Role of the interphase in the flow stability of reactive coextruded multilayer polymers

Polymer Engineering and Science, April, 2009 by Khalid Lamnawar, Abderrahim Maazouz

INTRODUCTION

Coextrusion is an industrial process used to form multi-layered sheets or films that are suitable for various products ranging from food packaging materials to reflective polarizers. The main problem is to simultaneously process polymers of different rheological properties. It is well known that under certain operating conditions, some defects can be observed inside the die, and these defects, mainly interfacial instabilities, affect the quality of the final product. They refers to two common types consisting of highly irregular or sometimes regular waviness, which appears in coex-truded structures at the polymer/polymer interface and the layer-to-layer uniformity (i.e., encapsulation caused by the tendency of the less viscous polymer to go to the region of high shear (i.e., the wall). Important theoretical and experimental advances with regard to the stability of compatible and incompatible polymers have, during the last decades, been achieved using a mechanical and numerical approach [EI Kissi et al. (1), Laure et al. (2)].

Yih (3) first studied the stability of Poiseuille flows of two Newtonian fluids submitted to very long waves. By using a linear stability theory, he was able to demonstrate that a difference in viscosity could lead to instabilities, even for materials with low Reynolds numbers. His analysis was extended by numerous authors to other shearing flows. Lee and White (4) have been predicted, using Reynolds lubrification theory, that the interface moves from its initial position on the side of the less viscous component to its final position on the side of the more viscous component. The authors carried out a theoretical and experimental analysis showing that with viscoelastic fluids, one must consider also normal stress effect on the mechanisms of interface distortion in the axisymmetrical two phase flows in the capillary die.

Although the subject is old in principle, we shall quickly list some investigations of close relevance to this article. For instance, the asymptotic methods developed by Hooper (5), Hooper and Boyd (6), or Yiantsios and Higgins (7). Numerical solutions have been proposed by Anturkar et al. (8), Rincon et al. (9), and Zatloukal (10), and a global overview of theoretical results is given by Joseph and Renardy (11). Several authors have carried out stability experiments mainly on polymeric liquids. Southern and Ballman (12) have been demonstrated that other parameter, in bicomponent polymer melt flow, should be taken into account as the wetting of one melt with the capillary wall. They concluded that the viscosity ratio dominates bicomponent flow with surface tension and elasticity ratios producing minor changes in the interface shape under very limited bicompo-nent flow conditions.

Furthermore, other investigations including Han (13), Khan and Han (14), Karagiannis et al. (15), and White et al. (16) have shown that the interfacial stability of multilayer flows can be determined by a number of factors including thickness, viscosity, density, elasticity ratios (in terms of [N.sub.1] or relaxations times ratios), and interfacial tension. The studies also demonstrated that the interfacial stability of multilayer flows was determined by a number of factors that were either essential (e.g., thickness, viscosity, and elasticity ratios) or rather uninfluential (e.g., density ratios and interfacial tension).

Hinch et al. (17) proposed a physical mechanism to explain how a jump in primary normal stress difference can cause instability whenever there is no jump in viscosity across the interface. It is important to note that the shear rate is not constant throughout thickness and a relevant parameter is rather the ratio between the viscosity and elasticity estimated at the interface. Moreover, in a more recent studies, several authors have shown that the mechanism of viscoelastic interfacial instabilities is due to a coupling of the perturbation velocity field and the polymeric stress gradient across the interface (Su and Khomami (18), Mohammed and Khomami (19)). The authors have shown that the relative contribution from elasticity stratification is a least on the some order as the contribution from the viscosity differences and is proportional to the jump in the first normal stress at the interface (i.e., the elasticity ratio at the some shear rate).

In addition, very comprehensive experiments have been carried out by Wilson and Khomami on both miscible and immiscible fluids (20), (21). Their facilities introduce temporally regular disturbances with controllable amplitudes and frequencies. The authors first investigated flows of immiscible fluids and found that theoretically predicted growth rates agreed with their experimental data. Subsequently, they considered a plane Poiseuille flow of a compatible polymer system. In such a system, there is no interfacial tension, and polymer chains are able to diffuse across the original interface, forming a diffuse interface (i.e., an interphase). In this case, growth rates were found to be much lower than those obtained for incompatible systems or for classic theoretical studies.