Study On The Behaviors Of Polyamine And Highly Cationic Starches In Controlling Micro-stickies In Waste Pulp

With the increasing use of recycled fibers and utilization of white water system with higher closure, micro-stickies tends to accumulate more severely than ever in the wet-end of modern papermaking processes. Microstickies deposit problems therefore increase, which deteriorates paper quality, lowers paper machine runnability, and increases production cost. One traditional and widely used way to control micro-stickies is to use chemical fixing agents which can "fix" micro-stickies onto fibers and take them out of system through sheet forming. Among the fixing agents used in alkaline papermaking conditions, polyamine (PA) is most reasonable, considering its performance to cost ratio; in addition, highly cationic starches (HCS), which can be easily made by modifying natural polymeric starches, may have special characteristics in micro-stickies control, if used as fixing agents. Theoretically, highly cationic starches not only have good affinity to pulp fibers, but also to the dissolved polysaccharides in the water phase of pulp suspension, therefore adding HCSs to pulp suspension may react selectively to some micro-stickies components, and after this, the products may still work as strengthening agents, solving the problem of paper strength reduction resulted from the fixation of hydrophobic micro-stickies onto fibers with the use of those traditional polyelectrolyte-based fixing agents.Thus, in this study, first polyamine fixing agents with different charge densities and molecular weights were prepared. It was found that polyamine with greatest molecular weight and charge density could be obtained at a suitable branching agent dosage, and this polyamine had the best micro-stickies controlling effects. Moreover, using pulp filtrate as the medium for the fixing agent addition and comparing the changes of theζpotential, colloidal particle size, charge demand, and turbidity of the filtrate thereafter, can be considered as a rapid method to screen the polyamine fixing agents.Then, normal, linear and branched starches were used as raw materials to prepare highly cationic starches with different configurations, charge densities and molecular weights (the latter was controlled by acid hydrolyzing the original HCS to different extents). Applying the HCSs into recycled pulps showed that compared with the normal HCSs, the linear and branched HCSs with lower molecular weights can be adsorbed onto fibers easier, but the adsorbed amount was less. HCSs with lower molecular weights had better effects in controlling the dissolved and colloidal substances. With respect to the strengthening properties, it was shown that tensile index can be maintained or even increased by all the HCSs while burst index changed very little; interestingly, both the degraded normal and linear HCSs had some tendency to decrease folding endurance and tearing index of paper sheets (tearing index was influenced to a large degree than folding endurance), the branched HCSs, either degraded or not, can maintain these two properties well.At last, the differences of PA and degraded HCS in controlling microstickies were compared by adding them into recycled old newspaper pulp as well as the solution of an oxidized starch as the model compound for those polysaccharide-based dissolved and colloidal substances. It was concluded that, based on same weight dosages, the HCSs had better affinity to fibers and better selectivity in removing the polysaccharide-based dissolved and colloidal substances, their comprehensive micro-stickies controlling effects were no worse than the optimized polyamine.To sum up, in this study, a technique for preparing and optimizing polyamine fixing agent was mastered, a rapid method to evaluate the effects of the fixing agent was developed, and the idea that HCSs with lower molecular weights and unique molecular configurations can not only control micro-stickies well but also strengthen sheet properties was proposed and proved. The results will be beneficial in improving the clean production level in using waste paper pulps for papermaking.