2-Cyanoacrylate adhesive composition

Abstract

To provide a 2-cyanoacrylate-based adhesive composition in which the adhesive strength of an adhesive composition containing a 2-cyanoacrylate compound selected from an alkyl-2-cyanoacrylate having 4 or more carbon atoms in the alkyl group and a 2-cyanoacrylate having an ether bond in the ester residue, to metals such as aluminum and copper is improved.SOLUTION: Provided is a 2-cyanoacrylate-based adhesive composition, which is obtained by containing a compound of (1) a carboxylic acid with a cyano group at the α-position having a particular structure represented by cyanoacrylic acid, cyanoacetic acid, and cyanocyclopropanoic acid, or (2) a carboxylic acid with an oxo group at the α-position having a particular structure represented by pyruvic acid, in the adhesive composition in an amount sufficient for the adhesive strength to the copper substrate measured in accordance with JIS K 6849 to be higher than that of the adhesive composition obtained by removing the carboxylic acid compound from the adhesive composition.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a 2-cyanoacrylate adhesive composition that has excellent adhesive strength to metals such as copper and aluminum. [Background technology]

[0002] 2-cyanoacrylate adhesive compositions, due to the unique anionic polymerization property of the main component 2-cyanoacrylate, initiate polymerization with weak anions such as traces of moisture adhering to the surface of the adherend, and can firmly bond various materials in a short time. For this reason, they are widely used as so-called instant adhesives in various industries such as electronics, electrical and automotive, leisure fields and ordinary households.

[0003] In recent years, 2-cyanoacrylate adhesive compositions have been used in the production of electric and electronic parts containing metal wiring, such as printed wiring boards and wire harnesses, and there is a demand for improved adhesion to metals. Patent Documents 1 to 3 disclose adhesive compositions that exhibit excellent adhesion speed to metals and thermoplastic elastomers with low polarity, as well as excellent appearance and storage stability of the cured product, by blending an onium salt having a specific structure with a 2-cyanoacrylate compound.

[0004] However, in recent years, there has been a demand for further improvement in the adhesion, particularly adhesive strength, of 2-cyanoacrylate adhesive compositions to metals, particularly to copper and aluminum used in the production of electric and electronic components. In particular, alkyl-2-cyanoacrylates with an alkyl group having 4 or more carbon atoms and 2-cyanoacrylates having an ether bond in the ester residue have superior flexibility in the cured body compared to alkyl-2-cyanoacrylates with an alkyl group having 3 or less carbon atoms, but have weak adhesive strength, so there is a demand for improving the adhesive strength.

[0005] Patent Document 4 proposes an impregnation composition that exhibits excellent impregnation and curing properties for wood-based materials and foamed materials by blending a 2-cyanoacrylate compound with a 2-cyanoacrylate-soluble carboxylic acid having a pKa of 1.2 to 4.0 and an anionic polymerization accelerator. However, this document only examines the improvement of the curing speed and does not disclose any improvement in adhesive strength, much less any improvement in adhesive strength to metals such as copper and aluminum. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] International Publication No. 2014 / 042082 [Patent Document 2] International Publication No. 2015 / 137156 [Patent Document 3] International Publication No. 2018 / 003682 [Patent Document 4] JP 2000-53924 A Summary of the Invention [Problem to be solved by the invention]

[0007] The object of the present invention is to provide a 2-cyanoacrylate-based adhesive composition that improves the adhesive strength to metals such as aluminum and copper of an adhesive composition containing an alkyl-2-cyanoacrylate having an alkyl group with 4 or more carbon atoms and a 2-cyanoacrylate having an ether bond in the ester residue, and is suitable for the production of electrical and electronic components that include wiring of such metals. [Means for solving the problem]

[0008] The inventors have discovered that by incorporating a specific carboxylic acid compound into an alkyl-2-cyanoacrylate in which the alkyl group has 4 or more carbon atoms, or a 2-cyanoacrylate having an ether bond in the ester residue, a 2-cyanoacrylate adhesive composition having superior adhesive strength to metals such as aluminum and copper compared to conventional compositions can be obtained, and have completed the present invention.

[0009] According to one aspect of the present invention, there is provided an adhesive composition comprising: The composition contains at least one 2-cyanoacrylate compound selected from the group consisting of alkyl-2-cyanoacrylates having an alkyl group with 4 or more carbon atoms and 2-cyanoacrylates having an ether bond in an ester residue, and at least one carboxylic acid compound selected from the group consisting of compounds represented by the following formula (1) and compounds represented by the following formula (2), The present invention provides a 2-cyanoacrylate adhesive composition containing the carboxylic acid compound in an amount sufficient to provide an adhesive strength to a copper substrate, measured in accordance with JIS K 6849, that is higher than that of an adhesive composition obtained by excluding the carboxylic acid compound.

[0010] [ka] (In formula (1), n ​​represents an integer of 1 or 2; each R1 independently represents a hydrogen atom or a hydrocarbon group having 1 to 13 carbon atoms; the bond a may be a single bond or a double bond; and two R1 may be bonded to each other to form a cyclic hydrocarbon group.)

[0011] [ka] (In formula (2), R2 represents a hydrocarbon group having 1 to 10 carbon atoms.)

[0012] According to a preferred embodiment of the present invention, the adhesive composition of the present invention contains the carboxylic acid compound in an amount sufficient to ensure that the adhesive strength to a copper substrate, measured in accordance with JIS K 6849, is at least 20% higher than that of an adhesive composition obtained by excluding the carboxylic acid compound from the adhesive composition. According to a preferred embodiment of the present invention, the adhesive composition of the present invention contains the carboxylic acid compound in an amount sufficient to ensure that the adhesive strength to an aluminum substrate, measured in accordance with JIS K 6849, is at least 3% higher than that of an adhesive composition obtained by excluding the carboxylic acid compound from the adhesive composition.

[0013] According to another preferred embodiment of the present invention, the adhesive composition of the present invention contains the carboxylic acid compound in an amount of 15 ppm or more per 100 parts by mass of the 2-cyanoacrylate compound. According to another preferred embodiment of the present invention, the adhesive composition of the present invention contains the carboxylic acid compound in an amount of 6000 ppm or less per 100 parts by mass of the 2-cyanoacrylate compound.

[0014] According to another preferred embodiment of the present invention, R1 in the formula (1) is each independently a hydrogen atom, a linear or branched alkyl group having 1 to 10 carbon atoms, a cycloalkyl group, an allyl group, or an aryl group, or two R1 are bonded to each other to form a cyclic hydrocarbon group. According to another preferred embodiment of the present invention, R2 in the formula (2) is a linear or branched alkyl group having 1 to 10 carbon atoms, a cycloalkyl group, an allyl group, or an aryl group. According to another preferred embodiment of the present invention, the alkyl-2-cyanoacrylate is at least one selected from the group consisting of 2-octyl-2-cyanoacrylate, 2-ethylhexyl-2-cyanoacrylate, n-octyl-2-cyanoacrylate, and n-hexyl-2-cyanoacrylate. According to another preferred embodiment of the present invention, the 2-cyanoacrylate having an ether bond in the ester residue is at least one selected from the group consisting of methoxyethyl-2-cyanoacrylate, ethoxyethyl-2-cyanoacrylate, and butoxyethyl-2-cyanoacrylate.

[0015] According to another aspect, the present invention provides a bonded article comprising at least a first metal part and a second metal part bonded thereto, the first part and the second part being bonded together via an adhesive, wherein the adhesive is the 2-cyanoacrylate-based adhesive composition of the present invention described above.

[0016] According to yet another aspect, the present invention provides a method for producing a bonded article, which comprises bonding two parts, at least one of which is made of metal, via the above-mentioned 2-cyanoacrylate adhesive composition of the present invention.

[0017] According to yet another aspect, the present invention provides an agent for improving the adhesive strength of a 2-cyanoacrylate adhesive composition to metal, the agent comprising at least one carboxylic acid compound selected from the group consisting of compounds represented by the above formula (1) and compounds represented by the above formula (2), wherein the 2-cyanoacrylate adhesive composition comprises at least one 2-cyanoacrylate compound selected from the group consisting of alkyl-2-cyanoacrylates having an alkyl group with 4 or more carbon atoms and 2-cyanoacrylates having an ether bond in the ester residue.

[0018] According to yet another aspect, the present invention provides a method for improving the adhesive strength of a 2-cyanoacrylate adhesive composition, comprising adding at least one 2-cyanoacrylate compound selected from the group consisting of alkyl-2-cyanoacrylates in which the alkyl group has 4 or more carbon atoms and 2-cyanoacrylates having an ether bond in the ester residue to a 2-cyanoacrylate adhesive composition in an amount sufficient to increase the adhesive strength of the adhesive composition to a copper substrate, as measured in accordance with JIS K 6849. Effect of the Invention

[0019] According to the present invention, at least one carboxylic acid compound selected from the group consisting of the compounds represented by the above formula (1) and the compounds represented by the above formula (2) is blended with an alkyl-2-cyanoacrylate having 4 or more carbon atoms in the alkyl group or a 2-cyanoacrylate having an ether bond in the ester residue, so that the adhesive strength of an adhesive composition containing the above 2-cyanoacrylate compound to metals such as aluminum and copper, particularly to copper, can be improved compared to conventional adhesive compositions, and the range of application of the above 2-cyanoacrylate compound can be expanded. It has also been revealed that the same carboxylic acid compound can be used to improve the adhesive strength of existing 2-cyanoacrylate-based adhesive compositions containing the above 2-cyanoacrylate to metals. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] Hereinafter, an embodiment of the present invention will be described, but the present invention is not limited thereto.

[0021] 1. 2-Cyanoacrylate compounds The 2-cyanoacrylate adhesive composition of the present invention contains, as the main adhesive component, at least one 2-cyanoacrylate compound selected from the group consisting of alkyl-2-cyanoacrylates having an alkyl group with 4 or more carbon atoms and 2-cyanoacrylates having an ether bond in the ester residue.

[0022] 1-1. Alkyl-2-cyanoacrylate with an alkyl group having 4 or more carbon atoms Various alkyl-2-cyanoacrylates can be used in which the alkyl group has 4 or more carbon atoms. The upper limit of the number of carbon atoms in the alkyl group corresponding to the ester residue of the alkyl-2-cyanoacrylate is not particularly limited, but can be, for example, 12 or less. Specific examples of such alkyl-2-cyanoacrylates include n-butyl-2-cyanoacrylate, sec-butyl-2-cyanoacrylate, isobutyl-2-cyanoacrylate (2-methylpropyl-2-cyanoacrylate), n-pentyl-2-cyanoacrylate, n-hexyl-2-cyanoacrylate, n-heptyl-2-cyanoacrylate, 1-methylpentyl-2-cyanoacrylate, n-octyl-2-cyanoacrylate, 2-octyl-2-cyanoacrylate, 2-ethylhexyl-2-cyanoacrylate, n-nonyl-2-cyanoacrylate, isononyl-2-cyanoacrylate, n-decyl-2-cyanoacrylate, isodecyl-2-cyanoacrylate, n-undecyl-2-cyanoacrylate, and n-dodecyl-2-cyanoacrylate. These may be used alone or in combination of two or more.

[0023] Among these, alkyl-2-cyanoacrylates having an alkyl group having 5 to 10 carbon atoms are preferred because of their high effect of improving the adhesive strength to copper according to the present invention, and specific examples thereof include at least one compound selected from the group consisting of n-pentyl-2-cyanoacrylate, n-hexyl-2-cyanoacrylate, n-heptyl-2-cyanoacrylate, 1-methylpentyl-2-cyanoacrylate, n-octyl-2-cyanoacrylate, 2-octyl-2-cyanoacrylate, 2-ethylhexyl-2-cyanoacrylate, n-nonyl-2-cyanoacrylate, isononyl-2-cyanoacrylate, and n-decyl-2-cyanoacrylate. Among these, at least one selected from the group consisting of 2-octyl-2-cyanoacrylate, 2-ethylhexyl-2-cyanoacrylate, n-octyl-2-cyanoacrylate, and n-hexyl-2-cyanoacrylate is more preferred.

[0024] 1-2. 2-Cyanoacrylate with an ether bond in the ester residue Examples of 2-cyanoacrylates having an ether bond in the ester residue include alkoxyalkyl-2-cyanoacrylates and cyclic alkyl ether 2-cyanoacrylates. In the present invention, the "ester residue" refers to the portion of the compound other than the ester bond, i.e., the portion represented by -R in the group represented by -CO-OR that is ester-bonded to 2-cyanoacrylate.

[0025] As the alkoxyalkyl-2-cyanoacrylate, the alkoxyalkyl group preferably has 2 to 12 carbon atoms, more preferably has 2 to 10 carbon atoms, and even more preferably has 2 to 8 carbon atoms. Specific examples of alkoxyalkyl-2-cyanoacrylates include methoxyethyl-2-cyanoacrylate, ethoxyethyl-2-cyanoacrylate, propoxyethyl-2-cyanoacrylate, isopropoxyethyl-2-cyanoacrylate, butoxyethyl-2-cyanoacrylate, hexyloxyethyl-2-cyanoacrylate, 2-ethylhexyloxyethyl-2-cyanoacrylate, butoxyethoxyethyl-2-cyanoacrylate, hexyloxyethoxyethyl-2-cyanoacrylate, 2-ethylhexyloxyethoxyethyl-2-cyanoacrylate, methoxypropyl-2-cyanoacrylate, methoxypropoxypropyl-2-cyanoacrylate, methoxypropoxypropoxypropyl-2-cyanoacrylate, ethoxypropyl-2-cyanoacrylate, and ethoxypropoxypropyl-2-cyanoacrylate. A specific example of the cyclic alkyl ether 2-cyanoacrylate is tetrahydrofurfuryl-2-cyanoacrylate. The 2-cyanoacrylate having an ether bond in the ester residue may be used alone or in combination of two or more kinds.

[0026] Among these, alkoxyethyl-2-cyanoacrylates having an alkoxy group with 1 to 8 carbon atoms, such as methoxyethyl-2-cyanoacrylate, ethoxyethyl-2-cyanoacrylate, and butoxyethyl-2-cyanoacrylate, are preferably used because they are easily available and have excellent stability, and ethoxyethyl-2-cyanoacrylate is particularly preferred.

[0027] 1-3. Other 2-cyanoacrylate compounds In the present invention, the above-mentioned 2-cyanoacrylate compound can be used in combination with other 2-cyanoacrylate compounds within the range that does not impair the effects of the present invention. Examples of such other 2-cyanoacrylate compounds include alkyl-2-cyanoacrylates having an alkyl group with 1 to 3 carbon atoms, cyclohexyl-2-cyanoacrylate, cyclohexenyl cyanoacrylate, phenyl-2-cyanoacrylate, chloroethyl-2-cyanoacrylate, allyl-2-cyanoacrylate, propargyl-2-cyanoacrylate, benzyl-2-cyanoacrylate, 2,2,2-trifluoroethyl-2-cyanoacrylate, hexafluoroisopropyl-2-cyanoacrylate, etc. These compounds may be used alone or in combination of two or more.

[0028] 2. Carboxylic acid compounds The 2-cyanoacrylate adhesive composition of the present invention contains at least one carboxylic acid compound selected from the group consisting of the compounds represented by the above formula (1) and the compounds represented by the above formula (2).

[0029] Specific examples of the compound represented by the above formula (1) include cyanoacrylic acid (2-cyanopropenoic acid), cyanoacetic acid, 2-cyanopropionic acid, 2-cyanobutanoic acid, 2-cyanopentanoic acid, 2-cyanohexanoic acid, 2-cyanoheptanoic acid, 2-cyanooctanoic acid, 2-cyanononanoic acid, 2-cyanodecanoic acid, 2-cyanododecanoic acid, 2-cyano-2-cyclohexylideneacetic acid, 2-cyano-2-cycloheptylacetic acid, 2-cyano-3-phenylpropionic acid, 2-cyano-3,3-diphenylacrylic acid, and 2-cyano-3-methyl-2-butenoic acid.

[0030] Among the compounds represented by the above formula (1), specific examples of compounds in which two R1 are bonded to each other to form a cyclic hydrocarbon group include compounds in which two R1 are bonded to each other to form a cyclic hydrocarbon group having 3 to 6 carbon atoms, such as 1-cyano-1-cyclopropanecarboxylic acid, 1-cyano-1-cyclobutanecarboxylic acid, 1-cyano-1-cyclopentanecarboxylic acid, and 1-cyano-1-cyclohexanecarboxylic acid.

[0031] Among these compounds, compounds in which R1 in the formula (1) is each independently a hydrogen atom, a linear or side-chain alkyl group having 1 to 10 carbon atoms, a cycloalkyl group, an allyl group, or an aryl group, or two R1 are bonded to each other to form a cyclic hydrocarbon group are preferred, and cyanoacrylic acid represented by the following formula (1-1), cyanoacetic acid represented by the following formula (1-2), and 1-cyano-1-cyclopropanecarboxylic acid represented by the following formula (1-3) are more preferred, since they improve adhesion to metals even when added in an extremely small amount.

[0032] [ka]

[0033] [ka]

[0034] [ka]

[0035] Specific examples of the compound represented by the above formula (2) include pyruvic acid (2-oxopropanoic acid), 2-oxobutyric acid (2-oxobutanoic acid), 2-oxovaleric acid (2-oxopentanoic acid), α-ketoisovaleric acid (3-methyl-2-oxobutanoic acid), 4-methyl-2-oxovaleric acid (4-methyl-2-oxopentanoic acid), 3-methyl-2-oxovaleric acid (3-methyl-2-oxopentanoic acid), phenylglyoxylic acid, 2-cyclohexyl-2-oxoacetic acid, and 2-oxo-4-phenyl-3-butenoic acid. Among these compounds, preferred are compounds in which R2 in the formula (2) is a linear or side-chain alkyl group, cycloalkyl group, allyl group, or aryl group having 1 to 10 carbon atoms, and more preferred is pyruvic acid represented by the following formula (2-1), since the addition of an extremely small amount of the compound improves adhesion to metals.

[0036] [ka]

[0037] 3. 2-Cyanoacrylate adhesive composition The present invention is based on the discovery that when the above-mentioned 2-cyanoacrylate compound contains a specific carboxylic acid compound, the adhesion to metals such as aluminum and copper is significantly improved compared to when the compound does not contain the carboxylic acid compound. Therefore, according to the present invention, the content of the carboxylic acid compound is set to an amount sufficient to make the adhesive strength to a copper substrate, measured in accordance with JIS K 6849, higher than that in the case where the carboxylic acid compound is not added.

[0038] Preferably, the content of the carboxylic acid compound in the adhesive composition of the present invention is an amount that provides a significant improvement in the adhesion of the 2-cyanoacrylate compound to aluminum substrates and / or copper substrates and exhibits a practical adhesive strength. In the present invention, a "significantly improved effect on adhesion" means, in the case of adhesion to copper substrates, that the adhesive strength to copper substrates, measured in accordance with JIS K 6849, is preferably 20% or more, and more preferably 30% or more higher than that of an adhesive composition excluding the carboxylic acid compound, and in the case of adhesion to aluminum substrates, that the adhesive strength to aluminum substrates, measured in accordance with JIS K 6849, is preferably 3% or more, and more preferably 5% or more higher than that of an adhesive composition excluding the carboxylic acid compound.

[0039] The appropriate content of the carboxylic acid compound in the adhesive composition of the present invention varies depending on the types of 2-cyanoacrylate compound and carboxylic acid compound, but it is preferable to set the lower limit of the content as a value that provides the above-mentioned "significant improvement effect in adhesion." The lower limit of the above content is usually 15 ppm or more, preferably 30 ppm or more, more preferably 300 ppm or more, even more preferably 500 ppm or more, even more preferably 1200 ppm or more, and particularly preferably 2100 ppm or more, relative to 100 parts by mass of the 2-cyanoacrylate compound. The upper limit of the content is not particularly limited as long as it is an amount that does not impair the practicality or economical efficiency of the adhesive of the present invention, and is preferably 6000 ppm or less. The content of the carboxylic acid compound in the adhesive composition of the present invention can be measured, for example, by liquid chromatography.

[0040] The 2-cyanoacrylate compound used in the present invention preferably has a purity of 99% or more, more preferably 99.5% or more. If the purity of the 2-cyanoacrylate is 99% or more, it is preferable because it has excellent instantaneous adhesion, storage stability, and adhesive heat resistance.

[0041] In order to improve storage stability, the 2-cyanoacrylate adhesive composition of the present invention may contain an anionic polymerization inhibitor such as sulfur dioxide, paratoluenesulfonic acid, methanesulfonic acid, propanesultone, or boron trifluoride complex, or a radical polymerization inhibitor such as hydroquinone, catechol, or pyrogallol. In addition, in order to improve adhesion speed, a curing accelerator such as polyethylene glycol derivatives, crown ether, or calixarene may be added. Furthermore, thickeners, plasticizers, fillers, elastomers, thixotropic agents, adhesion agents, crosslinking agents, dyes, fragrances, and the like may be added depending on the purpose. The amount of these additives added may be appropriately selected within a range that does not impair the effect of improving adhesive strength by adding the carboxylic acid compound of the present invention and the practicality or economics of the adhesive of the present invention.

[0042] 4. Joints and their manufacturing methods The 2-cyanoacrylate adhesive composition of the present invention has excellent adhesive strength to metals, and can be suitably used as an adhesive for bonding two parts, at least one of which is made of metal, in a manufacturing method for bonding the two parts. Therefore, according to the present invention, a bonded product can be manufactured that includes at least a first part made of metal and a second part bonded thereto, and the first part and the second part are bonded via the 2-cyanoacrylate adhesive composition of the present invention. In this case, the 2-cyanoacrylate adhesive composition of the present invention may be directly bonded to the metal surface of the first part, or may be bonded to a metal surface that has been previously treated with a primer. The metal constituting the first part is preferably aluminum or copper. The material of the second part is not particularly limited and can be appropriately selected from metal, plastic, rubber, wood, concrete, etc.

[0043] The bonded article of the present invention may, for example, be a laminate in which two sheet-like parts are superimposed and bonded together, or a laminate in which a linear part is covered with another part and bonded together. For example, the bonded article may be an article in which a metal part is attached to a resin molded article. More specifically, the bonded article may include a printed wiring board and a laminate thereof, a fire harness, a connector, a coated electric wire, etc. EXAMPLES

[0044] The present invention will be specifically described below based on examples. Note that the present invention is not limited to these examples. In the following, "parts" and "%" mean "parts by mass" and "% by mass", respectively, unless otherwise specified.

[0045] Examples 1 to 26 and Comparative Examples 1 to 5 The components shown in Tables 1 and 2 were mixed in the proportions shown in Tables 1 and 2 by a conventional method to prepare adhesive compositions, which were then evaluated as described below.

[0046] evaluation The tensile adhesive strength of the obtained adhesive composition to an aluminum substrate and a copper substrate at room temperature was measured in accordance with JIS K 6861. The increase rate of adhesive strength was also calculated relative to a comparative example not containing a carboxylic acid compound. The results are shown in Tables 1 and 2.

[0047] [Table 1]

[0048] [Table 2]

[0049] As shown in Table 1, in the cases of Examples 1 to 26 containing a carboxylic acid compound, the adhesive strength to a copper substrate showed an increase of 30% or more compared to the comparative example, and the adhesive strength to an aluminum substrate was also increased compared to the comparative example. [Industrial Applicability]

[0050] The 2-cyanoacrylate adhesive composition of the present invention has excellent adhesive strength to metals such as aluminum and copper, and is therefore suitable for the manufacture of electrical and electronic components that include metal wiring, and can also be widely used in other industrial fields where metal is used as an adherend.

Claims

1. 1. An adhesive composition comprising: The composition contains at least one 2-cyanoacrylate compound selected from the group consisting of alkyl-2-cyanoacrylates having an alkyl group with 4 or more carbon atoms and 2-cyanoacrylates having an ether bond in an ester residue, and at least one carboxylic acid compound selected from the group consisting of compounds represented by the following formula (1): The 2-cyanoacrylate adhesive composition contains the carboxylic acid compound in an amount of 600 ppm or more and 6,000 ppm or less per 100 parts by mass of the 2-cyanoacrylate compound. 【Chemistry 1】 (In formula (1), n ​​represents an integer of 1 to 2, and R １ each independently represents a hydrogen atom or a hydrocarbon group having 1 to 13 carbon atoms (except when all of R 1 are hydrogen atoms), the bond a may be a single bond or a double bond, and two R １ may be bonded to each other to form a cyclic hydrocarbon group.

2. R in the formula (1) １ are each independently a hydrogen atom, a linear or branched alkyl group having 1 to 10 carbon atoms, a cycloalkyl group, an allyl group, or an aryl group (excluding the case where all of R 1 are hydrogen atoms), １ The 2-cyanoacrylate adhesive composition according to claim 1, wherein are bonded to each other to form a cyclic hydrocarbon group.

3. The 2-cyanoacrylate adhesive composition according to claim 1 or 2, wherein the alkyl-2-cyanoacrylate is at least one selected from the group consisting of 2-octyl-2-cyanoacrylate, 2-ethylhexyl-2-cyanoacrylate, n-octyl-2-cyanoacrylate, and n-hexyl-2-cyanoacrylate.

4. A bonded article comprising at least a first metal component and a second metal component bonded thereto, the first metal component and the second metal component being bonded together via an adhesive, the adhesive being the 2-cyanoacrylate adhesive composition according to any one of claims 1 to 3.

5. A method for producing a bonded article, comprising bonding two parts, at least one of which is made of metal, via the 2-cyanoacrylate adhesive composition according to any one of claims 1 to 3.

6. 1. An agent for improving the adhesive strength of a 2-cyanoacrylate adhesive composition to metal, comprising at least one carboxylic acid compound selected from the group consisting of compounds represented by the following formula (1): wherein the 2-cyanoacrylate adhesive composition comprises at least one 2-cyanoacrylate compound selected from the group consisting of alkyl-2-cyanoacrylates having an alkyl group with 4 or more carbon atoms and 2-cyanoacrylates having an ether bond in the ester residue, and the carboxylic acid compound is prepared so as to be added in an amount of 600 ppm or more and 6000 ppm or less per 100 parts by mass of the 2-cyanoacrylate compound: 【Chemistry 3】 (In formula (1), n ​​represents an integer of 1 to 2, and R １ each independently represents a hydrogen atom or a hydrocarbon group having 1 to 13 carbon atoms (except when all of R 1 are hydrogen atoms), the bond a may be a single bond or a double bond, and two R １ may be bonded to each other to form a cyclic hydrocarbon group.

7. A method for improving the adhesive strength of a 2-cyanoacrylate adhesive composition, comprising adding at least one 2-cyanoacrylate compound selected from the group consisting of alkyl-2-cyanoacrylates having an alkyl group with 4 or more carbon atoms and 2-cyanoacrylates having an ether bond in the ester residue, to the 2-cyanoacrylate adhesive composition, in an amount of 600 ppm or more and 6,000 ppm or less per 100 parts by mass of the 2-cyanoacrylate compound: 【Chemistry 5】 (In formula (1), n ​​represents an integer of 1 to 2, and R １ each independently represents a hydrogen atom or a hydrocarbon group having 1 to 13 carbon atoms (except when all of R 1 are hydrogen atoms), the bond a may be a single bond or a double bond, and two R １ may be bonded to each other to form a cyclic hydrocarbon group.

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