Review on the effects of injection moulding parameters on the electrical resistivity of carbon nanotube filled polymer parts

The electrical resistivity of injection moulded carbon nanotube filled polymer parts is a result of the process history and the raw material. Key factors of the process are temperature and shear history of the melt. Various process parameters affect the aforementioned factors. Articles reviewed in here discuss effects of injection moulding and related processes, such as melt compounding. Furthermore, articles have been reviewed which describe the effect of shear, composition and temperature on the structure of the conductive CNT network. These effects are then related to the solidification of the part, resulting in the variation of the percolated network of CNTs. The variation of the network structure results in a variation of the electrical resistivity within the part.     

The role of disorder on the AC and DC electrical conductivity of vapour grown carbon nanofibre/epoxy composites

Four dispersion methods were used for the preparation of vapour grown carbon nanofibre (VGCNF)/epoxy composites. It is shown that each method induces certain levels of VGCNF dispersion and distribution within the matrix, and that these have a strong influence on the composite electrical properties. A homogenous VGCNF dispersion does not necessarily imply higher electrical conductivity. In fact, it is concluded that the presence of well distributed clusters, rather than a fine dispersion, is more important for achieving larger conductivities for a given VGCNF concentration. It is also found that the conductivity can be described by a weak disorder regime.     

The effect of molecular chain polarity on electric field-induced aligned conductive carbon nanotube network formation in polymer melt

Two ethylene–vinyl acetate (EVA) copolymers containing 10 and 25 wt.% vinyl acetate (EVA10 and EVA25) were utilized to explore the effect of molecular polarity on the formation of conductive carbon nanotube (CNT) network in EVA melt under an electric field. Because of the different interfacial energy, it was supposed to be stronger molecular chain-CNT interaction in CNT/EVA25 than that in CNT/EVA10. The critical time for conductive CNT network formation decreased with annealing temperature, filler loading and EVA polarity. The activation energy of conductive CNT network formation (93.9 kJ/mol) in CNT/EVA10 is lower than that (104.7 kJ/mol) in CNT/EVA25. By a thermodynamic percolation model, the percolation threshold at the equilibrium state was about 0.19 vol.% for CNT/EVA10, while it rose to 0.27 vol.% for CNT/EVA25. Morphological observation showed a high degree of CNT alignment in CNT/EVA10 compared to CNT/EVA25 after application of an electric field. The results suggested the strong CNT–EVA chain interaction and higher viscosity of polymer matrix limited the CNT alignment and the conductive network tended to form easily in EVA melt with a low chain polarity.     

Electrical resistance-based strain sensing in carbon nanotube/polymer composites under tension: Analytical modeling and experiments

This paper presents an analytical and experimental study on the strain sensing behavior of carbon nanotube (CNT)-based polymer composites. Tensile tests were conducted on CNT/polycarbonate composites and the responses in the electrical resistance were measured during the tests. An analytical model incorporating the electrical tunneling effect due to the matrix material between CNTs was also developed to describe the electrical resistance change as a result of mechanical deformation. The model deals with the inter-nanotube matrix deformation at the micro/nanoscale due to the macroscale deformation of the nanocomposites. A comparison of the analytical predictions and the experimental data showed that the proposed model captures the sensing behavior. In addition, the effect of the micro/nanoscale structures on the strain induced resistance change was discussed to provide useful information for designing CNT-based polymer composites with high strain sensing capability.     

Influence of different carbon nanotubes on the electrical and mechanical properties of melt mixed poly(ether sulfone)-multi walled carbon nanotube composites

Commercial Udel® poly(ether sulfone) (PSU) was filled with three different commercially available multiwalled carbon nanotubes (MWCNTs) by small scale melt mixing. The MWCNTs were as grown NC 7000 and two of its derivatives prepared by ball milling treatment. One of them was unmodified (NC 3150); the other was amino modified (NC 3152). The main difference beside the reactivity was the reduced aspect ratio of NC 3150 and NC 3152 caused by ball milling process. All PSU/MWCNT composites with similar filler content were prepared under fixed processing conditions and comparative analysis of their electrical and mechanical properties were performed and were correlated with their microstructure, characterized by optical microscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). A non-uniform MWCNT dispersion was observed in all composites. The MWCNTs were present in form of agglomerates in the size of 10–60 μm whereas the deagglomerated part was homogeneously distributed in the PSU matrix. The differences in the agglomeration states correlate with the variations of properties between different PSU/MWCNT composites. The lowest electrical percolation threshold of 0.25–0.5 wt.% was observed for the shortened non-functionalized MWCNT composites and the highest for amine-modified MWCNT composites (ca. 1.5 wt.%). The tensile behavior of the three composites was only slightly altered with CNT loading as compared to the pure PSU. However, the elongation at break showed a reduction with MWCNT loading and the reduction was least for composite with best MWCNT dispersion.     

Utilization of polymer degradation to modify electrical properties of poly(l-lactide)/poly(methyl methacrylate)/carbon filler composites

This research attempts to utilize polymer degradability in modifying electrical properties of poly(l-lactide) (PLLA)/poly(methyl methacrylate) (PMMA)/carbon fillers composites. Three kinds of carbon particles, i.e. carbon black, vapor-grown carbon fiber, and carbon nanotube, were compounded with PLLA/PMMA blend, followed by hydrolytic degradation of the composites, resulted in degradation of PLLA molecular chain from the surface of samples, with PMMA and carbon particles remained undegraded. By controlling degradation rate, it was possible to prepare samples with low surface resistivity, yet at the same time exhibited high value of volume resistivity. It was also found that final electrical properties of degraded composites depend on the size and the shape of the fillers.     

Effect of block-copolymer dispersants on properties of carbon nanotube/epoxy systems

The effects of the addition of eight different block copolymers on the dispersion stability of multi-walled carbon nanotubes (MWCNTs) are reported. Suspensions of CNTs in different components of an epoxy system have been prepared using a tip sonicator and different amounts of block copolymers. The resistance to sedimentation of MWCNTs in various media was systematically investigated by using a centrifugation technique. Block copolymers that result in dispersions of MWCNTs in epoxy and hardener stable for more than 1 week have been obtained. Dispersions using a single or a combination of two different dispersing agents have been used for the fabrication of MWCNT nanocomposites. The effect of different preparation routes and use of block copolymers on the tensile properties and surface resistivity of the composites have been evaluated. The results obtained have been related with the dispersion stability of the MWCNTs in the epoxy components.     

Evolution of dispersion of carbon nanotubes in Polyamide 11 matrix composites as determined by DC conductivity

Double-walled Carbon NanoTubes (DWCNTs) have been dispersed in a Polyamide 11 (PA11) matrix by two routes: in the solvent way, Polyamide 11 was first dissolved in its solvent to ensure a liquid state dispersion of carbon nanotubes by ultrasonic way; in the melt mixing way, an optimization of the extrusion parameters, such as mixing time, mixing speed, mixing temperature and screw rotation direction allow to reach satisfactory dispersion. Dispersion and percolation threshold have been compared thanks to the evolution of DC conductivity with carbon nanotubes weight fraction in Polyamide 11.     

Enhancement of delamination fatigue resistance in carbon nanotube reinforced glass fiber/polymer composites

We show that the addition of small volume fractions of multi-walled carbon nanotubes (CNTs) to the matrix of glass–fiber composites reduces cyclic delamination crack propagation rates significantly. In addition, both critical and sub-critical inter-laminar fracture toughness values are increased. These results corroborate recent experimental evidence that the incorporation of CNTs improve fatigue life by a factor of two to three in in-plane cyclic loading. We show that in both the critical and sub-critical cases, the degree of delamination suppression is most pronounced at lower levels of applied cyclic strain energy release rate, ΔG. High-resolution scanning electron microscopy of the fracture surfaces suggests that the presence of the CNTs at the delamination crack front slows the propagation of the crack due to crack bridging, nanotube fracture, and nanotube pull-out. Further examination of the sub-critical fracture surfaces shows that the relative proportion of CNT pull-out to CNT fracture is dependent on the applied cyclic strain energy, with pull-out dominating as ΔG is reduced. The conditions for crack propagation via matrix cracking and nanotube pull-out and fracture are studied analytically using fracture mechanics theory and the results compared with data from the experiments. It is believed that the shift in the fracture behavior of the CNTs is responsible for the associated increase in the inter-laminar fracture resistance that is observed at lower levels of ΔG relative to composites not containing CNTs.     

Influence of screw configuration,residence time,and specific mechanical energy in twin-screw extrusion of polycaprolactone/multi-walled carbon nanotube composites

Melt processing of thermoplastic-based nanocomposites is the favoured route to produce electrically conductive or electrostatic dissipative polymer composites containing carbon nanotubes (CNT). As these properties are desired at low filler fractions, a high degree of dispersion is required in order to benefit from the intrinsic CNT properties. This study discusses the influence of screw configuration, rotation speed, and throughput on the residence time and specific mechanical energy (SME) and the resulting macroscopic CNT dispersion in polycaprolactone (PCL) based masterbatches containing 7.5 wt.% multi-walled carbon nanotubes (MWNT) using an intermeshing co-rotating twin-screw extruder Berstorff ZE25.     

Effect of silica coating thickness on the thermal conductivity of polyurethane/SiO2 coated multiwalled carbon nanotube composites

Silica coated multiwalled carbon nanotubes (SiO₂@MWCNTs) with different coating thicknesses of ∼4 nm, 30–50 nm, and 70–90 nm were synthesized by a sol–gel method and compounded with polyurethane (PU). The effects of SiO₂@MWCNTs on the electrical properties and thermal conductivity of the resulting PU/SiO₂@MWCNT composites were investigated. The SiO₂ coating maintained the high electrical resistivity of pure PU. Meanwhile, incorporating 0.5, 0.75 and 1.0 wt% SiO₂@MWCNT (70–90 nm) into PU, produced thermal conductivity values of 0.287, 0.289 and 0.310 W/mK, respectively, representing increases of 62.1%, 63.3% and 75.1%. The thermal conductivity of PU/SiO₂@MWCNT composites was also increased by increasing the thickness of the SiO₂ coating.     

Multifunctional properties of high volume fraction aligned carbon nanotube polymer composites with controlled morphology

Advanced composites, such as those used in aerospace applications, employ a high volume fraction of aligned stiff fibers embedded in high-performance polymers. Unlike advanced composites, polymer nanocomposites (PNCs) employ low volume fraction filler-like concepts with randomly-oriented and poorly controlled morphologies due to difficult issues such as dispersion and alignment of the nanostructures. Here, novel fabrication techniques yield controlled-morphology aligned carbon nanotube (CNT) composites with measured non-isotropic properties and trends consistent with standard composites theories. Modulus and electrical conductivity are maximal along the CNT axis, and are the highest reported in the literature due to the continuous aligned-CNTs and use of an unmodified aerospace-grade structural epoxy. Rule-of-mixtures predictions are brought into agreement with the measured moduli when CNT waviness is incorporated. Waviness yields a large (10×) reduction in modulus, and therefore control of CNT collimation is seen as the primary limiting factor in CNT reinforcement of composites for stiffness. Anisotropic electron transport (conductivity and current-carrying capacity) follows expected trends, with enhanced conductivity and Joule heating observed at high current densities.     

Processing and electrical properties of carbon nanotube/vinyl ester nanocomposites

Vinyl ester resins are often utilized in advanced naval composite structures due to the relatively low viscosity of the resin and the capability to cure at ambient temperatures. These qualities facilitate the production of large naval composite structures using resin infusion techniques. Vinyl ester monomer was synthesized from the epoxy resin to overcome processing challenges associated with volatility of the styrene monomer in vinyl ester resin. In this research we have investigated the use of a calendering approach for dispersion of multi-walled carbon nanotubes in vinyl ester monomer, and the subsequent processing of nanotube/vinyl ester composites. The high aspect ratios of the carbon nanotubes were preserved during processing and enabled the formation of a conductive percolating network at low nanotube concentrations. An electrical percolation threshold below 0.1 wt.% carbon nanotubes in vinyl ester was observed. Formation of percolating carbon nanotube networks at low concentration holds promise for the utilization of carbon nanotubes as in situ sensors for detecting deformation and damage in advanced naval composites.     

Strain-sensitive Raman spectroscopy and electrical resistance of carbon nanotube-coated glass fibre sensors

This paper presents the development of glass fibres coated with nanocomposites consisting of carbon nanotubes (CNTs) and epoxy. Single glass fibres with different CNT content coating are embedded in a polymer matrix as a strain sensor for composite structures. Raman spectroscopy and electrical response of glass fibres under mechanical load are coupled for in situ sensing of deformation in composites. The results show that the fibres with nanocomposite coating exhibit efficient stress transfer across the fibre/matrix interface, and these with a higher CNT content are more prone to fibre fragmentation at the same matrix strain. A relationship between the fibre stress and the change in electrical resistance against the fibre strain is established. The major finding of this study has a practical implication in that the fibres with nanocomposite coating can serve as a sensor to monitor the deformation and damage process in composites.     

Electrospun aligned PLLA/PCL/functionalised multiwalled carbon nanotube composite fibrous membranes and their bio/mechanical properties

Aligned poly(L-lactide) (PLLA)/poly(ε-caprolactone) (PCL)/functionalized multiwalled carbon nanotube (F-MWNT) composite fibrous membranes were fabricated by electrospinning. Their morphology, mechanical properties, in vitro degradation and biocompatibility were studied. With a collector rotation speed of 3000 rpm, the electrospun fibers are highly aligned and the F-MWNTs are oriented along the fiber axis, reinforcing the electrospun fibrous membranes. When the F-MWNTs are incorporated, the PLLA/PCL/F-MWNT composite fibers become thinner due to the increased electrical conductivity. However, when the F-MWNTs are increased to 3.75 wt.%, the higher viscosity and aggregation of F-MWNTs have lead to the formation of beads and a wider diameter distribution in the electrospun fibers. Also, the electrospun fibers having smaller diameter, larger porosity and lower crystallinity induced by F-MWNTs have improved the bio-degradation of the PLLA/PCL/F-MWNT fibrous membranes, which have no toxic effects on the proliferation of adipose-derived stem cells.     

A novel approach to fabricate high volume fraction nanocomposites with long aligned carbon nanotubes

Conventional micro-fiber-reinforced composites provide insight into critical structural features needed for obtaining maximum composite strength and stiffness: the reinforcements should be long, well aligned in a unidirectional orientation, and should have a high reinforcement volume fraction. It has long been a challenge for researchers to process CNT composites with such structural features. Here we report a method to quickly produce macroscopic CNT composites with a high volume fraction of millimeter long, well aligned CNTs. Specifically, we use the novel method, shear pressing, to process tall, vertically aligned CNT arrays into dense aligned CNT preforms, which are subsequently processed into composites. Alignment was confirmed through SEM analysis while a CNT volume fraction in the composites was calculated to be 27%, based on thermogravimetric analysis data. Tensile testing of the preforms and composites showed promising mechanical properties with tensile strengths reaching 400 MPa.     

Electrical resistance change and crack behavior in carbon nanotube/polymer composites under tensile loading

This paper studies the electrical and mechanical responses of cracked carbon nanotube (CNT)-based polymer composites. Tensile tests were performed on single-edge cracked plate specimens of the nanocomposites at room temperature and liquid nitrogen temperature (77 K), and the electrical resistance change of the specimens was monitored. An analytical model based on the electrical conduction mechanism of CNT-based composites was also developed to predict the resistance change resulted from crack propagation. The crack induced resistance change was calculated, and a comparison of the analytical predictions against the experimental data was made to validate the applicability of the model. In addition, the fracture properties of the nanocomposites were assessed in terms of the J-integrals using an elastic-plastic finite element analysis.     

Augmented fatigue performance and constant life diagrams of hierarchical carbon fiber/nanofiber epoxy composites

We report the results of an extensive multi-stress ratio experimental study on the axial fatigue behavior of an all-carbon hierarchical composite laminate, in which carbon nanofibers (CNFs) are utilized alongside traditional micron-sized carbon fibers. Primary carbon fibers were arranged in matrix-dominated biax ±45° lay-ups in order to establish matrix and matrix/fiber interaction based performance. CNFs were matrix dispersed by three-roll calender milling. Results indicate that the CNF-reinforced composites collectively possess improved fatigue and static properties over their unmodified counterparts. Large mean lifetime improvements of 150–670% were observed in fully compressive, tensile and tensile dominated loadings. Enhancements are attributed to the high interface density and damage shielding effect of the CNFs within the matrix. Further improvements are believed to occur when the nanofibers arrest and redistribute small scale, slowly propagating matrix cracks at low applied stresses. These results highlight the ability of a nanometer-sized reinforcing phase to actively participate and enhance matrix properties while moving toward a cost effective alternative to current material solutions.     

Strain sensing in polymer/carbon nanotube composites by electrical resistance measurement

In this work multiwall carbon nanotubes (MWCNTs) dispersed in a polymer matrix have been used for strain sensing of the resulting nanocomposite under tensile loading. This was achieved by measuring the relative electrical resistance change (ΔR/R₀) in conductive polyvinylidenefluoride (PVDF)/MWCNTs nanocomposites prepared by melt-mixing with varying filler content from 0.5 wt.% to 8 wt.%. Two main parameters were systematically studied. The PVDF/MWCNTs mixing procedure that results in a successful MWCNTs dispersion, and the effect of MWCNTs content on material’s sensing behaviour. The samples were subjected to tensile loading and the longitudinal strain was monitored together with the longitudinal electrical resistance. The results showed that MWCNTs dispersed in insulating PVDF matrix have the potential to be used as a sensitive network to monitor the strain levels in polymer/carbon nanotube nanocomposites as the deformation level of each sample was being reflected by the resistance changes.     

TiO2-nanorod decorated carbon nanotubes for high-permittivity and low-dielectric-loss polystyrene composites

Polymer composites with high permittivity and low dielectric loss are highly desirable in electronic and electrical industry. Adding conductive fillers could significantly increase the permittivity of a polymer. However, polymer composites containing conductive fillers often exhibit very high dielectric loss due to their large electrical conduction or leakage currents. In this work, by engineering TiO₂-nanorod-decorated multi-walled carbon nanotubes (TD-CNTs), polystyrene (PS) composite with high permittivity and low dielectric loss have been successfully prepared. The composite containing of 17.2 vol.% TD-CNTs has a permittivity of 37 at 1 kHz, which is 13.7 times higher than that of the pure PS (2.7), while the dielectric loss still remains at a low value below 0.11. The dielectric properties of the composites are closely related to the length of CNTs and the loading level of TiO₂-nanorods on the CNT surfaces.