Phosphorus-nitrogen flame retardant as well as preparation method and application thereof

Abstract

The invention discloses a phosphorus-nitrogen flame retardant and a preparation method and application thereof.The preparation method of the flame retardant comprises the steps that phosphoric acid is added into a PEPA solution, and after a heating reaction, the mixture is cooled to the room temperature; and adding urea, carrying out secondary heating reaction, and washing, separating and drying after the reaction is finished. The prepared flame retardant not only can significantly improve the flame retardant property and thermal stability of cotton fabrics, but also has good environmental friendliness and practical application potential, and provides a new idea for development of high-performance flame-retardant cotton fabrics.

Description

Technical Field

[0001] The present invention relates to a phosphorus-nitrogen flame retardant, its preparation method and application, belonging to the field of flame retardant technology. Background Art

[0002] As an important natural fiber, cotton fabric is widely used in the fields of clothing, home textiles and industry due to its excellent breathability, comfort and renewability. However, its high flammability severely limits its safety in flammable environments. Although traditional halogen-containing flame retardant technologies improve the flame retardant performance of fabrics, they release toxic gases during decomposition, causing serious environmental pollution and health problems. Therefore, the development of environmentally friendly and efficient flame retardant technologies has become the research focus.

[0003] In recent years, a variety of innovative flame retardant technologies have been proposed. For example, Zhao et al. (2024) developed a phosphorus-nitrogen-based flame retardant applied to Lyocell fiber, with the limiting oxygen index (LOI) reaching 35.8%. However, the copper nanoparticles used in the synthesis have long-term stability and environmental risks. Xu et al. (2024) studied the synergistic effect of single-walled carbon nanotubes and ammonium polyphosphate, and developed a halogen-free nano-coating suitable for cotton fabric, with an LOI of 27.5%, and the peak heat release rate (pHRR) and total heat release (THR) were reduced by 92.22% and 58.44% respectively, but the flame retardant performance still needs to be improved. In addition, Liu et al. (2024) developed a bio-based coating based on plant acid and guanosine, which demonstrated excellent flame retardant performance, but its complex synthesis process limits large-scale application.

[0004] PEPA (2,6,7-trioxa-1-phosphabicyclo[2.2.2]octane-4-methanol-1-oxide) has attracted much attention due to its high flame retardant performance and excellent thermal stability. For example, Zhang Yang et al. (2022) improved the flame retardant performance of epoxy resin through PEPA-MPcs, increasing the LOI by 19%, and reducing the pHRR and THR by 64.3% and 48.6% respectively. Li Lei et al. (2023) developed PZS-MXene containing PEPA for flame retardant enhancement of polypropylene. At a 25wt% addition amount, the LOI reached 32.7%, and the pHRR and THR were reduced by 74.5% and 33.0% respectively. The high phosphorus content of PEPA can quickly form a protective layer through dehydration and carbonization reactions, effectively isolating heat and oxygen.

[0005] However, there are still some drawbacks when using PEPA as a flame retardant. For example, its thermal stability is insufficient, and it is prone to decomposition at high temperatures, resulting in a decline in flame retardant performance. In addition, the carbon layer formed by PEPA alone is not dense and stable enough, and its carbonization ability is limited, making it difficult to fully exert the heat insulation and oxygen isolation effects of the intumescent flame retardant system. Summary of the Invention

[0006] The object of the present invention is to provide a phosphorus-nitrogen flame retardant, its preparation method and application. This flame retardant not only significantly improves the flame retardancy and thermal stability of cotton fabrics, but also has good environmental friendliness and practical application potential, providing new ideas for the development of high-performance flame-retardant cotton fabrics.

[0007] To achieve the above object, the technical solution adopted by the present invention is as follows:

[0008] A preparation method of a phosphorus-nitrogen flame retardant is to add phosphoric acid to a PEPA solution, heat and react, and then cool to room temperature; then add urea and heat and react for the second time. After the reaction is completed, it can be washed, separated and dried.

[0009] Preferably, the molar ratio of PEPA, phosphoric acid and urea is (0.1-0.5):(0.1-0.5):(0.2-1).

[0010] Preferably, the preparation method of the PEPA solution is to mix PEPA and DMF in a ratio of (0.1-0.5) mol:(150-1000) mL, and react at 65-95 °C for 1-3 h under stirring.

[0011] Preferably, the conditions for the first heating reaction are: 120-180 °C, 1-4 h;

[0012] The conditions for the second heating reaction are: 130-180 °C, 2-5 h;

[0013] The drying temperature is 60-80 °C.

[0014] A phosphorus-nitrogen flame retardant is prepared by any of the above methods.

[0015] The application of the phosphorus-nitrogen flame retardant prepared by any of the above methods in the flame retardant finishing of fabrics is specifically to dissolve dicyandiamide in the phosphorus-nitrogen flame retardant solution, then immerse the cotton fabric in the above solution, heat and stir, and then take it out and dry it;

[0016] Among them, in the phosphorus-nitrogen flame retardant solution, the concentration of the phosphorus-nitrogen flame retardant is 50-500 g / L;

[0017] The addition amount of dicyandiamide is 3-8 wt% of the mass of the phosphorus-nitrogen flame retardant solution.

[0018] Preferably, the conditions for heating and stirring are: 60-90 °C, 0.5-3 h;

[0019] The drying temperature is: 70-90 °C.

[0020] Preferably, the cotton fabric has also been subjected to surface impurity removal treatment and mercerization treatment before immersion;

[0021] Among them, the method of surface impurity removal treatment is as follows:

[0022] Immerse the cotton fabric in an aqueous NaOH solution with a bath ratio of 1:(20 - 70) at 80 - 100 °C and a concentration of 5 - 20 g / L for 80 - 120 min, take it out, wash and dry it;

[0023] The method of mercerization treatment is: Immerse the cotton fabric after surface impurity removal treatment in an aqueous NaOH solution with a concentration of 20 - 30% for 3 - 10 min, take it out, wash it to neutrality and then dry it.

[0024] The beneficial effects of the present invention are as follows:

[0025] PEPA - AP forms stable chemical bonds on the surface of the cotton fabric. XRD analysis shows that the crystal structure of cellulose is not damaged. The treatment of PEPA - AP significantly improves the flame retardancy of the cotton fabric, and the limiting oxygen index (LOI) is increased. The vertical flammability test (VFT) shows that the treated sample has no after - burning and smoldering phenomena, and the char length is significantly reduced. The CCT test shows good fire - proof performance.

[0026] Thermogravimetric analysis (TGA) shows that the char residue rate of the cotton fabric treated with PEPA - AP is significantly increased at 800 °C. TG - FTIR analysis shows that the flame retardant effectively reduces the generation of combustible gases by releasing non - combustible gases such as NH3, CO2 and PO·, and at the same time promotes the formation of a dense charred layer. The flame retardant mechanism of PEPA - AP mainly includes the synergistic effect of the gas phase and the condensed phase: in the gas phase, the released PO· radicals react with H· and OH· to inhibit the combustion chain reaction; in the condensed phase, the phosphate ester groups promote the dehydration and carbonization reaction of cellulose to form a protective charred layer, thereby isolating the transfer of heat and oxygen. Description of the Drawings

[0027] Figure 1 It is the preparation process and chemical reaction formula of the flame retardant PEPA - AP;

[0028] Figure 2 It is the finishing process and chemical reaction formula of PEPA - AP - CF;

[0029] Figure 3 It is the FT - IR spectrum of PEPA - AP;

[0030] Figure 4 It is the FT - IR spectra of pure cotton (Contrl cotton) and flame - retardant cotton fabric (PEPA - AP - CF);

[0031] Figure 5 It is the XRD spectra of pure cotton and flame - retardant cotton fabric PEPA - AP - CF3;

[0032] Figure 6 XPS spectra of cotton fabrics, (a) XPS spectra of Control cotton, PEPA-AP-CF1 and PEPA-AP-CF3; (b) C1s peak fitting spectrum of PEPA-AP-CF3; (c) N1s peak fitting spectrum of PEPA-AP-CF3; (d) P2p peak fitting spectrum of PEPA-AP-CF3;

[0033] Figure 7 SEM images of cotton fabrics, (a1–a3) Control cotton and (b1–b3) PEPA-AP-CF3 fabrics; EDS element distribution maps of cotton fabrics, (e) Control cotton and (f) PEPA-AP-CF3 fabrics;

[0034] Figure 8 Vertical burning test results of fabrics, vertical burning test results of Control cotton, PEPA-AP-CF1, PEPA-AP-CF2 and PEPA-AP-CF3 fabrics;

[0035] Figure 9 Digital photos of cotton fabrics after 50 kW / m 2 CCT, (a) Control cotton, (b) PEPA-AP-CF1, (c) PEPA-AP-CF2 and (d) PEPA-AP-CF3 fabrics;

[0036] Figure 10 Curves of HRR (a), THR (b), TSP (c) and SPR (d) of cotton fabric samples in the cone calorimeter test;

[0037] Figure 11 TG (a) and DTG (b) curves of cotton fabrics in nitrogen atmosphere;

[0038] Figure 12 TG-FTIR spectra of Control cotton (a) and PEPA-AP-CF3 (b); FTIR spectra of char residues of Control cotton (c) and PEPA-AP-CF3 (d) after calcination at different temperatures;

[0039] Figure 13 SEM images of cotton fabrics after combustion, (a1–a3) Control cotton and (b1–b3) PEPA-AP-CF3 fabrics; EDS element distribution maps of cotton fabrics after combustion, (e) Control cotton and (f) PEPA-AP-CF3 fabrics;

[0040] Figure 14Raman spectra of cotton fabrics after combustion, (a) Control cotton, (b) PEPA-AP-CF1, and (c) PEPA-AP-CF3 fabrics;

[0041] Figure 15 EDS spectra and elemental distribution maps of PEPA-AP-CF3 fabric before (a) and after (b) combustion;

[0042] Figure 16 Flame retardant mechanism of PEPA-AP-CF cotton fabric;

[0043] Figure 17 Test result graphs of air permeability of Control cotton, PEPA-AP-CF1, PEPA-AP-CF2, and PEPA-AP-CF3 fabrics;

[0044] Figure 18 Test result graphs of whiteness of Control cotton, PEPA-AP-CF1, PEPA-AP-CF2, and PEPA-AP-CF3 fabrics;

[0045] Figure 19 Test result graphs of softness of cotton fabrics, (a) Control cotton, (b) PEPA-AP-CF1, (c) PEPA-AP-CF2, and (d) PEPA-AP-CF3 fabrics. Specific implementation mode

[0046] Synthesis of flame retardant PEPA-AP

[0047] Caged 2,6,7-trioxa-1-phosphabicyclo[2.2.2]octane-4-methanol-1-oxide (PEPA) is a novel flame retardant intermediate containing P-H bonds. First, PEPA is dehydrated through an esterification reaction, and then the final product PEPA-AP is obtained through an amination reaction.

[0048] The preparation process and chemical reaction formula of flame retardant PEPA-AP are as Figure 1 shown. The preparation process is as follows:

[0049] A mixed solution of PEPA (0.2 mol, 36.33 g) and N,N-dimethylformamide solvent (300 ml) was placed in a 500 ml three-necked flask equipped with a magnetic stirrer, stirred evenly and reacted in a water bath at 80 °C for 2 h to obtain a white turbid solution; then phosphoric acid (0.2 mol, 19.8 g) was added dropwise to the flask at room temperature, and the temperature was raised to 140 °C and reacted in an oil bath for 2 h to obtain a colorless transparent solution. Then the three-necked flask was taken out and cooled to room temperature, and then urea (0.4 mol, 26.4 g) was added, and the reaction was carried out in an oil bath at 150 °C for 3.5 h. During the stirring process, white solid was continuously generated until no more white solid was precipitated, and the reaction was stopped. The obtained white solid was washed three times with ethanol, and then the excess solution was removed by vacuum filtration. The white solid was transferred to a petri dish and dried in an oven at 70 °C to finally obtain 28.29 g of white powder (yield about 77%).

[0050] Pretreatment of cotton fabric

[0051] The treatment process of the flame-retardant cotton fabric is as follows:

[0052] (1) Pretreatment of cotton fabric: 10 g of sodium hydroxide was dissolved in 1000 ml of distilled water to obtain a pretreatment solution for cotton fabric. The pretreatment solution was heated in a water bath to 100 °C, and the cotton fabric and the pretreatment solution were in a bath ratio of 1:50 (mass ratio), and the cotton fabric was completely immersed in the pretreatment solution and reacted for about 90 min to remove the impurities attached to the surface of the cotton fabric. Finally, the treated cotton fabric was taken out and washed 3-4 times with clean water, paved flat with a silica gel pad, sealed and placed in an oven, dried at 80 °C and then taken out for standby.

[0053] (2) Mercerization of cotton fabric: The pretreated and dried cotton fabric was completely immersed in a 25% NaOH aqueous solution and reacted for about 5 min, and then the surface of the cotton fabric was rinsed with distilled water until it was neutral. Finally, the cotton fabric was placed in an oven and dried at 70 °C for 3 h and then taken out for standby.

[0054] Finishing of flame-retardant cotton fabric

[0055] The finishing process and chemical reaction formula of the flame-retardant cotton fabric are as Figure 2As shown. The obtained white powder was made into solutions with concentration gradients of 100 g / L, 200 g / L, and 300 g / L. 5 wt% of dicyandiamide (as a plasticizer to increase the heat resistance of cotton fabrics) was added as a catalyst according to different concentration ratios. The solution was transferred to a 500 mL beaker and placed on a digital display constant temperature magnetic stirrer until it was completely dissolved. Subsequently, three pieces of cotton fabric were cut according to the required size and placed in the solution to be completely immersed. Stirring treatment was carried out at 75 °C for 1.5 h. Finally, the treated cotton fabric was taken out, flattened with a silica gel pad, placed in an oven and dried at 80 °C for 3 h, and then taken out and placed in a sealed bag. According to the different concentrations of the solution, the treated cotton fabrics were named as follows: the sample treated with the 100 g / L solution was named PEPA-AP-CF1, the sample treated with the 200 g / L solution was named PEPA-AP-CF2, and the sample treated with the 300 g / L solution was named PEPA-AP-CF3.

[0056] Testing and Characterization

[0057] Limiting Oxygen Index Test (LOI)

[0058] The LOI of the fabric samples was tested according to the national standard GB / T 5454-1997 method and measured using an HC-2 type oxygen index instrument (Nanjing Jiangning Analytical Instrument Factory). The sample size was 150×58 mm 2 , the flame length was set to 2 cm, and each sample was tested 3 times and the average value was taken. The flame retardant performance of the flame retardant fabric was evaluated by measuring the oxygen index.

[0059] Vertical Flame Test (VFT)

[0060] The VFT of the fabric samples was tested according to the ASTM D6413-08 standard using a CZF-3 type vertical flame tester (Nanjing Jiangning Analytical Instrument Factory). The sample size was 300×89 mm 2 , the flame length of the igniter was set to 4 cm, the ignition time was 12 s, and the afterflame time, smoldering time, char length, and whether the sample burned were recorded. The test results were used to evaluate the combustion performance of the fabric.

[0061] Cone Calorimeter Test (CCT)

[0062] Using an FTT0007 cone calorimeter (FTT Company, UK), the flame retardant treated cotton fabric was tested under a radiant heat flux of 35 kW / m 2 . According to the ASTM E1354 standard, a fabric sample with a size of 100 mm×100 mm was placed on an aluminum foil and the four sides were wrapped and fixed. The foam coated surface and the back were randomly arranged, and the test heat flux was 50 kW / m 2, record parameters such as the heat release rate, total heat release, and ignition time of the fabric to evaluate the thermal stability and combustion behavior of the fabric.

[0063] Thermogravimetry-Fourier transform infrared spectroscopy (TG-FTIR)

[0064] TG-FTIR analysis of the samples was carried out using a TGA4000 thermogravimetric analyzer coupled with a Spectrum Two Fourier transform infrared spectrometer (PerkinElmer Enterprise Management (Shanghai) Co., Ltd.). Approximately 20 mg of the sample was weighed, the temperature was raised from 50 °C to 700 °C at a heating rate of 10 °C / min, and the nitrogen flow rate was 50 mL / min. The volatile gases generated by the decomposition of the sample were transported through a heated transfer line (250 °C) to the FTIR gas cell, and the infrared spectra of the volatile products were recorded at a resolution of 4 cm-1 in the range of 4000 - 500 cm-1.

[0065] Air permeability test

[0066] An YG461N digital fabric air permeability tester (Nantong Hongda Experimental Instrument Co., Ltd.) was used to test the air permeability of the cotton fabric treated with the flame retardant. According to the national standard GB / T 5453-1997 method, a medium-sized air nozzle (No. 4) was used, and each sample was tested 10 times at different positions and the average value was taken. The air permeability of the fabric was measured (mm / s) to evaluate the air permeability performance of the fabric.

[0067] Flexibility Test

[0068] The flexibility test was carried out according to the standard of Part 11 of IS 7016. During the test, cotton fabric samples with dimensions of 160 × 40 mm 2 were prepared. The short sides of the samples were aligned to form a circular structure, placed on a horizontal surface, and the flexibility of the samples was evaluated by measuring the height of the circle.

[0069] Whiteness Test

[0070] The whiteness test was carried out using an intelligent digital whiteness meter (WSB-3A, Nantong Hongda Experimental Instrument Co., Ltd.). During the test, the sample was placed in the test area of the instrument, and the whiteness value of the sample was determined according to the standard operating procedure of the instrument. The results were expressed as whiteness value (%). The measurement range, sensitivity, and repeatability of the instrument ensured the accuracy of the test data, which was used to evaluate the visual whiteness characteristics of the sample.

[0071] Contact angle measurement

[0072] The contact angle of the flame-retardant treated cotton fabric was measured at 25 °C using a DSA 30 contact angle measuring instrument (KRÜSS, Germany). A 10 μL water droplet was used for the measurement. The larger the contact angle, the better the hydrophobicity of the fabric. Each sample was measured 5 times to ensure data accuracy.

[0073] PEPA-AP Structure Analysis

[0074] The FT-IR spectra of the flame retardant PEPA-AP are as Figure 3 shown, showing the infrared absorption spectra of PEPA-AP and PEPA. By comparing the absorption peaks of the two, the chemical structure changes of PEPA during the modification process can be verified. The synthesis process of PEPA-AP is as Figure 1 shown. First, PEPA was preliminarily phosphorylated with phosphoric acid (H3PO4), and then reacted with an amino compound to form the target product. In the spectrum, the absorption peak near 3440 cm -1 corresponds to the stretching vibration of the -OH group, indicating that part of the -OH group is retained in both PEPA and PEPA-AP; the absorption peak near 2920 cm -1 belongs to the stretching vibration of -C-H, indicating the presence of an alkyl structure in the sample. In the spectrum of PEPA-AP, a new absorption peak appears near 2270 cm -1 , which belongs to the characteristic vibration of the -N=C=O group, confirming the introduction of the isocyanate group, which is consistent with the Figure 1 synthesis steps. In addition, the absorption peaks near 1680 cm -1 and 1250 cm -1 correspond to the stretching vibrations of C=O and P=O respectively, indicating the formation of a phosphate ester structure in PEPA-AP; multiple absorption peaks in the range of 1000 - 1200 cm -1 correspond to the characteristic vibrations of P-O-C and C-O-C, further verifying the success of the phosphorylation reaction. Combining the FT-IR spectra and the synthesis process, it can be clearly confirmed that PEPA was successfully modified by phosphorylation and amination to produce the target product PEPA-AP, and functional groups such as phosphate ester and isocyanate were introduced, laying a chemical foundation for the improvement of its flame retardant performance.

[0075] PEPA-AP-CF Structure Analysis

[0076] The FT-IR spectra of pure cotton fabric (Control cotton), PEPA-AP-CF1, PEPA-AP-CF2 and PEPA-AP-CF3 are as Figure 4 shown. By comparing the spectra, it can be found that in all samples, 3335 cm -1 and 2892 cm -1The absorption peaks correspond to the stretching vibrations of -O-H and -C-H in cellulose, indicating that the basic structure of cellulose in cotton fabric is retained during the modification process. In the PEPA-AP-CF3 sample, new absorption peaks appear at 2192 cm -1 and 2149 cm -1 . These peaks are attributed to the stretching vibration of P-OH, confirming the successful introduction of phosphorus groups into the sample. In addition, the absorption peak at 1642 cm -1 corresponds to the bending vibration of O-H, the absorption peak at 1554 cm -1 is attributed to the stretching vibration of N-H, the absorption peak at 1452 cm -1 is attributed to the P-C bond, and the absorption peak at 1208 cm -1 is attributed to the stretching vibration of P=O. These characteristic absorption peaks further verify the successful introduction of nitrogen- and phosphorus-based chemical groups during the modification process. At the same time, the absorption peak at 1030 cm -1 is attributed to the stretching vibration of P-O-C, and the absorption peak at 811 cm -1 corresponds to the deformation vibration of N-H. These characteristic absorption peaks indicate that PEPA-AP has been successfully bonded to cotton fibers through chemical bonding, achieving the goal of modification. In summary, the appearance of these infrared characteristic absorption peaks clearly demonstrates that PEPA-AP forms a stable chemical structure on the surface of cotton fabric, providing a structural basis for the improvement of its flame retardancy performance.

[0077] The XRD spectra of pure cotton fabric (Control cotton) and PEPA-AP-CF3 are as Figure 5 shown. It can be seen from the figure that the crystal structure of cellulose is retained after the flame retardant treatment, and characteristic diffraction peaks appear at 14.78°, 16.4°, 22.6° and 34.16°, corresponding to the (1-10), (110), (200) and (004) crystal planes of cellulose. In addition, in the treated sample PEPA-AP-CF3, new diffraction peaks at 23.87°, 26.6° and 45.3° appear, and these peaks are attributed to the (021), (420) and (400) crystal planes respectively. The appearance of these new peaks indicates that the PEPA-AP flame retardant has crystal characteristics and has been successfully attached to the cotton fabric. These results further illustrate that the flame retardant binds well to cotton fibers at the molecular level while not destroying the original crystal structure of cellulose.

[0078] Figure 6 shows the XPS spectra of pure cotton fabric (Control cotton), PEPA-AP-CF1 and PEPA-AP-CF3 and the results of related high-resolution spectral analysis. Among them, Figure 6In the a region of the wide-scan XPS spectrum, only characteristic peaks of C1s and O1s were shown in the pure cotton fabric, while characteristic peaks of P 2p and N1s were newly added in the PEPA-AP-CF1 and PEPA-AP-CF3 samples. Among them, the P 2p peak of PEPA-AP-CF3 was located at 133.7 eV and the N 1s peak was located at 400.5 eV, indicating that phosphorus and nitrogen groups had been successfully introduced into the flame-retarded cotton fabric. In the high-resolution C 1s spectrum of PEPA-AP-CF3 ( Figure 6 in the b region), characteristic peaks of 284.7 eV (C-C), 286.5 eV (C=O), and 288.5 eV (C-O-C) could be observed, and new peak positions of 285.9 eV (C-N) and 287.0 eV (C=O) were added. The changes in these peak positions indicated that new chemical bonds had been introduced to the surface of the cotton fabric during the flame-retardant treatment. In the high-resolution N1s spectrum ( Figure 6 in the c region), characteristic peaks of 398.8 eV (C=N), 399.8 eV (C-N), and 401.3 eV (N-H) further verified that nitrogen elements were bonded to the surface of the cotton fabric in the form of chemical bonds, while the weaker N1s signal in PEPA-AP-CF1 originated from residual NH4 + or other precursor substances. In the high-resolution P 2p spectrum ( Figure 6 in the d region), characteristic peaks of 133.7 eV (P=O) and 132.4 eV (P-O-C) were observed, indicating that the phosphorus-containing compound had been successfully bonded to the cotton fabric and formed stable chemical bonds with the fiber structure.

[0079] Morphology analysis of PEPA-AP-CF

[0080] Figure 7 SEM images and EDS element distribution maps of the pure cotton fabric and the PEPA-AP flame-retarded cotton fabric (PEPA-AP-CF3) were shown. The SEM image of the pure cotton fabric ( Figure 7 in the a1–a3 regions) showed that the fiber surface was smooth and arranged neatly, maintaining the natural morphology of cotton fibers without any modification or coating treatment. In contrast, the SEM image of the PEPA-AP-treated cotton fabric ( Figure 7 in the b1–b3 regions) showed that the fiber surface was rough and covered with obvious particles, indicating that the PEPA-AP flame retardant had been successfully deposited and uniformly adhered to the fiber surface while maintaining the structural integrity of the fiber. The EDS element distribution ( Figure 7 in the e–f regions) further verified this. The control cotton fabric ( Figure 7 in the e region) only showed a uniform distribution of carbon (C) and oxygen (O), which are the main components of cotton fibers; while the flame-retarded cotton fabric ( Figure 7In the f region, in addition to carbon and oxygen, a uniform distribution of nitrogen (N) and phosphorus (P) was also shown. This indicates that the PEPA-AP flame retardant was successfully bound to the cotton fibers, and the PEPA-AP flame retardant was successfully bound to the surface of the cotton fibers through chemical bonds, and its flame retardant elements (such as phosphorus and nitrogen) were uniformly distributed.

[0081] Flame retardancy analysis

[0082] Table 1 Test results of LOI and VFT of cotton fabric samples.

[0083]

[0084] The flame retardancy of pure cotton fabric (Control cotton) and PEPA-AP treated cotton fabric was comprehensively evaluated by the limiting oxygen index (LOI) and vertical flammability test (VFT), and the test results are as Figure 8 shown in Table 1. The pure cotton fabric showed poor flame retardancy, with an LOI value of only 18%, a damage length of 300 mm in the vertical flammability test, a continuous burning time of 12 s, and a smoldering time of 80 s, and it had no flame retardancy at all. In contrast, the samples treated with PEPA-AP showed significantly improved flame retardancy. Among them, the weight gain rate of PEPA-AP-CF1 was 25 ± 0.5%, the LOI value was increased to 42%, the continuous burning and smoldering times were both 0, and the damage length was significantly reduced to 80 mm; the weight gain rate of PEPA-AP-CF2 was further increased to 30 ± 0.4%, the LOI value was increased to 46%, and the damage length was shortened to 61 mm; the weight gain rate of PEPA-AP-CF3 reached 34 ± 0.8%, the LOI value was significantly increased to 58%, there was no continuous burning or smoldering phenomenon, and the damage length was further reduced to 56 mm. These results indicate that with the increase of the concentration of the flame retardant, the flame retardancy of the cotton fabric is significantly improved, and there is a good positive correlation among the weight gain rate, LOI value and vertical flammability performance. At the same time, the flame propagation is effectively inhibited, proving that PEPA-AP is an efficient flame retardant modifier.

[0085] Digital photos of the charred morphologies formed by pure cotton fabric (Control cotton), PEPA-AP-CF1, PEPA-AP-CF2 and PEPA-AP-CF3 fabrics after the CCT test are as Figure 9 shown. As can be seen from Figure 9 region a, the pure cotton fabric underwent severe degradation under high temperature, and the charred structure was loose and fragile, showing poor thermal stability and very little char residue. In contrast, Figure 9 region b shows that the charred layer of PEPA-AP-CF1 is tighter than that of the pure cotton fabric, indicating an improvement in its thermal protection performance. Further, Figure 9In region c, it is shown that PEPA-AP-CF2 forms a denser and more continuous char layer, significantly enhancing the thermal protection performance. Figure 9 The char layer of PEPA-AP-CF3 in region d is the most stable and uniform, showing excellent thermal stability and flame retardancy. In summary, PEPA-AP treatment significantly improves the thermal stability and flame retardancy of cotton fabrics, and the flame retardant effect is further enhanced with the increase of PEPA-AP content.

[0086] The cone calorimeter was used to evaluate the combustion behavior of pure cotton fabric (Controlcotton), PEPA-AP-CF1, PEPA-AP-CF2, and PEPA-AP-CF3, and the test results are as Figure 10 and Table 2 show. Under the heat flux condition of 50 kW / m 2 , the ignition time (TTI) of pure cotton fabric is only 8 s, showing fast ignition characteristics. Its peak heat release rate (PHRR) reaches 53.69 kW / m 2 , the total heat release (THR) is 1.89 MJ / m 2 , and the total smoke production (TSP) is 0.045 m 2 / kg. These results indicate that untreated cotton fabric releases a large amount of heat and smoke during combustion, showing poor fire resistance. In contrast, the samples treated with PEPA-AP show significantly improved flame retardancy.

[0087] The ignition times of PEPA-AP-CF1, PEPA-AP-CF2, and PEPA-AP-CF3 are extended to 164 s, 176 s, and 182 s respectively, significantly delaying the ignition time. At the same time, their PHRR values are reduced to 14.42 kW / m 2 , 7.98 kW / m 2 and 7.82 kW / m 2 respectively, which are reduced by 73.1%, 85.1%, and 85.4% compared with pure cotton fabric. The THR values also decrease to 1.64 MJ / m 2 , 1.53 MJ / m 2 and 1.01 MJ / m 2 respectively, reducing by 13.2%, 19.0%, and 46.6% respectively. In terms of smoke generation, the TSP values of the samples treated with PEPA-AP change, and the TSP value of PEPA-AP-CF3 increases to 0.078 m 2 / kg, showing a certain increase compared with 0.045 m 2 / kg of pure cotton fabric. This increase is attributed to the thermal decomposition process of the flame retardant, which releases ammonia, water vapor, and acidic gases. These gases dilute oxygen and isolate the combustion zone, effectively delaying the ignition time and reducing the heat release.

[0088] To further evaluate the flame retardancy performance, the flame growth index (FGI) and the fire retardancy performance index (FPI) were calculated, and the results are shown in Table 2. The FGI value of the PEPA-AP treated sample decreased significantly, and the FGI value of PEPA-AP-CF3 was 0.043 kW / m 2 ·s, indicating that the flame propagation speed decreased significantly. At the same time, the FPI value of PEPA-AP-CF3 increased significantly to 23.274 m 2 ·s / kW, while the FPI value of the pure cotton fabric was only 0.149 m 2 ·s / kW, reflecting the extension of the ignition delay time and the improvement of the flame safety performance.

[0089] It can be seen from this that the PEPA-AP coating significantly improved the overall flame retardancy performance of the cotton fabric, effectively enhancing the safety protection ability in the fire scene by extending the ignition time, significantly reducing the heat release rate and the amount of smoke generated, and slowing down the flame propagation.

[0090] Table 2 Main data of the cone calorimetry test of cotton fabric samples.

[0091]

[0092] Thermal performance analysis

[0093] Table 3 Main data of the TG test of cotton fabric samples in N2 atmosphere

[0094]

[0095]

[0096] As Figure 11 shown in and Table 3, the thermal stability of the cotton fabric was evaluated by thermogravimetric analysis (TGA) and derivative thermogravimetry (DTG) in a nitrogen atmosphere. Figure 11 Regions a and b in the figure are the TG and DTG curves respectively, showing the differences in the thermal degradation behavior between the pure cotton fabric (Controlcotton) and the PEPA-AP-CF sample. The initial decomposition temperature (T 5％ ) of the pure cotton fabric was 219.7 °C, the maximum decomposition rate temperature (T max ) was 312.2 °C, the maximum decomposition rate (R max ) was -0.54% / min, and the residual mass at 800 °C was only 7.5 wt%, indicating poor thermal stability and low carbonization ability.

[0097] In contrast, the T 5％Slightly decreased, in the range of 197.2 °C to 202.6 °C, which is mainly due to the early decomposition of the phosphorus-containing groups in the sample. However, at 800 °C, the residual masses of PEPA-AP-CF1, PEPA-AP-CF2, and PEPA-AP-CF3 increased significantly, reaching 38.6 wt%, 39.9 wt%, and 41.2 wt% respectively, indicating that the treated samples have higher thermal stability and significantly enhanced carbonization ability. The DTG curve further shows that the PEPA-AP-CF3 sample has the highest residual mass, and R max significantly decreased, indicating that it has the most effective carbonization ability and significantly improves the thermal stability of the cotton fabric.

[0098] The catalytic carbonization mechanism of the PEPA-AP coating promotes the formation of a carbon layer through the decomposition of phosphoric acid or polyphosphoric acid, effectively isolating the heat and oxygen transfer during the combustion process. At the same time, the stable carbon layer formed by the decomposition of the phosphorus-containing groups in PEPA-AP further enhances the thermal barrier effect of the material. The higher residual mass at 800 °C highlights the synergistic flame retardant mechanism of the flame retardant, indicating that the treated samples have excellent flame retardant properties. It can be seen that the PEPA-AP coating significantly improves the thermal stability and flame retardant properties of the cotton fabric through catalytic carbonization, verifying the excellent flame retardant effect of the treated samples.

[0099] To analyze the flame retardant mechanism of this system, Figure 12 shows the 3D spectra (a-b region) and FTIR spectra (c-d region) of pure cotton fabric (Control cotton) and PEPA-AP-CF3 at different temperatures by TG-FTIR. The 3D spectrum of pure cotton fabric by TG-FTIR ( Figure 12 region a) shows that typical volatile products are released during the thermal decomposition process, including 2350 cm -1 (CO2), 2900 cm -1 (C-H stretching vibration), 3500 cm -1 (O-H bending vibration), and 1100 cm -1 (C-O-C stretching vibration). The release of these volatile products indicates that cellulose undergoes rapid thermal degradation at high temperatures, and a large amount of combustible gas is generated in the absence of an effective carbonization protection mechanism.

[0100] In contrast, the 3D spectrum of TG-FTIR obtained from Figure 12 region b shows significantly different characteristics for PEPA-AP-CF3. In addition to the common CO2 and C-H peaks, 3300 cm -1 (N-H stretching vibration) and 1200 cm -1The newly added absorption peak of (P-O-P stretching vibration). The presence of these nitrogen- and phosphorus-containing groups indicates that they play a key role in promoting carbonization formation and reducing the release of combustible gases, thus providing enhanced flame retardant protection.

[0101] Further analysis of the FTIR spectra of pure cotton fabrics at different temperatures ( Figure 12 in region c) reveals that the peak intensities of 2350 cm -1 (CO2) and 2900 cm -1 (C-H stretching vibration) increase significantly above 500 °C, indicating accelerated cellulose decomposition and the release of a large amount of combustible gases, including ether compounds (confirmed by the C-O-C absorption peak at 1100 cm -1 ). The FTIR spectrum of PEPA-AP-CF3 ( Figure 12 in region d), on the other hand, shows a completely different thermal degradation behavior. Between 400 °C and 600 °C, characteristic absorption peaks of 1200 cm -1 (P-O-P stretching vibration) and 3300 cm -1 (N-H stretching vibration) appear, indicating that the phosphorus-containing groups form a protective carbonized layer through catalytic action, while the nitrogen-containing groups effectively inhibit the release of combustible gases. Compared with pure cotton fabrics, the peak intensities of CO2 and C-H in PEPA-AP-CF3 are significantly reduced, indicating that its flame retardant performance is significantly improved through the synergistic effect of phosphorus and nitrogen. The carbonization catalytic action of the phosphorus-containing groups and the combustible gas inhibition action of the nitrogen-containing groups together constitute an efficient flame retardant protection mechanism, making PEPA-AP-CF3 perform excellently in flame retardant performance.

[0102] Residual carbon morphology analysis

[0103] The SEM images and EDS element distributions of pure cotton fabric (Control cotton) and PEPA-AP-CF3 after combustion are as Figure 13 shown. The pure cotton fabric undergoes severe degradation after combustion, leaving only a small part of white ash and forming a loose and discontinuous carbonized layer ( Figure 13 in regions a1–a3). In contrast, the PEPA-AP-CF3 fabric maintains good structural integrity after combustion, forming a dense and continuous carbonized layer with visible micropores and particles ( Figure 13 in regions b1–b3).

[0104] Figure 13 Region e in Figure 13The EDS elemental distribution of PEPA-AP-CF3 in the f region shows that the sample is rich in phosphorus and nitrogen elements. The presence of these elements promotes the formation of a stable char layer, thus significantly enhancing the flame retardancy of the fabric. It can be seen that the flame retardant treatment of PEPA-AP-CF3 not only effectively improves the structure and compactness of the char layer, but also greatly enhances the flame retardant protection effect by introducing phosphorus and nitrogen elements.

[0105] Figure 14 The results of evaluating the graphitization degree of char residues of pure cotton fabric (Control cotton), PEPA-AP-CF1 and PEPA-AP-CF3 by Raman spectroscopy are shown. In the spectrum, two significant peaks appear at about 1360 cm -1 and 1590 cm -1 respectively, corresponding to the D band (disordered carbon) and the G band (graphitized carbon). The ratio of the area of the D peak to the G peak (I D / I G ratio) is an important index for evaluating the graphitization degree. The lower the I D / I G value, the higher the graphitization degree.

[0106] The I D / I G ratio of pure cotton fabric is 3.73, indicating a low graphitization degree and a loose and disordered char residue structure. In contrast, the I D / I G ratios of PEPA-AP-CF1 and PEPA-AP-CF3 are 2.23 and 2.09 respectively, significantly lower than that of pure cotton fabric, indicating that their char structures have a higher graphitization degree. PEPA-AP-CF3 has the highest graphitization degree, which indicates that the synergistic effect of its dense char structure and flame retardant elements in the condensed phase plays a key role in enhancing the flame retardancy of the fabric. It can be seen that the PEPA-AP treatment not only improves the char structure of cotton fabric, increases the graphitization degree of char residues, but also significantly enhances the flame retardancy of the fabric through the formation of this highly graphitized char layer.

[0107] Elemental analysis

[0108] By analyzing the pre-combustion ([ Figure 15 region a in Figure 15By analyzing the EDS spectra and elemental distribution maps of the b region in China, it can be observed that significant changes have occurred in the elemental content during the combustion process. Comparing the spectra, it can be found that the content of oxygen element has decreased from 51.41% before combustion to 38.39% after combustion, while the content of phosphorus element has increased significantly, from 1.93% to 12.61%, indicating that phosphorus is enriched in the char layer during combustion, enhancing the thermal stability and barrier properties of the char layer. At the same time, the content of carbon element has increased from 23.78% to 25.20%, while the content of nitrogen element has decreased slightly, from 22.88% to 23.80%, reflecting the roles of carbon and nitrogen in the formation of the char layer and the gas-phase flame retardant effect. In addition, the elemental distribution map shows that the distribution of phosphorus and oxygen elements in the char residue after combustion is dense and uniform, indicating that phosphorus and nitrogen have significantly improved the flame retardant performance of the PEPA-AP-CF3 fabric through the synergistic effect of the condensed phase and the gas phase during the flame retardant process. These results indicate that the flame retardant elements play a key role in forming a protective char layer and reducing the transfer of heat and oxygen, providing strong support for the improvement of flame retardant performance.

[0109] Flame Retardant Mechanism Analysis

[0110] Figure 16 The flame retardant mechanism of PEPA-AP-CF is demonstrated, and its flame retardant behavior during combustion is described in detail. When PEPA-AP-CF is exposed to a heat source or flame, the material undergoes thermal decomposition, releasing non-combustible gases, including NH3, H2O, CO2, and PO· (phosphorus oxides, such as PO and PO2). On the one hand, these non-combustible gases dilute the oxygen concentration in the combustion zone, reducing the amount of oxygen required for combustion, thereby inhibiting flame propagation; on the other hand, the release of these gases helps to isolate the contact between oxygen and the combustible substrate, reducing the persistence of combustion. In addition, a large amount of heat is absorbed during the material decomposition process, generating an endothermic effect, which reduces the surface temperature of the material and delays further thermal degradation.

[0111] At the same time, the phosphorus groups in PEPA-AP play a catalytic role during combustion, promoting the dehydration reaction and carbonization reaction of cellulose, and quickly generating a dense and stable char layer. This char layer acts as a physical barrier, effectively isolating the direct contact between the underlying material and heat and oxygen, preventing further combustion, and enhancing the fire resistance of the material.

[0112] In addition, the flame retardancy of PEPA-AP also benefits from the synergistic effect between phosphorus and nitrogen. In the gas phase, phosphorus oxide radicals (such as PO· and PO2·) can react with active radicals (such as H· and OH·) in the combustion reaction, disrupting the combustion chain reaction, reducing the generation of flammable gases, and further diluting the oxygen concentration in the combustion zone, thus effectively inhibiting the propagation of the flame. In the condensed phase, the synergistic effect of phosphorus and nitrogen promotes the formation of a char layer and enhances the density and thermal stability of the char layer, thereby effectively blocking the transfer of heat and oxygen.

[0113] In summary, the flame retardancy mechanism of PEPA-AP-CF can be attributed to the combined effects of the following aspects: dilution by non-combustible gases, endothermic effect during the thermal decomposition process, catalytic carbonization by phosphorus groups, and phosphorus-nitrogen synergistic effect. Under the dual action of the gas phase and the condensed phase, PEPA-AP-CF can significantly improve the flame retardancy of materials, slow down the combustion process, and form a strong fire protection effect.

[0114] Air permeability analysis

[0115] The air permeability test results of pure cotton fabric (Control cotton), PEPA-AP-CF1, PEPA-AP-CF2, and PEPA-AP-CF3 are as Figure 17 shown. It can be seen that with the introduction of the flame retardant treatment, the air permeability of the fabric gradually decreases. The pure cotton fabric has the highest air permeability, reaching 265.11 mm / s, which is due to its fiber structure having a high air permeability. After being treated with the flame retardant PEPA-AP, the air permeability of the fabric decreases. The air permeability of PEPA-AP-CF1 is 220.78 mm / s, showing a slight decrease compared to pure cotton, indicating that the attachment of the flame retardant has a certain impact on the fiber air permeability. With the increase in the content of the flame retardant, the air permeability of PEPA-AP-CF2 and PEPA-AP-CF3 further decreases, to 216.35 mm / s and 208.62 mm / s respectively. This downward trend is attributed to the increase in the content of the flame retardant making the fabric surface more dense, thus hindering air circulation to a certain extent. However, even PEPA-AP-CF3 with the lowest air permeability still maintains an air permeability of 208.62 mm / s, indicating that the flame retardant treatment has a limited impact on the air permeability of the fabric and can still meet the requirements of practical applications. It can be seen that with the increase in the content of PEPA-AP, the air permeability decreases, but the overall air permeability of the fabric is still at a relatively high level. This shows that the introduction of PEPA-AP has a small impact on the air permeability while improving the flame retardancy of the fabric.

[0116] Whiteness analysis

[0117] The whiteness test results of the pure cotton fabric (Control cotton), PEPA-AP-CF1, PEPA-AP-CF2, and PEPA-AP-CF3 are as follows Figure 18 shown. It can be seen that the pure cotton fabric has the highest whiteness of 77.44%, which is the original optical property exhibited by the fabric without flame retardant treatment. After being treated with the flame retardant PEPA-AP, the whiteness of the fabric decreased slightly. The whiteness of PEPA-AP-CF1, PEPA-AP-CF2, and PEPA-AP-CF3 was 76.85%, 75.56%, and 76.50% respectively. Although the whiteness decreased slightly with the introduction of the flame retardant, this change was small, indicating that the treatment with the flame retardant had a limited impact on the fabric appearance. In particular, the whiteness of PEPA-AP-CF3 remained at a relatively high level (76.50%), indicating that even under high-concentration flame retardant treatment, the visual appearance of the fabric could still be well retained. It can be seen that the PEPA-AP treatment significantly improved the flame retardant performance of the fabric while having a small impact on the whiteness of the fabric, indicating its high applicability in practical applications where good appearance characteristics need to be maintained.

[0118] Softness analysis

[0119] The softness test results of the pure cotton fabric (Control cotton), PEPA-AP-CF1, PEPA-AP-CF2, and PEPA-AP-CF3 are as follows Figure 19 shown. As can be seen from the figure, with the introduction of the PEPA-AP flame retardant and the increase in its content, the softness of the fabric changed significantly. The pure cotton fabric ( Figure 19 area a in it) showed good flexibility and could form a nearly closed circular structure after folding, with a height of 0.97 cm, indicating that it retained the original soft characteristics without treatment. Figure 19 The softness of PEPA-AP-CF1, PEPA-AP-CF2, and PEPA-AP-CF3 after flame retardant treatment shown in areas b, c, and d in it decreased and did not form a closed circular structure. This indicates that the introduction of the flame retardant changed the flexibility of the fabric fibers, resulting in a decrease in softness. Although the flexibility decreased slightly, each flame retardant-treated fabric still retained a certain bending ability, indicating that the flame retardant treatment had a small impact on the softness of the fabric and was still within an acceptable range.

[0120] The above is only the preferred embodiment of the present invention patent. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention patent, several improvements and retouches can still be made, and these improvements and retouches should also be regarded as the protection scope of the present invention patent.

Claims

1. A method for preparing a phosphorus-nitrogen flame retardant, characterized in that: The method comprises adding phosphoric acid to the PEPA solution, heating the solution for reaction and then cooling the solution to room temperature; then adding urea and heating the solution for reaction again; and washing, separating and drying the solution after the reaction is completed.

2. The method for preparing a phosphorus-nitrogen flame retardant according to claim 1, characterized in that: The molar ratio of PEPA, phosphoric acid and urea is (0.1-0.5):(0.1-0.5):(0.2-1).

3. The method for preparing a phosphorus-nitrogen flame retardant according to claim 1, characterized in that: The preparation method of the PEPA solution is as follows: PEPA and DMF are mixed in a ratio of (0.1-0.5) mol: (150-1000) mL, and the mixture is reacted at 65-95° C. for 1-3 hours under stirring.

4. The method for preparing a phosphorus-nitrogen flame retardant according to claim 1, characterized in that: Heating reaction conditions 120-180°C, 1-4h; The conditions for the secondary heating reaction are: 130-180°C, 2-5h; The drying temperature is 60-80℃.

5. A phosphorus-nitrogen flame retardant, characterized in that: The method is prepared by any one of claims 1 to 4.

6. Use of the phosphorus-nitrogen flame retardant prepared by the method according to any one of claims 1 to 4 in flame retardant finishing of fabrics.

7. The use according to claim 6, characterized in that: The method is to dissolve dicyandiamide in a phosphorus-nitrogen flame retardant solution, and then immerse the cotton fabric in the solution, heat and stir it, and then take it out and dry it; Wherein, in the phosphorus-nitrogen flame retardant solution, the concentration of the phosphorus-nitrogen flame retardant is 50-500 g / L; The added amount of dicyandiamide is 3-8wt% of the mass of the phosphorus-nitrogen flame retardant solution.

8. The use according to claim 7, characterized in that: The conditions for heating and stirring are: 60-90°C, 0.5-3h; The drying temperature is: 70-90℃.

9. The use according to claim 6, characterized in that: The cotton fabric was also subjected to the steps of surface cleaning and mercerization before immersion; Among them, the method of surface impurity removal is: The cotton fabric is immersed in a 80-100°C, 5-20g / L NaOH aqueous solution according to a bath ratio of 1:(20-70) for 80-120 minutes, and then taken out, washed and dried.

10. The use according to claim 9, characterized in that: The mercerizing method is: immerse the cotton fabric after surface impurity removal treatment in a 20-30% NaOH aqueous solution for 3-10 minutes, take it out, wash it to neutrality and then dry it.

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