5.2 Classification, Structure and Properties

Fig. 5-3 illustrates a classification of cooling lubricants based on DIN 51 385. The key differentiator is whether or not the cooling lubricant contains water in its ap­plication state.

5.2.1 Oils

In principle, cooling lubricants are always composed of a suitable base fluid, to which various active substance (additives) may be added according to the respec­tive application and requirements. Base fluids should conform with both the par­ticular conditions of the respective machining process and present-day toxicologi­cal requirements. Currently, mineral oils, hydrocrack oils, polyalphaolefines and ester-based oils are used are base fluids.

Mineral oil raffinates extracted directly from crude oil are still the most domi­nant hydrocarbon base fluids, primarily for economic reasons.

Hydrocrack oils are mineral oil raffinates refined with hydrogen. Compared with mineral oils, they have a smaller aromate content and, as a result of fewer un­saturated molecules, higher oxidation stability. Furthermore, they are character­ised by a higher viscosity index and improved lubricity. Moreover, by means of a more homogeneous distribution of molecule size, they are clearly less prone to vaporisation than mineral oils [REHB00, REHB01].

Polyalphaolefines (PAO) are synthetic hydrocarbons having a definite and con­sistent molecular structure. The consistent composition leads to a significantly smaller inclination towards vaporisation than mineral or hydrocrack oils. The poor solubility of many additives in non-polar PAO can be disadvantageous [N. N.96, REHB00, REHB01].

Ester is formed from alcohols and fatty acids as a product of a dehydration re­action. Natural esters from renewable raw materials usually have the disadvantage of poor resistance to aging. Therefore, chemically modified plant-based esters, which are much more oxidation-stable and hydrolysis-stable, are favoured as cool­ing lubricants. By selecting appropriate fatty acids and alcohols, we can obtain specific properties, such as viscosity and vaporisation. Among the mentionable advantages of ester oils are less vaporisation loss, high flashpoints, a very good purifying effect, good air separation behaviour, little foaming tendency and its po­larity and resultantly good boundary lubricity [FREI97, FREI98, LUTH02, SCHU98].

In contrast to water-based fluids, which have usually an amount of water clearly exceeding 90 %, oil has the advantage of better lubrication. Their kine­matic viscosity is, depending on its specifications, up to nearly 100 times more than that of water at a temperature of 40 °C.

Oils have good corrosion properties and are practically sterile from manufac­ture. Biocides for reducing fungal infestation and rust inhibitors to protect the ma­chine and the workpiece are therefore generally unnecessary [JUST83]. On the other hand, an anti-froth agent must be applied under certain conditions in order to suppress surface foam. A disadvantage of oil is that the cooling effect is relatively poor because of its low specific heat capacity and heat conductivity (table 5-1).

Oils used as cooling lubricants can lead to considerable oil mist formation, es­pecially at high cutting speeds. This oil mist can contain respirable aerosols [HOER88], which increase workplace stress. Beyond this, oil mist is easily flam­mable, so the machines have to be sufficiently enclosed for reasons of safety. This also includes the installation of an effective exhaust system. Moreover, further safety guards are necessary, for example, in order to prevent an explosive increase in temperature at the machining location in case of a sudden decrease in the amount of cooling lubricant.

On the other hand, non-water-based cooling lubricants have the advantage of longer lifespans. Because of their resistance to bacteriae, a significantly longer service life can be obtained by means of sufficient filtering of sediments than is the case for water-miscible products.