Date of Award

4-1997

Degree Name

Bachelor of Science

Department

Paper Science and Engineering

First Advisor

Dr. Dewei Qi

Abstract

There has been conflicting data regarding the effects of recycling on pulps. Strength properties go down for chemical pulps after being recycled, but the strength properties go up for mechanical pulps after recycling. The objective of the thesis will be to study the effects of varying the drying temperature on the recycled fiber properties of thermomechanical pulp.

A never-dried thermomechanical pulp furnish was obtained, from which three samples were separated out. Pulp sample one was air dried at approximately 77°F, sample two was dried at a temperature of 155°F, and the third sample was dried at a temperature of 250°F. Handsheets were made, dried, conditioned and tested for each of the three pulp samples. The paper was then soaked, disintegrated and dried at the same temperature as before. This continued to three recycles. It was imperative that the pulps remained free of contaminants and dried at the same temperature each time.

Strength properties were tested and the following key results were obtained. Density of all three samples increased slightly due to the unravelling of the fibers. The breaking length decreased for the samples dried at 155 and 250°F up to the second recycle, the sample that was dried at 250°F had the largest decrease in breaking length. The 250°F sample had an initial breaking length of 3.36km and this dropped to a value of 2.82 (km) after the second recycle. This is in comparison to an initial of 2.97 (km) and a value of 2.81 (km) after the second recycle for the 155°F sample. The air dried sample did not show an increase or decrease in overall breaking length strength. Upon examination of the wet breaking length data, it was shown that because of the 250°F samples underwent crystallization and did not absorb as much water as the 155°F or air dried sample, they had the highest wet breaking strength. The value of the wet breaking length for 250°F after three recycles was .132 (km) as compared to .122 (km) and .0859 (km) for the 155°F and the air dried samples, respectively. Scott Bond test showed a slight decrease in bond strength up through the second recycle and the zero span test showed that the fiber strength increased slightly. This explains that the loss of breaking length strength for the samples is due to loss of bond area and strength and not loss of fiber strength. The data supports the hypothesis that drying at higher temperatures does have the greatest negative effect on the recyclability of thermomechanical pulp. However, and more importantly, the data also shows that as the number of recycles increases, the effect of temperature on the sheet properties decreases.

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