Next Article in Journal
Characteristics and Typical Influential Factors of Wildfire Caused by High-Voltage Transmission Line Breakage Faults
Previous Article in Journal
Cooling Performance of a Nano Phase Change Material Emulsions-Based Liquid Cooling Battery Thermal Management System for High-Capacity Square Lithium-Ion Batteries
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
Fire
Volume 7
Issue 11
10.3390/fire7110372
fire-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite question_answer Discuss in SciProfiles thumb_up
...
Endorse textsms
...
Comment
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

Enhancing Flame Retardancy in Epoxy Resin with Clever Self-Assembly Method for Optimizing Interface Interaction via Well-Dispersed Cerium Oxide on Piperazine Pyrophosphate

by
Jiajun Zhao
1,
Zhengqian Wu
1,
Yutong Hong
1,
Hongyu Li
1,
Junbo Qian
1,
Kailiang Wu
1 and
Yan Xia
1,2,*
1
Institute of Fire Safety Materials, School of Materials Science and Engineering, NingboTech University, Ningbo 315100, China
2
Ningbo Dacheng Advanced Material Co., Ltd., Ningbo 315300, China
*
Author to whom correspondence should be addressed.
Fire 2024, 7(11), 372; https://doi.org/10.3390/fire7110372
Submission received: 3 September 2024 / Revised: 13 October 2024 / Accepted: 18 October 2024 / Published: 23 October 2024
Browse Figures
Versions Notes

Abstract

:
Developing flame-retardant epoxy resins (EPs) is essential to broaden their industrial applications, as their inherent flammability restricts their widespread use. In this study, commercial cerium oxide (CeO2) nanoparticles were modified with oleic acid and successfully assembled onto the surface of pyrophosphate piperazine (PAPP) through a simple solvophobic effect, constructing an integrated superstructure flame retardant, CeO2@PAPP, with enhanced performance integration. Compared to traditional simple blends, the EP composite with 10 wt% CeO2@PAPP displayed superior flame retardancy, thanks to the more subtle synergistic effects between flame retardant components and their favorable interface interactions. The EP composite achieved a UL-94 V-0 rating and increased the limiting oxygen index (LOI) to 34.2%. Significant reductions of 56.3% in peak heat release rate (PHRR) and 38.2% in total heat release (THR) were observed. Furthermore, total smoke release (TSR), carbon monoxide yield (COPR), and carbon dioxide yield (CO2PR) decreased by 52.2%, 50.2%, and 67.3%, respectively. Through comprehensive and detailed characterization, it was discovered that the assembled integrated CeO2@PAPP flame retardant can perform better in both the gas phase and condensed phase, resulting in superior flame-retardant properties. This study offers an effective strategy for developing highly flame-retardant EPs, thereby expanding their potential applications across various industries.

1. Introduction

Epoxy resin (EP) is a widely used polymer material known for its excellent electrical insulation, corrosion resistance, mechanical, and adhesive properties [1]. It holds an indispensable position in various industries, including electronics [2], construction [3], aerospace [4], and automotive manufacturing [5]. However, a major limitation of EP is its inherent flammability, causing it to continue burning even after the flame is removed, severely restricting its applications in many fields [6]. Therefore, developing flame retardant strategies to enhance the fire resistance of EP is crucial for expanding its use across multiple industries.
Extensive research has reported on the improvement of the flame retardancy of EP through the incorporation of either additive or inherent flame retardants [7]. Comparatively, the direct addition of flame retardants to the EP matrix is favored for several advantages, such as ease of processing, cost-effectiveness, and the ability to tailor the flame retardancy to specific requirements [8]. The simplest and most direct strategy is to directly blend multiple high-efficiency flame retardants, such as halogenated compounds [9], phosphorus-based additives [10], and nanomaterials [11] into the EP matrix. Although this approach has been successful, it also presents challenges. For instance, it is worth noting that, although halogenated additives are widely used as representative flame retardants, they release toxic gases during combustion, making them unsuitable for industrial applications. This is a key reason driving the search for alternative flame retardants, such as P-N systems, to replace halogenated ones. Another example is metal-based flame retardants, such as metal oxides and hydroxides (SiO2 [12], Mg(OH)2 [13], FeOX [14,15], etc.), which can release non-combustible gases during pyrolysis or adhere to the matrix surface. They can even catalyze the carbonization of the polymer, promoting the formation of a compact carbon layer, thereby demonstrating superior flame-retardant performance from multiple aspects. Additionally, it is important to highlight that single-component flame retardants often have limited effectiveness, requiring large quantities to achieve the desired flame resistance. However, adding large amounts of flame retardants can lead to uneven dispersion and agglomeration, which severely impacts the material’s mechanical properties [16]. To address these challenges, studies have shown that combining different flame retardants can result in enhanced flame-retardant properties [17]. Common combinations include the co-administration of inorganic and organic flame retardants [18], compounds with different elements [19], metal-organic frameworks (MOFs) with other functional components [20,21], and so on. The synergistic effects of such combinations are attributed to reasons such as enhanced thermal stability, improved char formation, and the promotion of a protective barrier against heat and oxygen [17,22]. Nonetheless, simply blending multiple flame retardants together often results in poor compatibility between the retardants and the resin [23], uneven dispersion of the retardants [24], and a lack of efficient synergy [25].
Based on this, an innovative self-assembly technology has been introduced in the preparation of flame retardants [26]. Assembling multiple components into a single entity not only enhances the dispersion and compatibility of the components but also facilitates the optimization of flame-retardant performance and the elucidation of flame-retardant mechanisms. Various novel and highly effective flame retardants have been prepared through self-assembly, such as Mg(OH)2@tin phytate@zinc tannate [27], UiO66-polydopamine-prussian blue analogues [28], graphene oxide@MCM-41 [29], sepiolite nanofiber@layered double hydroxides [30], etc. These integrated structures allow for full exploitation of the synergistic effects between components, demonstrating superior flame-retardant properties compared to simple blends. However, countless research results demonstrate that surface modification of components is essential to achieve effective assembly between components [31,32]. Due to limitations in more effective or universal surface modification or assembly methods, research on assembling flame retardants with excellent flame retardant performance, such as rare earth flame retardants, has faced challenges [33]. Despite the potential of the assembly method, it still faces challenges such as process complexity, poor structural controllability of flame retardants, and difficulties in large-scale production. Therefore, there is an urgent need to explore a simple and effective strategy for preparing high-performance flame retardants with controllable structures, particularly by using commercially available flame retardants as materials.
Herein, we introduce a novel approach to flame retardancy by surface-modifying cerium oxide (CeO2) nanoparticles (NPs) with oleic acid (OA) and uniformly assembling them on the surface of pyrophosphate piperazine (PAPP) to create a composite flame retardant, CeO2@PAPP. This method significantly enhances the flame retardancy of EP by controlling the distribution of CeO2 on the PAPP surface, thereby imparting excellent flame-retardant properties to the EP. The flame-retardant modification effect of CeO2@PAPP on EP was investigated through vertical burning tests, limiting oxygen index measurements, and cone calorimetry, providing insights into the flame-retardant mechanism. This work not only presents a novel composite flame retardant but also contributes to the understanding of high flame-retardant EP composite fabrication, offering new avenues for the development of safer and more reliable materials.

2. Results and Discussion

2.1. Synthesis and Characterization of CeO2@PAPP

Figure 1a illustrates the preparation process of CeO2@PAPP and EP composites. First, CeO2 was modified with OA, which formed a dense layer of OA molecules on the surface of CeO2 through coordination with the Ce element. This modification enabled the OA-coated CeO2 (OA@CeO2) to disperse well in the non-polar solvent n-hexane, thanks to the good dispersibility of OA in n-hexane. Next, OA@CeO2 was assembled onto the surface of PAPP to prepare CeO2@PAPP. When PAPP was added to the system dispersed with n- hexane, the polarity difference between PAPP and n-hexane prevented it from dispersing well. With ultrasonic assistance, the introduction of OA@CeO2 to the system led to its spontaneous assembly onto the PAPP surface, thereby reducing the surface energy of the entire system and significantly improving its dispersibility in EP resin. This process facilitated the subsequent blending to obtain well-structured EP composites.
The morphologies of CeO2, PAPP, and CeO2@PAPP were examined using SEM measurements, as depicted in Figure 1b–d. As evident from Figure 1b and Figure S1, the CeO2 NPs have an approximate diameter of 20–50 nm and exhibit significant aggregation, forming large agglomerates. The aggregation of NPs into large structures can be attributed to their strong intermolecular forces, which hinder individual dispersion. Conversely, the PAPP displays a smooth surface characterized by larger granular structures, approximately several micrometers in size, with an uneven distribution, a common challenge in the commercialization process (as depicted in Figure 1c and Figure S2). Upon the assembly of CeO2, it was observed that the previously aggregated CeO2 NPs now uniformly coat the surface of PAPP (illustrated in Figure 1d and Figure S3). The uniform arrangement of surface NPs, as observed in the magnified view in the upper-right corner of Figure 1d, provides indirect evidence of the successful functionalization of CeO2 with oleic acid (OA). This is further supported by the dispersion state of the raw material before and after modification with OA and the self-assembly process (Figure 1e and Figure S4). After the aggregated CeO2 dissolves completely, it can stably disperse in n-hexane, indicative of the successful modification of CeO2 by OA (referred to as OA@CeO2). Subsequently, when white PAPP powder is added to the assembled system, it settles to the bottom of the system before assembly is complete. Once the assembly process is fully carried out, resulting in CeO2@PAPP, no further precipitation occurs at the bottom of the system. It is worth noting that due to the relatively large size of PAPP containing CeO2, it does not remain stable in the system for an extended period of time. After a period of settling, the supernatant becomes clear and the precipitate forms a light-yellow powdery layer, further confirming the successful assembly of CeO2 onto PAPP, indicated as CeO2@PAPP.
The composition of PAPP, CeO2, and CeO2@PAPP were characterized in detail using X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Figure 1f showed the patterns of PAPP, CeO2 (PDF#34-0394) [34], and CeO2@PAPP. Compared to the individual diffraction patterns of PAPP and CeO2, the diffraction pattern of CeO2@PAPP exhibits peaks corresponding to both PAPP and CeO2, further confirming the successful assembly of CeO2 on PAPP. It can be clearly observed that the introduction of CeO2 onto PAPP had almost no effect on the crystal structure of PAPP. The chemical states of CeO2@PAPP were investigated using XPS. As shown in Figure 1g, the XPS spectrum of CeO2@PAPP reveals the peaks of the following elements: C, N, O, P, and Ce. The appearance and intensity of the peaks [35], particularly the characteristic peak of Ce, confirm the successful introduction and distribution of CeO2. Further, elemental analysis (EDS) combined with an energy spectrum evaluated the elemental composition of CeO2@PAPP, as shown in Figure 1h. The elemental mapping images of CeO2@PAPP indicate that all these elements are uniformly distributed, belonging, respectively, to PAPP and CeO2. It is noteworthy that the uniform distribution of the Ce element fully demonstrates the successful assembly of CeO2 on PAPP. However, as a contrasting experiment, when we attempted to assemble unmodified CeO2 onto the surface of PAPP using the same method, the CeO2 exhibited clear phase separation from PAPP and failed to adhere to its surface (Figure S5). This highlights the necessity and indispensability of the OA modification mentioned above. These phenomena underscore the successful integration of the modified CeO2 onto the PAPP surface, resulting in the CeO2@PAPP superstructure as a flame retardant, achieving the desired assembly effect. These results not only validate the successful assembly but also provide strong support for the analysis of the material’s structure and composition.

2.2. Thermal Degradation Behaviors of EP Composites

Thermogravimetric analysis (TGA) was utilized to investigate the thermal degradation behavior of EP with and without flame retardants (Table S1) under a nitrogen atmosphere. The TGA and derivative thermogravimetric (DTG) curves are presented in Figure 2, and the relevant thermal decomposition parameters, including T5% (the temperature at which 5% mass loss occurs), Tmax (the temperature corresponding to the maximum mass loss rate), and char residue yield, are summarized in Table S2.
As shown in Figure 2, all EP composites exhibit a single-stage thermal decomposition process. The T5% and Tmax values for pure EP and the EP composite containing 10 wt% CeO2 are approximately 350 °C and 370 °C, respectively. In contrast, with the introduction of PAPP, the T5% and Tmax values of the flame-retarded EP composites decrease slightly, to approximately 327 °C and 346 °C. This reduction is attributed to the relatively lower decomposition temperature of PAPP. Nevertheless, the char residue yields of EP, 10PAPP/EP, 10CeO2/EP, 10CeO2@PAPP/EP, and 10CeO2/PAPP/EP at 800 °C were 16.8%, 26.2%, 23.4%, 28.4%, and 26.8%, respectively. Notably, the 10CeO2/EP sample achieved a char residue yield of 23.4%, primarily due to the catalytic charring effect of CeO2 during pyrolysis and its retention in the residue, as CeO2 does not decompose. Additionally, the incorporation of PAPP significantly improved the char residue yield of the EP composites. This enhancement is attributed to the decomposition of pyrophosphate in PAPP, which produces pyrophosphoric and phosphoric acids during heating, catalyzing the charring of the EP matrix. Moreover, the char residue yield of 10CeO2@PAPP/EP, which involves assembled flame retardants, reached 28.4%, the highest among all samples. This indicates that the assembly of flame retardants promotes better synergistic effects between the components, resulting in enhanced flame-retardant performance compared to simple blending.

2.3. Flame Retardant Performance Analysis

The combustion performance of EP and its composites was initially investigated through UL-94 and LOI tests. As shown in Figure 3a and described in Table S3, the LOI value of pure EP was only 26.2%, the lowest among all samples, and it did not possess any UL-94 rating, indicating a high level of flammability [36]. Even though the addition of 10 wt% PAPP resulted in a slight increase in the LOI value, the EP composite only achieved a UL-94 V-2 rating. This suggests that PAPP adds some flame retardant properties to EP, but its effectiveness is limited [37], possibly due to its large particle size and hygroscopic nature, which hinders uniform dispersion within the EP matrix. In contrast, the EP composite reinforced with 10 wt% CeO2 exhibited significantly weaker flame retardant properties compared to those of PAPP, with an LOI value of only 27.2%, and it also did not obtain a UL-94 rating, contradicting the expectation that rare earth-based flame retardants should have excellent flame retardant properties [33]. The observed poor flame retardancy may be due to the untreated commercial CeO2 NPs agglomerating, leading to uneven dispersion in the resin and hindering their full potential [38]. Therefore, even with a simple combination of PAPP and CeO2, their flame-retardant properties are limited, with an LOI value of only 27.5% and achieving only a UL-94 V-2 rating, similar to pure PAPP, indicating that simple mixing does not produce a synergistic flame-retardant effect. Fortunately, when 10 wt% CeO2@PAPP is added, the EP composite shows the highest LOI value among all samples, reaching 34.2%, which is a 30.5% increase compared to pure EP, and easily obtains a UL-94 V-0 rating. Compared to simple blending, the main difference lies in the fact that in this case, CeO2 is uniformly assembled on the surface of PAPP and exhibits better dispersibility. This allows for more effective utilization and, more importantly, enables the synergistic flame-retardant effect between PAPP and CeO2 to be fully realized, resulting in superior flame-retardant properties.
We also utilize a cone calorimeter to evaluate the flame-retardant properties of composite materials. This device is highly effective for analyzing the combustion behavior of polymers during large-scale fires [39]. Figure 3b–i illustrate the variations in heat release rate (HRR), total heat release (THR), smoke production rate (SPR), total smoke production (TSP), total smoke release per unit area (TSR), carbon monoxide production rate (COPR), and carbon dioxide production rate (CO2PR) for different composite materials during combustion. Detailed parameters are summarized in Table S4. As seen in Figure 3, the values of these parameters for EP materials without flame retardants are significantly higher compared to those with flame retardants. For instance, the peak heat release rate (PHRR) of pure EP reaches 941 kW/m2, with a THR value of 91.9 MJ/m2—both among the highest observed across all samples. Surprisingly, the parameters for the EP composite containing 10 wt% CeO2 are nearly identical to those of pure EP, with some values even slightly increasing. This unexpected result may be due to the severe aggregation of CeO2 (as shown in Figure 1b and Figure S1), leading to its local excess and poor dispersion, which compromised its flame-retardant effectiveness.
In contrast, the introduction of the PAPP flame retardants into EP resulted in a noticeable improvement across all parameters, though the degree of impact varied. Among them, the single-component PAPP showed the least effective flame-retardant performance, while the combined CeO2 and PAPP mixture performed better, and the assembled CeO2@PAPP flame retardant exhibited the best results. Specifically, when 10 wt% PAPP was added, the PHRR value decreased to 660 kW/m2. This value further dropped to 527 kW/m2 in samples blended with PAPP and CeO2 and reached as low as 411 kW/m2 in samples containing CeO2@PAPP—a 56.3% reduction compared to pure EP and the lowest among all EP composites. It is worth noting that the main reason for the appearance of multiple peaks in the HRR curve is the multi-stage thermal decomposition behavior of epoxy resin and the synergistic effect of flame retardants at different temperature ranges (Figure 3a). A similar trend was observed for the other parameters. As depicted in Figure 3c, compared to pure EP, the THR of 10CeO2@PAPP/EP is reduced by 38.2%, marking the lowest among all samples and significantly lower than the other two PAPP-containing EP samples. Figure 3d shows that compared to pure EP, which has a PSPR of 0.308 m2/s, the value significantly decreases by 45.8% with the introduction of CeO2@PAPP. Additionally, the values of TSR, TSP, COPR, and CO2PR for EP composites containing the assembled CeO2@PAPP also dropped significantly by 52.2%, 50.2%, and 67.3%, respectively (Figure 3e–h). This indicates that the addition of CeO2@PAPP not only successfully improves the flame-retardant performance of epoxy resin composite materials, but also significantly reduces the generation of harmful gases and smoke during combustion, which is of great significance for ensuring the safety of people’s lives and property and environmental protection.
The addition of flame retardants not only impacts various parameters during combustion but also significantly improves the char residue yield. As shown in Figure 3i, the char residue yield of pure EP after combustion is only 17.2 wt%. With the addition of 10 wt% PAPP, this increases to 40.7 wt%. Notably, after adding the blended PAPP and CeO2, the char residue yield rises significantly to 41.6 wt%. However, the introduction of the assembled CeO2@PAPP flame retardant further increases the residual content to 44.5 wt%, marking a 158.7% rise compared to pure EP. This significant increase in char residue yield with the assembled CeO2 on PAPP indicates a better catalytic charring effect during combustion. Moreover, the dense carbon layer formed acts as an effective physical barrier, blocking heat exchange, preventing the diffusion of toxic gases, and impeding oxygen entry, thereby enhancing the flame retardant performance [40]. By comparing and analyzing various parameters, it can be concluded that assembling two flame retardants provides superior flame-retardant performance that cannot be achieved with single-component or simple blending methods.

2.4. Gas Phase Mechanism

To investigate the gas-phase flame-retardant mechanism of CeO2@PAPP, TG-IR tests were conducted on pure EP (containing DDM curing agent) and 10CeO2@PAPP/EP samples under air conditions, and the resulting infrared spectra are shown in Figure 4. From the figure, it can be observed that the characteristic absorption peaks of gas-phase products for the 10CeO2@PAPP/EP sample are significantly reduced compared to the pure EP sample (see Figure 4a,b), indicating that CeO2@PAPP suppresses the release of volatiles during the thermal degradation of the epoxy network. Furthermore, Figure 4c reveals that the gas-phase decomposition products of EP and 10CeO2@PAPP/EP samples primarily include H2O (O-H, 3643 cm−1), hydrocarbons (3011 cm−1), CO2 (2303–2356 and 671 cm−1), CO (2183 cm−1), carbonyl compounds (1743 cm−1), aromatic compounds (1420–1575 cm−1), and aliphatic ethers (1183 cm−1), consistent with the results reported in the literature. Notably, at the Tmax of both samples, the intensity of the absorption peaks of the gas-phase products is also markedly lower for the 10CeO2@PAPP/EP sample than for pure EP. This suggests that CeO2@PAPP promotes the formation of polyaromatic structures during the thermal degradation of the epoxy matrix, leading to the retention of more char in the condensed phase, which is further corroborated with the condensed-phase mechanism analysis discussed below. Consequently, the release of volatile combustibles into the gas phase is reduced. Therefore, the enhanced flame-retardant performance of the 10CeO2@PAPP/EP sample can be attributed primarily to the increased char formation during combustion and the reduced emission of flammable volatiles into the gas phase.
To further elucidate the pyrolysis mechanism and its contribution to flame retardancy, a Py-GC/MS test was conducted, with the results presented in Figure 5. Notably, the 10CeO2@PAPP/EP sample released significantly fewer fragments compared to pure EP (containing DDM curing agent), as evident from the GC spectra. This suggests that a rapid charring reaction occurred in 10CeO2@PAPP/EP, which in turn reduced the release of pyrolysis fragments. This effect is attributed to the better dispersion of the integrated flame retardant in the resin and the improved synergistic effect between the flame-retardant components. If the charring reaction had not occurred promptly, a large amount of pyrolysis products would have been released, as seen in the pure EP sample. Additionally, the peak positions clearly reveal that the pyrolysis fragments released by the two flame retardants were different. The results indicate that the low char yield of pure EP is due to the breakdown of its primary structure during pyrolysis, leading to the release of large quantities of small molecular aromatic compounds into the gas phase. In contrast, the aggregation of CeO2 and PAPP in the assembled flame retardant facilitated a superior synergistic effect, promoting a rapid charring reaction and enabling the material to exhibit a condensed-phase flame-retardant effect during combustion.

2.5. Condensed Phase Mechanism

To reveal the morphological and structural traits of residual carbon post-combustion in a variety of EP composites, Figure 6 displays the digital photographs and SEM images of the combustion residues. The pristine EP yields a sparse and fragmented carbon layer, with a residual carbon height of approximately 14 mm, as shown in Figure 6a1,a2. The integration of PAPP in the 10PAPP/EP composite leads to an expanded carbon layer, with the height of the residual carbon extending to 20 mm, as depicted in Figure 6b1–b3. Conversely, the EP composite enriched with CeO2 exhibits near-complete combustion, leaving a residual carbon height of merely 11 mm, as illustrated in Figure 6c1–c3. Remarkably, the 10CeO2@PAPP/EP composite forms an expanded carbon layer characterized by denser residues, fewer cracks, and reduced porosity, culminating in a more coherent structure, with the carbon layer height reaching 24 mm (Figure 6d1–d3). This phenomenon may be ascribed to the help of the assembled CeO2, which catalyzes the carbonization reaction, augmenting the char residue yield and thus fostering the emergence of a continuous, dense, and less porous protective carbon layer during combustion. However, the 10CeO2/PAPP/EP composite, with a residual carbon height of 17 mm (Figure 6e3), does not exhibit a heightened expansion of the residual carbon layer compared to the PAPP/EP composite. This may be due to the relatively lower proportion of PAPP in the flame-retardant blend and the less pronounced synergistic effect between CeO2 and PAPP, potentially due to their poor dispersion.
For a deeper comprehension of the microstructure of the char residues of EP and its composites, SEM analysis was conducted. The char residue of the pristine EP exhibits a notably loose structure with numerous cracks, indicative of its inadequate covering and barrier capabilities against oxidizing gases during combustion, thereby lacking superior flame retardancy [41], as observed in Figure 6a4. However, the addition of PAPP results in a denser internal residual carbon in PAPP/EP, characterized by fewer cracks and voids (Figure 6b4). Despite the char residue of CeO2/EP being denser than that of the original EP, the presence of many large, visible pores on the surface of the residual carbon suggests it does not possess effective barrier and flame-retardant properties (Figure 6c2). In contrast to other control groups, the CeO2@PAPP/EP composite demonstrates dense and continuous char residues devoid of cracks (Figure 6d4), capable of acting as a barrier that effectively isolates oxygen and prevents the sustained burning of the internal polymer [42]. On the contrary, the char residue layer of CeO2/PAPP/EP exhibits noticeable wrinkles and some holes (Figure 6e4), which undermine its flame retardancy and increase smoke emission compared to the CeO2@PAPP/EP composite. It is evident that the reconfiguration of conventional flame retardants through self-assembly methods can substantially enhance and refine their performance.

2.6. Combined Mechanism

Based on the above analysis, the flame-retardant mechanism of the CeO2@PAPP/EP composite is proposed, as illustrated in Figure 7. In the condensed phase, CeO2@PAPP decomposes upon heating into phosphoric acid and other phosphorus-containing derivatives, which accelerate the dehydration reaction of EP [43]. This leads to cyclization or cross-linking on the matrix surface, delaying heat release through isolation and releasing CeO2 NPs during combustion. These CeO2 NPs may enhance the graphitization degree of the char layer [44], forming a more continuous and dense expanded char layer, thereby improving the flame retardancy of the epoxy resin. In the gas phase, NH3 and H2O gases generated from the thermal decomposition of PAPP not only promote the expansion of the char layer but also dilute the concentration of oxygen and flammable gases [45]. Simultaneously, gaseous phosphorus compounds can serve as flame retardants, inhibiting the combustion process. During combustion, the CeO2@PAPP composite releases free radicals (e.g., HPO· and PO·) that inhibit chain reactions, thereby preventing the combustion process [46]. Meanwhile, as a rare-earth element, CeO2 has unfilled 4f orbitals that can provide vacant sites to capture free radicals produced during combustion [47], such as hydrogen radicals (·H) and hydroxyl radicals (·OH), thereby inhibiting chain reactions. However, as extensively reported in the literature, simple blending of two flame retardants usually results in weak interactions between them, leading to only limited improvements in flame retardancy [48]. In contrast, assembling CeO2 onto the surface of piperazine pyrophosphate may improve its compatibility with the epoxy resin matrix, thereby more effectively exerting synergistic effects during combustion, as further shown by the comparison with literature results (Table S5) [49,50,51,52,53,54]. Consequently, this not only promotes the formation of a higher-quality and denser char layer during combustion but also enables uniformly distributed CeO2 in the resin to more effectively form a physical barrier. This dual effect enhances the isolation of oxygen and heat, slowing down the thermal degradation of the material. In conclusion, the combined effects of the physical barrier, dilution of non-combustible gases, and free radical capture markedly improve the fire safety of EP.

3. Materials and Methods

3.1. Materials

Bisphenol-A type EP (DGEBA, epoxy value of 0.53 mol/100 g) was provided by Yueyang Baling Petrochemical Co., Ltd., Yueyang, China. Piperazine Pyrophosphate (PAPP) was afforded by the Shanghai Research Institute of Chemical Industry Co., Ltd., Shanghai, China. Cerium Oxide (CeO2, 20–50 nm, spherical, 99.5%) and oleic acid (OA, AR) were purchased from Maclean Biochemical Technology Co., Ltd. (Shanghai, China). N-hexane (AR) and 4,4′-diaminodiphenylmethane (DDM, AR) were provided by Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All agents were used without further purification.

3.2. Instruments and Characterization

The surface morphology of the flame retardant and char was characterized using a field emission scanning electron microscope (SEM, Zeiss-Ultra 55, 3 kV). The composition of the flame retardant was analyzed using energy dispersive spectroscopy (EDS). The crystal structure of the material was obtained using powder X-ray diffraction (XRD) with a Bruker D4 Phaser X-ray diffractometer. Elemental analysis was conducted using X-ray photoelectron spectroscopy (XPS) with a Perkin Elmer PHI-5000C ESCA system. The digital photos were taken with an iPhone 14 (Apple Inc., Cupertino, CA, USA).
Thermogravimetric analysis (TGA) was conducted on a TA TGA 550 analyzer with a heating rate of 10 °C/min from 25 °C to 800 °C in a flow of nitrogen atmosphere.
Thermogravimetric analysis/infrared spectrometry (TG-IR) was recorded with TA TGA55 equipment (TA Instruments, New Castle, DE, USA), which was interfaced with a Nicolet iS50 (Thermo Fisher Scientific, Waltham, MA, USA) in nitrogen condition at a heating rate of 10 °C /min, and the experimental temperature ranged from 25 °C to 800 °C.
The Pyrolysis-Gas Chromatography Mass Spectrometry (Py-GC/MS) analysis was recorded on EGA3030D (Frontier Laboratories, Koriyama, Fukushima, Japan) and GCMS-QP2020NX (Agilent, Santa Clara, CA, USA) to determine the pyrolysis products of EP composites under a helium flow.
The flame-retardant properties of EP composites were evaluated using the limiting oxygen index (LOI), vertical burning UL-94 test, and cone calorimeter test. The LOI test (HC-2, Nanjing Jiangning Analytical Instrument Co., Ltd., Nanjing, China) was conducted with sample dimensions of 130 × 6.5 × 3 mm3, following the GB/T2406.2-2009 standard. The vertical burning test (UL-94, CZF-111, Nanjing Jiangning Analytical Instrument Co., Ltd., China) was performed with sample dimensions of 130 × 13 × 3 mm3, according to the ASTM D3801 standard. The cone calorimeter test (Cone, Fire Testing Technology, East Grinstead, UK) was conducted with sample dimensions of 100 × 100 × 3 mm3, according to the ISO-5660 standard, with a heat flux of 50 kW/m2.

3.3. Fabrication of the Modified CeO2 (OA@CeO2)

To prepare the OA@CeO2, typically 1 g of CeO2 is added to a glass vial containing 30 mL of n-hexane and vigorously stirred until the CeO2 is dispersed in the solvent. Then, 7.5 mL of OA is added to the dispersed system, the vial is sealed, and it is placed in an ultrasonic bath for 1 h (shaking periodically) to ensure uniform dispersion and prevent any precipitation. Excess OA is then removed via centrifugation and repeated washing with hexane, usually repeated 3 times, resulting in OA-modified CeO2, denoted as OA@CeO2. Finally, the sample is dried in an oven set at 70 °C until all solvents evaporate, resulting in a dry powder.

3.4. Fabrication of CeO2@PAPP

To prepare the CeO2@PAPP flame retardant, the previously prepared OA@CeO2 was first uniformly dispersed in n-hexane through ultrasonication. Following this, 5 g of PAPP was added, and ultrasonication was continued for 1 h with periodic manual shaking to facilitate the assembly process and ensure even distribution of OA@CeO2 on the surface of PAPP until all particles were completely dispersed in the solvent. The resulting mixture was then separated via centrifugation and washed with n-hexane once. Finally, the washed product was separated and dried in an oven at 70 °C to produce the CeO2@PAPP flame retardant.

3.5. Preparation of EP Composites

The detailed formulas of EP composites are listed in Table S1. To prepare pure EP composites (referred to as EP), follow these steps: add 65 g of EP to a flask and place the flask in a water bath at 80 °C. Stir the EP at 700 rpm for 30 min until it has good flowability. Next, add 17.05 g of DDM to the stirred EP and continue stirring at 700 rpm for 10 min to ensure thorough mixing and uniformity. Transfer the well-mixed mixture to a preheated PTFE mold, maintaining the mold temperature at 80 °C. Cure the mold at 120 °C for 2 h, then increase the temperature to 160 °C and cure for an additional 3 h to complete the curing process.
To prepare the EP composites containing 10 wt% PAPP (referred to as 10PAPP/EP), follow these steps: mix 9.12 g of PAPP with 65 g of EP resin in a flask and place the flask in a water bath at 80 °C. Mechanically stir the mixture at 700 rpm for 30 min until a uniform EP composite mixture is formed. Next, add 17.05 g of DDM to the mixture and continue stirring at 700 rpm for 10 min to ensure thorough mixing and uniformity. Transfer the well-mixed mixture to a preheated PTFE mold, maintaining the mold temperature at 80 °C. Cure the mold at 120 °C for 2 h, then increase the temperature to 160 °C and cure for an additional 3 h to complete the curing process.
To prepare the EP composites containing 10 wt% CeO2 (referred to as 10CeO2 /EP), follow these steps: mix 9.12 g of CeO2 with 65 g of EP resin in a flask and place the flask in a water bath at 80 °C. Mechanically stir the mixture at 700 rpm for 30 min until a uniform EP composite mixture is formed. Next, add 17.05 g of DDM to the mixture and continue stirring at 700 rpm for 10 min to ensure thorough mixing and uniformity. Transfer the well-mixed mixture to a preheated PTFE mold, maintaining the mold temperature at 80 °C. Cure the mold at 120 °C for 2 h, then increase the temperature to 160 °C and cure for an additional 3 h to complete the curing process.
To prepare the EP composites containing 10 wt% CeO2@PAPP (referred to as 10CeO2@PAPP/EP), the following steps are typically required: combine 9.12 g of CeO2@PAPP with 65 g of EP resin in a flask; then, place the flask in a water bath at 80 °C. Stir the mixture mechanically at 700 rpm for 30 min until a homogeneous EP composites blend is formed. Next, add 17.05 g of DDM to the mixture and continue stirring at 700 rpm for an additional 10 min to ensure thorough mixing and homogeneity. Transfer the well-mixed mixture to a preheated PTFE mold, maintaining the mold temperature at 80 °C. Cure the mold at 120 °C for 2 h, then increase the temperature to 160 °C and cure for an additional 3 h to complete the curing process.
To prepare the EP composites containing 10 wt% CeO2/PAPP (referred to as 10CeO2/PAPP/EP), follow these steps: mix 1.52 g of CeO2, 7.6 g of PAPP, and 65 g of EP resin in a flask, and place the flask in a water bath at 80 °C. Mechanically stir the mixture at 700 rpm for 30 min until a uniform EP composite mixture is formed. Next, add 17.05 g of DDM to the mixture and continue stirring at 700 rpm for 10 min to ensure thorough mixing and uniformity. Transfer the well-mixed mixture to a preheated PTFE mold, maintaining the mold temperature at 80 °C. Cure the mold at 120 °C for 2 h, then increase the temperature to 160 °C and cure for an additional 3 h to complete the curing process.

4. Conclusions

In this study, we have successfully fabricated the CeO2@PAPP composite via a self-assembly technique involving the solvophobic effect and integrated it into EP to greatly enhance the fire retardancy of the resulting EP composites. The incorporation of 10 wt% CeO2@PAPP into EP resulted in a UL-94 V-0 classification and an elevated LOI value of 34.2%. Notably, the PHRR and THR of the 10CeO2@PAPP/EP composite were remarkably reduced by 56.3% and 38.2%, respectively, compared to the neat EP. Furthermore, the smoke suppression capabilities were enhanced, as evidenced by a substantial decrease in the SPR, TSP, COPR, and CO2PR by 45.8%, 52.2%, 50.2%, and 67.3%, respectively. The char residue rate of 10CeO2@PAPP/EP reached a remarkable 44.5%, which is a 158.7% increase over that of pure EP. The achievement of outstanding flame-retardant performance depends on the assembled flame retardants being able to better leverage the synergistic effect between the two components. This optimization and enhancement of flame retardancy is achieved through dual-phase action in both the condensed and gas phases, significantly surpassing the performance of simple blending or single-component flame retardants. The findings pave the way for an innovative strategy in the advancement of EP composites with significantly enhanced flame retardancy.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/fire7110372/s1, Table S1: The formulations of EP composites; Table S2: The characteristic data of thermal stability of EP composites; Table S3: Combustion behavior of EP; Table S4: The cone calorimetry data of EP and its composites; Table S5: Summary of the flame retardant properties of various FRs in EP composites; Figure S1: (a–d) The SEM images of CeO2 NPs at different magnifications; Figure S2: (a–d) The SEM images of PAPP at different magnifications; Figure S3: (a–d) The SEM images of CeO2@PAPP at different magnifications; Figure S4: Photographs of different kinds of flame retardants. Figure S5: SEM and EDS results of the CeO2 assembled with PAPP through ultrasonication without OA modification.

Author Contributions

J.Z.: Methodology, Investigation, Writing—original draft; Z.W.: Methodology, Investigation; Y.H.: Investigation, Validation; H.L.: Validation; J.Q.: Validation; K.W.: Validation; Y.X.: Methodology, Supervision, Writing—review and editing. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Scientific Research Foundation of NingboTech University (grant number 20220920Z0221), the General Scientific Research Project of Zhejiang Education Department (grant number 20240702Z0305), and the National College Students’ Innovation and Entrepreneurship Training Program (grant numbers 202413022017, 202413022021).

Data Availability Statement

The authors confirm that the data supporting the findings of this study are available within this article and its Supplementary Materials.

Acknowledgments

The authors thank Wang Sun at Department of Macromolecular Science at Shanghai, Fudan University for SEM testing for this work.

Conflicts of Interest

Author Yan Xia was employed by Ningbo Dacheng Advanced Material Co., Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

References

  1. Wen, Y.; Chen, C.; Ye, Y.; Xue, Z.; Liu, H.; Zhou, X.; Zhang, Y.; Li, D.; Xie, X.; Mai, Y.W. Advances on thermally conductive epoxy-based composites as electronic packaging underfill materials-a review. Adv. Mater. 2022, 34, 2201023. [Google Scholar] [CrossRef] [PubMed]
  2. Hu, Y.; Chen, C.; Wen, Y.; Xue, Z.; Zhou, X.; Shi, D.; Hu, G.; Xie, X. Novel micro-nano epoxy composites for electronic packaging application: Balance of thermal conductivity and processability. Compos. Sci. Technol. 2021, 209, 108760. [Google Scholar] [CrossRef]
  3. Xiang, Q.; Xiao, F. Applications of epoxy materials in pavement engineering. Constr. Build. Mater. 2020, 235, 117529. [Google Scholar] [CrossRef]
  4. Zhang, C.; Ling, Y.; Zhang, X.; Liang, M.; Zou, H. Ultra-thin carbon fiber reinforced carbon nanotubes modified epoxy composites with superior mechanical and electrical properties for the aerospace field. Compos. Part A Appl. Sci. Manuf. 2022, 163, 107197. [Google Scholar] [CrossRef]
  5. Sun, Z.; Li, J.; Yu, M.; Kathaperumal, M.; Wong, C. A review of the thermal conductivity of silver-epoxy nanocomposites as encapsulation material for packaging applications. Chem. Eng. J. 2022, 446, 137319. [Google Scholar] [CrossRef]
  6. Shi, X.; Li, X.; Liu, Q.; Wu, S.; Xie, W.; Zhao, N.; De La Vega, J.; Chen, M.; Wang, D. Constructing Co-decorated layered double hydroxide via interfacial assembly and its application in flame-retardant epoxy resin. Compos. Commun. 2023, 43, 101712. [Google Scholar] [CrossRef]
  7. Jiao, D.; Zhao, H.; Sima, H.; Cheng, C.; Liu, B.; Zhang, C. Engineering flame retardant epoxy resins with strengthened mechanical property by using reactive catechol functionalized DOPO compounds. Chem. Eng. J. 2024, 485, 149910. [Google Scholar] [CrossRef]
  8. Liu, B.W.; Zhao, H.B.; Wang, Y.Z. Advanced Flame-Retardant Methods for Polymeric Materials. Adv. Mater. 2022, 34, e2107905. [Google Scholar] [CrossRef]
  9. Dogan, M.; Dogan, S.D.; Savas, L.A.; Ozcelik, G.; Tayfun, U. Flame retardant effect of boron compounds in polymeric materials. Compos. Part B Eng. 2021, 222, 109088. [Google Scholar] [CrossRef]
  10. Huo, S.; Song, P.; Yu, B.; Ran, S.; Chevali, V.S.; Liu, L.; Fang, Z.; Wang, H. Phosphorus-containing flame retardant epoxy thermosets: Recent advances and future perspectives. Prog. Polym. Sci. 2021, 114, 101366. [Google Scholar] [CrossRef]
  11. Teles, F.; Martins, G.; Antunes, F. Fire retardancy in nanocomposites by using nanomaterial additives. J. Anal. Appl. Pyrolysis 2022, 163, 105466. [Google Scholar] [CrossRef]
  12. Zheng, P.; Zhao, H.; Zhou, Y.; Yuan, Q.; Liu, Q. P/N-containing SiO2 flame retardant particles formed by in-situ hydrolysis for significantly improve the flame retardancy and toughness of epoxy resins. Compos. Commun. 2024, 49, 101989. [Google Scholar] [CrossRef]
  13. Mohammed Fayyadh, S.; Ben Ahmed, A. A comparative study between the use of nanoparticles of magnesium oxide and zinc oxide as coating for polymeric surfaces a flame retardant and corrosion resistance. Mater. Chem. Phys. 2024, 314, 128899. [Google Scholar] [CrossRef]
  14. Baghdadi, Y.N.; Youssef, L.; Bouhadir, K.; Harb, M.; Mustapha, S.; Patra, D.; Tehrani Bagha, A.R. Thermal and mechanical properties of epoxy resin reinforced with modified iron oxide nanoparticles. J. Appl. Polym. Sci. 2021, 138, e50533. [Google Scholar] [CrossRef]
  15. Zhang, L.; Yang, D.; Li, Z.; Zhai, Z.; Li, X.; de La Vega, J.; Wang, D. Ultrafine iron oxide decorated mesoporous carbon nanotubes as highly efficient flame retardant in epoxy nanocomposites via catalytic charring effect. Sustain. Mater. Technol. 2024, 39, e00845. [Google Scholar] [CrossRef]
  16. Yuan, Y.; Yu, B.; Wang, W. The influence of poorly-/well-dispersed organo-montmorillonite on interfacial compatibility, fire retardancy and smoke suppression of polypropylene/intumescent flame retardant composite system. J. Colloid. Interface. Sci. 2022, 622, 367–377. [Google Scholar] [CrossRef]
  17. He, W.; Song, P.; Yu, B.; Fang, Z.; Wang, H. Flame retardant polymeric nanocomposites through the combination of nanomaterials and conventional flame retardants. Prog. Mater. Sci. 2020, 114, 100687. [Google Scholar] [CrossRef]
  18. Afolabi, O.A.; Ndou, N. Synergy of hybrid fillers for emerging composite and nanocomposite materials-a review. Polymers 2024, 16, 1907. [Google Scholar] [CrossRef]
  19. Yang, Y.; Wang, D.; Jian, R.; Liu, Z.; Huang, G. Chemical structure construction of DOPO-containing compounds for flame retardancy of epoxy resin: A review. Prog. Org. Coat. 2023, 175, 107316. [Google Scholar] [CrossRef]
  20. Song, K.; Pan, Y.; Zhang, J.; Song, P.; He, J.; Wang, D.; Yang, R. Metal-organic frameworks-based flame-retardant system for epoxy resin: A review and prospect. Chem. Eng. J. 2023, 468, 143653. [Google Scholar] [CrossRef]
  21. Nabipour, H.; Wang, X.; Song, L.; Hu, Y. Organic-inorganic hybridization of isoreticular metal-organic framework-3 with melamine for efficiently reducing the fire risk of epoxy resin. Compos. Part B Eng. 2021, 211, 108606. [Google Scholar] [CrossRef]
  22. Lei, Y.; Zhao, X.; Xu, L.; Li, H.; Liang, J.; Yeoh, G.H.; Wang, W. Natural flame retardant minerals for advanced epoxy composites. Fire 2024, 7, 308. [Google Scholar] [CrossRef]
  23. Rong, H.; Guo, Y.; Chen, Z.; Chen, T.; Yu, Y.; He, C.; Zhou, J.; Liu, X.; Zhang, Q.; Bu, Y.; et al. Synergistic effect of phenyl phosphoric acid derivatives and DOPO on multifunctional epoxy resin: Fire safety, mechanical properties, transparency and hydrophobicity. Polym. Degrad. Stabil. 2023, 216, 110471. [Google Scholar] [CrossRef]
  24. Chen, Y.; Peng, C.; Liu, X.; Fu, Z.; Guo, B.; Du, W.; Xu, Y.; Zeng, B.; Chen, G.; Luo, W.; et al. Enter the multifunctional trifluoromethylated epoxy Resin: Excellent flame retardant, better dielectric, low thermal expansion, etc. Chem. Eng. J. 2024, 481, 148405. [Google Scholar] [CrossRef]
  25. Chen, W.; Liu, P.; Liu, Y.; Liu, Z. Recent advances in Two-dimensional Ti3C2Tx MXene for flame retardant polymer materials. Chem. Eng. J. 2022, 446, 137239. [Google Scholar] [CrossRef]
  26. Fang, F.; Ran, S.; Fang, Z.; Song, P.; Wang, H. Improved flame resistance and thermo-mechanical properties of epoxy resin nanocomposites from functionalized graphene oxide via self-assembly in water. Compos. Part B Eng. 2019, 165, 406–416. [Google Scholar] [CrossRef]
  27. Meng, W.; Dong, Y.; Li, J.; Cheng, L.; Zhang, H.; Wang, C.; Jiao, Y.; Xu, J.; Hao, J.; Qu, H. Bio-based phytic acid and tannic acid chelate-mediated interfacial assembly of Mg(OH)2 for simultaneously improved flame retardancy, smoke suppression and mechanical properties of PVC. Compos. Part B Eng. 2020, 188, 107854. [Google Scholar] [CrossRef]
  28. Zhang, J.; Li, Z.; Shao, Z.; Zhang, L.; Wang, D. Hierarchically tailored hybrids via interfacial-engineering of self-assembled UiO-66 and prussian blue analogue: Novel strategy to impart epoxy high-efficient fire retardancy and smoke suppression. Chem. Eng. J. 2020, 400, 125942. [Google Scholar] [CrossRef]
  29. Li, Z.; González, A.J.; Heeralal, V.B.; Wang, D. Covalent assembly of MCM-41 nanospheres on graphene oxide for improving fire retardancy and mechanical property of epoxy resin. Compos. Part B Eng. 2018, 138, 101–112. [Google Scholar] [CrossRef]
  30. Zhang, H.; Hu, X.; Liu, Y.; Zhang, S.; Wu, Z. Convenient synthesis of one-dimensional a-SEP@LDH via self-assembly towards simultaneously improved fire retardance, mechanical strength and thermal resistance for epoxy resin. Compos. Part B Eng. 2021, 216, 108857. [Google Scholar] [CrossRef]
  31. Wang, W.; Zhang, M.; Pan, Z.; Biesold, G.M.; Liang, S.; Rao, H.; Lin, Z.; Zhong, X. Colloidal inorganic ligand-capped nanocrystals: Fundamentals, status, and insights into advanced functional nanodevices. Chem. Rev. 2021, 2, 4091–4162. [Google Scholar] [CrossRef] [PubMed]
  32. Boles, M.A.; Engel, M.; Talapin, D.V. Self-assembly of colloidal nanocrystals: From intricate structures to functional materials. Chem. Rev. 2016, 116, 11220–11289. [Google Scholar] [CrossRef]
  33. Wang, C.; Gong, K.; Yu, B.; Zhou, K. Rare earth-based flame retardants for polymer composites: Status and challenges. Compos. Part B Eng. 2023, 265, 110935. [Google Scholar] [CrossRef]
  34. Du, B.; Chen, N.; Zhang, G.; Chen, Y.; Gao, B.; Liu, L.; Zhao, Y. Enhanced ultraviolet aging resistance of epoxy resins through surface enrichment achieved by fluorinated graphene oxide@ CeO2. Compos. Sci. Technol. 2024, 253, 110655. [Google Scholar] [CrossRef]
  35. Zhu, M.; Jia, P.; Yang, G.; Song, L.; Hu, Y.; Wang, B. Synergistic effects of core-shell structured piperazine pyrophosphate microcapsules on fire safety and mechanical property in styrenic thermoplastic elastomer. J. Colloid. Interface. Sci. 2024, 653, 1112–1122. [Google Scholar] [CrossRef] [PubMed]
  36. Xia, Y.; Hong, Y.; Zhang, L.; Chai, J.; Wang, B.; Guo, Z.; Li, J.; Huo, S.; Fang, Z. Controllable self-assembly of carbon nanotubes on ammonium polyphosphate as a game-changer for flame retardancy and thermal conductivity in epoxy resin. Macromol. Rapid Commun. 2024, e2400356. [Google Scholar] [CrossRef]
  37. Xiao, F.; Fontaine, G.; Bourbigot, S. Improvement of flame retardancy and antidripping properties of intumescent polybutylene succinate combining piperazine pyrophosphate and zinc borate. ACS Appl. Polym. Mater. 2022, 4, 1911–1921. [Google Scholar] [CrossRef]
  38. Qu, Z.; Wu, K.; Meng, W.; Nan, B.; Hu, Z.; Xu, C.; Tan, Z.; Zhang, Q.; Meng, H.; Shi, J. Surface coordination of black phosphorene for excellent stability, flame retardancy and thermal conductivity in epoxy resin. Chem. Eng. J. 2020, 397, 125416. [Google Scholar] [CrossRef]
  39. Meinier, R.; Sonnier, R.; Zavaleta, P.; Suard, S.; Ferry, L. Fire behavior of halogen-free flame retardant electrical cables with the cone calorimeter. J. Hazard. Mater. 2018, 342, 306–316. [Google Scholar] [CrossRef]
  40. Liu, Q.; Zhao, Y.; Gao, S.; Yang, X.; Fan, R.; Zhi, M.; Fu, M. Recent advances in the flame retardancy role of graphene and its derivatives in epoxy resin materials. Compos. Pt. A.-Appl. Sci. Manuf. 2021, 149, 106539. [Google Scholar] [CrossRef]
  41. Zhu, M.; Liu, L.; Wang, Z. Iron-phosphorus-nitrogen functionalized reduced graphene oxide for epoxy resin with reduced fire hazards and improved impact toughness. Compos. Part B Eng. 2020, 199, 108283. [Google Scholar] [CrossRef]
  42. Yang, X.; Dong, X.; Liu, M.; Xing, H.; Liu, J.; Chang, H.; Lin, T. Efficient flame-retarded ethylene vinyl acetate composite containing microencapsulated expandable graphite and polyphosphoric acid. Polym. Degrad. Stabil. 2024, 227, 110904. [Google Scholar] [CrossRef]
  43. Wang, W.; Chen, G.; Wu, S.; Liu, Y.; Wang, Q. Solvent-free synthesis of phosphate-containing imidazole fluid for flame retardant one-component epoxy resin with long pot life, low curing temperature and fast curing rate. J. Appl. Polym. Sci. 2023, 140, e53509. [Google Scholar] [CrossRef]
  44. Gao, Z.; Zhang, B.; Yang, C.; Wu, Y. Fabrication of CeO2/carbon molecular sieving membranes for enhanced O2/N2 gas separation. Appl. Surf. Sci. 2024, 649, 159127. [Google Scholar] [CrossRef]
  45. Li, S.; Liu, Y.; Liu, Y.; Wang, Q. Synergistic effect of piperazine pyrophosphate and epoxy-octavinyl silsesquioxane on flame retardancy and mechanical properties of epoxy resin. Compos. Part B-Eng. 2021, 223, 109115. [Google Scholar] [CrossRef]
  46. Zhu, M.; Jia, P.; Sun, P.; Yu, F.; Yang, G.; Hu, Y.; Yu, B.; Song, L.; Wang, B. Core-shell structure antioxidant microencapsulated piperazine pyrophosphate towards improving service performance and fire safety of styrenic thermoplastic elastomer. Compos. Part A-Appl. Sci. Manuf. 2023, 174, 107732. [Google Scholar] [CrossRef]
  47. Yin, L.; Zhang, S.; Huang, Y.; Yan, C.; Du, Y. Cerium contained advanced materials: Shining star under electrocatalysis. Coord. Chem. Rev. 2024, 518, 216111. [Google Scholar] [CrossRef]
  48. Zhu, Y.; Wu, W.; Xu, T.; Xu, H.; Zhong, Y.; Zhang, L.; Ma, Y.; Sui, X.; Wang, B.; Feng, X.; et al. Preparation and characterization of polyphosphazene-based flame retardants with different functional groups. Polym. Degrad. Stabil. 2022, 196, 109815. [Google Scholar] [CrossRef]
  49. Huang, Z.; Wang, Z. Synthesis of a bio-based piperazine phytate flame retardant for epoxy resin with improved flame retardancy and smoke suppression. Polym. Adv. Technol. 2021, 32, 4282–4295. [Google Scholar] [CrossRef]
  50. Wang, X.; Hu, Y.; Song, L.; Xing, W.; Lu, H.; Lv, P.; Jie, G. Flame retardancy and thermal degradation mechanism of epoxy resin composites based on a DOPO substituted organophosphorus oligomer. Polymer 2010, 51, 2435–2445. [Google Scholar] [CrossRef]
  51. Senanayake, R.B.; Gan, H.; Liu, D.; Zhang, J.; Basnayake, A.P.; Heitzmann, M.T.; Varley, R.J. Partially bio-derived phosphazene-tannic acid microspheres as fire retardant additives for an epoxy tannic acid resin system. Compos. Pt. B-Eng. 2024, 287, 111831. [Google Scholar] [CrossRef]
  52. Yang, P.; Ren, M.; Chen, K.; Liang, Y.; Lü, Q.; Zhang, T. Synthesis of a novel silicon-containing epoxy resin and its effect on flame retardancy, thermal, and mechanical properties of thermosetting resins. Mater. Today Commun. 2019, 19, 186–195. [Google Scholar] [CrossRef]
  53. Wen, N.; Zeng, W.; Yang, Y.; Yang, Z.; Li, H.; Li, X.; Li, Q.; Ding, H.; Lei, Z. Preparation of the Intrinsic Flame-Retardant Curing Agent of Inorganic Epoxy Resin Containing Nitrogen and Phosphorus. J. Inorg. Organomet. Polym. Mater. 2022, 32, 412–422. [Google Scholar] [CrossRef]
  54. Shi, X.; Luo, S.; Du, X.; Li, Q.; Cheng, S. Improvement the flame retardancy and thermal conductivity of epoxy composites via melamine polyphosphate-modified carbon nanotubes. Polymers 2022, 14, 3091. [Google Scholar] [CrossRef] [PubMed]
Fire 07 00372 g001
Figure 1. (a) Schematic diagrams for preparation of CeO2@PAPP. SEM images of flame retardants: (b) CeO2, (c) PAPP, and (d) CeO2@PAPP. (e) Photographs of CeO2, PAPP, and CeO2@PAPP before and after ligand modification and self-assembly process. (f) XRD pattern, (g) XPS spectrum, and (h) EDS results of the CeO2@PAPP.
Figure 1. (a) Schematic diagrams for preparation of CeO2@PAPP. SEM images of flame retardants: (b) CeO2, (c) PAPP, and (d) CeO2@PAPP. (e) Photographs of CeO2, PAPP, and CeO2@PAPP before and after ligand modification and self-assembly process. (f) XRD pattern, (g) XPS spectrum, and (h) EDS results of the CeO2@PAPP.
Fire 07 00372 g001
Fire 07 00372 g002
Figure 2. TG curves (a) and DTG curves (b) of epoxy samples under a nitrogen atmosphere.
Figure 2. TG curves (a) and DTG curves (b) of epoxy samples under a nitrogen atmosphere.
Fire 07 00372 g002
Fire 07 00372 g003
Figure 3. (a) LOI value and UL-94 result and cone calorimetric data of EP and its composites: (b) HRR, (c) THR, (d) SPR, (e) TSP, (f) TSR, (g) COPR, (h) CO2PR, and (i) residue yield.
Figure 3. (a) LOI value and UL-94 result and cone calorimetric data of EP and its composites: (b) HRR, (c) THR, (d) SPR, (e) TSP, (f) TSR, (g) COPR, (h) CO2PR, and (i) residue yield.
Fire 07 00372 g003
Fire 07 00372 g004
Figure 4. TG-IR characterization of pyrolysis products. 3D TG-IR images of (a) pure EP and (b) 10CeO2@PAPP/EP. (c) IR spectra of pure EP and 10CeO2@PAPP/EP at the temperature of the maximum loss rate (Tmax).
Figure 4. TG-IR characterization of pyrolysis products. 3D TG-IR images of (a) pure EP and (b) 10CeO2@PAPP/EP. (c) IR spectra of pure EP and 10CeO2@PAPP/EP at the temperature of the maximum loss rate (Tmax).
Fire 07 00372 g004
Fire 07 00372 g005
Figure 5. Total ion chromatogram of (a) EP and (b) 10CeO2@PAPP/EP at 800 °C.
Figure 5. Total ion chromatogram of (a) EP and (b) 10CeO2@PAPP/EP at 800 °C.
Fire 07 00372 g005
Fire 07 00372 g006
Figure 6. Digital photographs and SEM images of the char residues for (a1–a4) EP, (b1–b4) 10PAPP/EP, (c1–c4) 10CeO2/EP, (d1–d4) 10CeO2@PAPP/EP, and (e1–e4) 10CeO2/PAPP/EP.
Figure 6. Digital photographs and SEM images of the char residues for (a1–a4) EP, (b1–b4) 10PAPP/EP, (c1–c4) 10CeO2/EP, (d1–d4) 10CeO2@PAPP/EP, and (e1–e4) 10CeO2/PAPP/EP.
Fire 07 00372 g006
Fire 07 00372 g007
Figure 7. Schematic representation for the flame-retardant mechanism of CeO2 @PAPP/EP composites.
Figure 7. Schematic representation for the flame-retardant mechanism of CeO2 @PAPP/EP composites.
Fire 07 00372 g007
Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Share and Cite

MDPI and ACS Style

Zhao, J.; Wu, Z.; Hong, Y.; Li, H.; Qian, J.; Wu, K.; Xia, Y. Enhancing Flame Retardancy in Epoxy Resin with Clever Self-Assembly Method for Optimizing Interface Interaction via Well-Dispersed Cerium Oxide on Piperazine Pyrophosphate. Fire 2024, 7, 372. https://doi.org/10.3390/fire7110372

AMA Style

Zhao J, Wu Z, Hong Y, Li H, Qian J, Wu K, Xia Y. Enhancing Flame Retardancy in Epoxy Resin with Clever Self-Assembly Method for Optimizing Interface Interaction via Well-Dispersed Cerium Oxide on Piperazine Pyrophosphate. Fire. 2024; 7(11):372. https://doi.org/10.3390/fire7110372

Chicago/Turabian Style

Zhao, Jiajun, Zhengqian Wu, Yutong Hong, Hongyu Li, Junbo Qian, Kailiang Wu, and Yan Xia. 2024. "Enhancing Flame Retardancy in Epoxy Resin with Clever Self-Assembly Method for Optimizing Interface Interaction via Well-Dispersed Cerium Oxide on Piperazine Pyrophosphate" Fire 7, no. 11: 372. https://doi.org/10.3390/fire7110372

APA Style

Zhao, J., Wu, Z., Hong, Y., Li, H., Qian, J., Wu, K., & Xia, Y. (2024). Enhancing Flame Retardancy in Epoxy Resin with Clever Self-Assembly Method for Optimizing Interface Interaction via Well-Dispersed Cerium Oxide on Piperazine Pyrophosphate. Fire, 7(11), 372. https://doi.org/10.3390/fire7110372

Article Metrics

Back to TopTop