The advanges of HPMC & MHEC in dry mix mortar products
November 22nd 2024
The greater the viscosity of hydroxypropyl methyl cellulose, the better the water retention. Viscosity is an important parameter of HPMC performance. At present, different HPMC manufacturers use different methods and instruments to measure the viscosity of HPMC. The main methods include HaakeRotovisko, Hoppler, Ubbelohde and Brookfield.
For the same product, the results of viscosity measured by different methods vary greatly, and some even multiply. Therefore, when comparing viscosity, it must be done between the same test method, including temperature, rotor, etc.
For particle size, the finer the particle, the better the water retention. After large particles of cellulose ether contact with water, the surface immediately dissolves and forms a gel to wrap the material to prevent the water molecules from continuing to infiltrate. Sometimes, even a long time stirring can not be evenly dispersed and dissolved, forming a cloudy flocculent solution or clumping. The water retention of cellulose ether is greatly affected, and the solubility is one of the factors of choosing cellulose ether.
Fineness is also an important performance index of methyl cellulose ether. MC for dry mortar requires powder, low water content, and fineness also requires 20%-60% particle size less than 63UM. Fineness affects the solubility of hydroxypropyl methyl cellulose ether. Coarser MC is usually granular and easily solubilized in water without clumping, but the dissolution rate is very slow and is not suitable for use in dry powder mortars.
In dry mortar, MC is dispersed among aggregate, fine filler, cement and other cementing materials, and only fine enough powder can avoid the appearance of methyl cellulose ether agglomerate when water is added. When the MC is dissolved in water, it is difficult to dissolve in dispersion. MC with coarser fineness is not only wasteful, but also reduces the local strength of mortar. When such dry mortar is constructed in a large area, it is shown that the curing speed of local dry mortar is significantly reduced, and cracking is caused by different curing time. For the shotcrete mortar with mechanical construction, because the mixing time is shorter, the fineness requirements are 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 solubability of MC will decrease accordingly, 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 more sticky the wet mortar will be, both during construction, the performance of sticky scraper and high adhesion to the substrate. But it is not helpful to increase the structural strength of wet mortar itself. When the construction, the performance of anti – droop is not obvious. On the contrary, some modified methyl cellulose ethers with medium and low viscosity have excellent performance in improving the structural strength of wet mortar.
The greater the amount of cellulose ether added in the mortar, the better the water retention performance, the higher the viscosity, the better the water retention performance.
HPMC fineness also has a certain impact on its water retention, generally speaking, for the same viscosity and fineness of different methyl cellulose ether, in the same amount of the case, the finer the fineness of the water retention effect is better.
The water retention of HPMC is also related to the temperature used, and the water retention of methyl cellulose ether decreases with the rise of temperature. But in the actual material application, many environments dry mortar will often be in the high temperature (higher than 40 degrees) under the condition of construction on the hot substrate, such as the exterior wall putty plaster under the sun in summer, which often accelerates the curing of cement and the hardening of dry mortar.
The decrease of water retention rate leads to the obvious feeling that both constructability and cracking resistance are affected. Under such conditions, it becomes particularly critical to reduce the influence of temperature factors. Although methylhydroxyethyl cellulose ether additive is considered to be at the forefront of technological development, its dependence on temperature will still lead to the weakening of the performance of dry mortar.
Increasing the dosage of methyl hydroxyethyl cellulose, the construction and cracking resistance still can not meet the needs of use. MC through some special treatment, such as improving the degree of etherification, can make its water retention effect in the case of high temperature to maintain a better effect, so that it can provide better performance under harsh conditions.
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