STRAIGHT TALK ON VARNISH
Part 2: Do Group II oils deserve their bad reputation?
Users often complain that before using Group II base-stock oils, they didn’t have a problem with varnish. As a result, Group II oils have a reputation for being varnish prone, leading users to search for alternatives. The market has responded, with many companies offering low-varnish versions, co-base stock oils, synthetic oils or even after-market additives for the more adventurous.
The truth is, Group II oils do have less capacity to hold breakdown products in solution than Group I. This is because breakdown products are polar in nature and on the spectrum of polarity, Group II base stocks are less polar (more non-polar) than Group I base stocks. Therefore, because “like dissolves like,” polar breakdown products are more likely to precipitate out of solution from more non-polar Group II oils than Group I resulting in varnish deposits.
To understand how these oils breakdown over time, we ran thermo-oxidative breakdown experiments in the lab subjecting each oil sample to peak turbine operating temperature (150°C), air and a copper catalyst. The results presented include acid number, RULER amine and phenol antioxidants, and MPC varnish potential (Fig. 1).