US20050150622A1

US20050150622A1 - Paper and paperboard production process and corresponding novel retention aids, and papers and paperboards thus obtained

Info

Publication number: US20050150622A1
Application number: US11/074,662
Authority: US
Inventors: Rene Hund; Christian Jehn-Rendu
Original assignee: SNF SA
Current assignee: SNF SA
Priority date: 1998-06-12
Filing date: 2005-03-09
Publication date: 2005-07-14
Also published as: JP2002518609A; FR2779752A1; EP1086276B1; DE69920014D1; AU4043899A; KR100694912B1; KR20010072606A; ATE275669T1; CA2334744C; EP1086276A1; CA2334744A1; WO1999066130A1; DE69920014T2; FR2779752B1

Abstract

The invention relates to an improved paper production process according to which a cross-linked polymer prepared in reverse-phase emulsion and sheared before introduction into the fibrous mass is used as the main retention aid, and then bentonite is used as the secondary retention aid (a dual type system). The two additions are not separated by any mandatory stage for intense shearing of the fibrous suspension (or mass). The paper manufacturer can therefore be free from the constraints of shearing the pulp. A distinctly improved retention of the fines is obtained, and also a distinct improvement in drainage. In addition, the bentonite content in the white water is reduced. Lastly, excellent formation is maintained.

Description

The present invention relates to the technical field of paper production and the polymers used in this field.
The invention relates to a process for the manufacture of a paper or paperboard with improved retention.
During the manufacture of paper, paperboard, or the like, it is well known to introduce into the pulp retention aids whose function is to retain a maximum of fines and fillers in the sheet. The beneficial effects that result from the utilization of a retention aid are essentially:

- increased production and reduction of manufacturing costs: energy savings, more reliable operation of the machine, higher yield in terms of fibers, fines, fillers and anionic finishing products, lower acidity in the circuit linked to a decrease in the use of aluminum sulfate, and hence a reduction in corrosion problems;
- an improvement in quality: better formation and better look-through, an improvement in the moisture content, the opacity, the gloss, and the absorptive capacity of the sheet, and a reduction in the porosity of the paper.

Long ago, it was proposed that bentonite be added to the pulp, possibly together with other mineral products such as aluminum sulfates or even synthetic polymers, notably polyethylene imine (see for example the documents DE-A-2 262 906 and U.S. Pat. No. 2,368,635).
In the document U.S. Pat. No. 3,052,595, it was proposed to associate the bentonite with a polyacrylamide of an essentially linear nature. This process met with competition from systems that were easier to use yet performed just as well. Moreover, even with the current linear polyacrylamides, the retention capacity is still insufficient.
In the document EP-A-0 017 353, it was proposed, for the retention of low-filler pulps (less than 5% fillers), to associate the bentonite with a nonionic or slightly anionic linear copolyacrylamide. This process has not been very widely used, since these polymers perform relatively poorly in terms of retention, especially that of pulps containing fillers, no doubt as a result of insufficient synergy between these copolymers and bentonite, which does not have much of a tendency to recoagulate.
Also known in the prior art are systems of retention aids for the manufacture of a sheet of paper, paperboard or the like, which comprise a combination of two retention aids, generally a main retention aid and a secondary retention aid. These are called “dual” systems.
Thus, in U.S. Pat. No. 4,753,710, it is recommended to use of a linear acrylic polymer of high molecular weight as the main retention aid, which is added to the fibrous mass, followed by an intense shearing, particularly in the mixing pump or “fan pump,” then an addition of bentonite, (which is a swelling clay) as the secondary retention aid. This document neither suggests nor describes any shearing of the polymer itself before introduction into the suspension to be flocculated.
Also known in the prior art are cross-linked retention aids as described, for example, in European patent 0 202 780, primarily for the treatment of water, and secondarily for paper. It is important to note that it uses a cross-linked product, which is added to the suspension to be flocculated, the flocs then being sheared during the paper production process, that is, sheared in and at the same time as the paper pulp. The flocs are thus transformed into flocs that are smaller and more shear resistant, therefore more tenacious. This document neither suggests nor describes any shearing of the polymer itself before introduction into the suspension to be flocculated.
Thus, according to the techniques of the prior art related to papermaking applications, between the flocculating agent and the fibrous mass of pulp, flocs of fairly large size are formed, and are then sheared so as to form flocs which, in the documents cited, are said to be smaller and more tenacious.
In the document EP-A-0 235 893, it was proposed to use essentially linear cationic polyacrylamides having molecular weights of greater than one million, of thirty million and higher. This results in the obtainment of a retention effect that is satisfactory, but is still deemed inadequate in the papermaking application; since the use of bentonite causes problems during water treatment, users select this system only if there are significant advantages.
U.S. Pat. No. 4,753,710 (the commercial product “HYDROCOL”™), already mentioned above, also describes the addition of a cationic polymer as the main retention aid, followed by an intense shearing stage, then an addition of bentonite as the secondary retention aid. Its drawbacks include the necessity to optimize the introduction point, which does not present any particular problem given all of the prior research on this subject, but which represents a limitation for the user in the papermaking industry, as well as the risk of an overdosage of the polymer, and which is necessary to reduce the uncontrolled or excessive degradation of the flocks by the shearing imposed.
In the notes presented at the lecture given in Seattle on Oct. 11-13, 1989, published under the title “Supercoagulation in the control of wet end chemistry by synthetic polymer and activated bentonite,” R. Kajasvirta described the mechanism of supercoagulation of activated bentonite in the presence of a cationic polyacrylamide, without specifying its exact nature. This process has the same drawbacks as those indicated above.
European patent 0 201 237 describes a flocculation process in which a polymer material is added to water to form an aqueous composition, and is used to flocculate the solid matter in suspension in an aqueous suspension, said polymer comprising a polymer of high molecular weight that is subjected to a shearing, this shearing being carried out before or during the flocculation and the polymer being required to have certain intrinsic properties, which are indicated in this patent.
According to this document, the polymer is a polymer of high molecular weight, formed from water-soluble monomers or from a mixture of such monomers, and the polymer is subjected to shearing. The process described in this patent is characterized in that it is possible to carry out the shearing before or during the flocculation. European patent 0 201 237 further indicates that the polymer used comprises a cross-linked water-swellable polymer which it is possible to shear to an intrinsic viscosity of at least 4 dl/g. It is also indicated that the aqueous composition containing the polymer material can be a stable and homogenous composition, the shearing in this case causing an increase in the intrinsic viscosity of at least 1 dl/g.
In this document, “stable and homogenous” designates a polymer composition that is stable when the polymer is at full equilibrium with the water, i.e., when it has reached its ultimate degree of solubility or swelling. The composition is also homogeneous in the sense that the polymer remains uniformly dispersed throughout the composition, without having a tendency to precipitate after several days.
This document specifically describes a number of applications for water treatment, which is clearly precisely the main application intended, and coal ore treatment.
This patent also mentions, very briefly and without providing an exemplary embodiment or even any precise instructions for implementation, an application to paper or paperboard production; it is merely indicated that the polymer can be added at an early stage of the pulp (fibrous mass) circulation line with a shearing along the flow line of the suspension, near the drainage stage or another water removal stage. The patent indicates that the shearing is carried out by pumping, hence by means of the “fan pump” or mixing pump effectively disposed in line in papermaking machines.
For the other applications, and especially for water treatment, the document also indicates that it is possible to carry out the shearing on the production line, as the suspension to be flocculated approaches a centrifuge, a filter press or a belt press, or another water removal stage. It is also indicated that the shear can be applied during a water removal stage that is conducted under a certain shear, preferably in a centrifuge or even in a filter press or a belt press.
Hence, this document only teaches a shearing of the flocs in the mixing pump or “fan pump” for the papermaking application. Moreover, it teaches that very low shear rates can be appropriate in the other applications, since filter presses and belt presses induce very low shear.
The invention eliminates the drawbacks mentioned above.
Its object is an improved process of the type in question, which incorporates operations comprised of adding to the suspension or fibrous mass to be flocculated, or paper pulp,

a) as the main retention aid, a (co)polyacrylamide that is cross-linked and is prepared in the form of a reverse phase or water-in-oil emulsion, this reverse-phase emulsion (“inverted” in water), or even the dried powder obtained from this reverse phase emulsion and redissolved in water, itself being sheared prior to introduction or injection into the fibrous mass, and
b) a second retention aid (a so-called “dual” system of the “microparticulate” type),
c) without a stage for intense shearing of the pulp between the additions a) and b), or with an “optional” shearing (as defined below) of the pulp between the two additions a) and b).

The second retention aid is bentonite, and in this field the reader is referred to the teaching of the above-mentioned U.S. Pat. No. 4,753,710, which can advantageously be replaced by a kaolin, preferably pre-treated by a polyelectrolyte, according to the teaching of French patent 95 13 051 filed in the name of the Applicant; and it would be useful for one skilled in the art to refer to these documents for the details of implementation, the usual additives, etc.
The addition of the polymer and that of the bentonite are not separated according to the invention by any mandatory stage for intense shearing of the pulp, for example at the level of the mixing pump known as the “fan pump,” contrary to the teaching of U.S. Pat. No. 4,753,710 and contrary to a vast body of prior art related to the addition point of the retention aid relative to the shearing stages existing in the machine, including U.S. Pat. No. 3,052,595; Unbehend, TAPPI Vol. 59, No. 10, October, 1976; Luner, 1984 Papermakers Conference or TAPPI, April, 1984, pp. 95-99; Sharpe, Merck and Co., Inc., Rahway, N.J., USA, around 1980, Chapter 5, “Polyelectrolyte Retention Aids”; Britt, TAPPI Vol. 56, October 1973, p. 46 ff.; Waech, TAPPI, March, 1983, p. 137; and U.S. Pat. No. 4,388,150 (Eka Nobel). According to the invention, it is in fact entirely preferred that there be no intercalary shearing of the pulp between the two additions.
This process according to the invention makes it possible to obtain a distinctly improved retention of fines and fillers without a reverse effect. An additional characteristic of this improvement is that the drainage properties are also improved, which is unexpected given the improvement of the retention, and excellent formation is maintained, which is also surprising.
The cross-linked polyacrylamide (or more generally the cross-linked (co)polymer) is introduced into the suspension or pulp to be flocculated in the form of the reverse phase water-in-oil emulsion derived from the synthesis, and itself “inverted” in water, or in the form of a solution in water, with about 5 g of polymer/liter, of the powder obtained by drying the reverse phase water-in-oil emulsion from the synthesis, said emulsion or said solution being sheared before introduction into the pulp or suspension to be flocculated, the dosage for the introduction being established at a rate of 0.03 to one per mill (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 mill, or 150 to 500 g/t.
In a way that is known to one skilled in the art, when the emulsion derived from the synthesis of the polymer is used directly, this water-in-oil polymer emulsion is diluted in water to obtain a polymer content on the order of 5 to 10 g/l, preferably close to 5 g/l, and is thus “inverted” by this dilution to form a solution, which is sheared according to the invention before its introduction into the pulp.
It is preferred according to the invention to use the sheared “inverted” emulsion, but Example 3 below shows that the results of the sheared solution of the powder obtained by drying the emulsion are equivalent.
According to the techniques of the prior art relative to papermaking applications, between the flocculating or retention aid and the fibrous mass of pulp, flocs of fairly large size are formed, which are then sheared so as to form flocs which, in the documents cited, are said to be smaller and more tenacious.
Moreover, the systems of the prior art of the dual system type require the use of an intense shearing between the addition of the cationic polymer and the addition of the second retention aid, the bentonite or a kaolin. Systems of this type can be classified as “microparticulate.”
The “dual” systems of the prior art were essentially composed of linear polymers with an addition of bentonite, or of a branched polyacrylamide or a starch, with an addition of colloidal silica, this last component being extremely expensive.
A known improvement of these processes is described in French patent 95 13 051 in the name of the Applicant, which relates to a dual system based on a polymer of the linear or branched polyacrylamide type and kaolin, kaolin being a non-swelling clay that does not have the drawbacks of bentonite, the kaolin being pre-treated in a preferred embodiment.
On the other hand, according to the present invention, a main retention aid is used, which is cross-linked and which is sheared before its introduction into the pulp, preferably in the form of a reverse phase water-in-oil emulsion, which leads directly to microflocs without going through the shearing of larger flocs involving the fibrous mass.
According to the invention, and without intending to be limited by any one theory, the Applicant in effect maintains that a microflocculation occurs directly as a result of the intense shearing carried out on the polymer itself before its injection into the fibrous mass of pulp, which is quite a different (and unexpected) process than reducing the size of large flocs (involving the fibrous mass) into smaller, more tenacious flocs, and which results in unforeseen improvements in the properties of the paper or paperboard sheet.
This selection of the cross-linked form, in a reverse phase emulsion (or in a solution of the powder obtained by drying the emulsion) before introduction into the fibrous mass, in combination with the subsequent addition of a second retention aid, but without any intercalary shearing of the pulp, makes it possible, in the papermaking application for the retention of fillers and fines, to reach a level of performance unequalled up to now.
The monomers used for the preparation of the (co)polymer can be nonionic, but generally at least some of the monomers used to form the polymer are ionic. The monomers are normally monomers with monoethylenic unsaturation, sometimes allylic monomers, but generally vinyl monomers. These are generally acrylic or methacrylic monomers.
Suitable nonionic monomers are acrylamide, methacrylamide, N-vinyl methyl acetamide or formamide, vinyl acetate, vinylpyrrolidone, methyl methacrylate or methacrylates of other acrylic esters, or of other esters with ethylenic unsaturation, or even of other vinyl monomers that are insoluble in water such as styrene or acrylonitrile.
Suitable anionic monomers are for example sodium acrylate, sodium methacrylate, sodium itaconate, 2-acrylamido-2-methylpropane sulfonate (AMPS), the sulfopropylacrylates or sulfopropylmethacrylates, or other water-soluble forms of these polymerizable sulfonic or carboxylic acids. It is possible to use a sodium vinylsulfonate or an allylsulfonate, or a sulfomethyl acrylamide.
Suitable cationic monomers are the dialkylaminoalkyl acrylates and methacrylates, particularly dialkylaminoethyl acrylate, as well as their acid salts or their quaternary products, and even the dialkylaminoalkylalkylacrylamides or methacrylamides, as well as their acid salts and the products of quaternization, for example methacrylamidopropyl trimethyl ammonium chloride and the Mannich products such as the quaternized dialkylaminomethylacrylamides. The alkyl groups in question are generally C₁-C₄alkyl groups.
The monomers can contain hydrophobic groups, for example as described in European patent 0 172 723, and in certain cases allylic ether monomers could be preferred.
For simplicity's sake, the term “(co)polyacrylamide” will be used herein to designate all of the combinations of these monomers, which are well known to one skilled in the art.
Advantageously, in practice, the cross-linked (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 by different acids and quaterinizing agents, benzyl chloride, methyl chloride, alkyl or aryl chloride, dimethyl sulfate, diallyldimethylammonium chloride (DADMAC), acrylamidopropyltrimethylammonium chloride (APTAC), and methacrylamidopropyltrimethylammonium chloride (MAPTAC).
The processes for synthesizing a reverse phase emulsion (co)polymer are well known to one skilled in the art. In this field, the reader is referred to the above-mentioned patents.
In a way that is also known, this (co)polymer is cross-linked by a cross linker constituted by a compound having at least two reagent groups chosen from the group comprising the double bonds, the aldehyde bonds or the epoxy bonds. These compounds are well known and are described, for example, in the document EP-A-0 374 458 (see also the Applicant's document FR-A-2 589 145).
As is known, a cross-linked polymer is a polymer which, in the branched chain, has groups or branchings disposed globally in three dimensions, resulting in practically insoluble products of infinite molecular weight; cross-linked polymers of this type having high molecular weights are well known as flocculating agents, for example as described in European patent 0 202 780 or European patent 0 201 237, whose teachings are equivalent.
The cross-linking can be carried out during or after the polymerization, for example by reaction of two soluble polymers having counter-ions, or by reaction on formaldehyde or a polyvalent metal compound. Often, the cross-linking is carried out during the polymerization by addition of a cross linker, and this method is clearly preferred according to the invention. These processes for polymerization with cross-linking are known.
The cross linkers that can be incorporated comprise ionic cross linkers such as polyvalent metal salts, formaldehyde, glyoxal, or preferably, covalent cross linkers that will copolymerize with the monomers, preferably monomers with diethylenic unsaturation (like the family of diacrylate esters such as the diacrylates of polyethylene glycol PEG) or polyethylenic unsaturation, of the type classically used for the cross-linking of water-soluble polymers, and particularly methylenebisacrylamide (MBA), or any of the other known acrylic cross linkers.
In practice, the cross linker is methylenebisacrylamide (MBA), introduced at a rate of five to two hundred (5 to 200) moles per million moles of monomers, preferably 5 to 50, preferably 10 or 20.
Advantageously, the quantity of cross-linked polyacrylamide introduced is between 0.03 per mill and one per mill (0.03% o and 1% o) or between thirty and one thousand grams of active polymer/ton of dry pulp (30 and 1000 g/t), preferably between 0.15 and 0.5 per mill (% o) of the quantity of dry pulp, or from 150 to 500 g/t; it was observed that if the quantity is less than 0.03% o, no significant retention is obtained; likewise, if this quantity exceeds 1% o, no proportional improvement is observed; however, unlike the linear cationic polyacrylamides, as described in the documents EP-A-0 017 353 and EP 0 235 893 mentioned in the preamble, there is no observed reverse dispersion effect by recirculation in the closed circuits of the excess polymer not retained in the sheet.
As stated above, it is important that cross-linked polymer be prepared in the form of a reverse phase emulsion in order to achieve the improvement of the invention.
This approach was condemned in the above-mentioned patent 574 335, in which it was indicated that if a branched polymer is used in emulsion, the indispensable presence of surfactants in these emulsions promotes the formation of foams during the production of the paper and the appearance of disparities in the physical properties of the finished paper (modification of the absorbency in the places where part of the oil phase of the emulsion is retained in the sheet).
Therefore, it was not obvious to consider, for a papermaking application, the reverse phase water-in-oil emulsions whose oil content is clearly high.
Moreover, in a French patent application not published on the filing date of the present application, it is indicated that it is important to stay within the field of branched polymers, and not to move into the field of cross-linked polymers, and it is also indicated that cross-linked emulsions are not known to provide any particular advantage in papermaking.
In another French patent application not published on the filing date of the present application, a cross-linked polymer in reverse phase emulsion or in solution and sheared before introduction is in fact used, but as the sole retention aid.
Bentonite, also known as “smectic swelling clay,” from the montmorillonite family, is well known and there is no need to describe it in detail here; these compounds, formed of microcrystallites, comprise surface sites having a high cation exchange capacity capable of retaining water (see for example the document U.S. Pat. No. 4,305,781, which corresponds to the document EP-A-0 017 353 mentioned above, and FR-A-2 283 102). For the examples below, a commercial bentonite CPB1, with a density of 900 kg/m³, a swelling capacity of 40 ml/g, a cation exchange capacity of 85 meq/100 g in the dry state, and an average size of <75 microns, was used. The use of this bentonite is not limiting. As indicated above, it is also possible to use a kaolin as the secondary retention aid.
Preferably, a semisodic 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%) of the dry weight of the fibrous suspension.
As a filler, it is possible to use kaolins, GCC or ground CaCO₃, precipitated CaCO₃or PCC, and the like.
According to the present invention, a cross-linked retention aid, prepared in the form of a reverse phase emulsion, is either used directly in the form of the synthetic emulsion (“inverted” as described above), or in the form of the solution of the powder obtained by drying said emulsion, the emulsion or the solution being sheared before its injection or introduction into the pulp to be flocculated, which leads directly to microflocs without going through the shearing of larger flocks involving the fibrous mass.
Without intending to be limited by any one theory, the Applicant in effect maintains that a microflocculation occurs directly when the intense-shearing is carried out on the polymer itself before its injection into the fibrous mass of pulp, which is quite a different process than reducing the size of large flocs (involving the fibrous mass) into smaller, more tenacious flocs, which results in unforeseen improvements in the properties of the paper or paperboard sheet.
It is noted that, contrary to the teaching of U.S. Pat. No. 4,753,710 (“HYDROCOL”™), a shearing under the flow line conditions (i.e., a shearing of the pulp) described in this document absolutely does not lead to the results of the invention.
For example, a shearing of the pulp in a pump of the “fan pump” type does not produce the anticipated result.
In this field, the reader is referred to the above examples.
On the other hand, it was discovered according to the invention that for the application related to the manufacture of a sheet of paper, paperboard, or the like, it is essential to carry out an intense shearing before the injection of the cross-linked polymer into the paper pulp or fibrous mass prior to its being flocculated, with no intercalary shearing of the pulp between the injection of the polymer (main retention aid) and the injection of the bentonite or the kaolin (secondary retention aid of the dual system).
One skilled in the art will realize, through the examination of the two series of examples A and B below, that the process according to the invention enables the paper manufacturer to be completely free of the constraint of the “intercalary” shearing of the pulp (i.e., the shearing of the pulp between the addition of the first and second retention aids). An intercalary shearing of this type is mandatory in, for example, the “HYDROCOL”™ system, when desiring obtain a compromise between the various properties of the paper, especially retention, drainage and formation, some of which are known to have been antagonistic in the prior art. However, these same examples show that, if an intercalary shearing occurs in the tests conducted on a product sheared before its introduction into the pulp, the properties of the paper are not appreciably diminished. Therefore, the invention chiefly relates to a process without intercalary shearing of the pulp, but also to a process comprising such an intercalary shearing, whether it be deliberate or imposed by the constraints of the existing equipment. The properties obtained will be better without this intercalary shearing, but if the injection point of the sheared polymer cannot be chosen freely by the paper manufacturer because of the existing equipment, the paper manufacturer can benefit from the excellent set of properties provided by the invention without having to modify its machine. Given below are comparative examples A/B which show that, if the shearing of the fibrous mass (i.e., after the addition of the sheared polymer) is carried out in a paper application, the results obtained are still entirely acceptable. This set of possibilities and findings is referred to in this document as “optional” shearing.
According to a variant of the invention, and as indicated above, it is possible to use a reverse phase emulsion of the polymer (“inverted” in water) or even the powder obtained from the emulsion by means of a known drying technique, such as for example “spray drying,” solvent precipitation, or agglomeration (PEG) and grinding (on this subject, see also the prior art, such as U.S. Pat. No. 5,696,228, WO 97/48 755 (U.S. Ser. No. 08/668,288) WO 97/48 750, WO 97/48 732, WO 97/34 945, WO 96/10589, U.S. Pat. Nos. 5,346,986, 5,684,107, EP 0 412 388, EP 0 238 050, U.S. Pat. No. 4,873,299, EP 0 742 231, WO 90/08789 or EP 0 224 923) which is redissolved in water, sheared, then used like an emulsion.
This variant of the method is very interesting since the dried product according to the invention behaves substantially like the emulsion, and this variant therefore provides a method for using dry products having the advantages of an emulsion, which it is not always possible to prepare by direct polymerization in the aqueous phase, in gel form or in solution.
According to the invention, however, it is preferable to use the reverse phase emulsion (inverted in water into a solution with 5-10 g/l) of the cross-linked polymer, with shearing prior to the injection into the pulp, of course.
Without intending to be limited by any one theory, the Applicant maintains that this is due to the fact that the cationic charge is not released.
According to the invention, laboratory shearing tests can be conducted, with a concentration on the order of 3-5 to 10-15 g of active material (i.e., the polymer)/liter of emulsion of the polymer, preferably between 5 and 10 g/l, in a piece of equipment known as an “Ultra Turrax”™, for example at 10,000 rpm, or in a household mixer of the “Moulinex”™ type, substantially at the same magnitude of rotation speed, for a duration that can last between 15-30 seconds and 2-5 minutes.
In the industry, there is existing equipment suitable for implementing the invention, for example high-pressure recirculation pumps or turbines, which are not referred to by the theoretical example of the document EP 0 201 237.
One skilled in the art will naturally know all the equipment that makes it possible to carry out an intense shearing of the polymer emulsion, diluted to an appropriate value as described below, without being limited to the above examples.
For the generalities of the production of a pulp for paper, paperboard or the like, as well as a list of the additives, fillers, etc., that are well known, it would be useful for one skilled in the art to refer to U.S. Pat. No. 4,753,710.
According to the invention, using an optimization within the scope of one skilled in the art, an ionic regain (IR as defined in European patent 0 201 237) of 40 to 50% is obtained, which can reach at least 60 or 70%, and even more, up to values greater or far greater than 100%.
Moreover, it is possible to adjust the shear so ad to favor, for the first time in this industry, one property of the paper over another, for example to promote retention slightly more than formation or drainage, or vice versa, or any of the various possible combinations, as will be seen by reading the examples that follow.
A normal dosage of the agent according to the invention is such that it leads to about 100 to 500 g of active material (polymer) per ton of fibrous matter to be treated.
According to the invention, it is possible to use a polymer having an intrinsic viscosity i.v. as low as 1 to 3, which becomes an intrinsic viscosity as high as 3-7 or 8 after the application of the shearing according to the invention.
Moreover, the system according to the invention is not expensive, and consequently it combines all of the advantages of the linear or cross-linked single-product systems with floc shearing and of the “dual” systems with two retention aids and also with floc shearing.
The cross-linked polymer in reverse phase emulsion (or in a solution of the redissolved powder), sheared according to the invention, is injected or introduced into the paper pulp (or fibrous mass to be flocculated), which is more or less diluted in accordance with the experience of one skilled in the art, and generally into the diluted paper pulp or “thin stock,” i.e., a pulp diluted to about 1.5% solid matter such as cellulose fibers, possible fillers, and the various additives commonly used in paper production.
The second retention aid, or secondary retention aid, such as bentonite or a preferably pretreated kaolin, is then added into said pulp without any intercalary shearing, or with an “optional” intercalary shearing, for example, in practice, between 5 and 30 seconds, preferably between about 10-20 s, but possibly up to 5 minutes, after the introduction into the pulp of the pre-sheared polymer in reverse phase emulsion (or in a solution of the redissolved polymer).
The following examples illustrate the invention without limiting its scope. FIGS. 1 and 2 represent the histograms corresponding to Tables (I) and (II).
The abbreviations have the meanings indicated below.

Test Column=type of polymer product used
%=dosage of the retention aid of the test column in % agent/dry pulp
% Ash=% by weight of ash (filler retention)
DXF=drainage according to the CSF (Canadian Standard Freeness) standard

X designates a “first pass” measurement.

Formation scale:

1 Excellent (homogeneous)

2 Good (nearly homogeneous)

3 Average (cloudy)

4 Poor (fleecy)

6 Very poor (mottled)

EXAMPLE 1

Production of a Cross-Linked Polymer in Reverse Phase Emulsion Form (PF 455 B)
In a reactor A, the constituents of the organic phase of the emulsion to be synthesized are mixed at the ambient 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 Produced is Prepared by Mixing:
385 g of acrylamide at 50%
73 g of ethyl acrylate trimethyl ammonium chloride (80%)
268 g of water
0.5 g of methylenebisacrylamide at 0.25%
0.75 ml of sodium bromate at 50 g l⁻¹
0.29 ml of Versenex at 200 g l⁻¹

The contents of B are mixed into A under agitation. After the mixture of the phases, the emulsion is sheared in the mixer for 1 minute in order to create the reverse phase emulsion. The emulsion is then degassed by means of a nitrogen bubbling; then after 20 minutes, the gradual addition of the metabisulfite causes the initiation followed by the polymerization.
Once the reaction is finished, a “burn out” (treatment with a bisulfite or metabisulfite to eliminate the residual monomer) is carried out in order to reduce the free monomer content.
The emulsion is then incorporated with its inverting surfactant in order to subsequently release the polymer in the aqueous phase. It is necessary to introduce 2 to 2.4% ethoxylated alcohol. The standard Brookfield viscosity of said polymer is 1.8 cps (viscosity measured at 0.1% in a 1 M NaCl solution at 25° C. at sixty rpm).

The results in terms of UL viscosity are the following:

Table of Example 1:

		NaH₂PO₂	UL
	MBA	ppm	Vis-	IR (1)	IVR (2)
Test	ppm	(*)	cosity	(%)	(%)	State

EM 140 CT	0	10	4.56	0	0	Linear
PF 455 B	10	0	1.80	80	100	Cross-
						linked
“HYDROCOL”	—	—	4.10	0	0	Linear
(TM)
CD3 (TM)

(*): sodium hypophosphite, transfer agent
(1): ionic regain IR in %
(2): intrinsic viscosity regain IVR in %
EM140CT: standard emulsion of very high molecular weight, containing no cross linker

It is noted that the linear products do not develop any ionic regain RI with shearing, and their intrinsic viscosity IV decreases (two of the IVR values are null).
The cross-linked product develops a high ionic regain and a very high IV regain.
Definitions of the Ionic Regains and Intrinsic Viscosity Regains:
Ionic regain IR=(X−Y)/Y×100

with X: ionicity after shearing in meq/g.
Y: ionicity before shearing in meq/g.
Intrinsic viscosity regain IVR=(V1−V2)/V2×100
with V1: intrinsic viscosity after shearing in dl/g
V2: intrinsic viscosity before shearing in dl/g

Some of the emulsions cited above will be the subjects of a study of effectiveness in retention and drainage in an automated sheet former at the Center for Paper Technology.
Procedure for Testing the Emulsions
Pulp Used:

mixture of 70% bleached hardwood kraft KF

10% bleached softwood kraft KR

20% mechanical pulp : PM

20% natural calcium carbonate.
Sizing in a neutral medium with 2% of an alkyl ketene dimer emulsion.
The pulp used is diluted to a consistency of 1.5%. A sample of 2.24 dry g of pulp, or 149 g of pulp at 150%, is taken, then diluted to 0.4% with clear water.
The 560 ml volume is introduced into the plexiglass cylinder of the (standard) automated sheet former, and the sequence is begun in accordance with the two procedures A and B.

Procedure A:

Intense Shearing of the Pulp at 150 rpm for 50 Seconds

- t=0 s, start of agitation at 1500 rpm (intense shearing).
- t=10 s, addition of the polymer (in the sheared state according to the invention when a cross-linked product is used).
- t=60 s, automatic reduction to 1000 rpm and, if necessary, addition of the bentonite.
- t=75 s, stopping of the agitation, formation of the sheet with vacuum under the wire, followed by reclamation of the white water.

Procedure B:

Simple Turbulence of the Pulp for 10 Seconds

- t=0 s, start of agitation, imposed at 800 rpm (no intense shearing).
- t=10 s, addition of the polymer (in the sheared state according to the invention when a cross-linked product is used).
- t=20 s, addition of bentonite, if necessary, still at 800 rpm.
- t=30 s, stopping of the agitation, formation of the sheet with vacuum under the wire, followed by reclamation of the white water.

The following operations are then carried out:

- measurement of the turbidity of the water under the wire.
- dilution of a beaker of thick stock for a new sheet with the reclaimed water under the wire.
- drying of the so-called 1st pass sheet.
- start of a new sequence for producing the so-called 2nd pass sheet.

After 3 passes, the products to be tested are changed.
The following analyses are then performed:

- measurement of the matter in suspension in the water under the wire (TAPPI standard T 656 cm/83)
- measurement of the ash in the sheets (TAPPI standard: T 211 om-93)
- measurement of turbidity 30′ after the fibers are deposited in order to learn the state of neutralization of the colloidal matter.
- measurement of the degree of drainability of the pulp with a Canadian Standard Freeness (CSF; TAPPI standard T 227 om-94).
  Comments on the Results: See the Comparative Table (I) Below Relative to Example 1

In Table (I), tests were conducted on various products in accordance with the two procedures (A) and (B).
Example 3 corresponds to a linear polymer similar to the “HYDROCOL”™ technique of the above-mentioned U.S. patent '710 (a linear polymer). The results are therefore similar to test 7, which corresponds to the technique of U.S. patent '710 likewise, tests 2 and 6 are comparable (a linear polymer, but without bentonite).
An examination of tests 3, 7 and 2, 6 confirms that bentonite provides advantageous performance levels.
Test 5 corresponds to a cross-linked polymer emulsion, sheared before injection into the pulp, and hence according to the invention, which leads to a extremely advantageous performance in terms of drainage (CSF Canadian Standard Freeness) while having excellent formation (an index of 1 as opposed to 2 for the other comparable tests).
This is quite surprising, since one skilled in the art knows that when drainage is successfully increased, formation is affected negatively. According to the invention, on the other hand, the formation is not affected.
Moreover, the clarity of the water under the wire is distinctly improved—note the very low turbidity of 122 as opposed to 134-159 (“HYDROCOL”™).
Thus, surprisingly, the use of a cross-linked (rather than linear) polymer, sheared before its injection, results in a distinct improvement in comparison with the “HYDROCOL”™ system, while freeing the paper manufacturer from the constraint of shearing the pulp between the two additions of polymer and of bentonite.
Tests 8 through 13 verified the effect obtained when attempting to eliminate the shearing of the pulp between the two additions, in contrast to U.S. patent '710. It may be seen that, in a “HYDROCOL”™ context, it is important to shear the pulp. In effect, it is possible to obtain a very high flocculation if the pulp is not sheared, but the formation suffers (indexes of 4 or 5), making it unusable.
If test 5 (A) (shearing of the pulp) is compared to test 11(B) (same test without shearing of the pulp), it may be seen that the invention (11) improves the drainage properties and provides good turbidity, while the formation remains excellent (an index of 2 rather than 1).
Lastly, if test 11 (the invention, strongly cross-linked) is compared to test 7 (“HYDROCOL”™, linear) it may be seen that the process according to the invention distinctly improves drainage (512 vs. 458), or +34%, with equal formation (an index of 2)—which is completely surprising, since this formation would have been expected to decrease sharply—improves turbidity (104 vs. 175) and improves filler retention (% ash X=% ash in the first pass) (100 vs. 90.4).

EXAMPLE 2

Production of a Cross-Linked Ethyl Acrylate Trimethyl Ammonium Chloride-Based in the Form of an Emulsion of the EM 240 BD Type:
In a reactor A, the constituents of the organic phase of the emulsion to be synthesized are mixed at the ambient temperature.
a) Organic Phase:

- 266 g of “Exxsol D 100”™
- 18 g of “Span 80”™
- 6 g of “Hypermer 2296”™
  b) In a Beaker B, the Phase of the Emulsion to be Produced is Prepared by Mixing:
- 438 g of acrylamide at 50%
- 186.5 g of ethyl acrylate trimethyl ammonium chloride (80%)
- 85 g of water
- 0.31 ml of methylenebisacrylamide at 6 g/l
- 1.50 ml of sodium bromate at 50 g/l
- 0.24 ml of Versenex at 200 g/l
- pH: 4

The contents of B are mixed into A under agitation. After the mixture of the phases, the emulsion is sheared in the mixer for 1 minute in order to create the reverse phase emulsion.
The emulsion is then degassed by means of a nitrogen bubbling; then after 20 minutes, the gradual addition of the metabisulfite causes the initiation followed by the polymerization.
Once the reaction is finished, a “burn” out is performed in order to reduce the free monomer content.
The emulsion is then incorporated with its inverting surfactant in order to release the polymer in the aqueous phase.

Table of Example 2:

MBA NaH₂PO₂ UL IR (1) IVR (2)

Test ppm ppm (*) Viscosity (%) (%) State

EM 240 CT 0 10 4.20 0 0 Linear

EM 240 BD 10 0 1.6 58 55 Cross-

linked

Procedure for Testing the Emulsions
(Procedure Identical to that of Example 1)
Comments on the Results: See the Comparative Table (II) Below Relative to Example 2
Example 2 leads to the same types of conclusions as Example 1.
According to the invention, the formation is maintained at an excellent level of 2. The drainage, filler retention and first pass retention are improved considerably.
If the series A and series B tests on a linear product of the “HYDROCOL”™ type with bentonite (15) are compared, it may be seen that without a shearing of the pulp, the formation drops from 2 to a disastrous value of 5; on the other hand, with a shearing of the pulp, the formation remains at the index 2. Consequently, for a linear product comparable to the “HYDROCOL”™ type, the shearing of the pulp is essential.
On the other hand, with a cross-linked product according to the invention, if the tests 17 and 21 are compared, it may be seen that, without a shearing of the pulp, the drainage is improved, and moreover, the filler retention on the wire (% ash) and the first pass retention (% Ret. X) are maintained. With a shearing of the pulp, the drainage (CSF) is admittedly slightly diminished, as are the retention and the turbidity, but the formation remains at a very good level (index 2). According to the invention, it is therefore entirely preferable not to shear the pulp between the addition of the sheared cross-linked polymer and the addition of the bentonite, but an intercalary shearing still results in a good combination of properties.
The invention is therefore compatible with all the existing papermaking equipment, including the equipment in which the injection point of the polymer cannot be chosen freely.
Moreover, the invention provides another important advantage relative to a very good formation of the sheet. As is known, formation indicates qualities of the sheet such as homogeneity and the like.
This advantage is attributable to the microflocculation produced by the agents sheared according to the invention.

EXAMPLE 3

Production of a Polymer in the Form of a Redissolved Powder (SD 455 B).
Example 1 is repeated in order to prepare the product PF 455 G in reverse phase emulsion.
This reverse phase emulsion is dried by means of a known spray drying technique; a white powder is obtained which is redissolved in water, to 5 g of polymer per liter.

This solution is then sheared in the “Ultra Turrax”™ as described above, under the same conditions as for the shearing of the reverse phase emulsion PF 455 B in Example 1 (inverted in water before shearing, of course).

Table of Example 3:

	MBA	NaH₂PO₂	UL	IR (1)	IVR (2)
Test	ppm	ppm (*)	Viscosity	(%)	(%)	State

EM140 CT	0	10	4.56	0	0	Linear
PF 455 B	10	0	1.80	80	100	Cross-
						linked
SD 455 B	10	0	1.85	85	100	Cross-
						linked

SD 455 B = a solution of the powder obtained by drying the reverse phase emulsion PF 455 B.
EM140CT = a standard emulsion of very high molecular weight, containing no cross linker.

Comments on the Results See the Comparative Table (III) Relative to Example 3.

An examination of Table (III) shows that the solution sheared before injection, SD 455 B, obtained by dissolution in water of 5 g of polymer/liter of the powder obtained by spray drying the emulsion PF 455 G, behaves substantially like the sheared emulsion itself.
The invention also relates to the novel retention aids described above, which consist of or comprise a sheared cross-linked polyacrylamide (or more generally a cross-linked acrylic (co)polymer) in reverse phase (or water-in-oil) emulsion (inverted in water), or in the form of the sheared solution of the powder obtained by drying said emulsion, as well as the processes for producing sheets of paper, paperboard or the like that use the agents according to the invention or the process according to the invention described above, and the sheets thus obtained.

Claims

1-34. (canceled)

35. Process for manufacturing a sheet of paper, paperboard or the like having improved retention and drainage properties, of the type which uses a dual system of an acrylic type polymer and bentonite or an optionally treated kaolin as the primary and secondary retention agents, respectively, characterized in that it incorporates operations comprised of adding to the suspension or fibrous mass to be flocculated, or paper pulp,

a) as the main retention agent, a (co)polyacrylamide that is cross-linked and exists in the form of a reverse phase or water-in-oil emulsion, or a solution of the powder obtained by drying said reverse phase emulsion, said emulsion or solution being sheared prior to introduction or injection into the fibrous mass; wherein said (co)polyacrylamide is 20 mole % cationic or less, and

b) then a second retention agent,

c) without a stage for intense shearing of the pulp between the additions a) and b), or with an “optional” shearing of the pulp between the additions a) and b);

wherein the active material concentration at the point in time when shear occurs is 3 to 15 g/l;

characterized in that the polymer used has an intrinsic viscosity i.v. as low as 1 to 3, which becomes an intrinsic viscosity i.v. as high as 3-7 or 8 after the application of the shearing; and characterized in that the quantity of cross-linked polyacrylamide or of cross-linked acrylic (co)polymer introduced in the form of a reverse phase water-in-oil emulsion, Areversed@ in water, or in the form of the solution of the powder obtained by drying said emulsion, is between 0.03 and 1% o, or between thirty and one thousand grams/ton (30 and 1000 g/t) of dry pulp, optionally between 0.15 and 0.5% o (or between 150 and 500 g/t).

36. Process for manufacturing a sheet of paper, paperboard or the like having improved retention and drainage properties, of the type which uses a dual system of an acrylic type polymer and bentonite or an optionally treated kaolin, as main and secondary retention agents, respectively, according to claim 35, characterized in the incorporates operations comprised of adding to the suspension or fibrous mass to be flocculated, or papers pulp,

a) as the main retention agent, a (co) polyacrylamide that is cross-linked and exists in the form of a reverse phase or water-in-oil emulsion, or a solution of the powder obtained by drying said reverse phase emulsion, said emulsion or solution being sheared prior to introduction or injection into the fibrous mass, and

b) then a second retention agent,

c) without a stage for intense shearing of the pulp between the additions a) and b).

37. Process for manufacturing a sheet of paper, paperboard or the like according to claim 35, characterized in that monomers chosen from among the nonionic monomers are used to prepare said (co) polymer, and/or in that at least some of the monomers used to form the polymer are ionic and/or in that the monomers are monomers with monoethylenic unsaturation, or allylic monomers or vinyl monomers, especially acrylic or methacrylic monomers, and/or in that the cross-linked acrylic (co) polymer prepared in reverse phase emulsion 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 by different acids and quaterinizing agents, benzyl chloride, methyl chloride, alkyl or aryl chloride, dimethyl sulfate, diallyldimethylammonium chloride (DADMAC), acrylamidopropyltrimethylammonium chloride (APTAC), and meth acrylamidopropyltrimethylammonium chloride (MAPTAC), and/or the cross-linked acrylic (co) polymer prepared in reverse phase emulsion is a copolymer of acrylamide and of ethyl acrylate trimethyl ammonium chloride, and/or in that the cross-linked acrylic (co) polymer prepared in reverse phase emulsion is cross-linked acrylic (co) polymer prepared in reverse phase emulsion is cross-linked by a cross-linking agent constituted by a polyfunctional compound having at least two reagent groups chosen from the group comprising the double bonds, the aldehyde bonds or the epoxy bonds, the cross-linking agents that can be incorporated comprising ionic cross-linking agents such as polyvalent metal salts, formaldehyde, glyoxal, or covalent cross-linking agents that will copolymerize with the monomers, preferably monomers with diethylenic unsaturation optionally the family of diacrylate esters and optionally the diacrylates of polyethylene glycol PEG or polyethylenic unsaturation, and optionally methylenebisacrylamide (MBA).

38. Process according to claim 35 characterized in that the cross-linked acrylic (co)polymer in reverse phase emulsion or in a solution of the powder obtained by drying the synthetic reverse phase emulsion is introduced into the paper pulp at a concentration of 0.03 to one per mill (0.03 to 1% o) by weight of the dry weight of the fibrous suspension of paper pulp, optionally 0.15 to 0.5 per mill (0.15 to 0.5% o).

39. Process according to claim 35 characterized in that the MBA is introduced at a concentration of 5 to 200 moles per million moles of monomers.

40. Process according to claim 35 characterized in that the bentonite is a semisodic 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.

41. Process according to claim 35, characterized in that the pulp used, which contains the filler, is diluted, then the polymer is added as the main retention agent, after which bentonite is added as the secondary retention agent.

42. Process according to claim 35, characterized in that the sheared cross-linked polymer in reverse phase emulsion form, “reversed” in water, or in the form of the sheared solution of the powder obtained by drying of said emulsion, is injected or introduced into the diluted paper pulp or fibrous mass to be flocculated or “thin stock,” optionally a pulp diluted to about 1.5% solid matter optionally including cellulose fibers, fillers, and various paper and paperboard additives, and the second retention agent, or secondary retention agent, such as bentonite or an optionally pretreated kaolin, is then added between 5 and 30 s after the introduction of the polymer.

43. Process for manufacturing a sheet of paper, paperboard or the like according to claim 35, characterized in that the shearing of the reverse phase emulsion of the polymer or of the solution obtained by the redissolution in water of the powder obtained by drying the synthetic reverse phase emulsion, is carried out before injection into the pulp, with a concentration of 3-5 to 10-15 g of active material optionally the polymer/liter of emulsion of the polymer, optionally between 5 and 10 g/l, in an “Ultra Turrax”™ machine, optionally at 10,000 rpm or in a household mixer of the “Moulinex”™ type, substantially at the same magnitude of rotation speed, for a duration that can last between 15-30 seconds and 2-5 minutes, or in that the shearing is carried out in high pressure recirculation pumps or turbines.

44. Process for manufacturing a sheet of paper, paperboard or the like according to claim 35, characterized in that the shearing the reverse phase emulsion “reversed” in water, or the solution of the dried powder of the synthetic emulsion, before injection into the pulp results in an ion regain IR of 40 to 50%, which can reach at least 60 or 70% and even more, up to values greater or far greater than 100%, with:

Ion regain IR=(X−Y)/Y×100

with X: ionicity after shearing in meg/g.

Y: ionicity before shearing in meg/g.

45. Novel retention agent for the manufacture of a sheet of paper, paperboard or the like, characterized in that it comprises a sheared cross-linked polyacrylamide or a cross-linked acrylic (co)polymer in reverse phase or water-in-oil emulsion, or in the form of the sheared solution of the powder obtained by drying said emulsion, and optionally characterized in that the cross-linking agent is a cross-linking agent constituted by a polyfunctional compound having at least two reagent groups chosen from the group comprising the double bonds, the aldehyde bonds or the epoxy bonds, the cross-linking agents that can be incorporated comprising ionic cross-linking agents such as polyvalent metal salts, formaldehyde, glyoxal, or covalent cross-linking agents that will copolymerize with the monomers, optionally monomers with diethylenic unsaturation the family of diacrylate esters which can include the diacrylates of polyethylene glycol (PEG) or polyethylenic unsaturation, and optionally methylenebisacrylamide (MBA), and in that the cross-linking agent is introduced at a rate of five to two hundred (5 to 200) moles per million moles of monomers characterized in that the polymer used has an intrinsic viscosity i.v. as low as 1 to 3, which becomes an intrinsic viscosity i.v. as high as 3-7 or 8 after the application of the shearing; and characterized in that the quantity of cross-linked polyacrylamide or of cross-linked acrylic (co)polymer introduced in the form of a reverse phase water-in-oil emulsion, “reversed” in water, or in the form of the solution of the powder obtained by drying said emulsion, is between 0.03 and 1% o, or between thirty and one thousand grams/ton (30 and 1000 g/t) of dry pulp, optionally between 0.15 and 0.5% o (or between 150 and 500 g/t).

46. Novel retention agent for the manufacture of a sheet of paper, paperboard or the like according to claim 45, characterized in that the cross-linked 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), quaterinized or alified by different acids and quaterinizing agents, benzyl chloride, methyl chloride, alkyl or aryl chloride, dimethyl sulfate, diallyldimethylammonium chloride (DADMAC), acrylamidopropyltrimethylammonium chloride (APTAC), and meth acrylamidopropyltrimethylammonium chloride (MAPTAC).

47. Retention agent according to claim 45, characterized in that it is sheared at a concentration on the order of 1-5 to 10-15 g of active material, optionally the polymer/liter of emulsion of the polymer, optionally between 5 and 10 g/l, in an “Ultra Turrax”™ machine optionally, at 10,000 rpm, or in a household mixer of the “Moulinex”™ type, substantially at the same magnitude of rotation speed, for a duration that can last between 15-30 seconds and 2-5 minutes, or in high-pressure recirculation pumps or turbines.

48. Retention agent according to claim 45, characterized in that it uses a polymer having an intrinsic viscosity i.v. as low as 1 to 3, which becomes an intrinsic viscosity as high as 3-7 or 8 after the application of the shearing, or in that an ion regain IR of 40 to 50% in obtained, which can reach at least 60 or 70% or even more, up to values greater or far greater than 100%, with:

Ion regain IR=(X−Y)/Y×100

with X: ionicity after shearing in meg/g.

Y: ionicity before shearing in meg/g.

49. A sheet of paper or paperboard prepared in a process utilizing a retention agent according to claim 45.

50. Sheet of paper, paperboard or the like, characterized in that it is obtained with the use of a process according to claim 35.

51. The process of claim 35 wherein the active material concentration at the point in time when shear occurs in 5 to 10 g/l.

52. The process according to claim 35 characterized in that the cross-linked acrylic (co)polymer in reverse phase emulsion or in a solution of the powder obtained by drying the synthetic reverse phase emulsion is introduced into the paper pulp at a concentration of 0.15 to 0.5 per mill (0.15 to 0.5% o) by weight of the dry weight of the fibrous suspension of paper pulp, optionally 0.15 to 0.5 per mill (0.15 to 0.5% o), or between 30 and 1000 g/t preferably 150 and 500 g/t.

53. The process according to claim 35 characterized in that the cross-linked acrylic (co)polymer in reverse phase emulsion or in a solution of the powder obtained by drying the synthetic reverse phase emulsion is introduced into the paper pulp at a concentration of 0.15 to 0.5 per mill (0.15 to 0.5% o) by weight of the dry weight of the fibrous suspension of paper pulp.

54. The process according to claim 39 characterized in that the MBA is introduced at a concentration of 5 to 50 moles per million moles of monomers.

55. The process according to claim 54 characterized in that the MBA is introduced at a concentration of 10 to 20 moles per million moles of monomers.

56. The process according to claim 42 characterized in that the second retention agent, or secondary retention agent or optionally pretreated kaolin, is added between 10-20 s after the introduction of the polymer.

57. The process according to claim 42 characterized in that the second retention agent, or secondary retention agent or optionally pretreated kaolin, is added up to about 5 minutes after the introduction of the polymer.

58. The novel retention agent of claim 45 characterized in that the cross-linking agent is introduced at a rate of 5 to 50 moles per million moles of monomers.

59. The novel retention agent of claim 58 characterized in that the cross-linking agent is introduced at a rate of 10 to 20 moles per million moles of monomers.

60. The novel retention agent of claim 59 characterized in that the cross-linking agent is introduced at a rate of 20 moles per million moles of monomers.