Influence of ultrasonic dual mixing on thermal and tensile properties of MWCNTs-epoxy composite

Multiwalled carbon nanotubes (MWCNTs) reinforced epoxy based composites were fabricated by using an innovative ultrasonic dual mixing (UDM) process consists of ultrasonic mixing with simultaneous magnetic stirring. The effect of addition of varying amount of MWCNTs on thermal stability and tensile properties of the epoxy based composite has been investigated. It is found that the thermal stability, tensile strength and toughness of the epoxy base improves with the increase of MWCNTs addition up to 1.5 wt.% and UDM processing at certain capacity of the system. Tensile tests and thermal gravimetric analysis (TGA) were performed on each group of composites containing different amount of MWCNTs to determine their mechanical and thermal properties respectively. The dispersion of 1.5 wt.% MWCNTs fillers in epoxy nanocomposites was studied by transmission electron microscopy (TEM) as well as by field emission scanning electron microscopy (FESEM) applied on their tensile fracture surface.     

A comparative study of the electrical and mechanical properties of epoxy nanocomposites reinforced by CVD- and arc-grown multi-wall carbon nanotubes

In this study, three different types of multi-wall carbon nanotubes (MWCNTs) were compared as nanostructured reinforcements in epoxy polymers: commercially available CVD-MWCNTs, synthesised in an industrial process, aligned-CVD-MWCNTs and arc-grown MWCNTs, both obtained from a lab-scale processes. The nanocomposite properties were characterised by means of electron microscopy, rheological, electrical and mechanical methods. Industrial CVD-MWCNTs are favourable for the implication of an electrical conductivity in the epoxy due to their high tendency to form conducting networks. The less entangled structure of aligned-CVD-MWCNTs turns out to be favourable for an easy dispersion and low viscosity in epoxy at similar conductivities compared to the CVD-MWCNTs. Additionally, they provide the highest increase in fracture toughness (∼17%). Arc-grown MWCNTs do not offer any electrical conductivity in epoxy without sufficient purification methods. Their high level of impurities and short length further complicate the transfer of their good electrical and mechanical properties into the nanocomposite.     

Fracture mechanisms of epoxy filled with ozone functionalized multi-wall carbon nanotubes

This work focused on the fracture mechanisms and reinforcing effects of ozone-treated multi-walled carbon nanotubes (MWCNTs) in epoxy matrix. Ozone functionalization of MWCNTs was found to be of help for a better dispersion and stronger interfacial bonding with epoxy matrix, which in turn improve the strength and fracture toughness of the resin. The MWCNT/epoxy composites showed complicated failure modes than the conventional fibrous composites, which have been quantitatively investigated and correlated with the fracture toughness of the nanocomposites studied.     

Modification of surface functionality of multi-walled carbon nanotubes on fracture toughness of basalt fiber-reinforced composites

In this work, we studied the influence of surface functionality of multi-walled carbon nanotubes (MWCNTs) on the mechanical properties of basalt fiber-reinforced composites. Acid and base values of the MWCNTs were determined by Boehm's titration technique. The surface properties of the MWCNTs were determined FT-IR, and XPS. The mechanical properties of the composites were assessed by measuring the interlaminar shear stress, fracture toughness, fracture energy, and impact strength. The chemical treatments led to a change of the surface characteristics of the MWCNTs and of the mechanical interfacial properties of MWCNTs/basalt fibers/epoxy composites. Especially the acid-treated MWCNTs/basalt fibers/epoxy composites had improved mechanical properties compared to the base-treated and non-treated MWCNTs/basalt fibers/epoxy composites. These results can probably be attributed to the improved interfacial bonding strength resulting from the improved dispersion and interfacial adhesion between the epoxy resin and the MWCNTs.     

Improving the mechanical properties of multiwalled carbon nanotube/epoxy nanocomposites using polymerization in a stirring plasma system

Uniform treatment of multiwalled carbon nanotubes by plasma treatment has been investigated using a custom-built stirring plasma system. A thin plasma polymer with high levels of amine groups has been deposited on MWCNTs using a combination of continuous wave and pulsed plasma polymerization of heptylamine in the stirring plasma system. Scanning electron microscopy showed that the plasma polymerization improved the dispersion and interfacial bonding of the MWCNTs with an epoxy resin at loadings of 0.1, 0.3 and 0.5 wt%. The flexural and thermal mechanical properties of plasma polymerized MWCNT/epoxy nanocomposites were also significantly improved while untreated MWCNT/epoxy nanocomposites showed an opposite trend. The epoxy with 0.5 wt% plasma polymerized MWCNTs had the greatest increase in flexural properties, with the flexural modulus, flexural strength and toughness increasing by about 22%, 17% and 70%, respectively.     

In situ preparation of reinforced polyimide nanocomposites with the noncovalently dispersed and matrix compatible MWCNTs

High performance MWCNT-reinforced polyimide nanocomposites were prepared through in situ polymerization route. 2,6-Diaminoanthraquinone (2,6-DAAQ) was selected to serve as both a diamine comonomer, and a noncovalent dispersant of MWCNTs through π–π interaction. The good dispersion of MWCNTs in both solution and polymer matrix, and high compatibility derived from the structural similarity have been proved to be particularly advantageous for the interfacial adhesion and load transfer from the polymer matrix to MWCNTs. The nanocomposites with 0.50–0.75 wt% MWCNTs had the most excellent thermal stability, thermomechanical and tensile properties, and the electrical conductivity also achieved a sharp increase at such low content. The load transfer efficiency was calculated based on both theoretical model and microscopical size measurement, and the results were rather reasonable within accepted error range.     

Fabrication and mechanical properties of well-dispersed multiwalled carbon nanotubes/epoxy composites

Multiwalled carbon nanotubes (MWCNTs)/epoxy nanocomposites were fabricated by using ultrasonication and the cast molding method. In this process, MWCNTs modified by mixed acids were well dispersed and highly loaded in an epoxy matrix. The effects of MWCNTs addition and surface modification on the mechanical performances and fracture morphologies of composites were investigated. It was found that the tensile strength improved with the increase of MWCNTs addition, and when the content of MWCNTs loading reached 8 wt.%, the tensile strength reached the highest value of 69.7 MPa. In addition, the fracture strain also enhanced distinctly, implying that MWCNTs loading not only elevated the tensile strength of the epoxy matrix, but also increased the fracture toughness. Nevertheless, the elastic modulus reduced with the increase of MWCNTs loading. The reasons for the mechanical property changes are discussed.     

Aligning carbon nanotubes using bulk acoustic waves to reinforce polymer composites

Bulk acoustic waves (BAWs) are used to align multi-walled carbon nanotubes (MWCNTs) in polymer composite materials. MWCNTs are first dispersed in the liquid state of a thermoset resin and aligned using standing BAWs. Cross-linking of the resin fixates the aligned MWCNTs in the polymer matrix material. We have quantified the alignment obtained with this method on the macro, micro, and nanoscale, and it is found to be similar to other alignment techniques such as stretching, slicing, and wet spinning. The elastic modulus and ultimate tensile strength of composite material specimens with aligned MWCNTs, fabricated using this technique, are evaluated and compared with specimens consisting of randomly oriented MWCNTs and resin material without MWCNTs. Different MWCNT loading rates are considered. The elastic modulus of composite material specimens with only 0.15 weight percent aligned MWCNTs is observed to be 44% higher than specimens with randomly oriented MWCNTs, and 51% higher than specimens without MWCNTs. However, further increasing the MWCNT loading rate does not significantly increase the elastic modulus and ultimate tensile strength, likely because of insufficient dispersion of MWCNTs in the thermoset matrix material.     

Interconnected multi-walled carbon nanotubes reinforced polymer-matrix composites

A modified method for interconnecting multi-walled carbon nanotubes (MWCNTs) was put forward. And interconnected MWCNTs by reaction of acyl chloride and amino groups were obtained. Scanning electron microscopy shows that hetero-junctions of MWCNTs with different morphologies were formed. Then specimens of pristine MWCNTs, chemically functionalized MWCNTs and interconnected MWCNTs reinforced epoxy resin composites were fabricated by cast moulding. Tensile properties and fracture surfaces of the specimens were investigated. The results show that, compared with pristine MWCNTs and chemically functionalized MWCNTs, the chemically interconnected MWCNTs improved the fracture strain and therefore the toughness of the composites significantly.     

Thermal conductive and electrical properties of polyurethane/hyperbranched poly(urea-urethane)-grafted multi-walled carbon nanotube composites

Hyperbranched poly(urea-urethane)-grafted multi-walled carbon nanotubes (HPU-MWCNTs) were incorporated in a polyurethane (PU) matrix based on poly(ethylene oxide-tetrahydrofuran) and aliphatic polyisocyanate resin as curing agent. The 9–12 nm thick HPU shell formed on the MWCNTs improved the dispersion of MWCNTs and enhanced the interfacial adhesion between the PU matrix and MWCNTs, leading to improvements in storage modulus and T_g of the composites and enhancement of the thermal stability of PU. Thus, composites with 0.5–1 wt% MWCNTs increased the thermal conductivity by about 60–70% when compared to, and retained the high electrical resistivity of, neat PU.     

Effect of moisture on elastic and viscoelastic properties of epoxy and epoxy-based carbon fibre reinforced plastic filled with multiwall carbon nanotubes

In this study, we investigated the peculiarities of moisture absorption and moisture-induced effects on the elastic and viscoelastic flexural properties of epoxy resin and carbon fibre reinforced plastic (CFRP) filled with multiwall carbon nanotubes (MWCNTs). Short-term cyclic creep-recovery tests of moistened epoxy and CFRP filled with MWCNTs revealed improvements in creep resistance for both materials. The addition of MWCNTs to the epoxy resin suppressed the moisture absorption by the material, causing a reduction in the diffusion coefficient by 31% and equilibrium moisture content by 15%. The addition of MWCNTs reduced the flexural strength of moistened epoxy and CFRP samples by approximately half, and also lowered the flexural modulus by ∼1.4 and ∼3 times, elastic strain by 1.25 and 1.04 times, viscoelastic strain by 1.39 and 1.03 times, and plastic strain by 2.68 and 1.60 times, respectively.     

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.     

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.     

Compressive properties of nanoparticle modified epoxy resin at different strain rates

Epoxy resins often exhibit high strength yet are often brittle, especially at high strain rates. Block copolymer modified epoxy resins have generated significant interest since it was demonstrated that the combination could lead to nanostructured thermosets through the self-assembly of the block copolymer. Such nanostructured epoxies exhibit increased ductility without the significant loss in yield strength exhibited by traditional rubber-modified epoxies. In this study, the effect of different nanoscale additives on the compressive yield strength of a model epoxy resin has been studied. In the first case, a block copolymer styrene-b-butadiene-b-polymethylmethacrylate (SBM) was added to the model epoxy resin. In the second case, carbon nanotubes (CNTs) were added. In the final case, both additives were mixed simultaneously with the epoxy resin. The compressive mechanical behavior of these materials has been investigated over a wide range of strain rates (0.001–3500 s⁻¹). The yield behavior was found to fit the cooperative yield model proposed by Fotheringham and Cherry.     

Reduction of thermal residual stresses of laminated polymer composites by addition of carbon nanotubes

This paper studies the effects of multi-walled carbon nanotubes (MWCNTs) on the thermal residual stresses in polymeric fibrous composites. Reinforced ML-506 epoxy nanocomposites with different amounts of homogeneously dispersed MWCNTs (0.1 wt.%, 0.5 wt.% and 1 wt.%) were fabricated using the sonication technique. Thermo-mechanical analysis and tensile tests of the specimens were carried out to characterize the thermal and mechanical properties of MWCNTs/epoxy composites. Due to the negative thermal expansion and high modulus of MWCNTs, addition of MWCNTs resulted in a great reduction of the coefficient of thermal expansion (CTE) of epoxy. The MWCNTs also moderately increased the Young’s modulus of the epoxy. Then, the effects of adding MWCNTs on micro and macro-residual stresses in carbon fiber (CF)/epoxy laminated composites were investigated using the energy method and the classical lamination theory (CLT), respectively. The results indicated that the addition of low amounts of MWCNTs leads to a considerable reduction in thermal residual stress components in both micro and macro levels.     

Fatigue behaviour of glass fibre reinforced epoxy composites enhanced with nanoparticles

Nanoparticle reinforcement of the matrix in laminates has been recently explored to improve mechanical properties, particularly the interlaminar strength. This study analyses the fatigue behaviour of nanoclay and multiwalled carbon nanotubes enhanced glass/epoxy laminates. The matrix used was the epoxy resin Biresin® CR120, combined with the hardener CH120-3. Multiwalled carbon nanotubes (MWCNTs) 98% and organo-montmorillonite Nanomer I30 E nanoclay were used. Composites plates were manufactured by moulding in vacuum. Fatigue tests were performed under constant amplitude, both under tension–tension and three points bending loadings. The fatigue results show that composites with small amounts of nanoparticles addition into the matrix have bending fatigue strength similar to the obtained for the neat glass fibre reinforced epoxy matrix composite. On the contrary, for higher percentages of nanoclays or carbon nanotubes addition the fatigue strength tend to decrease caused by poor nanoparticles dispersion and formation of agglomerates. Tensile fatigue strength is only marginally affected by the addition of small amount of particles. The fatigue ratio in tension–tension loading increases with the addition of nanoclays and multi-walled carbon nanotubes, suggesting that both nanoparticles can act as barriers to fatigue crack propagation.     

Influence of geometries of multi-walled carbon nanotubes on the pore structures of Buckypaper

Influence of geometries, i.e. lengths and diameters, of multi-walled carbon nanotubes (MWCNTs) on pore sizes and distributions as well as resin impregnation quality of Buckypaper was studied. Three kinds of MWCNTs with different lengths and diameters were used to fabricate Buckypapers by the process of vacuum assisted filtration. Scanning electron microscopy (SEM) and N₂ adsorption results show that the Buckypaper made of MWCNTs with longer lengths and thinner diameters has the smallest mean pore diameters and best pore size distribution. The results also show that MWCNTs with larger diameters are more inclined to form pore structures with larger pore sizes, and MWCNTs with shorter lengths are more inclined to form pore structures with much wider pore size distribution. Microstructure characterization of the Buckypaper/epoxy composites also exhibited phenomenon of the same rules.     

Evaluation of interfacial shear stress between multi-walled carbon nanotubes and epoxy based on strain distribution measurement using Raman spectroscopy

This study investigated the stress recovery of aligned multi-walled carbon nanotubes (MWCNTs) embedded in epoxy using Raman spectroscopy, and evaluated interfacial shear stress between MWCNTs and epoxy using shear-lag analysis. To this end, ultralong aligned MWCNTs (3.8 mm long) were embedded in epoxy to obtain Raman spectra at multiple points along the MWCNTs. Downshift of the G′-band due to tensile strain was measured from the nanotube end to the center, and the strain distribution of embedded MWCNTs was evaluated successfully. Interfacial shear stress was then estimated by minimizing the error between the shear-lag analysis and measured strain distribution. The maximum interfacial shear stress between the embedded MWCNTs and epoxy was 10.3–24.1 MPa at the failure strain of aligned MWCNT-reinforced epoxy composites (0.46% strain). Furthermore, the interfacial shear stress between an individual MWCNT and epoxy was investigated.     

Tribological performance of carbon nanotube–graphene oxide hybrid/epoxy composites

An investigation is conducted on the effect of the hybrid of multi-wall carbon nanotubes (MWCNTs) and graphene oxide (GO) nanosheets on the tribological performance of epoxy composites at low GO weight fractions of 0.05–0.5 phr. The MWCNT amount is kept constant at 0.5 phr, which is typical for CNT/epoxy composites with enhanced mechanical properties. Friction and wear tests against smooth steel show that the introduction of 0.5 phr MWCNTs into the epoxy matrix increases the friction coefficient and decreases the specific wear rate. When testing the tribological performance of MWCNT/GO hybrids, it is shown that at a high GO amount of 0.5 phr, the friction coefficient is decreased below that of the neat matrix whereas the wear rate is increased above that of the neat matrix. At an optimal hybrid formulation, i.e., 0.5 phr MWCNTs and 0.1 phr GO, a further increase in the friction coefficient and a further reduction in the specific wear rate are observed. The specific wear rate is reduced by about 40% down to a factor of 11 relative to the neat epoxy when the GO content is 0.1 phr.     

Preparation,characterization and properties of polycaprolactone diol-functionalized multi-walled carbon nanotube/thermoplastic polyurethane composite

Multi-walled carbon nanotubes (MWCNTs) were chemically functionalized to prepare thermoplastic polyurethane (PU) composites with enhanced properties. In order to achieve a high compatibility of functionalized MWCNTs with the PU matrix, polycaprolactone diol (PCL), as one of PU’s monomers, was selectively grafted on the surface of MWCNTs (MWCNT–PCL), while carboxylic acid groups functionalized MWCNTs (MWCNT–COOH) and raw MWCNTs served as control. Both MWCNT–COOH and MWCNT–PCL improved the dispersion of MWCNTs in the PU matrix and interfacial bonding between them at 1 wt% loading fraction. The MWCNT–PCL/PU composite showed the greatest extent of improvement, where the tensile strength and modulus were 51.2% and 33.5% higher than those of pure PU respectively, without sacrificing the elongation at break. The considerable improvement in both mechanical properties and thermal stability of MWCNT–PCL/PU composite should result from the homogeneous dispersion of MWCNT–PCL in the PU matrix and strong interfacial bonding between them.