WO2025205577A1 - Photocurable resin composition - Google Patents

Abstract

The present invention provides a photocurable resin composition that exhibits less shrinkage stress during curing, that applies less stress on an adherend during curing, and that can produce a cured product which has superior adhesiveness, and in which fogging due to moisture is reduced and yellowing occurring in a high-temperature atmosphere is reduced. A photocurable resin composition according to the present invention is characterized by comprising: a (meth)acrylic monomer (A) that is water‐insoluble and that has a glass transition temperature Tg of 50°C or higher; and a polyfunctional (meth)acrylic oligomer (B) that has a polydiene-based skeleton having a hydrogenation rate of 90% or more.

Description

Photocurable resin composition

The present invention relates to a photocurable resin composition.

Photocurable resin compositions have traditionally been used as adhesives for glass and synthetic resins. Photocurable resin compositions are cured by exposure to light, thereby exhibiting adhesive properties. Photocurable resin compositions generate shrinkage stress during photocuring.

If the shrinkage stress of a photocurable resin composition during photocuring is large, stress is applied to the adherend that is bonded using the photocurable resin composition, causing distortion. In the optical field, such as optical lenses, if optical distortion occurs in the adherend, it will lead to a decrease in product quality, so there is a need to reduce the stress applied to the adherend and thereby reduce the occurrence of optical distortion.

Furthermore, in recent years, the use of optical lenses in automobiles has increased with the spread of autonomous driving and related management systems. The interior of an automobile is greatly affected by temperature and humidity, and it is particularly important that the quality of the cured product of the photocurable resin composition does not deteriorate even in harsh, humid and hot environments.

Patent Document 1 discloses a photocurable adhesive composition containing (a) an oligomer having a photocurable functional group, (b) a long-chain hydrocarbon-based (meth)acrylate monomer, and (c) a cyclic (meth)acrylate monomer.

JP 2012-1648 A

However, the above-mentioned photocurable adhesive compositions have problems in that they undergo large shrinkage stresses during curing, which causes significant stress on the adherend during curing, and the cured product has poor adhesion. Furthermore, when used over long periods in harsh environments such as the interior of an automobile, the cured product of the photocurable adhesive composition can become cloudy, and when exposed to high summer temperatures, it can yellow.

The present invention provides a photocurable resin composition that exhibits low shrinkage stress during curing and imparts minimal stress to the adherend upon curing. It also provides a photocurable resin composition that has excellent adhesion, is resistant to fogging due to humidity, and can produce a cured product with reduced yellowing due to high-temperature environments.

The photocurable resin composition of the present invention contains a water-insoluble (meth)acrylic monomer (A) having a glass transition temperature Tg of 50°C or higher, and a polyfunctional (meth)acrylic oligomer (B) having a polydiene skeleton with a hydrogenation rate of 90% or higher.

The photocurable resin composition of the present invention has low shrinkage stress during curing, and therefore exerts little stress on the adherend during curing. The photocurable resin composition of the present invention can produce a cured product with excellent adhesive properties.

The photocurable resin composition of the present invention reduces moisture absorption, making it less susceptible to clouding due to moisture, and can produce cured products with reduced yellowing due to high-temperature environments.

In the numerical ranges described in this specification in stages, the upper or lower limit of a numerical range in one stage can be arbitrarily combined with the upper or lower limit of a numerical range in another stage. In the numerical ranges described in this specification, the upper or lower limit of that numerical range may be replaced with a value shown in an example or a value that can be unambiguously derived from an example. In this specification, numbers connected with "~" mean a numerical range that includes the numbers before and after "~" as the upper and lower limits.

The photocurable resin composition of the present invention contains a water-insoluble (meth)acrylic monomer (A) having a glass transition temperature Tg of 50°C or higher, and a polyfunctional (meth)acrylic oligomer (B) having a polydiene skeleton with a hydrogenation rate of 90% or higher.

In the present invention, "(meth)acrylic" refers to having an acryloyl group [formula (1)] or a methacryloyl group [formula (2)] in the molecule. In formulas (1) and (2), *1 represents a bond and represents a single bond.

[(Meth)acrylic Monomer (A) Having a Glass Transition Temperature Tg of 50°C or Higher and Being Water-Insoluble]
The photocurable resin composition contains a (meth)acrylic monomer (A) (hereinafter sometimes simply referred to as "(meth)acrylic monomer (A)") that has a glass transition temperature Tg of 50°C or higher and is water-insoluble.

The photocurable resin composition contains a (meth)acrylic monomer (A). By using the (meth)acrylic monomer (A) in combination with the polyfunctional (meth)acrylic oligomer (B) described below, the photocurable resin composition produces a cured product with excellent adhesive properties. Furthermore, the cured product of the photocurable resin composition exhibits reduced cloudiness due to moisture and reduced yellowing due to high-temperature environments.

The glass transition temperature Tg of the (meth)acrylic monomer (A) is 50°C or higher, preferably 90°C or higher, more preferably 120°C or higher, more preferably 150°C or higher, more preferably 160°C or higher, and more preferably 170°C or higher. The glass transition temperature Tg of the (meth)acrylic monomer (A) is preferably 250°C or lower, more preferably 230°C or lower, more preferably 210°C or lower, more preferably 200°C or lower, and more preferably 190°C or lower. When the glass transition temperature Tg of the (meth)acrylic monomer (A) is 50°C or higher, the photocurable resin composition produces a cured product with superior adhesive properties. Furthermore, the cured product of the photocurable resin composition exhibits reduced cloudiness due to humidity and reduced yellowing due to high-temperature environments. When the glass transition temperature Tg of the (meth)acrylic monomer (A) is 250°C or lower, the stress applied to the adherend during curing of the photocurable resin composition is reduced, which is preferable. The glass transition temperature Tg of (meth)acrylic monomer (A) refers to the temperature measured in accordance with JIS K7121:1987.

The (meth)acrylic monomer (A) is water-insoluble. Because the (meth)acrylic (A) is water-insoluble, the photocurable resin composition produces a cured product with excellent adhesive properties, and further, the cured product of the photocurable resin composition exhibits reduced cloudiness due to moisture.

"(Meth)acrylic monomer (A) is water-insoluble" means that when an equal volume of (meth)acrylic monomer (A) is added to 100 g of pure water at 20°C and mixed at a stirring speed of 100 rpm, and the mixture is allowed to stand for one hour, the (meth)acrylic monomer (A) and water separate into two layers.

The (meth)acrylic monomer (A) may be any monomer that has a glass transition temperature Tg of 50°C or higher and is water-insoluble, and examples thereof include (meth)acrylate monomers having a monocyclo ring, (meth)acrylate monomers having a bicyclo ring, and (meth)acrylate monomers having a tricyclo ring. The (meth)acrylic monomers (A) may be used alone or in combination of two or more. In the present invention, (meth)acrylate refers to acrylate or methacrylate.

In the (meth)acrylic monomer (A), the cyclic skeleton such as a monocyclo ring, bicyclo ring, or tricyclo ring is preferably a saturated alicyclic skeleton. When the (meth)acrylic monomer (A) has a saturated alicyclic skeleton, the photocurable resin composition produces a cured product with superior adhesive properties, and further, the cured product of the photocurable resin composition exhibits reduced cloudiness due to moisture and reduced yellowing due to high-temperature atmospheres. An alicyclic structure is a structure in which carbon atoms are bonded in a ring and does not have aromaticity.

Examples of (meth)acrylates having a monocyclo ring include cyclohexyl (meth)acrylate, morpholine (meth)acrylate, benzyl (meth)acrylate, phenoxyethyl (meth)acrylate, and 4-tert-butylcyclohexyl (meth)acrylate, with 4-tert-butylcyclohexyl (meth)acrylate being preferred.

Examples of (meth)acrylate monomers having a bicyclo ring include dicyclopentenyloxyethyl (meth)acrylate, dicyclopentanyl (meth)acrylate, dicyclopentenyl (meth)acrylate, and isobornyl (meth)acrylate. Of the (meth)acrylate monomers having a bicyclo ring, isobornyl (meth)acrylate and dicyclopentanyl (meth)acrylate are more preferred, with isobornyl methacrylate and dicyclopentanyl methacrylate being even more preferred. This is because the photocurable resin composition produces a cured product with superior adhesive properties, and furthermore, the cured product of the photocurable resin composition exhibits reduced cloudiness due to moisture and reduced yellowing due to high-temperature environments.

Examples of (meth)acrylate monomers having a tricyclo ring include (meth)acrylate compounds having an adamantane skeleton.

The (meth)acrylic monomer (A) is preferably a (meth)acrylate monomer having a bicyclo ring. The bicyclo ring preferably has an alicyclic skeleton, and more preferably a saturated alicyclic skeleton. This is because the photocurable resin composition produces a cured product with superior adhesive properties, and further, the cured product of the photocurable resin composition exhibits reduced cloudiness due to moisture and reduced yellowing due to high-temperature environments.

The (meth)acrylic monomer (A) may be polyfunctional. When the (meth)acrylic monomer (A) is polyfunctional, the photocurable resin composition has a small shrinkage stress during curing, and the stress applied to the adherend is small. Examples of the polyfunctional (meth)acrylic monomer (A) include tricyclodecane dimethanol di(meth)acrylate and ethoxylated bisphenol A di(meth)acrylate. The polyfunctional (meth)acrylic monomer (A) preferably has two or more (meth)acryloyl groups in the molecule. The (meth)acryloyl group refers to an acryloyl group (CH ₂ ═CHCO—) or a methacryloyl group (CH ₂ ═C(CH ₃ )CO—).

[Polyfunctional (meth)acrylic oligomer (B) having a polydiene skeleton with a hydrogenation rate of 90% or more]
The photocurable resin composition contains a polyfunctional (meth)acrylic oligomer (B) having a polydiene skeleton with a hydrogenation rate of 90% or more (hereinafter, may be simply referred to as "polyfunctional (meth)acrylic oligomer (B)").

Because the photocurable resin composition contains the polyfunctional (meth)acrylic oligomer (B), the photocurable resin composition has low shrinkage stress during curing and imparts little stress to the adherend. Furthermore, the photocurable resin composition produces a cured product with excellent adhesive properties, and the cured product of the photocurable resin composition exhibits reduced cloudiness due to moisture and reduced yellowing due to high-temperature environments.

The hydrogenation rate of the polyfunctional (meth)acrylic oligomer (B) is 90% or more, preferably 91% or more, more preferably 92% or more, more preferably 93% or more, and even more preferably 94% or more. When the hydrogenation rate of the polyfunctional (meth)acrylic oligomer (B) is 90% or more, yellowing caused by a high-temperature atmosphere can be reduced in the cured product of the photocurable resin composition. The hydrogenation rate of the polyfunctional (meth)acrylic oligomer (B) is the proportion of hydrogen added to unsaturated bonds contained in the molecule, and when all unsaturated bonds are hydrogenated, the hydrogenation rate is 100%.

The polyfunctional (meth)acrylic oligomer (B) having a hydrogenated polydiene skeleton refers to a (meth)acrylate having a hydrogenated conjugated diene skeleton in the molecule. The hydrogenated conjugated diene refers to, for example, a compound obtained by hydrogenating a conjugated diene. Examples of hydrogenated conjugated dienes include hydrogenated polybutadiene and hydrogenated polyisoprene, with hydrogenated polybutadiene being preferred.

The polyfunctional (meth)acrylic oligomer (B) has a hydrogenated polydiene skeleton within the molecule. Therefore, the photocurable resin composition exhibits reduced shrinkage stress during curing, reducing the stress it exerts on the adherend. Furthermore, the photocurable resin composition produces a cured product with excellent adhesive properties. Furthermore, the cured product of the photocurable resin composition exhibits reduced cloudiness due to moisture and reduced yellowing due to high-temperature environments.

The polyfunctional (meth)acrylic oligomer (B) is polyfunctional, which reduces the shrinkage stress of the photocurable resin composition during curing and reduces the stress applied to the adherend. It is preferable that the polyfunctional (meth)acrylic oligomer (B) has two or more (meth)acryloyl groups in its molecule.

The (meth)acrylate having a hydrogenated conjugated diene skeleton in the molecule is preferably a polyfunctional (meth)acrylate having two or more (meth)acryloyl groups at the terminal or side chain of the molecule. The polyfunctional (meth)acrylate refers to a compound having two or more (meth)acryloyl groups in the molecule. The (meth)acryloyl group refers to an acryloyl group ( _CH2 =CHCO-) or a methacryloyl group ( _CH2 =C( _CH3 )CO-).

It is preferable that the polyfunctional (meth)acrylic oligomer (B) having a hydrogenated polydiene skeleton does not have a conjugated diene structure (a conjugated diene structure in which a double bond is separated by one single bond).

As the polyfunctional (meth)acrylic oligomer (B) having a hydrogenated polydiene skeleton, urethane (meth)acrylate is preferred.

Examples of urethane (meth)acrylates include hydrogenated 1,2-polybutadiene terminal urethane (meth)acrylate (for example, "TEAI-1000" manufactured by Nippon Soda Co., Ltd.).

Here, urethane (meth)acrylate refers to a urethane (meth)acrylate having a urethane bond within the molecule, obtained by reacting (for example, by polycondensation) a polyol compound, an organic polyisocyanate compound, and a hydroxy (meth)acrylate.

Examples of polyol compounds include ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, polyethylene glycol, propylene glycol, dipropylene glycol, polypropylene glycol, butylene glycol, 1,4-butanediol, polybutylene glycol, 1,5-pentanediol, 1,6-hexanediol, 1,8-octanediol, 1,9-nonanediol, 3-methyl-1,5-pentanediol, 2,4-diethyl-1,5-pentanediol, 2,2-butylethyl-1,3-propanediol, neopentyl glycol, cyclohexanedimethanol, hydrogenated bisphenol A, polycaprolactone, trimethylolethane, trimethylolpropane, polytrimethylolpropane, pentaerythritol, polypentaerythritol, sorbitol, mannitol, glycerin, polyglycerin, poly Examples of suitable polyols include polyhydric alcohols such as tetramethylene glycol; polyether polyols having at least one structure of polyethylene oxide, polypropylene oxide, or ethylene oxide/propylene oxide block or random copolymer; polyester polyols, which are condensates of polyhydric alcohols or polyether polyols with polybasic acids such as maleic anhydride, maleic acid, fumaric acid, itaconic anhydride, itaconic acid, adipic acid, and isophthalic acid; caprolactone-modified polyols such as caprolactone-modified polytetramethylene polyol; polyolefin polyols, polycarbonate polyols, polybutadiene polyols, polyisoprene polyols, and hydrogenated conjugated diene polyols (e.g., hydrogenated polybutadiene polyols and hydrogenated polyisoprene polyols); and silicone polyols such as polydimethylsiloxane polyols. Polydiene polyols are preferred because they have a hydrogenated conjugated diene skeleton.

Among polydiene polyols, hydrogenated conjugated diene polyols are preferred because they have a hydrogenated conjugated diene skeleton. As hydrogenated conjugated diene polyols, hydrogenated polybutadiene polyols and hydrogenated polyisoprene polyols are more preferred, with hydrogenated polybutadiene polyols (hydrogenated polybutadiene polyols) being more preferred. Among hydrogenated polybutadiene polyols, hydrogenated 1,2-polybutadiene polyols are preferred.

Organic polyisocyanate compounds are not particularly limited, and examples include aromatic, aliphatic, cyclic aliphatic, and alicyclic polyisocyanates. Examples of organic polyisocyanate compounds include tolylene diisocyanate (TDI), diphenylmethane diisocyanate (MDI), hydrogenated diphenylmethane diisocyanate (H-MDI), polyphenylmethane polyisocyanate (crude MDI), modified diphenylmethane diisocyanate (modified MDI), hydrogenated xylylene diisocyanate (H-XDI), xylylene diisocyanate (XDI), and hexamethylene diisocyanate (HM Suitable polyisocyanates include tolylene diisocyanate (TDI), trimethylhexamethylene diisocyanate (TMXDI), tetramethylxylylene diisocyanate (m-TMXDI), isophorone diisocyanate (IPDI), norbornene diisocyanate (NBDI), and 1,3-bis(isocyanatomethyl)cyclohexane (H6XDI), as well as trimer compounds of these polyisocyanates and reaction products of these polyisocyanates with polyols. Of these, tolylene diisocyanate (TDI), hydrogenated xylylene diisocyanate (H-XDI), and isophorone diisocyanate (IPDI) are preferred, with tolylene diisocyanate (TDI) and isophorone diisocyanate (IPDI) being more preferred, and isophorone diisocyanate (IPDI) being even more preferred.

Hydroxy(meth)acrylate refers to a (meth)acrylate having a hydroxyl group. Examples of hydroxy(meth)acrylates include hydroxyalkyl(meth)acrylates such as 2-hydroxyethyl(meth)acrylate, 2-hydroxypropyl(meth)acrylate, and 2-hydroxybutyl(meth)acrylate, as well as 2-hydroxyethyl(meth)acryloylphosphate, 2-(meth)acryloyloxyethyl-2-hydroxypropyl phthalate, glycerin di(meth)acrylate, 2-hydroxy-3-(meth)acryloyloxypropyl(meth)acrylate, caprolactone-modified 2-hydroxyethyl(meth)acrylate, pentaerythritol tri(meth)acrylate, dipentaerythritol penta(meth)acrylate, and caprolactone-modified 2-hydroxyethyl(meth)acrylate, with hydroxyalkyl(meth)acrylates being preferred. Preferred hydroxyalkyl (meth)acrylates are 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, and 2-hydroxybutyl (meth)acrylate, with 2-hydroxyethyl (meth)acrylate being more preferred.

The average molecular weight of the polyfunctional (meth)acrylic oligomer (B) is preferably 6,000 or more, more preferably 10,000 or more, more preferably 15,000 or more, and more preferably 20,000 or more. The average molecular weight of the polyfunctional (meth)acrylic oligomer (B) is preferably 50,000 or less, more preferably 40,000 or less, and more preferably 30,000 or less. When the average molecular weight of the polyfunctional (meth)acrylic oligomer (B) is 6,000 or more, the stress applied to the adherend during curing of the photocurable resin composition is reduced, which is preferable.

The average molecular weight of the polyfunctional (meth)acrylic oligomer (B) is a value measured as follows. The average molecular weight of the polyfunctional (meth)acrylic oligomer (B) is measured by gel permeation chromatography (GPC) under the following conditions using tetrahydrofuran as the solvent, with a calibration curve prepared using commercially available standard polystyrene. For example, it can be measured using the following measuring device and under the following measuring conditions.

Measurement equipment: ACQUITY APC system manufactured by Waters Measurement conditions: Column: HSPgel™ HR MB-M manufactured by Waters
Mobile phase: tetrahydrofuran 0.5 mL/min Detector: RI detector Standard material: polystyrene SEC temperature: 40°C

The content of the polyfunctional (meth)acrylic oligomer (B) in the photocurable resin composition is preferably 40 parts by mass or more, more preferably 45 parts by mass or more, and more preferably 50 parts by mass or more, per 100 parts by mass of the (meth)acrylic monomer (A). The content of the polyfunctional (meth)acrylic oligomer (B) in the photocurable resin composition is preferably 150 parts by mass or less, more preferably 130 parts by mass or less, more preferably 110 parts by mass or less, and more preferably 90 parts by mass or less, per 100 parts by mass of the (meth)acrylic monomer (A). When the content of the polyfunctional (meth)acrylic oligomer (B) is 40 parts by mass or more, the shrinkage stress during curing of the photocurable resin composition is reduced, the stress applied to the adherend is reduced, and the cloudiness due to moisture of the cured product of the photocurable resin composition and yellowing in a high-temperature atmosphere are reduced. When the content of the polyfunctional (meth)acrylic oligomer (B) is 150 parts by mass or less, the adhesiveness of the cured product of the photocurable resin composition is improved.

[(Meth)acrylic Monomer (C) Having a Glass Transition Temperature Tg of 0°C or Lower]
The photocurable resin composition preferably contains a (meth)acrylic monomer (C) (hereinafter, sometimes simply referred to as "(meth)acrylic monomer (C)") having a glass transition temperature Tg of 0° C. or lower. The glass transition temperature Tg of the (meth)acrylic monomer (C) is measured in the same manner as that of the (meth)acrylic monomer (A).

By including a (meth)acrylic monomer (C) whose glass transition temperature Tg is 0°C or lower in the photocurable resin composition, the shrinkage stress during curing of the photocurable resin composition is further reduced, further reducing the stress applied to the adherend.

The glass transition temperature Tg of the (meth)acrylic monomer (C) is preferably 0°C or lower, more preferably -10°C or lower, more preferably -20°C or lower, more preferably -30°C or lower, more preferably -40°C or lower, more preferably -50°C or lower, and more preferably -60°C or lower. The glass transition temperature Tg of the (meth)acrylic monomer (C) is preferably -80°C or higher, more preferably -70°C or higher. When the glass transition temperature Tg of the (meth)acrylic monomer (C) is 0°C or lower, the shrinkage stress during curing of the photocurable resin composition is further reduced, and the stress applied to the adherend is further reduced.

(Meth)acrylic monomers (C) having a glass transition temperature Tg of 0°C or lower are not particularly limited, and examples include normal octyl (meth)acrylate, isooctyl (meth)acrylate, dodecyl (meth)acrylate, isodecyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, isostearyl (meth)acrylate, butyl (meth)acrylate, ethoxy-diethylene glycol (meth)acrylate, phenoxyethyl (meth)acrylate, and phenoxy-polyethylene glycol (meth)acrylate. Normal octyl (meth)acrylate, dodecyl (meth)acrylate, isodecyl (meth)acrylate, and 2-ethylhexyl (meth)acrylate are preferred, with normal octyl (meth)acrylate being more preferred, and normal octyl acrylate being even more preferred. The (meth)acrylic monomers (C) may be used alone or in combination of two or more.

The content of (meth)acrylic monomer (C) in the photocurable resin composition is preferably 3 parts by mass or more, more preferably 5 parts by mass or more, more preferably 8 parts by mass or more, more preferably 10 parts by mass or more, and more preferably 12 parts by mass or more, per 100 parts by mass of (meth)acrylic monomer (A). The content of (meth)acrylic monomer (C) in the photocurable resin composition is preferably 35 parts by mass or less, more preferably 30 parts by mass or less, and more preferably 25 parts by mass or less, per 100 parts by mass of (meth)acrylic monomer (A). When the content of (meth)acrylic monomer (C) is 3 parts by mass or more, stress applied to the adherend during curing of the photocurable resin composition is further reduced. When the content of (meth)acrylic monomer (C) is 35 parts by mass or less, clouding due to moisture in the cured product of the photocurable resin composition is further reduced, and yellowing in a high-temperature atmosphere is further reduced.

[Photopolymerization initiator]
The photocurable resin composition may contain a photopolymerization initiator. When the photocurable resin composition contains a photopolymerization initiator, the photocurable resin composition can be easily photopolymerized and cured.

The photopolymerization initiator is not particularly limited, and examples thereof include α-hydroxyketone-based photopolymerization initiators, acylphosphine oxide-based photopolymerization initiators, thioxanthone-based photopolymerization initiators, and triazine-based photopolymerization initiators. These initiators have excellent compatibility with the (meth)acrylic monomer (A) and polyfunctional (meth)acrylic oligomer (B), excellent curing properties of the photocurable resin composition, and can reduce shrinkage stress during curing of the photocurable resin composition, thereby reducing stress on the adherend. Therefore, α-hydroxyketone-based photopolymerization initiators and acylphosphine oxide-based photopolymerization initiators are preferred, with α-hydroxyketone-based photopolymerization initiators being more preferred. Photopolymerization initiators may be used alone or in combination of two or more types.

α-Hydroxyketone photopolymerization initiators are not particularly limited, and examples include 1-hydroxycyclohexyl phenyl ketone, 1-[4-(2-hydroxyethoxy)-phenyl]-2-hydroxy-2-methyl-1-propan-1-one, 2-hydroxy-1-{4-[4-(2-hydroxy-2-methyl-propionyl)-benzyl]phenyl}-2-methyl-propan-1-one, and 2-hydroxy-2-methyl-1-phenyl-propan-1-one.

Examples of acylphosphine oxide photopolymerization initiators include 2,4,6-trimethylbenzoyldiphenylphosphine oxide and bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide.

An example of a thioxanthone-based photopolymerization initiator is 2,4-diethylthioxanthone.

Examples of triazine-based photopolymerization initiators include 2-[2-(furan-2-yl)vinyl]-4,6-bis(trichloromethyl)-1,3,5-triazine, 2-[2-(5-methylfuran-2-yl)vinyl]-4,6-bis(trichloromethyl)-1,3,5-triazine, 2-[(4-methoxyphenyl)vinyl]-4,6-bis(trichloromethyl)-1,3,5-triazine, and 2-[(3,4-dimethoxyphenyl)vinyl]-4,6-bis(trichloromethyl)-1,3,5-triazine.

The content of the photopolymerization initiator in the photocurable resin composition is preferably 0.5 parts by mass or more, and more preferably 0.7 parts by mass or more, per 100 parts by mass of the (meth)acrylic monomer (A). The content of the photopolymerization initiator in the photocurable resin composition is preferably 5 parts by mass or less, more preferably 3 parts by mass or less, and even more preferably 2 parts by mass or less, per 100 parts by mass of the (meth)acrylic monomer (A). When the content of the photopolymerization initiator is within the above range, the shrinkage stress during curing of the photocurable resin composition can be further reduced, thereby reducing the stress applied to the adherend.

[Additives]
The photocurable resin composition may contain a silane coupling agent, a thixotropic agent, an adhesion-imparting resin, a plasticizer, non-thermally expandable fine particles, a dye, a pigment, a flame retardant, a surfactant, and the like, within the range that does not impair the physical properties of the composition.

The photocurable resin composition preferably contains a silane coupling agent. The silane coupling agent is not particularly limited, and examples include vinyltrimethoxysilane, vinyltriethoxysilane, (3-acryloxypropyl)trimethoxysilane, (3-methacryloxypropyl)trimethoxysilane, (3-methacryloxypropyl)triethoxysilane, (3-methacryloxyoctyl)trimethoxysilane, and (3-mercaptopropyl)trimethoxysilane. Alkoxysilanes are preferred, trialkoxysilanes are more preferred, trimethoxysilane and triethoxysilane are more preferred, and vinyltriethoxysilane, (3-acryloxypropyl)trimethoxysilane, and (3-methacryloxypropyl)trimethoxysilane are preferred.

The content of the silane coupling agent in the photocurable resin composition is preferably 1 part by mass or more, more preferably 1.5 parts by mass or more, and even more preferably 2 parts by mass or more, per 100 parts by mass of the (meth)acrylic monomer (A). The content of the silane coupling agent in the photocurable resin composition is preferably 10 parts by mass or less, more preferably 8 parts by mass or less, and even more preferably 6 parts by mass or less, per 100 parts by mass of the (meth)acrylic monomer (A).

[Photocurable resin composition]
The method for producing the photocurable resin composition is not particularly limited, and the photocurable resin composition can be produced, for example, by uniformly mixing the (meth)acrylic monomer (A), the polyfunctional (meth)acrylic oligomer (B), and compounds contained as necessary in a general manner, preferably under reduced pressure.

Generally, if the flexibility of a photocurable resin composition is improved in order to reduce the stress it exerts on an adherend during curing, the adhesiveness of the cured product of the photocurable resin composition will decrease and the cured product of the photocurable resin composition will be more susceptible to fogging due to moisture. As such, reducing the stress it exerts on an adherend during curing of the photocurable resin composition and improving the adhesiveness and reducing fogging due to moisture in the cured product of the photocurable resin composition are contradictory goals, and it is difficult to achieve both of these goals at the same time.

However, because the photocurable resin composition contains both the (meth)acrylic monomer (A) and the polyfunctional (meth)acrylic oligomer (B), it is possible to achieve the above-mentioned conflicting goals, namely, reducing the stress applied to the adherend when the photocurable resin composition is cured, while improving the adhesion of the cured product of the photocurable resin composition and reducing cloudiness due to moisture. Furthermore, it is also characterized by being able to reduce the yellowing of the cured product of the photocurable resin composition caused by a high-temperature atmosphere.

The tensile modulus of elasticity of the cured product of the photocurable resin composition is preferably 50 MPa or more, more preferably 100 MPa or more, more preferably 150 MPa or more, and more preferably 200 MPa or more. The tensile modulus of elasticity of the cured product of the photocurable resin composition is preferably 900 MPa or less, more preferably 800 MPa or less, more preferably 700 MPa or less, and more preferably 600 MPa or less. When the tensile modulus of elasticity of the cured product of the photocurable resin composition is 50 MPa or more, the adhesiveness of the cured product of the photocurable resin composition can be improved, clouding due to moisture in the cured product of the photocurable resin composition can be further reduced, and yellowing due to high-temperature atmospheres can be further reduced. When the tensile modulus of elasticity of the cured product of the photocurable resin composition is 900 MPa or less, shrinkage stress during curing of the photocurable resin composition can be further reduced. The tensile modulus of elasticity of the cured photocurable resin composition is the value measured in accordance with JIS K7161-1 under conditions of a dumbbell thickness of 1.0 mm and a tensile speed of 10 mm/min.

The maximum stress of the cured product of the photocurable resin composition is preferably 5 MPa or more, more preferably 6 MPa or more, and even more preferably 7 MPa or more. The maximum stress of the cured product of the photocurable resin composition is preferably 20 MPa or less, more preferably 16 MPa or less, and even more preferably 15 MPa or less. A maximum stress of 5 MPa or more in the cured product of the photocurable resin composition can improve the adhesion of the cured product of the photocurable resin composition, further reduce clouding due to moisture in the cured product of the photocurable resin composition, and further reduce yellowing due to high-temperature environments. A maximum stress of 20 MPa or less in the cured product of the photocurable resin composition can further reduce shrinkage stress during curing of the photocurable resin composition, thereby reducing stress on the adherend. The maximum stress of the cured product of the photocurable resin composition is the value measured in accordance with JIS K7161-1 under conditions of a dumbbell thickness of 1.0 mm and a tensile speed of 10 mm/min.

In the cured product of the photocurable resin composition, the curing shrinkage stress is preferably 0.06 MPa or less, more preferably 0.05 MPa or less. If the curing shrinkage stress is 0.06 MPa or less, the shrinkage stress during curing of the photocurable resin composition is small, reducing the stress applied to the adherend and reducing the occurrence of optical distortion in the adherend. The curing shrinkage stress of the photocurable resin composition is a value measured at room temperature of 23°C in accordance with JIS K6941. For example, it can be measured by curing under the following conditions using an apparatus commercially available from Acroedge Co., Ltd. under the trade name "Custron".
Light source: Panasonic LED-Aicure (wavelength: 365 nm)
Irradiation intensity: 65mW/cm ²
Irradiation time: 300 seconds

[Method for bonding adherends using photocurable resin composition]
The photocurable resin composition can be suitably used to bond and integrate adherends together. The adherend is not particularly limited, and examples thereof include thin-layer glass, optical glass, optical lenses, prisms, lenses, silicon wafers, semiconductor mounting components, and synthetic resin molded products (e.g., synthetic resin sheets), with optical lenses being preferred. The material of the optical lens is not particularly limited, and may be glass or synthetic resin, with glass being preferred. The photocurable resin composition can be suitably used as an adhesive for optical adherends, particularly thin-layer glass, optical glass, prisms, and optical lenses. The adherends may be of the same type or different types.

The following describes the procedure for bonding and integrating adherends using a photocurable resin composition. First, a laminate is produced by placing the adherends in a state where the photocurable resin composition is interposed between the opposing surfaces of the adherends (lamination process). A laminate may also be produced by laminating three or more adherends together with the photocurable resin composition interposed between the opposing surfaces of the adherends.

Because the photocurable resin composition is a one-component type, the complicated process of mixing two components is not required to create a laminate, making it easy to manufacture the laminate.

Next, the entire resulting laminate is irradiated with radiation in the stacking direction of the laminate, curing the photocurable resin composition sandwiched between the opposing surfaces of the adherends that make up the laminate and producing a cured product (polymer) (curing process). This cured product can bond the adherends together.

Because photocurable resin compositions have low shrinkage stress when cured, they can reduce the stress applied to two adherends bonded via the photocurable resin composition, thereby reducing distortion in the adherends.

The cured product produced by curing the photocurable resin composition has excellent adhesive properties, making it possible to firmly bond two adherends together via the cured product.

The peak wavelength of the radiation is preferably 500 nm or less, as this provides excellent curing properties for the photocurable resin composition. The radiation is preferably light with a peak wavelength of 320 nm or more. By using radiation with a peak wavelength of 320 nm or more, the photocurable resin composition can be cured deeply to produce a cured product.

Furthermore, the cured product of the photocurable resin composition that bonds and integrates the adherends has low water absorption, reducing the occurrence of clouding due to humidity and reducing yellowing caused by high-temperature environments.

As a result, the cured product of the photocurable resin composition can maintain a state in which adherends are firmly bonded together for a long period of time without becoming cloudy or yellowing, even in harsh environments such as the interior of an automobile.

The present invention will be explained in more detail below using examples, but the present invention is not limited to these. Specific numerical values for blending ratios (content ratios), physical property values, parameters, etc. used in the following description can be replaced with the upper limit values (numeric values defined as "equal to or less than") or lower limit values (numeric values defined as "equal to or greater than") of the corresponding blending ratios (content ratios), physical property values, parameters, etc. described in the "Description of Embodiments."

The following compounds were used in the examples and comparative examples.

[(Meth)acrylic monomer (A)]
Isobornyl methacrylate (glass transition temperature Tg: 180°C, average molecular weight: 222)
Dicyclopentanyl methacrylate (glass transition temperature Tg: 175°C, average molecular weight: 220)

[Polyfunctional (meth)acrylic oligomer (B)]
- Polyfunctional urethane acrylate oligomer having a 1,2-polybutadiene skeleton with a hydrogenation rate of 95% (polyfunctional urethane acrylate oligomer having a hydrogenated 1,2-polybutadiene skeleton)
A first reactant was obtained by reacting isophorone diisocyanate with 2-hydroxyethyl acrylate in a molar ratio of 1:1. A hydrogenated 1,2-polybutadiene with a diol compound at both ends (weight average molecular weight: 25,000) was reacted with the first reactant in a molar ratio of 1:2.
- Urethane acrylate oligomer having a polyester skeleton (manufactured by Negami Chemical Industrial Co., Ltd., product name "UN-7600", does not have a polydiene skeleton)
Methacrylate oligomer 1 having an unhydrogenated polyisoprene skeleton (manufactured by Kuraray Co., Ltd., product name "UC-102M", does not have a hydrogenated polydiene skeleton)
Methacrylate oligomer 2 having an unhydrogenated polyisoprene skeleton (manufactured by Kuraray Co., Ltd., product name "UC-203M", does not have a hydrogenated polydiene skeleton)

[(Meth)acrylic monomer (C)]
・Normal octyl acrylate

[Photopolymerization initiator]
1-hydroxycyclohexyl phenyl ketone (peak wavelength: 330 nm)

[Silane coupling agent]
(3-methacryloxypropyl)trimethoxysilane

(Examples 1 to 3, Comparative Examples 1 to 4)
The (meth)acrylic monomer (A), the polyfunctional (meth)acrylic oligomer (B), the (meth)acrylic monomer (C), and the silane coupling agent in the predetermined amounts shown in Table 1 were each supplied into a reaction vessel and mixed uniformly to prepare a mixed solution.

Next, the specified amount of photopolymerization initiator shown in Table 1 was added to the mixed liquid and mixed until the photopolymerization initiator was completely dissolved in the mixed liquid, yielding a photocurable resin composition.

The tensile modulus, maximum stress, cure shrinkage stress, water absorption, and yellowing index of the resulting photocurable resin composition were measured using the methods described below, and the results are shown in Table 1.

(Tensile modulus)
Using the obtained photocurable resin composition, a dumbbell-shaped test piece (Type 3 specified in JIS K6251) measuring 50 mm in length, 25 mm in width, and 1.0 mm in thickness was prepared. The test piece was irradiated with ultraviolet light having a peak wavelength of 365 nm at an irradiation intensity of 65 mW/cm for ² minutes to cure the photocurable resin composition and thus the test piece.

The tensile modulus of the cured test specimens was measured in accordance with JIS K7161-1 under conditions of a dumbbell thickness of 1.0 mm and a tensile speed of 10 mm/min.

(maximum stress)
Using the obtained photocurable resin composition, a dumbbell-shaped test piece (Type 3 specified in JIS K6251) measuring 50 mm in length, 25 mm in width, and 1.0 mm in thickness was prepared. The test piece was irradiated with ultraviolet light having a peak wavelength of 365 nm at an irradiation intensity of 65 mW/cm for ² minutes to cure the photocurable resin composition and thus the test piece.

Using the cured test specimens, the maximum stress of the test specimens was measured in accordance with JIS K7161-1 under conditions of a dumbbell thickness of 1.0 mm and a tensile speed of 10 mm/min. The smaller the maximum stress, the smaller the shrinkage stress of the photocurable resin composition during curing.

(Curing shrinkage stress)
The curing shrinkage stress of the obtained photocurable resin composition was measured at room temperature of 23° C. in accordance with JIS K6941 using a measuring device commercially available from Acroedge Co., Ltd. under the trade name "Custron." It can be determined that the smaller the curing shrinkage stress, the smaller the shrinkage stress of the photocurable resin composition during curing.

(Water absorption rate)
The photocurable resin composition thus obtained was used to prepare a planar square test piece measuring 20 mm on a side and 0.5 mm thick. The test piece was irradiated with ultraviolet light having a peak wavelength of 365 nm at an irradiation intensity of 65 mW/ ^cm for 2 minutes to cure the photocurable resin composition and the test piece. The mass _W0 of the test piece was measured.

A cylindrical water bath with a square base measuring 22.5 cm on each side and a height of 17 cm was prepared, and water was poured into the water bath to a depth of 4 cm. A 200-mesh stainless steel wire mesh was placed horizontally in the water bath, 11 cm vertically above the water surface, and a steam-permeable cloth was placed on top of the wire mesh. The cured test specimen was placed on top of the cloth.

Next, the upper opening of the water bath was completely closed with aluminum foil, and the water in the water bath was heated to 96°C and maintained at this temperature for 4 hours. After 4 hours had passed, the test piece was removed from the water bath and its mass _W1 was measured. The water absorption (%) was calculated based on the following formula:
Water absorption rate (%) = 100 x (W ₁ - W ₀ )/W ₀

(yellowing degree)
The photocurable resin composition thus obtained was used to prepare a plate-shaped test piece measuring 50 mm in length, 25 mm in width, and 1.0 mm in thickness. The test piece was irradiated with ultraviolet light having a peak wavelength of 365 nm at an irradiation intensity of 65 mW/ ^cm for 2 minutes to cure the photocurable resin composition and the test piece.

The initial transmittance T ₀ of the test piece at wavelengths of 450 nm and 400 nm was measured using an ultraviolet-visible spectrophotometer (manufactured by Shimadzu Corporation, trade name "UV-3600 Plus").

Next, the test piece was placed in an oven at 135°C and heated for 100 hours. The test piece was removed from the oven and left in an atmosphere at 25°C for 24 hours. The post-heating transmittance _T1 of the test piece after heating was measured using an ultraviolet-visible spectrophotometer (manufactured by Shimadzu Corporation, trade name "UV-3600 Plus"). The rate of change in transmittance was calculated based on the formula shown below. It can be determined that the smaller the rate of change, the lower the degree of yellowing.
Rate of change (%) = 100 x (T ₀ - T ₁ )/T ₀

CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims priority based on Japanese Patent Application No. 2024-57837, filed March 29, 2024, the disclosure of which is incorporated herein by reference in its entirety.

The photocurable resin composition of the present invention has low shrinkage stress during curing, and therefore exerts little stress on the adherend during curing. The photocurable resin composition of the present invention can produce a cured product with excellent adhesive properties. The photocurable resin composition of the present invention can produce a cured product that is less susceptible to moisture absorption, making it less susceptible to fogging due to moisture, and that is less susceptible to yellowing due to high-temperature atmospheres. Therefore, even in harsh environments such as the interior of an automobile, the cured product of the photocurable resin composition can maintain a state in which adherends are firmly bonded together for long periods of time without discoloration such as fogging or yellowing.

Claims (6)

A photocurable resin composition characterized by containing (A) a water-insoluble (meth)acrylic monomer having a glass transition temperature Tg of 50°C or higher, and (B) a polyfunctional (meth)acrylic oligomer having a polydiene skeleton with a hydrogenation rate of 90% or higher. The photocurable resin composition according to claim 1, characterized in that the weight-average molecular weight of the polyfunctional (meth)acrylic oligomer (B) is 6,000 or more. The photocurable resin composition according to claim 1 or 2, characterized in that the polyfunctional (meth)acrylic oligomer (B) is contained in an amount of 40 to 150 parts by mass per 100 parts by mass of the (meth)acrylic monomer (A). The photocurable resin composition according to claim 1 or 2, characterized in that it contains 3 to 30 parts by mass of a (meth)acrylic monomer (C) having a glass transition temperature Tg of 0°C or lower per 100 parts by mass of the (meth)acrylic monomer (A). The photocurable resin composition according to claim 1 or 2, characterized in that the cured product has a maximum stress of 5 to 20 MPa in a tensile test and a tensile modulus of elasticity of 50 to 900 MPa after curing. The photocurable resin composition according to claim 1 or 2, characterized in that it is used as an adhesive for optical glass.