Concepts and evidence of interdigitation in the strength development of paper and sheets formed from highly fibrillated cellulose: A review

This review considers published evidence supporting ways in which self-assembly and interdigitated structures that emerge during sheet formation contribute to mechanical strength and structural development in nanopaper and conventional paper-like sheets. Interdigitation is defined here as a form of three-dimensional connectivity within fibrous networks, including elements of parallel interaction, minor weaving, physical entanglement, and inter-diffusion among fibers, fibrils, and nano-scale cellulosic features of multiple length scales. Particular attention is given to out-of-plane fiber orientations and the persistence of three-dimensional connectivity during sheet formation, features that are not fully captured by idealized two-dimensional network models. The review considers mechanical, rheological, microscopic, and processing-related studies to assess how hydrodynamic conditions, flocculation, consolidation history, and drying influence the formation and effectiveness of interdigitated structures. The collected evidence suggests that interdigitation is an inherent feature of papermaking over a broad range of fiber and fibril dimensions, and that it can be strongly influenced by processing conditions. In addition to highlighting the topic of interdigitation, this review also reveals a need for more detailed theoretical consideration, as well as focused experimental work.