HPMC (Hydroxypropyl Methylcellulose) is used in liquid soap primarily to improve rheological properties, thicken, stabilize, and enhance the user experience. As a water-soluble, nonionic cellulose ether, HPMC is widely used in daily chemical applications, particularly in liquid cleansers and personal care products.

1. The Role of HPMC in Liquid Soap

1.1. Thickening
Liquid soap requires an appropriate viscosity to avoid being too thin, which can lead to a poor user experience. HPMC forms a uniform network structure in the aqueous phase, thereby increasing the viscosity of the system and imparting ideal flow and feel to the soap.

1.2. Stabilizing Suspension and Preventing Separation
Liquid soaps often contain fragrances, oils, or insoluble particles. The viscosity and colloid-protective properties of HPMC prevent sedimentation and segregation, ensuring the soap remains uniform during storage and use.

1.3. Moisturizing and Skin Care
HPMC inherently retains water. Its inclusion in the formulation can reduce dryness after washing, making liquid soap gentler and more skin-friendly.

1.4. Improving the Usage Experience
HPMC solution has a silky, smooth feel, enhancing the lathering and application experience while avoiding the cheap, “watery” feel of soap.

2. HPMC Addition Method

To ensure the performance of HPMC in liquid soap, proper dispersion and dissolution methods are required:

2.1. Pre-Dispersion Method
Slowly disperse HPMC powder in hot water at approximately 70°C. HPMC will not dissolve immediately, but will be evenly dispersed.

As the mixture cools to room temperature, HPMC will gradually dissolve, forming a transparent or translucent solution.

This method is suitable for large-scale production and avoids clumping.

2.2. Dry Mixing Method
First, evenly mix HPMC with other powdered ingredients (such as salt, fragrance carrier, etc.), then add to water and stir to dissolve. This method effectively reduces clumping of HPMC in water.

2.3. Solution Method
You can also prepare an HPMC aqueous solution of a certain concentration (e.g., 2%-5%) and then add it to the liquid soap base in the appropriate proportion.
This method is convenient and suitable for small-batch experiments or recipe adjustments.

3. Dosage and Formulation Recommendations

3.1. Addition Range
The typical dosage of HPMC in liquid soap is 0.2%-1.0%.
If only slight thickening is required, a lower concentration is sufficient; if high viscosity or a suspending effect is desired, the dosage can be increased appropriately.

3.2. Combination with Other Additives
It can be combined with surfactants (such as SLES and APG) to improve foam richness and stability.
It can be combined with humectants such as glycerin and propylene glycol to enhance water retention.
Adding salt (NaCl) can adjust the solution viscosity, but care should be taken to avoid salting out and causing turbidity.

4. Precautions

4.1. Dispersion Speed
HPMC powder can easily form lumps if added directly and quickly to water. It should be added slowly while stirring.

4.2. Water Quality Requirements
Deionized or soft water is recommended to avoid excessive calcium and magnesium ions that may affect dissolution.

4.3. pH Range
HPMC is relatively stable to acids and alkalis, with an applicable pH range of 4 to 8, making it ideal for liquid soap systems.

4.4. Temperature Effects
HPMC solutions may become turbid or gel at high temperatures (>70°C). Prolonged high-temperature treatment should be avoided.

5. Application Results Summary

Appropriate use of HPMC in liquid soap can significantly improve the following properties:

Significant thickening effect, excellent flowability, and hand feel;
Prevents settling of fragrances, pigments, or particles, improving product stability;
Enhances foam richness and uniformity;
Improves skin feel and reduces post-wash tightness;
Improves product appearance (transparency and uniformity), increasing market competitiveness.

The use of HPMC in liquid soaps goes beyond simple thickening to ensure stability, user experience, and skincare benefits. By selecting the appropriate grade and using a scientific addition method, formulators can achieve higher quality and added value while maintaining cost control.