Mechanical properties of aligned multi-walled carbon nanotube/epoxy composites processed using a hot-melt prepreg method

This study examined the mechanical properties of aligned multi-walled carbon nanotube (CNT)/epoxy composites processed using a hot-melt prepreg method. Vertically aligned ultra-long CNT arrays (forest) were synthesized using chemical vapor deposition, and were converted to horizontally aligned CNT sheets by pulling them out. An aligned CNT/epoxy prepreg was fabricated using hot-melting with B-stage cured epoxy resin film. The resin content in prepreg was well controlled. The prepreg sheets showed good drapability and tackiness. Composite film specimens of 24-33 μm thickness were produced, and tensile tests were conducted to evaluate the mechanical properties. The resultant composites exhibit higher Young’s modulus and tensile strength than those of composites produced using conventional CNT/epoxy mixing methods. For example, the maximum elastic modulus and ultimate tensile strength (UTS) of a CNT (21.4 vol.%)/epoxy composite were 50.6 GPa and 183 MPa. These values were, respectively, 19 and 2.9 times those of the epoxy resin.     

Mechanical property enhancement of aligned multi-walled carbon nanotube sheets and composites through press-drawing process

A solid-state drawing and winding process was done to create thin aligned carbon nanotube (CNT) sheets from CNT arrays. However, waviness and poor packing of CNTs in the sheets are two main weaknesses restricting their reinforcing efficiency in composites. This report proposes a simple press-drawing technique to reduce wavy CNTs and to enhance dense packing of CNTs in the sheets. Non-pressed and pressed CNT/epoxy composites were developed using prepreg processing with a vacuum-assisted system. Effects of pressing on the mechanical properties of the aligned CNT sheets and CNT/epoxy composites were examined. Pressing with distributed loads of 147, 221, and 294 N/m showed a substantial increase in the tensile strength and the elastic modulus of the aligned CNT sheets and their composites. The CNT sheets under a press load of 221 N/m exhibited the best mechanical properties found in this study. With a press load of 221 N/m, the pressed CNT sheet and its composite, respectively, enhanced the tensile strength by 139.1 and 141.9%, and the elastic modulus by 489 and 77.6% when compared with non-pressed ones. The pressed CNT/epoxy composites achieved high tensile strength (526.2 MPa) and elastic modulus (100.2 GPa). Results show that press-drawing is an important step to produce superior CNT sheets for development of high-performance CNT composites.     

Effects of stretching on mechanical properties of aligned multi-walled carbon nanotube/epoxy composites

Composites based on epoxy resin and differently aligned multi-walled carbon nanotube (MWCNT) sheets have been developed using hot-melt prepreg processing. Aligned MWCNT sheets were produced from MWCNT arrays using the drawing and winding technique. Wavy MWCNTs in the sheets have limited reinforcement efficiency in the composites. Therefore, mechanical stretching of the MWCNT sheets and their prepregs was conducted for this study. Mechanical stretching of the MWCNT sheets and hot stretching of the MWCNT/epoxy prepregs markedly improved the mechanical properties of the composites. The improved mechanical properties of stretched composites derived from the increased MWCNT volume fraction and the reduced MWCNT waviness caused by stretching. With a 3% stretch ratio, the MWCNT/epoxy composites achieved their best mechanical properties in this study. Although hot stretching of the prepregs increased the tensile strength and modulus of the composites considerably, its efficiency was lower than that of stretching the MWCNT sheets.     

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.     

Effect of carbon nanotube (CNT) content on the mechanical properties of CNT-reinforced aluminium composites

The interest in carbon nanotubes (CNTs) as reinforcements for aluminium (Al) has been growing considerably. Efforts have been largely focused on investigating their contribution to the enhancement of the mechanical performance of the composites. The uniform dispersion of CNTs in the Al matrix has been identified as being critical to the pursuit of enhanced properties. Ball milling as a mechanical dispersion technique has proved its potential. In this work, we use ball milling to disperse up to 5 wt.% CNT in an Al matrix. The effect of CNT content on the mechanical properties of the composites was investigated. Cold compaction and hot extrusion were used to consolidate the ball-milled Al–CNT mixtures. Enhancements of up to 50% in tensile strength and 23% in stiffness compared to pure aluminium were observed. Some carbide formation was observed in the composite containing 5 wt.% CNT. In spite of the observed overall reinforcing effect, the large aspect ratio CNTs used in the present study were difficult to disperse at CNT wt.% greater than 2, and thus the expected improvements in mechanical properties with increase in CNT weight content were not fully realized.     

Mechanical and electrical property improvement in CNT/Nylon composites through drawing and stretching

The excellent mechanical properties of carbon nanotubes (CNTs) make them the ideal reinforcements for high performance composites. The misalignment and waviness of CNTs within composites are two major issues that limit the reinforcing efficiency. We report an effective method to increase the strength and stiffness of high volume fraction, aligned CNT composites by reducing CNT waviness using a drawing and stretching approach. Stretching the composites after fabrication improved the ultimate strength by 50%, 150%, and 190% corresponding to stretch ratios of 2%, 4% and 7%, respectively. Improvement of the electrical conductivities exhibited a similar trend. These results demonstrate the importance of straightening and aligning CNTs in improving the composite strength and electrical conductivity.     

Properties of composites of carbon nanotube fibres

Composites have set the standard for high strength materials for several decades. With the discovery of nanotubes, new possibilities for reinforced composites have arisen, with potential mechanical properties superior to those of currently available materials. This paper reports the properties of epoxy matrix reinforced with fibres of carbon nanotubes (CNTs) which, in many ways, are similar to standard composites reinforced with commercial fibres. The composites were formed by the back diffusion of the uncured epoxy into an array of aligned fibres of CNTs. The fibre density and volume fraction were measured from thermogravimetric analysis (TGA). Properties in tension and compression were measured, and the level of fibre–matrix interaction analysed fractographically. The results show the significant potential for this route to CNT reinforcement.     

CNT reinforced light metal composites produced by melt stirring and by high pressure die casting

Light metal matrix composites are of great interest due to their potential for reducing CO₂ emission through lightweight design e.g. in the automotive sector. Carbon nanotubes can be considered as ideal reinforcements, due to their high strength, high aspect ratio and thermo-mechanic properties. In this research, CNT reinforced light metal composites were produced by melt stirring and by high pressure die casting, which can be both easily scaled up. The light metal composites showed significantly improved mechanical properties already at small CNT contents. The influence of CNT concentration on the composites was also studied.     

In situ polymerized nanocomposites: Polystyrene/CNT and Poly(methyl methacrylate)/CNT composites

Carbon nanotubes (CNTs) were incorporated into polystyrene (PS) and poly(methyl methacrylate) (PMMA) matrices via in situ emulsion and emulsion/suspension polymerization methods. The polymerizations were carried out using various initiators, surfactants, and carbon nanotubes to determine their influence on polymerization and on the properties of the composites. The loading of CNTs in the composites varied from 0 to 15 wt.%, depending on the CNTs used. Morphology and dispersion of the CNTs were analyzed by transmission and scanning electron microscopy techniques. The dispersion of multi-walled carbon nanotubes (MWCNT) in the composites was excellent, even at high CNT loading. The mechanical properties, and electrical and thermal conductivities, of the composites were also analyzed. Both electrical and thermal conductivities were improved.     

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.     

Mechanical performance of CNT-filled glass fiber/epoxy composite in in-situ elevated temperature environments emphasizing the role of CNT content

Carbon nanotubes (CNTs) are one of the prime choice nano-filler reinforcement for fibrous polymeric composites. But the stability of the CNT/polymer interface is yet to be ensured for elevated temperature engineering applications. Present study deals with the assessment of elevated temperature durability of glass fiber/epoxy (GE) composite with various level of multi walled carbon nanotube (MWCNT) loading. Flexural testing at room temperature revealed that addition of 0.1% MWCNT yielded maximum strength (+32.8% over control GE) and modulus (+11.5% over control GE) amongst all the CNT modified composite systems. Further, MWCNT–GE composites resulted in accelerated degradation of mechanical performance with increasing temperature as compared to GE composite. Dynamic mechanical thermal analysis (DMTA) was carried out to study the viscoelastic behavior of all composites over a range of temperature. The design parameters were evaluated by Weibull probability function. Fractographic analysis figured out various failure modes in all composites at various temperatures.     

Improvement in mechanical properties of recycled GF prepreg with CNT reinforced composites using a spray coating method

Dispersion and shape of nanoparticles, as well as interfacial conditions, add significantly to difficulties in composite manufacture. In the work reported here, an innovative method of recycling composites using out-of-date prepreg was investigated in which the carbon nanotube (CNT) on the prepreg was optimally coated. Nanocomposites utilizing the out-of-date prepreg were coated with CNT and fabricated by a sheet molding method. CNT nanofillers were observed to be uniformly dispersed on epoxy prepreg by spray coating. The mechanical and interfacial properties of these CNT coated nanocomposites were improved over those of more conventionally manufactured carbon fiber/epoxy composites. The CNT nanofillers were embedded at the epoxy and fiber interface, as a result of etching of the epoxy prepreg surface by a CNT dispersion solution which enhanced interfacial reactivity.     

New alignment procedure of magnetite–CNT hybrid nanofillers on epoxy bulk resin with permanent magnets

The main properties of epoxy composites reinforced with aligned carbon nanotubes (CNTs) have been studied. The alignment was carried out in a specific designed device applying a weak magnetic field (0.3 T) with permanent magnets. CNTs were modified with magnetite nanoparticles (Fe₃O₄) functionalized, in a one-stage-process which does not require use of strong acids or aggressive treatments which could affect the structural integrity of CNTs. The study by transmission electron microscopy confirmed that the Fe₃O₄ nanoparticles were closely bonded over CNT surfaces. The thermo-mechanical and tensile properties of composites measured were higher than neat epoxy resin and were similar for both composites: reinforced with neat CNTs and magnetite–CNT hybrid nanofillers. The electrical behaviour indicates a high anisotropy for aligned composites, showing an increase of one order of magnitude for the electrical conductivity in the direction of aligned nanotubes.     

Evaluation of optimal dispersion conditions for CNT reinforced epoxy composites using cyclic voltammetry measurements

The optimum dispersion time of nanoparticles is important for obtaining uniform dispersion of fillers or other additives in a matrix. In this study, the optimal dispersion time of carbon nanotube (CNT) in a matrix was investigated using cyclic voltammetry (CV), measurement for different dispersion methods and times. In addition, the mechanical properties of CNT composites manufactured using different dispersion methods were evaluated by tensile and flexural tests. The CV and mechanical test results were correlated to the dispersion condition of CNT in the composites. It was found that tip-type sonication resulted in better dispersion than bath-type sonication. Improved CNT dispersion resulted in composites with both enhanced CV measurements and improved mechanical properties. In the study reported here, improvements in dispersion were generally accompanied by higher electrical currents. This suggests that the CV measurement method is an effective tool for determining optimal dispersion times, for different CNT dispersion processes.     

Effect of surfactant treatment on thermal stability and mechanical properties of CNT/polybenzoxazine nanocomposites

The mechanical and thermo-mechanical properties of polybenzoxazine nanocomposites containing multi-walled carbon nanotubes (MWCNTs) functionalized with surfactant are studied. The results are specifically compared with the corresponding properties of epoxy-based nanocomposites. The CNTs bring about significant improvements in flexural strength, flexural modulus, storage modulus and glass transition temperature, T_g, of CNT/polybenzoxazine nanocomposites at the expense of impact fracture toughness. The surfactant treatment has a beneficial effect on the improvement of these properties, except the impact toughness, through enhanced CNT dispersion and interfacial interaction. The former four properties are in general higher for the CNT/polybenzoxazine nanocomposites than the epoxy counterparts, and vice versa for the impact toughness. The addition of CNTs has an ameliorating effect of lowering the coefficient of thermal expansion (CTE) of polybenzoxazine nanocomposites in both the regions below and above T_g, whereas the reverse is true for the epoxy nanocomposites. This observation has a particular implication of exploiting the CNT/polybenzoxazine nanocomposites in applications requiring low shrinkage and accurate dimensional control.     

Enhancement of mechanical performance of epoxy/carbon fiber laminate composites using single-walled carbon nanotubes

Carbon nanotubes (CNT) in their various forms have great potential for use in the development of multifunctional multiscale laminated composites due to their unique geometry and properties. Recent advancements in the development of CNT hierarchical composites have mostly focused on multi-walled carbon nanotubes (MWCNT). In this work, single-walled carbon nanotubes (SWCNT) were used to develop nano-modified carbon fiber/epoxy laminates. A functionalization technique based on reduced SWCNT was employed to improve dispersion and epoxy resin-nanotube interaction. A commercial prepregging unit was then used to impregnate unidirectional carbon fiber tape with a modified epoxy system containing 0.1 wt% functionalized SWCNT. Impact and compression-after-impact (CAI) tests, Mode I interlaminar fracture toughness and Mode II interlaminar fracture toughness tests were performed on laminates with and without SWCNT. It was found that incorporation of 0.1 wt% of SWCNT resulted in a 5% reduction of the area of impact damage, a 3.5% increase in CAI strength, a 13% increase in Mode I fracture toughness, and 28% increase in Mode II interlaminar fracture toughness. A comparison between the results of this work and literature results on MWCNT-modified laminated composites suggests that SWCNT, at similar loadings, are more effective in enhancing the mechanical performance of traditional laminated composites.     

Ablative and mechanical evaluation of CNT/phenolic composites by thermal and microstructural analyses

Highly ablation resistant carbon nanotube (CNT)/phenolic composites were fabricated by the addition of low concentrations of CNTs. Tensile and compressive mechanical properties as well as ablation resistance were significantly improved by the addition of only 0.1 and 0.3 wt% of uniformly dispersed CNTs. An oxygen–kerosene-flame torch and a scanning electron microscope (SEM) were used to evaluate the ablative properties and microstructures. Thermal gravimetric analysis (TGA) revealed that the ablation rate was lower for the 0.3 wt% CNT/phenolic composites than for neat phenolic or the composite with 0.1 wt% CNTs. Ablation mechanisms for all three materials were investigated using TGA in conjunction with microstructural studies using a SEM. The microstructural studies revealed that CNTs acted as an ablation resistant phase at high temperatures, and that the uniformity of the CNT dispersion played an important role in this ablation resistance.     

Reinforcing brittle and ductile epoxy matrices using carbon nanotubes masterbatch

In this study, the mechanical and thermal properties of epoxy composites using two different forms of carbon nanotubes (powder and masterbatch) were investigated. Composites were prepared by loading the surface-modified CNT powder and/or CNT masterbatch into either ductile or brittle epoxy matrices. The results show that 3 wt.% CNT masterbatch enhances Young’s modulus by 20%, tensile strength by 30%, flexural strength by 15%, and 21.1 °C increment in the glass transition temperature (by 34%) of ductile epoxy matrix. From scanning electron microscopy images, it was observed that the CNT masterbatch was uniformly distributed indicating the pre-dispersed CNTs in the masterbatch allow an easier path for preparation of CNT-epoxy composites with reduced agglomeration of CNTs. These results demonstrate a good CNT dispersion and ductility of epoxy matrix play a key role to achieve high performance CNT-epoxy composites.     

The effective properties and local aggregation effect of CNT/SMP composites

A micromechanics model of the thermomechanical constitutive behavior and micro-structural inhomogeneity of carbon nanotubes (CNTs)/shape memory polymer (SMP) composites is presented. It is assumed that the CNTs are elastic and the SMP obeys a thermomechanical constitutive law. The effective properties of CNT/SMP composites are examined using a micro-mechanics method. The effect of CNT aggregation in the composite, frequently encountered in real engineering situations, is studied. The degree of aggregation is described by an aggregation coefficient, and the effective properties of SMP composites with aggregated CNTs are calculated using a stepping scheme. It is shown that the degree of CNT aggregation dramatically influences the effective properties of the CNT/SMP composites. A homogeneous microstructure leads to maximum levels of effective composite properties.     

Formation and mechanical characterisation of SU8 composite films reinforced with horizontally aligned and high volume fraction CNTs

Carbon nanotube (CNT) reinforced composites have been identified as promising structural materials for the mechanical components of microelectromechanical systems (MEMS), potentially leading to advanced performance. High alignment and volume fraction of CNTs in the composites are the prerequisites to achieve such desirable mechanical characteristics. In particular, horizontal CNT alignment in composite films is necessary to enable high longitudinal moduli of the composites which is crucial for the performance of microactuators. A practical process has been developed to transfer CNT arrays from vertical to horizontal alignment which is followed by in situ wetting, realign and pressurized consolidation processes, which lead to a high CNT volume fraction in the range of 46-63%. As a result, SU8 epoxy composite films reinforced with horizontally aligned CNTs and a high volume faction of CNTs have been achieved with outstanding mechanical characteristics. The transverse modulus of the composite films has been characterised through nanoindentation and the longitudinal elastic modulus has been investigated. An experimental transverse modulus of 9.6 GPa and an inferred longitudinal modulus in the range of 460-630 GPa have been achieved, which demonstrate effective CNT reinforcement in the SU8 matrix.