DEPOSIT CONTROL IN PAPERMAKING SYSTEMS WORLD PULP&PAPER 92 of the dissolved fraction of extractives, triggered by water evaporation, temperature changes, or chemical precipitation (hardness), represent an important mechanism of deposit formation. These kinds of deposits typically show further down the paper machine in dryers, calendar stacks, and in the transfer rolls of printing presses. It is clear that properly selected and stable pH is pertinent in preventing deposits formed via two mechanisms – agglomeration of colloidal fraction and precipitation of soluble fraction of the natural pitch. Repulsive, stabilising force between anionic pitch particles depends strongly on the system conductivity. The impact of conductivity on all aspects of papermaking was recently discussed4 . It is important to remember that at change in charge extends between pH=3 and pH=7; it is steepest between pH=4 and pH=6 – well within the range of the papermaking process. Thus, it is important to maintain a stable pH when running the acid operation. Due to an increasing level of anionic charge, salts of resin acids and fatty acids become more soluble with increasing pH. For resin acids3, 50%- 70% of total content becomes soluble in water at around pH=7. Fatty acids are significantly less soluble, and their pH-solubility curves are shifted up by about 2 pH units. It would require around pH=9 for fatty acids to reach the solubility of resin acids observed at pH=7. Thus, resin acids have a higher tendency to accumulate in the papermaking water systems when fatty acids are more effectively removed with the formed web. Changes in solubility high conductivity levels, the natural tendency for agglomeration increases and system demands require a quicker and more decisive action. This is the future of papermaking with the trend to water system closure. STICKIES Stickies is a collective term describing synthetic hydrophobic materials introduced to the papermaking system with coated broke and recycled pulps. There is a significant amount of literature that describes the chemistry of different types of stickies. A higher melting point, lack of emulsifying components, and lower shear stresses of re-pulpers – as compared to refiners used in mechanical pulping – explain why stickies firstly originated from recycled material; secondly, have a lower tendency to present themselves in the colloidal form, and thirdly, enter the papermaking process Repulsive, stabilising force between anionic pitch particles depends strongly on the system conductivity Figure 2. Dissociation carboxylic acid function – pH dependence of anionic charge.