The transient normal force response in a concentrated suspension of spherical particles upon startup of shear, following a period of rest, was found to depend on the direction in which shear was restarted. When shear was restarted in the same direction, the normal force signal rapidly grew to its positive steady-state value. However, when shear was restarted in the opposite direction, the normal force signal was initially negative and decreased, within the response time of the instrument, to a negative minimum, from which it gradually increased, passing through zero, to its positive steady-state value. This is believed to be the first experimental confirmation of this phenomenon, which had been suggested by the numerical simulations of previous investigators.

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ABSTRACT:The high rate shear flow behavior and the morphology of five different oil-extended polypropylene (PP)/ethylene-propylene-diene monomer (EPDM) thermoplastic vulcanizate blends were investigated with the melt flow rate (MFR) of the PP varying from 0.7 to 20. The ratio of rubber to PP is 70 : 30 in three of the thermoplastic vulcanizates (TPVs) and 50 : 50 in the other two TPVs. The distribution of the high-temperature oil between the PP melt and the rubber is a key parameter because this will affect the viscosity of the PP/oil medium. The object of this study was to estimate the matrix composition in each of the TPVs at processing temperatures and to compare the shear viscosity of the effective matrix with that of the TPV. To this end, several PP/oil mixtures were prepared and their viscosity curves were correlated with the neat PP melt viscosity curves by means of shift factors varying with oil concentration. The oil distribution between the PP and rubber phases was estimated from TEM micrographs of the TPV blends. The results show that the PPs are mixed with oil to different proportions in the different TPVs and the viscosity curves of these mixtures exhibit the same trends in magnitude as the corresponding TPV viscosity curves. Hence, the shear flow of TPVs can be understood more readily in terms of the effective PP/oil medium flow behavior than in terms of the neat PP melt flow.

Accurate rheological measurements are needed in order to correlate the rheological behavior of solder pastes to their performance in the surface mount technology (SMT) process. In this paper, we summarize our efforts to measure the rheological properties of solder pastes, and outline the difficulties in obtaining their true rheological properties. In particular, we show that the rheological measurements of solder pastes are affected by "slip" at the test-apparatus surfaces and by shear fracture within the sample which have not been taken into consideration heretofore.

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