Field of the Invention
The present invention relates to non-polymer thickened multigrade engine oils based on synthetic hydrocarbons. More specifically, SAE 5W-30, SAE 10W-30 and SAE 15W-40 engine oils derived from hydrogenated decene-1 oligomers and which do not contain viscosity index improvers are provided.
Description of the Prior Art
SAE 10W-30 is probably the engine oil viscosity grade most widely recommended by manufacturers for gasoline passenger car service. SAE 5W-30 engine oils for gasoline passenger car service have in recent years also been gaining in use. For diesel truck operation, SAE 15W-40 is the most widely recommended engine oil viscosity grade.
All of the above oils are multigrade or cross-graded which, in general terms, means that they are acceptable for use in either a summer or winter environment. More precisely, these oils must meet the SAE J300 JUN87 specifications. For an SAE 10W-30 oil, this means that the oil has a viscosity of 3500 centipoise or below at -20.degree.C. as determined in accordance with ASTM D-2602 and a viscosity between 9.3 and 12.5 centistokes at 100.degree.C. as determined in accordance with ASTM D-445. Additionally, the formulated oil would have a borderline pumping temperature (ASTM D-3829) of -25.degree.C. or below and a stable pour point (FTMS 791b-203) of -30.degree.C. or below. SAE 15W-40 oils would have a maximum viscosity of 3500 centipoise at -15.degree.C., a viscosity between 12.5 and 16.3 at 100.degree.C., borderline pumping temperature of -20.degree.C. or below, and a stable pour point of -25.degree.C. or below. Similar viscosity specifications are defined by SAE J300 JUN87 for 5W-30 oils and for other multigrade oils.
In addition to satisfying the viscosity criteria, multigrade engine oils must also meet certain service classifications of the American Petroleum Institute (API). This is accomplished by the addition of appropriate performance additives to the oil. It should be noted that it is the formulated oil, i.e. the base oil with all of the performance additives, which must meet the SAE J300 JUN87 viscosity criteria.
To obtain multigrade motor oils using petroleum base stocks, it is also necessary to add a viscosity index (VI) improver. VI improvers are polymeric materials, such as ethylene-propylene copolymers, hydrogenated styrene-diene block copolymers, polyalkyl methacrylates, polyisobutylenes, ethylene-vinyl acetate copolymers or the like, which modify the rate of change of viscosity of the basestock with temperature when added thereto. While the polymeric VI improvers are necessary to achieve cross-grading with petroleum basestocks, the addition of these polymers is not without problem.
It is well documented in the prior art that the high molecular weight polymeric VI improvers can undergo shear, i.e., breakdown, under conditions of thermal and mechanical stress. Breakdown of the VI improver alters the viscosity characteristics of the formulated motor oil and can also contribute to the formation of sludge and engine deposits. Field studies have shown, for example, that a SAE 15W-40 diesel engine oil can drop to SAE 15W-30 after only several thousand miles of service. This presents a very real problem with heavy duty over-the-road trucks where it is not uncommon to accumulate 30,000 miles between service intervals. Breakdown of VI improvers is also a problem with gasoline engines, particularly in view of the longer drain intervals which are now being promoted and the fact that today's smaller engines operate at higher RPM's and higher temperatures. The general problems associated with the breakdown of polymeric VI improvers is discussed by W. Wunderlich and H. Jost in their entitled "Polymer Stability in Engines", Society of Automotive Engineers, Inc. SAE-429, Paper No. 780372.
One approach to overcoming the problems associated with the use of VI improvers is to develop improved polymers which are more resistant to shear under conditions of thermal and mechanical stress. While the development of new polymeric thickeners is a viable approach, it would be even more desirable and advantageous if VI improvers could be totally eliminated from multigrade motor oil formulations.
European Pat. application Nos. 88,453; 119,069; and 119,070 disclose multigrade lubricants which are combinations of synthetic fluids having different viscosities. The lubricants consist of blends of high viscosity ethylene-alphaolefin copolymers with lower viscosity synthetic hydrocarbons, such as an alkylated benzene or polyalphaolefin, or ester, such as a monoester, diester or polyester. 5W-40 and 10W-40 oils indicated as being suitable for use as diesel crankcase lubricants obtained by blending different synthetic products are disclosed.
U.S. Pat. No. to R.E. Pratt discloses base oils for motor oil uses comprised predominantly of tetramer (C40) and pentamer (C.sub.50) fractions. U.S. Pat. No. 4,282,392 to Cupples et al discloses hydrogenated mixtures of 1-decene oligomers with improved viscosity-volatility properties by virtue of high proportions of tetramer. There is no indication in either of the references to the preparation of multrigrade engine oils.
It would be highly desirable and advantageous if multigrade engine oils suitable for most passenger car and diesel truck service could be obtained using a synthetic hydrocarbon basestock without a VI improver. This would preclude compatability problems and eliminate the heretofore described breakdown problems caused by high shear conditions.