Aqueous solutions used for grinding purposes consist, like oil-in-water emulsions, of over 90 % water. The concentrate is devoid of mineral oil, consisting, for example, of polymers or salts [MANG76]. No emulsifiers are required to obtain a fine and even distribution of the concentrate, as the latter is dissolved molecularly, as is the case in solutions of water and common salt. Because no emulsifiers are needed, no foam problems arise, and the resistance to microorganisms is better than that of emulsions. Solutions have, however, less favourable lubrication properties than emulsions. Moreover, they tend to rinse off cutting oils from the guide bars and to form sticky residues [ZWIN79].
Since pure base fluids are usually not capable of meeting the requirements made on cooling lubricants for grinding applications, additives are used. These influence various cooling lubricant properties and should be activated by the high temperatures and pressures associated with grinding processes [BRIN00, IRRE96, SCHU02]. Additives can be subdivided into the following groups [HIPL00]:
• additives that change the physical properties of the cooling lubricant (e. g. viscosity index improvers)
• additives that change the chemical properties of the cooling lubricant (oxidation inhibitors)
• tribologically active additives, i. e. additives that change the friction conditions (polar additives, friction modifiers (FM), anti-wear (AW) and extreme pressure (EP) additives.
In the group of polar additives are included above all fatty substances derived from plants and animals as well as synthetic esters [N. N.92]. We speak also refer to greased oils in this context. Polar fat molecules deposit comparatively firmly on the metal surface due to electrostatic adsorption. Due to the dipolar character of the molecules, several shear sensitive layers with good pressure resistance are formed.
With metals that act as catalysts, there is an additional chemical reaction with the metal surface under the influence of the heat and pressure increased caused by the process, especially in the case of fatty acids. The resulting metallic soaps act as highly viscous, semisolid lubrication films. The temperature range in which the polar additives are active already ends at about 130 °C.
At higher loads, for instance when working with materials that are hard to machine, EP additives in the form of phosphorus (up to ca. 700 °C) or sulphur compounds (until above 1000 °C) are used. Chorine, previously often used (up to ca. 500 °C), is no longer used for environmental reasons. In its place, polymer additives and, to some extent, esters can be used [LING88].