Adhesive composition

Abstract

The present invention provides an adhesive composition that offers excellent reworkability and durability, prevents uneven brightness, and, when an optical film is bonded to a thin liquid crystal cell via an adhesive layer, prevents curling of the liquid crystal display panel and exhibits good resistance to dents. [Solution] An adhesive composition comprising an acrylic polymer with a weight-average molecular weight of 1.5 million to 3 million, obtained by copolymerizing only (A) an alkyl (meth)acrylate monomer having C8 to C18 C atoms in the alkyl group, (B) an alkyl (meth)acrylate monomer having C1 to C5 C atoms in the alkyl group, (C) a copolymerizable vinyl monomer containing a hydroxyl group, and (D) a copolymerizable vinyl monomer containing an aromatic group, without containing a copolymerizable vinyl monomer containing a carboxyl group, a crosslinking agent, and a silane coupling agent, wherein the crosslinking agent is a trifunctional or more isocyanate compound.

Description

Technical Field

[0001] The present invention relates to an adhesive composition and an adhesive film. More specifically, it relates to an adhesive composition for bonding an optical film incorporated in a display panel, and an adhesive film using the same. In addition, the present invention provides an adhesive composition having excellent reworkability and durability, and having good curl resistance (bending) properties such that curling of the display panel, which is a cause of luminance unevenness such as color shading, does not occur, and an adhesive film using the same. Furthermore, the adhesive layer using the adhesive composition according to the present invention is also excellent in dent resistance when a load is applied.

Background Art

[0002] In recent years, as displays (image display devices) for displaying images, displays of various display methods such as liquid crystal displays and organic EL displays have become widely popular. Optical films having various functions are incorporated in the display panels constituting these displays using an adhesive layer. Hereinafter, a liquid crystal display panel used in a liquid crystal display will be described as a representative example. For example, a general liquid crystal display panel has a structure in which liquid crystal is enclosed between two glass substrates, and polarizing plates are bonded to the surfaces of the glass substrates using an adhesive layer.

[0003] In addition, in a liquid crystal display panel, in addition to a polarizing plate, optical films such as a retardation plate, a brightness enhancement film, a lens film for a display, an anti-reflection film, a hard coat film, and a transparent conductive film for a touch panel are incorporated. Typically, the thermal shrinkage rates of optical films incorporated into liquid crystal display panels are not uniform in the vertical and horizontal directions. Furthermore, when multiple optical films are laminated, the thermal shrinkage rates of each optical film differ. As a result, changes in the display's operating environment (temperature, humidity) cause dimensional changes in the optical films incorporated into the liquid crystal display panel. This leads to curling and deformation of the liquid crystal display panel. In this process, the adhesive layer used to bond the optical films in the liquid crystal display panel is subjected to stress due to the dimensional changes of the optical films. For example, when a liquid crystal display panel curls, it can cause problems such as uneven brightness, including variations in color intensity, in the displayed image.

[0004] Furthermore, among liquid crystal display panels used in various applications, automotive liquid crystal display panels (for in-car monitors, car navigation systems, etc.) are subjected to even more stringent durability testing conditions (high temperature: e.g., 105°C, high temperature and high humidity conditions: e.g., 85°C, 85%RH, repeated high and low temperature cycles, etc.) compared to liquid crystal display panels for televisions and personal computers. As a result, dimensional changes and stress generation in the optical film incorporated into the liquid crystal display panel are more likely to occur.

[0005] Furthermore, in recent years, LCD panels have become thinner in order to be lighter, and the problem of the LCD panel curling due to the inability to withstand the dimensional changes and stress of the laminated optical film has become apparent. Therefore, there is a need to suppress or prevent defects such as curling (curl resistance) under more demanding display conditions.

[0006] As an example of a conventional technique for improving unevenness such as the color intensity of displayed images, it has been disclosed that an adhesive layer with a reduced gel fraction due to crosslinking is used, for example, by using a high molecular weight acrylic polymer (Patent Document 1). It is said that using this adhesive layer not only satisfies the requirements for heat resistance and moisture resistance, but also alleviates stress caused by dimensional changes in components such as polarizing films, thereby significantly suppressing light leakage and color unevenness.

[0007] Furthermore, as an alternative to conventional techniques for improving color unevenness in displayed images, it has been disclosed that an adhesive layer having a viscosity coefficient within a specific range is used, which is made by copolymerizing a high molecular weight polymer obtained by copolymerizing acrylic monomers having aromatic rings. It is said that by using this adhesive layer, an optical adhesive composition can be obtained in which no color unevenness or whitening occurs in the liquid crystal element.

[0008] Furthermore, as another conventional method for improving unevenness such as color intensity in displayed images, it has been disclosed that an adhesive layer with a low gel fraction is used, employing an acrylic polymer having a specific range of branching degrees. (Patent Document 3) It is said that an adhesive layer using this adhesive composition can suppress warping of the glass substrate when the glass substrate and polarizing plate constituting the liquid crystal display panel are bonded together via the adhesive layer.

[0009] Incidentally, the invention described in Patent Document 1 is based on the technical concept of resolving unevenness in the displayed image by adjusting the weight-average molecular weight, and specifies that the weight-average molecular weight of the acrylic polymer in the adhesive composition forming the adhesive layer is between 500,000 and 2,000,000. However, the acrylic polymers used in Examples 1 to 6 of Patent Document 1 have a weight-average molecular weight in the range of 700,000 to 1,400,000, and the effects of the invention have not been demonstrated for those with a weight-average molecular weight exceeding 1,400,000. Furthermore, the adhesive layer of the invention described in Patent Document 1 preferably has a gel fraction of 1 to 50%, and in Examples 1, 2, 4, and 5 of Patent Document 1, the gel fraction is less than 40%. In order to confirm the effectiveness of the invention described in Patent Document 1, a follow-up test was conducted, and it was confirmed that the adhesive layer of the invention described in Patent Document 1 is still insufficient in terms of durability and heat resistance.

[0010] Furthermore, the invention described in Patent Document 2 is based on the technical concept of relieving distortion by causing an adhesive layer having a viscosity coefficient within a specific range to shift, thereby solving the problem of color unevenness and white spots in liquid crystal elements. Furthermore, while the adhesive layer of the invention described in Patent Document 2 is said to have a weight-average molecular weight of 1 million to 2 million for the copolymer of acrylic polymers, the weight-average molecular weight of the copolymers used in Examples 1 to 11 is 1.2 million to 1.27 million, and the effects of the invention have not been demonstrated for copolymers with a weight-average molecular weight exceeding 1.27 million. Therefore, there are concerns that the adhesive layer of the invention described in Patent Document 2, like the adhesive layer of the invention described in Patent Document 1, may be insufficient in terms of heat resistance and durability.

[0011] Furthermore, the invention described in Patent Document 3 is based on the technical concept of using an adhesive composition containing an acrylic polymer having a specific range of branching degrees to form an adhesive layer with a low gel fraction, thereby easing stress while maintaining durability through the entanglement of branched chains. Furthermore, in Examples 1 to 14 of Patent Document 3, the weight-average molecular weight of the (meth)acrylic copolymer used is 400,000 to 1,300,000, and the gel fraction of the adhesive layer is 30% by mass or less. Therefore, even if the adhesive layer of the invention described in Patent Document 3 has excellent stress relaxation properties, there are concerns that its durability may be insufficient due to its low gel fraction.

[0012] As described above, in conventional technology, it has been difficult to simultaneously address the reworkability, durability, and curl resistance of the adhesive layer used to bond the optical film incorporated into the liquid crystal display panel when reducing brightness unevenness such as color gradation in image display devices. [Prior art documents] [Patent Documents]

[0013] [Patent Document 1] Japanese Patent Publication No. 2004-091500 [Patent Document 2] Japanese Patent Publication No. 2007-169329 [Patent Document 3] International Publication No. 2015 / 141382 [Overview of the project] [Problems to be Solved by the Invention]

[0014] In view of the above problems, an object of the present invention is to provide an adhesive composition having excellent reworkability and durability, and having good curl resistance (bending) properties in which curling of a display panel, which is a cause of uneven luminance such as color shading, does not occur, and an adhesive film using the same. Another object of the present invention is to provide an adhesive layer having excellent dent resistance when a load is applied. [Means for Solving the Problems]

[0015] In order to solve such problems, the present invention provides an adhesive composition containing an acrylic polymer, a crosslinking agent, and a silane coupling agent, wherein the acrylic polymer is a copolymerized product of at least one alkyl (meth) acrylate monomer having 8 to 18 carbon atoms in the alkyl group, at least one alkyl (meth) acrylate monomer having 1 to 5 carbon atoms in the alkyl group, at least one copolymerizable vinyl monomer containing a hydroxyl group, and at least one copolymerizable vinyl monomer containing an aromatic group, without containing a copolymerizable vinyl monomer containing a carboxyl group, and having a weight average molecular weight of 1.5 million to 3 million, and the crosslinking agent is a trifunctional or higher isocyanate compound. The adhesive composition of the present invention uses an adhesive layer formed by crosslinking the adhesive composition for bonding a liquid crystal display panel and an optical film, so that the adhesive layer relaxes the shrinkage stress generated with the dimensional change of the optical film, reduces the curl generated in the liquid crystal display panel, and can form an adhesive layer having excellent reworkability, high durability, and dent resistance when a load is applied.

[0016] ​

[0017] The reason why the adhesive layer formed by crosslinking the adhesive composition of the present invention relaxes the shrinkage stress generated in the optical film to be laminated, reduces the curl generated in the liquid crystal display panel, and exhibits excellent reworkability, high durability, and dent resistance is not clear. As a possible reason, the stress relaxation due to the flexibility of the long-chain alkyl side chains based on the (A) alkyl (meth) acrylate monomer having an alkyl group with 8 to 18 carbon atoms that constitutes the acrylic polymer of the adhesive composition, the expression of hardness by the combination with other copolymerization components in the acrylic polymer, and further the improvement of the cohesive force by the crosslinkable component are presumed to enable the expression of the above characteristics.

[0018] The acrylic polymer is based on a total of 100 parts by weight of the above (A), (B), and (D), at least one kind of the (A) alkyl (meth) acrylate monomer having an alkyl group with 8 to 18 carbon atoms is 20 to 69 parts by weight, at least one kind of the (B) alkyl (meth) acrylate monomer having an alkyl group with 1 to 5 carbon atoms is 30 to 79 parts by weight, at least one kind of the (C) copolymerizable vinyl monomer containing a hydroxyl group is more than 0 parts by weight and 5 parts by weight or less, The (D) acrylic polymer is a copolymerized acrylic polymer in which at least one kind of the copolymerizable vinyl monomer containing an aromatic group is copolymerized at a ratio of 1 to 50 parts by weight, In the adhesive composition, it is preferable that the trifunctional or higher isocyanate compound is contained at a ratio of 0.001 to 0.05 parts by weight with respect to a total of 100 parts by weight of the above (A), (B), and (D).

[0019] Preferably, at least one copolymerizable vinyl monomer containing a hydroxyl group (C) is selected from the group of compounds consisting of 8-hydroxyoctyl (meth)acrylate, 6-hydroxyhexyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, 2-hydroxyethyl (meth)acrylate, N-hydroxy(meth)acrylamide, N-hydroxymethyl (meth)acrylamide, and N-hydroxyethyl (meth)acrylamide.

[0020] Preferably, at least one of the alkyl (meth)acrylate monomers having C8 to C18 carbon atoms in the alkyl group (A) is selected from 2-ethylhexyl acrylate, isooctyl acrylate, isononyl acrylate, and isodecyl acrylate, and at least one of the alkyl (meth)acrylate monomers having C1 to C5 carbon atoms in the alkyl group (B) is selected from the group consisting of n-butyl acrylate, t-butyl acrylate, isobutyl acrylate, n-butyl methacrylate, and methyl acrylate.

[0021] Preferably, at least one copolymerizable vinyl monomer containing the aromatic group (D) is selected from benzyl (meth)acrylate and phenoxyethyl (meth)acrylate.

[0022] Furthermore, it is preferable that the silane coupling agent is a silane coupling agent having at least one functional group selected from epoxy groups, mercapto groups, and acid anhydride groups.

[0023] It is preferable that the molecular weight dispersion (Mw [weight-average molecular weight] / Mn [number-average molecular weight]) of the acrylic polymer is 6.0 or less.

[0024] Preferably, a polarizing plate with a thickness of 140 μm, using PET (polyethylene terephthalate film: 20 μm thick) as a protective film for the polarizing element, and an alkali-free glass with a thickness of 0.4 mm are bonded together via an adhesive layer formed by crosslinking the adhesive composition. When the polarizing plate with the PET surface is peeled from the glass surface at a peeling speed of 300 mm / min in the 180° direction in accordance with JIS-Z0237, the adhesive force is 4 N / 25 mm or less. More preferably, the adhesive force is 1 to 4 N / 25 mm.

[0025] Furthermore, it is preferable that the thixotropy index (TI value) of the adhesive composition solution obtained by dissolving the adhesive composition in a solvent is 1.25 or less.

[0026] Furthermore, the present invention provides an adhesive layer having a thickness of 20 μm and dent resistance of 10 g or more, which is obtained by crosslinking the adhesive composition.

[0027] Furthermore, the present invention provides an adhesive film in which an adhesive layer formed by crosslinking the adhesive composition is laminated on one side of a resin film.

[0028] Preferably, the adhesive film is an adhesive film used to bond a polarizing plate to a substrate.

[0029] Furthermore, the present invention provides an adhesive film having a release film / adhesive layer / release film configuration, wherein an adhesive layer formed by crosslinking the adhesive composition is formed on one side of the release film.

[0030] Furthermore, the present invention provides an optical film with an adhesive layer, wherein an adhesive layer formed by crosslinking the adhesive composition is laminated on at least one surface of the optical film, and the configuration is release film / adhesive layer / optical film.

[0031] Furthermore, the present invention provides a polarizing plate with an adhesive layer, wherein an adhesive layer formed by crosslinking the adhesive composition is laminated on one side of the polarizing plate, and the configuration is release film / adhesive layer / polarizing plate.

[0032] Furthermore, the present invention provides a liquid crystal display panel that uses the polarizing plate with the adhesive layer. [Effects of the Invention]

[0033] According to the present invention, it is possible to provide an adhesive composition that can form an adhesive layer having excellent reworkability and durability, without the occurrence of brightness unevenness such as color variations, and with good curl resistance (bending resistance). Furthermore, when an optical film is laminated to a thin liquid crystal cell via an adhesive layer formed by crosslinking the adhesive composition of the present invention, it is possible to provide an adhesive composition that can form an adhesive layer that has good curl resistance (bending resistance) so as not to cause curling in the liquid crystal display panel, while also having excellent dent resistance when a load is applied to the adhesive layer. Furthermore, it is possible to provide an adhesive composition with good coating properties. [Modes for carrying out the invention]

[0034] The present invention will be described below based on preferred embodiments.

[0035] The adhesive composition of this embodiment is an adhesive composition containing an acrylic polymer, a crosslinking agent, and a silane coupling agent, wherein the acrylic polymer is an acrylic polymer with a weight-average molecular weight of 1.5 million to 3 million obtained by copolymerizing (A) at least one alkyl (meth)acrylate monomer having C8 to C18 carbon atoms in the alkyl group, (B) at least one alkyl (meth)acrylate monomer having C1 to C5 carbon atoms in the alkyl group, (C) at least one copolymerizable vinyl monomer containing a hydroxyl group, and (D) at least one copolymerizable vinyl monomer containing an aromatic group, without containing a copolymerizable vinyl monomer containing a carboxyl group, and the crosslinking agent is an isocyanate compound with three or more functions.

[0036] The acrylic polymer in the adhesive composition of this embodiment is a copolymer obtained by copolymerizing (A) at least one alkyl (meth)acrylate monomer having C8 to C18 C atoms in the alkyl group, (B) at least one alkyl (meth)acrylate monomer having C1 to C5 C atoms in the alkyl group, (C) at least one copolymerizable vinyl monomer containing a hydroxyl group, and (D) at least one copolymerizable vinyl monomer containing an aromatic group, without including a copolymerizable vinyl monomer containing a carboxyl group. In this specification, (meth)acrylate is a general term for acrylate and methacrylate.

[0037] The alkyl(meth)acrylate monomers having C8 to C18 carbon atoms in the (A) alkyl group constituting the acrylic polymer in the adhesive composition of this embodiment include octyl(meth)acrylate, isooctyl(meth)acrylate, 2-ethylhexyl(meth)acrylate, nonyl(meth)acrylate, isononyl(meth)acrylate, decyl(meth)acrylate, isodecyl(meth)acrylate, undecyl(meth)acrylate, isoundecyl(meth)acrylate, dodecyl(meth)acrylate, isododecyl(meth)acrylate, and tridecyl(meth)acrylate. At least one compound selected from the group consisting of acrylate, isotridecyl(meth)acrylate, tetradecyl(meth)acrylate, isotetradecyl(meth)acrylate, pentadecyl(meth)acrylate, hexadecyl(meth)acrylate, heptadecyl(meth)acrylate, octadecyl(meth)acrylate, myristyl(meth)acrylate, isomiristyl(meth)acrylate, cetyl(meth)acrylate, isocetyl(meth)acrylate, stearyl(meth)acrylate, and isostearyl(meth)acrylate is included. The alkyl group of the alkyl(meth)acrylate monomer may be linear, branched, or cyclic (alicyclic).

[0038] When the number of carbon atoms in the alkyl(meth)acrylate monomer contained in the acrylic polymer is 7 or less, for example, in an acrylic polymer containing (B) but not (A) below, the flexibility of the alkyl group forming the side chain becomes insufficient, making it difficult for the curl-resistant effect of the adhesive layer formed by crosslinking the adhesive composition to be exhibited, which is undesirable. On the other hand, when the number of carbon atoms in the alkyl group is large, the flexibility becomes excessive, making the adhesive layer more prone to indentation under load, and crystallization of the alkyl group is more likely to occur, which can reduce the transparency of the adhesive layer, and is therefore undesirable.

[0039] (A) The alkyl (meth)acrylate monomer having C8 to C18 carbon atoms in the alkyl group is preferably at least one of the alkyl (meth)acrylate monomers having C8 to C10 carbon atoms in the alkyl group, and more preferably at least one selected from 2-ethylhexyl acrylate, isooctyl acrylate, isononyl acrylate, and isodecyl acrylate.

[0040] In the adhesive composition of this embodiment, the acrylic polymer can be copolymerized in a ratio of 20 to 69 parts by weight to 100 parts by weight of a total of (A) at least one alkyl (meth)acrylate monomer having C8 to C18 in the alkyl group, (B) at least one alkyl (meth)acrylate monomer having C1 to C5 in the alkyl group, and (D) at least one copolymerizable vinyl monomer containing an aromatic group. The lower limit of the copolymerization ratio of (A) is preferably 25 parts by weight or more, and particularly preferably 35 parts by weight or more. The upper limit of the copolymerization ratio of (A) is preferably 65 parts by weight or less, and particularly preferably 55 parts by weight or less.

[0041] The alkyl(meth)acrylate monomer having C1 to C5 carbon atoms in the (B) alkyl group constituting the acrylic polymer in the adhesive composition of this embodiment can be at least one selected from the group of compounds consisting of methyl(meth)acrylate, ethyl(meth)acrylate, propyl(meth)acrylate, isopropyl(meth)acrylate, butyl(meth)acrylate, isobutyl(meth)acrylate, t-butyl(meth)acrylate, pentyl(meth)acrylate, and isopentyl(meth)acrylate. The alkyl group of the alkyl(meth)acrylate monomer may be linear, branched, or cyclic (alicyclic). (B) The alkyl(meth)acrylate monomer having C1 to C5 carbon atoms in the alkyl group is preferably at least one selected from the group consisting of n-butyl acrylate, t-butyl acrylate, isobutyl acrylate, n-butyl methacrylate, and methyl acrylate. (B) It is also preferable to use two or more alkyl (meth)acrylate monomers having C1 to C5 carbon atoms in the alkyl group in combination. For example, it is preferable to use butyl acrylate, and in particular, a combination of n-butyl acrylate and methyl acrylate.

[0042] In the adhesive composition of this embodiment, the acrylic polymer can be copolymerized in a ratio of 30 to 79 parts by weight to 100 parts by weight of a total of (A) at least one alkyl (meth)acrylate monomer having C8 to C18 in the alkyl group, (B) at least one alkyl (meth)acrylate monomer having C1 to C5 in the alkyl group, and (D) at least one copolymerizable vinyl monomer containing an aromatic group. The lower limit of the copolymerization ratio of (B) is preferably 40 parts by weight or more, and particularly preferably 50 parts by weight or more. The upper limit of the copolymerization ratio of (B) is preferably 75 parts by weight or less, and particularly preferably 65 parts by weight or less.

[0043] Examples of copolymerizable vinyl monomers containing a hydroxyl group that constitute the acrylic polymer in the adhesive composition of this embodiment include hydroxyl group-containing (meth)acrylic acid esters such as 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, 6-hydroxyhexyl (meth)acrylate, and 8-hydroxyoctyl (meth)acrylate, as well as hydroxyl group-containing (meth)acrylamides such as N-hydroxy(meth)acrylamide, N-hydroxymethyl (meth)acrylamide, and N-hydroxyethyl (meth)acrylamide. At least one selected from this group of compounds is preferred. In particular, the copolymerizable vinyl monomer containing a hydroxyl group (C) is preferably at least one selected from the group of compounds consisting of 8-hydroxyoctyl(meth)acrylate, 6-hydroxyhexyl(meth)acrylate, 4-hydroxybutyl(meth)acrylate, 2-hydroxyethyl(meth)acrylate, N-hydroxy(meth)acrylamide, N-hydroxymethyl(meth)acrylamide, and N-hydroxyethyl(meth)acrylamide.

[0044] The acrylic polymer in the adhesive composition of this embodiment may contain, in proportion to 100 parts by weight of the total of (A) at least one alkyl (meth)acrylate monomer having C8 to C18 C atoms in the alkyl group, (B) at least one alkyl (meth)acrylate monomer having C1 to C5 C atoms in the alkyl group, and (D) at least one copolymerizable vinyl monomer containing an aromatic group, (C) at least one copolymerizable vinyl monomer containing a hydroxyl group in a proportion of more than 0 parts by weight and 5 parts by weight or less, preferably 0.05 to 2.5 parts by weight, particularly preferably 0.1 to 2.0 parts by weight, and more preferably 0.5 to 1.5 parts by weight.

[0045] As the copolymerizable vinyl monomer containing an aromatic group (D) that constitutes the acrylic polymer in the adhesive composition of this embodiment (hereinafter sometimes abbreviated as "aromatic group-containing monomer"), one can be used that has a polymerizable functional group having an unsaturated double bond, such as a (meth)acryloyl group or a vinyl group, and also has an aromatic group or a heterocycle. Examples of aromatic group-containing monomers include styrene monomers such as styrene and α-methylstyrene; vinyltoluene monomers such as vinyltoluene and α-vinyltoluene; heterocyclic vinyl monomers such as vinylpyridine, vinylpiperidone, vinylpyrimidine, vinylpiperazine, vinylpyrazine, vinylpyrrole, vinylimidazole, vinyloxazole, and vinylmorpholine; alkyl(meth)acrylates containing aromatic rings such as phenoxyethyl(meth)acrylate and benzyl(meth)acrylate; and heterocyclic (meth)acrylic monomers such as N-acryloylmorpholine, N-acryloylpiperidine, N-methacryloylpiperidine, and N-acryloylpyrrolidine. At least one compound selected from this group is preferred. Of the heterocycles exemplified above, piperidone, piperazine, morpholine, piperidine, and pyrrolidine are saturated heterocycles that do not contain unsaturated bonds within the ring, while pyridine, pyrimidine, pyrazine, pyrrole, imidazole, and oxazole are unsaturated heterocycles (heteroaromatic rings) that contain unsaturated bonds within the ring. The above aromatic group-containing monomers may also be monomers that do not contain hydroxyl groups and carboxyl groups. In particular, the copolymerizable vinyl monomer containing the (D) aromatic group is preferably at least one selected from benzyl (meth)acrylate and phenoxyethyl (meth)acrylate.

[0046] (D) Copolymerizable vinyl monomers containing aromatic groups are preferably used to obtain an adhesive layer with a high refractive index. Furthermore, in the adhesive composition of this embodiment, the copolymerizable vinyl monomer containing aromatic groups (D) is copolymerized together with other components constituting the acrylic polymer, thereby giving the adhesive layer formed by crosslinking the adhesive composition a certain degree of hardness and exhibiting dent resistance.

[0047] The acrylic polymer in the adhesive composition of this embodiment may contain, in a total of 100 parts by weight of (A) at least one alkyl (meth)acrylate monomer having C8 to C18 C atoms in the alkyl group, (B) at least one alkyl (meth)acrylate monomer having C1 to C5 C atoms in the alkyl group, and (D) at least one copolymerizable vinyl monomer containing an aromatic group, at least one copolymerizable vinyl monomer containing an aromatic group may be included in a proportion of 1 to 50 parts by weight, preferably 3 to 40 parts by weight, particularly preferably 5 to 20 parts by weight, with an upper limit of 15 parts by weight or less for the copolymerization ratio.

[0048] The acrylic polymer in the adhesive composition of this embodiment is a copolymer obtained by copolymerization without containing a copolymerizable vinyl monomer containing a carboxyl group. The polymerization method for the copolymer used as the acrylic polymer in the adhesive composition of this embodiment is not particularly limited, and known polymerization methods such as solution polymerization and emulsion polymerization can be used as appropriate.

[0049] In this embodiment, the glass transition temperature (hereinafter referred to as Tg) of the copolymer used as the acrylic polymer is preferably -70°C or higher and -20°C or lower. If Tg is below -70°C, foaming resistance (durability) and dent resistance decrease, and if Tg exceeds -20°C, curl resistance decreases and brightness unevenness is more likely to occur, which is undesirable. Furthermore, the Tg of the copolymer used as the acrylic polymer in the adhesive composition of this embodiment is more preferably -65°C to -30°C.

[0050] Furthermore, the weight-average molecular weight of the copolymer used as the acrylic polymer in the adhesive composition of this embodiment can be between 1.5 million and 3 million. It is preferably in the range of 1.75 million to 2.75 million, and particularly preferably in the range of 2 million to 2.5 million. If the weight-average molecular weight of the copolymer used as an acrylic polymer is below the above lower limit, its durability will be insufficient, which is undesirable.

[0051] Furthermore, it is preferable that the degree of molecular weight dispersion (Mw [weight-average molecular weight] / Mn [number-average molecular weight]) of the copolymer used as the acrylic polymer in the adhesive composition of this embodiment is 6.0 or less. In addition to the weight-average molecular weight of the copolymer used as an acrylic polymer being between 1.5 million and 3 million, the dispersion of the copolymer's molecular weight is 6.0 or less. This improves, for example, the peeling durability after being left at high temperatures exceeding 100°C, and suppresses peeling and foaming of the adhesive layer. The degree of dispersion of the molecular weight of the copolymer is more preferably 5.2 or less, and even more preferably 4.5 or less.

[0052] In the adhesive composition of this embodiment, a trifunctional or higher isocyanate compound is used as the crosslinking agent. The trifunctional or higher isocyanate compound can be any polyisocyanate compound having at least three isocyanate (NCO) groups in one molecule. Examples include biuret-modified and isocyanurate-modified diisocyanates (compounds having two NCO groups in one molecule) such as hexamethylene diisocyanate, isophorone diisocyanate, diphenylmethane diisocyanate, tolylene diisocyanate, and xylylene diisocyanate, as well as adducts (polyol-modified) with trivalent or higher polyols (compounds having at least three OH groups in one molecule) such as trimethylolpropane and glycerin. In particular, the isocyanate compound with three or more functions is preferably a polyisocyanate compound having at least three isocyanate (NCO) groups in one molecule, and more preferably at least one selected from the group of compounds consisting of isocyanurate forms of hexamethylene diisocyanate compounds, isocyanurate forms of isophorone diisocyanate compounds, adduct forms of hexamethylene diisocyanate compounds, adduct forms of isophorone diisocyanate compounds, burette forms of hexamethylene diisocyanate compounds, and burette forms of isophorone diisocyanate compounds. The isocyanate compound with three or more functions as a crosslinking agent may be included in a proportion of 0.001 to 0.05 parts by weight per 100 parts by weight of the total of (A), (B), and (D).

[0053] In the adhesive composition of this embodiment, a silane coupling agent can be included in the adhesive composition. Silane coupling agents include vinyl group-containing silane coupling agents such as vinyltrimethoxysilane, vinyltriethoxysilane, and vinyltris(2-methoxyethoxy)silane; methacryloxy group-containing silane coupling agents such as 3-methacryloxypropyltrimethoxysilane, 3-methacryloxypropyltriethoxysilane, 3-methacryloxypropylmethyldimethoxysilane, and 3-methacryloxypropylmethyldiethoxysilane; acetoxy group-containing silane coupling agents such as vinyltriacetoxysilane; acryloxy group-containing silane coupling agents such as 3-acryloxypropyltrimethoxysilane and 3-acryloxypropyltriethoxysilane; 3-glycidoxypropyltrimethoxysilane, 3-glycidoxypropyltriethoxysilane, 3-glycidoxypropylmethyldimethoxysilane, 3-glycidoxypropylmethyldiethoxysilane, and 2- Examples include epoxy group-containing silane coupling agents such as (3,4-epoxycyclohexyl)ethyltrimethoxysilane; chloro group-containing silane coupling agents such as vinyltrichlorosilane; mercapto group-containing silane coupling agents such as 3-mercaptopropyltrimethoxysilane, 3-mercaptopropyltriethoxysilane, 3-mercaptopropylmethyldimethoxysilane, and 3-mercaptopropylmethyldiethoxysilane; acid anhydride group-containing silane coupling agents such as 3-trimethoxysilylpropylsuccinic anhydride; amino group-containing silane coupling agents such as 3-aminopropyltrimethoxysilane, 3-aminopropyltriethoxysilane, and N-2-(aminoethyl)-3-aminopropyltrimethoxysilane; and isocyanate group-containing silane coupling agents such as 3-isocyanatetopropyltrimethoxysilane and 3-isocyanatetopropyltriethoxysilane. These silane coupling agents can be used individually or in combination of two or more types, and it is preferable to add them in an amount of 0.001 to 5 parts by weight per 100 parts by weight of the adhesive composition. As the silane coupling agent, it is preferable to use a silane coupling agent having at least one functional group selected from epoxy groups, mercapto groups, and acid anhydride groups. The silane coupling agent may be included in amounts of 0.01 to 0.1 parts by weight per 100 parts by weight of the total of (A), (B), and (D).

[0054] As the adhesive composition of this embodiment, an adhesive composition solution can be used in which the thixotropy index (TI value) = [viscosity at 2 rpm] / [viscosity at 20 rpm], which is the ratio of the viscosity measured at a rotor speed of 2 rpm to the viscosity measured at 20 rpm using a BH type viscometer (manufactured by Toki Sangyo Co., Ltd.) at 25°C, is 1.25 or less. The above TI value is a value that evaluates the shear rate dependence of viscosity in the viscosity characteristics of the adhesive composition solution, and is a value that serves as a measure of so-called "thixotropic viscosity." The TI value can be calculated by measuring the viscosity of the adhesive composition solution using a BH-type viscometer, changing the rotor rotation speed to 2 rpm and 20 rpm, and taking the ratio of the values. When the TI value is high, the viscosity of the coating material, which consists of an adhesive composition, decreases with changes in shear rate. For example, in the process of coating a resin film with a coating material consisting of an adhesive composition, the viscosity of the coating material decreases due to shear deformation, resulting in poor coating performance.

[0055] In this embodiment, an adhesive composition solution with a TI value of 1.25 or less can be used. If the TI value exceeds 1.25, the coating properties of the adhesive composition deteriorate, and stable coating cannot be achieved. Generally, in acrylic polymers with a molecular weight of 1 million or more obtained by polymerizing alkyl (meth)acrylate monomers with C8 or more carbon atoms in the alkyl group as a monomer component, thixotropic viscosity is exhibited due to the entanglement of polymer chains based on long-chain alkyl groups with C8 or more carbon atoms, and the above-mentioned TI value tends to be high, resulting in poor coating properties when used as an adhesive composition paint. In the adhesive composition of this embodiment, an adhesive composition solution within the above TI value range can be obtained by adjusting the type and content ratio of monomer components constituting the acrylic polymer of the adhesive composition, adjusting the polymerization method of the copolymer, and adjusting the concentration of the acrylic polymer when dissolved in a solvent such as ethyl acetate. The adhesive composition solution of this embodiment achieves stable coating properties because, despite the weight-average molecular weight of the copolymer used as the acrylic polymer being 1.5 million or more, the TI value is 1.25 or less.

[0056] The adhesive composition of this embodiment may appropriately incorporate known additives other than those mentioned above, such as antioxidants, antistatic agents, surfactants, curing accelerators, plasticizers, tackifiers, fillers, crosslinking catalysts, crosslinking retarders, curing retarders, processing aids, anti-aging agents, and UV absorbers. These may be used individually or in combination of two or more.

[0057] The adhesive layer formed by crosslinking the adhesive composition of this embodiment can be formed by applying the adhesive composition to a substrate or release film and then crosslinking the adhesive composition. The gel fraction of the adhesive layer after crosslinking is preferably 40-70%, more preferably 42-68%, and most preferably 45-65%. If the gel fraction of the adhesive layer is outside this range, it is undesirable because it reduces heat resistance and durability.

[0058] When using the crosslinked adhesive layer of the adhesive composition of this embodiment for bonding between layers of optical components, a thin adhesive layer is desirable. The thickness of the adhesive layer is preferably 1 to 25 μm, more preferably 5 to 25 μm, and even more preferably 5 to 20 μm. Furthermore, in order to reduce the reflection of light rays at the interface between the adhesive layer and the optical component, it is desirable to minimize the difference in refractive index between the optical component and the adhesive layer.

[0059] Furthermore, it is preferable that the dent resistance of the 20 μm thick adhesive layer, obtained by crosslinking the adhesive composition of this embodiment, is 10 g or more. Furthermore, it is preferable that the adhesive force when a 140 μm thick polarizing plate with PET (polyethylene terephthalate; 20 μm thick) as a surface substrate is bonded to an alkali-free glass with a thickness of 0.4 mm via an adhesive layer formed by crosslinking the adhesive composition, and when the polarizing plate with PET as the surface substrate is peeled from the glass surface at a peeling speed of 300 mm / min in the 180° direction in accordance with JIS-Z0237, is 4 N / 25 mm or less. It is even more preferable that the adhesive force is 1 to 4 N / 25 mm, and particularly preferable that it is 1.2 to 3.8 N / 25 mm.

[0060] Furthermore, an adhesive film formed by laminating an adhesive layer formed by crosslinking the adhesive composition of this embodiment can be manufactured by forming the adhesive layer on one side of a substrate, an optical film, and a release film. For the substrate film used to form the adhesive layer and the release film (separator) protecting the adhesive surface, resin films such as polyester film can be used. The substrate film can be treated on the side opposite to the side where the adhesive layer of the resin film is formed with antifouling treatments such as silicone-based or fluorine-based release agents or coatings, silica fine particles, or antistatic treatments such as application or kneading of antistatic agents.

[0061] The release film has a release agent applied to the side that comes into contact with the adhesive surface of the adhesive layer, such as a silicone-based or fluorine-based release agent. A "release film / adhesive layer / release film" configuration can also be achieved by aligning the release-treated sides of the release film with the adhesive layers of a single optical film adhesive layer. In this case, the release films on both sides can be peeled off sequentially or simultaneously to expose the adhesive surface, allowing for bonding with optical components such as optical films. Examples of optical films include polarizing films, phase difference films, anti-reflective films, anti-glare films, ultraviolet absorbing films, infrared absorbing films, optical compensation films, brightness-enhancing films, lens films, hard coat films, and transparent conductive films for touch panels.

[0062] The adhesive layer obtained by crosslinking the adhesive composition of this embodiment has excellent heat resistance and durability, making it suitable for applications in which a polarizing film and a liquid crystal display panel are bonded in a configuration of polarizing film / adhesive layer / liquid crystal display panel, and further, for applications in which a phase difference film and a polarizing film or an organic EL panel are bonded in a configuration of polarizing film / adhesive layer / phase difference film / adhesive layer / organic EL panel.

[0063] The adhesive layer and adhesive film obtained by crosslinking the adhesive composition of this embodiment can be used for laminating various optical films incorporated into image display devices, various optical films for touch panels, various optical films for electronic paper, various optical films incorporated into liquid crystal display panels, and various optical films incorporated into organic EL panels. Furthermore, an adhesive layer-equipped optical film can be obtained in which the adhesive layer is laminated on at least one surface of these optical films. When the adhesive layer is protected by a release film, the adhesive layer can be exposed by peeling off the release film, such as in an "optical film / adhesive layer" configuration, and by laminating it with other optical films, a configuration such as an "optical film / adhesive layer / optical film" can be obtained in which an adhesive layer is used between layers.

[0064] The adhesive layer obtained by crosslinking the adhesive composition of this embodiment is suitably used for laminating various optical films incorporated into image display devices. The adhesive layer obtained by crosslinking the adhesive composition of this embodiment can also be suitably used in configurations such as "polarizing film / adhesive layer", "polarizing film / adhesive layer / release film", "polarizing film / adhesive layer / phase difference film / adhesive layer", and "polarizing film / adhesive layer / phase difference film / adhesive layer / release film".

[0065] Examples of image display devices mentioned above include, but are not limited to, liquid crystal display devices (liquid crystal display panels), organic EL display devices (organic EL panels), touch panels, and electronic paper. While the following explanation will use organic EL panels as a representative example, the adhesive layer and adhesive film of this embodiment, based on these concepts, can also be applied to other image display devices such as liquid crystal display panels. As an image display device, it can be applied to LCD display panels for in-vehicle monitors and car navigation systems, particularly those used in automobiles and other vehicles where high durability is required.

[0066] Polycarbonate compounds are used as materials for the phase difference film incorporated into organic EL panels, and materials with inverse wavelength dispersion (birefringence increases with longer wavelengths) are preferred. Polycarbonate compounds are resins that contain carbonate bonds (-O-CO-O-) in the repeating units of the polymer, and various types of chemical structures are known for being contained between each carbonate bond, such as aromatic and aliphatic types. There may be one or more repeating units. The phase difference of the phase difference film may be 1 / 4λ or 1 / 2λ (1 / 4 or 1 / 2 times the wavelength λ). In recent years, organic EL panels have required circular polarizers to prevent external light reflection. These circular polarizers consist of two layers: a linear polarizer and a 1 / 4λ phase difference film. Furthermore, the phase difference film surface is laminated to the organic EL panel, resulting in drawbacks such as poor durability and foam resistance due to the polycarbonate resin used. The adhesive layer obtained by crosslinking the adhesive composition of this embodiment has extremely high utility as an optical film for polarizing films and organic EL panels that incorporate a 1 / 4λ phase difference film made of polycarbonate resin.

[0067] Generally, polarizing films use a polyvinyl alcohol (PVA) layer containing iodine molecules (I2) as the polarizer, with resin films made of materials with excellent optical properties (such as transparency), such as triacetylcellulose (TAC), COP (cycloolefin polymer), acrylic resin, or polypropylene (PP), laminated on both sides as protective layers (sometimes called protective films for the polarizer). Furthermore, the protective resin film may also function as a phase difference film. In addition, for applications requiring high durability, dye-based polarizers may be used instead of iodine-based ones.

[0068] Organic EL panels include those with an organic light-emitting diode (OLED) structure, in which multiple functional layers such as a hole injection layer, hole transport layer, light-emitting layer, electron transport layer, and electron injection layer are formed and stacked between positive and negative electrodes to create a light-emitting element. Examples of electrodes include light-transmitting transparent electrode layers and opaque metal electrode layers. Flexible organic EL panels can also be manufactured, and electrodes, wiring, and functional layers can be formed on a flexible substrate such as resin by foil, plating, printing, vapor deposition, sputtering, etc. [Examples]

[0069] The present invention will be specifically described below with reference to examples.

[0070] <Manufacturing of acrylic polymers> [Example 1] Nitrogen gas was introduced into a reaction apparatus equipped with a reaction vessel, stirrer, thermometer, reflux condenser, and nitrogen inlet tube to replace the air in the reaction vessel with nitrogen gas. Then, 50 parts by weight of isononyl acrylate, 35 parts by weight of butyl acrylate, 10 parts by weight of methyl acrylate, 0.2 parts by weight of 8-hydroxyoctyl acrylate, and 5 parts by weight of benzyl acrylate were added to the reaction vessel along with 60 parts by weight of solvent (ethyl acetate). Subsequently, 0.1 parts by weight of azobisisobutyronitrile was added dropwise over 2 hours as a polymerization initiator, and the reaction was carried out at 65°C for 6 hours to obtain the acrylic polymer solution of Example 1. [Examples 2-8 and Comparative Examples 1-5] Except for the types and proportions of each monomer constituting the acrylic polymer being as described in groups (A) to (D) of Table 1, acrylic polymer solutions for Examples 2 to 8 and Comparative Examples 1 to 5 were obtained in the same manner as in Example 1 above.

[0071] <Manufacturing of adhesive compositions, adhesive layers, and adhesive films> [Example 1] As described above, 0.05 parts by weight of xylylene diisocyanate (XDI) adduct (Takenate® D-110N; manufactured by Mitsui Chemicals, Inc.) and 0.05 parts by weight of silane coupling agent (KBM-403; 3-glycidoxypropyltrimethoxysilane; manufactured by Shin-Etsu Chemical Co., Ltd.) were added to the acrylic polymer solution of Example 1 in a reaction vessel and stirred to obtain the adhesive composition of Example 1. This adhesive composition was applied to a release film (silicone resin-coated polyethylene terephthalate (PET) film) to a thickness of 20 μm after drying, and then dried at 90°C for 5 minutes to form an adhesive layer. Subsequently, a release-treated release film was placed on top of the dried adhesive layer to create a layer structure consisting of release film / adhesive layer / release film. Further aging (curing) was performed for 7 days at 23°C and 50% RH to obtain the adhesive film of Example 1. [Examples 2-8 and Comparative Examples 1-5] Except for the types and proportions of additives other than acrylic polymers being as described in groups (G) and (H) of Table 1, respectively, adhesive compositions, adhesive layers, and adhesive films of Examples 2 to 8 and Comparative Examples 1 to 5 were obtained in the same manner as in Example 1 above.

[0072] [Table 1]

[0073] In Table 1, the total amount of alkyl (meth)acrylate monomers with C8 to C18 alkyl groups in group (A), alkyl (meth)acrylate monomers with C1 to C5 alkyl groups in group (B), and copolymerizable vinyl monomers containing aromatic groups in group (D) was set to 100 parts by weight. The addition ratios of groups (A) to (D), (G), and (H) are indicated in parts by weight after the abbreviations or names. Group (C) is copolymerizable vinyl monomers containing hydroxyl groups, but Comparative Example 3 also includes copolymerizable vinyl monomers containing carboxyl groups. Group (G) is isocyanate compounds with three or more functions, and group (H) is a silane coupling agent.

[0074] Table 2 shows the compound names corresponding to the abbreviations of each component shown in Table 1. Note that Coronate® L is a trade name of Tosoh Corporation. TDI means tolylene diisocyanate, and XDI means xylylene diisocyanate. The silane coupling agents KBM-403, X-12-967C (3-trimethoxysilylpropyl succinic anhydride), and X-41-1805 (mercapto group-containing oligomer-type silane coupling agent) are trade names of Shin-Etsu Chemical Co., Ltd.

[0075] [Table 2]

[0076] <Test Methods and Evaluation> Using the adhesive films obtained in Examples 1-8 and Comparative Examples 1-5, the release film (silicone resin-coated PET film) was peeled off to expose the adhesive layer, which was then used as various measurement and test samples.

[0077] <Evaluation of adhesive strength> A 140 μm thick polarizing plate with a 20 μm thick PET surface substrate was bonded to an 8-inch, 0.4 mm thick alkali-free glass via an adhesive layer obtained from the adhesive films of Examples 1-8 and Comparative Examples 1-5, and left for 1 hour. Subsequently, in accordance with JIS-Z0237, the peel strength when the polarizing plate with the PET surface substrate was peeled from the glass surface at a peeling speed of 300 mm / min in the 180° direction was defined as the adhesive strength (N / 25 mm).

[0078] <Evaluation of durability and brightness uniformity> A 140 μm thick polarizing plate with a 20 μm thick PET surface substrate was laminated to an 8-inch, 0.4 mm thick alkali-free glass using the adhesive films described in Examples 1-8 and Comparative Examples 1-5. After storage for 500 hours at 105°C, the "durability" was evaluated based on the presence or absence of defects such as peeling and foaming, and the presence or absence of "brightness unevenness," such as color intensity, when placed on a backlight was evaluated. In the durability evaluation, "×" was used if defects such as peeling or foaming of the adhesive layer occurred, and "○" was used if there were no defects at all. In the brightness unevenness evaluation, "×" was used if brightness unevenness was observed, and "○" was used if there was no unevenness.

[0079] <Evaluation of Panel Curl> A sample panel was prepared by interposing a 20 μm thick adhesive layer, formed by crosslinking an adhesive composition on both sides of an 8-inch, 0.4 mm thick alkali-free glass sheet, with a 140 μm thick polarizing plate using PET as the surface substrate. The panel was then laminated in a cross-nicol configuration. The panel was left at 80°C Dry for 120 hours, after which the curl of the panel was measured. The numerical value represents the height of the panel edge when the panel is placed on a flat surface. A panel was considered to be in good condition with no curl if the amount of curl was 0.5 mm or less.

[0080] <Evaluation of TI value of adhesive composition solution> A BH-type viscometer (manufactured by Toki Sangyo Co., Ltd.) was used to measure the viscosity of an 18% adhesive composition solution (solvent: ethyl acetate) adjusted to 25°C at rotor speeds of 2 rpm and 20 rpm. The thixotropy index (TI value) was calculated from the viscosity values ​​measured at each rotation speed as follows: thixotropy index = [viscosity at 2 rpm] / [viscosity at 20 rpm]. If the TI value exceeded 1.25, it was considered that the thixotropic viscosity was significant and that the coating properties were not good.

[0081] <Evaluation of average molecular weight and molecular weight dispersion (Mw / Mn) of acrylic polymers> Gel permuration chromatography (GPC) was used to calculate the weight-average molecular weight (Mw), number-average molecular weight (Mn), and dispersion (Mw / Mn) of acrylic polymers in terms of polystyrene.

[0082] <Evaluation of dent resistance> A 20 μm thick adhesive layer sheet, formed by crosslinking an adhesive composition, was subjected to a load applied to its surface by contacting a 5 mm diameter steel ball. The resulting indentation was visually observed and evaluated. The load was increased by 5 g increments, and the indentation was observed. The maximum load at which no indentation occurred was recorded and used as a measure of indentation resistance. If no indentation occurred at a load of 10 g, the sheet was considered to have good indentation resistance.

[0083] Table 3 shows the evaluation results.

[0084] [Table 3]

[0085] The adhesive layers of Examples 1 to 8, which are specific examples of the adhesive composition of the present invention, satisfy all the inventive features described in Claim 1 of this application and have appropriate adhesive strength considering reworkability. Furthermore, in durability tests at relatively high temperatures, no peeling, lifting, or foaming occurred, demonstrating good durability, and no brightness unevenness occurred. In other words, the adhesive layer formed from the adhesive composition of the present invention can exhibit excellent heat resistance and durability when used for bonding optical films such as polarizing films to liquid crystal display panels. Furthermore, in a laminate formed by bonding an optical film and a liquid crystal display panel with an adhesive layer formed from the adhesive composition of the present invention, even when left at high temperatures that cause dimensional changes in the optical film, the panel maintains a state of very little curling, and the adhesive layer itself exhibits excellent resistance to dents. This indicates that the present invention provides an adhesive composition that forms an adhesive layer with unprecedented performance, where the adhesive layer formed from the adhesive composition of the present invention has the ability to prevent the effects of deformation and stress on the optical film, which is prone to shrinkage and associated stress depending on environmental conditions, from being transmitted to the panel side, while the adhesive layer itself also has the characteristic of being resistant to deformation.

[0086] Furthermore, regarding the acrylic polymer, the adhesive layer using the adhesive composition of Comparative Example 1, which (A) has a low content of alkyl (meth)acrylate monomers with C8 to C18 C18 alkyl groups and (D) does not contain at least one copolymerizable vinyl monomer containing an aromatic group, had high adhesive strength and did not peel or foam after high-temperature treatment in durability evaluation, but exhibited brightness unevenness and panel curling, and did not have good performance.

[0087] Regarding the acrylic polymer, the adhesive layer using the adhesive composition of Comparative Example 2, which does not contain at least one copolymerizable vinyl monomer containing an aromatic group (D), has a weight-average molecular weight below the numerical range specified in claim 1 of this application, and does not contain a silane coupling agent, had insufficient durability, resulting in uneven brightness and panel curling, and poor resistance to dents. Furthermore, the adhesive composition solution exhibited strong thixotropic viscosity and poor coating properties.

[0088] In Comparative Example 3, the adhesive composition of the acrylic polymer, which contained a copolymerizable vinyl monomer containing a carboxyl group instead of a copolymerizable vinyl monomer containing a hydroxyl group, and had an excessive amount of crosslinking agent, exhibited good dent resistance in the adhesive layer, but poor durability including adhesive strength, brightness unevenness, and panel curling. Furthermore, the thixotropic viscosity of the adhesive composition solution was high, resulting in poor coating properties.

[0089] Furthermore, in the adhesive layer using the adhesive composition of Comparative Example 4, which (B) has a low content of alkyl (meth)acrylate monomers with C1 to C5 alkyl groups, (D) does not contain at least one copolymerizable vinyl monomer containing an aromatic group, and uses an acrylic polymer whose weight-average molecular weight is below the numerical range specified in Claim 1 of this application, and does not contain a silane coupling agent, the brightness uniformity and panel curling were good, but in the durability evaluation, peeling and foaming were observed after high-temperature treatment, and the dent resistance was also poor. The coating properties were also not good.

[0090] Furthermore, regarding the acrylic polymer, in the adhesive layer using the adhesive composition of Comparative Example 5, which does not contain at least one alkyl (meth)acrylate monomer with C1 to C5 carbon atoms in the alkyl group, the durability evaluation was unsatisfactory, as peeling and uneven brightness occurred after high-temperature treatment.

[0091] Thus, the adhesive layers using the adhesive compositions of Comparative Examples 1 to 5 did not have sufficient durability, and were unable to achieve both suppression of panel curling and resistance to dents.

Claims

[Claim 1] An adhesive composition containing an acrylic polymer, a crosslinking agent, and a silane coupling agent, The aforementioned acrylic polymer (A) At least one alkyl (meth)acrylate monomer having C8 to C18 in the alkyl group, in an amount of 20 to 50 parts by weight, (B) At least one alkyl (meth)acrylate monomer having C1 to C5 carbon atoms in the alkyl group, in an amount of 40 to 60 parts by weight, (C) At least one copolymerizable vinyl monomer containing a hydroxyl group, (D) At least one copolymerizable vinyl monomer containing an aromatic group, in amounts of 5 to 20 parts by weight, and only, The weight of the (C) copolymerizable vinyl monomer containing a hydroxyl group is such that when the total of (A) at least one alkyl (meth)acrylate monomer having C8 to C18 in the alkyl group, (B) at least one alkyl (meth)acrylate monomer having C1 to C5 in the alkyl group, and (D) at least one copolymerizable vinyl monomer containing an aromatic group is 100 parts by weight, the content of the (C) copolymerizable vinyl monomer containing a hydroxyl group is 0.5 to 1.5 parts by weight. An acrylic polymer with a weight-average molecular weight of 1.5 million to 3 million, copolymerized without containing a copolymerizable vinyl monomer containing a carboxyl group. The alkyl(meth)acrylate monomer having C8 to C18 carbon atoms in the alkyl group (A) is at least one selected from 2-ethylhexyl acrylate, isooctyl acrylate, isononyl acrylate, and isodecyl acrylate. The alkyl(meth)acrylate monomer having C1 to C5 carbon atoms in the (B) alkyl group is at least one selected from the group consisting of n-butyl acrylate, t-butyl acrylate, isobutyl acrylate, n-butyl methacrylate, and methyl acrylate. The (C) copolymerizable vinyl monomer containing a hydroxyl group is at least one selected from the group of compounds consisting of 8-hydroxyoctyl (meth)acrylate, 6-hydroxyhexyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, 2-hydroxyethyl (meth)acrylate, N-hydroxy(meth)acrylamide, N-hydroxymethyl (meth)acrylamide, and N-hydroxyethyl (meth)acrylamide. The methyl acrylate content is 3 to 30 parts by weight when the total of (A) at least one alkyl (meth)acrylate monomer having C8 to C18 in the alkyl group, (B) at least one alkyl (meth)acrylate monomer having C1 to C5 in the alkyl group, and (D) at least one copolymerizable vinyl monomer containing an aromatic group is 100 parts by weight. An adhesive composition in which the crosslinking agent is a trifunctional or more isocyanate compound contained in a proportion of 0.001 parts by weight or more and 0.05 parts by weight or less per 100 parts by weight of the total of (A) at least one alkyl (meth)acrylate monomer having C8 to C18 carbon atoms in the alkyl group, (B) at least one alkyl (meth)acrylate monomer having C1 to C5 carbon atoms in the alkyl group, and (D) at least one copolymerizable vinyl monomer containing an aromatic group.

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