PRODUCTION OF HYDROPHOBIC PAPER

FIELD OF THE INVENTION

[0001]The present invention relates to the field of hydrophobic paper production. In particular, a method of producing hydrophobic paper, using a sizing additive comprising a water-soluble chemically modified kraft lignin and a multivalent metal salt is described. The invention further relates to a hydrophobic fiber-based product, such as a paper-product, obtainable by such method, and to the use of the sizing additive and metal salt in the production of hydrophobic fiber-based products, such as paper products.

BACKGROUND

[0002]Lignin is one of the two major components of lignocellulose in plants. Structurally, it is a complex polymer made up of three phenyl-propanoid monomers. These monomers are commonly known as p-coumaryl alcohol, coniferyl alcohol and sinapyl alcohol and are present in varying proportions in the lignin of different plants. Lignin in wood behaves as an insoluble, three-dimensional network (cf. Ullmann's Encyclopedia of Industrial Chemistry, 6^th ed. 2003, Vol. 19, p. 512-513). Two main types of lignin are commercially available as a by-product of papermaking.

[0003]Lignosulfonates (also known as sulfite lignins) are byproducts of sulfite pulping. In this process, wood chips are treated with solutions of sulfite and bisulfite ions. Kraft lignins (also known as sulfate lignins) are obtained from black liquor, a byproduct of kraft pulping. In this process, wood chips are treated with so-called white liquor, a mixture of sodium hydroxide (NaOH), and sodium sulfide (Na₂S) and hot water. Kraft lignins are precipitated from black liquor with sulfuric acid, hydrochloric acid, or carbon dioxide. Lignosulfonates and kraft lignins differ greatly in their physical and chemical properties. For example, kraft lignins are known to be insoluble in water, unless the water is strongly alkaline, i.e., has a pH of >10.5.

[0004]Sizing agents, such as alkenyl succinic anhydride (ASA) or alkyl ketene dimer (AKD), are commonly used in the paper making industry as components in sizing dispersion formulations, for obtaining paper products with reduced tendency when dry to absorb liquid, and for improving printing properties.

[0005]WO 2002/033172 A1 describes a sizing dispersant system comprising sodium lignosulfonate, which can be used to obtain water-repellant properties in the paper.

[0006]WO 2018/169459 A1, the content of which is herein incorporated by reference, relates to a method of preparing a sizing boost additive comprising a lignin oil/polysaccharide blend, wherein the lignin oil is obtained by base catalyzed depolymerization of lignin.

[0007]The sizing boost additive can be used for the production of hydrophobic paper together with a hydrophobization agent such as ASA or AKD.

[0008]WO 2019/207048 A1, the content of which is herein incorporated by reference, relates to a method of producing hydrophobic paper, using a blend of a depolymerized lignin, which does not significantly dissolve in aqueous solution at approximately neutral pH and room temperature, and an auxiliary component as sizing additive.

[0009]WO 2017/192281 A1 relates to a composition and method for imparting paper and paperboard with resistance to aqueous penetrants using a sizing additive comprising a renewable biopolymer in combination with a water-soluble, hydroxylated polymer, e.g. starch. The renewable biopolymer is an alkaline solution or dispersion of crude or purified lignin. WO 2017/192281 A1 does not disclose a sizing additive comprising water-soluble chemically modified lignin as defined hereinbelow.

[0010]The use of hydrophobization agents (“sizing agents”) such as ASA or AKD leads to a substantial increase of costs in the paper manufacturing process. Further, papers manufactured with hydrophobization agents are subject to size reversion resulting in an undesired increase in water-absorption after prolonged heat or UV light exposure. Moreover, with ecologically sustainable production becoming more and more important, the use of fossil origin compounds like ASA and AKD is less desired. Other methods of the prior art mentioned above are relatively complex, thus increasing costs as well.

BRIEF SUMMARY OF THE INVENTION

[0011]Against this background, it is an object of the present invention to provide a novel sizing additive which can contribute to eliminate the use of traditional hydrophobization agents such as ASA, AKD and/or rosin in the manufacture of hydrophobic paper products, and to improve properties in such paper products. It was a further object to provide a novel method for producing hydrophobic paper.

[0012]This objective is solved by the invention as defined herein.

[0015]An advantage of the method according to the invention, using a chemically modified kraft lignin, such as for example maleic anhydride-modified Lignoboost®-lignin, as hydrophobicity/sizing agent does not require any addition of ASA or AKD. The inventors have surprisingly found that the method of the invention enables use of practically the whole lignin fraction, which leads to a very high yield. Further, the hydrophobization method of the invention is less complex than methods from the prior art. By chemically modifying the lignin so as to be charged and water-soluble, the lignin can be directly added to a pulp suspension of a paper machine together with the metal salt. There is no need for emulsifying actions, or similar chemical preparations. Another advantage, especially from a cost perspective, is that due to the high yield there is basically no residue product to handle.

[0016]In a further aspect, the invention relates to a hydrophobic fiber-based product, such as a hydrophobic paper product, which is obtainable by a method as described herein.

[0017]In yet a further aspect, the invention relates to the use of water-soluble chemically modified kraft lignin, particularly water-soluble chemically modified kraft lignin having an average molecular weight of about 3000 g/mol to about 10000 g/mol, which at a concentration of 100 g/L has a soluble fraction of at least 90 wt % at neutral pH and room temperature as the sole sizing additive, together with a multivalent salt and optionally further together with a cationic polymer, particularly a cationic polysaccharide, in the production of a hydrophobic fiber-based product, particularly a hydrophobic paper product.

DETAILED DESCRIPTION OF THE INVENTION

[0018]As mentioned hereinabove, the present invention particularly relates to a new way of making a paper product hydrophobic (also referred to as “sizing”). To this end, a so-called sizing additive (sometimes also referred to as “sizing agent” herein) comprising chemically modified kraft lignin, is used. Sizing takes place in the paper mill by addition of the modified lignin to a lignocellulosic pulp suspension.

[0019]Accordingly, the invention relates to a method of producing a hydrophobic fiber-based product, particularly a hydrophobic paper product, comprising a step of adding (i) a sizing additive comprising a water-soluble chemically modified kraft lignin; and (ii) a multivalent metal salt; to a lignocellulosic pulp suspension at the wet end of a paper manufacturing process. In the context of the invention, the water-soluble chemically modified kraft lignin at a concentration of 100 g/L has a soluble fraction of at least 90 wt % at neutral pH and room temperature.

[0020]In certain embodiments, the invention relates to a method of producing a hydrophobic fiber-based product, particularly a hydrophobic paper product, comprising a step of adding (i) a sizing additive comprising a water-soluble chemically modified kraft lignin; and (ii) a multivalent metal salt; to a lignocellulosic pulp suspension at the wet end of a paper manufacturing process, wherein the lignin has an average molecular weight of about 3000 g/mol to about 10000 g/mol, and wherein the water-soluble chemically modified kraft lignin of the sizing additive (i) is added in an amount of about 0.5 kg/t dry pulp to about 15 kg/t dry pulp. In the context of the invention, the water-soluble chemically modified kraft lignin at a concentration of 100 g/L has a soluble fraction of at least 90 wt % at neutral pH and room temperature,

[0021]In certain embodiments, the hydrophobic fiber-based product according to the invention is a hydrophobic paper product, for example a hydrophobic kraft paper, kraft liner, test liner, carton board, sack paper, flexible paper.

[0022]In one embodiment, the hydrophobic fiber-based product is hydrophobic paper, which is used in packaging applications.

[0023]The fiber source can be virgin chemical pulp (kraft pulp, sulfite pulp etc.), virgin semichemical, virgin mechanical (e.g., RMP, TMP, SGW, CTMP) or recycled fibers. Apart from pulp fibers, the hydrophobic paper may contain an inorganic filler. Suitable inorganic fillers for hydrophobic papers include, but are not limited to ground calcium carbonate (GCC), precipitated calcium carbonate (PCC), kaolin, titanium dioxide, talc, alumina, silica and silicates.

[0024]In one embodiment the hydrophobic paper is composed of several layers, of which at least one layer contains a sizing additive as described herein, i.e., a water-soluble chemically modified kraft lignin.

[0025]In one embodiment the hydrophobic paper product is made of unbleached fibers (having a dark color that matches the brown color of the sizing additive according to the invention).

[0026]In certain embodiments of the invention, the sizing additive essentially consists of a water-soluble chemically modified kraft lignin, which, at a concentration of 100 g/L has a soluble fraction of at least 90 wt % at neutral pH and room temperature. In this context, “essentially consists of” means that at least about 95 wt %, such as at least 95 wt %, particularly at least about 97 wt %, at least about 98 wt %, at least about 99 wt %, or even at least about 99.5 wt % of the total sizing additive is water-soluble chemically modified kraft lignin.

[0027]In certain preferred embodiments, the sizing additive consists of a water-soluble chemically modified kraft lignin, which, at a concentration of 100 g/L has a soluble fraction of at least 90 wt % at neutral pH and room temperature.

[0028]The term “hydrophobic” refers to a surface exhibiting water repelling properties. The term “hydrophobic paper” as used herein relates to paper which has been subjected to internal sizing during its manufacture. “Water-soluble” as used herein means that at a concentration of 100 g/L the fraction of the chemically modified kraft lignin soluble in water is at least 90 wt % of the total mass of lignin at neutral pH and room temperature, i.e., only 10 wt % or less of the lignin remain in solid state. In certain embodiments, the soluble fraction at a concentration of 100 g/L, neutral pH and room temperature may be at least 95 wt % or even at least 97 wt % of the total mass of lignin.

[0029]As used herein, a “chemically modified kraft lignin” is a kraft lignin that has been subjected to a chemical reaction with a suitable reactant, leading to a covalent modification of the kraft lignin. The kraft lignin according to the invention may have been obtained by separating it (e.g., Lignoboost® procedure) from black liquor after kraft pulping. A common procedure, as used in the LignoBoost®, LignoForce, and Liquid Lignin processes, is to precipitate the kraft lignin from the black liquor by lowering the pH. Carbon dioxide is commonly added for this purpose. The precipitate is separated from the liquor, and the lignin-rich solid is typically further purified (washed, optionally dried) or modified downstreams. A reactant is suitable if, after the reaction, the obtained modified kraft lignin has a high solubility in water. That is, the modified kraft lignin after the reaction has a soluble fraction of at least 90 wt % at a concentration of 100 g/L neutral pH and room temperature. Unless indicated otherwise, the pressure in the context of the invention is atmospheric pressure. Suitable reactants for the modification of kraft lignin according to the invention include, but are not limited to esterification reagents, etherification reagents, oxidation reagents and sulfonation reagents. In certain embodiments, the reactant to modify kraft lignin is an esterification reagent.

[0030]An exemplary sulfonation reagent according to the invention is sodium sulfite. Exemplary oxidation reagents according to the invention are oxygen, peroxides and ozone. Exemplary esterification reagents according to the invention are dicarboxylic acid anhydrides.

[0031]In the context of the present invention, “neutral pH” means a pH in the range of from about 7.0 to about 7.5, particularly 7.0 to 7.4, more particularly 7.0 to 7.2. “Room temperature” or “ambient temperature” as used herein means a temperature in the range of from about 20° C. to about 25° C., particularly 20° C. to 25° C.

[0032]The chemically modified kraft lignin according to the invention particularly may have a charge density of ionic groups of at least 0.4 mmol/g, particularly at least 0.6 mmol/g, or at least 0.8 mmol/g, as measured by polyelectrolyte titration at pH 7. The ionic groups may be particularly selected from the group consisting of carboxyl, sulfonate, sulfate, phosphate and amine groups. In certain embodiments, the charge preferably exceeds 0.4 mmol/g measured by polyelectrolyte titration at pH 7 (for further details on the measurement, see section “Charge Measurements” in Examples A1-A6 below).

[0033]As indicated above, esterification reagents, etherification reagents, oxidation reagents and sulfonation reagents are suitable to obtain a chemically modified kraft lignin according to the invention. Depending on the reagent used, different chemically modified lignins may be obtained. In some embodiments, when one or more dicarboxylic acid anhydride is used as esterification reagent, the chemically modified kraft lignin is a dicarboxylic acid anhydride-modified kraft lignin. In other embodiments, when a sulfonation reagent is used, the chemically modified kraft lignin is sulfonated kraft lignin. In yet other embodiments, when an oxidation reagent is used, the chemically modified kraft lignin is oxidized kraft lignin. In yet other embodiments, when an etherification reagent is used, the chemically modified kraft lignin is etherified kraft lignin.

[0034]Where, in the context of the invention, the chemically modified kraft lignin is a dicarboxylic acid anhydride-modified kraft lignin, a variety of anhydrides can advantageously be used. In particular, the dicarboxylic acid anhydride-modified kraft lignin may be a kraft lignin modified with a cyclic acid anhydride. In certain embodiments, the dicarboxylic acid anhydride may be a cyclic acid anhydride having a molecular weight of 220 g/mol or less. The cyclic anhydride may particularly be selected from the group consisting of maleic anhydride; succinic anhydride; citraconic anhydride; itaconic anhydride; 2,3-dimethylmaleic anhydride; ethylsuccinic anhydride; 2,2 dimethylsuccinic anhydride; phthalic anhydride; quinolinic anhydride; diacetyl-tartaric anhydride; tetramethylsuccinic anhydride; diglycolic anhydride; and glutaric anhydride.

[0035]In particular embodiments, the dicarboxylic acid anhydride has a boiling point of 100° C. or more at atmospheric pressure.

[0036]In further particular embodiments, the cyclic anhydride may be selected from the group consisting of maleic anhydride, succinic anhydride and glutaric anhydride. In certain preferred embodiments, the cyclic anhydride is maleic anhydride. Accordingly, the dicarboxylic acid anhydride-modified kraft lignin is, according to certain preferred embodiments of the invention, maleic anhydride (MA)-modified kraft lignin.

[0037]In these embodiments, the MA-modified kraft lignin may have been treated with an amount of, e.g., 1-50 wt %, particularly 1-20 wt %, preferably about 2-10 wt %, such as 2-10 wt % maleic anhydride relative to lignin. In exemplary embodiments, the MA-modified kraft lignin has been treated with an amount of about 10 wt % maleic anhydride relative to lignin.

[0038]According to certain embodiments, it may be useful that the chemically modified kraft lignin has a certain average molecular weight. Accordingly, the lignin may have an average molecular weight of at least about 3000 g/mol. In particular, the chemically modified kraft lignin may have an average molecular weight of at least about 4000 g/mol. In particular embodiments, the chemically modified kraft lignin may have an average molecular weight of about 3000 g/mol to about 10000 g/mol, or about 3000 g/mol to about 8000 g/mol, or about 4000 g/mol to about 6000 g/mol, such as 3000 g/mol to 10000 g/mol, or 3000 g/mol to 8000 g/mol, or 4000 g/mol to 6000 g/mol. In further particular embodiments, the chemically modified kraft lignin may have an average molecular weight of about 4000 g/mol to about 10000 g/mol, or about 5000 g/mol to about 10000 g/mol, or about 5000 g/mol to about 9000 g/mol, such as 4000 g/mol to 10000 g/mol, or 5000 g/mol to 10000 g/mol, or 5000 g/mol to 9000 g/mol. In examples, the chemically modified kraft lignin may have an average molecular weight of about 5000 g/mol to about 9000 g/mol, such as 5000 g/mol to 9000 g/mol.

[0039]It is further encompassed by the invention that the sizing additive (i) is essentially free of depolymerized lignin. In this context, “essentially free of” means that the sizing additive contains less than 6 wt %, particularly less than 3 wt % of depolymerized lignin. As used herein, “depolymerized lignin” means lignin that (i) has a low molecular weight, i.e., 400 to at most 2500 g/mol, and (ii) has been further decomposed, beyond the effect of the kraft process.

[0040]In the method of producing a hydrophobic fiber-based product, such as a hydrophobic paper product, according to the invention, the water-soluble chemically modified kraft lignin of the sizing additive (i) may particularly be added in an amount of about 0.5 kg/t dry pulp to about 15 kg/t dry pulp. More particularly, it may be added in an amount of about 1 kg/t dry pulp to about 10 kg/t dry pulp, or in an amount of about 1 kg/t dry pulp to about 6 kg/t dry pulp. In certain embodiments, the water-soluble chemically modified kraft lignin of the sizing additive (i) may be added in an amount of about 1.5 kg/t dry pulp to about 10 kg/t dry pulp, or in an amount of about 2 kg/t dry pulp to about 6 kg/t dry pulp.

[0041]In addition to the sizing additive, a multivalent metal salt is added to the lignocellulosic pulp suspension from which the hydrophobic paper shall be produced. In the context of the invention, “multivalent” means a metal salt wherein the metal has a cation valence of greater than 1. Particularly, the multivalent metal salt is at least one salt selected from a di- or trivalent metal salt. More particularly, the multivalent metal salt is at least one trivalent metal salt.

[0042]Nonlimiting examples of cations forming divalent metal salts are Ca²⁺, Cu²⁺, Fe²⁺, Mg²⁺, Mn²⁺, and Zn²⁺. In exemplary embodiments, the divalent metal salt is an iron (II) salt. Nonlimiting examples of cations forming trivalent metal salts are Al³⁺, Bi³⁺, Fe³⁺, and Mn³⁺, particularly Al³⁺ and Fe³⁺. In exemplary embodiments, the trivalent metal salt is an aluminum (III) salt.

[0043]According to some embodiments, the multivalent metal salt is at least one aluminum salt, at least one iron salt, or any mixture thereof.

[0044]In specific embodiments of the method according to the invention, the multivalent metal salt is an aluminum salt selected from an aluminum chloride salt, an aluminum sulfate salt, and/or any combination thereof.

[0045]An exemplary aluminum chloride salt according to the invention is polyaluminum chloride (PAC). The general structure of PAC is [Al₂(OH)_nCl_6-n]_m where (1≤n≤5, m≤10). For example, a commercially available PAC salt (e.g., a Fennofloc PAC product), having a content of pure Al calculated as 9 wt %, may be added.

[0046]An exemplary aluminum sulfate salt according to the invention is alum (Al₂(SO₄)₃·n H₂O). For example, an alum salt such as WiAL having a content of pure Al calculated as 4 wt % may be added.

[0047]In certain embodiments of the method according to the invention, the aluminum salt is added to the lignocellulosic pulp in an amount corresponding to 0.01 to 1 kg Al/t dry pulp. In other embodiments, the aluminum salt is added in an amount corresponding to 0.05 to 0.5 kg Al/t dry pulp.

[0048]In certain embodiments of the method according to the invention, the aluminum salt is a PAC solution, added to the lignocellulosic pulp in an amount of about 0.1 to about 10 kg/t dry pulp, such as in an amount of 0.1 to 10 kg/t dry pulp. In other embodiments, the PAC solution is added in an amount of about 0.5 to about 5 kg/t, or about 0.5 to about 3 kg/t, or about 1 to about 3 kg/t, or about 1 to about 5 kg/t dry pulp, such as in an amount of 0.5 to 5 kg/t, or 0.5 to 3 kg/t, or 1 to 3 kg/t, or 1 to 5 kg/t dry pulp.

[0049]In specific embodiments of the method according to the invention, the multivalent metal salt is an iron salt selected from iron (III) salts, preferably soluble types including iron(III) chloride, iron(III) sulfate, iron(III) nitrate, and iron(III) citrate, and/or any combination thereof.

[0050]A benefit of the aluminum or iron salt addition is that the hydrophobic effect achieved by the modified lignin has long term effect, being more resistant to UV-light.

[0051]In the method according to the invention, the sizing additive may be added first and then the multivalent metal salt is added to the pulp. There may also be embodiments where the sizing additive is added after the multivalent metal salt, or simultaneously with the multivalent metal salt.

[0052]The chemically modified kraft lignin and multivalent metal salt may for example be added in a weight ratio of from about 1:3 to about 10:1, or from about 1:3 to about 3.5:1, such as 1:3, 1:2.5, 1:1, 2.5:1, 3.33:1, 3.5:1 or 4:1. Exemplary ratios of lignin to dry aluminum salt are 1:3, 1:2.5, 1:1, 3.33:1, or 4:1.

[0053]To attach the anionic lignin polymers to the negatively charged fiber surface, it is possible to use a cationic polymer. Accordingly, in addition to the sizing additive and the multivalent metal salt, the method according to the invention may further comprise a step of adding a cationic polymer to the lignocellulosic pulp suspension at the wet end of a paper manufacturing process.

[0054]The cationic polymer functions as a retention agent. This polymer can be selected from a variety of compounds. According to particular embodiments, the cationic polymer may be selected from cationic polysaccharides, polyethyleneimines, poly-diallyldimethylammonium chloride (polyDADMAC), polyamines and cationic polyacrylamides (CPAM), and/or any combination thereof. An exemplary cationic polymer that may be used is cationic starch or gelatinized cationic starch.

[0055]In certain embodiments, the cationic polymer, if present, may be added in the form of a complex with the water-soluble chemically modified kraft lignin. In other embodiments, the cationic polymer, if present, may be added separately from the water-soluble chemically modified kraft lignin. When the cationic polymer is added separately from the kraft lignin, it is particularly added at a different entry point than the kraft lignin into the process.

[0056]The cationic polymer may generally be added in an amount of, e.g., about 0.01 to about kg/t dry pulp. In exemplary embodiments, the cationic polymer may be added in an amount of about 0.01 to about 0.2 kg/t dry pulp, or about 3 to about 5 kg/t dry pulp, or about 3 to about 10 kg/t dry pulp, or in an amount of about 10 kg/t dry pulp. For instance, in some embodiments, the cationic polymer is CPAM and is added in an amount of about 0.01 to about 0.2 kg/t dry pulp, particularly about 0.2 kg/t dry pulp. In other embodiments, the cationic polymer is cationic starch and is added in an amount of about 3 to about 5 kg/t dry pulp, or about 3 to about 10 kg/t dry pulp.

[0057]The weight ratio of the lignin and cationic polymer may be chosen from a wide range. In particular embodiments, the water-soluble chemically modified kraft lignin and cationic polymer, e.g., cationic polysaccharide, are added in a weight ratio of from about 1:10 to about 1:1. Exemplary ratios of lignin to cationic polymer according to the invention are 1:10, 1:3.33 or 1:1.

[0058]Optionally, the method according to the invention may further comprise a step of adding at least one further additive, which may be selected from dry-strength agents, wet-strength agents, enzymes, and/or any combination thereof.

[0059]Suitable dry-strength agents include acrylamide copolymers, starch, guar gum, polyamines, polyvinyl alcohol, and/or any combination thereof.

[0060]Suitable wet-strength agents include urea-formaldehyde (UF), melamine-formaldehyde (MF), polyaminoamide-epichlorohydrin (PAE), glyoxylated polyacrylamide, polyamines, and/or any combination thereof.

[0061]Suitable enzymes include cellulases, hemicellulases, xylanases, amylases, laccases, peroxidases, and/or any combination thereof.

[0062]The lignocellulosic pulp suspension to which the sizing additive is added may be selected from any suitable type of pulp suspension, e.g. a kraft pulp suspension, a recycled fiber pulp suspension or a suspension comprising a mixture of kraft pulp and recycled fibers, e.g. recycled fibers in an amount of up to about 50%, about 60%, about 70% or about 80% (w/w) as well as a suspension of bleached pulp, a suspension of chemi-thermomechanical pulp (CTMP), a suspension of sulfite pulp, a suspension of mechanical pulp (MP), a suspension of semi chemical pulp and/or any combination thereof.

[0063]The pH of the lignocellulosic pulp suspension may be in the range of from about 5 to about 10, particularly in the range of from about 7 to about 8.5.

[0064]One of the advantages of the method according to the invention is, as already mentioned, that when using a sizing additive comprising, essentially consisting of, or consisting of the chemically modified kraft lignin as described above, it is not required to rely on compounds of fossil origin, such as alkenyl succinic anhydride (ASA) or alkyl ketone dimer (AKD). Accordingly, in preferred embodiments of the invention, no further hydrophobization agent in addition to water-soluble chemically modified kraft lignin is used in carrying out the method. In particular, no ASA, no AKD and no rosin is used in carrying out the method.

[0065]In a further aspect, the present invention relates to a hydrophobic fiber-based product, obtainable by the method described herein. The hydrophobic fiber-based product particularly is a hydrophobic paper product.

[0066]After addition of the modified kraft lignin and the (optional) cationic polymer, the fiber suspension is formed to a sheet of paper and dried. The hydrophobicity/sizing effect can be measured by a Cobb test, which determines the amount of water absorbed into the surface by a paper sample in a set period of time, usually 60 or 1800 seconds (Cobb₆₀ or Cobb₁₈₀₀). Water absorbency is quoted in g/m². Paper samples can also be tested after exposure of UV-light under standardized conditions. A standard procedure to perform the Cobb test is according to norm ISO 535.

[0067]The below table shows Cobb values for sheets made of unbleached kraft pulp with addition of MA-modified kraft lignin (type L3 lignin, denoted in accordance with the Example section) and cationic starch. By means of the invention, low Cobb values are achieved, which means high hydrophobicity.

[0068]In particular embodiments of the invention, the hydrophobic fiber-based product, e.g., hydrophobic paper product, is characterized in that it has a Cobb 60 value of 40 g/m² or less, particularly 30 g/m² or less, as determined according to ISO 535.

[0069]In further particular embodiments of the invention, the hydrophobic fiber-based product, e.g., hydrophobic paper product, is characterized in that it has a Cobb 1800 value of 100 g/m² or less, particularly 80 g/m² or less, as determined according to ISO 535.

[0070]In further particular embodiments of the invention, the hydrophobic fiber-based product, e.g., hydrophobic paper product, is characterized in that it has a Cobb 1800_{UV 48h} value of 110 g/m² or less, particularly 100 g/m² or less, as determined according to ISO 535 after the UV treatment.

[0071]In further particular embodiments of the invention, the hydrophobic fiber-based product, e.g., hydrophobic paper product, is characterized in that it has at least two of the above-mentioned Cobb values. Accordingly, the product may have a Cobb 60 value of 40 g/m² or less, particularly 30 g/m² or less, and a Cobb 1800 value of 100 g/m² or less, particularly 80 g/m² or less; or a Cobb 60 value of 40 g/m² or less, particularly 30 g/m² or less, and a Cobb 1800_{UV 48h} value of 110 g/m² or less, particularly 100 g/m² or less; or a Cobb 1800 value of 100 g/m² or less, particularly 80 g/m² or less, and a Cobb 1800_{UV 48h} value of 110 g/m² or less, particularly 100 g/m² or less; each as determined according to ISO 535.

[0072]In still further particular embodiments of the invention, the hydrophobic fiber-based product, e.g., hydrophobic paper product, is characterized in that it has a Cobb 60 value of 40 g/m² or less, particularly 30 g/m² or less; and a Cobb 1800 value of 100 5 g/m² or less, particularly 80 g/m² or less; and a Cobb 1800_{UV 48h} value of 110 g/m² or less, particularly 100 g/m² or less; each as determined according to ISO 535.

[0073]In preferred embodiments of the invention, the hydrophobic fiber-based product, e.g., hydrophobic paper product, is free of a conventional sizing additive comprising an ASA, an AKD, a rosin and any reaction product thereof. In particular, the invention enables hydrophobic fiber-based products free of conventional sizing additives of fossil origin, such as ASA and AKD.

[0074]In a further aspect, the invention relates to the use of water-soluble chemically modified kraft lignin as defined hereinbefore, which at a concentration of 100 g/L has a soluble fraction of at least 90 wt % at neutral pH and room temperature as the sole sizing additive, together with a multivalent salt as defined hereinbefore in the production of a hydrophobic fiber-based product, e.g., a hydrophobic paper product. In certain embodiments, the water-soluble chemically modified kraft lignin and multivalent metal salt are used together with a cationic polymer, particularly a cationic polysaccharide, as defined hereinbefore.

[0075]In addition to the advantages already mentioned, advantages of the invention further include a high yield (about 90-100%) due to the very high solubility of the sizing agent. By means of the present invention, solubility is so high that there is normally no need to separate and remove the non-soluble fraction. Furthermore, the method is cost efficient, since lower dosage of compounds for achieving hydrophobicity is needed.

[0076]As mentioned, a further advantage is that a biobased (mainly or completely fossil-free) product can be used as sizing agent. Finally, the method according to the invention involves a straightforward procedure of adding the sizing agent, directly to the fiber suspension in the paper process.

LEGENDS TO THE FIGURES

[0134]FIG. 1: Graphical representation of results of solubility tests (for details, see also Table 5 below). FIG. 1 illustrates the relationship of lignin concentration and insoluble residue, relative fraction. A dashed line is inserted to indicate acceptance level for a water-soluble lignin product, maximum 10% solid residues.

[0135]FIG. 2: FIG. 2A-2B illustrate added amount of lignin and Cobb value for papers treated with modified kraft lignin and reference samples. The samples of FIG. 2A are unexposed, whereas the samples of FIG. 2B are UV-treated for 48 hours. Target values (limits) are indicated with lines.

EXAMPLES

Molecular Weight of the Lignin Feedstocks

[0144]The analysis method was based on size exclusion chromatography (SEC) with a UV detector (256 nm). It was calibrated with polystyrene sulfonate standards with known molecular weights (122-679 000 g/mol). The eluent was a borate buffer with 10% methanol and pH 10.3. Each lignin sample (50-100 mg) was dissolved in 0.5 ml 1 M sodium hydroxide and further diluted with eluent solution, followed by adjustment of pH.

Example A: Synthesis of Chemically Modified Kraft Lignins

Examples A1-A6: Synthesis of Maleic Anhydride (MA) Lignin Esters (A1-A6)

Reaction Conditions

[0145]Lignin was pre-dried at 70° C. for at least 24h. Lignin and acetone were mixed into a slurry in a stirred vessel. MA was added. A reflux condenser was mounted to the vessel and the temperature was increased to the setpoint (23-56° C.).

Molecular Weight of MA Lignin

[0151]Due to the sensitivity of the ester bond of the MA lignin to hydrolysis, the average molecular weight was estimated by calculations based on increase in charged substituent.

[0152]The carboxylic content of unmodified NBSK kraft lignin has been reported in literature to be about 0.39-0.59 meq/g (Nagy et al. Green Chemicals 12, 31, 2010).

[0153]The charge densities of the MA lignin esters prepared is up to 1.7 meq/g (cf. Table 2). Based on these values, the net charge increase is up to about 1.3 meq/g. Assuming that each additional charge corresponds to a substituent of maleic acid entity carrying a free carboxylate group with a corresponding sodium ion, the relative weight increase can be calculated from the charge increase and the molecular weight of this substituent. The molecular weight of the substituent is 120 g/mol and the calculation predicts an approximate relative weight increase of 16 wt %.

[0154]In the examples using Kraft lignin L1 and L2 as starting materials, this implies that the MA modified lignins would have an average molecular weight of 5500 g/mol and 5300 g/mol, respectively.

Charge Measurements

[0155]The completion of the reaction was analyzed by measuring the charge density of the modified lignin. Solutions for charge measurements were prepared at 1 g/L lignin concentration with an addition of 10 mM NaHCO₃ buffer. Each solution was pre-treated by boiling it on a hotplate for 10 min. The pH was then adjusted to 7 and the concentration was adjusted with deionized water. The charge was measured by polyelectrolyte titration against a standard polyDADMAC on a Mütek Particle Charge detector from BTG a Voith Company.

Examples A7-A8: Synthesis of Oxidized Kraft Lignin

[0156]A stock solution of lignin was prepared by dissolving 15 wt % lignin in 1 M NaOH (aq). A pre-determined amount of the lignin solution (see Table 3) was transferred to a 1 L autoclave. Still at room temperature, the autoclave was charged with 5 bar oxygen pressure, after which the autoclave was closed. Oxidation was performed by heating the autoclave to 80° C. and holding this temperature throughout the reaction time (either or 120 min, see Table 3). The autoclave was allowed to cool down to room temperature before the pressure was released and the product was collected.

[0157]The completion of the reaction was analyzed by measuring the charge density of the modified lignin. Polyelectrolyte titration, as described above for examples A1-A6, was used.

Examples A9-A10: Synthesis of Sulfonated Kraft Lignin

[0158]Sodium hydroxide, sodium sulfite and water were charged into a 5 L double-mantled autoclave equipped with an internal circulation loop. Table 4 shows the amounts and reaction conditions used in each experiment. The reactant mixture was pre-heated to 70° C. before the lignin was added in portions. The dispersion was homogenized under internal circulation for 15 min under atmospheric condition. The autoclave was then closed and the heating sequence was initiated. Reaction time was calculated from the moment that the dispersion had reached the target temperature for reaction. At the end of the reaction the product was cooled by feeding cold water through the mantle.

[0159]The completion of the reaction was analyzed by measuring the charge density of the modified lignin. Polyelectrolyte titration, as described above for examples A1-A6, was used.

Example B: Solubility Tests

[0160]Modified lignin products A1, A2, A3, A7 and A9 were tested for their solubility in water. This was studied as a function of the total lignin concentration, ranging from 1-100 g/L. The studied parameter was the relative fraction of insoluble lignin residues, as obtained from the mass of insoluble lignin residues divided by the total lignin content.

[0161]The acceptance level for a water-soluble lignin product has been set at maximum 10% solid residues, as evaluated at a total lignin concentration of 100 g/L.

[0162]The relative fraction of soluble lignin can be calculated according to the formula:

Fraction of soluble lignin=100%−Fraction of insoluble lignin residue

[0163]A summary of the solubility tests is shown in Table 5.

Pre-Treatment

[0164]Prior to the solubility test, the lignin samples were pre-treated by a sequence of adding a NaHCO₃ buffer, heating the solution to 100° C. for 10 min, cooling it to room temperature, and adjusting the pH to 7.

[0167]As reference, solubility tests were performed with neat kraft lignin L1 after pre-treatment with carbonate buffer and heat (Examples B18-B20).

Separation

[0171]The dry mass of each fraction was determined gravimetrically after oven drying. The weight fractions of non-dissolved and dissolved matter were obtained by dividing the mass of each fraction by the total dry mass.

[0172]The preparation of the water solution (pre-treatment) and the separation sequence will now be described with reference to examples B1 and B2.

Example B1

[0173]A water solution containing 10 g/L Lignin product A1 and a buffer, in this example 10 mM NaHCO₃, was prepared in an E-flask. Deionized water was used for dilution. The solution was heated on a hot plate at atmospheric conditions under magnetic stirring. After reaching boiling temperature it was boiled for 10 min (“heating time” in Table 5). The solution was cooled to room temperature, the pH was adjusted to 7, and the lignin concentration was readjusted by adding deionized water to compensate for vapor losses. Separation sequence Filtration: An aliquot of 20 mL of the prepared solution was vacuum-filtrated over a filter paper (Munktell, SE; No. 3) to recover the insoluble residue on the filter paper and the soluble lignin in the filtrate. Both lignin fractions were dried in an oven at 105° C. and the dry mass was analyzed.

Example B2

[0174]Separation sequence Centrifugation: Example B1 was repeated, except that the separation sequence was as follows. An aliquot of 20 mL of the prepared solution was transferred and divided between two centrifugation tubes. Centrifugation was performed at 5000 rpm for 20 min (Sorvall RC-5C, rotor SS-34). The water phase and the precipitated solids were separated manually. Both lignin fractions were dried in an oven at 105° C. and the dry mass was analyzed.

[0175]FIG. 1 is a diagram illustrating the relationship of lignin concentration and insoluble residue. A dashed line is inserted to indicate acceptance level for a water-soluble lignin product, maximum 10% solid residues.

[0176]It is evident from Table 5 and FIG. 1 that all examples of modified kraft lignin used in accordance with the present invention clearly fulfil the criteria for being water-soluble. The references, on the other hand, have a solid fraction in the order of about 20% or more. The results show a very high solubility of the modified kraft lignin. The fraction of solid residue is in general increasing as the concentration of lignin used increases.

Example C: Results on Hydrophobization

[0177]Cobb data for a set of lignin products.

General Procedure for Preparing Lignin Samples for Hydrophobization Tests

[0178]A solution of 1 wt % modified lignin and 10 mM NaHCO₃ buffer was prepared using deionized water for the dilution. The solution was heated to 100° C. after which it was boiled for 10 min at atmospheric pressure. After cooling to room temperature, the solution was adjusted to pH 7.2±0.2 by addition of dilute HCl(aq). The water loss during heating was compensated for by adding deionized water to the final target volume.

Hydrophobization of Paper with Modified Lignin

[0183]The wet sheet was put between two blotting papers and pressed to a density of about 650 kg/m³. The sheets were dried on an STFI dryer (RISE Innventia, SE) for 18 min at 105° C. under restrained conditions. To ensure that drying was fully complete, the sheets were further heated in an oven at 105° C. for 10 min.

[0184]To simulate an aging effect, the sized paper was exposed to UV-light for 48h. This test was run in a viewing cabinette from JustNormlicht, DE. Two types of fluorescent tubes were used to get a combination of D65 and UV light exposure. The tubes had a power of 36 W and 18 W, respectively (JustNormlicht, DE/Philips, NL). The paper samples were positioned at 100 mm distance from the light sources.

Tests on the Paper Hydrophobization Effect

[0185]The hydrophobic effect, i.e. the water repellency of the sheets, was measured using a Cobb test apparatus according to ISO 535. In these tests, the water uptake after 1800 s contact time was determined gravimetrically. Results from the Cobb tests are shown in Table 6 and FIG. 2A-2B.

[0186]FIG. 2A-2B illustrate added amount of lignin and Cobb value for paper treated with modified kraft lignin and reference samples. The samples of FIG. 2A are unexposed, whereas the samples of FIG. 2B are UV-treated for 48 hours. Target values (limits) are indicated with lines. According to the UN specification for dangerous goods (ST/SG/AC.10/1/Rev.22, section 6.1.4.12), the Cobb value of papers for this use must not exceed 155 g/m², indicated with a solid line. A desirable target Cobb value of papers for this use would be 110 or less, as indicated with a dashed line.

[0187]The results show an excellent hydrophobation effect (low Cobb-values) for paper products made from a pulp suspension to which a water-soluble chemically modified kraft lignin has been added in accordance with the present invention.

[0188]In particular, the size reversal effect after UV treatment meets the target values, see FIG. 2B.

[0189]The achieved hydrophobization results and size reversal effects are well in line with current commercial hydrophobization agents, such as ASA, AKD, and rosin sizes.