Wet-Stacking Lamination of Multilayer Mechanically Fibrillated Cellulose Nanofibril (CNF) Sheets with Increased Mechanical Performance for Use in High-Strength and Lightweight Structural and Packaging Applications

Mechanically fibrillated cellulose nanofibril (CNF) sheets of varying thicknesses were fabricated by using a wet stacking lamination technique without the use of solvents other than water or binders. The use of this technique allowed for the creation of multilayer structures with a working area of 117 mm by 117 mm and thickness of up to 0.547 ± 0.03 mm in under 2 h, which represents the shortest total processing time reported for such thickness of CNF sheets. To highlight the capabilities of utilizing wet stacking, the thickest reported 100% pure multilayer CNF sheet with a thickness of 1.65 ± 0.02 mm was produced. To gauge the effect of processing parameters on the mechanical performance of the produced sheets, thickness (85-547 μm thick), pressing time (35 min, 1 h, and 2 h), pressing pressure (0-5.17 MPa), and loading rate (4 min, 2 min, and 20 s) were varied. Tensile testing results show that the ultimate strength increased as the thickness increased and subsequently reached a plateau at a value of 207 ± 2.51 MPa at a critical thickness between 85 ± 2 and 153 ± 4 μm. A slight decrease in ultimate strength to a value of 184 ± 10.9 MPa was seen for the thicker 547 μm (0.547 mm) specimens. The specific strength was comparable to 2024 aluminum (T3 tempered) due to the relatively low density of CNF. The apparent toughness (work of failure) of the sheets was found to be on average 3.53 ± 0.36 MJ/m3, which is about 6 times greater than the reported value for poly(styrene). Because of their improved mechanical properties, these sheets could serve in high-strength and low-density structural applications where aluminum alloys (2024 and 6061) and packing materials/containers where commodity polymers like poly(styrene) are currently used.