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. 2026 Sep 30;11(40):62211–62225. doi: 10.1021/acsomega.6c09407

Biomaterial-Based Cu2O@Chitosan/Phosphorylated Cellulose Core–Shell Flame Retardant Fabricated via Layer-by-Layer Assembly for Enhanced Fire Safety of Epoxy Resin

Xiaotao Zhang †, Kaihua Lu ‡, Yushi Lu ‡, Hongqi Shi §,*
PMCID: PMC13670252  PMID: 42889838

Abstract

Epoxy resins (EPs) are widely used in advanced engineering applications but suffer from inherent flammability and rapid heat release during combustion. A biomaterial-based core–shell flame retardant, Cu2O@(CH/PT), has been fabricated using a mild layer-by-layer (LbL) assembly strategy by alternately depositing chitosan (CH) and phosphorylated cellulose (PT) onto Cu2O particles. Scanning electron microscopy (SEM) and Fourier-transform infrared (FTIR) analyses provided complementary morphological and chemical evidence for the surface deposition and assembly of CH and PT on Cu2O particles. The resulting Cu2O@(CH/PT) was incorporated into epoxy resin blends to investigate its effects on thermal degradation and combustion behavior. The incorporation of 8 wt % Cu2O@(CH/PT) increased the residual mass of the EP blends at 700 °C during thermal degradation from 7.79% to 20.62% and increased the limiting oxygen index (LOI) for combustion from 23.3% to 31.0%. Microscale combustion calorimetry results showed that the peak heat release rate (PHRR), total heat release (THR), and heat release capacity (HRC) for combustion of EP-8 were reduced by 45.1%, 33.4%, and 44.0%, respectively, compared with the corresponding values for combustion of neat EP. The enhanced flame-retardant performance may be associated with the cooperative effects of the Cu2O catalytic core and the biomaterial-based phosphorus/nitrogen-containing CH/PT shell, where PT promotes condensed-phase charring, CH contributes to the development of protective char structures, and Cu2O facilitates the stabilization of carbonaceous residues. A sustainable strategy for constructing biomaterial-based core–shell Cu2O flame retardants through LbL assembly to improve the fire safety of epoxy resin blends has been developed.

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1. Introduction

Epoxy resins (EPs) are widely used in advanced engineering fields, including electronic devices, aerospace structures, coatings, and adhesives, owing to their excellent mechanical properties, chemical resistance, dimensional stability, and strong adhesion. − However, the inherent flammability of epoxy resins remains a critical limitation for their applications in fire-sensitive environments. Typically, neat epoxy resins exhibit limiting oxygen index (LOI) values of approximately 20–25% during combustion tests, indicating their susceptibility to ignition and sustained combustion. , Moreover, epoxy combustion is usually accompanied by rapid thermal degradation, intensive heat release, and the generation of large amounts of combustible volatile products, which can accelerate flame propagation and increase fire hazards. , Therefore, improving the fire safety of epoxy resins through effective flame-retardant strategies has become an important focus for developing safer high-performance polymer systems.

Historically, halogen-containing flame retardants have been widely employed in epoxy resins due to their high flame inhibition efficiency, mainly through gas-phase radical-quenching processes. , However, concerns regarding their environmental persistence, potential toxicity, and release of corrosive and harmful halogen-containing decomposition products during combustion have gradually restricted their practical applications. , As a result, increasing attention has been directed toward environmentally friendly halogen-free flame-retardant systems, among which phosphorus- and nitrogen-containing flame retardants have emerged as promising alternatives due to their ability to promote char formation, generate protective barrier layers, and regulate combustible volatile products through condensed- and gas-phase effects. , Nevertheless, individual organic flame-retardant systems often face challenges in achieving efficient and comprehensive flame-retardant performance. , For typical phosphorus-containing systems, phosphorus species can effectively promote cationic cross-linking and char formation; however, an insufficient supply of carbon sources or gas-generating components may limit the formation of a continuous and stable protective char layer. − Similarly, nitrogen-containing flame retardants may evolve gaseous species that dilute the fuel load in the gas phase and may also contribute to char formation, but their carbonization capability is often insufficient when used alone. Therefore, the flame-retardant efficiency of single-component systems is frequently restricted by the imbalance among acid sources, carbon sources, and gas sources, highlighting the importance of constructing multifunctional flame-retardant systems with cooperative interactions.

Beyond conventional organic flame-retardant systems, transition-metal-containing materials have attracted considerable attention as catalytic additives for improving the flame retardancy of epoxy resins. Owing to their catalytic activity, transition-metal oxides can promote dehydrogenation, aromatization, and cross-linking reactions during polymer degradation, thereby facilitating the formation of more stable carbonaceous structures and enhancing the protective effect of char layers. , Among various transition-metal oxide catalysts, Cu2O has been recognized as a promising catalytic component due to its suitable catalytic activity, thermal stability, and reported ability to enhance the stability of carbonaceous residues. , However, metal-oxide-based flame-retardant systems still face intrinsic limitations. Acting mainly as catalytic sites, metal oxides do not inherently provide sufficient carbon sources, gas-generating species, or the organic components required for constructing an integrated protective char layer. , Although Cu2O can improve char-layer structure through catalytic effects, it mainly functions as a catalytic modifier that promotes cross-linking rather than as a carbon-forming component. Relying solely on these catalytic components is, therefore, generally insufficient for the formation of a continuous, compact, and highly effective protective barrier during combustion. Accordingly, integrating catalytic metal oxide centers into organic carbon-forming components represents a promising strategy for developing cooperative flame-retardant systems with enhanced condensed-phase protection.

On the basis of this concept, recent efforts have increasingly focused on constructing multicomponent flame-retardant systems by integrating metal oxide catalysts with phosphorus- and nitrogen-containing char-forming components. , The integration of catalytic metal species with phosphorus- and nitrogen-containing components provides complementary functions, where metal oxides facilitate the formation and stabilization of carbonaceous structures through catalytic effects, while phosphorus-containing species contribute mainly to condensed-phase reactions and char formation, and nitrogen-containing components may also generate volatile species that dilute combustible products in the gas phase. , Such strategies have demonstrated improved flame-retardant performance compared to individual flame-retardant components. However, many reported that multicomponent flame-retardant systems are still constructed through relatively simple physical mixing or a simple combination of different functional additives. Although this approach is convenient and straightforward, the random distribution of individual components and weak interfacial interactions between different phases often limit the effective utilization of cooperative effects. , In particular, for catalytic flame-retardant systems, the spatial separation between catalytic metal sites and organic carbon-forming components may hinder efficient catalytic interactions during thermal degradation, thereby restricting the formation of compact and stable protective char layers. , Therefore, achieving precise structural integration of catalytic components and carbon-forming constituents, rather than simply combining different functional additives, is essential for maximizing cooperative flame-retardant effects. Constructing well-defined architectures with intimate interfacial coupling between inorganic catalytic centers and organic flame-retardant components, therefore, represents a promising strategy for developing highly efficient epoxy flame-retardant systems.

To achieve efficient integration of different flame-retardant components, various structural integration strategies have been developed for constructing multifunctional flame-retardant architectures. Techniques such as in situ polymerization, sol–gel processing, and chemical grafting have been extensively explored to improve interfacial compatibility and promote the integration of different functional components within flame-retardant systems. , However, these approaches generally involve complicated reaction procedures, strict processing conditions, and multiple synthesis steps, which may hinder precise structural regulation of flame-retardant fillers and restrict their practical applications. In contrast, layer-by-layer (LbL) assembly technology provides a facile and versatile platform for constructing multifunctional core–shell structures through sequential deposition of oppositely charged components under mild aqueous conditions, enabling precise spatial organization of different functional species. , By controlling the deposition sequence and number of assembled layers, LbL assembly enables tunable interfacial structures and adjustable compositions, offering significant advantages for constructing well-defined core–shell functional architectures. − Therefore, LbL assembly represents an effective strategy for achieving intimate integration between catalytic inorganic components and organic flame-retardant constituents, providing a promising pathway for constructing cooperative flame-retardant additives for epoxy systems.

Considering the limitations of metal-oxide-based catalytic systems, the insufficient interactions among multicomponent flame-retardant constituents, and the relatively complicated fabrication of conventional structurally integrated systems, the development of efficient and sustainable flame-retardant architectures remains highly desirable. A biomaterial-based Cu2O@(CH/PT) flame-retardant system was designed and prepared through a mild and controllable layer-by-layer (LbL) assembly strategy by sequentially depositing phosphorus- and nitrogen-containing polyelectrolytes on Cu2O particles. In this architecture, Cu2O serves as a catalytic center to promote the formation and stabilization of carbonaceous residues during polymer degradation, while phosphorylated cellulose (PT), derived from renewable cellulose resources, acts as a phosphorus-rich carbon-forming component to facilitate condensed-phase cross-linking and charring. Meanwhile, chitosan (CH), a naturally derived nitrogen-containing polymer, contributes additional carbon sources and nitrogen-containing structures, assisting in the construction of a robust carbon framework. The sequential surface assembly of these components is expected to improve their interfacial contact and facilitate cooperative char-forming processes, thereby promoting the formation of protective char layers and improving the fire safety of epoxy resins containing these additives. To validate this cooperative design strategy, the constructed flame-retardant fillers were incorporated into epoxy resin, and the thermal stability and flame-retardant performance of the blends were systematically investigated. Possible cooperative flame-retardant processes were inferred from the thermal degradation characteristics and residual char analysis. This design provides a sustainable approach for integrating catalytic metal oxides with renewable biomaterial-derived phosphorus- and nitrogen-containing components to improve the fire safety of the epoxy blends.

2. Experimental Section

2.1. Materials

Copper­(II) sulfate pentahydrate (CuSO4·5H2O, ≥99.0%), sodium hydroxide (NaOH, ≥96.0%), glucose (C6H12O6, ≥99.5%), cetyltrimethylammonium bromide (CTAB, ≥99.0%), microcrystalline cellulose, urea (≥99.0%), phosphoric acid (H3PO4, 85 wt %), chitosan (degree of deacetylation: 80–95%, viscosity: 50–800 mPa·s), anhydrous ethanol (C2H5OH, ≥99.7%), hydrochloric acid (HCl, 36–38 wt %), 4,4′-diaminodiphenylmethane (DDM), and chloroform (CHCl3, ≥99.0%) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Epoxy resin (diglycidyl ether of bisphenol A, DGEBA; commercial designation E-44) was obtained from Shandong Yousuo Chemical Technology Co., Ltd. (Shandong, China). Deionized water (resistivity: 18.2 MΩ·cm) was used throughout all experiments.

2.2. Preparation of Cu2O Core Particles

Cu2O core particles were synthesized via a wet chemical reduction method using glucose as the reducing agent. In a typical procedure, 12.5 g of CuSO4·5H2O was dissolved in 200 mL of deionized water under magnetic stirring at room temperature to form a clear blue solution. Separately, 5.0 g of CTAB was dissolved in 200 mL of deionized water under stirring until complete dissolution. The CTAB solution was then slowly added to the copper sulfate solution under continuous stirring, and the resulting mixture was stirred for an additional 15 min.

Subsequently, 10.0 g of NaOH was dissolved in 100 mL of deionized water. The resulting NaOH solution was added dropwise to the CTAB–copper sulfate mixture over 20 min under vigorous stirring at room temperature, yielding a uniform dark blue suspension. The suspension was then heated to 95 °C and maintained at this temperature for 15 min. Meanwhile, 15.0 g of glucose was dissolved in 100 mL of deionized water. Subsequently, the glucose solution was added dropwise to the reaction system over 15 min at 95 °C under magnetic stirring. After completion of the addition, the reaction was allowed to proceed for an additional 30 min at 95 °C. The mixture was then allowed to cool naturally to room temperature.

The resulting precipitate was collected by centrifugation and washed three times with deionized water and twice with anhydrous ethanol. Finally, the product was dried in an oven at 70 °C for 12 h, yielding brick-red Cu2O particles.

2.3. Synthesis of Phosphorylated Cellulose

Microcrystalline cellulose powder (15 g) and urea (45 g) were mixed with 150 mL of 85 wt % phosphoric acid in a three-necked flask equipped with a reflux condenser. The reaction mixture was stirred in an oil bath at 120 °C for 3 h. After the reaction, the mixture was allowed to cool to room temperature, and the resulting phosphorylated cellulose product was collected by centrifugation. The precipitate was repeatedly washed with deionized water until the supernatant reached a neutral pH to remove residual phosphoric acid, followed by a final wash with anhydrous ethanol. The resulting product was dried in an oven at 60 °C for 12 h to obtain phosphorylated cellulose (denoted as PT). The schematic illustration of the phosphorylation reaction is presented in Figure .

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Schematic illustration of the phosphorylation of cellulose.

2.4. Preparation of Polyelectrolyte Solutions

Chitosan powder (5.0 g) was dispersed in deionized water (995 mL) and magnetically stirred at room temperature for 30 min. A 1 mol/L HCl solution was added dropwise with stirring to adjust the pH to approximately 4.0. The mixture was further stirred at room temperature for 24 h to obtain a homogeneous 0.5 wt % chitosan (CH) solution.

Phosphorylated cellulose (20.0 g) was dispersed in deionized water (980 mL) to form a dispersion. Under stirring at room temperature, the pH was adjusted to approximately 7.0 using 1 mol L–1 of HCl or 1 mol L–1 of NaOH. The dispersion was stirred for 30 min and ultrasonicated for 10 min to obtain a stable 2 wt % phosphorylated cellulose (PT) dispersion.

2.5. Preparation of Cu2O@(CH/PT) Core–Shell Flame Retardant

Cu2O core particles were surface-modified using a layer-by-layer (LbL) assembly process. Prior to assembly, a total of 3.0 g of Cu2O core particles was dispersed in an alkaline solution (pH ≈ 9) and stirred for 20 min to facilitate the generation of a negatively charged Cu2O surface. The particles were then collected by centrifugation, thoroughly washed with deionized water until the supernatant reached a neutral pH, and immediately redispersed in the chitosan (CH) solution. The suspension was ultrasonicated for 5 min and subsequently magnetically stirred for 15 min to ensure homogeneous dispersion and facilitate the adsorption of positively charged CH onto the negatively charged Cu2O surface. After centrifugation, the particles were washed three times with deionized water to remove loosely adsorbed CH.

The resulting CH-coated particles were redispersed in the phosphorylated cellulose (PT) dispersion, ultrasonicated for 5 min, and then magnetically stirred for 15 min to promote the electrostatic adsorption of the negatively charged PT layer. The particles were collected by centrifugation and washed three times with deionized water, completing one CH/PT bilayer deposition cycle. The above assembly cycle was repeated eight times, as preliminary optimization indicated that this condition provided a favorable balance between particle dispersibility and coating uniformity. Finally, the resulting powder was dried in an oven at 75 °C for 12 h to obtain the Cu2O@(CH/PT) core–shell flame retardant, denoted as Cu2O@(CH/PT). The overall preparation process is schematically illustrated in Figure .

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Schematic illustration of the layer-by-layer (LbL) assembly process for the preparation of Cu2O@(CH/PT) core–shell flame retardant.

2.6. Preparation of Epoxy Blends

A predetermined amount of Cu2O@(CH/PT) core–shell particles was dispersed in an appropriate amount of chloroform by ultrasonication and magnetic stirring to obtain a homogeneous suspension. Subsequently, 40.0 g of bisphenol A-type epoxy resin (E-44) was added to the suspension, and the mixture was stirred in an oil bath at 100 °C for 3 h to remove the chloroform and promote the uniform dispersion of the core–shell particles within the epoxy matrix. After the chloroform had evaporated, 8.8 g of premelted 4,4′-diaminodiphenylmethane (DDM) was added while maintaining an epoxy resin-to-DDM mass ratio of 4.55:1. The resulting mixture was further stirred at 100 °C for 5 min to ensure thorough mixing.

The mixture was then poured into a mold and cured in an oven at 100 °C for 2 h, followed by curing at 150 °C for an additional 2 h. After curing, the samples were allowed to cool naturally to room temperature and were then demolded to obtain the EP/Cu2O@(CH/PT) blends. By varying the loading of Cu2O@(CH/PT) core–shell particles, epoxy blends containing 0, 2, 5, and 8 wt % Cu2O@(CH/PT), based on the combined mass of epoxy resin and DDM, were prepared. The neat epoxy resin was denoted as EP, while the epoxy blends containing 2, 5, and 8 wt % Cu2O@(CH/PT) were designated as EP-2, EP-5, and EP-8, respectively. For the comparative experiments, two additional control formulations were prepared at a total additive loading of 8 wt %. EP-8-Cu2O contained uncoated Cu2O, whereas EP-8-(Cu2O/CH/PT) contained a physical mixture of Cu2O, CH, and PT. The nominal Cu2O/CH/PT mass ratio in the physical mixture was estimated from the mass increase of Cu2O after LbL modification and the relative solid contents of the CH solution and PT suspension used for LbL assembly.

3. Results and Discussion

3.1. Structural and Morphological Characterization of Cu2O Core Particles

The morphology and elemental distribution of the synthesized Cu2O core particles were characterized by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS), as shown in Figure . The low-magnification SEM image (Figure a) shows that the Cu2O particles exhibit relatively regular polyhedral morphologies with well-defined boundaries and no severe agglomeration. The high-magnification SEM image (Figure b) further reveals distinct particle edges and relatively smooth surfaces, with particle sizes of approximately 500–600 nm. The EDS elemental mappings (Figure c,d) reveal relatively uniform distributions of Cu and O throughout the scanned region, supporting the homogeneous elemental distribution of the synthesized particles.

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SEM images and EDS elemental mappings of Cu2O particles: (a) low-magnification SEM image; (b) high-magnification SEM image; (c) Cu elemental mapping; and (d) O elemental mapping.

The crystal structure of the synthesized product was examined by X-ray diffraction (XRD), and the resulting diffraction pattern is shown in Figure . All diffraction peaks observed in the 2θ range of 10°–80° can be well indexed to the reference diffraction pattern of cubic Cu2O (PDF No. 77–0199). The characteristic peaks at 2θ = 29.6°, 36.4°, 42.4°, 61.5°, 73.6°, and 77.6° correspond to the (110), (111), (200), (220), (311), and (222) crystal planes of Cu2O, respectively. The sharp and well-defined diffraction peaks indicate the high crystallinity of the synthesized Cu2O particles. Notably, no characteristic diffraction peaks attributable to CuO or metallic Cu were detected over the scanned range, supporting the high phase purity of the synthesized Cu2O particles within the detection limit of XRD.

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XRD pattern of the synthesized Cu2O particles compared with the standard diffraction pattern of cubic Cu2O (PDF No. 77–0199).

The Fourier-transform infrared (FTIR) spectrum of the synthesized Cu2O particles is presented in Figure . A characteristic absorption band at approximately 630 cm–1 is assigned to the Cu–O stretching vibration, which is consistent with the presence of Cu–O bonds in Cu2O. A broad absorption band centered at approximately 3440 cm–1 is attributed to the O–H stretching vibration of surface-adsorbed water and hydroxyl-containing species, indicating the presence of hydroxyl-containing species on the particle surface. Under alkaline conditions, these surface hydroxyl groups may contribute to the formation of negatively charged sites on Cu2O particles, thereby facilitating subsequent electrostatic interaction with positively charged chitosan during LbL assembly. Apart from the aforementioned characteristic bands, no other prominent absorption bands were observed between 500 and 4000 cm–1, indicating that no obvious impurity-related functional groups were detected.

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FTIR spectrum of Cu2O particles.

3.2. Characterization of Phosphorylated Cellulose

To evaluate the structural changes associated with cellulose phosphorylation, pristine cellulose (denoted as Cel) and phosphorylated cellulose (denoted as PT) were characterized by FTIR, and the resulting spectra are presented in Figure . The highlighted region contains the main spectral changes associated with the introduction of phosphate-containing groups. Compared with pristine cellulose, PT exhibits a new characteristic absorption band at approximately 1240 cm–1, which is assigned to the stretching vibration of PO groups in phosphate-containing structures. In addition, the absorption band near 1050 cm–1 becomes more pronounced after phosphorylation, which may result from overlap between phosphate-related P–O–C vibrations and the intrinsic C–O–C vibrations of cellulose. A distinct absorption band also appears at approximately 980 cm–1, which is associated with phosphate-related P–O vibrations and may involve contributions from P–O–C and/or P–OH groups. The appearance and enhancement of these phosphate-related absorption bands provide evidence for the incorporation of phosphate-containing groups into the cellulose structure.

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FTIR spectra of pristine cellulose (Cel) and phosphorylated cellulose (PT).

Meanwhile, the characteristic absorption bands associated with the cellulose backbone, including the O–H stretching vibration at approximately 3340 cm–1, the C–H stretching vibration at approximately 2895 cm–1, the CH2 bending vibration at approximately 1430 cm–1, and the β-glycosidic linkage vibration at approximately 897 cm–1, remain identifiable after phosphorylation. These results indicate that the main structural features of the cellulose backbone were retained following the phosphorylation treatment. Collectively, the FTIR results suggest that phosphate-containing groups were successfully introduced into the cellulose structure while the main structural characteristics of the cellulose backbone were retained.

3.3. Characterization of Cu2O@(CH/PT) Core–Shell Flame Retardant

Figure a,b shows the SEM images of Cu2O@(CH/PT) particles after LbL modification. Compared with the relatively smooth surface of pristine Cu2O (Figure b), the modified particles exhibit a noticeably rougher and more heterogeneous surface morphology. At a higher magnification, irregular surface textures and localized deposits can be clearly observed, providing morphological evidence for the surface modification of Cu2O after the sequential CH/PT assembly process.

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SEM images of Cu2O@(CH/PT) particles and fracture surfaces of epoxy blends: (a) low-magnification SEM image of Cu2O@(CH/PT) particles; (b) high-magnification SEM image of Cu2O@(CH/PT) particles; (c) fracture surface of neat EP; and (d) fracture surface of EP-8.

Figure c,d further compares the fracture surfaces of neat EP and EP-8 to evaluate the dispersion of Cu2O@(CH/PT) and its interfacial characteristics with the epoxy matrix. Neat EP exhibits a relatively homogeneous fracture surface (Figure c). In contrast, the fracture surface of EP-8 shows dispersed particle-related features throughout the matrix without obvious large-scale agglomeration (Figure d). The matrix surrounding most dispersed particles remains relatively continuous, suggesting a generally acceptable interfacial contact between Cu2O@(CH/PT) and the epoxy matrix.

To further investigate the chemical characteristics of the surface-modified layer, FTIR spectra of Cu2O before and after LbL assembly were recorded, as shown in Figure . Pristine Cu2O exhibits a characteristic absorption band at approximately 630 cm–1, corresponding to the Cu–O stretching vibration, together with a broad band near 3400 cm–1 associated with the O–H stretching vibration of surface-adsorbed water and hydroxyl-containing species. After LbL assembly, the characteristic Cu–O absorption band at approximately 630 cm–1 remains observable, indicating that the Cu–O bonding feature was retained during surface modification. In addition, several new or enhanced absorption bands appear at approximately 3340, 2899, 1640, 1430, 1160, 1109, 1058, and 980 cm–1. The broad band near 3340 cm–1 and the band at approximately 2899 cm–1 are attributed to O–H/N–H stretching and C–H stretching vibrations, respectively, associated with the CH/PT coating. The band near 1640 cm–1 is mainly attributed to the amide I vibration of chitosan, with possible contribution from adsorbed water, while the band near 1430 cm–1 is primarily attributed to CH2 deformation vibrations associated with the polysaccharide components. The absorption bands at approximately 1160, 1109, 1058, and 980 cm–1 are associated with phosphate-related P–O vibrations and may overlap with characteristic vibrations of the polysaccharide framework. These newly emerged and enhanced absorption bands are consistent with the characteristic functional groups of chitosan and phosphorylated cellulose. Together with the surface morphological changes observed by SEM, these spectral features provide complementary evidence for the deposition and assembly of CH and PT on the Cu2O surface, supporting the formation of a CH/PT-modified layer.

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FTIR spectra of Cu2O and Cu2O@(CH/PT).

3.4. Thermal Degradation Behavior and Condensed-Phase Residue Formation

To investigate the effect of Cu2O@(CH/PT) on the thermal degradation behavior and condensed-phase residue formation of epoxy resin, TG and DTG analyses were performed under a nitrogen atmosphere for neat EP and its blends containing different loadings of Cu2O@(CH/PT). The TG and DTG curves are presented in Figure , and the corresponding thermal degradation parameters, including T 5%, T 50%, T max, and residual mass at 700 °C, are summarized in Table .

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(a) TG and (b) DTG curves of neat EP and EP blends containing different amounts of Cu2O@(CH/PT).

1. Thermal Stability Parameters of EP and EP Blends.

Sample T 5% (°C) T 50% (°C) T max (°C) Residual mass at 700 °C (%)
EP 144.45 345.12 344.2 7.79
EP-2 140.78 344.22 343.72 10.86
EP-5 135.33 347.83 343 13.87
EP-8 132.64 350.81 343.04 20.62
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As shown in Figure a, all samples exhibit a similar single major weight-loss stage, indicating that the incorporation of Cu2O@(CH/PT) does not introduce an additional major degradation stage. With increasing loading of Cu2O@(CH/PT), the residual mass at high temperatures gradually increases. Specifically, the residual mass at 700 °C increases from 7.79% for neat EP to 20.62% for EP-8 (Table ), indicating that the incorporation of Cu2O@(CH/PT) markedly enhances the formation and/or retention of condensed-phase residues during thermal degradation. This enhancement may be associated with the combined contributions of the PT, CH, and Cu2O components and may favor the formation of more stable condensed-phase residues during thermal degradation. ,,

From the DTG curves in Figure b, all samples exhibit a dominant degradation peak, and no additional degradation peak is observed after the incorporation of Cu2O@(CH/PT). The maximum degradation temperature (T max) remains nearly unchanged at approximately 343–344 °C (Table ), indicating that Cu2O@(CH/PT) does not significantly alter the primary degradation behavior of the epoxy matrix. Only slight variations in peak intensity and width are observed with increasing loading, suggesting that the additive mainly influences condensed-phase residue formation rather than substantially changing the overall degradation pathway.

The thermal degradation parameters further supported the above analysis. As summarized in Table , the introduction of Cu2O@(CH/PT) results in a slight decrease in T 5%, whereas T 50% shows an overall increasing trend with increasing additive loading. The decrease in T 5% may be associated with the early decomposition of the phosphorus-containing PT component within the CH/PT shell, which may generate phosphorus-containing species and promote early condensed-phase reactions before the major degradation of the EP matrix. , Meanwhile, the increase in T 50% at higher additive loadings, together with the progressively increased residual mass, suggests that the accumulated condensed-phase residues may contribute to improved resistance to further thermal degradation. Therefore, Cu2O@(CH/PT) mainly affects the later stages of thermal degradation by promoting condensed-phase residue formation and retention, rather than substantially altering the primary degradation behavior of the epoxy matrix.

3.5. Limiting Oxygen Index

The flame retardancy of neat EP and its blends containing different amounts of Cu2O@(CH/PT) was evaluated by limiting oxygen index (LOI) tests according to ASTM D2863, and the results are summarized in Table . Neat EP exhibits an LOI value of 23.3%, indicating its inherent flammability. With the incorporation of Cu2O@(CH/PT) at loadings of 2, 5, and 8 wt %, the LOI values increase to 25.2%, 27.6%, and 31.0%, respectively, demonstrating a clear loading-dependent improvement in flame retardancy. Notably, the LOI value of EP-8 reaches 31.0%, demonstrating the effectiveness of Cu2O@(CH/PT) in improving the flame retardancy of epoxy blends. According to commonly reported LOI evaluation criteria, LOI values above approximately 30% are generally associated with reduced flammability and improved resistance to sustained combustion. , The increasing LOI values are consistent with the enhanced condensed-phase residue formation observed in the TG analysis, where the residual mass at 700 °C increases from 7.79% for neat EP to 20.62% for EP-8 (Table ). These results suggest that the improved flame retardancy of EP blends may be associated with the enhanced formation and retention of condensed-phase residues promoted by the Cu2O@(CH/PT) system.

2. Limiting Oxygen Index (LOI) Values of Neat EP and EP Blends.

Sample LOI (%)
EP 23.3
EP-2 25.2
EP-5 27.6
EP-8 31.0
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3.6. Microscale Combustion Calorimetry Analysis

To quantitatively evaluate the effect of Cu2O@(CH/PT) on the combustion behavior of epoxy resin, microscale combustion calorimetry (MCC) was performed on neat EP and its blends containing different amounts of Cu2O@(CH/PT). The heat release rate (HRR) and total heat release (THR) curves are presented in Figure , and the corresponding combustion parameters, including peak heat release rate (PHRR), peak heat release temperature (T p), total heat release (THR), and heat release capacity (HRC), are summarized in Table .

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(a) HRR and (b) THR curves of neat EP and its blends containing different loadings of Cu2O@(CH/PT).

3. MCC Parameters of Neat EP and EP Blends Containing Different Amounts of Cu2O@(CH/PT).

Sample PHRR (W/g) T p (°C) THR (kJ/g) HRC (J/(g·°C))
EP 280.971 367.94 28.519 297.377
EP-2 253.328 387.71 27.319 270.584
EP-5 237.649 388.88 26.825 250.404
EP-8 154.168 375.66 18.996 166.605
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As shown in Figure a, neat EP exhibits a sharp and intense heat release peak with a PHRR value of 280.97 W/g and a T p of 367.9 °C. With increasing Cu2O@(CH/PT) content from 2 to 8 wt %, the PHRR values gradually decrease to 253.33, 237.65, and 154.17 W/g, corresponding to reductions of 9.8%, 15.4%, and 45.1%, respectively, compared with neat EP. These reductions indicate that the incorporation of Cu2O@(CH/PT) effectively reduces the heat release intensity of the epoxy blends during combustion.

As presented in Figure b, the THR values decrease from 28.52 kJ/g for neat EP to 27.32, 26.83, and 19.00 kJ/g for EP-2, EP-5, and EP-8, respectively, corresponding to reductions of 4.2%, 5.9%, and 33.4% compared with neat EP. The decreasing trend in THR is consistent with the increased residual mass at 700 °C observed in TG analysis (Table ), suggesting that Cu2O@(CH/PT) may promote the formation of more stable condensed-phase residues during combustion. Among all samples, EP-8 exhibits the lowest THR value, which is consistent with its lowest PHRR value, indicating that a higher content of Cu2O@(CH/PT) results in more pronounced suppression of heat release.

In addition, the HRC value decreases from 297.4 J/(g·°C) for neat EP to 166.6 J/(g·°C) for EP-8, further supporting the effectiveness of Cu2O@(CH/PT) in reducing the combustion intensity of the epoxy blends.

Notably, the peak heat-release temperature of all epoxy blends filled with Cu2O@(CH/PT) is higher than that of neat EP (367.94 °C), reaching 387.71 °C, 388.88 °C, and 375.66 °C for EP-2, EP-5, and EP-8, respectively. The delayed peak heat-release behavior reveals that the incorporation of Cu2O@(CH/PT) delays the occurrence of the main heat-release event in epoxy blends. This phenomenon may be attributed to the cooperative contributions of the CH/PT shell, in which the phosphorus-containing PT component promotes condensed-phase cross-linking and charring reactions, while the CH/PT polysaccharide backbones provide additional carbon sources for residue formation. This interpretation is consistent with the lower T 5% values observed in TG measurements, suggesting that the CH/PT shell participates in the initial thermal degradation process and promotes early condensed-phase reactions. The resulting protective char layer may hinder heat transfer and restrict the release of combustible degradation products, thereby shifting the maximum heat-release event to a higher temperature. Overall, the MCC results demonstrate that Cu2O@(CH/PT) effectively suppresses heat release and delays the main heat-release event of epoxy blends, supporting its improved flame-retardant performance. These effects may be closely associated with the condensed-phase protection provided by the Cu2O@(CH/PT) system.

3.7. Visual Flame Combustion Observation

To visually compare the combustion behaviors of EP and its blends containing Cu2O@(CH/PT), sample strips of EP, EP-2, EP-5, and EP-8 (80 mm × 6.5 mm × 3 mm) were horizontally fixed on a heat-resistant holder. The central region of each strip was directly exposed to an alcohol lamp flame for 10 s. After removal of the flame source, the subsequent combustion behavior and final char residues were visually recorded. Since EP-8 exhibited the highest LOI value and the lowest PHRR, THR, and HRC values among all blends, it was selected as a representative sample for visual combustion comparison. Figure shows the combustion behaviors of neat EP and EP-8 immediately after flame removal and 20 s after flame removal. The EP strip continued to burn after removal of the ignition source (Figure a,b), with a visible flame remaining after 20 s. In contrast, the flame intensity of the EP-8 strip was markedly reduced after removal of the ignition source and nearly extinguished within 20 s (Figure c,d). These observations indicate that the incorporation of Cu2O@(CH/PT) significantly improves the flame-retardant behavior of epoxy blends by reducing flame persistence after removal of the external flame source.

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Digital photographs of EP and EP-8 during horizontal flame combustion: (a) EP immediately after flame removal, (b) EP at 20 s after removal, (c) EP-8 immediately after removal, and (d) EP-8 at 20 s after removal.

Figure presents the char residues of EP, EP-2, EP-5, and EP-8 after flame extinction. For neat EP, only a small amount of loose and discontinuous char residue remains at the ignited end, while the remaining surface exhibits limited carbonaceous deposits. In contrast, EP-2, EP-5, and EP-8 exhibit progressively increased char residues on the combustion-exposed surfaces. With increasing Cu2O@(CH/PT) loading, the continuity and integrity of the residual char are gradually enhanced. Among all samples, EP-8 exhibits the most continuous and intact char residue, covering most of the combustion-exposed region. This observation is consistent with the increased residual mass at 700 °C observed in TG analysis (Table ) and the reduced heat release behavior observed in MCC measurements (Table ), suggesting that the improved flame retardancy of the epoxy blends may be associated with the enhanced formation and retention of condensed-phase residues and the development of a more continuous protective char layer.

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Digital photographs of residual char from EP and EP blends after flame extinction.

3.8. Proposed Flame-Retardant Processes

To gain deeper insight into the flame-retardant processes of Cu2O@(CH/PT) in epoxy resin, additional analyses were performed to clarify the roles of the different components and the involvement of condensed- and gas-phase processes.

To clarify the influence of the LbL-assembled structure on the thermal degradation and heat-release behaviors, neat EP, EP-8-Cu2O, the physically blended EP-8-(Cu2O/CH/PT), and EP-8-Cu2O@(CH/PT) were selected as comparative systems and evaluated by TG and MCC. As shown in Figure a, the residual mass at 700 °C increased from 7.79% for neat EP to 16.09% for EP-8-Cu2O and further to 18.49% for EP-8-(Cu2O/CH/PT), while EP-8-Cu2O@(CH/PT) exhibited the highest residual mass of 20.62%. The increase observed for EP-8-Cu2O indicates that Cu2O contributes to the formation and/or retention of condensed-phase residues, whereas the further increase for EP-8-(Cu2O/CH/PT) suggests additional contributions from CH and PT. More importantly, the higher residue of EP-8-Cu2O@(CH/PT) compared with the corresponding physical blend indicates that the LbL-assembled structure is more favorable for the formation and stabilization of condensed-phase residues than simple physical mixing.

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Comparative thermal and combustion behaviors of EP, EP-8-Cu2O, EP-8-(Cu2O/CH/PT), and EP-8-Cu2O@(CH/PT): (a) TG curves and (b) MCC heat release rate curves.

A consistent tendency was observed in the MCC results (Figure b). The PHRR of EP-8-(Cu2O/CH/PT) was 219.970 W/g, whereas that of EP-8-Cu2O@(CH/PT) decreased markedly to 154.168 W/g. Since the two systems contain the same types of flame-retardant components with comparable nominal compositions, the lower heat release of the LbL-assembled system suggests that its improved performance cannot be attributed solely to the coexistence of Cu2O, CH, and PT. Instead, the closer interfacial contact and more organized component arrangement associated with the LbL-assembled architecture may facilitate more effective cooperative contributions of these components to condensed-phase residue formation and heat release suppression.

Given the enhanced char formation observed above, Raman spectroscopy was subsequently employed to evaluate the structural characteristics of condensed-phase residues. The residual chars of neat EP and EP-8 collected after the horizontal flame combustion test were analyzed for comparison. As shown in Figure , both samples exhibit the characteristic D and G bands of carbonaceous materials. The I D/I G value decreased from 3.52 for neat EP to 2.98 for EP-8, indicating a relatively higher degree of structural ordering in the carbonaceous residue after the incorporation of Cu2O@(CH/PT). This change suggests that Cu2O@(CH/PT) not only promotes char formation but also favors the development of a more-ordered carbonaceous structure. Together with the higher residual mass observed in TG, the Raman results provide additional condensed-phase evidence for the enhanced formation and stabilization of protective residues in EP-8.

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Raman spectra and D/G peak fitting of the residual chars: (a) neat EP and (b) EP-8.

To examine the possible gas-phase contribution during thermal degradation, TG-FTIR analysis was performed for neat EP and EP-8 under a nitrogen atmosphere to characterize the evolved volatile products. As shown in Figure , the FTIR spectra were compared at comparable mass-loss levels of 10%, 30%, and 50%. The spectra of the evolved volatile products from both samples exhibit generally similar characteristic absorption regions, mainly associated with O–H/H2O-containing species, aliphatic C–H-containing products, CO2, aromatic species, C–O/C–O–C-containing species, and aromatic C–H vibrations. These similarities suggest that the incorporation of Cu2O@(CH/PT) does not markedly alter the major types of volatile products generated during the inert pyrolysis of the epoxy matrix.

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TG-FTIR spectra of gaseous products evolved from (a) neat EP and (b) EP-8 under a nitrogen atmosphere at mass-loss levels of 10%, 30%, and 50%.

For EP-8, an additional weak absorption feature potentially attributable to N-containing volatile species can be observed, particularly at the higher-mass-loss stage. However, no pronounced additional absorption features attributable to nitrogen-containing gaseous products are evident in the TG-FTIR spectra. These observations suggest that the evolution of N-containing species may provide a limited gas-phase contribution during thermal degradation, although this contribution appears to be supplementary rather than being predominant. Combined with the enhanced char formation and the increased structural ordering of the residual char discussed above, the TG-FTIR results further support the importance of condensed-phase processes in the flame-retardant behavior of Cu2O@(CH/PT).

On the basis of the above results, the possible flame-retardant processes of Cu2O@(CH/PT) in epoxy resin can be proposed as follows. During thermal degradation and combustion, the phosphorus-containing groups in PT may promote condensed-phase cross-linking and char formation, while the polysaccharide structures of PT and CH provide additional carbonaceous sources for the development of the protective residue. Meanwhile, the Cu2O core may facilitate the development and stabilization of carbonaceous structures through catalytic effects, thereby contributing to a more abundant and structurally ordered char layer. This protective residue may act as a physical barrier to reduce the heat feedback and restrict the transport of combustible degradation products. In addition, the weak absorption feature attributed to N-containing volatile species in the TG-FTIR spectra may be associated with the CH component and suggests a possible supplementary gas-phase contribution during thermal decomposition. Overall, the available evidence suggests that the flame-retardant action of Cu2O@(CH/PT) is mainly associated with condensed-phase char formation and stabilization, accompanied by a relatively limited gas-phase contribution. The proposed flame-retardant processes are schematically illustrated in Figure .

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Schematic illustration of the proposed flame-retardant processes of Cu2O@(CH/PT) in EP blends.

3.9. Performance Comparison

To further evaluate the performance of the present Cu2O@(CH/PT) system in the context of recent epoxy flame-retardant research, its flame-retardant performance was compared with representative systems reported in recent literature, as summarized in Table . The comparison includes additive loading, LOI, UL-94 rating, calorimetry method, and the reductions in PHRR and THR. As shown in Table , the present Cu2O@(CH/PT) system achieves an LOI of 31.0% at a moderate loading of 8 wt %, together with reductions of 45.1% in PHRR and 33.4% in THR, as determined by MCC. Although some reported systems exhibit higher LOI values or larger reductions in individual heat-release parameters, the Cu2O@(CH/PT) system exhibits a relatively balanced performance in terms of additive loading, flammability improvement, and heat-release suppression. In addition, the integration of renewable biomaterial-derived chitosan and phosphorylated cellulose with a Cu2O core through a core–shell architecture provides a distinctive structural design. Nevertheless, compared with several recently reported systems exhibiting high flame-retardant efficiency at lower additive levels, the present formulation still has room for further optimization to reduce the required additive loading and improve flame-retardant efficiency. Moreover, while the current evaluation mainly focuses on thermal degradation, flammability, heat-release behavior, and condensed-phase residue formation, further evaluation of mechanical properties, smoke suppression, and toxicity-related performance would be valuable for assessing the comprehensive performance and practical applicability of the Cu2O@(CH/PT)/EP system.

4. Comparison of Flame-Retardant Performance of the Present System with Recently Reported Epoxy Flame-Retardant Systems.

Flame-retardant system Loading (wt % or phr) LOI (%) UL-94 rating Calorimetry method PHRR reduction (%) THR reduction (%) refs
Cu2O@(CH/PT) 8.0 wt % 31.0 MCC 45.1 33.4 This work
CeO2@PAPP 10.0 wt % 34.2 V-0 Cone 56.3 38.2
DAP@SiO2 12.0 wt % 33.2 V-0 Cone 55.7 24.3
PADP@BM 10.0 wt % 37.0 V-0 Cone 39.9 31.5
P–N@MXene 6.0 wt % 29.8 V-0 Cone 58.1 45.5
N–Si–CQDs/Sodi 6.25 wt % 29.0 V-0 Cone 33.0 10.0
DPM-H + APP 8.0 wt % 29.3 V-0 MCC 53.3 32.1
PB-E 3.9 wt % V-0 MCC 40.0 8.6
VHE 20.0 wt % 32.1 V-0 Cone 33.8 27.5
SCP 2.0 wt % 29.2 V-1 Cone 26.1
HCP-2 5.0 wt % MCC 49.9 20.6
HBFR 7.5 wt % 31.5 V-0 Cone 31.3 8.9
AEPO 10 phr 35.5 V-0 Cone 25.2 29.8
PSFR + HNTs 18.8 wt % MCC 43.9 19.5
NP-CQD + APP 8.0 wt % 33.2 V-0 MCC 48.1 22.8
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Beyond the performance evaluation itself, practical implementation of the present system also requires consideration of the fabrication scalability and material stability. The current aqueous LbL process involves repeated deposition and washing steps, while the preparation of phosphorylated cellulose adds processing complexity and cost, and the hydrophilic CH/PT shell may exhibit moisture sensitivity during storage and processing. These challenges may be mitigated through continuous or semicontinuous LbL assembly, reduction of unnecessary deposition cycles, optimization of the phosphorylation degree and reagent utilization during PT preparation, and controlled drying, storage, or mild postassembly surface treatment to improve moisture resistance. Further validation of these strategies under larger batch processing conditions will be necessary to assess their feasibility for practical epoxy formulations.

4. Conclusions

A biomaterial-based Cu2O@(CH/PT) flame retardant was prepared through a layer-by-layer (LbL) assembly strategy by alternately depositing chitosan (CH) and phosphorylated cellulose (PT) onto the Cu2O particles. SEM and FTIR analyses provided complementary morphological and spectroscopic evidence of the surface modification of Cu2O and the deposition of CH/PT-containing layers. The deposited CH/PT components provided phosphorus- and nitrogen-containing functionalities at the Cu2O surface, which may contribute to the enhanced flame-retardant performance of the epoxy resin.

The effects of Cu2O@(CH/PT) on the thermal degradation and combustion behaviors of epoxy blends were systematically investigated. TG analysis demonstrated that the incorporation of Cu2O@(CH/PT) markedly increased condensed-phase residue formation, with the residual mass at 700 °C increasing from 7.79% for neat EP to 20.62% for EP-8. Meanwhile, the LOI value increased from 23.3% to 31.0% with the addition of 8 wt % Cu2O@(CH/PT). MCC results further revealed that EP-8 exhibited reductions of 45.1%, 33.4%, and 44.0% in PHRR, THR, and HRC, respectively, compared with neat EP. The improved flame-retardant performance may be associated with the cooperative contributions of the Cu2O core and CH/PT shell, in which phosphorus-containing PT promotes condensed-phase charring reactions, while CH contributes to the development of protective char structures, and Cu2O contributes to the stabilization of carbonaceous residues through catalytic effects during thermal degradation. The resulting protective condensed-phase residues may hinder heat transfer and the release of combustible degradation products, thereby reducing the combustion intensity of epoxy blends.

Overall, these findings demonstrate a feasible aqueous strategy for constructing biomaterial-based core–shell flame retardants through aqueous LbL assembly. The integration of Cu2O catalytic centers with renewable biomaterial-derived chitosan and phosphorylated cellulose demonstrates potential for developing epoxy blends with improved fire safety.

Acknowledgments

The works described in this paper are substantially supported by the Suqian Sci&Tech Program (Grant No. K202434) and the National Natural Science Foundation of China (Grant No. 52376133).

The authors declare no competing financial interest.

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