From physical mixtures to block copolymer: Impose outstandingly toughening and flame retardant effect to polypropylene

To explore new flame retardant molecules with higher flame retardancy and toughening effect, a series of non-halogen block copolymer flame retardants PAPO-MP were designed and synthesized. Importantly, compared with the control sample 24(7PAPP/3MPP)/76PP in which the intumescent flame retardant was physically blended, the mechanical properties of 24(7PAPO-3MP)/76PP with the block copolymer was greatly increased. Its impact strength and elongation at break of 24(7PAPO-3MP)/76PP were 55.1 kJ/m2 and 140.6%, increased by 166% and 965% in contrast to those of 24(7PAPP/3MPP)/76PP, respectively. Because the block copolymerized structure led to the low crystalline degree, 7PAPO-3MP can change its particle shape from lamellar to water droplet morphology during thermal processing, which contributed to better compatibility and stronger interaction with PP matrix under the action of maleic anhydride grafted polypropylene, thereby dramatically enhancing the mechanical properties of PP composites. When the addition amounts of 7PAPO-3MP were 20 wt% and 24 wt% in polypropylene (PP) composite, the two flame retardant PP composites all passed UL94 V-0 rating and had the LOI values of 28.1% and 30.2%, respectively. Their peak of heat release rate values was all decreased by over 81% compared with that of neat PP. Moreover, their total heat release and average effective heat of combustion were also decreased and brought higher residue yields. Moreover, the more release of P-containing components in block copolymer also imposed more effective gas-phase quenching effect and flame inhibition effect during combustion. The research on the block copolymer in PP composites revealed a superior way to construct high-performance non-halogen polyolefin materials.