What are high-viscosity and low-viscosity hydroxypropyl methylcellulose?

High-viscosity and low-viscosity hydroxypropyl methylcellulose (HPMC) are two cellulose derivatives with different molecular structures, named for their different viscosities in aqueous solution. They are widely used in construction, medicine, food, cosmetics and other industries, and have functions such as thickening, film formation, water retention and bonding.

1. Definition

Hydroxypropyl methylcellulose is a non-ionic cellulose ether generated by the reaction of cellulose with methyl chloride and propylene oxide under alkaline conditions. The difference between high-viscosity and low-viscosity HPMC is mainly due to their molecular weight differences. The larger the molecular weight of HPMC, the higher its viscosity in solution; conversely, the smaller the molecular weight, the lower the viscosity. Therefore, high-viscosity HPMC refers to hydroxypropyl methylcellulose with a larger molecular weight, which exhibits a higher viscosity in aqueous solution; while low-viscosity HPMC is a cellulose ether with a smaller molecular weight, and the solution formed after dissolution has a lower viscosity.

2. Preparation

The preparation process of HPMC mainly involves the etherification reaction of cellulose with methyl chloride and propylene oxide under alkaline conditions. In order to obtain HPMC with different viscosities, the molecular weight of the product can be controlled by adjusting the reaction conditions, such as reaction temperature, time and the amount of etherifying agent. Generally speaking, extending the reaction time or increasing the temperature will make the molecular chain longer, thereby obtaining high-viscosity HPMC; while reducing the reaction intensity is conducive to the preparation of low-viscosity HPMC.

3. Properties

High-viscosity HPMC and low-viscosity HPMC differ in some basic properties:

Solubility: Both are water-soluble cellulose ethers, but they show different viscosities after dissolution. High-viscosity HPMC takes longer to dissolve, and the solution will be more viscous; while low-viscosity HPMC dissolves quickly and forms a thinner solution.

Thermogelation: HPMC will undergo thermogelation at a certain temperature, that is, the solution will change from liquid to gel state as the temperature rises. The thermogelation temperature of high-viscosity HPMC is low, and it is easy to form a stable gel; while the thermogelation temperature of low-viscosity HPMC is relatively high, and the gel strength is weaker.

Water retention: Due to the different molecular structures, high-viscosity HPMC has better water retention and is suitable for occasions that require long-term moisture retention; low-viscosity HPMC has slightly poor water retention.

4. Applications

High and low viscosity HPMC are widely used in many fields due to their unique properties, especially in the construction, medicine, food, cosmetics and other industries, playing different roles.

(1) Construction industry

High viscosity HPMC: In construction, high viscosity HPMC is often used in dry mortar, tile adhesives, exterior wall coatings and other products. It has strong thickening and water retention capabilities, which can effectively prevent water loss in mortar or coatings and ensure the operability of the materials during construction. At the same time, high viscosity HPMC helps to improve the bonding strength of mortar and avoid sag during construction.

Low viscosity HPMC: Low viscosity HPMC is mainly used in building materials that require good fluidity, such as self-leveling mortar or putty powder. Its main function is to improve the wettability and dispersibility of the material, help the ingredients to be more uniform during mixing, and do not affect the fluidity of the material.

(2) Pharmaceutical industry

High viscosity HPMC: In the pharmaceutical field, high viscosity HPMC is often used as a coating material for tablets and a matrix material for sustained-release preparations. It has good film-forming properties and can form a dense protective film to prevent the drug from getting wet or oxidized by air. In addition, high-viscosity HPMC can also control the release rate of the drug in the digestive tract to achieve a sustained or controlled release effect.

Low-viscosity HPMC: Low-viscosity HPMC is more commonly used in liquid preparations such as drug suspensions and capsule fillers. Because it dissolves quickly and does not affect the fluidity or bioavailability of the drug, it is suitable for liquid drug formulations.

(3) Food industry

High-viscosity HPMC: In the food industry, high-viscosity HPMC is used as a thickener and stabilizer and is commonly found in products such as dairy products, jellies, and sauces. It can provide excellent adhesion and maintain the texture and shape of the product, especially in frozen foods, where it can prevent water loss and improve the preservation effect.

Low-viscosity HPMC: Low-viscosity HPMC is mostly used in beverages, food coatings, and other products that require better fluidity in the food industry to improve fluidity and stability.

(4) Cosmetics Industry

High-viscosity HPMC: In cosmetics, high-viscosity HPMC is often used in skin creams, facial masks and other products, mainly as a thickener and film-forming agent to help the product form a protective film on the skin and lock in moisture.

Low-viscosity HPMC: Low-viscosity HPMC is more commonly used in liquid cosmetics such as shampoo and shower gel, because it does not affect the fluidity of the product too much and can increase the stability of the foam.

Although high-viscosity and low-viscosity hydroxypropyl methylcellulose have the same source, their performance and application are significantly different due to their different molecular weights. High-viscosity HPMC is mainly used in applications that require thickening, water retention and film formation, while low-viscosity HPMC is more suitable for applications that require fluidity and dispersibility. When choosing the appropriate type of HPMC, it is usually necessary to determine it based on specific application requirements, material properties and processing conditions.