Facile Constructing Inorganic Phosphorus/boron-layered Double Hydroxide Complexes for Highly Efficient Fire-safety Epoxy Resin

Facile Constructing Inorganic Phosphorus/boron-layered Double Hydroxide Complexes for Highly Efficient Fire-safety Epoxy Resin | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Facile Constructing Inorganic Phosphorus/boron-layered Double Hydroxide Complexes for Highly Efficient Fire-safety Epoxy Resin Zhu-Bao Shao, Tian-Ci Wang, Xiang Song, Jing Cui, Longxiang Zhu, and 1 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-1999162/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Concerning inorganic flame retardants, the facile fabrication and high-efficiency fire safety without compromising the mechanical property of matrix are still significant challenges. Here, nano-layered double hydroxide containing boron constructed on the surface of ammonium polyphosphate complexes (B-LDH@APP) is prepared by a simple in-situ coprecipitation technology to reduce the fire hazard and improve mechanical performances of epoxy resin (EP). The as-obtained 4B-LDH@APP/EP achieves the UL-94 V-0 rating and presents superior flame-safety performance. With respect to the 4APP/EP, the fire growth rate, the peak heat release rate, and the peak smoke production rate of 4B-LDH@APP/EP decrease by 77.8%, 57.3%, and 52.6%, respectively. The reason is mainly contributed to excellent synergistic flame-retardant effect among boron, LDH, and APP, which can accelerate the generation of compact charring residual with good microstructure during combustion of B-LDH@APP/EP composites. Furthermore, B-LDH@APP slightly affects the mechanical performances of EP matrix due to the upgraded interfacial interaction. Polymer-matrix composites (PMCs) Flame retardancy Mechanical properties Thermal properties Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 1. Introduction Epoxy resin (EP) is flammable and has high smoke and heat production after ignition, [ 1 – 3 ] which raises the flame hazard and essentially restricts application in some fields, such as railway vehicles and aerospace industries. Nowadays, flame-retardant epoxy resin composites are rapidly developed so as to meet the high fire-safety requirements. [ 4 , 5 ] For the commercial flame-retardant epoxy resin composites, both organic phosphorous flame retardants, such as DOPO and DOPO-derives, [ 6 , 7 ] and inorganic flame retardants, such as ATH and ammonium polyphosphate (APP), etc. [ 8 , 9 ] are used to enhance the fire performance of epoxy resin composites. However, the complex prepared process, the massive use of organic solvents in preparation as well as the low smoke suppression capacity of the DOPO-based flame retardants during combustion are still a challenge. [ 10 ] Oppositely, inorganic flame retardants can effectively reduce the heat and smoke production of EP composites, but the flame-retardant efficiency is unsatisfactory and high loadings are needed to obtain the well flame retardancy of EP, [ 11 – 13 ] resulting in the negative impacts on the mechanical performances. To balance the flame-retardant efficiency and mechanical properties of inorganic flame retardants, some organic-inorganic or inorganic-inorganic flame retardants are prepared and investigated. As one of the commercially inorganic flame retardants, APP is widely utilized to be main or synergistic agent of intumescent flame retardants in the majority of polymer materials, for example, polypropylene, [ 14 , 15 ] ethylene vinyl acetate [ 16 , 17 ] as well as epoxy resins [ 18 – 20 ] resulting from the friendly environment and flame-retardant synergism. Similar to other inorganic flame retardants, how to raise the efficiency of APP in flame retardant EP composites attracts the researchers’ attention. Recently, inspired by synergistic charring catalysis effect, plenty of work on the incorporation of APP and metal elements has been done, for instance, APP and/or iron, [ 21 ] zirconium, [ 22 ] zinc, [ 23 ] aluminum, [ 24 ] cobalt, [ 25 ] nickel elements, [ 26 ] etc. Our previous work also illustrates that the iron-modified APP can both obviously decrease the heat release and diminish the smoke product of EP. [ 27 ] However, the less compatibility between APP and metal elements in EP is unfavorable to meet further utilization. Layer double hydrosites (LDHs) with the ordered and well-defined porous structure can be used as the main component of flame retardant for its smoke absorption, catalytic charring, and high thermal stability. [ 28 – 30 ] Meanwhile, the nano-effect of LDHs can improve the mechanical performance of matrix. Especially, several published literature have put the focus on the synergistic effect of LDHs and APP on the thermoplastic and thermoset. [ 31 – 34 ] However, it is noted, the traditional blending strategy is unsatisfactory to further enhance the comprehensive performances of polymer substrates for the poor compatibility and dispersion between LDHs and APP. [ 35 ] Hence, the efficient combined strategy between APP and LDHs requires to be investigated. Herein, a feasible strategy of in-situ growing LDH nanoparticles on the surface of APP is constructed to obtain LDH@APP core-shell hybrid flame retardants. Based on the above, several kinds of LDH@APP core-shell hybrids are prepared through an ionic exchange and self-assemble method by etching APP, which is the precursor of LDH. Besides, it is proven that the LDH nanoparticles were growing up at the surface of APP since the metal cations are chemically introduced through ionic exchange in advance. Furthermore, inorganic boron acid is introduced to improve the charring formation of B-LDH@APP hybrid. As shown in the fire tests, the modified epoxy resin composite containing 4 wt% of B-LDH@APP hybrids shows the lowest heat release value and higher flame-retardant performance than pure APP modified epoxy resin. Meanwhile, B-LDH@APP displays little impact on the mechanical performances of cured EPs owing to the upgraded interfacial interaction between hybrids and EP matrix. 2. Experimental 2.1. Materials Epoxy resin (E-51) is obtained from Nantong Xingchen Synthetic Material Co., Ltd., China; Ammonium polyphosphate (form II) is obtained from Taifeng New-Type Flame Retardants Co., Ltd., China; Cobalt (II) nitrate hexahydrate (Co(NO 3 ) 2 ⋅6H 2 O) and Nickel (II) nitrate hexahydrate (Ni(NO 3 ) 2 ⋅6H 2 O), 4,4´-Diaminodiphenylmethane (DDM), ethanol, and ammonium hydroxide are obtained from China National Pharmaceutical Group Co., Ltd. 2.2. Synthesis of LDH@APP hybrids Typically, 10 g APP was immersed in the 100 mL ethanol with 5 g Co(NO 3 ) 2 ⋅6H 2 O and stirred for 4 h at 25°C. Subsequently, the solution with 5 g Ni(NO 3 ) 2 ⋅6H 2 O and 100 mL ethanol was added and continued stirring for 30 min. After that, the pH was adjusted to 9-9.5 by ammonium hydroxide, and kept reacting for 4 h. Finally, the product was centrifuged, washed, and dried. The synthesis was described in Fig. 1 . 2.3. Synthesis of B-LDH@APP hybrids The previously prepared route of B-LDH@APP was the same as that of LDH@APP before centrifugation and washing. Subsequently, 3 g of boric acid was added into the mixed solvent, adjusting the pH value to about 9 with ammonium hydroxide and stirring at room temperature for 12 h. The product was centrifuged, washed, and dried. The synthesis was described in Fig. 1 . 2.4. Preparation of flame-retardant EP composites The flame retardants were mixed with epoxy resin in a three-roll mixer for 30 min to obtain a uniformly dispersed system. After that, DDM was injected into the collected mixture and stirred at 90°C for 20 min. The as-obtained epoxy mixture was decanted into mold and cured at 120 for 2 h and 150°C for 2 h, respectively. The formulations of the pure EP and flame-retardant EP composites were presented in Table 1 . Table 1 Formulations of B-LDH@APP/EPs and corresponding EPs Samples EP (g) DDM (g) APP (g) LDH@APP (g) B-LDH@APP (g) Pure EP 50.0 14.1 - - - 4APP/EP 50.0 14.1 2.67 - - 4LDH@APP/EP 50.0 14.1 - 2.67 - 1B-LDH@APP/EP 50.0 14.1 - - 0.65 2B-LDH@APP/EP 50.0 14.1 - - 1.31 3B-LDH@APP/EP 50.0 14.1 - - 1.98 4B-LDH@APP/EP 50.0 14.1 - - 2.67 2.5. Characterization The particle sizes were carried on Bettersizer laser particle size analyzer; Elemental loadings of samples were investigated by inductively coupled plasma-atomic emission spectrometry (USA). X-ray diffraction (XRD) were recorded on a diffractometer (XPERT-PRO) with a CuKα (λ = 0.1542 nm) radiation. Scanning electron microscopy (SEM) were employed using FEI Helios NanoLab 600i apparatus. Thermogravimetric analysis (TGA) was obtained by TA 5500 with 10 K/min. The limiting oxygen index (LOI) and vertical burning test were performed on FTT instruments (UK) with ASTM D 2863 − 2013 and ASTM D 3801, respectively. The combustion behavior of samples was experimented by cone calorimeter (UK) with the sample of 100.0 × 100.0 × 3.0 mm 3 according to ISO 5660-1 under the heat radiation of 50 kW/m 2 . Tensile and flexural tests were performed on INSTRON 5967 apparatus according to standard ASTM D638 and ASTM D790 under a speed of 2 mm/min, respectively. Raman spectra were conducted on Nicolet N5700. X-ray photoelectron spectroscopy (XPS) test was carried out an Axis Supra + instrument (Japan). 3. Results And Discussion 3.1. Characterization of LDH@APP hybrids XRD is a commonly utilized method to study the structures and compositions of inorganic hybrids. The XRD spectra of LDH, APP, and B-LDH@APP are shown in Fig. 2 a. Apparently, the signals reveal that B-LDH@APP hybrid is consisted of two substances with respective crystal structures, which match APP and LDH. The strong peaks related to the APP in B-LDH@APP suggest that the crystal APP is remained after modification. XPS is carried out further confirming the structure and composition of LDH@APP and B-LDH@APP hybrids, and the results are shown in Fig. 2 b. Clearly, both Co and Ni elements appear at the surface of LDH@APP and B-LDH@APP. Moreover, with respect to LDH@APP, a new peak of B-LDH@APP located at 190–193 eV is related to the boron element, [ 36 ] suggesting that the B-LDH@APP is successfully synthesized. Additionally, the weak peak of phosphorus element also proves that the APP core is covered by a large amount of LDH nanoparticles, which agree well with those analyzed from XRD patterns. The SEM images of APP, LDH@APP, and B-LDH@APP are presented in Fig. 3 . APP microparticles exhibit a regular shape with smooth surface morphologies in Fig. 3 a and a´ . After modification, the LDH@APP and B-LDH@APP display rough morphologies (Fig. 3 b and c ) and a large amount of regular and homogeneous LDH nanostructures appear on the surface of them in the enlarged images (Fig. 3 b ´ and c´) , while the growth of LDH and boric acid-LDH hybrid does not deteriorate the overall morphology of APP. Moreover, for B-LDH@APP, the anion exchange reaction also does not affect the structure of LDH. Those phenomena reveal that LDH and B-LDH are successfully grown on the surfaces of pure APP. Besides, the uniform distribution of cobalt, nickel, and boron elements of B-LDH@APP in Fig. S1 further proves the successful preparation. In addition, the average particle sizes of B-LDH@APP increase from 17.64 µm (APP) to 19.82 µm. Meanwhile, according to the ICP-OES test, the phosphorus content in B-LDH@APP decreases to 30.5 wt% from 31.8 wt% of APP, the cobalt, nickel, and boron contents increase to 1.2 wt%, 0.6 wt% and 0.7 wt%, respectively. Hence, these results indicate that B-LDH@APP complex is successfully prepared. It can be predicted that the nanoparticles (B-LDH) will be mainly active at the interface of EP and APP. TG test is conducted to characterize the thermal properties of APP, LDH@APP, and B-LDH@APP. As presented in Fig. 4 , pure APP displays two-step thermal degradation from 250°C to 500°C and from 500°C to 700°C, respectively. After modification, the initial degradation of both hybrids of LDH@APP and B-LDH@APP occurs at a lower temperature than pure APP. The phenomena may result from some crystal water of LDH and the acceleration between LDH and APP, [ 37 ] which can be beneficial to the char formation in the early combustion stage of the flame-retardant materials. Moreover, the LDH@APP and B-LDH@APP are tended to be slow thermal decomposition after 300°C, especially, the B-LDH@APP does not display a fast mass loss with respect to pure APP after 450°C, only 0.14 wt%/min at the peak of mass loss is detected during 500–700°C, thus yields the residual of 55.5 wt% at 700°C, higher than that of LDH@APP (36.2 wt%) and APP (27.9 wt%). The results indicate that the possible synergistic effect among APP, LDH, and boron elements can play an important role in thermal stability of high temperature. [ 38 ] 3.2. Thermal stability of flame-retardant EPs The thermal properties of EP, 4APP/EP, 4LDH@APP/EP, and B-LDH@APP/EPs are investigated by TGA, and the curves and characteristic data are summarized in Fig. 5 and Table 2 . Here, cured EPs containing APP and different LDH@APP hybrids present similar thermal-decomposition behavior. The initial decomposition temperature and the maximum weight-loss rates of cured EPs decrease compared with pure EP, and only one weight-loss step exists until 700°C. Interestingly, for B-LDH@APP/EP composites, increasing the contents of B-LDH@APP, the maximum weight loss rates obviously decrease, and the residuals increase ( Fig. 5 c and d) . Accordingly, with the introduction of LDH@APP and B-LDH@APP, the resultant EPs present a lower maximum weight loss rate (0.78%/min of 4LDH@APP/EP and 0.76%/min of 4B-LDH@APP/EP) and higher residual (23.2 wt% of 4LDH@APP/EP and 26.7 wt% of 4B-LDH@APP/EP) ( Fig. 5 a and b) . Here, it is notable noticed that the residual of 4LDH@APP/EP and 4B-LDH@APP/EP are 7.9% and 24.2% higher than that of 4APP/EP, respectively, higher than the theoretical calculation increments (0.3% and 1.1%) according to TG results of Fig. 4 . The phenomena suggest that both the LDH@APP and B-LDH@APP can do well work on the residuals generation of EP comparing with APP. Furthermore, for B-LDH@APP, the charring effect of B-LDH@APP/EPs is superior to LDH@APP/EPs, which nicely matches the thermal-degradation results of LDH@APP and B-LDH@APP. Table 2 Thermogravimetric data of pure EP, APP/EP, LDH@APP/EP and B-LDH@APP/EPs in N 2 atmosphere Samples T 5% a (°C) R max b (%/min) W 700 c (wt%) Pure EP 340 -1.46 15.5 4APP/EP 326 -1.15 21.5 4LDH@APP/EP 321 -0.78 23.2 1B-LDH@APP/EP 329 -1.16 20.3 2B-LDH@APP/EP 321 -0.86 22.4 3B-LDH@APP/EP 320 -0.80 25.0 4B-LDH@APP/EP 318 -0.76 26.7 a The temperature where 5 wt% of weight was lost. b The maximum weight loss rate. c The residual weight at 700°C. 3.3 Fire retardant performance The fire-retardant efficiency of different LDH@APP hybrids in resulted EP composites is evaluated by LOI and UL-94 tests. For pure EP, the LOI value is 25.7%, and it burns fiercely and spreads to the clamp continuously during the vertical burning process, which is the main challenge of inorganic flame retardants applied in epoxy composites. As presented in Table 3 , the introduction of the APP brings little flame retardancy, and the LOI only increases to 26.7%. While the 4LDH@APP/EP enables the LOI value of 28.7% and reaches the UL-94 V-1 level, illustrating evident enhancement on flame resistance. Notably, B-LDH@APP/EPs present well self-extinguishing performances, increasing the loadings of B-LDH@APP helps to increase LOI values and decrease the self-extinguishing time, leading to better fire safety. Especially, the cured EP obtains the LOI value of 29.5% and the UL-94 V-0 level with 4 wt% B-LDH@APP. The comparison results imply that the introduction of the boron element on the LDH@APP can do better work on the flame safety of EP. Table 3 LOI and UL-94 results of B-LDH@APP/EPs and corresponding EPs Sample LOI (%) UL-94 (3.2 mm) Rating Time (t 1 + t 2 , s) Pure EP 25.7 NR > 50 4APP/EP 26.7 NR > 50 4LDH@APP/EP 28.7 V1 12–18 1B-LDH@APP/EP 27.0 NR 15–30 2B-LDH@APP/EP 28.0 V1 14–25 3B-LDH@APP/EP 28.8 V1 11–15 4B-LDH@APP/EP 29.5 V0 4–8 The fire performances of cured EPs are assessed by a cone calorimeter test. Pure epoxy resin and 4APP/EP exhibit high flammability since the fire spreads fast and the cured EP rapid reach the heat release rate peaks (pHRR) after ignition in Fig. 6 and Table 4 . Detailly, the pHRR of pure EP displays 1194 kW/m 2 within 115 s and the pHRR of 4APP/EP slightly decreased to 1129 kW/m 2 within 105 s. Instead, the modified APP hybrids present high flame-retardant efficiency not only in reducing the pHRR of EP composites, but also in the suppression of fire growing at the early stage since the time to pHRR of 4LDH@APP/EP and 4B-LDH@APP/EP are clearly increased. Especially, the time to pHRR of 4B-LDH@APP/EP delays to 201 s. It should be noticed that pure APP can also accelerate the formation of an intumescent char layer of epoxy resin because the epoxy resin can play some role as a charring agent as well. However, the remarkable reduction of pHRR and total heat release (THR) are only detected in 4LDH@APP/EP and 4B-LDH@APP/EP composites, which should be attributed to the physical insulation efficacy of a high-quality intumescent char layer composed of the inorganic degradation products of LDH@APP and B-LDH@APP, thus interrupt efficiently the diffusion of fuel and oxygen. [ 39 , 40 ] The apparent proofs are the mass-loss rate in Fig. 6 c and the increment of char residues after cone calorimeter test in Table 4 . It is found that the mass-loss rates of 4LDH@APP/EP and 4B-LDH@APP/EP are decreased dramatically, especially after ignition, and the 4B-LDH@APP/EP exhibits the lowest mass loss rates among them, implying the best fire safety. Accordingly, 4LDH@APP/EP has 19.5 wt% residues left, which is higher than 16.9 wt% of 4APP/EP. Furthermore, the char residue of 4B-LDH@APP/EP increased to 25.3 wt%, indicating that the small amount of boron acid acts as a cementing agent to accelerate the integrated and continuous residual. Additionally, the fire growth rates (FIGRAs) are introduced to illustrate the fire safety of APP hybrids in flame-retardant epoxy resins in Table 4 , where the FIGRAs of 4LDH@APP/EP and 4B-LDH@APP/EP are 4.2 kW/m 2 .s and 2.4 kW/m 2 .s, respectively, far lower than pure EP of 10.4 kW/m 2 .s and 4APP/EP of 10.8 kW/m 2 .s, implying more time for people to flee from the fire. [ 41 , 42 ] Moreover, the smoke production, including the peak smoke production rate (pSPR) and total smoke production (TSP), is presented as Fig. 7 and Table 4 . Similar to heat release, it should be noted that lower pSPR and TSP values of 4LDH@APP/EP and 4B-LDH@APP/EP composites are obtained than that of pure EP and 4APP/EP. Furthermore, the time to pSPR of 4LDH@APP/EP and 4B-LDH@APP/EP (Fig. 7 a) and total smoke production rate (Fig. 7 b) are apparently increased, demonstrating that LDH and B-LDH modified APP hybrids are more effective to suppress smoke production of epoxy resins than APP individually. Interestingly, the B-LDH@APP does well work on restraining both the heat release and smoke production of epoxy resin in comparison with LDH@APP, implying the introduction of boron element can enhance the fire safety of EP composites. Table 4 Cone calorimeter data of pure EP, 4APP/EP, 4LDH@APP/EP, and 4B-LDH@APP/EP Sample Pure EP 4APP/EP 4LDH@APP/EP 4B-LDH@APP/EP TTI (s) 67 67 66 63 Peak HRR (kW/m 2 ) 1194 1129 701 482 Time to PHRR (s) 115 105 165 201 FIGRA (kW/m 2 .s) 10.4 10.8 4.2 2.4 THR (MJ/m 2 ) 83.0 81.6 69.2 71.0 Peak SPR (m 2 /s) 0.36 0.38 0.27 0.18 TSP (m 2 ) 29.2 29.3 22.7 25.4 Residue (%) 8.1 16.9 19.5 25.3 3.4 Fire-retardant mechanism According to the combustion behaviors, the char residual plays a key role in enhancing the fire safety of 4LDH@APP/EP and 4B-LDH@APP/EP. Thereby, the digital photos of char residuals after cone calorimeter test are displayed as Fig. 8 . Some broken and fragile char residuals of pure EP are left (Fig. 8 a). With the addition of APP, an expanded char layer is obtained due to the ammonia and other non-flammable gases release during combustion. However, some obvious large holes can be observed at the surface of the char layer that provides exchange channels for the combustible volatiles deriving from the inner matrix and the heat feedback from the flame (Fig. 8 b), [ 43 , 44 ] resulting in the low efficiency of pure APP in epoxy resin. As for 4LDH@APP/EP and 4B-LDH@APP/EP composites, two similar compact and high intumescent char layers are obtained, respectively. Furthermore, the char micromorphology of 4LDH@APP/EP and 4B-LDH@APP/EP composites is continuous and compact. It is speculated that LDH and B-LDH have a positive function in facilitating the high-quality intumescent char residual of EP composites. The microstructures of residues are further investigated by SEM and Raman tests. In Fig. 9 a-b, the residues of pure EP and APP/EP represent discontinuous and loose honeycomb structures. After incorporation of LDH@APP, the compact structures increase and little cracks can be observed (Fig. 9 c). Interestingly, the residues of 4B-LDH@APP/EP are composed of dense cobweb-like microstructures (Fig. 9 d), which can provide a better physical barrier to decrease the heat exchange obviously. Additionally, The Raman curves of these char residues (Fig. 9 a ´-d´ ) show two representative peaks at 1350 cm − 1 and 1570 cm − 1 , which are assigned to the D band (disordered graphite or glassy carbons) and G band (graphitized carbons), respectively, due to sp 2 -hybridized carbon’s vibration. [ 45 , 46 ] The ratio of integrated intensities of D and G bands (I G /I D ) is usually applied to assess the graphitization degree of the char residual. A lower ratio of I G /I D corresponds to a higher graphitization degree of the char residue, which can provide a better protective shielding effect for the underlying matrix. [ 47 ] Here, 4B-LDH@APP/EP composite exhibits the lowest I D /I G value of 2.39 among the three samples (3.37 for pure EP, 3.02 for 4APP/EP, and 2.67 for 4LDH@APP/EP composites), revealing the highest graphitization degree in the char residue and glassy-state boric oxide derived from the decomposition of boron acid plays a crucial part in promoting the charred quality of epoxy resins. Additionally, the elemental compositions of charring residual of corresponding EPs after cone calorimeter test are confirmed by XPS, where P, Co, and Ni elements of 4LDH@APP/EP and 4B-LDH@APP/EP appear in Fig. 10 a besides the N, O, and C elements. Meanwhile, as depicted in Fig. 10 b, 3 .2 wt% boron is left on the residual of 4B-LDH@APP/EP, demonstrating the formation of structures containing boron in the residual. Combined with the microstructure of the charring residual, it indicates that the co-existence of P, Co, Ni, and B can accelerate the compactness of charring residual with good microstructure generated during combustion of B-LDH@APP/EP composites, and thus evokes satisfactory fire-safety effect. According to the above analysis, the flame-retardant mechanism is presented as follows. As only a little residue is formed, EP will occur to rapid thermal decomposition and generate considerable heat and smoke. Generally, 4 wt% APP alone owns the limited charring formed ability for EP. While LDH@APP and B-LDH@APP are incorporated, respectively, the excellent synergistic flame-retardant effect among LDH and APP are fabricated from the catalytic charring among LDH, APP, and EP, and the “barrier” effect of LDH, [ 48 – 50 ] especially, for B-LDH@APP, the improving charring formation resulting from the existence of boron, which further enhances the compactness and thermal stability at high temperature of the charring residual. 3.5. Mechanical performance The tensile and flexural strength properties are examined to evaluate the impact of LDHs modified APP hybrids on the mechanical performances of cured EPs (Table 5 ). Pure EP shows a tensile strength of 75.8 MPa and flexural strength of 100.4 MPa. With the introduction of APP, the tensile strength and flexural strength of 4APP/EP decrease to 57.8 MPa and 71.2 MPa due to the less interfacial interaction between APP and EP. However, for LDH@APP and B-LDH@APP, the mechanical performances improve compared with APP/EP, especially, the tensile strength and flexural strength of 4B-LDH@APP/EP increase to 65.5 MPa and 82.2 MPa. The phenomena come from the increasing specific surface area after modification, which leads to more interfacial action sites, enhancing the interfacial interaction of B-LDH@APP and EP. Interestingly, the 4(B-LDH + APP)/EP presents a flexural strength of 75.3 MPa and tensile strength of 59.6 MPa. Consequently, the comparison results illustrate that the in-situ inorganic nano-hybrid strategy is beneficial in maintaining the mechanical performances of epoxy resin resulting from the improved interfacial interaction. Table 5 Mechanical properties of B-LDH@APP/EPs and corresponding EPs Sample Tensile strength (MPa) Flexural strength (MPa) Pure EP 75.8 ± 3.4 100.4 ± 3.3 4APP/EP 57.8 ± 3.1 71.2 ± 2.9 4LDH@APP/EP 64.4 ± 3.8 81.6 ± 3.7 1B-LDH@APP/EP 74.3 ± 3.5 93.4 ± 3.3 2B-LDH@APP/EP 69.4 ± 3.6 88.2 ± 2.8 3B-LDH@APP/EP 67.9 ± 2.7 84.4 ± 2.1 4B-LDH@APP/EP 65.5 ± 3.0 82.2 ± 2.9 4(B-LDH + APP)/EP 59.6 ± 3.2 75.3 ± 3.1 4. Conclusions In this work, the incorporating inorganic phosphorus/boron-layered double hydroxide complexes (B-LDH@APP) is successfully prepared through a simple ionic exchange and self-assemble method. B-LDH@APP slightly affected the mechanical performances of the EP matrix due to the upgraded interfacial interaction. With respect to pure APP and LDH@APP, the investigation of the fire performance shows that B-LDH@APP presents an apparently synergistic effect on smoke and heat suppression property in EP. With the introduction of boron acid, the char residue of 4B-LDH@APP/EP further enhances the charring formation, then more effectively blocking heat and fuel. Accordingly, the challenge of epoxy resin in V-0 class of UL-94 testing is achieved. In conclusion, combining B-LDH and APP opens potential doors to decrease the flame hazard of EP composites. Declarations Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgment This work was partly support by the National Key Research Development Program of China (2021YFB3700201) References Chen ZW, Guo Y, Chu YP, Chen TT, Zhang QW, Li CX, Jiang JC, Chen T, Yu Y, Liu LX. Solvent-free and electron transfer-induced phosphorus and nitrogen-containing heterostructures for multifunctional epoxy resin. Compos. Part B-Eng. 2022; 240: 109999. 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Additional Declarations No competing interests reported. Supplementary Files floatimage11.jpeg Graphical Abstract SupplementaryMaterial.docx Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-1999162","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":132676611,"identity":"978182b1-3d46-4e12-9389-69e0420fc2af","order_by":0,"name":"Zhu-Bao Shao","email":"","orcid":"","institution":"Qingdao University","correspondingAuthor":false,"submittingAuthor":false,"prefix":"","firstName":"Zhu-Bao","middleName":"","lastName":"Shao","suffix":""},{"id":132676612,"identity":"ab31d3ed-0675-4b66-976e-19d0baaa8bdb","order_by":1,"name":"Tian-Ci Wang","email":"","orcid":"","institution":"Qingdao University","correspondingAuthor":false,"submittingAuthor":false,"prefix":"","firstName":"Tian-Ci","middleName":"","lastName":"Wang","suffix":""},{"id":132676613,"identity":"20498a1d-b0a3-41ef-9464-3a34c717cda2","order_by":2,"name":"Xiang Song","email":"","orcid":"","institution":"Qingdao University","correspondingAuthor":false,"submittingAuthor":false,"prefix":"","firstName":"Xiang","middleName":"","lastName":"Song","suffix":""},{"id":132676618,"identity":"b0f161d6-4351-48bc-a808-bd688049961a","order_by":3,"name":"Jing Cui","email":"","orcid":"","institution":"Qingdao University","correspondingAuthor":false,"submittingAuthor":false,"prefix":"","firstName":"Jing","middleName":"","lastName":"Cui","suffix":""},{"id":132676620,"identity":"9ede3ee3-b046-4f9f-9334-8e49acb7a7c8","order_by":4,"name":"Longxiang Zhu","email":"","orcid":"","institution":"Qingdao 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19:29:14","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-1999162/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-1999162/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":25948626,"identity":"b3a4afab-6b2c-4a45-8da8-9d27e1a92d53","added_by":"auto","created_at":"2022-09-01 19:21:32","extension":"jpeg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":112428,"visible":true,"origin":"","legend":"\u003cp\u003eThe synthetic route of LDH@APP and B-LDH@APP.\u003c/p\u003e\u003cp\u003e\u003cbr\u003e\u003c/p\u003e","description":"","filename":"floatimage1.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-1999162/v1/529c44a36869e752151c1c0e.jpeg"},{"id":25948624,"identity":"cfcf5dda-3ec2-4c33-9d56-158c72146b76","added_by":"auto","created_at":"2022-09-01 19:21:32","extension":"jpeg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":84478,"visible":true,"origin":"","legend":"\u003cp\u003eXRD patterns of LDH, APP and B-LDH@APP (a) and XPS survey spectra (b).\u003c/p\u003e\u003cp\u003e\u003cbr\u003e\u003c/p\u003e","description":"","filename":"floatimage2.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-1999162/v1/1c8ef07f7b76ce3f4449bd80.jpeg"},{"id":25949873,"identity":"b2c9e514-eef5-4cc1-8c98-25d7da04cab3","added_by":"auto","created_at":"2022-09-01 19:31:32","extension":"jpeg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":203290,"visible":true,"origin":"","legend":"\u003cp\u003e\tThe morphology of pure APP (a, a´), LDH@APP (b, b´) and B-LDH@APP (c, c´)\u003c/p\u003e","description":"","filename":"floatimage3.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-1999162/v1/a370a194aa27b8fc686b92dc.jpeg"},{"id":25949259,"identity":"7d51b840-d24b-4a7e-bf21-d1ec9731fc9f","added_by":"auto","created_at":"2022-09-01 19:26:32","extension":"jpeg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":58474,"visible":true,"origin":"","legend":"\u003cp\u003eTG (a) and DTG (b) curves of APP, LDH@APP and B-LDH@APP.\u003c/p\u003e\u003cp\u003e\u003cbr\u003e\u003c/p\u003e","description":"","filename":"floatimage4.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-1999162/v1/0adc76b2092567c90b78287f.jpeg"},{"id":25950431,"identity":"3e483c58-bd73-45f6-9d90-0bdcfe519683","added_by":"auto","created_at":"2022-09-01 19:36:32","extension":"jpeg","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":159407,"visible":true,"origin":"","legend":"\u003cp\u003eTG (a, c) and DTG (b, d) curves of pure EP, APP/EP, LDH@APP/EP and B-LDH@APP/EPs.\u003c/p\u003e\u003cp\u003e\u003cbr\u003e\u003c/p\u003e","description":"","filename":"floatimage5.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-1999162/v1/9899b5263e35e93810e7b4f5.jpeg"},{"id":25949265,"identity":"143edff7-a4a0-448c-bf46-8704cbeaa7dc","added_by":"auto","created_at":"2022-09-01 19:26:33","extension":"jpeg","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":101096,"visible":true,"origin":"","legend":"\u003cp\u003eThe heat release rate (a), total heat release (b), and mass loss (c) curves of pure EP, 4APP/EP, 4LDH@APP/EP, and 4B-LDH@APP/EP.\u003c/p\u003e\u003cp\u003e\u003cbr\u003e\u003c/p\u003e","description":"","filename":"floatimage6.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-1999162/v1/eae56b2411d99cadb837855b.jpeg"},{"id":25949876,"identity":"8fadc15a-9f10-4a93-81c9-639c8247cee3","added_by":"auto","created_at":"2022-09-01 19:31:33","extension":"jpeg","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":88493,"visible":true,"origin":"","legend":"\u003cp\u003eThe smoke production rate (a) and total smoke production (b) curves of pure EP, 4APP/EP, 4LDH@APP/EP, and 4B-LDH@APP/EP.\u003c/p\u003e\u003cp\u003e\u003cbr\u003e\u003c/p\u003e","description":"","filename":"floatimage7.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-1999162/v1/fed6dfbca288a05a2a6bd4cf.jpeg"},{"id":25950741,"identity":"184a596d-b480-4cd4-b6d1-9c28ee11601b","added_by":"auto","created_at":"2022-09-01 19:41:32","extension":"jpeg","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":124977,"visible":true,"origin":"","legend":"\u003cp\u003eTop view and front view of char residual of pure EP (a, a´), 4APP/EP (b, b´), 4LDH@APP/EP (c, c´) and 4B-LDH@APP/EP (d, d´) after cone calorimeter test.\u003c/p\u003e\u003cp\u003e\u003cbr\u003e\u003c/p\u003e","description":"","filename":"floatimage8.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-1999162/v1/1ab7046d0fdc86a74a2387de.jpeg"},{"id":25948631,"identity":"bfe75c4e-1d6d-4c56-b69b-c9f5b461f718","added_by":"auto","created_at":"2022-09-01 19:21:33","extension":"jpeg","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":281827,"visible":true,"origin":"","legend":"\u003cp\u003eThe SEM images and Raman spectra of char residual of pure EP (a, a´), 4APP/EP (b, b´), 4LDH@APP/EP (c, c´) and 4B-LDH@APP/EP (d, d´) after cone calorimeter test.\u003c/p\u003e\u003cp\u003e\u003cbr\u003e\u003c/p\u003e","description":"","filename":"floatimage9.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-1999162/v1/a1ae671a1bfc50967256f7d1.jpeg"},{"id":25948634,"identity":"bc117431-cda7-4b2f-abaf-a214a29fdde3","added_by":"auto","created_at":"2022-09-01 19:21:33","extension":"jpeg","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":112295,"visible":true,"origin":"","legend":"\u003cp\u003eThe XPS curves of char residual of pure EP, 4APP/EP, 4LDH@APP/EP and 4B-LDH@APP/EP (a); the B 1s spectrum of 4B-LDH@APP/EP, and contents of 4APP/EP, 4LDH@APP/EP and 4B-LDH@APP/EP (b) after cone calorimeter test.\u003c/p\u003e\u003cp\u003e\u003cbr\u003e\u003c/p\u003e","description":"","filename":"floatimage10.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-1999162/v1/b02a34b61d201f44739905d1.jpeg"},{"id":26153549,"identity":"d1513df1-e730-4257-ac26-862ea41f30c4","added_by":"auto","created_at":"2022-09-07 05:29:24","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1144550,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-1999162/v1/c3759b6a-db36-430d-9688-a807922e310a.pdf"},{"id":25949263,"identity":"e004db34-4e4d-4250-bd24-bc9fd65a36f0","added_by":"auto","created_at":"2022-09-01 19:26:33","extension":"jpeg","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":95049,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eGraphical Abstract\u003c/strong\u003e\u003c/p\u003e\u003cp\u003e\u003cbr\u003e\u003c/p\u003e","description":"","filename":"floatimage11.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-1999162/v1/403ab902a5127c2cce5b5351.jpeg"},{"id":25948630,"identity":"92f12126-1a76-4b6b-aadc-a37faa7b8864","added_by":"auto","created_at":"2022-09-01 19:21:32","extension":"docx","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":344357,"visible":true,"origin":"","legend":"","description":"","filename":"SupplementaryMaterial.docx","url":"https://assets-eu.researchsquare.com/files/rs-1999162/v1/212ab87e205ad65f2e4f31ac.docx"}],"financialInterests":"No competing interests reported.","formattedTitle":"Facile Constructing Inorganic Phosphorus/boron-layered Double Hydroxide Complexes for Highly Efficient Fire-safety Epoxy Resin","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eEpoxy resin (EP) is flammable and has high smoke and heat production after ignition, [\u003cspan additionalcitationids=\"CR2\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e] which raises the flame hazard and essentially restricts application in some fields, such as railway vehicles and aerospace industries. Nowadays, flame-retardant epoxy resin composites are rapidly developed so as to meet the high fire-safety requirements. [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e, \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e] For the commercial flame-retardant epoxy resin composites, both organic phosphorous flame retardants, such as DOPO and DOPO-derives, [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e, \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e] and inorganic flame retardants, such as ATH and ammonium polyphosphate (APP), etc. [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e, \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e] are used to enhance the fire performance of epoxy resin composites. However, the complex prepared process, the massive use of organic solvents in preparation as well as the low smoke suppression capacity of the DOPO-based flame retardants during combustion are still a challenge. [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e] Oppositely, inorganic flame retardants can effectively reduce the heat and smoke production of EP composites, but the flame-retardant efficiency is unsatisfactory and high loadings are needed to obtain the well flame retardancy of EP, [\u003cspan additionalcitationids=\"CR12\" citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e] resulting in the negative impacts on the mechanical performances. To balance the flame-retardant efficiency and mechanical properties of inorganic flame retardants, some organic-inorganic or inorganic-inorganic flame retardants are prepared and investigated.\u003c/p\u003e \u003cp\u003eAs one of the commercially inorganic flame retardants, APP is widely utilized to be main or synergistic agent of intumescent flame retardants in the majority of polymer materials, for example, polypropylene, [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e, \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e] ethylene vinyl acetate [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e, \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e] as well as epoxy resins [\u003cspan additionalcitationids=\"CR19\" citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e] resulting from the friendly environment and flame-retardant synergism. Similar to other inorganic flame retardants, how to raise the efficiency of APP in flame retardant EP composites attracts the researchers\u0026rsquo; attention. Recently, inspired by synergistic charring catalysis effect, plenty of work on the incorporation of APP and metal elements has been done, for instance, APP and/or iron, [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e] zirconium, [\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e] zinc, [\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e] aluminum, [\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e] cobalt, [\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e] nickel elements, [\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e] etc. Our previous work also illustrates that the iron-modified APP can both obviously decrease the heat release and diminish the smoke product of EP. [\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e] However, the less compatibility between APP and metal elements in EP is unfavorable to meet further utilization.\u003c/p\u003e \u003cp\u003eLayer double hydrosites (LDHs) with the ordered and well-defined porous structure can be used as the main component of flame retardant for its smoke absorption, catalytic charring, and high thermal stability. [\u003cspan additionalcitationids=\"CR29\" citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e] Meanwhile, the nano-effect of LDHs can improve the mechanical performance of matrix. Especially, several published literature have put the focus on the synergistic effect of LDHs and APP on the thermoplastic and thermoset. [\u003cspan additionalcitationids=\"CR32 CR33\" citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e] However, it is noted, the traditional blending strategy is unsatisfactory to further enhance the comprehensive performances of polymer substrates for the poor compatibility and dispersion between LDHs and APP. [\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e] Hence, the efficient combined strategy between APP and LDHs requires to be investigated.\u003c/p\u003e \u003cp\u003eHerein, a feasible strategy of in-situ growing LDH nanoparticles on the surface of APP is constructed to obtain LDH@APP core-shell hybrid flame retardants. Based on the above, several kinds of LDH@APP core-shell hybrids are prepared through an ionic exchange and self-assemble method by etching APP, which is the precursor of LDH. Besides, it is proven that the LDH nanoparticles were growing up at the surface of APP since the metal cations are chemically introduced through ionic exchange in advance. Furthermore, inorganic boron acid is introduced to improve the charring formation of B-LDH@APP hybrid. As shown in the fire tests, the modified epoxy resin composite containing 4 wt% of B-LDH@APP hybrids shows the lowest heat release value and higher flame-retardant performance than pure APP modified epoxy resin. Meanwhile, B-LDH@APP displays little impact on the mechanical performances of cured EPs owing to the upgraded interfacial interaction between hybrids and EP matrix.\u003c/p\u003e"},{"header":"2. Experimental","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1. Materials\u003c/h2\u003e \u003cp\u003eEpoxy resin (E-51) is obtained from Nantong Xingchen Synthetic Material Co., Ltd., China; Ammonium polyphosphate (form II) is obtained from Taifeng New-Type Flame Retardants Co., Ltd., China; Cobalt (II) nitrate hexahydrate (Co(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e\u0026sdot;6H\u003csub\u003e2\u003c/sub\u003eO) and Nickel (II) nitrate hexahydrate (Ni(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e\u0026sdot;6H\u003csub\u003e2\u003c/sub\u003eO), 4,4\u0026acute;-Diaminodiphenylmethane (DDM), ethanol, and ammonium hydroxide are obtained from China National Pharmaceutical Group Co., Ltd.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2. Synthesis of LDH@APP hybrids\u003c/h2\u003e \u003cp\u003eTypically, 10 g APP was immersed in the 100 mL ethanol with 5 g Co(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e\u0026sdot;6H\u003csub\u003e2\u003c/sub\u003eO and stirred for 4 h at 25\u0026deg;C. Subsequently, the solution with 5 g Ni(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e\u0026sdot;6H\u003csub\u003e2\u003c/sub\u003eO and 100 mL ethanol was added and continued stirring for 30 min. After that, the pH was adjusted to 9-9.5 by ammonium hydroxide, and kept reacting for 4 h. Finally, the product was centrifuged, washed, and dried. The synthesis was described in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e2.3. Synthesis of B-LDH@APP hybrids\u003c/h2\u003e \u003cp\u003eThe previously prepared route of B-LDH@APP was the same as that of LDH@APP before centrifugation and washing. Subsequently, 3 g of boric acid was added into the mixed solvent, adjusting the pH value to about 9 with ammonium hydroxide and stirring at room temperature for 12 h. The product was centrifuged, washed, and dried. The synthesis was described in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003e2.4. Preparation of flame-retardant EP composites\u003c/h2\u003e \u003cp\u003eThe flame retardants were mixed with epoxy resin in a three-roll mixer for 30 min to obtain a uniformly dispersed system. After that, DDM was injected into the collected mixture and stirred at 90\u0026deg;C for 20 min. The as-obtained epoxy mixture was decanted into mold and cured at 120 for 2 h and 150\u0026deg;C for 2 h, respectively. The formulations of the pure EP and flame-retardant EP composites were presented in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eFormulations of B-LDH@APP/EPs and corresponding EPs\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"6\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSamples\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eEP (g)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eDDM (g)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eAPP (g)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eLDH@APP (g)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eB-LDH@APP (g)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePure EP\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e50.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e14.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e4APP/EP\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e50.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e14.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e2.67\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e4LDH@APP/EP\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e50.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e14.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e2.67\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e1B-LDH@APP/EP\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e50.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e14.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e0.65\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e2B-LDH@APP/EP\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e50.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e14.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e1.31\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e3B-LDH@APP/EP\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e50.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e14.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e1.98\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e4B-LDH@APP/EP\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e50.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e14.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e2.67\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003e2.5. Characterization\u003c/h2\u003e \u003cp\u003eThe particle sizes were carried on Bettersizer laser particle size analyzer; Elemental loadings of samples were investigated by inductively coupled plasma-atomic emission spectrometry (USA). X-ray diffraction (XRD) were recorded on a diffractometer (XPERT-PRO) with a CuKα (λ\u0026thinsp;=\u0026thinsp;0.1542 nm) radiation. Scanning electron microscopy (SEM) were employed using FEI Helios NanoLab 600i apparatus. Thermogravimetric analysis (TGA) was obtained by TA 5500 with 10 K/min. The limiting oxygen index (LOI) and vertical burning test were performed on FTT instruments (UK) with ASTM D 2863\u0026thinsp;\u0026minus;\u0026thinsp;2013 and ASTM D 3801, respectively. The combustion behavior of samples was experimented by cone calorimeter (UK) with the sample of 100.0 \u0026times; 100.0 \u0026times; 3.0 mm\u003csup\u003e3\u003c/sup\u003e according to ISO 5660-1 under the heat radiation of 50 kW/m\u003csup\u003e2\u003c/sup\u003e. Tensile and flexural tests were performed on INSTRON 5967 apparatus according to standard ASTM D638 and ASTM D790 under a speed of 2 mm/min, respectively. Raman spectra were conducted on Nicolet N5700. X-ray photoelectron spectroscopy (XPS) test was carried out an Axis Supra\u0026thinsp;+\u0026thinsp;instrument (Japan).\u003c/p\u003e \u003c/div\u003e"},{"header":"3. Results And Discussion","content":"\u003cdiv class=\"Section2\" id=\"Sec9\"\u003e\n \u003ch2\u003e\u003cstrong\u003e3.1. Characterization of LDH@APP hybrids\u003c/strong\u003e\u003c/h2\u003e\n \u003cp\u003eXRD is a commonly utilized method to study the structures and compositions of inorganic hybrids. The XRD spectra of LDH, APP, and B-LDH@APP are shown in Fig. \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003ea. Apparently, the signals reveal that B-LDH@APP hybrid is consisted of two substances with respective crystal structures, which match APP and LDH. The strong peaks related to the APP in B-LDH@APP suggest that the crystal APP is remained after modification. XPS is carried out further confirming the structure and composition of LDH@APP and B-LDH@APP hybrids, and the results are shown in Fig. \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003eb. Clearly, both Co and Ni elements appear at the surface of LDH@APP and B-LDH@APP. Moreover, with respect to LDH@APP, a new peak of B-LDH@APP located at 190\u0026ndash;193 eV is related to the boron element, [\u003cspan class=\"CitationRef\"\u003e36\u003c/span\u003e] suggesting that the B-LDH@APP is successfully synthesized. Additionally, the weak peak of phosphorus element also proves that the APP core is covered by a large amount of LDH nanoparticles, which agree well with those analyzed from XRD patterns.\u003c/p\u003e\n \u003cp\u003eThe SEM images of APP, LDH@APP, and B-LDH@APP are presented in Fig. \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e. APP microparticles exhibit a regular shape with smooth surface morphologies in Fig. \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ea and \u003cstrong\u003ea\u0026acute;\u003c/strong\u003e. After modification, the LDH@APP and B-LDH@APP display rough morphologies (Fig. \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003eb and \u003cstrong\u003ec\u003c/strong\u003e) and a large amount of regular and homogeneous LDH nanostructures appear on the surface of them in the enlarged images (Fig. \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003eb\u003cstrong\u003e\u0026acute;\u003c/strong\u003e and \u003cstrong\u003ec\u0026acute;)\u003c/strong\u003e, while the growth of LDH and boric acid-LDH hybrid does not deteriorate the overall morphology of APP. Moreover, for B-LDH@APP, the anion exchange reaction also does not affect the structure of LDH. Those phenomena reveal that LDH and B-LDH are successfully grown on the surfaces of pure APP. Besides, the uniform distribution of cobalt, nickel, and boron elements of B-LDH@APP in \u003cstrong\u003eFig. S1\u003c/strong\u003e further proves the successful preparation. In addition, the average particle sizes of B-LDH@APP increase from 17.64 \u0026micro;m (APP) to 19.82 \u0026micro;m. Meanwhile, according to the ICP-OES test, the phosphorus content in B-LDH@APP decreases to 30.5 wt% from 31.8 wt% of APP, the cobalt, nickel, and boron contents increase to 1.2 wt%, 0.6 wt% and 0.7 wt%, respectively. Hence, these results indicate that B-LDH@APP complex is successfully prepared. It can be predicted that the nanoparticles (B-LDH) will be mainly active at the interface of EP and APP.\u003c/p\u003e\n \u003cp\u003eTG test is conducted to characterize the thermal properties of APP, LDH@APP, and B-LDH@APP. As presented in Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003e, pure APP displays two-step thermal degradation from 250\u0026deg;C to 500\u0026deg;C and from 500\u0026deg;C to 700\u0026deg;C, respectively. After modification, the initial degradation of both hybrids of LDH@APP and B-LDH@APP occurs at a lower temperature than pure APP. The phenomena may result from some crystal water of LDH and the acceleration between LDH and APP, [\u003cspan class=\"CitationRef\"\u003e37\u003c/span\u003e] which can be beneficial to the char formation in the early combustion stage of the flame-retardant materials. Moreover, the LDH@APP and B-LDH@APP are tended to be slow thermal decomposition after 300\u0026deg;C, especially, the B-LDH@APP does not display a fast mass loss with respect to pure APP after 450\u0026deg;C, only 0.14 wt%/min at the peak of mass loss is detected during 500\u0026ndash;700\u0026deg;C, thus yields the residual of 55.5 wt% at 700\u0026deg;C, higher than that of LDH@APP (36.2 wt%) and APP (27.9 wt%). The results indicate that the possible synergistic effect among APP, LDH, and boron elements can play an important role in thermal stability of high temperature. [\u003cspan class=\"CitationRef\"\u003e38\u003c/span\u003e]\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv class=\"Section2\" id=\"Sec10\"\u003e\n \u003ch2\u003e3.2. Thermal stability of flame-retardant EPs\u003c/h2\u003e\n \u003cp\u003eThe thermal properties of EP, 4APP/EP, 4LDH@APP/EP, and B-LDH@APP/EPs are investigated by TGA, and the curves and characteristic data are summarized in Fig. \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003e and Table \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e. Here, cured EPs containing APP and different LDH@APP hybrids present similar thermal-decomposition behavior. The initial decomposition temperature and the maximum weight-loss rates of cured EPs decrease compared with pure EP, and only one weight-loss step exists until 700\u0026deg;C. Interestingly, for B-LDH@APP/EP composites, increasing the contents of B-LDH@APP, the maximum weight loss rates obviously decrease, and the residuals increase \u003cstrong\u003e(\u003c/strong\u003eFig. \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003ec and \u003cstrong\u003ed)\u003c/strong\u003e. Accordingly, with the introduction of LDH@APP and B-LDH@APP, the resultant EPs present a lower maximum weight loss rate (0.78%/min of 4LDH@APP/EP and 0.76%/min of 4B-LDH@APP/EP) and higher residual (23.2 wt% of 4LDH@APP/EP and 26.7 wt% of 4B-LDH@APP/EP) \u003cstrong\u003e(\u003c/strong\u003eFig. \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003ea and \u003cstrong\u003eb)\u003c/strong\u003e. Here, it is notable noticed that the residual of 4LDH@APP/EP and 4B-LDH@APP/EP are 7.9% and 24.2% higher than that of 4APP/EP, respectively, higher than the theoretical calculation increments (0.3% and 1.1%) according to TG results of Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003e. The phenomena suggest that both the LDH@APP and B-LDH@APP can do well work on the residuals generation of EP comparing with APP. Furthermore, for B-LDH@APP, the charring effect of B-LDH@APP/EPs is superior to LDH@APP/EPs, which nicely matches the thermal-degradation results of LDH@APP and B-LDH@APP.\u003c/p\u003e\n \u003cdiv class=\"gridtable\"\u003e\n \u003cdiv align=\"char\" class=\"colspec\"\u003e\u003cbr\u003e\u003c/div\u003e\u0026nbsp;\u003ctable border=\"1\" id=\"Tab2\"\u003e\n \u003ccaption language=\"En\"\u003e\n \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e\n \u003cdiv class=\"CaptionContent\"\u003e\n \u003cp\u003eThermogravimetric data of pure EP, APP/EP, LDH@APP/EP and B-LDH@APP/EPs in N\u003csub\u003e2\u003c/sub\u003e atmosphere\u003c/p\u003e\n \u003c/div\u003e\n \u003c/caption\u003e\n \u003ccolgroup cols=\"4\"\u003e\u003c/colgroup\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eSamples\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eT\u003csub\u003e5%\u003c/sub\u003e\u003csup\u003ea\u003c/sup\u003e (\u0026deg;C)\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eR\u003csub\u003emax\u003c/sub\u003e\u003csup\u003eb\u003c/sup\u003e(%/min)\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eW\u003csub\u003e700\u003c/sub\u003e\u003csup\u003ec\u003c/sup\u003e (wt%)\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003c/thead\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003ePure EP\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e340\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e-1.46\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e15.5\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e4APP/EP\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e326\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e-1.15\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e21.5\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e4LDH@APP/EP\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e321\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e-0.78\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e23.2\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1B-LDH@APP/EP\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e329\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e-1.16\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e20.3\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e2B-LDH@APP/EP\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e321\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e-0.86\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e22.4\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e3B-LDH@APP/EP\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e320\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e-0.80\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e25.0\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e4B-LDH@APP/EP\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e318\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e-0.76\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e26.7\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003ctfoot\u003e\n \u003ctr\u003e\n \u003ctd colspan=\"4\"\u003e\u003csup\u003ea\u003c/sup\u003eThe temperature where 5 wt% of weight was lost.\u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd colspan=\"4\"\u003e\u003csup\u003eb\u003c/sup\u003eThe maximum weight loss rate.\u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd colspan=\"4\"\u003e\u003csup\u003ec\u003c/sup\u003eThe residual weight at 700\u0026deg;C.\u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tfoot\u003e\n \u003c/table\u003e\n \u003c/div\u003e\n\u003c/div\u003e\n\u003cdiv class=\"Section2\" id=\"Sec11\"\u003e\n \u003ch2\u003e3.3 Fire retardant performance\u003c/h2\u003e\n \u003cp\u003eThe fire-retardant efficiency of different LDH@APP hybrids in resulted EP composites is evaluated by LOI and UL-94 tests. For pure EP, the LOI value is 25.7%, and it burns fiercely and spreads to the clamp continuously during the vertical burning process, which is the main challenge of inorganic flame retardants applied in epoxy composites. As presented in Table\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e, the introduction of the APP brings little flame retardancy, and the LOI only increases to 26.7%. While the 4LDH@APP/EP enables the LOI value of 28.7% and reaches the UL-94 V-1 level, illustrating evident enhancement on flame resistance. Notably, B-LDH@APP/EPs present well self-extinguishing performances, increasing the loadings of B-LDH@APP helps to increase LOI values and decrease the self-extinguishing time, leading to better fire safety. Especially, the cured EP obtains the LOI value of 29.5% and the UL-94 V-0 level with 4 wt% B-LDH@APP. The comparison results imply that the introduction of the boron element on the LDH@APP can do better work on the flame safety of EP.\u003c/p\u003e\n \u003cdiv class=\"gridtable\"\u003e\u0026nbsp;\u003ctable border=\"1\" id=\"Tab3\"\u003e\n \u003ccaption language=\"En\"\u003e\n \u003cdiv class=\"CaptionNumber\"\u003eTable 3\u003c/div\u003e\n \u003cdiv class=\"CaptionContent\"\u003e\n \u003cp\u003eLOI and UL-94 results of B-LDH@APP/EPs and corresponding EPs\u003c/p\u003e\n \u003c/div\u003e\n \u003c/caption\u003e\n \u003ccolgroup cols=\"4\"\u003e\u003c/colgroup\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth align=\"left\" rowspan=\"2\"\u003e\n \u003cp\u003eSample\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" rowspan=\"2\"\u003e\n \u003cp\u003eLOI (%)\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colspan=\"2\"\u003e\n \u003cp\u003eUL-94 (3.2 mm)\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eRating\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eTime (t\u003csub\u003e1\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;t\u003csub\u003e2\u003c/sub\u003e, s)\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003c/thead\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003ePure EP\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e25.7\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eNR\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u0026gt;\u0026thinsp;50\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e4APP/EP\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e26.7\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eNR\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u0026gt;\u0026thinsp;50\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e4LDH@APP/EP\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e28.7\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eV1\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e12\u0026ndash;18\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1B-LDH@APP/EP\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e27.0\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eNR\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e15\u0026ndash;30\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e2B-LDH@APP/EP\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e28.0\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eV1\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e14\u0026ndash;25\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e3B-LDH@APP/EP\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e28.8\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eV1\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e11\u0026ndash;15\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e4B-LDH@APP/EP\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e29.5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eV0\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e4\u0026ndash;8\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n \u003c/div\u003e\n \u003cp\u003eThe fire performances of cured EPs are assessed by a cone calorimeter test. Pure epoxy resin and 4APP/EP exhibit high flammability since the fire spreads fast and the cured EP rapid reach the heat release rate peaks (pHRR) after ignition in Fig. \u003cspan class=\"InternalRef\"\u003e6\u003c/span\u003e and Table \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003e. Detailly, the pHRR of pure EP displays 1194 kW/m\u003csup\u003e2\u003c/sup\u003e within 115 s and the pHRR of 4APP/EP slightly decreased to 1129 kW/m\u003csup\u003e2\u003c/sup\u003e within 105 s. Instead, the modified APP hybrids present high flame-retardant efficiency not only in reducing the pHRR of EP composites, but also in the suppression of fire growing at the early stage since the time to pHRR of 4LDH@APP/EP and 4B-LDH@APP/EP are clearly increased. Especially, the time to pHRR of 4B-LDH@APP/EP delays to 201 s. It should be noticed that pure APP can also accelerate the formation of an intumescent char layer of epoxy resin because the epoxy resin can play some role as a charring agent as well. However, the remarkable reduction of pHRR and total heat release (THR) are only detected in 4LDH@APP/EP and 4B-LDH@APP/EP composites, which should be attributed to the physical insulation efficacy of a high-quality intumescent char layer composed of the inorganic degradation products of LDH@APP and B-LDH@APP, thus interrupt efficiently the diffusion of fuel and oxygen. [\u003cspan class=\"CitationRef\"\u003e39\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e40\u003c/span\u003e] The apparent proofs are the mass-loss rate in Fig. \u003cspan class=\"InternalRef\"\u003e6\u003c/span\u003ec and the increment of char residues after cone calorimeter test in Table \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003e. It is found that the mass-loss rates of 4LDH@APP/EP and 4B-LDH@APP/EP are decreased dramatically, especially after ignition, and the 4B-LDH@APP/EP exhibits the lowest mass loss rates among them, implying the best fire safety. Accordingly, 4LDH@APP/EP has 19.5 wt% residues left, which is higher than 16.9 wt% of 4APP/EP. Furthermore, the char residue of 4B-LDH@APP/EP increased to 25.3 wt%, indicating that the small amount of boron acid acts as a cementing agent to accelerate the integrated and continuous residual. Additionally, the fire growth rates (FIGRAs) are introduced to illustrate the fire safety of APP hybrids in flame-retardant epoxy resins in Table \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003e, where the FIGRAs of 4LDH@APP/EP and 4B-LDH@APP/EP are 4.2 kW/m\u003csup\u003e2\u003c/sup\u003e.s and 2.4 kW/m\u003csup\u003e2\u003c/sup\u003e.s, respectively, far lower than pure EP of 10.4 kW/m\u003csup\u003e2\u003c/sup\u003e.s and 4APP/EP of 10.8 kW/m\u003csup\u003e2\u003c/sup\u003e.s, implying more time for people to flee from the fire. [\u003cspan class=\"CitationRef\"\u003e41\u003c/span\u003e,\u0026nbsp;\u003cspan class=\"CitationRef\"\u003e42\u003c/span\u003e]\u003c/p\u003e\n \u003cp\u003eMoreover, the smoke production, including the peak smoke production rate (pSPR) and total smoke production (TSP), is presented as Fig. \u003cspan class=\"InternalRef\"\u003e7\u003c/span\u003e and Table \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003e. Similar to heat release, it should be noted that lower pSPR and TSP values of 4LDH@APP/EP and 4B-LDH@APP/EP composites are obtained than that of pure EP and 4APP/EP. Furthermore, the time to pSPR of 4LDH@APP/EP and 4B-LDH@APP/EP (Fig. \u003cspan class=\"InternalRef\"\u003e7\u003c/span\u003ea) and total smoke production rate (Fig. \u003cspan class=\"InternalRef\"\u003e7\u003c/span\u003eb) are apparently increased, demonstrating that LDH and B-LDH modified APP hybrids are more effective to suppress smoke production of epoxy resins than APP individually. Interestingly, the B-LDH@APP does well work on restraining both the heat release and smoke production of epoxy resin in comparison with LDH@APP, implying the introduction of boron element can enhance the fire safety of EP composites.\u003c/p\u003e\n \u003cdiv class=\"gridtable\"\u003e\n \u003cdiv align=\"left\" class=\"colspec\"\u003e\u003cbr\u003e\u003c/div\u003e\u0026nbsp;\u003ctable border=\"1\" id=\"Tab4\"\u003e\n \u003ccaption language=\"En\"\u003e\n \u003cdiv class=\"CaptionNumber\"\u003eTable 4\u003c/div\u003e\n \u003cdiv class=\"CaptionContent\"\u003e\n \u003cp\u003eCone calorimeter data of pure EP, 4APP/EP, 4LDH@APP/EP, and 4B-LDH@APP/EP\u003c/p\u003e\n \u003c/div\u003e\n \u003c/caption\u003e\n \u003ccolgroup cols=\"5\"\u003e\u003c/colgroup\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eSample\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003ePure EP\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003e4APP/EP\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003e4LDH@APP/EP\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003e4B-LDH@APP/EP\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003c/thead\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eTTI (s)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e67\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e67\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e66\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e63\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003ePeak HRR (kW/m\u003csup\u003e2\u003c/sup\u003e)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1194\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1129\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e701\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e482\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eTime to PHRR (s)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e115\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e105\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e165\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e201\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eFIGRA (kW/m\u003csup\u003e2\u003c/sup\u003e.s)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e10.4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e10.8\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e4.2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e2.4\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eTHR (MJ/m\u003csup\u003e2\u003c/sup\u003e)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e83.0\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e81.6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e69.2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e71.0\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003ePeak SPR (m\u003csup\u003e2\u003c/sup\u003e/s)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.36\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.38\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.27\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.18\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eTSP (m\u003csup\u003e2\u003c/sup\u003e)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e29.2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e29.3\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e22.7\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e25.4\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eResidue (%)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e8.1\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e16.9\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e19.5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e25.3\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n \u003c/div\u003e\n\u003c/div\u003e\n\u003cdiv class=\"Section2\" id=\"Sec12\"\u003e\n \u003ch2\u003e3.4 Fire-retardant mechanism\u003c/h2\u003e\n \u003cp\u003eAccording to the combustion behaviors, the char residual plays a key role in enhancing the fire safety of 4LDH@APP/EP and 4B-LDH@APP/EP. Thereby, the digital photos of char residuals after cone calorimeter test are displayed as Fig. \u003cspan class=\"InternalRef\"\u003e8\u003c/span\u003e. Some broken and fragile char residuals of pure EP are left (Fig. \u003cspan class=\"InternalRef\"\u003e8\u003c/span\u003ea). With the addition of APP, an expanded char layer is obtained due to the ammonia and other non-flammable gases release during combustion. However, some obvious large holes can be observed at the surface of the char layer that provides exchange channels for the combustible volatiles deriving from the inner matrix and the heat feedback from the flame (Fig. \u003cspan class=\"InternalRef\"\u003e8\u003c/span\u003eb), [\u003cspan class=\"CitationRef\"\u003e43\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e44\u003c/span\u003e] resulting in the low efficiency of pure APP in epoxy resin. As for 4LDH@APP/EP and 4B-LDH@APP/EP composites, two similar compact and high intumescent char layers are obtained, respectively. Furthermore, the char micromorphology of 4LDH@APP/EP and 4B-LDH@APP/EP composites is continuous and compact. It is speculated that LDH and B-LDH have a positive function in facilitating the high-quality intumescent char residual of EP composites.\u003c/p\u003e\n \u003cp\u003eThe microstructures of residues are further investigated by SEM and Raman tests. In Fig. \u003cspan class=\"InternalRef\"\u003e9\u003c/span\u003ea-b, the residues of pure EP and APP/EP represent discontinuous and loose honeycomb structures. After incorporation of LDH@APP, the compact structures increase and little cracks can be observed (Fig. \u003cspan class=\"InternalRef\"\u003e9\u003c/span\u003ec). Interestingly, the residues of 4B-LDH@APP/EP are composed of dense cobweb-like microstructures (Fig. \u003cspan class=\"InternalRef\"\u003e9\u003c/span\u003ed), which can provide a better physical barrier to decrease the heat exchange obviously. Additionally, The Raman curves of these char residues (Fig. \u003cspan class=\"InternalRef\"\u003e9\u003c/span\u003ea\u003cstrong\u003e\u0026acute;-d\u0026acute;\u003c/strong\u003e) show two representative peaks at 1350 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and 1570 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, which are assigned to the D band (disordered graphite or glassy carbons) and G band (graphitized carbons), respectively, due to sp\u003csup\u003e2\u003c/sup\u003e-hybridized carbon\u0026rsquo;s vibration. [\u003cspan class=\"CitationRef\"\u003e45\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e46\u003c/span\u003e] The ratio of integrated intensities of D and G bands (I\u003csub\u003eG\u003c/sub\u003e/I\u003csub\u003eD\u003c/sub\u003e) is usually applied to assess the graphitization degree of the char residual. A lower ratio of I\u003csub\u003eG\u003c/sub\u003e/I\u003csub\u003eD\u003c/sub\u003e corresponds to a higher graphitization degree of the char residue, which can provide a better protective shielding effect for the underlying matrix. [\u003cspan class=\"CitationRef\"\u003e47\u003c/span\u003e] Here, 4B-LDH@APP/EP composite exhibits the lowest I\u003csub\u003eD\u003c/sub\u003e/I\u003csub\u003eG\u003c/sub\u003e value of 2.39 among the three samples (3.37 for pure EP, 3.02 for 4APP/EP, and 2.67 for 4LDH@APP/EP composites), revealing the highest graphitization degree in the char residue and glassy-state boric oxide derived from the decomposition of boron acid plays a crucial part in promoting the charred quality of epoxy resins.\u003c/p\u003e\n \u003cp\u003eAdditionally, the elemental compositions of charring residual of corresponding EPs after cone calorimeter test are confirmed by XPS, where P, Co, and Ni elements of 4LDH@APP/EP and 4B-LDH@APP/EP appear in Fig. \u003cspan class=\"InternalRef\"\u003e10\u003c/span\u003ea besides the N, O, and C elements. Meanwhile, as depicted in Fig. \u003cspan class=\"InternalRef\"\u003e10\u003c/span\u003eb, \u003cstrong\u003e3\u003c/strong\u003e.2 wt% boron is left on the residual of 4B-LDH@APP/EP, demonstrating the formation of structures containing boron in the residual. Combined with the microstructure of the charring residual, it indicates that the co-existence of P, Co, Ni, and B can accelerate the compactness of charring residual with good microstructure generated during combustion of B-LDH@APP/EP composites, and thus evokes satisfactory fire-safety effect.\u003c/p\u003e\n \u003cp\u003eAccording to the above analysis, the flame-retardant mechanism is presented as follows. As only a little residue is formed, EP will occur to rapid thermal decomposition and generate considerable heat and smoke. Generally, 4 wt% APP alone owns the limited charring formed ability for EP. While LDH@APP and B-LDH@APP are incorporated, respectively, the excellent synergistic flame-retardant effect among LDH and APP are fabricated from the catalytic charring among LDH, APP, and EP, and the \u0026ldquo;barrier\u0026rdquo; effect of LDH, [\u003cspan class=\"CitationRef\"\u003e48\u003c/span\u003e\u0026ndash;\u003cspan class=\"CitationRef\"\u003e50\u003c/span\u003e] especially, for B-LDH@APP, the improving charring formation resulting from the existence of boron, which further enhances the compactness and thermal stability at high temperature of the charring residual.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv class=\"Section2\" id=\"Sec13\"\u003e\n \u003ch2\u003e3.5. Mechanical performance\u003c/h2\u003e\n \u003cp\u003eThe tensile and flexural strength properties are examined to evaluate the impact of LDHs modified APP hybrids on the mechanical performances of cured EPs (Table \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003e). Pure EP shows a tensile strength of 75.8 MPa and flexural strength of 100.4 MPa. With the introduction of APP, the tensile strength and flexural strength of 4APP/EP decrease to 57.8 MPa and 71.2 MPa due to the less interfacial interaction between APP and EP. However, for LDH@APP and B-LDH@APP, the mechanical performances improve compared with APP/EP, especially, the tensile strength and flexural strength of 4B-LDH@APP/EP increase to 65.5 MPa and 82.2 MPa. The phenomena come from the increasing specific surface area after modification, which leads to more interfacial action sites, enhancing the interfacial interaction of B-LDH@APP and EP. Interestingly, the 4(B-LDH\u0026thinsp;+\u0026thinsp;APP)/EP presents a flexural strength of 75.3 MPa and tensile strength of 59.6 MPa. Consequently, the comparison results illustrate that the in-situ inorganic nano-hybrid strategy is beneficial in maintaining the mechanical performances of epoxy resin resulting from the improved interfacial interaction.\u003c/p\u003e\n \u003cdiv class=\"gridtable\"\u003e\u0026nbsp;\u003ctable border=\"1\" id=\"Tab5\"\u003e\n \u003ccaption language=\"En\"\u003e\n \u003cdiv class=\"CaptionNumber\"\u003eTable 5\u003c/div\u003e\n \u003cdiv class=\"CaptionContent\"\u003e\n \u003cp\u003eMechanical properties of B-LDH@APP/EPs and corresponding EPs\u003c/p\u003e\n \u003c/div\u003e\n \u003c/caption\u003e\n \u003ccolgroup cols=\"3\"\u003e\u003c/colgroup\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eSample\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eTensile strength (MPa)\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eFlexural strength (MPa)\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003c/thead\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003ePure EP\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e75.8\u0026thinsp;\u0026plusmn;\u0026thinsp;3.4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e100.4\u0026thinsp;\u0026plusmn;\u0026thinsp;3.3\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e4APP/EP\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e57.8\u0026thinsp;\u0026plusmn;\u0026thinsp;3.1\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e71.2\u0026thinsp;\u0026plusmn;\u0026thinsp;2.9\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e4LDH@APP/EP\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e64.4\u0026thinsp;\u0026plusmn;\u0026thinsp;3.8\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e81.6\u0026thinsp;\u0026plusmn;\u0026thinsp;3.7\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1B-LDH@APP/EP\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e74.3\u0026thinsp;\u0026plusmn;\u0026thinsp;3.5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e93.4\u0026thinsp;\u0026plusmn;\u0026thinsp;3.3\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e2B-LDH@APP/EP\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e69.4\u0026thinsp;\u0026plusmn;\u0026thinsp;3.6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e88.2\u0026thinsp;\u0026plusmn;\u0026thinsp;2.8\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e3B-LDH@APP/EP\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e67.9\u0026thinsp;\u0026plusmn;\u0026thinsp;2.7\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e84.4\u0026thinsp;\u0026plusmn;\u0026thinsp;2.1\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e4B-LDH@APP/EP\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e65.5\u0026thinsp;\u0026plusmn;\u0026thinsp;3.0\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e82.2\u0026thinsp;\u0026plusmn;\u0026thinsp;2.9\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e4(B-LDH\u0026thinsp;+\u0026thinsp;APP)/EP\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e59.6\u0026thinsp;\u0026plusmn;\u0026thinsp;3.2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e75.3\u0026thinsp;\u0026plusmn;\u0026thinsp;3.1\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n \u003c/div\u003e\n\u003c/div\u003e"},{"header":"4. Conclusions","content":"\u003cp\u003eIn this work, the incorporating inorganic phosphorus/boron-layered double hydroxide complexes (B-LDH@APP) is successfully prepared through a simple ionic exchange and self-assemble method. B-LDH@APP slightly affected the mechanical performances of the EP matrix due to the upgraded interfacial interaction. With respect to pure APP and LDH@APP, the investigation of the fire performance shows that B-LDH@APP presents an apparently synergistic effect on smoke and heat suppression property in EP. With the introduction of boron acid, the char residue of 4B-LDH@APP/EP further enhances the charring formation, then more effectively blocking heat and fuel. Accordingly, the challenge of epoxy resin in V-0 class of UL-94 testing is achieved. In conclusion, combining B-LDH and APP opens potential doors to decrease the flame hazard of EP composites.\u003c/p\u003e"},{"header":"Declarations","content":" \u003cp\u003e \u003cstrong\u003eDeclaration of Competing Interest\u003c/strong\u003e \u003cp\u003eThe authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.\u003c/p\u003e \u003c/p\u003e\u003ch2\u003eAcknowledgment\u003c/h2\u003e \u003cp\u003eThis work was partly support by the National Key Research Development Program of China (2021YFB3700201)\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eChen ZW, Guo Y, Chu YP, Chen TT, Zhang QW, Li CX, Jiang JC, Chen T, Yu Y, Liu LX. Solvent-free and electron transfer-induced phosphorus and nitrogen-containing heterostructures for multifunctional epoxy resin. Compos. 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S. 2022; 157: 106912.\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Polymer-matrix composites (PMCs), Flame retardancy, Mechanical properties, Thermal properties","lastPublishedDoi":"10.21203/rs.3.rs-1999162/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-1999162/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eConcerning inorganic flame retardants, the facile fabrication and high-efficiency fire safety without compromising the mechanical property of matrix are still significant challenges. Here, nano-layered double hydroxide containing boron constructed on the surface of ammonium polyphosphate complexes (B-LDH@APP) is prepared by a simple in-situ coprecipitation technology to reduce the fire hazard and improve mechanical performances of epoxy resin (EP). The as-obtained 4B-LDH@APP/EP achieves the UL-94 V-0 rating and presents superior flame-safety performance. With respect to the 4APP/EP, the fire growth rate, the peak heat release rate, and the peak smoke production rate of 4B-LDH@APP/EP decrease by 77.8%, 57.3%, and 52.6%, respectively. The reason is mainly contributed to excellent synergistic flame-retardant effect among boron, LDH, and APP, which can accelerate the generation of compact charring residual with good microstructure during combustion of B-LDH@APP/EP composites. Furthermore, B-LDH@APP slightly affects the mechanical performances of EP matrix due to the upgraded interfacial interaction.\u003c/p\u003e","manuscriptTitle":"Facile Constructing Inorganic Phosphorus/boron-layered Double Hydroxide Complexes for Highly Efficient Fire-safety Epoxy Resin","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2022-09-01 19:21:30","doi":"10.21203/rs.3.rs-1999162/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"f1f29f16-7461-43b7-b125-e1019c25cfcb","owner":[],"postedDate":"September 1st, 2022","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2022-09-07T05:29:19+00:00","versionOfRecord":[],"versionCreatedAt":"2022-09-01 19:21:30","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-1999162","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-1999162","identity":"rs-1999162","version":["v1"]},"buildId":"ehx78VzkSd0WSzXnipQa-","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}