Interface distortion

One of the challenges in coextrusion is to control the uniformity of the individual layers. It is well known that when two fluids with different viscosity flow side by side the interface will distort because the fluid with the lowest viscosity has a tendency to flow to the high shear rate regions. As a result, the low-viscosity material will tend to encapsulate the high-viscosity material. This situation is illustrated in Fig. 9.42. 

The extent of the interface distortion will depend on the length of the flow channel. If the channel is long enough the high-viscosity fluid will be completely encapsulated as shown in Fig. 9.42. If the channel is short the interface distortion will show an intermediate configuration as shown in time ti or t2 in Fig. 9.42. The process of viscous encapsulation was studied by Gifford, as discussed in Section 12.4.2 see Fig. 12.18. 

The distortion shown in Fig. 9.42 is driven by viscosity differences. However, even when coextruding polymers with exactly the same viscosity, interfacial distortion has been observed [43]. 

Clearly, there are other mechanisms by which interfacial distortion occurs. Dooley and Hughes [43] performed careful experiments to illustrate the extent of interface distortion in coextrusion of polymers with the same flow characteristics. In their analysis they attribute the interface distortion to normal stress differences within the polymer melt, and they used a finite element program capable of handling viscoelastic fluids to predict the distortion within the fluid. This issue is discussed further in Section 12.4.2 see Figs. 12.19 to 12.21. 

Svabik, Samsonkova, and Perdikoulias [45] proposed another explanation for the interface distortion in coextrusion of fluids with equal viscosity. They performed three-dimensional flow analysis of coextrusion flow and found that even in coextrusion with Newtonian fluids with equal viscosity layer distortion takes place. Obviously, this type of distortion cannot be caused by normal stress differences since these do not occur in Newtonian fluids. Also, with the viscosities being equal the distortion cannot be caused by viscosity differences. The predicted layer distortion is schematically illustrated in Fig. 9.43. The authors call the distortion resulting from purely viscous flow geometrical encapsulation. 

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There are basically three different techniques for coextrusion. The first employs feed block dies where the various melt streams are combined in a relatively small cross-section before entering the die. The advantage of this system is simplicity and low cost. Existing dies can be used with little or no modification. Disadvantages are that the flow properties of the different polymers have to be quite close to avoid interface distortion. There is no individual thickness control of the various layers, only an overall thickness control. Figure 9.37 shows a schematic of a feed block sheet die. 

It appears that there are several mechanisms for interface distortion. One is distortion caused by viscosity differences (viscous encapsulation), another is caused by normal stress differences in the fluid (elastic encapsulation), and a third is caused by normal velocity differences within the fluid (geometrical encapsulation). Obviously, the distortion will increase when viscosity differences are large and when normal stress differences play a significant role. 

There are two approaches to minimizing the interface distortion. One is to reduce the length over which the different melt streams flow together. This is done in multimanifold dies where the different melt streams are combined just before the exit of the die. Another approach is to modify the initial configuration of the layers in such a way that the final layer configuration is the one desired. This approach is called profiling, and this is a method frequently used in feed block coextrusion systems to achieve uniform layer distribution at the exit of the die. This principle is illustrated in Fig. 9.44. 

Interface Distortion from Viscoeiasticity. While matching the viscosities of adjacent layers has proven to be very important, the effect of polymer viscoelasticity on layer thickness imiformity is also important (20-24). 

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 pronoimced 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 because of the differences in elasticity between the layers in flat dies. The effect is typically not observed in tubular dies. 

The advantage of the feed block system is that it is simple, inexpensive, and allows many layers to be combined. The main drawback is that the flow properties of the different plastics have to be quite close to avoid interface distortion. This limits the choice of materials that can be combined through feed block coextrusion. 

In the multimanifold system each plastic has its own entrance and manifold in the coextrusion die. The different melt streams are combined just before they exit the die, so that minimum interface distortion can occur. The advantage of the multimanifold system is that plastics with widely different flow properties can be combined. As a result, there is a wide choice of materials that can be combined through this extrusion technique. The disadvantage is that the design of the die is more complicated and therefore more expensive. 

It should be stressed that in this case the relative amount of (total) water was constantly decreased upon the addition of pentanol (and dodecane). Nevertheless, when the pentanol concentration reached a certain value, some of the (bound) water became free. This phenomenon may be interpreted by assuming that the pentanol molecules residing at the interface distort the three-dimensional tetrahe-... 

The Cheerios effect is due to the horizontal component of the capillary force generated by the interface distortion. In order to find the horizontal component, the shape of the air-liquid interface deformed by objects or walls needs to be determined. Let be a function describing the shape (vertical position) of the interface ... 

When two cylinders are apart at a large separation, the capillary interaction between the two cyhnders is weak. One can assume that the interface on the cylinder surface is modeled by a linear equation and is a superposition of the interfaces distorted by the two isolated cylinders [15] ... 

The effect of alcohol on the distribution of (total) water between the free and bound states was exemplified [2,9] by the swelling of a fixed amount of a 1 1 (by weight) C,2(EO)8-water mixture with increasing amounts of a 1 1 (wt/wt) solution of pentanol + dodecane. The relative amount of (total) water constantly diminished upon the addition of pentanol + dodecane, but when the alcohol concentration reached a threshold value, some of the (boimd) water became free (Fig. 11). The same phenomenon was observed in the system phosphatidylcholine (25 wt%)-tricaprylin + alcohol (60 wt% molar ratio of 1 5, respectively)-water (15 wt%). The alcohols used were ethanol, butanol, and hexanol, and the free water content increased in that order [72], The same inverse dependence on the alcohol hydrophiUcity was observed for sucrose ester-based microemulsions [29]. It seems that alcohol molecules adsorbed at the interface distort the three-dimensional network of... 

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