Supramolecular structure assessment of alkali modified eucalyptus kraft pulp for gentle hygienic fluff pulp applications

In this paper, the effect of elemental chlorine free fully bleached eucalyptus kraft pulp alkali treatment with 25% w/v of sodium hydroxide (NaOH) solution on the absorption behavior of the resultant air-laid absorbent pads is investigated. The results showed that after an hour of alkali treatment at room temperature, fiber charge (based on carboxyl group content) decreased to more than half. As a result, eucalyptus fibers have lost some of their hydrophilicity, as shown by the contact angle and water retention value (WRV) measurements, where the contact angle increased and the WRV decreased. Parallel cellulose I is converted into antiparallel and thermodynamically stable cellulose II by removing hemicellulose from inter-fibrillar spaces and fibrils' surfaces. In line with these molecular changes, fibril aggregation occurred. This was confirmed by the increase in hydrogen bond intensity and decrease in total crystallinity index calculated from ATR-FTIR data like the wrinkled surface of modified eucalyptus fiber from FESEM images. Also, fiber biometry measurements revealed a decrease in weighted average length and linear density making the air-laid absorbent pad thinner. A strike-through time decrease for artificial urine was achieved by improving the wet resiliency of fiber networks after alkali treatment. Essentially, after wetting, structural stability, and preserved porosity compensated for hydrophilicity loss, which played a crucial role in the performance of an air-laid fiber network.