Plastics — Determination of the viscosity of polymers in dilute solution using capillary viscometers — Part 1: General principles
1Key Takeaways
International Standard ISO 1628-1:2024(en) Plastics — Determination of the viscosity of polymers in dilute solution using capillary viscometers — Part 1: General principles The document specifies general conditions for determining the reduced viscosity, intrinsic viscosity and K-value of organic polymers i…
2Expert Interpretation
An in-depth interpretation of the ISO 1628-1:2024 international standard on the technical specifications for measuring the viscosity of dilute polymer solutions by capillary viscometer, covering core contents such as the comparison between the effluent time method and the pressure difference method, instrument selection guide, key points for solution preparation and K value calculation, providing a standardized operating framework for the characterization of polymer materials.
Standard Technology Evolution and Background
Compared to the 2021 version, the main updates of the fifth edition of ISO 1628-1 (2024) include: adding the pressure difference method as an automated alternative, making the calculation of the K value an independent chapter (9.2), and adding an introduction to the principle of the method. The standard continues the technical cooperation with CEN and achieves the coordination of European and international standards through the Vienna Agreement.
Definition of Core Concepts
| Term | Definition | Unit |
|---|---|---|
| Relative Viscosity (ηr) | The ratio of the viscosity of a solution to the viscosity of the solvent | Dimensionless |
| Intrinsic Viscosity [η] | The limiting value of the relative viscosity at infinite dilution | m3/kg |
| K value | The empirical parameter relating viscosity to concentration | Dimensionless |
Method comparison and selection
The effluent time method uses an Ubbelohde viscometer to measure the effluent time ratio of the solution to the solvent by gravity drive, and the kinetic energy correction term is required to be less than 3% of the solvent viscosity. The standard recommends that the inner diameter of the capillary be selected based on the kinematic viscosity of the solvent (Table 1), such as dichloromethane (0.33 mm2/s) corresponding to an inner diameter of 0.36-0.58 mm.
The differential pressure method uses a series of double capillaries to synchronously measure the differential pressure ratio of the solvent to the solution through a pressure sensor, which has a higher degree of automation and reduces solvent consumption by 20-30%. Its instrument constant Kv is calculated by formula (5), and the standard requires that the temperature fluctuation be controlled within ±0.2K.
Key Operation Points
- Solution Preparation: The concentration should make the viscosity ratio 1.2-2.0, and the recommended unit is kg/m3; parameters such as solvent pretreatment, dissolution temperature/time, and stabilizer should be indicated
- Temperature Control: The effluent time method at 25°C requires an accuracy of ±0.05K, and a thermometer with a reading of 0.02°C is recommended
- Viscometer Cleaning: It needs to be treated with chromic acid solution, distilled water, and acetone in sequence, and the drying temperature should be ≤100°C
Data Analysis Method
The intrinsic viscosity can be calculated using the Huggins equation (11):
[η] = I/(1 + k'cI)
Where k' is a constant of 0.2-0.3, which needs to be determined by the two-point method (Formula 12). The K value is converted according to the Fikentscher formula (13) and is related to the viscosity-average molecular weight.
Implementation recommendations
- Samples with high pigment content need to be centrifuged before measurement
- Automated equipment should be regularly calibrated for flow rate stability (error <5%)
- In case of dispute, the Ubbelohde viscometer data shall prevail
- The differential pressure method recommends a flow rate range of 0.5-3.0 cm3/min