JP2002201451A - Thermally active, photocationically polymerizable liquid adhesive and bonding method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a photocationically polymerizable adhesive exhibiting adhesive force and heat resistance in a short time, having a long pot life and easy to establish optimum conditions for photoirradiation intensity and photoirradiation time and to provide a bonding method using a photocationically polymerizable adhesive, easy to establish optimum conditions for photoirradiation intensity and photoirradiation time and permitting a long pot life of the adhesive after photoirradiation and exhibition of adhesive force and heat resistance in a short time. SOLUTION: The thermally active, photocationically polymerizable liquid adhesive comprises an epoxy group-containing compound (1), a hydroxy group- containing compound (2) and a photocationic polymerization catalyst (3), the molar ratio of the epoxy group to the hydroxy group present in the adhesive (epoxy group:hydroxyl group) being (100:0.1) to (100:4).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a photocationically polymerizable liquid adhesive in which a curing reaction is activated by heating, and a method for joining members using the same.

[0002]

2. Description of the Related Art In recent years, adhesives have been widely used for joining members such as housing members, automobile members, and building members. For this reason, adhesives used for these applications are being improved. In such adhesives, in recent years, the time required for the bonding operation has been reduced,
For simplicity of the operation, an adhesive that exhibits sufficient adhesive strength and heat resistance immediately after lamination is required. As adhesives exhibiting such performance, adhesives of a solvent type, a two-component mixed type, a moisture-curable hot melt type, a thermosetting type and the like have been known.

However, a large amount of solvent is volatilized in a short time in a solvent-type adhesive, so that a large-scale ventilation device excellent in ventilation capacity is required. In addition, since the adhesive is heated to a high temperature in order to volatilize the solvent in a short time, there is a problem that the solvent-type adhesive is not suitable for bonding heat-sensitive members together.

[0004] The two-component mixed type adhesive has a drawback that the operation becomes complicated since it is necessary to mix the main agent and the curing agent before application. In addition, when sufficient adhesive strength and heat resistance are to be developed in a short time, there is a problem that the pot life of the adhesive is shortened.

[0005] Moisture-curable hot-melt adhesives do not allow moisture to penetrate into the center of the adhesive, so that it is difficult to cure the entire adhesive in a short time, and high heat resistance is exhibited in a short time. There was a problem that it was difficult.

[0006] Since the thermosetting adhesive needs to be heated at a high temperature in order to cure the adhesive, there is a problem that it is not suitable for bonding members that are sensitive to heat.

Therefore, there is a demand for an adhesive capable of expressing sufficient adhesive strength and heat resistance in a short time without the need for extensive ventilation equipment or complicated work.
No. 012 proposes a photocurable adhesive utilizing a photoreaction as a new adhesive.

[0008] Such a photo-curing adhesive begins to cure as soon as light capable of initiating a curing reaction is applied, and the entire composition is quickly cured. Is expressed. In addition, since the photoreaction is used, it has an advantage that it can be applied to bonding of heat-sensitive members.

However, photo-radical polymerizable adhesives such as photo-curable acrylic adhesives utilizing a radical reaction are mainly used as photo-curable adhesives, and photo-cationic polymerizable adhesives utilizing a cationic reaction. The problem is that if the initial light irradiation intensity is too strong, the adhesive surface will be covered with the cured film and sufficient adhesion will not be obtained, and if the light irradiation intensity is too weak, the adhesive will develop in a short time However, it has a disadvantage that it is difficult to set the optimum light irradiation intensity and light irradiation time, and it has been rarely used so far.

[0010] Both the photo-curable adhesive and the photo-radical polymerizable adhesive gradually cure after irradiation with light, and the adhesive gels in several minutes to several tens of minutes. There was also a problem that the working time could not be lengthened.

[0011]

DISCLOSURE OF THE INVENTION In view of the above-mentioned circumstances, the present invention provides an adhesive force and heat resistance in a short time, has a long pot life, and has an optimum condition of light irradiation intensity and light irradiation time. It is an object of the present invention to provide a cationic photopolymerizable adhesive which can easily set the adhesiveness. Further, the present invention provides a bonding method using a cationic photopolymerizable adhesive, in which the optimum conditions of light irradiation intensity and light irradiation time can be easily set, and the pot life of the adhesive after light irradiation can be increased. It is an object of the present invention to provide a bonding method that can develop adhesive strength and heat resistance in a short time.

[0012]

The heat-activatable photocationically polymerizable liquid adhesive of the present invention comprises a compound having an epoxy group (1), a compound having a hydroxyl group (2), and a photocationic polymerization catalyst (3). A thermoactive photocationically polymerizable liquid adhesive comprising: an epoxy group and a hydroxyl group, wherein the molar ratio of the epoxy group to the hydroxyl group is 100: 0.1 to 100: 4.
It is characterized by being. Hereinafter, the present invention will be described in detail.

The heat-activatable photo-cationic polymerizable liquid adhesive of the present invention initiates a cationic reaction by light irradiation, but hardens at a normal temperature, but hardens rapidly when the adhesive is heated. Adhesive that is liquid at room temperature.

If the molar ratio between the epoxy group and the hydroxyl group contained in the adhesive is adjusted so as to increase the molar ratio of the hydroxyl group,
Curing of the adhesive at room temperature is strongly suppressed, and the pot life of the adhesive is prolonged. In addition, if the molar ratio of the epoxy group and the hydroxyl group contained in the adhesive is adjusted so that the molar ratio of the hydroxyl group becomes small, curing is promoted by heating the adhesive at a low temperature for only a short time, and sufficient time can be obtained in a short time. Develops adhesive strength and heat resistance.

According to the present invention, a thermoactive photocationically polymerizable liquid adhesive exhibiting a desired curing behavior is obtained by adjusting the molar ratio of epoxy group to hydroxyl group contained in the adhesive to an appropriate ratio.

The molar ratio of the epoxy group to the hydroxyl group contained in the heat-active photocationically polymerizable liquid adhesive of the present invention is epoxy group: hydroxyl group = 100: 0.1 to 100: 4. If the ratio of the hydroxyl group is less than 0.1, the adhesive is cured during or immediately after the light irradiation, and a sufficient adhesive force cannot be obtained. On the other hand, if the ratio of hydroxyl groups is more than 3, the adhesive cannot be sufficiently cured only by heating at a low temperature.

The heat-activatable photocationically polymerizable liquid adhesive of the present invention emits ultraviolet light having a wavelength of 365 nm from 0.5 to 5 J / c.
The reaction rate of the epoxy group after irradiation with m ² is preferably 15% or less. If it exceeds 15%, the curing of the adhesive is too fast to obtain a sufficient adhesive strength. The heat-active photocationically polymerizable liquid adhesive of the present invention has a wavelength of 365.
An adhesive is preferred in which the reaction rate of epoxy groups after irradiation with 0.5 to 5 J / cm ² of UV light of 0.5 nm and further heating at 60 to 120 ° C. for 30 to 180 seconds is 50% or more. If it is less than 50%, it will not be possible to cure sufficiently by heating at a low temperature.

As the compound (1) having an epoxy group used in the present invention, an epoxy resin is preferably used. Examples of the epoxy resin include bisphenol A type epoxy resin, bisphenol F type epoxy resin, phenol novolak type epoxy resin, cresol novolak type epoxy resin, glycidyl ether type epoxy resin, glycidylamine type epoxy resin, alicyclic type epoxy resin and the like. .

As the compound (1) having an epoxy group,
Oligomers having an epoxy group can also be suitably used. As the oligomer having an epoxy group, a bisphenol A type epoxy oligomer is preferable.

As the compound (1) having an epoxy group,
A copolymer obtained by copolymerizing a monomer or oligomer having an epoxy group with another copolymer component can also be suitably used. As the copolymer, glycidyl (meth)
A copolymer obtained by copolymerizing an acrylate monomer with any other monomer is suitable.

Further, as the compound (1) having an epoxy group, an epoxy-modified resin modified with a monomer or oligomer having an epoxy group can also be suitably used. As the epoxy-modified resin, glycidylated polyester, glycidylated polyurethane, glycidylated acryl, and the like are preferable. Among them, an epoxy-modified acrylic resin obtained by reacting an acrylic polymer having a hydroxyl group with epichlorohydrin, and an epoxy-modified polybutadiene resin obtained by reacting a polybutadiene having a carboxyl group at a molecular end with a polyfunctional epoxy monomer or oligomer And the like are preferred.

As the compound (1) having an epoxy group, a chelate-modified epoxy resin and a polyol-modified epoxy resin can also be suitably used. The compound (1) having an epoxy group may be a single compound or a mixture of two or more compounds.

The compound (1) having an epoxy group is represented by the following general formula [1]:

[0024]

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(Wherein m and n are natural numbers and m + n is 5 or less; R ¹ and R ^{2 each} independently represent hydrogen or a methyl group) in an amount of 10% by weight. It is preferable to contain the above. An adhesive containing the above compound in an amount of 10% by weight or more becomes a soft cured product, so that the peel strength is increased and the impact resistance is excellent.

When m + n is 5 or less, the above compound cures the adhesive only by heating at a low temperature for a short time, so that members which are weak to heat can be bonded to each other. m + n
If it is 6 or more, it is necessary to heat at a high temperature in order to cure the adhesive in a short period of time, which is not preferable for laminating heat-sensitive members.

The compound (2) having a hydroxyl group used in the present invention includes polyether polyol, polyester polyol, polycarbonate polyol, polymer polyol, rosinol and the like. These may be used alone or in combination of two or more.

More preferably, it is a mixture of any one selected from polyester polyol, polycarbonate polyol, polymer polyol and rosinol with a polyether polyol.

As the polyether polyol, a polymer obtained by subjecting an alkylene oxide to ring-opening polymerization in the presence of a low molecular weight compound having two or more active hydrogens is preferable. As the low molecular weight compound having two or more active hydrogens, for example, polyhydric alcohol, diamine and the like are preferable. Examples of polyhydric alcohols include diols such as bisphenol A, ethylene glycol, propylene glycol, butylene glycol, and 1,6-hexanediol, and triols such as glycerin and trimethylolpropane.

Examples of the diamine include ethylene diamine and butylene diamine. Examples of the alkylene oxide include ethylene oxide, propylene oxide, butylene oxide, amylene oxide, hexylene oxide, and tetrahydrofuran.

Examples of the polyester polyol include polybasic acids such as adipic acid, azelaic acid, sebacic acid, terephthalic acid, isophthalic acid and succinic acid, and bisphenol A, ethylene glycol, 1,2-propylene glycol, 1,4 -Polymers obtained by dehydration condensation of polyhydric alcohols such as butanediol, diethylene glycol, 1,6-hexane glycol and neopentyl glycol, and castor oil, and hydroxycarboxylic acids such as reaction products of castor oil and ethylene glycol; Examples include condensation reaction products with polyhydric alcohols.

As the polycarbonate polyol, for example, a trade name PNOC-100 provided by Kuraray Co., Ltd.
0, PNOC-2000, Kuraray polyol C-109
0, Kuraray polyol C-2090 and the like.

Examples of the polymer polyol include a graft polymer obtained by graft-polymerizing a radical polymerizable monomer such as acrylonitrile and styrene to a polyether polyol or a polyester polyol, or methyl (meth) acrylate to a polyether polyol or a polyester polyol. Acrylic polyol, 1,2-polybutadiene polyol, 1,4-polybutadiene polyol, and hydrogenated products thereof obtained by graft-polymerizing the above.

The weight average molecular weight of the polymer polyol is preferably from 100 to 200,000.

Examples of the rosinol include a rosin ester obtained by reacting a rosin with a polyhydric alcohol and a modified rosin having a hydroxyl group such as an epoxy-modified rosin obtained by reacting a rosin with an epoxy compound.

Examples of the rosin include pimaric acid-type resin acids such as abietic acid, neoabietic acid, dehydroabietic acid, and levopimaric acid, hydrogenated hydrogenated rosins thereof, and disproportionated rosins obtained by disproportionating them. Can be Specific examples of these rosins include KE-359, SR
-200, SR-30PX, KE-601, KE-61
5-3, KE-624 modified (all, manufactured by Arakawa Chemical Co., Ltd.) and the like.

The cationic photopolymerization catalyst (3) used in the present invention may be an ionic catalyst or a nonionic catalyst as long as it generates an acid upon irradiation with light. Examples of the ionic catalyst include onium salts such as aromatic diazonium salts, aromatic halonium salts, and aromatic sulfonium salts, and organic metal complexes such as iron-allene complexes, titanocene complexes, and arylsilanol-aluminum complexes.

The nonionic catalyst includes nitrobenzyl ester, sulfonic acid derivative, phosphate ester,
Phenolsulfonic acid ester, diazonaphthoquinone,
N-hydroxyimide sulfonate and the like can be mentioned.

The cationic photopolymerization catalyst (3) is preferably used in an amount of 0.0001 mol or more per 1 mol of the epoxy group when the heat-activated cationic photopolymerizable adhesive is used.

In constituting the heat-activated photocationically polymerizable liquid adhesive of the present invention, other resins may be further added. Examples of these other resins include polybutadiene acrylonitrile rubber having a terminal carboxyl group. CTBN), acrylic rubber, nitrile rubber (NB
R), rubbery resins such as styrene butadiene rubber (SBR), butyl rubber, nitrile rubber, isoprene rubber and the like. Epoxy resins to which these resins are added in advance can also be used.

The heat-active photocationically polymerizable liquid adhesive of the present invention may further contain a filler, a tacky oligomer, a silane coupling agent, a thixotropic agent, a plasticizer, a stabilizer and the like. . The heat-active photocationically polymerizable liquid adhesive of the present invention is not particularly limited as long as it has a viscosity capable of performing an application operation.
It is preferably 5,000,000 cps, more preferably 500 to 100,000 cps.

The heat-active photocationically polymerizable liquid adhesive of the present invention is applied to the surface of at least one of the members, irradiated with ultraviolet light having an intensity capable of initiating the cationic polymerization reaction, and then adhered to the other member. A method of joining members to be combined and heated and pressed is also one of the present inventions. In applying the heat-activated photocationically polymerizable liquid adhesive of the present invention, the method of applying to the surface of the member is not particularly limited. For example, a roll coater, a bar coater, a spray coater, a screen coater, a gravure Coater,
The coating can be performed on the member using a coating machine such as a flow coater or a spin coater.

As a device for irradiating the adhesive applied to the member with ultraviolet rays, for example, a mercury lamp, a chemical lamp, a black light lamp, a metal halide lamp and the like can be used. In these devices, the lamp may be provided with a filter.

The intensity of the ultraviolet light to be irradiated is not particularly limited as long as it can start the cationic polymerization reaction, but it is preferable to irradiate ultraviolet light of 365 nm with an intensity of 0.5 to 5 J / cm ² .

The member irradiated with ultraviolet rays is then bonded to the other member at an arbitrary time within the usable time range of the adhesive. The method for bonding the members is not particularly limited, and the members may be bonded manually or may be bonded using a device such as a press machine.

When the bonded members are hot-pressed, they may be heated at the same time as the bonding, or they may be pressed once at the time of bonding and then hot-pressed. The method of joining the members in this manner is also one of the present invention.

[0047]

The present invention will be described in more detail with reference to the following Examples and Comparative Examples, but the present invention is not limited to these Examples.

Examples 1 to 5 and Comparative Examples 1 to 4 Each raw material as shown in Table 1 was stirred and mixed in a dry atmosphere until uniform, to obtain a liquid adhesive. The units of the numerical values in Table 1 are parts by weight.

[0049]

[Table 1]

The following evaluation was performed on the obtained adhesive. (Measurement of the reaction rate of the epoxy group) A pentite steel sheet (7 cm × 10 cm × 0.8 cm) was placed in a 20 ° C. atmosphere at 200 ° C.
The adhesive was uniformly applied at a coating amount of g / m ² . An IR spectrum measuring device (“FTIR-8900”, manufactured by Shimadzu Corporation) using a pentite steel plate coated with an adhesive as a test piece
Was used to measure the IR spectrum of the adhesive. here,
The relative intensity (C1) of the absorption peak intensity (B1) of the epoxy group present at 914 cm ^-1 was calculated based on the absorption peak intensity (A1) of the benzene ring existing at 1607 cm ^-1 .

Next, an ultra-high pressure mercury lamp ("Jet Light-2300", manufactured by Oak Manufacturing Co., Ltd.) was used to make a 365 nm
Immediately after irradiating with ultraviolet light at an intensity of 40 mW for 60 seconds,
An IR spectrum was measured. Here, the relative intensity (C2) of the absorption peak intensity (B2) of the epoxy group present at 914 cm ^-1 was calculated based on the absorption peak intensity (A2) of the benzene ring existing at 1607 cm ^-1 . Then, based on the following equation (Equation 1), the reaction rate of the epoxy group immediately after the irradiation with ultraviolet rays was calculated.

Reaction rate (%) of epoxy group = (C1−C2) / C1 × 100 (Equation 1) (C1 = B1 / A1, C2 = B2 / A2)

Next, a pentite steel sheet to which the adhesive was applied was prepared again, and the IR spectrum was measured. Then, the sheet was irradiated with ultraviolet rays, allowed to stand at 20 ° C. and 50% RH for 30 minutes, and then the IR spectrum was measured. Here, based on the absorption peak intensity (A3) of the benzene ring existing at 1607 cm ⁻¹ , 9
Absorption peak intensity of epoxy group existing at 14 cm ^-1 (B
The relative intensity (C3) of 3) was calculated. Then, based on the following equation (Equation 2), the reaction rate of the epoxy group 30 minutes after the irradiation with the ultraviolet rays was calculated.

Reaction rate (%) of epoxy group = (C1−C3) / C1 × 100 (Equation 2) (C1 = B1 / A1, C3 = B3 / A3)

Next, the pentite steel sheet to which the adhesive was applied was prepared again, the IR spectrum was measured, and ultraviolet rays were irradiated. The pentite steel sheet to which the adhesive was applied was placed on a hot plate heated to 80 ° C. IR after 1 minute
The spectrum was measured. Here, the relative intensity (C4) of the absorption peak intensity (B4) of the epoxy group existing at 914 cm ^-1 was calculated based on the absorption peak intensity of the benzene ring existing at 1607 cm ^-1 (A4). Then, based on the following equation (Equation 3), the reaction rate of the epoxy group after being irradiated with ultraviolet rays and heated to 80 ° C. was calculated.

Reaction rate (%) of epoxy group = (C1−C4) / C1 × 100 (Equation 3) (where C1 = B1 / A1, C4 = B4 / A4)

The reaction rates of the epoxy groups obtained by calculation are shown in Table 2. The unit of the numerical values in Table 2 is%.

[0058]

[Table 2]

(Measurement of pot life) An adhesive was uniformly applied to a pentite steel sheet at an application amount of 200 g / m ² in an atmosphere of 20 ° C. and 50% RH. Then, ultraviolet light of 365 nm was irradiated at an intensity of 40 mW for 60 seconds using an ultra-high pressure mercury lamp. Then, it was placed in an atmosphere of 20 ° C. and 50% RH, and the adhesive was observed. Here, the time required for a film to be formed on the surface of the adhesive or for the adhesive to harden and become a gel was measured, and was defined as the pot life.

(Measurement of Adhesive Strength between Penteite Steel Plate and Paper Honeycomb) An adhesive was applied at 200 g / m ^{2 to two} pentite steel plates (7 cm × 10 cm × 0.8 cm) in an atmosphere of 20 ° C. and 50% RH. A uniform amount was applied. Next, each adhesive applied to the pentite steel plate was irradiated with ultraviolet rays of 365 nm at an intensity of 40 mW for 60 seconds using an ultrahigh pressure mercury lamp. Then, these were pasted on both sides of a paper honeycomb (7 cm x 7 cm x 3 cm),
Two steel plates (15 cm × 15 cm × 1) heated to 0 ° C.
cm) and heated and pressed at 80 ° C. for 1 minute. Then, after the paper honeycomb was cooled down to room temperature, the two pentite steel sheets bonded to the upper and lower surfaces of the paper honeycomb were grasped and pulled in the vertical direction to peel off the pentite steel sheets from the paper honeycomb. Here, the state of the surface of the peeled pentite steel plate was observed, and the material breakage rate of the paper honeycomb was obtained.

(Measurement of adhesive strength between pentite steel plate and pentite steel plate) Length 15 cm, width 2.5 cm, thickness 0.8 c
A pentite steel plate having a length of 10 cm and a width of 5 cm from an end of the pentite steel plate is prepared by bending a length of 5 cm from the end of the pentite steel plate.
Two steel plate parts with a 5 cm plane were made. 20
The adhesive was uniformly applied to the flat portions (the flat surface having a length of 10 cm and a width of 2.5 cm) of the two steel plate parts at an application amount of 200 g / m ² in an atmosphere of 50 ° C. and 50% RH. Each adhesive applied to the pentite steel plate was irradiated with ultraviolet rays of 365 nm at an intensity of 40 mW for 60 seconds using an ultra-high pressure mercury lamp. These were bonded together in a state where the flat portions (planes having a length of 10 cm and a width of 2.5 cm) of the steel plate parts were put together. The flat part of the bonded steel part is 80
Two steel plates (15cm × 15cm × 1c)
m), the mixture was heated and pressed at 80 ° C. for 1 minute.
Then, after the components were cooled to room temperature, the two steel plate parts were grasped and pulled in the vertical direction at a pulling speed of 50 mm / min, and the tensile adhesive force (unit: N / cm) was measured.

(Evaluation of heat resistance) A pentite steel plate (2.5
cm × 15 cm × 0.8 cm). 20 ° C, 5
Under an atmosphere of 0% RH, the adhesive is applied at a distance of only 1.25 cm from one end of the pentite steel sheet to 200 g / m
The coating amount of ² was applied uniformly. The adhesive applied to the pentite steel plate was irradiated with UV light of 365 nm at an intensity of 40 mW for 60 seconds using an ultra-high pressure mercury lamp. One end of the pentite steel sheet to which the adhesive was applied was attached so that one end of a pentite steel sheet having the same size and not applied with the adhesive was overlapped with the part to which the adhesive was applied. This was heated to 80 ° C. by two steel plates (15 cm × 15 c
(m × 1 cm) and heated and pressed at 80 ° C. for 1 minute. Then, the specimen was placed until cooled to room temperature to obtain a test piece in which two pentite steel sheets were bonded together at the center with an adhesive. The test piece was suspended in an oven at 150 ° C. with a 5 kg weight hanging on one end and the other end facing up. The test specimen was observed after one hour. here,
If the two bonded pentite steel sheets were not peeled off, it was evaluated that the heat resistance was good. Table 3 shows the results of the pot life, the adhesive strength between the pentite steel sheet and the paper honeycomb, the adhesive strength between the pentite steel sheet and the pentite steel sheet, and the heat resistance obtained by the measurement.

[0063]

[Table 3]

Examples 6 to 9 As shown in Table 4, each raw material was stirred and mixed in a dry atmosphere until it became uniform to obtain a liquid adhesive. The units of the numerical values in Table 4 are parts by weight.

[0065]

[Table 4]

The obtained adhesive was evaluated in the same manner and measured, and the obtained results were shown in Table 5 (unit:%) and Table 6.
It was shown to.

[0067]

[Table 5]

[0068]

[Table 6]

[0069]

According to the present invention, adhesive strength and heat resistance are developed in a short time, and a long pot life is provided. Optimal conditions of light irradiation intensity and light irradiation time can be easily set. Further, in the bonding method using the photocationically polymerizable adhesive, the optimum conditions of the light irradiation intensity and the light irradiation time can be easily set. The working life of the adhesive after light irradiation can be lengthened. Adhesive strength and heat resistance can be developed in a short time.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) C09J 201/06

Claims (5)

[Claims] 1. A compound having an epoxy group (1), a compound having a hydroxyl group (2), and a cationic photopolymerization catalyst (3)
A thermoactive photocationically polymerizable liquid adhesive comprising: an epoxy group and a hydroxyl group in a molar ratio of epoxy group: hydroxyl group = 100: 0.1 to 100: 4. Type cationic photopolymerizable liquid adhesive. 2. An ultraviolet ray having a wavelength of 365 nm of 0.5 to 5 J / c.
The reaction rate of the epoxy group after m ² irradiation is 15% or less, and the reaction rate of the epoxy group after heating at 60 to 120 ° C. for 30 to 180 seconds is 50% or more. Item 2. The heat-activated photocationically polymerizable liquid adhesive according to Item 1. 3. The compound (1) having an epoxy group is represented by the following general formula [1]: (Wherein, m and n are natural numbers, and m + n is 5 or less. R ¹ and R ^{2 each} independently represent hydrogen or a methyl group.) The heat-activated photocationically polymerizable liquid adhesive according to claim 1 or 2, wherein 4. The compound (2) having a hydroxyl group is a mixture of at least one selected from the group consisting of a polyester polyol, a polycarbonate polyol, a polymer polyol, and rosinol, and a polyether polyol. The heat-activated photocationically polymerizable liquid adhesive according to claim 1, 2 or 3. 5. The heat-activated photocationically polymerizable liquid adhesive according to claim 1, 2, 3 or 4, applied to at least one of the surfaces of the member, and irradiated with ultraviolet light having a strength capable of initiating a cationic polymerization reaction. A method for joining members, which comprises irradiating, followed by bonding to the other member, and hot pressing.