Drying Behavior of Cellulose Fibers Characterized by Thermal Analysis
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Date
2007-05-08
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Abstract
The objective of this research is to understand the drying behavior of cellulose fibers characterized by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). A parameter termed "hard-to-remove (HR) water content" was defined as the ratio of water mass to fiber mass at the transition between the constant and falling rate drying zones of an isothermal TGA experiment. The HR water content showed a linear relationship with water retention value. TGA and DSC were used to explain the linear relationship. During drying, free water was observed to evaporate first, followed by trapped water, freezing bound water and then non-freezing bound water, with some overlap. Trapped water can be described as water bound to the fibers, but difficult to evaporate. For pilot papermachine samples, all the water in the wet web entering the dryer section was HR water, with no free water detected. Based on the results, a qualitative drying model of cellulose fibers was proposed with regard to decreasing moisture ratio. Changes in the pore size distribution during drying were determined using DSC measurements and the Gibbs-Thomson equation. Larger pores collapsed first followed by the sequential collapse of smaller pores, indicating that pore wall collapse resistance is critical. The average pore size for bleached softwood was calculated to be about 80 nm and decreased with drying. A constant pore size of about 20 nm was observed at moisture ratios below 0.3 g⁄g, which corresponds to one-to-two layers of non-freezing bound water. The heat of vaporization of water associated with cellulose fibers was determined using modulated DSC and showed a steep increase at moisture ratios below 0.3 g⁄g, indicating that a higher energy is required to evaporate non-freezing bound water.
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Keywords
drying, cellulose, thermal analysis
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Degree
PhD
Discipline
Wood and Paper Science
