The Alkaline Immersion Production Method of Hydroxypropyl Methyl Cellulose (HPMC)

1. What is alkaline immersion production method

The production method of hydroxypropyl methylcellulose, the alkaline immersion production method is also known as the step-by-step method. This production method in China originated in the 1980s. The raw material of hydroxypropyl methylcellulose is refined cotton, and in some places, resin is also used for the production of hydroxypropyl methylcellulose. In addition, there are elements such as methyl, propylene oxide and methyl chloride on the composition of hydroxypropyl methylcellulose. The content of methyl group determines the thickening effect of cellulose ether to a certain extent. Therefore, these three substances are indispensable raw materials in the production process of hydroxypropyl methylcellulose.

The alkaline immersion production method involves refined cotton or wood pulp being impregnated with liquid caustic soda, squeezed to remove excess alkaline liquid to obtain the alkaline cellulose, then solvent and etherifying agent are added, and the etherification reaction is carried out at a certain temperature and pressure. At the end of the reaction based on the desired degree of etherification, the finished product is then neutralized, washed, dried, crushed, and the like.

The alkaline immersion  is as follows:

First, non-crushed flaky cellulose is added to an exclusive-used impregnating machine. The alkalization can be performed in more than six times the volume of an alkaline liquid. After removing the excess alkaline liquid, the cellulose is then fed into a vertical reactor with a strong agitating device and dispersed in about eight times the volume of methyl chloride. A certain amount of propylene oxide is then added for the chemical reaction to take place. Upon the completion of the reaction, the reacted methyl chloride, propylene oxide and byproduct dimethyl ether are recovered. The finished product is obtained through hot water washing, drying, crushing, and sieving.

The flowchart of the production method is shown in the following figure:

2. The main equipment utilized in the alkaline immersion production method:

3. The main parameters of the alkaline immersion production method:

4. The Advantages and disadvantages of alkaline immersion production method

The HPMC produced by the alkaline immersion production method has good stability as it undergoes the full 12-hour reaction. The number of impurities in the finished product is very small. Thus, the HPMC in high purity can be obtained.

However, the production method requires a large amount of methyl chloride as a dispersant and the desired reaction pressure is up to 3.0MPa. Upon the completion of the reaction, a large quantity of methyl chloride and dimethyl ether needs to be recovered. The raw material consumption and the amount of chemical oxygen demand (COD) in the sewage are high. The quality of the finished product derived from the production method is guaranteed. However, the price of the finished product is high because of the high cost of manpower, materials, and time.

And the operation of the alkaline leaching method requires intermittent operation, and the quality of the product is not stable. The improved method is to use continuous alkalization and etherification, purification technology. In the next article, melacoll will introduce you to another production method of hydroxypropyl methylcellulose - the liquid phase production method, also known as the slurry process. We will share with you a brief introduction to the process flow and advantages and disadvantages of this production method.