Adhesive composition

A curable adhesive composition comprising

• (a) a 2-cyanoacrylic acid ester,
• (b) a polymer compatible with the 2-cyanoacrylic acid ester, said polymer having a number average molecular weight in a range of 5,000 to 500,000,
• (c) a multifunctional 2-cyano-3-vinylacrylic acid ester having 2 or more 2-cyano-3-vinylacryloyl groups, wherein the number average molecular weight of the multifunctional 2-cyano-3-vinyl-acrylic acid ester (c) is from 1000 to 50000, and
• (d) an onium salt represented by the following general formula (1):
• C+A− (1)
• wherein, C+ represents an onium cation and A− represents an anion comprising a hydrogen sulfate anion, a hydrogen sulfite anion, a sulfonate anion represented by R1SO3− wherein R1 represents an alkyl group, a perfluoroalkyl group having 1 to 10 carbon atoms, a vinyl group, an aryl group;
• a perfluoroaryl group, a halogen atom, or a bis(substituted sulfonyl)imide anion represented by (R2SO2)2N− wherein R2 represents an alkyl group, a perfluoroalkyl group having 1 to 10 carbon atoms, or an aryl group, and
• wherein the contents of the components (b), (c), and (d) are from 2 to 50 parts by mass, from 1 to 200 parts by mass, and from 0.01 to 5 parts by mass, respectively, relative to 100 parts by mass of the component (a).

A cured product comprising a cured adhesive composition, wherein, before curing, the adhesive composition is curable and comprises

• (a) a 2-cyanoacrylic acid ester,
• (b) a polymer compatible with the 2-cyanoacrylic acid ester,
• (c) a multifunctional 2-cyano-3-vinylacrylic acid ester having 2 or more 2 cyano-3-vinylacryloyl groups, and
• (d) an onium salt represented by the following general formula (1):
• C+A− (1)
• in which C+ represents an onium cation and A− represents an anion provided that polymerization of the 2-cyanoacrylic acid ester is avoided, and wherein the amounts of the components (b), (c) and (d) are from 2 to 50 parts by mass, from 1 to 200 parts by mass, and from 0.01 to 5 parts by mass, respectively, relative to 100 parts by mass of the component (a), and
• said cured product has a continuous phase resulting from the (b) polymer and has a dispersed phase dispersed within said continuous phase, said dispersed phase resulting from the polymerization product of the 2-cyanoacrylic acid ester (a), and said dispersed phase exhibiting a rate of change of the maximum particle diameter of not more than twice after thermal treatment at 6000.