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Transparent TEMPO Oxidized Cellulose Nanofibril (TOCNF) Composites with Increased Toughness and Thickness by Lamination
TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl) cellulose nanofibrils (TOCNF) are polysaccharide nanomaterials that are extracted mainly from wood, plants and other biomass sources. TOCNF suspensions of negatively charged fibrils can be cast to produce totally transparent films with exceptional mechanical properties. Nevertheless, the inherent brittleness and the high stresses generated within the films during drying processes, makes the production of thick materials difficult and reduce the potential usage of TOCNFs films in industrial applications.
Hence, in this study lamination of TOCNF films with room temperature curable epoxy was used to combat brittleness, increase thickness, and produce a more damage tolerant material. The effect of the volume fraction and layer thickness of both phases, the number of layers, and the overall total thickness of the laminate on the tensile and flexural properties were investigated. Lamination was successful at increasing the toughness and thickness of TOCNF composites, resulting in an increased work of fracture (WOF) that was associated with fracture retardation by crack digression. The ultimate tensile strength (UTS) and Young’s modulus were higher for laminates with low volume fractions of epoxy, and with increasing number of TOCNF layers.
To further increase the mechanical properties of TOCNF materials, water-soluble polymers were screened as strengthening aids through solution casting. Polyvinyl alcohol (PVA) and poly(2-ethyl-2-oxazoline) (PEOX) were found as reinforcement agents for various types of cellulose nanofibrils (CNFs) films. Mechanical property increases of 99% in elastic modulus, 93% in UTS and 134% WOF were reported for TOCNF with 0.44 mmol/g carboxylate groups and 15 wt.% PVA. PEOX had a higher elastic modulus increase of 113% over PVA, yet lower UTS and WOF increases were found at 63% and 28%, respectively. Additionally, increases in UTS and elastic modulus were also seen in mechanically fibrillated CNF and TOCNFs with higher carboxylate contents (1.5 mmol/g). The toughening mechanism was attributed to the formation of strong hydrogen bonding between the CNFs and the hydrophilic polymers added. The presence of such mechanisms was indirectly confirmed by tensile testing, zeta potential and rheology.
Finally, an enhanced version of TOCNF laminates was fabricated using TOCNF films with PVA strengthening aid and four different epoxy formulations as interlayer. Flexural testing showed a correlation between the presence of stronger layers in the laminate with a higher flexural strength, bending modulus, and WOF. Different modes of fracture within the laminates occurred based on epoxy type. A stiffer epoxy generated a reduced mechanical response and substantial intralayer damage. On the other hand, a more ductile epoxy increased the WOF of the laminates, inducing a higher delamination at the interface. The addition of a silane coupling agent (APTES) resulted in a higher compatibility between the TOCNF and epoxy. In general, laminates with stronger TOCNF layers (TOCNF + PVA) and increased adhesion (APTES), showed a flexural strength increase of 61%, a bending modulus increment of 80% and the same WOF when compared with the original laminates. Finally, impact testing of TOCNF materials was performed, the specific energy to rupture of laminates was comparable to those achieved by acrylic and borosilicate glass, while maintaining a higher or similar specific strength to glass. Laminates maintained good transparency and low haziness to the naked eye.