Internal surface interactions in the plasma treatment of fine bore fluoropolymer tubings

An experimental investigation was carried out to study the modification of the inner surface of short fluoroelastomer tubings (1.5 mm internal diameter) by plasma treatments. The maximum level of wettability achievable was found to decrease with distance from the tube entrance and was attributed primarily to the rapid decay in plasma energy through collisions with the inner walls of the tubing. The effect was even more pronounced at higher gas pressures and with heavier gas ions, i.e. using argon instead of air. The critical length, defined as the length of the tubing affected by the plasma treatment, was found to increase with treatment time until it reached a limiting value. A characteristic retardation time was defined from a model which predicts an exponential relationship between critical length and treatment time. Moreover, from a plot of the fractional reduction in water contact angle against the reduced distance, an estimate was obtained of the time required for the reactions with the surface of the tubing to reach steady state conditions.