Patent Description:
In manufacture of cellulosic fibre webs, such as paper, board and the like, the properties of the produced webs are often improved by addition of various chemicals during and after the cellulosic fibre web formation. For example, surface sizing is commonly used to improve strength and/or water resistance of the produced cellulosic fibre webs, or otherwise enhance the surface properties of such fibre webs. 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 whereafter the web is dried.

There is a general trend and desire to decrease the use of materials which are based on non-renewable sources, especially this applies to petroleum-based raw materials. Within the paper and board industry there is an effort to decrease the use of synthetic polymers in manufacture of cellulosic fibre webs in order to further increase the sustainability of the overall process. Sizing agents used in the surface sizing often comprise, or are based on, synthetic polymers, such as poly(styrene acrylate). It would be desirable to reduce the amount of synthetic polymers, in particular styrene, in paper and board making. At the same time there is a need to obtain at least similar, preferably better, surface sizing results for the sized paper, board or the like in view of the strength, water resistance and/or other surface properties.

Rosin, as well as its derivatives, could be seen as a sustainable alternative for synthetic polymers. Aqueous dispersions of rosin and its derivatives have already been used as hydrophobation agents in papermaking. The use of rosin, however, conventionally requires use of alum. Furthermore, the softening point of rosin and its derivatives is generally too high for present surface sizing applications in order to obtain satisfactory surface sizing results. The particle size of rosin dispersions is also often too large for obtaining good surface sizing results. It has been noted that by mechanical mixing and other corresponding techniques it is hard to obtain a rosin dispersion with a small particle size. <CIT> discloses a rosin-based emulsion sizing agent obtained by emulsifying an emulsion of a reinforced resin in the presence of a polymeric dispersant, which is an anionic copolymer or a salt thereof. The reinforced rosin and copolymeric salt are added in a subsequent (third) step and no polysaccharide is present. <CIT> discloses blends of fumarated rosin and polyacrylamide. <CIT> discloses surface sizing composition comprising a metal salt and an aqueous polymer dispersion, wherein the aqueous polymer dispersion is an aqueous polymer dispersion obtainable by free radical emulsion copolymerizing ethylenically unsaturated monomers.

Consequently, there is a need for more sustainable alternative which is suitable for surface sizing of cellulosic fibre webs.

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

Another object of the present invention is to provide a polymer dispersion which is more sustainable and which provides good surface sizing results when used for surface sizing of a cellulosic fibre web.

Yet another object of the present invention is to provide a polymer dispersion which preferably has a small particle size.

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

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

A typical polymer dispersion according to the present invention comprises polymer particles dispersed in an aqueous continuous phase, wherein the polymer particles are obtained by a radical polymerisation of one or more feeds of vinyl monomers in an aqueous polymerisation medium comprising polysaccharide, wherein the vinyl monomers comprise at least one alkyl (meth)acrylate, wherein a rosin component is dissolved into at least one of the feeds of the vinyl monomers before the radical polymerisation of the vinyl monomers.

A typical use according to the present invention of the polymer dispersion according to the invention is for surface sizing of a cellulosic fibre web, such paper, board or the like, preferably in an amount of <NUM> - <NUM>/t, more preferably <NUM> - <NUM>/t, given as dry cellulosic fibre web.

A typical method according to the present invention for producing a polymer dispersion comprising polymer particles in an aqueous continuous phase for surface sizing of a cellulosic fibre web, such as paper, board or the like, comprises.

Now it has been surprisingly found that by dissolving a rosin component to a vinyl monomer solution before the radical polymerisation of those vinyl monomers in an aqueous polymerisation medium, it is possible to obtain a polymer dispersion, which is more sustainable as it produced by using less synthetic petroleum-based monomers. The polymer dispersion is also able to show good or even improved surface sizing results when it is applied on the surface of the cellulosic fibre web in surface sizing. When the rosin component is first dissolved in vinyl monomer solution, the radical polymerisation of the vinyl monomers is conducted in the presence of the rosin component, which becomes a part of the polymer particles. It is assumed that the rosin component becomes permanently incorporated into the structure of the polymer particles formed by the radical polymerisation of the vinyl monomers feed(s). Without having a theoretical explanation of the reactions and mechanisms involved, it has been observed that the obtained polymer dispersion is able to provide effective sizing effect without tackiness problems when rosin component is dissolved into at least one of the feeds of the used monomers and thus present during the polymerisation.

Typical polymer dispersion according to the present invention comprises polymer particles dispersed in an aqueous continuous phase. The polymer dispersion may comprise polymer particles, which have a particle size D50 ≤<NUM>, preferably ≤<NUM>, more preferably ≤<NUM>, even more preferably ≤<NUM>, sometimes even ≤ <NUM>. The particle size D50 for the polymer particles of the dispersion may be, for example, in a range of <NUM> - <NUM>, preferably <NUM> - <NUM>, more preferably <NUM> - <NUM>, even more preferably <NUM> - <NUM>, sometimes even <NUM> - <NUM>. The polymer dispersion may comprise polymer particles, which have a particle size D90 ≤<NUM>, preferably ≤<NUM>, more preferably ≤<NUM>, even more preferably ≤<NUM>. The particle size D90 for the polymer particles of the dispersion may be, for example, in a range of <NUM> - <NUM>, preferably <NUM> - <NUM>, more preferably <NUM> - <NUM>, even more preferably <NUM> - <NUM>. All particle sizes are measured by using Zetasizer Nano ZS, Malvern. In the present context the particle size D50 refers to the value for <NUM>th percentile of a volume based distribution and the particle size D90 refers to the value for <NUM>th percentile of a volume based distribution. It was unexpectedly observed that when the rosin component is dissolved into the vinyl monomer solution(s) before the radical polymerisation, the obtained polymer dispersion has a low particle size D50 and D90 value. The obtained values indicate also that the particle size distribution is relatively narrow. All this is advantageous in view of the surface sizing results.

The polymers in the dispersion may have a weight average molecular weight Mw in the range <NUM> - <NUM><NUM>/mol, preferably <NUM> - <NUM><NUM>/mol, more preferably <NUM><NUM> - <NUM><NUM>/mol, analysed from the final dispersion. The polymers in the dispersion may have a number average molecular weight Mn in the range <NUM> - <NUM><NUM>/mol, preferably <NUM> - <NUM><NUM>/mol, more preferably <NUM> - <NUM><NUM>/mol, analysed from the final dispersion. The weight average and number average molecular weights may be determined, for example, by size exclusion chromatography.

The polymer dispersion, as dry, may have a glass transition temperature Tg in a range of <NUM> - <NUM>, preferably <NUM> - <NUM>, more preferably <NUM> - <NUM> or even more preferably <NUM> - <NUM>.

The polymer dispersion according to the present invention is obtainable by a radical polymerisation, preferably by a free radical polymerisation, of one or more feeds of vinyl monomers. The vinyl monomers comprise at least one alkyl (meth)acrylate. The polymer dispersion may be obtained by radical polymerisation of one type of alkyl (meth)acrylate. Alternatively, the polymer dispersion may be obtained by a radical polymerisation of several, such as two, three or more, different vinyl monomers, of which at least one is alkyl (meth)acrylate. Preferably the polymer dispersion is obtained by a radical polymerisation of feed(s) of two or three different vinyl monomers of which at least one is alkyl (meth)acrylate. The different vinyl monomers may be fed as separate feeds, or one feed of vinyl monomers may comprise a mixture of two, three or more different vinyl monomers.

The vinyl monomer(s), of which at least one is alkyl (meth)acrylate, is/are obtained in solution form, i.e. liquid form, or prepared to form solution(s). The vinyl monomer solution(s) are fed to the aqueous polymerisation medium as one or more feeds of vinyl monomers. The resin component is dissolved to at least one of the vinyl monomer solution(s) or feeds before the start of the radical polymerisation of the vinyl monomers. At least one solution or feed of the vinyl monomers thus comprises a rosin component. Vinyl monomers to be used in the radical polymerisation are in form of a vinyl 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 monomer feeds, are preferably minimised. Preferably, each vinyl monomer to be used in the radical polymerisation is in form of a monomer solution or feed, which is essentially free of water. Preferably the vinyl monomer solution or feed is also essentially free of other solvents, e.g. organic solvents. In the present context the term "essentially free" means that the vinyl monomer solution comprises less than <NUM> weight-%, preferably less than <NUM> weight-%, more preferably less than <NUM> weight-%, of water and/or other solvents. It has been found that the rosin component can be effectively and uniformly dissolved into the vinyl monomer solution or feed or at least one of the vinyl monomer solutions or feeds before the radical polymerisation in the aqueous medium. The vinyl monomer solution(s) or feed(s) function as a solvent for the rosin component. The rosin component is preferably completely dissolved into the vinyl monomer solution(s) or feed(s), and after the dissolution preferably no solid or semi-solid rosin components can be observed. By dissolving the rosin component into the feed of vinyl monomer(s), it is possible to guarantee that the rosin component is uniformly present in the polymerisation and becomes well incorporated with the polymer structure. Furthermore, it is possible to avoid use of additional organic solvents in the process, as the vinyl monomer solution(s) or feed(s) function as a solvent for the rosin component. This makes the manufacture of the polymer dispersion simple and fast when there is no need to remove additional solvents from the aqueous phase of the polymer dispersion after the radical polymerisation. The use of vinyl monomer solution(s) or feed(s) as a solvent for the rosin component may also provide advantages in the quality of the obtained polymer dispersion, e.g. reduced tackiness.

At least one feed of the vinyl monomer solution(s) is fed with at least one polymerisation initiator into an aqueous polymerisation medium and the radical polymerisation of one or more feeds of the vinyl monomers is conducted in the presence of the rosin component. At the present it is assumed that, without wishing to be bound by a theory, that the rosin component may be integrated at least partly within the polymer structure formed during the polymerisation. The rosin component may thus preferably become an inseparable part of the polymer particles formed.

According to one embodiment of the invention the aqueous polymerisation medium may comprise an additional solvent during the radical polymerisation. The aqueous polymerisation medium may comprise at most <NUM> %, preferably at most <NUM> %, more preferably at most <NUM> %, of an additional solvent other than water during the polymerisation. The additional solvent may be an alcohol, such as ethanol or isopropanol. The additional solvent may be removed, for example by distillation, from the polymer dispersion after the polymerisation has been completed. According to one preferable embodiment the obtained polymer dispersion may comprise at most <NUM> %, preferably at most <NUM> %, more preferably at most <NUM> %, of an additional solvent other than water.

According to one especially preferred embodiment the aqueous polymerisation medium is free of other solvents than water, i.e. the aqueous polymerisation medium is free of organic solvents, such as alcohols, e.g. ethanol and isopropanol.

In the present context the term "rosin component" denotes rosin and its derivatives. The rosin component is insoluble in water and therefore it is dissolved in the solution of the vinyl monomers before the radical polymerisation. The rosin component may be a mixture of different rosins. The rosin component used in the present invention may preferably comprise rosin and/or rosin derivative, such as one or more rosin esters, dimerised rosins, polymerised rosins, hydrogenated rosins, fortified rosins and unfortified rosins. According to one preferable embodiment of the present invention the 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 rosin component may be a mixture of tall oil rosin and gum oil rosin. The use of different rosins or their mixtures as the rosin component may provide a possibility to influence the properties of the obtained polymer dispersion, at least to a certain degree.

According to one preferable embodiment of the invention the 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. Maleic acid and fumaric acid are being preferred. It has been observed that fortified rosin is very effectively dissolved in the vinyl monomer solution, which makes the process of producing the polymer dispersion easy and efficient.

According to one preferable embodiment of the present invention the rosin component may have a softening point in a range of <NUM> - <NUM> , preferably <NUM> - <NUM>, more preferably <NUM> - <NUM>, even more preferably <NUM> - <NUM>. 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 or added into the solution of the vinyl monomer(s). The rosin component may be dissolved in the vinyl monomer solution at a temperature of <NUM> - <NUM> or <NUM> - <NUM>. In general, the dissolving of the rosin component may be achieved without external heating of the vinyl monomer solution or feed. The dissolving time can be easily determined by few experiments, and it usually depends on the monomers used, rosin component used as well as the amount of the rosin component to be dissolved. Typical dissolution time varies between <NUM> - <NUM>.

The final polymer dispersion may comprise the rosin component in an amount of <NUM> - <NUM> weight-%, preferably <NUM> - <NUM> weight-%, more preferably <NUM> - <NUM> weight-%, calculated from the total weight of the vinyl monomers and the rosin component, as dry. The present invention allows the use of the rosin component within wide limits, which enables the flexible creation of polymer dispersions with different properties.

According to one embodiment of the invention the polymer dispersion may comprise <NUM> - <NUM> weight-%, preferably <NUM> - <NUM> weight-%, more preferably <NUM> - <NUM> weight-%, even more preferably <NUM> - <NUM> weight-%, of the rosin component, calculated from the total weight of the vinyl monomers and the rosin component, as dry. As the rosin component is dissolved to the vinyl monomer solution before the polymerisation, it is possible to increase the amount of rosin in the obtained polymer dispersion, without compromising the quality of the polymer dispersion, for example particle size of the dispersion, and the surface sizing effects obtained.

According to another embodiment of the invention the polymer dispersion may comprise <NUM> - <NUM> weight-%, preferably <NUM> - <NUM> weight-%, more preferably <NUM> - <NUM> weight-%, of the rosin component, calculated from the total weight of the vinyl monomers and the rosin component, as dry. Even when used in relatively small amounts, the rosin component effectively influences the properties of the obtained polymer dispersion. It is presently speculated that the rosin component may act as a chain transfer agent during the radical polymerisation and thus controls the structure and molecular weight of the polymer formed by the radical polymerisation.

The vinyl monomers may comprise or consist of alkyl (meth)acrylates, which can be selected from as C1-C18 alkyl (meth)acrylates, preferably C1-C12 alkyl (meth)acrylates, more preferably C1-C4 alkyl (meth)acrylates, and any of their mixtures. Vinyl monomers 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 <NUM>-butyl acrylate and the corresponding butyl methacrylates; n-pentyl or neopentyl acrylate and the corresponding pentyl methacrylates; <NUM>-hexyl or <NUM>-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. Preferably vinyl monomers 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 <NUM>-butyl acrylate and the corresponding butyl methacrylates; methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, propyl acrylate or propyl methacrylate. It is possible that the vinyl monomers may comprise or consists of a mixture of at least two isomeric butyl acrylates, e.g. a mixture of n-butyl acrylate and methyl methacrylate or a mixture of n-butyl acrylate and tert-butyl acrylate.

According to one embodiment of the present invention the vinyl monomers may comprise at least one first monomer (a) which is selected from alkyl (meth)acrylates, such as C1-C18 alkyl (meth)acrylates, preferably C1-C12 alkyl (meth)acrylates, more preferably C1-C4 alkyl (meth)acrylates, and any of their mixtures. Suitable first monomer (a) may be, for example, 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 <NUM>-butyl acrylate and the corresponding butyl methacrylates; n-pentyl or neopentyl acrylate and the corresponding pentyl methacrylates; <NUM>-hexyl or <NUM>-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. According to one preferable embodiment the first monomer (a) is selected from C1-C4-alkyl acrylates, C1-C4-alkyl methacrylates or any of their mixtures, e.g. n-butyl, iso-butyl, tert-butyl or <NUM>-butyl acrylate and the corresponding butyl methacrylates; methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, propyl acrylate or propyl methacrylate. The first monomer (a) may be a mixture of at least two isomeric butyl acrylates. For example, the first monomer (a) may be a mixture of n-butyl acrylate and methyl methacrylate or a mixture of n-butyl acrylate and tert-butyl acrylate.

According to one embodiment of the present invention the vinyl monomers may further comprise at least one second monomer (b) which may be selected from styrene, substituted styrenes, such as α-methylstyrene, vinyltoluene, ethylvinyltoluene, chloromethylstyrene, and any of their mixtures.

The polymer dispersion may be obtained by radical polymerisation of one or more feeds of vinyl monomers, which comprise <NUM> - <NUM> weight-%, preferably <NUM> - <NUM> weight-%, more preferably <NUM> - <NUM> weight-% or <NUM> - <NUM> weight-%, of the first monomer (a), and/or <NUM> - <NUM> weight-%, preferably <NUM> - <NUM> weight-%, more preferably <NUM> - <NUM> weight-% or <NUM> - <NUM> weight-%, of the second monomer (b), calculated from the total weight of the monomers, as dry.

According to one preferable embodiment of the present invention the polymer dispersion is obtained by radical polymerisation of at least one feed of vinyl monomers, which comprise alkyl (meth)acrylates, for example as defined above, in absence of styrene and substituted styrene monomers. The polymer dispersion may thus be obtained without a second monomer (b) selected from styrene, substituted styrenes, such as α-methylstyrene, vinyltoluene, ethylvinyltoluene, chloromethylstyrene, and any of their mixtures. The obtained polymer dispersion may thus be free of structural units originating from monomers selected from styrene, substituted styrenes, such as α-methylstyrene, vinyltoluene, ethylvinyltoluene, chloromethylstyrene, and any of their mixtures. It has been observed that when the radical polymerisation is conducted in the presence of the rosin component, the obtained polymer dispersion obtains glass transition temperature which makes it suitable for use in surface sizing of paper, board or the like. Thus it is possible to even completely replace the styrene in the polymer dispersion, which thus becomes free of styrene residuals.

The obtained polymer dispersion may have a solids content of at least <NUM> weight-%, preferably at least <NUM> weight-%, sometimes even at least <NUM> weight-%. According to one embodiment the solids content of the polymer dispersion may be in a range of <NUM> - <NUM> weight-%, preferably <NUM> - <NUM> weight-%, more preferably <NUM> - <NUM> weight-%.

The monomer solution, i.e. feed of vinyl monomers, or the aqueous polymerisation medium may contain regulating agents useful for the polymerisation, such as chain transfer agents. The regulating agent(s) may be introduced to the polymerisation medium simultaneously, but separately, with the vinyl monomer(s). Alternatively, or in addition, the regulating agent(s) may be introduced as mixture with the monomer solution(s) or feeds. When a regulating agent is used, the amount may be <NUM> - <NUM> weight-%, preferably <NUM> - <NUM> weight-%, more preferably <NUM> - <NUM> weight-%, calculated from the weight of the vinyl monomers. Sometimes the aqueous polymerisation medium may comprise <NUM> - <NUM> weight-%, preferably <NUM> - <NUM> weight-% or <NUM> - <NUM> weight-% of regulating agent(s). According to an embodiment the polymerisation is performed without addition and/or use of a regulating agent. Suitable regulating agents may be, for example, sulphur containing organic compounds, such as mercaptans, di- and polysulphides, sulphides or esters of thio- and dithiocarboxylic acids; halogen compounds, alcohols; or aldehydes. According to one preferable embodiment the regulating agent may be terpene-containing compounds, such as terpinolene.

It is possible that the continuous aqueous phase of the polymer dispersion further comprises a stabilizer. The stabilizer can be selected from synthetic stabilators, natural stabilators, electrostatically charged stabilators, and surface active stabilators. The stabilator may be anionic, cationic, amphoteric or non-ionic. The stabilizing effect may, for example, be based on steric stabilisation, electrosteric stabilisation, electrostatic stabilisation or pickering stabilisation.

The aqueous polymerisation medium comprises polysaccharide. Polysaccharide is added to the aqueous polymerisation medium before the start of the polymerisation and the polysaccharide is present during the radical polymerisation. The polysaccharide may function as a protective colloid for the polymer particles in the obtained polymer dispersion. It has been observed that the presence of the polysaccharide provides unexpectedly stabile dispersions with narrow particle size distributions, even if the polymerisation is conducted in the presence of the rosin component. The polysaccharide may be added in amount of <NUM> - <NUM> weight-%, preferably <NUM> - <NUM> weight-%, more preferably <NUM> - <NUM> weight-%, calculated from the total weight of the polymer dispersion, as dry. According to one preferable embodiment, the polysaccharide is selected from a group comprising polysaccharide derivatives, degraded polysaccharides, degraded polysaccharide derivatives and any of their mixtures. The polysaccharide may be selected, for example, from starch, substituted starches, cellulose, substituted celluloses, hemicelluloses, substituted hemicelluloses, chitosan, glucan derivatives, dextrin, degraded starch and any of their mixtures, preferably degraded starch. Preferably the polysaccharide is essentially water-soluble. The polysaccharide has preferably an average molecular weight Mn in a range of <NUM> - <NUM><NUM>/mol. The polysaccharide, such as starch, or degraded starch, may be anionic, cationic, amphoteric or non-ionic, preferably anionic.

According to one embodiment the polymer dispersion may comprise one or more surfactants which may function as stabilizers.

A water-soluble redox system comprising an oxidant and a reducing agent may be used for initiating the radical polymerisation of the feed of vinyl monomers. The oxidant of the redox system may be selected from peroxides, such as hydrogen peroxide, sodium peroxodisulphate, potassium peroxodisulphate, ammonium peroxodisulphate, dibenzoyl peroxide, dilauroyl peroxide, di-tert-butyl peroxide, tert-butyl hydroperoxide, <NUM>,<NUM>,<NUM>,<NUM>-tetramethylbutyl peroxy-<NUM>-ethylhexanoate, cumyl hydroperoxide or bis-cyclohexyl peroxydicarbonate. The reducing agent of the redox system may be selected from sodium sulphite, sodium pyrosulphite, sodium bisulphite, sodium dithionite, sodium hydroxymethanesulphinate or ascorbic acid, or metal salt such as cerium, manganese or iron(II) salt. According to one preferable embodiment the radical polymerisation may be carried out by using a graft-linking water-soluble redox initiator system comprising hydrogen peroxide and a metal salt. The metal salt, such as iron(II) salt may be added to the aqueous polymerisation medium before the start of the polymerisation, while hydrogen peroxide is added in simultaneously but separately with the addition of monomers.

The polymer particles for the polymer dispersion are formed directly by the radical polymerisation of the monomers in the aqueous polymerisation medium comprising polysaccharide. The radical polymerisation may be carried out by a feed process, where the one or more feeds of vinyl monomers are fed into the aqueous polymerisation medium during a polymerisation time, or by a batch process, where the whole feed of vinyl monomers is added into the aqueous polymerisation medium at once in the beginning of the polymerisation, preferably by a feed process. 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 at least one vinyl monomer and the free radical initiator are metered uniformly into the aqueous polymerisation medium, preferably comprising degraded starch, 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 <NUM> - <NUM>, preferably <NUM> - <NUM>, more preferably <NUM> - <NUM>.

The obtained polymer dispersion may have a viscosity of ≤<NUM> mPas, preferably ≤<NUM> mPas, more preferably ≤<NUM> mPas. The viscosity may be in a range of <NUM> - <NUM> mPas, preferably <NUM> - <NUM> mPas, more preferably <NUM> - <NUM> mPas. All the viscosity values are measured at <NUM>, with Brookfield LVDV viscometer, in a small sample adapter with spindle <NUM>, measured at solids content of <NUM> weight-%.

According to one embodiment of the invention the polymer dispersion is used together with an aluminium compound, such as alum or polyaluminium chloride, in the surface sizing. Use of alum is, however, not necessary, and sufficient surface sizing results can be easily obtained with the polymer dispersion of the present invention, even in absence of alum.

The polymer dispersion according to the present invention is especially suitable for use in surface sizing of cellulosic webs. The surface sizing compositions may, in addition to the polymer dispersion, further comprise surface sizing starch and 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.

The polymer dispersion may be applied on a cellulosic fibre web, such paper, board or the like, in an amount of <NUM> - <NUM>/t, preferably <NUM> - <NUM>/t, given as dry cellulosic fibre web. The polymer dispersion according to the present invention is 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.

Some embodiments of the present invention are more closely described in the following non-limiting examples.

The following methods have been used in the examples to characterise dispersion properties.

The particle size measurements of the polymer dispersions were done by using Malvern Zetasizer Nano-device. The particle size measurements of the rosin dispersion were done by Malvern MasterSizer <NUM>.

The solids content was measured using a Mettler Toledo Halogen moisture analyser.

The viscosities were measured at <NUM>, with Brookfield LVDV viscometer, in a small sample adapter with spindle <NUM>, <NUM> rpm.

The molecular weights were determined by size-exclusion chromatography (SEC) using an Agilent <NUM> HPLC system equipped with integrated autosampler, degasser, column oven and refractive index detector. Eluent was N,N-dimethylformamide (DMF) with <NUM>/l lithium chloride. Flow rate was <NUM>/min at <NUM> (column oven and RI detector). Column set consisted of three Polymer Standard Service GRAM columns (1000Å + <NUM> x 30Å columns). Samples were freeze-dried prior to the analysis. Injection volume was <NUM>µl with a sample concentration of <NUM>/ml. For conventional column calibration, narrow molecular weight distribution poly(styrene) standards (Polymer Standards Service) were used to calibrate the system over Mw range <NUM> - <NUM>/mol. Calibration curve was created using a GPC Addon software by Agilent.

The glass transition temperatures were measured from freeze dried samples using a differential scanning calorimeter Mettler Toledo DSC <NUM>+.

The tall oil rosin was commercially obtained and it had characteristic value of a softening point approximately <NUM> and rosin acid content <NUM> %.

The fortified rosins were fumarated tall oil rosins made of the tall oil rosin. Fortified Rosin <NUM> had softening point approximately <NUM> and Fortified Rosin <NUM> had softening point approximately <NUM>.

<NUM> of an oxidatively degraded potato starch (Perfectamyl A <NUM>) was dispersed with stirring in <NUM> of demineralized water in a <NUM> glass reactor with a cooling/heating jacket under a nitrogen atmosphere. The starch was dissolved by heating the mixture to <NUM> and cooking at <NUM> for <NUM> minutes. After starch dissolution was complete, <NUM> of <NUM> % strength aqueous solution of ferrous (II) sulphate heptahydrate was added into the reactor. After <NUM> minutes <NUM> of <NUM> % strength hydrogen peroxide was added. After <NUM> minutes in <NUM>, the starch degradation was complete.

During starch degradation, in a separate vessel, a mixture was made by blending together <NUM> tert-butyl acrylate, <NUM> of n-butyl acrylate and <NUM> of the tall oil rosin. The tall oil rosin was dissolved into the monomers.

After cooling the temperature of the reactor with degraded starch to <NUM>, the chemical feeds were started simultaneously. <NUM> of monomer and tall oil rosin mixture (solution) was fed during <NUM> minutes. <NUM> of <NUM> % solution of hydrogen peroxide was fed during <NUM>. The reactor temperature was kept at <NUM> during the feeds and <NUM> minutes after for post-polymerisation. Then the mixture was cooled to <NUM> and <NUM> of <NUM> % strength tert-butyl hydroperoxide solution was added dropwise into the reactor. The temperature was kept at <NUM> for further <NUM>. Thereafter, the obtained polymer dispersion was cooled to <NUM> and <NUM> of <NUM> % strength ethylenediaminetetraacetic acid sodium salt (EDTA-Na) solution was added, followed by pH adjustment to <NUM> with <NUM> % strength sodium hydroxide solution and cooling to room temperature. Filtration was performed using a <NUM> filter cloth. A finely divided polymer dispersion was obtained. The characteristics of the polymer dispersion is given in Table <NUM>.

The polymer dispersion in Example <NUM> was prepared using the same procedure as in Example <NUM>, with the exceptions that the amount of tert-butyl acrylate was <NUM>, amount of n-butyl acrylate was <NUM> and the amount of the tall oil rosin was <NUM>. Amounts of other materials and the reaction conditions were kept the same as in Example <NUM>. The characteristics of the polymer dispersion is given in Table <NUM>.

During starch degradation, in a separate vessel, a mixture was made by blending together <NUM> tert-butyl acrylate, <NUM> of n-butyl acrylate, <NUM> of styrene and <NUM> of Fortified Rosin <NUM>. The Fortified Rosin <NUM> was dissolved into the monomers. Keeping the temperature of the reactor with the degraded starch at <NUM>, the chemical feeds were started. <NUM> of monomer and fortified tall oil rosin mixture (solution), as well as <NUM> of dilution water as separate feed were fed during <NUM> minutes. Feed of <NUM> of <NUM> % solution of hydrogen peroxide was started simultaneously with the monomer and water feeds and this feed lasted for <NUM>. The reactor temperature was kept at <NUM> during the feeds and <NUM> minutes after for post polymerization. Then the mixture was cooled to <NUM> and <NUM> of <NUM> % strength tert-butyl hydroperoxide solution was added dropwise into the reactor. The temperature was kept at <NUM> for further <NUM>. Thereafter, the dispersion was cooled to <NUM> and <NUM> of <NUM> % strength ethylenediaminetetraacetic acid sodium salt (EDTA-Na) solution was added, followed by pH adjustment to <NUM> with <NUM> % strength sodium hydroxide solution and cooling to room temperature. Filtration was performed using a <NUM> filter cloth. A finely divided polymer dispersion was obtained. The characteristics of the polymer dispersion is given in Table <NUM>.

During starch degradation, in a separate vessel, a mixture was made by blending together <NUM> tert-butyl acrylate, <NUM> of styrene and <NUM> of the tall oil rosin. The tall oil rosin was dissolved into the monomers.

Keeping the temperature of the reactor with degraded starch at <NUM>, the chemical feeds were started. <NUM> of monomer and tall oil rosin mixture (solution), as well as <NUM> of dilution water as separate feed were fed during <NUM> minutes. Feed of <NUM> of <NUM> % solution of hydrogen peroxide was started simultaneously with the monomer and water feeds and this feed lasted for <NUM>. The reactor temperature was kept at <NUM> during the feeds and <NUM> minutes after for post-polymerisation. Then the mixture was cooled to <NUM> and <NUM> of <NUM> % strength tert-butyl hydroperoxide solution was added dropwise into the reactor. The temperature was kept at <NUM> for further <NUM>. Thereafter, the dispersion was cooled to <NUM> and <NUM> of <NUM> % strength ethylenediaminetetraacetic acid sodium salt (EDTA-Na) solution was added, followed by pH adjustment to <NUM> with <NUM> % strength sodium hydroxide solution and cooling to room temperature. Filtration was performed using a <NUM> filter cloth. A finely divided polymer dispersion was obtained. The characteristics of the polymer dispersion is given in Table <NUM>.

During starch degradation, in a separate vessel, a mixture was made by blending together <NUM> tert-butyl acrylate, <NUM> of n-butyl acrylate, <NUM> of styrene and <NUM> of Fortified Rosin <NUM>. The Fortified Rosin <NUM> was dissolved into the monomers.

Keeping the temperature of the reactor with the degraded starch at <NUM>, the chemical feeds were started. <NUM> of monomer and fortified rosin mixture (solution), as well as <NUM> of dilution water as separate feed were fed during <NUM> minutes. Feed of <NUM> of <NUM> % solution of hydrogen peroxide was started simultaneously with the monomer and water feeds and this feed lasted for <NUM>. The reactor temperature was kept at <NUM> during the feeds and <NUM> minutes after for post-polymerisation. Then the mixture was cooled to <NUM> and <NUM> of <NUM> % strength tert-butyl hydroperoxide solution was added dropwise into the reactor. The temperature was kept at <NUM> for further <NUM>. Thereafter, the dispersion was cooled to <NUM> and <NUM> of <NUM> % strength ethylenediaminetetraacetic acid sodium salt (EDTA-Na) solution was added, followed by pH adjustment to <NUM> with <NUM> % strength sodium hydroxide solution and cooling to room temperature. Filtration was performed using a <NUM> filter cloth. A finely divided polymer dispersion was obtained. The characteristics of the polymer dispersion is given in Table <NUM>.

During starch degradation, in a separate vessel, a mixture was made by blending together <NUM> tert-butyl acrylate, <NUM> of n-butyl acrylate, <NUM> of styrene and <NUM> of the tall oil rosin. The tall oil rosin was dissolved into the monomers.

Keeping the temperature of the reactor with degraded starch at <NUM>, the chemical feeds were started. <NUM> of monomer and tall oil rosin mixture (solution), as well as <NUM> of dilution water as separate feed were fed during <NUM> minutes. Feed of <NUM> of <NUM> % solution of hydrogen peroxide was started simultaneously with the monomer and water feeds and this feed lasted for <NUM>. The reactor temperature was kept at <NUM> during the feeds and <NUM> minutes after for post polymerization. Then the mixture was cooled to <NUM> and <NUM> of <NUM> % strength tert-butyl hydroperoxide solution was added dropwise into the reactor. The temperature was kept at <NUM> for further <NUM>. Thereafter, the dispersion was cooled to <NUM> and <NUM> of <NUM> % strength ethylenediaminetetraacetic acid sodium salt (EDTA-Na) solution was added, followed by pH adjustment to <NUM> with <NUM> % strength sodium hydroxide solution and cooling to room temperature. Filtration was performed using a <NUM> filter cloth. A finely divided polymer dispersion was obtained. The characteristics of the polymer dispersion is given in Table <NUM>.

During starch degradation, in a separate vessel, a mixture was made by blending together <NUM> tert-butyl acrylate, <NUM> of n-butyl acrylate and <NUM> of Fortified Rosin <NUM>. The Fortified Rosin <NUM> was dissolved into the monomers.

After cooling the temperature of the reactor with the degraded starch to <NUM>, the chemical feeds were started simultaneously. <NUM> of monomer and fortified rosin mixture (solution) was fed during <NUM> minutes. <NUM> of <NUM> % solution of hydrogen peroxide was fed during <NUM>. The reactor temperature was kept at <NUM> during the feeds and <NUM> minutes after for post-polymerisation. Then the mixture was cooled to <NUM> and <NUM> of <NUM> % strength tert-butyl hydroperoxide solution was added dropwise into the reactor. The temperature was kept at <NUM> for further <NUM>. Thereafter, the dispersion was cooled to <NUM> and <NUM> of <NUM> % strength ethylenediaminetetraacetic acid sodium salt (EDTA-Na) solution was added, followed by pH adjustment to <NUM> with <NUM> % strength sodium hydroxide solution and cooling to room temperature. Filtration was performed using a <NUM> filter cloth. A finely divided polymer dispersion was obtained. The characteristics of the polymer dispersion is given in Table <NUM>.

<NUM> of an oxidatively degraded potato starch (Perfectamyl A <NUM>) was dispersed with stirring into <NUM> of demineralized water in a <NUM> glass reactor with a cooling/heating jacket under a nitrogen atmosphere. The starch was dissolved by heating the mixture to <NUM> and stirring at <NUM> for <NUM> minutes. After starch dissolution was complete, <NUM> of <NUM> w-% strength aqueous solution of ferrous (II) sulphate heptahydrate was added into the reactor. After <NUM> minutes <NUM> of <NUM> w-% strength hydrogen peroxide was added. After <NUM> minutes, the starch degradation was complete.

During the starch degradation <NUM> tert-butyl acrylate, <NUM> n-butyl acrylate and <NUM> <NUM>-dodecyl mercaptan were mixed.

After cooling the temperature of the reactor with the degraded starch to <NUM>, the chemical feeds were started simultaneously. The mixture of the monomers and <NUM>-dodecyl mercaptan was fed during <NUM> minutes. <NUM> of <NUM> w-% solution of hydrogen peroxide was fed during <NUM>. The reactor temperature was kept at <NUM> during the feeds and <NUM> minutes after for post-polymerisation. Then the mixture was cooled to <NUM> and <NUM> of <NUM> w-% strength tert-butyl hydroperoxide solution was added into the reactor. The temperature was kept at <NUM> for further <NUM>. Thereafter, the polymer dispersion was cooled to <NUM> and <NUM> of <NUM> w-% strength ethylenediaminetetraacetic acid sodium salt solution was added, followed by pH adjustment to <NUM> with sodium hydroxide solution and cooling to room temperature. Filtration was performed using a <NUM> filter cloth. A finely divided polymer dispersion was obtained. The characteristics of the polymer dispersion is given in Table <NUM>.

<NUM> of an oxidatively degraded potato starch (Perfectamyl A <NUM>) was dispersed with stirring into <NUM> of demineralized water in a <NUM> glass reactor with a cooling/heating jacket under a nitrogen atmosphere. The starch was dissolved by heating the mixture to <NUM> and stirring at <NUM> for <NUM> minutes. After starch dissolution was complete, <NUM> of <NUM> w-% strength aqueous solution of ferrous (II) sulphate heptahydrate was added into the reactor. After <NUM> minutes <NUM> of <NUM> w-% strength hydrogen peroxide was added. After <NUM> minutes, the starch solution was diluted with addition of <NUM> demineralized water.

During the starch degradation <NUM> tert-butyl acrylate, <NUM> n-butyl acrylate, <NUM> styrene, and <NUM> <NUM>-dodecyl mercaptan were mixed.

After cooling the temperature of the reactor with the degraded starch to <NUM>, the chemical feeds were started simultaneously. The mixture of the monomers and <NUM>-dodecyl mercaptan was fed during <NUM> minutes. <NUM> of <NUM> w-% solution of hydrogen peroxide was fed during <NUM>. The reactor temperature was kept at <NUM> during the feeds and <NUM> minutes after for post polymerization. Then the mixture was cooled to <NUM> and <NUM> of <NUM> w-% strength tert-butyl hydroperoxide solution was added into the reactor. The temperature was kept at <NUM> for further <NUM>. Thereafter, the polymer dispersion was cooled to <NUM> and <NUM> of <NUM> w-% strength ethylenediaminetetraacetic acid sodium salt solution was added, followed by pH adjustment to <NUM> with sodium hydroxide solution and cooling to room temperature. Filtration was performed using a <NUM> filter cloth. A finely divided polymer dispersion was obtained. The characteristics of the polymer dispersion is given in Table <NUM>.

Sizing performance of the surface size compositions were tested on an internally unsized recycled fibre linerboard which had base weight of <NUM>/m<NUM>. The sheets were run through Mathis horizontal pond size press type <NUM> at <NUM>/min (<NUM> Bar). The temperature of surface size composition and the size press nip was adjusted to <NUM>. The sheets were dried at <NUM> using an AMC drum dryer at speed <NUM>, giving drying time of <NUM> minutes. Sizing efficiency was determined by measuring Cobb<NUM> sizing degree according to standard ISO <NUM>.

Surface size compositions were prepared by dissolving starch first into water according to its common starch cooking instruction. The dissolved starch in solution form was then blended with a polymer dispersion, as defined in Table <NUM>.

Tests were done with <NUM> % solution of Raisamyl <NUM> starch.

The results are shown in Table <NUM>. It is seen that the sizing efficiency is not decreased even if the amount of the polymer in the sizing composition is decreasing. It is also seen that alum is not needed for good sizing efficiency.

Surface size compositions were prepared as in Application Example <NUM>. Alum, when used, was added to the surface size composition in amount of <NUM> weight-%, calculated from dry starch, prior to the surface sizing.

In comparative tests <NUM> and <NUM>, a rosin dispersion was prepared by dispersing in water a fumarated tall oil rosin, which had softening point approximately <NUM>. The rosin dispersion had a particle size D90 of <NUM> and particle size D50 of <NUM>, measured on Mastersizer. The rosin content of the rosin dispersion was <NUM> w-% of its dry solids.

The results of Application Example <NUM> are given in Table <NUM>. It can be seen that polymer dispersion where the tall oil rosin is dissolved in the monomers before polymerisation provides better sizing results than the rosin dispersed in water.

Claim 1:
Polymer dispersion, which comprises polymer particles dispersed in an aqueous continuous phase, wherein the polymer particles are obtained by a radical polymerisation of one or more feeds of vinyl monomers in an aqueous polymerisation medium comprising polysaccharide, wherein the vinyl monomers comprise at least one alkyl (meth)acrylate, characterised in that a rosin component is dissolved into at least one of the feeds of the vinyl monomers before the radical polymerisation of the vinyl monomers.