Intrinsic Flame Retardancy in Flexible Polyurethane Foams Enabled by a Bio-Based P–N Reactive Polyol and a Silicon-Functionalized Triazine Charring Agent

Intrinsic Flame Retardancy in Flexible Polyurethane Foams Enabled by a Bio-Based P–N Reactive Polyol and a Silicon-Functionalized Triazine Charring Agent | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Intrinsic Flame Retardancy in Flexible Polyurethane Foams Enabled by a Bio-Based P–N Reactive Polyol and a Silicon-Functionalized Triazine Charring Agent Lugui Lan, Yanping Mo, Yuhui Xie, Feng Wu, Dong Feng, Yang Meng, and 2 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-9046312/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract The development of sustainable and intrinsically flame-retardant polymeric materials remains a critical challenge due to the reliance on migratory, halogen-containing additives. In this work, a bio-based phosphorus–nitrogen–silicon (P–N–Si) synergistic flame-retardant system was rationally designed for flexible polyurethane foams (FPUFs) using renewable feedstocks. A reactive phosphorus–nitrogen polyol (PMESO), derived from epoxidized soybean oil and phytic acid, was chemically incorporated into the polyurethane network to impart intrinsic flame retardancy, while a silicon-functionalized triazine charring agent (KHT) was introduced to reinforce condensed-phase protection. Thermal analysis revealed that PMESO promoted early dehydration and catalytic charring, significantly increasing char yield, whereas KHT further enhanced char compactness and thermal stability under oxidative conditions. Fire performance evaluation demonstrated that the combined system markedly improved flame retardancy, as evidenced by increased limiting oxygen index values, pronounced self-extinguishing behavior, and substantial reductions in heat release and combustion efficiency. Comprehensive char characterization confirmed the formation of a dense, continuous, and P–N–Si–enriched char layer, which effectively acted as a physical barrier against heat and oxygen transfer. In addition, TG–IR and Py–GC/MS analyses indicated the release of nonflammable and flame-inhibiting species into the gas phase, providing auxiliary suppression of flame propagation. Overall, this study demonstrates how a reactive, bio-derived P–N–Si strategy can minimize the use of migratory flame retardants while simultaneously enhancing fire safety and thermal stability. The results highlight a sustainable and chemically informed pathway for designing safer polyurethane materials. Flexible polyurethane foam Bio-based reactive polyols P–N–Si synergistic flame retardancy Condensed- and gas-phase mechanisms Sustainable flame retardants Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Highlights • A bio-based reactive P–N polyol enabled intrinsic flame retardancy in FPUFs. • Silicon-functionalized triazine enhanced char compactness and thermal stability. • The P–N–Si system showed strong condensed- and gas-phase synergistic effects. • Heat release and smoke production were significantly reduced during combustion. • A sustainable strategy was proposed for flame-retardant polyurethane foams. 1. Introduction Flexible polyurethane foams (FPUFs) are widely used in upholstered furniture, cushioning and vibration-damping materials, and automotive interiors owing to their excellent resilience, low density, and favorable tensile properties [1–3]. Conventionally, FPUFs are synthesized from petroleum-derived isocyanates and polyols. However, increasing concerns regarding the depletion of fossil resources and greenhouse gas emissions have stimulated extensive research into polyurethane foams based on bio-derived and renewable feedstocks [4]. Among various renewable resources, plant oils have attracted considerable attention as sustainable alternatives for polyol production due to their low cost, low toxicity, biodegradability, and abundant availability [5, 6]. Soybean oil is regarded as one of the most promising candidates for partially replacing petroleum-based polyols because of its high annual yield, low price, and high degree of unsaturation (iodine value ≈ 4.6) [7]. Considerable efforts have been devoted to the development of soybean oil-based polyols for polyurethane foams. For instance, G.S et al. [8] used soybean-based polyols to replace petroleum-based polyols in the production of polyurethane foam. Ji et al. [9]synthesized a soybean oil-based polyol (Polyol-P) via phenol ring-opening of epoxidized soybean oil and reported improved compressive strength, thermal stability, and glass transition temperature (Tg) when the Polyol-P content reached 25 wt%. Herrán et al. [10]further demonstrated that polyurethane foams derived from epoxidized soybean oil and lactic acid exhibited an enhanced specific compressive strength of 7.8 kPa·kg⁻¹·m³. These studies collectively highlight the feasibility of substituting petroleum-based polyols with soybean oil-based counterparts. Nevertheless, the inherently high flammability of soybean oil-based FPUFs severely restricts their practical applications and poses significant fire hazards [11]. Therefore, improving the flame retardancy of bio-based FPUFs is crucial for expanding their industrial use [12]. The most common strategy for enhancing the flame retardancy of bio-based FPUFs involves incorporating flame retardants into the polymer matrix either physically or chemically [13]. However, many commercially available flame retardants, particularly halogenated compounds, suffer from toxicity, environmental persistence, and potential health risks, although they are still widely used in industry [14]. Phytic acid (PA), a naturally occurring bio-based compound abundant in legume seeds and cereal germ, is another promising environmentally benign flame retardant owing to its high phosphorus content, renewability, and non-toxicity [15]. However, the poor compatibility of PA with polymer matrices limits its flame-retardant efficiency when used directly [16]. Notably, PA contains multiple phosphate groups with high chemical reactivity [17], enabling its use as a reactive precursor for flame-retardant polymer design. For example, Zhang et al. [18]synthesized a PA-based flame retardant using glycidyl methacrylate (GMA), which simultaneously improved the flame retardancy and mechanical properties of polylactic acid (PLA). Nevertheless, GMA is petroleum-derived and relatively expensive, which compromises the overall sustainability of the system [19]. Shi et al. [20]developed multifunctional flame-retardant polyurethane foam laminates by combining PA, expandable graphite, and γ-aminopropyltrimethoxysilane, achieving remarkable reductions in peak heat release rate and peak smoke production rate by 77.5% and 81.8%, respectively. These studies indicate that chemically incorporating flame-retardant elements such as phosphorus, nitrogen, and silicon into polyurethane segments is an effective strategy to achieve durable flame retardancy [21–23]. Despite these advances, the development of fully bio-based, reactive flame-retardant polyols that simultaneously balance flame retardancy, smoke suppression, and mechanical integrity remains highly challenging. In this work, a bio-based reactive flame-retardant polyol was rationally designed to construct an intrinsically flame-retardant flexible polyurethane foam with enhanced thermal stability and fire resistance. Phytic acid and ethanolamine were first reacted to generate a phosphorus–nitrogen (P–N) containing polyester polyol, which was subsequently employed to ring-open epoxidized soybean oil (ESO), yielding a P–N functionalized soybean oil-based polyether polyol (PMESO). The obtained PMESO was used to partially replace conventional petroleum-derived polyether polyols in flexible polyurethane foams, enabling the covalent incorporation of flame-retardant elements into the polymer backbone. On this basis, a silicon-modified triazine-based charring agent (KHT) was further introduced to construct a phosphorus–nitrogen–silicon (P–N–Si) synergistic flame-retardant system. The effects of PMESO content on the cellular morphology, mechanical properties, thermal degradation behavior, and flame-retardant performance of the resulting flexible polyurethane foams were systematically investigated. Particular emphasis was placed on elucidating the thermal degradation pathways and flame-retardant mechanisms of the P–N–Si system by thermogravimetric analysis, cone calorimetry, and char residue analysis. The synergistic roles of phosphorus, nitrogen, and silicon in regulating condensed-phase char formation and gas-phase flame inhibition were comprehensively discussed. 2. Experimental section 2.1 Materials ESO, commercial polyether polyols (polyether polyol 1030, polyether polyol 1621, and polyether polyol 3628), silicone surfactant (silicone oil 8002), cell opener (1900), and modified isocyanate (8019, a mixture of polyol-modified diphenylmethane diisocyanate and polymethylene polyphenyl isocyanate, with an –NCO content of 26.0–27.0%) were supplied by Jining Beidal Trading Co., Ltd. and used as received. The polyether polyol 1030 exhibited a hydroxyl value of 230–250 mg KOH g⁻¹, while polyether polyol 1621 and polyether polyol 3628 exhibited hydroxyl values of 33–39 mg KOH g⁻¹ and 28 ± 3 mg KOH g⁻¹, respectively. Triethylenediamine (A33), dibutyltin dilaurate (DBTDL, T12), and the physical blowing agent 141B were supplied by Dongguan Guang Siyuan Polyurethane Materials Co., Ltd. and used as industrial-grade materials. Phytic acid (PA), monoethanolamine (MEA), trichlorotriazine (TCT), 3-aminopropyl methyldimethoxysilane (KH554), and acetonitrile were purchased from Aladdin Industrial Corporation and used without further purification. Deionized water and ultrapure water were prepared in the laboratory. All chemicals were used as received without further purification. 2.2 Synthesis of phosphorus–nitrogen reactive polyol (PMESO) The PMESO was synthesized via a two-step process. First, PA and MEA were subjected to an esterification reaction to obtain a phosphorus–nitrogen-containing polyester intermediate (PM). Subsequently, PM was reacted with ESO through the ring-opening reaction of epoxy groups, yielding the P–N functionalized polyol PMESO. The progress of the reaction was monitored by FTIR spectroscopy, and the reaction was considered complete when the characteristic epoxy absorption bands disappeared. The resulting PMESO was used directly for foam preparation without further purification. The process is schematically presented in Scheme 1a , and furthre details can be found in S1 of the Supporting Information. 2.3 Preparation of silicon-modified triazine charring agent (KHT) The KHT was prepared by completely substituting the chlorine atoms of TCT with KH554. Through nucleophilic substitution reactions, a triazine-based charring agent with high silicon content was obtained. The resulting product was collected, washed, and dried prior to use. The process is schematically presented in Scheme 1b , and furthre details can be found in S1 of the Supporting Information. 2.4 Fabrication of flexible polyurethane foams FPUFs were prepared using a one-shot free-rise foaming method (Scheme 1c ). PMESO partially replaced the commercial polyether polyol at different loadings, and the resulting foams were denoted as FPUF/10P, FPUF/20P, FPUF/30P, and FPUF/40P, where the number indicates the PMESO content (php) relative to the total polyol component. Based on the optimized PMESO formulation, KHT was further incorporated into FPUF/40P at different contents to obtain FPUF/ [email protected] , FPUF/ [email protected] , and FPUF/ [email protected] , where the number indicates the KHT content (php). After thorough mixing, the reacting mixture was poured into an open mold and allowed to rise freely at ambient temperature. The obtained foams were cured at room temperature prior to characterization. The whole foaming process is schematically presented in Scheme 1c , and the detailed formulations of all samples are provided in the Supporting Information ( Table S1 ) 2.5 Characterization Fourier transform infrared (FTIR) spectra were recorded using a TENSOR II spectrometer (Bruker, USA) in the range of 500–4000 cm⁻¹. 1 H NMR and 31 P NMR spectra were collected using an AVANCE III HD nuclear magnetic resonance spectrometer (Bruker, Germany) with dimethyl sulfoxide (DMSO) as the solvent. X-ray photoelectron spectroscopy (XPS) analysis was conducted using a VG ESCALAB MK II spectrometer (VG Scientific, UK). The cellular morphology of the foams and residual char was observed using a TESCAN MIRA LMS scanning electron microscope (TESCAN, Czech Republic). Mechanical properties were measured using a 104B universal testing machine (Shenzhen Wance Testing Equipment Co., Ltd.) according to the relevant standards. Thermogravimetric analysis (TGA) was performed using an STA 449 F3 thermal analyzer (Netzsch, Germany) under nitrogen and air atmospheres. Samples with a mass of 6–15 mg were heated at a rate of 20°C min⁻¹ with a gas flow rate of 50 mL min⁻¹. Limiting oxygen index (LOI) measurements were conducted using a COI oxygen index tester (Motis Fire Technology, China) according to GB/T 2406–1993. UL-94 horizontal burning tests were performed using a CZF-6 flammability tester (Nanjing Jiangning Analytical Instrument Co., Ltd.) in accordance with ISO 9772:2012. Cone calorimetry tests were carried out using a BT300-2J cone calorimeter (Suzhou Phoenix Quality Inspection Instrument Co., Ltd.) following ISO 5660-1 at an external heat flux of 35 kW m⁻². Raman spectroscopy of residual char was performed using an AploRA PLUS Raman spectrometer (HORIBA, France) with a laser wavelength of 532 nm. TG–IR analysis was conducted using a TA 8000 thermogravimetric analyzer coupled with a Frontier FTIR spectrometer (PerkinElmer, USA). Pyrolysis–gas chromatography/mass spectrometry (Py–GC/MS) analysis was carried out using an EGA/PY-3030D pyrolyzer (Frontier Laboratories, Japan) coupled with a GCMS-QP2020NX system (Shimadzu, Japan). 3. Results and discussion 3.1 Structure confirmation of PMESO and KHT The chemical structures of the PMESO and the KHT were confirmed by FTIR, 1 H NMR, and XPS analyses, as shown in Fig. 1 . As shown in Fig. 1 a, the characteristic epoxy absorption bands of ESO disappear completely after the ring-opening reaction, accompanied by the emergence of broad hydroxyl absorption and phosphorus-related bands in the PMESO spectrum, indicating successful epoxy ring opening [24] and functionalization. The 1 H NMR spectra (Fig. 1 b) further confirm the structural evolution, where the disappearance of epoxy proton signals and the appearance of new hydroxyl-bearing methine and methylene resonances demonstrate the effective incorporation of phosphorus–nitrogen functionalities into the polyol backbone [25]. The FTIR spectrum of KHT (Fig. 1 c) exhibits characteristic absorptions associated with triazine structures as well as silicon-containing bonds [23, 26], indicating successful modification. The presence of Si and N elements is further confirmed by XPS survey analysis (Fig. 1 d). Additional structural evidence, including FTIR spectra of intermediate products, XRD patterns, and high-resolution XPS spectra, is provided in the Supporting Information ( Figure S1 and S2 ). These results collectively confirm the successful synthesis of PMESO and KHT, providing a reliable structural basis for subsequent investigations of their flame-retardant behavior. 3.2 Effect of reactive PMESO and subsequent KHT incorporation on cellular morphology and mechanical integrity The pore structure is a key factor influencing the properties of flexible polyurethane foam, as it is affected by bubble nucleation and growth during the foaming process [27]. The influence of reactive PMESO and subsequent KHT incorporation on the cellular morphology of flexible polyurethane foams was examined by SEM analysis (Fig. 2 ). Pristine FPUF exhibits relatively irregular cell structures with a broad cell size distribution. Upon incorporation of PMESO at an optimized content (FPUF/40P), the foam displays more uniform and well-defined cellular morphology, suggesting improved bubble stabilization during the foaming process [28]. This behavior can be attributed to the increased polarity and hydrogen-bonding capability introduced by the P–N-containing polyol [6]. With further incorporation of KHT, the cellular structure evolves depending on the KHT content. At low and moderate KHT loadings (FPUF/ [email protected] and FPUF/ [email protected] ), the foams maintain relatively uniform cell structures, whereas excessive KHT addition (FPUF/ [email protected] ) leads to a slight enlargement and distortion of cells, likely due to increased viscosity and localized heterogeneity during foam formation. The mechanical properties of the foams were evaluated through tensile and cyclic compression–unloading tests [12] (Fig. 3 ). The mechanical properties of foam primarily depend on tensile strength, elongation at break, and compressive strength [29]. Compared with pristine FPUF, FPUF/40P exhibits comparable elongation and mechanical resilience, indicating that partial replacement of petroleum-based polyols by PMESO does not compromise mechanical integrity. The incorporation of KHT leads to a gradual change in mechanical response, with moderate KHT contents preserving acceptable elasticity and energy dissipation behavior. These results demonstrate that the FPUF/40P formulation provides a robust structural platform for further flame-retardant modification. Supporting Information ( Figure. S2–S4 ) further illustrates the evolution of cellular morphology, mechanical properties, and macroscopic appearance of FPUFs containing different PMESO contents, confirming that PMESO contents beyond 40 php adversely affect the foaming process. Therefore, FPUF/40P was selected as the optimal base formulation for subsequent KHT incorporation. These results indicate that appropriate incorporation of PMESO and KHT enables flame-retardant modification without sacrificing the cellular structure and mechanical integrity of flexible polyurethane foams. 3.3 Thermal degradation behavior of FPUFs containing PMESO and KHT The thermal degradation behavior of flexible polyurethane foams with different formulations was investigated by TGA under nitrogen and air atmospheres, as shown in Fig. 4 , and the corresponding characteristic parameters are summarized in Table 1 . Under a nitrogen atmosphere (Fig. 4 a and 4 b), pristine FPUF exhibits a typical two-stage degradation process [29, 30] with an initial decomposition temperature ( T ₅ % ) of 317.7°C and a maximum mass loss rate ( T max₁ ) at 395.4°C, leaving a limited char residue of 8.95 wt% at 800°C. In contrast, incorporation of the reactive P–N polyol (FPUF/40P) leads to a pronounced change in the degradation profile. Although the T ₅ % value decreases markedly to 118.8°C, the char yield increases significantly to 15.73 wt%, indicating that the phosphorus–nitrogen functionalities promote early dehydration and carbonization during thermal decomposition [31]. Meanwhile, the T max₁ value remains at a comparable level (386.5°C), suggesting that the main-chain degradation of the polyurethane matrix is not severely disrupted. With further incorporation of the silicon-modified triazine charring agent, the char-forming ability of the foams is further enhanced. As the KHT content increases from 0.5 to 3.0 wt%, the char yield under nitrogen atmosphere gradually rises from 16.21 wt% to 17.41 wt%, which is almost twice that of pristine FPUF. Notably, the T max₁ values of the KHT-containing foams remain in a narrow range of 387.8–389.5°C, indicating that the P–N–Si synergistic system mainly affects the carbonization process rather than accelerating the catastrophic degradation of the polymer backbone [32]. Similar trends are observed under air atmosphere (Fig. 4 c and 4 d). Pristine FPUF shows almost complete oxidation of the char, leaving a negligible residue of only 0.16 wt% at 800°C. In contrast, FPUF/40P exhibits a substantially increased char residue of 4.05 wt%, demonstrating improved resistance to oxidative degradation. Upon incorporation of KHT, the residual char further increases, reaching 5.71 wt% for FPUF/ [email protected] . This enhancement can be attributed to the formation of thermally stable silicon-containing structures, which effectively retard char oxidation at elevated temperatures [20]. It is worth noting that the early mass loss observed for PMESO- and KHT-containing foams is associated with the decomposition of phosphorus-containing groups and triazine-based structures. However, this early-stage decomposition plays a beneficial role by triggering rapid char formation, which subsequently protects the underlying polymer from further thermal and oxidative attack. Overall, these results demonstrate that the combined incorporation of PMESO and KHT effectively regulates the thermal degradation pathway of flexible polyurethane foams, leading to promoted char formation and enhanced char stability, particularly under oxidative conditions, which is critical for improved flame retardancy. Table 1 TG data of of FPUFs with different formulations Sample N 2 AIR T 5% (℃) T max1 (℃) C wt 800 (%) T 5% (℃) T max1 (℃) C wt 800 (%) KHT 219.9 276.7 4.49 219.1 279.9 7.92 Pristine FPUF 317.7 395.4 8.95 318.1 389.91 0.16 FPUF/40P 118.8 386.5 15.73 116.2 379.7 4.05 FPUF/ [email protected] 148.9 389.5 16.21 129.4 359.6 4.18 FPUF/ [email protected] 136.4 388.1 17.22 140.3 369.5 4.22 FPUF/ [email protected] 122.2 387.8 17.41 174.2 376.0 5.71 3.4 Fire behavior and intrinsic flame retardancy of PMESO/KHT-modified FPUFs The intrinsic flame-retardant performance of flexible polyurethane foams was first evaluated by LOI measurements and UL-94 horizontal burning tests [33], and the results are shown in Fig. 5 . Pristine FPUF exhibits a low LOI value of 18.8%, reflecting its inherent flammability. After incorporation of the reactive phosphorus–nitrogen polyol, the LOI value of FPUF/40P increases markedly to 22.6%, demonstrating that the covalent introduction of P–N functionalities significantly enhances the intrinsic flame retardancy of the foam. With further incorporation of the silicon-modified triazine charring agent (KHT), the LOI values show a gradual increase to 23.1%, 23.2%, and 23.4% for FPUF/ [email protected] , FPUF/ [email protected] , and FPUF/ [email protected] , respectively. Although the increment becomes less pronounced at higher KHT contents, the consistently elevated LOI values indicate a positive contribution of the P–N–Si synergistic system to flame inhibition [34]. The UL-94 horizontal burning behavior further reveals the effect of KHT on flame propagation and self-extinguishing performance (Fig. 5 b). While pristine FPUF and FPUF/40P show continuous flame propagation along the sample surface, the incorporation of KHT induces clear self-extinguishing behavior. For FPUF/ [email protected] , the burning rate is significantly reduced compared with FPUF/40P, and the sample self-extinguishes after reaching the first scale mark within 30 s, leaving a rigid and intact char without bending or dripping. Notably, FPUF/ [email protected] exhibits the most pronounced flame inhibition, where the flame retreats and extinguishes almost immediately after removal of the ignition source at 30 s. For all KHT-containing samples, no molten dripping is observed on the flame-exposed surface during the UL-94 test. These observations indicate that KHT effectively enhances the condensed-phase flame-retardant performance of PMESO-modified FPUFs. The improved self-extinguishing behavior can be attributed to the synergistic charring effect between PMESO and KHT. During combustion, PMESO promotes dehydration and early char formation, while the silane-modified triazine structure of KHT facilitates the formation of thermally stable Si–C and Si–O structures at the flame-exposed surface through interactions with the thermal degradation products of the polyurethane matrix. These silicon-containing structures contribute to carbon fixation and the formation of a robust protective layer, thereby inhibiting flame propagation. To further evaluate the fire behavior under forced flaming conditions, cone calorimetry tests were performed, and the corresponding heat and smoke release parameters are summarized in Fig. 6 and Table 2 . Pristine FPUF ignites rapidly with a time to ignition (TTI) of 7 s and exhibits a high peak heat release rate (PHRR) of 210.38 kW m⁻², together with a large total heat release (THR) of 32.50 MJ m⁻², indicating severe fire hazards. After incorporation of PMESO, the PHRR of FPUF/40P decreases significantly to 152.01 kW m⁻², corresponding to a reduction of approximately 28% compared with pristine FPUF. Meanwhile, the THR is dramatically reduced to 12.99 MJ m⁻², confirming that the reactive P–N polyol effectively suppresses combustion intensity and heat output by promoting char formation and limiting heat feedback. The introduction of KHT further suppresses heat release behavior. Among the investigated samples, FPUFUF/ [email protected] exhibits the lowest PHRR value of 124.09 kW m⁻², representing a reduction of nearly 41% relative to pristine FPUF. Although the PHRR slightly increases at higher KHT contents, all P–N–Si-modified foams maintain substantially lower PHRR and THR values than pristine FPUF, demonstrating the effectiveness of the synergistic flame-retardant system. Smoke production behavior was also analyzed in detail. Pristine FPUF shows a relatively low total smoke production (TSP) of 2.97 m², but this is accompanied by a high average effective heat of combustion (Avg EHC) of 21.03 MJ kg⁻¹, indicating efficient flaming combustion. After incorporation of PMESO, the Avg EHC decreases markedly to 13.94 MJ kg⁻¹, suggesting a significant inhibition of gas-phase combustion efficiency. Although the TSP of FPUF/40P increases to 4.29 m², this increase can be attributed to incomplete combustion and the release of char-derived particulates rather than intensified flaming [35]. Table.2. Data of FPUFs with different formulations obtained by cone calorimetry tests Sample TTI (s) t p (s) PHRR (kW/m 2 ) THR (MJ/m 2 ) TSP (m 2 ) Avg EHC (MJ/Kg) Pristine FPUF 7 37 210.38 32.504 2.967 21.032 FPUF/40P 9 29 152.01 12.985 4.285 13.935 FPUF/ [email protected] 5 33 124.09 12.757 3.395 12.922 FPUF/ [email protected] 5 26 140.77 12.487 4.426 13.250 FPUF/ [email protected] 4 29 138.63 10.723 2.735 14.722 Notably, the incorporation of KHT effectively regulates smoke release behavior. FPUF/ [email protected] and FPUF/ [email protected] exhibit reduced TSP values of 3.40 m² and 2.74 m², respectively, compared with FPUF/40P. This improvement is associated with the formation of a denser and more stable P–N–Si char layer, which suppresses the release of smoke-forming volatiles during combustion. Meanwhile, the Avg EHC values of KHT-containing foams remain significantly lower than that of pristine FPUF, confirming sustained inhibition of combustion efficiency. Overall, the combined LOI, UL-94, and cone calorimetry results demonstrate that the PMESO/KHT system effectively enhances the flame retardancy and fire safety of flexible polyurethane foams. The P–N–Si synergistic system suppresses heat release, promotes self-extinguishing behavior, and regulates smoke production through coordinated condensed-phase protection and gas-phase combustion inhibition. 3.5 Condensed-phase flame-retardant mechanism To elucidate the condensed-phase flame-retardant mechanism of the PMESO/KHT system, the morphology and chemical structure of the residual char after cone calorimetry tests were systematically analyzed. The digital photographs of the residual char (Fig. 7 ) clearly reveal distinct differences among the samples. After combustion, pristine FPUF leaves only fragmented and fragile mesh-like residues, and the aluminum foil wrapping the sample is completely penetrated, indicating the absence of an effective protective char layer. In contrast, FPUF/40P forms a relatively continuous char layer; however, obvious pores and defects are observed on the char surface, which can be attributed to gas-phase release during combustion. Upon incorporation of KHT, this porous structure is significantly suppressed. Notably, FPUF/ [email protected] produces a thick, rigid, and mechanically robust char layer tightly adhered to the aluminum foil without fragmentation. The disappearance of pores and the appearance of a compact and glossy char surface demonstrate a markedly enhanced condensed-phase protective effect. The internal microstructure of the residual char was further examined by SEM at a magnification of 200× (Fig. 7 ). The residual char of pristine FPUF is characterized by numerous spherical particulates and an uneven surface, reflecting insufficient carbonization. FPUF/40P shows partial improvement; however, fractured openings and uneven regions are still present. In contrast, the incorporation of KHT leads to a dense, smooth, and continuous block-like char structure with significantly reduced pores and surface irregularities. The elimination of spherical particulates and fragmented openings indicates that KHT effectively promotes char densification and suppresses the release of volatile degradation products. FTIR analysis of the residual char ( Figure S5 ) provides further insight into the chemical composition of the char layer. Compared with the residual chars of pristine FPUF and FPUF/40P, the char of KHT-containing samples exhibits a significantly intensified absorption band at 3377 cm⁻¹, corresponding to hydroxyl groups, indicating the formation of a larger amount of hydroxyl-containing species. Meanwhile, the characteristic absorption bands associated with –CH₃ and –CH₂ stretching vibrations at 2960 cm⁻¹, 2924 cm⁻¹, and 2854 cm⁻¹ are enhanced. The disappearance of the absorption band at 2222 cm⁻¹ (HC≡–R stretching vibration), together with the appearance and shift of absorption bands corresponding to C ≡ N stretching (2310 cm⁻¹), symmetric bending vibration of (NH₄)⁺ (1707 cm⁻¹), and tertiary amine C–N stretching (1044 cm⁻¹), indicate the formation of various nitrogen-containing compounds in the residual char due to the introduction of KHT. In addition, a new absorption band appears at 881 cm⁻¹, which can be assigned to the symmetric stretching vibration of Si–O–Si, confirming the direct participation of silicon-containing species in char formation. These results demonstrate a synergistic effect between PMESO and KHT in promoting the formation of a more complete and chemically complex char structure. Raman spectroscopy was employed to evaluate the structural ordering and continuity of carbon atoms in the residual char (Fig. 8 ). The intensity ratio of the D band to the G band (I D /I G ) [36], which reflects the degree of disorder and graphitization, decreases significantly upon incorporation of KHT. Specifically, the I D /I G value decreases from 4.26 for pristine FPUF to 3.04 for KHT-containing samples, corresponding to a reduction of 28.64%, and is 13.14% lower than that of FPUF/40P. The pronounced decrease in the I D /I G ratio indicates enhanced graphitization and increased structural compactness of the residual char, confirming that KHT further improves char densification beyond the effect of PMESO alone. The elemental composition of the residual char was further analyzed by EDS (Fig. 8 ). Phosphorus is uniformly distributed over the char surface, with the P content reaching 13.5 wt%, indicating effective retention of phosphorus-containing species in the condensed phase. Meanwhile, the contents of silicon and oxygen increase to 1.2 wt% and 44.2 wt%, respectively, compared with pristine FPUF. These results suggest that the incorporation of KHT promotes the formation of phosphorus- and silicon-containing char structures during combustion. The resulting P–O–C and Si–O–C networks form a robust physical barrier that effectively retards heat transfer and mass diffusion. Notably, the nitrogen content in the residual char decreases from 8.4 wt% in FPUF/40P to 3.5 wt% after KHT incorporation, which can be attributed to the generation of nonflammable nitrogen-containing gases derived from triazine structures. This phenomenon suggests that KHT not only contributes to condensed-phase protection but also participates in gas-phase flame retardancy by diluting combustible volatiles. Overall, the combined morphological, spectroscopic, and compositional analyses demonstrate that the PMESO/KHT system exhibits a pronounced condensed-phase flame-retardant mechanism. The synergistic effects of phosphorus–nitrogen catalytic charring, triazine-induced carbonization, and silicon-assisted carbon fixation promote the formation of a dense, continuous, and thermally stable char layer. This char layer acts as an effective physical barrier, suppressing heat transfer, mass diffusion, and oxygen penetration, thereby playing a crucial role in enhancing the flame retardancy of flexible polyurethane foams. 3.6 Gas-phase flame-retardant behavior To further elucidate the gas-phase flame-retardant mechanism of the PMESO/KHT system, the volatile products released during thermal degradation were analyzed by thermogravimetric analysis coupled with infrared spectroscopy (TG–IR) and pyrolysis gas chromatography–mass spectrometry (Py–GC/MS), as shown in Fig. 9 and summarized in Tables S2–S4 . The TG–IR spectra of the evolved gases (Fig. 9 a–c) reveal clear differences in the composition and evolution behavior of volatile products among pristine FPUF, FPUF/40P, and FPUF/ [email protected] . Compared with pristine FPUF and FPUF/40P, FPUF/ [email protected] exhibits significantly stronger and sharper absorption bands at 3015 cm⁻¹ (olefinic C═H stretching), 2972 cm⁻¹ and 2930 cm⁻¹ (–CH₃ asymmetric stretching), and 2864 cm⁻¹ (–CH₂ stretching), indicating the formation of a larger amount of small-molecule alkanes, carbonyl compounds, and aliphatic ester fragments containing C–O or C═O groups. These observations suggest that the introduction of KHT promotes fragmentation into low-molecular-weight species during thermal degradation. Meanwhile, the absorption bands at 3364 cm⁻¹ and 3468 cm⁻¹, attributed to –OH stretching vibrations, become more pronounced at elevated temperatures (around 305°C) for KHT-containing samples. This enhancement indicates intensified dehydration and carbonization reactions triggered by the thermal decomposition of KHT, leading to the release of additional water vapor into the gas phase. The evolution of water vapor not only facilitates char consolidation in the condensed phase but also dilutes flammable gaseous products, thereby reducing combustion intensity. Notably, the absorption intensity of the CO₂ characteristic peak at 2310 cm⁻¹ is weakened after the incorporation of KHT, which is consistent with the reduced CO₂ production rate observed in cone calorimetry tests. This result implies that the gas-phase oxidation reactions are partially suppressed in the presence of the P–N–Si synergistic system. In addition, several nitrogen-related characteristic absorption bands become more prominent for KHT-containing foams. The absorption bands located near 1376 cm⁻¹ and 928 cm⁻¹, associated with N═N and NH₃ stretching vibrations, together with the band at 1514 cm⁻¹ corresponding to –NH vibrations in aromatic compounds, exhibit increased intensities compared with pristine FPUF and FPUF/40P. These results indicate that the incorporation of KHT promotes the generation of nitrogen-containing nonflammable gases in the gas phase, which dilute the combustible atmosphere and suppress flame propagation. More importantly, a pronounced absorption band appears at 1273 cm⁻¹, corresponding to the P═O stretching vibration, with significantly enhanced intensity for FPUF/ [email protected] . This result demonstrates that the presence of KHT facilitates the release and volatilization of phosphorus-containing species into the gas phase. These phosphorus-derived radicals can effectively scavenge flame-propagating radicals (e.g., H· and OH·), thereby interrupting chain reactions in the flame and inhibiting fire spread [37]. To further identify the chemical nature of gaseous degradation products, Py–GC/MS analysis was conducted at 500°C , and the resulting chromatograms are shown in Fig. 9 d–f, with detailed peak assignments listed in Tables S2–S4 . For FPUF/ [email protected] , the appearance of CO₂, butane, and other small hydrocarbon fragments indicates that the incorporation of flame retardants does not alter the fundamental degradation pathway of FPUF, which still follows an intramolecular rearrangement mechanism. However, compared with FPUF/40P, FPUF/ [email protected] generates a higher proportion of small-molecule hydrocarbons, such as CO₂, propylene, ethylene oxide, and propylene oxide, at short retention times. These results further corroborate the TG–IR observations. In addition, a variety of nitrogen-containing aromatic and aliphatic compounds, including substituted aniline derivatives, are detected at longer retention times for KHT-containing foams. The generation of these nitrogen-rich, nonflammable species further supports the role of KHT in gas-phase dilution and flame inhibition. Moreover, silicon-containing compounds, such as octylsilane and 1,4-bis(dimethylsilyl)butane, are detected exclusively in the degradation products of FPUF/ [email protected] . These results indicate that silicon-containing fragments derived from KHT can volatilize into the gas phase and participate in flame suppression by diluting combustible gases and interacting synergistically with nitrogen-containing species. Overall, the TG–IR and Py–GC/MS analyses demonstrate that the PMESO/KHT system exhibits a pronounced gas-phase flame-retardant mechanism. The synergistic release of phosphorus-containing radicals, nitrogen-derived nonflammable gases, water vapor, and silicon-containing fragments effectively dilutes the combustible atmosphere and interrupts flame-propagating reactions. Together with the condensed-phase char-forming effect, these gas-phase actions contribute to the overall P–N–Si synergistic flame retardancy of flexible polyurethane foams. 3.7 Overall P–N–Si synergistic flame-retardant mechanism Based on the comprehensive analysis of thermal degradation behavior, fire performance, residual char characteristics, and evolved gaseous products, an overall P–N–Si synergistic flame-retardant mechanism for PMESO/KHT-modified flexible polyurethane foams is proposed, as schematically illustrated in Fig. 10 . Upon exposure to heat or flame, the reactive phosphorus–nitrogen polyol (PMESO) undergoes early-stage decomposition, which initiates dehydration and promotes rapid char formation. This process is evidenced by the reduced initial decomposition temperature in TGA analysis and the significantly increased char yield under both nitrogen and air atmospheres. The phosphorus-containing structures facilitate the formation of phosphoric and polyphosphoric species, which catalyze the carbonization of the polyurethane matrix and suppress melt dripping during combustion. Meanwhile, nitrogen-containing moieties contribute to the generation of thermally stable char precursors and inert gaseous products, laying the foundation for both condensed- and gas-phase flame-retardant actions. With the incorporation of KHT, the flame-retardant mechanism is further reinforced through multiple synergistic pathways. In the condensed phase, the triazine ring structure enhances carbonization efficiency, while the grafted silane functionalities promote the formation of thermally stable Si–O–C and Si–C networks during combustion. These silicon-containing structures act as effective carbon-fixation centers, leading to the formation of a dense, continuous, and mechanically robust char layer. As confirmed by SEM, Raman spectroscopy, and EDS analysis, the resulting char exhibits enhanced compactness, increased graphitization degree, and enriched phosphorus and silicon contents, which collectively improve the barrier effect against heat transfer, mass diffusion, and oxygen penetration. Simultaneously, a pronounced gas-phase flame-retardant effect is activated. TG–IR and Py–GC/MS analyses reveal that the PMESO/KHT system promotes the release of multiple flame-inhibiting species into the gas phase. Phosphorus-containing radicals volatilized from PMESO effectively scavenge flame-propagating radicals, thereby interrupting chain reactions in the flame zone. Nitrogen-containing nonflammable gases and ammonia derivatives derived from the triazine structure dilute the concentration of combustible gases and oxygen, suppressing flame intensity. In addition, the thermal decomposition of KHT generates water vapor and silicon-containing fragments, which further dilute the combustible atmosphere and contribute to flame inhibition. The combined action of these species leads to reduced combustion efficiency, as evidenced by the decreased average effective heat of combustion and suppressed CO₂ evolution. The coordinated action of the condensed-phase barrier effect and gas-phase flame inhibition results in significantly reduced heat release, improved self-extinguishing behavior, and regulated smoke production, as demonstrated by LOI, UL-94, and cone calorimetry tests. Importantly, this synergistic P–N–Si system achieves effective flame retardancy at relatively low additive loadings while maintaining acceptable mechanical integrity and cellular structure of the flexible polyurethane foams. In summary, the PMESO/KHT system establishes a highly efficient and sustainable flame-retardant strategy for flexible polyurethane foams through a well-balanced P–N–Si synergistic mechanism. The integration of catalytic charring, carbon fixation, and gas-phase radical suppression provides comprehensive fire protection, highlighting the potential of bio-based, reactive flame-retardant systems for advanced polymeric materials. 4. Conclusion In this work, a sustainable P–N–Si synergistic flame-retardant system was successfully developed for flexible polyurethane foams by integrating a reactive bio-based phosphorus–nitrogen polyol (PMESO) with a silicon-functionalized triazine charring agent (KHT). The reactive incorporation of PMESO imparted intrinsic flame retardancy while preserving the cellular structure and mechanical integrity of the foams, whereas the introduction of KHT further enhanced fire safety through effective condensed- and gas-phase synergy. Thermogravimetric analysis revealed that the PMESO/KHT system regulated the thermal degradation pathway by promoting dehydration and carbonization, leading to increased char yield, particularly under oxidative conditions. Fire performance evaluations demonstrated markedly improved flame retardancy, including increased LOI values, clear self-extinguishing behavior in UL-94 tests, and significantly reduced heat release and combustion efficiency in cone calorimetry, accompanied by effective smoke suppression at appropriate KHT loadings. Char characterization confirmed the formation of a dense, continuous, and thermally stable char layer resulting from the combined effects of phosphorus–nitrogen catalytic charring, triazine-induced carbonization, and silicon-assisted carbon fixation. In addition, TG–IR and Py–GC/MS analyses indicated that the release of phosphorus-containing species, nitrogen-derived nonflammable gases, and dehydration-related volatiles contributed to gas-phase flame inhibition. Overall, this P–N–Si synergistic strategy offers an effective and sustainable route to enhancing flame retardancy and fire safety of flexible polyurethane foams. The results provide mechanistic insights into multi-element synergistic flame-retardant systems and highlight a promising approach for the design of advanced bio-based flame-retardant polymeric materials. Declarations CRediT authorship contribution statement Lugui Lan: Investigation, Writing - Original Draft. Yanping Mo: Investigation, Data curation, Writing - Original Draft. Yuhui Xie: Writing – review & editing, Methodology, Resources, Funding acquisition, Supervision. Feng Wu: Investigation, Validation, Resources, Writing – review & editing. Dong Feng: Validation, Resources, Writing – review & editing. Yang Meng: Investigation, Writing – review & editing. Yi Mei: Funding acquisition, Writing – review & editing. Delong Xie: Writing – review & editing, Funding acquisition, Project administration. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments The authors are grateful for financial support from the National Natural Science Foundation of China (No. 22268025), the Key Research and Development Program of Yunnan Province (No. 202403AA080003). Funding Declaration This work was supported by the National Natural Science Foundation of China (No. 22268025) and the Key Research and Development Program of Yunnan Province (No. 202403AA080003). Data Availability Data will be made available upon request. Appendix A. Supporting information Supplementary data associated with this article can be found in the online version. 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Xu Z, Gao M, Zhao Q, Zhang C, Zhang J, Cheng M, Xu J, Li T, Cheng C, (2024) A new method for preparing permanent flame-retardant lyocell fibre: Preparation of flame-retardant fibres by phosphorylated MTT/lyocell blended fibres, Cellulose 31(7) 4565-4580.http://doi.org/10.1007/s10570-024-05882-0. Scheme 1 Scheme 1 is available in the supplementary files section. Additional Declarations No competing interests reported. Supplementary Files Supplementaryinfromation.docx Scheme1.docx Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-9046312","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":602148585,"identity":"eced08a5-7732-42f8-b5f8-912040cd6bf1","order_by":0,"name":"Lugui Lan","email":"","orcid":"","institution":"Kunming University of Science and Technology","correspondingAuthor":false,"prefix":"","firstName":"Lugui","middleName":"","lastName":"Lan","suffix":""},{"id":602148586,"identity":"e2ba35d6-ba66-4f7a-8669-12da090ca3c0","order_by":1,"name":"Yanping Mo","email":"","orcid":"","institution":"Kunming University of Science and 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Technology","correspondingAuthor":false,"prefix":"","firstName":"Yi","middleName":"","lastName":"Mei","suffix":""},{"id":602148602,"identity":"3040467e-57d9-4eae-bde6-061df9d108d2","order_by":7,"name":"Delong Xie","email":"","orcid":"","institution":"Kunming University of Science and Technology","correspondingAuthor":false,"prefix":"","firstName":"Delong","middleName":"","lastName":"Xie","suffix":""}],"badges":[],"createdAt":"2026-03-06 05:38:06","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-9046312/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-9046312/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":104235710,"identity":"5442bd7e-7427-4487-acde-f3d57f4b801c","added_by":"auto","created_at":"2026-03-09 13:22:34","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":229310,"visible":true,"origin":"","legend":"\u003cp\u003eStructural confirmation of the reactive flame-retardant components PMESO and KHT: (a) FTIR spectra of ESO and PMESO, (b) ¹H NMR spectra of ESO and PMESO, (c) FTIR spectrum of KHT, and (d) XPS survey spectrum of KHT.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-9046312/v1/763231de855a9cd0d2ee25d4.png"},{"id":104235712,"identity":"6fac3354-b280-4a71-9596-edbf5d8a64f9","added_by":"auto","created_at":"2026-03-09 13:22:34","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":582938,"visible":true,"origin":"","legend":"\u003cp\u003eSEM images of cellular morphology of (a) pristine FPUF, (b) FPUF/40P, (c) FPUF/[email protected], (d) FPUF/[email protected], and (e) FPUF/[email protected].\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-9046312/v1/eb97b4b7fbfcf41ad428ddfd.png"},{"id":104235711,"identity":"eae8bb5e-e9dd-4226-a1cf-d65b4e50552a","added_by":"auto","created_at":"2026-03-09 13:22:34","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":134015,"visible":true,"origin":"","legend":"\u003cp\u003eThe elongation-strength curves (a) and the cycle of the compression and unloading curves (b) of FPUFs with different formulations, showing the effect of reactive P–N polyols and P–N–Si modification on structural integrity.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-9046312/v1/0fac6866d19464e2a6689679.png"},{"id":104405084,"identity":"e2728d4c-ab1d-4865-9ffc-7fd3a9c02404","added_by":"auto","created_at":"2026-03-11 12:21:43","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":263672,"visible":true,"origin":"","legend":"\u003cp\u003eThe\u003cstrong\u003e \u003c/strong\u003eTG and DTG curves of FPUFs with different formulations under a nitrogen (a \u0026amp; b) and air (c \u0026amp; d) atmosphere, highlighting differences in thermal degradation behavior and char yield.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-9046312/v1/ec0c7d316307560e3396c089.png"},{"id":104235715,"identity":"a4c103ea-7d05-4080-835c-e5b2beeff498","added_by":"auto","created_at":"2026-03-09 13:22:34","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":377153,"visible":true,"origin":"","legend":"\u003cp\u003eIntrinsic flame-retardant performance of flexible polyurethane foams: (a) (LOI) values and (b) UL-94 horizontal burning test of FPUFs with different formulations.\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-9046312/v1/108702b9e10c56a724186a1f.png"},{"id":104235714,"identity":"2099a964-3628-4fea-bad7-d40f4d8f8b71","added_by":"auto","created_at":"2026-03-09 13:22:34","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":246564,"visible":true,"origin":"","legend":"\u003cp\u003eCone calorimetry results of FPUFs with different formulations: (a) THR curves, (b) HRR curves, (c) SPR curves, (d)TSR curves, (e) COP curves, and (f) CO\u003csub\u003e2\u003c/sub\u003eP curves.\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-9046312/v1/4c51bc181f4b8f5f1b2d1c71.png"},{"id":104235719,"identity":"468af144-d618-46a6-b4e4-b4a8494054b1","added_by":"auto","created_at":"2026-03-09 13:22:34","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":615815,"visible":true,"origin":"","legend":"\u003cp\u003ePhotographs and SEM images of residual char after CC test of FPUFs with different formulations.\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-9046312/v1/71c554a8bc6057b60545fde9.png"},{"id":104235722,"identity":"59081057-dc97-4bb0-9309-af7888bc98ab","added_by":"auto","created_at":"2026-03-09 13:22:34","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":249685,"visible":true,"origin":"","legend":"\u003cp\u003eCharacterization of residual char after CC test of Pristine FPUF, FPUF/40P and FPUF/[email protected]: (a–c) elemental distribution of P, N, and Si obtained from SEM/EDS test, and (d-f) Raman spectra, illustrating the condensed-phase flame-retardant mechanism.\u003c/p\u003e","description":"","filename":"8.png","url":"https://assets-eu.researchsquare.com/files/rs-9046312/v1/2c8da818c94d63f71da38c72.png"},{"id":104235721,"identity":"eac4001f-7711-45e1-82ee-77ee82b7aa32","added_by":"auto","created_at":"2026-03-09 13:22:34","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":188573,"visible":true,"origin":"","legend":"\u003cp\u003eGas-phase analysis during thermal degradation of Pristine FPUF, FPUF/40P and FPUF/[email protected], obtained by TG–IR (a-c) and Py–GC/MS (d-f), revealing differences in volatile products and flame-inhibiting species.\u003c/p\u003e","description":"","filename":"9.png","url":"https://assets-eu.researchsquare.com/files/rs-9046312/v1/dba7d02991f7330da41eadb3.png"},{"id":104404774,"identity":"fb14e0f9-2f58-49be-b994-99664159f538","added_by":"auto","created_at":"2026-03-11 12:21:03","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":1083329,"visible":true,"origin":"","legend":"\u003cp\u003eProposed flame-retardant mechanism of the P–N–Si synergistic system in FPUFs, illustrating the coordinated action between PMESO and KHT in the condensed and gas phases.\u003c/p\u003e","description":"","filename":"10.png","url":"https://assets-eu.researchsquare.com/files/rs-9046312/v1/7b6f297f93673e57f51c133c.png"},{"id":104782857,"identity":"01587884-8f56-49a7-b5d3-108d2ea99a1b","added_by":"auto","created_at":"2026-03-17 07:57:53","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":5044434,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-9046312/v1/fe522205-0ea7-415d-8210-dddd78c537d4.pdf"},{"id":104404954,"identity":"371dc450-e360-4c20-8c74-8317a7443b67","added_by":"auto","created_at":"2026-03-11 12:21:26","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":15476265,"visible":true,"origin":"","legend":"","description":"","filename":"Supplementaryinfromation.docx","url":"https://assets-eu.researchsquare.com/files/rs-9046312/v1/c3a42ad30885bbf771d9bf00.docx"},{"id":104235716,"identity":"f20d5925-97b0-4be4-91bd-ad912e07ad0d","added_by":"auto","created_at":"2026-03-09 13:22:34","extension":"docx","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":10973703,"visible":true,"origin":"","legend":"","description":"","filename":"Scheme1.docx","url":"https://assets-eu.researchsquare.com/files/rs-9046312/v1/7eb10621898386c1fd288b01.docx"}],"financialInterests":"No competing interests reported.","formattedTitle":"Intrinsic Flame Retardancy in Flexible Polyurethane Foams Enabled by a Bio-Based P–N Reactive Polyol and a Silicon-Functionalized Triazine Charring Agent","fulltext":[{"header":"Highlights","content":"\u003cp\u003e\u0026bull; A bio-based reactive P\u0026ndash;N polyol enabled intrinsic flame retardancy in FPUFs.\u003c/p\u003e\u003cp\u003e\u0026bull; Silicon-functionalized triazine enhanced char compactness and thermal stability.\u003c/p\u003e\u003cp\u003e\u0026bull; The P\u0026ndash;N\u0026ndash;Si system showed strong condensed- and gas-phase synergistic effects.\u003c/p\u003e\u003cp\u003e\u0026bull; Heat release and smoke production were significantly reduced during combustion.\u003c/p\u003e\u003cp\u003e\u0026bull; A sustainable strategy was proposed for flame-retardant polyurethane foams.\u003c/p\u003e"},{"header":"1. Introduction","content":"\u003cp\u003eFlexible polyurethane foams (FPUFs) are widely used in upholstered furniture, cushioning and vibration-damping materials, and automotive interiors owing to their excellent resilience, low density, and favorable tensile properties [1\u0026ndash;3]. Conventionally, FPUFs are synthesized from petroleum-derived isocyanates and polyols. However, increasing concerns regarding the depletion of fossil resources and greenhouse gas emissions have stimulated extensive research into polyurethane foams based on bio-derived and renewable feedstocks [4]. Among various renewable resources, plant oils have attracted considerable attention as sustainable alternatives for polyol production due to their low cost, low toxicity, biodegradability, and abundant availability [5, 6].\u003c/p\u003e \u003cp\u003eSoybean oil is regarded as one of the most promising candidates for partially replacing petroleum-based polyols because of its high annual yield, low price, and high degree of unsaturation (iodine value\u0026thinsp;\u0026asymp;\u0026thinsp;4.6) [7]. Considerable efforts have been devoted to the development of soybean oil-based polyols for polyurethane foams. For instance, G.S \u003cem\u003eet al.\u003c/em\u003e [8] used soybean-based polyols to replace petroleum-based polyols in the production of polyurethane foam. Ji \u003cem\u003eet al.\u003c/em\u003e [9]synthesized a soybean oil-based polyol (Polyol-P) via phenol ring-opening of epoxidized soybean oil and reported improved compressive strength, thermal stability, and glass transition temperature (Tg) when the Polyol-P content reached 25 wt%. Herr\u0026aacute;n \u003cem\u003eet al.\u003c/em\u003e [10]further demonstrated that polyurethane foams derived from epoxidized soybean oil and lactic acid exhibited an enhanced specific compressive strength of 7.8 kPa\u0026middot;kg⁻\u0026sup1;\u0026middot;m\u0026sup3;. These studies collectively highlight the feasibility of substituting petroleum-based polyols with soybean oil-based counterparts. Nevertheless, the inherently high flammability of soybean oil-based FPUFs severely restricts their practical applications and poses significant fire hazards [11]. Therefore, improving the flame retardancy of bio-based FPUFs is crucial for expanding their industrial use [12].\u003c/p\u003e \u003cp\u003eThe most common strategy for enhancing the flame retardancy of bio-based FPUFs involves incorporating flame retardants into the polymer matrix either physically or chemically [13]. However, many commercially available flame retardants, particularly halogenated compounds, suffer from toxicity, environmental persistence, and potential health risks, although they are still widely used in industry [14]. Phytic acid (PA), a naturally occurring bio-based compound abundant in legume seeds and cereal germ, is another promising environmentally benign flame retardant owing to its high phosphorus content, renewability, and non-toxicity [15]. However, the poor compatibility of PA with polymer matrices limits its flame-retardant efficiency when used directly [16]. Notably, PA contains multiple phosphate groups with high chemical reactivity [17], enabling its use as a reactive precursor for flame-retardant polymer design. For example, Zhang \u003cem\u003eet al.\u003c/em\u003e [18]synthesized a PA-based flame retardant using glycidyl methacrylate (GMA), which simultaneously improved the flame retardancy and mechanical properties of polylactic acid (PLA). Nevertheless, GMA is petroleum-derived and relatively expensive, which compromises the overall sustainability of the system [19]. Shi \u003cem\u003eet al.\u003c/em\u003e [20]developed multifunctional flame-retardant polyurethane foam laminates by combining PA, expandable graphite, and γ-aminopropyltrimethoxysilane, achieving remarkable reductions in peak heat release rate and peak smoke production rate by 77.5% and 81.8%, respectively. These studies indicate that chemically incorporating flame-retardant elements such as phosphorus, nitrogen, and silicon into polyurethane segments is an effective strategy to achieve durable flame retardancy [21\u0026ndash;23]. Despite these advances, the development of fully bio-based, reactive flame-retardant polyols that simultaneously balance flame retardancy, smoke suppression, and mechanical integrity remains highly challenging.\u003c/p\u003e \u003cp\u003eIn this work, a bio-based reactive flame-retardant polyol was rationally designed to construct an intrinsically flame-retardant flexible polyurethane foam with enhanced thermal stability and fire resistance. Phytic acid and ethanolamine were first reacted to generate a phosphorus\u0026ndash;nitrogen (P\u0026ndash;N) containing polyester polyol, which was subsequently employed to ring-open epoxidized soybean oil (ESO), yielding a P\u0026ndash;N functionalized soybean oil-based polyether polyol (PMESO). The obtained PMESO was used to partially replace conventional petroleum-derived polyether polyols in flexible polyurethane foams, enabling the covalent incorporation of flame-retardant elements into the polymer backbone. On this basis, a silicon-modified triazine-based charring agent (KHT) was further introduced to construct a phosphorus\u0026ndash;nitrogen\u0026ndash;silicon (P\u0026ndash;N\u0026ndash;Si) synergistic flame-retardant system. The effects of PMESO content on the cellular morphology, mechanical properties, thermal degradation behavior, and flame-retardant performance of the resulting flexible polyurethane foams were systematically investigated. Particular emphasis was placed on elucidating the thermal degradation pathways and flame-retardant mechanisms of the P\u0026ndash;N\u0026ndash;Si system by thermogravimetric analysis, cone calorimetry, and char residue analysis. The synergistic roles of phosphorus, nitrogen, and silicon in regulating condensed-phase char formation and gas-phase flame inhibition were comprehensively discussed.\u003c/p\u003e"},{"header":"2. Experimental section","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1 Materials\u003c/h2\u003e \u003cp\u003eESO, commercial polyether polyols (polyether polyol 1030, polyether polyol 1621, and polyether polyol 3628), silicone surfactant (silicone oil 8002), cell opener (1900), and modified isocyanate (8019, a mixture of polyol-modified diphenylmethane diisocyanate and polymethylene polyphenyl isocyanate, with an \u0026ndash;NCO content of 26.0\u0026ndash;27.0%) were supplied by Jining Beidal Trading Co., Ltd. and used as received. The polyether polyol 1030 exhibited a hydroxyl value of 230\u0026ndash;250 mg KOH g⁻\u0026sup1;, while polyether polyol 1621 and polyether polyol 3628 exhibited hydroxyl values of 33\u0026ndash;39 mg KOH g⁻\u0026sup1; and 28\u0026thinsp;\u0026plusmn;\u0026thinsp;3 mg KOH g⁻\u0026sup1;, respectively. Triethylenediamine (A33), dibutyltin dilaurate (DBTDL, T12), and the physical blowing agent 141B were supplied by Dongguan Guang Siyuan Polyurethane Materials Co., Ltd. and used as industrial-grade materials. Phytic acid (PA), monoethanolamine (MEA), trichlorotriazine (TCT), 3-aminopropyl methyldimethoxysilane (KH554), and acetonitrile were purchased from Aladdin Industrial Corporation and used without further purification. Deionized water and ultrapure water were prepared in the laboratory. All chemicals were used as received without further purification.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2 Synthesis of phosphorus\u0026ndash;nitrogen reactive polyol (PMESO)\u003c/h2\u003e \u003cp\u003eThe PMESO was synthesized via a two-step process. First, PA and MEA were subjected to an esterification reaction to obtain a phosphorus\u0026ndash;nitrogen-containing polyester intermediate (PM). Subsequently, PM was reacted with ESO through the ring-opening reaction of epoxy groups, yielding the P\u0026ndash;N functionalized polyol PMESO. The progress of the reaction was monitored by FTIR spectroscopy, and the reaction was considered complete when the characteristic epoxy absorption bands disappeared. The resulting PMESO was used directly for foam preparation without further purification. The process is schematically presented in Scheme \u003cspan refid=\"Sch1\" class=\"InternalRef\"\u003e1a\u003c/span\u003e, and furthre details can be found in S1 of the Supporting Information.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e2.3 Preparation of silicon-modified triazine charring agent (KHT)\u003c/h2\u003e \u003cp\u003eThe KHT was prepared by completely substituting the chlorine atoms of TCT with KH554. Through nucleophilic substitution reactions, a triazine-based charring agent with high silicon content was obtained. The resulting product was collected, washed, and dried prior to use. The process is schematically presented in Scheme \u003cspan refid=\"Sch1\" class=\"InternalRef\"\u003e1b\u003c/span\u003e, and furthre details can be found in S1 of the Supporting Information.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003e2.4 Fabrication of flexible polyurethane foams\u003c/h2\u003e \u003cp\u003eFPUFs were prepared using a one-shot free-rise foaming method (Scheme \u003cspan refid=\"Sch1\" class=\"InternalRef\"\u003e1c\u003c/span\u003e). PMESO partially replaced the commercial polyether polyol at different loadings, and the resulting foams were denoted as FPUF/10P, FPUF/20P, FPUF/30P, and FPUF/40P, where the number indicates the PMESO content (php) relative to the total polyol component.\u003c/p\u003e \u003cp\u003eBased on the optimized PMESO formulation, KHT was further incorporated into FPUF/40P at different contents to obtain FPUF/[email protected], FPUF/[email protected], and FPUF/[email protected], where the number indicates the KHT content (php). After thorough mixing, the reacting mixture was poured into an open mold and allowed to rise freely at ambient temperature. The obtained foams were cured at room temperature prior to characterization. The whole foaming process is schematically presented in Scheme \u003cspan refid=\"Sch1\" class=\"InternalRef\"\u003e1c\u003c/span\u003e, and the detailed formulations of all samples are provided in the Supporting Information (\u003cb\u003eTable \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e\u003c/b\u003e)\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003e2.5 Characterization\u003c/h2\u003e \u003cp\u003eFourier transform infrared (FTIR) spectra were recorded using a TENSOR II spectrometer (Bruker, USA) in the range of 500\u0026ndash;4000 cm⁻\u0026sup1;. \u003csup\u003e1\u003c/sup\u003eH NMR and \u003csup\u003e31\u003c/sup\u003eP NMR spectra were collected using an AVANCE III HD nuclear magnetic resonance spectrometer (Bruker, Germany) with dimethyl sulfoxide (DMSO) as the solvent. X-ray photoelectron spectroscopy (XPS) analysis was conducted using a VG ESCALAB MK II spectrometer (VG Scientific, UK). The cellular morphology of the foams and residual char was observed using a TESCAN MIRA LMS scanning electron microscope (TESCAN, Czech Republic). Mechanical properties were measured using a 104B universal testing machine (Shenzhen Wance Testing Equipment Co., Ltd.) according to the relevant standards. Thermogravimetric analysis (TGA) was performed using an STA 449 F3 thermal analyzer (Netzsch, Germany) under nitrogen and air atmospheres. Samples with a mass of 6\u0026ndash;15 mg were heated at a rate of 20\u0026deg;C min⁻\u0026sup1; with a gas flow rate of 50 mL min⁻\u0026sup1;. Limiting oxygen index (LOI) measurements were conducted using a COI oxygen index tester (Motis Fire Technology, China) according to GB/T 2406\u0026ndash;1993. UL-94 horizontal burning tests were performed using a CZF-6 flammability tester (Nanjing Jiangning Analytical Instrument Co., Ltd.) in accordance with ISO 9772:2012. Cone calorimetry tests were carried out using a BT300-2J cone calorimeter (Suzhou Phoenix Quality Inspection Instrument Co., Ltd.) following ISO 5660-1 at an external heat flux of 35 kW m⁻\u0026sup2;. Raman spectroscopy of residual char was performed using an AploRA PLUS Raman spectrometer (HORIBA, France) with a laser wavelength of 532 nm. TG\u0026ndash;IR analysis was conducted using a TA 8000 thermogravimetric analyzer coupled with a Frontier FTIR spectrometer (PerkinElmer, USA). Pyrolysis\u0026ndash;gas chromatography/mass spectrometry (Py\u0026ndash;GC/MS) analysis was carried out using an EGA/PY-3030D pyrolyzer (Frontier Laboratories, Japan) coupled with a GCMS-QP2020NX system (Shimadzu, Japan).\u003c/p\u003e \u003c/div\u003e"},{"header":"3. Results and discussion","content":"\u003cdiv id=\"Sec9\" class=\"Section2\"\u003e \u003ch2\u003e3.1 Structure confirmation of PMESO and KHT\u003c/h2\u003e \u003cp\u003eThe chemical structures of the PMESO and the KHT were confirmed by FTIR, \u003csup\u003e1\u003c/sup\u003eH NMR, and XPS analyses, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea, the characteristic epoxy absorption bands of ESO disappear completely after the ring-opening reaction, accompanied by the emergence of broad hydroxyl absorption and phosphorus-related bands in the PMESO spectrum, indicating successful epoxy ring opening [24] and functionalization. The \u003csup\u003e1\u003c/sup\u003eH NMR spectra (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb) further confirm the structural evolution, where the disappearance of epoxy proton signals and the appearance of new hydroxyl-bearing methine and methylene resonances demonstrate the effective incorporation of phosphorus\u0026ndash;nitrogen functionalities into the polyol backbone [25]. The FTIR spectrum of KHT (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ec) exhibits characteristic absorptions associated with triazine structures as well as silicon-containing bonds [23, 26], indicating successful modification. The presence of Si and N elements is further confirmed by XPS survey analysis (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ed). Additional structural evidence, including FTIR spectra of intermediate products, XRD patterns, and high-resolution XPS spectra, is provided in the Supporting Information (\u003cb\u003eFigure \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e and S2\u003c/b\u003e). These results collectively confirm the successful synthesis of PMESO and KHT, providing a reliable structural basis for subsequent investigations of their flame-retardant behavior.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec10\" class=\"Section2\"\u003e \u003ch2\u003e3.2 Effect of reactive PMESO and subsequent KHT incorporation on cellular morphology and mechanical integrity\u003c/h2\u003e \u003cp\u003eThe pore structure is a key factor influencing the properties of flexible polyurethane foam, as it is affected by bubble nucleation and growth during the foaming process [27]. The influence of reactive PMESO and subsequent KHT incorporation on the cellular morphology of flexible polyurethane foams was examined by SEM analysis (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). Pristine FPUF exhibits relatively irregular cell structures with a broad cell size distribution. Upon incorporation of PMESO at an optimized content (FPUF/40P), the foam displays more uniform and well-defined cellular morphology, suggesting improved bubble stabilization during the foaming process [28]. This behavior can be attributed to the increased polarity and hydrogen-bonding capability introduced by the P\u0026ndash;N-containing polyol [6]. With further incorporation of KHT, the cellular structure evolves depending on the KHT content. At low and moderate KHT loadings (FPUF/[email protected] and FPUF/[email protected]), the foams maintain relatively uniform cell structures, whereas excessive KHT addition (FPUF/[email protected]) leads to a slight enlargement and distortion of cells, likely due to increased viscosity and localized heterogeneity during foam formation.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe mechanical properties of the foams were evaluated through tensile and cyclic compression\u0026ndash;unloading tests [12] (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). The mechanical properties of foam primarily depend on tensile strength, elongation at break, and compressive strength [29]. Compared with pristine FPUF, FPUF/40P exhibits comparable elongation and mechanical resilience, indicating that partial replacement of petroleum-based polyols by PMESO does not compromise mechanical integrity. The incorporation of KHT leads to a gradual change in mechanical response, with moderate KHT contents preserving acceptable elasticity and energy dissipation behavior. These results demonstrate that the FPUF/40P formulation provides a robust structural platform for further flame-retardant modification.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eSupporting Information (\u003cb\u003eFigure. S2\u0026ndash;S4\u003c/b\u003e) further illustrates the evolution of cellular morphology, mechanical properties, and macroscopic appearance of FPUFs containing different PMESO contents, confirming that PMESO contents beyond 40 php adversely affect the foaming process. Therefore, FPUF/40P was selected as the optimal base formulation for subsequent KHT incorporation. These results indicate that appropriate incorporation of PMESO and KHT enables flame-retardant modification without sacrificing the cellular structure and mechanical integrity of flexible polyurethane foams.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003e3.3 Thermal degradation behavior of FPUFs containing PMESO and KHT\u003c/h2\u003e \u003cp\u003eThe thermal degradation behavior of flexible polyurethane foams with different formulations was investigated by TGA under nitrogen and air atmospheres, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e, and the corresponding characteristic parameters are summarized in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. Under a nitrogen atmosphere (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea and \u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb), pristine FPUF exhibits a typical two-stage degradation process [29, 30] with an initial decomposition temperature (\u003cem\u003eT\u003c/em\u003e₅\u003csub\u003e%\u003c/sub\u003e) of 317.7\u0026deg;C and a maximum mass loss rate (\u003cem\u003eT\u003c/em\u003e\u003csub\u003emax₁\u003c/sub\u003e) at 395.4\u0026deg;C, leaving a limited char residue of 8.95 wt% at 800\u0026deg;C. In contrast, incorporation of the reactive P\u0026ndash;N polyol (FPUF/40P) leads to a pronounced change in the degradation profile. Although the \u003cem\u003eT\u003c/em\u003e₅\u003csub\u003e%\u003c/sub\u003e value decreases markedly to 118.8\u0026deg;C, the char yield increases significantly to 15.73 wt%, indicating that the phosphorus\u0026ndash;nitrogen functionalities promote early dehydration and carbonization during thermal decomposition [31]. Meanwhile, the \u003cem\u003eT\u003c/em\u003e\u003csub\u003emax₁\u003c/sub\u003e value remains at a comparable level (386.5\u0026deg;C), suggesting that the main-chain degradation of the polyurethane matrix is not severely disrupted. With further incorporation of the silicon-modified triazine charring agent, the char-forming ability of the foams is further enhanced. As the KHT content increases from 0.5 to 3.0 wt%, the char yield under nitrogen atmosphere gradually rises from 16.21 wt% to 17.41 wt%, which is almost twice that of pristine FPUF. Notably, the \u003cem\u003eT\u003c/em\u003e\u003csub\u003emax₁\u003c/sub\u003e values of the KHT-containing foams remain in a narrow range of 387.8\u0026ndash;389.5\u0026deg;C, indicating that the P\u0026ndash;N\u0026ndash;Si synergistic system mainly affects the carbonization process rather than accelerating the catastrophic degradation of the polymer backbone [32].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eSimilar trends are observed under air atmosphere (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ec and \u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ed). Pristine FPUF shows almost complete oxidation of the char, leaving a negligible residue of only 0.16 wt% at 800\u0026deg;C. In contrast, FPUF/40P exhibits a substantially increased char residue of 4.05 wt%, demonstrating improved resistance to oxidative degradation. Upon incorporation of KHT, the residual char further increases, reaching 5.71 wt% for FPUF/[email protected]. This enhancement can be attributed to the formation of thermally stable silicon-containing structures, which effectively retard char oxidation at elevated temperatures [20]. It is worth noting that the early mass loss observed for PMESO- and KHT-containing foams is associated with the decomposition of phosphorus-containing groups and triazine-based structures. However, this early-stage decomposition plays a beneficial role by triggering rapid char formation, which subsequently protects the underlying polymer from further thermal and oxidative attack. Overall, these results demonstrate that the combined incorporation of PMESO and KHT effectively regulates the thermal degradation pathway of flexible polyurethane foams, leading to promoted char formation and enhanced char stability, particularly under oxidative conditions, which is critical for improved flame retardancy.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eTG data of of FPUFs with different formulations\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"8\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eSample\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colspan=\"4\" nameend=\"c5\" namest=\"c2\"\u003e \u003cp\u003eN\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colspan=\"3\" nameend=\"c8\" namest=\"c6\"\u003e \u003cp\u003eAIR\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003ctr\u003e \u003cth align=\"left\" colspan=\"2\" nameend=\"c3\" namest=\"c2\"\u003e \u003cp\u003e\u003cem\u003eT\u003c/em\u003e\u003csub\u003e5%\u003c/sub\u003e(℃)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u003cem\u003eT\u003c/em\u003e\u003csub\u003emax1\u003c/sub\u003e(℃)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eC\u003csub\u003ewt\u003c/sub\u003e\u003csup\u003e800\u003c/sup\u003e(%)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003e\u003cem\u003eT\u003c/em\u003e\u003csub\u003e5%\u003c/sub\u003e(℃)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\"\u003e \u003cp\u003e\u003cem\u003eT\u003c/em\u003e\u003csub\u003emax1\u003c/sub\u003e(℃)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c8\"\u003e \u003cp\u003eC\u003csub\u003ewt\u003c/sub\u003e\u003csup\u003e800\u003c/sup\u003e(%)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c2\" namest=\"c1\"\u003e \u003cp\u003eKHT\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e219.9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e276.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e4.49\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e219.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e279.9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e7.92\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c2\" namest=\"c1\"\u003e \u003cp\u003ePristine FPUF\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e317.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e395.4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e8.95\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e318.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e389.91\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e0.16\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c2\" namest=\"c1\"\u003e \u003cp\u003eFPUF/40P\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e118.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e386.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e15.73\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e116.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e379.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e4.05\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c2\" namest=\"c1\"\u003e \u003cp\u003eFPUF/[email protected]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e148.9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e389.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e16.21\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e129.4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e359.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e4.18\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c2\" namest=\"c1\"\u003e \u003cp\u003eFPUF/[email protected]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e136.4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e388.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e17.22\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e140.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e369.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e4.22\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c2\" namest=\"c1\"\u003e \u003cp\u003eFPUF/[email protected]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e122.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e387.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e17.41\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e174.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e376.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e5.71\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec12\" class=\"Section2\"\u003e \u003ch2\u003e3.4 Fire behavior and intrinsic flame retardancy of PMESO/KHT-modified FPUFs\u003c/h2\u003e \u003cp\u003eThe intrinsic flame-retardant performance of flexible polyurethane foams was first evaluated by LOI measurements and UL-94 horizontal burning tests [33], and the results are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e. Pristine FPUF exhibits a low LOI value of 18.8%, reflecting its inherent flammability. After incorporation of the reactive phosphorus\u0026ndash;nitrogen polyol, the LOI value of FPUF/40P increases markedly to 22.6%, demonstrating that the covalent introduction of P\u0026ndash;N functionalities significantly enhances the intrinsic flame retardancy of the foam. With further incorporation of the silicon-modified triazine charring agent (KHT), the LOI values show a gradual increase to 23.1%, 23.2%, and 23.4% for FPUF/[email protected], FPUF/[email protected], and FPUF/[email protected], respectively. Although the increment becomes less pronounced at higher KHT contents, the consistently elevated LOI values indicate a positive contribution of the P\u0026ndash;N\u0026ndash;Si synergistic system to flame inhibition [34].\u003c/p\u003e \u003cp\u003eThe UL-94 horizontal burning behavior further reveals the effect of KHT on flame propagation and self-extinguishing performance (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eb). While pristine FPUF and FPUF/40P show continuous flame propagation along the sample surface, the incorporation of KHT induces clear self-extinguishing behavior. For FPUF/[email protected], the burning rate is significantly reduced compared with FPUF/40P, and the sample self-extinguishes after reaching the first scale mark within 30 s, leaving a rigid and intact char without bending or dripping. Notably, FPUF/[email protected] exhibits the most pronounced flame inhibition, where the flame retreats and extinguishes almost immediately after removal of the ignition source at 30 s. For all KHT-containing samples, no molten dripping is observed on the flame-exposed surface during the UL-94 test. These observations indicate that KHT effectively enhances the condensed-phase flame-retardant performance of PMESO-modified FPUFs. The improved self-extinguishing behavior can be attributed to the synergistic charring effect between PMESO and KHT. During combustion, PMESO promotes dehydration and early char formation, while the silane-modified triazine structure of KHT facilitates the formation of thermally stable Si\u0026ndash;C and Si\u0026ndash;O structures at the flame-exposed surface through interactions with the thermal degradation products of the polyurethane matrix. These silicon-containing structures contribute to carbon fixation and the formation of a robust protective layer, thereby inhibiting flame propagation.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eTo further evaluate the fire behavior under forced flaming conditions, cone calorimetry tests were performed, and the corresponding heat and smoke release parameters are summarized in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e and \u003cb\u003eTable\u0026nbsp;2\u003c/b\u003e. Pristine FPUF ignites rapidly with a time to ignition (TTI) of 7 s and exhibits a high peak heat release rate (PHRR) of 210.38 kW m⁻\u0026sup2;, together with a large total heat release (THR) of 32.50 MJ m⁻\u0026sup2;, indicating severe fire hazards. After incorporation of PMESO, the PHRR of FPUF/40P decreases significantly to 152.01 kW m⁻\u0026sup2;, corresponding to a reduction of approximately 28% compared with pristine FPUF. Meanwhile, the THR is dramatically reduced to 12.99 MJ m⁻\u0026sup2;, confirming that the reactive P\u0026ndash;N polyol effectively suppresses combustion intensity and heat output by promoting char formation and limiting heat feedback.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe introduction of KHT further suppresses heat release behavior. Among the investigated samples, FPUFUF/[email protected] exhibits the lowest PHRR value of 124.09 kW m⁻\u0026sup2;, representing a reduction of nearly 41% relative to pristine FPUF. Although the PHRR slightly increases at higher KHT contents, all P\u0026ndash;N\u0026ndash;Si-modified foams maintain substantially lower PHRR and THR values than pristine FPUF, demonstrating the effectiveness of the synergistic flame-retardant system. Smoke production behavior was also analyzed in detail. Pristine FPUF shows a relatively low total smoke production (TSP) of 2.97 m\u0026sup2;, but this is accompanied by a high average effective heat of combustion (Avg EHC) of 21.03 MJ kg⁻\u0026sup1;, indicating efficient flaming combustion. After incorporation of PMESO, the Avg EHC decreases markedly to 13.94 MJ kg⁻\u0026sup1;, suggesting a significant inhibition of gas-phase combustion efficiency. Although the TSP of FPUF/40P increases to 4.29 m\u0026sup2;, this increase can be attributed to incomplete combustion and the release of char-derived particulates rather than intensified flaming [35].\u003c/p\u003e \u003cp\u003e \u003cb\u003eTable.2.\u003c/b\u003e Data of FPUFs with different formulations obtained by cone calorimetry tests\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"No\" id=\"Taba\" border=\"1\"\u003e \u003ccolgroup cols=\"7\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSample\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eTTI\u003c/p\u003e \u003cp\u003e(s)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u003cem\u003et\u003c/em\u003e\u003csub\u003ep\u003c/sub\u003e\u003c/p\u003e \u003cp\u003e(s)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003ePHRR\u003c/p\u003e \u003cp\u003e(kW/m\u003csup\u003e2\u003c/sup\u003e)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eTHR (MJ/m\u003csup\u003e2\u003c/sup\u003e)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eTSP\u003c/p\u003e \u003cp\u003e(m\u003csup\u003e2\u003c/sup\u003e)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\"\u003e \u003cp\u003eAvg EHC\u003c/p\u003e \u003cp\u003e(MJ/Kg)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePristine FPUF\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e37\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e210.38\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e32.504\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e2.967\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e21.032\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eFPUF/40P\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e29\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e152.01\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e12.985\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e4.285\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e13.935\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eFPUF/[email protected]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e33\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e124.09\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e12.757\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e3.395\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e12.922\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eFPUF/[email protected]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e26\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e140.77\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e12.487\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e4.426\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e13.250\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eFPUF/[email protected]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e29\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e138.63\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e10.723\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e2.735\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e14.722\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eNotably, the incorporation of KHT effectively regulates smoke release behavior. FPUF/[email protected] and FPUF/[email protected] exhibit reduced TSP values of 3.40 m\u0026sup2; and 2.74 m\u0026sup2;, respectively, compared with FPUF/40P. This improvement is associated with the formation of a denser and more stable P\u0026ndash;N\u0026ndash;Si char layer, which suppresses the release of smoke-forming volatiles during combustion. Meanwhile, the Avg EHC values of KHT-containing foams remain significantly lower than that of pristine FPUF, confirming sustained inhibition of combustion efficiency. Overall, the combined LOI, UL-94, and cone calorimetry results demonstrate that the PMESO/KHT system effectively enhances the flame retardancy and fire safety of flexible polyurethane foams. The P\u0026ndash;N\u0026ndash;Si synergistic system suppresses heat release, promotes self-extinguishing behavior, and regulates smoke production through coordinated condensed-phase protection and gas-phase combustion inhibition.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec13\" class=\"Section2\"\u003e \u003ch2\u003e3.5 Condensed-phase flame-retardant mechanism\u003c/h2\u003e \u003cp\u003eTo elucidate the condensed-phase flame-retardant mechanism of the PMESO/KHT system, the morphology and chemical structure of the residual char after cone calorimetry tests were systematically analyzed. The digital photographs of the residual char (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e) clearly reveal distinct differences among the samples. After combustion, pristine FPUF leaves only fragmented and fragile mesh-like residues, and the aluminum foil wrapping the sample is completely penetrated, indicating the absence of an effective protective char layer. In contrast, FPUF/40P forms a relatively continuous char layer; however, obvious pores and defects are observed on the char surface, which can be attributed to gas-phase release during combustion. Upon incorporation of KHT, this porous structure is significantly suppressed. Notably, FPUF/[email protected] produces a thick, rigid, and mechanically robust char layer tightly adhered to the aluminum foil without fragmentation. The disappearance of pores and the appearance of a compact and glossy char surface demonstrate a markedly enhanced condensed-phase protective effect. The internal microstructure of the residual char was further examined by SEM at a magnification of 200\u0026times; (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e). The residual char of pristine FPUF is characterized by numerous spherical particulates and an uneven surface, reflecting insufficient carbonization. FPUF/40P shows partial improvement; however, fractured openings and uneven regions are still present. In contrast, the incorporation of KHT leads to a dense, smooth, and continuous block-like char structure with significantly reduced pores and surface irregularities. The elimination of spherical particulates and fragmented openings indicates that KHT effectively promotes char densification and suppresses the release of volatile degradation products.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFTIR analysis of the residual char (\u003cb\u003eFigure S5\u003c/b\u003e) provides further insight into the chemical composition of the char layer. Compared with the residual chars of pristine FPUF and FPUF/40P, the char of KHT-containing samples exhibits a significantly intensified absorption band at 3377 cm⁻\u0026sup1;, corresponding to hydroxyl groups, indicating the formation of a larger amount of hydroxyl-containing species. Meanwhile, the characteristic absorption bands associated with \u0026ndash;CH₃ and \u0026ndash;CH₂ stretching vibrations at 2960 cm⁻\u0026sup1;, 2924 cm⁻\u0026sup1;, and 2854 cm⁻\u0026sup1; are enhanced. The disappearance of the absorption band at 2222 cm⁻\u0026sup1; (HC\u0026equiv;\u0026ndash;R stretching vibration), together with the appearance and shift of absorption bands corresponding to C\u0026thinsp;\u0026equiv;\u0026thinsp;N stretching (2310 cm⁻\u0026sup1;), symmetric bending vibration of (NH₄)⁺ (1707 cm⁻\u0026sup1;), and tertiary amine C\u0026ndash;N stretching (1044 cm⁻\u0026sup1;), indicate the formation of various nitrogen-containing compounds in the residual char due to the introduction of KHT. In addition, a new absorption band appears at 881 cm⁻\u0026sup1;, which can be assigned to the symmetric stretching vibration of Si\u0026ndash;O\u0026ndash;Si, confirming the direct participation of silicon-containing species in char formation. These results demonstrate a synergistic effect between PMESO and KHT in promoting the formation of a more complete and chemically complex char structure.\u003c/p\u003e \u003cp\u003eRaman spectroscopy was employed to evaluate the structural ordering and continuity of carbon atoms in the residual char (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e). The intensity ratio of the D band to the G band (I\u003csub\u003eD\u003c/sub\u003e/I\u003csub\u003eG\u003c/sub\u003e) [36], which reflects the degree of disorder and graphitization, decreases significantly upon incorporation of KHT. Specifically, the I\u003csub\u003eD\u003c/sub\u003e/I\u003csub\u003eG\u003c/sub\u003e value decreases from 4.26 for pristine FPUF to 3.04 for KHT-containing samples, corresponding to a reduction of 28.64%, and is 13.14% lower than that of FPUF/40P. The pronounced decrease in the I\u003csub\u003eD\u003c/sub\u003e/I\u003csub\u003eG\u003c/sub\u003e ratio indicates enhanced graphitization and increased structural compactness of the residual char, confirming that KHT further improves char densification beyond the effect of PMESO alone.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe elemental composition of the residual char was further analyzed by EDS (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e). Phosphorus is uniformly distributed over the char surface, with the P content reaching 13.5 wt%, indicating effective retention of phosphorus-containing species in the condensed phase. Meanwhile, the contents of silicon and oxygen increase to 1.2 wt% and 44.2 wt%, respectively, compared with pristine FPUF. These results suggest that the incorporation of KHT promotes the formation of phosphorus- and silicon-containing char structures during combustion. The resulting P\u0026ndash;O\u0026ndash;C and Si\u0026ndash;O\u0026ndash;C networks form a robust physical barrier that effectively retards heat transfer and mass diffusion. Notably, the nitrogen content in the residual char decreases from 8.4 wt% in FPUF/40P to 3.5 wt% after KHT incorporation, which can be attributed to the generation of nonflammable nitrogen-containing gases derived from triazine structures. This phenomenon suggests that KHT not only contributes to condensed-phase protection but also participates in gas-phase flame retardancy by diluting combustible volatiles.\u003c/p\u003e \u003cp\u003eOverall, the combined morphological, spectroscopic, and compositional analyses demonstrate that the PMESO/KHT system exhibits a pronounced condensed-phase flame-retardant mechanism. The synergistic effects of phosphorus\u0026ndash;nitrogen catalytic charring, triazine-induced carbonization, and silicon-assisted carbon fixation promote the formation of a dense, continuous, and thermally stable char layer. This char layer acts as an effective physical barrier, suppressing heat transfer, mass diffusion, and oxygen penetration, thereby playing a crucial role in enhancing the flame retardancy of flexible polyurethane foams.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec14\" class=\"Section2\"\u003e \u003ch2\u003e3.6 Gas-phase flame-retardant behavior\u003c/h2\u003e \u003cp\u003eTo further elucidate the gas-phase flame-retardant mechanism of the PMESO/KHT system, the volatile products released during thermal degradation were analyzed by thermogravimetric analysis coupled with infrared spectroscopy (TG\u0026ndash;IR) and pyrolysis gas chromatography\u0026ndash;mass spectrometry (Py\u0026ndash;GC/MS), as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e and summarized in \u003cb\u003eTables S2\u0026ndash;S4\u003c/b\u003e. The TG\u0026ndash;IR spectra of the evolved gases (Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003ea\u0026ndash;c) reveal clear differences in the composition and evolution behavior of volatile products among pristine FPUF, FPUF/40P, and FPUF/[email protected]. Compared with pristine FPUF and FPUF/40P, FPUF/[email protected] exhibits significantly stronger and sharper absorption bands at 3015 cm⁻\u0026sup1; (olefinic C═H stretching), 2972 cm⁻\u0026sup1; and 2930 cm⁻\u0026sup1; (\u0026ndash;CH₃ asymmetric stretching), and 2864 cm⁻\u0026sup1; (\u0026ndash;CH₂ stretching), indicating the formation of a larger amount of small-molecule alkanes, carbonyl compounds, and aliphatic ester fragments containing C\u0026ndash;O or C═O groups. These observations suggest that the introduction of KHT promotes fragmentation into low-molecular-weight species during thermal degradation.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eMeanwhile, the absorption bands at 3364 cm⁻\u0026sup1; and 3468 cm⁻\u0026sup1;, attributed to \u0026ndash;OH stretching vibrations, become more pronounced at elevated temperatures (around 305\u0026deg;C) for KHT-containing samples. This enhancement indicates intensified dehydration and carbonization reactions triggered by the thermal decomposition of KHT, leading to the release of additional water vapor into the gas phase. The evolution of water vapor not only facilitates char consolidation in the condensed phase but also dilutes flammable gaseous products, thereby reducing combustion intensity. Notably, the absorption intensity of the CO₂ characteristic peak at 2310 cm⁻\u0026sup1; is weakened after the incorporation of KHT, which is consistent with the reduced CO₂ production rate observed in cone calorimetry tests. This result implies that the gas-phase oxidation reactions are partially suppressed in the presence of the P\u0026ndash;N\u0026ndash;Si synergistic system.\u003c/p\u003e \u003cp\u003eIn addition, several nitrogen-related characteristic absorption bands become more prominent for KHT-containing foams. The absorption bands located near 1376 cm⁻\u0026sup1; and 928 cm⁻\u0026sup1;, associated with N═N and NH₃ stretching vibrations, together with the band at 1514 cm⁻\u0026sup1; corresponding to \u0026ndash;NH vibrations in aromatic compounds, exhibit increased intensities compared with pristine FPUF and FPUF/40P. These results indicate that the incorporation of KHT promotes the generation of nitrogen-containing nonflammable gases in the gas phase, which dilute the combustible atmosphere and suppress flame propagation. More importantly, a pronounced absorption band appears at 1273 cm⁻\u0026sup1;, corresponding to the P═O stretching vibration, with significantly enhanced intensity for FPUF/[email protected]. This result demonstrates that the presence of KHT facilitates the release and volatilization of phosphorus-containing species into the gas phase. These phosphorus-derived radicals can effectively scavenge flame-propagating radicals (e.g., H\u0026middot; and OH\u0026middot;), thereby interrupting chain reactions in the flame and inhibiting fire spread [37].\u003c/p\u003e \u003cp\u003eTo further identify the chemical nature of gaseous degradation products, Py\u0026ndash;GC/MS analysis was conducted at \u003cb\u003e500\u0026deg;C\u003c/b\u003e, and the resulting chromatograms are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003ed\u0026ndash;f, with detailed peak assignments listed in \u003cb\u003eTables S2\u0026ndash;S4\u003c/b\u003e. For FPUF/[email protected], the appearance of CO₂, butane, and other small hydrocarbon fragments indicates that the incorporation of flame retardants does not alter the fundamental degradation pathway of FPUF, which still follows an intramolecular rearrangement mechanism. However, compared with FPUF/40P, FPUF/[email protected] generates a higher proportion of small-molecule hydrocarbons, such as CO₂, propylene, ethylene oxide, and propylene oxide, at short retention times. These results further corroborate the TG\u0026ndash;IR observations.\u003c/p\u003e \u003cp\u003eIn addition, a variety of nitrogen-containing aromatic and aliphatic compounds, including substituted aniline derivatives, are detected at longer retention times for KHT-containing foams. The generation of these nitrogen-rich, nonflammable species further supports the role of KHT in gas-phase dilution and flame inhibition. Moreover, silicon-containing compounds, such as octylsilane and 1,4-bis(dimethylsilyl)butane, are detected exclusively in the degradation products of FPUF/[email protected]. These results indicate that silicon-containing fragments derived from KHT can volatilize into the gas phase and participate in flame suppression by diluting combustible gases and interacting synergistically with nitrogen-containing species.\u003c/p\u003e \u003cp\u003eOverall, the TG\u0026ndash;IR and Py\u0026ndash;GC/MS analyses demonstrate that the PMESO/KHT system exhibits a pronounced gas-phase flame-retardant mechanism. The synergistic release of phosphorus-containing radicals, nitrogen-derived nonflammable gases, water vapor, and silicon-containing fragments effectively dilutes the combustible atmosphere and interrupts flame-propagating reactions. Together with the condensed-phase char-forming effect, these gas-phase actions contribute to the overall P\u0026ndash;N\u0026ndash;Si synergistic flame retardancy of flexible polyurethane foams.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec15\" class=\"Section2\"\u003e \u003ch2\u003e3.7 Overall P\u0026ndash;N\u0026ndash;Si synergistic flame-retardant mechanism\u003c/h2\u003e \u003cp\u003eBased on the comprehensive analysis of thermal degradation behavior, fire performance, residual char characteristics, and evolved gaseous products, an overall P\u0026ndash;N\u0026ndash;Si synergistic flame-retardant mechanism for PMESO/KHT-modified flexible polyurethane foams is proposed, as schematically illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e. Upon exposure to heat or flame, the reactive phosphorus\u0026ndash;nitrogen polyol (PMESO) undergoes early-stage decomposition, which initiates dehydration and promotes rapid char formation. This process is evidenced by the reduced initial decomposition temperature in TGA analysis and the significantly increased char yield under both nitrogen and air atmospheres. The phosphorus-containing structures facilitate the formation of phosphoric and polyphosphoric species, which catalyze the carbonization of the polyurethane matrix and suppress melt dripping during combustion. Meanwhile, nitrogen-containing moieties contribute to the generation of thermally stable char precursors and inert gaseous products, laying the foundation for both condensed- and gas-phase flame-retardant actions.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eWith the incorporation of KHT, the flame-retardant mechanism is further reinforced through multiple synergistic pathways. In the condensed phase, the triazine ring structure enhances carbonization efficiency, while the grafted silane functionalities promote the formation of thermally stable Si\u0026ndash;O\u0026ndash;C and Si\u0026ndash;C networks during combustion. These silicon-containing structures act as effective carbon-fixation centers, leading to the formation of a dense, continuous, and mechanically robust char layer. As confirmed by SEM, Raman spectroscopy, and EDS analysis, the resulting char exhibits enhanced compactness, increased graphitization degree, and enriched phosphorus and silicon contents, which collectively improve the barrier effect against heat transfer, mass diffusion, and oxygen penetration.\u003c/p\u003e \u003cp\u003eSimultaneously, a pronounced gas-phase flame-retardant effect is activated. TG\u0026ndash;IR and Py\u0026ndash;GC/MS analyses reveal that the PMESO/KHT system promotes the release of multiple flame-inhibiting species into the gas phase. Phosphorus-containing radicals volatilized from PMESO effectively scavenge flame-propagating radicals, thereby interrupting chain reactions in the flame zone. Nitrogen-containing nonflammable gases and ammonia derivatives derived from the triazine structure dilute the concentration of combustible gases and oxygen, suppressing flame intensity. In addition, the thermal decomposition of KHT generates water vapor and silicon-containing fragments, which further dilute the combustible atmosphere and contribute to flame inhibition. The combined action of these species leads to reduced combustion efficiency, as evidenced by the decreased average effective heat of combustion and suppressed CO₂ evolution. The coordinated action of the condensed-phase barrier effect and gas-phase flame inhibition results in significantly reduced heat release, improved self-extinguishing behavior, and regulated smoke production, as demonstrated by LOI, UL-94, and cone calorimetry tests. Importantly, this synergistic P\u0026ndash;N\u0026ndash;Si system achieves effective flame retardancy at relatively low additive loadings while maintaining acceptable mechanical integrity and cellular structure of the flexible polyurethane foams.\u003c/p\u003e \u003cp\u003eIn summary, the PMESO/KHT system establishes a highly efficient and sustainable flame-retardant strategy for flexible polyurethane foams through a well-balanced P\u0026ndash;N\u0026ndash;Si synergistic mechanism. The integration of catalytic charring, carbon fixation, and gas-phase radical suppression provides comprehensive fire protection, highlighting the potential of bio-based, reactive flame-retardant systems for advanced polymeric materials.\u003c/p\u003e \u003c/div\u003e"},{"header":"4. Conclusion","content":"\u003cp\u003eIn this work, a sustainable P\u0026ndash;N\u0026ndash;Si synergistic flame-retardant system was successfully developed for flexible polyurethane foams by integrating a reactive bio-based phosphorus\u0026ndash;nitrogen polyol (PMESO) with a silicon-functionalized triazine charring agent (KHT). The reactive incorporation of PMESO imparted intrinsic flame retardancy while preserving the cellular structure and mechanical integrity of the foams, whereas the introduction of KHT further enhanced fire safety through effective condensed- and gas-phase synergy. Thermogravimetric analysis revealed that the PMESO/KHT system regulated the thermal degradation pathway by promoting dehydration and carbonization, leading to increased char yield, particularly under oxidative conditions. Fire performance evaluations demonstrated markedly improved flame retardancy, including increased LOI values, clear self-extinguishing behavior in UL-94 tests, and significantly reduced heat release and combustion efficiency in cone calorimetry, accompanied by effective smoke suppression at appropriate KHT loadings. Char characterization confirmed the formation of a dense, continuous, and thermally stable char layer resulting from the combined effects of phosphorus\u0026ndash;nitrogen catalytic charring, triazine-induced carbonization, and silicon-assisted carbon fixation. In addition, TG\u0026ndash;IR and Py\u0026ndash;GC/MS analyses indicated that the release of phosphorus-containing species, nitrogen-derived nonflammable gases, and dehydration-related volatiles contributed to gas-phase flame inhibition. Overall, this P\u0026ndash;N\u0026ndash;Si synergistic strategy offers an effective and sustainable route to enhancing flame retardancy and fire safety of flexible polyurethane foams. The results provide mechanistic insights into multi-element synergistic flame-retardant systems and highlight a promising approach for the design of advanced bio-based flame-retardant polymeric materials.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003eCRediT authorship contribution statement\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eLugui Lan:\u003c/strong\u003e Investigation, Writing - Original Draft. \u003cstrong\u003eYanping Mo:\u003c/strong\u003e Investigation, Data curation, Writing - Original Draft.\u0026nbsp;\u003cstrong\u003eYuhui Xie:\u003c/strong\u003e Writing \u0026ndash; review \u0026amp; editing, Methodology, Resources, Funding acquisition, Supervision.\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003e\u003cstrong\u003eFeng Wu:\u0026nbsp;\u003c/strong\u003eInvestigation, Validation, Resources, Writing \u0026ndash; review \u0026amp; editing. \u003cstrong\u003eDong Feng:\u0026nbsp;\u003c/strong\u003eValidation, Resources, Writing \u0026ndash; review \u0026amp; editing. \u003cstrong\u003eYang Meng:\u0026nbsp;\u003c/strong\u003eInvestigation, Writing \u0026ndash; review \u0026amp; editing. \u003cstrong\u003eYi Mei:\u003c/strong\u003e Funding acquisition, Writing \u0026ndash; review \u0026amp; editing. \u003cstrong\u003eDelong Xie:\u003c/strong\u003e Writing \u0026ndash; review \u0026amp; editing, Funding acquisition, Project administration.\u003c/p\u003e\n\u003cp\u003eDeclaration of Competing Interest\u003c/p\u003e\n\u003cp\u003eThe authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.\u003c/p\u003e\n\u003cp\u003eAcknowledgments\u003c/p\u003e\n\u003cp\u003eThe authors are grateful for financial support from the National Natural Science Foundation of China (No. 22268025), the Key Research and Development Program of Yunnan Province (No. 202403AA080003).\u003c/p\u003e\n\u003cp\u003eFunding Declaration\u003c/p\u003e\n\u003cp\u003eThis work was supported by the National Natural Science Foundation of China (No. 22268025) and the Key Research and Development Program of Yunnan Province (No. 202403AA080003).\u003c/p\u003e\n\u003cp\u003eData Availability\u003c/p\u003e\n\u003cp\u003eData will be made available upon request.\u003c/p\u003e\n\u003cp\u003eAppendix A. Supporting information\u003c/p\u003e\n\u003cp\u003eSupplementary data associated with this article can be found in the online version. The Supporting Information provides additional structural, morphological, mechanical, thermal, and gas-phase characterization data that support the discussions and conclusions presented in the main text.\u003c/p\u003e\n\u003cp\u003eDeclaration of generative AI and AI-assisted technologies in the writing process\u003c/p\u003e\n\u003cp\u003eDuring the preparation of this work the authors used ChatGPT solely to improve language and readability. After using this tool, the authors reviewed and edited the content as needed and take full responsibility for the content of the publication.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eShen C, Shao R, Wang W, Wu X, Zhou B, Zhao L, Siddique A, Xu Z, (2024) Progress of flame retardant research on flexible polyurethane foam, Eur. Polym. 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In this work, a bio-based phosphorus\u0026ndash;nitrogen\u0026ndash;silicon (P\u0026ndash;N\u0026ndash;Si) synergistic flame-retardant system was rationally designed for flexible polyurethane foams (FPUFs) using renewable feedstocks. A reactive phosphorus\u0026ndash;nitrogen polyol (PMESO), derived from epoxidized soybean oil and phytic acid, was chemically incorporated into the polyurethane network to impart intrinsic flame retardancy, while a silicon-functionalized triazine charring agent (KHT) was introduced to reinforce condensed-phase protection. Thermal analysis revealed that PMESO promoted early dehydration and catalytic charring, significantly increasing char yield, whereas KHT further enhanced char compactness and thermal stability under oxidative conditions. Fire performance evaluation demonstrated that the combined system markedly improved flame retardancy, as evidenced by increased limiting oxygen index values, pronounced self-extinguishing behavior, and substantial reductions in heat release and combustion efficiency. Comprehensive char characterization confirmed the formation of a dense, continuous, and P\u0026ndash;N\u0026ndash;Si\u0026ndash;enriched char layer, which effectively acted as a physical barrier against heat and oxygen transfer. In addition, TG\u0026ndash;IR and Py\u0026ndash;GC/MS analyses indicated the release of nonflammable and flame-inhibiting species into the gas phase, providing auxiliary suppression of flame propagation. Overall, this study demonstrates how a reactive, bio-derived P\u0026ndash;N\u0026ndash;Si strategy can minimize the use of migratory flame retardants while simultaneously enhancing fire safety and thermal stability. The results highlight a sustainable and chemically informed pathway for designing safer polyurethane materials.\u003c/p\u003e","manuscriptTitle":"Intrinsic Flame Retardancy in Flexible Polyurethane Foams Enabled by a Bio-Based P–N Reactive Polyol and a Silicon-Functionalized Triazine Charring Agent","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-03-09 13:22:28","doi":"10.21203/rs.3.rs-9046312/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"aaceca84-e8c3-474d-a7fd-2b006212a7e8","owner":[],"postedDate":"March 9th, 2026","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2026-03-16T15:28:27+00:00","versionOfRecord":[],"versionCreatedAt":"2026-03-09 13:22:28","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-9046312","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-9046312","identity":"rs-9046312","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}