WORLD PULP&PAPER 94 effective; therefore, an early fixation is recommended. Fixation doesn’t offer any strong bond between contaminant particles and fibre. Some level of re-dispersion of fixed particles will always take place. The final, polishing fixation step is recommended with a typical application point at the suction side of Machine Chest pump or the Stuff Box accepts (still to the thick stock addition point). The effectiveness of this polishing treatment strongly depends on the proper, fast mixing of fixative with the stock and requires specialised mixing equipment such as Nalco Pareto™ nozzle. Excellent results were observed with the introduction of polishing treatment in many applications8. FIXATIVES Typically, fixatives included low molecular weight and high charge materials operating through the charge neutralisation mechanism. Historically, in acid papermaking, Alum served as a pitch control agent acting through the charge neutralisation and suppression of the double layer thickness by trivalent aluminum ion (Al+3) present at low pH levels. Alum remains a very important pitch control agent in regions utilising wood species with extremely high pitch levels, such as southern pine in the South Eastern USA. Dependence on high dosages of alum, and its incompatibility with Calcium Carbonate filler remains a major barrier for neutral pH conversions in these regions. Initial concerns about corrosion, followed by a trend towards neutral and alkaline papermaking brought about the development of synthetic polymeric coagulants as fixative agents. These materials are typically 100 mol% cationic (every monomer is positively charged). The charge density of these polymers depends on the molecular mass of the individual monomer and can typically vary between 4-15 mEq/g of product. Cationic charge of these polymers originates from various forms of amine functionality. It is important to realise that only quaternary amine functionality, maintains charge level at higher pH and remains the only option for both neutral and especially alkaline papermaking. Loss of charge was observed when transition to a higher pH of operation was carried out using a fixative package used at acid pH; this resulted in significant losses of efficiency due to deposit outbreaks8,9. Most versions of these typical coagulants, are based on DADMAC, EPI-DMA and PEI chemistry. These polymers are typically of low molecular weight (100,000 – 1,000,000 g/mol) and are sold as 20-50% solutions in water. Common coagulants operate via charge neutralisation and some contribution of mosaic adhesion (patch adhesion) mechanisms. HYBRIDS As discussed earlier in this paper, chemistry, size, and charge differences between different sources of hydrophobic contamination, allowed us to question the one-size-fits-all approach to their control. In the mid-1990’s, Nalco Water launched significant efforts to explore a different approach to the fixative application. The main intention was to move away from charge neutralisation as the major mechanism of fixation. Our initial effort was focused upon increasing molecular weight of poly- DADMAC products. Low reactivity of DADMAC monomer and the natural difficulty to propagate the growth of polymeric chain due to electrostatic repulsion resulted in the limited success of this approach. Additionally, such homo-polymers of DADMAC would still have a high charge density and potentially be quite expensive. Synthesis of co-polymers of DADMAC and acrylamide led us to the introduction of novel fixative materials. Since these polymers occupy an intermediate position between typical coagulants and flocculants and operate via a mix of charge neutralisation and flocculation, they are known as HYBRID coagulants. The first generation of HYBRID chemistry included two products with 30% mole charge densities and two different molecular weights described by reduced specific viscosity (RSV) values in the range of 3.5-6 g/l. The development, chemistry and first application of such was described in a paper published with a newsprint mill in Canada using TMP and low 000level of deinked pulp10. Other early application of this first generation of HYBRID chemistry for pitch control resulted in a 7% efficiency increase in one of the newsprint mills in Australia8,9. HYBRIDS were found to be very effective in pitch, stickies, and coated broke control applications and have been used in all regions of the world. Additionally, HYBRID chemistry became a part of the retention programs, used as a single polymer (mainly in newsprint applications), as a part of a dual polymer program (fine paper and board) with typical flocculants, and to boost specific filler retention by pre-treatment of fillers. DEPOSIT CONTROL IN PAPERMAKING SYSTEMS Only quaternary amine functionality, maintains charge level at higher pH