Dipak Khastgir

Effect of processing parameters, applied pressure and temperature on the electrical resistivity of rubber-based conductive composites

Abstract Carbon black- and short carbon fibre (SCF)-filled conductive composites were prepared from ethylene vinyl acetate (EVA), ethylene propylene diene (EPDM) rubber and their 50:50 blend. The electrical resistivity of carbon black- and SCF-filled composites were measured under different conditions. The electrical conductivity of filled polymer composites is due to the formation of a continuous conductive network in the polymer matrix. These conductive networks involve specific arrangement of conductive elements so that the electrical paths are formed for free movement of electrons. It was found that electrical conductivity of filled conductive composites depends on different processing parameters like mixing time, rotor speed, mixing temperature, vulcanization time and pressure and service conditions like applied pressure and temperature. The results of different experiments have been discussed in light of break down and formation of the continuous conductive network.

Electrical and mechanical properties of conducting carbon black filled composites based on rubber and rubber blends

The electrical and mechanical properties of new conductive rubber composites based on ethylene-propylene-diene rubber, acrylonitrile butadiene rubber (NBR), and their 50/50 (weight ratio) blend filled with conductive black were investigated. The threshold concentrations for achieving high conductivity are explained on the basis of the viscosity of the rubber. The electrical conductivity increases with the increase in temperature whereas the activation energy of conduction decreases with an increase in filler loading and NBR concentration in the composites. The electrical hysteresis and electrical set are observed during the heating-cooling cycle, which is mainly due to some kind of irreversible change occurring in the conductive networks during heating. The mechanisms of conduction in these systems are discussed in the light of different theories.

Carbon fibre filled conductive composites based on nitrile rubber (NBR), ethylene propylene diene rubber (EPDM) and their blend

Abstract The conductivity of nitrile rubber (NBR), ethylene–propylene–diene rubber (EPDM) and 50/50 NBR/EPDM blend has been measured against the loading of conductive carbon fibre to check the percolation limit for each system. The volume resistivity of all fibre–rubber composites increases with the increase in temperature, and the rate of increase in resistivity against temperature depends on the loading of carbon fibre and the nature of the base polymer. The change in volume resistivity during the heating and cooling cycle does not follow the same route, leading to the phenomena of electrical hysteresis and electrical set. The current–voltage relation is linear at room temperature, but becomes nonlinear at higher temperature. Some mechanical properties of these composites are also measured. The applicability of different theoretical models to predict the modulus and conductivity of these systems has been tested. Deviations between theoretical and experimental values are also discussed.

Pressure-sensitive electrically conductive nitrile rubber composites filled with particulate carbon black and short carbon fibre

The electrical conductivity of pressure-sensitive nitrile rubber composites, containing different loadings of particulate carbon black filler and short carbon fibre, have been studied. The conductivity of composites increases with increasing of filler concentration as well as with increased applied pressure up to a certain limit. The composites containing particulate fillers register low conductivity as compared to composites containing short carbon fibres, due to easy formation of an interconnecting network in the latter case. The effect of the orientation of short carbon fibre with respect to an applied electric field has also been studied. The pressure dependence of composites with transversely oriented carbon fibres with respect to electric fields is higher than that of composites with longitudinally oriented carbon fibres. The results are interpreted on the basis of the formation of interconnecting continuous conducting networks.

Conductive rubber composites from different blends of ethylene-propylene-diene rubber and nitrile rubber

Conductive rubber composites were derived from different blends of ethylene-propylene-diene monomer (EPDM) rubber and acrylonitrile butadiene rubber (NBR) containing acetylene black. The electrical and mechanical properties of these composites were measured. The percolation limit for achieving high conductivity of conductive filler depends on the viscosity of the blend. The higher the viscosity, the higher is the percolation limit. The conductivity rises with increasing temperature, and the activation energy of conduction increases with the decrease in the loading of conductive filler and percentage of NBR in the blend. Electrical hysteresis and an electrical resistivity difference during the heating-cooling cycle are observed for these systems, which is mainly due to some kind of irreversible change occurring in the conductive networks during heating. The mechanisms of conduction of these systems were discussed in the light of different theories. It was found that the degree of reinforcement by acetylene black in blends compares with those in the pure components NBR and EPDM. This is due to incompatibility of two elastomers in the blend.

Correlation between morphology with dynamic mechanical, thermal, physicomechanical properties and electrical conductivity for EVA-LDPE blends

Abstract Morphology, dynamic mechanical analysis, differential scanning calorimetry (d.s.c.), physicomechanical properties and electrical resistivity were studied on different blends of ethylene vinyl acetate (EVA) (28% VA content) and low density polyethylene (LDPE). An interpenetrating polymer network (IPN) is formed with a minimum of 50 wt% EVA in the blend. The glass transition temperatures change with blend compositions according to the Fox equation or the Gordon-Taylor relation. This indicates intermiscibility (compatibility). On curing, tensile strength goes up dramatically, and a maximum tensile value is obtained for 50:50 blend composition. The electrical resistivity of the blends is explained by a simple model, and can be correlated to morphology.

Electrical conductivity of carbon black and carbon fibre filled silicone rubber composites

Electrically conductive silicone rubber composites have been prepared through incorporation of conductive acetylene black and short carbon fibre (SCF). The percolation limit for the attainment of high conductivity is found to be relatively less for silicone rubber based composites compared to EPDM or NBR based composites reported earlier. Percolation limit is found to be lower for SCF-filled systems (7.5 phr) compared to black-filled ones (14 phr). Both black- and SCF-filled systems exhibit an increase in resistivity with the increase in temperature (PCT effect). This PCT effect may be explained in terms of differences in the thermal expansion between the rubber matrix and the conductive filler. However, resistivity-versus-temperature plots are not identical during the heating-cooling cycle, leading to some hysteresis and electrical set. The current-voltage relationship is linear (Ohmic in nature) at room temperature but becomes non-linear (non-Ohmic) at elevated temperatures. The resistivity of these composites is measured under different conditions such as on applying pressure and being subjected to different mechanical stress and strain over the specimens. An effort has been made to correlate the effect of different parameters on electrical resistivity with the change in the conductive network structure under different conditions. Elektrisch leitende Silikonkautschuk-Verbundwerkstoffe wurden durch Fullen mit leitfahigem Acetylenrus bzw. kurzen Kohlenstoffasern (SCF) hergestellt. Die Perkolationsgrenze fur das Erreichen einer hohen Leitfahigkeit liegt bei Silikonkautschuk-Verbundwerkstoffen relativ niedrig im Vergleich zu den bereits beschriebenen Verbundwerkstoffen auf EPDM- oder NBR-Basis. Die Perkolationsgrenze ist bei den SCF-gefullten (7.5 phr) Systemen niedriger als bei den mit Rus (14 phr) gefullten. Sowohl die rusgefullten als auch die SCF-gefullten Systeme zeigen eine Erhohung des spezifischen Widerstands mit der Temperatur (PCT-Effekt). Dieser PCT-Effekt kann mit dem unterschiedlichen thermischen Ausdehnungsverhalten von Kautschukmatrix und Fullstoff erklart werden. Die Temperatur/Widerstands-Meskurven bleiben jedoch wahrend eines Heiz-Kuhl-Durchgangs nicht gleich, was sich in Hysteresekurven und Differenzen zwischen Anfangs- und Endwert des spezifischen Widerstands zeigt. Die Beziehung zwischen Strom und Spannung ist bei Raumtemperatur linear (Ohmsches Gesetz), weicht jedoch bei hoheren Temperaturen vom linearen Verhalten ab. Der spezifische Widerstand dieser Verbundwerkstoffe wurde unter verschiedenen Bedingunn untersucht, unter Druck oder bei unterschiedlichen Spannungs- oder Dehnungszustanden der Probekorper. Ein Zusammenhang zwischen der Wirkung der verschiedenen Parameter auf den elektrischen Widerstand und der Strukturanderung des leitfahigen Netzwerks unter verschiedenen Bedingungen wurde untersucht.

The change in conductivity of a rubber-carbon black composite subjected to different modes of pre-strain

Conductivity of conductive rubber composites changes significantly when subjected to mechanical stress and strain. Electrical properties of different pre-strained samples derived from EPDM, 5050 NBR/EPDM blend and NBR rubbers, were measured. It was found that electrical resistivity of strained samples depends on strain amplitude (% elongation), frequency of stress-strain cycle, and also number of stress-strain cycles. Samples were strained in three different instruments: an Instron UTM; a Monsanto fatigue to failure tester; and a Goodrich flexometer. Under different conditions, electrical properties of strained and unstrained (original) samples were measured. It was found that there is similarity in the change of modulus and electrical resistivity against degree of strain and frequency of strain for different samples. The results of different experiments have been discussed in light of breakdown and formation of the carbon black-rubber structure.

Reinforcement of EPDM-based ionic thermoplastic elastomer by carbon black

Abstract High abrasion furnace carbon black improves the physical properties of zinc sulfonated ethylenepropylenediene terpolymer of high (75 wt%) ethylene content. Properties studied include hardness, stress-strain characteristics, tear strength, hysteresis and abrasion resistance. Scanning electron photomicrographs of the tear fractured and abraded surfaces show changes in failure mode of the polymer on incorporation of carbon black. Results of dynamic mechanical analyses and dielectric thermal analyses show that carbon black reinforces the ionomers, presumably through weak rubber—filler interaction involving the backbone chains and strong interaction between the active sites of the filler surface and the ionic aggregates present in the ‘multiplets’ and ‘clusters’. Reprocessability studies in the Monsanto Processability Tester (MPT) show that the carbon black filled polymer can be reprocessed like a thermoplastic elastomer and there was no fall in properties even after three cycles of extrusion through the MPT.

Electrical properties of natural rubber nanocomposites: effect of 1-octadecanol functionalization of carbon nanotubes

This article reports the results of studies on the effect of 1-octadecanol (abbreviated as C18) functionalization of carbon nanotubes (CNT) on electrical properties of natural rubber (NR) composites. Dispersion of CNT in NR matrix was studied by transmission electron microscopy (TEM) and electrical resistivity measurements. Fourier transform infra red spectrometry (FTIR) indicates characteristic peaks for ether and hydrocarbon in the case of C18 functionalized CNT. Dielectric constant increases with respect to the filler loading for both unmodified and functionalized CNTs, the effect being less pronounced in the case of functionalized CNT due to its better dispersion in the matrix. Stress–strain plots suggest that the mechanical integrity of the NR/CNT composites, measured in terms of tensile strength, increases on C18 functionalization of the nanofiller. TEM reveals that the functionalization causes improvement in dispersion of CNT in NR matrix, which is corroborated by the increase in electrical resistivity in the case of the functionalized CNT/NR composites.