Aspects of the Melt Spinning of Fibres from Carbon Nanotube-Nylon Nanocomposites

Carbon nanotubes (CNT) have experienced growing popularity in the past two decades. The field of advanced composite materials has developed quite an interest in using them for high performance applications, such as into thermoplastic fibres. However, a review of the literature showed that there was a lack of understanding between the rheological behavior of CNT-filled polymer and the processability of such materials through melt spinning. In this work, a methodology to incorporate carbon nanotubes (CNT) into thermoplastic fibres and to relate mechanical properties, fibre quality and viscosity was developed. Multi-walled nanotubes (MWNT) were combined into a polyamide 12 (PA12) matrix through melt compounding and twin-screw extrusion. Pellets containing 0 wt%, 0.5 wt%, 1.0 wt%, 2.0 wt%, 5.0 wt% and 10.0 wt% MWNT were produced. Their rheological behaviour was investigated and spinnability and processability criteria were developed based on the loss factor and the relative viscosity. They both predicted that masterbatches containing more than 2.0 wt% CNT would be unsuitable for the production of high quality MWNT/PA12 fibres. The pellets were subsequently melt spun with a capillary rheometer at winding speeds of 41 m/min and 152 m/min. The tensile properties of as-spun filaments were measured with a micro-tensile testing machine. The results showed that the maximum Young's modulus was reached between 0 wt% and 1.0 wt% CNT, exhibiting an increase of 17%. Morphological observations revealed that there was a link between the decrease of elastic modulus and loss of surface quality for filaments containing more than 1.0 wt% MWNT. To further improve the fibres' mechanical properties, post-drawing parameters were systematically investigated: temperature, drawing speed and elongation. The best improvements in terms of elastic modulus and tensile stress were measured for the following post-drawing conditions: 140C and 500% elongation, regardless of drawing speed. The elastic modulus (E) and tensile stress values of MWNT/PA12 fibres were improved by at least 300% after post-drawing. Compared to pure PA12 fibres post-drawn under the same conditions, E increased by up to 45% and the tensile stress by up to 62%, for fibres containing 5.0 wt% MWNT. It was confirmed through electron microscopy and X-ray diffraction that these enhancements were caused by uniform nanotube dispersion, the improvement of surface quality and the alignment of the polymer chains along the fibre axis, controlled by the post-drawing temperature and elongation. Control of the mechanical properties with the post-drawing parameters shows that this method gives the opportunity to tailor CNT-fibres to meet specific demands.