Hollow phenolic microspheres (HPMs) with closed hollow structure were introduced into polypropylene/intumescent flame retardant (PP/IFR) composites.On the one hand
induced by IFR in PP
intumescent char was formed; on the other hand
hollow carbon microsphere was formed from HPMs during combustion
and it would be embedded in the backbone of the intumescent char layer.Thus a hierarchically porous char was constructed to regulate the intumescent char layer and to improve the flame retardancy of PP/IFR composites.The flame retardant properties of PP/IFR composites were measured by using the limiting oxygen index (LOI) and vertical burning (UL-94).The thermal degradation behaviors were tested by thermal gravimetric analysis (TGA).The surface temperature was detected by Infrared Thermal Imager during the test of UL-94.The dispersion behaviors of IFR and HPMs and char morphology were observed by scanning electron microscopy (SEM).The results showed that no more than 1 wt% HPMs was dispersed uniformly into PP.The PP composites passed UL-94 V0 test at total addition of 18 wt% IFR and 1 wt% HPMs
while the samples containing 20 wt% IFR or HPMs could not be classified in UL-94 test.This indicated good synergistic effect between IFR and HPMs in PP composites.Observing from the photos after burning
a few char residues were formed for PP/IFR and PP/HPMs but the char volume for PP/IFR/HPMs was increased evidently.TGA results showed that char residues for PP/IFR/HPMs were more than that for other samples.Moreover
HPMs modified the intumescent char morphology well.It caused a dense surface of the char and induced the inside char to form hierarchical porous structure.The intumescent char layer with high quality decreased the superficial temperature quickly
prevented fuel and heat transfer from outside to inside
causing the flame extinguished.It was because of the improved char quantity
modified char quality and hierarchically porous char
which resulted in the increased flame retardant efficiency of PP/IFR/HPMs composites.
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