Date of Award

5-2014

Level of Access

Campus-Only Thesis

Degree Name

Master of Science (MS)

Department

Chemical Engineering

Advisor

Joseph Genco

Second Committee Member

Douglas Bousfield

Third Committee Member

Hemant Pendse

Abstract

Yield stress provides a good indication of the network strength of pulp suspension and is important in the design and operation of paper machines. Yield stress is defined as the shear stress required for flow to take place within a particle suspension and is measured by a variety of techniques. Consequently, literature data are often not in agreement. The objective of the thesis was to thoroughly characterize a broad range of pulps using standardized techniques in an effort to elucidate the fundamental fiber properties that contribute to network strength.

To meet the objective of this study, yield stress was determined for twenty-five industrially important pulps of varying furnish type and origin. Each pulp was fully characterized by measuring its average fiber length, its width, aspect ratio, coarseness or mass per unit length, and the number of kinks and level of curl in the fibers. Industrially important pulp properties were also measured and included water retention value, pulp freeness, and fine particle and ash content in the samples. The effects of process conditions such as pulp mixing and refining were also explored in controlled trials. Lastly, a generalized correlation was developed that relates yield stress to key fiber physical properties.

Fiber length was found to be the most important fiber physical property contributing to network strength, and was likely due to the fact that longer fibers form an increased number of inter-fiber contacts in the suspension. Of the fiber lengths measured, the weight-weighted fiber length was found to correlate most strongly with the increase in yield stress, which suggests that the longer fibers in suspension are contributing most heavily to the overall strength of the fiber network. The fine particles in suspension were shown to provide a negligible contribution to the overall strength of the fiber network, as they are too small to contribute significantly to the necessary inter-fiber contacts. An increase in fines content therefore results in a considerable reduction in network strength at a given mass consistency.

The yield stress of hardwood and softwood Kraft furnishes mixed in various proportions was shown to display an ideal mixing relationship, in which the resulting yield stress was simply the weighted average yield stress of the two pulps mixed together. The mixtures of thermomechanical pulp and softwood Kraft did not exhibit ideal mixing. Rather the resulting yield stress was more strongly influenced by the addition of softwood Kraft. High intensity refining was found to result in a decrease in yield stress when compared to low intensity refining. This observation was attributed to fiber shortening and loss of fiber stiffness during the high intensity refining.

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