Purdue University
`Purdue e-Pubs
`
`International Compressor Engineering Conference
`
`School of Mechanical Engineering
`
`1998
`
`The Development of Lubricants for Automotive A/
`C Systems
`W. L. Brown
`Union Carbide Corporation
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`Follow this and additional works at: http://docs.lib.purdue.edu/icec
`
`Brown W. L. "The Development of Lubr cants for Automot ve A/C Systems" (1998). International Compressor Engineering Conference.
`Paper 1247.
`http://docs.l b.purdue.edu/ cec/1247
`
`Th s docu e as bee ade ava ab e
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`oug Pu due e-Pubs, a se v ce o e Pu due U ve s y L b a es. P ease co ac epubs@pu due.edu o
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` a d o CD-ROM d ec y o e Ray W. He
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`ck Labo a o es a ttps://e g ee
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`g.pu due.edu/
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`Page 1 of 7
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`Arkema Exhibit 1117
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`THE DEVELOPMENT OF LUBRICANTS FOR AUTOMOTIVE A/C SYSTEMS
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`William L. Brown
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`Union Carbide Corporation
`771 Old Saw Mill River Road
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`Tarrytown, NY
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`ABSTRACT
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`The lubricant is a critical component of an automotive A/C system. Besides providing good
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`lubricity, it must exhibit sufficient solubility in the refrigerant to ensure lubricant return to the compressor.
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`Stability in the presence of the refrigerant, metals, and common contaminants such as air, water, and
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`mineral oil is critical. The lubricant must also be compatible with the hoses and other elastomeric parts.
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`lubricants for use in
`This paper will focus on the development of polyalkylene glycol (PAG)
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`automotive A/C systems.
`it will review the properties of PAG lubricants that has led to their use with R-
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`134a, including their good solubility, stability, and lubricity. The tradeoffs between competing properties
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`such as lubricity and high temperature solubility will also be discussed.
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`INTRODUCTION
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`in 1987 the Montreal Protocol set in motion a program to eliminate the use of chlorofluorocarbons
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`because of their adverse affect on the earth's ozone layer. The automotive industry chose refrigerant R-
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`134a, a hydrofluorocarbon, as a replacement
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`R-134a was chosen because of
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`thermodynamic similarities to Fl-12 and because it is nonflammable and exhibits low toxicity.
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`In automotive air
`A major problem with Fl-134a was its poor solubility in mineral oil lubricants.
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`conditioners, the lubricant travels through the system with the refrigerant.
`In order to ensure that sufficient
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`amounts of lubricant return to the compressor, the lubricant must exhibit good solubility in the refrigerant,
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`especially in the cold evaporator. Because of their good solubility, stability, and lubricity in R-134a, PAG
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`lubricants were chosen by the automotive industry for use in the new, ozone-friendly A/C systems.
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`LUBRICANT REQUIREMENTS FOR AUTOMOTIVE A/C SYSTEMS
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`The primary function of a refrigeration lubricant is to provide good compressor lubrication. There
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`are three lubrication regimes that exist within a compressor: hydrodynamic; elastohydrodynamic (EHL);
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`and boundary or extreme pressure (EP). Automotive A/C compressors are designed to run under
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`hydrodynamic or EHL lubrication regimes where the moving metal surfaces are always separated by a
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`lubricant film. However, metal-metal contact can occur during break-in or excessive loading, requiring
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`boundary or EP lubrication. Particulate contamination of the A/C system can also cause metal-metal
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`contact, as can lubricant starvation. For these reasons, the A/C lubricant must be able to provide all three
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`types of lubrication inside the compressor.
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`A/C system lubricants must meet a number of requirements in addition to providing good lubricity.
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`it is critical that the lubricant be sufficiently soluble in the refrigerant to ensure adequate return to the
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`compressor. Low pour points are needed to enable lubricant flow through the cold evaporator, and to
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`allow trouble-free operation during the winter. Refrigeration lubricants must be stable in the refrigerant
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`and in the presence of common contaminants such as air, water, residual petroleum oil, and refrigerant
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`Fl-12. Lubricant compatibility with the elastomers, plastics, and molecular sieves used in the A/C system
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`is also necessary.
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`Page 2 of 7
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`243
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`Page 2 of 7
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`POLYALKYLENE GLYCOL LUBRICANTS
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`Polyalkylene glycols are synthetic lubricants which were invented in the early 1940's during a joint
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`development project between Union Carbide Corporation and the Mellon Research Institute [1]. Since
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`then they have found use as compressor, gear, and food grade lubricants, metalworking and hydraulic
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`fluids, fiber lubricants, heat transfer fluids, and aqueous quenchants [2, 3].
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`Polyalkylene glycols are made from the polymerization of the monomers ethylene oxide (EO) and
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`propylene oxide (PO). PAGs are synthesized by adding the oxide monomers to an alcohol starter in the
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`presence of a catalyst (Figure 1). The molecular weight, and thus the viscosity of the PAG is controlled by
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`the oxide to starter ratio. The reaction proceeds until all of the oxide monomers are consumed [2]. Higher
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`molecular weight monomers such as butylene oxide can be used to make polyalkylene glycols, but
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`virtually all commercially available PAGs are made from either P0 or mixtures of EO and PO [4].
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`Polyalkylene glycols are unique among synthetic lubricants in that their solubility characteristics
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`can be tailored to meet the needs of a specific application. This can be done by changing the starter, the
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`E0 to PO ratio in the monomer feed, and the end groups (Figure 2).
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`DEVELOPING A POLYALKYLENE GLYCOL REFRIGERATION LUBRICANT
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`The automotive industry chose PAG refrigeration lubricants for use with refrigerant Fl—134a for a
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`number of reasons. This section will examine some of the factors and tradeoffs that need to be
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`considered when developing a PAG refrigeration lubricant.
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`R-1 34a Solubility
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`Polyalkylene glycols exhibit excellent low temperature solubility in refrigerant Fl-184a which helps
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`the lubricant maintain good flow properties in the cold evaporator and compressor suction lines. Good low
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`temperature flow properties are necessary to ensure sufficient lubricant return to the compressor.
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`As the temperature of a solution of polyalkylene glycol in Ft-134a increases, the solubility of the
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`PAG decreases. Eventually a temperature will be reached where the solution splits into a two phase
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`mixture. The high temperature solubility of PAGs increases with decreasing lubricant viscosity. The
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`solubility curves can also be affected by the PAG's starter, end group, and the E0 to PO ratio used during
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`manufacture. Figure 3 shows the solubility curves of a family of PAGs that vary only in viscosity.
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`It is important to note that in the high temperature insolubility region, the two layers that form do
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`not consist of pure PAG and pure R-134a. Instead they form a lubricant-rich phase and a refrigerant-rich
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`phase. As can be seen in Figure 4, the composition of the two phases can be obtained from the
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`intersection of the horizontal temperature tie line with the solubility curve of the polyalkylene glycol [4].
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`Early concerns about the high temperature insolubility of PAGs leading to heat transfer and
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`lubricant flow problems in the hot condenser have not been realized. However, this high temperature
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`insolubility does pose a problem in TXV A/C systems where a sight glass is used to determine if sufficient
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`refrigerant is present [5].
`If the PAG lubricant has a low separation temperature, it will cause the sight
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`glass to appear cloudy regardless of the amount of refrigerant present. This could cause an unwary
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`technician to overfill an A/C system.
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`Much work has been done over the last decade to improve the high temperature solubility of
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`PAGS in R-134a so that they can be used in TXV A/C systems that employ sight glasses. It is generally
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`believed that the 3 percent solubility of the PAG must be greater than 60°C if a sight glass is to be used.
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`In hotter climates, however, a higher separation temperature is probably needed.
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`in Figure 3.
`The large influence of PAG viscosity on high temperature solubility is evident
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`Changes in starter, monomer composition, and end groups have yielded small improvements, but no
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`Page 3 of 7
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`major breakthroughs. As a result, low viscosity PAGs in the neighborhood of 50 cSt at 40°C have been
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`used successfully in TXV A/C systems that employ sight glasses to determine refrigerant charge.
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`However, it has not been possible to use compressors that require a higher viscosity PAG lubricant (75-50
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`cSt at 40°C) in NC systems that use sight glasses.
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`One reason for the good lubricity of polyalkylene glycols is their polarity. Because every third
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`atom on the polymer‘s backbone is an oxygen, PAGs have a high affinity for metal surfaces. This natural
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`attraction to metal surfaces results in the formation of a protective layer which generally results in better
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`wear and scuffing characteristics when compared to nonpolar, hydrocarbon-based lubricants [6].
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`The high viscosity indexes (Vls) of PAGs also helps to account for their good performance in
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`Fl-134a A/C systems. The higher a lubricant's Vl, the smaller the change in viscosity from a given change
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`in temperature. PAGs have the highest Vls of any Fl-134a soluble lubricant, typically ranging from 180 to
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`230.
`In contrast, the mineral oils used with Fl-12 have Vls of less than 100. The effect of VI on a
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`lubricant's high and low temperature viscosities are shown in Figure 5. The high V|'s of PAGs means that
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`they will be more fluid at low temperatures, and still be viscous enough to lubricant the hot compressor.
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`The high temperature insolubility of PAGs in Fl-134a is actually beneficial in the hot compressor.
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`-The advantages of using a lubricant that is insoluble in the gas being compressed are well known among
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`the users of hydrocarbon gas compressors [7]. These benefits include better viscosity retention, reduced
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`cylinder wear, and reduced lubricant loss with the discharged gas.
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`While PAG base fluids perform better than mineral oils in many common lubricity tests, they
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`cannot by themselves make up for the loss of boundary or EP lubricity that was provided by the chlorine in
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`the Fl-12 [8, 9, 10]. For this reason, all widely used PAG refrigeration lubricants incorporate additional
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`additives to enhance their lubricity [8, 11]. However, it is important to remember that lubricity enhancers,
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`as well as all other additives in the lubricant, must be soluble in the refrigerant and compatible with all A/C
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`system components [10].
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`The lubricity of PAG lubricant candidates is often evaluated using common bench tests such as
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`the 4~Ba|l and Pin & V-Block tests. However, more specialized tests will often be run to attempt to more
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`closely simulate a particular wear mechanism that occurs in a given type of compressor. When testing a
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`lubricant for use in a swash plate compressor, a test employing sliding steel and aluminum contacts such
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`as the one described by Komatsuzaki might be used [8].
`If the lubricant is being developed for use in a
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`wobble plate compressor, an oscillating ring-on-block test could be employed to simulate the wear
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`mechanism that occurs in the swaged socket plate [12].
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`Pour Point
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`The pour point of a refrigeration lubricant should be below -30°C, preferably below -40°C. The
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`pour point of PAGs starts to increase when the polymerized EO content exceeds 50 weight percent.
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`lsoviscous EO/PO copolymers containing 50 percent polymerized E0 or less all have roughly equivalent
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`pour points. The pour points for these PAGs range from -50°C to -40°C for ISO viscosity ranges of 50 to
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`150 respectively. Since all commercially available PAG refrigeration lubricants contain 0 to 50 percent
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`polymerized E0 (50 to 100 percent polymerized PO), achieving an acceptably low pour point has not been
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`flit!
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`The excellent stability of dry PAG monols and ether capped PAGs in R-134a are well documented
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`[4, 8, 9, 11, 13, 14]. Neither the PAG's molecular weight or the EO/PO ratio of the monomer feed had any
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`significant effect on the stability of these lubricants in the presence of Ft-134a. Literature references
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`regarding the stability of dry, ester capped PAC-is are mixed [8, 11].
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`PAGs are hygroscopic by nature and will absorb water from the air. While this tendency can be
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`reduced by decreasing the amount of polymerized E0 in the polymer and by capping the hydroxyl end
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`groups, all PAGs are hygroscopic relative to mineral oils [4]. For this reason, much work has been done
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`on the stability of PAGs in the presence of Fl~134a and various amounts of water. Komatsuzaki found that
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`the water did not significantly effect the stability of a polypropylene glycol monoether, an ester capped
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`polypropylene glycol monoether, and an ether capped polypropylene glycol in the presence of R-134a [8].
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`Kaneko also found that polyalkylene glycol monols and ether capped PAGs were very stable in the
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`presence of R-134a and water regardless of molecular weight or the ratio of polymerized E0 to P0.
`In
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`contrast to Komatsuzaki, Kaneko found that the presence of water led to the hydrolysis of an acetoxy
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`capped polypropylene glycol monomethyl ether [11].
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`PAGs oxidize when held at
`Air is also a common contaminant in automotive A/C systems.
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`elevated temperatures in the presence of air [8, 11, 13]. There does not appear to be much effect of PAG
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`structure on oxidative stability [11]. However, the addition of antioxidants greatly improves the oxidative
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`stability of all polyalkylene glycol lubricants [8, 11]. For this reason antioxidants may be formulated into
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`PAG refrigeration lubricants.
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`PAGS are chemically
`Mineral oils are also commonly found in automotive A/C systems.
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`compatible with mineral oils in that they do not undergo any adverse chemical or physical reactions upon
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`mixing [4]. The solubility of mineral oils in PAG lubricants varies significantly with the composition of the
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`mineral oil and the structure of the polyalkylene glycol. Napthenic oils are more soluble in PAGs than
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`paraffinic oils. PAGs exhibit better oil solubility as the percentage of polymerized PO increases. Alkyl
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`capping also improves the oil solubility of a PAG. However, the oil solubility of PAGs does not appear to
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`effect their performance as refrigeration lubricants. PAGs with very limited oil solubility have been widely
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`used in both new R-134a A/C systems and retrofit vehicles where relatively large amounts of mineral oil
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`PAGs are not very stable in the
`in retrofit applications, refrigerant R-12 is always present.
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`presence of high concentrations of R-12 [4, 14]. However, when the concentration of Fl-12 is below 2
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`percent, as recommended in SAE Standard J1661, PAGs have proven to be quite stable [4]. The R-12
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`stability of PAGs can also be greatly enhanced through the use of stabilizers. These additives are often
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`incorporated in PAG lubricants for protection in the event of gross Fl-12 contamination [4].
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`Elastomer and Desiccant Comgatibility
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`A refrigeration lubricant must be compatible with the elastomers that are used in the A/C system.
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`In general, PAGs are compatible with most common elastomers [2, 4, 5]. However,
`it is important to
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`consider the effect of Fl-134a in combination with the lubricant before selecting an elastomer. A number
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`of studies show that H—NBFi, NBR, EPDM and chloroprene rubber all show good compatibility in the
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`PAG/Fl-134a combination [5, 9, 15]. Neoprene has also been shown to perform well, but its use at high
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`temperatures is limited [4, 15]. The structure of the PAG lubricant does not appear to have a significant
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`effect on elastomer compatibility [5, 9].
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`Desiccant compatibility with the refrigerant/lubricant pair is also important. Stability tests showed
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`that fresh or new XH—5 desiccant (used in R-12 A/C systems) is not compatible with Ft-134a. Further tests
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`did show good compatibility between the PAG/Fl-134a pair and desiccants XH-6, XH-7, and XH-9 [5, 14].
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`XH-7 molecular sieves are used in most new Fl-134a dryers because of their good physical characteristics
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`and relative low cost. Subsequent testing has shown that aged XH-5 molecular sieves can be used with
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`R-134a in retrofit applications.
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`SYSTEM TESTING OF A NEW PAG REFRIGERATION LUBRICANT
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`Once a lubricant candidate has met all of the chemical and physical requirements, it is ready for
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`system testing. Typically a lubricant will first undergo evaluation in a series of compressor stand tests
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`designed to simulate a variety of driving conditions.
`If the lubricant performs well in these tests, it will then
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`be charged into vehicles for fleet trials. Upon successful completion of the fleet tests, the lubricant is
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`ready for commercial use.
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`CONCLUSIONS
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`Pclyalkylene glycols are the automotive industry's lubricant of choice for use with refrigerant
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`Fl-134a. PAGs are unique among synthetic lubricants in the degree to which their chemical and physical
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`properties can be modified by changes in molecular weight, starter, end groups, and monomer selection.
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`The properties of PAGs such as R-134a solubility, hygroscopicity, and pour point are very dependent on
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`the lubricant's structure. Other properties, such as stability and elastomer and desiccant compatibility are
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`relatively unaffected by the chemical makeup of the polyalkylene glycol, although the poor hydrolytic
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`stability of ester capped PAGs has prevented them from gaining any widespread commercial acceptance.
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`The good response of PAGs to additives has led to significant improvements in their performance.
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`The use of antioxidants and stabilizers has greatly enhanced the stability of PAG lubricants in the
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`presence of air and residual R-12. Lubricity additives have helped the Ft-134a/PAG pair to overcome the
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`loss of extreme pressure lubricity that had formerly been provided by the chlorine in refrigerant R-12.
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`lubricity,
`To conclude, polyalkylene glycols can be synthesized that provide good inherent
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`solubility, and stability in R-134a. These properties, in combination with their good additive response, has
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`allowed the formulation of PAG based lubricants that provide excellent lubricity and system compatibility in
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`automotive air conditioners that use refrigerant Ft-134a.
`
`REFERENCES
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`1. Matlock, P. L. and Clinton, N. A. (1993). Pclyalkylene Glycols, Synthetic Lubricants and High-
`
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`Performance Functional Fluids (Shubkin, Ft. L.- editor), Marcel Deker, Inc., New York, NY
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`2. Brown, W. L. (1994). Pclyalkylene Glycols, Handbook of Lubrication and Tribology, Vol III (Boozer,
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`E. R.- editor), CRC Press lnc., Boca Raton, FL
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`3. UCON® Fluids and Lubricants, (1992). Union Carbide Corporation, Danbury, CT, Tech Literature
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`4. Brown, W. L. (1993). SAE Technical Paper Series 932904, Warrendale, PA
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`5. Refrigerant changes for NC systems (1991). Automotive Engineering, Vol 99, Number 2, pp 25-29
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`6. Konishi, T., Klaus, E. _E., and Duda, J. L. (1996). Tribology Trans., Vol. 39, 4, 811-818)
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`7. Short, G. D. and Miller, J. W. (1993). Compressors and Pumps, Synthetic Lubricants and High-
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`Performance Functional Fluids (Shubkin, R. L.- editor), Marcel Deker, lnc., New York, NY
`
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`8. Komatsuzaki, 8., Homma, Y., Kawashima, K., and ltoh, Y. (1991). Lub. Eng., Vol47, 12, 1018-1025
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`9. Yamada, A., Sonoda, Y., Arakawa, Y. (1992). SAE Technical Paper Series 920216, Warrendale, PA
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`10. Komatsuzaki, 8., Homma, Y. (1991). Lubrication Engineering, March, 193-198
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`11. Kaneko, M., Konishi, T., Kawaguchi, Y., Takagi, M. (1995). SAE Technical Paper Series 951052,
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`Warrendale, PA
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`12. Tseregounis, S. l. (1995). STLE Preprint, No. 95-AM-1C-1
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`13. Sundaresan, S. G., Findenstadt, W. Ft. (1992). ASHRAE Transactions 1992, Vol 98, Pt. 1
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`14. SUVA® Trans NC for Mobile Air Conditioning, DUPONT, Wilmington, DE, ARDT-31
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`15. UCON® Stationary Refrigeration Lubricant 68, (1995). Union Carbide Corporation, Danbury, CT, Tech
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`Literature UC-348A
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`Polyalkylene Glycols
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`Figure 1
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`MODOITIETS:
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`Ethylene Oxide (E0)
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`and
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` Propylene Oxide (PO)
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`FIGURES
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`Figure 2
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`PAG End Groups
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`R’
`R‘
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`PAG Monol: H-O"(CH:-CH-O)‘-CH1*-CH-OH
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`0l
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`PAG Ester Cap:
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`Fl-O-(CH1-CH-0).-CH3-CH~O-C-Fl‘
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`R‘l
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`PAG Ether Cap:
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`H-O-(CH2-CH-O)‘-CH2-CH-O-R‘
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`R-nlk i starter; we Fl'- lj, aim
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`Figure 4
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`PAG Phase Concentrations
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`100
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`B0
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`so
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`-
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`I
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`R0 "(CH:CH 2-OHCH-CH 2-O)-H
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`PAG Polymer Synthesis:
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`catyl.
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`ROH * EO 0 PO
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`Figure 3
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`Solubility of PAGS in R—134a
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`Figure 5
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`Viscosity Index
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`'3
`0
`"‘
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`ISO 100 FAB
`ISO 50 PAG
`ISO 100 OH
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`20
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`40
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`B0
`60
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`Temperature C
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`03-IE-I-D-Iflfiafl-I
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`‘V’
`5
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`1
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`O
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`ISO 100 FAQ: VI-220
`ISO 60 FAQ: VI-210
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`I50 ion on vi-75
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`'
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`.
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`.
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`..iji_ i?i_
`zljlu '
`i
`so
`70
`so
`so
`20
`so
`40
`so
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`96 PAG in Fi-134a
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`ion
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`—‘|-‘-3° 130 PAG +Phaae Concentrations
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`_l
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`49
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`20
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`O
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`0
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`1o
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`DI'l€-IIHIUSG-I
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`90
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`100
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`30
`70
`so
`40
`so
`so
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`WW PAG in PAGIFI-1343 Mixture
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`ISO 35 PN3
`ISO 50 PM)
`"
`ISO 100 PAG
`|
`ISO 135 FAQ
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