Permanent viscosity loss

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. 

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... 

It is desirable that the plasticizer compounded with a polymer be permanently retained. Loss of plasticizer changes the properties of a given formulation, and can be produced by volatilization, extraction or migration. The volatility of a plasticizer in a formulation can be related to the surface area, thickness of the polymeric material, and viscosity (e.g. molecular weight) of the plasticizer itself. 

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. 

However, in such a high concentration regime we can no longer represent the relaxation times (Equation (5.92)) in terms of the intrinsic viscosity. In the low frequency limit, because there is no permanent crosslinking present, the loss modulus divided by the frequency should equate with ... 

The electric properties of polymers are also related to their mechanical behavior. The dielectric constant and dielectric loss factor are analogous to the elastic compliance and mechanical loss factor. Electric resistivity is analogous to viscosity. Polar polymers, such as ionomers, possess permanent dipole moments. These polar materials are capable of storing... 

The most widely employed synthetic oil field polymer is polyacrylamide. At elevated temperatures, hydrolysis occurs on the labile amide groups. Charge repulsion, in the newly formed carboxylate groups brings on an increase in viscosity. The viscosity increase can be permanent in pure water, but in brines, such as those found in oil producing reservoirs, the viscosity increase is usually followed by a dramatic loss of viscosity. 

The viscosity of the copolymers degrades by an order of magnitude on increase of the salt concentration to 10,000 ppm. Analysis of the sensitivity of mobility control agents to rotational shear suggests that some permanent loss of viscosity would occur for the polyacrylamide, but not the polysaccharide, at the well-bore. 

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. 

---