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Article

Eco-Friendly Flame-Retardant Construction Composites Based on Bio-Based TPU, Recycled Rice Husk, and Ammonium Polyphosphate

by
Chen-Feng Kuan
1,
Chane-Yuan Yang
1,
Hsu-Chiang Kuan
2,
Min-Chin Chung
1 and
Yeng-Fong Shih
3,*
1
Department of Chemical and Materials Engineering, National Chin-Yi University of Technology, Taichung City 413310, Taiwan
2
Department of Chemical and Materials Engineering, Southern Taiwan University of Science and Technology, Tainan City 71005, Taiwan
3
Department of Applied Chemistry, Chaoyang University of Technology, Taichung City 413310, Taiwan
*
Author to whom correspondence should be addressed.
Buildings 2025, 15(18), 3420; https://doi.org/10.3390/buildings15183420
Submission received: 3 August 2025 / Revised: 6 September 2025 / Accepted: 17 September 2025 / Published: 22 September 2025
(This article belongs to the Section Building Materials, and Repair & Renovation)
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Abstract

This study explores the use of agricultural waste rice husk powder (RH) as a sustainable alternative to the petrochemical-derived carbon source, pentaerythritol (PER), in expandable flame retardants. RH is combined with halogen-free ammonium polyphosphate (APP), which serves as both an acid and a gas source. The resulting APP/RH system is incorporated into bio-based thermoplastic polyurethane (Biobased TPU) to prepare a halogen-free, flame-retardant composite material consistent with circular economy principles and environmental sustainability. The optimal APP-to-RH ratio in bio-based TPU was determined to be 2:1, with the best flame-retardant performance observed in the composite containing 20 wt% APP/RH. This formulation achieved a limiting oxygen index (LOI) of 27% and a UL-94 V-0 rating, indicating excellent flame resistance. Thermogravimetric analysis (TGA) showed a significant increase in char residue—from 0.51 wt% in pure TPU to 26.1 wt%—demonstrating improved thermal stability. Further characterization using cone calorimetry, thermogravimetric analysis–Fourier transform infrared spectroscopy (TGA-FTIR), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy confirmed that the addition of APP/RH significantly enhances the flame-retardant properties of the TPU composite. Consequently, the application of TPU in construction materials can be advanced through improved fire safety performance and alignment with sustainability goals.

1. Introduction

With the advancement of industrial technology, numerous types of polymer materials have been developed and are now widely used in diverse fields [1]. Among them, thermoplastic polyurethane (TPU) is recognized for its excellent mechanical properties, chemical stability, and ease of processing. In addition, bio-based TPU has recently gained considerable attention as a sustainable alternative, leveraging renewable resources like succinic acid and phytic acid to partially replace diisocyanates, thereby contributing to net-zero carbon emissions [2,3]. These advantages have led to its broad application in the construction industry, including cable sheathing, decorative materials, and protective coatings. However, its linear structure makes it highly flammable and prone to dripping in certain conditions, preventing it from meeting the required safety standards [4].
To improve the flame retardancy of TPU, various flame retardants can be incorporated. Among these, intumescent flame retardants (IFRs) have gained considerable attention for halogen-free flame retardation, owing to their low toxicity and low smoke emission. IFRs generally consist of three components: a carbon source, an acid source, and a gas source. The carbon source, usually a carbon-containing compound, undergoes acid-catalyzed dehydration to form a cross-linked char layer, which insulates the material and prevents internal combustion. This process also stabilizes carbon in the condensed phase, supporting efforts toward net-zero carbon emissions. The acid source, typically inorganic or carboxylic acids, reacts with surface monomers on the polymer through condensation reactions to form carbon-rich derivatives. Some acid ions migrate into the gas phase, where they quench free radicals and promote the formation of water molecules, thereby contributing to flame inhibition. The gas source decomposes thermally to release gases that expand the char layer, producing a microporous carbon structure. These gas molecules reduce heat and oxygen contact with the underlying material, while also diluting the energy and free radicals present in the flame, thereby mitigating the release of toxic gases during combustion [5,6]. However, the irregular pore formation and relatively low bonding energy of the carbon layer can result in thermal instability at elevated temperatures. Therefore, careful selection of the acid and gas sources, along with their ratios and synergy with the carbon source, is essential for optimizing flame-retardant performance.
Driven by increasing environmental concerns such as net-zero emissions, circular economy, and sustainable development, the development of bio-based and halogen-free flame retardants has received considerable attention in recent years [7,8,9,10,11,12]. Biomass-based IFRs are particularly promising due to their low toxicity, low smoke emission, and broad application potential in transportation, aerospace, building and electrical engineering, and electronics [13,14]. Recent studies have demonstrated that bio-based flame retardants derived from renewable resources such as starch, alcohols, polyols, proteins, cellulose, lignin, chitosan, tea saponins, phytic acid, and cyclodextrins can significantly enhance fire safety in polymers [15]. Beyond conventional biomass, agricultural wastes such as rice husk, tea residues, and spent coffee grounds have also been valorized as sustainable carbon or synergistic sources in TPU-based composites, showing improved char formation, reduced smoke release, and enhanced mechanical compatibility [2,3,15]. Comprehensive reviews have further highlighted the potential of bio-based flame retardants in polymer systems, particularly focusing on their mechanisms, thermal behavior, and synergistic effects with other additives [16]. In line with global trends toward sustainable development, bio-based flame retardants and agricultural-waste-derived additives are emerging as practical and eco-friendly alternatives for improving polymer fire safety while reducing environmental impact.
Ammonium polyphosphate (APP), due to its phosphorus (P) and nitrogen (N) content, can be used either as a standalone flame retardant or as the acid and gas source in conventional intumescent flame-retardant (IFR) systems. It exhibits synergistic effects when combined with carbon-based materials or other flame-retardant components, leading to high flame-retardant efficiency while maintaining relatively low cost. With the progressive phase-out of halogenated flame retardants, APP-based systems have attracted increasing attention as a green and sustainable alternative, consistent with environmental and regulatory requirements [17,18].
With industrialization, substantial economic growth and improvements in human living standards have been achieved. However, this progress has also caused significant environmental degradation in recent years. Key concerns include dependence on petroleum-based raw materials and the acceleration of global warming, among other issues. Consequently, the efficient use of natural resources, recycling of waste materials, and reduction in fossil fuel reliance have become essential strategies for achieving sustainable development [19,20].
Rice husk (RH), a byproduct of rice production, is rich in cellulose, hemicellulose, lignin, and silica, making it a sustainable biomass resource. Its high content of carbon-containing hydroxyl compounds enables RH to serve as a renewable alternative to petroleum-based carbon sources, functioning effectively as a green carbonization agent. Moreover, the silica present in RH promotes the formation of stable carbon structures during combustion, thereby enhancing char integrity and inhibiting unwanted reactions. Due to these properties, RH shows strong potential as a bio-based carbon additive in intumescent flame-retardant systems, supporting a circular economy through waste valorization and sustainable material reuse [16,21].
Aramwit et al. incorporated RH and coconut fiber into gypsum to develop composite materials and evaluated their mechanical and thermal properties under varying additive ratios. The results demonstrated that RH significantly enhanced the mechanical performance, with up to a 187% increase in bending strength compared to pure gypsum at lower loading levels. With higher RH content, the inorganic components and carbonaceous structure contributed to improved heat resistance and flame retardancy of the composite. This study highlights the potential for sustainable reuse of agricultural waste, promoting the development of eco-friendly building materials [22].
The present study leverages the synergistic flame-retardant effects of green APP and the naturally occurring nitrogen, phosphorus, and silicon elements found in agricultural waste RH powder to enhance flame retardancy. These components are incorporated into bio-based TPU to develop a halogen-free, flame-retardant composite material. This approach aims to promote circular economy principles and contribute to the goal of net-zero carbon emissions.

2. Materials and Methods

2.1. Materials

Bio-based thermoplastic polyurethane (TPU), grade G1085AEU-J2, was supplied by GREAT EASTERN RESINS INDUSTRIAL Co., Ltd. (Taichung, Taiwan). Ammonium polyphosphate (APP, (NH4PO3)n, n > 1000) was obtained from I-TAI CHEMICALS INC. (New Taipei City, Taiwan). Rice husk (RH) was sourced from Miaoli, Taiwan.

2.2. Preparation of Composite Materials

Before sample preparation, a feasibility assessment of the raw material proportions was conducted. Based on the optimized ratio of APP to RH (2:1), the additives were incorporated into TPU. Agricultural waste RH was first ground using a grinding machine for 10 min and then sieved through a 25-mesh screen to obtain a uniform particle size of approximately 0.5 mm. Subsequently, TPU granules, APP powder, and RH powder were dried in a constant-temperature oven at 100 °C for 12 h to remove residual moisture. A measured 50 g of dried TPU granules was placed into a plastic mixer and pre-mixed at 165 °C and 60 rpm until homogeneous. APP and RH, in the ratio of 2:1, were then added to the TPU at 5 wt%, 10 wt%, 15 wt%, and 20 wt% total filler content, and mixed for an additional 10 min. The resulting composite mixtures were transferred to a hot press and molded at 165 °C for 10 min. After hot pressing, the samples were cooled to room temperature under constant pressure and then demolded. The detailed formulation ratios are presented in Table 1. Finally, the molded TPU/APP/RH composites were post-cured in an oven at 40 °C for 24 h to obtain the final samples.

2.3. Characterization and Property Test

Thermogravimetric Analysis (TGA) was performed using a Q500 TGA analyzer (TA Instruments, New Castle, DE, USA) to investigate the decomposition behavior of the materials under a nitrogen atmosphere. Thermal stability was assessed by recording mass loss resulting from thermal decomposition.
Thermal Analysis–Fourier Transform Infrared Spectroscopy (TA-FTIR) was conducted using a Netzsch 209 F3 (NETZSCH, Selb, Germany) coupled with a Bruker Tensor II spectrometer (Bruker Corporation, Billerica, MA, USA). This technique was employed to analyze volatile decomposition products, such as oligomers and gaseous molecules, providing insights into the mechanism and effectiveness of gas-phase flame retardation.
The flammability properties of the composite materials were evaluated using a horizontal vertical burning tester (UL-94, ATLAS Fire Science Products, Mount Prospect, IL, USA) in accordance with ASTM D3801 [23] standards. Test specimens had dimensions of 125 ± 5 mm × 12.5 ± 0.5 mm × 1.25 ± 0.5 mm.
The limiting oxygen index (LOI) was determined using an ATLAS LOI tester (ATLAS (AMETEK® MOCON®), Mount Prospect, IL, USA) in a controlled oxygen/nitrogen atmosphere. This test quantifies the minimum oxygen concentration required to sustain combustion when the sample is ignited from the top.
Cone calorimetry testing (CCT) was employed to measure ignition behavior and combustion characteristics under external heat flux. Combustion gases were collected through a cone-shaped heater exhaust system and analyzed using a sampling probe positioned below the exhaust duct.
The microstructure of fracture surfaces and char residues was analyzed using field emission scanning electron microscopy (FESEM) with a JEOL JSM-6700F microscope (JEOL Ltd., Tokyo, Japan), allowing high-resolution morphological observations.
X-ray photoelectron spectroscopy (XPS) was conducted using a Ulfima IV system (Rigaku Americas Corporation, Austin, TX, USA) to analyze changes in surface chemical composition, oxidation states, and electronic structures. This technique provides detailed information on elemental distribution and chemical bonding states on the material surface.
Raman spectroscopy was conducted with a Renishaw Raman spectrometer (Renishaw plc, Wotton-under-Edge, UK) to identify carbon structures within the char. Characteristic peaks at 1350 cm−1 (D-band) and 1580 cm−1 (G-band) were analyzed, and the ID/IG ratio was used to assess the degree of graphitization. A lower ID/IG ratio indicates higher graphitization, contributing to improved thermal stability and carbon quality.

3. Results and Discussion

3.1. TGA Analyses

The effect of ammonium polyphosphate (APP)/rice husk (RH) incorporation on the thermal degradation behavior of thermoplastic polyurethane (TPU) was investigated by analyzing the mass loss profiles at varying additive concentrations, as illustrated in Figure 1 and Table 2. As shown in Figure 1a, the char yield (C.Y.) of pure TPU is only 0.51 wt%, indicating limited thermal stability. However, with the addition of APP/RH at 5 wt%, 10 wt%, 15 wt%, and 20 wt%, the residual char content increases significantly, reaching up to 26.1 wt% at 20 wt%, thereby demonstrating an effective enhancement in thermal stability and char-forming ability.
The initial decomposition temperature at 5% weight loss (Td5) for pure TPU is 294 °C, marking the onset of thermal degradation. Upon incorporating APP/RH, Td5 decreases with increasing additive content, reaching 272 °C at 20 wt%. This reduction in Td5 is attributed to the early thermal decomposition of phosphorus-based groups and ammonium ions in APP, which generate polyphosphoric acid, phosphoric acid, and ammonia. These products promote dehydration and carbonization of the TPU matrix, facilitating the formation of a protective char layer [24].
As shown in Figure 1b, the derivative thermogravimetric (DTG) curve indicates that pure TPU exhibits a maximum decomposition rate of −12.16 wt%/min at a peak degradation temperature of 385 °C. With the incorporation of flame retardants, the TPU/APP/RH 20% composite displays an increased maximum decomposition rate of −13.96 wt%/min, with the peak decomposition temperature shifting downward to 312 °C. This shift confirms the early degradation of phosphate groups from APP, which induces the formation of a protective carbonaceous layer at lower temperatures, thereby shielding the underlying TPU matrix. Additionally, the silicon content in RH decomposes at higher temperatures to form thermally stable silica, which migrates to the surface and integrates into the char structure. This silica layer further acts as a barrier, preventing thermal degradation at elevated temperatures. The observed improvement in thermal stability is attributed to the synergistic flame-retardant effect of phosphorus, silicon, and nitrogen elements, which collectively enhance char formation, provide thermal shielding, and suppress further decomposition of the composite material [25].
Table 2 shows that the initial decomposition temperature (Td5) decreases significantly with the addition of flame retardants. Meanwhile, the maximum decomposition rate (Rmax) increases as the APP/RH content rises. This trend is attributed to the greater release of ammonia and water at lower temperatures due to the presence of APP/RH. Moreover, the generation of low-molecular-weight phosphoric acid species facilitates the dehydration of TPU, accelerating char formation and causing increased mass loss in the early stages of decomposition. The C.Y. at 800 °C increases progressively with higher APP/RH loadings. This improvement is primarily due to the presence of phosphorus and nitrogen in the flame-retardant system. Phosphorus compounds promote carbonization, leading to greater char retention at elevated temperatures. The synergistic effect between phosphorus and nitrogen is crucial in improving the flame-retardant performance, forming a protective barrier that shields the TPU matrix from heat and oxygen, thereby suppressing combustion and enhancing thermal stability at high temperatures [15]. The synergistic effect between phosphorus and nitrogen plays a crucial role in improving flame-retardant performance. During combustion, nitrogen-containing groups in APP decompose to release inert gases such as NH3 and N2, which dilute oxygen and combustible volatiles in the flame zone, thereby suppressing flame propagation. Meanwhile, phosphorus promotes char formation in the condensed phase. The combined action of phosphorus-induced char and nitrogen-derived gases forms a protective barrier that shields the TPU matrix from heat and oxygen, resulting in suppressed combustion and enhanced thermal stability at elevated temperatures [25].

3.2. Toxicity Analyses

Thermogravimetric Analysis–Fourier Transform Infrared Spectroscopy

From Figure 2, the thermal decomposition pathways of neat TPU and TPU containing APP/RH can be elucidated. The characteristic features observed between 100 and 200 °C are attributable to scission of TPU molecular chains and the decomposition of lignin in the rice husk. In Figure 2A, the gaseous products generated during TPU decomposition exhibit characteristic peaks in the ranges of 3500–3570 cm−1 and 3050–2850 cm−1 [26], corresponding to the stretching vibrations of –OH groups (from water) and the symmetric/asymmetric stretching vibrations of hydrocarbons, respectively. The 2500–2000 cm−1 range [27] shows absorption bands attributed to the stretching and bending vibrations of CO2 and CO (carbonyl groups). Peaks in the 1750–1500 cm−1 range [17] correspond to the symmetric stretching of carbonyl groups, while those between 1050 and 1250 cm−1 are associated with ether group vibrations [26,27]. Furthermore, peaks in the 600–750 cm−1 range are characteristic of HCN [17]. Pure TPU continues to degrade between 700 and 800 °C, reflecting its lower thermal stability and the ongoing decomposition of polyols and related segments. As shown in Table 2, the char yield (C.Y.) at 800 °C was only 0.51 wt% for neat TPU, whereas it increased markedly to 20.4 wt% for TPU/APP/RH 10% and 26.1 wt% for TPU/APP/RH 20%. These results clearly demonstrate that CY is significantly enhanced by the incorporation of the RH-based IFR, providing quantitative evidence of the improved thermal stability of the composites compared with pure TPU.
In contrast, in TPU/APP/RH (Figure 2B), the earlier decomposition of APP produces phosphorus oxides and polyphosphates, which catalyze interchain dehydration, promote char formation, and thereby enhance thermal stability. Around 300 °C, new characteristic peaks (PN) in the range of 1050–1250 cm−1 and NH3 at 840–950 cm−1 indicate the decomposition of APP. This process triggers the premature cleavage of –NCO groups (2275 cm−1) in TPU, which subsequently react with H2O to produce CO2, thereby reducing the generation of toxic gases such as isocyanic acid, carbon monoxide, and hydrogen cyanide, compared to pure TPU [28]. TG-IR analysis reveals that the flame-retardant mechanism of APP involves the dehydration and carbonization of both TPU and rice husk. Concurrently, the decomposition of ammonium ions releases ammonia, which expands and facilitates the formation of a protective carbon layer, thereby suppressing the release of volatile compounds. This is evidenced by the reduced intensities of the characteristic peaks for H2O (3500–3570 cm−1), carbonyl groups (1743 cm−1), and aromatic compounds (1548 cm−1). In the gas phase, the release of ammonia and water vapor contributes to flame inhibition by diluting the concentrations of combustible gases (e.g., hydrocarbons) and oxygen, thereby highlighting the effective flame-retardant action of the APP/RH system in both the gas and condensed phases [29,30].

3.3. Flame-Retardant Properties

3.3.1. UL-94 and LOI Testing

As shown in Figure 3 and Table 3, neat TPU did not achieve a flame-retardant rating and exhibited severe dripping during combustion, indicating poor flame resistance. However, the incorporation of APP and RH significantly improved both the dripping behavior and extinguishing time. In particular, the addition of 5% APP/RH reduced the extinguishing time from over 30 s to less than 10 s, although dripping continued to propagate the flame. As the APP/RH content increased, the dripping phenomenon was progressively suppressed. At a concentration of 20% APP/RH, a stable and expanded char layer formed on the surface of the composite, effectively preventing melt dripping and resulting in a UL-94 V-0 rating.
As illustrated in Figure 4, pure TPU continued to burn with pronounced dripping after the ignition source was removed, ultimately self-extinguishing after approximately 30 s. This behavior highlights its poor flame-retardant performance. In contrast, the TPU/APP/RH 20% composite rapidly extinguished the flame following the removal of the ignition source in both ignition cycles. A substantial release of gas was observed from the bottom of the sample, and dripping was completely suppressed. These results indicate that, with an appropriate amount of rice husk serving as a carbon source, the synergistic effects of phosphorus (P) and nitrogen (N) from APP and silicon (Si) from rice husk play a vital role in flame retardancy. Specifically, the reaction between H+ ions from APP-derived phosphoric acid and hydroxyl groups increases the viscosity of the molten phase, promoting dehydration, carbonization, and the formation of a protective carbon layer. Concurrently, the release of non-combustible gases dilutes flammable volatiles, thereby inhibiting sustained combustion and achieving effective flame retardancy [29].
Table 3 shows that the LOI of pure TPU is 22%, indicating a high susceptibility to ignition and poor flame resistance. With the incorporation of APP/RH, the LOI value increases progressively with additive content. At 20% APP/RH, the composite achieves a flame-retardant threshold with an LOI of 27%. This improvement is primarily attributed to the synergistic action of oxides present in APP and rice husk, which facilitates the formation of a carbonized protective layer on the material surface, thereby inhibiting flame propagation. In the APP/RH system, the rice husk serves as an effective carbon source, while phosphorus and nitrogen from APP, together with silicon from the rice husk, exhibit excellent synergistic effects during combustion. Specifically, the protons (H+) from phosphoric acid in APP can react with hydroxyl groups, increasing the melt viscosity and promoting dehydration. This process facilitates the carbonization of the matrix and the formation of a stable char layer. At the same time, the decomposition also generates non-flammable gases, which help to dilute combustible volatiles and suppress continuous combustion. Through these combined effects, a cohesive carbonized protective layer is formed, thereby significantly enhancing the flame retardancy of the composites. The incorporation of APP and RH effectively establishes an expanded flame-retardant system, improving the overall flame-retardant efficiency of the TPU composite [31].

3.3.2. Cone Calorimetry Analysis

This study employed a commonly used heat flux condition to evaluate the fire behavior of TPU composite materials, with results summarized in Figure 5 and Table 4. The time to ignition (TTI) for pure TPU was 39 s, while that of the TPU/APP/RH composite was slightly reduced to 34 s. This reduction is attributed to the initial thermal decomposition of hydrocarbon-containing components in ammonium polyphosphate (APP) and rice husk, which trigger earlier degradation reactions under heat exposure [32].
As shown in Figure 5a,b, pure TPU exhibits a peak heat release rate (pHRR) of 593 kW/m2 at 86 s, with a total heat release (THR) of 29.3 MJ/m2. After incorporating APP and RH, the peak shifts earlier to 48 s, with the pHRR reduced to 344 kW/m2 and THR to 27 MJ/m2. This corresponds to a 42.1% reduction in pHRR and a 7.8% reduction in THR compared to pure TPU. The observed improvements are primarily due to the thermal decomposition of APP, which promotes dehydration and carbon layer formation. This results in a more robust, thicker, and continuous char barrier that restricts heat and mass transfer, thereby protecting the underlying TPU matrix [33].
In this study, the Fire Growth Index (FGI) and Fire Performance Index (FPI) were utilized to evaluate the overall fire risk of the materials. The FGI, defined as the ratio of pHRR to tpHRR (time to peak heat release rate), reflects the rapidity of fire development; a lower FGI value indicates a slower heat release rate and a longer time to reach the peak, implying improved fire control. The FPI, calculated as the ratio of TTI to pHRR, is an indicator of fire safety, where a higher value suggests delayed ignition and reduced fire growth rate, providing more time for evacuation. The results show that TPU/APP/RH 20% exhibits higher FGI and FPI values of 7.2 and 0.099, respectively, compared to those of neat TPU (6.9 and 0.066). These findings confirm that the incorporation of APP and rice husk facilitates the formation of a protective carbonized layer, which effectively retards heat release, thereby reducing fire propagation risk and enhancing overall fire safety [33].
Smoke production analysis, as shown in Figure 5c,d, reveals that pure TPU exhibits a peak smoke production rate (pSPR) of 0.171 m2/s2 and a total smoke production (TSP) of 6.1 m2. With the addition of APP/RH (20%), the pSPR decreased to 0.106 m2/s2, indicating improved instantaneous smoke suppression. However, the TSP increased slightly to 7.9 m2, which is attributed to the release of phosphorus-containing radicals during polyphosphate decomposition. These radicals extend combustion duration by scavenging free radicals in the gas phase, which may lead to increased suspended particle formation. Nonetheless, the substantial reduction in peak smoke generation demonstrates the composite’s ability to mitigate acute smoke hazards, a critical factor in fire safety.
As shown in Figure 5e and Table 4, both the Maximum Average Rate of Heat Emission (MARHE) and Effective Heat of Combustion (EHC) decreased when APP and RH were incorporated into TPU. For pure TPU, the MARHE and EHC values were 228.8 kW/m2 and 20.3 MJ/kg, respectively. In contrast, the TPU/APP/RH 20% composite exhibited values of 194.4 kW/m2 and 16.6 MJ/kg, representing reductions of 15% and 18.1%, respectively. The decrease in EHC reflects the contribution of gas-phase flame retardancy, wherein phosphorus-containing compounds released during APP decomposition capture active free radicals in the flame. This inhibits combustion by diluting flammable gases, confirming the gas-phase flame-inhibition effect of APP. Meanwhile, the reduction in MARHE is attributed to polyphosphates promoting the thermal decomposition of lignin, hemicellulose, and cellulose in RH, enhancing dehydration reactions and facilitating the formation of a cross-linked carbonaceous structure. This process favors the transformation of the composite into a thermally stable char that remains in the condensed phase as residue [29].
As shown in Figure 5f and Table 4, the weight residue of TPU increased from 22.6 wt% to 34.8 wt% with the addition of APP and RH, confirming their effectiveness in enhancing char formation.
Furthermore, the Flame Retardancy Index (FRI) quantifies the overall flame-retardant performance of a material by incorporating multiple parameters from cone calorimetry. It is defined as Equation (1) [34]:
Flame   retardancy   index   ( FRI )   = T H R × p H R R R T T I n e a t   p o l y m e r T H R × p H R R R T T I c o m p o s i t e
The calculated FRI for TPU/APP/RH 20% is 1.6, which is greater than 1, indicating a significant improvement in flame-retardant performance compared to pure TPU. An FRI value above 1 signifies enhanced fire safety behavior relative to the reference material, confirming the effectiveness of the APP/RH flame-retardant system.
In summary, TPU/APP/RH 20% demonstrates significant reductions in pHRR, THR, and MARHE compared to pure TPU, with values of 343.77 kW/m2, 27 MJ/m2, and 194.4 kW/m2, respectively. These results demonstrate that incorporating APP/RH effectively enhances the flame resistance of the TPU matrix.

3.4. Morphology

3.4.1. SEM

Figure 6a,b show the pre-combustion surface morphology of TPU/APP/RH 5% and TPU/APP/RH 20%, respectively. As the APP/RH content increases, fine particles become more uniformly dispersed within the TPU matrix, although slight aggregation is still observed. The post-combustion images in Figure 6a′ indicate that TPU/APP/RH 5% forms a thin and loosely structured micro-expanded char layer. The insufficient concentration of flame-retardant components leads to incomplete protection, allowing combustible gases and heat to penetrate the interior and sustain flame propagation. In contrast, Figure 6b′ shows that TPU/APP/RH 20% develops a significantly expanded and denser char layer. This improved performance results from the phosphorus compounds facilitating char formation, ammonia release promoting intumescent expansion, and silicon migration from the RH improving thermal stability. Together, these effects block heat and mass transfer, suppress the release of combustible gases, and prevent further flame spread, thereby achieving higher flame retardancy [20].

3.4.2. Elemental Analysis Before and After Combustion

Figure 7 and Table 5 present the elemental composition of TPU/APP/RH composites before and after combustion. As shown in Figure 7a, the pre-combustion analysis of TPU/APP/RH 5% reveals the presence of five primary elements: carbon (C), oxygen (O), nitrogen (N), silicon (Si), and phosphorus (P). These correspond to the aminoethyl ester groups in TPU and the hemicellulose, cellulose, and lignin in RH. The Si content originates from silica in RH, while P is derived from ammonium polyphosphate (APP). The respective weight percentages are C: 59.62%, O: 34.54%, N: 4.91%, Si: 0.06%, and P: 0.88%. With an increase in APP/RH content to 20 wt%, Figure 7b shows a proportional rise in P, Si, and N contents, reaching 4.33%, 0.25%, and 8.01%, respectively. Meanwhile, the percentages of C and O slightly decrease to 53.63% and 33.77%, indicating that the added flame retardants shift the overall elemental balance toward flame-retardant components.
Post-combustion analyses in Figure 7a′,b′ reveal notable changes in elemental distribution. For TPU/APP/RH 5%, the C, N, and P contents were 59.62%, 4.91%, and 0.88%, respectively, whereas for TPU/APP/RH 20% they shifted to 53.63%, 8.01%, and 4.33%. After combustion, the C, N, and P contents of TPU/APP/RH 5% were 58.06%, 4.66%, and 2.14%, while those of TPU/APP/RH 20% were 52.06%, 5.17%, and 4.24%. The reduction in C and N contents for both composites was attributed to the release of volatile products such as NH3 during thermal degradation. In contrast, the increase in P content underscores the critical role of APP in the synergistic flame-retardant mechanism. APP promotes premature degradation and dehydration of the TPU matrix, leading to the formation of a stable P-containing char layer. In conjunction with N species, these processes suggest the possible generation of P–N–C linkages within the char residue, which strengthened the carbonized structure and improved flame retardancy. Moreover, the variation in Si content indicates the migration of silica to the material surface, where it further contributes to the development of a protective barrier layer. For TPU/APP/RH 20%, both P and Si contents increase after combustion—from 2.14% and 0.14% (pre-combustion) to 4.24% and 0.20%, respectively. This suggests the formation of a P–N–C structured carbon layer, and the migration of silica to the material surface, contributing to a protective barrier layer. These mechanisms significantly enhance thermal stability, oxidation resistance, and flame retardancy, while slowing the combustion rate and protecting the underlying substrate. Additionally, the overall reduction in flame-retardant elements before and after combustion reflects the consumption or loss of these components during condensed-phase reactions, further supporting their active participation in flame-retardant mechanisms [35].

3.5. Char Residue Analysis

3.5.1. XPS Analysis of Flame-Retardant Mechanism

X-ray photoelectron spectroscopy (XPS) was employed to investigate the chemical bonding states in TPU/APP/RH 5% and 20% composites, both at room temperature and after combustion at 600 °C for 30 min. This analysis aimed to confirm the flame-retardant mechanism driven by the phosphorus–nitrogen synergistic system and to examine the bonding changes induced by the incorporation of flame retardants into the TPU matrix. In addition, the antioxidant behavior and structural evolution before and after combustion were evaluated. As shown in the XPS survey spectra (Figure 8), the elemental compositions of pure TPU, TPU/APP/RH 5%, TPU/APP/RH 20%, and their post-combustion residues were analyzed. In Figure 8a, the main elements observed in pure TPU include carbon (C), nitrogen (N), and oxygen (O)—primarily originating from the isocyanate functional groups. Figure 8b,c demonstrate that with the incorporation of APP/RH at 5% and 20%, two additional elements—phosphorus (P) and silicon (Si)—appear in the XPS spectra, confirming the successful introduction of flame-retardant additives at a 2:1 mixing ratio into the TPU matrix.
Figure 8d presents the XPS results of TPU/APP/RH 20% after exposure to 600 °C for 30 min, simulating high-temperature combustion. A comparison between Figure 8c,d reveals a notable increase in the P2s peak intensity following combustion. This enhancement is attributed to the rapid catalytic action of phosphorus at elevated temperatures, which promotes the formation of a phosphorus-rich char layer during the early stages of thermal degradation. Meanwhile, silicon migrates toward the surface under high-temperature conditions, contributing to the formation of a silica-based protective char. These synergistic effects reinforce the barrier properties, slow thermal degradation, and strengthen the flame-retardant performance of the composite [29].
The evolution of chemical bond types in the C1s and O1s spectra before and after high-temperature oxidation was further analyzed to clarify the flame-retardant mechanism of TPU/APP/RH composites. Figure 9 and Figure 10 present the high-resolution XPS spectra of C1s and O1s at room temperature and after combustion at 600 °C for 30 min, respectively. In Figure 9a,c, the C1s spectra of TPU/APP/RH composites at room temperature exhibit the presence of four main chemical bonds: C–C/C–H (284.8 eV), C–N (285.7 eV), C–O (286.5 eV), and C=O (287.3 eV), consistent with the molecular structures of TPU and the APP/RH additives. After high-temperature combustion, the C1s spectra in Figure 9b,d reveal a new peak at 284.5 eV, corresponding to C=C bonds, which indicates graphitization of the char layer. Moreover, the intensities of C=O and C–O bonds increase due to thermal oxidation, while some C–O bonds are associated with the formation of C–O–P linkages derived from APP decomposition. The observed increase in C–N bond intensity is attributed to the decomposition of ammonium groups in APP, followed by the bonding with carbon during char formation [36,37].
Figure 10a presents the O1s spectra of the composites. At room temperature, three distinct oxygen-containing bonds are identified: P=O (531.5 eV), =O– (531.9 eV), and –O (533.1 eV) [38]. After high-temperature treatment, significant changes occur in the O1s spectrum, reflecting oxidation and decomposition reactions. New bonds such as P–O, P–OH, C–O–P, and Si–O emerge at 531.9 eV, 532.5 eV, and 533.1 eV, respectively. These bonds originate from APP decomposition and acid generation, which promote the formation of a cross-linked, phosphorus-containing carbonaceous network. The emergence of these structures confirms the development of heterocyclic char, which improves thermal stability and flame retardancy [39]. In the P2p spectrum shown in Figure 10b, a key component of the composite—APP (ammonium polyphosphate)—is identified. The characteristic O=P–O bond appears at 134 eV. During high-temperature oxidation, this bond decomposes into inorganic phosphorus compounds such as P2O5 and P2O74−, which catalyze the formation of a char layer. These compounds exhibit peaks at 135 eV and 133.8 eV, respectively [40,41]. This transformation results from the thermal decomposition of the O=P–O bonds in APP.
As shown in Figure 11a, the N1s spectra of TPU/APP/RH 20% revealed the presence of N-containing species after high-temperature treatment, including N–H (400.2 eV [42]) originating from the incorporated flame retardants, pyridine-N (399 eV [43]), and pyrrole-N (400.5 eV [43]). The formation of pyridine-N and pyrrole-N was attributed to electron delocalization and dipole polarization [44]. Among them, pyridine-N bonds are more thermally stable, thereby enhancing the stability of the carbonaceous char [45]. These findings indicate the generation of nitrogen-doped hybrid carbon structures.
Following combustion at 600 °C for 30 min, the Si2p spectra (Figure 11b) revealed the characteristic SiO2 bond at 103.6 eV [29]. In addition, a –P(=O)–O–Si bond appeared at 104.3 eV [29]. This bond formation suggests that, during high-temperature combustion, the SiO2 glass formed on the composite surface reacts with phosphorus species to form –P(=O)–O–Si linkages, producing a synergistic flame-retardant effect.
At the molecular level, the interactions involve the generation of stable heteroatom bonds (pyridine-N, pyrrole-N, –P–O–Si) that reinforce the integrity of the carbonized matrix through enhanced cross-linking and electron delocalization. At the microstructural level, these molecular interactions manifest as the development of a cohesive, silica-reinforced phosphorus–nitrogen-rich char layer that effectively isolates heat, oxygen, and volatile products. This dual mechanism explains how phosphorus–silicon–nitrogen synergy contributes to the stabilization of char structures at the molecular scale and to the formation of a compact protective barrier at the microstructural scale, thereby enhancing the flame retardancy of TPU composites.
To further investigate the differences in chemical structure between TPU/APP/RH 5% and TPU/APP/RH 20% before and after high-temperature oxidation, a quantitative analysis was performed by calculating the area ratios of specific bond types in the C1s XPS spectra. The Cox/Ca ratio—where Cox represents oxidized carbons (C=O, C–O) and Ca denotes aliphatic/aromatic carbons (C–C, C–H, C=C)—serves as a reliable indicator of the antioxidant capacity of the material [46]. The results presented in Table 6 indicate that, at room temperature, the Cox/Ca ratios for TPU/APP/RH composites containing 5% and 20% flame retardant are 0.69 and 0.70, respectively. After exposure to 600 °C for 30 min, these values decrease to 0.57 and 0.53, respectively. This reduction in the Cox/Ca ratio after combustion suggests that the incorporation of APP/RH flame retardants improves the oxidation resistance of the composite by inhibiting the formation of oxygen-containing functional groups at elevated temperatures. Moreover, the lower Cox/Ca ratio in the TPU/APP/RH 20% sample compared to the 5% sample after high-temperature treatment confirms that higher flame-retardant loading provides superior antioxidant performance and enhanced thermal stability [47]. These findings demonstrate the dual role of APP/RH as both a flame retardant and a stabilizing additive against oxidative degradation.

3.5.2. Raman Spectra

In this study, Raman spectroscopy with an excitation wavelength of 633 nm was employed to scan the range of 1000 to 2000 cm−1 to investigate changes in the carbon crystalline structure of the composite materials after high-temperature annealing. The evolution of the D-band (disorder band) and G-band (graphitic band) content was examined. Figure 12a–d illustrate the changes in the D- and G-bands of TPU/APP/RH composites after exposure to a high-temperature furnace at 600 °C for 1 and 5 min [47,48]. As shown in Figure 12 and summarized in Table 7, the ID/IG ratios of the D- and G-bands for TPU/APP/RH 5% and TPU/APP/RH 20% were analyzed. After 1 min of heat treatment, the ID/IG ratios were 2.35 and 1.25, respectively. Following 5 min of annealing, these values decreased to 1.35 and 0.57, respectively, indicating a significant increase in the degree of graphitization in the carbon layer.
Although APP, as a single flame retardant, decomposes into phosphoric acid that promotes carbon layer formation, the resulting carbon is generally limited in both quantity and structural integrity. In contrast, RH, serving as a carbon source, contributes to greater carbon yield with increased APP content. Moreover, the quenching of free radicals by the flame retardant and the release of ammonia during ammonium ion decomposition in the condensed phase further promote graphitization. These synergistic effects facilitate the formation of a more stable and thermally resistant carbon layer, thereby enhancing the thermal stability of TPU/APP/RH composite materials [29].

3.6. TPU/APP/RH Flame-Retardant Mechanism

The proposed flame-retardant mechanism of the TPU/APP/RH composite, illustrated in Figure 13, involves both condensed-phase and gas-phase processes, driven by the synergistic effects of phosphorus, nitrogen, and silicon. In the condensed phase, the addition of APP and RH significantly modifies the thermal degradation pathway of TPU. Upon heating, APP decomposes at relatively low temperatures, releasing ammonia (NH3) and water vapor, and generating phosphoric acid or polyphosphoric acid. These acidic species catalyze the dehydration and carbonization of both the TPU matrix and the lignocellulosic components in RH, leading to early-stage char formation. The resulting phosphorus-rich residues enhance the system’s char-forming ability, producing a cohesive and dense carbonaceous layer that acts as a thermal and physical barrier, insulating the underlying material from heat and oxygen.
Meanwhile, RH—rich in silicon-containing compounds—thermally decomposes at elevated temperatures to yield silicon dioxide (SiO2). This silica migrates to the surface of the composite, reinforcing the char layer and forming a ceramic-like barrier. The resulting structure improves the integrity and stability of the residual char, thereby enhancing protection against thermal degradation. The phosphorus–silicon–nitrogen synergy effectively enhances the flame-retardant performance and thermal resistance of the TPU matrix.
In the gas phase, the release of nonflammable gases such as NH3 and H2O contributes to the dilution of flammable volatiles and reduction of available oxygen, thereby suppressing combustion reactions in the flame zone. Additionally, the evolution of gases facilitates the formation of an intumescent char layer, which expands into a foamed, thermally insulating barrier that further reduces heat and mass transfer during combustion. In summary, the phosphorus–nitrogen interaction from APP promotes char formation and flame inhibition, while the silicon species from RH enhance the char stability and mechanical reinforcement. This multi-phase and multi-element synergy provides the TPU/APP/RH composites with enhanced flame retardancy, improved thermal stability, and reduced generation of flammable gases, making them promising candidates for sustainable flame-retardant thermoplastic applications.

4. Conclusions

TPU composite materials were prepared using rice husk (RH) and ammonium polyphosphate (APP) as additive flame retardants, with an optimal RH:APP ratio of 1:2. The incorporation of APP/RH significantly increased the char yield from 0.51 wt% in pure TPU to 26.1 wt%. While pure TPU failed to meet flame-retardant standards and exhibited an LOI of only 22%, the APP/RH-modified composites achieved a UL-94 V-0 rating and a higher LOI of 27%, indicating significantly improved flame-retardant performance.
TG-IR analysis revealed that the decomposition of APP/RH generated inert gases such as nitrogen and ammonia, which acted as diluents in the gas phase. Simultaneously, a condensed-phase barrier was formed, serving as both a heat shield and a physical barrier that blocked combustible gases from reaching the substrate. This dual action effectively suppressed combustion.
SEM and EDS analyses of the TPU/APP/RH composites after burning confirmed the enrichment of flame-retardant elements (N, Si, and P), which facilitated the formation of a denser and more expanded char layer. This structure effectively hindered the transfer of heat and flammable volatiles, contributing to flame suppression.
Overall, the study demonstrates the effective carbonization behavior of the APP/RH system, which provides flame-retardant protection through both gas-phase dilution and condensed-phase insulation. Furthermore, the use of rice husk offers a sustainable and eco-friendly approach to enhancing TPU fire safety, supporting its application in the construction industry.

Author Contributions

Conceptualization, C.-F.K.; methodology, C.-F.K. and H.-C.K.; software, Y.-F.S.; validation, C.-Y.Y., H.-C.K., M.-C.C. and Y.-F.S.; formal analysis, C.-F.K., M.-C.C. and Y.-F.S.; investigation, H.-C.K.; resources, C.-F.K.; data curation, C.-F.K., C.-Y.Y. and M.-C.C.; writing—original draft preparation, C.-Y.Y.; writing—review and editing, C.-Y.Y. and Y.-F.S.; visualization, M.-C.C.; project administration, Y.-F.S.; funding acquisition, C.-F.K. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by National Science and Technology Council of Taiwan, R.O.C., grant number 113-2637-E-167-001.

Data Availability Statement

Data are contained within the article.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. (a) TGA and (b) DTG curves of TPU/APP/RH composites in N2.
Figure 1. (a) TGA and (b) DTG curves of TPU/APP/RH composites in N2.
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Figure 2. TG-IR spectra of volatilized products during the thermal degradation process of (A) neat TPU and (B) TPU/APP/RH 20%.
Figure 2. TG-IR spectra of volatilized products during the thermal degradation process of (A) neat TPU and (B) TPU/APP/RH 20%.
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Figure 3. Effect of various APP/RH contents on LOI and UL-94 of pure TPU and its flame composites.
Figure 3. Effect of various APP/RH contents on LOI and UL-94 of pure TPU and its flame composites.
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Figure 4. Video screenshots of (a) pure TPU and (b) TPU/APP/RH 20% during UL-94 test.
Figure 4. Video screenshots of (a) pure TPU and (b) TPU/APP/RH 20% during UL-94 test.
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Figure 5. CCT results of pure TPU and TPU/APP/RH 20%: (a) heat release rate (HRR); (b) total heat release rate (THRR); (c) smoke production rate (SPR); (d) total smoke production (TSP); (e) average rate of heat emission (ARHE) and (f) weight curves.
Figure 5. CCT results of pure TPU and TPU/APP/RH 20%: (a) heat release rate (HRR); (b) total heat release rate (THRR); (c) smoke production rate (SPR); (d) total smoke production (TSP); (e) average rate of heat emission (ARHE) and (f) weight curves.
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Figure 6. SEM micrographs of composites (a) TPU/APP/RH 5% (before burning) (×1000); (a′) TPU/APP/RH 5% (after burning) (×1000); (b) TPU/APP/RH 20% (before burning) (×1000) and (b′) TPU/APP/RH 20% (after burning) (×1000).
Figure 6. SEM micrographs of composites (a) TPU/APP/RH 5% (before burning) (×1000); (a′) TPU/APP/RH 5% (after burning) (×1000); (b) TPU/APP/RH 20% (before burning) (×1000) and (b′) TPU/APP/RH 20% (after burning) (×1000).
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Figure 7. EDS of composites (a) TPU/APP/RH 5% (before burning) (×1000); (a′) TPU/APP/RH 5% (after burning) (×1000); (b) TPU/APP/RH 20% (before burning) (×1000) and (b′) TPU/APP/RH 20% (after burning) (×1000).
Figure 7. EDS of composites (a) TPU/APP/RH 5% (before burning) (×1000); (a′) TPU/APP/RH 5% (after burning) (×1000); (b) TPU/APP/RH 20% (before burning) (×1000) and (b′) TPU/APP/RH 20% (after burning) (×1000).
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Figure 8. XPS survey of (a) pure TPU at RT, (b) TPU/APP/RH 5% at RT, (c) TPU/APP/RH 20% at RT, and (d) TPU/APP/RH 20% at 600 °C.
Figure 8. XPS survey of (a) pure TPU at RT, (b) TPU/APP/RH 5% at RT, (c) TPU/APP/RH 20% at RT, and (d) TPU/APP/RH 20% at 600 °C.
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Figure 9. C1s spectra of (a) TPU/APP/RH 5% RT, (b) TPU/APP/RH 5% under air atmosphere at 600 °C for 30 min, (c) TPU/APP/RH 20% RT, and (d) TPU/APP/RH 20% under air atmosphere at 600 °C for 30 min.
Figure 9. C1s spectra of (a) TPU/APP/RH 5% RT, (b) TPU/APP/RH 5% under air atmosphere at 600 °C for 30 min, (c) TPU/APP/RH 20% RT, and (d) TPU/APP/RH 20% under air atmosphere at 600 °C for 30 min.
Buildings 15 03420 g009
Buildings 15 03420 g010
Figure 10. (a) O1s spectra of TPU/APP/RH 20% and (b) P2p spectra of TPU/APP/RH 20% under an air atmosphere at 600 °C for 30 min.
Figure 10. (a) O1s spectra of TPU/APP/RH 20% and (b) P2p spectra of TPU/APP/RH 20% under an air atmosphere at 600 °C for 30 min.
Buildings 15 03420 g010
Buildings 15 03420 g011
Figure 11. (a) N1s spectra of TPU/APP/RH 20% (b) Si2p spectra of TPU/APP/RH 20% under air atmosphere at 600 °C for 30 min.
Figure 11. (a) N1s spectra of TPU/APP/RH 20% (b) Si2p spectra of TPU/APP/RH 20% under air atmosphere at 600 °C for 30 min.
Buildings 15 03420 g011
Buildings 15 03420 g012
Figure 12. The Raman spectra of char products from (a) TPU/APP/RH 5% at 600 °C 1 min, (b) TPU/APP/RH 5% at 600 °C 5 min, (c) TPU/APP/RH 20% at 600 °C 1 min, and (d) TPU/APP/RH 20% at 600 °C 5 min.
Figure 12. The Raman spectra of char products from (a) TPU/APP/RH 5% at 600 °C 1 min, (b) TPU/APP/RH 5% at 600 °C 5 min, (c) TPU/APP/RH 20% at 600 °C 1 min, and (d) TPU/APP/RH 20% at 600 °C 5 min.
Buildings 15 03420 g012
Buildings 15 03420 g013
Figure 13. Proposed flame-retardant mechanism of TPU/APP/RH composites.
Figure 13. Proposed flame-retardant mechanism of TPU/APP/RH composites.
Buildings 15 03420 g013
Table 1. Formulations of TPU/APP/RH composites.
Table 1. Formulations of TPU/APP/RH composites.
SampleTPU (wt%)APP (wt%)RH (wt%)
Pure TPU10000
TPU/APP/RH 5%953.331.67
TPU/APP/RH 10%906.673.33
TPU/APP/RH 15%85105
TPU/APP/RH 20%8013.336.67
Table 2. Thermal properties of TPU with various contents of APP/RH.
Table 2. Thermal properties of TPU with various contents of APP/RH.
SampleTd5 a (°C)Tmax b (°C)Rmax c (wt%/min)C.Y. (wt%)
Neat TPU294385−12.160.51
TPU/APP/RH 5%277337−9.7212.2
TPU/APP/RH 10%276328−9.8620.4
TPU/APP/RH 15%278320−12.0924.9
TPU/APP/RH 20%272312−13.9626.1
a The temperature at which the weight loss of the sample reaches 5%. b Corresponds to the temperature of the maximum degradation rate. c Corresponds to the maximum thermal degradation rate.
Table 3. The flame retardant of TPU/APP/RH composites by UL-94 and LOI values.
Table 3. The flame retardant of TPU/APP/RH composites by UL-94 and LOI values.
SampleUL-94LOI (%)
t1 (s)t2 (s)DrippingRanking
Pure TPUBC a-YesFail22
TPU/APP/RH 5%3.8 ± 0.11.6 ± 0.2YesV-224
TPU/APP/RH 10%0.9 ± 0.11.9 ± 0.1YesV-225
TPU/APP/RH 15%0.8 ± 0.10.7 ± 0.1YesV-126
TPU/APP/RH 20%0.6 ± 0.10.9 ± 0.1NOV-027
a burn to the clamp.
Table 4. Cone calorimetry data for pure TPU and TPU/APP/RH 20% composites at 35 kW/m2.
Table 4. Cone calorimetry data for pure TPU and TPU/APP/RH 20% composites at 35 kW/m2.
SamplePure TPUTPU/APP/RH 20%
TTI (s)3934
pHRR (kw/m2)593344
tpHRR (s)8648
THR (MJ/m2)29.327
EHC (MJ/Kg)20.316.6
FRI-1.6
pSPR (m2/s2)0.1710.106
TSP (m2)6.17.9
MARHE (kw/m2)228.8194.4
Weight residue (wt%)22.634.8
FPI (m2s/kw)0.0660.099
FGI (kw/m2s)6.97.2
Table 5. EDS data for TPU/APP/RH composites.
Table 5. EDS data for TPU/APP/RH composites.
SampleElements (wt%)
CONSiP
TPU/APP/RH 5% (before burning)59.6234.544.910.060.88
TPU/APP/RH 5% (after burning)58.0635.014.660.142.14
TPU/APP/RH 20%(before burning)53.6333.778.010.254.33
TPU/APP/RH 20% (after burning)52.0638.335.170.204.24
Table 6. The values of Cox/Ca of composites at RT and 600 °C.
Table 6. The values of Cox/Ca of composites at RT and 600 °C.
SampleTemperature
RT600 °C
TPU/APP/RH 5%0.690.57
TPU/APP/RH 20%0.700.53
Table 7. Area ratios of Raman shifts for TPU/APP/RH composite chars.
Table 7. Area ratios of Raman shifts for TPU/APP/RH composite chars.
SampleD-BandG-BandID/IG
(1350 cm−1)(1580 cm−1)
TPU/APP/RH 5%1min3249.41385.02.35
5 min78,841.458,192.21.35
TPU/APP/RH 20%1min79,809.163,712.41.25
5 min105,413.1185,994.70.57
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Kuan, C.-F.; Yang, C.-Y.; Kuan, H.-C.; Chung, M.-C.; Shih, Y.-F. Eco-Friendly Flame-Retardant Construction Composites Based on Bio-Based TPU, Recycled Rice Husk, and Ammonium Polyphosphate. Buildings 2025, 15, 3420. https://doi.org/10.3390/buildings15183420

AMA Style

Kuan C-F, Yang C-Y, Kuan H-C, Chung M-C, Shih Y-F. Eco-Friendly Flame-Retardant Construction Composites Based on Bio-Based TPU, Recycled Rice Husk, and Ammonium Polyphosphate. Buildings. 2025; 15(18):3420. https://doi.org/10.3390/buildings15183420

Chicago/Turabian Style

Kuan, Chen-Feng, Chane-Yuan Yang, Hsu-Chiang Kuan, Min-Chin Chung, and Yeng-Fong Shih. 2025. "Eco-Friendly Flame-Retardant Construction Composites Based on Bio-Based TPU, Recycled Rice Husk, and Ammonium Polyphosphate" Buildings 15, no. 18: 3420. https://doi.org/10.3390/buildings15183420

APA Style

Kuan, C.-F., Yang, C.-Y., Kuan, H.-C., Chung, M.-C., & Shih, Y.-F. (2025). Eco-Friendly Flame-Retardant Construction Composites Based on Bio-Based TPU, Recycled Rice Husk, and Ammonium Polyphosphate. Buildings, 15(18), 3420. https://doi.org/10.3390/buildings15183420

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