Análise de efeitos viscosos no escoamento de polipropileno em extrusoras

Abstract

Polypropylene (PP) is a thermoplastic polymer that finds many industrial applications, being extrusion one of the commonest processing techniques. Because of the high viscosity of the PP melt, very high mechanical stresses can be associated with the extruding operation. It can lead to an erosion in the screw and it may be necessary to carry out the maintenance of the screw or the operation may become infeasible. In the present work a multivariable rheological model based on the Cross equation was initially built and used to represent rheological curves of some comercial PP grades. Then, the proposed model was included in the Computational Fluid Dynamics software Polyflow (ANSYS) in order to evaluate the viscous effects associated with the PP flow in an extruder through simulations. It was concluded that thermal effects can be very significant during the PP extrusion process and that the shear stress profiles can be very sensitive to modification of the molar mass distributions of the analyzed PP grades. As a consequence, the viscous stresses are larger in the screw thread than in its cylindrical body. These results were also confirmed by the temperature profiles achieved through mathematical modeling considering lumped parameters and they suggest that both the operation conditions and the polymer grade can simultaneously affect the level of screw wear in the extruder, which is larger in the screw thread.