Lube-Tech
3.1 Chemistries of hydraulic fluids Base oil composition exhibits some of the greatest contributions to finished fluid properties such as viscosity – temperature behaviour, pour point, thermal resistance and oxidative stability, compatibility with seals. Nevertheless, some of these properties can be enhanced with the use of additives in combination with base oils; the formulators search for the best synergic effects between additives themselves and base oils.
Hydraulic fluids have many chemical functions, summarised in the table below:
PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE
No.137 page 3
- Additives can clash, forming insoluble reaction products
- Additives can neutralise, dominate or alter the performance of other additives.
Without going too much into details, we can already list some examples of incompatibilities based on base oils and additive chemistry. - Water-based hydraulic fluids with non-water based fluids
- Mineral oil based fluids with environmentally- acceptable lubricants will impact the environmental properties (biodegradability and eco-toxicity) as well as viscosity – temperature behaviour
- HM fluids may not be compatible with each other if the type of antiwear differs
- HFDR with mineral or ester-based hydraulic fluids - HEPG fluids contain polyglycols which may be incompatible with mineral-based hydraulic fluids.
- Hydraulic fluid heavy metal-free and hydraulic fluid heavy metal-containing (generally zinc). Both additive systems are most often incompatible with each other.
Table 1.
3.2 Main incompatibilities depending on base oils and additives chemistries The maintenance of the fluid composition is critically important if optimal performance has to be guaranteed.
The general classification of lubricant incompatibility are: - Mixtures of some base oils can reject additive solubility
- Base oils can chemically react or reject other base oils
- Base oils can react with foreign additives, forming transformation products
The chemical functions change with tribo-chemical reactions due to heat, friction, wear, oxidation, pressure and mechanical stress so it becomes even much more complex with lubricants in use.
4. Evaluation of HF compatibility step-by-step There are many risk factors related to lubricant cross-contamination and this occurs for a variety of reasons such as: - The reduction of the number of lubricants in use - Change in lubricant supplier (agreement not renewed)
- New machine application or reliability requirements triggers the need to switch to a lubricant with enhanced performance or modified properties
- Accidental mixing of lubricants caused by untrained personnel or packaging / labelling error
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