Focus on Cellulose ethers

How to control the thickening and thixotropy of cellulose ether?

The thickening effect of cellulose ether depends on: the degree of polymerization of cellulose ether, solution concentration, shear rate, temperature and other conditions. The gelling property of the solution is unique to alkyl cellulose and its modified derivatives. The gelation properties are related to the degree of substitution, solution concentration and additives. For hydroxyalkyl modified derivatives, the gel properties are also related to the modification degree of hydroxyalkyl. For low viscosity MC and HPMC, 10%-15% solution can be prepared, medium viscosity MC and HPMC can be prepared 5%-10% solution, and high viscosity MC and HPMC can only prepare 2%-3% solution, and usually the viscosity classification of cellulose ether is also graded with 1%-2% solution.
High-molecular-weight cellulose ether has high thickening efficiency, and polymers with different molecular weights have different viscosities in the same concentration solution. The target viscosity can only be achieved by adding a large amount of low molecular weight cellulose ether. Its viscosity has little dependence on the shear rate, high viscosity reaches the target viscosity, and the required addition amount is small, and the viscosity depends on the thickening efficiency. Therefore, to achieve a certain consistency, a certain amount of cellulose ether (concentration of the solution) and solution viscosity must be ensured. The gel temperature of the solution also decreases linearly with the increase of the concentration of the solution, and gels at room temperature after reaching a certain concentration. The gelling concentration of HPMC is relatively high at room temperature.
Consistency can also be adjusted by choosing particle size and choosing cellulose ethers with different degrees of modification. The so-called modification is to introduce a certain degree of substitution of hydroxyalkyl groups on the skeleton structure of MC. By changing the relative substitution values of the two substituents, that is, the DS and MS relative substitution values of the methoxy and hydroxyalkyl groups that we often say. Various performance requirements of cellulose ether can be obtained by changing the relative substitution values of the two substituents.
High-viscosity cellulose ether aqueous solution has high thixotropy, which is also a major characteristic of cellulose ether. Aqueous solutions of MC polymers usually have pseudoplastic and non-thixotropic fluidity below their gel temperature, but Newtonian flow properties at low shear rates. Pseudoplasticity increases with the molecular weight or concentration of cellulose ether, regardless of the type of substituent and the degree of substitution. Therefore, cellulose ethers of the same viscosity grade, no matter MC, HPMC, HEMC, will always exhibit the same rheological properties as long as the concentration and temperature are kept constant. Structural gels are formed when the temperature is raised, and highly thixotropic flows occur. High concentration and low viscosity cellulose ethers show thixotropy even below the gel temperature. This property is of great benefit to the adjustment of leveling and sagging in the construction of building mortar.
It needs to be explained here that the higher the viscosity of cellulose ether, the better the water retention, but the higher the viscosity, the higher the relative molecular weight of cellulose ether, and the corresponding decrease in its solubility, which has a negative impact on the mortar concentration and construction performance. The higher the viscosity, the more obvious the thickening effect on the mortar, but it is not completely proportional. Some medium and low viscosity, but the modified cellulose ether has better performance in improving the structural strength of wet mortar. With the increase of viscosity, the water retention of cellulose ether improves.

Post time: Mar-20-2023
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