A novel type of microencapsulated
ammonium polyphosphate (MAPP)
with a triallyl cyanurate (TAC)/SiO2 double-layered shell
was synthesized through in-situ polymerization, followed by the thiol–ene
photograted reaction. With a double bond in the TAC outer shell, MAPP
could be utilized as a highly efficient flame retardant for intumescent
flame-retarded natural rubber (NR). The chemical structure, morphology,
and performance of MAPP were characterized by X-ray photoelectron
spectroscopy, Fourier-transform infrared spectroscopy, water contact
angle (WCA), scanning electron microscopy, and thermogravimetric analysis.
MAPP exhibited a hydrophobic feature with a WCA of 101° because
of its organic outer shell, which made MAPP well dispersed in the
NR matrix as well as enhanced the compatibility between the MAPP and
NR matrix. The resulting intumescent flame-retardant NR compounds
not only achieved excellent flame-retardant performance but also obtain
improved mechanical properties because of the presence of 3D-cross-linking
networks between the MAPP and NR matrix. The NR/MAPP compounds also
presented good water resistance because of the microencapsulation
of APP with an SiO2 inner shell, which provided a waterproof
barrier for APP. Based on the cone calorimetric results, the NR/MAPP
compounds also presented a decline in total heat release and heat
release rate but an improvement in ignition time due to the fact that
both the SiO2 inner layer and TAC outer shell could enhance
the formation of a high-strength thermally stable char layer during
combustion, thus preventing heat transmission and diffusion. The significant
enhancement in flame-retardant performance was principally ascribed
to the synergistic char-forming effect derived from the APP core and
TAC/SiO2 shell. This work provides a new idea for development
of APP-based flame-retardant additives and also explores their potential
applications in intumescent flame-retardant polymeric systems.
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