Rationale
A rotational viscometer calculates the viscosity of a sample by measuring the torque subjected to it as it rotates in the sample. The test conditions are typically in the low shear rate range, which is suitable for evaluating the flow behavior of resins under static or low shear conditions. The structure of the instrument is relatively simple, easy to operate, and can provide a curve of viscosity with shear rate, the common formula is: η = τ / γ̇, where η represents the viscosity, τ is the shear stress, and γ̇ is the shear rate.
Capillary rheometers force the sample through a capillary tube of known diameter and length to obtain rheological data by measuring pressure drop and flow rate. It is capable of simulating resin processing processes (such as extrusion or injection) at higher shear rates and temperatures, providing flow information closer to actual processing conditions. Its basic formula is based on Hagen-Poissouer law, which can be expressed as: τ_w = (ΔP * R) / (2L), where τ_w is the shear stress of the pipe wall, ΔP is the pressure difference, R is the capillary radius, and L is the length.
Comparison of test conditions and scope of application
Rotational viscometers are typically suitable for low shear rate scenarios (e.g., 0.01 to 100 s⁻¹) and are suitable for testing resin flow during storage, transportation, or preliminary mixing stages. It requires less sample and is fast to test, making it often used for rapid screening in quality control or R&D. However, due to the low shear rate, it may not fully reflect the behavior during high shear machining.
Capillary rheometers cover a wider range of shear rates (up to 10⁶ s⁻¹) and can simulate the flow of resin in high-shear processes (e.g., injection molding, extrusion). It can also be tested at high temperatures, closer to the actual production environment. However, the instrument is relatively complex to operate, and the sample preparation and cleanup take a long time, and a large sample volume is required.
Application Analysis
Rotational viscometers mainly output viscosity-shear rate curves, temperature-dependent data, etc., which help to understand the Newtonian or non-Newtonian properties of resins (such as shear thinning behavior). These data are valuable for adjusting formulations to improve low shear flow.
Capillary rheometers provide a more comprehensive range of rheological characteristics, including shear viscosity, melt flow rate, elastic effects (such as outlet expansion), and flow instability data. This information is crucial for optimizing processing parameters (e.g., temperature, pressure) and predicting the quality of the final product. For example, by analyzing flow curves, the risk of degradation of resins at high shear can be assessed.
Select a suggestion
When choosing an instrument, consider the purpose of the test: if you are concerned about the behavior of the resin under low shear or static conditions, a rotational viscometer is a suitable choice; If you need to simulate a high-shear machining process, you should use a capillary rheometer. Sample characteristics (e.g., viscosity range, thermal stability) and test cost (time, sample volume) are also decision factors.
When using a rotary viscometer, attention should be paid to rotor selection and temperature control to avoid measurement errors. For capillary rheometers, calibrate the capillary size regularly and consider inlet pressure loss corrections to ensure data accuracy. Both instruments are subject to standard operation to ensure comparability and reliability of results.
Quote the instructions
The content of this paper refers to the basic theory of rheology, the processing and testing standards of polymer materials, and the principles and application descriptions of viscometers and rheometers in the instrument technical literature. Specifically, it involves the derivation of the torque-viscosity relationship of the rotary viscometer and the correction method of the Hagen-Poissouer law of the capillary rheometer in non-Newtonian fluids.