Lube-Tech
Films of similar viscosity lubricants were tested at 120°C to 140°C [4] and showed very different trends, Figure. 5.
Figure 5. Differences in oxidative decomposition intensity of several types of commercial lubricants, using 500 µm initial film thickness and temperatures between 120 to 140 °C
Mineral diesel engine oil of SAE 40 grade vaporised faster than gas engine oils [6] of similar viscosities at 40°C. Both samples from different batches showed very good repeatability, although initially their colour was somewhat different. Two mineral lubricants, coded GA4 and GP7 for powerful natural gas engines in electrical generators, behaved quite differently. Although GP7 was vaporising faster initially, it did not show much change in the decomposition rate. As a product from competing manufacturer, GA4 appeared less volatile during early stages. However, its vaporisation accelerated significantly afterwards and eventually exceeded the emissions of GP7. Considering similar ambient viscosities of all three brands of mineral oil lubricants (one diesel and two generator oils), the differences in long-term vaporisation are very perplexing. Not much is known about the specific basestock for each lubricant, so it would be speculative to try interpreting the underlying chemical reasons. Nevertheless, it is very likely that not just basestocks themselves, but also formulation additives had a significant effect on vaporisation trends.
When synthetic transmission oils of ISO VG100 were compared, Figure. 5, their long-term vaporisation also
PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE
No.116 page 4
showed different trends. Decomposition rate for 6X3 stayed similar, while the rate accelerated for CG3. It should also be pointed out that decomposition reactions did not arrive to the complete stop even after 3000 hrs of degradation. Being more viscous, these lubricants contained higher molar mass and the vaporisation rate was lower than that of engine oils. Nevertheless, the volatile emissions continued increasing at the end of the tests.
If lubricant loses most of its volume due to decomposition, it is hard to consider it as ‘forgiving,’ because lower amount of liquid usually translates into lubricant starvation. Flash points or NOACK tests can only measure the rate of evaporation, caused by low molar mass components. Long-term vaporisation from thin films is a much more realistic means to describe the ability of lubricant to remain in liquid under severe degradation conditions. It must be admitted that very long testing durations are needed. However, if possible losses might be very costly, the best lubricant must be selected from several options. Often it should be reasonable to run several months-worth of thin film tests with all the candidate lubricants to find the most ‘forgiving’ one. In fact, per customer report, one electrical generator running GA4, experienced problems with its engine pistons and had to be stopped for maintenance, which was quite costly. Just one confirmation could not be considered as a proof, but this still gave more credibility to GP7 as more “forgiving” product than GA4.
Formation of oil-insoluble residues Probably even more important for a “forgiving lubricant” is to ensure that it does not turn solid after prolonged degradation and it is capable of easily dissolving in freshly supplied oil. Lubricant solidification can occur through several mechanisms. Crystallisation is only important at low temperatures, which is not relevant here. The main degradation- related mechanism of solidification is oxidative polymerisation, which forms higher molar mass
LUBE MAGAZINE NO.145 JUNE 2018 33
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64
