Date of Award

4-1997

Degree Name

Bachelor of Science

Department

Paper Science and Engineering

First Advisor

Dr. Dewei Qi

Abstract

The objectives of this thesis were 1) to determine the effects of recycling and increased drying temperature on the hygroexpansivity of the fibers and 2) to investigate the role of beating on the hygroexpansivity of the fibers.

A never-dried thermomechanical pulp was obtained and separated into three samples. Handsheets were produced, dried, and placed in a conditioning room. Sample one was air dried (77°F), sample two was dried at 155°F and sample three was dried at 250°F, respectively. Five handsheet samples at each varying temperature were taken and the hygroexpansivity was tested and calculated for each sheet using the equation BC= ΔL/L. B is the hygroexpansivity coefficient, C is the moisture content determined by W-Wo/Wo, where W is final weight of saturated sample after hygroexpansivity test and Wo is initial O.D. weight of sample. ΔL/L is defined as the strain of the sample. After testing the hygroexpansivity of the sheets, the samples were repulped in a British Disintegrator to resemble the recycling process. Once disintegrated, a series of handsheets were again made for each respective temperature, continuing for three recycles.

The effects of beating on the hygroexpansivity of the sheet were also investigated. The freeness of the thermomechanical pulp was initially determined and handsheets were made and dried at 250°F, then tested for hygroexpansivity. Once tested, the sheets were disintegrated following the aforementioned process. The pulp was then taken to a PFI mill and beaten to the initial freeness of the pulp before recycling occurred. Again, after the series of recycling and beating, the sheets were tested for hygroexpansivity. Four recycles were completed.

It was determined that as the number of recycles and drying temperature increased, the hygroexpansivity of the sheets decreased, becoming more dimensionally stable. Recycling and drying caused an increase of the crystalline region and hardening of the fibers. The fiber absorbed less water, therefore, causing the sheet to react less to the rising humidity.

The effects of beating on the hygroexpansivity were very peculiar. In the past, when chemical pulps have been tested the hygroexpansivity of the pulp increased with beating due to fibrillization of the fibers exposing the region of the fiber that is hydrophilic causing dimensional instability. For the mechanical pulp tested, as beating was instituted, the hygroexpansivity of the sheet actually decreased, meaning increased dimensional stability. This trend was true for each of the recycles, which is exactly opposite of what is believed. This may be attributed to the lignin and hemicelluloses present in mechanical pulps, which are not present in chemical pulps.

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