Refrigeration Lubricant Based on Polyolester for Use With HFCs and Prospect of Its Application With R-22 (Part 1) Tribological Characteristics

Tribological behavior was evaluated for polyolester base oil and the formulated oils in some HFCs and HCFC refrigerants environment. HFCs as well as R-22 showed the boundary lubricating effect depending upon the solubility of gas in the oil. Thus, higher gas pressure of the refrigerants improved the wear resistance. A synergistic effect on antiwear properties was found when using an aryl phosphate together with a sulfur-phosphorus type additive whose perfonnance was comparable to that with the fonnulated alkylbenzene in R-22. The surface analysis showed that the improvement of antiwear performance was because of promoting the formation of a reaction film containing both iron phosphate from aryl phosphate and the decomposition products of polyolester on the sliding surfaces. INTRODUCTION In accordance with the predetermined schedules of the international treaty based upon Montreal protocol, the production of chlorofluorocarbons (CFCs) was totally banned by the end of year 1995 and that of the hydrochlorofluorocarbons (HCFCs) has been phased out to meet future regulatory legislation requirements. Instead, hydroflourocarbons (HFCs) having no ozone depleting potential have been regarded as alternative refrigerants. Since the replacement of CFCs seems to have been successfully completed in household refrigerators and automotive air conditioners, the current major concern is application of HFCs to air conditioners and low temperature refrigeration equipments. In refrigeration compressor applications, one of the key issues is the lubricant performances which are primarily miscibility with HFCs, thermal and chemical stability and tribological properties. Polyolesters (POEs) have been regarded as one of the most prominent candidates of refrigeration lubricant for HFCs. However, it has been pointed out that POE has the drawback of hydrolysis which results in sludge formation and wear enhancement in compressors [1]. Thus, POE lubricants have to be optimized from a viewpoint of chemical structure of base oil and the additive fonnulations. The present paper describes the tribological performance of the developed refrigeration lubricant based upon· a specified combination of POE and the additive packages. OILS TESTED AND SOLUBILITY OF Oll.JREFRIGERANT MIXTURE The base oils of lubricants tested are a pentaerythritol type ester POE(A32) and an alkylbenzene AB having a branched alkyl substituent. Some properties ofbase oils and compositions of additive-containing POEs are listed in Tables 1 and 2, respectively. All the formulated POEs contain in common a small amount of a phenol type antioxidant and an epoxide type stabilizer whose performance is described in part 2 report. In addition, an aryl phosphate (AP) and a sulfur-phosphorus (S-P) type antiwear additive were separately or together added to improve the wear resistance, in which POE-C contained as much AP as POE-A and as much S-Pas POE-B. First, the viscosity of oil/re:frigerant mixture was measured with a falling piston type viscometer as shown in Fig. I. Figure 2 shows the viscosity as well as the solubility of refrigerant gas in the oil under a constant gas pressure and temperature. The oil/refrigerant mixture showed the lower viscosity than the oil in the air for each oil. Decline of viscosity ofPOE 'With re:fiigerant was more significant for R~22 than for HFCs, and the viscosity ofPOE/HFC was