Temporary viscosity loss

Temporary viscosity loss All polymer solutions are non-Newtonian, that is, shear stress is not directly proportional to shear rate. There are several types of non-Newtonian behaviour, but VI improvers at high temperature exhibit only one pseudoplasticity, or shear-thinning. 

As with permanent viscosity loss, temporary viscosity loss is a function of molecular weight, thus the higher the molecular weight, the greater the temporary viscosity loss. It is also important to note that engine oils based on VI improvers which exhibit no permanent viscosity loss in service are still generally non-Newtonian, i.e. the molecules are distorted but there is insufficient energy to break chemical bonds. 

Fig. 5.10 Temporary viscosity loss vs. permanent viscosity loss, after [71]...

Primary polymer structure may account for some of the viscosity loss behaviour, but VI improvers which function as associative thickeners are a major confounding factor. When the physically associated multi-polymer structures enter a shear field, they can dissociate into their separate molecular species. These smaller individual polymers are low enough in molecular weight that they degrade either slowly or not at all. When the molecules leave the shear field, they associate again so that there is little or no permanent loss of viscosity. However, when in the shear field, the contribution to viscosity is from the smaller, distorted individual molecules. The net result is a system which exhibits a high temporary viscosity loss relative to its low permanent viscosity loss. 

Combined permanent and temporary viscosity loss Whilst much of the work that has been done in this area has focused on the isolated effects of permanent or temporary viscosity loss, equipment in the field obviously sees the combined effects. Limited work has been reported on the net effect, but effort in this area appears to be increasing. 

On the other hand, there are some changes that take place reversibly. These correspond to temporary viscosity losses associated with a change in the spatial conformation of the chains in solution. Increasing the shear rate lines up the molecules in the flow field and causes the drag on the molecules to decrease. This causes a viscosity loss as discussed earlier in the paper. 

The addition of electrolytes to a solution decreases the viscosity of a fresh polymer solution as will be discussed in the next section. High temperatures less than the thermodegrading conditions (less than 70-100°C depending on the polymer) cause a drop in the viscosity. This is also a reversible change associated with the change in the conformation of the chains in solution. The temperature effect on solution viscosity will be discussed. The effects of salt, temperature, severe shear, and aging will be discussed from the standpoint of viscosities and chain conformations in solution. First of all, the factors causing temporary viscosity losses will be presented. 

Temporary viscosity loss and its relationship to journal bearing performance, SAE 1978, Paper No. 780374. 

Nanomaterials were used by Zhang et al. (4) as a replacement for polymer-based viscosity modifiers for automotive lubricants. Compared with traditional polymer-based viscosity modifiers, nanomaterials induce a more even viscosity increase across engine operating temperature ranges. In addition, nanomaterials provide a viscosity modifier that exhibits temporary shear loss that can contribute to fuel economy. 

Substantial amounts of EPM are also used as viscosity modifiers in lubrication oils. Molecular weight, molecular weight distribution, ethylene propylene ratio and in particular sequence distribution are important parameters to meet the desired performance. They markedly influence the thickening efficiency, low temperature properties, temporary and permanent viscosity loss due to shear, and engine performance as a whole. Much work aims at modification of the EPM... 

There is the assumption here that the permeability is constant, and in fact there is no permanent loss of permeability resulting from polymer flow. In any case, the mobility parameter must be treated as an entity since there is no way of separating the apparent viscosity variable from a hypothesized temporary reduction in permeability. 

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