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. 2a. 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. 2b. 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. 3a and a´. After modification, the LDH@APP and B-LDH@APP display rough morphologies (Fig. 3b and c) and a large amount of regular and homogeneous LDH nanostructures appear on the surface of them in the enlarged images (Fig. 3b´ 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. 5c 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. 5a 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 N2 atmosphere
Samples
|
T5%a (°C)
|
Rmaxb(%/min)
|
W700c (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
|
aThe temperature where 5 wt% of weight was lost. |
bThe maximum weight loss rate. |
cThe 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 (t1 + t2, 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/m2 within 115 s and the pHRR of 4APP/EP slightly decreased to 1129 kW/m2 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. 6c 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/m2.s and 2.4 kW/m2.s, respectively, far lower than pure EP of 10.4 kW/m2.s and 4APP/EP of 10.8 kW/m2.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. 7a) and total smoke production rate (Fig. 7b) 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/m2)
|
1194
|
1129
|
701
|
482
|
Time to PHRR (s)
|
115
|
105
|
165
|
201
|
FIGRA (kW/m2.s)
|
10.4
|
10.8
|
4.2
|
2.4
|
THR (MJ/m2)
|
83.0
|
81.6
|
69.2
|
71.0
|
Peak SPR (m2/s)
|
0.36
|
0.38
|
0.27
|
0.18
|
TSP (m2)
|
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. 8a). 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. 8b), [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. 9a-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. 9c). Interestingly, the residues of 4B-LDH@APP/EP are composed of dense cobweb-like microstructures (Fig. 9d), which can provide a better physical barrier to decrease the heat exchange obviously. Additionally, The Raman curves of these char residues (Fig. 9a´-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 sp2-hybridized carbon’s vibration. [45, 46] The ratio of integrated intensities of D and G bands (IG/ID) is usually applied to assess the graphitization degree of the char residual. A lower ratio of IG/ID 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 ID/IG 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. 10a besides the N, O, and C elements. Meanwhile, as depicted in Fig. 10b, 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
|