Focus on Cellulose ethers

Factors affecting the water retention of hydroxypropyl methylcellulose

The higher the viscosity of hydroxypropyl methylcellulose, the better the water retention performance. Viscosity is an important parameter for HPMC performance. Currently, different HPMC manufacturers use different methods and instruments to measure the viscosity of HPMC. The main methods are Haake Rotovisko, Hoppler, Ubbelohde and Brookfield.

For the same product, the viscosity results measured by different methods vary greatly, and some are even twice as different. Therefore, when comparing viscosities, be sure to do it between the same test methods, including temperature, spindle, etc.

Regarding particle size, the finer the particles, the better the water retention. When the large particles of cellulose ether come into contact with water, the surface immediately dissolves to form a gel, which wraps the material and prevents the continuous penetration of water molecules. . It largely affects the water retention effect of its cellulose ether, and solubility is one of the factors in selecting cellulose ether. Fineness is also an important performance indicator of methylcellulose ether. The MC used in dry mortar is required to be powdery, with low moisture content, and the fineness also requires 20% to 60% of the particle size to be less than 63um. Fineness affects the solubility of hydroxypropyl methylcellulose ether. Coarse MC is usually granular and easily soluble in water without caking, but the dissolution rate is very slow, so it is not suitable for use in dry mortar. In dry mortar, MC is dispersed in cementitious materials such as aggregate, fine filler and cement. Only powders that are fine enough will prevent methylcellulose ether from clumping when mixed with water. When MC adds water to dissolve the aggregates, it is difficult to disperse and dissolve. MC with a coarser fineness not only causes waste, but also reduces the local strength of the mortar. When this type of dry mortar is constructed over a large area, the curing speed of the local dry mortar is significantly reduced, and cracking occurs due to different curing times. For spray mortar using mechanical construction, higher fineness is required due to shorter mixing time.

Generally speaking, the higher the viscosity, the better the water retention effect. However, the higher the viscosity and molecular weight of MC, the corresponding decrease in solubility, which has a negative impact on the strength and construction performance of the mortar. The higher the viscosity, the more obvious the thickening effect of the mortar, but it is not proportional. The higher the viscosity, the stickier the wet mortar. It will stick to the scraper during construction and has high adhesion to the substrate. But it does little to increase the structural strength of the wet mortar itself. During the construction process, the anti-sag performance was not obvious. In contrast, some low-viscosity but modified methylcellulose ethers have excellent properties in improving the structural strength of wet mortars.

The greater the amount of cellulose ether added in the mortar, the better the water retention performance, and the higher the viscosity, the better the water retention performance.

The fineness of HPMC also has a certain impact on its water retention. Generally speaking, for methyl cellulose ethers with the same viscosity but different fineness, when the addition amount is the same, the finer the fineness, the better the water retention effect.

The water retention of HPMC is also related to the use temperature. The water retention of methylcellulose ether decreases as the temperature increases. However, in actual material applications, dry mortar is often constructed on hot substrates with high temperatures (above 40 degrees) in many environments, such as putty plastering on exterior walls under the summer sun, which often accelerates the solidification of cement and the discoloration of cement. hardening. Dry mortar. The decrease in water retention makes it clear that workability and crack resistance will be affected, and it is particularly critical to reduce the influence of temperature factors under such conditions. Although methylhydroxyethylcellulose ether additives are currently considered to be at the forefront of technological development, their dependence on temperature can still lead to a weakening of dry mortar properties. Although the dosage of methylhydroxyethylcellulose (Xia formula) was increased, the processability and crack resistance still could not meet the needs of use. Through some special treatments, such as increasing the degree of etherification, etc., MC can maintain better water retention at higher temperatures, thereby providing better performance under harsh conditions.

Post time: Feb-06-2024
WhatsApp Online Chat !