Servicio de Información Comunitario sobre Investigación y Desarrollo - CORDIS

Molecular models for slip

In this part of the project the occurrence of slip near the die wall has been thoroughly studied. A program has been written to numerically simulate the dynamical behaviour of the polymer molecules in a thin layer near the wall. This yields the so-called slip law, i.e. the dependence of the slip velocity on the shear stress at the wall. Special attention has been paid to the interaction between polymer chains attached onto the wall and bulk chains that may are entangle with them. It is shown that the entanglement/disentanglement mechanism dominates the slip law.

One of the findings of this study is that the slip law is usually a nonmonotonous curve. This remarkable result is used to develop a model that includes this type of slip law. The model clearly shows that a nonmonotonous slip law inevitably leads to the occurrence of spurt.

Moreover, the model allows prediction of the critical extrusion rate at which spurt will manifest itself and makes it possible to calculate its characteristics, such as period and amplitude. This part of the project has led to a program, which may predict spurt behaviour as a function of the extruder geometry and the slip law. Furthermore, a simulation program has been developed to calculate the slip law. A start is made with unraveling the mechanism behind the sharkskin instability. It could be that the slip law is also an important ingredient for modeling sharkskin.

Reported by

Eindhoven University of Technology
P.O.Box 513
5600 MB Eindhoven
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