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. This shows large differences in the shear thinning tendencies of different engine oil VMs. 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 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. One of the tested lubricants does not show this time temperature superposition collapse. 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. This unusual response may originate from the presence on the VM molecules of two structurally and chemically different components. 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 [1].

In a companion paper, the temporary shear thinning behaviour of a series of viscosity-modifier (VM)-containing blends was studied over a wide shear rate and temperature range [Marx et al. in Tribol Lett, https ://doi.org/10.1007/s1124 9-018-1039-5]. It was found that for almost all VMs the resulting data could be collapsed on a single viscosity versus reduced strain rate curve using time-temperature superposition. This made it possible to derive a single equation to describe the viscosityshear rate behaviour for each VM blend. In the current paper, these shear thinning equations are used in a Reynolds-based hydrodynamic lubrication model to explore and compare the impact of different VMs on the film thickness and friction of a lubricated, isothermal journal bearing. It is found that VMs reduce friction and especially power loss markedly at high shaft speeds, while still contributing to increased hydrodynamic film thickness at low speeds. The model indicates that VMs can contribute to reducing friction in two separate ways. One is via shear thinning. This occurs especially at high bearing speeds when shear rates are large and can result in a 50% friction reduction compared to the equivalent isoviscous oil at low temperatures for the blends studied. The second is via their impact on viscosity index, which means that for a set viscosity at high temperature the low-shear-rate (and thus the high shear rate) viscosity of a high-VI oil, and consequently its hydrodynamic friction, will be lower at low temperatures than that of a low-VI oil. The identification and quantification of these two alternative ways to reduce friction should assist in the design of new, fuel-efficient VMs.

The friction properties of a range of viscosity modifier-containing oils in an engine bearing have been studied in the hydrodynamic regime using a combined experimental and modelling approach. The viscometric properties of these oils were previously measured and single equations derived to describe how their viscosities vary with temperature and shear rate (Marx et al. Tribol Lett 66:92, 2018). A journal bearing machine has been used to measure the friction properties of the test oils at various oil supply temperatures, while simultaneously measuring bearing temperature using an embedded thermocouple. This shows the importance of taking account of thermal response in journal bearings since the operating oil film temperature is often considerably higher than the oil supply temperature. For Newtonian oils, friction coefficient measurements made over a wide range of speeds, loads and oil supply temperatures collapse onto a single Stribeck curve when the viscosity used in determining the Stribeck number is based on an effective oil film temperature. Journal bearing machine measurements on VM-containing oils show that these give lower friction than a Newtonian reference oil. A thermo-hydrodynamic model incorporating shear thinning has been used to explore further the frictional properties of the VM-containing oils. These confirm the findings of the journal bearing experiments and show that two key factors determine the friction of the engine bearing; (i) the low shear rate viscosity of the oil at the effective bearing temperature and (ii) the extent to which the blend shear thins at the high shear rate present in the bearing.

• The composition of bitumen affects the storage stability of hot SBS modified bitumens.• Aromatic content of 40% and solid phase volume lower than 22% improve the stability.• The increase of solid phase content hinders the dispersion of the SBS rich phase.• Changes of the physical properties at 160 e C are related to the biphasic morphology. Keywords:SBS copolymer Polymer modified bitumen Thermal stability Phase separation Structure Polymer modified bitumens, PMBs, are usually prepared at high temperature and subsequently stored for a period of time, also at high temperature. The stability of PMBs, in these conditions, has a decisive influence in order to obtain the adequate performances for practical applications. In this article the attention is focused in the analysis of the factors that determine the stability of styrene-butadienestyrene copolymer (SBS)/sulfur modified bitumens when the mixtures are maintained at high temperature. Bitumens from different crude oil sources were used to prepare SBS/sulfur modified bitumens. Changes in the values of viscosity, softening point, as well as in the morphology of PMB samples, stored at 160 °C, were related to the bitumen chemical composition and to the amount of asphaltene micelles present in the neat bitumen used in their preparation.