Electrical conductivity and rheology of carbon‐filled 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 carbon‐filled thermoplastic matrix material to be extruded and molded into a bipolar plate. In this study, varying amounts of two different types of carbon, one carbon black and one synthetic graphite, were added to Vectra A950RX liquid crystal polymer. The resulting single filler composites were then tested for electrical conductivity and rheological properties. The electrical conductivity followed that typically seen in polymer composites with a percolation threshold at 4 vol % for carbon black and at 15 vol % for synthetic graphite. 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 and (n ? 1) = ?0.7 for highly filled composite materials. Viscosity increased with increasing filler volume fraction for all shear rates. The viscosity–enhancement effect was more rapid for the composites containing carbon black when compared with those containing synthetic graphite. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 2680–2688, 2006     

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     

Effects of carbon fillers on tensile and flexural properties in polypropylene‐based resins

A potential application for conductive resins is in bipolar plates for use in fuel cells. The addition of carbon filler can increase the electrical and thermal conductivities of the polymer matrix but will also have an effect on the tensile and flexural properties, important for bipolar plates. In this research, three different types of carbon (carbon black, synthetic graphite, and carbon nanotubes) were added to polypropylene and the effects of these single fillers on the flexural and tensile properties were measured. All three carbon fillers caused an increase in the tensile and flexural modulus of the composite. The ultimate tensile and flexural strengths decreased with the addition of carbon black and synthetic graphite, but increased for carbon nanotubes/polypropylene composites due to the difference in the aspect ratio of this filler compared to carbon black and synthetic graphite. Finally, it was found that the Nielsen model gave the best prediction of the tensile modulus for the polypropylene based composites. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

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     

Synergistic effects of carbon fillers in electrically and thermally conductive liquid crystal polymer based resins

Adding conductive carbon fillers to insulating thermoplastic resins increases composite electrical and thermal conductivity. Often, as much 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, varying amounts of three different carbons (carbon black, synthetic graphite particles, and carbon fiber) were added to Vectra A950RX Liquid Crystal Polymer. The resulting single filler composites were tested for electrical resistivity (1/electrical conductivity) and thermal conductivity. In addition, the effects of single fillers and combinations of two different carbon fillers were studied via a factorial design. The results indicated that for the composites containing only single fillers, synthetic graphite, followed by carbon fiber, cause a statistically significant decrease in composite electrical resistivity. Composites containing only synthetic graphite, followed by carbon black, and then carbon fiber cause a statistically significant increase in thermal conductivity. For the combinations of two different fillers, the composites containing carbon black/synthetic graphite and synthetic graphite/carbon fiber had a statistically significant and positive effect on thermal conductivity. It is possible that thermally conductive pathways are formed that “link” these carbon fillers, which results in increased composite thermal conductivity. POLYM. COMPOS., 2008. © 2008 Society of Plastics Engineers     

Non‐linear electrical conductivity of urea‐formaldehyde‐cellulose loaded with powders of different carbon fillers

This article reports on the making and characterization of composite materials prepared by compression molding of a commercial grade thermosetting resin of urea‐formaldehyde filled with α‐cellulose in powder form mixed successively with carbon black, synthetic graphite, and activated carbon. The morphology of the constituents and the composites has been characterized by optical microscopy. The porosity effect has been discussed from density measurements. Furthermore, it has been shown that the hardness of the samples remains almost constant with the increase of filler concentration. The electrical conductivity shows clearly a non‐linear behavior. The observed values are lower than 10^?11 S/cm, unless the filler content reaches the percolation threshold beyond which the conductivity increases markedly by as much as ten orders of magnitude, indicating insulator‐conductor phase transition. The conduction threshold depends on the filler nature. The results have been interpreted by means of the statistical percolation theory. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 990–996, 2005     

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     

Synergistic Effects of Carbon Fillers of Phenolic Resin Based Composite Bipolar Plates on the Performance of PEM Fuel Cell

The most essential and costly component of polymer electrolyte membrane fuel cells is the bipolar plate. The production of suitable composite bipolar plates for polymer electrolyte membrane fuel cell with good mechanical properties and high electrical conductivity is scientifically and technically very challenging. This paper reports the development of composite bipolar plates using exfoliated graphite, carbon black, and graphite powder in resole‐typed phenol formaldehyde. The exfoliated graphite with maximum exfoliated volume of 570 ± 10 mL g⁻¹ used in this study was prepared by microwave irradiation of chemically intercalated natural flake graphite in a few minutes. The composite plates were prepared by varying exfoliated graphite content from 10 to 35 wt.% in phenolic resin along with fixed weight percentage of carbon black (5 wt.%) and graphite powder (3 wt.%) by compression molding. The composite plates with filler weight percentage of 35/5/3/exfoliated graphite/carbon black/graphite powder offer in‐plane and trough‐plane electrical conductivities of 374.42 and 97.32 S cm⁻¹, bulk density 1.58 g cm⁻³, compressive strength 70.43 MPa, flexural strength 61.82 MPa, storage modulus 10.25 GPa, microhardness 73.23 HV and water absorption 0.22%. Further, I–V characteristics notify that exfoliated graphite/carbon black/graphite powder/resin composite bipolar plates in unit fuel cell shows better cell performance compared exfoliated graphite/resin composite bipolar plates. The composite plates own desired mechanical properties with low bulk density, high electrical conductivity, and good thermal stability as per the U.S. department of energy targets at low filler concentration and can be used as bipolar plates for proton exchange membrane fuel cells.     

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     

Nielsen thermal conductivity model for single filler carbon/polypropylene composites

In this study, three different carbon fillers (Thermocarb TC‐300 synthetic graphite, Ketjenblack EC‐600 JD carbon black, and Hyperion Catalysis International's FIBRIL? carbon nanotubes) were added to a polypropylene matrix to produce single filler composites with filler concentrations of up to 80 wt % synthetic graphite (61.6 vol %), 15 wt % carbon black (8.1 vol %), and 15 wt % carbon nanotubes (7.4 vol %). The through‐plane thermal conductivity for each formulation was measured. For the synthetic graphite, carbon black, and carbon nanotubes composites, the Nielsen model was applied to the experimental through‐plane thermal conductivity data. The Nielsen Model presented in this work showed very good agreement with experimental data. The model parameters were similar to those used in the literature for these fillers in other polymers. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

In‐plane thermal conductivity modeling of carbon‐filled liquid crystal polymer–based resins

Adding conductive carbon fillers to insulating thermoplastic resins increases composite electrical and thermal conductivity. In this study, varying amounts of three different carbons (carbon black, synthetic graphite particles, and carbon fiber) were added to Vectra A950RX liquid crystal polymer. The in‐plane thermal conductivity of the resulting single filler composites was tested. The results showed that adding synthetic graphite particles caused the largest increase in the in‐plane thermal conductivity of the composite. The composites were modeled using ellipsoidal inclusion problems to predict the effective in‐plane thermal conductivities at varying volume fractions with only physical property data of the constituents. The synthetic graphite and carbon black were modeled using the average field approximation with ellipsoidal inclusions and the model showed good agreement with the experimental data. The carbon fiber polymer composite was modeled using an assemblage of coated ellipsoids and the model showed good agreement with the experimental data. POLYM. COMPOS., 2011. © 2010 Society of Plastics Engineers     

A new equation for predicting electrical conductivity of carbon‐filled polymer composites used for bipolar plates of fuel cells

In this article, a statistical‐thermodynamic formula based on a new approach has been developed to predict electrical conductivity of carbon‐filled composites used for bipolar plate of proton exchange membrane fuel cell. In this model, based on percolation threshold phenomenon, it is assumed that the relationship between electrical conductivity of composite and filler volume fraction follows a sigmoidal equation. Afterwards, the four effective factors on composite conductivity including filler electrical conductivity, filler aspect ratio, wettability, as well as interface contact resistance are replaced upon constant parameters of sigmoidal function. In order to test the model, some single‐filler composites have been manufactured by using the phenolic resin as binder and graphite (G), expanded graphite (EG), and carbon fiber (CF) as fillers. The fitting quality is measured by R‐square, adjusted R‐square, SSE, and RMSE parameters. The results showed that there is a noteworthy agreement between the model and the experimental data. Compared to the other models, this model can be used for more types of fillers. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

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     

Synergistic effects of multiple carbon fillers on the rheology of liquid crystal polymer based resins

Adding conductive carbon fillers to insulating thermoplastic resin increases composite electrical and thermal conductivity. Often, as much 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, varying amounts of three different carbons (carbon black, synthetic graphite particles, and carbon fiber) were added to Vectra A950RX liquid crystal polymer. The rheology of resulting single filler composites was tested. In addition, the rheological properties of composites containing combinations of two different carbon fillers were studied via a factorial design. In all cases, viscosity increased with increasing filler volume fraction for all shear rates. Over the range of shear rates studied, the viscosity followed a shear‐thinning power law model. The factorial design results indicated that each of the single fillers and all of the two filler combinations caused a statistically significant increase in composite viscosity at a shear rate of 1,000 s⁻¹. The composites containing carbon black and synthetic graphite caused the largest increase in viscosity. It is possible that the highly branched, high surface area structure of carbon black ‘links’ with the synthetic graphite particles, which results in increased composite viscosity. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers.     

Highly filled graphene‐benzoxazine composites as bipolar plates in fuel cell applications

This research emphasizes on the development of highly filled graphene‐polybenzoxazine composites and investigates thermal, electrical, and mechanical properties of the obtained composites for bipolar plate applications. The composition of graphene loading was achieved to be in the range of 10–60 wt%. The experimental results revealed that at the maximum graphene content of 60 wt% (44.8 vol%) in the polybenzoxazine, storage moduli at room temperature of the composites were considerably enhanced with the amount of the graphene, that is, from 5.9 GPa of the neat polybenzoxazine to about 25.1 GPa at 60 wt% of graphene. Glass transition temperatures (T_g) of the obtained composites were observed to be 174–188°C and the values substantially increased with increasing the filler contents. At 60 wt% of graphene content, thermal conductivity, as high as 8.0 W/mK, is achieved for the graphene‐filled polybenzoxazine. Furthermore, the flexural modulus and flexural strength of the composites were found to be as high as 18 GPa and 42 MPa, respectively. Water absorption of graphene filled‐composite is relatively low with the value of only about 0.06% at 24 h of water immersion. Additionally, electrical conductivity was measured to be 357 S/cm at maximum loading of the graphene. Therefore, the graphene‐filled composites based on polybenzoxazine are highly attractive as bipolar plates for polymer electrolyte membrane fuel cells applications. POLYM. COMPOS., 37:1715–1727, 2016. © 2014 Society of Plastics Engineers     

Effect of filler size on thermophysical and electrical behavior of nanocomposites based on expanded graphite nanoparticles filled in low‐density polyethylene matrix

Various aspects of electrical and thermophysical properties of nanocomposites based on low‐density polyethylene matrix filled with nanostructuralized expanded graphite (EG) and standard, microsized graphite are presented in this article. A periodical method developed in the laboratory was used to measure simultaneously thermal conductivity, specific heat, and diffusivity of composites at room temperature. The effect of micro‐ and nanosized fillers on the final thermophysical and electrical behavior is investigated. It was found that the electrical conductivity of composites strongly depends not only on the filler content but also on the filler size. When the microsized graphite was used, the percolation concentration of the filler was found to be 15 vol%, whereas the percolation concentration of the filler in nanocomposites filled with EG of large sizes was significantly lower. Similarly, it was shown that the graphite significantly improves the thermophysical behavior of composites filled with micro‐ and nanofiller sizes. The thermal conductivity measured values were also compared with some theoretical models for the prediction of the thermal conductivity. POLYM. COMPOS., 2012. © 2013 Society of Plastics Engineers     

Effect of filler dispersion on the electrical conductivity and mechanical properties of carbon/polypropylene composites

Conducting carbon/polypropylene composites containing a mixed filler of crystalline natural graphite powder, carbon black, and multiwalled carbon nanotubes have been prepared. The effect of the filler dispersion on the electrical conductivity and mechanical properties has been studied. Keeping the graphite content constant and increasing the ratio of carbon black to nanotubes, the conductivity and the flexural modulus increased linearly at different graphite contents. Graphite aggregates on the cross sections were analyzed by optical microscopy to characterize the dispersion of graphite. The results of the optical microscopy studies showed that the dispersion of graphite is affected by the composition and amount of the nanofiller system. Based on the observations simplified morphological models were set up, that allowed to explain the change in the electrical and mechanical properties of the composites of different compositions. POLYM. COMPOS., 34:1195–1203, 2013. © 2013 Society of Plastics Engineers