Hydroxypropyl methylcellulose (HPMC) is a semi-synthetic product with a molecular weight range of 10,000 to 1,500,000. It can be prepared by two methods: (1) treating cotton lint or wood pulp fibers with caustic soda, followed by reaction with chloromethane and propylene oxide, and then refining and pulverizing; (2) treating suitable grade methylcellulose with sodium hydroxide, reacting with propylene oxide at high temperature and pressure to the desired degree, and then refining.
The refined cotton cellulose is treated with alkali solution at 35-40℃ for half an hour, pressed, pulverized, and aged appropriately at 35℃ to ensure the average degree of polymerization of the resulting alkali cellulose is within the required range. The alkali cellulose is then placed in an etherification reactor, with propylene oxide and chloromethane added sequentially, and etherified at 50-80℃ for 5 hours, with a maximum pressure of approximately 1.8 MPa. Then, appropriate amounts of hydrochloric acid and oxalic acid are added to hot water at 90℃ to wash the material, causing it to expand in volume. The material is dehydrated using a centrifuge, washed until neutral, and dried with hot air at 130°C until the moisture content is below 5%. Finally, it is pulverized and passed through a 20-mesh sieve to obtain the finished product.
