Hydraulic fluids oxidation stability

Polyalphaolefin hydraulic fluids have many advantages over the mineral oil counterparts including low temperature flow characteristics, lower volatility, and oxidative stability (Chrisope and Landry 1993 Shubkin 1993). Certain polyalphaolefins maintain good operational characteristics and have been proposed for use in hydraulic systems in U.S. military aircraft (Kinkead et al. 1992b). 

Antioxidant technology To indicate the oxidation and thermal stability of a variety of ZnDTP-containing and ashless anti-wear hydraulic fluids, TOST and Cincinnati Milacron data - both relevant tests for this type of fluid - have been compiled and summarised. Table 4.9, from both published [80, 81] and in-house results. The data show that oxidation resistance and thermal stability of the premium fluids, both ashless and stabilised ZnDTP, are comparable and clearly outperform the normal-grade fluids. The normal ashless grade at 0.4-0.5% additives is superior to the normal ZnDTP-based grade, especially for thermal stability, which may be due to the use of thermally unstable secondary ZnDTPs. 

Table 4.9 Oxidation and thermal stability of various anti-wear hydraulic fluids...

The example given in Table 4.10 illustrates the outstanding oxidation resistance of a rapeseed oil-based environmentally acceptable hydraulic fluid, containing a non-water-extractable phenolic antioxidant [83]. A similar type of non-water-extractable phenolic antioxidant provided excellent oxidation stability, TOST life >10,000 h, in a food-grade hydraulic fluid based on white oil [84]. All additives and the white oil are cleared by the (US)FDA. 

Table 4.10 Oxidation stability of a rapeseed oil-based hydraulic fluid...

Over the years, detergents have found equally important usage in many other types of lubricants, including gear oils, farm tractor hydraulic fluids, automatic transmission fluids and industrial oils. The performance properties that detergents impart to these lubricating fluids include rust and corrosion prevention, improvement in lubricant elastomeric seal compatibility, enhancement of sludge control under oxidative conditions and improvement of wet clutch and wet brake friction level and stability. 

The phosphoric acid triesters possess excellent anti-wear properties, see Chapters 3 and 6, and sufficient oxidative stability. Their disadvantage is their ability to hydrolyse, which depends significantly on the structure of the ester short chain alkyl esters and aryl esters hydrolyse more rapidly than the long-chain alkyl esters. The triaryl versions of phosphate esters are relatively unique among hydraulic fluids because of their self-extinguishing properties, the fluid does not create enough energy to support its own combustion in a fire [65]. 

Polychlorinated biphenyls (PCB) are noted for their stability. They are resistant to heat, oxidation and attack by strong acids and bases. As a result of these extraordinary properties PCB have been used widely and successfully as dielectric fluids, fire retardants, heat transfer agents, hydraulic fluids, plasticizers and in other applications (A) ... 

Automatic transmission fluid (ATF) performs many functions including lubrication, power transfer, hydraulic control, and cooling [73], This fluid is extremely complex consisting of a base mineral oil (consisting of paraffinic, naphthenic, and aromatic hydrocarbons) and an additive package. Newer ATF compositions consist of synthetic base materials. The additives prevent fluid oxidation, corrosion, loss of lubricity, and foaming. Even though antioxidants, inhibitors, and stabilizers are added to the ATF, the base oil... 

Phosphate esters are produced from phosphorus oxychloride with various alcohols or phenols, or combinations of these hydroxyl compounds. These fluids generally have good thermal and oxidative stabilities and fire-resistancy. However, because of their high polarity, poor Vl-pour point balance, facile hydrolysis and inferior elastomer and paint compatibility, their use in general lubrication is limited. The major use for phosphate esters is in fire-resistant hydraulic oils. 

Saturated branched fatty acids, such as isostearic acid, are important intermediates in various industrial products, including lubricants, surfactants, detergents, cosmetics, biodiesel, and hydraulic fluids. Saturated branched fatty acids and their alkyl esters have important advantages over natural linear fatty acids they have an excellent oxidative stability compared to unsaturated fatty acids and have a more desirable liquidity compared to linear saturated fatty acids. Furthermore, branched saturated fatty acids show a desirable low viscosity and improved formulation flexibifity [330]. As naturally occurring branched fatty acids are rare, catalytic skeletal isomerization of common linear fatty acids is required. 

Uses Antiwear additive, EP agent, antioxidant, stabiiizer, anlifoam and demulsifier for hydraulic fluids corrosion inhibitor for steel and yellow metal Features Imparts extended oil life and pump durability exc. thermal/oxidation stability... 

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