SYNTHETIC-BASED OILS Machinery's Handbook, 31st Edition Table 8. Polyalkylene Glycols (PAGs)
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Positive Features
Negative Features
Low pour point (to –60 ° F or –51 °C) High viscosity index, VI > 150 High viscosity range Fair seal compatibility Excellent biodegradability Do not produce sludge or varnish
Cost (4–8 × mineral oil cost) Poor mineral oil compatibility Poor PAO and synthetic ester based oil compatibility
Di-Basic Acid Esters (Di-Esters): Due to their high shear VI stability under extreme tem perature, di-esters, Table 9, have become very popular in the aerospace industry. Formu lated from the reaction between alcohol and acid-laden oxygen, di-esters originally saw primary use in jet engine oils, but are now used mainly in high temperature compressor oils. Table 9. Di-Basic Acid Esters (Di-esters) Positive Features Negative Features Low pour point (to –80 ° F or –62 °C) High viscosity index, VI > 150 High viscosity range Good mineral oil compatibility Good additive solvency Cost (4–8 × mineral oil cost) Poor hydrolytic stability Poor seal compatibility Fair mineral oil compatibility Poor corrosion stability Polyol-Esters: Due to their increased thermal stability over di-esters, polyol-esters, Table 10, have now taken over as the preferred oils for gas turbines, jet engines, and 2-cycle oil applications. Table 10. Polyol-Esters Positive Features Negative Features Low pour point (to –95 ° F or –71 °C) High viscosity index, VI >160 High viscosity range Good oxidation stability Good mineral oil compatibility Good anti-wear properties Good additive solvency Cost (10–15 × mineral oil cost) Poor hydrolytic stability Poor seal compatibility Fair mineral oil compatibility Poor corrosion stability Silicones: Silicone lubricants, Table 11, are semi-inorganic compounds formulated to provide the stability of inorganic products, yet retain the versatility of organic products. Although they have poor lubricity, silicone lubricants find favor in lightly loaded instru ment bearings and oils and situations requiring high temperature change and compatibil- ity with plastics. Additives are added to the base stocks to enhance lubricant performance, just as with mineral oils. Table 11. Silicone Positive Features Negative Features Low pour point (to –95 ° F or –71 °C) High viscosity index, VI > 250 Very high Viscosity range Very high flash point Good seal compatibility High Cost (30–100 × mineral oil cost) Poor lubricity Poor seal compatibility Poor mineral oil compatibility Poor biodegradability Poor additive solvency Temperature Effects on Oil.— Changes in temperature effect an oil’s viscosity and its ability to maintain a load carrying hydrodynamic film as depicted in Fig. 7. With the exception of silicone-based fluids, which unfortunately have poor lubricating quali - ties, most oils suffer a dramatic drop in viscosity once the temperature surpasses 100 ° F (38 ° C).
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