hydroxypropyl methylcellulose HPMC in Plastering MortarFebruary 2nd 2023
MC is methyl cellulose, which is made into cellulose ether by a series of reactions after the refined cotton is treated with alkali, using methyl chloride as the etherification agent. Generally, the degree of substitution is 1.6~2.0, and the solubility varies with the degree of substitution. Belongs to non-ionic cellulose ether.
(1) The water retention of methyl cellulose depends on its addition, viscosity, particle fineness and dissolution rate. Generally, if the addition amount is large, the fineness is small, and the viscosity is large, the water retention rate is high. Among them, the addition amount has the greatest influence on the water retention rate, and the viscosity is not proportional to the water retention rate. The dissolution rate mainly depends on the degree of surface modification of the cellulose particles and the particle fineness. Among the above cellulose ethers, methyl cellulose and hydroxypropyl methyl cellulose have higher water retention rates.
(2) Methyl cellulose is soluble in cold water, and it will be difficult to dissolve in hot water. Its aqueous solution is very stable in the range of pH=3~12. It has good compatibility with starch, guar gum, etc. and many surfactants. When the temperature reaches the gelation temperature, gelation will occur.
(3) Changes in temperature will seriously affect the water retention rate of methyl cellulose. Generally, the higher the temperature, the worse the water retention. If the mortar temperature exceeds 40°C, the water retention of methyl cellulose will be significantly worsened, which will seriously affect the workability of the mortar.
(4) Methyl cellulose has a significant effect on the workability and adhesion of the mortar. The “adhesion” here refers to the adhesive force felt between the worker’s application tool and the wall substrate, that is, the shear resistance of the mortar. The adhesion is large, the shear resistance of the mortar is large, and the force required by the workers in the use process is also large, and the workability of the mortar is poor. In cellulose ether products, the adhesion of methyl cellulose is at a medium level.
HPMC is hydroxypropyl methylcellulose, which is a mixture of non-ionic cellulose made from refined cotton after alkalization, using propylene oxide and methyl chloride as etherification agents, through a series of reactions ether. The degree of substitution is generally 1.2 to 2.0. Its properties are different by the ratio of methoxy content and hydroxypropyl content.
(1) Hydroxypropyl methylcellulose is easily soluble in cold water, and it will be difficult to dissolve in hot water. But its gelation temperature in hot water is significantly higher than that of methyl cellulose. The dissolution in cold water is also greatly improved compared to methyl cellulose.
(2) The viscosity of hydroxypropyl methyl cellulose is related to its molecular weight. The higher the molecular weight, the higher the viscosity. Temperature also affects its viscosity, as the temperature increases, the viscosity decreases. But its high viscosity has a lower temperature effect than methyl cellulose. The solution is stable when stored at room temperature.
(3) Hydroxypropyl methylcellulose is stable to acid and alkali, and its aqueous solution is very stable in the range of pH=2~12. Caustic soda and lime water do not have much effect on its performance, but alkali can speed up its dissolution rate and increase viscosity. Hydroxypropyl methylcellulose is stable to general salts, but when the concentration of the salt solution is high, the viscosity of the hydroxypropyl methylcellulose solution tends to increase.
(4) The water retention of hydroxypropyl methylcellulose depends on its addition amount, viscosity, etc. The water retention rate under the same addition amount is higher than that of methyl cellulose.
(5) Hydroxypropyl Methyl Cellulose can be mixed with water-soluble polymer compounds to form a uniform and higher viscosity solution. Such as polyvinyl alcohol, starch ether, vegetable glue, etc.
(6) The adhesion of hydroxypropyl methylcellulose to mortar construction is higher than that of methylcellulose.
(7) Hydroxypropyl methyl cellulose has better enzyme resistance than methyl cellulose, and the possibility of enzymatic degradation in solution is lower than that of methyl cellulose.