construction hpmc-What are the factors that affect the water retention of hydroxypropyl methylcellulose?
The higher the viscosity of hydroxypropyl methylcellulose, 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 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 even double the difference. Therefore, when comparing viscosity, be sure to do it between the same test method, including temperature, rotor, etc.
For particle size, the finer the particle, the better the water retention. After the large particles of cellulose ether come into contact with water, the surface dissolves immediately to form a gel, which wraps the material to prevent the continuous infiltration of water molecules. , which greatly affects the water retention of its cellulose ether, and solubility is one of the factors for choosing cellulose ether. Fineness is also an important performance index of methyl cellulose ether. The MC used for dry powder mortar requires powder with low water content, and the fineness also requires 20%-60% of the particle size to be less than 63um, and the fineness affects the hydroxyl group. Solubility of propyl methylcellulose ether. Coarse MC is usually granular, which is easy to disperse and dissolve in water without agglomeration, but the dissolution rate is very slow, so it is not suitable for use in dry powder mortar. In dry powder mortar, MC is dispersed among the cementitious materials such as aggregates, fine fillers and cement. Only fine enough powder can avoid the agglomeration of methyl cellulose ether when mixing with water. When MC is added with water to dissolve the agglomerates, it is difficult to disperse and dissolve. MC with coarser fineness is not only wasteful, but also reduces the local strength of the mortar. When such dry powder mortar is constructed in a large area, the solidification speed of the local dry powder mortar is obviously reduced, and cracking caused by different curing times occurs. The mechanically constructed spray mortar has higher requirements on fineness due to the shorter stirring time.
Generally speaking, the higher the viscosity, the better the water retention effect. However, the higher the viscosity and the higher the molecular weight of MC, the corresponding reduction in its solubility, which has a negative impact on the strength and construction properties of the mortar. The higher the viscosity, the more obvious the thickening effect on the mortar, but it is not proportional. The higher the viscosity, the more sticky the wet mortar will be. During construction, it is shown as sticking to the scraper and the adhesion to the substrate is high, but it does not help the increase of the structural strength of the wet mortar itself. During construction, it is shown as anti-sagging performance. It is not obvious, on the contrary, the larger the addition amount of some medium and low viscosity but modified methyl cellulose ethers 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 influence on its water retention. Generally speaking, for methyl cellulose ethers with the same viscosity but different fineness, in the case of the same addition amount, the finer the finer the better the water retention.
The water retention of HPMC is also related to the temperature used. The water retention performance of methyl cellulose ether decreases with the increase of temperature, but in practical material applications, dry powder mortar is often exposed to high temperature (above 40°C) in many environments. It is applied to hot substrates under the conditions of heat, such as exterior wall putty plastering under the sun in summer, which often accelerates the curing of cement and the hardening of dry mortar, and the decrease in water retention rate leads to a clear feeling of workability and crack resistance. All properties are affected, and it is especially critical to reduce the effect of temperature factors under such conditions. Although methyl hydroxyethyl cellulose ether additive is currently considered to be at the forefront of technological development, its dependence on temperature can still lead to the weakening of dry mortar performance, despite increasing the amount of methyl hydroxyethyl cellulose (summer formula), Workability and crack resistance can not meet the needs of use. Through some special treatments on MC, such as increasing the degree of etherification, etc., its water retention effect can be maintained at a higher temperature, and it can provide a good performance under worse conditions.