1. Field of the Invention
This invention relates to a polymeric additive which, when added to an oil will increase its viscosity, particularly at higher temperatures, and to oil compositions comprising said polymeric additive. More particularly, this invention relates to a polymeric additive of the triblock variety and to lubricating oil compositions comprising the same.
2. Prior Art
As is well known, the viscosities of lubricating oils vary with temperature and, since lubricating oils generally incur a relatively broad temperature range during use, it is important that the oil not be too viscous (thick) at low temperatures nor too fluid (thin) at higher temperatures. As is also well known, variation in the viscosity-temperature relationship of an oil is indicated by the so-called viscosity index (VI). The higher the viscosity index, the less the change in viscosity with temperature. In general, the viscosity index is a function of the oils viscosity at a given lower temperature and a given higher temperature. The given lower temperature and the given higher temperature have varied over the years but are fixed at any given time in an ASTM test procedure (ASTM D2270). Currently, the lower temperature specified in the test is 40.degree. C. and the higher temperature specified in the test is 100.degree. C.
Heretofore, several methods have been proposed for improving the rheological properties of lubricating oil compositions. Generally, these methods involve the use of one or more polymeric additives. Such methods wherein the polymeric additive is a linear or branched chain polymer are taught, for example, in U.S. Pat. Nos. 3,554,911; 3,668,125; 3,772,196; 3,775,329 and 3,835,053. The polymeric additives taught in this series of U.S. patents are, generally, hydrogenated, substantially linear polymers of conjugated dienes which may, optionally, also contain monomeric units of a monoalkenyl aromatic hydrocarbon. Polymers of the type disclosed in this series of U.S. patents are typically prepared via the anionic solution polymerization of the monomers followed by hydrogenation. The polymers may be random, tapered or block. The process for preparing the polymers involves polymerizing a conjugated diene and, optionally, a monoalkenyl aromatic hydrocarbon in solution and in the presence of an anionic initiator to form an unsaturated, so-called living polymer. The polymeric product is, thereafter, selectively hydrogenated so as to eliminate a significant portion of the ethylenic unsaturation in the polymer after its preparation. A selectively hydrogenated block copolymer comprising a single polystyrene block and a single hydrogenated isoprene polymer block, which block copolymer is within the scope of the teaching of U.S. Pat. No. 3,772,196, is available commercially and is commonly used as a VI improver. The VI improvers taught in U.S. Pat. No. 3,775,329 are tapered copolymers which may be coupled. The coupled polymers may, of course, approach a triblock copolymer but such copolymers will comprise two segments which are, in effect, copolymer blocks. These copolymer blocks will, of course, reduce the effectiveness of the tapered copolymers as VI improvers.
More recently, it has been discovered that certain so-called star-shaped polymers, such as those disclosed in U.S. Pat. Nos. 4,116,917 and 4,156,673 can be effectively used as VI improvers in lubricating oil compositions. The polymeric additives taught in these patents are, generally, hydrogenated star-shaped polymers wherein the arms are either homopolymers or copolymers of conjugated dienes or copolymers of one or more conjugated dienes and one or more monoalkenyl aromatic hydrocarbons or a mixture of such arms. The hydrogenated star-shaped polymers may be prepared by first polymerizing the arms, then coupling the arms with a suitable coupling agent and thereafter hydrogenating the star-shaped polymer product. A star-shaped polymer wherein all of the arms are homopolymers of isoprene, which star-shaped polymer is within the scope of the teaching of both U.S. Pat. Nos. 4,116,917 and 4,141,847, is commercially available and is commonly used as a VI improver.
As is further well known in the prior art, thickening efficiency of the polymeric additive is an important, and frequently the principal, consideration in its selection for use as a VI improver. Particularly, polymeric additives which significantly increase the high temperature kinematic viscosity without significantly increasing the low temperature kinematic viscosity are sought. In general, the thickening efficiency of any given polymeric additive will vary with polymer composition and structure but will, generally, increase with increased molecular weight. The ability of the polymeric additive to maintain an increase in viscosity after the same has been subjected to mechanical shear is also an important consideration in the selection of a polymeric additive for use as a VI improver. In general, lower molecular weight polymeric additives exhibit better mechanical shear stability than do the high molecular weight polymeric additives. Improved mechanical shear stability is, then, generally achieved at the expense of thickening efficiency although additional polymer may be used to offset any loss of thickening efficiency.
Another property which is frequently considered in the selection of a particular polymeric additive for use as a viscosity index improver is the high temperature, high shear rate (HTHSR) viscosity of the oil blend comprising the polymeric viscosity index improver. Heretofore, higher HTHSR viscosities were, generally, sought, although, as a practical matter, the HTHSR viscosity value associated with the desired balance of thickening efficiency and mechanical shear stability was generally accepted. ln general these HTHSR viscosity values have been relatively high but, generally, not as high as may be required to insure the maintenance of a relatively thick layer of oil in many areas of application.
As is still further known in the prior art, linear diblock copolymers comprising a single polystyrene block and a single polyisoprene block, such as taught in U.S. Pat. No. 3,772,196, can be prepared having relative thickening efficiencies and permanent shear stabilities ranging from a good thickening efficiency but poor permanent shear stability to poor thickening efficiency but good mechanical shear stability. Molecular weight of the copolymer is, of course, the principle controlling variable. The diblock copolymers generally have low HTHSR viscosity values, however, at all molecular weights. Star-shaped polymers having a plurality of polyisoprene arms, such as taught in U.S. Pat. Nos. 4,116,917 and 4,156,673, on the other hand, offer improved mechanical shear stability and, generally, offer higher HTHSR viscosities values but have poor thickening efficiencies. Neither of these type polymers is, then, well suited for use as a VI improver in applications where good thickening efficiency, good mechanical shear stability and a high HTHSR viscosity is required or at least is desirable. The need, then, for an improved polymeric VI improver which will provide a good balance between thickening efficiency and mechanical shear stability while at the same time affording higher HTHSR viscosity values is believed to be readily apparent.