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cellulose materials are known to be good oxygen barriers, 4,30,31 especially at low relative humidity, oxygen permeability measurements were performed before and after pressing. The results are presented in Table 1 and, as expected, the non- modified papers did not provide an oxygen barrier but the papers formed from beaten fibres with a degree of modifi- cation greater than 24% did indeed provide an oxygen barrier. Since greaseproof papers do not provide any measurable gas barrier, 32 this is, to the best of our knowledge, the first report of a non-coated paper produced by conventional papermaking methods that constitutes a gas barrier. Hot pressing did not improve the (non-existing) barrier pro- perties of the reference paper; but, pressing of the modified paper sheets tended to further decrease the oxygen per- meability. As long as the pressing does not cause any damage to the fibre network, it is expected that an increase in density (Fig. 4) and hence a decrease in void fraction should lower the permeability. Interestingly, pressing of papers made of the most modified non-beaten fibres transformed the paper from a non-barrier to a barrier. This not only shows that hot press- ing of the modified papers leads to a smaller void fraction, but also supports the above-mentioned observation of a synergetic e ff ect between beating and chemical modification (since the modified beaten fibres provided a barrier already before pressing). The oxygen permeability of all the papers/films exhibiting oxygen-barrier properties increased with increasing relative humidity. This is a well-known phenomenon, not only for cel- lulose and dialcohol cellulose but for polysaccharides in general. 7,31,33,34 It is also interesting to note that the barrier films made from beaten fibres with a degree of oxidation of 24% had a lower permeability than films made from beaten fibres having a 40% degree of oxidation. Apparently 24% oxi- dation is enough to soften the fibres enough to facilitate a film without interconnected pores, providing a gas barrier, and pre- sumably due to the higher degree of crystallinity (Fig. 2) these
films also exhibit lower oxygen permeability than the more modified samples. Regardless, the oxygen permeability at 80% RH was, both for the 24 and 40% oxidised papers, still below a level allowing the material to be used in, for example, packa- ging applications, especially considering the rapid dewatering and the presumably good and fast processability in a paper machine, which is a prerequisite for economic large-scale material production. Mechanical performance Dialcohol cellulose in its pure form is very ductile, but unfor- tunately it is also very weak. 8,9,20 However, our earlier studies have shown that it was possible to combine both ductility and strength by a heterogeneous modification of CNFs in a core – shell structure. 6,7 Fig. 7 shows that this approach can be taken even further by the extended modification described here. As shown in Fig. 7b, an increasing degree of modification results in a steadily increasing strain-at-break whereas the ultimate strength, Young ’ s modulus, yield stress and hardening modulus pass through a maximum at a degree of modification of 10 – 25%, reflecting the increasing influence of the more ductile, but weaker, dialcohol cellulose. It is also worth emphasising that the chemical modification has a dramatic e ff ect on the tensile strength of the papers made from fibres subjected to the lowest degree of modification (13%), where the strength is about three times greater than that of the untreated reference. Interestingly, the toughness defined as the work of fracture, i.e. the area under the stress – strain curve, reached about 21 MJ m − 3 for the most modified papers, which surpasses not only all kinds of conventional paper grades by about an order of magnitude but also most nanopapers and nanocomposites made from CNFs. 35,36 This emphasises the remarkable properties of this new material, which would presumably facilitate the advanced hydroforming and deep-drawing of the papers into complex 3D structures.
Table 1 Oxygen permeability of oxidised-reduced papers/ fi lms before and after hot pressing. Permeability values are means of four measurements given with 95% con fi dence limits
Permeability (ml μm (m 2 kPa24h) − 1 )
Degree of oxidation (%)
Average sample thickness (μm)
Beaten fibres
Pressed/non-pressed
50%RH
80%RH
Reference
Non-pressed
0 0
185 ± 15 158 ± 13 139 ± 1 115 ± 4 118 ± 5 99 ± 2 118 ± 4 113 ± 5
Over range Over range Over range Over range
Not measured Not measured Not measured Not measured
Pressed
6 h oxidation
Non-pressed
13 13 24 24 40 40 20 20 36 36
Pressed
<0.6 a <0.5 a <0.6 a <0.6 a
11.8 ± 0.9 9.6 ± 0.9 22.9 ± 2.1 20.5 ± 2.0
12 h oxidation
Non-pressed
Pressed
24 h oxidation
Non-pressed
Pressed
Non-beaten fibres 12 h oxidation
Non-pressed
144 ± 2 118 ± 3 118 ± 1 107 ± 4
Over range Over range Over range
Not measured Not measured Not measured
Pressed
24 h oxidation
Non-pressed
< 0.6 a
23.4 ± 3.4
Pressed
a Below the detection limit of the instrument.
3330 | Green Chem. , 2016, 18 , 3324 – 3333
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