Effects of CNT diameter on mechanical properties of aligned CNT sheets and composites

Drawing, winding, and pressing techniques were used to produce horizontally aligned carbon nanotube (CNT) sheets from free-standing vertically aligned CNT arrays. The aligned CNT sheets were used to develop aligned CNT/epoxy composites through hot-melt prepreg processing with a vacuum-assisted system. Effects of CNT diameter change on the mechanical properties of aligned CNT sheets and their composites were examined. The reduction of the CNT diameter considerably increased the mechanical properties of the aligned CNT sheets and their composites. The decrease of the CNT diameter along with pressing CNT sheets drastically enhanced the mechanical properties of the CNT sheets and CNT/epoxy composites. Raman spectra measurements showed improvement of the CNT alignment in the pressed CNT/epoxy composites. Research results suggest that aligned CNT/epoxy composites with high strength and stiffness are producible using aligned CNT sheets with smaller-diameter CNTs.     

Mechanical properties of epoxy composites filled with silane-functionalized graphene oxide

In this work, the effects of as-produced GO and silane functionalized GO (silane-f-GO) loading and silane functionalization on the mechanical properties of epoxy composites are investigated and compared. Such silane functionalization containing epoxy ended-groups is found to effectively improve the compatibility between the silane-f-GO and the epoxy matrix. Increased storage modulus, glass transition temperature, thermal stability, tensile and flexural properties and fracture toughness of epoxy composites filled with the silane-f-GO sheets are observed compared with those of the neat epoxy and GO/epoxy composites. These findings confirm the improved dispersion and interfacial interaction in the composites arising from covalent bonds between the silane-f-GO and the epoxy matrix. Moreover, several possible fracture mechanisms, i.e. crack pinning/deflection, crack bridging, and matrix plastic deformation initiated by the debonding/delamination of GO sheets, were identified and evaluated.     

多壁碳纳米管/聚醚砜环氧树脂复合材料的制备及力学性能

环氧树脂具有优异的热性能及力学性能,但本身脆性较大。为制备低成本、高性能的环氧树脂体系,使用聚醚砜（PES）和多壁碳纳米管（MWCNT）对环氧树脂进行增韧,制备了不同PES含量的PES-环氧树脂共混物,讨论了PES含量对环氧树脂力学性能的影响;采用熔融法,并配合使用机械搅拌、高剪切分散和超声分散制备了MWCNT/PES-环氧树脂复合材料,测试了其拉伸性能及断裂韧性,用SEM观察了MWCNT在树脂中的分散状态以及拉伸试样的断口形貌。结果表明：MWCNT的加入能够提高PES-环氧树脂体系的综合力学性能,且当MWCNT含量为0.7wt%时,树脂体系的综合力学性能最好;低PES含量下,小于1.0wt%的MWCNT的加入使材料力学性能超过用20.0wt%PES改性的环氧树脂;PES与MWCNT对环氧树脂具有协同增韧作用。     

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.     

Reinforcement effects of MWCNT and VGCF in bulk composites and interlayer of CFRP laminates

The reinforcement effects of two nanofillers, i.e., multi-walled carbon nanotube (MWCNT) and vapor grown carbon fiber (VGCF), which are used at the interface of conventional CFRP laminates, and in epoxy bulk composites, have been investigated. When using the two nanofillers at the interface between two conventional CFRP sublaminates, the Mode-I interlaminar tensile strength and fracture toughness of CFRP laminates are improved significantly. The performance of VGCF is better than that of MWCNT in this case. For epoxy bulk composites, the two nanofillers play a similar role of good reinforcement in Young’s modulus and tensile strength. However, the Mode-I fracture toughness of epoxy/MWCNT is much better than that of epoxy/VGCF.     

Structure and properties of highly oriented polyoxymethylene/multi-walled carbon nanotube composites produced by hot stretching

Highly-oriented polyoxymethylene (POM)/multi-walled carbon nanotube (MWCNT) composites were fabricated through solid hot stretching technology. With the draw ratio as high as 900%, the oriented composites exhibited much improved thermal conductivity and mechanical properties along the stretching direction compared with that of the isotropic samples before drawing. The thermal conductivity of the composite with 11.6 vol.% MWCNTs can reach as high as 1.2 W/m K after drawing. Microstructure observation demonstrated that the POM matrix had an ordered fibrillar bundle structure and MWCNTs in the composite tended to align parallel to the stretching direction. Wide-angle X-ray diffraction results showed that the crystal axis of the POM matrix was preferentially oriented perpendicular to the draw direction, while MWCNTs were preferentially oriented parallel to the draw direction. The strong interaction between the POM matrix and the MWCNTs hindered the orientation movement of molecules of POM, but induced the orientation movement of MWCNTs.     

The impact of fluorinated MWCNT additives on the enhanced dynamic mechanical properties of e-beam-cured epoxy

Multi-walled carbon nanotubes were embedded into e-beam-cured epoxy resin to improve the mechanical properties of epoxy resin. The surfaces of these carbon nanotubes were modified using a fluorination treatment to improve their dispersion and adhesion in epoxy resin. The dynamic mechanical properties of epoxy/carbon nanotube composites were investigated at various heating rates and frequencies. As an effect of fluorination treatment, the semi-ionic bond of C–F on the surface of multi-walled carbon nanotubes played an important role in the improved dispersion and adhesion of carbon nanotubes into the epoxy resin. The storage modulus and loss modulus of the composites increased with higher applied frequency. The activation energy of the composites was increased by the effects of a higher heating rate due to the slow heat transfer in the epoxy/carbon nanotube composites. Eventually, the dynamic mechanical properties of the investigated epoxy were significantly improved by the carbon nanotubes dispersed therein via the fluorination treatment.     

Properties of graphene nanopowder and multi-walled carbon nanotube-filled epoxy thin-film nanocomposites for electronic applications: The effect of sonication time and filler loading

Graphene nanopowder (GNP) and multi-walled carbon nanotube (MWCNT)-filled epoxy thin-film composites were fabricated using ultrasonication and the spin coating technique. The effect of sonication time (10, 20 and 30 min) and GNP loading (0.05–1 vol%) on the tensile and electrical properties of GNP/epoxy thin-film composites was investigated. The addition of GNP decreased the material’s tensile strength and modulus. However, among the tested samples, the GNP/epoxy composites produced using 20 min of sonication time had a slightly higher tensile strength and modulus, with a lower electrical percolation threshold volume fraction. The effect of sonication time was supported by morphological analysis, which showed an improvement in GNP dispersion with increased sonication time. However, GNP deformation was observed after a long sonication time. The GNP/epoxy composites at different filler loadings showed higher electrical properties but slightly lower tensile properties compared with the MWCNT/epoxy composites fabricated using 20 min of sonication time.     

The poly(urethane-ionic liquid)/multi-walled carbon nanotubes composites

In the paper, a novel kind of imidazolium based poly(urethane-ionic liquid)/multi-walled carbon nanotubes (PUIL/MWCNT) composites was facilely prepared by uncovalent ways. The imidazolium based ionic liquid (IL) greatly improved the dispersion of pristine MWCNTs in PUIL by the π-cation interaction formed between the imidazolium cation and the π-electron of MWCNTs. The PUIL/MWCNT composites showed obviously increased modulus, glass transition temperature and tensile strength in comparison with PU/MWCNT composites. The thermal and mechanical properties of the PUIL/MWCNT composites presented significant increase with low load of the MWCNTs. It indicated the interactions between PUIL and MWCNTs played an important role to enhance the performances of the composites.     

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.     

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.     

The effect of multiwall carbon nanotubes on the in-plane shear behavior of epoxy glass fiber reinforced composites

In this study, we investigate how multi-wall carbon nanotubes (MWCNTs) affect the in-plane shear mechanical behavior of glass fiber/epoxy composite. These multi-scale composites are fabricated using vacuum infusion: pristine MWCNT and amino-functionalized MWCNT are incorporated into epoxy resins at concentrations of both 0.1 and 0.3 wt.% and are subsequently evaluated. The MWCNT are mixed into the resins by mechanical stirring and sonication prior to resin infusion, and the MWCNT distribution in the cured laminate is then evaluated by performing a heat conduction assessment. Monotonic and cyclic quasi-static room temperature in-plane shear tests are performed following the ASTM D 4255 standard. The initial shear modulus, the deterioration of the shear modulus during plastic deformation and material hardening are evaluated. Incorporating MWCNT into the resins did not affect the parameters investigated under the imposed conditions.     

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.     

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.     

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.     

Mechanical and electrical properties of carbon fiber composites with incorporation of graphene nanoplatelets at the fiber–matrix interphase

In this study, carbon fibers (CFs) were coated with graphene nanoplatelets (GnP), using a robust and continuous coating process. CFs were directly immersed in a stable GnP suspension and the coating conditions were optimized in order to obtain a high density of homogeneously and well-dispersed GnP. GnP coated CFs/epoxy composites were manufactured by a prepreg and lay-up method, and the mechanical properties and electrical conductivity of the composites were assessed. The GnP coated CFs/epoxy composites showed 52%, 7%, and 19% of increase in comparison with non-coated CFs/epoxy composites, for 90° flexural strength, 0° flexural strength and interlaminar shear strength, respectively. Meanwhile, incorporating GnP in the CF/epoxy interphase significantly improved the electrical conductivity through the thickness direction by creating a conductive path between the fibers.     

Toughness,flexural, damping and interfacial properties of hybridized GFRE composites with MWCNTs

As the improved damping in fiber-reinforced composites can affect the other mechanical properties, therefore, the aim of this work is to investigate the effect of multiwall carbon nanotube (MWCNT) on the interfacial bond strength, flexural strength and stiffness, toughness and damping properties of hybridized glass-fiber reinforced epoxy (GFRE) composites. Nanophased epoxy resin was used to hybridize unidirectional and quasi-isotropic GFRE composites with [0/±45/90]_s and [90/±45/0]_s stacking sequences. Results from the interfacial characterizations of the hybridized composites showed improvement up to 30% compared to the control laminates. Hybridization of GFRE laminates with MWCNTs leads to decreasing the flexural and storage moduli, increasing flexural strength, toughness, natural frequencies and damping ratio. A high correlation coefficient of 0.9985 was obtained between static flexural and dynamic storage moduli. The highest flexural strength, flexural and storage moduli and natural frequency of quasi-isotropic laminate were observed for [0/±45/90]_s stacking sequence and vice versa for damping ratio.     

The reinforcing effect of graphene nanosheets on the cryogenic mechanical properties of epoxy resins

The reinforcing effect of graphene in enhancing the cryogenic tensile and impact properties of epoxy composites is examined at a weight fraction of 0.05–0.50%. The micro-structure and cryogenic mechanical properties of the graphene/epoxy composites are investigated using scanning electron microscopy, transmission electron microscopy, small-angle X-ray scattering and mechanical testing techniques. The results show that the graphene dispersion in the epoxy matrix is good at low contents while its aggregation takes place and becomes severer as its content increases. And the cryogenic tensile and impact strength at liquid nitrogen temperature (77 K) of the composites are effectively improved by the graphene addition at proper contents. The cryogenic Young’s modulus increases almost linearly with increasing the graphene content. Moreover, the results for the mechanical properties at room temperature (298 K) of the graphene/epoxy composites are also presented for the purpose of comparison.     

Effect of surface functionalization on mechanical properties and decomposition kinetics of high performance polyetherimide/MWCNT nano composites

In this investigation, Polyetherimide (PEI) reinforced with multi-walled carbon nanotube (MWCNT) using novel melt blending technique. Surface of MWCNTs are modified by acid treatment as well as by plasma treatment. PEI nano composites with 2 wt% treated MWCNT shows about 15% improvement in mechanical properties when compared to unfilled PEI. The thermal decomposition kinetics of PEI/MWCNT nano composites has been critically analyzed by using Coats – Redfern model. The increase in activation energy for thermal degradation by 699 kJ/mol for 2 wt% MWCNT implies improvement in the thermal properties of PEI. Studies under Fourier Transform Infrared Spectroscopy (FTIR) and Transmission Electron Microscopy (TEM) depict significant interfacial adhesion with uniform dispersion of MWCNT in polymer matrix due to surface functionalization. 0.5 wt% chemically modified MWCNT shows typical alignment of MWCNT. There is a significant improvement in mechanical properties and thermal properties for surface functionalized MWCNT reinforced.     

Synchronous effects of multiscale reinforced and toughened CFRP composites by MWNTs-EP/PSF hybrid nanofibers with preferred orientation

MWNTs-EP/PSF (polysulfone) hybrid nanofibers with preferred orientation were directly electrospun onto carbon fiber/epoxy prepregs and interlaminar synchronously reinforced and toughened CFRP composites were successfully fabricated. With MWNTs-EP loading increasing, the oriented nanofibers were obtained accompanying with enhanced alignment of inner MWNTs-EP. Flexural properties and interlaminar shear strength of composites were improved with increasing MWNTs-EP loadings, whereas fracture toughness attained maximum at 10 wt% MWNTs-EP loading and then decreased. Based on these results, multiscale schematic modeling and mechanism schematic of hybrid nanofibers reinforced and toughened composites were suggested. Due to the preferred orientation of nanofibers, MWNTs-EP was inclined to align vertically to carbon fiber direction along the in-plane of interface layer. The proposed network structures, containing four correlative phases of MWNTs-EP/PSF sphere/carbon fiber/epoxy matrix, contributed to simultaneous improvement of strength and toughness of composites, which was realized by crack pinning, crack deflection, crack bridging and effective load transfer.