FIELD OF THE INVENTION

The present invention relates to surface sizing of cellulosic products, such as paper, and in particular to a core-shell polymer particles suitable for surface sizing.

BACKGROUND OF THE INVENTION

Sizing is a technique which is typically used to control absorption and/or penetration of paper and other cellulosic products by fluids such as water, water-borne compositions, and inks. Paper and other cellulosic products may be sized to a variety of degrees and for a variety of purposes.

Sizing can be carried out before cellulosic product is completely formed by adding an internal sizing agent directly to aqueous pulp slurry to coat the fibers of the pulp or after formation of the finished cellulosic products by applying sizing agents to at least one surface of the product thus filling the ca- pillaries of the product. Surface sizing is generally less expensive than internal sizing, as almost the entire sizing agent is retained on the surface of the treated product.

Many different materials have been employed commercially as surface sizes including synthetic products based on styrene-maleic anhydride (SMA) copolymers and naturally occurring materials such as wax and as internal sizes such as alkenyl succinic anhydride, alkyl ketene dimer or rosin. While there are a variety of known surface sizing systems, a continual need exists for sizing systems that provide improved sizing performance (e.g. lower water absorptive- ness as determined by the Cobb method) over known sizing systems. BRIEF DESCRIPTION OF THE INVENTION

It is thus an object of the present invention to provide a polymer for surface sizing so as to alleviate the disadvantages of the known sizing systems. The objects of the invention are achieved by core-shell polymer particles, a surface sizing agent comprising the same and methods for use thereof which are characterized by what is stated in the independent claims. The preferred embodiments of the invention are disclosed in the dependent claims.

The invention is based on the realization that polymer particles having a core-shell structuring with a specific formulation have a sizing efficiency better than polymer particles containing the same monomers in a random polymer structure. The polymer composition of the present core-shell polymer particles may be made with an emulsifier-free polymerization.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following the invention will be described in greater detail by means of preferred embodiments with reference to the drawings, in which

Figure 1 shows water absorptiveness of paper surface sized with first batch of different sizing compositions; and

Figure 2 shows water absorptiveness of paper surface sized with second batch of different sizing compositions. DETAILED DESCRIPTION OF THE INVENTION

The present invention provides core-shell polymer particles for surface sizing of paper and other cellulosic products, wherein the core polymer and the shell polymer of the core/shell polymer particle are polymerized from monomers comprising at least 40%, preferably at least 50%, by weight of monomers selected from branched C3-io-alkyl (meth)acrylate, such as terf-butyl acrylate (t- BA), styrene, linear Ci-io-alkyl (meth)acrylate, such as n-butyl acrylate (n-BA), and acrylonitrile (ACN), based on the total weight of the monomers of the core/shell polymer particle;

provided that the polymer of the core/shell polymer particle comprises at least 40%, preferably at least 50%, by weight of branched C3-6-alkyl (meth)acrylate and at least 50%, preferably at least 60%, even more preferably at least 70%, by combined weight of branched C3-io-alkyl (meth)acrylate and styrene, linear Ci-io-alkyl (meth)acrylate, and/or acrylonitrile, based on the total weight of the monomers of the core/shell polymer particle.

Preferably the core polymer and the shell polymer of the core/shell polymer particles are polymerized from monomers selected from branched C3- 6-alkyl (meth)acrylate, such as terf-butyl acrylate and/or terf-butyl methacrylate, styrene, linear C3-6-alkyl acrylate, such as n-butyl acrylate, and acrylonitrile; more preferably from monomers selected from terf-butyl acrylate, n-butyl acry- late, and acrylonitrile.

When monomer ratio of the branched C3-io-alkyl (meth)acrylate is reduced below 40% by weight the hydrophobation performance of the polymer is significantly reduced.

As compared to particle composition of random polymer particles the core-shell structuring of the provided polymer particles provides improved sizing performance. With the core-shell structuring it is possible also to replace some of the expensive branched C3-io-alkyl (meth)acrylate with one or more monomers) selected from of styrene, linear Ci-io-alkyl (meth)acrylate, such as n-butyl acrylate, and acrylonitrile, and still maintain the sizing efficiency. In addition, it has been discovered that the best results are obtained when the branched C3- 10-alkyl (meth)acrylate, in particular terf-butyl acrylate and/or terf-butyl methac- rylate, is located in the shell of the polymer particle.

In an example of the present invention the core/shell polymer particle comprises (a) a core polymer polymerized from monomers comprising at least 25% by weight, of monomers selected from branched C3-io-alkyl (meth)acrylate, such as terf-butyl acrylate and/or terf-butyl methacrylate, styrene, linear Ci-io- alkyl (meth)acrylate, such as n-butyl acrylate, and acrylonitrile; and (b) a shell polymer polymerized from monomers comprising at least 25%, more preferably at least 30%, even more preferably at least 50%, by weight, of branched C3-10- alkyl (meth)acrylate, based on total weight of the monomers of the core/shell polymer particle, and wherein the shell polymer preferably encapsulates the core polymer.

Preferably the core/shell polymer particle comprises at least 50% by weight branched C3-io-alkyl (meth)acrylate and at least 75% by combined weight of branched C3-io-alkyl (meth)acrylate and acrylonitrile, based on the total weight of the monomers of the core/shell polymer particle. In particular the core/shell polymer comprises at least 60%, more preferably at least 75%, by weight, of branched C-3-io-alkyl (meth)acrylate, preferably branched C3-6-alkyl (meth)acry- late, in particular terf-butyl acrylate and/or terf-butyl methacrylate, based on the total weight of the monomers of the core/shell polymer particle.

The best sizing performance is obtained when shell of the core/shell polymer particle, i.e. the shell polymer, comprises at least 50%, preferably at least 60%, more preferably at least 75%, by weight, of branched C-3-io-alkyl (meth)acrylate, preferably branched C3-6-alkyl (meth)acrylate, in particular tert- butyl acrylate and/or terf-butyl methacrylate. based on the total weight of the shell polymer monomers.

The core of the core/shell polymer particle, i.e. the core polymer, preferably comprises 50% to 100%, more preferably 80 to 100%, by weight, of monomers selected from branched C-3-io-alkyl (meth)acrylate, such as terf-butyl acry- late and/or terf-butyl methacrylate, styrene, linear Ci-io-alkyl (meth)acrylate, such as n-butyl acrylate, and acrylonitrile, and any mixtures thereof, based on the total weight of the core polymer monomers. Presence of acrylonitrile allows lower monomer ratio of the branched C3-io-alkyl (meth)acrylate than the presence of linear Ci-io-alkyl (meth)acrylates, although a portion of branched C3-10- alkyl (meth)acrylates can be replaced by of linear Ci-io-alkyl (meth)acrylates without significant change in the sizing efficiency.

The monomers of the core/shell polymer particle can be provided in different combinations provided that polymer particles having a "core-shell" type construct are obtained. Thus the monomer composition can be altered as a gradient or in sequences, preferably in sequence.

Ratio of the weight of the core polymer monomers to the weight of the shell polymer monomers is preferably from 75:25 to 25:75. With styrene-free core-shell polymers, a thicker shell, in particular a thicker terf-butyl acrylate shell, improves the sizing performance. For optimal performance the shell polymer has a glass transition temperature from 35 to 75°C, more preferably from 40 to 65°C. Further, for optimal performance the core polymer has a glass transition temperature below 75°C, preferably below 65°C.

In a particularly advantageous example of the present core/shell polymer particle, the branched C3-io-alkyl (meth)acrylate monomer is branched C3- 6-alkyl (meth)acrylate, such as such as terf-butyl acrylate and/or terf-butyl meth- acrylate, more preferably branched C3-6-alkyl (meth)acrylate, most preferably terf-butyl acrylate.

The core polymer and the shell polymer of the core/shell polymer particle are typically polymerized from monomers in presence of degraded starch or water-soluble natural or synthetic polymer. The degraded starch typically has molecular weight M _n of 500 to 10000. The monomers are preferably dispersion polymerized, such as suspension polymerized or emulsion polymerized, to form the desired core/shell polymer particle. Preferably the core/shell polymer particles are obtained by a radically initiated dispersion polymerization in presence of degraded starch, and/or water-soluble natural or synthetic polymer, preferably degraded starch. The term "total weight of the monomers of the core/shell polymer particle" refers thus only to the amount of the polymerizable monomers and excludes the amount of degraded starch and water-soluble natural or synthetic polymer in the composition. Typically the amount of degraded starch and/or water-soluble natural or synthetic polymer varies from 10 to 60%, preferably 15 to 40%, per weight of the weight of the total solids content of the core/shell polymer particles i.e. the sum of monomers and degraded starch and water-soluble natural or synthetic polymer is 100%. Polymerization of the monomers is preferably preformed in the absence of low molecular weight emulsifiers, in particularly those selected from sodium alkanesulfonate, sodiumdodecylsulfate, sodium do- decylbenzenesulfonate, sulfosuccinic esters, fatty alcohol polyglycol ethers, al- kylaryl polyglycol ethers and the like.

The methods for core/shell polymerization are generally known to a person skilled in the art. Preparation of the core/shell polymer particles may be done for example by dispersing degraded potato starch in heated water and then feeding desired monomers in two steps: first core monomer(s) and then shell monomer(s) with a simultaneous feed of hydrogen peroxide. After cooling and quenching the reaction, filtration provides a finely divided dispersion comprising desired core/shell polymer particles.

The polymerization may be performed in the presence of a graft-link- ing water-soluble redox system to initiate polymerization. Suitable systems include redox systems comprising hydrogen peroxide and heavy metal ions such as cerium, manganese or iron(ll) salts.

The particle size distribution D50 of the core/shell polymer particles is preferably below 1 10 nm.

Further provided herein is a surface sizing composition comprising core-shell polymer particles as defined herein. The surface sizing composition is typically provided in an aqueous liquid vehicle, as an aqueous solution or dispersion, although small amounts of water-soluble or water miscible organic solvents) may also be present. The surface sizing composition solution typically includes, along with the sizing compounds, starch, which can be anionic, non- ionic, amphoteric, or cationic, preferably anionic starch. The starch concentration is preferably from 1 % to 30%, more preferably from 5 to 25% and the sizing agent, i.e. the core-shell polymer particle, concentration is from 0.1 to 20% by weight, preferably 0.5 to 5.0% by weight, based on the weight of dry starch.

The specific techniques used to size paper and other cellulosic products such as cardboard, include, but are not limited to, those techniques that are commonly employed in papermaking to apply the sizing composition to the cellulose-based product. For instance, the aqueous sizing composition may be applied to the surface of the paper using a puddle or film size press or a size press by using a calender or a doctor knife blade. Alternatively, the sizing composition may be sprayed onto the paper web or be applied by dipping the paper into the aqueous surface sizing composition. Paper or other cellulosic product treated with the surface sizing solution is then dried at elevated temperatures, typically temperature of the paper is from 80 to 1 10°C. Drying the paper web is sufficient to bring the surface size and surface strength to full development.

The sizing composition disclosed herein also may be used in conjunction with or serially with other additives conventionally used in the production of paper and other cellulosic products. Such additional additives commonly known in the art include, but are not limited to, colorants, inorganic pigments and fillers, anti-curl agents, additional conventional components such as surfactants, plasticizers, humectants, defoamers, UV absorbers, light fastness enhancers, polymeric dispersants, dye mordants, optical brighteners, and leveling agents.

The present invention further provides a method of surface sizing a cellulosic product, in particular paper, comprising applying to at least one surface of the cellulosic product a sizing composition comprising core-shell polymer particles as defined herein. Further accordingly provided herein is a paper surface-sized with core-shell polymer particles as defined herein or a surface sizing composition as defined herein.

The paper or other cellulosic product onto which the surface sizing composition is applied may vary widely and is independent of the kind of pulp used to make the paper. The surface sizing compositions disclosed herein are suitable for the preparation of sized paper of any thickness and of any kind and thus applicable to papers or cardboards obtained from any specific paper pulp and mixtures thereof.

The paper or other cellulosic product also may contain additives such as fillers, dyestuffs, paper strengthening agents, drainage rate improvers, and internal sizing agents.

Water absorptiveness of paper surface sized with the present surface size composition can be determined using the Cobb 60 method, ISO 535:1991 (E), at 23°C, 50% relative humidity EXAMPLES Example 1

57.25 g of an oxidatively degraded potato starch (Perfectamyl A 4692) was dispersed with stirring in 329 g of demineralizer water in a 1 L glass reactor with a cooling/heating jacket under a nitrogen atmosphere. The starch was dissolved by heating the mixture to 85°C and cooking at 85°C for 30 minutes. After starch dissolution was complete, 28.6 g of 0.66 % strength aqueous solution of ferrous (II) sulfate heptahydrate was added in to the reactor. After 10 minutes 3.3 g of 30% strength hydrogen peroxide was added. After 10 minutes, the starch degradation was complete. Then 167.4 g of heated demin- eralizer water was added to the reactor. After the temperature had stabilized to 85°C, the chemical feeds were started. Monomers were fed in two steps: 83.7 g of mixture of n-butyl acrylate and acrylonitrile (1 :1 weight) was fed first during 60 minutes. Then 83.7 g of tert-butyl acrylate was fed during 60 minutes. 48.8 g of 10 % solution of hydrogen peroxide was fed simultaneously with the monomer feeds during 120 min. The reactor temperature was kept at 85°C during the feeds and 15 minutes after for postpolymerization. Then the mixture was cooled to 60°C and 5.2 g of 1 1 .7% strength tert-butyl hydroperoxide solution was added dropwise into the reactor. The temperature was kept at 60°C for further 60 min. Thereafter, cooling was effected to 40°C and 5.9 g of 10% strength ethylenedi- aminetetraacetic acid sodium salt (EDTA-Na) solution was added, followed by pH adjustment to 6 with 25% strength sodium hydroxide solution and cooling to room temperature. Filtration was performed using a 100 m filter cloth. A finely divided dispersion with a solid content of 26.5% is obtained.

Example 2

Example 1 was repeated with 83.7 g of a mixture of n-butyl acrylate and styrene (1 :1 weight) in the first monomer feed and 83.7 g of tert-butyl acrylate in the second monomer feed. The first feed was fed in 60 minutes and the second in 60 minutes. A finely divided dispersion with a solid content of 26.7% is obtained. Example 3

Example 1 was repeated with 41 .9 g of n-butyl acrylate in the first monomer feed and 125.6 g of tert-butyl acrylate in the second monomer feed. The first feed was fed in 30 minutes and the second in 90 minutes. A finely divided dispersion with a solid content of 26.7% is obtained. Example 4

Example 1 was repeated with 41 .9 g of n-butyl acrylate in the first monomer feed and 125.6 g of tert-butyl acrylate in the second monomer feed. Half of the first feed was fed in 15 minutes and the second half in 45 minutes. The second feed was started 15 minutes after the first feed. 1/3 of the second feed was fed in 45 minutes simultaneously with the first monomer feed. After this 2/3 of the second monomer feed was fed in 60 minutes. A finely divided dispersion with a solid content of 26.8% is obtained.

Example 5

60.6 g of an oxidatively degraded potato starch (Perfectamyl A 4692) was dispersed with stirring in 348 g of deminerahzer water in a 1 L glass reactor with a cooling/heating jacket under a nitrogen atmosphere. The starch was dissolved by heating the mixture to 85°C and cooking at 85°C for 30 minutes. After starch dissolution was complete, 30.2 g of 0.66% strength aqueous solution of ferrous (II) sulfate heptahydrate was added in to the reactor. After 10 minutes 3.3 g of 30% strength hydrogen peroxide was added. After 10 minutes, the starch degradation was complete. Then 177.1 g of heated deminerahzer water was added to the reactor. After the temperature had stabilized to 85°C, the chemical feeds were started. Monomers were fed in two steps: 106.3 g of tert- butyl acrylate was fed first during 72 minutes. 70.8 g of mixture of n-butyl acrylate and styrene (22:78 weight) in the first monomer feed and 106.3 g tert-butyl acrylate in the second monomer feed. The first feed was fed in 48 minutes and the second in 72 minutes. 48.8 g of 10% solution of hydrogen peroxide was fed simultaneously with the monomer feeds during 120 min. The reactor tempera- ture was kept at 85°C during the feeds and 15 minutes after for postpolymeriza- tion. Then the mixture was cooled to 60°C and 5.5 g of 1 1 .7% strength tert-butyl hydroperoxide solution was added dropwise into the reactor. The temperature was kept at 60°C for further 60 min. Thereafter, cooling was effected to 40°C and 6.2 g of 10% strength ethylenediaminetetraacetic acid sodium salt (EDTA- Na) solution was added, followed by pH adjustment to 6 with 25% strength sodium hydroxide solution and cooling to room temperature. Filtration was performed using a 100 μιτι filter cloth. A finely divided dispersion with a solids content of 26.5% is obtained.

Example 6

60.6 g of an oxidatively degraded potato starch (Perfectamyl A 4692) was dispersed with stirring in 348 g of deminerahzer water in a 1 L glass reactor with a cooling/heating jacket under a nitrogen atmosphere. The starch was dissolved by heating the mixture to 85°C and cooking at 85°C for 30 minutes. After starch dissolution was complete, 30.2 g of 0.66% strength aqueous solution of ferrous (II) sulfate heptahydrate was added in to the reactor. After 10 minutes 3.5 g of 30% strength hydrogen peroxide was added. After 10 minutes, the starch degradation was complete. Then 177.1 g of heated demineralizer water was added to the reactor. After the temperature had stabilized to 85°C, the chemical feeds were started. Monomers were fed in two steps: 35.4 g of tert- butyl acrylate was fed first during 24 minutes. Then 141 .7 g of mixture of n- butyl acrylate and styrene (1 :3 weight) was fed during 96 minutes. 51 .6 g of 10 % solution of hydrogen peroxide was fed simultaneously with the monomer feeds during 120 min. The reactor temperature was kept at 85°C during the feeds and 15 minutes after for postpolymerization. Then the mixture was cooled to 60°C and 5.5 g of 1 1 .7% strength tert-butyl hydroperoxide solution was added dropwise into the reactor. The temperature was kept at 60°C for further 60 min. Thereafter, cooling was effected to 40°C and 6.2 g of 10% strength ethylenedi- aminetetraacetic acid sodium salt (EDTA-Na) solution was added, followed by pH adjustment to 6 with 25% strength sodium hydroxide solution and cooling to room temperature. Filtration was performed using a 100 m filter cloth. A finely divided dispersion with a solid content of 26.5% is obtained.

Example 7

60.6 g of an oxidatively degraded potato starch (Perfectamyl A 4692) was dispersed with stirring in 348 g of demineralizer water in a 1 L glass reactor with a cooling/heating jacket under a nitrogen atmosphere. The starch was dissolved by heating the mixture to 85°C and cooking at 85°C for 30 minutes. After starch dissolution was complete, 30.2 g of 0.66% strength aqueous solution of ferrous (II) sulfate heptahydrate was added in to the reactor. After 10 minutes 3.3 g of 30% strength hydrogen peroxide was added. After 10 minutes, the starch degradation was complete. Then 177.1 g of heated demineralizer water was added to the reactor. After the temperature had stabilized to 85°C, the chemical feeds were started. Monomers were fed in two steps: 106.3 g of tert- butyl acrylate was fed first during 72minutes. Then 70.8 g of mixture of n-butyl acrylate and styrene (1 :2 weight) was fed during 48 minutes. 48.8 g of 10 % solution of hydrogen peroxide was fed simultaneously with the monomer feeds during 120 min. The reactor temperature was kept at 85°C during the feeds and 15 minutes after for postpolymerization. Then the mixture was cooled to 60°C and 5.5 g of 1 1 .7% strength tert-butyl hydroperoxide solution was added drop- wise into the reactor. The temperature was kept at 60°C for further 60 min. Thereafter, cooling was effected to 40°C and 6.2 g of 10% strength ethylenedi- aminetetraacetic acid sodium salt (EDTA-Na) solution was added, followed by pH adjustment to 6 with 25% strength sodium hydroxide solution and cooling to room temperature. Filtration was performed using a 100 m filter cloth. A finely divided dispersion with a solid content of 25.9% is obtained.

Comparative example 1 (CE1)

57.25 g of an oxidatively degraded potato starch (Perfectamyl A 4692) was dispersed with stirring in 329 g of demineralizer water in a 1 L glass reactor with a cooling/heating jacket under a nitrogen atmosphere. The starch was dissolved by heating the mixture to 85°C and cooking at 85°C for 30 minutes. After starch dissolution was complete, 28.6 g of 0.66% strength aqueous solution of ferrous (II) sulfate heptahydrate was added in to the reactor. After 10 minutes 3.3 g of 30% strength hydrogen peroxide was added. After 10 minutes, the starch degradation was complete. Then 167.4 g of heated demin- eralizer water was added to the reactor. After the temperature had stabilized to 85°C, the chemical feeds were started. 167.5 g of a mixture of n-butyl acrylate and tert-butyl acrylate (1 :3 weight) was fed during 120 minutes. 48.8 g of 10% solution of hydrogen peroxide was fed simultaneously with the monomer feed during 120 min. The reactor temperature was kept at 85°C during the feeds and 15 minutes after for postpolymerization. Then the mixture was cooled to 60°C and 5.2 g of 16.7% strength tert-butyl hydroperoxide solution was added drop- wise into the reactor. The temperature was kept at 60°C for further 60 min. Thereafter, cooling was effected to 40°C and 5.9 g of 10% strength ethylenedi- aminetetraacetic acid sodium salt (EDTA-Na) solution was added, followed by pH adjustment to 6 with 25% strength sodium hydroxide solution and cooling to room temperature. Filtration was performed using a 100 μιτι filter cloth. A finely divided dispersion with a solid content of 26.8% is obtained.

Test Examples

A series of surface sizes and formulates were tested for the surface size application using an internally unsized, recycled fiber linerboards with base weight of ca. 120 and 100 g/m ² . Mathis size presses were used in these tests. The surface size was added to surface size starch (C ^* film 0731 1 ) solution at 15% reference to base weight 120 g/m ² or test series example number 1 and 18% solids content reference to base weight 100 g/m ² or test series example number 2. Hydrophobic polymers were added at concentrations of 1 .5, 3.0 and 4.5 weight-%. Sizing tests were carried out at 70°C temperature. Temperature of the size press nip was measured with Reatec NO1 temperature indicator strips and temperature of the water bath for size press rolls was adjusted to obtain the desired temperature. The sheets were run through a horizontal pond size press at 2 m/min (2 Bar). The sheets were dried at 95°C 1 min/side either using a contact dryer or a drum dryer. Temperature of the dryers was adjusted using Reatec NO82 temperature indicator strips. The sizing efficiency was determined by measuring Cobb60 sizing degree according to standard ISO 535:1991 (E), at 23°C and 50% relative humidity.

The particle sizes of the samples were measured using Zetasizer Nano-device. The glass transition temperatures of the samples were measured from freeze dried samples using a differential scanning calorimeter (DSC). The solids contents of the new surface sizes were measured using a Mettler Toledo Halogen moisture analyzer.

The compositions of the tested sizes are shown in Tablel and Table 2. Figure 1 and 2 show water absorptiveness of paper surface sized with the different sizing compositions. Of the tested monomer compositions comprising terf-butyl acrylate in the shell polymer gave the best results.

Table 1. Sizing tests with base liner number 1 , basis weight of 120 g/m ² and core-shell polymers CE1 , 1 , 2, 3 and 4.

^* excluding starch

Table 2 Sizing tests with base liner number 2, basis weight of 100 g/m ² and core- shell polymers 5, 6, and 7.

^* excluding starch

It will be obvious to a person skilled in the art that, as the technology advances, the inventive concept can be implemented in various ways. The in- vention and its embodiments are not limited to the examples described above but may vary within the scope of the claims.