Flame-retardant treatment of lignocellulosic materials and uses of the resulting flame-retardant lignocellulosic materials

Description

Technical field

[0001] The present disclosure relates to the field of chemistry and more particularly to the field of fireproofing. The invention relates in particular to a new bio-sourced and solvent-free fireproofing process. The invention relates more particularly to a process for fireproofing lignocellulosic materials by grafting phytic acid onto said lignocellulosic materials. It also relates to the new fireproofed lignocellulosic materials thus obtained and their uses in the manufacture of hard (such as panels) or flexible (woven or non-woven) fireproofed materials. Prior art

[0002] Fireproofing refers to a set of treatments carried out to improve the fire resistance of a material. Fireproofing therefore makes a naturally flammable material incombustible or less combustible by impregnating it with fire-retardant substances.

[0003] Flame retardant methods using phytic acid have already been described in the prior art.

[0004] Thus, the publication by Yang Zhou et al. (1) describes that phytic acid is an excellent dopant for improving the flame resistance of paper-based composites. Phytic acid is used as a mixture (not grafted) in a synthetic polymer (polyaniline). The latter is used as a surface treatment on paper to give it electrical conductivity and flame retardant properties.

[0005] The publication by Lucie Costes et al. (2) studies the effects of different cellulose / phosphorus combinations on the flame retardant properties of polylactide (PLA), a synthetic polymer whose raw material is bio-sourced. The tests showed that the combinations of phosphorylated microcrystalline cellulose (MCC-P) / aluminum phytate or nanocrystalline cellulose (NCC) / aluminum phytate gave good results. In this publication, the cellulose is chemically modified in molten urea at 140°C with phosphoric acid, washed with a sodium hydroxide solution and then precipitated in methanol. Composites based on PLA, modified cellulose and aluminum salt of phytic acid (ungrafted) were produced by extrusion.

[0006] The publication by Yu-Yang Gao et al. (3) describes the use of a phytic acid salt, obtained by reacting phytic acid and piperazine, with polypropylene to improve the flame resistance of the latter. Phytic acid is used with a synthetic amine and added as an additive (without grafting) to a petroleum-derived polymer.

[0007] The publication by Xiao-hui Liu et al. (4) describes the use of an ammonium salt of phytic acid (PAA: phytic acid ammonium) to improve the flame resistance of "Lyocell" fibers (which are regenerated cellulose fibers). In this publication, phytic acid is first combined with urea and then precipitated in N,N-dimethylformamide. The resulting salt is reacted under heat with the regenerated cellulose in the presence of dicyanadiamide in water. US 2020 / 056057 A1 describes the use of phytic acid and urea to improve the flame resistance of textiles.

[0008] However, the processes described in these publications are not bio-sourced because they use solvents and / or chemical compounds during the preparation of the flame-retardant material or because the starting material to be flame-retarded is often a compound obtained by chemical synthesis. In addition, phytic acid is generally not covalently grafted to the flame-retardant material and is only present on the surface of the flame-retardant material. Summary

[0009] There is therefore currently a need to develop new bio-sourced fireproofing processes, particularly to meet an increasingly pressing demand from manufacturers for fire-resistant, non-toxic and bio-sourced materials.

[0010] One of the aims of the invention is thus to develop a bio-sourced and solvent-free process. The term "bio-sourced process" means a process that uses bio-sourced material, at the level of the starting material but if possible also at the level of the flame retardant.

[0011] A bio-sourced material is a material derived from plant or animal biomass. Biomass refers to all living matter, i.e., material of biological origin, with the exception of geological or fossil materials. Biomass is essentially composed of carbon (approximately 50%), oxygen (approximately 40%), hydrogen (approximately 6%), a small amount of nitrogen (0.4 to 1.2%), and minerals (calcium, silicon, potassium).

[0012] According to the invention, the starting material to be flame retarded, namely a lignocellulosic material, is bio-sourced. Lignocellulosic material is understood to mean a material consisting of three major components: cellulose, hemicelluloses and lignin. Lignocellulosic material represents the vast majority of biomass. On average, lignocellulosic material contains 40-60% cellulose, 20-40% hemicelluloses and 10-25% lignin. The lignocellulosic material flame retarded according to the method of the invention therefore contains such proportions of cellulose, hemicellulose and lignin.

[0013] According to one embodiment of the invention, the flame retardant, namely phytic acid, is bio-sourced.

[0014] The present invention more particularly relates to a fireproofing method according to claim 1.

[0015] Concerning the percentages of phytic acid and urea in the aqueous solution as defined above, it is also possible to cite from 0.5% to 7% of phytic acid and from 1% to 22% of urea, said percentages being expressed in mass relative to the total mass of the aqueous solution.

[0016] According to one embodiment of the invention, the lignocellulosic material used in the fireproofing process is in the form of fibers and / or particles. In the invention, the term "fibers" more particularly designates natural plant fibers and the term "particles" designates wood particles. Natural plant fibers group together different categories of fibers according to their botanical origins (wood or plants) and their locations in the plant. For cultivated plants, a distinction is thus made between bast fibers, which are extracted from the outer part of the stem in the bast, fibers extracted from the trunk or stems, fibers extracted from leaves, fibers extracted from seeds or fruits, and straws.

[0017] According to another embodiment of the invention, the lignocellulosic material more particularly used in the process of the invention is in the form of plant fibers such as wood fibers and / or plant fibers, and in particular bast fibers, or in the form of wood particles.

[0018] Wood and / or plant fibers have a diameter less than or equal to 500 µm and a length ranging from 1 mm to 50 cm. For information, bast fibers can be very long (up to 50 cm) while wood fibers are much shorter and generally have lengths of a few millimeters.

[0019] The term "wood particles" within the meaning of the invention refers to wood fragments and / or sawdust. The wood particles can have variable shapes and when they are spherical they have a diameter of less than 5 mm.

[0020] According to one embodiment of the invention: the plant fibers are more particularly bast fibers chosen from the group comprising hemp, flax, ramie and jute fibers, the wood fibers and / or particles are more particularly chosen from the group comprising spruce, ash, birch, poplar, beech and oak fibers and / or particles.

[0021] According to an advantageous embodiment of the invention, the lignocellulosic material to be flame retarded is first subjected to a steam explosion. Thus, the lignocellulosic material used at the start of the flame retardant process is chosen from the group comprising steam-exploded plant fibers, and in particular steam-exploded bast fibers, steam-exploded wood fibers and steam-exploded wood particles.

[0022] Steam explosion is a thermo-mechanical-chemical treatment that consists of heating the lignocellulosic material to a high temperature (160-270°C) by injecting saturated steam under high pressure (10-50 bars) for a short time (a few seconds to a few minutes) and then performing a sudden expansion to atmospheric pressure. This results in a significant destructuring of the lignocellulosic material. A steam explosion installation is composed of a steam generator that feeds a reactor that will be subjected to sudden depressurization. During depressurization, the material is ejected from the reactor and recovered at a spark gap. Steam explosion treatment allows the lignocellulosic material to be fractionated in order to recover the different polymer fractions constituting the lignocellulosic material.More specifically, steam explosion reduces the hemicellulose content of lignocellulosic material. This treatment also refines the size of plant fibers and / or wood particles, in order to produce micro- and nanofibers and / or micro- and nanoparticles.

[0023] According to the method of the invention, the steam-exploded plant fibers, and in particular the steam-exploded bast fibers, have a diameter of less than 100 µm, and preferably less than 50 µm, and a length ranging from 1 cm to 10 cm, and preferably from 3 cm to 5 cm.

[0024] Steam-exploded wood particles, when spherical in shape, have a diameter of less than 3 mm.

[0025] According to another embodiment of the invention, the lignocellulosic material to be flame-retarded, preferably previously steam-exploded, is bleached. The purpose of bleaching is to delignify the lignocellulosic material, namely to remove all or part of the lignin from the lignocellulosic material, which results in a profound modification of the fiber and the production of a white lignocellulosic material. The bleaching of the cellulosic material is carried out using chlorite ions or a peroxide.

[0026] The lignocellulosic material used at the start of the fireproofing process of the invention can therefore be: "crude" (i.e. it has not undergone steam explosion and / or bleaching), "exploded" (it has undergone steam explosion but not bleaching), "bleached" (it has undergone bleaching but not steam explosion), "bleached exploded" (it has undergone explosion and bleaching). When lignocellulosic material is exploded bleached, it is first subjected to steam explosion and then to bleaching. Except for a possible steam explosion or possible bleaching, the lignocellulosic material used at the start of the fireproofing process of the invention has not undergone any other chemical treatment.

[0027] According to another embodiment of the invention, phytic acid is obtained from rapeseed meal. "Meal" refers to the solid residue obtained after extraction of oil from the seeds or fruits of oilseed plants. The meals contain relatively high proportions of phytic acid (3% to 6%), which constitutes a natural phosphorus reserve for the seeds of the plants. Rapeseed is the most widely cultivated oilseed resource in France. Rapeseed meal is an interesting industrial raw material due to its vegetable protein content and its attractive price. It is thus used in animal feed. However, phytic acid is an anti-nutritive agent that has a negative impact on the quality of the meal for animal feed applications.Extracting phytic acid from rapeseed meal (which contains 5 to 6% phytic acid) is therefore an excellent way to both use rapeseed meal in the food industry and to obtain phytic acid.

[0028] According to the invention, the process for extracting phytic acid from rapeseed meal comprises the following steps: bringing the rapeseed meal into contact with an acidic aqueous solution while stirring, centrifugation and recovery of the aqueous phase, ultrafiltration of the aqueous phase, recovery of the aqueous phase resulting from the ultrafiltration, freeze-drying of the aqueous phase resulting from the ultrafiltration in order to obtain a freeze-dried product comprising phytic acid.

[0029] The phytic acid lyophilisate thus obtained is then used in combination with urea in order to prepare an aqueous solution comprising from 0.5% to 10% of phytic acid and from 1% to 30% of urea, and preferably from 3% to 7% of phytic acid and from 15% to 22% of urea, said percentages being expressed by mass relative to the total mass of the aqueous solution, said aqueous solution being used in the impregnation step of the process of the invention. Concentrations of 0.5% to 7% of phytic acid and from 1% to 22% of urea, said percentages being expressed by mass relative to the total mass of the aqueous solution, are also suitable within the scope of the invention.

[0030] The flame retardant agent in the process of the invention is phytic acid. However, it is used in combination with urea, which has the role of increasing the accessibility of cellulose (from the lignocellulosic material) by swelling and of protecting the lignocellulosic material from degradation during the cooking stage.

[0031] According to an advantageous embodiment of the method of the invention, the step of impregnating the lignocellulosic material is carried out at room temperature: by soaking in the aqueous solution of phytic acid and urea, or by spraying with the aqueous solution of phytic acid and urea, said aqueous solution of phytic acid and urea being as defined above.

[0032] According to the invention, when the impregnation is carried out by soaking, the impregnation time is from 5 minutes to 1 hour, and preferably from 15 minutes to 1 hour. In the case of impregnation by soaking, the method of the invention further comprises, at the end of the impregnation step, and before the drying step, a step of recovery, preferably by filtering, of the impregnated lignocellulosic material.

[0033] According to an advantageous embodiment of the invention, when the impregnation step is carried out by spraying, then it is not necessary to carry out a drying step before the cooking step. Indeed, the lignocellulosic material obtained at the end of the spray impregnation step comprises a humidity ranging from 5% to 20%.

[0034] When the impregnation step is carried out by soaking, it is then necessary to dry before the cooking step.

[0035] The step of drying the impregnated lignocellulosic material is carried out at a temperature ranging from 20°C to 60°C, and preferably from 40°C to 60°C, for a period ranging from 5 minutes to 18 hours, and preferably from 15 minutes to 30 minutes. As an indication, the drying is carried out in an oven or by forced air.

[0036] The step of cooking the impregnated and optionally dried lignocellulosic material is carried out at a temperature ranging from 140°C to 200°C, and preferably from 140°C to 160°C, for a duration ranging from 15 minutes to 5 hours, and preferably from 30 min to 2 hours.

[0037] The firing step can be carried out in a continuous tunnel kiln. The continuous tunnel kiln is a conveyor kiln or a passage kiln allowing continuous firing, the lignocellulosic material being conveyed into the kiln on a belt. Thus, according to another advantageous embodiment of the method of the invention, the drying and firing steps are carried out continuously.

[0038] According to yet another embodiment, the method of the invention comprises, at the end of the cooking step, a washing and / or filtration step. The washing and / or filtration step makes it possible to eliminate molecules not fixed by strong bonding (i.e. by covalent bonding) on ​​the lignocellulosic material.

[0039] The method of the invention in fact advantageously allows covalent grafting of phosphorus originating from phytic acid both to the surface and to the core of the lignocellulosic material. The method of the invention is therefore further characterized in that, in the flame-retardant lignocellulosic material, the phosphorus originating from phytic acid is grafted by covalent bonding to the surface and to the core of said flame-retardant lignocellulosic material.

[0040] According to an advantageous embodiment of the invention, the fireproofing method of the invention does not color the fireproofed lignocellulosic material.

[0041] According to another advantageous embodiment, the method of the invention is further characterized in that it is solvent-free. It also does not require the use of water.

[0042] Advantageously, the process of the invention only involves impregnation, drying and cooking steps. There is no solution step. With the possibility of continuous processing, it is also easy to industrialize. The fireproofing process of the invention is environmentally friendly due to the use of bio-sourced, inert and non-toxic materials. It is also simple to implement and economical. It also makes it possible to recover agricultural co-products that are currently very little used, namely phytic acid obtained from rapeseed cake.

[0043] The lignocellulosic material obtained according to the fireproofing process of the invention no longer ignites, it no longer allows the flame to propagate and it chars when burned.

[0044] According to another aspect of the invention, there is provided a flame-retardant lignocellulosic material obtainable according to the process as described above.

[0045] The present invention also relates to a flame-retardant lignocellulosic material, characterized in that it comprises a phosphorus content ranging from 0.1% to 10%, preferably ranging from 0.3% to 3%, said percentages being expressed by mass relative to the total mass of the flame-retardant lignocellulosic material, and in that the phosphorus is grafted by covalent bonding to the surface and to the core of said flame-retardant lignocellulosic material.

[0046] The flame-retardant lignocellulosic material as defined above is further characterized in that it has a total heat release on combustion ("THR") ranging from 0.5 to 11 KJ / g as measured using a combustion microcalorimeter according to ASTM D7309, said standard referring to pyrolysis in an anaerobic environment at 1K / s up to 750°C.

[0047] The flame-retardant lignocellulosic material of the invention, as defined above, is preferably obtained from a lignocellulosic material chosen from the group comprising plant fibers (bast fibers such as hemp, flax, ramie and jute fibers), wood fibers, wood particles (spruce, ash, birch, poplar, beech, oak) and mixtures thereof.

[0048] According to the invention, the flame-retardant lignocellulosic material of the invention is preferably obtained from a steam-exploded lignocellulosic material.

[0049] According to another aspect of the invention, there is provided the use of a flame-retardant lignocellulosic material as described above or as obtained according to the process as described above.

[0050] The present invention thus also relates to the use of a flame-retardant bio-sourced lignocellulosic material as described above or as obtained according to the process as described above, for the manufacture of: of flame-retardant composite materials based on plant fibres, of woven or non-woven flame-retardant flexible materials based on plant fibres, and in particular textiles, of flame-retardant materials based on wood fibres and / or wood particles, and in particular flame-retardant wood panels.

[0051] Examples of flame-retardant composite materials based on plant fibers include flame-retardant composite materials based on plant fibers (especially wood fibers) and thermoplastic resins. Such composite materials find applications in the construction and transportation sectors.

[0052] Examples of flexible flame-retardant materials woven from plant fibres include furnishing textiles, technical textiles for the home, technical textiles (sportswear, for professionals (firefighters, etc.). Examples of flexible flame-retardant non-woven materials made from plant fibres include geotextiles and flexible membranes for construction and transport.

[0053] According to an advantageous embodiment of the invention, when the flame-retardant lignocellulosic material is obtained from steam-exploded wood fibers and / or steam-exploded wood particles, then said flame-retardant lignocellulosic material can advantageously be used to produce glue-free and resin-free flame-retardant wood panels, by simply pressing the lignocellulosic material. The invention therefore also relates to the use as defined above, for the manufacture of wood panels, characterized in that the flame-retardant lignocellulosic material is obtained from steam-exploded wood fibers and / or steam-exploded wood particles, and in that the resulting flame-retardant wood panels are free of glue and resin.

[0054] The invention also relates to fireproof wood panels, free of glue and resin, manufactured from a fireproof lignocellulosic material itself obtained from steam-exploded wood fibers and / or steam-exploded wood particles. Such panels are therefore perfectly environmentally friendly. Brief description of the drawings

[0055] Other features, details and advantages will become apparent upon reading the detailed description below, and upon analyzing the attached drawings, in which: Fig. 1 [ Fig. 1] shows different curves of the heat rate release (HRR) as a function of temperature, for steam-exploded and bleached hemp fibers, obtained with the pyrolysis combustion flow calorimeter (PCFC). Curve 1 relates to control hemp fibers, which are not flame-retarded (and which thus comprise a phosphorus content of 0%). Curve 2 relates to hemp fibers flame-retarded according to the method of the invention, which comprise a phosphorus content of 0.31%. Curve 3 relates to hemp fibers flame-retarded according to the invention, which comprise a phosphorus content of 0.49%. Curve 4 relates to hemp fibers flame-retarded according to the invention, which comprise a phosphorus content of 0.62%. Curve 5 relates to hemp fibers flame-retarded according to the invention, which comprise a phosphorus content of 2.14%. Fig.2 [ Fig.2] is a phosphorus map for steam-exploded and bleached hemp fibers, flame-retarded according to the process of the invention. Fig. 3 [ Fig. 3] shows different curves of the heat rate release (HRR) as a function of temperature, for exploded and bleached hemp fibers, obtained with the pyrolysis combustion flow calorimeter (PCFC). Curve 1 relates to control hemp fibers, which are not flame-retarded (and which thus comprise a phosphorus content of 0%). Curve 2 relates to hemp fibers flame-retarded according to the invention, which comprise a phosphorus content of 0.12%. Curve 3 relates to hemp fibers flame-retarded according to the invention, which comprise a phosphorus content of 0.33%. Curve 4 relates to hemp fibers flame-retarded according to the invention, which comprise a phosphorus content of 0.66%. Curve 5 relates to hemp fibers flame-retarded according to the invention, which comprise a phosphorus content of 1.53%.Curve 6 relates to flame-retardant hemp fibers according to the invention, which comprise a phosphorus content of 1.97%. Curve 7 relates to flame-retardant hemp fibers according to the invention, which comprise a phosphorus content of 2.40%. Fig. 4 [ Fig. 4] shows different curves of the heat rate release (HRR) as a function of temperature, for exploded and unbleached hemp fibers, obtained with the pyrolysis combustion flow calorimeter (PCFC). Curve 1 relates to control hemp fibers, which are not flame retardant. These control fibers comprise a phosphorus content of 0.07%. Curve 2 relates to hemp fibers flame retardant according to the invention, which comprise a phosphorus content of 0.15%. Curve 3 relates to hemp fibers flame retardant according to the invention, which comprise a phosphorus content of 0.24%. Curve 4 relates to hemp fibers flame retardant according to the invention, which comprise a phosphorus content of 0.53%. Curve 5 relates to hemp fibers flame retardant according to the invention, which comprise a phosphorus content of 0.93%.Curve 6 relates to flame-retardant hemp fibers according to the invention, which comprise a phosphorus content of 1.48%. Curve 7 relates to flame-retardant hemp fibers according to the invention, which comprise a phosphorus content of 1.83%. Fig. 5 [ Fig.5] shows different curves of the heat rate release (HRR) as a function of temperature, for raw hemp fibers (i.e., unexploded by steam and unbleached) obtained with the pyrolysis combustion flow calorimeter (PCFC). Curve 1 relates to raw control hemp fibers, which are not flame-retarded (and which include a phosphorus content of 0%). Curve 2 relates to hemp fibers flame-retarded according to the invention, using 1.57% phytic acid and 5% urea. Curve 3 relates to hemp fibers flame-retarded according to the invention, using 3.13% phytic acid and 10% urea. Curve 4 relates to hemp fibers flame-retarded according to the invention, using 6.26% phytic acid and 20% urea. Fig. 6 [ Fig. 6] shows different curves of the heat rate release (HRR) as a function of temperature, obtained with the pyrolysis combustion flow calorimeter (PCFC), for flame-retardant and raw (i.e. unexploded and unbleached) hemp fibers, for flame-retardant, exploded and unbleached hemp fibers, and for flame-retardant, exploded and bleached hemp fibers. These fibers are all flame-retarded with the same level of phytic acid (3.13%). Curve 1 relates to raw hemp fibers, flame-retarded according to the invention, using 3.13% phytic acid and 10% urea. Curve 2 relates to exploded and unbleached hemp fibers, flame-retarded according to the invention, using 3.13% phytic acid and 10% urea. Curve 3 relates to exploded and bleached hemp fibers, flame retarded according to the invention, using 3.13% phytic acid and 10% urea. Fig. 7 [ Fig. 7] shows different curves of the heat rate release (HRR) as a function of temperature, for spruce particles, obtained with the pyrolysis combustion flow calorimeter (PCFC). Curve 1 relates to steam-exploded spruce particles (not having been subjected to any treatment other than that of steam explosion), which comprise a phosphorus content of 0%. Curve 2 relates to steam-exploded spruce particles which have then been subjected respectively to a drying and cooking step as described in the method of the invention but which have not been subjected to a step of impregnation with a solution comprising phytic acid. Said spruce particles comprise a phosphorus content of 0%. Curve 3 relates to steam-exploded spruce particles, fireproofed according to the method of the invention, which comprise a phosphorus content of 2.14%. Fig. 8 [ Fig. 8] shows different curves of the heat rate release (HRR) as a function of temperature, for spruce particles, obtained with the pyrolysis combustion flow calorimeter (PCFC). Curve 1 relates to steam-exploded spruce particles (control: non-fireproofed particles). Curve 2 relates to steam-exploded spruce particles, whose impregnation step of the fireproofing process is carried out by spraying with 160mL of an aqueous solution comprising 10% phytic acid and 10% urea, and whose cooking time of the impregnated spruce particles is 30 minutes. Curve 3 relates to steam-exploded spruce particles, the impregnation step of the fireproofing process of which is carried out by soaking using 400 mL of an aqueous solution comprising 6.26% phytic acid and 20% urea, and the cooking time of the impregnated and dried spruce particles is 120 minutes. Fig. 9[ Fig. 9 ] is a 31< P NMR spectrum of hemp fibers grafted to phosphorus by a covalent bond. Description of the embodiments

[0056] The drawings and description below contain, for the most part, elements of a certain character. They may therefore not only serve to better understand this disclosure, but also contribute to its definition, if necessary. Examples Example 1 Preparation of phytic acid-based impregnation solutions

[0057] Extraction of phytic acid from rapeseed mealRapeseed meal, which is used as raw material, is suspended in an aqueous solution having a pH of 2 (± 0.05) stabilized by the alternate addition of sodium chloride and hydrogen chloride. The extraction is carried out at room temperature by applying mechanical stirring at a speed of 300 rpm for one hour. The aqueous phase is then separated from the solid phase at room temperature by centrifugation with a speed of 15,000 rpm for 30 minutes followed by a filtration step. The obtained filtrate is purified by a semi-automatic ultrafiltration system (Akta Flux 6, GE Healthcare, Chicago, IL, USA), using hollow fibers with a cutoff threshold of 3 kDa and a surface area of ​​4,800 cm 2 < . The filtrate is more specifically added to the retentate tank and circulated for 10 min while keeping the trans-membrane pressure at 1.5 bar and the feed flow rate at 2 L.min -1< and the stirring speed at 50 rpm then freeze-dried. This produces a freeze-dried product comprising phytic acid which is used in the next step of preparing the impregnation solutions. Preparation of aqueous solutions of phytic acid and urea

[0058] Aqueous solutions of phytic acid and urea are prepared from the phytic acid lyophilisate obtained in the previous step and urea. The urea used is marketed by the company Sigma-France in the form of urea granules with a purity of ≥ 99.5% (ReagentPlus ®< ). 4 aqueous impregnation solutions comprising various mass percentages of phytic acid and urea are prepared, namely: 1 / a solution comprising 0.63% phytic acid and 2% urea, 2 / a solution comprising 1.57% phytic acid and 5% urea, 3 / a solution comprising 3.13% phytic acid and 10% urea, 4 / a solution comprising 6.26% phytic acid and 20% urea. As already indicated, the percentages are expressed in mass relative to the total mass of the aqueous impregnation solution. The solution described in 1 / is prepared by dissolving 2 g of urea and 7.3 g of phytic acid lyophilisate in a volume of 100 mL of water. The solution described in 2 / is prepared by dissolving 5 g of urea and 18.2 g of lyophilisate of phytic acid in a volume of 100 mL of water. The solution described in 3 / is prepared by dissolving 10 g of urea and 36.4 g of phytic acid lyophilisate in a volume of 100 mL of water. The solution described in 4 / is prepared by dissolving 20 g of urea and 72 g of phytic acid lyophilisate in a volume of 100 mL of water.

[0059] Other aqueous solutions of phytic acid and urea are prepared from commercial phytic acid and urea. The phytic acid used is that marketed by the company Sigma Aldrich under the name "phytic acid solution". More specifically, it is a solution with a density of 1.432 g / mL and a phytic acid mass concentration of 50%. The urea used is that described in the previous paragraph. 7 aqueous impregnation solutions comprising various mass percentages of phytic acid and urea are prepared, namely: 1 / a solution comprising 0.00% phytic acid and 0% urea, 2 / a solution comprising 0.32% phytic acid and 1% urea, 3 / a solution comprising 0.63% phytic acid and 2% urea, 4 / a solution comprising 1.57% phytic acid and 5% urea, 5 / a solution comprising 3.13% phytic acid and 10% urea, 6 / a solution comprising 4.70% phytic acid and 15% urea, 7 / a solution comprising 6.26% phytic acid and 20% urea. As already indicated, the percentages are expressed in mass relative to the total mass of the aqueous impregnation solution. The solution described in 1 / is a water solution with a volume of 100 mL. The solution described in 2 / is prepared by dissolving 1 g of urea and 0.44 mL of phytic acid in a volume of 100 mL of water. The solution described in 3 / is prepared by dissolving 2 g of urea and 0.88 mL of phytic acid in a volume of 100 mL of water. The solution described in 4 / is prepared by dissolving 5 g of urea and 2.19 mL of phytic acid in a volume of 100 mL of water. The solution described in 5 / is prepared by dissolving 10 g of urea and 4.37 mL of phytic acid in a volume of 100 mL of water. The solution described in 6 / is prepared by dissolving 15 g of urea and 6.56 mL of phytic acid in a volume of 100 mL of water. The solution described in 7 / is prepared by dissolving 20 g of urea and 8.74 mL of phytic acid in a volume of 100 mL of water. Example 2 Process for fireproofing hemp fibers that have undergone steam explosion and bleaching treatment Material

[0060] The industrial hemp (Cannabis sativa L.) fibers used in this study were grown at La Chanvrière in Bar-sur-Aube, France. The stems were cut a few centimeters above the ground and harvested without retting in the fall of 2016. The fibers were isolated, dried, and stored in a dry, well-ventilated area throughout the study.

[0061] The NaOH sodium hydroxide used in steam explosion treatment is purchased from the company Sigma-France.

[0062] The aqueous solutions of phytic acid and urea used are as described in Example 1.

[0063] Steam explosion treatment of hemp fibers Steam explosion refining of hemp fibers involves impregnating raw hemp fibers with an aqueous solution containing 8% sodium hydroxide for 15 hours at room temperature without stirring. The fibers are then subjected to a steam explosion at 190°C for 4 minutes. After the explosion, the fibers are washed with distilled water to remove all traces of sodium hydroxide, then dried under a hood for 24 hours. Bleaching treatment of exploded fibers

[0064] Bleached hemp fibers are obtained using a delignification treatment based on acetic acid and sodium chlorite.

[0065] The exploded fibers obtained in the previous step are placed in a reflux flask with deionized water (83.3 mL / g of fibers), glacial acetic acid (1.998 mL / g of fibers) and sodium chlorite (1.998 mg / g of fibers). The mixture is heated at 70°C for 6 h. The addition of sodium chlorite and glacial acetic acid is repeated 2 times. A whitish solid residue consisting essentially of holocellulose is then obtained. After cooling, the bleached fibers are vacuum filtered and washed excessively with deionized water until the pH of the filtrate is neutralized. The fibers are dried overnight at room temperature. Grafting of phytic acid according to the process of the invention

[0066] A quantity of 10 g of hemp fibers thus exploded and bleached is impregnated in 100 mL of each of the 4 aqueous solutions of phytic acid and urea as defined in example 1, of which the phytic acid is extracted from rapeseed cakes, without stirring, for one hour at room temperature.

[0067] The impregnated fibers are then dried in a ventilated oven at a temperature of 60°C for 15 hours until the fibers have a humidity level of 30%.

[0068] The grafting step is completed by baking the dried fibers at a temperature of 150°C for 2 hours in a continuous tunnel oven.

[0069] At the end of treatment, the phosphorylated fibers are washed thoroughly with distilled water then filtered under vacuum before being dried under a hood for 48 hours. Elementary analyses

[0070] Inductively coupled plasma mass spectrometry (ICP-MS) is used to determine the phosphorus element content in the samples. A quantity of 0.15 g of the grafted fibers as obtained according to the flame retardant process of the invention is mixed with 5 mL of a nitric acid solution in a glass tube. Several tubes are thus prepared and then placed in a mineralization unit (Ultrawave system) and left to react for 30 min at 230°C and 110 bar. The samples were then left for 10 min at this temperature and pressure until complete mineralization. After mineralization, the samples are diluted with 50 mL of demineralized water and filtered before being analyzed. For ICP analysis, a standard range of 50, 100, 500, 1000, 5000, 10000 and 20000 ppb (parts per billion) is prepared using a commercial mono-elemental phosphorus solution of 1000 ppm concentration.A blank (water + 10% nitric acid) is injected before the passage of the various prepared standards. Several wavelengths are chosen for the analysis of phosphorus, presenting the least possible interference with the other elements that could be found in the sample. A calibration line is thus drawn by analyzing the different standards. The samples to be analyzed are then injected and if ever they are not in the correct range of the standards, they can be re-diluted. With the values ​​obtained thanks to the ICP we go back to the masses of phosphorus compared to what was weighed in each tube during the mineralizations.

[0071] A "Thermo Finnigan Flash EA 112 Series" is used for the elemental analysis of nitrogen, carbon, oxygen and hydrogen. The combustion of the samples (1.5 mg) to be analyzed is carried out at high temperature (1000°C) in the presence of tungstic anhydride, under an oxidizing atmosphere for 15 s. This decomposition gives CO 2 , H 2 O, SO 2 , NO x which is reduced to N2 in the presence of copper. These gaseous products are then analyzed by gas chromatography. The results are recorded and analyzed by the "Eager 300" software which directly calculates the percentage of each element present in the compound. Combustion microcalorimetry (PCFC: “Pyrolysis Combustion Flow Calorimetry)

[0072] A combustion microcalorimeter (Fire Testing Technology) is used to study the fire behavior of the samples of the invention at the micrometer scale (2-4 mg). The samples are pyrolyzed with a temperature rise rate of 1°C / s under a nitrogen flow from 90 to 750°C (anaerobic pyrolysis - method A according to ASTM D7309). The pyrolysis gases are transported to a combustion chamber in the presence of a flow of N 2 / O 2 (80 / 20). Under such conditions, all gases are fully oxidized. The heat release (HRR) is calculated according to the Huggett relationship. According to this relationship, 1 kg of oxygen consumed corresponds to 13.1 MJ of heat release. Each test is performed twice to ensure reproducibility of the measurements. Peak heat release (pHRR), temperature at pHRR (TpHRR), total heat release (THR), heat of combustion (Δh) and final residue rate (%) are determined. Flame tests on flame-retardant hemp fibers

[0073] A non-standardized fire test is performed to quickly and easily assess the flammability of fibers. Raw and pre-treated fibers glued vertically to an aluminum support are ignited by a lighter. Three behaviors are thus distinguished: flame propagation without residue (ignition), flame propagation with residue formation, and non-flame propagation (self-extinguishing). The residue is also weighed after complete combustion. The initial mass of each fiber is subtracted from the mass of the residue to calculate the percentage of mass loss (% residue). Results

[0074] The results obtained are shown in Table 1 below as well as in Figures 1 and 2 . “Phytic acid m / m” means the mass percentage of phytic acid used in the aqueous solution for impregnating the fibers. “Cooking time” means the duration of the cooking carried out during the grafting step of the flame retardant process of the invention. “%P” means the percentage of phosphorus covalently grafted to the flame retardant lignocellulosic material (hemp fibers in this example). “%N” means the percentage of nitrogen from urea. “HRR” means the heat rate release (W / g). “pHRR” means the maximum value of the HRR peak (W / g). “T pHRR” means the peak temperature (°C). “THR” means the total heat released (kJ / g). “%residue” means the percentage of charred residue after the flame-retardant lignocellulosic material (hemp fibers in this example) has been burned. [Table 1] Phytic acid (m / m) Cooking time (hours) %P %N T pHRR (°C) pHRR (W / g) THR (kJ / g) % residue 0,00 2 0,00 0,05 359,5 248,0 13,8 1,8 0,63 2 0,31 0,05 279,5 104,5 7,4 18,5 1,57 2 0,49 0,05 273,5 91,5 5,0 22,5 3,13 2 0,62 3,16 268,0 32,5 4,5 25,4 6,26 2,14 0,93 242,0 34,5 1,3 28,6 Comments and conclusion

[0075] Table 1 is illustrated by the Figure 1 It is observed that the higher the percentage of phosphorus grafted onto the hemp fibers (2.14%), the lower the maximum value of the heat release peak (p HRR) (34.5 W / g) (versus 248 W / g for the non-flame retardant control hemp fibers).

[0076] The observation is the same for the total heat released ("THR") which is 1.3 kJ / g for hemp fibers with a phosphorus graft of 2.14% versus 13.8 kJ / g for the non-flame retardant control hemp fibers.

[0077] Conversely, the higher the percentage of phosphorus grafted onto the hemp fibers, the higher the percentage of charred residue after the fireproofed lignocellulosic material has been burned.

[0078] There Figure 2 shows that phosphorus is uniformly distributed on the surface and core of steam-exploded and flame-retardant hemp fibers.

[0079] The hemp fibers flame-retardant according to the process of the invention, when ignited, therefore release much less heat than non-flame-retardant hemp fibers and char much more.

[0080] The test described above is repeated a second time, under the conditions described above, but this time using the 7 aqueous impregnation solutions comprising commercial phytic acid. A quantity of 10 g of hemp fibers, exploded and bleached according to the conditions defined above, is impregnated in 100 mL of each of the 7 aqueous solutions of phytic acid (Sigma Aldrich) and urea as defined in Example 1, without stirring, for one hour at room temperature. The results are given in Table 2 below. Comments and conclusion

[0081] Table 2 is illustrated by the Figure 3It is observed that the higher the percentage of phosphorus grafted onto the hemp fibers (2.40%), the lower the maximum value of the heat release peak (p HRR) (39.2 W / g) (versus 312 W / g for the non-flame retardant control hemp fibers).

[0082] The observation is the same for the total heat released ("THR") which is 1.7 kJ / g for hemp fibers with a phosphorus graft of 2.40% versus 12.7 kJ / g for the non-flame retardant control hemp fibers.

[0083] Conversely, the higher the percentage of phosphorus grafted onto the hemp fibers, the higher the percentage of charred residue after the fireproofed lignocellulosic material has been burned.

[0084] The hemp fibers flame-retardant according to the process of the invention, when ignited, therefore release much less heat than non-flame-retardant hemp fibers and char much more. Noticed

[0085] There are some variations in the phosphorus content grafted onto the hemp fibers between Tables 1 and 2, while the phytic acid concentrations in the aqueous impregnation solutions are the same. These variations can be explained by the different origin of the phytic acid used (rapeseed cakes versus commercial origin). In addition, hemp fibers are natural fibers that exhibit variability in their morphology. However, despite these differences, satisfactory grafting of phosphorus onto the hemp fibers is observed in all cases, which increases with the percentage of phytic acid present in the impregnation solution, which demonstrates the effectiveness of the flame retardant process of the invention. Example 3 Process for fireproofing steam-exploded but unbleached hemp fibers

[0086] This example is a comparative example of Example 2. In this example, the hemp fibers have only undergone steam explosion treatment and have not undergone bleaching. The exploded and unbleached hemp fibers are flame retarded according to the process of the invention, using respectively the 7 aqueous solutions of phytic acid (commercial - Sigma Aldrich) and urea as defined in Example 1. The results obtained are given in Table 3 below.

[0087] [Table 3] Phytic acid m / m Urea m / m P % N % PCFC Fire test pHRR TpHRR THR Resi of % Ignition Spread self-extinguishing W / g °C kJ / g 0,00 0 0,07 <0,05 300,8 358,6 12,4 7 Yes Yes No 0,32 1 0,15 0,05 260,9 322,0 11,1 8 Yes Yes No 0,63 2 0,24 0,31 180,4 295,9 8,90 12 Yes Yes No 1,57 5 0,53 0,66 86,30 266,1 5,40 25 Yes Yes Yes 3,13 10 0,93 0,78 57,80 255,9 3,40 29 Yes No - 4,70 15 1,48 1,07 46,30 250,5 2,50 36 No No - 6,26 20 1,83 1,40 42,90 237,6 2,30 34 No No - Comments and conclusion

[0088] Table 3 is illustrated by the Figure 4The same observations can be made for exploded and unbleached (fireproofed) hemp fibres as for exploded and bleached (fireproofed) hemp fibres, namely: the higher the percentage of phosphorus grafted onto the hemp fibres (1.83%), the lower the maximum value of the heat release peak (p HRR) (42.90W / g) (versus 300.8 W / g for the control exploded (non-fireproofed) hemp fibres).

[0089] The observation is the same for the total heat released ("THR") which is 2.30 kJ / g for exploded hemp fibers with a phosphorus graft of 1.83% versus 12.4 kJ / g for the control exploded hemp fibers (not flame retardant).

[0090] Conversely, the higher the percentage of phosphorus grafted onto the hemp fibers, the higher the percentage of charred residue after the fireproofed lignocellulosic material has been burned.

[0091] For the same phytic acid level used (3.13%), we note a more significant decrease in the HRR peak of exploded and bleached hemp fibers (50.00 W / g versus 57.80 W / g for exploded and unbleached hemp fibers). This decrease is accompanied by a shift in the peak temperature and the total heat released. The observation is the same for the different phytic acid levels, thus showing a more significant decrease in the HRR peak, the pHRR temperature and the total heat released for exploded and bleached (flame retardant) hemp fibers than for exploded and unbleached (flame retardant) hemp fibers.

[0092] It is noted, however, that the results obtained according to Example 2 are even better than those of Example 3 (see the data in Tables 2 and 3 respectively). It can therefore be concluded that, for hemp fibers, the preliminary steam explosion step, followed by the bleaching step, makes it possible to improve the effectiveness of the flame-retardant process of the invention. Example 4 Process for fireproofing hemp fibers that have not undergone steam explosion and no bleaching

[0093] In this example, the hemp fibers have not undergone any prior steam explosion and bleaching treatment. The raw hemp fibers, namely unexploded and unbleached, are flame retarded according to the process of the invention, using respectively 4 aqueous solutions of phytic acid (commercial - Sigma Aldrich) and urea as defined in Example 1, which comprise 0.00%, 1.57%, 3.13% and 6.26% of phytic acid.

[0094] The results obtained in terms of thermal properties are given in Table 4 below. [Table 4] % phytic acid % urea Cooking time (h) Raw Hemp TpHRR (°C) pHRR (W / g) THR (kJ / g) % residue 0 0 0 357 148,0 11,6 14 1,57 5 2 276,1 131,7 7,0 20 3,13 10 2 258,2 78,0 4,4 29 6,26 20 2 260,3 45,6 2,5 33 Comments and conclusion

[0095] Table 4 is illustrated by the Figure 5 The same observations can be made for raw (unexploded, unbleached) flame-retardant hemp fibers (example 4) as for flame-retardant, exploded and unbleached hemp fibers (example 3) and flame-retardant, exploded and bleached hemp fibers (example 2). These observations are as follows: the higher the percentage of phytic acid used on the hemp fibers (6.26%) (and therefore the higher the rate of phosphorus grafted onto the fibers), the lower the maximum value of the peak heat release (p HRR) (45.60 W / g) (versus 148.0 W / g for the control exploded hemp fibers (0% phytic acid)).

[0096] The same observation is made for the total heat released ("THR") which is 2.5 kJ / g for exploded hemp fibers flame-retardant with 6.26% phytic acid versus 11.6 kJ / g for the control exploded hemp fibers (0% phytic acid).

[0097] Conversely, the higher the percentage of phytic acid used on hemp fibers, the higher the percentage of charred residue after the flame-retardant lignocellulosic material has been burned.

[0098] In conclusion, it has been shown in examples 2, 3 and 4 (see tables 2 to 4 and figures 3 to 5 ) that the flame retardant process of the invention is effective both on raw plant fibers, namely unexploded and unbleached (example 4), as well as on plant fibers exploded and unbleached (example 3) and on plant fibers exploded and bleached (example 2). Example 5 Highlighting the advantages of steam explosion treatment and bleaching on hemp fibers to be flame retarded

[0099] This example is a comparison of examples 2, 3 and 4. In this example, the raw hemp fibers (unexploded and unbleached), the exploded and unbleached hemp fibers (example 3) and the exploded and bleached hemp fibers (example 2), are flame retarded according to the process of the invention, using the same aqueous solution of 3.13% phytic acid and 10% urea as defined in example 1.

[0100] The comparison of the results obtained in terms of thermal properties are given in Table 5 below. [Table 5] Hemp fibers TpHRR (°C) pHRR (W / g) THR (kJ / g) % residue Raw (unexploded, unbleached) 258,2 78,0 4,4 29 Exploded but not bleached 255,9 57,8 3,4 29 Exploded and bleached 247,0 50,00 2,9 35 Comments and conclusion

[0101] The results in Table 5 are illustrated by the Figure 6. For the same phytic acid level used (3.13%), we note a more significant decrease in the HRR peak of exploded and bleached hemp fibers (50 W / g versus 57.8 W / g for exploded hemp fibers and 78 W / g for raw hemp fibers). This decrease is accompanied by a shift in the peak temperature and the total heat released. The observation is the same for the different phytic acid levels, thus showing a more significant decrease in the HRR peak, the pHRR temperature and the total heat released for exploded and bleached (flame retardant) hemp fibers than for exploded and unbleached (flame retardant) hemp fibers than for raw (unexploded, unbleached) (flame retardant) hemp fibers.

[0102] In conclusion, the flame retardant process of the invention is effective both on raw plant fibers, namely unexploded and unbleached, as well as on unexploded and unbleached plant fibers, and on exploded and bleached plant fibers. Thus, the steam explosion step followed by the bleaching step makes it possible to improve the effectiveness of the flame retardant process of the invention. Example 6 Process for fireproofing spruce particles by dipping Material

[0103] The phytic acid used in this example is that marketed by the company Sigma Aldrich under the name “phytic acid solution”, as defined in example 1 (solution having a density of 1.432 g / mL and a mass concentration of phytic acid of 50%).

[0104] The purity of urea granules used is the same as that of Example 1. The concentrated sulfuric acid used in the steam explosion process is marketed by the company Sigma Aldrich and is mixed with demineralized water to prepare a mass concentration of 2%. Preparation of an aqueous solution of phytic acid and urea

[0105] The aqueous impregnation solution is prepared by mixing 30 g of urea, 13 mL of a commercial aqueous phytic acid solution and 100 mL of distilled water. The aqueous impregnation solution thus obtained comprises 6.26% by mass of phytic acid and 20% by mass of urea relative to the total mass of the aqueous solution. Steam explosion treatment of spruce fibers

[0106] Spruce heartwood particles (2-5 mm in diameter) are placed in a 2% sulfuric acid solution (2% by mass in water), in a solid / liquid ratio of 1 / 5 for 4 hours at room temperature and without stirring. Steam explosion is carried out at a temperature of 200°C for a residence time of 5 min. Following the explosion, the exploded spruce particles are washed with water. Grafting of phytic acid according to the process of the invention

[0107] The exploded spruce particles (80 g) are impregnated for one hour by soaking in 400 mL of the previously prepared aqueous solution of phytic acid and urea (comprising 6.26% phytic acid and 20% urea). The spruce particles are air-dried at room temperature for 18 h and then placed in an oven at 150°C for 2 hours.

[0108] The spruce particles are then characterized by the techniques previously described, in particular by combustion microcalorimetry (PCFC: “Pyrolysis Combustion Flow Calorimetry”) and by the flame test.

[0109] The results obtained are described in Table 6 below. [Table 6] Phytic acid (m / m) Cooking time (hour) %P %N T pHRR (°C) pHRR (W / g) THR (kJ / g) % residue Curve 1 0 0,00 0,00 381 167 11,95 15,2 0,00 (exploded) Curve 2 2 0,00 0,00 381 164 12,50 15,4 0,00 (exploded, dried and cooked) Curve 3 2 2,14 0,93 280 55 4,05 34,3 6,26 (exploded and fireproof) Comments and conclusion

[0110] Table 6 is illustrated by the Figure 7 The heat release is approximately the same for exploded spruce particles (curve 1) and exploded, dried and fired, but not impregnated spruce particles (curve 2).

[0111] The fireproofing treatment of the invention on the spruce particles previously exploded with steam drastically reduces the maximum value of the heat release peak (p HRR) (55 W / g) (versus 167 W / g and 164 W / g for the exploded spruce particles (curve 1) and exploded, dried and cooked but not impregnated spruce particles (curve 2) respectively).

[0112] The same observation is made for the total heat released ("THR") which is 4.05 kJ / g for exploded and fireproofed spruce particles including a phosphorus graft of 2.14% versus 11.95 kJ / g for exploded spruce particles (curve 1) and 12.50 kJ / g for exploded, dried and cooked but not impregnated spruce particles (curve 2).

[0113] Conversely, the percentage of charred residue after the fire-retardant lignocellulosic material has been burned increases and is 34.3% for exploded and fire-retardant spruce particles versus 15.2% for exploded spruce particles (curve 1) and 15.4% for exploded, dried and cooked but not impregnated spruce particles (curve 2).

[0114] The spruce particles fireproofed according to the method of the invention, when ignited, therefore release much less heat than the non-fireproof spruce particles and char much more. Example 7 Process for fireproofing spruce particles by spraying

[0115] This example is a comparative example of Example 6. The conditions of this example are those described in Example 6, the difference being that the step of impregnating the exploded spruce particles (80 g) is carried out by spraying (and not by dipping) using 160 mL of an aqueous solution comprising 10% phytic acid and 10% urea. The spruce particles thus impregnated are directly (without drying) placed in an oven at 150°C for 30 minutes (instead of 2 hours in Example 6). As already indicated, when the impregnation is carried out by spraying, it is not necessary to dry before spraying. Note: In Example 6, 400 mL of an aqueous impregnation solution comprising 6.26% phytic acid and 20% urea are used. This means that 25.04 g of phytic acid (6.26x400 / 100) and 80 g of urea (20x400 / 100) are used for the soaking impregnation step.In this example (spray impregnation), 160 mL of an aqueous impregnation solution comprising 10% phytic acid and 10% urea are used. This means that 16 g of phytic acid (10x160 / 100) and 16 g of urea (10x160 / 100) are used for the spray impregnation step. The amounts of phytic acid and urea are therefore lower in the spray impregnation step than in the dip impregnation step.

[0116] The results obtained in terms of thermal properties are given in Table 7 below. [Table 7] Phytic acid (%) Cooking time (minutes) TpHRR (°C) pHRR (W / g) THR (kJ / g) % residue Curve 1 0 381 167 11,95 15,2 0% (exploded) (witness) Curve 2 30 292,2 83,5 7,3 29,7 6,26% (exploded and pulverized) Curve 3 120 280 55 4,05 34,3 6,26% (exploded and soaked) Comments and conclusion

[0117] Table 7 is illustrated by the figure 8. Although the amount of phytic acid used by spraying is significantly lower than with soaking (-36%), the HRR curves in PCFC of the figure 8 show an efficiency signaled by a significant drop in pHRR values ​​from 167 W / g (control) to 83.5 W / g accompanied by a shift towards lower temperatures (381°C versus 292.2°C).

[0118] The same observation is made for the total heat released (THR) which is 11.95 kJ / g for untreated spruce particles versus 7.3 kJ / g for those fireproofed by spraying.

[0119] Conversely, for spray-treated particles, the percentage of carbon residue after flameproofing is double that of untreated particles.

[0120] In conclusion, the HRR curves in PCFC of the figure 8show the effectiveness of spraying the flame retardant solution on the flame retardancy of spruce particles, even though the quantities of phosphorus are lower with spraying. The spray impregnation step of the flame retardancy process of the invention makes it possible to significantly reduce the quantity of reagents used while retaining interesting thermal properties. Example 8 Covalent grafting of phytic acid onto flame-retardant lignocellulosic material

[0121] As described above, the grafting of phytic acid onto lignocellulosic material is covalent in nature (strong and durable bond). Covalent grafting onto hemp fibers is highly resistant to successive washings with water. This grafting was confirmed by solid-state NMR (see figure 9) with the detection of a signal with a shoulder attributed to the grafted phosphate group (ratio 1 / 6). Confirmation was provided by measuring the relaxation times of the phosphorus atoms (P1 = 6.8 seconds, P2 = 10.9 seconds). The longer relaxation time for grafted P2 is explained by a lower mobility. List cited documents Non-patent literature

[0122] For all useful purposes, the following non-patent element(s) is (are) cited: (1): Yang Zhou et al., Carbohydrate Polymers, 115 (2015) 670-676; (2): Lucie Costes et al., European Polymer Journal, 74 (2016) 218-228; (3): Yu-Yang Gao et al., Polymer Degradation and Stability, 161 (2019) 298-308; and (4): Xiao-hui Liu et al., Cellulose, 25, 799-811 (2018).

Claims

1. A process for the flame retardancy of a biobased lignocellulosic material, characterized in that it comprises the following steps: - optionally steam explosion and / or bleaching of the biobased lignocellulosic material, - impregnation of the biobased lignocellulosic material, optionally steam-exploded and / or bleached, in or with an aqueous solution comprising from 0.5% to 7% phytic acid and from 1% to 22% urea, said percentages being expressed by mass relative to the total mass of the aqueous solution, - optionally drying the impregnated biobased lignocellulosic material until it has a moisture content ranging from 5% to 20%, said percentages being expressed by mass relative to the total mass of the impregnated biobased lignocellulosic material, - cooking the impregnated and optionally dried biobased lignocellulosic material, at a temperature ranging from 140°C to 200°C, and preferably from 140°C to 160°C, for a time ranging from 15 minutes to 5 hours and preferably from 30 minutes to 2 hours, the flame-retarded biobased lignocellulosic material thus obtained comprising a content of phosphorus originating from phytic acid ranging from 0.1% to 10%, preferably ranging from 0.3% to 3%, said percentages being expressed by mass relative to the total mass of the flame-retarded biobased lignocellulosic material, said process being further characterized in that the biobased lignocellulosic material used at the start has not undergone any chemical treatment other than that of possible steam explosion and / or bleaching.

2. The process as claimed in claim 1, characterized in that the biobased lignocellulosic material is in the form of plant fibers such as wood fibers and / or plant fibers, and in particular bast fibers, or in the form of wood particles.

3. The process as claimed in claim 2, characterized in that: - the plant fibers are bast fibers chosen from the group comprising hemp, flax, ramie and jute fibers, - the wood fibers and / or particles are chosen from the group comprising spruce, ash, birch, poplar, beech and oak fibers and / or particles.

4. The process as claimed in any one of claims 1 to 3, characterized in that the biobased lignocellulosic material is chosen from the group comprising steam-exploded plant fibers, and in particular steam-exploded bast fibers, steam-exploded wood fibers and steam-exploded wood particles.

5. The process as claimed in claim 4, characterized in that the steam-exploded plant fibers, and in particular the steam-exploded bast fibers, have a diameter of less than 100 µm, and preferably less than 50 µm, and a length ranging from 1 cm to 10 cm, and preferably from 3 cm to 5 cm.

6. The process as claimed in any one of claims 1 to 5, characterized in that the biobased lignocellulosic material is bleached.

7. The process as claimed in any one of claims 1 to 6, characterized in that the phytic acid used is obtained from a rapeseed oil cake.

8. The process as claimed in any one of claims 1 to 7, characterized in that the step of impregnation of the biobased lignocellulosic material is performed at room temperature: - by soaking in the aqueous solution of phytic acid and urea, or - by spraying with the aqueous solution of phytic acid and urea.

9. The process as claimed in any one of claims 1 to 8, characterized in that the step of drying of the impregnated biobased lignocellulosic material is performed at a temperature ranging from 20°C to 60°C, and preferably from 40°C to 60°C, for a time ranging from 5 minutes to 18 hours, and preferably ranging from 15 minutes to 30 minutes.

10. The process as claimed in any one of claims 1 to 9, characterized in that, in the flame-retarded biobased lignocellulosic material, the phosphorus originating from the phytic acid is grafted by covalent bonding to the surface and to the core of said biobased flame-retarded lignocellulosic material.

11. The use of the flame-retarded biobased lignocellulosic material as obtained according to the process of any one of claims 1 to 10, for the manufacture of: - flame-retarded composite materials based on plant fibers, - flexible flame-retarded woven or nonwoven materials based on plant fibers, and notably textiles, - flame-retarded materials based on wood fibers and / or wood particles, and notably flame-retarded wood panels.

12. The use as claimed in claim 11, for the manufacture of flame-retarded wood panels, characterized in that the flame-retarded biobased lignocellulosic material is obtained from steam-exploded wood fibers and / or steam-exploded wood particles, and in that the flame-retarded wood panels obtained are free of glue and resin.