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Edson Jose Soares, Roney Leon Thompson, Andre Machado
Measuring the yielding of waxy crude oils considering its time-dependency and apparent-yield-stress nature
Appl. Rheol. 23:6 (2013) 62798 (11 pages) ►

Production in reservoirs located in deep and ultra-deep water that contain waxy crude oils faces a huge obstacle imposed by the low temperatures of the environment. When the waxy crude oil is subjected to a temperature below the Gelation Temperature, as in the case investigated in the present work, it exhibits a variety of non-Newtonian features: elasticity, plasticity, viscous effects, and time-dependency, which renders to this material a highly complex behavior. A crucial feature that is frequently ignored when the determination of the yield stress is being carried out, is the time-dependency nature of these materials. We demonstrate how significantly different values of yield stress can be obtained if this character of the material is neglected. We use the asphaltenes properties as inhibitors of wax formation and propose a protocol to capture yield-stress parameters. One important conclusion is that waxy crude oils can be classified as apparent-yield-stress fluids, and not (true-) yield-stress materials, with the presence of a dynamic and a static yield-stresses.

► Cite this publication as follows:
Soares EJ, Thompson RL, Machado A: Measuring the yielding of waxy crude oils considering its time-dependency and apparent-yield-stress nature, Appl. Rheol. 23 (2013) 62798.

Joana Peralta, Loic Hilliou, Hugo Silva, Ana Machado, Jorge Pais, Joel Oliveira
Rheological Quantification of Bitumen Aging: Definition of a New Sensitive Parameter
Appl. Rheol. 20:6 (2010) 63293 (12 pages) ►

Bitumen is undoubtedly the most important material in the construction and rehabilitation of flexible road pavements. By increasing the temperature, bitumen changes from brittle solid, to viscoelastic solid and finally to Newtonian fluid. The rheological characteristics of bitumen also vary greatly due to aging,which is a phenomenon initiated in the phases of production and application of bituminous (asphalt) mixtures and continued during the life of road pavements. The aim of this work is to study several rheological parameters in order to evaluate if they are able to quantify the aging of various types of bitumens. Four bitumens from the same distillation column in the refinery, but with different penetration grades, were aged by using the RTFOT method, which simulates the aging of the bitumen during the asphalt mixture production and pavement construction. The original (base) and aged bitumens were characterized with conventional tests used in the paving industry, and the results were compared with the rheological characteristics obtained with small amplitude oscillatory shear tests. Aging was assessed at high temperatures (110 to 180^oC) through the activation energy computed from the temperature dependence of the Newtonian viscosity. However, the comparison of the characteristic relaxation times extracted from master curves measured at medium/low temperatures (between 25 to 80^oC), proved to be the most sensitive indicator of bitumen aging.

► Cite this publication as follows:
Peralta J, Hilliou L, Silva H, Machado A, Pais J, Oliveira J: Rheological Quantification of Bitumen Aging: Definition of a New Sensitive Parameter, Appl. Rheol. 20 (2010) 63293.

J.M.Maia, O.S.Carneiro, A.V.Machado, J.A.Covas
On-Line Rheometry for Twin-Screw Extrusion (Along the Extruder) and its Applications
Appl. Rheol. 12:1 (2002) 18-24 ►

Due to a number of practical difficulties, both in- and on-line measurements of the rheological properties of complex systems during extrusion are usually performed at the end of the extruder, under very specific experimental conditions. This makes this type of instruments more useful for quality control than for process optimisation, since information about the influence of the geometry and/or processing conditions on the evolution of the material characteristics inside the extruder is not easily gathered. Recently, however, the authors have developed an on-line capillary rheometry system that overcomes most of the existing problems and allows small amounts of sample to be tested in very near real time, along the extruder. The present work aims at illustrating the usefulness of this concept for the study of physical compounding processes and some reactive systems. Two very different systems will be used for that purpose: a reactive extrusion process (the peroxide-induced thermal degradation of polypropylene) and the dispersive mixing involved in the preparation of thermoplastic/carbon fibre composites.

► Cite this publication as follows:
Maia JM, Carneiro OS, Machado AV, Covas JA: On-Line Rheometry for Twin-Screw Extrusion (Along the Extruder) and its Applications, Appl. Rheol. 12 (2002) 18.

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