WO1999063159A1

WO1999063159A1 - Method for making paper and cardboard and retention and dewatering agents

Info

Publication number: WO1999063159A1
Application number: PCT/FR1999/001278
Authority: WO
Inventors: René Hund; Christian Jehn-Rendu
Original assignee: Snf S.A.
Priority date: 1998-06-04
Filing date: 1999-06-01
Publication date: 1999-12-09
Also published as: DE69912954T2; JP2002517626A; ATE254693T1; KR100685533B1; EP1092064B2; JP4699608B2; EP1092064B1; DE69912954T3; DE69912954D1; KR20010052553A; EP1092064B9; FR2779452B1; AU4043999A; CA2334196A1; FR2779452A1; EP1092064A1; CA2334196C

Abstract

The invention concerns an improved method for making paper which consists in using as main retention agent a branched polymer prepared in invert emulsion and bentonite as secondary retention agent (dual type system). The two additions are separated by a step for shearing the fibrous suspension (or mass). The invention enables to obtain a highly improved retention and also highly improved dewatering. Moreover, it enables to reduce the bentonite content in the white water.

Description

PROCESS FOR PRODUCING PAPER AND CARDBOARD AND RETENTION AND DRIPPING AGENTS

The present invention relates to the technical sector of manufacturing paper and polymers used in this context.

The invention relates to a process for the production of retention paper or board, and other improved properties.

During the production of paper, cardboard or the like, it is well known to introduce retention agents into the pulp, the function of which is to retain a maximum of fines and fillers in the sheet. The beneficial effects that result from the use of a retention agent are essentially:

- the increase in production and the decrease in manufacturing costs: energy saving, smoother running of the machine, higher yield of fibers, fines, fillers and anionic finishing products, lower acidity in the circuit linked to a reduction in the use of alumina sulphate and therefore lessening of the corrosion problems;

- improving quality: better training and better appearance; improvement in the humidity of the sheet, the opacity, the smoothness, the absorbency and reduction of the porosity of the paper.

It has long been proposed to add bentonite to the pulp, which can optionally be added to other mineral products, such as aluminum sulphates, or even synthetic polymers, in particular polyethyleneimine (see for example documents DE-A-2 262

906 and US-A-2,368,635).

In document US Pat. No. 3,052,595, it has been proposed to combine bentonite with a polyacrylamide of essential linear characteristic. This process has found itself in competition with systems that are easier to implement while also being efficient. Furthermore, even with current linear polyacrylamides, the retention power is still insufficient.

In document EP-A-0 017 353, it has been proposed, for the retention of lightly loaded pastes (at most 5% of fillers) to combine with bentonite a linear nonionic copolyacrylamide which is weakly anionic. This process has hardly been developed, since these polymers are relatively ineffective in terms of retention, in particular of filled pastes, no doubt due to an insufficient synergy between these copolymers and bentonite which has little tendency to recoagulate.

In document EP-A-0 235 893, it has been proposed to use cationic polyacrylamides of molecular weight greater than one million, thirty million and more, essentially linear. In this way, a retention effect is obtained which is certainly satisfactory, but which is still considered insufficient in the paper application, because, the use of bentonite causing difficulties during the subsequent treatment of the effluents leaving the machine, the users only select this system. '' in the event of significant benefits.

In the notes presented during a course in Seattle, October 11-13, 1989, and published under the title "Supercoagulation in the control of wet end chemistry by synthetic polymer and activated bentonite", R. Kajasvirta described the mechanism of the supercoagulation of activated bentonite in the presence of a cationic copolyacrylamide, without specifying the exact nature thereof. This process has the same drawbacks as above.

Finally, EP 0 574 335 has made an important improvement by proposing the use of polymers (in particular polyacrylamides) branched in the form of a powder.

The invention overcomes the drawbacks mentioned above.

It relates to an improved process of the type in question, which consists in adding to the suspension or fibrous mass or pulp to be flocculated, as main retention agent, an agent which consists of, or comprises, a branched polyacrylamide which is characterized by what it was prepared in reverse emulsion or water-in-oil, and bentonite as a second retention agent (system called "dual", of the type also called "microparticulate"). By the term "is in reverse emulsion" or similar terms, relating to the polymer used (ie as injected or introduced into the flocculating paste) according to the invention, the skilled person will understand that denotes the reverse water-in-oil emulsion which is dissolved in water before its injection or introduction into the mass or flocculating paste (this dissolution in water causes what is called the "inversion" of the initial water-in-oil reverse emulsion; these methods are well known to those skilled in the art).

The polymer and bentonite additions are separated by a shearing step, for example at the level of the so-called "fan pump" mixing pump. In this area, reference will be made to the description of patent USP 4, 753, 710 as well as to a very large prior art dealing with the point of addition of the retention agent with respect to the shearing steps existing on the machine, in particular USP 3,052,595, Unbehend, TAPPI Vol. 59, N ° 10, October 1976, Luner, 1984 Papermakers Conférence or Tappi, April 1984, pp 95-99, Sharpe, Merck and Co Inc, Rahway, NJ, USA, around 1 980, Chapter 5 "polyelectrolyte retention aids" , Britt, Tappi Vol. 56, October 1973, p 46 ff. and Waech, Tappi, March 1983, pp 137, or the USP 4,388.1 50

(Eka Nobel).

Reference will also be made to patent USP 4,753,710, for all that relates to the generalities relating to the manufacture of paper, the usual additives used, and similar details.

It is possible to replace bentonite, as a secondary retention agent, with a kaolin as described in patent application FR 95 13051 of the Applicant, this kaolin being preferably pretreated with a polyelectrolyte. Those skilled in the art can refer to this patent

FR 95 13051.

This process makes it possible to obtain a significantly improved retention of fines and fillers, without any opposite effect. The drainage properties are also improved, which is an additional characteristic of this improvement. The branched polyacrylamide (or more generally the branched (co) polymer) is introduced into the suspension, very preferably, in the form of a water-in-oil reverse emulsion, at a rate of 0.03 to one per thousand (0.03 to 1% o, or 30 to 1000 g / t) by weight of active material (polymer) relative to the dry weight of the fibrous suspension, preferably 0.15 to 0.5 per thousand, or 150 at 500 g / t.

In a manner known to a person skilled in the art, the reverse polymer emulsion is diluted with water and inverted (dissolved) by this dilution before its introduction, as described above.

This selection of the reverse emulsion form allows, in the paper application for the retention of fillers and fines, to reach a level of performance unmatched before. The use of branched polymers also makes it possible to better retain the bentonite on the sheet, as described in the aforementioned patent EP 574 335 and therefore to limit its negative effects on the subsequent treatment of the effluents leaving the machine. In addition, the choice of this branched polyacrylamide increases the fixing power of the bentonite on the sheet, consequently brings about a synergy, therefore a re-coagulation which reduces the bentonite content in the white waters.

It will be understood that it is essential according to the invention that the polymer is prepared by a reverse water-in-oil emulsion polymerization. On the other hand, this polymer can then be used (ie injected or introduced into the mass or paste to be flocculated) either in the form - preferably - of this reverse emulsion after its dissolution in water, or in the form of a powder obtained by drying (in particular by spray drying or “spray drying”) of the reverse emulsion of the polymerization, then redissolution of this powder in water, for example at a concentration of the order of 5 g of active polymer / liter, the solution thus obtained is then injected into the paste at substantially the same dosages of polymer.

Advantageously, in practice, the branched (co) polyacrylamide is a cationic copolymer of acrylamide and of an unsaturated cationic ethylenic monomer, chosen from the group comprising dimethylaminoethyl acrylate

(ADAME), dimethylaminoethyl methacrylate (MADAME), quaternized or salified with different acids and quaternizing agents, benzyl chloride, methyl chloride, alkyl- or aryl chlorides, dimethylsulfate, dimethyldiallylammonium chloride (DADMAC), acrylamidopropyltrimethylammonium chloride (APTAC), and methacrylamidopropyltrimethylammonium chloride (MAPTAC),

In known manner, this copolymer is branched by a branching agent consisting of a compound having at least two reactive groups chosen from the group comprising double bonds, aldehyde bonds or epoxy bonds. These compounds are well known and are described for example in document EP-A-0 374 458 (see also document FR-A-2 589 145 of the Applicant).

As is known, a branched polymer ("branched") is a polymer which has on the chain branches, groups or branches arranged globally in a plane and not in the three directions, unlike a crosslinked polymer ( "Cross-linked"); such branched, high molecular weight polymers are well known as flocculating agents. These branched polyacrylamides are distinguished from crosslinked polyacrylamides by the fact that in the latter, the groups are arranged three-dimensionally to lead to products which are practically insoluble and of infinite molecular weight.

The branching may preferably be carried out during (or optionally after) the polymerization, for example by reaction of two soluble polymers having counterions, or by reaction on formaldehyde or a polyvalent metal compound. Often, the branching takes place during the polymerization by addition of a branching agent, and this solution will be clearly preferred according to the invention. These branched polymerization methods are well known.

The branching agents which can be incorporated include ionic branching agents such as polyvalent metal salts, formaldehyde, glyoxal, or alternatively, and preferably, covalent crosslinking agents which will copolymerize with the monomers, preferably diethylenically unsaturated monomers (such as the family of diacrylate esters such as polyethylene diacrylates glycols PEG) or polyethylene, of the type conventionally used for the crosslinking of water-soluble polymers, and in particular methylenebisacrylamide (MBA) or any of the other known acrylic branching agents. These agents are often identical to the crosslinking agents, but crosslinking can be avoided, when it is desired to obtain a branched polymer and not crosslinked, by optimizing the polymerization conditions such as concentration of polymerization, type and quantity of transfer agent, temperature, type and amount of initiators, and the like.

In practice, the branching agent is methylene bis acrylamide (MBA), introduced at a rate of five to two hundred (5 to 200) moles per million moles of monomers, preferably 5 to 50.

Advantageously, the amount of branched polyacrylamide introduced into the suspension to be flocculated is between thirty and a thousand grams of active polymer / tonne of dry paste (30 and 1000 g / l), or between 0.03 per thousand and one per thousand, of preferably 150 to 500 g / l; it has been observed that if the quantity is less than 0.03% o (0.03 per thousand), no significant retention is obtained; similarly, if this quantity exceeds 1% o (1 per thousand), no proportional improvement is observed; however, unlike linear cationic polyacrylamides, as described in documents EP-A-0 017 353 and EP 0 235 893 referred to in the preamble, there is no reverse effect of dispersion by recirculation in the closed circuits of the excess polymer not retained on the sheet. Preferably, the amount of branched polyacrylamide introduced is between 0.15 and 0.5 per thousand (0.15 and 0.5% o) of the amount of dry pulp, ie between 150 g / t and

500 g / t.

As already said, it is important that the branched polymer be prepared in the form of a reverse emulsion (water-in-oil) to carry out the improvement of the invention. Such emulsions and their preparation process are well known to those skilled in the art.

This approach was condemned in the aforementioned patent EP 0 574 335, where it was indicated that if a branched emulsion polymer is used, the indispensable presence in these emulsions of surfactants promotes the formation of foams during the manufacture of paper and the appearance of disparities in the physical properties of the finished paper (modification of the absorbance at the places where part of the oil phase of emulsion is retained on the sheet).

It was therefore not obvious to consider a fortiori the reverse water-in-oil emulsions whose oil content is obviously high.

The invention was all the more difficult to achieve since it is important to remain in the field of branched polymers and not to pass into the field of crosslinked polymers. However, we know that technically, especially on an industrial production scale, the border between the two zones is very easily crossed, in a way that is irreversible. As the branching zone is very narrow, we measure the difficulty of developing the invention, and it is the merit of the Applicant to have attacked the use of this technology in the field of the manufacture of paper, which poses special problems and has very strict quality requirements.

The risk of failure was all the greater, which may explain the fact that this technology was not used, as crosslinked emulsions are not known to provide a particular advantage in paper.

Compared with linear polymers, the branched powdered polymers of the abovementioned EP 0 574 335 had already brought significant progress with regard to the properties and the process of papermaking. The improvement was around 20 to 40% depending on the properties.

With the present branched emulsions, an improvement of the order of 50 to 60% is achieved, which was not foreseeable since it was known, on the other hand, that the crosslinked products did not work.

According to the invention, a “moderately branched” polymer will be used in a preferred, but not limiting manner, for example at 10 ppm of branching agent relative to the active material. As already indicated above, the polymer may be used either in the form of its reverse synthetic emulsion, dissolved or “inverted” in water, or in the form of the solution in water of the powder obtained by drying of said synthetic emulsion, in particular by spray drying. Spray drying is a process also known to those skilled in the art. Reference will be made to the tests below to verify that the results are comparable.

Bentonite, also known as "swecting smectic clay", from the montmorillonite family, is well known and does not need to be described here in detail; these compounds, formed of microcrystallites, have sites on the surface with a high cation exchange capacity capable of retaining water (see for example document US-A-4 305 781, which corresponds to document EP-A-0 017 353 , mentioned above, and patent FR-A-2,283,102).

Preferably, a semi-sodium bentonite is used, which is introduced just upstream from the headbox, at a rate of 0.1 to 0.5 percent (0.1 to 0.5%) (0.1 0.5 percent) of the dry weight of the fibrous suspension.

As filler, kaolins, "G CC" or ground CaC03, precipitated CaC03 or "PCC", and the like may be used.

The injection or introduction of the branched polymer in reverse emulsion according to the invention is carried out before a shearing step in the paper pulp (or fibrous mass to be flocculated) more or less diluted according to the practice of a person skilled in the art, and generally in diluted pulp or "thin stock", ie a pulp diluted to about 0.7 - 1.5% of solid materials such as cellulose fibers, any fillers, and the various usual additives in papermaking.

According to a variant of the invention, with fractional introduction, part of the branched emulsion polymer, according to the invention, will be introduced at the stage of preparation of the thick stock or “thick stock” at approx. 5% or more solids, or even in the preparation of the thick dough before a shearing step. The following examples illustrate the invention without, however, limiting its scope.

EXAMPLE 1

Manufacture of a branched polymer in the form of a water-in-oil reverse emulsion

In a reactor A, the constituents of the organic phase of the emulsion to be synthesized are mixed at room temperature.

a) - Organic phase - 252 g of Exxsol D100

- 18 g of Span 80

- 4 g of Hypermer 2296

b) - In a beaker B, the aqueous phase of the emulsion to be prepared is prepared by mixing:

- 385 g of 50% acrylamide

- 73 g of ethyl trimethyl ammonium acrylate chloride 80%

- 268 g of water

- 0.5 g of methylene bis acrylamide at 0.25% - 0.75 ml of sodium bromate at 50 g H

- 20 ppm sodium hypophosphite compared to the active ingredient

- 0.29 ml of Versenex at 200 g I ^-1

The contents of B are mixed in A without stirring. After mixing the phases, the emulsion is sheared in the mixer (mixer) for 1 minute in order to create the reverse emulsion. The emulsion is then degassed by bubbling nitrogen then after 20 minutes, the gradual addition of metabisulfite leads to initiation and then polymerization. Once the reaction is complete, a “bum out” is carried out (metabisulfite treatment) in order to reduce the content of free monomer. The emulsion is then incorporated with its reverse surfactant in order to subsequently release the polymer in the aqueous phase. It will be necessary introduce 2 to 2.4% ethoxylated alcohol. The standard Brookfield viscosity of the said polymer will be 4.36 cps (viscosity taken at 0.1% in a 1 M NaCl solution at 25 ° C at sixty revolutions per minute)

According to a variation in the MBA content from 5 to 20 ppm, the results in UL viscosity are as follows:

Table of Example 1:

FO 4198: branched powder containing 20 ppm of transfer agent and 5 ppm of branching agent.

( ^* ): sodium hypophosphite, transfer agent. (1): ionic recovery in%.

(2): increase in intrinsic viscosity in%.

EM140CT: standard emulsion of very high molecular weight containing no branching agent. EM140L: standard high molecular weight emulsion containing no branching agents. EM140LH: medium molecular weight emulsion containing no branching agents. EM140BD: crosslinked emulsion containing no transfer agent and

5 ppm of crosslinking agent. SD 102: R 102 emulsion spray-dried, and powder obtained dissolved in water to 5 g of active polymer / liter.

It is noted that the linear products do not develop an ionic recoveryRI and see their intrinsic viscosity IV decrease under the effect of a strong shear (two of the values of IV are negative); branched emulsion products develop an ionic boost RI, but no IV (values <= 0); crosslinked products develop a strong ionic boost and a very strong IV boost.

Definitions of ion regains and intrinsic viscosity regains:

Ionic regain RI = (X-Y) / Y x 100 with X: ionicity after shear in meq / g.

Y: ionicity before shearing in meq / g.

Increase in intrinsic viscosity R IV = (V1 - V2) / V2 x 100

with V1: intrinsic viscosity after shearing in dl / g

V2: intrinsic viscosity before shearing in dl / g

Part of the emulsions mentioned above will be the subject of a draining retention efficiency study on an "automatic retention form" from the Technical Paper Center.

Emulsion test procedure

Pulp used: mixture of 70% KF bleached hardwood kraft

10% KR bleached softwood kraft 20% mechanical pulp PM

20% natural calcium carbonate.

Bonding in neutral medium with 2% of a dimeric ketene alkyl emulsion. The paste used is diluted to a consistency of 1.5%. 2.24 g of dry paste are taken, ie 149 g of 150% paste and then diluted to 0.4% with clear water.

The volume of 560 ml is introduced into the plexiglass cylinder of the automated form and the sequence is started. - 1 = 0 s, stirring start at 1500 rpm. - 1 = 10 s, addition of the polymer.

- t = 60 s, automatic reduction to 1000 rpm and addition, if necessary, of bentonite. - t = 75 s, stirring stopped, formation of the sheet with the vacuum under canvas then recovery of white water. The following operations are then carried out:

- measurement of the turbidity of the water without canvas.

- dilution of a beaker of thick dough for a new leaf with the water under canvas collected.

- drying of the sheet known as the 1st pass.

- start of a new sequence in order to make the so-called 2nd pass sheet.

After 3 passes, we change the products to be tested.

The following analyzes are then carried out:

- measurement of suspended matter in canvas (TAPPI standard: T 656 cm / 83) - measurement of leaf ash, (TAPPI standard: T 21 1 om - 93)

- measurement of the turbidity 30 'after the fibers are deposited in order to know the state of the ionic medium.

- measurement of the degree of dripability of the dough with a Canadian Standard Freeness (CSF; Norm TAPPI T 227 om - 94). Notes for Tables (I) and (II) below: X = measure said on the first pass.

R1 = so-called second pass measurement (1st recycling) R2 = so-called third pass measurement (2nd recycling)

Ash% =% by weight of ash retained (= charge retention) on the sheet / weight of the sheet.

Comments on the results: cf. Tables (I) and (II) below relating to Example 1 and FIG. 1 to 10 which represent the corresponding histograms

Crosslinked polymers are of no interest in the flocculation and retention of fines and fillers despite the high shear rate applied during the process to the fibrous mass (and not applied to the polymer itself), here 1500 rpm, which is characteristic of this type of microparticulate retention system. They show a low capture of colloidal charges and materials because no reduction in turbidity is observed.

The combination with bentonite does not significantly improve the retention efficiency and only slightly improves the drainage efficiency.

For the linear polymer, its behavior follows the trend, improving the retention of charges and fines.

The combination according to the invention of a branched polymer in reverse emulsion and of bentonite provides a net gain in charge retention and in total retention, and appears to be superior to the known linear polymer / bentonite system. The coagulation power is higher for a branched emulsion polymer, which results in an excellent reduction in turbidity to 30 '(30 min.).

Test R 52 and test R 102 show that the invention makes it possible to obtain branched products having higher UL viscosities than those accessible by gel polymerization as described in patent EP 0 574 335. Any attempt to reach such very advantageous UL viscosity values by a gel polymerization route with powder drying would lead to a product that is completely insoluble and therefore totally unusable in industry.

The SD 102 test shows that the polymer used in the form of a solution in water of the powder obtained by drying the inverse emulsion of the synthesis of the polymer behaves like the polymer used in the form of the solution in the water of said reverse synthetic emulsion. In particular, no degradation of the polymer is observed during the spray drying step.

The R 52 test can be usefully compared to the FO 4198 test

(powder) because the polymers have the same chemistry, therefore the same cationicity, and the same% of MBA, while the R 52 of the invention is much higher than the powder in terms of drainage and retention (96.3 to be compared with 87.6); we will also compare the NTU turbidity after 30 min. , 32 compared to 75 NTU units.

Such UL viscosity values in particular lead to very improved drainage.

The invention therefore also relates to a new retention agent for the manufacture of a sheet of paper, cardboard or the like, which consists of an acrylic (co) polymer as described above, branched, in reverse emulsion, and which is characterized in that its UL viscosity is> 3, or> 3.5 or> 4. Said agent can be used either as an inverted emulsion with water, or in solution of the powder obtained by drying the emulsion, as described above. EXAMPLE 2

Manufacture of a branched polymer based on acrylamide propyl tri-methyl ammonium chloride (APTAC) in the form of a water-in-oil reverse emulsion:

a) - Organic phase: - 252 g of exxsol D100

- 18 g of Span 80

- 4 g of Hypermer 2296.

b) - In a beaker B, the phase of the emulsion to be prepared is prepared by mixing

- 378 g of 50% acrylamide

- 102.2 g of acrylamide-propyl trimethyl ammonium chloride (60%)

- 245.7 g of water

- 0.5 g of methylene bis acrylamide at 0.25%

- 0.75 ml of sodium bromate at 50 g / l

- 20 ppm sodium hypophosphite compared to the active ingredient

- 0.29 ml of Versenex at 200 g / l

The contents of B are mixed in A with stirring. After mixing the phases, the emulsion is sheared in the mixer for 1 minute to create the reverse emulsion. The emulsion is then degassed by bubbling nitrogen then after 20 minutes, the gradual addition of metabisulfite leads to initiation and then polymerization.

When the reaction is complete, a “burn out” is carried out in order to reduce the content of free monomer.

The emulsion is then incorporated with its inverting surfactant in order to subsequently release the polymer in the aqueous phase. Table of Example 2:

EK 190: standard emulsion of acrylamide co-polymer and acrylamide-propyl trimethyl ammonium chloride, linear.

Emulsion test procedure

(identical to that of Example 1)

Comments on the results: cf. Table (III) below relating to Example 2 _ and Figs. 11 to 20 which represent the corresponding histograms

The results call for the same comments as those of Example 1 and confirm the great interest of the present invention.

The invention also relates to the new retention agents described above, characterized in that they consist of, or comprise, at least one (co) polymer of the type described, branched, prepared in reverse emulsion, intended to cooperate with a secondary retention agent after an intermediate step of shearing of the paper pulp, as well as the processes for manufacturing sheets of paper, cardboard or the like, using the agents according to the invention or the method according to the invention, and the sheets of paper, cardboard and the like thus obtained.

Said agent can be used either as an inverted emulsion with water, or in solution of the powder obtained by drying the emulsion, as described above.

Table (I): comparative table of the results of Example 1 (polymer alone)

(in italics: branched polymer) (FO 4198 is also a powder, obtained by gel polymerization)

Table (II): comparative table of the results of Example 1 (polymer with bentonite)

Table (III): comparative table of the results of Example 2

Claims

1. Process for the manufacture of a sheet of paper or cardboard or the like, having improved retention and drainage characteristics, of the type according to which a dual system of acrylic polymer and bentonite or a kaolin optionally treated as agents is used primary and secondary, respectively, retention, the introductions of which are separated by a step of shearing the suspension or fibrous mass or paper pulp, characterized in that said polymer is a branched acrylic (co) polymer and prepared in the form of '' a water-in-oil reverse emulsion, used either as an inverted water emulsion or in solution of the powder obtained by drying the emulsion.

2. Method according to claim 1, characterized in that the branched acrylic (co) polymer prepared in reverse emulsion is introduced into the paper pulp at a concentration of 0.03 to one per thousand (0.03 to 1% o) by weight, ie from 30 to 1000 g / t, of the dry weight of the fibrous suspension of paper pulp, preferably 0.15 to 0.5 per thousand (0.15 to 0.5% o) or from 150 to 500 g / t-

3. Method according to claim 1 or 2, characterized in that the branched acrylic (co) polymer prepared in reverse emulsion is a cationic copolymer of acrylamide and of an unsaturated cationic ethylenic monomer, chosen from the group comprising acrylate of dimethylaminoethyl (ADAME), dimethylaminoethyl methacrylate (MADAME), quaternized or salified with different acids and quaternizing agents, benzyl chloride, methyl chloride, alkyl- or aryl chlorides, dimethylsulfate, dimethyldiallylammonium chloride (DADMAC), acrylamidopropyltrimethylammonium (APTAC), and methacrylamidopropyltrimethylammonium chloride (MAPTAC).

4. Method according to any one of claims 1 to 3, characterized in that the acrylic (co) polymer branched in reverse emulsion is branched by a branching agent constituted by a polyfunctional compound having at least two reactive groups chosen from the group comprising double bonds, aldehyde bonds or epoxy bonds.

5. Method according to any one of claims 1 to 4, characterized in that the (co) acrylic polymer branched in reverse emulsion is branched by a branching agent consisting of methylenebisacrylamide (MBA).

6. Method according to claim 5, characterized in that the MBA is introduced at a concentration of 5 to 200 moles per million moles of monomers.

7. Method according to claim 5 or 6, characterized in that the bentonite is a semi-sodium bentonite, used at a rate of 0.1 to 0.5 percent (0.1 to 0.5%) of the dry weight of the fibrous suspension.

8. Method according to claim 5, 6 or 7, characterized in that the paste used, containing the filler is diluted, then the polymer is added as main retention agent, a shearing step is carried out as for example in the pump. mixture or "fan pump" then add bentonite as a secondary retention agent.

9. Method according to claim 8, characterized in that the quantity of branched polyacrylamide (or more generally of branched acrylic (co) polymer) introduced either in reverse water-in-oil emulsion reversed with water, or in solution of the powder obtained by drying the emulsion, is between 0.03 and 1% o, or between thirty and a thousand grams / ton (30 and 1000 g / t) of dry paste

10. Method according to claim 8 or 9, characterized in that the amount of branched polyacrylamide (or more generally of branched acrylic (co) polymer) introduced either in reverse water-in-oil inverse emulsion reversed with water, or in solution of the powder obtained by drying the emulsion, is between 0.15 and 0.5% o (i.e. between 150 and 500 g / t)

1 1. Method according to claim 8, 9 or 10, characterized in that the bentonite is replaced by kaolin, optionally pretreated with a polyelectrolyte, as a secondary retention agent.

12. Method according to any one of claims 1 to 1 1, characterized in that the injection or introduction of the branched polymer prepared in reverse emulsion is carried out (either in reverse emulsion with water, or in solution of the powder obtained by drying the emulsion) before a shearing step in the paper pulp (or fibrous mass to be flocculated) more or less diluted according to the practice of a person skilled in the art, and generally in the diluted paper pulp or “thin stock ”, Ie a pulp diluted to approximately 0.7 - 1.5% of solid matter such as cellulose fibers, possible fillers, and the various usual additives for papermaking.

13. Method according to any one of claims 1 to 12, characterized in that part of the branched polymer emulsion is introduced at the stage of preparation of the thick paste or "thick stock" at approx. 5% or more solids, or even in the preparation of the thick dough before a shearing step.

14. New retention agent for the manufacture of a sheet of paper, cardboard or the like, characterized in that it comprises a branched polyacrylamide (or more generally a branched acrylic (co) polymer) in reverse emulsion (or water-in - oil) (either in dissolved inverse emulsion or "inverted" with water, or in solution of the powder obtained by drying the inverse emulsion, in particular by spray drying).

15. New retention agent for the manufacture of a sheet of paper, cardboard or the like, according to claim 14, characterized in that the branching agent is a branching agent consisting of a polyfunctional compound having at least two reactive groups chosen from the group comprising double bonds, aldehyde bonds or epoxy bonds, in particular methylene bis acrylamide (MBA).

16. New retention agent for the manufacture of a sheet of paper, cardboard or the like, according to claim 15, characterized in that the agent branching is introduced at a rate of five to two hundred (5 to 200) moles per million moles of monomers, preferably 5 to 50.

17. New retention agent for the manufacture of a sheet of paper, cardboard or the like, according to claim 14, 15 or 16, characterized in that the branched polyacrylamide is a cationic copolymer of acrylamide and of a cationic ethylenic monomer unsaturated, chosen from the group comprising dimethylaminoethyl acrylate (ADAME), dimethylaminoethyl methacrylate (MADAME), quaternized or salified by different acids and quaternizing agents, benzyl chloride, methyl chloride, alkyl- or aryl chlorides, dimethylsulfate, chloride dimethyldiallylammonium (DADMAC), acrylamide propyltrimethylammonium chloride (APTAC), and methacrylamidopropyltrimethylammonium chloride (MAPTAC).

18. New retention agent for the manufacture of a sheet of paper, cardboard or the like, according to any one of claims 14 to 17, characterized in that its viscosity UL is> 3.

19. New retention agent for the manufacture of a sheet of paper, cardboard or the like, according to any one of claims 14 to 18, characterized in that its viscosity UL is> 3.5.

20. New retention agent for the manufacture of a sheet of paper, cardboard or the like, according to any one of claims 14 to 19, characterized in that its viscosity UL is> 4.

21. New retention agent for the manufacture of a sheet of paper, cardboard or the like, according to any one of Claims 14 to 20, characterized in that it is "moderately branched", for example at 10 ppm of branching agent relative to to the active ingredient.

22. Sheet of paper, cardboard or the like, characterized in that it is obtained with the use of a retention agent according to any one of claims 14 to 21.

23. Sheet of paper, paperboard or the like, characterized in that _it is obtained using a method according to _one of claims 1 to 13.