BIO- strenghtening agent for pulp fiber related paper substrate

Abstract

A bio-strengthening agent for pulp fiber related paper substrate is disclosed comprising modified chitosan, the modified chitosan being the reaction product of chitosan and dialdehyde. The strengthening agent is applied in the process of manufacturing pulp fiber related products such as paper, tissues, napkin, paperboard, linear board, paper-based food containers and paper-based cups to increase wet and dry strength of the products. The synthesized modified chitosan is made from renewable bio-based raw material that and is biodegradable.

Description

February 20, 2026 Archroma IP GmbH 120949P915PCBIO- STRENGHTENING AGENT FOR PULP FIBER RELATED PAPER SUBSTRATE

[0001] The invention relates to a bio-strengthening agent comprising modified chitosan, the modified chitosan being the reaction product of chitosan and a dialdehyde preferably glyoxal, a method of preparing said bio-strengthening agent, and a process of using said additive in the paper making process to increase wet and dry strength of paper substrate such as linear board, container board, paper packaging, tissue and napkin.BACKGROUND OF THE INVENTION

[0002] During the manufacture of paper or paperboard a cellulosic fiber suspension, paper pulp, is formed which is subsequent drained, pressed and dried to form paper substrates. For many applications, it is desired to produce paper substrate products which have high paper strength. Paper strength dependents on the shape of the cellulosic fibers used in the paper making process, the strength of the individual fibers, the number and strength of bond between the fibers. Strength additives are known to enhance paper strength. Such additives can for example be- unmodified, cationic or anionic starch,- vegetable gums,- acrylamide polymers- carboxymethyl cellulose- polyvinyl alcohol- glyoxylate polyacrylamine polymersSuch additives are believed to enhance entanglement of cellulose fibers or to increase the bond strength between individual fibers, thus improving the overall paper strength. Increased paper strength is associated with an enhanced tensile strength, and increased burst-, fold- and tear resistance which are also relevant for increasing the production rate of paper products. Chitosan has also been considered as a paper strengthening agent. Chitosan has gained interest as a paper strengthening agent due to their amino functionality and bio-derivability.February 20, 2026 Archroma IP GmbH 120949P915PC

[0003] Synthetic petroleum-based glyoxylate polyacrylamide (GPAM) and other conventional synthetic paper strength additives include polyamide epichlorohydrin (PAE). Urea-formaldehyde resins are used in a variety of paper, and tissue products to improve dry and wet strength. For example, glyoxylate polyacrylamide (GPAM) may increase the wet strength of household tissue products when it is in contact with water. The widespread use of these petroleum-based wet-strength additives in the paper-making industry has raised environmental and health concerns due to their non-biodegradability and their creating of microplastic that contaminates drinking water and food. The compositions and methods disclosed herein address these and other associated issues.

[0004] In the publication “Laleg; “Wet-web strength increase by chitosan”; Nordic Pulp and Paper Res. J., 6(3), 99-103 (1991)”, chitosan was considered an efficient wet web strengthening aid for mechanical pulps and suggesting that the amino groups could form Schiff’s bases and react with aldehyde groups onto cellulose.

[0005] The publication “Chen et al.; "Combination of Glyoxal and Chitosan as the Crosslinking System to Improve Paper Wet Strength"; BioResources 8(4), 6087-6096 (2013)”, discloses a crosslinking system of glyoxal and chitosan as a wet strength agent improving wet strength of softwood kraft pulp prepared handsheets.

[0006] CN 104420395 A discloses a dry strengthening agent for papermaking, comprising phosphate ester starch amphoteric starch, amphoteric polyacrylamide, hydroxypropyl chitosan, glyoxal modified polyvinyl alcohol, and an anti-crosslinking agent.

[0007] CN 113638264 A discloses a dry strength agent for papermaking comprising soy protein flour, quaternary ammonium type alkyl starch ether, dialdehyde modified chitosan and nanocrystalline cellulose constituents.

[0008] The solution presented in the art are in some aspects still not satisfactory regarding dry strength and temporarily wet strength characteristics of paper substrates.OBJECTS OF THE INVENTION

[0009] Based on the prior art there is still the need to provide alternative paper strengthening agents improving dry and wet strength of paper substrates, in particularFebruary 20, 2026 Archroma IP GmbH 120949P915PCwherein these strengthening agents are based on bio-derivable resources and are biodegradable.SUMMARY OF THE INVENTION

[0010] The invention is set forth in the appended claims. In various aspects the present disclosure refers to a bio-strengthening agent, a method for making a bio-strengthening agent, a method for making a paper substrate, a paper substrate and the use of a biostrengthening agent in the paper making process.

[0011] In particular, the subject of the application can be summarized via the following items 1 to 21 as follows:

[0012] Item 1: A bio-strengthening agent for pulp fiber related paper substrate comprising water and modified chitosan, the modified chitosan being the reaction product of chitosan and a dialdehyde and having a degree of nitrogen substitution (DSN-AW) of more than 0 to 1.0.

[0013] Item 2: The bio-strengthening agent of item 1 wherein the amount of the modified chitosan in the bio-strengthening agent is from 0.01 to 5 wt. percent based on the total weight of the bio-strengthening agent.

[0014] Item 3: The bio-strengthening agent of any of the preceding items, wherein the amount of water in the bio-strengthening agent is from 80 to 99.9 wt. percent based on the total amount of the bio-strengthening agent.

[0015] Item 4: The bio-strengthening agent of any of the preceding items, wherein the bio-strengthening agent is acidified with an acidifying agent to have a pH of 2 to 3, preferably, wherein the acidifying agent is hydrochloric acid.

[0016] Item 5: The bio-strengthening agent of any of the preceding items, wherein the dialdehyde is a dialdehyde reactant of formular I:Formula IFebruary 20, 2026 Archroma IP GmbH 120949P915PCwherein R1is a bifunctional organic residue comprising linear Ci to C12 alkyl residue, or unsaturated C2 to C2 aliphatic residue, orbranched C3 to C12 alkyl or aliphatic residue, orcyclic Cs to C12 residue, oraromatic Ce to C12 residue, orwherein the dialdehyde is glyoxal.

[0017] Item 6:. The bio-strengthening agent of any of the preceding items, wherein the bio-strengthening agent is at least partly biodegradable; preferably wherein the biostrengthening agent is characterized by a degree of biodegradability of equal to or greater than about 94%, and wherein the degree of biodegradability refers to aerobic biodegradability in soil as determined in accordance with ISO 17556:2003 E.

[0018] Item 7: A method for making a bio-strengthening agent of at least one of items 1 to 6 comprising the steps:a) dissolving chitosan in acidified water to obtain an acidic aqueous chitosan solution,(b) adding a dialdehyde to said acidic aqueous chitosan solution and(c) reacting said chitosan and dialdehyde at a temperature of 1°C to 200°C for 0.01 to 1500 minutes to obtain an aqueous composition comprising a modified chitosan,characterized in thatthe weight ratio of chitosan to dialdehyde in step (b) is from 1:1 to 1:100.

[0019] Item 8: The method of item 7, wherein the amount of chitosan in the chitosan solution in step (a) is from 0.1 wt.% to 30 wt.%, the weight percent being based on the total weight of the chitosan solution.

[0020] Item 9: The method of items 7 or 8, wherein the method comprises mixing, or agitating, or stirring, or shaking, or sonicating, or combinations thereof in at least one of or all of step (a), (b) and (c).

[0021] .Item 10: The method of any of the preceding items 7 to 9, wherein the water in step (a) is acidified with hydrochloric acid.February 20, 2026 Archroma IP GmbH 120949P915PC

[0022] Item 11: The method of any of the preceding items 7 to 10 wherein the dialdehyde residue is a residue originating from the compound of formular I:Formula Iwherein R1is a bifunctional organic residue comprisinglinear Ci to C12 alkyl residue, orunsaturated C2 to C12 aliphatic residue, orbranched C3 to C12 alkyl or aliphatic residue, orcyclic Cs to C12 residue, oraromatic Ce to C12 residue, orwherein the dialdehyde is glyoxal.

[0023] Item 12: A method for making a paper substrate comprising the steps:adding a bio-strengthening agent of at least one of items 1 to 6 or as manufactured according to at least one of the items 7 to 11, to pulp slurry, making a paper substrate by molding the mixed pulp slurry into paper sheet and drying.

[0024] Item 13: The method of item 12 wherein the paper sheet is dried at 50 to 150 °C.

[0025] Item 14: The method of item 12 or 13 wherein the amount of the biostrengthening agent in the pulp slurry is 0.1 to 10 wt.%, preferably from 0.1 to 5 wt.%, further preferable from 0.1 to 2 wt.%, or 0.1 to 1 wt% based on the dried pulp.

[0026] Item 15: A paper substrate comprising a modified chitosan as defined in any items 1 to 6, or a modified chitosan as manufactured according to at least one of the items 7 to 11, in an amount of 0.01 wt.% to 5 wt.% based on the total weight of the paper substrate.

[0027] Item 16: The paper substrate according to item 15 comprising from 90 wt.% to 99.99 wt. % of pulp fiber based on the total weight of the paper substrate.

[0028] Item 17: Use of a bio-strengthening agent according to at least one of items 1 to 6 or as manufactured according to at least one of the items 7 to 11 in the paper making process.February 20, 2026 Archroma IP GmbH 120949P915PC

[0029] Item 18: A method for making tissue paper comprising the steps of: adding a bio-strengthening agent as defined in any one of items 1 to 6, or as manufactured according to at least one of the items 7 to 11 to a pulp slurry, and forming tissue paper sheet in accordance with the TAPPI T400 method.

[0030] Item 19: The method of item 18, wherein the tissue paper sheet is dried at a temperature of 50 to 150 °C, preferably 105 to 110 °C.

[0031] Item 20: The method of items 18 or 19, wherein the amount of the biostrengthening agent in the pulp slurry is from 0.1 to 10 wt.%, preferably from 0.1 to 5 wt.%, preferably from 0.1 to 2 wt.%, and most preferably from 0.1 to 1 wt.%, wherein the wt% are based on the dry pulp.

[0032] Item 21 A tissue paper substrate comprising a modified chitosan as defined in any one of items 1 to 6, or as manufactured according to at least one of the items 7 to 11 , in an amount of 0.01 wt.% to 5 wt.% based on the total weight of the tissue paper substrate.

[0033] Item 22: The tissue paper substrate according to item 21 , comprising from 90 wt.% to 99 wt.% pulp fiber based on the total weight of the tissue paper substrate.SHORT DESCRIPTION OF THE FIGURES

[0034] Fig. 1, depicting dry tensile strength of paper hand sheet non-treated (Control), treated with the bio-strengthening agent according to the aspects of the present disclosure (GC-0.25%, GC-0.5%) and treated with commercial paper strength agent (GPAM-0.25%, GPAM-0.5%) treated paper hand sheet.

[0035] Fig. 2, depicting wet tensile strength of paper hand sheet non-treated (Control), treated with the bio-strengthening agent according to the aspects of the present disclosure (GC-0.25%, GC-0.5%) and treated with commercial paper strength agent (GPAM-0.25%, GPAM-0.5%) treated paper hand sheet.

[0036] Fig. 3 depicting bursting strength of paper hand sheet non-treated (Control), treated with the bio-strengthening agent according to the aspects of the presentFebruary 20, 2026 Archroma IP GmbH 120949P915PCdisclosure (GC-0.25%, GC-0.5%) and treated with commercial paper strength agent (GPAM-0.25%, GPAM-0.5%) treated paper hand sheet.

[0037] Fig. 4 depicting wet and dry tensile index of tissue samples.

[0038] Fig. 5 depicting bulk and absorbency of tissue samples.DETAILED DESCRIPTION OF THE INVENTION

[0039] In a first aspect the present disclosure refers to a bio-strengthening agent for pulp fiber related paper substrate comprising water and modified chitosan, the modified chitosan being the reaction product of chitosan and a dialdehyde and wherein the chitosan has a degree of nitrogen substitution (DSN-AICI) of more than 0 to 1.0, preferably 0.1 to 1.0.

[0040] The bio-strengthening agent imparts good or even improved dry, wet and / or burst strength to paper substrate, when applied in the paper making process to pulp slurry or when applied to paper substrate via impregnation. Besides that, chitosan is derivable from bio resources and the modified chitosan included in said bio-strengthening agent is to the most part biodegradable.

[0041] Within the present application the term “bio-strengthening agent’ refers to an agent that is suited to impart dry- and / or wet- and / or burst strength to paper substrate compared to a control paper substrate not comprising this agent. The dry and wet strength being determined according to the test method TAPPI T 220 “Physical testing of pulp handsheets”. The bursting strength being determined according to the test method TAPPI T 403 “Busting strength of paper”. The term “bio” refers to the use of natural, i.e. plant based or animal-based resources and the use of these resources for the making of the bio-strengthening agent. For example in that respect chitosan as used in the making of the bio-strengthening agent of the present disclosure can be derived from natural chitin or from fungi. The term “bio” does also refer to the ability of bio-degradability of the biostrengthening agent.

[0042] Within the present application the term “pulp fibe is to be understood to mean aFebruary 20, 2026 Archroma IP GmbH 120949P915PCfibrous lignocellulosic material prepared chemically, semi-chemically or mechanically from cellulosic fibers from wood fiber crops or wastepaper.

[0043] “Chitosan” is linear polysaccharide composed of randomly distributed p-(1— >4)-linked D-glucosamine repeating units:

[0044] The term "modified chitosan” as used in the context of the present application is understood to be a chemical reaction product of chitosan and dialdehyde. In a preferred embodiment, one aldehyde group of the dialdehyde reacts with the amino group (-NH2) of a repeating unit of chitosan to form imine-structures.

[0045] The term “degree of nitrogen substitution” herein abbreviated with (DSN-AW) designates the ratio of the number of amino groups present in the chitosan reacted with an aldehyde function of the dialdehyde reactant to the total number of nitrogen atoms present in chitosan. That means that if all amino groups have reacted with an aldehyde function of the dialdehyde reactant the DSN-AW is 1.0. In case half of the amino groups of the chitosan have reacted with an aldehyde function of the dialdehyde reactant the DSN-Aid is 0.5. The number of amino groups in chitosan and in the modified chitosan can be determined by spectroscopic methods, for example via IR (infra-red)-spectroscopy or NMR spectroscopy or via titration. The degree of nitrogen substitution (DSN-AW) is determined according to a method disclosed in Xue-Li Liu, Chun-Feng Zhu, Han-Chun Liu & Jia-Ming Zhu. Quantitative analysis of degree of substitution / molar substitution of etherified polysaccharide derivatives. DESIGNED MONOMERS AND POLYMERS.2022, VOL. 25, NO. 1, 75-88. The modified chitosan comprised in the bio-strengthening agent has a DSN-Aid of more than 0, or at least 0.05, or at least 0.10, or at least 0.15, or at least 0.20, or at least 0.25, or at least 0.30, or at least 0.35, or at least 0.40, or at least 0.45, or at least 0.50, or at least 0.55, or at least 0.60, or at least 0.65, or at least 0.70, or at least 0.75, or at least 0.80, or at least 0.85, or at least 0.90, or at least 0.95, or 1.0 and / orFebruary 20, 2026 Archroma IP GmbH 120949P915PC

[0046] at most 1.0, or at most 0.95, or at most 0.90, or at most 0.85, or at most 0.80, or at most 0.75, or at most 0.70, or at most 0.65, or at most 0.60, or at most 0.55, or at most 0.50, or at most 0.45, or at most 0.40, or at most 0.35, or at most 0.30, or at most 0.25, or at most 0.20, or at most 0.15, or at most 0.10, or at most 0.05. In particular, the DSn-Ald of modified chitosan in the bio-strengthening agent can be more than 0 to at most 1.0, or at least 0.05 to at most 0.95, or at least 0.10 to at most 0.90, or at least 0.15 to at most 0.85, or at least 0.20 to at most 0.80, or at least 0.25 to at most 0.75, or at least 0.30 to at most 0.70, or at least 0.5 to at most 1.0, or at least 0.6 to at mosti .0, or at least 0.7 to at most 1.0, or at least 0.8 to at most 1.0 or at least 0.9 to at most 1.0, or at least 0.55 to at most 0.95, or at least 0.65 to at most 0.95, or at least 0.75 to at most 0. 95 or at least 0.85 to at most 0.95. High degrees of substitution are preferred.

[0047] It is possible that the bio-strengthening agent of the present invention is acidified with a water-soluble acid preparable hydrochloric acid to yield a pH value of 2 to 3. The hydrochloric acid can be added during the production of the bio-strengthening agent in form of a diluted or concentrated aqueous solution.

[0048] The amount of modified chitosan in the bio-strengthening agent can be at least 0.02 wt%, or at least 0.05 wt%, or at least 0.10 wt%, or at least 0.15 wt%, or at least 0.20 wt%, or at least 0.25 wt%, or at least 0.30 wt%, or at least 0.35 wt%, or at least 0.40 wt%, or at least 0.45 wt%, or at least 0.50 wt%, or at least 0.55 wt%, or at least 0.60 wt%, or at least 0.65 wt%, or at least 0.70 wt%, or at least 0.75 wt%, or at least 0.80 wt%, or at least 0.85 wt%, or at least 0.90 wt%, or at least 0.95 wt%, or at least 1.00 wt%, or at least 1.05 wt%, or at least 1.10 wt%, or at least 1.15 wt%, or at least 1.20 wt%, or at least 1.25 wt%, or at least 1.30 wt%, or at least 1.35 wt%, or at least 1.40 wt%, or at least 1.45 wt%, or at least 1.50 wt%, or at least 1.55 wt%, or at least 1.60 wt%, or at least 1.65 wt%, or at least 1.70 wt%, or at least 1.75 wt%, or at least 1.80 wt%, or at least 1.85 wt%, or at least 1.90 wt%, or at least 1.95 wt%, or at least 2.00 wt%, or at least 2.05 wt%, or at least 2.10 wt%, or at least 2.15 wt%, or at least 2.20 wt%, or at least 2.25 wt%, or at least 2.30 wt%, or at least 2.35 wt%, or at least 2.40 wt%, or at least 2.45 wt%, or at least 2.50 wt and / or

[0049] at most 5.0 wt%, or at most 4.95 wt%, or at most 4.90 wt%, or at most 4.85 wt%, or at most 4.80 wt%, or at most 4.75 wt%, or at most 4.70 wt%, or at most 4.65 wt%, orFebruary 20, 2026 Archroma IP GmbH 120949P915PCat most 4.60 wt%, or at most 4.55 wt%, or at most 4.50 wt%, or at most 4.45 wt%, or at most 4.40 wt%, or at most 4.35 wt%, or at most 4.30 wt%, or at most 4.25 wt%, or at most 4.20 wt%, or at most 4.15 wt%, or at most 4.10 wt%, or at most 4.05 wt%, or at most 4.00 wt%, or at most 3.95 wt%, or at most 3.90 wt%, or at most 3.85 wt%, or at most 3.80 wt%, or at most 3.75 wt%, or at most 3.70 wt%, or at most 3.65 wt%, or at most 3.60 wt%, or at most 3.55 wt%, or at most 3.50 wt%, or at most 3.45 wt%, or at most 3.40 wt%, or at most 3.35 wt%, or at most 3.30 wt%, or at most 3.25 wt%, or at most 3.20 wt%, or at most 3.15 wt%, or at most 3.10 wt%, or at most 3.05 wt%, or at most 3.00 wt%, or at most 2.95 wt%, or at most 2.90 wt%, or at most 2.85 wt%, or at most 2.80 wt%, or at most 2.75 wt%, or at most 2.70 wt%, or at most 2.65 wt%, or at most 2.60 wt%, or at most 2.55 wt%, or at most 2.50 wt%, or at most 2.45 wt%, or at most 2.40 wt%, or at most 2.35 wt%, or at most 2.30 wt%, or at most 2.25 wt%, or at most 2.20 wt%, or at most 2.15 wt%, or at most 2.10 wt%, wherein the wt% are based on the total weight of the bio-strengthening agent.

[0050] The amount of modified chitosan in the bio-strengthening agent is from 0.02 to 5 wt.%, or from 0.1 to 5 wt.%, or from 0.2 to 5 wt.%, or from 0.1 to 4 wt.% or from 0.1 to 3 wt.%, or from 0.1 to 2 wt.% or from 0.1 to 1 wt.%, or from 0.1 to 0.9 wt.%, or from 0.1 to 0.8 wt.%, or from 0.1 to 0.7 wt.%, or from 0.1 to 0.6 wt.% based on the total weight of the bio-strengthening agent.

[0051] The amount of water in the bio-strengthening agent ranges from 80 to 99.9 wt.%, or 85 to 99.9 wt. % or from 95 to 99.9 wt.% the weight percent being based on the total weight of the bio-strengthening agent.

[0052] The molecular weight of the chitosan used for forming the modified chitosan is from about 1,000 g / mol to about 2,000,000 g / mol, or from 5,000 g / mol to 1,000,000 g / mol or from 5,000 g / mol to 300,000 g / mol. The molecular weight is determined via gel permeation chromatography and laser light scattering.

[0053] The bio-strengthening agent may comprise further additives for improving stability and reactivity of the bio-strengthening agent. Suitable additives are preservatives like potassium sorbate or 1 ,2-benzisothiazol-3(2H)-one. The bio-strengthening agent may also comprise unreacted dialdehyde or side products, which can be removed before or after the use of the bio-strengthening agent in the paper making process.February 20, 2026 Archroma IP GmbH 120949P915PC

[0054] It is possible that the bio-strengthening agent consists of water, modified chitosan, acid, and optionally unreacted dialdehyde and further optionally side products originating from the reaction of chitosan and dialdehyde. It is further possible that no other additives are present in the bio-strengthening agent, like starch and / or soybean powder and / or lactic acid and / or nano-cellulose or a mixture thereof.

[0055] The bio-strengthening agent may not contain any strengthening agents derived from non-natural / non-bio sources. In particular, the bio-strengthening agent according to the invention may not comprise any synthetic strengthening agents like polyacrylamide, polyvinyl alcohol or the like.

[0056] The dialdehyde residue is a residue originating from the dialdehyde reactant of formula I:Formula Iwherein R1is a bifunctional organic residue comprisinglinear Ci to C12 alkyl residue, orunsaturated C2 to C12 aliphatic residue, orbranched C3 to C12 alkyl or aliphatic residue, orcyclic Cs to C12 residue, oraromatic Cs to C12 residue, orwherein the dialdehyde is glyoxal (C2H2O2).Without being bound to theory, the modified chitosan in the bio-strengthening agent is the reaction product of chitosan and dialdehyde. It is believed that the amino groups of the chitosan react with one aldehyde group of the dialdehyde to form imine-structures. Short alkyl chain length of R1like C2 to Cs or Ci to C3 or Ci to C2 are preferred, i.e. linear C2 to Cs alkyl residue, or unsaturated C2 to Cs aliphatic residue, or branched C2 to Cs alkyl or aliphatic residue; or linear Ci to C3 alkyl residue, or unsaturated Ci to C3 aliphatic residue, or branched Ci to C3 alkyl or aliphatic residue; or linear Ci to C2 alkyl residue. Most preferred is if R1is notFebruary 20, 2026 Archroma IP GmbH 120949P915PCpresent, i.e. the carbon atoms carrying the aldehyde functions are directly bonded to each other; i.e. if the dialdehyde is glyoxal.

[0057] It is possible that the bio-strengthening agent of the present disclosure is characterized in that the bio-strengthening agent is biodegradable. Biodegradability is determined according to ISO 17556:2003 E.

[0058] Preferably, the biodegradability of the bio-strengthening agent is equal to or greater than about 94%, wherein the degree of biodegradability refers to aerobic biodegradability in soil as determined in accordance with ISO 17556:2003 E.

[0059] In a second aspect the present disclosure refers to a method for making the biostrengthening agent according to at least one embodiment of the first aspect of the present disclosure, the method comprising the steps:(a) dissolving chitosan in acidified water to obtain an acidic aqueous chitosan solution,(b) adding a dialdehyde to said acidic aqueous chitosan solution and(c) reacting said chitosan and dialdehyde at a temperature of 1 °C to 200°C for 0.01 to 1500 minutes to obtain an aqueous composition comprising the modified chitosan, characterized in thatthe weight ratio of chitosan to dialdehyde in step (b) is from 1:1 to 1:100, the weight ratio being based on the total weight of the aqueous composition.

[0060] The reaction temperature can be adjusted to 20 to 150 °C or 20 to 100°C or 20 to 80 °C. Preferably the reaction temperature is adjusted to 20 to 60°C.

[0061] The reaction time can be adjusted to 10 min to 10 hours, or 10 min to 5 hours, or 10 min to 3 hours, or 30 min to 2 hours, preferably to 30 min to 1.5 hours.

[0062] The amount of the chitosan in the chitosan solution in step (a) is 0.1 to 30 wt.%, wherein the wt% are based on the total weight of the chitosan solution obtained in step (a). In further embodiments the amount of chitosan in the chitosan solution in step (a) is 0.1 to 25 wt.%, 0.1 to 20 wt. %, 0.1 to 15 wt.%, or 1 to 15 wt.% or 2 to 10 wt.% or 2 to 8 wt. %, or 0.1 to 8 wt.% or 0.1 to 7 wt. %, or 0.1 to 6 wt.%, or. 0.1 to 5 wt.%, or 0.1 to 4 wt.%, or 0.1 to 3 wt.%, or 0.1 to 2 wt. %, or 0.1 to 3 wt. %, or 0.1 to 2 wt. % or 0.1 to 1 wt. % or 0.1 to 0.5 wt.%, wherein the wt% are based on the total weight of the chitosanFebruary 20, 2026 Archroma IP GmbH 120949P915PCsolution obtained in step (a).

[0063] The weight ratio of chitosan to dialdehyde in step (b) is from 1:1 to 1:100, or from 1:1 to 1:50, or from 1:1 to 1:40, or from 1:1 to 1:30, or from 1:1. to 1:20, or from 1:1 to 1:10, or from 1:1 to 1:5, or from 1:1 to 1:2, or from 1:1 to 2:1, or from 1:1 to 3:1, or from 1 :1 to 5:1 , or from 1 :1 to 8:1.

[0064] The method of the second aspect comprises mixing, or agitating, or stirring, or shaking, or sonicating, or combinations thereof in at least one of or all of step (a), (b) and (c).

[0065] The method of the second aspect of the present disclosure, comprises that the water in step (a) is acidified with an acidifying agent, preferably hydrochloric acid. Acidification is preferably performed with aqueous hydrochloric acid having a hydrochloric acid content of 0.01 wt% to 5 wt.%, or 0.02 wt.% to 4 wt.% or 0.03 wt.% to 3 wt.%.

[0066] The method of the second aspect of the present disclosure comprises that the dialdehyde added in step (b) is a dialdehyde of compound of Formula I as defined above with regard to the first aspect of the invention, preferably the dialdehyde is glyoxal.

[0067] In particular, the dialdehyde reactant of formula I is used in the method according to the invention:Formula Iwherein R1is a bifunctional organic residue comprisinglinear Ci to C12 alkyl residue, orunsaturated C2 to C12 aliphatic residue, orbranched C3 to C12 alkyl or aliphatic residue, orcyclic Cs to C12 residue, oraromatic Ce to C12 residue, orwherein the dialdehyde is glyoxal (C2H2O2).

[0068] Short alkyl chain length of R1like C2 to Cs or Ci to C3 or Ci to C2 are preferred, i.e. linear C2 to Cs alkyl residue, or unsaturated C2 to Cs aliphatic residue, or branched C2 toFebruary 20, 2026 Archroma IP GmbH 120949P915PCCs alkyl or aliphatic residue; or linear Ci to C3 alkyl residue, or unsaturated Ci to C3 aliphatic residue, or branched Ci to C3 alkyl or aliphatic residue; or linear Ci to C2 alkyl residue. Most preferred is if R1is not present, i.e. the carbon atoms carrying the aldehyde functions are directly bonded to each other; i.e. if the dialdehyde is glyoxal.

[0069] In one embodiment no constituents other than water, chitosan, dialdehyde and acid are used for method for making the bio-strengthening agent.

[0070] In a third aspect the present disclosure refers to a method for making a paper substrate comprising the steps: (a) adding a bio-strengthening agent of the first aspect of the present disclosure to a pulp slurry, (b) making a paper substrate by molding the mixed pulp slurry into paper sheet and drying.

[0071] The pulp slurry can have a water to pulp fiber weight ratio of from 99.5 : 0.5 or from 99.6 : 0.4 preferably 99.7 : 0.3.

[0072] Drying can be performed at 50 to 150 °C, or 50 to 140 °C, or 50 to 130°C, or 20 to 120 °C, or 30 to 120°C, or 40 to 120°C, or 40 to 130 °C, or 50 to 130 °C o 80 to 130 °C, or 100 to 130°, preferably 110 to 120°C.

[0073] The amount of the bio-strengthening agent in the pulp slurry can be 0.1 to 10 wt.%, preferably from 0.1 to 5 wt.%, further preferable from 0.1 to 2 wt.%, or 0.1 to 1 wt.%, or 0.1 to 0.5 wt.% based on the dried pulp.

[0074] “Dried pulp” within the context of the present application means drying at 100 °C to 120 °C until weight constancy is reached.

[0075] In a fourth aspect, the present disclosure refers to a paper substrate comprising the modified chitosan in an amount of 0.01 wt.% to 5 wt.%, or from 0.01 to 4wt%, or from 0.01 to 3wt% or from 0.01 to 2wt%, or from 0.01 to 1 wt%, based on the total weight of the paper substrate.

[0076] Further, the paper substrate contains from 90 to 99.99 wt% or from 90 wt% to 99wt% pulp fiber, based on the total weight of the paper substrate.

[0077] In a fifth aspect, the present disclosure refers to the use of a bio-strengthening agent according one of the embodiments of the first aspect of the present disclosure in paper making processes, preferably in a process from making tissue paper in accordance with TAPPI T400 method.February 20, 2026 Archroma IP GmbH 120949P915PC

[0078] Within the context of the present disclosure, the term “paper” or “paper substrate” refers to linear board, container board, paper packaging, tissue, napkin etc.

[0079] The use includes contacting a fiber web with the bio-strengthening agent according to the present disclosure in a process comprising the steps: (a) forming a plurality of pulp fibers into paper substrate such like, tissue, paper, paperboard, (b) contacting at least a portion of the fiber web with the bio-strengthening agent according the first aspect of the present disclosures to form a impregnated fiber web and (c) drying the pulp fiber web.

[0080] Alternatively, the use of the fifth aspect includes, contacting a fiber web with the bio-strengthening agent according to the present disclosure in a process comprising the steps: (a) forming a plurality of pulp fibers into paper substrate by mixing the biostrengthening agent of the first aspect of the present disclosure with an aqueous pulp fiber slurry and (b) drying the pulp fiber to form paper products.

[0081] In a sixth aspect the present disclosure refers to a method for making a tissue paper substrate comprising the steps: adding a bio-strengthening agent of of the first aspect of the present disclosure and forming tissue paper sheet in accordance with the TAPPI 400 method.

[0082] The pulp slurry can have a water to pulp fiber weight ratio of from 99.5 : 0.5 or from 99.6 : 0.4 preferably 99.7 : 0.3.

[0083] the obtained tissue paper sheet can be dried and drying is performed at 50 to 150 °C, or 50 to 140 °C, or 50 to 130°C, or 20 to 120 °C, or 30 to 120°C, or 40 to 120°C, or 40 to 130 °C, or 50 to 130 °C o 80 to 130 °C, or 100 to 130°, preferably 110 to 120°C, or 105 to 110°C.

[0084] the amount of the bio-strengthening agent in the pulp slurry is 0.1 to 10 wt.%, preferably from 0.1 to 5 wt.%, further preferable from 0.1 to 2 wt.%, or 0.1 to 1 wt.%, or 0.1 to 0.5 wt.%, wherein the wt% are based on the dried pulp

[0085] In a seventh aspect, the present disclosure refers to a tissue paper substrate comprising the modified chitosan in an amount of 0.01 wt.% to 5 wt.%, or from 0.01 to 4wt%, or from 0.01 to 3wt% or from 0.01 to 2wt%, or from 0.01 to 1 wt%, based on the total weight of the tissue paper substrate.

[0086] The tissue paper substrate can comprise from 90 wt.% to 99.99 wt.% or fromFebruary 20, 2026 Archroma IP GmbH 120949P915PC90wt% to 99wt% pulp fiber based on the total weight of the tissue paper substrate.

[0087] It is to be understood that the definitions, reagents, compounds, definitions etc. given above for the various aspects of the invention are to be combined with all aspects of the invention independently under which aspects the disclosure of the same was made.EXAMPLESExample 1: - Method for forming a chitosan solution and a bio-strengthening agent comprising a modified chitosan

[0088] Chitosan, in an amount as will be indicated in the following, was dissolved in acidified water at 20 to 60°C. The water having a pH value of pH= 2 to 3, preferably 2.5. Aqueous hydrochloric acid was used for acidification. Once the chitosan had completely dissolved a glyoxal solution (40wt.% in water) was added to the chitosan solution at weight ratios chitosan: glyoxal indicated below at 20 to 60°C, preferably 32°C for one to 3 hours under stirring to produce a modified chitosan solution. Without being bound to theory, it is believed that inter alia the following reaction steps and intermediates occur within the described reaction steps between chitosan and glyoxal:February 20, 2026 Archroma IP GmbH 120949P915PC

[0089] It is believed that the dialdehyde binds to the amino groups of chitosan with one aldehyde function under formation of an imine structure. It is further believed that the second aldehyde function of the dialdehyde remains essentially unreacted and assists in the strengthening of paper substrate because of chemical bonding when added to a pulp slurry or when applied in an impregnation process to paper substrate and dried.The following samples had been prepared:Sample 1 - preparation of chitosan solution: 1 liter of chitosan solution was prepared by using 30 g chitosan and 970 g water acidified with hydrochloric acid to pH= 2 to 3.Sample 2 - preparation of bio-strengthening agent according to the invention: 1 liter of a solution comprising the modified chitosan was prepared by using 30 g chitosan, 150 g of a 40 wt.% aqueous glyoxal solution, and 820 g water acidified with hydrochloric acid to pH= 2 to 3.February 20, 2026 Archroma IP GmbH 120949P915PCSample 3 - preparation of bio-strengthening agent according to the invention: 1 liter of a solution comprising the modified chitosan was prepared by using 40 g chitosan, 100 g of a 40 wt.% aqueous glyoxal solution, and 860 g water acidified with hydrochloric acid to pH= 2 to 3.Example 2 - Preparation of paper hand sheet samples

[0090] The bio-strengthening agent of Sample 2 was mixed with pulp slurry comprising 99.7 wt.% water and 0.3% pulp fiber in an amount of 0.25 wt.% and 0.5 wt.% based on the dried pulp separately and stirred for 2 minutes before making two paper hand sheet samples. The paper hand sheets were prepared with a hand sheet molding machine according to the test method described in TAPPI T220. The paper hand sheet samples were dried in air oven at 110-120° C for 5 minutes.Example 3 - Analysis of the physical properties of paper hand sheet samples

[0091] Analysis of the physical properties of paper hand sheet made in accordance with Example 2: The dried paper hand sheet samples were characterized in terms of strength properties such as dry and wet tensile strength and bursting strength. The results are shown in Figs. 1, 2 and 3. Commercial glyoxylate polyacrylamide (GPAM) was used as a reference to compare with the hand sheets of Example 2 (GC). As a control sample (Control), the pulp was used without adding paper strength additive. The dry and wet tensile strength were determined according to the test method TAPPI T 220, the bursting strength was determined according to the test method TAPPI T 403. The paper hand sheets were prepared according to the Method TAPPI T 205 and were further conditioned as described in the method TAPPI T 402.

[0092] The dry and wet strength of paper hand sheets produced with the biostrengthening agent according to the invention is shown in Figures 1 and 2. Both Figures 1 and 2 indicate that the wet and dry strength of GC treated paper hand sheets are significantly higher than that of the control sample without using any bio-strengthening additive. It is also observed from Figures 1 and 2 that the dry and wet strength of GCFebruary 20, 2026 Archroma IP GmbH 120949P915PCtreated paper hand sheets were comparable or even improved to that of commercial GPAM treated paper hand sheets. It is further observed from Figure 3 that the GO treated paper hand sheets were significantly increased in bursting strength compared to the Control and were even higher than that of GPAM treated paper hand sheets.

[0093] The dry strength of the GC examples (GC-0.25% and GC-0.5%) worked according to the aspects of the present disclosure exhibit a dry tensile strength of 20.6 Ib / in and 23.1 Ib / in and a burst strength of 21.4 Psi and 24.9 Psi respectively, which is about 22-40% and 30-50% higher than the Control sample without including a bio-strengthening agent, as determined in accordance with TAPPI Standard Method.

[0094] The wet strength of the GC examples worked according to the aspects of the present disclosure have a wet tensile strength of 0.97 Ib / in (GC-0.25%) and 2.4 Ib / in (GC-0.5%) respectively, which is about 136-485% higher than the Control sample without biostrengthening agent.Example 4 - Preparation of tissue paper samples

[0095] The bio-strengthening agent of Sample 3 was mixed with a pulp slurry consisting of 99.7 wt.% water and 0.3 wt.% pulp fiber. The bio-strengthening agent was added at levels of 0.15 wt.%, 0.3wt%, 0.45wt%, 0.6 wt% and 0.75 wt.% (based on the dry pulp) separately and stirred for 15 minutes before forming tissue paper samples

[0096] The tissue paper samples were prepared using a lab-based napkin and tissue-paper-making equipment (DSF) was used, and the tissue paper was prepared according to the TAPPI T400 method using the DSF equipment. Creped and non-creped tissue paper samples were produced and were dried on a drum dryer at 105-110 °C.Example 5 - Analysis of the physical properties of tissue paper sheet samples

[0097] The dried tissue paper samples of Example 4 (“GC-treated samples”) were characterized for strength properties, including dry and wet tensile strength, bulkiness, and water absorbency. The results are presented in Figures 4 and 5 (3 lb / ton= 0.15wt%, 6 lb / ton= 0.30wt%, 9 lb / ton= 0.45wt%, 12 lb / ton= 0.60wt%, 15 lb / ton= 0.75wt%). A control sample (Control) was prepared using same pulp without any paper-strength additive. TheFebruary 20, 2026 Archroma IP GmbH 120949P915PCdry and wet tensile strengths were determined according to TAPPI test methods T494 and T456. The dry and wet strength, bulkiness and water absorbency of tissue paper sheets produced with the bio-strengthening agent according to the present invention is shown in Figures 4 and 5. These figures indicate that the wet and dry strength (see Fig.4), as well as the bulkiness and water absorbency (see Fig. 5; 0 [cm2 / g] / [g / g] for the control sample, which is not displayed in Fig. 5; high values for bulkiness and water absorbency are desired; best results in both cases for the samples with a 0.15wt%, 0.3wt% and 0.45wt% load) of GC-treated tissue paper, are significantly higher than those of the control sample prepared without any strengthening additive. The dry strength of the GC-treated tissue sheet samples (GC-0.15% to GC-0.75%) prepared according to the present disclosure exhibits a dry tensile index (Nm / g) of 22.35, 21.07, 23.42, 25.11 , and 27.67, respectively, which is approximately 15.72-43.29% higher than the control sample without a bio-strengthening agent, as determined in accordance with TAPPI Standard Method T494.

[0098] The wet strength of the GC-treated tissue sheet samples prepared according to the present disclosure exhibits a wet tensile index (Nm / g) of 0.60, 0.73, 1.05, 1.72, and 1.73, respectively, which is approximately 500-1630% higher than the control sample without a bio-strengthening agent, as determined in accordance with TAPPI Standard Method T456.

Claims

February 20, 2026 Archroma IP GmbH 120949P915PCCLAIMS1. A bio-strengthening agent for pulp fiber related paper substrate comprising water and modified chitosan, the modified chitosan being the reaction product of chitosan and a dialdehyde and having a degree of nitrogen substitution (DSN-A ) of more than 0 to 1.0; preferablywherein the amount of the modified chitosan in the bio-strengthening agent is from 0.01 to 5 wt. percent based on the total weight of the bio-strengthening agent.

2. The bio-strengthening agent of claim 1, wherein the amount of water in the biostrengthening agent is from 80 to 99.9 wt. percent based on the total amount of the bio-strengthening agent.

3. The bio-strengthening agent of claim 1 or 2, wherein the bio-strengthening agent is acidified with an acidifying agent to have a pH of 2 to 3, preferably, wherein the acidifying agent is hydrochloric acid.

4. The bio-strengthening agent of any of the preceding claims, wherein the dialdehyde is a dialdehyde reactant of formular I:Formula Iwherein R1is a bifunctional organic residue comprisinglinear Ci to C12 alkyl residue, orunsaturated C2 to C2 aliphatic residue, orbranched C3 to C12 alkyl or aliphatic residue, orcyclic Cs to C12 residue, oraromatic Ce to C12 residue, orwherein the dialdehyde is glyoxal.February 20, 2026 Archroma IP GmbH 120949P915PC5. The bio-strengthening agent of any of the preceding claims, wherein the biostrengthening agent is at least partly biodegradable; preferably wherein the biostrengthening agent is characterized by a degree of biodegradability of equal to or greater than about 94%, and wherein the degree of biodegradability refers to aerobic biodegradability in soil as determined in accordance with ISO 17556:2003 E.

6. A method for making a bio-strengthening agent of at least one of claims 1 to 5 comprising the steps:(a) dissolving chitosan in acidified water to obtain an acidic aqueous chitosan solution,(b) adding a dialdehyde to said acidic aqueous chitosan solution and(c) reacting said chitosan and dialdehyde at a temperature of 1°C to 200°C for 0.01 to 1500 minutes to obtain an aqueous composition comprising a modified chitosan, characterized in thatthe weight ratio of chitosan to dialdehyde in step (b) is from 1 :1 to 1 :100; preferably wherein the method comprises mixing, or agitating, or stirring, or shaking, or sonicating, or combinations thereof in at least one of or all of step (a), (b) and (c).

7. The method of claim 6, wherein the amount of chitosan in the chitosan solution in step (a) is from 0.1 wt.% to 30 wt.%, the weight percent being based on the total weight of the chitosan solution; preferablywherein the water in step (a) is acidified with hydrochloric acid.

8. The method of claim 6 or 7, wherein the dialdehyde residue is a residue originating from the compound of formular I:Formula Iwherein R1is a bifunctional organic residue comprisingFebruary 20, 2026 Archroma IP GmbH 120949P915PClinear Ci to C12 alkyl residue, orunsaturated C2 to C12 aliphatic residue, orbranched C3 to C12 alkyl or aliphatic residue, orcyclic Cs to C12 residue, oraromatic Ce to C12 residue, orwherein the dialdehyde is glyoxal.

9. A method for making a paper substrate comprising the steps:adding a bio-strengthening agent of at least one of claims 1 to 5 or as manufactured according to at least one of the claims 6 to 8, to pulp slurry, making a paper substrate by molding the mixed pulp slurry into paper sheet and drying; preferably wherein the paper sheet is dried at 50 to 150 °C.

10. The method of claim 9, wherein the amount of the bio-strengthening agent in the pulp slurry is 0.1 to 10 wt.%, preferably from 0.1 to 5 wt.%, further preferable from 0.1 to 2 wt.%, or 0.1 to 1 wt% based on the dried pulp.

11. A paper substrate comprising a modified chitosan as defined in any claims 1 to 5, or a modified chitosan as manufactured according to at least one of the claims 6 to 8, in an amount of 0.01 wt.% to 5 wt.% based on the total weight of the paper substrate; preferablywherein the paper substrate comprises from 90 wt.% to 99.99 wt. % of pulp fiber based on the total weight of the paper substrate.

12. Use of a bio-strengthening agent according to at least one of claims 1 to 5 or as manufactured according to at least one of the claims 6 to 8 in the paper making process.

13. A method for making tissue paper comprising the steps of: adding a biostrengthening agent as defined in any one of claims 1 to 5, or as manufacturedFebruary 20, 2026 Archroma IP GmbH 120949P915PCaccording to at least one of the claims 6 to 8 to a pulp slurry, and forming tissue paper sheet in accordance with the TAPPI T400 method; preferablywherein the tissue paper sheet is dried at a temperature of 50 to 150 °C, more preferably at a temperature of 105 to 110 °C.

14. The method of claims 13, wherein the amount of the bio-strengthening agent in the pulp slurry is from 0.1 to 10 wt.%, preferably from 0.1 to 5 wt.%, preferably from 0.1 to 2 wt.%, and most preferably from 0.1 to 1 wt.%, wherein the wt% are based on the dry pulp.

15. A tissue paper substrate comprising a modified chitosan as defined in any one of claims 1 to 5, or as manufactured according to at least one of the claims 6 to 8, in an amount of 0.01 wt.% to 5 wt.% based on the total weight of the tissue paper substrate; preferablywherein the tissue paper substrate comprises from 90 wt.% to 99 wt.% pulp fiber based on the total weight of the tissue paper substrate.

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