Method for producing paper and cardboard

1. A method for producing paper, cardboard or similar materials, which comprises, separately or in a mixture, adding to a fibrous suspension:

at least one main retention agent comprising a cationic (co)polymer, and at least one dual retention agent comprising, a crosslinked cationic (co)polymer obtained in the form of a water-in-water type emulsion or standard reverse emulsion other than microemulsion, and placed in solution with gentle stirring prior to its introduction into the fibrous suspension, said crosslinked cationic (co)polymer having a UL viscosity of between 1.3 and 2.7 cps, and an apparent cationicity ratio of between 25 and 75%,

and, optionally, before or after the dual agent or the main retention agent, one or more tertiary retention agent(s) selected from the group consisting of mineral particles and organic polymers carrying anionic charges,

wherein the at least one main retention agent and the at least one dual retention agent have an overall cationic charge.

2. The method as claimed in claim 1 , wherein the dual retention agent is placed in solution with stirring between 50 and 500 rpm.

3. The method as claimed in claim 1 , wherein the dual retention agent is obtained by polymerization or copolymerization, in standard reverse emulsion other than microemulsion, of at least one cationic monomer and, optionally, nonionic monomers, in the presence of at least one crosslinking agent.

4. The method as claimed in claim 3 , wherein:

a) the cationic monomers are selected from the group consisting of dimethylaminoethyl acrylate (ADAME) and/or dimethylaminoethyl methacrylate (MADAME) quaternized or salified, dimethyldiallylammonium chloride (DADMAC), acrylamidopropyltrimethylammonium chloride (APTAC) and/or methacrylamidopropyltrimethylammonium chloride (MAPTAC);

b) the nonionic monomers are selected from the group consisting of acrylamide, methacrylamide, N-isopropylacrylamide, N—N dimethylacrylamide, N-vinylformamide, N-vinyl acetamide, N-vinyl pyrrolidone, vinylacetate, acrylate esters, allyl alcohol and/or unsaturated anionic ethylenic monomers having a carboxylic function (e.g. acrylic acid, methacrylic acid, and salts thereof, etc.), having a sulfonic acid function (e.g. 2-acrylamido-2-methylpropane sulfonic acid (AMPS), methallyl sulfonic acid and salts thereof, etc.); and

c) the crosslinking agent is selected from the group consisting of methylene bisacrylamide (MBA), ethylene glycol diacrylate, polyethylene glycol dimethacrylate, diacrylamide, cyanomethylacrylate, vinyloxyethylacrylate or methacrylate, triallylamine, formaldehyde, glyoxal, compounds of the glycidylether type such as ethyleneglycol diglycidylether, or epoxy resins and derivatives thereof.

5. The method as claimed in claim 1 , wherein the dual retention agent is introduced into the suspension at the rate of 30 g/t to 1000 g/t by weight of active material (crosslinked cationic (co)polymer) of the dry weight of the fibrous suspension.

6. The method as claimed in claim 1 , wherein the main retention agent comprises a cationic (co)polymer:

of at least one nonionic monomer selected from the group consisting of acrylamide, methacrylamide, one of their substituted derivatives, N-vinylformamide, N-vinyl acetamide, and N-vinylpyrrolidone.

7. The method as claimed in claim 1 , wherein the main retention agent is a cationic (co)polymer obtained:

by Hofmann degradation or Hofmann reaction on acrylamide based (co)polymers,

by hydrolysis reaction on a (co)polymer of N-vinylformamide and/or N-vinyl acetamide,

or by Mannich reaction on acrylamide based polymers.

8. The method as claimed in claim 1 , wherein the quantity of main retention agent introduced into the suspension to be flocculated is between thirty and one thousand grams of active polymer per tonne of dried pulp (30 and 1000 g/t).

9. The method as claimed in claim 1 , wherein the main retention agent and the dual retention agent form a mixture.

10. The method as claimed in claim 1 , wherein the dual retention agent is introduced after a pressure screen.

11. The method as claimed in claim 1 , wherein the mineral particles are selected from the group consisting of bentonites, hectorites, smectites, montmorillonites, nontronites, saponites, sauconites, hormites, attapulgites sepiolites, silicates, silicoaluminates borosilicates, zeolites, kaolinites, colloidal silicas and precipitated silica.

12. The method as claimed in claim 1 , wherein, with or without tertiary agent, a coagulant is added to the fibrous suspension prior to the addition of the main retention agent, said coagulant being selected from the group consisting of inorganic coagulants and organic coagulants.

13. The method as claimed in claim 1 , wherein the dual retention agent has a cationicity lower than 4 meq/g.

14. The method as claimed in claim 1 , wherein the main retention agent is a cationic polymer of natural origin, selected from the group consisting of derivatives of starch or of guar gum.

15. The method as claimed in claim 1 , wherein said UL viscosity is between 1.5 and 2.4 cps.

16. The method as claimed in claim 2 , wherein said stirring is between 70 and 200 rpm.

17. The method as claimed in claim 5 , wherein said rate is 150 g/t to 500 g/t by weight of active material (crosslinked cationic (co)polymer) of the dry weight of the fibrous suspension.

18. The method as claimed in claim 6 , wherein the main retention agent further comprises at least one unsaturated cationic ethylenic monomer, selected from the group consisting of the monomers of the type of dialkylaminoalkyl (meth)acrylate, dialkylaminoalkyl (meth)acrylamide, diallylamine, methyldiallylamine and their quaternary ammonium or acidic salts.

19. The method according to claim 12 wherein said organic coagulant is chosen from the group consisting of (a) polymers based on diallyldimethyl ammonium chloride (DADMAC), (b) polyethyleneimine, (c) quaternary polyamines produced by condensation of a primary or secondary amine with epichlorhydrin, (d) vinylamine polymers and (e) dicyandiamide resins.