Many vehicles, such as aircraft, cars, trucks, etc., have one or more hydraulic systems which are drives or transmission systems that use pressurized hydraulic fluid to power hydraulic machinery. For example, early vehicles had hydraulic brake systems. As vehicles became more sophisticated, newer systems with hydraulic power were developed. Hydraulic systems in, for example, an aircraft provide for the operation of vehicle components, such as landing gear, flaps, flight control surfaces, and brakes.
A hydraulic system has a power generating device (pump) reservoir, accumulator, heat exchanger, and filtering system. System operating pressure may vary from a couple hundred pounds per square inch (psi) in small vehicles and rotorcraft to 5,000 psi in large vehicles.
Hydraulic system fluids (“hydraulic fluids”) flow through components of the hydraulic system during use to transmit and distribute forces to various components of the hydraulic system. If a number of passages exist in a system, pressure can be distributed through the various components of the system. Hydraulic operations have only negligible loss due to fluid friction.
If incompressibility and fluidity were the only qualities required, most liquids that are not too thick could be used in a hydraulic system. However, other properties should be considered when selecting a desired hydraulic fluid for a particular hydraulic system.
One of those properties is viscosity, which is a resistance of the fluid to flow. A liquid such as gasoline that has a low viscosity flows easily, while a liquid such as tar that has a high viscosity flows slowly. Viscosity increases as temperature decreases. A liquid for a given hydraulic system should have enough viscosity to give a good seal at pumps, valves, and pistons, but should not be so thick that it offers resistance to flow, leading to power loss and higher operating temperatures which may promote wear of hydraulic system components. A fluid that is not viscous enough can wear moving parts or parts that have heavy loads.
Another property pertinent to hydraulic fluids is the fire point of the fluid, which is the temperature at which a substance gives off vapor in sufficient quantity to ignite and continue to burn when exposed to a spark or flame. Like a flash point, a high fire point is desirable of hydraulic liquids.
Known hydraulic fluids do not possess ideal properties as discussed above. Polyalphaolefin-based hydraulic fluids are fire-resistant but have a high viscosity and are limited to use down to −40° F. Phosphate ester-based (Skydrol®) hydraulic fluids are not entirely fire-resistant and under certain conditions, they burn. Furthermore, polyalphaolefin-based hydraulic fluids and phosphate ester-based hydraulic fluids do not mix with each other. Furthermore, fluorocarbon-based hydraulic fluids tend to degrade paint and titanium couplings on the hydraulic lines of a hydraulic system. There is also a movement to ban production of chlorocarbon-based and fluorocarbon-based hydraulic fluids because of their toxicity and poor biodegradability. For example, chloroparaffins are stable in soil and persist in soil for years, having a half-life (T1/2) of at least months to years.
Furthermore, synthesis of hydraulic fluids tends to be laborious and cost intensive. Conventional reactions, such as the Arbuzov reaction, do not yield hydraulic fluids having ideal properties as described above. An Arbuzov reaction proceeds by reacting a primary alkyl halide with a phosphite to form a primary phosphono-substituted product. The Arbuzov reaction does not proceed readily using primary, secondary, or tertiary fluoro alkane starting material or using secondary or tertiary chloro-, bromo-, iodo-alkane starting materials.
Therefore, there is a need in the art for new and improved hydraulic fluids and methods of making hydraulic fluids.