Layer Nonuniformity in Coextrusion Processes

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. The viscosities of non-Newtonian polymers depend on the extrusion temperature and shear rate, both of which are factors that may vary within the coextrusion die. The shear rate dependence is further complicated because it is determined by the position and thickness of a polymer layer in the melt stream. The die or feedblock that has the best design does not necessarily ensure a commercially acceptable product. Layered melt streams flowing through a coextrusion die can spread nonuniformly or can become unstable, which can lead to layer nonuniformities — and even intermixing of layers — under certain conditions.

The causes of layer deformation are related to non-Newtonian flow properties of polymers and viscoelastic interactions. Previous studies have shown that variations in layer thicknesses during coextrusion processes can arise from many causes, with several of the primary ones being interlayer instability, viscous encapsulation, and elastic layer rearrangement.

1.2.3.1 Interlayer 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

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instability can cause thickness nonuniformities in the individual layers while still maintaining a product that has constant thickness. These instabilities result in irregular interfaces — and even layer intermixing in severe cases. At very low flow rates, the interface is smooth, as indicated in Figure 1-10(a). At moderate output rates, low-amplitude waviness of the interface is observed [see Figure 1-10(b]]; this waviness is barely noticeable to the eye and may not interfere with the function of the multilayer film. At higher output rates, layer distortion becomes more severe [Figure 1-10(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, which commonly is called “zigzag instability”, has been observed in tubular blown-film, multimanifold, and feedblock/single manifold dies. This instability develops in the die land; its onset can be correlated with a critical interfacial shear stress for a particular polymer system [9]. The most important variables that influence this instability are the 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; the interfacial stress is used to correlate variables for a particular system. Interfacial instability in a number of coextruded polymer systems has been correlated experimentally with their viscosity ratios and elasticity ratios [10], and a simplified rheology review is available [11]. Other studies have focused on viscosity differences [12–14], surface tension [15], critical stress levels [9,16,17], viscosity model parameters [18–20], and elasticity [21–29]. Other types of instabilities may exist: for example, a problem has been observed in the feedblock coextrusion of axisymmetric sheets [30]. A wavy interface is also characteristic of this instability, but the wave pattern is more regular when viewed from the surface. The instability, which commonly is called “wave instability”,

originates in the die, well ahead of the die land; the internal die geometry influences both the severity and the pattern. For a given die geometry, the severity of instability increases with structure asymmetry; some polymers are more susceptible to unstable flow than are 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 [31]. Figure 1-11 provides examples of both zigzag and wave instabilities.

No complete predictive theory exists that explains these complicated rheological interactions, but the accumulated experience of polymer producers, equipment suppliers, and experienced fabricators provides guidance in polymer selection.

1.2.3.2 Viscous Encapsulation

The importance of viscosity matching for layer uniformity was first studied in the capillary flow of two polymers for bicomponent fibril production [32–35]. Two polymers, which are 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 (Figure 1-12); it is possible for the lower-viscosity polymer to totally encapsulate the higher-viscosity polymer. Nature seeks the path of least resistance. The degree of interfacial distortion that is due to the viscosity mismatch depends on the extent of the difference in viscosity, the shear rate, and the residence time. 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, but good die design is required to obtain concentric layers.

1.2.3.3 Elastic Layer Rearrangement

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While matching the viscosities of adjacent layers has proven to be a very important requirement, the effect of polymer viscoelasticity on layer thickness uniformity is also an important consideration in the coextrusion process [36–40]. It has been shown that, in a die that can distort the interface between layers, polymers that are comparatively high in elasticity produce secondary flows normal to the primary flow direction (Figure 1-13). 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 that is due to elastic effects. Coextruding a structure that contains layers of polymers with alternating low and high levels of elasticity can cause interface distortion that is due to the differences in elasticity between the layers in the flat dies. Typically, this effect is not observed in tubular dies.

1.2.3.4 Solution Method for Layer Deformation Problem

Zigzag-type interfacial instability can be reduced or eliminated by increasing the skin-layer thickness, increasing the die gap, reducing the total rate, or decreasing the 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, it is the skin layer polymer viscosity that is decreased. In feedblock coextrusion, the resultant viscosity mismatch imposed by this remedy can cause variations in layer thickness. 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 layer deformation has been published [41]. Care should also be taken when designing the joining geometry in a feedblock or die. 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 them in a

perpendicular manner. The layers should also merge into a channel that is of an appropriate height so that it does not force one layer to flow into the other. Wave instability is related to the extensional viscosities of the individual layers. This finding implies that all of the design criteria mentioned previously for layer joining are also important for this type of instability, as is the spreading of the layers in a film or sheet die. 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 that have high extensional viscosities, the die should be designed to spread the layers slowly and at a uniform rate; this process will help minimize wave pattern instabilities.