Study of Permanent Shear Thinning of VM Polymer Solutions

Abstract

The ultrashear viscometer (USV) has been adapted and employed to investigate the permanent shear thinning of polystyrene solutions in a series of phthalate ester base fluids. The permanent shear stability index based on viscosities measured at 10 6 s -1 , PSSI(10 6 ), has been found to be a convenient way to express the magnitude of permanent shear thinning. When comparing permanent shear thinning at various shear rates in the USV, it is very important to take account of the different times of shear that are present at different shear rates. The PSSI(10 6 ) value divided by the total time of shear is then a useful way of quantifying and comparing permanent shear thinning rates. Tests using polystyrene in different viscosity base fluids have shown that this rate of permanent shear thinning depends on shear stress and not shear rate and varies linearly with polymer concentration. The rate of permanent shear thinning also varies exponentially with shear stress, suggestive of a stress-promoted polymer breakdown process. By using a small volume of test fluid in the USV and solvent extraction after a test, it has proved possible to obtain molecular weight distributions of polymer after shear using gel permeation chromatography (GPC). This indicates that the polymer breakdown process is different at low and high polymer concentrations, with molecule fragmentation at low polymer concentration but mid-chain scission at high concentration. A key feature of the USV is that, unlike other methods currently used to measure permanent shear thinning behaviour of engine oils, it subjects the test fluid to well-defined, controllable high shear conditions. Coupled with the use of GPC, this makes it possible for the first time to relate quantitatively the permanent shear thinning of engine oils to shear conditions and to polymer degradation response.

Stress-augmented thermal activation: Tribology feels the force

“…Then, in the 1940s it was found that very high molecular weight (MW) polymers in solution could show such chain rupture when subjected to high shear rates, resulting in permanent viscosity loss and limiting the molecular weight of polymers that could be used in lubricants [121]. A short review of permanent viscosity loss of viscosity modifiers has recently been provided by Marx et al [122], while a general review of the research on the mechanochemistry of polymers up to the late 1980s is given by Sohma [123]. In 1940 Kauzmann and Eyring, at the end of their paper on polymer viscosity, suggested that one mechanism of shear thinning was rupture of polymers caused by cleavage of their backbone C-C bonds due to mechanical force [14].…”

Review of Viscosity Modifier Lubricant Additives

“…where KV base is the kinetic viscosity of the base oil at 100°C. A recent study suggested that PSSI may be a better way to describe permanent viscosity loss because it could, under some conditions, be related theoretically to polymer concentration and molecular weight [18]. [15].…”

Shear Thinning and Hydrodynamic Friction of Viscosity Modifier-Containing Oils. Part I: Shear Thinning Behaviour

“…In practice most engine oil VMs show such breakdown at shear rates above 10 6 s −1 , so that successive viscosity measurements made on the same sample at high shear rate show a progressive reduction in viscosity as permanent shear thinning accumulates. This phenomenon can be utilised to study the permanent shear thinning of polymer solutions [13]. To address this problem in the current study, a new polymer sample was used for each combination of shear rate and temperature by flushing the previously used sample 10 times with fresh polymer solution.…”

“…24. It is evident that all fit the data quite closely so it is thus not possible to use the data measured in (13)…”