Points of interest for the application of biolubricants
Switching to biolubricants is not always a matter of simply replacing lubricants. What’s more, biolubricants do not all have the same technical and ecological performance. In hydraulic applications, the main testing criteria to determine whether biolubricants are suitable include:
- Lubricating properties, viscosity and viscosity temperature gradient;
- Pressure resistance (Vickers-Eaton/FZG – Forschungsstelle für Zahnräder und Getriebebau);
- Oxidating stability (aging resistance);
- Air separation abilities, foam behaviour and hydrolytic properties;
- Corrosion protection Fe/Cu;
- Paint damage;
- Behaviour of sealing materials at normal temperatures and temperatures exceeding 80°C
- Acryl Nitrile Butadiene Rubber, NBR;
- Polyurethane, AU/EU;
- Hydrogenated Nitrile Butadiene Rubber, HNBR;
- Ethylene Propylene Diene Monomer, EPDM (variety of synthetic rubber or elastomers);
- Fluorocarbon Rubber, FPM (e.g. Viton®, Fluorel®).
- It is recommended to test the tolerance of elastomers when fluids with an ISO VG 15 and 22 (ISO viscosity grades for industrial lubricants) up to a maximum temperature of 60°C are used.
Several of these testing criteria are explained further in the considerations listed below. References are also made to a number of suppliers and professional literature.
In order to prevent any problems and to successfully introduce biolubricants in both existing and new installations, vehicles or equipment, it is important to pay attention to:
1. Oxidation and thermal stability
Types of oil with a low oxidation stability are likely to oxidise faster when exposed to higher temperatures. When oil oxidises, acid and sludge are produced. The sludge may accumulate on critical parts of the equipment and disrupt the lubricating and cooling functions of the fluid. Pure plant oils are generally not very resistant to oxidation, even though vegetable oils with high oleic acid levels possess an acceptable oxidation stability for a large variety of applications. Synthetic esters are more stable than most vegetable oils. A small number of esters even have a higher oxidation stability than mineral-based lubricants.
2. Behaviour at low temperatures
At lower temperatures, the liquidity of (pure) vegetable oils does not compare well to that of other, synthetic biolubricants. When oil solidifies, its lubricating properties are severely affected.
3. Compatibility with system components
Biolubricants are not always compatible with certain types of paint, varnish, gland packings and metal. A chemical reaction may occur, causing the lubricant to swell. This particularly applies to polyglycols, as well as synthetic esters to a lesser degree. Extensive hands-on experience with vegetable oil-based lubricants revealed comparatively few problems with gland packings and types of paint. Vegetable oil-based lubricants tend to be compatible with steel and copper alloys. Additionally, they offer excellent protection against oxidation.
4. Compatibility with filters
Vegetable oils and synthetic esters do not require special filter elements. When switching to biolubricants, it is recommended to examine the filters after a few hours of use, since biolubricants tend to remove remnants of mineral oil from the system and transport them to the filters.
5. Hydrolytic behaviour
The hydrolytic behaviour – how well the substance is separated into its components in the presence of water – requires close attention, especially when considering the application of synthetic esters.
When using vegetable oils this is less of a concern. The hydrolytic stability of biolubricants available on the market is enhanced by additives.
Foamy oil can be a serious problem in lubricating and hydraulic systems. The lubricating and hydraulic properties of oils are severely affected by excessive foaming. However, laboratory experiments show that most biolubricants do not cause foaming problems.
7. Mineral oil residues
When switching to biolubricants, it is recommended to drain all mineral oil from the system and, if possible, to clean the entire system. Vegetable oils allow for a maximum residue of 2%.