Novel additives for improving the wet strength and dry strength of paper

Abstract

The invention relates to a process for the treatment of paper consisting of the application to the paper either of a cationic resin, PAE, and of a latex containing acid functional groups, the resin and the latex being applied simultaneously but separately, or of a stable mixture prepared beforehand containing the PAE and a latex stabilized by a nonionic surfactant.

Description

[0001] The invention relates to the manufacture of paper and in particular to the manufacture of paper having good wet strength.

[0002] For some applications, in particular absorbent papers, wallpapers, paper for bottle labels and currency paper, the paper has to. retain, in the wet state, some of its dry strength. The wet strength of the paper, denoted hereafter by WS, generally expressed in %, is calculated from the equation 1 WS = BL wet BL dry × 100

[0003] where

[0004] BLwet is the breaking length of a wet strip of paper,

[0005] Bldry is the breaking length of a dry strip of paper.

[0006] In general, a paper exhibits wet strength as soon as its WS exceeds 5%.

[0007] Those skilled in the art have known for a long time how to improve the WS of paper. In general, they incorporate, into the mass of cellulosic pulp (also called fibrous suspension or paper pulp), resins able to react with the cellulose fibers, such as cationic resins that can spontaneously crosslink onto the fiber at neutral pH, such as, for example, resins of the polyamidopolyamine-epichlorohydrin type, denoted hereafter by PAE, of low molecular weight, which are well known to those skilled in the art.

[0008] However, it is known that the improvement in WS due to PAE reaches saturation with the level of PAE introduced into the pulp, since the amount of PAE that can be adsorbed on the cellulose fibers is limited. This is all the more so the lower the surface density of negative charges that the fibers have, as is the case, for example, with cotton pulps. Thus, it is difficult to reach the 20 to 25% optimum WS values obtained in a chemical pulp, whereas in certain specific applications a WS of the order of 40% is required.

[0009] It has therefore been attempted to improve the wet strength by combining the PAE resin with various substances called wet-strength promoters, which are generally polymers of natural origin or derived from natural polymers, having a relatively pronounced anionic or amphoteric character which includes, among others, sodium carboxymethylcellulose (H. Espy, 1983 Papermakers Research J., pp. 191-195 and E. Strazdins 1994, Wet Strength Resins and their applications, Ed by L. Chan, TAPPI Press pp. 78-79) and modified guar gum (patents CA 808,531 and U.S. Pat. No. 5,318,669). These products are sold in pulverant form. Putting them into aqueous solution is a lengthy and tricky operation, which has to be carried out prior to their use in the wet end of the paper machine. These solutions are very sensitive to bacterial degradation and do not keep under the ambient temperature conditions in production workshops.

[0010] U.S. Pat. No. 5,200,036 proposes a solution based on the use of a PAE modified by reaction with the acid functional group carried by a radical polymerizable monomer. The free double bond of the monomer is then copolymerized with a monomer mixture in order to form a crosslinked monocompound. This solution may not be effected since the PAE crosslinks at neutral pH and at room temperature and even more quickly the higher the temperature, see for example the book “Applications of Wet End Paper Chemistry” by O. Au and I. Thorn, Ed. Blackie Academic and Professional. It therefore seems reasonable that the PAE crosslinks during the copolymerization and is therefore no longer completely reactive for reacting with the cellulose fibers.

[0011] The Applicant has now discovered that it is possible to improve both the wet strength and the dry strength of paper by virtue either of a special paper treatment process using a cationic resin and a wet-strength promoter based on an aqueous dispersion of a polymer containing acid functional groups or of a special stable composition based on a cationic resin and on a wet-strength promoter based on an aqueous dispersion of a polymer containing acid functional groups and stabilized by a nonionic surfactant.

[0012] The aqueous dispersion in the context of the invention is often denoted by the term “latex”.

[0013] One of the subjects of the invention is a process for the treatment of paper consisting of the application to the paper of a cationic resin and of an aqueous dispersion of particles of a thermoplastic polymer having a diameter ranging from 30 to 500 nm, these particles being stabilized by a polymeric or nonpolymeric surfactant.

[0014] The aqueous dispersion of the invention contains:

[0015] from 0.5 to 10% by weight with respect to the total weight of the particles of units obtained by the polymerization of at least one monomer A carrying at least one acid functional group; and

[0016] from 90 to 99.5% by weight with respect to the total weight of the particles of units obtained by the polymerization of at least one monomer B copolymerizable with A and chosen from the group consisting of vinyl, styrene, (meth)acrylic and diene monomers.

[0017] The polymer concentration of the aqueous dispersion has no effect on the process.

[0018] The cationic resins used for implementing the invention are low-molecular-weight resins having an azetidinium structure which are crosslinkable at neutral pH on the cellulose fiber. Resins of polyamido-polyamine-epichlorohydrin (PAE) structure constitute a typical example of the cationic resins and are, moreover, commonly employed by those skilled in the art. These resins (PAE) are obtained by the condensation of adipic acid with diethylenetriamine, followed by a condensation on epichlorohydrin.

[0019] The monomers (A) having acid functional groups may belong, partially or completely, either to the polymeric surfactant or to the dispersed thermoplastic polymer.

[0020] When it is desired to incorporate all the acid functional groups into the polymeric surfactant, the aqueous dispersion is prepared by the radical emulsion polymerization, within the solution of surfactant which is an anionic or amphoteric polymer, of a hydrophobic monomer or of an overall hydrophobic mixture of monomers, the composition of which may be adjusted in order to obtain a polymer with a glass transition temperature Tg chosen in advance. As explained below, these anionic or amphoteric polymers may be the natural polymers mentioned above; they may also be polymers or copolymers having, on the one hand, a monomer chosen from the family of acrylic acid, methacrylic acid and maleic anhydride and, on the other hand, a monomer chosen from the family of styrene, vinyl or acrylic or methacrylic ester monomers, for example and nonlimitingly, a styrene/maleic anhydride copolymer, a styrene/acrylic acid copolymer, a methyl methacrylate/acrylic acid copolymer or a styrene/butyl acrylate copolymer.

[0021] On the other hand, when all the acid functional groups are carried by the dispersed polymer, the aqueous dispersion is prepared by the radical emulsion polymerization, in the presence of at least one surfactant, of a monomer mixture containing:

[0022] from 0.5 to 10% by weight of at least one monomer A containing acid functional groups chosen from the group consisting of unsaturated monocarboxylic or &agr;, &bgr;-dicarboxylic acids, such as acrylic acid, methacrylic acid, itaconic acid, maleic acid or derivatives thereof;

[0023] from 90 to 99.5% by weight of at least one monomer B chosen from the group consisting of vinyl, styrene, C1-C8 (meth)acrylic ester and diene monomers.

[0024] According to a preferred form of the invention, the monomer A represents from 0.5 to 5% by weight.

[0025] The preferred monomer A of the invention is a acrylic acid or methacrylic acid.

[0026] The monomer mixture represents in practice from 5 to 60% by weight of the aqueous solution, however, as described above, the monomer concentration has no effect on the paper treatment process.

[0027] The monomer(s) A and the monomer(s) B are chosen and their respective amounts defined according to the properties and the nature that it is desired to give the intended polymer. For example, the glass transition (Tg) of a polymer may be estimated in advance by the following law: 2 1 Tg i = ∑ i ⁢ Tg i ∑ i ⁢ w i ⁢ Tg i

[0028] Tgi being the glass transition temperature of the homopolymer obtained by polymerization of the monomer i and

[0029] wi being the weight fraction of the monomer i.

[0030] Thus, according to the intended application, the Tg of the polymer is fixed and the monomers (i) chosen accordingly.

[0031] The surfactant(s) is (are) chosen from the group consisting:

[0032] of ionic or nonionic micromolecular surfactants. The ionic surfactants may be anionic, cationic or amphoteric. Usually anionic surfactants are used, such as sodium dodecylbenzene sulfonate or ethoxylated fatty alcohol sulfates, etc. The nonionic surfactants are chosen from the family of ethoxylated alkyl phenols or that of ethoxylated fatty alcohols;

[0033] of polymeric surfactants, such as copolymers having, on the one hand, a monomer chosen from the family of acrylic acid, methacrylic acid or maleic anhydride and, on the other hand, a monomer chosen from the family of styrene, vinyl or acrylic or methacrylic ester monomers, for example and nonlimitingly, a styrene/maleic anhydride copolymer, a styrene/acrylic acid copolymer, a methyl methacrylate/acrylic acid copolymer or a styrene/butyl acrylate copolymer.

[0034] Preferred forms of these latices have been disclosed in French Patent Applications FR-A-96/08226 and 96/08875 (Elf Atochem S.A.).

[0035] The invention is implemented by introducing the cationic resin and the latex into the aqueous suspension of cellulose fibers, said suspension often being called a paper pulp. The resin and the latex may be introduced successively in no particular order, or else at the same time, thereby considerably simplifying the plant.

[0036] The amounts used according to the invention are, with regard to the resin, from 0.25 to 3% by weight of resin dry matter with respect to the weight of dry fibers and from 0.25 to 3% with regard to the latex.

[0037] It is preferable according to the invention to maintain a latex dry matter/resin dry matter ratio of 0.5 to 2.

[0038] Thus, wet strengths, expressed as WS, of 25 to 40% are obtained. The dry strength of the papers obtained by this process is also increased by 10 to 40% over the untreated paper.

[0039] To simplify the paper treatment processes like those described above, attempts have been made to reduce the number of operations by effecting a single introduction of a mixture containing the PAE and a latex into the paper machine. However, such a mixture cannot be envisaged on an industrial scale since the PAE is preserved in an acid medium and the latex is not stable in an acid medium.

[0040] The Applicant has also discovered that it is possible, under certain conditions, to prepare a stable and ready-to-use mixture containing the PAE and a latex. This is because the Applicant has discovered that, when the latex is stabilized by a nonionic surfactant or by a predominantly nonionic mixture consisting of ionic surfactants and nonionic surfactants, it is possible to mix it with the cationic resin without the composition obtained changing or becoming unstable during storage.

[0041] Another subject of the invention is an aqueous composition for improving the wet strength and dry strength of paper, containing:

[0042] from 5 to 20% by weight of a cationic resin as defined above and in particular PAE and

[0043] from 5 to 40% by weight of a thermoplastic polymer dispersed in the form of particles having a diameter of between 30 and 500 nm,

[0044] wherein said polymer contains acid functional groups and wherein the particles are generally stabilized by 0.1 to 5% by weight of at least one cationic or amphoteric nonionic surfactant or else or a mixture of ionic and nonionic surfactants.

[0045] The cationic resins used for implementing the invention are low-molecular-weight resins having an azetidinium structure which are crosslinkable at neutral pH on the cellulose fiber. The resins of polyamido-polyamine-epichlorohydrin structure constitute a typical example of the cationic resins and are, moreover, commonly used by those skilled in the art. These resins (PAE) are obtained by the condensation of adipic acid with diethylenetriamine, followed by a condensation on epichlorohydrin.

[0046] The aqueous dispersions, or latices, consist of a dispersion of particles having a diameter of between 30 and 500 nm of thermoplastic polymers containing acid functional groups and stabilized by a macromolecular or non-macromolecular surfactant. These latices are obtained by the radical emulsion polymerization, in the presence of at least one ionic or nonionic surfactant or else of a predominantly nonionic mixture of surfactants, of a monomer mixture containing:

[0047] from 0.5 to 10%, and preferably from 0.5 to 5%, by weight of at least one monomer A containing acid functional groups, chosen from the group consisting of unsaturated monocarboxylic or &agr;, &bgr;-dicarboxylic acids, such as acrylic acid, methacrylic acid, itaconic acid, maleic acid and derivatives thereof;

[0048] from 90 to 99.5%, and preferably from 95 to 99.5%, by weight of at least one monomer B chosen from the group consisting of vinyl, styrene, C1-C8 (meth)acrylic ester and diene monomers.

[0049] The preferred monomer A of the invention is acrylic acid.

[0050] The monomer mixture generally represents from 5 to 60% by weight of the aqueous solution, however the monomer concentration has no effect on the stability of the composition nor on the paper treatment process employing such a composition.

[0051] The monomer(s) A and the monomer(s) B are chosen and their respective amounts defined according to the properties and nature that it is desired to give the end polymer.

[0052] The nonionic surfactant is chosen from the group consisting of ethoxylated alkyl phenols, such as ethoxylated nonyl phenol and/or ethoxylated fatty alcohols.

[0053] The compositions described above are stable over time. They may be stored and used as they are, in a process for improving the wet strength and dry strength of paper.

[0054] One of the subjects of the invention is a process for improving the wet strength and dry strength of paper consisting of the introduction of the stable aqueous composition described above into the aqueous suspension of cellulose fibers.

[0055] In order to implement the invention, only the additive dry matter/fiber dry matter ratio counts. The concentration of the fibrous suspension is not known in advance, and whatever the initial solids content of the mixture those skilled in the art will know how to dilute the latter so as to introduce the appropriate amount thereof into the pulp.

[0056] The following examples illustrate the invention without limiting its scope:

EXAMPLES

[0057] a—preparation of a latex

[0058] Introduced into a glass reactor, heated by a jacket and stirred using a mechanical stirrer, are 438 g of water and 3.3 g of sodium dodecylbenzene sulfonate. This mixture is heated to 82° C. and 5% of the pre-emulsion and 10% of the catalytic solution as a batch are introduced.

[0059] After 30 minutes of reaction, the remainder of the pre-emulsion and the remainder of the catalytic solution are added over 4 hours using metering pumps so as to maintain a constant flow rate. The reaction mixture is then left for 1 hour at 82° C. for the purpose of reducing the content of residual monomers.

[0060] Catalytic solution

[0061] 68.20 g of water

[0062] 3.30 g of sodium persulfate

[0063] Pre-emulsion

[0064] 554 g of water

[0065] 13.2 g of sodium dodecylbenzene sulfonate

[0066] 641.3 g of butyl acrylate

[0067] 425.7 g of methyl methacrylate

[0068] 33 g of methacrylic acid.

[0069] Under these conditions, a dispersion (DAF 25) is obtained which has the following physico-chemical characteristics:

[0070] S.C. (solids content) =49.2 %

[0071] pH =2.6

[0072] Particle size =105 nm.

[0073] The dispersion DAF 36 is obtained under similar conditions by substituting the dodecylbenzene sulfonate with a C12-C14 fatty alcohol ethoxylated to 30 mol of ethylene oxide in amounts sufficient to obtain a stable dispersion without a coagulator. The characteristics of this dispersion are the following:

[0074] S.C. =50.6%

[0075] pH =2.3

[0076] Particle size =350 nm.

[0077] b—treatment of the paper

[0078] In these examples, the papers are obtained using the general process which consists of producing handsheets on a FRANCK apparatus using a refined pulp having a certain degree Schopper (standing for the degree Schopper-Riegler or °SR, see the NF Q 50-003 standard, determination of the drainability, Schopper-Riegler method).

[0079] Added with stirring to the fibrous suspension consisting of 10 g/l of fibers in water, the pH of which is, if required, adjusted to 8.0 with dilute sulfuric acid or with sodium hydroxide, is the composition of wet-strength agents or according to a first mode, the latex, followed approximately one minute later by the PAE (the order of introduction is of no consequence: the order in which the PAE and the latex are introduced may be reversed, or they may be introduced simultaneously), or, according to a second mode, the stable aqueous composition containing the latex and the PAE. Whatever mode is used, the stirring is continued for approximately three minutes. The handsheet, with a mean grammage of about 65 g/m2 is then produced, by draining the suspension on a metal mesh, the handsheet being dewatered and dried for 5 minutes at 95° C. The PAE resin is crosslinked by putting the handsheets in an oven at a temperature of 105° C. for 7 minutes.

[0080] Two strips (test samples) of paper 180 mm in length and 15 mm in width were cut from each handsheet. The first serves for determining the strength of the dry paper. The tests, employed are tensile strength tests carried out in accordance with the NF Q 03-004 standard. The tensile tests are carried out on an ADAMEL Lhomargy apparatus, set at a speed of 50 mm/min. The force F expressed in newtons that had to be applied in order to break the strip allows the breaking length to be evaluated, this being calculated in meters from the equation: BL (m)=1/9.81×F×width-1×grammage-1, in which the width is taken in meters and the grammage in kilograms per square meter.

[0081] The displayed BL measurements on which the WS value is calculated are averages of five tensile tests carried out on five test specimens, each coming from these five separate handsheets.

[0082] The tensile tests on sheets of dry paper are carried out after a minimum of 24 hours conditioning at 23° C. and 50% humidity. The tensile tests on strips of wet paper are carried out according to the NF Q 03-056 standard on strips which, unless indicated otherwise, were immersed for 1 hour in town water at a constant temperature of 25° C. dewatered and then tested according to a strict procedure described in the standard.

[0083] The water absorbency of the paper is estimated according to the Cobb test, TAPPI T441-OM90 standard, which consists in measuring the amount of water absorbed by the paper over a period of 60 seconds. The result, called Cobb60, is expressed in grams of water per m2 of paper.

[0084] The PAE resin used is a resin with a 14% solids content, stabilized at 2.5-3.5 pH (CECA, R4947D).

[0085] The latices used are:

[0086] DAF 25; this is a dispersion with a 50.9% solids content, based on a butyl acrylate (BuA)/methyl methacrylate (MMA) /methacrylic acid terpolymer of 58.3/38.7/3 composition with a glass transition of 10° C., stabilized by an anionic dispersant (dodecylbenzene sulfonate).

[0087] DAF 36; this is a dispersion with a 50.6% solids content, based on a BuA/MMA/AA terpolymer of 58.3/38.7/3 composition having a Tg of 10° C., stabilized by a nonionic dispersant (C12-C14 fatty alcohol ethoxylated to 30 mol of ethylene oxide on average).

[0088] The latices (Starcote®) experimented on here, and indicated as latex A, latex B and latex C, are latices whose polymeric dispersant is a water-soluble styrene/maleic anhydride copolymer commercially available under the name SMA® Resins (ELF ATOCHEM N.A./ELF ATOCHEM S.A.). These resins are resins with a low molecular mass of between 500 and 10,000 and having an acid number at the very most equal to 500. SMA 3000 is a resin with a styrene/maleic anhydride molar ratio of 3; SMA 2625 is a resin with a styrene/maleic anhydride molar ratio of 2 and is esterified with propanol or any other suitable alcohol mixture, to a degree of esterification of between 75 and 100% of the semiester. By way of comparison, a latex D was also prepared, this being an ordinary anionic latex in which the dispersant is a surfactant, namely sodium dodecyl sulfate (SDS). The table below gives the compositions of these latices. The components in the table are expressed as dry matter. The solids content of these latices is around 25%. 1 Hydrophobic Specific monomer Polymeric latex Butyl dispersant Surfactant composition Styrene acrylate SMA 3000 SMA 2625 SDS Tg TMF latex A 46 54 40 25° C. 55° C. latex B 84 16 40 80° C. latex C 46 54 40 26° C. 21° C. latex D 46 54 4 25° C.

Example 1

[0089] Composite Latex/PAE Combination on Wood Fibers

[0090] The example illustrates the limit that may be expected from a treatment by a PAE resin alone, according to the prior art, of a chemical cellulosic pulp consisting of chemically bleached conifer fibers (from Cellulose du Rhone d'Aquitaine) and refined to 25° SR. The wet-strength treatment according to the prior art consisted in the bulk addition of PAE resin in various proportions up to 2.2% (by weight of dry product assayed with respect to the dry fibers). The table below gives wet-strength values as a function of the PAE content. 2 PAE content Wet breaking length (% by weight) (m) 0  516 0.42 1252 0.63 1480 0.84 1642 1.00 1720 1.26 1837 1.68 1851 2.00 1940 2.20 2005

[0091] The saturation effect may clearly be seen in this table above 1-1.5% of PAE.

Example 2

[0092] The pulp of Example 1 was used and successively treated with the PAE resin, by the mixed latex A and by a combination of the latex A and PAE. The combination of the PAE with the standard latex D is given as a counter-example; the PAE and latex contents used are both expressed as the amount of dry matter with respect to the fibers. 3   Additives added to the Handsheet Handsheet  WS Cobb60   fibers BLdry BLwet  (%) (g/m2)   Control handsheet 5960 m 177 m   3%   1% PAE 6110 m 1720 m  28.2  98   2% PAE 6390 m 1940 m  30.4  96   1% latex A 5950 m 157 m   2.6 136   1% latex A + 1% PAE 8027 m 2505 m  31.2  28   1% latex B + 1% PAE 7757 m 2304 m  29.7  95   1% latex C + 1% PAE 8367 m 2620 m  31.2  22   1% latex D + 1% PAE 7176 m 1528 m  21.3  38

[0093] It may be seen that the latex by itself has no appreciable effect on the WS. Although the combination of an ordinary anionic latex with a PAE increases the WS, the improvement is, however, limited. The result of the mixed latex/PAE treatment gives quite a different level than that which would be due to simply juxtaposing the effects of the components of the mixed latex or to that of the PAE alone. It should be noted, in passing, that the dry strength of the paper obtained is substantially increased.

[0094] In the light of these results, it will be understood that it is just as easy, by the choice of mixed latex, and more specifically by that of its film-forming temperature, and according to the process of the invention, to obtain papers which are highly water resistant and exhibit no hydrophobicity, such as absorbent papers (film-forming temperature at least equal to 60° C.) as to produce papers which are highly water resistant and have a Cobb60 of about 20 g/m2, and which are therefore easily printable, such as papers for labels and posters, or wallpapers (polymer of the latex with film formation at room temperature).

Example 3

[0095] mixed latex/PAE combination on cotton fibers

[0096] The example illustrates the limit which is reached in an ordinary treatment by a PAE resin alone, according to the prior art, of a chemical cellulosic pulp consisting of chemically bleached plant fibers refined to a high degree Schopper and the possibility provided by the treatment of the invention to exceed it, a result that cannot be obtained with the means of the prior art. In the case of the present example, the fibers are cotton fibers refined to 60° SR. The wet-strength treatment according to the prior art consisted in adding, to the bulk, PAE resin with various proportions up to 3.5% (PAE assayed as solid product with respect to the dry fibers). The amount of mixed latex added is expressed as amount of dry matter with respect to the dry fibers. The table shows the variation in the breaking lengths as a function of the PAE content. 4   Additives added to the Handsheet Handsheet WS   fibers BLdry BLwet (%) Cobb60   0.8% PAE 3920 m 720 m 18.4   1.7% PAE 3935 m 950 m 24.1 85   3.5% PAE 4015 m 1055 m 26.3   2% latex A + 3.5% PAE 4673 m 2010 m 43   1% latex B + 1.7% PAE 4405 m 1650 m 37.2 64   1% latex C + 1.7% PAE 4890 m 1819 m 39.7 20

[0097] It will have been noted that the mixed latex/PAE combination makes it possible to substantially increase the WS values at the same time as it makes it possible to obtain very low Cobb values.

Example 4

[0098] In this example, the fibrous suspension was treated only with 1 % PAE.

Example 5

[0099] In this example, the fibrous suspension was treated successively with 1% PAE and with 1% latex DAF 25.

Example 6

[0100] For this example, a stable composition, of PAE and latex DAF 36, was firstly produced. The amount of each compound in the dry matter is 50%. Secondly, the fibrous suspension was treated with 2% of the stable composition.

[0101] The results obtained are summarized in the following table: 5 BLwet WS BLdry Example Cationic resin Latex (m) (%) (m) 4 1% PAE — 1810 23 7930 5 1% PAE 1% DAF 25 2830 36 8380 6 (*) 1% PAE 1% DAF 36 2550 32 10080 (*) In this example, the latex/PAE mixture was produced prior to the treatment of the fibrous suspension.

Claims

1. A process for the treatment of paper consisting of the application to the paper of a cationic resin and of an aqueous dispersion of a polymer dispersed in the form of particles having a diameter ranging from 30 to 500 nm, these particles being stabilized by a polymeric or nonpolymeric surfactant, wherein said dispersion contains:

from 0.5 to 10% by weight with respect to the total weight of the particles of units derived by the polymerization of at least one monomer (A) carrying at least one acid functional group; and

from 90 to 99.5% by weight with respect to the total weight of the particles of units derived from the polymerization of at least one monomer (B) which is emulsion polymerizable, and copolymerizable with A, such as vinyl, meth(acrylic), styrene and diene monomers, the monomers (A) being carried, partially or completely, either by the polymeric surfactant or by the dispersed polymer.

2. The process as claimed in claim 1, wherein the dispersion contains from 0.5 to 5% by weight of units coming from the monomer (A).

3. The process as claimed in claim 1 or 2, wherein (A) is chosen from the group containing unsaturated monocarboxylic or &agr;, &bgr;-dicarboxylic acids, such as acrylic acid, methacrylic acid, maleic acid or derivatives thereof, such as maleic anhydride.

4. The process as claimed in claim 3, wherein A is acrylic acid.

5. The process as claimed in claim 3, wherein A is maleic anhydride.

6. The process as claimed in any one of the preceding claims, wherein the surfactant is chosen from the group consisting of:

ionic surfactants, such as sodium dodecylbenzene sulfonate or ethoxylated fatty alcohol sulfates;

nonionic surfactants, such as ethoxylated fatty alcohols; and

polymeric surfactants, such as polymers having, on the one hand, a monomer A and, on the other hand, a monomer chosen from the family of styrene, vinyl or (meth)acrylic ester monomers, or modified natural polymers, such as oxidized starch.

7. The process as claimed in claim 6, wherein the surfactant is a styrene/maleic anhydride copolymer.

8. The process as claimed in claim 6, wherein the surfactant is a methyl methacrylate/acrylic acid copolymer.

9. The process as claimed in claim 6, wherein the surfactant is a styrene/acrylic acid copolymer.

10. The process as claimed in claim 1, wherein the surfactant is oxidized starch.

11. The process as claimed in one of the preceding claims, in which the dispersed polymers of the aqueous composition come from the radical emulsion polymerization, within the surfactant solution, of an overall hydrophobic mixture of monomers, the composition of which may be adjusted so as to obtain a polymer having one or more glass transition temperatures Tg of between −20° C. and +100° C.

12. The process as claimed in claim 11, wherein the monomers to be polymerized are chosen from vinyl, styrene, (meth)acrylic and diene monomers, and unsaturated &agr;, &bgr;-dicarboxylic acids and derivatives thereof.

13. The process as claimed in claim 1, in which the useful amounts of wet-strength agents, expressed by weight of dry matter with respect to the mass of dry fibers, are from 0.1 to 5%, preferably from 0.5 to 3%, of aqueous dispersion and from 0.1 to 5%, preferably from 0.5 to 4%, of PAE resin, respectively.

14. An aqueous composition containing:

from 5 to 15% by weight of a cationic resin such as PAE and

from 5 to 15% by weight of a thermoplastic polymer dispersed in the form of particles having a diameter of between 30 and 500 nm wherein said polymer contains from 0.5 to 10% by weight of units derived from the polymerization of least one monomer (A) containing an acid functional group and wherein the particles are stabilized by a nonionic surfactant or by a mixture of ionic and nonionic surfactants.

15. The composition as claimed in claim 14, wherein the monomer A is chosen from the group containing unsaturated monocarboxylic or &agr;, &bgr;-dicarboxylic acids, such as acrylic acid, methacrylic acid, maleic acid, maleic acid and derivatives thereof, such as maleic anhydride.

16. The composition as claimed in claim 15, wherein the monomer A is acrylic acid.

17. The composition as claimed in claim 15, wherein the monomer A is maleic anhydride.

18. The composition as claimed in one of claims 14 to 17, wherein the nonionic surfactant is chosen from the group consisting of ethoxylated nonyl phenols and ethoxylated fatty alcohols.

19. The composition as claimed in one of claims 14 to 18, wherein the thermoplastic polymer is obtained by the emulsion polymerization, within the surfactant solution, of a monomer mixture containing:

from 0.5 to 10% of at least one monomer A and

from 99.5 to 90% of at least one monomer B, the composition of the mixture being defined so as to obtain a thermoplastic polymer having one or more glass transition temperatures (Tg) of between −20° C. and 100° C.

20. A process for the treatment of paper consisting of the application to the paper of the composition of claims 14 to 19, in which the useful amounts of the wet-strength agents, expressed by weight of dry matter with respect to the mass of dry fibers, is from 0.1 to 10% and preferably from 0.2 to 5%.

21. A process for manufacturing paper having wet strength, in which a PAE resin and a promoter, consisting of a latex based on a very fine dispersion of styrene/butyl acrylate thermoplastic copolymers in an aqueous solution of a styrene/maleic anhydride copolymer, are introduced into a chemical cellulosic pulp consisting of plant fibers refined to a high degree Schopper-Riegler, especially cotton fibers, successively or simultaneously.

22. A process for the manufacture of hydrophobic papers having wet strength, in which a PAE resin and a promoter consisting of a latex based on a very fine dispersion of styrene/butyl acrylate thermoplastic copolymers in an aqueous solution of a styrene/maleic anhydride copolymer, this latex having a minimum film-forming temperature of between 0 and +60° C. and having a glass transition temperature of between −20 and +60° C., are introduced into the cellulosic pulp, successively or simultaneously.

23. The process for manufacturing paper having wet strength, in which a PAE resin and a promoter consisting of a very fine dispersion of thermoplastic polymers of a butyl acrylate/methyl methacrylate/methacrylic acid terpolymer of 58.3/38.7/3 composition are introduced into a chemical cellulosic pulp consisting of plant fibers refined to a high degree Schopper-Riegler, especially cotton fibers, successively or simultaneously.

24. The process for manufacturing hydrophobic papers having wet strength, in which a PAE resin and a promoter, consisting of a very fine dispersion of thermoplastic polymers having a minimum film-forming temperature of between 0 and +60° C. and having a glass transition temperature of between −20 and +60° C, are introduced into the cellulosic pulp, successively or simultaneously.

25. The process for manufacturing hydrophobic papers having wet strength, in which a composition, the dry matter of which is composed of 50% of PAE and 50% of a butyl acrylate/methyl methacrylate/acrylic acid terpolymer of 58.3/38.7/3 composition, is introduced into the cellulosic pulp.