Pulp hysteresis

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The hysteresis of cellulose is based on an incomplete reversibility of sorption and desorption processes .

The formation of cellulose normally occurs with the participation of water. The binding sites (especially OH groups ) of the cellulose chain are therefore surrounded by a hydration shell . If the cellulose is now dried, these points can enter into inter- and intramolecular bonds ( hydrogen bonds ), which can be more or less weak to strong (20 kJ / mol). Due to the energetically most favorable arrangement, these are rather immobile. If the cellulose is now moistened again, only those places can be occupied again by the water that have not yet formed a bond or whose bond is very weak. Very strong bonds are retained. This means that the amount of water absorbed is less than the amount originally given off.

This effect plays an important role when comparing pulp samples. If a cellulose sample comes from a dry climate, it will never reach the same moisture content as a sample that comes from a humid climate.

The effect cannot be completely eliminated even with a longer residence time in a certain climate. However, the higher the temperature, the smaller the differences.

The characteristic hysteresis curve of cellulose represents a range of possible water absorption at a certain relative humidity . The sorption curve indicates the minimum, the desorption curve the maximum possible moisture content at a certain relative humidity.