Due to the multiplying effect of power-law coefficient and consistency index, an HDPE and a PP at identical shear rates and slightly different temperatures can have a difference in viscosity where the HDPE is three times as viscous as the PP. Melt-index and melt-flow measurements by capillary rheometer are at very low shear rates, where shear thinning is almost non-existent. As the list of polymers in the table shows, the calculated viscosities from the chart at shear rates of 100 sec -1 do not fall in the same order as the power-law coefficients because of that consistency-index effect.Īs a result, two polymers of similar melt index or melt flow can have vastly different viscosities at the elevated shear rates during processing.
The shear-thinning characteristics of various polymers are often categorized solely by the power-law coefficient, but the consistency index can have just as significant effect on the final viscosity and has to be considered.
