Standard method
The melt viscosity of hot melt adhesives is a key parameter in their processing and application, directly affecting the coating, wetting and bonding properties. Rotational viscometers are commonly used instruments to measure this parameter, and the principle is to determine viscosity by the resistance subjected to rotation of the rotor in the molten sample. This method is designed to provide a standardized set of operating procedures to ensure repeatability and comparability of measurement results.
Instruments
Measurements are made using a rotational viscometer suitable for high-temperature measurements, equipped with a temperature control unit (e.g. thermostatic bath or electric heating jacket) and a suitable rotor (usually cylindrical or tapered). The sample is a hot melt adhesive to be tested, and its melting temperature range needs to be understood in advance. Auxiliary tools include sample containers, thermometers, cleaning utensils, etc. All instruments should be calibrated before use.
Sample preparation
Sufficient hot melt samples are placed in a clean and dry container to fully melt and remove air bubbles at the specified melting temperature according to the material properties. Fill the molten sample evenly into the measuring cup of the viscometer to ensure that the liquid level covers the rotor marking line and does not introduce air bubbles. After loading, let it sit for a while to equilibrate the sample temperature with the measurement system.
Measurement conditions
The measurement temperature should be set according to the actual application or standard requirements of the hot melt adhesive, with a common range between 120°C and 200°C. The temperature stability should be controlled within ±0.5°C. The selection of rotor model and speed takes into account the expected viscosity range, usually pre-tested at lower speeds, and then selected at a speed that keeps the torque within the recommended range for formal measurement. Each condition should be stable for at least 1 min after readings.
Measurement steps
The measurement system with the sample is heated up to the target temperature and remains stable. Lower the rotor to the specified depth. Start the rotation and record the viscosity reading once the torque value has stabilized. Variable rotational speed measurements are used to investigate the rheological properties of the sample (e.g., shear thinning behavior). After the measurement is completed, the rotor is raised and the sample is cleaned up in time.
Data processing
The torque value directly output by the viscometer needs to be converted into a viscosity value according to the rotor parameters and speed. For Newtonian fluids, the viscosity η calculation formula is:
η = K × (T / ω)
where K is the instrument constant, T is the torque, and ω is the angular velocity. Non-Newtonian fluids need to indicate the shear rate. The report should contain the following information: sample identification, measurement temperature, rotor model, rotational speed, shear rate (if applicable), viscosity value and units, measurement date and environmental conditions.
Notes:
Before measuring, confirm that the temperature sensor is in the correct position. The sample should be completely melted and homogeneous, avoiding packing or degradation. The cleanliness of the rotor and measuring cup can affect the results and should be thoroughly cleaned after each measurement. For samples from different batches or formulations, it is recommended to repeat the measurement three times to average the value. Instruments should be calibrated regularly against reference materials.
FAQs
| Temperature fluctuations | Causes viscosity readings to drift |
| Bubbles or impurities | Causes fluctuations in measured values |
| Improper choice of rotor | Torque is out of range or sensitivity is insufficient |
| Sample thermal history differences | Affects the melt structure and deviates from the results |
| Incomplete cleaning | Residue interferes with subsequent measurements |
Summary
When measuring melt viscosity with a rotary viscometer, rigorous sample preparation, precise temperature control, appropriate instrument parameters, and standardized operating procedures are the basis for reliable data. This method provides a common framework that can be adapted according to relevant product specifications or industry standards for specific applications.
References
ASTM D3236-15， Standard Test Method for Apparent Viscosity of Hot Melt Adhesives and Coating Materials.
GB/T 2794-2013, Determination of viscosity of adhesives.
ISO 2555， Plastics — Resins in the liquid state or as emulsions or dispersions — Determination of apparent viscosity by the Brookfield Test method.