«Viscoelastic flow effects in multilayer polymer coextrusion Dooley, J. DOI: 10.6100/IR555718 Published: 01/01/2002 Document Version Publisher’s ...»
Figure 1-24. A four-channel interfacial surface generator with vanes to control layer thickness In order to efficiently generate many layers in a multilayer structure, the style of feedblock described in Figure 1-9 could be combined with the interfacial surface generator concept described in Figure 1-24. This combination would allow the inherent advantages of each device (i.e., the many uniform layers from the feedblock and the quick multiplication of layers by the interfacial surface generator) to combine to produce many microlayers with good layer uniformity.
Another process that has been used in conjunction with the coextrusion of very thin layers is the injection molding process (30). This process has been called the Lamellar Injection Molding and uses multilayer generation in conjunction with layer multiplication prior to injection molding. Figure 1-25 shows the nozzle of an injection molding machine that can inject a multilayer structure into the mold shown that contains different geometric shapes within the molding cavity.
Figure 1-25. Lamellar Injection Molding process
Figure 1-26 shows the results of injecting a multilayer structure into the mold described above. Photographs were taken of the layered structure at each of the locations indicated in the figure. Note that the multilayer structure maintains its integrity throughout each of the geometries in the mold.
Figure 1-26. Layered structures developed in the Lamellar Injection Molding process
One difficulty that has been observed with processing polymeric structures containing very thin layers is instability in the layers near the walls of the processing equipment which can lead to the very thin layers fracturing and breaking up. If maintaining the layered structure is critical for the final properties (optical applications, for instance), even a small number of broken layers can significantly affect the final properties of the film or sheet.
One technique that has been used to minimize the amount of instability in multilayered structures with very thin layers is the use of protective boundary layers (31). Protective boundary layers are thicker layers that are added to the surface of microlayer stacks to move the very thin layers away from the wall of the processing equipment where the highest stresses on the layers occur. By moving the very thin layers into a region of the channel with lower stresses, the breaking up of the layers is avoided.
Figure 1-27 shows one method in which protective boundary layers have been added to an interfacial surface generator to minimize layer break up. This diagram shows a four-channel interfacial surface generator in which protective boundary layers are added at positions “A” and “B”. Position “A” is prior to the splitting of the multilayer structure while position “B” is near the point at which they are rejoined.
A B Figure 1-27. A four-channel interfacial surface generator with protective boundary layers Figure 1-28 shows how a protective boundary layer can be added to the multilayer structure at point “A” in Figure 1-27 prior to splitting the channel into four sections. This allows the structures in each of the four channels to have protective boundary layers so that the layers will not breakup as they flow through the device. This diagram also shows how needle valves can be used to regulate the flow of the protective boundary layer material to each section of the multilayer structure before it is split into four sections.
Figure 1-28. Protective boundary layer addition in an interfacial surface generator
Figure 1-29 shows how the protective boundary layer material is added at point “B” in Figure 1-27 as the layers are rejoined together. This application of the protective boundary layer material at this point protects the very thin layers near the wall as they flow out of the interfacial surface generator and into the die. Note in this figure how protective boundary layers are also present between the four stacks of multilayer structures. This is due to the splitting and stacking of the multilayer structure that already had protective boundary layers applied prior to the splitting.
Figure 1-29. Surface protective boundary layer addition in an interfacial surface generator
1.8 Conclusions An overview has been given on how coextrusion can be used in many different processes to form multilayered polymer structures. Uniform layer thicknesses are normally required in a coextruded structure in order to produce an optimum product. The next chapter will deal with layer deformation in coextruded structures that may produce structures that are different than what is desired.
Chapter 2 Layer deformation in coextrusion processes 2 Layer deformation in coextrusion processes
2.1 Introduction Polymer rheology information is critical for designing coextrusion dies and feedblocks.
The flow characteristics of the polymer must be considered when selecting materials for coextruded products.
Viscosities of non-Newtonian polymers are dependent on extrusion temperature and shear rate, both of which may vary within the coextrusion die. The shear rate dependence is further complicated in that it is determined by the position and thickness of a polymer layer in the melt stream. A polymer used as a thin surface layer in a coextruded product experiences higher shear rate than it would if it were positioned as a central core layer. There are several types of flow instabilities that have been observed in coextrusion.
The best designed die or feedblock does not necessarily ensure a commercially acceptable product. Layered melt streams flowing through a coextrusion die can spread nonuniformly or can become unstable leading to layer nonuniformities and even intermixing of layers under certain conditions, The causes of these instabilities are related to non-Newtonian flow properties of polymers and viscoelastic interactions.
2.2 Interfacial distortions in coextrusion 2.2.1 Interfacial distortion due to flow instability Interfacial instability is an unsteady-state process in which the interface location between layers varies locally in a transient manner. Interface distortion due to flow instability can cause thickness nonuniformities in the individual layers while still maintaining a constant thickness product. These instabilities result in irregular interfaces and even layer intermixing in severe cases.
At very low flow rates, the interface is smooth as is shown in Figure 2-1(a). At moderate output rates, low amplitude waviness of the interface is observed (see Figure 2-1(b)), which is barely noticeable to the eye and may not interfere with the functionality of the multilayer film.
At higher output rates, the layer distortion becomes more severe (Figure 2-1(c)). If a large amplitude waveform develops in the flowing multilayer stream within the die, the velocity gradient can carry the crest forward and convert it into a fold. Multiple folding results in an extremely jumbled, intermixed interface. This type of instability, commonly called zig-zag instability, has been observed in tubular-blown film dies, multimanifold dies, and feedblock /single manifold dies.
Figure 2-1. Interlayer instability
This instability develops in the die land, and its onset can be correlated with a critical interfacial shear stress for a particular polymer system (32). The most important variables influencing this instability are skin-layer viscosity, skin-to-core thickness ratio, total extrusion rate, and die gap. Although the interfacial shear stress does not cause instability, elasticity is related to shear stress, and interfacial stress is used to correlate variables for a particular system.
Interfacial instability in a number of coextruded polymer systems has been experimentally correlated with viscosity ratios and elasticity ratios (33), and a simplified rheology review has been given (34). Other studies have looked at viscosity differences (35-37), surface tension (38), critical stress levels (32, 39, 40), viscosity model parameters (41-43), and elasticity (44-52).
Other types of instabilities may exist: for example, a problem has been observed in feedblock coextrusion of axisymmetric sheet (53). A wavy interface is also characteristic of this instability but the wave pattern is more regular when viewed from the surface. The instability, commonly called wave instability, originates in the die, well ahead of the die land and internal die geometry influences both the severity and pattern. For a given die geometry the severity of instability increases with structure asymmetry and some polymers are more susceptible to unstable flow than others. It has been suggested that this type of instability may be related to the extensional rheological properties of the polymers used in the coextruded structure (54).
Examples of both zig-zag and wave instabilities are shown in Figure 2-2.
No complete predictive theory exists for these complicated rheological interactions, but the accumulated experience of polymer producers, equipment suppliers, and experienced fabricators provides guidance in polymer selection.
2.2.2 Interfacial distortion from viscosity mismatch The importance of viscosity matching for layer uniformity was first studied in capillary flow of two polymers in bicomponent fiber production (55-58). Two polymers introduced side by side into a round tube experience interfacial distortion during flow if the viscosities are mismatched. The lower viscosity polymer migrates to regions of highest shear (at the wall) and tends to encapsulate the higher viscosity polymer. It is possible for the low viscosity polymer to encapsulate the higher viscosity polymer totally. Nature seeks the path of least resistance. The degree of interfacial distortion due to viscosity mismatch depends on the extent of viscosity difference, shear rate, and residence time.
Layer nonuniformities in feedblock fed flat dies occur for the same reason when there is a large enough viscosity mismatch. Low viscosity polymer migrates to wet the die wall. For unencapsulated layers, this migration starts in the die manifold as the layered stream spreads, resulting in increased layer thickness for low viscosity polymer at the edges of the film or sheet.
If unencapsulated low viscosity polymer is a core layer, it not only becomes thicker at the edges, but may even wrap around higher viscosity skin layers at the film edges.
Tubular blown-film dies are more tolerant of viscosity mismatch because the layers are arranged concentrically, i.e., there are no ends. Since streamlines cannot cross each other, further migration cannot occur. However, good die design is required to obtain concentric layers.
2.2.3 Interface distortion from viscoelasticity While matching the viscosities of adjacent layers has proven to be very important, the effect of polymer viscoelasticity on layer thickness uniformity is also important (59-63).
It has been shown that polymers that are comparatively high in elasticity produce secondary flows normal to the primary flow direction in a die that can distort the layer interface.
This effect becomes more pronounced as the width of a flat die increases. Appropriate shaping of the die channels can minimize the effect of layer interface distortion due to elastic effects.
Coextruding a structure that contains layers of polymers with low and high levels of elasticity can cause interface distortion due to the differences in elasticity between the layers in flat dies. The effect is typically not observed in tubular dies.
2.3 Solution methods for interfacial stability problems The zig-zag type of interfacial instability can be reduced or eliminated by increasing skin-layer thickness, increasing die gap, reducing total rate, or decreasing skin polymer viscosity.
These methods may be used singly or in combination. These remedies reduce interfacial shear stress, and stable flow results when it is below the critical stress for the polymer system being coextruded. Most often skin layer polymer viscosity is decreased. In feedblock coextrusion the resultant viscosity mismatch imposed by this remedy can cause variations in layer thickness as discussed earlier. Shaped skin layer feedslots are then typically used to compensate and produce a uniform product. A review of techniques used to minimize this type of instability has been given previously (64).
Care should also be taken when designing the joining geometry in a feedblock or die. In order to minimize instabilities, the layers should have similar velocities at the merging point.
The joining of the layers should occur in a geometry that is as parallel as is realistically possible rather than joining in a perpendicular manner. The layers should also merge into a channel that is of an appropriate height that does not force one layer to flow into the other.
Wave instabilities are related to the extensional viscosities of the individual layers. This implies that all of the previously mentioned design criteria for layer joining are important for this type of instability as well. In addition, the spreading of the layers in a film or sheet die is also important. Since this type of instability is related to extensional viscosity, the rate at which the layers are stretched in the die will affect the forces in each layer. In structures containing materials with high extensional viscosities, the die should be designed to spread the layers slowly and at a uniform rate. This will help minimize wave pattern instabilities.
2.4 Conclusions An overview of layer distortions in coextruded structures has been presented. The next chapter will deal specifically with layer distortions relating to structures with different viscosities in the coextruded layers.
Chapter 3 Layer uniformity in coextrusion for structures with layers with different viscosities 3 Layer uniformity in coextrusion for structures with layers with different viscosities
3.1 Introduction Good layer uniformity in coextruded structures is usually a requirement for producing a structure with uniform properties. Many variables can affect layer uniformity. One of the most important variables is the viscosity of the polymer in each layer. This chapter will deal with coextrusion of structures with layers of different viscosities.
Visualization of viscous encapsulation3.2