Preparation of flame-retardant cellulose paper via spray coating with lignin, phytic acid, and sodium silicate

Kweon, S. W., Lee, Y. J., Lee, T.-J., and Kim, H. J. (2025). "Preparation of flame-retardant cellulose paper via spray coating with lignin, phytic acid, and sodium silicate," BioResources 20(2), 3808–3825.

Abstract

A flame-retardant treatment for cellulosic paper was applied by spraying the paper with combinations of lignin, phytic acid, and sodium silicate. Lignin enhanced the flame retardancy in the condensed phase, while phytic acid provided dual-phase flame resistance. Sodium silicate further improved thermal stability by forming silica gel through its reaction with phytic acid. The limiting oxygen index increased from 16.8% to 22.0%, and in vertical flame tests, treated paper self-extinguished within 1.5 s, whereas untreated paper burned completely in 12.0 s. Thermogravimetric analysis revealed enhanced thermal stability, with treated paper retaining 36.5% residual char at 900 °C compared to 0% in untreated paper. Despite the relatively low coating coverage from the spray deposition method, the synergistic interaction of phytic acid, lignin, and silica gel effectively compensated by promoting dense char formation and thermal insulation. Fire retardancy was attributed to phytic acid-catalyzed lignin and silica composite formation in the char layer, enhancing structural stability and shielding efficiency. Raman spectroscopy confirmed improved graphitization (I_D/I_G = 1.20), while scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX) and Fourier transform infrared spectroscopy (FT-IR) verified phosphorus and silica retention. This treatment showed potential for high-performance flame-retardant cellulose materials in packaging, construction, and other industries.

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**Preparation of Flame-Retardant Cellulose Paper via Spray Coating with Lignin, Phytic Acid, and Sodium Silicate**

Soon Wan Kweon , Yong Ju Lee, Tai-Ju Lee,* and Hyoung Jin Kim *

DOI: 10.15376/biores.20.2.3808-3825

Keywords: Cellulose; Flame retardancy; Condensed phase; Gas phase; Spray coating; Self-extinguishing; Sulfonated kraft lignin; Sodium silicate; Silica gel

Contact information: Department of Forest Products and Biotechnology, Kookmin University, 77 Jeongneung-ro, Seongbuk-gu, Seoul 02707 Republic of Korea;

* Corresponding authors: leetj@kookmin.ac.kr, hyjikim@kookmin.ac.kr

Graphical Abstract

INTRODUCTION

Cellulose derived from wood is the most abundant natural polymer (Figueiredo et al. 2010). Wood-based cellulose has served as a primary raw material in the pulp and paper industry, contributing to the production of biodegradable, recyclable, and cost-effective paper materials (Laftah and Wan 2016). However, the highly flammable nature of cellulosic fibers hinders their broad application (Saba et al. 2016). Paper-based products show high flammability even after the incorporation of flame-retardant additives into the bulk of the fiber wall, either during the paper-making process or as a posttreatment of paper (Köklükaya et al. 2015). The most widely used flame-retardant additives contain elements such as phosphorus and halogens or inorganic compounds such as silicates, hydroxides, and hydrotalcites (Nassar et al. 1999; Katović et al. 2009; Zheng et al. 2019; Jeong et al. 2022). Generally, the flame retardancy of polymeric materials can be improved through the incorporation of various flame-retardant systems that act in the condensed phase, gas phase, or both. Figure 1 shows the mechanisms of flame retardance in the condensed and gas phases. In the condensed phase, flame retardants function by forming a thermally insulating char layer to protect the underlying material from fire. This char layer acts as a barrier, inhibiting the transmission of heat to the underlying polymer matrix. It also retards the release of combustible gases, which act as fuels in the combustion zone, during degradation (Yang et al. 2020).

Fig. 1. Combustion model for a flame-retardant material

Lignin, an abundant polyphenolic compound derived from biomass, particularly plant fibers, has garnered considerable interest in multiple areas. It possesses notable thermal stability, with a peak degradation temperature of approximately 350 °C in a nitrogen atmosphere (Costes et al. 2017). The degradation temperature of lignin can vary depending on several factors, including its molecular weight, functional group composition, and the presence of linkages such as β–O–4 ether bonds, which are more susceptible to thermal cleavage (Fan et al. 2017; Prieur et al. 2017). Additionally, its aromatic chemical structure contributes to a high char yield after decomposition, which reaches approximately 40% at 900 °C (Ferry et al. 2015). These properties make lignin a promising flame-retardant additive. At 250 °C, weak bonds such as the β–O–4 ether linkages in lignin are cleaved, and phenolic radicals are continuously produced. These phenolic radicals may further fragment to form other small compounds or radicals or participate in repolymerization via free-radical reactions (Fan et al. 2017). At the same time, the cleaving of strong bonds in lignin, such as the C–C bonds, with the removal of most functional groups results in the carbonization of the remaining aromatic rings in the char and the formation of a condensed ring network (Mousavioun et al. 2010; Song et al. 2011; Bertini et al. 2012; Ferry et al. 2017; Zhang et al. 2021). Flame-retardant effects in the gas phase are realized through the generation of active species that can act as free-radical scavengers in the combustion zone (Yang et al. 2020). In particular, polymeric materials such as halogen- or phosphorus-containing compounds can interrupt the combustion reaction of flammable volatiles in the gas phase.

Phytic acid (PA) is a phosphate-rich compound primarily derived from nuts, oilseeds, and cereals, where it serves as the main storage form of phosphorus in plant seeds, playing a crucial role in seed development and germination by providing essential nutrients (Reddy et al. 1982; Graf and Eaton 1990). It consists of six phosphate groups attached to a six-carbon ring, as shown in Fig. 2. In the condensed phase, PA produces a flame-retardant effect through two key mechanisms. First, acids generated during the pyrolysis of PA facilitate the swift dehydration and carbonization of oxygen-containing polymeric materials, resulting in the formation of a dense coke layer. This coke layer acts as an effective barrier, inhibiting the release of oxygen, heat, and volatile combustible gases. Second, PA transforms into polyphosphoric acid, which is a highly thermally stable and nonvolatile flame-retardant material. Polyphosphoric acid covers the surface of the polymer material, promoting the consolidation of the carbon layer and further improving flame retardance (Kong et al. 2020; Yuan et al. 2021; Zhang et al. 2021; Liu et al. 2023). In the gas phase, PA decomposes and generates phosphorus-containing radicals, such as HPO−, PO−, and HPO₂−. These radicals can scavenge reactive HO and H radicals formed during the combustion of the polymeric material, suppressing the combustion chain reaction and thereby reducing the flammability of the substrate (Liu et al. 2018; Cheng et al. 2020; Liu et al. 2023).

Fig. 2. Structural formula of phytic acid

Inorganic silica (SiO₂), known for its distinctive layered structure, is abundantly available in nature and has a chemical structure allowing intercalation. This makes it an economically advantageous material for the production of polymer nanocomposites. SiO₂ is commonly synthesized from a sodium silicate (Na₂SiO₃, water glass) precursor solution via the sol–gel method, which requires an acid (Niu et al. 2014). Low pH results in the formation of silicic acid, which further condensates/polymerizes to form a solid SiO₂ gel (Lin et al. 2023). Additionally, it has been found that the presence of silicon in flame retardants can improve the thermal stability of the char layer formed during combustion (Ni et al. 2011).

Studies on the manufacturing of eco-friendly flame-retardant composite materials have been steadily increasing in number in recent years. Liu et al. (2018) enhanced the flame resistance of lyocell fibers through the application of PA–ammonium, resulting in an increase in the limiting oxygen index (LOI) value. Xu et al. (2019) explored the flame-retardant properties of ammonium phosphite-treated paper, reporting a reduction in char length from 210 to 45 mm. Furthermore, thermogravimetric analysis (TGA) revealed that the treated samples had a lower initial decomposition temperature and yielded more residue than the untreated samples. Chen et al. (2021) utilized PA modified with dicyandiamide to improve the flame retardancy of cellulosic paper. Notably, at a concentration of the modified PA of 20%, the char length of the paper decreased from 12.5 to 4.1 cm, accompanied by a remarkable increase in the LOI value, from 19.6% to 41.5%. In other studies, Li et al. (2019) examined flame-retardant wood nanofibers containing lignin, and Lin et al. (2023) investigated the potential of a PA–silica hybrid system as a flame retardant.

The objective of the present study was to impart flame retardance to paper by spray coating it with a system prepared from lignin, PA, and sodium silicate. Specifically, this study investigates how PA facilitates the formation of thermally stable silica gels from sodium silicate and how this process, in conjunction with the char-forming properties of lignin, contributes to flame retardance. By clarifying these mechanisms, the present work provides a novel approach to integrating bio-based and inorganic components for the development of sustainable and efficient flame-retardant solutions.

EXPERIMENTAL

Materials

The Quantitative filter paper (mass per unit area: 80 g/m²) obtained from Hangzhou Whatman-Xinhua Filter Paper Co., Ltd. (China) was used as base paper. All the paper samples were dried at 105 °C for 24 h before use. Filter paper was selected because other types of wood pulp contain small amounts of lignin, which forms a lignin matrix. In such cases, it is challenging to effectively embed flame-retardant polymers within the fiber structure (Kalia et al. 2011; Li et al. 2019).

Sulfonated kraft lignin (average M_w ∼10,000, 4% sulfur, pH 10.5, soluble in water, particle size 125 to 180 μm) was purchased from Sigma-Aldrich (Stockholm, Sweden). Sulfonated kraft lignin is produced through a hot alkaline (sulfate) process and then modified via sulfonation. A 50% (w/w) aqueous solution of PA was also purchased from Sigma-Aldrich. Technical-grade aqueous solution of sodium silicate (27.6% SiO₂ and 8.5% Na₂O) was acquired from Daejung Chemical Co. (Republic of Korea).

Preparation of the Flame-Retardant Paper

The flame-retardant coating solution was diluted to a concentration of 10% with deionized water and spray-coated on both sides of the base paper 15 times, as shown in Fig. 3. Table 1 lists the compositions of the coating solutions. The control sample that was not spray-coated was named C, the sample treated only with sodium silicate was named S, the sample treated only with lignin was named L, the sample treated with lignin and PA was named LP, and the sample treated with PA and sodium silicate was named PS. The sample named LPS was prepared through a sequential spray-coating process, involving an initial coating of lignin followed by a separate coating with a mixture of phytic acid and sodium silicate. This method was designed to promote optimal interaction between the components, enhancing the synergistic flame-retardant effect.

The compositions of the flame-retardant coating solutions were determined in advance based on the pH of the mixture, to maintain the pH at 6.5 to 7.0. In the preliminary experiment, where the paper was treated only with PA, the strength of the sheet decreased, and it became brittle because of the acidity of PA. The coated papers were dried at 110 °C for 5 min using an automatic sheet press (HANTECH, Korea) at a pressure of 410 ± 10 kPa. The manufacturing process employed herein for the production of flame-retardant paper is schematically shown in Fig. 3.

Table 1. Formulations of Flame-Retardant Coatings

* two-step (L, PS) process employed

Fig. 3. Schematic of the manufacturing of flame-retardant paper

Characterization and Tests

Flammability test

Herein, flame retardance was evaluated using LOI and UL-94 tests. LOI is defined as the minimum oxygen concentration (vol%) required to sustain the continuous combustion of a material in a flowing mixture of oxygen and nitrogen for 3 min or until 5 cm of the sample is consumed (Yang et al. 2020). The LOI value was calculated using Eq. 1. In the LOI test, the sample was positioned vertically at the center of a glass chimney and ignited with a burner placed above the sample. Five tests were performed for each sample type to obtain reliable results. A higher LOI value indicates better flame retardance.

(1)

The vertical flame test was conducted according to the ASTM D 3801–19 standard, which aligns with the UL-94 test method for evaluating the flammability of materials. The vertical flammability test is commonly used for evaluating the flammability of paper (Chen et al. 2021). In particular, the UL-94 vertical burning test is the most frequently used test for measuring the ignitability and flame spread of materials (Yang et al. 2020). Herein, samples were tested according to an international standard, IEC 60695-11-10 (2013), with five tests performed in parallel. The samples used for this test had dimensions of 125 mm × 13 mm. In the results of this test, afterflame time indicates the duration of flame application, and afterglow time is the duration required for the fire glow to completely disappear.

Scanning electron microscopy (SEM)-energy dispersive x-ray (EDX) analysis

The surface morphologies and the elemental distribution of the uncoated reference and spray-coated paper samples were observed using SEM-EDX (JSM 7401F, JEOL Ltd., Japan) after sputtering with Pt. The acceleration voltage of the electron beam was set to 5 kV for SEM and 10 kV for EDX.

Thermogravimetric analysis (TGA)

The thermal stabilities of the uncoated reference and spray-coated paper samples were investigated using TGA (Perkin Elmer TGA8000TM, USA). The sample mass was 5 g, and the measurements were performed at a constant heating rate of 20 °C/min from 40 °C to 900 °C in N₂ supplied at a flow rate of 20 mL/min. The differential thermogravimetry (DTG) curves were obtained from TGA curves using R software (R Core Team, ver. 4.3.0, Auckland, New Zealand), with smoothing performed using a smoothing algorithm.

Attenuated total reflection infrared (ATR-IR) spectroscopy

The chemical structures of the uncoated reference and spray-coated insulating materials were investigated using ATR-IR spectroscopy (Nicolet iS50, Thermo Fisher Scientific, USA). Each spectrum was recorded in the range of 4,000 to 400 cm⁻¹ at a resolution of 4 cm⁻¹, employing 32 scans. The absorption of air was recorded and used as the reference standard.

Raman spectroscopy analysis

The char residues of the uncoated reference and spray-coated insulating materials were determined using Raman spectroscopy (LabRam Soleil, HORIBA Scientific, France). Each spectrum was obtained in the backscattering geometry configuration using a 532 nm argon laser in the scan range of 300 to 3000 cm⁻¹.

RESULTS AND DISCUSSION

Flame-Retardant Behavior Analysis

Flammability analysis

Figure 4 shows the measured LOI. LOI has been considered an important indicator of the flammability of paper (Xu et al. 2019; Chen et al. 2021; Liu et al. 2022). A higher LOI value indicates a higher O₂ concentration required to sustain combustion, suggesting higher fire resistance (Martinka et al. 2022). According to the results, the LOI of sample C was 16.8%, whereas coated samples showed higher LOI values. Sample LPS exhibited the highest LOI value (22.0%), suggesting the best flame resistance of this sample under a small flame scenario.

Although the LPS sample demonstrated superior flame retardancy, additional analysis revealed that the total coating weight (Table 1) alone did not directly correlate with the LOI values (R² = 0.42), indicating that the enhanced performance was primarily due to the synergistic effect of the combined flame-retardant components rather than the total mass of applied material. This synergistic interaction contributed significantly to the improved flame resistance, highlighting the importance of the chemical composition and component interaction rather than coating weight alone. This observation emphasizes that achieving optimal flame retardancy requires a well-designed combination of functional additives rather than relying solely on increasing coating mass.

Fig. 4. Limiting oxygen index values of the paper samples

Table 2 shows the results of the vertical flammability test, listing both afterflame time and afterglow time. For both parameters, a shorter time indicates better flame retardance.

As shown in Table 2, the reference sample C completely burned when ignited in air. Samples S and L, coated with single materials, showed better flame resistance than sample C but still exhibited low flame retardance. Samples PS and LPS, coated with two or more materials, showed improved flame resistance. In particular, sample LPS exhibited a very short afterflame time and no afterglow time, indicating a considerably decreased combustion rate and excellent flame retardance. At the same time, sample LP did not self-extinguish after ignition. This was attributed to the relatively low availability of PA, resulting in a weak gas-phase flame-retardant effect. These results suggest that achieving excellent flame retardance likely requires a synergetic effect in both the gas and condensed phases (Yang et al. 2020).

Table 2. Results of the Vertical Flammability Test of the Paper Samples

Figures 5 and 6 show digital photographs of the samples during and after the vertical flammability test, respectively. Upon exposure to flame, the uncoated sample C burned rapidly and completely, without any residual char residue. At the same time, sprayed samples exhibit notably lower rate and degree of combustion. The combustion degree was analyzed using the Free Form function in the image processing and analysis software PicMan (WaferMasters, Inc., Dublin, CA, USA). As a result, samples PS and LPS show degrees of combustion lower by approximately 95.3% and 97.4%, respectively, than the untreated sample C.

Fig. 5. Combustion behavior of paper samples

Fig. 6. Digital photographs of paper samples after UL-94 burning tests

Surface Morphology Analysis

The SEM images of samples C, PS, and LPS are shown in Fig. 7. At elevated temperatures, PA releases phosphorous/polyphosphate, which accelerates the carbonization of the fiber matrix and forms a protective char layer (Liu et al. 2023). Additionally, the reduction in pH facilitates the conversion of sodium silicate into silicic acid (Niu et al. 2014; Schlomach and Kind 2004), which subsequently polymerizes into a robust silica network. This silica gel is not only deposited on the fiber surface but also infiltrates the interior of the fiber network, reinforcing the structural integrity of the material.

Fig. 7. SEM images of the paper samples: samples (a) C, (b) PS, and (c) LPS before the vertical flammability test and samples (d) C, (e) PS, and (f) LPS after the vertical flammability test

Figure 7(d–f) shows the morphologies of samples C, PS, and LPS after the vertical flammability test. The untreated sample C burned completely, leaving a small amount of ash residue (Fig. 7(d)). In contrast, as shown in Fig. 7(e, f), samples PS and LPS retained their residual carbonized frame after combustion, maintaining their original texture and shape. This structural retention suggests that the thermally stable silica gel formed from sodium silicate effectively stabilized the char layer and improved the heat shielding properties of the material. In particular, sample LPS (Fig. 7(f)) exhibited the strongest protective effect, primarily attributed to the phosphorylation of lignin. Phosphorus improves the thermal stability of lignin, thereby accelerating dehydration and decarboxylation reactions and increasing the amount of char residue at high temperatures (Prieur et al. 2017). It follows that the combined presence of silica gel and phosphorylated lignin contributed to enhanced char formation. In summary, the formation of lignin-silica hybrids not only prevented the release of volatile decomposition products, but also enhances the residual char, which significantly improves the flame-retardant performance of the material.

Thermal Stability Analysis

The TGA curves of paper samples are shown in Fig. 8. This figure compares the weight loss and thermal stability of samples C, PS, and LPS over a temperature range of 0 to 900 °C. The reference sample (sample C) exhibited three stages of thermal decomposition. The first stage, beginning at 40 °C and concluding at 120 °C, is associated with the removal of bound water, whereas the second stage, occurring between 250 and 420 °C, corresponds to the thermal decomposition of wood components, particularly hemicellulose and cellulose (Lin et al. 2021). The third stage, occurring between 500 and 600 °C, involves further thermal decomposition of cellulose and other organic compounds.

Fig. 8. TGA and DTG curves of C, PS, and LPS

Sample C showed the highest weight loss in the second stage, at 401.4 °C, with a mass residue of 0 wt.% at 900 °C. The addition of flame retardants such as lignin, PA, and sodium silicate to cellulose considerably affected its thermal behavior, which was confirmed by a decrease in the primary decomposition temperature and an increase in the mass of residue. Samples PS and LPS decomposed at 333.1 and 320.1 °C, respectively. This was attributed primarily to the effect of PA, which promotes the formation of dense char at lower temperatures, thereby preventing further thermal decomposition of the material (Patra et al. 2020; Mokhena et al. 2022). Furthermore, the condensation of phosphate groups leads to the formation of polyphosphate, serving as a protective insulation layer (LeVan 1984). Silica gel acts as a noncombustible inert material, increasing the final char residue (Bahari et al. 2019). Notably, the mass of sample LPS stabilizes after decomposition, exhibiting the highest char residue of 36.5 wt.% at 900 °C. Although the spray-coated flame-retardant layer has a relatively low coverage compared to bulk treatments, the synergistic effect of lignin, phytic acid, and silica gel compensates for this limitation by enabling the formation of a thermally stable, dense char layer. This char effectively insulates the underlying cellulose structure, slowing down thermal degradation and enhancing overall flame resistance. These results support the conclusion that sample LPS possesses the highest thermal stability, explaining its excellent flame-retardant properties.

Flame-Retardant Mechanism and Structure Analysis

Flame-retardant mechanisms of silica and lignin–silica composites

Figure 9(a) shows the mechanism of silica formation from sodium silicate as a precursor in the PS (Lin et al. 2023). Silica acts as a physical barrier, suppressing smoldering and flaming combustion by preventing flammable materials from escaping the matrix and preventing the contact of the matrix with O₂ (Rowel 1984). Furthermore, during combustion, P radicals generated from PA effectively scavenge O and H radicals, thereby halting the progression of combustion (Liu et al. 2023). Protons derived from PA expedite the dehydration of hydroxyl groups within paper, promoting the formation of char residue. Concurrently, the release of water molecules leads to a reduction in oxygen concentration or partial pressure within the combustion zone. The dehydration of phosphate, leading to the generation of polyphosphate, fosters the formation of protective vitreous layers that impede further combustion (Lin et al. 2023).

Figure 9(b) shows the reaction scheme of the formation of lignin–silica hybrids in sample LPS (Zhang et al. 2012). The superior flame-retardant properties of sample LPS were attributed to enhanced char formation induced by the lignin–silica composite. In addition, the phosphate group present in PA exhibits remarkable efficacy in diminishing the heat release of lignocellulosic materials.

Fig. 9. (a) Mechanism of silica formation, (b) the formation of the lignin–silica hybrids and (c) flame retardancy by spray coating

Elemental Distribution Analysis

Figure 10 exhibits the EDX spectra and elemental maps of samples PS and LPS before and after the vertical flammability test. The surface of samples PS and LPS contained C, O, Na, Si, and P. The elements C and O are mainly derived from cellulose and lignin, and to a lesser extent from PA (Lin et al. 2023). P originated from PA. Na and Si mostly originated from sodium silicate. The presence of S and other elements was attributed to kraft pulping, sulfonation, and isolation of lignin.

The O and P contents of sample PS before the vertical flammability test (Fig. 10(a)) are 45.2% and 1.56%, and those in sample LPS (Fig. 10(b)) are 40.6% and 1.86%, respectively. Notably, after the vertical flammability test, samples PS and LPS (Fig. 10(e, f)) exhibit O contents of 13.8% and 20.4% and P contents of 4.35% and 6.40%, respectively. The char residue of these samples mainly consists of phosphoric complexes enriched with P, O, and aromatic char (Liu et al. 2018; Le et al. 1997). After the vertical flammability test, the P content of sample LPS was considerably higher than before the vertical flammability test. The dense and rigid carbonized layer observed in Fig. 7(f) and the surface chemical composition in Fig. 10(f, h) indicate that the lignin–silica hybrids modified with PA exerted flame-retardant effect through condensation and gas-phase mechanisms. These results explain the best flame retardancy of sample LPS.

Fig. 10. (a, b, e, and f) EDX spectra and (c, d, g, and h) elemental distribution maps of (a and c) PS before the vertical flammability test, (b and d) LPS before the vertical flammability test, (e and g) PS after the vertical flammability test, and (f and h) LPS after vertical flammability test

Chemical Structure and Residual Char Structure Analysis

The chemical structures of paper samples were characterized via ATR-IR analysis (Fig. 11(a, b)). The raw IR spectra (Fig. 11(a)) show a key absorption peak at 1240 cm⁻¹, which was attributed to hydroxyl (–OH) groups and ether (C–O–C) bonds in cellulose (Poletto et al. 2012). A peak at 1080 cm⁻¹ corresponds to C–O–C bonds present in either lignin or cellulose, and a peak at 935 cm⁻¹ is associated with P–O–P stretching vibrations, indicating the presence of phosphate esters or polyphosphates (Zheng et al. 2015). To enhance the resolution and interpretability of the ATR-IR spectra, the raw spectra were processed using the Savitzky–Golay filter (Savitzky and Golay 1964) with a 5^th-order polynomial and subjected to second derivative analysis. The processed spectra (Fig. 11(b)) reveal additional absorption peaks that were not as prominent in the original spectrum. These include a peak at 1638 cm⁻¹, corresponding to O–P–O stretching vibrations (Zhang et al. 2014), peaks in the range of 1575 to 1450 cm⁻¹, characteristic of the lignin fingerprint region (Reyes and Terrazas 2017), a peak at 1111 cm⁻¹ related to Si–O–Si vibrations (Siuda et al. 2019; Saleh et al. 2020; Sujan et al. 2020), and a peak at 1000 cm⁻¹ attributed to ether (C–O–C) bonds (Li et al. 2020). For pure PA, absorption peaks near 1009 and 891 cm⁻¹ correspond to the stretching vibrations of the PO⁻ and P–OH structures (Pan et al. 2009; Jiang et al. 2012). However, in the spectra of samples coated with PA–silica hybrid, these peaks are not observed, indicating that PA molecules were integrated into the silica network.

Fig. 11. ATR-IR and Raman spectra of samples C, PS, and LPS: (a) ATR-IR raw spectra, (b) second derivative spectra in the range of 1800 to 800 cm⁻¹, (c) Raman spectra of sample PS after the vertical flammability test, (d) Raman spectra of sample LPS after the vertical flammability test

Raman spectroscopy is an essential tool for characterizing the structure of carbonaceous materials and analyzing the structure of residual char. Therefore, herein, it was used to investigate the mechanisms of the flame-retardant effects (Fig. 11(c, d)). Gaussian fitting was employed for peak analysis (Hou et al. 2002; Yu et al. 2015), where the G-band at 1574 cm⁻¹ represents the organized graphitized structure of the carbon layer, and the D-band at 1354 cm⁻¹ corresponds to disordered carbon in the materials. Typically, the ratio of the peak area of the D-band to that of the G-band (I_D/I_G) is used as an indicator of the graphitization degree of residual char (Yu et al. 2015). This ratio indicates the microcrystalline size of the carbon layer, with higher values corresponding to smaller carbon structures, improved shielding efficiency, and enhanced thermal stability. The graphitized char layer acts as a barrier to mass and heat transfer, suppresses the release of volatile organic compounds, and reduces heat release rate during combustion (Hou et al. 2002; Yu et al. 2015). As shown in Fig. 11(c, d), the introduction of the flame-retardant additives through spray coating enhanced the flame retardancy of paper. The I_D/I_G values of samples PS and LPS are 1.14 and 1.20, respectively. These results suggest that the coating layer stabilized the structure of the residual char, improved the shielding efficiency, and provided a notable flame-retardant effect. This also indicates that the degree of graphitization of the carbon layer was higher in the sample containing lignin. These findings, consistent with the results of the flammability test, SEM-EDX, and TGA, demonstrate that the integration of flame-retardant additives, particularly lignin, significantly enhanced the structural stability and graphitization degree of the residual char, thereby improving thermal shielding and flame-retardant properties.

CONCLUSIONS

This study investigated the flame retardancy of cellulose paper enhanced by a composite coating of lignin, phytic acid (PA), and sodium silicate. Through limiting oxygen index (LOI) and vertical flammability tests, significant improvements in fire resistance were confirmed, while scanning electron microscopy (SEM) and attenuated total reflection – infrared (ATR-IR) analyses demonstrated the uniform distribution of flame-retardant additives on the fiber surface. Elemental analysis revealed a notable increase in phosphorus content (from 1.86% to 6.40%) after combustion, indicating the formation of a stable char layer that contributed to enhanced flame resistance.
Themogravimetric analysis (TGA) results showed that while the lignin-phytic acid-sodium silicate (LPS) sample began thermal decomposition at a lower temperature compared to other samples, it produced the highest char residue (36.5 wt.%) at 900 °C. This significant char formation was attributed to the synergistic effect of PA, lignin, and sodium silicate, which facilitated the formation of thermally stable silica gel and a dense char layer, effectively insulating the material against heat. Although the LPS sample demonstrated lower initial thermal stability, its superior char yield contributed to enhanced flame resistance. Raman spectroscopy further confirmed the improved graphitization of the char layer in the LPS sample, with an I_D/I_G ratio of 1.20, indicating enhanced shielding efficiency and improved thermal protection.
Despite the low coating coverage due to the spray deposition method, the developed flame-retardant system exhibited excellent fire resistance. The synergistic effect between PA, lignin, and silica gel compensated for the limited coating thickness by promoting efficient char formation and thermal insulation. The combined condensed-phase and gas-phase flame-retardant effects, including polyphosphate layer formation and silica gel barrier action, highlight the potential of this eco-friendly and sustainable flame-retardant system for improving the fire resistance of cellulose-based materials, offering promising applications in safety-critical industries. Future research could explore optimizing the coating process for enhanced uniformity and durability, as well as investigating the system’s long-term performance under various environmental conditions. Furthermore, this flame-retardant system holds promising potential for industrial applications in safety-critical sectors such as packaging, construction, and textiles, where improved fire resistance is essential.

ACKNOWLEDGMENTS

This study was carried out with the support of ‘R&D Program for Forest Science Technology (Project No. 2023473B10-2425-EE02)’ provided by Korea Forest Service(Korea Forestry Promotion Institute).

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Article submitted: February 10, 2025; Peer review completed: March 1, 2025; Revised version received and accepted: March 12, 2025; Published: April 3, 2025;

DOI: 10.15376/biores.20.2.3808-3825