The Difference In Physical And Chemical Properties of HPMC and HEMC(2)
Gel temperature is an important indicator of cellulose ether. The aqueous solution of cellulose ether has thermal gel properties. As the temperature increases, the viscosity continues to decrease. When the temperature of the solution reaches a certain value, the cellulose ether solution is no longer transparent, but forms a white colloid, and finally loses its viscosity. The gel temperature test means that the cellulose ether sample is slowly heated in a water bath from a 0.2% concentration of cellulose ether solution until the solution appears white or even white gel, and the viscosity is completely lost. The temperature of the solution is the gel temperature of the cellulose ether.
The ratio of methoxy, hydroxypropyl and HPMC has a certain effect on the water solubility, water holding capacity, surface activity and gel temperature of the product. Generally speaking, HPMC with high methoxy group content and low hydroxypropyl content has good water solubility and good surface activity, but the gel temperature is low: increasing the hydroxypropyl content and reducing the methoxy group content can increase the gel temperature. But excessive hydroxypropyl content will lower the gel temperature, reduce water solubility and surface activity. Therefore, cellulose ether manufacturers must strictly control the group content to ensure product quality and stability.
3. Application of construction industry
HPMC and HEMC have similar functions in building materials. It can be used as a dispersant, water-retaining agent, thickener, binder, etc. It is mainly used for the molding of cement mortar and gypsum products. Used in cement mortar to increase its cohesiveness and workability, reduce flocculation, increase viscosity and shrinkage, and have the functions of retaining water, reducing water loss on the concrete surface, increasing strength, preventing cracks and weathering of water-soluble salts. It is widely used in cement, plaster, mortar and other materials. It can be used as a film-forming agent, thickener, emulsifier and stabilizer for latex coatings and water-soluble resin coatings. It has good abrasion resistance, uniformity and adhesion, and improves surface tension, acid-base stability and compatibility with metallic pigments. Due to its good viscosity storage stability, it is especially suitable for emulsion coatings as a dispersant. All in all, although the system is small in volume, it has good effects and is widely used.
The gel temperature of cellulose ether determines its thermal stability in application. The gel temperature of HPMC is usually between 60°C and 75°C, depending on different types, group content and production processes of different manufacturers. Due to the characteristics of the HEMC group, it has a higher gel temperature, usually above 80°C, so its stability at high temperatures is due to HPMC. In practical applications, in the hot summer construction environment, the water holding capacity of HEMC with the same viscosity and dosage is better than HPMC. Especially in the south, mortar is sometimes applied at high temperatures. The cellulose ether of the low-temperature gel will lose its thickening and water-holding effects at high temperatures, thereby accelerating the hardening of the cement mortar and directly affecting the construction and crack resistance.
Because there are more hydrophilic groups in the structure of HEMC, it has better hydrophilicity. Under the same product dosage and the same viscosity, the water retention rate of HEMC in mortar is slightly higher than that of HEMC. In addition, the vertical flow resistance of HEMC is relatively good. The application of HPMC for tile adhesive will be better.
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