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Viscosity driver of coextrusion melt disturbance
April 17, 2008
In my last post, I spoke of making sure the outer layer was lower viscosity than the inner layer in a two- or three-layer coextrusion. How do I know this to be true? Well, let’s look at what is happening in the flow of a polymer melt. There is a phenomenon called encapsulation, which happens in the flow of two melts in a pipe which demonstrates the viscosity behavior or misbehavior if we don’t set about things right.
If I take two polymer streams and coextrude them with the lower viscosity material in the center of the flow (Figure 1), this is what happens. First, the center flow moves to the wall and then it begins to coat the wall until it completely surrounds the higher viscosity polymer. When this happens the pressure drop decreases, and it takes less energy to pump the two polymers through the pipe. This is why they put water in oil when they pump it as the water coats the wall of the pipe and lowers the horsepower requirements for pumping the water. This is why ethanol cannot be delivered like oil, because the water dissolves in the ethanol, and the pumps are undersized for pumping without water lubrication.
Figure 1
Now, we can look at why it happens. Figure 2 shows the viscosity as a function of shear rate for a HDPE resin. In Figure 2, we see that the viscosity drops as shear rate increases, i.e. the polymer shear thins.
Figure 2
In Figure 3, we have the velocity profile for the flow of a HDPE polymer which is perfectly homogeneous in temperature and composition.
Figure 3
Now the slope of the velocity profile is the shear rate and at the center of the flow the shear rate is zero, while at the wall the slope is highest and the shear rate at the wall is highest. Understanding that the viscosity is a maximum, when the shear rate is zero, we see clearly that for a single polymer flow, the viscosity at the wall is lower than that away from the wall because the polymer is shear thinning.
So, when we coextrude and layer polymers on purpose, we should take care to put the low-viscosity polymer on the outside, or we will get an encapsulation flow and drive a flow inversion which will give a melt disturbance of the third kind.
Posted by Eldridge M. Mount on April 17, 2008 | Comments (0)