In situ microfibrillar morphology and properties of polypropylene/polyamide/carbon black composites prepared through multistage stretching extrusion

Isotactic polypropylene/polyamide/carbon black (PP/PA/CB) composites with microfibrillar morphology were designed and prepared using a multistage stretching extruder with an assembly of laminating-multiplying elements (LMEs). CB was selectively located in PA. With the increase of LME number from zero to seven, the conductive PA/CB phase was found to experience an elongating-breaking-elongating process. This morphological development resulted in the strong dependence of electrical resistivity on the LME number. When no LME was used, PP/PA/CB materials with 2.0, 3.0, or 4.0 wt% (1.0, 1.6, and 2.1 vol%) CB employed were insulators (resistivity: 10¹⁰ Ω cm) due to their droplet morphology. With the introduction of LMEs, a conductive network was formed because of the microfibrillation of the conductive PA/CB phase; these materials became conductors (resistivity: 10⁴–10⁶ Ω cm). The percolation threshold can lower to 1.5 wt% (0.9 vol%). The low resisticity and percolation threshold cannot be obtained through the conventional method.