Lube-Tech PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE


types of base stocks, both mineral oil derived (including naphthenic oils) and a selection of synthetic base stocks (PAO, PAGs and Esters).


Each base stock type has its own unique set of characteristics which makes it the preferred choice in certain applications. In recent years there has been a decline in the availability of Group I but this has been more than compensated for by the increase in production of Group II and Group III base stocks (including GTL), as the demand for lower viscosity engine oils has increased.


Naphthenic oils find use in greases, metalworking fluids, transformer oils and process oils and PAOs in a wide range of high performance oils including but by no means limited to automotive transmission fluids, engine oils, industrial gear oils and incidental food contact lubricants. Polyalkylene glycols find their niches in fire resistant hydraulic fluids, metalworking and metal quenching fluids, industrial gear oils, incidental food contact lubricants and a range of other high performance industrial applications.


Esters are used in an enormous array of applications including engine oils, transmission fluids, refrigeration lubricants, aviation lubricants, air compressors, chain lubricants, incidental food contact applications, metalworking fluids, greases, hydraulic fluids, transformer fluids and many more.


Table 2 actually does not do justice to the variety of ester base fluids in commercial use today, nor does it explain how properties can be controlled through modest changes in the chemistry of the ester, some of which will be explained in the following sections.


Kinematic Viscosity at 40°C and Viscosity Index Mineral oil derived base stocks are basically hydrocarbons and there are only so many hydrocarbon structures that can be refined or chemically synthesised which limits the variety and diversity of base fluids that can be produced. With regard to viscosity at 40O


C, this means that Group I base stocks are


typically limited to the ISO range 10 – 460, Group II to ISO 22 – ISO 100 and Group III to ISO 22 – ISO 46. Naphthenic oils are limited to ISO 10 – 460.


Synthetic base stocks have a much wider viscosity range. PAOs can be produced with kinematic viscosities typically ranging from 5 – 10,000 mm2


/s at 40O C, although in basic chemical composition


terms PAO is a hydrocarbon of defined linearity, branching and molecular weight. Polyalkylene glycols (PAGs) have the broadest viscosity range of the synthetic base stocks considered here. PAGs can be produced in the viscosity range of 10 – 165,000 mm2


/s


and of the all the product types shown PAGs are unique in that they are the only synthetic base stock type that can be chemically synthesised to be soluble in water, a characteristic that can be controlled through the ethylene oxide content in the product. By varying the type of oxides and the molecular weight of the molecules one can dictate whether the PAGs are water-soluble or oil-soluble, or indeed insoluble in both water and oil.


Vegetable and animal esters are very limited in their viscosity range (typically ISO 32 and 46), which is governed by the type of crop from which they are derived.


34 LUBE MAGAZINE NO.129 OCTOBER 2015


Hydrocarbon base stocks and PAG base stocks are a mixture of different chemical compounds and / or oligomers. In terms of volatility characteristics, it is the lower molecular weight molecules that evaporate most easily and are essentially responsible for the temperature at which a specific base fluid will begin to evaporate and the amount of evaporation that takes place at a given temperature.


Many esters have a distinct advantage over other base stocks in that through the appropriate choice of raw materials it is possible to make esters having the same viscosity at a given temperature as other base stocks but with a very narrow molecular weight distribution. This means that esters have the lowest volatility properties of all the mineral oil derived and synthetic base stocks described in table 1.


Whilst progress is being made in the development of lower volatility Group II and Group III base stocks, figure 2 shows the relative Noack volatility performance characteristics of Group III and PAO base stocks and how they compare to the superior Noack volatility properties of ester base fluids.


No.100 page 3


Synthetic oleochemical and petrochemical esters are the most diverse and flexible of all the base fluids, due to the fact that there is such a wide variety of raw materials to choose from and which can be combined in almost unlimited ways. This means that it is possible to produce many synthetic esters with the same viscosity properties at 40O


C (typically in the range of


2 – 50,000 mm2/s) but with very different physical and chemical characteristics.


In terms of Viscosity Index (VI), much progress has been made in the development of Group II and Group III base oils compared to Group I base oils. The higher Viscosity Index of Group II and Group III base oils (120+) is a highly prized characteristic in the formulation of engine oils especially. Viscosity index for PAOs is higher again at around 130.


The chemistry of the ester dictates the Viscosity Index of ester base fluids. Aromatic esters such as the trimellitates typically have VIs in the region of 80, polyol esters made using short chain acids (for example C5 – C10 acids) are typically in the VI range 110 – 130 (similar to Group I, GroupII and Group III base stocks), diesters and polyol esters made using longer chain fatty acids can have VIs in the range 130 – 200 and polymeric esters can have VIs ranging from 150 to 300. Indeed, some of the very high viscosity polymeric esters can be used as VI booster additives.


Volatility Volatility is a critical characteristic for automotive and industrial applications alike. Some applications call for high volatility, for example, aluminium cold rolling uses an annealing process to remove lubricant, whereas other applications call for very low levels of volatility, for example, engine oils and high temperature chain lubricants.


Volatility is a function of chemistry, molecular weight and molecular weight distribution.


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