Preparation method of sodium carboxymethyl cellulose
2024.01.23
CMC Na was prepared using a two-step method using cellulose as raw material. Firstly, there is the alkalization process of cellulose. Cellulose reacts with sodium hydroxide to form alkaline cellulose, which then reacts with chloroacetic acid to form CMC Na, known as etherification reaction.
The reaction system must be alkaline. This process belongs to the Williamson ether synthesis method. The reaction mechanism is nucleophilic substitution. The reaction system is alkaline and is accompanied by some side reactions in the presence of water, such as the generation of by-products such as sodium hydroxyacetate and hydroxyacetic acid. Due to the presence of side reactions, the consumption of bases and etherification agents will increase, thereby reducing the etherification efficiency; At the same time, sodium hydroxyacetate, hydroxyacetic acid, and more salt impurities will be generated in the side reactions, resulting in a decrease in the purity and performance of the products. To suppress side reactions, it is not only necessary to use alkali reasonably, control the amount of water system, alkali concentration, and stirring method, with the aim of sufficient alkalization, but also to consider the requirements of the product for viscosity and substitution degree, comprehensively consider factors such as stirring speed and temperature control, improve etherification rate, and suppress side reactions.
According to the different etherification media, the industrial production of CMC Na can be divided into two categories: water-based method and solvent method. The method of using water as the reaction medium is called water-based method, which is used to produce alkaline medium to low-grade CMC Na. The method of using organic solvents as reaction media is called solvent method, which is suitable for producing mid to high grade CMC Na. Both of these reactions are carried out in a kneading machine and belong to the kneading process, which is currently the main method for producing CMC Na.
Water based method
The water-based method is an earlier industrial production process that involves reacting alkaline cellulose with etherifying agents under free alkali and water conditions. During the alkalization and etherification processes, there are no organic media in the system. The requirements for water-based method equipment are relatively simple, with low investment and cost. The disadvantage is the lack of a large amount of liquid medium, and the heat generated by the reaction increases the temperature, accelerating the speed of side reactions, resulting in low etherification efficiency and poor product quality. This method is used to prepare mid to low grade CMC Na products, such as detergents, textile sizing agents, etc.
Solvent method
The solvent method, also known as the organic solvent method, is characterized by the fact that the alkalization and etherification reactions are carried out under the condition of an organic solvent as the reaction medium (diluent). According to the amount of reaction diluent used, it can be divided into kneading method and slurry method. The reaction process of solvent method, like that of water method, also consists of two stages: alkalization and etherification, except that the reaction media for these two stages are different. The solvent method eliminates the inherent processes of alkali soaking, pressing, crushing, aging, etc. in the water based method, and alkalization and etherification are all carried out in the kneading machine. The disadvantage is relatively poor temperature controllability, space requirements, and high cost. Of course, strict control of system temperature, feeding time, etc. is required for production with different equipment layouts, in order to prepare products with excellent quality and performance.
