High-efficient Flame Retardancy and Low Mechanical Performance Impact in Polypropylene Blend Based on Surface Aggregation and Si-P-N Single-molecule Synergy

Flammability and droplet behavior are the key obstacle to the development of polypropylene (PP) materials. Traditional flame retardants (FRs) for this are inefficient and often lead to strong damage in mechanical performance, due to their whole-part distribution and incompatibility in PP blends. To address this, in this work, novel intumescent flame retardants with Si-P-N single-molecule synergy (SMIFRs) were prepared and introduced in PP by melt-blending process. Owing to the low-surface-energy properties of Si, it’s confirmed that SMIFRs could mostly aggregate on the surface, demonstrating high-efficient flame retardancy and obviously enhanced mechanical properties in PP blend. More specifically, with just 3.3 wt% phosphorus content, PP blend can achieve V-0 rating without any melt-droplet phenomenon, exhibiting increased limiting oxygen index (LOI) value by 58% and reduced peak heat release rate (pHRR) by 81%, compared with pure PP. Notably, just because of the surface aggregation that not only decreases the content of SMIFRs but also lowers its internal distribution in PP, the damage of mechanical properties of PP blend is greatly weakened (elongation is 57% higher than the blend prepared by mixed FRs at the same ratio), demonstrating optimal mechanical properties compared with flame-retardant PP blends reported before. This mechanism based on surface aggregation and single-molecule synergy would provide a novel strategy for PP to acquire both high-efficient flame retardancy and good mechanical performance.