19. What is the difference between HPMC and MC?
——Answer: MC is methyl cellulose, which is made of cellulose ether by treating refined cotton with alkali, using methane chloride as etherification agent, and going through a series of reactions. Generally, the degree of substitution is 1.6~2.0, and the solubility is also different with different degrees of substitution. It belongs to non-ionic cellulose ether.
(1) The water retention of methyl cellulose depends on its addition amount, 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 amount of addition has the greatest impact on the water retention rate, and the level of viscosity is not directly proportional to the level of water retention rate. The dissolution rate mainly depends on the degree of surface modification of cellulose particles and particle fineness. Among the above cellulose ethers, methyl cellulose and hydroxypropyl methyl cellulose have higher water retention rates.
(2) Methylcellulose 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 occurs.
(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 reduced, seriously affecting the construction of the mortar.
(4) Methyl cellulose has a significant effect on the construction and adhesion of mortar. The “adhesion” here refers to the adhesive force felt between the worker’s applicator tool and the wall substrate, that is, the shear resistance of the mortar. The adhesiveness is high, the shear resistance of the mortar is large, and the strength required by the workers in the process of use is also large, and the construction performance of the mortar is poor. Methyl cellulose adhesion is at a moderate level in cellulose ether products.
HPMC is hydroxypropyl methylcellulose, which is a non-ionic cellulose mixed ether made from refined cotton after alkalization, using propylene oxide and methyl chloride as etherification agents, and through a series of reactions. The degree of substitution is generally 1.2~2.0. Its properties are different due to the different ratios of methoxyl content and hydroxypropyl content.
(1) Hydroxypropyl methylcellulose is easily soluble in cold water, and it will encounter difficulties in dissolving in hot water. But its gelation temperature in hot water is significantly higher than that of methyl cellulose. The solubility in cold water is also greatly improved compared with methyl cellulose.
(2) 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 have little effect on its performance, but alkali can speed up its dissolution and increase its viscosity. Hydroxypropyl methylcellulose is stable to common salts, but when the concentration of salt solution is high, the viscosity of hydroxypropyl methylcellulose solution tends to increase.
(3) Hydroxypropyl methylcellulose can be mixed with water-soluble polymer compounds to form a uniform and higher viscosity solution. Such as polyvinyl alcohol, starch ether, vegetable gum, etc.
(4) The adhesion of hydroxypropyl methylcellulose to mortar construction is higher than that of methylcellulose.
(5) Hydroxypropyl methylcellulose has better enzyme resistance than methylcellulose, and its solution is less likely to be degraded by enzymes than methylcellulose.
20. What should be paid attention to in the actual application of the relationship between the viscosity and temperature of hydroxypropyl methylcellulose?
——Answer: The viscosity of HPMC is inversely proportional to the temperature, that is to say, the viscosity increases as the temperature decreases. The viscosity of a product we usually refer to refers to the test result of its 2% aqueous solution at a temperature of 20 degrees Celsius.
In practical applications, it should be noted that in areas with large temperature differences between summer and winter, it is recommended to use a relatively low viscosity in winter, which is more conducive to construction. Otherwise, when the temperature is low, the viscosity of the cellulose will increase, and the hand feel will be heavy when scraping.
Medium viscosity: 75000-100000 mainly used for putty
Reason: good water retention
High viscosity: 150000-200000 Mainly used for polystyrene particle thermal insulation mortar rubber powder and vitrified microbead thermal insulation mortar.
Reason: The viscosity is high, the mortar is not easy to fall off, sag, and the construction is improved.
But generally speaking, the higher the viscosity, the better the water retention. Therefore, considering the cost, many dry powder mortar factories replace medium and low viscosity cellulose (20000-40000) with medium-viscosity cellulose (20000-40000) to reduce the amount of addition.
Post time: Mar-27-2023