«Viscoelastic flow effects in multilayer polymer coextrusion Dooley, J. DOI: 10.6100/IR555718 Published: 01/01/2002 Document Version Publisher’s ...»
For a typical experiment, the coextrusion line was run for a minimum of thirty minutes to ensure that steady-state conditions had been reached. The normal extrusion rate was approximately 8.6 kg/hr for the polystyrene resin that would give a wall shear rate in the range of 30 to 40 reciprocal seconds. Both the polystyrene and polyethylene resins were extruded at 204oC while the polycarbonate resin was extruded at 260oC. Variable-depth thermocouples were placed in the melt streams from each extruder just prior to the entry into the feedblock to ensure that the temperatures of the materials were the same prior to being joined together. When steady state was reached, the extruders were stopped simultaneously and the coextruded material was cooled while still in the die channel. After it had cooled to room temperature, the frozen polymer "heel" was removed from the die and examined. This procedure allowed the major deformations of the interface to be analyzed.
4.4 Elastic layer rearrangement in a square channel
The initial experiment conducted consisted of extruding the two-layer coextruded structure through the feedblock with no die channel attached. This experiment was run to ensure that the 80/20 (substrate layer thickness/cap layer thickness) structure was produced with a flat interface prior to introducing the coextruded structure into a die channel. The coextruded structure produced is shown in Figure 4-3. This figure shows that an 80/20 structure was indeed produced with a flat interface produced between the two layers.
The next experiment consisted of running an 80/20 structure of polycarbonate resin into the square channel geometry die. The structure produced near the exit of the channel is shown in Figure 4-4. This figure shows that the interface in the polycarbonate structure is fairly flat and very little deformation has occurred.
Figure 4-4. Two-layer polycarbonate structure near the exit of the square channel When a two-layered polystyrene structure was extruded through the square channel die, the resulting structure produced is shown in Figure 4-5. This structure is significantly different than that produced by the polycarbonate resin since there is extensive deformation of the interface. This structure shows that material has flowed up along the die walls to the corners and then it turns and flows toward the center of the channel. Simultaneously, material that was nearer the center of the channel is being pushed up towards the top of the channel.
Figure 4-5. Two-layer polystyrene structure near the exit of the square channel
This interface shape is obviously not due to viscous encapsulation since identical materials are present in each layer so there can be no difference in viscosity to drive the viscous encapsulation. It is hypothesized that this interface shape is the result of elastic forces that produce secondary flows in the square geometry. These secondary flows would be present in a direction perpendicular to the main flow direction and be driven by second normal stress differences. These secondary flows have been discussed previously for polymer flows by White (56, 66) and coworkers. They noted that second normal stress differences could influence the interface shape along with the viscosity difference. They showed experimentally that in bicomponent tube flow, the less viscous layer always encapsulated the more viscous layer regardless of the first and second normal stress differences between the two materials. It should be noted, however, that all of their studies were done in channels with circular cross sections.
However, since the secondary normal forces are small and difficult to measure, it has been assumed in the past that they could be ignored (67). Studies have been done that show that these secondary flows are produced by differences in the normal forces (61-63) and are not due to viscous effects. These differences in normal forces are produced when a viscoelastic material flows through a non-radially symmetric channel.
Figure 4-6 shows the predicted secondary flows in a square channel for an elastic material. The predicted flow patterns for a viscoelastic fluid flowing through a square channel have been shown previously to contain eight recirculation zones or vortices, two each per quadrant (61-63, 68-72) as is shown in Figure 4-6. These flow patterns appear to correspond well with the interface deformation shown for the polystyrene resin in Figure 4-5. These flows would cause material to move up the walls and then turn back towards the center of the channel.
These secondary flows would also cause the material near the center to be pushed upwards toward the top of the channel. Numerical predictions of these secondary flow patterns will be covered in a subsequent chapter.
Figure 4-6. Secondary flow patterns for an elastic material in a square channel
One interesting aspect of the elastic layer rearrangement shown in Figure 4-5 is that it is a phenomenon that continues indefinitely as an elastic material flows down a square channel. This is different from viscous encapsulation since the driving force for viscous encapsulation tends to decrease as the materials flow down the channel as the less viscous material encapsulates the more viscous material and an energetically preferred state is reached. The constant layer rearrangement in an elastic material is shown in Figure 4-7 for a two-layer polystyrene structure.
This figure shows the progression of the elastic layer rearrangement as the structure flows down the channel by showing cuts at axial distances from the entry of 5, 20, 30, 40, 50, and 58 cm.
The cut in the upper left hand corner was taken from the sample near the entry to the square channel while the cut in the lower right hand corner was taken from the sample near the exit of the square channel. This figure shows the steady deformation of the layer interface as it progresses down the channel.
Figure 4-7. Progression of a two-layer polystyrene structure as it flows down a square channel Figure 4-8 shows a sample taken near the exit of the square channel for a two-layered structure composed of polyethylene. This figure shows results similar to those obtained for the polystyrene resin. However, the layer rearrangement for the polyethylene sample is not quite as extensive as the deformation in the polystyrene sample. This is consistent with the results of the measurements of the resins’ storage moduli. If the storage modulus is used as an indication of elasticity of the resin, the polystyrene is the most elastic, the polyethylene is of intermediate elasticity, and the polycarbonate is the least elastic. This order of level of elasticity also corresponds to the amount of layer rearrangement observed in these samples with polystyrene showing the most rearrangement and the polycarbonate the least amount of layer rearrangement.
Figure 4-8. Two-layer polyethylene structure near the exit of the square channel
4.5 Elastic layer rearrangement in a circular channel As described in the previous section, the layer rearrangements observed were hypothesized to be driven by secondary flows in the square channel caused by second normal stress differences. One way to test this hypothesis would be to extrude the materials through a channel with a radially symmetric geometry that would produce no secondary flows. This was done by extruding the three resins through a die with a circular cross-section. Very little layer rearrangement was observed in each case as compared to the rearrangement seen in the square and teardrop channels. Figure 4-9 shows a cross-sectional cut of the polyethylene resin near the exit of the die with the circular cross-section. The layer interface location in this sample is very similar to the interface location observed at the beginning of the channel implying that the interface did not move substantially as the structure flowed down the channel. Similar behavior was also observed for the polycarbonate and polystyrene resins even though they have substantially different viscoelastic properties compared to the polyethylene resin.
Figure 4-9. Two-layer polyethylene structure near the exit of the circular channel
4.6 Elastic layer rearrangement in a rectangular channel One question that arises from the experimental studies in the square channel is what effect would changing from a square geometry to a rectangular geometry have on the location and magnitude of the secondary flows. This was studied experimentally by building a rectangular channel with an aspect ratio (width to height) of 4 to 1.
Figure 4-10 shows a sample cut from near the exit of the 4:1 rectangular channel when running polycarbonate resin in each layer. As can be seen in the figure, there is very little deformation in the layer interface over most of the width of the sample, as was true in the square channel as well.
Figure 4-10. Two-layer polycarbonate structure near the exit of the rectangular channel Figure 4-11 shows the results when processing a polyethylene structure through the rectangular die channel. This figure shows that significant layer rearrangement has again occurred in the polyethylene sample, as was the case in the square channel. However, it can be seen that the interface remains fairly flat over a significant portion of the center of the rectangular channel, which did not occur in the square channel. The large layer rearrangement in the rectangular channel occurs primarily near the edges of the channel. The same general flow patterns that were observed in the square channel are also present in the rectangle as is evidenced by the flow up the walls along the edges of the channel and the upward movement on the substrate layer near the center of the channel.
Figure 4-11. Two-layer polyethylene structure near the exit of the rectangular channel Figure 4-12 shows the progression of the layer rearrangement as the polystyrene structure flows down the rectangular channel from left (near the channel entrance) to right (near the channel exit). This progression is similar but not identical to the progression seen in the square channel for polystyrene (Figure 4-7). This figures shows that the elastic layer rearrangement continues as the structure flows down the channel. It also shows that the secondary flows are affected by the change in aspect ratio from 1 for the square to 4 for the rectangle. The recirculation zones (which produce layer rearrangement) observed in the rectangular channel for the polystyrene resin are similar to those seen in the square channel but are elongated along the major axis (channel width direction) of the rectangle. This channel geometry appears to cause the vortex associated with the major axis to increase in size while decreasing the minor axis vortex, correspondingly. Although the vortices in the rectangular channel have shifted positions compared to those in the square channel, they are still present in the polystyrene sample. The main difference between the square and rectangular samples appears to be the location and magnitude of the vortices found in each quadrant. However, a substantial portion of the rectangular channel near the center maintains a flat layer interface in contrast to the square channel. This implies that rectangular channels with higher aspect ratios will give more uniform layers than channels with lower aspect ratios near unity. Like previous results, the polycarbonate resin did not show much movement of the layers.
Figure 4-13 shows the progression of the layer rearrangement for a polyethylene structure flowing down the rectangular channel from left (near the channel entrance) to right (near the channel exit). This progression is similar to the flow seen in the rectangular channel for polystyrene resin (Figure 4-12). The polyethylene sample once again shows the secondary flows in the corner of the channel. This sample also shows how the white cap layer flows along the centerline back towards the center of the channel.
4.7 Elastic layer rearrangement in a teardrop shaped channel In experiments similar to those done with the square and rectangular channels, two layered structures were also processed through a teardrop shaped channel. The flow through a teardrop shaped channel is very important industrially because many commercial film and sheet dies use teardrop shaped channels in their distribution manifolds. Originally, many of the manifolds in film and sheet dies were circular in cross-sectional shape, as is shown in Figure 4a). However, the transition from a circular manifold to a rectangular shaped land region caused some flow difficulties because of the abrupt change in geometry, especially in coextruded structures. This difficulty was overcome by using a tapered transition from the circular manifold to the rectangular land area, thus producing a teardrop shaped channel. Figure 4-14(b) shows the area of the channel that would be removed to produce the shape in Figure 4-14(c). Because the cross-sectional shape shown in Figure 4-14(c) is difficult to cut across the entire width of a large distribution manifold, many times the back of the manifold (in the circular section) is cut with a flat section as is shown in Figure 4-14(d). This cross-sectional shape is the familiar teardrop shape that is used in many film and sheet die distribution manifolds.
Figure 4-14. Die manifold geometry development from circular to teardrop shape Figure 4-15 shows the location of the interface near the exit of the teardrop channel for the polycarbonate resin. This sample shows a smooth interface with very little layer movement as the material flows down the channel. In contrast, a section cut from the two-layer polyethylene sample near the exit of the die is shown in Figure 4-16 while a cut from the twolayer polystyrene sample is shown in Figure 4-17.
Figure 4-15. Two-layer polycarbonate structure near the exit of the teardrop channel Figure 4-16. Two-layer polyethylene structure near the exit of the teardrop channel Figure 4-17. Two-layer polystyrene structure near the exit of the teardrop channel The results obtained from the teardrop channel are consistent with those seen in the square channel in that the most layer movement was observed in the polystyrene sample, the least movement in the polycarbonate sample, and the polyethylene sample was intermediate.