All issues / Volume 2 (2008) / Issue 12 (December)
The variation of electrical resistivity as will as the mechanical properties of PVC (polyvinylchloride)-NBR (acrylonitrile butadiene rubber) based conductive composites filled with different concentrations of graphite were studied. These samples were studied as function of the constant deformation fatigue test. When the specimen was subjected to a large number of rapidly repeating strain cycles, and different strain amplitudes, the conductivity, σ(T), shows an initial rapid fall followed by dynamic equilibrium. Increasing the number of cycles and strain amplitudes, the conductivity remains almost constant over the temperature range 30–140°C. The equilibrium state between destruction and reconstruction of graphite particles has been detected for all strains of certain values of strain cycles (1000, 2000, 3000, and 4000 cycles for 30% strain amplitude). A preliminary study was done to optimize the possibility to use Conductive Polymer Composites (CPC) as a strain sensor and to evaluate its performance by an intrinsic physico-mechanical modification measurement. The electromechanical characterization was performed to demonstrate the adaptability and the correct functioning of the sensor as a strain gauge on the fabric. The coefficient of strain sensitivity (K) was measured for 50 phr graphite/PVCNBR vulcanized at 3000 number of strain cycles and 30% strain amplitude. There was a broad maximum of K, with a peak value of 82, which was much higher, compared to conventional wire resistors. A slight hysteresis was observed at unloading due to plasticity of the matrix. A good correlation exists between mechanical and electrical response to the strain sensitivity. Mechanical reinforcement was in accordance with the Quemada equation  and Guth model  attested to good particle-matrix adhesion. It was found that the viscous component of deformation gradually disappeared and the hardening occurred with increasing strain cycles. The modulus, fracture strength, and elongation at break increased with increasing filler volume fraction up to 40 phr of graphite particles.
Thermoplastic vulcanizates (TPVs) are a special class of thermoplastic elastomers, which are produced by simultaneously mixing and crosslinking a rubber with a thermoplastic polymer at an elevated temperature. Peroxide-cured TPVs based on blends of silicone rubber and thermoplastic Engage of two different types, mainly ethylene-octene and ethylenebutene copolymers at different blend ratios have been developed. A detailed comparative study of ethylene-octene vs. ethylene-butene based TPVs are mainly focused in this paper. These TPVs exhibit very good overall mechanical and electrical properties. With increasing amount of Engage in the blends at a fixed concentration of peroxide and coagent, tensile strength, modulus and hardness of the TPVs were found to increase considerably. Ageing characteristics and recyclability of silicone rubber based TPVs are also found excellent. Rheological studies confirm the pseudoplastic nature of these TPVs.
The present work deals with the effect of conductive carbon black (Ensaco 350G) on the physico-mechanical and electrical properties of chlorosulfonated polyethylene (CSM) rubber vulcanizates. The physico-mechanical properties like tensile strength, tear strength, elongation at break, compression set, hardness and abrasion resistance have been studied before and after heat ageing. Up to 30 parts per hundred rubber (phr) filler loading both tensile and tear strength increases beyond which it shows a decreasing trend whereas modulus gradually increases with the filler loading. Incorporation of carbon black increases the hysteresis loss of filled vulcanizates compared to gum vulcanizates. Unlike gum vulcanizate, in filled vulcanizates the rate of relaxation shows increasing trend. The bound rubber content is found to increase with increase in filler loading. Dielectric relaxation spectra were used to study the relaxation behavior as a function of frequency (100 to 106 Hz) at room temperature. Variation in real and imaginary parts of electric modulus has been explained on the basis of interfacial polarization of fillers in the polymer medium. The percolation limit of the conductive black as studied by ac conductivity measurements has also been reported.
Acrylonitrile-butadiene rubber (NBR) and blends of poly(3-thiopheneacetic acid)/ acrylonitrile-butadiene rubber, P3TAA/NBR, were fabricated, and the electrorheological properties, dielectric, and electrical conductivities were investigated . The electrorheological properties were determined under an oscillatory shear mode in a frequency range of 0.1 to 100 rad/s at various electric field strengths, from 0 to 2 kV/mm, at a fixed 27°C to observe the effects of acrylonitrile content (ACN) in the rubber systems and the conductive particle concentration in the blends. For the pure rubber systems, the storage modulus response (ΔG′) is linearly dependent on its dielectric constant (ε′), and increases with the ACN content. For the NBR/P3TAA blends, the storage modulus response varies nonlinearly with the dielectric constant. The bending responses of the rubbers and the blends were investigated in a vertical cantilever fixture. For the pure rubber system, the bending angle and the dielectrophoresis force vary linearly with electric field strength. For the blend system, the bending angle and the dielectrophoresis force vary nonlinearly with electric field strength.
The effect of blending short and long chains (bimodality) silicone prepolymer, in the presence of a filler (17.2 and 30.2% w/w) on tensile properties such as ultimate tensile strength (UTS), percent elongation at break (% Eb), 100% modulus and elastic modulus has been investigated. The content of short chain prepolymer was varied from 0–69% and 0–58% for 17.2 and 30.2% filled silicones respectively. It was found out that the tensile properties were enhanced for the low filled (17.2%) silicone networks while in the case of highly filled (30.2%) silicone networks; the bimodality adversely affected the tensile properties such as UTS, % Eb. It is also observed that the optimum in these properties was recorded at 95 mol% as compared to 70 mol% of short chain prepolymer for unfilled silicone system. The phenomenon of improvement in mechanical properties due to bimodality in this system has been discussed.
Regenerated Bombyx mori silk fibers were spun from hexafluoro-iso- propanol solution of silk fibroin sponge in methanol used as a coagulant solvent and then elongated in water. The stress-strain curves of the regenerated fibers changed dramatically depending on the draw ratio and the structure was studied by 13C CP/MAS NMR and X-ray diffraction methods. The patterns of 13C CP/MAS NMR spectra of two regenerated fibers with different draw ratios (1× and 3×) and native silk fiber are all β-sheet structure although the fraction of random coil/distorted β-turn decreases in the order of 1×, 3× and native fiber gradually. On the other hand, azimuthal scans of their X-ray fiber patterns changed remarkably with increasing the draw ratio. This indicates that long-range orientation of the fibroin chain changes remarkably during the drawing process, but the short-range local structure does not change significantly. Regenerated silk fiber with a draw ratio of 3× is a fiber with high strength which is comparable with that of natural silk fiber. The regenerated fiber is also more degradable than natural silk fiber in enzyme solution in vitro.
In this study, the strength and erosive characteristics of CaCO3 filled unsaturated polyester/glass fiber (UPR/GFR) composite are evaluated. Samples of UPR with 40, 50 and 60 wt% content of CaCO3 and different CaCO3 particle sizes of 1, 2, 3, 5 and 10 micron were prepared and tested under tensile loading, indentation and erosion conditions. The tensile strength, hardness and erosion wear rate of unsaturated polyester/glass fiber (UPR composite)/CaCO3 composite were obtained and evaluated. The results showed that the higher is the percentage of CaCO3 in the composite and the smaller is the CaCO3 particle size, the higher is the strength and the erosive resistance of the glass fiber reinforced/unsaturated polyester composite (UPR-GFR). Furthermore, the highest erosion wear rate is at 90° impingement angle. Finally the results show that the erosive wear of CaCO3 content UPR/GFR composite in a brittle manner.