Relation between water retention and viscosity of HPMC

HPMC water retention: in building materials, especially in the production of dry mortar, cellulose ether plays an irreplaceable role, especially in the production of special mortar (modified mortar), is an indispensable important part. The important role of water-soluble cellulose ether in mortar mainly has three aspects, one is excellent water retention ability, the second is the impact on the consistency and thixotropy of mortar, and the third is the interaction with cement. The water retention effect of cellulose ether depends on the water absorption of the base, the composition of the mortar, the thickness of the mortar, the water requirement of the mortar, and the setting time of the set material. HPMC itself water retention comes from the solubility of cellulose ether itself and dehydration. It is well known that cellulose molecular chains, although they contain a large number of highly hydrating OH groups, are themselves insoluble in water because cellulose structures are highly crystalline. The hydration capacity of the hydroxyl group alone is not enough to pay for the strong hydrogen bonds and van der Waals forces between molecules. Therefore, when substituents are introduced into molecular chains, they not only destroy hydrogen chains, but also destroy inter-chain hydrogen bonds due to the wedging of substituents between adjacent chains. The larger the substituents, the greater the distance between molecules. The larger the effect of hydrogen bond destruction is, the solution enters after the expansion of cellulose lattice, and the cellulose ether becomes water-soluble, forming a high-viscosity solution. When the temperature rises, the hydration of the polymer is weakened and the water between the chains is expelled. When the dewatering is sufficient, the molecules begin to clump together, forming a three-dimensional network of gels.

The factors affecting the water retention of mortar include cellulose ether viscosity, addition amount, particle fineness and service temperature.

The higher the viscosity of HPMC, the better the water retention performance. Viscosity is an important parameter of cellulose properties. Currently, different cellulose manufacturers use different methods and instruments to determine the viscosity of cellulose. The main methods include Haake Rotovisko, Hoppler, Ubbelohde and Brookfield. For the same product, the viscosity results measured by different methods vary greatly, and some are even exponentially different. Therefore, when comparing viscosity, it must be carried out between the same test methods, including temperature, rotor, etc.

Generally speaking, the higher the viscosity, the better the water retention effect. But the higher the viscosity, the higher the molecular weight of cellulose, its solubility will be reduced, which has a negative impact on the strength of mortar and construction performance. The higher the viscosity, the more obvious the thickening effect of mortar, but it is not proportional to the relationship. The higher the viscosity, the more viscous wet mortar will be, both in the construction, as the adhesion scraper and high adhesion to the substrate. But the increase of structural strength of wet mortar itself is not helpful. When the construction, the performance of anti sagging performance is not obvious.