Many different types of materials are employed as functional fluids, and functional fluids are utilized in a wide variety of applications. Thus, such fluids have been utilized as electronic coolants, diffusion pump fluids, lubricants, damping fluid, power transmission and hydraulic fluids, heat transfer fluids and heat pump fluids. A particularly important application of such functional fluids has been their utilization as hydraulic fluids and lubricants in aircraft, requiring successful operation of such fluids over a wide temperature range, and fire resistant fluids.
Functional and hydraulic fluids employed in many industrial applications and particularly hydraulic fluids for aircraft must meet a number of important requirements. Thus, such hydraulic fluids particularly for aircraft use, should be operable over a wide temperature range, should have good stability at relatively high temperatures and preferably have lubricating characteristics. In addition to having the usual combination of properties making it a good lubricant or hydraulic fluid, such fluid should also have relatively low viscosity at extremely low temperatures and an adequately high viscosity at relatively high temperatures, and must have adequate stability at the high operating temperatures of use. Further, it is of importance that such fluids be compatible with and not adversely affect materials including metals and non-metals such as seals of the system in which the fluid is employed. It is also important in aircraft hydraulic fluids and lubricants that such fluids have as high a fire resistance as possible to prevent ignition if such fluids are accidentally or as result of damage to the hydraulic system, sprayed onto or into contact with surfaces of materials of high temperature. Another important property for application of a hydraulic fluid in aircraft is the provision of a low density fluid to increase pay load.
Hydraulic fluids in commercial jet aircraft are exposed to temperatures ranging from below -40.degree.F to over 200.degree.F. Within these temperature extremes, it is necessary for the fluid to maintain a reasonably low viscosity when cold, and yet not become too thin when hot.
In presently available commercial functional or hydraulic fluids, phosphate esters are among the most commonly employed base stocks, of which tributyl phosphate and dibutyl phenyl phosphate are widely used components. Both of the latter phosphates are too thin at high temperatures, and their use alone would result in rapid wear of moving parts. Other phosphate esters, such as tricresyl phosphate, for example, which provide the requisite high temperature viscosity become too thick to be useful at low temperatures. Even mixtures of various phosphate esters such as those noted above do not provide the required viscosity characteristics at both low and high temperatures. Accordingly, it has been the practice to achieve the required wide viscosity range required for aircraft hydraulic fluids by adding to a thin base stock, such as phosphate ester or mixtures thereof, a small proportion, e.g., up to 10%, of a polymeric material such as polyalkyl acrylates or methacrylates, whose solubility characteristics in the base stock are chosen so that the polymeric material thickens the fluid more at high temperatures than at low temperatures, and thus functions as a viscosity index (VI) improver.
However, during use, fluids of the above type containing polymers such as the polyalkyl acrylates or methacrylates, tend to decompose due to the shearing forces of the mechanical components encountered in an aircraft hydraulic system, on the fluid, and producing acids. This results in a high degree of acid build-up during use, which is detrimental in causing corrosion of metal surfaces with which the fluid is in contact and also causes further decomposition of the fluid.
In U.S. Pat. No. 3,637,507, improved acid stability of such functional or hydraulic fluids is achieved by the addition of monoepoxides and particularly certain diepoxides to hydraulic fluids containing a phosphate ester and particularly polyalkyl methacrylate viscosity index improver. However, in currently manufactured fluids of this type, the functionality of the epoxide is severly limited by the reaction which epoxides can have with acrylate and methacrylate viscosity index improver. Thus, although diepoxides such as the 3,4-epoxycyclohexylmethyl 3,4-epoxycyclohexane carboxylate disclosed in the above patent are preferred over the monoepoxide in the above fluids, the concentration of the latter diepoxide must be kept low since the reaction of polyfunctional epoxides with acrylate and methacrylate type viscosity index improvers leads to the formation of harmful insoluble deposits in aircraft hydraulic systems. Thus, it is particularly noteworthy that in all of the examples of the above patent, and employing polyalkyl methacrylate viscosity index improver, the concentration of the epoxide component, including the above-noted preferred diepoxide of the patent, is employed in a concentration ranging only from about 0.5 to about 1%. The necessity for using such low concentrations of such diepoxides in order to avoid formation of undesirable deposits in the fluid, due to reaction with acrylate and methacrylate viscosity index improvers, limits the effectiveness of such epoxides in preventing acid build-up.
The present invention relates to functional fluid compositions having good fire resistance and desirable viscosity characteristics at both high and low temperatures, and is particularly directed to functional fluid compositions having the above-noted properties and consisting essentially of a phosphorus compound, especially a phosphate ester, a particular viscosity index improver other than a polyacrylate or polymethacrylate, and an epoxide compound of a type which permits use of a high concentration of such epoxide in conjunction with said viscosity index improver, to effectively prevent formation of harmful insoluble deposits in the fluid while effectively functioning as an acid absorber to prevent acid build-up.