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density of a cellulose-based material but also of good barrier properties. In this work, a spectrophotometer equipped with an integrating sphere was used to measure both direct and di ff use transmittance. The visual appearance and optical transmittance of the di ff erent papers are shown in Fig. 5, where Fig. 5a shows how the fibre modification transforms the papers from being opaque to being fairly transparent. This transparency of the modified samples was further improved by hot pressing, especially for the beaten, most modified fibres which resulted in a more or less homogeneously transparent paper/film. Fig. 5 (and Fig. S3 † ) also shows that the non- beaten fibres were not as susceptible to the hot pressing as the beaten fibres, indicating a synergetic e ff ect between beating and the oxidation-reduction treatment of the fibre, where both treatments soften the fibre wall and facilitate a good consolidation
changes a ff ecting the thermoplasticity. This also implies that the mechanical properties of the non-pressed samples are con- trolled predominantly by the properties of the fibre wall, and not by the fibre network. Altogether, the data in Fig. 3 indicate that the material can be heat processed in di ff erent ways, such as hot pressing. To further study and utilise the thermoplastic features of the modified cellulose materials, samples were hot pressed between two smooth steel plates. The optimal pressing con- ditions, in terms of visual appearance and light transmittance, were found to be 16 MPa and 150 °C for 2 min (all the con- ditions tested can be found in the ESI † ). Fig. 4 shows the material density before and after hot pressing of the di ff erently modified materials. The density of the modified papers/films was high already before hot pressing, ranging from 1100 to 1300 kgm − 3 , which by far exceeds the density of conventional papers, including greaseproof papers, 21 and was similar to the density of fibre materials subjected to extended hot pressing at high pressures (45 MPa for 20 min). 22 The densities of the papers presented here is, to the best of our knowledge, sur- passed only by so-called nanopapers made from CNFs or CNCs, which can have densities of about 1500 kg m − 3 , i.e. close to that of solid cellulose. 23,24 However, compared to nanopapers, the current papers were formed and dewatered by conventional papermaking methods in a matter of a few tens of seconds whereas it takes hours to dewater (or solvent cast) a nanopaper. 25,26 When the papers were hot pressed, the density increased by 10 – 20% to densities greater than 1400 kg m − 3 for the papers with a degree of oxidation greater than 24%, i.e. densities not far from that of solid cellulose. This indicates that the modified fibres are soft and flexible and form a novel, highly consolidated type of paper, more resembling a transpar- ent film. Another indicator of a highly consolidated paper is a high optical transmittance, which is an indicator not only of a high
Fig. 5 Optical appearance of non-pressed and pressed samples; (a) scanned image, (b) total transmittance and haze, i.e. percentage of di ff use transmittance, measured at a wavelength of 550 nm (full spectra can be found in the ESI † ). The average sample thickness was 183 and 150 μm for the non-pressed and pressed reference, respectively, and the thickness of all the modi fi ed samples was in the range of 120 – 150 μm before pressing and 100 – 120 μm after pressing.
Fig. 4 Density before and after hot pressing for 2 min at 150 °C and 16 MPa. Values are means of at least eight measurements and are given with 95% con fi dence limits.
3328 | Green Chem. , 2016, 18 , 3324 – 3333
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