The Effects of the Tribo-Conditions and Antiwear Additives on the Degradation of an Ester Oil and Analyses of Degradation Products

The effects of tribo-conditions and antiwear additives on the degradation of an ester oil in refrigerant have been experimentally evaluated and the degradation products were analyzed. We have used hand-made pin and Vblock apparatus in order to control temperature, load, rotating speed, and environmental gas. The degree of degradation of the ester oil was determined by the amount of sludge on the friction surface. Chemical analyses of sludge were performed by using pyrolysis gas chromatograph-mass spectrometry and thermogravimetry. It was found that the amount of sludge increases as the amount of antiwear additives increases under the lower load (10 kgf), and has a minimum in the addition range 0 to 2.5% of antiwear additives under the higher load (20 and 30 kgf ). This result indicates that the antiwear film reduces the amount of the degradation products of the ester oil and inorganic products under the higher load. The amount of sludge increases as rotating speed and oil temperature increase. These results suggest that the surface temperature of contact and frictional resistance increase as the rotating speed and the oil temperature respectively increase. INTRODUCTION The various factors such as tribo-conditions, fresh surface of metal, exoelectron, kinds of base oil and additives, environmental gas, and oil temperature have influence on the degradation of lubricants on friction surface. It has been considered that the refrigeration oil is degraded on friction surface in the compressor for refrigerators and air-conditioners by these factors. It is very important to evaluate the amount of degradation products (sludge) of refrigeration oil from the viewpoint of the long-term reliability of refrigerators and air-conditioners because the cycle-pass of refrigerant are chocked with sludge. There have been many investigations of the degradation of lubricants ; reaction rate<1), reaction mechanism<2), heat stability<3),analyses of oxidation products<4).(5)_ However, the degradation of lubricants in these investigations was regarded as static chemical reaction, and there have been only a few investigations (6),(!) of dynamic( mechanochemical) degradation on friction surface. In addition, there are no investigation that the degree of the degradation of lubricants is determined by the amount of sludge. This investigation was carried out to evaluate the effects of tribo-conditions and antiwear additives on the degradation of an ester oil in refrigerant. The degradation of an ester oil was also considered by the analyses of sludge and friction surface. EXPERIMENTAL DETAILS Materials and test condition Table 1 shows the properties of the ester oiL The additives are an antioxidant, a water and acid captor, and antiwear additives. The antioxidant is butyl hydroxy toluen. The water and acid captor is cyclohexene type compound. The antiwear additives are phenyl sulfur-phosphate type compound(Atype) and alkylphenyl phosphate type compound (Btype). The refrigerant is HFC 134a. We have used hand-made pin and V-block apparatus (8), shown in Fig.1, to evaluate the effects of tribe-conditions and antiwear additives on the degradation of the ester oil. The rotating pin is MoNiCr (cast iron) and V-block is SKH51 (steel, nitrided). The test specimen and conditions are summarized in Table 2.