Tensile yield behavior of PP/SEBS blends

Abstract

Tensile yield behavior of the blends of polypropylene (PP) and styrene–ethylene butylene–styrene block copolymer (SEBS) is studied in blend composition range 0–25 wt % SEBS. Three sets of samples, (i) solution-blended compression-molded (SBCM), (ii) melt-blended compression-molded (MBCM), and (iii) melt-blended injection-molded (MBIM), were studied to investigate the relative merits of solution blending and melt blending and the effect of subsequent mixing during injection moulding. Systematic changes with varying blend composition were found in stress–strain behavior in the yield region, viz., in yield stress, yield strain, width of yield peak, and work of yield. Growth of shear bands before necking also showed some systematic variation with blend composition. Shapes and sizes of dispersed-phase (SEBS) domains at various blend compositions were studied by scanning electron microscopy. Analysis of yield stress data on the basis of the various expressions of first power and two-thirds power laws of blend composition dependence and the porosity model (i.e., the exponential law) led to consistent results from all expressions about the variation of stress concentration effect in these sample sets; the stress concentration effect increased in the following order: MBIM < SBCM < MBCM. Furthermore, in addition to revealing relative suitability of the various expressions to the present system, this analysis also showed a transition around the blend composition 5 wt % SEBS from a continuous to a discontinuous structure. Solution blending produces lower degree of discontinuity in the structure of this two-phase blend than the melt blending, and this discontinuity in melt blended samples is reduced on subsequent mixing during injection-molding process.