Determination of the Noack Evaporation Loss of Lubricants by TGA - METTLER TOLEDO

Determination of the Noack Evaporation Loss of Lubricants by TGA

Abstract 

For quality and environmental reasons, lubricants for engines and other applications must only exhibit a low evaporation rate. The loss of volatile components from an oil increases its viscosity and leads to increased oil consumption, coking and wear. The Noack method is a widely used standard test method for measuring the evaporation loss from lubricating oils. According to the ILSAC GF-3 and API-SL specifications the evaporation loss must not be greater than 15%.

The ASTM standard test method D6375 for the determination of the evaporation loss of lubricating oils by the Noack method [1] uses thermogravimetric analysis, TGA.

 This method yields the same results as other standard test methods (e.g. ASTM D5800 [2], DIN 51581-1 [3], JPI-5S-41-93 [4]). This article describes how the Noack evaporation loss is determined in comparison to a reference oil sample using TGA.

 

Introduction

The increase in the usable lifetime of lubricants coupled with faster oil circulation rates, longer oil change intervals and lower lubricant consumption means that lubricants are subjected to greater stress. Higher temperatures coupled with smaller oil volumes and higher performance lead to a constant increase in the demands placed on the performance and quality of the lubricants. To ensure that the lubricants are properly used, they must be properly specified and classified.

The specifications describe the physical properties of engine oils such as the viscosity, evaporation loss and shear stability. Performance behavior is also tested in engine tests. This includes wear protection and cleanliness as well as the influence on fuel consumption and the changes in the engine oil during operation due to viscosity changes (thickening). The classification is provided by organizations such as ILSAC, API or SAE (see the table of acronyms).

One of the commonly used specifications is the evaporation loss. The low molecular mass constituents of an engine oil, which consists of fractions of different hydrocarbons with different chain lengths and molecular masses, can evaporate under increased thermal stress. This usually leads to an increase in the viscosity of the lubricant. At the same time, the solubility of the additives in the base oil is affected. 

The evaporation is important for all lubricant groups (e.g. also for synthetic oils) if they are used at higher temperatures. For example with engine oils, evaporation losses can occur through high temperatures at the piston rings and elsewhere. These losses lead to undesirable oil thickening and increased oil consumption. 

 

The Noack Evaporation Loss Test According to ASTM D6375

The Noack test to quantitatively determine the evaporation loss of oils under standard conditions was introduced many years ago. For example, the DIN 51581 [3] test method measures the evaporation loss over a period of one hour at 250 °C under vacuum (2 mbar).

Noack Reference Time and Volatility 

The ASTM D6375 standard thermogravimetric test method was developed [5] to combine the advantages of the gas chromatographic method [6] with the realistic conditions of the traditional Noack test. The method is quicker and more reliable than both and can be performed with less sample material.

According to the ASTM D6375 method, a sample is heated rapidly in a crucible to 249 °C and held isothermally for 30 min at this temperature during which time the TGA curve is recorded. The Noack evaporation loss is the loss in mass up to the Noack reference time. This time is determined beforehand under the same experimental conditions with a Noack reference oil. In this method, it is important that the sample temperature quickly rises to a value between 247 and 249 °C but does not overshoot. To simulate the traditional Noack method, a sample is usually heated to 220 °C at 100 K/min and then to 249 °C at 10 K/min. The sample mass (ms) to be used is determined from the internal diameter of the crucible (d) using the following equation:

Conclusions

Over the past years, the demands placed on lubricants in many application areas have changed significantly. Thermal oxidation stability, low tendency to evaporate and their influence on our natural and working environments have become very important. The innovative development of modern lubricants and their proper application have far-reaching economical consequences. Lubricants (base liquid and additives) that have been optimized for the different tasks, for example with low evaporation losses,

  • save energy
  • reduce service intervals
  • minimize wear
  • increase engine service life
  • increase oil change intervals (lifetime)

and result in considerable economical savings. The determination of the evaporation loss by thermogravimetry is therefore an important step in the qualification of lubricant. The METTLER TOLEDO TGA system with sample robot and automated evaluation provides high sample throughput and rapid pass/fail assessment of the oil in question.

Determination of the Noack Evaporation Loss of Lubricants by TGA | Thermal Analysis Application No. UC 252 | Application published in METTLER TOLEDO Thermal Analysis UserCom 25