Showing papers by "Dipak Khastgir published in 1998"

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

Abstract: 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

Abstract: Conductivity of conductive rubber composites changes significantly when subjected to mechanical stress and strain. Electrical properties of different pre-strained samples derived from EPDM, 50 50 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.

Phase modification of SEBS block copolymer by different additives and its effect on morphology, mechanical and dynamic mechanical properties

Abstract: The objective of this study is to examine the phase modification of styrene-ethylene butylene-styrene (SEBS) block copolymer by different additives and its influence on morphology and mechanical, and dynamic mechanical properties. The additives chosen are the coumarone-indene (CI), phenol-formaldehyde (PF), paraffin hydrocarbon (PAHY) resins, as well as aromatic oil (AO), polystyrene (PS), polypropylene (PP), ethylene vinyl acetate (EVA) (VA 28 and 45%), and ethylene propylene diene monomer ( EPDM ) rubber. It is interesting to note that of all the additives, PP has the most prominent effect. The mechanical properties of SEBS polymer are enhanced to a large extent by PP. The value of tan δ maximum of SEBS at both the low and the high temperature transitions is decreased. All the resins and PS increase the storage modulus and the tensile modulus of the SEBS polymer. CI resin and AO modify the hard and soft phases of SEBS polymer. AO, EPDM rubber, and EVA lower the mechanical strength of the SEBS polymer. The results are explained on the basis of morphology studied with the help of scanning electron microscopy.

Influence of block molecular weight on the dielectric properties of segmented polyamides

Abstract: Dielectric measurements of segmented polyamides consisting of polyether and polyamide structural units have been carried out over a wide range of temperatures and frequencies. The polymers exhibit different peaks - α, β and γ in the log (tan 6) temperature curves. With decreasing hard block molecular weight, the α and β transitions shift to lower temperatures, but the γ transition remains unaltered. The α transition becomes weaker, while the γ transition shifts to higher temperatures when the soft segment molecular weight is increased. The dielectric constant at 100°C at a frequency of 1000 Hz decreases with decreasing hard segment molecular weight. However, the variation of dielectric constant with soft segment molecular weight is irregular. The results of dielectric studies are explained with the help of the segmented structure of the polyether-ester-amides.