Process for preparing a sheet material with improved on-machine retention

A process using paper-making techniques for producing a fibrous sheet comprising fibers, an organic binder, a non-binding mineral filler and a flocculant, designed to improve the retention of the mineral filler in the sheet, sheet formation and the sheet's physical properties, which comprises the following steps in the following order:

• dispersing a non-binding mineral filler in an aqueous medium at a concentration from 150 to 600 g/l,
• mixing an anionic organic binder selected from the group consisting of native starches, chemically, enzymatically or thermally modified starches, dextrines, casein, animal glue, vegetable proteins, cellulose esters, alginates carboxylated or uncarboxylated styrene-butadiene latexes, acrylic latexes, styrene acrylics, vinyl acetates, neoprene Iatexes, acrylonitrile latexes and vinyl chloride latexes with the dispersed filler under agitation, continuously in a mixer in order to obtain an aqueous dispersion of mineral filler particles coated with binder which, after dilution, has a concentration from 50 to 200 g/l before flocculation;
• carrying out continuously ionic destabilization of said aqueous dispersion of mineral filler particles coated with binder by means of a cationic flocculant selected from the group consisting of aqueous solutions of polyethylenimine, polyamido-amine, cross-linked polyalkylamine, modified polyacrylamides, polyaluminium chloride and quaternary ammonium compounds in order to obtain improved flocculation control and good uniformity of the floc particle size, which favors formation, appearance, uniformity of surface and dimensional stability of the sheet;
• then continuously introducing the mineral filler/organic binder flocs in aqueous suspension into the refined pulp storage vat, conveying the mixture thus prepared to the head box and subsequently subjecting it to the draining, wet pressing and drying usual treatments of the paper manufacturing process for obtaining the fibrous sheet
• wherein the amount of binder lies between 1 and 40 parts by weight per 100 parts by weight of fibers and filler.