OIL COMPOSITION COMPRISING A POLYMERIC VISCOSITY INDEX IMPROVER

Abstract: A triblock copolymer VI improver comprising two terminal hydrogenated polyisoprene blocks and a central polymeric block containing predominantly monoalkenyl aromatic hydrocarbon monomer units. Oil compositions comprising such triblock copolymer VI improvers exhibit a good balance between thickening efficiency and mechanical shear stability combined with relatively high HTHSR viscosities.

Description: This invention relates to oil composition comprising a polymeric additive which, when added to said oil, increase its viscosity, particularly at higher temperatures. More particularly, this invention relates to lubricating oil compositions comprising a polymeric additive of the triblock variety. As is well known, the viscosities of lubricating oils vary with temperature and, since lubricating oils are generally exposed to a relatively broad temperature range during use, it is important that the oil be not too viscous (thick) at low temperatures nor too fluid (thin) at higher temperatures. As is also well known, 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 oil's viscosity at a defined lower temperature and a defined higher temperature. The defined lower temperature and the defined 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°C and the higher temperature specified in the test is 100°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 US-A-3,554,911; 3,668,125 (equivalent to FR-A-2 070 827); 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. Thus, for example, FR-A-2 070 827 discloses a lubricating oil composition comprising a major proportion of a mineral lubricating oil and as a viscosity index improver a minor proportion of a block copolymer having at least three polymer blocks C and D, which blocks consist of 1 or more blocks C and 1 or more blocks D, blocks C being a monovinyl arene polymer, or when more than 1 block C is present a monovinyl arene polymer which has been at least partially hydrogenated and/or a polyvinylcyclohexane, the blocks C having average molecular weights of 5,000 to 50,000, preferably 9,000 to 35,000, and blocks D being an alpha-monoolefin polymer or a conjugate diene polymer which has been at least partially hydrogenated, the blocks D having average molecular weights of 10,000 to 1,000,000, preferably 15,000 to 200,000. 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 US-A-3,772,196, is available commercially and is commonly used as a VI improver. The VI improvers taught in US-A-3,775,329 are tapered copolymers which may be coupled, forming two segments which are, in effect, copolymer blocks; these copolymer blocks 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 US-A-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 in which the arms are either homopolymers or copolymers of conjugated dienes or copolymers of one of 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 US-A-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, the thickening efficiency of the polymeric additive is an important, and frequently the principle, consideration in its selection for use an a VI improver. In particular it is desirable to have polymeric additives which significantly increase the high temperature kinematic viscosity without significantly increasing the low temperature kinematic viscosity. 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 subjection to mechancial 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 additives. Improved mechanical shear stability is, therefore, 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 containing the polymeric viscosity index improver. Heretofore, higher HTHSR viscosities have been sought, although, as a practical matter, the HTHSR viscosity value has normally been accepted as that dictated by the desired balance of thickening efficiency and mechanical shear stability. In general, these HTHSR viscosity values have been relatively high, but not always as high as may be required to ensure 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 US-A-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, although these diblock copolymers generally have low HTHSR viscosity values at all molecular weights. On the other hand, star-shaped polymers having a plurality of polyisoprene arms, such as taught in US-A-4,116,917 and 4,156,673 offer improved mechanical shear stability and generally offer higher HTHSR viscosities, but poor thickening efficiencies. Hence, neither of these types of polymers is perfectly suited for use as a VI improver in applications which require a combination of good thickening efficiency and mechanical shear stability while at the same time affording higher HTHSR viscosity values. It has now been discovered that many of the aforementioned deficiencies of the prior art VI improvers can be offset with the VI improvers of the present invention and improved lubricating oil compositions prepared therewith. According to the present invention, therefore, there are provided lubricating oil compositions containing a triblock copolymer comprising two terminal polymeric blocks and a central polymeric block, the terminal blocks predominantly comprising hydrogenated isoprene monomer units and the central block predominantly comprising monoalkenyl aromatic hydrocarbon monomer units, wherein at least 80% of the original olefinic unsaturation is hydrogenated and less than 20% of the aromatic unsaturation is hydrogenated. As indicated more fully hereinafter the triblock copolymer preferably contains from 50 to 82 wt%, especially 55% to 75% and in particular 63% to 72%, hydrogenated isoprene monomer units and from 50 to about 18 wt%, especially 45% to 25% and in particular 37% to 28%, monoalkenyl aromatic hydrocarbon monomer units, and preferably each block is a homopolymer. As used herein, the term "predominantly" in conjugation with polymer block monomer composition means that the block comprises at least 90 wt% of the specified monomer. In general, any of the methods well known in the prior art may be used to produce the triblock copolymers which are subsequently selectively hydrogenated, and which are then useful as VI improvers in the present invention. Suitable methods include, but are not limited to, those described in US-A-3,231,635; 3,265,765 and 3,322,856. In general, these triblock copolymers may be represented by the general formula: I-A-I wherein I is a polymeric block comprising predominantly isoprene monomer units and A is a polymeric block comprising predominantly one or more monoalkenyl aromatic hydrocarbon monomer units. In general, polymers of this type can be prepared by a sequential polymerisation in the presence of an organo metallic compound, particularly an organo metallic compound containing an alkali metal atom. Particularly preferred organo metallic compounds for use in preparing the triblock copolymers useful in the present invention with such sequential polymerization technique are hydrocarbon radicals bonded to a single lithium atom. Suitable hydrocarbon compounds containing a single lithium atom include unsaturated compounds such as allyllithium, methallylithium and the like; aromatic compounds such as phenyllithium, the tolyllithiums, the xylylithiums, the naphlylithiums and the like; and alkyllithiums such as methyllithium, ethyllithium, propyllithium, butyllithium, amyllithium, hexyllithium, 2-ethylhexyllithium, n-hexadecyllithium and the like. Secondary butyllithium is a most preferred initiator for use in the present invention. Suitable solvents useful in the preparation of the triblock copolymers include hydrocarbons such as paraffins, cycloparaffins, aromatics and alkyl-substituted aromatics containing from 4 to 10 carbon atoms per molecule. Suitable solvents particularly include benzene, toluene, cyclohexane, methylcyclohexane, n-butane, n-hexane, n-heptane, and the like. Once polymerization of the first isoprene polymer block has been completed, the monoalkenyl aromatic hydrocarbon polymeric block may be formed by adding one or more monoalkenyl aromatic hydrocarbon monomers to the solution containing the living isoprene polymer blocks and continuing the polymerization until polymerization of the monoalkenyl aromatic hydrocarbon monomer is substantially complete. Suitable monoalkenyl aromatic hydrocarbon monomers include aryl-substituted olefins such as styrene, various alkyl-substituted styrenes, alkoxy-substituted styrenes, vinyl naphthalene, vinyl toluene and the like, with homopolymer blocks derived from styrene being especially preferred. In general, the amount of monoalkenyl aromatic hydrocarbon monomer added will be controlled such that the desired molecular weight of the monoalkenyl aromatic hydrocarbon polymer block is obtained. Once preparation of the monoalkenyl aromatic hydrocarbon monomer polymeric block has been completed, the second terminal isoprene polymeric block can be prepared by adding additional isoprene, or a monomer mixture comprising predominantly isoprene, to the solution containing the living diblock copolymer prepared in the previous steps. Polymerization of the added monomer is then continued until polymerization of the added monomer is at least substantially complete. The molecular weight of the second terminal isoprene polymer block can be controlled by controlling the amount of monomer added during this third step of the triblock copolymer preparation. The I-A-I triblock copolymers which may be subsequently hydrogenated for use as VI improvers may also be prepared by first polymerizing the monoalkenyl aromatic hydrocarbon block with an anionic initiator comprising two lithium atoms and thereafter growing the terminal polyisoprene blocks. The I-A-I triblock copolymers may also be prepared by polymerizing a first isoprene block and then a monoalkenyl aromatic hydrocarbon polymer block having a molecular weight equal to 1 / 2 that sought in the final polymer and then coupling the resulting diblock copolymer using techniques well known in the prior art. In general, preparation of the triblock copolymers which will be subsequently selectively hydrogenated, may be completed at a temperature within the range from about -150° to about 300°C, may be carried out in an inert atmosphere, preferably nitrogen, and may be carried out under pressure, for example, at a pressure within the range from about 0.5 to about 10 bars. The concentration of initiator used in the polymerization reaction may vary over a relatively wide range but will be controlled according to established procedures in combination with the amount of monomer used so as to produce triblock copolymers having the desired molecular weights. The isoprene polymer blocks of the triblock copolymers preferably have weight average molecular weights, as determined by gel permeation chromatograph (GPC), within the range from 30,000 to 150,000, especially 40,000 to 125,000. The monoalkenyl aromatic hydrocarbon polymer blocks preferably have weight average molecular weights, as determined by GPC, within the range from 15,000 to 125,000, especially 35,000 to 85,000. The triblock copolymers produced via the methods described above will still contain metal atoms, particularly alkali metal atoms and preferably lithium atoms, when all of the monomer has been polymerized. These metal sites may be deactivated by adding water; an alcohol such as methanol, ethanol, isopropanol, 2-ethylhexanol and the like; or a carboxylic acid such as formic acid, acetic acid and the like. Other compounds are, of course, known in the prior art to deactivate the active or living metal atom sites and any of these known compounds may also be used. Alternatively, the living triblock copolymer may simply be hydrogenated to deactivate the metal sites. The triblock copolymers are selectively hydrogenated for use as VI improvers, using any of the techniques known in the prior art to be suitable for selective hydrogenation of olefinic unsaturation. The hydrogenation conditions employed will be sufficient to ensure that at least 80%, preferably at least 95%, and most preferably at least 98% of the original olefinic unsaturation is hydrogenated. The hydrogenation conditions are also selected so as to ensure that less than 20%, preferably less than 10% and most preferably less than 5% of the aromatic unsaturation is hydrogenated. These known techniques involve the use of a suitable catalyst, particularly a catalyst or catalyst precursor comprising a Group VI or Group VIII metal atom. Suitable catalysts are described in GB-A-1,030,306, and in US-A-3,700,633. The process taught in US-A-3,700,633 is particularly preferred for hydrogenating the triblock copolymers which are then useful as VI improvers in accordance with this invention. In this process, hydrogenation of the polymer is accomplished in the same solvent as was used during the polymerization with a catalyst comprising the reaction product of an aluminium alkyl and a nickel or cobalt carboxylate or alkoxide. In general, hydrogenation is accomplished at a temperature within the range from about 25°C to about 175°C at a hydrogen partial pressure below 750 KPa, and usually within the range from 35 to 200 KPa. In general, contacting times within the range from about 5 minutes to about 8 hours will be sufficient to permit the desired degree of hydrogenation and the selectively hydrogenated triblock copolymer is suitably recovered either as crumbs using known techniques or used directly as a solution. The selectively hydrogenated triblock copolymers useful as VI improvers in this invention may be added to a variety of oils to produce oil compositions generally having improved viscosity index characteristics. For example, the selectively hydrogenated triblock copolymers may be added to fuel oils such as middle distillate fuels, synthetic and natural lubricating oils, crude oils and industrial oils. The concentration of the selectively hydrogenated triblock copolymer in such oils may vary between wide limits with amounts within the range from 0.05 to about 2.5 wt% being preferred. Oil compositions prepared with the selectively hydrogenated triblock copolymers useful as VI improvers in this invention may also contain other additives such as anti-corrosive additives, antioxidants, detergents, pour point depressants, one or more additional VI improvers and the like. Typical additives which are useful in the oil compositions such as those of this invention and their description will be found, for example, US-A-3,772,196 and 3,835,083. The invention is illustrated in the following examples. Example 1 In this example, a selectively hydrogenated triblock copolymer comprising terminal selectively hydrogenated isoprene homopolymer blocks and a central polystyrene block was prepared by sequentially polymerizing isoprene-styrene-isoprene in the presence of a s-butyllithium initiator, using techniques well know to those skilled in the art of anionic polymerization. The weight average molecular weight (as determined by GPC) of the central polystyrene block of the triblock copolymer thus produced was 66,000, and that of the terminal isoprene blocks was 43,000. The polyisoprene/polystyrene/polyisoprene triblock copolymer was next hydrogenated in cyclohexane in the presence of a catalyst prepeared by combining Ni(octoate)₂ and A1 (Et)₃ so as to saturated at least 98% of the ethylenic unsaturation originally contained in the polyisoprene blocks. Example 2-15 and and 20-25 In these examples, a selectively hydrogenated triblock copolymer containing polyisoprene blocks and a central polystyrene block was prepared using the same procedure as was used in Example 1 except that the amount of isoprene, secondary butyllithium and styrene was varied so as to produce triblock copolymers wherein the weight average molecular weight of the polymeric blocks were as shown in Table 1. Ex. No. Weight Av. Mol. wt. of: Polystyrene block Polyisoprene blocks 2 55 000 43 000 3 43 000 41 000 4 110 000 85 000 5 101 000 43 000 6 81 000 61 000 7 105 000 59 000 8 79 000 80 000 9 86 000 42 000 10 54 000 83 000 11 36 000 83 000 12 25 000 70 000 13 31 000 84 000 14 21 000 79 000 15 39 000 105 000 20 25 000 33 000 21 32 000 43 000 22 40 000 52 000 23 47 000 62 000 24 54 000 72 000 25 62 000 81 000 Example 16 In this example, a selectively hydrogenated triblock copolymer containing terminal polyisoprene blocks and a central polystyrene block was prepared using a different technique well known to those skilled in the art of anionic polymerization. The polymer in this Example was prepared by sequentially polymerizing isoprene and styrene with a s-butyllithium initiator and then coupling two living diblock copolymer to produce a triblock copolymer. The weight average molecular weight of the styrene block in the triblock copolymer thus produced was 59,000 and the weight average molecular weight of each polyisoprene block in the triblock copolymer was about 58,000, as determined by GPC. Example 17 In this example, a selectively hydrogenated triblock copolymer containing terminal polyisoprene blocks and a central polystyrene block was prepared using the same procedure as was used in Example 16 except that the amount of isoprene, secondary butyllithium and styrene was varied so as to produce a triblock copolymer wherein the weight average molecular weight of the styrene block was 42,000 and the weight average molecular weight for each polyisoprene block was about 112,000, as determined by GPC. Example 18 In this example, a selectively hydrogenated triblock copolymer containing terminal polyisoprene blocks and a central polystyrene block was prepared using the same procedure as was used in Example 16 except that the amount of isoprene, secondary butyllithium and styrene was varied so as to produce a triblock copolymer wherein the weight average molecular weight of the styrene block was 22,000 and the weight average molecular weight of each polyisoprene block was about 100,000, as determined by GPC. Example 19 In this example, a selectively hydrogenated triblock copolymer containing terminal polyisoprene blocks and a central polystyrene block was prepared using the same procedure as was used in Example 16 except that the amount of isoprene, secondary butyllithium and styrene was varied so as to produce a triblock copolymer wherein the weight average molecular weight of the styrene block was 9,000 and the weight average molecular weight of each polyisoprene block was about 100,000, as determined by GPC. Comparison Example 26 In this example, a diblock copolymer comprising a single polystyrene block and a single polyisoprene block was prepared. The diblock was prepared using the same procedure as was used in Example 1 except that the polymerization was stopped after polymerization of the styrene block was completed. The diblock copolymer was recovered as a crumb by precipitating with an alcohol. The weight average molecular weight of the polystyrene block was about 35,000 and the weight average molecular weight of the polyisoprene block was 91,000. Comparison Example 27 In this example, a diblock compolymer containg a single polyisoprene block and a single polystyrene block was prepared using the same method as was used in Example 26 except that the amount of isoprene, s-butyllithium and styrene was varied so as to produce a diblock copolymer wherein the weight average molecular weight of the polystyrene block was about 35,000 and the average molecular weight of the polyisoprene block was about 65,000. Examples 28-54 In these examples, 1.7 wt% of each of the polymers prepared in Examples 1-27 was added to an HIV 100N oil. The kinematic viscosity at 100°C and the mechanical shear stability of each blend and the viscosity index was determined using ASTM D2270 and the mechanical shear stability was determined using ASTM D3945. The results for each blend as well as the weight average molecular weight of each of the polymers are summarized in the following Table 2. As will be appparent from the data summarized in the foregoing Table 2, the triblock copolymers lose a significantly lower percentage of viscosity in the DIN test as the molecular weight increases than do the diblock copolymers (cf., e.g. Examples 30 and 53; 47 and 54 and 42 and 53). As will also be apparent from the data summarized in the preceding Table 2, those polymers containing lower molecular weight polystyrene blocks are, generally, less effective as VI improvers (cf., e.g., Examples 39 and 41 and 42). Examples 55-81 In these examples the selectively hydrogenated block copolymers produced in Examples 1-27 were used as VI improvers in a 10W-40 multigrade lubricating oil composition. The base stock used in the preparation of the multigrade lubricating oil composition was a blend of an HVI 100N oil and an HVI 250N oil. The amount of HVI 250N oil was varied so as to provide a lubricating oil composition having a viscosity with the range from 3.2 to 3.5 PaS measured in the cold cranking simulator (see SAE J-300, Apr. 84) at -20°C. The multigrade lubricating oil compositions prepared in these examples also contained 7.75 wt% of a commercially available additive package (Lubrizol 7573) and 0.3 wt% Acryloid 160. For each oil composition, the following properties were measured: the kinematic viscosities at 100°C, the viscosity index, the cold cranking simulator (CCS) viscosity at -20°C, the high temperature high shear rate viscosity at 150°C with a shear rate of 1 x 10⁶ seconds⁻¹ using the tapered bearing simulator (TBS) according to ASTM D4683 and the mechanical shear stability using the DIN test according to ASTM D3945. The polymer used in each Example, the amount thereof added to the oil composition, the percent of HV1 250N oil used in the lubricating oil composition and all other results are summarized in the table 3 below: From the data summarized in the foregoing Table 3, it is apparent that when molecular weight is taken into consideration, the triblock VI improvers of this invention (Examples 1-25) exhibit a better balance of thickening efficiency, mechanical shear stability and HTHSR viscosity when compared to the prior art diblock polymer VI improvers (Examples 26-27). As will also be apparent from the data summarized in the preceding Table 3, the VI improvers of this invention, generally, give oil compositions having higher HTHSR viscosities than the prior art diblock copolymers.

Claims: Claims for the following Contracting States : AT, BE, CH, DE, FR, GB, GR, IT, LI, LU, NL, SE A lubricating oil composition containing a triblock copolymer comprising two terminal polymeric blocks and a central polymeric block, the terminal blocks predominantly comprising hydrogenated isoprene monomer units and the central block predominantly comprising monoalkenyl aromatic hydrocarbon monomer units, wherein at least 80% of the original olefinic unsaturation is hydrogenated and less than 20% of the aromatic unsaturation is hydrogenated. Composition as claimed in claim 1 wherein the triblock copolymer contains from 50 to 82 wt% hydrogenated isoprene monomer units and from 50 to 18 wt % monoalkenyl aromatic hydrocarbon monomer units. Composition as claimed in claim 1 or 2, wherein said monoalkenyl aromatic hydrocarbon is styrene. Composition as claimed in claim 1, 2 or 3 wherein the weight average molecular weight of the hydrogenated isoprene blocks is within the range from 30,000 to 150,000 and the weight average molecular weight of the monalkenyl aromatic hydrocarbon polymer block is within the range from 15,000 to 125,000. Composition as claimed in claim 4 wherein the weight average molecular weight of the hydrogenated isoprene blocks is within the range of from 40,000 to 125,000 and the weight average molecular weight of the monoalkenyl aromatic hydrocarbon polymer block is within the range from about 35,000 to 85,000. Composition as claimed in any one of the preceding claims, wherein the triblock copolymer is present in an amount in the range from 0.05 to 2.5 wt%. Claims for the following Contracting State : ES A process for preparing a lubricating oil composition which comprises mixing a lubricating base oil and a triblock copolymer comprising two terminal polymeric blocks and a central polymeric block, the terminal blocks predominantly comprising hydrogenated isoprene monomer units and the central block predominantly comprising monoalkenyl aromatic hydrocarbon monomer units, wherein at least 80% of the original olefinic unsaturation is hydrogenated and less than 20% of the aromatic unsaturation is hydrogenated. A process as claimed in claim 1 wherein the triblock copolymer contains from 50 to 82 wt% hydrogenated isoprene monomer units and from 50 to 18 wt % monoalkenyl aromatic hydrocarbon monomer units. A process as claimed in claim 1 or 2, wherein said monoalkenyl aromatic hydrocarbon is styrene. A process as claimed in claim 1, 2 or 3 wherein the weight average molecular weight of the hydrogenated isoprene blocks is within the range from 30,000 to 150,000 and the weight average molecular weight of the monalkenyl aromatic hydrocarbon polymer block is within the range from 15,000 to 125,000. A process as claimed in claim 4 wherein the weight average molecular weight of the hydrogenated isoprene blocks is within the range of from 40,000 to 125,000 and the weight average molecular weight of the monoalkenyl aromatic hydrocarbon polymer block is within the range from about 35,000 to 85,000. A process as claimed in any one of the preceding claims, wherein the triblock copolymer is present in an amount in the range from 0.05 to 2.5 wt%.

Inventors: HANDLIN, DALE LEE, OLSON, DANFORTH HAROLD
Applicants: SHELL INTERNATIONALE RESEARCH MAATSCHAPPIJ B.V.
Application year: 1988
Main classification: C10M143
IPC codes: C10M143, C10M159, C08F297, C10N60, C10N30