Sodium Carboxymethyl Cellulose

Sodium Carboxymethyl Cellulose (CMC) is an important cellulose derivative, which is widely used in food, medicine, daily chemicals, petroleum, papermaking and other fields. Its production process involves a variety of chemical reactions and operation steps, this article will explain in detail the production process, raw material selection, equipment requirements and operation points of sodium carboxymethyl cellulose, and analyze various influencing factors in the production process.

First, the principle of production

Sodium carboxymethyl cellulose is a product obtained by alkalizing and carboxymethylation reaction of natural cellulose, and its basic reaction is the etherification reaction between the hydroxyl group on the cellulose and sodium chloroacetate to form carboxymethyl cellulose. The carboxymethyl group (—CH₂COOH) in its chemical structure gives CMC good water solubility and high viscosity.

Second, the selection of raw materials

The main raw materials for the production of CMC include cellulose, sodium hydroxide, chloroacetic acid and water, and other auxiliary materials such as ethanol and isopropanol are sometimes used. The choice of raw materials has a direct impact on the quality and production efficiency of CMC.

1. Cellulose

Cellulose is the basic raw material for the production of CMC, and lignocellulose, cotton cellulose or vegetable cellulose are usually used. The purity and polymerization (DP) of cellulose affect the performance of the final product. Cellulose with higher purity can improve the substitution degree and water solubility of CMC, while cellulose with lower DP is beneficial for the production of CMC products with lower viscosity.

2. Sodium hydroxide

Sodium hydroxide (NaOH) is a strong base used in the alkalizing process of cellulose. The concentration and dosage of NaOH have an important impact on the alkalinization effect of cellulose and the progress of carboxymethylation reaction.

3. Chloroacetic acid

Chloroacetic acid (ClCH₂COOH) is the main reagent for etherification reactions, which is used to introduce carboxymethyl groups. The purity, order of addition, and reaction conditions of chloroacetic acid affect the degree of substitution and reaction yield of CMC.

4. Water

Water is the solvent in the reaction and is also an important medium for washing solid products after the neutralization reaction. Water has high purity requirements, and deionized or distilled water is often used to reduce the influence of impurities on the reaction.

Third, the production process

The production process of sodium carboxymethyl cellulose includes alkalization, etherification, dehydration, drying, crushing and other steps. The operating conditions and parameters at each step have a direct impact on the performance of the final product.

1. Pretreatment of cellulose

Before producing CMC, cellulose needs to be pre-treated to remove impurities and adjust the physical properties of cellulose (e.g., particle size, density, etc.). Common pretreatment methods include washing, steaming, and grinding.

• Water washing: Some soluble impurities in cellulose (such as lignin, hemicellulose, etc.) can affect the production of CMC, so these impurities need to be removed by water washing.
• Cooking: Cooking cellulose at high temperature and high pressure can further remove some insoluble impurities and increase the reactivity of cellulose.
• Grinding: In order to increase the contact area of cellulose with lye and chloroacetic acid, it is usually necessary to grind the cellulose to obtain a smaller, uniform cellulose powder.

2. Alkalizing of cellulose

Alkalinization is a critical step in the production of CMC, which causes the hydroxyl group (—OH) on the cellulose chain to react with sodium hydroxide to form sodium alk (—ONa), which provides an active site for subsequent etherification reactions.

• Conditions for alkalinization reaction: Cellulose is mixed with a certain amount of NaOH, and the alkalinization reaction is usually carried out at 50~80 °C. The reaction time is typically 30 minutes to 2 hours, depending on the type of cellulose and the degree of substitution (DS) required.
• Control of alkalinization reaction: The temperature, concentration of the alkali and the reaction time need to be controlled during the reaction to ensure sufficient alkalinization without excessive damage to the cellulose molecular structure.

3. Etherification reaction

The alkalized cellulose reacts with sodium chloroacetate to form sodium carboxymethyl cellulose. At the heart of this process is the reaction of the carboxymethyl group (—CH₂COOH) in chloroacetic acid with the sodium alkyl group on cellulose to form a carboxymethyl ether bond.

• Reaction conditions: The etherification reaction is usually carried out immediately after the end of alkalinization, the reaction temperature is generally controlled at 60~90 °C, and the reaction time is 1~5 hours. The temperature and time of the reaction affect the degree of substitution and molecular weight of the final product.
• Addition of Chloroacetic Acid: Chloroacetic acid is usually added in the form of a sodium salt (sodium chloroacetate, ClCH₂COONa) to improve the efficiency of the reaction and reduce the occurrence of side reactions.
• Reaction by-products: By-products such as sodium chloride (NaCl) are produced in this reaction and need to be removed in subsequent steps.

4. Wash with neutralization

At the end of the reaction, the alkaline substances and by-products (e.g., NaCl) in the system need to be removed by washing.

• Neutralization step: After the etherification reaction, it is usually necessary to neutralize the residual base in the reaction mixture to neutralization. Commonly used neutralizers include hydrochloric acid, acetic acid and other weak acids.
• Washing: After neutralization, the crude CMC product is washed with a large amount of deionized water or ethanol to remove the salts and unreacted sodium chloroacetate to ensure the purity of the product.
• Dehydration: The washed product contains a large amount of water and most of the water must be removed by mechanical dehydration or centrifugation.

5. Dry

After dehydration, CMC products still contain a certain amount of moisture and need to be further dried.

• Drying Equipment: Commonly used drying equipment includes vacuum dryers, fluidized bed dryers, or spray dryers. The drying temperature is generally controlled at 60~80°C to avoid product degradation caused by excessive temperature.
• Drying time: The drying time is determined according to the particle size and moisture content of the product, usually 2~5 hours. The moisture content after drying is generally controlled below 5%.

6. Crushing and sifting

After drying, the CMC block material needs to be crushed into a homogeneous powder for subsequent packaging and use.

• Crushing equipment: Commonly used crushing equipment includes ball mills, winnowing crushers, etc. The particle size after crushing is generally controlled between 80 mesh and 200 mesh.
• Sieving: In order to ensure the uniformity of the product particles, the crushed product needs to be sieved to remove particles that are too large or too small.

7. Packaging & Storage

The crushed and sieved CMC powder can be packaged. The packaging materials are usually moisture-proof and corrosion-proof, such as multi-layer kraft paper bags or polyethylene composite film bags.

• Storage conditions: CMC is prone to moisture absorption, so the storage environment needs to be kept dry to avoid moisture affecting the quality of the product. The storage temperature is generally controlled at 15~25°C, and the humidity is below 50%.

Fourth, the key factors affecting the production process

In the production process of CMC, there are several factors that affect the quality and production efficiency of the final product.

1. Choice of cellulose

The purity and molecular weight of cellulose have an important impact on the performance of CMC. High-purity cellulose enables the production of high-quality CMC products, while low-molecular-weight cellulose facilitates the production of CMC with lower viscosity.

2. Dosage of sodium hydroxide

The dosage and concentration of NaOH directly affect the effect of alkalinization reaction. Too much NaOH can lead to excessive degradation of cellulose, which can affect the performance of the final product; If the dosage is too small, it may lead to incomplete alkalinization, which will affect the subsequent etherification reaction.

3. Amount of chloroacetic acid added and reaction conditions

The amount of chloroacetic acid added determines the degree of substitution of CMC

（DS）。 The higher the DS, the higher the water solubility and viscosity of the CMC. Reaction temperature and time also have an important influence on the process of etherification reaction. Too high a temperature or too long a reaction time can lead to the degradation of cellulose and reduce the molecular weight and viscosity of the product.

4. Control of the reaction environment

The moisture content, temperature, and pressure in the reaction environment all affect the progress of the reaction. Especially in etherification reactions, too high a moisture content can dilute the reactants and reduce the reaction efficiency.

5. Washing & Drying

The washing step is effective in removing reaction by-products, but excessive washing may lead to CMC dissolution and affect product yield. When drying, the temperature needs to be controlled to avoid the degradation of CMC caused by high temperatures, and at the same time ensure that the dried product has a low moisture content.

Fifth, product quality control

The quality control of sodium carboxymethyl cellulose mainly includes the following aspects:

1. Degree of substitution (DS)

DS is an important indicator to measure the performance of CMC products, the higher the DS, the better the water solubility and thickening performance of CMC. Usually DS is controlled between 0.7~1.2.

2. Viscosity

The viscosity of CMC is one of its most important physical properties, and the viscosity is closely related to the molecular weight of cellulose and DS. The viscosity of CMC is usually measured by a viscometer and characterized by the viscosity of the solution.

3. Purity

Purity refers to the amount of cellulose derivatives in CMC. Higher purity means fewer impurities, helping to improve the effectiveness of the product.

4. Moisture content

The moisture content of CMC should be strictly controlled within a certain range, usually no more than 5%.