The Viscosity index

We must perforce devote some time and attention to the concept known as the vlscoilty Index, which has become embedded in the terminology of technological lubrication with some unfortunate connotations. Whereas the Walther equation and the ASTM viscosity-temperature charts are frankly empirical devices used to linearize the viscosity-temperature relations for convenience and utility, the viscosity index is an attempt to impart the mystique of quality by assigning an evaluative aspect to the 

As an example let us take an oil with a viscosity of 7.30 cs at 100 C and 68.06 cs at 40 C. A value of 84.53 cs is obtained from the abscissa where the 7.30 cs line intersects the Series L curve, as shown in Fig. 4-17. A value of 68.06 cs for H is obtained similarly from the Series H curve. Substitution into Eqn 4-36 gives 50.3 as the viscosity index of this particular oil. 

Suppose we have two oils with the same viscosity at 40 C but with different viscosities as 100 C. Then the individual lines for the 100 C viscosities will intersect the Series L and H curves to give different values of L and H for the two oils and consequently their viscosity indices will be different. 

Detailed directions for computing the viscosity index are published by the American Society for Testing and Materials [26]. For oils of viscosity not greater than 70 cs at 100 C and viscosity index not over 100 there is a table of values for L and H. For oils of viscosity greater than 70 cs at 100 C and for oils of viscosity index above 100, special computational formulas have been developed. These tables and formulas were adopted by ASTM in 1977. Prior to that the two fixed temperatures for the viscosity index were 100 and 210 F. Examination of Eqn 4-36 reveals that the value of the viscosity index does not depend on the units in which the viscosities are reported, since the factor for conversion to centistokes occurs in both the numerator and the denominator of the formula. The temperature interval and the viscosity-temperature characteristics of the series of reference oils are the two principal basic influences on the general nature of the viscosity index. 

It is apparent in Fig. 4-18 that viscosity increaseis with pressure most strongly for the naphthenic oil and least for the synthetic sebacate fluid. It is also apparent that for the most part increase of viscosity with pressure does not follow the exponential relation 

---

The temperatures 100°F and 210°F (37.8°C, 98.9°C) have been selected because they were initially used in the ASTM procedure for calculating the viscosity index of petroleum cuts (ASTM D 2270). 

The viscosity index is an empirical number, determined from the kinematic viscosities at 40 and 100°C it indicates the variation in viscosity with temperature. 

VI additives to improve the viscosity index polymethacrylates, polyacrylates, olefin polymers. 

The viscosity of a hydrocarbon mixture, as with all liquids, decreases when the temperature increases. The way in which lubricant viscosities vary with temperature is quite complex and, in fact, charts proposed by ASTM D 341 or by Groff (1961) (Figure 6.1) are used that provide a method to find the viscosity index for any lubricant system. Remember that a high viscosity index corresponds to small variation of viscosity between the low and high... 

The function of viscosity additives is to improve the viscosity index so as to obtain multigrade oils. The problem is to use materials that, by only slightly increasing the low temperature viscosity, are capable of counterbalancing the decrease in viscosity when the temperature increases. 

Additives for improving the viscosity index are added in concentrations of five to ten weight per cent of the oil. 

These products have molecular weights between 2000 and 10,000, well below those of additives improving the viscosity index (100,000). They are added in very small concentrations (0.01 to 0.3 weight percent) and at these concentrations they can lower the pour point 30°C. 

Polymers for improving the viscosity index of the copolymethacrylate type can be made into dispersants by copolymerization with a nitrogen monomer. The utilization of these copoiymers allows the quantity of dispersant additives in the formulation to be reduced. 

The absolute viscosities of the perfluorinated inert Hquids are higher than the analogous hydrocarbons but the kinematic viscosities are lower due to the higher density of the perfluorinated compounds. The viscosity index, ie, the change in viscosity with temperature, is generally higher for the perfluorinated Hquids than for hydrocarbons. 

Alkylated aromatics have excellent low temperature fluidity and low pour points. The viscosity indexes are lower than most mineral oils. These materials are less volatile than comparably viscous mineral oils, and more stable to high temperatures, hydrolysis, and nuclear radiation. Oxidation stabihty depends strongly on the stmcture of the alkyl groups (10). However it is difficult to incorporate inhibitors and the lubrication properties of specific stmctures maybe poor. The alkylated aromatics also are compatible with mineral oils and systems designed for mineral oils (see Benzene Toulene Xylenes and ethylbenzene). ... 

Lubricants. Petroleum lubricants continue to be the mainstay for automotive, industrial, and process lubricants. Synthetic oils are used extensively in industry and for jet engines they, of course, are made from hydrocarbons. Since the viscosity index (a measure of the viscosity behavior of a lubricant with change in temperature) of lube oil fractions from different cmdes may vary from +140 to as low as —300, additional refining steps are needed. To improve the viscosity index (VI), lube oil fractions are subjected to solvent extraction, solvent dewaxing, solvent deasphalting, and hydrogenation. Furthermore, automotive lube oils typically contain about 12—14% additives. These additives maybe oxidation inhibitors to prevent formation of gum and varnish, corrosion inhibitors, or detergent dispersants, and viscosity index improvers. The United States consumption of lubricants is shown in Table 7. 

Although the viscosity index is useful for characterizing petroleum oils, other viscosity—temperature parameters are employed periodically. Viscosity temperature coefficients (VTCs) give the fractional drop in viscosity as temperature increases from 40 to 100°C and is useful in characterizing behavior of siHcones and some other synthetics. With petroleum base stocks, VTC tends to remain constant as increasing amounts of VI improvers are added. Constant B in equation 9, the slope of the line on the ASTM viscosity—temperature chart, also describes viscosity variation with temperature. 

An example of an appHcation of hydrocracking is in lubricating oils, where it is used to improve the viscosity index, color, and color stabiHty to reduce polymer formation (storage stabiHty) and to decrease the neutralization number (acidity) (61). 

On the other hand, liquid propane also has a high affinity for paraffinic hydrocarbons. Propane deasphalting removes asphaltic materials from heavy lube oil base stocks. These materials reduce the viscosity index of lube oils. In this process, liquid propane dissolves mainly paraffinic hydrocarbons and leaves out asphaltic materials. Higher extraction temperatures favor better separation of the asphaltic components. Deasphalted oil is stripped to recover propane, which is recycled. 

The conformational mobility of a chromophoric main-chain polymer is often connected to its electronic structure. Therefore, changes in the UV-visible absorption spectra and/or chiroptical properties are spectroscopically observable as thermo-, solvato-, piezo-, or electrochromisms. It is widely reported that o-conjugating polysilanes exhibit these phenomena remarkably clearly.34 However, their structural origins were controversial until recently, since limited information was available on the correlation between the conformational properties of the main chain, electronic state, and (chir)optical characteristics. In 1996, we reported that in various polysilanes in tetrahydrofuran (THF) at 30°C, the main-chain peak intensity per silicon repeat unit, e (Si repeat unit)-1 dm3 cm-1, increases exponentially as the viscosity index, a, increases.41 Although conventional viscometric measurements often requires a wide range of low-dispersity molecular-weight polymer samples, a size exclusion chromatography (SEC) machine equipped with a viscometric detector can afford... 

The UV and CD spectra of 117 and 121 (V) are shown in Figure 48. Considering 117, at —40°C a negative Cotton effect, coincident with the UV absorption, is evident, and at —5 °C a positive Cotton effect, coincident with the UV absorption (both of which are slightly red-shifted with respect to the —40°C profiles), is observed. It is thus apparent that 117 underwent a helix-helix transition at some temperature between —5 and —40°C. In contrast, the Cotton effects of 121(V) were positive at all temperatures, indicating that no helix-helix transition occurred. Similarly to 121, 88 did not undergo a helix-helix transition. These results are due to the different stiffness of the molecules, which is quantified by the viscosity index, a. 

WOCI4 is combined with Ph Sn (ratio WOCI4 Plx Sn = 1 2) in 1,4-dioxane/benzene to afford poly(phenylacetylene) efficiently, whose reaches 1.1 x 10 ([77] 1.23 dL g ) and whose m-content is 7i% High polymer yields can be achieved even in the case of a high monomer/catalyst ratio, 1260. The viscosity index, a, of poly(phenylacetylene) formed by this catalyst was determined to be 0.61, indicating a sufficiently flexible chain. 

Functions of properties such as the viscosity index or the viscosity gravity constant may be useful. However, the values of a function for a series of fractions of the same oil may be of approximately equal magnitude or may not be serial, as illustrated by the viscosity index (Table I). 

The viscosity index improver was prepared after reacting with W-phenyl-p-phenylenediamine. 

Before 1925, there were a few compounded oils made for special purposes, such as lubrication of marine engines and steam cylinders, but additives were not used in automotive crankcase oils. In the 1930 s, chemical compounds made by condensation of chlorinated paraffin wax with naphthalene were found to lower the pour points of oils. Pour depressants (9) apparently are adsorbed on small wax crystals which separate from oils when they are chilled. The protective adsorbed layer of additive prevents the normal interlacing of larger wax crystals which forms a gel. In 1934 polymerized unsaturated hydrocarbons first came into large scale commercial use to lower the temperature coefficient of viscosity of oils. Other compounds for increasing the viscosity index of oils have since become common. 

In the petroleum industry a dimensionless number termed the viscosity index has been used to describe the temperature dependence of a fluid s kinematic viscosity. The calculation of viscosity index involves the use of published look-up tables [388], In terms of relative changes, a higher viscosity index represents a smaller decrease in viscosity with increasing temperature. 

In a fully synthetic oil, there is almost certainly some mineral oil present. The chemical components used to manufacture the additive package and the viscosity index improver (VI) contain mineral oil. When all these aspects are considered, it is possible for a "fully synthetic" engine oil to surpass mineral oil (Shubkin, 1993). Synthetic oils fall into general ASTM classification (a) synthetic hydrocarbons (poly-a-olefins, alkylated aromatics, cycloaliphatics) (b) organic esters (dibasic acid esters, polyol esters, polyesters) (c) other fluids (polyalkylene glycols, phosphate esters, silicates, silicones, polyphenyl esters, fluorocarbons). 

Lubricants are formulated products composed of a base stock, which is either a mineral or synthetic oil, and various specialty additives designed for specific performance needs. Additive levels in lubricants range from 1 to 25% depending on the application. Synthetic base stocks are oligomers of small molecules, synthesized to a defined molecular weight. Important performance indicators include viscosity index which measures the viscosity index behavior over a temperature range, oxidative stability, and pour point. The performance of synthetic and mineral oils (Morse, 1998 Shubkin, 1993) is summarized in Table 2.7. 

Viscosity index (VI) improver (a) Demonstrate that the viscosity index as plotted in Figure 2.6 illustrates the viscosity index improver requirement, (b) Show that multiplication of polymeric viscosity index improver makes it possible to utilize it in several SAE viscosity grades, e.g., SAE 10, SAE 20, SAE 30, SAE 40, SAE 50. 

Viscosity index, the customary basis for comparison of change of viscosity in hydrocarbon oils, becomes less satisfactory when applied to silicone oils, because the viscosity index varies with viscosity as well as with the temperature coefficient. In silicone oils the variation of viscosity with temperature is too small in relation to the viscosity itself. A true viscosity-temperature coefficient (VTC) has been proposed as a more satisfactory criterion [see Wilcock, Mechanical Engineering 66, 739 (1944)1. 

---