Principle
A capillary rheometer is an instrument designed based on the principles of fluid mechanics to measure the flow characteristics of materials under controlled temperature and shear conditions. Its core principle is to melt the material under test in a heating chamber and subsequently push the melt through a capillary tube of known geometry at a constant speed or pressure through a piston or plunger. During the flow process, the instrument accurately measured the pressure drop and volumetric flow, and calculated the apparent shear stress and apparent shear viscosity of the material at different shear rates according to the basic rheological relationship such as the Hagen-Poissouf equation. For non-Newtonian fluids such as hot melt adhesives, their viscosity often varies with shear rate, so obtaining a complete shear viscosity profile is critical to understanding their processability (e.g., coating, spraying, extrusion) and application performance.
Test methodology
Determining the shear viscosity curve of hot melt adhesive requires a systematic operation process. First, set the test temperature of the instrument according to the typical application temperature range of the hot melt adhesive, such as 120°C, 140°C, 160°C, etc. Fill the barrel with an appropriate amount of sample that has been preheated to test temperature, compact and remove air bubbles, and let it stand at a constant temperature for a period of time to ensure that the sample is completely melted and the temperature is uniform. Then, select a set of test speeds that cover the actual machining shear rate range (e.g., piston down speed) and test them from low to high. At each set speed, the molten hot melt is pushed through the capillary die (usually the length-to-diameter ratio L/D is 10:1 to 30:1 to reduce the effect of inlet pressure loss), and after the flow is stable, the stable pressure value and the corresponding volumetric flow data are recorded. After the test, the necessary inlet pressure loss correction (e.g., Bagley correction) is performed to obtain the true shear stress and shear rate.
Curve analysis
Once the raw pressure and flow data are obtained, they are calculated using a series of formulas to obtain the viscosity curve. The core calculation formula is as follows:
Apparent shear rate γ̇_a = (4Q) / (πR³)
Apparent shear stress τ_a = (ΔP R) / (2L)
Apparent viscosity η_a = τ_a / γ̇_a
where Q is the volumetric flow, R is the capillary radius, L is the capillary length, and ΔP is the pressure difference between the two ends of the capillary. For non-Newtonian fluids, a non-Newtonian correction of the shear rate (i.e., Rabinovich correction) is required to obtain the true shear rate γ̇. Finally, the flow curve or viscosity curve of the hot melt adhesive at this temperature is drawn with the shear rate (usually taken in logarithmic coordinates) as the abscissa and the true viscosity (usually taken as the logarithmic coordinate) as the ordinate. By analyzing the shape of the curve, it is possible to determine whether the hot melt adhesive exhibits rheological behaviors such as shear thinning (pseudoplastic), Newtonian, or shear thickening, and the viscosity value at a specific shear rate can be directly read.
Influencing factors
The accuracy and repeatability of the measurement results are affected by multiple factors. The primary factor is the precision and uniformity of temperature control, and small temperature fluctuations can significantly affect the viscosity value. Secondly, the geometric size (diameter to diameter ratio) of the capillary die mouth should match the viscosity range of the sample with the expected shear rate, and the length-to-diameter ratio is large enough to obtain more accurate data. The sample preparation process needs to ensure that the charging is consistent, free of bubbles, and the constant temperature time is sufficient. During the test, the possibility of thermal degradation should be paid attention to, and the high temperature residence time should be shortened as much as possible for sensitive materials. In addition, the calibration status of the instrument, especially the accuracy of the pressure and displacement sensors, is the basis for reliable data. When analyzing data, it is important to determine whether inlet correction and non-Newtonian correction are necessary according to standard methods, which is especially important for high-fill or strong non-Newtonian hot melt adhesives.
Application significance
The shear viscosity profile of a hot melt adhesive is a core characterization of its rheological properties and is directly related to the actual application process. For example, in high-speed coating or spraying processes, viscosity at high shear rates determines ease of operation and uniformity of coating; During low-speed filling or bonding, viscosity at low shear rates affects the initial and holding stickiness. By measuring viscosity profiles at different temperatures, processing temperature windows can be optimized, pumping resistance can be predicted, and formulation development can be assisted in formulation development to adjust product flowability. A number of domestic and foreign standards involve the determination of polymer melt rheological properties using capillary rheometers, providing a methodological framework for testing. The tests are to be performed in a professional laboratory environment by trained personnel to ensure data comparability and validity.
Typical example
| Test temperature typical range | 80°C - 180°C |
| Commonly used capillary diameters | 0.5 mm - 2.0 mm |
| Common Aspect Ratio (L/D) | 10:1, 20:1, 30:1 |
| Shear rate test range | 10 s⁻¹ - 10000 s⁻¹ |
| Typical viscosity range of hot melt adhesives (160°C) | 10 Pa·s - 1000 Pa·s |
| Key output parameters | Viscosity curve, non-Newtonian index, activation energy |
Cited Literature
1. ASTM D3835-16, Standard Test Method for Determination of Rheological Properties of Polymer Melts by Capillary Rheometer.
2. ISO 11443:2021, Plastics - Determination of the rheological properties of plastic melts using capillary and slit rheometers.
3. Polymer melt rheology and its applications, Chemical Industry Press.