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Cellulose (2017) 24:1759–1773
Scheme 1 Structures and modification of used polysaccharides. (O)GG (oxidized) guar gum galactomannan, GGM galactoglucomannan
during composite production. The mechanical perfor- mance of laminated nanopaper-epoxy composites as a function of temperature was evaluated by means of DMTA in three point bending mode. The nanopaper-epoxy composites, as exemplarily shown for a BC and a CNF-OGG 80 composite in Fig. 1, exhibited the typical behavior of stiff, high glass transition thermosets; the modulus decreases slightly with increasing temperature and tan d stays constant at a very small value. The CNF nanopaper based composites exhibited slightly lower modulus compared to BC nanopaper based composites with a stronger decrease at elevated temperatures. It was shown that these composites can be used up to almost their degradation temperature (see below) without passing the glass transition region of the epoxy matrix. The pure, cured epoxy resin follows the same principal trend up to 170 C, but has a significantly lower storage modulus. The storage modulus at 20 C was 2.54 GPa compared to around 17 GPa for nanopaper composites. However, at 170 C the storage modulus of the epoxy resin decreased significantly, while tan d increased indicating the onset of the glass transition. The glass transition temperature (T g ), taken as the maximum of tan d , was 196 C. Thus the introduction of nanopapers into an epoxy resin matrix not only improved mechanical properties, but also increased the application temperature range significantly. No T g of neither BC nor CNF composites could be detected up to the onset of the degradation temperature around
250 C, at which the composites still had a storage modulus of around 10 GPa. The storage modulus at 20 C was used to assess the influence of different WSPS on the mechanical performance of laminated nanocomposites (Table 1). As already shown for (modified) nanopapers (Luce- nius et al. 2014), also the modulus of the composites depended on the type of polysaccharide introduced into the nanopaper. Composites containing an unmod- ified CNF nanopaper reinforcement had a storage modulus of 20 GPa, whereas for composites contain- ing WSPS modified CNF nanopapers it was slightly lower, indicating the capacity of WSPS to act as Fig. 1 Storage modulus E 0 and loss factor tan d as function of temperature for cured epoxy resin films ( blue full line ), an epoxy-CNF-OGG 80 composite ( black dotted line ) and an epoxy-BC composite ( green dashed line ). (Colur figure online)
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