hydroxypropyl methylcellulose HPMC in Plastering MortarFebruary 2nd 2023
HPMC endows wet mortar with excellent viscosity, which can significantly increase the bonding ability between wet mortar and base, and improve the anti-sagging performance of mortar. It is widely used in plastering mortar, external wall insulation system and tile bonding mortar. The thickening effect of cellulose can also increase the homogeneity and dispersion resistance of fresh cement-based materials, prevent the stratification, segregation and bleeding of mortar and concrete, and can be used in fiber concrete, underwater concrete and self-compacting concrete.
The viscosity of cement-based materials increased by hydroxypropyl methylcellulose comes from the viscosity of cellulose ether solution. The viscosity of cellulose generally refers to the viscosity value of cellulose ether solution with a certain concentration (such as 2%), which is measured by a specified measuring instrument (such as a rotary viscometer) at a specified temperature (such as 20℃) and shear rate (or rotation rate, such as 20 rpm).
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 base materials, and the stronger the anti-sag and anti-dispersion ability. However, if its viscosity is too large, it will affect the fluidity and operability of cement-based materials (such as sticking plaster knives in 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 with high fluidity require lower viscosity of cellulose ether.
In addition, the thickening effect of hydroxypropyl methylcellulose will increase the water demand of cement-based materials, thus increasing the output of mortar.
The viscosity of hydroxypropyl methylcellulose solution depends on the following factors:
1. Molecular weight (or degree of polymerization) and concentration of cellulose ether, solution temperature, shear rate and test method.
2. The higher the polymerization degree and molecular weight of cellulose ether, the higher the viscosity of its aqueous solution;
3. The higher the content (or concentration) of cellulose ether, the higher the viscosity of its aqueous solution. However, attention should be paid to selecting the appropriate content when using it, so as to avoid too high content, which will affect the working performance of mortar and concrete;
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 solution is usually pseudoplastic, which has the property of shear thinning. The higher the shear rate, the lower the viscosity.
Therefore, the cohesiveness of mortar will be reduced due to the external force, which is beneficial to the scraping construction of mortar, and enables mortar to have good workability and cohesiveness at the same time. However, when the concentration and viscosity of cellulose ether solution are very low, it will show Newtonian fluid characteristics. When the concentration increases, the solution will gradually show pseudoplastic fluid characteristics, and the higher the concentration, the more obvious the pseudoplasticity.