Preparation And Application Of Cross-Linked Amphoteric Polyacrylamide

In recent years, the development trends in pulp and paper industry made the demand for ideal paper strengthening agent increasingly growing. The trends included the papermaking system changed from acid to neutral or alkaline, elevated paper-machine speed, and the increased use of fillers, high-yield pulps and recycled fibers and further closed water circuit. In order to ensure the output and quality of the paper production, novel papermaking additives clearly need to be developed to meet the demand for more cost-effective strengthening agents. The cross-linked AmPAM with a high molecular weight, low viscosity, and more binding points became a new direction in research and development of novel enhancing agent.According to mechanism of free radical polymerization, a series of cross-linked AmPAM with different molecular weights and charge density were prepared as water solution by using acrylamide (AM),2-methacryloxyethyl trimethyl ammonium chloride (DMC), and acrylic acid (AA) as comonomers, N,N-methylene bis-acrylamide (MBA) and sodium methylallyl sulfonate (SMAS) as molecular structure modification additives, ammonium persulfate as initia. The results showed that AmPAM with a good strengthening effect could be synthesized under the following polymerization conditions:reaction temperature 70-75℃, the charge of initiator 0.2 wt%, reaction pH value 3,the dosage of cross-linking monomer MBA 0.09wt%, the dosage of cationic monomer DMC15wt%, the dosage of anionic monomer AA 2.2wt%, the dosage of chain transfer agent SMAS 1.1 wt%, reaction concentration 20%. The properties of the cross-linked AmPAM prepared under the above conditions were the cationic charge density of 0.78～0.81 mmol·g-1, the molecular weight of 35×104, the pH value of 3-4 and the stable quality.The pilot scale experiment was conducted according to the synthesis process set up in the laboratory. The properties of cross-linked AmPAM accorded with the demands of paper mills, and the cross-linked AmPAM had good strengthening effect which was as good as that of cross-linked AmPAM obtained in the laboratory. Through pilot trail, the feasible process conditions for AmPAM production were presented to meet the industrialization application.The prepared cross-linked AmPAM was used for the handsheet strenghening experiments. The results showed that the mechanical strength of paper was remarkably enhanced. The tensile index and breaking length of the handsheets increased by 25-35% and 800-1000m, respectively, compared with those of handsheets without addition of AmPAM. And the change of the relative molecular weight of the AmPAM had a greater impact on the strenghening effect for the bleached pulp, while the influence of changes in the charge density of the AmPAM on the strenghening effect for OCC pulp was more apparent.Impacts of white water cycling and filler on the application effect of cross-linked AmPAM were investigated. The results showed that, with the increasing in the amount of filler, the strenghening effect of the cross-linked AmPAM was gradually decreased, and the retention of filler was also reduced. And with the increasing of the times of white water cycling, the paper strength of OCC pulp increased continuously, but the paper strength of the mixed pulp stock (20%filler) essentially unchanged and the filler retention increased significantly.