The Relationship Between Water Retention and Viscosity of Cellulose Ethers
Release Time:
2021-12-03
Source:
Water-retention properties of cellulose ethers: In the production of construction materials, particularly dry-mix mortars, cellulose ethers play an irreplaceable role; in the manufacture of special mortars (modified mortars), they are an indispensable and essential component. The key functions of water-soluble cellulose ethers in mortars can be summarized in three main aspects: first, their excellent water-retention capability; second, their influence on mortar consistency and thixotropy; and third, their interaction with cement. The water-retention performance of cellulose ethers depends on factors such as the absorbency of the substrate, the composition of the mortar, the thickness of the mortar layer, the mortar’s water demand, and the setting time of the binding material. As for the intrinsic water-retention capacity of cellulose ethers themselves, it stems from their solubility in water and their dehydrating effect. It is well known that although cellulose molecular chains contain a large number of highly hydrophilic hydroxyl groups, cellulose itself is insoluble in water due to its highly crystalline structure. The hydrophilic capacity of the hydroxyl groups alone is insufficient to overcome the strong intermolecular hydrogen bonds and van der Waals forces. Consequently, cellulose swells but does not dissolve in water. However, when substituent groups are introduced into the molecular chain, these groups not only disrupt the hydrogen-bonding network but also interfere with interchain hydrogen bonds by wedging themselves between adjacent chains; the larger the substituent group, the greater the intermolecular spacing, the more pronounced the disruption of hydrogen bonding, and the more extensive the swelling of the cellulose crystal lattice. Once the crystal lattice has expanded sufficiently to allow solvent penetration, the cellulose ether becomes water-soluble and forms a high-viscosity solution. As temperature rises, macromolecular hydration weakens, and water is progressively expelled from the interchain spaces. When dehydration becomes sufficiently complete, the polymer chains begin to aggregate, forming a three-dimensional network gel that precipitates out.
Factors influencing the water retention of mortar include cellulose ether viscosity, dosage, particle fineness, and application temperature.
The higher the viscosity of a cellulose ether, the better its water-retention performance. Viscosity is a crucial parameter for characterizing cellulose properties; currently, different cellulose manufacturers employ various methods and instruments to determine viscosity, with the main techniques including the Haake Rotovisko, Hoppler, Ubbelohde, and Brookfield viscometers. For the same product, viscosity measurements obtained using different methods can vary substantially—sometimes by as much as several-fold. Therefore, when comparing viscosities, it is essential to use the same test method throughout, ensuring that conditions such as temperature and rotor type are consistent.
In general, the higher the viscosity, the better the water-retention performance. However, as viscosity increases, the molecular weight of cellulose also rises, leading to a corresponding decline in its solubility, which can negatively affect the strength and workability of mortar. While higher viscosity does produce a more pronounced thickening effect on mortar, this relationship is not strictly linear. Increased viscosity makes wet mortar more sticky, resulting in greater adhesion to the trowel and to the substrate during application; yet it provides only limited improvement in the structural integrity of the wet mix itself, as evidenced by poor resistance to sagging. Conversely, certain modified methyl cellulose ethers with medium to low viscosities demonstrate outstanding performance in enhancing the structural strength of wet mortar.
