In addition to the concentrates, water composite cooling lubricants contain a high percentage of water. The properties of this cooling lubricant group are, therefore, crucially influenced by the quality of the mixing water used, upon which special requirements must be placed concerning the nitrate, chloride, sulphate, and phosphate content, total hardness, pH-value, and the microbial resilience (Figure 10.1) [Moller and Boor 1986, Leiseder 1991, Konig et al. 1993, Pfeiffer 1993].
The most important base materials of water-immiscible and water composite cooling lubricants (water and mineral oil) have fundamentally differing thermal physical properties (Table 10.2). Hence, the capacity of a cooling lubricant to carry away thermal energy from the grinding process through heat absorption strongly depends on its water or mineral oil content. The cooling effect of cooling lubricants is, first of all, defined by their heat conductivity, evaporation heat, specific heat, and wetting capacity. Due to their high water fraction, cooling lubricant solutions are characterized by an efficient cooling effect. Compared to water-immiscible cooling lubricants, oil-in-water emulsions have a good cooling effect, too, which decreases in favor of a higher lubricating effect if the oil fraction is increased [Zwingmann 1979, Konig 1980, Eckhardt 1983, VDI-Richtlinie 3396 1983]. The cooling effect of water-immiscible cooling lubricants is also strongly influenced by viscosity. Low-viscosity cooling lubricants penetrate tight gaps much faster and are, therefore, better at dissipating heat.
10.4.2 Rinsing Capacity
The rinsing or washing capability of cooling lubricants depends on viscosity and wetting capacity. The surface tension against air is a measure of the wetting capacity of liquids. At a surface tension against air of approximately oo = 30 mN/m, the wetting capacity of mineral oil is superior to water. Through the addition of detergents, however, the surface tension of water of approximately oo = 72 mN/m can be reduced to oo = 30 mN/m, too [Zwingmann 1960]. Generally, with decreasing
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TABLE 10.2
Source: From Mang 1983, VDI-Richtlinie 3396 1983, Moller and Boor 1986. With permission. |
viscosity, water-immiscible cooling lubricants exhibit better washing capabality [Mang 1983, Spur 1983]. Due to the low viscosity, water-miscible cooling lubricants show a superior rinsing capacity compared to water-immiscible products [Spur 1983].
10.4.3 Lubricating Capability
The lubricating capacity of a cooling lubricant first of all depends on the additives it contains. Also, viscosity influences the lubricating capacity of water-immiscible and water-miscible cooling lubricants. The kinematic viscosity of mineral oil is 15 times higher than that of water, and grinding oils generally have a better lubricating capacity than water composite cooling lubricants [Kohblanck 1956, Zwingmann 1979]. The lubricating effect of oil-in-water emulsions depends on the oil fraction it contains and increases with a larger percentage of oil. Since cooling lubricant solutions are free of mineral oil, they have a poorer lubricating capacity than emulsions [Eckhardt 1983]. Due to the high temperatures and pressures in the contact zone, the pressure absorption capacity of water and mineral oil is not sufficient concerning the formation of a stable lubricating film. For this reason, additives are added to state-of-the-art cooling lubricants to improve the lubricating properties.