An investigation concerning the interactions between cationic starch and peroxide bleached thermornechanical pulp (TMP) has been carried out. The influence of electrolytes, pH, temperature, a fixing agent and charge density of the starch is discussed. The adsorption of cationic starch onto dispersed and dissolved wood substances and unwashed fibres was reduced by an increase in NaCl concentration. The adsorption onto washed fibres, however, showed a maximum with increasing NaCl concentration. The adsorption onto dispersed and dissolved substances and unwashed fibres increased and reached a plateau level as function of increasing pH.
Onto washed fibres, the adsorption was reduced as pH was increased from 5 to 8. An increase in temperature affected only the adsorption onto washed fibres . A fixing agent retained the dispersed and disolved wood substances more effectively than the ordinary cationic starch but the amount of adsorbed cationic starch remained unaffected by the presence of the fixing agent. Cationic starch of
medium charge density (degree of substitution, D.S ., : 0.035) aggregated turbidsubstances more effectively than cationic starches of low charge density (D.S . 0.015) and of high charge density (D.S . 0.05) .