Thickening and Thixotropy of Cellulose Ethers
Release Time:
2021-12-27
Source:
Thickening and Thixotropy of Cellulose Ethers: The second function of cellulose ethers—thickening—is influenced by factors such as the degree of polymerization, solution concentration, shear rate, and temperature. The gelation behavior of these solutions is a property unique to alkyl celluloses and their modified derivatives, and it depends on the degree of substitution, solution concentration, and the presence of additives. For hydroxyalkyl-modified derivatives, gelation characteristics are also affected by the extent of hydroxyalkylation. For low-viscosity celluloses and HPC, solutions with concentrations of 10%–15% can be prepared; for medium-viscosity celluloses and HPC, solutions with concentrations of 5%–10% are suitable; whereas high-viscosity celluloses and HPC can only be formulated at 2%–3%. In fact, viscosity grades for cellulose ethers are typically defined based on 1%–2% solutions. High-molecular-weight cellulose ethers exhibit high thickening efficiency; at the same concentration, polymers with different molecular weights display distinct viscosities, which can be related to molecular weight by the following equation: [η] = 2.92 × 10⁻² (DPₙ)⁰·⁹⁰⁵, where DPₙ represents the number-average degree of polymerization. Lower-molecular-weight cellulose ethers require larger dosages to achieve the desired viscosity. Their viscosity is relatively insensitive to shear rate, so achieving the target viscosity with high-viscosity formulations requires smaller addition levels; the magnitude of viscosity, in turn, is determined by thickening efficiency. Therefore, to attain a specified consistency, it is essential to ensure an appropriate dosage of cellulose ether (i.e., solution concentration) and maintain the desired solution viscosity. Furthermore, the gelation temperature of the solution decreases linearly with increasing concentration, and once a certain threshold is reached, gelation occurs at room temperature. For HPC, the concentration required for gelation at room temperature is higher.
The consistency can also be adjusted by selecting the particle size and by choosing cellulose ethers with different degrees of modification. By “modification,” we mean introducing hydroxyalkyl groups into the cellulose backbone to a specified degree of substitution. By varying the relative substitution levels of the two substituents—commonly referred to as the DS (degree of substitution) of methoxy groups and the ms (molar substitution) of hydroxyalkyl groups—we can tailor the cellulose ether to meet a wide range of performance requirements.
Relationship between consistency and modification: The addition of cellulose ethers affects the water demand of mortar, altering the water-to-cement ratio and thereby producing a thickening effect; the higher the dosage, the greater the water requirement.
Cellulose ethers used in powdered construction materials must dissolve rapidly in cold water and impart appropriate viscosity to the system. If, even under a specified shear rate, the product remains in a flocculent or gel-like clumped form, it is considered nonconforming or of poor quality.
