Strength composition and method for dissolving the strength composition

Abstract

The present invention relates to a strength composition for manufacture of paper, board or the like. The composition is in form of a dry particulate mixture and has a dry solids content of at least 80 weight-%. The composition comprises cationic starch, having a degree of substitution, DS, >0.12, preferably >0.14, and an anionic polymeric component, wherein the composition has a net anionic charge density in a range from −0.30 to −2.0 meq / g, when dissolved in water, at pH 7. The invention also relates a method for dissolving the strength composition.

Description

[0001] The present invention relates to a strength composition, its use and to a method for dissolving the strength composition according to preambles of the enclosed independent claims.

[0002] In manufacture of cellulosic fibrous webs, such as paper, board or the like, strength properties of the final fibrous web are improved by adding different chemicals to the fibre stock before formation of the fibrous web. It is well known to use synthetic polymers for improving the strength properties of the fibrous web. Many of the synthetic polymers are supplied as solutions, dispersions or emulsions, which means that their transport always involves transport of water or other liquid phase, which in uneconomical from logistics point of view. Storage of solutions, dispersion or emulsions may also be problematic, for example due to microbial growth. Consequently, polymers supplied as solutions, dispersions or emulsions are not optimal in view of transport and / or storage.

[0003] It is also well known to use natural polymers, such as starch, for improving the strength properties of the fibrous webs. Starch can be supplied in dry form, as particulate powder, which is traditionally dissolved by on-site cooking using elevated temperature and / or pressure. The cooking of starch involves occupational risks, when the operators must be involved with steam, hot liquids and / or pressure. Furthermore, if the cooking process is not appropriately conducted, the starch might form gel or gel particles, cause dirtying of the equipment and / or quality defects in the produced fibrous webs.

[0004] In general, on-site mixing of various chemicals in liquid form is cumbersome. Therefore there is a need for chemical composition which can provide the fibrous web with the desired strength properties, and which is easy and economical to transport, have good storage stability and which is easy to transform into liquid form for application to the fibrous stock.

[0005] An object of this invention is to minimise or even eliminate the disadvantages existing in the prior art.

[0006] An object is also to provide a strength composition which is easy to dissolve, and which provides effective increase in strength properties of the formed fibrous web.

[0007] A further object of this invention is to provide a strength composition has a good storage stability and which is easy to handle in on-site conditions.

[0008] These objects are attained with the invention having the characteristics presented below in the characterising parts of the independent claims. Some preferable embodiments are disclosed in the dependent claims.

[0009] The embodiments mentioned in this text relate, where applicable, to all aspects of the invention, even if this is not always separately mentioned.

[0010] A typical strength composition according to the present invention for manufacture of paper, board or the like, wherein the composition is in form of a dry particulate mixture, having a dry solids content of at least 80 weight-%, comprises

[0011] cationic starch, having a degree of substitution, DS, >0.12, preferably >0.14,

[0012] an anionic polymeric component,wherein the composition has a net anionic charge density in a range from −0.30 to −2.0 meq / g, when dissolved in water, at pH 7.

[0013] Typical use according to the present invention of the strength composition according to the invention is for improving strength properties of a paper, board or the like.

[0014] A typical method for dissolving a strength composition according to the present invention, having a dry solids content of at least 80 weight-%, comprises

[0015] providing a mixture of the strength composition and water,

[0016] passing the mixture through a first high shear treatment to a residence reactor, where the mixture is kept at temperature of <80° C.,

[0017] removing the mixture from the residence reactor and optionally diluting it to a concentration of 1-5 weight-%, calculated as dry solids.

[0018] Now it has been surprisingly found out that a strength composition comprising cationic starch having degree of substitution of >0.12 and an anionic polymeric component can be produced in form of a dry particulate mixture, having dry solids content of at least 80 weight-%, i.e. in powder form. The high cationicity of the cationic starch makes it easy to dissolve, even without high temperature and / or pressure. Furthermore, it has been surprisingly found that the strength composition in powder form is homogenous and stable, i.e. there is no significant risk for segregation or separation of the components (particles) during transport and storage. The strength composition according to the present invention is also resistant to caking or agglomerate formation during storage, even in high humidity conditions, which is unexpected. It is speculated that in the dry particulate mixture, the particles of the anionic polymeric component are surrounded by the particles of cationic starch, possibly due to the high cationicity of the starch, which makes the particulate mixture to resist particle separation and / or caking. The properties of the cationic starch and the anionic polymeric component may thus produce the unexpected advantages of reduced caking, agglomeration and component (particle) separation.

[0019] The strength composition according to the present invention is in form of a dry particulate mixture. This means that the strength composition is in solid form, e.g. powder, in contrast to liquid form, and comprises discrete particles. The strength composition has a dry solids content of at least 80 weight-%, preferably at least 82 weight-%, more preferably at least 84 weight-%, even more preferably at least 85 weight-%. The dry solids content of the strength composition may be in a range of 80-98 weight-%, preferably 82-98 weight-%, more preferably at least 84-95 weight-%, even more preferably at least 85-95 weight-%.

[0020] The strength composition according to the present invention has a net anionic charge density in a range from −0.30 to −2.0 meq / g, when dissolved in water, at pH 7. According to one preferable embodiment the strength composition may have the charge density in the range from −0.4 to −1.6 meq / g, preferably from −0.55 to −1.5 meq / g, when dissolved in water, at pH 7. The charge density is measured by titration with Mütek PCD 03. The net anionic charge density provides the strength composition with optimal interaction with the fibres and thus the final fibre web with the desired strength properties.

[0021] Cationic starch, which is suitable for use as a component in the strength composition, has a degree of substitution, DS, of at least 0.12, preferably at least 0.14, sometimes the degree of substitution may even be at least 0.15 or at least 0.16. This means that the cationic starch has a high cationicity which provides several advantages for the strength composition. The high cationicity increases the solubility of the starch, thus effectively minimising the risk of gel formation during the dissolving of the strength composition. High cationicity of the cationic starch may also have a positive effect to the properties of the strength composition itself as well as to the strength properties obtained in the final paper, board or the like. According to one preferable embodiment the cationic starch may have the degree of substitution in a range of 0.12-0.3, preferably 0.13-0.27, more preferably 0.14-0.25, for example in a range of 0.15-0.22 or 0.16-0.22. It is highly unexpected that even if the composition comprises cationic starch with high cationicity, the composition shows good stability, i.e. resistance to caking or agglomerate formation, even in humid and warm conditions.

[0022] Cationic starch may be obtained by cationising starch by any suitable method. Preferably cationic starch is obtained by using 3-chloro-2-hydroxypropyl-trimethylammonium chloride or 2,3-epoxypropyltrimethylammonium chloride for cationisation. It is also possible to cationise starch by using cationic acrylamide derivatives, such as (3-acrylamidopropyl)-trimethylammonium chloride. Various methods for cationisation of starch are known as such for a person skilled in the art.

[0023] Cationic starch used in the strength composition may originate from potato, waxy potato, rice, waxy corn, sweet potato, arrowroot or tapioca starch, or any combination thereof. Preferably cationic starch is potato starch or waxy potato starch, more preferably potato starch. In the present invention, the amylopectin content of the cationic starch is not decisive parameter. It has been observed that the cationic starch may have an amylopectin content<80 weight-%, such as less than 75 weight-% or less than 70 weight-%. For example, the amylopectin content of the cationic starch may be in a range from 60 weight-% to less than 80 weight-% or from 65 weight-% to less than 80 weight-%. It is speculated, without wishing to be bound by a theory, that the high cationicity of the cationic starch provides similar advantages in interaction with the anionic polymeric component that were earlier associated with amylopectin content of starch.

[0024] According to one preferable embodiment, the cationic starch may have a solubilization temperature of ≤90° C., preferably ≤80° C., more preferably ≤70° C. or ≤65° C., even more preferably ≤60° C. The solubilisation temperature of cationic starch may be in a range of 20-90° C., preferably 25-80° C., more preferably 30-70° C. or 35-65° C., even more preferably 40-60° C. When the cationic starch has relatively low or low solubilisation temperature, the dissolving of the strength composition can be performed in a safe occupational manner, without subjecting the process operators for risks associated with dissolving procedures at high temperatures. Furthermore, using cationic starch that enables dissolving the strength composition at low temperature provides significant energy savings for the process. In the present context the term “solubilization temperature” denotes the temperature, at which the cationized starch is deemed to fully dissolve in water. The starch is deemed fully dissolved in water, when 25 g of starch is mixed with 1 litre of water, heated to the desired temperature, and mixed for 60 min while maintaining the desired temperature. After this the obtained solution is filtered through 100 micron steel mesh. If more than 0.5 weight-% of material, calculated as dry from the starting starch weight, is retained on the mesh, the starch has not been fully dissolved. If no material is retained on the mesh, the starch has been fully dissolved.

[0025] The cationic starch may be non-degraded starch. In the present context “non-degraded starch” denotes starch which is essentially untreated by oxidative, thermal, enzymatic and / or acid treatment in a manner that would cause hydrolysis or breakage of glycosidic bonds or degradation of starch molecules or units. The cationic starch may comprise starch units, i.e. starch molecules, of which at least 70 weight-%, preferably at least 80 weight-%, have a weight average molecular weight MW over 20 000 000 g / mol, preferably over 50 000 000 g / mol.

[0026] The anionic polymeric component of the strength composition may be either a synthetic polymer or an anionically derivatized polysaccharide. According to one preferable embodiment the anionic polymeric component is anionically derivatized polysaccharide, as it provides the possibility make the strength composition more sustainable, and to minimise or even completely avoid the use of synthetic petroleum-based polymers.

[0027] The anionic polymeric component may comprise cationically charged groups, as long as the anionic polymeric component is net anionic.

[0028] When the anionic polymeric component is anionically derivatized polysaccharide, it is preferably carboxymethylated cellulose. Preferably the anionic polymeric component may be carboxymethyl cellulose.

[0029] According to one preferable embodiment the anionic polymeric component of the strength composition is an anionically derivatized polysaccharide, such as carboxymethylated cellulose. In that case, the strength composition may comprise 40-70 weight-%, preferably 45-65 weight-%, more preferably 50-60 weight-%, of starch, and / or 30-60 weight-%, preferably 35-55 weight-%, more preferably 40-50 weight-%, of the anionically derivatized polysaccharide.

[0030] According to one embodiment of the invention the anionic polymeric component may be carboxymethylated cellulose, preferably carboxymethyl cellulose, which may have a degree of carboxymethyl substitution>0.3, preferably >0.4 or >0.45, more preferably >0.5. The degree of carboxymethyl substitution may be in a range of 0.4-1.2, more preferably 0.45-1.0 or 0.5-0.9. It has been observed that the degree of carboxymethyl substitution provides enhanced water-solubility for the strength composition in particular form, especially when mixed with the cationic starch having the degree of substitution>0.12. In one preferable embodiment the carboxymethylated cellulose may have a degree of carboxymethyl substitution in the range of 0.5-0.9, which provides essentially complete water-solubility for the carboxymethyl cellulose.

[0031] The carboxymethylated cellulose, preferably carboxymethyl cellulose, which is used in the strength composition, may have a charge density value less than −1.6 meq / g, preferably less than −1.5 meq / g, more preferably less than −2 meq / g, when measured at pH 7. The charge density value may be, for example, in a range from −4.7 to −2.1 meq / g, more preferably from −4.1 to −2.3 meq / g, even more preferably from −3.8 to −2.5 meq / g, when dissolved in water, measured at pH 7. All measured charge density values are calculated per weight as dry.

[0032] According to one embodiment of the invention the anionically derivatized polysaccharide comprises carboxymethylated cellulose, preferably carboxymethyl cellulose, which may have viscosity in the range of viscosity in a range of 200-20 000 mPas, preferably 400-15 000 mPas, more preferably 500-10 000 mPas, measured from 2 weight-% aqueous solution at 25° C., by using Brookfield LV DV1, as defined in the experimental section.

[0033] The anionic polymeric component in the strength composition may alternatively be a synthetic polymer, such an anionic copolymer of (meth)acrylamide. The strength composition may comprise 25-55 weight-%, preferably 30-50 weight-%, more preferably 35-45 weight-%, of starch, and / or 45-75 weight-%, preferably 50-70 weight-%, more preferably 55-65 weight-%, of the synthetic polymer.

[0034] When the anionic polymeric component is a synthetic polymer, it is preferably an anionic copolymer of (meth)acrylamide. According to one embodiment the anionic polymeric component may be an anionic copolymer of (meth)acrylamide, obtained by polymerisation of (meth)acrylamide and at least one anionic monomer, which is selected from unsaturated mono- or dicarboxylic acids or their salts, such as acrylic acid, methacrylic acid, maleic acid, itaconic acid, crotonic acid, isocrotonic acid, and any of their mixtures. Preferably, the synthetic polymer component is prepared by radical polymerisation of acrylamide and acrylic acid. The synthetic polymer component may be obtained, for example by gel polymerisation. It is possible that the anionic copolymer of (meth)acrylamide may comprise cationically charged groups as long as the copolymer is net anionic.

[0035] Preferably the synthetic polymer of the strength composition may be an anionic copolymer of (meth)acrylamide, which has an anionicity of 6-50 mol-%, preferably 8-40 mol-%, more preferably 10-30 mol-%. The anionicity relates to the amount of structural units in the synthetic polymer component which originate from anionic monomers. The anionicity of the copolymer is able to advance stronger complex formation with the cationic starch.

[0036] The synthetic polymer of the strength composition may be an anionic copolymer of (meth)acrylamide, which has a weight average molecular weight MW of ≥1 500 000 g / mol, preferably ≥2 000 000 g / mol, more preferably ≥2 500 000 g / mol. The weight average molecular weight may be, for example, in a range of 2 000 000-15 000 000 g / mol, preferably 2 500 000-10 000 000 g / mol, more preferably 3 000 000-8 000 000 g / mol, even more preferably 2 500 000-8 000 000 g / mol. The weight average molecular weights can be determined by measuring the standard viscosity of the polymer and then estimating the weight average molecular weight from a correlation curve based on experimental measurements. In general, the standard viscosity of the polymer gives an indication of the length and / or weight of the polymer chains of the polymer. Standard viscosity (SV) is measured at 0.1 weight-% polymer content in an aqueous 1 M NaCl solution, using Brookfield LV viscometer equipped with UL adapter, at 25° C., using UL Adapter Spindle and rotational speed 60 rpm. A correlation curve can be made by determining standard viscosity (SV) and intrinsic viscosity (IV) of the polymer. The weight average molecular weight (MW) is calculated from IV results using Mark-Houwink-Sakurada constants K=2.57×10-4 dl / g and a=0.67, which are fitted for conditions 1 M NaCl at 25° C., for range 1.1×106-1.5×107 g / mol (Dautzenberg et al., Polyelectrolytes. Formation, Characterization and Application, Carl Hanser Verlag, Munich Vienna New York, 1994, pp. 222-223). The weight average molecular weights obtained by this method are directive. As a general guideline, a correlation between standard viscosity, intrinsic viscosity and weight average weight molecular weight of a cationic copolymer of (meth)acrylamide may be assumed, as shown in Table 1.TABLE 1General correlation between standard viscosity, intrinsicviscosity and weight average weight molecular weight.StandardIntrinsicWeight AverageViscosity, SVViscosity, IVMolecular Weight, MW[mPas][dl / g][g / mol]24.8 1 300 0002.56.7 3 400 00038.5 5 500 0003.510.4 7 600 000412.3 9 800 0004.514.112 000 000516.014 000 0005.517.916 000 000619.718 000 0006.521.620 000 000723.523 000 000

[0037] The strength composition according to the present invention is used for improving strength properties of a paper, board or the like.

[0038] The strength composition may be dissolved relatively easily even at low temperature. The strength composition and water can be mixed to form a mixture, where the concentration of the strength composition is 0.3-8 weight-%, preferably 0.5-6 weight-%, more preferably 1-5 weight-% or 2-5 weight-%, calculated as dry active solids. The strength composition may preferably be mixed with warm water having a temperature in a range of 15-80° C., more preferably 20-70° C., more preferably 25-65° C. or 30-60° C. Alternatively, the strength composition may be mixed with cold water, having a temperature of <15, and the obtained mixture can be heated to the temperature in a range of 15-80° C., more preferably 20-70° C., more preferably 25-65° C. or 30-60° C. before the mixture is passed through a first high shear treatment. In some embodiments, the strength composition may be mixed with cold water and passed through the first high shear mixer without preceding external heating. Mixing with warm water or heating the mixture is not necessary, but it may speed up and / or promote the dissolution of the strength composition.

[0039] The obtained mixture of the strength composition and water is passed through the first high shear treatment to a residence reactor. In the present context “high shear treatment” denotes a treatment where the mixture is subjected to a high circumferential speed, high shear rate, high shear forces and high energy dissipation. High shear treatment may involve the use of a high shear dispergator, which are widely used in energy intensive processes such as homogenization, dispersion, emulsification and grinding. The high energy dispergators suitable for use in the high shear treatment may be selected from rotor-stator dispergators, such as a colloid mill, Cavitron or Supraton dispergators; rotor-rotor dispergators, such as Atrex dispergators; friction grinders, such as Masuko supermasscolloiders; homogenizers; fluidizers, such as micro-fluidizer, macrofluidizer or fluidizer-type homogenizer; or any type of milling device such as bead mill. In some devices the mechanical energy to the starch is transferred using specific media, such as beads in the bead mill. Preferably the high shear treatment comprises a high shear dispergator selected from rotor-stator dispergator or a rotor-rotor dispergator. The high shear dispergator of the high shear treatment may have a power output from 5 to 150 kWh for 1 ton of mixed feed to be treated.

[0040] The high shear treatment may be continuous or batchwise.

[0041] In the residence reactor the mixture is kept at temperature of ≤80° C., preferably in a range of 15-80° C., more preferably 20-70° C., more preferably 25-65° C. or 30-60° C. The mixture may have a residence time in a range of 1-60 min, preferably 1-45 min, more preferably 5-35 min in the residence reactor. The residence reactor may be any suitable reactor, such as tubular reactor or batch reactor.

[0042] The mixture is removed from the residence reactor and optionally diluted to a concentration of 1-5 weight-%, preferably 2-5 weight-%, calculated as dry active solids. According to one preferable embodiment the mixture may be removed from the residence reactor through a second high shear treatment. In this manner complete and effective dissolution of the strength composition into the water is ensured and the strength composition is ready for use in paper and board making applications.EXPERIMENTAL

[0043] Some embodiments of the present invention are described in the following non-limiting example.Equipment and Methods Used in Analysis

[0044] Dry solids content was determined by using Mettler Toledo HR73, at 150° C.

[0045] Viscosity was determined by using Brookfield LV DV1, equipped with small sample adapter, at 25° C., using spindle S31. The highest feasible rotation speed for the spindle was used.

[0046] pH was determined by using a calibrated pH-meter.

[0047] Charge density was determined at pH 7.0, adjusted with 1 weight-% aqueous NaOH solution, by charge titration using polydiallyldimethylammonium solution as titrant. Mütek PCD-03 was used for end point detection.Example 1 Dissolving of Dry Strength Composition

[0048] Strength composition in dry particulate form, i.e. powder mixture, comprising cationic starch and carboxymethyl cellulose was made by mixing 56 g cationic starch (DS 0.16, bound nitrogen 1.2 w-%, dry content 85%) and 44 g carboxymethyl cellulose, sodium salt (DS 0.80, dry content 89 w-%) for 60 min in a Glas-Col 099A RD20 rotary shaker. Dry content of obtained powder mixture was 87 w-%.

[0049] 59 g of the obtained powder mixture was dosed in 5 min time in 2500 g de-ionized water in 5-liter pre-heated metal kettle under mixing of Kady LT 2000 rotor-stator high speed dispergator. Metal kettle was pre-heated with 60° C. water before dissolution procedure. Water temperature at start was 60° C. Frequency of Kady dispergator was 10 Hz for the first 2 min; 15 Hz for the next 2 min; and 20 Hz for the last 1 min. When all of the powder mixture was dosed, the mixture was mixed with 20 Hz frequency for 1 min. The lumps which were sticked on the kettle walls were removed with spatula, and the mixture was mixed for 2 min with 25 Hz frequency. Temperature was 58° C. after the dissolution procedure. The formed solution had white colour and did not contain any lumps. Dry content of the solution was 2%, viscosity at 25° C. was 2300 cP, pH was 6.0, charge density was-1.2 meq / g, measured by Mütek at pH 7.0.

[0050] The results of Example 1 show that it is possible to dissolve the net anionic strength composition of cationic starch and carboxymethyl cellulose at temperature below 60° C. Reasonably low dissolution temperature is beneficial in process safety and energy consumption point of view.Example 2 Separation Test for Dry Particulate Strength Composition

[0051] It is a well-known phenomenon that a separation of particles may occur in dry compositions comprising a mixture of different particles. The size difference, shapes, densities, and friction of the particles will have an impact on the separation tendency of the particles. The separation of particles leads to inhomogeneous composition, where the concentration of different particles varies in different parts of the composition body, e.g. in a storage vessel, such as big bag. Typically the separation may be due to vibrations occurring during transportation conditions.

[0052] In Example 2 the phase separation tendency of the dry particulate strength composition was studied by comparing the particle distributions in the composition before and after a standardized vibration test. The standardized vibration test is intended to simulate the behaviour of the composition during transport in big bags.

[0053] A strength composition in form of a dry particulate mixture was prepared by mixing a cationic starch (DS>0.14) and an anionic polymer (carboxymethyl cellulose). The obtained strength composition was net anionic, having a charge density in a range from −1.3 to −0.9 meq / g.

[0054] The freshly prepared strength composition was analysed as follows: the sample was screened through five (5) screens having decreasing mesh size from 1 mm to 0.09 mm as given in Table 1. Four (4) parallel samples were analysed. The results for the analyses are given in Table 1.TABLE 1Sieve residues (as %) for the freshlyprepared strength composition.Sieve sizeSample 1Sample 2Sample 3Sample 4Average[mm]Sieve residue at sieve, given as % of total amount15.24.65.24.64.90.852.52.32.52.22.40.4518.518.519.118.218.60.220.220.120.620.320.30.099.99.19.39.29.4under 0.0943.645.043.245.144.2

[0055] It can be seen from Table 1 that the particle size distribution was rather wide as about 5% of the particles were larger than 1 mm in diameter and about 47% of the particles larger than 0.2 mm. The finest fraction smaller than 0.09 mm represented about 44% of the total strength composition amount powder amount. However, the consistent results for the parallel samples indicate that the used analysis method was reliable for characterization of the particle size distribution of the strength composition samples taken from a larger body / bigger batch of the strength composition.

[0056] The separation tendency of the particles in the dry strength composition was studied by using a vibrating screen device (Fritsch analysette, type 03.502). The induced vibrations simulate the conditions during a transport of the dry strength composition. 800 g of the strength composition was packed in a tightly sealed plastic bag to simulate a typical powder big bag. The strength composition in the tightly sealed plastic bag was placed on top of the vibrating screen device with the topside of the plastic bag facing upwards. The vibrating screen device was set on continuous vibration with amplitude of 5 (of maximum 10) for 1 hour. Thereafter, the tightly sealed plastic bag was carefully removed from the vibrating screen device. Two samples, one from the top of the plastic bag and one from the bottom of the plastic bag were carefully taken and analysed with the same screen setup used above. The results are given in Table 2.TABLE 2Sieve residues (as %) for the strengthcomposition after vibration test.Sieve sizeTop of the bagBottom of the bag[mm]Sieve residue at sieve, given as % of total amount14.94.60.852.52.40.4517.717.50.219.219.70.0910.110.3under 0.0945.245.2

[0057] It is seen from the results of Table 2 that the particle size distributions for the strength composition samples taken from the top and the bottom of the tightly sealed plastic bag were very similar and consistent with the size distribution of the freshly prepared strength composition, as given in Table 1. This indicates that no significant separation of particles had occurred, even if the strength composition was subjected to conditions simulating transport.

[0058] 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.

Claims

1. A strength composition for manufacture of paper, board or the like, wherein the composition is in form of a dry particulate mixture, having a dry solids content of at least 80 weight-%, the composition comprisingcationic starch, having a degree of substitution, DS, >0.12, preferably >0.14,an anionic polymeric component,wherein the composition has a net anionic charge density in a range from −0.30 to −2.0 meq / g, when dissolved in water, at pH 7.

2. The composition according to claim 1, wherein the cationic starch has the degree of substitution in a range of 0.12-0.3, preferably 0.13-0.27, more preferably 0.14-0.25.

3. The composition according to claim 1, wherein the cationic starch has a solubilization temperature of ≤90° C., preferably ≤80° C., more preferably ≤70° C. or ≤65° C., even more preferably ≤60° C.

4. The composition according to claim 1, wherein the anionic polymeric component is an anionically derivatized polysaccharide, such as carboxymethylated cellulose.

5. The composition according to claim 4, wherein the anionically derivatized polysaccharide is a carboxymethyl cellulose havinga degree of carboxymethyl substitution>0.3, preferably in a range of 0.4-1.2, more preferably 0.45-1.0, even more preferably 0.5-0.9, and / ora charge density value less than −1.6 meq / g, preferably in a range of from −4.7 to −2.1 meq / g, more preferably from −4.1 to −2.3 meq / g, even more preferably from −3.8 to −2.5 meq / g, when dissolved in water, measured at pH 7 and / ora viscosity in a range of 200-20 000 mPas, preferably 400-15 000 mPas, more preferably 500-10 000 mPas, measured from 2 weight-% aqueous solution at 25° C., by using Brookfield LV DV1.

6. The composition according to claim 4, wherein the strength composition comprises 40-70 weight-%, preferably 45-65 weight-%, more preferably 50-60 weight-%, of starch, and / or 30-60 weight-%, preferably 35-55 weight-%, more preferably 40-50 weight-%, of the anionically derivatized polysaccharide.

7. The composition according to claim 1, wherein the anionic polymeric component is a synthetic polymer, such an anionic copolymer of (meth)acrylamide.

8. The composition according to claim 7, wherein the synthetic polymer is an anionic copolymer of (meth)acrylamide havingan anionicity of 6-50 mol-%, preferably 8-40 mol-%, more preferably 10-30 mol-%, and / ora weight average molecular weight MW>1 500 000 g / mol, preferably in a range of 2 000 000-15 000 000 g / mol, more preferably 2 500 000-10 000 000 g / mol, more preferably 2 500 000-8 000 000 g / mol.

9. The composition according to claim 1, wherein the strength composition comprises 25-55 weight-%, preferably 30-50 weight-%, more preferably 35-45 weight-%, of starch, and / or 45-75 weight-%, preferably 50-70 weight-%, more preferably 55-65 weight-%, of the synthetic polymer.

10. The composition according to any of claim 1, wherein the strength composition has the charge density in the range from −0.4 to −1.6 meq / g, preferably from −0.55 to −1.5 meq / g, when dissolved in water, at pH 7.

11. (canceled)12. A method for dissolving a strength composition according to claim 1, having a dry solids content of at least 80 weight-%, the method comprisingproviding a mixture of the strength composition and water,passing the mixture through a first high shear treatment to a residence reactor, wherein the mixture is kept at temperature of <80° C.,removing the mixture from the residence reactor and optionally diluting it to a concentration of 1-5 weight-%, calculated as dry solids.

13. The method according to claim 12, wherein the mixture is removed from the residence reactor through a second high shear treatment.

14. The method according to claim 12, wherein the temperature in the residence reactor is kept in a range of 15-80° C., more preferably 20-70° C., more preferably 25-65° C.

15. The method according to claim 12, wherein the mixture has a residence time in a range of 1-60 min, preferably 1-45 min, more preferably 5-35 min in the residence reactor.