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BC and the onset of the first degradation step occurred at 320 C. This difference can be explained by a higher degree of crystallinity of BC (72 ± 1%) compared to CNF (60 ± 5%) (O¨ sterberg et al. 2013), which is due to the absence of residues of lignin and hemicelluloses (Lee et al. 2012c). A char residue at 600 C of 12 and 13% was found for OGG- 50 and OGG-80 modified CNF, respectively. For GGM modified CNF it was 16% and BC and unmodified CNF had a char residue of 17%. In air atmosphere again around 5 and 2%, respec- tively, of moisture was removed between 30 and 150 C for CNF and BC nanopapers, respectively. The first degradation step, attributed to the degradation of low-molecular weight glycosidic compounds (Cheng et al. 2009; Seifert et al. 2004), occurred around 250 C for all the CNF nanopapers tested. For BC this temperature was higher (300 C), similar to results reported in literature (Lee et al. 2012c). The second degradation step, attributed to the degradation of pyran structures, started for all types of CNF and BC nanopapers around 450 C. Only minor deviations were found for modified CNF nanopapers; all modi- fied nanopapers were completely degraded at 500 C. The thermal decomposition of composites is shown in Fig. 5. Similar to the pure nanopapers, a one-step degradation regime was observed for all types of composites in inert atmosphere (Fig. 5, left). This demonstrated that the overall thermal behavior was mainly governed by the nanopapers, which constitute
allowing predicting the composites properties. It was found that addition of only 2 wt% of GGM for the preparation of CNF nanopaper resulted in doubling the work of fracture of the final composite. This result was obtained without any chemical pretreatment or syn- thetic polymers. Since the extraction of GGM from wood is scalable (Leppa¨nen et al. 2011) this result is quite interesting. To produce even higher strength, co- polymers with grafted soft chains like CMC-g-PEG could be used, but that would at the same time increase the complexity of the system. It is further noteworthy that the nanopapers used also significantly improved the thermal stability of the epoxy composites.
Thermal behavior of nanopapers and nanopaper composites
The thermal degradation behavior of BC and CNF nanopapers alone was tested in both nitrogen (Fig. 4, left) and air (Fig. 4, right) atmosphere. A one-step degradation regime was observed for all types of nanopapers in inert atmosphere. During the initial testing phase between 30 and 150 C, no significant difference was found for the different CNF nanopa- pers. Around 5% of moisture was removed for (modified) CNF nanopapers and the onset of the thermal degradation took place at around 275–280 C. This degradation step was attributed to cleavage of glycosidic linkages of cellulose (USDA 1970). A smaller amount of moisture (2%) was removed from
Fig. 4 TGA under nitrogen ( left ) andair ( right ) of CNF and BC nanopapers. CNF ( green full line ), CNF ? GGM( blue dashed line ), CNF ? OGG-50 ( orange dash-dotted line ),
CNF ? OGG-80 ( red dash-double dotted line ) and BC ( black narrow dashed line ). (Colur figure online)
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