Failure analysis of cellulose nanocomposites
The SEM micrographs of the fractured surfaces of the anisotropic CNF composites along with the pure bio-epoxy samples are shown in Fig. 3.10. The SEM micrographs of pure bio-epoxy show regular cracks, indicating a typical brittle fracture. On the other hand, the SEM images for the anisotropic CNF samples show a uniform fractured surface interspersed with holes or channels. The smooth part of the fractured surface indicates that the mode of failure is a cleavage similar to the brittle fracture of metals under tensile tests. Fractured surfaces clearly shows that the fibers clusters have been pulled out and the resulting channels have appeared on the fractured surface. Such fiber pull-out acts as the dominant toughening mechanism for the anisotropic CNF composites. Because of the weaker interfacial bonds between the CNF and bio-epoxy, all SEM figures show smooth surfaces on the channels. Also, the channels, being almost of the same size as the holes in the honeycomb-shaped aligned pore structure of the anisotropic CNF aerogels, are perhaps created from the “pull-out” of these individual “cylinders” of bio-epoxy.
Bubbles were created in the bio-epoxy resin during its mixing prior to its infusion into the mold and also during its vacuum-induced suction through the mold. The presence of bubbles and the resulting low-interfacial bond between the CNF solid phase and bio-epoxy resin may be the reason for having a lower ultimate stress in CNF composites compared to the pure bio-epoxy.
Figure 3.10 SEM images of fractured surfaces of samples with different CNF volume fractions and different magnifications: (A) Pure bio-epoxy 100X; (B) Pure bio-epoxy 1000X;
(C) 1% CNF 100X; (D) 1% CNF 1000 X ; (E) 1.3% CNF 100 X ; (F) 1.3% CNF 1000X.
The SEM micrographs of the fractured surfaces of the isotropic CNF composites are shown in Fig. 3.11. The silylated sample shows much less void-content compared to the nonsilylated sample—the latter displays more numerous round voids. This difference can be recognized to have come about due to the better wettability of CNF solid phase by the resin due to the silane treatment. The silane agent also improves the mechanical entanglement between the CNF reinforcement and bioepoxy matrix because of better bonding between them during the curing process. Both these effects work together to create a better CNF composite.
Figure 3.11 SEM micrographs of: (A) nonsilylated CNF composite; (B) silylated CNF composite.
Source: Journal of Carbohydrate Polymer, vol. 17 pp. 282—293.