"KINETIC OF LIQUID TRANSPORT IN ETHYLENE VINYL ACETATE BEAD"

In order to investigate the kinetic of the diffusion of a liquid in a polymer matrix, the study of the diffusion oF the n-heptane in ethylene vinyl acetate is carried out. The polymer considered has a spherical shape. The kinetics of absorption and desorption of the Iiquid are carried out experimentally by weighing the bead at successive times. Thus the experimental curve is compared to the calculated curves using the analytical and numerical solutions. Two numerical solutions, based on finite differences are suggested: 1. Numerical solution for the bead without swelling 2. Numerical solution for bead with swelling and subsequent change in dimensions. The second purpose seems being the better and give a good agreement with the experience since the diffusivity (experimental and calculated) are of the same magnitude. Key word: diffusion, numerical analysis, ethylene vinyl acetate, polymer, matter transfer. INTRODUCTION When the n-heptane is in contact with the polymer, the liquid enters into the polymer by diffusing between the polymer chains. In the case of ethylene vinyl acetate, with the massic proportion of vinyl acetate equal to 28% and the melt index of 40 (EVAc 28-40), the polymer is in the caoutchouc state and the diffusion of liquid is of Fickian type1 (M = k t ). Where Mt is the amount of liquid transferred at time t. The process of matter transfer is controlled by transient diffusion. The study of this phenomenon is of high interest since it helps us to understand and eventually to predict the liberation of drugs in stomach2,3,4,5 from the matrix of galenic shape. Results of this study can also apply to the liberation of fertilizer6,7,8] in the soil from a spherical geometrical form. The methodology followed in this study is the next: 1. The bead immersed in n-heptane is weighed at successive times and the curve Mt = f ( t ) is drawn for short times (Mt < 0.2) . From the slope of this curve we determine the experimental diffusivity. 2. Refine the diffusivity by the model calculations. 3. Compare the experimental curve Mt / M = f (t), where M is the amount of liquid in the bead at saturation, to those