WO2024261409A1

WO2024261409A1 - Quick and direct lignin-esterification for varying applications

Info

Publication number: WO2024261409A1
Application number: PCT/FI2024/050348
Authority: WO
Inventors: Sahar BABAEIPOUR; Paula NOUSIAINEN; Monika ÖSTERBERG; Alexander HENN
Original assignee: Aalto University Foundation Sr
Priority date: 2023-06-22
Filing date: 2024-06-24
Publication date: 2024-12-26

Abstract

This disclosure provides esterified lignin and a method of producing it using non-halogenated carboxylic acids, with or without a minor amount of acid anhydrides. The present disclosure also provides lignin particles and a method of producing them from the esterified lignin, and the use of the lignin particles for antifogging coatings and for photonic coating. The disclosure presents a way to achieve a lignin that performs better in multiple applications, and therefore promotes its utilization in solid products instead of incineration. By promoting the use of lignin more widely, the invention may increase the value of lignocellulosic bio-based resources and provide new renewable products that can help reduce modern societies' dependency on fossil-based resources.

Description

Quick and Direct Lignin-Esterification for Varying Applications

FIELD OF THE INVENTION

The invention belongs to the field of wood modification for a diverse range of applications, including specialty antifogging coatings, coloring agents, and renewable plastic fillers. The invention relates to a new method to replace lignin’s hydroxyl groups with acetyl groups and other ester groups with short side-chains. The method uses carboxylic acids as acetylation agent and can be catalyzed with acid. Procedure does not require distillation during the reaction, in contrast to other Fischer-esterification procedures. The selectivity of the modification can be controlled by the addition of acetic anhydrides. The resulting lignin can be used to make lignin particles at high concentrations.

BACKGROUND OF THE INVENTION

Lignin is an abundant side-stream from pulp production and is mostly incinerated for energy recovery. There is a lot of interest regarding lignin’s potential as raw material to replace fossil-based polymers in plastics and adhesives, but lignin’s amphiphilic properties and branched heterogeneous structure makes it difficult to apply. Its poor water solubility has also been an obstacle for its use in consumer goods, where volatile or alkaline solvents cannot be used. Water-based dispersions of lignin nanoparticles emerged as a potential solution for this problem, but the process of making lignin particles has challenges of its own. For example, it is difficult to produce stable lignin particles at concentrations above 10 wt.%. The low initial concentration used to create the particles results in a need to increasing the concentration post-production. Producing particles at high initial lignin concentrations lead to large particle sizes and lower yields, which means that the lignin-particle synthesis has to balance the particle size and yield with the energy that is required to concentrate the dispersions. Large particle sizes can be a drawback in applications where the performance depends on the surface-phenomena and surfaceinteractions, because large particle sizes mean a smaller total surface area. To name only a few examples, small particle-sizes are beneficial for emulsion-stabilizers and reinforcing agents in composites. Therefore, small particles can many times be more desirable, but they are also more costly to produce.

Lignin’s molecular weight and hydrophilicity also affects the particle-size. It has recently been shown that the particle sizes can be altered by using different lignin fractions (1). However, fractionation processes require large amounts of solvents, and fractionation does not necessarily remove the need for downstream processing and modification. Hydroxyl groups can be derivatized by esterification, which can reduce hydrogen bonding and therefore lead to smaller particles. Lignin esterification is most often done with halogenated carboxylic acids or acid anhydrides as they are more reactive (2). Anhydrides and halogenated carboxylic acids are nevertheless expensive compared to unmodified carboxylic acids, and are in addition harmful and corrosive which causes environmental issues and additional economic problems for the overall process. This was one of the reasons for using unmodified lignin to make particles when that technology emerged. Consequently, the processes would have to be more sustainable if lignin particles were to be made from acetylated lignin.

Direct esterification and acetylation have achieved limited success, but indirect esterification and acetylation (meaning esterification using halogenated carboxylic acids or anhydrides) can be done successfully in many different ways. US20080317661A1 describes a quick method that acetylates lignin using acetic anhydride preferably with an organic amine, such as pyridine, as a catalyst (3). CN111333860A describes the lignin esterification with acetic anhydride in deep eutectic solvents without requiring pyridine as catalyst (4). JP2019073625A describes a method to acetylate lignocellulosic biomass using isopropenyl acetate in the ionic liquid l-ethyl-3-methylimidazolium acetate (EmimAc), acting both as catalyst and solvent (5). CNH3042008A describes ligninesterification of an activated lignin using halogenated fatty acids with organic amines, such as triethylamine (6). The previously presented inventions solve small steps in the esterification procedures, but they come with unique challenges. Deep eutectic solvents and ionic liquids are relatively new classes of solvents that are not as widely used or available as conventional organic solvents. Safety and stability aspects are challenges for both. Pyridine and acetic anhydride or halogenated carboxylic acids can be used to quickly esterify lignin, but all of the previously mentioned chemicals are rather expensive and come with safety challenges. In addition, reaction times for these methods are long despite the aggressive chemicals. Direct lignin esterification has also seen some advances, but challenges remain. For example, lignin has been acetylated directly with acetic acid at elevated temperatures using microwave reactors (7). However, micro wave reactors are expensive and face challenges in scale-up that conventional systems are not affected by. Lignin has also been chemoselectively esterified using an excess of carboxylic acid without catalysts at elevated temperatures, but with long reaction times (8).

This invention tackles prior challenges in lignin- esterification. The invention is an improved method to esterify lignin in a conventional Fischer esterification system without needing other organic solvents than acetic acid. The esterification can be done in minutes at only 60 °C with a high yield. Esterified lignin has already been studied in composites, and this process provides a good alternative to the pyridine-catalyzed esterification methods. The esterified lignin made using this process can be used to prepare optically clear spherical lignin particle dispersions at high concentrations. This unlocks completely new areas for lignin particle dispersions. For example, the clarity makes lignin particles suitable for optical applications, like antifogging coatings or structural colors. The high concentration that can be used to prepare lignin particles from esterified lignin also reduces the need to concentrate the dispersions after synthesis. SUMMARY OF THE INVENTION

It is the aim of this disclosure to provide esterified lignin and a method of producing it using nonhalogenated carboxylic acids with or without acid anhydrides. The present disclosure also provides lignin particles and a method of producing them from the esterified lignin, and the use of the lignin particles for antifogging coatings and for photonic coating.

The present disclosure provides esterified lignin. The disclosure also provides a method of synthesis of the esterified lignin, wherein first, a reagent mixture, comprising one or more acid catalyst(s) and one or more carboxylic acid(s), is prepared or is provided. Optionally one or more acid anhydrides can be added to the reagent mixture. Next, the reagent mixture is mixed and heated, followed by the addition of dried lignin powder to the reagent mixture, and allowing the esterification reaction to take place. Then, the reaction is stopped and the solid lignin-containing phase and the liquid non-lignin containing phase are isolated. The solid lignin-containing phase is washed and finally, the esterified lignin is recovered.

Furthermore, the present disclosure provides lignin particles and a method of preparing the lignin particles from the achieved esterified lignin. The method comprises first re-dissolving the esterified lignin in a solvent system comprising of one or more solvent(s). Next, the lignin is precipitated into water dispersible particles, the solvents are removed from the particle dispersion, and optionally the particle dispersion is concentrated. Finally, the lignin particles are recovered. The present disclosure further provides the use of the (esterified) lignin particles for antifogging coating and for photonic coating.

The lignin used in the invention may originate from various sources and may be isolated using various processes. The invention brings the following key benefits. Firstly, the invention allows the use of carboxylic acids instead of halogenated carboxylic acids for esterification. Halogenated carboxylic acids are more expensive than regular carboxylic acids and are more hazardous and reactive. The invention also does not require the use of pyridine, which is expensive compared to acid catalyst, such as sulfuric acid. The reaction described by the invention is also quick even at moderately low temperatures. The invention promotes the use of lignin in a variety of applications. By promoting the use of lignin more widely, the invention may increase the value of lignocellulosic bio-based resources and provide new renewable products that can help reduce modem societies’ dependency on fossil-based resources.

The reduced hydroxyl group content is useful in non-polar polymer matrices, which makes the lignin useful also without further being modified as filler in polymer blends or other polymer dispersions, like in all-purpose and technical adhesives. In contrast to other esterification methods, the reaction presented within this disclosure does not require pyridine or halogenated carboxylic acids. The amount of anhydrides can also be very small, which is useful to be able to recycle all reagents used in the modification reaction and therefore can make the system more sustainable compared to other available methods. The disclosed method presents a way to achieve a lignin that performs better in multiple applications, and therefore promotes its utilization in solid products instead of incineration. The invention promotes the use of renewable materials and is in alignment with the United Nation’s sustainable development goals.

The objects of the invention are achieved by the methods, products and uses characterized by what is stated in the independent claims. The preferred embodiments of the invention are disclosed in the dependent claims. Other objects, details and advantages of the present invention will become apparent from the following detailed description.

BRIEF DESCRIPTION OF DRAWINGS

Figure 1. Shows a simplified reaction scheme of lignin acetylation, meaning lignin that has been esterified using acetic acid as esterification agent, presenting the reaction mechanism using acetic acid and acetic anhydride.

Figure 2. Results of Fourier transform infrared (FT-IR) absorbance and nuclear magnetic resonance analysis of acetylated lignins, (a, b) The full spectrum and (b) the fingerprint region of the Fourier- Transformed infrared spectra of lignins acetylated with different reaction times, and non-acetylated lignin as reference. The spectra were baseline corrected and normalized according to the aromatic C-C stretching band (1508 cm-1). (c) 31P NMR spectra of acetylated lignins.

Figure 3. Heteronuclear Single Quantum Coherence (HSQC) and hydroxyl group content of acetylated and non-acetylated lignins, (a, b) HSQC spectra of lignin acetylated with (a) acetic anhydride and (b) acetic acid for 10 minutes at 30 °C and 60 °C respectively. The area is zoomed in to show signals from the aliphatic sidechains, (c) The hydroxyl-group content of different acetylated lignins obtained from 3 IP NMR spectra presented in Figure 2 c. The reaction time was 10 minutes for all samples in the figure, and the temperature and reagent are specified in the figure legend.

Figure 4. shows the precipitation of water dispersible lignin particles with fully and partially acetylated lignin compared to unmodified lignin, (a) The appearance of the dispersions 2 hours after precipitation. In case of aggregate formation, the height of the aggregated mass is indicated, (b-c) The particle size and zeta-potential of particles made with different initial concentrations of (b) fully acetylated, (c) partially acetylated, and (d) unmodified kraft lignin. The asterisk (*) is used next to measurement points where significant sedimentation of large aggregates skew results to smaller particle-size, (e) Atomic force microscopy images of fully-, partially-, and non-acetylated lignin particles made with an initial concentration of 10 g/1. The particle diameters were obtained with the NanoScope Analysis 3.0 software.

Figure 5. shows the appearance, hydrophilicity, and antifogging properties of acetylated lignin particle layers, (a, b) Animated model of the water-interactions of (a) uncoated and (b) coated glass, (c, d) Photo demonstrating temperature-induced condensation of water onto (c) uncoated and (d) coated glass, (e, f) The steam-induced condensation of water onto (e) uncoated and (f) coated glass, (g, h) Water contactangles on (g) uncoated and (h) coated glass.

Figure 6. Scanning electron microscopy images of (a, b) monolayers and (c, d) multilayers of esterified lignin particles (a, c) before and (b ,d) after exposure to steam, showing crack-formation in multilayers and preserved structures in monolayers, (e) Animated model of the cracking mechanism. The coating thickness is 0.4 pl/cm2 in (a) and (b) and 20 pl/cm2 in (c) and (d). (k)

Figure 7. shows the use of thin films of partially acetylated lignin particles as photonic coatings, (a) An illustration of the preparation method and (b) a simplified illustration of the principles behind thin films interference. The image shows an example of destructive interference. The red arrows show the optical path difference, (c) Heights curves of the layers obtained via atomic force microscopy measurements. The heights were corrected according to slope of the surface, (d) Pictures of samples from above and at an angle of 40°.

Figure 8. shows the effect of (a) sulfuric acid : acetic acid ratio, (b) reaction time, and (c) temperature on the acetylation reaction, measured using FT-IR absorbance. The standard condition in a-c was a reaction time of 10 minutes, a temperature of 60 °C, and a sulfuric acid : acetic acid ratio of 0.5 : 3 (v:v), and one parameter was changed at a time. The spectra were baseline corrected and normalized according to the aromatic C-C stretching band (1508 cm-1).

Figure 9. shows the effect of acetic anhydride : acetic acid ratio on the acetylation reaction, measured using FT-IR absorbance. The reaction time was 10 minutes, the temperature was 60 °C, and a sulfuric acid : acetic acid ratio of 0.5 : 3 (v:v) was used. The spectra were baseline corrected and normalized according to the aromatic C-C stretching band (1508 cm-1).

DETAILED DESCRIPTION OF EMBODIMENTS

The invention is described in more detail in the following description with reference to some embodiments, which shall not be regarded as limiting. In this description and claims, the unit of temperature expressed as degrees C correspond to °C.

With the context of this specification, the term ’’esterification” refers to the grafting of matter via an ester structure. For example, the grafting of an acetyl group (derived from acetic acid or acetic anhydride) to a lignin molecule via the lignin’s hydroxyl groups, with an ester bond form between the two original molecules. Within the context of this specification, acetylation refers to an esterification reaction, wherein an acetyl group is introduced into a chemical compound.

With the context of this specification, the term ’’lignin particles” refers to both lignin particles as such as well as lignin particle dispersion.

The synthesis of esterified lignin

A method for synthesis of esterified lignin is disclosed. The method comprises first providing or preparing a reagent mixture, comprising one or more acid catalyst(s) and one or more carboxylic acid(s). Optionally one or more acid anhydrides can be added to the reagent mixture. Next, the reagent mixture is mixed and heated, followed by the addition of dried lignin powder to the reagent mixture, and allowing the esterification reaction to take place. Then, the reaction is stopped and the solid lignincontaining phase and the liquid non-lignin containing phase are isolated. The solid lignin-containing phase is washed and finally, the esterified lignin is recovered.

In one embodiment, a carboxylic acid of choice (or a mixture of multiple carboxylic acids) is mixed with an acid catalyst (or a mixture of multiple acid catalysts) to form a reagent mixture. If multiple catalysts or carboxylic acids are used, there is no specific order that the chemicals need to be combined in. The carboxylic acid(s) can be miscible with the acid catalyst(s). In one embodiment, the carboxylic acid(s) is selected from acetic acid, propionic acid, butanoic acid, or pentanoic acid, or others. Nonlinear organic acids can also be used. The acid catalyst(s) can be any acid that is able to protonate the chosen carboxylic acid(s). The acid catalyst can be dry and contain maximally 15 wt.% water. Alternatively, the acid catalyst can contain 2 - 5 wt.% water. In one embodiment the acid catalyst is selected from sulfuric acid, hydrochloric acid or p-toluenesulfonic acid. In another embodiment, the ratio of the acid catalyst(s) to the carboxylic acid(s) ratio is between 0.005 to 0.8 mol : mol, or 0.02 to 0.5 mol : mol. In another embodiment, the ratio of the acid catalyst(s) to the carboxylic acid(s) is between 0.02 to 0.45 mol : mol, or 0.02 to 0.25 mol : mol, or 0.07 to 0.25 mol : mol.

In a further embodiment, one or several acid anhydrides can be added to the formed reagent mixture. In one embodiment, the acid anhydride is selected from acetic anhydride, propionic anhydride, pentanoic anhydride, acetyl butyrate, or propanyol butyrate. In a further embodiment, the ratio of the acid anhydride(s) to the carboxylic acid(s) is between 0.002 to 1 mol : mol, between 0.01 to 1 mol : mol, or 0.002 to 0.3 mol : mol. In another embodiment, the ratio of the acid anhydride(s) to the carboxylic acid(s) is 0.005 to 0.01 mol : mol, or 0.02 to 0.15 mol : mol.

The reagent mixture is then properly mixed and heated to the desired reaction temperature. In one embodiment, the reagent mixture is heated to a temperature between 5 to 150 degrees C, or between 10 to 150 degrees C. In another embodiment, the reagent mixture is heated between 20 to 80 degrees C, or between 30 to 100 degrees C, or between 50 to 70 degrees C.

Dried lignin is then added to the reagent mixture. The lignin used in the invention may originate from various sources and may be isolated using various processes. In one embodiment, the dried lignin has a moisture content of less than 20 w.%. In another embodiment, the dried lignin has a moisture content of less than 7 wt.%. The lignin can be powdery and not include large clumps and can be added evenly while the solution is being mixed to avoid the formation of clumps. In the synthesis of the esterified lignin any lignin that has aliphatic or phenolic hydroxyl groups may be used as a raw material. Some examples of suitable lignin types include kraft lignin, organosolv lignin, alkali lignin, enzymatic lignin, acid hydrolysis lignin, etc., different types of modified lignins, such as lignin particles, fractionated lignins, or depolymerized lignins, are also suitable, ft is advantageous that the lignin is unmodified after its isolation. The lignin may or may not be purified. In one embodiment, the lignin concentration in the reagent mixture is between 1 to 40 wt.%, or 1 to 50 wt.%. In another embodiment, the lignin concentration in the reagent mixture is between 10 to 30 wt.%, or 5 to 30 wt.%, or 5 to 20 wt.%. Optionally, the lignin’s hydroxyl group content does not exceed the amount of carboxylic acid and anhydride in the reagent mixture. In one embodiment, the esterification reaction of lignin takes place at 10 to 150 degrees C. In another embodiment, the esterification reaction of lignin takes place at 30 to 100 degrees C. In a further embodiment, the esterification reaction of lignin takes place for 3 to 120 minutes, or 3 to 60 minutes. In a further embodiment, the esterification reaction of lignin takes place for 5 to 60 minutes, or for 5 to 30 minutes, or for 5 to 20 minutes. In another embodiment, the reaction is stopped by precipitating the esterified lignin into suitable solvent. The stopping and precipitation may occur simultaneously. In another embodiment, the solvent is selected from water or water supplemented with NaOH. The need of other solvents and supplements vary depending on the lignin content and catalyst to reagent ratio. The solvent may optionally be cooled down to temperatures between 1 - 20 °C to increase the precipitation efficiency.

The liquid and solid phase is then separated by a suitable method. In one embodiment, the solid lignincontaining phase and the liquid non-lignin containing phase are isolated by a method selected from filtering, decanting or centrifugation of the suspension. In one embodiment, when NaOH is not used in the precipitation water, the carboxylic acid(s) and the acid catalyst(s) are recovered for recycling through removal of water from the non-lignin containing phase by a method selected from distillation or solvent-extraction. The esterified lignin is then washed to remove the acidic solvents. In one embodiment, the washing method is selected from centrifugation, re-dispersion and dilution, or washing over a filter, or dialysis, or a method of the likes. In another embodiment, the washing comprises one or more washing rounds. In another embodiment, the washing liquid from the washing step is recovered to recycle the reagents present in the washing liquid. The esterified lignin can optionally be neutralized during or after washing. In one embodiment, the esterified lignin is neutralized using a base selected from NaOH, Ca(OH)2, or ammonium hydroxide. In a further embodiment, the esterified lignin concentration can be increased by centrifugation or by pressing, or any method of the likes. In an additional embodiment, the esterified lignin is dried at ambient conditions, or at a temperature of 10 to 100 degrees C and/or a pressure from ambient pressure to 1 mbar. The esterified lignin, produced by the above-described method, is provided. The esterified lignin may be used as raw material for epoxidation reactions, as hardener for epoxies, and thus be used for coatings or adhesives.

The use of esterified lignin to prepare lignin particles

A method of preparation of lignin particles from the esterified lignin is disclosed. The method is a modified version of the method described in W02020109671A1(9) or US11524974B2 (10), and is adjusted according to the used lignin. The method comprises re-dissolving the esterified lignin in a solvent system comprising of one or more solvent(s), precipitation of the lignin into water dispersible particles, and removal of the solvents from the particle dispersion. Optionally, the particle dispersion may be concentrated. Finally, the lignin particles are recovered.

The esterified lignin is dissolved in a suitable solvent system. To make hybrid particles, other components may be co-dissolved along with the lignin. The solvent system may require only one organic solvent, or it may require a mixture of solvents. In one embodiment, the one or more solvent(s) is selected from acetone, tetrahydro furan, dimethyl sulfoxide, 1,4-dioxane, chloroform, ethanol, methanol, and solvents of the likes. In one embodiment, the solvent system includes water. In a further embodiment, the concentration of the esterified lignin in the solvent system is between 1 - 60 wt.%. In another embodiment, the concentration of the esterified lignin in the solvent system is between 5 - 30 wt.%. The water-content that is most suitable depends on the degree of esterification and the length and character of the ester side-chains. The lignin is mixed in the solvent system until properly dissolved. The lignin is then precipitated. In one embodiment, the precipitation of the lignin into water dispersible particles is achieved by adding the lignin solution to stirring water. In another embodiment, the precipitation of the lignin into water dispersible particles is achieved by adding water into stirring lignin solution. In another embodiment, the amount of water is 1.0 - 5 times the weight of the solution weight. In another embodiment, the amount of water is between 1.5 - 3 time, the weight of the solution weight. The lignin can be stirred in water for 1 to 30 minutes. Alternatively, the lignin can be stirred in water for 2 to 10 minutes. The solvents can then be removed from the particle dispersion. In one embodiment, the removal of the solvents from the particle dispersion is achieved by a method selected from distillation or dialysis. In one embodiment, one or more solvent(s) can be added to reduce surface tension of the particle dispersion. In another embodiment, the solvent(s) is selected from ethanol, methanol, or acetone. In a further embodiment, when acetone is present in the particle dispersion, some or all of the acetone may be left in the dispersion to reduce surface tension of the particle dispersion. The dispersion can be concentrated. In one embodiment, the concentration of the particle dispersion is achieved by a method selected from ultrafiltration, centrifugation followed by the removal of the supernatant and re-dispersion, or distillation. The particles can further be modified. In a further embodiment, the lignin particles are further modified by cross-linking with epoxies, or laccases, or other suitable chemical or enzymatic cross-linking methods.

The lignin particles, produced by the method described above, are disclosed. In one embodiment, the lignin particles are hybrid particles, consisting of esterified lignin and one or more other compounds. In another embodiment, the one or more compounds are unmodified or modified. In a further embodiment, the esterified lignin is used to create hybrid lignin particles together with unmodified lignin or another compounds. The esterified lignin particles may be used for any applications that regular lignin particles can be used. For example, they can be used to stabilize Pickering emulsions, prepare hybrid particles for multiple applications (such as phase change materials or antibacterial agents), adsorb and remove viruses and bacteria from aqueous mixtures, strengthen composites, act as rheology modifier in gels, and more. The esterified lignin particles may be used as such or as lignin particle dispersion. In the following embodiments of uses the term “lignin particle” covers both the lignin particles as such as well as lignin particle dispersion.

The use of (esterified) lignin particles for antifogging coatings and for photonic coating

Use of lignin particles, produced by the method described above, for antifogging coating is disclosed. Additionally, use of lignin particles, produced by the method described above, for photonic coating is disclosed. The lignin particles are deposited on a surface to be used as antifogging coating or photonic coating. Both types of coatings, for which the lignin particles are used, are water-based. The esterified lignin particles can form transparent particle dispersions when prepared according to the above described methods of synthesis of esterified lignin and preparation of lignin particles from the esterified lignin. Thus, they are suitable for both types of coating. In one embodiment, the lignin particles used in either of the coatings are hybrid particles, consisting of lignin and one or more other compounds. In one embodiment, the one or more compounds are unmodified or modified. In one embodiment, the esterified lignin is used to create hybrid lignin particles together with unmodified lignin or another compound (or other compounds), which hybrid particles can be used for either of the coatings. The particles can further be modified. In a further embodiment, the lignin particles used in either of the coatings are further modified by cross-linking with epoxies, or laccases, or other suitable chemical or enzymatic means.

For antifogging coatings, it is desirable if the lignin particles are not seemingly visible on the coated surface, especially if the surface is transparent. Very small lignin particles with sizes 1-1000 ran, preferably 1-300 nm, can be optically less apparent. Thus, the esterified lignin particles prepared from esterified lignin are suitable for use in antifogging coating. Alternatively, for antifogging coatings other lignin particles prepared by other mean may be used as well. Optionally, the lignin particle dispersion can contain organic solvents, and the main dispersant can be water, and the particles in the dispersion can stay solid and not dissolve in the dispersion. The dispersion’s pH can vary as long as the particles remain stable and do not dissolve. The lignin dispersion is first spread on the desired surface e.g. using a textile cloth, a rubber film, a rod, or by spraying. The dispersion can also be deposited as droplets or simply poured onto the surface, forming a pool, which can then be allowed to dry on the surface. The surface’s material can be smooth and optionally does not absorb water. The dispersion’s concentration can vary. For example, the concentration can be between 0.1 - 100 g/1, or between 0.5 - 10 g/1. For photonic coatings, the lignin particle dispersion can be prepared from esterified or non-esterified lignin, and can be cross-linked using epoxies (e.g., as described in W02021240071 Al) or using other chemical or enzymatic means. The photonic coatings work best on slightly reflective non-transparent surfaces. Examples of suitable surface-materials are metal (e.g. various steels, iron, copper, aluminum, various metal alloys e.g., like bronze or brass, and other metallic materials or others of the likes), on polymers (such as polyurethanes, cured combinations of epoxies and hardeners, such as bisphenol diglycidyl ether and triethylenetetramine or any surface polymerized using epoxy ring-opening polymerization, acrylic lacquers, thermoplastics like polyethylene or polypropylene, and other thermosets or thermoplastics that can be used to prepare even surfaces), on composites (such as fiber- or glass-reinforced polymer composites, like fiberglass or others of the likes with thermosetting or thermoplastic polymer matrices), glass surfaces and various other oxides (e.g., silicon dioxide wafers), and other materials of the likes. The antifogging coating can be prepared by simply depositing lignin particle dispersion onto the substrate. The necessary concentration depends on the particle size. The concentrations for the lignin particle dispersions can be between 0.01 - 40 wt.%, or 0.05 - 5 wt.%. The color can be controlled using more deposition methods, like layer-by-layer deposition, spin-coating, rod-coating, spraying, or other coating techniques of the likes. The lignin can be modified, for example esterified as described above, or non-modified.

The following examples are given to further illustrate the invention without, however, restricting the invention thereto.

EXAMPLES

Example 1. Synthesis of esterified lignin with a sulfuric acid : acetic acid ratio of 0.16 : 1 mol : mol at 70 degrees C using centrifugation to retrieve the solids

3 ml acetic acid is mixed with 400 pl sulfuric acid while being stirred. The temperature is increased to 70 degrees C. Then, 0.3 g dry lignin is added evenly added to the mixture to avoid the formation of clumps. The reaction is allowed to take place for 5 minutes. The solution is precipitated in 20 ml deionized water, cooled to 10 degrees C. The suspension is agitated with careful movements to mix the water and the acid solvents. The suspension is centrifuged and the supernatant is collected. 50 ml more deionized water is added, and the esterified lignin is re-dispersed. The mixture is then centrifuged using the same procedure twice more, and the esterified lignin is optionally dried using vacuum, heat, or in ambient conditions.

Example 2. Synthesis of esterified lignin with a sulfuric acid : acetic acid ratio of 0.08 : 1 mol : mol at 80 degrees C using filtration to retrieve the solids

3 ml acetic acid is mixed with 200 pl sulfuric acid while being stirred. The temperature is increased to 80 degrees C. Then, 0.3 g dry lignin is added evenly added to the mixture to avoid the formation of clumps. The reaction is allowed to take place for 20 minutes. The solution is precipitated in 50 ml deionized water, cooled to 10 degrees C. The suspension is agitated with careful movements to mix the water and the acid solvents. The suspension is filtered with under-pressure using a Buchner funnel, and the solvents are collected and isolated. The esterified lignin is then washed with 100 ml deionized water, added with a rate of 50 ml/minute. The esterified lignin optionally dried using vacuum, heat, or in ambient conditions.

Example 3. Synthesis of esterified lignin with a sulfuric acid : propionic acid ratio of 0.1 : 1 mol : mol at 80 degrees C using filtration to retrieve the solids

3 ml propionic acid is mixed with 200 pl sulfuric acid while being stirred. The temperature is increased to 80 degrees C. Then, 0.3 g dry lignin is added evenly added to the mixture to avoid the formation of clumps. The reaction is allowed to take place for 20 minutes. The solution is precipitated in 50 ml deionized water, cooled to 10 degrees C. The suspension is agitated with careful movements to mix the water and the acid solvents. The suspension is filtered with under-pressure using a Buchner funnel, and the solvents are collected and isolated. The esterified lignin is then washed with 100 ml deionized water, added with a rate of 50 ml/minute. The esterified lignin optionally dried using vacuum, heat, or in ambient conditions.

Example 4. Synthesis of esterified lignin with a sulfuric acid : butyric acid ratio of 0.16 : 1 mol : mol at 70 degrees C using filtration to retrieve the solids

3 ml butanoic acid is mixed with 300 pl sulfuric acid while being stirred. The temperature is increased to 70 degrees C. Then, 0.3 g dry lignin is added evenly added to the mixture to avoid the formation of clumps. The reaction is allowed to take place for 20 minutes. The solution is precipitated in 50 ml deionized water, cooled to 10 degrees C. The suspension is agitated with careful movements to mix the water and the acid solvents. The suspension is filtered with under-pressure using a Buchner funnel, and the solvents are collected and isolated. The esterified lignin is then washed with 100 ml deionized water, added with a rate of 50 ml/minute. The esterified lignin optionally dried using vacuum, heat, or in ambient conditions. Example 5. Synthesis of esterified lignin with a sulfuric acid : acetic acid ratio of 0.16 : 1 mol : mol and a small amount of acetic anhydride

3 ml acetic acid is mixed with 400 .1 sulfuric acid while being stirred. 100 pl acetic anhydride is added to the mixture. The temperature is increased to 80 degrees C. Then, 0.3 g dry lignin is added evenly added to the mixture to avoid the formation of clumps. The reaction is allowed to take place for 20 minutes. The solution can then be precipitated and washed following any of the procedures described in the examples above.

Example 6. Synthesis of esterified lignin with precise control over the selectivity and type of ester side chain

3 ml propionic acid is mixed with 400 pl sulfuric acid while being stirred. The temperature is increased to 60 degrees C. Then, 0.3 g dry lignin is added evenly added to the mixture to avoid the formation of clumps. The reaction is allowed to take place for 5 minutes. 500 pl acetic anhydride is added to the mixture, and the reaction is allowed to proceed for another 20 minutes. The solution can then be precipitated and washed following any of the procedures described in the examples above.

Example 7. The use of esterified lignin to prepare small lignin particles with an initial lignin concentration of 100 g/1

Lignin is first esterified according to Example 1. Synthesis of esterified lignin with a sulfuric acid : acetic acid ratio of 0.16 : 1 mol : mol at 70 degrees C using centrifugation to retrieve the solids. The esterified lignin is dried using vacuum. Then, the esterified lignin is dissolved in 3 ml of a mixture of 75 wt.% acetone and 25 wt.% deionized water. The solution is stirred for 1 hour. The solution is then added to 9 ml deionized water under vigorous stirring. The dispersion is allowed to stir for 15 minutes. The solvents can then removed by distilling the solution at 40 degrees C and 100 mbar for 20 minutes or by dialysis, or using another suitable method. Certain solvents, like ethanol, can be added later to reduce the dispersions surface tension. Some or all of the acetone can also be left in the dispersion for the same purpose.

Example 8. The use of esterified lignin to prepare small lignin particles with an initial lignin concentration of 10 g/1

Lignin is first esterified according to Example 1. Synthesis of esterified lignin with a sulfuric acid : acetic acid ratio of 0.16 : 1 mol : mol at 70 °C using centrifugation to retrieve the solids. The esterified lignin is dried using vacuum. Then, the esterified lignin is dissolved in 30 ml of a mixture of 75 wt.% acetone and 25 wt.% deionized water. The solution is stirred for 1 hour. The solution is then added to 90 ml deionized water under vigorous stirring. The dispersion is allowed to stir for 15 minutes. The solvents can then be removed by distilling the solution at 40 degrees C and 100 mbar for 20 minutes or by dialysis, or another suitable method. Certain solvents, like ethanol, can be added later to reduce the dispersions surface tension. Some or all of the acetone can also be left in the dispersion for the same purpose.

Example 9. The use of esterified lignin particles for antifogging coatings applied using a rubber film

The dispersion prepared in Example 8 is diluted to 2 g/1. The dispersion is then evenly spread on a glass surface using a non-absorbing latex film. A spread of 0.4 pl/cnr is used, and the dispersion is spread across the glass surface until all the liquid has evaporated.

Example 10. The use of esterified lignin particles for antifogging coatings applied using spraying and a textile cloth

The dispersion prepared in Example 8 is diluted to 2 g/1. The dispersion is sprayed with a manual spraying flask until the glass surface is completely covered. A cellulose textile cloth is used to even out the dispersion droplets and wipe the surface. The textile should not be so big that it can absorb all the dispersion, ft is beneficial if the textile is small enough to be significantly wetted by the liquid while the dispersion is being wiped to guarantee that not all of the liquid is absorbed into the textile.

Example 11. The use of lignin particles for antifogging coatings applied using spraying and a textile cloth

A lignin dispersion from unmodified kraft lignin is made according to the general principles described in W02020109671A1(9), or US11524974B2 (10). The particles can be prepared as described in Example 8, but with non-modified lignin instead of esterified lignin. The dispersion is diluted to 2 g/1. The dispersion is sprayed with a manual spraying flask until the glass surface is completely covered. A cellulose textile cloth is used to even out the dispersion droplets and wipe the surface. The textile should not be so big that it can absorb all the dispersion, ft is beneficial if the textile is small enough to be significantly wetted by the liquid while the dispersion is being wiped to guarantee that not all of the liquid is absorbed into the textile.

Example 12. The use of esterified lignin particles as photonic coating on silica using layer-by-layer deposition with poly-L-lysine as cationic anchor

Prior to the layer-by-layer assembly, the silica substrates are pretreated by immersion in 1 M NaOH for 5 seconds, rinsing with Milli-Q-water and acetone, and then ozonized in a UV-ozoniser for 20 minutes. 0.1 % poly-L-lysine (PLL) is used as cation, and a 0.5 wt.% lignin particle dispersion acetylated as described in Example 1 and made into particle as described in Example 8 is used as anion. PLL is first deposited onto the silica substrate by submerging the silica in PLL solution for one minute. The wafer is then carefully rinsed with deionized water and dried by pressurized air. Then, the wafer is submerged in lignin dispersion solution for two minutes to add PLL onto the particles, and rinsed and dried as previously described. These steps are alternated 20 times to achieve a blue color. The immersion time can be longer, for example, 10 - 30 minutes, for the first 3 - 5 bilayers to ensure complete deposition of PLL and lignin particles.

REFERENCES

1. Wang J, Chen W, Y ang D, Fang Z, Liu W, Xiang T, et al. Correction to: Monodispersed Lignin Colloidal Spheres with Tailorable Sizes for Bio-Photonic Materials. Small. 2022;18(30):2200671.

2. Moreno A, Liu J, Gueret R, Hadi SE, Bergstrom L, Slabon A, et al. Unravelling the Hydration Barrier of Lignin Oleate Nanoparticles for Acid- and Base-Catalyzed Functionalization in Dispersion State. Angew Chemie - Int Ed. 2021;60(38).

3. Eckert RC, Abdullah Z. Carbon fibers from kraft softwood lignin. United States of America; US20080317661A1, 2008.

4. Liu Z, Li Y, Ming H. Improved method for acetylation of alkali lignin. China; CN111333860A, 2020.

5. Suzuki S, Takahashi K. Method for producing cellulose derivative, hemicellulose derivative, and lignin derivative. Japan: Kanazawa University, Kanazawa Institute of Technology; JP2019073625A, 2017.

6. Huifang Z, Peng QS, Daliang G, Jing L. Alkali lignin micro/nanosphere/paper-based adsorption material, preparation method thereof and application thereof in treatment of dye wastewater. China; CN113042008A, 2021.

7. de Oliveira DR, Avelino F, Mazzetto SE, Lomonaco D. Microwave-assisted selective acetylation of Kraft lignin: Acetic acid as a sustainable reactant for lignin valorization. Int J Biol Macromol. 2020;164:1536-44.

8. Liu LY, Hua Q, Renneckar S. A simple route to synthesize esterified lignin derivatives. Green Chem. 2019;21 (13):3682— 92.

9. Osterberg M, Sipponen MH, Kostiainen MA, Akras L, Riviere G, Zhang X. Lignin particle based hydrogel and the method for preparation of lignin colloidal particles by solvent evaporation process. Finland; W02020109671A1, 2019.

10. Lintinen K, Bangalore Ashok RP, Leskinen T, Xiao Y, Osterberg M, Kostiainen M, et al. Aqueous lignin dispersions and methods of preparing the same. Finland; US 11524974B2, 2018.

Claims

1. A method of synthesis of esterified lignin, wherein the method comprises the steps of: a) providing or preparing of a reagent mixture, which reaction mixture comprises one or more acid catalyst(s) and one or more carboxylic acid(s); and optionally one or more acid anhydride(s); b) mixing and heating of the reagent mixture; c) addition of dried lignin powder to the reagent mixture and allowing the esterification reaction to take place; d) stopping the reaction and isolation of the solid lignin-containing phase and the liquid non-lignin containing phase; e) washing the solid lignin-containing phase; and f) recovering the esterified lignin.

2. The method of claim 1, wherein the acid catalyst is selected from sulfuric acid, hydrochloric acid or p-toluenesulfonic acid.

3. The method of any of claim 1 to 2, wherein the carboxylic acid is selected from acetic acid, propionic acid, butanoic acid, or pentanoic acid.

4. The method of any of the preceding claims, wherein the acid anhydride is selected from acetic anhydride, propionic anhydride, pentanoic anhydride, acetyl butyrate, or propanyol butyrate.

5. The method of any of the preceding claims, wherein the ratio of acid catalyst(s) to the carboxylic acid(s) ratio is between 0.005 to 0.8 mol : mol, preferably between 0.02 to 0.45 mol : mol.

6. The method of any of the preceding claims, wherein the ratio of the acid anhydride(s) to the carboxylic acid(s) is between 0.002 to 1 mol : mol, preferably 0.005 to 0.01 mol : mol.

7. The method of any of the preceding claims, wherein the reagent mixture in step b) of claim 1 is heated to a temperature between 5 to 150 degrees C, preferably between 20 to 80 degrees C.

8. The method of any of the preceding claims, wherein the dried lignin in step c) of claim 1 has a moisture content of less than 20 w.%, preferably less than 7 wt.%.

9. The method of any of the preceding claims, wherein the lignin concentration in the reagent mixture is between 1 to 40 wt.%, preferably between 10 to 30 wt.%.

10. The method of any of the preceding claims, wherein the reaction in step c) of claim 1 takes place for 3 to 120 minutes, preferably 5 to 60 minutes, at 10 to 150 degrees C, preferably 30 to 100 degrees C.

11. The method of any of the preceding claims, wherein the reaction in step d) of claim 1 is stopped by precipitating the esterified lignin into suitable solvent.

12. The method of claim 11, wherein the solvent is selected from water or water supplemented with NaOH.

13. The method of any of the preceding claims, wherein the solid lignin-containing phase and the liquid non-lignin containing phase in step d) of claim 1 are isolated by a method selected from filtering, decanting or centrifugation of the suspension.

14. The method of any of the preceding claims, wherein the washing method in step e) of claim 1 is selected from centrifugation, re-dispersion and dilution, or washing over a filter, or dialysis, wherein the washing comprises one or more washing rounds.

15. The method of any of the preceding claims, wherein the washing liquid from step e) of claim 1 is recovered to recycle the reagents present in the washing liquid.

16. The method of any of the preceding claims, wherein the esterified lignin is neutralized using a base selected from NaOH, Ca(OH)2, or ammonium hydroxide.

17. The method of any of the preceding claims, wherein the esterified lignin concentration can be increased by centrifugation or by pressing.

18. The method of any of the preceding claims, wherein the esterified lignin in step f) of claim 1 is dried at ambient conditions or at a temperature of 10 to 100 degrees C and/or a pressure from ambient pressure to 1 mbar.

19. Esterified lignin produced by the method of any of claims 1 to 18.

20. A method of preparation of lignin particles from the esterified lignin achieved by the method of any of claim 1 to 18, wherein the method comprises the steps of a) re-dissolving the esterified lignin in a solvent system comprising of one or more solvent(s); b) precipitation of the lignin into water dispersible particles; c) removal of the solvents from the particle dispersion; d) optionally concentration of the particle dispersion; and e) recovering the lignin particles.

21. The method of claim 20, wherein the one or more solvent(s) in step a) is selected from acetone, tetrahydro furan, dimethyl sulfoxide, 1,4-dioxane, chloroform, ethanol, methanol.

22. The method of any of claim 20 to 21, wherein the solvent system in step a) includes water.

23. The method of any of claim 20 to 22, wherein the concentration of the esterified lignin in the solvent system is between 1 - 60 wt.%, preferably 5 - 30 wt.%

24. The method of any of claim 20 to 23, wherein the precipitation of the lignin into water dispersible particles in step b) of claim 20 is achieved by adding the lignin solution to stirring water, or by adding water into stirring lignin solution, wherein the amount of water is 1.0 - 5 times, preferably between 1.5 - 3 time, the weight of the solution weight.

25. The method of any of claim 20 to 24, wherein the removal of the solvents from the particle dispersion in step d) of claim 20 is achieved by a method selected from distillation or dialysis.

26. The method of any of claim 20 to 25, wherein one or more solvent(s) are added to reduce surface tension of the particle dispersion, wherein the solvent(s) is selected from ethanol, methanol, or acetone. l ' l. The method of any of claim 20 to 25, wherein when acetone is present in the particle dispersion, some or all of the acetone may be left in the dispersion to reduce surface tension of the particle dispersion.

28. The method of any of claim 20 to 27, wherein the concentration of the particle dispersion in step e) of claim 20 is achieved by a method selected from ultrafiltration, centrifugation followed by the removal of the supernatant and re-dispersion, or distillation.

29. The method of any of claim 20 to 28, wherein the lignin particles are further modified by crosslinking with epoxies, or laccases.

30. Lignin particles produced by the method of any of claims 20 to 29. 31. The lignin particles of claim 30, wherein the lignin particles are hybrid particles, consisting of lignin and one or more other compounds, wherein the compounds are unmodified or modified; and wherein the esterified lignin is used to create hybrid lignin particles together with unmodified lignin or another compound.

32. Use of lignin particles of any of claim 29 to 31 for antifogging coating. 33. Use of lignin particles of any of claim 29 to 31 for photonic coating.