Thickening and thixotropy of hydroxypropyl methylcellulose (HPMC)

The thickening effect of hydroxypropyl methylcellulose depends on the polymerization degree of hydroxypropyl methylcellulose, solution concentration, shear rate, temperature and other conditions. The gelation characteristics of solutions are unique to alkyl cellulose and its modified derivatives. The gelation characteristics are related to the degree of substitution, the concentration of solution and additives. For hydroxy alkyl modified derivatives, the gel properties are also related to the degree of hydroxyl alkylation modification. MC and HPMC with low viscosity can prepare 10% - 15% solution, MC and HPMC with medium viscosity can prepare 5% - 10% solution, while MC and HPMC with high viscosity can only prepare 2% - 3% solution. Generally, the viscosity classification of hydroxypropyl methylcellulose is also based on 1% - 2% solution.

High molecular weight hydroxypropyl methylcellulose has high thickening efficiency. In the same concentration solution, different molecular weight polymers have different viscosities. The relationship between viscosity and molecular weight can be expressed as follows[ η]= two point nine two × 10-2 (DPN) 0.905, DPN has a high average degree of polymerization. The target viscosity can only be achieved by adding more hydroxypropyl methylcellulose with low molecular weight. Its viscosity has little dependence on the shear rate. When the high viscosity reaches the target viscosity, the amount required is small, and the viscosity depends on the thickening efficiency. Therefore, in order to achieve a certain consistency, a certain amount of hydroxypropyl methylcellulose must be added (solution concentration) and solution viscosity. The gel temperature of the solution decreases linearly with the increase of the concentration of the solution. After reaching a certain concentration, the gel temperature is room temperature, and the gelation concentration of HPMC is higher at room temperature.

The consistency can also be adjusted by selecting particle size and hydroxypropyl methylcellulose with different degrees of modification. The so-called modification is to introduce hydroxyalkyl with a certain degree of substitution into the skeleton structure of MC. By changing the relative substitution values of the two substituents, that is, the DS and MS relative substitution values of methoxy and hydroxyalkyl. By changing the relative substitution values of the two substituents, the requirements of various properties of hydroxypropyl methylcellulose are obtained.

Hydroxypropyl methylcellulose used in powdery building materials must be dissolved rapidly in cold water and can provide the system with appropriate consistency. If given a certain shear rate, it is still colloidal block, which is unqualified or of poor quality.