WORLD PULP&PAPER 91 Dissociation of the most abundant acid function in papermaking systems – carboxylic group – provides a more anionic charge at a higher pH. In the case of weak acid functions, dissociation is centered around their pKa value and practically full transition between uncharged carboxylic acid form (-COOH) and anionic carboxylate salt form (-COOH) takes place between pH=pKa-2 and pH=pKa+2, as shown in Figure 2. The anionic charge of carboxylic functions on the surface of colloidal particle provides repulsive forces responsible for its electrostatic stablisation. With pKa=5 for the typical carboxylic group, this significant extractives present in the systems exposed to alkaline treatment show increased sensitivity to pH and hardness changes stablise colloidal pitch particle. This explains the higher stability of softwood pitch colloidal when compared to hardwood pitch that lacks the resin acid component. Lack of resin acid fractions in hardwoods is also responsible for the more difficult emulsification of pitch in the pulping process, impaired de-resination and, consequently, higher pitch content of hardwood pulps. In typical fibre mix in wood-free grades, hardwood pulp stream needs to be looked at first, if pitch issues on the machine are to be investigated. Exposure of the pitch to alkaline conditions – in the Kraft process and in the hydrogen peroxide bleaching – can lead to very critical alkaline hydrolysis reactions that will change the composition and stability of pitch particles. First, the hydrolysis of ester fractions of extractives increases the acid content (resin acids in softwood and fatty acids in softwood and hardwood) in the pitch particle. Secondly, hydrolysis and consequent removal of hemicelluloses reduces steric stabilisation of pitch particles. As a result, extractives present in the systems exposed to alkaline treatment show increased sensitivity to pH and hardness changes2. In these cases, controlling the level and stability of pH and paying close attention to hardness generation, becomes a pre-requisite to the success of any deposit control program. Figure 1. Colloidal pitch particle – structure and origin of charge.