- Composite materials demand constant improvements in mechanical and flame retardant (FR) properties. The goal of this project is to study the effect of additives on these properties in polypropylene composites containing glass fibers, pentabromobenzyl acrylate (PBBMA) as a primary FR and magnesium hydroxide as a secondary FR. Optimal composition is reached by means of statistical design of experiments (DOE) rather than by “trial and error” approach. The DOE approach allows minimization of the number of experiments, investigation of the influence of each additive and the mutual interactions between additives. It also allows here, prediction of optimal sample properties better than 8% from experimental values. The optimal composition exhibits improved mechanical and FR properties. Both FRs reduce the impact strength while enhancing flame retardancy. Glass fibers increase the modulus, but have only a moderate effect on the impact strength due to poor adhesion with PP. The interpretation of the effect of glass fibers on the flammability is inconclusive.