Hydroxypropyl methylcellulose endows wet mortar with excellent viscosity, which can significantly increase the bonding ability between wet mortar and the base layer, and improve the anti-sag performance of mortar. It is widely used in plastering mortar, external wall insulation system and brick bonding in mortar. The thickening effect of cellulose ether can also increase the homogeneity and anti-dispersion ability of freshly mixed cement-based materials, prevent delamination, segregation and bleeding of mortar and concrete, and can be used in fiber concrete, underwater concrete and self-compacting concrete .
Hydroxypropyl methylcellulose increases the viscosity of cement-based materials from the viscosity of cellulose ether solution. The index of “viscosity” is usually used to evaluate the viscosity of cellulose ether solution. The viscosity of cellulose ether generally refers to the cellulose ether solution with a certain concentration (such as 2%). Speed (or rotation rate, such as 20 rpm), the viscosity value measured with a specified measuring instrument (such as a rotational viscometer).
Viscosity is an important parameter to evaluate the performance of cellulose ether. The higher the viscosity of hydroxypropyl methylcellulose solution, the better the viscosity of cement-based materials, the better the adhesion to the substrate, and the better the anti-sagging and anti-dispersion capabilities. Strong, but if its viscosity is too large, it will affect the fluidity and operability of cement-based materials (such as sticking a plastering knife during plastering mortar construction). Therefore, the viscosity of cellulose ether used in dry-mixed mortar is usually 15,000~60,000 mPa. S-1, self-leveling mortar and self-compacting concrete, which require higher fluidity, require lower viscosity of cellulose ether.
In addition, the thickening effect of hydroxypropyl methylcellulose will increase the water demand of cement-based materials, thereby increasing the yield of mortar.
The viscosity of hydroxypropyl methylcellulose solutions depends on the following factors:
1. Molecular weight (or degree of polymerization) and concentration, solution temperature, shear rate and test method of cellulose ether.
2. The higher the degree of polymerization of cellulose ether, the greater the molecular weight, and the higher the viscosity of its aqueous solution;
3. The higher the dosage (or concentration) of cellulose ether, the higher the viscosity of its aqueous solution, but care should be taken to select the appropriate dosage when using it, so as not to affect the performance of mortar and concrete if the dosage is too high;
4. Like most liquids, the viscosity of cellulose ether solution will decrease with the increase of temperature, and the higher the concentration of cellulose ether, the greater the influence of temperature;
Cellulose ether solution is usually a pseudoplastic, which has the property of shear thinning, and the higher the shear rate is, the lower the viscosity is.
Therefore, the cohesion of mortar will be reduced due to external force, which is beneficial to the scraping construction of mortar, so that mortar can have good workability and cohesion at the same time. However, the cellulose ether solution will exhibit Newtonian fluid characteristics when the concentration is very low and the viscosity is small. When the concentration increases, the solution will gradually exhibit pseudoplastic fluid characteristics, and the higher the concentration, the more obvious the pseudoplasticity.
Post time: Jan-09-2023