WO1999064487A1

WO1999064487A1 - Anionic agent for paper surface sizing

Info

Publication number: WO1999064487A1
Application number: PCT/IT1998/000158
Authority: WO
Inventors: Gino Giuliani
Original assignee: Compagnia Commerciale Italiana S.R.L.
Priority date: 1998-06-12
Filing date: 1998-06-12
Publication date: 1999-12-16
Also published as: AU7930598A

Abstract

The invention relates to an anionic agent comprising a solution copolymer obtained from copolymerization between an unsaturated cyclic anhydride, a maleic anhydride for example, modified by reaction with an epoxy and alkene, styrene for example. The invention also relates to a process for obtaining said anionic agent. This anionic agent can be used for paper surface sizing.

Description

S P E C I F I C A T I O N ANIONIC AGENT FOR PAPER SURFACE SIZING

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FIELD OF THE INVENTION The present invention relates to an anionic agent comprising a modified maleic anhydride-alkene copolymer. In addition, the present invention relates to a process for obtaining said anionic agent, and to its use for paper surface sizing in a size press. In the paper sizing field, anionic agents are known which comprise copolymers obtained by radical copolymerization between the maleic anhydride derivatives and the styrene derivatives. These anionic agents are generally employed during a paper working cycle as surface sizing agents for the paper in a size press. The products based on these anionic agents that are presently put on the market cannot be successfully used for paper weight reduction. In fact, application of usual anionic agents comprising maleic anhydride-styrene copolymers during the paper working cycle does not involve the desired effect of reducing the paper weight, the paper use modalities being the same. In addition, it is required that the paper weight reduction should not bring about a sharp reduction in the mechanical properties of the paper itself. As a matter of fact, this reduction in the mechanical properties would limit the use of the paper material previously processed in a size press by means of sizing agents of the above mentioned type. In addition, application of common anionic agents present on the market, employed for reducing paper weight, gives the paper sheet an important reduction as regards its hydrophilic qualities. This reduction in the physico- chemical absorption capabilities of the treated sheet impairs and limits use of these sheets for normal printing procedures in offices provided with ink-jet printers. In fact, following treatment with a common anionic agent comprising a maleic anhydride-styrene copolymer there is a great reduction in the Cobb test value.

A Cobb apparatus is used for determining water absorption by paper and therefore indirectly, the sizing degree of the paper itself. The obtained parameter gives a Cobb test measure on a paper sheet and expresses the water absorption as grams of absorbed water for each square metre of paper. The paper sheet plasticization involves the further drawback of preventing paper recycling and consequently regeneration of the cellulose material, in that the anionic agents employed in paper sizing hardly detach from the cellulose material during the regenerating steps.

Therefore, availability of anionic agents devoid of the known art drawbacks is greatly felt.

In particular, anionic agents capable of simultaneously meeting several different requirements are needed. More specifically, an increase in the sizing effect is required as well as an increase in the mechanical properties, so that a reduction in the paper weight may be obtained, the paper use conditions being the same. In addition, it is required that a surface application of the anionic agents by a size press should not involve "plasticization" of the material. In fact, a possible plasticization of the treated paper would involve a reduction in the Cobb test value, that would not ensure a good physico-chemical absorption capability of the treated paper material and would not allow a correct printing procedure.

SUMMARY OF THE INVENTION It is an aim of the present invention to supply new anionic agents capable of reducing the paper weight, the use modalities and mechanical properties of said paper being the same. It is a further aim of the invention to envisage a process enabling anionic agents to be obtained which, in addition to having the above mentioned properties, are also characterized by the fact that they reduce the Cobb test parameter to values included in a range adapted to allow an appropriate use of the paper for printing.

A still further aim of the invention is to enable use of said anionic agents in the paper working cycle for surface sizing of paper sheets by means of a size press, or use of said anionic agents as co-additives directly in the slurry.

The foregoing and still further aims that will become more apparent in the progress of the following detailed description are obtained by the present disclosure in which the Applicant has surprisingly found it useful to modify an unsaturated cyclic anhydride to be used in copolymerization with an alkene monomer, by introduction of particular mono- and/or polyfunctional compounds into the anhydride. Accordingly, it is an object of the present invention to provide an anionic agent comprising a solution copolymer, said copolymer being obtained by copolymerization of an alkene monomer with a compound obtained by reacting: - 20-50% by moles of an unsaturated cyclic anhydride or derivates thereof;

- 30-60% by moles of one or more alcohols; and

- 5-20% by moles of a mono- and/or polyfunctional epoxy derivative; wherein part of the starting anhydride in said copolymer is esterified and part of the starting anhydride is converted into an imide.

It is a further object of the invention to provide a process for preparing said anionic agent. In particular, this process comprises the following steps: a) solubilizing an unsaturated cyclic anhydride or derivatives thereof in one or more organic solvents; b) adding a solution comprising one or more alcohols and a mono- and/or polyfunctional epoxy derivative to the solution thus obtained, so as to obtain a compound originating from the reaction of:

- 20-50% by moles of an unsaturated cyclic anhydride or derivates thereof;

- 30-60% by moles of one or more alcohols; and

- 5-20% by moles of a mono- and/or polyfunctional epoxy derivative; c) adding a radical initiator and an alkene monomer to the reaction medium of the preceding step, in an amount adapted to carry out a copolymerization; d) adding a base until a complete solubilization of the obtained copolymer is reached.

A still further object of the invention is to provide use of said anionic agent for treatment of a paper surface by a method involving use of a size press.

In accordance with the present invention a copolymer is obtained between an alkene monomer and an unsaturated cyclic anhydride (a maleic anhydride, for example) suitably modified by introduction of mono- and/or polyfunctional epoxy compounds into the anhydride. The alkene monomer is selected from: - cyclic and/or straight-chain unsaturated olefin hydrocarbons containing carbon atoms in a number included between 4 and 14, preferably between 4 and 10, such as for example: indene, cyclopentadiene, dicyclopentadiene, divinylacetate, isobutylene, etc.; - benzene derivatives with one or more olefin substituents having a number of carbon atoms included between 1 and 8, such as α-methyl styrene, ortho-, meta- and para-methyl styrene or ortho-, meta- and para-divinyl benzene; preferably styrene. The unsaturated cyclic anhydride is preferably selected from those having a five-atom ring; the derivatives of the unsaturated cyclic anhydride contain at least one alkyl-chain substituent containing 1 to 8 carbon atoms; preferably the type of unsaturated cyclic anhydride used is maleic anhydride. The copolymer between the alkene monomer and the compound resulting from the reaction between the unsaturated cyclic anhydride or derivatives thereof, one or more alcohols and a mono- and/or polyfunctional epoxy derivative is obtained by radical polymerization, by use of radical initiators of the peroxide or hydroperoxide type, such as for example: benzoyl peroxide, tert- butylhydroperoxide, cumene hydroperoxide; or azo compounds such as: 2,2'-azo-di (2-methyl propanenitrile) also referred to as azodiisobutyronitrile or AIBN, 2,2'- azo-bis (2-methylbutyronitrile) also called AMBN, 2,2'- azo-bis (N,N' -dimethylene isobutyramide) dihydrochloride, 2, 2 ' -azo-bis (2-amidinepropane) dihydrochloride. Preferably, the initiator used in the present invention is to be found in the AIBN and AMBN types, and within the peroxide range, such as benzoyl peroxide and tert-butyl hydroperoxide, for example.

The amount of the radical initiator is preferably in a ratio of 0.005-0.017 moles of initiator per mole of alkene monomer. An amount less than 0.005 moles leads to a copolymerization yield not exceeding 85%; an amount higher than 0.017 moles leads to a high yield but has a tendency to cause styrene homopolymerization.

Practically, when starting from maleic anhydride and making said anhydride react with a solution containing one or more alcohols and a mono- and/or polyfunctional epoxy derivative, a compound is formed that can be referred to as a modified maleate. Formation of the modified maleate takes place in a polar and/or apolar organic solvent or mixtures thereof, in aliphatic or aromatic solvents such as: methyl ethyl ketone(2- butanone) , methyl isobuthyl ketone (4-methyl-2-pentanone) , ethyl acetate, alcohols, toluol, xylol, Solvesso® 100 and 150 and para-cymene (4-isopropyl toluene).

Alcohols to be used are selected from those having a number carbon atoms in a range of 1 to 10, preferably 1 to 8, such as: methyl alcohol, ethyl alcohol, isopropyl alcohol, butyl alcohol, hexyl alcohol, heptyl alcohol, octyl or benzyl alcohol.

The epoxy derivative preferably has a functionality intended as number of contained epoxy groups included between 1 and 4; and it is preferably selected from aliphatic, cycloaliphatic or aromatic mono- and/or polyfunctional epoxy derivatives. The equivalent weight of these mono- and polyfunctional epoxy compounds is preferably included between 10 and 1800, more preferably between 100 and 1400. These epoxy derivatives can be liquids or solids and/or mixtures thereof. They can be solubilized in alcohol solvents, polar and/or apolar organic solvents and/or mixtures thereof, depending on the equivalent weight of the considered epoxide groups. Among the onofunctional epoxy derivatives the following ones are mentioned:

- para tert-butylphenol monoglycidyl ether, 2- ethylhexyl monoglycidyl ether alcohol, phenyl glycidyl ether, but preferably polyfunctional epoxidic groups are employed, such as: 1, 4-butanediol diglycidyl ether, 1,6- hexanediol diglycidyl ether, 4, 4 '-isopropyliden diphenol diglycidyl ether (bisphenol A), bisphenol F (an isomeric mixture) diglycidyl ether;

- the upper homologous series of the condensation of bisphenol A diglycidyl ether with the bisphenol A itself having an equivalent weight included between 450 and 700;

- the upper homologues series of the condensation of bisphenol F (epoxy-phenol novolac resin) diglycidyl ether with the bisphenol F itself and/or with bisphenol A; - other products such as epoxy-cresol novolac resin, glycerol triglycidyl ether, trimethylol propane triglycil ether, tetraphenylol ethane tetraglycil ether, para- aminophenol triglycidyl ether, paramethylene dianiline tetraglycidyl ether; cycloaliphatic resins such as bis-3,4- epoxycyclohexyl carboxylate, vinyl cyclohexyl dioxide and others.

Of the above mentioned mono- and polyfunctional epoxy derivatives the preferred ones are para tert- butylphenol monoglycidyl ether, 2-ethylhexyl monoglycidyl ether alcohol, 1, 6-hexanediol diglycidyl^"ether, 4,4'- isopropyliden diphenol diglycidyl ether (bisphenol A) , bisphenol F (an isomeric mixture) diglycidyl ether and vinyl ciclohexyl dioxide.

The solution consisting of an alcoholic mixture with the mono- and/or polyfunctional epoxy derivative comprises the two constituents in the following percentages: - alcohol 50-80% by moles,

- epoxide 50-20% by moles.

The molar ratio between alcohol and epoxide can be increased, but the resulting product could jeopardize recycling of the treated paper material. The maleic anhydride is solubilized in one or more organic solvents and is brought into contact with a solution consisting of one or more alcohols selected from the above mentioned ones, and of a mono- and/or polyfunctional epoxy derivative selected from the above mentioned ones.

The maleic anhydride ring opens because it can react both with one or more alcohols and with the mono- and/or polyfunctional epoxy derivative, which are both present in the reaction medium. Alcohols and/or the mono- and/or polyfunctional epoxy derivative open the ring and bond with the anhydride, thereby esterifying one or both ends thereof. In this way a modified maleate is formed.

In fact, maleic anhydride is modified following esterification with one or more alcohols and/or with the mono- and/or polyfunctional epoxy derivative.

The esterification degree of the starting maleic anhydride can reach 50% by moles and is preferably included between 20 and 40% by moles.

The modified maleate thus formed is maintained under stirring with an organic solvent and is subsequently reacted with the alkene monomer, styrene for example, by radical copolymerization reaction using the above mentioned compounds, such as peroxides and hydroperoxides, as radial initiators for the reaction. The reaction time of the modified maleate with the alkene monomer for making the copolymer can preferably vary from 30 to 300 minutes and more preferably from 60 to 180 minutes. This time gap is strictly connected with the activity of the radical initiator employed in copolymerization.

The copolymerization reaction is preferably conducted at a temperature included between 55°C and 170°C, more preferably between 65°C and 140°C.

There is a straight correlation between the temperature at which copolymerization is carried out and the molecular weight of the copolymer obtained as final product. In fact, the higher the copolymerization temperature is, the lower the molecular weight of the copolymer. The copolymer of the present invention is obtained in the presence of aliphatic or aromatic solvents, in order to obtain a solution or a suspension. For example, the used solvents among the aliphatic ones are: ethylethyl ketone (2-butanone) , methyl isobutyl ketone (4-methyl-2-pentanone) , ethyl acetate, alcohols, and among aromatic solvents: toluol, xylol, Solvesso® 100 and 150 and para-cymene(4-isopropyl toluene).

The copolymer is obtained by reaction of the alkene monomers and maleic anhydride monomers in a molar ratio of 1:1 to 5:1, preferably 3:1. The obtained copolymer practically has an alternated structure between alkene monomers and maleic anhydride monomers, said anhydride being esterified both with the alcoholic portion and the epoxy portion.

Alternation in the copolymer may be equimolecular between the alkene monomers and the esterified maleic anhydride monomers, or there may be a prevalence of the styrene monomers or the esterified anhydride monomers.

The obtained copolymer as previously set forth, is salified to bring it to a solution state, causing it to react with a base. For the purpose an ammonia and/or aliphatic amine solution having carbon atoms in a number included between 1 and 20, preferably 1 and 14, can be used, such as methylamine, ethylamine, isopropylamine, butylamine, hexylamine, heptylamine, cyclohexylamine, dodecylamine, etc.; or cycloaliphatic amines having carbon atoms in a number included between 7 and 12, such as isophorone diamine.

Polyamines such as ethylenediamine, diethylenetriamine, triethylenetetramine, tetraethylenepentaamine, pentaethylenehexaamine, ethanolamine can be also employed.

Preferably a water solution of ammonium hydroxide is employed.

For the reaction, a base amount adapted to enable the final copolymer to be completely solubilized is required.

With the addition of said bases, the acidic groups of the maleic anhydride are partly converted into imides, preferably in an amount of at lest 50% by moles of the starting maleic anhydride, more preferably in an amount of 50-70% by moles of the starting anhydride. Possibly, addition of said bases can also partly convert the acidic groups of the maleic anhydride into carboxy-carbamic groups.

The anionic agent of the present invention can also be used in the paper industry compatibly with the products of normal use such as: paraffins, esterified fatty acid derivatives, modified starches, optical blue- colouring and bleaching agents, mineral fillers and pigments. The anionic agent of the present invention can be added to the cellulose slurry during paper production, but in a more efficient manner can be applied in the subsequent surface paper treatment.

The pH at which one operates in using the anionic agent of the present invention is the one normally employed in paper production. The anionic agent of the present invention has a dry content, also including the salified copolymer, of 10-25% by weight, preferably 14- 22% by weight. By "dry" it is herein intended the part dissolved in the anionic agent comprising the salified copolymer and other possible secondary products formed during preparation of said anionic agent.

The anionic agent of the present invention is used for paper processing in such proportions that the dry content of the anionic agent is included between 0.01 and 3% by weight, and more preferably between 0.1 and 1% by weight referred to the paper weight.

Use of the anionic agent in the paper working cycle enables a reduction of 10-20% in the treated paper weight, the use conditions being the same.

The below examples are only given for demonstration purposes and must not be interpreted as a limitation of the present invention. Example No. 1 (control) For the copolymer preparation, 240 g of maleic anhydride and 220 g of isopropyl alcohol are loaded in a reactor and solubilization is carried out at a temperature - of 90-95°C. A radical-initiator solution consisting of 1.4 g of tert-butylhydroperoxide and 30 g of isopropyl alcohol and 240 g of styrene monomer are added to the mixture in a period of time of 2 to 3 hours. The whole mixture is maintained at a temperature of 95- 100°C for further two hours.

After this period of time has elapsed, 150 g of 30% ammonia and 300 g of water are added, keeping at the same temperature for further six hours.

After this period of time, temperature is lowered to 40°C and the copolymer is treated with a 10% ammonia solution so that, after about 30 minutes, an almost clear solution of the anionic agent is obtained which has a dry content of 18-20% by weight and a pH of 8.5-9.5. Example No . 2

A solution is prepared by solubilization of 120 g of maleic anhydride with 300 g of a toluene/ethyl acetate

(90:10) mixture. 70 g of an alcoholic mixture consisting of 55% by weight of isopropyl alcohol, 10% by weight of butyl alcohol and 35% by weight of 1,6-epoxy hexanediol are added to this solution. The mixture is maintained at room temperature for 1 hour and then heated to 120°C over two hours. This solution is put in a reactor containing 800 g of a toluene/ethyl acetate mixture (90:10) preferably heated to a temperature of 90 to 95°C, and 240 g of styrene and 1.5 g of a radical initiator (tert- butylhydroperoxide) , dissolved in 30 g of alcohol are added thereto. The time interval for this addition is 3-4 hours during which temperature must be maintained constant. When addition is over, a temperature elevation to 100-105°C is carried out and the reaction mixture is maintained under these conditions for further 5 hours. Subsequently, the solvent is distilled and during distillation 450 g of 30% ammonia and 500 g of water are added. At the end of distillation of the azeotropic mixture, cooling at 45°C is carried out and the copolymer is treated with a 10% ammonia solution so as to obtain an anionic agent solution having a dry content of 15-18% by weight and a pH of 8.5-9.5. Example No. 3

A solution consisting of 120 g of maleic anhydride dissolved in 300 g of a toluene/ethyl acetate solvent

(90:10) is prepared. 60 g of an alcoholic mixture consisting of 53% by weight of isopropyl alcohol, 9% by weight of butyl alcohol and 38% by weight of ortho- cresol-monoglycidyl ether are added to the solution. Mixture is maintained at room temperature for 1 hour and then heated to 120 °C for 2 hours. This solution is then measured in a reactor containing 800 g of toluene heated to 95°C, simultaneously with 240 g of styrene and 1.5 g of a radial initiator (para-tert-butylhydroperoxide) , dissolved in 15 g of alcohol, during a time interval of about 3 hours taking care that the temperature should be maintained constant at a value of 90-100°C.

When measuring is over, temperature is increased to 100°C and these conditions are maintained for further 5 hours. Then the solvent is distilled and during distillation 450 g of 30% ammonia and 500 g of water are added. At the end of distillation of the azeotropic mixture, cooling to 45°C is carried out and the copolymer is treated with a 10% ammonia solution so as to obtain an anionic agent solution having a dry content of 18-20% by weight and a pH of 8.5-9.5. Example No. 4

A maleic anhydride solution is prepared by solubilization of 120 g of said anhydride into 300 g of a mixture consisting of toluene/ethyl acetate (90:10). 70 g of a mixture consisting of 55% by weight of isopropyl alcohol, 10% by weight of butyl alcohol and 35% by weight of a solid epoxy resin having a molecular weight included between 900 and 950 are added to the solution. This mixture has been maintained at room temperature for 1 hour and then heated to 120°C for further 2 hours.

This solution is then measured in a reactor containing 800 g of toluene heated to about 90°C, simultaneously with 240 g of styrene and 3.5 g of a radical initiator (para-tert-butylhydroperoxide) dissolved in 15 g of alcohol, during a period of time of about 3 hours, keeping the temperature between 90 and 95°C. When measuring is over, temperature is increased to 105°C and these conditions are maintained for about 5 hours . Subsequently solvent is distilled and during distillation 250 g of a 30% ammonia solution and 500 g of water are added, the evaporated water being integrated with distilled water.

At the end further 200 g of a 30% ammonia solution diluted with 400 g of water are added. The same temperature is maintained for further 5 hours and after this lapse of time a slight vacuum (residual pressure 500 mmHg) is applied for 10 minutes.

Cooling at a temperature of 60°C and dilution with water is carried out, thereby obtaining an almost clear solution of anionic agent having a dry content of 14-16% by weight and a pH included between 8.5 and 9.5. Example No. 5

A solution consisting of maleic anhydride is prepared by solubilization of 120 g of said anhydride into 300 g of a toluene/ethyl acetate mixture (90:10).

70 g of a mixture consisting of 55% by weight of isopropyl alcohol, 10% by weight of butyl alcohol and 35% by weight of 4, 4 '-isopropyl-iden diphenol diglycidyl ether (bisphenol A) are added to the solution. This mixture has been maintained at room temperature for 1 hour and then heated to 120°C for further 2 hours.

This solution is then measured in a reactor containing 800 g of a toluene/ethyl acetate mixture (90:10), heated to 80-105°C, simultaneously with 240 g of styrene and 3.5 g of a radical initiator (tert- butylhydroperoxide) in 15 g of an isopropyl and butyl alcohol mixture, during a lapse of time of about 3 hours, keeping the temperature between 90 and 95°C.

When measuring is over, temperature is increased to 105°C and these conditions are maintained for 5 hours. Then solvent is distilled and during distillation 250 g of a 30% ammonia solution and 500 g of water are added. During distillation reintegration of the evaporated water occurs. At the end, further 200 g of a 30% ammonia solution and 400 g of water are added. The mixture is allowed to stand at the same temperature for further 5 hours. After this period of time a slight vacuum (residual pressure 250 mmHg) is applied for 10 minutes. Cooling at a temperature of 60°C and dilution with water are carried out so that an almost clear solution of an anionic agent is obtained which has a dry content of 14-16% by weight and a pH of 8.5-9.5. Example No . 6 A solution of maleic anhydride is prepared by solubilizing 120 g of said anhydride into 300 g of a toluene/ethyl acetate mixture (90:10).

70 g of a mixture consisting of 55% by weight of isopropyl alcohol, 10% by weight of butyl alcohol and 35% by weight of phenol novolac diglycidylether (bisphenol F) are added to the solution. This mixture has been maintained at room temperature for 1 hour and then heated to about 120°C for further 2 hours.

This solution is then measured in a reactor containing 800 g of a toluene/ethyl acetate mixture (90:10), heated to about 90°C; simultaneously 240 g of styrene and 3.5 g of a radical initiator (tert-butyl hydroperozide) in 15 g of an isopropyl and butyl alcohol are added, in a period of time of 2 to 3 hours, keeping the temperature between 90 and 95°C. When measuring is over, temperature is increased to 105°C and these conditions are maintained for 5 hours. Then solvent is distilled and during distillation 250 g of a 30% ammonia solution and 500 g of water are added. During distillation the evaporating water is reintegrated. At the end, further 200 g of a 30% ammonia solution diluted with 400 g of water are added.

The mixture is allowed to stand at the same temperature for further 5 hours. After this period of time a slight vacuum (residual pressure 250 mmHg) is applied for 10 minutes. Cooling at a temperature of 60°C and dilution with water are carried out, thereby obtaining an almost clear solution of an anionic agent having a dry content of 14-16% by weight and a pH of 8.5- 9.5. Application Tests

Determination of the Cobb test value (60) : Test A

A surface-untreated paper having a weight of 80 g/m² prepared following the usual methods is impregnated by means of a size press with a preparation containing:

- 7% by weight of a hot-water-soluble starch;

- an anionic-agent amount, obtained from Examples 1 to 6, sufficient for introducing 1.2% by weight of dry content into the preparation; - a sufficient amount of water for reaching 100% by weight.

The preparation amount absorbed by the paper is capable of determining, when drying has occurred, an increase in the paper weight included between 45 and 55%. Paper is dried at a temperature of 110°C and subsequently the Cobb test value (60) is determined following the standard methodology.

Example No. Absorbed water g/m²

1 (control ) 40-44 g/m²

2 24-26 g/m² 3 26-28 g/m²

4 18-20 g/m²

5 18-20 g/m²

6 18-20 g/m² Test B A surface-untreated paper having a weight of 60 g/m², prepared following the usual methods is impregnated by means of a size press with a preparation as set forth herebelow:

- 8% by weight of a hot-water-soluble starch; - an anionic-agent amount, obtained from Examples 1 to 6, sufficient for introducing 1.2% by weight of dry content into the preparation;

- a sufficient amount of water for reaching 100% by weight. The preparation amount absorbed by the paper is capable of determining, when drying has occurred, an increase in the paper weight included between 45 and 55%.

Paper is dried at a temperature of 110°C and subsequently the Cobb test value (60) is determined following the standard methodology.

Example No . Absorbed water g/m²

1 ( cont rol ) 36 -40 g/m²

2 20 g/m²

3 22 g/m²

4 18 g/m²

5 18 g/m²

6 18-20 g/m²

Claims

C L A I M S

1. An anionic agent comprising a solution copolymer, said copolymer being obtained by copolymerization of an alkene monomer with a compound obtained by reacting:

- 20-50% by moles of an unsaturated cyclic anhydride or derivates thereof;

- 30-60% by moles of one or more alcohols; and

2. The anionic agent as claimed in claim 1, characterized in that the anhydride converted into imide in said copolymer is at least 50% by moles of the starting anhydride.

3. The anionic agent as claimed in claim 1, characterized in that no more than 50% of the starting anhydride is esterified.

4. The anionic agent as claimed in claim 1, characterized in that the alkene monomer is selected either from cyclic and/or straight-chain unsaturated olefin hydrocarbons containing a number of carbon atoms included between 4 and 10, or from benzene derivatives with one or more olefin substituents having a number of carbon atoms included between 1 and 8.

5. The anionic agent as claimed in claim 4, characterized in that the alkene monomer is selected from ╬▒-methyl styrene, ortho-, meta- and para-methyl styrene, ortho-, meta- and para-divinyl benzene or styrene.

6. The anionic agent as claimed in claim 1, characterized in that the unsaturated cyclic anhydride or a derivative thereof is selected from those anhydrides having a five atoms cyclic ring containing at least one substituent with an alkyl-chain having a number of carbon atoms included between 1 and 8.

7. An anionic agent as claimed in claim 6, characterized in that the unsaturated cyclic anhydride is maleic anhydride.

8. The anionic agent as claimed in claim 1, characterized in that said one or more alcohols comprise at least one alcohol selected from those with a number of carbon atoms included between 1 and 8.

9. The anionic agent as claimed in claim 8, characterized in that alcohol is selected from methyl alcohol, ethyl alcohol, butyl alcohol, isopropyl alcohol or benzyl alcohol and mixtures thereof.

10. The anionic agent as claimed in claim 1, characterized in that the mono- and/or polyfunctional epoxy derivative is selected from aliphatic, cycloaliphatic or aromatic derivatives.

11. The anionic agent as claimed in claim 10, characterized in that the mono- and/or polyfunctional epoxy derivative is selected from para tert-butylphenol monoglycidyl ether, 2-ethylhexyl monoglycidyl ether alcohol, 1, 6-hexanediol diglycidyl ether, 4,4'- isopropyliden diphenol diglycidyl ether (bisphenol A) , bisphenol F (an isomeric mixture) diglycidyl ether and vinyl ciclohexyl dioxide.

12. The anionic agent as claimed in claim 1, characterized in that the mono and/or polyfunctional epoxy derivative has a functionality included between 1 and 4 and an equivalent weight included between 100 and 1400.

13. The anionic agent as claimed in claim 1, characterized in that the copolymer has a molar ratio between alkene monomers and maleic anhydride monomers included between 1:1 and 5:1.

14. The anionic agent as claimed in claim 1, characterized in that the dry content also comprising the salified copolymer is 10-25% by weight.

15. A process for preparing an anionic agent comprising a solution copolymer of an unsaturated cyclic anhydride or derivatives thereof and an alkene monomer, wherein said process comprises the following steps: a) solubilizing said unsaturated cyclic anhydride or derivatives thereof in one or more organic solvents; b) adding a solution comprising at least one or more alcohols and a mono- and/or polyfunctional epoxy derivative to the solution thus obtained, so as to obtain a compound originating from the reaction of:

- 20-50% by moles of an unsaturated cyclic anhydride or derivates thereof;

- 30-60% by moles of one or more alcohols; and

16. Use of the anionic agent as claimed in claim 1 for paper treatment in a working cycle of said paper.

17. The use as claimed in claim 16, characterized in that the anionic agent is employed for surface treatment of the paper with a method involving a size press.

18. The use as claimed in claim 17, characterized in that the anionic agent is employed in such proportions that the dry content of the anionic agent is included between 0.01 and 3% by weight, referred to the paper weight.