The present invention relates to a method for reducing starch content of an aqueous phase removed from fibre stock preparation, especially stock preparation of recycled fibre material and/or broke, according to the preamble of the enclosed independent claim.

Paper, board and other cellulosic webs are often surface sized and/or coated with compositions that contain starch in order to obtain improved surface and/or other properties for the produced webs as well as for the products made from such webs. When these products are then recycled and repulped, the obtained fibre stock from the repulping process may contain significant amounts of starch originating from the compositions applied on the surface of the original webs in the previous production process. This starch is often poorly retained on the fibres as it has no charge or has a slightly anionic charge. It is easily enrichened to the water circulation of the pulping and fibre stock preparation processes, from where it may follow together with the effluent to the wastewater treatment.

Starch is also added as a dry strength agent and as a component of an internal size to the fibre stocks which are used for production of paper, board and the other cellulosic webs. Addition of starch is done in order to improve the properties of the formed cellulosic webs, e.g. to increase the strength properties of the formed cellulosic webs.

In the water circulation of the stock preparation starch may become a suitable nutrient for various microbial organisms, which increases the risk for microbial growth, slime and/or biofilm formation. Microbial organisms may further negatively affect both the functioning of the chemistry of papermaking and/or the quality of the end product. For example, microbial organisms may produce organic acids, which lower the pH of the process that may successively lead to dissolution of calcium compounds and increased risk for formation of deposits. Presence of microorganisms may also lead to formation of large stickies, which spoil the quality of the final product and may cause runnability problems. Especially harmful is the presence of microbial organisms in the production of paper or board intended for packaging, particularly food or beverage packaging, where the presence of microbial organisms may destroy the product quality of the produced packages and make them unsuitable for preserving foodstuffs, even if the packages would be visually defect-free. Abundant growth of microorganisms in the paper or board production may further cause severe odour problems.

Furthermore, there is significant loss in yield of the manufacturing process when starch present in the fibre stock is destroyed by microbes. Loss of starch must be compensated by equivalent addition of fibre material, in order to maintain the same solids content for the fibre stock. The loss of starch may also decrease the strength of the produced paper or board, which have to be compensated by supplementary starch addition or by addition of other strength chemicals.

Typically various biocide regimes are used in manufacture of production of paper, board and the other cellulosic webs, in order to reduce or eliminate the problems associated with high starch content of the fibre stock and microorganisms. However, biocide regimes do not improve the starch retention to the fibres.

It is known to use chemical auxiliaries to retain starch liberated at pulping to the fibres. For example, EP 2817453 discloses a method where an inorganic coagulant is added to a pulp flow in order to interact with the starch having a low molecular weight. A polymer flocculant is then added separately to a flow comprising interacted coagulant agent for forming starch agglomerates, which are then retained to the fibres and/or to the formed web.

However, there is a need for new effective ways to retain starch to the fibres already during the stock preparation, especially when recycled fibre material and/or broke is used. If the starch is allowed to escape with the aqueous phase which is removed from the fibre stock manufacture, e.g. at the thickening of the fibre stock, the removed starch ends up to the water treatment process and increases the COD (chemical oxygen demand) load of the water to be treated. Microbial degradation of starch during stock preparation also increases carbon dioxide emissions from the process. Consequently, the efficiency of the starch trapping to the fibres should be improved already at the early stages of the manufacture of paper, board, tissue or the like.

An object of this invention is to minimise or even eliminate the disadvantages existing in the prior art.

An object of the invention is to provide a method with which starch is effectively and cost-efficiently associated with fibres already during fibre stock preparation.

These objects are attained with the invention having the characteristics presented below in the characterising part of the independent claim.

Some preferred embodiments of the present invention are presented in the dependent claims.

All the described embodiments and advantages apply to all aspects of the present invention, even if not always explicitly stated so.

A typical method according to the present invention for reducing starch content of an aqueous phase, which is removed from a fibre stock preparation in a manufacturing process of paper, board, tissue or the like, the fibre stock preparation comprises a thickening step, where a fibre stock comprising cellulosic fibres originating from recycled fibre material and/or broke as well as starch dispersed in an aqueous phase is thickened from a first concentration to a second concentration by removing a part of the aqueous phase from the fibre stock, wherein a cationic polymer obtained by copolymerisation of (meth)acrylamide and at least 25 mol-% of solely cationic monomer(s), having a standard viscosity SV of at least 1 .7 mPas, is added to the fibre stock at the latest at the thickening step in order to associate the starch with the cellulosic fibres of the fibre stock. Now it has been surprisingly found that a cationic polymer obtained by copolymerisation of (meth)acrylamide and at least 25 mol-% of solely cationic monomers is able to associate starch with great effectiveness with the cellulosic fibres already at the fibre stock preparation stage, provided that the cationic polymer has a standard viscosity of at least 1 .7 mPas. It is assumed, without wishing to be bound by any theory, that the amount of cationic monomers and the size of the polymer provide optimal polymer structure which is able to both physically and chemically trap the starch and associate the starch with the fibres. When the cationic polymer is added to the fibre stock during the fibre stock preparation, at the latest at the thickening step, an unexpected reduction in starch content in the aqueous phase, which was removed from the fibre stock preparation, was observed. Effective removal of starch reduces microbial load in the overall process, especially in waste water treatment. Furthermore, it may be possible to reduce use of biocides in the process, and for example see a reduction in scale formation in the waste water treatment of the process.

In the present context the expression “associate with” means and is synonymous with that the starch present in the aqueous phase of the fibre stock interacts with the cationic polymer and the cellulosic fibres. The interaction may be based on physical entanglement of the starch and the polymer structure, wherein the starch is “trapped” or “caught” by the polymer structure, and/or the interaction may be based on chemical interactions, where the starch and/or fibres may be bound to each other, e.g. by electrostatic forces. The association of the starch and fibres, elicited by the cationic polymer, makes it possible for the cellulosic fibres to carry the starch forward in the manufacturing process, inhibit its removal from the process with the aqueous phase and finally enable its retention to the final web to be formed.

In the present context it is understood that the cellulosic fibres in the fibre stock may originally be produced by any suitable pulping method, i.e. they may originate from chemical pulping, mechanical pulping or chemi-mechanical pulping. The cellulosic fibres may usually be wood-based fibres, but it is possible that at least some of them are non-wood-based fibres, e.g. cellulosic fibres originating from annual plants. The fibre stock usually comprises a significant amount of recycled fibres or fibres originating from broke. For example, the fibre stock may comprise from 60 weight- %, preferably from 75 weight-% or from 90 weight-%, up to 95 weight-% or up to 100 weight-%, of recycled fibres or fibres originating from broke, calculated from dry weight of the fibre stock.

The cationic polymer used in the present invention is obtained by copolymerization of (meth)acrylamide, preferably acrylamide, and cationic monomers. In some embodiments the cationic copolymer may be obtained by copolymerization of (meth)acrylamide, cationic monomers and <1 mol-%, preferably <0.5 mol-%, more preferably <0.1 mol-%, of anionic monomers. According to one preferable embodiment, the cationic copolymer is free of anionically charged structural units, i.e. the copolymerisation is performed in the absence of anionic monomers. The polymer thus preferably consists of structural units that originate from non-ionic monomers, i.e. (meth)acrylamide, and solely from cationic monomers, even in that case a minor amount of anionically charged groups may be formed to the polymer structure during polymer preparation, e.g. during drying.

The cationic polymer may be obtained by copolymerisation of (meth)acrylamide and one or more of cationic monomers. The cationic polymer is obtained by copolymerisation of (meth)acrylamide and at least 25 mol-%, preferably at least 30 mol-%, more preferably at least 35 mol-% of solely cationic monomer(s). For example, the cationic polymer may be obtained by copolymerising 5 - 75 mol-%, preferably 20 - 75 mol-%, more preferably 30 - 70 mol-%, even more preferably 40

- 70 mol-%, of (meth)acrylamide, preferably acrylamide, and 25 - 95 mol-%, preferably 25 - 80 mol-%, more preferably 30 - 70 mol-%, even more preferably 30

- 60 mol-% of cationic monomer(s). According to one embodiment, the cationic polymer may be obtained by copolymerising 40 - 75 mol-%, preferably 45 - 75 mol- %, of (meth)acrylamide, preferably acrylamide, and 25 - 60 mol-%, preferably 25 - 55 mol-%, of cationic monomer(s). It has been observed that when at least 25 mol- % of cationic monomers is present in the polymerisation, the obtained cationic polymer is provided with good ability to associate with the starch present in the aqueous phase of the fibre stock, e.g. through electrostatic forces, and at the same time its ability to interact also with the anionically charged fibres is improved. According to one embodiment of the invention the cationic monomer(s) may be selected from 2-(dimethylamino)ethyl acrylate (ADAM), [2-(acryloyloxy)ethyl] trimethylammonium chloride (ADAM-CI), 2-(dimethylamino)ethyl acrylate benzylchloride, 2-(dimethylamino)ethyl acrylate dimethylsulphate, 2- dimethylaminoethyl methacrylate (MADAM), [2-(methacryloyloxy)ethyl] trimethylammonium chloride (MADAM-CI), 2-dimethylaminoethyl methacrylate dimethylsulphate, [3-(acrylamido)propyl] trimethylammonium chloride (APTAC), or [3-(methacrylamido)propyl] trimethylammonium chloride (MAPTAC). Preferably the cationic monomer(s) may be selected from 2-(dimethylamino)ethyl acrylate (ADAM), [2-(acryloyloxy)ethyl] trimethylammonium chloride (ADAM-CI), and [3- (acrylamido)propyl] trimethylammonium chloride (APTAC).

The cationic polymer has a standard viscosity SV of at least 1.7 mPas, preferably at least 2.5 mPas, more preferably at least 3.0 mPas. According to one embodiment the standard viscosity of the cationic polymer may be in a range of 1 .7 - 7.0 mPas, preferably 2.5 - 6.0 mPas, more preferably 3.0 - 5.0 mPas. Sometimes the standard viscosity of the cationic polymer may be in a range of 3.0 - 7.0 mPas, preferably 3.5 - 6.0 mPas, more preferably 4.0 - 5.5 mPas or 4.5 - 5.5 mPas. Standard viscosity is measured at 0.1 weight-% polymer content in an aqueous 1 M NaCI solution, using Brookfield LV viscometer equipped with UL adapter, at 25 °C, using UL Adapter Spindle and rotational speed 60 rpm. In general, the standard viscosity of the polymer gives an indication of the length and/or weight of the polymer chains of the polymer. It has been observed that when the standard viscosity SV of the cationic polymer is at least 1.7 mPas, the polymer is able to effectively associate with the starch present in the aqueous phase of the fibre stock. It is assumed that the cationic polymer has an improved ability to physically trap the starch and to interact at the same time with the starch as well as the anionically charged cellulosic fibres of the fibre stock.

A general relationship between the standard viscosity of the cationic polymer and its average molecular weight is given in Table A. Table A General relationship between the standard viscosity and the average molecular weight.

The relationship shown in Table 1 is based on standard viscosity and intrinsic viscosity measurements and using Mark-Houwink-Sakurada constants K=2.57-10’ ⁴ dl/g and a=0.67.

The cationic polymer may be in liquid form or in dry form, preferably in dry form as a particulate material. If the cationic polymer is in dry form, it is dissolved before its addition to the fibre stock. Irrespective if the polymer is in liquid form or dry form, it is usually diluted with water to a suitable dosing concentration before addition to the fibre stock. The dosing concentration may be <10 weight-%, for example 0.01 - 10 weight-% or 0.01 - 3 weight-%, sometimes 0.1 - 3 weight-% or 0.5 - 3 weight-%. The cationic polymer used in the present invention may be obtained by any suitable polymerisation method for copolymerisation of (meth )acrylam ide and cationic monomers. The cationic polymer may be obtained suspension polymerisation, such as solution polymerisation or gel polymerisation; dispersion polymerisation; or emulsion polymerisation. Preferably the cationic polymer is obtained by solution polymerisation or gel polymerisation.

The cationic polymer is added to the fibre stock at the latest at the thickening step of the fibre stock preparation, where a fibre stock comprising cellulosic fibres originating from recycled fibre material and/or broke is thickened from a first concentration to a second concentration by removing a part of the aqueous phase from the fibre stock in order to associate the starch with the cellulosic fibre material. The thickening step is usually the last stage of the stock preparation, whereafter the fibre stock is transferred optionally through various storage towers or the like and stock blending to the short circulation of the paper or board machine. The thickening step usually employs a thickener, such as a disc filter, vacuum disc filter, gravity thickener or the like. Thickening step for recycled fibre material may typically employ a disc filter as a thickener and for broke the thickener may be a gravity thickener. The first concentration for a fibre stock at the stock inlet of the thickener may be 0.6 - 1.4 weight-%, calculated as dry solids. The second concentration at the stock outlet of the thickener may be 2 - 13 weight-%, depending on the thickener used. For example, the second concentration at the stock outlet of the thickener may be 8 - 13 weight-%, as dry solids, for a disc filter, or the second concentration at the stock outlet of the thickener may be 2 - 6 weight-%, as dry solids, for a gravity thickener. According to one preferable embodiment, the cationic polymer may be added to the fibre stock at the stock inlet or through a separate feed connection into the thickener. The cationic polymer can be added, for example, to the fibre stock at the stock inlet of the thickener or to the disc filter chamber.

Alternatively, the cationic polymer may be added to the fibre stock at one of the steps preceding the thickening step in the fibre stock preparation, e.g. a screening step and/or fibre fractioning step of the fibre stock preparation. In the present context, fibre stock preparation includes all process steps needed for forming recycled fibre material in form of an aqueous fibre suspension, which after optional dilution with water is suitable for use for manufacture of paper, board, tissue or the like. Fibre stock preparation includes also broke handling, where fibre containing waste which is generated prior to completion of the manufacturing process is repulped. For example, the cationic polymer may be added to the fibre stock directly after a pulping step, where the recycled fibre material or broke is transformed into a fibre stock. In one embodiment, the cationic polymer may be added into a dump chest. It is advantageous, but not necessary, that the cationic polymer is added at a dosage location that allows some time for the association between the polymer, starch and the fibres. However, it has been observed that the addition at the latest at the thickener still provides a significant reduction in starch content of the aqueous phase removed from the fibre stock preparation.

It is possible that the cationic copolymer is added to the fibre stock in one or more dosage locations. For example, the cationic polymer may be added to the dump chest and at the inlet of the thickener. If the cationic polymer is added to the fibre stock in multiple dosage locations, the dosage amount may vary between the different locations. The cationic polymer may be added in a first dosage location in a first amount and in a subsequent dosage location in a subsequent amount, the first amount and the subsequent amount being different from each other. For example, the cationic copolymer may be added to the fibre stock in two or more dosage locations, preferably in different amounts in each dosage location. It is also possible to add cationic polymer in multiple dosage locations, the dosage amount being constant at each dosage location.

According to one embodiment of the invention the cationic polymer is added at least one additional dosage location, situated after the thickening step and before a wire section of a web forming machine. This means that it is possible to add at least one additional dose of the cationic polymer to the fibre stock after the thickening step at an additional dosage location, which is situated after the thickening step and before a wire section of a web forming machine. The cationic polymer added after the thickening step and before the wire section is preferably the same cationic polymer which is added to the fibre stock at the latest at the thickening step. For example, the additional dose of the same cationic polymer may be added to a storage tower or it may be added to a thick stock, preferably having a consistency >3 weight-%, preferably 3 - 6 weight-%. According to a preferred embodiment the additional dose of the same cationic polymer is added to the cellulosic fibre stock before a dilution step, where the fibre stock is diluted to a third concentration of <2 weight-%. According to one embodiment, the additional dose of the same cationic polymer may be even added to the thin stock having concentration <2 weight-%, in which case the addition of the cationic polymer may provide in addition of starch retention even advantageous effects in total retention and/or drainage. In one embodiment of the invention, the fibre stock preparation comprises a fibre fractioning step, where for example a long fibre fraction is separated from a short fibre fraction, and the cationic polymer is added at least to the short fibre fraction. Each of the separated fibre fractions may be separately thickened in separate thickening steps, and the cationic polymer may be added separately to the separate fibre fractions. In general, the fibre length of long fibre fraction is longer than the fibre length of short fibre fraction measured by Kajaani FSA analyzer using length weighted distribution. For example, for a recycled fibre material the long fibre fraction may have a fibre length in a range of 1 .2 - 1 .9 mm and the short fibre fraction may have a fibre length in a range of 0.8 - 1 .1 mm. Cationic polymer may be added both to the long fibre fraction and to the short fibre fraction, or only one of the fractions. If cationic polymer is added to both fibre fractions, it is possible that the cationic polymer is added in different doses in the long fibre fraction and in the short fibre fraction, depending for example on the starch content of the fibre fractions. Preferably the cationic polymer is added at least to the short fibre fraction. According to one preferable embodiment the cationic polymer is added to both the long fibre fraction and the short fibre fraction, wherein the cationic polymer is added to the short fibre fraction in an amount that is higher than the amount of the cationic polymer added to the long fibre fraction. This means that the dose of the cationic polymer to the short fibre fraction is higher that the dose of the cationic polymer to the long fibre fraction.

According to one preferable embodiment of the invention an amylase enzyme inhibitor and/or at least one biocide or biocidal agent is added to the fibre stock before or after the addition of the cationic polymer, when the cationic polymer is added to the fibre stock at the latest at the thickening step. Preferably at least an amylase enzyme inhibitor is added to the fibre stock before the thickening step, before or after the addition of the cationic polymer.

The fibre stock comprises cellulosic fibres which originate from recycled fibre material and/or broke dispersed in an aqueous phase. The method according to the present invention is especially suitable for a fibre stocks where cellulosic fibres comprise at least 50 weight-%, preferably at least 70 weight-%, more preferably 100 weight-%, of recycled fibre material, calculated from total amount of fibres, as dry. According to one embodiment the recycled cellulosic fibre material comprises at least 40 weight-%, preferably at least 50 weight-%, of fibre material originating from old corrugated containers (OCC).

The fibre stock further comprises starch, which is dispersed with the fibres in the aqueous phase of the fibre stock. The fibre stocks comprising cellulosic fibres originating from recycled fibre material often comprise significant amount of starch, which may originate, for example, from surface sizing of the original paper or board. According to one embodiment the fibre stock may comprise starch at least 2 weight- %, preferably at least 2.4 weight-%, more preferably at least 3 weight-%, calculated from the dry solid matter, before the addition of the cationic copolymer. The fibre stock may comprise starch up to 10 weigh-% or up to 20 weight-%, calculated from the dry solid matter, before the addition of the cationic copolymer. Especially, if an effective biocide regime, amylase enzyme inhibitor addition or the like is employed in the fibre stock preparation process, the starch content of the fibre stock may become high, if not associated with the cellulosic fibres of the fibre stock by using the present invention.

The starch dispersed in the aqueous phase of the fibre stock may be low molecular weight starch, such as oxidized starch or degraded starch. The starch may have, for example, a weight average molecular weight in the range of 30 000 - 5 000 000 g/mol, typically 50 000 - 2 000 000 g/mol. The starch is usually non-ionic or slightly anionic, for example with a charge density from -0.25 to 0 meq/g, or from -0.1 to 0 meq/g, measured at pH 7.

According to one embodiment the cationic polymer may be added in total amount of 0.2 - 1 .5 kg/ton, preferably 0.3 - 1 .2 kg/ton, more preferably 0.4 - 1 kg/ton, even more preferably 0.5 - 0.8 kg/ton. If cationic polymer is added in several dosage locations, the total amount is the sum of additions in each location.

According to one preferable embodiment of the present invention, the method is free of addition steps of synthetic organic coagulant or inorganic coagulant, such as aluminium compounds, iron compounds, bentonite and colloidal silica. This means that no synthetic organic coagulants or inorganic coagulants are added in the stock preparation before or at the latest at the thickening step.

Some embodiments are described more closely in the following schematical nonlimiting figures, where

Figure 1 shows a conventional stock preparation process without chemical additions;

Figure 2 shows a conventional stock preparation process with biocide and amylase enzyme inhibitor additions; and

Figure 3 shows a stock preparation process where cationic polymer is added according to one embodiment of the present invention.

Figure 1 shows a conventional stock preparation process without chemical additions. The full arrows in Figure 1 indicate the fibre stock flow through the stock preparation process and the dash lines indicate water flows recycled within or removed from the stock preparation process. The various stock preparation stages and apparatuses are indicated with following reference signs: coarse screening 1 , dump tower 2, fine screening 3, thickening 4; stand pipe 5, storage tower 6, filtrate tank 7, and pulper water tower 8. It is assumed that the fibre stock flow A comprises 100 parts of starch when entering the stock preparation process. The numbers above the arrows indicate the amount of starch (in parts) in the fibre stock flow at that location and the percentages in each stock preparation stage/apparatus indicate the loss percentage for starch in that stage/apparatus. For example, it is seen that before fine screening stage 3 the fibre stock flow comprises 135.7 parts of starch. The starch loss within the fine screening stage 3 is 2%, which means that after the fine screening stage 3 the fibre stock flow comprises 133 parts of starch. It is seen from Figure 1 that if no chemicals are added at stock preparation, of 100 parts of incoming starch only 5.3 parts remain after the storage tower 6. This is a significant loss of useful starch material, and it may also lead significant load on water treatment, here indicated as relative effluent COD value.

Figure 2 shows a conventional stock preparation process with biocide and amylase enzyme inhibitor additions. Same stock preparation stages and apparatuses are indicated with same reference signs as in Figure 1. Sludge thickening step is indicated with reference number 9. In the stock preparation process of Figure 2 the amylase enzyme inhibitor addition to coarse screening stage 1 is indicated with arrow I. Amylase enzyme inhibitor is added in order to reduce the degradation of starch by amylase enzyme. Addition of one or more biocides into the fibre stock flow before storage tower 6 is indicated with arrow B. It can be seen that the addition of amylase enzyme inhibitor and biocide(s) reduce the loss of starch within the stock preparation process. It is calculated that of 100 parts of starch entering the stock preparation process approximately 19.5 parts remain in the fibre stock flow after the storage tower. This is a clear improvement to the situation of Figure 1 , but still a significant amount of starch is lost in the stock preparation process.

Figure 3 shows a stock preparation process where cationic polymer is added according to one embodiment of the present invention. Same stock preparation stages and apparatuses are indicated with same reference signs as in Figures 1 and 2. At least one biocide and amylase enzyme inhibitor are added in the same manner as in Figure 2, indicated by arrows I and B. Furthermore, a cationic polymer obtained by copolymerisation of (meth)acrylamide and at least 25 mol-% of solely cationic monomer(s) is added to the fibre stock flow immediately before the thickening stage 4. It can be seen that the addiction of the polymer unexpectedly increases the amount of starch in the fibre stock flow after the storage tower 6 to 56.5 parts. In practice this implies a major improvement in the process and enables significant savings due to increased starch retention as well as reduced COD load in the water treatment.

EXPERIMENTAL Some embodiments of the present invention are described in the following nonlimiting examples.

Example 1

Example 1 demonstrates the effect of high cationic polyacrylamide, when dosed to a fibre stock before the thickening stage, for improving the retention of starch on the fibres.

The fibre stock was slushed and diluted as follows:

OCC (Old Corrugated Container) material from a European board mill was soaked for 5 minutes at 2.5 % consistency at 85 °C in artificial process water having conductivity of 4 mS/cm, pH 7. The ratio of salts in the artificial process was 70 % calcium acetate, 20 % sodium sulfate and 10% sodium bicarbonate. After 5 min soaking, while still hot, the disintegration was performed with laboratory disintegrator, 30000 rotations, wherein a test fibre stock was obtained. Amylase enzyme inhibitor (FennoSpec 1200, 100 ppm) and biocide (FennoSan GL10, 100 ppm) were added to the fibre stock after disintegration. Before the experiments the fibre stock was cooled to a room temperature (approx. 22 °C) and diluted to consistency of 1 .25 %, with the artificial process water described above.

The filtering at a thickener was modelled by using a Dynamic Drainage Analyzer, DDA, equipment. DDA parameters used were

- Wire: 0.25 mm openings

- Vacuum: 300 bar

- Follow-up time: 20 s

The test polymers used were cationic polyacrylamides obtained by polymerisation of acrylamide and [2-(acryloyloxy)ethyl] trimethylammonium chloride. Their properties are shown in Table 1 , where charge value gives the amount of cationic monomer used in the polymerisation and SV is the standard viscosity of the test polymer, measured as described elsewhere in this application. The test polymer was dosed to 500 ml of fibre stock prepared as described above (1 .25 % consistency), 60 min before start of the filtering. Thus obtained sample was mixed in a beaker using gentle mixing. 60 s before the start of the filtering the sample was poured into DDA’s vessel and mixing at 1000 rpm was started. The mixing was stopped when the filtering was started.

After filtering of the sample, 25 ml of the DDA filtrate was added to 10 ml HCI (cone. 10 weight-%). The mixture stirred for 10 min in 50 ml beaker with a magnetic stirrer and filtered by gravitation in a funnel with black ribbon filter paper. 1 ml of the filtrated mixture was added to 8.5 ml of deionized water, followed by addition of 0.5 ml iodine reagent (7.5 g/l of KI + 5 g/l of h). Starch content in the filtrated mixture was determined by a spectrophotometer Hach Lange DR 900 by measuring absorbance value at 610 nm, 30 seconds after the addition of iodine reagent.

Non-ionic degraded starch (C*film 0731 1 ) was used as a reference to make a calibration equation for the starch content.

In the present context, the term “starch retention” is used when the starch reduction in the DDA filtrate is compared to the starch amount in the water phase of the fibre stock before filtration, and the term “starch retention improvement” is used to describe the increase of starch retention obtained by test polymer addition in comparison to the corresponding measurement without any test polymer addition (0-test). Even in the 0-test a starch retention of a few percentages was typically found.

Starch retention and starch retention improvement were determined by using equations (1 ) and (2):

Starch retention = (Abs _puip - Abstest)/Abs _pUiP x 100% (1 ) where

Abs _puip is the absorbance value of the water phase of the fibre stock sample before DDA filtration, without any test polymer addition; Abstest is the absorbance value of the DDA filtrate of the same fibre stock sample after DDA filtration, alternatively with a test polymer addition to the sample or no test polymer addition to the sample (O-test).

Starch retention improvement = (Abso-test - Abs _p0 iymer)/Abso-test x 100% (2) where

Abso-test is the absorbance value of the DDA filtrate of a sample without test polymer addition (O-test) after DDA filtration; and

AbSpoiymer is the absorbance value of the DDA filtrate of a sample with test polymer addition after DDA filtration

The measured absorbance value results and the improvements in starch retention, calculated from the absorbance value results and by using the equations (1 ) and (2), are shown in Table 1 . Fibre stock without any test polymer addition (O-test) gave absorbance value 1 .122, corresponding to starch content of 780 mg/l.

Table 1 Test polymer properties, absorbance values and starch retention results of Example 1 . given kg of polymer per ton dry pulp

It can be seen from Table 1 that cationic polymers obtained by polymerising acrylamide and 38 mol-% or more of cationic monomer clearly improve the starch retention. During the experiments it was also observed that some of the tested polymers gave improved drainage time in filtration. This indicates that filtering rate of thickener might also be improved when tested polymers are added before the thickening stage.

Example 2

Example 2 demonstrates the effect of high cationic polyacrylamide, when dosed to a fibre stock before the thickening stage, for improving the retention of starch on the fibres.

The fibre stock was prepared in the same way as in Example 1 , except that the slushing was done at 2.0 % consistency and the fibre stock was diluted to 1.2 % consistency for DDA experiments.

The test polymers were cationic polyacrylamides cationic polyacrylamides obtained by polymerisation of acrylamide and [2-(acryloyloxy)ethyl] trimethylammonium chloride. Their properties are shown in Table 2, where charge value gives the amount of cationic monomer used in the polymerisation and SV is the standard viscosity of the test polymer, measured as described elsewhere in this application.

The same DDA parameters were used as in Example 1 . 700 ml of fibre stock, consistency 1.2 %, was poured into DDA’s vessel 120 s before the start of the filtering and mixing at 500 rpm was started. Test polymer was dosed 40 s before the start of the filtering. The used test polymer dosage was 0.6 kg/ton dry fibre stock.

The starch content and starch retention improvement were determined in the same manner as described in Example 1. The measured absorbance value results and the calculated improvement in starch retention are shown in Table 2. Fibre stock without any chemical addition (O-test) gave absorbance value 0.926.

It can be seen from Table 2 that cationic polymers obtained by polymerising of acrylamide and high amounts of cationic monomers (30 mol-% and above) clearly improve the retention of starch from the water phase to the fibre stock. An improved drainage rate was also again observed.

Table 2 Test polymer properties, absorbance values and starch retention results of Example 2.

Example 3

Example 3 demonstrates the effect of a high cationic polyacrylamide to the starch retention on fibres, when the polymer is dosed before the thickening stage of fibre stock and used together with a retention system in the sheet forming stage.

The fibre stock was prepared in the same way as in Example 1 and same OCC material was used as the raw material. Consistency of the fibre stock was 1 .25 %, conductivity 4 mS/cm and pH 7.

The test polymers were cationic polyacrylamides obtained by polymerisation of acrylamide and [2-(acryloyloxy)ethyl] trimethylammonium chloride. Their properties are shown in Table 3, where charge value gives the amount of cationic monomer used in the polymerisation and SV is the standard viscosity of the test polymer, measured as described elsewhere in this application.

Thickening stage was modelled by adding the test polymer to the fibre stock, dosage 600 g polymer/ton dry fibre stock, 60 min before the start of the sheet forming. The test polymer was dosed to 200 ml of fibre stock (1 .25 % consistency). The fibre stock was then mixed for 60 min in a beaker using gentle mixing. The sheet forming stage was modelled by using DDA equipment, same DDA parameters as in Example 1 were used. 60 s before the sheet forming the sample was poured into DDA’s vessel and mixing at 500 rpm was started. To model the short circulation stage of a paper/board mill, the fibre stock was diluted 30 seconds before the sheet forming to a consistency of 0.5 % with artificial process water (as described in Example 1 ) which also contained ground calcium carbonate GCC in amount of 1 g/l, and the mixing was increased to 1000 rpm.

In the experiments the retention system was cationic polyacrylamide Poly-1 , dosage 250 g/ton dry fibre stock, and silica microparticles, added 15 s (Poly-1 ) and 10 s (microparticles) before the sheet forming.

The starch content and starch retention improvement were determined in the same manner as described in Example 1. The measured absorbance value results and the calculated improvement in starch retention are shown in Table 3. Fibre stock without any chemical addition (O-test) gave absorbance value 0.425.

Table 3 Test polymer properties, absorbance values and starch retention results of Example 3.

* Retention polymer, in brackets, added to 0.5 % consistency stock

It can be seen from Table 3 that the use of a retention polymer alone (test 3-1 ) did not have any significant impact on starch retention, as starch retention improved only 2 %. Addition of cationic polymer at thickening stage, however, improves starch retention significantly, even over 30 %. It can also be seen that a high charge for the cationic polymer is beneficial for starch retention. Example 4

Example 4 demonstrates the effect of a high cationic polyacrylamide to the starch retention on fibres, when the polymer is dosed before thickening stage of the fibre stock and also used as a retention polymer in a retention system in the sheet forming stage.

The fibre stock was prepared in the same way as in Example 1 and the same OCC material was used as the raw material. Consistency of the fibre stock was 1 .25 %, conductivity 4 mS/cm and pH 7.

The test polymers were cationic polyacrylamides obtained by polymerisation of acrylamide and [2-(acryloyloxy)ethyl] trimethylammonium chloride. Their properties are shown in Table 4, where charge value gives the amount of cationic monomer used in the polymerisation and SV is the standard viscosity of the test polymer, measured as described elsewhere in this application.

Thickening stage and sheet forming stage were modelled in the same manner as in Example 3.

The retention system in the experiments included the same test polymer that was added at the thickening stage, dosage 200 g /ton dry fibre stock, and silica microparticles, added 15 s (polymer) and 10 s (microparticles) before the sheet forming.

The starch content and starch retention improvement were determined in the same manner as described in Example 1. The measured absorbance value results and the calculated improvement in starch retention are shown in Table 4. Fibre stock without any chemical addition (O-test) gave absorbance value 0.398.

Furthermore, Table 4 shows total starch retention values, which indicate retention of all the materials in the fibre stock, including fibre material, fillers, starch, etc. The total retention experiments were performed with Dynamic Drainage Jar (DDJ). Similarly, as with DDA, thickening stage was modelled by adding the test polymer 60 min before the start of the experiment to 200 ml of fibre stock (1.25 % consistency). The fibre stock was then mixed for 60 min in a beaker using gentle mixing. The sheet forming stage was modelled by using DDJ equipment. 60 s before end of the experiment, the fibre stock sample was poured into DDJ’s vessel and mixing at 500 rpm was started. To model the short circulation stage at paper/board mill, pulp was diluted to consistency of 0.5 % (5 g/l) 30 seconds before end of the experiment with artificial process water (as described in Example 1 ) containing 1 g/l of ground calcium carbonate GCC, and mixing was increased to 1000 rpm. 100 ml of filtrate was collected from DDJ. From the filtrate the consistency was measured by filtrating the filtrate through weighed black ribbon filtrate paper. Then, filtrate paper was dried and weighed for the consistency calculation (equation 3):

Cfiltrate ⁼ (matter filtration ^_ mfilter paper)/(V0lUITie Of the Sample) (3)

By using the obtained consistency value, a total retention was calculated by using equation 4:

Total retention — (Cpuip ^_ Cfiitrate)/Cpuip x 100% (4)

In equations (3) and (4), m denotes mass and C denotes consistency.

Table 4 Test polymer properties, absorbance values, starch retention and total retention results of Example 4.

* Retention polymer, in brackets, added to 0.5 % consistency stock It can be seen from Table 4 that the use of retention system alone (test 4-1 ) did not have a significant impact on starch retention. Addition of the cationic polymer with high charge both at the thickening stage and the sheet forming stage, however, improves the starch retention significantly. It can also be seen that a higher charge for the cationic polymer is beneficial for starch retention. Furthermore, it can be seen that even if the cationic test polymers provided also a slight increasing effect on the total retention, the starch retention was improved significantly more than the total retention. This shows that the present invention where the polymer is dosed before thickening stage specifically improves starch retention.

Example 5

Example 5 demonstrates the effect of a high cationic polyacrylamide to the starch retention on fibres, when the polymer is dosed before thickening stage of the fibre stock. The effect of the molecular weight of the cationic polymer on the starch retention was studied.

The fibre stock was prepared in the same way as in Example 1 and the same OCC material was used as the raw material. Consistency of the fibre stock was 1 .25 %, conductivity 4 mS/cm and pH 7.

The test polymers with the names “Poly-X” were cationic polyacrylamides obtained by gel polymerisation of acrylamide and [2-(acryloyloxy)ethyl] trimethylammonium chloride, and with a high molecular weight (SV >3 mPas). The test polymer SPoly was a cationic polyacrylamide obtained by solution polymerisation of acrylamide and [2-(acryloyloxy)ethyl] trimethylammonium chloride, and with a lower molecular weight (SV 1.2 mPas). The test polymer PVAm was a commercial vinylamine copolymer. The properties of the test polymers are shown in Table 5, where charge value gives the amount of cationic monomer used in the polymerisation of cationic polyacrylamides and SV is the standard viscosity of the test polymer, measured as described elsewhere in this application.

The experiments were conducted in the same manner using DDA as in Example 1 . The starch content and starch retention improvement were determined in the same manner as described in Example 1. The measured absorbance value results and the calculated improvement in starch retention are shown in Table 5. Fibre stock without any chemical addition (O-test) gave absorbance value 0.977.

Table 5 Test polymer properties, absorbance values and starch retention results of Example 5.

It is seen from Table 5 that test polymers having a higher charge were able to produce improved starch retention in comparison to test polymers with charge under 20 mol-%. However, a high charge of the test polymer does not alone guarantee a high starch retention, but molecular weight of the polymer also has to be high enough. It is seen from Table 5 that the commercial vinylamine copolymer, PVAm, and cationic polyacrylamide SPoly are polymers with high cationic charge, 30 mol- % and 46 mol%, respectively, but their molecular weights are rather low. The starch retention improvement obtained with these polymers was significantly lower. It can be thus concluded that both the cationic charge and high enough molecular weight of the polymer are important for obtaining the desired high starch retention.

Even if the invention was described with reference to what at present seems to be the most practical and preferred embodiments, it is appreciated that the invention shall not be limited to the embodiments described above, but the invention is intended to cover also different modifications and equivalent technical solutions within the scope of the enclosed claims.