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The viscosity index of an oil or an oil formulation is an important physical parameter. Viscosity index improvers, VIIs, are comprised of five main classes of polymers: polymethylmethacrylates (PMAs), olefin copolymers (OCPs), hydrogenated poly(styrene-co-butadiene or isoprene) (HSD/SIP/HRIs), esterified polystyrene-co-maleic anhydride (SPEs) and a combination of PMA/OCP systems. The chemistry, manufacture, dispersancy and utility of each class are described. The comparative functions, properties, thickening ability, dispersancy and degradation of VIIs are discussed. Permanent and temporary shear thinning of VII-thickened formulations are described and compared. The end-use performance and choice of VI improvers is discussed in terms of low-and high-temperature viscosities, journal bearing oil film thickness, fuel economy, oil consumption, high-temperature pumping efficiency and deposit control. Discussion of future developments concludes that VI improvers will evolve to meet new challenges of increased thermaloxidative degradation from increased engine operating temperatures, different base stocks of either synthetic base oils or vegetable oil-based, together with alcohol-or vegetable oil-based fuels. VI improvers must also evolve to deal with higher levels of fuel dilution and new types of sludge and also enhanced low-temperature requirements. IntroductionThe 'viscosity index', VI, was an important measure of quality early in the history of the lubricants industry, indicating an oil's potential applications over a wide range of temperatures, described in Section 1.3.2. Pennsylvania grade oils,~100 VI, were the standard against which all others were measured. Hydrogenation and solvent extraction processes were developed to upgrade lubricants from poorer quality crude oils, but the practical VI ceiling for 1930s refinery technology was~110-115.Early workers found that small amounts of rubber dissolved into mineral oil substantially raised VI; however, high levels of unsaturation in the polymer led to oxidation and sludge formation. This was overcome by using a synthetic polymer prepared from gasoline light ends [1], and similar behaviour was later described for 153 R.M. Mortier et al. (eds.), Chemistry and Technology of Lubricants, 3rd edn.,

A constant quest but a difficult goal is replacement of expensive and lengthy engine tests with more cost effective bench tests. This paper will report on development of a bench test, the ROBO, as a potential replacement for the Sequence IIIGA, an oxidative oil thickening engine test now part of ILSAC GF-4 specifications. Sequence IIIGA, a gasoline fueled engine running under severe conditions, is used to condition oils prior to determining TP-1 MRV viscosity, a cold temperature and low shear rate measurement. After Sequence IIIGA conditioning, candidate oils must meet SAE J300 pumpability limits at no more than 5°C warmer than its original SAE W grade. The ROBO bench procedure has been developed to potentially replace Sequence IIIGA engine aging by mimicking, to the degree possible, oxidative conditions found in the engine conditioning procedure. ROBO conditions were developed by employing the three Sequence IIIG ASTM matrix oils and comparing their used oil rheologies from multiple Sequence IIIG and ROBO runs. The results from the final ROBO conditions compare well not only for TP-1 MRV but also for KV 40°C and CCS. To further test the correlation of bench to engine conditioning, a set of diverse oils (various SAE viscosity grades and DI chemistries) have been examined; these viscometric data from ROBO and Sequence IIIG also compare favorably with only a few anomalies. Finally, the status of industry acceptance of ROBO will be discussed.

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