Cellulose (2017) 24:1759–1773
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Fig. 5 TGA under nitrogen ( left ) and air ( right ) of CNF and BC-epoxy composites. CNF ( green full line ), CNF ? GGM ( blue broad dashed line ),CNF ? OGG-50 ( orange dash-dotted
line ),CNF ? OGG-80( red dash-double dotted line ),BC( black narrow dashed line ) and epoxy resin ( black dotted line ). (Colur figure online)
the majority of the composites. However, during the initial degradation phase between 30 and 150 C, only around 3% (compared to 5% for the CNF nanopapers) of moisture was removed for CNF composites. This was attributed to the increased overall hydrophobicity caused by the epoxy matrix. For BC composites this value (2%) was similar to the pure BC nanopaper. The onset of thermal degradation of CNF composites occurred at around 270 C, comparable to the CNF nanopapers. For BC composites, the degradation commenced at 300 C, which was slightly lower than for pure BC nanopapers. Final degradation was found to be somewhat different for the different sets of polysaccharide phases within the CNF nanopaper. A char residue at 600 C of 19–22% was found for the various types of composites, thus being slightly higher than for the pure nanopapers and the epoxy resin alone. In air atmosphere (Fig. 5, right), regarding removal of moisture, the same tendencies for differences between nanopapers and composites were found as in nitrogen. The first degradation step occurred at 270 C (compared to 250 C for CNF nanopapers), demonstrating the influence of the protecting epoxy matrix. Between (modified) CNF samples, no obvious difference was observed. For BC composites the onset temperatures of the first degradation step occurred at 293 C, which was similar to pure BC nanopapers. Also for the onset of the second degradation step around 430 C, hardly any deviations were found for the composites containing modified CNF nanopapers.
For BC composites the onset of the second degrada- tion step occurred at 450 C, similar to those of the pure nanopapers and higher compared to CNF com- posites, which again was ascribed to the higher degree of crystallinity of BC. All the composites were completely degraded around 500 C. The cured epoxy resin starts decomposing at 350 C and was com- pletely degraded at 650 C. In Fig. 6, the degradation of composites containing GGM modified CNF and BC, respectively, in inert atmosphere is contrasted to the degradation of the corresponding nanopapers. Furthermore, a theoretical degradation graph is displayed, constructed from the degradation of the epoxy resin and the nanopapers, assuming a resin content of 20%. Only minor differ- ences between theory and experiment can be observed. The composites apparently start to degrade at slightly lower temperatures compared to the nanopaper, whereas a small increase would be theoretically expected. This might be due to thermal damage the nanopaper experiences during the composite produc- tion at 180 C. Furthermore, a higher than expected amount of char residue was found. This can be explained by mutual protection of both nanopaper and epoxy resin. Figure 7 shows the degradation behavior of the pure and modified nanopapers and composites con- taining GGM modified CNF and BC nanopapers in air together with the theoretical degradation graph con- structed from the degradation of the epoxy resin and
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