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C.V.G. Esteves: Oxygen deligni fi ed fi bers as a greener alternative for tissue paper
Figure3: Water retention value for unbleached oxygen deligni fi ed pulps and fully bleached pulp as a function of A) PFI-re fi ning, B) kappa number and C) total fi ber charge.
the pulps, however, it might impact the paper drying section, leading to higher energy requirements. However, the in- crease in the water retention value also leads to a decrease in the re fi ning required to a certain tensile strength, which in turn will lead to lower energy consumption.
3.4 Absorption and capillarity properties
To evaluate the absorption and capillarity properties of the studied pulps, the basket method and Klemm test were used, respectively. The absorption capacity can be de fi ned as a measurement of the fi ber ability to absorb water (g water/g fi ber) until it gets saturated (Schuchard and Berg 1991). The absorption quanti fi cation also considers the fi ber capacity of holding water, since there is a dewatering period before measuring the wet weight. The absorption capacity for the analyzed samples is presented in Figure 4. As expected, the absorption capacity is directly related to the paper bulk despite the large span in kappa number, ranging from 0 (REF) to 47. The commercial fully bleached pulp and the pulps K61_O and K61_OO presented very similar results for the absorp- tion capacity (Figure 4) while the oxygen deligni fi ed pulps with the highest kappa number (K109_O and K109_OO) and the lowest (K29_O10) after the cooking step presented lower absorption capacity. Absorbency can be signi fi cantly increased through the increase in the product bulk (Sedin and Vomho ff 2017). Bulk is therefore an important property of tissue paper, as it regulates the paper absorption, paper network porosity and softness sensation. The oxygen deligni fi ed pulps (K61_O25 and K61_OO18) presented
similar absorption capacity as the reference pulp with slightly higher bulk. The oxygen deligni fi ed pulps K109_O47, K109_OO35 and K29_O10 presented a much lower bulk when compared to the other pulps and, consequently lower absorption capacity. For the unre- fi ned samples, K109_O47 and K29_O10, the bulk and ab- sorption capacity are very similar, even though the lignin content is signi fi cantly di ff erent. Contrary to what happens with the water retention value (Figure 3A), when re fi ning is applied the water ab- sorption capacity of the pulps decreases – Figure 4. While the water retention value is mainly related to the water ab- sorption in the fi ber wall, the absorption capacity is related to the water absorption within the fi ber network. The external and internal fi brillation created by the re fi ning leads to a greater swelling potential by increasing the water penetration between the fi brils seen as water retention Figure 4: Absorption capacity 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.
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