Base fluids for lubricants
Different base oils for metalworking!
Non-water-miscible cooling metalworking and industrial lubricants consist mainly of hydrocarbons to which various additives are added to improve the high-pressure, lubrication, wetting and ageing properties.
Mineral oil fractions are traditionally used as hydrocarbons. However, new high-performance metalworking processes, increase demands on resiliency, and oil change intervals in hydraulics and transmissions. Also new products should not burden people’s health and the environment. To meet these requirements, modern hydrocarbon base fluids that are both more efficient and less problematic than mineral oils in respect to the above are being used.
Mineral oil raffinates
The hydrocarbons obtained directly from crude oil currently still make up the majority of the base fluids used, due to the lower price. With the help of various physical (distillation, extraction) and chemical (hydrogenation) processes, mineral oil fractions are obtained from crude oil which consists predominantly of saturated hydrocarbons (paraffin), branched, unbranched (iso- or n-paraffin) or cyclic structure (naphthenic). Due to reasons of oxidation stability, admixtures of unsaturated hydrocarbons (olefins, acetylenes) as well as unsaturated cyclic and polycyclic aromatics are undesirable for toxicological reasons. These substances are largely, but not completely, removed during the refining process. The broad spectrum of structure and size of the hydrocarbon molecules of a mineral oil fraction causes a comparatively high tendency to evaporate. This tendency to evaporation cannot be positively influenced by the addition of additives.
One method to refine petroleum hydrocarbons is the production of hydrocracking oils. For this purpose, crude paraffin or the residue from vacuum distillation is split into shorter molecules in the presence of hydrogen and with the use of catalysts at temperatures of up to 450°C and pressures of up to 300 bars. At the same time, the aromatic compounds contained are split and hydrogenated. Hydrocrack oils therefore have a lower aromatic content than mineral oils. Since this process produces compounds with more favorable properties (paraffin) from compounds with poor viscosity-temperature properties and low lubricity (naphthenic, aromatics), the products produced in such a way have a higher viscosity index and better lubricity than mineral oils. Another advantage of hydrocracking oils is their lower evaporation and fogging rate due to the more homogeneous molecular size distribution, which can reduce hydrocarbon emissions at the workplace.
Gas-to-Liquid (GTL) oils
These base oils are not produced from crude oil, but are synthesized from gas (often methane gas). In the gas-to-liquid process, natural gas is converted into a synthesis gas by adding steam and oxygen. The produced synthesized gas is the base product for the Fischer-Tropsch process, which in turn converts the synthesized gas into liquid hydrocarbons. This is done by a catalytic process in which carbon dioxide, carbon monoxide and hydrogen are converted. The resulting base oils rank qualitatively between hydrocracking oils and polyalphaolefins. These base oils are often referred to as Group III+ oils.
Comparable to premium engine oils, industrial lubricants and coolants with minimal evaporation, a high flash point at low viscosity, a high viscosity index and excellent shear and ageing stability can only be achieved by using synthetic polyalphaolefins (PAO´s). The thermal decomposition of mineral oil in the presence of hydrogen produces ethene, from which a linear olefin, usually 1-decene, is produced with the help of catalysts by the olefin synthesis. This is oligomerized in a further catalyzed reaction step, i.e. two, three or more olefin molecules are bound together. By this procedure nearly pure hydrocarbons of a certain polymerization grade are produced as well as unsaturated compounds, which are responsible for the oxidation or aging of the products. Due to the consistent composition, the evaporation rate of PAO´s is considerably lower compared to comparable mineral or hydrocrack oils, and the tendency to misting is also reduced.
In addition to hydrocarbons, carboxylic acid esters are also used as base liquids. These can be divided into synthetic products and those of natural origin (vegetable oils, animal fats). Their tendency to evaporate is even lower compared to PAO´s. Natural esters are well biodegradable, but their ageing properties are low. The oxidation stability of lubricants based on synthetic esters is considerably better when selected appropriately, since saturated esters with a high degree of purity are available. However, these compounds are often less biodegradable. Also the stability against hydrolysis (in the presence of water the ester splits into alcohol and acid) can be improved by using sterically hindered synthetic esters. A general problem with ester-based lubricants is their limited compatibility with elastomers and paints. In particular, the low-viscosity esters used for low-viscosity metalworking oils or spindle oils behave very aggressively towards many sealing and insulating materials.