Effects of drying and pressing on the pore structure in the cellulose fibre wall studied by 1H and 2H NMR relaxation

A H-1 and H-2 NMR relaxation method was used to investigate the influence of drying and pressing on the pore size and pore size distribution in the cellulose fibre wall. The investigation was made in the moisture interval in which cellulose fibres normally shrink, i.e. from a moisture ratio of about 1.5 g water/g fibre to dry fibres. When the moisture content of a fibre sample was decreased by drying or pressing, the pores decreased in size and the pore size distribution became narrower. It was found that there were only small differences at a given moisture content between the pore size distributions of samples prepared by drying and by pressing. The results also indicate that the pore shrinkage in cellulose fibres during pressing or drying is a process in which the cell wall pores of a wet cellulose fibre successively shrink as the moisture content decreases. It was observed that, at low moisture contents, pressing and drying resulted in different H-1 NMR spin-lattice relaxation profiles. This is discussed in terms of morphology differences in the fibre matrix. The mobility of the protons in the solid phase influences the liquid H-1 NMR spin-lattice relaxation in heterogeneous systems through magnetization transfer. We have also studied the effects of hornification in recycled pulps.