Low temperature viscosity measurements of lubricants with rotational viscometers, and why you need them

Lubricants play a pivotal role in minimizing friction and wear between interacting components, thereby facilitating the efficient operation and extension of the lifespan of machinery. However, their performance can be adversely affected by low temperatures. The low-temperature viscosity of lubricants is particularly critical as it determines the lubricant’s ability to flow and provide sufficient protection in environments with temperatures as low as -40 °C. Under these extreme conditions, it is imperative for lubricants to maintain a sufficiently low viscosity to ensure effective flow characteristics.

Viscosity assessment at the lowest operational temperature is essential to guarantee adequate lubrication of critical components. Moreover, cold start behavior in car motors or generators using oil can be simulated. Standardized quality control tests ensure proper flow and pumpability of the test medium and identify flow problems, which, in the case of lubricants, can be caused by flow-limited and/or air-binding behavior. Flow-limited behavior is associated with the oil’s viscosity and air-binding behavior is associated with gelation. Determining the viscosity and gelation point at low temperatures helps minimize flow problems.
This article will examine two primary standards for measuring low-temperature viscosity of lubricants, specifically ASTM D2983 and ASTM D5133.

ASTM D2983 [1] – distinguishing between procedures

ASTM D2983 utilizes a rotational viscometer to determine the suitability of fluids such as automatic transmission fluids, gear oils, hydraulic fluids, and other lubricants for use at low temperatures, covering a viscosity range of 300 mPa·s to 900,000 mPa·s. The standard describes four different procedures, A, B, C, D, each requiring a different configuration. Procedure D is widely regarded as the automated test method as the heating, cooling and measurement of the sample are performed in one without the need for further operator intervention and with only one instrument configuration. A, B and C require more intervention as the test samples have to be externally pre-heated before they are transferred to the temperature control unit.
The most important part of the test is the cooling of the sample and subsequent measurement. Procedures A, B, and C rely on a mechanical refrigeration cooling technology which requires the use of CFC (Chlorofluorocarbons) or HFC (Hydrofluorocarbons) refrigerants for the compressors, and, in the case of B and C, also the use of liquid cooling baths (usually methanol or ethanol). Procedure D, on the other hand, utilizes the air counter-cooled Peltier technology. The instruments of procedure A, B and C feature multiple positions for test cells and require at least one reference fluid sample to be tested at the beginning of each run (two reference samples with procedure A). Procedure D typically features one position for a test cell without requiring reference oil tests for each test-run. The precision statement for A, B and C is as follows: Repeatability 13.5 % and reproducibility 18.1 %, while for procedure D it is 8.4 % and 9.7 %, respectively.   
 
Figure 1: Anton Paar’s setup (ViscoQC 300 + Peltier Temperature Device PTD 175) for ASTM D2983 procedure D or ASTM D8210

Rotational viscometer

Procedures A, B and C require a rotational viscometer having a torque range between 0.0670 mN·m and 0.0680 mN·m. Procedure D needs a programmable rotational viscometer that has a torque range between 0.0670 mN·m and 0.1800 mN·m.
For viscosity measurements, the viscometer must have at least the following speeds available: 0.6 rpm, 1.5 rpm, 3.0 rpm, 6.0 rpm, 12.0 rpm, 30.0 rpm, 60.0 rpm and 120 rpm (120 rpm is desirable for procedure A to C, and mandatory for procedure D).
Temperature control units
The choice of temperature control unit depends on the test procedure. Samples are cooled as follows:
•    Procedure A: with an air bath to test temperature
•    Procedure B: with a mechanical refrigerated programmable liquid bath
•    Procedure C: with a mechanical refrigerated constant temperature liquid bath by means of a simulated air cell
•    Procedure D: with a thermo-electric temperature-controlled chamber in a range from -45 °C to +90 °C

Measuring system

Viscometer spindle:

Procedures A, B, C, and D require a cylindrical viscometer spindle with the same geometry.
•    An uninsulated steel spindle should be used only for procedure A
•    A composite spindle with lower thermal conductivity must be used for procedure C
•    A spindle with insulation on top (Figure 2) is required for procedure D
 
Figure 2: Insulated steel spindle for ASTM D2983 procedure D

Test tubes:

•    Procedures A and B: standard test tube with approx. 25 mm ID and 115 mm in length, and 30 mL of sample volume
•    Procedure C: SimAir Stator with 15 mm ID, and 15 mL of sample volume
•    Procedure D:  test tube with approx. 25 mm OD and 150 mm in length, and 20 mL of sample volume

General procedure and reported results

After pre-heating (50 °C) and subsequent low-temperature conditioning of the sample, measurements are taken in 3-minute steps at increasing speeds. The viscosity value to be documented (together with the respective percent torque, spindle speed and test temperature) is at the speed with the highest possible torque, which is still below 80% of the viscometers torque range, but more than 20%. Spindle speed data is needed to ensure that different laboratories use the same shear rates.

What is ASTM D8210? And what is the connection to ASTM D2983? [2]
In addition to ASTM D2983, there is another very similar standard: ASTM D8210. ASTM D8210 describes a test method which is equivalent to procedure D from ASTM D2983. In ASTM D8210, this test procedure is called “Option A – Standard Thermal Conditioning”. Procedure D from ASTM D2983 and Option A from ASTM D8210 include the following main steps:
1. Preheating the sample to 50 °C
2. Cooling to room temperature
3. Cooling to test temperature according to Newton’s cooling law
4. Keeping at test temperature for 865 minutes
5. Viscosity measurement at several speeds
The difference between these standards is that ASTM D8210 additionally describes an automated test method with a reduced thermal conditioning phase. This procedure is called “Option B – Abbreviated thermal conditioning”. The holding time at test temperature before the viscosity measurement starts is reduced from 865 min to 265 min. Shortening the thermal conditioning time can result in a lower viscosity value than that measured with the standard method.

ASTM D5133 - Temperature-scanning technique unveils gelation behaviorof lubricants

An oil’s pumpability behavior at low temperature is of particular interest since a catastrophic number of air-binding failures occurred in 1980 due to gelation, with numerous car engines damaged in the winter because of bad engine oil. Consequently, a test method indicating gelation during slow cooling of the oil over a wide low-temperature range was required. Engine oils must not show any gelling within the exposed temperature range if engine failure is to be avoided. In response, the ASTM D5133 test method was developed. Today, the use of highly paraffinic base oils and vegetable oils is increasing rapidly. These oils are prone to gelation and may have a higher low-temperature gelation point, which is why their effects in new engine oil formulations must be studied carefully to avoid flow-limited or air-binding failure. Products sold in countries with cold temperatures, in particular, need to be tested. [3]

Overview of the test method

The sample is preheated to +90 °C for 1.5 h to 2.0 h. This step should remove the ‘memory’ of the oil. An oil’s thermal history can influence its future behavior including gelation properties. The temperature is then reduced to -5 °C and the sample is held at that level for 15 min to 30 min for temperature equilibration. A temperature ramp from -5 °C to -40 °C with a cooling rate of 1 °C/h is initiated. During the temperature ramp, the sample is exposed to a continuous low shear rate of approx. 0.2 s-1 (0.3 rpm). The measurement ends when the temperature reaches -40 °C, or when the viscosity exceeds 40,000 mPa·s, which is considered a critical pumpability viscosity for engines. The test report shows the Gelation Index, which is the maximum value of the incremental ratio over the temperature range scanned when the incremental decrease in temperature is one Kelvin.
The following equation is used:
Equation 1: Formula for determining the Gelation Index

The temperature at which the Gelation Index occurs is called the Gelation Index Temperature (T2). By plotting the Gelation Index values (y-axis) against the temperature (x-axis), the gelation of an oil can be detected with a peak [4], indicating a structural build up and air-binding behavior of the oil at that temperature range.
Figure 3: Gelated vs non-gelated engine oil

Additionally, the Gelation Index temperature and critical pumpability temperature are reported, as well as the temperatures
associated with the following viscosities: 5,000 mPa·s, 10,000 mPa·s, 20,000 mPa·s, 30,000 mPa·s and 40,000 mPa·s.

Requirements for a viscosity measurement according to ASTM D5133
•    Rotational viscometer: For the measurement, a rotational viscometer capable of measuring at least 45,000 mPa·s is required
•    Measuring system: A special cylindrical measuring spindle and test tube must be used. A spindle with a length of 65.5 mm (±0.1 mm) and a diameter of 18.40 mm (±0.02 mm) is required. The critical diameter of the test tube is 22.05 mm (±0.02 mm)
•    Temperature device: A temperature device which can perform a temperature ramp with a cooling rate of 1 °C/h is required. For the temperature ramp, a range from -5 °C to -40 °C is required. For the sample, pre-treatment at a temperature of +90 °C is necessary. The same or a separate temperature device can be used for the sample pre-treatment. Direct control of the temperature device via the viscometer significantly simplifies the operation. The Peltier temperature device is a piece of state-of-the-art technology which has many advantages compared to traditional
liquid temperature devices for this application. Such advantages include:
o    Precise sample temperature control that ensures the highest viscosity accuracy
o    No additional space in the lab needed for a thermostat or oven
o    Minimum maintenance thanks to air cooling
o    No flammable cooling liquids

Figure 4: Air cooled Peltier temperature device with insulation system

What is ASTM D7110? And what is the connection to ASTM D5133?

Standard ASTM D7110 is used for viscosity measurements of used and soot-containing engine oil [5]. The test procedure is very similar to ASTM D5133, but differs regarding the cooling rate for the temperature ramp. According to ASTM D7110, a cooling rate of 3 °C/h instead of 1 °C/h is required. Additionally, the device setup must include a source for dry air or nitrogen. The top of the test tube must be flooded with a low dry-air flow / gas atmosphere of approximately 10 mL/min to 20 mL/min during the measurement. This action prevents condensation and freezing of water on the oil surface.
In contrast to ASTM D5133, the test result of ASTM D7110 must contain the temperature value at 5,000 mPa·s, 15,000 mPa·s, 25,000 mPa·s, 40,000 mPa·s and 60,000 mPa·s.
Note: Other test methods also address the pumpability problem of engine oils at low temperatures: ASTM D3829 and D4684. However, the temperature cooling procedure and shear rate are different, and this can lead to significantly different test results.

References

1. https://www.astm.org/Standards/D2983.htm
2. https://www.astm.org/Standards/D8210.htm
3. Selby, T. and Miller, G., 2008. Thermal History of the Engine Oil and Its Effects on Low-Temperature Pumpability and Gelation Formation. SAE Technical Paper Series.
4. https://www.astm.org/Standards/D5133.htm
5. https://www.astm.org/Standards/D7110.htm