Hard-to-remove water in cellulose fibers characterized by thermal analysis: A model for the drying of wood-based fibers

Abstract

A new parameter, hard-to-remove (HR) water content, was recently defined as the moisture ratio at the onset point between the constant and falling rate zones in an isothermal cellulose drying process and was further investigated with differential scanning calorimeter (DSC) characterization in this study. The linear relationship between the HR water content and the water retention value (WRV) was explained by the combined results of thermogravimetric analysis (TGA) and DSC. The drying order of different classes of water was quantitatively verified. We found that free water was evaporated first, followed by trapped water, freezing-bound water, and then nonfreezingbound water, with some overlap. This was found for bleached, unbleached, and mechanical pulp. It was also found that the HR water mass is a combination of trapped water, freezing-bound water, and nonfreezing-bound water. Trapped water could be described as water that is not bound to the fibers, but is difficult to evaporate. For the pilot paper machine samples, all the water in the wet web after pressing and 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 is proposed with regard to decreasing moisture ratio. This model is valid for different types of wood-based cellulose fibers.