Method for measuring ink viscosity with a rotational viscometer

Principle

Rotational viscometer is a viscosity measurement instrument based on the principle of rotational shear, which is widely used in rheological characteristic analysis in ink, coating, food and other industries. Its core working principle is that a rotor rotates through the ink sample to be tested, measuring the torque required to maintain a specific rotational speed or overcome fluid resistance, thereby calculating the viscosity value of the ink. According to international standards such as ASTM D2196 and ISO 2555, the calculation of viscosity (η) is typically based on the following formula: η = K × (T/ω), where K is the instrument constant, T is the measured torque, and ω is the rotational angular velocity. This method can effectively reflect the flow behavior of inks at different shear rates, providing key data for production process control.

Preparation

Adequate preparation is required to ensure the accuracy and repeatability of measurement results. First, select a rotor and speed combination that matches the expected viscosity range of the ink, please refer to the recommended table in the instrument manual. Secondly, the ink sample should be left in a constant temperature environment, usually controlled at 23±0.5°C to eliminate the influence of temperature fluctuations on viscosity. The sample container should be clean and dry to avoid contamination. Finally, calibrate the rotary viscometer and verify it with a standard viscosity solution to ensure that the instrument status meets the measurement requirements.

Measurement steps

The measurement process is subject to a standardized process. Pour an appropriate amount of ink sample into the container, and the liquid level should cover the rotor marking line. Install the rotor gently to avoid introducing air bubbles. Start the instrument so that the rotor rotates at a predetermined speed and record the data after the torque reading stabilizes. Multi-point measurements at different shear rates are recommended to plot the flow curve of the ink. During operation, it should be noted that the rotor immersion depth should be consistent; The measurement time should not be too long to prevent the volatilization of ink solvents; Clean the rotor thoroughly after each measurement to avoid residue affecting subsequent results.

Analysis of results

Once the raw data is obtained, the viscosity value is calculated based on the instrument constants and measurement conditions. For inks with non-Newtonian fluid properties, their shear thinning or thickening behavior can be analyzed. Typical data processing includes calculating average viscosity, standard deviation to assess repeatability, and evaluating ink suitability in combination with flow curves. For example, offset printing inks typically require low viscosity at high shear and maintain high viscosity at low shear to ensure print quality. The following are the viscosity reference ranges for common ink types:

Note: The above ranges are based on ambient temperature measurements and actual values may vary depending on the formulation and shear rate.

Influencing factors

Multiple factors can interfere with measurements. Temperature fluctuations are the main source of error and need to be controlled using thermostats. Improper rotor selection can result in data being out of instrument range or insufficient resolution. Bubbles or particulate matter in the ink can cause fluctuations in readings, and degassing or filtration is recommended before measurement. Additionally, the instrument's maintenance status, such as worn bearings or shaft calibration deviations, can affect long-term stability. Regular calibration and maintenance in accordance with standard procedures is key to ensuring data reliability.

Application:

Rotational viscometer measurements provide a quantitative basis for ink development and production. During the R&D phase, viscosity analysis optimizes the resin and solvent ratio to improve leveling and transferability. In production, in-line or offline viscosity monitoring helps control batch consistency and reduce printing failures such as flying ink or dot enlargement. Combined with environmental standards such as GB/T 13217.4, this method supports the ink industry to achieve standardized quality control, improving product applicability and production efficiency.

Cited Literature

ASTM D2196-20, Standard Test Methods for Rheological Properties of Non-Newtonian Materials by Rotational Viscometer.

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.

GB/T 13217.4-2020, Ink viscosity test method.