Paper strengthening agent and paper strengthening method

There is provided a paper strengthening agent comprising a polymer dispersion manufactured by polymerizing a monomer containing (meth)acrylic acid in an aqueous solution and precipitating the produced polymer as fine particles, and which is added to a paper layer after a paper layer is formed, wherein (1) the concentration of the (meth)acrylic acid (co)polymer in said polymer dispersion is at least 5 wt%, (2) the average particle diameter of the (meth)acrylic acid (co)polymer particles in said polymer dispersion is 1 to 100 mu m, (3) the viscosity of said polymer dispersion is 30 to 5000 mPa.s immediately after and one month after the production, (4) the polymer particles can be recognized when said polymer dispersion is diluted 10 times with distilled water and the diluted dispersion is magnified by a factor of 400 and observed with a microscope, and (5) when the pH of said diluted dispersion is adjusted to 7.0 by adding alkali, said polymer particles are dissolved to form an aqueous solution. The paper strengthening agent prepared in this manner allows to obtain paper having increased paper strength by diluting with water, coating or impregnating onto a dry paper surface or wet paper surface, and drying the treated paper.