(317t) Transient Diffusion of Gases in Polymer Foams Having Non-Uniform Density

Transient gas diffusion through closed-cell polymeric foams varies with boundary and initial conditions, which affect the local foam concentration for both constant and variable diffusion coefficients. In order to determine the solution for transient gas diffusion with variable diffusion coefficients, a new approach, implicit numerical solution, has been adopted using the Thomas algorithm to solve the tri-diagonal matrices. The one-dimensional diffusion equation, neglecting accumulation and depletion of volatile solute in the non-crystalline polymer fraction was used to develop the model. The results produced are equivalent to those predicted by the Fourier series solution for a polymeric foam having uniform density. The implicit numerical model was validated using gravimetric desorption measurement experiments.

BACKGROUND

A critical literature review on various properties and the diffusion process of rigid cellular foams indicated that the study on thermal aging and evaluation of the physical properties had been confined to the closed-cell foams of constant density. Various models for predicting the physical parameters of rigid cellular foams and experimental methods for the measurement of these parameters have been developed at isothermal conditions for foams having uniform density and cell structure. The prediction of aging characteristics for foams having locally variable density has not been developed comprehensively. The need for further development of a model for the prediction of physical properties for the foams having variable density should be considered along with validation methods.

OBJECTIVES OF PAPER

The purpose of this paper is to discuss a mathematical model developed to predict the transient local gas pressure with variable diffusion coefficients, resulting from inward diffusion of air components and outward diffusion of a blowing agent at defined environmental conditions using a finite difference numerical solution.