Date of Award

5-2004

Level of Access Assigned by Author

Campus-Only Thesis

Degree Name

Master of Science (MS)

Department

Chemical Engineering

Advisor

Joseph M. Genco

Second Committee Member

David J. Neivandt

Third Committee Member

Hemant P. Pendse

Abstract

Fine particle retention is a critical process in forming paper and directly affects process economics and the quality of the paper being produced on the paper machine. Starch/polyacrylamide (PAM) gel complexes have recently been shown to be highly effective as retention aids in papermaking. In the gel preparation process, cationic starch is reacted with cationic PAM in an automated starch cooker. StarchPAM gels are thought to function primarily by "bridging flocculation" or simultaneous adsorption and entrapment of fine particles within the gel structures. Retention aids that function by bridging depend strongly on the charge density and molecular weight of the polymer. The polymers often suffer reductions in their effectiveness due to shear forces present on industrial paper machines that degrade their molecular weight. Likewise the effectiveness of polymers by bridging depends upon the residence time that the polymer has to interact with the particles being collected. The objective of the research was to determine the effects of shear rate and mixing time on the ability of the starch/PAM gels to retain fine particles under papermaking conditions. To meet this objective, starch/PAM gels were prepared in a batch starch cooker and used in retention experiments conducted in a drainage apparatus that is capable of draining stock samples under conditions of known shear rate. StarchPAM gels and commercial retention aids were added to pulp furnishes to aid in the filler retention. The drainage velocities produced in the apparatus give rise to drainage forces that simulate the forces present on industrial paper machines. The results for the filler retention were compared to suitable control systems. Several elements were found to be important in the formation of starch/PAM gels to achieve consistent retention results. The PAM needed to be fully uncoiled to facilitate adequate hydrolysis of the PAM and its subsequent bonding to the starch molecules to form the gels. Accurate measurement of the starch to PAM ratio and mixing were found to be important variables in determining the level of residual cationic charge on the starchPAM complex and producing uniform gels. The shear rate and mixing time were found to have an impact on the filler retention for both the starch/PAM gels and for the separate addition control system. The starchPAM gel outperformed the separate addition control system at all shear rates evaluated. This was thought to result from higher molecular weights and greater resistance to shear forces due to the coulombic forces holding the starch/PAM gel together. A maximum occurs in the filler retention curve as a function of shear rate at a fixed mixing time. Low retention found at low stirring speeds was thought to be caused by insufficient mixing of the starch1PAM gels. Low retention found at high stirrer speeds was thought to be cause by shear forces that lowered the molecular weight of the polymer. The maximum in the filler retention curves was though to occur from a shear rate high enough to cause good mixing but low enough not to cause degradation of the starchPAM gels.

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