Biodegradable hydraulic fluids: Rheological behaviour at low temperatures of several oleochemically derived synthetic esters
✍ Scribed by Ridderikhoff, H. ;Oosterman, J.
- Publisher
- John Wiley and Sons
- Year
- 2005
- Tongue
- English
- Weight
- 698 KB
- Volume
- 21
- Category
- Article
- ISSN
- 0265-6582
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✦ Synopsis
One of the most important aspects of a hydraulicfluid used in mobile systems (e.g., excavators, wheel loaders) is the viscometric behaviour of the fluid at low temperatures. It has been found that the pour point of the base fluid does not predict this behaviour satisfactorily, and, as a result, the Swedish standard for these products (SSl5.5434) requires that the kinematic viscosity of a hydraulicfluid remains stable at low temperatures (-20 or -30°C) during a period of 72 h.
However, in rheological studies under dynamic conditions, it has now been found that certain oleochemically derived esters do not show Newtonian flow behaviour at these low temperatures. The authors have found not only a shear-dependent behaviour of these esters but also a time-dependent behaviour, especially afer (high) energy input from an external source to simulate pumping of the fluid.
Given the non-Newtonian behaviour of these esters, the kinematic viscosity test method with an Ubbelohde tube was found to be inappropriate for the determination of the low-temperature viscosity because this method assumes Newtonian behaviour. Dynamic viscosity measurements at higher shear rates (e.g., cold cranking simulator) resulted in much better reproducibility and may offer the best method of assessment.