Drying behavior of cellulose fibers characterized by thermal analysis

The objective of this research is to understand the drying behavior of cellulose fibers characterized by TGA and 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 zones from isothermal TGA experiments. At specific experimental conditions, the HR water content showed a linear relationship with water retention value (WRV). This relationship was explained by the combined results of TGA and DSC. The drying order of different classes of water existed in cellulose fibers was quantitatively verified and found that free water was evaporated first followed by trapped water, freezing bound water and then non-freezing bound water, with some overlap. The HR water mass was a combination of trapped water, freezing bound water, and non-freezing bound water. The existence of trapped water could be described as water that is not bound to the fibers, but is difficult to evaporate. For the pilot papermachine samples, all the water in wet web entering the dryer section was found to be entirely 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 pore size distribution during the drying were determined using DSC based on the Gibbs-Thomson equation. It was observed that larger pores collapse first followed by the sequential collapse of smaller pores, indicating that pore wall collapse resistance is the primary factor. The average measured pore size for bleached kraft softwood was calculated about 80 nm and reduced 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 a 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.