Hydroxypropyl methylcellulose is a non-ionic cellulose mixed ether prepared by alkalizing purified cotton through a series of reactions using propylene oxide and methyl chloride as etherifying agents. Hydroxypropyl methylcellulose is a nonionic cellulose ether with a white powder appearance, odorless and tasteless. The degree of substitution is usually 1.2 to 2.0. Its properties vary by the ratio of methoxyl content and hydroxypropyl content.
1. Features of hydroxypropyl methylcellulose:
1. Hydroxypropyl methylcellulose is soluble in cold water, but it will encounter difficulties in hot water. But its gelation temperature in hot water is significantly higher than that of methyl cellulose. There is also a great improvement over methylcellulose when dissolved in cold water.
2. The viscosity of hydroxypropyl methylcellulose and its molecular weight, the molecular weight is high viscosity. Temperature also affects its viscosity, with temperature, viscosity decreases. But its viscosity is lower than methyl cellulose. This solution is storage stable at room temperature.
3. The water retention of hydroxypropyl methylcellulose depends on its addition amount, viscosity, etc. The water retention rate of the same amount is higher than that of methylcellulose.
4. Hydroxypropyl methylcellulose is stable to acid and alkali, and the aqueous solution is very stable in the pH range of 2 to 12. Caustic soda and lime water have little effect on their properties, but alkali accelerates their dissolution rate and slightly increases viscosity. Hydroxypropyl methylcellulose is stable to common salts, but when the concentration of the salt solution is high, the viscosity of the hydroxypropyl methylcellulose solution tends to increase.
5. Hydroxypropyl methylcellulose can be mixed with water-soluble polymer compounds evenly, and the solution with high viscosity. Such as polyvinyl alcohol, starch ether, vegetable glue, etc.
6. Hydroxypropyl methylcellulose has better resistance to enzymes than methylcellulose, and its solution enzymatic degradation possibility is lower than that of methylcellulose.
7. The bond of hydroxypropyl methyl cellulose on the mortar structure is higher than that of methyl cellulose.
Second, the synthesis of hydroxypropyl methylcellulose:
The purified cotton fibers are treated in alkaline solution at 35-40°C for half an hour, pressed, the cellulose is pulverized, and aged at 35°C, the average degree of polymerization of the obtained alkaline fibers is within the desired range. Put the alkali fiber into the etherification kettle, add propylene oxide and methyl chloride in turn, and etherify at 50-80°C for 5 hours, the maximum pressure is about 1.8MPa. Then add an appropriate amount of hydrochloric acid and oxalic acid to the hot water at 90°C to wash the material to expand the volume. Dehydrate with a centrifuge. Wash to neutrality, when the water content is less than 60%, the flow rate of hot air at 130°C drops below 5%. The final crushing is completed over a 20-mesh sieve.