Dendritic copolymers from P-, N- and Si-based monomer and melamine phosphate generate thermal deformation toughening and a rapid charring flame retardant effect in polypropylene

In this work, a novel dendritic copolymer PNSi-co-MP as an intumescent flame retardant was fabricated using a silicon-, phosphorus- and nitrogen-containing monomer (PNSi) and melamine phosphate (MP). Random copolymerization way disrupted the regularity of monomer arrangement, resulting in the thermal deformation behavior of PNSi-co-MP being different from the self-polymerized macromolecule (MPNSi) / melamine polyphosphate (MPP) system. Consequently, PNSi-co-MP was distributed reasonably evenly in the polypropylene (PP) matrix, and some of the PNSi-co-MP particles deformed to 200–300 nm from 5 to 10 μm. More importantly, the toughness of PNSi-co-MP/PP was dramatically increased compared with MPNSi/MPP/PP. Specifically, 16PNSi-co-4MP/PP exhibited a 278% higher elongation at break and a 33.3% higher unnotched impact strength compared to 16MPNSi/4MPP/PP. Furthermore, PNSi-co-MP exhibited a rapid charring effect in PP. In a cone calorimeter test, the maximum combustion intensity of the composite was inhibited in only 17 s by the PNSi-co-MP molecule. Compared with the pure PP, the composite with 20 wt% 12PNSi-co-8MP showed a 93.6% lower peak heat release rate and an 87.2% lower smoke production rate, indicating its excellent flame retardancy. The composites containing copolymer molecules also performed much higher flame retardancy than MPNSi/MPP/PP sample, although their chemical composition was almost the same except for the linking form of flame-retardant monomers. PNSi-co-MP simultaneously endowed PP with a higher limited oxygen index, glow wire flammability index, glow wire ignition temperature, residue yield, and fire safety performance than MPNSi/MPP. The reason for this was that the copolymerization strategy aggregated the flame-retardant monomers into one molecule, thus effectively promoting the charring reaction. Accordingly, this work provided a new method for preparing flame-retardant molecules with comprehensive performance through molecular composition and structural design.