Research subject: Polypropylene (PP) is a cost-effective, highly processable polymer; however, its intrinsic flammability limits its use in several industrial applications. Among flame-retardant strategies, intumescent systems are prominent, yet they typically require high loadings to be effective, which can reduce melt flow and mechanical properties. In this study, ammonium polyphosphate (APP) was employed as the primary flame retardant and zinc borate (ZnB) as a synergist to determine the optimal formulation and to evaluate their effects on the flammability, melt flow behavior, and mechanical properties of PP.
Research approach: PP compounds were prepared using a twin-screw extruder containing 20–30 wt% APP and 3 wt% ZnB. Test specimens were obtained by injection molding. UL-94 vertical flammability test, melt flow index (MFI), tensile testing, and impact resistance evaluations were conducted to analyze processing–performance interactions.
Main results: APP alone improved flame resistance, but, even at 30 wt%, was limited to a V-1 rating. Adding 3 wt% ZnB produced a suitable synergistic effect. Formulations containing 25 wt% APP and 3 wt% ZnB achieved a UL-94 V-0 rating, characterized by rapid self-extinguishing and reduced melt dripping, attributed to the formation of a dense char layer. The MFI of the compounds decreased at additive loadings above 25 wt%. While APP reduced the impact resistance of PP, incorporating ZnB into the formulation with 25 wt% APP improved impact strength by 12% and more than doubled the elongation at break, while maintaining a similar range of tensile strength. Overall, the APP/ZnB system in a PP matrix offers a favorable balance between flame retardancy and mechanical properties, making it a suitable option for both industrial and environmentally friendly applications.