EP1963470B1

EP1963470B1 - Additive package for high temperature synthetic lubricants

Info

Publication number: EP1963470B1
Application number: EP06845688A
Authority: EP
Inventors: Dale D. Carr; Thomas G. Schaefer; Jeffrey A. Hutter
Original assignee: Hatco Corp
Current assignee: Hatco Corp
Priority date: 2005-12-16
Filing date: 2006-12-18
Publication date: 2013-02-27
Anticipated expiration: 2026-12-18
Also published as: CN101331216A; RU2008129100A; US20070184989A1; WO2007075531A2; EP1963470A2; JP2009520079A; WO2007075531A3; CN101331216B

Description

BACKGROUND OF THE INVENTION

This invention relates generally to high temperature lubricant fluids and, more particularly to an additive package including at least an oligomeric aromatic amine antioxidant and a borated compound for use with a polyol ester to provide a lubricant fluid suitable for use in applications operating at temperatures in excess of 250°C.
There are continuing demands for lubricant compositions suitable to operate at high temperature in excess of 250°C. Such lubricants must provide lubrication and antiwear protection. In addition, they must be stable in the high temperature environment, or decompose harmlessly without forming hard, varnish-like deposits or unacceptable amounts of smoke. Many industrial processes involve operation of open chain and drive gear assemblies that are associated with ovens, furnaces, kilns and other hot equipment. Such chain and drive gear assemblies are used in the manufacture of textiles, wallboard, corrugated metal, paper and plastic film.
In addition to not forming deposits or varnish and possessing stability at high temperatures, the lubricants must perform under high load, be compatible with all materials in contact with the lubricant and be low in volatility. Existing commercial lubricants for chain and drive gear operations, which are based on vegetable oils or other glycerol-based esters and mineral oil, lack sufficient high-temperature stability. Polyolefins or polyacid esters also lack the necessary high-temperature stability. All these lubricants are prone to varnish formation and are characterized by relatively high volatility, as well as severe compatibility problems with silicone elastomers.
Grease is a lubricating oil that contains a thickener to prevent the oil from leaking out of the area requiring lubrication. Conventional greases use a mineral oil or PAO based lubricating oil and a fatty acid-metal salt, clay, PTFE, or polyurea thickening system. The stability of the lubricating oil is a major factor in determining the suitability of a grease for a particular application. Difficult applications such as the constant velocity joints of front wheel drive automobiles can overstress conventional greases by exceeding their temperature limitations.
One such high temperature chain and drive gear lubricant is described in U.S. Patent 5,151,205 to Calpon, Jr. While the Calpon patent describes a wide variety of synthetic polyalphaolefin based oils and ester based oils, the described compositions include a polyalphaolefin base oil, an ester oil solubulizer and 2-4 weight % of a polybutene tackifier. The composition is promoted for reducing smoking in chain and drive gear assemblies operated at high temperatures. However, as shown in comparative testing below such lubricants based on these polyalphaolefins tend to evaporate under high temperature exposure and are not fully satisfactory. Presently, no 100% polyalphaolefin based chain lubricants are fully satisfactory in this respect.
Additional high temperature lubricant compositions are set forth in U.S. Patent No. 6,436,881 that issued on August 20, 2002. The lubricants described therein include a polyol ester base stock formed from a major proportion of dipentaerythritol and a mixture of C₅ to C₁₂ monocarboxylic acids. The base stock is mixed with an additive package that includes a viscosity improver, antioxidants, extreme pressure/antiwear agents and a corrosion inhibitor. This lubricant is acceptable for many applications such as static chain oils, where pools of lubricants are subject to continual heating. XP 13007534 "Fluid bearing spindle motor with special lubricant" discloses a system of additives comprising oligomers of dioctylphenylamine and phenylnaphtylamine, which improves oxidative stability of light duty FDB oils.
It remains desirable to provide high temperature oils that are suitable for use in high temperature chain oil environments and that exhibit reduced evaporation rates under high temperature conditions and avoid the varnish/deposits shortcomings of some commercially available chain oil lubricants. It is also desirable to provide greases that exhibit improved oxidative stability and a higher drop point than current commercially available greases.

SUMMARY OF THE INVENTION

Generally speaking, in accordance with the invention, an improved additive package for a synthetic polyol ester lubricant fluid to be used as a high temperature oil or a component in a grease is provided. The additive package includes an effective amount of at least one oligomeric amine antioxidant and at least one borated extreme pressure/antiwear agent and may include a corrosion inhibitor. The polyol ester fluid is formed by reacting pentaerythritol and a monocarboxylic acid having from 5 to 12 carbon atoms including at least 60 WT % iso C9 acid. After addition of the additive package in accordance with the invention, the resulting lubricant fluid does not require the addition of a viscosity index improver to be suitable for use in high temperature chain oil and grease applications. Preferably, the pentaerythritol is dipentaerythritol.
The additive package is added in an amount between 5 to 15 percent by weight of the lubricant fluid to provide an oil having a viscosity at 40°C of at least 120 cSt and preferably at 100°C of no less than about 15.0 cSt.
An object of the invention is to provide an improved synthetic polyol ester lubricant fluid including an additive package including at least one oligomeric amine antioxidant according to claims 4 and 8 and at least one borated extreme pressure agent suitable for use in high temperature chain oil and grease applications.
A further object of the invention is to provide an improved polyol ester lubricant additive package that includes at least an oligomeric amine antioxidant and a borated extreme pressure agent.
Yet another object of the invention is to provide an improved high temperature polyol ester synthetic lubricant including a dipentaerythritol ester and an additive package that has reduced weight loss when subject to heat for extended periods of time.
Still another object of the invention is to provide an improved polyol ester lubricant oil for high temperature application that does not form hard varnish and undesirable deposits when subject to high temperature.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.
The invention accordingly comprises a composition of matter possessing the characteristics, properties and the relation of components that will be exemplified in the compositions hereinafter described, and the scope of the invention will be indicated in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a fuller understanding of the invention, reference is had to the following description taken in connection with the accompanying drawing(s), in which:
FIG. 1 is a photograph of panel coking tests for polyol ester based lubricant fluids in accordance with the invention and a lubricant of a polyol and an additive package formulated in accordance with U.S. Patent No. 6,436,881 .

DESCRIPTION OF PREFERRED EMBODIMENTS

The additive package in accordance with the invention suitable for use in 100% polyol ester high temperature lubricant fluids includes at least: (i) an oligomeric aromatic amine, and (ii) a borated extreme pressure agent.
The components of the additive package in accordance with the invention are as follows:

(1) Antioxidant: The preferred antioxidant is an oligomeric aromatic amine available from R.T. Vauderbilt Company as Vanlube 9317. This is the reaction product of alkylated diphenylamine, alkylated phenylnaphthylamine and an ester base stock. The antioxidant is present in an amount between about 1 to 10 weight percent, based on the total weight of the lubricant. Preferably, between about 3 to 8 weight percent and most preferably about 3 to 5 weight percent is included.
(2) Extreme Pressure/Antiwear Agents: The preferred extreme pressure agent found to be particularly suitable for use with the preferred antioxidant is a borated additive. These borated additives are borated amines, potassium tetraborate, borates of Group 1a alkali metals, borates of Group 2a alkaline earth metals, stable borates of transition metals such as zinc, copper and tin and boric acid. Mild antiwear agents, such as methylene bis (dithiocarbamates) and dialkyl dithiophosphate esters where the alkyls for these range from C₁ to C₈ as well as higher alkylated (C₉ to C₁₂) triphenyl phosphorothionate, tripheny phosphorothronate, tranryl phosphorates, and mixtures thereof may also be included. The extreme pressure agent is present in an amount between about 0.1 to 5 weight percent, and preferable between about 1 to 4 weight percent. Most preferably, between about 1.5 to 3 weight percent is present.
(3) Corrosion Inhibitor: Corrosion inhibitors may be included, and include heterocyclic nitrogen compounds such as benzothiazole, benzotriazole, tolyltriazole and aminotriazole or mixtures thereof. Generally, the corrosion inhibitor is present in an amount between about 0.01 to 1.0 weight percent.

The polyol esters suitable for high temperature application are the reaction product of pentaerythritol or dipentaerythritol, with a monocarboxylic acid or monocarboxylic acid mixture of C₅ to C₁₂ acids. Preferably, the polyol is a dipentaerythritol that is at least about 85 weight percent diPE and may include about 5 percent monopentaerythritol, and 10 percent tripentaerythritol plus higher pentaerythritols. The acid mixture includes at least about 60 weight percent iso-C₉ acid.
The viscosity of the lubricating fluid base stock is at least about 120 cSt at 40°C. For the preferred fluid compositions to be suitable in lubricants such as oils and greases there is no need to add a viscosity index modifier.
The monocarboxylic acid found particularly suitable for use in preparing the high temperature fluid may be isononanoic (3, 5, 5-trimethylhexanoic) acid ("iso-C₉ acid") for high viscosity lubricants and greases or mixtures of C₇ and C_8-10 normal monocarboxylic acids and iso-C₉ acid.. It is one of the preferred embodiment of the invention that the acid be only iso-C₉ acid. Mixtures including heptanoic (C₇) acid and caprylic/capric (C_8-10) acid with the iso-C₉ acid are within the scope of the invention. Preferred acid mixtures include between at least about 60 to 70 weight percent iso-C₉ acid with the balance being C₇ and C_8-10 straight chain acids.
It is possible to vary the dipentaerythritol and acid composition to provide an ester composition having a minimum viscosity at 40°C of at least about 140 cSt. The viscosity of the polyol ester at 100°C should be between about 15 to 30 cSt and have a viscosity index in the range of about 60 to 120. In one preferred embodiment of the invention, the polyol ester is dipentaerythritol isononanate having a viscosity at 40°C of about 360 to 400 cSt. In another preferred embodiment the acid mixture is varied to include C₇ and C_8-10 straight chain acids and iso-C₉ acid to yield polyols having a viscosity at 40°C of between 120 and 170 cSt, and most preferably about 150 cSt.
The additive package in accordance with the invention is added to the synthetic polyol ester base stock to form a lubricant fluid. The additive package includes between about 3-8 weight percent antioxidant in combination with about 1-4 weight percent of a borated extreme pressure/antiwear agent, based on the total weight of the composition. In addition, a minor effective amount of a corrosion inhibitor may also be added to the lubricant fluid. This yields a lubricant fluid having a density at 15.6°C of 960 to 990kg/m³ (8.0 to 8.25 lbs./gal), a total acid number of about 0.30 to 0.40, a pour point of less than about -15°C and a flash point of at least about 285°C. Generally, the lubricant fluids in accordance with the invention will include between about 5 to 10 parts by weight of the additive package and 100 parts by weight of the desired polyol ester base stock.
The lubricating fluid of the polyol ester and additive package is suitable for use directly as an oil lubricant. The oil lubricant is particularly well suited for use in high temperature chain oil applications. The lubricating fluid can also be utilized to form a grease. In this case, a thickening agent to immobilize the fluid is included in the composition. The concentration of thickener determines the consistency and general properties of the finished product and may be included in amounts ranging from 20 to 30 weight percent of the total composition.
A wide variety of thickener types are suitable. These include metallic soap thickeners, complex metallic soap thickeners, and non-soap thickeners. The non-soap thickeners are organo-clay, polyurea; and PTFE. Metallic soap thickeners are usually complex metallic soaps. These include aluminum, calcium, barium and lithium complexes.
The non-soap thickeners are the organo-clay greases, PTFE (Teflon) and silica gel greases. When polyurea greases are utilized, the oils are mixed with suitable amines and isocyanides or disocyanates making the polyurea greases particularly suitable for high temperature applications.
A grease in accordance with the invention includes between about 65 to 85 weight percent polyol ester, between about 15 to 35 percent polyurea or other suitable thickener and the additive package of between about 1 to 8 percent oligomeric aromatic amine antioxidant and about 0.10 to 4 percent borated extreme pressure agent. It has been found that such greases exceed 600 hours in the bearing life test at 1,000 rpm and 169 °C (325 °F) as described in ASTM D-3527.
Ester based grease is prepared by charging a standard grease manufacturing kettle having milling and recirculation capabilities with about 40 parts of ISO 400 grade polyol ester, 21 parts of the polyurea thickening agent components and 0.5 parts water and heated to 225° to 235°F. Heating is continued to 375° to 385°F then shut the heat off and agitated for 15 to 20 minutes. Three parts of a boron extreme pressure agent are added, and the oil is milled with the cooling oil on for at least 3 to 4 hours. After 1 to 1.2 hours of milling, the penetration of the grease is adjusted to a value of 240 to 260 by adding up to 22 parts of ISO 170 grade polyol ester while milling. When the mixture temperature is 107-113°C (225°C to 235°F), 0.05 parts corrosion inhibitor and 2 parts of an antiwear additive are added with continue milling. The penetration is adjusted to the range of 240 to 260 using ISO 170 polyol ester as necessary. Milling is stopped with continued recirculation of the mixture. When the temperature is 190°F, 1 part of a secondary antiwear additive, 4 parts oligomeric amine anti-oxidant, and 0.5 parts of a corrosion inhibitor are added. The batch is mixed for 25 to 35 minutes and adjusted .to a penetration range of 265 to 295 to complete the grease.
When preparing a lubricant to operate at high temperatures, it is important that the lubricant not only provide the desired viscosity properties at operating temperatures, but also provide improved thermal stability. Accordingly, incorporation of an additive package to protect oxidation and corrosion and boundary surface wear will result in a highly desirable lubricant. In addition, any polyol ester based lubricant must exhibit compatibility with materials it contacts.
The polyol ester lubricating fluid is prepared by placing the desired amount of polyol and carboxylic acid into a reaction vessel and conducting an esterification reaction to form the ester. The carboxylic acid component is present in the reaction mixture in an excess of about 5 to 10 weight percent for the amount of polyol. The excess carboxylic acid is used to force the reaction to completion. The excess is not critical to carrying out the reaction, except that the smaller the excess, the longer the reaction time. After the esterification reaction is complete, the excess acid is removed by stripping and refining. Generally, the esterification reaction is carried out in the presence of a conventional catalyst. For example, tin, titanium, zirconium or tungsten-based catalysts designed for high temperature systems are suitable. Uncatalyzed esterification may also be carried out.
High temperature lubricant fluids are prepared by mixing an additive package with the ester reaction product. The additive package includes at least the oligomeric amine antioxidant and the borated extreme pressure and antiwear agent together with a corrosion inhibitor. Additional additives such as an antifoam agent, detergents, hydrolytic stabilizers and metal deactivators may also be included.
In a preferred embodiment of the invention, the antioxidant is an oligomeric aromatic amine, such as the reaction product of alkylated diphenylamine, an alkylated phenylnaphthylamine and an ester base stock, in amounts between about 3 to 8 parts by weight based on 100 parts of fluid. The extreme pressure and antiwear agent is a borated compound selected from the group of borated amines, potassium tetraborate, borates of Group la alkali metals, borates of Group 2a alkaline earth metals, stable borates of transition metals, such as zinc, copper and tin, and boric acid. The borated agents may be included in amounts between about 1 to 4 parts by weight. A corrosion inhibitor, such as a benzotriazole may be added in minor amounts between about 0.01 to 0.10 parts by weight.
After mixing the selected polyol ester with the additive package, the lubricant fluid should have a viscosity at 40°C between about 120 to 400 cSt depending on the viscosity of the ester. The viscosity at 100°C should be between about 15 to 30 cSt. Preferably, the viscosity index is between about 80 to 130 the pour point is below about -15°C and the flash point is in excess of about 285°C. A polyol ester of desired viscosity can be prepared by blending a high viscosity polyol ester with a lower viscosity polyol ester.
The invention will be better understood with reference to the following examples. All percentages are set forth in percentages by weight, except where molar quantities are indicated. These examples are presented for purposes of illustration only, and are not intended to be construed in a limiting sense.

EXAMPLE 1

A dipentaerythritol hexaisononanoate ester was prepared in a reaction vessel equipped with a mechanical stirrer, thermocouple, thermoregulator, Dean-Stark trap, condenser, nitrogen sparge and vacuum source. The following materials were charged to the reactor:

INGREDIENT AMOUNT gms (moles)

Dipentaerythritol 1225 g (4.8m)

Isononanoic acid 5175 g (32.75m)
The reaction mixture was heated to 185°-190°C with agitation. The water-of-reaction was collected in and removed from the Dean-Stark trap. The temperature was gradually raised over 5-6 hours to about 230°C with application of vacuum to maintain reflux. This removed the reaction water and returned the acid collected in the trap to the reactor. These conditions were maintained to a point where the hydroxyl number of the reaction mixture was less than 3.0. The bulk of the excess acid was then removed by vacuum distillation together with nitrogen sparge and then residual acidity was removed with alkali.

The resulting product was dried and filtered to obtain 5000g of ester product having the following properties:

TABLE I

		Run 1	Run 2
Viscosity, cSt
	@ 100°C :	27	26
	@ 40°C :	403	395
Viscosity Index :		90	86
Pour Point, °C :		-18	-23
Flash Point, C.O.C., °C:		291	288
Total Acid No., mgKOH/g:		.02	.01
Density @ 15.6°C (60°F.), lb/gal		8.06	8.07

EXAMPLE 2

A dipentaerythritol ester having a viscosity of about 150 cSt at 40°C can be prepared in a vessel in the same manner as described for the ester in Example 1. In this case, the following materials are charged to the reactor:

INGREDIENT AMOUNT gms (moles)

Dipentaerythritol 1225 g (4.8m)

Heptanoic acid 597 g (4.59m)

Caprylic/capric acid 1094 g (7.06m)

Isononanoic acid 3283 g (20.78m)
The reaction mixture is heated to 185°-190°C with agitation. The water-of-reaction is collected in and removed from the Dean-Stark trap. The temperature is gradually raised over 5-6 hours to about 230°C with application of vacuum to maintain reflux. This removes the reaction water and returns the acid collected in the trap to the reactor. These conditions are maintained to a point where the hydroxyl number of the reaction mixture is less than 3.0. The bulk of the excess acid is then removed by vacuum distillation together with nitrogen sparge and then residual acidity was removed with alkali.
The resulting product is dried and filtered to obtain 5000g of an ester product having a viscosity at 40°C of about 150.

EXAMPLE 3

An additive package of an oligomeric amine antioxidant and borated antiwear agent in accordance with the invention was added to the polyol esters prepared in Example 1 to formulate the lubricating fluid. The esters of Runs 1 and 2 from Example 1 were formulated as follows:

TABLE II

	Lubricating Fluid
Component	Weight %
Polyol ester	92.95
Oligomeric amine antioxidant	3.00
Borate antiwear agent	2.00
Secondary antiwear agents	2.00
Corrosion inhibitor	0.05
Anti-foaming agent	5 ppm

The resulting lubricant fluids had the following physical properties:

TABLE III

		Run

3	Run 4
Viscosity, cSt
	@ 100°C :	25.6	25.3
	@ 40°C:	386	384
Viscosity Index :	86	87
Pour Point, °C :	-20 (-5)	-37 (-35)
Flash Point, C.O.C., °C(°F):	310(590)	310 (590)
Acid Valve, mgKOH/g:	.36	.38

EXAMPLE 4

In order for a lubricant fluid to be acceptable in the high temperature applications, it must have low volatility and not form deposits or varnish when exposed to high temperatures for extended periods of time. To test the high temperature volatility of the lubricant fluid, samples of the lubricant fluids of Example 2 were placed into an oven at high temperature for an extended period of time and the weight loss was measured.
In this test, 2 grams of lubricant fluid was placed in an aluminum weighing dish having an internal diameter of 70mm. The aluminum dish was placed in a ventilated oven at 288°C (550°F) and weighed after 5 hours.
The results are as follows: TABLE IV

Oven Evaporation Loss, 5 hours at 288°C

Run

3 Run 4

Evaporation Loss, wt% 44 42

EXAMPLE 5

In order for a lubricant fluid to be acceptable in the high temperature applications, it must have superior oxidative and thermal stability. To evaluate the high temperature oxidative stability the formulations of Example 2 were tested by the FED-STD-791 Method 5308, Oxidation and Corrosion Stability. In this test 100 ml of test oil is held at 425°F (218°C) for 72 hours in the presence of four different metals and a sparge of dry air. At the end of the test period, the test oil is evaluated for viscosity change, acidity change, and sediment/sludge formation. The test results are shown in Table V. TABLE V

425°F 72 hour OCS Test

Run

3 Run 4

% Viscosity Change +12.5 +7.4

Acid Value change, mgKOH/g 1.29 0.12

Sediment/sludge, mg/100ml 1.9 3.2

COMPARATIVE EXAMPLE 6

A polyol ester high temperature lubricant prepared following the procedures of Example 2 in U.S. Patent No. 6,436,881 was prepared with the following components.

TABLE VI

Lubricant		Parts by Weight
Polyol ester of		100.00
	Example 1-U.S.
	Patent 6,436,881 *
Viscosity index improver		5.60 (2.8% polymer)
Antioxidant		4.50
Extreme pressure and antiwear agents		2.25
Corrosion inhibitor		0.05
*Polyol Ester Base Stock:
	Dipentaerythritol	1225g (4.8m)
	Heptanoic acid	750g (5.77m)
	Caprylic/capric acid (acid no. 361.5)	750g (4.83m)
	Isononanoic acid	3500g (22.15m)

EXAMPLE 7

A lubricant fluid prepared in accordance with Example 3 was compared to the lubricant prepare in accordance with comparative Example 6 in a bench panel test. In this test, a stainless steel panel is electrically heated by means of two heaters which are inserted into holes in the panel. The temperature is monitored by means of a thermocouple. The panel is placed on a slight incline and heated to 540° F. The lubricant to be tested is dropped onto the heated panel for 20 hours in the presence of 0.1-0.2 lpm humid air flow. The characteristics are observed and shown in FIG 1. The lubricant contacts the panel near the top of the incline and is observed as a central dark band. The lubricant then tends to thin out as it travels towards the pointed end of the heated panel. It is along the oil-air-metal interface that the degradation of the lubricant is best observed.
The results of the panel test for the lubricants of Runs 3 and 4 prepared in accordance with the fluid of Example 2 showed essentially no degradation along the oil-air-metal interface. In comparison; the panel test results for a polyol ester lubricant in accordance with Example A in U.S. Patent No. 6,463,881 shows observable carbonization along the oil-air-metal interface.

EXAMPLE 8

A grease using the polyol ester of Example 1 is prepared in accordance with the invention as follows.
A standard grease manufacturing kettle having milling and recirculation capabilities is charged with 40 parts of ISO 400 grade polyol ester, 8.95 parts of an aromatic diisocyanate, 9.1 parts of a tallowalkylamine, and 0.95 parts of ethylene diamine (an alkyldiamine). The mixture is heated to 225° to 235°F and then add 0.5 parts water is added. Heating is continued to 375° to 385°F, then shut the heat off and the batch is agitated for 15 to 20 minutes.
Three parts of a boron containing extreme pressure agent is added and then the oil is milled with the cooling oil on for 3 to 4 hours minimum. After 1 to 1.2 hours of milling, the penetration of the grease is adjusted to a value of 240 to 260 by adding up to 22 parts of ISO 170 grade polyol ester while milling. When the mixture temperature is 225° to 235°F 0.05 parts of a corrosion inhibitor and 2 parts of an antiwear additive are added on and milling is continued. The penetration is adjusted to the range of 240 to 260 using ISO 170 polyol ester as necessary. Milling is stopped, but recirculation of the mixture is continued. When the temperature is 190°F maximum 1 part of a secondary antiwear additive is added with 4 parts polymeric amine anti-oxidant, and 0.5 parts of a corrosion inhibitor b. The batch is mixed for 25 to 35 minutes and then penetration is adjusted to the range of 265 to 295.
The grease prepared including the borated antiwear agent and aromatic amine antioxidant exhibits the desired high temperature properties sought herein. It will thus be seen that the objects set forth above, among those made apparent from the preceding description, are efficiently attained and, since certain changes may be made in the above composition of matter without departing from the scope of the invention, it is intended that all matter contained in the above description shall be interpreted as illustrative and not in a limiting sense.

Claims

A synthetic ester based lubricant fluid, comprising
(A) an ester base stock that is the reaction product of:
(i) pentaerythritol, and

(ii) a monocarboxylic acid mixture including at least 60 weight percent iso-C₉ acid; and

(B) an additive package comprising:
at least one oligomeric aromatic amine antioxidant which is the reaction product of alkylated diphenylamine, an alkylated phenylnaphthylamine and an ester base stock, and

at least one borated extreme pressure/antiwear agent;

wherein the additive package is present in an amount between 5 to 15 weight percent, based on the total weight of the fluid;
the lubricant fluid having a viscosity at 40°C of at least 120 cSt.
The additive package of claim 1 wherein the borated extreme pressure/antiwear agent is at least one of a borated amine, potassium tetraborate, borates of Group 2a alkaline earth metals, stable borates of transition metals, including zinc, copper and tin, and boric acid.
The synthetic lubricant fluid of claims 1-2, wherein the antioxidant is present in an amount between 1 to 10 weight percent, based on the total weight of the fluid.
The synthetic lubricant fluid of claims 1-2, wherein the borated antiwear agent is present in an amount between 0.1 to 5 weight percent based on the total weight of the fluid.
The synthetic lubricant fluid of claims 1-2, wherein the polyol is at least 85 weight percent pentaerythritol, preferably is about 98 percent pentaerythritol.
The synthetic lubricant fluid of claims 1-2, further including an effective amount of a corrosion inhibitor, preferably benzotriazole.
The synthetic lubricant fluid of claims 1-2, wherein the balance of the monocarboxylic acid is normal C₇ to C₁₀ acids and wherein the acid mixture is at least 66 percent iso-C₉ acid.
A synthetic ester base lubricant fluid, comprising
(A) an ester base stock that is the reaction product of:
(i) dipentaerythritol, and

(ii) a monocarboxylic acid mixture including at least about 60 weight percent iso-C₉ acid and straight chain C₇ and C_8-10 acids; and

(B) an additive package including
at least one oligomeric aromatic amine antioxidant which is the reaction product of alkylated diphenylamine, an alkylated phenylnaphthylamine and an ester base stock, and a borated extreme pressure/antiwear agent;
to yield the lubricant fluid having a viscosity at 40°C of at least 120 cSt;
wherein the lubricant fluid includes between 5 to 10 parts by weight of the additive package and 100 parts by weight of the desired polyol ester base stock.
The synthetic ester lubricant of claim 8, wherein the dipentaerythritol is at least 85 weight percent dipentaerythritol.
The synthetic ester lubricant fluid of claim 8, wherein the antioxidant is present in an amount between 3 to 8 weight percent, based on the total weight of the fluid.
The synthetic ester lubricant fluid of claim 8, wherein the borated antiwear agent is present in an amount between 0.5 to 4 weight percent based on the total weight of the fluid.
The synthetic ester lubricant fluid of claims 1 or 8, further including a thickening agent to form a grease, which thickening agent is preferably a polyurea.