Use of an anionic composition and a method for surface sizing

Abstract

The present invention relates to a use of an anionic composition for surface sizing of a cellulosic fibrous web, preferably comprising recycled cellulosic fibres. The anionic composition comprises an anionically derivatized polysaccharide and cationized degraded starch, and has a charge density from -3.5 meq / g to -0.2 meq / g, measured at pH 7. The surface size composition is applied on at least one large surface of the cellulosic fibrous web.

Description

[0001] USE OF AN ANIONIC COMPOSITION AND A METHOD FOR SURFACE SIZING

[0002] The present invention relates to a use of an anionic composition and a method for surface sizing of a cellulosic fibrous web according to the preambles of the enclosed independent claims.

[0003] Recycling of used products to provide raw material for the manufacture of new products has been a standard practice in the manufacture of cellulosic fibrous webs, such as paper and board, already for decades. The increase in recycling cycles results in decline in the quality of recycled cellulosic fibres, as each recycling cycle negatively affects the fibre properties, e.g. by causing hornification and shortening of the length of the recycled cellulosic fibres. This leads to reduced strength properties for the cellulosic fibrous webs produced from the recycled fibres. Overall, the quality of the recycled cellulosic fibres has steadily declined as the cellulosic fibres undergo more recycling cycles and the amount of recycled fibres in the fibre stock has increased. At the same time the demand for high-performing and durable cellulosic fibrous webs to be used as packaging materials is steadily increasing. The fibre strength loss of the stock raw material must thus be compensated by other measures in the web production, typically by chemical measures, such as application of a higher amount of starch at the size press, as described below. However, starch applied at the size press is lost in the recycling and results in poor process yield and higher COD load in the effluent water. Consequently, there is a permanent need to improve the dry strength properties of cellulosic fibrous webs.

[0004] The dry strength properties of the cellulosic fibrous webs can be improved either by using chemical agents applied into the fibre stock before formation of the fibrous web or by applying chemical agents on the formed fibrous web. For example, surface sizing where starch solution is applied on the surface of the fibrous web is commonly used to improve the dry strength properties of the produced fibrous web, e.g. board. Even if dry strength properties of the cellulosic fibrous web are thus improved, heavy surface sizing where high amount of starch is applied on the fibrous web may lead to score cracking during converting of the fibrous web, i.e. the web may become strong but brittle. Excessive use of starch may be associated with microbial problems, too. Especially in repulping, starch used in surface sizing becomes a burden in the water treatment process as it follows to the web making process with the recycled cellulosic fibres but remains in the water circulation. There the starch presents a nutrition source to unwanted microbial growth. This leads to higher COD load in the effluent water. Furthermore, the use of starch itself can be considered problematic, as it often requires agricultural resources that could be used for food or feed production instead. Consequently, there is an interest to reduce the use of starch in surface sizing, without compromising the dry strength properties obtained.

[0005] Polyelectrolyte complexes comprising a cationic component and an anionic component have been added into the cellulosic fibre stocks for improving the dry strength properties of cellulosic fibrous webs. Good strength results have been obtained, but sometimes when large dosages are needed for achieving the desired strength, problems with the web drainage may occur. Therefore, there is still need for new ways of improving the strength properties of fibrous webs.

[0006] The object of the present invention is to minimize or even eliminate the disadvantages existing in the prior art.

[0007] One object of the present invention is to improve the dry strength properties of the cellulosic fibrous webs and reduce the starch usage in surface sizing.

[0008] Another object of the present invention is to provide a method for surface sizing of cellulosic fibrous web with an anionic composition, which is easy to use and provides effective strength improvement.

[0009] An additional advantage of the present invention is the facilitation of the surface sizing of cellulosic fibrous webs by reduction of the risk of starch retrogradation and gel formation during handling of the sizing composition.

[0010] These objects are attained with the invention having the characteristics presented below in the characterising part of the independent claim. Some preferred embodiments of the invention are presented in the dependent claims.

[0011] The embodiments mentioned in this text relate, where applicable, to all aspects of the invention, both the use as well as the method, even if this is not always separately mentioned.

[0012] Typical use of an anionic composition according to the present invention is for surface sizing of a cellulosic fibrous web, preferably comprising recycled cellulosic fibres, wherein the anionic composition comprises

[0013] - an anionically derivatized polysaccharide, and

[0014] - cationized degraded starch, wherein the anionic composition has a charge density from -3.5 meq / g to -0.2 meq / g, measured at pH 7.

[0015] A typical method according to the present invention for surface sizing of a cellulosic fibrous web, preferably comprising recycled cellulosic fibres, comprises

[0016] - obtaining a cellulosic fibrous web, and

[0017] - applying a surface size composition on at least one large surface of the cellulosic fibrous web, wherein the surface size composition comprises an anionic composition comprising

[0018] - an anionically derivatized polysaccharide, and

[0019] - cationized degraded starch, wherein the anionic composition has a charge density from -3.5 meq / g to - 0.2 meq / g, measured at pH 7.

[0020] Now it has been surprisingly found out that an anionic composition comprising an anionically derivatized polysaccharide and a cationized degraded starch provides improved strength properties when the anionic composition is used as a surface size composition for surface sizing of cellulosic fibrous web, by applying the surface size on at least one large surface of the fibrous web. It is assumed that the anionically derivatized polysaccharide and the cationized degraded starch are able to interact with each other and form a polyelectrolyte complex which makes the anionic composition, when dissolved in water, stable for a prolonged time without viscosity increase and eventual gel formation. The anionic composition also provides good dry strength properties for the cellulosic fibrous web when the composition is used for surface sizing while reducing the amount of starch needed for surface sizing. The use of anionic composition for surface sizing thus reduces the amount of starch in water circulations: not only less starch is used from the beginning, but it has also been unexpectedly observed that the formed polyelectrolyte complex is at least partially retained with fibres at recycling, thus minimising the starch load in recycled fibres, in broke system and in water systems.

[0021] The anionic composition according to the present invention is easy to use and provides a clever way for the desired strength improvement. The cationized degraded starch can be considered as “masked” by the anionically derivatized polysaccharide in the anionic composition, by formation of a polyelectrolyte complex The fibre surfaces in the cellulosic fibrous web are anionic, which means that the interaction with the anionic composition is minimised at the surface size application step, when the surface size comprising the anionic composition is applied on the surface of the fibrous web. This enables the penetration of the polyelectrolyte complex of the anionic composition into the structure of the cellulosic fibrous web. The good penetration results in high pick-up of the surface size composition and increases the bonding between fibres also in the middle of the fibrous web, leading to improved burst strength, short span compression test (SCT) strength or Concora medium test (CMT) strength.

[0022] The use of anionic composition according to the present invention thus maintains or improves especially the strength properties of the cellulosic fibrous web, selected from one or more of the following: short span compression test (SCT) strength, burst strength, internal bond (Scott bond) strength, ring crush test (RCT) strength and Concora medium test (CMT) strength.

[0023] In the present context the term “cellulosic fibrous web” denotes webs formed from fibres comprising cellulose as their structural component, irrespective the method of manufacturing and / or pulping. The term thus encompasses fibres originating from wood and non-wood sources, such as hardwood, softwood, grasses, hemp, bagasse, cotton, and the like. The fibres of the cellulosic fibrous webs may be obtained by any available pulping technique, such as mechanical pulping, chemical pulping, chemi-mechanical pulping or semi-chemical pulping. The fibres of the cellulosic fibrous webs may be recycled fibres, obtained by contained board or mixed waste pulping or recycled fibre deinking. The fibres of the cellulosic fibrous web may comprise from 80 to 100 weight-% or 85 to 95 weight-%, of recycled fibres, calculated from the total dry weight of cellulosic fibres comprised in the cellulosic fibrous web.

[0024] The surface size composition comprising or consisting of the anionic composition may be applied on at least one large surface of the cellulosic fibrous web by using any known surface sizing technique. The surface size composition can be applied by using a pond type size press, flooded size press, metering film size press, such as gate roll film press or rod metering film size press, hard nip size press, spray application, foam application, or curtain size application. The application temperature may be 50 - 80 °C. The surface size composition comprising or consisting of the anionic composition is preferably added to the large surface, which forms the outer large surface of the cellulosic fibrous web. The anionic composition has suitable properties, such as viscosity, making it easily applicable on the cellulosic fibrous web for obtaining a thin layer size layer with good coverage, preferably on the outer surface of the cellulosic fibrous web.

[0025] According to one embodiment the anionic composition may have a viscosity in a range of 4 - 80 mPas, preferably 5 - 70 mPas, more preferably 8 - 60 mPas, measured from 2 weight-% aqueous solution at 25 °C, pH 7, by using Brookfield LV. It has been surprisingly observed that the anionic composition is able to retain its initial viscosity for a prolonged time, up to 21 hours or more. It is assumed that due to the polyelectrolyte complex formation between the anionically derivatized polysaccharide and the cationic starch, the anionic composition does not undergo significant starch retrogradation under cooling and / or storage. This means that the risk of gel formation is minimised, and the anionic composition provides operational easiness. It is speculated that the presence of anionically derivatized polysaccharide inhibits the retrogradation, i.e. ability of the cationic starch to orientate and recover its semi-crystalline structure resulting in gel formation. The use of anionic composition thus significantly simplifies the surface sizing process, as elevated temperatures and strictly controlled storage times, which are usually needed for avoidance of retrogradation of the starch-based surface size compositions, are not required.

[0026] The anionic composition according to the present invention comprises anionically derivatized polysaccharide and cationized degraded starch in amounts that provide the composition with anionic net charge. The anionic net charge of the composition improves the penetration of the composition to the cellulosic fibrous web in the surface sizing and the interaction of the composition with the cellulosic fibres within the fibrous web. Furthermore, the use of the composition with anionic net charge can be beneficial in further processing of the fibrous webs. For example, when the fibrous webs of liner and fluting are combined at a corrugator by using a glue or starch to form a corrugated board, the layer of anionic composition on the surface of the web(s) of liner and / or fluting facilitates more bond formation and the strength between the layers. The anionic composition of the present invention has a charge density from -3.5 meq / g to -0.2 meq / g, measured at pH 7, when dissolved in water. The charge density is measured by titration with Mutek PCD 03. According to one preferable embodiment the anionic composition may have the charge density in the range from -3.5 meq / g to -0.5 meq / g, preferably from -2.5 meq / g to -0.8 meq / g, more preferably from -2.0 meq / g to -1 .0 meq / g, sometimes from -2.0 meq / g to -1 .1 meq / g, measured at pH 7, when dissolved in water.

[0027] The anionic composition used for surface sizing comprises at least one anionically derivatized polysaccharide. The anionically derivatized polysaccharide can be any suitable polysaccharide that has an anionic net charge. In the present context the term “anionically derivatized polysaccharide” is understood to refer to a polysaccharide which comprises one or more anionic groups covalently bound to the polysaccharide structure formed from monosaccharide units bound together by glycosidic linkages. The covalently bound anionic groups can be identical to or different from each other. The covalently bound anionic groups can be selected from carboxyl, sulphate, sulphonate, phosphonate or phosphate groups, including their salts forms, and any combinations thereof. Anionic groups are preferably introduced to the structure of the polysaccharide by chemical modification, such as carboxymethylation, oxidation, sulphation, sulphonation or phosphorylation.

[0028] According to one embodiment, the anionically derivatized polysaccharide can be selected from anionic starch, carboxymethylated polysaccharides, such as carboxymethylated celluloses, carboxymethylated a-(1 ,3-glucan) polymers or carboxymethylated xylans, anionic hemicelluloses or any of their mixtures. The anionically derivatized polysaccharide is at least partially, preferably fully, water- soluble. Preferably the anionically derivatized polysaccharide is selected from anionic starch, carboxymethylated cellulose or carboxymethylated a-(1 ,3-glucan) polymers.

[0029] According to one embodiment, the anionically derivatized polysaccharide is anionic starch, preferably with a relatively low anionicity. The anionically derivatized polysaccharide may be anionic starch, which has a degree of anionic substitution in a range of 0.01 - 0.05, for example 0.015 - 0.04. The anionic starch may be starch originating from potato, waxy potato, rice, corn, waxy corn, sweet potato, arrowroot, pea or tapioca. Preferably the anionic starch is potato starch. The anionic starch is preferably anionic degraded starch, obtained by oxidative degradation. The anionic starch may have an intrinsic viscosity in a range of 0.1 - 0.9 dl / g, preferably 0.1 - 0.5 dl / g, more preferably 0.15 - 0.4 dl / g.

[0030] In the present context, the intrinsic viscosity values are determined as follows. Sample for measurement is dissolved at 1 % (based on active content) in DI water under magnetic stirring at 60 °C for 60 minutes. Obtained solution is diluted to 0.5% with 2 M NaCI, to obtain a stock solution in 1 M NaCI. The stock solution is filtered through 200 pm sieve. The sieved stock solution is diluted with 1 M NaCI to obtain a series of measurement solutions with suitable concentrations, e.g. 0.08 g / dl, 0.1 . g / dl, 0.125 g / dl. 0.15 g / dl, 0.17 g / dl. This procedure is known for a person skilled in measuring intrinsic viscosities. The diluted solutions were mixed by turning the in flask several times (e.g. 15 times), then mixing was continued for 10 min at 100 rpm with magnetic stirrer, and the pH was measured. Thereafter, the viscosity measurements were done immediately. Intrinsic viscosities were measured with Ubbelohde capillary viscometer at 25°C, at pH 5 - 7.

[0031] According to one preferable embodiment, the anionically derivatized polysaccharide is carboxymethylated polysaccharide, preferably carboxymethylated cellulose or carboxymethylated a-(1 ,3-glucan) polymer, more preferably carboxymethylated cellulose. The carboxymethylated cellulose suitable for use in the present invention may be obtained by any process known in the art. The carboxymethylated cellulose may be purified carboxymethylated cellulose or technical grade carboxymethylated cellulose. The use of carboxymethylated polysaccharide, especially carboxymethylated cellulose or carboxymethylated a-(1 ,3-glucan) polymer, in the anionic composition is advantageous, as it reduces the amount of starch in the surface size composition and used in surface sizing. Furthermore, the carboxymethylated cellulose and a-(1 ,3-glucan) polymers induce less microbial growth in the manufacture of the cellulosic fibrous web, thus reducing microbial problems in process water circulation as well as process water treatment.

[0032] The anionically derivatized polysaccharide may be carboxymethylated polysaccharide, such as carboxymethylated cellulose or a-(1 ,3-glucan) polymer, preferably carboxymethylated cellulose, which has a degree of carboxymethyl substitution 0.2 - 1 .2, preferably 0.45 - 1 .0, more preferably 0.45 - 0.65. Preferably the degree of carboxymethyl substitution is 0.4 or more, preferably 0.5 or more. The degree of carboxymethyl substitution provides the carboxymethylated polysaccharide at least a partial water-solubility, preferably a full water-solubility, which is beneficial in dissolving the anionic composition and forming the surface size composition.

[0033] In the present context, water-solubility means that when anionically derivatized polysaccharide, such as anionic starch, carboxymethylated cellulose or carboxymethylated a-(1 ,3-glucan) polymer, is dissolved in excess of water, the anionically derivatized polysaccharide contains at most 30 weight-%, preferably at most 20 weight-%, more preferably at most 10 weight-%, of water-insoluble material. The water-solubility can be determined by dissolving the anionically derivatized polysaccharide to 200 ml of water at 25 °C to form 1 weight-% solution of polysaccharide, pouring the obtained solution through 200 pm mesh, diameter about 20 cm, and washing the insolubles on the mesh for 3 minutes with water (25 °C, about 10 litres) by using a hand shower. The insolubles from the mesh are dried at 110 °C for four hours and weighted. The amount of insoluble material is then calculated as percentage from the original dry weight of the dissolved anionically derivatized polysaccharide.

[0034] The anionically derivatized polysaccharide may comprise carboxymethylated polysaccharide, such as carboxymethylated cellulose or carboxymethylated a-(1 ,3- glucan) polymer, preferably carboxymethylated cellulose, which has an intrinsic viscosity in a range of 1 - 7 dl / g, preferably 1.5 - 6 dl / g, more preferably 2 - 5 dl / g. The intrinsic viscosity is measured as described above. The low intrinsic viscosity of the carboxymethylated polysaccharide indicates that the molecular weight of the carboxymethylated polysaccharide is relatively low.

[0035] According to one embodiment, the anionically derivatized polysaccharide may have a linear structure.

[0036] In addition to the anionically derivatized polysaccharide, the anionic composition comprises cationized degraded starch. The amount of anionic groups of the anionically derivatized polysaccharide in the anionic composition is higher than the amount of cationic groups of cationized degraded starch, in order to ensure that the anionic composition has a net anionic charge as desired. The anionic composition may comprise 10 - 90 weight-%, preferably 20 - 80 weight-%, more preferably 25 - 75 weight-%, even more preferably 40 - 75 weight-% or 50 - 70 % weight-%, of the anionically derivatized polysaccharide, preferably carboxymethylated cellulose, calculated from the total dry weight of the anionically derivatized polysaccharide and cationized degraded starch. This means that it is possible to replace a large amount of starch in the surface size composition by a non-food polysaccharide, which decreases the microbial problems in the manufacturing process of the cellulosic fibrous web and makes the anionic composition more sustainable (use of less food / feed grade material).

[0037] The cationized degraded starch, which is used for the anionic composition, may be potato starch, waxy potato starch, rice starch, corn starch, waxy corn starch, sweet potato starch, arrowroot starch, pea starch or tapioca starch. Preferably the cationized degraded starch is potato starch. Cationized degraded starch may be obtained by cationising starch by any suitable method, known as such for a person skilled in the art, typically by using 3-chloro-2-hydroxypropyltrimethylammonium chloride or 2,3-epoxypropyltrimethylammonium chloride as cationising agent. The cationization may be carried out before or after the degradation of starch.

[0038] According to one embodiment of the invention, the cationized degraded starch may have a degree of cationic substitution in a range of 0.01 - 0.2, preferably 0.015 - 0.1 , more preferably 0.02 - 0.05 or 0.02 - 0.044. In comparison to conventional surface size starches, the cationized degraded starch of the anionic composition has higher degree of cationic substitution. This provides the anionic composition with desired strength properties by enhancing the formation of the bonds between the anionically derivatized polysaccharide and the cationized degraded starch, while maintaining the composition’s ability to penetrate into the cellulosic fibrous web. In comparison to furnish applications, cationized degraded starch having relatively low cationicity is preferred in the present invention. Too high cationic substitution has a negative effect on penetration of the surface size into the cellulosic fibrous web.

[0039] The term “cationized degraded starch” encompasses both cationized degraded starch or degraded cationized starch. The degradation of starch, either before or after cationization, effectively enables the penetration into the cellulosic fibrous web and guarantees viscosity properties for the anionic composition, required in surface sizing. The starch can be oxidatively, thermally or enzymatically degraded starch. Preferably, starch is degraded by oxidative treatment, for example by hypochlorite or peroxide treatment. The oxidative degradation treatment may introduce some anionically charged groups to the cationized degraded starch, but it maintains its cationic net charge, even after oxidative treatment. The cationic component comprises a branched and / or crosslinked structure.

[0040] The cationized degraded starch may have an intrinsic viscosity in a range of 0.1 - 0.9 dl / g, preferably 0.1 - 0.5 dl / g, more preferably 0.15 - 0.45 dl / g. The intrinsic viscosity may be measured as described above. The cationized degraded starch has a low intrinsic viscosity, which indicates the low molecular weight of the used cationized degraded starch. The intrinsic viscosity may be considered indicative as the degradation degree of the starch.

[0041] The anionic composition may comprise 10 - 90 weight-%, preferably 20 - 80 weight- %, more preferably 25 - 75 weight-%, even more preferably 25 - 60 weight-% or 30 - 50 % weight-%, of the cationized degraded starch, calculated from the total dry weight of the anionically derivatized polysaccharide and cationized degraded starch.

[0042] The anionic composition may comprise anionic charges to cationic charges in a ration from 1.5:1 to 30:1 , preferably from 3:1 to 25:1 , more preferably from 5:1 to 20:1 , determined at pH 7.

[0043] The anionic composition is preferably in form of a dry particulate material. This means that the anionic composition is in solid form, e.g. in form of a powder or fine particulate material and comprises or consists of discrete solid particles. The solid form of the anionic composition makes it easy to transport and store, and reduces risk for microbial attacks, e.g. during storage at elevated temperatures. The anionic composition may have a dry solids content of at least 80 weight-%, preferably at least 85 weight-%, more preferably at least 87 weight-% The dry solids content of the anionic composition may be in a range of 80 - 99 weight-%, preferably 85 - 97 weight-% more preferably 87 - 95 weight-%, calculated from the total weight of the composition. Typically, the remaining, up to 100 weight-%, is atmospheric moisture. Preferably all components of the anionic components can be dissolved at a same dissolving temperature. The dissolving temperature may be in a range of 40 - 140 °C, preferably 45 - 100 °C, more preferably 50 - 80 °C, sometimes 65 - 80 °C. According to one embodiment of the invention the anionic composition may comprise one or more auxiliary agents. The auxiliary agents may include anionic hydrophobation agents, cationic hydrophobation agents, polyaluminium chloride, polyvalent metal salts, such as zirconium salts, polyvinyl alcohols and lignin-based substances, such as lignosulphonates, lignocarbohydrate complexes.

[0044] According to one preferable embodiment, the anionic composition is free of polymer latices.

[0045] The anionic composition is used for surface sizing of a cellulosic fibrous web, preferably cellulosic fibrous web comprising recycled fibres, by application on at least one surface of the cellulosic fibrous web. The anionic composition is especially suitable for surface sizing of cellulosic fibrous web, such as board, which comprise recycled cellulosic fibres. The cellulosic fibrous web, preferably comprising recycled fibres, may be fluting, test liner, liner, folding boxboard, white line chipboard, gypsum board liner, core board, solid bleached sulphate board, solid unbleached sulphate board or liquid packaging board. The cellulosic fibre web may have grammage from 65 g / m2to 600 g / m2, preferably from 70 g / m2to 250 g / m2. When the surface size composition comprising or consisting of the anionic composition is applied on at least one large surface of the cellulosic fibrous web, the cellulosic fibrous web may have a dryness of >25 weight-%, preferably >40 weight-% or >50 weight-%, calculated from the total weight of the fibrous web, especially when the surface size composition is applied on a large surface forming the outer surface of the cellulosic fibrous web.

[0046] The surface size composition comprising or consisting of the anionic composition is applied at least on one large surface, or both large surfaces, of a cellulosic fibrous web. The surface size composition may comprise, in addition to the anionic composition, hydrophobic sizing agents, such as acrylate-based polymer dispersions, alkenyl succinic acid, alkyl ketene dimer, as well as conventional enzymatically degraded surface size starch. According to one embodiment, the surface size composition is free of synthetic hydrophobic sizing agents, such as acrylate-based polymer dispersions, alkenyl succinic acid and / or alkyl ketene dimer. According to one embodiment the surface size composition used for surface sizing of the cellulosic fibrous web comprising recycled fibres may comprise 1 .5 - 20 weight-%, preferably 2 - 15 weight-%, of the anionic composition comprising anionically derivatized polysaccharide and cationized degraded starch, given as dry. According to one preferable embodiment of the invention, the surface size composition further comprises enzymatically degraded surface size starch. The anionic composition enables reducing the amount of enzymatically degraded surface size starch in the surface sizing. Surface sizing with a surface size composition comprising the anionic composition and enzymatically degraded surface size starch may provide pick-up of 1 - 10 %, preferably 2 - 7 %, more preferably 2.5 - 4.5 %, of the basepaper weight.

[0047] Preferably the surface sizing composition comprising or consisting of the anionic composition is free of inorganic mineral particles, such as coating pigments of calcium carbonate, kaolin, talc or the like.

[0048] The surface size composition comprising or consisting of the anionic composition is applied in an amount of 1 - 20 kg / ton, preferably 2 - 15 kg / ton, given as per ton cellulosic fibrous web, as dry. For example, the surface size composition comprising or consisting of the anionic composition may be applied in amount of 0.05 - 15 g / m2, preferably 0.1 - 12 g / m2.

[0049] According to one embodiment, the surface size composition comprising or consisting of the anionic composition may provide pick-up of 0.1 - 2 %, preferably 0.2 - 1 .5 %, of the basepaper weight.

[0050] EXPERIMENTAL

[0051] Some embodiments of the invention are more closely described in the following nonlimiting examples- Example 1 : Preparation and dissolving of anionic composition

[0052] Example 1 generally demonstrates the preparation of anionic composition according to the present invention in dry particulate form and process for its dissolution into water.

[0053] The anionic composition was prepared by first mixing carboxymethylated cellulose and cationized degraded starch. Both the carboxy methylated cellulose and the cationized degraded starch were in powder form. For example, 2.4 g of carboxymethylated cellulose (as dry) was combined with 1.6 g of cationized degraded starch (as dry). The powders were mixed thoroughly to ensure homogeneous distribution and mixing of the powders. In this way, a powder mix of the anionic composition can be obtained. The ratios of carboxymethylated starch and cationized degraded starch can be adjusted as needed, as long as the anionic composition has the charge density from -3.5 meq / g to -0.2 meq / g, measured at pH 7.

[0054] An exemplary way of dissolving the anionic composition is as follows. 196 ml of deionized water was measured into a beaker, and it was stirred with a magnetic stirrer. For example, to prepare a 2 % solution of anionic composition, 4 g of powder mix of the anionic composition (given as dry, humidity adjusted) was weighed and added slowly into the beaker under stirring. The used amount of anionic composition can be adjusted according to desire. The obtained mixture of anionic composition and water was then heated to 80 °C and kept at that temperature under constant stirring for 30 minutes to dissolve the anionic composition and to form the polyelectrolyte complex. After 30 minutes stirring the solution was cooled down.

[0055] It was observed that the obtained solution was visibly more turbid than the solutions where the components were dissolved separately. This was given as indication of the formation of the polyelectrolyte complex.

[0056] Application example 1 The basepaper used in application example 1 was recycled fibre based unsized European testliner basepaper with 10% ash (525 °C, Standard ISO 1762). The grammage of the testliner basepaper was 100 g / m2.

[0057] In application example 1 the testliner basepaper was surface sized with a surface size composition comprising only surface size starch (reference) or with a surface size composition comprising surface size starch and an anionic composition according to the invention.

[0058] The used surface size starch was dextrin starch (C*Film 07311 , Cargill). This thermally degraded dextrin starch simulates enzymatically degraded wheat starch with similar viscosity.

[0059] The anionic compositions comprising anionically derivatized polysaccharide and cationized degraded starch were prepared according to Table 1 a. The components of the anionic composition were dissolved to deionised water at 2 weight-% concentration by cooking at 60 °C for 60 min (anionically derivatized polysaccharide) or cooking at 97 °C for 30 min (cationized degraded starch). After dissolution, the anionically derivatized polysaccharide and cationized degraded starch were mixed, as specified and in ratios given in Table 1 a, to a homogenous anionic composition for use in surface sizing. The weight percentages for the components are given as dry.

[0060] Table 1 a Anionic compositions used in Application Example 1 . Viscosities of the obtained anionic compositions were measured at 25 °C, 2 weight- % concentration, Brookfield LV DVI SSA viscometer, with maximum allowed rpm speed with a spindel guided by the instrument. The charge density was measured by titration with Mutek PCD 03. The properties of the anionic compositions are given in Table 1 b.

[0061] Table 1 b Properties of the anionic compositions used in Application Example 1 .

[0062] Surface sizing of the testliner basepaper was done with laboratory size-press (Mathis) and drum-dried. Size-press and drying parameters are listed in Table 2. Surface sizing solution was kept at temperature 70 °C before use and approximately 170 g of formulation was used for surface sizing in each trial. Five (5) board sheets of A5 size were surface sized per trial. The size-press was washed between the trials.

[0063] Table 2 Laboratory size-press and drying parameters used in application example 1 .

[0064] After surface sizing and drying, the sized sheets were taken into climate-controlled room (RH 50%, 23 °C) for conditioning. After conditioning, the basis weight and the strength properties of the sheets were measured according to Table 3. The measured strength values were indexed by dividing the measured strength value with the sheet basis weight. Geometrical mean (GM) for a strength value is the square root of the product of the strength in machine direction (MD) and the strength in cross direction (CD), x StrengthMD. Pick-up for a sized sheet indicates the increase in the air conditioned basis weight of the sheet, in %, measure before and after sizing in the size press.

[0065] Table 3 Sheet testing devices and standard methods used. Sizing trials and their results are presented in Table 4. Trial 1 was a 0-test with an unsized testliner basepaper. Trials 2 - 4 are reference trials, where the surface sizing is performed by using only surface size starch. Trials 5 - 8 are according to invention, where the surface size composition comprised in addition to surface size starch also anionic composition according to the invention.

[0066] Table 4 The results of application example 1 .

[0067] *dry

[0068] **viscosity of the surface sizing composition, measured with Brookfield LV, at 60 °C. From the results of Table 4 it can be seen that when the surface size composition comprises 2 - 3 weight-% of PEC5 or PEC6 (trials 5 - 8 ), both the burst strength and SCT strength properties are improved at 5% pick-up in comparison to reference trial 2 with 100% surface size starch at the same pick-up. In order to obtain similar burst and SCT strength results as in trials 5 - 8 by using only surface size starch, it was necessary to increase the surface size starch concentration in the surface sizing composition and the pick-up. In practice this means that by using the anionic composition of the present invention, it is possible to significantly reduce the amount of the surface size starch needed for surface sizing. It is also possible to operate at lower concentrations, which makes the handling of the solutions easier.

[0069] Application Example 2

[0070] The testliner basepaper and the anionic composition PEC6 were the same as in application example 1. The same surface sizing method and test methods were used as in application example 1 .

[0071] The surface size starch used in application example 2 was enzymatically degraded native corn starch. The used corn starch was degraded as follows:

[0072] 1.3 litres of native corn starch at 16 weight-% concentration was degraded with 63 pL thermoalfaamylase enzyme in 2 I beaker with propeller stirring. Enzyme dosage was adjusted based on achieved starch viscosity, because the enzyme activity was not exactly known. Degradation was conducted at 90 °C for 13 min. Degradation reaction was stopped by elevating the temperature to 125 °C by pumping the starch solution through an oil bath with 100 ml / min flow rate. The achieved surface size starch viscosity was 10 mPas, measured with Brookfield LV at 15 weight-% concentration, and 60 °C.

[0073] An additional reference polymer in application example 2 was a synthetic commercial strength polymer, which was a glyoxylated cationic polyacrylamide (GPAM). Sizing trials and their results are presented in Table 5. Trial 2-1 was a O-test with an unsized testliner basepaper. Trials 2-2, 2-3 and 2-4 are reference trials, where the surface sizing is performed by using only surface size starch. Trials from 2-5 to 2-8 are according to invention, where the surface size composition comprised in addition to surface size starch also anionic composition according to the invention.

[0074] Table 5 Results of application example 2.

[0075] *dry

[0076] **viscosity of the surface sizing composition, measured with Brookfield LV at 60 °C.

[0077] It can be seen from Table 5 that the surface size composition comprising anionic composition PEC-6 achieves higher burst strength and SCT strength than surface size starch alone at 5% pick-up, demonstrated in trials 2-3 vs. 2-6. The surface size composition comprising PEC6 achieved even higher burst strength and SCT strength at 5% pick-up than surface size composition comprising conventional GPAM. When the surface size composition consisted solely of PEC6 (trial 2-7), it is seen that same burst strength and SCT strength is achieved only at 0.9% pick-up than at 5% pick-up, when surface size composition consisted solely of surface size starch (trial 2-3). This shows great potential for material savings, and at the same reduces COD load originating from recycled board starch. The lower amount of starch in broke reduces risks for microbial problems at board machine, which improves runnability and reduces need for washing shutdowns. Possibility to achieve the same strength properties with lower pick-up also makes it possible to save energy needed in the drying section, as there is less water to evaporate from the sized fibrous web. For mills where the drying capacity is at the present the production limiting factor, the reduced need for evaporation makes it possible to increase the production speed, and thus the amount of produced fibrous web.

[0078] Application example 3

[0079] The basepaper used in application example 3 was recycled fibre based unsized North American testliner basepaper with 7% ash (525 °C, Standard ISO 1762). The grammage of the testliner basepaper was 100 g / m2.

[0080] Application example 3 was performed at pilot metering film size press (Valmet OptiSizer) with infrared dryers as post dryers after sizing. The running speed was 700 m / min and target amounts of surface size composition were adjusted by rod pressure evenly to both sides of the fibrous web.

[0081] The surface size starch used in application example 3 was enzymatically degraded native corn starch, degraded (as described above) to about 20 mPas viscosity, measured with Brookfield LV at 60 °C, at 15 weight-% concentration.

[0082] PEC6 was in form of dry particulate mixture, formed according to example 1 and prepared from components as indicated in Table 1. The dry particulate mixture of PEC6 was dissolved in water by cooking at 60 °C, for 60 min.

[0083] After surface sizing and drying, the samples of sized web were taken into climate- controlled room (RH 50%, 23 °C) for conditioning at least for 4 hours. After conditioning, the following properties were measured: basis weight (ISO 536), tensile strength (ISO 1924-3), SCT strength (ISO 9895) and burst strength (ISO 2758). For interpretation of the results, a reference trendline was made by running trials with a surface size composition consisting of 100% of surface size starch at three different pick-up levels (trials from 3-2 to 3-4), and reading the strength results from the trendline at matching pick-up points. The results of application example 3 are shown in Table 6. Trial 3-1 was a O-test with an unsized testliner basepaper.

[0084] It can be seen from Table 6 that the burst strength, the SCT strength and tensile strength are all improved at 4% pick-up when the surface size composition comprised the anionic composition PEC6. It is obvious that starch savings can be achieved by using the anionic composition PEC6 in surface sizing, as the strength properties can be maintained, even if the pick-up is reduced from 5% to 4%. In practical industrial scale, this reduction produces significant savings both in used starch amounts and needed drying energy amount when seen on annual level.

[0085] Table 6 Results of application example 3.

[0086] *dry

[0087] Application Example 4

[0088] The testliner basepaper was the same as used in application example 1 .

[0089] Surface sizing of the testliner basepaper was done in the same manner and with same parameters as in application example 1 , except that the size press temperature was 60 °C. The sized sheets were handled after surface sizing in the same manner as in application example 1 , and the basis weight and the strength properties of the sheets were measured according to Table 3. Concora medium test (CMT) strength was measured according to standard ISO 9895. The surface size starch, used as reference, was dextrin starch (50:50 w-% mixture of C*Film 07311 and C*Film 07312, Cargill). Dextrin starch was dissolved in water at 97°C for 30 min.

[0090] Cationized oxidized starch (DS 0.04), used as the reference, was dissolved in water at 80°C for 30 min.

[0091] The anionic compositions comprising different ratios of cationized oxidatively degraded starch (DS 0.04) and carboxymethylated cellulose (DS 0.50) were prepared as given in Table 7. The cationized oxidatively degraded starch and carboxymethylated cellulose were mixed together to form powder mixtures, which were dissolved in water for 30 min at 80 °C, according to principles described in example 1. Table 7 shows the dissolution concentrations for the surface size compositions, as well as the dry solids of the surface size composition at the size press, after optional dilution with water.

[0092] Table 7 Compositions used for surface sizing in application example 4

[0093] *dry

[0094] **viscosity of the surface sizing composition, measured with Brookfield LV, at 60 °C.

[0095] In application example 4 the testliner basepaper was surface sized with a surface size composition consisting of surface size starch, i.e. dextrin starch (trial 4-2, reference) or of the cationized oxidatively degraded starch (trial 4-3, reference) or with a surface size composition comprising an anionic composition according to the invention (trials 4-4 to 4-8). Results of the application example 4 are shown in Table 8. Trial 4-1 was a O-test with an unsized testliner basepaper. It can be seen from results of Table 8 that when the surface size composition comprises anionic composition according to the invention (trials 4-4 to 4-8), similar or even better SCT strength, CMT strength and burst strength properties can be obtained at significantly lower pick-up levels when compared to references trials 4-

[0096] 2 and 4-3. For example, SCT GM index of 24.2 Nm / g was obtained in reference trial 4-2 at 5.8% pick-up, whereas a SCT GM index of 24.6 Nm / g was obtained in trial 4- 4, at 1 .7% pick-up. Table 8 Results of application example 4.

[0097] Even if the invention was described with reference to what at present seems to be the most practical and preferred embodiments, it is appreciated that the invention shall not be limited to the embodiments described above, but the invention is intended to cover also different modifications and equivalent technical solutions within the scope of the enclosed claims.

Claims

CLAIMS1. Use of an anionic composition for surface sizing of a cellulosic fibrous web, preferably comprising recycled cellulosic fibres, the anionic composition comprising- an anionically derivatized polysaccharide, and- cationized degraded starch, wherein the anionic composition has a charge density from -3.5 meq / g to -0.2 meq / g, measured at pH 7.

2. Use according to claim 1 , characterised in that the anionic composition has the charge density in the range from -3.5 meq / g to -0.5 meq / g, preferably from -2.5 meq / g to -0.8 meq / g, more preferably from -2.0 meq / g to -1 .0 meq / g, measured at pH 7.

3. Use according to claim 1 or 2, characterised in that the anionic composition has a viscosity in a range of 4 - 80 mPas, preferably 5 - 70 mPas, more preferably 8 - 60 mPas, measured from 2 weight-% aqueous solution at 25 °C, pH 7, by using Brookfield LV.

4. Use according to claim 1 , 2 or 3, characterised in that the anionically derivatized polysaccharide is selected from a group consisting of anionic starch and carboxymethylated polysaccharide, such as carboxymethylated cellulose or carboxymethylated a-(1 ,3-glucan) polymer.

5. Use according to claim 4, characterised in that the anionically derivatized polysaccharide is carboxymethylated polysaccharide, which has- a degree of carboxymethyl substitution 0.2 - 1 .2, preferably 0.45 - 1 .0, more preferably 0.45 - 0.65, and / or- an intrinsic viscosity in a range of 1 - 7 dl / g, preferably 1.5 - 6 dl / g, more preferably 2 - 5 dl / g.

6. Use according to claim 4, characterised in that the anionically derivatized polysaccharide is anionic starch, which has- a degree of anionic substitution in a range of 0.01 - 0.05, and / or- an intrinsic viscosity in a range of 0.1 - 0.9 dl / g, preferably 0.1 - 0.5 dl / g, more preferably 0.15 - 0.4 dl / g.

7. Use according to any of preceding claims 1 - 6, characterised in that the cationized degraded starch is oxidatively degraded starch.

8. Use according to any of preceding claims 1 - 7, characterised in that the cationized degraded starch has- a degree of substitution in a range of 0.01 - 0.2, preferably 0.015 - 0.1 , more preferably 0.02 - 0.05 and / or- - an intrinsic viscosity in a range of 0.1 - 0.9 dl / g, preferably 0.1 - 0.5 dl / g, more preferably 0.15 - 0.45 dl / g.

9. Use according to any of preceding claims 1 - 8, characterised in that in the anionic composition comprises anionic charges and cationic charges in a ratio from 1 .5:1 to 30:1 , preferably from 3:1 to 25:1 , more preferably from 5:1 to 20:1 , at pH 7.

10. Use according to any of preceding claims 1 - 9, characterised in that the anionic composition comprises- 10 - 90 weight-%, preferably 20 - 80 weight-%, more preferably 25 - 75 weight- %, of the anionically derivatized polysaccharide, calculated from the dry weight of the anionically derivatized polysaccharide and cationized degraded starch.

11. Use according to any of preceding claims 1 - 10, characterised in that the composition is in form of a dry particulate material.

12. Use according to any of preceding claims 1 - 11 , characterised in that the anionic composition is free of inorganic mineral particles.

13. Method for surface sizing of a cellulosic fibrous web, preferably comprising recycled cellulosic fibres, the method comprising- obtaining a cellulosic fibrous web, and- applying a surface size composition on at least one large surface of the cellulosic fibrous web, wherein the surface size composition comprises an anionic composition comprising- an anionically derivatized polysaccharide, and - cationized degraded starch, wherein the anionic composition has a charge density from -3.5 meq / g to -0.2 meq / g, measured at pH 7.

14. Method according to claim 13, characterised in that the surface size composition further comprises enzymatically degraded starch.

15. Method according to claim 13 or 14, characterised in that the surface size is applied in an amount of 1 - 20 kg / t, preferably 2 - 15 kg / t, preferably by using a pond type size press; flooded size press; metering film size press, such as gate roll film press or rod metering film size press; hard nip size press; spray application; foam application; or curtain size application, more preferably by using metering film size press, hard nip size press or spray application.

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