Effect of rubber functionality on mechanical and fracture properties of impact-modified nylon 6,6/polypropylene blends

Tensile and fracture properties of PA6,6/PP blends with 20wt% styrene-ethylene/butylene-styrene triblock copolymers (SEBS) grafted with different levels of maliec anhydride (MA) were studied. It was found that tensile strength and stiffness increased with MA level in the 75/25 PA6,6/PP blends but an opposite trend occurred in the 50/50 PA6,6/PP regime. The opposite trend in tensile properties arose from the migration of maleated copolymers from PP phase to the nylon domains when a phase inversion occurred in the 50/50 regime where PP became the dominant phase. An intrinsic strengthening behavior in the interpenetrating network formed by the SEBS copolymers was suggested as the cause for stiffening in the nylon-rich regime but weakening in the PP continuous phase as MA level increased. Fracture toughness of the PA6,6/PP blends containing 20 wt% SEBS was dramatically altered by the MA level. It was believed that two competing factors based on the Ramberg-Osgood framework, viz., yield stress (σ_y) and yield strain (ε_y), were responsible for the toughness increase in PA6,6/PP system. When σ_y was maintained relatively high and steady, ε_y became the overriding factor in 75/25 PA6,6/PP blends; whereas in the case of relatively low and drastic reduction in σ_y, toughness became driven by σ_y alone.