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C.V.G. Esteves: Oxygen deligni fi ed fi bers as a greener alternative for tissue paper
water will rise. Micropores are present in native fi bers, but during pulping the lignin – hemicellulose matrix is gradually removed creating some pores in the cell wall, called mac- ropores (Brännvall et al. 2021). The micro and macropores will in fl uence the water absorption capacity of the fi bers, once they in fl uence the fi ber capillary pressure (Beuther et al. 2010). Bigger pores tend to decrease the capillary forces and the holding capacity (Joutsimo and Asikainen 2013), while smaller pores are capable of retaining more liquid at higher pressures than larger pores due to greater capillary force (Brodin and Theliander 2012). When fi bers undergo re fi ning, the capillary pressure tends to decrease, probably due to internal and external fi brillation. As fi ber interactions get stronger, the sheet thickness is reduced by the increase in sheet densi fi cation, and consequently, the open voids available in the sheet in which water can be held will be reduced (Kullander 2012). The decrease in the capillary pressure with the increase in fi brillation was suggested before (Morais et al. 2021). The higher the bulk, the higher the volume of pores inside the paper network. Figure 6 shows the di ff erent speeds in the capillary rise for the studied pulps. For unre fi ned pulps, the oxygen deligni fi ed K29_O10 and K61_O25 had the fastest water rise (Figure 6A). The pulps with slower water rising were the oxygen deligni fi ed pulps with higher kappa numbers, K109_O47 and K109_OO35. The faster capillarity rise seen for the K61_O and K61_OO is probably due to the more open structure of the paper network since both pulp presented the highest bulk.
value (Hartman 1985; Kang 2007), but simultaneously, the increase in fi brillation also leads to better fi ber bonding which promotes a denser paper structure, leading to a lower absorption values (Lumiainen 2000). Figure 5 shows the capillary rise behavior of the di ff erent pulps obtained by the Klemm method. Similar to the results in Figure 4, the pulps K61_O25 and K61_OO18 presented the most similar results when compared to the reference fully bleached pulp. Capillary rise depends on the cohesion and adhesion forces of the water molecules that will rise along the fi ber walls or within the pores (Morais and Curto 2022). The size of those pores will de fi ne the capillary pressure inside the fi bers and the fi ber network, determining how fast the Figure 5: Water capillarity rise distance for unbleached oxygen deligni fi ed pulps and fully bleached pulp as a function of bulk (20 g/m 2 laboratory tissue handsheets were used). Decrease in bulk for each pulp sample is obtained by increased re fi ning.
Figure6: Rate of water capillarity rise for unbleached oxygen deligni fi ed pulps and fully bleached pulp for A) unre fi ned and B) 4000 PFI-re fi nedpulps. (20g/m 2 laboratory tissue handsheets were used).
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