Using cellulose as raw material, CMC-Na is prepared by a two-step method. The first is the alkalization process of cellulose. After cellulose reacts with sodium hydroxide to generate alkali cellulose, alkali cellulose reacts with chloroacetic acid to generate CMC-Na, which is called 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 in the presence of water, some side reactions are accompanied by the generation of by-products such as sodium glycolate and glycolic acid. Due to the presence of side reactions, the consumption of alkali and etherification agent will increase, thereby reducing the etherification efficiency; at the same time, sodium glycolate, glycolic acid and more salt impurities will be generated in the side reactions, resulting in reduced purity and performance of the product. In order to inhibit side reactions, it is necessary not only to use alkali reasonably, control the amount of water system, alkali concentration and stirring method, with the purpose of sufficient alkalization, but also to consider the product's requirements for viscosity and substitution degree, comprehensively consider factors such as stirring speed and temperature control, increase the etherification rate, and inhibit 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-based method. The method using water as the reaction medium is called the water-based method, which is used to produce alkaline medium and low-grade CMC-Na. The method using organic solvents as the reaction medium is called the solvent method, which is suitable for the production of medium and high-grade CMC-Na. Both reactions are carried out in a kneader, which belongs to the kneading process and is currently the main method for producing CMC-Na.
Water-based method
The water-based method is an earlier industrial production process. This method is to react alkali cellulose with an etherifying agent under the conditions of free alkali and water. During the alkalization and etherification process, there is no organic medium in the system. The water-based method has relatively simple equipment requirements, low investment and low cost. The disadvantage is that there is a lack of a large amount of liquid medium, and the heat generated by the reaction increases the temperature, which accelerates the speed of side reactions, resulting in low etherification efficiency and poor product quality. This method is used to prepare medium and low-grade CMC-Na products, such as detergents, textile sizing agents, etc.
Solvent method
The solvent method is also called organic solvent method. Its main feature is that the alkalization and etherification reactions are carried out under the condition that the organic solvent is used as the reaction medium (diluent). According to the amount of reaction diluent used, it is divided into kneading method and slurry method. The solvent method has the same reaction process as the water-based method, and is also composed of two stages: alkalization and etherification, but the reaction medium of these two stages is different. The solvent method omits the inherent processes of alkali leaching, pressing, crushing, aging, etc. of the water-based method. Alkalization and etherification are both carried out in a kneader. The disadvantage is that the temperature controllability is relatively poor, and the space requirements and costs are high. Of course, for the production of different equipment layouts, the system temperature, feeding time, etc. must be strictly controlled to prepare products with excellent quality and performance.
