MHEC Used In Tile Adhesive

MC stands for methyl cellulose, which is a cellulose ether made from purified cotton through alkali treatment using chloromethane as the etherification agent, followed by a series of reactions. The degree of substitution is generally 1.6-2.0, and different degrees of substitution result in different solubilities. It belongs to non-ionic cellulose ethers. 1. Methyl cellulose's water retention depends on the amount added, viscosity, particle size, and dissolution rate. Generally, a higher amount, smaller particle size, and higher viscosity result in better water retention. Among these cellulose ethers, methyl cellulose and hydroxypropyl methyl cellulose have higher water retention. 2. Methyl cellulose is soluble in cold water but has difficulty dissolving in hot water. Its aqueous solution is stable within the pH range of 3-12. It has good compatibility with starch, guar gum, and many surfactants. Gelation occurs when the temperature reaches the gelation temperature. 3. Temperature variation significantly affects the water retention of methyl cellulose. Generally, higher temperatures result in poorer water retention. If the temperature of the mortar exceeds 40°C, the water retention of methyl cellulose decreases significantly, which adversely affects the workability of the mortar. 4. Methyl cellulose has a noticeable impact on the workability and adhesion of mortar. "Adhesion" refers to the adhesion force between the worker's application tool and the wall substrate, i.e., the shear resistance of the mortar. A higher adhesion leads to higher shear resistance, requiring more force from the worker during application and resulting in poorer workability. Among cellulose ether products, methyl cellulose has a moderate level of adhesion. HPMC stands for Hydroxypropyl Methyl Cellulose. It is a non-ionic cellulose ether derived from refined cotton through alkalization, using epichlorohydrin and chloromethane as etherification agents in a series of reactions. The degree of substitution is generally between 1.2 and 2.0. Its properties vary with the ratio of methoxy content to hydroxypropyl content. (1) Hydroxypropyl Methyl Cellulose is soluble in cold water, but it can be difficult to dissolve in hot water. However, its gelation temperature in hot water is significantly higher than that of methyl cellulose. Its solubility in cold water is greatly improved compared to methyl cellulose. (2) The viscosity of Hydroxypropyl Methyl Cellulose depends on its molecular weight, with higher molecular weight leading to higher viscosity. Temperature also affects its viscosity, with viscosity decreasing as temperature rises. However, its viscosity is less affected by temperature compared to methyl cellulose. Its solution is stable when stored at room temperature. (3) Hydroxypropyl Methyl Cellulose exhibits stability in acids and alkalis, and its aqueous solution is highly stable within the pH range of 2 to 12. It is minimally affected by sodium hydroxide and lime water, although alkalis can accelerate its dissolution and slightly increase its viscosity. It demonstrates stability in general salts, but at higher salt concentrations, the viscosity of Hydroxypropyl Methyl Cellulose solution tends to increase. (4) The water retention capacity of Hydroxypropyl Methyl Cellulose depends on factors such as the dosage and viscosity, and at the same dosage, its water retention rate is higher than that of methyl cellulose. (5) Hydroxypropyl Methyl Cellulose can be mixed with water-soluble high molecular weight compounds to form homogeneous solutions with higher viscosity. Examples include polyvinyl alcohol, starch ethers, and plant gums. (6) Hydroxypropyl Methyl Cellulose exhibits higher adhesion in mortar construction compared to methyl cellulose. (7) Hydroxypropyl Methyl Cellulose has better resistance to enzymatic degradation compared to methyl cellulose, and its solution is less likely to undergo enzymatic degradation.

HPMC is widely used in industries such as construction materials, coatings, synthetic resins, ceramics, pharmaceuticals, food, textiles, agriculture, cosmetics, and tobacco. HPMC can be classified into architectural grade, food grade, and pharmaceutical grade based on its application. Currently, most domestically produced HPMC falls under the architectural grade category. In the architectural grade, a large amount of HPMC is used in putty powder, accounting for approximately 90% of its usage, while the rest is used in cement mortar and adhesives.

In the application of HPMC in putty powder, it plays three roles: thickening, water retention, and facilitating construction. Thickening: Cellulose can thicken the mixture, maintain uniform suspension, and prevent sagging. Water retention: It slows down the drying process of putty powder and assists in the reaction of lime and calcium in water. Construction: Cellulose acts as a lubricant, improving the workability of the putty powder. HPMC does not participate in any chemical reactions; it only serves as an auxiliary agent. When putty powder is mixed with water and applied to the wall, a chemical reaction occurs because new substances are formed. However, if the putty powder is scraped off the wall, ground into powder, and reused, it is not suitable because a new substance (calcium carbonate) has already formed. The main components of lime and calcium powder are Ca(OH)2, CaO, and a small amount of CaCO3. The reaction can be represented as: CaO + H2O = Ca(OH)2 — Ca(OH)2 + CO2 = CaCO3 ↓ + H2O. Under the action of water and carbon dioxide in the air, lime and calcium carbonate are formed. HPMC only assists in water retention and the better reaction of lime and calcium; it does not participate in any reactions itself.

The powder loss in putty is mainly related to the quality of the lime powder and has little to do with HPMC. Low calcium content in lime powder and an improper ratio of CaO and Ca(OH)2 in lime powder can both cause powder loss. If there is a slight relationship with HPMC, it would be that poor water retention of HPMC can also contribute to powder loss.