Surface sizing composition and use thereof

Abstract

The present invention relates to a surface sizing composition for surface sizing of paper, paperboard, etc. The composition comprises at least an aluminum compound and an anionic lignin-carbohydrate complex.

Description

Technical Field

[0001] According to the preambles of the attached independent claims, the present invention relates to an aqueous surface size composition for paper, board or the like and the use thereof. Background Art

[0002] Various properties of paper or paperboard can be modified by surface sizing. For example, surface sizing can be used to increase the hydrophobicity of the paper / paperboard surface or to improve the strength properties and printability of the paper / paperboard. In conventional surface sizing, the sizing composition is applied in the form of a solution to the surface of an at least partially dried fiber web.

[0003] Typically, surface sizing compositions mainly contain starch, which enhances strength properties and smoothes the substrate surface. However, starch is hydrophilic, which may result in a reduced hydrophobicity of the sizing surface, and this may result in an undesirable trade-off between the strength obtained and the reduced hydrophobicity. This means that in order to improve or maintain the water resistance of the sizing surface, it is necessary to add one or more hydrophobic agents to the surface sizing composition or to increase the water resistance of the substrate itself, for example by using internal sizing. In the latter case, the hydrophobic agent is added to the raw material itself before the paper or paperboard is formed.

[0004] In addition to starch, many components used in sizing compositions for surface sizing are synthetic polymers or other synthetic compounds. Synthetic compounds are used in surface sizing, in particular for improving the hydrophobicity of the surface. In the paper and paperboard industry, the recent general trend towards more sustainable production processes and products has also emphasized the importance of using materials derived from renewable resources. This trend has increased the interest in finding new alternatives to synthetic components, especially hydrophobic agents for surface sizing compositions. For example, lignin has been studied for surface treatment, but its commercial application has been limited due to its weak hydrophobic effect. Therefore, there is still a need for new surface sizing compositions with components from renewable resources that will provide the desired properties for surface sizing. Summary of the invention

[0005] It is an object of the present invention to minimize or possibly even eliminate the disadvantages existing in the prior art.

[0006] It is also an object to provide a surface sizing composition comprising a hydrophobic agent obtained from renewable resources.

[0007] Another object of the present invention is to provide a surface sizing composition which is easy to prepare and which provides good surface sizing results, in particular in terms of strength properties and hydrophobic effect.

[0008] These objects are achieved by the invention having the features presented below in the characterizing part of the independent claim. Some preferred embodiments are disclosed in the dependent claims.

[0009] The features recited in the dependent claims and the embodiments described in the description may be freely combined with one another, unless explicitly stated otherwise.

[0010] The exemplary embodiments presented herein and their advantages relate to all aspects of the invention by way of applicable parts, even though this is not always mentioned individually.

[0011] A typical aqueous surface sizing composition for sizing paper, paperboard, etc. according to the invention comprises at least an aluminum compound and an anionic lignin-carbohydrate complex.

[0012] A typical use of the surface sizing composition according to the present invention is the surface sizing of a sheet-like fibrous substrate comprising cellulose fibers, such as paper, paperboard, and the like.

[0013] It has now surprisingly been found that a surface sizing composition comprising an aluminum compound and an anionic lignin-carbohydrate complex, wherein the lignin and the carbohydrate are directly covalently bound to each other, provides unexpectedly improved hydrophobicity to the sized paper or paperboard surface. The surface sizing composition thus obtained provides at least similar, sometimes even improved sizing results compared to surface sizing compositions using conventional synthetic polymer-based hydrophobing agents. The present invention provides the possibility of effectively reducing the use of petroleum-based synthetic hydrophobing agents and increasing the use of substances derived from renewable resources. The present invention provides the possibility of using materials from renewable resources to improve the hydrophobicity of paper and paperboard without the need for additional oil-based hydrophobing agents.

[0014] Anionic lignin-carbohydrate complexes suitable for the present invention are natural polymer complexes comprising lignin and carbohydrates (preferably hemicellulose) covalently bound to each other. Therefore, lignin-carbohydrate complexes are conjugates of lignin and carbohydrates, which are irreversibly bound to each other into an overall structure. Without permanently destroying the structure of the complex, the lignin and carbohydrates of the complex cannot be separated from each other. Anionic lignin-carbohydrate complexes can have branched structures. For example, lignin or carbohydrates can form the backbone structure of the complex, and another component, carbohydrate or lignin, can form a side group covalently bound to the backbone structure.

[0015] Lignin-carbohydrate complexes can be formed by lignin and one or more carbohydrates such as hemicellulose. The carbohydrates in the lignin-carbohydrate complexes can preferably be formed by monosaccharides, such as galactose, glucose, mannose, arabinose and / or xylose or fragments thereof or residues thereof, which are covalently bound to lignin. The exact amount of monosaccharides in the lignin-carbohydrate complexes and their relative proportions depend on the wood species, such as hardwood / softwood, from which the pulping process has been used and from which the lignin-carbohydrate complexes are derived. Monosaccharides can be present in the lignin-carbohydrate complexes as sugar residues covalently bound to lignin.

[0016] The lignin-carbohydrate complex may contain anionic functional groups, which may be sulfonic acid groups, carboxyl groups and / or phenolic groups. The lignin-carbohydrate complex may contain, for example, sulfonic acid groups of >1300-1700 μmol / g, preferably 1400-1600 μmol / g; carboxyl groups of 300-500 μmol / g, preferably 350-450 μmol / g; and / or phenolic groups of 125-250 μmol / g, preferably 150-225 μmol / g.

[0017] Lignin-carbohydrate complexes suitable for the present invention can be obtained, for example, from a side stream of a pulping process. In one embodiment, suitable lignin-carbohydrate complexes can be obtained by enzymatic treatment of lignin-carbohydrate materials derived from a pulping process. For example, lignin-carbohydrate complexes can be separated from a side stream of a wood pulping process by filtration (such as membrane filtration) by separating lignin-carbohydrate materials, and obtained by enzymatic processing of the separated lignin-carbohydrate materials preferably using laccase. Alternatively, lignin-carbohydrate complexes can be separated from lignocellulosic materials (such as wood or pulp) using well-known separation and fractionation methods. For example, lignin-carbohydrate complexes can be separated by fractionating lignin from an industrial process, such as kraft pulping or sulfite pulping. Suitable lignin fractionation methods include, for example, solvent fractionation or precipitation classification. In solvent fractionation, various organic solvents and binary mixtures thereof can be used, such as acetone-hexane, acetone-water, ethanol-water, propylene glycol monomethyl ether-water. This fractionation method is described, inter alia, in Int. J. Biol. Macromolecules 106 (2018) 979-987.

[0018] According to a preferred embodiment of the present invention, the anionic lignin-carbohydrate complex is an anionic lignin sulfonate-carbohydrate complex. It can be obtained, for example, by membrane filtration of a prehydrolysis mixture from a sulfite pulping process of wood, and then by enzymatic oxidation treatment, preferably by laccase treatment. Preferably, the filtered prehydrolysis mixture is obtained from the sulfite pulping process of wood. The prehydrolysis mixture may contain wood-based (wood-based, wood-based) components and pulping chemicals. Suitable anionic lignin sulfonate-carbohydrate complexes are disclosed in BioResources 13 (4), 7606-7627, 2018, and they are commercially available from Ecohelix AB, Sweden.

[0019] The anionic lignin-carbohydrate complex may have an anionic charge density of less than -0.2 meq / g, preferably less than -0.5 meq / g, more preferably less than -0.85 meq / g, measured at pH 7. The anionic charge density of the complex may be from -0.2 meq / g to -2.5 meq / g, preferably from -0.5 meq / g to -2.4 meq / g, more preferably from -0.85 meq / g to -2.3 meq / g, measured at pH 7. Sometimes the anionic charge density of the complex may be from -0.5 meq / g to -1.75 meq / g, preferably from -0.85 to -1.5 meq / g, measured at pH 7. The anionic charge density of the complex may even be from -2.0 meq / g to -2.3 meq / g, preferably from -2.1 to -2.2 meq / g, measured at pH 7. All charge density values ​​are given on a dry basis and were measured using a Mütek particle charge detector.

[0020] The lignin-carbohydrate complex may have a weight average molecular weight of >3500 g / mol, preferably >4000 g / mol, more preferably >5000 g / mol. The lignin-carbohydrate complex may have a weight average molecular weight MW in the range of 3500-90000 g / mol, preferably 4000-80000 g / mol, more preferably 5000-70000 g / mol.

[0021] According to a preferred embodiment, the lignin-carbohydrate complex may preferably have a relatively high molecular weight. Without wishing to be bound by theory, it is believed that high molecular weight provides at least some of the surprising effects that have been observed. The lignin-carbohydrate complex may have a weight average molecular weight MW of>12000g / mol, preferably>15000g / mol, more preferably>20000g / mol or>25000g / mol. The lignin-carbohydrate complex may have a weight average molecular weight MW in the range of 8000-50000g / mol or 10000-45000g / mol, preferably 12000-40000g / mol or 15000-37000g / mol. Sometimes, the lignin-carbohydrate complex may have a weight average molecular weight MW in the range of 20000-45000g / mol, preferably 25000-40000g / mol, more preferably 25000-35000g / mol or 25000-27000g / mol. According to a specific embodiment, the lignin-carbohydrate complex may have a weight average molecular weight MW in the range of 15000-120000 g / mol or 20000-90000 g / mol, preferably 25000-80000 g / mol, more preferably 30000-70000 g / mol.

[0022] The lignin-carbohydrate complex may comprise lignin and carbohydrates (preferably hemicellulose) in a ratio of 90:10 to 10:90, preferably 80:20 to 20:80, more preferably 75:25 to 25:75 (lignin: carbohydrate). According to one embodiment of the invention, the lignin-carbohydrate complex may comprise at least 10% by weight, preferably at least 15% by weight of carbohydrates (preferably hemicellulose), calculated on the total dry weight of the complex. The lignin-carbohydrate complex may comprise carbohydrates in the range of 10-40% by weight, 10-30% by weight or 15-25% by weight, calculated on the total dry weight of the complex.

[0023] The surface sizing composition may contain anionic lignin-carbohydrate complexes in an amount of 5-100 wt %, preferably 9-99 wt %, more preferably 15-98 wt %, even more preferably 18-85 wt %, and sometimes even 25-65 wt %, calculated on the dry solids content of the surface sizing composition. It is believed that possible trace impurities, such as inorganic salts associated with the lignin-carbohydrate complexes, may be contained in the weight of the anionic lignin-carbohydrate complexes.

[0024] The surface sizing composition generally has a solid content of 1-30% by weight, preferably 3-25% by weight, more preferably 5-15% by weight, even more preferably 7-13% by weight, calculated as dry solids. The surface sizing composition may have a solid content of 6-14% by weight, preferably 7-11% by weight, calculated as dry solids. The surface sizing composition comprises water as solvent and preferably consists of water as solvent.

[0025] The viscosity of the surface sizing composition is <200 mPas, preferably <100 mPas, more preferably <50 mPas, measured at 25° C. using a Brookfield LVDV viscometer at 60 rpm at <50 mPas, 30 rpm at 50-200 mPas in a small sample adapter with 18 spindles. The viscosity provides suitable application results when conventional application techniques are used.

[0026] The surface sizing composition may also contain at least one aluminum compound. Suitable aluminum compounds may be selected from, for example, polyaluminium chloride (polyaluminium chloride), alum and aluminum sulfate. The surface sizing composition may contain an aluminum compound in an amount of 0.01-6, preferably 0.04-4, more preferably 0.07-2, even more preferably 0.1-1.6, given as % by weight of aluminum ions in the dry solids of the surface sizing composition. The surface sizing composition may contain an aluminum compound, in particular polyaluminium chloride in an amount of 0.02-1.5, preferably 0.05-1.35, even more preferably 0.09-1.30 or 0.15-0.99, given as % by weight of aluminum ions in the dry solids of the surface sizing composition.

[0027] In some embodiments, the surface sizing composition may be free of aluminum compounds. It has been observed that in the absence of aluminum compounds, the surface sizing composition can still provide improved strength performance results even though the hydrophobic effect may be reduced.

[0028] The polyaluminium chloride in the form of a solution suitable for use in the surface sizing composition according to the present invention may contain aluminium (as Al) in an amount of 4-15 wt %, preferably 6-14 wt %, more preferably 8-13 wt %, even more preferably 8.5-10 wt %. 3+ The polyaluminium chloride in the form of a solution may contain Al2O3 in an amount of 1-25 wt%, preferably 5-22 wt%, more preferably 7-20 wt%, even more preferably 15-18 wt%. The alkalinity of the polyaluminium chloride solution may be 25-85%, preferably 25-75%, more preferably 30-65%, even more preferably 35-50%. The polyaluminium chloride may contain chloride (in the form of Cl2O3) in an amount of 7-25 wt%, 15-25 wt%, or 19-23 wt%. - in the form of ).

[0029] According to a preferred embodiment, in addition to the lignin-carbohydrate complex and the optional aluminum compound, the surface sizing composition also comprises a polysaccharide, preferably starch or a starch derivative. Starch can be any starch suitable for surface sizing of paper or paperboard, such as potato, waxy potato, rice, corn, waxy corn, wheat, barley or cassava starch. The starch used is preferably degraded starch, such as degraded potato, cassava, corn or wheat starch. In principle, any degraded starch is suitable for the present invention, and the starch used can be degraded by any suitable method known in the art or their combination. For example, degraded starch can be obtained by subjecting starch to chemical, thermal or enzymatic degradation, preferably chemical or enzymatic degradation. Chemical degradation includes acidic and oxidative degradation, preferably oxidative degradation. Hypochlorite, peroxydisulfate, hydrogen peroxide or their mixture can be used as an oxidant.

[0030] Starch can be nonionic starch or net anionic starch. Net anionic starch can contain cationic groups as long as its net charge is anionic. The surface sizing composition can contain starch in an amount of 0-95% by weight, preferably 1-91% by weight, more preferably 2-85% by weight, even more preferably 15-82% by weight, and sometimes even preferably 30-75% by weight, calculated based on the dry solid content of the surface sizing composition. For example, the surface sizing composition can contain starch in an amount of 0-80% by weight, preferably 10-75% by weight, more preferably 35-70% by weight, calculated based on the dry solid content of the surface sizing composition.

[0031] It has been observed that even though the amount of starch in the surface sizing composition may be reduced, the strength effect obtained is at least similar or even improved.

[0032] The surface sizing composition may also include additional components such as brighteners, defoamers, biocides and / or crosslinking agents such as glyoxal. The surface sizing composition may also include additional hydrophobic agents such as poly(styrene acrylate), AKD or rosin. Additional hydrophobic agents are preferably anionic or amphoteric with net anionicity.

[0033] The surface sizing composition preferably contains no inorganic mineral fillers and / or inorganic mineral pigments.

[0034] According to one embodiment, the surface sizing composition according to the invention improves the strength properties of the final paper or paperboard, in particular the compressive strength expressed as SCT strength, even under high humidity conditions. The compressive strength, expressed as SCT strength of paper, paperboard, etc., is measured according to ISO 9895 standard (2008) when exposed to standard environmental conditions (+23°C, relative humidity 50%) or when exposed to environmental conditions in which the relative humidity is ≥80%, preferably ≥85%, more preferably ≥90% and the temperature is ≥30°C, preferably ≥35°C, more preferably ≥38°C. The short-span compression test (SCT) strength can be used to predict the compression resistance of the final product, such as a paperboard box. Boxes made of containerboard are subjected to high loads when stacked during storage and transportation, which makes the compressive strength (i.e. SCT strength) possibly one of the more important strength properties of paper or paperboard. The strength properties of paper and board tend to become particularly critical when the product is exposed to environmental conditions where the relative humidity is ≥ 80%, preferably ≥ 85%, more preferably ≥ 90% and the temperature is ≥ 30°C, preferably ≥ 35°C, more preferably ≥ 38°C.

[0035] The surface sizing composition according to the invention is suitable for the surface sizing of all cellulosic products, in particular all paper and board grades. Cellulosic products, such as paper or board, may be unsized or they may be internally presized, for example by adding an internal sizing agent, such as alkyl ketene dimer, alkenyl succinic anhydride or rosin, to the fiber raw material before web formation.

[0036] The surface sizing composition according to the invention is particularly suitable for paper or board which may be based on 100% virgin fibres, on 100% recycled fibres or on any possible blend between virgin fibres and recycled fibres. The surface sizing composition according to the invention is particularly suitable for the surface sizing of board made from recycled fibres. For example, the fibres in the raw material may comprise at least 80% recycled fibres, preferably at least 90% recycled fibres, sometimes even 100% recycled fibres. All percentages are given in % by weight, calculated on the dry total fibre weight. The recycled fibres may come from old corrugated board and / or mixed paper grades. Preferably, a mixture of old corrugated board and mixed paper grades is used for the recycled fibres.

[0037] The surface sizing composition according to the present invention can be applied to the surface of paper or paperboard using any available suitable technique, and is generally used for surface sizing of paper, paperboard, etc. For example, the surface sizing composition can be provided to a cellulose-based product, such as a paper or paperboard web, in the form of a liquid or foam. For example, the aqueous surface sizing composition can be applied to the surface of paper or paperboard by using a paddle sizing machine, a film sizing machine, or a sizing machine using a roller or a doctor blade in a sizing application. Alternatively, the surface sizing composition can be sprayed onto a paper or paperboard web or applied by dipping the paper or paperboard into an aqueous surface sizing composition. Cellulose products, such as paper or paperboard, treated with a surface sizing solution according to the present invention are dried at elevated temperatures, typically the temperature of the paper or paperboard can be 80 to 110°C.

[0038] According to one embodiment of the present invention, the surface sizing composition may be applied at a rate of at least 0.5 g / m 2 / surface, preferably at least 1.3g / m 2 / surface, more preferably 1.3-5.0g / m 2 / surface, more preferably 1.4-3.2g / m 2 The amount of sizing agent applied to the surface of the fiber web is given as dry solids. These application amounts are particularly preferred for surface sizing of pads and grooves. DETAILED DESCRIPTION

[0039] Experimental Some embodiments of the invention are described in the following non-limiting examples.

[0040] The sizing performance of the surface sizing composition is 140g / m 2 The paper was tested on internal unsized linerboard of a basis weight of 1.50 mm. The temperature of the surface sizing composition was set at 65°C. The paper was run through a Mathis horizontal pool size press model 5607 at 2 m / min (2 bar). The temperature of the size press nip was set at 65°C. In Examples 1 and 2, the paper was dried at 95°C for 1 minute per side using a PTI laboratory paper dryer. In Example 3, the paper was dried at 95°C using an AMC drum dryer at speed 50 for 1.5 minutes.

[0041] The sizing efficiency is determined by measuring the Cobb 60 sizing degree according to standard ISO 535. The compressive strength in the cross direction (CD) is studied using the short span compression test (SCT) method designed based on standard ISO 9895:1989 (Short span test for compressive strength of paper and board). However, the number of deviations from the standard or parallel measurements is usually 8 instead of 20. For normal SCT measurements, the samples are pre-conditioned at RH 50% and 23°C according to standard ISO 187:1990 (Standard atmosphere for conditioning and testing of paper, board and pulp and procedures for monitoring atmosphere and sample conditioning). For tropical SCT measurements, the samples are pre-conditioned in a climate cabinet at RH 90% and 38°C for at least 4 hours, usually overnight.

[0042] The surface sizing composition is prepared by heating the lignin-hemicellulose complex solution to the sizing temperature and mixing the aluminum compound into the lignin-hemicellulose complex solution before surface sizing. In the embodiment in which starch is used in the surface sizing composition, the starch is first dissolved in water according to its common starch cooking instructions, and then the dissolved starch in solution form is blended with the lignin-hemicellulose complex solution at the sizing temperature. In Examples 1 and 2, the starch is C*film07312, and in Examples 3 and 4, the starch is Raisamyl 01121. Before surface sizing, the aluminum compound is added to the mixture of lignin-hemicellulose complex and starch.

[0043] Example 1

[0044] Test and reference surface sizing compositions having a dry solids content of 8 wt% were prepared as described above. Test composition 3 had a dry solids content of 12 wt%.

[0045] In reference compositions 2 and 3, lignin sulfonate (Ufoxane, Borregaard, Norway) was used instead of lignin-hemicellulose complex obtained from Ecohelix AB, Sweden (MW about 31000 g / mol, charge density about -2.15, pH 7).

[0046] The components of the surface sizing composition are given in Table 1. The weight % of each component is given as weight % of dry solids.

[0047] Table 1 Surface sizing composition used in Example 1.

[0048] Lignin-hemicellulose complex starch Lignin sulfonate Test composition 1 100 - - Test composition 2 33 67 - Test composition 3 33 67 - Reference composition 1 - 100 - Reference composition 2 - - 100 Reference composition 3 - 67 33

[0049] The surface sizing composition was tested as is and with the addition of a polyaluminium solution PAC at a dosage of 1.3, 2.5 and 3.9 wt% of solid content. The polyaluminium solution used contained aluminium Al 3+9 wt%; Al2O3 17 wt%; Chloride Cl - 21 wt%; basicity 42%. The sizing results are given in Table 2.

[0050] Table 2. Sizing properties of the surface sizing composition of Example 1

[0051]

[0052] Example 2

[0053] A surface sizing composition having a dry solids content of 12 wt % was prepared according to the above description.

[0054] In reference composition 4, lignin sulfonate (Ufoxane, Borregaard, Norway) was used instead of lignin-hemicellulose composite (same as Example 1). The components of the surface sizing composition are given in Table 3. The weight % of each component is given as weight %, calculated on total dry solids.

[0055] The surface sizing composition was tested as is and with the addition of a 39 wt % aluminum sulfate solution at dosages of 3.6, 7.3 and 10.9 wt % of the solids content of the surface sizing composition. The sizing results are given in Table 4.

[0056] Table 3 Surface sizing composition of Example 2

[0057] Lignin-hemicellulose complex starch Lignin sulfonate Test composition 4 33 67 Reference composition 4 67 33

[0058] Table 4 Sizing properties of the surface sizing composition of Example 2

[0059]

[0060] Example 3

[0061] Example 3 investigated the effect of a lignin-hemicellulose complex (Ecohelix AB, Sweden) as a partial replacement for starch in a surface sizing composition.

[0062] The following surface sizing composition having a dry solids content of 12 wt% was prepared according to the above description:

[0063] Size A: 100% starch, reference

[0064] Compound B: 90% starch, 10% lignin-hemicellulose composite

[0065] Compound C: 80% starch, 20% lignin-hemicellulose composite

[0066] Compound D: 70% starch, 30% lignin-hemicellulose composite

[0067] The proportions of the components in the surface sizing composition are given as weight % of dry solids.

[0068] The strength and hydrophobicity results obtained are given in Table 5.

[0069] Table 5 Sizing performance results of the surface sizing composition of Example 3

[0070]

[0071] Example 4

[0072] Example 4 investigates the effect of lignin-hemicellulose complex as a partial replacement of starch in a surface sizing composition together with polyaluminium chloride (PAC). Lignin-hemicellulose complex and polyaluminium chloride are as defined in Example 1.

[0073] The following surface sizing composition having a dry solids content of 12 wt% was prepared according to the above description:

[0074] Compound A1: 100% starch, reference

[0075] Compound B1: 100% starch, 2% PAC

[0076] Compound C1: 90% starch, 10% lignin-hemicellulose complex, 2% PAC

[0077] Compound D1: 80% starch, 20% lignin-hemicellulose complex, 2% PAC

[0078] Compound E1: 70% starch, 30% lignin-hemicellulose complex, 2% PAC

[0079] Compound F1: 90% starch, 10% lignin-hemicellulose complex, 1% PAC

[0080] Compound G1: 80% starch, 20% lignin-hemicellulose complex, 1% PAC

[0081] Compound H1: 70% starch, 30% lignin-hemicellulose complex, 1% PAC

[0082] The proportions of the components in the surface sizing composition are given as weight % of dry solids.

[0083] The strength and hydrophobicity results obtained are given in Table 6.

[0084] Table 6 Sizing performance results of the surface sizing composition of Example 4

[0085]

[0086] The project leading to this application has received funding from the Biobased Industries Consortium (JU) under grant agreement No 837866. JU is supported by the European Union’s Horizon 2020 research and innovation programme and the Biobased Industries Consortium.

[0087] Even though the present invention has been described with reference to what currently appear to be the most practical and preferred embodiments, it should be understood that the present invention should not be limited to the above-described embodiments, but is intended to also cover different modifications and equivalent technical solutions within the scope of the appended claims.

Claims

1. A surface sizing composition for surface sizing of paper or paperboard, wherein: The composition comprises at least: - polysaccharides selected from starch, - Aluminium compounds, and - anionic lignin-carbohydrate complexes, wherein lignin and one or more carbohydrates are covalently bound to each other, the carbohydrates in the anionic lignin-carbohydrate complexes are formed by monosaccharides, the anionic lignin-carbohydrate complexes have a weight average molecular weight MW of > 12000 g / mol, the anionic lignin-carbohydrate complexes have a lignin:carbohydrate ratio of 90:10 to 10:90, The surface sizing composition comprises the anionic lignin-carbohydrate complex in an amount of 9-99 wt %, calculated on the dry solids content of the surface sizing composition, The composition comprises the aluminum compound in an amount of 0.1 to 6% by weight given as aluminum ions in the dry solids of the surface sizing composition, The composition comprises the starch in an amount of 15-91 wt % calculated on the dry solid content of the surface sizing composition.

2. The composition according to claim 1, characterized in that The anionic lignin-carbohydrate complex comprises anionic functional groups selected from sulfonic acid groups, carboxyl groups and / or phenolic groups.

3. The composition according to claim 1, characterized in that The anionic lignin-carbohydrate complex has an anionic charge density measured at pH 7 of less than -0.2 meq / g.

4. The composition according to claim 3, characterized in that The anionic lignin-carbohydrate complex has the anionic charge density measured at pH 7 of -0.2 meq / g to -2.5 meq / g.

5. The composition according to claim 1 or 2, characterized in that The anionic lignin-carbohydrate complex comprises residues of galactose, glucose, mannose, arabinose, xylose covalently bound to lignin.

6. The composition according to claim 1 or 2, characterized in that The aluminum compound is selected from polyaluminum chloride, alum and aluminum sulfate.

7. The composition according to claim 1 or 2, characterized in that The composition has a solids content of 1-30 wt % calculated as dry solids.

8. Use of the surface sizing composition according to any one of claims 1 to 7 for surface sizing of a sheet-like fiber substrate selected from paper or paperboard.

9. The use according to claim 8, characterized in that The composition is used for surface sizing of paperboard containing recycled fibers.

10. Use of the anionic lignin-carbohydrate complex as claimed in any one of claims 1 to 7 as a hydrophobic agent in a surface sizing composition for paper or board as claimed in any one of claims 1 to 7, wherein the carbohydrates in the anionic lignin-carbohydrate complex are formed from monosaccharides.

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