Tensile and thermomechanical properties of short carbon fiber reinforced polyamide 6 composites

In this study, carbon fiber (CF) reinforced polyamide 6 (PA6) composites were prepared by using melt mixing method. Effects of fiber length and content, on the mechanical, thermal and morphological properties of CF reinforced PA6 composites were investigated. Fiber length distributions of composites were also determined by using an image analyzing program. It was seen that the maximum number of fibers were observed in the range of 0–50 μm. Mechanical test results showed that, increasing CF content increased the tensile strength, modulus and hardness values but decreased strain at break values of composites. DSC results showed that T_g and T_m values of composites were not changed significantly with increasing CF content and length. However, heat of fusion and the relative degree of crystallinity values of composites decreased with ascending CF content. DMA results revealed that storage modulus and loss modulus values of composites increased with increasing CF content.     

Heterogeneously structured conductive carbon fiber composites by using multi-scale silver particles

This paper reports a new approach to enhance the through-thickness thermal conductivity of laminated carbon fabric reinforced composites by using nanoscale and microscale silver particles in combination to create heterogeneously structured continuous through-thickness thermal conducting paths. High conductivity of 6.62 W/(m K) with a 5.1 v% silver volume fraction can be achieved by incorporating these nanoscale and microscale silver particles in EWC-300X/Epon862 composite. Silver flakes were distributed within the inter-tow area, while nanoscale silver particles penetrated into the fiber tows. The combination of different sizes of silver fillers is able to effectively form continuous through-thickness conduction paths penetrating fiber tows and bridging the large inter-tow resin rich areas. Positive hybrid effects to thermal conductivity were found in IM7/EWC300X/sliver particle hybrid composites. In addition, microscale fillers in resin rich areas showed less impact on tensile performance than nanoscale particles applied directly on fiber surface.     

Hybrid multi-scale epoxy composite made of conventional carbon fiber fabrics with interlaminar regions containing electrospun carbon nanofiber mats

Herein we report the development and evaluation of hybrid multi-scale epoxy composite made of conventional carbon fiber fabrics with interlaminar regions containing mats of electrospun carbon nanofibers (ECNs). The results indicated that (1) the interlaminar shear strength and flexural properties of hybrid multi-scale composite were substantially higher than those of control/comparison composite without ECNs; in particular, the interlaminar shear strength was higher by ∼86%; and (2) the electrical conductivities in both in-plane and out-of-plane directions were enhanced through incorporation of ECNs, while the enhancement of out-of-plane conductivity (∼150%) was much larger than that of in-plane conductivity (∼20%). To validate the data reduction procedure, a new shear stress formula was formulated for composite laminates, which took into account the effect of layup and inter-layers. The study suggested that ECNs could be utilized for the development of high-performance composites, particularly with the improved out-of-plan properties (e.g., interlaminar shear strength).     

Comparison of drying methods for the preparation of carbon fiber felt/carbon nanotubes modified epoxy composites

Carbon fiber felt with carbon nanotubes (CNTs) were prepared by immersing three-dimensional (3D) felt into CNT aqueous solution (with dispersant) followed by removing water with different drying methods. Epoxy resin was then introduced into the felt to obtain 3D fiber felt/CNTs modified epoxy composites. This paper highlights the effect of drying method on macro-morphologies of the felt, morphological dispersion of CNTs and some relevant properties of the composites, including electrical conductivity and flexural performance. The results demonstrate that compared to the commonly used heat drying method, freeze drying technique possesses obvious advantages for the fabrication of fiber felt/CNT modified epoxy composites.     

Magnetically-sensitive shape memory polyurethane composites crosslinked with multi-walled carbon nanotubes

Magnetically-sensitive polyurethane composites, which were crosslinked with multi-walled carbon nanotubes (MWCNTs) and were filled with Fe₃O₄ nanoparticles, were synthesized via in situ polymerization method. MWCNTs pretreated with nitric acid were used as crosslinking agents. Because of the crosslinking of MWCNTs with polyurethane prepolymer, the properties of the composites with a high content of Fe₃O₄ nanoparticles, especially the mechanical properties, were significantly improved. The composites showed excellent shape memory properties in both 45 °C hot water and an alternating magnetic field (f = 45 kHz, H = 29.7 kA m⁻¹). The shape recovery time was less than one minute and the shape recovery rate was over 95% in the alternating magnetic field.     

Functionally graded carbon nanofiber/phenolic nanocomposites and their mechanical properties

For the first time, functionally graded carbon nanofiber/phenolic nanocomposites were designed and fabricated. The effect of compositional gradients on the flexural properties of functionally graded carbon nanofiber/phenolic composite beams was evaluated. Samples with four compositional gradients as well as a non-graded nanocomposite with the same total carbon nanofiber content and geometry were fabricated using a combination of powder stacking and compression molding techniques. Analytical and finite element models were both performed to investigate the effects of compositional gradients, boundary conditions, and external loadings on flexural properties of nanocomposite beams. Close agreement was observed between analytical solutions, finite element analyses and experiment. The morphology of the fracture surfaces was examined using a scanning electron microscope. The results showed that the flexural properties of carbon nanofiber/phenolic nanocomposites can be greatly improved by controlling the carbon nanofiber content across the thickness of the samples.     

The influence of nano/micro hybrid structure on the mechanical and self-sensing properties of carbon nanotube-microparticle reinforced epoxy matrix composite

Nano/micrometer hybrids are prepared by chemical vapor deposition growth of carbon nanotubes (CNTs) on SiC, Al₂O₃ and graphene nanoplatelet (GNP). The mechanical and self-sensing behaviors of the hybrids reinforced epoxy composites are found to be highly dependent on CNT aspect ratio (AR), organization and substrates. The CNT–GNP hybrids exhibit the most significant reinforcing effectiveness, among the three hybrids with AR1200. During tensile loading, the in situ electrical resistance of the CNT–GNP/epoxy and the CNT–SiC/epoxy composites gradually increases to a maximum value and then decreases, which is remarkably different from the monotonic increase in the CNT–Al₂O₃/epoxy composites. However, the CNT–Al₂O₃ with increased AR ⩾ 2000 endows the similar resistance change as the other two hybrids. Besides, when AR < 3200, the tensile modulus and strength of the CNT–Al₂O₃/epoxy composites gradually increase with AR. The interrelationship between the hybrid structure and the mechanical and self-sensing behaviors of the composites are analyzed.     

Electrical properties of polyvinylidene fluoride (PVDF)/multi-walled carbon nanotube (MWCNT) semi-transparent composites: Modelling of DC conductivity

Transparent conductive composites can be achieved from PVDF–MWCNT at very low concentration of MWCNT. These composites show different degree of UV–Visible radiation absorption depending on MWCNT concentration in composites. The composition dependent dielectric properties and AC conductivity were also measured for these composites. Properties like AC conductivity, dielectric constant and loss are increasing with filler concentration. The variations of DC conductivity against composition and temperature are also reported. The electrical hysteresis and electrical set are observed for PVDF–MWCNT composites when subjected to heating–cooling cycle. The validity of different theoretical models depicting percolation threshold with respect to DC conductivity was tested for these composites.     

Synergistic effect of boron nitride flakes and tetrapod-shaped ZnO whiskers on the thermal conductivity of electrically insulating phenol formaldehyde composites

Tetrapod-shaped zinc oxide (T-ZnO) whiskers and boron nitride (BN) flakes were employed to improve the thermal conductivity of phenolic formaldehyde resin (PF). A striking synergistic effect on thermal conductivity of PF was achieved. The in-plane thermal conductivity of the PF composite is as high as 1.96 W m⁻¹ K⁻¹ with 30 wt.% BN and 30 wt.% T-ZnO, which is 6.8 times higher than that of neat PF, while its electrical insulation is maintained. With 30 wt.% BN and 30 wt.% T-ZnO, the flexural strength of the composite is 312.9% higher than that of neat PF, and 56.2% higher that of the PF composite with 60 wt.% BN. The elongation at break is also improved by 51.8% in comparison with that of the composite with 60 wt.% BN. Such a synergistic effect results from the bridging of T-ZnO whiskers between BN flakes facilitating the formation of effective thermal conductance network within PF matrix.     

Structural and mechanical properties of cellulose composites made of isolated cellulose nanofibers and poly(vinyl alcohol)

A composite of cellulose-nanofibers (Cel-F)/polyvinyl alcohol (PVA) was made through a developed water-jet nano-isolation process called the Star Burst processing (SB). The structural and the mechanical properties of the pure Cel-F and the Cel-F/PVA composites were analyzed for comparison. The microstructural analyses revealed the step-by-step nano-isolation procedures of the SB processing, eventually constructing nanofibers with the minimum diameter of ∼23 nm. It was also found that the crystallinity of Cel-F was rapidly increased by 14% at the early stage of the SB process, subsequently becoming almost constant, irrespective of the number of the SB treatments. Additionally, Cel-F were homogenously dispersed in PVA matrix after 40 SB treatments. Young’s modulus of the resulting composite was increased by 48%. The results were in good agreement with the outcome of the short-fiber composite theory, indicating a highly potential use of the SB-processed cellulose nanofibers as new reinforcement materials.     

Effect of cellulose nano fiber (CNF) on fatigue performance of carbon fiber fabric composites

The effect of cellulose nano fibers (CNF): micro-fibrillated cellulose and bacteria cellulose fibers were investigated on the fatigue life of carbon fiber (CF) fabric/epoxy (EP) composites. Epoxy used as the matrix was physically modified with CNF in advance before fabricating the laminates. The high cycle fatigue strength was significantly improved at 0.3 wt% CNF. There exists an appropriate CNF content which makes the fatigue life longest. An increase of adhesive strength between CF and matrix results due to physical modification with CNF. The adhesive strength much increases with increasing the CNF content. Almost no interfacial debonding occurs at 0.8 wt% CNF content when CF breakage takes place. On the other hand, some debonding occurs along CFs from the breaking point at 0.3 wt% CNF. Debonding is more significant in the case of no CNF addition to the matrix. An appropriate interfacial strength brought at 0.3 wt% CNF is the key of fatigue life extension.     

Review of the mechanical properties of carbon nanofiber/polymer composites

In this paper, the mechanical properties of vapor grown carbon nanofiber (VGCNF)/polymer composites are reviewed. The paper starts with the structural and intrinsic mechanical properties of VGCNFs. Then the major factors (filler dispersion and distribution, filler aspect ratio, adhesion and interface between filler and polymer matrix) affecting the mechanical properties of VGCNF/polymer composites are presented. After that, VGCNF/polymer composite mechanical properties are discussed in terms of nanofibers dispersion and alignment, adhesion between the nanofiber and polymer matrix, and other factors. The influence of processing methods and processing conditions on the properties of VGCNF/polymer composite is also considered. At the end, the possible future challenges for VGCNF and VGCNF/polymer composites are highlighted.     

Effect of silica coating thickness on the thermal conductivity of polyurethane/SiO2 coated multiwalled carbon nanotube composites

Silica coated multiwalled carbon nanotubes (SiO₂@MWCNTs) with different coating thicknesses of ∼4 nm, 30–50 nm, and 70–90 nm were synthesized by a sol–gel method and compounded with polyurethane (PU). The effects of SiO₂@MWCNTs on the electrical properties and thermal conductivity of the resulting PU/SiO₂@MWCNT composites were investigated. The SiO₂ coating maintained the high electrical resistivity of pure PU. Meanwhile, incorporating 0.5, 0.75 and 1.0 wt% SiO₂@MWCNT (70–90 nm) into PU, produced thermal conductivity values of 0.287, 0.289 and 0.310 W/mK, respectively, representing increases of 62.1%, 63.3% and 75.1%. The thermal conductivity of PU/SiO₂@MWCNT composites was also increased by increasing the thickness of the SiO₂ coating.     

Hybrid carbon fiber composite lattice truss structures

Carbon fiber reinforced polymer (CFRP) composite sandwich panels with hybrid foam filled CFRP pyramidal lattice cores have been assembled from linear carbon fiber braids and Divinycell H250 polymer foam trapezoids. These have been stitched to 3D woven carbon fiber face sheets and infused with an epoxy resin using a vacuum assisted resin transfer molding process. Sandwich panels with carbon fiber composite truss volumes of 1.5–17.5% of the core volume have been fabricated, and the through-thickness compressive strength and modulus measured, and compared with micromechanical models that establish the relationships between the mechanical properties of the core, its topology and the mechanical properties of the truss and foam. The through thickness modulus and strength of the hybrid cores is found to increase with increasing truss core volume fraction. However, the lattice strength saturates at high CFRP truss volume fraction as the proportion of the truss material contained in the nodes increases. The use of linear carbon fiber braids is shown to facilitate the simpler fabrication of hybrid CFRP structures compared to previously described approaches. Their specific strength, moduli and energy absorption is found to be comparable to those made by alternative approaches.     

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.     

The effect of graphene presence in flame retarded epoxy resin matrix on the mechanical and flammability properties of glass fiber-reinforced composites

The effect of adding graphene in epoxy containing either an additive (MP) or reactive-type (DOPO) flame retardant on the thermal, mechanical and flammability properties of glass fiber-reinforced epoxy composites was investigated using thermal analysis; flexural, impact, tensile tests; cone calorimetry and UL-94 techniques. The addition of MP or DOPO to epoxy had a thermal destabilization effect below 400 °C, but led to higher char yield at higher temperatures. The inclusion of 10 wt% flame retardants slightly decreased the mechanical behavior, which was attributed to the poor interfacial interactions in case of MP or the decreased cross-linking density in case of DOPO flame retarded resin. The additional graphene presence increased flexural and impact properties, but slightly decreased tensile performance. Adding graphene further decreased the PHRR, THR and burning rate due to its good barrier effect. The improved fire retardancy was mainly attributed to the reduced release of the combustible gas products.     

Preparation of continuous carbon nanotube networks in carbon fiber/epoxy composite

In order to optimize carbon nanotube (CNT) dispersion state in fiber/epoxy composite, a novel kind of CNT organization form of continuous networks was designed. The present work mainly discussed the feasibility of preparing continuous CNT networks in composite: Fiber fabric was immersed into CNT aqueous solution (containing dispersant) followed by freeze drying and pyrolysis process, prior to epoxy infusion. The morphologies of fabric with CNTs were observed by Scanning Electron Microscope. The relationship between CNT networks and flowing epoxy resin was studied. Properties of composite, including out-of-plane electrical conductivity and interlaminar shear strength (ILSS), were measured. The results demonstrated that continuous and porous CNT networks formed by entangled CNTs could be assembled in fiber fabric. Most part of them were preserved in composite due to the robustness of network structures. The preserved CNT networks significantly improved out-of-plane electrical conductivity, and also have an effect on ILSS value.     

Effect of fiber loading on the properties of treated cellulose fiber-reinforced phenolic composites

The effect of fiber loading on the properties of treated cellulose fiber-reinforced phenolic composites was evaluated. Alkali treatment of the fibers and reaction with organosilanes as coupling agents were applied to improve fiber–matrix adhesion. Fiber loadings of 1, 3, 5, and 7 wt% were incorporated to the phenolic matrix and tensile, flexural, morphological and thermal properties of the resulting composites were studied. In general, mechanical properties of the composites showed a maximum at 3% of fiber loading and a uniform distribution of the fibers in such composites was observed. Silane treatment of the fibers provided derived composites with the best thermal and mechanical properties. Meanwhile, NaOH treatment improved thermal and flexural properties, but reduced tensile properties of the materials. Therefore, the phenolic composite containing 3% of silane treated cellulose fiber was selected as the material with optimal properties.     

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.     

Simultaneously enhanced cryogenic tensile strength and fracture toughness of epoxy resins by carboxylic nitrile-butadiene nano-rubber

Epoxy resins are important matrices for composites. Carboxylic nitrile-butadiene nano-rubber (NR) particles are employed to improve the tensile strength and fracture toughness at 77 K of diglycidyl ether of bisphenol-F epoxy using diethyl toluene diamine as curing agent. It is shown that the cryogenic tensile strength and fracture toughness are simultaneously enhanced by the addition of NR. Also, the fracture toughness at room temperature (RT) is enhanced by the addition of NR. On the other hand, the tensile strength at RT first increases and then decreases with further increasing the NR content up to 5 phr. 5 phr NR is the proper content, which corresponds to the simultaneous enhancements in the tensile strength and fracture toughness at RT. Moreover, the comparison of mechanical properties between 77 K and room temperature indicates that the tensile strength, Young’s modulus and fracture toughness at 77 K are higher than those at RT but the failure strain shows the opposite results. The results are properly explained by the SEM observation.