TWI859668B - Adhesive sheet and method for manufacturing the same, and image display device - Google Patents

Abstract

The adhesive sheet (5) of the present invention has a haze of 1% or less, an adhesion to glass of 2.0 N/10 mm or more, a glass transition temperature of -3°C or less, and a shear storage elastic modulus of 0.16 MPa or more at 25°C. The adhesive sheet (5) is obtained, for example, by coating a composition comprising an acrylic monomer and/or a partial polymer thereof and urethane (meth)acrylate in a layer on a substrate and then irradiating the composition with active light for photocuring.

Description

Adhesive sheet and method for manufacturing the same, and image display device

The present invention relates to an adhesive sheet and a manufacturing method thereof. Furthermore, the present invention relates to an image display device using the adhesive sheet.

Liquid crystal display devices or organic EL display devices are widely used as various image display devices such as mobile phones, smart phones, car navigation devices, computer monitors, and televisions. In order to prevent damage to the image display panel caused by impact from the outer surface, a transparent front plate such as a transparent resin plate or a glass plate (also called a "cover window" etc.) is sometimes provided on the viewing side of the image display panel. In addition, in recent years, components having a touch panel on the viewing side of the image display panel have become popular.

As a method of disposing a front transparent component such as a front transparent plate or a touch panel in front of an image display panel, an "interlayer filling structure" is proposed in which the image display panel and the front transparent component are bonded together via an adhesive sheet. Sometimes an adhesive sheet is provided between the touch panel and the front transparent plate. In the interlayer filling structure, since the gaps between the components are filled with an adhesive, the refractive index difference at the interface is reduced, thereby suppressing the reduction in visibility due to reflection or scattering. In addition, in the interlayer filling structure, the components are bonded and fixed by the adhesive sheet, so there is an advantage that it is less likely to cause the front transparent component to be peeled off due to impact such as falling, compared with the case where the front transparent component is only fixed by the housing. In particular, by using an adhesive sheet with a thicker thickness, there is a tendency to improve impact resistance. Since a thicker adhesive sheet can be formed with a uniform thickness, the interlayer filling adhesive sheet is widely used in solvent-free photocurable adhesives (for example, see Patent Document 1 and Patent Document 2). [Prior Art Document] [Patent Document]

[Patent document 1] Japanese Patent Publication No. 2014-125524 [Patent document 2] International Publication No. 2013/161666

[The problem the invention is trying to solve]

Previously, the size of the front transparent member such as the cover window is larger than the display panel, and the front transparent member and the housing are bonded together using adhesive tape or the like in the area outside the outer periphery of the display panel. That is, if the front transparent member is bonded to the housing, it is fixed by bonding to the surface of the display panel using an adhesive sheet for interlayer filling.

In recent years, with smartphones and other mobile devices as the center, the display device has been promoted to have narrow edges or no borders. With narrow edges or no borders, the size of the display panel 10 is equal to or larger than the size of the front transparent component 7. In this structure, the housing 9 and the front transparent component 7 cannot be fixed by adhesive tapes, etc., and the front transparent component 7 must be fixed only by the interlayer filling adhesive sheet 5 (refer to Figure 2). As a result, the interlayer filling adhesive sheet requires a higher bonding force and is required not to be peeled off due to impacts such as falling in a wide temperature range.

Furthermore, when the size of the display panel is equal to or larger than the size of the front transparent component 7, a resin material is sometimes used to seal the gap 90 between the housing 9 and the front transparent component 7. For example, a molten resin material is flowed into the gap 90, cooled to room temperature, and the resin is solidified, thereby sealing with the resin material. If a high-temperature resin flows into the gap 90, the front transparent component 7, the housing 9, and the adhesive sheet 5 become high temperatures near the gap 90 and are cooled when the resin solidifies. When dimensional strain occurs in the front transparent component and the housing due to such temperature changes, the adhesive sheet 5 is also required to have bonding durability against strain stress so as not to cause peeling between the adherends.

The adhesive sheet for interlayer filling disclosed in Patent Document 1 and the like lacks low-temperature adhesion or impact resistance due to its high glass transition temperature. On the other hand, the adhesive sheet with low glass temperature disclosed in Patent Document 2 has low adhesion durability against strain stress, and it is difficult to have both low-temperature impact resistance and durability against strain stress during heating and cooling of resin seals.

In view of the above situation, the purpose of the present invention is to provide an adhesive sheet with excellent impact resistance and bonding durability against strain stress in a wide temperature range. [Technical means for solving the problem]

The present invention relates to an adhesive sheet formed of an adhesive agent containing a base polymer in a sheet shape. The haze of the adhesive sheet is 1% or less. The glass transition temperature of the adhesive sheet is preferably -3°C or less. The shear storage elastic modulus _{G'25 °C} of the adhesive sheet at a temperature of 25°C is preferably 0.16 MPa or more. The peak value of the loss tangent of the adhesive sheet is preferably 1.5 or more.

As the base polymer contained in the adhesive sheet, for example, an acrylic polymer chain crosslinked by a urethane chain segment is used. In order to meet the above-mentioned various properties, the content of the urethane chain segment is preferably 3 to 30 parts by weight relative to 100 parts by weight of the acrylic polymer chain. The weight average molecular weight of the urethane chain segment is preferably 4,000 to 50,000.

The base polymer in which the acrylic polymer chain is crosslinked by the urethane chain segments is obtained, for example, by copolymerizing the monomer component constituting the acrylic polymer chain with a urethane (meth)acrylate having (meth)acrylic groups at at least two terminals. As the multifunctional urethane (meth)acrylate, a urethane di(meth)acrylate having (meth)acrylic groups at both terminals is preferred.

The weight average molecular weight of the urethane (meth) acrylate is preferably 4000 to 50000. The glass transition temperature of the urethane (meth) acrylate is preferably below 0°C.

The adhesive sheet comprising a base polymer in which acrylic polymer chains are cross-linked by urethane chain segments is obtained, for example, by coating a composition comprising acrylic monomers and/or partial polymers thereof and urethane (meth) acrylate in a layer on a substrate and then irradiating the composition with active light for photocuring. The adhesive composition preferably contains 3 to 30 parts by weight of urethane (meth) acrylate relative to 100 parts by weight of the total of acrylic monomers and partial polymers thereof.

The adhesive sheet of the present invention is used, for example, for bonding transparent components in an image display device in which the transparent components are arranged on the visual side surface. For example, the image display device is formed by fixing the front transparent component to the visual side surface of the image display panel via the above-mentioned adhesive sheet. [Effect of the invention]

The adhesive sheet of the present invention has a lower glass transition temperature and a larger shear storage elastic modulus, so it can have both impact resistance such as falling and bonding durability against strain stress in a wide temperature range. The adhesive sheet of the present invention is used to bond image display devices such as cover windows on the viewing side surface, and has excellent bonding reliability, and can also correspond to narrow edges or no frame.

Fig. 1 is a diagram showing an adhesive sheet in which release films 1 and 2 are temporarily attached to both sides of an adhesive sheet 5. Fig. 2 is a cross-sectional view showing an example of a configuration of an image display device in which a front transparent plate 7 is fixed using an adhesive sheet.

[Physical properties of adhesive sheet] The adhesive sheet of the present invention is formed of an adhesive in a sheet shape. The adhesive sheet is a transparent adhesive sheet with a haze of 1.0% or less. The adhesive sheet preferably has a shear storage modulus _{G'25 ℃} at ₂₅ ℃ of 0.16 MPa or more. By having the G'25 _℃ of the adhesive sheet be 0.16 MPa or more, the bonding reliability is improved. From the viewpoint of improving the bonding reliability at high temperatures, the shear storage modulus G'80 _℃ of the adhesive sheet at ₈₀ ℃ is preferably 0.11 MPa or more.

On the other hand, from the viewpoint of making the adhesive sheet have appropriate viscosity and ensuring wettability, the G' _{25 ℃} of the adhesive sheet is preferably 1 MPa or less, more preferably 0.5 MPa or less, and further preferably 0.4 MPa or less. From the same viewpoint, the G' _{80 ℃} of the adhesive sheet is preferably 0.6 MPa or less, more preferably 0.4 MPa or less, and further preferably 0.3 MPa or less.

The glass transition temperature of the adhesive sheet is preferably below -3°C. The glass transition temperature of the adhesive sheet is preferably above -20°C, more preferably above -15°C, and further preferably above -13°C. When the glass transition temperature is within the above range, the adhesive sheet has appropriate viscosity even in a low temperature region, and thus tends to have excellent impact resistance.

The peak value of the loss tangent tanδ of the adhesive sheet is preferably 1.5 or more, more preferably 1.6 or more, and further preferably 1.7 or more. An adhesive sheet having a larger peak value of tanδ tends to have greater adhesive behavior and excellent impact resistance.

The shear storage modulus G', glass transition temperature, and tanδ peak value of the adhesive sheet are obtained by viscoelastic measurement at a frequency of 1 Hz. The glass transition temperature is the temperature (peak temperature) at which tanδ is extremely large. Tanδ is the ratio G"/G' of the storage modulus G' and the loss modulus G". The storage modulus G' is equivalent to the part stored as elastic energy when the material is deformed, and is an indicator of the degree of hardness. The larger the storage modulus of the adhesive sheet, the higher the adhesion retention force and the tendency to suppress peeling caused by strain. The loss modulus G" is equivalent to the energy lost by internal friction when the material is deformed, indicating the degree of viscosity. The larger the tanδ, the stronger the viscosity tends to be, the deformation behavior becomes liquid, and the rebound energy tends to decrease.

The upper limit of the peak value of tan δ of the adhesive sheet is not particularly limited, but is generally 3.0 or less. From the viewpoint of adhesive retention, the peak value of tan δ is preferably 2.7 or less, and more preferably 2.5 or less.

The adhesion of the adhesive sheet is preferably 2 N/10 mm or more, more preferably 2.5 N/10 mm or more, and further preferably 3 N/10 mm or more. When the adhesion of the adhesive sheet is within the above range, the adhesive sheet can be prevented from peeling off from the adherend when stress caused by strain or impact caused by falling or the like occurs. The adhesion is obtained by using a glass plate as the adherend and performing a peeling test at a tensile speed of 60 mm/min and a peeling angle of 180°. Unless otherwise specified, the adhesion is a measured value at 25°C.

The adhesive strength of the adhesive sheet at 65° C. is preferably 1 N/10 mm or more, more preferably 1.5 N/10 mm or more, and further preferably 2 N/10 mm or more.

There is no particular limitation on the thickness of the adhesive sheet, and it can be set according to the type or shape of the adherend. When a component with a printed step is set as the adherend, it is preferred that the thickness of the adhesive sheet is greater than the thickness of the printed step. The thickness of the adhesive sheet used for bonding the front transparent plate (cover window) is preferably 30 μm or more, more preferably 40 μm or more, and further preferably 50 μm or more. By increasing the thickness of the adhesive sheet, there is a tendency for the step absorption and impact resistance to improve. There is no particular limitation on the upper limit of the thickness of the adhesive sheet. From the perspective of the productivity of the adhesive sheet, it is preferably 500 μm or less, more preferably 300 μm or less, and further preferably 250 μm or less.

[Composition of Adhesive] As long as the adhesive sheet of the present invention satisfies the above-mentioned characteristics, the composition of the adhesive is not particularly limited, and polymers such as acrylic polymers, silicone polymers, polyesters, polyurethanes, polyamides, polyvinyl ethers, vinyl acetate/vinyl chloride copolymers, modified polyolefins, epoxy polymers, fluorine polymers, natural rubbers, synthetic rubbers, and other rubber-based polymers can be appropriately selected for use.

In particular, acrylic adhesives containing acrylic polymers as base polymers are preferably used because of their excellent optical transparency, moderate adhesive properties such as wettability, cohesion and adhesion, and excellent weather resistance and heat resistance. Among them, acrylic base polymers having a structure in which acrylic polymer chains are cross-linked by urethane chain segments are preferred.

[Base polymer] By crosslinking the acrylic polymer chain with the urethane chain segment, a higher bonding strength can be exerted at a low glass transition temperature. The base polymer preferably has a urethane chain segment content of 3 parts by weight or more relative to 100 parts by weight of the acrylic polymer chain.

If the amount of urethane chain segments is excessively increased, the viscosity of the adhesive may decrease and the impact resistance may decrease as the crosslinking density increases. Also, if the amount of urethane chain segments is excessively increased, the transparency of the adhesive sheet may decrease and the haze may increase. Therefore, the amount of urethane chain segments in the base polymer is preferably 30 parts by weight or less, and more preferably 25 parts by weight or less, relative to 100 parts by weight of the acrylic polymer chain.

<Acrylic polymer chain> The acrylic polymer chain contains (meth)acrylic acid alkyl ester as the main monomer component. In this specification, "(meth)acrylic acid" means acrylic acid and/or methacrylic acid.

As the alkyl (meth)acrylate, it is preferred to use an alkyl (meth)acrylate having an alkyl group with a carbon number of 1 to 20. The alkyl (meth)acrylate may have a branched or cyclic alkyl group.

Specific examples of the alkyl (meth)acrylate having a chain alkyl group include methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, isobutyl (meth)acrylate, sec-butyl (meth)acrylate, tert-butyl (meth)acrylate, pentyl (meth)acrylate, isopentyl (meth)acrylate, neopentyl (meth)acrylate, hexyl (meth)acrylate, heptyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, octyl (meth)acrylate, isooctyl (meth)acrylate, oct ... (meth)acrylate, octadecyl (meth)acrylate, isooctadecyl (meth)acrylate, undecyl (meth)acrylate, dodecyl (meth)acrylate, isotridecyl (meth)acrylate, tetradecyl (meth)acrylate, isotetradecyl (meth)acrylate, pentadecyl (meth)acrylate, hexadecyl (meth)acrylate, heptadecyl (meth)acrylate, octadecyl (meth)acrylate, isooctadecyl (meth)acrylate, nonadecyl (meth)acrylate,

Specific examples of the alkyl (meth)acrylate having an alicyclic alkyl group include: cycloalkyl (meth)acrylates such as cyclopentyl (meth)acrylate, cyclohexyl (meth)acrylate, cycloheptyl (meth)acrylate, and cyclooctyl (meth)acrylate; (meth)acrylates having a dicyclic aliphatic hydrocarbon ring such as isobutyl (meth)acrylate; and (meth)acrylates having a tricyclic or more aliphatic hydrocarbon ring such as dicyclopentyl (meth)acrylate, dicyclopentyloxyethyl (meth)acrylate, tricyclopentyl (meth)acrylate, 1-adamantyl (meth)acrylate, 2-methyl-2-adamantyl (meth)acrylate, and 2-ethyl-2-adamantyl (meth)acrylate.

The amount of the alkyl (meth)acrylate is 40% by weight or more, more preferably 50% by weight or more, and further preferably 60% by weight or more, relative to the total amount of the monomer components constituting the acrylic polymer chain. From the viewpoint of making the glass transition temperature (Tg) of the polymer chain within an appropriate range, the amount of the alkyl (meth)acrylate having a carbon 4-10 chain alkyl group in the acrylic polymer chain is preferably 30% by weight or more, more preferably 40% by weight or more, and further preferably 45% by weight or more, relative to the total amount of the monomer components constituting the acrylic polymer chain. Furthermore, the so-called monomer components constituting the acrylic polymer chain refer to those excluding the urethane (meth)acrylate and the like constituting the urethane chain segment.

The acrylic polymer chain may contain a hydroxyl-containing monomer or a carboxyl-containing monomer as a constituent monomer component. The acrylic polymer chain has a hydroxyl-containing monomer as a constituent monomer component, which improves the transparency of the adhesive sheet and suppresses the tendency of whitening in a high temperature and high humidity environment.

As the monomer containing a hydroxyl group, there can be listed (meth)acrylates such as 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, 6-hydroxyhexyl (meth)acrylate, 8-hydroxyoctyl (meth)acrylate, 10-hydroxydecyl (meth)acrylate, 12-hydroxylauryl (meth)acrylate, or (meth)acrylate (4-hydroxymethylcyclohexyl)-methyl ester. Among them, the acrylic polymer chain is preferably a (meth)acrylate containing a hydroxyalkyl group having 4 to 8 carbon atoms as a constituent monomer component from the viewpoint of having a higher compatibility with the urethane chain segment and improving the transparency of the adhesive sheet.

The amount of the hydroxyl-containing monomer is preferably 1 to 35 wt %, more preferably 3 to 30 wt %, and even more preferably 5 to 25 wt % relative to the total amount of monomer components constituting the acrylic polymer chain.

Examples of carboxyl group-containing monomers include acrylic acid monomers such as (meth)acrylic acid, carboxyethyl (meth)acrylate, and carboxypentyl (meth)acrylate, and itaconic acid, maleic acid, fumaric acid, and crotonic acid.

The acrylic polymer chain may contain nitrogen-containing monomers as constituent monomer components. Examples of nitrogen-containing monomers include vinyl monomers such as N-vinylpyrrolidone, methylvinylpyrrolidone, vinylpyridine, vinylpiperidone, vinylpyrimidine, vinylpiperidone, vinylpyrrolidone, vinylpyrrole, vinylimidazole, vinyloxazole, vinylmorpholine, (meth)acryloylmorpholine, N-vinylcarboxamides, N-vinylcaprolactam, or cyano-containing acrylic monomers such as acrylonitrile and methacrylonitrile.

The acrylic polymer chain has a tendency to increase the cohesive force of the adhesive and improve the adhesion retention at high temperatures by containing highly polar monomers such as hydroxyl-containing monomers and carboxyl-containing monomers as constituent monomer components. On the other hand, if the content of highly polar monomers is too large, the glass transition temperature may increase, and the adhesion or impact resistance at low temperatures may decrease. Therefore, the amount of highly polar monomers (the total of hydroxyl-containing monomers, carboxyl-containing monomers, and nitrogen-containing monomers) relative to the total amount of monomer components constituting the acrylic polymer chain is preferably 3 to 40% by weight, more preferably 5 to 35% by weight, and further preferably 10 to 30% by weight. Furthermore, the amount of the nitrogen-containing monomer is preferably 1 to 25 wt %, more preferably 2 to 20 wt %, and even more preferably 3 to 15 wt % relative to the total amount of monomer components constituting the acrylic polymer chain.

The acrylic polymer chain may include monomers containing anhydride groups, caprolactone adducts of (meth)acrylic acid, monomers containing sulfonic acid groups, monomers containing phosphoric acid groups, vinyl monomers such as vinyl acetate, vinyl propionate, styrene, and α-methylstyrene; acrylic monomers containing cyano groups such as acrylonitrile and methacrylonitrile; monomers containing epoxy groups such as glycidyl (meth)acrylate; glycol acrylate monomers such as polyethylene glycol (meth)acrylate, polypropylene glycol (meth)acrylate, methoxyethylene glycol (meth)acrylate, and methoxypolypropylene glycol (meth)acrylate; acrylate monomers such as tetrahydrofuranmethyl (meth)acrylate, fluoro(meth)acrylate, polysilicone (meth)acrylate, or 2-methoxyethyl (meth)acrylate, etc., as monomer components other than the above.

The acrylic polymer chain may contain multifunctional monomers or oligomers. Multifunctional compounds are compounds that contain two or more polymerizable functional groups having unsaturated double bonds such as (meth)acryloyl or vinyl groups in one molecule. Examples of multifunctional compounds include polyethylene glycol di(meth)acrylate, polypropylene glycol di(meth)acrylate, polybutylene glycol di(meth)acrylate, bisphenol A ethylene oxide modified di(meth)acrylate, bisphenol A propylene oxide modified di(meth)acrylate, alkanediol di(meth)acrylate, tricyclodecanedimethanol di(meth)acrylate, ethoxylated isocyanuric acid triacrylate, pentaerythritol tri(meth)acrylate, pentaerythritol di(meth)acrylate, and 1,1,2-dimethoxy-1,1-diol-1,1-diol-2,1-diol-3,1-diol-4,1-diol-5,1-diol-6,1-diol-7,1-diol-8,1-diol-9,1-diol-10,1-diol-11,1-diol-12,1-diol-13,1-diol-14,1-diol-15,1-diol-16,1-diol-17,1-diol-18,1-diol-19,1-diol-20,1-diol-19,1-diol-21,1-diol-11 acrylate, trihydroxymethylpropane tri(meth)acrylate, di-trihydroxymethylpropane tetra(meth)acrylate, ethoxylated pentaerythritol tetra(meth)acrylate, pentaerythritol tetra(meth)acrylate, dipentaerythritol poly(meth)acrylate, dipentaerythritol hexa(meth)acrylate, neopentyl glycol di(meth)acrylate, glycerol di(meth)acrylate, epoxy (meth)acrylate, butadiene (meth)acrylate, isoprene (meth)acrylate, and the like.

The acrylic polymer chain introduces a branch structure (cross-linked structure) into the polymer chain by including a multifunctional monomer as a constituent monomer component. As described later, in the adhesive of the present invention, a cross-linked structure is introduced into the acrylic polymer chain by a carbamate-based chain segment. If the amount of cross-linked structure introduced is increased by using a multifunctional monomer component other than the carbamate-based chain segment, the low-temperature adhesion of the adhesive sometimes decreases. Therefore, the amount of the multifunctional compound (except acrylic carbamate) relative to the total amount of the monomer components constituting the acrylic polymer chain is preferably 3% by weight or less, more preferably 1% by weight or less, further preferably 0.5% by weight or less, and particularly preferably 0.3% by weight or less.

Among the above-mentioned monomer components, the acrylic polymer chain preferably has the highest content of (meth) alkyl acrylate. The type of the monomer (main monomer) with the highest content among the constituent monomers of the acrylic polymer chain can easily influence the characteristics of the adhesive sheet. For example, when the main monomer of the acrylic polymer chain is a (meth) alkyl acrylate having a chain alkyl group with a carbon number of 6 or less, the peak value of tanδ increases, and the impact resistance tends to be improved. In particular, when a _C4 alkyl acrylate such as butyl acrylate is the main monomer, the peak value of tanδ tends to increase. The amount of the (meth) alkyl acrylate having a chain alkyl group with a carbon number of 6 or less is preferably 30 to 80% by weight, more preferably 35 to 75% by weight, and further preferably 40 to 70% by weight, relative to the total amount of the monomer components constituting the acrylic polymer chain. In particular, it is preferred that the content of butyl acrylate as a constituent monomer component is within the above range.

The theoretical Tg of the acrylic polymer chain is preferably -50°C or higher. The theoretical Tg of the acrylic polymer chain is preferably -10°C or lower, more preferably -20°C or lower, and further preferably -25°C or lower. The theoretical Tg is calculated from the glass transition temperature Tg _i of the homopolymer of the monomer components constituting the acrylic polymer chain and the weight fraction _Wi of each monomer component using the following Fox formula. 1/Tg=Σ( _Wi /Tg _i )

Tg is the glass transition temperature of the polymer chain (unit: K), _Wi is the weight fraction of monomer component i constituting the chain segment (copolymerization ratio based on weight), and Tg _i is the glass transition temperature of the homopolymer of monomer component i (unit: K). As the glass transition temperature of the homopolymer, the value described in the Polymer Handbook, 3rd edition (John Wiley & Sons, Inc., 1989) can be used. For the Tg of the homopolymer of the monomer not described in the above literature, the peak temperature of the loss tangent (tanδ) measured by dynamic viscoelasticity can be used.

<Urethane chain segment> Urethane chain segment is a molecular chain with urethane bonds. Both ends of the urethane chain segment are covalently bonded to the acrylic polymer chain to introduce a cross-linking structure into the acrylic polymer chain.

(Structure of urethane chain segment) Urethane chain segment is typically a polyurethane chain formed by reacting diol with diisocyanate. From the perspective of obtaining an adhesive having both low-temperature adhesion and high-temperature retention, the molecular weight of the polyurethane chain in the urethane chain segment is preferably 4000 to 50000, more preferably 4500 to 40000, and further preferably 5000 to 30000.

The greater the molecular weight of the polyurethane chain in the urethane chain segment, the longer the distance between the crosslinking points of the acrylic polymer chain. In the case where the molecular weight of the polyurethane chain is too small and the distance between the crosslinking points is short, the storage elastic modulus increases with the increase in cohesion. As a result, the viscosity of the adhesive sheet decreases, tanδ decreases, and therefore there is a tendency for the impact resistance to decrease. In the case where the molecular weight of the polyurethane chain is too large and the distance between the crosslinking points is long, sometimes the storage elastic modulus is small and the adhesion retention is insufficient. When the molecular weight of the polyurethane chain is within the above range, the adhesive has appropriate cohesion, so it can have both impact resistance and adhesion retention.

The diols used for forming the polyurethane chain include low molecular weight diols such as ethylene glycol, diethylene glycol, propylene glycol, butanediol, and hexanediol; and high molecular weight polyols such as polyester polyols, polyether polyols, polycarbonate polyols, acrylic polyols, epoxy polyols, and caprolactone polyols.

Polyether polyols are obtained by ring-opening addition polymerization of polyols and alkylene oxides. Examples of alkylene oxides include ethylene oxide, propylene oxide, butylene oxide, styrene oxide, tetrahydrofuran, etc. Examples of polyols include the above-mentioned diols, glycerol, trihydroxymethylpropane, etc.

Polyester polyol is obtained by reacting a polybasic acid with a polyol in a manner that the alcohol equivalent is in excess relative to the carboxylic acid equivalent. The polybasic acid component and the polyol component constituting the polyester polyol are preferably a combination of a dibasic acid and a diol.

Examples of the dibasic acid component include aromatic dicarboxylic acids such as phthalic acid, isophthalic acid, and terephthalic acid; alicyclic dicarboxylic acids such as hexahydrophthalic acid, tetrahydrophthalic acid, 1,3-cyclohexanedicarboxylic acid, and 1,4-cyclohexanedicarboxylic acid; aliphatic dicarboxylic acids such as oxalic acid, succinic acid, malonic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, decanedicarboxylic acid, dodecanedicarboxylic acid, and octadecanedicarboxylic acid; and anhydrides and lower alcohol esters of these dicarboxylic acids.

The diol component includes ethylene glycol, 1,2-propylene glycol, 1,3-propylene glycol, 1,3-butylene glycol, 1,4-butylene glycol, neopentyl glycol, pentylene glycol, 1,6-hexanediol, 1,8-octanediol, 1,10-decanediol, diethylene glycol, triethylene glycol, polyethylene glycol, dipropylene glycol, polypropylene glycol, 1,4-cyclohexanedimethanol, 1,4-cyclohexanediol, bisphenol A, bisphenol F, hydrogenated bisphenol A, hydrogenated bisphenol F, and the like.

Examples of polycarbonate polyols include polycarbonate polyols obtained by condensing a diol component with carbonyl chloride; polycarbonate polyols obtained by ester exchange condensation of a diol component with carbonic acid diesters such as dimethyl carbonate, diethyl carbonate, dipropyl carbonate, diisopropyl carbonate, dibutyl carbonate, ethyl butyl carbonate, ethylene carbonate, propylene carbonate, diphenyl carbonate, and dibenzyl carbonate; copolymer polycarbonate polyols obtained by using two or more polyol components; polycarbonate polyols obtained by esterifying the above polycarbonate polyols with a carboxyl group-containing compound; polycarbonate polyols; polycarbonate polyols obtained by subjecting the above-mentioned various polycarbonate polyols to an etherification reaction with a hydroxyl-containing compound; polycarbonate polyols obtained by subjecting the above-mentioned various polycarbonate polyols to an ester exchange reaction with an ester compound; polycarbonate polyols obtained by subjecting the above-mentioned various polycarbonate polyols to an ester exchange reaction with a hydroxyl-containing compound; polyester-based polycarbonate polyols obtained by subjecting the above-mentioned various polycarbonate polyols to a condensation reaction with a dicarboxylic acid compound; copolymerized polyether-based polycarbonate polyols obtained by copolymerizing the above-mentioned various polycarbonate polyols with an alkylene oxide, etc.

Polyacrylic polyol is obtained by copolymerizing (meth)acrylate with a monomer component having a hydroxyl group. Examples of monomers having a hydroxyl group include hydroxyalkyl (meth)acrylates such as 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, 2-hydroxybutyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, and 2-hydroxypentyl (meth)acrylate; (meth)acrylate monoesters of polyols such as glycerol and trihydroxymethylpropane; and N-hydroxymethyl (meth)acrylamide. Examples of (meth)acrylates include methyl (meth)acrylate, butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, and cyclohexyl (meth)acrylate.

Polyacrylic acid polyol may contain monomer components other than the above as copolymer components. Examples of copolymer monomer components other than the above include: unsaturated monocarboxylic acids such as (meth) acrylic acid; unsaturated dicarboxylic acids such as maleic acid and their anhydrides and mono- or diesters; unsaturated nitriles such as (meth) acrylonitrile; unsaturated amides such as (meth) acrylamide and N-hydroxymethyl (meth) acrylamide; vinyl esters such as vinyl acetate and vinyl propionate; vinyl ethers such as methyl vinyl ether; α-olefins such as ethylene and propylene; halogenated α,β-unsaturated aliphatic monomers such as vinyl chloride and vinylidene chloride; α,β-unsaturated aromatic monomers such as styrene and α-methylstyrene, etc.

The diisocyanate used in the formation of the polyurethane chain may be either an aromatic diisocyanate or an aliphatic diisocyanate. Examples of the aromatic diisocyanate include 1,5-naphthalene diisocyanate, 4,4'-diphenylmethane diisocyanate (MDI), 4,4'-diphenyldimethylmethane diisocyanate, tetramethyldiphenylmethane diisocyanate, 1,3-phenylene diisocyanate, 1,4-phenylene diisocyanate, 2-chloro-1,4-phenylene diisocyanate, 2,4-toluene diisocyanate, 2,6-toluene diisocyanate, xylylene diisocyanate, 4,4'-diphenyl ether diisocyanate, 4,4'-diphenyl sulfone diisocyanate, 4,4'-diphenyl sulfone diisocyanate, and 4,4'-biphenyl diisocyanate. Examples of the aliphatic diisocyanate include butane-1,4-diisocyanate, hexamethylene diisocyanate, 2,2,4-trimethylhexamethylene diisocyanate, 2,4,4-trimethylhexamethylene diisocyanate, cyclohexane-1,4-diisocyanate, isophorone diisocyanate, dicyclohexylmethane-4,4'-diisocyanate, 1,3-bis(isocyanatemethyl)cyclohexane, and methylcyclohexane diisocyanate.

As diisocyanates, derivatives of isocyanate compounds may also be used. Examples of derivatives of isocyanate compounds include dimers of polyisocyanates, trimers of isocyanates (isocyanurates), polymeric MDI, adducts with trihydroxymethylpropane, biuret modified products, allophanate modified products, urea modified products, etc. As diisocyanate components, urethane prepolymers having an isocyanate group at the end may be used.

(Introduction of cross-linked structure of acrylic polymer chain using carbamate chain segment) A cross-linked structure using carbamate chain segment can be introduced into acrylic polymer chain by using a compound having a functional group copolymerizable with a monomer component constituting acrylic polymer chain at the end of a polyurethane chain, or using a compound having a functional group reactive with a carboxyl group, a hydroxyl group, etc. contained in an acrylic polymer chain at the end of a polyurethane chain. Since it is easy to introduce cross-linking points uniformly into an acrylic polymer chain, and the acrylic polymer chain has excellent compatibility with the carbamate chain segment, it is preferable to use a carbamate di(meth)acrylate having (meth)acryloyl groups at both ends of a polyurethane chain to introduce a cross-linked structure using a carbamate chain segment. For example, the monomer components constituting the acrylic polymer chain can be copolymerized with urethane di(meth)acrylate to introduce a crosslinking structure into the acrylic polymer chain using the urethane chain segment.

Urethane di(meth)acrylate having (meth)acryl groups at both ends is obtained, for example, by using a (meth)acrylic compound having a hydroxyl group in addition to a diol component during the polymerization of a polyurethane. From the viewpoint of controlling the chain length (molecular weight) of the urethane chain segment, it is preferred to react a diol with a diisocyanate in an excess of isocyanate to synthesize an isocyanate-terminated polyurethane, and then add a (meth)acrylic compound having a hydroxyl group to react the terminal isocyanate group of the polyurethane with the hydroxyl group of the (meth)acrylic compound.

By reacting a polyol with a polyisocyanate compound in an excess amount of the polyisocyanate compound, a polyurethane chain having an isocyanate group at the terminal is obtained. To obtain an isocyanate-terminated polyurethane, a diol component and a diisocyanate component are preferably used in an NCO/OH (equivalent ratio) of 1.1 to 2.0, more preferably 1.15 to 1.5. The diol component and the diisocyanate component may be mixed in approximately equal amounts and reacted, and then the diisocyanate component may be added.

Examples of the (meth)acrylic acid compound having a hydroxyl group include hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate, hydroxybutyl (meth)acrylate, hydroxyhexyl (meth)acrylate, hydroxymethacrylamide, hydroxyethylacrylamide, and the like.

As the urethane (meth)acrylate, commercial products sold by Arakawa Chemical Industries, Ltd., Shin-Nakamura Chemical Industries, Ltd., Toagosei, Kyoeisha Chemical, Nippon Kayaku, Nippon Gosei Chemical Industries, Ltd., Negami Industries, Daicel Allnex, etc. can be used. The weight average molecular weight of the urethane (meth)acrylate is preferably 4,000 to 50,000, more preferably 4,500 to 40,000, and further preferably 5,000 to 30,000.

The glass transition temperature of urethane (meth) acrylate is preferably below 0°C, more preferably below -10°C, and further preferably below -20°C. By using a low Tg urethane (meth) acrylate, when a cross-linking structure is introduced through a urethane chain segment to improve the cohesive force of the base polymer, an adhesive having excellent low-temperature adhesion can be obtained. The lower limit of the glass transition temperature of urethane (meth) acrylate is not particularly limited, but from the viewpoint of obtaining an adhesive having excellent high-temperature retention, it is preferably above -100°C, more preferably above -90°C, and further preferably above -80°C.

When urethane (meth) acrylate is used, in the case where the acrylic polymer chain is cross-linked by the urethane chain segment, the glass transition temperature of the urethane chain segment of the base polymer is substantially equal to the glass transition temperature of the urethane (meth) acrylate.

<Preparation of base polymer> The polymer that uses a urethane chain segment to introduce a crosslinking structure into an acrylic polymer chain can be polymerized using various known methods. When urethane (meth) acrylate is used as a component of the urethane chain segment, the monomer component used to constitute the acrylic polymer chain can be copolymerized with urethane (meth) acrylate.

The amount of urethane (meth) acrylate used is preferably 3 to 30 parts by weight, more preferably 4 to 25 parts by weight, relative to 100 parts by weight of the monomer component used to constitute the acrylic polymer chain. By adjusting the amount of urethane (meth) acrylate used, a base polymer having a urethane chain segment content within the above range can be prepared. When the content of the urethane chain segment is too small, the adhesion retention of the adhesive sheet tends to decrease due to the decrease in the cohesiveness of the base polymer. When the content of the urethane chain segment is too large, the viscosity of the adhesive sheet tends to decrease and the impact resistance tends to decrease as the cohesiveness of the base polymer increases.

As a polymerization method for the base polymer, photopolymerization is preferred. Since the polymer can be prepared without using a solvent during photopolymerization, it is not necessary to dry and remove the solvent when forming the adhesive sheet, and an adhesive sheet with a larger thickness can be formed uniformly.

When preparing the base polymer, the total amount of the monomer components and the urethane (meth) acrylate used to introduce the crosslinking structure to constitute the acrylic polymer chain can be reacted at once, or the polymerization can be carried out in multiple stages. As a method of carrying out polymerization in multiple stages, it is preferred to polymerize the monofunctional monomers constituting the acrylic polymer chain to form a prepolymer composition (preliminary polymerization), add a polyfunctional compound such as urethane di(meth) acrylate to the slurry of the prepolymer composition, and polymerize the prepolymer composition and the polyfunctional monomer (formal polymerization). The prepolymer composition is a polymer containing a low degree of polymerization and a partial polymer of unreacted monomers.

By pre-polymerizing the components of the acrylic polymer, the branching points (cross-linking points) of polyfunctional compounds such as urethane di(meth)acrylate can be uniformly introduced into the acrylic polymer chain. Alternatively, a mixture of a low molecular weight polymer or a partial polymer and unpolymerized monomer components (adhesive composition) can be coated on a substrate and then formally polymerized on the substrate to form an adhesive sheet.

Since low-polymerization-degree compositions such as prepolymer compositions have low viscosity and excellent coating properties, by coating a mixture of the prepolymer composition and a multifunctional compound, i.e., an adhesive composition, on a substrate and then performing formal polymerization, the productivity of the adhesive sheet can be improved and the thickness of the adhesive sheet can be made uniform.

[Adhesive sheet] As described above, a low-polymerization degree prepolymer composition is prepared by preliminary polymerization, an adhesive composition to which a multifunctional compound is added to the prepolymer composition is applied in layers on a substrate, and the adhesive composition on the substrate is polymerized (formal polymerization) to obtain an adhesive sheet.

<Preliminary polymerization> The prepolymer composition can be prepared, for example, by polymerizing a composition of a monomer component constituting an acrylic polymer chain and a polymerization initiator. The prepolymer-forming composition can contain a polyfunctional compound (polyfunctional monomer or polyfunctional oligomer). For example, a portion of the polyfunctional compound that is a raw material for the polymer can be contained in the prepolymer-forming composition, and after the prepolymer is polymerized, the remaining portion of the polyfunctional compound can be added for formal polymerization.

The prepolymer-forming composition preferably contains a photopolymerization initiator. Examples of the photopolymerization initiator include benzoin ether-based photopolymerization initiators, acetophenone-based photopolymerization initiators, α-ketol-based photopolymerization initiators, aromatic sulfonyl chloride-based photopolymerization initiators, photoactive oxime-based photopolymerization initiators, benzoin-based photopolymerization initiators, benzyl-based photopolymerization initiators, benzophenone-based photopolymerization initiators, ketal-based photopolymerization initiators, thioxanthone-based photopolymerization initiators, and acylphosphine oxide-based photopolymerization initiators.

During polymerization, a chain transfer agent or a polymerization inhibitor (polymerization retardant) may be used for the purpose of molecular weight adjustment. Examples of the chain transfer agent include thiols such as α-butylglycerol, lauryl mercaptan, glycidyl mercaptan, butyl acetic acid, 2-butyl ethanol, thioglycolic acid, 2-ethylhexyl thioglycolate, 2,3-dibutyl-1-propanol, and α-methylstyrene dimer.

The prepolymer-forming composition may contain a chain transfer agent, etc., in addition to the monomer and the polymerization initiator, if necessary. The polymerization initiator or chain transfer agent used in the prepolymerization is not particularly limited, and for example, the above-mentioned photopolymerization initiator or chain transfer agent may be used.

The polymerization rate of the prepolymer is not particularly limited. From the perspective of the viscosity suitable for coating on the substrate, it is preferably 3 to 50% by weight, and more preferably 5 to 40% by weight. The polymerization rate of the prepolymer can be adjusted to the desired range by adjusting the type or amount of photopolymerization initiator, the irradiation intensity and irradiation time of active light such as UV light, etc. The polymerization rate of the prepolymer is calculated according to the following formula based on the weight before and after heating at 130°C for 3 hours. The polymerization rate of the adhesive sheet is also calculated by the same method. Polymerization rate (%) = weight after drying/weight before drying × 100

<Preparation of adhesive composition> The adhesive composition is prepared by mixing urethane (meth) acrylate, and the remaining part of the monomer component constituting the acrylic polymer chain, a polymerization initiator, a chain transfer agent, and other additives in the above-mentioned prepolymer composition. The adhesive composition preferably has a viscosity suitable for coating on a substrate (for example, about 0.5 to 20 Pa・s). The viscosity of the adhesive composition can be set to an appropriate range by adjusting the polymerization rate of the prepolymer, the amount of urethane (meth) acrylate added, the composition, molecular weight, and amount of other components (such as oligomers). Thickening additives can also be used for the purpose of adjusting the viscosity.

The polymerization initiator or chain transfer agent used in the main polymerization is not particularly limited, and for example, the above-mentioned photopolymerization initiator or chain transfer agent can be used. In the case where the polymerization initiator is not deactivated and remains in the prepolymer composition during the preliminary polymerization, the addition of the polymerization initiator used in the main polymerization can be omitted.

(Oligomer) The adhesive composition may include various oligomers for the purpose of adjusting the adhesion or viscosity of the adhesive sheet. As the oligomer, for example, one having a weight average molecular weight of about 1,000 to 30,000 is used. As the oligomer, acrylic oligomers are preferred because of their excellent compatibility with acrylic base polymers.

The acrylic oligomer contains an alkyl (meth)acrylate as a main monomer component. Among them, as a monomer component, preferably, an alkyl (meth)acrylate having a chain alkyl group (chain alkyl (meth)acrylate) and an alkyl (meth)acrylate having an alicyclic alkyl group (alicyclic alkyl (meth)acrylate) are included. Specific examples of the chain alkyl (meth)acrylate and the alicyclic alkyl (meth)acrylate as monomer components of the acrylic polymer chain are as exemplified above.

The glass transition temperature of the acrylic oligomer is preferably above 20°C, more preferably above 30°C, and further preferably above 40°C. By using a low-Tg base polymer with a crosslinked structure introduced by a urethane chain segment and a high-Tg acrylic oligomer together, the adhesive retention of the adhesive sheet tends to be improved. The upper limit of the glass transition temperature of the acrylic oligomer is not particularly limited, but is generally below 200°C, preferably below 180°C, and more preferably below 160°C. The glass transition temperature of the acrylic oligomer is calculated by the above-mentioned Fox formula.

Among the exemplified (meth)acrylic acid alkyl esters, methyl methacrylate is preferred as the (meth)acrylic acid chain alkyl ester due to its high glass transition temperature and excellent compatibility with the base polymer. Dicyclopentanyl acrylate, dicyclopentanyl methacrylate, cyclohexyl acrylate, and cyclohexyl methacrylate are preferred as the (meth)acrylic acid cycloalkyl ester. That is, the acrylic oligomer preferably contains one or more selected from the group consisting of dicyclopentanyl acrylate, dicyclopentanyl methacrylate, cyclohexyl acrylate, and cyclohexyl methacrylate, and methyl methacrylate as constituent monomer components.

The amount of the cyclic alkyl (meth)acrylate is preferably 10 to 90% by weight, more preferably 20 to 80% by weight, and further preferably 30 to 70% by weight relative to the total amount of the monomer components constituting the acrylic oligomer. The amount of the chain alkyl (meth)acrylate is preferably 10 to 90% by weight, more preferably 20 to 80% by weight, and further preferably 30 to 70% by weight relative to the total amount of the monomer components constituting the acrylic oligomer.

The weight average molecular weight of the acrylic oligomer is preferably 1000 to 30000, more preferably 1500 to 10000, and further preferably 2000 to 8000. By using an acrylic oligomer having a molecular weight within this range, the adhesion or adhesion retention of the adhesive tends to be improved.

Acrylic oligomers are obtained by polymerizing the above-mentioned monomer components by various polymerization methods. Various polymerization initiators can be used in the polymerization of acrylic oligomers. In addition, chain transfer agents can also be used for the purpose of adjusting the molecular weight.

When the adhesive composition contains an oligomer component such as an acrylic oligomer, its content is preferably 0.5 to 20 parts by weight, more preferably 1 to 15 parts by weight, and further preferably 2 to 10 parts by weight relative to 100 parts by weight of the above-mentioned base polymer. When the content of the oligomer in the adhesive composition is within the above range, there is a tendency to improve the adhesion at high temperature and the high temperature retention.

(Silane coupling agent) For the purpose of adjusting the bonding strength, a silane coupling agent can be added to the adhesive composition. When a silane coupling agent is added to the adhesive composition, the amount added is usually about 0.01 to 5.0 parts by weight, preferably about 0.03 to 2.0 parts by weight, relative to 100 parts by weight of the base polymer.

(Crosslinking agent) The base polymer may have a crosslinking structure other than the above-mentioned multifunctional compound as needed. By including a crosslinking agent in the adhesive composition, a crosslinking structure can be introduced into the base polymer. As a crosslinking agent, there can be listed compounds that react with functional groups such as hydroxyl or carboxyl groups contained in the polymer. As specific examples of the crosslinking agent, there can be listed: isocyanate crosslinking agents, epoxy crosslinking agents, oxazoline crosslinking agents, aziridine crosslinking agents, carbodiimide crosslinking agents, metal chelate crosslinking agents, etc.

If the amount of crosslinking structure other than the urethane chain segment is increased, the viscosity and impact resistance may decrease. Therefore, the amount of the crosslinking agent used is preferably 3 parts by weight or less, more preferably 2 parts by weight or less, and further preferably 1 part by weight or less, relative to 100 parts by weight of the base polymer.

(Other additives) In addition to the components listed above, the adhesive composition may include additives such as adhesive agents, plasticizers, softeners, anti-degradation agents, fillers, colorants, ultraviolet absorbers, antioxidants, surfactants, antistatic agents, etc.

<Coating and formal polymerization of adhesive composition> After coating the adhesive composition in layers on the substrate, photocuring is performed by irradiating active light. When performing photocuring, it is preferred to attach a cover sheet to the surface of the coating layer, and irradiate active light while sandwiching the adhesive composition between two sheets to prevent polymerization hindrance caused by oxygen.

Any appropriate substrate can be used as the base material and the cover sheet used to form the adhesive sheet. The base material and the cover sheet can be a release film having a release layer on the contact surface with the adhesive sheet.

As the film substrate of the release film, a film comprising various resin materials is used. Examples of the resin material include polyester resins such as polyethylene terephthalate and polyethylene naphthalate, acetate resins, polyether sulfone resins, polycarbonate resins, polyamide resins, polyimide resins, polyolefin resins, (meth)acrylic resins, polyvinyl chloride resins, polyvinylidene chloride resins, polystyrene resins, polyvinyl alcohol resins, polyarylate resins, polyphenylene sulfide resins, etc. Among them, polyester resins such as polyethylene terephthalate are particularly preferred. The thickness of the film substrate is preferably 10 to 200 μm, more preferably 25 to 150 μm. The material of the release layer includes: polysilicone release agent, fluorine release agent, long chain alkyl release agent, fatty amide release agent, etc. The thickness of the release layer is generally about 10 to 2000 nm.

As a method of applying the adhesive composition on the substrate, various methods are used, such as roller coating, contact roller coating, gravure coating, reverse coating, roller brush coating, spray coating, dip roller coating, rod coating, scraper coating, air knife coating, curtain coating, lip coating, die-mouth coating, etc.

The adhesive composition coated on the substrate in a layer is irradiated with active light to carry out formal polymerization. During the formal polymerization, the unreacted monomer components in the prepolymer composition react with the urethane (meth) acrylate to obtain a base polymer having a cross-linked structure in which the acrylic polymer chain is introduced by the urethane chain segment.

The active light can be selected according to the type of polymerizable components such as monomers or urethane (meth) acrylates, or the type of photopolymerization initiators, and generally ultraviolet light and/or short-wave visible light are used. The cumulative light amount of the irradiated light is preferably about 100 to 5000 mJ/ ^cm2 . The light source used for light irradiation is not particularly limited as long as it can irradiate the light of the wavelength range in which the photopolymerization initiator contained in the adhesive composition has sensitivity. It is preferred to use LED light sources, high-pressure mercury lamps, ultra-high-pressure mercury lamps, metal halogen lamps, xenon lamps, etc. The polymerization rate of the adhesive sheet after formal polymerization is preferably 97% or more, more preferably 98% or more, and further preferably 99% or more. To increase the polymerization rate, the adhesive sheet can be heated after light curing to volatilize residual monomers or unreacted polymerization initiators.

By laminating the release films 1 and 2 on the surface of the adhesive sheet 5, as shown in Fig. 1, an adhesive sheet with release films temporarily attached to both sides is obtained. The release film used as the base material or cover sheet when the adhesive sheet is formed can be directly used as the release films 1 and 2.

When release films 1 and 2 are provided on both sides of the adhesive sheet 5, the thickness of one release film 1 may be the same as or different from the thickness of the other release film 2. The peeling force when the adhesive sheet 5 is peeled off from the release film temporarily attached to one side may be the same as or different from the peeling force when the adhesive sheet 5 is peeled off from the release film temporarily attached to the other side. When the peeling forces of the two are different, the release film 2 (light peeling film) with a relatively smaller peeling force is peeled off from the adhesive sheet 5 first, and the first adherend is bonded thereto, while the release film 1 (heavy peeling film) with a relatively larger peeling force is peeled off, and the workability is excellent when bonding thereto is performed with the second adherend.

[Image display device] The adhesive sheet of the present invention can be used to bond various transparent components or opaque components. The type of the bonded body is not particularly limited, and can be exemplified by various resin materials, glass, metal, etc. The adhesive sheet of the present invention is suitable for bonding optical components such as image display devices due to its high transparency. In particular, the adhesive sheet of the present invention is suitable for bonding transparent components such as the front transparent plate or touch panel to the viewing side surface of the image display device due to its excellent bonding durability and impact resistance.

FIG2 is a cross-sectional view showing an example of a laminated structure of an image display device in which a front transparent plate 7 is bonded to a viewing side surface of an image display panel 10 via an adhesive sheet 5. The image display panel 10 has a polarizing plate 3 bonded to a viewing side surface of an image display unit 6 such as a liquid crystal unit or an organic EL unit via an adhesive sheet 4. The front transparent plate 7 may be provided with a step using a printed layer 76 or the like at the periphery of one surface of a transparent flat plate 71. For example, a transparent resin plate such as an acrylic resin or a polycarbonate resin, or a glass plate is used for the transparent plate 71. The transparent plate 71 may have a touch panel function. As the touch panel, a touch panel of any method such as a resistive film method, an electrostatic capacitor method, an optical method, an ultrasonic method, etc. is used.

The polarizing plate 3 and the front transparent plate 7 disposed on the surface of the image display panel 10 are bonded via the adhesive sheet 5. The bonding order is not limited, and the bonding of the adhesive sheet 5 of the image display panel 10 may be performed first, or the bonding of the adhesive sheet 5 of the front transparent plate 7 may be performed first. In addition, the bonding of the two may be performed simultaneously. From the viewpoint of the workability of bonding, it is preferred to peel off one release film (light-peel release film) 2, bond the exposed surface of the adhesive sheet 5 to the image display panel 10, and then peel off the other release film 1 (heavy-peel release film) and bond the exposed surface of the adhesive sheet to the front transparent plate 7.

After the adhesive sheet 5 and the front transparent plate 7 are bonded together, degassing may be performed to remove bubbles at the interface between the adhesive sheet 5 and the flat plate 71 of the front transparent plate 7, or near the non-flat portion such as the printed layer 76. As a degassing method, appropriate methods such as heating, pressurization, and decompression may be used. For example, bonding may be performed while suppressing the mixing of bubbles under decompression and heating, and then pressurization may be performed simultaneously with heating by autoclave treatment or the like in order to suppress delayed foaming. When degassing is performed by heating, the heating temperature is generally about 40 to 150°C. When pressurization is performed, the pressure is generally about 0.05 MPa to 2 MPa.

When there is a gap 90 between the housing 9 and the front transparent plate 7, it is preferable to fill the gap 90 with a resin material or the like for sealing. As described above, the adhesive sheet 5 has a large shear storage elastic modulus, so it has excellent bonding reliability in a wide temperature range. Therefore, when the temperature change during sealing by the resin material or the like generates stress and strain at the bonding interface of the adhesive sheet, it is also possible to suppress peeling under the bonding interface. In addition, the adhesive sheet 5 has a low glass transition temperature and a large peak value of tanδ, so it has excellent impact resistance in a wide temperature range, and it is difficult to produce peeling caused by impacts such as falling.

[Optical film with adhesive sheet] The adhesive sheet of the present invention can be used as an adhesive film for fixing the adhesive sheet to an optical film, etc., in addition to the form of temporarily sticking release films on both sides as shown in FIG1. For example, in the form shown in FIG3, a release film 1 is temporarily stuck on one side of the adhesive sheet 5, and a polarizing plate 3 is fixed on the other side of the adhesive sheet 5. In the form shown in FIG4, an adhesive sheet 4 is further provided on the polarizing plate 3, and a release film 2 is temporarily stuck thereon.

In this way, when the adhesive sheet is pre-bonded with an optical film such as a polarizing plate, the release film 1 temporarily bonded to the surface of the adhesive sheet 5 can be peeled off and bonded to the transparent component in front. Example

The present invention is described in more detail with the following embodiments and comparative examples, but the present invention is not limited to these embodiments.

[Preparation of acrylic oligomer] 60 parts by weight of dicyclopentyl methacrylate (DCPMA), 40 parts by weight of methyl methacrylate (MMA), 3.5 parts by weight of α-isobutylene glycol as a chain transfer agent, and 100 parts by weight of toluene as a polymerization solvent were mixed and stirred at 70°C for 1 hour under a nitrogen atmosphere. Next, 0.2 parts by weight of 2,2'-azobisisobutylonitrile (AIBN) was added as a thermal polymerization initiator, and after reacting at 70°C for 2 hours, the temperature was raised to 80°C and reacted for 2 hours. Thereafter, the reaction solution was heated to 130°C, and toluene, chain transfer agent and unreacted monomers were dried and removed to obtain a solid acrylic oligomer. The weight average molecular weight of the acrylic oligomer is 5100.

[Example 1] (Polymerization of prepolymer) As monomer components for forming a prepolymer, 52.8 parts by weight of butyl acrylate (BA), 10.9 parts by weight of cyclohexyl acrylate (CHA), 9.7 parts by weight of N-vinyl-2-pyrrolidone (NVP), 14.8 parts by weight of 4-hydroxybutyl acrylate (4HBA), and 11.8 parts by weight of isostearyl acrylate (ISTA), and a photopolymerization initiator (0.035 parts by weight of "Irgacure 184" manufactured by BASF and 0.035 parts by weight of "Irgacure 651" manufactured by BASF) were mixed, and then irradiated with ultraviolet light until the viscosity (BH viscometer No. 5 rotor, 10 rpm, measurement temperature 30°C) reached about 20 Pa·s, and polymerization was carried out to obtain a prepolymer composition (polymerization rate; about 9%).

(Preparation of photocurable adhesive composition) Add 7 parts by weight of terminal acrylic modified polyether urethane ("UV-3300B" manufactured by Nippon Gosei Kagaku Kogyo) as urethane (meth) acrylate, 3 parts by weight of terminal acrylic modified polyester urethane ("UV-3010B" manufactured by Nippon Gosei Kagaku Kogyo), 5 parts by weight of the above acrylic oligomer, 0.05 parts by weight of Irgacure 184 and 0.57 parts by weight of Irgacure 651 as photopolymerization initiator, and α-methylstyrene dimer ("Nofiner" manufactured by NOF Corporation) as chain transfer agent to the above prepolymer composition. MSD): 0.1 parts by weight, and "KBM403" manufactured by Shin-Etsu Chemical as a silane coupling agent): 0.3 parts by weight, and then they are evenly mixed to prepare an adhesive composition.

(Adhesive sheet preparation) A 75 μm thick polyethylene terephthalate (PET) film ("DIAFOIL MRF75" manufactured by Mitsubishi Chemical) with a silicone release layer on the surface is used as a base (also serving as a heavy release film). The above-mentioned photocurable adhesive composition is applied to the base to a thickness of 150 μm to form a coating layer. A 75 μm thick PET film ("DIAFOIL MRE75" manufactured by Mitsubishi Chemical) with a silicone release treatment on one side is attached to the coating layer as a cover sheet (also serving as a light release film). The laminate was irradiated with ultraviolet light from the side of the cover sheet using a black light with an intensity of 5 mW/ ^cm2 adjusted in the irradiation surface directly below the light for photocuring, thereby obtaining an adhesive sheet with a thickness of 150 μm and a polymerization rate of 99%.

[Examples 2 to 5, Comparative Examples 1 to 8] The monomer composition of the prepolymer, the type and amount of the multifunctional compound (acrylic urethane and/or multifunctional acrylate), acrylic oligomer, photopolymerization initiator, and chain transfer agent added to the adhesive composition were changed as shown in Table 1. In addition, the photocurable adhesive composition was prepared in the same manner as in Example 1, and was coated on the substrate and photocured to obtain an adhesive sheet.

[Evaluation] ＜Weight average molecular weight＞ The weight average molecular weight (Mw) of acrylic oligomers and urethane (meth)acrylates was measured using a GPC (gel permeation chromatography) device (product name "HLC-8120GPC") manufactured by Tosoh. The sample used for the measurement was a 0.1 wt% solution of a base polymer dissolved in tetrahydrofuran and filtered using a 0.45 μm membrane filter. The GPC measurement conditions are as follows. (Measurement conditions) Column: G7000HXL＋GMHXL＋GMHXL manufactured by Tosoh Corporation Column size: 7.8 mmφ×30 cm each (total column length: 90 cm) Column temperature: 40℃・Flow rate: 0.8 mL/min Injection volume: 100 μL Eluent: Tetrahydrofuran Detector: Differential refractometer (RI) Standard sample: Polystyrene

<Storage elastic modulus, glass transition temperature, and tanδ peak value of adhesive sheets> Ten adhesive sheets with a thickness of about 1.5 mm were laminated as test samples. Dynamic viscoelasticity measurements were performed using the "Advanced Rheometric Expansion System (ARES)" manufactured by Rheometric Scientific under the following conditions. (Test conditions) Deformation mode: Torsion Test frequency: 1 Hz Heating rate: 5°C/min Shape: Parallel plate 7.9 mmϕ

The shear storage modulus is obtained by reading the storage modulus G' at each temperature from the measurement results. The temperature (peak temperature) at which the loss tangent (tanδ) is extremely large is set as the glass transition temperature of the adhesive sheet. In addition, the value of tanδ (peak value) at the glass transition temperature is read.

<Adhesion> The self-adhesive sheet was peeled off from the light peeling film, and a PET film with a thickness of 50 μm was laminated. After being cut into 10 mm wide × 100 mm long, the heavy peeling film was peeled off and pressed onto a glass plate using a 5 kg roller to prepare a specimen for adhesion measurement. After the adhesion measurement specimen was kept at 25°C or 65°C for 30 minutes, the specimen was peeled off from the glass plate using a tensile testing machine at a tensile speed of 300 mm/min and a peeling angle of 180° to measure the peeling force.

<Haze> Use a test piece of adhesive sheet bonded to alkali-free glass (total light transmittance 92%, haze 0.4%) with a thickness of 800 μm, and measure the haze using a haze meter ("HM-150" manufactured by Murakami Color Technology Laboratory). The haze of the adhesive sheet is calculated by subtracting the haze of the alkali-free glass (0.4%) from the measured value.

<Interlayer Adhesion> (Preparation of Test Samples) The adhesive sheet was cut into a size of 75 mm × 45 mm, and the light peeling film was peeled off from the adhesive sheet. The adhesive sheet was bonded to the center of a 500 μm thick glass plate (100 mm × 50 mm) by a roller laminating machine (pressure between rollers: 0.2 MPa, feed speed: 100 mm/min). After that, the heavy peeling film was peeled off, and a 30 μm thick black ink was printed on the periphery of a frame-shaped 500 μm thick glass plate (50 mm × 100 mm), and the adhesive sheet was bonded by vacuum pressing (surface pressure 0.3 MPa, pressure 100 Pa). The ink printing area of the glass plate is 5 mm from both ends in the short side direction and 15 mm from both ends in the long side direction. The 5 mm area from the ends of the four sides of the adhesive sheet contacts the black ink layer. The sample was treated in an autoclave (50℃, 0.5 MPa) for 30 minutes.

After the above sample was kept in an environment of 60°C for 30 minutes, a 200 μm thick polystyrene sheet was inserted between the two glass plates from the end of the adhesive sheet to a distance of 1 mm, as shown in Figure 5A, and kept for 10 seconds. The end of the adhesive sheet was observed using a digital microscope with a magnification of 20 times. The one with stripe-shaped bubbles (see Figure 5B) or the peeling of the adhesive sheet from the glass plate was set as NG, and the one without bubbles or glass was set as OK.

<Impact resistance> Except that the size of the glass plate without the printed layer of black ink is changed to 100 mm × 70 mm, the glass plates are bonded to both sides of the adhesive sheet and the autoclave is processed in the same manner as the preparation of the sample for the interlayer adhesion test. As shown in FIG6, with the glass plate 7 with the printed layer 76 as the lower side, the two ends of the short side direction of the test sample 95 are placed on the configuration table 93 with a spacing of 60 mm, and the upper surface of the end of the glass plate 5 without the printed layer is fixed on the table 80 with an adhesive tape (not shown). After the test sample 95 fixed with adhesive tape on the table 93 is kept in an environment of -5°C for 24 hours, it is taken out at room temperature and within 40 seconds, a metal ball 97 with a mass of 11 g is dropped from a height of 300 mm on the glass plate 7 to conduct an impact resistance test.

In the impact resistance test, in order to fix the falling position of the metal ball, a cylindrical guide 99 is used to drop the metal ball 97 from the corner of the inner edge of the frame of the printed area of the printed layer 76 at a distance of 10 mm in the short side direction and the long side direction. The test is performed twice, and the test in which the glass plate is not peeled off in either test is set as OK, and the test in which the glass plate is peeled off in either or both of the two tests is set as NG.

[Evaluation results] The formulation of the adhesive composition used in the preparation of each adhesive sheet and the evaluation results of the adhesive sheet are shown in Table 1. In Table 1, each component is described by the following abbreviations. <Acrylic monomer> BA: Butyl acrylate 2HEA: 2-ethylhexyl acrylate CHA: Cyclohexyl acrylate NVP: N-vinyl-2-pyrrolidone 4HBA: 4-hydroxybutyl acrylate 2HEA: 2-hydroxyethyl acrylate ISTA: Isostearyl acrylate

<Acrylic urethane> UV-3300B: "UV-3300B" manufactured by Nippon Synthetic Chemical Industry (weight average molecular weight of about 12,000, diacrylate polyether urethane with a glass transition temperature of -30°C) 3400: diacrylate polyether urethane with a weight average molecular weight of about 3,400) UA-4200: "UA-4200" manufactured by Shin-Nakamura Chemical Industry (weight average molecular weight of about 1,000 diacrylate polyether urethane) UN-350: "Artresin UN-350" manufactured by Negami Industry (weight average molecular weight of about 12,500, diacrylate polyester urethane with a glass transition temperature of -57°C) UV-3010B: "UV-3010B" manufactured by Nippon Synthetic Chemical Industry (weight average molecular weight of about 11,000 diacrylate polyester urethane) Monoacrylate urethane: Monoacrylate polyether urethane with a weight average molecular weight of about 1300) ＜Multifunctional acrylate＞ HDDA: Hexanediol diacrylate ＜Photopolymerization initiator＞ Irg651: Irgacure651 (2,2-dimethoxy-1,2-diphenylethane-1-one) Irg184: Irgacure184 (1-hydroxy-cyclohexyl-phenyl-ketone)

[Table 1] Embodiment 1 Embodiment 2 Comparison Example 1 Comparison Example 2 Comparison Example 3 Embodiment 3 Embodiment 4 Embodiment 5 Comparison Example 4 Comparison Example 5 Comparison Example 6 Comparative Example 7 Comparative Example 8 Composition Prepolymer composition BA 52.8 52.8 52.8 52.8 52.8 60.5 68.7 68.7 57.0 54.0 - - - 2EHA - - - - - - - - - - 56.0 56.0 52.0 CHA 10.9 10.9 10.9 10.9 10.9 12.4 - - 12.0 11.0 - - - NVP 9.7 9.7 9.7 9.7 9.7 10.2 16.9 16.9 - 15.0 13.0 13.0 29.0 4HBA 14.8 14.8 14.8 14.8 14.8 16.9 14.4 14.4 23.0 20.0 20.0 20.0 8.0 2HEA - - - - - - - - 8.0 - 11.0 11.0 11.0 ISTA 11.8 11.8 11.8 11.8 11.8 - - - - - - - - Urethane diacrylate UV-3300B 7 10 - 15 - 5 - - - - - - - 3400 - - 10 - - - - - - - - - - UA-4200 - - - - - - - - - - - 15 - UN-350 - - - - - 5 3.5 5 - - - - - UV-3010B 3 10 - 15 - - - - - - - - - Urethane Monoacrylate - - - - 5 - - - - - - - - Multifunctional acrylate HDDA - 0.2 - 0.2 - - - - 0.12 0.12 2 0.12 0.3 Acrylic oligomer 5 5 5 5 5 5 5 5 - - 5 5 - Photopolymerization initiator Irg651 0.57 0.57 0.55 0.57 0.55 0.55 0.55 0.55 0.05 0.05 0.55 0.55 0.55 Irg184 0.07 0.07 0.05 0.07 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 Chain transfer agent Nofiner MSD 0.1 0.3 0.2 0.2 0.2 0.2 0.2 0.2 - - 0.2 0.2 0.2 Silane coupling agent KBM-403 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 Reviews Tg(℃) -5 -6 -5 -7 -5 -5 -5 -5 -20 1 -6 -4 -1 tanδ 2.0 1.9 1.8 1.8 1.8 2.0 1.8 1.8 2.4 2.1 1.2 1.6 1.6 G' _{25 ℃} (MPa) 0.23 0.22 0.19 0.25 0.14 0.20 0.19 0.21 0.12 0.21 0.21 0.24 0.17 G' _{80 ℃} (MPa) 0.18 0.15 0.16 0.18 0.08 0.15 0.11 0.12 0.10 0.14 0.18 0.20 0.13 25℃ Adhesion force (N/10 mm) 4 5 1 3 13 10 10 8 6 5 0.4 0.3 6 65℃ Adhesion force (N/10 mm) 2 2 0.3 1 8 2 3 3 2 2 0.1 0.1 2 Fog(%) 0.4 0.4 0.4 2.0 0.3 0.4 0.4 0.4 0.2 0.2 0.4 0.6 0.5 Layer indirectness OK OK NG OK NG OK OK OK NG OK NG NG OK Drop impact durability OK OK NG OK OK OK OK OK OK NG NG NG NG

In Examples 1 and 2, in which an adhesive composition in which urethane diacrylate or the like is added to a prepolymer composition obtained by preliminarily polymerizing an acrylic monomer having butyl acrylate as a main monomer, both interlayer adhesion and drop impact durability are good.

In Comparative Example 1 using low molecular weight urethane diacrylate, the adhesive sheet has a weak adhesion to the adherend, poor interlayer adhesion and drop impact durability. In Comparative Example 2 where the amount of urethane diacrylate added is increased, the haze of the adhesive sheet is high and the transparency is reduced. In Comparative Example 3 using monourethane diacrylate, the shear storage elastic modulus of the adhesive sheet is low and the adhesion durability is poor.

In Example 3, Example 4, and Example 5 in which the composition of the acrylic monomer in the prepolymer-forming composition was changed, similarly to Examples 1 and 2, good bonding properties were also exhibited.

In Comparative Example 4, which lowered the glass transition temperature by adjusting the composition of acrylic monomers without using urethane materials, the G' _{25 ℃} and G' _{80 ℃} of the adhesive sheet were small and the bonding reliability was poor. In Comparative Example 5, which improved the cohesion by increasing the ratio of polar monomers (NVB and 4HBA) in the acrylic monomer component, the adhesion was good, the glass transition temperature was high, and the impact resistance was reduced. The same tendency was also seen in Comparative Example 8.

In Comparative Example 6, in which the ratio of multifunctional acrylate in the adhesive composition is increased, the peak value of tanδ is small and the viscosity is low, so the adhesion is insufficient and the impact resistance is poor. The same tendency can be seen in Comparative Example 7, in which a low molecular weight diacrylate urethane is introduced into the crosslinking structure. In these comparative examples, the main monomer of the acrylic polymer chain is _C8 alkyl acrylate (2-ethylhexyl acrylate), which is also compared with Examples 1 to 5 in which the main monomer is _C4 alkyl acrylate (butyl acrylate), and it is believed that one of the reasons is that tanδ is smaller.

According to these results, it can be seen that the adhesive sheet including a base polymer having a cross-linked structure introduced into an acrylic polymer chain using a diacrylate urethane having a specific molecular weight also shows a higher shear storage elastic modulus at a low glass transition temperature and a larger tan δ, and thus can have both adhesion durability and impact resistance.

1,2: Release film 3: Polarizing plate 4: Adhesive sheet 5: Adhesive sheet 6: Image display unit 7: Front transparent plate 9: Housing 10: Image display panel 100: Image display device

FIG1 is a cross-sectional view showing an example of the structure of an adhesive sheet of a release film. FIG2 is a cross-sectional view showing an example of the structure of an image display device. FIG3 and FIG4 are cross-sectional views showing an example of the laminated structure of an optical film of an adhesive sheet. FIG5A is a photograph showing the situation of the interlayer adhesion test, and FIG5B is an observation photograph of a sample in which stripe-shaped bubbles are generated in the interlayer adhesion test. FIG6 is a schematic diagram showing the configuration of the sample in the impact resistance test.

1: Release film

2: Release film

5: Adhesive sheet

Claims (6)

An adhesive sheet, which is an adhesive formed in sheet form, and contains an acrylic base polymer having a crosslinking structure introduced into an acrylic polymer chain, and an acrylic oligomer, and has a haze of less than 1%, an adhesion to glass of more than 2.0 N/10 mm, a glass transition temperature of less than -3°C, and a shear storage elastic modulus at a temperature of 25°C of more than 0.16 MPa. The acrylic base polymer is obtained by copolymerizing a monomer component constituting the acrylic polymer chain with a urethane di(meth)acrylate having (meth)acrylic groups at both ends, and a crosslinking structure utilizing the urethane di(meth)acrylate is introduced into the acrylic polymer chain. The weight average molecular weight of the above-mentioned urethane di(meth)acrylate is 4000-50000, and the glass transition temperature is below 0°C. The amount of the above-mentioned urethane di(meth)acrylate is 3-30 parts by weight relative to 100 parts by weight of the monomer components constituting the above-mentioned acrylic polymer chain. The weight average molecular weight of the above-mentioned acrylic oligomer is 1000-30000, and the glass transition temperature is above 20°C. Relative to 100 parts by weight of the above-mentioned acrylic base polymer, the above-mentioned acrylic oligomer is contained in an amount of 0.5-20 parts by weight. The adhesive sheet of claim 1, wherein the peak value of the loss tangent is greater than 1.5. The adhesive sheet of claim 1 or 2, wherein the polymerization rate of the adhesive is greater than 97%. An adhesive sheet with a release film attached comprises the adhesive sheet as claimed in any one of claims 1 to 3, and release films temporarily adhered to both sides of the adhesive sheet. A method for manufacturing an adhesive sheet, which is a method for manufacturing an adhesive sheet as claimed in any one of claims 1 to 3, and After a composition comprising an acrylic monomer and/or a partial polymer thereof, urethane di(meth)acrylate, and an acrylic oligomer is applied in a layer on a substrate, the composition is irradiated with active light for photocuring, The weight average molecular weight of the urethane di(meth)acrylate is 4000 to 50000, and the glass transition temperature is below 0°C, The weight average molecular weight of the acrylic oligomer is 1000 to 30000, and the glass transition temperature is above 20°C, In the composition, relative to a total of 100 parts by weight of the acrylic monomer and its partial polymer, the content of the urethane di(meth)acrylate is 3 to 30 parts by weight, and the content of the acrylic oligomer is 0.5 to 20 parts by weight. An image display device fixes a front transparent component on the viewing side surface of an image display panel by means of an adhesive sheet as described in any one of claims 1 to 3.