Limiting oxygen index (LOI) is one of the important indicators to
characterize the combustion performance of polymers. LOI refers to the
volume fraction concentration of oxygen in a mixture of oxygen and
nitrogen when it is just able to support its combustion. In general,
polymers with LOI greater than 27% can be considered flame retardant
polymers.[43] The results (Table 1 ) show
that DABPPI has an LOI of 40.9% and is a good flame retardant polymer.
MMT-PI has an even higher LOI of 67.9%. It can be observed that the
addition of MMT exhibits better performance on the flame retardant
behavior of the developed bioinspired composites. Together, MMT and PI
built a “wall” in the composite, preventing gas from entering and
exiting. This “wall” had a good oxygen barrier, making it difficult
for combustion-supporting gas to enter the interior of the material.
Likewise, the combustible gas thermally decomposed inside was hard to
release to the outside to maintain combustion. Benefiting from the
strong interaction between MMT and PI, the composite presented a nearly
perfect “brick-and-mortar” structure, and the LOI was greatly
improved.
DABPPI burned quickly after being ignited in the LOI test, and its film
frame burned out after the test (Fig. S3 in Supporting
Information). Whereas, after combustion, MMT-PI still retained its basic
framework due to its good flame retardancy. This means that if it is
applied in fireproof coatings, like exterior wall coatings, it may
further delay the damage to the wall, thus giving people more time to
escape.