Mini-Encyclopedia of Papermaking Wet-End
Part Two: Definitions and Concepts
Extensional viscosity is of concern to papermakers because it can explain certain instances of slow drainage. The most familiar type of viscosity is shear viscosity, and it can be measured in a number of ways that involve the relative flow of a liquid and a solid object or wall. Likewise, extensional viscosity of a fluid can be evaluated by experiments in which the liquid converges or diverges. Such experiments have shown that solutions of polyelectrolytes tend to have very high levels of extensional viscosity. Makers of wet-laid nonwoven fabrics take advantage of this effect to achieve good formation of mats composed of long, slender fibers. Papermakers try to avoid having any residual polyelectrolyte in the solution phase, since usually they cannot afford the loss in drainage rate.
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