Cellulose ether derivatives are a class of chemically modified natural cellulose polymers. Due to their excellent water solubility, viscosity adjustment performance and sensitivity to external conditions such as temperature and pH, they are widely used in building materials, coatings, medicines, foods and cosmetics. The viscosity control function of cellulose ether is one of the core characteristics of its wide application in many industrial and daily applications.

1. Structure and classification of cellulose ethers
Cellulose ether derivatives are prepared from natural cellulose through etherification reaction. Cellulose is a polymer compound formed by glucose monomers connected by β-1,4-glycosidic bonds. The preparation process of cellulose ether usually involves reacting the hydroxyl (-OH) part of cellulose with an etherification agent to generate cellulose derivatives with different substituents (such as methoxy, hydroxyethyl, hydroxypropyl, etc.).

Depending on the substituent, common cellulose ether derivatives include methyl cellulose (MC), hydroxyethyl cellulose (HEC), hydroxypropyl methyl cellulose (HPMC), carboxymethyl cellulose (CMC), etc. These different types of cellulose ethers have different solubility and viscosity adjustment properties. The number and position of substituents not only affect the water solubility of cellulose ethers, but also directly relate to their viscosity forming ability in aqueous solutions.

2. Viscosity formation mechanism
The viscosity regulating effect of cellulose ethers mainly comes from their dissolution in water and the extension behavior of molecular chains. When cellulose ethers are dissolved in water, polar groups form hydrogen bonds with water molecules, causing the cellulose molecular chains to unfold in water, resulting in water molecules being “entangled” around cellulose molecules, increasing the internal friction of water, and thus increasing the viscosity of the solution.

The magnitude of viscosity is closely related to the molecular weight, substituent type, degree of substitution (DS) and degree of polymerization (DP) of cellulose ethers. Generally, the larger the molecular weight of cellulose ethers and the longer the molecular chain, the higher the viscosity of the solution. At the same time, different substituents affect the hydrophilicity of cellulose ether molecules, and thus affect their solubility and viscosity in water. For example, HPMC has good water solubility and viscosity stability due to its hydroxypropyl and methyl substituents. CMC, however, has a higher viscosity because it introduces negatively charged carboxyl groups, which can interact more strongly with water molecules in aqueous solution.

3. Effect of external factors on viscosity
The viscosity of cellulose ether depends not only on its own structure, but also on external environmental factors, including temperature, pH value, ion concentration, etc.

3.1 Temperature
Temperature is an important factor affecting the viscosity of cellulose ether solution. Generally, the viscosity of cellulose ether solution decreases with increasing temperature. This is because increasing temperature accelerates molecular motion, weakens the interaction between molecules, and causes the curling degree of cellulose molecular chains in water to increase, reducing the binding effect on water molecules, thereby reducing viscosity. However, some cellulose ethers (such as HPMC) exhibit thermal gelation characteristics within a specific temperature range, that is, as the temperature increases, the solution viscosity increases and eventually forms a gel.

3.2 pH value
The pH value also has a significant effect on the viscosity of cellulose ether. For cellulose ethers with ionic substituents (such as CMC), the pH value affects the charge state of the substituents in the solution, thereby affecting the interaction between molecules and the viscosity of the solution. At higher pH values, the carboxyl group is more ionized, resulting in stronger electrostatic repulsion, making the molecular chain easier to unfold and increasing viscosity; while at lower pH values, the carboxyl group is not easily ionized, the electrostatic repulsion is reduced, the molecular chain curls, and the viscosity decreases.

3.3 Ion concentration
The effect of ion concentration on the viscosity of cellulose ether is particularly obvious. Cellulose ether with ionic substituents will be affected by the shielding effect of external ions in solution. As the ion concentration in the solution increases, the external ions will weaken the electrostatic repulsion between cellulose ether molecules, making the molecular chain curl more tightly, thereby reducing the viscosity of the solution. Especially in a high-salt environment, the viscosity of CMC will decrease significantly, which is of great significance for application design.

4. Viscosity control in application fields
Cellulose ether has been widely used in many fields due to its excellent viscosity adjustment performance.

4.1 Building materials
In building materials, cellulose ether (such as HPMC) is often used in dry-mixed mortar, putty powder, tile adhesive and other products to adjust the viscosity of the mixture and enhance the fluidity and anti-sagging properties during construction. At the same time, it can also delay the evaporation of water, improve the water retention of materials, and thus improve the strength and durability of the final product.

4.2 Coatings and inks
Cellulose ethers act as thickeners and stabilizers in water-based coatings and inks. By adjusting the viscosity, they ensure the leveling and adhesion of the coating during construction. In addition, it can also improve the anti-splashing of the coating, reduce sagging, and make the construction more uniform.

4.3 Medicine and food
In the fields of medicine and food, cellulose ethers (such as HPMC, CMC) are often used as thickeners, emulsifiers or stabilizers. For example, HPMC, as a coating material for tablets, can achieve a sustained release effect of drugs by controlling the dissolution rate. In food, CMC is used to increase viscosity, improve taste, and extend the shelf life of food.

4.4 Cosmetics
The application of cellulose ethers in cosmetics is mainly concentrated in products such as emulsions, gels and facial masks. By adjusting the viscosity, cellulose ethers can give the product appropriate fluidity and texture, and form a moisturizing film on the skin to increase comfort during use.

Cellulose ether derivatives can effectively control the viscosity of solutions through their unique molecular structure and responsiveness to the external environment. This has led to their wide application in many fields such as construction, medicine, food, and cosmetics. With the continuous development of science and technology, the functions of cellulose ethers will be further expanded to provide more precise viscosity control solutions for more fields.