Showing papers in "Polymer Composites in 2001"
TL;DR: In this article, the structure-property relationship of kenaf fiber reinforced polypropylene (PP) and its impact copolymers was investigated and a significant improvement in impact strengths was observed when the maleated polypropylenes (MAPP) was used in the composites.
Abstract: Although lignocellulosic, fiber-thermoplastics composites have been used for several decades, recent economic and environmental advantages have resulted in significant commercial interest in the use of these fibers for several applications. Kenaf is a fast growing annual growth plant that is harvested for its bast fibers. These fibers have excellent specific properties and have potential to be outstanding reinforcing fillers in plastics. This paper reports the structure-property relationships of kenaf fiber reinforced polypropylene (PP) and its impact copolymers. The use of maleated polypropylenes (MAPP) is important to improve the compatibility between the fiber and matrix. A significant improvement in impact strengths was observed when the MAPP was used in the composites. Results also indicate that the impact copolymer blends with coupling agent have better high temperature moduli and lower creep compliance than the uncoupled systems. The coupling agent also changes the crystallization and melting behavior of these blends. Because of the better adhesion between the polymer molecules and kenaf fibers, the coupled samples have more restricted molecules than the uncoupled blends. As a result, the crystallization of the coupled high molecular weight blends is slower than the uncoupled blends, resulting in a lower crystallization temperature (T C ) and reduced crystallinity. For the lower molecular weight blends, the coupling agent enhances the crystallization of polymer matrix and results in a higher crystallization temperature and increased crystallinity of the coupled blend. The coupled blends also have more defects in the polymer crystals, and the crystallinity of coupled blends is also lower than the uncoupled blends. This could explain the lower melting temperatures of the coupled samples as compared to uncoupled samples.
185 citations
TL;DR: In this article, fiber surface modification was effected through dewaxing, alkali treatment, aqueous graft copolymerization of methyl methacrylate (MMA) onto 5% alkali treated coir for different extents using CuSO 4 - NaIO 4 combination as an initiator system and cyanoethylation with a view to improve the mechanical performance of coir-polyester composites.
Abstract: Coir, an important lignocellulosic fiber, can be incorporated in polymers like unsaturated polyester in different ways for achieving desired properties and texture. But its high level of moisture absorption, poor wettability and insufficient adhesion between untreated fiber and the polymer matrix lead to debonding with age. In order to improve the above qualities, adequate surface modification is required. In our present work, fiber surface modification was effected through dewaxing, alkali (5%) treatment, aqueous graft copolymerization of methyl methacrylate (MMA) onto 5% alkali treated coir for different extents using CuSO 4 - NaIO 4 combination as an initiator system and cyanoethylation with a view to improve the mechanical performance of coir-polyester composites. Mechanical properties like tensile strength (TS), flexural strength (FS) and impact strength (IS) of the composites as a function of fiber loading and fiber surface modification have been evaluated. Composites containing 25 wt% of fiber (untreated) improved tensile and flexural strength by 30% and 27% respectively in comparison to neat polyester. The work of fracture (impact strength) of the composite with 25 wt% fiber content was found to be 967 J/m. The elongation at break of the composites exhibits an increase with the introduction of fiber. All types of surface modification result in improved mechanical properties of the composites. Significant improvement in mechanical strength was also observed for composites prepared from 5% PMMA grafted fiber.
102 citations
TL;DR: In this paper, through-thickness measurements were recorded to experimentally investigate the through thickness flow and to validate a closed form solution of the resin flow during the vacuum assisted resin transfer molding process (VARFM).
Abstract: Through-thickness measurements were recorded to experimentally investigate the through thickness flow and to validate a closed form solution of the resin flow during the vacuum assisted resin transfer molding process (VARFM). During the VART'M process, a highly permeable distribution medium is incorporated into the preform as a surface layer and resin is inftised Into the mold, under vacuum. During Infusion, the resin flaws preferentially across the surface and simultaneously through the thickness of the preform, giving rise to a three dimensional-flow front. The time to fill the mold and the shape of the flow front, which plays a key role in dry spot formation, are critical for the optimal manufacture of large composite parts. An analytical model predicts the flow times and flow front shapes as a function of the properties of the preform, distribution media and resin. It was found that the flow front profile reaches a parabolic steady state shape and the length of the region saturated by resin is proportional to the square root of the time elapsed. Experimental measurements of the flow front in the process were carried out using embedded sensors to detect the flow of resin through the thickness of the preform layer and the progression of flow along the length of the part. The time to fill the part, the length of flow front and its shapes show good agreement between experiments and the analytical model. The experimental study demonstrates the need for control and optimization of resin injection during the manufacture of large parts by VARTM.
98 citations
TL;DR: In this article, the authors developed ORW and ORW3 models for fiber-fiber interaction coefficients, where principal values of the 4th order tensor are assumed in terms of polynomial expansions of the eigenvalues of the 2nd order tensors, and unknown parameters are determined by a least square fitting technique with assumed exact solutions.
Abstract: We developed improved models (called ORW and ORW3) of existing “orthotropic fitted” closure approximations (ORF or ORL) for use with a wide range of fiber-fiber interaction coefficients. Closure approximation refers to the approximation of a higher order tensor in terms of a lower order tensor. Principal values of the 4th order tensor are assumed in terms of polynomial expansions of the eigenvalues of the 2nd order tensor. Unknown parameters are determined by a least-square fitting technique with assumed exact solutions. Flow data for the optimal fitting were designed to cover the entire domain of the orientation triangle as uniformly as possible to eliminate non-physical oscillation. Shear/biaxial stretching combined flow turns out to play an important role in covering the orientation triangle, thereby increasing the accuracy of fiber orientation prediction. When tested for a variety of flow cases, neither ORW nor ORW3 shows any of the non-physical oscillatory behaviors that ORF and ORL frequently suffer from.
97 citations
TL;DR: In this paper, an alkaline treatment was performed in order to improve the adhesion and the compatibility of the fiber with the matrix, and the effect of the treatment on the tensile properties and morphology was determined.
Abstract: Biocomposites were produced using polycaprolactone and starch as matrix, and sisal fibers as reinforcement The matrix is a biodegradable commercial product called MaterBi-Z, which is based on a polycaprolactone and starch system The relationship between processing conditions and properties is reported An alkaline treatment was performed in order to improve the adhesion and the compatibility of the fiber with the matrix The effect of the treatment on the tensile properties and morphology was determined Fiber content enhances the tensile properties of the biodegradable composite The experimentally observed tensile properties (modulus and tensile strength) of short sisal fiber reinforced MaterBi-Z matrix composites with different fiber loading are compared with the existing theories of reinforcement SEM photomicrographs of the fractured composite surfaces are also analyzed
94 citations
TL;DR: In this article, the preparation and characterization of composite materials prepared by compression molding of a mixture of aluminum flakes and nylon 6 powder was investigated, and the electrical conductivity, density, hardness and morphology of composites were investigated.
Abstract: This work is concerned with the preparation and characterization of composite materials prepared by compression molding of a mixture of aluminum flakes and nylon 6 powder. The electrical conductivity, density, hardness and morphology of composites were investigated. The electrical conductivity of the composites is < 10 -11 S/cm unless the metal content reached the percolation threshold, beyond which the conductivity increased markedly by as much as 10 11 . The volume fraction of conductive filler at the percolation threshold was calculated from experimental data, by fits to functions predicted by the percolation theory. Decreasing the average particle diameter of filler leads to increased percolation threshold (it varies from 23 to 34 vol% for the three different fillers studied) and decreased maximal conductivity of composites. The density of the composites was measured and compared with values calculated assuming different void levels within the samples. Furthermore, it is shown that for certain sizes of particle filler, the hardness decreases initially with the increase of metal concentration, possibly because of poor surface contact with the nylon matrix, but, starting from a certain value, there is a hardness increase. For the smallest particle filler, the hardness of samples is not influenced by the presence of the filler.
81 citations
TL;DR: In this paper, both microwave curing and thermal curing of 24.5 mm (1 inch) glass/epoxy laminates were investigated through the development of a numerical process simulation and conducting experiments in processing in a conventional autoclave and a microwave furnace, and both numerical and experimental results show that volumetric heating due to microwaves promotes an inside-out cure and can dramatically reduce the overall processing time.
Abstract: In conventional processing, thermal gradients cause differential curing of thick laminates and undesirable outside-in solidification. To reduce thermal gradients, thick laminates are processed at lower cure temperatures and heated with slow heating rates, resulting in excessive cure times. Microwaves can transmit energy volumetrically and instantaneously through direct interaction of materials with applied electromagnetic fields, The more efficient energy transfer of microwaves can alleviate the problems associated with differential curing, and the preferred inside-out solidification can be obtained. In this work, both microwave curing and thermal curing of 24.5 mm (1 inch) thick-section glass/epoxy laminates are investigated through the development of a numerical process simulation and conducting experiments in processing thick laminates in a conventional autoclave and a microwave furnace. Outside-in curing of the autoclave-processed laminate resulted in visible matrix cracks, while cracks were not visible in the microwave-processed laminate. Both numerical and experimental results show that volumetric heating due to microwaves promotes an inside-out cure and can dramatically reduce the overall processing time.
78 citations
TL;DR: In this article, the four carbon fillers investigated included a PAN-based carbon fiber (milled, 200μ long), an electrically conductive carbon black, vapor grown graphitic nanotubes, and Thermocarb (high quality synthetic milled graphite).
Abstract: Increasing the thermal and electrical conductivity of typically insulating polymers, such as nylon 6,6, opens new markets. A thermally conductive resin can be used for heat sink applications. An electrically conductive resin can be used in static dissipative and Electromagnetic Interference/Radio Frequency Interference shielding applications. This research focused on performing compounding runs followed by injection molding and testing (tensile properties, volumetric electrical resistivity, and through-plane thermal conductivity) of carbon filled nylon 6,6. The four carbon fillers investigated included a PAN-based carbon fiber (milled, 200μ long), an electrically conductive carbon black, vapor grown graphitic nanotubes, and Thermocarb (high quality synthetic milled graphite). Formulations were produced and tested that contained varying amounts of a single carbon filler. Combinations of fillers were also investigated via conducting half of a 2 4 factorial design. It was determined that Thermocarb has the largest effect on the thermal conductivity. Increasing Thermocarb increases thermal conductivity. For conductive resins containing only a single filler type, nanotubes caused the electrical resistivity (ER) to decrease the most. For the half fraction factorial design formulations that contain at least one filler type at the higher level, the ER of the conductive resin ranged from 0.1 to 0.3 ohm-cm.
75 citations
TL;DR: The mechanical performance of natural fiber reinforced polymers is often limited owing to a weak fiber-matrix interface as discussed by the authors, and melamine-formaldehyde resins are well known to have a stro...
Abstract: The mechanical performance of natural fiber reinforced polymers is often limited owing to a weak fiber-matrix interface. In contrast, melamine-formaldehyde (MF) resins are well known to have a stro ...
69 citations
TL;DR: In this article, a review of preformed particle modification of high performance thermosetting resins and composite systems is presented, which consists of thermoplastic and rubber preformed particles with no size limitations.
Abstract: A review is presented that focuses on preformed particle modification of high performance thermosetting resins and composite systems. The modifiers reviewed consist of thermoplastic and rubber preformed particles with no size limitations. Both organic and inorganic preformed polymer particles are considered but not glass or hollow spheres. In this text, preformed particles are defined as those which do not require phase separation and remain in the shape in which they were added to the neat resin or composite. Therefore, these particles may be developed prior to the resin formulation and then added to the thermosetting resin or developed in situ (during resin formulation) before the resin is catalyzed or cured. This technical review of preformed particle modification of thermosetting resins and composite systems summarizes the utilization of these materials and their performance.
69 citations
TL;DR: In this article, a design sensitivity analysis is used to optimize the applied wall temperature vs. time in autoclave curing for thermoset matrix composites, which minimizes they cure time and obeys a maximum temperature constraint in the composite.
Abstract: A design sensitivity analysis is used to optimize the applied wall temperature vs. time in autoclave curing for thermoset matrix composites. The calculation minimizes they cure time and obeys a maximum temperature constraint in the composite. The transient, coupled thermal and cure problem is solved by a finite element method. Design sensitivity information is extracted efficiently from this primal analysis, based on an analytical, direct differentiation approach. The sensitivities are then used with gradient-based optimization techniques to systematically improve the curing process. The optimal cure cycles for different numbers of temperature dwells may be similar (for a 2 mm thick part) or very different (for a 4 cm thick part), depending on the nature of the problem. In the latter case a large reduction of cure time is obtained when a three-dwell cure cycle is used, and the optimizer has more flexibility to adjust the cure cycle. This systematic optimization approach provides a powerful and practical means of optimizing composite manufacturing processes.
TL;DR: In this paper, a study was conducted to characterize chemical and physical changes in polymer matrix resins following exposure to water, alkaline and saline environments at ambient and elevated temperatures for extended periods of time.
Abstract: One of the obstacles hindering the acceptance of polymer composites in civil engineering applications is the susceptibility of the polymeric matrix to degradation that is initiated by moisture, temperature, and corrosive chemical environments. The objective of this study was to characterize chemical and physical changes in polymer matrix resins following exposure to these environments. Resin systems studied were vinyl ester and isophthalic polyester, both of which are proposed for use in construction applications. Unreinforced free films were exposed to water, alkaline and saline environments at ambient and elevated temperatures for extended periods of time. Changes in strength and thermophysical properties were evaluated through tensile testing, dynamic mechanical thermal analysis (DMTA) and thermogravimetric analysis (TGA). Chemical degradation of the polymers was characterized using Fourier transform infrared (FTIR) spectroscopy. Energy dispersive X-ray (EDX) analysis of specimens following exposure was carried out to determine if ion diffusion into the bulk polymer occurred. Only minor changes in the glass transition temperatures of the polymers were observed after prolonged exposure at elevated temperature, but more substantial changes were noted in tensile strength, particularly in the case of the isophthalic polyester. Examination of the polymers following immersion in salt solution and alkaline solution showed essentially no ionic penetration into the bulk, with the exception of specimens that were visibly degraded. Spectroscopic analysis of chemical structure prior to and following exposure revealed varying degrees of ester hydrolysis.
TL;DR: In this paper, the authors investigated the primary factors governing the isothermal unsaturated flow through dual-scale porous media and found that the magnitude of the sink effect was a function of the capillary number.
Abstract: In Resin Transfer Molding (RPM), which is a process to manufacture polymer composites, the impregnation of fibrous reinforcement In the form of mats by a thermosetting resin is modeled as the flow of a Newtonian liquid through a single length-scale porous medium. While this approach is sufficiently accurate for random fiber-mats, it can lead to appreciable errors when applied to woven, braided, or stitched fiber-mats that contain two length scales. This work investigates the primary factors governing the isothermal unsaturated flow through such dual-scale porous media. Two studies were conducted to better understand this phenomenon: the first experimenatally investigated the flow, while the second theoretically modeled the flow and identified important parameters affecting such a flow with the help of dimensionless analysis. In the first study, one-dimensional constant injection rate experiments were performed using various fiber mats. The unsaturated flow behavior of various mats was characterized using a constant “sink” term in the continuity equation. Results indicated that for a given fiber-mat, the magnitude of the sink effect was a function of the capillary number. In the second study, a numerical model was developed to describe flow through dual-scale preforms in which the two flow domains, the inter- and intra-tow regions, were coupled. We identified a dimensionless number called the sink effect index ψ that characterizes the magnitude of liquid absorption by the tows and is a function of the relative resistance to flow in the tow and inter-tow regions, and the packing density of the tows. The parametric study of this index with the help of numerical simulations reveals its influence on the flow and identifies the distinct transient and steady-state flow regimes.
TL;DR: In this paper, the effect of different processing factors on the joint strength of ultrasonically welded composites, including weld time, weld pressure, amplitude of vibration, hold time, hold pressure, and geometry of energy director, was reported.
Abstract: Ultrasonic welding of thermoplastic composites has become an important process in industry because of its relatively low cost and resultant high quality joints. An experimental study, based on the Taguchi orthogonal array design, is reported on the effect of different processing factors on the joint strength of ultrasonically welded composites, including weld time, weld pressure, amplitude of vibration, hold time, hold pressure, and geometry of energy director. Three materials were used in the study: virgin polypropylene, and 10% and 30% glass-fiber filled polypropylene composites. Experiments were carried out on a 2000-Watt ultrasonic welding unit. After welding, the joint strength of the composites was determined by a tensile tester. For the factors selected in the main experiments, weld time, geometry of energy director and amplitude of vibration were found to be the principal factors affecting the joint property of ultrasonically welded thermoplastic composites. Glass-fiber filled polymers required less energy for successful welding than the non-filled polymer. The joint strength of welded parts increased with the fiber content in the composites. In addition, a triangular energy director was found to weld parts of the highest strength for virgin polypropylene and 10% glass-fiber filled polypropylene composites, while a semi-circular energy director was found to weld the highest strength parts for 30% glass-fiber filled composites.
TL;DR: In this paper, a nonlinear resonance acoustical spectroscopy (SIMONRAS) was applied to measure the linear and nonlinear dynamical properties of polyester composites as a funtion of exposure time.
Abstract: The Chemical resistance of pultruded polyester composites to various chemical immersion solutions is investigated as a function of the exposure time, and the observations from static tests are compared with the results from nonlinear dynamic vibration experiments. The mechanical properties were measured according to ASTM standards, both in flexural and tensile tests. Barcol hardness and sorption measurements provide complemntary information. In addition, a novel nondestructive (NDE) method, called Single Mode Nonlinear Resonance Acoustic Spectroscopy (SIMONRAS), was applied to measure the linear and nonlinear dynamical properties of the samples as a funtion of exposure time. This new NDE method focuses on the strain amplitude dependence of the resonance frequency while driving a sample at relatively low excitation levels. The obtained relative frequency shift is a measure of the internal microstructural properties of the material. The correlation between this nonlinearity parameter and the mechanical properties is extremely good, which implies that the SIMONRAS technique can be applied to predict the chemo-mechanical degradation of composites in a nondestructive manner.
TL;DR: In this paper, a numerical tool for predicting the evolution of internal and residual stresses during the processing of thermoplastic matrix composites has been developed based on a finite element formulation, the model accounts for the anisotropy, viscoelasticity and heterogeneity of the materials and represents mechanisms of both stress generation and stress relaxation.
Abstract: A numerical tool for predicting the evolution of internal and residual stresses during the processing of thermoplastic matrix composites has been developed Based on a finite element formulation, the model accounts for the anisotropy, viscoelasticity and heterogeneity of the materials and represents mechanisms of both stress generation and stress relaxation The viscoelastic properties are described by a linear thermoviscoelastic formulation The model allows the buildup of stresses during processing to be monitored, in particular when the material is cooling through its transition temperatures, and enables the prediction of stress release and the resulting part warpage on demolding Its use is demonstrated for unidirectional and crossply polyetherimide/glass fiber (PEI/GF) laminates processed by compression molding, and the influence of cooling conditions on stress levels is shown
TL;DR: In this article, the effects of different fiber surface treatments and fiber amounts on the performance of resulting bio-composites are investigated, and the remarkable achievement of the present investigation is that a low strength coir fiber, through optimal surface modifications, on reinforcement with BAK show an encouraging level of mechanical properties.
Abstract: Biocomposites are prepared from a cheap, renewable natural fiber, coir (coconut fiber) as reinforcement with a biodegradable polyester amide (BAK 1095) matrix. In order to have better fiber-matrix interaction the fibers are surface modified through alkali treatment, cyanoethylation, bleaching and vinyl grafting. The effects of different fiber surface treatments and fiber amounts on the performance of resulting bio-composites are investigated. Among all modifications, cyanoethylated coir-BAK composites show better tensile strength (35.50 MPa) whereas 7% methyl methacrylate grafted coir-BAK composites show significant improvement in flexural strength (87.36 MPa). The remarkable achievement of the present investigation is that a low strength coir fiber, through optimal surface modifications, on reinforcement with BAK show an encouraging level of mechanical properties. Moreover, the elongation at break of BAK polymer is considerably reduced by the incorporation of coir fibers from nearly 400% (percent elongation of pure BAK) to 16-24% (coir-BAK biocomposites). SEM investigations show that surface modifications improve the fiber-matrix adhesion. From biodegradation studies we find that after 52 days of soil burial, alkali treated and bleached coir-BAK composites show significant weight loss. More than 70% decrease in flexural strength is observed for alkali treated coir-BAK composites after 35 days of soil burial. The loss of weight and the decrease of flexural strength of degraded composites are more or less directly related.
TL;DR: In this paper, the effect of position of the aramid layer on the impact properties of hybrid composites was investigated using a driven dart impact tester, which was attributed to the fact that the flexible layer at the impacted surface in thin laminates can experience larger deformation.
Abstract: Aramid fiber/glass fiber hybrid composites were prepared to examine the effect of stacking sequence on the impact behavior of thin laminates. The effect of position of the aramid layer on the impact properties of hybrid composites was investigated using driven dart impact tester. The delamination area and fracture surface of hybrid composites were analyzed for correlation with impact energy. The addition of glass layer to aramid layer reduced the impact resistance of hybrid composite due to the restriction in the deformation of aramid layer. The position of aramid layer resulted in variations in the impact behavior of hybrid composites. When the aramid layer was at the impacted surface, the composite exhibited a higher impact energy. This was attributed to the fact that the flexible layer at the impacted surface in thin laminates can experience larger deformation. In three-layer composites, the aramid fiber-reinforced composite (AAA) exhibited the highest total impact energy due to high impact energy per delamination area (1EDA) in spite of low delamination area. Aramid fiber and glass fiber-reinforced composites showed a different impact behavior according to the change of thickness. This was attributed to the difference in the energy absorption at interface between laminae.
TL;DR: In this article, the authors focused on the study of mechanical properties, thermal stability and rheological behavior of HDPE/graphite composite and showed that with an increase of the graphite content, Young's modulus increases and the elongation at break and impact strength decrease.
Abstract: Having been treated with coupling agent and pan-milling, graphite is incorporated into HDPE to make a HDPE/graphite composite with enhanced thermal conductivity as well as good mechanical properties and processibility. This paper focuses on the study of mechanical properties, thermal stability and rheological behavior of HDPE/graphite composite. The experimental results show that with increase of the graphite content, Young's modulus of HDPE/graphite increases and the elongation at break and impact strength decrease. However, when the graphite content is 35% in HDPE/graphite, the elongation at break and impact strength still remain 22.4% and 85.8J/m, respectively. Also, the yield strength increases with the increase of the graphite content, and reaches the maximum at 55% graphite content, and reduces afterwards. The crystallization temperature and thermal stability of HDPE/graphite increase with the increase of the graphite content. The melt viscosity of the filled HDPE remains almost unchanged, but the shear sensitivity increases, and the temperature sensitivity decreases with the increase of the graphite content. By optimizing the experimental conditions, a HDPE/graphite composite with fairly good comprehensive properties such as enhanced thermal conductivity and stability, good mechanical properties, and processability could be prepared, which has potential application in the field of heat transfer.
TL;DR: In this paper, the effect of the reinforcement structure on the overall compaction behavior of the resulting stack is investigated. But, the authors focus on the impact of the flow enhancement layer on the compaction response.
Abstract: The Vacuum infusion molding process is gaining increased popularity for its relatively short cycle time, low equipment cost and low labor requirements. Furthermore, for large and complex structures, it offers a high potential of integration in addition to the complete elimination of volatile organic compound (V.O.C.) emissions, which are considered to be a major health concern for employees within the composite industry. Successful implementation of the process, especially for large structures such as boat hulls where large amounts of raw materials are involved, requires the completion of the impregnation phase in the shortest time possible. This can be achieved by the integration of a highly permeable layer to the reinforcement stack. However, this will strongly affect the compaction behavior of the resulted stack. This paper investigates the effect of the reinforcement structure on the overall compaction behavior. More specifically, the investigation is oriented toward the effect of the flow enhancement layer on the compaction response.
TL;DR: In this article, raw and dewaxed jute felt composites were prepared with resol and lignin modified phenol formaldehyde resin, and the thermal stability of the composites was assessed by DSC and TGA.
Abstract: Raw and dewaxed jute felt composites were prepared with resol and lignin modified phenol formaldehyde resin. Four different types of lignin modified resins were used by replacing phenol with lignin. The lignin modified resins were prepared from purified lignin obtained from paper industry waste black liquor. To investigate bonding between jute and resin, IR spectroscopy of jute felts and composites was carried out. The thermal stability of the composites was assessed by DSC and TGA. It was found that the lignin resin jute composite is thermally more stable than resol composite. XRD of jute felt and composite shows that the crystallinity of the jute fiber increases after composite preparation. The lignin resin composites were tested for water absorption and thickness swelling, and it was found that the results are comparable with those of resol jute composite. Composites prepared from lignin phenol formaldehyde resin with 50% phenol replacement has shown 75% tensile strength retention to that of pure resol jute composite.
TL;DR: In this paper, the contributions of the so-called interface effects on the overall viscoelastic behavior of SBR reinforced by precipitated silica modified by model sizing agents are evaluated by using micromechanical models in a reverse mode.
Abstract: The contributions of the so-called “interface effects” on the overall viscoelastic behavior of SBR reinforced by precipitated silica modified by model sizing agents are evaluated by using micromechanical models in a reverse mode. According to the nature of the coupling agent, two kinds of “interface effects” can be distinguished: (i) an overall change in the viscoelastic behavior of the polymer matrix, when the sizing agent is a chemical promotor acting as additional ties of the SBR network, and (ii) the formation of an actual mesophase, when direct interactions between SBR chains and silica could occur, as for example, when the coating agent of silica is an organosilane based on short aliphatic chains. With increasing the length of the aliphatic chains of the coating agent (LOS agent), some a plasticisizing effect of the polymer is detected by DSC. It can affect either the entire polymer matrix or a confined region at the close vicinity of silica aggregates. According to this assumption, the bimodal viscoelastic behavior of the LOS-mesophase revealed by the modeling indicates that in addition to a local plasticisizing effect induced by this agent, some direct interactions could occur between silica surface and SBR constraining the mobility of rubber molecules.
TL;DR: In this paper, the phase morphology and the crystallization behavior of the nanocomposites were investigated, using DSC, DMTA, XRD and SEM, and it was found that the treated clay content and dynamic processing time affect the viscosity of the EVOH/clay mixtures.
Abstract: Ethylene-vinyl alcohol copolymer (EVOH)/clay nanocomposites were prepared via a dynamic melt-intercalation process. The phase morphology and the crystallization behavior of the nanocomposites were investigated, using DSC, DMTA, XRD and SEM. It was found that the treated clay content and dynamic processing time affect the viscosity of the EVOH/clay mixtures: higher clay contents and longer mixing times result in higher torque/viscosity levels. This is due to the increased interaction of the molten polar matrix (EVOH) with the treated organosilicate surface. Under the dynamic high shearing forces, the polymer penetrates the clay agglomerates/aggregates, intercalates within the organoclay galleries, and finally causes delamination. Thermal analysis of the EVOH/clay nanocomposites showed that the melting temperature, crystallization temperature and heat of fusion of the EVOH matrix, sharply decrease with increasing both, the clay content and processing time. The intercalation level was characterized by X-ray diffraction (XRD), which verified an increased gallery height. The DMTA spectra showed that longer processing times resulted in higher damping (E intensity) levels of the EVOH/clay composites, indicating higher fractions of the EVOH amorphous phase. However, no T g changes were seen in spite of the high polymer/treated clay interaction levels, which may be attributed to a plasticizing effect of the low molecular weight organic cations.
TL;DR: In this paper, a two-layer built-in model is proposed to represent particulate-filled composites and a reasonable agreement is found between the predicted tensile strength and the experimental results found in the literature.
Abstract: Based on Christen sen and Lo's (11) three-layer sphere model, a two-layer built-in model is proposed to represent particulate-filled composites. Following Papanico-laou and Bakos' (14) procedure for a particle embedded in an infinite matrix model and using the rule-of-mixtures approach, formulations estimating the tensile strength of particulate-filled composites are developed. Unlike Fapanicolaou and Bakos' formulations, the formulations developed in the present paper can characterize the effect of particle size, particle size distribution, and particle clustering on the tensile strength of the composites. A reasonable agreement is found between the predicted tensile strength and the experimental results found in the literature. Parameters affecting the tensile strength of particulate-filled composites are discussed via the calculated results.
TL;DR: In this paper, binary images were generated from low contrast OCT data through image de-noising, contrast enhancement and feature recognition, and the resulting data were input to a lattice-Boltzmann model for permeability prediction.
Abstract: Knowledge of the permeability tensor in liquid composite molding is important for process optimization. Unfortunately, experimental determination of permeability is difficult and time consuming. Numerical calculation of permeability from a model reinforcement can circumvent experimentation. However, permeability predictions often rely on a model reinforcement that does not accurately mimic the actual microstructure. A rapid, nondestructive technique called optical coherence tomography (OCT) can image the microstructure of a composite in minutes. Actual microstructural information can be then used to improve the accuracy of the model and therefore the predicted permeability. Additionally, the influence on fiber volume fraction and microstructural variability on permeability can be systematically studied.
In this work, binary images were generated from the low contrast OCT data through image de-noising, contrast enhancement and feature recognition. The resulting data were input to a lattice-Boltzmann model for permeability prediction. The influence of the fiber volume fraction, tow surface area, average mean free channel path, and variable microstructure are discussed in terms of their individual and synergistic effects on permeability. The calculated axial and transverse permeabilities from the images show very good agreement with the experimental values.
TL;DR: In this paper, the extrusion conditions and properties of VGCF reinforced polypropylene were investigated using a Leistritz twin-screw extruder at temperatures ranging from 215°C to 250°C.
Abstract: Vapor grown carbon fiber (VGCF) is a new and inexpensive carbon fiber produced by vapor deposition of hydrocarbons on metal catalysts. The ability to fabricate VGCF reinforced thermoplastic composites by extrusion provides the potential to exploit VGCF into new commercial markets. This study investigates the extrusion conditions and properties of VGCF reinforced polypropylene. Extrusion was carried out using a Leistritz twin-screw extruder. Polypropylene reinforced with 0% to 12.5% volume fraction VGCF was extruded successfully at temperatures ranging from 215°C to 250°C. The extrusion pressure increased and the flow rate decreased as the percentage of fiber was increased. Increases in tensile strength and modulus were observed by the addition of VGCF. However, the void content of the composite samples increased with fiber content.
TL;DR: In this article, continuous carbon fiber polymer-matrix composites and their joints, as studied by DC electrical measurements, are reviewed, and the quality of composite-composite joints obtained by adhesion or fastening is revealed by resistance measurements.
Abstract: Continuous carbon fiber polymer-matrix composites and their joints, as studied by DC electrical measurements, are reviewed. The resistance gives information on the microstructure and allows the self-sensing of strain, damage and temperature. In the case of composites with dissimilar fibers in adjacent laminae, the Seebeck effect allows temperature sensing, using the interface between laminae as a thermocouple junction. The resistance in the through-thickness direction can be apparently negative, due to entropy-driven electron backflow. The longitudinal resistance allows sensing of the glass transition and melting of the thermoplastic polymer matrix. The quality of composite-composite joints obtained by adhesion or fastening, and of composite-concrete joints obtained by adhesion, is revealed by resistance measurements.
TL;DR: In this article, the interfacial adhesion between four different forms of jute fibers (sliver, bleached, mercerized and untreated) and polyolefinic matrices (LDPE and PP) was studied, as a critical factor affecting the mechanical behavior of these composites.
Abstract: The interfacial adhesion between four different forms of jute fibers (sliver, bleached, mercerized and untreated) and polyolefinic matrices (LDPE and PP) was studied, as a critical factor affecting the mechanical behavior of these composites. The fiber-matrix adhesion was estimated by means of the critical fiber length (l c ) and the stress transfer ability parameter (T); such parameters were obtained by Single Fiber Composite (SFC) tests. Tests were carried out to evaluate the mean tensile strength of the fibers, the mean critical fiber lengths and the stress transfer ability parameter for every fiber-matrix combination, according to Weibull's statistical method. Thermal-mechanical characterization of the fibers was also carried out to evaluate the resistance to processing conditions. A limited degradation of strength was observed, which, however, does not preclude the use of jute fibers as reinforcing means in polyolefin based composites. It was found that the adhesion was better in PP-jute composites than in LDPE-jute composites. In both cases the results showed that the sliver jute and the untreated jute had better adhesion to both matrices than had the bleached and the mercerized fibers. With both matrices the interface adhesion was in the order: mercerized < bleached < untreated = sliver.
TL;DR: In this paper, the heat transfer phenomena occurring during the autoclave manufacturing cycle have been analyzed, and the assumption of a negligible through-the-thickness thermal gradient led to simplified energy balance equations.
Abstract: In autoclave technology, polymer based composites are manufactured under the application of pressure and heat. The heat transferred between the energy carrying fluid and the bag-composite-tool element activates exothermic curing reactions, leading to composite consolidation. The convective heat transfer mechanism is the most relevant aspect controlling the rate of chemical and physical transformations associated with composite curing. Moreover, the fluidodynamic regime that results from the interactions between the autoclave and the tool geometry, even if totally predictable in theory, is unattainable in practice. In this study, the heat transfer phenomena occurring during the autoclave manufacturing cycle have been analyzed. The assumption of a negligible through-the-thickness thermal gradient led to simplified energy balance equations. In this case, the thermal evolution of the manufacturing elements has been completely determined by two parameters: the global convective heat exchange coefficient, setting the rate of the heat transfer between the autoclave environment and the bag-composite-tool element, and the adiabatic temperature rise, establishing the relevance of the polymerization exotherm. A scaling analysis has been performed in order to identify the dimensionless parameters controlling the autoclave process. The developed semitheoretical methodology has been extensively tested by comparison with experimental data from an industrial autoclave.
TL;DR: In this article, the combined effect of varying loading rate and test temperature on the mode II interlaminar fracture properties of AS4/carbon fiber reinforced PEEK has been investigated, showing that increasing the crosshead displacement rate has been shown to increase the value of G IIc by up to 25%.
Abstract: The combined effect of varying loading rate and test temperature on the mode II interlaminar fracture properties of AS4/carbon fiber reinforced PEEK has been investigated. End notch flexure tests have shown that this thermoplastic-based composite system offers a very high value of interlaminar fracture toughness at room temperature. Increasing the test temperature leads to a reduction in the mode II interlaminar fracture toughness of the composite, with the value at 150°C being approximately one half of the room temperature value. In contrast, increasing the crosshead displacement rate has been shown to increase the value of G IIc by up to 25%. A more detailed understanding of the effect of varying temperature and loading rate on the failure mechanisms occurring at the crack tip of these interlaminar fracture specimens has been achieved using the double end notch flexure (DENF) geometry. Here, extensive plastic flow within the crack tip region was observed in all specimens. It is believed that the rate sensitivity of G IIc reflects the rate-dependent characteristics of the thermoplastic resin.