Please use this identifier to cite or link to this item: http://localhost:8081/xmlui/handle/123456789/1285
Title: STUDIES OF THE EFFECT OF BASE OIL COMPOSITION ON ADDITIVE RESPONSE
Authors: Adhvaryu, Atanu
Keywords: CHEMISTRY;BASE OIL COMPOSITION;ADDITIVE RESPONSE;BASE OIL
Issue Date: 1997
Abstract: Base oil is the precursor for manufacturing of lubricating oil. In early 1920s any oil was acceptable for lubrication if it adhered to certain physico-chemical characteristics. But with increase in diversity and severity of application, a specific grade base oil along with blended additive package is required to meet desired product specification. It is widely recognized that the performance of a finished lubricant is sensitive to the properties of the base stock used to formulate the product. The two basic criteria applied in accessing base stocks are : first, the hydrocarbon composition should be such that they perform satisfactorily in the intended application; second, the base stock should be responsive to additives to attain better performance at reasonable cost. The specifications ofa good base oil demand excellent thermal stability, low pour point, high viscosity index etc., resulting in low sludge deposits, metal corrosion, oil thickening and wear/tear of metal parts. Certainly, the most critical performance aspect of base oil is oxidative resistance - a base stock hydrocarbon composition dependent phenomena. Hence, more than ever before, a complete molecular level understanding of the hydrocarbon types of the base oil is required to ensure its performance characteristics as well as additive response. In the present study the effect of base oil composition on additive response is undertaken. An in-depth understanding of the molecular composition and their participation in thermo-oxidative degradation is reported. This is required to know the extent and severity of oxidation in a dynamic system. It has been established by now that hydrocarbon type information is inadequate to explain the base oil molecule-additive interactions. The behavior of aromatic molecules depend not only on the number of rings but also on the number, size and position of alkyl substitution on these rings. The NMR is the only technique which provide informations on these lines. The study undertaken shows at the molecular level the participation and the influence of base oil hydrocarbons during thermal oxidation and additive response. The contents of the thesis are divided into five chapters. Chapter 1 reviews the molecular composition of base oils with special emphasis on the hydrocarbon types and their properties. This chapter also gives a brief account of the conventional method of base oil manufacture followed by some of the recent advances in base oil refining e.g., hydrorefining, catalytic wax isomerization etc. A brief survey of various synthetic lubricants, their advantages and disadvantages in use are reported. This chapter also includes literature survey on various additives and their reactivity under operating conditions. Compositional studies of base oils using various chromatographic and spectroscopic techniques have been reported. Special emphasis has been given to characterization using NMR and FTIR techniques in the study of base oils. Chapter 2 deals with the detailed characterization of base oils. Physico-chemical tests were carried out using different standard procedures. The various base oils are separated column chromatographically into three distinct hydrocarbon types, such as saturates, aromatics and polars. They are then characterized in detail using FT NMR (!H and 13C) and FTIR techniques to compute their average structural parameters and aromatic substitution profile. These structural data can explain the variation in physico-chemical properties of various base oils, their oxidative tendencies and response towards antioxidant used. Multi-component regression analysis between NMR derived parameters and physico-chemical properties of several other base oils belonging to wide viscosity ranges are carried out and the equations are established on the basis of these correlations. These Ill equations can minimize experimental time and are extremely useful for rapid analysis of quality of hase oils. Bench top oxidation of base oils and their chromatographic fractions are discussed in Chapter 3. Effect of polar doping on synthetic and base oil during oxidation is also studied. The apparatus used is a static reactor containing the sample at 120°C in which dry 02 is passed at the rate of 1 l/hr for 48 hrs in presence of activated Cu-wire catalyst. 'H and ' CNMR spectroscopy is used to study structural changes and rearrangements occurring in the hydrocarbon matrix during oxidation. It is observed that cyclisation through Dickmann condensation is the reason for increase in % CN, indicating the formation of cyclic saturated compounds which is further identified through FTIR studies. In the aromatic fraction, significant alkylation is observed from increase in % Cara|k value. The decrease in average chain length (ACL) is found to be maximum in aromatic fraction than in saturates. This phenomena suggest that alkylation is mainly by alkyl groups generated from the cleavage of long alkyl substitution on aromatic rings. Significant amount of condensation of aromatic rings in the base oil is observed during oxidation. The increase in % Carb and fc corroborate this phenomena. The C NMR overlaid spectra of the oxidized base oilontWe neat one also showssharp peaks at 128-130 ppm from condensed aromatic structures, supporting the above observation. It is also observed that both saturate and aromatic fractions decrease in yield on oxidation to produce polar products which subsequently polymerize to increase viscosity of the system. This decrease is the. significant inAaromatic fraction suggesting that the polar compounds have their origin mainly from the aromatic fraction. *H and 13C NMR studies on the base oil have shown that all saturate types (n-and iso-paraffins)except naphthenes degrade at the same and slow rate, while the naptheno-aromatics and polyaromatic structures show high susceptibility towards oxidation as indicated from their polar yield. IV In Chapter 4 detailed kinetic studies using DSC under programmed mode are reported on base oils having different molecular composition, synthetic mixtures and additive doped base oils. Several thermodynamic parameters like Activation energy (Ea), Rate constant (k), Exponential factor (Z) and Half life period (t1/2) are computed and their variations are explained in terms of molecular composition andNMRderived structural parameters of base oils. It is observed that oxidation of base oils usually follow first order kinetics and Arrhenius equation can be applied to such system to calculate various thermodynamic parameters. The study reveals that polars affect the rate of oxidation reaction significantly and antioxidant additive decreases the rate of reaction. However, the failure to follow the theoretical relation between Ea and kcould possibly be due to large changes in the entropy of complex molecular composition of base oils during oxidation. It is observed that molecular level study of different hydrocarbons present in the base oil matrix can be useful in understanding their oxidative and kinetic behavior both under neat and additive doped conditions. Chapter 5 deals with the oxidation of column chromatographically separated saturates and aromatics as well as neat base oils with various doses ofantioxidant additives (ZDDP) over extended period. The oxidized materials are later chromatographically separated and characterized using FT-NMR ( H and C) and FTIR techniques. Structural changes and rearrangement in the hydrocarbon matrix, in general, and carbon-skeleton in particular, are studied in terms of variation observed in the NMR derived structural parameters. FTIR technique is used to quantify the extent of oxidation and identify the oxidation products from the spectral range 1815-1650 cm"1 using deconvolution technique. The study revealsthat saturates exhibit good response to anti-oxidants and thus the oxidation of alkyl groups on aromatics could be reduced by addition of additives. Further, alkyl benzenes give good solvency to the additive molecule for their effective role in oxidation inhibition. However, the mechanism in actual base oil suggests a mixed effect. The study revealsthat moLecularfypes the response ofa particular hydrocarbon type is dependent on the presence ofother; the net response is the sum total of all the responses put together. It is noticed that the inhibitor depletion rate is influenced by n- and iso-paraffins, alkylbenzenes (deduced in terms of%Cnp, %Cip and %Caralk from C NMR studies) and presence of sulfur heteroatomics, which decrease the rate of acid formation and the rate of post-inhibited oxidation. Similarly various other structural parameters obtained through NMR studies (i.e. % C;lr, % Caralk, % Csal, % CN etc.) could be used to explain the results obtained from DSC onset temperature measurement of various base oils. This study provides in-depth information on the mechanism of oxidation as well as additive response observed in base oils having different molecular composition in terms of their structural parameters. Important aspects like condensation-polymerization, cyclisation, radical initiated alkylation, rearrangement in the carbon skeleton of base oils, their separated fractions and formation of different oxygenated compounds following oxidation have been discussed in light of their average structural parameters. Computation of kinetic as well as thermodynamic parameters of different base oils and their relative variation is explained in terms of their structural details at the molecular level. The response of antioxidant additive has been discussed in terms of the composition of base oils.
URI: http://hdl.handle.net/123456789/1285
Other Identifiers: Ph.D
Research Supervisor/ Guide: Singh, I. D.
Sharma, C. L.
metadata.dc.type: Doctoral Thesis
Appears in Collections:DOCTORAL THESES (chemistry)

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