HPMC Viscosity Testing Guide

1. Introduction

Viscosity is a key quality indicator for hydroxypropyl methylcellulose (HPMC), directly affecting its thickening, water retention, and film-forming properties, and determining its application effectiveness in construction, coatings, pharmaceuticals, and food industries. Standardized viscosity testing helps ensure batch consistency, compliance with specifications, and fulfillment of end-use requirements.

Currently, commonly used testing methods include the Brookfield rotational viscometer method (refer to relevant standards such as ASTM D2363 and ASTM D1347), the capillary viscometer method (refer to ISO 1628-1 and ISO 3105), and the falling ball method. Among these, the Brookfield rotational method is the most widely used in industrial quality control due to its ease of operation and wide applicable viscosity range.

2. Principle of the Brookfield Viscosity Method

This method determines viscosity by measuring the fluid resistance (torque) experienced by a rotor rotating at a constant speed. The magnitude of the resistance is directly proportional to the liquid viscosity. Results are typically expressed in millipascal-seconds (mPa·s) or centipoises (cP) and determined at specified temperatures (usually 20°C or 25°C) and concentrations (usually 2%).

3. Equipment and Materials

Brookfield rotational viscometer (equipped with an appropriate rotor, such as the LV series for low viscosity, and the RV series for medium to high viscosity)

Constant temperature water bath (temperature control accuracy ±0.1°C)

Analytical balance (accuracy 0.01 g)

Graded beaker

Deionized water

Stirring device (e.g., magnetic stirrer)

Temperature measuring device

Timer

4. Sample Preparation

The accuracy of sample preparation is crucial to the test results. A 2% (w/w) concentration is generally recommended for testing; the concentration can be reduced for higher viscosity grades.

Preparation Steps:

Accurately weigh an appropriate amount of HPMC powder.

To prevent clumping, the powder can be first dispersed in hot water (80–90°C), stirred evenly, and then diluted with cold water to volume; alternatively, a dry-mixing-assisted dispersion method can be used.

Allow the solution to stand for full hydration (usually 30 minutes to 2 hours), gently stirring during this period to ensure homogeneity.

5. Test Procedure

Heat the sample to the specified temperature (e.g., 20°C ± 0.1°C).

Select a suitable rotor and speed based on the estimated viscosity.

Immerse the rotor to the specified depth, start the instrument, and record the viscosity value after the reading stabilizes.

It is recommended to repeat the test three times to confirm reproducibility. In quality control, a deviation of ±5% is generally allowed.

6. Capillary Viscometer Method (Alternative)

This method is suitable for research or testing of low-viscosity samples. The kinematic viscosity is calculated by measuring the time it takes for the solution to flow through a standard capillary, and then converted to dynamic viscosity by combining this with density. Strict temperature control and operational consistency are essential.

7. Influencing Factors

Concentration: Weighing errors significantly affect high-viscosity samples.

Temperature: Strict temperature control is essential; even small fluctuations can cause noticeable viscosity changes.

Shear Rate: HPMC solutions are typically pseudoplastic fluids; viscosity decreases with increasing rotational speed.

Dissolution and Hydration: Incomplete dissolution or insufficient hydration will lead to lower results.

Time and pH: Viscosity may increase slightly with standing time; extreme pH environments may cause polymer degradation.

8. Data Report

The report should include the following information:

Sample identification (name, batch number)

Test date and personnel

Test method and standard (e.g., ASTM D2363)

Test concentration and temperature

Instrument and rotor model, speed

Viscosity results (units and average values)

Any deviations from standard operating procedures

Example: HPMC, batch number 240801, 2% solution, 20°C, Brookfield RVT viscometer, rotor #4 @ 20 rpm, average viscosity 85,000 mPa·s.

9. Quality Control

Manufacturers typically specify an acceptable viscosity range (e.g., nominal value ±10%). Batches exceeding this range must be retested and the cause analyzed.

10. Safety and Handling

Wearing a dust mask is recommended when handling powder.

Clean the rotor and container promptly after testing.

HPMC solutions are biodegradable; discharge should comply with local environmental regulations.

Conclusion

Viscosity testing is an important quality control method for evaluating HPMC performance. The Brookfield rotation method is the preferred method due to its reliability and ease of operation. Strict adherence to sample preparation, temperature control, and standardized procedures is crucial for obtaining accurate and reproducible results.