Development of refrigeration oil for use with R32.

R32 has been used as an air-conditioner refrigerant in Japan since 2012. Although its GWP is slightly higher than the limit prescribed in the EU's F-gas regulation, the high COP of R32 systems is a key driving force behind their adoption. For refrigeration oils, there are two main issues with regard to use with R32. The first is miscibility with R32, because the current refrigeration oils designed for R410A have poor miscibility with R32. The second issue is lubricity. Generally, when the refrigerant has good miscibility with the refrigeration oil, dissolved refrigerant will reduce the viscosity of the refrigerant-oil mixture. Lower viscosity results in lower lubricity, which ultimately leads to wear of sliding parts, reduced compressor durability, insufficient sealing, or a lower COP. Several refrigeration oils miscible with R32 have been proposed. However, we experienced problems with these refrigeration oils in R32 systems, such as excessive wear on the surface of the main shaft. These problems were not seen in R410A systems. We assumed there was some mechanism acting on the sliding parts in R32 systems that was different from that seen in the current refrigerant systems. We focused on the viscosity of the refrigerant-oil mixture, which could be related to lubricity. We found that the viscosities of R32-refrigeration oil mixtures were dramatically lower than those of R410A-refrigeration oil mixtures. Normally, the sliding parts are under hydrodynamic lubrication, so this result led us to surmise that oil films were not being formed sufficiently on the surfaces due to the extreme low viscosity of the mixture. We shifted our goal for development of a refrigeration oil for R32 to focus on balancing miscibility and lubricity, which are normally in a trade-off relationship. Through various studies, we eventually found a new base oil to meet our goal, making it possible to achieve not only high viscosity in refrigerant-oil mixtures, but also good miscibility with R32. Its excellent lubricity and miscibility were also confirmed in various compressor tests.