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Qin, S., Liu, Y., Shi, X., Shen, X., Chu, D., and Liu, S. (2024). “Phytic acid-based NP fire retardant and its effect on combustion property of poplar wood,” BioResources 19(1), 955-972.


To enhance the synergistic effect of phosphorus (P) and nitrogen (N) on flame retardant property, four different phytic acid-based NP flame retardants (FR-PAN) were manufactured using phytic acid and urea with various molar ratios, ranging from 1:3 to 1:12. The FR-PAN water solution was used to impregnate poplar wood under vacuum condition, and the thermal degradation performance of the FR-PAN treated wood were investigated. Compared to untreated wood, the PAN-6 (molar ratio is 1:6) group showed a reduction of 57.1% in total heat release and 80.0% in total smoke release. In the combustion, due to the introduction of P and N, FR-PAN generates O=P/C-O/C-P/C-N bonds, forming highly graphitized char residues, which effectively isolate the entry of oxygen and heat and play a good protective role in the condensed phase. Morphological and chemical analysis of the residual char layer revealed that the introduction of P and N elements formed a more stable hybrid structure, significantly improving the thermal stability of the char layer. Among them, the PAN-6 group exhibited the highest char layer stability, indicating optimal synergistic effects of nitrogen and phosphorus under these conditions.


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Phytic Acid-based NP Fire Retardant and its Effect on Combustion Property of Poplar Wood

Shenglei Qin,1† Yangguang Liu,1† Xin Shi, Xiaoshuang Shen, Demiao Chu,* and Shengquan Liu *

To enhance the synergistic effect of phosphorus (P) and nitrogen (N) on flame retardant property, four different phytic acid-based NP flame retardants (FR-PAN) were manufactured using phytic acid and urea with various molar ratios, ranging from 1:3 to 1:12. The FR-PAN water solution was used to impregnate poplar wood under vacuum condition, and the thermal degradation performance of the FR-PAN treated wood were investigated. Compared to untreated wood, the PAN-6 (molar ratio is 1:6) group showed a reduction of 57.1% in total heat release and 80.0% in total smoke release. In the combustion, due to the introduction of P and N, FR-PAN generates O=P/C-O/C-P/C-N bonds, forming highly graphitized char residues, which effectively isolate the entry of oxygen and heat and play a good protective role in the condensed phase. Morphological and chemical analysis of the residual char layer revealed that the introduction of P and N elements formed a more stable hybrid structure, significantly improving the thermal stability of the char layer. Among them, the PAN-6 group exhibited the highest char layer stability, indicating optimal synergistic effects of nitrogen and phosphorus under these conditions.

DOI: 10.15376/biores.19.1.955-972

Keywords: Nitrogen; Phosphorus; Coordination; Wood flame retardant; Phytic acid; Graphitization degree

Contact information: School of Forestry & Landscape Architecture, Anhui Agricultural University, Hefei 230036, China; 1†: Authors are co-first authors;

*Corresponding authors:;


As a natural and environmentally friendly biopolymer group, wood has extensive use in the construction industry, and the demand for wood is rapidly increasing in many industrial sectors (Chen et al. 2020; Tang et al. 2021). The inherent flammability of wood (Chen et al. 2019; Zheng et al. 2022) poses a significant risk to human life and restricts the application of wood products. To improve the fire safety of wood, various flame retardants, such as phosphorus (Xia et al. 2014; Li et al. 2019; Song et al. 2020), nitrogen (Attia et al. 2016; Yu et al. 2020; Zhang et al. 2020), boron (Yang et al. 2016; Cheng et al. 2020), and silicon-based compounds (Zhu et al. 2019; Zhang et al. 2021), have been introduced through coating or impregnation techniques (Fan et al. 2022). Among these, intumescent flame retardants have gained popularity due to their low smoke production, low toxicity, excellent flame retardancy (Liu et al. 2011; Lin et al. 2021), and cost-effectiveness during combustion. Traditional intumescent flame retardants typically consist of ammonium polyphosphate as the dehydrating agent, pentaerythritol as the char-forming agent, and melamine as the blowing agent (Grancaric et al. 2015; Liu et al. 2020). Compared with non-intumescent flame retardant coatings, intumescent flame retardants are increasingly preferred due to their superior performance characteristics and low cost.

There have been numerous studies on nitrogen and phosphorus-based flame retardants. To effectively reduce the release of smoke particles, the presence of a thick char layer as a physical barrier is essential (Tong et al. 2023). This protective char layer not only hinders rapid pyrolysis of the underlying wood and reduces the formation of gaseous products (Liu et al. 2023), but it also restricts the transfer of smoke precursors or particles (Giraud et al. 2013). Many compounds are used in combination with phytic acid to enhance smoke suppression efficiency by improving the stability of char residues (Zhang et al. 2022). Li et al. (2021) incorporated hydrolyzed collagen (HC) and glycerol (GL) with phytic acid in the wood cell cavity to create an expansive flame-retardant system, resulting in a significant increase in the residue char of the flame-retardant wood. Furthermore, the introduction of heteroatoms to form cross-linked structures in residues char has improved the flame retardancy and smoke suppression properties of wood composites (Wang et al. 2021). Yang et al. (2013) developed a phytic acid (PA)-silica hybrid system. The phytate-silica hybrid sol (PS) facilitated the generation of coherent and cross-linked residues char during combustion. The results showed that the total heat release and total smoke release of the PS/wood composite were 49.8% and 81.2%, which were lower than those of untreated wood, respectively. The flame retardant efficiency of the aforementioned system is relatively low, however, requiring a high flame retardant load (up to 51%) to meet the desired flame retardant performance, which significantly affects the practical application of phytic acid-based flame retardants (Dhineshbabu et al. 2014). The limited enhancement effect of the flame retardant system on the stability of residues char is the main reason behind its poor flame retardant effect.

Most studies have shown that nitrogen and phosphorus synergies exist in expansive flame retardants, but the effect of the ratio of the two on the synergies has not been considered. Only a few studies have shown that the different ratio of char source and acid source in intumescent flame retardants have different effects on flame retardancy (Wang et al. 2022). More importantly, to obtain good flame retardant properties, it is usually preferred to incorporate strong acid flame retardants into the wood matrix, but the penetration of strong acid phosphorous flame retardants into the wood structure may damage the complete structure of the wood (Cheng et al. 2015). It may also affect the mechanical properties of the wood. This inspired the authors to change the ratio of acid source (P) and gas source (N), and dip N and P co-flame retardant into wood cells to enhance N and P co-action.

This study explored the effect of the additional amount of air source and acid source on the flame retardant performance of intumescent flame retardants. The experiments incorporated bio-based organic acid PA as dehydrating agent, six char rings on phytic acid groups as a char forming agent, and urea (GL) as blowing agent. This combination was used to prepare a water-soluble nitrogen and phosphorus flame retardant through the complex reaction. These flame retardants were dissolved in water. Negative pressure vacuum impregnation was used to prepare environmentally friendly flame retardant wood. The smoke suppression, thermal stability, and combustion behavior of environmentally friendly flame retardant wood were analyzed, and the flame retardant mechanism of condensed phase and gas phase was explored. This environmentally friendly flame retardant system will provide a new way for sustainable and simple flame retardant treatment of wood.



Poplar wood were collected from Fuyang Forest Farm, Anhui Province (Variety: poplar cultivar ‘Jianghuai No.1′, age 7 years, air-dried wood; the 3.3 to 5.3 m section was selected).

Phytic acid (PA, C6H18O24P6, 50% aqueous solution), ethanol (C2H6O, 99.7%), and urea (CH4N2H, 99%) were purchased from McLean Company (Shanghai, China).

Preparation of FR-PAN and Modified Wood

Preparation of flame retardants

A certain amount of 50% phytic acid solution was heated while stirring magnetically in a 250 mL three-neck flask at a speed of 300 r/min, using an oil bath at 120 °C. A reflux device was installed to achieve uniform heat distribution and prevent the reagent from drying. When the temperature of the system reached 60 °C, 60.06 g of urea was added (molar ratio of PA to urea =1:3, 1:6, 1:10, 1:12). It was reacted at 120 °C for 5 h to obtain a yellow oily transparent liquid. Then it was cooled to room temperature and frozen at -80 °C for 24 h. It was freeze-dried at -80 °C for 24 h to obtain white flame-retardant-PAN (FR-PAN-3, FR-PAN-6, FR-PAN-10, FR-PAN-12). The term FR-PAN-3 stands for PA-based NP flame retardant with a molar ratio of PA to urea of 1:3.

Dipping of poplar specimens

The wood sample was dried at 103 °C for 8 h until it reached a constant mass (m0). The wood was impregnated with 20% PA, FR-PAN-3, FR-PAN-6, FR-PAN-10, and FR-PAN-12 solutions under vacuum at -0.1 MPa for 2 h, then dried in an oven at 103 °C for 24 h to a constant weight (m). Treated and untreated wood was dried and balanced at the same temperature (25 ± 2 °C) and humidity (65 ± 3%) for a week. The weight gain rate of the poplar sample was calculated using Eq. 1 (Chen et al. 2021).

WPG(%) = (m m0)/m0∗ 100% (1)

Fig. 1. Preparation process

Fire Retardancy of the FR-PAN Modified Wood

The limiting oxygen index (LOI) of 10 samples in each group was tested, and the results were averaged. The samples were tested according to the GB/T2406.2-2009 standard. The sample size was 130 mm × 6.5 mm × 3 mm. For the smoke density test (SDR) according to GB/T8627-2007 standard, the sample size was 25 mm × 25 mm × 5 mm, and each group has 15 replicates. For the cone calorimetry test (CCT), the cone calorimeter (FTT00007, FTT, UK) was used to measure the combustion of the treated wood according to ISO5660-2, the sample size was is 100 mm × 100 mm × 20 mm, and radiation power was is 50 kW/m2 . The indexes including heat release rate (HRR), total heat release (THR), smoke release rate (SPR), total smoke release (TSR), fire spread index (FGI), and residual mass (Residue) were taken into consideration.

Thermogravimetric and Thermodynamic Analysis

To explore the influence of FR-PAN on the thermal degradation process of wood, three different heating rates were employed under the N2 atmosphere, and its activation energy was calculated according to the Kissinger method to study its degradation changes. The thermal degradation behavior of flame retardants was measured from 30 to 700 °C using a thermogravimetric analyzer (TGA Q5000, TA Instruments, USA) under N2 atmosphere at heating rates of 5, 10, or 20 °C /min. The activation energy (Ea) required for the decomposition of the wood sample was calculated from the TG curve (5, 10, 20 °C/min) according to Eq. 2 (Hendrix et al. 1972),


where A and Tpi are the characteristic temperatures corresponding to the frequency factor and the maximum weight loss rate, respectively. R is the gas constant and β is the heating rate. For different heating rates, the activation energy Ea and frequency factor A are obtained by the slope and intercept of the curve for 1/Tpi.

Residue Char Analysis

The residual char was used after the CCT. After 20 s of gold plating, the surface 5 mm char layer was observed in cross-section with SEM. The surface morphology of the sample was observed by cold field emission scanning electron microscope (Hitachi S-4800, Hitachi). X-ray photoelectron spectroscopy (XPS) was conducted through an XSAM 800 spectrometer (Kratos Co., Stretford, UK =1486.6eV), using Al Kα excitation radiation, 12 kV operating voltage, and 15 mA working current selected for this test. A Raman microscope (Renishaw in Via, UK) excited at 514.5 nm was used to measure the peak cumulative area ratio R at 1350 cm−1 (peak D) and 1580 cm−1 (peak G). The smaller the R-value, the higher the graphitization degree of the residual char. Six kinds of residue char powder were laid in the grooves of monocrystal silicon sheet and tested by polycrystalline X-ray diffractometer (XRD-6, Beijing Spectrometer). The operating voltage of the instrument was 36 kV, the current was 20 mA, and the scanning range was 2θ=5° to 70°. The scanning speed was 4°/min. The degree of graphitization was calculated using CuKa1 radiation (λ= 0.15406 nm) and according to the Mering and Maire equations (Eqs. 3 and 4). The distance d(002) can be calculated from Bragg Eq. 4,



where 0.3440 nm is the layer spacing of a completely ungraphitized char layer, and 0.3354 nm is the ideal layer spacing of graphite crystals. The quantity d(002) is the layer spacing of the main characteristic peak of graphite (002 crystal face). λ is the incident wavelength, θ is the diffraction angle of the 002-crystal face, and the g value indicates the degree of graphitization of this char layer.


Fire Retardancy

LOI and SDR analysis

The fire safety of poplar treated with FR-PAN was assessed by conducting tests of LOI and SDR. Table 1 presents the impact of FR-PAN on the LOI value and SDR of Poplar. The untreated group exhibited an LOI value of only 18.6%, indicating high flammability and emphasizing the necessity for flame retardant treatment. The introduction of phosphorus and nitrogen in PAN-6 groups resulted in a significant increase in LOI to 49.6%, representing a remarkable enhancement of 172% compared to the untreated group. Table 1 reveals that PA treatment led to a smoke density grade of 43.5. However, incorporation of urea reduced smoke emission, as evidenced by the lower smoke density grade (32.4) observed in the PAN-6 group, demonstrating a reduction by 72% compared to PA treated. The findings highlight the synergistic effect between phytic acid and urea, wherein the introduction of urea significantly improves poplar’s flame retardancy while inhibiting smoke release. However, excessive addition of urea can lead to increased smoke release. It is well known that the smoke is very harmful to personnel safety in a fire, and the PAN-6 group shows a lower SDR, which will reduce the harm of smoke to personnel.

Table 1. Weight Gain Rates of the Modified Wood and their LOI and SDR

CCT Analysis

CONE testing is commonly employed for laboratory-scale evaluation of fire parameters and is widely acknowledged as an effective method for regulating the burning behavior of real fires. During the test, various parameters were determined (refer to Fig. 2): Heat release rate (HRR), Total heat release (THR), Total smoke rate (TSR), Smoke production rate (SPR), and residual mass, with corresponding data listed in Table 2. As depicted in Fig. 2, at the initial stage of the test, the untreated wood group exhibited a sharp increase in HRR due to surface wood combustion. After 200 s, a char layer formed on the surface leading to isolation of oxygen and heat, resulting in a constant HRR value. At around 480 s, rupture of the surface char layer caused renewed pyrolysis from internal wood components, reaching a PHRR of 372 kW/m2 at 569s (Table 2). After 800 s, the heat release decreased because the wood smoldered, and the final char residue was 5.9%.

As shown in Fig. 2 (b), it is evident that the flame retardant treatment significantly altered the THR and TSR of poplar wood. The addition of urea in PAN-6 group resulted in a remarkable reduction of 57.1% and 82% in THR compared to untreated group and PA treated group, respectively, indicating the effective suppression of heat release by urea addition. Table 2 demonstrates a trend where the THR decreased with increasing urea content. This can be attributed to the molten state of urea at temperatures around 150 °C due to its polar nature, which facilitates enhanced contact between ammonium phytate and cellulose/hemicelluloses (Cheng et al. 2022). At temperatures above 160 °C, decomposition reactions occur, leading to ammonia release from urea decomposition as well as pyrophosphoric acid formation from FR-PAN degradation catalyzing dehydration reactions converting cellulose/hemicellulaceous residues. Additionally, pyrophosphoric acid promotes expansive six-char ring structures on phytic acid groups, effectively providing thermal insulation properties while inhibiting smoke emission and toxic gas production (Hendrix et al. 1972). The wood combustion process begins at the ignition point temperature, heat accumulation was necessary to reach the ignition point temperature, and a lower THR will reduce the fire hazard. The PAN-6 group exhibited an impressive final residue char amount, that was about 4.9 times higher than that observed for untreated group at the end of the experiment (32.4%). The increase of residual char not only means a decrease in total heat release, but also a continuous and dense char layer has a certain mechanical strength, which can delay or even change the collapse of indoor furniture and building structures, which was very important in fire safety. These findings demonstrate excellent flame retardancy performance achieved through FR-PAN-6 treatment.

Fig. 2. HRR(a), THR(b), TSR(c), and SPR(d) plots of wood samples

Table 2. Combustion Parameters of Modified Wood Samples

Note: PHRR (peak of heat release rate)

As shown in Fig. 2 (d), the SPR of the treated group was significantly reduced compared to untreated and PA-treated groups. Specifically, the SPR of the PAN-6 group was 0.0081m2/s, which was 50.3% and 42.95% lower than that of untreated and PA treated group. Furthermore, Fig. 2 (c) illustrates that TSR of PAN-6 group decreased by 80.0% and 30.8% compared to untreated and PA treated group. The addition of urea improved the smoke suppression performance of flame-retardant treated group by decreasing the PHRR value initially and then increasing it later. When there was excess urea in the system, urea decomposes at 170 ℃ to generate NH3 and HCNO (Eq. 5) (Ana et al. 2015), and ammonia expands the char-containing char layer, which can form a physical barrier to isolate the entry of oxygen and heat. This will protect the substrate and reduce heat release. On the other hand, the HCNO generated reacts with the hydroxyl groups in cellulose and hemicellulose to form a carbamate group (Eq. 6) (Geake et al. 2020), which causes a decrease in the crystallinity of cellulose and accelerates the decomposition of glycosyl units (Hassan et al. 2010). This results in reduced heat release and smoke release. When the urea content was excessive, the expansive structure was destroyed, and the thermal and oxygen insulation ability of the surface char layer was reduced, resulting in the reduction of flame retardant and smoke suppression performance. Overall, when considering THR and TSR, treated groups with added urea demonstrate superior flame retardant properties compared to both untreated samples and those treated with PA alone. Notably, a molar ratio of phytic acid to urea at 1:6 resulted in an expansion structure that efficiently suppressed smoke generation while minimizing heat release.

CO(NH2)2 → NH3 ↑+HCNO (5)

R1-N=C=O+R2-OH → R1-NH-CO-O-R2 (6)

Based on this mechanism, Fig. 3 shows the plot of THR relative to mass loss, as they relate to the basic properties of the group such as combustion efficiency and effective heat of combustion of volatiles. There was a linear fit to the mean value, and the slope of the poplar curve treated with phytic acid and ammonium phytate changed significantly. This effect may be related to the gas phase effect, while the large decrease in the mass loss value indicates the flame retardant effect of the condensed phase. The effective combination of gas-solid flame retardants was mainly attributed to the P-N synergistic effect of the FR-PAN. The corresponding parameters are shown in the table, among which the slope of PAN-6 group was the smallest, which means that its P-N synergistic effect was the best.

Fig. 3. Fitting of moss loss and total heat release of the FR-PAN modified wood

TGA and Thermodynamic Analysis

The pyrolysis process of poplar wood can be observed in Fig. 4 (a). As shown, the process can be divided into three stages. The first stage (30 to 237 °C) involved evaporation of water and heat-sensitive volatiles from the poplar wood, resulting in a mass loss of 4%. During the second stage (237 to 394 °C), approximately 70% mass loss took place as cellulose and hemicellulose decompose extensively, forming small molecular organic matter (Xiong et al. 2019). The third stage (394 to 700 °C) exhibited a mass loss of about 11%, indicating completion of half fiber and cellulose decomposition. Lignin primarily decomposes into aldehydes and forms char at this stage, resulting in a final residue char level of 15.0%.

As shown in Fig. 4(b), the degradation process of treated groups also can be categorized into three stages. In the initial stage (30 to 150 °C), there was a mass loss of 5%, mainly attributed to water volatilization and release of heat-sensitive substances. The second stage (150 to 290 °C) experienced an approximate mass loss of 45%, with an initial decomposition temperature falling between untreated wood and PA-treated group. The initial decomposition temperature of the untreated group in Table 3 exhibited an initial increase followed by a decrease upon addition of urea, with PAN-6 group displaying the highest maximum initial decomposition temperature at 170.4 °C. This higher temperature suggests an improved thermal stability of the treated group.

In Fig. 4 (b), a distinct peak was observed at 190 °C on the DTG curve, which differed from both the untreated and PA-treated groups. Within this temperature range, heat absorption by the ammonium phosphate group leads to the generation of ammonium dihydrogen phosphate, while ammonium pyrophosphate releases non-combustible gases such as CO2, NH3, and water vapor (Chen et al. 2010). These sequential reactions inhibit combustible gas production. Furthermore, the DTG curve for the PAN-6, PAN-10, and PAN-12 groups showed strong peaks at 258, 253, and 249 °C, respectively, indicating the phosphorylation reaction on C-6 hydroxyl groups of dehydrated glucose (Puziy et al. 2006). During the third stage (290 to 700 °C), a mass loss of approximately 19% occurs primarily due to lignin degradation and char formation. Among all the five flame retardant treated wood examined here, PAN-6 group exhibited a residue char level of about 41.6% at 700 °C representing the highest value thereby indicating an enhanced thermal stability of its residue char. As shown in Table 3, with increasing urea concentration there is a gradual decrease in activation energy during thermal decomposition for treated groups. This trend can be attributed to high urea concentrations. After calculation, it was found that the amount of urea added had a certain effect on the activation energy of the reaction, and when the urea content was excessive, it would cause the activation energy to decrease, which was consistent with the findings of Nam et al. (2011).

The above discussion demonstrates that the flame retardancy and thermal stability of treated groups were significantly enhanced compared to untreated and PA-treated group, which can be attributed to the incorporation of urea.

Fig. 4. Thermal behavior of untreated and treated wood, TG (a) and DTG (b) curves

Table 3. Thermogravimetric Data and Thermodynamic Parameters of Wood Samples in N2

Note: Ea = reaction activation energy, unit KJ/mol

T5% = initial decomposition temperature, unit °C

Percentage of residue char at 700 °C, unit %

Tmax = The characteristic temperature corresponds rate, unit °C

Residual Char Graphitization Analysis

SEM analysis

As shown in Fig. 5, the action mechanism of flame retardants, the residue char in wood samples after combustion was analyzed. Figure 5 presents the residual cell wall thickness of both treated and untreated wood samples. When fully burned, the untreated group exhibited minimal residual cell wall thickness of only 0.9 um. In contrast, the PAN-6 group formed a protective expansion structure during combustion that restricted heat and gas transfer and prevented rapid pyrolysis within the wood, resulting in highly graphitized residue char production. The residual cell wall thickness measured 2.09 um for treated group, which was twice that observed in untreated group. There was a relationship between urea addition and residual cell wall thickness in the treatment group. Specifically, there was pattern of an initial increase followed by a decrease, consistent with CCT results.

Fig. 5. Morphology of residues char obtained from CCT. a, b, and c represent the control, PA, and PAN-3 group, and d, e, and f represent PAN-6, PAN-10, and PAN-12 group. (Insert: Residual thickness of the cell wall).

Raman and XRD Analysis

Raman spectroscopy was employed to evaluate char graphitization degree as more graphite structures within char residues indicate higher thermal stability (Lin et al. 2018). Figure 6 displays two prominent peaks observed at 1350 cm-1 (disordered char structure or D-band) and 1580 cm-1 (graphitized char structure or G-band). The peak area ratio (R=ID/IG) between D-band and G-band is commonly used as an indicator for graphitization degree assessment. As shown in Fig. 6, for untreated group, ID/IG value measured at 3.32. In comparison, the flame-retarded group exhibited a significantly reduced ID/IG value (1.91), suggesting that FR-PAN contributes to highly graphitized char formation. Table 4 illustrates ID/IG values for both treated and untreated group. The values followed a trend similar to scanning electron microscopy rule regarding residual cell wall thickness.

Table 4. Residual Char Graphitization Parameters

The 2θ values of the untreated and treated groups, as presented in Table 4, are recorded as follows: 22.58°, 23.02°, 23.12°, 23.14°, 23.56°, 23.54°, and 22.68° respectively. Correspondingly, the d(002) values for these groups were measured as: 0410, 0.392, 0.385, 0.384, and 0.375 nm, respectively. The g values obtained for both treated and untreated group were found to be -5.74, -4.89, -4.77, -4.60, -3.87, and -3.91, respectively. It has been shown that FR-PAN can contribute to the formation of highly graphitized char. Additionally, the graphitization degree of residue char initially increased with the addition of urea but then decreased. This trend is consistent with the findings from laser Raman testing.

Fig. 6. Raman spectra of different coal and coke residues. a, b, and c represent the control, PA, PAN-3 group, and d, e, f represent PAN-6, PAN-10, PAN-12 group.

XPS Analysis

Elemental composition analysis was conducted using the XPS technique. Figure 7 displays high-resolution XPS spectra of C1S. Peak assignments along with their corresponding contributions can be found in Table 5. Notably, the C content in untreated wood was observed to be higher than that in treated group because only the black characeous portion of the residue was sampled. Moreover, a significant increase in peak intensity at around 285.2 to 285.8 eV was observed after flame retardant treatment. This suggests a greater presence of C-P/C-N/C-O-P structures being formed (Wang et al. 2020). Comparatively, the concentration of C-P/C-N/C-O-P within PAN-6 group char residues (19.0%) exceeded that present in untreated wood (16.5%). This change in C1s peak confirms the introduction of phosphorus into the char skeleton during combustion (Freitas 2006).

In the O1S spectrum, P=O, which denotes the existence of polyphosphates in the residues char, can be seen in the O1S spectrum. P-O-C, which denotes the creation of cross-linked structures, can be seen in the O1S spectra of the flame-retardant treated group. P=O (13.6%) and C-O-P (6.2%) concentrations in the residues char of the PAN-6 group were much greater than those of the untreated groups. As shown in Table 5, the matching peaks of pyrrole/pyridinium nitrogen (N-5) and pyridinium nitrogen (N-6) occurred in the XPS energy spectra of the residues char following flame retardant treatment (Théotime et al. 2022). N-Q concentration in the coke of groups treated with flame retardants (2.67%) was significantly greater than that of the residue char of PA treated group (0.86%), which indicates that more N is injected into the graphite structure. Nitrogen doping in the char structure considerably improves the stability of the residue char.

Fig. 7. XPS scans of different residues char at CCT

The P-C/P-O/P=O structure and P2O5 showed peaks in the P2P spectrum (Table 5) at around 134 and 135 eV, respectively. Compared to untreated group and PA treated group, treated group has a higher P-C/P-O/P=O structural content (Théotime et al. 2023). The amount of P (88.1%) in the residual char was accounted for by the P-C/P-O/P = O structure present in the treated group char in the PAN-6 group, which had a 3.97% content. The amount of untreated group residue char was less than 0.5%. High stability the PAN-6 group charring layer can considerably increase wood flame resistance (Yu et al. 2021). In general, the addition of P and N elements during the combustion of flame-retardant-treated group can significantly enhance the flame-retardant performance. The highest content and best residual char stability were produced when the molar ratio of phytic acid to urea was 1:6, which resulted in the formation of P = O, P-C/P-O/P=O, and other hybrids.

The introduction of phytic acid in the combustion process led to the successive generation of metaphosphoric acid, phosphoric acid, polymetaphosphoric acid, and pyrophosphoric acid. Finally the glycan unit is dehydrated into charcoal under the action of phosphorus-containing compounds to generate polyphosphate (Fig. 7), and P and N are introduced into the char layer (Maoyuan et al. 2019; Li et al. 2020). Doping P and N can improve the thermal oxidation stability of the char layer. The doping of heteroatoms reduces the reactivity of the char active center (Liew et al. 2016). It also plays a role in inhibiting the gasification of the char layer, enhances the structural integrity of the char layer, and promotes the formation of porous thermal insulation char layer in the condensed phase, thereby enhancing the flame retardancy and smoke suppression. The application of adsorption, filtration and indoor insulation groups will be expanded.

Table 5. Peak Assignments for C1S, O1S, N1S, and P2P Spectra of Different Coke Char Residues Obtained from CCT

*The data were analysed by Avantage software

Flame-Retardant Mechanism

According to the above analysis, the flame retardant mechanism of FR-PAN may be summarized as follows. In the combustion process, FR-PAN is decomposed by heat to produce NH3 and H2O, which will dilute the combustible gas in the gas phase. In addition, the phytic acid group will also generate P·and PO·radicals, which will inhibit the combustion chain reaction. When urea is present in the flame retardant system, urea becomes molten when heated at 150 ℃ to 170 ℃, promotes the contact between FR-PAN and hemicellulose and cellulose, and accelerates the phosphoric acid to catalyzed the dehydration of hemicellulose and cellulose into char. In addition, NH3 and HCNO produced by the decomposition of urea, and the NH3 will promote the formation of a char layer with an expansive structure, which will contribute a good flame retardant effect in the condensed phase. The dehydration will reduce the crystallinity of cellulose and promote the decomposition of cellulose. Finally, nitrogen and phosphorus are introduced into the char layer, and the addition of heteroatoms will improve the thermal oxidation stability of the char layer, which can effectively inhibit the release of combustibles and smoke. The results showed that when the molar ratio of phytic acid to urea was 1:6, the flame retardancy and smoke suppression performance of this treated group was the best.