Polymer dispersion and its use

Abstract

The present invention relates to a polymer dispersion and its use. The dispersion comprises polymer in particle form dispersed in an aqueous liquid phase. The polymer particles are obtained by a radical polymerisation of monomers in a presence of a dissolved rosin component, in an aqueous polymerisation medium comprising a polymeric stabilizer. The monomers comprise a first monomer (i) selected from alkyl (meth)acrylates and a charge-carrying second monomer (ii) having a cationic charge, when measured at pH 3, selected from ethylenically unsaturated tertiary amines.

Description

[0001] POLYMER DISPERSION AND ITS USE

[0002] The invention relates to a polymer dispersion and its use according to preambles of the enclosed independent claims.

[0003] Surface sizing is commonly used to enhance the properties of cellulosic fibre webs, such as paper, board and the like. In surface sizing an aqueous solution or an aqueous dispersion comprising one or more sizing agents is applied on the surface of the cellulosic fibre web before the web is dried. Typically, an improved strength and / or water resistance is obtained by surface sizing. Surface sizing agents often comprise synthetic polymers, such as poly(styrene acrylate). In view of sustainability and recyclability there is a general trend and desire to decrease the use of materials which are based on non-renewable sources. Especially there is a desire to reduce the use of petroleum-based synthetic polymers.

[0004] Rosin, as well as its derivatives, have been used as hydrophobation agents in papermaking. For example, WO 2022 / 219245 discloses a polymer dispersion, which can be used for surface sizing of a cellulosic fibre web. The polymer dispersion is obtainable by a radical polymerisation of one or more vinyl monomers. A rosin component is dissolved into at least one of the vinyl monomers before the radical polymerisation. The sizing results achieved with the polymer dispersions are promising, but the production of the polymer dispersions may be challenging, as it has been observed that precipitates may form in the polymerization process. Such precipitates should be removed from the polymer dispersion before its use in sizing, which makes the production process more complicated. It would be desirable to reduce the amount of precipitates formed in the polymerization, in order to fully utilise the advantages of the polymer dispersion.

[0005] An object of this invention is to minimise or possibly even eliminate any disadvantages existing in the prior art. Another object of the present invention is to provide a sustainable polymer dispersion which provides good surface sizing results when used for surface sizing of a cellulosic fibre web.

[0006] Yet another object of the present invention is to provide a polymer dispersion which can be obtained with a minimal formation of precipitates during its polymerisation and which preferably has a small particle size.

[0007] These objects are attained with the invention having the characteristics presented below in the characterising parts of the independent claims.

[0008] Some preferred embodiments of the invention are presented in the dependent claims. All the described embodiments and advantages apply to all aspects of the present invention, i.e. the polymer dispersion, its use and the method for its manufacture, when applicable, even if not always explicitly stated so.

[0009] Atypical polymer dispersion according to the present invention comprises a polymer in particle form, dispersed in an aqueous liquid phase, wherein the polymer in particle form is obtained by a radical polymerisation of monomers comprising

[0010] - a first monomer (i) selected from alkyl (meth)acrylates, and

[0011] - a charge-carrying second monomer (ii) having a cationic charge, when measured at pH 3, selected from ethylenically unsaturated tertiary amines, in a presence of a dissolved rosin component, in an aqueous polymerisation medium comprising a polymeric stabilizer.

[0012] A typical use according to the present invention of a polymer dispersion according to the invention is for surface sizing of a cellulosic fibre web, such paper, board, nonwoven, textile or the like.

[0013] In a typical method according to the present invention for manufacturing a polymer dispersion comprising a polymer in particle form dispersed in an aqueous liquid phase, the polymer is obtained by a radical polymerisation of - a first monomer feed comprising a first monomer (i) selected from alkyl (meth)acrylates, and

[0014] - a second monomer feed comprising a charge-carrying second monomer (ii) having a cationic charge, when measured at pH 3, selected from ethylenically unsaturated tertiary amines, in an aqueous polymerisation medium comprising a polymeric stabilizer, wherein a rosin component is dissolved at least one of the monomer feeds before start of the radical polymerisation.

[0015] Now it has been surprisingly found that an inclusion of a charge-carrying second monomer (ii), having a cationic charge and selected from ethylenically unsaturated tertiary amines, improves the properties of the polymer dispersion obtained by a radical polymerisation in the presence of a dissolved rosin component. The improvement can be observed both during the preparation of the polymer dispersion and during its application in surface sizing. Especially, it was found that the amount of precipitates formed during the radical polymerisation may be unexpectedly reduced when the charge-carrying second monomer (ii) having a cationic charge and selected from ethylenically unsaturated tertiary amines takes part in the radical polymerisation in the presence of the dissolved rosin component. Reducing the amount of precipitates provides significant improvements in process economy: the amount of active polymer increases in the dispersion; the produced polymer dispersion have a higher quality which improves its applicability to various end uses; and there is no need for additional steps of precipitate removal from the polymer dispersion and less waste is formed. At the same time, the other properties of the polymer and the polymer dispersion, such as small particle size, high solids content and appropriate viscosity, are maintained or even improved. The obtained polymer dispersion also shows good surface sizing results. The present invention thus provides an unexpectedly enhanced polymer and polymer dispersion, providing incentives to further increase the use of materials based on renewable sources in the polymerisation. In the present context, the term “charge-carrying second monomer” denotes a monomer which has either a cationic charge when measured at pH 3 or an anionic charge when measured at pH 8.5.

[0016] The expressions “polymer in particle form” and ’’polymer particles” are synonymous in the present context, and fully interchangeable.

[0017] The polymer dispersion of the present invention may have a cationic or anionic net charge, preferably a cationic net charge, when measured at pH 3. Net charge indicates the total charge of the polymer dispersion, after taking account all individual charges, i.e. both positive and negative charges, of all components in the polymer dispersion. The charge may be determined as charge density, measured by using particle charge detector, as described in the experimental part.

[0018] The polymer in particle form, i.e. polymer particles of the polymer dispersion, is obtained by the radical polymerisation of monomers comprising at least one first monomer (i) selected from alkyl (meth)acrylates, and at least one charge-carrying second monomer (ii) having a cationic charge and selected from ethylenically unsaturated tertiary amines. The polymer particles may be obtained by polymerisation of monomers comprising one or more first monomers (i) and one or more charge-carrying monomers, wherein at least one of the charge-carrying monomers is the charge-carrying second monomer (ii) having a cationic charge, selected from ethylenically unsaturated tertiary amines. In addition to the chargecarrying second monomer (ii), which is cationic and selected from ethylenically unsaturated tertiary amines, the charge-carrying monomers may comprise an additional charge-carrying monomer (iii) which is cationic, and / or an additional charge-carrying monomer (iv), which is anionic, as described later. Consequently, the obtained polymer and the polymer particles may be cationic, anionic or amphoteric. Preferably, all charge-carrying monomers are cationic, and the obtained polymer as well as the polymer particles have a cationic net charge.

[0019] It is possible that the polymer particles of the polymer dispersion may be obtained by polymerisation of monomers comprising at least one first monomer (i), at least one charge-carrying second monomer (ii) having a cationic charge and selected from ethylenically unsaturated tertiary amines, and at least one anionic additional charge-carrying monomer (iv). The polymerisation may preferably be performed at an acidic pH, for example at pH 3 - 5, and the obtained polymer, polymer particles and the polymer dispersion show amphoteric characteristics and may have either cationic or anionic net charge, when measured at pH 3.

[0020] The net charge of the polymer particles and the polymer dispersion may further depend on the charge and amounts of the rosin component and / or the polymeric stabilizer present during the radical polymerisation. Preferably, the polymer particles and the polymer dispersion have a cationic net charge, measured at pH 3.

[0021] The polymer particles of the polymer dispersion may be cationic and have a zeta potential >0 mV, measured at pH 3. According to one preferable embodiment the polymer particles may have the zeta potential in a range of 1 - 60 mV, preferably 2

[0022] - 15 mV, more preferably 3 - 12 mV, even more preferably 4 - 10 mV, measured at pH 3. The zeta potential may be measured by using Zetasizer Nano ZS, Malvern, as described in the experimental part. The zeta potential of the polymer particles provides good electrostatic stabilization of the polymer dispersion, thus improving its handling and storage life, while still maintaining at least good surface sizing performance obtainable with the polymer dispersion.

[0023] The polymer dispersion preferably has a cationic charge density, measured at pH 3. The charge density may be measured by using particle charge detector, Mutek™ PCD-05, as described in the experimental part. According to one advantageous embodiment, the polymer dispersion may have the charge density in a range of 0.01

[0024] - 0.60 meg / g, preferably 0.02 - 0.25 meq / g, more preferably 0.02 - 0.19 meq / g, measured at pH 3. The charge density values are given based on dry weight of the dispersion.

[0025] As described above, in some embodiments the polymer and the polymer particles may be anionic. The polymer particles may have a zeta potential <0 mV, measured at pH 3. For example, the polymer particles may have the zeta potential in a range from -60 mV to -1 mV, preferably from -30 mV to -5 mV, more preferably from -25 mV to -10 mV, measured at pH 3. The zeta potential may be measured by using Zetasizer Nano ZS, Malvern, as described in the experimental part. The anionic polymer dispersion may have a charge density in a range from -0.2 meq / g to -0.01 meq / g, preferably from -0.15 to -0.01 meq / g, more preferably from -0.03 meq / g to - 0.01 meq / g, measured at pH 3. The charge density may be measured by using particle charge detector, Mutek™ PCD-05, as described in the experimental part and the results are given based on dry weight.

[0026] According to the present invention, the monomers for obtaining the polymer particles by the radical polymerisation comprise the charge-carrying second monomer (ii) having a cationic charge, measured at pH 3, selected from ethylen ically unsaturated tertiary amines. The ethylenically unsaturated tertiary amines are relatively hydrophobic, which means that they are present in the same phase with the first monomers (i) and the dissolved rosin component. This enables effective polymerisation of the charge-carrying second monomer (ii) and the first monomer (i) in the presence of the hydrophobic rosin component. The rosin component may thus become incorporated to the structure of the formed polymer particles of the polymer dispersion. The charge-carrying second monomer (ii) is an ethylenically unsaturated tertiary amine, which may be selected from a group consisting of N,N-dialkylamino- alkylacrylamides, N,N-dialkylaminoalkylmethacrylamides, N,N-dialkylaminoalkyl acrylates, N,N-dialkylaminoalkyl methacrylates, such as N,N-di(C1 -C4-alkyl)amino C1 -C4-alkyl acrylates, N,N-di(C1 -C4-alkyl)amino C1 -C4-alkyl methacrylates, N,N- di(C1-C4-alkyl)amino C1 -C4-alkylacrylamides, N,N-di(C1-C4-alkyl)amino C1 -C4- alkyl methacrylamides, and optionally any of their mixtures. Preferably the cationic charge-carrying second monomer (ii) may be an ethylenically unsaturated tertiary amine selected from N,N-dimethylamino C1 -4-alkylacrylates, N,N-dimethylamino C1 -4-alkylmethacrylates and corresponding acrylamides, and optionally any mixtures thereof. More preferably the cationic charge-carrying second monomer (ii) may be an ethylenically unsaturated tertiary amine selected from 2- (dimethylamino)ethyl methacrylate, dimethylaminoethyl acrylate, dimethylaminopropyl methacrylate, dimethylaminopropyl acrylate, dimethylaminopropyl acrylamide, and dimethylaminopropyl methacrylamide, and optionally any mixtures thereof. According to one preferable embodiment the charge-carrying second monomer (ii) may be 3-dimethylaminopropyl methacrylamide or 2- (dimethylamino)ethyl methacrylate or any of their mixtures.

[0027] According to one embodiment, the monomers for obtaining the polymer particles by the radical polymerisation may comprise an additional charge-carrying monomer (iii) having a cationic charge, when measured at pH 3, and selected from one or more ethylenically unsaturated secondary amines, ethylenically unsaturated quaternary amines, and any of their mixtures. For example, the additional charge-carrying monomer (iii) having a cationic charge may be an ethylenically unsaturated quaternary amine selected from a group consisting of quaternary salts of N,N,N- trialkylaminoalkylacrylamides, N,N,N-trialkylaminoalkylmethacrylamides, N,N,N- trialkylaminoalkyl acrylates, N,N,N-trialkylaminoalkyl methacrylates, such as N,N,N- tri(C1 -C4-alkyl)amino C1 -C4-alkylacrylates, N,N,N-tri(C1 -C4-alkyl)amino C1 -C4- alkylmethacrylates, N,N,N-tri(C1 -C4-alkyl)amino C1 -C4-alkylacrylamides, N,N,N- tri(C1 -C4-alkyl)amino C1 -C4-alkylmethacrylamides. Preferably, the additional charge-carrying monomer (iii) having a cationic charge may be selected from quaternary salts of N,N,N-trimethylamino C1 -C4-alkylacrylates and N,N,N- trimethylamino C1 -C4-alkylmethacrylates with mineral acid, such as quaternary salts of N,N,N-trimethylamino ethyl (meth)acrylates with HCI, for example [2- (methacryloyloxy)ethyl]trimethylammonium chloride and [2-(acryloyloxy)ethyl]- trimethylammonium chloride.

[0028] The additional charge-carrying monomer (iii) having a cationic charge may further be selected from 2-(dimethylamino)ethyl acrylate benzylchloride, 2- (dimethylamino)ethyl acrylate dimethylsulphate, 2-dimethylaminoethyl methacrylate dimethylsulphate, and diallyldimethylammonium chloride.

[0029] According one embodiment, the monomers for obtaining the polymer particles by the radical polymerisation may comprise an additional charge-carrying monomer (iv) having an anionic charge, when measured at pH 8.5. The anionic additional chargecarrying monomer (iv) may comprise at least one carboxylate and / or sulphonate group. The anionic additional charge-carrying monomer (iv) may be, for example, carboxylic acid, such as acrylic acid, methacrylic acid, 2-sulphoethyl methacrylate, styrene sulphonic acid, or any of their mixtures.

[0030] The first monomer (i) used for obtaining the polymer in particle form, i.e. the polymer particles, by radical polymerisation, is selected from alkyl (meth)acrylates. Preferably, the first monomer (i) may be selected from C1 -C18 alkyl (meth)acrylates, more preferably from C1 - C12 alkyl (meth)acrylates, even more preferably C1 - C4 alkyl (meth)acrylates or any mixtures thereof. The first monomer (i) may be selected from methyl acrylate; methyl methacrylate; ethyl acrylate; ethyl methacrylate; n-propyl or iso-propyl acrylate and corresponding propyl methacrylates; n-butyl, iso-butyl, tert-butyl or 2-butyl acrylate and the corresponding butyl methacrylates; n-pentyl or neopentyl acrylate and the corresponding pentyl methacrylates; 2-hexyl or 2-ethylhexyl acrylate and corresponding methacrylates; n-octyl or isooctyl acrylate and corresponding methacrylates; decyl acrylate; decyl methacrylate; dodecyl acrylate; dodecyl methacrylate; lauryl acrylate; lauryl methacrylate; stearyl acrylate; stearyl methacrylate, or any mixtures thereof. Preferably, the first monomer (i) may be selected from C1-C4-alkyl acrylates, C1 - C4-alkyl methacrylates or any of their mixtures, e.g. n-butyl, iso-butyl, tert-butyl or 2-butyl acrylate and the corresponding butyl methacrylates; methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, propyl acrylate or propyl methacrylate. It is possible, and often preferred, that the first monomers (i) may comprise or consists of a mixture of at least two isomeric butyl (meth)acrylates, e.g. a mixture of n-butyl acrylate and sec-butyl acrylate or a mixture of n-butyl acrylate and tert-butyl acrylate.

[0031] According to one preferable embodiment of the invention the polymer particles of the polymer dispersion, i.e. polymer in particle form, dispersed in an aqueous liquid phase, may be obtained by radical polymerisation of monomers comprising or consisting of

[0032] - 85 - 99.9 weight-%, preferably 89 - 99 weight-%, more preferably 91 - 96 weight- %, of the first monomer (i), and / or - 0.01 - 15 weight-%, preferably 1 - 11 weight-%, more preferably 4 - 9 weight-%, of the charge-carrying second monomer (ii) having a cationic charge and selected from ethylenically unsaturated tertiary amines, calculated from the total weight of the first monomers (i) and second monomers (ii), as dry.

[0033] It is possible that the polymer particles of the polymer dispersion, i.e. polymer in particle form, dispersed in an aqueous liquid phase, may be obtained by radical polymerisation of monomers comprising or consisting of

[0034] - 80 - 99.9 weight-%, preferably 82 - 99 weight-%, more preferably 85 - 96 weight- %, of the first monomer (i), and / or

[0035] - 0.01 - 20 weight-%, preferably 1 - 18 weight-%, more preferably 4 - 15 weight-%, of the charge-carrying second monomer (ii) having a cationic charge and selected from ethylenically unsaturated tertiary amines, calculated from the total weight of the first monomers (i) and second monomers (ii), as dry. These monomer amounts may be beneficial for obtaining polymer dispersions with minimal content of precipitates.

[0036] The radical polymerisation of the monomers is carried out in the presence of the dissolved rosin component. The rosin component may preferably be dissolved at least one of the feeds of the used monomers before the radical polymerisation of the monomers in the aqueous polymerisation medium. This means that the rosin component is at least dissolved in the first monomer (i) and / or in the charge-carrying second monomer (ii). As both the first monomer (i) and the charge-carrying second monomer (ii) are relatively hydrophobic, the rosin component dissolves well into these monomers and the risk for insoluble rosin component and formation of precipitates are minimised. It is possible that part of the rosin component is dissolved into the optional additional charge-carrying monomers (iii) and / or (iv), if they are present. When the radical polymerisation is carried out the presence of the dissolved rosin component, the rosin component becomes an integral part of the polymer particles of the polymer dispersion. It is assumed that when dissolved in the monomers, at least part of the rosin component becomes permanently incorporated into the structure of the polymer in particle form, formed by the radical polymerisation of the monomers. Alternatively, or in addition a part of the rosin component may be present in the aqueous polymerisation medium, as a solution or as dispersed.

[0037] In the present context the term “rosin component” denotes rosin and its derivatives. The rosin component is typically relatively insoluble in water and therefore the rosin component is preferably dissolved in at least one of the monomers / monomer feeds before the radical polymerisation, as described above. The dissolved rosin component may even be a mixture of different rosins. According to one embodiment the dissolved rosin component may be selected from rosin, rosin derivatives, such as rosin esters, dimerised rosins, polymerised rosins, hydrogenated rosins, fortified rosins and unfortified rosins, or any of their mixtures. According to one preferable embodiment the dissolved rosin component may be selected from a group consisting of tall oil rosin, wood rosin, gum rosin, their derivatives, and any of their mixtures. For example, the dissolved rosin component may be a mixture of tall oil rosin and gum oil rosin. According to one preferable embodiment, the rosin component has a low amount of free fatty acids. The rosin component may comprise <5 weight-%, preferably <4 weight-%, more preferably <3 weight-%, sometimes even <2 weight-%, of free fatty acids, calculated from the total weight of the rosin component.

[0038] According to one preferable embodiment of the present invention the dissolved rosin component may be a fortified rosin. Fortified rosins are obtained by adducting an unsaturated carboxylic acid to a rosin. Suitable carboxylic acids are, for example, fumaric acid, acrylic acid, maleic acid or itaconic acid. Fortified rosins obtained by adducting maleic acid or fumaric acid to the rosin are being preferred. It has been observed that fortified rosin is very effectively dissolved in one of the monomer feeds used in the radical polymerisation. According to one preferable embodiment the fortified rosins comprise from 7 - 60 weight-%, preferably 14 - 52 weight-%, more preferably 20 - 45 weight-%, of the rosin acids, such as abietic acid, neoabietic acid, palustric acid, pimaric acid, isopimaric acid or any of their combinations. Analysis of the fortified rosin acids can be conducted by using known chromatographic methods, for example, gas chromatography. The polymer dispersion comprising the polymer in particle form dispersed in aqueous liquid phase, is obtainable by a radical polymerisation, preferably by a free radical polymerisation, of one or more monomer feeds. One monomer feed may comprise one or more first monomers (i) and / or one or more charge-carrying second monomers (ii), and optional additional charge carrying monomers (iii) and / or (iv), which are described in this application and used for obtaining the polymer in particle form. Alternatively, each monomer feed may comprise only one monomer, i.e. each used monomer has its own separate feed. The rosin component may be dissolved in any or each of these monomers feeds, as described above. Monomer feeds used in the radical polymerisation are in form of a monomer solution(s), which may contain small amounts of water and / or other solvents. However, the amounts of water and / or other solvents in the monomer solution(s), and consequently in the monomer feeds, are preferably minimised. Preferably, all monomer feeds are essentially free of water and of other solvents, e.g. organic solvents. In the present context the term “essentially free” means that the monomer feed comprises less than 1 weight-%, preferably less than 0.5 weight-%, more preferably less than 0.1 weight-%, of water and / or other solvents. The monomer feed(s) function as a solvent for the rosin component.

[0039] The rosin component may have a softening point in a range of 15 - 150 °C, preferably 40 - 140 °C, more preferably 55 - 130 °C, even more preferably 75 - 125 °C. According to one embodiment the rosin component is essentially free of monocyclic terpene compounds. The rosin component may be in form of a liquid or solid when it is dissolved into one or more of the monomer feeds. The rosin component may be dissolved at a temperature of 15 - 40 °C or 20 - 40 °C. In general, the dissolving of the rosin component into one or more of the monomer feeds may be achieved without external heating.

[0040] The dissolved rosin component may be present during the radical polymerisation of the monomers in an amount of 0.01 - 70 weight-%, preferably 1 - 60 weight-%, more preferably 1 .5 - 49 weight-%, calculated from the total weight of the monomers and the rosin component, as dry. When the dissolved rosin component is present during the polymerisation of the monomers, preferably as dissolved to at least one of the monomers / monomer feeds before the radical polymerisation, it is easy to increase the amount of rosin in the obtained polymer dispersion. For example, the polymer in particle form may be obtained by the radical polymerisation of the monomers in the presence of 5 - 70 weight-%, preferably 7.5 - 49 weight-%, more preferably 10 - 35 weight-%, even more preferably 15 - 34 weight-%, of the dissolved rosin component, calculated from the total weight of the monomers and the rosin component, as dry. Presence of the charge-carrying second monomer (ii) which has a cationic charge and is selected from ethylenically unsaturated tertiary amines makes it possible to increase the amount of the rosin component while avoiding formation of precipitates in the polymerisation.

[0041] According to another embodiment of the invention the polymer dispersion may be obtained by the radical polymerisation of the monomers in the presence of 0.05 - 4.5 weight-%, preferably 0.5 - 3.5 weight-%, more preferably 1 .5 - 3.0 weight-%, of the dissolved rosin component, calculated from the total weight of the monomers and the rosin component, as dry. When the used in relatively small amounts, the dissolved rosin component may act as a chain transfer agent during the radical polymerisation and thus have an impact on the structure and molecular weight of the polymer formed by the radical polymerisation.

[0042] The polymer dispersion comprises polymer particles, i.e. polymer in particle form, dispersed in an aqueous liquid phase, which forms the continuous phase of the dispersion. The presence of dissolved rosin component during the radical polymerisation does not negatively influence the properties of the formed polymer and the polymer particles of the polymer dispersion. The polymer dispersion may comprise polymer in particle form having a particle size D50 <200 nm, preferably <120 nm, more preferably <80 nm, even more preferably <55 nm, sometimes even < 45 nm. The particle size D50 for the polymer particles of the dispersion may be, for example, in a range of 10 - 200 nm, preferably 15 - 120 nm, more preferably 20 - 80 nm, even more preferably 25 - 55 nm, sometimes even 25 - 45 nm. The polymer dispersion may comprise polymer in particle form having a particle size D90 <500 nm, preferably <200 nm, more preferably <140 nm, even more preferably <100 nm. The particle size D90 for the polymer particles of the dispersion may be, for example, in a range of 20 - 500 nm, preferably 25 - 200 nm, more preferably 30 - 140 nm, even more preferably 35 - 100 nm. All particle sizes are measured by using Zetasizer Nano ZS, Malvern. In the present context the particle size D50 refers to the value for 50thpercentile of a volume based distribution and the particle size D90 refers to the value for 90thpercentile of a volume based distribution. The obtained values indicate that the particle size distribution for the polymer dispersion is relatively narrow, which is advantageous in view of its use in the surface sizing.

[0043] The polymer dispersion comprises an aqueous liquid phase, which forms the continuous phase of the dispersion. Preferably the aqueous liquid phase of the polymer dispersion is identical to the aqueous polymerisation medium of the radical polymerisation. According to one embodiment of the invention the aqueous polymerisation medium may comprise an additional solvent. The aqueous polymerisation medium may comprise at most 50 %, preferably at most 35 %, more preferably at most 15 %, of an additional solvent other than water during the polymerisation. The additional solvent may be an organic acid, such as acetic acid, or an alcohol, such as ethanol or isopropanol. The additional solvent may be removed, for example by distillation, from the polymer dispersion after the radical polymerisation of the monomers has been completed. According to one preferable embodiment the aqueous liquid phase of the obtained polymer dispersion may comprise at most 10 weight-%, preferably at most 5 weight-%, more preferably at most 1 weight-%, of an additional solvent other than water, calculated from the total weight of the solvent. The use of additional solvent may be advantageous if the rosin component or part of it is directly dissolved into the aqueous polymerisation medium.

[0044] According to one preferred embodiment, however, the aqueous polymerisation medium is free of other solvents than water, i.e. the aqueous polymerisation medium is free of organic solvents, such as organic acids, e.g. acetic acid, and alcohols, e.g. ethanol and isopropanol. This means that the aqueous polymerisation medium, and consequently the aqueous liquid phase of the polymer dispersion is water or consists of water. The aqueous polymerisation medium comprises a polymeric stabilizer. The polymeric stabilizer can be selected from synthetic polymeric stabilizers or natural polymeric stabilizers. The polymeric stabilizer may be anionic, cationic, amphoteric or non-ionic. Preferably the polymeric stabilizer is non-ionic or anionic, more preferably the polymeric stabilizer is anionic. The polymeric stabilizer is added to the aqueous polymerisation medium before the start of the radical polymerisation and the polymeric stabilizer is present during the radical polymerisation of the monomers. The polymeric stabilizer may function as a protective colloid for the formed polymer particles in the obtained polymer dispersion.

[0045] According to one embodiment the polymeric stabilizer is a synthetic polymeric stabilizer selected from polyvinyl alcohols, polyvinyl acetate-co-vinyl alcohols, anionic polyacrylates, cationic polyacrylates, anionic poly(styrene acrylates) and cationic polystyrene acrylates).

[0046] According to one preferable embodiment the polymeric stabilizer is a natural polymeric stabilizer, which is a polysaccharide or its derivative. When the polymeric stabilizer is a polysaccharide, it may be added in amount of 10 - 45 weight-%, preferably 15 - 40 weight-%, more preferably 20 - 35 weight-%, calculated from the total weight of the polymer dispersion, as dry. According to one preferable embodiment, the polymeric stabilizer is selected from a group comprising polysaccharides, polysaccharide derivatives, degraded polysaccharides, degraded polysaccharide derivatives and any of their mixtures. Preferably the polysaccharide used as the polymeric stabilizer is essentially water-soluble. The polymeric stabilizer may be selected, for example, from starches, substituted starches, cellulose, substituted celluloses, cellulose derivatives, hemicelluloses, substituted hemicelluloses, chitosan, glucan derivatives, dextrin, degraded starches and any of their mixtures. Preferably the polymeric stabilizer is degraded starch, for example having a degree of polymerisation in a range of 10 - 750 glucose units and / or a weight average molecular weight MW in range of 500 - 250 000 g / mol. The polysaccharide used as a polymeric stabilizer, such as degraded starch, has preferably a number average molecular weight Mn in a range of 500 - 10 000 g / mol. The number average molecular weights may be determined, for example, by size exclusion chromatography. The polysaccharide, such as starch or degraded starch, used as polymeric stabilizer, may be anionic, cationic, amphoteric or non-ionic, preferably anionic.

[0047] According to one embodiment the polymer dispersion may comprise one or more surfactants which may function as co-stabilizers. It has been observed that the presence of a surfactant as co-stabilizer further reduces the amount of precipitates formed during the radical polymerisation. Further, the use of co-stabilizer provides stabile dispersions with narrow particle size distributions. According to the one embodiment the surfactant is selected from non-ionic and anionic surfactants. Suitable surfactants are, for example, polyethylene glycol monostearate, disodium lauryl phenyl ether disulphonate, sodium lignosulphonate and sodium naphthalene sulphonate. In general, surfactants having a hydrophilic-lipophilic balance (HLB) value in a range from 8 - 18, preferably 10 - 18, can be used as co-stabilizer due to their ability to promote oil-in-water emulsion stability. If used, the surfactant may be present in amount of 0.5 - 3 weight-%, preferably 0.8 - 2.5 weight-%, calculated from the total dry weight of the monomers.

[0048] According to one preferable embodiment, the aqueous polymerisation medium comprises a polysaccharide as a polymeric stabilizer and a non-ionic or anionic surfactant as a co-stabilizer. Preferably, a natural polymeric stabilizer is used in combination with a surfactant. Preferable combinations of natural polymeric stabilizer and the surfactant comprise anionic starch, preferably anionic degraded starch, with a surfactant selected from polyethylene glycol monostearate, disodium lauryl phenyl ether disulphonate, sodium lignosulphonate or with sodium naphthalene sulphonate.

[0049] According to one embodiment of the invention, the polymer in particle form, i.e. polymer particles of the polymer dispersion, may be obtained by radical polymerisation of monomers comprising at least one first monomer (i), at least one charge-carrying second monomer (ii) having a cationic charge and selected from ethylenically unsaturated tertiary amines, and at least one third monomer (v). The first monomer (i) and the charge-carrying second monomer (ii) have been described above and are different from each other. The third monomer (v) is different from the first monomer (i) and the charge-carrying second monomer (ii). Preferably the at least one third monomer (v) is selected from styrene, substituted styrenes, such as a-methylstyrene, vinyltoluene, ethylvinyltoluene, chloromethylstyrene, divinylbenzene, and any of their mixtures. More preferably the third monomer (v) is selected from styrene, substituted styrenes, such as a-methylstyrene, vinyltoluene, ethylvinyltoluene, chloromethylstyrene, and any of their mixtures. The amount of the third monomer (v) may be 0.05 - 30 weight-% or 1 - 30 weight-%, preferably 1 - 15 weight-%, more preferably 2 - 10 weight-%. It is possible that the amount of the third monomer (v) may preferably be 5 - 15, more preferably 5 - 10 weight-%, calculated from the total weight of all monomers, as dry. The amount of the third monomer (v) may be, for example, in a range of 1 - 30 weight-%, preferably 2 - 20 weight-%, more preferably 2 - 10 weight-%, of the third monomer (v), calculated from the total weight of monomers (i), (ii) and (v), as dry.

[0050] According to one preferable embodiment of the present invention the polymer dispersion is obtained by radical polymerisation of monomers consisting of at least one first monomer (i) and at least one charge-carrying second monomer (ii). The radical polymerisation is thus free of optional additional charge-carrying second monomers (iii) and (iv), as well as of third monomer (v). Especially, the polymer in particle form is obtained by radical polymerisation in the absence of styrene and substituted styrenes.

[0051] Further, the radical polymerisation of the first monomer (i) and the charge-carrying second monomer (ii), the optional additional charge-carrying monomers (iii) and (iv), as well as the optional third monomer (v), may be conducted in a presence of a crosslinking agent. The presence of crosslinking agent is fully optional, but it may provide improved properties for the obtained polymer particles and the polymer dispersion. According to one embodiment, the radical polymerisation of the monomers (i) and (ii), and optional monomers (iii), (iv) and / or (v), is free of crosslinking agents. The polymer in particle form in the polymer dispersion may have a weight average molecular weight Mw in the range 1000 - 100 000 g / mol, preferably 5000 - 80 000 g / mol, more preferably 8000 - 40000 g / mol, analysed from the final dispersion. The polymer in particle form in the polymer dispersion may have a number average molecular weight Mn in the range 1000 - 20 000 g / mol, preferably 2000 - 15 000 g / mol, more preferably 3000 - 10000 g / mol, analysed from the final dispersion. The weight average and number average molecular weights may be determined, for example, by size exclusion chromatography.

[0052] The polymer in particle form in the polymer dispersion, as dry, may have a glass transition temperature Tgin a range of 10 - 90 °C, preferably 25 - 80 °C, more preferably 35 - 75 °C or even more preferably 38 - 70 °C.

[0053] The obtained polymer dispersion may have a solids content of at least 10 weight- %, preferably at least 20 weight-%, sometimes even at least 25 weight-%. According to one embodiment the solids content of the polymer dispersion may be in a range of 10 - 60 weight-%, preferably 20 - 55 weight-%, more preferably 25 - 45 weight- %.

[0054] The polymer in particle form, i.e. the polymer particles for the polymer dispersion, are formed directly by the radical polymerisation of the monomers in the aqueous polymerisation medium comprising rosin component. The radical polymerisation may be carried out by a feed process, where the one or more feeds of the monomers are fed into the aqueous polymerisation medium during a polymerisation time, or by a batch process, where the complete feeds of the monomers are added into the aqueous polymerisation medium at once in the beginning of the radical polymerisation. Feed process is preferred. A continuous polymerisation process in a stirred kettle cascade or a flow tube is also possible. In a preferred feed process, the continuous feed(s) of the monomers and the free radical initiator are metered uniformly into the aqueous polymerisation medium in a stirred reactor. During the entire preparation and polymerisation process, thorough mixing with the aid of any suitable stirring or mixing units is maintained so that the added monomer feed(s) and other components are homogeneously distributed as rapidly as possible. The radical polymerisation may be performed at a polymerisation temperature in a range of 50 - 100 °C, preferably 60 - 90 °C, more preferably 70 - 90 °C.

[0055] The obtained polymer dispersion may have a viscosity of <500 mPas, preferably <200 mPas, more preferably <50 mPas. The viscosity may be in a range of 1 - 500 mPas, preferably 1 - 200 mPas, more preferably 2 - 50 mPas. All the viscosity values are measured at 25 °C, with Brookfield LVDV viscometer, in a small sample adapter with spindle 18, measured at solids content of 25 weight-%.

[0056] The polymer dispersion according to the present invention is especially suitable for use in surface sizing of cellulosic fibre webs. The surface sizing compositions may, in addition to the polymer dispersion, further comprise surface sizing starch and / or other additives conventionally used in surface sizing of paper, board and other cellulosic products. Such additives commonly known in the art include, but are not limited to, dispersing agents, antifoaming agents, colorants, inorganic pigments and fillers, anti-curl agents, anti-static agents, additional conventional components such as surfactants, plasticizers, humectants, defoamers, UV absorbers, light fastness enhancers, polymeric dispersants, dye mordants, optical brighteners, levelling agents, rheology modifiers, and strength additives. The additives may be used to further enhance the sizing performance which is obtained with polymer dispersion according to the present invention.

[0057] The polymer dispersion may be applied on a cellulosic fibre web, such paper, board, nonwoven, textile or the like, in an amount of 0.1 - 10 kg / t, preferably 0.5 - 6 kg / t, given as dry cellulosic fibre web. The polymer dispersion according to the present invention is especially suitable for surface sizing of all paper and paper board qualities. The polymer dispersion according to the present invention is particularly suitable for surface sizing of cellulosic fibre webs which comprise recycled fibres.

[0058] EXPERIMENTAL Some embodiments of the invention are described more closely in the following nonlimiting examples. All percentages in the examples are weight-%, if not otherwise stated.

[0059] Methods

[0060] Following properties and measurement methods have been used in the examples to characterize the dispersions. pH: measured with laboratory pH meter at 25 °C.

[0061] Particle size: measured by using Malvern Zetasizer Nano-device.

[0062] Solids content: measured using a Mettler Toledo Halogen moisture analyser. Viscosity: measured at 25 °C, with Brookfield LVDV viscometer, in a small sample adapter with spindle 18, 60 rpm, at solids concentration about 25 weight-% Zeta-potential: measured by using Zetasizer Nano ZS, Malvern. 2g of sample was diluted with 248 g deionized water, and the pH was adjusted to pH 3.

[0063] Charge density: measured by using particle charge detector, Mutek™ PCD-05. The samples were diluted with deionized water and the pH adjusted to pH 3. The anionic samples were titrated with a cationic titrant and the cationic samples were titrated with anionic titrant. The charge density values are given based on dry weight of the sample.

[0064] Comparative Example 1 : Preparation of Polymer Dispersion Without Cationic Monomer

[0065] 94.5 g of an oxidatively degraded potato starch (Perfectamyl A 4692) was dispersed with stirring in 557 g of deionized water under a nitrogen atmosphere in a 1 L glass reactor having a cooling / heating jacket. Prior the addition of the starch, the pH of the deionized water was adjusted to pH 3 with 5 % sulfuric acid solution. The starch was dissolved by heating the mixture to 95 °C and cooking at 95 °C for 30 min. After the starch dissolution was complete, 28.6 g of 0.74 % aqueous solution of ferrous(ll) sulphate heptahydrate was added into the reactor. After 10 min 3.4 g of 30 % hydrogen peroxide was added. After 40 min in 95 °C, the starch degradation was complete. During starch degradation, in a separate vessel, a mixture was made by blending together 101.9 g terf-butyl acrylate, 5.7 g of n-butyl acrylate, 5.7 g of styrene, 1.8 g of 80 % divinylbenzene and 54.7 g of a fortified tall oil rosin. The fortified tall oil rosin was dissolved into the monomers in the mixture.

[0066] After cooling the temperature of the reactor with degraded starch to 85 °C, the chemical feeds were started simultaneously. 169.8 g of the mixture (solution) of the monomers and the fortified tall oil rosin was fed during 120 min. 74.7 g of 5.5 % solution of hydrogen peroxide was ted during 165 min. The reactor temperature was kept at 85 °C during the feeds and 15 min after for post-polymerisation. Then the mixture was cooled to 60 °C and 5.2 g of 11.7 % terf-butyl hydroperoxide solution was added dropwise into the reactor. The temperature was kept at 60 °C for further 60 min. Thereafter, the obtained polymer dispersion was cooled to 50 °C and 5.9 g of 10 % ethylenediaminetetraacetic acid sodium salt (EDTA-Na) solution was added, followed by pH adjustment to 3.8 with sodium hydroxide solution and cooling to room temperature. Filtration was performed using a 100 pm filter cloth, the wet filter residue was weighted with the scale. A finely divided polymer dispersion was obtained. The polymer dispersion had a zeta potential value of -14.6 mV and a charge density value of -0.065 meq / g. The characteristics of the polymer dispersion is given in Table 1 .

[0067] Examples 1 - 7: Preparation of Polymer Dispersion from Cationic Chargecarrying Monomers, in the Presence of Fortified Tall Oil Rosin

[0068] Example 1

[0069] 94.5 g of an oxidatively degraded potato starch (Perfectamyl A 4692) was dispersed with stirring in 557 g of deionized water under a nitrogen atmosphere in a 1 L glass reactor having a cooling / heating jacket. Prior addition of the starch, the pH of the deionized water was adjusted to pH 3 with 5 % sulfuric acid solution. The starch was dissolved by heating the mixture to 95 °C and cooking at 95 °C for 30 min. After the starch dissolution was complete, 28.6 g of 0.74 weight-% aqueous solution of ferrous(ll) sulphate heptahydrate was added into the reactor. After 10 min 3.4 g of 30 % hydrogen peroxide was added. After 40 min of the hydrogen peroxide addition, in 95 °C, the starch degradation was complete.

[0070] During starch degradation, in a separate vessel, a mixture was made by blending together 97.5 g terf-butyl acrylate, 5.7 g of n-butyl acrylate, 5.7 g of styrene, 2.2 g of 3-dimethylaminopropyl methacrylamide, 2.2 g of 2-(dimethylamino)ethyl methacrylate, 1.8 g of 80 % divinylbenzene and 54.7 g of a fortified tall oil rosin. The fortified tall oil rosin was dissolved into the monomers in the mixture.

[0071] After the temperature of the reactor with degraded starch was cooled to 85 °C, the chemical feeds were started simultaneously. 169.8 g of the mixture (solution) of the monomer and the fortified tall oil rosin was fed during 120 min. 74.7 g of 5.5 % solution of hydrogen peroxide was ted during 165 min. The reactor temperature was kept at 85 °C during the feeds and 15 min after the completion of the feeds for postpolymerisation. Then the mixture was cooled to 60 °C and 5.2 g of 11 .7 % terf-butyl hydroperoxide solution was added dropwise into the reactor. The temperature was kept at 60 °C for further 60 min. Thereafter, the obtained polymer dispersion was cooled to 50 °C and 5.9 g of 10 % ethylenediaminetetraacetic acid sodium salt (EDTA-Na) solution was added, followed by cooling to room temperature. Filtration was performed using a 100 pm filter cloth, the wet filter residue was weighted with the scale. A finely divided polymer dispersion was obtained. The characteristics of the polymer dispersion are given in Table 1 .

[0072] Example 2

[0073] 94.5 g of an oxidatively degraded potato starch (Perfectamyl A 4692) was dispersed with stirring in 557 g of deionized water under a nitrogen atmosphere in a 1 L glass reactor having a cooling / heating jacket. Prior the addition of the starch, the pH of the deionized water was adjusted to pH 3 with 5 % sulfuric acid solution. The starch was dissolved by heating the mixture to 95 °C and cooking at 95 °C for 30 min. After the starch dissolution was complete, 28.6 g of 0.74 weight-% aqueous solution of ferrous(ll) sulphate heptahydrate was added into the reactor. After 10 min 3.4 g of 30 % hydrogen peroxide was added. After 40 min in 95 °C, the starch degradation was complete. During starch degradation, in a separate vessel, a mixture was made by blending together 96.3 g terf-butyl acrylate, 5.7 g of n-butyl acrylate, 5.7g of styrene, 5.7 g of 2-(dimethylamino)ethyl methacrylate, 1 .8 g of 80 % divinylbenzene and 54.7 g of a fortified tall oil rosin. The fortified tall oil rosin was dissolved into the monomers. The tall oil rosin was the same as used in Example 1 .

[0074] After the temperature of the glass reactor with degraded starch was cooled to 85 °C, the chemical feeds were started simultaneously. 169.8 g of the mixture (solution) of the monomers and fortified tall oil rosin was fed during 120 min. 74.7 g of 5.5 % solution of hydrogen peroxide was ted during 165 min. The reactor temperature was kept at 85 °C during the feeds and 15 min after for post-polymerisation. Then the mixture was cooled to 60 °C and 5.2 g of 11.7 % terf-butyl hydroperoxide solution was added dropwise into the reactor. The temperature was kept at 60 °C for further 60 min. Thereafter, the obtained polymer dispersion was cooled to 50 °C and 5.9 g of 10 % ethylenediaminetetraacetic acid sodium salt (EDTA-Na) solution was added, followed by cooling to room temperature. Filtration was performed using a 100 pm filter cloth, the wet filter residue was weighted with the scale. Afinely divided polymer dispersion was obtained. The characteristics of the polymer dispersion is given in Table 1 .

[0075] Example 3:

[0076] 94.5 g of an oxidatively degraded potato starch (Perfectamyl A 4692) was dispersed with stirring in 557 g of deionized water under a nitrogen atmosphere in a 1 L glass reactor having a cooling / heating jacket. Prior the addition of the starch, the pH of water was adjusted to pH 3 with 5 % sulfuric acid solution. The starch was dissolved by heating the mixture to 95 °C and cooking at 95 °C for 30 min. After starch dissolution was complete, 28.6 g of 0.74 % aqueous solution of ferrous(ll) sulphate heptahydrate was added into the reactor. After 10 min 3.4 g of 30 % hydrogen peroxide was added. After 40 min in 95 °C, the starch degradation was complete.

[0077] During starch degradation, in a separate vessel, a mixture was made by blending together 96.3 g terf-butyl acrylate, 5.7 g of n-butyl acrylate, 5.7 g of styrene, 5.7 g of 3-dimethylaminopropyl methacrylamide, 1 .8 g of 80% divinylbenzene and 54.7 g of the fortified tall oil rosin. The fortified tall oil rosin was dissolved into the monomers in the mixture. The fortified rosin was the same as in the earlier examples.

[0078] After the temperature of the reactor with degraded starch was cooled to 85 °C, the chemical feeds were started simultaneously. 169.8 g of the mixture (solution) of monomers and the fortified tall oil rosin was fed during 120 min. 74.7 g of 5.5 % solution of hydrogen peroxide was ted during 165 min. The reactor temperature was kept at 85 °C during the feeds and 15 min after for post-polymerisation. Then the mixture was cooled to 60 °C and 5.2 g of 11.7 % terf-butyl hydroperoxide solution was added dropwise into the reactor. The temperature was kept at 60 °C for further 60 min. Thereafter, the obtained polymer dispersion was cooled to 50 °C and 5.9 g of 10 % ethylenediaminetetraacetic acid sodium salt (EDTA-Na) solution was added, followed by cooling to room temperature. Filtration was performed using a 100 pm filter cloth, the wet filter residue was weighted with the scale. Afinely divided polymer dispersion was obtained. The characteristics of the polymer dispersion is given in Table 1 .

[0079] Example 4

[0080] The polymer dispersion in Example 4 was prepared using the same procedure as described in Example 1 , with the exceptions that the amount of terf-butyl acrylate was 94 g and the amount of 3-dimethylaminopropyl methacrylamide was 7.9 g. The amounts of the other materials and the reaction conditions were otherwise kept the same as given in Example 1 . The characteristics of the polymer dispersion is given in Table 1 .

[0081] Example 5

[0082] The polymer dispersion in Example 5 was prepared using the same procedure as described in Example 1 , with the exceptions that the amount of terf-butyl acrylate was 90.6 g and the amount of 3-dimethylaminopropyl methacrylamide was 11 .3 g. The amounts of the other materials and the reaction conditions were otherwise kept the same as given in Example 1. The characteristics of the polymer dispersion is given in Table 1 . Example 6

[0083] 94.5 g of an oxidatively degraded potato starch (Perfectamyl A 4692) was dispersed with stirring in 557 g of deionized water under a nitrogen atmosphere in a 1 L glass reactor having a cooling / heating jacket. Prior addition of the starch the pH of water was adjusted to pH 3 with 5 % sulfuric acid solution. The starch was dissolved by heating the mixture to 95 °C and cooking at 95 °C for 30 min. After the starch dissolution was complete, 28.6 g of 0.74 % aqueous solution of ferrous(ll) sulphate heptahydrate was added into the reactor. After 10 min 4.7 g of 30 % hydrogen peroxide was added. After 60 min in 95 °C, the starch degradation was complete.

[0084] During starch degradation, in a separate vessel, a mixture was made by blending together 97.5 g terf-butyl acrylate, 5.7 g of n-butyl acrylate, 5.7 g of styrene, 2.2 g of 3-dimethylaminopropyl methacrylamide, 2.2 g of 2-(dimethylamino)ethyl methacrylate, 1.8g of 80% divinylbenzene and 54.7 g of the fortified tall oil rosin. The fortified tall oil rosin was dissolved into the monomers. The fortified rosin was the same as in the earlier examples.

[0085] After cooling the temperature of the reactor with degraded starch to 85 °C, the chemical feeds were started simultaneously. 169.8 g of the mixture (solution) of the monomers and fortified tall oil rosin was fed during 60 min. 22.4 g of 5.5 % solution of hydrogen peroxide was fed during 80 min, followed by feeding 52.3 g of the same concentration hydrogen peroxide solution during 85 min. The reactor temperature was kept at 85 °C during the feeds and 15 min after for post-polymerisation. Then the mixture was cooled to 60 °C and 5.2 g of 11 .7 % terf-butyl hydroperoxide solution was added dropwise into the reactor. The temperature was kept at 60 °C for further 60 min. Thereafter, the obtained polymer dispersion was cooled to 50 °C and 5.9 g of 10 % ethylenediaminetetraacetic acid sodium salt (EDTA-Na) solution was added, followed by cooling to room temperature. Filtration was performed using a 100 pm filter cloth, the wet filter residue was weighted with the scale. Afinely divided polymer dispersion was obtained. The polymer dispersion had a zeta potential value of 5.06 mV and a charge density value of 0.023 meq / g, at pH 3. The characteristics of the polymer dispersion is given in Table 1 . Example 7: Preparation of Polymer Dispersion in the Presence of a Non-ionic Surfactant

[0086] 85 g of an oxidatively degraded potato starch (Perfectamyl A 4692) was dispersed with stirring in 494 g of deionized water under a nitrogen atmosphere in a 1 L glass reactor with a cooling / heating jacket. Prior addition of the starch, the pH of water was adjusted to pH 3 with 5 % sulfuric acid solution. The starch was dissolved by heating the mixture to 95 °C and cooking at 95 °C for 30 min. After starch dissolution was complete, 25.8 g of 0.83 % aqueous solution of ferrous(ll) sulphate heptahydrate was added into the reactor. After 10 min 3.4 g of 30 % hydrogen peroxide was added. After 45 min in 95 °C, the starch degradation was complete. 9.3 g of 25.8 % polyethylene glycol monostearate (n=40) solution was added into the glass reactor containing the degraded starch.

[0087] During starch degradation, in a separate vessel, a mixture was made by blending together 87.8 g terf-butyl acrylate, 5.1 g of n-butyl acrylate, 5.1 g of styrene, 2 g of 3-dimethylaminopropyl methacrylamide, 2 g of 2-(dimethylamino)ethyl methacrylate, 1.7 g of 80 % divinylbenzene and 49.3 g of a fortified tall oil rosin. The fortified tall oil rosin was dissolved into the monomers in the mixture.

[0088] After the temperature of the glass reactor with degraded starch was cooled to 85 °C, the chemical feeds were started simultaneously. 152.9 g of the mixture (solution) of the monomers and the fortified tall oil rosin was fed during 60 min. 20.2 g of 5.5 % solution of hydrogen peroxide was fed during 80 min, followed by feeding 47.1 g of the same concentration hydrogen peroxide solution during 85 min. The reactor temperature was kept at 85 °C during the feeds and 15 min after for postpolymerisation. Then the mixture was cooled to 60 °C and 4.5 g of 12 % terf-butyl hydroperoxide solution was added dropwise into the reactor. The temperature was kept at 60 °C for further 60 min. Thereafter, the obtained polymer dispersion was cooled to 50 °C and 5.3 g of 10 % ethylenediaminetetraacetic acid sodium salt (EDTA-Na) solution was added, followed by cooling to room temperature. Filtration was performed using a 100 pm filter cloth, the wet filter residue was weighted with the scale. A finely divided polymer dispersion was obtained. The characteristics of the polymer dispersion is given in Table 1 .

[0089] Table 1 Characteristics of the polymer dispersions of Examples 1 - 7 and

[0090] Comparative Example 1

[0091] * for 1 kg product

[0092] ** from 1 kg product

[0093] *** in comparison to Comparative example 1

[0094] It can be seen from Table 1 that the polymer dispersion of Comparative Example 1 , prepared without any charge-carrying monomers, cationic or anionic, has a filter residue of 1 .4 weight-%, calculated from the total weight of the obtained dispersion product. When charge-carrying monomers are present in the polymerisation, the precipitate amount in the obtained polymer dispersion is reduced by 16 - 78 %, providing dispersions with only 0.3 - 1 .1 weight-% of filter residue, calculated from total weight of the obtained product. The reduction seems to be dependent on the ratio of charge-carrying monomer used. The filter residue decreased when the ratio of cationic monomer increased, see examples 1 - 5.

[0095] It can be seen from Table 1 that the filter residue can also be reduced when a surfactant was present during the polymerization. It can be seen from Example 7 that the filter residue was reduced 29 weight-% when a non-ionic surfactant was present during the polymerization.

[0096] Application Example Sizing performance of the surface size compositions were tested on an internally unsized recycled fibre linerboard which had base weight of 140 g / m2. The sheets were run through Mathis horizontal pond size press type 5607 at 2 m / min (2 Bar). The temperature of surface size composition and the size press nip was adjusted to 60 °C. The sheets were dried at 95 °C using an AMC drum dryer at speed 50, giving drying time of 1 .5 minutes. Sizing efficiency was determined by measuring Cobb60 sizing degree according to standard ISO 535.

[0097] Sizing performance of the polymer dispersions was tested with and without polyaluminum chloride.

[0098] Surface size compositions were prepared by dissolving starch (Raisamyl 01121 ) first into water according to conventional starch cooking procedure. The dissolved starch, in solution form, was then mixed with a polymer dispersion to be tested. For each polymer dispersion three different dosages 1 weight-%, 2 weight-%, 3 weight- % were tested, the percentages calculated from weight of dry starch. The starch concentration was 8 weight-%. The tested polymer dispersions can be seen in Table 2 as well as the sizing results, given as Cobb60 g / m2values. Cobb60 indicates water absorption capacity of the sample, smaller values are better. The polymer dispersions used in the Application Example are the same as prepared in the preceding examples 1 , 3, 5, 6 and 7. Polyaluminum chloride (PAC), when used as indicated in Table 2, was added to the surface size composition in amount of 1 weight-%, calculated from dry starch, prior to the surface sizing.

[0099] It can be seen from Table 2 that the polymer dispersion according to the invention, obtained by polymerisation of the alkyl (meth)acrylates and a charge-carrying second monomer, provides at least similar or even better sizing performance that the polymer dispersion prepared in the presence of rosin component but without a charge carrying monomer.

[0100] 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.

[0101] Table 2 Sizing performance results of the Application Example. * weight-% of starch as dry solids

Claims

CLAIMS1. A polymer dispersion comprising a polymer in particle form, dispersed in an aqueous liquid phase, wherein the polymer in particle form is obtained by a radical polymerisation of monomers comprising- a first monomer (i) selected from alkyl (meth)acrylates, and- a charge-carrying second monomer (ii) having a cationic charge, when measured at pH 3, selected from ethylenically unsaturated tertiary amines, in a presence of a dissolved rosin component, in an aqueous polymerisation medium comprising a polymeric stabilizer.

2. Polymer dispersion according to claim 1 , characterised in that the ethylenically unsaturated tertiary amine is selected from a group consisting of N,N-dialkylamino- alkylacrylamides, N,N-dialkylaminoalkylmethacrylamides, N,N-dialkylaminoalkyl acrylates, N,N-dialkylaminoalkyl methacrylates, and any of their mixtures.

3. Polymer dispersion according to claim 1 or 2, characterised in that the first monomer (i) is selected from C1 -C18 alkyl (meth)acrylates, preferably C1-C12 alkyl (meth)acrylates, more preferably C1 -C4 alkyl (meth)acrylates, or any mixtures thereof.

4. Polymer dispersion according to claim 1 , 2 or 3, characterised in that the dissolved rosin component is selected from rosin, rosin derivatives, such as rosin esters, dimerised rosins, polymerised rosins, hydrogenated rosins, fortified rosins and unfortified rosins, or any of their mixtures.

5. Polymer dispersion according to any of preceding claims 1 - 4, characterised in that the monomers comprise an additional charge-carrying monomer (iii) having a cationic charge, when measured at pH 3, selected from one or more ethylenically unsaturated secondary amines, ethylenically unsaturated quaternary amines, and any of their mixtures.

6. Polymer dispersion according to any of preceding claims 1 - 5, characterised in that the monomers comprise an additional charge-carrying monomer (iv) having an anionic charge, when measured at pH 8.5, which additional charge-carrying monomer (iv) comprises at least one carboxylate and / or sulphonate group.

7. Polymer dispersion according to any of claims 1 - 6, characterised in that the polymer particles have a zeta potential in a range of 1 - 60 mV, preferably 2 - 15 mV, more preferably 3 - 12 mV, even more preferably 4 - 10 mV, measured at pH 3.

8. Polymer dispersion according to any of preceding claims 1 - 7, characterised in that the polymer in particle form is obtained by the radical polymerisation of monomers comprising- 80 - 99.9 weight-%, preferably 82 - 99 weight-%, more preferably 85 - 96 weight- %, of the first monomer (i), and / or- 0.01 - 20 weight-%, preferably 1 - 18 weight-%, more preferably 4 - 15 weight-%, of the charge-carrying second monomer (ii) having a cationic charge and selected from ethylenically unsaturated tertiary amines, calculated from the total weight of the first monomers (i) and second monomers (ii), as dry.

9. Polymer dispersion according to any of preceding claims 1 - 8, characterised in that the polymer in particle form is obtained by the radical polymerisation of monomers comprising- 85 - 99.9 weight-%, preferably 89 - 99 weight-%, more preferably 91 - 96 weight- %, of the first monomer (i), and / or- 0.01 - 15 weight-%, preferably 1 - 11 weight-%, more preferably 4 - 9 weight-%, of the charge-carrying second monomer (ii) having a cationic charge and selected from ethylenically unsaturated tertiary amines, calculated from the total weight of the first monomers (i) and second monomers (ii), as dry.

10. Polymer dispersion according to any of preceding claims 1 - 9, characterised in that the polymer in particle form is obtained by the radical polymerisation in the presence of 0.01 - 70 weight-%, preferably 1 - 60 weight-%, more preferably 1 .5 - 49 weight-%, of the dissolved rosin component, calculated from the total weight of the monomers and the rosin component, as dry.11 . Polymer dispersion according to claim 10, characterised in that the polymer in particle form is obtained by the radical polymerisation in the presence of 5 - 70 weight-%, preferably 7.5 - 49 weight-%, more preferably 10 - 35 weight-%, even more preferably 15 - 34 weight-%, of the dissolved rosin component, calculated from the total weight of the monomers and the rosin component, as dry.

12. Polymer dispersion according to any of preceding claims 1 - 11 , characterised in that the monomers comprise a third monomer (v) which is selected from styrene, substituted styrenes, such as a-methylstyrene, vinyltoluene, ethylvinyltoluene, chloromethylstyrene, divinylbenzene, and any of their mixtures.

13. Polymer dispersion according to claim 12, characterised in that the monomers comprise 1 - 30 weight-%, preferably 2 - 20 weight-%, more preferably 2 - 10 weight-%, of the third monomer (v), calculated from the total weight of the monomers (i), (ii) and (v), as dry.

14. Polymer dispersion according to any of the preceding claims 1 - 13, characterised in that the polymeric stabilizer is selected from polysaccharides, polysaccharide derivatives, degraded polysaccharides, degraded polysaccharide derivatives and any of their mixtures.

15. Polymer dispersion according to any of the preceding claims 1 - 14, characterised in that the polymer dispersion comprises polymer in particle form having a particle size D50 <200 nm, preferably <120 nm, more preferably <80 nm, even more preferably <55 nm, and / or a particle size D90 <500 nm, preferably <200 nm, more preferably <140 nm, even more preferably <100 nm.

16. Polymer dispersion according to any of the preceding claims 1 - 15, characterised in that the polymer in particle form has a weight average molecular weight Mw in the range 1000 - 100000 g / mol, preferably 5000 - 80000 g / mol, more preferably 8000 - 40 000 g / mol.

17. Use of a polymer dispersion according to any of claims 1 - 16 for surface sizing of a cellulosic fibre web, such paper, board, non-woven, textile or the like.

18. Method for manufacturing a polymer dispersion comprising a polymer in particle form dispersed in an aqueous liquid phase, wherein the polymer is obtained by a radical polymerisation of- a first monomer feed comprising a first monomer (i) selected from alkyl (meth)acrylates, and- a second monomer feed comprising a charge-carrying second monomer (ii) having a cationic charge, when measured at pH 3, selected from ethylenically unsaturated tertiary amines, in an aqueous polymerisation medium comprising a polymeric stabilizer, wherein a rosin component is dissolved at least one of the monomer feeds before start of the radical polymerisation.

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