Relationship between morphology and electrical properties in PP/MWCNT composites: Processing-induced anisotropic percolation threshold

• Published in: Materials chemistry and physics 2016-09, Vol.180, p.284-290
• Main Authors: Cesano, F., Zaccone, M., Armentano, I., Cravanzola, S., Muscuso, L., Torre, L., Kenny, J.M., Monti, M., Scarano, D.
• Format: Article
• Language: English
• Subjects: Anisotropy; Composite materials; Direct current; Electrical conductivity; Electrical properties; Electron microscopy (STEM, TEM and SEM); Injection molding; Mathematical models; Morphology; Multi wall carbon nanotubes; Polypropylenes
• ISSN: 0254-0584; 1879-3312
• DOI: 10.1016/j.matchemphys.2016.06.009

Multi-walled carbon nanotubes (MWCNTs)/polypropylene composites were prepared by melt-mixing, by varying the MWCNT content from 1 to 7 wt%, and samples were manufactured by injection moulding technique. DC electrical characterization was performed by the two-probe method in the three main directions: longitudinal and transversal to the flux of the material during the mould filling, and in the through-thickness direction. Moreover, a dedicated setup was adopted to measure the electrical resistance at different depths of the specimen cross-sectional areas. Two different electrical percolation thresholds, calculated at about 2 wt% and 3 wt% of MWCNTs (longitudinally/transversely to the mould filling flux and in the through-thickness directions, respectively), were found. In order to investigate the role of the structure/morphology of the composites on the electrical properties, samples have been cryofractured, chemically etched and characterized by means of scanning electron microscopy. As a result, the observed anisotropic electrical behaviour was associated with the different network morphology, which was detected in the cross-sectional area, caused by the injection moulding process. Based on the observed through-thickness electrical behaviour, a phenomenological DC conduction model has been developed, describing the sample as a multilayer system, being the external layers (skin) less conductive than the internal region (core). This model, combined with the bulk electrical tests, can be considered as a valuable mathematical tool to foresee the electrical behaviour of MWCNT-based composites for designing new industrial injection-moulded components. [Display omitted] •(1–7 wt%) MWCNTs/polypropylene composites are made by injection moulding technique.•The mould temperature is affecting the anisotropic electrical properties.•The anisotropic properties are connected with CNTs dispersion/aggregation.•External layers (skin) are less conductive than the internal region (core) of samples.•A model predicting electrical behaviors of the skin and core regions is proposed.