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Fig. 8 t -Peel force-displacement curves for the separation of two strips made of the most modi fi ed fi bres fused together by hot pressing for 2 min at 150 °C and 16 MPa.
Hot pressing of the papers a ff ected the mechanical pro- perties in the opposite way to an increasing degree of oxi- dation, i.e. the strain-at-break decreased and the strength, modulus, yield stress and hardening modulus increased in papers that had been hot pressed (Fig. 7). There are at least two possible explanations of this observation: heat-induced chemical degradation, supported by the earlier sample failure seen in Fig. 3 for the modified samples, or a significant decrease in the free volume in the material. 37 Regardless of mechanism, the hot pressed material still shows a very high ductility for a wood-fibre-based network. From the DMTA data (Fig. 3) and the SEM micrographs (Fig. 6) it can be concluded that the modified cellulose has an increased molecular mobility. This implies that it is possible to fuse two separate pieces by hot pressing. Therefore two paper strips placed on top of each other were hot pressed, fol- lowed by a t -peel test to quantify the force needed to separate the strips. A force of about 110 N m − 1 was needed for separ- ation (Fig. 8), showing that it was indeed possible to “ weld ” two modified papers by hot pressing. Besides showing good adhesion, the merged strips were highly transparent (Fig. S4 † ). These features demonstrate how the material, in combination with 3D forming, could, for example, be a biorenewable alternative to the often di ffi cult-to-open heat-sealed blister and clamshell packages made of plastic. Conclusions Cellulose fibres were heterogeneously converted to di ff erent contents of dialcohol cellulose, where the modified cellulose forms an amorphous shell surrounding an intact core of highly ordered cellulose of each nanofibril. Papers and films
Fig. 7 Tensile data for oxidised-reduced papers/ fi lms; (a) average stress – strain curves, (b) tensile strength, strain-at-break, Young ’ s modulus, yield stress and hardening modulus as functions of the degree of oxidation. Values are the means of ten test pieces given with 95% con fi dence limits.
Green Chem. , 2016, 18 , 3324 – 3333 | 3331
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