Flocculation of fiber suspensions studied by Rheo-OCT

When dealing with papermaking fiber suspensions, particle flocculation takes place even before the paper web is formed. The particle flocculation depends on several aspects, including particle mass concentration (consistency), particle collisions, electrochemical interactions promoted by chemical additives, etc. Due to its impor-tance, fiber suspension flocculation has been studied for a long time in papermaking, and several methods have been developed for this purpose. The traditional techniques include, for example, focused beam reflectance micros-copy (FBRM) and high-speed video imaging (HSVI). Recently, a new optical method, optical coherence tomography (OCT), has emerged for flocculation analysis. The advantages of OCT are the possibility to study opaque suspensions, its micron-level resolution, and its high data acquisition speed. The OCT measurements can be combined with rheological (Rheo) measurements, allowing simul-taneous measurement of both the time evolution of the floc size and the suspension viscosity. In this work, we used this approach, Rheo-OCT, to study the flocculation of suspensions of various papermaking furnishes. We analyzed the time evolution of the floc size and the fiber suspension viscosity when the studied paper-making suspensions were treated with highly refined furnish (HRF) — a furnish that contained a significant amount of micofibrillated cellulose (MFC)-type fibrils — and/or chemical additives. Such studies can lead to a better under-standing of the impact of flocculation on the produced paper web in terms of qualities like formation, drainage potential, and strength behavior.