Hi tachi Ltd., Japan. and of the mineral oil, which has nomiscibility, for reference. Mineral

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1 LUBR CANTS FOR HFC-134a AUTOMOB LE A R COND T ONERS S. Komatsuzaki and A. Kishi Hi tachi Ltd., Japan NTRODUCT ON Due to the ozone layer depletion issue, the refrigerant CFC-12(CC12Fz), which was now widely used in(domestic refrigerators and) automobile air conditioners, is likely to be replaced by HFC-134a(CH2FCFB) in the near future. This change, in turn, brings about the problems of finding suitable materials for automobile air conditioners which are compartible with HFC-134a. n particular, the choice of the lubricant is crucial, since the re1 iabi lity of compressor cannot be secured without suitable lubricant. This presentat ion examines the properties of candidate lubricants for HFC -134a compressors and ways of improving their properties to meet various requirement. The properties dealt here are: miscibility with the refrigerant, chemical stability, and lubricity. A particular emphasis is placed on the lubrication problems, which arises when CFC-12 is replaced by HFC-134a. Last but not least, the durability tests were carried out by using the improved lubricant. RESULTS AND D SCUSS ON Miscibi ity Miscibility is the most important property, because, for example, when widely used mineral oil is applied to HFC-134a using system, the seizure of sliding parts occurs easily, due to the poor oil return to the compressor. Many kinds of base oils were evaluatead. Figure 1. shows the miscibility of the typical base oils which have miscibility with HFC-l34a, and of the mineral oil, which has nomiscibility, for reference. Mineral oil is completely immiscible with HFC-134a in the range between -6O'c and +80C. Base oils with good miscibility were fluorosilicone, PAG, ester. Fluorosilicone had the best miscibility among the samples tested here. Lubr i c i ty Lubricity of refrigerator oils was evaluated by Falex tester. Load is applied at the rate of about 2kN/nin. Figure 2 shows the relation between load and friction coefficient of three base oils, fluorosililcone, PAG, 563

2 and ester, which have good miscibility with HFC-134a. Mineral oil is for reference. Load was increased until there was a sharp rise of friction coefficient due to seizure. Although f luorosi 1 i lcone had excel lent miscibility, it had the worset lubricity among three, and the friction coefficient increased conspicuously at a low load. n comparison with mineral oil, PAG had much higher seizure load, and ester oil had a little higher seizure load. Those results were obtained in air, not under ref r i ge r ant env i r onment. b: Next, lubricity of ester oil and PAG under refrigerant environment was examined with the home-made apparatus with the pressurized chamber in which the rubbing was carried out. The results were shown in Table 1. Lubricity became low under HFC-134a environment, than under CFC-18 environment. However, in the case of PAG or ester oil improved by an EP agent, the seizure did not occur even after the load was over 4.0kN under HFC-134a environment, which is close to the seizure load of the CFC-12/ mineral oil mixture. n this way, it was found that lubricity under HFC -134a can be enhansed to that under CFC-18 environment when suitable EP agents are added to the lubricants. Stabi i ty Problems in chemical stability with PAG and ester oil are as follows: PAG *t deteriorates when combined with chloride. *t is prone to oxidation. Ester oi 1 *t is prone to hydrolysis. PAG deteriorates when combined with chloride. Figure 3 shows that PAG has a drastic color change in the sealed tube test due to deterioration when combine with CFC-12, containing chlorine in the molecule. At present CFC -12/mineral oil combination is used in the compressors,of automobile air conditioners. f HFC-l34a/PAG replaces CFC-lZ/mineral oi 1 in the compressors, a small amount of CFC-12 may remain in the system. Then the residual CFC-12 will give a harmful effect on the stability of PAG. This deterioration is ascribable to the production of HC1 caused by decomposition of CFC-12. Therefore, the addition of HC1 deactivator to PAG improves the stability as is shown in the figure. PAG has another problem: it is prone to oxidation. Air-leak into the system causes oxidation of lubricant. The oxidation inhibitor improves the stability likewise. Ester oil has stronger affinity with moisture than mineral oil; moreover it cannot avoid the hydrolysis due to its chemical structure. This causes a problem when moisture leaks into the system through hoses: fatty acid produced by hydrolysis may cause corrosion wear of rubbing parts. However, 564

3 this harmful fatty acid can be deactivated by additive(figure 4). Durabi i ty test PAG based and ester based oils were found to be promising candidates to be used with HEX-134a. n order to assure their practical use, the durability tests were carried out, finally, under severe conditions as fo 1 1 ows : 1. running at high discharge pressure 2. running under high rotation and high discharge pressure 3. on/off running under high discharge pressure 4. running under a small amount of lubricant 5. cyclic running at high and low rotation speed 6. others After the tests, wear of sliding parts and lubricant deterioration were examined, and were compared with those of CFC-l2/mineral oil case. Both the wear and deterioration were approximately the same. The accelerated durability tests indicated that HFC-134a compressors which used improved PAGbased or ester-based lubricants had the same level of reliability as the conventional compressors f i 1 led with CFC-12 and mineral oi 1. Although the definite conclusion must come after the field tests, which are now being carried out, it seems quite likely that PAG-based and ester -based lubricants can be applied to HFC-134a compressors. Of the two, PAG-based lubricants are now regarded as the most promising. However, it is quite possible that ester-based lubricants may come into use if their performance can be improved by additives or by modification of chemical structure. Misci bl e Mineral oils i mmiscible L PAGs le Esters Perflubropolyethe c Fluorosilicones -60 t40 t20 0 t20 t40 t60 t80 t100 Temper at u re ("C) Figure 1 Miscible temperature range of base oils with HFC-134a f 565

4 ~~ ~ ~ Ref r i gerant/ ubr i cant Se i zu re oad (kn) CFC-l2/mineral oi 4.0 HFC- 134a/PAG 3.2 HFC-l34a/PAG t EP agent 4.0 HFC-l34a/ester o i HFC-l34a/ester oil t EP agent 4.0 Tests were carried out by increasing the applied load from 0.4 kn, stepwise at the rate of 0.2kN/min. 1.2 > Ester oil Mir PAG Fluorosilicone - 5 t, 1 // Figure 2 Relation between applied load and friction coefficient in the Falex test 566

5 Addition of HC1 deactivator -w v ' Heating time (h) Figure 3 Additive effect of HC deactivator on color change of CFC-l2/PAG mixture in sealed tube test 0.5 n M \ X 0 M E! Ld 0.1 W a -4 0, -z 0.01 Addition of de ac t i vat o r Water content in oil (ppm) Figure 4 Relation between water content and acid value in an ester oil 567