Principle
The basic principle of determining the viscosity of a liquid with a rotary viscometer is based on Stokes' law and the principle of internal friction. The instrument rotates at a uniform speed in the liquid to be measured through a rotor, and the viscous resistance of the liquid to the rotor surface creates a corresponding torque. This torque value is directly proportional to the viscosity of the liquid. The absolute viscosity of the liquid can be calculated by measuring the torque required to maintain a constant rotation speed of the rotor and based on the instrument constants and rotor geometric parameters. For non-Newtonian fluids such as resin fluids, the viscosity value is usually related to the shear rate, so the shear conditions at the time of measurement need to be specified in the report.
Instrument preparation
Before starting the measurement, make sure the rotational viscometer is in good working order. First, check that the instrument is placed horizontally to avoid gravity interfering with the measurement. Secondly, according to the estimated viscosity range of the resin solution, select the appropriate rotor model and speed combination. In general, samples with higher viscosity should be selected with a small rotor and lower speed, and vice versa. Before formal testing, the instrument must be calibrated using a standard viscosity solution, which should be consistent with the temperature of subsequent sample testing. Record the calibration factor and correct it when setting or subsequent calculations in the instrument parameters.
Sample preparation
The representativeness and processing of resin sample directly affect the accuracy of the measurement results. The sample should be thoroughly stirred to eliminate concentration gradients due to sedimentation or delamination. The diameter of the container containing the sample needs to be large enough to ensure that the boundary effect of the vessel wall on the flow is negligible as the rotor rotates (the inner diameter of the vessel is usually required to be no less than 2 times the diameter of the rotor). When loading, the level of the liquid level should be completely submerged in the marking line of the rotor and avoid the introduction of air bubbles. If the sample contains visible bubbles, it should be left to defoam or removed by gentle vacuuming.
Process control
The container with the sample is placed in a constant temperature bath or temperature-controlled platform of the viscometer so that the sample temperature reaches and stabilizes at the target temperature (e.g., 25°C±0.1°C). Temperature control is key to viscosity determination, as the viscosity of resin fluids is often sensitive to temperature changes. Start the instrument so that the rotor rotates at a set speed. Once the torque reading has stabilized (usually 30 seconds to several minutes, depending on the rheological properties of the sample), the stable viscosity value is recorded. For non-Newtonian resins, it is recommended to measure the viscosity values at multiple shear rates at the same temperature by changing the rotor or changing the rotational speed to characterize its rheological behavior.
Data logging
The following information should be recorded for each measurement: sample identification, rotor model, rotational speed (rpm), shear rate (if the instrument can display or calculate), measurement temperature, stabilized viscosity reading (mPa·s or Pa·s), and measurement duration. If the instrument displays the viscosity value directly, check whether the calibration factor has been applied. For cases where manual calculations are required, the viscosity η can be calculated by the following formula: η = K × τ/ω, where K is the instrument constant, τ is the torque, and ω is the angular velocity. The same condition recommends repeated measurements at least three times, taking the arithmetic mean as the reported outcome, and calculating the standard deviation to assess repeatability.
FAQs
Problems such as drift readings, abnormal values, or poor repeatability may be encountered during the measurement. Reading drift can result from temperature incomplete stabilization, chemical reactions in the sample, or structural recovery; Abnormal values may be due to improper rotor selection, insufficient sample volume, or the presence of bubbles; Poor repeatability may be related to sample uniformity, loading techniques, or instrument status. Routine maintenance requires keeping the rotor clean, thoroughly washed and dried with a suitable solvent to prevent residue from affecting the next measurement. Regularly inspect the rotor and connecting shaft for deformation or wear. Instruments should be stored in a dry, clean environment and subjected to periodic professional testing as recommended by the manufacturer.
Notes:
When operating the rotational viscometer, pay attention to the general electrical safety of the laboratory. When using flammable resin liquids, keep them away from fire sources and operate in a well-ventilated area. Some resin monomers or solvents can be irritating, and appropriate PPE is recommended. The rotor has kinetic energy when rotating at high speed, so it should be ensured that it is securely installed and that loading and unloading operations should be carried out after the rotor is completely stopped to prevent mechanical injury. Discarded resin samples should be disposed of in accordance with laboratory chemical waste management regulations.
References
ASTM D2196-20, Standard Test Methods for Rheological Properties of Non-Newtonian Materials by Rotational Viscometer.
GB/T 2794-2013, Determination of viscosity of adhesives.
ISO 2555:2018, Plastics — Resins in the liquid state or as emulsions or dispersions — Determination of apparent viscosity by a single cylinder type rotational viscometer method.
Instrument operation manual and books related to the basics of rheological testing technology.