What is the principle of a capillary rheometer?

Capillary rheometers offer advantages such as a simple structure, a wide temperature adjustment range, minimal material consumption, short measurement times, and a wide range of shear rates. They have been widely used in polymer processing testing and research. Because capillary rheometers require less sample to test the rheological properties of polymer materials and facilitate formulation design, they offer both time-saving and economical testing and analysis. They have been crucial in plastics processing performance research and formulation design.

Composition and Principle

A capillary rheometer is a device for testing the rheological properties of materials based on plasticization analysis. Its distinguishing feature is its ability to study the rheological behavior of materials under conditions close to those of real processing. A capillary rheometer primarily consists of a drive system, temperature control system, software, and various accessories.

Drive System

The capillary rheometer's drive system primarily provides power and control for each functional unit. Its chamber, a cylindrical plasticizer, is a crucial component of the rheometer. During polymer processing, different dies are typically selected based on the shear strength requirements of the test material. By recording the changes in material temperature, apparent shear stress, and viscosity with shear rate during testing, the material's dispersion properties, flow behavior, and thermal stability can be studied during processing. A rheological curve can also be generated, objectively demonstrating the material's plasticization process.

Temperature Control System

The capillary rheometer's temperature control system primarily uses a sensor to measure the chamber temperature, converting the signal into a signal. This signal is then input into a temperature display and control instrument. After comparison with the set signal, the actuator output variable is calculated, thereby varying the heating chamber's heat output to achieve temperature control. When the chamber temperature exceeds a specified range, the temperature control system automatically adjusts the temperature downward. When the chamber temperature falls below this range, the temperature control system automatically adjusts the temperature upward to ensure proper connection between the system hardware and the sensor.

Software System

The capillary rheometer's software system primarily consists of two modules: automatic calibration and experimental testing. The automatic calibration module utilizes an integrated modular design, enabling automatic sensor calibration and 1:3 model recognition, making it easy to operate and understand. The experimental testing module primarily configures experimental conditions according to a program, providing simultaneous display of measurement parameters, curves, and tables. Furthermore, experimental data can be automatically read by analyzing specific points on the shear rate vs. viscosity curve.

Operating Principle

The instrument's core mechanical structure consists of a heated barrel, a precision piston drive system, and a replaceable capillary die. During testing, a pre-prepared granular or flake sample is placed and compacted in the precisely temperature-controlled heated barrel. A computer-controlled servo drive system applies a constant piston speed (for steady-state testing) or constant pressure, thereby extruding the sample. After the material is heated and melted in the barrel, it is forced by the piston through a precision capillary tube with a specific aspect ratio (L/D).

The core physical phenomena of this process occur within the capillary tube. As the material is squeezed and passes through the narrow capillary tube, it experiences intense shear. Based on the principles of fluid mechanics, a given piston speed generates a corresponding pressure within the system, which is monitored in real time by a high-precision pressure sensor mounted at the barrel end or on the piston. Simultaneously, the extrudate velocity is precisely recorded. These two directly measured quantities—pressure (ΔP) and volumetric flow rate (Q)—form the foundation for all subsequent calculations.

Secondly, the conversion of these raw data into rheological parameters relies on a rigorous set of mathematical models for fluid mechanics. Volumetric flow rate (Q) and capillary geometry (radius R, length L) are used to calculate the apparent shear rate (γ̇_app = 4Q/πR³). The pressure drop (ΔP) and capillary dimensions are used to calculate the shear stress at the tube wall (τ_w = ΔPR/2L). By measuring at a range of different shear rates, the flow curve for the material at a specific temperature—the relationship between shear stress and shear rate—is obtained.

However, due to the inlet effect (convergent flow and elastic deformation of the material upon entering the capillary) and the potential slip of the fluid at the tube wall, the directly calculated result is an "apparent" shear viscosity. To obtain a material's true shear viscosity (η), two key corrections must be performed:

Inlet pressure drop correction (Bagley correction): By using a set of capillaries of varying lengths but identical radius, the pressure-flow rate data is extrapolated to zero capillary length to eliminate the additional pressure loss caused by the inlet effect, thereby obtaining the true shear stress acting on the capillary itself.

Non-Newtonian correction (Rabinowitsch correction): Given that polymer melts are typically non-Newtonian fluids with velocity profiles that differ from those of Newtonian fluids, the apparent shear rate must be corrected to obtain the true shear rate.

After these corrections, the true shear viscosity (η = τ / γ̇) of the material at various shear rates can be accurately calculated, allowing the viscosity-shear rate curve—a key curve characterizing the material's processing properties—to be plotted.

The operating principle of a capillary rheometer is to simulate the extrusion process, accurately measure pressure and flow, and, based on fluid mechanics theory and necessary non-Newtonian corrections, convert these macroscopic mechanical signals into microrheological parameters that reveal the internal flow and deformation patterns of the material.

It can not only be used to study the shear-thinning behavior of materials and determine processing windows, but also to evaluate melt elasticity by analyzing phenomena such as outlet swell, providing an indispensable scientific basis for product formulation development, process optimization, and quality control.

Application Analysis

Working Curve

The operating principle of a capillary rheometer is the plasticization process of a material from granular (or powdered) to melt under the influence of temperature, pressure, and other factors. It measures the dynamic rheological phenomena of the material's transition from a glassy state to a viscous flow state.

During the experiment, the operating parameters of the capillary rheometer are first set. The appropriate amount of material is weighed according to the test conditions. Once the temperature reaches the set conditions, the appropriate die and pressure sensor are installed. After the temperature stabilizes, the material is added and the test begins. This experiment yields a curve showing how the viscosity and apparent shear stress of the material change with shear rate at a specific temperature. Simultaneously, the computer screen displays the dynamic rheological curve of the material's temperature, apparent shear rate, apparent shear stress, and apparent viscosity.

Relationship between Apparent Viscosity and Shear Rate

A capillary rheometer measures viscosity at a specific temperature. If the apparent viscosity remains constant with changes in shear rate, the fluid is considered Newtonian; if the viscosity changes with changes in shear rate, the fluid is a typical non-Newtonian fluid. Generally speaking, under constant temperature and pressure, the viscosity of most melts decreases with increasing shear rate. However, different materials have varying degrees of sensitivity to shear rate (shear stress). At very low and very high shear rates, the apparent viscosity barely changes with shear rate.

Relationship between Apparent Viscosity and Temperature

Apparent viscosity is a function of either shear rate or temperature. Therefore, studying the effect of temperature on viscosity is only meaningful when the shear rate is constant. Generally speaking, increasing temperature inevitably accelerates intermolecular motion, reducing entanglement between molecular chains and increasing the distance between molecules, resulting in a decrease in viscosity. However, if the temperature is too low, the melt viscosity increases, making flow difficult, moldability poor, and elasticity high, which can also affect the stability of the product.

Apparent Viscosity and Viscous Flow Activation Energy

Within the experimental temperature range, the viscous flow activation energy decreases with increasing shear rate. This is because external shear stress disrupts the entanglements between macromolecules, increasing the range of chain segment movement, increasing the distance between molecules, and weakening intermolecular forces. This results in a lower barrier to intrachain rotation and less energy required for molecules to migrate over the forces of surrounding molecules, resulting in a lower viscous flow activation energy.

Combining the effect of temperature on viscosity, it can be found that within the normal processing range of a material, increasing the shear rate has a similar effect on viscosity as increasing the temperature. However, from a process perspective, simply increasing the temperature or shear rate to improve a material's flow properties is inappropriate. Excessively high shear rates not only fail to significantly improve fluid flow but can also cause excessive power loss and equipment wear, as well as problems such as flashing and increased internal stress in the product. Excessively high temperatures, on the other hand, can cause defects such as product deformation, resulting in reduced performance and impaired usability.

2025-10-27 14:07