The study of the rheological behavior of molten polymers and the existence of correlations with their structure provide a valuable and essential tool for improving the quality of products and the yield of transformation processes. This approach is possible in two ways:
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optimize the choice of a material for a given processing technique and application;
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optimize a process for a material and an application.
This last point has gained importance since the 2000s with the development of process simulation tools and software for which the determination of a behavior law (viscous in most cases) is a must.
The fundamental concepts and results presented in the article ” Linear viscoelasticity of polymer melts of this treatise related to behavior in situations of infinitesimal deformations. However, during the transformation of thermoplastic polymers, from the granule to the finished object, the deformation is obviously macroscopic and this situation, combined with the viscoelastic behavior of the polymer, generates a set of manifestations that are difficult to interpret within the restricted framework of viscoelasticity. linear.
In the case of fluids and in the context of current possibilities in terms of modeling, taking into account the viscous behavior in shear is often sufficient, at least for common applications (implementation by injection of thermoplastics, spreading of paintings…). Thus, within this framework, the constitutive equations, linking all the possible states of stress to all the possible states of deformation, can be simplified in the particular form of flow laws. These laws then consider only the particular context of constraint or deformation of the fluid (in shear) and use only part of the parameters of the material intervening in the constitutive equation.
However, a certain number of characteristics of an elastic nature as well as the existence of processes clearly involving different kinematics (elongation in particular) show the limits of such an approach and invite us to approach these aspects of the behavior of viscoelastic fluids which are molten polymers.
This article proposes to describe some phenomena observed in situations of simple, but nonlinear flow, of molten polymers, the tools for analyzing behavior in shear and elongation (rheometers), the links with the structure of polymers and some examples of applications to implementation.
The analysis of the viscoelastic behavior in a situation of low or large deformations in the case of simple flows finally makes it possible to understand the origin of a certain number of phenomena appearing in complex flows. These phenomena of viscoelastic nature are sometimes at the origin of a certain number of characteristic defects appearing on the products resulting from forming in the molten state and the understanding of the causes of their appearance makes it possible to hope to overcome these disadvantages and these limiting factors of productivity or quality. We will study here more particularly the swelling at the exit of the die and the defects of extrudates.
Finally, in view of this set of particularities, the prediction of the nonlinear viscoelastic behavior of molten polymers obviously cannot be satisfied with purely viscous behavior laws. The mathematical formalism as well as the basic notions which lead to the formulation of more complex, but more realistic constitutive equations, will finally be approached.
The reader will benefit from also consulting the article Linear viscoelasticity of polymer melts [AM 3 620] of this treaty.
