The advanges of HPMC & MHEC in dry mix mortar products
November 22nd 2024
The important role of water-soluble hydroxypropyl methyl cellulose ether in mortar is mainly in three aspects, one is excellent water retention capacity, the other is the effect on the consistency and thixotropy of mortar, and the third is the interaction with cement.
The water retention effect of hydroxypropyl methyl cellulose ether(HPMC) depends on the water absorption of the base layer, the composition of the mortar, the layer thickness of the mortar, the water demand of the mortar, and the setting time of the coagulating material. The water retention of hydroxypropyl methylcellulose ether(HPMC) itself comes from the solubility and dehydration of cellulose ether itself.
As we all know, although the hydroxypropyl methylcellulose(HPMC) molecular chain contains a large number of OH groups with strong hydration, it is not soluble in water itself, because the cellulose structure is highly crystalline. The hydration ability of the hydroxyl group alone is not enough to pay for the strong intermolecular hydrogen bonds and van der Waals forces. Therefore, it only swells and does not dissolve in water.
When a substituent is introduced into the molecular chain, not only the substituent destroys the hydrogen chain, but also the interchain hydrogen bond is destroyed due to the wedging of adjacent interchain substituents. The larger the substituent, the greater the intermolecular distance. The greater the effect of destroying the hydrogen bond, after the cellulose lattice is expanded, the solution enters, and the cellulose ether becomes water-soluble, forming a high-viscosity solution.
When the temperature increases, the hydration of the polymer weakens, and the water between the chains is expelled. When the dehydration is sufficient, the molecules begin to aggregate, forming a three-dimensional network structure and the gel folds out. Factors that affect the water retention of mortar include cellulose ether viscosity, addition amount, particle fineness and use temperature.
The higher the viscosity of hydroxypropyl methylcellulose ether(HPMC), the better the water retention performance and the viscosity of the polymer solution. Depends on the molecular weight of the polymer (degree of polymerization), but also by the molecular structure chain length and chain morphology. The distribution of the species and number of substituents also directly affects its viscosity range. The viscosity of the polymer solution depends on the molecular weight of the polymer.
The viscosity and concentration of cellulose ether solutions are related to applications in various fields. Therefore, each cellulose ether has many different viscosity specifications, and the adjustment of viscosity is mainly achieved by the degradation of alkali cellulose, that is, the breaking of cellulose molecular chains.
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