Destabilization of colloidal wood pitch by calcium ions in TMP suspensions

Sticky deposits caused by wood pitch remain a severe problem in papermaking when using mechanical pulps, such as thermomechanical pulp (TMP). Formation of deposits on paper machines is dependent on aggregation of colloidal pitch. The stability of colloidal wood pitch droplets towards electrolyte-induced aggregation by CaCl ₂ was determined in suspensions of unbleached and peroxide-bleached TMP. The colloidal stability was not significantly affected by addition of unbleached or peroxide-bleached TMP, in the absence of CaCl ₂ Addition of CaCl ₂ caused destabilization of colloidal pitch at both pH 5 and pH 8, and to a higher degree if TMP was present. The colloidal stability of pitch was much higher at pH 8 than at pH 5. Addition of CaCl ₂ at pH 8 resulted in formation of calcium-resinates, which can be very detrimental to the papermaking process. Calcium-resinates do not have affinity to TMP or colloidal pitch droplets, and are therefore difficult to control in a process. Calcium ions were deemed the cause of destabilization and aggregation of colloidal pitch in these experiments. The concentration of free calcium should be kept as low as possible to prevent uncontrolled aggregation of pitch.