Viscosity index improver polymers or viscosity modifiers (VMs) have been used as engine oil additives since the mid-1930s. In a companion paper, the master shear thinning curves obtained in this paper are used to explore how VMs impact film thickness and friction in a steadily loaded, isothermal journal bearing. It is found that for almost all of the VMs, time–temperature superposition enables the shear thinning behaviour at all temperatures to be described by a single equation. By using three different viscometers, full flow curves of viscosity versus shear rate are obtained at several temperatures and fitted to various shear thinning equations. This paper describes measurement of the temporary shear thinning behaviour of a range of engine oil viscosity modifier (VM) additive solutions. It is well known that their blends exhibit shear thinning at the high shear rates present in lubricated contacts and that the resulting reduction in viscosity leads to thinner lubricant films and lower hydrodynamic friction than predicted in the absence of shear thinning. Viscosity modifier additives are used to increase the viscosity index of lubricants and are key components of most crankcase engine oils. This unusual response may originate from the presence on the VM molecules of two structurally and chemically different components. This fluid also exhibits extremely high viscosity index and shear thins more easily at high than at low temperature, unlike all the other solutions tested. One of the tested lubricants does not show this time temperature superposition collapse. This enables shear thinning equations to be derived that are able to predict the viscosity of a given oil at any shear rate and temperature within the range originally tested.
It has been found that viscosity versus shear rate data at different temperatures can be collapsed onto a single master curve using time–temperature superposition based on a shear rate shift factor. This shows large differences in the shear thinning tendencies of different engine oil VMs. Viscosity versus shear rate curves have been measured up to 10 7 s −1 for a range of VM solutions and fully formulated oils of known composition at several temperatures.
0 kommentar(er)
