Epoxy resin composition, prepreg, and fiber-reinforced composite material

An epoxy resin composition comprising components [A], [B], [C], and [D], meeting the following requirements [d], [e], and [f], and also meeting the following requirement 1 and requirement 2:

• [A]:
• epoxy resin, wherein component [A] contains a tri- or higher functional epoxy resin,
• [B]:
• dicyandiamide,
• [C]:
• aromatic urea,
• [D]:
• borate ester,
• [d]:
• 0.005≤(content of component [D]/content of component [C])≤0.045,
• [e]:
• 0.9≤(number of moles of active groups in component [A]/number of moles of active hydrogen in component [B])≤1.3,
• [f] 12≤(content of component [A]/content of component [C])≤26,
• requirement 1:
• in dielectric measurement at 80° C., the time period from the start of the measurement until the cure index reaches 10% is 120 minutes or more, and
• requirement 2:
• in dielectric measurement at 150° C., the time period from the start of the measurement until the cure index reaches 70% is 120 seconds or less,
• wherein the following component [A1], used as the tri- or higher functional epoxy resin, accounts for 55 to 100 parts by mass in 100 parts by mass of component [A]:
• [A1]:
• an epoxy resin as represented by formula (I) or an epoxy resin as represented by formula (II):
• wherein in formula (I), R1, R2, and R3 are each independently a hydrogen atom or a methyl group, and n is an integer of 1 or more, and
• wherein in formula (II), n is an integer of 1 or more.

An epoxy resin composition comprising components [A], [B], [C], and [D], meeting the requirements [d], [e], and [f], and also meeting the following requirement 3 and requirement 4:

• [A]:
• epoxy resin, wherein component [A] contains a tri- or higher functional epoxy resin,
• [B]:
• dicyandiamide,
• [C]:
• aromatic urea,
• [D]:
• borate ester,
• [d]:
• 0.005≤(content of component [D]/content of component [C])≤0.045,
• [e]:
• 0.9≤(number of moles of active groups in component [A]/number of moles of active hydrogen in component [B])≤1.3,
• [f] 12≤(content of component [A]/content of component [C])≤26,
• requirement 3:
• The temperature at which the epoxy resin composition exhibits the lowest viscosity when the temperature is raised from 40° C. to 250° C. at a rate of 5° C./minute in dynamic viscoelasticity measurement is 110° C. or more and 140° C. or less, and
• requirement 4:
• the difference between the heat generation onset temperature (T0) and the heat generation offset temperature (T1) during the period in which the epoxy resin composition is heated from 30° C. to 300° C. in a differential scanning calorimeter at a constant rate of 5° C./min is 25° C. or less,
• wherein the following component [A1], used as the tri- or higher functional epoxy resin, accounts for 55 to 100 parts by mass in 100 parts by mass of component [A]:
• [A1]:
• an epoxy resin as represented by formula (I) or an epoxy resin as represented by formula (II):
• wherein in formula (I), R1, R2, and R3 are each independently a hydrogen atom or a methyl group, and n is an integer of 1 or more, and
• wherein in formula (II), n is an integer of 1 or more.

An epoxy resin composition comprising components [A], [B], [C], and [D], meeting the following requirements [d], [e], and [f], and also meeting the following requirement 5 and requirement 6:

• [A]:
• epoxy resin, wherein component [A] contains a tri- or higher functional epoxy resin,
• [B]:
• dicyandiamide,
• [C]:
• aromatic urea,
• [D]:
• borate ester,
• [d]:
• 0.005≤(content of component [D]/content of component [C])≤0.045,
• [e]:
• 0.9≤(number of moles of active groups in component [A]/number of moles of active hydrogen in component [B])≤1.3,
• [f] 12≤(content of component [A]/content of component [C])≤26,
• requirement 5:
• in Curelastometer measurement at 150° C., the demolding index calculated by dividing the maximum torque (TH) by the volume of the sample is 0.40 N·m/cm3 or more and 1.50 N·m/cm3 or less, and
• requirement 6:
• in Curelastometer measurement at 150° C., the time tm(70) from the start of the measurement until torque reaches 70% of the maximum torque is 150 seconds or less,
• wherein the following component [A1], used as the tri- or higher functional epoxy resin, accounts for 55 to 100 parts by mass in 100 parts by mass of component [A]:
• [A1]:
• an epoxy resin as represented by formula (I) or an epoxy resin as represented by formula (II):
• wherein in formula (I), R1, R2, and R3 are each independently a hydrogen atom or a methyl group, and n is an integer of 1 or more, and
• wherein in formula (II), n is an integer of 1 or more.