Lubricants

Abstract: The invention relates to lubricants and aims to provide a satisfactory lubricant for industrial and automotive requirements based on vegetable oil rather than mineral oil. The lubricating oil of the invention comprises a palm oil liquid fraction derivative, known as a palm olein, and containing a minor amount of an ester additive to retard crystallisation of the palm olein. The ester additive is preferably an ester of sorbitan and stearic acid, e.g. sorbitan tristearate and may be present in an amount of from 0.04 to 0.24% by weight.

Description: This invention relates to lubricants and particularly to lubricants based on vegetable oil. Automotive and industrial lubricating oils have hitherto been largely based on mineral oils. Despite the wide availability of vegetable oils and the apparent attractions of using such renewable and environmentally-friendly source materials, the use of vegetable oils as lubricant base fluids has been highly restricted. They have not found wide applicability in high-performance modem lubricants. Rape seed oil and castor oil, for example, have been and are used in lubricants in specific applications. We have now found that it is possible to provide satisfactory high-performance lubricants based on palm oil. Accordingly, in one aspect, the invention provides a lubricating oil comprising a palm olein with a minor amount of an ester additive to retard crystallisation of the palm olein, wherein the ester additive is an ester of sorbitan and stearic acid and the palm olein has an Iodine Value of from 62 to 67. The palm olein will normally be used as a base fluid in a lubricant containing a variety of other ingredients conventionally used, e.g. dispersants, anti-oxidants and detergents. In another aspect the invention provides the use as a lubricating oil of a composition comprising a palm olein containing an ester additive to retard crystallisation of the palm olein, wherein the ester additive is an ester of sorbitan and stearic acid and the palm olein has an Iodine Value of from 62 to 67. For example, the ester additive may be sorbitan monostearate, sorbitan tristearate or polyoxyethylene sorbitan tristearate, sorbitan tristearate being particularly preferred. Palm olein is a liquid fraction derivative obtained when the palm oil is fractionated into solid and liquid fractions. It typically has Iodine Values (IV) in the range from about 55 to 67. Iodine Value is an indication of the degree of unsaturation of the oil, the higher the value the greater the unsaturation. See Institute of Petroleum method IP84 for a detailed explanation of its measurement. Palm olein has been widely used, particularly in the Far East and South America, as a food/edible oil, particularly for frying. It is well known that clear palm olein of IV approximately 55 becomes cloudy as its temperature drops below about 20°C. This is believed to be due to crystallisation of some of the molecules, primarily triglycerides, within the palm olein. It is also known that such crystallisation may be retarded by suitable additives such as, inter alia, sorbitan esters as described above. The actual temperature and form of crystallisation depends on a number of factors including the IV of the olein used, the rate of cooling and whether the test method is isothermal or temperature programmed. For example, if test methods are equal, a specific palm olein of relatively high IV might start to become cloudy due to crystallisation at, say, 15°C, whereas one of lower IV, e.g. 55, might start at 20°C. Similarly, two identical oleins may crystallise at different temperatures if subjected to different temperature regimes. It has previously been suggested that palm oleins of IV about 55 or 56 that include a crystallisation retarder (hereafter referred to as "stabilised") will remain clear for extended periods when stored under air-conditioned supermarket-type conditions at about 20°C. This appears to have been based partly, at least, on constant temperature test methods comparing stabilised and "control" palm oleins held at a constant temperature, say 20°C, over a period of several days. We have found, however, that such isothermal tests are not a useful indicator of the stability required for a lubricant, which will encounter a much greater climatic variability (above and below 20°C) under its use and storage conditions. We have, therefore, devised a uniformly repeatable test cycle in which palm olein in stabilised or natural (control) form can be slowly and repeatedly cooled and then warmed between desired minimum and maximum temperatures, say 12°C and 23°C, 24 hours being taken for a full cycle from maximum to minimum and back to maximum. Each 24 hour cycle is then repeated over a number of days, typically for three weeks. We have found that such slow cycles force precipitation in the palm olein differently from "shock" temperature changes or single temperature testing. Hence, we have found that for use in a lubricating oil, the stabilised palm olein should have a IV of from 62 to 67. Palm oleins of lower IV have been found to be unsuitable in that, even when stabilised, they are likely to undergo undesirable crystallisation causing cloudiness and unsuitability for the intended purpose, during normal changes m temperature in a tropical climate. It had also previously been suggested (see The DSC Thermal Analysis of Crystallisation Behavious in Palm Oil, II, by K. Kawamura, J. American Oil Chem. Soc., 1980, Vol 57, No. 1, pages 48-52) that the stabiliser (crystallisation retarder) was preventing or hindering formation of triglyceride seed crystals in the palm olein. We believe, however, that in the temperature cycles of our above-mentioned test procedure and hence, in the temperature cycles more likely to be actually encountered in use as lubricative oils, both diglyceride and triglyceride seed crystals tend to form and hence, both need hindering by the stabiliser. Thus, although not wishing to be limited by any particular theory, it is thought likely that stabilisers such as sorbitan tristearate can be used to sterically hinder the formation of both tri- and di- glyceride crystals. Again, we believe this may be an important factor requiring the choice of more specific palm oleins for use in lubricating oils. Thus, we have now surprisingly found that such stabilised palm oleins may have useful application as lubricants, particularly in relatively hot climates where minimum temperatures are unlikely to fall below about 12°C. Thus, for example, they may be of particular value in tropical regions of countries such as Brazil, Malaysia, Singapore, Indonesia and Thailand. The lubricants with which this invention is concerned may find application as automotive or industrial lubricants. As automotive lubricants they may find applicability in a wide range of uses covering two-stroke and four-stroke engine lubricants, gear oil lubricants and transmission fluids. As industrial lubricants, they may find applicability as hydraulic oils, chain saw lubricants, gear oils and metal-working. The amounts of ester additive required for satisfactory performance can be very small. For example, 0.04% to 0.24% by weight based on the amount of palm olein may be found sufficient, 0.04% to 0.18% being especially useful. The actual preferred amount will depend on the specific ester used, the palm olein used, indicated for example, by its Iodine Value and, possibly, the type and amount of any other additives that may be included for specific purposes.

Claims: A lubricating oil comprising a palm olein with a minor amount of an ester additive to retard crystallisation of the palm olein, wherein the ester additive is an ester of sorbitan and stearic acid and the palm olein has an Iodine Value of from 62 to 67. A lubricating oil according to Claim 1, in which the palm olein is the base fluid of the lubricant, which additionally contains one or more dispersants, anti-oxidants and detergents. A lubricating oil according to Claim 1 or 2, in which the ester is sorbitan monostearate, sorbitan tristearate, or polyoxyethylene sorbitan tristearate. A lubricating oil according to any one of the preceding claims, in which the ester additive is present in an amount of from 0.04% to 0.24% by weight. A lubricating oil according to Claim 4, in which the ester additive is present in an amount of up to 0.18% by weight. The use as a lubricating oil of a composition comprising palm olein containing an ester additive to retard crystallisation of the palm olein wherein the ester additive is an ester of sorbitan and stearic acid and the palm olein has an Iodine Value of from 62 to 67. The use as a lubricating oil of a composition according to Claim 6, in which the palm olein is the base fluid of the lubricant, which additionally contains one or more dispersants, anti-oxidants and detergents. The use as a lubricating oil of a composition according to Claim 6 or 7, in which the ester is sorbitan monostearate, sorbitan tristearate, or polyoxyethylene sorbitan tristearate. The use as a lubricating oil of a composition according to Claim 6, 7 or 8, in which the ester additive is present in an amount of from 0.04% to 0.24% by weight. The use as a lubricating oil of a composition according to Claim 9, in which the ester additive is present in an amount of up to 0.18% by weight.

Inventors: DOBBIN PENELOPE J, WOOD VICTORIA M E, DOBBIN, PENELOPE J., WOOD, VICTORIA M.E.
Applicants: CASTROL LTD, CASTROL LIMITED
Application year: 1993
Main classification: C10M101
IPC codes: C10M101, C10M169