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.