DESC:FIELD OF THE INVENTION
[001] The present invention relates to a method of preparing dithiopyrophosphate, wherein the yield is at least 90%. The invention also relates to a fire retardant viscose stable fibre (FR-VSF) and to a method of preparing the same.

DESCRIPTION OF THE BACKGROUND ART
[002] Viscose fibre is widely used in the manufacturing of textile clothing and decorative fabrics due to its excellent hygroscopicity, breathability, and dye ability. It is a regenerated cellulose and can be produced using cellulose sources, such as, wood or bamboo based pulp and cotton linter, indicating its sustainability, biodegradability, and low cost. However, the disadvantage of viscose fibre is its flammability. Limiting oxygen index (LOI) value for viscose fibre is about 19%, which indicates that it is highly flammable. Therefore, it is necessary to reduce the flammability of viscose fibre to ascertain the personal safety under fire circumstance.

[003] Various methods have been developed to reduce the flammability of viscose fibre and rayon through the application of flame retardants. The preferred method to make flame retardant viscose fibre is to add the flame retardant additives to the viscose solution and produce viscose fibre through the wet spinning method. In recent years, many nanoclay materials have been developed as flame retardants to provide an economically acceptable and environment-friendly solution. However, the introduction of nanoclay materials always caused major strength loss. The halogen flame retardant has excellent flame-retardant properties, but it tends to release toxic gases when it is burned, which can result in environmental pollution and personal injury. Non-halogen flame retardants containing phosphorus, nitrogen or silicon are widely employed in flame-retardant process of textile materials as lower toxic additives.

[004] It is well known that the organophosphorus compounds are environment friendly and have been used as flame retardant dope additive for viscose rayon fiber. Specifically, NL7508884A discloses the application of compound of formula I as flame retardants for textile applications.

[005] US 2010/0190402 A1 discloses the use of organophosphoric additive for viscose dope to impart flame retardant properties. The fibers of US’402 exhibits 28-30% of LOI after adding of 10-20% of organophosphoric compound as an additive.

[006] Dithiopyrophosphate is currently manufactured by Clariant and sold under the trade name Exolit 5060® DP. Presently, the flame-retardant viscose fibres for textile applications are manufactured by Lenzing, Austria (Viscose FR®) is the phosphorus based flame-retardant fibre developed from cellulose. Viscose FR® is made by incorporating ~25% organophosphorus-based material as a flame-retardant additive.

[007] Jilin Chemical Fiber Group Co., Ltd., in the year 2014, reported the preparation of phosphorodithioic acid neopentyl diester using mixed solution of dripping solvent triethylamine or cyclohexane and trimethylamine, as a fire retardant additive for viscose dope, which exhibited 28.2% of LOI.

[008] NL7508884A reports a process for production of organic dithiopyrophosphates of formula I involve the following: a) Reaction of 1, 3-diol with thiophosphoryl chlorides to get phosphochloridothionate; b) phosphochloridothionate is further hydrolysed with water in presence of a base to get the resulting cyclic dithiopyrophosphates. In the prior art different solvents or mixture of solvents are used for the two steps and the intermediate product are separated. The base used in the process activates the reaction. TWI427082B, US 8,324,418 B2, and WO 2010/049083 A1 discloses the use of 3-methylpyridine both as solvent and base to increase the yield of the final product from 75-85 %.

[009] REN Yuan-Lin et al disclose a novel phosphorus, nitrogen, and sulfur containing fire retardant, N, N'-bis (2-thio-5,5-dimethyl-1,3,2-dioxaphosphorinane) ethane (DDPSN), by reacting neopentyl glycol with thiophosphoryl chloride to obtain 2-thio-2-chloro-5,5-dimethyl-1,3,2-dioxaphosphorinane (DDSP). The obtained DDSP was then reacted with ethylenediamine.

[010] Koutu and Sharma reported pyridine as the base and benzene as the solvent for the first step to get the compound (I). Benzene is regarded as a carcinogen and now its application at industrial scale is banned.

[011] Efforts are being made to develop a process with an increased yield of the dithiopyrophosphate.

SUMMARY OF THE INVENTION
[012] According to an embodiment of the invention, there is provided a method of preparing dithiopyrophosphate in high yield of at least 90%, the method comprising the steps of:
a. adding 4-(dialkylamino)pyridine and a base to a solution of neopentyl glycol in an organic solvent, and adding thiophosphoryl chloride at 60 to 80 °C for 1.5 to 3h followed by mixing at 70 to 90 °C for 3 to 5h;
b. cooling down the mixture of step (a), adding water to the mixture, separating and collecting the organic layer of the mixture, evaporating the organic solvent to obtain crude phosphochloridothionate;
c. purifying the phosphochloridothionate by recrystallization using an organic solvent;
d. hydrolyzing the purified phosphochloridothionate of step (c) by dissolving in a solvent, a base, and water to obtain a solution, followed by adding 4-(dialkylamino)pyridine to the solution and refluxing for 4 to 6 h; and
e. cooling and filtering the solution of step (d), and then purifying the crude filtered solids by recrystallization using a solvent to obtain dithiopyrophosphate in a yield of at least 90%.

[013] According to another embodiment of the invention, there is provided a fire retardant viscose stable fibre (FR-VSF) having Limiting oxygen index (LOI) of 27.5 to 28.5%, denier of 1.3 to 1.4, and tenacity of 2.2 to 2.3 g/d, comprising finely ground dithiopyrophosphate as flame retardant prepared by the method above, said dithiopyrophosphate having particle size less than 500nm and being blended into the viscose in a range of 10 to 20% (w/w).

[014] According to yet another embodiment of the invention, there is provided a method of preparing fire retardant viscose staple fibre (FR-VSF) having Limiting oxygen index (LOI) of 27.5 to 28.5%, denier of 1.3 to 1.4, and tenacity of 2.2 to 2.3 g/d, the method comprising mixing 10 to 40% by weight dithiopyrophosphate, prepared by the method above, with 0.2 to 10% by weight polyoxyethylene alkylphenol ether phosphate with water, grinding the mixture to obtain a suspension and mixing the suspension with viscose dope, wherein the amount of dithiopyrophosphate in the suspension is 10 to 20% by weight with respect to cellulose in the viscose dope.

BRIEF DESCRIPTION OF THE DRAWINGS
[015] Figure 1 is a scheme of synthesis of dithiopyrophosphate according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION
[016] In the following detailed description, reference is made to the accompanying drawings that form a part hereof. The embodiments of the invention are described in sufficient detail to enable those skilled in the art to practice the invention and it is understood that other embodiments may be utilized and that logical processual changes may be made without departing from the spirit or scope of the invention. To avoid detail not necessary to enable those skilled in the art to practice the embodiments described herein, the description may omit certain information known to those skilled in the art. The following detailed description is, therefore, not to be taken in a limiting sense and the scope of the illustrative embodiments are defined only by the appended claims.

[017] As used herein, the singular forms "a," "an," and "the" include plural reference unless the context clearly dictates otherwise.

[018] The terms “preferred” and “preferably” refer to embodiments of the invention that may afford certain benefits, under certain circumstances. However, other embodiments may also be preferred, under the same or other circumstances. Furthermore, the recitation of one or more preferred embodiments does not imply that other embodiments are not useful, and is not intended to exclude other embodiments from the scope of the invention

[019] As used herein, the terms “comprising” “including,” “having,” “containing,” “involving,” and the like are to be understood to be open-ended, i.e. to mean including but not limited to.

[020] As used herein, “Limiting oxygen index (LOI)” refers to the minimum oxygen concentration (in vol%) that is necessary to sustain a stable combustion of the specimen after ignition.

[021] Figure 1 illustrates the scheme for the preparation of dithiopyrophosphate in accordance with the present invention. According to an embodiment of the invention, there is provided a method of preparing dithiopyrophosphate in high yield of at least 90%, the method comprising the steps of:
a. adding 4-(dialkylamino)pyridine and a base to a solution of neopentyl glycol in an organic solvent, and adding thiophosphoryl chloride at 60 to 80 °C for 1.5 to 3h followed by mixing at 70 to 90 °C for 3 to 5h;
b. cooling down the mixture of step (a), adding water to the mixture, separating and collecting the organic layer of the mixture, evaporating the organic solvent to obtain crude phosphochloridothionate;
c. purifying the phosphochloridothionate by recrystallization using an organic solvent;
d. hydrolyzing the purified phosphochloridothionate of step (c) by dissolving in a solvent, a base, and water to obtain a solution, followed by adding 4-(dialkylamino)pyridine to the solution and refluxing for 4 to 6 h; and
e. cooling and filtering the solution of step (d), and then purifying the crude filtered solids by recrystallization using a solvent to obtain dithiopyrophosphate in a yield of at least 90%.

[022] In an embodiment of the invention, the 4-(dialkylamino)pyridine is 4-(dimethylamino)pyridine, 4-(diethylamino)pyridine or a combination thereof. Preferably, in step (a) 0.01 to 0.05 equivalents of 4-(dialkylamino)pyridine are used relative to neopentyl glycol and 0.01 to 0.05 equivalents of 4-(dialkylamino)pyridine are used relative to thiophosphoryl chloride.

[023] In an embodiment of the invention, the 4-(dialkylamino)pyridine is 4-(dimethylamino)pyridine, 4-(diethylamino)pyridine or a combination thereof. Preferably, in step (d) 0.01 to 0.05 equivalents of 4-(dialkylamino)pyridine are used relative to phosphochloridothionate.

[024] The organic solvent in step (a) and (e) can be toluene, while the organic solvent in step (c) can be petroleum ether and the solvent in step (d) can be 1,4-dioxane. Preferably, the base is pyridine.

[025] According to another embodiment of the invention, there is provided a fire retardant viscose stable fibre (FR-VSF) having a Limiting oxygen index (LOI) of 27.5 to 28.5%, denier of 1.3 to 1.4, and tenacity of 2.2 to 2.3 g/d, comprising finely ground dithiopyrophosphate as flame retardant prepared by the method above, said dithiopyrophosphate having a particle size less than 500nm and being blended into the viscose in a range of 10 to 20%w/w.

[026] According to yet another embodiment of the invention, there is provided a method of preparing fire retardant viscose staple fibre (FR-VSF) having Limiting oxygen index (LOI) of 27.5 to 28.5%, denier of 1.3 to 1.4, and tenacity of 2.2 to 2.3 g/d, the method comprising mixing 10 to 40% by weight dithiopyrophosphate, prepared by the method above, with 0.2 to 10% by weight polyoxyethylene alkylphenol ether phosphate with water, grinding the mixture to obtain a suspension and mixing the suspension with viscose dope, wherein the amount of dithiopyrophosphate in the suspension is 10 to 20% by weight with respect to cellulose in the viscose dope.

[027] The 4-(dialkylamino)pyridine, as a catalyst, has increased the reactivity. The yield of cyclic phosphochloridothionate has also increased from 70% to >96%. In an embodiment, said cyclic phosphochloridothionate is isolated and purified using crystallization from non-polar solvents, such as n-alkanes or their mixtures or petroleum ether.

[028] The application of 4-(dialkylamino)pyridine, more particularly, 4-(dimethylamino) pyridine, as a catalyst, enhance the reactivity of the process and the yield of the desired product, dithiopyrophosphate from 65% to >93%. Therefore, the overall product yield achieved in this invention is ~90% which is significantly higher than the prior art.

[029] The intermediate hydroxy-product [II] shown in Figure 1 transforms to dithiopyrophosphate under reflux of step (d).

[030] The following experimental examples are illustrative of the invention but not limitative of the scope thereof:

Example 1: Conventional method
[031] Pyridine (16.2 ml, 0.2 mol) was added to the solution of neopentyl glycol (10.41 g, 0.1 mol) in toluene (60 ml). To this mixture thiophosphoryl chloride (16.94 g, 0.1 mol) was added drop by drop with constant stirring at 60-80 °C over a period of 2h. The reaction mixture was stirred at 80 °C for 5h. The reaction mixture was cooled to ambient temperature and added 400 ml water. The organic layer was collected and evaporated to complete dryness under reduced pressure to get the crude product. The crude product was recrystallized from petroleum ether to get 10.5 g of phosphochloridothionate. Yield 68 %, m. p. 88 °C, Elemental analyses (%): observed C, 29.84; H, 5.06; P, 14.33; S, 16.19, calculated C, 29.94; H, 5.02; P, 15.44; S, 15.98.

[032] To the freshly synthesized phosphochloridothionate (2 g, 10 mmol) dissolved in dioxane (5.4 ml) was added pyridine (1.4 ml, 16 mmol) and water (0.7 ml, 36 mmol) and allowed to stir for 6h under reflux. The reaction mixture was cooled at room temperature and added 3 ml water, stirred for 30 min, and the resulting solid was filtered. The crude product was crystallized from toluene to get 1.1 g of dithiopyrophosphate. Yield 65 %; m. p. 226.31 °C; Elemental analyses (%): observed C, 34.54; H, 5.76; P, 16.06, S, 18.39 calculated C, 34.68; H, 5.82; P, 17.89; S, 18.52.

Example 2: Conventional method
[033] Pyridine (16.2 ml, 0.2 mol) was added to the solution of neopentyl glycol (10.62 g, 0.1 mol) in toluene (60 ml). To this mixture, thiophosphoryl chloride (16.94 g, 0.1 mol) was added drop by drop with constant stirring at 60-80 °C over a period of 2h. The reaction mixture was stirred at 80 °C for 5h. The reaction mixture was cooled to room temperature and added 400 ml water. The organic layer was collected and evaporated to complete dryness under reduced pressure to get. Without further purification, pyridine (13 ml, 0.16 mol) and water (6.3 ml, 0.36 mol) were added the solution of phosphochloridothionate in dioxane (50 ml) The reaction mixture was allowed to stir for 6h under reflux. After the completion of reaction, the reaction mixture was allowed to cool at room temperature and added 30 ml water, stirred for 30 min and the resulting solid was filtered. The crude product was recrystallized from toluene to get 8.3g of dithiopyrophosphate. Yield 48 %; m. p. 226.3 °C; Elemental analyses (%): observed C, 34.48; H, 5.87; P, 16.16; S, 18.42 calculated C, 34.68; H, 5.82; P, 17.89; S, 18.52.

Example 3: Method according to an embodiment of present invention
[034] Pyridine (16.2 ml, 0.2 mol) and 4-(dimethylamino)pyridine (DMAP) (0.122 g, 0.001 mol) was added to the solution of neopentyl glycol (10.41 g, 0.1 mol) in toluene (60 ml). To this mixture thiophosphoryl chloride (16.93 g, 0.1 mol) was added drop by drop with constant stirring at 60-80 °C over a period of 2h. The reaction mixture was stirred at 80 °C for 4h. The reaction mixture was cooled at room temperature and added 400 ml water. The organic layer was collected and evaporated to complete dryness under reduced pressure to get the crude product. The crude product was recrystallized from petroleum ether to get 15 g of phosphochloridothionate. Yield 97%, m. p. 88.2 °C, Elemental analyses (%): observed C, 29.92; H, 5.04; P, 15.33; S, 16.19, calculated C, 29.94; H, 5.02; P, 15.44; S, 15.98.

[035] To the freshly synthesised phosphochloridothionate (2 g, 10 mmol) dissolved in dioxane (5.4 ml) was added pyridine (1.4 ml, 16 mmol), DMAP (0.012 g, 0.10 mmol) and water (0.7 ml, 36 mmol). The reaction mixture was stirred for 5h under reflux and then allowed to cool at room temperature. After that 3 ml water was added to the reaction mixture, stirred for 30 min and the resulting solid was filtered. The crude product was recrystallized from toluene to get 1.6 g of dithiopyrophosphate. Yield 92.5 %; m. p. 226.4; Elemental analyses (%): observed C, 34.64; H, 5.70; P, 17.86; S, 18.49 calculated C, 34.68; H, 5.82; P, 17.89; S, 18.52.

Example 4: Method according to an embodiment of the present invention
[036] Pyridine (6016 g, 76 mol) and 4-(dimethylamino)pyridine (42.2 g, 0.347 mol) were added to the solution of neopentyl glycol (3600 g, 34.56 mol) in toluene (15925 g). To this mixture thiophosphoryl chloride (5855 g, 34.56 mol) was added drop by drop with constant stirring at 60-80 °C over a period of 2h. The reaction mixture was stirred at 80 °C for 4h. Further, the reaction mixture was cooled to room temperature, and added 5000×3 ml water. The organic layer was collected and evaporated to complete dryness under reduced pressure to get phosphochloridothionate as a crude product. Without further purification, phosphochloridothionate was dissolved in dioxane (18292 g) and to this solution pyridine (4380 g, 55.36 mol), 4-(dimethylamino)pyridine (42.3 g, 0.346 mol) and water (2242 g, 124.56 mol) was added and allowed to stir for 5h under reflux. The reaction mixture was allowed to cool to room temperature and added 10000×3 ml of cold water, stirred for 30 min, and the resulting solid was filtered. The crude product was recrystallized from toluene to get 5065 g dithiopyrophosphate. Yield 91%; m. p. 226.25 °C; Elemental analyses (%): observed C, 34.61; H, 5.79; P, 17.32; S, 18.21 calculated C, 34.68; H, 5.82; P, 17.89; S, 18.52.

Example 5: Method according to an embodiment of present invention
[037] Pyridine (24064 g, 304.2 mol) and 4-(dimethylamino)pyridine (DMAP) (168.8 g, 1.38 mol) was added to the solution of neopentyl glycol (14400 g, 138.24 mol) in toluene (63700 g). To this mixture, thiophosphoryl chloride (23420 g, 138.25 mol) was added drop by drop with constant stirring at 60-80 °C over a period of 2h and allowed to stir at 80 °C for 4h. The reaction mixture was cooled to room temperature and added 20000×3 ml water. The organic layer was collected and evaporated to complete dryness under reduced pressure to get phosphochloridothionate as crude product. Without any purification, phosphochloridothionate was first dissolved in dioxane (73168 g), and to the solution, pyridine (17520 g, 221.44 mol), 4-(dimethylamino)pyridine (169.2 g, 1.38 mol) and water (8968 g, 498.2 mol) was added. The reaction mixture was allowed to stir for 5h under reflux and allowed to cool at room temperature. Further 40000×3 ml of water was added to the reaction mixture and stirred for 30 min and the resulting solid was filtered. The crude product was recrystallized from toluene to get 20040 g dithiopyrophosphate. Yield 90%; m. p. 226.1 °C; Elemental analyses (%): observed C, 34.51; H, 5.69; P, 17.45; S, 18.44 calculated C, 34.68; H, 5.82; P, 17.89; S, 18.52.

Example 6: Dispersion of dithiopyrophosphate in water and fibre spinning
[038] The synthesiszed fire retardant, dithiopyrophosphate is a white solid, insoluble in water, and very stable towards strong acids and alkalis. For the application of fire retardant in the viscose process, solid microcrystallite dithiopyrophosphate is ground into a fine powder, having a particle size of less than 500nm.

[039] Dithiopyrophosphate (10-40%) and polyoxyethylene alkylphenol ether phosphate (0.2-10%) were diluted with water and grinded in planetary ball mill using zirconia beads (0.2 to 0.4 mm) for a duration of 120 min to obtain fire retardant additive as suspension. The homogenous slurry of finely dispersed additives was mixed thoroughly with viscose dope, wherein the amount of additive in the slurry was kept at 10-20% by weight with respect to cellulose. The viscose solution containing fire retardant additive was extruded through a spinneret using a conventional process and regenerated in a spin bath comprised of sulphuric acid. The viscose fibres were drawn from spin bath using winders. Winder godet speeds were set at 30-50 rpm to obtain the flame-retardant fibre having a denier of 1.3.

Example 7: Determination of flame retardancy of viscose fibre
[040] The fibre with flame retardant additives was prepared following the above examples were cut into short fibres and transformed to a non-woven fibre sheet of 5 gm, having an area 16x16 cm2 using a sheet-making machine. The resulting fibres were tested for limiting oxygen index (LOI) as per standard method (IS 13501: 1992 RA 2013). The fibre properties and LOI of control viscose fibre and flame-retardant viscose fibre (FR-VSF) were summarized in Table 1. The results showed that with the nano suspension of fire-retardant additives, the LOI of flame-retardant viscose fibre can be achieved closed to 27.7% with lower loading of 15%.

[041] Table 1. LOI values of flame-retardant viscose fibre sheet
Sample FR additive (%, w/w) Denier Tenacity (g/d) Elongation
(%) Average LOI (%)
FR-VSF-1 15 1.31 2.28 16.8 27.7
FR-VSF-2 20 1.34 2.24 16.1 28.2
Control VSF 0 1.30 2.82 18.5 17.1
[042] The above examples are non-limiting. The invention is defined by the claims that follow.

,CLAIMS:1. A method of preparing dithiopyrophosphate in a high yield of at least 90%, the method comprising the steps of:
a. adding 4-(dialkylamino)pyridine and a base to a solution of neopentyl glycol in an organic solvent, and adding thiophosphoryl chloride at 60 to 80 °C for 1.5 to 3h followed by mixing at 70 to 90 °C for 3 to 5h;
b. cooling down the mixture of step (a), adding water to the mixture, separating and collecting the organic layer of the mixture, evaporating the organic solvent to obtain crude phosphochloridothionate;
c. purifying the phosphochloridothionate by recrystallization using an organic solvent;
d. hydrolyzing the purified phosphochloridothionate of step (c) by dissolving in a solvent, a base, and water to obtain a solution, followed by adding 4-(dialkylamino)pyridine to the solution and refluxing for 4 to 6 h; and
e. cooling and filtering the solution of step (d), and then purifying the crude filtered solids by recrystallization using a solvent to obtain dithiopyrophosphate in a yield of at least 90%.
2. The method as claimed in claim 1, wherein the 4-(dialkylamino)pyridine is 4-(dimethylamino)pyridine, 4-(diethylamino)pyridine or a combination thereof.
3. The method as claimed in 1, wherein in step (a) 0.01 to 0.05 equivalents of 4-(dialkylamino)pyridine is used relative to neopentyl glycol and 0.01 to 0.05 equivalents of 4-(dialkylamino)pyridine is used relative to thiophosphoryl chloride.
4. The method as claimed in 1, wherein in step (d) 0.01 to 0.05 equivalents of 4-(dialkylamino)pyridine is used relative to phosphochloridothionate as obtained from step (c).
5. The method as claimed in claim 1, wherein the organic solvent in step (a) or step (e) or both steps is toluene.
6. The method as claimed in claim 1, wherein the organic solvent in step (c) is petroleum ether.
7. The method as claimed in claim 1, wherein the solvent in step (d) is 1,4-dioxane.
8. The method as claimed in claim 1, wherein the base is pyridine.
9. A fire retardant viscose stable fibre (FR-VSF) having Limiting oxygen index (LOI) of 27.5 to 28.5%, denier of 1.3 to 1.4, and tenacity of 2.2 to 2.3 g/d, comprising finely ground dithiopyrophosphate as flame retardant prepared by the method of any of claims 1 to 8, said dithiopyrophosphate having a particle size less than 500nm and being blended into the viscose in a range of 10 to 20%w/w.
10. A method of preparing fire retardant viscose staple fibre (FR-VSF) having Limiting oxygen index (LOI) of 27.5 to 28.5%, denier of 1.3 to 1.4, and tenacity of 2.2 to 2.3 g/d, the method comprising mixing 10 to 40% by weight dithiopyrophosphate, prepared by the method of any of claims 1 to 8, with 0.2 to 10% by weight polyoxyethylene alkylphenol ether phosphate with water, grinding the mixture to obtain a suspension and mixing the suspension with viscose dope, wherein the amount of dithiopyrophosphate in the suspension is 10 to 20% by weight with respect to cellulose in the viscose dope.
Dated this 23rd day of January 2023