Efforts to improve the performance of natural mineral oil based lubricants by the synthesis of oligomeric hydrocarbon fluids have been the subject of important research and development in the petroleum industry for at least fifty years and have led to the relatively recent market introduction of a number of superior polyalpha-olefin (PAO) synthetic lubricants, primarily based on the oligomerization of alpha-olefins or 1-alkenes. In terms of lubricant property improvement, the thrust of the industrial research effort on synthetic lubricants has been toward fluids exhibiting useful viscosities over an extended range of temperature, i.e.,improved viscosity index, while also showing good lubricity, thermal and oxidative stability and pour point equal to or better than mineral oils. These new synthetic lubricants may exhibit lower friction and hence increase the mechanical efficiency of the equipment in which they are used, for example, mechanical loads such as worm gears, gear sets, and traction drives as well as in engines and they may do so over a wider range of operating conditions than mineral oil lubricants.
Notwithstanding their generally superior properties, PAO lubricants are often formulated with additives to enhance those properties for specific applications. The more commonly used additives include oxidation inhibitors, rust inhibitors, metal passivators, antiwear agents, extreme pressure additives, pour point depressants, detergent-dispersants, viscosity index (VI) improvers, foam inhibitors and the like, as described, for example, in Kirk-Othmer "Encyclopedia of Chemical Technology", 3rd edition, Vol. 14, pp. 477-526, to which reference is made for a description of such additives and their use. Significant improvements in lubricant technology have come from improvements in additives.
Recently, high VI lubricant compositions comprising high viscosity index polyalpha-olefins (referred to here as HVI-PAO) have been disclosed in U.S. Pat. Nos. 4,827,064 and 4,827,073. The process for making these materials comprises, briefly, oligomerizin a C.sub.6 -C.sub.20 1-alkene feedstock such as 1-decene with a reduced valence state Group VIB metal catalyst, preferably a reduced chromium oxide on a porous silica support, to produce high viscosity, high VI, liquid hydrocarbon oligomers which have a characteristic structure with a branch ratio less than 0.19. The process is distinctive in that little isomerization of the olefinic bond occurs compared to known oligomerization methods to produce polyalpha-olefins using Lewis acid catalyst. A similar process using coordination catalysts to prepare high polymers from 1-alkenes, especially chromium catalyst on a silica support, is described by Weiss et al. in J. Catalysis 88, 424-430 (1984) and in DE-OS 3,427,319 and this process and its products are discussed in more detail below.
The high VI oligomers are characterized by good flow properties, usually having a pour point below -15.degree. C. Lubricants produced by the process cover the full range of viscosities from low viscosity lubricants such as 5cS fluids to higher viscosity lubricant additives useful as VI improvers, for instance, oligomers having a viscosity of 1,000 cS or more, as described in Application Ser. No. 07/345,606, now U.S. Pat. No. 5,012,020 to which reference is made for a description of these high viscosity materials and their preparation. These high viscosity oligomers, too, exhibit a remarkably high VI and low pour point even at high viscosity. The as-synthesized HVI-PAO oligomer has olefinic unsaturation associated with the last of the recurring monomer units in the structure and accordingly, the oligomer will usually be subjected to a final hydrogenation treatment in order to reduce residual unsaturation to make a final, fully stable product.
Considering the abundance of C.sub.2 to C.sub.5 alpha-olefins in the petroleum refinery, and their low cost, it has long been recognized that they could be a preferred source of low cost lubricant if they could be oligomerized to provide high viscosity index lubricant in good yield with a manageable, regenerable, non-corrosive catalyst. Prior application Ser. No. 07/345,061 now U.S. Pat. No. 4,990,709 discloses a process for making oligomers of C.sub.3 -C.sub.5 olefins using the reduced metal oxide catalysts. The olefin oligomers produced by the process are characterized by a unique structure which confers particularly useful properties on the products. In conventional Ziegler oligomerization of alpha olefins the oligomers produced contain a high degree of structural regularity, or regio-regularity, as exhibited by a preponderance of head-to-tail bonding in the oligomerization of these alpha olefins. In the products from Ziegler catalyzed oligomerization not more than twenty percent of the repeating units are linked by regular head-to-head and tail-to-tail bonding. In the olefin oligomers produced from the reduced metal oxide catalysts, however, at least twenty percent of the repeating units are bonded by irregular or head-to-head or tail-to-tail connections. These C.sub.3 -C.sub.5 alpha-olefin oligomers therefore have a regio-irregularity of at least twenty percent, usually from 20 to 40 percent, and in most cases, not more than 60 percent (where 100% regio-regularity corresponds with all head-to-tail connections for the recurring oligomeric units). Thus, in most cases, from 60 to 80 percent of the recurring connections in the oligomer are linked by regular head-to-tail bonding.