Synthesis of a New Phosphorus/Nitrogen Durable Flame Retardant for Cotton Fabrics

Phosphorus/nitrogen ame retardant ammonium three phosphoric acid glycerol ester (FR) with reactive -P-O − NH 4+ groups was synthesized from glycerol, phosphoric acid and urea. At high temperature, the -P-O − NH 4+ group in FR is decomposed into -P-O − H + group. Under the action of catalyst dicyandiamide, -P-O − H + forms phosphonic anhydride. Phosphonic anhydride can dehydrate and condense with the hydroxyl group on the 6-position carbon atom in the glucose ring of cotton ber, forming a rm P-O-C bond, thus xing FR molecule rmlyon cotton ber. XRD suggested that the nishing process only slightly affected the cotton ber structure and the surface morphology, elemental composition of char residues in cotton fabrics were tested by scanning electron microscope (SEM) and energy dispersive spectrometer (EDS). Conrmed that FR was grafted well with slight damage to the cotton structure. When the treated cotton with 25.3% weight gain(WG) in FR, the treated cotton has self extinguishing property and passed UL-94 vertical burning V-0 classication, the limiting oxygen index (LOI) was improved from 17.0–40.5%, and the LOI can still reach 30.9% after 50 laundering cycles, the cone calorimetry(CONE) analysis indicated that the peak heat release rate (PHRR) of the treated cotton was reduced from 190.3 kW/m 2 to 17.9 kW/m 2 , and the total heat release (THR) was reduced from 2.8 MJ/m 2 to 1.8 MJ/m 2 , The thermogravimetric(TG) and Differential scanning calorimeter (DSC) showed that the addition of FR inhibited the initial thermal degradation temperature of the treated cotton under heating conditions, and TG showed that the initial thermal degradation temperature of treated cotton in nitrogen and air was 225.9 ℃ and 221.8 ℃ , respectively, which was lower than that of untreated cotton. The mechanical properties are in the usable range, which showed that treated cotton have excellent


Introduction
Due to its good hygroscopicity, biodegradability and natural skin friendly characteristics, cotton fabric is widely used in industrial re clothing, military protective clothing, home decoration, building industry (such as building decoration materials, wallboard materials, fences, etc.) and automotive industry (such as doors, door dashboard, etc.) (Shahidi et al. 2013; Emam et al. 2019). However, cotton fabric is very easy to be ignited, LOI value is about 17.0% (Liu et al. 2018b). The ame spreads quickly. It can be treated by ame retardant nishing (Pethsangave et al. 2017) and blending with other ame retardant bers (Mohamed et al. 2014). Common ame retardant elements include phosphorus, halogen, nitrogen, silicon, magnesium, aluminum, boron, sulfur, etc. Halogen ame retardant has good re prevention effect.
However, chlorine and bromine atoms will release halogen free radicals or hydrogen halide, which will produce corrosive and toxic smoke in the degradation or burning process, which has been banned in many countries (Abou-Okeil et al. 2013).
At present, N-hydroxymethyl phosphoryl Propionamide(CP) is the main commercial phosphorus/nitrogen ame retardant widely used in cotton fabric ame retardant (Fang et al. 2016). It has the synergistic effect of solid phase barrier, gas phase dilution and endothermic cooling (Liu et al. 2018a), high e ciency and low toxicity. The treated cottonc shows excellent ame retardancy. However, the nished fabric is affected by the hydrolysis of N-hydroxy methyl group and the crosslinking agent of polyhydroxy methyl melamine, which contains a lot of free formaldehyde and seriously endangers people's health. Therefore, it is a very important challenge to nd the active groups of new ame retardants (Manfredi et al. 2018).
Phosphorus containing compounds are considered to be the most promising durability ame retardants for cotton (Alongi et al. 2013). At high temperature, phosphorus containing compounds can be decomposed into phosphoric acid/polyphosphoric acid and other acidic uid substances. These acidic intermediates can react with the matrix materials, catalyze the dehydration reaction of materials and promote the carbonization of materials. At the same time, these phosphoric acid compounds can still maintain the liquid state and strong acidity at 864 ℃, forming a protective lm on the surface of the ber, preventing oxygen and heat from penetrating into the deep ber, thus blocking the burning of the material (Rosace et al. 2017). This is also the reason why the ame retardancy of phosphorus containing compounds is better than that of acidic ame retardant compounds (such as sulfates, borax, nitrates, etc.) (Molaba et al. 2016). In addition, Urea including nitrogen during combustion can release NH 3 , N 2 and other non combustible gases, dilute the air. At the same time, it has excellent carbonization ability and thermal stability (Sharma et al. 2017). Therefore, phosphorus/nitrogen ame retardant are selected to synthesize target ame retardants for ame retardant nishing of cotton fabric (Mohammad et al. 2015).
In recent years, some durability, halogen free and environmentally friendly ame retardants and technologies have been reported. Various surface treatments such as LBL self-assembly (Carosio et al.  (Annalisa et al. 2016)used layer by layer self-assembly method to nish 30 layers of DNA chitosan double-layer structure on cotton fabric, and then cured by UV, which gave the cotton fabric good self extinguishing property, reduced the initial pyrolysis temperature and maximum pyrolysis rate temperature of cotton fabric, and increased the amount of coke. The main problem to be solved by this method is to maintain the friction and washing durability of the treated fabric. Sol-gel technology is considered to be a multipurpose synthesis method based on hydrolysis and condensation two step reaction. However, under acidic pH conditions, the mechanical properties of cotton fabric often drop sharply (Kundu et al. 2018; Nabipour et al. 2020). Zhang (Zhang et al. 2017) proposed a simple and economical method to prepare cotton fabric with hydrophobic and ame retardant properties through sol-gel reaction and self-assembly technology. However, due to the lack of low surface energy and micro nano structure, the WCA of the fabric is about 120 ℃, which can not meet the requirements of hydrophobicity. Nano materials are environmentally friendly, non-toxic and other characteristics, and have also been used in ame retardants. However, the main disadvantage is poor washing durability, which leads to unstable ame retardancy (Malucelli et al. 2014;Lin et al. 2019). So far, environmentally friendly and washable ame retardant coatings are a great challenge for the scienti c and industrial circles. Some studies show that (Wang et al. 2016) chemical grafting is considered to be an effective method to prepare durability ame retardant.
In this study, FR was successfully grafted onto cotton fabric by chemical grafting with simple heating, soaking and rolling processes, and it has certain chemical stability. The treated cotton with durable, ame retardant, smoke suppression and environmental protection is realized. The raw materials used in this method are safe and easy to obtain, and the process ow is simple. TG was used to analyze the thermal stability and oxidation resistance of cotton fabrics before and after nishing. XRD and FT-IR were used to characterize the crystal structure and chemical structure of cotton fabrics before and after nishing. SEM-EDS was used to analyze the surface morphology and element composition of carbon residue of cotton fabrics before and after nishing. LOI, vertical burning and CONE were used to test the combustion properties of cotton fabrics before and after nishing. Finally, the exibility and mechanical properties of cotton fabrics before and after nishing were evaluated.

Synthesis of FR
Put 0.1 mol glycerol into a 250 ml beaker, add 0.3 mol phosphoric acid into the beaker, stir and raise the temperature to 110 ℃, and react for 25 h. At this time, the reaction system changes from colorless transparent mixture to brown viscous liquid. Then, reduce the reaction temperature to 80 ℃, slowly add 0.3 mol urea into the beaker, stir and raise the temperature to 110 ℃ to continue the reaction, and stop the reaction when the pH value of the mixture reaches 6. The white solid ame retardant FR was obtained after the crude product was extracted with anhydrous ethanol to remove impurities and dried. The yield was 70.0%. The structure of FR was analyzed by 13

FR nishing of cotton fabric
Finishing prescription: soak cotton fabric in FR solution with a spec c concentration, and add a certain amount of dicyandiamide as catalyst in the solution. Finishing process: Dipping (soaking at 60 ℃ for 20 min, twice dipping and twice rolling, so that the liquid carrying rate reaches about 80%) → pre-drying (80 ℃, 10 min) → baking (160 ℃, 2 min) → water washing → drying. Scheme 2 describes the chemical grafting reaction between cotton fabric and FR. Scheme 3 describes the FR nishing process of cotton fabric. At 160 ℃, some -P-O -NH 4 + groups may also form phosphoramide group O = P-NH 2 .
WG was calculated in accordence with Eq. (1): In which: W 1 is the mass of untreated cotton, g; W 2 is the mass of treated cotton, g.  In conclusion, the infrared spectrum of the synthesized product was consistent with the characteristic group of the target product, which indicates that FR has been successfully synthesized.

LOI and durability analysis of untreated cotton and treated cotton
The WG value of the cotton treated with different FR concentrations change are presented in Fig. 2 respectively. The cotton fabric treated with 400g/L FR solution achieves a large WG of 33.7%. If the concentration of FR is too high, the WG% of treated cotton will also increase, which will affect the physical properties such as handle and original comfort.
The LOI value represents the minimum oxygen concentration required to maintain the ame state when the sample is burned in oxygen and nitrogen atmosphere (Dutkiewicz et al. 2018). As shown in Fig. 2 . Therefore, the cotton fabric with 25.3% WG FR or more can be regarded as durability treated cotton. At the same time, the LOI value of the treated cotton decreases with the increase of laundering cycles. That is why the unreacted FR and the covalent bond P-O-C in some FR are washed away in the washing process. FR has the active group of -P-O-NH 4 + , which can combine with cellulose through P-O-C covalent bond. In addition, FR can react with -OH on cellulose to form P-O-C bond at the same time. Only when the P-O-C covalent bond between FR and cellulose is completely hydrolyzed can FR be removed from the treated cotton. However, due to the polymerization of ame retardant molecules to a certain extent, it is di cult to hydrolyze all P-O-C covalent bonds. Therefore, the cotton fabric treated with ame retardant has excellent durability.

Thermal and thermo-oxidative analysis of FR, untreated cotton and treated cotton
Thermal and thermo-oxidative processes of FR, untreated cotton and treated cotton have been investigated by TGA. T 5% and T 10% were temperature at 5%, 10% mass loss, T max was the maximum thermal degradation rate temperature. R max was the maximum thermal degradation rate.
The TG and DTG curves and data of FR, untreated cotton and treated cotton in N 2 atmosphere are presented in Fig. 3(a) and 3(b) and Table 1., For FR, T 5% was 101.6 ℃, which was caused by water evaporation and oligomer decomposition. T 10% was 126.8 ℃, and weight loss was 33.1%. The rapid weight loss in this stage was due to the fact that the phosphoric acid released from FR molecular fracture can promote the dehydration and carbonization of FR. T max of FR was 161.9 ℃, and the R max was 0.6%/ ℃. The pyrophosphoric acid produced by dehydration of phosphoric acid promotes the carbonization of FR, and then C-C, P = O, -O-break a lot, releasing volatile substances such as NH 3 , H 2 O and other small molecular products. The residual carbon rate was 21.0% at 600 ℃. These results indicated that FR has good thermal stability. For untreated cotton and treated cotton, in the initial stage, the fabric has a little weight loss, which was due to the loss of bound water due to the heating of the fabric. The main weight loss of the fabric was between 250 ℃-350 ℃, and the mass loss rate increases rapidly in this stage. This was mainly due to the depolymerization of cellulose macromolecules to produce L-glucose, which was further decomposed to form pyrolysis products and coke (Hussain et al. 2019). When the temperature continues to rise, the cellulose burning residue will continue to dehydrate and release water and carbon dioxide, and the mass loss rate will be signi cantly reduced, and the char residues content will be higher and higher (Pan et al. 2018 That is why the treated cotton decomposes to form phosphoric acid when heated, and then polymerizes to form polyphosphoric acid, which inhibits the production of L-glucose and further improves the thermal stability. FR dehydrate the cellulose and promote the formation of char residues. The char residues were covered on the surface of the ber to isolate heat and inhibit the further decomposition of the ber. The char residues rate of treated cotton at 600 ℃was 28.6%, while that of untreated cotton was only 18.9%, which indicates that improve the thermal stability of cotton fabric. The thermo-oxidative processes of FR, untreated cotton and treated cotton curves and data are presented in Fig. 3(c) and 3(d) and Table.1. For FR, T 5% was 92.2 ℃, T 10% was 136.1 ℃,which indicated that FR began to decompose at lower temperatures in air atmosphere. T max of FR was 179.1 ℃, and the R max was 0.6%/℃. In the range of 200-500 ℃, the weight loss of FR was slow. The residual carbon rate was 35.9% at 600 ℃. For untreated cotton and treated cotton, Below 100 ℃, it belongs to the dehydration stage of water molecules in cellulose. The second degradation occurs when the temperature rises to about 200 ℃. When the temperature is kept at 262.6 ℃, the weight loss rate reaches the maximum, and the weight loss is 30.6%. The weight loss rate of the untreated cotton was the highest at 372 ℃, The weight loss was 72%. In addition, the residue of treated cotton is 12.6% at 600 ℃ in air atmosphere, while the residue of untreated cotton is 1.0%. Compared with the untreated cotton, the initial decomposition temperature of treated cotton decreased and the residual content increased signi cantly in both nitrogen and air atmospheres. The above test results show that FR can promote the thermal decomposition of cotton fabric, form a carbon slag protective layer to protect the deep ber and reduce the decomposition rate of cellulose matrix. It also shows that FR presents a typical condensed phase ame retardant mechanism.

XRD patterns of Untreated cotton and treated cotton
The crystal structures of untreated cotton and treated cotton were determined by XRD analysis. As shown in Fig The peak position and shape of untreated cotton and treated cotton were very similar, which means that the plane spacing between crystal planes does not change, and the nishing process has little effect on the structure of treated cotton. The results show that the crystal structure of cellulose was slightly expanded or layered by the dehydration reaction of FR and cotton fabric cellulose (Lee et al. 2018), and the diffraction peak intensity of treated cotton was slightly lower than that of untreated cotton. This may be due to the penetration of dicyandiamide and FR into the amorphous region of the ber during the process of cotton fabric soaking in ame retardant nishing solution, and these small molecules were grafted in the subsequent high temperature grafting reaction In this region, the content of cellulose in the treated cotton decreases, which affects the spatial chemical structure of the ber. That is to say, the proportion of crystalline region of treated cotton grafted with FR decreases, which leads to the decrease of diffraction peak intensity.

FT-IR patterns of untreated cotton and treated cotton
The FT-IR of untreated cotton and treated cotton are presented in Fig. 5

Surface morphology and elemental composition of char residues from untreated cotton and treated cotton
In order to better study the ame retardant effect of cotton fabric, the morphology of carbon residue in cotton fabric more valuable than that of raw cotton. SEM and EDS were used to study the surface morphology and element composition of fabric char residues. The SEM of untreated cotton char residues with different magni cation are shown in Fig. 6(a) and (b), the ber structure was obviously damaged, and the SEM of treated cotton char residues with different magni cation are shown in Fig. 6(c) and (d), the ber structure can still remain intact, indicating that FR can effectively protect the structure of the fabric after burning, and there are obvious particles on the surface of the treated cotton after burning, indicating that FR is successfully attached to the treated cotton. As shown in Fig. 6(e) and (f), EDS shows that phosphorus is evenly distributed on the surface of the char residues of the treated cotton, and the carbon content was increased compared with that before combustion, which indicates that FR can improve the ame retardant performance of cotton fabric by promoting carbon formation.

Vertical burning analysis of untreated cotton and treated cotton
The vertical burning method is used to evaluate the burning performance of cotton fabric. Figure 7 shows the electronic photos of untreated cotton and treated cotton after vertical burning. As shown in Table 2, the treated cotton with 8.5%, 19.0%, 25.3% and 33.7% WG FR solution have carbon length equal to reach 50 mm, 42 mm, 38 mm and 33 mm. The untreated cotton was ignited immediately in the vertical burning test, the ame spread rapidly and nally completely burned out, the after-ame time(t 1 ) and after-glow time(t 2 ) were 7.0s and 11.2s respectively, while the ame diffusion speed of treated cotton decreased, the t 1 and t 2 were 0, and the treated cotton went out immediately after leaving the ame. The results show that the treated cotton has good ame retardancy. When the re source was removed, with the increase of FR concentration, the length of carbon decreases, and the t 1 and t 2 are both 0. The carbon length increased after 50 LCs. When the treated cotton with 25.3% WG, the treated cotton has self extinguishing property, and passed UL-94 V-0 classi cation of vertical burning test. The mechanism of ame retardant is that phosphorus element can strengthen the system, which can be transformed into phosphorus/base acid in the condensed phase, thus accelerating the carbonization of cotton fabric. The carbonization layer can protect the treated cotton fabric from heat transfer and the release of combustible volatiles. Active factors such as PO· can also be released in the gas phase to terminate the free radical reaction. At the same time, small molecules such as CO 2 , H 2 O and NH 3 can be produced during the burning and decomposition of treated cotton. The reduction of the contact between the treated cotton and O 2 and other combustibles indicates that the treated cotton has good ame retardant properties and plays a ame retardant effect in both condensed phase and gas phase.
SEM of char residues of untreated cotton and treated cotton under different magni cation is shown in Fig. 8. Figure 8(a), (b)show the char residue of untreated cotton indicated that the ber structure was obviously destroyed, Fig. 8(c), (d)show the char residues of treated cotton after burning was continuous and uniform, and the structure was relatively complete. That is why FR containing phosphorus decomposes into phosphoric acid during heating, and phosphoric acid forms polyphosphoric acid at high temperature, which plays the role of dehydration, thus inhibiting the formation of L-glucose and further dehydrating and carbonizing cellulose. The continuous carbon layer formed insulates the contact between internal ber and oxygen, slows down the rate of thermal decomposition reaction (Naebe et al. 2016), and makes the structure of residual carbon relatively complete. The surface was rough with particles, which indicates that FR was successfully attached to the treated cotton. Table 2 Vertical burning data of untreated cotton and treated cotton.

WG (%)
LCs t 1 (s) t 2 (s) Char length(mm) Self-extinguishing property It can be seen from Fig. 9(d) and (e) that the ber structure of the untreated cotton has been destroyed after CONE, while the structure of the treated cotton was complete. The results show that FR not only improves the thermal stability of treated cotton, but also protects the internal structure of the ber. And there are ne particles on the surface of the ber, which is because the excessive FR molecules can not graft with the active groups in the cellulose molecules, but adhere to the surface of the ber in the form of coating at high temperature. The relevant data of CONE are shown in Table 3. Time to ignite(TTI) and time to PHRR (T PHRR ) of treated cotton were 28.7s and 30.3s, respectively, which were higher than those of untreated cotton (13.0s and 20.6s), which means that treated cotton is not easy to be ignited in the same environment, and people have a greater chance to escape from re. The re growth rate (FGR, FGR = PHRR/T PHRR ) of treated cotton is 0.6 kW/(m 2 ·s), while that of untreated cotton is 9.2 kW/(m 2 ·s), which indicates that the slower the burning growth rate of treated cotton, the lower the re risk . The average mass loss rate (av-MLR) decreased from 1.23 g/s to 0.55 g/s, indicating that the treated cotton released less heat in the burning process. FR can promote the formation of carbon layer of cellulose to isolate heat and make the burning di cult to spread. The char residues rate of treated cotton was 29.4%, and that of untreated cotton was 5.6%. It also shows that FR promotes the dehydration of cellulose into carbon and improves the ame retardancy of treated cotton. In addition, the total smoke rate (TSR) was signi cantly increased, because some ame-retardant gases containing NH 3 , H 2 O and CO 2 were released at high temperature, which diluted the O 2 concentration on the surface of the treated cotton, making the treated cotton burning insu ciently.

DSC analysis of untreated cotton and treated cotton
The thermal stability of the fabric was further studied by DSC. As shown in Fig. 10(a), the total heat released by the treated cotton was 43.7 mW, which was lower than 36.3 mW of the untreated cotton. Figure 10(b) shows the thermal degradation rate of the untreated cotton and treated cotton. As shown in Table 4, The T max and R max of untreated cotton were 300.0 ℃ and 0.6%/℃ respectively, and the T max and R max of treated cotton were 276.9 ℃ and 0.8%/℃ respectively. These effects may be attributed to the lower pyrolysis temperature of FR, the formation of polyphosphoric acid or pyrophosphoric acid compounds in the decomposition process, which promotes the dehydration and carbonization of cotton ber, and makes the treated cotton have a lower maximum exothermic rate temperature. At 400 ℃, the char residues of treated cotton was 32.8%, which is higher than that of untreated cotton by 7.7%. Compared with the untreated cotton, the treated cotton has better thermal stability. 3.10 Flexibility,whiteness and yellowness index analysis of untreated cotton and treated cotton The exibility, whiteness and yellowness index of untreated cotton and treated cotton are shown in Fig. 11 and Table 5. With the increase of the WG in FR, the ring height of the untreated cotton was 10.0 mm, and thus it has good exibility. The ring height of treated cotton exibility increased slowly, which indicates that FR has a negative effect on the exibility of cotton fabric. The whiteness decreased slowly and the yellowness increased slightly, which may be due to the decomposition of -P-O -H + by FR in the process of high temperature baking. Although there was a catalyst to catalyze the grafting reaction, the -P-O -H + group obtained in the future will make the cellulose in a strong acidic environment, destroy part of the bronectin, and make the treated cotton whiteness decreased and yellowness increased. When the treated cotton with 25.3% WG, the treated cotton have good exibility and whiteness. But when the treated cotton reaches 33.7% WG, the ring height of treated cotton exibility increased obviously and the whiteness of treated cotton decreased obviously.  Fig. 12 (a), water absorbability Fig. 12 (b), tensile strength Fig. 12 (c) and elongation at break Fig. 12 (d) are important indices to measure the comfort of the cotton fabric.
Therefore, the properties of the treated cotton were tested. There is no signi cant difference in vapor transmissibility and water absorbability between treated cotton and untreated cotton, which indicates that ame retardant nishing does not affect the warp and weft density of cotton fabric. The tensile strength and elongation at break of fabric can re ect the external resistance of fabric. When the treated cotton with 25.3% WG, the tensile strength of treated cotton in warp and weft direction decreased by 18.3% and 28.4% respectively, the decrease of tensile strength was due to the high temperature treatment of the cotton fabric during the grafting reaction, which causes some damage to the strength of the cotton ber. In addition, during the chemical grafting reaction, when the cotton ber was in the high temperature acidic environment, part of the glycosidic bond of cellulose was oxidized into C = O group, and the number of damaged nodes in the ber increases, which leads to the decrease of tensile strength and fracture of the treated cotton. The change of elongation at break was small, which was due to a FR molecule with multiple reactive groups can bond with multiple cellulose macromolecules. This strong and close cross-linking reaction reduces the deformation ability of the fabric under the action of external force. Therefore, it has little effect on the elongation at break of cotton fabric. But when the treated cotton reaches 33.7% WG, the tensile strength of the cotton fabric decrease obviously, So the best treated cotton was with 25.3% WG.

Conclusion
In this study, reactive phosphorus/nitrogen FR with -P-O-NH 4 + group was synthesized by two-step method. FR has the advantages of halogen-free, formaldehyde free, environmental protection and high e ciency. XRD results show that FR has little effect on the crystal structure of cotton fabric. FT-IR results indicated that the P-O-C covalent bond formed gave cotton fabric good ame retardancy and durability.
SEM-EDS showed that FR not only successfully nished cotton fabric, but also effectively protected the structure of cotton ber in the combustion zone. When the cotton fabric treated with 25.3% WG, The LOI of the treated cotton was improved to 39.5%, without after-time or after-glow. TG and DSC analysis showed that FR changed the decomposition path of cotton fabric at high temperature, TG showed that the char residues rate of treated cotton was improved from 18.9-28.6% at 600 ℃. CONE results indicated that PHRR and THR decreased by 90.6% and 35.7% respectively. The mechanical property of the treated cotton slightly decreased but retained more than 70%. It is considered that FR can be used as an e cient, ame retardance and durability for cotton fabric, and it has ame retardant effect in condensed phase and gas phase at the same time. Declarations