Rabinowitsch expression

Polymer melts are frequendy non-Newtonian. In this case the earlier expression given for the shear rate at the capillary wall does not hold. A correction factor (3n + 1)/4n, called the Rabinowitsch correction, must be appHed in such a way that equation 21 appHes, where 7 is the tme shear rate at the wall and nis 2l power law factor (eq. 22) determined from the slope of a log—log plot of the tme shear stress at the wad, T, vs 7. For a Newtonian hquid, n = 1. A tme apparent viscosity, Tj, can be calculated from equation 23. 

The Rabinowitsch correction accounts for the fact that the true shear rate is often larger (because of shear thinning) than the apparent shear rate for non-Newtonian materials. Hence, for any non-Newtonian fluid the expression for the wall shear rate is given not by Eq. (17.11), but as... 

The analogous expressions for AP and yw for non-Newtonian fluids depend on the rheological model describing the fluid. The shear rate at the wall 7W is given by the Rabinowitsch-Mooney equation (86, 87), which in its general form is independent of the rheology of the fluid ... 

The flow behavior of non-Newtonian fluids is usually described by expressing either shear rate or viscosity as a function of shear stress. Absolute viscometers, either capillary or rotational, are used to perform the necessary measurements. In the capillary viscometer, the flow rate is measured as a function of applied pressure. Apparent viscosities calculated by means of Poiseuille s relation [Eq. (9)], are converted to true viscosities using the Weissen-berg-Rabinowitsch correction... 

In an effort to solve the Rabinowitsch and Mooney equations presented in Equations 5-15 and 5-16, Chilton and Stainsby (1998) proposed to express the pressure drop for a Herschel-Bulkley fluid as ... 

The shear rate at the die wall is expressed by the Rabinowitsch-Weissenberg [55] equation for steady laminar flow of a time-independent fluid as,... 

This rheometer is also similar to the one described in section 3.2.1 except for two differences. Firstly, the capillary used is of very short length and secondly, the polymer is extruded by the use of dead weights (i.e. constant pressure) rather than constant plunger speed. This instrument, popularly known as the Melt Flow Indexer, is very popular in the thermoplastics industry due to its ease of operation and low cost, which more than compensates for ite lack of sophistication. The parameter measured through the melt flow indexer contains mixed information of the elastic and viscous effects of ttie pol)nner. Further, no end loss corrections have been developed for this capillary equipment nor can the melt flow index be easily related to the Weissenberg-Rabinowitsch shear rate expression. 

A general expression for viscosity from capillary data is due to Weissenberg and Rabinowitsch ... 

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