Refrigeration oil, R32, Lubricity
R32 has been applied as an air-conditioner refrigerant in Japan for a few years. Although its GWP is slightly higher than the limit of F-gas restriction, high COP is a key driving force for R32 system. Regarding refrigeration oil applied to R32, there are two main issues. First issue is miscibility with R32, because incumbent refrigeration oil for R410A is rather immiscible to R32. Second issue is lubricity. When refrigerant is miscible well with refrigeration oil, generally, dissolved refrigerant reduce viscosity of refrigerant-oil mixture. Lower viscosity results in, lower lubricity, and finally, it would cause wear of sliding parts, compressor durability shortage, insufficient sealing, or reduced COP. At the earlier stage, several refrigeration oils miscible with R32 have been proposed. However, we have experienced wearing problems on surface of a main shaft in R32 system with such refrigeration oils, for example. Those problems were not found in R410A systems. We assumed there is some different mechanism on sliding parts in R32 systems comparing with current refrigerant systems. We focused on viscosity of refrigerant – oil mixture, which could be correlated to lubricity. Then, we found that viscosity of R32 – refrigeration oil mixture is extremely lower than that of R410A – refrigeration oil. Together with results of wearing problem in sliding parts, which are not worn by fluid lubrication in general, we assumed that oil film is formed insufficiently on the surface due to the extreme low viscosity of the mixture. Our goal of development of refrigeration oil for R32 was reset to balance miscibility and lubricity, which is trade-off normally. Through various studies, finally, we found a new base oil to meet our goal, which makes us possible to achieve not only high viscosity of the mixture, but also good miscibility with R32. Its excellent characteristics on lubricity and miscibility are also confirmed by several compressor tests.