Translate this page into:
Design of core-shell chitosan-based phase change microcapsules towards improving the thermal management capability and fire safety of polyurethane
*Corresponding author: E-mail address: caoqing526@163.com (Q. Cao)
-
Received: ,
Accepted: ,
Abstract
The flammability properties of rigid polyurethane foam (RPUF) and phase change materials (PCMs) limit its applicability. This research employed organic montmorillonite (OMMT) and chitosan (CS) as shell materials, with paraffin serving as the core material, to effectively produce innovative OMMT CS phase change microcapsules (OMMT/CS/PCM). The integration of OMMT/CS/PCM into RPUF markedly improved the flame-retardant characteristics of the polyurethane. The performance was extensively assessed using differential scanning calorimetry (DSC) and cone calorimetry. Experimental data reveal that at a content of 30 wt% OMMT/CS/PCM, the phase change latent heat of RPUF-30% OMMT/CS/PCM is 7.514 J g-1, the limiting oxygen index (LOI) is 32.4%, and the flame retardancy index (FRI) varies from 1 to 20, indicating superior flame-retardant characteristics and a significant reduction in fire risk. OMMT and CS establish a barrier effect that is essential in reducing the diffusion of flammable gases and impeding heat transfer, thereby markedly enhancing the flame-retardant characteristics of the composite material. This work offers novel insights into the utilization of PCMs in RPUF.
Keywords
flame retardant mechanisms
flame retardant qualities
OMMT/CS/PCM
rigid polyurethane foam
thermal management capabilities
1. Introduction
Under the backdrop of intensifying global temperature rise and tightening energy resources, low-carbon development has become a shared objective for nations worldwide. China has explicitly set its “dual carbon” goals (carbon peak and carbon neutrality), aiming to drive green transformation in economic and social sectors through technological innovation and industrial upgrading. Against this backdrop, PCMs have emerged as a key material in energy storage and temperature control due to their ability to absorb and release heat during phase transitions. They demonstrate significant application potential in building energy efficiency, industrial waste heat utilization, and electronic device thermal management. However, PCMs still face numerous challenges in practical applications, particularly in high-temperature or fire environments. Their insufficient thermal stability and flame-retardant properties may lead to reduced thermal energy storage efficiency or even pose safety hazards. Simultaneously, traditional flame-retardant materials predominantly rely on halogen-containing compounds, which release toxic gases during combustion, posing severe hazards to the environment and human health. Consequently, developing phase change composite materials that combine low-carbon, high-efficiency thermal energy storage with superior flame-retardant properties has become a crucial research direction for achieving green, low-carbon development.
Among numerous studies, the thermal stability and heat transfer properties of phase change microcapsules have remained key research focuses. In experimental investigations of microcapsule performance, many scholars have explored preparation methods and characteristics through various combinations of core and shell materials, with paraffin and alkane-based materials being the most commonly used cores. Varshney et al. [1,2] prepared microcapsules using n-eicosane as the core material and wall materials such as Poly (Sty-co-BA) and phenolic resin. They investigated the effects of core-to-shell mass ratio and stirring speed on the thermal properties and morphology of the microcapsules. Han, Zou et al. [3,4] prepared novel phase-change microcapsules with excellent thermal conductivity by using paraffin wax (PW) as the core material and lignin/melamine-urea-formaldehyde, melamine-formaldehyde, or urea-formaldehyde resins as the shell material, incorporating various types of highly thermally conductive particles. They investigated the effects of different process conditions on the microcapsule properties. Wang et al. [5] employed a two-step method to prepare nano-encapsulated PCMs with n-tetradecane as the core material and a melamine-urea-formaldehyde-titanium dioxide composite as the shell material. Hu, Zhao et al. [6,7] prepared phase change capsules using n-octadecane as the core material and styrene-divinylbenzene copolymer and calcium fluoride (CaF2) as the shell material. They investigated the effects of core-shell mass ratio and emulsifier dosage on the performance of the phase change capsules.
In addition to the aforementioned organic wall materials, some researchers have employed inorganic materials for microcapsule preparation. For instance, Zhang, Parsamanesh et al. [8] synthesized phase-change microcapsules with silica shells. Yamada et al. [9] syntaxed n-tetradecane-CaCO3 microcapsules at different pH values using inorganic calcium carbonate as the shell material. Jiang et al. [10] prepared PS/CaCO₃/HDA microencapsulated phase change materials (MEPCMs) using hexadecylamine (HDA) as the core material and polystyrene (PS) and calcium carbonate (CaCO₃) as wall materials via ultrasonic emulsification. Microcapsules with high thermal conductivity and thermal stability were developed by encapsulating PW with CaCO3 and incorporating carbon-based materials as additives [11]. Zhao et al. [12] encapsulated n-octadecane within a titanium dioxide-doped styrene-divinylbenzene copolymer (SDB) shell to prepare microcapsules with dual thermal storage and photocatalytic functions. Zhang et al. [13] developed phase change capsules featuring PW cores and a double-layered silica-iron oxide (SiO₂/Fe₃O₄) shell structure. Inorganic materials have been extensively studied due to their excellent thermal conductivity and chemical stability, but their brittleness and high production costs limit their applications. Therefore, the development of composite inorganic shell materials featuring low cost, superior thermal properties, and stability is indispensable.
Studies on phase change microencapsulated polyurethane composites have focused on the effect of microcapsule loading on the latent heat of phase change, structure, mechanical properties, and thermal conductivity of polyurethane foams [14,15]. Zhao et al. [16] applied in-situ polymerization and hydrogen-bonded assembly techniques to prepare MXene-modified microcapsule phase change materials (MFPCM@MXene), which were incorporated into RPUF. The optimized RPUF-3%MFPCM@MXene composite demonstrated enhanced fire safety and thermal insulation properties. Huang et al. [17] developed MFPCM@MXene@PBA composites via a layered assembly method and incorporated them into RPUF. Experiments demonstrated that RPUF materials containing 5% MFPCM@MXene@PBA exhibited significantly enhanced flame retardancy (LOI value reaching 27.15%), while their total heat release (THR) rate and total smoke volume decreased by 23.3% and 39.4%, respectively, compared to the original RPUF material. Li et al. [18] encapsulated n-octadecane (C18) within a melamine-formaldehyde resin shell, followed by depositing Prussian blue analog (PBA) onto its surface. The resulting MFPCM@PBA was incorporated into RPUF. Test results indicate that compared to unmodified RPUF, RPUF-5% MFPCM @ PBA exhibits a LOI of 25.6%, with a 34.4% reduction in total heat release (THR) and a 46.7% decrease in total smoke production (TSP). Furthermore, after 500 s of continuous heating, the core temperature of RPUF-5%MFPCM@PBA was 20°C lower than that of unmodified RPUF. Overall, extensive research has been conducted in academia on various types of core-shell materials, focusing on the thermal stability, chemical stability, and thermal conductivity of phase change microcapsules. However, there is still a relative lack of research on phase change microencapsulated composites, especially their effect on the flame retardant properties of composites, which needs to be explored in depth.
This paper focuses on the synergistic innovation of low-carbon PCMs and flame retardant technology. Building upon prior research on the preparation and properties of chitosan (CS)/phase change microcapsules (CS/PCM) [19] and montmorillonite modification studies [20], we investigate the influence of organic montmorillonite (OMMT)/CS/phase change microcapsules (OMMT/CS/PCM) on the properties of polyurethane composite materials. PW with a melting point around 56°C was selected due to its broad phase-change temperature range, excellent heat storage capacity, stable phase-change behavior, abundant availability, and cost-effectiveness. CS is a relatively scarce natural alkaline polysaccharide. It is non-toxic, environmentally friendly, readily biodegradable, non-polluting, and chemically stable. Soluble in acidic environments, it serves as an ideal capsule wall material [21]. Montmorillonite (MMT) is a natural layered silicate mineral and an environmentally friendly flame-retardant material [22].
However, the interlayers of montmorillonite contain a large number of inorganic ions, resulting in poor compatibility with organic compounds. Organically modified montmorillonite layers can act as barriers within polymers, slowing and hindering the migration of combustible small molecules, produced by the degradation of polymer chains during combustion, toward the combustion interface. Simultaneously, they impede the penetration of external oxygen into the material, making it difficult for oxidation reactions to proceed fully, thereby achieving a flame-retardant effect. OMMT and CS, natural, eco-friendly materials, were selected as wall materials due to their excellent biocompatibility, degradability, and flame-retardant properties. By combining PCM with RPUF and integrating OMMT and CS, a composite material of OMMT/CS/phase change microcapsules/polyurethane (OMMT/CS/PCM/RPUF) composite material. This innovation simultaneously fulfills thermal energy storage and temperature regulation requirements while significantly enhancing flame retardancy, offering novel insights and solutions for sustainable energy material development.
2.Materials and Methods
2.1. Materials
CS and glacial acetic acid were analytical grade reagents supplied by Sinopharm Chemical Reagent Co., Ltd. Glutaraldehyde and petroleum ether were analytical grade reagents supplied by Meilin Technology Co., Ltd. Arabic gum was chemical grade reagent supplied by Ussuo Chemical Technology Co., Ltd. Polyether polyol R4110 was an industrial-grade product supplied by Jiahua Chemical Co., Ltd. Catalyst A-33, dichlorofluoroethane, silicone oil, and crosslinking agent DMP-30 were industrial-grade reagents supplied by Shanghai Bai’ang Chemical Technology Co., Ltd. Industrial-grade polyisocyanate was provided by Wanhua Chemical Group Co., Ltd. The OMMT/CS/phase change microcapsule (OMMT/CS/PCM) was developed in-house.
2.2. Preparation method
Place a designated quantity of OMMT in a vacuum drying oven and dry it at 45°C. Thereafter, combine OMMT with CS/PCM at a 1:1 mass ratio, as detailed in the preparation method of CS/PCM illustrated in Figure 1(a) [19]. Use a mixing stirrer to continuously stir at a specific speed for 30 min, ensuring thorough homogenization of OMMT with CS/PCM to prepare the OMMT/CS/phase change microcapsules (OMMT/CS/PCM). The preparation method and capsule formation process have been illustrated in Figure 1(b).
- Preparation process of OMMT/CS/PCM.
The synthesized OMMT/CS/PCM was subsequently utilized in RPUF to fabricate OMMT/CS/PCM/RPUF composites. To differentiate the composite materials containing various phase change microcapsules in the performance characterization diagram, distinct symbols were employed to indicate the phase change microcapsules. OMMT/CS/phase change microcapsules are referred to as OMMT/CS/PCM. The OMMT/CS/phase change microcapsule/polyurethane composites are referred to as OMMT/CS/PCM/RPUF. Various quantities of OMMT/CS/PCM were integrated into RPUF to produce RPUF composites, labeled as RPUF-5%, RPUF-10%, RPUF-15%, RPUF-20%, RPUF-25%, and RPUF-30%.
3. Results and Discussion
3.1. Surface morphology and structure analysis
3.1.1. SEM analysis
Scanning electron microscope (SEM) studies were conducted to examine the morphological properties of RPUF composites, including varying amounts of OMMT/CS/PCM, as illustrated in Figure 2. The surface of OMMT/CS/PCM microcapsules prepared with a 1:1 mass ratio of OMMT to CS/PCM exhibits relatively uniform OMMT distribution, as shown in Figure 2(a) and (b). The particle sizes of the microcapsule samples vary, influenced by the size of the CS/PCM microcapsules, exhibiting dispersion with particle diameters primarily concentrated between 100 μm and 400 μm.
- SEM of composite materials (a) CS/PCM; (b) OMMT/CS/PCM; (c) RPUF; (d) RPUF-10% OMMT/CS/PCM; (e) RPUF-20% OMMT/CS/PCM; (f) RPUF-30% OMMT/CS/PCM; (g) Aperture of RPUF-10% OMMT/CS/PCM; (h) Aperture of RPUF-20% OMMT/CS/PCM; (i) Aperture of RPUF-30% OMMT/CS/PCM. (The red circles in (d-f) represent the positions of OMMT/CS/PCM within the RPUF composite material).
The incorporation of OMMT/CS/PCM results in a steady rise in the aperture of the OMMT/CS/PCM/RPUF composite, as shown in Figure 2(c), (d), (e), and (f). The average aperture of RPUF-30% OMMT/CS/PCM was 505.81 μm, representing a 21.37% increase compared to the average aperture of RPUF-10% OMMT/CS/PCM, as shown in Figure 2(g), (h), and (i). However, compared to CS/PCM/RPUF composites with the same microcapsule content, the pore size decreased [19]. This is attributed to the interlayer bonding force of OMMT, which hinders the dispersion of OMMT/CS/PCM, and the strong interaction between OMMT/CS/PCM and the matrix. When the OMMT/CS/PCM content reached 30%, the pore size increased, significant voids formed in the layered stacking, phase separation intensified, and the compressive strength of the RPUF composite decreased.
3.1.2. FT-IR analysis
The film samples were pulverized and compacted with potassium bromide powder, and the spectral range was 4000 cm⁻1 to 500 cm⁻1. Figure 3 displays Fourier transform infrared (FT-IR) pictures of several materials. At 3544.91 cm⁻1 of OMMT, absorption peaks indicative of stretching vibrations of the hydroxy group and the hydroxy group on the montmorillonite surface are observed. The absorption maxima at 2848 cm⁻1 and 2918 cm⁻1 correspond to the stretching vibration absorption of methyl (CH₃) and methylene (CH₂), respectively. The absorption peak at around 2275 cm⁻1 corresponds to the stretching vibration of the N=C=O group, while the vibration peak of the unsaturated hydroxyl group is observed near 2010 cm⁻1. The distinctive peak of OMMT at approximately 1070 cm⁻1 corresponds to the absorption of Si-O-Si, and the characteristic peak of RPUF becomes pronounced following the addition of OMMT/CS/PCM. In the range of 1618 cm⁻1 to 1735 cm⁻1, a multi-peak superposition was seen. A mild C=N stretching vibration absorption peak is observed at about 1618 cm⁻1, whereas a P=O stretching vibration peak is present near 1680 cm⁻1, with an enhancement of the distinctive peak of OMMT in this region. The absorption peaks of paraffin at 1380 cm-1 and 1470 cm-1 correspond to the bending vibration absorption peaks of methyl -CH3 and methylene-CH2 groups. The absorption peak of RPUF at approximately 1195 cm⁻1 corresponds to the stretching vibration of the carbamate C-O bond in RPUF. The flexural vibration peak of the C-H bond occurs about 620 cm⁻1. The FT-IR spectrum of OMMT/CS/PCM exhibits the distinctive peaks of PW and CS/PCM, alongside the typical peaks of OMMT. The FT-IR spectrum of RPUF, including OMMT/CS/PCM, predominantly exhibits the typical peaks of OMMT, paraffin, and CS/PCM.
- (a) FT-IR of composite materials; (b) Enlarged view marked with a red circle of (a).
3.2. Analysis of mechanical properties
The density and maximum compressive strength of RPUF composites containing different amounts of OMMT/CS/PCM have been shown in Table 1. Both the density and maximum compressive strength of RPUF composites with added OMMT/CS/PCM exhibit a trend of first increasing and then decreasing as the amount of OMMT/CS/PCM added increases. At lower OMMT/CS/PCM content levels, the surface OMMT within the OMMT/CS/PCM exhibits compatibility with the RPUF matrix, forming favorable interactions. However, as the OMMT/CS/PCM content increases, the microcapsules expand the internal spacing within the RPUF matrix, leading to a decline in the mechanical properties of the composite material.
| Parameter | The content of OMMT/CS/PCM (%) | ||||||
|---|---|---|---|---|---|---|---|
| 0 | 5 | 10 | 15 | 20 | 25 | 30 | |
| Average density (kg/m3) | 47.5 | 49.3 | 48.4 | 47.5 | 46.1 | 45.8 | 44.7 |
| Maximum compression strength (KPa) | 293 | 294.9 | 278.6 | 274.7 | 254.9 | 244.9 | 238.2 |
3.3. Analysis of TG
The thermogravimetric (TG) analysis was conducted in a nitrogen atmosphere with a temperature range of 30°C to 750°C, a heating rate of 10°C min-1, and a flow rate of 50 mL/min. The TG test of OMMT/CS/PCM yielded TG diagrams for paraffin, CS/PCM, and OMMT/CS/PCM at various mass ratios, as seen in Figure 4.
- TG and DTG of composite materials (a) TG; (b) DTG.
Figure 4 illustrates that the weight loss process of the composite material primarily occurs in three stages: weight loss of the phase change microcapsule core material, degradation of the residual phase change microcapsule core material and the RPUF hard segment, and thermal degradation of the phase change microcapsule wall material and the soft segment in the RPUF. Due to varying amounts of added OMMT/CS/PCM, the second stage exhibits significant differences across the composite materials. Weight loss commences at approximately 230°C and concludes near 330°C, with the rate of decrease plummeting from 95% to approximately 40%.
The incorporation of CS enhanced the thermal stability of microcapsules. Further modification with OMMT resulted in a higher initial decomposition temperature for the OMMT/CS/PCM system, attributed to the physical barrier effect of OMMT layers. As the addition of OMMT/CS/PCM increases, the char yield of the composite in the high-temperature zone improves. This indicates that the incorporation of microcapsules, particularly the OMMT and CS components, effectively promotes carbonization of the RPUF matrix at elevated temperatures. This forms a thermally stable protective layer, thereby inhibiting further polymer decomposition and vaporization.
3.4. Analysis of phase change parameters
Differential scanning calorimetry (DSC) tests were conducted on RPUF composites containing varying amounts of OMMT/CS/PCM. The tests were performed at a heating rate of 10°C min-1 under N2 atmosphere, with results shown in Figure 5(a).
- DSC curves and activation energy fitting plots. (a) DSC of RPUF with different contents of OMMT/CS/PCM; (b) DSC of OMMT/CS/PCM; (c) DSC of RPUF - 20% OMMT/CS/PCM; (d) DSC of RPUF - 30% OMMT/CS/PCM; (e) Activated energy fitting graph (1 is Kissinger method, vertical axis ln(β/Tp2); 2 is Ozawa method, vertical axis ln (β); 3 is Starink method, vertical axis ln (β/Tp1.8).
The DSC curve of pure RPUF exhibited no melting or endothermic peaks, indicating its lack of phase-change heat storage capability; thus, it is not displayed in the figure. RPUF composites containing OMMT/CS/PCM exhibited distinct endothermic peaks. The DSC curves of RPUF composites with varying OMMT/CS/PCM contents resembled those of pure OMMT/CS/PCM, sharing similar phase transition onset and peak temperatures. The peak area during melting progressively increased with rising OMMT/CS/PCM content. DSC analysis indicates that the phase change latent heat of RPUF increases to 7.514 J g-1 after incorporating 30% OMMT/CS/PCM.
DSC analysis was conducted on the OMMT/CS/PCM and RPUF containing OMMT/CS/PCM composites under nitrogen atmosphere at different heating rates: 5°C min-1, 10°C min-1, 15°C min-1, and 20°C min-1. Experiments were conducted at a flow rate of 50 mL min-1, as shown in Figures 5(b-e).
The phase transition temperatures of OMMT/CS/PCM at varying heating rates were 56.04°C, 57.24°C, 58.6°C, and 59.46°C, respectively. With an increase in the heating rate, the heat absorption peak of the OMMT/CS/PCM DSC curve elevates, the initial phase transition temperature ascends, the peak temperature rises, and the phase transition process moves towards higher temperatures. This is mostly due to an increased heating rate, resulting in a more significant heat transmission delay. Theoretical analysis of phase transition dynamics utilized the Kissinger formula (eqs. 1), Ozawa formula (eqs. 2), and Starink formula (eqs. 3) to compute the activation energy and pre-exponential factor of phase transitions [19]. Table 2 displays the activation energy values computed using the three approaches. Ea represents the mean of the three activation energies, measured in kJ mol-1. Ea1 , Ea2, and Ea3 represent the activation energies derived from the Kissinger, Ozawa, and Starink equations (eqns 1-3), respectively.
| Sample |
Ea1 kJ mol-1 |
Ea2 kJ mol-1 |
Ea3 kJ mol-1 |
Ea kJ mol-1 |
lgA | R2 |
|---|---|---|---|---|---|---|
| OMMT/CS/PCM | 357.7 | 345.6 | 357.0 | 353.5 | 57.1 | 0.981 |
| RPUF - 20% OMMT/CS/PCM | 1603.2 | 1529.4 | 1597.8 | 1576.8 | 254.0 | 0.99 |
| RPUF - 30% OMMT/CS/PCM | 2292.7 | 2186.2 | 2284.8 | 2254.5 | 363.2 | 0.984 |
A denotes the pre-factor, measured in min⁻1 or s⁻1; E signifies the activation energy, expressed in kJ mol⁻1; R represents the universal gas constant, valued at 8.314 J mol-1 K-1; T indicates the thermodynamic temperature in K; β refers to the heating rate, in K min⁻1; and Tp is the peak phase transition temperature of the composite material at the specified heating rate, in K.
Table 2 demonstrates that the RPUF composite comprising OMMT/CS/PCM possesses markedly higher activation energy compared to pure OMMT/CS/PCM. The activation energy of RPUF composites increases incrementally with elevated OMMT/CS/PCM content. This issue arises because, during composite production, the OMMT/CS/PCM surface accumulates significant composite material, which obstructs the thermal motion of the molecules involved in the phase transition. The external layer of OMMT in the composite offers partial thermal insulation during temperature increases. The activation energy necessary for phase change escalates with the content of OMMT/CS/PCM, thereby augmenting its effect on both the phase transition temperature and the latent heat of the composite. Figure 4(e) and Table 2 provide similar fitting findings, with average variance coefficients nearing 0.99, thus affirming the dependability of the experimental data.
3.5. Analysis of thermal conductivity testing
Thermal conductivity was measured using a plate-type thermal conductivity tester (PBD-12-4P). Samples of 150 mm × 150 mm × 10 mm were tested at both 30°C and 60°C. Table 3 shows the thermal conductivity of RPUF at 30°C and 60°C with varying additions of phase change microcapsules.
| Parameter | The content of OMMT/CS/PCM (%) | |||||||
|---|---|---|---|---|---|---|---|---|
| 0 | 5 | 10 | 15 | 20 | 25 | 30 | ||
| Thermal conductivity (W/m·K) | 30°C | 0.0252 | 0.0317 | 0.0328 | 0.0335 | 0.0338 | 0.0341 | 0.0344 |
| 60°C | 0.0294 | 0.0380 | 0.0394 | 0.0405 | 0.0410 | 0.0416 | 0.0423 | |
The thermal conductivity of the RPUF composite increased steadily with increasing phase change microcapsule content at both 30°C and 60°C. At 30°C, the thermal conductivity of the RPUF composite containing 20% OMMT/CS/PCM was 0.0338 W m-1 K-1), representing a 33.98% improvement over pure RPUF at the same temperature. At 60°C, the thermal conductivity of the RPUF composite with 20% OMMT/CS/PCM addition reached 0.0410 W m-1 K-1, representing a 39.58% improvement over pure RPUF. The addition of OMMT/CS/PCM significantly enhances the thermal conductivity of RPUF. This improvement primarily stems from the inclusion of phase-change microcapsules, which increase the foaming ratio of the composite material, fill internal pores, and thereby elevate thermal conductivity. As the temperature rises, molecular thermal motion within the material intensifies, leading to increased thermal conductivity at 60°C.
3.6. Analysis of flame retardant performance
3.6.1. Cone calorimetry analysis
The experiment was performed under an irradiation flux of 35 kW m-2, with an airflow rate of 24 L s-1, oriented horizontally, with a sample dimension of 100 mm × 100 mm × 25 mm. Figures 6(a–d) shows the changes in heat release rate (HRR), THR, smoke production rate (SPR), and total smoke release (TSR) for RPUF that has been improved with phase change microcapsules. Table 4 presents the cone calorimeter data.
- Curves of HRR, THR, MLR and SPR. (a) HRR; (b) THR; (c) SPR; (d) TSR.
| Sample | TTI /(s) | PHRR/(kW/m2) | tPHRR/(s) | THR/(MJ/m2) | PSPR/(g/s) | TCOP/TCO2P | FIGRA | FRI |
|---|---|---|---|---|---|---|---|---|
| RPUF | 2 | 279.6 | 20 | 17.6 | 0.126 | 0.057 | 2455.8 | |
| RPUF - 10% OMMT/CS/PCM | 3 | 170.9 | 25 | 13.9 | 0.076 | 0.055 | 793.9 | 3.1 |
| RPUF - 20% OMMT/CS/PCM | 4 | 92.9 | 30 | 11.9 | 0.055 | 0.052 | 276.0 | 8.9 |
| RPUF - 30% OMMT/CS/PCM | 5 | 79.7 | 30 | 10.0 | 0.038 | 0.049 | 159.2 | 15.4 |
In comparison to pure RPUF composites (PHRR 279.6 kW m-2, THR 17.6 MJ m-2), the PHRR of OMMT/CS/PCM/RPUF decreased to 79.7 kW m-2 following the incorporation of 30% OMMT/CS/PCM. The HRR and THR of the RPUF composite containing OMMT/CS/PCM are significantly diminished, demonstrating a pronounced flame-retardant action and substantially lowering fire hazards. The fire growth rate index (FIGRA) of RPUF composites containing OMMT/CS/PCM decreased by 67.67%, 88.76%, and 93.51%, respectively, resulting in a substantial enhancement of fire safety. The FRI of the RPUF composite containing 30% OMMT/CS/PCM is 15.42, indicating superior flame-retardant capability.
The SPR and TSR curves indicate that the TSR value of the RPUF composite is inferior to that of pure RPUF following the incorporation of OMMT/CS/PCM. After 50 s, the TSR change curve approaches a plateau. The PSPR value of pure RPUF is 0.126 g s-1, while the PSPR value of the RPUF-30% OMMT/CS/PCM is 69.75% lower compared to pure RPUF. Meanwhile, the ratio of carbon monoxide production to carbon dioxide production (TCOP/TCO2P) decreased by 14.03% compared to pure RPUF. This variation occurs because OMMT/CS/PCM absorbs a portion of the emitted heat, hence decelerating the heat release rate. CS and OMMT in phase change microcapsules contain nitrogen, which means that when they burn, they produce an inert gas that reduces the amount of oxygen in the combustion area. Simultaneously, OMMT can emit crystalline water during combustion, absorb heat and evaporate, and possess a catalytic impact on carbon production to impede the transfer of heat and smoke.
3.6.2. LOI analysis
The LOI was assessed utilizing the SH5706A oxygen index tester on items measuring 120 mm × 13 mm × 13 mm. Figure 7 displays the test findings. The LOI of RPUF composites increased significantly with elevated OMMT/CS/PCM concentration, whereas the flame retardancy of OMMT/PCM/RPUF composites exhibited substantial enhancement. The LOI of RPUF-30% OMMT/CS/PCM attained 32.4%, reflecting increases of 70.53% and 24.13% relative to pure RPUF and RPUF composites with 30% CS/PCM, respectively.
- LOI of composite materials.
The incorporation of OMMT/CS/PCM into RPUF composites can improve their flame-retardant properties. The heat absorption of the core material in the phase-change microcapsule mitigates the rise in material temperature, consequently enhancing the material’s flame-retardant characteristics. CS in the shell material of the phase-change microcapsule has a notable flame-retardant effect, with a layer of OMMT present on the surface of OMMT/CS/PCM. OMMT possesses a lamellar structure that facilitates the development of a carbon layer and inhibits heat transfer. Consequently, the flame-retardant efficacy of RPUF composites incorporating OMMT/CS/PCM surpasses that of RPUF composites containing only CS/PCM.
3.7. Flame retardant mechanism
3.7.1. SEM analysis of residue
Figure 8(a-f) displays the SEM images of RPUF residues post-combustion with varying OMMT/CS/PCM compositions. The carbon layer structure of the RPUF composite comprising OMMT/CS/PCM is evident during cone burning. This results from the exposure of the phase-change microcapsule core material during combustion and the thermal degradation of OMMT in the wall material, which produces polyphosphoric acid. This polyphosphoric acid facilitates the dehydration of organic compounds such as OMMT/CS/PCM. The resultant phosphorus-oxygen compound exhibits thermal stability at elevated temperatures and produces heat during burning, thereby diminishing the oxygen requirement for combustion to a degree, which enhances the sample’s flame-retardant characteristics. The surface of pure RPUF residue carbon exhibits numerous irregular pores and cracks. However, as the addition of OMMT/CS/PCM increases, the post-combustion carbon layer structure of OMMT/PCM/RPUF demonstrates relatively continuous, robust, and stable characteristics, as shown in Figure 8(c) and (f). structure may delay and inhibit the exchange of oxygen, heat, and substances [23-26].
- SEM of residual materials after combustion of composite materials. (a)RPUF - 10% OMMT/CS/PCM, Scale bar: 300 μm; (b) RPUF - 20% OMMT/CS/PCM, Scale bar: 300 μm; (c) RPUF - 30% OMMT/CS/PCM, Scale bar: 300 μm; (d) RPUF - 10% OMMT/CS/PCM, Scale bar: 100 μm; (e) RPUF - 20% OMMT/CS/PCM, Scale bar: 100 μm; (f) RPUF - 30% OMMT/CS/PCM, Scale bar: 100 μm
3.7.2. Raman spectrum examination of residue
The remaining material following the cone calorimetry test was analyzed using a Thermo Scientific DXR Raman spectrometer for sample characterization. The scanning wavenumber range was from 500 cm⁻1 to 2000 cm⁻1. Figure 9 illustrates the Raman spectra of the residue.
- Raman of residue after cone combustion.
Graphitized carbon (D-peak) and disordered carbon (G-peak) were identified at around 1370 cm⁻1 and 1580 cm⁻1 in RPUF containing OMMT/CS/PCM, respectively. At doses of OMMT/CS/PCM of 10%, 20%, and 30%, the ID/IG values of RPUF were 0.986, 0.915, and 0.865, respectively. With the rise in OMMT/CS/PCM content, the D-peak and G-peak values of RPUF composites progressively rose, whereas the ID/IG ratios steadily declined. The incorporation of flame retardants enhances the carbonization of materials, increases the degree of graphitization, elevates the thermal stability of carbon residues, and diminishes substrate quality loss and heat release.
3.7.3. Mechanism of flame retardancy
Figure 10 illustrates the flame-retardant mechanism of the OMMT/CS/PCM/RPUF composite. Upon exposure to fire, the composite’s interior organic molecules combust and degrade, emitting a substantial quantity of non-flammable gases, including CO₂ and NO₂. Concurrently, the hydroxyl groups in the CS/PCM shell undergo dehydration to produce carbon, which absorbs heat and releases NH₃, diluting the combustible gases and O₂ in the gas phase, thereby improving the flame-retardant efficacy of the RPUF composite. The burning of the OMMT/CS/PCM/RPUF composite produces carbides, with OMMT serving a catalytic function in carbide synthesis [27-29]. OMMT decomposes upon heating during combustion to produce polyphosphate. The phosphorus-oxygen combination facilitates the dehydration of organic materials. OMMT/CS/PCM’s resistance to decomposition at elevated temperatures not only promotes the development of a carbon layer but also enhances the structural integrity of that layer. This significantly impedes the transport of heat and O₂ during combustion, retards the diffusion of combustible substances, and obstructs the combustion and decomposition of composite materials, hence offering an efficient barrier effect. This barrier effect elevates the carbon residue of the composite on the one hand. Conversely, during the combustion of the composite, the inorganic residue and the C-N bond established by CS in CS/PCM facilitate the development of a protective carbon layer, diminish smoke production and emission, efficiently obstruct the transfer of heat, combustible gases, and oxygen, and enhance the flame retardant efficacy of the composite.
- Flame retardant mechanism.
In summary, OMMT/CS/PCM significantly improves the thermal management capability and fire safety of RPUF composites through the following synergistic mechanism. In terms of thermal management, the paraffinic core inside the microcapsules absorbs and releases a large amount of latent heat during the phase change process, which can effectively regulate the ambient temperature of the composites and play the role of a thermal buffer, thus enhancing the thermal management capability of the materials. This enhances the thermal management capability of the material. In terms of fire safety, CS in the shell layer decomposes at high temperatures to produce non-combustible gases, which dilute the concentration of oxygen and combustible gases, thus playing the role of a gas-phase barrier. OMMT and CS synergistically form a dense and stable “barrier shell layer.” This physical barrier not only blocks the transfer of heat and combustible volatiles but also protects the internal paraffin core material, slowing down its thermal decomposition and combustion process and playing the role of the condensation phase. The thermal buffering effect of the PCM delays the overall composite material from reaching the thermal decomposition temperature, buying critical time for the early formation of the fire barrier (OMMT/CS shell layer). After the formation of the barrier, the leakage and combustion of the paraffin are in turn inhibited, thus realizing the synergistic enhancement of thermal regulation and fire safety.
4. Conclusions
In this study, OMMT/CS/PCM composites with excellent performance were prepared by the dry mixing method, and then OMMT/CS/PCM/RPUF composites were prepared by adding them to RPUF, and the properties of the composites were tested and analyzed. The experimental results showed that the density and compressive strength of RPUF composites showed an increasing and then decreasing trend with the addition of OMMT/CS/PCM. The addition of OMMT/CS/PCM significantly increased the carbonization rate of RPUF composites in the high-temperature region and effectively suppressed their high-temperature degradation. The composites containing OMMT/CS/PCM showed obvious heat absorption peaks. The limiting oxygen index and the enthalpy of phase transition of RPUF composites gradually increased with the increase of OMMT/CS/PCM addition, and the activation energy of RPUF composites also gradually increased. When 30% OMMT/CS/PCM was added, the PHRR of OMMT/CS/PCM/RPUF composites decreased to 79.73, and the PSPR value was 69.75% lower than that of pure RPUF, and the PSPR value of TCOP/TCO2P also decreased by 14.03%. The fire growth rate index (FIGRA) of RPUF composites with added OMMT/CS/PCM also showed a decreasing trend, and the fire resistance performance was significantly improved. Meanwhile, the FRI of the composites ranged from 1 to 20, which indicated that all the composites had excellent flame-retardant properties. The flame retardant mechanism of OMMT/CS/PCM/RPUF composites consisted of gas-phase dilution flame retardant and condensation-phase barrier effects. The addition of OMMT/CS/PCM synergistically enhanced the thermal management capability and fire safety of RPUF composites.
Although this study successfully prepared the OMMT/CS/PCM/RPUF composite material and verified its excellent thermal and flame-retardant properties, certain limitations remain. To address these limitations, future research will continue in the following directions: (1) Optimize the composition ratio and preparation process to enhance mechanical strength while ensuring flame retardancy and thermal performance; (2) Developing scalable fabrication techniques to explore industrial-scale production feasibility and optimize raw material costs; (3) Conducting long-term performance testing under sustained thermal/mechanical stresses and multi-factor coupled conditions, while exploring practical application scenarios in construction, electronics, and other fields.
Acknowledgments
This research was funded by the National Natural Science Foundation of China, grant numbers NO. 52204224 and NO. 51904032; and the doctoral scientific research initiation fund project of Shandong University of Aeronautics, grant number NO. 2024Y35.
CRediT authorship contribution statement
Qing Cao: Writing-original draft. Wenjie Guo: Data analysis & Experimental studies. Xinlei Jia: Design & Data analysis. Lanjuan Xu: Experimental studies & Manuscript preparation. Yingying Hu: Data analysis & Manuscript preparation.
Declaration of competing interest
There are no conflicts of interest.
Data availability
All data in the manuscript may be obtained from the corresponding author.
Use of artificial intelligence (AI)-assisted technology for manuscript preparation
The authors confirm that there was no use of AI-assisted technology for assisting in the writing of the manuscript and no images were manipulated using AI.
References
- Synthesis and characterization of microencapsulated phase change material with phenol-formaldehyde resin shell for thermal energy storage. Solar Energy Materials and Solar Cells. 2022;243:111789. https://doi.org/10.1016/j.solmat.2022.111789
- [Google Scholar]
- Fabrication and evaluation of eicosane/poly (styrene-co-butylacrylate) microencapsulated phase change materials through ultrasonicated mini-emulsion technique. Chemical Engineering Journal. 2024;500:156994. https://doi.org/10.1016/j.cej.2024.156994
- [Google Scholar]
- Preparation and optimization of thermochromic microcapsules as a ternary system of crystal violet lactone: bisphenol A: decanol encapsulated with urea formaldehyde resin in a UV-curable primer. Polymers (Basel). 2025;17:851. https://doi.org/10.3390/polym17070851
- [Google Scholar]
- Synthesis and characterization of microencapsulated paraffin with melamine-urea-formaldehyde shell modified with lignin. International Journal of Biological Macromolecules. 2024;261:129640. https://doi.org/10.1016/j.ijbiomac.2024.129640
- [Google Scholar]
- Nanoencapsulated n-tetradecane phase change materials with melamine–urea–formaldehyde–TiO2 hybrid shell for cold energy storage. Colloids and Surfaces A: Physicochemical and Engineering Aspects. 2022;636:128162. https://doi.org/10.1016/j.colsurfa.2021.128162
- [Google Scholar]
- Preparation and characterization of n-octadecane @ calcium fluoride microencapsulated phase change materials. Solar Energy Materials and Solar Cells. 2022;237:111571. https://doi.org/10.1016/j.solmat.2021.111571
- [Google Scholar]
- Microencapsulated phase change n-Octadecane with high heat storage for application in building energy conservation. Applied Energy. 2023;329:120284. https://doi.org/10.1016/j.apenergy.2022.120284
- [Google Scholar]
- Systematic investigation on preparation and characterization of silica shell microencapsulated phase change materials based on sodium silicate precursor. Colloids and Surfaces A: Physicochemical and Engineering Aspects. 2023;667:131328. https://doi.org/10.1016/j.colsurfa.2023.131328
- [Google Scholar]
- Thermal energy storage performance enhancement of microencapsulated n-Tetracosane with a polymorph-modulated calcium carbonate shell. Energy & Fuels. 2023;37:3152-3158. https://doi.org/10.1021/acs.energyfuels.2c03564
- [Google Scholar]
- Research on the low-temperature waste heat storage performance of a novel microencapsulated phase change material with a double-shell encapsulation of hexadecylamine. Journal of Energy Storage. 2024;80:110253. https://doi.org/10.1016/j.est.2023.110253
- [Google Scholar]
- Self-assembled synthesis of microencapsulated paraffin wax phase change materials with excellent thermal properties of calcium carbonate shell. Journal of Enhanced Heat Transfer. 2023;30:1-18. https://doi.org/10.1615/JEnhHeatTransf.2022045579
- [Google Scholar]
- Phase change n-Octadecane microencapsulated in titanium dioxide nanoparticle-doped polymer for photothermal conversion and photocatalysis. Solar Energy. 2023;254:73-87. https://doi.org/10.1016/j.solener.2023.03.012
- [Google Scholar]
- Enhancing phase change fiber composites with double shell capsules: Magnetic response and photothermal conversion studies. Materials Chemistry and Physics. 2024;328:130008. https://doi.org/10.1016/j.matchemphys.2024.130008
- [Google Scholar]
- Development of rigid polyurethane foam incorporating phase change material for a low‐temperature thermal energy storage application. Polymer International. 2023;72:490-499. https://doi.org/10.1002/pi.6492
- [Google Scholar]
- Phase-change materials as cryo-shock absorbers in rigid polyurethane cryogenic insulation foams. Polymers (Basel). 2025;17:729. https://doi.org/10.3390/polym17060729
- [Google Scholar]
- MXene-doped microencapsulated phase change materials for rigid polyurethane foam with enhanced flame retardancy and thermal insulation properties. Polymer Degradation and Stability. 2025;240:111505. https://doi.org/10.1016/j.polymdegradstab.2025.111505
- [Google Scholar]
- Hierarchical MFPCM@ MXene@PBA nanohybrids towards high-efficiency flame retardancy and thermal regulation performance for microencapsulated phase change materials in rigid polyurethane foam. Energy. 2025;336:138387. https://doi.org/10.1016/j.energy.2025.138387
- [Google Scholar]
- Microencapsulated phase change materials functionalized with PBA for enhancing the flame retardancy of thermally regulated rigid polyurethane foam. Polymer Degradation and Stability. 2025;242:111702. https://doi.org/10.1016/j.polymdegradstab.2025.111702
- [Google Scholar]
- Thermal stability and flame retardancy of rigid polyurethane foam composites filled with phase-change microcapsule. Materials. 2024;17:888. https://doi.org/10.3390/ma17040888
- [Google Scholar]
- Influence of organic intercalation modification agent on montmorillonite. New Chemical Materials. 2017;45:197-199.
- [Google Scholar]
- Recent advances in chitosan-based applications-a review. Materials. 2023;16:2073. https://doi.org/10.3390/ma16052073
- [Google Scholar]
- Functionalized montmorillonite/epoxy resin nanocomposites with enhanced thermal and mechanical properties. RSC Advances. 2024;14:31251-31258. https://doi.org/10.1039/d4ra03125c
- [Google Scholar]
- Development of multifunctional highly-efficient bio-based fire-retardant poly(lactic acid) composites for simultaneously improving thermal, crystallization and fire safety properties. International Journal of Biological Macromolecules. 2022;215:646-656. https://doi.org/10.1016/j.ijbiomac.2022.06.158
- [Google Scholar]
- Fabrication of novel renewable furan-based phosphorus and its applications in poly (lactic acid): Thermal, flammability, crystallization and mechanical properties. Polymer Degradation and Stability. 2022;203:110060. https://doi.org/10.1016/j.polymdegradstab.2022.110060
- [Google Scholar]
- Constructing high fire-safety and ultra-long warning smart polyurethane: Preparation, structure, property and mechanism. Construction and Building Materials. 2025;482:141530. https://doi.org/10.1016/j.conbuildmat.2025.141530
- [Google Scholar]
- Boron nitride based nanostructure inspired flame retarded thermoplastic polyurethane composites: Comprehensive evaluation and mechanism investigation. Construction and Building Materials. 2024;457:139478. https://doi.org/10.1016/j.conbuildmat.2024.139478
- [Google Scholar]
- Recent progress on flame retardance of polymer/layered silicate nanocomposites. Journal of Matericals Engineering. 2015;43:104-112.
- [Google Scholar]
- Flame retardant properties of polymer nanocomposites based on new layered structure nanoparticles. Flame Retardant Nanocomposites. 2024;4:117-158. https://doi.org/10.1002/pi.6492
- [Google Scholar]
- Flame‐retardant thermoplastic polyurethane‐based nanocomposites: Research status, prospects and challenges. Polymers for Advanced Technologies. 2025;36:70227. https://doi.org/10.1002/pat.70227
- [Google Scholar]