In-situ Kinetic Studies on Direct Fluorination of Thin Polyethylene Films with QCM

Abstract

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 F2 and PE molecules, and the 3rd step controlled by the diffusion rate of dissolved F2 in FPE. The complete fluorination of PE films of ca. 2—10 μm 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 F2 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 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.

Surface modification of polymers by direct fluorination: A convenient approach to improve commercial properties of polymeric articles

“…Thickness δ F of the fluorinated layer was found to be proportional to t (HDPE and PP [30]), (p F ؒt) 0.5 (PE [31]), (PS, polycarbonate, and PMMA [26]), t (polymethylpentene [32]). At the initial stage, the mass increase of PE was not constant and tended to increase, but in several min the mass of the film was increased proportionally to t 0.5 [19]. Unfortunately, calculated in [19], the activation energy of the rate of fluorination is not valid, because the mathematical model only took into account the diffusion of fluorine through the fluorinated layer to the untreated one, but the consumption of fluorine inside the reaction zone between fluorinated and untreated layers was not taken into account.…”

“…At the initial stage, the mass increase of PE was not constant and tended to increase, but in several min the mass of the film was increased proportionally to t 0.5 [19]. Unfortunately, calculated in [19], the activation energy of the rate of fluorination is not valid, because the mathematical model only took into account the diffusion of fluorine through the fluorinated layer to the untreated one, but the consumption of fluorine inside the reaction zone between fluorinated and untreated layers was not taken into account. In [33], the mass increase of LDPE, HDPE, PVF, PVDF, PP, and ethylene-propylene (EP) copolymer was very small at the initial stage (1-2 min), but then the mass of the film was increased proportionally to t 0.5 .…”

“…Investigations of the kinetics of growth of the thickness δ F of fluorinated layer were performed by several researches, but there is no data in the available literature on "in situ" measurements (excluding kinetics of the mass increase during fluorination of PE [19]). To measure the δ F value, the fluorination was interrupted.…”

“…At present, the majority of research [1,17,19,34,40,41] considers the process of the direct fluorination of polymers as a radical-chain one. But such an affirmation was based only on the fact that long-life radicals were detected in the fluorine-treated polymers, but no radicals taking part in the processes of chain initiation and propagation were detected except by research carried out by our group [2,4,6,7,[41][42][43].…”

Surface fluorinated epoxy resin for high voltage DC application

“…The presence of fluorine element from the EDX spectrum comes from the formation of fluoride groups that are bound to happen because bond energy of C-F bonds is much higher than the bond of C-H and C-OH of pristine epoxies [121]. During a directfluorination treatment on hydrogen-carbon based polymeric materials, C-H and C-OH bonds are disrupted while the conjugated double bonds are saturated and followed by the formation of C-F, C-F₂ and C-F₃ groups [122,123]. This experiment was conducted using Leica microscope and Renishaw Raman RM1000 system with a 785 nm CW diode laser of 25 mW.…”