Thermal and mechanical properties of phenolic-based composites reinforced by carbon fibres and multiwall carbon nanotubes

The thermal, mechanical and ablation properties of carbon fibre/phenolic composites filled with multiwall carbon nanotubes (MWCNTs) were investigated. Carbon fibre/phenolic/MWCNTs were prepared using different weight percentage of MWCNTs by compression moulding. The samples were characterized by scanning electron microscopy (SEM), flexural tests, thermal gravimetric analysis and oxyacetylene torch tests. The thermal stability and flexural properties of the nanocomposites increased by increasing MWCNTs content (wt% ⩽1), but they decreased when the content of MWCNTs was 2 wt%. The linear and mass ablation rates of the nanocomposites after modified with 1 wt% MWCNTs decreased by about 80% and 52%, respectively. To investigate the material post-test microstructure, a morphological characterization was carried out using SEM. It was shown that the presence of MWCNTs in the composite led to the formation of a strong network char layer without any cracks or opening.     

Interfacially reinforced methylphenylsilicone resin composites by chemically grafting multiwall carbon nanotubes onto carbon fibers

Both silane and multiwall carbon nanotubes (CNTs) were grafted successfully onto carbon fibers (CFs) to enhance the interfacial strength of CFs reinforced methylphenylsilicone resin (MPSR) composites. The microstructure, interfacial properties, impact toughness and heat resistance of CFs before and after modification were investigated. Experimental results revealed that CNTs were grafted uniformly onto CFs using 3-aminopropyltriethoxysilane (APS) as the bridging agent. The wettability and surface energy of the obtained hybrid fiber (CF-APS-CNT) were increased obviously in comparison with those of the untreated-CF. The CF-APS-CNT composites showed simultaneously remarkable enhancement in interlaminar shear strength (ILSS) and impact toughness. Moreover, the interfacial reinforcing and toughening mechanisms were also discussed. In addition, Thermogravimetric analysis and thermal oxygen aging experiments indicated a remarkable improvement in the thermal stability and heat oxidation resistance of composites by the introduction of APS and CNTs. We believe the facile and effective method may provide a novel interface design strategy for developing multifunctional fibers.     

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.     

Pristine and amino functionalized carbon nanotubes reinforced glass fiber epoxy composites

Multi-phase composites have been studied by incorporating carbon nanotubes (CNTs) as a secondary reinforcement in an epoxy matrix which was then reinforced with glass fiber mat. Different types of CNTs e.g. amino functionalized carbon nanotubes (ACNT) and pristine carbon nanotubes (PCNT) were homogeneously dispersed in the epoxy matrix and two-ply laminates were fabricated using vacuum-assisted resin infusion molding technique. The issues related to CNT dispersion and interfacial bonding and its affect on the mechanical properties have been studied. An important finding of this study is that PCNT scores over ACNT in composites prepared under certain conditions. This is a very significant finding since PCNT is available at a much lower cost than ACNT.     

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.     

Unidirectional carbon fibre reinforced polyamide-12 composites with enhanced strain to tensile failure by introducing fibre waviness

Unidirectional (UD) carbon fibre reinforced polymers offer high specific strength and stiffness but they fail in a catastrophic manner with little warning. Gas-texturing and non-constrained annealing were used to introduce fibre waviness into UD polyamide 12 composites produced by wet-impregnation hoping to produce composites with a more gradual failure mode and increased failure strain. Both methods increased the variation of fibre alignment angle compared to the control samples. The composites containing wavy fibres exhibited a stepwise, gradual failure mode under strain controlled uniaxial tension rather than a catastrophic failure, observed in control samples. Gas-texturing damaged the fibres resulting in a decrease of the tensile strength and strain to failure, which resulted in composites with lower tensile strength and ultimate failure strain than the control composites. Non-constrained annealing of carbon fibre/PA-12 produced wavy fibre composites with ultimate failure strain of 2%, significantly higher than 1.6% of the control composite.     

Influence of plastic deformation on single-fiber push-out tests of carbon fiber reinforced epoxy resin

In our study we present a procedure to measure and analyze single-fiber push-out force–displacement curves on carbon fiber reinforced polymers using a cyclic loading–unloading scheme. The measured cyclic force–displacement curves allow an energy-based evaluation of the interfacial failure, taking into account elastic, plastic and other dissipative energy contributions. Experimental and modeling results demonstrate that a deviation of the push-out curve from linear behavior does not correspond to crack opening but to a plastic deformation of the matrix. Evaluating the plastic energy yields a linear increase of the total plastic energy after a certain indenter displacement. This linear increase is attributed to stable crack propagation. Back-extrapolation of the linear part to zero total plastic energy using a linear regression yields the initiation of crack growth. It is concluded that for ductile matrix materials like polymers, a reliable interpretation of push-out data has to take into account plastic material deformation.     

Effect of water ageing on nanoindentation response of single hemp yarn/epoxy composites

This paper aims to evaluate the influence of water ageing on nanoindentation response of the in situ components of hemp/epoxy composites. Specific samples have been tested, made of single hemp yarn composites with two different epoxy resins. Analysis of indentation points depending on their location in the yarn microstructure has been performed. Measurements showed the influence of neighbouring fibres on the reduced modulus in the confined resin. Water induces a decrease in nanoindentation properties, and maximum decrease in reduced modulus takes place in the interfacial zone, which shows the significant degradation of the fibre/matrix interface caused by water ageing. Evolutions in the indentation properties induced by water are similar for both partially bio-based and fully synthetic epoxy resin composites. All these results bring insights into the influence of water ageing of each constituent of hemp fibre reinforced composites and give interesting data for developing eco-composite materials.     

A strategy for improving mechanical properties of a fiber reinforced epoxy composite using functionalized carbon nanotubes

Carbon fiber reinforced epoxy composite laminates are studied for improvements in quasi static strength and stiffness and tension-tension fatigue cycling at stress-ratio (R-ratio) = +0.1 through strategically incorporating amine functionalized single wall carbon nanotubes (a-SWCNTs) at the fiber/fabric-matrix interfaces over the laminate cross-section. In a comparison to composite laminate material without carbon nanotube reinforcements there are modest improvements in the mechanical properties of strength and stiffness; but, a potentially significant increase is demonstrated for the long-term fatigue life of these functionalized nanotube reinforced composite materials. These results are compared with previous research on the cyclic life of this carbon fiber epoxy composite laminate system reinforced similarly with side wall fluorine functionalized industrial grade carbon nanotubes. Optical and scanning electron microscopy and Raman spectrometry are used to confirm the effectiveness of this strategy for the improvements in strength, stiffness and fatigue life of composite laminate materials using functionalized carbon nanotubes.     

One-step amino-functionalization of milled carbon fibre for enhancement of thermo-physical properties of epoxy composites

Development of new chemical approaches for preparation of engineered carbon-based fillers is critical for high-performance applications. Herein, an efficient method for covalent functionalization of polyacrylonitrile-based carbon fibre through azo radical addition under mild condition is demonstrated. In this way, isobutyronitrile radicals in situ produced from thermal decomposition of 2,2′-azobisisobutyronitrile (AIBN), were covalently grafted on milled carbon fibre (MCF) surface, assisted by microwave irradiation, as evidenced by FTIR, Raman, and TGA analysis. The grafted isobutyronitriles on MCF surface (n-MCF) were applied for further MCF amino-functionalization (a-MCF) via nucleophilic reaction of an amine-rich compound. Then, both pure MCF and a-MCF were incorporated into epoxy matrix; and its curing process and thermo-physical properties were investigated using DSC, rheometry, DMA, TGA, and flexural analysis. The T_g and flexural strength of epoxy/a-MCF composites, compared to epoxy/MCF, increased by ∼3.5% and ∼10.2%, resulting from good adhesion between a-MCF and epoxy matrix which confirmed by SEM observations.     

Interfacial shear strength of a glass fiber/epoxy bonding in composites modified with carbon nanotubes

In recent years, carbon nanotubes (CNTs) grown on fibers have attracted a lot of interest as an additional reinforcing component in conventional fiber-reinforced composites to improve the properties of the fiber/matrix interface. Due to harsh growth conditions, the CNT-grafted fibers often exhibit degraded tensile properties. In the current study we explore an alternative approach to deliver CNTs to the fiber surface by dispersing CNTs in the fiber sizing formulation. This route takes advantage of the developed techniques for CNT dispersion in resins and introduces no damage to the fibers. We focus on unidirectional glass fiber/epoxy macro-composites where CNTs are introduced in three ways: (1) in the fiber sizing, (2) in the matrix and (3) in the fiber sizing and matrix simultaneously. Interfacial shear strength (IFSS) is investigated using single-fiber push-out microindentation. The results of the test reveal an increase of IFSS in all three cases. The maximum gain (over 90%) is achieved in the composite where CNTs are introduced solely in the fiber sizing.     

Preparation, morphology and properties of acylchloride-grafted multiwall carbon nanotubes/fluorinated polyimide composites

A fluorinated polyimide (PI) was synthesized by a two-step reaction from 4,4′-(hexafluoroisopropylidene) diphthalic anhydride and 2,2′-bis(trifluoromethyl)-4,4′diaminobiphenyl. A series of PI composites with various mass fractions of multi-walled carbon nanotubes (MWNTs) were prepared by either an in situ polymerization or blending process. To increase the chemical compatibility of carbon nanotubes with the PI matrix, MWNTs were treated with an acid mixture and sulfoxide chloride by turns. Results show that the dispersion of the MWNTs is highly improved in the PI by modification. The modified MWNTs are dispersed homogeneously in the matrix, while the structures of the PI and MWNTs are stable in the preparation process. The thermal stability of the nanocomposites is slightly lower than that of the pure PI. With incorporating MWNTs, the storage modulus and glass transition temperature of the composite films enhanced comparing to that of PI matrix. The dielectric constants of the composites increase sharply, which is favorable to their practical use in anti-static materials and embedded capacitors.     

Effect of imidazolium phosphate and multiwalled carbon nanotubes on thermal stability and flame retardancy of polylactide

A series of composites based on polylactide (PLA), have been prepared by melt-blending with multiwalled carbon nanotubes (MWNT) and Tri(1-hydroxyethyl-3-methylimidazolium chloride) phosphate (IP) functionalized MWNT (MIP). The morphology, thermal stability and burning behavior of the composites were investigated by Field Emission Scanning Electron Microscopy (FESEM), Thermogravimetric Analysis (TGA) and Cone Calorimeter Test (CCT), respectively. Significant improvement in fire retardant performance was observed for the PLA/MIP composite from CCT (reducing both the heat release rate and the total heat release) and TGA (increasing the char residue) compared to PLA/MWNT. SEM and Raman spectroscopy were utilized to explore the surface morphology and chemical structure of the char residues. It revealed that the catalytic charring effect of IP, the physical crosslinking effect of MWNT, and the combined effect of both IP and MWNT (forming continuous and compact char layers) were very efficient in improving the flame retarding properties of PLA/MIP composite.     

Large strain cyclic fatigue testing of unidirectional carbon fibre reinforced epoxy resin

The fatigue behaviour of a unidirectional carbon fibre reinforced epoxy resin subjected to large cyclic strains has been investigated. The development of a specific testing apparatus and procedure was found to be necessary so as to successfully test the unidirectional material in fatigue. A loop configuration has been used which allowed a gradual transfer of load from the testing machine to the specimen to be achieved without producing stress concentrations in the clamps. The results of the study reveal the existence of a fatigue mechanism for unidirectional carbon fibre reinforced epoxy resin which becomes evident under large cyclic strains, greater than 0.7%. This effect was seen to be a function of the loading level and whether the material was aged in humid conditions or not. Ageing lowered the threshold level for the onset of fatigue. The failure mechanisms involved have been revealed by scanning electron microscopy.     

Silane modification on mesoporous silica coated carbon nanotubes for improving compatibility and dispersity in epoxy matrices

A simple synthetic method for placing a mesoporous silica coating on multi-wall carbon nanotubes (CNTs@MS) was developed to improve the surface compatibility with regard to a polar epoxy matrix. In addition, the mesoporous silica shell with silanol groups on the CNTs provides a platform to attach silane molecules (e.g. 3-glycidoxypropyltrimethoxysilane, GPTMS) that enable the CNTs@MS to be incorporated into the epoxy matrix at a content of up to 20 wt.%. The viscosities of the CNTs@MS- and GPTMS-modified-CNTs@MS–epoxy composites are much lower than that of the CNTs–epoxy, and then the voids in the GPTMS-modified-CNTs@MS–epoxy composites are most significantly reduced. The effects of the CNTs@MS and GPTMS-modified CNTs@MS on the mechanical and thermal properties of the epoxy composite are investigated. The results show that the GPTMS-modified CNTs@MS improved the filler–epoxy matrix interaction, and has better compatibility in epoxy than the CNTs@MS. As the surface compatibility and interaction strength increase in the epoxy matrix, the enhancement in storage modulus, thermal conductivity and reduction in the coefficient of thermal expansion are in the following order: GPTMS-modified CNTs@MS > CNTs@MS ≫ CNTs.     

Long-term moisture effects on the interfacial shear strength between surface treated carbon fiber and epoxy matrix

In recent years, carbon fiber reinforced polymer (CFRP) composites have found increasing applications in marine and offshore area, where the CFRP components are subjected to a persistent attack of moisture. The performance degradation of composites under those critical service conditions becomes a key issue. In this work, silane coating and multiwalled carbon nanotubes were applied on carbon fibers to enhance the fiber/matrix interfacial bonding strength. The long-term effects of moisture on the interfacial shear strength (IFSS) of the composites in underwater environments, such as de-ionized water and simulated seawater, have been studied using single fiber microbond method. The silane coating and carbon nanotube-modified silane coating are found to contribute 14.5% and 26.3% increase in IFSS of the CFRP in dry air, and well maintain this improvement during a 120-day immersion test in de-ionized water and simulated seawater.     

Effect of carbon nanotubes on the mechanical properties and crystallization behavior of poly(ether ether ketone)

Pristine carbon nanotubes (CNTs) and noncovalently functionalized carbon nanotubes (f-CNTs) were used to prepare poly(ether ether ketone) (PEEK) composites (CNTs/PEEK and f-CNTs/PEEK) via melt blending. Noncovalently functionalized multiwalled nanotubes were synthesized using hydrogen-bonding interactions between sulfonic groups of sulfonated poly(ether ether ketone) (SPEEK) and carboxylic groups of nanotubes treated by acid (CNTs–COOH). The effects of these two kinds of nanotubes on the mechanical properties and crystallization behavior of PEEK were investigated. CNTs improved mechanical properties and promoted the crystallization rate of PEEK as a result of heterogeneous nucleation. Better enhancement of mechanical properties appeared in the f-CNTs/PEEK composites, which is ascribed to the good interaction between f-CNTs and PEEK. However, the strong interaction of f-CNTs and PEEK chains decreased the crystallization rate of PEEK for high content of f-CNTs.     

Processing parameters and characterisation of flax fibre reinforced engineering plastic composites with flame retardant fillers

This work studies the possibility of compounding natural fibres (flax) into engineering plastics (PA6 and PB6) and comparing the results with counterpart glass fibre composites. The problem in compounding is the difficulty to compound the fibres with such polymers of high melting temperatures without decomposing the natural fibre thermally. Preliminary experiments are tried to define the possible processing window using the kneader namely temperature, compounding time and shear rate. Fibre content is tried in range of 0–50 wt.% with 10% step. The mixing temperature covers the range around the melting temperature ‘Tm’ [Tm−20, Tm+20]°C. The use of pre-melting temperature in compounding would utilise the energy evolving by fibres mutual rubbing. Compounding time is optimised at the minimum level. Shearing rate is tried at 25, 50, 75 and 100 rpm. Optimum conditions are defined to be 210–230 °C and 200–210 °C for PBT and PA6 respectively. Shearing rate is also defined to lie within 25–50 rpm.Two different additives of non-organic mineral and organic phosphate flame retardants are tried with the prepared composites either alone or in combination with each other. The loading of flame retardants is limited to 20 wt.% in order to leave a space for natural fibres as well as the polymer and to keep in turn the overall composite mechanical properties. A mix of 1:1 ratio between the both types of retardants is needed to reach V0 flame retardation level. Mechanical properties are even improved 30% in E-modulus and 4% in strength with respect to composites without flame retardants. However, the injection moulding is reported to be difficult because of the high viscosity and the parameters should be optimised regarding the desired flame retardance level and the required mechanical properties as well as keeping the fibres not damaged.     

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.     

Electrical and thermal properties of polyamide 12 composites with hybrid fillers systems of multiwalled carbon nanotubes and carbon black

Hybrid filler systems of multiwalled carbon nanotubes (MWCNTs) and carbon black (CB) were incorporated into two types of polyamide 12 (PA12) using small-scale melt mixing in order to identify potential synergistic effects on the interaction of these two electrical conductive fillers. Although no synergistic effects were observed regarding the electrical percolation threshold, at loadings well above the percolation threshold higher volume conductivities were obtained for samples containing both, MWCNT and CB, as compared to single fillers. This effect was more pronounced when using a higher viscous PA12 matrix. The formation of a co-supporting network can be assumed. The combined use of CB and MWCNTs improved the macrodispersion of MWCNT agglomerates, which can be assigned as a synergistic effect. DSC measurements indicated an effect of the nanofiller on crystallisation temperatures of PA12; however this was independent of the kind or amount of the carbon nanofiller.