Current theories about fiber-fiber bonding in wet paper webs leave room for various concepts and interpretations. Up to now, it has not been possible to prove any of them by images showing the actual condition of the web. The reason for this is the preparation of samples, which tends to impair the bonding structure of fibers but has been necessary to achieve the desired resolution so far.<br/>To solve this problem, handsheets of different moisture levels were formed from unrefined and refined pulps. Specimens were prepared by cryofixation and subsequent freeze-drying to image the convergence of fibers by means of high-resolution Field Emission-Scanning Electron Microscopy (FE-SEM) technology.<br/>The images show the behavior of microfibrils in the fiber-water gel. Partly detached microfibrils protrude from fibers into the surrounding water and interact with neighboring fibrils and fibers. In the specimens made from refined pulp, the interaction is not limited to single microfibrils – portions of the S1 and S2 wall layers are capable of moving freely in the fiber-water gel to form contact areas with the surrounding fibrous material as well. We can therefore assume that van der Waals forces exist between fibers already in the wet mat. Acting like hook and loop fasteners, they create strong cohesive bonds within the fiber structure.<br/>The images reveal the processes involved in the convergence of fibers and microfibrils even when acquired at high moisture levels, thus confirming a number of models that had previously been merely suppositional.

Making fiber-fiber bonds visible

249th National Meeting (2015)

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