By Peter Keith Brian Hodges
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7 Fluoroethers Fluoroalkylethers and perfluoropolyethers (PFPE) are characterized by extreme chemical inertness and a high degree of oxidation stability, thermal stability and radiation resistance. In addition these fluids display good lubricating properties, low pour points, excellent fire resistance, compatibility with most plastics, elastomers and metals, and biological inertness. Negative aspects are high cost, relatively low bulk moduli, poor solvent ability for additives and immiscibility with other fluids.
Inadequate solubility may thus hinder an additive in fulfilling the envisaged function, whilst excessive solubility would be unfavourable with respect to, for example, the desired adsorption of surface-active agents on metal surfaces. Hydraulic fluids intended for low or high temperature applications are often based on synthetic base oils with very different additive compatibility compared to mineral base stocks. Nevertheless, the manufacturing processes for synthetic oils usually permit the physical and performance properties of a specific fluid type to be adjusted Additives 23 to an optimum combination.
In - 3 . 9 ' i ' . .. ,,,' , - , ' . . 1 Fig. . . 4 Significance of viscosity for operation of a typical hydrostatic system. 3). This balance of mechanica] and volumetric considerations is best achieved by practical rig tests, and will often be found to lie nearest the limiting viscosity, ensuring adequate lubrication of the hydraulic pump. The various types of hydraulic pumps present dissimilar viscosity requirements, and winter conditions in, for example, Canada and the Nordic countries frequently represent a considerable challenge with respect to the selection of hydraulic fluids for satisfactory operation of outdoor equipment.