HPMC vs HEMC (1)
HPMC vs HEMC (3)

2 Physical and chemical properties

The physicochemical properties of the VITO ether industry testing standards include: appearance, fineness, dry weight loss, sulphate ash, solution pH, solution light transmission, solution viscosity, gel temperature, group content, etc. (excluding mortar application experiments).
Appearance, fineness, dry weight loss, sulphate ash, solution pH and light transmission, viscosity, etc. are related to the product type and function, which vary from manufacturer to manufacturer and are not discussed here.

2.1 Cellulose ether moiety content

Due to the differences in HPMC and HEMC substituents. Cellulose ether specimens can be reacted by heating in a closed reactor. Catalyzed by adipic acid, the substituted alkoxy groups are quantitatively cleaved by hydriodic acid to produce the corresponding iodosubstituted alkanes. The reaction products are extracted with o-xylene and the extract is injected into a gas chromatograph for the separation of the components. The time taken to separate out allows the identification of hydroxypropoxy and hydroxyethoxy. It is also quantified by internal standard method and the amount of the component to be measured in the specimen is calculated. The separation time for hydroxyethoxy is intermediate between methoxy and hydroxypropoxy. And the type of group is determined by comparing the separation times of the standard solutions. The time to peak is used to determine the group type and the peak area to calculate the group content.

In general, the methoxy content of HPMC is between 16% and 30% and hydroxypropoxy can be between 4% and 32%. The methoxy content of HEMC is between 22% and 30% and hydroxypropoxy 2% to 14%.

2.2 Gel temperature

The gel temperature is an important indicator of cellulose ether. Cellulose ether aqueous solution has thermal gelation characteristics, with the increase of temperature, the viscosity decreases continuously. When the solution temperature reaches a certain value, the cellulose ether solution is no longer homogeneous and transparent, but forms a white gel, and eventually loses viscosity.

The gel temperature test method refers to: the cellulose ether sample is prepared into a 0.2% concentration of cellulose ether solution. Slowly heating in a water bath until the solution appears white turbid or even white gel, and completely loses viscosity. At which time the temperature of the solution is the gel temperature of cellulose ether. Eight cellulose ethers were randomly selected from domestic and international products to test the gel temperature. The result is that the overall gel temperature of HEMC is slightly higher than that of HPMC. In general, the gel temperature of HPMC is between 60°C and 75°C. And that of HEMC is between 75°C and 90°C.
The ratio of methoxy, hydroxypropoxy content of HPMC has an influence on the water solubility, water retention capacity, surface activity and gel temperature of the product. Usually, HPMC with high methoxy content and low hydroxypropoxy content has good water solubility and surface activity, but low gel temperature. Appropriately increasing the hydroxypropoxy content and decreasing the methoxy content can improve the gel temperature. But too high hydroxypropoxy content will lower the gel temperature and make the water solubility and surface activity worse. Therefore, cellulose ether manufacturers must ensure product quality and stability by strictly controlling the content of the groups.