Wettability of kraft pulps-effect of surface composition and oxygen plasma treatment

Wettability and ESCA studies have been carried out using unextracted and extracted kraft pulps with different amounts of lignin. The effect of oxygen plasma treatment was investigated. The wettability was studied by placing a drop of water on a sheet made from the pulp and following the decrease in drop volume and apparent contact angle with time. The first reading is taken after about 0.1 s. Extrapolation to zero time gives the initial contact angle. The initial absorption rate was also determined. Good correlation between the initial contact angle and the oxygen to carbon atomic ratio was found for the pulps without extractives. Pulps containing extractives are very hydrophobic, but an oxygen plasma treatment renders these pulps as hydrophilic as the purest pulps. It is concluded that the oxygen plasma treatment oxidizes the lignin and the extractives but reduces the pure cellulose surface. The oxidized extractives appear to act as wetting agents. The initial absorption rate is strongly dependent on the amount of extractives in the pulp. An oxygen plasma treatment improves the absorption rate, especially if the pulp contains high amounts of extractives.