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In this paper, polypropylene (PP) filled with different levels of multiwalled carbon nanotubes (MWCNTs) manufactured by injection molding was closed-loop recycled in order to investigate the effect of recycling and reprocessing on its rheological, electrical, and mechanical properties. It was found that the PP/MWCNT composites keep the flow performance after mechanical recycling. Moreover, the stress and strain at break increase after one reprocessing cycle (mechanical recycling and injection molding), whereas no statistically significant changes in electrical conductivity, Young's modulus, and tensile strength of the PP/MWCNT composites filled with 1, 3, and 5 wt.% were observed.

In this study, the structure-property relationships in thermoplastic polyurethane (TPU) filled with multi-walled carbon nanotubes (MWCNTs) were investigated. Firstly, the contribution of MWCNTs to the melt shear viscosity and the pressure-volume-temperature (pVT) behavior was investigated. Secondly, injection-molded samples and 2 mm diameter filaments of TPU/MWCNT composites were fabricated and their mechanical and electrical properties analyzed. It was found that the melt processability of TPU/MWCNT composites is not affected by the addition of a small amount (1–5 wt.%) of MWCNTs, all composites displaying shear-thinning at high shear rates. The mechanical and electrical properties of the TPU/MWCNT composites were substantially enhanced with the addition of MWCNTs. However, the conductivity values of composites processed by injection molding were two and three orders of magnitude lower than those of composites processed by extrusion, highlighting the role of melt shear viscosity on the dispersion and agglomeration of nanotubes.

The objective of this paper was to investigate the electrical and rheological behaviors of polypropylene (PP) filled with 1.0, 3.0 and 5.0 wt.-% multi-wall carbon nanotubes (MWCNTs). The flow behavior was analyzed in terms of the melt flow index measured at temperatures relevant for the injection molding process and the flow activation energy was calculated using an Arrhenius type equation. The electrical behavior of PP/MWCNTs composites was examined by DC resistance measurements on injection molded samples. The experimental results have shown that the incorporation of MWCNTs effectively enhances the electrical conductivity of the injection molded PP/MWCNTs composites. The composites under analysis can be classified as semi-conductors with the conducting network arranged in 4 dimensions, i.e. the critical exponent of the power-law dependence of the conductivity on the wt.-% MWCNTs is 2.37. The increased conductivity is explained by the orientation of the MWCNTs along the melt flow and the increased nanotubes-to-nanotubes contact after the formation of the percolation network.

In this work, friction spot stir welding (FSSW) is applied to join high-density polyethylene/multi-walled carbon nanotube (HDPE/MWCNTs) composites. Injection-molded coupons were welded with a single lap-shear configuration under different welding conditions (tool rotational speed, plunge depth, and dwell time). By analyzing the lap-shear tensile load and the fracture surface of the welded joints, it is found that the weld attributes (e.g. weld area and maximum lap-shear tensile load) increase with increasing dwell time, tool rotational speed, and plunging depth. The maximum lap-shear tensile load increases with nanotube loading up to a threshold, followed by a decreasing trend at nanotube loading higher than 1.0 wt.%. It is hypothesized that the bonding mechanism for FSSW of HDPE/MWCNT composites is mainly through the co-crystallization across the interface. When more nanotubes are involved in the welding zone (>1.0 wt.%), saturation of nucleation is reached, the positive effect on the crystallization is vanished, and consequently the overall mechanical properties decrease. Interface failure of the welded joints and bulk fracture originated from the upper coupon within the weld nugget perimeter were identified as the two main failure mechanisms.