Hydroxypropyl methylcellulose (HPMC) is an important chemical material widely used in construction, medicine, food, coatings and other fields. To judge the quality of HPMC products, it is necessary to conduct a comprehensive evaluation from multiple aspects such as its physical and chemical properties, appearance characteristics and specific application effects.

1. Appearance characteristics
Color and state: High-quality HPMC is usually white or off-white powder or granules, with uniform color and no obvious impurities on the surface. Too dark color or spots may indicate insufficient raw material purity or poor production process control.
Odor: High-quality HPMC has no obvious odor. If there is any odor, it may indicate the presence of impurities or chemical residues in the production process.

2. Physical and chemical performance indicators
Viscosity: Viscosity is an important parameter of HPMC, which directly affects its performance in application. It is usually tested by a rotational viscometer or a Brookfield viscometer. The viscosity of high-quality HPMC should be stable, and the error range between the test value and the nominal value is small (generally not more than ±10%).
Degree of substitution: The performance of HPMC is closely related to the degree of methoxy and hydroxypropyl substitution. The methoxy content is usually 19-30%, and the hydroxypropyl content is 4-12%. Too low or too high substitution degree will affect the solubility and viscosity stability of the product.
Moisture content: The moisture content is usually no more than 5%. Too high moisture content will affect the storage stability and application effect of HPMC.
Ash content: Ash content mainly reflects the content of inorganic impurities in HPMC. The ash content of high-quality products should be less than 1%.
Solubility: HPMC should have good solubility, be easy to disperse in cold water, and form a transparent and uniform colloidal solution. If obvious particles or flocculent precipitation appear during the dissolution process, it means that the product quality is poor.

3. Functional performance
Water retention: In construction applications, the water retention of HPMC directly affects the construction performance. The water retention rate in cement mortar or gypsum is determined experimentally (generally required to be above 90%) to judge its quality.
Thickening performance: HPMC can significantly increase the viscosity of the system in solution, and the thickening effect should be uniform and stable. If stratification or viscosity decreases, it may indicate poor product stability.
Film-forming properties: HPMC has good film-forming properties and should have certain flexibility and transparency after film formation. Uneven or fragile film formation indicates poor product quality.
Thermal stability: High-quality HPMC should maintain good performance at higher temperatures and is not prone to decomposition or a significant drop in viscosity.

4. Testing methods and standards
Laboratory testing: Use viscometers, spectrometers, ash analyzers and other equipment to accurately measure the physical and chemical properties of HPMC to ensure that the product meets the standards.
Application testing: Add HPMC to a specific application system (such as cement mortar or paint) and test its water retention, dispersibility, thickening and other properties by simulating actual use conditions.
International standards: High-quality HPMC products usually meet relevant standards such as ISO, USP, EP, etc. These standards have clear requirements for product purity, performance and safety.

5. Product stability
Long-term storage performance: High-quality HPMC should be able to maintain stable physical and chemical properties and application performance during storage. Accelerated aging experiments are used to test its performance changes under high temperature and high humidity environments.
Salt resistance: Salt substances may exist in some application environments. High-quality HPMC should be able to maintain good solubility and viscosity in salt solutions.

6. Supplier’s production process and quality control
Raw material selection: The production of high-quality HPMC requires high-purity cellulose raw materials, and the quality of raw materials has a significant impact on the final product.
Production process: Modern production process and strict quality control are the guarantee of high-quality HPMC. High-quality suppliers should have stable production lines and complete testing equipment.
Batch stability: By comparing the performance of different batches of products, it can be judged whether the supplier’s production process is stable.

7. User feedback and market reputation
Customer evaluation: The actual application effect and feedback of users are important references for judging the quality of HPMC products.
Market recognition: HPMC products of well-known brands or widely used in multiple industries are usually of more reliable quality.

8. Precautions
When purchasing HPMC, the specific application requirements should be clarified and products of appropriate specifications should be selected. At the same time, the applicability of the product can be further confirmed through small batch tests to avoid losses caused by performance mismatch.