时间：2022-09-20 11:57:35
Abstract:There are three predominant aspects of the rheological requirements: maximum possible high temperature thickening, minimum possible low-temperature thickening, and high shear stability for automatic transmission fluids. High temperature thickening by VI improvers is largely a function of molecular size and high solubility to give extended polymer coil size. On the other hand, the desirable characteristic of minimum low-temperature thickening is a function of somewhat the opposite characteristic; namely relatively poor solubility leading to minimal polymer coil size and thus low viscosity contribution. In addition to not thickening fluids at cold temperatures VI improvers should also provide control of wax-gelation which, if unfettered, can lead to congealed fluids. Finally the third property of high shear stability is in direct conflict with the desirable characteristic of large molecular size for effective high temperature thickening. In this paper we shall attempt to sort out these conflicting properties and provide some insight into how we approach design of VI improvers for the highly demanding world of automatic transmission fluid rheology.
Key Words: automatic transmission fluid; VI improver; shear stability; low-temperature flow
中图分类号：TE624.82 文献标识码：A
0 Introduction
Previously, we reported on various ATF Viscosity Index improver properties and effects including: shear stability, oxidation stability, and to a lesser degree, low-temperature rheology［1-2］.
Relative to shear stability, we had concluded that of various bench shear stability tests one could order their severity in the following manner: KRL>JASO sonic>FZG>ASTM 12.5 minute sonic>ASTM 3 minute sonic>FISST>Bosch Injector.While KRL was found to be much more severe than actual field service, FZG and 3 minute sonic correlated quite well with field service as shown by excellent coefficients of determination(R-square values).
The low-temperature rheology study has now been expanded and reported upon in this paper. We have conducted a more detailed examination of polymer compositional effects in API Group Ⅰ, Group Ⅱ, Group Ⅲ base stocks.These investigations cover both major low-temperature rheological phenomena: wax-gels and excessive viscosity. We have investigated polymer compositional effects as they relate to control of wax-gel matrix structures which, of course, can have enormously negative rheological effects at low-temperature. In addition to these well recognized wax related phenomena we have also investigated relative polymer solubility, which is to say polymer coil size in solution, and the attendant effects on viscosity contribution. Minimization of polymer coil size-consistent with adequate solubility-is desirable in order to achieve minimal viscosity contribution at cold temperatures.
1 Technical Background-Viscosity Index Improvers
Also there were short discussions of the technical nature of VI improvers ［1-2］.
VI improvers are high molecular weight polymers made from monomers selected so as to impart solubility in the base stocks to be used in specific applications. Either of two major chemical families are used; one being hydrocarbons, for example ethylene-propylene copolymers; while the second type contains ester functionality, for example poly(methacrylates) (PMA) and others. The ester containing chemistries dominate ATF applications because of their superior low-temperature properties. Dispersancy may be added to the base chemistry through incorporation of a polar monomer that typically contains nitrogen.
本文为全文原貌 未安装PDF浏览器用户请先下载安装 原版全文
The amount of thickening provided by a VII is directly related to the hydrodynamic volume in solution occupied by the randomly coiled polymer chain. Coil size is related to molecular weight, as a first approximation, but more exactly to the cube of the root mean square end-to-end distance of the polymer backbone. Molecular weight of the polymer backbone is an important parameter in understanding not only thickening effects but also shearing effects. Unfortunately, high molecular weight polymers are subject to mechanical shearing which results in loss of molecular weight (reduction of polymer end-to-end distance) and consequently in loss of viscosity contribution. Very high shear stresses, turbulent flow or cavitation present in lubricated equipment can lead to extreme polymer coil distortion and a concentration of enough vibrational energy to cause polymer rupture. Statistically, the rupture occurs near the middle of the polymer chain leading to two smaller molecules whose sum of hydrodynamic volumes is lower than that of the single parent molecule.
Higher molecular weight polymers are more susceptible to shearing while sufficiently low molecular weight polymers may not shear at all depending upon the severity of the equipment. Since the shearing process generates lower molecular weight species, which are eventually no longer susceptible to further degradation, the shearing process is self-limiting and the viscosity of the fluid will eventually become stable［3］.
Loss of viscosity after shearing is normally quantified by the following equation:
where: ηi is the initial viscosity, and ηs is the sheared oil viscosity. A more meaningful calculation that removes variations in fresh oil and base oil/DI viscosities and allows direct data comparison is
where: SSI is Shear Stability Index and ηo is the base oil viscosity including all additives except VI improver. It is presumed that all viscosity degradation comes from shearing of the VII only. Base oil and normal DI components are most often of sufficiently low molecular weight that they are not affected by mechanical shear. However, there may sometimes be minor shearing of DI component(s), typically high molecular weight dispersants, as noted in our previous study［1］.
Thermal and oxidative effects are usually assumed to not be a factor in laboratory bench shearing devices. However, there may be a minor role for oxidative or thermal degradation as suggested by Hillman, et al.［4］ who investigated mechanisms for degradation in the Kurt Orbahn and FZG shear tests.
Polymers are known to undergo chemical reactions involving oxidative and thermal processes when under severe enough conditions. The major oxidative reaction leads to random scissions along the polymer backbone resulting in loss of molecular weight. Since the site of an oxidative scission is totally random, the consequence for loss of viscosity depends on whether the breakage is near the middle of the molecule or near the end. Scission near the end of a molecule results in only minor reduction of molecular weight and thus minor if any impact on viscosity. Conversely, scission near the middle of a polymer chain can inflict the same viscometric consequences as mechanical shear.
本文为全文原貌 未安装PDF浏览器用户请先下载安装 原版全文
In practical terms, VI improver stability is highly dependent on molecular size and the severity of the mechanical shearing environment, while oxidative stability is a function of the polymer existed environment and the protection afforded by the antioxidants normally present in the additive package.
2 Low-Temperature Viscosity and Viscosity Index Improvers
It is important to note that low-temperature flow failures can occur by either of two mechanisms.The first is the inability of oil to flow because of a gel structure normally created by the crystallization of wax at cold temperatures. In the arena of engine oil lubrication this behavior is known as ‘air binding’ as air is pumped through the engine rather than liquid because liquid oil is bound within the wax-gel structure. The second failure mode is simply very high viscosity which prevents the oil from being pumped or from flowing in a timely manner. This behavior, often referred to as ‘flow limited behavior’, is what might be called the ‘native viscosity’ of the fluid.In other words, it is the viscosity of the fluid, absent wax-gel effects. One can obtain best case estimates of a fluids native viscosity by extrapolating higher temperature viscosity behavior to lower temperatures as though the fluid were entirely Newtonian. (which it may not be).These low-temperature flow behaviors and their potential consequences on equipment durability and performance are well documented in engine oils; but both phenomena can apply equally well to the behavior and consequences of mineral oil lubricants in transmissions.
To control the wax related viscosity problem, it is necessary to employ additives that control crystallization of paraffins or other wax like substances in mineral oil based lubricants. Paraffin wax in oil is in part removed by refinery dewaxing but it is uneconomical and unnecessary to completely remove the wax. The remaining wax is controlled by the addition of Pour Point Depressant additives or more typically in transmission fluids by VI improvers that are constructed to act as Pour Point Depressants in addition to providing thickening and high Viscosity Index contribution. In either case these additives are added to lubricant formulations in order to control wax crystallization and prevent the formation of wax-gel structures. These additives function by virtue of co-crystallization of their long, linear (wax like) alkyl side chains at cold temperatures with waxes present in mineral oil; thereby disrupting further crystal growth through steric hindrance from the large size (high molecular weight) of the VII/PPD molecules. A typical PPD molecular weight value would be on the order of about 80,000 AMU and VI improvers are generally even higher; this depends on the required shear stability.
It is obviously desirable to minimize VI improver viscosity contribution at cold temperatures (the opposite applies at warm temperatures). While it is clearly important to maintain solubility of the additive, one can influence VI improver viscosity contribution by minimizing its solubility and thus its coil size at colder temperatures. As stated previously, VI improvers containing ester functionality dominate the ATF field because of their excellent low-temperature viscosity effects. It is because of the polarity of the ester groups, attached to the polymer backbone, in combination with non polar solvent-mineral oil-that at cold temperatures give poorly solvated hence small polymer coils. At warm temperatures, ester containing polymeric molecules are usually well solvated, have extended coil sizes and thus much higher viscosity contribution. This behavior is of course often categorized as high Viscosity Index. It is important to note for our studies reported herein that the choice of the alkyl group attached to the ester can accentuate coil size contraction; short alky ester side chains make for greater polarity.
本文为全文原貌 未安装PDF浏览器用户请先下载安装 原版全文
So, good low-temperature flow properties are a consequence of having an acceptably low ‘native’ viscosity as well as the absence of a wax-gel structure, which would inhibit flow. Both properties are highly influenced by VI improver.
3 ATF Low-Temperature Viscosity Measurement in the Laboratory
Laboratory low-temperature flow testing is invariably defined in automatic transmission fluid specifications by ASTM D 2983 ‘Determination of Apparent Viscosity by Brookfield Viscometer’［5-6］.The test is a low-shear rate procedure with a shock cooling profile extending down from room temperature to typically -40 ℃ over 16 to 18 hours followed by measurement at the same temperature. Some automatic transmission fluid specifications call for additional measurements at other warmer temperatures but essentially all ATF specifications do call for a -40 ℃ measurement. The actual viscosity measurement can be conducted with the aid of a balsa wood box to enclose the sample, in its glass tube, and insulate it while the spindle is attached and the measurement made. Alternatively, the sample container may be moved to a liquid bath to maintain the desired test temperature within the sample during spindle attachment and measurement. The actual Brookfield measurement is conducted at a very low shear rate, and it is quite important to do so. The low spindle speed and the large gap between rotor and stator provide conditions where wax-gel structure, if present, is not destroyed by the act of measurement itself. Clearly, this is of large importance in order to understand and measure the effect of wax-gel on viscosity.
The low-temperature Brookfield viscosity test is not so sophisticated as those employed in the field of engine oils. Engine oil low-temperature, low-shear rate test procedures, such TP-1 Mini Rotary Viscometer and Scanning Brookfield, employ slow cooling in order to better allow crystal growth that might lead to wax-gel structure should it be a fluid problem. The ASTM D 2983 Brookfield procedure seemingly avoids providing good crystal growth possibilities with a rapid temperature cool down. Nevertheless, the procedure has been employed since the latter part of the 1950s to evaluate low-temperature performance of automotive lubricants and its utility is probably related to the choice of the severe test temperature, at least for ATF, of -40 ℃. This test temperature is well beyond most real world applications of ATF but using it (in combination with ever more stringent limits) has probably provided a significant safety margin despite its unsophisticated cooling profile.
A last thought on low-temperature rheology testing is the importance of measuring viscosity on fully formulated fluids. Certainly, base stock and VI improver choice are of great importance in both native viscosity and wax related effects. Not as well known is that Detergent/Inhibitor (D/I) package components may often influence low-temperature rheology. For instance, higher molecular weight D/I components can thicken oil, most notably oligomeric dispersants. Perhaps not as well appreciated is the fact that D/I components may contribute wax-like behavior and influence wax-gel properties either in a positive or negative manner. A prominent example would be detergents with long, linear alkyl chains attached to sulfonate or phenate aromatic ring structures. These wax-like alkylates influence low-temperature properties in somewhat unpredictable ways. There may be positive contribution (they may act as weak Pour Point Depressants) or negative contribution (contribute gel characteristics).In either event it is important to account for this activity and choice of the appropriate VI improver (or Pour Point Depressant) to control wax crystallization.
本文为全文原貌 未安装PDF浏览器用户请先下载安装 原版全文
4 ATF Low-Temperature Viscosity Needs and Requirements
Central to modern automatic transmission systems is the need for efficient low-temperature performance from the transmission fluid. A properly controlled transmission fluid low-temperature rheology can offer reduced clutch engagement response time, reduced cranking loads for the power plant, better fuel economy, and a synergy with the general transmission engineering. Accordingly low-temperature rheology requirements for Automatic Transmission Fluids have become much stricter. Current ATF specification of -40 ℃ Brookfield viscosity limit requires 20,000 mPa•s maximum and many cite even lower limits. These new requirements have put a bright spotlight on low-temperature viscometric performance and the choice of premium base stocks and VI improvers to meet the challenge. For this reason we have conducted a study of VI improver compositional effects in wax crystallization control and minimum coil sizing. These studies have been conducted in a range of base stock compositions from API Group Ⅰ through API Group Ⅲ.
In order to help meet the ever more demanding low-temperature viscometric requirements for transmission fluids, the base oil or blend of base oils used for this application have evolved to employ specialized base stocks accompanied by a general decrease in viscosity (often for fuel economy reasons) and an increase in oxidative stability. The base stocks being employed are solvent extracted API Group Ⅰ oils, hydrocracked, catalytically dewaxed API Group Ⅱ oils, and wax hydroisomerized or severely hydrocracked API Group Ⅲ oils. These oils are distinct from one another in their chemistry, solvency for additives, and wax type/concentrations; not to mention their contribution to final fluid economics.
5 Customizing Viscosity Index Improver Technology
While it is necessary that the transmission fluid and its additive package be tailored to the engineering requirements of the transmission system, it is likewise important that the Viscosity Index improver be specifically tailored to the base oil or blend of base oils used for the fluid. It is the job of the fluid′s Viscosity Index improver to impart not only high temperature viscosity at a reasonable shear stability level but also to afford minimal low-temperature viscosity contribution, and in ideal cases to influence the inherent waxes from the fluid′s base oil or blend of oils.
This customizing of the Viscosity Index improver is necessary given the characteristics described previously. Especially critical is the Viscosity Index improver/base oil or base oil blend solvency relationship and wax relationships. This is to say that a Viscosity Index improver that performs efficiently in one oil system may not necessarily perform as efficiently or even well in another. This is demonstrated in Table 1.
From the above it is evident that VII A has been designed for use in an API Group Ⅰ fluid with VII B being designed for rather general application across the three API oil Groups, and lastly VII C demonstrating best performance in an API Group Ⅲ oil.
本文为全文原貌 未安装PDF浏览器用户请先下载安装 原版全文
Engineering a Viscosity Index improver to yield minimal low-temperature viscosity contribution for specific base oil or blends thereof may be achieved through appropriate Wax Interaction Factor (WIF) definition and by achieving an optimal Minimum polymer Coil Size (MCS). The Wax Interaction Factor is a relative indicator that depends on those compositional components of the Viscosity Index improver that are able to interact with the paraffinic elements of the base oil(s) and thus control the nucleation and crystal growth processes. The Minimum polymer Coil Size, on the other hand, is a reference to the compositional elements of the Viscosity Index improver that influence the polymer′s hydrodynamic volume in solution-ultimately a function of polymer solubility dynamics.
In general the composition of a poly alkyl methacrylate (PAMA) Viscosity Index improver consists of varying proportions of different alkyl chain length methacrylate monomer units.Alkyl chain length of the methacrylate is in reference to the R group as seen:
Typically these proportions include a short alkyl chain length component, usually C1 through C4, a mid length alkyl chain, C8 through C15, perhaps the incorporation of higher alkyl chain length monomers, C16 through C22, and/or other functional monomeric moieties-methacrylate or otherwise.It is the judicious proportion and the selection of these monomers that allow for optimization of the Viscosity Index improver′s low-temperature performance.
Taking a closer look at the compositional elements of PAMA Viscosity Index improvers, we can find that their relative insolubility is a function of monomers polarity in relationship to the mineral oil solvent.The mid-length alkyl chain monomers are, in general, quite soluble for the given system. For this class of monomer it is more a case of the isomeric structure of the monomer unit that lends functionality for low-temperature to the VII polymer. The longer chain alkyl units are less important in terms of insolubility but offer interaction advantage with the paraffinic elements of the oil(s). As mentioned, other monomers may be incorporated for specific functionality-usually apart from low-temperature performance. Such function building necessitates a broader understanding and attention to the overall polymer system balance.
Given the breadth of compositional units and their respective impact upon performance for PAMA Viscosity Index improvers, it is apparent that in order to achieve desired performance in the various API base oil Groups a careful selection of monomers in exacting ratios must be made. As stated, the Wax Interaction Factor is largely influenced by the longer alkyl chain monomers. The nature of such an influence may be best considered as a co-crystallization of the polymer′s long alkyl chain with the paraffins from the oil during the onset of the respective crystallization of various paraffin chain lengths. This co-crystallization interrupts the formation of macro wax structures and ultimately limits the growth of the system′s viscosity. On the other hand, the short chain monomers have no direct influence on the WIF but do affect the polymer′s hydrodynamic volume through decreased solubility and therefore impact the measured dynamic viscosity. This response relates proportionally to the MCS.
本文为全文原貌 未安装PDF浏览器用户请先下载安装 原版全文
Table 2 presents a comparison of Viscosity Index improvers with varied ratios of long and short alkyl chain components. These Viscosity Index improvers are of generally equal polymer shear stability and concentration in the bulk state. Apparent from the data is how crucial is the need for balancing the WIF modifying component against the MCS modifying one. Equally evident, is the strong effect the MCS has on controlling the low-temperature performance.
The mid-length alkyl monomers are an equally essential part of building a PAMA Viscosity Index improver for reasons of economics, solubility, overall viscosity contribution, and their apparent role in WIF activity. This class of PAMA monomer may be categorized according to isomeric structure: branched and linear. By evaluating prepared Viscosity Index improvers with increasing concentrations of branched isomers (trading for the equal concentrations of the linear-all else held equal) one may see in table 3 that there is a general Brookfield viscosity sensitivity in the API Group Ⅰ fluid while being rather indiscriminate in the API Group Ⅱ, and a varied response in the API Group Ⅲ fluid. Moreover, the data indicate a deterioration of low-temperature performance, and similarly the WIF, from increased concentration of the branched monomer. This is seen in Table 3.
After considering the low-temperature impact from those components that influence the WIF and MCS it becomes obvious that a delicate optimization of these must be made for modern and future base stocks used in this application. Such careful engineering of the Viscosity Index improver′s long, mid, and short length alkyl components can yield a desired low-temperature fluidity response. Table 4 demonstrates how such a balance impacts this performance.
In this table no one Viscosity Index improver may claim the best low-temperature performance in all the fluids. It is through the customization of the Viscosity Index improver polymer composition that the balance of the MCS and WIF characteristics, and in the end the low-temperature viscosity contribution, may be made.
6 Conclusions
•Given today′s stringent rheological requirements in ATF specifications, no single Viscosity Index improver is capable of efficiently serving all groups of base stocks.
•ATF low-temperature viscometrics may be controlled through a carefully engineered Viscosity Index improver.
•The ability of the Viscosity Index improver to influence the base oil′s inherent waxes, its Wax Interaction Factor, may be optimized through manipulation of the polymer′s composition.
•In addition to controlling waxes from the ATF′s base oil, the Viscosity Index improver can influence low-temperature viscometrics via maintaining a Minimum Coil Size-a function of polymer solubility.
•A thoughtfully constructed Viscosity Index improver that is customized for the specific base oil of use is the best way to achieve outstanding low-temperature performance from an ATF.
References:
［1］ Kinker B. Automatic Transmission Fluid Shear Stability Testing And Viscosity Index Improver Trends［A］. International Symposium on the Tribology of Vehicle Transmissions［C］. Yokohamo:1998.
［2］ Kinker B. Automatic Transmission Fluid Shear Stability Testing And Oxidative Stability［A］. International Symposium on the Tribology of Vehicle Transmissions［C］. Toyota:2001.
［3］ Mortimer R M. Chemistry and Technology of Lubricants［M］. Blackie Academic & Professional, 1997:158-165.
［4］ Hillman D E, Morris R R, Paul J I, et al. Comparison of the Modes of Degradation of Viscosity Index Improvers in the Kurt Orbahn and FZG Tests by Gel Permeatation Chromatography［C］. Materials Quality Assurance Directorate Technical Paper.
［5］ ASTM D 2983 Standard Test Method for Low-Temperature Viscosity of Lubricants Measured by Brookfield Viscometer［S］.
［6］ Alexander D L, Rein S W. Relationship between Engine Oil Bench Shear Stability Tests ［C］. SAE paper 872047.
本文为全文原貌 未安装PDF浏览器用户请先下载安装 原版全文