Carbon Nanotube Epoxy Research Articles

The effect of choline-based ionic liquid on CNTs' arrangement in epoxy resin matrix

This work focuses on the effect of ionic liquids (ILs) on the arrangement of carbon nanotubes (CNTs) in CNTs–epoxy composites. Electrical conductivity experiments on epoxy–CNTs–ILs suggested that, compared to Ethaline, Oxaline and Reline, Glyceline had a more distinguishable effect on the CNTs' arrangement in the epoxy–CNTs–Glyceline (CG) composites based on scanning electron microscope results. The electrical conductivity of CG composites was improved by increasing the weight content of Glyceline up to 50 times that of CNTs (Gly/CNTs=50), and for weight ratios higher than 50, a downward trend was observed. According to the proposed model, the increase in electrical conductivity seems to be a result of the formation of liquid vesicles which push the CNTs closer outside the Glyceline vesicles. Conversely, the drop in the electrical conductivity can be due to the enlargement of the vesicles, which dissolve CNTs and reduce their content in the epoxy. In addition, the tensile strength of CG composites increased by ratio of Gly/CNTs=10 followed by a significant drop thereafter.Thermal stability of the CG composites was seen to decrease as Glyceline content is increased in the composite. This reduction was attributed to the early decomposition of Glyceline vesicles present inside the epoxy matrix.

Piezoresistive characterization of multi-walled carbon nanotube-epoxy based flexible strain sensitive films by impedance spectroscopy

In this study, we investigate the piezoresistive properties of flexible, strain sensitive multi-walled carbon nanotubes (MWCNTs)/epoxy composites. The deformation over the sensor area was tested by digital image correlation (DIC) under quasi-static uniaxial tension. The piezoresistive characteristics of the films were investigated quantitatively by electrochemical impedance spectroscopy (EIS) over a wide range of frequencies from 40 Hz to 110 MHz. Scanning electron microscopy (SEM) images confirmed that MWCNTs/epoxy composites with different CNT concentrations have a good homogeneity and dispersion. Additionally, in order to tailor the piezoresistivity of the strain sensor, an RC equivalent circuit was derived based on the impedance responses and the corresponding parameters were extracted under tensile strain. Compared with traditional strain gauges, higher sensitivity is obtained in particular at the concentrations close to the percolation threshold (13.6 for 0.3 wt.%). Due to the tunneling effect, a non-linear piezoresistivity is observed at low concentrations. It was found that sensors with 1 wt.% shows the highest linearity with a correlation coefficient of 0.999. The standard deviation of the cyclic readings was found to be 0.05%, indicating a high repeatability.

The sonication effect on CNT-epoxy composites finally clarified

Carbon nanotube-epoxy composites were prepared using amino-functionalized CNTs and sonication as a mixing process. Different times and sonication powers were used for preparing composites in order to study how the sonication process may influence the curing reaction of both systems: neat epoxy resin and amino-CNT/epoxy composite.The curing reaction was investigated with differential scanning calorimetry and the results were associated with analysis of gel permeation chromatography. The results showed that the effect of CNTs on the cure behavior of the epoxy resin depends on the sonication power. The sonication of neat resin with a 150 W powered device led to a molar mass reduction of the resin and an increase in the cure enthalpy. The CNT addition to this system reduced the cure enthalpy. However, when neat epoxy resin was sonicated with a 200 W powered device, the molar mass did not decrease (i.e. it was increased or was not changed) and the cure enthalpy did not increase (essentially it decreased or did not change). The CNT addition to such solutions did not reduce (i.e. it was increased or did not change) the cure enthalpy, which is a contrary result from that obtained with a 150 W powered device. POLYM. COMPOS., 2015. © 2015 Society of Plastics Engineers

Dispersion evaluation, processing and tensile properties of carbon nanotubes-modified epoxy composites prepared by high pressure homogenization

In this paper, high pressure homogenization (HPH) served as the main dispersion method for making carbon nanotubes (CNTs)-modified composite materials. A new quantitative approach was proposed to evaluate the dispersion of CNTs in epoxy resin by combining microscopic images with maximum likelihood estimation theory. The results were in good agreement with those obtained from optical observation and ultraviolet–visible (UV–vis) absorbance measurement. Moreover, the dispersion process specific to HPH processing was analyzed with the aid of the proposed approach. The changes of the morphologies and lengths of CNTs in HPH were illustrated. It was found that, in the first few cycles, the lengths of CNTs fell significantly and the entanglements remained severe. In the following cycles, individual CNTs were gradually separated from agglomerates and the lengths experienced a slow decline. After homogeneous dispersion, the transverse tensile properties of CNTs-modified carbon fiber reinforced epoxy composites (CFRP) got obviously ameliorated.

Determination of arbutin and bergenin in Bergeniae Rhizoma by capillary electrophoresis with a carbon nanotube-epoxy composite electrode

This report describes the fabrication and the application of a novel carbon nanotube (CNT)-epoxy composite electrode as a sensitive amperometric detector for the capillary electrophoresis (CE). The composite electrode was fabricated on the basis of the in situ polycondensation of a mixture of CNTs and 1,2-ethanediamine-containing bisphenol A epoxy resin in the inner bore of a piece of fused silica capillary under heat. It was coupled with CE for the separation and detection of arbutin and bergenin in Bergeniae Rhizoma, a traditional Chinese medicine, to demonstrate its feasibility and performance. The two phenolic constituents were well separated within 10min in a 45cm capillary length at a separation voltage of 12kV using a 50mM borate buffer (pH 9.2). The CNT-based detector offered higher sensitivity, significantly lower operating potential, satisfactory resistance to surface fouling, and lower expense of operation, indicating great promise for a wide range of analytical applications. It showed long-term stability and reproducibility with relative standard deviations of less than 5% for the peak current (n=15).

Toward improved mechanical performance of multiscale carbon fiber and carbon nanotube epoxy composites

A novel class of multiscale epoxy composites was developed containing carbon fibers (CFs) and multiwalled carbon nanotubes (MWCNTs) to explore their mutual effect on the mechanical performance of composites. The loading of CFs in composites was kept constant at ∼60 wt%, while the contents of MWCNTs were increased from 0.5 wt% to 2.0 wt%. MWCNTs were functionalized through acid treatment before incorporating into epoxy matrix to promote dispersion quality. The developed composites were characterized microstructurally by scanning electron microscopy and mechanically by tensile, flexural, edgewise compression, and hardness tests. Homogeneous dispersion of MWCNTs was observed until their loading of 1.5 wt%, which enhanced the mechanical performance of composites. Hardness increased up to 47% while tensile, flexural, and edgewise compressive moduli increased to 40%, 16.3%, and 164%, respectively. Moreover, tensile, flexural, and edgewise compressive strengths showed rises of 45%, 15.2%, and 43%, respectively. The fracture strain increased in both the tensile and flexural tests while it decreased in edgewise compressive tests. Increasing the MWCNTs in composites to 2.0 wt% produced their agglomerates and reversed the rising trend in mechanical properties. POLYM. COMPOS., 2015. © 2015 Society of Plastics Engineers

Temperature-compensated force/pressure sensor based on multi-walled carbon nanotube epoxy composites.

In this study, we propose a multi-walled carbon nanotube epoxy composite sensor for force and pressure sensing in the range of 50 N–2 kN. A manufacturing procedure, including material preparation and deposition techniques, is proposed. The electrode dimensions and the layer thickness were optimized by the finite element method. Temperature compensation is realized by four nanocomposites elements, where only two elements are exposed to the measurand. In order to investigate the influence of the filler contents, samples with different compositions were prepared and investigated. Additionally, the specimens are characterized by cyclical and stepped force/pressure loads or at defined temperatures. The results show that the choice of the filler content should meet a compromise between sensitivity, temperature influence and noise behavior. At constant temperature, a force of at least 50 N can be resolved. The measurement error due to the temperature influence is 150 N in a temperature range of −20°C−50°C.

Atomistic investigation of the interfacial mechanical characteristics of carbon nanotube reinforced epoxy composite

In this work, we investigate the interfacial mechanical characteristics of carbon nanotube (CNT) reinforced epoxy composite using molecular dynamics (MD) simulations. The MD simulations were carried out by developing two pull-out models; namely, the displacement-load model and the velocity-load model. The second-generation force field – polymer consistent force field (PCFF) – is used in the current MD simulations. The effects of various parameters, such as epoxy density, length and diameter of a CNT, the CNT–epoxy interfacial thickness, LJ cut-off distance and capping conditions of a CNT on the interfacial mechanical characteristics have also been investigated and discussed. The results from the present pull-out model, which are validated with earlier studies, reveal that (i) the interfacial shear strength (ISS) of the CNT-reinforced epoxy composite is improved with the increase in the epoxy density, (ii) the ISS of the CNT-reinforced epoxy composite decreases with the increase in the values of length and diameter of a CNT, and the thickness of CNT–epoxy matrix interface, (iii) the LJ cut-off distance has marginal effect on the ISS and the pull-out force of CNT, and (iv) incorporation of an end cap in the simulations results high initial pull-out peaks, which well correlate with the experimental findings.

Mechanical properties of CNT–Bisphenol E cyanate ester-based CFRP nanocomposite developed through VARTM process

This paper reports on the effect of multiwall carbon nanotubes (CNTs) without and with chemical functionalization on the mechanical properties of Bisphenol E cyanate ester resin (BECy) based carbon fibre reinforced plastic (CFRP) laminated composites. BECy with its low viscosity, low moisture uptake and superior mechanical properties is selected for its application in CFRP laminates through the cost-effective Vacuum Assisted Resin Transfer Moulding (VARTM) process. However, unlike CNT–epoxy–CFRP composites, processing and performance issues such as dispersion of CNTs, chemical bonding with resin, functionalization effects, effects on mechanical properties, etc. for BECy–CNT–CFRP composite system are not well reported. The objective of this study is to improve the mechanical properties of BECy resin with small additions of CNTs and functionalized CNTs in CFRP laminates. CNTs and fCNTs are infused into BECy using ultrasonication and standard mixing methods. Improvements in Young’s modulus and strength in tension, compression, shear, flexure and interlaminar shear strength are analysed. It is observed that addition of 0.5 wt% CNTs effected for maximum mechanical properties of the resin and 1 wt% CNTs for the mechanical properties of CNT–CFRP nanocomposite. Further, improvements obtained with fCNTs are marginal. Dispersion behaviour and effect of CNTs/fCNTs in load transfer corroborated with SEM pictures are presented. The enhanced mechanical properties realized in VARTM processing of BECy-CFRP laminate indicate clear advantage of CNT based modification of the process.

Study of the influence of single-wall carbon nanotube dispersion techniques upon the properties of epoxy resin-based nanocomposites

A comparison of two techniques, ultrasonication and the controlled wave treatment, for the dispersion of ultra-small concentrations of carbon nanotubes in epoxy resin has been carried out with specially designed instruments. Studies with optical microscopy and physicomechanical and thermomechanical tests showed that both techniques provide almost equal degrees of nanotube dispersion in the matrix and dependences of the basic physicomechanical properties of cured epoxy polymers on the nanofiller concentrations. However, the difference between the two techniques has been shown at the study of the structural properties of the cured epoxy polymers (the molecular weight of the internode chains of the polymeric network, and the vitrification temperature), which are more stable in the case of the wave treatment. Thus, wave treatment is a promising dispersion technique, and it is applicable at the industrial scale because it is almost unlimited by the size and capacities of the equipment.

Hygro-Thermo-Electric Properties of Carbon Nanotube Epoxy Nanocomposites with Agglomeration Effects

In the present study, the effective electric, thermal, and moisture properties of carbon nanotube (CNT) epoxy composites are derived by considering the agglomeration effect of CNT concentrations in the epoxy matrix. In this direction, the Voigt and Reuss homogenization method is adopted in the derivations. It is well known from experiments that the CNT thermal and electrical conductivities and the epoxy hygro-thermal expansion coefficients have significant effects on the behavior of CNT nanocomposites. Moreover, it has been experimentally proved that the agglomeration of CNTs in the matrix with high and low concentrations of the CNTs certainly affects the resistivity and, hence, the thermal expansion properties. Therefore, the effective elastic, thermal, electrical, and moisture properties for the randomly distributed CNTs in the matrix has been derived in terms of the agglomeration volume fractions of CNTs. In the effective relations, a single agglomeration parameter is considered to be active for a given potential. The results of variation in the hygro-electro-thermal properties due to change in CNT volume fraction as well as agglomeration parameters have been presented. The results and observation show that CNT agglomeration has a strong influence on the effective hygro-thermo-electric properties of the nanocomposites.

Pull-out simulations of interfacial properties of amine functionalized multi-walled carbon nanotube epoxy composites

In this work the effect of amine functionalization of carbon nanotubes (CNTs) on the interfacial bonding characteristics between amine functionalized CNTs and LY556 resin has been investigated using molecular dynamics (MD) simulations. By analyzing the interfacial shear stress of nano composites (reinforced with single, double and triple walled CNTs), the effect of functionalization with Ethylene-di-amine (E-NH2) functional group has been examined. This study investigates the effect of functionalization with up to ten E-NH2 groups on the potential energy, pull-out energy and interfacial shear strength of different functionalized CNTs structures with reference to pristine CNTs. MD simulation proves that the interfacial bonding and shear stress between the CNTs and LY556 resin matrix depends on the degree of functionalization of CNTs. Thus optimum functionalization of CNT surface can be an effective way to improve the load transfer between the CNTs and resin. An artificial neural network based non-linear regression model is also developed for energy required to pullout different walled CNTs from the epoxy matrix. It has been observed that ANN modeling produced best results for MWCNT pullout as compared to SWCNT and DWCNT. This study will assist in the modeling, simulation and design of advanced nanotube reinforced polymer composites for potential real life applications, particularly in military structural applications.

Impact of Electric Field Application During Curing on Epoxy-Carbon Nanotube Nanocomposite Electrical Conductivity

In the present work, the conductivity of multi-walled carbon nanotube (MWCNT) and epoxy resin matrix nanocomposites produced under the action of a 100-V/cm external sinusoidal electric field is studied, for five MWCNT concentration values. The results showed that the AC electrical conductivities of the nanocomposite samples exhibited a large increase compared to those of pure epoxy resin samples. The conductivity at 40 Hz increased from ~ 10−9 S/m for the pure resin samples to 10−2–10−3 S/m for materials with 0.30 wt% MWCNT concentration, while a further increase of up to ~ 10−1 S/m was achieved on the samples produced under the effects of a sinusoidal field. This phenomenon was attributed to a spatial alignment of the MWCNT inside the epoxy matrix and/or the bending of close MWCNT due to the electrostatic force, which created efficient electric current conduction paths along the MWCNT. A proposed new method estimated the value of the current percolation threshold, yielding a result of 0.016 wt% for the sample that was not subjected to the electric field, which then decreased by approximately one order of magnitude for the samples subjected to the electric field.

Anomalous behavior of thermal stability of amino-carbon nanotube–epoxy nanocomposite

Carbon nanotube–epoxy nanocomposite was obtained using amino-carbon nanotube and its thermal stability was compared with neat resin. The results showed that at lower temperatures (below 360℃), nanocomposite is more stable than neat resin. However, at higher temperatures (above 380℃), the contrary was observed: neat resin is more stable. In order to understand this anomalous behavior, a complete kinetic study of the cure process was performed. The precure degree of the nanocomposite was higher than that of the neat resin. However, both samples showed virtually the same final cure degree. The cure heat and activation energy were very similar for both samples. Nevertheless, the difference in thermal stability between nanocomposite and neat resin is not related to the cure process. This behavior must be associated with the good adhesion of amino-carbon nanotube–epoxy and to the presence of the functional groups linked to the carbon nanotube surface, especially oxygen molecules.

Quantitative nanomechanical characterization of the van der Waals interfaces between carbon nanotubes and epoxy

Interfacial interactions between carbon nanotubes (CNTs) and polymer matrices play a critical role in the bulk mechanical performance of CNT-reinforced polymer nanocomposites, but their mechanisms remain elusive after over a decade of research. Here we present an in situ electron microscopy nanomechanical study of the non-covalent van der Waals interfaces between individual CNTs and epoxy resins in conjunction with atomistic simulations. By pulling out individual double-walled CNTs from Epon 828 films inside a high resolution electron microscope, the nanomechanical measurements capture the shear lag effect on CNT–epoxy interfaces. The maximum pull-out load of CNT–epoxy interfaces is found to be about 44% higher than the recently reported value for CNT–poly(methyl methacrylate) (PMMA) interfaces that were characterized using the same experimental technique and the same batch of dispersed CNTs. The higher interfacial strength of CNT–epoxy interfaces is partially attributed to the forced molecular deformations of aromatic rings in epoxy chains in the vicinity of the binding interface, which is supported by molecular dynamics simulations of the CNT–polymer interfacial interactions. The research findings contribute to a better understanding of the local load transfer on the tube–polymer interface and the tube’s reinforcing mechanism, and ultimately the optimal design and performance of nanotube-reinforced polymer nanocomposites.

Single-walled carbon nanotube–epoxy composites for structural and conductive aerospace adhesives

Single-walled carbon nanotubes (SWCNTs) were incorporated at low loading (up to ∼1wt%) into an unfilled aerospace-grade epoxy system, to impart electrical conductivity while maintaining structural bonding capability, as a route for development of a structural and conductive adhesive. At these low SWCNT loadings the tensile properties were maintained or improved, while strength decreased in a higher loading case. The structural bonding performance of composite-to-composite joints, evaluated in lap-shear and peel tests, was reasonably maintained for adhesives containing 0.5wt% or 1wt% SWCNTs. In the case of the 0.5wt% SWCNT–adhesive, peel and lap-shear strength were unchanged while the addition of 1wt% resulted in 30% increase of peel strength but the lap-shear strength was reduced by 10–15%. For 1wt% SWCNT–adhesives, conductivities as high as 10−1Sm−1 and typically ∼10−3Sm−1 were achieved. Joint electrical resistance measured between aluminum adherends was larger than predicted by the bulk conductivity, but was reduced by a post-treatment step resulting in apparent joint conductivities within one order of magnitude of the bulk samples.

Simultaneous ultrasonication‐assisted internal mixing to prepare MWCNTs‐filled epoxy composites with increased strength and thermal conductivity

The uniform dispersion of carbon nanotubes in epoxy resin is one of the key factors to achieve the composites with desirable mechanical and physical property enforcement. However, the widely used dispersion methods have their own respective limitations in pursuing satisfactory nanotube dispersion. Herein, a new dispersion approach, based on the synergetic effect of combining high speed internal mixing with running simultaneously continuous ultrasonication treatment, has been proposed. The dispersion of nanotubes was carried out in a high speed internal mixer, consisting of twin kneading block structured rotors and an integrated ultrasonic horn, which was intercalated into the central position between the twin rotors. At first, the FEM simulation was conducted to optimize the kneading element assembly and illuminate the geometry influence of the ultrasonic horn intercalation on the mixing flow. Afterwards, to confirm the feasibility of the approach, pristine MWCNTs (P‐CNTs), oxidation modified MWCNTs (M‐CNTs) and M‐CNTs/multilayer graphene nanoplatelets (MGPs) hybrid are dispersed into epoxy resin. The dispersion of each sample in its liquid epoxy state is investigated under transparent optical microscopy. More characterizations, including SEM, TG/DTA, tensile test, and thermal conductivity measurements, were conducted on the cured composites. Competitive reinforcements on mechanical tensile property and thermal conductivity were observed. Especially, at a 1.5 wt% M‐CNTs/MGPs hybrid content, the composite mechanical tensile strength and thermal conductivity were 47% and 30% higher than those of neat epoxy. This preliminary study demonstrates the feasibility and practicability of the proposed approach to achieving good MWCNTs dispersion and distribution in epoxy resin. POLYM. COMPOS., 37:870–880, 2016. © 2014 Society of Plastics Engineers

Creep Behavior Study for Carbon Fiber Nano-Composites

The surface modification of carbon nanotubes (CNTs) has been recently observed to influence the distribution of CNTs in epoxy resin and the mechanical properties and electrical conductivities of these CNTs. Accordingly, the treatment of CNTs to with organic acids to oxidize them generates functional groups on the surface of CNTs. This investigation studies the consequent enhancement of the mechanical properties and electrical conductivities of CNTs. The influence of adding various proportions of CNTs to the epoxy resin on the mechanical properties and electrical conductivities of the composites thus formed is investigated, and the strength of the material is tested at different temperatures.The test results also indicate that mechanical strength and electrical conductivity increase with the amount of CNTs added to the composites. Different coefficients of expansion of the matrix, fiber and CNTs, are such that overexpansion of the matrix at high temperature results in cracking in it.Moreover, the creep behaviors of carbon fiber (CF) /epoxy resin thermosetting composites and CNTs/CF/ epoxy resin composites were tested and analyzed at different stresses, orientations of fiber, temperatures and humidities. The creep exhibits only two stages-primary creep and steady-state creep. The effects of creep stress, creep time, and humidity on the creep of composites that contain various proportion of CNTs were investigated at various temperatures.Additionally, increasing the number of cycles in cyclic creep tests at room temperature resulted in a decrease in creep strain even at a high temperature of 55°C. Possible room temperature creep mechanisms have been proposed and discussed. With increasing number of creep tests, the creep strain decreased due to strain hardening which occurred during creep. Creep strain is believed to increase with applied stress, creep time, humidity, temperature and degree of the angle θ between the orientation of fiber and the direction of the applied stress.Finally, the test results of creep strain of CF/epoxy resin composites and CNTs/CF/epoxy resin composites tested under various conditions can be smoothly fitted by the fitting curves of Findley power law.

Ultralow percolation threshold of single walled carbon nanotube-epoxy composites synthesized via an ionic liquid dispersant/initiator

Uniform dispersion of single walled carbon nanotubes (SWNTs) in an epoxy was achieved by a streamlined mechano-chemical processing method. SWNT-epoxy composites were synthesized using a room temperature ionic liquid (IL) with an imidazolium cation and dicyanamide anion. The novel approach of using ionic liquid that behaves as a dispersant for SWNTs and initiator for epoxy polymerization greatly simplifies nanocomposite synthesis. The material was processed using simple and scalable three roll milling. The SWNT dispersion of the resultant composite was evaluated by electron microscopy and electrical conductivity measurements in conjunction with percolation theory. Processing conditions were optimized to achieve the lowest possible percolation threshold, 4.29 × 10−5 volume fraction SWNTs. This percolation threshold is among the best reported in literature yet it was obtained using a streamlined method that greatly simplifies processing.

Evaluation of mechanical properties of untwisted carbon nanotube yarn for application to composite materials

Abstract Carbon nanotubes (CNTs) with superior mechanical properties have been of interest as reinforcement for polymer composites. However, the length of individual CNTs is limited. As a solution, yarns spun by twisting together multi-walled carbon nanotubes (MWCNTs) have been reported. In this study, untwisted CNT yarns were prepared by a non-conventional method drawing CNTs through a die. The MWCNTs in these yarns are held together by strong van der Waals forces that arise due to the interactions on the long and smooth surfaces of the MWCNTs. Here, mechanical properties of untwisted CNT yarn were studied by tensile tests. The strength of the CNT yarn was increased by increasing the apparent density of the yarn. The CNT yarns showed high tensile strength of 1 GPa and elastic modulus of 79 GPa at a yarn diameter of 35 μm. The interfacial shear strength between the CNT yarn and epoxy resin was studied by the microdroplet method, and it was very low. The wettability of the CNT yarn was affected by a type of curing agent. A unidirectional composite of epoxy resin and CNT yarn was prepared by the pultrusion molding method. Mechanical properties of the unidirectional composite were affected by the type of curing agent.