Effect of carbon fiber surface functionality on the moisture absorption behavior of carbon fiber/epoxy resin composites

Polyacrylonitrile (PAN)‐based carbon fibers were electrochemically oxidized in aqueous ammonium bicarbonate with increasing current density. The electrochemical treatment led to significant changes of surface physical properties and chemical structures. The oxidized fibers showed much cleaner surfaces and increased levels of oxygen functionalities. However, it was found that there was no correlation between surface roughness and the fiber/resin bond strength, i.e. mechanical interlocking did not play a major role in fiber/resin adhesion. Increases in surface chemical functionality resulted in improved fiber/resin bonding and increased interlaminar shear strength (ILSS) of carbon fiber reinforced epoxy composites. The relationship between fiber surface functionality and the hydrothermal aging behavior of carbon fiber/epoxy composites was investigated. The existence of free volume resulted from poor wetting of carbon fibers by the epoxy matrix and the interfacial chemical structure were the governing factors in the moisture absorption process of carbon fiber/epoxy composites. Copyright © 2016 John Wiley & Sons, Ltd.     

Thermomechanical behaviors and dielectric properties of polyaniline‐doped para‐toluene sulfonic acid/epoxy resin composites

Composites based on conductive organic/inorganic fillers dispersed in insulating matrix have been widely investigated because of their widespread applications such as electromagnetic shielding, electrostatic discharge, and sensors. In this context, novel composite materials based on epoxy resin matrix charged with polyaniline (PANI)‐doped para‐toluene sulfonic acid were elaborated. Fourier transform infrared spectroscopy, X‐ray diffraction and scanning electron microscopy were used to check the structure and the morphology of the samples. Viscoelastic behavior and thermal stability of the composites were explored by dynamic mechanical thermal analysis and thermogravimetric analysis. It was shown that the PANI particles exhibited a partial crystalline structure and were homogeneously dispersed in epoxy matrix. Consequently, this structure affected the thermal stability and viscoelastic properties of the composites. Furthermore, the dielectric and electrical properties were investigated up to 1 MHz. Measurements of dielectric properties revealed that with loading fillers in matrix, the dielectric parameters increased to high values at low frequency then decreased at values around 40 and 32 of real and imaginary parts, respectively, at 1 MHz with 15% of PANI content. Copyright © 2011 John Wiley & Sons, Ltd.     

Effects of processing conditions on rheological,thermal, and electrical properties of multiwall carbon nanotube/epoxy resin composites

We report on the effect of processing conditions on rheology, thermal and electrical properties of nanocomposites containing 0.02–0.3 wt % multiwall carbon nanotubes in an epoxy resin. The influence of the sonication, the surface functionalization during mixing, as well as the application of external magnetic field (EMF) throughout the curing process was examined. Rheological tests combined with optical microscopy visualization are proved as a very useful methodology to determine the optimal processing conditions for the preparation of the nanocomposites. The Raman spectra provide evidence for more pronounced effect on the functionalized with hardener compositions, particularly by curing upon application of EMF. Different chain morphology of CNTs is created depending of the preparation conditions, which induced different effects on the thermal and electrical properties of the nanocomposites. The thermal degradation peak is significantly shifted towards higher temperatures by increasing the nanotube content, this confirming that even the small amount of carbon nanotubes produces a strong barrier effect for the volatile products during the degradation. The ac conductivity measurements revealed lower values of the percolation threshold (pc) in the range of 0.03–0.05 wt %. CNTs for the nanocomposites produced by preliminary dispersing of nanotubes in the epoxy resin, compared to those prepared by preliminary functionalization of the nanotubes in the amine hardener. This is attributed to the higher viscosity and stronger interfacial interactions of the amine hardener/CNT dispersion which restricts the reorganization of the nanotubes. The application of the EMF does not influence the pc value but the dc conductivity values (σ_dc) of the nanocomposites increased at about one order of magnitude due to the development of the aforementioned chain structure. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

Interfacial adhesion and shear behaviors of aramid fiber/polyamide 6 composites under different thermal treatments

Aramid fiber (AF) reinforced by polyamide (PA) composites are excepted to have good interfacial matching due to their similar chemical interactions of hydrogen bonding. Thus, polarizing optical microscope (POM), transverse fiber bundle (TFB) test, and droplet micro-debonding technique were respectively performed to characterize interfacial crystallization, adhesion and shear behaviors of AF/PA6 composites with different thermal treatments. Both interface adhesion and AF fibrillation are enhanced with decreasing cooling rate or increasing annealing temperature due to the increased interfacial transcrystallization interaction. However, fast cooled interface also presents a high interfacial shear strength (IFSS) due to favorable normal residual stress. The apparent IFSS is believed to be a result of competition between crystallization enhancing interfacial interaction, interfacial mismatching aggravating debonding, and an uncertain residual stress positive or negative for load transfer. TFB failure mechanism including AF fibrillation and kinking are schematically presented. Fibrillation strength of AF is found to follow Weibull distribution evaluated by droplet micro-debonding technique.     

Effects of thermal volume expansion on positive temperature coefficient effect for carbon black filled polymer composites

Polymeric positive temperature coefficient (PTC) materials have been prepared by incorporating carbon black (CB) into two different polymer matrices, crystalline high density polyethylene (HDPE) and amorphous polystyrene (PS). The effects of thermal volume expansion on the electrical properties of conductive polymer composites were studied. The volume fraction of conductive particles behaves like a switch from insulator to conductor in the polymeric PTC composite. Our mathematical model and experimental model have proved that the abrupt resistivity increase at PTC transition range and at the percolation curve close to the critical volume fraction for both polymeric PTC composites have the same conductive mechanism. The thermal expansion is one of the key factors responsible for the PTC effect and can be seen by comparing the PTC transition curves from model predictions and experiment. Furthermore, the model predicts PTC curves of CB/PS composite more successfully than it does for the CB/HDPE composite, and the reasons for this are also discussed. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 3078–3083, 2007     

Preparation of (Fe)MIL-101 on short carbon fibers to improve the flame retardancy,smoke suppression,and mechanical of epoxy resin

Nano (Fe)MIL-101 particles were grafted on the short carbon fibers (SCFs) by in situ growth method to prepare (Fe)MIL-101@SCFs. The flame-retarded composites of epoxy resin (EP) were fabricated with combination of (Fe)MIL-101@SCFs and ammonium polyphosphate (APP). The composites showed good flame retardancy, smoke suppression, and mechanical properties simultaneously. The main heat release rate peak of the flame-retarded composites was reduced and delayed evidently in comparison with pristine EP. The high amount of residual char with coherent and dense structure was formed owing to the catalytic char formation of (Fe)MIL-101 as well as the strengthening action of SCF. The improvement in mechanical properties of the flame-retarded composite was due to the reinforcement effects of (Fe)MIL-101@SCFs and its action of interfacial adjustment. This research solved the contradiction between the flame retardancy and mechanical properties of EP, and proposed a new method to prepare the mechanically reinforced and flame retardant EP.     

The numerical simulation of the mechanical failure behavior of shear thickening fluid/fiber composites: A review

This study reveals the finite element modeling of mechanical failure behavior of shear thickening fluid (STF)/fiber composites under impact. Numerical analysis and finite element modeling of the rheological properties of non-Newtonian fluid, STF are introduced. This review summarizes the model coupling methods in finite element modeling and the mechanical failure behavior prediction models of STF/fiber composites under impact. Further, the influencing factors on the accuracy of mechanical failure simulation models are analyzed. Factors such as the friction between fibers, shear rate, filler particles in the fibers, hysteresis effect and the boundary conditions should be considered in simulating the shear thickening effect of the composites.     

The effects of wood-flour on combustion and thermal degradation behaviors of PVC in wood-flour/poly(vinyl chloride) composites

The effects of wood-flour on combustion and thermal degradation behaviors of PVC in wood-flour/poly (vinyl chloride) composites (WF-PVC) were investigated by using cone calorimeter (CONE) and TGA. The results show that thermal degradation behavior of WF-PVC composites has obvious characteristics of that of PVC. Interactions occur between wood-flour and PVC during the combustion and thermal degradation of WF-PVC composites. The thermal degradation of wood-flour can be accelerated by pure PVC. Moreover, the char formation can be raised by adding wood-flour to PVC. Compared with PVC at all flaming stage, when heat flux is kept at 50 kW m⁻², the average heat release rate (av-HRR), the total heat release (THR), the total smoke production (TSP) and the average specific extinction area (av-SEA) of WF-PVC composites are respectively reduced by 44%, 9.2%, 25.8% and 29.9%. In WF-PVC composites, the wood-flour has remarkable effects on the properties of heat release and smoke release of PVC.     

Preparation,characterization and thermal degradation study of poly(amide imide)s based on tri-component mixture of PMDA/BTDA,diamines and acid chloride

A series of poly(amide imide)s (PAIs) having alternate (amide–amide) and (imide–imide) units (polymers 1–14 and 22–35), and random distribution of amide-imide linkages (polymers 15–21 and 36–42) were prepared by low temperature solution polymerization of benzene-1,2,4,5-tetracarboxylic dianhydride (PMDA)/benzophenone-3,3′,4,4′-tetracarboxylic dianhydride (BTDA), diamines (cyclic and aromatic) and acid chloride in dimethylforamide. All the polymers were readily soluble in polar aprotic solvents with inherent viscosities in the range of 0.134–0.878. The process of cycloimidization of poly(amide amic acid)s (PAAs) to PAIs was investigated by TGA and FT-IR techniques at four different temperatures i.e., 175, 200, 225, and 260 °C. The rate of cycloimidization was calculated by taking into account the theoretical weight loss (W_T), obtained from [n × M_w (H₂O)/M_w (R_U)] W, where M_w (H₂O) molecular weight of water, W weight of PAA taken for TGA, M_w (R_U) the molecular weight of repeat unit of PAA, n number of water molecules eliminated per repeat unit of PAA upon cycloimidization. For a particular diamine, the extent of percentage cycloimidization at the end of the isothermal heating was higher for PAAs containing trimellitic anhydride chloride (TMAc) unit, irrespective of the nature of the dianhydride and diamine. Thermal and thermooxidative degradation of PAIs was investigated by TGA in nitrogen and oxygen atmosphere. The initial decomposition temperatures (IDT) of polymers are above 260 °C, and vary widely (from 260 to 501 °C) depending upon the structure of the polymer backbone. PAIs containing TMAc exhibited higher thermal stability as compared to those polymers having diacid chloride units, in both N₂/O₂ atmospheres.