WO2019005767A1 - Antioxidant composition for polyalkylene glycols - Google Patents

Abstract

The effectiveness of a diarylamine antioxidant in a polyalkylene glycol base stock is improved by using an effective amount of a trialkanolamine antioxidant enhancing agent.

Description

ANTIOXIDANT COMPOSITION FOR POLYALKYLENE GLYCOLS

Field

The present disclosure relates to a polyalkylene glycol composition. The disclosure more particularly relates to such compositions comprising certain antioxidants and amines.

Background

Polyalkylene glycols (PAGs) often provide lubricant formulations with improved performance properties, including high viscosity index, high load carrying capabilities and deposit control. However, due to their increased susceptibility to oxidative decomposition relative to hydrocarbon fluids, PAGs require more robust antioxidant packages. To date, readily available antioxidants have proven inadequate for stabilizing PAGs in applications with high temperatures and/or challenging oxidation conditions, e.g. use in gear oils or engine oils.

It would be desirable to have a PAG lubricant composition with improved antioxidant performance and improved stability.

Summary

The composition of this disclosure is such a composition, and comprises a PAG base stock, an effective amount of a diarylamine antioxidant, and an effective amount of a trialkanolamine antioxidant enhancing agent.

Surprisingly, the lifetime of a diarylamine antioxidant in a PAG lubricant is greatly extended as a result of the inclusion of a trialkanolamine as described herein, thereby enabling significant improvement in the useful lifetime of P AG-containing lubricants.

Brief Description of the Drawings

Fig. 1 shows the change in weight fraction of an antioxidant in four lubricant compositions over time.

Detailed Description

The composition of the disclosure comprises a PAG, an antioxidant, and an antioxidant enhancing agent. The composition of the disclosure exhibits improved antioxidant lifetime, which leads to improved lubricant composition lifetime. Improved antioxidant lifetime is determined by measuring the concentration of the antioxidant in the composition over time and comparing that concentration to a comparative composition that has no antioxidant enhancing agent. A composition that maintains an antioxidant concentration over a longer time than the comparative composition has an improved lifetime.

As used herein, the term "an effective amount" refers to an amount that is sufficient to accomplish the purpose of a given component of the composition. For example, an effective amount of an antioxidant is an amount that, during the use of the composition, is sufficient to provide antioxidant properties or functionality to the composition in which the antioxidant is employed. As a further example, an effective amount of an antioxidant enhancing agent is an amount that, during the use of the composition, is sufficient to extend the useful lifetime of the antioxidant properties or functionality to the composition in which the antioxidant and the agent are employed. As used herein, the term "major amount" means more than 50 wt.%, based on 100 wt.% of a composition.

As used herein, the terms "a," "an," "the," "at least one," and "one or more" are used interchangeably. The terms "comprises" and "includes" and variations thereof do not have a limiting meaning where these terms appear in the description and claims. Thus, for example, "a" material can be interpreted to mean "one or more" materials, and a composition that "includes" or "comprises" a material can be interpreted to mean that the composition includes things in addition to the material.

Unless stated to the contrary, implicit from the context, or customary in the art, all parts and percentages are based on weight and all test methods are current as of the filing date of this disclosure.

The composition of the disclosure comprises a major amount of a polyalkylene glycol base stock, an effective amount of a diarylamine antioxidant, and an effective amount of a trialkanolamine antioxidant enhancing agent.

Suitable polyalkylene glycol fluids include homopolymers and copolymers of alkylene oxides. The PAG may be a polyalkylene glycol or modified polyalkylene glycol. Especially suitable are homopolymers and copolymers of ethylene oxide, butylene oxide and propylene oxide. In embodiments of this disclosure wherein the poly ether is a modified polyalkylene glycol, the modified polyalkylene glycol is an end-capped polyalkylene glycol. The end-capped polyalkylene glycol preferably includes a non- reactive end-capping moiety selected from a group consisting of a) an alkyl ether, the alkyl ether having an alkyl moiety that contains from one to 30 carbon atoms, b) an aromatic ether, c) an ester, and d) a sterically hindered active hydrogen, hydrocarbyl or hydrocarboxy group. Mixtures of PAGs may be employed.

PAGs suitable for use in the composition of this disclosure are, in some nonlimiting embodiments, selected from homopolymers and random and block copolymers prepared by alkoxylating an alcoholic initiator. Examples of alcoholic initiators include monoalcohols, diols, e.g. glycols, triols and the like. The alkoxylation may be based on alkylene oxides such as, for example, ethylene oxide, propylene oxide, butylene oxide and/or mixtures thereof. Examples of commercially available PAGs include, for example, UCON LB fluids, UCON HB fluids, UCON OSP fluids, and SYNALOX fluids. In one embodiment, the overall alkylene oxide unit content of the PAG preferably ranges from 30 wt.% to 95 wt.% ethylene oxide units based on the total PAG weight, the remainder being propylene oxide units. The ethylene oxide unit content more preferably ranges from 40 wt.% to 85 wt.%, and still more preferably from 45 wt.% to 70 wt.%, based on the total PAG weight, the remainder being propylene oxide units. In another embodiment, the overall butylene oxide unit content preferably ranges from 30 wt.% to 95 wt.%, based on the total PAG weight, the remainder being propylene oxide units. The butylene oxide unit content more preferably ranges from 40 wt.% to 85 wt.%, and still more preferably from 45 wt.% to 70 wt.%, based on the total PAG weight, the remainder being propylene oxide units. In another embodiment, the overall propylene oxide unit content is 100 % based on the total PAG weight. The PAG may be initiated using initiators that are monols, diols, triols, tetrols, higher polyfunctional alcohols, or combinations thereof. Nonlimiting examples of initiators include n-butanol, dodecanol, monoethylene glycol, diethylene glycol, monopropylene glycol, dipropylene glycol and glycerol. Mixtures of PAGs may be employed.

By way of illustration, but not by limitation, preparation of a suitable PAG may be by any means or method known to those skilled in the art. For example, ethylene oxide and propylene oxide may be polymerized to form random PAG copolymers by simultaneous addition of the oxides to an initiator such as ethylene glycol, butanol or propylene glycol using, for example, a base catalyst, such as potassium hydroxide, to facilitate the polymerization.

PAGs are well-known and many are commercially available. One may purchase a PAG base stock. For example, SYNALOX and UCON lubricant fluids are available from The Dow Chemical Company. In one embodiment of the disclosure, the base stock is of lubricating viscosity. In some nonlimiting embodiments, a PAG having a kinematic viscosity (KV) in the ISO viscosity range of 22 to 1000, that is, a KV of from 22 cSt to 1,000 cSt at 40 °C, may be particularly effective, although a viscosity ranging from 32 cSt to 68 cSt at 40 °C may be selected for many applications. It may also be desirable to select a copolymer PAG base stock that is water soluble, rather than water insoluble, as a water soluble base fluid may provide improved friction control in certain applications.

The kinematic viscosity of the base stock advantageously is in the range of about 10 cSt to about 3000 cSt at 40 °C depending, of course, upon the intended use of the ultimate composition. For example, for gearboxes the preferred kinematic viscosity of the fluid will be in the range of about 32 to about 3200 cSt at 40 °C.

The lubricant composition of the disclosure has a viscosity index or VI, determined as detailed below, that preferably lies above 120, more preferably above 140 and, still more preferably, above 150. VI's in excess of 400, while known, are rare. Skilled artisans recognize that VI indicates how a lubricant viscosity changes with temperature. For example, a low VI, e.g. 100, suggests that fluid viscosity will vary considerably when it is used to lubricate a piece of equipment that operates over a wide range of temperatures, such as from 20 °C. to 100 °C. Skilled artisans also recognize that as VI increases, lubricant performance also tends to improve. Based upon that recognition, skilled artisans prefer higher VI values, e.g. 150, over lower VI values, e.g. 100. For purposes of comparison, typical base stock VI ranges are as follows: API Group I II and III base stocks have a VI range of 80-140; API Group IV polyalphaolefins have Vis of 120 for PAO 4 to 170 for PAO 100; synthetic esters have Vis of 120 to 200 and polyalkylene glycols have Vis of 120 to 300.

The diarylamine antioxidant serves to decrease the rate of oxidation of the

PAG base stock. In one embodiment of the disclosure, the diarylamine antioxidant is an alkylated or nonalkylated diarylamine, or a nonalkylated or alkylated phenyl alpha- naphthylamine. In one embodiment of the disclosure, the antioxidant is a diphenylamine of the formula:

where Rl and R2 are independently H or alkyl of from 2 to 20 carbon atoms. It is preferred that Rl and R2 are independently alkyl groups of from 4 to 8 carbon atoms. In one embodiment of the disclosure, the antioxidant is an alkylated or nonalkylated phenyl alpha-naphthylamine of the formula:

where R3, R4, and R5 are independently H or alkyl of from 2 to 20 carbon atoms. It is preferred that R3, R4, and R5 are independently alkyl groups of from 4 to 8 carbon atoms.

Diarylamine antioxidants are well-known to those skilled in the art and many are commercially available. Examples of diarylamine antioxidants include, for example, IRGANOX L 57, IRGANOX L 67, IRGANOX L 06, IRGANOX L 01, NAUGALUBE APAN, NAUGALUBE 438, NAUGALUBE 438L, NAUGALUBE 640, VANLUBE NA, VANLUBE SS, VANLUBE 81 , NAUGARD PANA, ADDITIN 7130 and VANLUBE 961.

The antioxidant enhancing agent serves to improve the effectiveness of the antioxidant. The antioxidant enhancing agent is a trialkanolamine, and preferably is triisopropanolamine or triethanolamine. Trialkanolamines are well-known to those skilled in the art and many are commercially available.

In various embodiments, the weight ratio of antioxidant to antioxidant enhancing agent is in the range of 3: 1 to 1 :3, or from 2: 1 to 1 :2, or about 1 : 1.

In one embodiment of the disclosure, the amount of the diarylamine antioxidant in the composition of the disclosure is from 0.1 wt.% to 5.0 wt.%, preferably about 0.5 to 4 wt.% and more preferably from 0.75 to 1.5 wt.%. The amount of the antioxidant enhancing agent in the composition of the disclosure is from 0.1 to 10 wt.%, or from 0.2 to 5 wt.%, or from 0.5 to 3 wt.%. All compositional ranges are based on 100 wt.% of the composition. The base stock forms the balance of the composition. In one embodiment, the amount of PAG ranges from 50 wt.% to 99 wt.%, or to 70 wt.%, or to 80 wt.%, or to 90 or 95 wt.%.

In one embodiment, the PAG is the only base stock. In one embodiment, in addition to the PAG, the lubricant composition may also include one or more conventional lubricants in addition to components specified above. Examples of conventional nonPAG base stocks are those in API Groups I to V. These are well-known to those skilled in the art and many are commercially available. In one embodiment, the conventional base stock is at least partially soluble in the PAG.

The lubricant composition may also include one or more conventional lubricant additives in addition to components specified above. Example of such additives include antifoamers such as polymethylsiloxanes, demulsifiers, oil-soluble copper compounds, corrosion inhibitors including, as examples, ferrous corrosion inhibitors, copper corrosion inhibitors and/or metal deactivators; pour point depressants, detergents such as calcium or magnesium overbased detergents, dyes, friction modifiers (e.g.

molybdenum dithiocarbamate, glycerol mono-oleate, UCON™ OSP fluids), phosphorus and sulfur containing extreme pressure/antiwear (EP/AW) additives, viscosity index improvers (e.g. olefin copolymers, polymethacrylates), dispersants (e.g. polyisobutylene succinimides), combinations thereof, and the like. The conventional lubricant additives, if present, typically range from 100 parts per million parts by weight ("ppmw') of the lubricant composition to 10 wt.%, based upon total lubricant composition weight. Many additives are well-known to those skilled in the art and are commercially available.

Examples of extreme pressure/antiwear (EP/AW) additives include alkyl- and aryl phosphate esters including mono-, di- and tri- phosphate esters and the amine salts of mono- and di- ester phosphates. DURAD 310M is an example an aryl phosphate ester, IRGALUBE 349 is an example of an amine phosphate. Esters of phosphorothionate such as IRGALUBE TPPT are also useful. Sulfurized olefins, esters, and fats are useful extreme pressure additives. Chlorinated paraffins and fatty acids can be used to provide EP properties. Zinc dialkyldithiophosphates (ZDDP) are also useful for anti-wear and as secondary antioxidants. Examples of yellow metal corrosion inhibitors include tolutriazole and 1H- Benzotriazole-l-methanamine, N,N-bis(2-ethylhexyl)-ar-methyl- (IRGAMET 39), benzotriazole and mercaptobenzothiazole. Examples of sulfur scavengers include dimercaptothiadiazole derivatives (King Industries K-CORR NF 410).

Examples of ferrous corrosion inhibitors include calcium

alkylnaphthalenesulfonate/carboxylate complex (Na Sul Ca 1089 from King Industries), and carbonated basic barium dinonylnaphthalenesulfonate (Na Sul 611).

The composition of the disclosure advantageously may be employed as a lubricant and may be used in the form of a liquid. In one embodiment, the composition of the disclosure is free of added phosphate esters.

This disclosure also includes a process for improving the lifetime of a diarylamine antioxidant in a PAG base stock, the process comprising admixing an effective amount of a trialkanolamine with the PAG base stock and the diarylamine antioxidant, and optionally one or more additives. The relative amounts of components, e.g. PAG base stock, nonalkylated or alkylated diarylamine antioxidant, antioxidant enhancing agent, and additives is discussed hereinabove. As shown in the following specific embodiments, the effective life of the PAG is improved by the addition of the antioxidant enhancing agent. Diarylamine antioxidant, and thus PAG, lifetime is determined by the modified D943 method, described hereinbelow, as determined by the time that the antioxidant

concentration becomes nondetectable, i.e. the antioxidant concentration as measured by the Reverse-Phase High Pressure Liquid Chromatography (HPLC) method described hereinbelow.

Specific Embodiments

Several PAG fluid formulations are prepared using the UCON HB or UCON LB fluid listed in Table 1 and are then tested according to a modified version of ASTM

D943, using T = 90°C, air bubbling, condensers and a copper-steel coil as described below.

Viscosity Test Method

Determine kinematic viscosity, in centistokes (cSt) and its metric equivalent, either square millimeters per second (mm²/sec) or lxlO^"6 square meters per second, at 40 °C. and 100 °C. using a Stabinger viscometer in accord with American Society for Testing and Materials (ASTM) D7042. Use the kinematic viscosities to calculate a VI in accord with ASTM D2270.

Pour Point Test Method

Measure lubricant pour point in accord with ASTM D97-87. Lubricant compositions of the present disclosure have a pour point (e.g. a temperature at which an oil ceases to flow) that is preferably -10 °C. or less, more preferably -15 °C. or less, even more preferably -20 °C. or less, still more preferably, -25 °C. or less and most preferably -27 °C. or less. The phrase "or less" means lower in temperature. For example, -15 °C. is less than -10 °C.

Oxidation Test Method: ASTM D943 Standard Test Method for Oxidation Characteristics of Inhibited Mineral Oils is modified as follows:

a) T = 90 °C for this modified method vs 95 °C.

b) D943 calls for Ch to be bubbled through the liquid. Compressed building air is bubbled instead of Ch.

c) D943 calls for water to be added to the lubricant at the beginning of the test. No water is added to the fluids in this modified method.

d) Cu-Steel catalyst coils are used as received from the supplier.

e) Samples are taken at intervals of 168 hours.

Because PAG-based fluids undergo rapid degradation once antioxidants are fully consumed, measuring antioxidant stability in a PAG-based fluid is an indirect means of measuring the fluid's expected thermo-oxi dative stability /lifetime.

Table 1 - Raw Materials

The antioxidant concentration is measured by Reverse-Phase High Pressure Liquid Chromatography (HPLC) using an Agilent 1100 HPLC system equipped with a Waters Atlantis dcl8 HPLC column (4.6 x 150 mm, 3 micron particles) and a diode array detector (DAD, detection at 287, 280, 260, and 253 nm). A mixed solvent system of 0.1 wt.% formic acid in deionized water and 0.1% formic acid in acetonitrile is used, with a gradient elution program (increasing from 0% to 100% of the 0.1 wt.% formic acid in acetonitrile solution) and a flow rate of 1.0 mL/min. Samples are prepared by treating 100 mg of the experimental lubricant with 5.0 mL acetonitrile and filtering the solution through a 0.2 micron PTFE syringe filter. Comparative Examples 1 and 2 - (Not embodiments of the disclosure)

UCON™ LB-285 fluid and UCON™ 50-HB-260 fluid containing 0.5 wt.% antioxidant (AO) (IRGANOX L57) are each tested according to the test methods described hereinabove for diarylamine antioxidant concentration. The weight fraction (WF) of antioxidant (AO) is calculated using HPLC data using the following equation:

_ Cmtc iifXitton of AO found in 8®&φΙ@ &£ Ums i

Cmwentrntton of AO found to sampk at time t = 0

The results are summarized in Figure 1.

Examples 1 and 2

Comparative Example 1 is repeated except that 1.0 wt.%

triisopropanolamine is added to the fluids. The results are summarized in Figure 1.

As shown in Figure 1, the inclusion of triisopropanolamine greatly increases the lifetime of the IRGANOX L57 antioxidant, leading to reduced rates of PAG oxidation. Increased antioxidant lifetime is indicative of improved PAG thermo-oxidative stability.

Comparative Example 3 - (Not an embodiment of the disclosure)

Comparative Example 1 is repeated except that the antioxidant is IRGANOX

L06. The results are summarized in Table 2.

Examples 3 and 4

Comparative Example 3 is repeated except that 1.0 wt.%

triisopropanolamine (TIP A) or 1.0 wt.% triethanolamine (TEA) are included in the fluid. The results are summarized in Table 2.

Table 2

*AO wt fraction reaches zero at 168 hr.

As shown in Table 2, the inclusion of either triisopropanolamine or triethanolamine increases the lifetime of the IRGANOX L06 antioxidant.

Comparative Example 4 (Not an embodiment of the disclosure) and

Examples 5 - 8

Comparative Example 1 is repeated except that the antioxidant is IRGANOX L57 in the presence of 0.0 (C.E. 4), 0.5 (Ex. 5), 1.0 (Ex. 6), or 3.0 (Ex. 7) wt.%

triisopropanolamine, or 1.0 wt.% triethanolamine (Ex. 8). The results are summarized in Table 3.

Table 3

As shown in Table 3, triisopropanolamine increases the lifetime of the IRGANOX L57 antioxidant when included at 0.5, 1.0 and 3.0 wt.%. Similarly, 1.0 wt.% triethanolamine also increases the antioxidant lifetime. In Table 3, the lifetime testing is stopped at 1,344 hours, when the antioxidant wt. fraction of C.E. 4 reaches zero. Comparative Examples 5 and 13 (Not embodiments of the disclosure) and Examples 9 - 12

Comparative Example 4 is repeated except that the LB-285 fluid is replaced with the UCON™ OSP-68 fluid and the test is continued for a longer period of time. The results are summarized in Table 4. For Comparative Example 13, Example 10 is repeated, except that the IRGANOX L57 antioxidant is excluded.

Table 4

*not measured

As shown in Table 4, triisopropanolamine increases the lifetime of the

IRGANOX L57 antioxidant when included at 0.5, 1.0 and 3.0 wt.%. Of the samples treated with triisopropanolamine, the sample treated with the highest initial concentration (Ex 11) contains the highest remaining AO weight fraction after 4872 hours. Similarly, 1.0 wt.% triethanolamine also increases the antioxidant lifetime. After 4872 hours, the sample treated with triethanolamine contains a higher remaining AO weight fraction than all of the triisopropanolamine-treated samples.

In Comparative Example 13, within 1 week of testing, the OSP-68 fluid undergoes rapid oxidative decomposition, indicating that triisopropanolamine is not an effective fluid stabilizer in the absence of the IRGANOX L57 antioxidant.

Comparative Examples 6 - 8

UCON™ LB-285 fluid containing 0.5 wt.% Irganox L57 antioxidant in the presence of a hindered alkylamine (PRIMENE™ 81R, 1.0 wt.%) or two polyetheramines (JEFF AMINE D230 or JEFF AMINE M600, 1.0 or 3.0 wt.%, respectively) are subjected to the oxidation lifetime test described hereinabove. The results are summarized in Table 5. Table 5

As shown in Table 5, a hindered alkylamine and two polyetheramines fail to increase the lifetime of the IRGANOX L57 antioxidant. The results achieved with triisopropanolamine and triethanolamine are surprisingly superior.

The data in Table 2 shows, by comparing Example 3 to C.E. 3, that the composition of Ex. 3 is about 10 times more stable than the composition of C.E. 3.

Similarly, the data in Table 3 shows a 13X improvement in stability.

The comparative lifetime of base stocks with antioxidant only versus base stocks with antioxidant and trialkanolamine can be calculated from the following equation using the antioxidant concentration data from the HPLC method:

Comparative lifetime = (time for AO in base stocks with AO and alkanolamine to be nondetectable) / (time for AO in base stocks with AO to be

nondetectable)

If the antioxidant data from base stocks with antioxidant and trialkanolamine are detectable then the estimated remaining lifetime of the mixture can be calculated from:

. _ Time of last AQ analysts

Remaining life - - -r

1—- . W.,.F„. AO at im _^ i— :——

w of last mtal sts

where Remaining life is substituted in the numerator of the Comparative life time equation.

As shown by the preceding examples and comparative examples, the lifetime of P AG-based lubricants containing diarylamine antioxidants is surprisingly improved by the addition of triisopropanolamine and triethanolamine as antioxidant enhancing agents.

Claims

WHAT IS CLAIMED IS:

1. A lubricant composition comprising a PAG base stock, an effective amount of a diarylamine antioxidant, and an effective amount of a trialkanolamine antioxidant enhancing agent.

2. The composition of claim 1 wherein the amount of trialkanolamine antioxidant enhancing agent is from 0.1 to 10 wt.%, and the amount of diarylamine antioxidant is from 0.2 to 5 wt.% of the composition, wherein the total wt.% of the composition is 100 wt.%.

3. The composition of any of the preceding claims comprising from 0.2 to 5 wt.% antioxidant, from 0.1 to 10 wt.% trialkanolamine antioxidant enhancing agent, from 0 to 5 wt.% of an additive, with the balance being the PAG base stock wherein the total wt.% of the composition is 100 wt.%.

4. The composition of any of the preceding claims wherein the amount of trialkanolamine antioxidant enhancing agent is from 0.2 to 5 wt.% or from 0.5 to 3 wt.%.

5. The composition of any of the preceding claims wherein the trialkanolamine antioxidant enhancing agent is triisopropanolamine or triethanolamine.

6. The composition of any of the preceding claims wherein the trialkanolamine antioxidant enhancing agent is triisopropanolamine.

7. The composition of any of claims 1 to 5 wherein the trialkanolamine antioxidant enhancing agent is triethanolamine.

8. A process for improving the lifetime of a diarylamine antioxidant in a PAG base stock, the process comprising admixing an effective amount of a trialkanolamine antioxidant enhancing agent with the PAG base stock and the antioxidant, and optionally one or more additives.

9. The process of claim 8 wherein the amount of the trialkanolamine antioxidant enhancing agent is from 0.1 to 10 wt.%, or from 0.2 to 5 wt.%, or from 0.5 to 3 wt.%, wherein the total wt.% of the composition is 100 wt.%.

10. The process of any of claims 8-9 wherein the trialkanolamine is triisopropanolamine or triethanolamine.