Improved through‐plane electrical conductivity in a carbon‐filled thermoplastic via foaming

Flow‐induced orientation of the conductive fillers in injection molding creates parts with anisotropic electrical conductivity where through‐plane conductivity is several orders of magnitude lower than in‐plane conductivity. This article provides insight into a novel processing method using a chemical blowing agent to manipulate carbon fiber (CF) orientation within a polymer matrix during injection molding. The study used a fractional factorial experimental design to identify the important processing factors for improving the through‐plane electrical conductivity of plates molded from a carbon‐filled cyclic olefin copolymer (COC) containing 10 vol% CF and 2 vol% carbon black. The molded COC plates were analyzed for fiber orientation, morphology, and electrical conductivity. With increasing porosity in the molded foam part, it was found that greater out‐of‐plane fiber orientation and higher electrical conductivity could be achieved. Maximum conductivity and fiber reorientation in the through‐plane direction occurred at lower injection flow rate and higher melt temperature. These process conditions correspond with foam flow during filling of the mold cavity, indicating the importance of shear stress on the effectiveness of a fiber being rotated out‐of‐plane during injection molding. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers     

Measurement and numerical simulation of three‐dimensional fiber orientation states in injection‐molded short‐fiber‐reinforced plastics

To determine three‐dimensional fiber orientation states in injection‐molded short‐fiber composites, a confocal laser scanning microscope (CLSM) is used. Since the CLSM optically sections the specimen, more than two images of the cross sections on and below the surface of the composite can be obtained. Three‐dimensional fiber orientation states can be determined by using geometric parameters of fiber images obtained from two parallel cross sections. For experiments, carbon‐fiber‐reinforced polystyrene is examined by the CLSM and geometric parameters of fibers on each cross‐sectional plane are measured by an image analysis. In order to describe fiber orientation states compactly, orientation tensors are determined at different positions of the prepared specimen. Three‐dimensional orientation states are obtained without any difficulty by determining the out‐of‐plane angles utilizing fiber images on two parallel planes acquired by the CLSM. Orientation states are different at different positions and show the shell–core structure along the thickness of the specimen. Fiber orientation tensors are predicted by a numerical analysis and the numerically predicted orientation states show good agreement with measured ones. However, some differences are found at the end of cavity. They may result from the fountain flow effects, which are not considered in the numerical analysis. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 500–509, 2003     

Effect of the fiber orientation on the sorption kinetics of seawater in an epoxy/glass composite: A free-volume microprobe study

The conventional gravimetric method and positron lifetime spectroscopy have been used to investigate the effect of glass fiber orientation on the diffusion behavior of seawater in epoxy-based composite samples with glass fiber orientations of 0 and 45°. The equilibrium mass uptake of seawater in 45 and 0° orientation composites has been found to be 2.77 and 1.57%, respectively. The diffusion process is non-Fickian in a 45° fiber oriented composite, whereas it is Fickian in a 0° oriented composite. Free-volume data for 45° fiber oriented composites indicates swelling upon the sorption of seawater leading to structural relaxation, and hence the diffusion becomes non-Fickian. On the other hand, a 0° fiber orientation sample exhibits no swelling, and this suggests that water diffusion to the fiber–resin interface through the resin matrix is impeded by the large number of bonds. A polymer–fiber interaction parameter determined from these results also further supports the idea that interface interaction in a 45° fiber oriented composite is less than that in a 0° fiber oriented composite. Positron and gravimetric results support this argument. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Prediction of elastic properties for curved fiber polymer composites

The objective of this article is to study the effect of fiber curvature on the elastic properties of a long‐fiber composite. The study was carried out using a unit cell homogenization approach and micromechanical modeling. In the first approach, a unit cell with a fiber bundle was defined and used in the analysis. Appropriate boundary conditions were prescribed to extract the elastic stiffness components. The second approach made use of the Eshelby‐Mori‐Tanaka model to compute the stiffness of the aligned fiber composite. Fiber curvature was then accounted for through the variation of fiber orientation within a prescribed range that corresponds to a given degree of fiber curvature. It was found that curved fibers significantly affect the composite properties since they lead to a significant stiffness reduction in the longitudinal direction while relatively small increase in stiffness is achieved in the transverse direction in the plane containing the fiber tow. POLYM. COMPOS., 2008. Published 2008 Society of Plastics Engineers     

Effect of fiber‐mat anisotropy on 1D mold filling in LCM: A numerical investigation

The 1D flow experiment is one of the most common methods to measure the saturated and unsaturated permeabilities as well as to study the unsaturated flow in liquid composite molding (LCM) processes used for manufacturing polymer composites. The effective permeability along a flow direction in an anisotropic fiber mat, which is a function of the principal components of the permeability tensor and the angle between the principal and flow directions, is based on the assumption of uniform 1D flow in the mat. In the present paper, the validity of such unidirectional flow assumption is shown to depend on three factors, i.e., the fiber‐mat aspect ratio, the anisotropy ratio (the ratio of the major and minor principal in‐plane permeabilities), and the angle of the principal permeability direction. A mold‐filling simulation for hard‐mold LCM processes based on the control volume/finite element formulation is used to investigate how these three factors affect the 1D flow. A new algorithm for applying the uniform inlet‐pressure condition prevalent in the 1D flow mold under the constant‐flow‐rate injection is developed to match the actual conditions in the mold. Our numerical results show that the three aforementioned factors have significant influence on the inlet‐pressure history as well as on the mold‐filling pattern. Maximum deviations from the unidirectional flow predictions in the inlet‐pressure history as well as the flow‐front progress is seen when the principal permeability direction is at an angle of 45° with respect to the flow direction; these deviations worsen with a decrease in the mat aspect ratio and with an increase in the anisotropy ratio. However, the inlet‐pressure history remains linear and the progress of the average flow‐front remains predictable by the 1D flow theory. POLYM. COMPOS., 2008. © 2008 Society of Plastics Engineers     

Enhanced compatibilization and orientation of polyvinyl alcohol/gelatin composite fibers using carbon nanotubes

Polyvinyl alcohol (PVA)/gelatin composite fibers containing carbon nanotubes (CNTs) had been prepared by wet‐spinning method. A remarkable increase of tensile strength of the PVA/gelatin fibers was achieved by adding small amount of CNT. The mechanism of reinforcement has been studied using a combination of differential scanning calorimetry (DSC), 2D wide‐angle X‐ray diffraction (2D‐WAXD) and scanning electron microscopy (SEM). SEM showed a decreased gelatin domain size by adding CNTs, suggesting a possible compatibilization effect between PVA and gelatin. On the other hand, an increased crystallinity and degree of orientation of PVA/gelatin fibers has been observed by adding CNTs. Thus, the increased compatibilization, crystallinity and degree of orientation in PVA/gelatin/CNTs composite fibers should be the reasons for the observed increase of mechanical properties. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Electromechanical characteristics of unidirectional glass fiber epoxy composites

Because crack detection for laminated composites in‐service is an effective method for improving structural reliability, various nondestructive methods have been studied to detect cracks in laminated composites. A recent study on the damage monitoring of composite materials using the piezoelectric method, which uses the piezoelectric characteristics of polymeric materials for the matrix of composite materials, concluded that the piezoelectric method can be a useful, nondestructive method for the damage monitoring of glass fiber epoxy composites. The dielectric and piezoelectric properties of composites are the basic properties used for the analysis of electromechanical coupling systems, along with the mechanical properties. In this research, therefore, the electromechanical characteristics of unidirectional glass fiber epoxy composites as well as their electric and mechanical characteristics were investigated. The dielectric, piezoelectric, and mechanical properties of unidirectional glass fiber epoxy composites were measured experimentally with respect to the fiber orientation, and the relationship between the piezoelectric and mechanical characteristics of unidirectional glass fiber epoxy composites was analyzed. POLYM. COMPOS., 2011. © 2011 Society of Plastics Engineers     

Statistical analysis of the mechanical properties of natural fibers and their composite materials. II. Composite materials

In this study, the tensile behavior of different natural fiber reinforced composite materials were analyzed. The statistical analysis used to study the natural fibers in the first article, has been extended to analyze the behavior of PP‐matrix composites, combining the probability density function estimation of fiber properties with the Halpin‐Tsai equation. In this case, the advanced statistical approach overestimates the mechanical properties of the composites, probably because of the poor matrix‐fiber adhesion between polypropylene and natural fibers in the real system. POLYM. COMPOS., 2008. © 2008 Society of Plastics Engineers     

A phenomenological study of the mechanical properties of long‐fiber filled injection‐molded thermoplastic composites

Tensile and flexural tests on specimens cut from rectangular injection‐molded plaques show that long‐fiber filled thermoplastic composites are complex, non‐homogeneous, anistropic material systems. Like all fiber‐filled materials, they exhibit through‐thickness nonhomogeneity as indicated by differences between tensile and flexural properties. The in‐plane orientation of fibers in through‐thickness layers causes the material to have in‐plane anisotropic properties. However, these long‐fiber filled materials exhibit an unexpectedly large level of in‐plane nonhomogeneity. Also, the effective mechanical properties of these materials are strongly thickness dependent. The thinnest plaques exhibit the largest differences between the flow and cross‐flow tensile properties. These differences decrease with increasing thickness. A methodology for part design with this class of materials is discussed.     

Quantitative 3D measurement of the nanostructural features that dictate mesoscale performance properties of nanocomposites

Mesoscale performance properties of nanocomposites are dictated by the nanoscale structure developed in the composite during processing, combining nanoscale contributions over a mesoscale volume. In this artilce, for the first time, we present a procedure that deduces the fully 3D process‐induced nanostructural features of carbon nanofiber/polymer composites responsible for the mesoscale performance. In particular, we have developed a method for obtaining the nanofiber orientation in 3D Euclidean space from 2D projections provided by a transmission electron microscope. The 3D Euler angles we obtain are used to construct orientation tensors, the measure of nanostructure that has the most significant influence on mesoscale performance properties. Our measurement method is benchmarked by using numerically generated 3D samples to compare orientation tensor components known a priori with those generated using our procedure. A significant contribution of the procedure is its ability to produce quantitative 3D measurements of the evolution of nanostructure in space and time. The method is successfully applied to observe the effect of extensional rheology on nanofiber orientation. It is shown that orientation tensor data obtained from our experimental method accurately fits the predictions of the fiber orientation evolution equation proposed by Folgar and Tucker. POLYM. COMPOS., 31:1495–1503, 2010. © 2009 Society of Plastics Engineers     

Mechanical properties of injection‐molded short‐fiber thermoplastic composites. Part 1: The elastic moduli and strengths of glass‐filled poly(butylene terephthalate)

The effects of processing and part geometry on the local mechanical properties of injection‐molded, 30 wt% short‐fiber‐reinforced filled poly(butylene terephthalate) (PBT) are characterized by mechanical tests on specimens cut from rectangular plaques of different thicknesses injection molded at several different processing conditions. Stiffness data from tensile tests at 12.7‐mm intervals on 12.7‐mm‐wide strips cut from injection‐molded plaques—both along the flow and cross‐flow directions—and flexural tests on these strips show consistency of plaque‐to‐plaque local properties. Also, in addition to the well‐known anisotropic properties caused by flow‐induced fiber orientation, injection‐molded short fiber composites exhibit in‐plane and through‐thickness nonhomogeneity—as indicated by in‐plane property variations, by differences between tensile and flexural properties, and by the flexural strength being significantly higher than the tensile strength. The sensitivity of these mechanical properties to process conditions and plaque geometry have also been determined: the flow‐direction tensile modulus increases with fill time, the differences between flow and cross‐flow properties decrease with increasing thickness, and both the flow and cross‐flow flexural moduli decrease with increasing plaque thickness. While the flexural modulus is comparable to the tensile modulus, the flexural strength is significantly higher than the tensile strength. POLYM. COMPOS., 26:428–447, 2005. © 2005 Society of Plastics Engineers     

Application and evaluation of the method of ellipses for measuring the orientation of long,semi‐flexible fibers

The method of ellipses (MoE) is a common experimental technique utilized to quantitatively determine the orientation state of a population of rigid fibers within a fiber–polymer composite. In this research, the validity of applying the MoE to long, semi‐flexible fiber systems in which the majority of fibers are flexible is discussed. The components of the orientation tensor were first determined for a composite formed by a homogenous, simple shear field. The minimum acceptable image analysis width, or bin width, for the selected geometry was found to be ∼5.5 mm, or 1.4 times the average fiber length. This modified bin width was then used to determine the orientation at multiple percentages of flow within an injection‐molded, center‐gated disc, and compared to orientation values obtained utilizing the traditional, 0.7‐mm bin width. The results show that the traditional, 0.7‐mm bin width is sufficient for analysis of the center‐gated geometry. This fortuitous result is attributed to the axisymmetric nature of the center‐gated geometry, and the highly transverse fiber alignment seen within the samples, especially at moderate to high percentages of flow. In more complex flows, it is expected that the conventional bin width will not apply. POLYM. COMPOS., 2013. © 2013 Society of Plastics Engineers     

Influence of out‐of‐plane fiber orientation on reaction‐to‐fire properties of carbon fiber reinforced polymer matrix composites

This work investigates the influence of the out‐of‐plane orientation of carbon fibers on the reaction‐to‐fire characteristics of polymer matrix composites. A deep insight into combustion processes is gained, which is necessary to fully understand and assess advantages of composites with out‐of‐plane fiber angles. Epoxy‐based Hexply 8552/IM7 specimens with primarily low fiber angles between 0° and 15° are characterized by cone calorimetry. Heat release during fire is greatly affected by the out‐of‐plane fiber angle because of the thermal boundaries created by the fibers. The advancement of the pyrolysis front during fire was determined from peak heat release rates and validated by temperature measurements along the back surface of the panels, representing a novel method of determining position‐dependent pyrolysis migration velocity. These measurements show a transverse shift in pyrolysis front velocity for increasing out‐of‐plane fiber angles. Pyrolysis pathways between the fiber boundaries facilitate faster combustion through the composite thickness, especially for increasing angles from 0° to 15°. It was determined that under the chosen conditions, the pyrolysis front advances approximately 4 times faster when propagating parallel to the fibers than perpendicular.     

Nonisothermal transient filling simulation of fiber suspended viscoelastic liquid in a center‐gated disk

The present study numerically investigates a fiber orientation in injection‐molded short fiber reinforced thermoplastic composite by using a rheological model, which includes the nonlinear viscoelasticity of polymer and the anisotropic effect of fiber in the total stress. A nonisothermal transient‐filling process for a center‐gated disk geometry is analyzed by a finite element method using a discrete‐elastic‐viscous split stress formulation with a matrix logarithm for the viscoelastic fluid flow and a streamline upwind Petrov–Galerkin method for convection‐dominated problems. The numerical analysis result is compared to the experimental data available in the literature in terms of the fiber orientation in center‐gated disk. The effects of the fiber coupling and the slow‐orientation kinetics of the fiber are discussed. Also, the effect of the injection‐molding processing condition is discussed by varying the filling time and the mold temperature. POLYM. COMPOS., 2011. © 2010 Society of Plastics Engineers     

Comparison of the guarded‐heat‐flow and transient‐plane‐source methods for carbon‐filled nylon 6,6 composites: Experiments and modeling

In this study, two different carbons (synthetic graphite particles and carbon fiber) were added to nylon 6,6, and the resulting composites were tested for thermal conductivity. The first goal of this work was to compare through‐plane thermal conductivity results from the guarded‐heat‐flow method and the transient‐plane‐source method. The results showed that both test methods gave similar through‐plane thermal conductivity results for composites containing 10–40 wt % synthetic graphite and for composites containing 5–40 wt % carbon fiber. The advantages of using the transient‐plane‐source method were that the in‐plane thermal conductivity was also measured and the experimental time was shorter than that of the guarded‐heat‐flow method. The second goal of this work was to develop and use a detailed finite‐element analysis to model heat transfer within a carbon‐filled nylon 6,6 composite sample for the transient‐plane‐source method and compare these results to actual experimental results. The results showed that the finite‐element model compared well with the actual experimental data. The finite‐element model could be used in the future as a design tool to predict the dynamic thermal response of different composite materials for many applications. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci, 2006     

Effect of chemical modifications on the thermal stability and degradation of banana fiber and banana fiber‐reinforced phenol formaldehyde composites

Banana fiber has been modified by treatments with sodium hydroxide, silanes, cyanoethylation, heat treatment, and latex treatment and the thermal degradation behavior of the fiber was analyzed by thermogravimetry and derivative thermogravimetry analysis. Both treated and untreated fibers showed two‐stage decomposition. All the treatments were found to increase the thermal stability of the fiber due to the physical and chemical changes induced by the treatments. The thermal degradation of treated and untreated banana fiber‐reinforced phenol formaldehyde composites has also been analyzed. It was found that the thermal stability of the composites was much higher than that of fibers but they are less stable compared to neat PF resin matrix. Composite samples were found to have four‐stage degradation. The NaOH treated fiber‐reinforced composites have very good fiber/matrix adhesion and hence improvement in thermal stability is observed. Though both silane treatments increased the thermal stability of the composite the vinyl silane is found to be more effective. Heat treatment improves the crystallinity of the fiber and decreases the moisture content, hence an improved thermal stability. The latex treatment and cyanoethylation make the fiber surface hydrophobic, here also the composite is thermally more stable than untreated one. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci 2008     

Properties and structure of MWNTs/cellulose composite fibers prepared by Lyocell process

Lyocell fiber is a new kind of regenerated cellulose fiber and expected to replace the Rayon fiber to be not only used in the textile field but also used in the fields of industry and aerospace after being modified. In this work, the multi‐walled carbon nanotubes (MWNTs)/Lyocell composite fibers were prepared under different draw ratios by dry‐wet spinning and their electrical properties, mechanical properties, and structure were investigated. It was found that an appropriate amount of MWNTs could be dispersed homogeneously in the Lyocell matrix and could improve the mechanical and thermal properties of composite fiber. The results of wide angle X‐ray diffraction (WAXD) showed that the MWNTs in the composite fiber almost aligned along the axis of the fibers and the orientation of MWNTs increased with the increasing draw ratio. Furthermore, it was found that more MWNTs content and lower draw ratio could improve the electrical conductance of the composite fiber. The composite fiber containing 5 wt % MWNTs has a volume conductivity of 8.8 × 10^?4 S/cm, which is five orders higher than that of pure Lyocell fiber. These results indicate that the MWNTs/Lyocell composite fiber has potential applications in the areas of precursor of carbon fiber and conductive fiber. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Jute fiber reinforced polypropylene produced by continuous extrusion compounding,part 1: Processing and ageing properties

This article addresses the processing and ageing properties of jute fiber reinforced polypropylene (PP) composites. The composite has been manufactured by a continuous extrusion process and results in free flowing composite granules, comprising up to 50 weight percent (wt %) jute fiber in PP. These granules have similar shape and diameter as commercially available PP granules. Rheological analysis shows that viscosity of the compounds follows the same shear rate dependency as PP and is on the same level as glass‐PP compounds. The mechanical properties show very little variation and exhibit strength and stiffness values at the upper range of competing natural fiber reinforced compounds for injection molding. The mechanical performance reduces gradually upon prolonged thermal loading and immersion in water. The low water diffusion coefficient of the 50 wt % jute‐PP composites indicates that the fibers are not forming a continuous network throughout the polymer. The jute fibers exhibit a stabilizing effect against ultra violet irradiation (UV) on PP polymer and, as a consequence, the mechanical properties of jute‐PP composites hardly decrease during an accelerated UV ageing test. Bacteria, fungi, and garden mold grow easily on the compound material, but only have a limited effect on mechanical properties. The resistance to growth of bacteria on the materials surface can be increased using a biostabilizer. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Numerical analysis of parametric effects on consolidation of angle‐bended composite laminates

In the previous study, the finite element formulation has been developed by our group based on two‐dimensional resin flow and fiber compaction model. Good agreement between simulations and experimental results was found under the one‐dimensional flow condition. In this article, the two‐dimensional model was used to simulate the consolidation of angle‐bended laminates with the convex tool in autoclave process. The effects of material properties on the consolidation were studied. It was found that the fiber bed shear modulus significantly affects the compaction behavior in the corner section of angle‐bended laminate, the fiber bed compaction property decide the laminate deformation, and the resin viscosity and fiber bed permeability affect the rate of laminate compaction and consolidation time. The angle‐bended T700/BMI QY8911‐Ilaminates were manufactured in autoclave process. The experimental data validate the numerical simulation method for the consolidation of the angle‐bended laminates. These results are greatly helpful for the optimization of processing parameters, improvement of composite parts quality, and reduction of the fabrication cost. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers