Sorption behaviour and conditioning time
For both, softwood and hardwood pulp, the development of pulp moisture content (MC) was measured for changes of RH between 0% RH and 30, 50 and 80% RH, respectively. Equilibrium moisture content was recorded after 12 h. The deviation from EMC (equilibrium moisture content), i.e. the progress in equilibration from 0% RH to 30, 50 and 80% RH, is plotted in Fig. 2. Table 1 shows the time needed to reach equilibrium moisture content as well as the percentage of EMC reached after 120, 240 and 480 min. From the results it can be seen that after 120 min more than 99.77% of the water adsorption is completed, after 480 min 99.98% of the adsorption has taken place.
In general, the time to reach equilibrium moisture content depends on the initial MC of the pulp, the final level of relative humidity and time needed for mass transport within the fibres. For the measurements performed within this study, the pulps started bone dry (equilibrated at 0% RH) and were subsequently conditioned and equilibrated to final levels of 30, 50 and 80% RH. Eventhough the pulp sample was continuously flushed with air throughout measurement process, one can assume that the sorption process is somewhat slower for the bulk pulp sample during the DVS measurement than for the individual fibre in the bond tester. Thus the equilibration time for the fibres and fibre joints in case of this study is probably faster than the ones obtained in the DVS measurements. According to the results obtained from the DVS measurements it can be concluded that after 120 min certainly more than 99.77% of the sorption has taken place. Therefore, the equilibration time of 120 min was found to be sufficient for the joints and fibres to reach an equilibrium state.
Table 1 Equilibration time
Single fibre tensile testing
Figure 3 and Table 2 show the box plots and the values for the breaking load obtained for single fibre tensile testing at different levels of relative humidity. The red line in the box plots represents the median value, the box size is the inter quartile range (IQR) i.e. the range comprising 50% of the data points. The whiskers are giving the minimum and maximum values, not considering outliers (+) which are defined to be outside one IQR from the range comprised by the box. These results are all showing fibers relaxed during reconditioning.
As can be seen in case of both softwood and hardwood, there is a decreasing trend in breaking load when moving toward higher or lower RH, having somewhat of a maximum around 50% RH.
Table 2 Breaking load of softwood and hardwood fibres tested at different RH
In case of softwoods, the breaking load is around 20% smaller when the fibres were tested either in elevated or decreased humidity. This difference, however, was not statistically significant. The only significant difference was obtained with softwood fibres conditioned under restraint at 30% RH (\(\alpha =0.01\), p value = 0.0047), i.e. without stress relaxation during equilibration as described in section 2.3. In case of hardwood fibres, the same behaviour as in case of non-restrained softwoods can be observed. For fibres tested at 30 and 80% RH, a reduction in breaking load of 29 and 38% compared to 50% relative humidity can be seen, respectively, but no statistically significant difference was obtained in either of the cases. Figure 4 shows a comparison of current values for softwood with values from previous studies. The values obtained in the current study have a similar trend like the most comprehensive study performed by Kersavage (1973), who found a maximum at 60% RH and a decrease when moving towards higher or lower RH.
Nevertheless, besides the wet samples in case of Kersavage (1973), none of the values from the previous or the current study show a statistically significant difference. Additionally, the majority of the studies performed focused only on the differences in fibre behaviour in the dry and wet state which rendered most of the studies incomparable. Leopold and Thorpe (1968) have attributed the change in breaking load to the breaking of hydrogen bonds within the fibre. In case of fibres with low internal cohesion (i.e. earlywood fibres) the strength decrease is due to hydrogen bond disruption, whereas in case of latewood fibres, the breaking of hydrogen bonds improves the stress distribution across the fibre surface. Wardrop (1951) also attributed increase in breaking load of wet fibres to more uniform stress distribution. Additionally, lower breaking load values in case of dry fibres could be attributed to the possible development of strength reducing flaws during drying (Russell et al. 1964). Higher occurrence of such weak spots, in combination with tension forces during conditioning, could result in a fibre that is already damaged prior to testing. Contrary to that, Klauditz et al. (1947) and Kersavage (1973) attributed higher strength of dry fibres to a closer contact and higher cell wall cohesiveness. The increase in strength is attributed to either the increase in cell wall cohesiveness present at lower MC (Kersavage 1973; Klauditz et al. 1947 and Leopold and Thorpe 1968) or an improvement of internal stress distribution at higher MC or RH (Russell et al. 1964; Wardrop 1951; Leopold and Thorpe 1968). In conclusion, there are two hypotheses existing in the literature.
Anyhow, it is possible that both mechanisms, better stress distribution and improved cell wall cohesiveness, coexist and compete at the same time, and the nonlinearity of their behaviour results in a fibre strength maximum at around 50% RH. Additionally, there is a third factor that plays a role in the outcome of the testing, the effect of restrained testing. A free standing fibre, when conditioned from 50 to 30% RH will shrink, rotate and twist. By gluing it in one plane, all three natural behaviors are inhibited. Due to these restrictions, the fibre cannot shrink freely, inner tension develops and the fibre becomes more brittle. Due to the tension forces created by the fibre shrinkage, it is possible that cracks in the wall are initiated even without any external load. The embrittlement, in combination with possible crack initiation would account for the loss in the load bearing capacity. An amplified effect of the embrittlement present at lower RH can be seen with softwood fibres which were tested “under restraint” and where the unloading sequence was not performed. Those fibres are the only ones that exhibited statistically significantly lower values than the fibres tested at 50% RH. In case of fibres tested at 80% RH, the opposite effect takes place and the moisture absorbed from the air act as a plasticizer. Since the fibre is still fixed during moisture absorption and therefore cannot move, it is possible that some internal bonds break, cellulose chains slip, and the load bearing capacity of the softened matrix diminish. Figure 5 shows the principle of the changes the fibres undergo upon variation in the relative humidity, the embrittlement which is present with low RH, and softening present at higher RH.
Nevertheless, had the fibres been conditioned in a unrestrained state, it is very probable that somewhat different results would have been obtained.
Fibre to fibre joint testing
Figure 6 and Table 3 show breaking load values of joints tested at three different levels of RH.
Softwood joints tested at 30 and 80% RH show lower values when compared to values obtained at 50% RH. Those tested at 30% RH show a 35% reduction in breaking load whereas bonds tested at 80% show a 52% reduction when compared to 50% RH. On the contrary, hardwoods seem to exhibit little or no change in the breaking load when exposed to a range of RH values. According to a t test, no statistically significant difference between bonds tested at 30 and 50% RH could be obtained in neither of the cases. However, there is a significant difference in case of softwood joints tested at 80% RH (\(\alpha = 0.05\), p value = 0.023).
Table 3 Breaking load of fibre to fibre joints tested at different RH
The reduction in breaking loads of joints tested at elevated or decreased RH is attributed to the increase of the dried-in stresses at 30% RH and to the softening of the fibres at 80% RH. Figure 7 illustrates the response of the joints towards changes in RH.
The drop in fibre–fibre joint strength at 30% RH can be attributed to an increase of stresses in the fibre–fibre bond. During drying of fibre–fibre bonds internal stresses are building in the bonding region (Sirviö 2008), because the fibres are shrinking laterally much more than longitudinally. These internal stresses are indicated by the arrows in Fig. 7. Same as in the case of individual fibres, the joints were also fixed in one plane and could therefore not swell or shrink freely. The dried-in stresses increase due to the shrinkage of the fibres in the cross direction, while the shrinkage in the longitudinal direction imposes additional tensile stress on the joint, which is indicated by the dashed red lines in Fig. 7. As stated earlier, it was not possible to perform the unloading procedure in case of joints, but the fact that they did not break gives an indication that the embrittlement evident in individual fibres was avoided through compensation of the crossing fibre bending.
In case of fibres tested at 80% RH, the strength decrease is attributed to the decrease in the dried in stresses and partial failure of the bond due to swelling. As the moisture gets absorbed, the fibres swell and soften which leads to partial failure of the fibre–fibre bond due to the reduced E-modulus of the fibres (Hirn and Schennach 2017).
So for the fibre–fibre bonds there are two mechanisms taking place when relative humidity changes: increase in internal stresses upon drying and weakening of the fibre–fibre bonds during moisturing. It seems that around 50% RH there is an optimum between these mechanisms leading to highest strength. Results similar to softwood kraft pulp, i.e. lower fibre–fibre breaking loads at 30 and 80% RH compared to 50% RH, have also been observed for softwood sulphite pulp (Jajcinovic 2017). In earlier work the fibre–fibre joints have been tested either fully wet or in laboratory conditions (Schniewind et al. 1964 and Russell et al. 1964) and the authors are not aware of experiments on fibre to fibre bond strength at varying relative humidity. Thus, according to this initial work in this field, the authors see a tendency that fibre to fibre joint strength is lower at 30 and 80% RH compared to 50% RH. Nevertheless, same as in the case of individual fibres, it is quite possible that different results would be obtained if the joints had been conditioned in a unrestrained state.
It must be mentioned that differences in the cooking-, bleaching- or drying process have a large impact on the water sorption of pulp fibers at different levels of relative humidity (Leuk et al. 2016). It can be expected that this will also affect the strength of the fibres and fibre–fibre joints.