What is the difference between hydroxypropyl methylcellulose and hydroxyethyl cellulose?

Hydroxypropyl methylcellulose (HPMC) and hydroxyethyl cellulose (HEC) are two common cellulose ethers, which are widely used in many fields such as construction, pharmaceuticals, food, and cosmetics. They are both obtained by chemical modification of natural cellulose, but there are significant differences in structure, performance and application. This article will discuss the difference between HPMC and HEC in detail, including their chemical structure, physicochemical properties, production process, application fields and performance comparison.

## 1. Chemical structure

### 1.1 Hydroxypropyl methylcellulose (HPMC)

HPMC is made by introducing hydroxypropyl and methyl groups into cellulose molecules. In its chemical structure, some hydroxyl groups (—OH) are replaced by methoxy groups (—OCH3) and hydroxypropoxy groups (—OCH2CH(OH)CH3). The degree of substitution (DS) and substitution uniformity of HPMC determine its solubility and viscosity and other properties.

### 1.2 Hydroxyethyl Cellulose (HEC)

HEC is made by introducing hydroxyethyl groups into cellulose molecules. In its chemical structure, some hydroxyl groups are replaced by hydroxyethoxy groups (—OCH2CH2OH). The degree of substitution (DS) and uniformity of substitution of HEC also affect its solubility, viscosity and water retention.

## 2. Physical and chemical properties

### 2.1 Solubility

- **HPMC**: HPMC dissolves in cold water and some organic solvents to form a transparent and stable colloidal solution. Its solubility is significantly affected by temperature, and gelation occurs after heating to a certain temperature.

- **HEC**: HEC can be dissolved in both cold and hot water to form a transparent viscous solution. Unlike HPMC, HEC has good solubility in a wide temperature range and is not prone to gelation.

### 2.2 Viscosity

- **HPMC**: The viscosity of HPMC is greatly affected by temperature and concentration. Usually, a high-viscosity solution can be formed at a lower concentration, which is suitable for applications requiring high viscosity.

- **HEC**: The viscosity of HEC solution is also affected by temperature and concentration, but compared with HPMC, HEC has a lower viscosity at low concentrations and is suitable for applications with medium viscosity requirements.

### 2.3 Water retention

- **HPMC**: It has excellent water retention and is particularly suitable for use in building materials to improve the water retention and construction performance of cement mortar and gypsum-based materials.

- **HEC**: It also has good water retention, but is slightly inferior to HPMC and is mainly used in applications that require medium water retention, such as coatings and cosmetics.

## 3. Production process

### 3.1 Production process of HPMC

The production of HPMC includes the etherification reaction of cellulose, which reacts cellulose with chloropropanol and methanol under alkaline conditions to form hydroxypropyl and methyl groups. The temperature, time and reactant ratio during the reaction need to be strictly controlled to ensure the degree of substitution and uniformity of the product.

### 3.2 HEC production process

HEC production also includes cellulose etherification reaction, in which cellulose is reacted with ethylene oxide under alkaline conditions to form hydroxyethyl groups. The temperature, pressure and reactant ratio in the production process also need to be strictly controlled to ensure the quality and performance of the product.

## 4. Application fields and performance comparison

### 4.1 Building materials

- **HPMC**: Widely used in cement mortar, tile adhesive, plaster mortar and gypsum-based materials, mainly used to improve the water retention, adhesion and construction performance of materials.

- **HEC**: Relatively less used in building materials, mainly used in water-based coatings and adhesives to provide thickening and water retention functions.

### 4.2 Coatings and paints

- **HPMC**: As a thickener and stabilizer, HPMC is used in coatings and paints to improve rheological properties and coating properties, and prevent coating sagging and precipitation.

- **HEC**: Widely used in coatings and paints, especially in water-based coatings, HEC provides good thickening and water retention effects, improving the construction and leveling properties of coatings.

### 4.3 Cosmetics and personal care products

- **HPMC**: In cosmetics and personal care products, HPMC is used for thickening, film formation and moisturizing, such as lotions, conditioners and eye creams.

- **HEC**: Widely used in cosmetics and personal care products as a thickener, stabilizer and moisturizer, such as shampoo, shower gel and lotion.

### 4.4 Pharmaceutical industry

- **HPMC**: In the pharmaceutical industry, HPMC is used for film coating, sustained release agent and adhesive of tablets, with good biocompatibility and stability.

- **HEC**: In the pharmaceutical industry, HEC is mainly used for thickening and stabilization of liquid drugs, such as oral solutions, suspensions and eye drops.

### 4.5 Food Industry

- **HPMC**: In the food industry, HPMC is used for thickening, stabilization and water retention, such as ice cream, jelly and dairy products.

- **HEC**: It is relatively rarely used in the food industry and is mainly used for thickening and stabilization of certain specific foods, such as sauces and beverages.

## 5. Performance Comparison Summary

### 5.1 Viscosity and Rheological Properties

- **HPMC**: It can form a high viscosity solution at low concentrations and is suitable for applications that require high viscosity. Its rheological properties are greatly affected by temperature and concentration.

- **HEC**: It forms a solution of moderate viscosity at medium concentrations, and its rheological properties are relatively stable, which is suitable for applications that require medium viscosity and stability.

### 5.2 Water Retention and Film Formation

- **HPMC**: It has excellent water retention and film formation, and is suitable for applications that require high water retention and film formation, such as building materials and cosmetics.

- **HEC**: Good water retention and film-forming properties, but slightly inferior to HPMC, suitable for applications such as coatings and personal care products that require moderate water retention and film-forming properties.

### 5.3 Solubility and thermal stability

- **HPMC**: Solubility is significantly affected by temperature, and gelation is prone to occur at high temperatures. Its thermal stability is average.

- **HEC**: Solubility is less affected by temperature, and it can maintain good solubility and stability in a wide temperature range. Its thermal stability is good.

## 6. Conclusion

In summary, although hydroxypropyl methylcellulose (HPMC) and hydroxyethyl cellulose (HEC) are both cellulose ethers, they have significant differences in chemical structure, physicochemical properties, production process, application fields and performance. HPMC is widely used in building materials, cosmetics and pharmaceutical industries due to its excellent water retention and film-forming properties; while HEC is mainly used in coatings, personal care products and some food industries due to its good solubility and stability. Understanding the difference between the two will help to select suitable cellulose ethers in different applications, optimize product performance and improve application effects.