FIGURE 5 Digital photos of MMT-PI after combustion test, (a ) is composite side and (b ) is substrate DABPPI side. SEM diagram of (c ) cross-section and (d ) surface of MMT-PI after combustion.
The flame retardancy mechanism of MMT-PI can be inferred by SEM images before and after combustion (Fig. 5 and Fig. S5 in Supporting Information). The layered structure of MMT-PI remained intact after burning, but the thickness increased by about 30 μm (the thickness before combustion was about 45 μm (Fig. S5)) (Fig. 5c ). This could be attributed to the following reasons. MMT is a solid acid catalyst that can rapidly carbonize PI.[44] The carbonized products were attached to MMT, which promotes the higher gas barrier properties of the composite surface. Thereby, combustible gas released by thermal decomposition inside composites was more difficult to escape, resulting in internal expansion and the formation of many bubbles on the surface (Fig. 5a, d ).[45]The combustible substances inside the composite were laborious in contact with oxygen, and the internal oxygen concentration was diluted and reduced, thereby enhancing the flame retardancy of MMT-PI.
In addition, the use of PI as the polymer matrix endows the bionic composite with relatively low WA% (see Supporting Information for details). In contrast, other commonly used polymer matrices for the preparation of bionic composites (such as PVA, CMC, etc.) have high water absorption or can be completely dispersed in water. Low water absorption is beneficial, when applied to fireproof exterior walls, having low water absorption will have less damage on snowy and rainy days.
Surprisingly, MMT-PAAS without thermal imidization also has fairly good flame retardant properties. We prepared MMT-PAAS bioinspired coating on a combustible PU foam. The combustion of PU foam and coating is then tested. PU foam burned out completely when it exploded into the fire (remove the ignition source as soon as the PU foam was ignited) (Fig. 6a, b ). However, for MMT/PAAS-PU, under the same conditions, only the surface exposed to the flame was charred and then self-extinguished, and the rest remained in its original shape (Fig. 6c, d ). After combustion, the MMT-PAAS expanded in the coating layer of MMT/PAAS-PU, but its layered structure was preserved (Fig. 6e, f ). This loose lamellar structure effectively isolated external oxygen and heat,[46] so that the wrapped PU will not be ignited, showing excellent flame retardant performance.