Matrix resin composition, prepreg and method for producing the same, and fiber-reinforced composite material

A method for producing a prepreg which contains reinforcing fibers and a matrix resin composition with the weight per square meter of the reinforcing fibers being 250-2,000 g/m2, the production method comprising the following steps (1)-(3):

• (1) a matrix resin composition blending step for obtaining a matrix resin composition having a viscosity of 12 Pa·s to 3,000 Pa·s at 30° C. by mixing an epoxy resin, a radically polymerizable unsaturated compound, an epoxy resin curing agent and a polymerization initiator that generates radicals, in said step the content of the radically polymerizable unsaturated compound relative to 100% by mass of the total of the epoxy resin and the radically polymerizable unsaturated compound being 10-25% by mass;
• (2) a matrix resin composition impregnating step for obtaining a prepreg precursor by impregnating a reinforcing fiber assembly being 250-2,000 g/m2 in weight with the matrix resin composition;
• and
• (3) a surface viscosity increasing step of obtaining a prepreg in which viscosity of the matrix resin composition is increased more in a superficial part than in a central part, by letting the polymerization initiator present in the superficial part of the prepreg precursor generate a radical, while not letting the polymerization initiator present in the central part generate a radical,
• wherein the step (2) includes the following steps (2-iv) to (2-vi):
• (2-iv) a matrix resin composition applying step comprising applying the matrix resin composition only to one face of a first reinforcing fiber sheet A1 composed of consecutive reinforcing fiber bundles or applying the matrix resin composition only to both a first face and a second face of the first reinforcing fiber sheet A1;
• (2-v) a reinforcing fiber matrix laminating step comprising laminating a second reinforcing fiber sheet A2 composed of consecutive reinforcing fiber bundles, and a third reinforcing fiber sheet A3 composed of consecutive reinforcing fiber bundles, onto the first face of the A1 fiber sheet and the second face of the A1 fiber sheet respectively, wherein the matrix resin composition is only present on the A1 fiber sheet;
• and
• (2-vi) a pressurizing step comprising impregnating the A1 fiber sheet, the A2 fiber sheet, and the A3 fiber sheet with the matrix resin composition by pressurizing the A1, the A2, and the A3 fiber sheets so that the matrix resin composition migrates outward from inside of the A1, the A2 and the A3 fiber sheets to obtain the prepreg precursor.