All issues / Volume 4 (2010) / Issue 4 (April)
Thermoplastic vulcanizates (TPVs) based on polypropylene (PP) with ethylene octene copolymer (EOC) and ethylene propylene diene rubber (EPDM) have been developed by coagent assisted dicumyl peroxide crosslinking system. The study was pursued to explore the influence of two dissimilar polyolefin polymers (EOC and EPDM) having different molecular architectures on the state and mode of dispersion of the blend components and their effects with special reference to morphological, thermal and mechanical characteristics. The effects of dynamic crosslinking of the PP/EOC and PP/EPDM have been compared by varying the concentration of crosslinking agent and ratio of blend components. The results suggested that the uncrosslinked and dynamically crosslinked blends of PP/EOC exhibit superior mechanical properties over PP/EPDM blends. From the hystersis experiments it was found that PP/EOC blends also perform better fatigue properties over PP/EPDM based blends. It was demonstrated that, the origin of the improved mechanical properties of EOC based blends is due to the combined effect of the unique molecular architecture with the presence of smaller crystals and better interfacial interaction of EOC phase with PP as supported by the results of thermal and fatigue analyses.
The long-term viscoelastic behaviour of self-reinforced polypropylene composites (SRPPC) was studied by short-term flexural creep tests at different temperatures. As reinforcement a fabric, woven from highly stretched split PP yarns, whereas as matrix materials α and β crystal forms of isotactic PP homopolymer and random copolymer (with ethylene) were selected and used. The composite sheets were produced by film-stacking method and compression moulded at different processing temperatures (5, 20, 35°C above the melting temperatures of the matrices) keeping the holding time and pressure constant. The manufactured specimens were subjected to isothermal creep tests at different temperatures ranging from –20 to 80°C under an applied load. The time-temperature superposition principle was verified for the creep data. An Arrhenius type relationship described the shift data obtained from the creep tests. It was found, that with improving consolidation (increasing processing temperature) the creep compliance decreased and good correlation was found between creep compliance and density/peel strength.
The effect of silane treatment of carboxylic-functionalized multi-walled carbon nanotubes (COOH-MWCNTs) on the thermal properties of COOH-MWCNTs/epoxy nanocomposites was studied by comparing the research results on differential scanning calorimetry and thermogravimetric analysis data of silane treated COOH-MWCNTs/epoxy system with those of as-received COOH-MWCNTs/epoxy system. At the initial curing stage, silane treatment of COOH-MWCNTs does not change the autocatalytic cure reaction mechanism of COOH-MWCNTs/diglycidyl ether of bisphenol-A glycidol ether epoxy resin/2-ethyl-4-methylimidazole (COOH-MWCNTs/DGEBA/EMI-2,4) system, however, silane treatment of COOH-MWCNTs has catalytic effect on the curing process, which could help to shorten pre-cure time or lower pre-temperature. Then, at the later curing stage, silane treatment of COOH-MWCNTs promotes vitrification, which would help to shorten post-cure time or lower post-temperature. Therefore, overall, silane treatment of COOH-MWCNTs could bring positive effect on the processing of epoxy nanocomposites. Furthermore, it was also found that silane treatment of COOH-MWCNTs does not affect the thermal degradation pattern of COOH-MWCNTs/DGEBA/EMI-2,4 system, however, decreases the thermal stability of COOH-MWCNTs/DGEBA/EMI-2,4 nanocomposites.
Some customers are reluctant to change, because the halogen-free solutions may have higher cost. This is one of the reasons that the synergistic effect is always the subject for researchers to pursue. The synergy between sepiolite and magnesium hydroxide (MH) in halogen-free flame retardant ethylene-vinyl acetate (EVA) copolymer was investigated in the paper through some common facilities, such as limiting oxygen index (LOI), UL-94 test, thermogravimetric analysis (TGA), differential thermal analysis (DTA) and cone calorimeter test (CCT). In the wake of the positive results from the LOI and UL-94 tests, the CCT data indicated not only the reduction of heat release rate (HRR) and mass loss rates (MLR), but also prolonged ignition time (TTI) and depressed smoke release (SR) were observed during combustion. Simultaneously, the tensile strength and Young’s modulus of the system were also much better improved with the increase of sepiolite added due to the hydrogen bonds between silanol groups attached to the sepiolite molecules and the ester groups of EVA. The synergistic mechanism has been discussed in the paper in terms of the barrier mechanism in the condensed phase.
The dielectric response and conductivity of polymer matrix-titanium carbide composites was examined by means of Broadband Dielectric Spectroscopy in the frequency range of 10–1–107 Hz and over the temperature range of 40–150°C, varying the filler content. Dielectric data were analyzed via the electric modulus formalism. Recorded relaxations were attributed to interfacial polarization, glass to rubber transition and local motions of polar side groups. Alternating current conductivity varies up to seven orders of magnitude with both frequency and temperature. Direct current conductivity increases with temperature, although the rate of its alteration does not remain constant in the examined temperature range. In the low temperature region (up to 60°C) increases at a higher rate, while right afterwards approaches rather constant values. Finally, in the high temperature range (above 90°C) conductivity raises again but at a lower rate. This behaviour adds functionality to the composites’ performance and could be exploited in developing self-current regulators.
The phenomenon of Stress Oscillation (SO) was studied in syndiotactic polypropylene and syndiotactic polypropylene nanocomposites with montmorillonite. The effect was provoked by varying the crosshead speed during tensile testing of thin stripes. The internal morphology of the stress oscillated specimens was studied by scanning electron microscopy on chemically etched samples revealing that cavitation prevails inside the opaque stripes of the yielded areas. Differential scanning calorimetry proved that there exists virtually no crystallinity differentiation between the characteristic alternating opaque/transparent stripes that mark the stress oscillation during necking. Finally a simple finite element model of the necking area of the specimens revealed non-uniform internal stress distributions of yield-point magnitudes.
In the present work, the effect of pristine layered silicate montmorillonite (MMT) nanoclay on the properties of silane-crosslinked LLDPE prepared by melt compounding is investigated. The effect of the sequence of feeding additives (nanoclay and grafting agent) into the mixer on gel content, thermal and mechanical properties were studied. Results demonstrate that the sequence of feeding additives influences the final properties of nanocomposites. For samples prepared by first grafting of silane on LLDPE followed by incorporation of nanoclay into the polymer matrix, the gel content and the rate of crosslinking increased, while the elongation at break decreased. For samples prepared by first mixing nanoclay into the LLDPE matrix followed by the grafting reaction, the rate of crosslinking and the tensile properties did not change significantly. The gel content increased with increasing content of nanoclay for both process routes due to an enhanced permeation of water molecules into the polymer matrix in the presence of polar montmorillonite particles. Wide angle X-ray scattering (WAXS) results proved the intercalation/partial exfoliation morphology of nanoclay in the silane-grafted LLDPE matrix. Differential scanning calorimetry (DSC) data showed multiple melting behaviour for crosslinked samples which is indicative for different crystalline structures of the sol and gel part of the LLDPE matrix.