PP/MWCNT injection-moulded components: an analytical study of electrical properties and morphology

Since their early synthetization in the 30s, the interest in polymers potentialities has continuously expanded. Nowadays, the majority of everyday devices could not exist without plastics. The combination of their light-weight, versatility, easy processability and relatively low cost makes these materials essential in several industrial sectors. Their applications, in fact, range from consumer goods to specialist devices realization. Moreover, their powerful properties can be easily exploited, in combination with the numerous manufacturing techniques, with which they are transformed. Among all, melt mixing and injection moulding processes, which are the most known and used technologies to modify polymers, are effortlessly adopted to create new materials. Nonetheless, it is reported in literature that both these techniques and their working conditions strongly affect the final properties of the final components, in particular for what concerns the injection moulding process. It is known, in fact, that a tough orientation of molecular chains and fillers occurs inside the molten material during this process. This phenomenon creates a marked anisotropic behaviour of the final properties of the components, with a consequent non-uniform distribution of properties. Studying this mechanism is important to understand how to modify and to optimize the processing conditions, in order to tailor the final properties of the prepared materials. This thesis aims to investigate the electrical anisotropic behaviour of injection-moulded polymeric MWCNT-based nanocomposites, correlating the variations of process conditions and the morphological and electrical anisotropic properties of thermoplastic nanocomposites. With this purpose, multi-walled carbon nanotubes/polypropylene nanocomposites were prepared and manufactured using melt mixing and injection moulding processes. The processing parameters were changed and the variations in the electrical behaviour and in the morphological structure of the nanocomposites were observed. During the melt mixing phase, the temperature of the screw profile and the MWCNTs feeding zone along the screw were modified. During the injection moulding phase, three main parameters were modulated, namely injection rate, temperature of the mould and temperature of the melt. Moreover, an innovative injection moulding process, i.e. the Heat&Cool technique, was exploited in order to vary the temperature of the mould, quickly increasing and decreasing it. The prepared specimens were characterized electrically through DC, AC and surface resistance measurements. A multi-direction electrical testing was used to evaluate the electrical percolation threshold in the three main spatial directions, i.e. longitudinal and transversal to the flux of the molten material inside the mould and in the through-thickness direction. The morphological structure of the sample was observed through both Optical and Scanning Electron Microscopy. The influence of the change of processing parameters on these properties was deeply studied. As main results, different levels of inhomogeneity was observed. An inhomogeneous processing-induced morphological skin-core structure in the thickness was detected. The electrical behaviour appeared non-uniform and this aspect seems to be correlated directly to the internal morphological structure of the injection-moulded parts. Then, an alteration in the electrical behaviour is also observable in different positions of the same injection-moulded sample, namely nearer to or farther from the injection gate. Finally, the change of the processing conditions appears to play a fundamental role in the formation of both the inhomogeneous morphological structure and the anisotropic electrical behaviour of the MWCNT-based nanocomposites. In fact, the increase of the temperature of the mould and of the injection rate act as the main responsible for the decrease of the electrical resistivity of the prepared nanocomposites.