Aggregation and destabilisation of colloidal wood pitch by calcium ions in TMP suspensions

The colloidal stability of wood pitch against aggregation and destabilisation by CaCl2 was assessed in suspensions of unbleached and peroxide-bleached thermomechanical pulp (TMP). The experi-ments were conducted at different pulp consistencies, at both pH 5 and pH 8, to determine pitch aggregation in acidic and neutral papermaking processes using TMP. The colloidal pitch had low affinity to TMP fibres in the absence of CaCl2. At pH 8, a higher concentration of CaCl2 was required to destabilise the colloidal pitch than at pH 5. Even low CaCl2-concentrations caused destabili-sation of colloidal pitch in suspensions of TMP at pH 5. More pitch was destabilised by CaCl2 in suspensions of unbleached TMP than in suspensions of peroxide-bleached TMP. Resin acids were to a large extent found as stable calcium-soaps in the water phase at pH 8, even at high concentrations of CaCl2. Water-soluble galactoglucomannans (GGM) stabilised pitch emulsions almost completely against calcium-induced aggregation both at pH 5 and 8. The colloidal stability of pitch in suspensions of TMP was not total, but was much higher when GGM was added, compared to when no GGM was added.