IN-SITU KINETIC-STUDIES ON DIRECT FLUORINATION OF THIN POLYETHYLENE FILMS WITH QCM

Fluorination of polyethylene (PE) films with fluorine gas was studied by using the quartz crystal microbalance technique from the interest for the preparation of wet-proofed carbon black with fluorinated polyethylene (FPE) film for application to gas-diffusion electrodes in fuel cells. The fluorination reaction proceeds clearly in three separated steps, i.e., the first transient step, the 2nd step controlled by the chemical reaction between F-2 and PE molecules, and the 3rd step controlled by the diffusion rate of dissolved F-2 in FPE. The complete fluorination of PE films of ca. 2-10 Fun in thickness can be achieved after sufficient reaction time. It was found that the fluorination at the 2nd step obeys the first order kinetics regarding F-2 concentration with the activation energy of only 34.3 kJ mol(-1). The fluorination at the 3rd step proceeded linearly with the square-root of the reaction time. Diffusion parameters S root D and apparent activation energies for the fluorination at the 3rd step were determined by solving Fick's diffusion equation based on those experimental data. Both of them were very large in comparison with that for un-reacting gases, which were ascribed to the fluorination step proceeded by the reactive diffusion process.