Polymers 2021 , 13 , 2485
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where the extra microporosity is clearly visible. This observation is in alignment with the fibre wall densities obtained from the CT analyses (see Section 3.1). 3.3. Lignin Inter-Diffusion Affecting Wet Tensile Strength The dependence of wet tensile strength index (wet tensile strength divided by the grammage) on pressing temperature seems to be defined by the activation energy for the inter-diffusion of lignin between fibre surfaces. The inter-diffusion is expected to be proportional to exp ( − E a RT ) [10], where E a is the activation energy, T is temperature, and R is gas constant. We obtained E a / R by plotting ln(Wet tensile strength index) vs. 1/Temper- ature (1/ T ) and taking the slope of the linear fitting line. In Figure 6, this is done first for TMP only (Figure 6a) and then for the whole data (Figure 6b) with different furnishes at temperatures exceeding 150 ◦ C. The relationship between ln(Wet tensile strength index) and1/ T seems quite linear in the range of 150–270 ◦ C for all pulps. This is striking taking into account that the press type and associated nip dwelling time are different below (cylinder press) and above (steel belt press) 200 ◦ C for the data. The above exponential temperature-dependence of lignin diffusion rate thus dominates over other factors when the level of wet tensile strength of pressed material is set by these processes.
( a ) ( b ) Figure 6. The logarithm of wet tensile strength index plotted against 1/Temperature when pressed with either cylinder press (T = 150 ◦ C, 190 ◦ C; 6 MPa) or steel belt press (T = 230 ◦ C, 270 ◦ C; 8MPa): ( a ) TMP sheets with preferred MD fibre orientation pressed at an initial solids content of 61%. The points represent an average of 10 data points and their confidence intervals. ( b ) Varied pulps and pressing conditions for standard laboratory sheets with uniform fibre orientation. Solids content varies in the range of 50–65%. The overall trend is described by a similar activation energy of 26 kJ mol − 1 as in ( a ). The temperature behaviour of TMP (Figure 6a) is quantitatively similar to that of the whole data (Figure 6b) with E a / R ≈ 3080 K, i.e., E a ≈ 26kJmol − 1 . This value is close to the value of 29 kJ mol − 1 obtained earlier for the diffusion of dissolved lignin from the interior of the chip to the bulk liquor, during the kraft pulping of Eucalyptus globulus wood [25]. Thus, the diffusion rate does not seem to be very sensitive to the type of lignin.
We studied the effect of lignin content of fibres by making similar plots for different pulps separately. A systematic increase in lignin content in the range of 0–12% was obtained for chemical kraft pulps by varying the cooking time. The results for these pulps were compared with similar data for CTMP (lignin content 27%) and TMP (lignin content 28%). Figure 7 shows both estimated E a and extrapolated wet strength at 1/ T = 0 for the different cases. Here the 1/ T = 0 limit, plotted on a logarithmic axis, describes the order of magnitude of wet strength achievable in hot-pressing. On the other hand, a low E a value seen for the smallest lignin contents indicates a relatively weak temperature dependence, which is generally coupled with a low 1/ T = 0 limit as well. It seems that at least c.a. 7% of
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