ASTM D5018 Standard Test Method for Shear Viscosity of Coal-Tar and Petroleum Pitches
12. Procedure for Viscosity Determination
12.1 The following general principles apply to all viscosity measurements determined by rotational methods:
12.1.1 Maximize rotor diameter.
12.1.2 Minimize gap width between rotor and cup.
12.1.3 Minimize end effects (use longest available rotor).
12.1.4 Prevent viscous heating (due to prolonged rotation of rotor at high revolutions per minute).
12.2 Select the proper rotor/cup combination that covers the anticipated viscosity range. A typical viscosity-temperature curve for various coal-tar pitches is given in Fig. 2.
12.3 Determine viscosity from the lowest to the highest test temperatures.
12.4 Turn rotor at low rotational speed during heat-up (after sample is melted) and when changing test temperatures.
12.5 Stabilize temperature for a minimum of 5 min but not more than 15 min before measuring viscosity.
12.6 Determine viscosity following manufacturer's procedure. Ensure that the scale reading has stabilized before recording the value; only record values that are between 10 % and 90 % of the maximum torque scale reading.
12.7 Increase temperature controller to next higher test temperature and repeat 12.4 through 12.7.
12.8 Record test temperature and corresponding scale readings and convert scale readings to viscosity using appropriate factors supplied by manufacturer or determined by calibration.
12.9 Pitch is generally regarded as a Newtonian liquid (the viscosity is independent of shear rate (rotational speed). To ensure that the pitch sample is in fact Newtonian, determine the viscosity at different rotational speeds at a given temperature. If viscosities at different rotational speeds are different (at same temperature), but are within 10 % of the average of the readings, report the average value. If the viscosities, at different rotational speeds but the same temperature, differ by more than 10 % from the average, the material is non-Newtonian and the viscosity for each shear rate (rotational speed) should be reported.
12.10 Occasionally, the pitch volume in the cup can decrease when heated to higher temperatures due to loss of moisture or entrapped air. Consequently, the rotor will not be completely immersed in pitch and the viscosity at higher temperatures will be lower than the true value. Also, the viscosity at lower temperatures will be inaccurate due to foaming. To ensure that this did not occur, check the level in the cup after the test is completed and the temperature is about 150 °C. If the rotor is not completely immersed, repeat the test taking care to expel any moisture or entrapped air prior to charging the cup. Moisture can be eliminated by drying at 50 °C, see 9.1. Entrapped air can be removed by carefully melting the pitch and stirring gently until no foaming is observed; avoid overheating the pitch, see 10.1. Also, entrapped air can be minimized by avoiding the use of fine pitch particles; use 6 mm x 12 mm sized particles, see 9.2.
13. Cleaning
13.1 After completion of test, cool to about 40 °C above SP, remove rotor and place rotor into a container of quinoline or other high-boiling aromatic solvent (in hood), or wipe rotor using toluene or quinoline.
13.2 Remove cup and pour out pitch; clean with quinoline, or other appropriate solvent, and toluene.
13.3 After the cup and rotor are clean and cool, rinse both with acetone to remove any oil residue.
