EP1740769B1 - Method for the production of paper and cardboard, corresponding novel retention and draining agents, and paper and cardboard thus obtained - Google Patents

Description

The invention relates to a method for the production of paper, cardboard or the like using at least three retention and dewatering agents, respectively a main agent, one or even two secondary agents and a tertiary agent. It also relates to paper or paperboard obtained by this method. Finally, it relates to the use of specific crosslinked anionic organic polymers as a tertiary retention agent.

Microparticulate retention systems are well known in the papermaking process. Their function is to improve the retention, drainage and formation during the manufacture of the sheet.

• le brevet US-A-3052595 , où il est proposé d'associer de la bentonite (microparticule inorganique) à un polyacrylamide linéaire (agent principal de rétention),
• le brevet EP-A- 235893 , qui décrit l'addition d'un polymère cationique de haut poids moléculaire (agent principal de rétention), puis une étape de cisaillement, puis l'addition de bentonite(agent secondaire de rétention). Selon ce brevet, le polymère doit être essentiellement linéaire (sans ajout volontaire d'agent de ramification),
• le brevet US-A-5180473 ainsi que l'article Watanabe & al. (Pulp Gijitsu Times, mars 1989, pages 17 à 21 ) sont relatifs à l'utilisation dans un système de type «dual» d'un polymère (agent principal de rétention) puis d'une microparticule organique (agent secondaire de rétention). Il est précisé (col.3, 1.65) que les microparticules (anioniques ou cationiques) doivent avoir une taille la plus uniforme possible et la plus fine possible.

It is known in the prior art:

• the patent US Patent 3052595 , where it is proposed to associate bentonite (inorganic microparticle) with linear polyacrylamide (main retention agent),
• the patent EP-A-235893 , which describes the addition of a cationic polymer of high molecular weight (main retention agent), then a shearing step, then the addition of bentonite (secondary retention agent). According to this patent, the polymer must be substantially linear (without the voluntary addition of branching agent),
• the patent US Patent 5180473 as well as the article Watanabe & al. (Pulp Gijitsu Times, March 1989, pages 17-21 ) relate to the use in a "dual" type system of a polymer (main retention agent) and then an organic microparticle (secondary retention agent). It is stated (col.3, 1.65) that the microparticles (anionic or cationic) must be as uniform as possible and as thin as possible.

It should be noted that, as is the case for retention systems using mineral microparticles, the person skilled in the art knows, as early as 1989, that the size of the organic microparticles is a direct indication of the power of particulate agglomeration and thus of retention. (allowing, in particular, to increase the availability of charged, anionic or cationic sites).

• le brevet EP-A-462365 , qui est relatif lui aussi à l'utilisation d'un polymère (agent principal de rétention) et d'une microparticule organique (agent secondaire de rétention), caractérisée en ce que la taille moyenne de la microparticule doit être inférieure à 750 nm,
• le document WO 02/33171 propose, dans la même lignée que le brevet ci-avant cité (p.7, 1.16+), l'emploi conjoint en plus du polymère organique cationique (agent principal de rétention), d'une particule inorganique (agent secondaire de rétention) et d'une particule organique anionique (agent tertiaire de rétention) qui doit être de taille inférieure à 750 nm. Il s'agit là d'un système trial, c'est à dire d'un système dans lequel il y a trois types d' agent de rétention,
• le document WO 98/29604 décrit l'utilisation d'un polymère cationique (agent principal de rétention) suivi d'un polymère organique anionique ramifié entièrement soluble dans l'eau et présentant une IV supérieure à 3 dl/g (agent secondaire de rétention).. D'après ce document, l'utilisation d'un polymère anionique ramifié en tant qu'agent de rétention secondaire, en lieu et place d'un polymère linéaire permettrait d'améliorer à la fois la formation et la rétention de la feuille. En outre, ce document ainsi que la division EP-B-1 167 392 distingue la notion de polymère ramifié et celle de polymère réticulé. Ainsi et en prenant en considération les enseignements du document EP-A-374 458 , un polymère obtenu par polymérisation en phase inverse d'un monomère anionique hydrosoluble, en présence d'un agent réticulant dont le taux est compris entre environ 6 et 25 ppm molaire, en l'absence d'agent de transfert, est nécessairement réticulé. Comme il ressort du document EP-B-1 167 392 , cela signifie donc qu'à un taux d'agent réticulant inférieur à 6 ppm molaire et en l'absence d'agent de transfert, le polymère anionique obtenu est ramifié, tandis que dans la fourchette de 6 à 25ppm molaire, la présence d'agent de transfert est nécessaire pour obtenir un polymère ramifié. Il s'ensuit que pour des taux d'agent réticulant supérieurs, le polymère est forcément réticulé s'il n'y a pas d'agent de transfert. Un autre critère permettant de distinguer les deux notions ramifié/réticulé serait le calcul de la tangente delta, paramètre décrit dans ces documents. En pratique, les polymères dont la tangente delta est inférieure à 0.7 sont réticulés alors qu'ils sont ramifiés dans le cas inverse. Ainsi les polymères divulgués notamment dans les exemples 57A à 59A du document EP-A-374 458 , obtenus en présence respectivement de 6.9, 11.6 et 23.2 ppm molaire d'agent réticulant et en l'absence d'agent de transfert, pour lesquels la tangente delta est égale respectivement à 0.54, 0.32 et 0.51 correspondent en réalité à des polymères incomplètement solubles dans l'eau, c'est à dire des polymères réticulés. Il est également envisagé dans le document WO98/29604 d'utiliser l'agent de rétention secondaire sous la forme d'un mélange de polymères anioniques. Dans une telle hypothèse, soit chacun des polymères reproduit les caractéristiques du polymère anionique spécifique (ramifié, entièrement soluble dans l'eau, viscosité supérieure à 3 dl/g et tangente delta supérieure à 0.7), soit le mélange global reproduit ces caractéristiques. Dans cette dernière hypothèse, il est bien évident qu'un mélange globalement ramifié ne pourra que contenir à tout le moins des polymères ramifiés et tout au plus un mélange de polymères ramifiés et de polymères linéaires, ce qui signifie par là même qu'un mélange de polymères réticulés avec des polymères linéaires ne peut conduire à l'obtention d'un mélange ramifié,
• les documents EP-A-1 228 273 et WO 01/34909 décrivent des systèmes proches de celui mentionné dans le document EP-A-950 138 si ce n'est qu'il s'agit d'un système trial. Plus précisément, il est prévu d'introduire de la bentonite ou un matériau siliceux, avant ou après le polymère anionique.

Following the teaching of Watanabe, then come:

• the patent EP-A-462365 which also relates to the use of a polymer (main retention agent) and an organic microparticle (secondary retention agent), characterized in that the average size of the microparticle must be less than 750 nm,
• the document WO 02/33171 proposes, in the same vein as the aforementioned patent (p.7, 1.16+), the joint use in addition to the cationic organic polymer (main retention agent), of an inorganic particle (secondary retention agent) and an anionic organic particle (tertiary retention agent) which must be less than 750 nm in size. This is a trial system, ie a system in which there are three types of retention agent,
• the document WO 98/29604 discloses the use of a cationic polymer (main retention agent) followed by an organic branched anionic polymer fully soluble in water and having an IV greater than 3 dl / g (secondary retention agent). this document, the use of a branched anionic polymer as a secondary retention agent, instead of a linear polymer would improve both the formation and retention of the sheet. In addition, this document as well as the division EP-B-1,167,392 distinguishes the notion of branched polymer and that of crosslinked polymer. So and taking into consideration the teachings of the document EP-A-374 458 , a polymer obtained by reverse phase polymerization of a water-soluble anionic monomer, in the presence of a crosslinking agent whose content is between about 6 and 25 molar ppm, in the absence of transfer agent, is necessarily reticle. As is apparent from the document EP-B-1,167,392 it therefore means that at a content of crosslinking agent of less than 6 ppm molar and in the absence of transfer agent, the anionic polymer obtained is branched, while in the range of 6 to 25 ppm molar, the presence of Transfer agent is necessary to obtain a branched polymer. It follows that for higher levels of crosslinking agent, the polymer is necessarily crosslinked if there is no transfer agent. Another criterion for distinguishing the two notions branched / crosslinked would be the calculation of the delta tangent, a parameter described in these documents. In practice, the polymers whose delta tangent is less than 0.7 are crosslinked while they are branched in the opposite case. Thus the polymers disclosed in particular in Examples 57A to 59A of the document EP-A-374 458 , obtained respectively in the presence of 6.9, 11.6 and 23.2 molar ppm of crosslinking agent and in the absence of transfer agent, for which the delta tangent is equal to 0.54, 0.32 and 0.51 respectively correspond in fact to incompletely soluble polymers. in water, ie crosslinked polymers. It is also envisaged in the document WO98 / 29604 to use the secondary retention agent in the form of a mixture of anionic polymers. In this hypothesis, either each of the polymers reproduces the characteristics of the specific anionic polymer (branched, fully soluble in water, viscosity greater than 3 dl / g and delta tangent greater than 0.7), or the overall mixture reproduces these characteristics. In the latter case, it is obvious that a globally branched mixture can only contain at least branched polymers and at most a mixture of branched polymers and linear polymers, which means that a mixture of of polymers crosslinked with linear polymers can not lead to the production of a branched mixture,
• the documents EP-A-1 228 273 and WO 01/34909 describe systems close to the one mentioned in the document EP-A-950138 except that it is a trial system. More specifically, it is planned to introduce bentonite or a siliceous material, before or after the anionic polymer.

The Applicant has found that the use of a crosslinked polymer of standard size as a retention agent, optionally mixed with an anionic or amphoteric polymer, instead of the other organic polymers of the prior art, gave very good results. at least in terms of training but also and in some cases, in terms of retention and draining.

• tout d'abord à ajouter à la suspension fibreuse au moins un agent de rétention principal constitué d'un (co)polymère cationique,
• puis éventuellement à cisailler les flocs obtenus,
  le procédé se caractérisant en ce qu'on ajoute ensuite à la suspension, séparément ou en mélange, dans un sens ou dans l'autre :
• au moins un agent de rétention secondaire choisi dans le groupe comprenant les dérivés de la silice et les polymères organiques anioniques ou amphotères,
• au moins un agent de rétention tertiaire constitué d'un polymère organique anionique réticulé, de taille supérieure ou égale à 1 micromètre et présentant une viscosité intrinsèque inférieure à 3 dl/g.

The subject of the invention is therefore a process for producing paperboard or the like which consists of:

• first of all adding to the fibrous suspension at least one main retention agent consisting of a cationic (co) polymer,
• then possibly shear the flocs obtained,
  the process being characterized in that one adds to the suspension, separately or in mixture, in one direction or the other:
• at least one secondary retention agent selected from the group consisting of silica derivatives and anionic or amphoteric organic polymers,
• at least one tertiary retention agent consisting of a crosslinked anionic organic polymer of size greater than or equal to 1 micrometer and having an intrinsic viscosity of less than 3 dl / g.

As can be seen from the reading of the prior art, a person skilled in the art had long been quite dissuaded from using as a secondary retention agent a compound of high size and / or of low specific surface area since these characteristics are associated with a low agglomeration power.

Those skilled in the art were dissuaded from using low molecular weight polymers (low intrinsic viscosity) because these are known to exhibit poor performance in retention and dewatering.

Because of this knowledge, which seems prohibitive, the risk of failure was important. This explains the fact that the technology of the invention, intended to use in a "microparticulate" system, a crosslinked anionic organic polymer of size greater than or equal to 1 micron and low intrinsic viscosity, has not been used.

One of the merits of the invention is to have developed a method of manufacturing paper pulp, according to which there is no specific constraint related to the process for preparing the tertiary agent, which is obtained by a conventional method of dispersion polymerization requiring no particular precaution as to the polymerization conditions.

For marketing reasons, it will be sought to provide the tertiary agent in its most concentrated form possible, preferably in dispersion, well known to those skilled in the art. This form having the advantage of not requiring the use of high amounts of surfactants. The tertiary agent is thus produced either in inverse emulsion or "water-in-oil", or in aqueous dispersion also called "water-in-water emulsion".

As already stated, the invention is directed to an improved process which consists in adding, to the suspension or fibrous mass or paper stock to be flocculated, as main retention agent, at least one cationic polymer, followed by the addition, in a mixture or no, at least one secondary retention agent and at least one tertiary anionic crosslinked organic retention agent different from the secondary agent, of size greater than or equal to 1 micron and of low intrinsic viscosity (less than 3 dl / boy Wut).

The additions of the main retention agent and the secondary and tertiary agents are separated or not by a shearing step, for example at the so-called "fan pump" mixing pump. Reference in this field to the description of the patent USP 4, 753, 710 as well as a very vast 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, No. 10, October 1976 , Luner, 1984 Papermakers Conference or Tappi, April 1984, pp 95-99 , Sharpe, Merck and Co Inc., Rahway, NJ, USA, circa 1980, Chapter 5 "Polyelectrolyte Retention Aids," Britt, Tappi Vol. 56, October 1973, p 46 ff . and Waech, Tappi, March 1983, pp. 137 , or the USP 4,388,150 . The method of the invention makes it possible to obtain a significantly improved retention without any reverse effect. An additional feature of this improvement is also improved the drainage properties without deteriorating the quality of formation of the sheet.

This selection of an anionic organic particle with a size greater than or equal to 1 micron, low intrinsic viscosity and crosslinked achieves a level of performance unequaled until now in the paper application for total retention and loads and the draining.

The method of the invention can take several embodiments.

In a first embodiment, only one secondary retention agent consisting of one or more silica derivatives, advantageously bentonite, is added to the suspension. In this case, the tertiary retention agent is added separately, at the same point or at a separate point, before or after the secondary agent.

In the remainder of the description and in the claims, the expression "a single secondary agent" means that said agent may contain several products provided they are of the same nature. This is for example the case when the secondary agent consists of one or more silica derivatives.

In a second embodiment, only one secondary retention agent consisting of one or more anionic or amphoteric organic polymers and different from the tertiary retention agent is added to the suspension. In such a case, the secondary and tertiary retention agents may constitute a mixture, which is then injected at one point. This will generally be the case when both products are in compatible physical forms.

In a third embodiment, two secondary retention agents, one or more derivatives of silica and one or more anionic or amphoteric organic polymers and different from the tertiary retention agent, are added to the suspension.

In this case and as before, essentially for reasons of simplification of the industrial process, when the tertiary agent and one of the secondary agents will be in a physical form making their mixture compatible, they will preferably be used in a mixture. Whatever the chosen solution, the objective is to reduce the number of injection pumps required for the process in order to simplify its implementation.

The main, secondary and tertiary retention agents will now be described in detail.

A / The main retention agent: the cationic polymer

• d'au moins un monomère éthylénique cationique non saturé, choisi dans le groupe comprenant les monomères de type dialkylaminoalkyl (meth)acrylate, dialkylaminoalkyl (meth)acrylamide, diallylamine, methyldiallylamine et leurs sels d'ammonium quaternaire ou d'acides. On citera en particulier l'acrylate de dimethylaminoethyl (ADAME) et/ou le methacrylate de dimethylaminoethyle (MADAME) quaternisés ou salifiés, le chlorure de dimethyldiallylammonium (DADMAC), le chlorure d'acrylamidopropyltrimethylammonium (APTAC) et/ou le chlorure de methacrylamidopropyltrimethylammonium (MAPTAC).
• éventuellement d'au moins un monomère non ionique choisi dans le groupe comprenant l'acrylamide et/ou le méthacrylamide et/ou un de leurs dérivés substitués et/ou N-isopropylacrylamide et/ou N-N-diméthylacrylamide et/ou la N-vinylformamide et/ou le N-vinyl acetamide et/ou la N-vinylpyrrolidone,
• éventuellement, d'au moins un monomère acrylique, allylique ou vinylique hydrophobe choisi dans le groupe comprenant les dérivés de l'acrylamide comme les N-alkylacrylamide par exemple le N-tert-butylacrylamide, l'octylacrylamide ainsi que les N,N-dialkylacrylamides comme le N,N-dihexylacrylamide et/ou les dérivés d'acide acrylique comme les alkyl acrylates et méthacrylates,
• éventuellement d'un agent de ramification/réticulation.

Advantageously, in practice, the main cationic retention agent is a cationic polymer based on:

• at least one unsaturated cationic ethylenic monomer selected from the group consisting of dialkylaminoalkyl (meth) acrylate, dialkylaminoalkyl (meth) acrylamide, diallylamine, methyldiallylamine and their quaternary ammonium or acid salts. In particular, mention may be made of dimethylaminoethyl acrylate (ADAME) and / or dimethylaminoethyl methacrylate (MADAME) quaternized or salified, dimethyldiallylammonium chloride (DADMAC), acrylamidopropyltrimethylammonium chloride (APTAC) and / or methacrylamidopropyltrimethylammonium chloride (MAPTAC).
• optionally at least one nonionic monomer selected from the group comprising acrylamide and / or methacrylamide and / or one of their substituted derivatives and / or N-isopropylacrylamide and / or NN-dimethylacrylamide and / or N-vinylformamide and / or N-vinyl acetamide and / or N-vinylpyrrolidone,
• optionally, at least one hydrophobic acrylic, allylic or vinyl monomer selected from the group comprising acrylamide derivatives such as N-alkylacrylamide, for example N-tert-butylacrylamide, octylacrylamide and N, N-dialkylacrylamides; such as N, N-dihexylacrylamide and / or acrylic acid derivatives such as alkyl acrylates and methacrylates,
• optionally a branching agent / crosslinking agent.

Optionally, the main retention agent may also be of amphoteric nature comprising, in combination with the cationic charges, anionic charges carried by anionic monomers, such as, for example, (meth) acrylic acid, acrylamidomethylpropane acid, sulfonic acid, itaconic acid, maleic anhydride, maleic acid, vinylsulfonic acid and their salts.

This polymer does not require the development of a particular polymerization process. It can be obtained by any of the polymerization techniques well known to those skilled in the art: gel polymerization, precipitation polymerization, emulsion polymerization (aqueous or inverse) followed or not by a distillation step and / or spray drying, suspension polymerization, solution polymerization ...

The branching and / or crosslinking may preferably be carried out during (or possibly after) the polymerization, in the presence of a branching / crosslinking agent and optionally of a transfer agent. The following is a nonlimiting list of branching / crosslinking agents: methylene bisacrylamide (MBA), ethylene glycol di-acrylate, polyethylene glycol dimethacrylate, diacrylamide, cyanomethylacrylate, vinyloxyethylacrylate or methacrylate, triallylamine, formaldehyde, glyoxal, glycidyl ether type compounds such as ethylene glycol diglycidyl ether, or epoxy or any other means well known to those skilled in the art for crosslinking.

In a well known manner, an optimization of the polymerization conditions (amount of branching agent / crosslinking agent used, concentration of the active ingredient at the polymerization, polymerization temperature, type and quantity of initiators, optional transfer agent) makes it possible to obtain depending on what is desired, either a branched polymer or a crosslinked polymer.

In practice, the branching / crosslinking agent is methylenebisacrylamide (MBA), introduced at a rate of five to ten thousand (5 to 10,000) parts per million by weight, preferably 5 to 1000.

Below is a non-exhaustive list of transfer agents: isopropyl alcohol, sodium hypophosphite, mercaptoethanol, etc.

The skilled person will choose the best combination based on his own knowledge and the present description, as well as examples that follow.

The cationic polymer is characterized in that it has an IV greater than 2 dl / g and without maximum limitation.

Advantageously, the amount of cationic polymer introduced into the suspension to flocculate is between thirty and thousand grams of active polymer per ton of dry pulp (30 and 3000 g / t), or between 0.003 percent and 0.3 percent. It has been observed that if the amount is less than 0.003%, no significant retention is obtained. Similarly, if this amount exceeds 0.3%, no noticeable improvement is observed. Preferably, the amount of main retention agent introduced is between 0.01 and 0.05 percent (0.01 and 0.05%) of the amount of the dry pulp, ie between 150 g / t and 500 g / t. g / t.

The injection or the introduction of the main retention agent according to the invention is carried out before a possible shearing step, in the pulp (or fibrous mass to be flocculated) more or less diluted according to the practice of man of the art, and generally in thin paper pulp or "thin stock", that is to say a paste diluted to about 0.7 - 1.5% of solids such as cellulose fibers, possible fillers, and various common additives in papermaking.

A variant of the invention relates to the fractional introduction, we introduce a portion of the cationic polymer, according to the invention, at the stage of preparation of the thick paste or "thick stock" to approx. 5% or more solids, or even in the preparation of the thick slurry before a shear step.

B / The secondary retention agent

• les dérivés de la silice comme par exemple les particules de silice dont les bentonites provenant d'hectorites, de smectites, de montmorillonites, de nontronites, de saponites, de sauconites, d'hormites, d'attapulgites et de sépiolites, les dérivés de type silicates, aluminosilicates ou borosilicates, les zéolites, les kaolinites, ou les silices colloïdales modifiées ou non. Ce type d'agents secondaires est, de préférence, introduit juste en amont de la caisse de tête, à raison de 0,01 à 0,5 pour cent (0,01 à 0,5 %)en poids sec par rapport au poids sec de la suspension fibreuse,
• les polymères organiques anioniques ou amphotères, réticulés, ramifiés ou linéaires et différents de l'agent tertiaire de rétention. Avantageusement, en pratique, il s'agit d'un (co)polymère de préférence linéaire d'au moins un monomère éthylénique anionique non saturé, choisi dans le groupe comprenant les monomères tels que, par exemple, l'acide (méth)acrylique, l'acide acrylamidomethylpropane sulfonique, l'acide itaconique, l'anhydride maléique, l'acide maléique, l'acide vinyl-sulfonique et leurs sels et présentant une viscosité UL supérieure à 2, de préférence supérieure à 4 et sans limitation maximum.

These agents preferably comprise, but not limited to, alone or in admixture:

• derivatives of silica such as silica particles whose bentonites from hectorites, smectites, montmorillonites, nontronites, saponites, sauconites, hormites, attapulgites and sepiolites, derivatives of the type silicates, aluminosilicates or borosilicates, zeolites, kaolinites, or colloidal silicas modified or not. This type of secondary agent is preferably introduced just upstream of the headbox, at a rate of 0.01 to 0.5 percent (0.01 to 0.5%) by dry weight relative to the weight. dry fibrous suspension,
• anionic or amphoteric organic polymers, crosslinked, branched or linear and different from the tertiary retention agent. Advantageously, in practice, it is a preferably linear (co) polymer of at least one unsaturated anionic ethylenic monomer selected from the group comprising monomers such as, for example, (meth) acrylic acid , acrylamidomethylpropanesulphonic acid, itaconic acid, maleic anhydride, maleic acid, vinylsulfonic acid and their salts and having a UL viscosity greater than 2, preferably greater than 4 and without any maximum limitation.

This type of secondary agent is preferably introduced just upstream of the headbox, at a rate of 30 to 1000 g / t of polymer active material relative to the dry weight of the fibrous pulp suspension. paper, preferably from 30 to 600 g / t.

C. Tertiary retention agent: crosslinked anionic organic dispersion of size greater than or equal to 1 micron and low intrinsic viscosity

In practice, the tertiary retention agent is an anionic organic polymer characterized in that it is crosslinked, having a particle size greater than or equal to 1 micron and a low intrinsic viscosity of less than 3 dl / g.

More particularly, the invention relates to dispersions of organic polymers with anionic units obtained in the form of a dispersion comprising, for example, from 10 to 80% by weight of at least one crosslinked anionic polymer of particle size greater than or equal to 1 micron and low intrinsic viscosity (less than 3 dl / g).

By the term "dispersion" or similar terms relating to the polymer used according to the invention, one skilled in the art will understand that it designates either a composition comprising a continuous oil phase, a discontinuous aqueous phase and at least one emulsifier of water-in-oil type, ie a composition comprising as a continuous phase a brine (water + salts) and at least one stabilizing agent.

The tertiary retention agents of the present invention are obtained using, during the polymerization, a crosslinking agent, well known to those skilled in the art, and preferably in the absence of transfer agent.

More specifically, the tertiary retention agents are obtained by polymerization (or respectively copolymerization, together throughout the text and claims: "polymerization") of at least one anionic monomer and optionally other nonionic or cationic monomers, in the presence of a crosslinking agent. They must have a global anionic charge.

Those skilled in the art will appreciate from their own knowledge or through routine testing, the polymerization conditions to be used to obtain a final polymer having an intrinsic viscosity as required.

In addition, it is also possible to concentrate the polymer by all the known techniques, for example by azeotropic distillation, precipitation, spray drying ...

In accordance with the present invention, it has been surprisingly discovered that a new family of anionic organic dispersions achieves a level of performance hitherto unsurpassed for retention and dewatering.

• 10-100 % molaire d'au moins un monomère possédant une charge anionique,
• 0-90 % molaire d'au moins un monomère possédant une charge neutre et/ou cationique,
• la concentration de polymérisation est de préférence comprise entre 20 et 50%,
• et d'un agent réticulant. De préférence le taux d'agent de réticulation est supérieure à 5 ppm, avantageusement 15 ppm.

According to a preferred embodiment, the copolymer is obtained from:

• 10-100 mol% of at least one monomer having an anionic charge,
• 0-90 mol% of at least one monomer having a neutral and / or cationic charge,
• the polymerization concentration is preferably between 20 and 50%,
• and a crosslinking agent. Preferably, the level of crosslinking agent is greater than 5 ppm, advantageously 15 ppm.

1. a/ les monomères éthyléniques anioniques non saturés possédant une fonction carboxylique (ex: acide acrylique, acide methacrylique, et leurs sels...), possédant une fonction acide sulfonique (ex : acide 2-acrylamide-2-méthylpropane sulfonique (AMPS) et leurs sels...)...
2. b/ les monomères non ioniques : acrylamide, methacrylamide, N-isopropylacrylamide, N-N diméthylacrylamide, N-vinylformamide, N-vinyl acétamide, N-vinyl pyrrolidone, vinylacetate, esters acrylate, alcool allylique...et/ou les monomères cationiques : on citera, en particulier et de façon non limitative, l'acrylate de dimethylaminoethyl (ADAME) et/ou le methacrylate de dimethylaminoethyle (MADAME) quaternisés ou salifiés, le chlorure de dimethyldiallylammonium (DADMAC), le chlorure d'acrylamidopropyltrimethylammonium (APTAC) et/ou le chlorure de methacrylamidopropyltrimethylammonium (MAPTAC).

Below is a nonlimiting list of the monomers that can be used:

1. a / unsaturated anionic ethylenic monomers having a carboxylic function (eg acrylic acid, methacrylic acid, and their salts ...), having a sulphonic acid function (eg 2-acrylamide-2-methylpropanesulphonic acid (AMPS) and their salts ...) ...
2. b / the nonionic monomers: acrylamide, methacrylamide, N-isopropylacrylamide, N, N-dimethylacrylamide, N-vinylformamide, N-vinylacetamide, N-vinylpyrrolidone, vinylacetate, acrylate esters, allyl alcohol, and / or the cationic monomers: mention, in particular and in a nonlimiting manner, quaternized or salified dimethylaminoethyl acrylate (ADAME) and / or dimethylaminoethyl methacrylate (MADAME), dimethyldiallylammonium chloride (DADMAC), acrylamidopropyltrimethylammonium chloride (APTAC) and / or or methacrylamidopropyltrimethylammonium chloride (MAPTAC).

It is important to note that, in combination with these monomers, it is also possible to use water-insoluble monomers such as acrylic, allylic or vinyl monomers having a hydrophobic group. When used, these monomers will be used in very small amounts, less than 20 mole%, preferably less than 10 mole%, and they will be chosen preferentially from the group comprising acrylamide derivatives. as N-alkylacrylamide for example N-tert-butylacrylamide, octylacrylamide and N, N-dialkylacrylamides such as N, N-dihexylacrylamide ... acrylic acid derivatives such as alkyl acrylates and methacrylates ...

The following is a nonlimiting list of crosslinkers: methylene bisacrylamide (MBA), ethylene glycol di-acrylate, polyethylene glycol dimethacrylate, diacrylamide, cyanomethylacrylate, vinyloxyethylacrylate or methacrylate, triallylamine, formaldehyde, glyoxal, glycidyl ether type compounds such as ethylene glycol diglycidyl ether, or epoxy or any other means well known to those skilled in the art for crosslinking.

The tertiary retention agent is introduced into the suspension, very preferably in a proportion of 30 g / t to 1000 g / t by weight of active ingredient (polymer) relative to the dry weight of the fibrous suspension, preferably 30 g / t at 600g / t.

As already indicated above, it is possible to use the polymer particle either in the form of a dispersion, dissolved or "inverted" in water, or in the form of a solution in water of the powder obtained by drying of said dispersion.

In an advantageous embodiment, a coagulant is added to the fibrous suspension, prior to the addition of the main retention agent.

As the person skilled in the art is well aware, the use of this type of product makes it possible to improve all the more the performance in retention at dosages (in active form) of 0.01 to 10 kg / t and preferably between 0. , 03 and 3 kg / t. Mention may be made in particular, and by way of examples, of coagulants chosen from the group comprising inorganic coagulants such as polyaluminium chloride (PAC), alumina sulphate, aluminum polychlorosulphate ..., or coagulants including - polymers based on diallyldimethyl ammonium chloride (DADMAC), - quaternary polyamines manufactured by condensation of a primary or secondary amine with epichlorohydrin, polymers having functions of vinylamine type or dicyandiamide type resins ... These coagulants may be used alone or as a mixture and are preferably added thick paste.

It will be noted that the addition of secondary and tertiary retention agents may be carried out in any order of introduction, mixed or not.

The invention also relates to the use of a crosslinked anionic organic polymer of size greater than or equal to 1 micron and having an intrinsic viscosity of less than 3 dl / g optionally mixed with one or more anionic or amphoteric organic polymers different from said polymer. crosslinked organic anionic agent as a retention agent in a process for producing paper, board or the like.

In a particular case, it also relates to a tertiary or secondary retention agent consisting of at least one crosslinked anionic organic polymer, of size greater than or equal to 1 micrometer and having an intrinsic viscosity of less than 3 dl / g, optionally mixing with one or more linear anionic organic polymers.

The following examples illustrate the invention without, however, limiting its scope.

1. Method of measuring particle size and intrinsic viscosity :

1. a / Particle size measurements were performed using a HORIBA LA-900 series apparatus by diffraction of laser light.
2. b / The intrinsic viscosity measurements were carried out according to the method as defined in ISO 1628/1 - October 1988 "Guidelines for the Standardization of Methods for Determining the Viscosity Index and the Viscosity Viscosity Index" polymers in dilute solution
3. c / UL viscosity measurement: the UL viscosity is measured using an LVT type Brookfield viscometer equipped with a UL adapter whose module rotates at 60 rpm (0.1% polymer by weight in a saline solution of sodium chloride 1M).
4. d / Training measures:
   • Table 3: Visual Evaluation (Frm). Scale of formation of the obtained leaves: 1: excellent, homogeneous; 2: good, melted; 3: medium, cloudy; 4: bad, sheepish.
   • Table 6: Measurement of the formation index (LT index) using a 2-dimensional epairmeter: the lower the value, the better the formation of the sheet.

2. Presentation of polymers A / The main retention agent: the cationic polymer

In the examples which follow, the following polymers are used: Senior Detention Officer Type Composition cationicity Presence of crosslinking agent UL Viscosity RI AP1 (polym.en gel) Powder AM / ADC 10 mol% NO 4.2 3% AP2 Inverse emulsion AM / ADC 10 mol% YES 3.5 35% AP3 Water / water emulsion AM / ADC 10 mol% NO 4.1 2% AP4 Water / water emulsion AM / ADC 10 mol% YES 3.2 38% AP5 Inverse emulsion AM / APTAC 20 mol% NO 3.6 2%

• AM : acrylamide
• ADC : acrylate de diméthylaminoéthyle quaternisé par le chlorure de méthyl
• APTAC : chlorure de acrylamidopropyltrimethylammonium
• Agent de ramification/réticulation : méthylène Bis acrylamide
• Regain ionique : RI = ((X-Y) / Y) x 100
  avec X : ionicité après cisaillement en meq/g
  Y : ionicité avant cisaillement en meq/g.

With:

• AM: acrylamide
• ADC: Dimethylaminoethyl acrylate quaternized with methyl chloride
• APTAC: acrylamidopropyltrimethylammonium chloride
• Branching Agent / Crosslinking: methylene bis acrylamide
• Ionic Regain: RI = ((XY) / Y) x 100
  with X: ionicity after shearing in meq / g
  Y: ionicity before shear in meq / g.

B / Tertiary retention agent: cross-linked anionic organic polymer dispersion

With the exception of E3, all the particles were prepared by the inverse emulsion polymerization technique on a basis of 30 mole percent acrylamide and 70 mole percent ammonium acrylate.

* E3 is produced in aqueous dispersion ("water-in-water" emulsion) 50 mole% acrylamide and 50 mole% acrylic acid. particles Reticant: MBA (mass ppm) Transfer Agent: Hypophosphite (mass ppm) Diam. Particles (microns) Intrinsic viscosity E1 50 0 1.0 2.5 X2 50 0 0.7 2.3 E3 * 50 0 5 2.8 E4 100 0 1.1 2.2 E5 10 0 1.0 2.9 E6 200 10 1.2 2.1 X7 Crosslinked anionic organic particle marketed by Ciba under the trade name Polyflex CP3 0.2 1.8 E8 5 0 1.0 2.5 X9 50 0 1.1 3.5 X10 0 100 1.3 2.8 X11 5 10 1.2 10

Examples E relate to the tertiary retention agents of the invention. Examples X are counterexamples.

3. Procedure for testing emulsions

• 70 % de kraft de feuillus blanchis
• 10 % de kraft de résineux blanchis
• 20 % de pâte mécanique
• 20 % de carbonate de calcium naturel.
• collage en milieu neutre avec 2 % d'une émulsion d'alkyle cétène dimère.

The various tests were carried out in a "Britt Jar" type container and with a paste consisting of a mixture of:

• 70% bleached hardwood kraft
• 10% bleached softwood kraft
• 20% of mechanical pulp
• 20% natural calcium carbonate.
• bonding in a neutral medium with 2% of a dimeric ketene alkyl emulsion.

The paste used is diluted to a consistency of 1.5%. Dry 2.24 g of pulp is taken, ie 149 g of 1.5% pulp and then diluted to 0.4% with clear water. A volume of 560 ml of this diluted paste is introduced into the plexiglass cylinder of the automated form and the sequence is started.

• T=0s : Ajout éventuel du coagulant
• T=70s : Ajout de l'agent principal de rétention
• T=80s : Ajout de l'agent secondaire de rétention
• T=85s : Ajout de l'agent tertiaire de rétention
• T=90s : Elimination des 20 premiers ml correspondant au volume mort, puis prélèvement de 100ml exactement pour filtration pour le test britt jar.

Britt Jar Sequence at 1000 rpm (RPM):

• T = 0s: Possible addition of coagulant
• T = 70s: Adding the Senior Detention Agent
• T = 80s: Add Secondary Retention Agent
• T = 85s: Addition of tertiary retention agent
• T = 90s: Elimination of the first 20 ml corresponding to the dead volume, then removal of 100ml exactly for filtration for the test britt jar.

• % FPAR : rétention des cendres première passe en pourcentage
• % FPR : rétention première passe en pourcentage (rétention totale)
• CSF : mesure du degré d'égouttabillité de la pâte (norme TAPPI T 2270M-94). Pour chacune de ces analyses, les valeurs les plus élevées correspondent aux meilleures performances.

The following analyzes are then carried out:

• % FPAR: retention of ashes first pass in percentage
• % RPF: retention first pass in percentage (total retention)
• CSF: measure of the degree of drainability of the paste (TAPPI standard T 2270M-94). For each of these analyzes, the highest values correspond to the best performances.

4. results

Tab.1: comparison of different retention systems No. Senior Detention Officer Agent (s) of detention Respective dosage (kg / t) % FPAR % FPR CSF secondary tertiary 0 White - 0.00 70.05 421 1 - AP1 - - 0.3 31,80 78.37 430 2 - AP1 bentonite - 0.3 / 2 46.74 81.46 451 3 - AP1 Colloidal silica - 0.3 / 2 34.94 78.39 443 4 - AP1 X7 - 0.3 / 0.4 40.46 78.80 422 5 - AP1 bentonite X7 0.3 / 2 / 0.4 57.70 82.80 471 6 - AP1 bentonite E8 0.3 / 2 / 0.4 60.5 0 83,00 479 7 - AP1 bentonite X9 0.3 / 2 / 0.4 59.71 79.91 473 8 - AP1 bentonite E1 0.3 / 2 / 0.4 66.06 84.18 492 9 - AP1 Colloidal silica E1 0.3 / 2 / 0.4 56.72 82.08 457 10 - AP1 X7 E1 0.3 / 0.4 / 0.4 65.41 84.25 463 11 Coagulant AP1 bentonite E1 1 / 0.3 / 2 / 0.4 85.49 91.45 611 12 - AP1 (PL + E1) added as a mixture (90/10) 0.3 / 0.4 85.89 91.54 550 13 Coagulant AP1 (PL + E1) added as a mixture (90/10) 0.3 / 0.4 90.23 94.12 577 PL: linear anionic copolymer (30 mole% sodium acrylate, 70 mole% acrylamide): UL viscosity: 8.2 (IV = 25).

The coagulant used in Table 1 is a polyaluminium chloride (dosage: 1 kg / T).

Comments of the results:

For a given main retention agent, the use in combination with a secondary retention agent of a tertiary retention agent of the invention improves overall and significantly the efficiency of the paper making process, in retention and in draining.

• l'association d'un agent tertiaire de l'invention avec de la bentonite (essai 8 / essais 2, 5, 6 et 7) permet d'obtenir des performances jusque là inégalées
• de même, l'association d'un agent tertiaire de l'invention avec une silice colloïdale (9 / 3) ou avec une microparticule organique telle que le Polyflex CP3 (10/4) aboutit à des niveaux de rétention et d'égouttage supérieurs à des systèmes comparables.

In particular, we note that:

• the combination of a tertiary agent of the invention with bentonite (test 8 / tests 2, 5, 6 and 7) makes it possible to obtain hitherto unequaled performances
• likewise, the combination of a tertiary agent of the invention with a colloidal silica (9/3) or with an organic microparticle such as Polyflex CP3 (10/4) results in higher retention and drainage levels to comparable systems.

Tests 5, 6, 7 and 8 demonstrate the essential role played by the criteria of size of dispersion and intrinsic viscosity. <b><u> Tab.2: Comparative study related to the nature of tertiary retention agents </ u></b> No. Senior Detention Officer Agent (s) of detention Respective dosage (kg / t) % FPAR % FPR CSF secondary tertiary 8 AP1 bentonite E1 0.3 / 2 / 0.4 66.06 84.18 492 14 AP1 bentonite E3 0.3 / 2 / 0.4 61.36 83.77 479 15 AP1 bentonite E4 0.3 / 2 / 0.4 62.83 83.89 485 16 AP1 bentonite E5 0.3 / 2 / 0.4 61.52 81.99 479 17 AP1 bentonite E6 0.3 / 2 / 0.4 63.02 83.97 487 18 AP1 bentonite X2 0.3 / 2 / 0.4 60.14 82,78 476 5 AP1 bentonite X7 0.3 / 2 / 0.4 57.70 82.80 471 6 AP1 bentonite E8 0.3 / 2 / 0.4 60.50 83,00 479 7 AP1 bentonite X9 0.3 / 2 / 0.4 59.71 79.91 473 19 AP1 bentonite X10 0.3 / 2 / 0.4 53.21 79.31 452

Comments of the results:

The tests demonstrate that the performance of the particles is not correlated with their size, which goes against the knowledge and expectations of those skilled in the art. Only the particles according to the invention, which combine both a size greater than 1 micron, a degree of crosslinking and an intrinsic viscosity of less than 3, make it possible to have a higher efficiency as regards the total retention and the retention of charges and to the draining. <b><u> Tab.3: Comparative study related to agent dosing </ u><u> secondary retention </ u></b> No. Senior Detention Officer Agent (s) of detention Respective dosage (kg / t) % FPAR % FPR CSF Frm secondary tertiary 0 White - 0.00 70.05 421 1 2 AP1 bentonite - 0.3 / 2 46.74 81.46 452 3 8 AP1 bentonite E1 0.3 / 2 / 0.4 66.06 84.18 492 3 20 AP1 bentonite E1 0.3 / 1 / 0.4 62.24 84.17 485 2 21 AP1 bentonite E1 0.3 / 2 / 0.2 58.94 82.47 473 3 22 AP1 bentonite E1 0.3 / 1 / 0.2 65.96 83.91 486 1

Comments of the results:

The use of an organic dispersion according to the invention as a tertiary retention agent makes it possible to reduce the dosage of the secondary retention agent without affecting the performance of the process.

In addition, and quite surprisingly, the formation of the sheet is not impaired and is in some cases even improved by the use of a tertiary agent of the invention, despite an increase in the performance of the invention. draining (which is most often done to the detriment of the formation of the leaf). <b><u> Tab.4: Comparative study related to the nature of the chief retention agent </ u></b> No. Senior Detention Officer Agent (s) of detention Respective dosage (kg / t) % FPAR % FPR CSF secondary tertiary 8 AP1 bentonite E1 0.3 / 2 / 0.4 66.06 84.18 492 23 AP2 bentonite E1 0.3 / 2 / 0.4 70,17 85.73 494 24 AP3 bentonite E1 0.3 / 2 / 0.4 65.10 84.03 489 25 AP4 bentonite E1 0.3 / 2 / 0.4 69.97 85.59 493 26 AP2 Colloidal silica E1 0.3 / 2 / 0.4 57.18 80.36 454 27 AP3 Colloidal silica E1 0.3 / 2 / 0.4 54.25 79.87 450 28 AP4 Colloidal silica E1 0.3 / 2 / 0.4 56.77 80,04 457 29 AP4 X7 E1 0.3 / 2 / 0.4 67.18 83.27 465 30 AP5 E1 bentonite 0.3 / 2 / 0.4 69.92 85.18 487

Comments of the results:

The organic particle according to the invention used as a tertiary retention agent is not significantly affected by the nature of the main retention agent (8, 21-23; 24-26; 27). The order of introduction of secondary and tertiary retention agents is also not a criterion of distinction (28).

In conclusion, despite a low intrinsic viscosity and a high particle size, greater than 1 micron, it is found that, in combination with another secondary retention agent, the combination according to the invention provides a net gain in charge retention and total retention and proves to be superior to pre-existing systems. <b><u> Tab.5: Comparative study related to the nature of the coagulant </ u></b> No. Coagulant Senior Detention Officer Agent (s) of detention Respective dosage (kg / t) % FPAR % FPR secondary tertiary 31 - AP1 bentonite E1 0.3 / 1 / 0.3 63.27 83.79 32 Polyamide* AP1 bentonite E1 0.4 / 0.3 / 1 / 0.3 85.04 91.50 33 PolyDADMAC # AP1 bentonite E1 0.4 / 0.3 / 1 / 0.3 85.51 90.52 34 Polyamine * + PAC (50/50) AP1 bentonite E1 0.4 / 0.3 / 1 / 0.3 86.91 90.66 35 PolyDADMAC # + PAC (50/50) AP1 bentonite E1 0.4 / 0.3 / 1 / 0.3 86.25 89.57 36 CAP AP1 bentonite E1 0.4 / 0.3 / 1 / 0.3 82.99 88.54 37 polyvinylamine AP1 bentonite E1 0.4 / 0.3 / 1 / 0.3 87.02 92.27 *: epichlorohydrin resin + dimethylamine
#: homopolymer of DADMAC

Comments of the results:

For a retention system of the given invention, the use of a coagulant significantly improves the efficiency of the papermaking process. <b><u> Tab.6: Comparative study related to the use of several secondary agents </ u></b> No. Senior Detention Officer Retention agents Respective dosage (kg / t) % FPA R % FPR Frm secondary tertiary 0 White - 0.00 70.05 175.9 38 AP1 bentonite E1 0.3 / 2 / 0.3 64.31 84,02 190.5 39 AP1 bentonite X7 0.3 / 2 / 0.3 53.12 81.67 193.2 40 AP1 bentonite X11 0.3 / 1 / 0.3 56.07 82,17 230.3 41 AP1 (Bentonite + PL) E1 0.3 / 2-0,27 / 0.03 86.39 91.01 209.2 42 AP1 bentonite (PL + E1) (80/20) 0.3 / 1 / 0.3 87.11 91.44 211.6 43 AP1 bentonite (PL + E1) (10/90) 0.3 / 1 / 0.3 75.23 86.45 196.7

• seule l'utilisation des polymères de l'invention permet d'obtenir à la fois des performances élevées aussi bien en rétention qu'en formation
• l'association des polymères réticulés de l'invention avec un deuxième agent secondaire de rétention de type polymère anionique linéaire, en plus de la bentonite, permet une amélioration notable de la rétention avec pour simple contrepartie une détérioration très limitée et acceptable industriellement de la formation de la feuille.

We observe that :

• only the use of the polymers of the invention makes it possible to obtain at the same time high performances both in retention and in formation
• the combination of the crosslinked polymers of the invention with a second linear anionic polymer-type secondary retention agent, in addition to bentonite, allows a noticeable improvement in the retention with, for a simple counterpart, a very limited and industrially acceptable deterioration of the formation. of the leaf.

This makes it possible to propose a multi-component retention system that is flexible and adjustable to the paper manufacturer's objectives (productivity performance and production quality).

It will be noted that the branched polymer (test 40) used as a tertiary agent has, as indisputably demonstrated in the preamble of this application, characteristics different from those of a crosslinked linear polymer + polymer mixture and leads to inferior results.

Claims (17)

1. Process for the manufacture of paper, board or similar products, which consist:

   - first of all in adding, to the fibrous suspension, at least one main retention aid composed of a cationic (co)polymer,

   - then optionally in shearing the floes obtained,

   - then adding to the suspension, separately or as a mixture, in any order:

   - at least one secondary retention aid chosen from the group comprising silica derivatives and anionic or amphoteric organic polymers,

   - at least one tertiary retention aid composed of a crosslinked anionic organic polymer with a size of greater than or equal to 1 micrometer,

   characterized in that the tertiary retention aid exhibits an intrinsic viscosity of less than 3 dl/g.
2. Process according to Claim 1, characterized in that a single secondary retention aid composed of one or more silica derivatives is added to the suspension.
3. Process according to Claim 1, characterized in that a single secondary retention aid composed of one or more anionic or amphoteric organic polymers which are different from the tertiary retention aid is added to the suspension.
4. Process according to Claim 1, characterized in that two secondary retention aids, respectively one or more silica derivatives and one or more anionic or amphoteric organic polymers which are different from the tertiary retention aid, are added to the suspension.
5. Process according either of Claims 3 and 4, characterized in that the anionic or amphoteric organic polymers are injected as a mixture with a tertiary retention aid.
6. Process according to Claim 1, characterized in that:

   - the tertiary retention aid is prepared in the form of a dispersion and is introduced into the paper pulp at a concentration of 30 to 1000 g/t by weight of active material of the polymer with respect to the dry weight of the paper pulp fibrous suspension, preferably of 30 to 600 g/t, and in that:

   - the secondary retention aid is:

   - a silica derivative chosen from the group comprising particles of silica including bentonites originating from hectorites, smectites, montmorillonites, nontronites, saponites, sauconites, hormites, attapulgites and sepiolites, derivatives of silicate, aluminosilicate or borosilicate type, zeolites, kaolinites, and modified or unmodified colloidal silicas, and is introduced in a proportion of 0.01 to 0.5 per cent (0.01 to 0.5%) by dry weight with respect to the dry weight of the fibrous suspension,

   - and/or an anionic or amphoteric organic polymer introduced in a proportion of 30 to 1000 g/t by weight of active material of the polymer with respect to the dry weight of the fibrous suspension, preferably of 30 to 600 g/t.
7. Process according to Claim 1, characterized in that the second secondary retention aid is a linear anionic organic polymer.
8. Process according to Claim 7, characterized in that the linear anionic organic polymer is a (co)polymer of at least one anionic unsaturated ethylenic monomer, chosen from the group comprising the monomers such as (meth)acrylic acid, acrylamidomethylpropanesulphonic acid, itaconic acid, maleic anhydride, maleic acid, vinylsulphonic acid and their salts, which exhibits a UL viscosity of greater than 2, preferably of greater than 4.
9. Process according to Claim 1, characterized in that the main retention aid is a cationic polymer based:

   - on at least one cationic unsaturated ethylenic monomer chosen from the group comprising monomers of the following types: dialkylaminoalkyl (meth)acrylate, dialkylaminoalkyl(meth)acrylamide, diallylamine, methyldiallylamine and their quaternary ammonium salts or acid salts,

   - and optionally on at least one nonionic monomer chosen from the group comprising acrylamide and/or methacrylamide and/or N-isopropylacrylamide and/or N,N-dimethylacrylamide and/or N-vinylformamide and/or N-vinylacetamide and/or N-vinylpyrrolidone, and/or on at least one anionic monomer chosen from the group comprising (meth)acrylic acid, acrylamidomethylpropanesulphonic acid, itaconic acid, maleic anhydride, maleic acid, vinylsulphonic acid and their salts;

   - optionally on at least one hydrophobic acrylic, allylic or vinyl monomer chosen from the group comprising acrylamide derivatives, such as N-alkylacrylamides, for example N-(tert-butyl)acrylamide or octylacrylamide, and N,N-dialkyl-acrylamides, such as N,N-dihexylacrylamide, and/or acrylic acid derivatives, such as alkyl acrylates and methacrylates,

   - and optionally on a branching/crosslinking aid.
10. Process according to Claim 1, characterized in that the main retention aid has an IV of greater than 2 dl/g.
11. Process according to Claim 1, characterized in that the tertiary retention aid is a polymer based:

    - on at least one anionic unsaturated ethylenic monomer chosen from the group comprising monomers having a carboxyl functional group and their salts, including acrylic acid or methacrylic acid, and/or monomers having a sulfonic acid functional group and their salts, including 2-acrylamido-2-methylpropanesulphonic acid,

    - optionally on at least one nonionic monomer chosen from the group comprising acrylamide and/or methacrylamide and/or N-isopropylacrylamide and/or N,N-dimethylacrylamide and/or N-vinylformamide and/or N-vinylacetamide and/or N-vinylpyrrolidone,

    - and on a crosslinking aid.
12. Process according to either one of Claims 9 and 11, characterized in that the branching/crosslinking aid is chosen from the group comprising methylenebisacrylamide (MBA), ethylene glycol diacrylate, polyethylene glycol dimethacrylate, diacrylamide, cyanomethyl acrylate, vinyloxyethyl acrylate or methacrylate, triallylamine, formaldehyde, glyoxal, compounds of glycidyl ether type, such as ethylene glycol diglycidyl ether, or epoxy compounds.
13. Process according to Claim 11, characterized in that the branching/crosslinking aid chosen is MBA and is introduced at a concentration of greater than or equal to 5 ppm (parts per million) by weight of monomers and preferably of greater than or equal to 15 ppm.
14. Process according to Claim 1, characterized in that the secondary retention aid is a semi-sodium bentonite used in a proportion of 0.1 to 0.5% by dry weight with respect to the dry weight of the fibrous suspension.
15. Process according to Claim 1, characterized in that the amount of main retention aid introduced is between 0.003 and 0.3% by weight of active material of the polymer with respect to the dry pulp, preferably between 0.01 and 0.05%.
16. Process according to Claim 1, characterized in that a coagulant is added to the suspension prior to the addition of the main retention aid.
17. Process according to Claim 16, characterized in that the coagulant is chosen from the group comprising polyaluminium chloride (PAC), aluminium sulphate, polyaluminium chlorosulphate..., and organic coagulants, including:

    - polymers based on diallyldimethylammonium chloride (DADMAC),

    - quaternary polyamines manufactured by condensation of a primary or secondary amine with epichlorohydrin, polymers exhibiting functional groups of vinylamine type or resins of dicyandiamide type.