Hydroxypropyl Methylcellulose (HPMC) is a modified cellulose ether widely used in pharmaceutical preparations, food additives, building materials, cosmetics and other fields. HPMC has thickening, film-forming, adhesion and other properties. The relationship between the viscosity and concentration of its aqueous solution is of great significance for various applications.
Viscosity characteristics of HPMC aqueous solution
Basic characteristics
HPMC forms a transparent or translucent viscous solution after dissolving in water. Its viscosity is affected not only by the concentration of HPMC, but also by factors such as molecular weight, substituent type and solution temperature.
Molecular weight: The larger the molecular weight of HPMC, the higher the solution viscosity. This is because the macromolecules form a more complex entangled structure in the solution, which increases the friction between molecules.
Substituent type: The ratio of methoxy and hydroxypropoxy substituents affects the solubility and viscosity of HPMC. Generally, when the methoxy content is higher, the solubility of HPMC is better and the viscosity of the solution is also higher.
Relationship between concentration and viscosity
Dilute solution stage:
When the concentration of HPMC is low, the interaction between molecules is weak and the solution exhibits Newtonian fluid properties, that is, the viscosity is basically independent of the shear rate.
At this stage, the viscosity of the solution increases linearly with increasing concentration. This linear relationship can be expressed by a simple viscosity equation:
| Concentration (%) | Viscosity (mPa·s) |
| 0.5 | 100 |
| 1.0 | 300 |
| 2.0 | 1000 |
| 5.0 | 5000 |
| 10.0 | 20000 |
It can be seen from the data that the viscosity of HPMC aqueous solution increases exponentially with the increase in concentration. This growth will appear on the graph as a steeply rising curve, especially in areas of high concentration.
Influencing factors
Effect of temperature
Temperature has a significant effect on the viscosity of HPMC solution. Generally speaking, an increase in temperature decreases the viscosity of a solution. This is because increased temperature causes increased molecular motion and weakens the interaction between molecular chains, thereby reducing viscosity.
Effect of shear rate
For high-concentration HPMC solutions, the viscosity is also affected by the shear rate. At high shear rates, the orientation of the molecular chains becomes more consistent and internal friction is reduced, resulting in a lower apparent viscosity of the solution. This phenomenon is called shear thinning.
Applications
In pharmaceutical preparations, HPMC is commonly used in tablet coatings, sustained-release dosage forms, and thickeners. Understanding how the viscosity of HPMC aqueous solutions changes with concentration is crucial for designing appropriate drug formulations. For example, in tablet coating, an appropriate HPMC concentration can ensure that the coating liquid has sufficient viscosity to cover the tablet surface, while not being too high to be difficult to handle.
In the food industry, HPMC is used as a thickener and stabilizer. Understanding the relationship between concentration and viscosity can help determine the optimal concentration to ensure the taste and texture of food.
The viscosity of HPMC aqueous solution has a significant positive correlation with concentration. It shows a linear increase in the dilute solution stage and an exponential increase in high concentration. This viscosity characteristic is crucial in a variety of industrial applications, and understanding and controlling the viscosity changes of HPMC is of great significance for process optimization and product quality improvement.