Crystallization,mechanical, thermal and rheological properties of polypropylene composites reinforced by magnesium oxysulfate whisker

Polypropylene (PP) composites containing magnesium oxysulfate whisker (MOSw) or lauric acid (LA) modified MOSw (LAMOSw) were prepared via melt mixing in a torque rheometer.The heterogeneous nucleating effect of LAMOSw was clearly observed in polarized light microscopy (PLM) pictures with the presence of an abundance of small spherulites.MOSw exhibited no nucleation effect and formed a few spherulites with large size.Compared with PP/MOSw composites,PP/LAMOSw exhibited better impact strength,tensile strength and nominal strain at break,ascribing to three possible reasons:(i) more β-crystal PP formed,(ii) better dispersity of LAMOSw in PP matrix and (iii) the plasticizing effect of LA.The results of dynamic mechanical thermal analysis (DMTA) indicated that brittleness of the PP matrix at low temperature was improved by the addition of LAMOSw,while the interfacial interactions between MOSw and PP matrix were actually weakened by LA,as evidenced by the higher tanδ values over the entire range of test temperatures.In terms of the rheological properties of the composites,both the η* and G'at low frequencies increase with the addition of MOSw or LAMOSw,indicating that the PP matrix was transformed from liquid-like to solid-like.However,a network of whiskers did not form because no plateau was found in the G'at low frequencies.With low filler content,LAMOSw produced a stronger solid-like behavior than MOSw mainly due to the better dispersion of the LAMOSw in PP matrix.However,for highly-filled composites,the rη* of PP/LAMOSw at low frequencies was smaller than that of PP/MOSw composite,since the particle-particle contact effect played a major role.