The viscosity achieved depends on the density of cross-links which is in turn controlled by the chemical equilibria of the
system, pH and temperature.
Tetrahydroxy borate anions in the pH range 8.5 - 12.5 form complexes with cis-diols. The borate-polymer reaction is a dynamic
equilibrium, with the cross-links forming and dissociating in the millisecond timescale. First, the tetrahydroxy borate
anion forms a bidentate monoester, and this reacts further to form a tetradentate diester (spiroboron complex - see Figs
B4 and H3).
 |
Enlarge Image |
| Fig H3: cross-linking of polyhydroxy polymers by borates |
|
|
These 2:1 complexes form a network of connected molecular chains that increase fluid viscosity.
The amount of complex in the solution is proportional to the tetrahydroxy borate anion concentration, itself dependent
upon pH and temperature.
When diluted to normal wash concentrations, the chemical equilibrium of the system requires that the complexes dissociate.
The cross-links are then broken, so reducing the viscosity.
|
