Engine oils play a significant role in maintaining the performance and longevity of internal combustion engines. The use of proper additives in the production of engine oils maintains engine cleanliness and impedes the formation of impurities. The dissolution of additives in the base oils leads to interaction with the base oil components. The composition of the base oil, as well as the degree of its purification, significantly affects the effectiveness of the additives in it [1, 2].

It is important to evaluate the interactions of additives with the hydrocarbon classes, such as polyaromatic hydrocarbons and tar-asphaltene substances. These classes of hydrocarbons are involved in the process of polycondensation and formation of high-temperature deposits. On the other hand, they determine the colloidal stability of additives [3, 4]. Polyaromatic hydrocarbons and resinous-asphaltene substances in the lubricants are in the state of associates which complicates the assessment of the interactions of additives with them [5-7].

Experimental Part

The high-temperature catalytic oxidation method was used to get the most informative indicators of lubricants operating at high-temperatures and to study the changes in the physicochemical properties of the lubricating oils [8-10]. The method simulates the oxidation (thermochemical transformation) of lubricants in the most stressed temperature part of the engine cylinder, which allows to test lubricants in the most severe operating conditions and predict its operation in the engine [10, 11].

In order to evaluate the high-temperature properties of base oils, six samples (three petroleum fractions, as well as their mixture and two synthetic base oils) were subjected to high-temperature catalytic oxidation for 180 min. at 230°C (Table 1.).

Table 1. High-temperature deposit forming tendency of base oils

Results and Discussion

Synthetic base oils (PAO-4 and PAO-8) possessed the least tendency to form deposits and destruction of the base stock. Increasing the boiling point of petroleum fractions leads to the rising of the deposits. The formation of the small amount of sediment in synthetic oils is due to the presence of free double bonds in PAO. The presence of branched hydrocarbons, as well as methylene groups undesirably affects the high-temperature properties of base oils [3, 12].

During the exploitation process aging of engine oil occurs which is accompanied by a change in the structural group composition. The alteration in the composition of the base stock was evaluated after three-hour oxidation of the residue >500^⸰C (Table 2).

The initial fraction contains 81.58 % of “desirable” components – naphthenic-paraffinic and monocyclic aromatic hydrocarbons. Hydrocarbons which are prone to sedimentation account for 18.42%. The following ratio is observed after the three-hour oxidation process: the “desirable components”- 64.85 %, deposit formers – 35.15% which comprise of 9.24 % asphaltenes and copper naphthenate.

Table 2. Structural group composition of residue >500^⸰C

In order to assess the effect of various groups of hydrocarbons on the formation of deposits the correlation coefficient (R) was determined for various hydrocarbon groups of petroleum fractions (Table 2).

The correlation coefficient characterizes the static relationship between two random variables. Assuming that a strict order relation is given on the values of the variables, then a negative correlation – is a correlation in which an increase in one variable is associated with a decrease in another variable, while the correlation coefficient can be negative; a positive correlation under such conditions – is a correlation in which a rise in one variable is associated with an increase in another variable, however, the correlation coefficient can be positive [13].

In the case of naphthenic-paraffinic hydrocarbons the correlation coefficient is negative, which indicates that with an increase in the content of these hydrocarbons, the amount of deposit in the oil decreases, however in the case of other groups of hydrocarbons, the correlation coefficient is positive. The closer the value of the correlation coefficient is to 1 the greater the influence of the group hydrocarbons has on the formation of high-temperature deposits [14].

Bicyclic hydrocarbons, polycyclic hydrocarbons and resins have the greatest deposit forming tendency (Fig.1).

Fig.1. Influence of structural-group composition of residue on the formation of high-temperature deposits:
1 – bicyclic aromatic hydrocarbons; 2 – polycyclic aromatic hydrocarbons; 3 – resins

Conclusion

The assessment of the oxidation of lubricants by the high-temperature catalytic oxidation method showed that as the base oil oxidizes, the amount of bicyclic, polycyclic hydrocarbons and resin increases, that causes the rising of the high temperature deposit forming tendency of lubricating oils. The branching degree of hydrocarbons and the presence of unsaturated bonds influence the formation process of high-temperature deposits in the synthetic base oils.

Lubricating oils play a significant role in the increasing of life expectancy of internal combustion engines. Moreover, proper lubrication is an important factor to maintain better performance of automotive engines [1].

Base oils and a variety of chemical additives are the essential components for formulation of automotive lubricants that impact the lubrication system of modern engines. Therefore, creation of the interaction with base oils and additives are crucial for the efficient use of engine oils [2, 3]. The base oil composition influences the effectiveness of the detergent-dispersant additives in automotive lubricants [4]. Detergent-dispersant additives are most common additives used in modern motor oils. Furthermore, they are essential component in order to formulate of any engine oils [4-6].

Commercial additives are a solution of the active substance in oil-solvent. Oil-solvent can be synthetic and petroleum oil [2, 7]. The composition of oil-solvent of commercial additives can contain paraffin-naphthenic, aromatic (monocyclic, bicyclic, polycyclic) hydrocarbons and resins [8]. Bicyclic, polycyclic hydrocarbons and asphaltene and resins components can decrease the effectiveness of the action of additives. They are in the form of associates in the base oil, therefore they complicate the interaction of base oils with additives [3, 7, 8].

Experimental Part

Two commercial additives, petroleum high alkaline calcium sulfonate and high alkaline calcium alkyl salicylate are used in the research.

Standard research methods were used for determination of alkalinity and kinematic viscosity of base oils. Analytical methods (change in electrical conductivity, determination of structural group composition of base oils) were used in order to evaluate the interactions between additives and base oil components [9-12].

The electrical conductivity method for measurement of hydrocarbon liquids was used to study intermolecular interactions in the additives colloidal systems and oil-oxidation products. Determination of electrical conductivity was carried out on an electrometric installation. Any slight change in colloidal systems is accompanied by a change in their donor-acceptor properties, that is reflected in the indicator of the electrical conductivity [9-11].

The determination of chemical group composition method is chromatographic method that provides the separation of the sample into 6 groups: paraffin-naphthenic hydrocarbons, monocyclic aromatic hydrocarbons, bicyclic aromatic hydrocarbons, polycyclic aromatic hydrocarbons, resins, asphaltenes. Moreover, the high-temperature catalytic oxidation method was used to study the changes in the physical and chemical properties of the base oils and to get the indicators of lubricants at high-temperatures [10, 11].

Results and Discussion

The solvent-oil of commercial additives contains resins, polycyclic and bicyclic aromatic hydrocarbons, which can reduce the effectiveness of the additives (active substance) [8]. The active substance was isolated from commercial additives prepared with petroleum oil-solvents using a steam solvent (precipitation and decantation). The isolated active substance was dissolved in PAOM synthetic oil and a “purified” additive was obtained.

“Purified” calcium sulfonate interacts with 40-45% of resins, while commercial calcium sulfonate practically does not interact with resins. The process of chemical interaction of the “purified” additive is influenced by a drop in electrical conductivity with an increase in the concentration of resins (Fig.1).

Fig. 1. Influence of resins on the electrical conductivity of calcium sulfonate

1 – synthetic oil-solvent; 2 – petroleum oil-solvent

A slight abrupt change in the electrical conductivity of commercial calcium sulfonate in the region of 40-80% resin concentration in the additive-resin mixture indicates a variation in the solvate layer of additives. However, the replacing of the oil-solvent of calcium salicylate causes not only the changing of reactivity of “purified” additive, but also its nature of the interaction (Fig. 2).

Fig. 2. Influence of resins on the electrical conductivity of calcium salicylate

1 – synthetic oil-solvent; 2 – petroleum based oil-solvent.

The chemical interaction with resins (20-22% in the mixture with an additive) occurs in the case of commercial calcium salicylate, that is indicated by a drop in electrical conductivity followed by a steady constant value of electrical conductivity. The interaction of “purified” calcium salicylate occurs in steps. At the first stage, as in the case of a commercial additive, chemical interaction occurs with resins (18-20%), that is indicated by a drop in electrical conductivity. A further increase in electrical conductivity in the resin concentration of 20–45% shows the occurrence of colloidal process-solubilization (second stage). So, the difference in the reactivity of additives depends on the composition of the oil-solvent.

Dissolution of additives in the base oil leads to the formation of a new solvate shell. Moreover, physical changes occur in the structure of the additives. These processes affect the colloidal stability, the effectiveness of the lubricant additives.

The greatest interaction occurs between additives and bicyclic, polycyclic aromatic hydrocarbons, resins. These groups of hydrocarbons are sources of high-temperature deposits, while their presence in the base oil increases the colloidal stability of the additives.

Sulfonates and salicylates additives contain “strongly” polar groups and under the action of an electric field they are deposited. Dissolution of such additives in the base oil causes the physical interaction between the polar components of the base oil (polycyclic aromatic and resinous substances). Furthermore, these interconnections lead to a decrease in the formation of high temperature oxidation and an improvement in the colloidal stability of the lubricant additives.

Conclusion

Commercial additives prepared on the base of synthetic oil-solvents perform their functions 20-30% more efficiently compared to similar additives prepared on the base of petroleum oil-solvents. Replacement of petroleum oil-solvents with synthetic oil-solvents leads to the increasing of the reactivity of sulfonates and salicylates additives.