Electrical conductivity modeling of carbon‐filled liquid‐crystalline polymer composites

Electrically conductive resins are needed for bipolar plates used in fuel cells. Currently, the materials for these bipolar plates often contain a single type of graphite powder in a thermosetting resin. In this study, various amounts of two different types of carbon, carbon black and synthetic graphite, were added to a thermoplastic matrix. The resulting single‐filler composites were tested for electrical conductivity, and electrical conductivity models were developed. Two different models, the Mamunya and additive electrical conductivity models, were used for both material systems. It was determined how to modify these models to reduce the number of adjustable parameters. The models agreed very well with experimental data covering a large range of filler volume fractions (from 0 to 12 vol % for the carbon black filled composites and from 0 to 65 vol % for the synthetic graphite filled composites) and electrical conductivities (from 4.6 × 10^?17 S/cm for the pure polymer to 0.5 S/cm for the carbon black filled composites and to 12 S/cm for the synthetic graphite filled composites). © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 3293–3300, 2006     

Electrical conductivity model evaluation of carbon fiber filled liquid crystal polymer composites

Electrically conductive resins may have applications as fuel cell bipolar plates. The current trend in this technology is a thermosetting polymer as the matrix containing high concentrations of various types of fillers. These fillers are carbon based and electrically conductive powders, particles, or fibers. In this study, we utilized two composite formulations of polyacrylonitrile fibers (Fortafil 243 and Panex 30) in a liquid crystal polymer (Vectra A950RX) with increasing concentrations. Electrical conductivity tests were performed and modified Mamunya and additive models were applied to the experimental data. These models fit the entire range of data for each composite tested. Four alternate models were also produced: linear, quadratic, exponential, and geometric, with a restricted range of electrical conductivity data greater than 10^?2 S/cm. The exponential and the geometric resulted in the best fits over this restricted data range. These particular models may allow researchers to extrapolate beyond the maximum filler concentrations studied here. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

FEM calculations of capillary rheometer flow for carbon‐filled liquid crystal polymer composites

There is an emerging market for conductive resins for use in fuel cell bipolar plates. This research focuses on developing a finite element model of a capillary rheometer. Comsol Multiphysics 3.2b was used to model the flow of a remeltable thermoplastic matrix material, Vectra A950RX Liquid Crystal Polymer, with varying amounts of either a carbon black or synthetic graphite filler, to obtain the velocity profile and pressure drop of these composites within the capillary. Previous experimental results have shown that the molten composites obey a shear‐thinning power law behavior. When comparing the model predicted pressure drops from the model with the experimental data, very good agreement was obtained. This signifies that the rheological behavior of the composites can be described by a power law relationship, using parameters specific to each composite. When comparing the modeled velocity profile with the theoretical profile, it was found for all composite formulations that the velocity becomes fully developed within a length of 0.05 times the diameter of the tube, independent of the power law parameters n and m. This work is a necessary first step in developing 2D or 3D mold filling simulations for fuel cell bipolar plate applications. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Thermal conductivity of carbon fiber/liquid crystal polymer composites

Thermally conductive resins are needed for bipolar plates in fuel cells. Currently, the materials used for these bipolar plates often contain a single type of graphite in a thermosetting resin. In this study, varying amounts of two different types of polyacrylonitrile based carbon fibers, Fortafil 243 and Panex 30, were added to a thermoplastic matrix (Vectra A950RX Liquid Crystal Polymer). The resulting single filler composites were tested for thermal conductivity and a simple exponential thermal conductivity model was developed for the square root of the product of the in‐plane and through‐plane thermal conductivity . The experiments showed that the through‐plane thermal conductivity was similar for composites up to 40 vol % fiber. However, at higher loadings, the Panex 30 samples exhibited higher thermal conductivity. The experiments also showed that the in‐plane thermal conductivity of composites containing Panex 30 was higher than those containing Fortafil 243 for all volume fractions studied. Finally, the model agreed very well with experimental data covering a large range of filler volume fraction (from 0 to 55 vol % for both single filler systems). The model can be used with existing through‐plane thermal conductivity models to predict in‐plane thermal conductivity. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 5456–5462, 2006     

Effects of carbon fillers on the rheology of highly filled liquid‐crystal polymer based resins

Adding conductive carbon fillers to insulating resins increases the composite electrical and thermal conductivity. Often, enough of a single type of carbon filler is added to achieve the desired conductivity while still allowing the material to be molded into a bipolar plate for a fuel cell. In this study, various amounts of three different carbons (carbon black, synthetic graphite particles, and carbon fiber) were added to Vectra A950RX liquid‐crystal polymer. The rheological properties of the resulting single‐filler composites were measured. In addition, the rheological properties of composites containing combinations of different carbon fillers were studied via a factorial design. In all cases, the viscosity increased with increasing filler volume fraction and followed a shear‐thinning power‐law model. The factorial design results indicated that each of the single fillers and all the filler combinations caused a statistically significant increase in the composite viscosity when compared at a shear rate of 500 s^?1 or at a stress of 10⁵ Pa. For composites containing synthetic graphite particles and/or carbon fiber, the viscosity variation with the volume fraction of carbon followed a modified Maron–Pierce equation. When compared at a constant volume fraction of carbon, composites containing carbon black showed viscosity enhancement above and beyond that shown by the other composites. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Thermal conductivity models for carbon/liquid crystal polymer composites

Thermally conductive resins are needed for bipolar plates in fuel cells. Currently, the materials used for these bipolar plates often contain a single type of graphite in a thermosetting resin. In this study, varying amounts of four different types of polyacrylonitrile carbon fillers (Ketjenblack carbon black, Thermocarb synthetic graphite, Fortafil 243 carbon fiber, and Panex 30 carbon fiber) were added to a thermoplastic matrix (Vectra A950RX Liquid Crystal Polymer), with the resulting resins tested for through‐plane and in‐plane thermal conductivity. There are two unique contributions of this work. The first contribution is the use of the Nielsen model for the through‐plane thermal conductivity as a function of the single filler volume fraction. The model fits the data for all composites well. The second contribution is the development of a new, accurate, empirical model to predict the in‐plane thermal conductivity for all resins containing synthetic graphite or carbon fiber. Both of these models will form the basis for the development of new thermal conductivity models for composites with multiple fillers for fuel cell bipolar plate applications. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

Effects of carbon fillers on the thermal conductivity of highly filled liquid‐crystal polymer based resins

The thermal conductivity of insulating polymers can be increased by the addition of conductive fillers. One potential market for these thermally conductive resins is for fuel cell bipolar plates. In this study, various amounts of three different carbon fillers (carbon black, synthetic graphite particles, and carbon fiber) were added to Vectra A950RX liquid crystal polymer. Because the resulting composites were anisotropic, they were tested for both through‐plane and in‐plane thermal conductivities. The effects of single fillers and combinations of the different fillers were studied via a factorial design. Each single filler caused a statistically significant increase in composite through‐plane and in‐plane thermal conductivities at the 95% confidence level, with synthetic graphite causing the largest increase. All of the composites containing combinations of the different fillers caused statistically significant increases in the composite through‐plane and in‐plane thermal conductivities. It is possible that thermally conductive pathways were formed that linked these carbon fillers, which resulted in increased composite thermal conductivity. Composites containing 70, 75, and 80 wt % synthetic graphite and the composite containing all three fillers (2.5 wt % carbon black, 65 wt % synthetic graphite, and 5 wt % carbon fiber) had in‐plane thermal conductivities of 20 W m^?1 K^?1 or higher, which is desirable for bipolar plates. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Electrical conductivity of carbon‐filled polypropylene‐based resins

Adding conductive carbon fillers to insulating thermoplastic resins increases composite electrical conductivity. Often, as much of a single type of carbon filler is added to achieve the desired conductivity and still allow the material to be molded into a bipolar plate for a fuel cell. In this study, various amounts of three different carbons (carbon black, synthetic graphite particles, and carbon nanotubes) were added to polypropylene resin. The resulting single‐filler composites were tested for electrical resistivity (1/electrical conductivity). The effects of single fillers and combinations of the different carbon fillers were studied via a factorial design. The percolation threshold was 1.4 vol % for the composites containing only carbon black, 2.1 vol % for those containing only carbon nanotubes, and 13 vol % for those containing only synthetic graphite particles. The factorial results indicate that the composites containing only single fillers (synthetic graphite followed closely by carbon nanotubes and then carbon black) caused a statistically significant decrease in composite electrical resistivity. All of the composites containing combinations of different fillers had a statistically significant effect that increased the electrical resistivity. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Thermal and electrical conductivity of carbon–filled liquid crystal polymer composites

The thermal and electrical conductivity of resins can be increased by adding conductive carbon fillers. One emerging market for thermally and electrically conductive resins is for bipolar plates for use in fuel cells. In this study, varying amounts of five different types of carbon, one carbon black, two synthetic graphites, one natural flake graphite, and one calcined needle coke, were added to Vectra A950RX Liquid Crystal Polymer. The resulting composites containing only one type of filler were then tested for thermal and electrical conductivity. The objective of this work was to determine which carbon filler produced a composite with the highest thermal and electrical conductivity. The results showed that composites containing Thermocarb TC‐300 synthetic graphite particles had the highest thermal and electrical conductivity. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99; 1552–1558, 2006     

Conducting polymer composites of zinc‐filled urea–formaldehyde

This article is concerned with the preparation and characterization of composite materials prepared by the compression molding of mixtures of zinc powder and urea–formaldehyde embedded in cellulose powder. The morphologies of the constituent, filler, and matrix were investigated by optical microscopy. The elaborated composites were characterized by density, which was compared with calculated values, and the porosity rate was deduced. Further, the hardness of samples remained almost constant with increasing metal concentration. The electrical conductivity of the composites was less than 10^?11 S/cm unless the metal content reached the percolation threshold at a volume fraction of 18.9%, beyond which the conductivity increased markedly, by as much as eight orders of magnitude. The obtained results interpreted well with the statistical percolation theory. The deduced critical parameters, such as the threshold of percolation, the critical exponent t, and the packing density coefficient were in good accord with earlier studies. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 96: 2011–2015, 2005     

Tensile modulus modeling of carbon‐filled liquid crystal polymer composites

Electrically and thermally conductive resins can be produced by adding conductive fillers to insulating polymers. Mechanical properties, such as tensile modulus, are also important. This research focused on performing compounding runs followed by injection molding and tensile testing of carbon‐filled Vectra liquid crystal polymer. The two carbon fillers investigated were Thermocarb synthetic graphite particles and Fortafil carbon fiber at varying filler amounts. The tensile modulus experimental results were compared to results predicted by several different models. It was found that the Halpin Tsai 2D Randomly Oriented fiber model provided the best fit to the experimental data. The degree of filler‐polymer adhesion was also studied with nanoscratch tests for synthetic graphite and carbon fiber fillers in three polymers: Vectra, nylon 6,6, and polycarbonate. The adhesion trends seen in the nanoscratch tests showed qualitative agreement with the tensile modulus, and should be considered in formulating advanced tensile modulus models. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers     

Synergistic effects of carbon fillers on tensile and flexural properties in liquid‐crystal polymer based resins

One emerging market for thermally and electrically conductive resins is bipolar plates for use in fuel cells. Adding carbon fillers to thermoplastic resins increases the composite thermal and electrical conductivity. These fillers have an effect on the composite tensile and flexural properties, which are also important for bipolar plates. In this study, various amounts of three different types of carbon (carbon black, synthetic graphite particles, and carbon fibers) were added to Vectra A950RX liquid‐crystal polymer. In addition, composites containing combinations of fillers were also investigated via a factorial design. The tensile and flexural properties of the resulting composites were then measured. The objective of this study was to determine the effects and interactions of each filler with respect to the tensile and flexural properties. The addition of carbon black caused the tensile and flexural properties to decrease. Adding synthetic graphite particles caused the tensile and flexural modulus to increase. The addition of carbon fiber caused the tensile and flexural modulus and ultimate flexural strength to increase. In many cases, combining two different fillers caused a statistically significant effect on composite tensile and flexural properties at the 95% confidence level. For example, when 40 wt % synthetic graphite particles and 4 wt % carbon black were combined, the composite ultimate tensile and flexural strength increased more than what would be expected from the individual additive effect of each single filler. It is possible that linkages were formed between the carbon black and synthetic graphite particles that resulted in improved ultimate tensile and flexural strength. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Manipulating the conductivity of carbon‐black‐filled immiscible polymer composites by insulating nanoparticles

The conductivity of an immiscible polymer blend system, microfibrillar conductive poly(ethylene terephthalate) (PET)/polyethylene (PE) composite (MCPC) containing carbon black (CB), was changed by the addition of insulating CaCO₃ nanoparticles. In MCPC, the PET forms microfibrils during processing and PE forms the matrix. The CB particles are selectively localized in the PET microfibrils. When the insulating CaCO₃ nanoparticles are added, they substitute for some of the conductive CB particles and obstruct the electron paths. As a result, the resistivity of the MCPC can be tailored depending on the insulating filler content. The resistivity‐insulating filler content curve displays a sluggish postpercolation region (the region immediately following the percolation region and in front of the equilibrium flat of the resistivity‐filler content curve), suggesting that the MCPC in the postpercolation region possesses an enhanced manufacturing reproducibility and a widened processing window. These features are of crucial importance in making sensor materials. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008.     

Electrical conductivity and rheology of carbon fiber/liquid crystal polymer composites

One emerging market for electrically conductive resins is for bipolar plates for use in fuel cells. Adding carbon fillers to thermoplastic resins increases composite electrical conductivity and viscosity. Current technology often adds as much of a single type of carbon filler as possible to achieve the desired conductivity, while still allowing the material to be extruded and molded into a bipolar plate. In this study, varying amounts of two different types of polyacrylonitrile (PAN) based carbon fiber (Fortafil 243 and Panex 30) were added to Vectra A950RX liquid crystal polymer. The resulting single fiber composites were then tested for electrical conductivity and rheological properties. The electrical conductivity followed the behavior typically seen in composites with a percolation threshold at 5 vol% for Fortafil 243 and at 13 vol% for Panex 30. Viscosity increased with increasing filler volume fraction for all shear rates, but was more rapid for the Fortafil 243 composites. Over the range of shear rates studied, the viscosity followed a shear‐thinning power law model with power‐law exponent (n – 1) = –0.5 for neat Vectra A950RX. Panex 30 had no effect on the power‐law exponent and Fortafil 243 changed (n – 1) to −0.6. POLYM. COMPOS., 28:168–174, 2007. © 2007 Society of Plastics Engineers     

Development of an additive equation for predicting the electrical conductivity of carbon‐filled composites

The electrical conductivity of polymeric materials can be increased by the addition of carbon fillers. The resulting composites can be used in applications such as electrostatic dissipation and interference shielding. Electrical conductivity models are often proposed to predict the conductivity behavior of these materials. The electrical conductivity of carbon‐filled polymers was studied here by the addition of three single fillers to nylon 6,6 and polycarbonate in increasing concentrations. The fillers used in this project were carbon black, synthetic‐graphite particles, and milled pitch‐based carbon fibers. Materials were extruded and injection‐molded into test specimens, and then the electrical conductivity was measured. Additional material characterization tests included optical microscopy for determining the filler aspect ratio and orientation. The filler and matrix surface energies were also determined. An updated model developed by Mamunya and others and a new additive model (including the constituent conductivities, filler volume fraction, percolation threshold, constituent surface energies, filler aspect ratio, and filler orientation) fit the electrical conductivity results well. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 2280–2299, 2003     

Extrudate swell behavior of polypropylene composites filled with microencapsulated red phosphorus

The microencapsulated red phosphorus (MRP) filled polypropylene (PP) composites were prepared using a twin‐screw extruder. The effects of load and temperature on the extrudate swell behavior of the PP/MRP composite melts were investigated by means of a melt flow indexer. The test temperatures and loads were varied from 180 to 205°C and from 2.16 to 12.5 kg, respectively. The results showed that the die‐swell ratio (B) of the composite melts increased roughly linearly with increasing load while decreased slightly with a rise of test temperature. The sensitivity of the die‐swell ratio of the composite melts to load was significant. When the test temperature or load was constant, the values of the B of the composite melts decreased slightly with increasing MRP weight fraction. The findings can provide useful information for processing of these composites. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Properties of a semi-crystalline and an amorphous thermotropic liquid crystalline polymer

The physical properties, thermal stability, rheology and tensile properties of a commercial semi-crystalline and an amorphous thermotropic liquid crystalline polymer (TLCP) have been investigated. Analysis by differential scanning calorimetry (DSC) and dynamic mechanical thermal analysis (DMTA) confirm the presence of a small melting endotherm and a glass transition in the former material. The as-received amorphous TLCP exhibits no obvious melting endotherm and a strong glass transition is detected. The flow and tensile properties of the semicrystalline polymer are dominated by the presence of the crystalline to nematic transition temperature. The properties of the amorphous TLCP appear to be governed by increasing mobility afforded by increasing temperature. Based on flow behaviour and further DSC analysis it has been shown that under appropriate annealing conditions the as-received amorphous TLCP can develop solid crystalline order.     

Electrical conductivity and mechanical performance of multiwalled CNT‐filled polyvinyl chloride composites subjected to tensile load

This article reports a study on the strain‐sensitive conductivity (tensoresistivity) and mechanical properties of polyvinyl chloride/multiwalled carbon nanotube (PVC/MWCNT) composites subjected to tensile loading at different strain rates for potential use in sensor‐enabled geosynthetics and other applications involving electrically conductive polymer composites. Results indicate that adding 0.5 wt % MWCNT to the composite results in 57% reduction in its ultimate (failure) strain and a fivefold increase in its tensile modulus while leaving its ultimate strength almost unchanged. Laser scanning confocal microscopy is used to investigate the microscopic failure mechanism of the composite and how it contributes to the strain‐sensitive conductivity of the composites. It is observed that tensile fractures are initiated from inside the largest bundles between 18% and 36% strain and continue through further fractal‐like fracturing in smaller bundles. Gauge factors (e.g., 3.17) comparable to or exceeding those of typical strain gauges are obtained for the composite, indicating its strong potential for structural performance monitoring and damage detection applications. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43665.     

Measuring and predicting in‐plane thermal conductivity of carbon‐filled nylon 6,6 polymer composites

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 both the through‐plane thermal conductivity k_thru and the in‐plane thermal conductivity k_in, using the transient plane source method. The first goal of this work was to use a finite element model to develop a procedure to accurately measure the material properties using this relatively new analytical procedure. Reproducible data can be obtained for nylon 6,6 polymer composites, by choosing a power dissipation (an input parameter to the transient plane source method) corresponding to a sensor temperature increase of 2 K above the initial temperature after 5 s. The second goal of this work was to develop a simple empirical model for the in‐plane thermal conductivity, which is easily measured with the transient plane source method. The results show that the product of the through‐plane and in‐plane thermal conductivities is a linear function of the volume percent ϕ. As the through‐plane thermal conductivity of these composites is accurately predicted with a modified Nielsen model, this empirical relationship can be used to estimate in‐plane thermal conductivities for a range of applications. POLYM. COMPOS. 27:1–7, 2006. © 2005 Society of Plastics Engineers     

Studies on morphology,mechanical, thermal,and rheological behavior of extrusion‐blended polypropylene and thermotropic liquid crystalline polymer

Polypropylene (PP) was melt‐blended in a single‐screw extruder with a thermotropic Vectra B‐950 liquid crystalline polymer (LCP) in different proportions. The mechanical properties of such blends were compared in respect of their Young's moduli, ultimate tensile strength (UTS), percent elongation at break, and toughness to those of pure PP. The thermal properties of these blends were studied by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The morphology was studied by using a polarizing light microscope (PLM) and a scanning electron microscope (SEM) while the rheological aspects of the blends and the pure PP were studied by a Haake Rheowin equipment. Mechanical analysis (tensile properties) of the blends showed pronounced improvement in the moduli and the UTS of the PP matrix in the presence of 2–10% of LCP incorporation. TGA of all the blends showed an increase in the thermal stability for all the blends with respect to the matrix polymer PP, even at a temperature of 410°C, while PP itself undergoes drastic degradation at this temperature. DSC studies indicated an increase in the softening range of the blends over that of PP. Morphological studies showed limited mixing and elongated fibril formation by the dispersed LCP phase within the base matrix (PP) at the lower ranges of LCP incorporation while exhibiting a tendency to undergo gross phase separation at higher concentrations of LCP, which forms mostly agglomerated fibrils and large droplets. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 767–774, 2003