Rheological scaling and bubble nucleation of a polymer-diluent solution in extrusion foaming
In this work, a new methodology is developed that describes the viscoelastic scaling of a polymer – physical foaming agent (PFA) solution in a detailed and internally consistent manner. The approach is new in that while previous researchers have largely focused on scaling down experimentally obtained high pressure polymer-PFA solution viscosity data onto a master curve for the viscosity of the undiluted polymer melt at a reference temperature and atmospheric pressure, we have generated the shear viscosity data required for our simulations by systematically scaling up the viscosity values obtained through a single set of experiments on the undiluted pure polymer melt at atmospheric pressure. Using the above data, simulations were run for the flow of a polymer – PFA solution through an extrusion foaming die with an abrupt axisymmetric contraction. The pressure drops across the die obtained through the simulations showed good qualitative agreement with experimental pressure drop measurements on the foaming extrusion die obtained previously in our laboratory. Field values of pressure, temperature and velocity were obtained at each point in the foaming die. Once the values of pressure and temperature were obtained along each point in the foaming die, classical nucleation theory for bubble nucleation, in the form developed originally by Zeldovich, was invoked to predict the local bubble nucleation rate downstream of the saturation surface. The hydrodynamic constraints to the nucleation rate were calculated using Kagan’s extension of the Zeldovich theory. Diffusional constraints were incorporated into the theory using the method suggested by Katz and his coworkers. The capillarity approximation was found not to be valid for bubble nucleation of CO2 in polymers; a correction on the lines suggested by Tolman was applied to get non-zero nucleation rates for the system.
School:The Ohio State University
School Location:USA - Ohio
Source Type:Master's Thesis
Keywords:polystyrene polymer processing nucleation rheological scaling
Date of Publication:01/01/2007