INTRODUCTION With the increased content and lower quality of recycled fibre, decreased and highly varying quality of recycling raw materials, a trend towards neutral pH in mechanical grades, and the increasing application of High Yield Pulps (HYP) in typical fine paper or board furnish, issues related to controlling pitch and stickies are becoming more and more important to the overall economic results of many paper mills. Based upon our observations, these deposition issues that typically affect the wet end of the paper machine, forming and press sections, have recently been found to form further down the process – in the dryer section and on calendar stack rolls. In some cases, these deposits can reach printing press operations, leading to significant printability complications. Systems operating with a blend of virgin, mechanical or chemical pulps and recycled fibre, experience a uniquely high propensity for deposition. Higher levels of accumulated, dissolved and colloidal substances can result in a lower stability of the fragile colloidal state of hydrophobic materials, thus making them prone to agglomeration and consequent uncontrollable deposition. In turn, this leads to increased levels of mill closure. The prevention of deposit outbreaks on either paper or board machines, an ability to operate with lower quality and cheaper furnish, and avoiding DEPOSIT CONTROL IN PAPERMAKING SYSTEMS WORLD PULP&PAPER 90 costly equipment upgrades, will remain three increasingly important factors in improving economic results. In many cases, these may be the sole deciders in the sustainability of a particular machine operation. Importance of these issues is reflected in recently-undertaken paper industry research that concluded in a better understanding of various extractive materials and the mechanism of their deposition. Factors affecting their stability and mechanisms explain the principles of operation of chemical programs. These efforts resulted in new chemical additives with optimised application strategies. This paper reviews some of the new research findings in the engineering of new, medium molecular weight / medium charge fixatives (known as HYBRID coagulants) and its mechanisms of operation. HYDROPHOBIC MATERIALS Water represents a highly structured network of individual molecules that is disrupted by any hydrophobic materials not able to participate in this network structure. Hydrophobic materials have been a part of the papermaking process whether it uses virgin or recycled pulps. In a thermodynamic sense, the destabilisation energy of the water network is proportional to the total surface area of hydrophobic materials. In the attempt to reduce this surface area, hydrophobic materials undergo spontaneous agglomeration. Agglomeration of hydrophobic materials is the single most important step towards uncontrollable deposition. Although natural wood pitch and man-made stickies come under the hydrophobic materials banner, there are many differences in their chemistry, morphology, and stability that need to be taken into consideration when developing new products or designing effective control programming into the machine. NATURAL PITCH Natural wood pitch is present in both virgin mechanical and chemical pulps. However, several washing stages and exposure to harsh chemical treatment can reduce the level and change the chemical composition of pitch in chemical pulps. In mechanical pulps, pitch enters the papermaking system predominantly as a colloidal dispersion (0.1-2μm), with the unimodal distribution of all wood pitch component within each colloidal particle1 (Figure 1). Pitch components with higher polarity, serve as stablisers for more hydrophobic components – as found in the inner core – which can interact with the water structure, and are located on the surface of such particles. In the case of softwood pitch, both fatty acids and resin acids, are available to deposition issues that typically affect the wet end of the paper machine, forming and press sections, have recently been found to form further down the process By Przemyslaw Pruszynski, Global Technical Specialist, and Ruben Manzanares, Senior Marketing Manager, Nalco Water, an Ecolab Company HYBRID Coagulants – expanding chemistry toolbox for deposit control in papermaking systems ™