Viscosity is an important parameter for the performance of cellulose ethers.
Generally speaking, the higher its viscosity, the better the water retention effect of the gypsum mortar. However, the higher the viscosity, the higher the molecular weight of the cellulose ether and the corresponding decrease in its solubility, which has a negative effect on the strength and construction properties of the mortar. The higher the viscosity, the more obvious the thickening effect on the mortar, but it is not a positive relationship.
The higher the viscosity, the stickier the wet mortar will be. During construction, this is reflected in the stickiness of the squeegee and the high adhesion to the substrate. However, it does not help much to increase the structural strength of the wet mortar itself. In addition, the sagging resistance of the wet mortar is not evident during construction. In contrast, some low to medium viscosity but modified methyl cellulose ethers have excellent performance in improving the structural strength of wet mortars.
Most of the building wall materials are porous structures and they are all strongly absorbent. The plaster building materials used for wall construction are easily absorbed by the wall after being modulated with water, resulting in a lack of water necessary for hydration of the plaster, which makes plaster construction difficult and reduces bond strength, resulting in cracks, drums, spalling and other quality problems. Improving the water retention of gypsum building materials can improve the quality of construction and the bond with the wall. Therefore, water retention agent has become one of the important admixtures for gypsum building materials.
The use of building powders such as plaster, bonding plaster, jointing plaster and plaster putty are manufactured with a plaster retarder to extend the construction time of the plaster paste in order to facilitate construction. The gypsum slurry needs to be kept on the wall for 1 to 2 h before it sets, due to the inclusion of retarders in the gypsum, which inhibit the hydration process of the hemihydrate gypsum. Therefore the gypsum slurry should be treated with water retention to avoid part of the water in the slurry being transferred to the wall. This causes a lack of water when the gypsum slurry hardens and incomplete hydration, causing separation and crusting at the joint between the gypsum and the wall. Adding water retention agent is to maintain the moisture contained in the plaster paste, to ensure the hydration of the plaster paste at the interface, thus ensuring the bond strength. Commonly used water retention 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 and rare earth powder can also be used to improve the water retention properties.
Regardless of what kind of water retention agent can delay the hydration speed of gypsum to varying degrees, when the amount of retardant remains unchanged, the water retention agent can generally retard the setting of 15-30min. therefore, the amount of retardant can be reduced appropriately.