Roles of anion of polyoxometalate-based ionic liquids in properties of intumescent flame retardant polypropylene

Three polyoxometalates-based ionic liquid (PIL) hybrid materials with different anions were prepared and introduced into polypropylene/intumescent flame retardants (PP/IFR) system to modify its flame retardant efficiency. It is found that the anions of PILs play great roles in the flame retardant properties of PP/IFR composites. The synergistic effect of [BMIm]3PW (PIL1) on the flame retardant properties of PP composites is better than that of [BMIm]4SiW (PIL3), and [BMIm]3PMo (PIL2) has the best suitability to PP/IFR composites. The PP composite with 15 wt% total amount of PIL2 and IFR has a LOI 28.0 and passes the UL-94 V-0 test without melt-dripping, while that for PP/IFR is only 23.7 and not classified at the same content. The morphology of char residue reveals that the PIL2 promotes the reaction between APP and PER, accelerates the formation of a compact and continuous charring layer and enhances the char accumulating degree that is the inherent reason for good flame retardant properties. However, PIL1 and PIL3 show poor synergistic effects on charring behavior. A good char layer can protect the inside matrix from the erosion of heat and gas, resulting in good flame retardant properties.