Nano-violet phosphorus/nano-crystalline cellulose composite films for fire-retardant coatings

Abstract

As fire safety issues receive increasing attention, the development of efficient flame-retardant materials has become particularly important. This study explores the preparation methods of nano-violet phosphorus (NVP) and nanocellulose (NCC) composite films and their superior performance in flame retardancy applications. NVP was successfully prepared using a liquid-phase exfoliation method and was then non-covalently self-assembled with NCC to form a novel composite film with good uniformity. The microstructure, mechanical properties, and flame-retardant performance of the composite film were comprehensively characterized. The results indicated that the NVP/NCC composite film with only 1 wt% NVP exhibited significantly enhanced self-extinguishing capability (<2 s) and a peak heat release rate (23.31 kW m⁻²) and total heat release (1.78 MJ m⁻²), indicating its effectiveness in delaying the combustion process under flame exposure. The introduction of NVP made the decomposition rate of the composite film slower and the decomposition behavior more stable in an aerobic environment. In addition, the presence of NVP delayed the dehydration process of NCC, and promoted the carbonization reaction, thereby reducing the release of H₂O, thus forming a protective carbon layer and removing active free radicals, significantly improving the thermal stability and safety of the composite. This work provides important theoretical foundations and practical guidance for the development of new flame-retardant materials.

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1. Introduction

Fires pose a serious threat to human society, and effective fireproof coating materials are critical for reducing fire-related losses. In practical applications, certain complex-shaped components impose higher demands on thermal insulation and flame-retardant materials. Because of their intricate shapes, these components make applications more challenging, resulting in weak regions for thermal insulation and flame resistance.^1,2 In recent years, the development of flame-retardant coating materials has benefited from their simplicity of application and their ability to enhance the fire resistance of substrate materials without being limited by space or shape.^3–5 These materials have been widely used in thermal management across industries such as construction, electronic devices, and transportation equipment.

Flame retardants are the core components of flame-retardant coating materials, directly impacting the physical, chemical, and fire resistance properties of the coatings. However, traditional flame retardants have significant drawbacks even as they enhance fire resistance. For example, halogenated flame retardants could emit harmful chemicals during burning, causing pollution in the environment.^6,7 While traditional phosphorus-based flame retardants (such as phosphates) are generally environmentally friendly, they can degrade the mechanical properties of polymers and exhibit poor thermal stability at high temperatures.^8,9 Inorganic flame retardants often require high addition levels to be effective, which increases the density of the material and lowers its mechanical performance.^10,11 Although organosilicon flame retardants perform well in terms of thermal stability, their flame-retardant effectiveness is limited when exposed directly to flames, and their costs are relatively high.^12,13 These limitations have driven the research and application of novel flame-retardant materials, particularly nanoflame retardants. Compared with traditional flame retardants, nanoflame retardants offer significant improvements in flame-retardant performance even with little doses, thanks to their ultra-small particle sizes and large surface areas.^14,15 They also preserve the mechanical qualities and functionality of thermal insulation materials without jeopardizing the interior structure. Furthermore, these materials pose minimal harm to human health and the environment during thermal decomposition. In recent years, nanolayered materials such as graphene, hexagonal boron nitride (h-BN), and molybdenum disulfide (MoS₂) have attracted widespread attention in the fields of flame retardancy and smoke suppression due to their layered structures and superior thermal stability.^16–18 Therefore, the continuous development of novel nanolayered flame-retardant materials to meet the fire protection needs of complex-shaped components has become a key focus of current research.

Among various layered materials, phosphorus-containing nanomaterials have demonstrated great potential due to their unique physical and chemical properties, particularly in the development of novel flame retardants. For example, layered phosphates (such as aluminum phosphate and calcium phosphate), layered double hydroxides containing phosphorus, and black phosphorus are notable examples.^19,20 Black phosphorus, an allotrope of phosphorus, can catalyze the formation of char and capture combustion free radicals.^21,22 Its distinctive layered structure serves as a barrier to heat and oxygen during combustion. Studies have shown that the uniform distribution of black phosphorus within a polyurethane matrix could significantly improve the material's flame-retardant properties. However, black phosphorus is highly prone to oxidation and degradation when exposed to water and oxygen, leading to the deterioration of its physical and chemical properties, which limits its application in flame-retardant systems. To address this issue, Cai et al. utilized electrostatic interactions to drive self-assembly and combined in situ radical polymerization to introduce ionic liquid-functionalized black phosphorus nanosheets into a polyurethane matrix.²³ This significantly reduced the peak heat release rate (pHRR) and total heat release (THR) of the material, while also effectively reducing the emissions of CO₂ and toxic CO. These findings indicate that black phosphorus, as a phosphorus-containing layered material, has significant potential for flame retardant applications.

Despite the promising flame-retardant properties of black phosphorus, its susceptibility to oxidation and degradation limits its broad practical application.^24,25 Although many researchers continue to focus on enhancing the oxidation resistance of black phosphorus to improve its potential applications in flame retardancy, this approach often introduces significant uncertainties during the modification stage, thus prompting researchers to explore other phosphorus-containing materials with higher thermal stability. In this context, violet phosphorus, a novel layered material, has drawn great attention. Our research team has reported, for the first time, the synthesis of a violet phosphorus single crystal, a new type of layered semiconductor.^26–29 Compared with black phosphorus, violet phosphorus not only retains the advantages of layered materials for flame retardant applications but also offers higher thermal stability, with a thermal decomposition temperature of 512 °C, significantly higher than 460 °C for black phosphorus.³⁰ However, no studies have yet explored the application of violet phosphorus in the flame retardant field.

Based on our previous work using the chemical vapor transport (CVT) method to prepare violet phosphorus crystals, this study takes a significant step forward by employing a liquid-phase exfoliation method to fabricate nano-violet phosphorus (NVP), enabling its integration into flame-retardant applications for the first time. This was followed by the non-covalent self-assembly of NVP with nanocrystalline cellulose (NCC), leveraging the unique synergy between NVP's inherent flame-retardant properties and NCC's mechanical strength to create innovative NVP/NCC composite films (Fig. 1). The resulting materials were systematically characterized for their microstructure, mechanical properties, and flame-retardant performance using transmission electron microscopy (TEM), scanning electron microscopy (SEM), nanoindentation, cone calorimetry, PyGC-MS, and limiting oxygen index (LOI) testing. Additionally, the composite film was coated onto a highly flammable sodium alginate–hyaluronic acid thermal insulation composite material to evaluate its practical application performance, demonstrating significant enhancement in fire resistance. The flame-retardant mechanism of NVP was elucidated through X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) analysis of the combustion products. To the best of our knowledge, this is the first report on exploring and establishing violet phosphorus as a functional material in the field of flame retardancy. This work not only introduces a novel application of NVP but also provides a convenient and scalable method for developing enhanced flame-retardant coatings, representing a significant step forward in improving the flame-retardant performance of thermal insulation composites.

Fig. 1 Process of synthesizing the NVP/NCC composite film for application of thermal insulation composites with a fire-retardant coating.

2. Methods

2.1 Materials

NCC was purchased from Zhongshan Naxiansi New Materials Co., Ltd, and violet phosphorus was synthesized in our laboratory. Sodium hyaluronate (1.0–1.8 MDa, SH) and D-glucono-δ-lactone were supplied by Aladdin, while sodium alginate (1% solution viscosity: 1000–1200 mPa s, SA) and cetyltrimethylammonium bromide (CTAB) were obtained from Macklin. The fiber/epoxy (EG) was sourced from crushed and sieved waste materials (50–100 mesh) obtained from scrap wind turbine blades.³¹

2.2 Preparation of NVP

Bulk violet phosphorus crystals were prepared using a CVT method previously reported by our group.²⁶ A measured amount of bulk violet phosphorus was ground thoroughly in a mortar and then transferred into a beaker, where it was dispersed in distilled water containing CTAB at a concentration of 1.8 mg mL⁻¹. After stirring the mixture thoroughly, the solution underwent ultrasonic treatment for 3 h using a sonicator with a power of 600 W. The resulting solution was performed through fractional centrifugation, and the supernatant was collected. The supernatant was further ultrasonically dispersed to obtain a uniform NVP solution.

2.3 Preparation of the NVP/NCC composite film

NCC aqueous solution (1.0 wt%) was prepared and thoroughly dispersed. The resulting NVP solution (2 mL) was then mixed with the NNC solution, and the mixture was subjected to ultrasonic treatment at 100 W and 20 °C to complete the self-assembly process. Finally, the mixed solution was poured into a glass petri dish with a diameter of 60 mm and allowed to dry naturally at room temperature, resulting in the NVP/NCC composite film. For the sake of comparative analysis, the NVP/NCC composite film obtained from this ratio is labeled as NVP_0.5/NCC. The composite films with NVP solution amounts doubled, tripled, and quadrupled relative to the aforementioned NVP solution are labeled as NVP₁/NCC, NVP_1.5/NCC, and NVP₂/NCC, respectively.

2.4 Preparation of SA-SH-EG@NVP

SA (1.4 g) and SH (0.44 g) were dissolved in distilled water (75 mL) to obtain an SA/SH mixed solution. Then, EG was dispersed in distilled water (30 mL), using an ultrasonic cell disrupter to break it down for 15 min. This dispersion was then added to the SA/SH mixed solution along with CaCO₃ (3.0 wt%), and the mixture was stirred at room temperature until uniform dispersion was achieved, resulting in the SA/SH/EG mixture. To achieve gelation, D-glucono-δ-lactone was added to regulate the crosslinking degree, and the resulting mixture was allowed to stand at 35 °C for 50 min to crosslink. Finally, the SA–SH–EG thermal insulation composite material was obtained by freeze-drying for 48 h. The mixed solution from the self-assembly of the NVP solution and NCC solution was coated onto the surface of the SA–SH–EG sample, which measured 30 mm × 100 mm × 100 mm, and then freeze-dried for 48 h to obtain the thermal insulation and flame-retardant composite material SA–SH–EG@NVP.

2.5 Characterization

The size and microstructure of NVP were characterized using transmission electron microscopy (TEM, JEOL JEM-F200). Scanning electron microscopy (SEM, ZEISS Sigma 300) was employed at a voltage of 5.00 kV to observe the morphology and structure of the violet phosphorus and films before and after combustion. Chemical surface characterization was performed by X-ray photoelectron spectrometry (XPS, Edinburgh FLS1000). The mechanical properties of the NVP/NCC composite film were characterized using a nanoindentation instrument at room temperature (28.3 °C) with a compression rate of 1 mm min⁻¹, resulting in measurements of the indentation hardness (H_IT, GPa) and equivalent elastic modulus (E, GPa). The flame resistance of the NVP/NCC composite film was evaluated through ignition experiments. Specifically, a flame was used to ignite the lower end of a film sample strip measuring about 3.2 cm in length and 1.7 cm in width at a 45° angle for 1 s, after which the flame was removed, and the combustion process of the film was recorded. A micro calorimeter (DEATAK MCC-3) was used to investigate the combustion performance of the film. The limiting oxygen index (LOI) was measured using an oxygen index tester (FTT0077) to characterize the flame-retardant performance. Additionally, the combustion performance of the bulk was tested using a cone calorimeter (FTT0007) under a radiant intensity of 35 kW m⁻². An X-ray diffractometer (XRD, D8 Advance) was used to characterize the components of the char layer. Pyrolysis-gas chromatography-mass spectrum (PyGC-MS, SDT 650 + Discovery MS) was used to characterize the gas components and contents of film pyrolysis in an air atmosphere.

3. Results and discussion

3.1 Microstructure and stability of NVP

Similar to the results reported in our previous studies, the purple phosphorus prepared in this work had a layered structure and was therefore a typical layered structure during the liquid-phase exfoliation process (Fig. 2a). After sufficient exfoliation time and subsequent gradient centrifugation, nanoscale NVP could be obtained. Fig. 2b shows the TEM image of NVP, along with its corresponding particle size distribution. The images indicate that the resulting NVP consisted of nanoscale particles with an average diameter of 23.1 nm (Fig. 2c). Selected area electron diffraction (SAED) was performed to characterize its structure and crystallinity. The diffraction pattern was obtained along the [001] crystallographic zone axis, showing a symmetrical distribution of diffraction points (Fig. 2d). The four closest diffraction points to the center form a rectangular shape, corresponding to the monoclinic crystal system of violet phosphorus (in the a–b plane). All diffraction points exhibit periodic arrangement, indicating that the nanoscale violet phosphorus obtained from the complex liquid-phase exfoliation environment retains high crystallinity. Moreover, the angle between adjacent diffraction points near the center is 90°, with an interplanar spacing of approximately 3.2 Å, corresponding to the (220) and (2–20) crystal planes of violet phosphorus.

Fig. 2 Typical layered structure of violet phosphorus during liquid-phase exfoliation (a); TEM image of NVP (b); average diameter of NVP (c); diffraction pattern of NVP (d); CTAB solution with no Tyndall effect (e); NVP solution with a typical Tyndall effect and good stability in the presence of CTAB (f).

The use of CTAB as a stabilizer to disperse NVP in the aqueous phase is feasible, based on previous studies that utilized CTAB to stabilize black phosphorus in water.³² CTAB could completely dissolve uniformly in the aqueous solution, and at the concentrations used in this experiment, it could not form noticeable micelles, thus avoiding the Tyndall effect (Fig. 2e). In contrast, the NVP aqueous solution exhibits a significant Tyndall effect, and after 1 week of standing, no sediment was observed at the bottom (Fig. 2f). This indicates that NVP can be stably dispersed in the aqueous solution through CTAB, which is one of the key factors for achieving good compatibility between NVP and NCC. In our previous study, XPS has been used to confirm the stability of violet phosphorus following liquid-phase exfoliation under an oxygen environment, which also established its advantages as a nano-flame retardant.²⁶

3.2 Flame retardant properties of the composite film

As a flame-retardant coating, the key performance of the NVP/NCC composite film is its ability to resist flames in open flame situations. To intuitively assess this performance, this study first verified the flame-retardant capabilities of the NVP/NCC composite film using ignition experiments and compared them with those of pure NCC film.

The experimental results indicate that the pure NCC film, due to the inherent flammability of NCC, burned rapidly upon exposure to flames, completely incinerating within 5 s with minimal residual ash (Fig. 3a). This is primarily because the hydroxyl groups in the NCC molecular structure are prone to pyrolysis at high temperatures, generating combustible gases such as water vapor, carbon dioxide, and volatile organic compounds. These gases ignite quickly upon contact with oxygen, leading to rapid heat release and exacerbating the combustion process.

Fig. 3 Ignition and combustion test of pure NCC film (a) and NVP/NCC composite films with increased NVP content (b)–(e).

In contrast, all composite films containing NVP showed much lower combustion intensity and rate, and the burning time was noticeably shortened after the flame was removed (Fig. 3b–e). Apart from the areas directly exposed to the flame, the other parts of these composite films remained intact, demonstrating excellent flame-retardant performance. Notably, samples with higher NVP content, such as NVP₁/NCC, NVP_1.5/NCC, and NVP₂/NCC, showed that the flames at the bottom of the films extinguished rapidly after the removal of the ignition source, with no additional flame propagation into other areas.

The flame-retardant mechanism of NVP in the composite films is primarily attributed to its nano-layered structure, which forms a physical barrier during combustion, effectively isolating oxygen and heat while reducing the generation of combustible gases.³³ Additionally, the chemical properties of NVP contribute to the formation of a stable char layer, preventing flame spread. Furthermore, as the NVP content increases, the self-extinguishing speed of the composite films accelerates significantly, indicating that a higher amount of NVP correlates with enhanced flame-retardant effectiveness.

In summary, NVP, as a flame retardant, not only significantly improves the flame-retardant performance of NCC-based composite films but also demonstrates considerable potential in suppressing flame spread and lowing combustion intensity. Given the practical need for lower amounts of flame retardant additives, future research has focused on NVP₁/NCC to explore the structural, property, and performance features at low additive amounts.

3.3 Microstructure and morphology of the composite film

To further investigate the microstructure of the NVP₁/NCC composite film and its mechanism of action in flame retardancy, a detailed analysis of its surface morphology and cross-sectional structure was conducted. The surface structure of the NVP₁/NCC composite film was dense (Fig. 4b), and its cross-section exhibited a tightly packed layered structure along the horizontal direction (Fig. 4c and d). Typically, this stacking arrangement, due to its small oxygen-containing space, contributes to enhancing the flame-retardant performance of the composite film. In the interface analysis of the NVP₁/NCC composite film (Fig. 4e), the distribution of P element was very uniform, and was unaffected by the varying thickness of the cross-section, indicating that NCC and NVP formed a uniform structure through non-covalent self-assembly. Additionally, the N element derived from CTAB was also uniformly distributed, demonstrating that CTAB played a key role in promoting the uniform dispersion of NVP. Experiments show that in the absence of surfactants, NVP tends to aggregate rapidly in the aqueous phase, while the presence of CTAB could effectively prevent this aggregation and enhance the compatibility between NVP and NCC.

Fig. 4 Images of the NVP₁/NCC composite film: digital image (a), SEM image of the surface (b), SEM image of the cross section with different magnifications (c) and (d), SEM mapping (e); images of the NVP₁/NCC composite film after burning: digital image (f), SEM image of the surface (g), SEM images of the cross-section with different magnifications (h) and (i).

In combustion experiments, the NVP₁/NCC composite film largely retained its original film morphology after burning (Fig. 4f and g). The film surface formed typical small protrusions, which were presumed to result from the expansion of the char layer due to gas release during combustion. The cross-sectional SEM images after combustion showed significant expansion and carbonization, while the surface became rougher and exhibited visible cracks (Fig. 4h and i). However, the layered structure of the film remained intact, indicating that the NVP₁/NCC composite film could maintain a certain degree of structural integrity under flame exposure.

3.4 Mechanical properties of the composite film

As a fire-retardant coating, the mechanical properties of the NVP/NCC composite film are crucial for the long-term protection and stability of thermal insulation materials. Coating the NVP/NCC composite film onto the surface of the insulation material before curing can form a fireproof layer. Therefore, this study first examined the mechanical properties of the NVP/NCC composite film.

The five F_m–h curves of the NVP₁/NCC composite film exhibited less deviation compared to the five F_m–h curves of the pure NCC film, indicating that the incorporation of NVP played a significant role in enhancing the uniformity of the film (Fig. 5a and b). During the drying process, the rapid removal of water molecules between cellulose molecular chains in the pure NCC film led to the destruction of the existing cellulose–water hydrogen bonds, resulting in the formation of numerous cellulose–cellulose hydrogen bonds that caused uneven shrinkage of the film. In contrast, NVP provided a flat and orderly template for the cellulose molecular chains, helping them arrange more regularly in the plane. Additionally, the CTAB molecules on the surface of NVP acted as a plasticizer, forming good interfacial compatibility with the cellulose molecular chains, effectively reducing stress concentration at the interface. Coupled with NVP's good thermal stability, these reduced adverse factors such as thermal degradation and shrinkage, further helping the film maintain structural stability during the dehydration and drying process, thus making the NVP/NCC composite film more uniform.

Fig. 5 Representative nanoindentation load–displacement curves for the NVP₁/NCC composite film (a) and pure NCC film (b), average H_IT (c) and E (d) of the NVP₁/NCC composite film and pure NCC film.

The average indentation hardness (H_IT) and equivalent elastic modulus (E) of the NVP₁/NCC composite film were 0.47 GPa and 12.85 GPa, respectively, while those of the pure NCC film were 0.56 GPa and 10.11 GPa, respectively (Fig. 5c and d). The average HIT of the NVP₁/NCC composite film decreased by approximately 18.9%, while the average E increased by 27.1%, showing a significant change. Moreover, the maximum load that the NVP₁/NCC composite film could withstand was 10 mN, compared to 5 mN for the pure NCC film, indicating that the NVP₁/NCC composite film achieved a good balance between strength and ductility. The NVP₁/NCC composite film could not only bear a larger overall load but also exhibited a certain degree of flexibility and ductility under localized stress. This means that in practical applications, the NVP₁/NCC composite film is suitable for high-strength situations while also absorbing and dispersing energy through microscopic deformation mechanisms, thereby preventing sudden fracture or failure of the material.

The incorporation of NVP allowed for relative sliding between the layers of the film under external forces, which effectively dispersed stress and reduced the likelihood of brittle fracture. Additionally, the interfacial interactions between NVP and NCC through hydrogen bonds and van der Waals forces further enhanced their bonding strength. This strong bond not only helped maintain the integrity of the film but also improved its capacity for deformation under stress. Consequently, the NVP₁/NCC composite film showed significantly enhanced resistance to both instantaneous and continuous external forces, indicating higher adaptability and reliability in practical applications.

3.5 Flame retardant mechanism

In order to evaluate the effect of NVP on the thermal stability of films, the thermal properties of NVP₁/NCC under oxygen conditions were investigated. As shown in Fig. S1 and S2 (ESI†), although the initial decomposition temperature of the composite film was slightly lower than that of the pure NCC film (possibly due to the decomposition behavior of CTAB), the introduction of NVP resulted in a more gradual decomposition rate and more stable decomposition behavior, demonstrating excellent high-temperature resistance. The residual mass of the NVP₁/NCC composite film at high temperatures significantly increased, rising from about 11% for the pure NCC film to 18–20%. This indicated that NVP promoted the carbonization reaction of NCC under high-temperature conditions, forming a more stable and compact carbon layer. This carbon layer effectively blocked the transfer of heat and oxygen, significantly suppressing further decomposition and combustion of the material.

The pyrolysis process and gas products of the pure NCC film and NVP₁/NCC composite film in air were characterized using PyGC-MS. In the PyGC-MS spectrum of the pure NCC film (Fig. S1, ESI†), the H₂O signal (18 AMU) was released more intensively during decomposition, with a higher peak intensity, indicating that the decomposition of NCC was accompanied by significant dehydration reactions. This was likely related to the dehydration and decomposition of hydroxyl groups (–OH) in the NCC molecular structure at high temperatures. In the PyGC-MS spectrum of the NVP₁/NCC composite film (Fig. S2, ESI†), the H₂O signal was more dispersed compared to that of the pure NCC film, and its intensity was reduced. This suggests that the introduction of NVP altered the decomposition pathway of NCC to some extent, potentially by promoting carbonization and reducing the release of water vapor. During the pyrolysis process of NCC, a series of dehydration reactions typically occurred, generating water vapor (H₂O) and char. The dehydration reaction consumed part of the heat and played a certain role in flame retardancy. In the pure NCC film, due to the absence of the synergistic effect of NVP, dehydration reactions predominantly occurred during the low-temperature decomposition stage. The introduction of NVP may promote the rapid carbonization of NCC, reducing dehydration reactions at the low-temperature stage, which made the release of H₂O more dispersed. Furthermore, the phosphates produced by the high-temperature decomposition of NVP may react with the hydroxyl groups in cellulose, forming stable carbonized products instead of further decomposing to release H₂O, which also reduces the intensity of H₂O release. The CO₂ release peak (44 AMU) in the pure NCC film was more concentrated and had a higher intensity, indicating that it rapidly decomposed to generate a large amount of CO₂ during pyrolysis. In the NVP₁/NCC composite film, the CO₂ release peak was relatively dispersed, and its peak intensity was lower, indicating that the pyrolysis process was delayed. This could be due to the carbonization-promoting effect of NVP, which increased the carbonization rate and reduced the release of volatile gases. In terms of the overall gas composition, the NVP₁/NCC composite film released less CO₂ during pyrolysis, indicating that more carbon remained in the solid phase, forming a stable carbon layer. This carbon layer effectively isolated heat and oxygen, significantly enhancing the flame retardancy.

To more accurately assess the fire resistance of the NVP₁/NCC composite film, this study conducted a detailed analysis using the Micro calorimeter. Fig. 6a and b show the HRR and THR curves for the pure NCC film and NVP₁/NCC composite film, respectively. The pure NCC film exhibited very high flammability during combustion, with a peak HRR (pHRR) of 59.4 W g⁻¹ and a THR of 5.4 kJ g⁻¹, indicating that the NCC film burned rapidly and released a substantial amount of heat. In contrast, the pHRR value of the NVP₁/NCC composite film decreased to 52.2 W g⁻¹, and the THR value decreased to 5.1 kJ g⁻¹, demonstrating that the incorporation of NVP effectively enhanced the flame-retardant performance of the composite material.

Fig. 6 HRR (a) and THR (b) curves of the pure NCC film and NVP₁/NCC composite film with temperature change; wide scan of the XPS survey spectrum (c) and P 2p high magnification XPS spectrum (d) of the NVP₁/NCC composite film after burning.

The key role of NVP in the composite material can be understood from several perspectives. Firstly, NVP possessed excellent thermal stability and may decompose at high temperatures to form phosphoric acid or polyphosphate substances, thereby creating a protective phosphate glass layer. This glass layer not only effectively blocked the ingress of oxygen but also slowed down the further combustion of the material. To confirm this mechanism, XPS was used to characterize the combustion products of the NVP₁/NCC composite film. As shown in Fig. 6c, the combustion products of the NVP₁/NCC composite film mainly contained C, O, N, and P elements. The N element originated from the CTAB used to stabilize NVP, while the P element came from the introduction of NVP. The chemical state of the P element in violet phosphorus is singular, with a binding energy of about 129.3 eV;²⁰ however, the significant increased binding energy of P in the combustion products indicated that P underwent oxidation, resulting in phosphorus-containing compounds, specifically phosphate substances (Fig. 6d). It is important to note that in the XPS spectrum of P 2p, no peaks corresponding to unoxidized NVP were observed. We boldly speculate that all NVP can fully exert its flame-retardant function. This is because, due to the size of the NVP, it is unlikely to maintain its original internal properties under high-temperature combustion conditions, as explained in a previous study reported by our collaborators.³⁴

Phosphate compounds play a crucial role in the flame-retardant process, as they can catalyze the carbonization of NCC, forming a dense char layer on the surface of the material, while further preventing the diffusion of heat and oxygen, thereby slowing down the combustion rate.^35,36 Additionally, the presence of the char layer significantly reduced the generation of combustible gases during the combustion reaction, further enhancing the flame-retardant performance of the material. These results indicate that the incorporation of NVP enhanced the thermal stability and carbonization ability of the composite film, significantly reducing heat release and flame spread during combustion, ultimately improving the overall flame-retardant performance of the material.

3.6 Application of the NVP₁/NCC composite film as a fire-retardant coating

Based on the excellent flame-retardant properties of the NCC/NVP composite film, this study further validated its flame-retardant effectiveness as a coating applied to the surface of the SA–SH–EG composite material. The SA–SH–EG composite material is a porous thermal insulation material prepared by embedding pulverized glass fiber/epoxy resin from scrap wind turbine blades into an interpenetrating network composed of SA and SH, followed by freeze-drying. Using the Hot Disk transient plane source method and referencing the international standard ISO22007, the thermal conductivity of this material was measured to be 0.041 W (m K)⁻¹, demonstrating outstanding thermal insulation performance compared to current insulation materials.³⁷ However, due to the natural biopolymeric nature of SA and SH, the composite material has an inherent drawback of flammability. While introducing flame retardants throughout the internal structure of the SA–SH–EG composite material could enhance its fire resistance, this approach is costly and economically inefficient. In contrast, applying a flame-retardant coating is a more cost-effective solution, as combustion typically initiates from the material's surface, and surface treatment can significantly improve fire resistance.

The flame-retardant performance of SA–SH–EG@NVP was characterized in detail through limiting oxygen index (LOI) tests and cone calorimetry. The results showed that the LOI value of the uncoated SA–SH–EG was 22.9%, while the LOI value of the coated SA–SH–EG@NVP increased to 23.7%. This indicates that the incorporation of NVP effectively enhanced the flame-retardant performance, and this improvement is closely related to the concentration of NVP in the NCC/NVP solution, as well as the thickness and depth of the coating. In this case, we used a very low concentration of NVP solution (<1.0 wt%) and applied only three simple brush coatings.

The curves for HRR, THR, CO₂ production (CO₂P), CO production (COP), smoke production rate (SPR), and total smoke release (TSP) of SA–SH–EG@NVP reflect its flame-retardant process. The HRR and THR curves indicate that during combustion, the HRR and THR values of SA–SH–EG@NVP rise sharply, with pHRR reaching only 23.31 kW m⁻² at 40 s, and THR being only 1.78 MJ m⁻², suggesting that the addition of NVP can effectively delay heat release (Fig. 7a and b).

Fig. 7 HRR (a), THR (b), CO₂P (c), COP (d), SPR (e), and TSP (f) of the SA–SH–EG and SA–SH–EG@NVP thermal insulation composite material.

Regarding gas emissions, the peak production rates of CO₂ and CO for SA–SH–EG@NVP are 0.02 g s⁻¹ and 0.005 g s⁻¹, respectively (Fig. 7c and d). Compared to the uncoated SA–SH–EG thermal insulation composite material, the introduction of NVP significantly suppressed CO₂ emissions by 66.66%, while the release of CO slightly increased. This phenomenon may be attributed to the formation of a dense char layer by NVP during combustion, which prevents oxygen penetration and reduces the generation of complete combustion gases (such as CO₂), while incomplete combustion leads to an increase in CO release.

The smoke production rate (SPR) of the SA–SH–EG thermal insulation composite material reached its peak at an earlier time and continued to fluctuate afterward without decreasing to 0, while the total smoke release (TSP) showed a continuous upward trend over a longer period (Fig. 7e and f). In contrast, the appearance of the maximal SPR of SA–SH–EG@NVP would delay and drop to 0 at around 40 s, while the TSP reached its maximum at about 30 s and no longer increased. These findings indicate that the presence of the NVP-based flame-retardant coating, even in small amounts, significantly enhanced the flame-retardant performance of the SA–SH–EG thermal insulation composite material, which is crucial for meeting the demands of special applications in practical environments. However, it is important to note that the increased release of CO suggested a need to balance flame-retardant performance with CO emissions in practical applications.

Although the LOI value of SA–SH–EG@NVP has increased, the improvement is not significant, and the material still falls within the combustible range. This is primarily due to the low amount of NVP added, as the flame-retardant efficiency of a retardant typically correlates positively with its concentration. If the NVP content is too low, it may not be sufficient to effectively absorb heat, suppress the generation of free radicals, and prevent the further spread of flame when SA–SH–EG is subjected to flame exposure. Additionally, the uneven distribution of NVP on the surface of SA–SH–EG@NVP may lead to certain areas lacking sufficient NVP to exert a flame-retardant effect, resulting in inconsistencies and limitations in flame-retardant performance. Flames may bypass regions with lower NVP concentrations and continue to spread. Therefore, future work needs to further optimize the coating process and material ratios to ensure the uniform distribution of NVP on the material's surface, thereby improving flame-retardant performance to meet practical application needs. It is worth noting that although the increase in the LOI value with a small amount of NVP is relatively modest, the combustion experiments demonstrate that the incorporation of NVP can significantly enhance the self-extinguishing capability of the material, which is particularly beneficial for coatings used in certain specialized environments.

Fig. 8 shows the char morphology of SA–SH–EG@NVP after cone calorimetry tests. It can be seen that there was no distinct black carbon layer for SA–SH–EG, while SA–SH–EG@NVP formed a gray char layer, which was attributed to the synergistic flame-retardant action of NVP in both the gas phase and condensed phase. Firstly, the presence of NVP can effectively block the heat and pyrolysis products released during the thermal decomposition of the aerogel, reducing their transfer between the flame and the aerogel matrix. In a high-temperature and oxygen-rich environment, the products of the thermal decomposition of NVP acts as a strong dehydrating agent, facilitating the formation of the char layer, which was generated from the dehydration of the carbonized polymer matrix. This could be explained by analyzing the composition of the char layer. The char layer obtained from the surface of the product of combustion in Fig. 8b has been characterized using XRD (Fig. S3, ESI†). Under the condition of sufficient oxygen and combustion time, the combustion products of the NVP₁/NCC composite film mainly include graphitic carbon (26.0°), solid sodium phosphate (20.9°) and solid NaCl (27.3°, 31.7°, 45.5°, 56.5°, 66.2°, 75.3°, 84.1°).^38,39 It should be noted that the presence of NaCl was due to the formation of Na⁺ in NCC and Cl⁻ in CTAB. During combustion, reactive hydrogen free radicals attack the strong bonds in the polymer chains, leading to polymer degradation. Therefore, eliminating reactive free radicals is crucial for suppressing polymer degradation. NVP may remove these reactive hydrogen free radicals through two pathways. First, during combustion, NVP releases reactive phosphorus-containing compounds that react with oxygen to eliminate free radicals (i.e., H˙ and HO˙).⁴⁰ Second, the thermal decomposition products of NVP act as excellent dehydrating agents, helping to form a protective char layer, and can also break down into active phosphorus-containing groups that further capture H˙ and OH˙ free radicals.^41,42 Thus, NVP, as a flame retardant, can significantly enhance the fire safety of composite materials.

Fig. 8 Combustion residues of SA–SH–EG (a) and SA–SH–EG@NVP (b) after cone calorimetry tests.

4. Conclusions

In this work, NVP was prepared using the liquid-phase exfoliation method, and the NVP/NCC composite film was prepared through a simple and convenient process for use as a flame-retardant coating. The NVP/NCC composite film exhibited significant advantages in flame-retardant performance, particularly in enhancing the self-extinguishing capability and delaying the combustion rate. Through its unique properties, NVP could form a stable char layer that effectively isolates oxygen and heat, reduce the generation of combustible gases, and inhibit the spread of flames. Additionally, the thermal decomposition products of NVP can capture reactive hydrogen free radicals, further reducing the degradation rate of the polymer and ensuring the structural integrity of the composite film under high-temperature conditions. Future research should focus on optimizing the amount and distribution of NVP to further enhance its flame-retardant performance and fulfill actual application requirements. By exploring the performance under different ratios and application environments, this study provides new insights into the development of flame-retardant coatings based on NVP, opening up broad prospects for the application and development of related materials.

Author contributions

Conceived the ideas of the work: L. Zhang and C. Du. Carried out the experiment: L. Zhang and T. Du. Involved in the data analysis: L. Zhang, T. Du, B. Wang, Z. Chang and C. Du. Wrote the final version of the manuscript: L. Zhang, T. Du and C. Du. Review: Y. Tang, C. Du, Y. Cheng and G. Zhang. Supervised this project: G. Zhang. All authors contributed to the discussions.

Data availability

The authors confirm that the data supporting the findings of this study are available within the article and/or its ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the Natural Science Basic Research Program of Shaanxi (Grant No. 2023-JC-QN-0104), the National Natural Science Foundation of China (Grant No. 12372114), and the Suzhou Industry Park Project (ZCXPT2023001). Moreover, C. Du, Y. Cheng and G. Zhang would like to thank Shiyanjia Lab (https://www.shiyanjia.com) for the SEM, TEM, nanoindentation, XPS, micro calorimeter, limiting oxygen index, and cone calorimeter characterization.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4nj04660a

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