15 Analytical Instrumentation 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 behavior
of 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 paraffi nic 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 fl ow-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 infl uence 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
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.
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:
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 fl ammable cooling liquids
Figure 4: Air cooled Peltier temperature device with insulation system What is ASTM D7110? And what 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 signifi cantly simplifi es the operation. The Peltier temperature device is a piece of state-of-the-art technology
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 fl ooded with a low dry-air fl ow / 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 signifi cantly 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
Author Contact Details Melina Much, Product Competence Viscometry • Anton Paar GmbH • Graz, Austria • Tel: +43 316 257-0 • Email:
info@anton-paar.com • Web:
www.anton-paar.com
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