Characterization of the ENR/PLA/CS/GP composites
SEM imaging analysis (Fig. 1) indicated that the brittle section of the ENR/PLA composite displayed a smooth surface and good compatibility. Similarly, after the addition of GP/CS, the brittle section of ENR/PLA/CS/GP composite is smooth and flat, and still shows good compatibility.
As depicted in Fig. 2, the addition of GP/CS resulted in a shift of the C = O and C-O-C stretching vibrations corresponding to the 1723cm− 1 and 1160cm− 1 absorption peaks in the sample towards lower wave numbers when compared to ENR/PLA composites. This shift could be attributed to the formation of hydrogen bonds between the amino group on CS/GP and the epoxy group on ENR[31, 32].
Figure 3(a) and 3(b) demonstrated the influence of GP on the tensile properties of ENR/PLA composites, while Fig. 3c and 3d illustrated the influence of CS. With the increase of GP content, the tensile strength of ENR/PLA/CS/GP composites exhibited an initial rise followed by a subsequent decline, in accordance with the behavior observed for ENR/PLA/CS/GP composites as CS content increases. The tensile strength of ENR/PLA/CS2/GP4 composites could achieve a maximum value of 4.22 MPa. The enhancement of tensile strength could be ascribed to the formation of hydrogen bonds between CS/GP and functional groups on ENR. However, the reason for the decrease of mechanical properties was that excessive GP/CS restricted the movement of ENR/PLA chain.
Self-healing behavior of the ENR/PLA/CS/GP composites
Figure 4(a) and 4(b) investigated the effects of time and temperature on the self-healing behavior of composite materials. The results demonstrated that prolonging repair time and increasing temperature could effectively enhance the tensile stress of ENR/PLA composites. During the experiment, it was found that too high healing time and temperature would lead to the deformation of the composite. Ultimately, a self-healing treatment at 130℃ for 3h was performed.
Figure 4(c) and 4(d) depicted the tensile stress and healing efficiency of composites with varying CS/GP contents at a temperature of 130℃ for 3 h. The tensile stress and healing efficiency of ENR/PLA/CS/GP composites exhibited an upward trend with increasing CS/GP content. Among all, ENR/PLA/CS2/GP4 composites demonstrated the most remarkable healing effect, achieving a healing rate as high as 78%.
In order to prove the healing behavior of ENR/PLA/CS/GP composites, the following experiments were carried out. The results presented in Fig. 5(a) demonstrate that the composites exhibited favorable tensile strain properties after healing. Additionally, Fig. 5(b) showed that the composite material maintained its structural integrity in an A-shape after healing. Moreover, Fig. 5(c) and 5(d) indicated that the composite could withstand a weight of 1.5KG after healing. Finally, Fig. 5(e) depicted the repair process of the composite material at different time intervals. The incision marked gradually faded after 0.5h of repair and were almost completely eliminated after 1h.
Figure 6 showed the self-healing mechanism of ENR/PLA/CS/GP composites. During the healing process, ENR and PLA chains diffused and interpenetrated at the interface. Then, the rearrangement of the cross-linked network could be activated by dynamic exchange between ENR and PLA chains[33]. Finally, the chain was completely entangled at the interface, which lead to the completion of the healing process. Hydrogen bonds were formed between amino groups on CS and GP and epoxy groups on ENR, which promoted the rearrangement of cross-linked networks, thus enhancing the healing properties of composites.
Flame retardancy of the ENR/PLA/CS/GP composites
The pyrolysis behavior of ENR/PLA/CS/GP composites in N2 atmosphere was studied by TG and DTG, as shown in Fig. 7. The ENR/PLA composite experienced 10%(T10%) and the maximum mass loss (Tmax) at 356.5℃ and 380.1℃, respectively. For ENR/PLA/CS/GP composites, decomposition temperatures appeared at 340.1℃(T10%) and 361.2℃(Tmax). The carbon residue content of ENR/PLA/CS/GP composites was higher than that of ENR/PLA composites. The observed phenomena can be attributed to the decomposition of GP, which causes the matrix to become carbonized and release gas, thereby accelerating the pyrolysis of composite materials [34]. CS, acting as a carbon forming agent, increases the amount of residual carbon in the composite material after pyrolysis [25].
A series of combustion parameters, such as heat release rate (HRR), total heat release rate (THR), total smoke production rate (TSP) and so on, were generated during the combustion of the composites by using a cone calorimeter. These parameters could be used to evaluate the combustion behavior of materials during combustion. Figure 8(a) showed that the content of GP increased from 2–5%, and the PHRR of composites gradually decreased. The PHRR of ENR/PLA/CS2/GP5 composite was 40.3% lower than that of ENR/PLA composite. Figure 8(b) showed that the content of CS increased from 1–4%, but the PHRR of the composite changed ignorable. In the combustion process, CS played the role of char-forming agent, but the content of GP was the key factor affecting the PHRR of ENR/PLA/CS/GP composites. Figure 8(c) and 8(d) showed that the addition of GP and CS effectively reduced the THR of the ENR/PLA composite. CS and GP promoted the formation of dense carbon layer of composites, thus reducing THR.
Table 1
Combustion parameters of ENR/ PLA composites.
samples | PHRR (KW·m− 2) | THR (MJ·m− 2) | TSP (m2) |
ENR/PLA | 2367.96 | 79.37 | 13.56 |
CS2/GP2/ENR/PLA | 1930.12 | 77.68 | 20.87 |
CS2/GP3/ENR/PLA | 1754.26 | 75.82 | 19.83 |
CS2/GP4/ENR/PLA | 1519.46 | 72.15 | 21.53 |
CS2/GP5/ENR/PLA | 1412.90 | 72.89 | 19.75 |
CS1/GP4/ENR/PLA | 1502.81 | 73.79 | 18.76 |
CS3/GP4/ENR/PLA | 1678.84 | 71.63 | 19.59 |
CS4/GP4/ENR/PLA | 1682.47 | 72.10 | 19.90 |
The carbon layer of the composites after cone calorimeter was studied. As shown in Fig. 9, it could be found that pure ENR/PLA was almost burned out. However, ENR/PLA/CS/GP composites had more residual carbon phases, which could effectively protect the substrate and prevent fire propagation.
The morphology of the residual carbon was studied by scanning electron microscopy (SEM) as shown in Fig. 10. The residual carbon of ENR/PLA composite was brittle and cracked, which was difficult to protect the internal material during combustion. The addition of CS and GP could promote the formation of continuous and compact char layers structure during the combustion process. These dense char layers acted as effective insulation, preventing the transfer of heat and oxygen.
As shown in Fig. 11, all the samples showed a D band at 1349 cm− 1 and a G band at 1590 cm− 1. The ratio of D peak to G peak of ENR/PLA/CS2/GP4 composite is smaller than that of ENR/PLA composite. Generally speaking, the smaller the ratio of peak D to peak G, the higher the degree of graphitization of the material, the more conducive to the improvement of flame-retardant effect.
As shown in Fig. 12, when the composite material received heat from a heat source, the GP in IFR undergoes deamination reaction due to its high decomposition activity in the low temperature range. In this process, GP decomposed to produce phosphoric acid. With the increase of temperature, the composites began to melt with the operation of IFR system. Pyrophosphoric acid formed by condensation polymerization of phosphoric acid promoted dehydration of carbon source into carbon. In this process, CS began to dehydrate to form a carbon layer and cover the surface of molten ENR/PLA[25]. The continuous dense carbon layer could isolate some oxygen and heat. The whole reaction process was accompanied by the reaction of free radicals. PO free radicals were produced in the gas phase, which could inhibit the combustion of free radical reaction. At the same time, inert gases such as ammonia also had the function of diluting combustible volatiles in gas phase[35].