On increasing wet-web strength with adhesive polymers

The strength of fiber-fiber joints is a critical component of the overall strength of paper. With freshly formed very wet pulp fiber webs, all evidence suggests there are no fiber-fiber crossings with significant joint strength. With water removal, a point will be reached where fiber-fiber adhesion starts to contribute to the overall wet-web strength. The literature reveals very few examples of polymers that increase fiber-fiber joint strength in freshly formed webs. Herein we summarize the literature and explain why it is so difficult to promote fiber-fiber wet adhesion with polymers. Nevertheless, ongoing research in areas as diverse as tissue engineering scaffolds and biomimetic adhesives gives clues to future developments. Advances in papermachine engineering have lessened the importance of wet-web strength. By contrast, a critical issue in many of the evolving nanocellulose technologies is the strength of objects first formed by aqueous processing, the green strength. For example, 3-D printed nanocellulose objects and ultralow density cellulosic aerogels can be destroyed by capillary forces during drying. There is a need for adhesives that strengthen freshly formed, wet lignocellulosic joints.