Views: 1 Author: Site Editor Publish Time: 2022-09-16 Origin: Site
HPMC hydroxypropyl methyl cellulose, non-ionic type, in dry mortar commonly used as water retention agent, is the most commonly used water retention material in mortar.
HPMC production process is mainly cotton fiber (domestic) after alkalization, etherization and the generation of a polysaccharide ether products. It has no charge and does not react with charged ions in the cementitious material, and its performance is stable. The price is also lower than other types of cellulose ether, so it is widely used in dry mixed mortar.
Hydroxypropyl methyl cellulose function: can make the new mortar thickened so as to have a certain wet viscosity, prevent segregation. (thickening) water retention is also the most important characteristic, which helps to maintain the amount of free water in the mortar, so that the mortar construction, cementitious materials have more time to hydrate. (water retention) its own gas, can introduce uniform small bubbles, improve the mortar construction.
The greater the viscosity of hydroxypropyl methyl cellulose ether, 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. That is, during construction, the anti-droop performance 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 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. In this regard, methyl hydroxyethyl cellulose ether additive is currently considered to be at the forefront of technological development. Although increase methyl hydroxyethyl cellulose dosage (summer formula), 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.
General HPMC has gel temperature, can be divided into 60 types, 65 types, 75 types. For ordinary ready-mixed mortar with river sand enterprises had better choose 75 HPMC with high gel temperature. HPMC dosage should not be too high, too high will make the mortar water demand increase, but also sticky, setting time is too long, affect the construction. Different mortar products choose different viscosity HPMC, do not casually use high viscosity HPMC. Therefore, although hydroxypropyl methylcellulose products are good, but the use of good only praise, choose the right HPMC is the primary responsibility of enterprise laboratory personnel. At present, there are a lot of illegal dealers in the compound HPMC, the quality is quite poor, the laboratory should choose some kind of cellulose, do a good job in the experiment, to ensure the stability of mortar products, do not covet cheap, cause unnecessary losses.