Mini-Encyclopedia of Papermaking Wet-End Chemistry
Part Two: Definitions and Concepts

Flocs: Hard vs. Soft

When papermakers use the word "flocs," they usually are referring to groups of fibers clumped together. Strong hydrodynamic forces, such as those present in fan pumps and pressure screens, can be very effective in redispersal of flocs. However, shear forces also cause fibers to bend. As the fibers straighten out, they may lock together again mechanically. Papermakers sometimes make the distinction between hard flocs and soft flocs. Hard flocs are those formed by very-high-mass retention aid polymers. Hard flocs a quite strong, but when broken, they are not able to form as strong flocs the next time. By contrast, soft flocs are readily broken by moderate levels of shear, and they are completely reversible. Factors causing soft flocs to form include colloidal forces of attraction and the previously mentioned effects of shear flow. It is worth keeping in mind that colloidal chemists and water-treatment specialists use the word "flocs" to denote voluminous or gel-like particles formed as a result of high-mass polyelectrolytes in the absence of fibers.

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This page is maintained by Martin hubbe, Associate Professor of Wood and Paper Science, NC State University, .