A study of the oxidative deterioration of mineral lubricating oils

The effect of oil composition on its behaviour under oxidative conditions, both by air-blowing and in an internal combustion engine, was the main purpose behind this research programme. Initially it was necessary to devise techniques for the accurate determination of molecular weight and chloroform insolubles, and, to modify various other methods of analysis so that they were suitable for this particular work. An ebullioscopic technique involving the use of a thermistor sensing element was used to determine molecular weight, but rather severe fluctuations were experienced in the meter readings. In an attempt to rectify this the cold finger condenser was replaced by a permanently fixed Liebig condenser - thus giving a constant volume of solvent in the ebulliometer. However, this was not the complete answer to the fluctuations, these being finally eliminated by the use of benzene (as solvent) which had been carefully re-distilled and dried over sodium wire. Apparently moisture was the principal factor causing the inaccuracies. Segregation of that part of the oil insoluble sludge derived from engine wear and blow-by particles was carried out by taking up the engine deteriorated oil in chloroform and filtering off the extraneous material. This was made possible by the ease with which the oil and the sludge produced by oxidation of the oil could be dissolved in this particular solvent. The minimum amount of chloroform required per gramme of deteriorated oil was established oil was the most efficient method of solvent removal, i.e. rotary film evaporation. This treatment of the used oil sample did not effect the structure or composition of either the oxidised or unoxidised portions. In this programme several oils of different compositions were oxidised by air-blowing at 200°C (a modified I.P, oxidation test), and by running under steady conditions in an internal combustion engine for various periods of time. In general, the pattern of deterioration by oxidation was similar in both systems, i.e. the oxidative deterioration followed the path of: hydrocarbons→rasins→oil soluble sludge →oil insoluble sludge. The oils used in this programme all behaved in a similar fashion under these conditions, but differed from one another in the amounts of various oxidation products produced. The extent to which any one oil followed the path of oxidation outlined above was apparently governed by the percentage of aromatic material contained in the original oil. Some evidence was obtained supporting the theory of "optimum aromaticity" in the range of 6-8% aromatics. This effect was very evident in oils oxidised by air-blowing but was less obvious when the some oils were deteriorated in the engine. In this work it was shown that a similar optimum effect existed when the ratio of aromatics : naphthenics was considered. At aromatic concentrations lower than 7 or 8% insufficient oxidation inhibiting materials, e.g. naphthols, were formed to prevent the formation of oil soluble sludge, and at this "optimum aromaticity" enough inhibitor was formed to limit the oxidation to the production of rosins. Where the aromatic content was high the amount of inhibitor formed was not even sufficient to prevent the oxidation process to proceed yet another step to oil insoluble sludge. Conditions in the engine appeared to favour oil soluble sludge formation but not the formation of oil insoluble sludge. It may have been that the metals present counteracted the effect of the inhibitors formed, thus allowing soluble sludge, and that insufficient time had been spent at engine conditions for insoluble sludge to be produced. In both methods of deterioration the same hydrocarbon types were oxidised to give similar oxidation products. The material which was removed upon oxidation and treatment with fuller's earth was found to be spread over the hydrocarbon types present in the original oil with a tendency for the heavier aromatic material to be preferentially attacked. This tendency was more apparent in the oils containing most aromatics. The tendency for hydrocarbons to be oxidised to resins, than to oil soluble sludge, and finally to oil insoluble sludge was made more apparent by the evidence found during this programme of work. A transfer of material apparently took place from rosins, via soluble sludge, to insoluble sludge. In addition, it was found that the acidity of the oil insoluble sludge was related to its insolubility in the oil - this may have been duo to the formation of hydroxy acids (by the oxidation of oil soluble sludge) which are known to be insoluble in mineral lubricating oils