The characterization of rheological properties of melt grafting polypropylene for foaming

Large enhancements of the melt strength of polypropylene (PP) were achieved by the introduction of specific unsaturated linear polyester (ULP) branches using melt grafting. The transient torque curves and optical rheology microscope images indicated that branching reactions took place and the ULP had been grafted onto the PP backbone. Shear rheological behaviors of three kinds of PP were investigated using rotational rheometer under dynamic shear mode with periodic shear rate. These PP samples are foamable PP (FPP) with sparse branches obtained by grafting ULP, commercial high melt strength PP (HMS PP) for foaming and conventional linear PP (EPS). It was found that the rheological properties of FPP, the HMS PP, were distinctly different from those of conventional PP. Storage modulus, steady state compliance and zero shear viscosity increased in comparison with EPS, while shear viscosity decreased. This result implied the presence of branching structures that was not revealed in conventional PP. In melt flow measurements, extrusion swell that was a prominent behavior of branching PP was observed also for FPP and PF. Compared to linear PP, FPP and PF showed distinct sag-resistant property and lower melt flow index. On the other hand, to estimate the extent of branching, a detailed method was applied using the obtained zero shear viscosity. The result showed that FPP was grafted by sparse ULP. From these results, it was found that FPP showed obvious enhancements in rheological behaviors similar to PF, although its melt strength was lower than that of PF due to the presence of shorter branching chains grafted on the backbone of FPP.