Bagley correction factor

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The Bagley correction factor m has to be determined in eiqierinients with capillaries of varying L/R. Its value falls in the range 1 < m < IS. 

Aside from the change in polydispersity, which was also observed by Moss, the greatest effect of multiple passes was observed in the swell and ESCR properties. The change in viscosity, as measured with a capillary rheometer, showed little effect regardless of the material. However, the complex viscosity at the lower frequencies and the elongational viscosity, as determined from the Bagley correction factor, did show some effect from multiple passes. 

If the capillary rheometer is used to compare different polymers, it is not necessary to go through the various correction procedures. However, if one wants to know the absolute values of the viscosity, it is important to apply the various correction factors. The most important corrections are the correction of the shear rate for non-Newtonian fluid behavior (often referred to as Rabinowitsch correction) and the correction of the shear stress for entrance effects (often referred to as Bagley correction). These are the most common corrections applied to capillary rheometers. Other corrections that are sometimes considered are corrections for viscous heating, corrections for the effect of pressure on viscosity, corrections for compressibility, correction for time effects, etc. If many corrections are applied to the data, the whole measurement and data analysis procedure can become very complex and time consuming. 

Figure 21. Bagley pressure correction term, Pe, versus shear stress, ai2 L-1, and BL-2. Data for BL-1 and BL-2 have been displaced horizontally by a factor of 5 and 10 respectively.

Shear viscosity-shear rate curves were measured using a Goettfert Rheograph 6000 triple bore capillary rheometer at 210°C. A die of 30/1 aspect ratio and 180° entrance angle was used. Apparent values were determined by controlling the drive speed and recording the entrance pressure. No corrections were made to the data to determine the Bagley and Rabinowitch factors. 

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