The Importance of Water Retention of HPMC
April 24th 2024
Viscosity is an important parameter of cellulose ether performance.
Generally speaking, the higher the viscosity, the better the water retention effect of gypsum mortar. However, the higher the viscosity, the higher the molecular weight of the cellulose ether, and the corresponding reduction in its solubility, which has a negative impact on the strength and construction performance of the mortar. The higher the viscosity, the more obvious the thickening effect on the mortar, but it is not proportional.
The higher the viscosity, the more viscous the wet mortar will be. During construction, it will be sticky to the scraper and have high adhesion to the substrate. However, it does not help much to increase the structural strength of the wet mortar itself. In addition, during construction, the sag resistance of wet mortar is not obvious. On the contrary, some medium and low viscosity but modified methyl cellulose ethers have excellent performance in improving the structural strength of wet mortar.
Most of the building wall materials are porous structures, and they all have strong water absorption. The gypsum building materials used for wall construction are added to the wall after being modulated by adding water, and the water is easily absorbed by the wall, resulting in the lack of water necessary for hydration of the gypsum, causing difficulties in plastering construction and reducing the bonding strength, resulting in cracks, Quality problems such as hollowing and peeling. Increasing the water retention of gypsum building materials can improve the construction quality and the adhesion to the wall. Therefore, water retaining agent has become one of the important admixtures for gypsum building materials.
Plastering plaster, bonding plaster, caulking plaster, gypsum putty and other building powder materials are used. In order to facilitate the construction, a gypsum retarder is added in the production to extend the construction time of the gypsum slurry. Because the plaster is mixed with people The retarder inhibits the hydration process of hemihydrate gypsum. This type of gypsum paste needs to be kept on the wall for 1 to 2 hours before it is set. Most of the walls have water absorption properties, especially brick walls and aerated concrete Lightweight new wall materials such as walls, porous insulation boards, etc., so the gypsum slurry should be treated with water to prevent part of the water in the slurry from being transferred to the wall, causing water shortage and incomplete hydration when the gypsum slurry hardens. Causes the separation and shelling of the joint between the plaster and the wall. Adding water-retaining agent is to maintain the moisture contained in the gypsum slurry, to ensure the hydration reaction of the gypsum slurry at the interface, and to ensure the bonding strength. Commonly used water-retaining agents are cellulose ethers, such as: methyl cellulose (MC), hydroxypropyl methyl cellulose (HPMC), hydroxyethyl methyl cellulose (HEMC) and so on. In addition, polyvinyl alcohol, sodium alginate, modified starch, diatomaceous earth, rare earth powder, etc. can also be used to improve water retention.
No matter what kind of water-retaining agent can delay the hydration speed of gypsum to varying degrees, when the amount of retarder does not change, the water-retaining agent can generally retard 15-30min. Therefore, the amount of retarder can be appropriately reduced.
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