Flame retardant lyocell fibers

Abstract

A flame retardant lyocell fiber and a process for preparing said flame retardant lyocell fiber is disclosed Said flame retardant lyocell fiber comprises lyocell fiber incorporating 20-50 wt% of silicic acid. The flame retardant lyocell fiber has a tenacity of at least 1.5 grams / denier and an elongation of at least 14% in a conditioned state.

Description

[0001] FLAME RETARDANT LYOCELL FIBERS

[0002] Field of Invention

[0003] The present invention relates to flame -retardant lyocell fibers suitable for woven and non-woven applications and to a process for manufacturing said fibers.

[0004] It is known to manufacture flame -retardant regenerated cellulosic fibers by treating the fibers with a flame retardant. Well-known flame retardants include sodium silicates, halogen-based organic compounds, and, more recently, organophosphorus compounds.

[0005] Among these, silica-based flame retardants are of particular interest due to their abundance, eco-friendliness, ease of handling, and cost-effectiveness. In the production of flame-retardant viscose fibers, for example, sodium silicate is typically added to the spinning solution. The resulting fiber acquires flameretardant properties during the spinning and subsequent regeneration steps, wherein sodium silicate is converted to silicic acid in a sulfuric acid bath.

[0006] However, sodium silicate finds limited or no application in imparting flame retardancy to solvent-spun regenerated fibers such as lyocell. This limitation arises because the lyocell process does not involve chemical regeneration in an acid bath, which is required for the conversion of sodium silicate to silicic acid. Therefore, sodium silicate cannot be utilized effectively in the lyocell production process.

[0007] Furthermore, the use of sodium silicate in the viscose process itself presents several disadvantages. Large quantities of sodium silicate are required to achieve an effective flame-retardant effect. This high loading is necessary to compensate for the loss of flame retardant due to leaching into the acid bath during regeneration. Such high loadings result in a reduction of the mechanical properties of the fiber. For example, with a silica loading of 30-35% in the fiber, the tenacity of the resulting viscose fiber is only about 1.5 grams per denier (g / den).

[0008] Additionally, known processes for preparing flame-retardant lyocell fibers often fail to achieve one or more desired mechanical properties, such as strength and durability. This limitation restricts the range of applications for lyocell fibers where both flame retardancy and robust mechanical performance are required.

[0009] Summary

[0010] A flame retardant lyocell fiber is disclosed. Said flame retardant lyocell fiber comprises lyocell fiber incorporating 20-50 wt% of silicic acid. The flame retardant lyocell fiber has a tenacity of at least 1.5 grams / denier and an elongation of at least 14% in a conditioned state.

[0011] A process for preparing a flame retardant lyocell fiber is also disclosed. Said process comprises the steps of: a. preparing a spinning solution comprising cellulose in aqueous N- Methylmorpholine N-oxide (NMMO) and a silica suspension, such that the spinning solution comprises 20-50 wt% silica based on weight of cellulose; b. extruding the spinning solution through a spinneret into a spinning bath to obtain a never dried lyocell fiber; c. applying a spin-finish to the never dried lyocell fiber; and d. drying to obtain the flame retardant lyocell fiber, such that the flame retardant lyocell fiber comprises lyocell fiber incorporating 20- 50 wt% of silicic acid and has a tenacity of at least 1.5 grams / denier and an elongation of at least 14% in a conditioned state. Brief Description of Drawings

[0012] Figure 1 presents a thermogravimetric analysis (TGA) plot comparing the thermal stability of flame-retardant lyocell fibers prepared in accordance with an exemplary embodiment, alongside unmodified lyocell fiber as a control.

[0013] Detailed Description

[0014] Reference will now be made in detail to embodiments of the present disclosure. The terminology used in the description presented herein is not intended to be interpreted in any limited or restrictive manner, simply because it is being utilized in conjunction with a detailed description of certain specific embodiments of the invention. Furthermore, embodiments of the invention may include several features, no single one of which is solely responsible for its desirable attributes, or which is essential to practicing the inventions herein described.

[0015] It will be understood by those skilled in the art that the foregoing general description and the following detailed description are explanatory of the invention and are not intended to be restrictive thereof.

[0016] The terms “a,” “an,”, and “the” are used to refer to “one or more” (i.e., to at least one) of the grammatical object of the article.

[0017] Reference throughout this specification to “an aspect”, “another aspect” or similar language 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.

[0018] The terms "comprises", "comprising", or any other variations thereof, are intended to cover a non-exclusive inclusion and are not intended to be construed as “consists of only”, such that a process or method that comprises a list of steps does not include only those steps but may include other steps not expressly listed or inherent to such process or method.

[0019] Likewise, the terms “having” and “including”, and their grammatical variants are intended to be non-limiting, such that recitations of said items in a list are not to the exclusion of other items that can be substituted or added to the listed items.

[0020] Also, any numerical range recited herein is intended to include all sub-ranges subsumed therein. For example, a range of "1 to 10" is intended to include any and all subranges between and including the recited minimum value of 1 and the recited maximum value of 10, that is, all subranges beginning with a minimum value equal to or greater than 1 and ending with a maximum value equal to or less than 10, and all subranges in between.

[0021] As used herein, the term “tenacity” is intended to mean the ultimate (breaking) force of the fiber (in gram-force units) divided by the denier.

[0022] As used herein, the term “elongation” is intended to mean elongation at break.

[0023] As used herein, the expression “Limiting oxygen index (LOT)” is defined as the minimum amount of oxygen in a nitrogen-oxygen mixture (in vol%) required to sustain flaming combustion of a material. LOT is tested as per ASTM D2863 and ISO 4589. Higher LOI values indicate lower flammability and better flame retard ancy.

[0024] As used herein, the expression “% of silica on cellulosic fibre” or “ash content” is defined as the percent weight left during ignition at a high temperature that causes organic contents to burn or volatilize. “% of silica on cellulosic fibre” is tested as per ASTM D2584-18. As used herein, the expression “sol-gel” reaction or method or conversion is defined as a method which involves two main reactions: (1) hydrolysis of the precursor in the acidic or basic mediums, and (2) polycondensation of the hydrolyzed products into a polymeric network.

[0025] In an aspect, a flame retardant lyocell fiber is disclosed. Said flame retardant lyocell fiber comprises lyocell fiber incorporating 20-50 wt% of silicic acid. The flame retardant lyocell fiber has a tenacity of at least 1.5 grams / denier and an elongation of at least 14% in a conditioned state, measured as per ASTM D 3822.

[0026] In another aspect, a process for preparing a flame retardant lyocell fiber is disclosed. Particularly, the disclosed process comprises the steps of: a. preparing a spinning solution comprising cellulose in aqueous N- Methylmorpholine N-oxide (NMMO) and a silica suspension, such that the spinning solution comprises 20-50 wt% silica based on weight of cellulose; b. extruding the spinning solution through a spinneret into a spinning bath to obtain a never dried lyocell fiber; c. applying a spin-finish to the never dried lyocell fiber; and d. drying to obtain the flame retardant lyocell fiber, such that the flame retardant lyocell fiber comprises lyocell fiber incorporating 20-50 wt% of silicic acid and has a tenacity of at least 1.5 grams / denier and an elongation of at least 14% in a conditioned state, measured as per ASTM D 3822.

[0027] It has been found by the present inventors that incorporating silicic acid or hydrated silica into lyocell fibers by treatment of cellulose with silica, followed by the conversion of silica to silicic acid under controlled alkaline conditions, imparts enhanced flame retardancy to lyocell fibers while overcoming the disadvantages associated with the use of sodium silicates as flame retardants. The resulting flame retardant lyocell fiber exhibits a tenacity of greater than 1.5 g / denier, as measured according to ASTM D 3822, and an elongation at break of at least 14% in a conditioned state, also measured as per ASTM D 3822. In some embodiments, the flame retardant lyocell fiber has a tenacity ranging from about 1.5 g / den to about 4.0 g / den. In some embodiments, the elongation at break ranges from about 14% to about 30% in the conditioned state.

[0028] Compared to prior art flame retardant lyocell fibers, the disclosed flame retardant lyocell fiber achieves effective flame retardancy with minimal silica loading, while maintaining mechanical properties that are comparable to or better than those of commercial-grade lyocell fibers lacking flame retardant treatment.

[0029] In an embodiment, the disclosed flame retardant lyocell fiber exhibits a Limiting Oxygen Index (LOI) value of at least 27, measured in accordance with ASTM D2863. In some embodiments, the LOI value ranges from about 27 to about 40.

[0030] In an embodiment, the disclosed flame retardant lyocell fiber has an ash content ranging from about 10% to about 50% by mass, as determined by ASTM D2584-18. In some embodiments, the ash content is within the range of about 18% to about 35% by mass.

[0031] In an embodiment, the silica suspension is added in an amount such that the spinning solution comprises silica in the range of about 20% to about 50% by weight relative to cellulose. In some embodiments, the silica suspension is added in an amount such that the spinning solution comprises silica between about 25% and about 35% by weight of cellulose. The silica suspension used in the present disclosure includes amorphous silica, including both synthetic as well as naturally occurring, such as diatomaceous earth, precipitated amorphous silica, amorphous silica gel, pyrogenic silica, fumed amorphous silica, fused amorphous silica, and colloidal amorphous silica. In an embodiment, the amorphous silica employed is of high purity and is substantially free of metal contaminants. In an embodiment, the amorphous silica is characterized by a particle size sufficiently small to ensure uninterrupted operation of the spinning process, thereby maintaining process efficiency and fiber quality.

[0032] In an embodiment, the silica suspension is a colloidal silica suspension. Colloidal silica suspension refers to a stable, alkaline, aqueous dispersion of colloidal silica, wherein the colloidal silica particles possess a mean particle size ranging from about 2 nm to about 100 nm. The colloidal silica suspension utilized in the present disclosure encompasses any known colloidal silica suspension, including commercially available products. Said colloidal silica suspension is formed by dissolving sodium silicate (TS^SiOs) in water, where it dissociates into sodium ions (Na+) and silicate ions (SiOs2). Passing this solution through a hydrogen-form ion exchange resin replaces Na+ions with H+ions, converting silicate ions into silicic acid (Si(OH)4), a weakly acidic and unstable species. To stabilize the resulting colloidal silica suspension at around pH 9, alkali sodium salts are added. In some embodiments, suitable colloidal silica suspension includes those available under brand names such as "Ludox®" (Grace Company), "Nalco®" (Nalco Chemical Company), "Levasil®" or "Bindzil®" (Akzo Nobel), "ORGANOSILICASOL" (Nissan Chemical), and "Highlink®" (Highlink OG range).

[0033] In another embodiment, the silica suspension comprises amorphous silica selected from the group consisting of precipitated silica and fumed silica. The precipitated silica is porous and has a moderate surface area ranging from about 150 to about 300 m2 / g. It is typically formed by acid treatment of sodium silicate and consists of aggregated particles with sizes ranging from about 10 nm to about 40 nm. The precipitated silica exhibits a moderate silanol group content and may contain residual salts resulting from the manufacturing process. The fumed silica is non-porous and characterized by a high surface area, up to about 250 m2 / g. It is produced via flame hydrolysis, which imparts a high density of silanol groups on the particle surface, thereby conferring strong chemical reactivity. The fumed silica particles are aggregated, with primary particle sizes ranging from about 5 nm to about 50 nm. In an embodiment, the silica suspension comprising precipitated or fumed silica is prepared by adding silica to an aqueous solution of NMMO, with the NMMO concentration ranging from 50% to 76% by weight. This dispersion ensures uniform distribution of silica particles within the spinning solution, facilitating effective incorporation into the lyocell fiber matrix during fiber formation.

[0034] In embodiments where the silica suspension is formed of amorphous silica selected from the group consisting of precipitated and fumed silica, a buffer solution is added during or after one of steps (a) to (d) to initiate the sol gel conversion of silica to silicic acid that impart flame retardant properties to the lyocell fibers. The alkaline environment created by the buffer facilitates the breaking of siloxane (Si- O-Si) bonds on the silica surface, generating silanol (Si-OH) groups that further hydrolyze to form silicic acid.

[0035] The buffer solution can be applied at any step of the process for preparing the lyocell fibers to optimize the process. In an embodiment, the buffer solution is added to the cellulose slurry or the spinning solution along with silica suspension, in step (a). In another embodiment, the buffer solution is applied by spraying the same on the extruded fibers or filaments, in step (b). In yet another embodiment, the treatment with the buffer solution is performed after applying a soft finish, in step (c). In other embodiment, the buffer solution is applied on the dried lyocell fibers or filaments, in step (d).

[0036] In an embodiment, the buffer solution is any buffer solution known in the art or developed in the future, capable of facilitating and maintaining a pH in the range of about 8 to about 10 to enable the conversion of silica to silicic acid. In some embodiments, the pH is adjusted more specifically to a range between about 8 and about 9. The buffer solution comprises one or more sodium salts. In some embodiments, the buffer solution includes at least one of sodium bicarbonate, sodium carbonate, and sodium sulfate. The concentration of the buffer solution may vary depending on the desired pH and process conditions; for example, the buffer solution may comprise approximately 0.1 wt% of sodium bicarbonate or sodium carbonate. In other embodiments, the buffer solution comprises sodium sulfate in a concentration range of about 0.05 wt% to about 0.1 wt%. The use of such buffer solutions ensures controlled pH conditions conducive to the sol-gel conversion of silica to silicic acid, thereby facilitating the formation of flame retardant lyocell fibers.

[0037] In an embodiment, when the silica suspension is a colloidal silica suspension, the process further comprises adding the above buffer solution during or after one of steps (a) to (d) to facilitate the conversion of any remaining silica to silicic acid. This ensures that any residual silica not initially converted during spinning or mixing steps is transformed into the active silicic acid form, which becomes uniformly incorporated into the cellulose matrix.

[0038] In an embodiment, an additive is added to the cellulose slurry or the spinning solution along with silica. In an embodiment, said additive is selected from the group consisting of a dispersant, a surfactant, a defoamer, and an emulsifier. Any known dispersant, a surfactant, a defoamer, and emulsifier can be used. In some embodiments, the dispersant is selected from the group consisting of sodium polyphosphate, sodium hexametaphosphate, sodium pyrophosphate, and a polymeric dispersant. In some embodiments, the surfactant is selected from the group consisting of sodium dodecyl sulfate (SDS), cetyltrimethylammonium bromide (CTAB), polyoxyethylene glycol alkyl ethers (such as Triton X-100), and polysorbates (such as Tween 80). In some embodiments, the defoamer is selected from the group consisting of silicone-based defoamers, mineral oil-based defoamers, polyether-based defoamers, and fatty alcohol-based defoamers. In some embodiments, the emulsifier is selected from the group consisting of lecithin, mono- and diglycerides, sorbitan esters, and polyglycerol esters.

[0039] In an embodiment, the spinning solution in step (a) is prepared by first adding silica suspension to a cellulose slurry, followed by removal of excess moisture from the cellulose slurry containing silica. In an alternate embodiment, the spinning solution in step (a) is prepared by initially preparing a cellulose slurry comprising aqueous NMMO, then removing the excess moisture from the cellulose slurry, and subsequently adding the silica suspension to obtain the spinning solution. In an embodiment, after preparing the spinning solution by addition of the silica suspension, the spinning solution is subjected to mixing at a temperature ranging from about 80°C to about 120°C under a vacuum pressure ranging from about 500 mm Hg to about 750 mm Hg.

[0040] In step (b), the spinning solution is extruded to obtain never-dried cellulosic filaments. In one embodiment, the extruded filaments are cut into staple fibers. Alternatively, in another embodiment, the spinning solution is extruded to obtain continuous filament yam.

[0041] In an embodiment, prior to applying a spin finish, the fibers or filaments obtained in step (b) are subjected to one or more washing steps to remove residual solvent.

[0042] In an embodiment, the disclosed flame-retardant lyocell fiber is incorporated into a woven fabric using a technique known in the art. In such an embodiment, the woven fabric comprises a weft and a warp, at least one of which consists of the flame-retardant lyocell fiber. In some embodiments, the woven fabric further comprises one or more other fiber types in addition to the flameretardant lyocell fiber, wherein the other fiber types are selected from the group consisting of meta-aramid, para-aramid, modacrylic, nylon, wool, cotton, silk, linen, flame-retardant modal, flame -retardant viscose, lyocell, modal, and viscose. In another embodiment, the flame-retardant lyocell fiber is incorporated into a nonwoven fabric. The nonwoven fabric is produced by forming a web of fibers and bonding the fibers together by mechanical, thermal, or chemical means known in the art. In some embodiments, the nonwoven fabric is produced by methods including, but not limited to, air-laying, carding and needling, hydroentangling, spun laying, or any other suitable nonwoven fabric manufacturing technique.

[0043] In order that this invention may be better understood, the following examples are set forth. These examples are for the purpose of illustration only and the exact compositions, methods of preparation and embodiments shown are not limiting the invention, and any obvious modifications will be apparent to one skilled in the art.

[0044] Examples:

[0045] Example 1: Preparation of exemplary flame retardant lyocell fiber

[0046] Process: A silica slurry (fumed silica) was prepared by mixing a polymeric dispersant (for example: Spredox® from Doxa Chemicals) and silica in NMMO. Cellulose solution was prepared by pre-mixing the cellulose for 30 minutes, followed by mixing cellulose with dissolving grade pulp in 50% weight ratio, in silica dispersed NMMO. A solution with cellulose concentration of 12.5% was prepared in 76 wt% silica dispersed NMMO. The ratio of added silica was 20-50 wt% based on weight of cellulose. The pre-mix comprising cellulose and NMMO was mixed thoroughly. After mixing, high shear was applied to prepare a cellulose - NMMO slurry at ~ 100°C. The slurry was subjected to temperature ~ 110°C and pressure of 650 mmHg of vacuum for removal of water as per the Cellulose - NMMO phase diagram known in the art.

[0047] The cellulose solution containing silica was extruded through suitable spinnerette at a range of temperatures 105°C ± 15°C depending on the viscosity of the solution. The cellulose fibers were regenerated after passing through a spinneret and an air gap into the spinning bath, having a concentration of NMMO of 20 to 22% in water.

[0048] In the present example, various flame retardant lyocell fibers were made using one the following techniques:

[0049] (i) The resultant flame-retardant cellulosic filaments is either continuous or is cut to the desired length, washed and dried. The dried lyocell fiber comprising silica was treated using a buffer solution containing 0.1 % sodium bicarbonate and 0.5 % sodium sulphate (pH: 8-10, temperature 40-60°C). Next, the treated fiber washed with water and dried at 100°C.

[0050] (ii) The never dried lyocell fiber comprising silica was bleached containing a buffer solution (pH: 8- 10, temperature 40-60°C) of 0.1 % sodium bicarbonate. Next, the treated fiber washed with water, oiled and dried at 100 °C.

[0051] (iii) The dried lyocell fiber prepared as demonstrated as in example 1 however salt 0.1% sodium bicarbonate is added along with silica in NMMO to enable buffering action while making lyocell spinning solution. The cellulose solution containing silica was extruded through suitable nozzles at a range of temperatures 105 °C ± 15 °C depending on the viscosity of the solution. The cellulose fibers were regenerated after passing through a spinneret and an air gap into the spinning bath, having a concentration of NMMO of 20 to 22% in water. The resultant flame -retardant cellulosic filament is either continuous or is cut to the desired length, washed. The dried lyocell fiber comprising silica was bleached, washed with water, oiled and dried at 100°C. Example 2: Comparison of mechanical properties of flame retardant lyocell fiber prepared in accordance with an embodiment of the present disclosure and an unmodified lyocell fiber (i.e. without a flame retardant)

[0052] To compare the mechanical properties of flame retardant lyocell fiber prepared in accordance with an embodiment of the disclosure and an unmodified lyocell fiber, various flame retardant lyocell fibers were formed using the process of Example 1 with 0-50 wt% loading of silica on fiber, as illustrated in Table 1.

[0053] These fibers were subsequently processed to form a fabric sheet using needle-punching in a nonwoven fabric manufacturing process. The resulting fabric sheets were subjected to a LOI test in accordance with ASTM D2863.

[0054] Table 1 provides the type of silica suspension used, technique (i), (ii) or (iii) used to prepare the linear density, tenacity, elongation % and LOI of the resultant flame retardant lyocell fibers.

[0055] Herein, the fiber having 0% of silica represents unmodified lyocell fiber. Flame retardant lyocell fibers (COMP1-COMP2) were formed using the process of Example 1 using fumed silica, but less than 20wt% silica loading. Flame retardant lyocell fibers (INV1-INV4) were formed using the process of Example 1 using colloidal silica. Flame retardant lyocell fibers (INV5-INV7) were formed using the process of Example 1 using fumed silica. Flame retardant lyocell fibers (INV8- INV9) were formed using the process of Example 1 precipitated silica.

[0056] Table 1: Comparison of properties of flame retardant lyocell fibers

[0057] It was observed that the flame retardant lyocell fiber of the present disclosure exhibited the required flame retardancy at lower silica loadings of 20- 50wt%, while simultaneously achieving a tenacity of at least 1.5 grams / denier and an elongation of at least 14% in a conditioned state.

[0058] Example 3: Measurement of % of silica or ash content on exemplary flame retardant lyocell fibers The inventive flame-retardant lyocell fibers INV 1, INV 3, INV 5, INV 6, and INV 7, along with the unmodified lyocell fiber, were prepared according to the treatment, washing, oiling, and drying steps as illustrated in Example 1. These fibers were then analyzed to determine the percentage of silica on the cellulosic fiber (also referred to as ash content) in accordance with ASTM D2584- 18. The results, representing the percentage of silica on the cellulosic fiber for each of the flame-retardant lyocell fibers (INV 1, INV 3, INV 5, INV 6, and INV 7) as well as the unmodified lyocell fiber, are summarized in Table 3 below.

[0059] Table 2: % of silica (or ash content) on cellulosic fibre of exemplary flame retardant lyocell fibers

[0060] The above results indicate the amount of silica that has been successfully incorporated into the fiber.

[0061] Example 4: TGA Measurement of exemplary flame-retardant lyocell fibers

[0062] Figure 1 presents a TGA plot comparing the thermal stability of flameretardant lyocell fibers INV 7, alongside unmodified lyocell fiber as a control. The TGA data were acquired by heating each sample from 0°C to 400°C at a rate of 1 °C per minute. The derivative thermogravimetric (DTG) curves, derived from the TGA data, are also overlaid on the plot. Each curve is clearly labeled. On the plot, mass is expressed as a percentage of the initial mass (%), and temperature is indicated in degrees Celsius (°C). The TGA and DTG plots together provide insight into how the flameretardant fibers behave when heated, helping to assess their thermal stability and the effectiveness of flame -retardant treatments compared to untreated lyocell fiber. It was observed from the TGA and DTG plots that the inventive flame-retardant lyocell fiber (INV 7) exhibited improved thermal stability compared to the unmodified lyocell fiber, indicating improved flame -retardant properties.

[0063] Example 5: Wash fastness of exemplary flame-retardant lyocell fibers

[0064] The flame retardancy of the inventive flame-retardant lyocell fiber was evaluated after subjecting the fiber to twenty laundering cycles in accordance with AATCC Test Method 135 (2018). Table 4 summarizes the LOI values obtained for the fiber following the laundering process, as determined by the AATCC test method.

[0065] Table 3: Wash fastness of flame retardant lyocell fiber

[0066] It was observed that there was no substantial decrease in the ash content and no significant change in the flame retardancy of the fibers after multiple washing cycles with water. This indicates that there is no leaching of silicic acid from the fibers following repeated washing, thereby demonstrating the durability and washfastness of the flame -retardant treatment.

[0067] Example 6: Dyeing fastness of exemplary flame-retardant lyocell fibers

[0068] The flame retardancy of the disclosed flame retardant lyocell fiber was measured after the fiber undergoes scouring and reactive dye treatment. Table 4 summarizes the LOI of fiber after scouring and reactive dye treatment.

[0069] Table 4: Dye fastness of exemplary flame-retardant lyocell fibers

[0070] It was observed that a slight decrease in flame retardancy occurred following scouring and reactive dye treatment. However, the flame retardancy, as measured by the LOI, remained above 27, thereby satisfying the established criterion for use as a flame -retardant fiber. This demonstrates the resilience of the flame-retardant properties of the disclosed lyocell fibers even after common textile processing treatments.

[0071] Example 6: LOI of exemplary Flame-Retardant Lyocell Fibers Obtained Using Colloidal Silica Suspension Upon Further Treatment with Buffer Solution

[0072] Three flame -retardant lyocell fiber samples were prepared following the process described for INV 3, wherein a colloidal silica suspension was incorporated into the spinning solution. Subsequently, each fiber sample underwent an additional treatment using one of the techniques (i), (ii), or (iii) as detailed in Example 1, involving the application of a buffer solution to facilitate the conversion of residual silica to silicic acid.

[0073] The LOI values of the flame-retardant lyocell fibers obtained after these further treatments are summarized in Table 5 below: Table 5: LOI of exemplary flame-retardant lyocell fibers

[0074] The results demonstrate that the additional buffer treatment effectively enhances the flame -retardant properties of the lyocell fibers, as indicated by the LOI values obtained.

[0075] Industrial Applicability

[0076] The disclosed flame-retardant lyocell fibers exhibit enhanced flame retardancy while retaining or improving their mechanical properties relative to unmodified lyocell fibers. In these fibers, the incorporation of a silica suspension is followed by the conversion of silica to silicic acid or hydrated silica under controlled alkaline conditions. The resulting silicic acid becomes incorporated into the cellulose matrix of the lyocell fibers, thereby imparting flame retardancy while maintaining or enhancing key mechanical properties such as tenacity and elongation.

[0077] This approach effectively addresses the limitations associated with traditional sodium silicate-based flame retardants, which typically require higher loading levels due to leaching from the cellulose matrix and often negatively impact fiber strength and flexibility. In contrast, the disclosed flame retardant lyocell fibers achieve a LOI of at least 27, indicating significantly improved resistance to combustion, at lower silica loadings to 20- 50wt%. Additionally, these fibers possess an ash content consistent with effective silica incorporation without compromising fiber integrity. The minimal silica loading necessary to achieve these properties ensures that the fibers retain the desirable mechanical properties, softness, moisture management, and sustainability characteristics inherent to lyocell fibers, rendering them suitable for a broad range of textile applications. The disclosed flameretardant lyocell fibers meet stringent performance requirements for demanding industries such as firefighting, military, and emergency services, where exposure to flames poses a significant hazard. They are also highly suitable for use in home textiles, protective clothing, and various technical textiles.

[0078] Furthermore, these fibers are environmentally friendly and exhibit superior physical and chemical robustness compared to known flame-retardant regenerated cellulose fibers. Their versatility extends to applications in upholstery, carpets, curtains, and other soft furnishings for both commercial and residential use.

[0079] Additionally, the disclosed fibers maintain mechanical properties compatible with modem textile processing techniques, including weaving, knitting, and nonwoven fabric production, facilitating their seamless integration into diverse industrial products. In some embodiments, the flame -retardant lyocell fibers can be blended with other flame -retardant or high-performance fibers — such as modacrylic, aramid, or wool — to tailor properties including strength, flexibility, and resistance to specific hazards, thereby expanding their utility across multiple sectors.

Claims

We Claim:

1. A flame retardant lyocell fiber comprising: lyocell fiber incorporating 20-50 wt% of silicic acid, the flame retardant lyocell fiber having a tenacity of at least 1.5 grams / denier and an elongation of at least 14% in a conditioned state, measured as per ASTM D 3822.

2. The flame retardant lyocell fiber as claimed in claim 1, having the tenacity of 1.5 to 4.0 grams / denier, measured as per ASTM D 3822.

3. The flame retardant lyocell fiber as claimed in claim 1, having the elongation of 14-30% in the conditioned state measured as per ASTM D 3822.

4. The flame retardant lyocell fiber as claimed in claim 1, having a Limiting oxygen index (LOI) value of at least 27, measured as per ASTM D2863 (limiting oxygen index test).

5. The flame retardant lyocell fiber as claimed in claim 1, having an ash content in a range of 10 to 50% by mass, measured as per ASTM D2584-18.

6. The flame retardant lyocell fiber as claimed in claim 1, wherein the flame retardant lyocell fiber is a staple fiber or a continuous filament.

7. A woven fabric comprising weft yams extending in the weft direction and warp yams extending in the warp direction, wherein the weft yarns and warp yams are interwoven to form the fabric, and wherein at least one of the weft yams and the warp yarns comprises the flame-retardant lyocell fiber as claimed in claim 1.

8. The woven fabric as claimed in claim 7, further comprising one or more fibre selected from the group consisting of meta-aramid, para-aramid, modacrylic, nylon, wool, cotton, silk, linen, flame retardant modal, flame retardant viscose, lyocell, modal and viscose fiber.

9. A nonwoven fabric comprising the flame-retardant viscose fiber as claimed in claim 1, wherein the nonwoven fabric is produced by forming a web of fibers and bonding the fibers together by mechanical, thermal, or chemical means.

10. A process for preparing a flame retardant lyocell fiber, the process comprising the steps of: a. preparing a spinning solution comprising cellulose in aqueous N- Methylmorpholine N-oxide (NMMO) and a silica suspension, such that the spinning solution comprises 20-50 wt% silica based on weight of cellulose; b. extruding the spinning solution through a spinneret into a spinning bath to obtain a never dried lyocell fiber; c. applying a spin-finish to the never dried lyocell fiber; and d. drying to obtain the flame retardant lyocell fiber, such that the flame retardant lyocell fiber comprises lyocell fiber incorporating 20-50 wt% of silicic acid and has a tenacity of at least 1.5 grams / denier and an elongation of at least 14% in a conditioned state, measured as per ASTM D 3822.

11. The process as claimed in claim 10, wherein the silica suspension is a colloidal silica suspension which when added to the spinning solution undergoes sol gel conversion to form silicic acid to enable the formation of the flame retardant lyocell fiber.

12. The process as claimed in claim 10, wherein the silica suspension is formed of amorphous silica selected from the group consisting of precipitated and fumed silica, and a buffer solution is added during or after one of steps (a) to (d) to initiate the sol gel conversion of silica to silicic acid to enable the formation of the flame retardant lyocell fiber.

13. The process as claimed in claim 12, wherein the buffer solution comprises one or more sodium salts selected from the group consisting of sodium sulfate, sodium bicarbonate and sodium carbonate.

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