LUBRICATING FLUID

Description

FIELD OF THE INVENTION

The present invention relates to a lubricating fluid which is suitably used as shock absorber fluid. The present invention further relates to the use of the lubricating fluid in a shock absorber.

BACKGROUND OF THE INVENTION

A shock absorber (sometimes referred to as a damper) is a mechanical device designed to smooth out or damp a sudden shock impulse and dissipate kinetic energy. Shock absorbers are an important part of automobile, motorbike or bicycle suspensions, aircraft landing gear, train suspensions, and the supports for many industrial machines. Large shock absorbers are also used in architecture and civil engineering to reduce the susceptibility of structures to earthquake damage and resonance.

Shock absorbers convert kinetic energy to heat energy, which can then be dissipated. Hydraulic shock absorbers are composed of a cylinder with a sliding piston inside. The cylinder is filled with a shock absorber fluid. This fluid-filled piston/cylinder combination is also referred to as a dashpot. In a vehicle the wheel suspension usually contains several shock absorbers, mostly in combination with pressure resilient means such as coil springs, leaf springs, or torsion bars. These springs are not shock absorbers as springs only store and do not dissipate or absorb energy. If a wheel is put into a horizontal motion, the spring will absorb the up- and downward force, and convert this into heat. The shock absorber, along with hysteresis in for instance the tyres of the wheel, dampens the motion of the unsprung weight up and down, thereby effectively damping the wheel bounce. This is achieved by converting the kinetic energy into heat through fluid friction due to the flow of the shock absorber fluid through a narrow orifice, such as an internal valve.

WO201063752 discloses fluids that may be used as shock absorber fluids that have high biological degradability and high compatibility with viscosity improvers, particularly at low temperatures. The fluids comprise a base oil composition and a viscosity index improver. The base oil composition comprises GTL base oil and an ester of a polyhydroxy compound.

The present inventors have sought to provide lubricating fluids suitable for use as shock absorber fluids having advantageous properties, e.g. better long term shear stability and/or better low temperature viscosity properties than commercially available shock absorber fluids.

SUMMARY OF INVENTION

The present inventors have found that lubricating fluids suitable for use as shock absorber fluids can be prepared from an advantageous combination of a GTL base oil, an alkylbenzene or alkylnaphthalene, and a viscosity modifier. Such a combination exhibits good shear stability and good low temperature viscosity.

Accordingly the present invention provides a lubricating fluid comprising:

• • (a) at least 40 wt %, based upon the weight of the lubricating fluid, of a GTL base oil;
  • (b) from 5 to 25 wt %, based upon the weight of the lubricating fluid, of alkylbenzene or alkylnaphthalene; and
  • (c) from 0.1 to 20 wt %, based upon the weight of the lubricating fluid, of a viscosity index improver;
    wherein the lubricating fluid has a viscosity index in the range of from 50 to 1000, and a pour point of below −30° C.

The lubricating fluid is suitable for use as a shock absorber fluid but may also find use as a fork oil or as an industrial lubricant such as a hydraulic fluid or a bearing and circulating oil.

The present invention further provides the use of a lubricating fluid according to the invention in a shock absorber.

The present invention yet further provides a vehicle comprising a lubricating fluid according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

The lubricating fluid comprises at least 40 wt %, based upon the weight of the lubricating fluid, of a GTL base oil. The lubricating fluid preferably comprises in the range of from 50 to 90 wt % of GTL base oil, based upon the weight of the lubricating fluid, more preferably of from 60 to 85 wt %.

The term “GTL base oil” is used to describe base oils that are synthesised by the Fischer-Tropsch method of converting natural gas to liquid fuel. They have a very low sulphur content and aromatic content compared with mineral oil base oils refined from crude oil and have a very high paraffin constituent ratio. The GTL base oil may be a mixture of several different GTL base oils, having different viscosities. Preferably the kinematic viscosity of the GTL base oil at 100° C. is in the range of from 2 to 10 mm²/s, more preferably in the range of from 2.5 to 7 mm²/s. The kinematic viscosity is suitably determined by ASTM D445. Suitable base oils, known at “GTL 4” and “GTL 3”, are available from Shell.

The lubricating fluid comprises from 5 to 25 wt %, based upon the weight of the lubricating fluid, of alkylbenzene or alkylnaphthalene. Suitably the alkylbenzene or alkylnaphthalene is a mixture of different alkylbenzene molecules and/or alkylnaphthalene molecules. The alkylbenzenes and/or alkylnaphthalenes may be mono- or poly-substituted but are preferably mono- or di-substituted. In a preferred embodiment, the lubricating fluid comprises from 5 to 25 wt % of a mixture of alkylbenzenes that are mono- or di-substituted with linear and/or branched alkyl groups, wherein the alkyl groups are C₆-C₂₀ alkyl groups, preferably C₉-C₁₅ alkyl groups. The kinematic viscosity at 40° C. of the alkylbenzene or alkylnaphthalene (suitably determined by ASTM D445) is suitably between 3 and 400=²/s, preferably between 3 and 50 mm²/s and more preferably between 3 and 10 mm²/s. The average relative molecular weight is suitably from 180 to 300, preferably from 200 to 280, and more preferably from 230 to 260.

The lubricating fluid comprises from 0.1 to 20 wt %, based upon the weight of the lubricating fluid, of a viscosity index improver. The lubricating fluid preferably comprises from 1 to 18 wt % of a viscosity index improver, more preferably from 3 to 10 wt %.

Viscosity index improvers (also known as VI improvers, viscosity modifiers, or viscosity improvers) provide lubricants with high- and low-temperature operability. These additives impart shear stability and acceptable viscosity at elevated temperatures and at low temperatures. Suitable viscosity index improvers include both low molecular weight and high molecular weight hydrocarbons, polyesters and viscosity index improver dispersants that can function as both a viscosity index improver and a dispersant. Typical molecular weights of these polymers are between about 10,000 to 1,000,000, more typically about 20,000 to 500,000 and even more typically between about 50,000 and 200,000. Examples of suitable viscosity index improvers are polymers and copolymers of methacrylate, butadiene, olefins, or alkylated styrenes.

Preferably, the viscosity index improver is poly methyl methacrylate (further referred to as PMMA), i.e. a copolymer of various chain length methyl and alkyl methacrylates. Particularly preferred PMMA viscosity index improvers are the commercially available Viscoplex viscosity improvers (Viscoplex is a tradename of the Röhm GmbH & CO. KG, Darmstadt, Germany).

The lubricating fluid suitably further comprises one or more additives that are typically used in shock absorber fluids. These additives can be added in the form of an additive package. A typical additive package includes oxidation inhibitors and anti-wear agents, but may also include dispersants, detergents, corrosion and rust inhibitors, metal deactivators, extreme pressure additives, anti-seizure agents, pour point depressants, wax modifiers, seal compatibility agents, friction modifiers, lubricity agents, anti-staining agents, chromophoric agents, anti-foaming agents and demulsifiers.

The lubricating fluid preferably comprises an antiwear additive. Suitable antiwear additives include metal-containing and metal-free alkylthiophosphates, such as zinc dialkyldithiophosphates, typically used in amounts of from about 0.4% by weight to about 1.4% by weight of the lubricating fluid.

The lubricating fluid preferably comprises an anti-foaming agent. Silicones and organic polymers are typical anti-foam agents. Preferably the anti-foaming agent is a low-silicon or no-silicon anti-foam, such as an acrylic copolymer or a fatty amine ethoxylate. The amount of anti-foaming agent is suitably less than 1 wt %, based up on the weight of the lubricating fluid, preferably less than 0.1 wt % and more preferably less than 0.05 wt %.

The lubricating fluid has a viscosity index in the range of from 50 to 1000, preferably in the range of from 100 to 600. Suitably the viscosity index is determined according to ASTM D2272. If the viscosity index is too low, the lubricating fluid is likely to be too viscous at low temperatures and too thin at higher temperatures, and then the lubricating fluid will not function effectively in a shock absorber.

The lubricating fluid has a pour point of below −30° C., preferably below −45° C. Suitably the pour point is determined according to ASTM D97. If the lubricating fluid has a higher pour point then the fluid would not flow in cold ambient conditions and a shock absorber containing the fluid would not function.

The lubricating fluid suitably has a kinematic viscosity at 40° C. of at least 7 mm²/s, preferably at least 10 mm²/s and more preferably at least 12 mm²/s. Suitably the kinematic viscosity at 40° C. is determined according to ASTM D445. Having such a viscosity is important if the lubricating fluid is to function effectively in a shock absorber.

The lubricating fluid suitably has a Brookfield viscosity at −40° C. of less than 2000 cP, more preferably less than 1500 cP and most preferably less than 1250 cP. Suitably the Brookfield viscosity at −40° C. is determined by ASTM D2983. Having such a viscosity is important if the lubricating fluid is to function effectively in a shock absorber.

The lubricating fluid suitably has a shear stability at 40° C., measured according to CEC L-45-99, of less than 10%, more preferably less than 5%, most preferably less than 3%. It is important that the lubricating fluid has the highest possible shear stability (and the lowest possible loss of shear stability under the test conditions) so that, when employed in a shock absorber, the lubricating fluid has the correct viscosity range for efficient operation for the longest possible period. If the lubricating composition has poor shear stability, it will shear down over time and the viscosity will soon be outside the required range.

The present invention is described below with reference to the following Example, which is not intended to limit the scope of the present invention in any way.

Example

A shock absorber fluid (Example 1) was prepared having the formulation shown in table 1:

	TABLE 1
	Component	Amount (wt %)

	GTL 4 base oil	10.00
	GTL 3 base oil	68.18
	Alkylbenzene	15.00
	VI improver	6.00
	Additive package for shock	0.75
	absorber fluid (contains
	dispersants, detergents, anti-
	wear additives, anti-oxidants)
	Ashless antiwear additive	0.05
	Low-silicon Antifoam	0.02

Both GTL base oils are available from Shell. The GTL 4 base oil had a viscosity at 100° C. (measured by ASTM D445) of between 3.80 and 4.20 cSt. The GTL 3 base oil had a viscosity at 100° C. (measured by ASTM D445) of 2.8 cSt. The alkylbenzene was a mixture of mono-substituted alkylbenzenes having a kinematic viscosity at 40° C. of between 3 and 5 mm²/s. The fluid was prepared by mixing and heating of all the components until a homogeneous mixture resulted.

Testing

The shock absorber fluid of Example 1 and two commercial shock absorber fluids (Comparative Example 1 and Comparative Example 2) were tested. Table 2 shows the properties that were tested, the test methods used and the results for Example 1, Comparative Example 1 and Comparative Example 2.

TABLE 2
			Comparative	Comparative
Property	Test Method	Example 1	Example 1	Example 2

Density at 15° C. (g/cm3)	ASTM D4052	0.8247	0.8611	0.836
Viscosity at 40° C. (cSt)	ASTM D445	13.086	14.18	14.5018
Viscosity at 100° C. (cSt)	ASTM D445	3.8561	4.0547	4.0913
Viscosity Index	ASTM D2272	210.7	206.4	202.3
Brookfield Viscosity at −40° C. (cP)	ASTM D2983	1020	2369	1800
Pour point (° C.)	ASTM D97	−60	<−45	<−45
Shear stability at 40° C. (% viscosity loss)	CEC L-45-99	2.70	13.50	6.02

The results show that all three shock absorber fluids had similar density, viscosities at 40° C. and 100° C. and viscosity index. The shock absorber fluid of the invention (example 1) had an improved Brookfield viscosity compared to the commercial shock absorber fluids (comparative example 1 and comparative example 2), and also had a better shear stability.