Improvements in mechanical and thermo-mechanical properties of e-glass/epoxy composites using amino functionalized MWCNTs

The prime objective of this work is to optimize the mechanical and thermo-mechanical properties of e-glass/epoxy composites by utilizing amino-functionalized multi-walled carbon nanotubes (MWCNTs–NH₂) through a combination of dispersion method. At first, 0.1–0.4 wt.% of MWCNT–NH₂ was integrated into SC-15 epoxy suspension using a combination of ultra-sonication and calendaring techniques. E-glass/epoxy nanocomposites were than fabricated at elevated temperature with the modified resin using hand layup and compression hot press. 3-Point flexural and dynamic mechanical analysis (DMA) results demonstrated a linearly increasing trend in properties from 0 to 0.3 wt.% loading. Micrographs of MWCNTs incorporated epoxy and e-glass/epoxy samples revealed uniform dispersion of MWCNTs in epoxy, good interfacial adhesion between CNTs and polymer, and improved interfacial bonding between fiber/matrix at 0.3 wt.% loading. An improved dispersion and hence an improved crosslink interaction between MWCNT–NH₂ and epoxy lead to the stronger shift of the mechanical and thermo-mechanical properties of the composites.     

Characterization of epoxy functionalized graphite nanoparticles and the physical properties of epoxy matrix nanocomposites

This work presents a novel approach to the functionalization of graphite nanoparticles. The technique provides a mechanism for covalent bonding between the filler and matrix, with minimal disruption to the sp² hybridization of the pristine graphene sheet. Functionalization proceeded by covalently bonding an epoxy monomer to the surface of expanded graphite, via a coupling agent, such that the epoxy concentration was measured as approximately 4 wt.%. The impact of dispersing this material into an epoxy resin was evaluated with respect to the mechanical properties and electrical conductivity of the graphite–epoxy nanocomposite. At a loading as low as 0.5 wt.%, the electrical conductivity was increased by five orders of magnitude relative to the base resin. The material yield strength was increased by 30% and Young’s modulus by 50%. These results were realized without compromise to the resin toughness.     

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.     

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.     

Covalent functionalization of graphene with organosilane and its use as a reinforcement in epoxy composites

Functionalized graphene nanosheets (f-GNSs) produced by chemically grafting organosilane were synthesized by a simple covalent functionalization with 3-aminopropyl triethoxysilane. The f-GNSs showed a larger thickness, but smaller width and than the un-treated graphene. The covalent functionalization of graphene with silane was favorable for their homogeneous dispersion in the polymer matrix even at a high nanofiller loading (1 wt.%). The initial thermal degradation temperature of epoxy composite was increased from 314 °C to 334 °C, at a f-GNS content of 1 wt.%. Meanwhile, the addition of 1 wt.% f-GNSs increased the tensile strength and elongation to failure of epoxy resins by 45% and 133%, respectively. This is believed to be attributed to the strong interfacial interactions between f-GNSs and the epoxy resins by covalent functionalization. The experimentally determined Young’s modulus corresponded well with theoretical simulation under the hypothesis that the graphene sheets randomly dispersed in the polymer matrix.     

Studies on the characterization of piassava fibers and their epoxy composites

This paper presents morphology, physical and strength properties of piassava fiber, a very rigid fiber having a potential to be used as composite reinforcement. Composites of continuous and aligned piassava fibers with and without alkali treatment dispersed in epoxy matrix were subjected to three point bend, tensile, and Izod impact tests. Composites with fibers above 20 vol.% showed an effective reinforcement behavior both in flexural and tensile tests, while the impact energy linearly increased for the amount of piassava fibers used in this study. Fractographic study revealed a relatively weaker fiber/matrix adhesion acting as preferential site for crack nucleation. Evidence was also found for crack arrest by the fiber above 20 vol.%. This, together with spiny surface protrusion in the piassava fibers, was found to be responsible for the reinforcement of the epoxy composites.     

The effect of alternate heating rates during cure on the structure-property relationships of epoxy/MMT clay nanocomposites

This paper investigates the effect of both the mixing technique and heating rate during cure on the dispersion of montmorillonite (MMT) clay in an epoxy resin. The combination of sonication and using a 10 °C/min heating rate during cure was found to facilitate the dispersion of nanoclay in epoxy resin. These processing conditions provided a synergistic effect, making it possible for polymer chains to penetrate in-between clay galleries and detach platelets from their agglomerates. As the degree of dispersion was enhanced, the flexural modulus and strength properties were found to decrease by 15% and 40%, respectively. This is thought to be due to individual platelets fracturing in the nanocomposite. Complementary techniques including X-ray diffraction (XRD), small angle X-ray scattering (SAXS), scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDX), transmission electron microscopy (TEM) and optical microscopy were essential to fully characterise localised and spatial regions of the clay morphologies.     

Synthesis,microstructure and mechanical properties of Yttria Stabilized Zirconia (3YTZP) – Multi-Walled Nanotube (MWNTs) nanocomposite by direct in-situ growth of MWNTs on Zirconia particles

In this research, Yttria Stabilized Zirconia (3YTZP) – carbon nanotube (CNT) composites are fabricated by direct in-situ growth of CNTs on the Zirconia particles, followed by densification via the Spark Plasma Sintering (SPS) technique. Scanning electron microscopy analysis of the 3YTZP-CNT powders shows uniform distribution of CNTs without the formation of agglomerates frequently seen with the traditional ex-situ mixing of CNTs in ceramic compositions. The samples were sintered to nearly 100% theoretical density and with a finer grain size microstructure. High Resolution Transmission Electron Microscopy (HRTEM) and Raman Spectroscopy confirm CNT retention in the sintered nanocomposites up to 1600 °C. The flexural strength increases from ∼260 MPa for samples without CNTs sintered at 1600 °C to ∼312 MPa for samples with ∼4 wt.% CNTs sintered at the same temperature. A corresponding increase in the indentation fracture toughness is also observed for samples with ∼4 wt.% CNTs sintered at 1600 °C as compared to samples sintered at the same temperature without CNTs.     

Nano-enhanced composite materials under thermal shock and environmental degradation: A durability study

Carbon nanotubes (CNTs) were incorporated at 0.5 wt% in epoxy resin using sonication at two different levels of amplitude (50% and 100% of 400 W nominal sonication power). The CNTs modified epoxy systems were used to manufacture carbon fibre reinforced laminates (CFRPs). All specimens were subjected to thermal shock and hygrothermal exposure. The presence of CNTs did not alter the water absorption profiles for the epoxy resin, but it resulted in a spectacular 40% reduction in the water uptake at equilibrium for the CFRPs. The interlaminar shear strength of the CFRPs was not significantly affected by the thermal shock cycles; however it was reduced by 50–60% after hygrothermal exposure. The addition of CNTs led to slightly lower interlaminar shear strength values in the as-manufactured state. However their presence did not accelerate the deterioration of the strength after the environmental exposure. Although the addition of CNTs did not significantly influence the thermomechanical properties of the resin, they were beneficial in the case of the CFRPs since (i) they enhanced the storage modulus and glass transition temperature and (ii) limited the deterioration of these properties after thermal shock and hygrothermal exposure. The amplitude level during sonication which determined the dispersion state and length of the CNTs had a clear effect on the durability of the studied systems.     

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 and thermal characterization of epoxy composites reinforced with random and quasi-unidirectional untreated Phormium tenax leaf fibers

The present work investigates tensile and flexural behavior of untreated New Zealand flax (Phormium tenax) fiber reinforced epoxy composites. Two series of laminates were produced using the same reinforcement content (20 wt%), arranged either as short fibers or quasi-unidirectional ones. Composites reinforced using quasi-unidirectional fibers showed higher modulus and strength both in tensile and flexural loading, when compared to neat epoxy resin. Short fiber composites, although still superior to epoxy resin both for tensile and flexural moduli, proved inferior in strength, especially as concerns tensile strength. These results have been supported by scanning electron microscopy (SEM), which allowed characterizing fiber–matrix interface, and by acoustic emission (AE) analysis, which enabled investigating failure mechanisms. In addition, thermal behavior of both untreated phormium fibers and composites has been studied by thermogravimetric analysis (TGA), revealing the thermal stability of composites to be higher than for phormium fibers and epoxy matrix alone.     

Method for reducing contact resistivity of carbon nanotube-containing epoxy adhesives for aerospace applications

Bonded joints prepared with conductive epoxy adhesive based on carbon nanotubes (CNTs) display an unusually high resistance mainly due to the high contact resistivity at the adhesive metal interface. A new method is proposed to reduce efficiently the contact resistivity by forcing a controlled amount of electric current through the bonded joint. Current treatment at 0.5 A/cm² of current densities for 30 s typically reduces to up to 10 times the contact resistivity. Apart from noble metals, all other metals are usually covered by a more or less conductive oxide layer. This oxide layer is the main cause for high contact resistivity. During the current treatment large gradients of electric field developed around the highly curved CNTs are capable of breaking down locally the oxide layer generating conductive channels. Static shear strength and fatigue resistance of the bonded joint are not affected by the current treatment.     

A facile method for preparing CNT-grafted carbon fibers and improved tensile strength of their composites

Carbon nanotube (CNT)-grafted carbon fibers (CFs) have emerged as new reinforcements for improving the mechanical properties of CF-reinforced composites but such enhancement in macroscale composites has not been realized. This paper reports a facile method for preparing CNT-grafted CFs and improving the tensile strength of their composites. A CNT/polyacrylonitrile solution was sprayed onto the surface of the CF woven fabrics, and the CNTs were grafted by a thermal treatment at 300 °C. CNT-grafted CF composites were fabricated using the CNT-grafted CF woven fabrics using a vacuum-assisted resin transfer molding process with epoxy resin. The CNT-grafted CF composite exhibited 22% enhancement in the tensile strength compared to that of the pristine CF composite. Fracture surfaces of the CNT-grafted CF composites showed that the grafted CNTs obstructed the propagation of micro-cracks and micro-delamination around the CFs and also yarn boundaries, resulting in improved tensile strength of CNT-grafted CF composites.     

Interlaminar and intralaminar reinforcement of composite laminates with aligned carbon nanotubes

Three-dimensional reinforcement of woven advanced polymer–matrix composites using aligned carbon nanotubes (CNTs) is explored experimentally and theoretically. Radially-aligned CNTs grown in situ on the surface of fibers in a woven cloth provide significant three-dimensional reinforcement, as measured by Mode I interlaminar fracture testing and tension-bearing experiments. Aligned CNTs bridge the ply interfaces giving enhancement in both initiation and steady-state toughness, improving the already tough system by 76% in steady state (more than 1.5 kJ/m² increase). CNT pull-out on the crack faces is the observed toughening mechanism, and an analytical model is correlated to the experimental fracture data. In the plane of the laminate, aligned CNTs enhance the tension-bearing response with increases of: 19% in bearing stiffness, 9% in critical strength, and 5% in ultimate strength accompanied by a clear change in failure mode from shear-out failure (matrix dominated) without CNTs to tensile fracture (fiber dominated) with CNTs.     

Effect of particle size and weight fraction on the flexural strength and failure mode of TiO2 particles reinforced epoxy

Effect of inclusions size and weight fraction on flexural strength and failure mode of composite containing SC-15 epoxy resin and TiO₂ particles has been studied in this investigation. The sizes of particles varied from macro (0.02 mm) to nano (5 nm) scale, and these particles were infused into the part-A of SC-15 through sonic cavitations and then mixed with part-B of SC-15 by using a high speed mechanical agitator. Three-point bending tests were performed on unfilled, 0.5 wt.%, 1.0 wt.% and 1.5 wt.% particles filled SC-15 epoxy to identify the loading effect on mechanical properties of the composites. Results show that 1.0 wt.% nanoparticles reinforced epoxy exhibit the highest mechanical performance. Higher than 1.0%, strength of composite decreased because of poor dispersion. Experimental results also shown that micro-sized particles have little effect on strength of epoxy at such low loading, and strength of composite increased as the size of particles decreased to nano scale. However, degradation in strength was found in 5 nm TiO₂/epoxy system due to agglomeration.     

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.     

Processing,characterization, and modeling of carbon nanotube-reinforced multiscale composites

Carbon fiber-reinforced epoxy composites modified with carbon nanotubes (CNTs) were fabricated and characterized. High-energy sonication was used to disperse CNTs in the resin, followed by infiltration of fiber preform with the resin/CNT mixture. The effects of sonication time on the mechanical properties of “multiscale” composites, which contain reinforcements at varying scales, were studied. A low CNT loading of 0.3 wt% in resin had little influence on tensile properties, while it improved the flexural modulus, strength, and percent strain to break by 11.6%, 18.0%, and 11.4%, respectively, as compared to the control carbon fiber/epoxy composite. While sonication is an effective method to disperse CNTs in a resin, duration, intensity, and temperature need to be controlled to prevent damages imposed on CNTs and premature resin curing. A combination of Halpin–Tsai equations and woven fiber micromechanics was used in hierarchy to predict the mechanical properties of multiscale composites, and the discrepancies between the predicted and experimental values are explained.     

Recycled carbon fibre for high performance energy absorption

This paper compares the mechanical properties of virgin and recycled woven carbon fibre prepreg and goes on to assess the potential for recycled carbon fibre reinforced plastic (rCFRP) to be used in high performance energy absorption structures. Three sets of material were examined: fresh containing virgin fibres and resin, aged which was an out of life but otherwise identical roll and recycled which contained recycled fibre and new resin. The compressive strength and modulus of rCFRP were approximately 94% of the values for fresh material. This correlated directly with the results from impact testing where rCFRP conical impact structures were found to have a specific energy absorption of 32.7 kJ/kg versus 34.8 kJ/kg for fresh material. The tensile and flexural strength of rCFRP were 65% of the value for fresh material. Tensile and flexural moduli of rCFRP were within 90% of fresh material and ILSS of rCFRP was 75% that of fresh.     

Interfacial load transfer in carbon nanotube/ceramic microfiber hybrid polymer composites

Growing carbon nanotubes (CNTs) on the surface of fibers has the potential to modify fiber–matrix interfacial adhesion, enhance the composite delamination resistance, and possibly improve its toughness and any matrix-dominated elastic property as well. In the present work aligned CNTs were grown upon ceramic fibers (silica and alumina) by chemical vapor deposition (CVD) at temperatures of 650 °C and 750 °C. Continuously-monitored single fiber composite (SFC) fragmentation tests were performed on pristine as well as on CNT-grown fibers embedded in epoxy. The critical fragment length, fiber tensile strength at critical length, and interfacial shear strength were evaluated. Significant increases (up to 50%) are observed in the fiber tensile strength and in the interfacial adhesion (which was sometimes doubled) with all fiber types upon which CNTs are CVD-grown at 750 °C. We discuss the likely sources of these improvements as well as their implications.     

Electrical and thermal property enhancement of fiber-reinforced polymer laminate composites through controlled implementation of multi-walled carbon nanotubes

Aligned carbon nanotubes (CNTs) are implemented into alumina-fiber reinforced laminates, and enhanced mass-specific thermal and electrical conductivities are observed. Electrical conductivity enhancement is useful for electrostatic discharge and sensing applications, and is used here for both electromagnetic interference (EMI) shielding and deicing. CNTs were grown directly on individual fibers in woven cloth plies, and maintained their alignment during the polymer (epoxy) infiltration used to create laminates. Using multiple complementary methods, non-isotropic electrical and thermal conductivities of these hybrid composites were thoroughly characterized as a function of CNT volume/mass fraction. DC and AC electrical conductivity measurements demonstrate high electrical conductivity of >100 S/m (at 3% volume fraction, ∼1.5% weight fraction, of CNTs) that can be used for multifunctional applications such as de-icing and electromagnetic shielding. The thermal conductivity enhancement (∼1 W/m K) suggests that carbon-fiber based laminates can significantly benefit from aligned CNTs. Application of such new nano-engineered, multi-scale, multi-functional CNT composites can be extended to system health monitoring with electrical or thermal resistance change induced by damage, fire-resistant structures among other multifunctional attributes.