Elongational Viscosity Analysis and Modeling of a Modified Polylactide for Foaming Applications

In the literature it is well known that foaming is a highly complex process which is influenced by e.g. the process parameters, the choice of polymer and its structure as well as by the blowing agent type and concentration. Due to the dependency of both, shear and elongational viscosity on such influencing factors, the melt viscosity is highly influenced during the foaming process. Thus, the viscosity is a crucial parameter for the foaming process. A collapse of single bubbles or even of the whole foam structure is often a result of a too low elongational viscosity and the associated melt strength. Therefore, polylactide (PLA) is difficult to foam. In this study the melt viscosity in shear and extensional flow, and the melt strength was increased by modifying PLA with dicumyl peroxide on a twin-screw extruder. Thus, the polymer chain structure was changed from linear to branched and strain hardening was observed during uniaxial viscosity measurements with a Sentmanat extension rheometer at different temperatures and Hencky strain rates. The uniaxial elongational viscosity of the modified PLA including the observed strain hardening was predicted at different temperatures and different Hencky strain rates with the molecular stress function model (MSF). Because of the good accuracy of the MSF model predictions in comparison to the uniaxial viscosity measurements, the equibiaxial viscosity was modeled as well since equibiaxial deformations mainly occur during the foaming process. Therefore, the elongational viscosity of modified polymers for foaming applications can be characterized and predicted.