Hydroxypropyl methyl cellulose(HPMC) mainly plays a role of water retention in cement & gypsum based drymix products, which can effectively improve the adhesion and sag resistance of the mortar.
Factors such as air temperature, temperature and wind pressure speed will affect the volatilization rate of water in cement or gypsum-based mortar. Therefore, in different seasons, there are some differences in the water retention effect of the same amount of HPMC products. In the specific construction, the water retention effect of the mortar can be adjusted by increasing or decreasing the amount of HPMC added. The water retention of HPMC under high temperature conditions is an important indicator to distinguish the quality of HPMC. Excellent HPMC series products can effectively solve the problem of water retention at high temperatures. In high-temperature seasons, especially in hot and dry areas and thin-layer construction on the sunny side, high-quality HPMC is required to improve the water retention of the mortar. High-quality HPMC has very good uniformity. Its methoxy and hydroxypropoxy groups are evenly distributed along the cellulose molecular chain, which can improve the ability of the oxygen atoms on the hydroxyl and ether bonds to associate with water to form hydrogen bonds , So that free water becomes bound water, thereby effectively controlling the evaporation of water caused by high temperature weather, and achieving high water retention.
High-quality HPMC can be uniformly and effectively dispersed in cement mortar and gypsum based mortar, and wrap all solid particles, and form a layer of wetting film. The water in the base is gradually released over a long period of time. The condensed material undergoes hydration reaction, thereby ensuring the bonding strength and compressive strength of the material. Therefore, in the high temperature summer construction, in order to achieve the effect of water retention, it is necessary to add high-quality HPMC products in sufficient quantities according to the formula, otherwise, insufficient hydration, reduced strength, cracking, hollowing and falling off will occur due to excessive drying. Problems, but also increase the difficulty of construction workers. As the temperature drops, the amount of HPMC added can be gradually reduced, and the same water retention effect can be achieved.
The water retention of HPMC is affected by following factors:
1. HPMC uniformity
The homogeneously reacted HPMC has uniform distribution of methoxy and hydroxypropoxy groups and high water retention rate.
2. HPMC gel temperature
Thermal gel temperature is high, the water retention rate is high; on the contrary, the water retention rate is low.
3. HPMC viscosity
When the viscosity of HPMC increases, the water retention rate also increases; when the viscosity reaches a certain level, the increase in water retention rate tends to be flat.
Viscosity: Brookfield RV, 2%
4.HPMC particle size
Hpmc with large particle size dissolves faster, and water retention can develop faster.
5. HPMC addition amount
The greater the amount of HPMC added, the higher the water retention rate and the better the water retention effect. In the range of 0.25-0.6%, the water retention rate increases rapidly with the increase of the addition amount; when the addition amount further increases, the increasing trend of the water retention rate slows down.
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