JP7480804B2 - Polyester-based pressure-sensitive adhesive, pressure-sensitive adhesive sheet for optical members, and polyester-based pressure-sensitive adhesive composition - Google Patents

Description

The present invention relates to a polyester-based adhesive, and more specifically to a polyester-based adhesive that has a high refractive index and excellent adhesion to various adherends, particularly polycarbonate materials, and further to an adhesive sheet for optical components, and a polyester-based adhesive composition for obtaining these.

Traditionally, polyester resins have been used in a wide range of applications, including films, PET bottles, fibers, toner, electrical components, and adhesives and pressure sensitive adhesives, due to their excellent heat resistance, chemical resistance, durability, and mechanical strength.

In recent years, display devices such as liquid crystal displays (LCDs) and input devices used in combination with such display devices, such as touch panels, have become widely used, and in the manufacture of these devices, transparent adhesive sheets are used to bond optical components such as optical films and substrates.

Furthermore, a light diffusion sheet is used to improve the visibility of a liquid crystal display device. The light diffusion sheet is made of a transparent resin material and a filler, and the difference in refractive index between the resin material and the filler is utilized to exhibit diffusibility. The optical members of the liquid crystal display device are assembled into the liquid crystal display device using an adhesive, and it is desirable for the refractive index of the adhesive layer to be higher in order to achieve light diffusion.
In addition, since the refractive index of an optical film is generally higher than that of an adhesive, reflection occurs at the interface, resulting in a problem of reduced light extraction efficiency of the display device. By reducing the difference in refractive index at such an interface, the interface reflection is reduced, and therefore, from the standpoint of light extraction efficiency, it is desirable for the adhesive to have a higher refractive index.

Acrylic resin adhesives are generally used to bond the optical components described above, but the refractive index of the adhesive layer is around 1.47 and does not have a high refractive index exceeding 1.50.
Therefore, in Patent Document 1, a polyester-based adhesive is proposed in place of the acrylic resin-based adhesive.

JP 2009-25575 A

However, in the technology disclosed in Patent Document 1, the adhesive composition contains a polyester resin with a relatively high refractive index, but a polyester resin with an even higher refractive index is required in terms of the diffusion efficiency of the light diffusion film and the light extraction efficiency.

In light of this background, the present invention aims to provide a polyester-based adhesive that has excellent adhesion to adherends, particularly polycarbonate materials, and a high refractive index, as well as an adhesive sheet for optical components, and a polyester-based adhesive composition for obtaining these.

However, in light of these circumstances, the inventors conducted extensive research and discovered a polyester-based adhesive that combines high adhesion and a high refractive index, which have been difficult to achieve in combination with other adhesive compositions containing polyester resins, and thus completed the present invention.

That is, a first gist of the present invention is a polyester-based pressure-sensitive adhesive comprising a pressure-sensitive adhesive composition [I] containing a polyester-based resin (A) and a crosslinking agent (C) , wherein the polyester-based resin (A) is a polyester-based resin (A) obtained by copolymerizing a copolymerization component consisting of a polyvalent carboxylic acid (A1) and a polyhydric alcohol (A2), the glass transition temperature of the polyester-based resin (A) is -40 to 40°C, the polyvalent carboxylic acid (A1) contains an aliphatic polyvalent carboxylic acid, and at least one of the polyvalent carboxylic acid (A1) and the polyhydric alcohol (A2) contains an aromatic compound, the ratio of the polyester-based resin (A) to the plasticizer (C) is such that the amount of reactive groups of the crosslinking agent (C) is 0.2 to 10 equivalents per equivalent of the hydroxyl group and/or carboxy group contained in the polyester-based resin (A), and the polyester-based pressure-sensitive adhesive has an adhesive strength (α) of 10 to 200 N/ 25mm and a refractive index of 1.530 to 1.650 as described below.
Adhesive strength (α): When a polyester-based adhesive is formed on a substrate to form an adhesive sheet, the sheet is attached to a polycarbonate plate adherend and autoclaved at 50°C and 0.5 MPa for 20 minutes. After 24 hours, the sheet exhibits a 180° peel strength (N/25 mm) at a peel speed of 300 mm/min against the adherend.

The second aspect of the present invention is an adhesive sheet for optical components having an adhesive layer made of the polyester-based adhesive.

The third gist of the present invention is a polyester-based pressure-sensitive adhesive composition that contains a polyester-based resin (A), a hydrolysis inhibitor (B) and a crosslinking agent (C), and the polyester-based resin (A) contains a structural portion derived from an aliphatic polyvalent carboxylic acid and a structural portion derived from an aromatic polyhydric alcohol.

In addition, a fourth aspect of the present invention is a polyester-based pressure-sensitive adhesive comprising a pressure-sensitive adhesive composition containing a polyester-based resin (A) and an inorganic oxide (E), the polyester-based pressure-sensitive adhesive having an adhesive strength (β) of 5 N/25 mm or more and a refractive index of 1.530 or more, as described below.
Adhesive strength (β): When a polyester-based adhesive is formed on a substrate to form an adhesive sheet, the sheet is attached to a glass plate adherend and left for 30 minutes at 23° C. and 50% RH. The 180-degree peel strength (N/25 mm) from the adherend is then measured at a peel speed of 300 mm/min.

In the present invention, when applying a polyester resin with a high glass transition temperature to an adhesive, it is necessary to eliminate as many rigid structures as possible, such as aromatic groups, and to lower the glass transition temperature and soften the resin by introducing an alkyl group-derived component. In addition, fine adjustment of the glass transition temperature is generally performed by adjusting aliphatic dicarboxylic acids and aromatic dicarboxylic acids, and the introduction of aromatic diols is discouraged. On the other hand, in order to increase the refractive index, it is effective to introduce a structure with high electron density, i.e., an aromatic ring, etc., but as mentioned above, the introduction of an aromatic ring is considered to be unsuitable for application to an adhesive. In the present invention, by selecting the acid component and alcohol component that constitute the polyester resin, a high refractive index and adhesive performance, which were previously contradictory properties, are compatible.

The polyester-based adhesive of the present invention has excellent adhesion to adherends, particularly polycarbonate materials, and has a high refractive index, and is further useful as an adhesive sheet for optical components.

The configuration of the present invention will be described in detail below, but these are merely examples of preferred embodiments.
In the present invention, the term "carboxylic acids" includes not only carboxylic acids but also carboxylic acid derivatives such as carboxylic acid salts, carboxylic acid anhydrides, carboxylic acid halides, and carboxylic acid esters.

The polyester-based pressure-sensitive adhesive of the present invention comprises a pressure-sensitive adhesive composition [I] containing a polyester-based resin (A), that is, is obtained by curing such a pressure-sensitive adhesive composition [I].
The polyester-based pressure-sensitive adhesive has an adhesive strength (α) of 10 N/25 mm or more, as measured below, and a refractive index of 1.530 or more.
Adhesive strength (α): When a polyester-based adhesive is formed on a substrate to form an adhesive sheet, the sheet is attached to a polycarbonate plate adherend and autoclaved at 50°C and 0.5 MPa for 20 minutes. After 24 hours, the sheet exhibits a 180° peel strength (N/25 mm) at a peel speed of 300 mm/min against the adherend.

The measurement of the adhesive strength (α) can be carried out, for example, as follows.

That is, the adhesive composition [I] is applied to a polyethylene terephthalate (PET) film having a thickness of 50 μm using an applicator, and dried at 100° C. for 3 minutes to form an adhesive composition layer (thickness 25 μm). The surface of the obtained adhesive composition layer is then covered with a release-treated PET film (release film) and aged at 40° C. for 10 days to obtain an adhesive sheet with a release film. The release film is peeled off, and the sheet is attached to a polycarbonate plate substrate by rolling it back and forth twice with a 2 kg roller, and autoclaved at 50° C. and 0.5 MPa for 20 minutes. After 24 hours, the 180-degree peel strength (N/25 mm) is measured at a peel speed of 300 mm/min to determine the adhesive strength (α).

In the present invention, the adhesive strength (α) of the polyester-based adhesive is 10 N/25 mm or more, preferably 15 N/25 mm or more, and particularly preferably 20 N/25 mm or more. If the adhesive strength (α) is too small below the above range, the adhesion will be poor, resulting in reduced adhesion reliability. The upper limit of the adhesive strength (α) is usually 200 N/25 mm, preferably 100 N/25 mm, and particularly preferably 80 N/25 mm.

In the present invention, the refractive index of the polyester-based adhesive is 1.530 or more, preferably 1.530 to 1.650, more preferably 1.540 to 1.640, particularly preferably 1.550 to 1.630, and particularly preferably 1.560 to 1.620. If the refractive index is too small below the above range, the effect of the present invention will not be achieved, and there is a risk of problems such as interface reflection and reduced brightness occurring when laminated with an optical film. If the refractive index is too large, it will be higher than the refractive index of the optical film, and there is a tendency for problems such as interface reflection and reduced brightness occurring when laminated with an optical film to occur.

The refractive index in this invention is the refractive index for the D line (589 nm) at 23°C, and is measured using an Abbe refractometer DR-M4 manufactured by Atago Co., Ltd.

In order to satisfy the above-mentioned adhesive strength and refractive index in the polyester-based adhesive of the present invention, it is preferable to form the adhesive using an adhesive composition [I] containing a polyester-based resin (A). Furthermore, from the viewpoints of adhesive strength and durability, the adhesive composition [I] preferably contains the following polyester-based resin (A), a hydrolysis inhibitor (B) and a crosslinking agent (C), and from the viewpoint of improving initial adhesion, it preferably further contains a plasticizer (D), and from the viewpoint of improving the refractive index, it preferably further contains an inorganic oxide (E).
Each component will be described in turn below.

<Polyester Resin (A)>
The polyester resin (A) used in the present invention is obtained by copolymerizing copolymerization components containing polyvalent carboxylic acids (A1) and polyhydric alcohols (A2) as constituent raw materials. Among them, it is preferable that the polyester resin (A) contains a structural portion derived from an aliphatic polyvalent carboxylic acid and a structural portion derived from an aromatic polyhydric alcohol in terms of the balance between adhesive strength and refractive index.

[Polyvalent Carboxylic Acids (A1)]
Examples of the polyvalent carboxylic acid (A1) used in the present invention include
Aliphatic dicarboxylic acids such as malonic acid, dimethylmalonic acid, succinic acid, glutaric acid, adipic acid, trimethyladipic acid, pimelic acid, 2,2-dimethylglutaric acid, azelaic acid, sebacic acid, fumaric acid, maleic acid, itaconic acid, thiodipropionic acid, diglycolic acid, or these carboxylic acids;
Aromatic dicarboxylic acids such as terephthalic acid, isophthalic acid, benzylmalonic acid, diphenic acid, 4,4'-oxydibenzoic acid, naphthalenedicarboxylic acid, or these carboxylic acids;
Alicyclic dicarboxylic acids such as 1,3-cyclopentanedicarboxylic acid, 1,2-cyclohexanedicarboxylic acid, 1,3-cyclopentanedicarboxylic acid, 1,4-cyclohexanedicarboxylic acid, 2,5-norbornanedicarboxylic acid, adamantanedicarboxylic acid, and these carboxylic acids;
and the like divalent carboxylic acids.
These may be used alone or in combination of two or more.

Among these, it is preferable to include aliphatic dicarboxylic acids, particularly aliphatic dicarboxylic acids having 4 to 12 carbon atoms, in order to impart a tacky feel, and it is even more preferable to include sebacic acids.

The content of such aliphatic dicarboxylic acids is preferably 20 mol % or more, more preferably 50 to 95 mol %, and even more preferably 70 to 90 mol %, based on the total content of polyvalent carboxylic acids (A1). If the content is too low, the glass transition temperature tends to be high and sufficient adhesive strength cannot be obtained.

Of the above, aromatic dicarboxylic acids can also be included to impart cohesive strength, such as isophthalic acids.

The content of such aromatic dicarboxylic acids is preferably 50 mol % or less, more preferably 5 to 40 mol %, and even more preferably 10 to 30 mol %, based on the total content of the polyvalent carboxylic acids (A1). If the content is too high, the glass transition temperature becomes high, and sufficient adhesive performance tends to be unavailable.

In the present invention, from the viewpoint of a balance of adhesive properties, aromatic dicarboxylic acids and aliphatic dicarboxylic acids can be used in combination as the polyvalent carboxylic acids (A1), and the content ratio (molar ratio) of aromatic dicarboxylic acids/aliphatic dicarboxylic acids is preferably 0/100 to 90/10, particularly preferably 1/99 to 49/51, and even more preferably 10/90 to 30/70.

[Sulfonate group-containing dicarboxylic acids (A1-1)]
In the present invention, from the viewpoint of improving compatibility when adding the inorganic oxide (E) described below, it is preferable to use a sulfonate group-containing dicarboxylic acid (A1-1) as the polyvalent carboxylic acid (A1). Such sulfonate group-containing dicarboxylic acid (A1-1) is not particularly limited as long as it is a monomer component having both two carboxyl groups and a sulfonate group in the molecule, and for example, phthalic acid, isophthalic acid, terephthalic acid, and mono- or diesters thereof containing an alkali metal sulfonate such as sodium sulfonate or potassium sulfonate are preferably used. These sulfonate group-containing dicarboxylic acids (A1-1) can be used alone or in combination of two or more kinds.

Specific examples of the sulfonate group-containing dicarboxylic acids (A1-1) include sodium dimethyl 4-sulfoisophthalate, sodium 5-sulfoisophthalate, sodium dimethyl 5-sulfoisophthalate, potassium dimethyl 4-sulfoisophthalate, potassium dimethyl 5-sulfoisophthalate, sodium 2-sulfoterephthalate, potassium 2-sulfoterephthalate, sodium dimethyl 2-sulfoterephthalate, potassium dimethyl 2-sulfoterephthalate, sodium diethylene glycol 2-sulfoterephthalate, etc. Among these, sodium dimethyl 5-sulfoisophthalate is preferably used.

The proportion of the sulfonate group-containing dicarboxylic acids (A1-1) in the polyvalent carboxylic acids (A1) is preferably 0.001 to 20 mol % relative to the total polyvalent carboxylic acids (A1), particularly preferably 0.01 to 10 mol %, further preferably 0.03 to 5 mol %, and particularly preferably 0.05 to 2 mol %. If the amount of the sulfonate group-containing dicarboxylic acids (A1-1) is too small, the compatibility with the inorganic oxide (E) described below when it is added tends to decrease, and the haze tends to increase (transparency tends to decrease), while if the amount is too large, the durability tends to decrease.

In addition, in order to increase the number of branching points in the polyester resin (A), polyvalent carboxylic acids having a valence of three or more can be used. Examples of such polyvalent carboxylic acids having a valence of three or more include trimellitic acid, pyromellitic acid, adamantanetricarboxylic acid, trimesic acid, and carboxylic acids thereof. Among these, it is preferable to use trimellitic acid because it is relatively difficult for gelation to occur.

The content of such trivalent or higher polyvalent carboxylic acids is preferably 10 mol % or less, and particularly preferably 0.1 to 5 mol %, based on the total polyvalent carboxylic acids (A1), in order to increase the cohesive strength of the adhesive. If the content is too high, gelation tends to occur easily during the production of the polyester resin (A).

[Polyhydric alcohol (A2)]
Examples of the polyhydric alcohol (A2) used in the present invention include
Aliphatic diols such as ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, dipropylene glycol, 1,3-propanediol, 2,4-dimethyl-2-ethylhexane-1,3-diol, 2-methyl-1,3-propanediol, 2,2-dimethyl-1,3-propanediol (neopentyl glycol), 2-ethyl-2-butyl-1,3-propanediol, 2-ethyl-2-isobutyl-1,3-propanediol, 1,3-butanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 3-methyl-1,5-pentanediol, and 2,2,4-trimethyl-1,6-hexanediol;
Alicyclic diols such as 1,2-cyclohexanedimethanol, 1,3-cyclohexanedimethanol, 1,4-cyclohexanedimethanol, spiroglycol, tricyclodecane dimethanol, adamantanediol, and 2,2,4,4-tetramethyl-1,3-cyclobutanediol;
Aromatic diols such as 4,4'-thiodiphenol, 4,4'-methylenediphenol, bisphenol S, bisphenol A, bisphenolfluorene, 4,4'-dihydroxybiphenyl, o-, m- and p-dihydroxybenzene, 2,5-naphthalenediol, and p-xylenediol;
and ethylene oxide, propylene oxide adducts thereof;
aromatic diols such as hexane, hexanediol ...

Furthermore, in the present invention, it is preferable that the aromatic diol contains a fluorene-based diol represented by the following general formula (1):

In formula (1), R ₁ is an alkylene group having 1 to 5 carbon atoms, such as a methylene group, an ethylene group, a propylene group, a butylene group, a pentylene group, etc. R ₂ , R ₃ , R ₄ and R ₅ are a hydrogen atom, an alkyl group having 1 to 5 carbon atoms, an aryl group or an aralkyl group, which may be the same or different from each other.

Examples of the alkyl group having 1 to 5 carbon atoms include a methyl group, an ethyl group, a propyl group, an isopropyl group, a butyl group, an isobutyl group, a pentyl group, a neopentyl group, and an isopentyl group.
The aryl group includes aryl groups having 6 to 10 carbon atoms, such as a phenyl group, a tolyl group, and an o-xylyl group.
The aralkyl group includes aralkyl groups having 7 to 20 carbon atoms, such as a benzyl group and a phenethyl group.

In the fluorene-based diol represented by the above general formula (1), R ₁ is preferably a methylene group or an ethylene group. R ₂ , R ₃ , R ₄ and R ₅ are preferably a hydrogen atom, a methyl group or an ethyl group, with a hydrogen atom being particularly preferred. As the fluorene-based compound, bisphenoxyethanolfluorene and bis(4-(2-hydroxyethoxy)phenyl)fluorene are preferably used from the viewpoint of availability and the like. One or more types of fluorene-based compounds can be used.
These dihydric alcohols may be used alone or in combination of two or more kinds.

In the present invention, among the polyhydric alcohols (A2), it is preferable to include an aromatic diol, since it can achieve a high refractive index, and it is further preferable to include multiple aromatic rings in one molecule. In particular, it is preferable to include the fluorene-based diol, since it can increase the refractive index while maintaining adhesion.

The content of aromatic diol in polyhydric alcohol (A2) is preferably 10 to 100 mol % relative to the entire polyhydric alcohol (A2), particularly preferably 15 to 80 mol %, further preferably 20 to 70 mol %, and especially preferably 25 to 60 mol %. If the content is too low, it tends to be difficult to obtain a sufficient refractive index, and if it is too high, the adhesive performance tends to decrease.

The content of aromatic diol is preferably 10% by weight or more, more preferably 15 to 85% by weight, particularly preferably 20 to 65% by weight, and even more preferably 25 to 60% by weight, based on the total polyester resin. If the content is too low, it tends to be difficult to obtain a sufficient refractive index, and if it is too high, the adhesive performance tends to decrease.

Furthermore, the content of the fluorene-based diol is preferably 10 to 90 mol %, more preferably 15 to 80 mol %, even more preferably 20 to 70 mol %, and even more preferably 25 to 60 mol %, based on the total polyhydric alcohol (A2). If the content is too low, it tends to be difficult to obtain a sufficient refractive index, and if it is too high, the adhesive performance tends to decrease.

The content of fluorene-based diols relative to the total aromatic diols is preferably 50 mol % or more, particularly 60 mol % or more, further 70 mol % or more, and especially 90 mol % or more, in order to obtain a sufficient refractive index. Of these, it is particularly preferable that all of the diols are fluorene-based diols.

Furthermore, in the present invention, among the polyhydric alcohols (A2), it is preferable that they contain an aliphatic diol or an alicyclic diol because of their excellent reactivity. Particularly preferable examples of the aliphatic diols are ethylene glycol, 1,3-propanediol, 2-methyl-1,3-propanediol, neopentyl glycol, 1,4-butanediol, 1,5-pentanediol, and 1,6-hexanediol, and examples of the alicyclic diols are 1,2-cyclohexanedimethanol, 1,3-cyclohexanedimethanol, and 1,4-cyclohexanedimethanol. It is particularly preferable that they contain an aliphatic diol because of the balance of flexibility and the development of adhesive performance.

The content of the aliphatic diol is preferably 1 to 90 mol % relative to the total polyhydric alcohol (A2), particularly preferably 10 to 85 mol %, further preferably 20 to 80 mol %, and especially preferably 30 to 75 mol %. If the content is too low, the adhesive performance tends to decrease, and if it is too high, the refractive index tends to decrease.

In addition, trihydric or higher polyhydric alcohols can be used to increase the number of branching points in the polyester resin (A). Examples of trihydric or higher polyhydric alcohols include pentaerythritol, dipentaerythritol, tripentaerythritol, glycerin, trimethylolpropane, trimethylolethane, 1,3,6-hexanetriol, and adamantanetriol.

The content of the trihydric or higher polyhydric alcohol is preferably 10 mol % or less, and more preferably 0.1 to 5 mol %, based on the total polyhydric alcohol (A2). If the content is too high, it tends to be difficult to produce the polyester resin (A).

The blend ratio of polyhydric carboxylic acids (A1) and polyhydric alcohol (A2) is preferably 1 to 2.5 equivalents of polyhydric alcohol (A2) per equivalent of polyhydric carboxylic acids (A1), and particularly preferably 1.1 to 2.0 equivalents. If the content ratio of polyhydric alcohol (A2) is too low, the acid value tends to be high, making it difficult to increase the molecular weight, and if it is too high, the yield tends to decrease.

The polyester resin (A) used in the present invention is produced by subjecting the polyvalent carboxylic acids (A1) and polyhydric alcohols (A2) selected from the above to a polycondensation reaction in the presence of a catalyst by a known method.

In a polycondensation reaction, an esterification reaction is carried out first, followed by a condensation reaction.

In such an esterification reaction, a catalyst is used, specifically, for example, titanium-based catalysts such as tetraisopropyl titanate and tetrabutyl titanate, antimony-based catalysts such as antimony trioxide, germanium-based catalysts such as germanium dioxide, and catalysts such as zinc acetate, manganese acetate, and dibutyltin oxide, and one or more of these are used. Among these, antimony trioxide, tetrabutyl titanate, germanium dioxide, and zinc acetate are preferred from the perspective of the balance between high catalytic activity and color.

The amount of the catalyst is preferably 1 to 10,000 ppm, more preferably 10 to 5,000 ppm, and even more preferably 20 to 3,000 ppm, based on the total copolymerization components. If the amount is too small, the polymerization reaction tends not to proceed sufficiently, and if the amount is too large, there is no advantage to shortening the reaction time, and side reactions tend to occur easily.

The reaction temperature during the esterification reaction is preferably 160 to 280°C, particularly preferably 180 to 250°C, and even more preferably 200 to 240°C. If the reaction temperature is too low, the reaction tends not to proceed sufficiently, and if it is too high, side reactions such as decomposition tend to occur easily. In addition, the pressure during the reaction is usually normal pressure.

After the esterification reaction, a condensation reaction is carried out.
The reaction conditions for the polycondensation reaction are as follows: the same amount of the catalyst as that used in the above esterification reaction is further mixed, and the reaction temperature is preferably 220 to 280° C., particularly preferably 230° C.
It is preferable to set the temperature at 270° C. or lower, gradually reduce the pressure in the reaction system, and ultimately carry out the reaction at 5 hPa or less.
If the reaction temperature is too low, the reaction will not proceed sufficiently, whereas if the reaction temperature is too high, side reactions such as decomposition will tend to occur.

Thus, the polyester resin (A) used in the present invention is obtained.
The glass transition temperature of the polyester resin (A) is preferably 40° C. or lower, particularly 30° C. or lower, and more preferably 20° C. or lower, from the viewpoint of the balance between adhesive properties and refractive index. If the glass transition temperature is too high, flexibility is lost, initial adhesiveness is reduced, and sufficient adhesive strength is not easily exerted with finger pressure, which tends to reduce workability. The lower limit of the glass transition temperature is usually −40° C., preferably −30° C., and more preferably −20° C.

The glass transition temperature (Tg) of the polyester resin (A) is a value measured using a differential scanning calorimeter DSC Q20 manufactured by TA Instruments.
The measurement temperature range was from -90°C to 100°C, and the temperature rise rate was 10°C/min.

In addition, the weight average molecular weight of the polyester resin (A) is preferably 8,000 to 200,000, particularly 10,000 to 180,000, and even more preferably 20,000 to 150,000, from the viewpoint of the cohesive strength of the adhesive. If the weight average molecular weight is too small, the adhesive will not have sufficient cohesive strength and will tend to have reduced heat resistance and mechanical strength, whereas if the weight average molecular weight is too large, the adhesive will lose flexibility, the initial adhesiveness will decrease, and it will tend not to exhibit sufficient adhesive strength even with finger pressure.

The weight average molecular weight above is the weight average molecular weight converted to the standard polystyrene molecular weight, and is measured using a high performance liquid chromatography (Tosoh Corporation, "HLC-8320GPC") with two columns in series: TSKgel SuperMultipore HZ-M (exclusion limit molecular weight: 2 x 106, theoretical plate number: 16,000 plates/column, packing material: styrene-divinylbenzene copolymer, packing particle size: 4 μm).

The acid value of the polyester resin (A) is preferably 10 mgKOH/g or less, more preferably 3 mgKOH/g or less, even more preferably 1 mgKOH/g or less, and even more preferably 0.5 mgKOH/g or less. If the acid value is too high, there is a concern that a metal layer or the like may be corroded on one side of the adhesive layer made of the polyester adhesive. For example, when a metal oxide thin film layer is formed, corrosion may occur, and the conductivity of the metal oxide film may tend to decrease.

The acid value of the polyester resin (A) is determined by neutralization titration based on JIS K 0070.

In the present invention, the refractive index of the polyester resin (A) alone is preferably 1.530 or more.

<Hydrolysis Inhibitor (B)>
The polyester-based pressure-sensitive adhesive composition [I] of the present invention preferably contains a hydrolysis inhibitor (B) together with the polyester-based resin (A). The hydrolysis inhibitor (B) is contained in order to ensure long-term durability.

The hydrolysis inhibitor (B) is not particularly limited, and may be any known compound. For example, it may be a compound that reacts with the carboxylic acid end group of the polyester resin (A) to bond with it. Specific examples of such compounds include compounds having functional groups such as carbodiimide groups, epoxy groups, and oxazoline groups. Among these, carbodiimide group-containing compounds are preferred because they are highly effective in eliminating the catalytic activity of protons derived from the carboxyl end groups.

As the carbodiimide group-containing compound, a known polycarbodiimide having one or more carbodiimide groups (-N=C=N-) in the molecule may usually be used, but in order to increase durability under high temperature and high humidity conditions, a compound having two or more carbodiimide groups in the molecule is preferable, and in particular a compound having three or more, further five or more, and especially seven or more. However, if the compound has 30 or more groups, the molecular structure tends to become too large, which is undesirable. It is also preferable to use a high molecular weight polycarbodiimide produced by subjecting a diisocyanate to a decarboxylation condensation reaction in the presence of a carbodiimide catalyst.

Examples of such high molecular weight polycarbodiimides include those obtained by subjecting the following diisocyanates to a decarboxylation condensation reaction.

Examples of such diisocyanates include 4,4'-diphenylmethane diisocyanate, 3,3'-dimethoxy-4,4'-diphenylmethane diisocyanate, 3,3'-dimethyl-4,4'-diphenylmethane diisocyanate, 4,4'-diphenylether diisocyanate, 3,3'-dimethyl-4,4'-diphenylether diisocyanate, 2,4-tolylene diisocyanate, 2,6-tolylene diisocyanate, 1-methoxyphenyl-2,4-diisocyanate, isophorone diisocyanate, 4,4'-dicyclohexylmethane diisocyanate, tetramethylxylylene diisocyanate, and the like. These can be used alone or in combination of two or more. Such high molecular weight polycarbodiimides can be synthesized or commercially available products can be used.

Commercially available carbodiimide group-containing compounds include, for example, the Carbodilite (registered trademark) series manufactured by Nisshinbo Chemical Inc. Among these, Carbodilite (registered trademark) V-01, V-03, V-05, V-07, and V-09 are preferred because of their excellent compatibility with organic solvents.

The epoxy compound is preferably, for example, a glycidyl ester compound or a glycidyl ether compound.

Specific examples of glycidyl ester compounds include benzoic acid glycidyl ester, t-Bu-benzoic acid glycidyl ester, p-toluic acid glycidyl ester, cyclohexane carboxylic acid glycidyl ester, pelargonic acid glycidyl ester, stearic acid glycidyl ester, lauric acid glycidyl ester, palmitic acid glycidyl ester, behenic acid glycidyl ester, versatic acid glycidyl ester, oleic acid glycidyl ester, linoleic acid glycidyl ester, linoleic acid glycidyl ester, behenolic acid glycidyl ester, stearolic acid glycidyl ester, terephthalic acid diglycidyl ester, isophthalic acid diglycidyl ester, Examples of such glycidyl esters include glycidyl esters of ...

Specific examples of glycidyl ether compounds include phenyl glycidyl ether, o-phenyl glycidyl ether, 1,4-bis(β,γ-epoxypropoxy)butane, 1,6-bis(β,γ-epoxypropoxy)hexane, 1,4-bis(β,γ-epoxypropoxy)benzene, 1-(β,γ-epoxypropoxy)-2-ethoxyethane, 1-(β,γ-epoxypropoxy)-2-benzyloxyethane, 2,2-bis-[р-(β,γ-epoxypropoxy)phenyl]propane, and bisglycidyl polyethers obtained by reacting bisphenols such as 2,2-bis-(4-hydroxyphenyl)propane and 2,2-bis-(4-hydroxyphenyl)methane with epichlorohydrin, and these can be used alone or in combination of two or more kinds.

The oxazoline compound is preferably a bisoxazoline compound. Specifically, for example, 2,2'-bis(2-oxazoline), 2,2'-bis(4-methyl-2-oxazoline), 2,2'-bis(4,4-dimethyl-2-oxazoline), 2,2'-bis(4-ethyl-2-oxazoline), 2,2'-bis(4,4'-diethyl-2-oxazoline), 2,2'-bis(4-propyl-2-oxazoline), 2,2'-bis(4-butyl-2-oxazoline), 2,2'-bis(4-hexyl-2-oxazoline), ), 2,2'-bis(4-phenyl-2-oxazoline), 2,2'-bis(4-cyclohexyl-2-oxazoline), 2,2'-bis(4-benzyl-2-oxazoline), 2,2'-p-phenylenebis(2-oxazoline), 2,2'-m-phenylenebis(2-oxazoline), 2,2'-o-phenylenebis(2-oxazoline), 2,2'-p-phenylenebis(4-methyl-2-oxazoline), 2,2'-p-phenylenebis(4,4 -dimethyl-2-oxazoline), 2,2'-m-phenylenebis(4-methyl-2-oxazoline), 2,2'-m-phenylenebis(4,4-dimethyl-2-oxazoline), 2,2'-ethylenebis(2-oxazoline), 2,2'-tetramethylenebis(2-oxazoline), 2,2'-hexamethylenebis(2-oxazoline), 2,2'-octamethylenebis(2-oxazoline), 2,2'-decamethylenebis(2-oxazoline), 2 Examples include 2,2'-ethylenebis(4-methyl-2-oxazoline), 2,2'-tetramethylenebis(4,4-dimethyl-2-oxazoline), 2,2'-9,9'-diphenoxyethanebis(2-oxazoline), 2,2'-cyclohexylenebis(2-oxazoline), and 2,2'-diphenylenebis(2-oxazoline), and among these, 2,2'-bis(2-oxazoline) is most preferred from the viewpoint of reactivity with polyester. These can be used alone or in combination of two or more kinds.

These hydrolysis inhibitors (B) are preferably low in volatility, and therefore it is preferable to use one with a high number average molecular weight, usually 300 to 10,000, and preferably 1,000 to 5,000.

Among the hydrolysis inhibitors (B), it is preferable to use a carbodiimide group-containing compound. In this case, the carbodiimide equivalent is preferably 50 to 10,000, particularly 100 to 1,000, and more preferably 150 to 500. The carbodiimide equivalent indicates the chemical formula weight per one carbodiimide group.

The content of the hydrolysis inhibitor (B) is preferably 0.01 to 10 parts by weight, more preferably 0.1 to 5 parts by weight, and even more preferably 0.2 to 3 parts by weight, per 100 parts by weight of the polyester resin (A). If the content is too high, turbidity tends to occur due to poor compatibility with the polyester resin (A), and if the content is too low, sufficient durability tends to be difficult to obtain.

The content of the hydrolysis inhibitor (B) is preferably optimized depending on the acid value of the polyester resin (A), and the molar ratio ((b)/(a)) of the total amount of functional groups (b) of the hydrolysis inhibitor (B) in the pressure-sensitive adhesive composition to the total acid value (a) of the polyester resin (A) in the pressure-sensitive adhesive composition [I] is preferably 0.5≦(b)/(a), particularly preferably 1≦(b)/(a)≦1,000, and further preferably 1.5≦(b)/(a)≦100.
If the content ratio of (b) to (a) is too high, the compatibility with the polyester resin (A) tends to decrease, and the adhesive strength, cohesive strength, and durability performance tend to decrease. Conversely, if the content ratio of (b) to (a) is too low, the moist heat resistance performance tends to decrease.

<Crosslinking Agent (C)>
The polyester-based pressure-sensitive adhesive composition [I] of the present invention contains the above-mentioned polyester-based resin (A) and preferably further contains a hydrolysis inhibitor (B). Usually, however, it is preferable for the composition to contain a crosslinking agent (C). By containing the crosslinking agent (C), the polyester-based resin (A) is crosslinked with the crosslinking agent (C) to give a composition with excellent cohesive strength, thereby improving the performance as a pressure-sensitive adhesive.

Examples of such crosslinking agents (C) include compounds having functional groups that react with the hydroxyl groups and/or carboxyl groups contained in the polyester resin (A), such as polyisocyanate-based compounds and polyepoxy-based compounds. Among these, it is particularly preferable to use polyisocyanate-based compounds, as they can achieve a good balance between initial adhesion, mechanical strength, and heat resistance.

Examples of such polyisocyanate compounds include polyisocyanates such as tetramethylene diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate, tolylene diisocyanate, diphenylmethane diisocyanate, hydrogenated diphenylmethane diisocyanate, xylylene diisocyanate, hydrogenated xylylene diisocyanate, tetramethylxylylene diisocyanate, 1,5-naphthalene diisocyanate, and triphenylmethane triisocyanate. Also included are adducts of the above polyisocyanates with polyol compounds such as trimethylolpropane, and biuret and isocyanurate forms of these polyisocyanate compounds. The above polyisocyanate compounds may also be used in which the isocyanate portion is blocked with phenol, lactam, or the like. These crosslinking agents (C) may be used alone or in a mixture of two or more.

The content of the crosslinking agent (C) can be appropriately selected depending on the molecular weight of the polyester resin (A) and the intended use. In general, the crosslinking agent (C) is preferably contained in a ratio such that the reactive group contained in the crosslinking agent (C) is 0.2 to 10 equivalents relative to 1 equivalent of the hydroxyl group and/or carboxyl group contained in the polyester resin (A), particularly preferably 0.5 to 5 equivalents, and further preferably 0.5 to 3 equivalents.
If the equivalent number of the reactive groups contained in the crosslinking agent (C) is too small, the cohesive strength decreases and sufficient heat resistance tends to be unavailable. If the equivalent number is too large, the flexibility decreases, the initial adhesion decreases, and sufficient adhesive strength cannot be obtained even with finger pressure.

When the crosslinking agent (C) is blended, it is preferable to simultaneously contain a catalyst.
Examples of such catalysts include metal catalysts such as tin-based catalysts, lead-based catalysts, bismuth-based catalysts, transition metal compounds such as iron, copper, titanium, zirconium, nickel, cobalt, and manganese-based catalysts, acetylacetonate complexes, and non-metal catalysts such as tertiary amine compounds. From the viewpoint of catalytic activity, organotin-based catalysts, organotitanium-based catalysts, and organozirconium-based catalysts are preferred.

<Plasticizer (D)>
Thus, a pressure-sensitive adhesive composition [I] is obtained which contains the polyester resin (A), and preferably further contains a hydrolysis inhibitor (B) and a crosslinking agent (C), and in the present invention, it is preferable to further contain a plasticizer (D). Such a plasticizer (D) is contained to ensure adhesion even if the glass transition temperature of the resin becomes too high due to the high refractive index.

Examples of such plasticizers (D) include alkyl esters of aromatic polycarboxylic acids and alkyl esters of aliphatic polycarboxylic acids. Among them, alkyl esters of aromatic polycarboxylic acids are preferred because they can exert a plasticizing effect without a significant decrease in the refractive index, and alkyl esters of aromatic polycarboxylic acids having a valence of three or more are even more preferred.

The content of the plasticizer (D) is preferably 2 to 30 parts by weight, more preferably 3 to 20 parts by weight, and even more preferably 4 to 10 parts by weight, per 100 parts by weight of the polyester resin (A). If the content is too high, the adhesive strength tends to decrease due to bleeding out of the plasticizer, and if the content is too low, the adhesive strength tends to decrease due to poor compatibility.

<Inorganic oxide (E)>
In the present invention, it is preferable to further contain an inorganic oxide (E), which has a relatively high refractive index and is therefore preferably contained for the purpose of increasing the refractive index of the adhesive layer. Furthermore, by blending the inorganic oxide (E), it is possible to improve the adhesive strength of the polyester-based adhesive.

That is, in a polyester-based pressure-sensitive adhesive composition containing a polyester-based resin (A) and an inorganic oxide (E), the polyester-based pressure-sensitive adhesive can have an adhesive strength (β) of 5 N/25 mm or more and a refractive index of 1.530 or more.
Adhesive strength (β): When a polyester-based adhesive is formed on a substrate to form an adhesive sheet, the sheet is attached to a glass plate adherend and left for 30 minutes at 23° C. and 50% RH. The 180-degree peel strength (N/25 mm) from the adherend is then measured at a peel speed of 300 mm/min.

When an inorganic oxide (E) is contained, the refractive index can be further increased, and the refractive index is 1.530 or more, preferably 1.560 or more, more preferably 1.600 or more, and particularly preferably 1.620 or more. The upper limit is usually 1.700, preferably 1.680, and more preferably 1.660.

There are no particular limitations on the inorganic oxide (E), but metal oxides are preferred. Among them, particles containing a metal oxide and an organic substance having a reactive functional group that modifies the surface of the metal oxide, more specifically, coated particles containing a metal oxide particle and an organic substance having a reactive functional group that modifies the surface of the particle (hereinafter also referred to as reactive modified metal oxide particles) are preferred. The reactive functional group may be in a state of interacting with the metal oxide through hydrogen bonds or the like, or may be in a state where it can interact with another substance without such a state. The inorganic oxide (E) is preferably inorganic oxide fine particles in a fine particle state, and examples of the form of the inorganic oxide fine particles include fine particle powder, paste, and sol, but is preferably a sol.

As the metal oxide, a metal oxide generally used as a filler in a resin is preferably used. Examples of such metal oxides include zirconium oxide ( _ZrO2 : zirconia), titanium oxide ( _TiO2 : titania), _{aluminum oxide (Al2O3 :} alumina), iron oxide ( _{Fe2O3 , Fe3O4 )} , copper oxide (CuO), zinc oxide ( _ZnO ), yttrium oxide ( _Y2O3 : _{yttria), niobium oxide (Nb2O5 )} , molybdenum oxide ( _MoO3 , _MoO2 ), indium oxide ( _In2O3 ), tin oxide ( _SnO2 ), tantalum oxide ( _Ta2O5 , _TaO2 ), tungsten oxide ( _WO3 , _WO2 ), lead oxide (PbO), bismuth oxide ( _Bi2O3 ), cerium oxide ( _{CeO2 )} , _{and the like} . Examples of the non-metallic oxide include ceria, antimony oxide (Sb ₂ O ₃ , Sb ₂ O ₅ ), tin-doped indium oxide (ITO), antimony-doped tin oxide (ATO), fluorine-doped tin oxide (FTO), phosphorus-doped tin oxide (PTO), zinc antimonate (AZO), indium-doped zinc oxide (IZO), aluminum-doped zinc oxide, and gallium-doped zinc oxide. Examples of the non-metallic oxide include silicon oxide (SiO ₂ : silica), which is generally used as a filler in resins, and boron oxide (B ₂ O ₃ ). These metal oxides and non-metallic oxides may be used alone or in combination. Among these, zirconium oxide (ZrO ₂ : zirconia) and titanium oxide (TiO ₂ : titania) are preferred from the viewpoint of easily obtaining a high refractive index adhesive.

Examples of reactive functional groups in organic substances having reactive functional groups include hydroxyl groups, phosphate groups, carboxyl groups, amino groups, alkoxy groups, isocyanate groups, acid halides, acid anhydrides, glycidyl groups, chlorosilane groups, and alkoxysilane groups. As organic substances having reactive functional groups, organic substances having isocyanate groups are particularly preferred because they can improve the stability between the metal oxide and the surrounding substances. Examples of organic substances having isocyanate groups include acryloxymethyl isocyanate, methacryloxymethyl isocyanate, acryloxyethyl isocyanate, methacryloxyethyl isocyanate, acryloxypropyl isocyanate, methacryloxypropyl isocyanate, and 1,1-bis(acryloxymethyl)ethyl isocyanate.

In the reactive modified metal oxide particles, the content of the organic material having a reactive functional group is preferably 1 to 40 parts by weight per 100 parts by weight of the metal oxide.

Reactive modified metal oxide particles can be obtained as a suspension in which the particles are dispersed in an organic solvent by mixing metal oxide particles, an organic substance having a reactive functional group, an organic solvent, and any additives that can be added as needed, and then subjecting the resulting mixture to a treatment such as ultrasonic treatment as needed.

Examples of organic solvents include ketones such as methyl ethyl ketone, methyl isobutyl ketone, acetone, and cyclohexanone; aromatic hydrocarbons such as benzene, toluene, xylene, and ethylbenzene; alcohols such as methanol, ethanol, isopropyl alcohol, n-butanol, and iso-butanol; ethers such as ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, diethylene glycol monomethyl ether, and diethylene glycol monoethyl ether; esters such as ethyl acetate, butyl acetate, ethyl lactate, γ-butyrolactone, propylene glycol monomethyl ether acetate, and propylene glycol monoethyl ether acetate; and amides such as dimethylformamide, N,N-dimethylacetoacetamide, and N-methylpyrrolidone. As the organic solvent, one of these may be used alone or two or more may be used in combination. An example of an optional additive that may be added to the mixture is a metal chelating agent.

When the reactive modified metal oxide particles are obtained as a suspension in which the particles are dispersed in an organic solvent, it is preferable from the viewpoint of ease of production, etc. to adjust conditions such as the amount of solvent so that the suspension contains 1 to 50% by weight of reactive modified metal oxide particles, and then directly subject this to the production of the adhesive composition.

When preparing the mixture, it is preferable to mix the components using a bead mill or the like. By doing so, secondary particles or higher order particles can be pulverized to the primary particle level, and the surface can be treated in the primary particle state, resulting in a uniform surface treatment.

The mixture can be further subjected to ultrasonic treatment if necessary. The ultrasonic treatment can be carried out using a device such as an ultrasonic cleaner, ultrasonic homogenizer, or ultrasonic disperser. By such treatment, a good suspension can be obtained.

The reactive modified metal oxide particles may be commercially available particles as they are. The commercially available particles may be provided as a slurry containing components such as a solvent and additives, and may be used as a material for the pressure-sensitive adhesive composition of the present invention in the form of a slurry containing such components as they are.
An example of a slurry of reactive modified metal oxide particles containing _ZrO2 as a metal oxide is the product name "NANON5 ZR-010" (manufactured by Solar Co., Ltd., solvent: methyl ethyl ketone, particle content ratio 30%, organic substance having a reactive functional group that modifies the surface: isocyanate having a polymerizable functional group, volume average particle diameter 15 nm). An example of a slurry of reactive modified metal oxide particles containing _TiO2 as a metal oxide is the product name "NOD-742GTF" (manufactured by Nagase ChemteX Corporation, solvent: polyethylene glycol monomethyl ether, particle content ratio 30%, volume average particle diameter 48 nm).

The volume average particle diameter of the inorganic oxide (E) is preferably 5 nm or more, more preferably 10 nm or more, particularly preferably 20 nm or more, while it is preferably 50 nm or less, more preferably 40 nm or less, particularly preferably 30 nm or less. By setting the particle diameter within the above numerical range, an adhesive layer with little coloring and high light transmittance can be obtained, and the particles can be easily dispersed. When the reactive modified metal oxide aggregates to form secondary particles or higher-order particles, the particle diameter range can be the primary particle diameter range. The particle diameter can be measured using a dynamic light scattering particle size distribution analyzer (Nanotrac Wave-EX150 manufactured by Nikkiso Co., Ltd.) by using the volume as the particle diameter standard.

The lower limit of the content of inorganic oxide (E) in the adhesive composition is preferably 10 parts by weight or more, particularly preferably 40 parts by weight or more, and more preferably 90 parts by weight or more, per 100 parts by weight of polyester resin (A). On the other hand, the upper limit of the content of inorganic oxide (B) is 500 parts by weight or less, preferably 400 parts by weight or less, and more preferably 300 parts by weight or less, per 100 parts by weight of polyester resin (A). If the content of inorganic oxide (B) is too low, the desired refractive index tends not to be obtained, and if it is too high, the adhesive strength tends to decrease.

In the present invention, in addition to the above-mentioned polyester resin (A), hydrolysis inhibitor (B), crosslinking agent (C), plasticizer (D), and inorganic oxide (E), conventionally known additives such as antioxidants, softeners, ultraviolet absorbers, stabilizers, antistatic agents, and tackifiers, as well as other powdery or particulate additives such as inorganic or organic fillers, metal powders, and pigments, may be blended within the range that does not impair the effects of the present invention.
From the viewpoints of durability and transparency, it is preferable that the composition is substantially free of the tackifier.

The polyester-based pressure-sensitive adhesive of the present invention is made of the above-mentioned pressure-sensitive adhesive composition [I], that is, is obtained by curing the pressure-sensitive adhesive composition [I].
In the polyester-based pressure-sensitive adhesive of the present invention, when adjusting the adhesive strength (α) to 10 N/25 mm or more and the refractive index to 1.530 or more, for example, (1) a method of blending a large amount of raw materials having an aromatic ring among polyvalent carboxylic acids and/or polyhydric alcohols in the raw material composition of the polyester-based resin (A), (2) a method of blending a large amount of raw materials having an aromatic ring among polyvalent carboxylic acids and/or polyhydric alcohols in the raw material composition of the polyester-based resin (A) and adding a plasticizer to the polyester-based resin (A) having a glass transition temperature of −20° C. to 40° C., (3) a method of adding an inorganic oxide (E) having a high refractive index to the polyester-based resin (A), or (4) a combination of these (1) to (3), etc. can be mentioned. In terms of being easy to balance between cost, refractive index, and adhesive performance, methods (1) and (2) are preferred, and method (2) is particularly preferred.
In order to further increase the refractive index, a combination of methods (2) and (3) is preferable.

It is preferable that such a polyester-based adhesive does not substantially contain acidic groups, and specifically, the acid value is preferably 10 mgKOH/g or less, particularly preferably 5 mgKOH/g or less, even more preferably 1 mgKOH/g or less, and especially preferably 0.5 mgKOH/g or less.
The acid value of the polyester-based pressure-sensitive adhesive can be determined in the same manner as the acid value of the polyester-based resin (A).

The polyester-based adhesive composition [I] of the present invention can be used for bonding various members, but is preferably used as an adhesive for optical members for bonding optical members. By laminating an adhesive layer of a polyester-based adhesive made of the polyester-based adhesive composition [I] on an optical member, the optical member with the adhesive layer can be obtained.

Examples of such optical members include transparent electrode films such as ITO electrode films and inorganic or organic conductive films such as polythiophene, polarizing plates, retardation plates, elliptical polarizing plates, optical compensation films, brightness improvement films, electromagnetic wave shielding films, near infrared absorbing films, and AR (anti-reflection) films. Among these, the ITO electrode film is particularly preferred because it is effective when the optical member is a transparent electrode film and can obtain high adhesive strength. In addition, the ITO electrode film is often formed as a thin film on a substrate such as glass or PET, but in the present invention, it is particularly preferred to use a film in which the ITO electrode film is formed as a thin film on a PET substrate.
The film is also suitable for use as a light extraction film provided on the light emitting surface of a surface emitter of an organic EL element, or as a light diffusion sheet for a liquid crystal display.

It is preferable that the optical member with the pressure-sensitive adhesive layer further has a release sheet on the surface of the pressure-sensitive adhesive layer opposite the optical member surface, and when used in practice, the release sheet is peeled off and the pressure-sensitive adhesive layer is attached to an adherend. It is preferable to use a silicone-based release sheet as such a release sheet.

The polyester-based adhesive of the present invention can be used as an adhesive sheet having an adhesive layer made of the adhesive of the present invention on one or both sides of a supporting substrate, and is particularly suitable as an adhesive sheet for optical components used for bonding optical components. In addition, it is also preferable to use a substrate-less double-sided adhesive sheet that does not have a supporting substrate, because of its excellent transparency and high adhesive strength relative to the thickness of the substrate. In the present invention, the term "sheet" is used to include the meanings of "film" and "tape".

<Adhesive sheet>
The pressure-sensitive adhesive sheet can be produced, for example, as follows.
Such an adhesive sheet can be produced according to a publicly known general method for producing an adhesive sheet. For example, the adhesive composition [I] is applied onto a substrate sheet, dried, a release sheet is attached to the adhesive layer surface on the opposite side, and cured as necessary to obtain an adhesive sheet of the present invention having an adhesive layer made of the polyester-based adhesive of the present invention on a substrate.

The adhesive sheet of the present invention can also be obtained by coating the above-mentioned adhesive composition [I] on a release sheet, drying it, laminating a substrate sheet to the opposite side of the adhesive layer, and curing it as necessary.

Alternatively, a substrate-less double-sided pressure-sensitive adhesive sheet can be produced by forming a pressure-sensitive adhesive layer on a release sheet and laminating a release sheet to the opposite side of the pressure-sensitive adhesive layer.
When using the resulting pressure-sensitive adhesive sheet or substrate-less double-sided pressure-sensitive adhesive sheet, the release sheet is peeled off from the pressure-sensitive adhesive layer, and the pressure-sensitive adhesive layer is attached to an adherend.

Examples of the substrate include polyester resins such as polyethylene naphthalate, polyethylene terephthalate, polybutylene terephthalate, and polyethylene terephthalate/isophthalate copolymer; polyolefin resins such as polyethylene, polypropylene, and polymethylpentene; polyethylene fluoride resins such as polyvinyl fluoride, polyvinylidene fluoride, and polyethylene fluoride; polyamides such as nylon 6 and nylon 6,6; vinyl polymers such as polyvinyl chloride, polyvinyl chloride/vinyl acetate copolymer, ethylene-vinyl acetate copolymer, ethylene-vinyl alcohol copolymer, polyvinyl alcohol, and vinylon; cellulose resins such as cellulose triacetate and cellophane; acrylic resins such as polymethyl methacrylate, polyethyl methacrylate, polyethyl acrylate, and polybutyl acrylate; polystyrene; polycarbonate; polyarylate; polyimide; and synthetic resin sheets such as cycloolefin polymers; metal foils such as aluminum, copper, and iron; paper such as fine paper and glassine paper; and woven and nonwoven fabrics made of glass fiber, natural fibers, and synthetic fibers. These substrate sheets can be used as a single layer or as a multi-layer structure in which two or more types are laminated.

Among these, substrate sheets made of polyethylene terephthalate and polyimide are particularly preferred, with polyethylene terephthalate being particularly preferred because of its excellent adhesion to the adhesive. Furthermore, polyethylene terephthalate having a metal thin film layer is preferred because it has excellent adhesion between the substrate and the adhesive, can stably maintain the substrate without corroding the metal thin film layer, and can significantly exert the effects of the polyester-based adhesive of the present invention.

In the present invention, it is also preferable to form an optical laminate in which an ITO electrode film is formed as a thin film on a PET substrate and the film has an adhesive layer on the PET side, the PET substrate and a polycarbonate-based film are laminated via the adhesive layer, and an acrylic film is further laminated (layer structure: ITO electrode film/PET substrate/adhesive layer/PC-based film/acrylic film).
In addition, it is preferable that the refractive index of the base film adjacent to the pressure-sensitive adhesive layer is 1.52 to 1.63, in terms of good light extraction properties when laminated with the pressure-sensitive adhesive of the present invention.

As the release sheet, for example, the various synthetic resin sheets, paper, cloth, nonwoven fabric, etc. exemplified above as the substrate can be used which have been subjected to a release treatment. As the release sheet, it is preferable to use a silicone-based release sheet.

The thickness of the substrate is preferably, for example, 1 to 1000 μm, particularly preferably 2 to 500 μm, and even more preferably 3 to 300 μm.

The adhesive composition [I] may be applied using, for example, a gravure roll coater, reverse roll coater, kiss roll coater, dip roll coater, bar coater, knife coater, spray coater, etc.

The conditions for the above curing treatment are usually room temperature to 70°C, and the time is usually 1 to 30 days. Specifically, the treatment can be performed under conditions such as 23°C for 1 to 20 days, preferably 23°C for 3 to 14 days, or 40°C for 1 to 10 days.

As for drying conditions, the drying temperature is preferably 60 to 140°C, and particularly preferably 80 to 120°C, and the drying time is preferably 0.5 to 30 minutes, and particularly preferably 2 to 5 minutes.

The thickness of the adhesive layer of the above-mentioned adhesive sheet and substrate-less double-sided adhesive sheet is preferably 2 to 500 μm, particularly preferably 5 to 200 μm, and even more preferably 10 to 100 μm. If the thickness of such an adhesive layer is too thin, the adhesive strength tends to decrease, while if it is too thick, it becomes difficult to apply uniformly and problems such as air bubbles entering the coating tend to occur. When considering impact absorption properties, it is preferable to make the thickness 50 μm or more.

The thickness of the adhesive layer is determined by subtracting the measured thickness of the components other than the adhesive layer from the measured thickness of the entire adhesive sheet using a Mitutoyo ID-C112B.

The gel fraction of the pressure-sensitive adhesive layer of the pressure-sensitive adhesive sheet is preferably 20% or more from the viewpoints of durability and adhesive strength, particularly preferably 30 to 90%, and further preferably 40 to 70%. If the gel fraction is too low, the cohesive strength decreases, and thus the durability tends to decrease.
If the gel fraction is too high, there is a concern that the adhesive strength will decrease due to an increase in cohesive strength.

The gel fraction is an index of the degree of crosslinking and is calculated, for example, by the following method.
That is, an adhesive sheet (without a separator) consisting of an adhesive layer formed on a polymer sheet substrate (e.g., a polyethylene terephthalate film, etc.) is wrapped in a 200-mesh SUS wire netting and immersed in toluene at 23°C for 24 hours, and the weight percentage of the insoluble adhesive component remaining in the wire netting is taken as the gel fraction, excluding the weight of the substrate.

Furthermore, such an adhesive sheet may be protected by providing a release sheet on the outside of the adhesive layer, if necessary. In addition, in an adhesive sheet in which the adhesive layer is formed on one side of a substrate, it is also possible to protect the adhesive layer by applying a release treatment to the side of the substrate opposite the adhesive layer, thereby utilizing the release-treated surface.

The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples as long as it does not depart from the gist of the invention. In the examples, "parts" and "%" are by weight.
The weight average molecular weight and glass transition temperature of the polyester resin in the following examples were measured according to the above-mentioned methods. The acid value of the polyester resin was measured by dissolving 1 g of the polyester resin in 30 g of a 7/3 (weight ratio) (toluene/methanol) mixed solvent and performing neutralization titration based on JIS K0070.

<Production of polyester resin (A)>
In the following Production Examples, the term "mol" refers to a molar ratio when the total amount of the polybasic carboxylic acid components is taken as 1.0 mol.

[Production Example 1: Production of Polyester Resin (A-1)]
In a reaction vessel equipped with a thermometer, a stirrer, a rectification column, a nitrogen inlet tube and a vacuum device, 42.9 parts (1.0 mole) of sebacic acid as a polyvalent carboxylic acid component (A1), 15.5 parts (0.7 moles) of neopentyl glycol, 13.4 parts (0.7 moles) of 1,4-butanediol as a polyol component (A2), and 0.4 parts (0.013 moles) of trimethylolpropane, 27.9 parts (0.3 moles) of bisphenoxyethanolfluorene, and 0.01 parts of tetrabutyl titanate as a catalyst were charged, and the temperature was gradually increased to an internal temperature of 250 ° C., and an esterification reaction was carried out over 4 hours. Thereafter, the internal temperature was increased to 260 ° C., and 0.01 parts of tetrabutyl titanate was charged as a catalyst, the pressure was reduced to 1.33 hPa, and a polymerization reaction was carried out over 3 hours to produce a polyester resin (A-1).
The resulting polyester resin (A-1) had a weight average molecular weight of 65,000, a glass transition temperature of −18° C., an acid value of 0.7 mgKOH/g, and a refractive index of 1.537.
The final component ratio was as follows: polycarboxylic acid component: sebacic acid = 100 mol %, polyhydric alcohol component: neopentyl glycol/1.4-butanediol/trimethylolpropane/bisphenoxyethanolfluorene = 33.4 mol %/36.5 mol %/0.8 mol %/29.3 mol %.

[Production Example 2: Production of Polyester Resin (A-2)]
In a reaction vessel equipped with a thermometer, a stirrer, a rectification column, a nitrogen inlet tube and a vacuum device, 40.1 parts (1.0 mole) of sebacic acid as a polyvalent carboxylic acid component (A1), 12.4 parts (0.6 moles) of neopentyl glycol, 12.4 parts (0.7 moles) of 1,4-butanediol as a polyol component (A2), 0.3 parts (0.013 moles) of trimethylolpropane, 34.7 parts (0.4 moles) of bisphenoxyethanolfluorene, and 0.01 parts of tetrabutyl titanate as a catalyst were charged, and the temperature was gradually increased to an internal temperature of 250° C., and an esterification reaction was carried out over 4 hours. Thereafter, the internal temperature was increased to 260° C., and 0.01 parts of tetrabutyl titanate was charged as a catalyst, the pressure was reduced to 1.33 hPa, and a polymerization reaction was carried out over 3 hours to produce a polyester resin (A-2).
The resulting polyester resin (A-2) had a weight average molecular weight of 36,000, a glass transition temperature of -4°C, an acid value of 1.0 mgKOH/g, and a refractive index of 1.553.
The final component ratio was as follows: polycarboxylic acid component: sebacic acid = 100 mol %, polyhydric alcohol component: neopentyl glycol/1.4-butanediol/trimethylolpropane/bisphenoxyethanolfluorene = 26.3 mol %/33.9 mol %/0.8 mol %/39.0 mol %.

[Production Example 3: Production of Polyester Resin (A-3)]
In a reaction vessel equipped with a thermometer, a stirrer, a rectification column, a nitrogen inlet tube and a vacuum device, 37.6 parts (1.0 mole) of sebacic acid as a polyvalent carboxylic acid component (A1), 9.7 parts (0.5 moles) of neopentyl glycol as a polyol component (A2), 11.7 parts (0.7 moles) of 1,4-butanediol, and 0.3 parts (0.013 moles) of trimethylolpropane, 40.7 parts (0.5 moles) of bisphenoxyethanolfluorene, and 0.01 parts of tetrabutyl titanate as a catalyst were charged, and the temperature was gradually increased to an internal temperature of 250 ° C., and an esterification reaction was carried out over 4 hours. Thereafter, the internal temperature was increased to 260 ° C., and 0.01 parts of tetrabutyl titanate was charged as a catalyst, the pressure was reduced to 1.33 hPa, and a polymerization reaction was carried out over 3 hours to produce a polyester resin (A-3).
The resulting polyester resin (A-3) had a weight average molecular weight of 41,000, a glass transition temperature of 14° C., an acid value of 1.2 mgKOH/g, and a refractive index of 1.566.
The final component ratio was as follows: polycarboxylic acid component: sebacic acid = 100 mol %, polyhydric alcohol component: neopentyl glycol/1.4-butanediol/trimethylolpropane/bisphenoxyethanolfluorene = 19.6 mol %/31.0 mol %/0.7 mol %/48.7 mol %.
Met.

[Production Example 4: Production of Polyester Resin (A-4)]
Thermometer, stirrer, distillation column, nitrogen inlet tube and vacuum device equipped reactor, polyvalent carboxylic acid component (A1) 40.3 parts (0.95 moles) of sebacic acid, 3.1 parts (0.05 moles) of dimethyl sodium sulfonyl isophthalate, polyol component (A2) 15.3 parts (0.7 moles) of neopentyl glycol, 13.2 parts (0.7 moles) of 1,4-butanediol and 0.4 parts (0.013 moles) of trimethylolpropane, 27.6 parts (0.3 moles) of bisphenoxyethanolfluorene, catalyst 0.01 parts of tetrabutyl titanate was charged, the temperature was gradually raised to 250 ° C., and esterification reaction was carried out over 4 hours. Thereafter, the internal temperature was raised to 260 ° C., tetrabutyl titanate 0.01 parts was charged as a catalyst, the pressure was reduced to 1.33 hPa, and polymerization reaction was carried out over 3 hours to produce polyester resin (A-4).
The resulting polyester resin (A-4) had a weight average molecular weight of 46,000, a glass transition temperature of -27°C, an acid value of 1.0 mgKOH/g, and a refractive index of 1.537.
The final component ratios were as follows: polycarboxylic acid component: sebacic acid/sodium sulfonyl isophthalate = 95 mol%/5 mol%, and polyhydric alcohol component: neopentyl glycol/1.4-butanediol/trimethylolpropane/bisphenoxyethanolfluorene = 33.4 mol%/36.5 mol%/0.8 mol%/29.3 mol%.

[Production Example 5: Production of Polyester Resin (A-5)]
A reaction vessel equipped with a thermometer, a stirrer, a rectification column, a nitrogen inlet tube, and a vacuum device was charged with 45.9 parts (0.8 mol) of sebacic acid, 7.1 parts (0.15 mol) of isophthalic acid, and 4.2 parts (0.05 mol) of dimethyl sodium sulfonylisophthalate as the polyvalent carboxylic acid component (A1), 26.6 parts (0.9 mol) of neopentyl glycol, 12.8 parts (0.5 mol) of 1,4-butanediol, 2.9 parts (0.087 mol) of 1,6-hexanediol, and 0.5 parts (0.013 mol) of trimethylolpropane as the polyol component (A2), and 0.01 part of tetrabutyl titanate as a catalyst, and the internal temperature was gradually raised to 250° C., and an esterification reaction was carried out over 4 hours. Thereafter, the internal temperature was raised to 260° C., 0.01 part of tetrabutyl titanate was charged as a catalyst, the pressure was reduced to 1.33 hPa, and a polymerization reaction was carried out over 3 hours to produce a polyester resin (A-5).
The resulting polyester resin (A-5) had a weight average molecular weight of 51,000, a glass transition temperature of −49° C., an acid value of 0.8 mgKOH/g, and a refractive index of 1.489.
The final component ratios were as follows: polycarboxylic acid components: sebacic acid/isophthalic acid/sodium sulfonylisophthalate = 80 mol %/15 mol %/5 mol %; and polyhydric alcohol components: neopentyl glycol/1.4-butanediol/1.6-hexanediol/trimethylolpropane = 58.7 mol %/34.1 mol %/6.2 mol %/1.0 mol %.

[Production Example 6: Production of Polyester Resin (A'-1)]
In a reaction vessel equipped with a thermometer, a stirrer, a rectification column, a nitrogen inlet tube and a vacuum device, 9.6 parts (0.2 moles) of isophthalic acid and 46.8 parts (0.8 moles) of sebacic acid as polyvalent carboxylic acid component (A1), 27.1 parts (0.9 moles) of neopentyl glycol, 13.0 parts (0.5 moles) of 1,4-butanediol, 3.0 parts (0.09 moles) of 1,6-hexanediol and 0.5 parts (0.013 moles) of trimethylolpropane as polyol component (A2), 0.01 parts of tetrabutyl titanate as catalyst were charged, and the temperature was gradually increased to an internal temperature of 250 ° C., and an esterification reaction was carried out over 4 hours. Thereafter, the internal temperature was increased to 260 ° C., 0.01 parts of tetrabutyl titanate was charged as a catalyst, the pressure was reduced to 1.33 hPa, and a polymerization reaction was carried out over 3 hours, producing a polyester resin (A'-1).
The resulting polyester resin (A'-1) had a weight average molecular weight of 70,000, a glass transition temperature of -50°C, an acid value of 0.4 mgKOH/g and a refractive index of 1.486.
The final composition was as follows: polycarboxylic acid component: sebacic acid/isophthalic acid = 80 mol%/20 mol%; polyhydric alcohol component: neopentyl glycol/1.4-butanediol/1.6-hexanediol/trimethylolpropane = 54.8 mol%/38.4 mol%/0.9 mol%/5.9 mol%.
Met.

[Production Example 7: Production of Polyester Resin (A'-2)]
In a reaction vessel equipped with a thermometer, a stirrer, a distillation column, a nitrogen inlet tube and a vacuum device, 22.9 parts (0.64 moles) of naphthalene dicarboxylic acid dimethyl ester (NDCM) as a polyvalent carboxylic acid component (A1), 65.8 parts (1.0 moles) of bisphenoxyethanol fluorene (BPEF) as a polyol component (A2), and 0.01 parts of tetrabutyl titanate as a catalyst were charged, and the temperature was gradually increased to an internal temperature of 250 ° C., and an esterification reaction was carried out over 4 hours. Subsequently, after cooling to 200 ° C., 11.3 parts (0.36 moles) of pyromellitic anhydride (PMAn) was charged as a carboxylic acid anhydride, and the reaction was carried out at 170 ° C. for 2 hours while dissolving, to produce a polyester resin (A'-2).
The resulting polyester resin (A'-2) had a weight average molecular weight of 2,000, a glass transition temperature of 173°C, an acid value of 32 mgKOH/g and a refractive index of 1.652.

<Hydrolysis Inhibitor (B)>
The following was prepared as the hydrolysis inhibitor (B).
(B-1) Carbodiimide group-containing hydrolysis inhibitor (manufactured by Nisshinbo Chemical Inc.; product name "Carbodilite V-07")

<Crosslinking Agent (C)>
The following was prepared as the crosslinking agent (C).
(C-1) Trimethylolpropane/tolylene diisocyanate adduct (manufactured by Nippon Polyurethane Industry Co., Ltd.; product name "Coronate L55E")

<Plasticizer (D)>
The following plasticizers were prepared:
(D-1) Trioctyl trimellitate (TOTM)
(D-2) Dioctyl phthalate (DOP)

<Inorganic oxide (E)>
The following was prepared as the inorganic oxide (E).
(E-1) AX-ZP-158-A (zirconium oxide dispersion liquid manufactured by Nippon Shokubai Co., Ltd.)
(E-2) NANON5 ZR-010 (zirconium oxide dispersion liquid manufactured by Solar Co., Ltd.)
(E-3) ZIRCOSTAR ZP-153 (zirconium oxide dispersion liquid manufactured by Nippon Shokubai Co., Ltd.)

<Catalyst>
The following catalysts were prepared:
・Dibutyltin dilaurate (DBTL)

<Antioxidants>
The following antioxidants were prepared:
- Hindered phenol-based antioxidant/Irganox 1010 (manufactured by BASF)

(Examples 1 to 14, Comparative Examples 1 to 3)
The polyester resin obtained above was diluted with ethyl acetate to a solid content concentration of 50%, and each component shown in Table 1 was blended with 200 parts of this polyester resin solution (solid content 100 parts) and stirred and mixed to obtain a polyester pressure-sensitive adhesive composition.
The obtained polyester-based pressure-sensitive adhesive composition was evaluated as follows.

<Production of adhesive sheets with release films>
Each of the polyester-based pressure-sensitive adhesive compositions obtained in Examples 1 to 14 and Comparative Examples 1 to 3 was applied onto a 50 μm-thick polyethylene terephthalate (PET) film using an applicator and dried at 100° C. for 3 minutes to form a pressure-sensitive adhesive composition layer (thickness 25 μm). Next, the surface of the obtained pressure-sensitive adhesive composition layer was covered with a release-treated PET film (release film) and subjected to aging treatment at 40° C. for 10 days to obtain a pressure-sensitive adhesive sheet with a release film.

The gel fraction, adhesive strength, and optical performance (refractive index, transparency) of this adhesive sheet with release film were evaluated as follows. The results are summarized in Table 1.

[Gel fraction (%)]
The release film was peeled off from the pressure-sensitive adhesive sheet with release film obtained above, wrapped in a 200-mesh SUS wire netting, and immersed in toluene at 23°C for 24 hours, and the weight percentage of the insoluble pressure-sensitive adhesive component remaining in the wire netting was calculated as the gel fraction, excluding the weight of the substrate.

[Adhesive strength (N/25 mm)]
The release film was peeled off from the pressure-sensitive adhesive sheet with release film obtained above, and the sheet was attached to a polycarbonate plate by rolling it back and forth twice with a 2 kg roller, and then autoclaved at 50°C and 0.5 MPa for 20 minutes. After 24 hours, the 180° peel strength (N/25 mm) was measured at a peel speed of 300 mm/min to determine the adhesive strength. An autograph (Shimadzu Corporation's "Autograph AGS-H 500N") was used to measure the adhesive strength.
The evaluation criteria for adhesive strength are as follows.
(Evaluation criteria)
○: 20N/25mm or more ×: Less than 20N/25mm

[Refractive index]
The adhesive sheet with release film obtained above was left to stand at 23° C. for 3 hours, the release film was peeled off, and the refractive index of the adhesive layer side was measured at D line (589 nm) using an Abbe refraction system DR-M4 manufactured by Atago Co., Ltd.

[transparency]
The transparency of the pressure-sensitive adhesive sheet with release film obtained above was visually observed.
○: transparent △: slightly cloudy ×: cloudy

From the above results, in Examples 1 to 14, adhesives with excellent adhesive strength and very high refractive index were obtained, whereas Comparative Examples 1 and 2 had low refractive indexes, and Comparative Example 3 had too high a glass transition point, so although it had a high refractive index, it did not obtain adhesive properties and was not satisfactory in both refractive index and adhesive strength.

In Examples 10 to 14 above, the initial adhesive strength was also evaluated as follows. The results are shown in Table 2.

[Initial adhesive strength (N/25 mm)]
The release film was peeled off from the pressure-sensitive adhesive sheet with release film obtained above, and the sheet was applied to a glass plate adherend by rolling it back and forth twice with a 2 kg roller under an environment of 23°C and 50% RH. After 30 minutes, the 180° peel strength (N/25 mm) was measured at a peel speed of 300 mm/min to determine the adhesive strength. An autograph (Shimadzu Corporation's "Autograph AGS-H 500N") was used to measure the adhesive strength. The evaluation criteria for adhesive strength are as follows.
(Evaluation criteria)
○: 5N/25mm or more △: 0.5N/25mm or more, less than 5N/25mm ×: less than 0.1N/25mm

The above results show that in Examples 10 to 14, the incorporation of inorganic oxide not only improved the refractive index, but also provided good initial adhesive strength.

(Reference Examples 1 to 6)
The polyester resin obtained above was diluted with ethyl acetate to a solid content concentration of 50%, and inorganic oxides were added to 200 parts of this polyester resin solution (solid content 100 parts) as shown in Table 3, and the mixture was stirred and mixed to obtain a polyester resin composition.
The resulting polyester resin composition was evaluated for refractive index and transparency as described below. The results are shown in Table 3.

[Refractive index]
The refractive index of the polyester resin composition obtained above was measured at D line (589 nm) using an Abbe refraction system DR-M4 manufactured by Atago Co., Ltd.

[transparency]
The transparency of the polyester resin composition obtained above was visually observed.
○: transparent △: slightly cloudy ×: cloudy

From the above results, in Reference Examples 1 to 5, by incorporating a sulfonate group-containing dicarboxylic acid as a copolymerization component of the polyester resin when incorporating an inorganic oxide, compatibility as well as refractive index was good, and transparency was excellent. Reference Example 6 had a good refractive index due to the incorporation of an inorganic oxide, but since it did not incorporate a sulfonate group-containing dicarboxylic acid as a copolymerization component, transparency was slightly inferior.

The polyester-based adhesive of the present invention can be used as an adhesive for electronic components, an adhesive for bonding optical components, an adhesive for soft polyvinyl chloride, etc. In particular, the polyester-based adhesive of the present invention has excellent adhesion to adherends, particularly polycarbonate materials, and has a high refractive index, so it can be suitably used for bonding optical components.

Claims (9)

A polyester-based pressure-sensitive adhesive comprising a pressure-sensitive adhesive composition [I] containing a polyester-based resin (A) and a crosslinking agent (C) ,
The polyester-based resin (A) is a polyester-based resin (A) obtained by copolymerizing a copolymerization component consisting of a polyvalent carboxylic acid (A1) and a polyhydric alcohol (A2),
The glass transition temperature of the polyester resin (A) is −40 to 40° C.,
The polycarboxylic acid (A1) contains an aliphatic polycarboxylic acid,
At least one of the polyvalent carboxylic acids (A1) and the polyhydric alcohol (A2) contains an aromatic compound,
the ratio of the polyester resin (A) to the plasticizer (C) is such that the amount of the reactive group contained in the crosslinking agent (C) is 0.2 to 10 equivalents per equivalent of the hydroxyl group and/or carboxyl group contained in the polyester resin (A);
A polyester-based pressure-sensitive adhesive characterized by having an adhesive strength (α) of 10 to 200 N/25 mm and a refractive index of 1.530 to 1.650.
Adhesive strength (α): When a polyester-based adhesive is formed on a substrate to form an adhesive sheet, the sheet is attached to a polycarbonate plate adherend and autoclaved at 50°C and 0.5 MPa for 20 minutes. After 24 hours, the sheet exhibits a 180° peel strength (N/25 mm) at a peel speed of 300 mm/min against the adherend. The polyester-based adhesive according to claim 1, characterized in that the adhesive composition [I] contains a polyester-based resin (A), a hydrolysis inhibitor (B) and a crosslinking agent (C), and the polyester-based resin (A) contains a structural portion derived from an aliphatic polyvalent carboxylic acid and a structural portion derived from an aromatic polyhydric alcohol. The polyester-based adhesive according to claim 1 or 2, characterized in that the adhesive composition [I] further contains a plasticizer (D). 4. The polyester-based pressure-sensitive adhesive according to claim 1 , wherein the polyhydric alcohol (A2) which is a constituent raw material of the polyester-based resin (A) contains an aromatic polyhydric alcohol. 5. The polyester-based pressure-sensitive adhesive according to claim 1 , wherein the polyester-based resin (A) has a weight average molecular weight of 8,000 to 200,000. 3. The polyester-based pressure-sensitive adhesive according to claim 2 , wherein the content of the hydrolysis inhibitor (B) is 0.01 to 10 parts by weight based on 100 parts by weight of the polyester-based resin (A). 4. The polyester-based pressure-sensitive adhesive according to claim 3 , wherein the content of the plasticizer (D) is 2 to 30 parts by weight based on 100 parts by weight of the polyester-based resin (A). The polyester-based pressure-sensitive adhesive according to any one of claims 1 to 7 , further comprising an inorganic oxide (E). 9. An adhesive sheet for optical members, comprising the polyester-based adhesive according to claim 1 formed on a substrate.