Process for producing a lignosulfonate polymer

Description

[0001] The invention relates to a process for producing a lignosulfonate polymer, a lignosulfonate polymer produced by a process according to the invention, and a growth substrate comprising a lignosulfonate polymer according to the invention.

[0002] Lignosulfonates are also known as ligninsulfonates and are salts of ligninsulfonic acid. Lignosulfonic acid is a water-soluble polymer consisting essentially of the monomer units coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol, with individual hydroxyl groups (-OH) substituted by sulfonyl groups (-SO 3 -< ).

[0003] Lignosulfonates are a byproduct of cellulose production using the sulfite process. Lignosulfonate solutions produced in this way are, for example, a waste product in paper production and are referred to in the art as crude liquor or brown liquor. In addition to lignosulfonates, crude liquor typically also contains coarse components, such as lignin fibers or particles, as well as low-molecular-weight components, such as salts or organic compounds.

[0004] Since crude lye is generated in large quantities as a waste product in industrial processes, it would be desirable to use it as a raw material for the manufacture of certain products. Crude lye is used, for example, in the preparation of wetting agents, dispersants, binders, and adhesives, although large quantities are also used simply for energy generation through combustion.

[0005] In the context of the present invention, lignosulfonate polymers are those lignosulfonate structures in which further crosslinking of the native lignosulfonate units has occurred. Lignosulfonate polymers are preferably essentially water-insoluble compared to lignosulfonates.

[0006] Different lignosulfonate polymers are known in the prior art: For example, the article entitled "Investigation of the Molecular Weight of Commercial Lignosulfonates by Laccase Catalysis", Biomacromolecules 2010, 11, 904-910, describes a process for producing a lignosulfonate polymer, but does not describe how water-insoluble polymers can be formed.

[0007] DE 3037992 A1 describes a process for producing a binder for chipboard.

[0008] DE 19700904 A1 describes an intermediate for the production of polymers from lignin derivatives. It describes the lignin derivatives being subjected to an enzymatic treatment.

[0009] The article entitled "Polymerization of Various Lignins via Immobilized Myceliophthora thermophila Laccase (MtL)", Polymers 2016, 8, 280 and the article entitled "Laccase mediated oxidation of industrial lignins: Is oxygen limiting?", Process Biochemistry 2015, 50, 1277-1283 describe lignosulfonate salts as starting materials for polymerization reactions.

[0010] The article entitled "Polymerisation of Kraft lignin from black liquors by laccase from Myceliophthora thermophila: Effect of operational conditions and black liquor origin", Biosource Technology 2013, 131, 288-294 does not describe the polymerisation of lignosulfonate, but of Kraft lignin from black liquor.

[0011] Using processes known in the prior art, it has not yet been possible to produce lignosulfonate polymers that are essentially entirely based on lignosulfonate. According to the prior art, it is necessary to crosslink the lignosulfonate units with crosslinkers, thereby forming copolymers. This allows the formation of lignosulfonate polymers with a sufficiently high average molecular weight to exhibit the desired chemical and physical properties. Such copolymerization requires the use of additional raw materials and is undesirable in terms of the economic viability of a production process for lignosulfonate polymers.

[0012] The object of the present invention is to overcome the problems of the prior art and to provide a process by which it is possible to create water-insoluble lignosulfonate polymers which are essentially completely based on lignosulfonate and which, in particular, do not require the use of cross-linking additives.

[0013] The object of the invention is achieved by the features of the patent claims. Preferred features are set forth in the dependent patent claims.

[0014] In the context of the present invention, the term "water-insoluble" means that a substance, in particular a lignosulfonate polymer, is essentially insoluble in water. In particular, "water-insoluble" or "essentially water-insoluble" can mean that less than 0.1 wt.% of the total mass of a substance in question is soluble in water at a temperature of 20°C.

[0015] The invention relates to a process for producing a water-insoluble lignosulfonate polymer from a solution containing a lignosulfonate precursor, namely from crude liquor, comprising the steps according to claim 1.

[0016] In the context of the present invention, the term "lignosulfonate precursor" refers to a mixture of water-soluble lignosulfonate oligomers. The lignosulfonate oligomers can be composed, in particular, of the partial structures or monomer units coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol, with individual hydroxyl groups being substituted by sulfonyl groups. An important chemical structural property of the lignosulfonate precursors according to the present invention is the presence of reactive phenolic OH groups, which can be reduced to an oxygen radical by the radical-forming enzymes or the radical-forming enzyme according to the invention.

[0017] In the context of the present invention, the term "radical-forming enzyme" can refer to any enzyme selected from laccases and / or peroxidases. The radical-forming enzymes are preferably obtained from a microbial culture, in particular from a fungal culture. Optionally, one or more radical-forming enzymes can be used to form the lignosulfonate radicals.

[0018] The gassing of the lignosulfonate precursor solution with oxygen, which is provided for in the second step of the process according to the invention, essentially refers to any method known in the prior art for supplying a solution with a gas. Suitable methods are well known to a person skilled in the art and include, for example, blowing oxygen through a sintered frit. If necessary, the availability of oxygen in the reaction mixture can be improved by vigorous stirring.

[0019] Optionally, the lignosulfonate precursors are water-soluble lignosulfonate oligomers with a molecular weight of more than 500 Da, preferably with a molecular weight of more than 5000 Da. It may therefore be provided that the lignosulfonate precursors do not contain any lignosulfonate-based molecules with a molecular weight of less than 500 Da, preferably less than 5000 Da.

[0020] Where appropriate, the solution is intended to be an aqueous solution.

[0021] The radical-forming enzyme is intended to be a laccase, a peroxidase, or a mixture of several of these enzymes. If necessary, mixtures of different laccases or peroxidases can be used.

[0022] It is intended that oxygen be added as a gas with a maximum admixture of 10 vol.%, preferably a maximum of 5 vol.%, of other gases. This means, for example, that the added oxygen may contain an admixture of approximately 2% nitrogen or other gases. However, it is preferred that oxygen be used in the form of pure oxygen without any significant admixture of other gases. Common impurities found in gaseous oxygen may be present in small quantities.

[0023] The first process step, namely the step of separating or selecting the lignosulfonate precursors, comprises the following steps: Filtering components with a particle diameter of less than 1 µm, in particular less than 100 nm, and / or separating low molecular weight components, in particular salts and low molecular weight organic compounds, preferably components with a molecular weight of less than 500 Da, preferably less than 5000 Da.

[0024] This makes it possible to obtain the molecular weight fraction preferred for the polymerization of the lignosulfonate precursor. On the one hand, the optional process steps can achieve the removal of coarse particles or fibers, and on the other hand, it can make it possible to remove any low-molecular-weight components that may interfere with the polymerization from the lignosulfonate precursor fraction.

[0025] Optionally, the pH of the solution in the second process step can be between 5 and 8, preferably between 6.5 and 7.5. The pH of the solution is preferably adjusted to the optimal activity range of the radical-forming enzyme used. The pH can be adjusted using any method known in the art, in particular by adding a basic or acidic substance. A basic substance can be, for example, sodium hydroxide solution (NaOH). An acidic substance can be, for example, hydrochloric acid (HCl).

[0026] Optionally, at least one additive is added during or after the polymerization, wherein the additive is preferably selected from elastomers, plasticizers, stabilizers, and polymers. Preferably, the addition of the additive does not result in copolymerization of lignosulfonate units with the additive, thus essentially resulting in no covalent bond between molecules of the additive(s) and the lignosulfonate polymer.

[0027] Optionally, the addition of additives can improve the properties of the resulting lignosulfonate polymer. For example, the tensile strength of the lignosulfonate polymer can be improved by adding plasticizers such as sorbitol, xylitol, or glycerin. Any substance known to a person skilled in the art as a plasticizer can be included as a plasticizer. Plasticizers can be present in the polymer in a proportion of 5% to 50% by weight.

[0028] Optionally, the process according to the invention additionally comprises the step of foaming the lignosulfonate precursor solution during polymerization to form a lignosulfonate polymer foam. To form a lignosulfonate polymer foam, for example, sodium carbonate or sodium bicarbonate in combination with an acid can be added to the reaction mixture. During the formation of carbon dioxide by the reaction of the carbonate / bicarbonate with the acid, the lignosulfonate precursor polymerizes, resulting in a solid or gel-like lignosulfonate polymer foam. Such lignosulfonate polymer foams can be used particularly advantageously for water storage.

[0029] Also described is a lignosulfonate polymer foam produced using a process according to the invention. Such a lignosulfonate polymer foam can optionally have a porosity between 20% and 90%. Porosity refers to the ratio of void volume to total volume.

[0030] Optionally, the method additionally comprises the step of pouring the lignosulfonate precursor solution during polymerization to form a lignosulfonate polymer material. In particular, it can be provided that the reaction mixture is poured while still in the liquid state into a layer with a substantially constant thickness, after which complete polymerization then takes place. After drying the polymer, a lignosulfonate polymer material, in particular a bioplastic, is obtained. Preferably, a plasticizer, such as sorbitol or glycerol, is added before complete polymerization to increase the tensile strength of the resulting polymer material.

[0031] Also described is a bioplastic made of lignosulfonate polymer material, which is produced by a process according to the invention.

[0032] To produce a bioplastic from lignosulfonate polymer material, the reaction solution produced in a process according to the invention can be poured. This means that the still-liquid reaction solution is poured into any desired shape. Such shapes can be, for example, foils, films, foams, or solid bodies of various geometries.

[0033] The thickness of a film or foil can be between 10 µm and 500 µm, preferably between 50 µm and 500 µm. However, the bioplastic produced by a process according to the invention can also have greater thicknesses to form a solid body.

[0034] Preferably, a material according to the invention additionally comprises a plasticizer, such as sorbitol or glycerin, in order to improve elasticity and tear resistance.

[0035] Where appropriate, the activity of the radical-forming enzyme after its addition to the reaction mixture is between 10 nkat / mL and 500 nkat / mL, in particular between 50 nkat / mL and 200 nkat / mL. The unit "nkat / mL" is a commonly used unit for catalytic activity in the field of enzymatic catalysis. 1 kat is the amount of enzyme that converts 1 mol of substrate in 1 second.

[0036] Also described is a lignosulfonate polymer produced by a process according to the invention. The lignosulfonate polymer produced by the process according to the invention is preferably a solid at room temperature, i.e., in particular at 25°C. Polymerization in the process according to the invention is therefore preferably carried out until a solid is obtained. The polymer thus obtained optionally has an average molecular weight of 200 kDa to 800 kDa, in particular of 300 kDa to 500 kDa.

[0037] Also described is a growth substrate for plants comprising between 0.1 wt.% and 5 wt.%, preferably between 0.5 wt.% and 2 wt.%, of a lignosulfonate polymer according to the invention. The stated amount refers to dry lignosulfonate polymer. Optionally, the growth substrate can contain between 1 wt.% and 40 wt.% of a hydrogel, i.e., a lignosulfonate polymer completely swollen in water.

[0038] Due to the good water-storing effect of the lignosulfonate polymers obtained by the process according to the invention, the lignosulfonate can preferably be used in a growth substrate for water storage. Such a growth substrate can comprise, in addition to the lignosulfonate polymer, soil, sand, clay, or other components typically found in a growth substrate for plants.

[0039] The lignosulfonate polymer can be contained in the growth substrate, particularly in the form of a lignosulfonate hydrogel. In the context of the present invention, a hydrogel refers in particular to a polymer that has a high water content or is saturated with water. Such a hydrogel can have a water content of at least 50%, preferably at least 80% or higher.

[0040] Also described is a water storage agent for a growth substrate, wherein the water storage agent comprises or consists of a lignosulfonate polymer according to the invention.

[0041] The lignosulfonate polymer, in particular the water storage agent, may optionally be in the form of a water-containing hydrogel or in dry form. In dry form, the polymer may be in the form of granules, a powder, or other known forms.

[0042] The water absorption capacity of the lignosulfonate polymer according to the invention may be up to 2000% based on the dry weight of the polymer.

[0043] The lignosulfonate polymer can be biodegradable. Thus, when used as a water storage agent for a growth substrate, it also serves as a fertilizer itself. Despite its biodegradability, its water storage capacity can be maintained for several years, for example, two to five years, in a natural environment, such as soil. Thus, when used as a water storage agent, the lignosulfonate polymer of the invention contrasts with conventional water storage agents, which are neither biodegradable nor have an inherent fertilizing effect.

[0044] If desired, in all embodiments of the process, the radical-forming enzyme can be replaced by another suitable radical-forming agent. Such a radical-forming agent can, for example, be a conventional chemical radical initiator.

[0045] The lignosulfonate polymer can optionally store not only water but also substances dissolved in water. Such substances can be salts, fertilizers, pesticides, other agrochemicals, and the like. The lignosulfonate polymer according to the invention can therefore be suitable for the continuous release of water and other substances. This can, for example, reduce the leaching of agrochemicals from the soil.

[0046] Further features of the invention emerge from the patent claims, the figures and the exemplary embodiments.

[0047] The invention is explained in detail below using exemplary embodiments. These embodiments are intended merely to illustrate the invention and do not limit its scope.

[0048] The attached figures serve to further illustrate the effects of the present invention. They show: Fig. 1 the viscosity curve as a function of the reaction time in a first process according to the invention; Fig. 2 the course of the average molecular weight as a function of the reaction time in the first process according to the invention; Fig. 3 the water absorption of a lignosulfonate polymer produced and dried by the process according to the invention; Fig. 4 the course of the relative humidity of growth substrates comprising a lignosulfonate polymer according to the invention; Fig. 5 the tensile strength of lignosulfonate polymer materials according to the invention; Fig. 6 the elongation at break of lignosulfonate polymer materials according to the invention. Example 1 - Preparation of a lignosulfonate polymer

[0049] The first embodiment describes the production of a water-insoluble lignosulfonate polymer, in particular a hydrogel containing a lignosulfonate polymer, by a process according to the invention.

[0050] The starting material for the process according to the invention according to the first embodiment is raw liquor, which is usually produced as a waste product.

[0051] In the first step of the process according to the invention, the raw liquor is filtered to remove solids such as fibers or particles. A filter with a mesh size of approximately 5 µm is used.

[0052] Subsequently, an ultrafiltration step is performed to remove salts, low-molecular-weight lignin, and other impurities from the raw liquor that could negatively impact polymerization or the quality of the polymer. Ultrafiltration steps are performed using membranes with different retention properties, namely 20 kDa, 5 nm, and 10 nm. This results in a sufficiently pure lignosulfonate precursor solution. Additionally, the lignosulfonate precursors are concentrated in the aqueous solution. In the embodiment described here, the concentration of the lignosulfonate precursors after all filtration steps is approximately 11 wt.%. The average molecular weight of the lignosulfonate precursors in the treated solution is approximately 26 kDa.

[0053] 2 L of the solution prepared as above are adjusted to a pH of approximately 7.0 by adding 1 M NaOH solution. Laccase from Myceliophthora thermophilais added until a final enzyme activity of approximately 160 nkat / mL is achieved. The reaction temperature in this embodiment is approximately 40°C. The reaction temperature can preferably be between 20°C and 70°C.

[0054] While stirring with a propeller stirrer (600 rpm), pure oxygen is blown into the solution through a sintered frit. The oxygen addition rate is approximately 200 mL / min. Every 30 minutes, a sample is taken from the reaction mixture, from which the viscosity and the average molecular weight are determined. The viscosity curve as a function of the reaction time is shown in Fig. 1 The course of the average molecular weight as a function of the reaction time is shown in Fig. 2 shown.

[0055] Out of Fig. 1 and Fig. 2It is clearly evident that both the viscosity of the solution and the molecular weight of the lignosulfonate polymers increase with the reaction time. This is an indicator of the successful polymerization of the lignosulfonate precursors.

[0056] After a reaction time of 250 minutes, stirring and the addition of oxygen are stopped. If the mixture is allowed to stand, a gel-like solid consisting of lignosulfonate polymer and water forms. This gel-like solid is also referred to as a hydrogel.

[0057] The resulting hydrogel is divided into approximately 20 g pieces and dried at 80°C for 12 hours in a drying cabinet. The weight reduction during drying is approximately 90 wt.%, which indicates that approximately 90 wt.% of the hydrogel is water. After complete drying, water is added to the polymer, and the polymer rapidly swells and absorbs water. No portions of the polymer are observed to dissolve in water; therefore, the resulting lignosulfonate polymer is water-insoluble.

[0058] The water absorption capacity of the dried polymer is tested by swelling the polymer pieces prepared as above. The water absorption curve is shown in Fig. 3 It can be seen that the amount of water absorption essentially reaches a plateau after a swelling time of 20 min, which is about 600% of the original mass of the dry polymer. Example 2 - Lignosulfonate polymer as water storage

[0059] In this example, the lignosulfonate polymer obtained in the first embodiment is used as a water reservoir in a plant growth substrate. The reswollen polymer from Example 1 is mixed with a sand-soil mixture in proportions of approximately 5, 10, and 20 wt.%. Additionally, a sand-soil mixture without added polymer is prepared as a control.

[0060] 200 g of the prepared growing media are placed in separate pots, with three pots of each substrate (sample and control) being prepared. The substrates are soaked with water for 20 hours. After this time, excess water is drained, and three bean seeds are sown in each pot.

[0061] From this point on, no more watering is carried out, and all pots are kept at the same humidity, temperature, and light intensity. Germination begins after about 10 days, with a germination rate of about 94% (17 out of 18 seeds) being observed. In addition to monitoring the growth of the seedlings, the relative humidity of the growing medium is measured at regular intervals. The course of the relative humidity of the substrate (average of three pots containing the same substrate) over time is shown in Fig. 3 While the relative humidity of the control substrate is already close to 0% after 11 days, drying out takes longer for the hydrogel-containing growth substrates. For the growth substrate with 20 wt% hydrogel, the relative humidity is still approximately 10% even after 50 days.

[0062] The Fig. 3The observation shown corresponds to the condition of the seedlings: While the plants in the control substrates began to wilt significantly after just 15 days, the plants in the hydrogel-containing substrates showed no visible signs of water deficiency over the entire test period of 50 days. Example 3 - Preparation of lignosulfonate polymer materials

[0063] For this embodiment, the lignosulfonate polymerization is carried out according to the inventive process described in the first embodiment. Before the polymerization is complete, different weight proportions of sorbitol or glycerol are added to the reaction mixture as a plasticizer, and the mixture is homogenized. Mixtures containing 25 wt.%, 28 wt.%, 33 wt.%, 40 wt.%, and 50 wt.% sorbitol or glycerol, respectively, are prepared.

[0064] After the oxygen addition and stirring are complete, the viscous reaction mixture is poured into plastic Petri dishes to a pouring height of approximately 1 mm, 2 mm, and 4 mm. The cast layers of the material are allowed to dry for approximately 30 minutes at 80°C. The mechanical properties of the material according to the invention are tested on the resulting layers.

[0065] After drying, the layers or films have thicknesses of approximately 0.10 mm to 0.40 mm.

[0066] Tests are carried out on the materials to determine their mechanical strength.

[0067] Fig. 5 shows the tensile strength of the obtained materials, Fig. 6 shows the elongation at break.

[0068] It can be seen that a higher plasticizer content results in lower tensile strength. With sorbitol, the tensile strength is higher than with the same glycerin content.

[0069] The elongation at break is essentially the opposite of the tensile strength, which Fig. 6 Samples with higher plasticizer content show greater elongation. Samples with glycerin show greater elongation than samples with sorbitol at the same levels.

Claims

1. A method for preparing a water-insoluble lignosulfonate polymer from a solution containing lignosulfonate precursors, namely from brown liquor, wherein the lignosulfonate precursor is a mixture of water-soluble lignosulfonate oligomers, the method comprising the following steps: a. separating the lignosulfonate oligomers and providing a solution with at least 5% by weight, preferably at least 10% by weight, of lignosulfonate oligomers, comprising filtering out components having a particle diameter of less than 1 µm, in particular of less than 100 nm, and removing low-molecular components with a molecular weight of less than 500 Da, b. adding an enzyme and gassing the lignosulfonate oligomer solution provided in step (a) with oxygen to form polymerizable lignosulfonate radicals, the enzyme being adapted to catalyze the formation of oxygen radicals at phenolic hydroxyl groups, the enzyme being a laccase, a peroxidase, or a mixture of several of these enzymes, oxygen being added as a gas with an admixture of not more than 10% by volume, preferably not more than 5 % by volume, of other gases, and c. polymerizing the lignosulfonate oligomers to form a water-insoluble lignosulfonate polymer.

2. The method according to claim 1, characterized in that the lignosulfonate oligomers have a molecular weight of more than 500 Da, preferably of more than 5000 Da.

3. The method according to claim 1 or 2, characterized in that the solution of step b) is an aqueous solution, wherein oxygen is injected into the solution by a sinter frit.

4. The method according to one of claims 1 to 3, characterized in that the pH of the solution in step b) is between 5 and 9, preferably between 6 and 8.

5. The method according to one of claims 1 to 4, characterized in that at least one additive is added during or after the polymerization, wherein said additive is preferably selected from elastomers, plasticizers, stabilizers and polymers.

6. The method according to one of claims 1 to 5, characterized in that the method additionally comprises the following step: - foaming the lignosulfonate oligomer solution during polymerization to form a lignosulfonate polymer foam.

7. The method according to one of claims 1 to 5, characterized in that the method additionally comprises the following step: - pouring the lignosulfonate oligomer solution during the polymerization of the lignosulfonate oligomer to form a lignosulfonate polymer material, in particular a lignosulfonate bioplastic.

8. The method according to one of claims 1 to 7, characterized in that the activity of the enzyme in step b) is between 10 nkat / mL and 500 nkat / mL, in particular between 50 nkat / mL and 200 nkat / mL.

9. The method according to one of claims 1 to 8, further comprising the following step: - forming a growth substrate for plants, which contains the water-insoluble lignosulfonate polymer in an amount between 0.1% by weight and 5% by weight, preferably between 1% by weight and 3% by weight.