Importance of Rheological parameters in Lubrication Control of lubricant viscosity for a specific SAE grade is exercised by SAE J 300 through stipulation of Kinematic Viscosity (KV), Cold Cranking Simulator (CCS), Borderline Pumping Temperature by Mini Rotary Viscometer (MRV), and High Temperature High Shear (HTHS) viscosities at specific temperatures. Lubricants are however non Newtonian fluids and their viscosities are affected with differing shear rates in engines. VI Improver polymers experience shearing and there is a resultant viscosity drop, the extent of which is dependent on the polymer type and its molecular weight which confer a characteristic represented by the Shear Stability Index (SSI) of the VI Improver. High SSI VI Improvers experience greater shear and thereby more viscosity loss than lower SSI VI Improvers. The ability of oil to retain its viscometric properties is particularly important in Heavy Duty Diesel Engine Oil (HDDEO) applications to prevent wear and maintain intended levels of oil pressure. In modern performance classifications such as API CJ-4 and ACEA E7, E6 and E9 the requirements of shear stability has been changed, and an oil’s KV100 must stay within the original viscosity grade as defined by SAE J300 after 90 cycles in the Kurt-Orbahn (KO) apparatus. While in API CI-4/CI-4+, ACEA E4 the shear stability requirements remain at 30 cycles, OEM requirements in that segment are aligned with the more stringent requirement of 90 cycles. This is the reason why different VI Improvers with more shear stability characteristics are specified for Top Tier HDDEO.
To improve fuel economy, many OEMs turned to increasing oil temperature to reduce oil viscosity. Though this may sound like a simple change, increasing the oil temperature accelerates oil oxidation, which in turn accelerates oil degradation, creating products that affect start-up performance in low temperature conditions. Increasing use of biofuels, enhance the problem with the formation of gummy materials and increased acid concentration. Modern performance specifications have instituted tests such as proprietary bench oxidation tests for OEMs and aged oil MRV (CEC-L-105) which is now required for all ACEA qualifications. The test method involves adding 5% biofuel to the finished oil, heating the mixture at 150°C for 3 days to age the oil and then measuring MRV. MRV limits are the same as those for fresh oil; no yield stress and MRV viscosity of less than 60,000 cP. As oil conditions become severe with higher operating temperatures and biofuel dilution, robust engine oil formulations need to be developed with VI Improvers having good low temperature characteristics.
Lower viscosity oils improve fuel economy but may require more polymer content to maintain viscosity at higher temperatures. Higher amounts of polymer can increase deposits. Longer drains also allow for more opportunity for the viscosity modifier polymer to shear. VI Improvers that have a good thickening efficiency which reduces the overall polymer concentration will then have better propensity of performance in modern engines.
Thus the attributes of an effective VI Improver may be summed up as that having, good low temperature characteristics, excellent shear stability and good thickening efficiency, as shown in Figure 1.
Selection of suitable VI Improver type The co-monomer composition OCP-VI Improvers (VIIs) has a profound influence on its rheological properties. Ethylene and propylene (EP) are the most common co-monomers used for OCP-VI Improvers. Among EP co-polymers, the performance for application in motor oils is highly dependent on the architecture of the ethylene and propylene segments within the polymer chain. An improper design can have undesired effects (Bansal et al)2
. Traditionally OCP VI Improvers have used Ziegler-Natta catalysts in the synthesis of polymers. However, the polymer industry is increasingly using Metallocene® manufacture of OCP polymers. Metallocene®
catalysts for the catalysts have
the advantage of greater control of the polymer architecture. Recent studies by Patel et al2
have reported the development of
high performance ethylene-propylene polymers with optimized properties for use in motor oils by controlling the branching, molecular weight, molecular weight distribution, branching, and ethylene / propylene segment distributions using Metallocene® catalysis. The resulting properties include high thickening efficiency, reduced viscosity loss after exposure to extended shear, as well as precise control over co-polymer microstructure for robust low temperature behaviour.
Figure 2
The high thickening efficiency of high ethylene OCP chemistry is also advantageous for its reduced demerit contribution in piston cleanliness tests. Comparative performance and benefits of this VI Improver is given in Figures 3, 4 and 5.
Figure 3
Figure 1
LUBE MAGAZINE NO.124 DECEMBER 2014
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