This invention relates to phosphine oxides, and in particular to liquefied mixtures of trialkylphosphine oxides useful as components in aircraft hydraulic fluids and in other formulations.

Modern aircraft are equipped with a number of hy- draulically actuated mechanisms such as brakes, power steering for taxiing, landing gear, control surfaces, that is, wing flaps, elevator, rudder and the like. In a large wide-bodied airliner, the quantity of hydraulic fluid that must be carried can be substantial upwards of about 175 gallons. The substitution of a hydraulic fluid with a specific gravity of 0.878 instead of 1.00 amounts to a weight savings of about 200 pounds over the commonly used phosphate ester based hydraulic fluids. A comparable product having a lower specific gravity would be of benefit to the aircraft industry.

It has now been discovered that liquid mixed tri¬ alkylphosphine oxides are excellent base stocks in the formulation of low density hydraulic fluids and the pro¬ vision of such compositions and their use in hydraulic devices and processes constitutes the principal objects and purposes of the invention.

The herein liquefied trialkylphosphine oxides are obtained by combining individual members to provide mix¬ tures which through mutual melting point depression re¬ main in the liquid state. Generally speaking, trialkyl¬ phosphine oxides, as single compounds, are highly crys¬ talline solids. Although they constitute a known chemi- cal series, trialkylphosphine oxides have not, so far as can be ascertained, been prepared and utilized as lique¬ fied mixtures.

In forming the liquefied trialkylphosphine oxide mixtures of the invention, the maximum number of in- dividual members should be used to insure that crystal¬ lization does not occur. Thus, trialkylphosphine oxides can be selected in which the alkyls are alike or dif-

ferent with respect to their isomeric configuration and number of carbon atoms. For instance, a mixture of tri¬ alkylphosphine oxides in which the number of carbon atoms per alkyl is 6 to 10 would include a large number of homologous and isomeric compounds. However, the synthesis of sufficient numbers of different structures in order to provide and maintain stable liquefied mix¬ tures can be arduous and time-consuming. It has been found that this laborious procedure can be circumvented and entirely satisfactory liquefied trialkylphosphine oxides obtained by phosphinating mixtures of α-olefins followed by oxidation of the trialkylphosphine mixture. The reaction is carried out in the presence of a free radical initiator as illustrated in the following scheme r

3RCH=CH ₂ + PH ₃ Azobisisobutyronitrile

P(CH ₂ CH ₂ R) ₃ + [0] P(CH ₂ CH ₂ R) ₃

0 wherein R is alkyl in an olefinic mixture. From the standpoint of economy and convenience, RCH=CH ₂ is pre¬ ferably an even numbered olefin since these are readily available as by-products from petroleum refineries. Typically, the reaction sequence is conducted by intro¬ ducing phosphine into a molar excess of the monoolefin mixture in the presence of the radical initiator under inert conditions at moderately elevated temperatures - about 80°C to about 120°C. The resulting intermediate phosphines are then oxidized, preferably with hydrogen peroxide, to the corresponding mixed phosphine oxide. Suitable radical initiators include any number of com¬ pounds which are photochemically or thermochemically decomposed to form free radicals under the reactive conditions. A preferred radical initiator is azobis¬ isobutyronitrile. Generally speaking, the reaction was carried out following the procedure for preparing indi¬ vidual trialkylphosphine oxides as set forth in ϋ. S. Patent No. 2,803,597 to Stiles and J. Org. Chem. 26,

5138 (1961) .

The reaction of phosphine with -olefinic mixtures in molar ratios of 1:1:1 results in a statistical dis¬ tribution of phosphine oxides, although variations in ^• the reactivities of the individual olefins in the mix¬ ture may alter the distribution to some extent. By varying the olefinic molar ratio, however, relatively large shifts in the distribution of products can be realized. For instance, it may be desirable to include a preponderance of a higher molecular weight olefin in the mixture in order to suppress the vapor pressure of the liquefied trialkylphosphine oxides to permit their use at elevated temperature. In Table 1 is listed the properties of various trialkylphosphine oxides. The hydraulic fluids of the invention will nor¬ mally contain very minor amounts, typically about 0.01% to about 5.0% by weight of various additives of the type normally incorporated in formulating hydraulic fluid compositions such as antioxidants, rust inhibitors, σor- rosion inhibitors, antifoam agents, antiwear agents, cavitation inhibitors, pour point depressants, and other special purpose additives.

Rust and corrosion inhibitors commonly employed include benzothiazole, benzot iazole, triethanola ine, phenothiazine, trialkyl phosphites, N-acrylsarcosines, propyl gallate, succinic acid and alkylsuccinic acids. Additives to inhibit foaming and cavitation include organosilicones, dialkyl carboxylic acid esters such as diethyl succinate or dioctyl sebacate. Antioxidants include dialkylthiodipropionate, for example, dilauryl- thiodipropionate, etc., organic amines, for example, diphenylamine, phenylnaphthylamine, hindered phenols, etc.

As previously pointed out, the herein trialkyl- phosphine oxides are characterized by exceptionally low density and are thus particularly suitable as the base stock in hydraulic fluids for aircraft. Whereas, the

OMPI

common commercial phosphate ester type hydraulic fluids, for example, tr ibutylphosphate and dibutylphenylphos- phate have a specific gravity of about at least 1.000, the trialkylphosphine oxides of the invention exhibit an average specific gravity of about 0.878. This trans¬ lates into substantial weight savings for large commer¬ cial airliners such as the 747 which has a hydraulic fluid capacity of about 175 gallons. Furthermore, the herein liquefied mixtures of trialkylphosphine oxides have one-thousand -fold lower volume resistivities than phosphate esters which should decrease flow of wall cur¬ rents induced by the high velocity flow through control valves thereby inhibiting erosion of hydraulic systems. The invention is further illustrated by the fol- lowing example :

Tri(C ₆ -C ₈ -C ₁₀ ) Phosphine Oxide Example 1 To a 1-liter 316 SS stirred autoclave evacuated to 10 mm Hg pressure was added 135.0 g (1.61 moles) of hexene-1, 188.0 g (1.68 moles) of octene-1, 241.0 g

(1.72 moles) of decene-1, and 0.4 g of azobis isobutyro¬ nitr ile. Phosphine (51.0 g, 1.50 moles) was then added and the reaction mixture heated with agitation to 85°C- 90°C and held at this temperature for a total of 10 hours. A toluene solution of azobis isobutyronitr ile, made by dissolving 1.6 g of azobis isobutyronitr ile in 50 g of toluene, was added in two equal portions after the first and second hours of reaction time.

At the end of the 10 hour reaction period, the reaction mixture was vacuum st ipped at 100°C and 30-50 mm Hg (4 to 6.7 kPa) to remove toluene and unreacted olefins and the residue taken up in 600 ml of isopropyl alcohol and oxidized by the dropwise addition of 122 ml of 30% hydrogen peroxide (1.08 moles). The oxidized mixture was vacuum stripped at 30°C and 5 mm Hg (667 Pa) to give 559 g of crude tr i (C ₆ -Cg-C ₁₀ ) phosphine oxide corresponding to 96.5% of theory based on phosphine.

The crude product, having an acid number of > 4 , was dissolved in 300 ml of petroleum ether and stirred for 3 hours with 250 ml of saturated aqueous ₂ C0 ₃ solution and 100 ml of saturated aqueous NaCl solution. ⁵ After phase separation and vacuum stripping at 30°C and

5 mm Hg (667 Pa), 378.5 g of liquid product was obtained corresponding to a 65% yield of tri (Cg-Cg-C^ ₀ ) phosphine oxide ( g/C /C- _jQ = 1.0) based on phosphine. Found: C, 73.93; H, 13.01; P, 8.19. Calculated for C ₂ 4 ^H 51 ^OP: C, ¹⁰ 74.61; H, 13.21; P, 8.03. Acid number was 0.5. Product was oil-soluble and water-insoluble. Viscosity index was 77. Pour point was +17°F. Specific gravity at 20/20°C was 0.878. Volume resistivity at 10 volts and 23°C was 10 ⁶ ohm-centimeter. ¹⁵ In addition to their use in aircraft hydraulic systems, the herein liquefied trialkylphosphine oxide mixtures exhibit antistatic properties as evidenced by the marked decrease in volume resistivit 'of thermo¬ plastic polymers in which such phosphine oxides have ²⁰ been incorporated. The liquefied trialkylphosphine oxide mixtures possess other useful and valuable characteristics. For instance, liquid decyl hexyl octylphosphine oxide, a representative member of the series obtained by reacting molar proportions of 25 1-hexene, 1-octene and 1-decene with phosphine followed by oxidation of the reaction mixture, is capable of plasticizing nylon. This is indeed surprising in view of nylon's inertness and incompatibility with the nor¬ mally used plastic izers. Even more surprising, however, ^Jυ are the properties of nylon plasticized with the herein liquid trialkylphosphine oxides. Particularly note¬ worthy in this regard is the unusually high luster of fibers spun from these novel plasticized nylon compo¬ sitions. Dyed fabrics manufactured from such trans- 5 parent fibers should, therefore, exhibit much greater color brightness compared with conventional nylon tex¬ tiles. Another surprising and further unexpected prop-

erty of the herein plasticized nylon is that during cold drawing, the resulting fibers become increasingly more extendable without any apparent loss of tensile strength. Normally, incorporation of plasticizer in a polymer causes a decrease - not an increase in physical strength.

Plastic films containing the liquid trialkylphos¬ phine oxides of the invention are rendered more trans¬ parent. For example, polyethylene film, which tends to be milky or hazy, was obtained in highly clarified form. Similar results were realized with nylon film.

TABLE I

PHYSICAL FORM AND SOLUBILITIES OF TRIALKYLPHOSPHINE

OXIDES

Trialkyl phosphine Physical Solubility

Oxide Form Ξ ₂ 0 Mineral Oil

^C 18 ^"C 24 Solid I I

^C U ^"C 14 Solid I S

^C 6- ^C 12 (Cg/C ₁₂ =0.5) Solid I s

^C 6- ^C 12 «VC ₁₂ = 2.0) Liquid I s

^C 6~ ^C 10 (C ₆ /C ₁₀ =2.0) Liquid I s

V ^C 10 (C ₈ /C ₁₀ =1.0) Liquid I s (partially cryst. )

^C 6~ ^C 8 (C _β /C ₈ =2.0) Liquid I s

^C ό- ^C 8 (Cg Cg-1.85) Liquid I s

^C 6~ ^C 8 ^"C 10

(c ₆ /c ₈ /c ₁₀ =ι.θ) Liquid I s

Sec C ₄ Solid I s

¹ Sun SSR 110.