JP7664049B2 - Adhesive tape - Google Patents

Description

The present invention relates to an adhesive tape.

Conventionally, adhesive tapes have been widely used to secure components in electronic devices. Specifically, adhesive tapes are used, for example, to adhere cover panels for protecting the surface of portable electronic devices to touch panel modules or display panel modules, or to adhere touch panel modules to display panel modules. In addition to high adhesiveness, adhesive tapes used to secure such electronic device components are required to have functions such as heat resistance, thermal conductivity, and impact resistance depending on the environment of the area where they are used (for example, Patent Documents 1 to 3).

JP 2015-052050 A JP 2015-021067 A JP 2015-120876 A

In recent years, electronic devices have become smaller, lighter, and less expensive, leading to widespread use of electronic devices that are worn or kept at hand, such as mobile phones, smartphones, and wearable devices. These electronic devices have become smaller and larger in screen size in recent years. This has led to a problem of noticeable yellowing of the display screen.
In view of the above-mentioned circumstances, an object of the present invention is to provide a pressure-sensitive adhesive tape having excellent yellowing resistance.

The present invention is an adhesive tape having an adhesive layer containing a (meth)acrylic copolymer, wherein the (meth)acrylic copolymer contains 60% by weight or more of structural units derived from an alkoxy group-containing (meth)acrylate.
The present invention will be described in detail below.

The present inventors have investigated the cause of yellowing when electronic devices are manufactured using adhesive tape, and have found that the adhesive tape used to bond the cover panel and touch sensor panel swells with sebum, increasing in width and thickness, causing the liquid crystal inside to be compressed and distorted, resulting in yellowing.
As a result of further intensive research, the inventors have found that in an adhesive tape having an adhesive layer containing a (meth)acrylic copolymer, yellowing can be prevented by using a (meth)acrylic copolymer containing a certain amount of structural units derived from an alkoxy group-containing (meth)acrylate, and have completed the present invention.

The pressure-sensitive adhesive tape of the present invention has a pressure-sensitive adhesive layer containing a (meth)acrylic copolymer.
The (meth)acrylic copolymer contains a structural unit derived from an alkoxy group-containing (meth)acrylate. The alkoxy group-containing (meth)acrylate has the property that the SP value is relatively high while the glass transition temperature is relatively low. The adhesive layer containing the (meth)acrylic copolymer containing the structural unit derived from the alkoxy group-containing (meth)acrylate is not easily swollen by sebum, and even if it swells, it shows stress relaxation properties that do not impose a load on the liquid crystal, and can exhibit excellent yellowing resistance. In addition, the adhesive layer can exhibit excellent conformability to unevenness.

Examples of the alkoxy group-containing (meth)acrylate include compounds represented by the following general formula (1):

In general formula (1), _R1 represents a hydrogen atom or a methyl group, _R2 represents an alkylene group having 2 to 4 carbon atoms, _R3 represents an alkyl group or aryl group having 1 to 10 carbon atoms, and n represents an integer of 1 to 10.

Specific examples of the alkoxy group-containing (meth)acrylate include 2-methoxyethyl acrylate (MOEA), ethyl carbitol acrylate (CBA), phenoxyethyl acrylate (PHEA), and methoxytriethylene glycol acrylate (MTG). Among these, 2-methoxyethyl acrylate (MOEA) is preferred because it provides particularly high resistance to yellowing.

The lower limit of the content of the structural units derived from the alkoxy group-containing (meth)acrylate in the (meth)acrylic copolymer is 60% by weight. By using a (meth)acrylic copolymer containing 60% by weight or more of the structural units derived from the alkoxy group-containing (meth)acrylate, the pressure-sensitive adhesive tape of the present invention can exhibit high yellowing resistance. The content of the structural units derived from the alkoxy group-containing (meth)acrylate is preferably 70% by weight or more, and more preferably 75% by weight or more. The upper limit of the content of the structural units derived from the alkoxy group-containing (meth)acrylate is not particularly limited, but is preferably 97% by weight, and more preferably 85% by weight.

The (meth)acrylic copolymer preferably further contains a structural unit derived from a (meth)acrylate having a glass transition temperature of 60° C. or higher and not containing a carboxyl group.
The adhesive layer containing the (meth)acrylic copolymer containing the structural unit derived from the alkoxy group-containing (meth)acrylate is not easily swollen by sebum, and even if it swells, it shows stress relaxation properties that do not impose a load on the liquid crystal, and can exhibit excellent yellowing resistance. However, since the adhesive layer having such excellent stress relaxation properties has high flexibility, there is a risk that it may easily foam due to the gas remaining when the cover panel and the touch sensor panel are bonded together. By using a (meth)acrylate having a glass transition temperature of 60°C or more in combination, the storage modulus of the adhesive layer at room temperature (23°C) and high temperature (140°C) can be relatively high, and foaming can be suppressed.
The reason why it is stated that the copolymer "does not contain a carboxyl group" is that if a (meth)acrylic copolymer containing a large number of structural units derived from a (meth)acrylate containing a carboxyl group is used, the resulting adhesive layer may be easily swollen by sebum, which may reduce resistance to yellowing.

Specific examples of the (meth)acrylate having a glass transition temperature of 60° C. or higher and not containing a carboxyl group include methyl methacrylate (MMA), isobornyl acrylate (IBOA), cyclohexyl methacrylate (CHMA), etc. Among these, methyl methacrylate (MMA) is preferred because it gives a pressure-sensitive adhesive layer with particularly excellent foaming resistance.
The (meth)acrylate having a glass transition temperature of 60° C. or higher and containing no carboxyl group preferably does not contain nitrogen, which can suppress swelling due to sebum and deterioration of yellowing resistance.

The preferred lower limit of the content of the structural units derived from (meth)acrylates having a glass transition temperature of 60°C or higher and not containing a carboxyl group in the (meth)acrylic copolymer is 3% by weight, and the preferred upper limit is 15% by weight. A pressure-sensitive adhesive layer having a glass transition temperature of 60°C or higher and containing structural units derived from (meth)acrylates having no carboxyl group within this range can exhibit excellent yellowing resistance and foaming resistance. The more preferred lower limit of the content of the structural units derived from (meth)acrylates having a glass transition temperature of 60°C or higher and not containing a carboxyl group is 5% by weight, and the more preferred upper limit is 10% by weight.

The (meth)acrylic copolymer preferably further contains a structural unit derived from a monomer having a crosslinkable functional group. When the structural unit derived from a monomer having a crosslinkable functional group is contained, the (meth)acrylic copolymer chains are crosslinked when a crosslinking agent is used in combination. In this case, the gel fraction can be adjusted by adjusting the degree of crosslinking.

Examples of the crosslinkable functional group include a hydroxyl group, a carboxyl group, a glycidyl group, etc. Among these, a hydroxyl group or a carboxyl group is preferred because it allows easy adjustment of the gel fraction of the pressure-sensitive adhesive layer.
In addition, amino groups, amide groups, nitrile groups, etc. are also crosslinkable functional groups, but if monomers having these crosslinkable functional groups are used, the adhesive layer may become more susceptible to swelling by sebum, which may reduce resistance to yellowing, and therefore it is preferable not to use them in the present invention.

Examples of the monomer having a hydroxyl group include (meth)acrylic acid esters having a hydroxyl group, such as 4-hydroxybutyl (meth)acrylate and 2-hydroxyethyl (meth)acrylate.
The monomer having a carboxyl group may, for example, be (meth)acrylic acid.
An example of the monomer having a glycidyl group is glycidyl (meth)acrylate.

The content of the constituent units derived from the monomer having a crosslinkable functional group in the (meth)acrylic copolymer is not particularly limited, but a preferred lower limit is 0.01% by weight and a preferred upper limit is 5% by weight.
In particular, when the crosslinkable functional group is a carboxyl group, the content of the constituent unit derived from the monomer having a carboxyl group is preferably 5% by weight or less. If the content of the constituent unit derived from the monomer having a carboxyl group exceeds 5% by weight, the pressure-sensitive adhesive layer becomes easily swollen by sebum, and the yellowing resistance may decrease.

The (meth)acrylic copolymer may further contain structural units derived from other alkyl (meth)acrylates such as methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, n-butyl (meth)acrylate, n-hexyl (meth)acrylate, etc., within the scope of not impairing the effects of the present invention. In addition, the (meth)acrylic copolymer may contain structural units derived from aromatic (meth)acrylates such as benzyl (meth)acrylate, phenoxyethyl (meth)acrylate, etc.
However, when a (meth)acrylate with a long chain length (specifically, 6 or more carbon atoms) is used, the SP of the resulting (meth)acrylic copolymer tends to be low, and the adhesive layer may be easily swollen by sebum. Therefore, as described later, it is preferable to select the type and amount of other alkyl (meth)acrylate within a range in which the SP value of the (meth)acrylic copolymer is 10.2 or more.

The (meth)acrylic copolymer preferably does not contain a structural unit derived from a nitrogen-containing (meth)acrylate. If the (meth)acrylic copolymer contains a structural unit derived from a nitrogen-containing (meth)acrylate, the pressure-sensitive adhesive layer may be easily swollen by sebum, and resistance to yellowing may be reduced.

The (meth)acrylic copolymer preferably has an SP value calculated by the Fedors method of 10.2 or more. The SP value of 10.2 or more makes it difficult to swell with sebum, and can exhibit excellent yellowing resistance. The SP value is more preferably 10.25 or more, and even more preferably 10.3 or more. The upper limit of the SP value is not particularly limited, but due to the difficulty of synthesis, the upper limit is about 10.6.
The SP value is called the solubility parameter and is an index that can express the ease of dissolution. In this specification, the Fedors method (R.F. Fedors, Polym. Eng. Sci., 14(2), 147-154 (1974)) is used to calculate the SP value. The SP value of the (meth)acrylic copolymer can be calculated based on the SP value of each single repeating unit in the copolymer, using the blending ratio (molar ratio).
The SP value of the (meth)acrylic copolymer can be adjusted by selecting the monomers used in the polymerization of the (meth)acrylic copolymer and the blending ratio of the monomers.

The (meth)acrylic copolymer preferably has a weight-average molecular weight of 250,000 or more. By making the (meth)acrylic copolymer have a weight-average molecular weight of 250,000 or more, the obtained adhesive tape can have a more excellent adhesive strength, and can also have an adhesive tape that is more excellent in resistance to sebum and yellowing resistance. The more preferable lower limit of the weight-average molecular weight of the (meth)acrylic copolymer is 300,000, the even more preferable lower limit is 400,000, and the particularly preferable lower limit is 500,000. The upper limit of the weight-average molecular weight of the (meth)acrylic copolymer is not particularly limited, but the preferable upper limit is 2 million, and the more preferable upper limit is 1.8 million.
The weight average molecular weight can be adjusted by the polymerization conditions (for example, the type or amount of the polymerization initiator, the polymerization temperature, the monomer concentration, etc.).

The method for preparing the (meth)acrylic copolymer is not particularly limited, and examples thereof include a method in which the (meth)acrylic monomer from which the above-mentioned structural units are derived is subjected to a radical reaction in the presence of a polymerization initiator. The polymerization method is not particularly limited, and a conventionally known method can be used. Examples include solution polymerization (boiling point polymerization or constant temperature polymerization), emulsion polymerization, suspension polymerization, bulk polymerization, and the like. Among these, solution polymerization is preferred because of its ease of synthesis.

When solution polymerization is used as the polymerization method, examples of the reaction solvent include ethyl acetate, toluene, methyl ethyl ketone, methyl sulfoxide, ethanol, acetone, diethyl ether, etc. These reaction solvents may be used alone or in combination of two or more kinds.

The polymerization initiator is not particularly limited, and examples thereof include organic peroxides and azo compounds.
Examples of the organic peroxide 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. These polymerization initiators may be used alone or in combination of two or more.

The pressure-sensitive adhesive layer preferably contains a crosslinking agent. When the (meth)acrylic copolymer contains a constituent unit derived from a monomer having a crosslinkable functional group, a crosslinked structure can be formed by the crosslinking agent.
The crosslinking agent is not particularly limited, and examples thereof include isocyanate-based crosslinking agents, aziridine-based crosslinking agents, epoxy-based crosslinking agents, metal chelate-type crosslinking agents, etc. Among these, isocyanate-based crosslinking agents and epoxy-based crosslinking agents are preferred.
When the pressure-sensitive adhesive tape of the present invention is used as an optically transparent pressure-sensitive adhesive tape, it is preferable to use a crosslinking agent that does not contain an aromatic ring from the viewpoint of weather resistance.

The amount of the crosslinking agent to be blended is preferably 0.01 parts by weight at the lower limit and 10 parts by weight at the upper limit, more preferably 0.1 parts by weight at the lower limit and 5 parts by weight at the upper limit, relative to 100 parts by weight of the (meth)acrylic copolymer.

The pressure-sensitive adhesive layer may contain a silane coupling agent. By containing a silane coupling agent, the adhesion to the adherend can be improved, and the resistance of the pressure-sensitive adhesive tape to sebum and alkaline cleaning agents can be further increased.

The above silane coupling agent is not particularly limited, and examples thereof include vinyltrimethoxysilane, vinyltriethoxysilane, γ-methacryloxypropyltrimethoxysilane, γ-methacryloxypropylmethyldimethoxysilane, γ-glycidoxypropyltrimethoxysilane, γ-glycidoxypropylmethyldimethoxysilane, γ-glycidoxypropylmethyldiethoxysilane, γ-glycidoxypropyltriethoxysilane, 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane, γ-aminopropyltrimethoxysilane, γ-aminopropyltriethoxysilane, γ-aminopropyltrimethylmethoxysilane, N-(2-aminoethyl)3-aminopropyltriethoxysilane, N-(2-aminoethyl)3-aminopropylmethyldimethoxysilane, γ-mercaptopropyltrimethoxysilane, γ-mercaptopropyltriethoxysilane, mercaptobutyltrimethoxysilane, γ-mercaptopropylmethyldimethoxysilane, and the like. Among these, gamma-glycidoxypropyltriethoxysilane and gamma-mercaptopropyltrimethoxysilane are preferred.

The content of the silane coupling agent is not particularly limited, but a preferred lower limit is 0.1 parts by weight and a preferred upper limit is 5 parts by weight relative to 100 parts by weight of the (meth)acrylic copolymer. When the content of the silane coupling agent is 0.1 parts by weight or more, resistance to sebum and alkaline cleaning agents can be further improved. When the content is 5 parts by weight or less, adhesive residue upon re-peeling can be suppressed. A more preferred lower limit of the content of the silane coupling agent is 1 part by weight and a more preferred upper limit is 3 parts by weight.

The adhesive layer may contain additives such as plasticizers, emulsifiers, softeners, fillers, pigments, and dyes, tackifiers such as rosin-based resins and terpene-based resins, and other resins, as necessary.

The pressure-sensitive adhesive layer preferably has a gel fraction of 70% or more and 99% or less. By having the gel fraction within this range, the resistance to sebum can be further improved and more excellent yellowing resistance can be exhibited. The more preferred lower limit of the gel fraction is 72%, and the more preferred lower limit is 75%. The more preferred upper limit of the gel fraction is 90%, and the more preferred upper limit is 80%.
In this specification, the "gel fraction" refers to the ratio, expressed as a percentage, of the weight of the pressure-sensitive adhesive layer after immersed in ethyl acetate and dried to the weight of the pressure-sensitive adhesive layer before immersion in ethyl acetate, as shown in the following formula:
Gel fraction (wt%)=100×(W ₂ −W ₀ )/(W ₁ −W ₀ )
(W ₀ : weight of the substrate, W ₁ : weight of the adhesive tape test piece before immersion in ethyl acetate, W ₂ : weight of the adhesive tape test piece after immersion in ethyl acetate and drying)

The thickness of the adhesive layer is not particularly limited, but the preferred lower limit is 5 μm and the preferred upper limit is 500 μm. When the thickness of the adhesive layer is 5 μm or more, the resulting adhesive tape can have better adhesion. When the thickness of the adhesive layer is 500 μm or less, the resulting adhesive tape can have better processability.

The pressure-sensitive adhesive layer is not particularly limited, and examples thereof include pressure-sensitive adhesive layers formed from a solvent-based pressure-sensitive adhesive composition, a hot melt-based pressure-sensitive adhesive composition, a water-dispersed pressure-sensitive adhesive composition, and an active energy ray-curable pressure-sensitive adhesive composition. That is, examples of the pressure-sensitive adhesive layer include a solvent-based pressure-sensitive adhesive layer, a hot melt-based pressure-sensitive adhesive layer, a water-dispersed pressure-sensitive adhesive layer, and an active energy ray-curable pressure-sensitive adhesive layer. Here, the solvent-based pressure-sensitive adhesive composition refers to a pressure-sensitive adhesive composition in a form containing a pressure-sensitive adhesive (pressure-sensitive adhesive layer-forming component) in an organic solvent. In addition, the water-dispersed pressure-sensitive adhesive composition refers to a pressure-sensitive adhesive composition in a form containing a pressure-sensitive adhesive in a solvent (aqueous solvent) mainly composed of water.
From the viewpoint of achieving favorable adhesive properties, the pressure-sensitive adhesive layer is preferably a non-active energy ray-curable pressure-sensitive adhesive layer, and more preferably a solvent-based pressure-sensitive adhesive layer.

The pressure-sensitive adhesive tape of the present invention may be a supported type having a substrate, or a non-supported type having no substrate. In the case of a supported type, the pressure-sensitive adhesive layer may be formed on one side of the substrate, or on both sides.

The substrate is not particularly limited, and examples thereof include polyolefin resin films such as polyethylene films and polypropylene films, polyester resin films such as PET films, ethylene-vinyl acetate copolymer films, polyvinyl chloride resin films, and polyurethane resin films. Other examples include polyolefin foam sheets such as polyethylene foam sheets and polypropylene foam sheets, and polyurethane foam sheets. Among these substrates, PET films are preferred. Furthermore, polyolefin foam sheets are preferred from the viewpoint of impact resistance.
As the substrate, a substrate printed in black to prevent light transmission, a substrate printed in white to improve light reflectivity, a substrate on which metal has been vapor-deposited, or the like can also be used.

The method for producing the pressure-sensitive adhesive tape of the present invention is not particularly limited. For example, when the pressure-sensitive adhesive tape of the present invention is a double-sided pressure-sensitive adhesive tape having a substrate, the following method can be mentioned.
First, a solution of acrylic adhesive a is prepared by adding a solvent to the (meth)acrylic copolymer and, if necessary, a crosslinking agent. The obtained solution of acrylic adhesive a is applied to the surface of the substrate, and the solvent in the solution is completely dried and removed to form an adhesive layer a. Next, a release film is superimposed on the formed adhesive layer a with its release-treated surface facing the adhesive layer a. Next, a release film other than the release film is prepared, and a solution of acrylic adhesive b is applied to the release-treated surface of this release film, and the solvent in the solution is completely dried and removed to prepare a laminated film in which adhesive layer b is formed on the surface of the release film. The obtained laminated film is superimposed on the back surface of the substrate on which adhesive layer a is formed, with adhesive layer b facing the back surface of the substrate to prepare a laminate. Then, the laminate is pressed by a rubber roller or the like to obtain an adhesive tape having adhesive layers on both sides of the substrate and the surface of the adhesive layer covered with a release film.

Alternatively, two sets of laminated films may be prepared in a similar manner, and these laminated films may be superimposed on both sides of a substrate with the adhesive layer of the laminated film facing the substrate to produce a laminate. By pressing this laminate with a rubber roller or the like, an adhesive tape having adhesive layers on both sides of the substrate and the surface of the adhesive layer covered with a release film may be obtained.

The adhesive tape of the present invention is not particularly limited in its applications, but because it has excellent resistance to yellowing, it can be particularly suitably used to fasten components of electronic devices that are frequently touched by human hands. Specifically, it can be suitably used to fasten together components that make up display devices, such as cover panels, touch panels, and touch sensors, used in display devices such as liquid crystal display elements and organic EL elements.

The shape of the adhesive tape of the present invention is not particularly limited and may be a square, rectangle, etc., but a frame-like shape is preferred when used to fasten components that make up a display device. The adhesive tape of the present invention can maintain high adhesive strength even in areas that are frequently touched by human hands, so frame-like adhesive tape can be preferably used even if it is narrow in width.

The present invention provides an adhesive tape with excellent yellowing resistance.

The following examples further illustrate aspects of the present invention, but the present invention is not limited to these examples.

Example 1
(1) Preparation of (meth)acrylic copolymer Ethyl acetate was added as a polymerization solvent into a reaction vessel, and after bubbling with nitrogen, the reaction vessel was heated while flowing in nitrogen to start reflux. Subsequently, a polymerization initiator solution in which 0.1 parts by weight of azobisisobutyronitrile was diluted 10 times with ethyl acetate was added into the reaction vessel as a polymerization initiator. Furthermore, 62.9 parts by weight of 2-methoxyethyl acrylate (MOEA), 14.5 parts by weight of methyl methacrylate (MMA), 0.3 parts by weight of acrylic acid (Aac), 2.9 parts by weight of hydroxyethyl acrylate (HEA), and 19.4 parts by weight of ethyl acrylate (EA) were added dropwise over 2 hours. After the dropwise addition was completed, a polymerization initiator solution in which 0.1 parts by weight of azobisisobutyronitrile was diluted 10 times with ethyl acetate was again added into the reaction vessel as a polymerization initiator, and a polymerization reaction was carried out for 4 hours to obtain a (meth)acrylic copolymer-containing solution.
The SP value of the (meth)acrylic copolymer calculated by the Fedors method is 10.27.
The weight average molecular weight of the obtained (meth)acrylic copolymer was determined by gel permeation chromatography (Waters, 2690 Separations Model) using a GPC LF-804 (Showa Denko KK) column, and was found to be 480,000.

(2) Production of Adhesive Tape To the obtained (meth)acrylic copolymer-containing solution, an isocyanate-based crosslinking agent, Coronate L-45 (manufactured by Tosoh Corporation), was added in an amount of 0.72 parts by weight in terms of solid content relative to the (meth)acrylic copolymer. The obtained solution was applied to a release-treated PET film having a thickness of 75 μm so that the thickness of the adhesive layer after drying would be 15 μm, and then dried at 110° C. for 5 minutes. This adhesive layer was transferred to a corona-treated PET film having a thickness of 50 μm as a substrate, and aged at 40° C. for 48 hours to obtain an adhesive tape.

(3) Measurement of gel fraction of adhesive layer The obtained adhesive tape was cut into a flat rectangular shape of 20 mm x 40 mm to prepare a test piece, and the weight was measured. The test piece was immersed in ethyl acetate at 23°C for 24 hours, and then removed from the ethyl acetate and dried at 110°C for 1 hour. The weight of the test piece after drying was measured, and the gel fraction was calculated using the following formula.
Gel fraction (wt%)=100×(W ₂ −W ₀ )/(W ₁ −W ₀ )
(W ₀ : weight of the substrate, W ₁ : weight of the adhesive tape test piece before immersion in ethyl acetate, W ₂ : weight of the adhesive tape test piece after immersion in ethyl acetate and drying)

(Examples 2 to 11, Comparative Examples 1 to 3)
An adhesive tape was obtained in the same manner as in Example 1, except that the types and amounts of monomers used and the amount of crosslinking agent were as shown in Table 1.
In the table, CBA stands for ethyl carbitol acrylate, PHEA stands for phenoxy acrylate, MA stands for methyl acrylate, nOA stands for n-octyl acrylate, DMAA stands for dimethylacrylamide, IBOA stands for isobornyl acrylate, CHMA stands for cyclohexyl methacrylate, and MTG stands for methoxytriethylene glycol acrylate.

(evaluation)
The pressure-sensitive adhesive tapes obtained in the Examples and Comparative Examples were evaluated as follows. The results are shown in Table 1.

(1) Yellowing resistance test A PET film measuring 50 mm x 100 mm and 100 μm thick was prepared, with a black printed layer of 2 mm width and 15 μm height provided on all four sides. The obtained adhesive tape was cut into a rectangular shape of 50 mm x 100 mm, one side of which was attached to the prepared PET film, and the other side was attached to a liquid crystal panel of 89 mm x 156 mm (manufactured by Innolux Co., Ltd.) to prepare a test piece. 10 mL of sebum liquid was dropped onto the end of the test piece, which was left at room temperature, and the presence or absence of yellowing at the end of the screen was visually confirmed. Those whose screen end did not turn yellow for 5 days or more were evaluated as "○", and those whose screen end turned yellow in less than 5 days were evaluated as "×".

(2) Anti-foaming test The obtained adhesive tape was cut into a rectangular shape of 50 mm x 125 mm, one side of which was attached to a PET film (50 mm x 125 mm, thickness 50 μm), and the other side was attached to an acrylic plate (50 mm x 125 mm, thickness 2 mm) to prepare a test piece. The test piece was left to stand for 30 minutes in an environment of 40 ° C. / 0.3 MPa, and the presence or absence of foaming in the test piece was visually confirmed. The test piece was evaluated as "◎" when no foaming was observed at the end of the test piece, "○" when only foaming with a diameter of less than 0.3 mm was observed, and "×" when foaming with a diameter of 0.3 mm or more was observed.

According to the present invention, a pressure-sensitive adhesive tape having excellent yellowing resistance can be provided.

Claims (6)

A pressure-sensitive adhesive tape having a pressure-sensitive adhesive layer containing a (meth)acrylic copolymer,
The (meth)acrylic copolymer contains 60% by weight or more of structural units derived from an alkoxy group-containing (meth)acrylate,
the (meth)acrylic copolymer further comprises a constituent unit derived from a monomer having a carboxyl group as a crosslinkable functional group, and a constituent unit derived from a (meth)acrylate having a glass transition temperature of 60° C. or higher and not containing a carboxyl group , the content of the constituent unit derived from the monomer having a carboxyl group being 5% by weight or less,
The (meth)acrylic copolymer does not contain a structural unit derived from a nitrogen-containing (meth)acrylate,
The pressure-sensitive adhesive layer has a gel fraction of 70% or more and 99% or less.
An adhesive tape characterized by: 2. The adhesive tape according to claim 1, wherein the alkoxy group-containing (meth)acrylate is a compound represented by the following general formula (1):

In general formula (1), _R1 represents a hydrogen atom or a methyl group, _R2 represents an alkylene group having 2 to 4 carbon atoms, _R3 represents an alkyl group or aryl group having 1 to 10 carbon atoms, and n represents an integer of 1 to 10. The adhesive tape according to claim 1 or 2, characterized in that the alkoxy group-containing (meth)acrylate is 2-methoxyethyl acrylate. The adhesive tape according to claim 1, 2 or 3, characterized in that the (meth)acrylate having a glass transition temperature of 60°C or higher and not containing a carboxyl group is methyl methacrylate. 5. The adhesive tape according to claim 1, 2 , 3 or 4 , wherein the (meth)acrylic copolymer has an SP value of 10.2 or more as calculated by the Fedors method. 6. The adhesive tape according to claim 1, 2, 3 , 4 or 5 , which is used for fixing parts of electronic devices.