Absorbent Materials from Kraft Pulp

Doctoral thesis, 2013

Superabsorbent polymers (SAP) have, since being introduced in the 1980s, become an important constituent in hygiene products, such as disposable diapers. The design of disposable diapers has changed over the years and today thinner products can be produced partly because of the excellent absorption and retention properties of the petrochemical-based SAP. The possibilities of producing alternative absorbent materials from a renewable resource, kraft pulp, are investigated in this thesis. In this work, a four step process was developed using kraft pulp from either softwood or hardwood as the raw material. In the process, kraft pulp fibers were treated with TEMPO oxidation followed by washing, disintegration of the fibers into microfibrillar cellulose (MFC), and porous absorbent foam was then obtained by freeze drying the MFC. Process conditions were evaluated to optimize the properties of the obtained foams. In an attempt to improve the mechanical stability of the foam, un-modified fibres were added before the drying step, thus forming a cellulosic composite material. Tests of absorption properties showed that the absorbent foams had a more rapid absorption rate, similar absorption capacity and about half the retention capacity as compared to commercial SAP. In the foam, wet integrity was provided by the aldehyde groups introduced by TEMPO oxidation possibly through the formation of hemiacetal bonds. A high dosage of primary oxidant and the use of wood from birch as the raw material were the most favourable for maximizing liquid retention in the foams. An increase in the oxidation temperature from 25˚C to 50˚C decreased the reaction time, reduced the energy demand to disintegrate the fibers, while the retention capacity remained more or less unchanged. Washing experiments showed that in comparison with washing reference pulp, oxidized pulps had a higher specific filtration resistance and thus a larger filter area or longer hold-up time was needed for oxidized pulps. The addition of CTMP fibres increased the liquid retention capacity of the TEMPO-oxidized MFC. At fibre contents around 85%, the retention capacities of the TEMPO-oxidized MFC were comparable with the capacity of commercial SAP. The preparation of foam on the kilogram-scale showed that a similar morphology to gram-scale experiments was not achieved and consequently liquid retention was lower for kilogram-scale foams.