Green Synthesis of Biobased P-N Coating Via Mechanochemistry Strategy: For High-Eciency Flame Retardant Finish of Cotton Fabric

Cotton fabric is widely used in many occasions, but it is ammable with high re risk. To meet the great re safety demands of cotton fabric, a novel lignocellulosic-based P-N synergistic (LFPN) ame-retardant coating with high eciency and environment friendly was developed via mechanochemistry strategy in the aqueous phase. The characterisation results showed the stable P-O-C bond formed to bind both lignocellulosic bre and ammonium polyphosphate (APP). Meanwhile, LFPN has an excellent dispersion in water with a nanometer-scale enveloping rod structure. The cotton fabric treated by the LFPN coating showed outstanding ame-retardant properties, the peak heat release rate (PHRR) was reduced by 77% and the residue mass was increased by 259% compared with control cotton fabric. And there was a self-extinction phenomenon during the ammability test of ame retardant cotton. Based on the analysis of the combustion and pyrolysis process, a gas-condensed two-phase ame retardant mechanism model was proposed, which could be used to explain the action process of LFPN for cotton fabric during combustion.

Introduction hemicellulose (L. Y. Zhang et al., 2020a; P. P. , and lignin  has been used to prepare various bio-based ame retardants, which has the characteristics of the wide source, low-cost, and renewable as the most abundant biomass resources on the earth (Dhyani and Bhaskar, 2018).
Lignocellulosic biomass-based ame retardants have thr inherent carbonization ability to produce stable char residue during the combustion process (Costes et al., 2017). However, there are still some issues of ine ciency, poor compatibility, and so on for these natural ame retardants. Lignocellulosic biomass is bene ted from abundant hydroxyl groups reactive sites ( improve the e ciency of modi cation, and separate complex biomass (Fiss et al., 2019). However, organic solvents, catalysts, phosphoric acid, or phosphoric anhydride are often used in the process of mechanochemistry modi cation to create reaction conditions, which is contrary to the original intention of using environment-friendly biomass ame retardant. Therefore, if we can use the characteristics, which can increase the temperature and pressure of the environment, of wet ball milling to create reaction conditions (Friscic et al., 2020;Andersen and Mack, 2018). The above problems may be improved and green preparation can be realized. At present, the use of the mechanochemistry approach in the aqueous phase to phosphorus and nitrogen functionalize of biomass is rarely reported.
In this work, the phosphorus-nitrogen functionalized lignocellulosic bre (LFPN) ame-retardant coating had been synthesized, which used lignocellulosic bre as lignocellulosic biomass, environment-friendly ammonium polyphosphate as phosphorus and nitrogen source, and deionized water as grinding aid, via mechanochemistry in aqueous phase approach. Then, the ame retardant nishing of cotton fabric was carried out by LFPN ame-retardant coating via the chemical grafting treated method. This bio-based coating endowed cotton fabrics with low-cost, high e ciency, and environmental ame retardant properties.

Synthesis of LFPN coating
LFPN was prepared by ball milling, and ball-mill QM-3SP2 used was provided by Nanjing Chi Shun Technology Development Co., Ltd. Lignocellulosic bre (31.25g), ammonium polyphosphate (62.5g) and deionized water (300mL) were added into a 500 mL stainless-steel jar with 750g of stainless-steel balls at room temperature. The mill process was planned to run with a ve min rest each 40 min at 30Hz, and the total milling time was set as 10 h. The resultant mixture after milling was collected. And the product was centrifuged at 5000 r/min for 20 min to separate macromolecular chain and impurity solids. Finally, the saffron yellow supernatant with 4% LFPN was collected. The synthesis of LFPN coating was shown in Fig. 1.

Flame retardant nishing of cotton fabric
The cotton fabric, which was rinsed and dried in advance, was immersed in the suspension containing LFPN of a certain concentration (i.e., 0.5wt%, 1wt%, 2wt%, and 4wt%). This process was carried out at 60 ℃ for 2 hours. And the treated cotton fabric was taken out to dry at 80 ℃ until to constant weight. The obtained cotton fabric was labelled as Cotton-LFPN-1 with 0.72 wt% LFPN, Cotton-LFPN-2 with 1.73 wt% LFPN, Cotton-LFPN-3 with 3.18 wt% LFPN, and Cotton-LFPN-4 with 6.87 wt% LFPN, respectively.
Materials characterization was shown in supporting information.

Results And Discussion
Structure and morphology analysis of LFPN TEM and SEM-EDS were applied to characterize the structure and morphology of LFPN, which was shown in Fig. 2. As shown in Fig. 2A, there were many nanometer particles, which were irregular sphere shapes, assembled around a certain structure became a cluster. In order to further explore the internal form of the cluster, TEM was carried out to get a 2D image of LFPN (Fig. 2b, c). Some rod nano bres can be clearly observed with the width of 50-105 nm, and were coated with a lot of nanometer particles. It can be seen from the Fig. 2c, irregular spherical particles were uniformly scattered on its surface, forming clusters. The phenomenon explains the aggregation of nanometer particles in the SEM image of LFPN. Figure 2d shows the chemical element distribution of LFPN by SEM-EDS image. The distribution of carbon elements showed a rod-like structure like TEM images, and phosphorus and nitrogen elements distributed evenly around carbon. Therefore, the possible structure of LFPN is the grafting of ammonium polyphosphate onto the surface of nanometer lignocellulosic bres cut by shear force.
Chemical structure analysis of LFPN The chemical composition and structure of LFPN were analyzed by FTIR, XRD, TGA, XPS, and dispersion stability test as shown in Fig. 3, respectively.
The FTIR spectra of LF, APP, and LFPN was shown in  (Cai et al., 2018). The results can explain the phenomenon after ball milling: a strong smell of ammonia can be smelled when the lid of the ball mill jar was opened. Overall, the FTIR spectra revealed a possibility that crosslinking type APP was reacted with the hydroxyl group on LF to form the P-O-C bond.
The XRD patterns of LF, LFPN, and APP were shown in Fig. 3b. There were typical and sharp peaks of APP ad LF, indicating both APP and LF have good crystallinity. However, the XRD pattern of LFPN showed an amorphous crystal form, which was completely distinct from LF and APP. This characteristic of LFPN is consistent with the results of SEM and TEM images. The XRD images showed that amorphous morphology was due to the encapsulation of clusters of particles. And it is proved that LFPN has a smaller particle size than LF or APP. Moreover, only weak peaks at 15.7° and 29.8° were corresponding to the characteristic peaks of LF and APP, respectively. In conclusion, LFPN was obtained by the chemical reaction between lignocellulosic bre and ammonium polyphosphate The XPS survey spectra were also used to characterize the structure of LFPN as shown in Fig. 3e. And, the peaks ascribed to O1s, N1s, C1s, and P2p can be easily found. What needs illustration is that the peak at 346.0 eV was assigned to Ca2p, which was introduced during the production and treatment of LF were many differences for the two subpeaks between LFPN and APP. Firstly, the ratio of NH 4 + to -NHcontent in APP is 1.55, while the ratio in LFPN is 6.46. Secondly, the peak intensity of N1s for LFPN was 30% lower than that of APP. Lastly, the P/N atomic ratio of LFPN is about 1.00, which is two times higher than that of APP. The above three points indicated that the chains of cross-linked APP were broken by the action of shear force in the process of the mill, greatly reducing the -NH-and some of the NH 4 + undergo hydrolysis, making APP with a small molecular chain react with the hydroxyl group in LF, and increasing the P/N atomic ratio in LFPN.
The transparent orange LFPN suspension showed good dispersion and stability in water (Fig. 3d). And there were no particles felled down after 24 hours. This phenomenon can be illustrated by two reasons: one is that LFPN has a smaller molecular weight than both APP and LF; the other is the introduction of hydrophilic groups such as phosphate groups into LF (Zabihi et al., 2020).
By combining SEM, TEM, FTIR, XRD, XPS, and dispersion stability test, the reaction mechanism can be speculated in Fig. 1. A long time and intermittent ball milling make the chains of LF and APP break into small molecules and release gases such as ammonia and water vapor, which results in a pressure increase in the jar. At the same time, the physical impact of the ball mill produces a lot of heat, and due to the heat dissipation ability of the stainless-steel jar is not as good as the agate jar, the temperature in the jar continuously increased. It can be easily veri ed by the boiling phenomenon after ball milling. It is easy that the multi-hydroxyl LF can react with APP to generate LFPN ame retardant with short-chain biobased phosphate ester in an environment of high temperature, high pressure, and strong shear force.
The thermal stability of LF, APP, and LFPN was analyzed by TGA. The TGA results of the samples under nitrogen atmosphere were carried out as Fig. 3c. The thermal degradation of APP proceeds by two stages: the temperature of the rst stage is between 300-500 ℃, which is due to the release of NH 3 accompanied by the formation of polyphosphoric acid; the other stage is between 500-700 ℃, corresponding to volatilization of metaphoric acid and P . As for LFPN, the pyrolysis process has some similarities with LF, but it is completely different. A weight loss was observed between 150-250 ℃ of LFPN, indicating the release of initial phosphoric acid and escape of ammonia due to the introduction of low polymeric degree ammonium phosphate structure. And the degradation process between 220-550 ℃ was caused by the slow decomposition of cellulose, hemicellulose, and lignin. It is worth noting that, compared to the DTG curve of LF, the decomposition rate of LFPN was signi cantly reduced after 220℃. This phenomenon may be that phosphoric acid promotes dehydration and cross-linking of P-O-C bond linked lignocellulosic bre to carbon formation. And the inference can be con rmed by the initial decomposition temperature (T 5% ).
T 5% of APP, LF and LFPN were 348.1 ℃, 319.4 ℃ and 236.9 ℃, respectively. T 5% demonstrated that the introduction of the ammonium phosphate structure and the formation of P-O-C bond promote the early decomposition of ammonium polyphosphate and the cross-linking with LF to form carbon char. The formation of the carbon char layer signi cantly reduces the decomposition rate of LF, resulting in a higher nal residual mass of LFPN.
In addition, it has been reported that the decomposition temperature for cotton fabric is about 250℃. In contrast, the decomposition temperature of LFPN is lower than 250℃, and LFPN has a good carbon formation effect, so it is very suitable for the ame retardant Characterization of Cotton-LFPN SEM-EDS was used to survey the surface morphology of Cotton and Cotton-LFPN, as shown in Fig. 4. As displayed in Fig. 4, both Cotton and Cotton-LFPN showed typical smooth cotton bre structure at low magni cation. At the same time, many ne particles on the surface of cotton-LFPN at high magni cation makes the surface rough. The EDS results of Cotton-LFPN (Fig. 4e) con rmed that the many small particles on the fabric surface were due to the formation of LFPN ame-retardant coating.
To understand the form of LFPN coating on the cotton surface, XRD and ATR-FTIR were used to study the fabric surface, which was shown in Fig. 5.
As is shown in Fig. 5a, XRD patterns of Cotton and Cotton-LFPN both showed the typical peaks of cotton fabric. The difference lies in the intensity of the diffraction peak. The peak strength of Cotton-LFPN is weaker than that of Cotton, indicating that there is a thin lm composed of amorphous material on the surface of cotton fabric. The phenomenon is corresponding to Fig. 4 and Fig. 3b.
As shown by ATR-FTIR spectra in Fig. 5b,

Thermostability of cotton fabric
The TGA curves for Cotton and Cotton-LFPN under N 2 were presented in Fig. 6, and the related data were summarized in Table 1. Both Cotton and Cotton-LFPN showed the evaporation of water before 110 ℃. . However, the pyrolysis of Cotton-LFPN was distinct from that of Cotton. As for the Cotton-LFPN compared with that of Cotton., the initial decomposition began about 38.8 ℃ earlier, and the char residue was increased from 16.6 % to 34.7%, respectively. Moreover, the maximum decomposition rate of Cotton-LFPN decreased by 27%, and the temperature at the maximum decomposition rate of Cotton-LFPN decreased from 359 ℃ to 296 ℃. Indicating that LFPN decomposed and formed a char layer, and then the phosphorus products after decomposition dehydrated cotton fabric into carbon char to inhibit further decomposition.
To sum up, the pyrosis of cotton-LFPN consists of two carbonization processes: one process is the formation of LFPN char layer with thermal insulation and shielding effect; the other process is the crosslinking carbonization process of cotton fabrics promoted by LFPN.

Flame-retardant Performance Analysis
In corresponding to investigate the ame retardant e ciency of LFPN for cotton fabric, the improved vertical ammability test (IVFT) and cone calorimeter test (CCT) were carried out, and the results were shown in Fig. 7.
As shown in Fig. 7a and video in the support information, both Cotton and Cotton-LFPN were ignited at 3s, but showed a completely different combustion behavior. The ame height and re spread speed of Cotton-LFPN were weaker than those of Cotton. Moreover, the ame was extinguished, which meant the  Especially for Coton-LFPN-2, the SF value was much higher than that of the control cotton fabric. This may be due to the crack of the surface charforming fabric, leading to the rapid escape of the isolated ue gas into ame zoom. As shown in Fig. 7c, in the case of Cotton-LFPN, there was a dense char residue layer formed, which can play a role as a physical barrier. In addition, the CO 2 /CO ratio can re ect the combustion e ciency. With the increase of LFPN loads, the CO 2 /CO ratio decreased gradually, which means the combustion e ciency decreased gradually. This indicated that the formation of a cross-linking char layer made the burning more inadequate. Moreover, the mass loss curves correspond to thermal stability results and can be explained by digital images of char residue layer after CCT (Fig. 7c). Excellent char-formation performance can effectively reduce the generation of ammable volatiles gases to delay combustion and selfextinguishing behavior (P. P. Li et al., 2019).
The CCT results were closely related to IVFT and showed the same effect, which indicated that cotton-LFPN has excellent ame-retardant performance and ame retardant e ciency.

Flame Retardant Mechanism
Condensed phase analysis. In order to deduce the ame-retardant mechanism of Cotton-LFPN, SEM, LRS and ATR-FTIR were used to investigate the structural change of condensed phase, as shown in Fig. 8.
As shown in Fig. 8d, the SEM image of char residues after CCT qualitatively con rmed the results of CCT. It was easily found that the bre structure of char for Cotton was incomplete, exhibited relatively loose and fragile. In contrast, a compact char residue with a complete and tight brous structure obtained from Cotton-LFPN can effectively insulate ammable gas into oxygen and delay heat transfer (Y. Yuan et al., 2020). Speci cally, some carbon tumors with spherical structure can be observed, indicating ductile char surface from the LFPN coating acted effective protection for cotton fabric from further combustion .
The LRS was conducted to further quantitative evaluation performance of char residues of Cotton and Cotton-LFPN after CCT, as depicted in Fig. 8a Gas phase analysis. In addition to the condensed phase, the gas phase analysis can further understand the combustion and pyrolysis process and infer the ame-retardant mechanism.
Effective heat of combustion (EHC), which means the ratio of heat release rate to the mass loss rate, is a valid measure to judge the ame-retardant mechanism. Because EHC can show the degree of involvement for ammable volatile gases in the combustion process (Tang et al., 2016;Dong et al., 2016). The EHC curves of Cotton and Cotton-LFPN was given in Fig. 9. Compared with the control cotton fabric, the EHC of treated cotton fabric was signi cantly reduced. This indicated Cotton-LFPN produced less-ammable volatile gases and gas phase ame-retardant effect existed.
It has been reported that the microscale-combustion calorimeter was an effective method to vertify the result of EHC ( obvious that the peaks at 2359, 2182, 1732, and 1506 cm − 1 correspond to CO 2 , CO of the thermal cracking process, carbonyl compounds, and aromatic compounds, respectively. From the FTIR spectrum of gas products at maximum decomposition rate (Fig. 10a), combustible gas of Cotton-LFPN such as hydrocarbon and carbonyl compounds were decreased compared with control cotton fabrics. Nay, the TG-IR spectra at a speci c temperature (Fig. 10b, c) of 100, 200, 300, 400, 500, 600 and 700 ℃ showed the intensity of pyrolysis gas for Cotton-LFPN is generally weaker than that for Cotton. Gas chromatogram (Fig. 10d) combined with mass spectrum can further con rm the infer of TG-IR. Compared with control fabrics, aldehydes, ketones, esters, and aromatic pyrolysis products of Cotton-LFPN were decreased except for the furans. This indicated that LFPN catalyzes the dehydration and carbonization of cotton fabrics, and inhibits the formation of L-glucose and the increase of pyrolysis products. It is also worth noting that the peaks at 3300, 1625, and 1500 cm − 1 , corresponding to the NH 3 absorptions, can be observed from Fig. 10c (Z. Z. Wang et al., 2009). And signi cant enhancement of the water absorption peak. This indicated there was gas phase ame-retardant mechanism in LFPN.
Based on the above analysis, the ame-retardant mechanism of LFPN could be speculated, and the possible process as illustrated in Fig. 11. The ame-retardant mechanism of LFPN can be divided into two parts: condensed phase and gas phase. The condensed phase ame-retardant mechanism could be separated into two aspects. On the one hand, LFPN ame-retardant coating was dehydrated and cyclized into graphitization LFPN char layer at high temperature due to the polyhydroxy structure and phosphaterich group of LFPN. LFPN char layer was endowed with a dense protective layer with the carbon tumor structure, which can isolate the contact between oxygen and combustible gas, suppress released smoke, shield the transfer of heat. On the other hand, LFPN ame-retardant coating released phosphoric acid and derivatives, catalyzed the cross-linking carbonization of cotton bres, and form a stable phosphoruscontaining ring structure, to prevent the further decomposition of cellulose. In the gas phase, the concentration of combustible gas was diluted by non-ammable gases such as NH 3 and H 2 O vapor which was released by LFPN ame retardant coating. Moreover, the self-extinguishing phenomenon proved that LFPN decomposed P· radicals which participated in the free radical competitive reaction and reduced the concentration of OH· in the ame region (Velencoso et al., 2018).

Conclusion
In this work, a novel lignocellulosic-based P-N synergistic (LFPN) ame-retardant coating had been successfully green synthesized by mechanochemical approach with lignocellulosic bre, ammonium polyphosphate, and water, without any organic solvent or organic auxiliaries. The LFPN had a nanometerscale enveloping rod structure which was riched in phosphorus and nitrogen elements. Meanwhile, the results of FTIR, XRD, and XPS showed the formation of the P-O-C bond and con rmed the chemical structure of LFPN. In addition, LFPN also had excellent thermal stability and water dispersion, which was con rmed by TGA and dispersion test. And the decomposition temperature of LFPN ame-retardant coating was matched with that of the cotton fabric. Therefore, cotton fabric was nished with LFPN ame-retardant coating. The layer of coating was fomed by LFPN ame-retardant coating on the surface of fabric through the chemical bonding. The PHRR, THR, SF, CO 2 /CO, and damaged area of Cotton-LFPN was decreased by 57.4%, 42.2%, 38.4%, 84.7%, and 66.7%, respectively. And the residual mass of Cotton-LFPN increased by 109.3% compared to that of neat cotton fabric. The above results indicated that LFPN-Cotton has superior ame retardant properties. SEM-EDS, ATR-FTIR, LRS, EHC, MCC, and TG-IR-GC/MS were used to illustrate the condensed phase and gas phase change of Cotton-LFPN during combustion. Basing on the above analysis, the ame-retardant mechanism of Cotton-LFPN was deduced. As for the condensed phase ame-retardant mechanism, there were two processes of carbonization: one is that LFPN was dehydrated and carbonized under the action of high temperature due to the polyphosphate groups, formed a stable protective carbon char layer which plays a role in physical barrier; the other is that phosphoric acid was released during the thermal decomposition of LFPN, which catalyzed the crosslinked cyclization of cotton fabric and prevented the further decomposition of cotton fabrics to release ammable substances such as aldehydes, ketones, ethers, and esters. As for the gas phase ameretardant mechanism, LFPN released non-ammable gases such as NH 3 and H 2 O which diluted the concentration of combustible gas in the combustion zone. Meanwhile, the production of P· radicals would interrupt the free radical reaction in the ame region. Thus, LFPN obtained through the green mechanization method with the environment-friendly, low toxicity, and high-e ciency ame retardant characteristics would have a broader application prospect.             TG-IR spectra of pyrolysis products of Cotton and Cotton-LFPN (a) at maximum decomposition rate, the TG-IR spectra at speci c temperature (b, c), gas chromatogram of pyrolysis products of Cotton and Cotton-LFPN at maximum decomposition rate.

Figure 11
Schematic illustration for ame retardant mechanism of Cotton-LFPN.

Supplementary Files
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