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C.V.G. Esteves: Oxygen deligni fi ed fi bers as a greener alternative for tissue paper
leading to better fi ber bonding (Gharehkhani et al. 2015; Kullander 2012; Maloney and Paulapuro 1999). However, for the fully bleached pulps, the increase in the tensile index seems to be more in fl uenced by the increase of sheet densi fi cation due to re fi ning, rather than the in- crease in the fi ber bonding strength. Oxygen deligni fi ed pulps presented higher strength for similar bulk compared to the reference pulp. This suggests that a less bonded area is needed to accomplish a certain strength. Nordström (2014) and Esteves et al. (2021b) have previ- ously reported similar results, where pulps with higher lignin content obtained a higher tensile index to a given low density. From Figure 7B, it is interesting to highlight that the oxygen deligni fi ed pulps have a signi fi cantly higher strength for similar absorption capacity. As seen for the other properties, the samples K61_O25 and K61_OO18 pre- sented the best strength and absorption performance, as shown in Figure 4.
Whenre fi ning is applied, the fastest water rise is for the commercial fully bleached pulp, while the oxygen deligni fi ed pulps land in lower water rising, especially for the K29_O10.
3.5 Mechanical properties
3.5.1 Dry tensile properties
The reference fully bleached pulp presented a much lower tensile index than the unbleached oxygen deligni fi ed pulps – Figure 7A. The di ff erence in the tensile index is even more signi fi cant for unre fi ned pulps. This is caused by the di ff erent chemistry of the fully bleached and unbleached fi bers and the harsher industrial treatment that the fully bleached pulp was subjected to. The oxygen deligni fi ed pulps obtained a higher tensile index. All the unre fi ned oxygen deligni fi ed pulps, ob- tained similar tensile index (between 74 and 83 Nm/g) and higher bulk (1.55 – 1.45cm 3 /g) when compared to the reference fully bleached pulp re fi ned with 2000 PFI-revolutions (72 Nm/g and80g/cm 3 ). For higher re fi ning levels, the unbleached pulps continued to present higher tensile index values than the reference pulp. The results showed that oxygen deligni fi cation pulps obtain similar absorption properties with much higher tensile strength than fully bleached pulps, with lower re fi ning energy required. The increase in paper strength was surely achieved by the increase in internal and external fi brillation in the re fi ning step. Re fi ning breaks some inner bonds in the fi bers wall, making them more fl exible and swellable,
3.5.2 Wet tensile properties
Cellulose-based materials such as tissue products are highly sensitive to humidity since cellulose molecules hold a large number of hydroxyl groups. Paper is, therefore, a hydrophilic material; when in contact with water or moisture, it starts to swell, breaking down the hydrogen bonds and losing its structure. Therefore, the wet strength becomes critical for tissue papers exposed to water and moisture.
Figure 7: Tensile index for the reference and oxygen deligni fi ed pulps as a function of A) bulk (60 g/m 2 laboratory tissue handsheets were used) and B) absorption capacity (20 g/m 2 laboratory tissue handsheets were used). The increase in tensile strength for each pulp sample is obtained by increased re fi ning.
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