LUBRICANTS FOR RECIPROCATING AIR COMPRESSORS

Abstract: Synthetic lubricants comprise 15 to 45 weight percent of an ester of a monohydric alcohol of 4 to 18 carbon atoms with an aromatic or alkane dicarboxylic acid having 4 to 18 carbon atoms blended with 85 to 55 weight percent of a polyether polyol having a number average molecular weight from 400 to 5000. The blends are preferably compounded with antioxidants, corrosion inhibitors, and metal deactivators to produce a superior lubricant for reciprocating air compressors. The compressors can be run for longer time intervals without downtime for valve maintenance.

Description: This invention concerns synthetic lubricants having as components a suitably inhibited blend of (1) an ester of a monohydric alcohol having 4 to 18 carbon atoms with one or more aromatic or alkane dicarboxylic acids and (2) one or more polyether polyols. Reciprocating air compressors having air cushioned valves are well known in the art. Using hydrocarbon lubricating oils to lubricate the pistons and piston rings of the foregoing air compressors and lubricate the bearings is well known. Due to the high temperature and pressure of the air, it has been found that these hydrocarbon oils break down, leave deposits, and prevent the valves from operating correctly in a relatively short time. This hydrocarbon oil breakdown requires manual repairs to clean the valves. One problem that results is carbon buildup on valves which can cause hot spots and air line fires. Synthetic esters made from dicarboxylic acids have been used to produce long-lasting compressor fluids, such as Anderol® 495 sold by Nuodex for rotary screw air compressors. The major component of Anderol® 495 is believed to be a dialkyl adipate. However, Anderol® 495 does not have sufficient high temperature viscosity for suitable lubrication of the pistons and cylinders of reciprocating air compressors. US-A-4,302,343 teaches that rotary screw air compressors can be lubricated with a blend of polyhydric alcohol esters and polyether polyols. However, these lubricants are relatively expensive and leave deposits on the valves of a reciprocating air compressor. Anderol® 500 (a dialkyl phthalate composition) is known to be useful in reciprocating air compressors. However, this synthetic ester has the disadvantage of having a high viscosity during start up at low temperatures. US-A-4,072,619 discloses polyesteralkylene glycol compositions wherein phenothiazine is incorporated into the alkylene glycols. These compositions, however, degrade in a relatively short time, i.e. 1000 hours. Synthetic lubricants comprising a major amount of a polyester and a minor amount of a polyglycol or monocapped polyglycol are known from GB-A-786950 (corresponding to DE-A-1057272); GB-A-860675 (corresponding to DE-A-1067553); GB-A-933,721; GB-A-986,066; and GB-A-1,162,818; however, these patented compositions are disclosed to be useful only in aircraft gas turbines where a different viscosity range is needed. GB-A-933721 (supra) discloses a lubricant composition consisting essentially of a blend of an ester mixture and one or more substantially water-insoluble polyoxyalkylene glycol esters. The ester mixture is formed as a result of the esterification of (1) one or more aliphatic monohydric C₈-C₁₃ alcohols; (2) one or more aliphatic C₆-C₁₀ dicarboxylic acids; and (3) one or more diols selected from ethylene glycol, propylene glycol, neopentyl glycol and polyethylene glycols having a molecular weight up to 500. The ethers are compounds of the formula RO(R₁O) n H, wherein R is C₁-C₁₀ alkyl, R₁ is ethylene or propylene and n is an integer and have a viscosity of 5-60 centistokes at 210°F (99°C). Although reference is made to an ether content of 2.5 to 85 weight percent, the preferred ether content is 0.5 to 25 weight percent and the exemplified compositions contain 5 to 18 weight percent. The compositions are intended to lubricate aviation gas turbine engines. Carswell et al (Lubrication Engineering, November, 1983, 584-589) discloses that a lubricant composition based on a polypropylene glycol and a pentaerythritol tetraester gives greatly extended useful life as a lubricant in rotary screw air compressors. A lubricant composition based on 24% disodecylazelate and 76% 1200 MW polypropylene glycol was rejected as an alternative because the ester was too volatile and was being depleted. Other esters rejected as too volatile include diisodecyladipate and di-2-ethylhexylazelate. It now has been found that a suitably inhibited blend of esters of aliphatic monohydric alcohols with one or more aromatic or alkane dicarboxylic acids has the required high temperature viscosity and stability to heat, air, and water to be improved lubricants for reciprocating air compressors. More specifically, the synthetic base lubricants of this invention have a lubricant composition comprising: (A) 15 to 45 weight percent of an ester of a monohydric alcohol having 4 to 18 carbon atoms with one or more aromatic or alkane dicarboxylic acids having 4 to 18 carbon atoms; and (B) 85 to 55 weight percent of one or more polyether polyols which have a flash point greater than 375°F (190°C) and which have the formula wherein: Z is the residue of a compound having 1 to 8 hydroxyl groups; R¹ is an alkylene radical having 2 to 4 carbon atoms; n is an integer which will give a number average molecular weight range from 400 to 5000 for the final compound; m is an integer having a value of from 1 to 8; and R² is hydrogen or an alkyl group of 1 to 6 carbon atoms. An additional aspect of the present invention comprises the above base lubricant with the addition of effective amounts of oxidation inhibitors, corrosion inhibitors, and metal or copper deactivators. A further aspect of the present invention comprises a method of lubricating air compressors using the inhibited lubricant. The neutral esters used in this invention are commercially available. Examples of suitable esters are the esters of monohydric alcohols having 4 to 18 carbons such as butanol, octanol, decanol, and others with aromatic dicarboxylic acids such as phthalic, terephthalic and isophthalic acids. Also useful are the esters of the above monohydric alcohols with alkanedioic acids having 4 to 18 carbons such as succinic, adipic, suberic, tetradecane-1,-14-dioic acid, and hexadecane-1,16-dioic acid. Examples of the polyether polyols or polyoxyalkylene polyols used in this invention are those derived from ethylene oxide, propylene oxide, 1-2 or 2-3 butylene oxide. The above oxides may be polymerized alone, i.e. homopolymerized,or in combination. The combined oxides may also be combined in a random or block addition. While some of the above compounds may be of a hydrophilic nature, those of a hydrophobic nature are preferred, such as those derived from propylene oxide, butylene oxides or combinations thereof. Examples of suitable capped polyoxyalkylene glycols are those derived from ethylene, propylene, and butylene oxides wherein the alkylene oxides are initiated from a compound having 1 to 8 active hydrogens in a known manner. The terminal hydroxyl groups may be further reacted with organic acids to form esters or with alkyl or aryl halides to form alkyl or aryl capped polyoxyalkylene glycols. These polyether polyols and their preparation are well known from the book "Polyurethanes" by Saunders and Frisch, Interscience Publishers (1962), pages 33-39. Examples of suitable initiator compounds which are employed to prepare the above polyether polyols are compounds having 1 to 8 active hydrogens such as, for example, water, methanol, ethanol, propanol, butanol, ethylene glycol, propylene glycol, butylene glycol, 1,6-hexanediol, glycerine, trimethylolpropane, pentaerythritol, sorbitol, sucrose, and mixtures thereof. Other initator compounds which are useful include monohydric phenols and dihydric phenols and their alkylated derivatives such as, for example, phenol, o -, m -, and p -cresol, guaiacol, saligenin, carvacrol, thymol, o - and p -hydroxy diphenyl, catechol, resorcinol, hydroquinone, pyrogallol, and phloroglucinol. The foregoing polyether polyols should have a flash point greater that 375°F (190°C) and preferably greater than 450°F (230°C). They also should have a number average molecular weight range from 400 to 5000, preferably in the range from 700 to 2500. The foregoing polyether polyols are blended to give a base lubricant composition containing 15 to 45 weight percent of the esters and 85 to 55 weight percent of the polyols, with the ranges 15 to 25 and 85 to 75 being the preferred ranges. respectively. The compositions of this invention are used in a reciprocating air compressor and are selected so as to have a viscosity in the range of 5 to 25 centistokes at 210°F (99°C) and preferably 6 to 16 centistokes at 210°F (99°C) and a pour point in the range of 0° to -65°F (-18 to -55°C). The final lubricant compositions of this invention may contain effective amounts of ashless additives, such as antioxidants, corrosion inhibitors, metal deactivators, lubricity additives, extreme pressure additives, dispersants, detergents, demulsifiers or other such additives as may be required. Examples of useful ashless antioxidants which can be used herein are phenyl naphthylamines, i.e., both alpha and beta-naphthyl amines; diphenyl amine; iminodibenzyl; p,p-dibutyldiphenylamine; p,p′-dioctyl-diphenylamine; and mixtures thereof. Other suitable antioxidants are hindered phenolics such as 6-t-butylphenol, 2,6-di-t-butylphenol and 4-methyl-2,6-di-t-butylphenol and the like. Examples of suitable ashless metal corrosion inhibitors are commercially available, such as Irgalube® 349 from Ciba-Geigy. This inhibitor compound is believed to be an aliphatic amine salt of phosphoric acid monohexyl ester. Other useful metal corrosion inhibitors are NA-SUL® DTA and NA-SUL® EDS from the White Chemical Company (diethylenetriamine dinonylnapthalene sulfonate and ethylenediamine dinonylnaphthalene sulfonate, respectively). Examples of suitable ashless copper metal deactivators, particularly cuprous metal deactivators, are imidazole, benzimidazole, pyrazole, benzotriazole, tolutriazole, 2-methylbenzimidazole, 3,5-dimethylpyrazole, and methylene bis-benzotriazole. An effective amount of the foregoing additives for use in a reciprocating air compressor is generally in the range from 0.1 to 5.0 percent by weight for the antioxidants, 0.1 to 5.0 percent by weight for the corrosion inhibitors, and 0.001 to 0.5 percent by weight for the metal deactivators. The foregoing weight percentages are based on the total weight of the polyether polyols and the esters. It is to be understood that more or less of the additives may be used depending upon the circumstances for which the final composition is to be used. The following preparation and examples are presented to illustrate but not limit the invention. Preparation A formulation consisting of the following blend was prepared. A) 11,489 pounds (5211 kg) (77.48%) of polypropylene glycol (number average molecular weight 1200) B) 2,872 pounds (1303 kg) (19.37%) of Mobil Ester DB-32(¹) C) 296 pounds (134 kg) (2.0%) of p,p′-dioctyl diphenylamine D) 148 pounds (67 kg) (1.0%) of Ciba-Geigy IRGALUBE® 349(²) E) 0.37 pounds (0.17 kg) (25 parts per million) of Dow Corning DC-200(³) F) 15 pounds (7 kg) (0.1%) of Mobil MOBILAD® C-402 (⁴) G) 7.4 pounds (3.4 kg) (0.05%) of Sherwin-Williams CORBRATEC® TT-100(⁵) Notes: (¹) diisooctyl adipate (²) an amine salt of phosphoric acid (corrosion inhibitor) (³) a silicone anti-foam (⁴) a high mol. wt. polyacrylate (demulsifier) (⁵) tolutriazole (copper deactivator) In a suitable vessel, the ester and additives were blended together. After sufficient agitation time, the ester/additive mixture was transferred to the vessel which holds the polyglycol. the mixture was heated to 80°C and agitated until the solution was clear. If the additives are ignored, the formulation contains 20 percent by weight of the ester and 80 percent by weight of the polypropylene glycol. The above fluid was tested for corrosion resistance in accordance with ASTM D-665 - procedure A and ASTM D-665 - procedure B. The fluid passed both tests. The fluid was found to have the following characteristics: Examples 1-14 The above fluid was placed in fourteen reciprocating air compressors made by different manufacturers. The valves in each compressor were checked intermittently over a long period of time as shown in Table I. The valves were found to be in excellent condition having no deposits or residues. The same compressors using petroleum oils had service deposits after 1000 to 4000 hours of operation which created reduced performance with the possibility of line fires. It will be appreciated that the invention includes lubricant compositions in which a mixture of esters and/or a mixture of polyether polyols of the specified classes are used instead of a single ester and polyether polyol as exemplified above.

Claims: Claims for the following Contracting States : BE, DE, FR, GB, IT, NL, SE The use as a base lubricant in a reciprocating air compressor of a composition comprising: (A) 15 to 45 weight percent of at least one ester of a monohydric alcohol having 4 to 18 carbon atoms with at least one aromatic or alkane dicarboxylic acid having 4 to 18 carbon atoms; and (B) 85 to 55 weight percent of at least one polyether polyol which has a flash point greater than 375°F (190°C) and which has the formula wherein: Z is the residue of a compound having 1 to 8 hydroxyl groups; R¹ is an alkylene radical having 2 to 4 carbon atoms; n is an integer which will give a number average molecular weight range from 400 to 5000 for the final compounds; m is an integer having a value of from 1 to 8; and R₂ is hydrogen or an alkyl group of 1 to 6 carbon atoms. A use as claimed in Claim 1, wherein the weight percent of said ester ranges from 15 to 25 and the weight percent of said polyol compound ranges from 75 to 85. A use as claimed in Claim 1 or Claim 2, wherein said ester is an ester of a monohydric alcohol with an alkanedioic acid. A use as claimed in Claim 1 or Claim 2, wherein said ester is an ester of a monohydric alcohol with an aromatic dicarboxylic acid. A use as claimed in any one of the preceding claims, wherein said polyol has a number average molecular weight range from 700 to 2500. A use as claimed in any one of the preceding claims, wherein said polyol is a polyoxyalkylene glycol. A use as claimed in Claim 6, wherein said polyoxyalkylene glycol is a homopolymer. A use as claimed in Claim 6, wherein said polyoxyalkylene glycol is a random copolymer. A use as claimed in Claim 6, wherein said polyoxyalkylene glycol is a block copolymer. A use as claimed in Claim 6, wherein said glycol is polypropylene glycol having a number average molecular weight of 1200. A use as claimed in Claim 10, which comprises 20 weight percent of said ester and 80 weight percent of said polypropylene glycol. The use of a lubricant in a reciprocating air compressor of a base lubricant as defined in any one of the preceding claims, containing one or more ashless additives. A use as claimed in Claim 12, wherein said ashless additives include: (A) 0.1 to 5.0 weight percent of an aromatic amine antioxidant; (B) 0.1 to 5.0 weight percent of a metal corrosion inhibitor; and (C) 0.001 to 0.5 weight percent of a cuprous metal deactivator. A use as claimed in Claim 12 or Claim 13, wherein said ashless additives include p,p'-dioctyldiphenyl amine, an aromatic amine salt of phosphoric acid monoester, and tolutriazole. A method of lubricating a reciprocating air compressor, wherein said compressor is continuously run for long time intervals without downtime for valve maintenance which comprises using as the lubricant a composition as defined in any one of the preceding claims. A lubricating composition for a reciprocating air compressor comprising a base lubricant as defined in any one of Claims 1 to 14 with the proviso that the base lubricant does not comprise 24% diisodecylazelate and 76% polypropylene glycol having a number average molecular weight of 1200. Claims for the following Contracting State : ES A method of lubricating a reciprocating air compressor, wherein said compressor is continuously run for long time intervals without downtime for valve maintenance which comprises using as the lubricant a composition having a base lubricant comprising: (A) 15 to 45 weight percent of at least one ester of a monohydric alcohol having 4 to 18 carbon atoms with at least one aromatic or alkane dicarboxylic acid having 4 to 18 carbon atoms; and (B) 85 to 55 weight percent of at least one polyether polyol which has a flash point greater than 375°F (190°C) and which has the formula wherein: Z is the residue of a compound having 1 to 8 hydroxyl groups; R¹ is an alkylene radical having 2 to 4 carbon atoms; n is an integer which will give a number average molecular weight range from 400 to 5000 for the final compounds; m is an integer having a value of from 1 to 8; and R₂ is hydrogen or an alkyl group of 1 to 6 carbon atoms. A method as claimed in Claim 1, wherein the weight percent of said ester ranges from 15 to 25 and the weight percent of said polyol compound ranges from 75 to 85. A method as claimed in Claim 1 or Claim 2, wherein said ester is an ester of a monohydric alcohol with an alkanedioic acid. A method as claimed in Claim 1 or Claim 2, wherein said ester is an ester of a monohydric alcohol with an aromatic dicarboxylic acid. A method as claimed in any one of the preceding claims, wherein said polyol has a number average molecular weight range from 700 to 2500. A method as claimed in any one of the preceding claims, wherein said polyol is a polyoxyalkylene glycol. A method as claimed in Claim 6, wherein said polyoxyalkylene glycol is a homopolymer. A method as claimed in Claim 6, wherein said polyoxyalkylene glycol is a random copolymer. A method as claimed in Claim 6, wherein said polyoxyalkylene glycol is a block copolymer. A method as claimed in Claim 6, wherein said glycol is polypropylene glycol having a number average molecular weight of 1200. A method as claimed in Claim 10, wherein said composition comprises 20 weight percent of said ester and 80 weight percent of said polypropylene glycol. A method claimed in any one of the preceding claims, wherein said composition further contains one or more ashless additives. A method as claimed in Claim 12, wherein said composition contains: (A) 0.1 to 5.0 weight percent of an aromatic amine antioxidant; (B) 0.1 to 5.0 weight percent of a metal corrosion inhibitor; and (C) 0.001 to 0.5 weight percent of a cuprous metal deactivator. A method as claimed in Claim 12 or Claim 13, wherein said ashless additives include p,p'-dioctyldiphenyl amine, an aromatic amine salt of phosphoric acid monoester, and tolutriazole. A process for preparing a lubricating composition, which comprises preparing in known manner (A) at least one ester of a monohydric alcohol having 4 to 18 carbon atoms with at least one aromatic or alkane dicarboxylic acid having 4 to 18 carbon atoms; and (B) at least one polyether polyol which has a flash point greater than 375°F (190°C) and which has the formula wherein: Z is the residue of a compound having 1 to 8 hydroxyl groups; R¹ is an alkylene radical having 2 to 4 carbon atoms; n is an integer which will give a number average molecular weight range from 400 to 5000 for the final compounds; m is an integer having a value of from 1 to 8; and R₂ is hydrogen or an alkyl group of 1 to 6 carbon atoms; and subsequently co-mingling or mixing said components in the proportions of 15 to 45 weight percent (A) and 85 to 55 weight percent (B) with the proviso that the base lubricant does not comprise 24% diisodecylazelate and 76% polypropylene glycol having a number average molecular weight of 1200. The use as a base lubricant in a reciprocating air compressor of a composition as defined in any one of Claims 1 to 14.

Inventors: APPLEMAN, THOMAS J., MCGRAW, PHILIP W., WARD, ELDON L.
Applicants: THE DOW CHEMICAL COMPANY
Application year: 1986
Main classification: C10M111
IPC codes: C10M111, C10M171