WORLD PULP&PAPER 93 mainly as larger particles with less anionic charge. Freshly re-pulped coated broke – the source of coating binder latexes such as Poly-vinyl Acetate (PVA) and Styrene-Butadiene Rubber (SBR) (especially in the systems operating with a high shear broke disintegration equipment) – may contain slightly higher levels of colloidal fraction and is often referred to as white pitch. The chemistry of stickies influences their behaviour and deposition tendency mainly through variation of their Tg values; this impacts tackiness and tendency to agglomerate. In the presence of well-operated screening equipment, the high limit of the size of stickies particles is defined by the selection of screen baskets and their operational practices5. Further agglomeration of an accepted fraction, leading to the formation of secondary stickies, is a function of their tackiness (temperature and Tg value) and frequency of collisions (shear, size, and concentration). SYSTEMS WITH MIXED HYDROPHOBIC CONTAMINANTS As mentioned earlier, furnishes containing both natural pitch and stickies contaminants present a unique challenge for papermakers. Most of the difficulties originate at the stage of blending virgin and recycled pulps with a potential of pH changes, colloidal destablisation, hardness increase, etc. It is very important to ensure that each individual stream blended into the machine furnish, is individually prepared prior to their blending, in order to avoid deposits further down the system. Understanding and acting upon pH level and its variation, the calcium carbonate filler level in the recycled furnish, the conductivity of each of the streams and their temperature are big steps towards the elimination (or reduction) of deposits on the machine. Recent research6 demonstrated that natural pitch components interact with the surface of synthetic stickies making them tackier and increase their rate of deposition. This scenario should be examined in the case of Light Weight Coated (LWC) grades using significant levels of mechanical pulps (TMP, SGW or PGW). In this case, natural pitch serves as a tackifier for coating latexes in the broke and the synergistically increase deposition potential. If this happens, wood pitch components and latexes are found in the chemical analysis of machine deposits or sheet defects. Market mechanical pulps (BCTMP, APMP) and on-site produced mechanical pulps (TMP, GWD) – so-called High Yield Pulps – are increasingly replacing chemical pulps in wood-free grades. This trend is driven by cost – either directly by pulp replacement or indirectly by allowing for higher filler content by correcting related bulk loss. This trend increases the need for better deposit control without impacting certain sensitivities of fine paper operations such as charge balance, impact on both the brightness and performance of dyes, and on optical brightening additives. DEPOSITS CONTROL STRATEGIES Fundamentals of deposit control strategies were laid down in the mid- 1990’s but the research effort in this area continues with the focus on better understanding deposition mechanisms, identifying external factors affecting deposition, and developing new monitoring techniques, chemical products, and application strategies7. The goal of these studies remained the development of the clear logic needed when creating an action plan to combat deposits. This logic must contain non-chemical considerations including raw material selection, furnish preparation, contaminant purging strategy, and overall wet end chemistry management. Many of these items come with a significant price tag and the overall economic picture often needs to be compromised. In many cases, access to higher quality furnish, new equipment, and increased purging rates, is limited. Thus, the residual level of contamination needs to be addressed chemically on the paper machine. For a successful on-machine chemical treatment, we need to maintain the focus on wet end chemistry (pH, hardness, conductivity), proper selection of chemical additives and their addition points, and mixing additives with the furnish. FIXATION Fixation is the most important approach to chemical deposit control. In general, fixation represents a process of attachment of detrimental substances to furnish elements with the goal of reducing their agglomeration and the effective removal from the system with the sheet. Fixation is typically performed earlier, after the last purging or refining stage: Attachment to the long fibre, rather than to fines or fillers, is preferred as it increases the chances of removing contaminants from the system, as fibres are essentially retained. All hydrophobic materials undergo agglomeration. Hydrodynamic shear in papermaking systems creates more stress on larger particles. Fixation of small particles is more furnishes containing both natural pitch and stickies contaminants present a unique challenge for papermakers.