Toughening behavior and synergistic flame retardant effect of a ternary block copolymer with amorphous morphology and thermal softening property in polypropylene

A novel intumescent flame retardant with ternary block copolymer style not only more effectively enhanced flame retardant efficiency but also dramatically increased mechanical properties of polypropylene (PP) composites compared with traditional compounds. The ternary block copolymers were synthesized by the dehydration reaction of polydiphenylsiloxane, piperazine pyrophosphate oligomer and melamine phosphate. Among them, the ternary block copolymer T4-0.8PSi with particular unit ratio presented best comprehensive effect in PP composites. The LOI value of PP composite T4-0.8PSi/PP with 20 wt% T4-0.8PSi reached 30.2 %, and UL94 level of 1.6 mm spline passed V-0 rating. Compared with binary block copolymer B1 and the mixtures B1/0.8PSi and Mix/0.8PSi with similar contents as T4-0.8PSi, T4-0.8PSi obviously reduced the peak value of heat release rate, total heat release, effective heat of combustion and total smoke release. Particularly, it dramatically increased the residue yield and promoted the formation of denser and compact char layer. The ternary block copolymer T4-0.8PSi with appropriate group ratio generated synergistic charring effect in PP. At the same time, the mechanical properties of T4-0.8PSi/PP were dramatically enhanced and the impact strength was increased by 14.5 % compared with B1. Compared with the two mixtures just mentioned, the impact strength of T4-0.8PSi/PP increased by 48.8 % and 103.3 %, and the elongation at break increased by 601 % and 2056 %, respectively. The silicon-containing unit in ternary block copolymer nearly eliminated crystallinity to form amorphous morphology, which made flame retardants deform to droplet-like particles during processing, thus resulting in the more contact area and stronger interaction between the ternary block copolymer and polymer matrix. The research provided a new way for developing polyolefin materials with both high flame retardancy and excellent mechanical properties.