«Viscoelastic flow effects in multilayer polymer coextrusion Dooley, J. DOI: 10.6100/IR555718 Published: 01/01/2002 Document Version Publisher’s ...»
The teardrop geometry, however, produced only six vortices or recirculation zones (58, 59) as compared to the eight observed in the square channel. This produced a flow pattern that is symmetric only about the centerline of the channel and is therefore different than the flow patterns seen in the square and rectangular channels. It appears from these results that a teardrop shaped channel produces a coextruded structure with more layer thickness non-uniformity than a rectangular channel. These results are, however, dependent on the comparative aspect ratios of the rectangular and teardrop channels. These results are very significant from a commercial perspective since a large portion of the sheet and film dies produced in the industry contain teardrop shaped distribution manifolds.
4.8 Elastic layer rearrangement in a rectangular manifold channel
One half of the rectangular manifold channel is shown in Figure 4-18 along with a polymer sample that had been solidified in the channel. The rectangular manifold channel was designed based on principles used in the design of coat-hanger style die manifolds to produce uniform output across the die exit and is meant to simulate half of a coat-hanger style die. The rectangular distribution manifold maintains a constant 4:1 aspect ratio down its entire length and starts with dimensions identical to those of the rectangular channel. This allows direct comparisons to be made between the two channels. The axial length of the die channel was
60.96 cm. This die channel was fabricated in two halves and bolted together so that it could easily be split apart for removal of the polymer sample.
Figure 4-18. Rectangular manifold die channel and solid polymer sample
When the three resins were extruded through the rectangular manifold channel, a significant amount of layer rearrangement occurred for the polystyrene and polyethylene samples but not in the polycarbonate sample. Figures 4-19, 4-20, and 4-21 show a series of crosssectional cuts made at 7.6 cm intervals down the rectangular manifold channel for the two-layer polycarbonate, polyethylene, and polystyrene samples, respectively. Again the cut on the left was made nearest to the feedblock while the cut on the right was near the end of the manifold.
The flow patterns for the polycarbonate sample in the rectangular manifold channel are shown in Figure 4-19. Note that the layer ratio at the entrance of the manifold is 80/20 (substrate / cap layer) for the two-layer sample and remains constant down the channel. The layer interface between the cap and substrate layer does remain substantially parallel as the flow progresses down the channel. This can be contrasted with the results of the two-layer polyethylene and polystyrene samples as are shown in Figures 4-20 and 4-21. In both of these cases, the interfaces do not remain flat but are distorted at the top of the manifold channels. Note also that although the layer ratio at the entrance of the manifolds is 80/20 (substrate / cap layer) for the two-layer samples, the ratio is approximately 50/50 by the time the structures reach the ends of the manifolds.
It appears that the layer profiles seen in the polyethylene and polystyrene resins in the rectangular manifold channel are produced by a combination of the vortices formed in the corners of a rectangular channel and the leakage of the material out the bottom of the channel.
The movements of layers in the top corners of the manifold are very similar to the movements seen in the rectangular channel shown in Figure 4-12. However, it appears that the further the material moves down the channel, the leakage flow becomes a larger percentage of the flow and pulls the layers that have moved in the upper section of the channel down towards the bottom of the channel. These figures show that the leakage flow comes primarily from the lower part of the channel. This combination of flow patterns will produce a fairly uniform layer pattern near the entry of the manifold but the layers near the end of the channel will be very distorted. This may be one mechanism to explain why coextruded structures of polymers with matching viscosities can sometimes produce products with non-uniform layer thicknesses. This could also explain why measurements of individual layer thicknesses in sheet products tend to show poorer distribution near the edges of the sheet compared to the center.
These results have many implications for the commercial coextrusion of viscoelastic polymers. Since the layer movements observed occurred without any differences in viscosity between the layers, this implies that layer rearrangement can occur in coextrusions in which the polymer viscosities are well matched. Also, since the layers continue to rearrange as the polymer flows down the channel, these results also imply that this effect will become more pronounced in sheet dies as they are scaled up to larger widths. The die used in these experiments would represent a die with a width of approximately 1.2 meters. As discussed earlier, commercial cast film dies have been built with widths up to 10 meters, or approximately eight times as wide as the die used in these experiments!
Chapter 5 Analyzing flow patterns during elastic layer rearrangement using different feedblock configurations 5 Analyzing flow patterns during elastic layer rearrangement using different feedblock configurations
5.1 Introduction The previous section dealt with the flow of two-layered structures through dies with different cross-sectional geometries. The two-layer structures show the interface deformations for the three materials studied but since there is only one interface, some interpretation of the experimental results is needed to understand how these results can be extended to cover the entire flow field in a particular geometry. For instance, Figure 4-7 shows the interface deformation for a two-layer polystyrene structure as it flows down the square channel. This figure certainly shows the layer movement in the upper half of the sample, but since the entire lower half is pigmented black, no additional information can be gained from this portion of the sample.
In order to address this lack of information about the rest of the flow area, samples were made in which a different feedblocks were developed that produced a variety of structures with alternating layers of the same material that were pigmented black and white. Among those feedblock designs were styles that produced multiple parallel layers (2, 27 and 165 layers), a matrix of 49 strands, and concentric rings with 2 or 13 layers. Schematic diagrams of the structures are shown in Figure 5-1.
Figure 5-1. Schematic diagrams of structures produced by different feedblock designs
5.2 Parallel layer feedblocks (27 and 165 layers) Samples were made in a unique feedblock that produced a structure with 27 alternating layers of the same material that were pigmented black and white for comparison to the 2 layered samples. A schematic diagram of a feedblock that was similar in style to the one used to produce this structure was shown previously in Figure 1-9. These 27-layered structures contained many more interfaces that could be examined so that more insight could be obtained on the total flow field. Figure 5-2 shows just such a structure containing twenty-seven polystyrene layers near the entry and exit of the square channel. By increasing the number of layers, it is obvious that the flow patterns observed in the upper half of the two-layered structure also occur in the lower half.
This many layers make it easier to observe the symmetry of the flow in the square channel.
Figure 5-2. Twenty-seven-layer polystyrene structure near the entry and exit of the square channel Since increasing the number of layers from 2 to 27 gave more information on the flow field, the next logical step would be to increase the number of layers even further to gain even more information. This was done by developing a feedblock that could produce coextruded structures with 165 alternating layers of the same material that could be pigmented to show the interfaces. A schematic diagram of the style of feedblock used to produce this structure was shown previously in Figure 1-9. An example of this structure at the entry and exit of the square channel when processing polystyrene is shown in Figure 5-3. Note that at the entry of the channel, all 165 layers are fairly uniform and parallel. Near the exit of the channel, these layers have rearranged significantly and the symmetry of the flow perpendicular to the main flow direction is obvious. Comparing Figure 5-3 with the predicted secondary flows shown in Figure 4-6 makes it much easier to visualize the secondary flow patterns. When compared to a two-layer structure (Figure 4-7) the 165-layer structure gives much more detail on the flow pattern in the channel.
Figure 5-3. One hundred and sixty five-layer polystyrene structure near the entry and exit of the square channel One important observation of the 165-layer structure is that some of the details of the flow in the channel are lost due to thinning of the layers. The thin, parallel layers at the beginning of the channel are thinned further and distorted by the layer rearrangement taking place in the channel and so some of the resolution of the layer interfaces is lost due to the extreme thinness of the layers. Structures with greater than 5000 alternating layers were produced to study the phenomenon of elastic layer rearrangement but details of the flow in the channel were not as easy to determine because of the extreme thinness of the individual layers.
Figure 5-4 shows a photomicrograph of approximately 100 layers of a structure that contained over five thousand layers. Note how thin the layers are in this 100-layer section. If the entire structure was shown in this illustration, these layers would be approximately fifty times thinner than they appear in this photomicrograph. This illustrates how too many layers can reduce the optical resolution of the system when trying to trace layer movement.
Figure 5-4. One hundred layers of a five thousand-layered structure It appears that 165 layers provide enough information on the entire flow field that structures with more layers are not necessary or desirable due to a lack of optical resolution.
Since the 165-layer structures give more information than the 2 or 27-layer structures but do not suffer the optical resolution problems of the 5000-layer structures, they will be used in the remainder of this section to illustrate the flow patterns in the various channel geometries.
Figure 5-5 shows the 165-layer polystyrene structure near the exit of the rectangular channel. This figure shows a similar flow pattern as was seen in Figure 4-12 for the two-layer polystyrene sample but gives more detail. The 165 layers show the extreme symmetry of this flow field. Note that the recirculation zones near the ends of the channel are small in comparison to those seen in the square channel. It should also be seen that the layers near the center of the channel remain parallel over a substantial width of the channel. This implies that a rectangular channel would be superior to a square channel for maintaining parallel layers in a coextruded structure.
Figure 5-5. One hundred and sixty five-layer polystyrene structure near the exit of the rectangular channel Figure 5-6 shows the 165-layer polystyrene structure near the exit of the teardrop channel. This figure also shows the symmetry that is evident in this flow field that could not be observed in the two-layer structure. It appears that there are only 6 recirculation zones in the teardrop channel compared to the 8 recirculation zones in the square channel. This figure also shows much more distortion in the layers than was present in the rectangular channel (Figure 5This implies that the rectangular channel would produce more uniform coextruded structures than the teardrop channel. This is significant since many of the commercial scale coextrusion dies contain teardrop shaped distribution manifold channels.
Figure 5-6. One hundred and sixty five-layer polystyrene structure near the exit of the teardrop channel Figure 5-7 shows a series of cross-sectional cuts made at 7.6 cm intervals down the rectangular manifold channel for the 165-layer polystyrene sample. The cut on the left was made nearest to the feedblock while the cut on the right was near the end of the manifold. This figure shows more of the details of the flow field than were seen in the two-layer structure (Figure 4Figure 5-8 shows an expanded view of a few of the cross-sectional cuts shown in Figure 5
Figure 5-8. Progression of a one hundred and sixty five-layer polystyrene structure as it flows down a rectangular manifold channel (expanded view) As was observed in the two-layer structure, it appears that the layer profiles seen in the polystyrene structure in the rectangular manifold channel are produced by a combination of the vortices formed in the corners of a rectangular channel and the leakage of the material out the bottom of the channel. The movements of layers in the top corners of the manifold are very similar to the movements seen in the rectangular channel shown in Figure 4-21. However, it appears that the further the material moves down the channel, the leakage flow becomes a larger percentage of the flow and pulls the layers that have moved in the upper section of the channel down towards the bottom of the channel. This figure also shows that the leakage flow comes primarily from the lower part of the rectangular channel. This combination of flow patterns will produce a fairly uniform layer pattern near the entry of the manifold but the layers near the end of the channel will be very distorted.
5.3 Multi-strand feedblock