Flow behavior of lubricating oil and grease at low temperatures
Lubricating greases are generally highly structured suspensions, consisting of a thickener (usually a metal soap) dispersed in mineral or synthetic oil. The thickener forms a percolating network, which traps the oil and confers the appropriate rheological behavior to the grease. This structure can be compared to that of a sponge soaked with lubricating oil.
The most prominent features of lubricating greases include their ability to flow under external forces, their mechanical stability during flow as well as their thermal susceptibility. Lubricating greases are supposed to work well in any climatic condition, from extreme cold to extreme hot.
For moderate and high temperatures (T ≥ 25 °C) test methods have been developed enabling to predict application properties of lubricants from laboratory tests (DIN 51810-1 and DIN 51810-2). However, these tests are not fully transferable to low temperatures, because below the pourpoint (DIN ISO 3016) the structure and thus also the application properties of lubricating oils and greases change drastically.