EP2870287A1

EP2870287A1 - Improved method for manufacturing paper using a cationic polymer obtained by hofmann degradation

Info

Publication number: EP2870287A1
Application number: EP13737326.2A
Authority: EP
Inventors: René Hund; Christophe Auriant
Original assignee: SPCM SA
Current assignee: SPCM SA
Priority date: 2012-07-09
Filing date: 2013-06-17
Publication date: 2015-05-13
Anticipated expiration: 2033-06-17
Also published as: CA2876609A1; FR2992981A1; US9303359B2; ES2590528T3; US20150176208A1; CA2876609C; FR2992981B1; WO2014009621A1; KR20150035723A; KR102123132B1; EP2870287B1; CN104395525A; BR112014031379A2

Abstract

A method for manufacturing a sheet of paper and/or of cardboard, according to which, in an apparatus comprising a dilution pump and a head box: - a cellulose-based fibrous suspension is prepared, - the white water is introduced into the thick paste; - the mixture is homogenized in the dilution pump; - the diluted paste is transferred to the head box; - the sheet is formed and then dried, characterized in that, before homogenization of the mixture in the dilution pump, a cationic copolymer obtained by "Hofmann" degradation reaction is introduced into the white water and/or the thick paste and/or the mixture formed by the white water and the thick paste.

Description

IMPROVED PAPER MANUFACTURING METHOD USING A CATIONIC POLYMER OBTAINED BY HOFMANN DEGRADATION

The invention relates to an improved process for the production of paper, cardboard or the like using at least one cationic polymer obtained by Hofmann degradation and making it possible to increase the level of filler in said paper and paperboard, while retaining properties of interesting physical resistance. It also relates to paper or paperboard obtained by this method.

Hofmann degradation polymers are chemical compounds commonly used in the papermaking industry. For example, the document WO2011 / 015783 describes, in particular, cationic (co) polymers derived from acrylamide obtained by Hofmann degradation. These compounds are added as dripping agents in so-called diluted pastes, or to improve dry strength performance, also in diluted pastes.

The composition of most fibrous suspensions used in the manufacture of paper contains, following a direct or indirect addition (using recycled paper), inorganic fillers such as clays, kaolin, calcium carbonate, or titanium dioxide. Industrially, the most used fillers are calcium carbonates, either in ground form (we speak of GCC for "Ground Calcium Carbonate"), or in precipitated form (it is called PCC for "Precipitated Calcium Carbonate") . At present, in view of the significant increase in the price of paper fibers, there is an increasing interest in substituting some of the fiber for the sheet with less expensive mineral fillers.

Conventionally, retention agents are used to increase the overall retention in the sheet (RPF: "First Pass Retention"), and in particular the retention of charges (FPAR: "First Pass Ash Retention"). Chemically, these retention agents are generally polymers of high molecular weight (i.e. greater than 1 million g / mol), such as acrylamide copolymers. These polymers can be combined with inorganic microparticulate compounds (bentonite, colloidal silica).

However, the increase in the charge rate, to the detriment of fibers, with this widespread technology, tends to deteriorate the physical properties of the paper. The amount of charges incorporated in the sheet is therefore limited because of the strength constraints. The retention agents conventionally used are added to the diluted dough, ie a fibrous suspension containing from 0.1 to 1.5% of dry matter. They make it possible to improve the charge retention, ie to optimize the quantity of charge used. Their role consists in particular to retain the charges in the paper and thus reduce the amount of charges discharged into the white water from the drainage of the sheet during its formation on the canvas.

WO2009 / 036271 discloses a process for increasing the filler content in the paper by pre-flocculation of the "slurry" of fillers (liquid paste) in the presence of two flocculants injected successively, and combined with an added global retention agent near the headbox. However, this technique is difficult to implement because of the multitude of compounds added in a well-defined sequence. The documents US2006 / 0024262 and US2009 / 0272506 describe a treatment using an amphoteric PolyVinylAmine (PVA) resulting from the hydrolysis of an N-vinylformamide base copolymer (NVF).

US2012073774A1 discloses a process involving the addition of a cationic polymer and an aqueous suspension of tackifier. The cationic polymer is preferably a polyvinylamine obtainable in particular by hydrolysis or by the Hofmann degradation reaction. These two compounds are typically integrated into the diluted dough. They make it possible to reduce the adhesion of the sheet of paper to the fabric during drying.

Although these methods make it possible to introduce into the sheet an advantageous loading rate while maintaining acceptable physical properties, they nevertheless have limits. There is therefore a need to further increase the amount of charges without deteriorating the physical performance of the paper.

The problem to be solved by the invention concerns, in particular, the optimum increase in the quantity of charges, or charge rates, in the sheets of paper or cardboards, while maintaining satisfactory physical performances.

The present invention provides an improved process for producing paper, board and the like comprising adding to a fibrous suspension at least one polymer obtained by Hofmann degradation characterized in that the polymer obtained by Hofmann degradation is cationic, and added before the dilution pump ("fan pump") of the thick stock (thick stock) with white water.

More specifically, the present invention relates to a method of manufacturing a sheet of paper and / or cardboard and the like, according to which, in an installation comprising a dilution pump and a headbox:

a cellulosic fibrous suspension is prepared in which charges are advantageously introduced, referred to as thick stock;

the white water coming from the drainage of the leaf is introduced into the thick paste;

the mixture thus obtained is homogenized in the dilution pump (fan pump); the thin stock is transferred from the homogenization to the headbox;

the sheet is formed;

the sheet is dried.

This process is characterized in that before homogenization of the mixture in the dilution pump, that is to say before the dilution pump, a cationic copolymer obtained by Hofmann degradation reaction is introduced into the white water and / or the thick paste, and / or the mixture formed by the white waters and the thick paste.

In view of the prior art, it is quite surprising to find that a cationic version of the polymer obtained by Hofmann degradation, when introduced into the process as mentioned above, can provide better performance than the amphoteric versions in terms of charge retention while maintaining very good properties of physical resistance.

The present invention also relates to the paper or paperboard obtained that can be obtained by this method. Without being bound to any theory, the Applicant considers that the cationic polymer obtained by Hofmann degradation can play the role of affinity activator between charges and fibers, which allows the charges to be retained quantitatively. in the sheet of paper at the time of formation of the paper network. Moreover, this very good affinity seems to reinforce the cohesion of the structure of the sheet of paper, thus conferring on it an unequaled physical resistance relative to the percentage of load present in the sheet. As previously mentioned, in a paper making process, cardboard or the like, the white water is added to the thick stock before the fan pump. Once mixed, the dough forms a diluted paste (dilute suspension, "thin stock") which at the output of the dilution pump goes to the head box where the wet sheet is formed before to be dried. Generally a shearing step is provided between the dilution pump and the headbox: it is the "pressure screen". The fillers are generally added in the form of slurry in the thick paste. However, these fillers may come from a raw material containing fillers, for example deinked pastes, broken / glued pasta, etc.

The thick paste, or thick fibrous suspension generally contains between 2 and 5% of dry matter. As already indicated, the cationic polymer obtained by Hofmann degradation can be introduced into the process in the thick paste and / or in the white waters and / or in the mixture of the two before the dilution pump.

Conventionally, fillers are added, especially in the form of slurry, before the dilution pump. They are in the thick paste and / or the white waters and / or the mixture of the two, and this, in one or in several times. The fillers are nevertheless most often advantageously added to the thick paste.

In a first embodiment, the polymer is added in the immediate vicinity of the point or points of introduction of the charges.

In a second embodiment, the cationic polymer is introduced at the same time as the charges. Advantageously, it is introduced in this case in the "slurry" of charges or during its preparation.

When the polymer is introduced into the white water, it is advantageously just before mixing with the thick paste.

"Slurry" filler is an aqueous dispersion containing fillers. Generally a "slurry" contains more than 10% by weight. The improved process according to the invention may further comprise the addition in the paper sequence of any other mineral compound or natural or synthetic polymer well known to those skilled in the art. The addition of at least one additive chosen from the group comprising coagulation agents (PAC (polyaluminium chloride), polyDADMAC, polyamine), retention agents (anionic, cationic or amphoteric polymers, bentonites, siliceous materials), dry-strength resins (DSR) (native starch, cationic starch, polyvinylamine) or drainage agents (polyethyleneimine). In a particular embodiment, the process according to the invention comprises the addition of at least one cationic polymer obtained by Hofmann degradation before the dilution pump, and at least one cationic polymer based on acrylamide in the diluted dough, that is to say after the dilution pump. Preferably, this cationic polymer based on acrylamide has a molecular weight greater than 1 million g / mol.

The amount of cationic polymer obtained by Hofmann degradation introduced according to the process of the invention is between 50 and 4000 grams of active polymer per tonne of dry pulp (g / t). Preferably, the amount introduced is between 100 g / t and 1000 g / t.

Hofmann's degradation is a reaction discovered by Hofmann at the end of the nineteenth century, which makes it possible to convert an amide into a primary amine by eliminating carbon dioxide. The reaction mechanism is detailed below. In the presence of a base (soda), a proton is torn from the amide.

O u 0

it _ / ^H OH- it -

R- C- N ^ τ, * R- C- N- H

H

The amidate ion formed then reacts with the active chlorine (Cl ₂ ) of the hypochlorite (eg: NaClO which is in equilibrium: 2 NaOH + Cl ₂ NaClO + NaCl + H ₂ O) to give an N-chloramide. The base (NaOH) removes a proton from chloramide to form an anion. The anion loses a chloride ion to form a nitrene which is transposed to isocyanate. By reaction between the hydroxide ion and the isocyanate, a carbamate is formed.

R-N = C = O + OH R-NH-C0 ₂

After decarboxylation (removal of CO ₂ ) from the carbamate, a primary amine is obtained

R- NH ₂ - C0 + - R- NH _£

- C0 ₂

For the conversion of all or part of the amide functions of an acrylamide (co) polymer in amine function, two main factors intervene (expressed in molar ratios). These are: - Alpha = (alkaline and / or alkaline earth / acrylamide hypohalide) and - Beta = (alkali and / or alkaline earth hydroxide / alkaline and / or alkaline earth hypohalide).

The cationic polymers obtained by Hofmann degradation implemented in the process according to the invention are advantageously chosen from the polymers described in document WO2011 / 015783.

They are obtained by Hofmann degradation on a precursor based on acrylamide or derivatives, otherwise called (co) polymer base, previously modified with at least one polyfunctional compound containing at least 3 identical or different heteroatoms and each having at least one mobile hydrogen.

The heteroatoms can be: N, S, O, P

The polyfunctional compounds may in particular be oligomers, polymers, or carbon chains containing at least three carbon atoms.

In an advantageous embodiment, the polyfunctional compound may be chosen from the group comprising polyethyleneimines (PEI), polyamines (primary or secondary), polyallylamines, polyamines amides (PAA), polythiols, polyalcohols, polyamides epichloridrine (PAE) and their mixtures.

In a preferred embodiment, the polyfunctional compound incorporated may be polyethyleneimine (PEI) or a polyamine amide (PAA). In practice, the polymer obtained at the end of the Hofmann reaction may be branched, due to a branching of the base polymer. In other words, it is the branched nature of the base copolymer which will confer its branched state on the final polymer. In a preferred embodiment, the polymer is obtained by Hofmann degradation reaction in the presence, as hypohalogenide, of an alkaline hypochlorite, preferably sodium hypochlorite.

According to another characteristic, the alpha hypohalide / nonionic monomer coefficient (expressed in molar ratios) used for the preparation of the polymers of the invention is greater than 0.3 or even 0.5, advantageously between 0.8 and 1. included.

According to another characteristic, the Hofmann degradation product is produced at a concentration greater than 4% by weight, preferably greater than 5%, advantageously greater than 7%.

In addition, the copolymer of the invention may have a cationic charge density of preferably greater than 2 meq / g and advantageously greater than 5 meq / g.

The polymer used in the process according to the invention is advantageously obtained by Hofmann degradation reaction on a base copolymer comprising:

at least 5 mole% of a nonionic monomer chosen from the group comprising acrylamide (and / or methacrylamide), N, N-dimethylacrylamide and / or acrylonitrile, preferably acrylamide,

at least 0.001 mol% of at least one additional polyfunctional compound selected from the group consisting of polyethyleneimine, polyamine (primary or secondary), polyallylamine, preferably polyethyleneimine polythiols, optionally at least:

an unsaturated cationic ethylenic monomer, preferably chosen from the group comprising dialkylaminoalkyl (meth) acrylamide, diallylamine, methyldiallylamine and their quaternary ammonium or acidic salt monomers. Mention may in particular be made of dimethyldiallylammonium chloride (DADMAC), acrylamidopropyltrimethylammonium chloride (APTAC) and / or methacrylamidopropyltrimethylammonium chloride (MAPTAC), and / or a nonionic monomer preferably chosen from the group comprising N-vinylacetamide. N-vinyl formamide, N-vinylpyrrolidone and / or vinyl acetate. Advantageously, the base polymer is branched and preferably consists of three types of compounds:

acrylamide,

polyethyleneimine, and

at least one unsaturated cationic ethylenic comonomer selected from the group comprising dialkylaminoalkyl (meth) acrylamide, diallylamine, methyldiallylamine and their quaternary ammonium or acid salts, preferably dimethyldiallylammonium chloride. It is important to note that, in combination with these monomers, it is also possible to use water-insoluble monomers such as acrylic, allylic or vinyl monomers having a hydrophobic group. In use, these monomers are employed in amounts generally less than 20 mol%, preferably less than 10 mol%. They may be chosen preferentially from the group comprising acrylamide derivatives such as N-alkylacrylamide, for example N-tert-butylacrylamide, octylacrylamide and Ν, Ν-dialkylacrylamides, such as N, N-dihexylacrylamide, etc.

In a preferred embodiment, the precursor based on acrylamide or derivatives (otherwise called base polymer on which the Hofmann degradation is carried out) incorporates, within itself, at least polyethyleneimine (PEI);

the alpha hypohalide / nonionic monomer coefficient used for the preparation of the polymers of the invention is between 0.8 and 1 inclusive;

the base copolymer is branched.

The branching may preferably be carried out during (or possibly after) the polymerization of the "base" copolymer, in the presence of a polyfunctional branching agent and optionally of a transfer agent. The following is a non-limiting list of branching agents: methylene bisacrylamide (MBA), ethylene glycol di-acrylate, polyethylene glycol dimethacrylate, diacrylamide, cyanomethylacrylate, vinyloxyethylacrylate or methacrylate, triallylamine, formaldehyde, glyoxal, glycidyl ether compounds such as ethylene glycol glycidyl ether, or epoxy or any other means well known to those skilled in the art for crosslinking. In practice, the branching agent is advantageously introduced at a rate of from five to fifty thousand (5 to 50,000) parts per million by weight relative to the active ingredient, preferably from 5 to 10,000, advantageously from 5 to 5,000. Advantageously, the branching agent is methylenebisacrylamide (MBA). The incorporation of the additional polyfunctional compound into the base copolymer can be carried out in the reaction medium, before or during the polymerization of the monomers constituting the (co) polymer base, or by any other grafting method on the finished base copolymer.

Preferably, the additional polyfunctional compound is mixed with a comonomer before polymerization.

The transfer agent may in particular be chosen in a non-limiting manner from the group comprising isopropyl alcohol, sodium hypophosphite, and mercaptoethanol.

The copolymer serving as a basis for the Hofmann degradation reaction does not require the development of a particular polymerization process. The main polymerization techniques, well known to those skilled in the art and which can be used are: precipitation polymerization, emulsion polymerization (aqueous or inverse) followed or not by a distillation step and / or "spray drying" "(Spray drying), and suspension polymerization or solution polymerization, both of which are preferred.

It is also possible to add in the base copolymer solution, before or during the Hofmann degradation reaction, certain compounds which are capable of reacting with the isocyanate functions of the polymer generated during degradation. In general, these are molecules carrying nucleophilic chemical functions such as hydroxyl or amine functions. As examples, the compounds in question can therefore be of the family: alcohols, polyols, polyamines, polyethyleneimines.

The incorporation of polyvalent cationic ion salts, as mentioned in the Applicant's document WO2010 / 061082, can also be carried out.

As already stated, the Hofmann reaction requires the conversion of amide functions to amino functions by involving two main factors (expressed in molar ratios):

Alpha = (alkaline and / or alkaline earth hypochlorite / (meth) acrylamide) Beta = (alkali and / or alkaline-earth hydroxide / alkaline and / or alkaline-earth hypochlorite) From a previously described "base" copolymer solution of concentration of between 10 and 40% by weight, preferably between 15 and 25%, the molar amount of total amide function is determined. The level of degradation Alpha is then chosen, which makes it possible to determine the dry quantity of alkaline and / or alkaline earth hypohalide and then the beta coefficient, which makes it possible to determine the dry quantity of alkaline hydroxide and / or alkaline-earthy.

A solution of hypohalide and alkali and / or alkaline earth hydroxide is then prepared from the alpha and beta ratios. According to the invention, the reagents preferably used are sodium hypochlorite (bleach) and sodium hydroxide (sodium hydroxide).

In order to stabilize the amine functions that will be produced, it may optionally be added to the reactor containing the base polymer, one (or possibly more) quaternary ammonium derivative as described in JP 57077398 and well known by the US Pat. one skilled in the art, it is indeed intended to avoid the reaction between the amine functions and residual amide functions. It will be noted, moreover, that the addition of these agents can be carried out separately, simultaneously, mixed or not, in any order of introduction and at one or more injection points.

The increase in cationicity of the base copolymer takes place during the so-called Hofmann degradation, by the use of an alkaline or alkaline-earth hypohalide.

Similarly, although prepared in solution, the polymers of the invention may also be provided in solid form. Under these conditions, the solid form contains not only the copolymer, but also a proportion of salt obtained at the end of the Hofmann degradation reaction. In practice, they are obtained inter alia by methods of drying the aforementioned solution. The main insulation techniques then used are those of spray drying (which consists of creating a cloud of fine droplets in a hot gas stream for a controlled period of time), drum drying, fluidized bed dryers, etc.

The incorporation of the cationic polymer obtained by Hofmann degradation will be done with the conventional means known to those skilled in the art.

The process according to the invention can be used with all types of pulp: virgin fiber pulps (kraft, bisulfite, etc.), recycled fibers, deinked pastes, mechanical and thermomechanical pulps, etc. With respect to fillers, it may be all types of fillers that may be selected from the group consisting of clays, kaolin, ground calcium carbonate (GCC), precipitated calcium carbonate (PCC), titanium dioxide, and mixtures thereof. The charges can be added in different forms, the shape "slurry" being the most encountered. They can be prepared with or without dispersant, out of or on paper manufacturing site.

The cationic polymer obtained by Hofmann degradation may be prepared near the paper machine.

The following examples illustrate the invention, but are not limiting in nature.

Polymer A:

The cationic polymer A is obtained by a Hofmann degradation reaction (alpha = 1) on a base copolymer (20% solution of base copolymer), of acrylamide (70 mol%) and of dimethyldiallyl ammonium chloride (DADMAC). (30% molar) branched (MBA: 600 ppm / active ingredient) modified with a polyethyleneimine polymer (BASF type of Polymin HM), at a level of 5% of active material.

To do this, the polyethyleneimine is mixed with the DADMAC monomer and the MBA in the reactor.

The acrylamide will be incorporated in continuous casting for 2 hours, in a reaction medium maintained at 85 ° C. The polymerization is catalyzed in the presence of SPS (sodium persulfate) and MBS (sodium metabisulfite), catalysts well known to those skilled in the art. The precursor polymer thus obtained has a viscosity of 5500 cps (25 ° C, Brookfield LV3, 12 rpm). The Hofmann degradation itself is carried out in the same manner as in Example 1 of the Applicant's document WO2010061082, by performing a complete Hofmann degradation. The acrylamide-derived cationic copolymer thus prepared has a bulk viscosity of 35 cps (25 ° C, Brookfield LV1, 60 rpm) and a concentration of 8.5% active material. Polymer B:

The cationic polymer B is obtained by a Hofmann degradation reaction (alpha = 1) on a base copolymer (20% of active material) of acrylamide (60 mol%), acrylic acid (10 mol%) and dimethyldiallyl ammonium chloride (DADMAC) (30% molar) branched (MBA: 600 ppm / active ingredient) modified with a polyethyleneimine polymer (BASF type Polymin HM), at a level of 5% active ingredient.

Acrylamide and acrylic acid will be incorporated in continuous casting for 2 hours, in a reaction medium maintained at 85 ° C. The polymerization is catalyzed in the presence of SPS and MBS, catalysts well known to those skilled in the art. The precursor polymer thus obtained has a viscosity of 4500 cps (25 ° C, Brookfield LV3, 12 rpm).

The Hofmann degradation itself is carried out in the same manner as in Example 1 of the Applicant's document WO2010061082, by performing a complete Hofmann degradation. The resulting acrylamide-derived cationic copolymer has a bulk viscosity of 55 cps (25 ° C, Brookfield LV1, 60 rpm) and a concentration of 9%.

These polymers will be compared to (1) a high molecular weight acrylamide / ADAME MeCl powder copolymer (FO 4190 PG1, SNF Floerger), standard retention agent, and (2) Luredur PR 8351 from BASF, an amphoteric copolymer based on PVA (from the hydrolysis of NVF), current reference as a charge retention agent and DSR performance maintainer.

Dry resistance evaluation procedure

Farmhouses of paper are realized with an automatic dynamic fermette.

The "slurry" of the dough is made by disintegrating dry pulp in order to obtain a final concentration of 3%.

The necessary amount of pulp is removed so as to finally obtain a sheet having a basis weight of 60 g / m ² . The concentrated paste is introduced into the vat of the dynamic form and stirred there. To this paste is added a "slurry" of fillers injected simultaneously (but separately) to the polymer A, B or Luredur PR 8351 from BASF. This paste is then diluted to a concentration of 0.32%.

In manual mode, the dough is pumped to the nozzle level to prime the circuit.

A blotter and training cloth are placed in the bowl of the dynamic formette before starting the rotation of the bowl at 900m / min and constructing the water wall. Potentially, a retention agent will be injected 10 seconds before the start of the production cycle of the sheet. The sheet is then made (in automatic mode) by 22 round trips of the nozzle projecting the paste into the wall of water. Once the water is drained and the automatic sequence is complete, the forming web with the formed fiber network is removed from the dynamically shaped bowl and placed on a table. A dry blotter is deposited on the side of the wet fiber mat and is pressed once with a roll. The whole is returned and the fabric is delicately separated from the fibrous mat. A second dry blotter is deposited and the sheet (between the two blotters) is pressed once under a press delivering 4 bars and is then dried on a dryer stretched for 9 min at 107 ° C. The two blotters are then removed and the sheet is stored overnight in a room with controlled humidity and temperature (50% relative humidity and 23 ° C). The dry strength properties of all the sheets obtained by this procedure are then evaluated.

The burst index is measured with a Messmer Buchel M 405 (average of 14 measurements).

Dry traction is measured in the machine direction with a Testometric AX tensile tester (average of 5 samples).

The rate of fillers in the sheet is measured using a muffle furnace according to a standard non-organic material measurement procedure (570 ° C for 5 hours)

The tests are carried out with a neutral pH paste of the following composition, by weight relative to the dry weight of the composition: (this composition exceeds 100% of material)

70%) of bleached hardwood kraft fibers

- 10% o of bleached softwood kraft fibers

20%) of mechanical pulp fibers made from pine 30% (by weight relative to the amount of fibers) of natural calcium carbonate are added to the dough.

Polymers used alone:

We can observe that polymer A provides more charge retention but also more DSR performance than Luredur PR 8351.

The polymer B, amphoteric, gives performance equivalent to Luredur PR 8351 but lower than the polymer A.

Polymers associated with a standard retention agent:

In a manner known to those skilled in the art, the simple use of a retention agent provides charge retention, but greatly deteriorates the physical performance.

In combination with a retention agent, the polymer A provides the highest amount of fillers in the paper sheet while maintaining good physical strength properties of the sheet.

Claims

1 / A method of manufacturing a sheet of paper and / or cardboard and the like, according to which, in an installation comprising a dilution pump and a headbox: - a fibrous cellulosic suspension, designated thick paste (thick stock) is prepared );

the mixture thus obtained is homogenized in the dilution pump (fan pump); - The thin stock is transferred from the homogenization to the headbox;

the sheet is formed;

we dry the leaf,

characterized in that, before homogenization of the mixture in the dilution pump, a cationic copolymer obtained by Hofmann degradation reaction is introduced into the white water and / or the thick paste and / or the mixture formed by the white water and the thick paste.

21. Process according to claim 1, characterized in that charges are introduced into the thick paste and the cationic polymer is introduced in the immediate vicinity of the point (s) of introduction of the charges.

3 / A method according to claim 1 characterized in that is introduced into the thick paste charges and in that the cationic polymer is introduced simultaneously with the charges.

4 / A method according to claim 3, characterized in that the charges are introduced in the form of a "slurry" and in that one introduces the cationic polymer in the "slurry" of charges or during its preparation.

5 / A method according to claim 1 characterized in that the cationic polymer is introduced into the white water, preferably just before introduction of said white water into the thick paste. 6 / A method according to one of claims 2 to 4, characterized in that the fillers are selected from the group comprising clays, kaolins, ground calcium carbonate (GCC), precipitated calcium carbonate (PCC), titanium dioxide, and mixtures thereof. 7 / A method according to one of the preceding claims, characterized in that the cationic copolymer is obtained by so-called Hofmann degradation reaction on a precursor based on acrylamide or derivatives, otherwise called (co) polymer base, previously modified with at least a polyfunctional compound containing at least 3 identical or different heteroatoms and each having at least one mobile hydrogen.

8 / A method according to claim 7, characterized in that the polyfunctional compound is selected from the group comprising polyethyleneimines (PEI), polyamines (primary or secondary), polyallylamines, polyamines amides (PAA), polythiols, polyalcohols epichloridin polyamides (PAEs) and mixtures thereof.

Process according to claim 7, characterized in that the (co) polymer base is branched by adding a polyfunctional branching agent and optionally a transfer agent.

10 / A method according to one of claims 7 to 9, characterized in that the coefficient alpha hypohalide / nonionic monomer (expressed in mole ratios) used for the preparation of the polymer is between 0.8 and 1 inclusive.