MANUFACTURING PROCESS OF A PAPER AND CARDBOARD

This invention relates to a method for manufacturing a sheet of paper or cardboard, comprising the addition of a water-soluble polymer P with a weight-average molecular weight greater than 750,000 Dalton to a fibrous suspension. This method comprises the following successive steps:preparing an aqueous solution A containing at least one water-soluble polymer P at a concentration C of between 0.1 and 0.5% by weight, said polymer P having a factor F(C) strictly greater than 4, with F(C)=Δ600/C, Δ600 being the slope to reach 90% of the viscosity developed by aqueous solution A at 600 seconds at 25° C., obtained from the viscosity curve of aqueous solution A as a function of time, at the given concentration C, at 25° C.,adding aqueous solution A to the fibrous suspension, at one or more injection points,forming a sheet of paper or cardboard.