SE1050985A1

SE1050985A1 - A paper or paperboard product and a process of manufacture of a paper or paperboard product

Info

Publication number: SE1050985A1
Application number: SE1050985A
Authority: SE
Inventors: Lars Axrup; Isto Heiskanen; Kaj Backfolk; Mika Riikonen
Original assignee: Stora Enso Oyj
Priority date: 2010-09-22
Filing date: 2010-09-22
Publication date: 2012-03-23
Also published as: WO2012039668A1; BR112013006626A2; CN103154366A; JP2013537942A; EP2619367A1; EP2619367A4; US20130180680A1

Abstract

17 Abstract The present invention relates to a paper or paperboard productcomprising a furnish wherein said furnish comprises a cationic polymer in anamount of above 1,5% by weight, an anionic polymer and microfibriliatedceiiuiose or highly refined puip with a SR value >80. The invention furtherreiates to a process for the production of said product.

Description

A PAPER OR PAPERBOARD PRODUCT AND A PROCESS FORPRODUCTION OF A PAPER OR PAPERBOARD PRODUCT.

Field of the invention The present invention relates to a paper or paperboard productcomprising a furnish which furnish comprises a cationic polymer, an anionicpolymer and microfibrillated cellulose or highly refined pulp with a SR value>80.

Background ln papermaking processes there is an ongoing concern to find ways to produce paper or paperboard at reduced cost without impairing the propertiesof the product, e.g. without decreasing the mechanical properties such asstrength of the product This could be done by process optimization or byusing alternative low cost raw materials and hence gradually replace therather expensive wood based fiber.

One way to reduce cost of a paper or paperboard product is to increasefiller content of the product and thus be able to reduce the amount of fibersused in the paper or paperboard. Besides being economically beneficial.fillers also improve the opacity and printability properties of the product.However, it requires that the cost of the filler is substantially lower than thecost of the fiber material. Also, large amount of fillers in the productdecreases the strength. Thus, there is a balance between the possibleamount of fillers added and the required strength of the paper or paperboardproduced.

Furthermore, during production of paper or paperboard there is a desireto produce a strong but yet low density product. When increasing the strengthof a paperboard the density normally increases. On the other hand, if thedensity is reduced the strength is normally decreased. There is thus abalance between the desired strength and the density of the paperboardproduct. lt is possible to compensate for the decrease in strength, caused forexample by addition of large amount of filler or by increased bulk, byimproving the fiber bonding properties between the fibers in the paper orpaperboard and thereby maintaining the strength. The predominant treatmentfor improving paper or paperboard strength, particularly dry strength, has sofar been to add a strength agent, preferably cationic starch, to the furnishprior to the sheet forming operation. Cationic starch moiecules added to thefurnish can adhere to the naturally anionic pulp fibers by eiectrostaticattraction or by hydrogen bonds and thus be retained in the wet fiber mat andremain in the final paper or paperboard. Amphoteric starches or even anionicstarches could also be used in order to achieve the same effect.

When adding large amounts of cationic starch to a papermaking furnish,in order to achieve high resulting paper or paperboard strength, two majorproblems arise. The first is that the cationic starch moiecules tend to occupyor screen the anionic charges on the cellulose fibers, thus setting a limit to theamount of cationic starch which can be fixed to the fibers. lf an excess ofcationic starch is added, only a portion of the added starch will be retained inthe sheet, and the rest will circulate in the paper or board machine whitewater system. A second problem is that fibers which are made cationic byexcessive cationic starch addition will not be able to adsorb other cationicadditives which are commonly added to the pulp slurry, such as sizing agentsand retention aids. Furthermore, high amounts of starch often causeproblems with runnability, microbiology and foaming during the productionprocess, which is partly related to the fact that fiber has a certain capacity toabsorb/adsorb starch molecules.

Another approach to improve dry strength by using starch is to adduncooked starch or partly cooked starch. ln this way, the starch gelatinizesduring wet end paper making process, pressing and drying section. Besidesits physical chemical character in the uncooked form or partly cooked formed,it has both better tendency to improve retention and to be fixed to fiber-fiberconnections.

Another way to be able to increase the amount of starch in a paper orpaperboard product is to alternately add cationic starch and an anionicpolymer to the furnish. ln this way a polyelectrolyte multilayer is formed and the amount of starch can be increased. This is for example described inWO2006/O41401. However, these multilayer concepts, i.e. layer-by-layeradsorption technique or polymer complexation, is rather difficult to implementin mill scale, especially when several multilayers are required in order toprovide sufficient strength and retention levels. This kind of concept is verysensitive to the concentration of polymers, order of addition, charge density ofpolymers and the system, temperature competitive adsorption, dissolvedcolloidal substances and dosages.

When adding several polymers, it is preferred that polymercomplexation occur on the fiber surface. ln order to achieve this, it isimportant that all polymers are fixed onto the fiber surface, otherwise anion-cation polymer complexation might occur in the liquid phase and this mightthen impact negatively on the paper machine runnability. lf retention is nothigh, particularly when using polymers from renewable resources, the impacton microbial activity in the wet-end might be very critical. ln this respect, it ispreferable that adsorption of all anionic-cationic polymeric additives are wellcontrolled.

There is still a need for a robust system having high retention ofpolymers when producing a paper or paperboard product with good strengthproperties.

Summary of the invention lt is an object of the present invention to provide a paper or paperboardwith improved strength and maintained or even reduced density.

Another object of the present invention is to provide a process which inan easy and cost efficient way will be able to produce a paper or paperboardwith improved strength and maintained or reduced density.

Yet another object of the invention is to ensure high load of cationicpolymer to the paper or paperboard without losing retention of the addedpolymer.

These objects and other advantages are achieved by the paper orpaperboard product according to claim 1. The present invention relates to a paper or paperboard product comprising a furnish which comprises a cationicpolymer and an anionic polymer wherein the furnish further comprisesmicrofibrillated cellulose or highly refined pulp with a SR value >80, and thatthe amount of cationic polymer is above 1,5% by weight by total weight of thefurnish. lt has been shown that a product made from a furnish comprisingcationic polymer, anionic polymer and microfibrillated cellulose (MFC) orhighly refined pulp with a SR value >8O will have increased strength since theamount of cationic polymer can be increased. Furthermore, even though thestrength of the product increases, it has been shown that the density of theproduct is not increased as much as expected. lt is thus possible to produce aproduct with decreased bulk but still with very good strength properties.

The furnish preferably comprises cellulosic fibers which may behardwood and/or softwood fibers. lt is preferred that the furnish comprises i-30% by weight (by total weight of the furnish) of microfibrillated cellulose or highly refined pulp with aSR value >80. The amount of cationic polymer, anionic polymer respectivelyMFC or highly refined pulp with a SR value >8O of the product depends onthe desired properties of the product and the end use of the product. Highamounts of cationic polymer will increase the strength of the product and ithas been shown that the combination of l\/lFC or highly refined pulp with a SRvalue >80, anionic polymer and cationic polymer makes it possible for theproduct to retain larger amounts of cationic polymer. lt is preferred that the cationic polymer is cationic starch or amphotericstarch. The anionic polymer is preferably carboxymethyl cellulose (CMC).

The product may be a multiply product comprising at least two paper orpaperboard plies. lt may be preferred that the product comprises at leastthree plies and that the ply located in the middle of the product comprises thefurnish comprising cationic polymer, anionic polymer and microfibrillatedcellulose or highly refined pulp with a SR value >80.

The invention further relates to a process for producing a paper orpaperboard product which process comprises the steps of providing a furnishcomprising fibers, adding more than 1,5% by weight (by total weight of the furnish) of cationic polymer to the furnish, adding an anionic polymer to thefurnish, adding microfibrillated cellulose or highly refined pulp with a SR value>80 to the furnish and thereafter conducting the furnish to a wire in order toform a web. lt is preferred that 1-30 % by weight (by total weight of the furnish),even more preferred 145% by weight of microfibrillated cellulose or highlyrefined pulp with a SR value >80 is added to the furnish.

The cationic polymer, the anionic polymer and MFC or highly refinedpulp with a SR value >80 are preferably added to the furnish separately. lt ispreferred that cationic polymer first is added to the furnish in a first step,which first step is followed by addition of MFC or highly refined pulp with a SRvalue >80 in a second step, which second step is followed by addition ofanionic polymer in a third step. lt is preferred that the cationic polymer isadded to the furnish in one step, ie. in a single addition point.

Detailed description The invention relates to a paper or paperboard product comprising afurnish which comprises a cationic polymer, an anionic polymer andmicrofibrillated cellulose or highly refined pulp with a SR value >80. Thefurnish comprises cationic polymer, preferably cationic staroh or amphotericstarch, in an amount of above 1,5% by weight (by total weight of the furnish),preferably above 2,0% by weight or even more preferably above 3,0% byweight. Consequently, a paper or paperboard product or a ply of the paper orpaperboard product produced from the furnish, will also comprise cationicpolymer of an amount above 15% by weight. lt has been shown that byadding cationic polymer, anionic polymer and microfibrillated cellulose orhighly refined pulp with a SR value >80 to a furnish, it is possible to increasethe amount of cationic polymer retained in the product produced of thefurnish. The increased amount of cationic polymer, such as starch, to thefurnish and thus also to a paper or paperboard product, increases thestrength of the product will increase. Both the z-strength, Scott- Bond andcompression strength has been shown to increase of a product producedaccording to the invention. Furthermore, the density of the product is notreduced as much as feared and it is thus possible to produce a very strongbut yet high bulk paper or paperboard product.

One reason for the improved retention of the added polymers and MFCto the furnish is that relatively large particle-polymer complexes are formed.The complexes formed according to invention is much larger compared to thepolymer complexes formed in PEM techniques according to prior art. Themicrofibrilled cellulose or highly refined pulp with a SR value >8O togetherwith the polymers used in the invention will form complexes which both areattached to the fibers by bonds and also due to the relative large size whichleads to that the complexes are retained in the fiber matrix. improvedretention will cause less BOD and COD problems in the white water.

Furthermore, it is also possible to adjust and control the charge of thefibers of the treated furnish, making sure that the fibers are anionic. ln thesame way it is also possible to adjust and control the total charge of thesystem. Since fibers normally are anionic, commonly used papermakingadditives and chemicals are designed to work together with the anionic fibers.Consequently, it is an advantage to be able to control the charge of the fibersin order to make them anionic and thus be able to use the normally usedpapermaking additives.

The furnish preferably comprises cellulosic fibers. The cellulosic fibersmay be hardwood and/or softwood fibers. The cellulosic fibers may bemechanically, chemimechanically and/or chemically treated. The fibers mayalso be bleached or unbleached. The furnish might also contain broke orrecycled paper or paperboard. The furnish might also contain fiber from non-wood based materials.

Microfibrillated cellulose or highly refined pulp with a SR value >8Ocomprises many negatively charged "sites" and large amount of open surfacewhich are formed during the production of MFC or highly refined pulp with aSR value >80. The addition of microfibrillated cellulose or highly refined pulpwith a SR value >8O to furnish will thus increase the sites to which a cationicpolymer can attach. By combining the addition of microfibrillated cellulose orhighly refined pulp with a SR value >8O and cationic polymer it is thuspossible to increase the amount of cationic polymer retained to the fibers andthus also in the paper or paperboard product. Furthermore, the addition of theanionic polymer is above all added in order to make sure that the fibers of the furnish is anionic after addition of the cationic polymer and l\/IFC or highlyrefined pulp with a SR value >80. The anionic polymer will of course also bindcationic polymer. The anionic respectively cationic polymer and the MFC orhighly refined pulp with a SR value >8O added to the furnish are interactingwith each other, thereby enabling a larger amount of polymers and MFC orhighly refined pulp with a SR value >8O to be attached to the fibers and thisresults in increased strength of the final paper or paperboard product.

The cationic polymer may be one or more chosen from the groupconsisting of: cationic starch, amphoteric starch, polyvinyl amine, chitosan,primary and secondary amines, polyethylene imines, PolyDADMAC,polyallylamine, cationic polysaccharlde , PVAm, polyethyleneimine (PEl),Polyethyleneoxide (PEO), polyamine, polyvinyl pyrrolidone, modified polyacrylamides (PAM) or similar polymers. The cationic polymer is preferably cationicstarch, which is advantageous because it results in a board or paper havingenhanced strength properties and is economically beneficial, due to its lowprice and easy availability.

The anionic polymer may be one or more chosen from the groupconsisting of: carboxymethyl cellulose (ClvlC), polyvinyl sulphate, anionicgalactoglucomannan, anionic starch, polyphosphoric acid, alginate polyacrylicacid, protein, anionic polyacryl amide, anionic silica, bentonite, papermakersalum and polymethacrylic acid, The anionic polymer is preferably CMC, sinceit interact well with cationic polymers as well as it is economically beneficiai,due to its low cost. lt is preferred that the anionic polymer is added in anamount so that the fibers of the furnish has the same charge as before anychemicals were added, i.e. back to the original charge of the fibers.

The ratio between added cationic starch and anionic CMC is preferably5:1 to 15:1, preferably 10:1, lf other polymers are used, the ratio may bedifferent, for example between 520,1 to 30:O,1 (cationic polymer: anionicpolymer). Optimal ratio depends on the charge difference of the addedpolymers, The ratio between the added cationic polymer and anionic polymerof the furnish is controlled and regulated in order to receive the optimalstrength and density of the paper or paperboard product.

An advantage with the present invention is that is possible to addcationic polymer in an amount above 1,5% by weight to a furnish in a singleaddition step. This is due to the combination of cationic polymer, anionicpolymer and MFC or highly refined pulp with a SR value >80. This can becompared to the polyelectrolyte multilayering techniques (PEM) described inprior art, where small amounts of cationic polymer is added in consecutivesteps. By this invention, a more robust and less complicated process for theproduction of a paper or paperboard product comprising high amounts ofcationic polymer.

However, it is possible to also add each of the components, i.e.cationic polymer, anionic polymer and MFC or highly refined pulp with a SRvalue >80 in more than one consecutive steps. The cationic polymer is thusadded in more than one step, i.e. at least two smaller amounts of cationicpolymer is added to the furnish, preferably in consecutive steps. The samemight go for the anionic polymer and MFC or highly refined pulp with a SRvalue >80 if necessary.

Microfibrillated cellulose (MFC) (also known as nanocellulose) is amaterial made from wood cellulose fibers or agricultural raw materials orwaste products, where the individual microfibrils have been partly or totallydetached from each other. MFC is normally very thin (~20 nm) and the lengthis often between 100 nm to 10 um. However, the microfibrils may also belonger, for example between 10-100 um but lengths up to 200um can also beused. Fibers that has been fibrillated and which have microfibrills on thesurface and microfibrils that are separated and located in a water phase of aslurry are included in the definition MFC.

MFC can be produced in a number of different ways. lt is possible tomechanically treat cellulosic fibers so that microfibrils are formed. Theproduction of nanocellulose or microfibrillated cellulose with bacteria isanother option. Electrospinning is another method for production ofmicrofibrillated cellulose or nanofibers. lt is also possible to producemicrofibrils from cellulose by the aid of different chemicals and/or enzymeswhich will break or dissolve the fibers. Another option is to use steam andpressure to breakup the intra and inter-fibril hydrogen bonds. Most common 2G the MFC is produced by combining the chemical or bio-chemical step with preor post-mechanical treatment in one or several steps.

One example of production of MFC is shown in WO2007091942 whichdescribes production of MFC by the aid of refining in combination withaddition of an enzyme. lt is also possible to modify the microfibrillated cellulose before additionto the furnish. ln this way it is possible to change its interaction and affinity toother substances. For example, by introducing more anionic charges to MFCthe stability of the fibril and fibril aggregates of the MFC are increased Howthe modification of the microfibrillated fibers is done depends, for example onthe other components present in the furnish. lt is also possible to add highly refined pulps to the furnish. Cellulosicpulps refined to >8O SR value may thus be used. lt is possible to use afraction with a SR value >8O of a refined pulp, for example a fines fraction. ltmay be preferred to use CTMP pulp which has been refined to >8O SR value.lt may be possible to refine an entire CTMP pulp of to use a fraction with a SRvalue >8O of the refined CTMP pulp. lt may be necessary to increase theamount of added highly refined pulp to the furnish compared to if MFC isadded in order to achieve the same strength effect. On the other hand, thedensification is reduced when highly refined pulp is added.

The chosen amount of cationic polymer, anionic polymer respectivelyMFC or highly refined pulp with a SR value >8O added to the furnish dependson the final product produced and the desired properties of the product. Highamounts of cationic polymer will increase the strength of the product.However, it is not possible to increase the amount of cationic polymer toomuch since other problems then may occur. For example, too high amountsof cationic polymer, for example cationic starch, might make the paper orpaperboard product sticky, which might make it difficult to remove the paperor paperboard web from the wire. Too high amounts of MFC or highly refinedpulp with a SR value >8O might increase the density of the paper orpaperboard product too much. This might not be advantageous when makingfor example a high quality paperboard that often needs to have a high bulk.Moreover, too high amounts of MFC or highly refined pulp with a SR value >80 may also cause dewatering problems since MFC or highly refined pulpwith a SR value >80 is a very fine material which easily absorbs water andincreased content will make it more difficult to dewater the product. lftheamount of anionic polymer is too large the added anionic polymer will not beable to bond to the cationic polymer, the fibers and/or MFC or highly refinedpulp with a SR value >80 with cationic charge. Consequently, free amounts ofanionic polymers will be present in the furnish, which might make dewateringof the furnish in order to form a fiber based web difficult.

The paper or paperboard product is preferably a multiply productcomprising at least two plies of paper or paperboard. lt may be preferred thatthe product comprises at least three plies and that the ply located in themiddle of the product comprises furnish comprising cationic polymer, anionicpolymer and microfibrillated cellulose. lt is also preferred that the furnishcomprises CTMP which then forms the middle ply of a paper or paperboardproduct. However, it is also possible that at least one outer ply of the productor even all plies of the product comprises furnish comprising cationic polymer,anionic polymer and MFC or highly refined pulp with a SR value >80. Forsome products it might be advantageous that at least one of the outer pliescomprises furnish comprising cationic polymer, anionic polymer and MFC orhighly refined pulp with a SR value >80. ln this way it is possible to increasethe strength and/or the bulk ofthis ply. Consequently, depending on the enduse of the product, it is decided which and how many of the plies that willcomprise furnish comprising cationic polymer, anionic polymer and MFC orhighly refined pulp with a SR value >80. lt is not necessary that the entire furnish in a ply of the paper or paperboard product comprises cationic polymer, anionic polymer and MFC orhighly refined pulp with a SR value >80, but it is preferred that the cationicpolymer, anionic polymer and MFC or highly refined pulp with a SR value >80are added to the majority of the furnish of the ply. However, the ply may alsocomprise other components, such as broke pulp which does not comprisecationic polymer, anionic polymer and MFC or highly refined pulp with a SRvalue >80.

The furnish may also contain various amounts of fillers to increase forexample runnability and cost-efficiency of the process and the produced 11 substrate. Other commonly used additives used in the production of paper orpaperboard can also be added.

The paperboard product is preferably a high quality paperboardproduct, such a liquid packaging board, graphical board or food serviceboard. The paper product is preferably a high quality paper, such as copypaper of grades A or B, graphical papers, LWC, SC or news paper for highspeed printing machines. However, other grades, such as corrugated boardor liner may also be produced.

The product according to the invention, i.e. the product made from afurnish being treated with a cationic polymer, MFC or highly refined pulp witha SR value >80 and an anionic polymer wherein the furnish comprises atleast 1,5% by weight of cationic polymer, is preferably a paperboard withincreased strength and with little or no densification. The paperboard ispreferably a multiply board wherein the furnish comprising cationic polymer,anionic polymer and MFC forms at least one piy of the product, wherein theboard preferably has a z-strength above 250 kPa, preferably between 250-400 kPa and even more preferably between 250-350 kPa and a density of theformed ply of between 300-550 kg/m3, preferably between 350-500 kg/m3.The middle ply of said paperboard product has preferably a z-strength ofabove 250 kPa and since the middle ply of a multiply paperboard productnormally is the weakest ply, the z-strength of the paperboard product is thusalso above 250 kPa.

The present invention further relates to a process for producing apaper or paperboard product which process comprises the steps of providinga furnish comprising fibers, adding a cationic polymer of an amount above1,5% by weight to the furnish, adding microfibrillated cellulose or highlyrefined pulp with a SR value >80 to the furnish, adding an anionic polymer tothe furnish and conducting the furnish to a wire in order to form a web. Theadditions of cationic polymer, anionic polymer and MFC or highly refined pulpwith a SR value >80 are preferably done in the machine chest or before thefan pump. lt may also be possible to add the components to the circulationwater which later on is added to the furnish. However, all practical points ofaddition for the cationic polymer, anionic polymer and MFC or highly refinedpulp with a SR value >80 can be used as long as there is enough time and 12 mixing of the cationic polymer, anionic polymer and MFC or highly refinedpulp with a SR value >8O with the furnish before it is conducted to the wire.

The cationic polymer, anionic polymer and MFC or highly refined pulpwith a SR value >8O are preferably added separately. lt is also preferred thatthat cationic polymer first is added to the furnish in a first step, which first stepis followed by addition of MFC or highly refined pulp with a SR value >8O in asecond step, which second step is followed by addition of anionic polymer ina third step, lt is preferred to first add the cationic polymer to the furnishwhich will adsorb to the fibers of the furnish. MFC or highly refined pulp with aSR value >8O is thereafter added and any free cationic polymer will thenabsorb to the added MFC or highly refined pulp with a SR value >80, Thefinal addition of anionic polymer is added in order to both adsorb free cationicpolymer (if any) of the furnish and above all in order to get the total charge ofthe furnish back to negative. lt is preferred that the fibers have negativecharge since many papermaking additives are cationic and will thus interactwell with the anionic fibers of the furnish.

The furnish is preferably mixed between the addition points, i.e. beforeaddition of another component. lt is preferred that the cationic polymer isadded in one step, i.e. one addition point, to the furnish. lt is also possible to mix the cationic polymer, anionic polymer and/orMFC or highly refined pulp with a SR value >8O before addition to the furnish.lt is thus possible to mix all three components before addition. lt is howeveralso possible to mix two of the components and add that mixture to thefurnish followed by addition of the third component. lt is preferred to mixcationic polymer and MFC or highly refined pulp with a SR value >8O and addthat to the furnish before addition of the anionic polymer, or mix anionicpolymer and MFC or highly refined pulp with a SR value >8O followed byaddition of the cationic polymer. 13 ExampleMaterial used: Chemo thermo mechanical pulp (CTMP) at 500 CSF.

Microfibrillated cellulose (MFC) was prepared by refining bleachedsoftwood kraft pulp of 4% consistency with edge load of 2 Ws/m to 28 SR.The pulp were thereafter enzymatically treated with Endoglucanase(Novozym 476) with the activity of 0,85 ECU/g. The enzymes were dosed tothe pulp and which thereafter was treated at 50°C for 2 hours, at pH 7. Afterthe enzymatic treatment, the pulp was washed and enzymes weredeactivated at 80°C for 30 min. The pulp was thereafter refined once more to90-95 SR and the refined pulp was then fluidized (Microfluidizer, Microfuidicscorp.) by Ietting pulp of 3% consistency pass through a 400pm Chamberfollowed by a 100pm chamber wherein the MFC used were formed.

Starch used was cationized starch, Raisamyl 70021, Ciba (now BASF).

Cl\/IC used was FinnFix30, CP-KelcoC-PAM used was Percol 292 NS, Ciba (now BASF).BMA used was Eka NP495, Eka Chemicals.

Procedure:The dried CTMP were soaked in water over night and then dispersedin hot water. The CTMP suspension was thereafter diluted to a concentration of 03%.

The produced MFC was also diluted to a concentration of 03% anddispersed using a kitchen mixer.

A formette sheet former was used to prepare the sheets for testing.The sheets were prepared according to the following procedure; Pulpsuspension measured to produce a 150gsm sheet was added to the stocktank. During agitation, cationic-starch if used, MFC if used, and CMC if used,were added in the mentioned order with 30 to 60 seconds between theadditions to ensure thorough mixing before addition of next component. After 14 30 seconds, 500g/t C-PAM was added and after another 30 seconds was300g/t BMA added to the stock and the sheet forming was thereafter started.

The formed sheet was wet pressed and dried while the shrinkage was constrained. The dried sheet was tested for structural density according toSCAN P 88:01 and z-strength according to SCAN P 80:88 Table 1: Shows the results of strength and density Sample Cationic CMC (kg/t) MFC Density z-strengthStarch STFI (kPa)(kg/t) (kg/m3)0 0 0 361 131Ref 30 3 0 359 21260 6 0 359 2430 0 30 399 162Sample 1 30 3 30 402 25260 6 30 394 3310 0 60 442 253Sample 2 30 3 60 434 34260 6 60 438 424 As can be seen from table 1, the strength of Sample 1 and 2 whenstarch, CMC and MFC was added is increased compared to if no MFC wasadded. Also, the density of the board is almost maintained, ie. it is notincreased much. ln view of the above detailed description of the present invention, othermodifications and variations will become apparent to those skilled in the art.However, it should be apparent that such other modifications and Variationsmay be effected without departing from the spirit and scope of the invention.

Claims

1. A paper or paperboard product comprising a furnish whichcomprises a cationic polymer and an anionic polymer characterized in that thefurnish further comprises microfibrillated cellulose or highly refined pulp with aSR value >80, and that the amount of cationic polymer is above 1,5% byweight (by total weight of the furnish).

2. The paper or paperboard product according to claim 1 characterizedin that the furnish comprises cellulosic fibers.

3. The paper or paperboard product according any of the precedingclaims characterized in that the furnish comprises 180% by weight ofmicrofibrillated cellulose or highly refined pulp with a SR value >80.

4. The paper or paperboard product according to any of the precedingclaims characterized in that the cationic polymer is cationic starch oramphoteric starch.

5. The paper or paperboard product according to any of the precedingclaims characterized in that the anionic polymer is carboxymethyl cellulose(CMC).

6. The paper or paperboard product according to any of the precedingclaims characterized in that the product is a multiply product comprising atleast two paper or paperboard plies.

7. The paper or paperboard according to claim 6 characterized in thatthe product comprises at least three plies and that the ply located in themiddle of the product comprises furnish comprising cationic polymer, anionicpolymer and microfibrillated cellulose or highly refined pulp with a SR value>80. 16

8. : The paper or paperboard according to any of the preceding claims,characterized in that the product is a multiply board wherein the furnishcomprising cationic polymer, anionic polymer and MFC forms at least one plyof the product, wherein the board has a z-strength of 250-400 kPa and adensity of the formed ply of between 300-550 kg/m3,

9. A process for producing a paper or paperboard product whichprocess comprises the steps of: -providing a furnish comprising fibers, -adding more than 1,5 % by weight of cationic polymer to the furnish,-adding microfibrillated cellulose or highly refined pulp with a SR value>80 to the furnish, -adding anionic polymer to the furnish and -conducting the furnish to a wire in order to form a web.

10. The process according to claim 9 characterized in that 1-30% byweight of microfibrillated cellulose or highly refined pulp with a SR value >80is added to the furnish.

11. The process according to any of claims 9-10 characterized in thatcationic polymer, the anionic polymer and MFC or highly refined pulp with aSR value >80 are added separately to the furnish.

12. The process according to any of the claims 9-11 characterized inthat cationic polymer first is added to the furnish in a first step, which first stepis followed by addition of MFC or highly refined pulp with a SR value >80 in asecond step, which second step is followed by addition of anionic polymer ina third step.

13. The process according to any of the claims 9-12 characterized inthat the cationic polymer is added in a single addition point.