The greater the viscosity of HPMC hydroxypropyl methyl cellulose, the better the water retention performance. Viscosity is an important parameter of HPMC performance. At present, different HPMC manufacturers use different methods and instruments to determine the viscosity of HPMC. The main methods are Haake Rotovisko, Hoppler, Ubbelohde and Brookfield, etc.
For the same product, the results of viscosity measured by different methods are very different, some are even multiple differences. Therefore, when comparing viscosity, it must be carried out between the same test method, including temperature, rotor, etc.
For particle size, the finer the particle, the better the water retention. Large particles of cellulose ether contact with water, the surface immediately dissolve and form a gel to wrap up the material to prevent water molecules from continuing to penetrate, sometimes long time stirring can not be evenly dispersed dissolved, the formation of a muddy flocculent solution or agglomerate. The solubility of cellulose ether is one of the factors to choose cellulose ether. Fineness is also an important performance index of methyl cellulose ether. MC for dry mortar requires powder, low water content, and fineness of 20%~60% particle size less than 63um. Fineness affects the solubility of HPMC hydroxypropyl methyl cellulose ether. Coarse MC is usually granular and can be easily dissolved in water without agglomerating, but the dissolution speed is very slow, so it is not suitable for use in dry mortar. In dry mortar, MC is dispersed between aggregate, fine fillers and cementing materials such as cement, and only powder that is fine enough can avoid clumping of methyl cellulose ether when mixing with water. When MC adds water to dissolve agglomerate, it is very difficult to disperse and dissolve it. MC with coarse fineness not only wastes, but also reduces the local strength of mortar. When such dry mortar is constructed in a large area, the curing speed of local dry mortar is significantly reduced, resulting in cracking caused by different curing time. For mechanical spraying mortar, because of the short mixing time, the fineness is higher.
Generally speaking, the higher the viscosity, the better the water retention effect. However, the higher the viscosity is, the higher the molecular weight of MC is, and the dissolution performance will decrease correspondingly, which has a negative impact on the strength and construction performance of mortar. 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 wet mortar will be more sticky, both construction, the performance of the sticky scraper and high adhesion to the base material. But it is not helpful to increase the structural strength of wet mortar. During construction, the anti-sag performance is not obvious. On the contrary, some low viscosity but modified methyl cellulose ethers have excellent performance in improving the structural strength of wet mortar.
The more cellulose ether is added to the mortar, the better water retention performance, the higher the viscosity, the better water retention performance.
HPMC fineness also has a certain effect on its water retention, generally speaking, for the same viscosity and different fineness of methyl cellulose ether, in the case of the same amount of addition, the finer the water retention effect is better.
The water retention of HPMC is also related to the temperature of use, and the water retention of methyl cellulose ether decreases with the rise of temperature. But in the actual material application, many environments of dry mortar often will be in high temperature (higher than 40 degrees) under the condition of construction in hot substrate, such as summer insolation of the exterior wall putty plastering, which often accelerated the solidification of cement and dry mortar hardening. The decrease of water retention rate leads to the obvious feeling that both constructability and cracking resistance are affected. In this condition, reducing the influence of temperature factors becomes particularly critical. Although the additive of methyl hydroxyethyl cellulose ether is considered to be at the forefront of technological development, its dependence on temperature will still lead to the weakening of the properties of dry mortar. Even with the increase of methyl hydroxyethyl cellulose dosage (summer formula), the construction and cracking resistance still cannot meet the needs of use. Through some special treatment of MC, such as increasing the degree of etherification, the water retention effect of MC can maintain a better effect under high temperature, so that it can provide better performance under harsh conditions.
Post time: May-18-2022