8.1 Rheometry
Rheology is a study of deformation and flow. In a simplistic way the deformation may be
associated with elasticity and the flow with viscosity. Then, rheology is synonymous to
viscoelasticity. Indeed, sometimes these terms are used interchangeably. An extensive
treatment has already been given for the viscoelastic behaviour of gum rubbers and
compounds in Chapter 6 and 7.
The reasons why rheology is selected as a separate chapter is as follows: the capillary
rheometer and the rotational rheometer, which had originally been designed for the
rheological measurements of liquids have been used for the observation of gum rubber
and compound behaviour. The question is what these measurements really mean, because
gum rubbers as well as compounds are not liquids but they are in the rubbery state.
However, in this chapter, the conventional practice of treating the material as if it were
liquid is followed. Not only is the viscosity-shear rate relationship discussed but also the
melt fracture, extrudate swell and slip. Shown in Figure 8.1 are ‘flow curves’ of NBR
samples, A, B, C, and D at 100 °C [1].
These curves are very much like the ones obtained with thermoplastic melts, exhibiting a
non-Newtonian behaviour. The only difference is that with the plastic melt the curves
become flatter, decreasing the negative slope with the decreasing shear rate. This suggests
that if the lower shear-rate-data were obtained, the extension of the curves of Figure 8.1
would become like those of the thermoplastics.
From this discussion the question arises, of whether or not the flow mechanism is the
same for plastic melt and rubber throughout the shear-rate range of non-Newtonian
flow. As pointed out with the curve A, the melt fracture occurs at a shear rate higher
than that indicated by the arrow. With the other three samples the melt fracture occurred
over the entire range of the shear rate. The appearance of the extrudates is depicted in
Figure 8.2. Onset of the melt fracture suggests some sort of a discontinuity.
المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .
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