Answers to some questions on hydroxypropyl methylcellulose (III)

17、What is the difference between HPMC and MC?

--Answer:MC is methyl cellulose, which is made from cellulose ether after a series of reactions by treating refined cotton with alkali and using methylene chloride as etherifier. The general degree of substitution is 1.6~2.0, and the solubility varies with the degree 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 speed. Generally, if the added 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 a greater impact on the water retention rate, the viscosity of the high and low water retention rate is not proportional to the high and low relationship. 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 rate.

(2) Methyl cellulose is soluble in cold water, hot water dissolution will encounter difficulties, and its aqueous solution is more stable in the range of pH=3~12. It has good compatibility with starch, guanidinium gum and many surfactants. When the temperature reaches the gelation temperature, gelation will occur.

(3) The change of temperature will seriously affect the water retention of methyl cellulose. Generally the higher the temperature, the worse the water retention. If the temperature of the mortar exceeds 40 ℃, the water retention of methyl cellulose will be significantly worse, seriously affecting the constructability of the mortar.

(4) Methyl cellulose has a significant impact on the constructability and adhesion of mortar. The "adhesion" refers to the adhesion between the applicator and the wall substrate, i.e. the shear resistance of the mortar. Adhesion is large, the shear resistance of the mortar is large, the force required by the workers in the process of application is also large, and the constructability of the mortar is poor. In the cellulose ether products methyl cellulose adhesion is at a medium level.

HPMC is hydroxypropyl methyl cellulose, which is a nonionic cellulose mixed ether made from refined cotton by alkalization treatment, using propylene oxide and chloromethane as etherifying agents, through a series of reactions. The degree of substitution is generally 1.2~2.0. Its properties are subject to the different ratio of methoxy content and hydroxypropyl content.

(1)Hydroxypropyl methyl cellulose is easily soluble in cold water, hot water dissolution will encounter difficulties. However, its gelatinization temperature in hot water is significantly higher than that of methyl cellulose. The dissolution in cold water is also greatly improved compared with methyl cellulose.

(2)The viscosity of hydroxypropyl methylcellulose is related to its molecular weight, large molecular weight is high viscosity. Temperature also affects its viscosity, temperature increases, viscosity decreases. However, the effect of high temperature on its viscosity is lower than that of methyl cellulose. Its solution is stable when stored at room temperature.

(3)Hydroxypropyl methyl cellulose is stable to acid and alkali, and its aqueous solution is more stable in the range of pH=2~12. Caustic soda and lime water, there is not much effect on its performance, but alkali can accelerate its dissolution rate and enhance the viscosity pin. Hydroxypropyl methylcellulose has stability to general salts, but salt solution concentration is high, hydroxypropyl methylcellulose solution viscosity has a tendency to increase.

(4)The water retention of hydroxypropyl methylcellulose depends on its additive amount, viscosity, etc. Its water retention under the same additive amount is higher than that of methylcellulose.

(5) Hydroxypropyl methyl cellulose can be mixed with water-soluble polymer compounds to become uniform, higher viscosity solution. Such as polyvinyl alcohol, starch ether, vegetable gum, etc..

(6) Hydroxypropyl methyl cellulose has higher adhesion to mortar construction than methyl cellulose.

(7) Hydroxypropyl methyl cellulose has better enzyme resistance than methyl cellulose, and the possibility of enzymatic degradation of its solution is lower than that of methyl cellulose.

18, the relationship between the viscosity of HPMC and temperature, what should be noted in practical applications?

--Answer:The viscosity of HPMC is inversely proportional to the temperature, and the viscosity increases with the decrease of temperature. What we usually say about the viscosity of a certain product refers to the result of testing its 2% aqueous solution at a temperature of 20 degrees Celsius.

In practical applications, the temperature difference between summer and winter areas, it should be noted that the recommended viscosity in the winter time is relatively low, which is more conducive to the construction. Otherwise, when the temperature is low, the viscosity of cellulose will increase, and the hand feeling will be heavy when scraping.

Medium viscosity:75000-100000 Mainly used for putty

Reason:Good water retention

High viscosity:150,000-200,000 mainly used for polystyrene granule insulation mortar powder and glass beads insulation mortar.

Reason: High viscosity, the mortar is not easy to fall off, hanging, improve the construction.

But in general, the higher the viscosity, the better the water retention, so many dry mortar factories take into account the cost, with medium viscosity cellulose (75,000-100,000) instead of low viscosity cellulose (20,000-40,000) to reduce the amount of additive.