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C.V.G. Esteves: Oxygen deligni fi ed fi bers as a greener alternative for tissue paper
the reference unre fi ned pulp was not possible to determine due to the lack of wet strength since no wet strength addi- tives were used in this study. The reference fully bleached pulp after 2000 and 4000 PFI revolutions did not present an increase in the wet tensile strength (Figure 8A). The unbleached pulps, on the other hand, had an almost linear increase of the wet tensile index with the re fi ning, obtaining between 200 and 400 % higher wet tensile than the reference fully bleached pulp for similar bulk values. When the wet tensile index is seen as a function of the water retention value, the relation is almost linear for the unbleached pulps (Figure 8B). The higher the pulp swelling the higher the wet tensile index. Figure 8C shows similar behaviour as seen in Figure 8A, since bulk and absorption capacity have a linear relationship. For similar absorption capacity (around 7 g/g), the pulps with the higher kappa number (K109_O47 and K109_OO35) present the highest wet strength (2.5 Nm/g). The increase in fi ber fi brillation and fi ber bonding, that is expected from re fi ning, seems to vanish when the paper is wetted for the commercial fully bleached pulp. The unre fi ned unbleached pulps presented wet tensile strength in the same range as the commercial fully bleached pulp when it was re fi ned with 2,000 and 4,000 revolutions, with much higher bulk. The higher wet strength seen for the unbleached pulps shows a great potential for towel papers, for example, where they can still have a high absorption capacity and higher strength. When the strength values are plotted as a function of the kappa number the relation is clear: higher lignin con- tent in the pulps leads to higher wet tensile strength (Figure 9). Some old laboratory studies have also shown a connection between high lignin content and increased wet strength (Gunnarsson 2012). Figure 9B shows the fairly linear relation of the pulps ’ relative strength and lignin content. These results show the potential of using pulps with high lignin content to increase the wet strength of the tissue products. The equilibrium between wet strength and ab- sorption capacity of unbleached tissue products can be altered by the oxygen deligni fi cation conditions and by the starting kappa number of the pulps.
Wet tensile strength is one of the important properties of tissue products since they should be resistant and maintain their structure when wetted. Figure 8 presents the wet tensile index for the oxygen deligni fi ed pulps and the reference fully bleached pulp. The wet tensile index for
3.6 Softness
Softness can be described as the human perception of something that is bulky and delicate with no sharp edges. However, it is also a quite complex phenomenon to evaluate and quantify. Softness depends on several factors, such as
Figure8: Wet tensile index for the reference and oxygen deligni fi ed pulps as a function of A) bulk, B) water retention value and C) absorption capacity. (20 g/m 2 laboratory tissue handsheets were used).
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