Background The formation (a measure of the basis weight variations of paper) of paper is a major quality parameter of paper. The formation affects the strength/fracture resistance as well as the printability of the paper sheet. In order to retain the filler on/between the wire(s), retention aids are used. There are a number of different types of retention aids used in paper manufacture, known to those skilled in paper manufacture. They may broadly be classified into one component or two component systems. One-component systems may in turn be classified into charge neutralisation agents, patch flocculants and bridging type of flocculants. Both anionic and cationic polymers are being used, although the latter are more commonly used. Two-component systems utilize two active retention aid components, commonly two polyelectrolytes, but inorganic components may also be used in combination with a polyelectrolyte. The two components may have the same charge, almost always cationic or the components may be oppositely charged, which is a more frequently used combination. Combinations using one cationic polyelectrolyte and anionic silica sol or montmorillonite clay have gained large use in the paper industry. Retention aids agglomerate/flocculate the fibres in the stock suspension and a clear disadvantage is that the formation of paper is deteriorated when retention aids are used. Thus there is a problem with the deterioration of paper formation when retention aids are used. In spite of the fact that considerable efforts have been spent in order to improve the formation at a given retention level, the experience is that most retention aid types have comparable formation compared at a given retention level.

Summary of the invention Through the use of fibres grafted with carboxymethylcellulose (CMC) or a derivative thereof or an amphoteric CMC derivative, it has now, surprisingly been found, that using a particular order of addition of chemicals, so may not only the retention be greatly improved, but also the formation at a given retention level. Another feature is that the retention system can be used at high electrolyte concentrations. Accordingly, the present invention solves one or more of the above problems by providing, according to a first aspect a method for manufacturing paper or paperboard, wherein the a) the pulp is treated with CMC or a derivative thereof or an amphoteric CMC derivative, thereby forming a stock , b) filler/pigment (preferably CaCO3) is treated with a cationically active polymer with an addition of approximately from 0.015 % to 1.5 % (based on filler/pigment; preferably CaCO3), preferably from 0.03 % to 0.6 %, most preferred approximately 0.3 % ; and c) the thus treated filler/pigment is added to the stock, comprising of at least 10 % treated fibres (up to 100%) according to step a). The present invention also provides, according to a second aspect, paper or paperboard obtainable by the above method according to the first aspect. According to a third aspect of the invention, use of paper or paper board according to the second aspect of the invention for the manufacture of liquid board, communication paper, packaging paper, liner or board is provided.

Detailed description of the invention It is intended throughout the present description that the expression "CMC or a derivative thereof, embraces in addition to carboxymethylcellulose, various derivatives thereof other than amphoteric CMC derivatives. A preferred molar degree of substitution (D. S.) may approximately be 0.3 - 1.3 and a preferred viscosity may approximately be 25 - 8,000 mPa at a concentration of 4%. A higher viscosity may be preferred, since it has become clear that the irreversibility of the adsorption is higher for higher molecular weights. It is intended throughout the present description that the expression "amphoteric CMC derivative", embraces derivatized carboxymethylcellulose which has become amphoteric. This is accomplished by cationizing said carboxymethylcellulose. Said CMC derivative has preferably a molar substitution degree between 0.0001 and 1.2. Further said CMC derivative is preferably cationized to a substitution degree between 0.001 and 1.0, preferably 0.01 and 0.4, and the cationization is preferably performed by the introduction of at least one ammonium function, preferably a secondary, tertiary or quaternary ammonium function, into the derivative. It is intended throughout the present description that the expression "cationically active polymer" embraces any cationically active polymer, and is preferably selected from the group consisting of cationic polyacrylamides, cationic polyamines, cationized polyamide amines, cationized natural products and cationic starches. Cationic polyacrylamides is preferably selected from the group consisting of dimethylaminoethyl methacrylate (DMAEM), dimethylaminoethyl acrylate (DMAEA), diethylaminoethyl acrylate (DEAEA), triacrylamide-3- methyl-butyl-tri-methyl-ammonium chloride, diethylaminoethyl methacrylate (DEAEM) or their quaternary ammonium forms made with dimethyl sulfate or methyl chlorides or Mannich reaction modified polyacrylamides. The cationic polyacrylamides may also be co-polymers and they may preferably be selected from diallylcyclohexylamine hydrochloride (DACHA HCI), diallyldimethylammonium chloride (DADMAC), high-molecular weight polydiallyldimethylammonium chloride (poly-DADMAC), methacrylamidopropyltrimethylammonium chloride (MAPTAC), allyl amine (ALA) and co¬ polymers of diallyldimethylammonium chloride (DADMAC). Most preferred said cationically active polymer is high-molecular weight polydiallyldimethylammonium chloride (poly- DADMAC). It is intended throughout the present description that the expression "filler/pigment" embraces CaCO3, clay, talc or any other filler/pigment known for a person skilled in the art of paper manufacturing. Preferably said filler/pigment is CaCO3. It is intended throughout the present description that the expression "electrolyte" embraces one or more electrolytes or mixtures thereof. Examples of electrolytes are given further below. The pulp may be treated with CMC or a derivative thereof or an amphoteric CMC derivative using any method for modifying the cellulosic fibres in the pulp with CMC or a derivative (e.g. by using a method as set out in WO 99/57370) thereof or an amphoteric CMC derivative. Preferably when the pulp is treated with CMC or a derivative thereof the following method is used in step a): cellulosic fibres are treated for at least 5 minutes with an aqueous electrolyte-containing solution, at acidic or basic conditions, of CMC or a derivative thereof, whereby the temperature during the treatment is at least 100 0C and at least one of the following conditions applies: A) the pH of the aqueous solution during the treatment lies in the interval of approximately 1.5-4.5; or B) the pH of the aqueous solution during the treatment is higher than approximately 11 ; or C) the concentration of the electrolyte in the aqueous solution lies in the interval of approximately 0.001 - 0.5 M if the electrolyte has monovalent cations, or in the range of approximately 0.0002-0.25 M if the electrolyte has divalent cations. Most preferred condition C applies together of either condition A or condition B. Said preferred method in step a) is disclosed in WO 01/21890. Preferably when the pulp is treated with an amphoteric CMC derivative the following method is used in step a): the cellulosic fibres are treated for at least 5 minutes with an aqueous electrolyte-containing solution of an amphoteric CMC derivative, whereby the temperature during the treatment is at least 50° C, and at least one of the following conditions apply: A) the pH of the aqueous solution during the treatment lies in the interval of approximately 1.5 - 4.5, preferably in the region 2 - 4; or B) the pH of the aqueous solution during the treatment is higher than approximately 11 ; or C) the concentration of the electrolyte in the aqueous solution lies in the interval of approximately 0.0001 - 0.05 M, preferably approximately 0.001 - 0.04 M, if the electrolyte has monovalent cations, or in the range of approximately 0.0002 - 0.1 M, preferably approximately 0.0005-0.02 M, if the electrolyte has divalent cations. Most preferred condition C applies together of either condition A or condition B. 8. Said CMC derivative has preferably a molar substitution degree between 0.0001 and 1.2. Further said CMC derivative is preferably cationized to a substitution degree between 0.001 and 1.0, preferably 0.01 and 0.4, and the cationization is preferably performed by the introduction of at least one ammonium function, preferably a secondary, tertiary or quaternary ammonium function, into the derivative. According to a further preferred embodiment of the first aspect of the present invention the cellulosic fibres are treated with CMC or a derivative thereof or an amphoteric CMC derivative with a concentration of approximately 0.5 - 20 mg/g, preferably approximately 0.5 - 5 mg/g, most preferred approximately 1 mg/g. According to a further preferred embodiment of the first aspect of the present invention the cellulosic fibres are treated for approximately 5 -180 minutes. According to a further preferred embodiment of the first aspect of the present invention the temperature during the treatment is in excess of approximately 100 0C, preferably at least approximately 120 0C, and most preferred up to approximately 150 0C. According to a further preferred embodiment of the first aspect of the present invention the cationically active polymer is selected from the group consisting of dimethylaminoethyl methacrylate (DMAEM), dimethylaminoethyl acrylate (DMAEA), diethylaminoethyl acrylate (DEAEA), triacrylamide-3-methyl-butyl-tri-methyl-ammonium chloride, diethylaminoethyl methacrylate (DEAEM) or their quaternary ammonium forms made with dimethyl sulfate or methyl chlorides or Mannich reaction modified polyacrylamides, diallylcyclohexylamine hydrochloride (DACHA HCI), diallyldimethylammonium chloride (DADMAC), high-molecular weight polydiallyldimethylammonium chloride (poly-DADMAC), methacrylamidopropyltrimethylammonium chloride (MAPTAC), allyl amine (ALA), cationized starch and co-polymers of diallyldimethylammonium chloride (DADMAC); preferably said cationically active polymer is high-molecular weight polydiallyldimethylammonium chloride (poly-DADMAC). The cellulosic fibres that may be used with the present invention include all types of wood-based fibres, such as bleached, half-bleached and unbleached sulphite, sulphate and soda pulps, together with unbleached, half-bleached and bleached mechanical, thermo- mechanical, chemo-mechanical and chemo-thermo-mechanical pulps, and mixtures of these. Both new fibres and recycled fibres can be used with the present invention, as can mixtures of these. Pulps from both softwood and hardwood trees can be used, as can mixtures of such pulps. Pulps that are not based on wood, such as cotton linters, regenerated cellulose, kenaf and grass fibres may also be used with the present invention. The pulp may have a consistency ranging from low consistency to high consistency. According to a further preferred embodiment of the first aspect of the present invention the electrolyte is selected from the group consisting of the following ions: Na+, Ca2+, Mg2+, K+, CI", SO42" and HCO/. The paper or paper board obtainable by the method according to the first aspect of the present invention may be used for the manufacture of liquid board, various communication papers, such as newsprint grades, supercalendered SC-grades and coated communication papers such as light weight coated papers (LWC), MWC (medium weight coated) and HWC (high weight coated) and various packaging papers, kraft- and test liners and various board grades from recycled boards, folding boxboards and solid bleached boards. Preferred features of each aspect of the invention are as for each of the other aspects mutatis mutandis. The prior art documents mentioned herein are incorporated to the fullest extent permitted by law. The invention is further described in the following examples which do not limit the scope of the invention in any way. Embodiments of the present invention are described in more detail with the aid of examples of embodiments, the only purpose of which is to illustrate the invention and are in no way intended to limit its extent.

Examples

Example A This example shows that by using a particular addition order of chemicals, a high retention may be obtained at high electrolyte cone. (CaCI2) provided the pulp used has been grafted with CMC. In this example the retention of filler, CaCO3 (HC-60-GG, Omya) to a bleached softwood kraft pulp (M-real, Husums Mills) was investigated. The bleached kraft pulp had been grafted with different amounts of carboxymethylcellulose (CMC, Finnfix WRH, Noviant) as described in a previous patent application (WO 01/21890). The filler/pigment had been pre-cationised with high-molecular weight (Mw= 440000) polydiallyldimethylammonium chloride (poly-DADMAC). The polymer dosage was 5 mg/g filler/pigment. The retention level was investigated using a baffled Britt Dynamic Drainage Jar (BDDJ) at a propeller speed of 750 rev./min. The stock consisted of 80% bleached softwood kraft pulp and 20 % filler. The results are shown in table 1 together with reference experiments. Table 1. The filler/pigment retention (BDDJ) in a stock consisting of 80% bleached softwood kraft pulp (Bl.Sa) and 20 % ground calcium carbonate. The fibre material had been grafted to various levels with CMC.

) Based on CaCO3

The results in table 1 show that by grafting of CMC onto the fibre material very significant retention effects can be obtained. The retention level cannot be improved by simply increasing the addition level of poly-DADMAC. It is well known that it is necessary to have a very high molecular weight of a cationic polyelectrolyte, when being used alone as a flocculant. This is typical for bridging type flocculants. The poly-DAMAC used had a high molecular weight but not sufficiently high to act as an efficient retention aid. By instead using a high molecular weight polyacrylamide (PL 1520 from Eka Chemicals, mole % cationic groups = 20%, Mw> 5 Millions, C-PAM) it is possible to improve the retention in electrolyte free systems - this fact is obvious from table 2. With an addition level of 0.1 % of C-PAM a retention level of 76% was obtained under electrolyte free conditions. A general problem with single component bridging type systems is their sensitivity to electrolytes. Hence in the presence of 0.1 M CaCI2, the retention drops to 0 %, while with the new process the retention is 47% at the same electrolyte cone. Table 2. The filler/pigment retention (BDDJ) in a stock consisting of 80% bleached softwood kraft pulp (Bl.Sa) and 20 % ground calcium carbonate. The fibre material had been grafted with CMC.

* ) Based on CaCO3

Example B This example shows how the use of the new process it is possible to improve the formation maintaining the retention. In this example, Finnish hand sheets were manufactured in the lab using a C-PAM (PL 1520) (reference series) together with a series of sheets, using the new process. The stock was in both examples subjected to shear in a Britt Dynamic Drainage Jar for 30 sec at 1000 rev/min before the stock was transferred to the hand sheet mould. In table 3 it can be seen that the formation is deteriorated the higher the retention for the reference samples. These values may be plotted as a graph showing the formation vs. retention and it will be recognized that there is a breaking point at a retention level of 35 %, above which the formation is severely deteriorated. For the new process, this breaking in the formation/retention relationship is being shifted towards 45% for the new process. Hence, it will be possible to produce paper with significantly improved formation at a given retention level using this new process. Table 3. Retention/formation for hand sheets made from a stock consisting of 80% bleached softwood Kraft pulp (Balsa) and 20 % ground calcium carbonate but different retention/formation strategies. The fibre material had been grafted with different amounts of CMC. The formation number is the grammage variation coefficient of the paper.

) Based on CaCO3

Example C This example shows how an amphoteric CMC derivative may be obtained. The CMC preparation may be made through a method comprising the addition of 2,3-epoxi-trimethyI ammonium propane chloride to CMC at slightly elevated temperature and under basic conditions, pH 7-12. This method gives rise to a product which can be summarized as follows:

CMC-O-CH2-CHOH-CH2-N-(CH3)S CI" (counter ion)

Thus an amphoteric CMC derivative is obtained by introducing into the CMC a quaternary ammonium function, through the hydroxyl groups of the CMC. The preparation may have a substitution degree, with respect to carboxy methyl groups of DS = 0.65 and with respect to cationic nitrogen groups a DS = 0.052. Obviously it is possible to use, instead of said tri- methyl ammonium chloride, a secondary (such as di-methyl amine) or a tertiary amine (or mixtures thereof which also may comprise substances comprising quaternary ammonium functions). By using either of said amines it is possible to obtain a cationic polymer, and as CMC comprises anionic moieties it is thus possible to obtain an amphoteric CMC derivative.

Various embodiments of the present invention have been described above but a person skilled in the art realizes further minor alterations, which would fall into the scope of the present invention. The breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents. For example, any of the above- noted methods can be combined with known methods. Other aspects, advantages and modifications within the scope of the invention will be apparent to those skilled in the art to which the invention pertains.