Effect of degree of substitution on viscosity of HPMC production
Hydroxypropyl methylcellulose (HPMC) is an important cellulose ether widely used in construction, medicine, food and other industries. Its viscosity is a key indicator to evaluate its performance and directly affects its use effect. In the production process of HPMC, the degree of substitution (DS, Degree of Substitution) and the molar degree of substitution (MS, Molar Substitution) are two important structural parameters, which have a significant impact on the viscosity of the product.
1. Degree of substitution and molecular structure of HPMC
HPMC is composed of cellulose skeleton and hydroxypropyl and methoxy substituents. The degree of substitution refers to the number of hydroxyl groups substituted by substituents on the cellulose molecule. Theoretically, there are three hydroxyl groups per anhydrous glucose unit (AGU), DS ranges from 0 to 3, while MS usually represents the number of ether groups on the unsubstituted hydroxyl group, which can exceed 3.
In the synthesis of HPMC, different degrees of substitution can be achieved by appropriately controlling the ratio of methoxy and hydroxypropoxy groups. Generally, the ratio of methoxy and hydroxypropoxy substitution directly determines the solubility, thermal gelability and viscosity of HPMC.
2. The influence mechanism of substitution degree on HPMC viscosity
Viscosity is the result of the three-dimensional network structure formed by HPMC molecules in solution and the interaction between molecules. The influence of substitution degree on viscosity is mainly reflected in the following aspects:
1) Intermolecular hydrogen bonding
A higher degree of substitution reduces the hydrogen bonding between cellulose molecules, weakens the interaction between molecular chains, and makes it easier to expand in the solution, thus increasing the viscosity of the solution. In particular, the introduction of hydroxypropyl group, due to its larger volume, can effectively increase the distance between molecular chains, thereby further increasing the viscosity.
2) Solubility
The type and proportion of substituents have a significant impact on the solubility of HPMC. When the degree of substitution is low, there are more hydroxyl groups in the cellulose molecules, and the hydrogen bonds between the molecules are relatively strong, making it difficult to fully dissolve, resulting in lower viscosity. As the degree of substitution increases, the solubility increases, and the molecular chains are more likely to form entangled structures in the solution, thereby increasing the viscosity.
3) Effect of molecular weight
Within a certain range, a higher degree of substitution will increase the molecular weight of HPMC, and the molecular weight has a direct relationship with the viscosity. Generally, the greater the molecular weight, the higher the viscosity. However, if the substitution is too high and the repulsive force between molecular chains is too strong, it may lead to insufficient molecular chain stretchability, which will have a negative impact on viscosity.
4) Temperature sensitivity
The viscosity of HPMC changes with temperature. The different degrees of substitution will affect the performance of HPMC at different temperatures, especially during the formation of thermal gels. HPMC with a high degree of substitution usually has a higher critical temperature, but its viscosity decreases slower and is more suitable for use under high temperature conditions. .
3. Optimization in production practice
In the production process of HPMC, by precisely controlling the degree of substitution of methoxy and hydroxypropyl groups, HPMC products of different viscosity grades can be produced. HPMC with a low degree of substitution has a relatively low viscosity and is suitable for applications requiring low viscosity, such as building mortar; while HPMC with a high degree of substitution has a higher viscosity and is more suitable for thickeners, sustained-release materials, etc.
Controlling reaction conditions (such as temperature, pH, reaction time) can also indirectly affect the degree of substitution, thereby adjusting the viscosity of HPMC. It should be noted that viscosity is not only affected by the degree of substitution, but also related to other factors such as solution concentration, shear rate, etc. Therefore, multiple factors need to be considered comprehensively in actual production to meet the needs of different application fields.
The degree of substitution has an important impact on the viscosity of HPMC, mainly affecting its viscosity properties by changing intermolecular forces, solubility, molecular weight, etc. Reasonable control of the degree of substitution is key in the production and application of HPMC. The type and proportion of substituents can be adjusted according to different application requirements to obtain ideal product performance.