WO2011042227A1

WO2011042227A1 - Method for producing wet-strengthened papers

Info

Publication number: WO2011042227A1
Application number: PCT/EP2010/059759
Authority: WO
Inventors: Alexander Erdtmann
Original assignee: Weiser Chemie + Technik Ug
Priority date: 2009-10-09
Filing date: 2010-07-07
Publication date: 2011-04-14
Also published as: DE102009044228A1; EP2486186A1; DE102009044228B4; EP2486186B1

Abstract

The present invention relates to a method for producing wet-strength papers by adding to a cellulose fibre slurry not only a conventional wet-strengthener having a charge density of less than 6 meq/g polymer, but also at least one further wet-strengthener having a high charge density of at least 5 meq/g polymer, for the wet-strength-compatible fixing of extraneous substances to the cellulose fibres. This wet-strengthener having a high charge density of at least 5 meq/g polymer carries one or more functional groups which, in the course of drying, form covalent bonds preferably with the reactive hydroxyl groups of the cellulose and/or with the extraneous substances.

Description

Process for the production of wet-strength papers

The present invention relates to a process for producing wet-strength papers, in which a cellulose pulp in addition to a conventional wet strength agent having a charge density of below 6 meq / g polymer, at least one further wet strength agent with a high charge density of at least 5 meq / g polymer for wet-strength fixing of impurities is added to the cellulose fibers. This wet strength agent with a high charge density of at least 5 meq / g polymer carries one or more functional groups which undergo covalent bonds during drying, preferably with the reactive hydroxyl groups of the cellulose and / or the impurities.

Paper is mainly used for printing and writing. Papermakers are often distorted, kinked, and compressed during manufacture and processing on the paper machine during painting, printing, folding, etc., so that they are exposed to forces in both the x and y or z directions. Thus, certain strengths are required so that it does not come to tears, plucking or dusting or holes. In addition to the purely physical wickerwork effect, the forces connecting the individual fibers (hydrogen bonds, van der Waals forces, covalent bonds and ionic interactions) play an important role in the strength of paper. The composition of the quantitatively most important papers (graphic papers) with high levels of Deinkingstoff, wood pulp and fillers leads to a reduction in the strength of the paper by low single fiber strength (with damage to the fiber wall) or poor fiber-fiber bonding (due to lack of hydrophilicity at a high Lignin content, low bond area at too high filler content or lack of plasticity due to the fiber morphology). For these reasons, it is practically essential to add strength enhancing additives.

In order to increase the strength of the dry paper, dry strength agents are frequently used, which are hydrophilic, water-based soluble, high molecular weight substances, such as unmodified or slightly modified starch (preferably from corn, wheat, potatoes), plant gums (such as guar gum or alginates), carboxymethyl cellulose (CMC) or synthetic polymers. These agents enhance hydrogen bonding or increase the number of bonds between the fibers at their crossing points. The classic dry strength agent, the starch, is characterized by its favorable price and is preferably applied by means of a glue film press or also added to the composition. Above all, surface application improves primary surface properties such as pick resistance, abrasion resistance, density and smoothness. The addition of starch to the mass increases the tensile strengths such as breaking strength, bursting strength and folding strength. Synthetic dry solids can be tailored to the specific application and, because of their high specific potency, they need not be dosed as high. Dry stabilizers which have proven to be especially different long-chain polyacrylamides, which differ in their degree of polymerization, as well as other copolymers (for example, with amines or acids). Polyacrylamides are able to form additional hydrogen bonds with the cellulose, which are not so easily dissolved by the penetration of water molecules and thus also have a certain wet strength. Likewise, polyvinyl alcohol, a synthetic polymer which is very similar to the starch due to the large number of hydroxyl groups, is used as the dry strength agent. It is formed by saponification of the acetyl groups from polyvinyl acetates. If not all acetyl groups are split off, copolymers with very adaptive properties are formed.

When dry, the tensile strength of the paper is mainly due to the hydrogen bonding between the cellulosic fibers. However, after penetration of water these fiber-fiber bonds are canceled and the paper almost completely loses its strength when wet. The strength of the paper is then only about 10% of the dry strength of the paper at most. The embedded water molecules increase the distance between the individual cellulose fibers, giving much of the strength get lost. However, for special applications, the so-called wet strength plays a very important role in addition to the dry strength. In particular, in the production of hygiene products such as tissue paper or toilet paper, filter papers, label papers or tea filters, coffee filters or banknote paper, it is important that these paper products still have some of their original strength even when wet or wet. Wet strength of paper and board products can only be achieved by using special wet strength agents. The wet strength agents are mainly added to the pulp suspension, but may also be added to the surface sizing agent or the coating slip as high molecular weight precondensates. These agents can be either cationic, anionic or non-ionic, resulting in special conditions of use. As a wet strength agent usually polymers or polyfunctional compounds are used, which react only after the drying of the paper with the OH groups of the pulp fibers and thereby bridge several fibers together. By physical addition of the wet bonding solid polymer bridges are first formed, whereby the reactive groups of the wet strength agent abut the fiber surface and come into close contact with the OH groups of the cellulose. Upon drying, the water evaporates and the reactivity of the crosslinking groups increases dramatically due to the elevated temperature, so that they react with the cellulose to form covalent compounds. This creates a three-dimensional network, which partially envelops the fibers in a thread-like manner and additionally connects them with each other. The rate of this reaction is mainly dependent on pH and temperature; The reaction is completed only after a few days to two weeks.

To improve the wet strength, products are predominantly used which undergo covalent crosslinking with the OH groups of the cellulose with their reactive groups by means of polycondensation: epoxy resins, polyamine resins, isocyanate resins, melamine-formaldehyde resins, urea-formaldehyde resins, formaldehyde resins, dialdehyde starch or glyoxal and glutaraldehyde. When Wet-strength agents are also any copolymers of the listed substances conceivable. The molecular weight of the conventional wet strength agent is usually between 1 0,000 and 500,000 g / mol and the charge density is at most 3 to 5 meq / g of polymer.

Under acidic conditions, the use of urea-formaldehyde resins and melamine-formaldehyde resins in particular has become established. At a pH below pH 5, predominantly urea-formaldehyde resins are used. Under neutral and basic conditions, which are preferred in the paper industry, polyamine-epichlorohydrin resins have proven useful. These have the advantage that they are versatile and are particularly cost-effective. Polyethyleneimines are also used as wet strength agents, although they contain no reactive groups. In addition, resins based on aldehyde-containing polymers are used, although the usefulness of these materials is limited due to the narrow pH range in which they can be used. Under acidic conditions, dialdehyde starch, glyoxal and polyacrylamide with glyoxal groups and their copolymers have been established. The use of wet strength agent achieves various positive effects. First, the covalent bonding results in increased strength of the paper in the damp or wet state. The fiber swelling is prevented in view of the three-dimensional networking. Due to the additional bonds between the fibers, the strength in the dry state is also increased, specifically the dry tensile strength, the bursting resistance, the erasure resistance and the pick resistance.

Since the cellulose fibers used in papermaking are negatively charged, the use of cationic wet strength agents has been proven. For this reason, cationic polymers which are based for example on polyamides or polyacrylamides have established themselves as wet-strength agents. These wet strength agents are commonly added in the manufacture of tissue paper in amounts of from 8 to 10 kg / t. In the prior art is the application of melamine-formaldehyde resins, inter alia, in US 4,461,858. Cationic polymers are predominantly based on polyamide-epichlorohydrin (PAE) and polyamidoamine-epichlorohydrin (PAAE) used (for example in US 2,926.1 16, US 3,733,290, US 4,605,702). In addition, the use of graft copolymers based on polyethylene oxide or polyethylene glycol segments and polyethyleneimine segments has become established, which is described, for example, in DE 10 2004 038 132 B3.

In the paper industry wet strength agents listed above are used for wet strength, which amounts to amounts of 0.2-5%, based on dry fibers. For label paper, the required quantities are usually much higher. Here, a single agent is always used to achieve the wet strength. However, wet strength agent use is adversely affected by so-called contaminants, fines, and small fiber debris. These mostly anionic substances enter into compounds with the cationic auxiliaries, but these do not contribute anything to the strength structure of the paper. Part of the wet strength agent used is thus lost to the contaminants. These impurities are mostly largely harmless in very small amounts, but interfere in higher concentrations. They also lead to the formation of deposits (resin deposits, polymer deposits, deposition of adhesive residue or slime), stains or holes in the paper product. In addition, these substances can lead to the precipitation of various additives, for example by the formation of calcium salts of organic acids (lime soaps) or starch-fat complexes. To avoid this, the contaminants are either removed or bound to the paper fibers. Approaches to problem solving are, above all, the mechanical separation of coarse-particle impurities by screening the material stream prior to papermaking or the discharge from the circulating water either by direct discharge into the wastewater or by cleaning with a technical "kidney". Another alternative is the fixation of the predominantly low molecular weight impurities in the paper by means of mostly partially hydrophobic cationic fixing agent, so that they are discharged with the paper from the system. The process water is thus relieved, since the content of dissolved substances and suspended matter is reduced. In addition, the impurity effect is reduced, in particular with regard to the impairment of the effectiveness of the other additives. Here, the fixing agents form flakes with the solids and suspended matter in the paper suspension. The forces between the fixing agent and the impurities are mainly based on electrostatic attraction and hydrogen bonding. Thus, a high charge density is advantageous for the electrostatic fixation of the contaminants. For this reason, typical fixatives are highly charged, but do not have a very high degree of polymerization. They can also consist of branched molecules that are particularly compact. Fixing agents are often cationic polymers, such as polyacrylamides (PAM) polyethylenimines (PE I) or polyvinylamines (PVAm), which have an average molecular weight of up to 5,000 g / mol and a high charge density of up to 15 meq / g polymer at pH 7. Typically, the positive charge varies with pH. In contrast to this, quaternary ammonium compounds always have a positive charge, regardless of the pH. A polymer in the chain of which such quaternary ammonium groups are contained is poly-diallyldimethylammonium chloride (DADMAC).

The object of the present invention is to provide a process for producing wet-strength papers which ensures increased wet strength with lower wet strength consumption. At the same time, impurities should not only be electrostatically fixed, but be bound covalently to the cellulose (wet-strength), so that these impurities no longer have a negative effect on the other additives.

This object is achieved by the method according to the invention, in which the cellulose pulp in addition to a conventional wet strength agent with a charge density of less than 6 meq / g polymer at least one other A wet strength agent having a high charge density of at least 5 meq / g of polymer is added for wet strength fixing of contaminants to the cellulosic fibers. This wet strength agent with a high charge density of at least 5 meq / g polymer carries one or more functional groups, ie the dry-covalent bonds of the same with the reactive hydroxy groups of the cellulose and / or accept the impurities.

The conventional wet strength agent with a charge density below 6 meq / g polymer preferably has a charge density of less than 5 meq / g polymer, more preferably between 0.2 and 4 meq / g polymer. The molecular weight is preferably 10,000 to 500,000 g / mol, more preferably 30,000 to 200,000 g / mol. Any conventional wet strength agent can be used here.

The wet strength agent having a high charge density of at least 5 meq / g of polymer preferably has a charge density of at least 6 meq / g of polymer, more preferably 7 meq / g of polymer. The molecular weight is preferably between 5,000 and 400,000 g / mol, more preferably between 10,000 and 100,000 g / mol. In this novel process, the contaminants and fines are first bound to the fibers as in a conventional contaminant fixative. However, the electrostatic fixing is replaced by permanent covalent bonds when drying the paper due to the high charge density of the wet strength agent with a high charge density of at least 5 meq / g polymer. This gives the paper a completely new, as yet unknown matrix of wet-strength bonds of short and long-chain molecules with high and low charge density, which causes previously unattained wet strength values. The fact that the impurities are covalently fixed to the cellulose fibers, they can no longer undesirable interactions with other paper additives. For this reason, significantly less wet strength is needed, with savings of up to 55% wet strength can be achieved. It is most advantageous to add the wet strength agent with a high charge density of at least 5 meq / g polymer first, the thick or already in the pulper or afterwards, and in a further step the wet strength agent with a charge density of less than 6 meq / g polymer , This early addition has the advantage that even at this stage all impurities and fiber residues can be bound to the cellulose fibers and not remain unbound in the paper machine system and free for bonds. However, it is also possible to add the wet strength agent with a high charge density of at least 5 meq / g polymer shortly before or after the addition of the wet strength agent with a charge density of less than 6 meq / g polymer, for example in a thinness agent. The efficiency of the wet strength agent with a high charge density of at least 5 meq / g polymer is further enhanced if shear forces still act on the fiber suspension after metering because it reacts faster with the cellulosic fibers than the wet strength agent with a charge density below 6 meq / g polymer.

It is further conceivable to mix the two wet strength agents (the wet strength agent with a high charge density of at least 5 meq / g polymer and the wet strength agent with a charge density below 6 meq / g polymer) before use. It may be advantageous if the two components are already offered as a ready mix. Alternatively, it is possible to mix the two wet strength agents directly on site in papermaking. It is particularly advantageous if the ratio between the wet strength agent with a high charge density of at least 5 meq / g polymer and the wet strength agent with a charge density of less than 6 meq / g polymer 1: 5 to 1: 20, preferably 1: 8 to 1 : 15, more preferably 1:10.

The process according to the invention has numerous advantages over the processes described in the prior art for the production of wet-strength papers: The paper machine system is efficiently freed from fillers, fines and contaminants. Screen stains caused by sticky contaminants (from waste paper stock, spine, glue, etc. stickies) or resin particles are reduced. The retention and drainage is improved because the contaminants can no longer interact with the additives. Thus, overall, the amount of additives used (retention, flocculation, wet strength, coating, defoamer) can be reduced. In addition, the solids content in clear water drops. In paper production, energy can also be saved in the form of steam, as the paper now dries better. The resulting paper product has a significantly increased wet strength and dry strength compared to untreated paper. By using smaller amounts of wet strength agent, the chlorine content in the paper is also drastically reduced, which is also desirable.

Of the wet strength agent with a high charge density of at least 5 meq / g polymer, only very small amounts are used. Less than 1% by weight of the wet strength agent with a high charge density of at least 5 meq / g of polymer is used, preferably less than 0.3% by weight, particularly preferably 0.08 to 0.12% by weight, based in each case on dry fibers.

The amount of added wet strength agent having a charge density of less than 6 meq / g polymer is between 1 and 2% by weight according to the processes described in the prior art. This amount can also be reduced by the method according to the invention. Wen less than 5% by weight of the wet strength agent with a charge density of less than 6 meq / g polymer are used, preferably 0.5 to 2% by weight, particularly preferably 0.8 to 1, 2% by weight, based in each case on dry fibers.

The wet strength agent with a high charge density of at least 5 meq / g polymer should bind mainly anionic substances firmly to the fibers. This requires a large number of func- tional groups and cationic charges. For the electrostatic fixing a high charge density in a small space is favorable. Therefore, typical fixatives are highly charged, but do not have a very high degree of polymerization, but can also consist of branched molecules. The wet strength agent used with a high charge density of at least 5 meq / g polymer preferably has one Charge density of at least 6 meq / g of polymer, more preferably of at least 7 meq / g of polymer. Since the cellulose fibers carry a negative charge, the use of strong cationic wet strength agents is preferable. It is preferable to use wet strength agents which are crosslinked and branched.

In the process of the present invention, to the wet strength agent having a charge density of below 6 meq / g of polymer, a wet strength agent having a high charge density of at least 5 meq / g of polymer is added which combines the properties of an impurity fixative and a wet strength agent. Thus, the contaminants are not only electrostatically fixed, but also covalently linked after drying of the paper with the cellulose fibers. For this reason, this wet strength agent with a high charge density of at least 5 meq / g polymer is always a copolymer bearing both highly charged groups and reactive groups capable of covalent linkage. As a component with a high charge density, all components of conventional fixatives come into question. These may preferably be acrylamide, amidoamine compounds, ethyleneimine compounds, vinylamine compounds, dimethylamine compounds or diallyldimethylammonium chloride (DADMAC). However, all substances come in questions that have comparable properties, in particular with regard to the charge distribution and the degree of polymerization. In addition to these highly charged groups, reactive groups can also be found in the copolymer that can form covalent bonds with the cellulose and the contaminant particles. Here, all functional groups are conceivable that are used in conventional wet strength agents. These are preferably epoxide compounds, polyamine compounds, isocyanate compounds, formaldehyde, methylolmelamine, methylolurea, dialdehyde starch or glyoxal and glutaraldehyde. Particular preference is given to using epichlorohydrin or polyamine. Other analogously prepared copolymers are conceivable which have similar properties to those mentioned above. To ensure the wet strength of the paper produced, any wet strength agent capable of forming covalent bonds to the cellulose can be used. Epoxy resins, polyamine resins, isocyanate resins, formaldehyde resins, melamine-formaldehyde resins, urea-formaldehyde resins, dialdehyde starch or glyoxal and glutaraldehyde are preferred here. Particularly preferred is the use of epichlorohydrin resin or polyamine resin. Again, all combinations of copolymers are conceivable.

Furthermore, a mixture of a wet strength agent with a high charge density of at least 5 meq / g polymer and at least one further wet strength agent with a charge density of less than 6 meq / g polymer for carrying out the method according to the invention is part of the invention. Here, the ratio between the wet strength agent having a high charge density of at least 5 meq / g polymer and the wet strength agent having a charge density of below 6 meq / g polymer is 1: 5 to 1:20, preferably 1: 8 to 1:15, more preferably 1:10. This mixture can be used instead of a conventional wet strength agent for the production of wet-strength papers. The use of this mixture has the advantage that less wet strength needs to be used than in a conventional process. Furthermore, with the aid of this mixture due to the wet strength agent with a high charge density of at least 5 meq / g polymer impurities can be covalently bonded to the cellulose fibers, so that they can no longer interact with other additives and no wet strength agent is wasted by binding to contaminants. Thus, up to 55% of wet strength agent can be saved.

The paper produced by the process according to the invention is likewise part of the invention. This paper has a significantly increased wet strength, making it ideal for use on tissue products such as toilet paper, sanitary crepe, laminate paper, coffee and tea filters. The paper produced by the process according to the invention receives a completely novel, hitherto unknown matrix in which a structure wet-strength bonds between short-chain and long-chain molecules, with high and low charge density. For the first time, the impurities are covalently bound to the cellulose fibers. In addition, the use of a wet strength agent having a high charge density of at least 5 meq / g of polymer bearing one or more functional groups that undergo covalent bonding with the cellulose during drying is included as an additive during the process of wet strength in the process of the invention , The use of this wet strength agent with a high charge density of at least 5 meq / g polymer allows for the first time the covalent bonding of contaminants to the cellulose fibers and thus a significant saving of the wet strength agent used. Surprisingly, it is also possible to use the wet strength agent having a high charge density of at least 5 meq / g of polymer bearing one or more functional groups which covalently bond with the cellulose during drying as an additive during the dry consolidation process (preferably by means of polyacrylamides or Polyvinyl alcohol). This use of the wet strength agent with a high charge density of at least 5 meq / g polymer is also part of the invention. The use of this wet strength agent with a high charge density of at least 5 meq / g polymer allows the covalent attachment of contaminants to the cellulose fibers and thereby a significant saving of the dry strength agent used. Thus, both increased dry and increased wet strength of the treated paper are achieved. The advantageous embodiments of the method according to claims 2 to 6 and 9 to 11 according to the invention can be applied analogously to the use of the wet strength agent with a high charge density of at least 5 meq / g polymer in the dry consolidation process. Examples:

The tested leaves (Buchesulfid / Fichtesulfid 50%, ground / unground 50%) were prepared according to DIN 54358.

The strengths were determined using a horizontal tensile tester that can determine both wet and dry strengths. For the determination of the wet strength according to EN 12625-4 + 5, a paper strip of 50 mm width was inserted into the pneumatic sample clamps, then lowered into a immersion tank filled with water (20 ° C.) and watered for 1 5 s. After the prescribed washing time, the sample was pulled out of the water basin and stretched until a break occurred. The dry strength was determined using a 15 mm wide paper strip according to DIN 531 12. The following parameters were determined during the measurements: tensile strength (kN / m), breaking length (km), tensile index (Nm / g), test time (s), breaking elongation (mm), relative elongation (%) and wet fracture force (N).

Wet strength agent 1 (with a charge density below 6 meq / g polymer):

Epichlorohydrin wet strength agent 2 (high charge density of at least 5 meq / g polymer): Strong cationic wet strength agent in an aqueous solution containing 55% by weight of a cross-linked epichlorohydrin-dimethylamine-ammonium terpolymer having a charge density of 7.27 meq / g polymer. Dry agent 1:

Polyacrylamide with glyoxal (1 -5% by weight) Auxiliary agent Dry strength Elongation Wet tensile strength Addition rate DIN 53112 EN 12625-4 / 5 without 25,05 100,00% 1,31% 100,00% 0,00 0,00%

Wet strength 2 26,18 104,51% 1,35% 103,05% 0,81 13,37% 0,1%

Wet strength 2 27.03 107.90% 1.41% 107.63% 1.80 29.70% 0.5%

Wet strength 2 31.96 127.58% 1.56% 119.08% 2.98 49.17% 0.8%

Wet strength agent 2 36.32 144.99% 1.86% 141.98% 3.19 52.64% 3.0%

Wet strength agent 1 53.42 213.25% 1, 46% 111, 45% 6.06 100.00% 1.9%

Table 1: Effect of wet strength agent with high charge density of at least 5 meq / g

Polymer with increasing concentration compared to the conventional wet strength agent (below 6 meq / g polymer)

Table 2: Effect of wet strength agent with high charge density of at least 5 meq / g

Polymer in combination with a conventional wet strength agent (below 6 meq / g polym Auxiliary agent Dry strength Elongation Wet tensile strength Addition rate DIN 53112 EN 12625-4 / 5 without 38,32 100,00% 1, 23% 100,00% 0 0,00%

Dry strength agent 1 43.03 1 12.29% 1, 24% 100.76% 0 0.00% 1.22%

Dry strength agent 1 59.97 156.50% 1, 44% 1 17.07% 4.51 71, 70% 1.22% +

Wet strength agent 2

0.1%

Dry strength agent 1 64.35 167.93% 1, 68% 136.59% 10.44 165.98% 1.22% +

Wet strength agent 1

0,95% +

Wet strength agent 2

0.1%

Wet strength 1 52.82 137.84% 1, 36% 1 10.57% 6.29 100.00% 1.9%

Table 3: Effect of wet strength agent with high charge density of at least 5 meq / g

Polymer in combination with a conventional dry strength agent

Claims

claims:

1 . A process for producing wet-strength papers, characterized in that a cellulose pulp in addition to a wet strength agent having a charge density of less than 6 meq / g polymer, at least one further wet strength agent with a high charge density of at least 5 meq / g polymer for wet-strength fixing of contaminants to the Cellulosic fibers are added which carries one or more functional groups, which form covalent bonds with the cellulose and / or the impurities during drying.

2. The method according to claim 1, characterized in that in a first step, the wet strength agent with a high charge density of at least 5 meq / g polymer is added to the pulp, and in a second step, the wet strength agent with a charge density of less than 6 meq / g polymer is added.

3. The method according to claim 1, characterized in that in a first step, wet strength agent with a charge density of less than 6 meq / g

Polymer is added to the pulp, and in a second step, the wet strength agent with a high charge density of at least 5 meq / g polymer is added.

4. The method according to claim 1, characterized in that the wet strength agent is mixed with a high charge density of at least 5 meq / g polymer before the treatment of the pulp with the wet strength agent with a charge density of less than 6 meq / g polymer.

5. The method according to claim 4, characterized in that the wet strength agent is mixed with a high charge density of at least 5 meq / g polymer before the treatment of the pulp with the wet strength agent with a charge density of less than 6 meq / g polymer, the ratio between the Wet strength agent with a high charge density of at least at least 5 meq / g polymer and the wet strength agent with a charge density of less than 6 meq / g polymer 1: 5 to 1: 20, preferably 1: 8 to 1: 15, particularly preferably 1: 10.

6. The method according to any one of claims 1 to 5, characterized in that less than 1% by weight wet strength agent with a high charge density of at least 5 meq / g polymer are used, preferably less than 0.3% by weight, particularly preferably 0, From 08 to 0.12% by weight, based in each case on dry fibers.

7. The method according to any one of claims 1 to 6, characterized in that less than 6% by weight of the wet strength agent with a charge density of less than 6 meq / g of polymer are used, preferably 0.2 to 5% by weight, particularly preferably 0, 8 to 1, 2% by weight, based in each case on dry fibers.

8. The method according to any one of claims 1 to 7, characterized in that the wet strength agent having a charge density of less than 6 meq / g polymer, preferably a charge density of less than 5 meq / g polymer, more preferably from 0.2 to 4 meq / g polymer having.

9. The method according to any one of claims 1 to 8, characterized in that the wet strength agent having a charge density of at least 5 meq / g polymer preferably has a charge density of more than 6 meq / g polymer, more preferably of more than 7 meq / g polymer ,

10. The method according to any one of claims 1 to 9, characterized in that it is the wet strength agent with a charge density of at least 5 meq / g polymer is a strong cationic wet strength, which is a copolymer of at least one monomer from each group I and II:

I. acrylamide, an amidoamine compound, an ethyleneimine compound, a vinylamine compound, a dimethylamine compound or dialkyldimethylammonium chloride (DADMAC) and II. An epoxide compound, a polyamine, an isocyanate compound, formaldehyde, methylolmelamine, methylolurea, dialdehyde starch or glyoxal and glutaraldehyde.

1 1. Method according to one of claims 1 to 10, characterized in that it is the wet strength agent having a charge density of at least 5 meq / g polymer is a strong cationic wet strength agent, which is a copolymer of at least one monomer from the groups I and II:

I. acrylamide, an amidoamine compound, an ethyleneimine compound, a vinylamine compound, a dimethylamine compound or dialkyldimethylammonium chloride (DADMAC) and

II. Epichlorohydrin or a polyamine.

12. The method according to any one of claims 1 to 1 1, characterized in that it is the wet strength agent having a charge density of less than 6 meq / g polymer is at least one of the following substances:

Epoxy resins, polyamine resins, isocyanate resins, formaldehyde resins, melamine-formaldehyde resins, urea-formaldehyde resins, dialdehyde starch or glyoxal and glutaraldehyde.

13. The method according to any one of claims 1 to 12, characterized in that it is the wet strength agent with a charge density of less than 6 meq / g polymer is an epichlorohydrin resin or a polyamine resin.

14. Mixture of a wet strength agent with a charge density of less than 6 meq / g polymer and at least one further wet strength agent with a charge density of at least 5 meq / g polymer for carrying out a process according to one of claims 1 to 13, characterized in that the ratio between the wet strength agent with a charge density of at least 5 meq / g polymer and the wet strength agent with a charge density of less than 6 meq / g polymer 1: 5 to 1: 20, preferably 1: 8 to 1: 15, particularly preferably 1: 10.

15. wet-strength paper, characterized in that it was prepared by a method according to any one of claims 1 to 13.

16. Use of a wet strength agent having a charge density of at least 5 meq / g of polymer which carries one or more functional groups which undergo covalent bonds with the cellulose during drying, as an additive during the process of wet compaction in a process according to any one of claims 1 to 13 for increasing the wet strength of paper and for wet-strength bonding of contaminants and / or fines to the cellulose fibers.

17. Use of a wet strength agent having a charge density of at least 5 meq / g of polymer carrying one or more functional groups which undergo covalent bonding with the cellulose during drying, as an additive during the dry consolidation process to increase the dry strength and wet strength of paper and paper for wet-strength bonding of contaminants and / or fines to the cellulose fibers.