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.     

Creep behaviour of injection moulded polyamide 6/organoclay nanocomposites by nanoindentation and cantilever-bending

The creep behaviour of injection moulded PA 6/organoclay nanocomposites was studied by depth-sensing nanoindentation and DMA cantilever-bending. The glass transitions of PA 6 and its nanocomposites were decreased below room temperature upon saturation with water so that the materials could be tested in the rubbery regime. For nanoindentation creep on the skin and core regions of injection moulded samples, whilst organoclay improves the creep resistance of PA 6, the enhancement is due to the decrease of the initial compliance at zero time but the time-dependent creep is actually increased. In contrast, for cantilever-bending creep, organoclay reduces the creep compliance and the time-dependent creep in PA 6. It is suggested that the organoclay imparts a constraint effect on the PA 6 molecular chains, restricting their mobility in the bulk compared to the surface and hence improving their resistance to creep. A modified Halpin-Tsai equation was used to model their creep behaviour under these two loading configurations and compared to experimental data.     

Fracture toughness and creep performance of PMMA composites containing micro and nanosized carbon filaments

Chopped carbon fiber (CCF), multiwall carbon nanotubes (MWCNTs) and carbon nanofibers (CNFs) were introduced to polymethylmethacrylate (PMMA) via melt compounding. Fracture toughness and creep performance of these composites are presented. A constant volume concentration of 1 vol.% was used. Dispersion was evaluated on both micro- and nano-scales. Flexural modulus was also measured.     

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.     

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.     

Hybrid effect of nanoparticles with carbon fibers on the mechanical and wear properties of polymer composites

Polyetheretherketone (PEEK) composites reinforced with carbon fibers (CFs) and nano-ZrO₂ particles were prepared by incorporating nanoparticles into PEEK/CF composites via twin-screw extrusion. The effects of nanoparticles on the mechanical and wear properties of the PEEK/CF composites were studied. The results showed that the incorporation of nano-ZrO₂ particles with carbon fiber could effectively enhance the tensile properties of the composites. The tensile strength and Young’s modulus of the composites increased with the increasing nano-ZrO₂ content. The enhancement effect of the particle was more significant in the hybrid reinforced composites. The compounding of the two fillers also remarkably improved the wear resistance of the composites under water condition especially under high pressures. It was revealed that the excellent wear resistance of the PEEK/CF/ZrO₂ composites was due to a synergy effect between the nano-ZrO₂ particles and CF. CF carried the majority of load during sliding process and prevented severe wear to the matrix. The incorporation of nano-ZrO₂ effectively inhibited the CF failures through reducing the stress concentration on the carbon fibers interface and the shear stress between two sliding surfaces. It was also indicated that the wear rates of the hybrid composites decreased with the increasing applied load and sliding distance under water lubrication. And low friction coefficient and low wear rate could be achieved at high sliding velocity.     

Morphological changes of carbon nanotubes in polyethylene matrices under oscillatory tests as determined by dielectrical measurements

By combining a high sensitive dielectric sensor into a parallel plate rheometer, the time evolution of the dielectric properties of polyethylene/carbon nanotube composites was measured in the molten state under oscillatory shear. Composites with single- (SWCNT) or multiwall (MWCNT) carbon nanotubes initially decrease its conductivity proportional to the oscillatory shear-strain applied. After this initial drop, some composites increase the conductivity under these non-quiescent conditions reflecting a possible shear-induced agglomeration process. The latter based on the complex permittivity spectrum showing a shortening in the CNT-CNT distances in these composites after shear. At concentrations below the electrical percolation threshold, the presence of both SWCNTs and MWCNTs reduces the DC conductivity of the molten matrices. This result shows that carbon nanotubes can act as a scavenger for impurities or additives present in commercial polyethylenes.     

Investigation of the electrical conductivity of HDPE composites filled with bundle-like MWNTs

Bundle-like multi-walled carbon nanotubes (MWNTs) were melting-mixed with high-density polyethylene (HDPE). MWNTs are distributed in the matrix mostly isolated. Agglomerates are observed at higher concentrations. The electric conductivity of the composites follows the theory of a percolation system, and the charge transport shows different models. The temperature dependence of the electrical conductivity for the composites shows that both positive (PTC) and negative (NTC) temperature coefficients appear near the melting temperature of matrix. The NTC behaviors within the temperature range below the melting point depend on the MWNTs concentration and the NTC intensity increases with the increasing MWNTs concentration.     

Temperature dependence of creep and recovery behaviors of polymer composites filled with chemically reduced graphene oxide

Resistance to time-dependent plastic deformation of polymer composite materials is a crucial requirement in their application for long-term durability and reliability. Herein, creep and recovery behaviors of polystyrene (PS) composites filled with various loadings of chemically reduced graphene oxide (CRGO) were investigated at different environment temperatures. As expected, incorporation of CRGO into PS polymer increases the thermal stability, glass transition temperature and elastic modulus, although the tensile strength of the composite has a slight decrease. It was found that the creep deformation and strain rate of PS polymer reduce with decreasing temperature and with increasing loading of CRGO. A significant improvement in the recovered strain of PS was also obtained after the presence of CRGO. Based on the analytical modelings (Burger’s model and Weibull distribution function) and experimental results, the role of CRGO on improving the creep and recovery performance of thermoplastics was proposed and discussed.     

Short-term creep behavior of a biodegradable polymer reinforced with wood-fibers

The short-time creep behavior at tensile and single cantilever mode of deformation for a series of biodegradable composites was thoroughly studied. The composites were based on a biodegradable polymer matrix consisted a blend of poly(butylene adipate-terephthalate) (PBAT) copolyester, produced by non-renewable resources, and Polylactic acid (PLA). The matrix was reinforced with three different wood fiber types, at 20 and 30 wt%. The experimental data were analyzed in terms of Findley's and Burger's viscoelastic models. The effect of stress and temperature and wood fiber type on the material's creep response was analytically studied, while the Burger's model parameters were related to the composites morphology. In all cases, the wood fibers improved the creep resistance of the composites.     

Characterization of kenaf fiber composites prepared with tributyl citrate plasticized cellulose acetate

Cellulose bio-composites have been formed with many matrix polymers; however, a cellulose based matrix offers advantages of enhanced compatibility and environmental conformity. The aim was to use cellulose acetate as matrix since it is readily soluble and more flexible compared with cellulose. Kenaf (Hibiscus cannabinus) was selected as the reinforcement material. Cellulose acetate was dissolved for solution impregnation of the fibers. Plasticized cellulose composites were prepared by addition of tributyl citrate (TBC) to cellulose acetate solutions prior to fiber impregnation. Composites were then compression molded for compaction and fabrication of the composites. The surface morphology, thermal stability and mechanical properties of the kenaf-cellulose acetate composites were characterized. Thermal stability of the composites was limited by evaporation of moisture and TBC, followed by elimination of acetic acid for cellulose acetate. Scanning electron microscopy provided a morphological examination of the composites. Mechanical property measurements demonstrated that modulus of cellulose acetate was increased by kenaf, though the composites were brittle without TBC plasticizer addition.     

Relation between repeated uniaxial compressive pressure and electrical resistance of carbon nanotube filled silicone rubber composite

Carbon nanotube filled polymer composite can be used as sensitive material of flexible pressure sensor. By using solution mixing method, carbon nanotubes are dispersed into silicone rubber matrix to fabricate the composite. The piezoresistivities of the composite with different carbon nanotube concentrations under repeated compressions are researched quantitatively. The monotonicity of the piezoresistivity is dependent on the content of carbon nanotube and the range of the applied pressure. The reproducibility error of the piezoresistivity decreases with the increase of the compression cycles. The experimental data of the piezoresistivity are fitted by the linear combination of two exponential functions. The piezoresistive mechanism is studied qualitatively by analyzing the changes in the carbon nanotube network.     

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.     

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.     

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.