(Brookfield Method)
Fatty Acids and Raw Materials
Determination of Physical Properties
1.TITLE
Determination of dynamic viscosity (Brookfield Method)
2. SCOPE
This APAG standard describes an application for the determination of the dynamic viscosity (or apparent viscosity). It is applicable to all fatty acids, fats and oils.
3. DEFINITION
3.1 DYNAMIC VISCOSITY:
The dynamic viscosity is the ratio between the applied shear stress and the rate of shear. It is a measure of the resistance to flow of the liquid (Notes 1 & 2).
3.2 NEWTONIAN LIQUID: A liquid is “Newtonian” if its viscosity is independent of the shear stress or shear rate.
Note 1:
The SI units for the dynamic viscosity are pascal.second or millipascal.second, but in practice poise or centipoise (1 cP = 10-3 Pa.s = mPa.s) are used.
Note 2:
The viscosity of a given product highly depends on the temperature. Thus, it should always be reported with the measurement temperature.
4. PRINCIPLE
In rotational viscometers, a sensing element -named spindle- is immersed in the fluid being studied and driven at a constant rotational frequency. The viscosity is related to the torque generated by the fluid resistance to the induced movement and is hence determined by measuring the tightening of the spiral spring attached to the spindle (Note 3).
Unless specified, the sample temperature is selected based on practical criteria:
Table 1: Selection of temperature according to the sample type
|
|
Type of samples
|
|
|
|
Fatty acids with a cloud point < 20 °C
|
20
|
|
|
Oils with a melting point < 20°C
|
20
|
|
Fatty acids with a cloud point > 20 °C
|
70
|
|
Oils with a melting point > 20 °C
|
70
|
Note 3:
Since the products under study are generally “non-Newtonian”, the measured viscosity depends on the velocity gradient applied during the measurement.
5. REAGENT
None
6. APPARATUS
Usual laboratory equipment, and in particular:
6.1 BROOKFIED VISCOMETER, (e.g. LVT-DV, DV-II+ or equivalent), covering a minimal viscosity range of 50 to 100.000 cP.
6.2 A SET OF SPINDLE AND SAMPLE CUP, e.g. respectively SC4 - 31 and SC4 - 13R.
6.3 WATERBATH (e.g. Colora W3 thermostat bath), to maintain the sample being tested at the required test temperature with an accuracy of ± 0.2 °C (as specified in Table 1).
6.4 THERMOMETER, graduated in 0.1 °C, suitable for measuring the temperature of the sample being tested.
7. SAFETY
The use of this method may involve hazardous chemicals and/or equipment, although the safety aspects have been omitted from this procedure.
Please study and be aware of the Material Safety Data Sheet and correct laboratory performance for the appropriate health and safety precautions that may apply to any of the chemicals and equipment prior to use.
For chemicals the CAS numbers have been included in reagent paragraph.
8. PROCEDURE
8.1 SAMPLING
8.1.1 Take the test sample in accordance with ISO 5555 – 1991 [14.3].
8.1.2 Prepare the sample in accordance with ISO 661 – 1989 [14.4].
8.2 DETERMINATION:
8.2.1 Adjust the waterbath to the required temperature (Table 1) and wait until the temperature has stabilised.
8.2.2 Bring the sample to approximately the required temperature. Then, fill with the product the sample cup of the viscometer to the prescribed height, taking care not to introduce any air bubbles (Notes 4 & 5).
8.3 Put the sample cup in the thermostated cup holder. Introduce slowly the spindle in the product and attach the spindle to the drive. Wait until the sample has reached the required temperature (± 0.2 °C).
8.4 Start the motor, select the rotational frequency recommended for the product being studied and wait until a constant reading is displayed. If the deflection is outside the range 20 – 95 full scale, select a lower / higher speed depending on whether the reading is on the higher / lower side speed of the range respectively (Note 6).
8.5 Repeat the measurement (8.4) and make a second reading.
8.6 After each determination, detach the spindle from the apparatus and wash it thoroughly with the appropriate solvent.
Note 4:
When the density of the product being studied is known at the test temperature, the amount of sample to be used for the test can be measured by using the mass instead of the volume.
Note 5:
The viscosity measurement is only valid when the product being studied is completely free of bubbles.
Note 6:
The best accuracy is in the range between 45 and 95 full scale.
9. CALCULATIONS
9.1 VISCOSITY:
The viscosity can be calculated using the following formula (Note 7):
ç = F x R
Where:
ç is the viscosity in mPa.s or Cp at the selected temperature (Table1),
F is the factor depending on the rotational frequency / spindle combination (Table 2),
R is the mean value of the two readings displayed by the viscometer.
Table 2:
Values of the factor F according to the rotational frequency for the SC4 – 31 spindle.
|
|
Rotational frequency (rpm)
|
F
|
|
|
60
|
5
|
|
|
30
|
10
|
|
12
|
25
|
|
6
|
50
|
|
3
|
100
|
|
1.5
|
200
|
|
0.6
|
500
|
|
0.3
|
1000
|
Note 7:
When using the Brookfield viscometer type DV-II+, the viscosity is directly displayed and the correction of the reading by the factor F should not be performed.
10. EXPRESSION OF THE RESULTS
Report the result in mPa.s or cP. Make reference to APAG-FA-004, quoting the temperature.
11. ACCURACY & PRECISION
The accuracy and precision are not defined in the ISO 2555:1989(E).
12. METHOD VALIDATION
There is no data available about both accuracy and precision (Note 8).
Note 8:
The precision, as measured by both repeatability and reproducibility, is generally evaluated by running interlaboratory testing.
13. HISTORIC
This first electronic version of APAG-FA-004 is equivalent to the 1987 version; only typographic changes have been made.
14. REFERENCE
14.1 Plastics
- Resins in the liquid state or as emulsions or dispersions
- Determination of apparent viscosity by the Brookfield Test Method. ISO 2555:1989(E).
14.2 ISO 5555 – 1991 – Animal and vegetable fats and oils – Sampling.
14.3 ISO 661 – 1989 - Animal and vegetable fats and oils – Preparation of the test sample.
15. ANNEXE
None
