Moisture effects on the behavior of graphite/polyimide composites

The effect of moisture on the material behavior of Magnamite® IM7 Graphite/Avimid® K3B thermoplastic polyimide composite laminates has been investigated. Laminates consisting of a 62 vol% fiber were laid up with several different stacking sequences: [90]₁₀ or unidirectional, [0₂/90]_S or thin cross-ply, [0₂/90₂/0₂]_S or thick cross-ply, [45/0/-45/90]_S or quasi-isotropic, and [0/90] laminates. In this study, the glass-transition temperature, T_g, and the intralaminar fracture toughness, G_IC, were measured for dry and moisture-saturated unidirectional samples. When laminates were saturated with moisture, the value of T_g was found to decrease from its baseline (dry) value, but recovered upon redrying the samples. This observation is consistent with the effects of moisture on the T_g of other polymer composites. Permanent toughness losses have not been observed in samples conditioned in room-temperature 75% and 100% relative humidity environments. However, during long-term conditioning of cross-ply and quasi-isotropic samples in liquid water, transverse cracks initiated in the absence of an applied mechanical load. Moisture uptake curves for conditioning in room-temperature liquid water, 80°C (176°F) liquid water, and at room temperature and 75% relative humidity were used to calculate Henry's Law constants and diffusion coefficients. Non-Fickian behavior, consisting of a postsaturation increase in moisture uptake, was observed in crossply and quasi-isotropic laminates and might be due to the observed transverse cracking.     

The effect of isothermal aging on transverse crack development in carbon fiber reinforced cross-ply laminates

An investigation into the effect of isothermal aging on the development of transverse cracks in cross-ply laminates of two high temperature composite systems was performed. The composite materials investigated were BASF X5260/640–800 and DuPont Avimid K/IM6. Changes in the glass transition temperature, composite weight loss, crack density, and mode I intralaminar fracture toughness were monitored during isothermal aging in air at 177°C for up to 2232 h. The two laminate configurations used in this study include two variations of the generic cross-ply configuration [0₂/90_n]_s, in which n equals 1 and 2. The results of this investigation show that a layer of degraded material forms at the surface of the X5260/640–800 bismaleimide laminates and that the thickness of the degraded layer increases with aging time. After 744 h of aging, transverse cracks form in the surface plies and an increasing crack density evolves as aging time is increased; however, transverse cracks do not form in the inner 90° ply groups with aging during the time period investigated. The Avimid K/IM6 thermoplastic polyimide laminates, which show evidence of cracking prior to aging, do not exhibit any significant change in crack density with aging. The results of the aging experiments also show that the bismaleimide system exhibits a weight loss of 1.5% and an increase in glass transition temperature from 250°C to 300°C after 2232 h of aging at 177°C, while the thermoplastic polyimide system shows a weight loss of only 0.05% and an increase in glass transition temperature from 280 to 285°C after 2232 h. Changes in the resistance to crack formation are also seen in these materials during aging. The mode I intralaminar fracture toughness, a measure of resistance to transverse crack formation, shows a 50% decrease after aging for 2232 h for the bismaleimide system, while the behavior exhibited by the thermoplastic polyimide shows little evidence of a reduction.     

Effect of multiple extrusions on the impact properties of polypropylene/clay nanocomposites

Polypropylene (PP)‐based polymer nanocomposites containing organically modified montmorillonite (OMMT) with and without maleic anhydride grafted PP, were compounded by twin‐screw extrusion. The extrusion process was repeated various numbers of times to increase the extruder residence time (T_R) and, through that, the particle dispersion. Rheological measurements fitted to a modified Carreau–Yasuda model defining a melt yield stress were used to indicate changes in the particle dispersion with regard to T_R. This analysis showed a monotonically increased dispersion of clay particles in the PP matrix with increasing extruder T_R. The small‐strain tensile properties were tested at both ambient (20°C) and elevated (90°C) temperatures, and no significant changes were observed in the tensile strength or modulus as a function of T_R. Instrumented Izod impact tests showed that the nanocomposite impact strength (σ_i) increased monotonically with increased T_R by 70% from least dispersed to best dispersed, which was still 20% below the level for neat PP. Both the fracture initiation energy and propagation energy increased with T_R, but the primary effect on σ_i came from the fracture propagation energy, which delivered 80% of the improvement. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Transient computational fluid dynamics modeling of pervaporation separation of aromatic/aliphatic hydrocarbon mixtures using polymer composite membrane

Pervaporation (PV) separation of toluene/n‐heptane mixtures was studied experimentally and theoretically by means of a molecular surface engineering (MSE) polymer composite membrane. A comprehensive mathematical model was developed to predict unsteady state transport of toluene and n‐heptane (nC₇) through the membrane. Conservation equations including continuity, and heat transfer equations were solved using finite element method (FEM). Computational fluid dynamics (CFD) technique was applied to solve the model equations. The model was then verified with PV experimental data. The simulation results were in good agreement with the experimental data. The simulation results revealed that the proposed model could provide a general simulation of transport in the PV process. POLYM. ENG. SCI., 2013. © 2012 Society of Plastics Engineers     

Investigation of production parameters in fracture behavior of hot-pressed Al2O3–SiC/graphite fibrous monolithic ceramics: Fibers orientation and cell boundary fraction

Fibrous monoliths (FMs) exhibit graceful failure in flexure and have higher toughness values. In this research, a mixture of Al₂O₃ and SiC as the core and graphite as the shell material of fibers were produced by extrusion-molding technique and after aligning along intended directions (0°, 90°, and 0°/90°) were sintered using the hot-pressing method at the temperature of 1500°C under pressure of 35 MPa for 1 hour. The significance of fibers orientation angle and the cell to cell boundary volume ratio in defining the fracture behavior of the FMs was detected. Because of the extensive crack interactions with graphite cell boundary such as crack deflection and delamination, with increasing cell boundary content from 25 to 30 vol%, the fracture toughness was enhanced. The highest flexural strength (184.8 ± 0.61 MPa) obtained from samples with 0° fibers orientation compared to 0°/90°. Since in the transverse plies (layers with 90° aligning), the properties of matrix phase are dominant, hence the strength in specimens with 0°/90° fibers orientation decreased considerably due to weak graphite matrix phase. In addition, the fracture toughness value increased up to 8.35 ± 0.74 MPa·m^1/2 for the unidirectional architecture of (0°) in comparison with cross-ply (0°/90°) architecture.     

Chemical kinetic modeling of i‐butane and n‐butane catalytic cracking reactions over HZSM‐5 zeolite

A chemical kinetic model for i‐butane and n‐butane catalytic cracking over synthesized HZSM‐5 zeolite, with SiO₂/Al₂O₃ = 484, and in a plug flow reactor under various operating conditions, has been developed. To estimate the kinetic parameters of catalytic cracking reactions of i‐butane and n‐butane, a lump kinetic model consisting of six reaction steps and five lumped components is proposed. This kinetic model is based on mechanistic aspects of catalytic cracking of paraffins into olefins. Furthermore, our model takes into account the effects of both protolytic and bimolecular mechanisms. The Levenberg–Marquardt algorithm was used to estimate kinetic parameters. Results from statistical F‐tests indicate that the kinetic models and the proposed model predictions are in satisfactory agreement with the experimental data obtained for both paraffin reactants. © 2011 American Institute of Chemical Engineers AIChE J, 58: 2456–2465, 2012     

Fatigue behavior of filament‐wound glass fiber reinforced epoxy composite tubes under tension/torsion biaxial loading

A study of filament‐wound glass fiber/epoxy composite tubes under biaxial fatigue loading is presented. The focus is placed on fatigue lives of tubular specimens under tension/torsion biaxial loading at low cycle up to 100,000 cycles. Filament‐wound glass‐fiber/epoxy tubular specimens with three different lay‐up configurations, namely [±35°]_n, [±55°]_n, and [±70°]_n lay‐ups, are subjected to in‐phase proportional biaxial cyclic loading conditions. The effects of winding angle and biaxiality ratio on the multiaxial fatigue performance of composites are discussed. Specimens are also tested under two cyclic stress ratio: R = 0 and R = −1. The experimental results reveal that both tensile and compressive loading have an influence on the multiaxial fatigue strength, especially for [±35°]_n specimens. A damage model proposed in the literature is applied to predict multiaxial fatigue life of filament‐wound composites and the predictions are compared with the experimental results. It is shown that the model is unsuitable for describing the multiaxial fatigue life under different cyclic stress ratios. POLYM. COMPOS. 28:116–123, 2007. © 2007 Society of Plastics Engineers     

A new technique for preparation of PDMS/ceramic nanocomposite membrane for gaseous hydrocarbons separation

A novel composite membrane using polydimethylsiloxane (PDMS) as a top active layer and ceramic nanocomposite as the support layer was developed for the gaseous hydrocarbons separation. For the fabrication of hybrid membranes, nanocomposite technology applied for manufacturing ceramic supports with controllable microstructures. Also, a new method was used for coating a uniform and no penetrated polymeric layer. Top layer of ceramic support with nanocomposite microstructures was fabricated using 5 wt % α‐Al₂O₃‐SiO₂ bidispersed suspensions with optimum weight fraction of second phase (SiO₂) based on the fractional collision frequency theory. PDMS selective layer was coated on the outer surface of the porous ceramic nanocomposite support by dip‐coating method. In this respect, the effect of several parameters such as pretreatment temperature, PDMS solution concentration, and number of coated polymeric layers on prepared layers morphology and hybrid membrane performance in the separation of condensable hydrocarbons (iso and n‐butane) from hydrogen were investigated. The results showed that the membranes fabricated at 140°C as pretreatment temperature and three polymeric layers by 7, 15, and 15 wt % PDMS concentration, respectively, had a high selectivity (>25 at 2 bar)) in C₄H₁₀/H₂ separation. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Modelling stiffness degradation due to matrix cracking in angleply composite laminates

Abstract

When a multidirectional composite laminate is subjected to in plane static or fatigue tensile loading, matrix cracks parallel to the fibres appear in the off axis plies long before catastrophic failure. Matrix cracking significantly reduces the laminate stiffness properties and triggers development of other harmful resin dominated damage modes such as delaminations. Concurrent matrix cracking in the adjacent off axis plies is an extremely complex problem to model analytically and has been analysed mostly using finite element methods.

The present paper is concerned with the theoretical modelling of stiffness reduction in cracked angleply [θ₁ /θ₂ ]_s laminates subjected to multiaxial in plane loading. A new approach based on the equivalent constraint model of the damaged ply and an improved two-dimensional shear lag method has been applied to model matrix cracking in angleply [θ₁ /θ₂ ]_s laminates. Stresses in the cracked ply are determined from a system of two coupled ordinary differential equations and used to calculate the in situ damage effective functions, which describe the stiffness loss. For angleply [±θ ]_s laminates with a cracked midlayer, it is found that the reduction due to matrix cracking of the laminate axial and transverse moduli is more significant than in crossply laminates, while for the shear modulus, the opposite is true. Matrix cracking in such laminates can result in an increase in the Poisson's ratio – a phenomenon not observed in crossply [0/90]_s laminates. In addition, matrix cracking in angleply [±θ ]_s laminates introduces coupling between extension and shear.     

Experimental and numerical investigation of fracture of ABS polymeric material for different sample's thickness using a new loading device

The fracture behavior of ABS (acrylonitrile butadiene styrene) polymeric material has been investigated under the full range of in‐plane loading conditions using a new loading device to obtain more reliable results. Loading conditions from pure mode‐I through various mixed‐mode I/II ratios up to pure mode‐II have been generated using the proposed new loading device for the same specimen geometry. From the experimentally measured critical loads, the mode‐I, mode‐II, and the various mixed‐mode I/II critical energy release rates have been determined at different loading angles from 0° to 90°. Using the FE results, nondimensional stress intensity factors were applied to the specimen. The primary objectives of this study were to develop a new loading device to determine the mixed‐mode fracture toughness K_IC and K_IIC of ABS polymeric material. Another goal was to obtain stress intensity and strain energy release rates solutions associated with the crack, and to examine effects of thickness and geometric variables, particularly under mixed‐mode loading conditions. It was found that the thickness of the 10 mm specimen satisfied the plane strain condition with average fracture toughness ≈4.32 MPa·m^1/2 under pure mode‐I loading and ≈1.42 MPa·m^1/2 for pure mode‐II loading. POLYM. ENG. SCI., 54:2086–2096, 2014. © 2013 Society of Plastics Engineers     

Ferroelectric–Dielectric Composites Model of Ba0.5Sr0.5TiO3/Mg2AO4(A = Ti,Si) for Tunable Application

The correspondence between the theoretical model and the experimental results of the dielectric response in two‐phase composites of Ba_0.5Sr_0.5TiO₃ and Mg₂AO₄ (A = Ti, Si)was studied. The Ba_0.5Sr_0.5TiO₃ (BST50)/Mg₂AO₄ composites in 2‐2 model structure consisting of BST50 layers and Mg₂AO₄ layers were fabricated by tape casting and multilayer technique. The 3‐0 model of the two‐phase composites is fabricated by conventional ball mill mixing and solid‐state reaction process. The ceramics samples with dense structure were obtained because the coefficient of thermal expansion (CTE) of Mg₂SiO₄ (12.84 ppm/°C) and Mg₂TiO₄ (12.11 ppm/°C) ceramic specimens are close to the pure BST50(13.15 ppm/°C) ceramic. The microstructure, dielectric, and tunable properties of 2‐2 and 3‐0 model composites were investigated. The experimental results agree well with the theoretical prediction in 2‐2 model. An important feature of 2‐2 model composites is that the DC field is efficiently applied to the high‐permittivity ferroelectric phase. With the increase in Mg₂AO₄ volume fraction q, the tunability of the composite remains almost unchanged whereas the permittivity greatly reduced in the 2‐2‐//model. These results show that the 2‐2‐//model sample is good candidates for the tunable devices.     

Delamination characteristics of multi‐directional carbon fiber/epoxy composites under high pressure

It was shown in a previous study that for unidirectional (0‐deg) graphite/epoxy composites, the fracture toughness under hydrostatic pressure increased 38% as hydrostatic pressure increased from 0.1 MPa to 200 MPa. This work investigates the compressive delamination behavior of multi‐directional graphite/epoxy laminated composites subjected to various hydrostatic pressures. Compressive delamination tests were performed under four hydrostatic pressure levels: 0.1, 100, 200, and 300 MPa Eighty‐eight‐ply dog‐bone type specimens with a single delamination at the center of the specimen were used. The stacking sequence applied was [0°/±45°/90°]_lls. The compliance and fracture load were determined from load‐displacement curves as a function of hydrostatic pressure. The results show that the compliance decreases with increasing pressure while fracture load increases with increasing pressure. The compressive delamination toughness, G_c, was determined from the compliance method as a function of applied hydrostatic pressure. The results also show that G_c is significantly affected by hydrostatic pressure and increases from 2.11 kJ/m² to 3.04 kJ/m2 (44% increase) as hydrostatic pressure increased from 0.1 MPa to 300 MPa. Visual examination of the fractured surface revealed that the increase of G_c is due to the suppression of micro‐cracks With increasing pressure. It was also found from SEM examination of delaminated surface that the G_c increase is due to more epoxy adhering to the fibers and more plastic deformation of epoxy material as applied hydrostatic pressure increases.     

Hot‐embossing experiments of polymethyl methacrylate across the glass transition temperature with variation in temperature and hold times

Hot‐embossing (HE) experiments were conducted on polymethyl methacrylate (PMMA) across its glass transition temperature from 92 to 142°C. The glass transition temperature (T_g) of the PMMA used in this study was ～ 102°C. The polymer samples were embossed to a depth of 0.8 mm (800 μm). The experiments were carried out at various temperatures for different hold times of 30, 90, and 180 sec during the embossing process. A few additional experiments were conducted at 142°C with cooling of the samples as well. The force required for embossing and the final depth of the embossed features were analyzed. Polymers, including PMMA, show significantly different material behavior around and above T_g. The same was seen in the aforementioned tests; the trends observed for the force as well as the final depth changed considerably around 122°C (T_g + 20). These findings will be used in developing material models for use in simulating the hot‐embossing process. POLYM. ENG. SCI., 2012. © 2012 Society of Plastics Engineers     

Oxide multilayer thermoelectric generators

Oxide multilayer thermoelectric generators (MLTEG) were fabricated, using the standard multilayer technology. Green tapes of p‐type La₂CuO₄ and n‐type Nd₂CuO₄ thermoelectric oxides were stacked with intermediate insulating glass layers. Electrical contacts between thermoelectric oxides were applied, using screen‐printing of AgPd paste, and multilayers were cofired at 1000°C. However, cofiring of four different materials turned out to be very challenging, and contact resistance problems frequently led to device malfunctions. We developed a new concept of a transversal multilayer thermoelectric generator (TMLTEG), which is characterized by a simple design. This generator is build up by stacking layers of a p‐ or n‐type thermoelectric oxide and printing stripes of AgPd paste onto the thermoelectric layers at an angle with respect to the temperature gradient. Transversal multilayer thermoelectric generators were fabricated using p‐type La₂CuO₄, or n‐type substituted CaMnO₃; cofiring of the multilayer stacks was performed at 1000°C. The TMLTEG based upon p‐type lanthanum cuprate exhibits a power output of 7.8 mW at ?T= 200 K in the low temperature range of 25‐135°C. Materials issues, cofiring characteristics, design and the thermoelectric performance of multilayer TEGs will be discussed.     

Fracture of adhesive joints and laminated composites

The mechanisms of resin controlled failure in adhesive joints and composites (delamination and transverse cracking) are examined. An in-situ failure model based on the fracture mechanics principles is applied here to describe the failure processes involved. The model centers on the crack tip plastic zone developed in the thin resin layer between the fibers or the adherends. The plastic zone in the resin layer is heavily influenced by a dominant slow varying stress distribution, approximated to be r^?m/2 dependent with m ? 1 (r is the distance from the crack tip). The adhesive or composite fracture toughness G*_IC can then be expressed as a function of several resin properties of comparable importance: modulus E, yield stress σ_y, resin G_IC and residual stress. The relative significance of the resin properties on the adhesive or composite fracture is discussed. The effects of temperature, loading rate, and resin toughening on such failure as a result of the corresponding variations in resin properties are also addressed.     

Hydrolysis of poly(hydroxybutyrate‐co‐hydroxyvalerate) nanoparticles

Poly(hydroxybutyrate‐co‐hydroxyvalerate) (PHBV) is a natural polyester known for its biocompatibility and biodegradability. The hydrolysis of PHBV nanoparticles (90–150 nm) and microparticles (33–58 µm) was investigated. Particles were formulated from preformed polymer(s) by miniemulsification/solvent evaporation technique to obtain nanoparticles or by emulsification/solvent evaporation technique to obtain microparticles. The morphology of the nanoparticles was studied by Field Emission Gun‐Scanning Electron Microscopy (FEG‐SEM). The kinetics of PHBV degradation was followed by gel permeation chromatography. After storage of PHBV nanoparticles for 25 days at 37 °C, the M_n and M_w of PHBV was reduced up to 85 and 80%, respectively. PHBV nanoparticles stored at 4 °C presented a much lower molecular weight reduction. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Incorporation of silicon dioxide nanoparticles at the carbon fiber‐epoxy matrix interphase and its effect on composite mechanical properties

Functionalized silicon dioxide nanoparticles (nano‐fSiO₂) were uniformly deposited on the surface of carbon fibers (CFs) using a coating process which consisted of immersing the fibers directly in a suspension of nano‐fSiO₂ particles and epoxy monomers in 1‐methyl‐2‐pyrrolidinone (NMP). The 0° flexural properties, 90° flexural properties, and Interlaminar shear strength (ILSS) mechanical properties of unidirectional epoxy composites made with nano‐fSiO₂+epoxy sized carbon fibers, with control fibers, and with epoxy‐only sized fibers were measured and compared. An obvious increase of the fiber/matrix adherence strength was obtained with the nano‐fSiO₂+epoxy coating. The nano‐fSiO₂+epoxy sized CF/epoxy composites showed a relative increase of 15%, 50%, and 22% in comparison to control fibers, for the Interlaminar shear strength, the 90^° flexural strength and the 90° flexural modulus, respectively, but little e difference was measured between the different systems for the 0° flexural properties. The observation of the fracture surfaces by scanning electron microscopy of composite fracture confirmed the improvement of the interfacially dependent mechanical properties. POLYM. COMPOS., 38:1474–1482, 2017. © 2015 Society of Plastics Engineers     

Microstructure,crystallization and dynamic mechanical properties of polyamide/clay nanocomposites after melt‐state annealing

Polyamide 6/clay (PA/clay) nanocomposites produced by melt‐compounding were treated under various melt‐state annealing processes. The effect of melt‐state annealing on the microstructure, crystallization, and dynamic mechanical properties was characterized by transmission electron microscope (TEM), modulated differential scanning calorimetry (MDSC), X‐ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and dynamic mechanical analysis (DMA). Clay layers were exfoliated in PA matrix. The crystalline transformation between α and γ‐crystalline phase was virtually dependent on the annealing process and clay loading. After melt‐state annealing between 230 and 250°C, clay induced the appearance of a new endothermic peak in PA/clay. PA/clay after melt‐state annealing exhibited a higher elastic modulus above T_g and a lower β relaxation below T_g as compared with the non‐annealed sample. FTIR analysis demonstrated that the melt‐state annealing caused strong hydrogen bonding interaction of amide groups with clay layers. POLYM. COMPOS., 2012. © 2012 Society of Plastics Engineers     

Mathematical model for the bulk polymerization of styrene using the symmetrical cyclic trifunctional initiator diethyl ketone triperoxide. I. Chemical initiation by sequential decomposition

In this study, we experimentally and theoretically investigated the use of the symmetrical cyclic trifunctional initiator diethyl ketone triperoxide (DEKTP) in the bulk polymerization of styrene. The experimental study consisted of a series of isothermal batch polymerizations at different temperatures (120 and 130°C) with different initiator concentrations (0.005, 0.01, and 0.02 mol/L). A mathematical model was developed to predict the evolution of the reacting chemical species and the produced molecular weight distributions. The kinetic model included chemical and thermal initiation, propagation, transfer to the monomer, termination by combination, and reinitiation reactions. The simulation results predict the concentration of diradicals, monoradicals, and polymeric chains, characterized by the number of undecomposed peroxide groups. The experimental results showed that at reaction temperatures of 120–130°C, initiation by DEKTP produced an increase in the polymerization rates (R_p's) and average molecular weights, depending on the initiator concentration, due to sequential decomposition. The mathematical model was adjusted and validated with the experimental data. The theoretical predictions were in very good agreement with the experimental results. Also, an optimum initiator concentration was observed that achieved high R_p's and high molecular weights simultaneously. For polymerization temperatures of 120–130°C, the optimum concentration was 0.01 mol/L. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Polyoxymethylene orientation by equal‐channel multiple angular extrusion

The effect of conditions and routes of deformation in the course of equal‐channel multiple‐angular extrusion (ECMAE) on physical and mechanical properties of polyoxymethylene (POM) have been studied. As deformation routes, Route C (shear planes are parallel, and the simple shear direction of every deformation zone is changed through 180°) and Route E (shear planes are turned through ±45° around the extrusion axis and the normal to the axis, and simple shear direction is changed through 180° or ±90° with respect to the deformation zone) were selected. It has been shown that ECMAE provides the increase of modulus of elasticity E more than twice, tensile strength σ_T increases in four times. At the same time, strain at break ε_b is reduced by 1.5%. The value of the achieved effects depends on the accumulated deformation and the selected deformation route. The best set of physical and mechanical characteristics was observed in the case of Route E. According to SEM data, Route C results in partial pore healing and E provides total pore healing both in longitudinal and transversal direction. The observed effects are related to orientation order formation, increase of cristallinity degree and reduction of structure imperfection of extrudates. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012