1766
Cellulose (2017) 24:1759–1773
the nanocomposites. Furthermore, the extent of the improvement could be estimated based on the mechanical properties of the nanopapers. The increase of the strength went hand in hand with a reduction of the modulus, as was already demonstrated in three point bending mode by DMTA (see above). While there was no detectable influence of the low DO OGG- 50 on the composite modulus, for the higher DO OGG- 80 grade a slight reduction compared to pure CNF and CNF/OGG-50 reinforced composites was observed. For GGM a small reduction of the tensile modulus was found. Modified CNF composites exhibited both enhanced Young’s modulus and tensile strength compared to a previous study (Lee et al. 2012c) utilizing CNF nanopapers that were vacuum-infused with a brittle epoxy resin. This improvement was enabled by modification of the CNF by adsorbed WSPS. The introduction of WSPS seems to affect crack propagation in the nanopapers and thus also in the composites. The higher modulus compared to the nanopapers can be explained by further compaction of the laminates and accordingly the nanopapers during composite manufacturing. The strain at break and work of fracture were determined for the composites from stress–strain- curves. For the nanopaper reinforcement modified with low DO OGG-50 grade again no detectable dif- ference was found compared to pure CNF, which was explained by a too low DO leading to an insufficient amount of hemiacetal bonds forming within the nanopapers. For OGG-80 modified nanopaper rein- forced composites the strain at break was significantly higher, which, in conjunction with higher tensile strength, resulted in significantly increased work of fracture. The same was found for GGM modified CNF nanopaper based composites. Bacterial cellulose nanopapers were also tested for their reinforcing ability in composites. BC is purer than wood derived CNF due to the absence of hemicelluloses and lignin (Klemm et al. 2011). Moreover, long, entangled and homogeneous fibrils are responsible for good mechanical performance (Paakko et al. 2008; Klemm et al. 2011; Lee et al. 2014b). Therefore a higher reinforcing ability com- pared to CNF can be anticipated. This assumption was proven correct; the tensile strength of the BC nanopa- per composites was the highest within this study with 151 MPa. However, BC composites had a lower tensile modulus compared to CNF nanopaper based
composites, due to lower packing efficiency caused by thicker BC fibrils. Furthermore, hemicelluloses pre- sent in CNF nanopapers, which are absent in BC, enable a better stress transfer between CNF fibrils (Iwamoto et al. 2008; Lee et al. 2012c; Gro¨ndahl et al. 2004). This was in good agreement with the DMTA results and literature (Lee et al. 2012c). BC composites even had higher tensile strength and modulus com- pared to pure BC nanopapers. This can be explained by further compaction of the nanopapers during composite manufacturing; i.e. the composites are thinner than 2 nanopaper layers. Moreover, a very high strain at break and thus work of fracture was measured for BC composites. The higher strain to break for BC composites can be explained by fewer physical crosslinking points between the BC nanofib- rils, allowing for realignment of the fibrils during tensile loading (Lee et al. 2012c) and the higher length of BC fibrils compared to CNF. The reorientation of the fibrils even within the composites was still possible since the resin did not fully impregnate the BC nanopapers because of the small pore dimensions and low nanopaper porosity of 33%. The fracture surfaces of the composites were inspected by SEM. In Fig. 3, the fracture surfaces of composites made from CNF, CNF/GGM, CNF/OGG- 50, CNF/OGG-80 and BC nanopapers are shown. The layered structure of the nanopapers can be easily seen. In the center of the composites, a pure, 5–10 l m thick epoxy resin phase can be observed. The resin spreads into both nanopapers, thus ensuring good adhesion between nanopapers and resin, as well as holding the nanopapers together. However, the larger part of the nanopapers was not impregnated, thus allowing for realignment of the fibrils within the nanopapers. This is particularly true for the nanopapers containing WSPS, in which those polysaccharides act as lubricant in between the nanocellulose fibrils. In the review by Lee et al. (2014b) poly- L -lactic acid (PLLA) was used as a benchmark because it is the best performing commercially available renewable bulk polymer. Compared to the PLLA standard, having a tensile modulus of 4 GPa and tensile strength 63 MPa, all the reinforced samples studied here well exceeded the mechanical properties of PLLA. If compared to other BC or CNF reinforced composites we note that especially the Young’s modulus obtained for the GGM modified CNF nanopaper laminates exceeded most of the previously reported results, but
123
Made with FlippingBook - Online catalogs