Fibers sized with a coating of amorphous polyetherketoneketone are useful in the preparation of reinforced polymers having improved properties, wherein the amorphous polyetherketoneketone can improve the compatibility of the fibers with the polymeric matrix.

A reinforcing fiber bundle base material has a reinforcing fiber bundle surface to which a sizing agent adheres, wherein a reinforcing fiber bundle has a fiber number per unit width of 600 fibers/mm or more and less than 1,600 fibers/mm while the reinforcing fiber bundle has a drape level of 120 mm or more and 240 mm or less.

Fibers sized with a coating of amorphous polyetherketoneketone are useful in the preparation of reinforced polymers having improved properties, wherein the amorphous polyetherketoneketone can improve the compatibility of the fibers with the polymeric matrix.

Fibers sized with a coating of amorphous polyetherketoneketone are useful in the preparation of reinforced polymers having improved properties, wherein the amorphous polyetherketoneketone can improve the compatibility of the fibers with the polymeric matrix.

Fibers sized with a coating of amorphous polyetherketoneketone are useful in the preparation of reinforced polymers having improved properties, wherein the amorphous polyetherketoneketone can improve the compatibility of the fibers with the polymeric matrix.

Fibers sized with a coating of amorphous polyetherketoneketone are useful in the preparation of reinforced polymers having improved properties, wherein the amorphous polyetherketoneketone can improve the compatibility of the fibers with the polymeric matrix.

Fibers sized with a coating of amorphous polyetherketoneketone are useful in the preparation of reinforced polymers having improved properties, wherein the amorphous polyetherketoneketone can improve the compatibility of the fibers with the polymeric matrix.