Cellulose (2017) 24:1759–1773
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Table 1 Results of DMTA and tensile tests of BC and (modified) CNF-epoxy composites: Storage modulus at 20 C, Young’s modulus, tensile strength, strain at break and work of fracture
Sample
Storage modulus (GPa)
Tensile strength (MPa)
Young’s modulus (GPa)
Strain at break (%)
Work of fracture (MJ m - 3 )
Epoxy
2.54 ± 0.05
36.1 ± 1.7
1.6 ± 0.1
4.16 ± 0.39
0.87 ± 0.12
Epoxy ? CNF
20.0 ± 2.4
84.6 ± 4.3
12.2 ± 0.5
0.88 ± 0.09
0.34 ± 0.03
Epoxy ? CNF/OGG-50
17.2 ± 1.0
89.8 ± 3.2
12.2 ± 0.5
0.78 ± 0.09
0.35 ± 0.03
Epoxy ? CNF/OGG-80
17.4 ± 0.6
107.1 ± 6.1
10.7 ± 0.4
1.23 ± 0.17
0.63 ± 0.09
Epoxy ? CNF/GGM 17.4 ± 0.2
116.8 ± 4.5
12.0 ± 1.0
1.17 ± 0.12
0.69 ± 0.11
Epoxy ? BC
18.5 ± 0.5
150.8 ± 9.3
9.0 ± 0.1
2.76 ± 0.29
2.68 ± 0.41
lubricant. For BC nanopapers, a storage modulus of 18.5 GPa was measured, thus being in between the CNF nanopapers with and without WSPS modification. In addition to DMTA, tensile tests were performed to determine the ultimate tensile strength, tensile modulus, strain at break and work of fracture. The work of fracture can be considered to be an indicator of the toughness of the composites. Representative stress–strain-curves from tensile tests of (modified) CNF nanopaper based composites are shown in Fig. 2 and all results are collected in Table 1. The ultimate tensile strength was determined to be 85 MPa for the epoxy composite reinforced by two pure CNF nanopaper layers, compared to 36 MPa for the pure epoxy resin. Addition of OGG with a DO of 50% to the CNF did not significantly influence the tensile strength, while a DO of 80% for OGG led to a significantly improved tensile strength of the nanocomposites. This dependency on the DO was as to be expected (Lucenius et al. 2014) and showed that only a sufficiently high DO leads to an improved tensile strength. This can be explained by the low total amount of oxidized galactosyls in the composite, whereby oxidized galactosyls create hemiacetal cross- links between hydroxyl and aldehyde groups of CNF and WSPS, which is likely to be the reason for stronger nanopapers as intermolecular crosslinks formed between fibrils. The higher the degree of oxidation was, the higher the tensile strength of CNF nanopapers (Lucenius et al. 2014) and correspondingly also of the composites. The effect was more pronounced for composites containing OGG-80 modified nanopaper reinforcements. Crosslinking has previously been shown to be beneficial for CNF composites (Lee et al. 2014a). Here it was shown that it is possible to control the mechanical properties of CNF
nanocomposites by controlling the composition and thus mechanical properties of nanopapers. As anticipated, the highest tensile strength among CNF composites was observed when using CNF/GGM nanopaper reinforcements. Accordingly, just as for the pure nanopapers, GGM exhibited the highest rein- forcing ability also within composites. This means that 80% of the original strength of the nanopapers was retained in the composites, which can be expected considering that a fibril fraction of 80 vol% was used to reinforce the resin. Thus the hypothesis of produc- ing better performance cellulose nanocomposites when using better nanopapers was proven to be correct. The introduction of only 2 wt% WSPS into a CNF nanopaper network resulted in nanopapers with improved mechanical properties and the preparation of high loading fraction CNF nanocomposites from these nanopapers led to increased tensile strength of Fig. 2 Representative stress–strain curves for composites with pure CNF ( black dotted line ), CNF/OGG-50 ( red dashed line ), CNF/OGG-80 ( blue dash-dotted line ) and CNF/GGM ( green full line ) reinforcement. The setting behavior (up to 0.2% strain) of the specimens were due to handling difficulties owing to the thin laminates. (Colur figure online)
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