3.1 Structure characterization
MXene was obtained by etching Al layer from MAX and then delaminated via sonicating (Fig. S1). The diffraction peak at 6.8° in XRD result (Fig. S2) and vibration peaks at 3450 cm− 1, 1368 cm− 1 (-OH), 550 cm− 1 (Ti-O) in FTIR result (Fig. S3) demonstrate the successful preparation of MXene [38]. The synthesis process of M-hydrogel can be divided into two parts: (Ⅰ) PVA, PA, and MXene were mixed in DI water and the mixture was stirred at 95 ℃ for 2 h to make pre-hydrogel. (Ⅱ) Then M-hydrogel could be obtained after the pre-hydrogel frozen in refrigerator overnight and thawed in room temperature. In our previous work, it has been proved PA could enhance mechanical performances of hydrogel by promoting gelation degree [39]. MXene here could make the same contribution as gelation agent which formed more hydrogen bond. To get the M-hydrogel-coated wood, the pre-hydrogel was applied on the surface of wood via blading and the thickness was controlled at 300 µm, then the wood with coating experienced a freeze-thaw cycle to gain M-hydrogel-coated wood (Fig. 1a). The component in hydrogel (PVA, PA, MXene) can formed hydrogen bonds with cellulose of wood, which insure the adhesion between M-hydrogel and wood. All samples are named Mx-hydrogel, in which x represents the content of MXene. The component details of different samples can be seen in Table 1.
The effect on hydrogel with the introduction of MXene was explored by FTIR spectra (Fig. 1b) and XPS (Fig. 1c). It can be seen a distinct characteristic peak in M0 and M0.3 at 962 cm− 1 and 983 cm− 1, which should be assigned to vibration peak of PA [40]. Compared with M0, there are new adsorption peaks appeared at 786 cm− 1 and 668cm− 1 in M0.3. These two characteristic peaks may be due to deformation vibrations when the hydroxyl groups (-OH) contained in PVA and PA interact with the functional group of MXene under the formation of hydrogen bond [41]. XPS shows that except for P (135 ev), C (284 ev), O (532 ev), the characteristic elements of Ti (450 ev) and F (686 ev) arise in M0.3 sample resulting from MXene component. Also, as is shown in Fig. 1h, the results of EDX of M0.3 hydrogel confirms that the ingredients are mixed evenly. All tests above prove the M-hydrogel is synthesized successfully. SEM images reveal the morphology of M-hydrogel-coated and uncoated wood. The surface of pure is rough and many strip cracks could be seen because of the plenty of wood fibers (Fig. 1d). It is noting that appearance of M-hydrogel-coated wood is smooth, and all cracks are sealed which would make a positive contribution to flame retardancy (Fig. e, f). The cross-section image in Fig. 1g shows the coating combined with wood closely at the thickness of 300 µm. It can not only ensure the flame retardancy, but also do no harm to the mechanical property of wood.
3.2 Mechanical and self-healing properties
The mechanical properties of M-hydrogel were evaluated by tensile tests. As is shown in Fig. 2a, with the higher content of MXene, the stress and strain of M-hydrogel increase significantly. The stress and strain of M0, M0.1, M0.2, M0.3 hydrogel could reach 0.076, 0.084, 0.105, 0.124 MPa and 150%, 220%, 280%, 310%, respectively. The enhancement in stress-strain curves of M-hydrogel can be ascribed to more hydrogen bonds and stronger polymer chain entanglement with the importation of MXene [42]. The functional groups in MXene can form hydrogen bonds with PVA and PA, resulting in more hydrogen bonds in M-hydrogel. Besides, MXene can act as gelation agent in M-hydrogel to heighten the entanglement of PVA chains. The two reasons are the main factors contributing to strengthen of M-hydrogel. The tensile elastic modulus and yield stress of M0, M0.1, M0.2, M0.3 hydrogel are 0.048, 0.047, 0.038, 0.036 MPa and 0.017, 0.018, 0.020, 0.023 MPa (Fig. 2b). Lower tensile elastic and higher tensile yield stress modulus means less stiffness of M-hydrogel and it can endure bigger deformation, which are coherent with the results of stress-strain curves. Furthermore, the M0.3 hydrogel could be twisted, knotted, and resist the weight of 500 g which reflect the excellent mechanical properties of M0.3 hydrogel (Fig. S4, S5). As a flame-retardant coating, adhesion is another important aspect to think over. We performed cross-hatching test to investigate adhesion of M0.3 hydrogel coating on wood. From Fig. 2c, there is evenly none coating falling off after the tape test. The result exhibits wonderful adhesion of M0.3 hydrogel coating on wood.
It should be considered that when crack appears, it would have adverse effect on the flame retardancy as dense coating is essential for fireproofing, so self-healing is important. To explore the self-healing principle of M-hydrogel, we cut a cuboid M0.3 hydrogel into two blocks, then the fracture section was contacted closely, and some water was sprayed. Finally, the two blocks of hydrogel self-healed into a whole hydrogel overnight in ambient environment (Fig. S6). The schematic illustration of M-hydrogel self-healing principle is demonstrated in Fig. 2d. When the hydrogel was cut into two pieces, the hydrogen bonds among hydrogel were destroyed. However, as dynamic and reversible non-covalent bonds, hydrogen bonds could be rebuilt when the fracture section contacted with water spraying, which ensure the self-healing property of M-hydrogel [43].
3.3 Water retention and swelling ratio
As a fireproof coating, water is crucial for M-hydrogel in flame retardancy. Conventional hydrogels lost its water easily due to evaporation [44]. In our previous work, we have proved that PV could improve the water retention of PVA hydrogel significantly by bring more hydrogen bonds between PVA and PA [45]. Here MXene was added under the same reason. It is gratifying that all M-hydrogel samples holds it weight over 90% after 15 days under the occasion of 25 ℃ and relative humidity of 60% (Fig. 2e). Further experiments were conducted to investigate the water adsorption capacity of M-hydrogels. The results in Fig. 2f exhibit that with the increasing content of MXene, water capacity of M-hydrogel also increases. The swelling ratio of M0, M0.1, M0.2, M0.3 hydrogel can reach 26.16%, 28.39%, 29.16% and 31.08%, respectively. This phenomenon should be assigned to more hydrogen bonds interaction owing to more MXene and the functional groups in MXene (-F, -OH) endows the M0.3 hydrogel stronger water adsorption capacity [46]. The photographs of M0 and M0.3 hydrogel under different water adsorption are listed in Fig. 2g. When immersed in water for the same time, M0.3 hydrogel is bigger than M0 hydrogel. Other M-hydrogel water capacity image are shown in Fig. S7. All M-hydrogels immersed in water exhibit the same phenomenon, which is coincident with results in Fig. 2f.
3.4 Flame retardancy
UL-94 tests performed to evaluate the flame-retardant rating of M-hydrogel-coated and uncoated wood and the t1, t2 is listed in Table 2. During the test, pure wood (Fig. 3a) was ignited after the first 10 s burning and cannot self-distinguished until burn out, showing its intrinsic flammability and fire hazard and the rating should be ranked to V2. Obviously, all M-hydrogel-coated wood did not be ignited even after the second burn for 10 s as shown in Fig. 3b, c, d, e, which demonstrate the excellent flame retardancy of M-hydrogel coatings. The photos of M-hydrogel-coated and uncoated wood after UL-94 test can be seen in Fig. S8. The related data of all M-hydrogel-coated wood in t1 and t2 are 0 s listed in Table 2, stating all M-hydrogel-coated wood can reach V0 rating.
Open fire tests were conducted to see the effect of fireproof coating intuitively. The samples fixed in clamp were controlled at 5 cm distance from the flame gun (Fig. 3f). In Fig. 3g, after 20 s ignition, there is more intense flame on the surface of pure wood compared with M-hydrogel-coated wood (M0.1 and M0.2 hydrogel -coated samples can be seen in Fig. S9). It is because when the temperature reaches a certain point, the internal moisture of wood evaporates completely, then the surface material begins to gradually decompose carbonized, producing combustible gases and forming a stable flame combustion. But the M-hydrogel coatings contain more water which could effectively delay the process and no damage can be seen from M0 and M0.3 hydrogel-coated wood. Also, the char layer formed by hydrogel coatings are denser than pure wood at 40 s. The SEM images are applied to further evaluate the structure of char layer. It is clearly that the char layer of pure wood (Fig. 3h) is loose and demonstrate fibrous dispersion showing poor heat resistance ability. Significantly, the char layer of M0 hydrogel coating (Fig. 3i) is dense and smooth with sheet layer structure. Though cracks still can be seen, it shows a better heat and O2 resistance capability. As for M0.3 hydrogel-coated wood, the char layer is the densest and expansion structure appeared, exhibiting the best heat and O2 resistance effect in open tests and best fireproof function (Fig. 3j). Moreover, the M-hydrogel fireproof coating could be extended application. When used on RPUF, it exhibits excellent flame retardancy, too (Fig. S10).
CCT test was performed to further evaluate the fire safety of M-hydrogel coating and the main parameters are listed in Table 2. According to the results, the time to ignition (TTI) of pure wood is 32 s while M-hydrogel coated wood of M0, M0.1, M0.2 and M0.3 are prolonged to 64 s, 74 s, 66 s and 69 s, respectively (Fig. 4c). The results show the function of water contained in hydrogel coatings to limit the fire spread from surface to inside of wood. It is further proofed in results of heat release rate (HRR) and total heat release (THR) in Fig. 4 (a, b). There are two peaks in HRR curves of pure wood. When the temperature reaches about 250 ℃, the combustion gases of H2 and CH4 generated along with carbonization of wood surface. These gases are ignited at ignition temperature under heat resource and then fire appears inside resulting in the first peak. When the fire spread to surface of the char layer, no combustion gases decomposed from wood and fuel is applied from volatile matter inside wood. At this circumstance, burning flame changes into flameless combustion and the second peak appears. Compared with pure wood, there are also two peaks in HRR curves of M0 hydrogel-coated wood, but the appearance time of the two peaks is later than pure wood. It is due to M0 hydrogel coating could insulate O2 from contact with wood under heating condition. It is worth noting that there is only one peak appears in HRR curves of M0.1, M0.2 and M0.3 hydrogel-coated wood. This may be attributed to the denser char layer formed with introduction of MXene which significantly increases the heat resistance ability of hydrogel coating, resulting in the burning flame and flameless combustion phenomenon occurs at the same time. Besides, the appearance time of peak heat release rate (pHRR) and total heat release (THR) of pure wood and M0, M0.1, M0.2 and M0.3 hydrogel-coated wood are 70, 75, 90, 95, 95 s and 23.96, 16.41, 14.65, 14.72, 15.30 MJ/m2, respectively. The results exhibit better fireproof ability of hydrogel coating with the introduction of MXene.
Table 2
The related data of UL-94 and CCT.
Sample
|
t1
(s)
|
t2
(s)
|
Rating
|
TTI
(s)
|
TpHRR
(s)
|
HRR (kW/m2)
|
THR (MJ/m2)
|
Pure wood
|
/
|
/
|
V2
|
32
|
70
|
109.25
|
23.96
|
M0
|
0
|
0
|
V0
|
64
|
75
|
64.15
|
16.41
|
M0.1
|
0
|
0
|
V0
|
74
|
90
|
56.54
|
14.65
|
M0.2
|
0
|
0
|
V0
|
66
|
95
|
58.36
|
14.72
|
M0.3
|
0
|
0
|
V0
|
69
|
95
|
63.80
|
15.30
|
t1, t2: The first and second after-flame time during UL-94 tests. TpHRR: The time of the appearance of pHRR.
Table 3
TGA results of M0 and M0.3 hydrogel.
Samples
|
T10% (℃)
|
TI (℃)
|
TII (℃)
|
TIII (℃)
|
TIV (℃)
|
TV (℃)
|
M0
|
130.56
|
130.12
|
162.25
|
310.28
|
435.42
|
644.41
|
M0.3
|
125.86
|
130.68
|
158.33
|
258.35
|
327.06
|
678.45
|
T10%: The temperature of the original mass loss reaches to 10%. TI, TII, TIII, TIV, TV: The temperature of the first, second, third, fourth, fifth peaks appear.
3.5 Thermal degradation behaviors and flame-retardant mechanism
As a critical property, the heat resistance ability of M-hydrogel is evaluated by infrared camera. The M-hydrogel-coated side of the samples is exposed to an open fire, while the temperature of back-side is recorded by an infrared camera and the results can be seen in Fig. 4d. For pure wood, the temperature of back-side rises to 150 ℃ quickly while M0 and M0.3 hydrogel-coated wood show an excellent heat resistance capacitation. At about 100 ℃, the temperature rising of M0 and M0.3 hydrogel-coated wood appears a significant postpone. This is because the water evaporation in hydrogel coating brings most heat. In addition, the temperature of M0.3 hydrogel-coated wood stays under 100 ℃ from 100 s to 175 s, which can be assigned to the denser char layer formed by M0.3 hydrogel coating resisting more heat.
Thermogravimetric analysis (TGA) and differential thermal gravity (DTG) results are supplied to explore the thermal property in Fig. 4 (e, f) and the related data are listed in Table 3. T10% appears at 130.56 and 125.86 ℃ of M0 and M0.3 hydrogels. The decomposition process of M0 and M0.3 hydrogel can be divided into 5 parts. TI and TII at 130.12, 162.25 and 130.68, 158.33 ℃ of M0 and M0.3 hydrogel can be ascribed to the dehydration of hydroxyl of PVA and PA. TIII and TIV at 310.28, 435.42 and 258.35, 327.06 ℃ of M0 and M0.3 hydrogel can be assigned to the decomposition of side chain and main chain of PVA. TV arises at 644.41 and 678.45 ℃ of M0 and M0.3 hydrogel can be explained as the decomposition process of phytate groups in PA [47]. When temperature reaches 800 ℃, the mass left of M0 and M0.3 hydrogel is 26.96% and 30.58%, which demonstrates the enhancement of thermal stability in hydrogel with the introduction of MXene.
Based on all results of the experiments above, the flame-retardant mechanisms can be attributed as following (Fig. 4g): (I) Cooling effect due to the large heat capacity and enthalpy of water evaporation. (II) PA as a phosphorus-based flame retardant, promotes the dehydration of wood into carbon and prolong the red-hot combustion process of wood. (III) Denser and harder char layer due to the introduction of MXene, which insulates most O2 and heat spread.