Interactions of oxide surfaces with dissolved and colloidal substances released in mechanical pulping

The objective of this work was to clarify the factors governing the formati on of surface layers on silica and aluminium oxide surfaces by adsorption o f dissolved and colloidal substances (DCS) released in mechanical pulping. The kinetics of adsorption as well as the interactions of the surface layer s were studied by reflectometry in stagnation point flow. The parameters of the study were the type and concentration of electrolyte (NaCl, CaCl2, 0.1 -100 mM), and pH (4.5 and 6.5). Model dispersions were prepared from xylan, DCS released from either unbleached thermomechanical pulp (TMP) or H2O2 bl eached TMP as well as model resin dispersions prepared from TMP. The critic al deposition concentrations (CDC) for DCS on SiO2 were in the range 0.2-0. 7 M of either NaCl or CaCl2, with the exception of DCS from bleached TMP, f or which the CDC occurred in 10 mM CaCl2 at pH 6.5. Bleached or unbleached DCS adsorbed on Al2O3 regardless of ionic strength or pH, but the factors c ontrolling the kinetics of adsorption depended on the pH of the electrolyte , and the colloidal stability of the DCS. Interactions of the deposited lay ers of DCS with synthetic cationic and anionic polymers and dispersed resin were also studied. The anionic polymer poly(vinyl sulphate) did not adsorb on DCS layers. The cationic polymer poly-DADMAC adsorbed on DCS regardless of ionic strength or pH. This resulted in formation of sequential layers o f DCS and poly-DADMAC. With one exception, dispersed resin did not deposit on the surface layers formed by DCS. However, at pH 6.5 when [Ca2+] greater than or equal to 10 mM, resin deposited on surface layers formed by H2O2 b leached DCS.