olzstreams.blogg.se - Calculating intrinsic viscosity from melt flow index

At apparent shear rates of 60 sec -1 and below, the flow and the pressure measurements are completely normal.

For apparent shear rates of 100 and 80 sec -1, we observed a stick-slip phenomenon on the extruded strands, which corresponds to the massive pressure oscillations recorded with the rheometer.

For high apparent shear rates from 2000 to 200 sec -1, the flow is slightly erratic and unstable as shown by the slight instability in pressure measurements.

The pressure measurements show very interesting patterns: The model predicts a pressure drop of about 6 MPa (870 psi), and a temperature increase of about 4° C (7.2 ° F) at the outlet of the mixer.įIG 5 Pressure measurements as a function of time at several different shear ratesįrom a capillary rheometer, showing severe flow instability for a highly filled compound. Figure 2 shows CFD results for the LDPE grade pressure through the mixer and resulting melt temperature change due to shear heating.

1) were designed and optimized for LDPE, which is a very shear-thinning polymer due to long-chain branching and broad MWD. To illustrate the last point, we calculated melt-temperature predictions with Computational Fluid Dynamics (CFD) in the mixing section of a single-screw extruder for two polymers with comparable viscosity, but vastly different shear-thinning behaviors. In particular, the shape of the viscosity curve as a function of shear rate has an effect on the flow distribution from an extrusion die, the melt pressure through a die, and melt temperature at the tip of a screw. Understanding how a given polymer grade shear-thins can help processors understand many extrusion issues, and therefore it is critical to learn how to “read” a viscosity curve.

For instance, a linear polymer like LLDPE will tend to shear-thin much less than a polymer with long-chain branching like LDPE. This viscosity decrease is called shear thinning, and it depends on polymer structure and molecular-weight distribution (MWD). Polymer melts are non-Newtonian fluids and exhibit a decrease in viscosity at an increasing shear rate. Recycled Polyamide for Consumer Goods and Engineering Plastics