At what temperature is HPMC easy to gel?

Hydroxypropyl Methylcellulose (HPMC) is a multifunctional polymer material widely used in construction, pharmaceuticals, food, cosmetics and other fields. HPMC exhibits different solubility and gelation properties at different temperatures, and its gelation temperature is crucial to its application performance. This article will discuss in detail the gelation temperature of HPMC and its influencing factors and applications.

### 1. Basic characteristics of HPMC

#### 1.1 Chemical structure and solubility

HPMC is made by reacting cellulose with propylene oxide and formaldehyde after alkali treatment. Its molecular structure contains methoxy (-OCH3) and hydroxypropoxy (-OCH2CHOHCH3) substituents. These substituents give HPMC unique solubility and gelation properties.

#### 1.2 Gel properties

The solubility and gelation of HPMC in water are closely related to temperature. At low temperatures, HPMC can dissolve in water to form a transparent solution. However, when the temperature rises to a certain range, HPMC will gel and form an opaque gel.

### 2. Gel temperature of HPMC

#### 2.1 Definition of gel temperature

Gelation temperature refers to the temperature range at which the HPMC solution changes from liquid to gel. This temperature range depends on factors such as the chemical structure, degree of substitution, molecular weight and solution concentration of HPMC.

#### 2.2 Factors affecting gel temperature

1. **Degree of Substitution (DS)**:

- **Methoxy degree of substitution**: The higher the methoxy degree of substitution of HPMC, the lower its gel temperature. This is because the methoxy group is hydrophobic. As the temperature rises, the hydrophobic interaction between HPMC molecules increases, prompting it to form a gel.

- **Hydroxypropoxy degree of substitution**: Hydroxypropoxy is hydrophilic. Increasing the hydroxypropoxy degree of substitution will increase the gel temperature of HPMC, because the hydrophilic group enhances the solubility of HPMC in water and inhibits gelation.

2. **Molecular weight**:

- The higher the molecular weight of HPMC, the lower its gel temperature. HPMC with high molecular weight has longer molecular chains, stronger intermolecular interactions, and is easy to form gels at lower temperatures.

3. **Solution concentration**:

- The higher the concentration of the HPMC solution, the lower its gel temperature. HPMC molecules in a high concentration solution are more likely to interact and form gels.

4. **Solution pH and additives**:

- The pH of the solution and certain additives (such as salts, surfactants, etc.) also affect the gel temperature of HPMC. Generally speaking, changes in pH affect the solubility of HPMC, while additives may adjust the gel temperature by changing the ionic strength or intermolecular interactions of the solution.

### 3. HPMC gel temperature range

Depending on the degree of substitution and molecular weight of HPMC, its gel temperature is generally between 50°C and 90°C. Specifically:

- **Low substitution HPMC** (high methoxy content): The gel temperature is lower, usually between 50°C and 70°C.

- **High substitution HPMC** (high hydroxypropoxy content): The gel temperature is higher, usually between 70°C and 90°C.

### 4. Gel temperature in the application of HPMC

#### 4.1 Building materials

In building materials, HPMC is mainly used as a thickener and water retainer for cement-based materials. Its gel temperature has an important influence on the construction performance and the quality of the final product. For example, in tile adhesives and putty powder, the appropriate gel temperature can ensure that the material has good workability and anti-sagging during the construction process.

- **Construction temperature control**: In order to ensure the best performance of HPMC during the construction process, the construction environment temperature should be controlled below the gel temperature of HPMC to prevent the material from gelling prematurely and affecting the construction effect.

#### 4.2 Pharmaceutical industry

In the pharmaceutical industry, HPMC is widely used in tablets, capsules and sustained-release preparations. Its gel temperature has an important influence on the release rate and solubility of drugs.

- **Controlled release preparations**: In controlled-release preparations, the gel properties of HPMC are used to control the release rate of drugs. The appropriate gel temperature ensures that the drug is gradually released in the in vivo environment, improving the bioavailability and efficacy of the drug.

#### 4.3 Food Industry

In the food industry, HPMC is used as a thickener, emulsifier and stabilizer, and its gel temperature affects the texture and stability of food.

- **Emulsifier stabilizer**: In dairy products and beverages, the gel properties of HPMC help stabilize the emulsified system and prevent stratification and precipitation. The appropriate gel temperature ensures that the product remains uniform and stable during storage and use.

#### 4.4 Cosmetics and personal care products

In cosmetics and personal care products, HPMC is used as a thickener, moisturizer and film former. Its gel temperature affects the feel and stability of the product.

- **Skin care products**: In skin care products, the gel properties of HPMC help form a soft and smooth film and provide good moisturizing effects. The appropriate gel temperature ensures that the product is easy to apply and absorb on the skin.

### 5. Regulation and optimization of HPMC gel temperature

#### 5.1 Regulation by degree of substitution

By adjusting the degree of substitution of methoxy and hydroxypropoxy groups of HPMC, its gel temperature can be precisely regulated to meet the needs of different applications.

#### 5.2 Regulation by molecular weight

Select HPMC products with appropriate molecular weight to achieve the required gel temperature and solution performance.

#### 5.3 Use of additives

By adding appropriate amounts of salt, surfactants or other functional additives, the gel temperature of HPMC can be adjusted to optimize its performance in specific applications.

### 6. Future research directions

#### 6.1 New modified HPMC

Develop modified HPMC with specific functions, such as by introducing special functional groups to improve its performance stability and application effect within a specific temperature range.

#### 6.2 Green and environmentally friendly HPMC

Promote the development of green and environmentally friendly preparation technology, reduce environmental pollution in the production process, and improve the sustainability of products.

#### 6.3 Multifunctional HPMC

Explore the cross-application of HPMC in multiple fields, such as combining nanotechnology and biotechnology to develop multifunctional HPMC products.

In summary, hydroxypropyl methylcellulose (HPMC) exhibits different solubility and gel properties at different temperatures, and its gel temperature is affected by many factors. By rationally regulating the degree of substitution, molecular weight and use of additives of HPMC, its performance can be optimized to meet the needs of different industries and applications. In the future, with the continuous deepening of research and continuous innovation of technology, the application prospects of HPMC will be broader, providing stronger support for the development of related industries.