Phase structure in polypropylene/PA6/SEBS blends

A range of blends based on 70% by weight of polypropylene (PP) with 30% by weight dispersed phase were produced via melt blending in a co-rotating twin screw extruder. The dispersed phase composition was varied from pure Polyamide 6 (PA6) over a range of PA6: SEBS (poly[styrene-b-(ethylene-co- butylene)-b-styrene]) ratios, using both reactive (maleic anhydride grafted) and non-reactive SEBS elastomers. The two-component PP/30%PA6 blend was unstable towards coalescence during melt processing and consequently exhibited a very coarse morphology and poor mechanical properties. Whereas a three-component PP/15%PA6/15%SEBS blend containing non-reactive SEBS exhibited two dispersed phases; a PA6 phase with a size-scale of 1-5 μm and an SEBS phase with a size-scale of ≤ 0.5 μm. As a result both tensile and impact properties were much improved compared to the 70/30 blend. The use of reactive SEBS-g-MA resulted in the formation of dispersed phases consisting of PA6 particles encapsulated with SEBS. Varying the fraction of SEBS-g-MA in the dispersed phase allowed a manipulation of the dispersed phase structure, to form either core-shell PA6/SEBS particles or larger, more complex agglomerated PA6/SEBS structures. The core-shell particles resulted in an increase in charpy impact strength of greater than an order of magnitude, compared to the PP matrix, and almost equivalent tensile properties. The larger agglomerated structures also generated very large increases in impact strength, of up to thirty-fold, but only at the expense of significant reductions in tensile modulus and yield stress.