WO1996027648A1

WO1996027648A1 - Power steering fluid with wide performance range

Info

Publication number: WO1996027648A1
Application number: PCT/US1996/002841
Authority: WO
Inventors: Gordan A. Alexander
Original assignee: Exxon Research & Engineering Company
Priority date: 1995-03-03
Filing date: 1996-02-26
Publication date: 1996-09-12

Abstract

A power steering fluid suitable for use in electric vehicles and having a wide performance range is disclosed comprising a mixture of a major portion of PAO 2 having a nominal viscosity of 2 cSt @ 100 °C and a minor portion of polymethacrylate having a weight average molecular weight in the range 50,000 to 300,000 and a shear stability index of <10.

Description

POWER STEERING FLUID WITH WIDE PERFORMANCE RANGE

Background of the Invention

Field of the Invention

This invention relates to power steering fluids containing synthetic base stock oils and polymeric viscosity index improvement additives.

Description of the Related Art

The use of synthetic base stocks in lubricating formulation is documented in the literature.

U.S. Patent 5,387,346 teaches an automatic transmission fluid consisting essentially of from 50 to 85 wt% of a mineral oil having a viscosity in the range 3 to 5 cSt @ 100°C, 5 to 35 wt% of poly alpha olefm having a viscosity in the range 1 to 8 cSt @ 100°C and from 8 to 15 wt% of a combina¬ tion of alkenyl succinimide dispersant, a viscosity index improver and an anti oxidant, the resulting lubricating composition being characterized in that it contains less than 7.0 wt% of the viscosity index improver and the composition has a Brookfield viscosity @ -40°C of no more than 20,000 cP. Viscosity index improvers include polymethacrylate, polyacrylates, styrene-maleic ester copolymers.

"Lubrication Fundamentals" J. George Willis, Chapter 4 "Synthetic Lubricants" pg. 75-87, Marcel Dekker Inc., New York, 1980, describes various synthetic lubricating oil base stocks. Poly alpha olefms are addressed in the section titled "Olef Oligomers, where it is indicated that such materials are widely used in automative lubricants, often combined with organic esters as base fluids in engine oils, gear oils, and hydraulic fluids.

Brief Description of the Invention

The present invention relates to a synthetic power steering fluid characterized as having kinematic viscosities of 5.2-5.6 cSt @ 100°C and 14 cSt max. @ 50°C, which gives a calculated value of 18.27 cSt max. @ 40°C and a VI of 245 minimum, Brookfield viscosities of 300 cP max. @ -18°C, 2000 cP max. @ -40°C, a shear stability defined as the change in viscosity at 100°C of +/-3%, a pour point of -48°C max., a flash point of 150°C min. and an oxidation stability of 800 minutes minimum- said power steering fluid comprising a mixture of synthetic base oil of about 2 cSt @ 100°C and polymethacrylate hav¬ ing a weight average molecular weight in the range 50,000 to 300,000 and a shear stability index of <10 wherein the base oil consists of poly alpha olefin. The poly alpha olefin comprises about 75-85 vol%, preferably 78-82 vol%, most preferably about 79-81 vol% of the power steering fluid combination, the poly¬ methacrylate comprising about 14-18 vol%, preferably about 15-17 vol%, most preferably 15-16 vol% of the power steering fluid, the balance, if any, compris¬ ing standard power steering fluid additives selected from detergent/dispersant inhibitors, metal deactivators, pour point depressants, rust inhibitors, anti- foaming agents, anti-wear agents, seal swelling agents, etc.

Detailed Description of the Invention

Power steering fluid useful in electric vehicles characterized in having kinematic viscosities of 5.2-5.6 cSt @ 100°C and 14 cSt maximum @ 50°C, which gives a calculated value of 18.27 cSt mix at 40°C and a VI of 245 minimum, Brookfield viscosities of 300 cP max. @ -18°C and 2000 cP max @ -40°C, a shear stability defined as the change in viscosity at 100°C of +/-3%, a pour point of -48°C max. a flash point of 150°C max. and an oxidation stability of 800 minutes minimum comprises a major portion of a 2 cSt @ 100°C base oil consisting of poly alpha olefin and a minor portion of a polymethacrylate having a weight average molecular weight in the range 50,000 to 300,000, preferably 60,000 to 200,000, most preferably about 100,000 and a shear stability index of <10.

The poly alpha olefin used is identified in the text and in the appended claims as poly alpha olefin 2 (or PAO 2), a material having a nominal viscosity of about 2 cSt @ 100°C and a viscosity index of about 97.

Poly alpha olefins are produced by oligomerizing low molecular weight olefin material, usually ethylene or C3-C12 alpha olefins into a final olefin material having a predetermined carbon number and degree of branchiness. Oligomerization produces dimers, trimers, tetramer, pentamers etc. of the olefin starting material. The viscosity of the final product is a function of its molecular weight. T e viscosity index and pour point of the final product is related to the mono olefin starting material carbon number, linearity, and posi¬ tion of unsaturation. Thus, because one can start with known materials, the mono olefins, it is possible to synthesize final products of specific or at least relatively narrow carbon number or range, thus securing products having specific performance and physical characteristics, e.g. viscosity, viscosity index, pour point, etc. Basic starting material building blocks are combined, oligomerized, into specific finished products. Following the oligomerization process the product is hydrogen stabilized (hydrogenated) to remove the olefinic nature of the product, thus rendering it relatively inert and resistant to oxidation and other forms of degradation common in lube oil applications. Those of ordinary skill in the art, therefore, have no difficulty in producing particular poly alpha olefins having specific performance and physical characteristics. Poly alpha olefins of nominally 2 cSt @ 100°C are known materials which are commercially available as PAO 2. PAO 2 is produced by the oligomerization of 1-decene, followed by hydrogenation of the total product. PAO 2 is recovered from the hydrogenated total product by distillation which fractionates the total product into specific viscosity grades. PAO 2 is primarily a dimer having an average mol. wt, g/mole of 287. Commercially available PAO 2 is nominally defined as a 2 cSt @ 100°C material. The viscosity index of the material is generally about 97 VI. By comparison, PAO-4 has a typical viscosity of about 3.8 cSt @ 100°C and a VI of about 1 14 while PAO 8 has a typical viscosity of about 8 cSt @ 100°C and a VI of about 133.

As previously stated, the present invention employs a base stock consisting of PAO 2, the material having a nominal viscosity of about 2 cSt @ 100°C and a nominal VI of about 97.

The power steering fluid of the present invention also comprises a polymethacrylate VI improver. As previously stated the polymethacrylate employed will have a weight average molecular weight in the range of from 50,000 to 300,000, preferably 60,000 to 200,000, most preferably about 100,000 and a Shear Stability Index (SSI) of <10, preferably <5, most preferably <3. Polymethacrylates are commercially available from many manu¬ facturers. Rohm and Haas produces a line of polymethacrylates under the trade name Acryloid®, as does Rohm under the trade name Viscoplex®.

The polymethacrylates are used in the power steering fluid of the present invention in an amount in the range 14-18 vol%, preferably 15-17 vol%, most preferably 15-16 vol%.

The power steering fluid may contain other typical, conventional, power steering fluid additives. Such additives include corrosion and rust inhibitors, anti-oxidants, detergents/dispersants, anti-foam agents, anti-wear agents, extreme pressure additives, friction modifiers, seal swelling agents, metal deactivators and flow improvers. Such additives are described in "Lubricants and Related Products" by Dieter lamann, Verlag Chemie, Deerfield Beach, Florida, 1984. The other conventional additives can be present in the formulated power steering fluid in an amount in aggregate of up to 8 vol%, preferably 5 vol%.

The target specification for power steering fluids for use in electric vehicles is given in Table 1.

A number of power steering fluid formulations were prepared using different PAO's and PAO/polyol ester mixtures as base oil and different polymethacrylates as additives. Each formulation contained, in addition, a minor amoimt of other typical power steering fluid additives, which do not affect die visiometric properties of the resulting formulated oil.

The results are summarized in Table 2.

TABLE 2 (continued)

I

-J I

(1) Acryloid 1019 - weight average molecular wt -- 100,000, SSI--1

(2) Acryloid HF 833 - weight average molecular wt -- 250,000 SSI-8

(3) Emery 2939 - polyol ester, 5 cSt @ 100°C, 125 VI

(4) Additive - for Aj-E2 anti wear; anti oxidant; seal sweller for I anti oxidant, anti wear, seal sweller (all at high treat levels then in A1-E2) and additionally containing antifoam agents and dye

Example A\ through E₂ are presented for comparison purposes.

From review of the data in Table 2, it is readily apparent that only Example I satisfies the target specifications recited in Table 1.

Examples Cι/C₂ and Dι/D₂ clearly show that it is not possible to employ P AO-4 as the base stock in formulating the power steering fluid of the present invention.

While PAO- is the typical base stock oil used in producing syn¬ thetic or part synthetic formulated oils, the data show that although PAO-4 has a VI of 114 (literature value), when it is combined with the polymethacrylate the blends have VI's of only 199-239. Beyond this, however, the 40°C viscosity grossly exceeds the maximum limit specified for the power steering fluid.

Examples E] E₂ show that using as the base stock PAO-4 or PAO-2 blended with a polyol ester does not produce base stocks which, when formulated with polymethacrylate results in formulations which meet or even suggest that the specification of Table 1 could be met. E\ which uses PAO-2/polyol ester base stocks falls short with respect to VI and viscosity at 100°C while E , which uses PAO-4/polyol ester base stock, grossly exceeds 40°C, 50°C and 100°C viscosities while failing to even approach minimally acceptable VI.

However, while one may conjecture that the viscosity target could be reached if PAO-2 were used, because of PAO-2 lower inherent VI (97 litera¬ ture value) one would not expect that any formulated product could or would meet the minimum VI specification. Rather, one would expect that, by starting with a material having a lower VI than that of PAO-4, the final formulated product made using PAO-2 as base stock would have a formulated VI even lower than that of the formulated product made using P AO-4 as base stock.

Examination of Examples Aj/A₂ and Bj/B₂ shows, however, that quite unexpectedly, the VI of a formulated product using P AO-2 as base stock exceeds the VI of a product made using PAO-4. The discoveries made in Example Aj/A₂ and B]/B₂ permitted the refinement presented in Example I, which corresponds to the formulation of the present invention.

Claims

CLAIMS:

1. A synthetic power steering fluid comprising 75 to 85 vol% of a synthetic base oil of about 2 cSt at 100°C, about 14 to 18 vol% of a polymeth¬ acrylate having a weight average molecular weight in the range 50,000 to 300,000 and a shear stability index of <10, and up to 8 vol% of conventional power steering fluid additives and wherein the synthetic base oil consists of poly alpha olefin-2.

2. The synthetic power steering fluid of claim 1 wherein the poly alpha olefin synthetic base oil comprises 78-82 vol% of the power steering fluid.

3. The synthetic power steering fluid of claim 1 wherein the poly alpha olefin synthetic base oil comprises 79-81 vol% of the power steering fluid.

4. The synthetic power steering fluid of claim 1, 2 or 3 wherein the polymethacrylate has a weight average molecular weight in the range 60,000 to 200,000.

5. The synthetic power steering fluid of claim 4 wherein the polymethacrylate has a weight average molecular weight of about 100,000.

6. The synthetic power steering fluid of claim 1, 2 or 3 wherein the polymethacrylate is present in an amount in the range 15-17 vol%.

7. The synmetic power steering fluid of claim 1, 2 or 3 wherein the polymethacrylate is present in an amount in the range 15-16 vol%.

8. The synthetic power steering fluid of claim 4 wherein the polymethacrylate is present in an amount in the range 15-17 vol%.

9. The synthetic power steering fluid of claim 5 wherein the polymethacrylate is present in an amount in the range 15-16 vol%.