- July 1, 2021
- Posted by: admin
- Category: News
Hydroxypropyl methyl cellulose-water retention is very important properties and it ‘s also concerned by many dry powder mortar manufacturers in China, especially in the south with high temperature. Hydroxypropyl methyl cellulose plays an irreplaceable role in the production of building materials, especially in the production of dry powder mortar, especially in the production of special mortar (modified mortar). The viscosity, content, ambient temperature and molecular structure of hydroxypropyl methylcellulose have great influence on its water retention.
Under the same conditions, the higher the viscosity of hydroxypropyl methylcellulose is, the better the water retention is, and the higher the content is, the better the water retention is, usually a small amount of hydroxypropyl methylcellulose can greatly improve the water retention of mortar. when the content reaches a certain extent, the increasing trend of water retention slows down.With the increase of ambient temperature, the water retention of cellulose ether usually decreases, but some modified hydroxypropyl methylcellulose also has better water retention at high temperature, and the water retention of hydroxypropyl methylcellulose with lower degree of substitution is better. The oxygen atoms on the hydroxyl groups and ether bonds of hydroxypropyl methylcellulose molecules will associate with water molecules to form hydrogen bonds, making free water into bound water, thus playing a good role in water retention.The interaction between water molecules and hydroxypropyl methylcellulose molecular chains enables water molecules to enter into the hydroxypropyl methylcellulose macromolecular chain and is strongly binding, thus forming free water, winding water, and improving the water retention of cement slurry.
Hydroxypropyl methylcellulose improved the rheological properties of fresh cement paste, porous network structure and osmotic pressure or film-forming properties of hydroxypropyl methylcellulose hindered the diffusion of water. Hydroxypropyl methyl cellulose-thickening and thixotropy. Hydroxypropyl methyl cellulose gives excellent viscosity to wet mortar which can significantly increase the bonding ability between wet mortar and base and improve the sagging resistance of mortar.
It’s widely used in plastering mortar, brick bonding mortar and external wall thermal insulation system.
The thickening effect of hydroxypropyl methylcellulose can also increase the anti-dispersion ability and homogeneity of fresh materials, prevent material delamination, segregation and bleeding and can be used in fiber reinforced concrete, underwater concrete and self-compacting concrete. The thickening effect of hydroxypropyl methyl cellulose on cement-based materials comes from the viscosity of hydroxypropyl methyl cellulose solution.Under the same conditions, the higher the viscosity of hydroxypropyl methyl cellulose is, the better the viscosity of the modified cement-based material is, but if the viscosity is too high, it will affect the fluidity and maneuverability of the material (such as sticky plastering knife).Self-leveling mortar and self-compacting concrete with high fluidity require low viscosity of hydroxypropyl methyl cellulose.
In addition, the thickening effect of hydroxypropyl methyl cellulose will also increase the water demand of cement-based materials and increase the output of mortar. The aqueous solution of hydroxypropyl methyl cellulose with high viscosity has high thixotropy, which is also a major characteristic of hydroxypropyl methyl cellulose.The aqueous solution of methyl cellulose usually has pseudoplasticity and non-thixotropic fluidity below its gel temperature, but it has Newtonian flow properties at low shear rate.The pseudoplasticity increases with the increase of molecular weight or concentration of hydroxypropyl methyl cellulose and has nothing to do with the type of substituents and the degree of substitution.
Therefore, the same viscosity grade of hydroxypropyl methyl cellulose, regardless of MC,HPMC,HEMC, as long as the concentration and temperature remain constant, always show the same rheological properties.