LYOCELL FIBER MODIFIED BY COMPOUND FLAME RETARDANT AND PREPARATIONS METHOD THEREOF

Description

FIELD

The present application belongs to the field of fiber manufacturing, and specifically, relates to a Lyocell fiber modified with a compound flame retardant and a preparation method thereof.

BACKGROUND

Lyocell fiber is a new type of regenerated cellulose fiber prepared by a solvent method, which is prepared by a dry-wet spinning process using N-methylmorpholine-N-oxide (NMMO) as a solvent. The waste in the preparing process of the Lyocell fiber can be naturally degraded. The NMMO solvent can also be efficiently recycled by 99.5%, and is non-toxic and does not pollute the environment. Lyocell fiber has excellent moisture absorption and breathability, good handle, comfortable wearing, and excellent mechanical properties, so it is widely applied in the clothing field.

However, Lyocell fiber is a flammable fiber and burns very easily. In order to expand the application of Lyocell fiber, Lyocell fiber is modified for retarding flame. Existing flame retardant modification technologies for Lyocell fiber are applied on the main basis of the blending process and the finishing process. In the finishing process, flame retardants are attached to fiber or fabrics by means of impregnation, baking, coating, spraying, etc, and there are high requirements for flame retardants in the method, but the finished fabric has a poor handle and is not resistant to water washing. In the blending process, flame retardants are added in the slurry or spinning solution to spin flame-retardant fibers. The method is simple, but usually has the problems such as large particle size of flame retardants, wide range of particle size, easy agglomeration of flame retardant particles, large loss of mechanical properties, and poor washing resistance of modified fibers. In addition, for the existing multiple series of flame retardant compounds, the costs of preparing them are high, and it is prone to environmental pollution during the preparation process.

Therefore, the present application is proposed.

SUMMARY

The present application is to overcome the deficiencies of the prior art and provide a modified Lyocell fiber with a compound flame retardant and a preparation method thereof. In the present application, Lyocell fiber is modified by a compound flame retardant. By controlling the particle size, the fiber spinning process can be smoothly run, and the compound flame retardant is uniformly dispersed inside the fiber. So the flame retardancy of the fiber is significantly improved, and the fiber is resistant to washing and has excellent durability of flame retardant. It is low in cost, zero-emission, and environmentally friendly in preparing Lyocell fiber.

In order to solve the above problem, the technical solutions adopted by the present application are the follows.

A first aspect of the present application is to provide a Lyocell fiber modified with a compound flame retardant, in which cellulose-based fiber as a matrix material in which a flame retardant and a synergistic agent are uniformly dispersed. A mass of the flame retardant is in a range of 40%-80% of a mass of cellulose in the fiber, and a mass of the synergistic agent is in range of 1-20% of the mass of cellulose in the fiber. A ratio of a particle size D₉₀ of the flame retardant and the synergistic to a diameter φ of fiber is less than 0.1.

As a solution, the mass of the flame retardant is in a range of 60%-80% of the mass of cellulose in the fiber, and the mass of the synergistic agent is in a range of 5-15% of the mass of cellulose in the fiber. The ratio of the particle size D₉₀ of the flame retardant and the synergistic agent to the diameter φ of fiber is less than 0.1. In the present application, D₉₀ being less than a certain value means that 90% of the particles have the particle size less than a certain particle size.

As a preferable solution, the ratio of the particle size D₉₀ of the flame retardant and the synergistic agent to the diameter φ of fiber is less than 0.08.

In the present application, the finished flame retardant and synergistic agent are compounded, and the ratio of the particle size D₉₀ of the flame retardant and synergistic agent to the diameter o of fiber is controlled to be less than 0.1, so that it can be ensured the fiber spinning process is smoothly run, and the flame retardant is added inside the fiber. On the one hand, by controlling the particle size of the compound flame retardant to modify Loycell fiber, the flame retardancy is significantly improved, and the limiting oxygen index (LOI) is increased from 17% to more than 27%. The fiber can still maintain a high LOI value after being washed, and has good durability of flame retardant. On the other hand, in the present application, the finished flame retardant and synergistic agent are compounded. Compared with the multi-series of the flame retardants such as containing nitrogen and phosphorus, the preparation is more flexible, no synthesis is required, the cost is lower, and it is more environmentally friendly not to produce pollutants. By controlling the addition ratio and particle size of the flame retardant and the synergistic agent, a good flame retardant effect is achieved. When the ratio of the particle size D₉₀ to the diameter φ of fiber is less than 0.08, the impact on fiber strength is lower and the durability of flame retardant is better.

In the present application, a cross-sectional diameter of the fiber with flame retardancy is about 17 microns. If the particle size of the flame retardant and the synergistic agent is too large, the flame retardant and the synergistic agent are likely to precipitate on the surface of the fiber, it is reduced in effective addition amount and the blending effect, and it is reduced in properties such as washing resistance. And the strength of the modified fiber is greatly reduced.

In a further solution, the flame retardant is selected from a nitrogen-based flame retardant.

Optionally, the flame retardant is one or more compounds selected from a group consisting of melamine, dicyandiamide, guanidine phosphate and derivatives thereof.

Optionally, an initial particle size D₉₀ of the flame retardant is less than 50 um.

In a further solution, the synergistic agent is one or more compounds selected from a group consisting of organic phosphorus compound, metal oxide, boron-containing compound and water-insoluble silicate.

Optionally, the organic phosphorus compound is one or more compounds selected from a group consisting of phosphate ester, tetrakis (hydroxymethyl) phosphonium chloride, tetrakis (hydroxymethyl) phosphonium chloride-urea condensate, tetrakis (hydroxymethyl) phosphonium sulfate, and tetrakis (hydroxymethyl) phosphonium sulfate-urea condensate.

The metal oxide is one or more compounds selected from a group consisting of antimony trioxide, zirconium oxide, titanium oxide, magnesium oxide, calcium oxide, aluminum hydroxide, and calcium hydroxide.

The boron-containing compound is selected from zinc borate.

The water-insoluble silicate is one or more compounds selected from a group consisting of calcium silicate, magnesium silicate, and aluminum silicate.

Optionally, an initial particle size D₉₀ of the synergistic agent is less than 50 um.

As a solution, the flame retardant is melamine cyanurate, and the synergistic agent is zinc borate. Alternatively, the flame retardant is melamine cyanurate, and the synergistic agent is zinc oxide.

In a further solution, for Lyocell fiber modified with the compound flame retardant, 15 g of the fiber is put in a laundry bag and washed with tap water in an ordinary pulsator washing machine (XQN35-188) of which a standard washing program of 38 min is as one wash, and dried in an oven at a temperature of 80° C. after washing. After being repeatedly washed 11 times to 12 times, the level of flame retardancy is maintained, and LOI value is more than 27%.

The Lyocell fiber with flame retardancy of the present application has excellent property of flame retardancy, good washing resistance, and excellent durability of flame retardancy. The flame retardant is uniformly and tightly mixed with the cellulose, and the Lyocell fiber has good mechanical properties and good handle.

A second objective of the present application is to provide a method for preparing Lyocell fiber modified with a compound flame retardant, including:

• • (1) premixing a flame retardant and a synergistic agent with NMMO solution to obtain a first mixture, wherein, in the first mixture, a ratio of a particle size D₉₀ of the flame retardant and the synergistic agent to a diameter φ of fiber is less than 0.1;
  • (2) the first mixture and cellulose fiber pulp being sequentially pre-mixed, stirred, and swelled to obtain a second mixture fully swollen, wherein, a mass of the flame retardant is in a range of 40-80% of an absolute dry mass of the cellulose fiber pulp, and a mass of the synergistic agent is in a range of 1-20% of the absolute dry mass of the cellulose fiber pulp;
  • (3) the cellulose fiber being completely dissolved to obtain a spinning solution after the second mixture is dehydrated, and the spinning solution being spun to obtain Lyocell fiber modified with the compound flame retardant.

In a further solution, the mass of the flame retardant is in a range of 60%-80% of a mass of cellulose in the fiber, and the mass of the synergistic agent is in a range of 5-15% of the mass of cellulose in the fiber. The ratio of particle size D₉₀ of the flame retardant and the synergistic agent to the diameter φ of the fiber is less than 0.1.

In a further solution, the flame retardant is a nitrogen-based flame retardant.

Optionally, the flame retardant is one or more compounds selected from a group consisting of melamine, dicyandiamide, guanidine phosphate and their derivatives thereof.

Optionally, the initial particle size D₉₀ of the flame retardant is less than 50 um.

In a further solution, the synergistic agent is one or more compounds selected from a group consisting of organic phosphorus compound, metal oxide, boron-containing compound and water-insoluble silicate.

Optionally, the synergistic agent is one or more compounds selected from a group consisting of phosphate ester, antimony trioxide, zirconium oxide, titanium oxide, magnesium oxide, calcium oxide, aluminum hydroxide, calcium hydroxide, zinc borate, calcium silicate, magnesium silicate, aluminum silicate, tetrakis (hydroxymethyl) phosphonium chloride, tetrakis (hydroxymethyl) phosphonium chloride-urea condensate, tetrakis (hydroxymethyl) phosphonium sulfate, and tetrakis (hydroxymethyl) phosphonium sulfate-urea condensate.

Optionally, an initial particle size D₉₀ of the synergistic agent particles is less than 50 μm.

In a further solution, in step (1), the flame retardant and the synergistic agent are first made into a flame retardant dispersion, and then premixed with NMMO solution to obtain the first mixture. A method for preparing the flame retardant dispersion includes: grinding a dispersion medium, an emulsifier, a dispersant, an antifoaming agent, a flame retardant, and a synergistic agent to obtain the flame retardant dispersion.

Optionally, a grinding apparatus is selected from one or a combination of a ball mill, a homogenizer, and a sand mill, and a grinding bead is selected from one or a combination of stainless steel bead, zirconia bead, and tungsten carbide bead.

As a specific solution, the zirconia bead is used for grinding in the sand mill at a speed of 1100 r/min to 1300 r/min for a grinding time of 1 h to 6 h.

In a further solution, mass parts of the components in the flame retardant dispersion include:

• • flame retardant: 10 parts-50 parts
  • synergistic agent: 1 part-20 parts
  • emulsifier: 0.2 parts-20 parts
  • dispersant: 0.2 parts-20 parts
  • antifoaming agent: 1 part-5 parts
  • dispersion medium as a remaining material, wherein the dispersion medium is selected from water or an aqueous solution of NMMO.

In a further solution, the emulsifier is one or more compounds selected from a group consisting of sodium dodecylbenzene sulfonate, sodium dodecyl sulfate, sodium dodecyl sulfonate, polyoxyethylene ether compound, styrene-maleic anhydride copolymer, polyoxyethylene sorbitan monooleate, and polyoxyethylene sorbitan trioleate.

Optionally, the dispersant is one or more compounds selected from a group consisting of sodium polyacrylate, sodium ethylene-bis-naphthalenesulfonate, fatty alcohol polyoxyethylene ether, α-olefin polyoxyethylene sulfonate, and sodium maleate.

Optionally, the antifoaming agent is one or more compounds selected from a group consisting of polyether modified polysiloxane, polyether siloxane copolymer, and water-based acrylic antifoaming agent.

In a further solution, in step (2), a temperature in a stirring process is in a range of 30° C. to 100° C., and a swelling duration is in a range of 5 min to 60 min.

Optionally, a pulp in the second mixture is in a uniform, fine, and microfibrous slurry state without a white core.

In a further solution, in step (3), by a dry-wet spinning process, the spinning solution is sequentially subjected to steps of extruding by spinneret, coagulating, drawing, washing, cutting, and drying to obtain Lyocell fiber modified with the compound flame retardant.

After adopting the above technical solutions, the present application have the following advantages compared with the prior art.

• • 1. In the present application, the finished flame retardant and the synergistic agent are used for compounding, and the ratio of the particle size D₉₀ of the flame retardant and the synergistic agent to the diameter φ of fiber is controlled to be less than 0.1, so the fiber spinning process can be smoothly run, and the flame retardant is added inside the fiber. By modifying Loycell fiber with a compound flame retardant having a certain particle size, the flame retardancy is significantly improved, and the limiting oxygen index (LOI) is increased from 17% to more than 27%. After being washed, the fiber can be still maintained a higher value of LOI, and has good durability of the flame retardancy.
  • 2. In the present application, the finished flame retardant and the synergistic agent are used for compounding. Compared with multi-series flame retardants such as containing nitrogen and phosphorus, the preparation is more flexible, no synthesis is required, the cost is lower, and it is more environmentally friendly not to produce pollutants. By controlling the addition ratio and particle size of the flame retardant and the synergistic agent, a good flame retardant effect is achieved. The present application, it is found in the experiment that the effects of different compounding systems on fiber strength and flame retardancy are different, and the initial LOI value of the fiber is higher than 27%. Among them, the compounding system of melamine cyanurate and zinc oxide has the best effect.
  • 3. According to the method of the present application, the flame retardant dispersion with excellent stability can be obtained, the particle size remains stable during the static process, and the compatibility of the flame retardant dispersion and NMMO solution is excellent. In the preparation process, there is no emission, and it is environmentally friendly. The flame retardancy of Lyocell fiber has excellent property of flame retardancy, good water washing resistance, and excellent durability of flame retardancy. The flame retardant is uniformly and tightly mixed with the cellulose, and the Lyocell fiber has good mechanical properties and good handle. The embodiments of the present application are further described in detail below in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are part of the present application and are used to provide a further understanding of the present application. The exemplary embodiments of the present application and their descriptions are used to explain the solutions of the present application, but do not improperly limit the present application. Obviously, the drawings described below are only some embodiments. For ordinary person skilled in the field, other drawings can be obtained based on these drawings without creative work. In the drawings:

FIG. 1 is a schematic diagram of the distribution of a flame retardant and a synergistic agent in a flame retardant dispersion of the present application;

FIG. 2 is a microscope image of a spinning solution for a flame retardant fiber of the present application;

FIG. 3 is a morphology of surface of Lyocell fiber modified by the dispersion with a particle size D₉₀ of 1.84 um in the present application;

FIG. 4 is a morphology of surface of Lyocell fiber modified by the dispersion with a particle size D₉₀ of 1.4 um in the present application;

FIG. 5 is a morphology of surface Lyocell fiber modified by the dispersion with a particle size D₉₀ of 0.7 um in the present application.

It should be noted that these drawings and descriptions are not intended to limit the scope of the present application in any way, but rather to illustrate the present application for those skilled in the art by referring to embodiments.

EMBODIMENTS

In order to make the purpose, solutions and advantages of the embodiments of the present application clearer, the solutions in the embodiments are clearly and completely described below in conjunction with the drawings in the present application. The following embodiments are used to illustrate the present application, but are not used to limit the scope of the present application.

It should be noted that, unless otherwise specified, the reagents used in the present application are all commercially available.

Test method of D90 is as follows: in which Laser particle size analyzer SYMPATEC-HELOS/BF is as the detection device, 1. diluting the dispersion 200-300 times with deionized water, 2. injecting an appropriate amount of deionized water into the sample pool to detect the blank background for reference, and 3. injecting an appropriate amount of the diluted dispersion into the sample pool for detection, and reading D₉₀ data.

Test method of fiber (μm) is as follows: in which Hitachi field emission scanning electron microscope (S-4700) is as the detection device, 1. preparing fiber samples; 2. observing the SEM morphology of the surface of the fibers. According to the scale of the SEM image, the surface widths of 20 fibers with regular morphology are read, and the average value is taken as the diameter of the test fiber.

Test method of fiber fineness (dtex) is carried out based on the test standard GB/T 14335-2008 as follows: in which the cut-middles method for the fiber bundle is used, 1. taking the fiber bundle with a certain number of fibers (about 2000) from the moisture-conditioned fiber samples, and making one ends of the fibers be flush with each other, and the fibers be straight and not stretched, 2. arranging 3-5 bundles in parallel, 3. cutting a certain length of samples from the middle of the arranged bundles with a cutter, wherein the cutter is vertical and there is no free fiber in the cut samples, 4. arranging and fixing the mid-high section samples in parallel, and using a projector to count them one by one, 5. adjusting the moisture of the counted samples, weighing them one by one, and calculating the linear density.

Test method for dry breaking strength (cN/dtex) is carried out based on the test standard GB/T 14337-2008 as follows: 1. conditioning the moisture of the fiber; 2. taking the fiber out of the sample, clamping one end of the fiber with a tension clamp specified in the standard, and placing the fiber in a clamper of an instrument for tensile testing, and obtaining the load when the sample breaks, 3. testing about 50 fibers (if the fiber is broken in or slipped out of the jaw of the clamper, it is removed and tested again), and taking the average value as the dry breaking strength.

Test method for Limiting oxygen index is carried out based on FZ/T 50016-2011 as follows: 1. fixing a sample vertically in a transparent combustion cylinder in which the mixed gas of oxygen and nitrogen flows upward, igniting the top of the sample and observing the combustion characteristics of the sample, and comparing the time of continuously burning the sample with the given criterion, 2. testing about 15 samples, 3. estimating the minimum oxygen concentration through testing with different concentrations.

The washing method includes: putting 15 g of fiber into a laundry bag, washing fiber with tap water in an ordinary pulsator washing machine (XQN35-188) of which a standard washing program of 38 min is as one wash, and drying in an oven at 80° C. after washing. The fiber is repeatedly washed for 12 times.

Preparation of Flame Retardant Dispersion

Embodiment 1

Preparation of Flame Retardant Dispersion

3 parts by weight of sodium dodecyl sulfate, 5 parts by weight of sodium ethylene-bis-naphthalene sulfonate, 2 parts by weight of polyether siloxane copolymer, and 75 parts by weight of water were mixed and stirred in a dispersion tank, and then 26.1 parts by weight of melamine cyanurate and 2.2 parts by weight of zinc borate were gradually added to obtain a mixture under a shear stirring of 4-6 m/s. After stirring evenly, the mixture was transferred to a sand mill and ground with zirconia beads at 1200 r/min for 2 h. Then, the dispersion was filtered to obtain a flame retardant dispersion with an effective component being flame retardant of 25%.

The effective component is the flame retardant and the synergistic agent, and the weight percentage of the effective components=(weight of effective components/total weight of all components)×100%.

Embodiment 2

Preparation of Flame Retardant Dispersion

4 parts by weight of sodium dodecylbenzene sulfonate, 7 parts by weight of fatty alcohol polyoxyethylene ether, 1 part by weight of polyether modified polysiloxane, and 148 parts by weight of water were mixed and stirred in a dispersion tank, and then 24.1 parts by weight of melamine, 2.2 parts by weight of zinc oxide and 2 parts by weight of antimony trioxide were gradually added to obtain a mixture under a shear stirring of 4-6 m/s. After stirring evenly, the mixture was transferred to a sand mill and ground with zirconia beads at 1200 r/min for 2 h. Then, the dispersion was filtered to obtain a flame retardant dispersion with an effective component being flame retardant of 15%.

Embodiment 3

Preparation of Flame Retardant Dispersion

0.2 parts by weight of sodium dodecyl sulfonate, 0.5 parts by weight of α-olefin polyoxyethylene sulfonate, 1 part by weight of polyether modified polysiloxane, and 87.5 parts by weight of water were mixed and stirred in a dispersion tank, and then 19.1 parts by weight of melamine cyanurate, 2.2 parts by weight of magnesium silicate and 1 part by weight of zinc borate were gradually added to obtain a mixture under a shear stirring of 4-6 m/s. After stirring evenly, the mixture was transferred to a sand mill and ground with zirconia beads at 1200 r/min for 2 h. Then, the dispersion was filtered to obtain a flame retardant dispersion with an effective component being flame retardant of 20%.

Embodiment 4

Preparation of Flame Retardant Dispersion

5 parts by weight of sodium dodecyl sulfonate, 5 parts by weight of sodium dodecyl sulfate, parts by weight of α-olefin polyoxyethylene sulfonate, 5 parts by weight of polyether modified polysiloxane, and 201 parts by weight of water were mixed and stirred in a dispersion tank, and then 19.1 parts by weight of melamine cyanurate, 2.5 parts by weight of calcium silicate and 1 part by weight of antimony trioxide were gradually added to obtain a mixture under a shear stirring of 4-6 m/s. After stirring evenly, the mixture was transferred to a sand mill and ground with zirconia beads at 1200 r/min for 2 h. Then, the dispersion was filtered to obtain a flame retardant dispersion with an effective component being flame retardant of 10%.

Embodiments and Comparative Examples of Preparation of Lyocell Fiber

Embodiment 5

A flame retardant dispersion with an effective component of 25 wt % was prepared according to the method of Embodiment 1, and D₉₀ less than 1.2 um was detected by a laser particle size analyzer. As shown in FIG. 1, particulate matter (flame retardant and synergistic agent) is in the dispersion state in NMMO, and it can be seen that the dispersion is uniform.

Cellulose pulp and a mixture of NMMO aqueous solution and a flame retardant dispersion were successively added into a reactor, and the total amount of the flame retardant and the synergistic agent is 75% of the absolute dry weight of the pulp (wherein the flame retardant is about 69% and the synergistic agent is about 6%). It is subjected to being stirred and swelled at a temperature of 75° C. for 25 min, then heated to a temperature of 106° C., vacuumed to 0.095 Mpa, and vacuum dehydrating for 40-60 min until the cellulose was completely dissolved to form a spinning solution. FIG. 2 shows a microscopic image of the spinning solution of a flame retardant fiber, and it can be seen that the particles are very evenly distributed in the spinning solution.

The spinning solution was extruded from a spinneret with 0.09 mm/27000 holes at a speed of 50 ml/min by a metering pump, stretched in air gap with a length of 25 mm, and then coagulated and precipitated into fiber in a coagulation bath of 20% NMMO at a temperature of 25° C. The residual NMMO in the fiber was washed by an ultrasonic cleaner for 30 min, and then the fiber was dried at a temperature of 105° C.

For the fiber obtained, 15 g of the fiber were put in a laundry bag and washed with tap water in an ordinary pulsator washing machine (XQN35-188) of which a standard washing program of 38 min is as one wash, and then dried in an oven at a temperature of 80° C. after washing. The fiber was repeatedly washed 11 times to 12 times. The limiting oxygen index of the fiber before and after being washed was tested according to the method specified in FZT 50016-2011, and the LOI values of the fiber before and after being washed were 34.6% and 33.2% respectively; the fiber fineness was 2.2 dtex, and the dry breaking strength was 2.25CN/dtex.

Embodiment 6

A flame retardant dispersion with an effective component of 15 wt % was prepared according to the method of Embodiment 2, and D₉₀ less than 1 um was detected by a laser particle size analyzer. Cellulose pulp and a mixture of NMMO aqueous solution and a flame retardant dispersion were added into a reactor in sequence, and the total amount of the flame retardant and the synergistic agent is 65% of the absolute dry weight of the pulp (wherein the flame retardant is about 55.4% and the synergistic agent is about 9.6%). It is subject to being stirred and swelled at a temperature of 80° C. for 20 min, then heated to a temperature of 105° C., vacuumed to 0.095 Mpa, and vacuum dehydrating for 40-60 min until the cellulose was completely dissolved. The spinning solution was extruded from a spinneret with 0.075 mm/2000 holes at a speed of 50 ml/min by a metering pump, stretched in air gap with a length of 25 mm, and then coagulated and precipitated into fiber in a coagulation bath of 20% NMMO at a temperature of 25° C. The residual NMMO in the fiber was washed by an ultrasonic cleaner for 30 min, and then the fiber was dried at a temperature of 105° C.

For the fiber obtained, 15 g of the fiber were put in a laundry bag, and washed with tap water in an ordinary pulsator washing machine (XQN35-188) of which a standard washing program of 38 min is as one wash, and then dried in an oven at a temperature of 80° C. after being washed. The fiber was repeatedly washed 11 times to 12 times. The limiting oxygen index of the fiber being washed was tested according to the method specified in FZ/T 50016-2011, and the LOI values of the fiber before and after being washed was 31.5% and 30.4% respectively; the fiber fineness was 2.2 dtex, and the dry breaking strength was 3.12CN/dtex.

Embodiment 7

A flame retardant dispersion with an effective component of 20 wt % was prepared according to the method of Embodiment 3, and D₉₀ less than 1 um was detected by a laser particle size analyzer. Cellulose pulp and a mixture of NMMO aqueous solution and a flame retardant dispersion were added into a reactor in sequence, and the total amount of the flame retardant and the synergistic agent is 55% of the absolute dry weight of the pulp (wherein the flame retardant is about 47% and the synergistic agent is about 8%). It is subject to being stirred and swelled at a temperature of 72° C. for 28 min, then heated to a temperature of 108° C., vacuumed to 0.095 Mpa, and vacuum dehydrating for 40-60 min until the cellulose was completely dissolved. The spinning solution was extruded from a spinneret with 0.09 mm/27000 holes at a speed of 50 ml/min by a metering pump, stretched in air gap with a length of 25 mm, and then coagulated and precipitated into fiber in a coagulation bath of 20% NMMO at a temperature of 25° C. The residual NMMO in the fiber was washed by an ultrasonic cleaner for 30 min, and then the fiber was dried at a temperature of 105° C.

The limitation oxygen index of the fiber was tested according to the method specified in FZ/T 50016-2011, and the LOI values of the fiber before and after being washed were 30.2% and 29.8% respectively; the fiber fineness was 2.2 dtex, and the dry breaking strength was 3.25CN/dtex.

Embodiment 8

A flame retardant dispersion with an effective component of 10 wt % was prepared according to the method of Embodiment 4, and D₉₀ less than 1 um was detected by a laser particle size analyzer. Cellulose pulp and a mixture of NMMO aqueous solution and a flame retardant dispersion were added into a reactor in sequence. The flame retardant and the synergistic agent is 45% of the absolute dry weight of the pulp (wherein the flame retardant is about 38% and the synergistic agent is about 7%). It is subject to being stirred and swelled at a temperature of 85° C. for 15 min, then heated to a temperature of 102° C., and vacuumed to 0.095Mpa, and vacuum dehydrating for 40-60 min until the cellulose was completely dissolved. The spinning solution was extruded from a spinneret with 0.09 mm/27000 holes at a speed of 50 ml/min by a metering pump, stretched in air gap with a length of 25 mm, and then coagulated and precipitated into fiber in a coagulation bath of 20% NMMO at a temperature of 25° C. The residual NMMO in the fiber was washed by an ultrasonic cleaner for 30 min, and then the fiber was dried at a temperature of 105° C.

The limitation oxygen index of the fiber was tested according to the method specified in FZ/T 50016-2011, and the LOI values of the fiber before and after being washed were 27.5% and 27%, respectively; the fiber fineness was 2.2 dtex, and the dry breaking strength was 3.19CN/dtex.

Comparative Example 1

The spinning solution was extruded from a spinneret with 0.09 mm/27000 holes at a speed of 50 ml/min by a metering pump, stretched in air gap with a length of 25 mm, and then coagulated and precipitated into fiber in a coagulation bath of 20% NMMO at a temperature of 25° C. The limiting oxygen index of the fiber was tested according to the method specified in FZT 50016-2011, and the LOI value was 17%; the fiber fineness was 2.2 dtex, and the dry breaking strength was 3.89cN/dtex.

The test results of the strength and the limitation oxygen index of the fiber in Embodiments 5-8 and Comparative Example 1 are shown in Table 1 and Table 2.

TABLE 1
Strength of the fiber modified with different
contents of the flame retardant

				Dry breaking
		φ fiber	Fineness	strength
	D90 (um)	(μm)	(dtex)	(cN/dtex)

Comparative	—	>15	2.2	3.89
Example 1
Embodiment 5	D90 < 1.2 um	>15	2.2	2.25
Embodiment 6	D90 < 1 um	>15	2.2	3.12
Embodiment 7	D90 < 1 um	>15	2.2	3.25
Embodiment 8	D90 < 1 um	>15	2.2	3.19

From the difference in strength values of the fiber between Comparative Example 1 and Embodiments 5-8, it can be seen that the dry breaking strength is significantly reduced due to the addition of the compound flame retardant in the fiber of the present application; and with the increase in the amount of the compound flame retardant, it is higher probability to generate stress defects inside the fiber, and the decrease range of the dry strength is also greater.

TABLE 2
Limiting Oxygen Index (LOI) values of the fiber modified
with different contents of the flame retardant

		LOI/% after
	LOI/%	12 washes

	Comparative	17	17
	Example 1
	Embodiment 5	34.6	33.2
	Embodiment 6	31.5	30.4
	Embodiment 7	30.2	29.8
	Embodiment 8	27.5	27

It can be seen from Embodiments 5-8 and Comparative Example 1 that the Lyocell fiber modified with the compound flame retardant of the present application still has good flame retardancy after being washed 12 times with water. When the flame retardant and synergistic agent is more than 45% of the absolute dry weight of the pulp, the LOI of the fiber with flame retardancy is higher than 27%, which reaches the higher level of the flame retardancy. The limiting oxygen index LOI value is increased as the increase in the amount of the compound of the flame retardant and the synergistic agent.

Comparative Example 2

Influence of Different Particle Sizes of the Flame Retardant and the Synergistic Agent

Referring to the components and method of Embodiment 1, the difference among the groups is in the grinding condition to control the particle size. The dispersion was filtered to obtain a flame retardant dispersion with an effective component being flame retardant of 25%. D₉₀ was detected by a laser particle size analyzer. Then, the fiber was prepared according to the steps and method of Embodiment 5, and the properties were tested. The difference conditions and the properties tested are shown in the table below.

	TABLE 3
					Dry breaking		LOI/%
	Grinding	D90	φ fiber	Fineness	strength		after 12
	conditions	(um)	(μm)	(dtex)	(cN/dtex)	LOI/%	washes

Group	1200 r/min	1.84	>19	3.0	1.93	29.8	24.4
1	grinding for 0.5 h
Group	1200 r/min	1.4	>15	3.72	2.2	31.5	26.1
2	grinding for 1 h
Group	1200 r/min	0.7	>15	3.58	2.34	33.6	32.7
3	grinding for 5 h

The result is analyzed as the follows. By controlling the grinding time, the flame retardant dispersions with different particle sizes were prepared. The difference in particle size has an impact on the mechanical properties and flame retardancy property of the fiber, especially on the water washing resistance of the fiber with the flame retardancy property. By reducing the particle size, it is reduced in the damage to the mechanical properties of the fiber caused by the flame retardant, etc., and improved in the water washing resistance of the fiber with the flame retardant property.

Specifically, the strength the fiber is affected by the fineness thereof. The fiber with high fineness is less drawn, which affects the degree of orientation (the higher the degree of orientation is, the better the mechanical properties is). Herein, the degree of orientation corresponding to the high fineness should be lower, and the mechanical strength should theoretically be lower. However, in Table 3, with the increase of the fineness, the strength of the fiber is accordingly increased, which is somewhat different from the theory. This is because of the change in particle size. It can be seen from the results in Table 3 that the degree of damage of the flame retardant and synergistic agent to the mechanical properties of the fiber is reduced by reducing the particle size.

FIG. 3 shows a morphology of the surface of Lyocell fiber modified by the flame retardant dispersion with the particle size D₉₀ of 1.84 um. FIG. 4 shows a morphology of the surface of Lyocell fiber modified by the flame retardant dispersion with the particle size D₉₀ of 1.4 um. FIG. 5 shows a morphology of the surface of Lyocell fiber modified by the flame retardant dispersion with the particle size D₉₀ of 0.7 um. FIGS. 3 to 5 show SEM morphologies of the fiber surfaces prepared from the flame retardant dispersions with different particle sizes. It is shown from the difference in morphologies that particles are avoided being accumulated and adhered on the fiber surface due to reducing in the particle size, so the LOI value is increased, and the washing resistance is improved.

When the ratio of the particle size D₉₀ in the flame retardant dispersion to the diameter φ of the fiber is more than 0.1, the particle size of the flame retardant and synergistic agent is too large. Firstly, the flame retardant and synergistic agent are easily precipitated on the fiber surface, so that it is reduced in the effective addition amount and the blending effect, and it is reduced in properties such as washing resistance. Secondly, the strength of the modified fiber is greatly reduced.

Comparative Example 3

Influence of Different Types of the Flame Retardant and the Synergistic Agent

Referring to the components, proportions and method of Embodiment 1, the difference among the groups is in types of the flame retardant and the synergistic agent. D₉₀ less than 1 μm was detected by a laser particle size analyzer, and then the dispersion was filtered to obtain a flame retardant dispersion with an effective component being flame retardant of 25%. The fiber was prepared according to the steps and method of Embodiment 5, and the properties were tested. The difference conditions and the properties tested are shown in the table below.

	TABLE 4
						Dry
						breaking		LOI/%
	Flame		Fineness	D90	φ fiber	strength	LOI/	after 12
	retardant	Synergistic agent	(dtex)	(um)	(μm)	(cN/dtex)	%	washes

Group	Melamine	Aluminum silicate	2.2	1.4	>15	2.10	28.1	26.5
1
Group	Melamine	Tetrakis	2.2	1.3	>15	2.37	30.3	27.4
2		(hydroxymethyl)
		phosphonium
		sulfate
Group	Melamine	Aluminum silicate	2.2	1.4	>15	2.16	29.5	28.4
3	cyanurate
Group	Melamine	Zinc oxide	2.2	1.3	>15	2.28	33.5	29.7
4	cyanurate
Group	Melamine	Zinc borate	2.2	1.2	>15	2.25	34.6	33.2
5	cyanurate

Analyzing the result: It is comprehensively analyzed the influence of different compounds on the fiber strength and the flame retardancy. For different compounds, the initial LOI values of the fibers are all higher than 27%, but there are differences in the reduction range after being washed with water. Group 5 shows the better properties, so the compound of the Group 5 is further optimized, namely the compound of the melamine cyanurate and zinc borate, and the fiber has high strength and good flame retardancy and water washing resistance.

The above preferred embodiments of the present application are not intended to limit the present application in any form. Although the present application has been disclosed the above preferred embodiments, it is not intended to limit the present application. Any person skilled in the art can use the above-mentioned teachings to make some changes or modifications to equivalent variations of the embodiments without departing from the scope of the solution of the present application. However, any equivalent changes and modifications made to the above embodiments based on the solution of the present application without departing from the substantive solution of the present application shall still fall within the scope of the present application.