EP4634266A1

EP4634266A1 - Flame retardant coatings and polymer additives comprising colloidal particles prepared from nitrogen-modified lignin

Info

Publication number: EP4634266A1
Application number: EP23825415.5A
Authority: EP
Inventors: Petri Widsten
Original assignee: VTT Technical Research Centre of Finland Ltd
Current assignee: VTT Technical Research Centre of Finland Ltd
Priority date: 2022-12-14
Filing date: 2023-12-12
Publication date: 2025-10-22
Also published as: FI20226103A1; FI130852B1; WO2024126895A1

Abstract

The present invention relates to flame retardant compositions comprising colloidal particles prepared from nitrogen-modified lignin, to a method of producing said flame retardant compositions, and to their use as flame retardant coatings or as polymer additives.

Description

FLAME RETARDANT COATINGS AND POLYMER ADDITIVES COMPRISING

COLLOIDAL PARTICLES PREPARED FROM NITROGEN-MODIFIED LIGNIN

FIELD

[0001] The present invention relates to flame retardant compositions comprising colloidal lignin particles prepared from nitrogen-modified lignin. The invention also relates to a method of producing said flame retardant compositions and to their use as flame retardant coatings or as polymer additives.

BACKGROUND

[0002] In the field of flame retardant chemicals and systems, using bio-based materials such as technical lignins as the carbon source has been the focus of research in recent years. Technical lignins are by-products of chemical pulping and other lignocellulosic biorefineries. However, technical lignins are insoluble in water at acidic and neutral pH, which prevents their application as aqueous coatings by practical methods such as spraying and dipping. In addition, their large and irregular micro-sized particles have limited contact area with the substrate. These characteristics would result in uneven coating appearance and poor adhesion if such large micro-sized particles were applied as aqueous coatings.

[0003] On the other hand, regular unmodified kraft lignin has been shown not to possess significant flame retardant properties. Previous research has shown that increasing the nitrogen content of lignin to >10% improves its ability to impart flame retardancy when compounded with other materials such as plastics (Widsten et al, 2020) or deposited onto cellulose (Pohler et al, 2022). A way to introduce nitrogen into the lignin structure was via Mannich reaction using urea and formaldehyde as the reagents.

[0004] Another solution for improving coating properties of lignin is to convert lignin to colloidal particles (CLPs), which have a large surface-to-mass ratio and uniform size distribution. CLPs which may be nanoparticles or small microparticles have attracted increased attention. The preparation and application of lignin nanoparticles was reviewed by Osterberg et al (2020). Stabilization of Pickering emulsion by lignin-based colloidal particles was disclosed by Dai et al (2019). WO 2020109671 Al disclosed a hydrogel comprising colloidal lignin particles prepared by solvent exchange for use as a media for 3D cell culture. Further, the use of an aqueous dispersion of the colloidal lignin particles as a rheology modifier in hydrogels, for water purification, for example as filters, packed columns, as flocculants, or for binding of viruses was suggested.

[0005] Some attempts to prepare CLP -based flame retardants have been made. Wu et al (2021) studied a functional lignin-based colloidal filler (lignin-diethylenetriamine/red phosphorus colloidal particles) for flame-retardant blend.

[0006] Despite the above mentioned and other attempts to prepare lignin-based flame retardants, there exists a need to obtain lignin-based flame retardant material with improved coating adhesion and uniformity as well as improved flame retardancy. Moreover, a cost-efficient and simple synthesis method for the preparation of such material would be needed for industrial applications.

SUMMARY OF THE INVENTION

[0007] The invention is defined by the features of the independent claims. Some specific embodiments are defined in the dependent claims.

[0008] In the present invention it has been found that lignin chemically modified to introduce nitrogen into its structure and then converted into a mixture of nano- and microsized CLPs (N-CLPs) ranging in size from 10 to 2000 nm forms a stable aqueous dispersion, which can be coated onto substrates to impart flame retardancy to the substrate. The flame retardant coatings comprising such nitrogen-modified CLPs improve coating adhesion and uniformity and the high nitrogen content improves flame retardancy.

[0009] According to a first aspect of the present invention, there is provided a flame retardant composition for coating of substrates, comprising an aqueous dispersion of N- CLPs, wherein the nitrogen-modified lignin comprises >7% nitrogen by weight of the modified lignin.

[0010] According to a second aspect of the present invention, there is provided a method of producing a flame retardant composition comprising N-CLPs, wherein the method comprises the steps of preparing nitrogen-modified lignin by incorporating nitrogen into the lignin structure to obtain nitrogen-modified lignin comprising >7% nitrogen by weight of the modified lignin; dissolving the nitrogen-modified lignin in an organic or aqueous organic solvent; converting the nitrogen-modified lignin to N-CLPs by adding the solution of nitrogen-modified lignin into an excess of water (antisolvent); optionally removing the organic solvent from the N-CLP dispersion to obtain an aqueous dispersion comprising the N-CLPs; and optionally concentrating or drying the obtained aqueous dispersion of N-CLPs.

[0011] A further aspect of the invention relates to a method of coating substrates for imparting them with flame retardant properties, comprising coating said substrates with a flame retardant composition according to the invention, optionally in concentrated or dried form.

[0012] Another aspect relates to the use of flame retardant compositions comprising aqueous dispersions of N-CLPs for preparing flame retardant coatings on substrates, wherein the modified lignin comprises >7% nitrogen by weight of the modified lignin.

[0013] A still further aspect of the invention relates to the use of flame retardant compositions comprising colloidal particles prepared from nitrogen-modified lignin as polymer additives, optionally after drying said compositions before their incorporation into the polymer.

[0014] Considerable advantages are obtained by the invention. First, the nanosize (<1000 nm) and/or small microsize (1001-2000 nm) provides improved coating adhesion and uniformity and the high nitrogen content improves flame retardancy. The aqueous dispersions of N-CLPs will be able to be coated onto substrates by spraying, brushing, rodcoating, dipping or by other suitable means. Additional adhesion promoters such as silicon chemicals and/or surface pre-treatments such as plasma or flame treatment are options whose implementation depends on the substrate characteristics and the intended end-use application.

[0015] Further, in the event of fire the N-CLP coatings form an insulating char layer on the surface of the burning materials. The nitrogen functionalities give off nitrogenbased gases that make the char layer more intumescent, improving its insulation properties.

[0016] Third, the invention provides practical ways to apply flame-retardant and biobased coatings with low impact on human health and the environment onto substrates from aqueous dispersions. Treatment of substrates already in use and regular replenishment of the coatings is also possible. Alternatively, the substrates may be coated before installation. [0017] Further features and advantages of the present technology will appear from the following description of some embodiments.

EMBODIMENTS

[0018] DEFINITIONS

[0019] In the present context, the term “lignin” comprises lignin from softwood, hardwood, grasses or combinations thereof. In particular, the lignin comprises technical lignins, preferably selected from softwood kraft lignin (SW KL), hardwood kraft lignin (HW KL) and heat-treated kraft black liquor and combinations thereof. Within this disclosure, “modified lignin” refers to nitrogen-modified lignin, i.e., lignin modified by amino, imine or other nitrogenous groups.

[0020] Within this disclosure, colloidal lignin particles (CLPs) include nanosize (<1000 nm) and small microsize (1001-2000 nm) colloidal lignin particles, ranging in size from 10 to 2000 nm.

[0021] As used herein, the terms “essentially solvent-free” or “solvent-free” mean that the referred material contains less than 5% of solvent.

[0022] In the present invention it has been found that flame retardant coatings fully composed of or containing N-CLPs as the main component provide enhanced flame retardancy properties to cellulosic substrate coated with an aqueous dispersion comprising said N-CLPs. Moreover, the novel aqueous dispersions of N-CLPs improve coating adhesion and uniformity of coatings.

[0023] The flame retardant compositions of the invention may comprise the novel aqueous dispersions of N-CLPs as the sole or main flame retardant agent. In some embodiments, the flame retardant composition may consist essentially of the aqueous dispersion of N-CLPs, preferably in concentrated form.

[0024] In embodiments, the flame retardant composition for coating of substrates comprises an aqueous dispersion of N-CLPs, wherein the nitrogen-modified lignin comprises >7% nitrogen by weight of the modified lignin.

[0025] Thus in one embodiment the method for producing the flame retardant composition comprises a step of preparing nitrogen-modified lignin by incorporating nitrogen into the lignin structure to obtain modified lignin comprising >7% nitrogen by weight of the modified lignin.

[0026] Modification of lignin by introducing nitrogen to the lignin structure is known. One practical route to nitrogen-functional lignin (N-lignin) is the Mannich reaction of lignin with formaldehyde and amine or urea (e.g., Zhang et al, 2012):

[0027] Thus in one embodiment the step of preparing nitrogen-modified lignin comprises a step of subjecting lignin to treatment with formaldehyde and urea to obtain nitrogen-modified lignin. [0028] Another possible route to nitrogen-modified lignin is the Schiff base reaction:

[0029] In both reactions nitrogenous groups are introduced into the lignin structure. The modified lignin comprises >7% nitrogen, preferably >10% nitrogen, by weight of the modified lignin. The upper limit of nitrogen modification is naturally dependent on the structure of lignin, i.e., on the number of reactive structures available for nitrogen modification.

[0030] The nitrogen-modified lignin is converted to N-CLPs, preferably by means of solvent-exchange or other methods, to obtain an aqueous dispersion of N-CLPs, wherein the nitrogen-modified lignin comprises >7% nitrogen by weight of the modified lignin. [0031] In one embodiment, the nitrogen-modified lignin is first dissolved in an organic or aqueous organic solvent, the solution is then added to an excess of water acting as an antisolvent to generate a N-CLP dispersion, and the organic solvent is then optionally removed from the dispersion to obtain a solvent-free or an essentially solvent-free aqueous dispersion comprising the N-CLPs.

[0032] The organic solvent is typically selected from acetone, tetrahydrofuran, N,N- dimethylformamide, and alcohols, such as ethanol, isopropanol and ethylene glycol. One preferred organic solvent is acetone.

[0033] In one embodiment the nitrogen-modified lignin is dissolved in an organic or aqueous organic solvent, which comprises at least 50% (v/v) of the organic solvent, preferably at least 60%, such as 60-80% of the organic solvent.

[0034] As stated above, after dissolving the nitrogen-modified lignin in the organic or aqueous organic solvent, the solution comprising the dissolved nitrogen-modified lignin is added into an excess of water to generate N-CLP dispersions.

[0035] In one embodiment the solution comprising the dissolved nitrogen-modified lignin is added into water in a volume ratio between the solution of dissolved nitrogen- modified lignin and water of 1 :2 - 1 :10, preferably in a volume ratio of about 1 :7, to generate N-CLPs. Basically, there is no upper limit for the amount of excess water except for practical reasons. As regards the solvent-to-water ratio, typically it is sufficient for the generation of particle dispersions that the proportion of solvent in the mixture after the lignin solution is added to water is no more than 30%, preferably below 30% by weight of the mixture.

[0036] After N-CLP generation, the organic solvent is optionally removed to obtain a solvent-free or an essentially solvent-free aqueous dispersion of N-CLPs. The organic solvent may be removed for example by evaporation or dialysis against water. In one embodiment, where acetone is used as organic solvent, the acetone can be removed by stirring the dispersion in an open vessel or the acetone can be recovered and reused in the process.

[0037] Optionally, the obtained aqueous dispersion of N-CLPs may be concentrated by known methods, such as by centrifugation. In high-speed centrifugation, the nano-sized particles tend to remain in the supernatant and the micro-sized particles in the precipitate (concentrate).

[0038] As measured by DLS (Dynamic Light Scattering), after concentration of N- CLP dispersions by high-speed centrifugation, the typical sizes of the nanoparticles in the supernatant and of the microparticles in the concentrate are ca. 300 nm and 1600 nm, respectively, when hardwood kraft lignin is used as the starting material. Naturally, the N- CLP sizes will depend on the type of lignin used, its extent of nitrogen modification, and the parameters of the N-CLP preparation process.

[0039] In embodiments, the N-CLPs may have a zeta- (Q potential value. Typically, the N-CLPs have a zeta-potential value, which is below -30 mV, preferably below -40 mV, such as -50 - -40 mV, more preferably below -50 mV. Zeta-potential data were obtained from electrophoretic mobility data by applying the Smoluchowski model as disclosed in the experimental section.

[0040] Thus the N-CLPs have a high negative surface charge. The small particle size and a highly negative zeta-potential of the N-CLPs are preferred in terms of adhesion and cohesion of the coatings.

[0041] In general, direct adhesion of nitrogen-modified CLP coatings to hydrophilic surfaces (cellulosic and lignocellulosic substrates such as cellulose, viscose and wood) is good, particularly for thin coatings. Some hydrophobic substrates such as plastics may benefit from previous oxidative activation of the surface by plasma or flame treatment to improve coating spreadability and adhesion. When thick flame retardant coatings are prepared from N-CLPs, the coatings may sometimes tend to show insufficient cohesion (particle-to-particle adhesion) and adhesion to the substrate. Optionally, adhesion may be enhanced by including adhesion promoters as one or several layers of multilayer N-CLP coatings. If they do not precipitate the N-CLPs (as might occur with proteins), they can also be added to the dispersion. Therefore, in some embodiments, adhesion promoters may be included in the aqueous dispersion comprising N-CLPs.

[0042] Optional adhesion promoters may include for example vinyl ethylene acetate, acrylates, silicones, polyurethanes and proteins. In one embodiment, the adhesion promoter is selected from plant-based proteins, animal-based proteins and combinations thereof. [0043] In some embodiments, the flame retardant composition may comprise binders to improve mechanical properties of the composition. Suitable binders include but are not limited to acrylic, styrene-acrylic or polyvinyl acetate dispersions or organosilicon copolymers.

[0044] In some embodiments, the flame retardant composition may be blended with a polymer matrix. The polymer is preferably selected from thermoplastic polymers and cellulosic polymers. In some embodiments the polymer may comprise polypropylene, polyamide, polylactic acid, polybutylene succinate, epoxy polymers and polyurethanes. When used as polymer additives, it may be advantageous to dry the flame retardant composition before its incorporation into the polymer.

[0045] As mentioned above, an embodiment of the invention relates to a method of treating substrates for imparting them with flame retardant properties, comprising coating said substrates with an aqueous dispersion comprising N-CLPs or with concentrated or dried N-CLPs.

[0046] The flame retardant performance and the fire behaviour of materials are assessed commonly by measuring the amount of thermal energy released during their combustion. Quantities such as peak heat release rate (PHR or PHRR) and total heat release (THR) can be measured using, e.g., a micro-scale combustion calorimeter (MCC) or a cone calorimeter (CCM). It should be noted, however, that similar quantities obtained using different test methods are not directly comparable due to the differences in the test conditions, specimen configurations and several other factors. In addition, the mass loss rate obtained from thermogravimetric analysis (TGA) can be used as a measure of thermal degradation performance and as an indicator of fire performance.

[0047] A char layer formed on the surface of the burning material is another factor illustrating the flame retardant performance of the flame retardant material. The char layer may isolate the substrate surface from oxygen and heat and prevent the volatilisation of degradation products. The FR performance of the char layer depends on the amount and density of the char. A thicker, more porous layer has better FR properties than a thinner, denser layer with the same amount of char. The formation of poorly burning gases, such as ammonia, may also be beneficial to fire behaviour as they can dilute the pyrolysis gases and promote intumescence of the char layer, thereby improving its insulation properties. [0048] As stated above, flame retardancy results largely from the char layer, which provides insulation against heat, oxygen and flammable volatilities. In the present invention the ability of lignin to form a char layer has been increased by nitrogen modification of lignin, while also improving the coating properties of lignin by converting the nitrogen-modified lignin to small N-CLPs, which have a large surface-to-mass ratio.

[0049] In one embodiment, the method of treating substrates for imparting them with flame retardant properties comprises applying on the substrate 2 to 200 g/m², for example 3 to 150 g/m², such as 40-50 g/m², of the flame retardant composition of the invention.

[0050] The application method may comprise any one of spraying, brushing, rodcoating and any other suitable means. In some embodiments the step of applying the flame retardant composition on the substrate may comprise brushing the flame retardant composition onto the substrate. In the case of polymer additives, the flame retardant composition in wet or dry form may be blended with the matrix material.

[0051] Typically, the flame retardant composition comprises at least 0.5 wt%, such as 1-5 wt%, of N-CLPs. However, preferably the aqueous dispersion comprising N-CLPs is concentrated to obtain a dispersion that is better suited for coating application, e.g., by brushing.

[0052] The N-CLPs consist essentially of nitrogen-modified lignin. In some embodiments, the N-CLPs comprise at least 95%, preferably at least 98% of modified lignin.

[0053] As mentioned above, in addition to the aqueous dispersion of N-CLPs, the flame retardant composition of the invention may comprise binders, adhesion promoters and/or other additives known to persons skilled in the art.

[0054] In one embodiment the method of coating substrates for imparting them with flame retardant properties may comprise the following steps:

- applying a first layer of the flame retardant composition comprising N-CLPs on the substrate;

- applying a layer of adhesion promoter on the first layer of the flame retardant composition comprising N-CLPs; - applying a second layer of the flame retardant composition comprising N-CLPs on the adhesion promoter layer; and

- optionally continuing to apply alternating layers of the flame retardant composition comprising N-CLPs and the adhesion promoter on the substrate, and wherein the flame retardant composition may consist of or consist essentially of the aqueous dispersion of N-CLPs, wherein the modified lignin comprises >7% nitrogen by weight of the modified lignin.

[0055] In some embodiments a hydrophobic substrate may be pretreated by plasma or flame treatment before coating the substrate with the aqueous N-CLPs or the flame retardant composition comprising the aqueous N-CLPs dispersion as the main or sole flame retardant agent.

[0056] The method of the invention is particularly suitable for the treatment of substrates of cellulosic and lignocellulosic materials. However, the method is applicable also on any other substrate where flame retardancy is desired.

[0057] It is to be understood that the embodiments of the invention disclosed are not limited to the particular structures, process steps, or materials disclosed herein, but are extended to equivalents thereof as would be recognized by those ordinarily skilled in the relevant arts. It should also be understood that terminology employed herein is used for the purpose of describing particular embodiments only and is not intended to be limiting.

[0058] Reference throughout this specification to one embodiment or an embodiment means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Where reference is made to a numerical value using a term such as, for example, about or substantially, the exact numerical value is also disclosed.

[0059] As used herein, a plurality of items, structural elements, compositional elements, and/or materials may be presented in a common list for convenience. However, these lists should be construed as though each member of the list is individually identified as a separate and unique member. Thus, no individual member of such list should be construed as a de facto equivalent of any other member of the same list solely based on their presentation in a common group without indications to the contrary. In addition, various embodiments and example of the present invention may be referred to herein along with alternatives for the various components thereof. It is understood that such embodiments, examples, and alternatives are not to be construed as de facto equivalents of one another but are to be considered as separate and autonomous representations of the present invention.

[0060] Furthermore, the described features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. In the following description, numerous specific details are provided, such as examples of lengths, widths, shapes, etc., to provide a thorough understanding of embodiments of the invention. One skilled in the relevant art will recognize, however, that the invention can be practiced without one or more of the specific details, or with other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the invention.

EXPERIMENTAL

[0061] Particle size by Dynamic Light Scattering. A Malvern Zetasizer Colloidal ZS90 Instrument (UK) was used to measure particle size distributions of CLPs and N- CLPs by dynamic light scattering (DLS). The instrument was fitted with a 633 nm He-Ne laser. The measurements were performed at 173° in the backscattering mode. The water viscosity utilized in measurements was 0.89 cP and reflective index 1.33 at 25 °C. Approximately 15 to 20 subruns, adjusted automatically by the equipment, were performed for each measurement. From three to five parallel measurements were done per one sample. The particle sizes are reported through intensity-based ^-average hydrodynamic diameter. Additionally, number-based and volume -based hydrodynamic diameters are measured and analysed.

[0062] ^-Potential. A Malvern Zetasizer Colloidal-ZS90 Instrument (UK) was utilized for the zeta-potential determination. Zeta-potential data were obtained from electrophoretic mobility data by applying the Smoluchowski model. The measurements were done using a dip cell probe DTS1070 and repeated three-five times for each sample to check the reproducibility. A voltage of <5 V was applied. [0063] Micro-scale combustion calorimetry (MCC). Micro-scale combustion calorimetry (MCC), also known as pyrolysis combustion flow calorimetry, is an experimental method for measuring the heat release rate of a small sample as a function of temperature. It reveals how much combustible gases evolve and how much energy is released in the pyrolysis of the specimen tested. The MCC method operates in similar manner thermogravimetric analysis. A small (~l-5 mg) sample is heated inside a furnace, following a constant heating rate (typically 20-60) K/min). The furnace atmosphere is either inert (N2) or oxidative (air). The released pyrolysis gases are conducted to the combustion chamber that has a high temperature and a high enough oxygen level to ensure complete combustion.

[0064] Peak heat release rate (PHR or PHRR), temperature at PHR (TPHR) and total heat release (THR) were determined by MCC in a nitrogen atmosphere at a heating rate of 1.4 K/s. The char yield was determined gravimetrically. Two replicate tests were performed for each material.

[0065] Preparation of nitrogen-modified lignin

[0066] Nitrogen-modified lignin was prepared by Mannich-reaction by treatment of hardwood kraft lignin with formaldehyde and urea using the method of Zhang et al (2012). The nitrogen content of the modified lignin was about 10% by weight.

[0067] Preparation of colloidal particles comprising nitrogen-modified lignin

[0068] The nitrogen-modified lignin was dissolved in an aqueous mixture comprising 60% (v/v) acetone. The dissolved nitrogen-modified lignin was then added into an excess of water with stirring to generate N-CLPs. The amount of acetone in the mixture comprising the solution of dissolved modified lignin, acetone and water was no more than 30% by weight of the mixture. The acetone was allowed to evaporate by stirring the dispersion to leave a solvent-free aqueous N-CLP dispersion.

[0069] Example 1

[0070] As can be seen in Table 1, the nitrogen modification increases the particle size of CLPs. For unmodified lignin (data provided here as a reference), the particles in supernatant as well as the concentrate are nano-sized, whereas for N-modified lignins, only the particles in the supernatant are nano-sized (< 1000 nm). The surface charge of the particles is also reduced (becomes more negative) due to the modification.

Table 1. Size and surface charge of particles in high-speed centrifugation fractions of CLPs prepared from reference unmodified hardwood kraft lignin (HW-KL) and N-modified HW- KL (N-HW-KL)

Sample Size (Mi), nm -potential, mV S/C¹ yield ratio

Supernatant Concentrate Supernatant Concentrate

HW-KL 103 ± 1 199 ± 4 -43.0 ± 1.2 -45.8 ± 1.3 10/90

N-HW-KL 325 ± 14 1590 ± 167 -48.9 ± 1.2 -48.5 ± 1.9 22/78

¹ Supernatant/ concentrate dry mass ratio; ²Size values >1000 nm have reduced reliability

[0071] Example 2

[0072] Microcombustion calorimetry (MCC) testing of N-CLP coating on cellulose (48 g/m²) showed that the CLPs prepared from N-modified HW kraft lignin gave better (lower) HRR and THR values and better (higher) char yield compared to uncoated cellulose. The nitrogen content of the N-modified HW kraft lignin was 10%. The results are shown in Table 2.

Table 2. FR properties of uncoated and N-CLPs coated cellulose by MCC

Coating PHRR, W/g THR, J/g Char residue, %

Uncoated cellulose sheet (ref) 207.2 ± 6.6 10195.6 ± 231.6 11.5 ± 1.1

N-CLP, 48 g/m² 177.4 ± 0.7 9700.9 ± 358.3 14.6 ± 1.3

[0073] Example 3

[0074] Cone calorimetry (CCM) testing of CLP and N-CLP coatings on wood (ca. 50 g/m² on one side of 10 x 10 cm spruce wood blocks) showed that the CLPs prepared from N-modified HW kraft lignin gave better (lower) HRR maxima and similar THR values compared to reference CLPs made from unmodified lignin. The results are shown in Table 3.

Table 3. FR properties of CLP and N-CLP coated wood by CCM

Coating HRR_maxi, W/g HRR_max2, W/g THR, J/g

CLP, 50 g/m² 188 ± 14 150 ± 2 106 ± 5 N-CLP, 50 g/m² 142 ± 7 143 ± 10 106 ± 3

[0075] While the forgoing examples are illustrative of the principles of the present invention in one or more particular applications, it will be apparent to those of ordinary skill in the art that numerous modifications in form, usage and details of implementation can be made without the exercise of inventive faculty, and without departing from the principles and concepts of the invention. Accordingly, it is not intended that the invention be limited, except as by the claims set forth below.

[0076] The verbs “to comprise” and “to include” are used in this document as open limitations that neither exclude nor require the existence of also un-recited features. The features recited in dependant claims are mutually freely combinable unless otherwise explicitly stated. Furthermore, it is to be understood that the use of "a" or "an", that is, a singular form, throughout this document does not exclude a plurality.

INDUSTRIAL APPLICABILITY

[0077] At least some embodiments of the present invention find industrial application in coating industry, in particular as flame retardant coatings. The invention provides easily applicable flame-retardant surface treatment for various surfaces, including building materials such as wood and wood-based materials and cellulose-based insulation.

ACRONYMS LIST

CCM cone calorimeter/calorimetry

CLP colloidal lignin particle

FR flame retardant

HW hardwood

MCC micro-combustion calorimetry

PHR or PHRR peak heat release rate

PP polypropylene

SW softwood TGA thermogravimetric analysis

THR total heat release

CITATION LIST

Non Patent Literature

Dai et al, Lignin-Based Nanoparticles Stabilized Pickering Emulsion for Stability Improvement and Thermal-Controlled Release of trans Resveratrol. ACS Sustainable Chem. Eng. 2019, 7, 13497-13504.

Pohler et al, Improved Fire Retardancy of Cellulose Fibres via Deposition of Nitrogen- Modified Biopolyphenols. Molecules 2022, 27, 3741.

Widsten et al, Modified and unmodified technical lignins as flame retardants for polypropylene. Holzforschung 2021, 75(6): 584-590.

Wu et al, A functional lignin-based nano filler for flame-retardant blend. Int. J. of Biol. Macromolecules, 2021, 190, 390-395.

Zhang et al, Modification of lignin and its application as char agent in intumescent flame retardant poly(lactic acid). Polym. Eng. Sci. 2012, 52: 2620-2626.

Osterberg et al, Spherical lignin particles: a review on their sustainability and applications. Green Chem., 2020, 22, 2712.

Patent literature

WO 2020109671 Al

Claims

CLAIMS:

1. A flame retardant composition for coating of substrates, comprising an aqueous dispersion of colloidal particles of nitrogen-modified lignin, wherein the nitrogen-modified lignin comprises >7% nitrogen by weight of the modified lignin.

2. The flame retardant composition according to claim 1, wherein the colloidal particles of nitrogen-modified lignin have an intensity-based average particle size distribution of <2000 nm as measured by DLS (Dynamic Light Scattering).

3. The flame retardant composition according to claim 1 or 2, wherein the nitrogen- modified lignin comprises >10% nitrogen by weight of the modified lignin.

4. The flame retardant composition according to any one of the preceding claims, wherein the nitrogen-modified lignin colloidal particles have a zeta potential of below -30 mV, preferably below -40 mV, such as -50 - -40 mV, more preferably below -50 mV.

5. The flame retardant composition according to any one of the preceding claims, wherein the lignin comprises lignin from softwood, hardwood, grasses or combinations thereof.

6. The flame retardant composition according to any one of the preceding claims, wherein the lignin comprises technical lignins selected from softwood kraft lignin, hardwood kraft lignin and heat-treated kraft black liquor and combinations thereof.

7. The flame retardant composition according to any one of the preceding claims, comprising adhesion promoters, such as vinyl ethylene acetate, acrylates, silicones, polyurethanes, proteins or combinations thereof, and optionally binders.

8. The flame retardant composition according to any one of the preceding claims blended with a polymer matrix, wherein the polymer is preferably selected from thermoplastic polymers and cellulosic polymers.

9. The flame retardant composition according to claim 8, wherein the polymer is selected from polypropylene, polyamide, polylactic acid, polybutylene succinate, epoxy polymers, polyurethanes and combinations thereof.

10. A method of producing a flame retardant composition comprising nitrogen-modified lignin colloidal particles (N-CLPs), the method comprising:

- preparing nitrogen-modified lignin by incorporating nitrogen into the lignin structure to obtain nitrogen-modified lignin comprising >7% nitrogen by weight of the modified lignin;

- dissolving the nitrogen-modified lignin in an organic or aqueous organic solvent,

- converting the nitrogen-modified lignin to N-CLPs by adding the solution of nitrogen-modified lignin into an excess of water (antisolvent) and by optionally removing the organic solvent to obtain an essentially solvent-free aqueous dispersion comprising the nitrogen-modified lignin colloidal particles, and

- optionally concentrating or drying the obtained aqueous dispersion of nitrogen- modified lignin colloidal particles.

11. The method according to claim 10, wherein the step of preparing nitrogen-modified lignin comprises subjecting lignin to treatment with formaldehyde and urea or an amine to obtain the nitrogen-modified lignin.

12. The method according to claim 10 or 11, wherein the organic solvent is selected from acetone, tetrahydrofuran, N,N-dimethylformamide, and alcohols, such as ethanol, isopropanol and ethylene glycol.

13. The method according to any one of claims 10 to 12, wherein the organic solvent is acetone.

14. The method according to any one of claims 10 to 13, wherein the nitrogen-modified lignin is dissolved in the organic or aqueous organic solvent, which comprises at least 50% (v/v) of the organic solvent, preferably at least 60%, such as 60-80% of the organic solvent.

15. The method according to any one of claims 11 to 14, which comprises the step of adding the solution comprising the dissolved nitrogen-modified lignin into an excess of water (the antisolvent) in a volume ratio between the solution of dissolved nitrogen- modified lignin and water of 1 :2 - 1 :10, preferably in a volume ratio of about 1 :7, to generate the N-CLPs.

16. The method according to claim 15, wherein the amount of the organic solvent in the obtained mixture comprising the dissolved nitrogen-modified lignin and the excess of water is below 30% by weight of the mixture.

17. The method according to any one of claims 10 to 16, wherein the organic solvent is removed, preferably by evaporation or dialysis.

18. A method of coating substrates for imparting them with flame retardant properties, comprising coating said substrates with a flame retardant composition according to any one of claims 1 to 9, optionally in concentrated or dried form.

19. The method according to claim 18, comprising applying on the substrate 2 to 200 g/m², for example 3 to 150 g/m², such as 40-50 g/m², of the flame retardant composition.

20. The method according to claim 18 or 19, wherein the method comprises:

- applying a first layer of the flame retardant composition on the substrate;

- applying a layer of adhesion promoter on the first layer of the flame retardant composition;

- applying a second layer of the flame retardant composition on the adhesion promoter layer; and

- optionally continuing to apply alternating layers of the flame retardant composition and the adhesion promoter on the substrate, and wherein the flame retardant composition consists of or consists essentially of the aqueous dispersion of nitrogen-modified lignin colloidal particles, wherein the modified lignin comprises >7% nitrogen by weight of the modified lignin.

21. The coating method according to any one of claims 18 to 20, comprising the step of applying the flame retardant composition on the substrate by spraying, brushing, rodcoating or by other suitable means.

22. The coating method according to any one of claims 18 to 21, comprising the step of applying the flame retardant composition on the substrate by brushing the flame retardant composition onto the substrate.

23. The method according to any one of claims 18 to 22, wherein the flame retardant composition comprises at least 0.5 wt%, preferably 1-5%, of colloidal particles of nitrogen-modified lignin, wherein said colloidal particles preferably consist essentially of nitrogen-modified lignin.

24. The method according to any one of claims 18 to 23, wherein a hydrophobic substrate is pretreated by plasma or flame treatment before applying the aqueous nitrogen-modified lignin colloidal particle dispersion on the substrate.

25. The method according to any one of claims 18 to 24 for treating cellulosic and lignocellulosic substrates such as cellulose or wood.

26. Use of flame retardant compositions comprising aqueous dispersions of nitrogen- modified lignin colloidal particles for preparing flame retardant coatings on substrates, wherein the nitrogen-modified lignin comprises >7% nitrogen by weight of the modified lignin and wherein the aqueous dispersions are applied on the substrates, preferably brushed, rod-coated, sprayed or applied by other means on the substrates, or the substrates are dipped into the aqueous dispersions.

27. Use of the flame retardant compositions according to any one of claims 1 to 9 as polymer additives, optionally after drying the flame retardant compositions before their incorporation into the polymer.