140°C, a heating rate of 5°C / min, a frequency of 1 Hz, and a strain of 0.08%. The glass transition temperature was determined as the temperature at which the maximum in the loss tangent (tanδ) due to micro-Brownian motion appeared. The resulting glass transition temperatures are shown in Table 1.

[0066] (4) Measurement of gel fraction of adhesive layer The release PET film on one side of the resulting adhesive tape was peeled off, and the tape was laminated to a 23 μm thick base PET film ("FE2002" manufactured by Futamura Chemical Co., Ltd.) and cut into a flat rectangular shape 20 mm wide and 40 mm long. The release PET film on the other side of the adhesive tape was then peeled off to prepare a test piece, and its mass was measured. The test piece was immersed in ethyl acetate at 23°C for 24 hours, then removed from the ethyl acetate and dried at 110°C for 1 hour. The mass of the dried test piece was measured, and the gel fraction was calculated using the following formula (I). The results are shown in Table 1. Gel fraction (mass%) = 100 × (W2 - W0) / (W1 - W0) (I) (W0: Mass of the substrate, W1: Mass of the test piece before immersion, W2: Mass of the test piece after immersion and drying)

[0067] (5) Measurement of b* of adhesive tape The resulting adhesive tape was cut to a width of 52 mm and a length of 76 mm. The release PET film on one side was removed and the tape was adhered to a 1.0 mm thick, 52 mm wide, and 76 mm long glass plate (Matsunami Glass Industry Co., Ltd., "Large Slide White Edge Polished No. 2 S9112"). The release PET film on the other side of the adhesive tape was then removed to prepare a test specimen. The b* (b* before heating) of the resulting test specimen was measured using a spectrophotometer at room temperature (20°C to 25°C). Separately, a test specimen prepared in the same manner was placed in a constant temperature and humidity oven at 60°C and 90% RH for 120 hours. The b* (b* after heating) of the heated test specimen was measured using a spectrophotometer at room temperature (20°C to 25°C). Δb* was calculated as the absolute difference between the b* value before heating and the b* value after heating. The spectrophotometer used was a CM-3700D (manufactured by Konica Minolta, Inc.) The obtained b* before heating, b* after heating, and Δb* are shown in Table 1.

[0068] (Examples 2 to 20, 22 to 26, Comparative Examples 1 to 6) Pressure-sensitive adhesive tapes were obtained in the same manner as in Example 1, except that the types and amounts of monomers constituting the acrylic copolymer and the types and amounts of each component contained in the pressure-sensitive adhesive composition were as shown in Tables 1 to 4, and the amount of polymerization initiator added was appropriately changed. Furthermore, the weight-average molecular weight of the acrylic copolymer, the glass transition temperature of the pressure-sensitive adhesive layer, the gel fraction of the pressure-sensitive adhesive layer, and the b*, b*, and Δb* of the pressure-sensitive adhesive tape before heating were measured in the same manner as in Example 1. The results are shown in Tables 1 to 4.

[0069] Example 21 A pressure-sensitive adhesive composition was prepared in the same manner as in Example 1, except that the types and amounts of each component contained in the pressure-sensitive adhesive composition were as shown in Table 2. The resulting pressure-sensitive adhesive composition was applied to the release-treated surface of a 75-μm-thick release PET film so that the thickness of the pressure-sensitive adhesive layer after drying would be 19 μm, and then dried at 110°C for 5 minutes to obtain a pressure-sensitive adhesive layer. The resulting pressure-sensitive adhesive layer was bonded to one side of a 12-μm-thick base PET film (manufactured by Toyobo Co., Ltd., "E5200"). Furthermore, a pressure-sensitive adhesive layer having the same composition and thickness was formed on the release-treated surface of another 75-μm-thick release PET film, which was then bonded to the other side of the base PET film and aged at 40°C for 48 hours to obtain a pressure-sensitive adhesive tape (support type, total thickness 50 μm excluding the release PET film) having a pressure-sensitive adhesive layer and a release PET film on both sides of the substrate. In addition, the weight average molecular weight of the acrylic copolymer, the glass transition temperature of the pressure-sensitive adhesive layer, the gel fraction of the pressure-sensitive adhesive layer, and the b* of the pressure-sensitive adhesive tape before heating, the b* of the pressure-sensitive adhesive tape after heating, and Δb* were measured in the same manner as in Example 1. The glass transition temperature of the pressure-sensitive adhesive layer was measured on the pressure-sensitive adhesive layer before it was bonded to the base PET film. The gel fraction of the pressure-sensitive adhesive layer was measured using a test piece obtained by cutting the pressure-sensitive adhesive tape into a flat rectangular shape 20 mm wide and 40 mm long and then peeling off the release PET films on both sides. The results are shown in Table 2.

[0070] <Evaluation> The pressure-sensitive adhesive tapes obtained in the examples and comparative examples were evaluated by the following methods, and the results are shown in Tables 1 to 4.

[0071] (Adhesion to the substrate) The resulting adhesive tape was cut into 25 mm wide strips, the release PET film on one side was peeled off, and the strip was bonded to a polycarbonate substrate (TAKIRON C.I., "PC1600") by rolling a 2 kg rubber roller back and forth at a speed of 300 mm / min. The strip was then left to stand at 23°C and 50% RH for 20 minutes to obtain a test specimen. The resulting test specimen was subjected to a tensile test using a tensile tester (A&D, "RTI-1310") in accordance with JIS Z 0237 at 23°C, a peel speed of 300 mm / min, and a peel angle of 180°, to measure the 180° peel strength. Adhesion to the adherend was evaluated according to the following criteria. ◎: 180° peel adhesive strength is 15N / 25mm or more ○: 180° peel adhesive strength is 10N / 25mm or more but less than 15N / 25mm ×: 180° peel adhesive strength is less than 10N / 25mm

[0072] (Color retention when exposed to high temperature and humidity environments) The resulting adhesive tape was cut to a width of 50 mm and a length of 50 mm, and the release PET film on one side was peeled off and the tape was bonded to a 50 μm thick base PET film (manufactured by Toyobo Co., Ltd., "E5200"). The release PET film on the other side of the adhesive tape was then peeled off and the tape was bonded to a polycarbonate substrate (manufactured by Takiron C.I., "PC1600") to obtain a test specimen. The resulting test specimen was placed in a constant temperature and humidity oven at 60°C and 90% RH and heated for 120 hours. The test specimen was then removed from the oven and visually inspected from the base PET film side while irradiating it with light. Color retention was evaluated according to the following criteria. ◎: No yellowing was observed ○: Slight yellowing was observed ×: Clear yellowing was observed

[0073] [Table 1]

[0074] [Table 2]

[0075] [Table 3]

[0076] [Table 4] [Industrial Applicability]

[0077] According to the present invention, it is possible to provide a pressure-sensitive adhesive tape that has excellent adhesion to an adherend and excellent color tone retention even when exposed to a high-temperature, high-humidity environment.

Claims

1. A pressure-sensitive adhesive tape having a pressure-sensitive adhesive layer formed using a pressure-sensitive adhesive composition, The pressure-sensitive adhesive composition contains a (meth)acrylic copolymer and a tackifying resin, the pressure-sensitive adhesive layer has a glass transition temperature of −10° C. or higher and 20° C. or lower; When the pressure-sensitive adhesive tape is heated for 120 hours in an environment of 60°C and 90% RH, the change in b* measured by measuring the color difference before and after heating is 2.00 or less. An adhesive tape characterized by:

2. The pressure-sensitive adhesive tape according to claim 1, wherein the total content of the tackifier resin in the pressure-sensitive adhesive composition is 20 parts by mass or less per 100 parts by mass of the (meth)acrylic copolymer.

3. 3. The pressure-sensitive adhesive tape according to claim 1, wherein the (meth)acrylic copolymer has a structural unit derived from 1-methylheptyl (meth)acrylate.

4. The pressure-sensitive adhesive tape according to claim 3 , wherein the (meth)acrylic copolymer further comprises a structural unit derived from a monomer having a crosslinkable functional group.

5. The pressure-sensitive adhesive tape according to claim 4 , wherein the monomer having a crosslinkable functional group comprises at least one selected from the group consisting of a carboxyl group-containing monomer and a hydroxyl group-containing monomer.

6. The pressure-sensitive adhesive tape according to claim 4 , wherein the (meth)acrylic copolymer contains structural units derived from the monomer having a crosslinkable functional group in an amount of 0.01% by mass or more and less than 20% by mass.

7. The pressure-sensitive adhesive tape according to claim 3, wherein the (meth)acrylic copolymer further comprises a structural unit derived from n-heptyl (meth)acrylate.

8. The pressure-sensitive adhesive tape according to claim 3, wherein the (meth)acrylic copolymer further comprises a structural unit derived from at least one monomer having a non-crosslinkable ether structure selected from the group consisting of a monomer having a cyclic ether structure other than an epoxy group or an oxetanyl group, and a monomer having an acyclic ether structure.

9. The pressure-sensitive adhesive tape according to claim 8 , wherein the (meth)acrylic copolymer contains more than 0% by mass and not more than 50% by mass of structural units derived from the monomer having a non-crosslinkable ether structure.

10. 3. The pressure-sensitive adhesive tape according to claim 1, wherein the (meth)acrylic copolymer has a weight average molecular weight of 700,000 or more and 1,500,000 or less.

11. The pressure-sensitive adhesive tape according to claim 1 or 2, wherein the pressure-sensitive adhesive composition further contains a crosslinking agent.

12. The pressure-sensitive adhesive tape according to claim 11, wherein the crosslinking agent comprises at least one selected from the group consisting of an isocyanate-based crosslinking agent and an epoxy-based crosslinking agent.

13. The pressure-sensitive adhesive tape according to claim 12 , wherein the crosslinking agent comprises the isocyanate-based crosslinking agent and the epoxy-based crosslinking agent.

14. 3. The adhesive tape according to claim 1, which has a 180° peel adhesive strength to a polycarbonate substrate of 10 N / 25 mm or more.

15. The adhesive tape according to claim 1 or 2, which is used for fixing electronic equipment parts or vehicle-mounted parts.

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