Cellulose (2017) 24:1759–1773
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Germany). A sample size of approximately 5 mg was used. The samples were heated from 30 to 650 Cat a heating rate of 10 C min - 1 and a gas flow rate of 25 mLmin - 1 . The onset of degradation was com- puted to be the temperature at which the mass loss rate was exceeding 0.2% per C.
respectively. The backbone of GG remained unmod- ified due to the selective oxidation of the galactosyls, facilitating good compatibility with cellulose. Fur- thermore, the viscosity of the OGG solutions was low at both degrees of oxidation, enabling good mixing with the CNF suspension. Extensive mixing was used to ensure homogeneous distribution of WSPS in the CNF suspension. Nanopapers with grammages of 60 g m - 2 were produced from CNF with and without 2 wt% of GGM or OGG, respectively, and from BC (50 g m - 2 ) using a simple papermaking process (Mautner et al. 2015). The final thickness of the nanopapers was 60 ± 5 l m for (modified) CNF and 50 ± 5 l m for BC nanopapers. The tensile strength (88 MPa) of unmodified CNF nanopapers improved by more than 50% by CNF modification with oxidized GG (135 MPa) or GGM (141 MPa). This took place at the expense of stiffness, as shown by the slight decrease of the modulus from 9 GPa for unmodified CNF to 7.9 GPa for oxidized GG. For GGM (8.7 GPa) on the other hand no significant modulus decrease was observed. This was expected for WSPS modified nanopapers, in which WSPS act as plasticizer between stiff CNF, enhancing the ductility of the nanopaper in accordance with previous results (Lucenius et al. 2014; Olszewska et al. 2013b). During film formation from aqueous suspen- sions, the WSPS form a water-swollen dissipative layer on the CNF surface (Lozhechnikova et al. 2014; Eronen et al. 2011), thus enhancing the dispersibility of CNF. This is crucial to avoid CNF aggregation and defects in the final nanopaper resulting in improved mechanical properties. For BC nanopapers, a Young’s modulus of 8.3 GPa and a tensile strength of 144 MPa were measured, which are values typically found for BC nanopapers prepared without prior removal of fibril aggregates (Lee et al. 2012c).
Results and discussion
A small amount of water-soluble polysaccharides (WSPS) as low as 2 wt% was introduced into the fibril network of non-pretreated CNF nanopapers and their influence on the thermal and mechanical performance of laminated nanopaper-epoxy composites evaluated. The polysaccharides studied were the mannans GGM and GG, which contain ca. 10 and 40% terminal a -D- galactosyl residues, respectively. These residues are directly attached to the C-6 of the mannosyl units of the backbone (Wielinga 2009). GG was hydrolyzed to reduce the molecular weight and enzymatically oxi- dized at the C-6 position of the mannosyl units to improve the mechanical performance (Lucenius et al. 2014). The effect of the polysaccharides used to modify the CNF and the differences between BC, constituting highly pure and crystalline cellulose, and modified CNF on the nanopaper and composite properties are discussed.
Structure and properties of the WSPS and nanopapers
WSPS GGM was utilized in its unmodified state, whereas low molecular weight GG was prepared by partial hydrolysis with b -mannanase (Lucenius et al. 2014). The M w of the hydrolyzed GG, as determined by size exclusion chromatography, was approximately 30 kDa as compared to 1000 kDa before hydrolysis (Wielinga 2009). Hydrolyzed GG was subsequently oxidized (Scheme 1) with galactose oxidase (GO), catalase and horseradish peroxidase (HRP) (Parikka et al. 2010). The degree of oxidation (DO) of oxidized galactosyls in OGG was 50% (OGG-50) and 80% (OGG-80), respectively. The total amount of oxidized carbohydrates in hydrolyzed GG was 20 and 31%, respectively, and the total relative amount of oxidized galactosyls in the final modified nanopaper was found to be 0.375 and 0.60% of total carbohydrates,
Multi-layer epoxy-nanopaper composites
Multi-layer composites were produced by a laminat- ing technique impregnating (modified) CNF as well as BC nanopapers with a two-component epoxy resin. Resulting CNF composites had thicknesses around 100 l m and a fibril content of around 80 vol%. For BC composites the thickness was around 90 l m and the fibril content around 80 vol%. The reduced thickness of the composites was explained by further compaction of the nanopapers in the compression step
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