Viscosity skin-layer

One interesting benefit that can be obtained with coextrusion is a more uniform temperature distribution in the material. This can be realized by coextruding a thin, low-viscosity outer layer over a high-viscosity inner layer. The highest shear rate and heat generation normally occurs at the wall. By having a low-viscosity material at the location of maximum shear rate, the heat generation is reduced at this point and a more even temperature profile is obtained. This is a useful technique for thermally unstable polymers. As discussed before in Section 7.5.3, this technique can also avoid the occurrence of shark skin and melt fracture type flow instabilities. 

This instability develops in the die land, and its onset can be correlated with a critical interfacial shear stress for a particular polymer system (2). 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. 

This type of interfacial instability can be reduced or eliminated by increasing skin-layer thickness, increasing die gap, reducing total rate, or decreasing skin-layer 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 used to compensate. 

HP-LCP is a promising matrix polymer for tribological applications because of its low melt viscosity, which is the typical namre of TLCPs, as well as high heat resistance and high strength and modulus. However, TLCPs have an intrinsic nature that results from the orientation of molecular chains in the skin layer of a molded article along MD, which generates fibrils easily from the surface. 

These data show that the thinner the skin layer is in the structure, the more the viscosity of the structure approaches that of the core material. This result is very helpful to a die designer or troubleshooter sinee the coextrasion die can be designed or the flow in the die analyzed using the rheology of the skin layer to approximate the rheology of the coextraded structure if the skin layer is thiek enough. 

The final set of experiments consisted of measnring the viscosity of encapsulated structures of Resins A and B in which the skin layer thicknesses are varied from 0 to 50% of the total structure. These experiments were ran at a constant shear rate of approximately 40 1/s. The results of these experiments are shown in Figures 9 and 10. 

Figure 9 shows the viscosity of encapsulated structures with a skin layer of Resin A on a core of Resin B as the skin layer thickness is varied from 0 to 50% of the structure. Since Resin A is less viscous than Resin B, a decrease in the structure viscosity would be expected as a layer of Resin A is added to the structure. Figure 9 shows that there is a significant drop in the viscosity of the structure even with just a 5% skin layer of Resin A added to the structure. 

Figure 8. Viscosity data for individual components and for coextraded encapsulated stractures with the skin layer being more viscous than the core layer.

Routh and Russel [10] proposed a dimensionless Peclet number to gauge the balance between the two dominant processes controlling the uniformity of drying of a colloidal dispersion layer evaporation of solvent from the air interface, which serves to concentrate particles at the surface, and particle diffusion which serves to equilibrate the concentration across the depth of the layer. The Peclet number, Pe is defined for a film of initial thickness H with an evaporation rate E (units of velocity) as HE/D0, where D0 = kBT/6jT ir- the Stokes-Einstein diffusion coefficient for the particles in the colloid. Here, r is the particle radius, p is the viscosity of the continuous phase, T is the absolute temperature and kB is the Boltzmann constant. When Pe 1, evaporation dominates and particles concentrate near the surface and a skin forms, Figure 2.3.5, lower left. Conversely, when Pe l, diffusion dominates and a more uniform distribution of particles is expected, Figure 2.3.5, upper left. 

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