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In this work, tension and fracture behaviors of cellulose nanopaper (CNP) made from two different preparation approaches are comparatively studied. The CNP are prepared by casting (or C-CNP) and filtration (or F-CNP) of CNF suspension. The resulting CNP are mechanically characterized using the vision-based full-field optical method of Digital Image Correlation. Tension tests show that F-CNP has a higher strength and greater nonlinearity than the C-CNP. The crack initiation and growth characteristics of the two types of CNP are investigated using optical measurements. The data are analyzed under small-scale-yielding conditions to quantify the fracture parameters such as stress intensity factors and energy release rates at crack initiation as well as during crack growth. The results indicate that both C-CNP and F-CNP show significant crack growth resistance in the post-crack initiation regime. The F-CNP particularly offers substantial resistance to crack growth relative to the C-CNP demonstrating that filtration is the preferred method to make CNP with higher tensile strength and better fracture properties.

Cellulose nanopapers, 2D flat and foldable film made of network-forming cellulose nanofibers, are strong, tough and biodegradable materials of great interest as they have potential to replace the conventional fossil based polymers in various applications. Although there are several cellulose nanopaper preparation methods described in the literature, no standard method is available for the preparation of uniform and smooth nanopapers within a practical time frame. In present study, TAPPI standard method of preparation of pulp handsheets was systemically modified/optimized to prepare flat and wrinkle free cellulose nanopapers in few hours of preparation time. Physical properties of these cellulose nanopapers such as tensile strength and water vapor transmission rate were comparable to the reported values in the literature. The proposed standard preparation method will enable the comparison of the physical properties of cellulose nanopapers prepared in different studies.