Measurement of the flow of molten polymers through short capillaries

Abstract

A method of treating experimental flow measurements on high polymers to obtain the basic shear diagram is described. Shear stress data are corrected to eliminate capillary end effects, and shear rate data are modified for the non‐Newtonian behavior of the materials. Differences in flow behavior of commercial low‐density polyethylenes are illustrated, and the effects of the two corrections are demonstrated. The method is applicable to other polymer melts. Although the data could be expressed by the power law, the fact that logarithmic plots of shear stress versus shear rate are not linear suggests that such relationships would be approximate. The data are expressed better by the threeconstant Powell‐Eyring equation, although the significance of the three parameters is uncertain. The need to obtain the rheological constants from the basic shear diagram rather than from an uncorrected or partially corrected capillary flow diagram is emphasized. In support of previous investigators, irregularities in the extrudate were observed at high shear stresses. However, it was not possible to determine at what pressure the roughness was initiated. The shear diagrams calculated from the experimental data do not exhibit any point of inflection as reported in some studies.

Engineering Analysis of Soy Dough Rheology in Extrusion

“…(1957) and Metzger and Brodkey (1963) enables the calculation of end correction .for capillary flow of non-Newtonian molten polymers. This method illustrates the correction of entrance effects and allows the comparison of data obtained from using different capillaries.…”

Steady State and Dynamic Oscillatory Shear Properties of Carbon Black Filled Elastomers

“…70,72 The correction method was used successfully in evaluating many polymer melts and solutions. [73][74][75][76][77][78][79][80] processes that occur during transit through the capillary. 81 If transit time was short in comparison to the relaxation processes, then pressure as a function of L/R would be linear.…”

“…70,105 These methods have proven to be successful for many polymer melts and solutions. [73][74][75][76][77][78][79] Pure rubbers exhibit a Newtonian plateau at low shear rates and shear thinning behavior as the shear rate is increased. 2,88 Highly loaded rubbers, with volume loading greater than 15%, show signs of a yield stress.…”

A Capillary Viscometer for Non-Newtonian Liquids

“…Unfortunately this law is not based on rheological fundamentals and is a purely empirical concept. In addition, the exponent n may change with rate of shear, as confirmed by some investigators (15,26). Thus, when viscometric data are needed for non-Newtonian liquids, a complete flow curve should be shown which includes at least the range of interest, although for future purposes, data beyond that range are preferable.…”

“…It may affect the readings and its extreme values should be estimated P C" (dT/dt) = Vap-D2 p Q.CPAT = VwDW where V is volume of capillary. ~CV Re (15) The mean value of shear rate should be taken. Therefore, if we design Dw for shear rate at the wall, an approximate value of Dx/2 is preferred.…”