Hydroxypropylmethylcellulose

Overview: referred to as HPMC, white or off-white fibrous or granular powder. There are many types of cellulose and are widely used, but we mainly contact with customers in the dry powder building materials industry. The most common cellulose refers to hypromellose.

Production process: The main raw materials of HPMC: refined cotton, methyl chloride, propylene oxide, other raw materials include flake alkali, acid, toluene, isopropanol, etc. Treat the refined cotton cellulose with alkali solution at 35-40℃ for half an hour, press, pulverize the cellulose, and properly age at 35℃, so that the average degree of polymerization of the obtained alkali fiber is within the required range. Put the alkali fibers into the etherification kettle, add propylene oxide and methyl chloride in turn, and etherify at 50-80 °C for 5 hours, with a maximum pressure of about 1.8 MPa. Then add an appropriate amount of hydrochloric acid and oxalic acid to the hot water at 90 °C to wash the material to expand the volume. Dehydrate with a centrifuge. Wash until neutral, and when the moisture content in the material is less than 60%, dry it with a hot air flow at 130°C to less than 5%. Function: water retention, thickening, thixotropic anti-sag, air-entraining workability, retarding setting.

Water retention: Water retention is the most important property of cellulose ether! In the production of putty gypsum mortar and other materials, cellulose ether application is essential. High water retention can fully react cement ash and calcium gypsum (the more fully the reaction, the greater the strength). Under the same conditions, the higher the viscosity of cellulose ether, the better the water retention (the gap above 100,000 viscosity is narrowed); the higher the dosage, the better the water retention, usually a small amount of cellulose ether can greatly improve the mortar’s performance. Water retention rate, when the content reaches a certain level, the trend of increasing water retention rate becomes slower; the water retention rate of cellulose ether usually decreases when the ambient temperature increases, but some high-gel cellulose ethers also have better performance under high temperature conditions. Water retention. The interdiffusion between water molecules and cellulose ether molecular chains enables water molecules to enter the interior of the cellulose ether macromolecular chains and receive strong binding force, thereby forming free water, entangling water, and improving the water retention of cement slurry.

Thickening, thixotropic and anti-sag: imparts excellent viscosity to wet mortar! It can significantly increase the adhesion between wet mortar and the base layer, and improve the anti-sagging performance of mortar. The thickening effect of cellulose ethers also increases dispersion resistance and homogeneity of freshly mixed materials, preventing material delamination, segregation and bleeding. The thickening effect of cellulose ethers on cement-based materials comes from the viscosity of cellulose ether solutions. Under the same conditions, the higher the viscosity of cellulose ether, the better the viscosity of the modified cement-based material, but if the viscosity is too large, it will affect the fluidity and operability of the material (such as sticky trowel and batch scraper). laborious). Self-leveling mortar and self-compacting concrete that require high fluidity require low viscosity of cellulose ether. In addition, the thickening effect of cellulose ether will increase the water demand of cement-based materials and increase the yield of mortar. High viscosity cellulose ether aqueous solution has high thixotropy, which is also a major characteristic of cellulose ether. Aqueous solutions of cellulose generally have pseudoplastic, non-thixotropic flow properties below their gel temperature, but Newtonian flow properties at low shear rates. Pseudoplasticity increases with increasing molecular weight or concentration of cellulose ether. Structural gels are formed when the temperature is increased, and high thixotropic flow occurs. Cellulose ethers with high concentrations and low viscosity exhibit thixotropy even below the gel temperature. This property is of great benefit to the construction of building mortar to adjust its leveling and sag. It should be noted here that the higher the viscosity of the cellulose ether, the better the water retention, but the higher the viscosity, the higher the relative molecular weight of the cellulose ether, and the corresponding decrease in its solubility, which has a negative impact on the mortar concentration and workability.

Cause: Cellulose ether has obvious air-entraining effect on fresh cement-based materials. Cellulose ether has both a hydrophilic group (hydroxyl group, ether group) and a hydrophobic group (methyl group, glucose ring), is a surfactant, has surface activity, and thus has an air-entraining effect. The air-entraining effect of cellulose ether will produce a “ball” effect, which can improve the working performance of the freshly mixed material, such as increasing the plasticity and smoothness of the mortar during operation, which is beneficial to the paving of the mortar; it will also increase the output of the mortar. , reducing the cost of mortar production; but it will increase the porosity of the hardened material and reduce its mechanical properties such as strength and elastic modulus. As a surfactant, cellulose ether also has a wetting or lubricating effect on cement particles, which together with its air-entraining effect increases the fluidity of cement-based materials, but its thickening effect will reduce the fluidity. The effect of flow is a combination of plasticizing and thickening effects. When the content of cellulose ether is very low, it is mainly manifested as plasticizing or water-reducing effect; when the content is high, the thickening effect of cellulose ether increases rapidly, and its air-entraining effect tends to be saturated, so the performance is increased. Thickening effect or increased water demand.

Setting retardation: Cellulose ether can delay the hydration process of cement. Cellulose ethers endow the mortar with various beneficial properties, and also reduce the early hydration heat release of the cement and delay the hydration kinetic process of the cement. This is unfavorable for mortar use in cold regions. This retardation is caused by the adsorption of cellulose ether molecules on hydration products such as C-S-H and ca(OH)2. Due to the increase in the viscosity of the pore solution, the cellulose ether reduces the mobility of ions in the solution, thereby delaying the hydration process. The higher the concentration of cellulose ether in the mineral gel material, the more pronounced the effect of hydration delay. Cellulose ethers not only retard setting, but also retard the hardening process of the cement mortar system. The retardation effect of cellulose ether depends not only on its concentration in the mineral gel system, but also on the chemical structure. The higher the degree of methylation of HEMC, the better the retardation effect of cellulose ether. The retardation effect is stronger. However, the viscosity of cellulose ether has little effect on the hydration kinetics of cement. With the increase of cellulose ether content, the setting time of mortar increases significantly. There is a good nonlinear correlation between the initial setting time of mortar and the content of cellulose ether, and the final setting time has a good linear correlation with the content of cellulose ether. We can control the operating time of the mortar by changing the content of cellulose ether. In the product, it plays the role of water retention, thickening, delaying cement hydration power, and improving construction performance. Good water retention capacity makes cement gypsum ash calcium react more completely, significantly increases wet viscosity, improves the bond strength of mortar, and at the same time can properly improve tensile strength and shear strength, greatly improving construction effect and work efficiency. Adjustable time. Improves spray or pumpability of mortar, as well as structural strength. In the actual application process, it is necessary to determine the type, viscosity, and amount of cellulose according to different products, construction habits, and environment.